# Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting
Mendel
Cooper

    thegrendel.abs@gmail.com

10
10 Mar 2014
978-1-4357-5219-1
6.5
05 Apr 2012
mc
'TUNGSTENBERRY' release
6.6
27 Nov 2012
mc
'YTTERBIUMBERRY' release
10
10 Mar 2014
mc
'PUBLICDOMAIN' release

This tutorial assumes no previous knowledge of scripting or programming, yet progresses rapidly toward an intermediate/advanced level of instruction . . . all the while sneaking in little nuggets of UNIX wisdom and lore. It serves as a textbook, a manual for self-study, and as a reference and source of knowledge on shell scripting techniques. The exercises and heavily-commented examples invite active reader participation, under the premise that `the only way to really learn scripting is to write scripts`.

This book is suitable for classroom use as a general introduction to programming concepts.

This document is herewith granted to the Public Domain. `No copyright!`

For Anita, the source of all the magic

## Introduction

Script: A writing; a written document. [Obs.]

--Webster's Dictionary, 1913 ed.

The shell is a command interpreter. More than just the insulating layer between the operating system kernel and the user, it's also a fairly powerful programming language. A shell program, called a script, is an easy-to-use tool for building applications by gluing together system calls, tools, utilities, and compiled binaries. Virtually the entire repertoire of UNIX commands, utilities, and tools is available for invocation by a shell script. If that were not enough, internal shell commands, such as testing and loop constructs, lend additional power and flexibility to scripts. Shell scripts are especially well suited for administrative system tasks and other routine repetitive tasks not requiring the bells and whistles of a full-blown tightly structured programming language.

### Shell Programming!

No programming language is perfect. There is not even a single best language; there are only languages well suited or perhaps poorly suited for particular purposes.

--Herbert Mayer

A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient at system administration, even if they do not anticipate ever having to actually write a script. Consider that as a Linux machine boots up, it executes the shell scripts in `/etc/rc.d` to restore the system configuration and set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a system, and possibly modifying it.

The craft of scripting is not hard to master, since scripts can be built in bite-sized sections and there is only a fairly small set of shell-specific operators and options

These are referred to as builtins, features internal to the shell.

to learn. The syntax is simple -- even austere -- similar to that of invoking and chaining together utilities at the command line, and there are only a few rules governing their use. Most short scripts work right the first time, and debugging even the longer ones is straightforward.

>         In the early days of personal computing, the BASIC language enabled
>         anyone reasonably computer proficient to write programs on an early
>         generation of microcomputers. Decades later, the Bash scripting
>         language enables anyone with a rudimentary knowledge of Linux or
>         UNIX to do the same on modern machines.

>         We now have miniaturized single-board computers with amazing
>         capabilities, such as the Raspberry Pi.
>         Bash scripting provides a way to explore the capabilities of these
>         fascinating devices.

A shell script is a quick-and-dirty method of prototyping a complex application. Getting even a limited subset of the functionality to work in a script is often a useful first stage in project development. In this way, the structure of the application can be tested and tinkered with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, Perl, or Python.

Shell scripting hearkens back to the classic UNIX philosophy of breaking complex projects into simpler subtasks, of chaining together components and utilities. Many consider this a better, or at least more esthetically pleasing approach to problem solving than using one of the new generation of high-powered all-in-one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to alter your thinking processes to fit the tool.

According to Herbert Mayer, a useful language needs arrays, pointers, and a generic mechanism for building data structures. By these criteria, shell scripting falls somewhat short of being useful. Or, perhaps not. . . .

When not to use shell scripts

- Resource-intensive tasks, especially where speed is a factor (sorting, hashing, recursion

  Although recursion is possible in a shell script, it tends to be slow and its implementation is often an ugly kludge.

  ...)

- Procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ or FORTRAN instead)

- Cross-platform portability required (use C or Java instead)

- Complex applications, where structured programming is a necessity (type-checking of variables, function prototypes, etc.)

- Mission-critical applications upon which you are betting the future of the company

- Situations where security is important, where you need to guarantee the integrity of your system and protect against intrusion, cracking, and vandalism

- Project consists of subcomponents with interlocking dependencies

- Extensive file operations required (Bash is limited to serial file access, and that only in a particularly clumsy and inefficient line-by-line fashion.)

- Need native support for multi-dimensional arrays

- Need data structures, such as linked lists or trees

- Need to generate / manipulate graphics or GUIs

- Need direct access to system hardware or external peripherals

- Need port or socket I/O

- Need to use libraries or interface with legacy code

- Proprietary, closed-source applications (Shell scripts put the source code right out in the open for all the world to see.)

If any of the above applies, consider a more powerful scripting language -- perhaps Perl, Tcl, Python, Ruby -- or possibly a compiled language such as C, C++, or Java. Even then, prototyping the application as a shell script might still be a useful development step.

We will be using Bash, an acronym

An acronym is an ersatz word formed by pasting together the initial letters of the words into a tongue-tripping phrase. This morally corrupt and pernicious practice deserves appropriately severe punishment. Public flogging suggests itself.

for Bourne-Again shell and a pun on Stephen Bourne's now classic Bourne shell. Bash has become a de facto standard for shell scripting on most flavors of UNIX. Most of the principles this book covers apply equally well to scripting with other shells, such as the Korn Shell, from which Bash derives some of its features,

Many of the features of ksh88, and even a few from the updated ksh93 have been merged into Bash.

and the C Shell and its variants. (Note that C Shell programming is not recommended due to certain inherent problems, as pointed out in an October, 1993 Usenet post by Tom Christiansen.)

What follows is a tutorial on shell scripting. It relies heavily on examples to illustrate various features of the shell. The example scripts work -- they've been tested, insofar as possible -- and some of them are even useful in real life. The reader can play with the actual working code of the examples in the source archive (`scriptname.sh` or `scriptname.bash`),

By convention, user-written shell scripts that are Bourne shell compliant generally take a name with a `.sh` extension. System scripts, such as those found in `/etc/rc.d`, do not necessarily conform to this nomenclature.

give them execute permission (`chmod u+rx scriptname`), then run them to see what happens. Should the source archive not be available, then cut-and-paste from the HTML or pdf rendered versions. Be aware that some of the scripts presented here introduce features before they are explained, and this may require the reader to temporarily skip ahead for enlightenment.

Unless otherwise noted, the author of this book wrote the example scripts that follow.

His countenance was bold and bashed not.

--Edmund Spenser

### Starting Off With a Sha-Bang

Shell programming is a 1950s juke box . . .

--Larry Wall

In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least, this saves the effort of retyping that particular sequence of commands each time it is invoked.

#### cleanup: A script to clean up log files in /var/log

    &ex1;

There is nothing unusual here, only a set of commands that could just as easily have been invoked one by one from the command-line on the console or in a terminal window. The advantages of placing the commands in a script go far beyond not having to retype them time and again. The script becomes a program -- a tool -- and it can easily be modified or customized for a particular application.

#### cleanup: An improved clean-up script

    &ex1a;

Now that's beginning to look like a real script. But we can go even farther . . .

#### cleanup: An enhanced and generalized version of above scripts.

    &ex2;

Since you may not wish to wipe out the entire system log, this version of the script keeps the last section of the message log intact. You will constantly discover ways of fine-tuning previously written scripts for increased effectiveness.

* * *

The
sha-bang (
#!) More commonly seen in the literature as she-bang or sh-bang. This derives from the concatenation of the tokens sharp (#) and bang (!). at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two-byte Some flavors of UNIX (those based on 4.2 BSD) allegedly take a four-byte magic number, requiring a blank after the ! -- `#! /bin/sh`. According to Sven Mascheck this is probably a myth.
magic number, a special marker that designates a file type, or in this case an executable shell script (type `man magic` for more details on this fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (the line following the sha-bang line), and ignoring comments. The #! line in a shell script will be the first thing the command interpreter (`sh` or `bash`) sees. Since this line begins with a #, it will be correctly interpreted as a comment when the command interpreter finally executes the script. The line has already served its purpose - calling the command interpreter. If, in fact, the script includes an extra #! line, then `bash` will interpret it as a comment.

    #!/bin/bash

    echo "Part 1 of script."
    a=1

    #!/bin/bash
    # This does *not* launch a new script.

    echo "Part 2 of script."
    echo $a  # Value of $a stays at 1.

    #!/bin/sh
    #!/bin/bash
    #!/usr/bin/perl
    #!/usr/bin/tcl
    #!/bin/sed -f
    #!/bin/awk -f

Each of the above script header lines calls a different command interpreter, be it `/bin/sh`, the default shell (`bash` in a Linux system) or otherwise.

This allows some cute tricks.

    #!/bin/rm
    # Self-deleting script.

    # Nothing much seems to happen when you run this... except that the file disappears.

    WHATEVER=85

    echo "This line will never print (betcha!)."

    exit $WHATEVER  # Doesn't matter. The script will not exit here.
                    # Try an echo $? after script termination.
                    # You'll get a 0, not a 85.

Also, try starting a `README` file with a `#!/bin/more`, and making it executable. The result is a self-listing documentation file. (A here document using cat is possibly a better alternative -- see ).

Using `#!/bin/sh`, the default Bourne shell in most commercial variants of UNIX, makes the script portable to non-Linux machines, though you sacrifice Bash-specific features. The script will, however, conform to the POSIX

Portable Operating System Interface, an attempt to standardize UNIX-like OSes. The POSIX specifications are listed on the Open Group site.

`sh` standard.

Note that the path given at the sha-bang must be correct, otherwise an error message -- usually Command not found. -- will be the only result of running the script. To avoid this possibility, a script may begin with a #!/bin/env bash sha-bang line. This may be useful on UNIX machines where bash is not located in `/bin`

#! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. The second example, above, requires the initial #!, since the variable assignment line, `lines=50`, uses a shell-specific construct. If Bash is your default shell, then the #! isn't necessary at the beginning of a script. However, if launching a script from a different shell, such as tcsh, then you will need the #!. Note again that `#!/bin/sh` invokes the default shell interpreter, which defaults to `/bin/bash` on a Linux machine.

This tutorial encourages a modular approach to constructing a script. Make note of and collect boilerplate code snippets that might be useful in future scripts. Eventually you will build quite an extensive library of nifty routines. As an example, the following script prolog tests whether the script has been invoked with the correct number of parameters.

    E_WRONG_ARGS=85
    script_parameters="-a -h -m -z"
    #                  -a = all, -h = help, etc.

    if [ $# -ne $Number_of_expected_args ]
    then
      echo "Usage: `basename $0` $script_parameters"
      # `basename $0` is the script's filename.
      exit $E_WRONG_ARGS
    fi

Many times, you will write a script that carries out one particular task. The first script in this chapter is an example. Later, it might occur to you to generalize the script to do other, similar tasks. Replacing the literal (hard-wired) constants by variables is a step in that direction, as is replacing repetitive code blocks by functions.

#### Invoking the script

Having written the script, you can invoke it by `sh scriptname`,

Caution: invoking a Bash script by `sh scriptname` turns off Bash-specific extensions, and the script may therefore fail to execute.

or alternatively `bash scriptname`. (Not recommended is using `sh <scriptname`, since this effectively disables reading from `stdin` within the script.) Much more convenient is to make the script itself directly executable with a chmod.

Either:

  `chmod 555 scriptname` (gives everyone read/execute permission) A script needs read, as well as execute permission for it to run, since the shell needs to be able to read it.

or

  `chmod +rx scriptname` (gives everyone read/execute permission)

  `chmod u+rx scriptname` (gives only the script owner read/execute permission)

Having made the script executable, you may now test it by `./scriptname`.

Why not simply invoke the script with `scriptname`? If the directory you are in ($PWD) is where `scriptname` is located, why doesn't this work? This fails because, for security reasons, the current directory (`./`) is not by default included in a user's $PATH. It is therefore necessary to explicitly invoke the script in the current directory with a `./scriptname`.

If it begins with a sha-bang line, invoking the script calls the correct command interpreter to run it.

As a final step, after testing and debugging, you would likely want to move it to `/usr/local/bin` (as root, of course), to make the script available to yourself and all other users as a systemwide executable. The script could then be invoked by simply typing `scriptname` [ENTER] from the command-line.

#### Preliminary Exercises

1. System administrators often write scripts to automate common tasks. Give several instances where such scripts would be useful.

2. Write a script that upon invocation shows the time and date, lists all logged-in users, and gives the system uptime. The script then saves this information to a logfile.

## Basics

### Special Characters

What makes a character special? If it has a meaning beyond its literal meaning, a meta-meaning, then we refer to it as a special character. Along with commands and keywords, special characters are building blocks of Bash scripts.

#### Special Characters Found In Scripts and Elsewhere

#

  #### Comments

  Lines beginning with a # (with the exception of #!) are comments and will not be executed.

      # This line is a comment.

  Comments may also occur following the end of a command.

      echo "A comment will follow." # Comment here.
      #                            ^ Note whitespace before #

  Comments may also follow whitespace at the beginning of a line.

           # A tab precedes this comment.

  Comments may even be embedded within a pipe.

      initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\
      # Delete lines containing '#' comment character.
                 sed -e 's/\./\. /g' -e 's/_/_ /g'` )
      # Excerpted from life.sh script

  A command may not follow a comment on the same line. There is no method of terminating the comment, in order for live code to begin on the same line. Use a new line for the next command.

  Of course, a quoted or an escaped # in an echo statement does not begin a comment. Likewise, a # appears in certain parameter-substitution constructs and in numerical constant expressions.

      echo "The # here does not begin a comment."
      echo 'The # here does not begin a comment.'
      echo The \# here does not begin a comment.
      echo The # here begins a comment.

      echo ${PATH#*:}       # Parameter substitution, not a comment.
      echo $(( 2#101011 ))  # Base conversion, not a comment.

      # Thanks, S.C.

  The standard quoting and escape characters (" ' \) escape the #.

  Certain pattern matching operations also use the #.

;

  #### Command separator [semicolon]

  Permits putting two or more commands on the same line.

      echo hello; echo there

      if [ -x "$filename" ]; then    #  Note the space after the semicolon.
      #+                   ^^
        echo "File $filename exists."; cp $filename $filename.bak
      else   #                       ^^
        echo "File $filename not found."; touch $filename
      fi; echo "File test complete."

  Note that the ; sometimes needs to be escaped.

;;

  #### Terminator in a case option [double semicolon]

      case "$variable" in
        abc)  echo "\$variable = abc" ;;
        xyz)  echo "\$variable = xyz" ;;
      esac

;;&
;&

  #### Terminators in a case option (version 4+ of Bash).

.

  #### dot command [period]

  Equivalent to source (see ). This is a bash builtin.

.

  #### dot, as a component of a filename

  When working with filenames, a leading dot is the prefix of a hidden file, a file that an ls will not normally show.

      bash$ touch .hidden-file
      bash$ ls -l
      total 10
       -rw-r--r--    1 bozo      4034 Jul 18 22:04 data1.addressbook
       -rw-r--r--    1 bozo      4602 May 25 13:58 data1.addressbook.bak
       -rw-r--r--    1 bozo       877 Dec 17  2000 employment.addressbook

      bash$ ls -al
      total 14
       drwxrwxr-x    2 bozo  bozo      1024 Aug 29 20:54 ./
       drwx------   52 bozo  bozo      3072 Aug 29 20:51 ../
       -rw-r--r--    1 bozo  bozo      4034 Jul 18 22:04 data1.addressbook
       -rw-r--r--    1 bozo  bozo      4602 May 25 13:58 data1.addressbook.bak
       -rw-r--r--    1 bozo  bozo       877 Dec 17  2000 employment.addressbook
       -rw-rw-r--    1 bozo  bozo         0 Aug 29 20:54 .hidden-file

  When considering directory names, a single dot represents the current working directory, and two dots denote the parent directory.

      bash$ pwd
      /home/bozo/projects

      bash$ cd .
      bash$ pwd
      /home/bozo/projects

      bash$ cd ..
      bash$ pwd
      /home/bozo/

  The dot often appears as the destination (directory) of a file movement command, in this context meaning current directory.

      bash$ cp /home/bozo/current_work/junk/* .

  Copy all the junk files to $PWD.

.

  #### dot character match

  When matching characters, as part of a regular expression, a dot matches a single character.

"

  #### partial quoting [double quote]

  "STRING" preserves (from interpretation) most of the special characters within STRING. See .

'

  #### full quoting [single quote]

  'STRING' preserves all special characters within STRING. This is a stronger form of quoting than "STRING". See .

,

  #### comma operator

  The comma operator

  An operator is an agent that carries out an operation. Some examples are the common arithmetic operators, `+ - * /`. In Bash, there is some overlap between the concepts of operator and keyword.

  links together a series of arithmetic operations. All are evaluated, but only the last one is returned.

      let "t2 = ((a = 9, 15 / 3))"
      # Set "a = 9" and "t2 = 15 / 3"

  The comma operator can also concatenate strings.

      for file in /{,usr/}bin/*calc
      #             ^    Find all executable files ending in "calc"
      #+                 in /bin and /usr/bin directories.
      do
              if [ -x "$file" ]
              then
                echo $file
              fi
      done

      # /bin/ipcalc
      # /usr/bin/kcalc
      # /usr/bin/oidcalc
      # /usr/bin/oocalc

      # Thank you, Rory Winston, for pointing this out.

,
,

  #### Lowercase conversion in parameter substitution (added in version 4 of Bash)

\

  #### escape [backslash]

  A quoting mechanism for single characters.

  `\X` escapes the character X. This has the effect of quoting X, equivalent to 'X'. The \ may be used to quote " and ', so they are expressed literally.

  See for an in-depth explanation of escaped characters.

/

  #### Filename path separator [forward slash]

  Separates the components of a filename (as in `/home/bozo/projects/Makefile`).

  This is also the division arithmetic operator.

`

  #### command substitution

  The ``command`` construct makes available the output of `command` for assignment to a variable. This is also known as backquotes or backticks.

:

  #### null command [colon]

  This is the shell equivalent of a NOP (no op, a do-nothing operation). It may be considered a synonym for the shell builtin true. The : command is itself a Bash builtin, and its exit status is true (0).

      :
      echo $?   # 0

  Endless loop:

      while :
      do
         operation-1
         operation-2
         ...
         operation-n
      done

      # Same as:
      #    while true
      #    do
      #      ...
      #    done

  Placeholder in if/then test:

      if condition
      then :   # Do nothing and branch ahead
      else     # Or else ...
         take-some-action
      fi

  Provide a placeholder where a binary operation is expected, see and default parameters.

      : ${username=`whoami`}
      # ${username=`whoami`}   Gives an error without the leading :
      #                        unless "username" is a command or builtin...

      : ${1?"Usage: $0 ARGUMENT"}     # From "usage-message.sh example script.

  Provide a placeholder where a command is expected in a here document. See .

  Evaluate string of variables using parameter substitution (as in ).

      : ${HOSTNAME?} ${USER?} ${MAIL?}
      #  Prints error message
      #+ if one or more of essential environmental variables not set.

  `Variable expansion / substring replacement`.

  In combination with the > redirection operator, truncates a file to zero length, without changing its permissions. If the file did not previously exist, creates it.

      : > data.xxx   # File "data.xxx" now empty.

      # Same effect as   cat /dev/null >data.xxx
      # However, this does not fork a new process, since ":" is a builtin.

  See also .

  In combination with the >> redirection operator, has no effect on a pre-existing target file (`: >> target_file`). If the file did not previously exist, creates it.

  This applies to regular files, not pipes, symlinks, and certain special files.

  May be used to begin a comment line, although this is not recommended. Using # for a comment turns off error checking for the remainder of that line, so almost anything may appear in a comment. However, this is not the case with :.

      : This is a comment that generates an error, ( if [ $x -eq 3] ).

  The : serves as a field separator, in `/etc/passwd`, and in the $PATH variable.

      bash$ echo $PATH
      /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games

  A colon is acceptable as a function name.

      :()
      {
        echo "The name of this function is "$FUNCNAME" "
        # Why use a colon as a function name?
        # It's a way of obfuscating your code.
      }

      :

      # The name of this function is :

  This is not portable behavior, and therefore not a recommended practice. In fact, more recent releases of Bash do not permit this usage. An underscore `_` works, though.

  A colon can serve as a placeholder in an otherwise empty function.

      not_empty ()
      {
        :
      } # Contains a : (null command), and so is not empty.

!

  #### reverse (or negate) the sense of a test or exit status [bang]

  The ! operator inverts the exit status of the command to which it is applied (see ). It also inverts the meaning of a test operator. This can, for example, change the sense of equal ( = ) to not-equal ( != ). The ! operator is a Bash keyword.

  In a different context, the ! also appears in indirect variable references.

  In yet another context, from the command line, the ! invokes the Bash history mechanism (see ). Note that within a script, the history mechanism is disabled.

*

  #### wild card [asterisk]

  The * character serves as a wild card for filename expansion in globbing. By itself, it matches every filename in a given directory.

      bash$ echo *
      abs-book.sgml add-drive.sh agram.sh alias.sh

  The * also represents any number (or zero) characters in a regular expression.

*

  #### arithmetic operator

  In the context of arithmetic operations, the * denotes multiplication.

  ** A double asterisk can represent the exponentiation operator or extended file-match globbing.

?

  #### test operator

  Within certain expressions, the ? indicates a test for a condition.

  In a double-parentheses construct, the ? can serve as an element of a C-style trinary operator. This is more commonly known as the ternary operator. Unfortunately, ternary is an ugly word. It doesn't roll off the tongue, and it doesn't elucidate. It obfuscates. Trinary is by far the more elegant usage.

  `condition``?``result-if-true``:``result-if-false`

      (( var0 = var1<98?9:21 ))
      #                ^ ^

      # if [ "$var1" -lt 98 ]
      # then
      #   var0=9
      # else
      #   var0=21
      # fi

  In a parameter substitution expression, the ? tests whether a variable has been set.

?

  #### wild card

  The ? character serves as a single-character wild card for filename expansion in globbing, as well as representing one character in an extended regular expression.

$

  #### Variable substitution (contents of a variable)

      var1=5
      var2=23skidoo

      echo $var1     # 5
      echo $var2     # 23skidoo

  <anchor id="varprefixref"</para>

  A $ prefixing a variable name indicates the value the variable holds.

$

  #### end-of-line

  In a regular expression, a $ addresses the end of a line of text.

${}

  #### Parameter substitution

$' ... '

  #### Quoted string expansion

  This construct expands single or multiple escaped octal or hex values into ASCII

  `A`merican `S`tandard `C`ode for `I`nformation `I`nterchange. This is a system for encoding text characters (alphabetic, numeric, and a limited set of symbols) as 7-bit numbers that can be stored and manipulated by computers. Many of the ASCII characters are represented on a standard keyboard.

  or Unicode characters.

$*
$@

  #### positional parameters

$?

  #### exit status variable

  The $? variable holds the exit status of a command, a function, or of the script itself.

$$

  #### process ID variable

  The $$ variable holds the process ID A PID, or process ID, is a number assigned to a running process. The PIDs of running processes may be viewed with a ps command. `Definition:` A process is a currently executing command (or program), sometimes referred to as a job. of the script in which it appears.

()

  #### command group

      (a=hello; echo $a)

  A listing of commands within parentheses starts a subshell.

  Variables inside parentheses, within the subshell, are not visible to the rest of the script. The parent process, the script, cannot read variables created in the child process, the subshell.

      a=123
      ( a=321; )

      echo "a = $a"   # a = 123
      # "a" within parentheses acts like a local variable.

  #### array initialization

      Array=(element1 element2 element3)

{xxx,yyy,zzz,...}

  #### Brace expansion

      echo \"{These,words,are,quoted}\"   # " prefix and suffix
      # "These" "words" "are" "quoted"

      cat {file1,file2,file3} > combined_file
      # Concatenates the files file1, file2, and file3 into combined_file.

      cp file22.{txt,backup}
      # Copies "file22.txt" to "file22.backup"

  A command may act upon a comma-separated list of file specs within braces.

  The shell does the brace expansion. The command itself acts upon the result of the expansion.

  Filename expansion (globbing) applies to the file specs between the braces.

  No spaces allowed within the braces unless the spaces are quoted or escaped.

  `echo {file1,file2}\ :{\ A," B",' C'}`

  `file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C`

{a..z}

  #### Extended Brace expansion

      echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z
      # Echoes characters between a and z.

      echo {0..3} # 0 1 2 3
      # Echoes characters between 0 and 3.

      base64_charset=( {A..Z} {a..z} {0..9} + / = )
      # Initializing an array, using extended brace expansion.
      # From vladz's "base64.sh" example script.

  The {a..z} extended brace expansion construction is a feature introduced in version 3 of Bash.

{}

  #### Block of code [curly brackets]

  Also referred to as an inline group, this construct, in effect, creates an anonymous function (a function without a name). However, unlike in a standard function, the variables inside a code block remain visible to the remainder of the script.

      bash$ { local a;
      	      a=123; }
      bash: local: can only be used in a
      function

      a=123
      { a=321; }
      echo "a = $a"   # a = 321   (value inside code block)

      # Thanks, S.C.

  The code block enclosed in braces may have I/O redirected to and from it.

  #### Code blocks and I/O redirection

      &ex8;

  #### Saving the output of a code block to a file

      &rpmcheck;

  Unlike a command group within (parentheses), as above, a code block enclosed by {braces} will not normally launch a subshell.

  Exception: a code block in braces as part of a pipe may run as a subshell.

      ls | { read firstline; read secondline; }
      #  Error. The code block in braces runs as a subshell,
      #+ so the output of "ls" cannot be passed to variables within the block.
      echo "First line is $firstline; second line is $secondline"  # Won't work.

      # Thanks, S.C.

  It is possible to iterate a code block using a non-standard for-loop.

{}

  #### placeholder for text

  Used after xargs `-i` (replace strings option). The {} double curly brackets are a placeholder for output text.

      ls . | xargs -i -t cp ./{} $1
      #            ^^         ^^

      # From "ex42.sh" (copydir.sh) example.

{} \;

  #### pathname

  Mostly used in find constructs. This is not a shell builtin.

  Definition: A pathname is a filename that includes the complete path. As an example, `/home/bozo/Notes/Thursday/schedule.txt`. This is sometimes referred to as the absolute path.

  The ; ends the `-exec` option of a `find` command sequence. It needs to be escaped to protect it from interpretation by the shell.

[ ]

  #### test

  Test expression between `[ ]`. Note that `[` is part of the shell builtin test (and a synonym for it), not a link to the external command `/usr/bin/test`.

[[ ]]

  #### test

  Test expression between [[ ]]. More flexible than the single-bracket [ ] test, this is a shell keyword.

  See the discussion on the [[ ... ]] construct.

[ ]

  #### array element

  In the context of an array, brackets set off the numbering of each element of that array.

      Array[1]=slot_1
      echo ${Array[1]}

[ ]

  #### range of characters

  As part of a regular expression, brackets delineate a range of characters to match.

$[ ... ]

  #### integer expansion

  Evaluate integer expression between $[ ].

      a=3
      b=7

      echo $[$a+$b]   # 10
      echo $[$a*$b]   # 21

  Note that this usage is deprecated, and has been replaced by the (( ... )) construct.

(( ))

  #### integer expansion

  Expand and evaluate integer expression between (( )).

  See the discussion on the (( ... )) construct.

> &> >& >> < <>

  #### redirection

  `scriptname >filename` redirects the output of `scriptname` to file `filename`. Overwrite `filename` if it already exists.

  `command &>filename` redirects both the `stdout` and the `stderr` of `command` to `filename`.

  This is useful for suppressing output when testing for a condition. For example, let us test whether a certain command exists.

      bash$ type bogus_command &>/dev/null

      bash$ echo $?
      1

  Or in a script:

      command_test () { type "$1" &>/dev/null; }
      #                                      ^

      cmd=rmdir            # Legitimate command.
      command_test $cmd; echo $?   # 0

      cmd=bogus_command    # Illegitimate command
      command_test $cmd; echo $?   # 1

  `command >&2` redirects `stdout` of `command` to `stderr`.

  `scriptname >>filename` appends the output of `scriptname` to file `filename`. If `filename` does not already exist, it is created.

  `[i]<>filename` opens file `filename` for reading and writing, and assigns file descriptor i to it. If `filename` does not exist, it is created.

  #### process substitution

  `(command)>`

  `<(command)`

  In a different context, the < and > characters act as string comparison operators.

  In yet another context, the < and > characters act as integer comparison operators. See also .

<<

  #### redirection used in a here document

<<<

  #### redirection used in a here string

<
>

  #### ASCII comparison

      veg1=carrots
      veg2=tomatoes

      if [[ "$veg1" < "$veg2" ]]
      then
        echo "Although $veg1 precede $veg2 in the dictionary,"
        echo -n "this does not necessarily imply anything "
        echo "about my culinary preferences."
      else
        echo "What kind of dictionary are you using, anyhow?"
      fi

\<
\>

  #### word boundary in a regular expression

  bash$ `grep '\<the\>' textfile`

|

  #### pipe

  Passes the output (`stdout`) of a previous command to the input (`stdin`) of the next one, or to the shell. This is a method of chaining commands together.

      echo ls -l | sh
      #  Passes the output of "echo ls -l" to the shell,
      #+ with the same result as a simple "ls -l".

      cat *.lst | sort | uniq
      # Merges and sorts all ".lst" files, then deletes duplicate lines.

  A pipe, as a classic method of interprocess communication, sends the `stdout` of one process to the `stdin` of another. In a typical case, a command, such as cat or echo, pipes a stream of data to a filter, a command that transforms its input for processing. Even as in olden times a philtre denoted a potion alleged to have magical transformative powers, so does a UNIX filter transform its target in (roughly) analogous fashion. (The coder who comes up with a love philtre that runs on a Linux machine will likely win accolades and honors.)

  `cat $filename1 $filename2 | grep $search_word`

  For an interesting note on the complexity of using UNIX pipes, see the UNIX FAQ, Part 3.

  The output of a command or commands may be piped to a script.

      #!/bin/bash
      # uppercase.sh : Changes input to uppercase.

      tr 'a-z' 'A-Z'
      #  Letter ranges must be quoted
      #+ to prevent filename generation from single-letter filenames.

      exit 0

  Now, let us pipe the output of `ls -l` to this script.

      bash$ ls -l | ./uppercase.sh
      -RW-RW-R--    1 BOZO  BOZO       109 APR  7 19:49 1.TXT
       -RW-RW-R--    1 BOZO  BOZO       109 APR 14 16:48 2.TXT
       -RW-R--R--    1 BOZO  BOZO       725 APR 20 20:56 DATA-FILE

  The `stdout` of each process in a pipe must be read as the `stdin` of the next. If this is not the case, the data stream will block, and the pipe will not behave as expected.

      cat file1 file2 | ls -l | sort
      # The output from "cat file1 file2" disappears.

  A pipe runs as a child process, and therefore cannot alter script variables.

      variable="initial_value"
      echo "new_value" | read variable
      echo "variable = $variable"     # variable = initial_value

  If one of the commands in the pipe aborts, this prematurely terminates execution of the pipe. Called a broken pipe, this condition sends a SIGPIPE signal.

>|

  #### force redirection (even if the noclobber option is set)

  This will forcibly overwrite an existing file.

||

  #### OR logical operator

  In a test construct, the || operator causes a return of 0 (success) if either of the linked test conditions is true.

&

  #### Run job in background

  A command followed by an & will run in the background.

      bash$ sleep 10 &
      [1] 850
      [1]+  Done                    sleep 10

  Within a script, commands and even loops may run in the background.

  #### Running a loop in the background

      &bgloop;

  A command run in the background within a script may cause the script to hang, waiting for a keystroke. Fortunately, there is a remedy for this.

&&

  #### AND logical operator

  In a test construct, the && operator causes a return of 0 (success) only if both the linked test conditions are true.

-

  #### option, prefix

  Option flag for a command or filter. Prefix for an operator. Prefix for a default parameter in parameter substitution.

  `COMMAND -[Option1][Option2][...]`

  `ls -al`

  `sort -dfu $filename`

      if [ $file1 -ot $file2 ]
      then #      ^
        echo "File $file1 is older than $file2."
      fi

      if [ "$a" -eq "$b" ]
      then #    ^
        echo "$a is equal to $b."
      fi

      if [ "$c" -eq 24 -a "$d" -eq 47 ]
      then #    ^              ^
        echo "$c equals 24 and $d equals 47."
      fi

      param2=${param1:-$DEFAULTVAL}
      #               ^

  `--`

  The double-dash `--` prefixes long (verbatim) options to commands.

  `sort --ignore-leading-blanks`

  Used with a Bash builtin, it means the end of options to that particular command.

  This provides a handy means of removing files whose names begin with a dash.

      bash$ ls -l
      -rw-r--r-- 1 bozo bozo 0 Nov 25 12:29 -badname

      bash$ rm -- -badname

      bash$ ls -l
      total 0

  The double-dash is also used in conjunction with set.

  `set -- $variable` (as in )

-

  #### redirection from/to `stdin` or `stdout` [dash]

      bash$ cat -
      abc
      abc

      ...

      Ctl-D

  As expected, `cat -` echoes `stdin`, in this case keyboarded user input, to `stdout`. But, does I/O redirection using `-` have real-world applications?

      (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)
      # Move entire file tree from one directory to another
      # [courtesy Alan Cox <a.cox@swansea.ac.uk>, with a minor change]

      # 1) cd /source/directory
      #    Source directory, where the files to be moved are.
      # 2) &&
      #   "And-list": if the 'cd' operation successful,
      #    then execute the next command.
      # 3) tar cf - .
      #    The 'c' option 'tar' archiving command creates a new archive,
      #    the 'f' (file) option, followed by '-' designates the target file
      #    as stdout, and do it in current directory tree ('.').
      # 4) |
      #    Piped to ...
      # 5) ( ... )
      #    a subshell
      # 6) cd /dest/directory
      #    Change to the destination directory.
      # 7) &&
      #   "And-list", as above
      # 8) tar xpvf -
      #    Unarchive ('x'), preserve ownership and file permissions ('p'),
      #    and send verbose messages to stdout ('v'),
      #    reading data from stdin ('f' followed by '-').
      #
      #    Note that 'x' is a command, and 'p', 'v', 'f' are options.
      #
      # Whew!

      # More elegant than, but equivalent to:
      #   cd source/directory
      #   tar cf - . | (cd ../dest/directory; tar xpvf -)
      #
      #     Also having same effect:
      # cp -a /source/directory/* /dest/directory
      #     Or:
      # cp -a /source/directory/* /source/directory/.[^.]* /dest/directory
      #     If there are hidden files in /source/directory.

      bunzip2 -c linux-2.6.16.tar.bz2 | tar xvf -
      #  --uncompress tar file--      | --then pass it to "tar"--
      #  If "tar" has not been patched to handle "bunzip2",
      #+ this needs to be done in two discrete steps, using a pipe.
      #  The purpose of the exercise is to unarchive "bzipped" kernel source.

  Note that in this context the - is not itself a Bash operator, but rather an option recognized by certain UNIX utilities that write to `stdout`, such as `tar`, `cat`, etc.

      bash$ echo "whatever" | cat -
      whatever

  Where a filename is expected, - redirects output to `stdout` (sometimes seen with `tar cf`), or accepts input from `stdin`, rather than from a file. This is a method of using a file-oriented utility as a filter in a pipe.

      bash$ file
      Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file...

  By itself on the command-line, file fails with an error message.

  Add a - for a more useful result. This causes the shell to await user input.

      bash$ file -
      abc
      standard input:              ASCII text

      bash$ file -
      #!/bin/bash
      standard input:              Bourne-Again shell script text executable

  Now the command accepts input from `stdin` and analyzes it.

  The - can be used to pipe `stdout` to other commands. This permits such stunts as prepending lines to a file.

  Using diff to compare a file with a section of another:

  `grep Linux file1 | diff file2 -`

  Finally, a real-world example using - with tar.

  #### Backup of all files changed in last day

      &ex58;

  Filenames beginning with - may cause problems when coupled with the - redirection operator. A script should check for this and add an appropriate prefix to such filenames, for example `./-FILENAME`, `$PWD/-FILENAME`, or `$PATHNAME/-FILENAME`.

  If the value of a variable begins with a -, this may likewise create problems.

      var="-n"
      echo $var
      # Has the effect of "echo -n", and outputs nothing.

-

  #### previous working directory

  A `cd -` command changes to the previous working directory. This uses the $OLDPWD environmental variable.

  Do not confuse the - used in this sense with the - redirection operator just discussed. The interpretation of the - depends on the context in which it appears.

-

  #### Minus

  Minus sign in an arithmetic operation.

=

  #### Equals

  Assignment operator

      a=28
      echo $a   # 28

  In a different context, the = is a string comparison operator.

+

  #### Plus

  Addition arithmetic operator.

  In a different context, the + is a Regular Expression operator.

+

  #### Option

  Option flag for a command or filter.

  Certain commands and builtins use the `+` to enable certain options and the `-` to disable them. In parameter substitution, the `+` prefixes an alternate value that a variable expands to.

%

  #### modulo

  Modulo (remainder of a division) arithmetic operation.

      let "z = 5 % 3"
      echo $z  # 2

  In a different context, the % is a pattern matching operator.

~

  #### home directory [tilde]

  This corresponds to the $HOME internal variable.

  `~bozo` is bozo's home directory, and `ls ~bozo` lists the contents of it. ~/ is the current user's home directory, and `ls ~/` lists the contents of it.

      bash$ echo ~bozo
      /home/bozo

      bash$ echo ~
      /home/bozo

      bash$ echo ~/
      /home/bozo/

      bash$ echo ~:
      /home/bozo:

      bash$ echo ~nonexistent-user
      ~nonexistent-user

~+

  #### current working directory

  This corresponds to the $PWD internal variable.

~-

  #### previous working directory

  This corresponds to the $OLDPWD internal variable.

=~

  #### regular expression match

  This operator was introduced with version 3 of Bash.

^

  #### beginning-of-line

  In a regular expression, a ^ addresses the beginning of a line of text.

^
^^

  #### Uppercase conversion in parameter substitution (added in version 4 of Bash)

Control Characters

  #### change the behavior of the terminal or text display.

  A control character is a CONTROL + key combination (pressed simultaneously).

  A control character may also be written in octal or hexadecimal notation, following an escape.

  Control characters are not normally useful inside a script.

  - `Ctl-A`

    Moves cursor to beginning of line of text (on the command-line).

  - `Ctl-B`

    `Backspace` (nondestructive).

  - `Ctl-C`

    `Break`. Terminate a foreground job.

  - `Ctl-D`

    Log out from a shell (similar to exit).

    `EOF` (end-of-file). This also terminates input from `stdin`.

    When typing text on the console or in an xterm window, `Ctl-D` erases the character under the cursor. When there are no characters present, `Ctl-D` logs out of the session, as expected. In an xterm window, this has the effect of closing the window.

  - `Ctl-E`

    Moves cursor to end of line of text (on the command-line).

  - `Ctl-F`

    Moves cursor forward one character position (on the command-line).

  - `Ctl-G`

    `BEL`. On some old-time teletype terminals, this would actually ring a bell. In an xterm it might beep.

  - `Ctl-H`

    `Rubout` (destructive backspace). Erases characters the cursor backs over while backspacing.

        #!/bin/bash
        # Embedding Ctl-H in a string.

        a="^H^H"                  # Two Ctl-H's -- backspaces
                                  # ctl-V ctl-H, using vi/vim
        echo "abcdef"             # abcdef
        echo
        echo -n "abcdef$a "       # abcd f
        #  Space at end  ^              ^  Backspaces twice.
        echo
        echo -n "abcdef$a"        # abcdef
        #  No space at end               ^ Doesn't backspace (why?).
                                  # Results may not be quite as expected.
        echo; echo

        # Constantin Hagemeier suggests trying:
        # a=$'\010\010'
        # a=$'\b\b'
        # a=$'\x08\x08'
        # But, this does not change the results.

        ########################################

        # Now, try this.

        rubout="^H^H^H^H^H"       # 5 x Ctl-H.

        echo -n "12345678"
        sleep 2
        echo -n "$rubout"
        sleep 2

  - `Ctl-I`

    `Horizontal tab`.

  - `Ctl-J`

    `Newline` (line feed). In a script, may also be expressed in octal notation -- '\012' or in hexadecimal -- '\x0a'.

  - `Ctl-K`

    `Vertical tab`.

    When typing text on the console or in an xterm window, `Ctl-K` erases from the character under the cursor to end of line. Within a script, `Ctl-K` may behave differently, as in Lee Lee Maschmeyer's example, below.

  - `Ctl-L`

    `Formfeed` (clear the terminal screen). In a terminal, this has the same effect as the clear command. When sent to a printer, a `Ctl-L` causes an advance to end of the paper sheet.

  - `Ctl-M`

    `Carriage return`.

        #!/bin/bash
        # Thank you, Lee Maschmeyer, for this example.

        read -n 1 -s -p \
        $'Control-M leaves cursor at beginning of this line. Press Enter. \x0d'
                   # Of course, '0d' is the hex equivalent of Control-M.
        echo >&2   #  The '-s' makes anything typed silent,
                   #+ so it is necessary to go to new line explicitly.

        read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a'
                   #  '0a' is the hex equivalent of Control-J, linefeed.
        echo >&2

        ###

        read -n 1 -s -p $'And Control-K\x0bgoes straight down.'
        echo >&2   #  Control-K is vertical tab.

        # A better example of the effect of a vertical tab is:

        var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a'
        echo "$var"
        #  This works the same way as the above example. However:
        echo "$var" | col
        #  This causes the right end of the line to be higher than the left end.
        #  It also explains why we started and ended with a line feed --
        #+ to avoid a garbled screen.

        # As Lee Maschmeyer explains:
        # --------------------------
        #  In the [first vertical tab example] . . . the vertical tab
        #+ makes the printing go straight down without a carriage return.
        #  This is true only on devices, such as the Linux console,
        #+ that can't go "backward."
        #  The real purpose of VT is to go straight UP, not down.
        #  It can be used to print superscripts on a printer.
        #  The col utility can be used to emulate the proper behavior of VT.

        exit 0

  - `Ctl-N`

    Erases a line of text recalled from history buffer Bash stores a list of commands previously issued from the command-line in a buffer, or memory space, for recall with the builtin history commands. (on the command-line).

  - `Ctl-O`

    Issues a newline (on the command-line).

  - `Ctl-P`

    Recalls last command from history buffer (on the command-line).

  - `Ctl-Q`

    Resume (`XON`).

    This resumes `stdin` in a terminal.

  - `Ctl-R`

    Backwards search for text in history buffer (on the command-line).

  - `Ctl-S`

    Suspend (`XOFF`).

    This freezes `stdin` in a terminal. (Use Ctl-Q to restore input.)

  - `Ctl-T`

    Reverses the position of the character the cursor is on with the previous character (on the command-line).

  - `Ctl-U`

    Erase a line of input, from the cursor backward to beginning of line. In some settings, `Ctl-U` erases the entire line of input, regardless of cursor position.

  - `Ctl-V`

    When inputting text, `Ctl-V` permits inserting control characters. For example, the following two are equivalent:

        echo -e '\x0a'
        echo <Ctl-V><Ctl-J>

    `Ctl-V` is primarily useful from within a text editor.

  - `Ctl-W`

    When typing text on the console or in an xterm window, `Ctl-W` erases from the character under the cursor backwards to the first instance of whitespace. In some settings, `Ctl-W` erases backwards to first non-alphanumeric character.

  - `Ctl-X`

    In certain word processing programs, Cuts highlighted text and copies to clipboard.

  - `Ctl-Y`

    Pastes back text previously erased (with `Ctl-U` or `Ctl-W`).

  - `Ctl-Z`

    Pauses a foreground job.

    Substitute operation in certain word processing applications.

    `EOF` (end-of-file) character in the MSDOS filesystem.

Whitespace

  #### functions as a separator between commands and/or variables.

  Whitespace consists of either spaces, tabs, blank lines, or any combination thereof.

  A linefeed (newline) is also a whitespace character. This explains why a blank line, consisting only of a linefeed, is considered whitespace.

  In some contexts, such as variable assignment, whitespace is not permitted, and results in a syntax error.

  Blank lines have no effect on the action of a script, and are therefore useful for visually separating functional sections.

  $IFS, the special variable separating fields of input to certain commands. It defaults to whitespace.

  `Definition:` A field is a discrete chunk of data expressed as a string of consecutive characters. Separating each field from adjacent fields is either whitespace or some other designated character (often determined by the $IFS). In some contexts, a field may be called a record.

  To preserve whitespace within a string or in a variable, use quoting.

  UNIX filters can target and operate on whitespace using the POSIX character class [:space:].

### Introduction to Variables and Parameters

Variables are how programming and scripting languages represent data. A variable is nothing more than a label, a name assigned to a location or set of locations in computer memory holding an item of data.

Variables appear in arithmetic operations and manipulation of quantities, and in string parsing.

#### Variable Substitution

The name of a variable is a placeholder for its value, the data it holds. Referencing (retrieving) its value is called variable substitution.

$

  Let us carefully distinguish between the name of a variable and its value. If `variable1` is the name of a variable, then `$variable1` is a reference to its value, the data item it contains.

  Technically, the name of a variable is called an lvalue, meaning that it appears on the left side of an assignment statment, as in `VARIABLE=23`. A variable's value is an rvalue, meaning that it appears on the right side of an assignment statement, as in `VAR2=$VARIABLE`. A variable's name is, in fact, a reference, a pointer to the memory location(s) where the actual data associated with that variable is kept.

      bash$ variable1=23

      bash$ echo variable1
      variable1

      bash$ echo $variable1
      23

  The only times a variable appears naked -- without the $ prefix -- is when declared or assigned, when unset, when exported, in an arithmetic expression within double parentheses (( ... )), or in the special case of a variable representing a signal (see ). Assignment may be with an = (as in `var1=27`), in a read statement, and at the head of a loop (`for var2 in 1 2 3`).

  Enclosing a referenced value in double quotes (" ... ") does not interfere with variable substitution. This is called partial quoting, sometimes referred to as weak quoting. Using single quotes (' ... ') causes the variable name to be used literally, and no substitution will take place. This is full quoting, sometimes referred to as 'strong quoting.' See for a detailed discussion.

  Note that `$variable` is actually a simplified form of `${variable}`. In contexts where the `$variable` syntax causes an error, the longer form may work (see , below).

  ##### Variable assignment and substitution

      &ex9;

  An uninitialized variable has a null value -- no assigned value at all (not zero!).

      if [ -z "$unassigned" ]
      then
        echo "\$unassigned is NULL."
      fi     # $unassigned is NULL.

  Using a variable before assigning a value to it may cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable.

      echo "$uninitialized"                                # (blank line)
      let "uninitialized += 5"                             # Add 5 to it.
      echo "$uninitialized"                                # 5

      #  Conclusion:
      #  An uninitialized variable has no value,
      #+ however it evaluates as 0 in an arithmetic operation.

  See also .

#### Variable Assignment

=

  the assignment operator (no space before and after)

  Do not confuse this with = and -eq, which test, rather than assign!

  Note that = can be either an assignment or a test operator, depending on context.

  ##### Plain Variable Assignment

      &ex15;

  ##### Variable Assignment, plain and fancy

      &ex16;

  Variable assignment using the $(...) mechanism (a newer method than backquotes). This is likewise a form of command substitution.

      # From /etc/rc.d/rc.local
      R=$(cat /etc/redhat-release)
      arch=$(uname -m)

#### Bash Variables Are Untyped

Unlike many other programming languages, Bash does not segregate its variables by type. Essentially, Bash variables are character strings, but, depending on context, Bash permits arithmetic operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits.

##### Integer or string?

    &intorstring;

Untyped variables are both a blessing and a curse. They permit more flexibility in scripting and make it easier to grind out lines of code (and give you enough rope to hang yourself!). However, they likewise permit subtle errors to creep in and encourage sloppy programming habits.

To lighten the burden of keeping track of variable types in a script, Bash does permit declaring variables.

#### Special Variable Types

Local variables

  Variables visible only within a code block or function (see also local variables in functions)

Environmental variables

  Variables that affect the behavior of the shell and user interface

  In a more general context, each process has an environment, that is, a group of variables that the process may reference. In this sense, the shell behaves like any other process.

  Every time a shell starts, it creates shell variables that correspond to its own environmental variables. Updating or adding new environmental variables causes the shell to update its environment, and all the shell's child processes (the commands it executes) inherit this environment.

  The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems.

      bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"

      bash$ du
      bash: /usr/bin/du: Argument list too long

  Note: this error has been fixed, as of kernel version 2.6.23.

  (Thank you, Stéphane Chazelas for the clarification, and for providing the above example.)

  If a script sets environmental variables, they need to be exported, that is, reported to the environment local to the script. This is the function of the export command.

  A script can `export` variables only to child processes, that is, only to commands or processes which that particular script initiates. A script invoked from the command-line cannot export variables back to the command-line environment. Child processes cannot export variables back to the parent processes that spawned them.

  `Definition:` A child process is a subprocess launched by another process, its parent.

Positional parameters

  Arguments passed to the script from the command line Note that functions also take positional parameters. : `$0`, `$1`, `$2`, `$3` . . .

  `$0` is the name of the script itself, `$1` is the first argument, `$2` the second, `$3` the third, and so forth. The process calling the script sets the `$0` parameter. By convention, this parameter is the name of the script. See the manpage (manual page) for `execv`. From the command-line, however, `$0` is the name of the shell.

      bash$ echo $0
      bash

      tcsh% echo $0
      tcsh

  After `$9`, the arguments must be enclosed in brackets, for example, `${10}`, `${11}`, `${12}`.

  The special variables $* and $@ denote all the positional parameters.

  ##### Positional Parameters

      &ex17;

  Bracket notation for positional parameters leads to a fairly simple way of referencing the last argument passed to a script on the command-line. This also requires indirect referencing.

      args=$#           # Number of args passed.
      lastarg=${!args}
      # Note: This is an *indirect reference* to $args ...

      # Or:       lastarg=${!#}             (Thanks, Chris Monson.)
      # This is an *indirect reference* to the $# variable.
      # Note that lastarg=${!$#} doesn't work.

  Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check `$0`, the name it was invoked by.

  If the the script is sourced or symlinked, then this will not work. It is safer to check $BASH_Source.

  There must also exist symbolic links to all the alternate names of the script. See .

  If a script expects a command-line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter.

      variable1_=$1_  # Rather than variable1=$1
      # This will prevent an error, even if positional parameter is absent.

      critical_argument01=$variable1_

      # The extra character can be stripped off later, like so.
      variable1=${variable1_/_/}
      # Side effects only if $variable1_ begins with an underscore.
      # This uses one of the parameter substitution templates discussed later.
      # (Leaving out the replacement pattern results in a deletion.)

      #  A more straightforward way of dealing with this is
      #+ to simply test whether expected positional parameters have been passed.
      if [ -z $1 ]
      then
        exit $E_MISSING_POS_PARAM
      fi

      #  However, as Fabian Kreutz points out,
      #+ the above method may have unexpected side-effects.
      #  A better method is parameter substitution:
      #         ${1:-$DefaultVal}
      #  See the "Parameter Substition" section
      #+ in the "Variables Revisited" chapter.

  ---

  ##### wh, whois domain name lookup

      &ex18;

  ---

  The `shift` command reassigns the positional parameters, in effect shifting them to the left one notch.

  `$1` <--- `$2`, `$2` <--- `$3`, `$3` <--- `$4`, etc.

  The old `$1` disappears, but `$0` (the script name) does not change. If you use a large number of positional parameters to a script, `shift` lets you access those past `10`, although {bracket} notation also permits this.

  ##### Using shift

      &ex19;

  The `shift` command can take a numerical parameter indicating how many positions to shift.

      #!/bin/bash
      # shift-past.sh

      shift 3    # Shift 3 positions.
      #  n=3; shift $n
      #  Has the same effect.

      echo "$1"

      exit 0

      # ======================== #

      $ sh shift-past.sh 1 2 3 4 5
      4

      #  However, as Eleni Fragkiadaki, points out,
      #+ attempting a 'shift' past the number of
      #+ positional parameters ($#) returns an exit status of 1,
      #+ and the positional parameters themselves do not change.
      #  This means possibly getting stuck in an endless loop. . . .
      #  For example:
      #      until [ -z "$1" ]
      #      do
      #         echo -n "$1 "
      #         shift 20    #  If less than 20 pos params,
      #      done           #+ then loop never ends!
      #
      # When in doubt, add a sanity check. . . .
      #           shift 20 || break
      #                    ^^^^^^^^

  The `shift` command works in a similar fashion on parameters passed to a function. See .

### Quoting

Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in the string from reinterpretation or expansion by the shell or shell script. (A character is special if it has an interpretation other than its literal meaning. For example, the asterisk * represents a wild card character in globbing and Regular Expressions).

    bash$ ls -l [Vv]*
    -rw-rw-r--    1 bozo  bozo       324 Apr  2 15:05 VIEWDATA.BAT
     -rw-rw-r--    1 bozo  bozo       507 May  4 14:25 vartrace.sh
     -rw-rw-r--    1 bozo  bozo       539 Apr 14 17:11 viewdata.sh

    bash$ ls -l '[Vv]*'
    ls: [Vv]*: No such file or directory

In everyday speech or writing, when we quote a phrase, we set it apart and give it special meaning. In a Bash script, when we quote a string, we set it apart and protect its literal meaning.

Certain programs and utilities reinterpret or expand special characters in a quoted string. An important use of quoting is protecting a command-line parameter from the shell, but still letting the calling program expand it.

    bash$ grep '[Ff]irst' *.txt
    file1.txt:This is the first line of file1.txt.
     file2.txt:This is the First line of file2.txt.

Note that the unquoted `grep [Ff]irst *.txt` works under the Bash shell. Unless there is a file named `first` in the current working directory. Yet another reason to quote. (Thank you, Harald Koenig, for pointing this out.

Quoting can also suppress echo's appetite for newlines.

    bash$ echo $(ls -l)
    total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh

    bash$ echo "$(ls -l)"
    total 8
     -rw-rw-r--  1 bo bo  13 Aug 21 12:57 t.sh
     -rw-rw-r--  1 bo bo  78 Aug 21 12:57 u.sh

#### Quoting Variables

When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape).

Encapsulating ! within double quotes gives an error when used from the command line. This is interpreted as a history command. Within a script, though, this problem does not occur, since the Bash history mechanism is disabled then. Of more concern is the apparently inconsistent behavior of \ within double quotes, and especially following an `echo -e` command.

    bash$ echo hello\!
    hello!
    bash$ echo "hello\!"
    hello\!

    bash$ echo \
    >
    bash$ echo "\"
    >
    bash$ echo \a
    a
    bash$ echo "\a"
    \a

    bash$ echo x\ty
    xty
    bash$ echo "x\ty"
    x\ty

    bash$ echo -e x\ty
    xty
    bash$ echo -e "x\ty"
    x       y

Double quotes following an echo sometimes escape \. Moreover, the `-e` option to echo causes the \t to be interpreted as a tab.

(Thank you, Wayne Pollock, for pointing this out, and Geoff Lee and Daniel Barclay for explaining it.)

Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see , above).

Use double quotes to prevent word splitting.

Word splitting, in this context, means dividing a character string into separate and discrete arguments.

An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators.

    List="one two three"

    for a in $List     # Splits the variable in parts at whitespace.
    do
      echo "$a"
    done
    # one
    # two
    # three

    echo "---"

    for a in "$List"   # Preserves whitespace in a single variable.
    do #     ^     ^
      echo "$a"
    done
    # one two three

A more elaborate example:

    variable1="a variable containing five words"
    COMMAND This is $variable1    # Executes COMMAND with 7 arguments:
    # "This" "is" "a" "variable" "containing" "five" "words"

    COMMAND "This is $variable1"  # Executes COMMAND with 1 argument:
    # "This is a variable containing five words"

    variable2=""    # Empty.

    COMMAND $variable2 $variable2 $variable2
                    # Executes COMMAND with no arguments.
    COMMAND "$variable2" "$variable2" "$variable2"
                    # Executes COMMAND with 3 empty arguments.
    COMMAND "$variable2 $variable2 $variable2"
                    # Executes COMMAND with 1 argument (2 spaces).

    # Thanks, Stéphane Chazelas.

Enclosing the arguments to an `echo` statement in double quotes is necessary only when word splitting or preservation of whitespace is an issue.

##### Echoing Weird Variables

    &weirdvars;

Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally. Consider single quotes (full quoting) to be a stricter method of quoting than double quotes (partial quoting).

Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result.

    echo "Why can't I write 's between single quotes"

    echo

    # The roundabout method.
    echo 'Why can'\''t I write '"'"'s between single quotes'
    #    |-------|  |----------|   |-----------------------|
    # Three single-quoted strings, with escaped and quoted single quotes between.

    # This example courtesy of Stéphane Chazelas.

#### Escaping

Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally.

With certain commands and utilities, such as echo and sed, escaping a character may have the opposite effect - it can toggle on a special meaning for that character.

##### Special meanings of certain escaped characters

used with `echo` and `sed`

\n

  means newline

\r

  means return

\t

  means tab

\v

  means vertical tab

\b

  means backspace

\a

  means alert (beep or flash)

\0xx

  translates to the octal ASCII equivalent of 0nn, where nn is a string of digits

  The `$' ... '` quoted string-expansion construct is a mechanism that uses escaped octal or hex values to assign ASCII characters to variables, e.g., `quote=$'\042'`.

  ##### Escaped Characters

      &escaped;

  A more elaborate example:

  ##### Detecting key-presses

      &bashek;

  See also .

\"

  gives the quote its literal meaning

      echo "Hello"                     # Hello
      echo "\"Hello\" ... he said."    # "Hello" ... he said.

\$

  gives the dollar sign its literal meaning (variable name following \$ will not be referenced)

      echo "\$variable01"           # $variable01
      echo "The book cost \$7.98."  # The book cost $7.98.

\\

  gives the backslash its literal meaning

      echo "\\"  # Results in \

      # Whereas . . .

      echo "\"   # Invokes secondary prompt from the command-line.
                 # In a script, gives an error message.

      # However . . .

      echo '\'   # Results in \

The behavior of \ depends on whether it is escaped, strong-quoted, weak-quoted, or appearing within command substitution or a here document.

                          #  Simple escaping and quoting
    echo \z               #  z
    echo \\z              # \z
    echo '\z'             # \z
    echo '\\z'            # \\z
    echo "\z"             # \z
    echo "\\z"            # \z

                          #  Command substitution
    echo `echo \z`        #  z
    echo `echo \\z`       #  z
    echo `echo \\\z`      # \z
    echo `echo \\\\z`     # \z
    echo `echo \\\\\\z`   # \z
    echo `echo \\\\\\\z`  # \\z
    echo `echo "\z"`      # \z
    echo `echo "\\z"`     # \z

                          # Here document
    cat <<EOF
    \z
    EOF                   # \z

    cat <<EOF
    \\z
    EOF                   # \z

    # These examples supplied by Stéphane Chazelas.

Elements of a string assigned to a variable may be escaped, but the escape character alone may not be assigned to a variable.

    variable=\
    echo "$variable"
    # Will not work - gives an error message:
    # test.sh: : command not found
    # A "naked" escape cannot safely be assigned to a variable.
    #
    #  What actually happens here is that the "\" escapes the newline and
    #+ the effect is        variable=echo "$variable"
    #+                      invalid variable assignment

    variable=\
    23skidoo
    echo "$variable"        #  23skidoo
                            #  This works, since the second line
                            #+ is a valid variable assignment.

    variable=\
    #        \^    escape followed by space
    echo "$variable"        # space

    variable=\\
    echo "$variable"        # \

    variable=\\\
    echo "$variable"
    # Will not work - gives an error message:
    # test.sh: \: command not found
    #
    #  First escape escapes second one, but the third one is left "naked",
    #+ with same result as first instance, above.

    variable=\\\\
    echo "$variable"        # \\
                            # Second and fourth escapes escaped.
                            # This is o.k.

Escaping a space can prevent word splitting in a command's argument list.

    file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7"
    # List of files as argument(s) to a command.

    # Add two files to the list, and list all.
    ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list

    echo "-------------------------------------------------------------------------"

    # What happens if we escape a couple of spaces?
    ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list
    # Error: the first three files concatenated into a single argument to 'ls -l'
    #        because the two escaped spaces prevent argument (word) splitting.

The escape also provides a means of writing a multi-line command. Normally, each separate line constitutes a different command, but an escape at the end of a line escapes the newline character, and the command sequence continues on to the next line.

    (cd /source/directory && tar cf - . ) | \
    (cd /dest/directory && tar xpvf -)
    # Repeating Alan Cox's directory tree copy command,
    # but split into two lines for increased legibility.

    # As an alternative:
    tar cf - -C /source/directory . |
    tar xpvf - -C /dest/directory
    # See note below.
    # (Thanks, Stéphane Chazelas.)

If a script line ends with a |, a pipe character, then a \, an escape, is not strictly necessary. It is, however, good programming practice to always escape the end of a line of code that continues to the following line.

    echo "foo
    bar"
    #foo
    #bar

    echo

    echo 'foo
    bar'    # No difference yet.
    #foo
    #bar

    echo

    echo foo\
    bar     # Newline escaped.
    #foobar

    echo

    echo "foo\
    bar"     # Same here, as \ still interpreted as escape within weak quotes.
    #foobar

    echo

    echo 'foo\
    bar'     # Escape character \ taken literally because of strong quoting.
    #foo\
    #bar

    # Examples suggested by Stéphane Chazelas.

### Exit and Exit Status

... there are dark corners in the Bourne shell, and people use all of them.

--Chet Ramey

The `
exit` command terminates a script, just as in a `C` program. It can also return a value, which is available to the script's parent process.

Every command returns an
exit status (sometimes referred to as a
return status or exit code). A successful command returns a 0, while an unsuccessful one returns a non-zero value that usually can be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a 0 exit code upon successful completion, though there are some exceptions.

Likewise, functions within a script and the script itself return an exit status. The last command executed in the function or script determines the exit status. Within a script, an `exit nnn` command may be used to deliver an nnn exit status to the shell (nnn must be an integer in the 0 - 255 range).

When a script ends with an `exit` that has no parameter, the exit status of the script is the exit status of the last command executed in the script (previous to the `exit`).

    #!/bin/bash

    COMMAND_1

    . . .

    COMMAND_LAST

    # Will exit with status of last command.

    exit

The equivalent of a bare `exit` is `exit $?` or even just omitting the `exit`.

    #!/bin/bash

    COMMAND_1

    . . .

    COMMAND_LAST

    # Will exit with status of last command.

    exit $?

    #!/bin/bash

    COMMAND1

    . . .

    COMMAND_LAST

    # Will exit with status of last command.

`
$?` reads the exit status of the last command executed. After a function returns, `$?` gives the exit status of the last command executed in the function. This is Bash's way of giving functions a return value.

In those instances when there is no return terminating the function.

Following the execution of a pipe, a `$?` gives the exit status of the last command executed.

After a script terminates, a `$?` from the command-line gives the exit status of the script, that is, the last command executed in the script, which is, by convention, `0` on success or an integer in the range 1 - 255 on error.

#### exit / exit status

    &ex5;

$? is especially useful for testing the result of a command in a script (see and ).

The !, the logical not qualifier, reverses the outcome of a test or command, and this affects its exit status.

#### Negating a condition using !

    true    # The "true" builtin.
    echo "exit status of \"true\" = $?"     # 0

    ! true
    echo "exit status of \"! true\" = $?"   # 1
    # Note that the "!" needs a space between it and the command.
    #    !true   leads to a "command not found" error
    #
    # The '!' operator prefixing a command invokes the Bash history mechanism.

    true
    !true
    # No error this time, but no negation either.
    # It just repeats the previous command (true).

    # =========================================================== #
    # Preceding a _pipe_ with ! inverts the exit status returned.
    ls | bogus_command     # bash: bogus_command: command not found
    echo $?                # 127

    ! ls | bogus_command   # bash: bogus_command: command not found
    echo $?                # 0
    # Note that the ! does not change the execution of the pipe.
    # Only the exit status changes.
    # =========================================================== #

    # Thanks, Stéphane Chazelas and Kristopher Newsome.

Certain exit status codes have reserved meanings and should not be user-specified in a script.

### Tests

Every reasonably complete programming language can test for a condition, then act according to the result of the test. Bash has the test command, various bracket and parenthesis operators, and the `if/then` construct.

#### Test Constructs

- An `if/then` construct tests whether the exit status of a list of commands is 0 (since 0 means success by UNIX convention), and if so, executes one or more commands.

- There exists a dedicated command called `[` (left bracket special character). It is a synonym for `test`, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false).

- With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that `[[` is a keyword, not a command.

  Bash sees `[[ $a -lt $b ]]` as a single element, which returns an exit status.

- The (( ... )) and let ... constructs return an exit status, according to whether the arithmetic expressions they evaluate expand to a non-zero value. These arithmetic-expansion constructs may therefore be used to perform arithmetic comparisons.

      (( 0 && 1 ))                 # Logical AND
      echo $?     # 1     ***
      # And so ...
      let "num = (( 0 && 1 ))"
      echo $num   # 0
      # But ...
      let "num = (( 0 && 1 ))"
      echo $?     # 1     ***

      (( 200 || 11 ))              # Logical OR
      echo $?     # 0     ***
      # ...
      let "num = (( 200 || 11 ))"
      echo $num   # 1
      let "num = (( 200 || 11 ))"
      echo $?     # 0     ***

      (( 200 | 11 ))               # Bitwise OR
      echo $?                      # 0     ***
      # ...
      let "num = (( 200 | 11 ))"
      echo $num                    # 203
      let "num = (( 200 | 11 ))"
      echo $?                      # 0     ***

      # The "let" construct returns the same exit status
      #+ as the double-parentheses arithmetic expansion.

  Again, note that the exit status of an arithmetic expression is not an error value.

      var=-2 && (( var+=2 ))
      echo $?                   # 1

      var=-2 && (( var+=2 )) && echo $var
                                # Will not echo $var!

- An `if` can test any command, not just conditions enclosed within brackets.

      if cmp a b &> /dev/null  # Suppress output.
      then echo "Files a and b are identical."
      else echo "Files a and b differ."
      fi

      # The very useful "if-grep" construct:
      # -----------------------------------
      if grep -q Bash file
        then echo "File contains at least one occurrence of Bash."
      fi

      word=Linux
      letter_sequence=inu
      if echo "$word" | grep -q "$letter_sequence"
      # The "-q" option to grep suppresses output.
      then
        echo "$letter_sequence found in $word"
      else
        echo "$letter_sequence not found in $word"
      fi

      if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED
        then echo "Command succeeded."
        else echo "Command failed."
      fi

- These last two examples courtesy of Stéphane Chazelas.

##### What is truth?

    &ex10;

##### Exercise

Explain the behavior of , above.

    if [ condition-true ]
    then
       command 1
       command 2
       ...
    else  # Or else ...
          # Adds default code block executing if original condition tests false.
       command 3
       command 4
       ...
    fi

When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the same line begins, the old one must terminate.

    if [ -x "$filename" ]; then

##### Else if and elif

elif

  `elif` is a contraction for else if. The effect is to nest an inner if/then construct within an outer one.

      if [ condition1 ]
      then
         command1
         command2
         command3
      elif [ condition2 ]
      # Same as else if
      then
         command4
         command5
      else
         default-command
      fi

The `if test condition-true` construct is the exact equivalent of `if [ condition-true ]`. As it happens, the left bracket, `[` , is a token A token is a symbol or short string with a special meaning attached to it (a meta-meaning). In Bash, certain tokens, such as `[` and . (dot-command), may expand to keywords and commands. which invokes the `test` command. The closing right bracket, `]` , in an if/test should not therefore be strictly necessary, however newer versions of Bash require it.

The `test` command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash script, `test` does not call the external `/usr/bin/test` binary, which is part of the sh-utils package. Likewise, `[` does not call `/usr/bin/[`, which is linked to `/usr/bin/test`.

    bash$ type test
    test is a shell builtin
    bash$ type '['
    [ is a shell builtin
    bash$ type '[['
    [[ is a shell keyword
    bash$ type ']]'
    ]] is a shell keyword
    bash$ type ']'
    bash: type: ]: not found

If, for some reason, you wish to use `/usr/bin/test` in a Bash script, then specify it by full pathname.

##### Equivalence of test, `/usr/bin/test`, [ ], and `/usr/bin/[`

    &ex11;

The [[ ]] construct is the more versatile Bash version of [ ]. This is the extended test command, adopted from ksh88.

* * *

No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and command substitution.

    file=/etc/passwd

    if [[ -e $file ]]
    then
      echo "Password file exists."
    fi

Using the `[[ ... ]]` test construct, rather than `[ ... ]` can prevent many logic errors in scripts. For example, the &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.

Arithmetic evaluation of octal / hexadecimal constants takes place automatically within a [[ ... ]] construct.

    # [[ Octal and hexadecimal evaluation ]]
    # Thank you, Moritz Gronbach, for pointing this out.

    decimal=15
    octal=017   # = 15 (decimal)
    hex=0x0f    # = 15 (decimal)

    if [ "$decimal" -eq "$octal" ]
    then
      echo "$decimal equals $octal"
    else
      echo "$decimal is not equal to $octal"       # 15 is not equal to 017
    fi      # Doesn't evaluate within [ single brackets ]!

    if [[ "$decimal" -eq "$octal" ]]
    then
      echo "$decimal equals $octal"                # 15 equals 017
    else
      echo "$decimal is not equal to $octal"
    fi      # Evaluates within [[ double brackets ]]!

    if [[ "$decimal" -eq "$hex" ]]
    then
      echo "$decimal equals $hex"                  # 15 equals 0x0f
    else
      echo "$decimal is not equal to $hex"
    fi      # [[ $hexadecimal ]] also evaluates!

Following an `if`, neither the `test` command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary.

    dir=/home/bozo

    if cd "$dir" 2>/dev/null; then   # "2>/dev/null" hides error message.
      echo "Now in $dir."
    else
      echo "Can't change to $dir."
    fi

The "if COMMAND" construct returns the exit status of COMMAND.

Similarly, a condition within test brackets may stand alone without an `if`, when used in combination with a list construct.

    var1=20
    var2=22
    [ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2"

    home=/home/bozo
    [ -d "$home" ] || echo "$home directory does not exist."

The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it returns an exit status of 1, or false. A non-zero expression returns an exit status of 0, or true. This is in marked contrast to using the `test` and [ ] constructs previously discussed.

##### Arithmetic Tests using (( ))

    &arithtests;

#### File test operators

##### Returns true if...

-e

  file exists

-a

  file exists

  This is identical in effect to -e. It has been deprecated,

  Per the 1913 edition of Webster's Dictionary:

      Deprecate
      ...

      To pray against, as an evil;
      to seek to avert by prayer;
      to desire the removal of;
      to seek deliverance from;
      to express deep regret for;
      to disapprove of strongly.

  and its use is discouraged.
-f

  file is a regular file (not a directory or device file)

-s

  file is not zero size

-d

  file is a directory

-b

  file is a block device

-c

  file is a character device

      device0="/dev/sda2"    # /   (root directory)
      if [ -b "$device0" ]
      then
        echo "$device0 is a block device."
      fi

      # /dev/sda2 is a block device.

      device1="/dev/ttyS1"   # PCMCIA modem card.
      if [ -c "$device1" ]
      then
        echo "$device1 is a character device."
      fi

      # /dev/ttyS1 is a character device.

-p

  file is a pipe

      function show_input_type()
      {
         [ -p /dev/fd/0 ] && echo PIPE || echo STDIN
      }

      show_input_type "Input"                           # STDIN
      echo "Input" | show_input_type                    # PIPE

      # This example courtesy of Carl Anderson.

-h

  file is a symbolic link

-L

  file is a symbolic link

-S

  file is a socket

-t

  file (descriptor) is associated with a terminal device

  This test option may be used to check whether the `stdin` `[ -t 0 ]` or `stdout` `[ -t 1 ]` in a given script is a terminal.

-r

  file has read permission (for the user running the test)

-w

  file has write permission (for the user running the test)

-x

  file has execute permission (for the user running the test)

-g

  set-group-id (sgid) flag set on file or directory

  If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup.

-u

  set-user-id (suid) flag set on file

  A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it.

  Be aware that suid binaries may open security holes. The suid flag has no effect on shell scripts.

  This is useful for executables (such as `pppd` and `cdrecord`) that need to access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root user.

      	      -rwsr-xr-t    1 root       178236 Oct  2  2000 /usr/sbin/pppd

  A file with the suid flag set shows an s in its permissions.

-k

  sticky bit set

  Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. On Linux systems, the sticky bit is no longer used for files, only on directories. If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. This restricts altering or deleting specific files in that directory to the owner of those files.

      	      drwxrwxrwt    7 root         1024 May 19 21:26 tmp/

  If a user does not own a directory that has the sticky bit set, but has write permission in that directory, she can only delete those files that she owns in it. This keeps users from inadvertently overwriting or deleting each other's files in a publicly accessible directory, such as `/tmp`. (The owner of the directory or root can, of course, delete or rename files there.)

-O

  you are owner of file

-G

  group-id of file same as yours

-N

  file modified since it was last read

f1 -nt f2

  file f1 is newer than f2

f1 -ot f2

  file f1 is older than f2

f1 -ef f2

  files f1 and f2 are hard links to the same file

!

  not -- reverses the sense of the tests above (returns true if condition absent).

##### Testing for broken links

    &brokenlink;

, , , , and also illustrate uses of the file test operators.

#### Other Comparison Operators

A binary comparison operator compares two variables or quantities. Note that integer and string comparison use a different set of operators.

##### integer comparison

-eq

  is equal to

  `if [ "$a" -eq "$b" ]`

-ne

  is not equal to

  `if [ "$a" -ne "$b" ]`

-gt

  is greater than

  `if [ "$a" -gt "$b" ]`

-ge

  is greater than or equal to

  `if [ "$a" -ge "$b" ]`

-lt

  is less than

  `if [ "$a" -lt "$b" ]`

-le

  is less than or equal to

  `if [ "$a" -le "$b" ]`

<

  is less than (within double parentheses)

  `(("$a" < "$b"))`

<=

  is less than or equal to (within double parentheses)

  `(("$a" <= "$b"))`

>

  is greater than (within double parentheses)

  `(("$a" > "$b"))`

>=

  is greater than or equal to (within double parentheses)

  `(("$a" >= "$b"))`

##### string comparison

=

  is equal to

  `if [ "$a" = "$b" ]`

  Note the whitespace framing the `=`.

  `if [ "$a"="$b" ]` is not equivalent to the above.

==

  is equal to

  `if [ "$a" == "$b" ]`

  This is a synonym for =.

  The == comparison operator behaves differently within a double-brackets test than within single brackets.

      [[ $a == z* ]]   # True if $a starts with an "z" (pattern matching).
      [[ $a == "z*" ]] # True if $a is equal to z* (literal matching).

      [ $a == z* ]     # File globbing and word splitting take place.
      [ "$a" == "z*" ] # True if $a is equal to z* (literal matching).

      # Thanks, Stéphane Chazelas

!=

  is not equal to

  `if [ "$a" != "$b" ]`

  This operator uses pattern matching within a [[ ... ]] construct.

<

  is less than, in ASCII alphabetical order

  `if [[ "$a" < "$b" ]]`

  `if [ "$a" \< "$b" ]`

  Note that the < needs to be escaped within a `[ ]` construct.

>

  is greater than, in ASCII alphabetical order

  `if [[ "$a" > "$b" ]]`

  `if [ "$a" \> "$b" ]`

  Note that the > needs to be escaped within a `[ ]` construct.

  See for an application of this comparison operator.

-z

  string is null, that is, has zero length

       String=''   # Zero-length ("null") string variable.

      if [ -z "$String" ]
      then
        echo "\$String is null."
      else
        echo "\$String is NOT null."
      fi     # $String is null.

-n

  string is not null.

  The `-n` test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see ) normally works, however, this is an unsafe practice. Always quote a tested string. As S.C. points out, in a compound test, even quoting the string variable might not suffice. `[ -n "$string" -o "$a" = "$b" ]` may cause an error with some versions of Bash if `$string` is empty. The safe way is to append an extra character to possibly empty variables, `[ "x$string" != x -o "x$a" = "x$b" ]` (the x's cancel out).

##### Arithmetic and string comparisons

    &ex13;

##### Testing whether a string is null

    &strtest;

##### zmore

    &ex14;

##### compound comparison

-a

  logical and

  exp1 -a exp2 returns true if both exp1 and exp2 are true.

-o

  logical or

  exp1 -o exp2 returns true if either exp1 or exp2 is true.

These are similar to the Bash comparison operators `&&` and `||`, used within double brackets.

    [[ condition1 && condition2 ]]

The `-o` and `-a` operators work with the test command or occur within single test brackets.

    if [ "$expr1" -a "$expr2" ]
    then
      echo "Both expr1 and expr2 are true."
    else
      echo "Either expr1 or expr2 is false."
    fi

But, as rihad points out:

    [ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ]   # true
    [ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ]   # (no output)
    # ^^^^^^^ False condition. So far, everything as expected.

    # However ...
    [ 1 -eq 2 -a -n "`echo true 1>&2`" ]       # true
    # ^^^^^^^ False condition. So, why "true" output?

    # Is it because both condition clauses within brackets evaluate?
    [[ 1 -eq 2 && -n "`echo true 1>&2`" ]]     # (no output)
    # No, that's not it.

    # Apparently && and || "short-circuit" while -a and -o do not.

Refer to , , and to see compound comparison operators in action.

#### Nested if/then Condition Tests

Condition tests using the if/then construct may be nested. The net result is equivalent to using the && compound comparison operator.

    a=3

    if [ "$a" -gt 0 ]
    then
      if [ "$a" -lt 5 ]
      then
        echo "The value of \"a\" lies somewhere between 0 and 5."
      fi
    fi

    # Same result as:

    if [ "$a" -gt 0 ] && [ "$a" -lt 5 ]
    then
      echo "The value of \"a\" lies somewhere between 0 and 5."
    fi

and demonstrate nested if/then condition tests.

#### Testing Your Knowledge of Tests

The systemwide `xinitrc` file can be used to launch the X server. This file contains quite a number of if/then tests. The following is excerpted from an ancient version of `xinitrc` (Red Hat 7.1, or thereabouts).

    if [ -f $HOME/.Xclients ]; then
      exec $HOME/.Xclients
    elif [ -f /etc/X11/xinit/Xclients ]; then
      exec /etc/X11/xinit/Xclients
    else
         # failsafe settings.  Although we should never get here
         # (we provide fallbacks in Xclients as well) it can't hurt.
         xclock -geometry 100x100-5+5 &
         xterm -geometry 80x50-50+150 &
         if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then
                 netscape /usr/share/doc/HTML/index.html &
         fi
    fi

Explain the test constructs in the above snippet, then examine an updated version of the file, `/etc/X11/xinit/xinitrc`, and analyze the if/then test constructs there. You may need to refer ahead to the discussions of grep, sed, and regular expressions.

### Operations and Related Topics

#### Operators

##### assignment

variable assignment

  Initializing or changing the value of a variable

=

  All-purpose assignment operator, which works for both arithmetic and string assignments.

      var=27
      category=minerals  # No spaces allowed after the "=".

  Do not confuse the = assignment operator with the = test operator.

      #   =  as a test operator

      if [ "$string1" = "$string2" ]
      then
         command
      fi

      #  if [ "X$string1" = "X$string2" ] is safer,
      #+ to prevent an error message should one of the variables be empty.
      #  (The prepended "X" characters cancel out.)

##### arithmetic operators

+

  plus

-

  minus

*

  multiplication

/

  division

**

  exponentiation

      # Bash, version 2.02, introduced the "**" exponentiation operator.

      let "z=5**3"    # 5 * 5 * 5
      echo "z = $z"   # z = 125

%

  modulo, or mod (returns the remainder of an integer division operation)

      bash$ expr 5 % 3
      2

  5/3 = 1, with remainder 2

  This operator finds use in, among other things, generating numbers within a specific range (see and ) and formatting program output (see and ). It can even be used to generate prime numbers, (see ). Modulo turns up surprisingly often in numerical recipes.

  ##### Greatest common divisor

      &gcd;

+=

  plus-equal (increment variable by a constant)

  In a different context, `+=` can serve as a string concatenation operator. This can be useful for modifying environmental variables.

  `let "var += 5"` results in `var` being incremented by `5`.

-=

  minus-equal (decrement variable by a constant)

*=

  times-equal (multiply variable by a constant)

  `let "var *= 4"` results in `var` being multiplied by `4`.

/=

  slash-equal (divide variable by a constant)

%=

  mod-equal (remainder of dividing variable by a constant)

  Arithmetic operators often occur in an expr or let expression.

  ##### Using Arithmetic Operations

      &arithops;

Integer variables in older versions of Bash were signed long (32-bit) integers, in the range of -2147483648 to 2147483647. An operation that took a variable outside these limits gave an erroneous result.

    echo $BASH_VERSION   # 1.14

    a=2147483646
    echo "a = $a"        # a = 2147483646
    let "a+=1"           # Increment "a".
    echo "a = $a"        # a = 2147483647
    let "a+=1"           # increment "a" again, past the limit.
    echo "a = $a"        # a = -2147483648
                         #      ERROR: out of range,
                         # +    and the leftmost bit, the sign bit,
                         # +    has been set, making the result negative.

As of version >= 2.05b, Bash supports 64-bit integers.

Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings.

    a=1.5

    let "b = $a + 1.3"  # Error.
    # t2.sh: let: b = 1.5 + 1.3: syntax error in expression
    #                            (error token is ".5 + 1.3")

    echo "b = $b"       # b=1

Use bc in scripts that that need floating point calculations or math library functions.

##### bitwise operators

The bitwise operators seldom make an appearance in shell scripts. Their chief use seems to be manipulating and testing values read from ports or sockets. Bit flipping is more relevant to compiled languages, such as C and C++, which provide direct access to system hardware. However, see vladz's ingenious use of bitwise operators in his base64.sh () script.

##### bitwise operators

<<

  bitwise left shift (multiplies by `2` for each shift position)

<<=

  left-shift-equal

  `let "var <<= 2"` results in `var` left-shifted `2` bits (multiplied by `4`)

>>

  bitwise right shift (divides by `2` for each shift position)

>>=

  right-shift-equal (inverse of <<=)

&

  bitwise AND

&=

  bitwise AND-equal

|

  bitwise OR

|=

  bitwise OR-equal

~

  bitwise NOT

^

  bitwise XOR

^=

  bitwise XOR-equal

##### logical (boolean) operators

!

  NOT

      if [ ! -f $FILENAME ]
      then
        ...

&&

  AND

      if [ $condition1 ] && [ $condition2 ]
      #  Same as:  if [ $condition1 -a $condition2 ]
      #  Returns true if both condition1 and condition2 hold true...

      if [[ $condition1 && $condition2 ]]    # Also works.
      #  Note that && operator not permitted inside brackets
      #+ of [ ... ] construct.

  && may also be used, depending on context, in an and list to concatenate commands.

||

  OR

      if [ $condition1 ] || [ $condition2 ]
      # Same as:  if [ $condition1 -o $condition2 ]
      # Returns true if either condition1 or condition2 holds true...

      if [[ $condition1 || $condition2 ]]    # Also works.
      #  Note that || operator not permitted inside brackets
      #+ of a [ ... ] construct.

  Bash tests the exit status of each statement linked with a logical operator.

  ##### Compound Condition Tests Using && and ||

      &andor;

  The && and || operators also find use in an arithmetic context.

      bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))
      1 0 1 0

##### miscellaneous operators

,

  Comma operator

  The `comma operator` chains together two or more arithmetic operations. All the operations are evaluated (with possible side effects. Side effects are, of course, unintended -- and usually undesirable -- consequences.

      let "t1 = ((5 + 3, 7 - 1, 15 - 4))"
      echo "t1 = $t1"           ^^^^^^  # t1 = 11
      # Here t1 is set to the result of the last operation. Why?

      let "t2 = ((a = 9, 15 / 3))"      # Set "a" and calculate "t2".
      echo "t2 = $t2    a = $a"         # t2 = 5    a = 9

  The comma operator finds use mainly in for loops. See .

#### Numerical Constants

A shell script interprets a number as decimal (base 10), unless that number has a special prefix or notation. A number preceded by a 0 is octal (base 8). A number preceded by 0x is hexadecimal (base 16). A number with an embedded # evaluates as BASE#NUMBER (with range and notational restrictions).

##### Representation of numerical constants

    &numbers;

#### The Double-Parentheses Construct

Similar to the let command, the `(( ... ))` construct permits arithmetic expansion and evaluation. In its simplest form, `a=$(( 5 + 3 ))` would set `a` to `5 + 3`, or `8`. However, this double-parentheses construct is also a mechanism for allowing C-style manipulation of variables in Bash, for example, `(( var++ ))`.

##### C-style manipulation of variables

    &cvars;

See also and .

#### Operator Precedence

In a script, operations execute in order of precedence: the higher precedence operations execute before the lower precedence ones. Precedence, in this context, has approximately the same meaning as priority

&opprectable;

In practice, all you really need to remember is the following:

- The My Dear Aunt Sally mantra (multiply, divide, add, subtract) for the familiar arithmetic operations.

- The compound logical operators, `&&`, `||`, `-a`, and `-o` have low precedence.

- The order of evaluation of equal-precedence operators is usually left-to-right.

Now, let's utilize our knowledge of operator precedence to analyze a couple of lines from the `/etc/init.d/functions file`, as found in the Fedora Core Linux distro.

    while [ -n "$remaining" -a "$retry" -gt 0 ]; do

    # This looks rather daunting at first glance.

    # Separate the conditions:
    while [ -n "$remaining" -a "$retry" -gt 0 ]; do
    #       --condition 1-- ^^ --condition 2-

    #  If variable "$remaining" is not zero length
    #+      AND (-a)
    #+ variable "$retry" is greater-than zero
    #+ then
    #+ the [ expresion-within-condition-brackets ] returns success (0)
    #+ and the while-loop executes an iteration.
    #  ==============================================================
    #  Evaluate "condition 1" and "condition 2" ***before***
    #+ ANDing them. Why? Because the AND (-a) has a lower precedence
    #+ than the -n and -gt operators,
    #+ and therefore gets evaluated *last*.

    #################################################################

    if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then

    # Again, separate the conditions:
    if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then
    #    --condition 1--------- ^^ --condition 2-----

    #  If file "/etc/sysconfig/i18n" exists
    #+      AND (-a)
    #+ variable $NOLOCALE is zero length
    #+ then
    #+ the [ test-expresion-within-condition-brackets ] returns success (0)
    #+ and the commands following execute.
    #
    #  As before, the AND (-a) gets evaluated *last*
    #+ because it has the lowest precedence of the operators within
    #+ the test brackets.
    #  ==============================================================
    #  Note:
    #  ${NOLOCALE:-} is a parameter expansion that seems redundant.
    #  But, if $NOLOCALE has not been declared, it gets set to *null*,
    #+ in effect declaring it.
    #  This makes a difference in some contexts.

To avoid confusion or error in a complex sequence of test operators, break up the sequence into bracketed sections.

    if [ "$v1" -gt "$v2"  -o  "$v1" -lt "$v2"  -a  -e "$filename" ]
    # Unclear what's going on here...

    if [[ "$v1" -gt "$v2" ]] || [[ "$v1" -lt "$v2" ]] && [[ -e "$filename" ]]
    # Much better -- the condition tests are grouped in logical sections.

## Beyond the Basics

### Another Look at Variables

Used properly, variables can add power and flexibility to scripts. This requires learning their subtleties and nuances.

#### Internal Variables

Builtin variables:

  variables affecting bash script behavior

`$BASH`

  The path to the Bash binary itself

      bash$ echo $BASH
      /bin/bash

`$BASH_ENV`

  An environmental variable pointing to a Bash startup file to be read when a script is invoked

`$BASH_SUBSHELL`

  A variable indicating the subshell level. This is a new addition to Bash, version 3.

  See for usage.

`$BASHPID`

  Process ID of the current instance of Bash. This is not the same as the $$ variable, but it often gives the same result.

      bash4$ echo $$
      11015

      bash4$ echo $BASHPID
      11015

      bash4$ ps ax | grep bash4
      11015 pts/2    R      0:00 bash4

  But ...

      #!/bin/bash4

      echo "\$\$ outside of subshell = $$"                              # 9602
      echo "\$BASH_SUBSHELL  outside of subshell = $BASH_SUBSHELL"      # 0
      echo "\$BASHPID outside of subshell = $BASHPID"                   # 9602

      echo

      ( echo "\$\$ inside of subshell = $$"                             # 9602
        echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL"      # 1
        echo "\$BASHPID inside of subshell = $BASHPID" )                # 9603
        # Note that $$ returns PID of parent process.

`$BASH_VERSINFO[n]`

  A 6-element array containing version information about the installed release of Bash. This is similar to `$BASH_VERSION`, below, but a bit more detailed.

      # Bash version info:

      for n in 0 1 2 3 4 5
      do
        echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
      done

      # BASH_VERSINFO[0] = 3                      # Major version no.
      # BASH_VERSINFO[1] = 00                     # Minor version no.
      # BASH_VERSINFO[2] = 14                     # Patch level.
      # BASH_VERSINFO[3] = 1                      # Build version.
      # BASH_VERSINFO[4] = release                # Release status.
      # BASH_VERSINFO[5] = i386-redhat-linux-gnu  # Architecture
                                                  # (same as $MACHTYPE).

`$BASH_VERSION`

  The version of Bash installed on the system

      bash$ echo $BASH_VERSION
      3.2.25(1)-release

      tcsh% echo $BASH_VERSION
      BASH_VERSION: Undefined variable.

  Checking $BASH_VERSION is a good method of determining which shell is running. $SHELL does not necessarily give the correct answer.

`$CDPATH`

  A colon-separated list of search paths available to the cd command, similar in function to the $PATH variable for binaries. The `$CDPATH` variable may be set in the local `~/.bashrc` file.

      bash$ cd bash-doc
      bash: cd: bash-doc: No such file or directory

      bash$ CDPATH=/usr/share/doc
      bash$ cd bash-doc
      /usr/share/doc/bash-doc

      bash$ echo $PWD
      /usr/share/doc/bash-doc

`$DIRSTACK`

  The top value in the directory stack

  A stack register is a set of consecutive memory locations, such that the values stored (pushed) are retrieved (popped) in reverse order. The last value stored is the first retrieved. This is sometimes called a LIFO (last-in-first-out) or pushdown stack.

  (affected by pushd and popd)

  This builtin variable corresponds to the dirs command, however `dirs` shows the entire contents of the directory stack.

`$EDITOR`

  The default editor invoked by a script, usually `vi` or `emacs`.

`$EUID`

  effective user ID number

  Identification number of whatever identity the current user has assumed, perhaps by means of su.

  The `$EUID` is not necessarily the same as the $UID.

`$FUNCNAME`

  Name of the current function

      xyz23 ()
      {
        echo "$FUNCNAME now executing."  # xyz23 now executing.
      }

      xyz23

      echo "FUNCNAME = $FUNCNAME"        # FUNCNAME =
                                         # Null value outside a function.

  See also .

`$GLOBIGNORE`

  A list of filename patterns to be excluded from matching in globbing.

`$GROUPS`

  Groups current user belongs to

  This is a listing (array) of the group id numbers for current user, as recorded in `/etc/passwd` and `/etc/group`.

      root# echo $GROUPS
      0

      root# echo ${GROUPS[1]}
      1

      root# echo ${GROUPS[5]}
      6

`$HOME`

  Home directory of the user, usually `/home/username` (see )

`$HOSTNAME`

  The hostname command assigns the system host name at bootup in an init script. However, the `gethostname()` function sets the Bash internal variable `$HOSTNAME`. See also .

`$HOSTTYPE`

  host type

  Like $MACHTYPE, identifies the system hardware.

      bash$ echo $HOSTTYPE
      i686

`$IFS`

  internal field separator

  This variable determines how Bash recognizes fields, or word boundaries, when it interprets character strings.

  $IFS defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a comma-separated data file. Note that $* uses the first character held in `$IFS`. See .

      bash$ echo "$IFS"

      (With $IFS set to default, a blank line displays.)

      bash$ echo "$IFS" | cat -vte
       ^I$
       $
      (Show whitespace: here a single space, ^I [horizontal tab],
        and newline, and display "$" at end-of-line.)

      bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'
      w:x:y:z
      (Read commands from string and assign any arguments to pos params.)

  Set `$IFS` to eliminate whitespace in pathnames.

      IFS="$(printf '\n\t')"   # Per David Wheeler.

  `$IFS` does not handle whitespace the same as it does other characters.

  ##### $IFS and whitespace

      &ifsh;

  (Many thanks, Stéphane Chazelas, for clarification and above examples.)

  See also , , and for instructive examples of using `$IFS`.

`$IGNOREEOF`

  Ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out.

`$LC_COLLATE`

  Often set in the <link linkend="sample-bashrc"<filename>.bashrc or `/etc/profile` files, this variable controls collation order in filename expansion and pattern matching. If mishandled, `LC_COLLATE` can cause unexpected results in filename globbing.

  As of version 2.05 of Bash, filename globbing no longer distinguishes between lowercase and uppercase letters in a character range between brackets. For example, `ls [A-M]*` would match both `File1.txt` and `file1.txt`. To revert to the customary behavior of bracket matching, set `LC_COLLATE` to `C` by an `export LC_COLLATE=C` in `/etc/profile` and/or `~/.bashrc`.

`$LC_CTYPE`

  This internal variable controls character interpretation in globbing and pattern matching.

`$LINENO`

  This variable is the line number of the shell script in which this variable appears. It has significance only within the script in which it appears, and is chiefly useful for debugging purposes.

      # *** BEGIN DEBUG BLOCK ***
      last_cmd_arg=$_  # Save it.

      echo "At line number $LINENO, variable \"v1\" = $v1"
      echo "Last command argument processed = $last_cmd_arg"
      # *** END DEBUG BLOCK ***

`$MACHTYPE`

  machine type

  Identifies the system hardware.

      bash$ echo $MACHTYPE
      i686

`$OLDPWD`

  Old working directory (OLD-Print-Working-Directory, previous directory you were in).

`$OSTYPE`

  operating system type

      bash$ echo $OSTYPE
      linux

`$PATH`

  Path to binaries, usually `/usr/bin/`, `/usr/X11R6/bin/`, `/usr/local/bin`, etc.

  When given a command, the shell automatically does a hash table search on the directories listed in the path for the executable. The path is stored in the environmental variable, `$PATH`, a list of directories, separated by colons. Normally, the system stores the `$PATH` definition in `/etc/profile` and/or `~/.bashrc` (see ).

      bash$ echo $PATH
      /bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin

  `PATH=${PATH}:/opt/bin` appends the `/opt/bin` directory to the current path. In a script, it may be expedient to temporarily add a directory to the path in this way. When the script exits, this restores the original `$PATH` (a child process, such as a script, may not change the environment of the parent process, the shell).

  The current working directory, `./`, is usually omitted from the `$PATH` as a security measure.

`$PIPESTATUS`

  Array variable holding exit status(es) of last executed foreground pipe.

      bash$ echo $PIPESTATUS
      0

      bash$ ls -al | bogus_command
      bash: bogus_command: command not found
      bash$ echo ${PIPESTATUS[1]}
      127

      bash$ ls -al | bogus_command
      bash: bogus_command: command not found
      bash$ echo $?
      127

  The members of the `$PIPESTATUS` array hold the exit status of each respective command executed in a pipe. `$PIPESTATUS[0]` holds the exit status of the first command in the pipe, `$PIPESTATUS[1]` the exit status of the second command, and so on.

  The `$PIPESTATUS` variable may contain an erroneous 0 value in a login shell (in releases prior to 3.0 of Bash).

      tcsh% bash

      bash$ who | grep nobody | sort
      bash$ echo ${PIPESTATUS[*]}
      0

  The above lines contained in a script would produce the expected `0 1 0` output.

  Thank you, Wayne Pollock for pointing this out and supplying the above example.

  The `$PIPESTATUS` variable gives unexpected results in some contexts.

      bash$ echo $BASH_VERSION
      3.00.14(1)-release

      bash$ $ ls | bogus_command | wc
      bash: bogus_command: command not found
       0       0       0

      bash$ echo ${PIPESTATUS[@]}
      141 127 0

  Chet Ramey attributes the above output to the behavior of ls. If ls writes to a pipe whose output is not read, then SIGPIPE kills it, and its exit status is 141. Otherwise its exit status is 0, as expected. This likewise is the case for tr.

  `$PIPESTATUS` is a volatile variable. It needs to be captured immediately after the pipe in question, before any other command intervenes.

      bash$ $ ls | bogus_command | wc
      bash: bogus_command: command not found
       0       0       0

      bash$ echo ${PIPESTATUS[@]}
      0 127 0

      bash$ echo ${PIPESTATUS[@]}
      0

  The pipefail option may be useful in cases where `$PIPESTATUS` does not give the desired information.

`$PPID`

  The `$PPID` of a process is the process ID (`pid`) of its parent process.

  The PID of the currently running script is `$$`, of course.

  Compare this with the pidof command.

`$PROMPT_COMMAND`

  A variable holding a command to be executed just before the primary prompt, `$PS1` is to be displayed.

`$PS1`

  This is the main prompt, seen at the command-line.

`$PS2`

  The secondary prompt, seen when additional input is expected. It displays as >.

`$PS3`

  The tertiary prompt, displayed in a select loop (see ).

`$PS4`

  The quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x [verbose trace] option. It displays as +.

  As a debugging aid, it may be useful to embed diagnostic information in `$PS4`.

      P4='$(read time junk < /proc/$$/schedstat; echo "@@@ $time @@@ " )'
      # Per suggestion by Erik Brandsberg.
      set -x
      # Various commands follow ...

`$PWD`

  Working directory (directory you are in at the time)

  This is the analog to the pwd builtin command.

      &wipedir;

`$REPLY`

  The default value when a variable is not supplied to read. Also applicable to select menus, but only supplies the item number of the variable chosen, not the value of the variable itself.

      &reply;

`$SECONDS`

  The number of seconds the script has been running.

      &seconds;

`$SHELLOPTS`

  The list of enabled shell options, a readonly variable.

      bash$ echo $SHELLOPTS
      braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs

`$SHLVL`

  Shell level, how deeply Bash is nested. Somewhat analogous to recursion, in this context nesting refers to a pattern embedded within a larger pattern. One of the definitions of nest, according to the 1913 edition of Webster's Dictionary, illustrates this beautifully: A collection of boxes, cases, or the like, of graduated size, each put within the one next larger. If, at the command-line, $SHLVL is 1, then in a script it will increment to 2.

  This variable is not affected by subshells. Use $BASH_SUBSHELL when you need an indication of subshell nesting.

`$TMOUT`

  If the $TMOUT environmental variable is set to a non-zero value `time`, then the shell prompt will time out after `$time` seconds. This will cause a logout.

  As of version 2.05b of Bash, it is now possible to use $TMOUT in a script in combination with read.

      # Works in scripts for Bash, versions 2.05b and later.

      TMOUT=3    # Prompt times out at three seconds.

      echo "What is your favorite song?"
      echo "Quickly now, you only have $TMOUT seconds to answer!"
      read song

      if [ -z "$song" ]
      then
        song="(no answer)"
        # Default response.
      fi

      echo "Your favorite song is $song."

  There are other, more complex, ways of implementing timed input in a script. One alternative is to set up a timing loop to signal the script when it times out. This also requires a signal handling routine to trap (see ) the interrupt generated by the timing loop (whew!).

  ##### Timed Input

      &tmdin;

  An alternative is using stty.

  ##### Once more, timed input

      &timeout;

  Perhaps the simplest method is using the `-t` option to read.

  ##### Timed read

      &tout;

`$UID`

  User ID number

  Current user's user identification number, as recorded in `/etc/passwd`

  This is the current user's real id, even if she has temporarily assumed another identity through su. `$UID` is a readonly variable, not subject to change from the command line or within a script, and is the counterpart to the id builtin.

  ##### Am I root?

      &amiroot;

  See also .

  The variables `$ENV`, `$LOGNAME`, `$MAIL`, `$TERM`, `$USER`, and `$USERNAME` are not Bash builtins. These are, however, often set as environmental variables in one of the Bash or login startup files. `$SHELL`, the name of the user's login shell, may be set from `/etc/passwd` or in an init script, and it is likewise not a Bash builtin.

      tcsh% echo $LOGNAME
      bozo
      tcsh% echo $SHELL
      /bin/tcsh
      tcsh% echo $TERM
      rxvt

      bash$ echo $LOGNAME
      bozo
      bash$ echo $SHELL
      /bin/tcsh
      bash$ echo $TERM
      rxvt

##### Positional Parameters

`$0`, `$1`, `$2`, etc.

  Positional parameters, passed from command line to script, passed to a function, or set to a variable (see and )

`$#`

  Number of command-line arguments The words argument and parameter are often used interchangeably. In the context of this document, they have the same precise meaning: a variable passed to a script or function. or positional parameters (see )

`$*`

  All of the positional parameters, seen as a single word

  `$*` must be quoted.

`$@`

  Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without interpretation or expansion. This means, among other things, that each parameter in the argument list is seen as a separate word.

  Of course, `$@` should be quoted.

  ##### arglist: Listing arguments with $* and $@

      &arglist;

  Following a `shift`, the `$@` holds the remaining command-line parameters, lacking the previous `$1`, which was lost.

      #!/bin/bash
      # Invoke with ./scriptname 1 2 3 4 5

      echo "$@"    # 1 2 3 4 5
      shift
      echo "$@"    # 2 3 4 5
      shift
      echo "$@"    # 3 4 5

      # Each "shift" loses parameter $1.
      # "$@" then contains the remaining parameters.

  The `$@` special parameter finds use as a tool for filtering input into shell scripts. The `cat "$@"` construction accepts input to a script either from `stdin` or from files given as parameters to the script. See and .

  The `$*` and `$@` parameters sometimes display inconsistent and puzzling behavior, depending on the setting of $IFS.

  ##### Inconsistent `$*` and `$@` behavior

      &incompat;

  The `$@` and `$*` parameters differ only when between double quotes.

  ##### `$*` and `$@` when `$IFS` is empty

      &ifsempty;

##### Other Special Parameters

`$-`

  Flags passed to script (using set). See .

  This was originally a ksh construct adopted into Bash, and unfortunately it does not seem to work reliably in Bash scripts. One possible use for it is to have a script self-test whether it is interactive.

`$!`

  PID (process ID) of last job run in background

      LOG=$0.log

      COMMAND1="sleep 100"

      echo "Logging PIDs background commands for script: $0" >> "$LOG"
      # So they can be monitored, and killed as necessary.
      echo >> "$LOG"

      # Logging commands.

      echo -n "PID of \"$COMMAND1\":  " >> "$LOG"
      ${COMMAND1} &
      echo $! >> "$LOG"
      # PID of "sleep 100":  1506

      # Thank you, Jacques Lederer, for suggesting this.

  Using `$!` for job control:

      possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }
      # Forces completion of an ill-behaved program.
      # Useful, for example, in init scripts.

      # Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.

  Or, alternately:

      # This example by Matthew Sage.
      # Used with permission.

      TIMEOUT=30   # Timeout value in seconds
      count=0

      possibly_hanging_job & {
              while ((count < TIMEOUT )); do
                      eval '[ ! -d "/proc/$!" ] && ((count = TIMEOUT))'
                      # /proc is where information about running processes is found.
                      # "-d" tests whether it exists (whether directory exists).
                      # So, we're waiting for the job in question to show up.
                      ((count++))
                      sleep 1
              done
              eval '[ -d "/proc/$!" ] && kill -15 $!'
              # If the hanging job is running, kill it.
      }

      #  -------------------------------------------------------------- #

      #  However, this may not not work as specified if another process
      #+ begins to run after the "hanging_job" . . .
      #  In such a case, the wrong job may be killed.
      #  Ariel Meragelman suggests the following fix.

      TIMEOUT=30
      count=0
      # Timeout value in seconds
      possibly_hanging_job & {

      while ((count < TIMEOUT )); do
        eval '[ ! -d "/proc/$lastjob" ] && ((count = TIMEOUT))'
        lastjob=$!
        ((count++))
        sleep 1
      done
      eval '[ -d "/proc/$lastjob" ] && kill -15 $lastjob'

      }

      exit

`$_`

  Special variable set to final argument of previous command executed.

  ##### Underscore variable

      #!/bin/bash

      echo $_              #  /bin/bash
                           #  Just called /bin/bash to run the script.
                           #  Note that this will vary according to
                           #+ how the script is invoked.

      du >/dev/null        #  So no output from command.
      echo $_              #  du

      ls -al >/dev/null    #  So no output from command.
      echo $_              #  -al  (last argument)

      :
      echo $_              #  :

`$?`

  Exit status of a command, function, or the script itself (see )

`$$`

  Process ID (PID) of the script itself.

  Within a script, inside a subshell, `$$` returns the PID of the script, not the subshell.

  The `$$` variable often finds use in scripts to construct unique temp file names (see , , and ). This is usually simpler than invoking mktemp.

#### Typing variables: `declare` or `typeset`

The declare or typeset builtins, which are exact synonyms, permit modifying the properties of variables. This is a very weak form of the typing

In this context, typing a variable means to classify it and restrict its properties. For example, a variable declared or typed as an integer is no longer available for string operations.

    declare -i intvar

    intvar=23
    echo "$intvar"   # 23
    intvar=stringval
    echo "$intvar"   # 0

available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts.

##### declare/typeset options

-r readonly

  (`declare -r var1` works the same as `readonly var1`)

  This is the rough equivalent of the `C` const type qualifier. An attempt to change the value of a readonly variable fails with an error message.

      declare -r var1=1
      echo "var1 = $var1"   # var1 = 1

      (( var1++ ))          # x.sh: line 4: var1: readonly variable

-i integer

      declare -i number
      # The script will treat subsequent occurrences of "number" as an integer.

      number=3
      echo "Number = $number"     # Number = 3

      number=three
      echo "Number = $number"     # Number = 0
      # Tries to evaluate the string "three" as an integer.

  Certain arithmetic operations are permitted for declared integer variables without the need for expr or let.

      n=6/3
      echo "n = $n"       # n = 6/3

      declare -i n
      n=6/3
      echo "n = $n"       # n = 2

-a array

      declare -a indices

  The variable `indices` will be treated as an array.

-f function(s)

      declare -f

  A `declare -f` line with no arguments in a script causes a listing of all the functions previously defined in that script.

      declare -f function_name

  A `declare -f function_name` in a script lists just the function named.

-x export

      declare -x var3

  This declares a variable as available for exporting outside the environment of the script itself.

-x var=$value

      declare -x var3=373

  The `declare` command permits assigning a value to a variable in the same statement as setting its properties.

##### Using declare to type variables

    &ex20;

Using the declare builtin restricts the scope of a variable.

    foo ()
    {
    FOO="bar"
    }

    bar ()
    {
    foo
    echo $FOO
    }

    bar   # Prints bar.

However . . .

    foo (){
    declare FOO="bar"
    }

    bar ()
    {
    foo
    echo $FOO
    }

    bar  # Prints nothing.

    # Thank you, Michael Iatrou, for pointing this out.

##### Another use for declare

The declare command can be helpful in identifying variables, environmental or otherwise. This can be especially useful with arrays.

    bash$ declare | grep HOME
    HOME=/home/bozo

    bash$ zzy=68
    bash$ declare | grep zzy
    zzy=68

    bash$ Colors=([0]="purple" [1]="reddish-orange" [2]="light green")
    bash$ echo ${Colors[@]}
    purple reddish-orange light green
    bash$ declare | grep Colors
    Colors=([0]="purple" [1]="reddish-orange" [2]="light green")

#### $RANDOM: generate random integer

Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin.

--John von Neumann

`$RANDOM` is an internal Bash function (not a constant) that returns a pseudorandom

True randomness, insofar as it exists at all, can only be found in certain incompletely understood natural phenomena, such as radioactive decay. Computers only simulate randomness, and computer-generated sequences of random numbers are therefore referred to as pseudorandom.

integer in the range 0 - 32767. It should not be used to generate an encryption key.

##### Generating random numbers

    &ex21;

##### Picking a random card from a deck

    &pickcard;

##### Brownian Motion Simulation

    &brownian;

Jipe points out a set of techniques for generating random numbers within a range.

    #  Generate random number between 6 and 30.
       rnumber=$((RANDOM%25+6))

    #  Generate random number in the same 6 - 30 range,
    #+ but the number must be evenly divisible by 3.
       rnumber=$(((RANDOM%30/3+1)*3))

    #  Note that this will not work all the time.
    #  It fails if $RANDOM%30 returns 0.

    #  Frank Wang suggests the following alternative:
       rnumber=$(( RANDOM%27/3*3+6 ))

Bill Gradwohl came up with an improved formula that works for positive numbers.

    rnumber=$(((RANDOM%(max-min+divisibleBy))/divisibleBy*divisibleBy+min))

Here Bill presents a versatile function that returns a random number between two specified values.

##### Random between values

    &randombetween;

Just how random is `$RANDOM`? The best way to test this is to write a script that tracks the distribution of random numbers generated by `$RANDOM`. Let's roll a `$RANDOM` die a few times . . .

##### Rolling a single die with RANDOM

    &randomtest;

As we have seen in the last example, it is best to reseed the `RANDOM` generator each time it is invoked. Using the same seed for `RANDOM` repeats the same series of numbers. The seed of a computer-generated pseudorandom number series can be considered an identification label. For example, think of the pseudorandom series with a seed of 23 as Series #23. A property of a pseurandom number series is the length of the cycle before it starts repeating itself. A good pseurandom generator will produce series with very long cycles. (This mirrors the behavior of the random() function in C.)

##### Reseeding RANDOM

    &seedingrandom;

The `/dev/urandom` pseudo-device file provides a method of generating much more random pseudorandom numbers than the `$RANDOM` variable. `dd if=/dev/urandom of=targetfile bs=1 count=XX` creates a file of well-scattered pseudorandom numbers. However, assigning these numbers to a variable in a script requires a workaround, such as filtering through od (as in above example, , and ), or even piping to md5sum (see ).

There are also other ways to generate pseudorandom numbers in a script. `Awk` provides a convenient means of doing this.

##### Pseudorandom numbers, using awk

    &random2;

The date command also lends itself to generating pseudorandom integer sequences.

### Manipulating Variables

#### Manipulating Strings

Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr command. This results in inconsistent command syntax and overlap of functionality, not to mention confusion.

##### String Length

${#string}

expr length $string

  These are the equivalent of strlen() in C.

expr "$string" : '.*'

      stringZ=abcABC123ABCabc

      echo ${#stringZ}                 # 15
      echo `expr length $stringZ`      # 15
      echo `expr "$stringZ" : '.*'`    # 15

##### Inserting a blank line between paragraphs in a text file

    &paragraphspace;

##### Length of Matching Substring at Beginning of String

expr match "$string" '$substring'

  $substring is a regular expression.

expr "$string" : '$substring'

  $substring is a regular expression.

      stringZ=abcABC123ABCabc
      #       |------|
      #       12345678

      echo `expr match "$stringZ" 'abc[A-Z]*.2'`   # 8
      echo `expr "$stringZ" : 'abc[A-Z]*.2'`       # 8

##### Index

expr index $string $substring

  Numerical position in $string of first character in $substring that matches.

      stringZ=abcABC123ABCabc
      #       123456 ...
      echo `expr index "$stringZ" C12`             # 6
                                                   # C position.

      echo `expr index "$stringZ" 1c`              # 3
      # 'c' (in #3 position) matches before '1'.

  This is the near equivalent of strchr() in C.

##### Substring Extraction

${string:position}

  Extracts substring from $string at $position.

  If the `$string` parameter is * or @, then this extracts the positional parameters, This applies to either command-line arguments or parameters passed to a function. starting at `$position`.

${string:position:length}

  Extracts $length characters of substring from $string at $position.

      stringZ=abcABC123ABCabc
      #       0123456789.....
      #       0-based indexing.

      echo ${stringZ:0}                            # abcABC123ABCabc
      echo ${stringZ:1}                            # bcABC123ABCabc
      echo ${stringZ:7}                            # 23ABCabc

      echo ${stringZ:7:3}                          # 23A
                                                   # Three characters of substring.

      # Is it possible to index from the right end of the string?

      echo ${stringZ:-4}                           # abcABC123ABCabc
      # Defaults to full string, as in ${parameter:-default}.
      # However . . .

      echo ${stringZ:(-4)}                         # Cabc
      echo ${stringZ: -4}                          # Cabc
      # Now, it works.
      # Parentheses or added space "escape" the position parameter.

      # Thank you, Dan Jacobson, for pointing this out.

  The position and length arguments can be parameterized, that is, represented as a variable, rather than as a numerical constant.

  ##### Generating an 8-character random string

      &randstring;

  If the `$string` parameter is * or @, then this extracts a maximum of `$length` positional parameters, starting at `$position`.

      echo ${*:2}          # Echoes second and following positional parameters.
      echo ${@:2}          # Same as above.

      echo ${*:2:3}        # Echoes three positional parameters, starting at second.

expr substr $string $position $length

  Extracts $length characters from $string starting at $position.

      stringZ=abcABC123ABCabc
      #       123456789......
      #       1-based indexing.

      echo `expr substr $stringZ 1 2`              # ab
      echo `expr substr $stringZ 4 3`              # ABC

expr match "$string" '\($substring\)'

  Extracts $substring at beginning of $string, where $substring is a regular expression.

expr "$string" : '\($substring\)'

  Extracts $substring at beginning of $string, where $substring is a regular expression.

      stringZ=abcABC123ABCabc
      #       =======

      echo `expr match "$stringZ" '\(.[b-c]*[A-Z]..[0-9]\)'`   # abcABC1
      echo `expr "$stringZ" : '\(.[b-c]*[A-Z]..[0-9]\)'`       # abcABC1
      echo `expr "$stringZ" : '\(.......\)'`                   # abcABC1
      # All of the above forms give an identical result.

expr match "$string" '.*\($substring\)'

  Extracts $substring at end of $string, where $substring is a regular expression.

expr "$string" : '.*\($substring\)'

  Extracts $substring at end of $string, where $substring is a regular expression.

      stringZ=abcABC123ABCabc
      #                ======

      echo `expr match "$stringZ" '.*\([A-C][A-C][A-C][a-c]*\)'`    # ABCabc
      echo `expr "$stringZ" : '.*\(......\)'`                       # ABCabc

##### Substring Removal

${string#substring}

  Deletes shortest match of $substring from front of $string.

${string##substring}

  Deletes longest match of $substring from front of $string.

      stringZ=abcABC123ABCabc
      #       |----|          shortest
      #       |----------|    longest

      echo ${stringZ#a*C}      # 123ABCabc
      # Strip out shortest match between 'a' and 'C'.

      echo ${stringZ##a*C}     # abc
      # Strip out longest match between 'a' and 'C'.

      # You can parameterize the substrings.

      X='a*C'

      echo ${stringZ#$X}      # 123ABCabc
      echo ${stringZ##$X}     # abc
                              # As above.

${string%substring}

  Deletes shortest match of $substring from back of $string.

  For example:

      # Rename all filenames in $PWD with "TXT" suffix to a "txt" suffix.
      # For example, "file1.TXT" becomes "file1.txt" . . .

      SUFF=TXT
      suff=txt

      for i in $(ls *.$SUFF)
      do
        mv -f $i ${i%.$SUFF}.$suff
        #  Leave unchanged everything *except* the shortest pattern match
        #+ starting from the right-hand-side of the variable $i . . .
      done ### This could be condensed into a "one-liner" if desired.

      # Thank you, Rory Winston.

${string%%substring}

  Deletes longest match of $substring from back of $string.

      stringZ=abcABC123ABCabc
      #                    ||     shortest
      #        |------------|     longest

      echo ${stringZ%b*c}      # abcABC123ABCa
      # Strip out shortest match between 'b' and 'c', from back of $stringZ.

      echo ${stringZ%%b*c}     # a
      # Strip out longest match between 'b' and 'c', from back of $stringZ.

  This operator is useful for generating filenames.

  ##### Converting graphic file formats, with filename change

      &cvt;

  ##### Converting streaming audio files to ogg

      &ra2ogg;

  A simple emulation of getopt using substring-extraction constructs.

  ##### Emulating getopt

      &getoptsimple;

##### Substring Replacement

${string/substring/replacement}

  Replace first match of $substring with $replacement. Note that $substring and $replacement may refer to either literal strings or variables, depending on context. See the first usage example.

${string//substring/replacement}

  Replace all matches of $substring with $replacement.

      stringZ=abcABC123ABCabc

      echo ${stringZ/abc/xyz}       # xyzABC123ABCabc
                                    # Replaces first match of 'abc' with 'xyz'.

      echo ${stringZ//abc/xyz}      # xyzABC123ABCxyz
                                    # Replaces all matches of 'abc' with # 'xyz'.

      echo  ---------------
      echo "$stringZ"               # abcABC123ABCabc
      echo  ---------------
                                    # The string itself is not altered!

      # Can the match and replacement strings be parameterized?
      match=abc
      repl=000
      echo ${stringZ/$match/$repl}  # 000ABC123ABCabc
      #              ^      ^         ^^^
      echo ${stringZ//$match/$repl} # 000ABC123ABC000
      # Yes!          ^      ^        ^^^         ^^^

      echo

      # What happens if no $replacement string is supplied?
      echo ${stringZ/abc}           # ABC123ABCabc
      echo ${stringZ//abc}          # ABC123ABC
      # A simple deletion takes place.

${string/#substring/replacement}

  If $substring matches front end of $string, substitute $replacement for $substring.

${string/%substring/replacement}

  If $substring matches back end of $string, substitute $replacement for $substring.

      stringZ=abcABC123ABCabc

      echo ${stringZ/#abc/XYZ}          # XYZABC123ABCabc
                                        # Replaces front-end match of 'abc' with 'XYZ'.

      echo ${stringZ/%abc/XYZ}          # abcABC123ABCXYZ
                                        # Replaces back-end match of 'abc' with 'XYZ'.

##### Manipulating strings using awk

<anchor id="awkstringmanip2"</para>

A Bash script may invoke the string manipulation facilities of awk as an alternative to using its built-in operations.

###### Alternate ways of extracting and locating substrings

    &substringex;

##### Further Reference

For more on string manipulation in scripts, refer to and the relevant section of the expr command listing.

Script examples:

#### Parameter Substitution

##### Manipulating and/or expanding variables

`${parameter}`

  Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less ambiguous ${parameter} form works.

  May be used for concatenating variables with strings.

      your_id=${USER}-on-${HOSTNAME}
      echo "$your_id"
      #
      echo "Old \$PATH = $PATH"
      PATH=${PATH}:/opt/bin  # Add /opt/bin to $PATH for duration of script.
      echo "New \$PATH = $PATH"

`${parameter-default}`
`${parameter:-default}`

  If parameter not set, use default.

      var1=1
      var2=2
      # var3 is unset.

      echo ${var1-$var2}   # 1
      echo ${var3-$var2}   # 2
      #           ^          Note the $ prefix.

      echo ${username-`whoami`}
      # Echoes the result of `whoami`, if variable $username is still unset.

  ${parameter-default} and ${parameter:-default} are almost equivalent. The extra : makes a difference only when `parameter` has been declared, but is null.

      &paramsub;

  The default parameter construct finds use in providing missing command-line arguments in scripts.

      DEFAULT_FILENAME=generic.data
      filename=${1:-$DEFAULT_FILENAME}
      #  If not otherwise specified, the following command block operates
      #+ on the file "generic.data".
      #  Begin-Command-Block
      #  ...
      #  ...
      #  ...
      #  End-Command-Block

      #  From "hanoi2.bash" example:
      DISKS=${1:-E_NOPARAM}   # Must specify how many disks.
      #  Set $DISKS to $1 command-line-parameter,
      #+ or to $E_NOPARAM if that is unset.

  See also , , and .

  Compare this method with using an and list to supply a default command-line argument.

`${parameter=default}`
`${parameter:=default}`

  If parameter not set, set it to default.

  Both forms nearly equivalent. The : makes a difference only when `$parameter` has been declared and is null, If $parameter is null in a non-interactive script, it will terminate with a 127 exit status (the Bash error code for command not found). as above.

      echo ${var=abc}   # abc
      echo ${var=xyz}   # abc
      # $var had already been set to abc, so it did not change.

`${parameter+alt_value}`
`${parameter:+alt_value}`

  If parameter set, use `alt_value`, else use null string.

  Both forms nearly equivalent. The : makes a difference only when `parameter` has been declared and is null, see below.

      echo "###### \${parameter+alt_value} ########"
      echo

      a=${param1+xyz}
      echo "a = $a"      # a =

      param2=
      a=${param2+xyz}
      echo "a = $a"      # a = xyz

      param3=123
      a=${param3+xyz}
      echo "a = $a"      # a = xyz

      echo
      echo "###### \${parameter:+alt_value} ########"
      echo

      a=${param4:+xyz}
      echo "a = $a"      # a =

      param5=
      a=${param5:+xyz}
      echo "a = $a"      # a =
      # Different result from   a=${param5+xyz}

      param6=123
      a=${param6:+xyz}
      echo "a = $a"      # a = xyz

`${parameter?err_msg}`
`${parameter:?err_msg}`

  If parameter set, use it, else print err_msg and abort the script with an exit status of 1.

  Both forms nearly equivalent. The : makes a difference only when `parameter` has been declared and is null, as above.

##### Using parameter substitution and error messages

    &ex6;

##### Parameter substitution and usage messages

    &usagemessage;

##### Parameter substitution and/or expansion

The following expressions are the complement to the `match` in `expr` string operations (see ). These particular ones are used mostly in parsing file path names.

##### Variable length / Substring removal

`${#var}`

  `String length` (number of characters in `$var`). For an array, `${#array}` is the length of the first element in the array.

  Exceptions:

  - `${#*}` and `${#@}` give the number of positional parameters.

  - For an array, `${#array[*]}` and `${#array[@]}` give the number of elements in the array.

  ##### Length of a variable

      &length;

`${var#Pattern}`
`${var##Pattern}`

  `${var#Pattern}` Remove from `$var` the shortest part of `$Pattern` that matches the front end of `$var`.

  `${var##Pattern}` Remove from `$var` the longest part of `$Pattern` that matches the front end of `$var`.

  A usage illustration from :

      # Function from "days-between.sh" example.
      # Strips leading zero(s) from argument passed.

      strip_leading_zero () #  Strip possible leading zero(s)
      {                     #+ from argument passed.
        return=${1#0}       #  The "1" refers to "$1" -- passed arg.
      }                     #  The "0" is what to remove from "$1" -- strips zeros.

  Manfred Schwarb's more elaborate variation of the above:

      strip_leading_zero2 () # Strip possible leading zero(s), since otherwise
      {                      # Bash will interpret such numbers as octal values.
        shopt -s extglob     # Turn on extended globbing.
        local val=${1##+(0)} # Use local variable, longest matching series of 0's.
        shopt -u extglob     # Turn off extended globbing.
        _strip_leading_zero2=${val:-0}
                             # If input was 0, return 0 instead of "".
      }

  Another usage illustration:

      echo `basename $PWD`        # Basename of current working directory.
      echo "${PWD##*/}"           # Basename of current working directory.
      echo
      echo `basename $0`          # Name of script.
      echo $0                     # Name of script.
      echo "${0##*/}"             # Name of script.
      echo
      filename=test.data
      echo "${filename##*.}"      # data
                                  # Extension of filename.

`${var%Pattern}`
`${var%%Pattern}`

  `${var%Pattern}` Remove from `$var` the shortest part of `$Pattern` that matches the back end of `$var`.

  `${var%%Pattern}` Remove from `$var` the longest part of `$Pattern` that matches the back end of `$var`.

Version 2 of Bash added additional options.

##### Pattern matching in parameter substitution

    &pattmatching;

##### Renaming file extensions:

    &rfe;

##### Variable expansion / Substring replacement

  These constructs have been adopted from ksh.

`${var:pos}`

  Variable var expanded, starting from offset pos.

`${var:pos:len}`

  Expansion to a max of len characters of variable var, from offset pos. See for an example of the creative use of this operator.

`${var/Pattern/Replacement}`

  First match of Pattern, within var replaced with Replacement.

  If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is, deleted.

`${var//Pattern/Replacement}`

  ##### Global replacement

  All matches of Pattern, within var replaced with Replacement.

  As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing, that is, deleted.

  ##### Using pattern matching to parse arbitrary strings

      &ex7;

`${var/#Pattern/Replacement}`

  If prefix of var matches Pattern, then substitute Replacement for Pattern.

`${var/%Pattern/Replacement}`

  If suffix of var matches Pattern, then substitute Replacement for Pattern.

  ##### Matching patterns at prefix or suffix of string

      &varmatch;

`${!varprefix*}`
`${!varprefix@}`

  Matches names of all previously declared variables beginning with `varprefix`.

      # This is a variation on indirect reference, but with a * or @.
      # Bash, version 2.04, adds this feature.

      xyz23=whatever
      xyz24=

      a=${!xyz*}         #  Expands to *names* of declared variables
      # ^ ^   ^           + beginning with "xyz".
      echo "a = $a"      #  a = xyz23 xyz24
      a=${!xyz@}         #  Same as above.
      echo "a = $a"      #  a = xyz23 xyz24

      echo "---"

      abc23=something_else
      b=${!abc*}
      echo "b = $b"      #  b = abc23
      c=${!b}            #  Now, the more familiar type of indirect reference.
      echo $c            #  something_else

### Loops and Branches

What needs this iteration, woman?

--Shakespeare, Othello

Operations on code blocks are the key to structured and organized shell scripts. Looping and branching constructs provide the tools for accomplishing this.

#### Loops

A loop is a block of code that iterates

Iteration: Repeated execution of a command or group of commands, usually -- but not always, while a given condition holds, or until a given condition is met.

a list of commands as long as the loop control condition is true.

##### for loops

`for `arg` in [list]`

  This is the basic looping construct. It differs significantly from its C counterpart.

      		for
      		arg
      		in
      		list
      		do
      		 command(s)
      		done

  During each pass through the loop, arg takes on the value of each successive variable in the list.

      for arg in "$var1" "$var2" "$var3" ... "$varN"
      # In pass 1 of the loop, arg = $var1
      # In pass 2 of the loop, arg = $var2
      # In pass 3 of the loop, arg = $var3
      # ...
      # In pass N of the loop, arg = $varN

      # Arguments in [list] quoted to prevent possible word splitting.

  The argument list may contain wild cards.

  If do is on same line as for, there needs to be a semicolon after list.

      		for
      		arg
      		in
      		list
      		;
      		do

  ##### Simple for loops

      &ex22;

  Each `[list]` element may contain multiple parameters. This is useful when processing parameters in groups. In such cases, use the set command (see ) to force parsing of each `[list]` element and assignment of each component to the positional parameters.

  ##### for loop with two parameters in each [list] element

      &ex22a;

  A variable may supply the `[list]` in a for loop.

  ##### Fileinfo: operating on a file list contained in a variable

      &fileinfo;

  The `[list]` in a for loop may be parameterized.

  ##### Operating on a parameterized file list

      &fileinfo01;

  If the `[list]` in a for loop contains wild cards (* and ?) used in filename expansion, then globbing takes place.

  ##### Operating on files with a for loop

      &listglob;

  Omitting the `in [list]` part of a for loop causes the loop to operate on $@ -- the positional parameters. A particularly clever illustration of this is . See also .

  ##### Missing `in [list]` in a for loop

      &ex23;

  It is possible to use command substitution to generate the `[list]` in a for loop. See also , and .

  ##### Generating the `[list]` in a for loop with command substitution

      &forloopcmd;

  Here is a somewhat more complex example of using command substitution to create the `[list]`.

  ##### A grep replacement for binary files

      &bingrep;

  More of the same.

  ##### Listing all users on the system

      &userlist;

  Yet another example of the `[list]` resulting from command substitution.

  ##### Checking all the binaries in a directory for authorship

      &findstring;

  A final example of `[list]` / command substitution, but this time the command is a function.

      generate_list ()
      {
        echo "one two three"
      }

      for word in $(generate_list)  # Let "word" grab output of function.
      do
        echo "$word"
      done

      # one
      # two
      # three

  The output of a for loop may be piped to a command or commands.

  ##### Listing the symbolic links in a directory

      &symlinks;

  The `stdout` of a loop may be redirected to a file, as this slight modification to the previous example shows.

  ##### Symbolic links in a directory, saved to a file

      &symlinks2;

  There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses.

  ##### A C-style for loop

      &forloopc;

  See also , , and .

  ---

  Now, a for loop used in a real-life context.

  ##### Using efax in batch mode

      &ex24;

  The keywords `do` and `done` delineate the for-loop command block. However, these may, in certain contexts, be omitted by framing the command block within curly brackets

      for((n=1; n<=10; n++))
      # No do!
      {
        echo -n "* $n *"
      }
      # No done!

      # Outputs:
      # * 1 ** 2 ** 3 ** 4 ** 5 ** 6 ** 7 ** 8 ** 9 ** 10 *
      # And, echo $? returns 0, so Bash does not register an error.

      echo

      #  But, note that in a classic for-loop:    for n in [list] ...
      #+ a terminal semicolon is required.

      for n in 1 2 3
      {  echo -n "$n "; }
      #               ^

      # Thank you, YongYe, for pointing this out.

`while`

  This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the number of loop repetitions is not known beforehand.

      		while
      		 condition
      		do
      		 command(s)
      		done

  The bracket construct in a while loop is nothing more than our old friend, the test brackets used in an if/then test. In fact, a while loop can legally use the more versatile double-brackets construct (while [[ condition ]]).

  As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.

      		while
      		 condition
      		;
      		do

  Note that the test brackets are not mandatory in a while loop. See, for example, the getopts construct.

  ##### Simple while loop

      &ex25;

  ##### Another while loop

      &ex26;

  A while loop may have multiple conditions. Only the final condition determines when the loop terminates. This necessitates a slightly different loop syntax, however.

  ##### while loop with multiple conditions

      &ex26a;

  As with a for loop, a while loop may employ C-style syntax by using the double-parentheses construct (see also ).

  ##### C-style syntax in a while loop

      &whloopc;

  Inside its test brackets, a while loop can call a function.

      t=0

      condition ()
      {
        ((t++))

        if [ $t -lt 5 ]
        then
          return 0  # true
        else
          return 1  # false
        fi
      }

      while condition
      #     ^^^^^^^^^
      #     Function call -- four loop iterations.
      do
        echo "Still going: t = $t"
      done

      # Still going: t = 1
      # Still going: t = 2
      # Still going: t = 3
      # Still going: t = 4

  Similar to the if-test construct, a while loop can omit the test brackets.

      while condition
      do
         command(s) ...
      done

  By coupling the power of the read command with a while loop, we get the handy while read construct, useful for reading and parsing files.

      cat $filename |   # Supply input from a file.
      while read line   # As long as there is another line to read ...
      do
        ...
      done

      # =========== Snippet from "sd.sh" example script ========== #

        while read value   # Read one data point at a time.
        do
          rt=$(echo "scale=$SC; $rt + $value" | bc)
          (( ct++ ))
        done

        am=$(echo "scale=$SC; $rt / $ct" | bc)

        echo $am; return $ct   # This function "returns" TWO values!
        #  Caution: This little trick will not work if $ct > 255!
        #  To handle a larger number of data points,
        #+ simply comment out the "return $ct" above.
      } <"$datafile"   # Feed in data file.

  A while loop may have its `stdin` redirected to a file by a < at its end.

  A while loop may have its `stdin` supplied by a pipe.

`until`

  This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is false (opposite of while loop).

      		until
      		 condition-is-true
      		do
      		 command(s)
      		done

  Note that an until loop tests for the terminating condition at the top of the loop, differing from a similar construct in some programming languages.

  As is the case with for loops, placing the do on the same line as the condition test requires a semicolon.

      		until
      		 condition-is-true
      		;
      		do

  ##### until loop

      &ex27;

How to choose between a for loop or a while loop or until loop? In `C`, you would typically use a for loop when the number of loop iterations is known beforehand. With Bash, however, the situation is fuzzier. The Bash for loop is more loosely structured and more flexible than its equivalent in other languages. Therefore, feel free to use whatever type of loop gets the job done in the simplest way.

#### Nested Loops

A nested loop is a loop within a loop, an inner loop within the body of an outer one. How this works is that the first pass of the outer loop triggers the inner loop, which executes to completion. Then the second pass of the outer loop triggers the inner loop again. This repeats until the outer loop finishes. Of course, a break within either the inner or outer loop would interrupt this process.

##### Nested Loop

    &nestedloop;

See for an illustration of nested while loops, and to see a while loop nested inside an until loop.

#### Loop Control

Tournez cent tours, tournez mille tours,

Tournez souvent et tournez toujours . . .

--Verlaine, Chevaux de bois

##### Commands affecting loop behavior

`break`
`continue`

  The `break` and `continue` loop control commands These are shell builtins, whereas other loop commands, such as while and case, are keywords. correspond exactly to their counterparts in other programming languages. The `break` command terminates the loop (breaks out of it), while `continue` causes a jump to the next iteration of the loop, skipping all the remaining commands in that particular loop cycle.

  ##### Effects of break and `continue` in a loop

      &ex28;

  The `break` command may optionally take a parameter. A plain `break` terminates only the innermost loop in which it is embedded, but a `break N` breaks out of `N` levels of loop.

  ##### Breaking out of multiple loop levels

      &breaklevels;

  The `continue` command, similar to `break`, optionally takes a parameter. A plain `continue` cuts short the current iteration within its loop and begins the next. A `continue N` terminates all remaining iterations at its loop level and continues with the next iteration at the loop, `N` levels above.

  ##### Continuing at a higher loop level

      &continuelevels;

  ##### Using continue N in an actual task

      &continuenex;

  The `continue N` construct is difficult to understand and tricky to use in any meaningful context. It is probably best avoided.

#### Testing and Branching

The `case` and `select` constructs are technically not loops, since they do not iterate the execution of a code block. Like loops, however, they direct program flow according to conditions at the top or bottom of the block.

##### Controlling program flow in a code block

`case (in) / esac`

  The `case` construct is the shell scripting analog to switch in `C/C++`. It permits branching to one of a number of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple if/then/else statements and is an appropriate tool for creating menus.

      		case
      		"$variable"
      		in
      		 "$condition1" )
      		 command
      		 ;;
      		 "$condition2" )
      		 command
      		 ;;
      		esac

  - Quoting the variables is not mandatory, since word splitting does not take place.

  - Each test line ends with a right paren `)`. Pattern-match lines may also start with a `(` left paren to give the layout a more structured appearance.

        case $( arch ) in   # $( arch ) returns machine architecture.
          ( i386 ) echo "80386-based machine";;
        # ^      ^
          ( i486 ) echo "80486-based machine";;
          ( i586 ) echo "Pentium-based machine";;
          ( i686 ) echo "Pentium2+-based machine";;
          (    * ) echo "Other type of machine";;
        esac

  - Each condition block ends with a double semicolon ;;.

  - If a condition tests true, then the associated commands execute and the `case` block terminates.

  - The entire `case` block ends with an `esac` (case spelled backwards).

  ##### Using case

      &ex29;

  ##### Creating menus using case

      &ex30;

  An exceptionally clever use of `case` involves testing for command-line parameters.

      #! /bin/bash

      case "$1" in
        "") echo "Usage: ${0##*/} <filename>"; exit $E_PARAM;;
                            # No command-line parameters,
                            # or first parameter empty.
      # Note that ${0##*/} is ${var##pattern} param substitution.
                            # Net result is $0.

        -*) FILENAME=./$1;;   #  If filename passed as argument ($1)
                            #+ starts with a dash,
                            #+ replace it with ./$1
                            #+ so further commands don't interpret it
                            #+ as an option.

        * ) FILENAME=$1;;     # Otherwise, $1.
      esac

  Here is a more straightforward example of command-line parameter handling:

      #! /bin/bash

      while [ $# -gt 0 ]; do    # Until you run out of parameters . . .
        case "$1" in
          -d|--debug)
                    # "-d" or "--debug" parameter?
                    DEBUG=1
                    ;;
          -c|--conf)
                    CONFFILE="$2"
                    shift
                    if [ ! -f $CONFFILE ]; then
                      echo "Error: Supplied file doesn't exist!"
                      exit $E_CONFFILE     # File not found error.
                    fi
                    ;;
        esac
        shift       # Check next set of parameters.
      done

      #  From Stefano Falsetto's "Log2Rot" script,
      #+ part of his "rottlog" package.
      #  Used with permission.

  ##### Using command substitution to generate the case variable

      &casecmd;

  A `case` construct can filter strings for globbing patterns.

  ##### Simple string matching

      &matchstring;

  ##### Checking for alphabetic input

      &isalpha;

`select`

  The `select` construct, adopted from the Korn Shell, is yet another tool for building menus.

      		select
      		variable
      		in list
      		do
      		 command
      		 break
      		done

  This prompts the user to enter one of the choices presented in the variable list. Note that `select` uses the `$PS3` prompt (#? ) by default, but this may be changed.

  ##### Creating menus using select

      &ex31;

  If `in list` is omitted, then `select` uses the list of command line arguments (`$@`) passed to the script or the function containing the `select` construct.

  Compare this to the behavior of a

      		for
      		variable
      		in list

  construct with the `in list` omitted.

  ##### Creating menus using select in a function

      &ex32;

  See also .

### Command Substitution

`Command substitution` reassigns the output of a command For purposes of command substitution, a `command` may be an external system command, an internal scripting builtin, or even a script function. or even multiple commands; it literally plugs the command output into another context. In a more technically correct sense, command substitution extracts the `stdout` of a command, then assigns it to a variable using the = operator.

The classic form of command substitution uses backquotes (`...`). Commands within backquotes (backticks) generate command-line text.

    script_name=`basename $0`
    echo "The name of this script is $script_name."

#### The output of commands can be used as arguments to another command, to set a variable, and even for generating the argument list in a for loop.

    rm `cat filename`   # filename contains a list of files to delete.
    #
    # S. C. points out that "arg list too long" error might result.
    # Better is              xargs rm -- < filename
    # ( -- covers those cases where filename begins with a - )

    textfile_listing=`ls *.txt`
    # Variable contains names of all *.txt files in current working directory.
    echo $textfile_listing

    textfile_listing2=$(ls *.txt)   # The alternative form of command substitution.
    echo $textfile_listing2
    # Same result.

    # A possible problem with putting a list of files into a single string
    # is that a newline may creep in.
    #
    # A safer way to assign a list of files to a parameter is with an array.
    #      shopt -s nullglob    # If no match, filename expands to nothing.
    #      textfile_listing=( *.txt )
    #
    # Thanks, S.C.

Command substitution invokes a subshell.

Command substitution may result in word splitting.

    COMMAND `echo a b`     # 2 args: a and b

    COMMAND "`echo a b`"   # 1 arg: "a b"

    COMMAND `echo`         # no arg

    COMMAND "`echo`"       # one empty arg

    # Thanks, S.C.

Even when there is no word splitting, command substitution can remove trailing newlines.

    # cd "`pwd`"  # This should always work.
    # However...

    mkdir 'dir with trailing newline
    '

    cd 'dir with trailing newline
    '

    cd "`pwd`"  # Error message:
    # bash: cd: /tmp/file with trailing newline: No such file or directory

    cd "$PWD"   # Works fine.

    old_tty_setting=$(stty -g)   # Save old terminal setting.
    echo "Hit a key "
    stty -icanon -echo           # Disable "canonical" mode for terminal.
                                 # Also, disable *local* echo.
    key=$(dd bs=1 count=1 2> /dev/null)   # Using 'dd' to get a keypress.
    stty "$old_tty_setting"      # Restore old setting.
    echo "You hit ${#key} key."  # ${#variable} = number of characters in $variable
    #
    # Hit any key except RETURN, and the output is "You hit 1 key."
    # Hit RETURN, and it's "You hit 0 key."
    # The newline gets eaten in the command substitution.

    #Code snippet by Stéphane Chazelas.

Using `echo` to output an unquoted variable set with command substitution removes trailing newlines characters from the output of the reassigned command(s). This can cause unpleasant surprises.

    dir_listing=`ls -l`
    echo $dir_listing     # unquoted

    # Expecting a nicely ordered directory listing.

    # However, what you get is:
    # total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
    # bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh

    # The newlines disappeared.

    echo "$dir_listing"   # quoted
    # -rw-rw-r--    1 bozo       30 May 13 17:15 1.txt
    # -rw-rw-r--    1 bozo       51 May 15 20:57 t2.sh
    # -rwxr-xr-x    1 bozo      217 Mar  5 21:13 wi.sh

Command substitution even permits setting a variable to the contents of a file, using either redirection or the cat command.

    variable1=`<file1`      #  Set "variable1" to contents of "file1".
    variable2=`cat file2`   #  Set "variable2" to contents of "file2".
                            #  This, however, forks a new process,
                            #+ so the line of code executes slower than the above version.

    #  Note that the variables may contain embedded whitespace,
    #+ or even (horrors), control characters.

    #  It is not necessary to explicitly assign a variable.
    echo "` <$0`"           # Echoes the script itself to stdout.

    #  Excerpts from system file, /etc/rc.d/rc.sysinit
    #+ (on a Red Hat Linux installation)

    if [ -f /fsckoptions ]; then
            fsckoptions=`cat /fsckoptions`
    ...
    fi
    #
    #
    if [ -e "/proc/ide/${disk[$device]}/media" ] ; then
                 hdmedia=`cat /proc/ide/${disk[$device]}/media`
    ...
    fi
    #
    #
    if [ ! -n "`uname -r | grep -- "-"`" ]; then
           ktag="`cat /proc/version`"
    ...
    fi
    #
    #
    if [ $usb = "1" ]; then
        sleep 5
        mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=02"`
        kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"`
    ...
    fi

Do not set a variable to the contents of a long text file unless you have a very good reason for doing so. Do not set a variable to the contents of a binary file, even as a joke.

#### Stupid script tricks

    &stupscr;

Notice that a buffer overrun does not occur. This is one instance where an interpreted language, such as Bash, provides more protection from programmer mistakes than a compiled language.

Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output of an echo command within the loop.

#### Generating a variable from a loop

    &csubloop;

Command substitution makes it possible to extend the toolset available to Bash. It is simply a matter of writing a program or script that outputs to `stdout` (like a well-behaved UNIX tool should) and assigning that output to a variable.

    #include <stdio.h>

    /*  "Hello, world." C program  */

    int main()
    {
      printf( "Hello, world.\n" );
      return (0);
    }

    bash$ gcc -o hello hello.c

    #!/bin/bash
    # hello.sh

    greeting=`./hello`
    echo $greeting

    bash$ sh hello.sh
    Hello, world.

The `$(...)` form has superseded backticks for command substitution.

    output=$(sed -n /"$1"/p $file)   # From "grp.sh"	example.

    # Setting a variable to the contents of a text file.
    File_contents1=$(cat $file1)
    File_contents2=$(<$file2)        # Bash permits this also.

The `$(...)` form of command substitution treats a double backslash in a different way than ``...``.

    bash$ echo `echo \\`

    bash$ echo $(echo \\)
    \

The `$(...)` form of command substitution permits nesting. In fact, nesting with backticks is also possible, but only by escaping the inner backticks, as John Default points out.

    word_count=` wc -w \`echo * | awk '{print $8}'\` `

    word_count=$( wc -w $(echo * | awk '{print $8}') )

Or, for something a bit more elaborate . . .

#### Finding anagrams

    &agram2;

Examples of command substitution in shell scripts:

### Arithmetic Expansion

Arithmetic expansion provides a powerful tool for performing (integer) arithmetic operations in scripts. Translating a string into a numerical expression is relatively straightforward using backticks, double parentheses, or let.

#### Variations

Arithmetic expansion with backticks (often used in conjunction with expr)

      z=`expr $z + 3`          # The 'expr' command performs the expansion.

Arithmetic expansion with double parentheses
and using let

  The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses -- `((...))` and `$((...))` -- and also by the very convenient let construction.

      z=$(($z+3))
      z=$((z+3))                                  #  Also correct.
                                                  #  Within double parentheses,
                                                  #+ parameter dereferencing
                                                  #+ is optional.

      # $((EXPRESSION)) is arithmetic expansion.  #  Not to be confused with
                                                  #+ command substitution.

      # You may also use operations within double parentheses without assignment.

        n=0
        echo "n = $n"                             # n = 0

        (( n += 1 ))                              # Increment.
      # (( $n += 1 )) is incorrect!
        echo "n = $n"                             # n = 1

      let z=z+3
      let "z += 3"  #  Quotes permit the use of spaces in variable assignment.
                    #  The 'let' operator actually performs arithmetic evaluation,
                    #+ rather than expansion.

  Examples of arithmetic expansion in scripts:

### Recess Time

This bizarre little intermission gives the reader a chance to relax and maybe laugh a bit.

>     	  Fellow Linux user, greetings! You are reading something which
>     	  will bring you luck and good fortune. Just e-mail a copy of
>     	  this document to 10 of your friends. Before making the copies,
>     	  send a 100-line Bash script to the first person on the list
>     	  at the bottom of this letter. Then delete their name and add
>     	  yours to the bottom of the list.

>     	  Don't break the chain! Make the copies within 48 hours.
>     	  Wilfred P. of Brooklyn failed to send out his ten copies and
>     	  woke the next morning to find his job description changed
>     	  to "COBOL programmer." Howard L. of Newport News sent
>     	  out his ten copies and within a month had enough hardware
>     	  to build a 100-node Beowulf cluster dedicated to playing
>     	  Tuxracer. Amelia V. of Chicago laughed at this letter
>     	  and broke the chain. Shortly thereafter, a fire broke out
>     	  in her terminal and she now spends her days writing
>     	  documentation for MS Windows.

>     	  Don't break the chain!  Send out your ten copies today!

Courtesy 'NIX "fortune cookies", with some alterations and many apologies

## Commands

Mastering the commands on your Linux machine is an indispensable prelude to writing effective shell scripts.

This section covers the following commands:

- . (See also source)

- ac

- adduser

- agetty

- agrep

- ar

- arch

- at

- autoload

- awk (See also Using `awk` for math operations)

- badblocks

- banner

- basename

- batch

- bc

- bg

- bind

- bison

- builtin

- bzgrep

- bzip2

- cal

- caller

- cat

- cd

- chattr

- chfn

- chgrp

- chkconfig

- chmod

- chown

- chroot

- cksum

- clear

- clock

- cmp

- col

- colrm

- column

- comm

- command

- compgen

- complete

- compress

- coproc

- cp

- cpio

- cron

- crypt

- csplit

- cu

- cut

- date

- dc

- dd

- debugfs

- declare

- depmod

- df

- dialog

- diff

- diff3

- diffstat

- dig

- dirname

- dirs

- disown

- dmesg

- doexec

- dos2unix

- du

- dump

- dumpe2fs

- e2fsck

- echo

- egrep

- enable

- enscript

- env

- eqn

- eval

- exec

- exit (Related topic: exit status)

- expand

- export

- expr

- factor

- false

- fdformat

- fdisk

- fg

- fgrep

- file

- find

- finger

- flex

- flock

- fmt

- fold

- free

- fsck

- ftp

- fuser

- getfacl

- getopt

- getopts

- gettext

- getty

- gnome-mount

- grep

- groff

- groupmod

- groups (Related topic: the $GROUPS variable)

- gs

- gzip

- halt

- hash

- hdparm

- head

- help

- hexdump

- host

- hostid

- hostname (Related topic: the $HOSTNAME variable)

- hwclock

- iconv

- id (Related topic: the $UID variable)

- ifconfig

- info

- infocmp

- init

- insmod

- install

- ip

- ipcalc

- iptables

- iwconfig

- jobs

- join

- jot

- kill

- killall

- last

- lastcomm

- lastlog

- ldd

- less

- let

- lex

- lid

- ln

- locate

- lockfile

- logger

- logname

- logout

- logrotate

- look

- losetup

- lp

- ls

- lsdev

- lsmod

- lsof

- lspci

- lsusb

- ltrace

- lynx

- lzcat

- lzma

- m4

- mail

- mailstats

- mailto

- make

- MAKEDEV

- man

- mapfile

- mcookie

- md5sum

- merge

- mesg

- mimencode

- mkbootdisk

- mkdir

- mkdosfs

- mke2fs

- mkfifo

- mkisofs

- mknod

- mkswap

- mktemp

- mmencode

- modinfo

- modprobe

- more

- mount

- msgfmt

- mv

- nc

- netconfig

- netstat

- newgrp

- nice

- nl

- nm

- nmap

- nohup

- nslookup

- objdump

- od

- openssl

- passwd

- paste

- patch (Related topic: diff)

- pathchk

- pax

- pgrep

- pidof

- ping

- pkill

- popd

- pr

- printenv

- printf

- procinfo

- ps

- pstree

- ptx

- pushd

- pwd (Related topic: the $PWD variable)

- quota

- rcp

- rdev

- rdist

- read

- readelf

- readlink

- readonly

- reboot

- recode

- renice

- reset

- resize

- restore

- rev

- rlogin

- rm

- rmdir

- rmmod

- route

- rpm

- rpm2cpio

- rsh

- rsync

- runlevel

- run-parts

- rx

- rz

- sar

- scp

- script

- sdiff

- sed

- seq

- service

- set

- setfacl

- setquota

- setserial

- setterm

- sha1sum

- shar

- shopt

- shred

- shutdown

- size

- skill

- sleep

- slocate

- snice

- sort

- source

- sox

- split

- sq

- ssh

- stat

- strace

- strings

- strip

- stty

- su

- sudo

- sum

- suspend

- swapoff

- swapon

- sx

- sync

- sz

- tac

- tail

- tar

- tbl

- tcpdump

- tee

- telinit

- telnet

- Tex

- texexec

- time

- times

- tmpwatch

- top

- touch

- tput

- tr

- traceroute

- true

- tset

- tsort

- tty

- tune2fs

- type

- typeset

- ulimit

- umask

- umount

- uname

- unarc

- unarj

- uncompress

- unexpand

- uniq

- units

- unlzma

- unrar

- unset

- unsq

- unzip

- uptime

- usbmodules

- useradd

- userdel

- usermod

- users

- usleep

- uucp

- uudecode

- uuencode

- uux

- vacation

- vdir

- vmstat

- vrfy

- w

- wait

- wall

- watch

- wc

- wget

- whatis

- whereis

- which

- who

- whoami

- whois

- write

- xargs

- xrandr

- xz

- yacc

- yes

- zcat

- zdiff

- zdump

- zegrep

- zfgrep

- zgrep

- zip

### Internal Commands and Builtins

A builtin is a `command` contained within the Bash tool set, literally built in. This is either for performance reasons -- builtins execute faster than external commands, which usually require forking off As Nathan Coulter points out, "while forking a process is a low-cost operation, executing a new program in the newly-forked child process adds more overhead." a separate process -- or because a particular builtin needs direct access to the shell internals.

When a command or the shell itself initiates (or spawns) a new subprocess to carry out a task, this is called forking. This new process is the child, and the process that forked it off is the parent. While the child process is doing its work, the parent process is still executing.

Note that while a parent process gets the process ID of the child process, and can thus pass arguments to it, the reverse is not true. This can create problems that are subtle and hard to track down.

#### A script that spawns multiple instances of itself

    &spawnscr;

Generally, a Bash builtin does not fork a subprocess when it executes within a script. An external system command or filter in a script usually will fork a subprocess.

A builtin may be a synonym to a system command of the same name, but Bash reimplements it internally. For example, the Bash `echo` command is not the same as `/bin/echo`, although their behavior is almost identical.

    #!/bin/bash

    echo "This line uses the \"echo\" builtin."
    /bin/echo "This line uses the /bin/echo system command."

A keyword is a reserved word, token or operator. Keywords have a special meaning to the shell, and indeed are the building blocks of the shell's syntax. As examples, for, while, do, and ! are keywords. Similar to a builtin, a keyword is hard-coded into Bash, but unlike a builtin, a keyword is not in itself a command, but a subunit of a command construct. An exception to this is the time command, listed in the official Bash documentation as a keyword (reserved word).

#### I/O

`echo`

  prints (to `stdout`) an expression or variable (see ).

      echo Hello
      echo $a

  An `echo` requires the `-e` option to print escaped characters. See .

  Normally, each `echo` command prints a terminal newline, but the `-n` option suppresses this.

  An `echo` can be used to feed a sequence of commands down a pipe.

      if echo "$VAR" | grep -q txt   # if [[ $VAR = *txt* ]]
      then
        echo "$VAR contains the substring sequence \"txt\""
      fi

  An `echo`, in combination with command substitution can set a variable.

  `a=`echo "HELLO" | tr A-Z a-z``

  See also , , , and .

  Be aware that `echo `command`` deletes any linefeeds that the output of command generates.

  The $IFS (internal field separator) variable normally contains \n (linefeed) as one of its set of whitespace characters. Bash therefore splits the output of command at linefeeds into arguments to `echo`. Then `echo` outputs these arguments, separated by spaces.

      bash$ ls -l /usr/share/apps/kjezz/sounds
      -rw-r--r--    1 root     root         1407 Nov  7  2000 reflect.au
       -rw-r--r--    1 root     root          362 Nov  7  2000 seconds.au

      bash$ echo `ls -l /usr/share/apps/kjezz/sounds`
      total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root root ...

  So, how can we embed a linefeed within an echoed character string?

      # Embedding a linefeed?
      echo "Why doesn't this string \n split on two lines?"
      # Doesn't split.

      # Let's try something else.

      echo

      echo $"A line of text containing
      a linefeed."
      # Prints as two distinct lines (embedded linefeed).
      # But, is the "$" variable prefix really necessary?

      echo

      echo "This string splits
      on two lines."
      # No, the "$" is not needed.

      echo
      echo "---------------"
      echo

      echo -n $"Another line of text containing
      a linefeed."
      # Prints as two distinct lines (embedded linefeed).
      # Even the -n option fails to suppress the linefeed here.

      echo
      echo
      echo "---------------"
      echo
      echo

      # However, the following doesn't work as expected.
      # Why not? Hint: Assignment to a variable.
      string1=$"Yet another line of text containing
      a linefeed (maybe)."

      echo $string1
      # Yet another line of text containing a linefeed (maybe).
      #                                    ^
      # Linefeed becomes a space.

      # Thanks, Steve Parker, for pointing this out.

  This command is a shell builtin, and not the same as `/bin/echo`, although its behavior is similar.

      bash$ type -a echo
      echo is a shell builtin
       echo is /bin/echo

`printf`

  The `printf`, formatted print, command is an enhanced `echo`. It is a limited variant of the C language `printf()` library function, and its syntax is somewhat different.

      	      printf
      	      format-string
      	      parameter

  This is the Bash builtin version of the `/bin/printf` or `/usr/bin/printf` command. See the `printf` manpage (of the system command) for in-depth coverage.

  Older versions of Bash may not support `printf`.

  #### printf in action

      &ex47;

  Formatting error messages is a useful application of `printf`

      E_BADDIR=85

      var=nonexistent_directory

      error()
      {
        printf "$@" >&2
        # Formats positional params passed, and sends them to stderr.
        echo
        exit $E_BADDIR
      }

      cd $var || error $"Can't cd to %s." "$var"

      # Thanks, S.C.

  See also .

`read`

  Reads the value of a variable from `stdin`, that is, interactively fetches input from the keyboard. The `-a` option lets `read` get array variables (see ).

  #### Variable assignment, using read

      &ex36;

  A `read` without an associated variable assigns its input to the dedicated variable $REPLY.

  #### What happens when read has no variable

      &readnovar;

  Normally, inputting a `\` suppresses a newline during input to a `read`. The `-r` option causes an inputted `\` to be interpreted literally.

  #### Multi-line input to read

      &readr;

  The `read` command has some interesting options that permit echoing a prompt and even reading keystrokes without hitting ENTER.

      # Read a keypress without hitting ENTER.

      read -s -n1 -p "Hit a key " keypress
      echo; echo "Keypress was "\"$keypress\""."

      # -s option means do not echo input.
      # -n N option means accept only N characters of input.
      # -p option means echo the following prompt before reading input.

      # Using these options is tricky, since they need to be in the correct order.

  The `-n` option to `read` also allows detection of the arrow keys and certain of the other unusual keys.

  #### Detecting the arrow keys

      &arrowdetect;

  The `-n` option to `read` will not detect the ENTER (newline) key.

  The `-t` option to `read` permits timed input (see and ).

  The `-u` option takes the file descriptor of the target file.

  The `read` command may also read its variable value from a file redirected to `stdin`. If the file contains more than one line, only the first line is assigned to the variable. If `read` has more than one parameter, then each of these variables gets assigned a successive whitespace-delineated string. Caution!

  #### Using read with file redirection

      &readredir;

  Piping output to a read, using echo to set variables will fail.

  Yet, piping the output of cat seems to work.

      cat file1 file2 |
      while read line
      do
      echo $line
      done

  However, as Bjön Eriksson shows:

  #### Problems reading from a pipe

      &readpipe;

  The gendiff script, usually found in `/usr/bin` on many Linux distros, pipes the output of find to a while read construct.

      find $1 \( -name "*$2" -o -name ".*$2" \) -print |
      while read f; do
      . . .

  It is possible to paste text into the input field of a read (but not multiple lines!). See .

#### Filesystem

`cd`

  The familiar `cd` change directory command finds use in scripts where execution of a command requires being in a specified directory.

      (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)

  [from the previously cited example by Alan Cox]

  The `-P` (physical) option to `cd` causes it to ignore symbolic links.

  `cd -` changes to $OLDPWD, the previous working directory.

  The `cd` command does not function as expected when presented with two forward slashes.

      bash$ cd //
      bash$ pwd
      //

  The output should, of course, be `/`. This is a problem both from the command-line and in a script.

`pwd`

  Print Working Directory. This gives the user's (or script's) current directory (see ). The effect is identical to reading the value of the builtin variable $PWD.

`pushd`
`popd`
`dirs`

  This command set is a mechanism for bookmarking working directories, a means of moving back and forth through directories in an orderly manner. A pushdown stack is used to keep track of directory names. Options allow various manipulations of the directory stack.

  `pushd dir-name` pushes the path dir-name onto the directory stack (to the top of the stack) and simultaneously changes the current working directory to dir-name

  `popd` removes (pops) the top directory path name off the directory stack and simultaneously changes the current working directory to the directory now at the top of the stack.

  `dirs` lists the contents of the directory stack (compare this with the $DIRSTACK variable). A successful `pushd` or `popd` will automatically invoke `dirs`.

  Scripts that require various changes to the current working directory without hard-coding the directory name changes can make good use of these commands. Note that the implicit `$DIRSTACK` array variable, accessible from within a script, holds the contents of the directory stack.

  #### Changing the current working directory

      &ex37;

#### Variables

`let`

  The `let` command carries out arithmetic operations on variables.

  Note that let cannot be used for setting string variables.

  In many cases, it functions as a less complex version of expr.

  #### Letting let do arithmetic.

      &ex46;

  The let command can, in certain contexts, return a surprising exit status.

      # Evgeniy Ivanov points out:

      var=0
      echo $?     # 0
                  # As expected.

      let var++
      echo $?     # 1
                  # The command was successful, so why isn't $?=0 ???
                  # Anomaly!

      let var++
      echo $?     # 0
                  # As expected.

      # Likewise . . .

      let var=0
      echo $?     # 1
                  # The command was successful, so why isn't $?=0 ???

      #  However, as Jeff Gorak points out,
      #+ this is part of the design spec for 'let' . . .
      # "If the last ARG evaluates to 0, let returns 1;
      #  let returns 0 otherwise." ['help let']

`eval`

  `eval arg1 [arg2] ... [argN]`

  Combines the arguments in an expression or list of expressions and evaluates them. Any variables within the expression are expanded. The net result is to `convert a string into a command`.

  The `eval` command can be used for code generation from the command-line or within a script.

      bash$ command_string="ps ax"
      bash$ process="ps ax"
      bash$ eval "$command_string" | grep "$process"
      26973 pts/3    R+     0:00 grep --color ps ax
       26974 pts/3    R+     0:00 ps ax

  Each invocation of eval forces a re-evaluation of its arguments.

      a='$b'
      b='$c'
      c=d

      echo $a             # $b
                          # First level.
      eval echo $a        # $c
                          # Second level.
      eval eval echo $a   # d
                          # Third level.

      # Thank you, E. Choroba.

  #### Showing the effect of eval

      &ex43;

  #### Using eval to select among variables

      &arrchoice;

  #### Echoing the command-line parameters

      &echoparams;

  #### Forcing a log-off

      &ex44;

  #### A version of rot13

      &rot14;

  Here is another example of using eval to evaluate a complex expression, this one from an earlier version of YongYe's Tetris game script.

      eval ${1}+=\"${x} ${y} \"

  uses eval to convert array elements into a command list.

  The eval command occurs in the older version of indirect referencing.

      eval var=\$$var

  The eval command can be used to parameterize brace expansion.

  The `eval` command can be risky, and normally should be avoided when there exists a reasonable alternative. An `eval $COMMANDS` executes the contents of COMMANDS, which may contain such unpleasant surprises as `rm -rf *`. Running an `eval` on unfamiliar code written by persons unknown is living dangerously.

`set`

  The `set` command changes the value of internal script variables/options. One use for this is to toggle option flags which help determine the behavior of the script. Another application for it is to reset the positional parameters that a script sees as the result of a command (`set `command``). The script can then parse the fields of the command output.

  #### Using set with positional parameters

      &ex34;

  More fun with positional parameters.

  #### Reversing the positional parameters

      &revposparams;

  Invoking `set` without any options or arguments simply lists all the environmental and other variables that have been initialized.

      bash$ set
      AUTHORCOPY=/home/bozo/posts
       BASH=/bin/bash
       BASH_VERSION=$'2.05.8(1)-release'
       ...
       XAUTHORITY=/home/bozo/.Xauthority
       _=/etc/bashrc
       variable22=abc
       variable23=xzy

  Using `set` with the `--` option explicitly assigns the contents of a variable to the positional parameters. If no variable follows the `--` it unsets the positional parameters.

  #### Reassigning the positional parameters

      &setpos;

  See also and .

`unset`

  The `unset` command deletes a shell variable, effectively setting it to null. Note that this command does not affect positional parameters.

      bash$ unset PATH

      bash$ echo $PATH

      bash$

  #### Unsetting a variable

      &uns;

  In most contexts, an undeclared variable and one that has been unset are equivalent. However, the ${parameter:-default} parameter substitution construct can distinguish between the two.

`export`

  The `export`

  To Export information is to make it available in a more general context. See also scope.

  command makes available variables to all child processes of the running script or shell. One important use of the `export` command is in startup files, to initialize and make accessible environmental variables to subsequent user processes.

  Unfortunately, there is no way to export variables back to the parent process, to the process that called or invoked the script or shell.

  #### Using export to pass a variable to an embedded awk script

      &coltotaler3;

  It is possible to initialize and export variables in the same operation, as in `export var1=xxx`.

  However, as Greg Keraunen points out, in certain situations this may have a different effect than setting a variable, then exporting it.

      bash$ export var=(a b); echo ${var[0]}
      (a b)

      bash$ var=(a b); export var; echo ${var[0]}
      a

  A variable to be exported may require special treatment. See .

`declare`
`typeset`

  The declare and typeset commands specify and/or restrict properties of variables.

`readonly`

  Same as declare -r, sets a variable as read-only, or, in effect, as a constant. Attempts to change the variable fail with an error message. This is the shell analog of the C language `const` type qualifier.

`getopts`

  This powerful tool parses command-line arguments passed to the script. This is the Bash analog of the getopt external command and the getopt library function familiar to C programmers. It permits passing and concatenating multiple options

  An option is an argument that acts as a flag, switching script behaviors on or off. The argument associated with a particular option indicates the behavior that the option (flag) switches on or off.

  and associated arguments to a script (for example `scriptname -abc -e /usr/local`).

  The `getopts` construct uses two implicit variables. `$OPTIND` is the argument pointer (OPTion INDex) and `$OPTARG` (OPTion ARGument) the (optional) argument attached to an option. A colon following the option name in the declaration tags that option as having an associated argument.

  A `getopts` construct usually comes packaged in a while loop, which processes the options and arguments one at a time, then increments the implicit `$OPTIND` variable to point to the next.

  1. The arguments passed from the command-line to the script must be preceded by a dash (`-`). It is the prefixed `-` that lets `getopts` recognize command-line arguments as options. In fact, `getopts` will not process arguments without the prefixed `-`, and will terminate option processing at the first argument encountered lacking them.

  2. The `getopts` template differs slightly from the standard while loop, in that it lacks condition brackets.

  3. The `getopts` construct is a highly functional replacement for the traditional getopt external command.

      while getopts ":abcde:fg" Option
      # Initial declaration.
      # a, b, c, d, e, f, and g are the options (flags) expected.
      # The : after option 'e' shows it will have an argument passed with it.
      do
        case $Option in
          a ) # Do something with variable 'a'.
          b ) # Do something with variable 'b'.
          ...
          e)  # Do something with 'e', and also with $OPTARG,
              # which is the associated argument passed with option 'e'.
          ...
          g ) # Do something with variable 'g'.
        esac
      done
      shift $(($OPTIND - 1))
      # Move argument pointer to next.

      # All this is not nearly as complicated as it looks <grin>.

  #### Using getopts to read the options/arguments passed to a script

      &ex33;

#### Script Behavior

`source`
. (dot command)

  This command, when invoked from the command-line, executes a script. Within a script, a `source file-name` loads the file `file-name`. Sourcing a file (dot-command) imports code into the script, appending to the script (same effect as the `#include` directive in a C program). The net result is the same as if the sourced lines of code were physically present in the body of the script. This is useful in situations when multiple scripts use a common data file or function library.

  #### Including a data file

      &ex38;

  File `data-file` for , above. Must be present in same directory.

      &ex38bis;

  If the sourced file is itself an executable script, then it will run, then return control to the script that called it. A sourced executable script may use a return for this purpose.

  Arguments may be (optionally) passed to the sourced file as positional parameters.

      source $filename $arg1 arg2

  It is even possible for a script to source itself, though this does not seem to have any practical applications.

  #### A (useless) script that sources itself

      &selfsource;

`exit`

  Unconditionally terminates a script. Technically, an `exit` only terminates the process (or shell) in which it is running, not the parent process. The `exit` command may optionally take an integer argument, which is returned to the shell as the exit status of the script. It is good practice to end all but the simplest scripts with an `exit 0`, indicating a successful run.

  If a script terminates with an `exit` lacking an argument, the exit status of the script is the exit status of the last command executed in the script, not counting the `exit`. This is equivalent to an `exit $?`.

  An `exit` command may also be used to terminate a subshell.

`exec`

  This shell builtin replaces the current process with a specified command. Normally, when the shell encounters a command, it forks off a child process to actually execute the command. Using the `exec` builtin, the shell does not fork, and the command exec'ed replaces the shell. When used in a script, therefore, it forces an exit from the script when the `exec`'ed command terminates. Unless the `exec` is used to reassign file descriptors.

  #### Effects of exec

      &ex54;

  #### A script that exec's itself

      &selfexec;

  An `exec` also serves to reassign file descriptors. For example, `exec <zzz-file` replaces `stdin` with the file `zzz-file`.

  The `-exec` option to find is not the same as the `exec` shell builtin.

`shopt`

  This command permits changing shell options on the fly (see and ). It often appears in the Bash startup files, but also has its uses in scripts. Needs version 2 or later of Bash.

      shopt -s cdspell
      # Allows minor misspelling of directory names with 'cd'
      # Option -s sets, -u unsets.

      cd /hpme  # Oops! Mistyped '/home'.
      pwd       # /home
                # The shell corrected the misspelling.

`caller`

  Putting a `caller` command inside a function echoes to `stdout` information about the caller of that function.

      #!/bin/bash

      function1 ()
      {
        # Inside function1 ().
        caller 0   # Tell me about it.
      }

      function1    # Line 9 of script.

      # 9 main test.sh
      # ^                 Line number that the function was called from.
      #   ^^^^            Invoked from "main" part of script.
      #        ^^^^^^^    Name of calling script.

      caller 0     # Has no effect because it's not inside a function.

  A `caller` command can also return caller information from a script sourced within another script. Analogous to a function, this is a subroutine call.

  You may find this command useful in debugging.

#### Commands

`true`

  A command that returns a successful (zero) exit status, but does nothing else.

      bash$ true
      bash$ echo $?
      0

      # Endless loop
      while true   # alias for ":"
      do
         operation-1
         operation-2
         ...
         operation-n
         # Need a way to break out of loop or script will hang.
      done

`false`

  A command that returns an unsuccessful exit status, but does nothing else.

      bash$ false
      bash$ echo $?
      1

      # Testing "false"
      if false
      then
        echo "false evaluates \"true\""
      else
        echo "false evaluates \"false\""
      fi
      # false evaluates "false"

      # Looping while "false" (null loop)
      while false
      do
         # The following code will not execute.
         operation-1
         operation-2
         ...
         operation-n
         # Nothing happens!
      done

`type [cmd]`

  Similar to the which external command, `type cmd` identifies cmd. Unlike `which`, `type` is a Bash builtin. The useful `-a` option to `type` identifies keywords and builtins, and also locates system commands with identical names.

      bash$ type '['
      [ is a shell builtin
      bash$ type -a '['
      [ is a shell builtin
       [ is /usr/bin/[

      bash$ type type
      type is a shell builtin

  The `type` command can be useful for testing whether a certain command exists.

`hash [cmds]`

  Records the path name of specified commands -- in the shell hash table

  Hashing is a method of creating lookup keys for data stored in a table. The data items themselves are scrambled to create keys, using one of a number of simple mathematical algorithms (methods, or recipes). An advantage of hashing is that it is fast. A disadvantage is that collisions -- where a single key maps to more than one data item -- are possible. For examples of hashing see and .

  -- so the shell or script will not need to search the $PATH on subsequent calls to those commands. When `hash` is called with no arguments, it simply lists the commands that have been hashed. The `-r` option resets the hash table.

`bind`

  The `bind` builtin displays or modifies readline The readline library is what Bash uses for reading input in an interactive shell. key bindings.

`help`

  Gets a short usage summary of a shell builtin. This is the counterpart to whatis, but for builtins. The display of help information got a much-needed update in the version 4 release of Bash.

      bash$ help exit
      exit: exit [n]
          Exit the shell with a status of N.  If N is omitted, the exit status
          is that of the last command executed.

#### Job Control Commands

Certain of the following job control commands take a job identifier as an argument. See the table at end of the chapter.

`jobs`

  Lists the jobs running in the background, giving the job number. Not as useful as ps.

  It is all too easy to confuse jobs and processes. Certain builtins, such as `kill`, `disown`, and `wait` accept either a job number or a process number as an argument. The fg, bg and `jobs` commands accept only a job number.

      bash$ sleep 100 &
      [1] 1384

      bash $ jobs
      [1]+  Running                 sleep 100 &

  1 is the job number (jobs are maintained by the current shell). 1384 is the PID or process ID number (processes are maintained by the system). To kill this job/process, either a `kill %1` or a `kill 1384` works.

  Thanks, S.C.

`disown`

  Remove job(s) from the shell's table of active jobs.

`fg`
`bg`

  The `fg` command switches a job running in the background into the foreground. The `bg` command restarts a suspended job, and runs it in the background. If no job number is specified, then the `fg` or `bg` command acts upon the currently running job.

`wait`

  Suspend script execution until all jobs running in background have terminated, or until the job number or process ID specified as an option terminates. Returns the exit status of waited-for command.

  You may use the `wait` command to prevent a script from exiting before a background job finishes executing (this would create a dreaded orphan process).

  ##### Waiting for a process to finish before proceeding

      &ex39;

  Optionally, `wait` can take a job identifier as an argument, for example, wait%1 or wait $PPID.

  This only applies to child processes, of course.

  See the job id table.

  Within a script, running a command in the background with an ampersand (&) may cause the script to hang until ENTER is hit. This seems to occur with commands that write to `stdout`. It can be a major annoyance.

      #!/bin/bash
      # test.sh

      ls -l &
      echo "Done."

      bash$ ./test.sh
      Done.
       [bozo@localhost test-scripts]$ total 1
       -rwxr-xr-x    1 bozo     bozo           34 Oct 11 15:09 test.sh
       _

  >         As Walter Brameld IV explains it:

  >         As far as I can tell, such scripts don't actually hang. It just
  >         seems that they do because the background command writes text to
  >         the console after the prompt. The user gets the impression that
  >         the prompt was never displayed. Here's the sequence of events:

  >         1. Script launches background command.
  >         2. Script exits.
  >         3. Shell displays the prompt.
  >         4. Background command continues running and writing text to the
  >            console.
  >         5. Background command finishes.
  >         6. User doesn't see a prompt at the bottom of the output, thinks script
  >            is hanging.

  Placing a `wait` after the background command seems to remedy this.

      #!/bin/bash
      # test.sh

      ls -l &
      echo "Done."
      wait

      bash$ ./test.sh
      Done.
       [bozo@localhost test-scripts]$ total 1
       -rwxr-xr-x    1 bozo     bozo           34 Oct 11 15:09 test.sh

  Redirecting the output of the command to a file or even to `/dev/null` also takes care of this problem.

`suspend`

  This has a similar effect to ControlZ, but it suspends the shell (the shell's parent process should resume it at an appropriate time).

`logout`

  Exit a login shell, optionally specifying an exit status.

`times`

  Gives statistics on the system time elapsed when executing commands, in the following form:

      0m0.020s 0m0.020s

  This capability is of relatively limited value, since it is not common to profile and benchmark shell scripts.

`kill`

  Forcibly terminate a process by sending it an appropriate terminate signal (see ).

  ##### A script that kills itself

      &selfdestruct;

  `kill -l` lists all the signals (as does the file `/usr/include/asm/signal.h`). A `kill -9` is a sure kill, which will usually terminate a process that stubbornly refuses to die with a plain `kill`. Sometimes, a `kill -15` works. A zombie process, that is, a child process that has terminated, but that the parent process has not (yet) killed, cannot be killed by a logged-on user -- you can't kill something that is already dead -- but `init` will generally clean it up sooner or later.

`killall`

  The `killall` command kills a running process by name, rather than by process ID. If there are multiple instances of a particular command running, then doing a killall on that command will terminate them all.

  This refers to the `killall` command in `/usr/bin`, not the killall script in `/etc/rc.d/init.d`.

`command`

  The `command` directive disables aliases and functions for the command immediately following it.

      bash$ command ls

  This is one of three shell directives that effect script command processing. The others are builtin and enable.

`builtin`

  Invoking `builtin BUILTIN_COMMAND` runs the command BUILTIN_COMMAND as a shell builtin, temporarily disabling both functions and external system commands with the same name.

`enable`

  This either enables or disables a shell builtin command. As an example, enable -n kill disables the shell builtin kill, so that when Bash subsequently encounters kill, it invokes the external command `/bin/kill`.

  The `-a` option to enable lists all the shell builtins, indicating whether or not they are enabled. The `-f filename` option lets enable load a builtin as a shared library (DLL) module from a properly compiled object file. The C source for a number of loadable builtins is typically found in the `/usr/share/doc/bash-?.??/functions` directory. Note that the `-f` option to `enable` is not portable to all systems. .

`autoload`

  This is a port to Bash of the ksh autoloader. With `autoload` in place, a function with an autoload declaration will load from an external file at its first invocation. The same effect as `autoload` can be achieved with typeset -fu. This saves system resources.

  Note that autoload is not a part of the core Bash installation. It needs to be loaded in with enable -f (see above).

##### Job identifiers

Notation | Meaning
--- | ---
`%N` | Job number [N]
`%S` | Invocation (command-line) of job begins with string S
`%?S` | Invocation (command-line) of job contains within it string S
`%%` | current job (last job stopped in foreground or started in background)
`%+` | current job (last job stopped in foreground or started in background)
`%-` | Last job
`$!` | Last background process

### External Filters, Programs and Commands

Standard UNIX commands make shell scripts more versatile. The power of scripts comes from coupling system commands and shell directives with simple programming constructs.

#### Basic Commands

##### The first commands a novice learns

`ls`

  The basic file list command. It is all too easy to underestimate the power of this humble command. For example, using the `-R`, recursive option, `ls` provides a tree-like listing of a directory structure. Other useful options are `-S`, sort listing by file size, `-t`, sort by file modification time, `-v`, sort by (numerical) version numbers embedded in the filenames, The `-v` option also orders the sort by upper- and lowercase prefixed filenames. `-b`, show escape characters, and `-i`, show file inodes (see ).

      bash$ ls -l
      -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt

      bash$ ls -lv
       total 0
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Chapter_headings.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:49 Preface.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter1.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter2.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter3.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter10.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter11.txt
       -rw-rw-r-- 1 bozo bozo 0 Sep 14 18:44 chapter12.txt

  The ls command returns a non-zero exit status when attempting to list a non-existent file.

      bash$ ls abc
      ls: abc: No such file or directory

      bash$ echo $?
      2

  ##### Using ls to create a table of contents for burning a CDR disk

      &ex40;

`cat`
`tac`

  `cat`, an acronym for concatenate, lists a file to `stdout`. When combined with redirection (> or >>), it is commonly used to concatenate files.

      # Uses of 'cat'
      cat filename                          # Lists the file.

      cat file.1 file.2 file.3 > file.123   # Combines three files into one.

  The `-n` option to `cat` inserts consecutive numbers before all lines of the target file(s). The `-b` option numbers only the non-blank lines. The `-v` option echoes nonprintable characters, using ^ notation. The `-s` option squeezes multiple consecutive blank lines into a single blank line.

  See also and .

  In a pipe, it may be more efficient to redirect the `stdin` to a file, rather than to `cat` the file.

      cat filename | tr a-z A-Z

      tr a-z A-Z < filename   #  Same effect, but starts one less process,
                              #+ and also dispenses with the pipe.

  `tac`, is the inverse of cat, listing a file backwards from its end.

`rev`

  reverses each line of a file, and outputs to `stdout`. This does not have the same effect as `tac`, as it preserves the order of the lines, but flips each one around (mirror image).

      bash$ cat file1.txt
      This is line 1.
       This is line 2.

      bash$ tac file1.txt
      This is line 2.
       This is line 1.

      bash$ rev file1.txt
      .1 enil si sihT
       .2 enil si sihT

`cp`

  This is the file copy command. `cp file1 file2` copies `file1` to `file2`, overwriting `file2` if it already exists (see ).

  Particularly useful are the `-a` archive flag (for copying an entire directory tree), the `-u` update flag (which prevents overwriting identically-named newer files), and the `-r` and `-R` recursive flags.

      cp -u source_dir/* dest_dir
      #  "Synchronize" dest_dir to source_dir
      #+  by copying over all newer and not previously existing files.

`mv`

  This is the file move command. It is equivalent to a combination of `cp` and `rm`. It may be used to move multiple files to a directory, or even to rename a directory. For some examples of using `mv` in a script, see and .

  When used in a non-interactive script, `mv` takes the `-f` (force) option to bypass user input.

  When a directory is moved to a preexisting directory, it becomes a subdirectory of the destination directory.

      bash$ mv source_directory target_directory

      bash$ ls -lF target_directory
      total 1
       drwxrwxr-x    2 bozo  bozo      1024 May 28 19:20 source_directory/

`rm`

  Delete (remove) a file or files. The `-f` option forces removal of even readonly files, and is useful for bypassing user input in a script.

  The rm command will, by itself, fail to remove filenames beginning with a dash. Why? Because rm sees a dash-prefixed filename as an option.

      bash$ rm -badname
      rm: invalid option -- b
       Try `rm --help' for more information.

  One clever workaround is to precede the filename with a -- (the end-of-options flag).

      bash$ rm -- -badname

  Another method to is to preface the filename to be removed with a `dot-slash` .

      bash$ rm ./-badname

  When used with the recursive flag `-r`, this command removes files all the way down the directory tree from the current directory. A careless `rm -rf *` can wipe out a big chunk of a directory structure.

`rmdir`

  Remove directory. The directory must be empty of all files -- including invisible dotfiles

  Dotfiles are files whose names begin with a dot, such as `~/.Xdefaults`. Such filenames do not appear in a normal `ls` listing (although an `ls -a` will show them), and they cannot be deleted by an accidental `rm -rf *`. Dotfiles are generally used as setup and configuration files in a user's home directory.

  -- for this command to succeed.

`mkdir`

  Make directory, creates a new directory. For example, `mkdir -p project/programs/December` creates the named directory. The -p option automatically creates any necessary parent directories.

`chmod`

  Changes the attributes of an existing file or directory (see ).

      chmod +x filename
      # Makes "filename" executable for all users.

      chmod u+s filename
      # Sets "suid" bit on "filename" permissions.
      # An ordinary user may execute "filename" with same privileges as the file's owner.
      # (This does not apply to shell scripts.)

      chmod 644 filename
      #  Makes "filename" readable/writable to owner, readable to others
      #+ (octal mode).

      chmod 444 filename
      #  Makes "filename" read-only for all.
      #  Modifying the file (for example, with a text editor)
      #+ not allowed for a user who does not own the file (except for root),
      #+ and even the file owner must force a file-save
      #+ if she modifies the file.
      #  Same restrictions apply for deleting the file.

      chmod 1777 directory-name
      #  Gives everyone read, write, and execute permission in directory,
      #+ however also sets the "sticky bit".
      #  This means that only the owner of the directory,
      #+ owner of the file, and, of course, root
      #+ can delete any particular file in that directory.

      chmod 111 directory-name
      #  Gives everyone execute-only permission in a directory.
      #  This means that you can execute and READ the files in that directory
      #+ (execute permission necessarily includes read permission
      #+ because you can't execute a file without being able to read it).
      #  But you can't list the files or search for them with the "find" command.
      #  These restrictions do not apply to root.

      chmod 000 directory-name
      #  No permissions at all for that directory.
      #  Can't read, write, or execute files in it.
      #  Can't even list files in it or "cd" to it.
      #  But, you can rename (mv) the directory
      #+ or delete it (rmdir) if it is empty.
      #  You can even symlink to files in the directory,
      #+ but you can't read, write, or execute the symlinks.
      #  These restrictions do not apply to root.

`chattr`

  `Ch`ange file `attr`ibutes. This is analogous to `chmod` above, but with different options and a different invocation syntax, and it works only on ext2/ext3 filesystems.

  One particularly interesting `chattr` option is `i`. A `chattr +i `filename`` marks the file as immutable. The file cannot be modified, linked to, or deleted, not even by root. This file attribute can be set or removed only by root. In a similar fashion, the `a` option marks the file as append only.

      root# chattr +i file1.txt

      root# rm file1.txt

      rm: remove write-protected regular file `file1.txt'? y
       rm: cannot remove `file1.txt': Operation not permitted

  If a file has the `s` (secure) attribute set, then when it is deleted its block is overwritten with binary zeroes. This particular feature may not yet be implemented in the version of the ext2/ext3 filesystem installed on your system. Check the documentation for your Linux distro.

  If a file has the `u` (undelete) attribute set, then when it is deleted, its contents can still be retrieved (undeleted).

  If a file has the `c` (compress) attribute set, then it will automatically be compressed on writes to disk, and uncompressed on reads.

  The file attributes set with `chattr` do not show in a file listing (`ls -l`).

`ln`

  Creates links to pre-existings files. A link is a reference to a file, an alternate name for it. The `ln` command permits referencing the linked file by more than one name and is a superior alternative to aliasing (see ).

  The `ln` creates only a reference, a pointer to the file only a few bytes in size.

  The `ln` command is most often used with the `-s`, symbolic or soft link flag. Advantages of using the `-s` flag are that it permits linking across file systems or to directories.

  The syntax of the command is a bit tricky. For example: `ln -s oldfile newfile` links the previously existing `oldfile` to the newly created link, `newfile`.

  If a file named `newfile` has previously existed, an error message will result.

  ##### Which type of link to use?

  As John Macdonald explains it:

  Both of these [types of links] provide a certain measure of dual reference -- if you edit the contents of the file using any name, your changes will affect both the original name and either a hard or soft new name. The differences between them occurs when you work at a higher level. The advantage of a hard link is that the new name is totally independent of the old name -- if you remove or rename the old name, that does not affect the hard link, which continues to point to the data while it would leave a soft link hanging pointing to the old name which is no longer there. The advantage of a soft link is that it can refer to a different file system (since it is just a reference to a file name, not to actual data). And, unlike a hard link, a symbolic link can refer to a directory.

  Links give the ability to invoke a script (or any other type of executable) with multiple names, and having that script behave according to how it was invoked.

  ##### Hello or Good-bye

      &hellol;

`man`
`info`

  These commands access the manual and information pages on system commands and installed utilities. When available, the info pages usually contain more detailed descriptions than do the man pages.

  There have been various attempts at automating the writing of man pages. For a script that makes a tentative first step in that direction, see .

#### Complex Commands

##### Commands for more advanced users

`find`

  -exec COMMAND \;

  Carries out COMMAND on each file that `find` matches. The command sequence terminates with ; (the ; is escaped to make certain the shell passes it to `find` literally, without interpreting it as a special character).

      bash$ find ~/ -name '*.txt'
      /home/bozo/.kde/share/apps/karm/karmdata.txt
       /home/bozo/misc/irmeyc.txt
       /home/bozo/test-scripts/1.txt

  If COMMAND contains {}, then `find` substitutes the full path name of the selected file for {}.

      find ~/ -name 'core*' -exec rm {} \;
      # Removes all core dump files from user's home directory.

      find /home/bozo/projects -mtime -1
      #                               ^   Note minus sign!
      #  Lists all files in /home/bozo/projects directory tree
      #+ that were modified within the last day (current_day - 1).
      #
      find /home/bozo/projects -mtime 1
      #  Same as above, but modified *exactly* one day ago.
      #
      #  mtime = last modification time of the target file
      #  ctime = last status change time (via 'chmod' or otherwise)
      #  atime = last access time

      DIR=/home/bozo/junk_files
      find "$DIR" -type f -atime +5 -exec rm {} \;
      #                          ^           ^^
      #  Curly brackets are placeholder for the path name output by "find."
      #
      #  Deletes all files in "/home/bozo/junk_files"
      #+ that have not been accessed in *at least* 5 days (plus sign ... +5).
      #
      #  "-type filetype", where
      #  f = regular file
      #  d = directory
      #  l = symbolic link, etc.
      #
      #  (The 'find' manpage and info page have complete option listings.)

      find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \;

      # Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files.
      # There a few extraneous hits. Can they be filtered out?

      # Possibly by:

      find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \
      | grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$'
      #
      #  [:digit:] is one of the character classes
      #+ introduced with the POSIX 1003.2 standard.

      # Thanks, Stéphane Chazelas.

  The `-exec` option to `find` should not be confused with the exec shell builtin.

  ##### Badname, eliminate file names in current directory containing bad characters and whitespace.

      &ex57;

  ##### Deleting a file by its inode number

      &idelete;

  The `find` command also works without the `-exec` option.

      #!/bin/bash
      #  Find suid root files.
      #  A strange suid file might indicate a security hole,
      #+ or even a system intrusion.

      directory="/usr/sbin"
      # Might also try /sbin, /bin, /usr/bin, /usr/local/bin, etc.
      permissions="+4000"  # suid root (dangerous!)

      for file in $( find "$directory" -perm "$permissions" )
      do
        ls -ltF --author "$file"
      done

  See , , and for scripts using `find`. Its manpage provides more detail on this complex and powerful command.

`xargs`

  A filter for feeding arguments to a command, and also a tool for assembling the commands themselves. It breaks a data stream into small enough chunks for filters and commands to process. Consider it as a powerful replacement for backquotes. In situations where command substitution fails with a too many arguments error, substituting `xargs` often works. And even when xargs is not strictly necessary, it can speed up execution of a command involving batch-processing of multiple files. Normally, `xargs` reads from `stdin` or from a pipe, but it can also be given the output of a file.

  The default command for `xargs` is echo. This means that input piped to `xargs` may have linefeeds and other whitespace characters stripped out.

      bash$ ls -l
      total 0
       -rw-rw-r--    1 bozo  bozo         0 Jan 29 23:58 file1
       -rw-rw-r--    1 bozo  bozo         0 Jan 29 23:58 file2

      bash$ ls -l | xargs
      total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan...

      bash$ find ~/mail -type f | xargs grep "Linux"
      ./misc:User-Agent: slrn/0.9.8.1 (Linux)
       ./sent-mail-jul-2005: hosted by the Linux Documentation Project.
       ./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version)
       ./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article
       ./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem
       . . .

  `ls | xargs -p -l gzip` gzips every file in current directory, one at a time, prompting before each operation.

  Note that xargs processes the arguments passed to it sequentially, one at a time.

      bash$ find /usr/bin | xargs file
      /usr/bin:          directory
       /usr/bin/foomatic-ppd-options:          perl script text executable
       . . .

  An interesting xargs option is `-n NN`, which limits to NN the number of arguments passed.

  `ls | xargs -n 8 echo` lists the files in the current directory in `8` columns.

  Another useful option is `-0`, in combination with `find -print0` or `grep -lZ`. This allows handling arguments containing whitespace or quotes.

  `find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f`

  `grep -rliwZ GUI / | xargs -0 rm -f`

  Either of the above will remove any file containing GUI. (Thanks, S.C.)

  Or:

      cat /proc/"$pid"/"$OPTION" | xargs -0 echo
      #  Formats output:         ^^^^^^^^^^^^^^^
      #  From Han Holl's fixup of "get-commandline.sh"
      #+ script in "/dev and /proc" chapter.

  The `-P` option to xargs permits running processes in parallel. This speeds up execution in a machine with a multicore CPU.

      #!/bin/bash

      ls *gif | xargs -t -n1 -P2 gif2png
      # Converts all the gif images in current directory to png.

      # Options:
      # =======
      # -t    Print command to stderr.
      # -n1   At most 1 argument per command line.
      # -P2   Run up to 2 processes simultaneously.

      # Thank you, Roberto Polli, for the inspiration.

  ##### Logfile: Using xargs to monitor system log

      &ex41;

  As in `find`, a curly bracket pair serves as a placeholder for replacement text.

  ##### Copying files in current directory to another

      &ex42;

  ##### Killing processes by name

      &killbyname;

  ##### Word frequency analysis using xargs

      &wf2;

`expr`

  All-purpose expression evaluator: Concatenates and evaluates the arguments according to the operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison, string, or logical.

  `expr 3 + 5`

    returns `8`

  `expr 5 % 3`

    returns 2

  `expr 1 / 0`

    returns the error message, expr: division by zero

    Illegal arithmetic operations not allowed.

  `expr 5 \* 3`

    returns 15

    The multiplication operator must be escaped when used in an arithmetic expression with `expr`.

  `y=`expr $y + 1``

    Increment a variable, with the same effect as `let y=y+1` and `y=$(($y+1))`. This is an example of arithmetic expansion.

  `z=`expr substr $string $position $length``

    Extract substring of $length characters, starting at $position.

  ##### Using expr

      &ex45;

  The : (null) operator can substitute for `match`. For example, `b=`expr $a : [0-9]*`` is the exact equivalent of `b=`expr match $a [0-9]*`` in the above listing.

      &ex45a;

The above script illustrates how `expr` uses the escaped parentheses -- \( ... \) -- grouping operator in tandem with regular expression parsing to match a substring. Here is a another example, this time from real life.

    # Strip the whitespace from the beginning and end.
    LRFDATE=`expr "$LRFDATE" : '[[:space:]]*\(.*\)[[:space:]]*$'`

    #  From Peter Knowles' "booklistgen.sh" script
    #+ for converting files to Sony Librie/PRS-50X format.
    #  (http://booklistgensh.peterknowles.com)

Perl, sed, and awk have far superior string parsing facilities. A short `sed` or `awk` subroutine within a script (see ) is an attractive alternative to `expr`.

See for more on using `expr` in string operations.

#### Time / Date Commands

##### Time/date and timing

`date`

  Simply invoked, `date` prints the date and time to `stdout`. Where this command gets interesting is in its formatting and parsing options.

  ##### Using date

      &ex51;

  The `-u` option gives the UTC (Universal Coordinated Time).

      bash$ date
      Fri Mar 29 21:07:39 MST 2002

      bash$ date -u
      Sat Mar 30 04:07:42 UTC 2002

  This option facilitates calculating the time between different dates.

  ##### Date calculations

      &datecalc;

  The date command has quite a number of output options. For example `%N` gives the nanosecond portion of the current time. One interesting use for this is to generate random integers.

      date +%N | sed -e 's/000$//' -e 's/^0//'
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      #  Strip off leading and trailing zeroes, if present.
      #  Length of generated integer depends on
      #+ how many zeroes stripped off.

      # 115281032
      # 63408725
      # 394504284

  There are many more options (try `man date`).

      date +%j
      # Echoes day of the year (days elapsed since January 1).

      date +%k%M
      # Echoes hour and minute in 24-hour format, as a single digit string.

      # The 'TZ' parameter permits overriding the default time zone.
      date                 # Mon Mar 28 21:42:16 MST 2005
      TZ=EST date          # Mon Mar 28 23:42:16 EST 2005
      # Thanks, Frank Kannemann and Pete Sjoberg, for the tip.

      SixDaysAgo=$(date --date='6 days ago')
      OneMonthAgo=$(date --date='1 month ago')  # Four weeks back (not a month!)
      OneYearAgo=$(date --date='1 year ago')

  See also and .

`zdump`

  Time zone dump: echoes the time in a specified time zone.

      bash$ zdump EST
      EST  Tue Sep 18 22:09:22 2001 EST

`time`

  Outputs verbose timing statistics for executing a command.

  `time ls -l /` gives something like this:

      real    0m0.067s
       user    0m0.004s
       sys     0m0.005s

  See also the very similar times command in the previous section.

  As of version 2.0 of Bash, `time` became a shell reserved word, with slightly altered behavior in a pipeline.

`touch`

  Utility for updating access/modification times of a file to current system time or other specified time, but also useful for creating a new file. The command `touch zzz` will create a new file of zero length, named `zzz`, assuming that `zzz` did not previously exist. Time-stamping empty files in this way is useful for storing date information, for example in keeping track of modification times on a project.

  The `touch` command is equivalent to `: >> newfile` or `>> newfile` (for ordinary files).

  Before doing a cp -u (copy/update), use `touch` to update the time stamp of files you don't wish overwritten.

  As an example, if the directory `/home/bozo/tax_audit` contains the files `spreadsheet-051606.data`, `spreadsheet-051706.data`, and `spreadsheet-051806.data`, then doing a `touch spreadsheet*.data` will protect these files from being overwritten by files with the same names during a `cp -u /home/bozo/financial_info/spreadsheet*data /home/bozo/tax_audit`.

`at`

  The `at` job control command executes a given set of commands at a specified time. Superficially, it resembles cron, however, `at` is chiefly useful for one-time execution of a command set.

  `at 2pm January 15` prompts for a set of commands to execute at that time. These commands should be shell-script compatible, since, for all practical purposes, the user is typing in an executable shell script a line at a time. Input terminates with a Ctl-D.

  Using either the `-f` option or input redirection (<), `at` reads a command list from a file. This file is an executable shell script, though it should, of course, be non-interactive. Particularly clever is including the run-parts command in the file to execute a different set of scripts.

      bash$ at 2:30 am Friday < at-jobs.list
      job 2 at 2000-10-27 02:30

`batch`

  The `batch` job control command is similar to `at`, but it runs a command list when the system load drops below `.8`. Like `at`, it can read commands from a file with the `-f` option.

  The concept of batch processing dates back to the era of mainframe computers. It means running a set of commands without user intervention.

`cal`

  Prints a neatly formatted monthly calendar to `stdout`. Will do current year or a large range of past and future years.

`sleep`

  This is the shell equivalent of a wait loop. It pauses for a specified number of seconds, doing nothing. It can be useful for timing or in processes running in the background, checking for a specific event every so often (polling), as in .

      sleep 3     # Pauses 3 seconds.

  The `sleep` command defaults to seconds, but minute, hours, or days may also be specified.

      sleep 3 h   # Pauses 3 hours!

  The watch command may be a better choice than `sleep` for running commands at timed intervals.

`usleep`

  Microsleep (the u may be read as the Greek mu, or micro- prefix). This is the same as `sleep`, above, but sleeps in microsecond intervals. It can be used for fine-grained timing, or for polling an ongoing process at very frequent intervals.

      usleep 30     # Pauses 30 microseconds.

  This command is part of the Red Hat initscripts / rc-scripts package.

  The `usleep` command does not provide particularly accurate timing, and is therefore unsuitable for critical timing loops.

`hwclock`
`clock`

  The `hwclock` command accesses or adjusts the machine's hardware clock. Some options require root privileges. The `/etc/rc.d/rc.sysinit` startup file uses `hwclock` to set the system time from the hardware clock at bootup.

  The `clock` command is a synonym for `hwclock`.

#### Text Processing Commands

##### Commands affecting text and text files

`sort`

  File sort utility, often used as a filter in a pipe. This command sorts a text stream or file forwards or backwards, or according to various keys or character positions. Using the `-m` option, it merges presorted input files. The info page lists its many capabilities and options. See , , and .

`tsort`

  Topological sort, reading in pairs of whitespace-separated strings and sorting according to input patterns. The original purpose of `tsort` was to sort a list of dependencies for an obsolete version of the ld linker in an ancient version of UNIX.

  The results of a tsort will usually differ markedly from those of the standard `sort` command, above.

`uniq`

  This filter removes duplicate lines from a sorted file. It is often seen in a pipe coupled with sort.

      cat list-1 list-2 list-3 | sort | uniq > final.list
      # Concatenates the list files,
      # sorts them,
      # removes duplicate lines,
      # and finally writes the result to an output file.

  The useful `-c` option prefixes each line of the input file with its number of occurrences.

      bash$ cat testfile
      This line occurs only once.
       This line occurs twice.
       This line occurs twice.
       This line occurs three times.
       This line occurs three times.
       This line occurs three times.

      bash$ uniq -c testfile
            1 This line occurs only once.
             2 This line occurs twice.
             3 This line occurs three times.

      bash$ sort testfile | uniq -c | sort -nr
            3 This line occurs three times.
             2 This line occurs twice.
             1 This line occurs only once.

  The `sort INPUTFILE | uniq -c | sort -nr` command string produces a frequency of occurrence listing on the `INPUTFILE` file (the `-nr` options to `sort` cause a reverse numerical sort). This template finds use in analysis of log files and dictionary lists, and wherever the lexical structure of a document needs to be examined.

  ##### Word Frequency Analysis

      &wf;

      bash$ cat testfile
      This line occurs only once.
       This line occurs twice.
       This line occurs twice.
       This line occurs three times.
       This line occurs three times.
       This line occurs three times.

      bash$ ./wf.sh testfile
            6 this
             6 occurs
             6 line
             3 times
             3 three
             2 twice
             1 only
             1 once

`expand`
`unexpand`

  The `expand` filter converts tabs to spaces. It is often used in a pipe.

  The `unexpand` filter converts spaces to tabs. This reverses the effect of `expand`.

`cut`

  A tool for extracting fields from files. It is similar to the `print $N` command set in awk, but more limited. It may be simpler to use cut in a script than awk. Particularly important are the `-d` (delimiter) and `-f` (field specifier) options.

  Using `cut` to obtain a listing of the mounted filesystems:

      cut -d ' ' -f1,2 /etc/mtab

  Using `cut` to list the OS and kernel version:

      uname -a | cut -d" " -f1,3,11,12

  Using `cut` to extract message headers from an e-mail folder:

      bash$ grep '^Subject:' read-messages | cut -c10-80
      Re: Linux suitable for mission-critical apps?
       MAKE MILLIONS WORKING AT HOME!!!
       Spam complaint
       Re: Spam complaint

  Using `cut` to parse a file:

      # List all the users in /etc/passwd.

      FILENAME=/etc/passwd

      for user in $(cut -d: -f1 $FILENAME)
      do
        echo $user
      done

      # Thanks, Oleg Philon for suggesting this.

  `cut -d ' ' -f2,3 filename` is equivalent to `awk -F'[ ]' '{ print $2, $3 }' filename`

  It is even possible to specify a linefeed as a delimiter. The trick is to actually embed a linefeed (RETURN) in the command sequence.

      bash$ cut -d'
       ' -f3,7,19 testfile
      This is line 3 of testfile.
       This is line 7 of testfile.
       This is line 19 of testfile.

  Thank you, Jaka Kranjc, for pointing this out.

  See also .

`paste`

  Tool for merging together different files into a single, multi-column file. In combination with cut, useful for creating system log files.

      bash$ cat items
      alphabet blocks
       building blocks
       cables

      bash$ cat prices
      $1.00/dozen
       $2.50 ea.
       $3.75

      bash$ paste items prices
      alphabet blocks $1.00/dozen
       building blocks $2.50 ea.
       cables  $3.75

`join`

  Consider this a special-purpose cousin of `paste`. This powerful utility allows merging two files in a meaningful fashion, which essentially creates a simple version of a relational database.

  The `join` command operates on exactly two files, but pastes together only those lines with a common tagged field (usually a numerical label), and writes the result to `stdout`. The files to be joined should be sorted according to the tagged field for the matchups to work properly.

      File: 1.data

      100 Shoes
      200 Laces
      300 Socks

      File: 2.data

      100 $40.00
      200 $1.00
      300 $2.00

      bash$ join 1.data 2.data
      File: 1.data 2.data

       100 Shoes $40.00
       200 Laces $1.00
       300 Socks $2.00

  The tagged field appears only once in the output.

`head`

  lists the beginning of a file to `stdout`. The default is `10` lines, but a different number can be specified. The command has a number of interesting options.

  ##### Which files are scripts?

      &scriptdetector;

  ##### Generating 10-digit random numbers

      &rnd;

  See also .
`tail`

  lists the (tail) end of a file to `stdout`. The default is `10` lines, but this can be changed with the `-n` option. Commonly used to keep track of changes to a system logfile, using the `-f` option, which outputs lines appended to the file.

  ##### Using tail to monitor the system log

      &ex12;

  To list a specific line of a text file, pipe the output of `head` to `tail -n 1`. For example `head -n 8 database.txt | tail -n 1` lists the 8th line of the file `database.txt`.

  To set a variable to a given block of a text file:

      var=$(head -n $m $filename | tail -n $n)

      # filename = name of file
      # m = from beginning of file, number of lines to end of block
      # n = number of lines to set variable to (trim from end of block)

  Newer implementations of `tail` deprecate the older `tail -$LINES filename` usage. The standard `tail -n $LINES filename` is correct.

  See also , and .

`grep`

  A multi-purpose file search tool that uses Regular Expressions. It was originally a command/filter in the venerable `ed` line editor: `g/re/p` -- global - regular expression - print.

      		grep pattern
      		file

  Search the target file(s) for occurrences of pattern, where pattern may be literal text or a Regular Expression.

      bash$ grep '[rst]ystem.$' osinfo.txt
      The GPL governs the distribution of the Linux operating system.

  If no target file(s) specified, `grep` works as a filter on `stdout`, as in a pipe.

      bash$ ps ax | grep clock
      765 tty1     S      0:00 xclock
       901 pts/1    S      0:00 grep clock

  The `-i` option causes a case-insensitive search.

  The `-w` option matches only whole words.

  The `-l` option lists only the files in which matches were found, but not the matching lines.

  The `-r` (recursive) option searches files in the current working directory and all subdirectories below it.

  The `-n` option lists the matching lines, together with line numbers.

      bash$ grep -n Linux osinfo.txt
      2:This is a file containing information about Linux.
       6:The GPL governs the distribution of the Linux operating system.

  The `-v` (or `--invert-match`) option filters out matches.

      grep pattern1 *.txt | grep -v pattern2

      # Matches all lines in "*.txt" files containing "pattern1",
      # but ***not*** "pattern2".

  The `-c` (`--count`) option gives a numerical count of matches, rather than actually listing the matches.

      grep -c txt *.sgml   # (number of occurrences of "txt" in "*.sgml" files)

      #   grep -cz .
      #            ^ dot
      # means count (-c) zero-separated (-z) items matching "."
      # that is, non-empty ones (containing at least 1 character).
      #
      printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz .     # 3
      printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$'   # 5
      printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^'   # 5
      #
      printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$'    # 9
      # By default, newline chars (\n) separate items to match.

      # Note that the -z option is GNU "grep" specific.

      # Thanks, S.C.

  The `--color` (or `--colour`) option marks the matching string in color (on the console or in an xterm window). Since grep prints out each entire line containing the matching pattern, this lets you see exactly what is being matched. See also the `-o` option, which shows only the matching portion of the line(s).

  ##### Printing out the From lines in stored e-mail messages

      &fromsh;

  When invoked with more than one target file given, `grep` specifies which file contains matches.

      bash$ grep Linux osinfo.txt misc.txt
      osinfo.txt:This is a file containing information about Linux.
       osinfo.txt:The GPL governs the distribution of the Linux operating system.
       misc.txt:The Linux operating system is steadily gaining in popularity.

  To force `grep` to show the filename when searching only one target file, simply give `/dev/null` as the second file.

      bash$ grep Linux osinfo.txt /dev/null
      osinfo.txt:This is a file containing information about Linux.
       osinfo.txt:The GPL governs the distribution of the Linux operating system.

  If there is a successful match, `grep` returns an exit status of 0, which makes it useful in a condition test in a script, especially in combination with the `-q` option to suppress output.

      SUCCESS=0                      # if grep lookup succeeds
      word=Linux
      filename=data.file

      grep -q "$word" "$filename"    #  The "-q" option
                                     #+ causes nothing to echo to stdout.
      if [ $? -eq $SUCCESS ]
      # if grep -q "$word" "$filename"   can replace lines 5 - 7.
      then
        echo "$word found in $filename"
      else
        echo "$word not found in $filename"
      fi

  demonstrates how to use `grep` to search for a word pattern in a system logfile.

  ##### Emulating grep in a script

      &grp;

  How can `grep` search for two (or more) separate patterns? What if you want `grep` to display all lines in a file or files that contain both pattern1 and pattern2?

  One method is to pipe the result of `grep pattern1` to `grep pattern2`.

  For example, given the following file:

      # Filename: tstfile

      This is a sample file.
      This is an ordinary text file.
      This file does not contain any unusual text.
      This file is not unusual.
      Here is some text.

  Now, let's search this file for lines containing both file and text . . .

      bash$ grep file tstfile
      # Filename: tstfile
       This is a sample file.
       This is an ordinary text file.
       This file does not contain any unusual text.
       This file is not unusual.

      bash$ grep file tstfile | grep text
      This is an ordinary text file.
       This file does not contain any unusual text.

  Now, for an interesting recreational use of grep . . .

  ##### Crossword puzzle solver

      &cwsolver;

  `egrep` -- extended grep -- is the same as `grep -E`. This uses a somewhat different, extended set of Regular Expressions, which can make the search a bit more flexible. It also allows the boolean | (or) operator.

      bash $ egrep 'matches|Matches' file.txt
      Line 1 matches.
       Line 3 Matches.
       Line 4 contains matches, but also Matches

  `fgrep` -- fast grep -- is the same as `grep -F`. It does a literal string search (no Regular Expressions), which generally speeds things up a bit.

  On some Linux distros, `egrep` and `fgrep` are symbolic links to, or aliases for `grep`, but invoked with the `-E` and `-F` options, respectively.

  ##### Looking up definitions in Webster's 1913 Dictionary

      &dictlookup;

  See also for an example of speedy fgrep lookup on a large text file.

  `agrep` (approximate grep) extends the capabilities of `grep` to approximate matching. The search string may differ by a specified number of characters from the resulting matches. This utility is not part of the core Linux distribution.

  To search compressed files, use `zgrep`, `zegrep`, or `zfgrep`. These also work on non-compressed files, though slower than plain `grep`, `egrep`, `fgrep`. They are handy for searching through a mixed set of files, some compressed, some not.

  To search bzipped files, use `bzgrep`.

`look`

  The command `look` works like `grep`, but does a lookup on a dictionary, a sorted word list. By default, `look` searches for a match in `/usr/dict/words`, but a different dictionary file may be specified.

  ##### Checking words in a list for validity

      &lookup;

`sed`
`awk`

  Scripting languages especially suited for parsing text files and command output. May be embedded singly or in combination in pipes and shell scripts.

`sed`

  Non-interactive stream editor, permits using many `ex` commands in batch mode. It finds many uses in shell scripts.

`awk`

  Programmable file extractor and formatter, good for manipulating and/or extracting fields (columns) in structured text files. Its syntax is similar to C.

`wc`

  wc gives a word count on a file or I/O stream:

      bash $ wc /usr/share/doc/sed-4.1.2/README
      13  70  447 README
      [13 lines  70 words  447 characters]

  `wc -w` gives only the word count.

  `wc -l` gives only the line count.

  `wc -c` gives only the byte count.

  `wc -m` gives only the character count.

  `wc -L` gives only the length of the longest line.

  Using `wc` to count how many `.txt` files are in current working directory:

      $ ls *.txt | wc -l
      #  Will work as long as none of the "*.txt" files
      #+ have a linefeed embedded in their name.

      #  Alternative ways of doing this are:
      #      find . -maxdepth 1 -name \*.txt -print0 | grep -cz .
      #      (shopt -s nullglob; set -- *.txt; echo $#)

      #  Thanks, S.C.

  Using `wc` to total up the size of all the files whose names begin with letters in the range d - h

      bash$ wc [d-h]* | grep total | awk '{print $3}'
      71832

  Using `wc` to count the instances of the word Linux in the main source file for this book.

      bash$ grep Linux abs-book.sgml | wc -l
      138

  See also and .

  Certain commands include some of the functionality of `wc` as options.

      ... | grep foo | wc -l
      # This frequently used construct can be more concisely rendered.

      ... | grep -c foo
      # Just use the "-c" (or "--count") option of grep.

      # Thanks, S.C.

`tr`

  character translation filter.

  Must use quoting and/or brackets, as appropriate. Quotes prevent the shell from reinterpreting the special characters in `tr` command sequences. Brackets should be quoted to prevent expansion by the shell.

  Either `tr "A-Z" "*" <filename` or `tr A-Z \* <filename` changes all the uppercase letters in `filename` to asterisks (writes to `stdout`). On some systems this may not work, but `tr A-Z '[**]'` will.

  The `-d` option deletes a range of characters.

      echo "abcdef"                 # abcdef
      echo "abcdef" | tr -d b-d     # aef

      tr -d 0-9 <filename
      # Deletes all digits from the file "filename".

  The `--squeeze-repeats` (or `-s`) option deletes all but the first instance of a string of consecutive characters. This option is useful for removing excess whitespace.

      bash$ echo "XXXXX" | tr --squeeze-repeats 'X'
      X

  The `-c` complement option inverts the character set to match. With this option, `tr` acts only upon those characters not matching the specified set.

      bash$ echo "acfdeb123" | tr -c b-d +
      +c+d+b++++

  Note that `tr` recognizes POSIX character classes. This is only true of the GNU version of `tr`, not the generic version often found on commercial UNIX systems.

      bash$ echo "abcd2ef1" | tr '[:alpha:]' -
      ----2--1

  ##### toupper: Transforms a file to all uppercase.

      &ex49;

  ##### lowercase: Changes all filenames in working directory to lowercase.

      &lowercase;

  ##### du: DOS to UNIX text file conversion.

      &du;

  ##### rot13: ultra-weak encryption.

      &rot13;

  ##### Generating Crypto-Quote Puzzles

      &cryptoquote;

  Of course, tr lends itself to code obfuscation.

      #!/bin/bash
      # jabh.sh

      x="wftedskaebjgdBstbdbsmnjgz"
      echo $x | tr "a-z" 'oh, turtleneck Phrase Jar!'

      # Based on the Wikipedia "Just another Perl hacker" article.

  ##### tr variants

  The `tr` utility has two historic variants. The BSD version does not use brackets (`tr a-z A-Z`), but the SysV one does (`tr '[a-z]' '[A-Z]'`). The GNU version of `tr` resembles the BSD one.

`fold`

  A filter that wraps lines of input to a specified width. This is especially useful with the `-s` option, which breaks lines at word spaces (see and ).

`fmt`

  Simple-minded file formatter, used as a filter in a pipe to wrap long lines of text output.

  ##### Formatted file listing.

      &ex50;

  See also .

  A powerful alternative to `fmt` is Kamil Toman's `par` utility, available from http://www.cs.berkeley.edu/~amc/Par/.

`col`

  This deceptively named filter removes reverse line feeds from an input stream. It also attempts to replace whitespace with equivalent tabs. The chief use of `col` is in filtering the output from certain text processing utilities, such as `groff` and `tbl`.

`column`

  Column formatter. This filter transforms list-type text output into a pretty-printed table by inserting tabs at appropriate places.

  ##### Using column to format a directory listing

      &colm;

`colrm`

  Column removal filter. This removes columns (characters) from a file and writes the file, lacking the range of specified columns, back to `stdout`. `colrm 2 4 <filename` removes the second through fourth characters from each line of the text file `filename`.

  If the file contains tabs or nonprintable characters, this may cause unpredictable behavior. In such cases, consider using expand and `unexpand` in a pipe preceding `colrm`.

`nl`

  Line numbering filter: `nl filename` lists `filename` to `stdout`, but inserts consecutive numbers at the beginning of each non-blank line. If `filename` omitted, operates on `stdin.`

  The output of `nl` is very similar to `cat -b`, since, by default `nl` does not list blank lines.

  ##### nl: A self-numbering script.

      &lnum;

`pr`

  Print formatting filter. This will paginate files (or `stdout`) into sections suitable for hard copy printing or viewing on screen. Various options permit row and column manipulation, joining lines, setting margins, numbering lines, adding page headers, and merging files, among other things. The `pr` command combines much of the functionality of `nl`, `paste`, `fold`, `column`, and `expand`.

  `pr -o 5 --width=65 fileZZZ | more` gives a nice paginated listing to screen of `fileZZZ` with margins set at 5 and 65.

  A particularly useful option is `-d`, forcing double-spacing (same effect as `sed -G`).

`gettext`

  The GNU `gettext` package is a set of utilities for localizing and translating the text output of programs into foreign languages. While originally intended for C programs, it now supports quite a number of programming and scripting languages.

  The `gettext` program works on shell scripts. See the info page.

`msgfmt`

  A program for generating binary message catalogs. It is used for localization.

`iconv`

  A utility for converting file(s) to a different encoding (character set). Its chief use is for localization.

      # Convert a string from UTF-8 to UTF-16 and print to the BookList
      function write_utf8_string {
          STRING=$1
          BOOKLIST=$2
          echo -n "$STRING" | iconv -f UTF8 -t UTF16 | \
          cut -b 3- | tr -d \\n >> "$BOOKLIST"
      }

      #  From Peter Knowles' "booklistgen.sh" script
      #+ for converting files to Sony Librie/PRS-50X format.
      #  (http://booklistgensh.peterknowles.com)

`recode`

  Consider this a fancier version of `iconv`, above. This very versatile utility for converting a file to a different encoding scheme. Note that recode is not part of the standard Linux installation.

`TeX`
`gs`

  `TeX` and `Postscript` are text markup languages used for preparing copy for printing or formatted video display.

  `TeX` is Donald Knuth's elaborate typsetting system. It is often convenient to write a shell script encapsulating all the options and arguments passed to one of these markup languages.

  Ghostscript (`gs`) is a GPL-ed Postscript interpreter.

`texexec`

  Utility for processing TeX and pdf files. Found in `/usr/bin` on many Linux distros, it is actually a shell wrapper that calls Perl to invoke Tex.

      texexec --pdfarrange --result=Concatenated.pdf *pdf

      #  Concatenates all the pdf files in the current working directory
      #+ into the merged file, Concatenated.pdf . . .
      #  (The --pdfarrange option repaginates a pdf file. See also --pdfcombine.)
      #  The above command-line could be parameterized and put into a shell script.

`enscript`

  Utility for converting plain text file to PostScript

  For example, `enscript filename.txt -p filename.ps` produces the PostScript output file `filename.ps`.

`groff`
`tbl`
`eqn`

  Yet another text markup and display formatting language is `groff`. This is the enhanced GNU version of the venerable UNIX `roff/troff` display and typesetting package. Manpages use `groff`.

  The `tbl` table processing utility is considered part of `groff`, as its function is to convert table markup into `groff` commands.

  The `eqn` equation processing utility is likewise part of `groff`, and its function is to convert equation markup into `groff` commands.

  ##### manview: Viewing formatted manpages

      &manview;

  See also .

`lex`
`yacc`

  The `lex` lexical analyzer produces programs for pattern matching. This has been replaced by the nonproprietary `flex` on Linux systems.

  The `yacc` utility creates a parser based on a set of specifications. This has been replaced by the nonproprietary `bison` on Linux systems.

#### File and Archiving Commands

##### Archiving

`tar`

  The standard UNIX archiving utility.

  An archive, in the sense discussed here, is simply a set of related files stored in a single location.

  Originally a Tape ARchiving program, it has developed into a general purpose package that can handle all manner of archiving with all types of destination devices, ranging from tape drives to regular files to even `stdout` (see ). GNU tar has been patched to accept various compression filters, for example: `tar czvf archive_name.tar.gz *`, which recursively archives and gzips all files in a directory tree except dotfiles in the current working directory ($PWD). A tar czvf ArchiveName.tar.gz * will include dotfiles in subdirectories below the current working directory. This is an undocumented GNU `tar` feature.

  Some useful `tar` options:

  1. `-c` create (a new archive)

  2. `-x` extract (files from existing archive)

  3. `--delete` delete (files from existing archive)

     This option will not work on magnetic tape devices.

  4. `-r` append (files to existing archive)

  5. `-A` append (tar files to existing archive)

  6. `-t` list (contents of existing archive)

  7. `-u` update archive

  8. `-d` compare archive with specified filesystem

  9. `--after-date` only process files with a date stamp after specified date

  10. `-z` gzip the archive

      (compress or uncompress, depending on whether combined with the `-c` or `-x`) option

  11. `-j` bzip2 the archive

  It may be difficult to recover data from a corrupted gzipped tar archive. When archiving important files, make multiple backups.

`shar`

  Shell archiving utility. The text and/or binary files in a shell archive are concatenated without compression, and the resultant archive is essentially a shell script, complete with #!/bin/sh header, containing all the necessary unarchiving commands, as well as the files themselves. Unprintable binary characters in the target file(s) are converted to printable ASCII characters in the output shar file. Shar archives still show up in Usenet newsgroups, but otherwise `shar` has been replaced by `tar`/`gzip`. The `unshar` command unpacks shar archives.

  The `mailshar` command is a Bash script that uses `shar` to concatenate multiple files into a single one for e-mailing. This script supports compression and uuencoding.

`ar`

  Creation and manipulation utility for archives, mainly used for binary object file libraries.

`rpm`

  The Red Hat Package Manager, or `rpm` utility provides a wrapper for source or binary archives. It includes commands for installing and checking the integrity of packages, among other things.

  A simple `rpm -i package_name.rpm` usually suffices to install a package, though there are many more options available.

  `rpm -qf` identifies which package a file originates from.

      bash$ rpm -qf /bin/ls
      coreutils-5.2.1-31

  `rpm -qa` gives a complete list of all installed rpm packages on a given system. An `rpm -qa package_name` lists only the package(s) corresponding to `package_name`.

      bash$ rpm -qa
      redhat-logos-1.1.3-1
       glibc-2.2.4-13
       cracklib-2.7-12
       dosfstools-2.7-1
       gdbm-1.8.0-10
       ksymoops-2.4.1-1
       mktemp-1.5-11
       perl-5.6.0-17
       reiserfs-utils-3.x.0j-2
       ...

      bash$ rpm -qa docbook-utils
      docbook-utils-0.6.9-2

      bash$ rpm -qa docbook | grep docbook
      docbook-dtd31-sgml-1.0-10
       docbook-style-dsssl-1.64-3
       docbook-dtd30-sgml-1.0-10
       docbook-dtd40-sgml-1.0-11
       docbook-utils-pdf-0.6.9-2
       docbook-dtd41-sgml-1.0-10
       docbook-utils-0.6.9-2

`cpio`

  This specialized archiving copy command (`c`o`p`y `i`nput and `o`utput) is rarely seen any more, having been supplanted by `tar`/`gzip`. It still has its uses, such as moving a directory tree. With an appropriate block size (for copying) specified, it can be appreciably faster than `tar`.

  ##### Using cpio to move a directory tree

      &ex48;

`rpm2cpio`

  This command extracts a `cpio` archive from an rpm one.

  ##### Unpacking an rpm archive

      &derpm;

`pax`

  The pax `p`ortable `a`rchive e`x`change toolkit facilitates periodic file backups and is designed to be cross-compatible between various flavors of UNIX. It was designed to replace tar and cpio.

      pax -wf daily_backup.pax ~/linux-server/files
      #  Creates a tar archive of all files in the target directory.
      #  Note that the options to pax must be in the correct order --
      #+ pax -fw     has an entirely different effect.

      pax -f daily_backup.pax
      #  Lists the files in the archive.

      pax -rf daily_backup.pax ~/bsd-server/files
      #  Restores the backed-up files from the Linux machine
      #+ onto a BSD one.

  Note that pax handles many of the standard archiving and compression commands.

##### Compression

`gzip`

  The standard GNU/UNIX compression utility, replacing the inferior and proprietary `compress`. The corresponding decompression command is `gunzip`, which is the equivalent of `gzip -d`.

  The `-c` option sends the output of `gzip` to `stdout`. This is useful when piping to other commands.

  The `zcat` filter decompresses a gzipped file to `stdout`, as possible input to a pipe or redirection. This is, in effect, a `cat` command that works on compressed files (including files processed with the older compress utility). The `zcat` command is equivalent to `gzip -dc`.

  On some commercial UNIX systems, `zcat` is a synonym for `uncompress -c`, and will not work on gzipped files.

  See also .

`bzip2`

  An alternate compression utility, usually more efficient (but slower) than `gzip`, especially on large files. The corresponding decompression command is `bunzip2`.

  Similar to the `zcat` command, `bzcat` decompresses a bzipped2-ed file to `stdout`.

  Newer versions of tar have been patched with `bzip2` support.

`compress`
`uncompress`

  This is an older, proprietary compression utility found in commercial UNIX distributions. The more efficient `gzip` has largely replaced it. Linux distributions generally include a `compress` workalike for compatibility, although `gunzip` can unarchive files treated with `compress`.

  The `znew` command transforms compressed files into gzipped ones.

`sq`

  Yet another compression (`sq`ueeze) utility, a filter that works only on sorted ASCII word lists. It uses the standard invocation syntax for a filter, `sq < input-file > output-file`. Fast, but not nearly as efficient as gzip. The corresponding uncompression filter is `unsq`, invoked like `sq`.

  The output of `sq` may be piped to `gzip` for further compression.

`zip`
`unzip`

  Cross-platform file archiving and compression utility compatible with DOS pkzip.exe. Zipped archives seem to be a more common medium of file exchange on the Internet than tarballs.

`unarc`
`unarj`
`unrar`

  These Linux utilities permit unpacking archives compressed with the DOS arc.exe, arj.exe, and rar.exe programs.

`lzma`
`unlzma`
`lzcat`

  Highly efficient Lempel-Ziv-Markov compression. The syntax of lzma is similar to that of gzip. The 7-zip Website has more information.

`xz`
`unxz`
`xzcat`

  A new high-efficiency compression tool, backward compatible with lzma, and with an invocation syntax similar to gzip. For more information, see the Wikipedia entry.

##### File Information

`file`

  A utility for identifying file types. The command `file file-name` will return a file specification for `file-name`, such as `ascii text` or `data`. It references the magic numbers found in `/usr/share/magic`, `/etc/magic`, or `/usr/lib/magic`, depending on the Linux/UNIX distribution.

  The `-f` option causes `file` to run in batch mode, to read from a designated file a list of filenames to analyze. The `-z` option, when used on a compressed target file, forces an attempt to analyze the uncompressed file type.

      bash$ file test.tar.gz
      test.tar.gz: gzip compressed data, deflated,
       last modified: Sun Sep 16 13:34:51 2001, os: Unix

      bash file -z test.tar.gz
      test.tar.gz: GNU tar archive (gzip compressed data, deflated,
       last modified: Sun Sep 16 13:34:51 2001, os: Unix)

      # Find sh and Bash scripts in a given directory:

      DIRECTORY=/usr/local/bin
      KEYWORD=Bourne
      # Bourne and Bourne-Again shell scripts

      file $DIRECTORY/* | fgrep $KEYWORD

      # Output:

      # /usr/local/bin/burn-cd:          Bourne-Again shell script text executable
      # /usr/local/bin/burnit:           Bourne-Again shell script text executable
      # /usr/local/bin/cassette.sh:      Bourne shell script text executable
      # /usr/local/bin/copy-cd:          Bourne-Again shell script text executable
      # . . .

  ##### Stripping comments from C program files

      &stripc;

`which`

  `which command` gives the full path to command. This is useful for finding out whether a particular command or utility is installed on the system.

  `$bash which rm`

      /usr/bin/rm

  For an interesting use of this command, see .

`whereis`

  Similar to `which`, above, `whereis command` gives the full path to command, but also to its manpage.

  `$bash whereis rm`

      rm: /bin/rm /usr/share/man/man1/rm.1.bz2

`whatis`

  `whatis command` looks up command in the whatis database. This is useful for identifying system commands and important configuration files. Consider it a simplified `man` command.

  `$bash whatis whatis`

      whatis               (1)  - search the whatis database for complete words

  ##### Exploring `/usr/X11R6/bin`

      &what;

  See also .

`vdir`

  Show a detailed directory listing. The effect is similar to ls -lb.

  This is one of the GNU fileutils.

      bash$ vdir
      total 10
       -rw-r--r--    1 bozo  bozo      4034 Jul 18 22:04 data1.xrolo
       -rw-r--r--    1 bozo  bozo      4602 May 25 13:58 data1.xrolo.bak
       -rw-r--r--    1 bozo  bozo       877 Dec 17  2000 employment.xrolo

      bash ls -l
      total 10
       -rw-r--r--    1 bozo  bozo      4034 Jul 18 22:04 data1.xrolo
       -rw-r--r--    1 bozo  bozo      4602 May 25 13:58 data1.xrolo.bak
       -rw-r--r--    1 bozo  bozo       877 Dec 17  2000 employment.xrolo

`locate`
`slocate`

  The `locate` command searches for files using a database stored for just that purpose. The `slocate` command is the secure version of `locate` (which may be aliased to `slocate`).

  `$bash locate hickson`

      /usr/lib/xephem/catalogs/hickson.edb

`getfacl`
`setfacl`

  These commands retrieve or set the `f`ile `a`ccess `c`ontrol `l`ist -- the owner, group, and file permissions.

      bash$ getfacl *
      # file: test1.txt
       # owner: bozo
       # group: bozgrp
       user::rw-
       group::rw-
       other::r--

       # file: test2.txt
       # owner: bozo
       # group: bozgrp
       user::rw-
       group::rw-
       other::r--

      bash$ setfacl -m u:bozo:rw yearly_budget.csv
      bash$ getfacl yearly_budget.csv
      # file: yearly_budget.csv
       # owner: accountant
       # group: budgetgrp
       user::rw-
       user:bozo:rw-
       user:accountant:rw-
       group::rw-
       mask::rw-
       other::r--

`readlink`

  Disclose the file that a symbolic link points to.

      bash$ readlink /usr/bin/awk
      ../../bin/gawk

`strings`

  Use the `strings` command to find printable strings in a binary or data file. It will list sequences of printable characters found in the target file. This might be handy for a quick 'n dirty examination of a core dump or for looking at an unknown graphic image file (`strings image-file | more` might show something like JFIF, which would identify the file as a jpeg graphic). In a script, you would probably parse the output of `strings` with grep or sed. See and .

  ##### An improved strings command

      &wstrings;

##### Comparison

`diff`
`patch`

  `diff`: flexible file comparison utility. It compares the target files line-by-line sequentially. In some applications, such as comparing word dictionaries, it may be helpful to filter the files through sort and `uniq` before piping them to `diff`. `diff file-1 file-2` outputs the lines in the files that differ, with carets showing which file each particular line belongs to.

  The `--side-by-side` option to `diff` outputs each compared file, line by line, in separate columns, with non-matching lines marked. The `-c` and `-u` options likewise make the output of the command easier to interpret.

  There are available various fancy frontends for `diff`, such as `sdiff`, `wdiff`, `xdiff`, and `mgdiff`.

  The `diff` command returns an exit status of 0 if the compared files are identical, and 1 if they differ (or 2 when binary files are being compared). This permits use of `diff` in a test construct within a shell script (see below).

  A common use for `diff` is generating difference files to be used with `patch` The `-e` option outputs files suitable for `ed` or `ex` scripts.

  `patch`: flexible versioning utility. Given a difference file generated by `diff`, `patch` can upgrade a previous version of a package to a newer version. It is much more convenient to distribute a relatively small diff file than the entire body of a newly revised package. Kernel patches have become the preferred method of distributing the frequent releases of the Linux kernel.

      patch -p1 <patch-file
      # Takes all the changes listed in 'patch-file'
      # and applies them to the files referenced therein.
      # This upgrades to a newer version of the package.

  Patching the kernel:

      cd /usr/src
      gzip -cd patchXX.gz | patch -p0
      # Upgrading kernel source using 'patch'.
      # From the Linux kernel docs "README",
      # by anonymous author (Alan Cox?).

  The `diff` command can also recursively compare directories (for the filenames present).

      bash$ diff -r ~/notes1 ~/notes2
      Only in /home/bozo/notes1: file02
       Only in /home/bozo/notes1: file03
       Only in /home/bozo/notes2: file04

  Use `zdiff` to compare gzipped files.

  Use `diffstat` to create a histogram (point-distribution graph) of output from `diff`.

`diff3`
`merge`

  An extended version of `diff` that compares three files at a time. This command returns an exit value of 0 upon successful execution, but unfortunately this gives no information about the results of the comparison.

      bash$ diff3 file-1 file-2 file-3
      ====
       1:1c
         This is line 1 of "file-1".
       2:1c
         This is line 1 of "file-2".
       3:1c
         This is line 1 of "file-3"

  The `merge` (3-way file merge) command is an interesting adjunct to diff3. Its syntax is `merge Mergefile file1 file2`. The result is to output to `Mergefile` the changes that lead from `file1` to `file2`. Consider this command a stripped-down version of patch.

`sdiff`

  Compare and/or edit two files in order to merge them into an output file. Because of its interactive nature, this command would find little use in a script.

`cmp`

  The `cmp` command is a simpler version of `diff`, above. Whereas `diff` reports the differences between two files, `cmp` merely shows at what point they differ.

  Like `diff`, `cmp` returns an exit status of 0 if the compared files are identical, and 1 if they differ. This permits use in a test construct within a shell script.

  ##### Using cmp to compare two files within a script.

      &filecomp;

  Use `zcmp` on gzipped files.

`comm`

  Versatile file comparison utility. The files must be sorted for this to be useful.

  `comm -options first-file second-file`

  `comm file-1 file-2` outputs three columns:

  - column 1 = lines unique to `file-1`

  - column 2 = lines unique to `file-2`

  - column 3 = lines common to both.

  The options allow suppressing output of one or more columns.

  - `-1` suppresses column `1`

  - `-2` suppresses column `2`

  - `-3` suppresses column `3`

  - `-12` suppresses both columns `1` and `2`, etc.

  This command is useful for comparing dictionaries or word lists -- sorted text files with one word per line.

##### Utilities

`basename`

  Strips the path information from a file name, printing only the file name. The construction `basename $0` lets the script know its name, that is, the name it was invoked by. This can be used for usage messages if, for example a script is called with missing arguments:

      echo "Usage: `basename $0` arg1 arg2 ... argn"

`dirname`

  Strips the `basename` from a filename, printing only the path information.

  `basename` and `dirname` can operate on any arbitrary string. The argument does not need to refer to an existing file, or even be a filename for that matter (see ).

  ##### basename and dirname

      &ex35;

`split`
`csplit`

  These are utilities for splitting a file into smaller chunks. Their usual use is for splitting up large files in order to back them up on floppies or preparatory to e-mailing or uploading them.

  The `csplit` command splits a file according to context, the split occuring where patterns are matched.

  ##### A script that copies itself in sections

      &splitcopy;

##### Encoding and Encryption

`sum`
`cksum`
`md5sum`
`sha1sum`

  These are utilities for generating checksums. A checksum is a number The checksum may be expressed as a hexadecimal number, or to some other base. mathematically calculated from the contents of a file, for the purpose of checking its integrity. A script might refer to a list of checksums for security purposes, such as ensuring that the contents of key system files have not been altered or corrupted. For security applications, use the `md5sum` (`m`essage `d`igest `5` check`sum`) command, or better yet, the newer `sha1sum` (Secure Hash Algorithm). For even better security, use the sha256sum, sha512, and sha1pass commands.

      bash$ cksum /boot/vmlinuz
      1670054224 804083 /boot/vmlinuz

      bash$ echo -n "Top Secret" | cksum
      3391003827 10

      bash$ md5sum /boot/vmlinuz
      0f43eccea8f09e0a0b2b5cf1dcf333ba  /boot/vmlinuz

      bash$ echo -n "Top Secret" | md5sum
      8babc97a6f62a4649716f4df8d61728f  -

  The `cksum` command shows the size, in bytes, of its target, whether file or `stdout`.

  The `md5sum` and `sha1sum` commands display a dash when they receive their input from `stdout`.

  ##### Checking file integrity

      &fileintegrity;

  Also see , , and for creative uses of the `md5sum` command.

  There have been reports that the 128-bit `md5sum` can be cracked, so the more secure 160-bit `sha1sum` is a welcome new addition to the checksum toolkit.

      bash$ md5sum testfile
      e181e2c8720c60522c4c4c981108e367  testfile

      bash$ sha1sum testfile
      5d7425a9c08a66c3177f1e31286fa40986ffc996  testfile

  Security consultants have demonstrated that even `sha1sum` can be compromised. Fortunately, newer Linux distros include longer bit-length `sha224sum`, `sha256sum`, `sha384sum`, and `sha512sum` commands.

`uuencode`

  This utility encodes binary files (images, sound files, compressed files, etc.) into ASCII characters, making them suitable for transmission in the body of an e-mail message or in a newsgroup posting. This is especially useful where MIME (multimedia) encoding is not available.

`uudecode`

  This reverses the encoding, decoding uuencoded files back into the original binaries.

  ##### Uudecoding encoded files

      &ex52;

  The fold -s command may be useful (possibly in a pipe) to process long uudecoded text messages downloaded from Usenet newsgroups.

`mimencode`
`mmencode`

  The `mimencode` and `mmencode` commands process multimedia-encoded e-mail attachments. Although mail user agents (such as pine or kmail) normally handle this automatically, these particular utilities permit manipulating such attachments manually from the command-line or in batch processing mode by means of a shell script.

`crypt`

  At one time, this was the standard UNIX file encryption utility.

  This is a symmetric block cipher, used to encrypt files on a single system or local network, as opposed to the public key cipher class, of which pgp is a well-known example.

  Politically-motivated government regulations prohibiting the export of encryption software resulted in the disappearance of `crypt` from much of the UNIX world, and it is still missing from most Linux distributions. Fortunately, programmers have come up with a number of decent alternatives to it, among them the author's very own cruft (see ).

`openssl`

  This is an Open Source implementation of Secure Sockets Layer encryption.

      # To encrypt a file:
      openssl aes-128-ecb -salt -in file.txt -out file.encrypted \
      -pass pass:my_password
      #          ^^^^^^^^^^^   User-selected password.
      #       aes-128-ecb      is the encryption method chosen.

      # To decrypt an openssl-encrypted file:
      openssl aes-128-ecb -d -salt -in file.encrypted -out file.txt \
      -pass pass:my_password
      #          ^^^^^^^^^^^   User-selected password.

  Piping openssl to/from tar makes it possible to encrypt an entire directory tree.

      # To encrypt a directory:

      sourcedir="/home/bozo/testfiles"
      encrfile="encr-dir.tar.gz"
      password=my_secret_password

      tar czvf - "$sourcedir" |
      openssl des3 -salt -out "$encrfile" -pass pass:"$password"
      #       ^^^^   Uses des3 encryption.
      # Writes encrypted file "encr-dir.tar.gz" in current working directory.

      # To decrypt the resulting tarball:
      openssl des3 -d -salt -in "$encrfile" -pass pass:"$password" |
      tar -xzv
      # Decrypts and unpacks into current working directory.

  Of course, openssl has many other uses, such as obtaining signed certificates for Web sites. See the info page.

`shred`

  Securely erase a file by overwriting it multiple times with random bit patterns before deleting it. This command has the same effect as , but does it in a more thorough and elegant manner.

  This is one of the GNU fileutils.

  Advanced forensic technology may still be able to recover the contents of a file, even after application of `shred`.

##### Miscellaneous

`mktemp`

  Create a temporary file

  Creates a temporary directory when invoked with the `-d` option.

  with a unique filename. When invoked from the command-line without additional arguments, it creates a zero-length file in the `/tmp` directory.

      bash$ mktemp
      /tmp/tmp.zzsvql3154

      PREFIX=filename
      tempfile=`mktemp $PREFIX.XXXXXX`
      #                        ^^^^^^ Need at least 6 placeholders
      #+                              in the filename template.
      #   If no filename template supplied,
      #+ "tmp.XXXXXXXXXX" is the default.

      echo "tempfile name = $tempfile"
      # tempfile name = filename.QA2ZpY
      #                 or something similar...

      #  Creates a file of that name in the current working directory
      #+ with 600 file permissions.
      #  A "umask 177" is therefore unnecessary,
      #+ but it's good programming practice nevertheless.

`make`

  Utility for building and compiling binary packages. This can also be used for any set of operations triggered by incremental changes in source files.

  The make command checks a `Makefile`, a list of file dependencies and operations to be carried out.

  The make utility is, in effect, a powerful scripting language similar in many ways to Bash, but with the capability of recognizing dependencies. For in-depth coverage of this useful tool set, see the GNU software documentation site.

`install`

  Special purpose file copying command, similar to cp, but capable of setting permissions and attributes of the copied files. This command seems tailormade for installing software packages, and as such it shows up frequently in `Makefiles` (in the make install : section). It could likewise prove useful in installation scripts.

`dos2unix`

  This utility, written by Benjamin Lin and collaborators, converts DOS-formatted text files (lines terminated by CR-LF) to UNIX format (lines terminated by LF only), and vice-versa.

`ptx`

  The `ptx [targetfile]` command outputs a permuted index (cross-reference list) of the targetfile. This may be further filtered and formatted in a pipe, if necessary.

`more`
`less`

  Pagers that display a text file or stream to `stdout`, one screenful at a time. These may be used to filter the output of `stdout` . . . or of a script.

  An interesting application of more is to test drive a command sequence, to forestall potentially unpleasant consequences.

      ls /home/bozo | awk '{print "rm -rf " $1}' | more
      #                                            ^^^^

      # Testing the effect of the following (disastrous) command-line:
      #      ls /home/bozo | awk '{print "rm -rf " $1}' | sh
      #      Hand off to the shell to execute . . .       ^^

  The less pager has the interesting property of doing a formatted display of man page source. See .

#### Communications Commands

Certain of the following commands find use in network data transfer and analysis, as well as in chasing spammers.

##### Information and Statistics

`host`

  Searches for information about an Internet host by name or IP address, using DNS.

      bash$ host surfacemail.com
      surfacemail.com. has address 202.92.42.236

`ipcalc`

  Displays IP information for a host. With the `-h` option, `ipcalc` does a reverse DNS lookup, finding the name of the host (server) from the IP address.

      bash$ ipcalc -h 202.92.42.236
      HOSTNAME=surfacemail.com

`nslookup`

  Do an Internet name server lookup on a host by IP address. This is essentially equivalent to `ipcalc -h` or `dig -x` . The command may be run either interactively or noninteractively, i.e., from within a script.

  The `nslookup` command has allegedly been deprecated, but it is still useful.

      bash$ nslookup -sil 66.97.104.180
      nslookup kuhleersparnis.ch
       Server:         135.116.137.2
       Address:        135.116.137.2#53

       Non-authoritative answer:
       Name:   kuhleersparnis.ch

`dig`

  `D`omain `I`nformation `G`roper. Similar to `nslookup`, dig does an Internet name server lookup on a host. May be run from the command-line or from within a script.

  Some interesting options to dig are `+time=N` for setting a query timeout to `N` seconds, `+nofail` for continuing to query servers until a reply is received, and `-x` for doing a reverse address lookup.

  Compare the output of `dig -x` with `ipcalc -h` and `nslookup`.

      bash$ dig -x 81.9.6.2
      ;; Got answer:
       ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649
       ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

       ;; QUESTION SECTION:
       ;2.6.9.81.in-addr.arpa.         IN      PTR

       ;; AUTHORITY SECTION:
       6.9.81.in-addr.arpa.    3600    IN      SOA     ns.eltel.net. noc.eltel.net.
       2002031705 900 600 86400 3600

       ;; Query time: 537 msec
       ;; SERVER: 135.116.137.2#53(135.116.137.2)
       ;; WHEN: Wed Jun 26 08:35:24 2002
       ;; MSG SIZE  rcvd: 91

  ##### Finding out where to report a spammer

      &spamlookup;

  ##### Analyzing a spam domain

      &isspammer;

  For a much more elaborate version of the above script, see .

`traceroute`

  Trace the route taken by packets sent to a remote host. This command works within a LAN, WAN, or over the Internet. The remote host may be specified by an IP address. The output of this command may be filtered by grep or sed in a pipe.

      bash$ traceroute 81.9.6.2
      traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets
       1  tc43.xjbnnbrb.com (136.30.178.8)  191.303 ms  179.400 ms  179.767 ms
       2  or0.xjbnnbrb.com (136.30.178.1)  179.536 ms  179.534 ms  169.685 ms
       3  192.168.11.101 (192.168.11.101)  189.471 ms  189.556 ms *
       ...

`ping`

  Broadcast an ICMP ECHO_REQUEST packet to another machine, either on a local or remote network. This is a diagnostic tool for testing network connections, and it should be used with caution.

      bash$ ping localhost
      PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.
       64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec
       64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec

       --- localhost.localdomain ping statistics ---
       2 packets transmitted, 2 packets received, 0% packet loss
       round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms

  A successful ping returns an exit status of 0. This can be tested for in a script.

        HNAME=news-15.net  # Notorious spammer.
      # HNAME=$HOST     # Debug: test for localhost.
        count=2  # Send only two pings.

      if [[ `ping -c $count "$HNAME"` ]]
      then
        echo ""$HNAME" still up and broadcasting spam your way."
      else
        echo ""$HNAME" seems to be down. Pity."
      fi

`whois`

  Perform a DNS (Domain Name System) lookup. The `-h` option permits specifying which particular whois server to query. See and .

`finger`

  Retrieve information about users on a network. Optionally, this command can display a user's `~/.plan`, `~/.project`, and `~/.forward` files, if present.

      bash$ finger
      Login  Name           Tty      Idle  Login Time   Office     Office Phone
       bozo   Bozo Bozeman   tty1        8  Jun 25 16:59                (:0)
       bozo   Bozo Bozeman   ttyp0          Jun 25 16:59                (:0.0)
       bozo   Bozo Bozeman   ttyp1          Jun 25 17:07                (:0.0)

      bash$ finger bozo
      Login: bozo                             Name: Bozo Bozeman
       Directory: /home/bozo                   Shell: /bin/bash
       Office: 2355 Clown St., 543-1234
       On since Fri Aug 31 20:13 (MST) on tty1    1 hour 38 minutes idle
       On since Fri Aug 31 20:13 (MST) on pts/0   12 seconds idle
       On since Fri Aug 31 20:13 (MST) on pts/1
       On since Fri Aug 31 20:31 (MST) on pts/2   1 hour 16 minutes idle
       Mail last read Tue Jul  3 10:08 2007 (MST)
       No Plan.

  Out of security considerations, many networks disable `finger` and its associated daemon. A daemon is a background process not attached to a terminal session. Daemons perform designated services either at specified times or explicitly triggered by certain events. The word daemon means ghost in Greek, and there is certainly something mysterious, almost supernatural, about the way UNIX daemons wander about behind the scenes, silently carrying out their appointed tasks.

`chfn`

  Change information disclosed by the `finger` command.

`vrfy`

  Verify an Internet e-mail address.

  This command seems to be missing from newer Linux distros.

##### Remote Host Access

`sx`
`rx`

  The `sx` and `rx` command set serves to transfer files to and from a remote host using the xmodem protocol. These are generally part of a communications package, such as `minicom`.

`sz`
`rz`

  The `sz` and `rz` command set serves to transfer files to and from a remote host using the zmodem protocol. Zmodem has certain advantages over xmodem, such as faster transmission rate and resumption of interrupted file transfers. Like `sx` and `rx`, these are generally part of a communications package.

`ftp`

  Utility and protocol for uploading / downloading files to or from a remote host. An ftp session can be automated in a script (see and ).

`uucp`
`uux`
`cu`

  `uucp`: UNIX to UNIX copy. This is a communications package for transferring files between UNIX servers. A shell script is an effective way to handle a `uucp` command sequence.

  Since the advent of the Internet and e-mail, `uucp` seems to have faded into obscurity, but it still exists and remains perfectly workable in situations where an Internet connection is not available or appropriate. The advantage of `uucp` is that it is fault-tolerant, so even if there is a service interruption the copy operation will resume where it left off when the connection is restored.

  ---

  `uux`: UNIX to UNIX execute. Execute a command on a remote system. This command is part of the `uucp` package.

  ---

  `cu`: `C`all `U`p a remote system and connect as a simple terminal. It is a sort of dumbed-down version of telnet. This command is part of the `uucp` package.

`telnet`

  Utility and protocol for connecting to a remote host.

  The telnet protocol contains security holes and should therefore probably be avoided. Its use within a shell script is not recommended.

`wget`

  The `wget` utility noninteractively retrieves or downloads files from a Web or ftp site. It works well in a script.

      wget -p http://www.xyz23.com/file01.html
      #  The -p or --page-requisite option causes wget to fetch all files
      #+ required to display the specified page.

      wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE
      #  The -r option recursively follows and retrieves all links
      #+ on the specified site.

      wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2
      #  The -c option lets wget resume an interrupted download.
      #  This works with ftp servers and many HTTP sites.

  ##### Getting a stock quote

      &quotefetch;

  See also and .

`lynx`

  The `lynx` Web and file browser can be used inside a script (with the `-dump` option) to retrieve a file from a Web or ftp site noninteractively.

      lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE

  With the `-traversal` option, `lynx` starts at the HTTP URL specified as an argument, then crawls through all links located on that particular server. Used together with the `-crawl` option, outputs page text to a log file.

`rlogin`

  Remote login, initates a session on a remote host. This command has security issues, so use ssh instead.

`rsh`

  Remote shell, executes command(s) on a remote host. This has security issues, so use `ssh` instead.

`rcp`

  Remote copy, copies files between two different networked machines.

`rsync`

  Remote synchronize, updates (synchronizes) files between two different networked machines.

      bash$ rsync -a ~/sourcedir/*txt /node1/subdirectory/

  ##### Updating FC4

      &fc4upd;

  See also .

  Using rcp, rsync, and similar utilities with security implications in a shell script may not be advisable. Consider, instead, using `ssh`, scp, or an `expect` script.

`ssh`

  Secure shell, logs onto a remote host and executes commands there. This secure replacement for `telnet`, `rlogin`, `rcp`, and `rsh` uses identity authentication and encryption. See its manpage for details.

  ##### Using ssh

      &remote;

  Within a loop, `ssh` may cause unexpected behavior. According to a Usenet post in the comp.unix shell archives, `ssh` inherits the loop's `stdin`. To remedy this, pass `ssh` either the `-n` or `-f` option.

  Thanks, Jason Bechtel, for pointing this out.

`scp`

  Secure copy, similar in function to `rcp`, copies files between two different networked machines, but does so using authentication, and with a security level similar to `ssh`.

##### Local Network

`write`

  This is a utility for terminal-to-terminal communication. It allows sending lines from your terminal (console or xterm) to that of another user. The mesg command may, of course, be used to disable write access to a terminal

  Since `write` is interactive, it would not normally find use in a script.

`netconfig`

  A command-line utility for configuring a network adapter (using DHCP). This command is native to Red Hat centric Linux distros.

##### Mail

`mail`

  Send or read e-mail messages.

  This stripped-down command-line mail client works fine as a command embedded in a script.

  ##### A script that mails itself

      &selfmailer;

`mailto`

  Similar to the `mail` command, `mailto` sends e-mail messages from the command-line or in a script. However, `mailto` also permits sending MIME (multimedia) messages.

`mailstats`

  Show mail statistics. This command may be invoked only by root.

      root# mailstats
      Statistics from Tue Jan  1 20:32:08 2008
        M   msgsfr  bytes_from   msgsto    bytes_to  msgsrej msgsdis msgsqur  Mailer
        4     1682      24118K        0          0K        0       0       0  esmtp
        9      212        640K     1894      25131K        0       0       0  local
       =====================================================================
        T     1894      24758K     1894      25131K        0       0       0
        C      414                    0

`vacation`

  This utility automatically replies to e-mails that the intended recipient is on vacation and temporarily unavailable. It runs on a network, in conjunction with `sendmail`, and is not applicable to a dial-up POPmail account.

#### Terminal Control Commands

##### Command affecting the console or terminal

`tput`

  Initialize terminal and/or fetch information about it from terminfo data. Various options permit certain terminal operations: `tput clear` is the equivalent of clear; `tput reset` is the equivalent of reset.

      bash$ tput longname
      xterm terminal emulator (X Window System)

  Issuing a `tput cup X Y` moves the cursor to the (X,Y) coordinates in the current terminal. A `clear` to erase the terminal screen would normally precede this.

  Some interesting options to tput are:

  - `bold`, for high-intensity text

  - `smul`, to underline text in the terminal

  - `smso`, to render text in reverse

  - `sgr0`, to reset the terminal parameters (to normal), without clearing the screen

  Example scripts using tput:

  Note that stty offers a more powerful command set for controlling a terminal.

`infocmp`

  This command prints out extensive information about the current terminal. It references the terminfo database.

      bash$ infocmp
      #       Reconstructed via infocmp from file:
       /usr/share/terminfo/r/rxvt
       rxvt|rxvt terminal emulator (X Window System),
               am, bce, eo, km, mir, msgr, xenl, xon,
               colors#8, cols#80, it#8, lines#24, pairs#64,
               acsc=``aaffggjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,
               bel=^G, blink=\E[5m, bold=\E[1m,
               civis=\E[?25l,
               clear=\E[H\E[2J, cnorm=\E[?25h, cr=^M,
               ...

`reset`

  Reset terminal parameters and clear text screen. As with `clear`, the cursor and prompt reappear in the upper lefthand corner of the terminal.

`clear`

  The `clear` command simply clears the text screen at the console or in an xterm. The prompt and cursor reappear at the upper lefthand corner of the screen or xterm window. This command may be used either at the command line or in a script. See .

`resize`

  Echoes commands necessary to set `$TERM` and `$TERMCAP` to duplicate the size (dimensions) of the current terminal.

      bash$ resize
      set noglob;
       setenv COLUMNS '80';
       setenv LINES '24';
       unset noglob;

`script`

  This utility records (saves to a file) all the user keystrokes at the command-line in a console or an xterm window. This, in effect, creates a record of a session.

#### Math Commands

##### Doing the numbers

`factor`

  Decompose an integer into prime factors.

      bash$ factor 27417
      27417: 3 13 19 37

  ##### Generating prime numbers

      &primes2;

`bc`

  Bash can't handle floating point calculations, and it lacks operators for certain important mathematical functions. Fortunately, `bc` gallops to the rescue.

  Not just a versatile, arbitrary precision calculation utility, `bc` offers many of the facilities of a programming language. It has a syntax vaguely resembling `C`.

  Since it is a fairly well-behaved UNIX utility, and may therefore be used in a pipe, `bc` comes in handy in scripts.

  Here is a simple template for using `bc` to calculate a script variable. This uses command substitution.

      	      variable=$(echo "OPTIONS; OPERATIONS" | bc)

  ##### Monthly Payment on a Mortgage

      &monthlypmt;

  ##### Base Conversion

      &base;

  An alternate method of invoking `bc` involves using a here document embedded within a command substitution block. This is especially appropriate when a script needs to pass a list of options and commands to `bc`.

      variable=`bc << LIMIT_STRING
      options
      statements
      operations
      LIMIT_STRING
      `

      ...or...

      variable=$(bc << LIMIT_STRING
      options
      statements
      operations
      LIMIT_STRING
      )

  ##### Invoking bc using a here document

      &altbc;

  ##### Calculating PI

      &cannon;

  See also .

`dc`

  The `dc` (`d`esk `c`alculator) utility is stack-oriented and uses RPN (Reverse Polish Notation). Like `bc`, it has much of the power of a programming language.

  Similar to the procedure with `bc`, echo a command-string to `dc`.

      echo "[Printing a string ... ]P" | dc
      # The P command prints the string between the preceding brackets.

      # And now for some simple arithmetic.
      echo "7 8 * p" | dc     # 56
      #  Pushes 7, then 8 onto the stack,
      #+ multiplies ("*" operator), then prints the result ("p" operator).

  Most persons avoid `dc`, because of its non-intuitive input and rather cryptic operators. Yet, it has its uses.

  ##### Converting a decimal number to hexadecimal

      &hexconvert;

  Studying the info page for `dc` is a painful path to understanding its intricacies. There seems to be a small, select group of dc wizards who delight in showing off their mastery of this powerful, but arcane utility.

      bash$ echo "16i[q]sa[ln0=aln100%Pln100/snlbx]sbA0D68736142snlbxq" | dc
      Bash

      dc <<< 10k5v1+2/p # 1.6180339887
      #  ^^^            Feed operations to dc using a Here String.
      #      ^^^        Pushes 10 and sets that as the precision (10k).
      #         ^^      Pushes 5 and takes its square root
      #                 (5v, v = square root).
      #           ^^    Pushes 1 and adds it to the running total (1+).
      #             ^^  Pushes 2 and divides the running total by that (2/).
      #               ^ Pops and prints the result (p)
      #  The result is  1.6180339887 ...
      #  ... which happens to be the Pythagorean Golden Ratio, to 10 places.

  ##### Factoring

      &factr;

`awk`

  Yet another way of doing floating point math in a script is using awk's built-in math functions in a shell wrapper.

  ##### Calculating the hypotenuse of a triangle

      &hypot;

#### Miscellaneous Commands

##### Command that fit in no special category

`jot`
`seq`

  These utilities emit a sequence of integers, with a user-selectable increment.

  The default separator character between each integer is a newline, but this can be changed with the `-s` option.

      bash$ seq 5
      1
       2
       3
       4
       5

      bash$ seq -s : 5
      1:2:3:4:5

  Both `jot` and `seq` come in handy in a for loop.

  ##### Using seq to generate loop arguments

      &ex53;

  A simpler example:

      #  Create a set of 10 files,
      #+ named file.1, file.2 . . . file.10.
      COUNT=10
      PREFIX=file

      for filename in `seq $COUNT`
      do
        touch $PREFIX.$filename
        #  Or, can do other operations,
        #+ such as rm, grep, etc.
      done

  ##### Letter Count"

      &lettercount;

  Somewhat more capable than seq, `jot` is a classic UNIX utility that is not normally included in a standard Linux distro. However, the source rpm is available for download from the MIT repository.

  Unlike seq, `jot` can generate a sequence of random numbers, using the `-r` option.

      bash$ jot -r 3 999
      1069
       1272
       1428

`getopt`

  The `getopt` command parses command-line options preceded by a dash. This external command corresponds to the getopts Bash builtin. Using `getopt` permits handling long options by means of the `-l` flag, and this also allows parameter reshuffling.

  ##### Using getopt to parse command-line options

      &ex33a;

  As Peggy Russell points out:

  It is often necessary to include an eval to correctly process whitespace and quotes.

      args=$(getopt -o a:bc:d -- "$@")
      eval set -- "$args"

  See for a simplified emulation of `getopt`.

`run-parts`

  The `run-parts` command

  This is actually a script adapted from the Debian Linux distribution.

  executes all the scripts in a target directory, sequentially in ASCII-sorted filename order. Of course, the scripts need to have execute permission.

  The cron daemon invokes `run-parts` to run the scripts in the `/etc/cron.*` directories.

`yes`

  In its default behavior the `yes` command feeds a continuous string of the character `y` followed by a line feed to `stdout`. A controlC terminates the run. A different output string may be specified, as in `yes different string`, which would continually output `different string` to `stdout`.

  One might well ask the purpose of this. From the command-line or in a script, the output of `yes` can be redirected or piped into a program expecting user input. In effect, this becomes a sort of poor man's version of expect.

  `yes | fsck /dev/hda1` runs `fsck` non-interactively (careful!).

  `yes | rm -r dirname` has same effect as `rm -rf dirname` (careful!).

  Caution advised when piping yes to a potentially dangerous system command, such as fsck or fdisk. It might have unintended consequences.

  The yes command parses variables, or more accurately, it echoes parsed variables. For example:

      bash$ yes $BASH_VERSION
      3.1.17(1)-release
       3.1.17(1)-release
       3.1.17(1)-release
       3.1.17(1)-release
       3.1.17(1)-release
       . . .

  This particular feature may be used to create a very large ASCII file on the fly:

      bash$ yes $PATH > huge_file.txt
      Ctl-C

  Hit `Ctl-C` very quickly, or you just might get more than you bargained for. . . .

  The yes command may be emulated in a very simple script function.

      yes ()
      { # Trivial emulation of "yes" ...
        local DEFAULT_TEXT="y"
        while [ true ]   # Endless loop.
        do
          if [ -z "$1" ]
          then
            echo "$DEFAULT_TEXT"
          else           # If argument ...
            echo "$1"    # ... expand and echo it.
          fi
        done             #  The only things missing are the
      }                  #+ --help and --version options.

`banner`

  Prints arguments as a large vertical banner to `stdout`, using an ASCII character (default '#'). This may be redirected to a printer for hardcopy.

  Note that banner has been dropped from many Linux distros, presumably because it is no longer considered useful.

`printenv`

  Show all the environmental variables set for a particular user.

      bash$ printenv | grep HOME
      HOME=/home/bozo

`lp`

  The `lp` and `lpr` commands send file(s) to the print queue, to be printed as hard copy.

  The print queue is the group of jobs waiting in line to be printed.

  These commands trace the origin of their names to the line printers of another era. Large mechanical line printers printed a single line of type at a time onto joined sheets of greenbar paper, to the accompaniment of a great deal of noise. The hardcopy thusly printed was referred to as a printout.

  bash$ `lp file1.txt` or bash `lp <file1.txt`

  It is often useful to pipe the formatted output from `pr` to `lp`.

  bash$ `pr -options file1.txt | lp`

  Formatting packages, such as groff and Ghostscript may send their output directly to `lp`.

  bash$ `groff -Tascii file.tr | lp`

  bash$ `gs -options | lp file.ps`

  Related commands are `lpq`, for viewing the print queue, and `lprm`, for removing jobs from the print queue.

`tee`

  [UNIX borrows an idea from the plumbing trade.]

  This is a redirection operator, but with a difference. Like the plumber's tee, it permits siphoning off to a file the output of a command or commands within a pipe, but without affecting the result. This is useful for printing an ongoing process to a file or paper, perhaps to keep track of it for debugging purposes.

                                   (redirection)
                                  |----> to file
                                  |
        ==========================|====================
        command ---> command ---> |tee ---> command ---> ---> output of pipe
        ===============================================

      cat listfile* | sort | tee check.file | uniq > result.file
      #                      ^^^^^^^^^^^^^^   ^^^^

      #  The file "check.file" contains the concatenated sorted "listfiles,"
      #+ before the duplicate lines are removed by 'uniq.'

`mkfifo`

  This obscure command creates a named pipe, a temporary first-in-first-out buffer for transferring data between processes. For an excellent overview of this topic, see Andy Vaught's article, Introduction to Named Pipes, in the September, 1997 issue of Linux Journal. Typically, one process writes to the FIFO, and the other reads from it. See .

      #!/bin/bash
      # This short script by Omair Eshkenazi.
      # Used in ABS Guide with permission (thanks!).

      mkfifo pipe1   # Yes, pipes can be given names.
      mkfifo pipe2   # Hence the designation "named pipe."

      (cut -d' ' -f1 | tr "a-z" "A-Z") >pipe2 <pipe1 &
      ls -l | tr -s ' ' | cut -d' ' -f3,9- | tee pipe1 |
      cut -d' ' -f2 | paste - pipe2

      rm -f pipe1
      rm -f pipe2

      # No need to kill background processes when script terminates (why not?).

      exit $?

      Now, invoke the script and explain the output:
      sh mkfifo-example.sh

      4830.tar.gz          BOZO
      pipe1   BOZO
      pipe2   BOZO
      mkfifo-example.sh    BOZO
      Mixed.msg BOZO

`pathchk`

  This command checks the validity of a filename. If the filename exceeds the maximum allowable length (255 characters) or one or more of the directories in its path is not searchable, then an error message results.

  Unfortunately, `pathchk` does not return a recognizable error code, and it is therefore pretty much useless in a script. Consider instead the file test operators.

`dd`

  Though this somewhat obscure and much feared `d`ata `d`uplicator command originated as a utility for exchanging data on magnetic tapes between UNIX minicomputers and IBM mainframes, it still has its uses. The `dd` command simply copies a file (or `stdin/stdout`), but with conversions. Possible conversions include ASCII/EBCDIC, EBCDIC (pronounced ebb-sid-ick) is an acronym for Extended Binary Coded Decimal Interchange Code, an obsolete IBM data format. A bizarre application of the `conv=ebcdic` option of `dd` is as a quick 'n easy, but not very secure text file encoder.

      cat $file | dd conv=swab,ebcdic > $file_encrypted
      # Encode (looks like gibberish).
      # Might as well switch bytes (swab), too, for a little extra obscurity.

      cat $file_encrypted | dd conv=swab,ascii > $file_plaintext
      # Decode.

  upper/lower case, swapping of byte pairs between input and output, and skipping and/or truncating the head or tail of the input file.

      # Converting a file to all uppercase:

      dd if=$filename conv=ucase > $filename.uppercase
      #                    lcase   # For lower case conversion

  Some basic options to `dd` are:

  - if=INFILE

    INFILE is the source file.

  - of=OUTFILE

    OUTFILE is the target file, the file that will have the data written to it.

  - bs=BLOCKSIZE

    This is the size of each block of data being read and written, usually a power of 2.

  - skip=BLOCKS

    How many blocks of data to skip in INFILE before starting to copy. This is useful when the INFILE has garbage or garbled data in its header or when it is desirable to copy only a portion of the INFILE.

  - seek=BLOCKS

    How many blocks of data to skip in OUTFILE before starting to copy, leaving blank data at beginning of OUTFILE.

  - count=BLOCKS

    Copy only this many blocks of data, rather than the entire INFILE.

  - conv=CONVERSION

    Type of conversion to be applied to INFILE data before copying operation.

  A `dd --help` lists all the options this powerful utility takes.

  ##### A script that copies itself

      &selfcopy;

  ##### Exercising dd

      &exercisingdd;

  To demonstrate just how versatile `dd` is, let's use it to capture keystrokes.

  ##### Capturing Keystrokes

      &ddkeypress;

  The `dd` command can do random access on a data stream.

      echo -n . | dd bs=1 seek=4 of=file conv=notrunc
      #  The "conv=notrunc" option means that the output file
      #+ will not be truncated.

      # Thanks, S.C.

  The `dd` command can copy raw data and disk images to and from devices, such as floppies and tape drives (). A common use is creating boot floppies.

  `dd if=kernel-image of=/dev/fd0H1440`

  Similarly, `dd` can copy the entire contents of a floppy, even one formatted with a foreign OS, to the hard drive as an image file.

  `dd if=/dev/fd0 of=/home/bozo/projects/floppy.img`

  Likewise, `dd` can create bootable flash drives and SD cards.

  `dd if=image.iso of=/dev/sdb`

  ##### Preparing a bootable SD card for the Raspberry Pi

      &rpsdcard;

  Other applications of `dd` include initializing temporary swap files () and ramdisks (). It can even do a low-level copy of an entire hard drive partition, although this is not necessarily recommended.

  People (with presumably nothing better to do with their time) are constantly thinking of interesting applications of `dd`.

  ##### Securely deleting a file

      &blotout;

  See also the dd thread entry in the bibliography.

`od`

  The `od`, or octal dump filter converts input (or files) to octal (base-8) or other bases. This is useful for viewing or processing binary data files or otherwise unreadable system device files, such as `/dev/urandom`, and as a filter for binary data.

      head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
      # Sample output: 1324725719, 3918166450, 2989231420, etc.

      # From rnd.sh example script, by Stéphane Chazelas

  See also and .

`hexdump`

  Performs a hexadecimal, octal, decimal, or ASCII dump of a binary file. This command is the rough equivalent of `od`, above, but not nearly as useful. May be used to view the contents of a binary file, in combination with dd and less.

      dd if=/bin/ls | hexdump -C | less
      # The -C option nicely formats the output in tabular form.

`objdump`

  Displays information about an object file or binary executable in either hexadecimal form or as a disassembled listing (with the `-d` option).

      bash$ objdump -d /bin/ls
      /bin/ls:     file format elf32-i386

       Disassembly of section .init:

       080490bc <.init>:
        80490bc:       55                      push   %ebp
        80490bd:       89 e5                   mov    %esp,%ebp
        . . .

`mcookie`

  This command generates a magic cookie, a 128-bit (32-character) pseudorandom hexadecimal number, normally used as an authorization signature by the X server. This also available for use in a script as a quick 'n dirty random number.

      random000=$(mcookie)

  Of course, a script could use md5sum for the same purpose.

      # Generate md5 checksum on the script itself.
      random001=`md5sum $0 | awk '{print $1}'`
      # Uses 'awk' to strip off the filename.

  The `mcookie` command gives yet another way to generate a unique filename.

  ##### Filename generator

      &tempfilename;

`units`

  This utility converts between different units of measure. While normally invoked in interactive mode, `units` may find use in a script.

  ##### Converting meters to miles

      &unitconversion;

`m4`

  A hidden treasure, `m4` is a powerful macro A macro is a symbolic constant that expands into a command string or a set of operations on parameters. Simply put, it's a shortcut or abbreviation. processing filter, virtually a complete language. Although originally written as a pre-processor for RatFor, `m4` turned out to be useful as a stand-alone utility. In fact, `m4` combines some of the functionality of eval, tr, and awk, in addition to its extensive macro expansion facilities.

  The April, 2002 issue of Linux Journal has a very nice article on `m4` and its uses.

  ##### Using m4

      &m4;

`xmessage`

  This X-based variant of echo pops up a message/query window on the desktop.

      xmessage Left click to continue -button okay

`zenity`

  The zenity utility is adept at displaying GTK+ dialog widgets and very suitable for scripting purposes.

`doexec`

  The `doexec` command enables passing an arbitrary list of arguments to a binary executable. In particular, passing `argv[0]` (which corresponds to $0 in a script) lets the executable be invoked by various names, and it can then carry out different sets of actions, according to the name by which it was called. What this amounts to is roundabout way of passing options to an executable.

  For example, the `/usr/local/bin` directory might contain a binary called aaa. Invoking `doexec /usr/local/bin/aaa list` would list all those files in the current working directory beginning with an a, while invoking (the same executable with) `doexec /usr/local/bin/aaa delete` would delete those files.

  The various behaviors of the executable must be defined within the code of the executable itself, analogous to something like the following in a shell script:

      case `basename $0` in
      "name1" ) do_something;;
      "name2" ) do_something_else;;
      "name3" ) do_yet_another_thing;;
      *       ) bail_out;;
      esac

`dialog`

  The dialog family of tools provide a method of calling interactive dialog boxes from a script. The more elaborate variations of `dialog` -- `gdialog`, `Xdialog`, and `kdialog` -- actually invoke X-Windows widgets.

`sox`

  The `sox`, or `so`und e`x`change command plays and performs transformations on sound files. In fact, the `/usr/bin/play` executable (now deprecated) is nothing but a shell wrapper for sox.

  For example, `sox soundfile.wav soundfile.au` changes a WAV sound file into a (Sun audio format) AU sound file.

  Shell scripts are ideally suited for batch-processing `sox` operations on sound files. For examples, see the Linux Radio Timeshift HOWTO and the MP3do Project.

### System and Administrative Commands

The startup and shutdown scripts in `/etc/rc.d` illustrate the uses (and usefulness) of many of these comands. These are usually invoked by root and used for system maintenance or emergency filesystem repairs. Use with caution, as some of these commands may damage your system if misused.

#### Users and Groups

`users`

  Show all logged on users. This is the approximate equivalent of `who -q`.

`groups`

  Lists the current user and the groups she belongs to. This corresponds to the $GROUPS internal variable, but gives the group names, rather than the numbers.

      bash$ groups
      bozita cdrom cdwriter audio xgrp

      bash$ echo $GROUPS
      501

`chown`
`chgrp`

  The `chown` command changes the ownership of a file or files. This command is a useful method that root can use to shift file ownership from one user to another. An ordinary user may not change the ownership of files, not even her own files. This is the case on a Linux machine or a UNIX system with disk quotas.

      root# chown bozo *.txt

  The `chgrp` command changes the group ownership of a file or files. You must be owner of the file(s) as well as a member of the destination group (or root) to use this operation.

      chgrp --recursive dunderheads *.data
      #  The "dunderheads" group will now own all the "*.data" files
      #+ all the way down the $PWD directory tree (that's what "recursive" means).

`useradd`
`userdel`

  The `useradd` administrative command adds a user account to the system and creates a home directory for that particular user, if so specified. The corresponding `userdel` command removes a user account from the system The `userdel` command will fail if the particular user being deleted is still logged on. and deletes associated files.

  The `adduser` command is a synonym for `useradd` and is usually a symbolic link to it.

`usermod`

  Modify a user account. Changes may be made to the password, group membership, expiration date, and other attributes of a given user's account. With this command, a user's password may be locked, which has the effect of disabling the account.

`groupmod`

  Modify a given group. The group name and/or ID number may be changed using this command.

`id`

  The `id` command lists the real and effective user IDs and the group IDs of the user associated with the current process. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables.

      bash$ id
      uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)

      bash$ echo $UID
      501

  The `id` command shows the effective IDs only when they differ from the real ones.

  Also see .

`lid`

  The lid (list ID) command shows the group(s) that a given user belongs to, or alternately, the users belonging to a given group. May be invoked only by root.

      root# lid bozo
       bozo(gid=500)

      root# lid daemon
       bin(gid=1)
        daemon(gid=2)
        adm(gid=4)
        lp(gid=7)

`who`

  Show all users logged on to the system.

      bash$ who
      bozo  tty1     Apr 27 17:45
       bozo  pts/0    Apr 27 17:46
       bozo  pts/1    Apr 27 17:47
       bozo  pts/2    Apr 27 17:49

  The `-m` gives detailed information about only the current user. Passing any two arguments to `who` is the equivalent of `who -m`, as in `who am i` or `who The Man`.

      bash$ who -m
      localhost.localdomain!bozo  pts/2    Apr 27 17:49

  `whoami` is similar to `who -m`, but only lists the user name.

      bash$ whoami
      bozo

`w`

  Show all logged on users and the processes belonging to them. This is an extended version of `who`. The output of `w` may be piped to grep to find a specific user and/or process.

      bash$ w | grep startx
      bozo  tty1     -                 4:22pm  6:41   4.47s  0.45s  startx

`logname`

  Show current user's login name (as found in `/var/run/utmp`). This is a near-equivalent to whoami, above.

      bash$ logname
      bozo

      bash$ whoami
      bozo

  However . . .

      bash$ su
      Password: ......

      bash# whoami
      root
      bash# logname
      bozo

  While `logname` prints the name of the logged in user, `whoami` gives the name of the user attached to the current process. As we have just seen, sometimes these are not the same.

`su`

  Runs a program or script as a `s`ubstitute `u`ser. `su rjones` starts a shell as user rjones. A naked `su` defaults to root. See .

`sudo`

  Runs a command as root (or another user). This may be used in a script, thus permitting a regular user to run the script.

      #!/bin/bash

      # Some commands.
      sudo cp /root/secretfile /home/bozo/secret
      # Some more commands.

  The file `/etc/sudoers` holds the names of users permitted to invoke `sudo`.

`passwd`

  Sets, changes, or manages a user's password.

  The `passwd` command can be used in a script, but probably should not be.

  #### Setting a new password

      &setnewpw;

  The `passwd` command's `-l`, `-u`, and `-d` options permit locking, unlocking, and deleting a user's password. Only root may use these options.

`ac`

  Show users' logged in time, as read from `/var/log/wtmp`. This is one of the GNU accounting utilities.

      bash$ ac
              total       68.08

`last`

  List last logged in users, as read from `/var/log/wtmp`. This command can also show remote logins.

  For example, to show the last few times the system rebooted:

      bash$ last reboot
      reboot   system boot  2.6.9-1.667      Fri Feb  4 18:18          (00:02)
       reboot   system boot  2.6.9-1.667      Fri Feb  4 15:20          (01:27)
       reboot   system boot  2.6.9-1.667      Fri Feb  4 12:56          (00:49)
       reboot   system boot  2.6.9-1.667      Thu Feb  3 21:08          (02:17)
       . . .

       wtmp begins Tue Feb  1 12:50:09 2005

`newgrp`

  Change user's group ID without logging out. This permits access to the new group's files. Since users may be members of multiple groups simultaneously, this command finds only limited use.

  Kurt Glaesemann points out that the newgrp command could prove helpful in setting the default group permissions for files a user writes. However, the chgrp command might be more convenient for this purpose.

#### Terminals

`tty`

  Echoes the name (filename) of the current user's terminal. Note that each separate xterm window counts as a different terminal.

      bash$ tty
      /dev/pts/1

`stty`

  Shows and/or changes terminal settings. This complex command, used in a script, can control terminal behavior and the way output displays. See the info page, and study it carefully.

  #### Setting an erase character

      &erase;

  #### secret password: Turning off terminal echoing

      &secretpw;

  A creative use of `stty` is detecting a user keypress (without hitting ENTER).

  #### Keypress detection

      &keypress;

  Also see and .

  #### terminals and modes

  Normally, a terminal works in the canonical mode. When a user hits a key, the resulting character does not immediately go to the program actually running in this terminal. A buffer local to the terminal stores keystrokes. When the user hits the ENTER key, this sends all the stored keystrokes to the program running. There is even a basic line editor inside the terminal.

      bash$ stty -a
      speed 9600 baud; rows 36; columns 96; line = 0;
       intr = ^C; quit = ^\; erase = ^H; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>;
       start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O;
       ...
       isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt

  Using canonical mode, it is possible to redefine the special keys for the local terminal line editor.

      bash$ cat > filexxx
      wha<ctl-W>I<ctl-H>foo bar<ctl-U>hello world<ENTER>
      <ctl-D>
      bash$ cat filexxx
      hello world
      bash$ wc -c < filexxx
      12

  The process controlling the terminal receives only 12 characters (11 alphabetic ones, plus a newline), although the user hit 26 keys.

  In non-canonical (raw) mode, every key hit (including special editing keys such as ctl-H) sends a character immediately to the controlling process.

  The Bash prompt disables both `icanon` and `echo`, since it replaces the basic terminal line editor with its own more elaborate one. For example, when you hit ctl-A at the Bash prompt, there's no ^A echoed by the terminal, but Bash gets a \1 character, interprets it, and moves the cursor to the begining of the line.

  Stéphane Chazelas

`setterm`

  Set certain terminal attributes. This command writes to its terminal's `stdout` a string that changes the behavior of that terminal.

      bash$ setterm -cursor off
      bash$

  The `setterm` command can be used within a script to change the appearance of text written to `stdout`, although there are certainly better tools available for this purpose.

      setterm -bold on
      echo bold hello

      setterm -bold off
      echo normal hello

`tset`

  Show or initialize terminal settings. This is a less capable version of `stty`.

      bash$ tset -r
      Terminal type is xterm-xfree86.
       Kill is control-U (^U).
       Interrupt is control-C (^C).

`setserial`

  Set or display serial port parameters. This command must be run by root and is usually found in a system setup script.

      # From /etc/pcmcia/serial script:

      IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'`
      setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ

`getty`
`agetty`

  The initialization process for a terminal uses `getty` or `agetty` to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is `stty`.

`mesg`

  Enables or disables write access to the current user's terminal. Disabling access would prevent another user on the network to write to the terminal.

  It can be quite annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi-user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions.

`wall`

  This is an acronym for write all, i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator's tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see ).

      bash$ wall System going down for maintenance in 5 minutes!
      Broadcast message from bozo (pts/1) Sun Jul  8 13:53:27 2001...

       System going down for maintenance in 5 minutes!

  If write access to a particular terminal has been disabled with `mesg`, then `wall` cannot send a message to that terminal.

#### Information and Statistics

`uname`

  Output system specifications (OS, kernel version, etc.) to `stdout`. Invoked with the `-a` option, gives verbose system info (see ). The `-s` option shows only the OS type.

      bash$ uname
      Linux

      bash$ uname -s
      Linux

      bash$ uname -a
      Linux iron.bozo 2.6.15-1.2054_FC5 #1 Tue Mar 14 15:48:33 EST 2006
       i686 i686 i386 GNU/Linux

`arch`

  Show system architecture. Equivalent to `uname -m`. See .

      bash$ arch
      i686

      bash$ uname -m
      i686

`lastcomm`

  Gives information about previous commands, as stored in the `/var/account/pacct` file. Command name and user name can be specified by options. This is one of the GNU accounting utilities.

`lastlog`

  List the last login time of all system users. This references the `/var/log/lastlog` file.

      bash$ lastlog
      root          tty1                      Fri Dec  7 18:43:21 -0700 2001
       bin                                     **Never logged in**
       daemon                                  **Never logged in**
       ...
       bozo          tty1                      Sat Dec  8 21:14:29 -0700 2001

      bash$ lastlog | grep root
      root          tty1                      Fri Dec  7 18:43:21 -0700 2001

  This command will fail if the user invoking it does not have read permission for the `/var/log/lastlog` file.

`lsof`

  List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, `lsof` may be piped to grep and/or awk to parse and analyze its results.

      bash$ lsof
      COMMAND    PID    USER   FD   TYPE     DEVICE    SIZE     NODE NAME
       init         1    root  mem    REG        3,5   30748    30303 /sbin/init
       init         1    root  mem    REG        3,5   73120     8069 /lib/ld-2.1.3.so
       init         1    root  mem    REG        3,5  931668     8075 /lib/libc-2.1.3.so
       cardmgr    213    root  mem    REG        3,5   36956    30357 /sbin/cardmgr
       ...

  The `lsof` command is a useful, if complex administrative tool. If you are unable to dismount a filesystem and get an error message that it is still in use, then running lsof helps determine which files are still open on that filesystem. The `-i` option lists open network socket files, and this can help trace intrusion or hack attempts.

      bash$ lsof -an -i tcp
      COMMAND  PID USER  FD  TYPE DEVICE SIZE NODE NAME
       firefox 2330 bozo  32u IPv4   9956       TCP 66.0.118.137:57596->67.112.7.104:http ...
       firefox 2330 bozo  38u IPv4  10535       TCP 66.0.118.137:57708->216.79.48.24:http ...

  See for an effective use of `lsof`.

`strace`

  `S`ystem `trace`: diagnostic and debugging tool for tracing system calls and signals. This command and `ltrace`, following, are useful for diagnosing why a given program or package fails to run . . . perhaps due to missing libraries or related causes.

      bash$ strace df
      execve("/bin/df", ["df"], [/* 45 vars */]) = 0
       uname({sys="Linux", node="bozo.localdomain", ...}) = 0
       brk(0)                                  = 0x804f5e4

       ...

  This is the Linux equivalent of the Solaris `truss` command.

`ltrace`

  `L`ibrary `trace`: diagnostic and debugging tool that traces library calls invoked by a given command.

      bash$ ltrace df
      __libc_start_main(0x804a910, 1, 0xbfb589a4, 0x804fb70, 0x804fb68 <unfinished ...&gt:
       setlocale(6, "")                                 = "en_US.UTF-8"
      bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale"
      textdomain("coreutils")                          = "coreutils"
      __cxa_atexit(0x804b650, 0, 0, 0x8052bf0, 0xbfb58908) = 0
      getenv("DF_BLOCK_SIZE")                          = NULL

       ...

`nc`

  The `nc` (netcat) utility is a complete toolkit for connecting to and listening to TCP and UDP ports. It is useful as a diagnostic and testing tool and as a component in simple script-based HTTP clients and servers.

      bash$ nc localhost.localdomain 25
      220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1;
       Thu, 31 Mar 2005 15:41:35 -0700

  A real-life usage example.

  #### Checking a remote server for identd

      &iscan;

  And, of course, there's Dr. Andrew Tridgell's notorious one-line script in the BitKeeper Affair:

      echo clone | nc thunk.org 5000 > e2fsprogs.dat

`free`

  Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or `Perl`. The `procinfo` command shows all the information that `free` does, and much more.

      bash$ free
                      total       used       free     shared    buffers     cached
         Mem:         30504      28624       1880      15820       1608       16376
         -/+ buffers/cache:      10640      19864
         Swap:        68540       3128      65412

  To show unused RAM memory:

      bash$ free | grep Mem | awk '{ print $4 }'
      1880

`procinfo`

  Extract and list information and statistics from the `/proc` pseudo-filesystem. This gives a very extensive and detailed listing.

      bash$ procinfo | grep Bootup
      Bootup: Wed Mar 21 15:15:50 2001    Load average: 0.04 0.21 0.34 3/47 6829

`lsdev`

  List devices, that is, show installed hardware.

      bash$ lsdev
      Device            DMA   IRQ  I/O Ports
       ------------------------------------------------
       cascade             4     2
       dma                          0080-008f
       dma1                         0000-001f
       dma2                         00c0-00df
       fpu                          00f0-00ff
       ide0                     14  01f0-01f7 03f6-03f6
       ...

`du`

  Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified.

      bash$ du -ach
      1.0k    ./wi.sh
       1.0k    ./tst.sh
       1.0k    ./random.file
       6.0k    .
       6.0k    total

`df`

  Shows filesystem usage in tabular form.

      bash$ df
      Filesystem           1k-blocks      Used Available Use% Mounted on
       /dev/hda5               273262     92607    166547  36% /
       /dev/hda8               222525    123951     87085  59% /home
       /dev/hda7              1408796   1075744    261488  80% /usr

`dmesg`

  Lists all system bootup messages to `stdout`. Handy for debugging and ascertaining which device drivers were installed and which system interrupts in use. The output of `dmesg` may, of course, be parsed with grep, sed, or awk from within a script.

      bash$ dmesg | grep hda
      Kernel command line: ro root=/dev/hda2
       hda: IBM-DLGA-23080, ATA DISK drive
       hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63
       hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4

`stat`

  Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files.

      bash$ stat test.cru
        File: "test.cru"
         Size: 49970        Allocated Blocks: 100          Filetype: Regular File
         Mode: (0664/-rw-rw-r--)         Uid: (  501/ bozo)  Gid: (  501/ bozo)
       Device:  3,8   Inode: 18185     Links: 1
       Access: Sat Jun  2 16:40:24 2001
       Modify: Sat Jun  2 16:40:24 2001
       Change: Sat Jun  2 16:40:24 2001

  If the target file does not exist, `stat` returns an error message.

      bash$ stat nonexistent-file
      nonexistent-file: No such file or directory

  In a script, you can use `stat` to extract information about files (and filesystems) and set variables accordingly.

      #!/bin/bash
      # fileinfo2.sh

      # Per suggestion of Joël Bourquard and . . .
      # http://www.linuxquestions.org/questions/showthread.php?t=410766

      FILENAME=testfile.txt
      file_name=$(stat -c%n "$FILENAME")   # Same as "$FILENAME" of course.
      file_owner=$(stat -c%U "$FILENAME")
      file_size=$(stat -c%s "$FILENAME")
      #  Certainly easier than using "ls -l $FILENAME"
      #+ and then parsing with sed.
      file_inode=$(stat -c%i "$FILENAME")
      file_type=$(stat -c%F "$FILENAME")
      file_access_rights=$(stat -c%A "$FILENAME")

      echo "File name:          $file_name"
      echo "File owner:         $file_owner"
      echo "File size:          $file_size"
      echo "File inode:         $file_inode"
      echo "File type:          $file_type"
      echo "File access rights: $file_access_rights"

      exit 0

      sh fileinfo2.sh

      File name:          testfile.txt
      File owner:         bozo
      File size:          418
      File inode:         1730378
      File type:          regular file
      File access rights: -rw-rw-r--

`vmstat`

  Display virtual memory statistics.

      bash$ vmstat
         procs                      memory    swap          io system         cpu
       r  b  w   swpd   free   buff  cache  si  so    bi    bo   in    cs  us  sy id
       0  0  0      0  11040   2636  38952   0   0    33     7  271    88   8   3 89

`uptime`

  Shows how long the system has been running, along with associated statistics.

      bash$ uptime
      10:28pm  up  1:57,  3 users,  load average: 0.17, 0.34, 0.27

  A load average of 1 or less indicates that the system handles processes immediately. A load average greater than 1 means that processes are being queued. When the load average gets above 3 (on a single-core processor), then system performance is significantly degraded.

`hostname`

  Lists the system's host name. This command sets the host name in an `/etc/rc.d` setup script (`/etc/rc.d/rc.sysinit` or similar). It is equivalent to `uname -n`, and a counterpart to the $HOSTNAME internal variable.

      bash$ hostname
      localhost.localdomain

      bash$ echo $HOSTNAME
      localhost.localdomain

  Similar to the `hostname` command are the `domainname`, `dnsdomainname`, `nisdomainname`, and `ypdomainname` commands. Use these to display or set the system DNS or NIS/YP domain name. Various options to `hostname` also perform these functions.

`hostid`

  Echo a 32-bit hexadecimal numerical identifier for the host machine.

      bash$ hostid
      7f0100

  This command allegedly fetches a unique serial number for a particular system. Certain product registration procedures use this number to brand a particular user license. Unfortunately, `hostid` only returns the machine network address in hexadecimal, with pairs of bytes transposed.

  The network address of a typical non-networked Linux machine, is found in `/etc/hosts`.

      bash$ cat /etc/hosts
      127.0.0.1               localhost.localdomain localhost

  As it happens, transposing the bytes of `127.0.0.1`, we get `0.127.1.0`, which translates in hex to `007f0100`, the exact equivalent of what `hostid` returns, above. There exist only a few million other Linux machines with this identical hostid.

`sar`

  Invoking `sar` (System Activity Reporter) gives a very detailed rundown on system statistics. The Santa Cruz Operation (Old SCO) released `sar` as Open Source in June, 1999.

  This command is not part of the base Linux distribution, but may be obtained as part of the sysstat utilities package, written by Sebastien Godard.

      bash$ sar
      Linux 2.4.9 (brooks.seringas.fr) 	09/26/03

      10:30:00          CPU     %user     %nice   %system   %iowait     %idle
      10:40:00          all      2.21     10.90     65.48      0.00     21.41
      10:50:00          all      3.36      0.00     72.36      0.00     24.28
      11:00:00          all      1.12      0.00     80.77      0.00     18.11
      Average:          all      2.23      3.63     72.87      0.00     21.27

      14:32:30          LINUX RESTART

      15:00:00          CPU     %user     %nice   %system   %iowait     %idle
      15:10:00          all      8.59      2.40     17.47      0.00     71.54
      15:20:00          all      4.07      1.00     11.95      0.00     82.98
      15:30:00          all      0.79      2.94      7.56      0.00     88.71
      Average:          all      6.33      1.70     14.71      0.00     77.26

`readelf`

  Show information and statistics about a designated elf binary. This is part of the binutils package.

      bash$ readelf -h /bin/bash
      ELF Header:
         Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
         Class:                             ELF32
         Data:                              2's complement, little endian
         Version:                           1 (current)
         OS/ABI:                            UNIX - System V
         ABI Version:                       0
         Type:                              EXEC (Executable file)
         . . .

`size`

  The `size [/path/to/binary]` command gives the segment sizes of a binary executable or archive file. This is mainly of use to programmers.

      bash$ size /bin/bash
         text    data     bss     dec     hex filename
        495971   22496   17392  535859   82d33 /bin/bash

#### System Logs

`logger`

  Appends a user-generated message to the system log (`/var/log/messages`). You do not have to be root to invoke `logger`.

      logger Experiencing instability in network connection at 23:10, 05/21.
      # Now, do a 'tail /var/log/messages'.

  By embedding a `logger` command in a script, it is possible to write debugging information to `/var/log/messages`.

      logger -t $0 -i Logging at line "$LINENO".
      # The "-t" option specifies the tag for the logger entry.
      # The "-i" option records the process ID.

      # tail /var/log/message
      # ...
      # Jul  7 20:48:58 localhost ./test.sh[1712]: Logging at line 3.

`logrotate`

  This utility manages the system log files, rotating, compressing, deleting, and/or e-mailing them, as appropriate. This keeps the `/var/log` from getting cluttered with old log files. Usually cron runs `logrotate` on a daily basis.

  Adding an appropriate entry to `/etc/logrotate.conf` makes it possible to manage personal log files, as well as system-wide ones.

  Stefano Falsetto has created rottlog, which he considers to be an improved version of `logrotate`.

#### Job Control

`ps`

  Process Statistics: lists currently executing processes by owner and PID (process ID). This is usually invoked with `ax` or `aux` options, and may be piped to grep or sed to search for a specific process (see and ).

      bash$  ps ax | grep sendmail
      295 ?	   S	  0:00 sendmail: accepting connections on port 25

  To display system processes in graphical tree format: `ps afjx` or `ps ax --forest`.

`pgrep`
`pkill`

  Combining the `ps` command with grep or kill.

      bash$ ps a | grep mingetty
      2212 tty2     Ss+    0:00 /sbin/mingetty tty2
       2213 tty3     Ss+    0:00 /sbin/mingetty tty3
       2214 tty4     Ss+    0:00 /sbin/mingetty tty4
       2215 tty5     Ss+    0:00 /sbin/mingetty tty5
       2216 tty6     Ss+    0:00 /sbin/mingetty tty6
       4849 pts/2    S+     0:00 grep mingetty

      bash$ pgrep mingetty
      2212 mingetty
       2213 mingetty
       2214 mingetty
       2215 mingetty
       2216 mingetty

  Compare the action of `pkill` with killall.

`pstree`

  Lists currently executing processes in tree format. The `-p` option shows the PIDs, as well as the process names.

`top`

  Continuously updated display of most cpu-intensive processes. The `-b` option displays in text mode, so that the output may be parsed or accessed from a script.

      bash$ top -b
        8:30pm  up 3 min,  3 users,  load average: 0.49, 0.32, 0.13
       45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped
       CPU states: 13.6% user,  7.3% system,  0.0% nice, 78.9% idle
       Mem:    78396K av,   65468K used,   12928K free,       0K shrd,    2352K buff
       Swap:  157208K av,       0K used,  157208K free                   37244K cached

         PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME COMMAND
         848 bozo      17   0   996  996   800 R     5.6  1.2   0:00 top
           1 root       8   0   512  512   444 S     0.0  0.6   0:04 init
           2 root       9   0     0    0     0 SW    0.0  0.0   0:00 keventd
         ...

`nice`

  Run a background job with an altered priority. Priorities run from 19 (lowest) to -20 (highest). Only root may set the negative (higher) priorities. Related commands are `renice` and `snice`, which change the priority of a running process or processes, and `skill`, which sends a kill signal to a process or processes.

`nohup`

  Keeps a command running even after user logs off. The command will run as a foreground process unless followed by &. If you use `nohup` within a script, consider coupling it with a wait to avoid creating an orphan or zombie process.

`pidof`

  Identifies process ID (PID) of a running job. Since job control commands, such as kill and renice act on the PID of a process (not its name), it is sometimes necessary to identify that PID. The `pidof` command is the approximate counterpart to the $PPID internal variable.

      bash$ pidof xclock
      880

  #### pidof helps kill a process

      &killprocess;

`fuser`

  Identifies the processes (by PID) that are accessing a given file, set of files, or directory. May also be invoked with the `-k` option, which kills those processes. This has interesting implications for system security, especially in scripts preventing unauthorized users from accessing system services.

      bash$ fuser -u /usr/bin/vim
      /usr/bin/vim:         3207e(bozo)

      bash$ fuser -u /dev/null
      /dev/null:            3009(bozo)  3010(bozo)  3197(bozo)  3199(bozo)

  One important application for `fuser` is when physically inserting or removing storage media, such as CD ROM disks or USB flash drives. Sometimes trying a umount fails with a device is busy error message. This means that some user(s) and/or process(es) are accessing the device. An `fuser -um /dev/device_name` will clear up the mystery, so you can kill any relevant processes.

      bash$ umount /mnt/usbdrive
      umount: /mnt/usbdrive: device is busy

      bash$ fuser -um /dev/usbdrive
      /mnt/usbdrive:        1772c(bozo)

      bash$ kill -9 1772
      bash$ umount /mnt/usbdrive

  The `fuser` command, invoked with the `-n` option identifies the processes accessing a port. This is especially useful in combination with nmap.

      root# nmap localhost.localdomain
      PORT     STATE SERVICE
       25/tcp   open  smtp

      root# fuser -un tcp 25
      25/tcp:               2095(root)

      root# ps ax | grep 2095 | grep -v grep
      2095 ?        Ss     0:00 sendmail: accepting connections

`cron`

  Administrative program scheduler, performing such duties as cleaning up and deleting system log files and updating the slocate database. This is the superuser version of at (although each user may have their own `crontab` file which can be changed with the `crontab` command). It runs as a daemon and executes scheduled entries from `/etc/crontab`.

  Some flavors of Linux run `crond`, Matthew Dillon's version of `cron`.

#### Process Control and Booting

`init`

  The `init` command is the parent of all processes. Called in the final step of a bootup, `init` determines the runlevel of the system from `/etc/inittab`. Invoked by its alias `telinit`, and by root only.

`telinit`

  Symlinked to `init`, this is a means of changing the system runlevel, usually done for system maintenance or emergency filesystem repairs. Invoked only by root. This command can be dangerous -- be certain you understand it well before using!

`runlevel`

  Shows the current and last runlevel, that is, whether the system is halted (runlevel `0`), in single-user mode (`1`), in multi-user mode (`2` or `3`), in X Windows (`5`), or rebooting (`6`). This command accesses the `/var/run/utmp` file.

`halt`
`shutdown`
`reboot`

  Command set to shut the system down, usually just prior to a power down.

  On some Linux distros, the `halt` command has 755 permissions, so it can be invoked by a non-root user. A careless halt in a terminal or a script may shut down the system!

`service`

  Starts or stops a system service. The startup scripts in `/etc/init.d` and `/etc/rc.d` use this command to start services at bootup.

      root# /sbin/service iptables stop
      Flushing firewall rules:                                   [  OK  ]
       Setting chains to policy ACCEPT: filter                    [  OK  ]
       Unloading iptables modules:                                [  OK  ]

#### Network

`nmap`

  `N`etwork `map`per and port scanner. This command scans a server to locate open ports and the services associated with those ports. It can also report information about packet filters and firewalls. This is an important security tool for locking down a network against hacking attempts.

      #!/bin/bash

      SERVER=$HOST                           # localhost.localdomain (127.0.0.1).
      PORT_NUMBER=25                         # SMTP port.

      nmap $SERVER | grep -w "$PORT_NUMBER"  # Is that particular port open?
      #              grep -w matches whole words only,
      #+             so this wouldn't match port 1025, for example.

      exit 0

      # 25/tcp     open        smtp

`ifconfig`

  Network interface configuration and tuning utility.

      bash$ ifconfig -a
      lo        Link encap:Local Loopback
                 inet addr:127.0.0.1  Mask:255.0.0.0
                 UP LOOPBACK RUNNING  MTU:16436  Metric:1
                 RX packets:10 errors:0 dropped:0 overruns:0 frame:0
                 TX packets:10 errors:0 dropped:0 overruns:0 carrier:0
                 collisions:0 txqueuelen:0
                 RX bytes:700 (700.0 b)  TX bytes:700 (700.0 b)

  The `ifconfig` command is most often used at bootup to set up the interfaces, or to shut them down when rebooting.

      # Code snippets from /etc/rc.d/init.d/network

      # ...

      # Check that networking is up.
      [ ${NETWORKING} = "no" ] && exit 0

      [ -x /sbin/ifconfig ] || exit 0

      # ...

      for i in $interfaces ; do
        if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then
          action "Shutting down interface $i: " ./ifdown $i boot
        fi
      #  The GNU-specific "-q" option to "grep" means "quiet", i.e.,
      #+ producing no output.
      #  Redirecting output to /dev/null is therefore not strictly necessary.

      # ...

      echo "Currently active devices:"
      echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'`
      #                            ^^^^^  should be quoted to prevent globbing.
      #  The following also work.
      #    echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })'
      #    echo $(/sbin/ifconfig | sed -e 's/ .*//')
      #  Thanks, S.C., for additional comments.

  See also .

`netstat`

  Show current network statistics and information, such as routing tables and active connections. This utility accesses information in `/proc/net` (). See .

  `netstat -r` is equivalent to route.

      bash$ netstat
      Active Internet connections (w/o servers)
       Proto Recv-Q Send-Q Local Address           Foreign Address         State
       Active UNIX domain sockets (w/o servers)
       Proto RefCnt Flags       Type       State         I-Node Path
       unix  11     [ ]         DGRAM                    906    /dev/log
       unix  3      [ ]         STREAM     CONNECTED     4514   /tmp/.X11-unix/X0
       unix  3      [ ]         STREAM     CONNECTED     4513
       . . .

  A `netstat -lptu` shows sockets that are listening to ports, and the associated processes. This can be useful for determining whether a computer has been hacked or compromised.

`iwconfig`

  This is the command set for configuring a wireless network. It is the wireless equivalent of `ifconfig`, above.

`ip`

  General purpose utility for setting up, changing, and analyzing IP (Internet Protocol) networks and attached devices. This command is part of the iproute2 package.

      bash$ ip link show
      1: lo: <LOOPBACK,UP> mtu 16436 qdisc noqueue
           link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
       2: eth0: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast qlen 1000
           link/ether 00:d0:59:ce:af:da brd ff:ff:ff:ff:ff:ff
       3: sit0: <NOARP> mtu 1480 qdisc noop
           link/sit 0.0.0.0 brd 0.0.0.0

      bash$ ip route list
      169.254.0.0/16 dev lo  scope link

  Or, in a script:

      &ipscript;

`route`

  Show info about or make changes to the kernel routing table.

      bash$ route
      Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
       pm3-67.bozosisp *               255.255.255.255 UH       40 0          0 ppp0
       127.0.0.0       *               255.0.0.0       U        40 0          0 lo
       default         pm3-67.bozosisp 0.0.0.0         UG       40 0          0 ppp0

`iptables`

  The `iptables` command set is a packet filtering tool used mainly for such security purposes as setting up network firewalls. This is a complex tool, and a detailed explanation of its use is beyond the scope of this document. Oskar Andreasson's tutorial is a reasonable starting point.

  See also shutting down iptables and .

`chkconfig`

  Check network and system configuration. This command lists and manages the network and system services started at bootup in the `/etc/rc?.d` directory.

  Originally a port from IRIX to Red Hat Linux, `chkconfig` may not be part of the core installation of some Linux flavors.

      bash$ chkconfig --list
      atd             0:off   1:off   2:off   3:on    4:on    5:on    6:off
       rwhod           0:off   1:off   2:off   3:off   4:off   5:off   6:off
       ...

`tcpdump`

  Network packet sniffer. This is a tool for analyzing and troubleshooting traffic on a network by dumping packet headers that match specified criteria.

  Dump ip packet traffic between hosts bozoville and caduceus:

      bash$ tcpdump ip host bozoville and caduceus

  Of course, the output of `tcpdump` can be parsed with certain of the previously discussed text processing utilities.

#### Filesystem

`mount`

  Mount a filesystem, usually on an external device, such as a floppy or CDROM. The file `/etc/fstab` provides a handy listing of available filesystems, partitions, and devices, including options, that may be automatically or manually mounted. The file `/etc/mtab` shows the currently mounted filesystems and partitions (including the virtual ones, such as `/proc`).

  `mount -a` mounts all filesystems and partitions listed in `/etc/fstab`, except those with a `noauto` option. At bootup, a startup script in `/etc/rc.d` (`rc.sysinit` or something similar) invokes this to get everything mounted.

      mount -t iso9660 /dev/cdrom /mnt/cdrom
      # Mounts CD ROM. ISO 9660 is a standard CD ROM filesystem.
      mount /mnt/cdrom
      # Shortcut, if /mnt/cdrom listed in /etc/fstab

  The versatile mount command can even mount an ordinary file on a block device, and the file will act as if it were a filesystem. Mount accomplishes that by associating the file with a loopback device. One application of this is to mount and examine an ISO9660 filesystem image before burning it onto a CDR.

  For more detail on burning CDRs, see Alex Withers' article, Creating CDs, in the October, 1999 issue of Linux Journal.

  #### Checking a CD image

      # As root...

      mkdir /mnt/cdtest  # Prepare a mount point, if not already there.

      mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest   # Mount the image.
      #                  "-o loop" option equivalent to "losetup /dev/loop0"
      cd /mnt/cdtest     # Now, check the image.
      ls -alR            # List the files in the directory tree there.
                         # And so forth.

`umount`

  Unmount a currently mounted filesystem. Before physically removing a previously mounted floppy or CDROM disk, the device must be `umount`ed, else filesystem corruption may result.

      umount /mnt/cdrom
      # You may now press the eject button and safely remove the disk.

  The `automount` utility, if properly installed, can mount and unmount floppies or CDROM disks as they are accessed or removed. On multispindle laptops with swappable floppy and optical drives, this can cause problems, however.

`gnome-mount`

  The newer Linux distros have deprecated `mount` and `umount`. The successor, for command-line mounting of removable storage devices, is `gnome-mount`. It can take the `-d` option to mount a device file by its listing in `/dev`.

  For example, to mount a USB flash drive:

      bash$ gnome-mount -d /dev/sda1
      gnome-mount 0.4

      bash$ df
      . . .
       /dev/sda1                63584     12034     51550  19% /media/disk

`sync`

  Forces an immediate write of all updated data from buffers to hard drive (synchronize drive with buffers). While not strictly necessary, a `sync` assures the sys admin or user that the data just changed will survive a sudden power failure. In the olden days, a `sync; sync` (twice, just to make absolutely sure) was a useful precautionary measure before a system reboot.

  At times, you may wish to force an immediate buffer flush, as when securely deleting a file (see ) or when the lights begin to flicker.

`losetup`

  Sets up and configures loopback devices.

  #### Creating a filesystem in a file

      SIZE=1000000  # 1 meg

      head -c $SIZE < /dev/zero > file  # Set up file of designated size.
      losetup /dev/loop0 file           # Set it up as loopback device.
      mke2fs /dev/loop0                 # Create filesystem.
      mount -o loop /dev/loop0 /mnt     # Mount it.

      # Thanks, S.C.

`mkswap`

  Creates a swap partition or file. The swap area must subsequently be enabled with `swapon`.

`swapon`
`swapoff`

  Enable / disable swap partitition or file. These commands usually take effect at bootup and shutdown.

`mke2fs`

  Create a Linux ext2 filesystem. This command must be invoked as root.

  #### Adding a new hard drive

      &adddrv;

  See also and .

`mkdosfs`

  Create a DOS FAT filesystem.

`tune2fs`

  Tune ext2 filesystem. May be used to change filesystem parameters, such as maximum mount count. This must be invoked as root.

  This is an extremely dangerous command. Use it at your own risk, as you may inadvertently destroy your filesystem.

`dumpe2fs`

  Dump (list to `stdout`) very verbose filesystem info. This must be invoked as root.

      root# dumpe2fs /dev/hda7 | grep 'ount count'
      dumpe2fs 1.19, 13-Jul-2000 for EXT2 FS 0.5b, 95/08/09
       Mount count:              6
       Maximum mount count:      20

`hdparm`

  List or change hard disk parameters. This command must be invoked as root, and it may be dangerous if misused.

`fdisk`

  Create or change a partition table on a storage device, usually a hard drive. This command must be invoked as root.

  Use this command with extreme caution. If something goes wrong, you may destroy an existing filesystem.

`fsck`
`e2fsck`
`debugfs`

  Filesystem check, repair, and debug command set.

  `fsck`: a front end for checking a UNIX filesystem (may invoke other utilities). The actual filesystem type generally defaults to ext2.

  `e2fsck`: ext2 filesystem checker.

  `debugfs`: ext2 filesystem debugger. One of the uses of this versatile, but dangerous command is to (attempt to) recover deleted files. For advanced users only!

  All of these should be invoked as root, and they can damage or destroy a filesystem if misused.

`badblocks`

  Checks for bad blocks (physical media flaws) on a storage device. This command finds use when formatting a newly installed hard drive or testing the integrity of backup media. The `-c` option to mke2fs also invokes a check for bad blocks. As an example, `badblocks /dev/fd0` tests a floppy disk.

  The `badblocks` command may be invoked destructively (overwrite all data) or in non-destructive read-only mode. If root user owns the device to be tested, as is generally the case, then root must invoke this command.

`lsusb`
`usbmodules`

  The `lsusb` command lists all USB (Universal Serial Bus) buses and the devices hooked up to them.

  The `usbmodules` command outputs information about the driver modules for connected USB devices.

      bash$ lsusb
      Bus 001 Device 001: ID 0000:0000
       Device Descriptor:
         bLength                18
         bDescriptorType         1
         bcdUSB               1.00
         bDeviceClass            9 Hub
         bDeviceSubClass         0
         bDeviceProtocol         0
         bMaxPacketSize0         8
         idVendor           0x0000
         idProduct          0x0000

         . . .

`lspci`

  Lists pci busses present.

      bash$ lspci
      00:00.0 Host bridge: Intel Corporation 82845 845
       (Brookdale) Chipset Host Bridge (rev 04)
       00:01.0 PCI bridge: Intel Corporation 82845 845
       (Brookdale) Chipset AGP Bridge (rev 04)
       00:1d.0 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #1) (rev 02)
       00:1d.1 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #2) (rev 02)
       00:1d.2 USB Controller: Intel Corporation 82801CA/CAM USB (Hub #3) (rev 02)
       00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev 42)

         . . .

`mkbootdisk`

  Creates a boot floppy which can be used to bring up the system if, for example, the MBR (master boot record) becomes corrupted. Of special interest is the `--iso` option, which uses `mkisofs` to create a bootable ISO9660 filesystem image suitable for burning a bootable CDR.

  The `mkbootdisk` command is actually a Bash script, written by Erik Troan, in the `/sbin` directory.

`mkisofs`

  Creates an ISO9660 filesystem suitable for a CDR image.

`chroot`

  CHange ROOT directory. Normally commands are fetched from $PATH, relative to `/`, the default root directory. This changes the root directory to a different one (and also changes the working directory to there). This is useful for security purposes, for instance when the system administrator wishes to restrict certain users, such as those telnetting in, to a secured portion of the filesystem (this is sometimes referred to as confining a guest user to a chroot jail). Note that after a `chroot`, the execution path for system binaries is no longer valid.

  A `chroot /opt` would cause references to `/usr/bin` to be translated to `/opt/usr/bin`. Likewise, `chroot /aaa/bbb /bin/ls` would redirect future instances of `ls` to `/aaa/bbb` as the base directory, rather than `/` as is normally the case. An `alias XX 'chroot /aaa/bbb ls'` in a user's `~/.bashrc` effectively restricts which portion of the filesystem she may run command XX on.

  The `chroot` command is also handy when running from an emergency boot floppy (`chroot` to `/dev/fd0`), or as an option to `lilo` when recovering from a system crash. Other uses include installation from a different filesystem (an rpm option) or running a readonly filesystem from a CD ROM. Invoke only as root, and use with care.

  It might be necessary to copy certain system files to a chrooted directory, since the normal `$PATH` can no longer be relied upon.

`lockfile`

  This utility is part of the `procmail` package (www.procmail.org). It creates a lock file, a semaphore that controls access to a file, device, or resource.

  `Definition:` A semaphore is a flag or signal. (The usage originated in railroading, where a colored flag, lantern, or striped movable arm semaphore indicated whether a particular track was in use and therefore unavailable for another train.) A UNIX process can check the appropriate semaphore to determine whether a particular resource is available/accessible.

  The lock file serves as a flag that this particular file, device, or resource is in use by a process (and is therefore busy). The presence of a lock file permits only restricted access (or no access) to other processes.

      lockfile /home/bozo/lockfiles/$0.lock
      # Creates a write-protected lockfile prefixed with the name of the script.

      lockfile /home/bozo/lockfiles/${0##*/}.lock
      # A safer version of the above, as pointed out by E. Choroba.

  Lock files are used in such applications as protecting system mail folders from simultaneously being changed by multiple users, indicating that a modem port is being accessed, and showing that an instance of Firefox is using its cache. Scripts may check for the existence of a lock file created by a certain process to check if that process is running. Note that if a script attempts to create a lock file that already exists, the script will likely hang.

  Normally, applications create and check for lock files in the `/var/lock` directory. Since only root has write permission in the `/var/lock` directory, a user script cannot set a lock file there. A script can test for the presence of a lock file by something like the following.

      appname=xyzip
      # Application "xyzip" created lock file "/var/lock/xyzip.lock".

      if [ -e "/var/lock/$appname.lock" ]
      then   #+ Prevent other programs & scripts
             #  from accessing files/resources used by xyzip.
        ...

`flock`

  Much less useful than the `lockfile` command is `flock`. It sets an advisory lock on a file and then executes a command while the lock is on. This is to prevent any other process from setting a lock on that file until completion of the specified command.

      flock $0 cat $0 > lockfile__$0
      #  Set a lock on the script the above line appears in,
      #+ while listing the script to stdout.

  Unlike `lockfile`, `flock` does not automatically create a lock file.

`mknod`

  Creates block or character device files (may be necessary when installing new hardware on the system). The `MAKEDEV` utility has virtually all of the functionality of `mknod`, and is easier to use.

`MAKEDEV`

  Utility for creating device files. It must be run as root, and in the `/dev` directory. It is a sort of advanced version of `mknod`.

`tmpwatch`

  Automatically deletes files which have not been accessed within a specified period of time. Usually invoked by cron to remove stale log files.

#### Backup

`dump`
`restore`

  The `dump` command is an elaborate filesystem backup utility, generally used on larger installations and networks. Operators of single-user Linux systems generally prefer something simpler for backups, such as `tar`. It reads raw disk partitions and writes a backup file in a binary format. Files to be backed up may be saved to a variety of storage media, including disks and tape drives. The `restore` command restores backups made with `dump`.

`fdformat`

  Perform a low-level format on a floppy disk (`/dev/fd0*`).

#### System Resources

`ulimit`

  Sets an upper limit on use of system resources. Usually invoked with the `-f` option, which sets a limit on file size (`ulimit -f 1000` limits files to 1 meg maximum). As of the version 4 update of Bash, the `-f` and `-c` options take a block size of 512 when in POSIX mode. Additionally, there are two new options: `-b` for socket buffer size, and `-T` for the limit on the number of threads. The `-t` option limits the coredump size (`ulimit -c 0` eliminates coredumps). Normally, the value of `ulimit` would be set in `/etc/profile` and/or `~/.bash_profile` (see ).

  Judicious use of `ulimit` can protect a system against the dreaded fork bomb.

      #!/bin/bash
      # This script is for illustrative purposes only.
      # Run it at your own peril -- it WILL freeze your system.

      while true  #  Endless loop.
      do
        $0 &      #  This script invokes itself . . .
                  #+ forks an infinite number of times . . .
                  #+ until the system freezes up because all resources exhausted.
      done        #  This is the notorious sorcerer's appentice scenario.

      exit 0      #  Will not exit here, because this script will never terminate.

  A `ulimit -Hu XX` (where XX is the user process limit) in `/etc/profile` would abort this script when it exceeded the preset limit.

`quota`

  Display user or group disk quotas.

`setquota`

  Set user or group disk quotas from the command-line.

`umask`

  User file creation permissions mask. Limit the default file attributes for a particular user. All files created by that user take on the attributes specified by `umask`. The (octal) value passed to `umask` defines the file permissions disabled. For example, `umask 022` ensures that new files will have at most 755 permissions (777 NAND 022).

  NAND is the logical not-and operator. Its effect is somewhat similar to subtraction.

  Of course, the user may later change the attributes of particular files with chmod. The usual practice is to set the value of `umask` in `/etc/profile` and/or `~/.bash_profile` (see ).

  #### Using umask to hide an output file from prying eyes

      &rot13a;

`rdev`

  Get info about or make changes to root device, swap space, or video mode. The functionality of `rdev` has generally been taken over by `lilo`, but `rdev` remains useful for setting up a ram disk. This is a dangerous command, if misused.

#### Modules

`lsmod`

  List installed kernel modules.

      bash$ lsmod
      Module                  Size  Used by
       autofs                  9456   2 (autoclean)
       opl3                   11376   0
       serial_cs               5456   0 (unused)
       sb                     34752   0
       uart401                 6384   0 [sb]
       sound                  58368   0 [opl3 sb uart401]
       soundlow                 464   0 [sound]
       soundcore               2800   6 [sb sound]
       ds                      6448   2 [serial_cs]
       i82365                 22928   2
       pcmcia_core            45984   0 [serial_cs ds i82365]

  Doing a `cat /proc/modules` gives the same information.

`insmod`

  Force installation of a kernel module (use `modprobe` instead, when possible). Must be invoked as root.

`rmmod`

  Force unloading of a kernel module. Must be invoked as root.

`modprobe`

  Module loader that is normally invoked automatically in a startup script. Must be invoked as root.

`depmod`

  Creates module dependency file. Usually invoked from a startup script.

`modinfo`

  Output information about a loadable module.

      bash$ modinfo hid
      filename:    /lib/modules/2.4.20-6/kernel/drivers/usb/hid.o
       description: "USB HID support drivers"
       author:      "Andreas Gal, Vojtech Pavlik <vojtech@suse.cz>"
       license:     "GPL"

#### Miscellaneous

`env`

  Runs a program or script with certain environmental variables set or changed (without changing the overall system environment). The `[varname=xxx]` permits changing the environmental variable `varname` for the duration of the script. With no options specified, this command lists all the environmental variable settings.

  In Bash and other Bourne shell derivatives, it is possible to set variables in a single command's environment.

      var1=value1 var2=value2 commandXXX
      # $var1 and $var2 set in the environment of 'commandXXX' only.

  The first line of a script (the sha-bang line) may use `env` when the path to the shell or interpreter is unknown.

      #! /usr/bin/env perl

      print "This Perl script will run,\n";
      print "even when I don't know where to find Perl.\n";

      # Good for portable cross-platform scripts,
      # where the Perl binaries may not be in the expected place.
      # Thanks, S.C.

  Or even ...

      #!/bin/env bash
      # Queries the $PATH enviromental variable for the location of bash.
      # Therefore ...
      # This script will run where Bash is not in its usual place, in /bin.
      ...

`ldd`

  Show shared lib dependencies for an executable file.

      bash$ ldd /bin/ls
      libc.so.6 => /lib/libc.so.6 (0x4000c000)
      /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)

`watch`

  Run a command repeatedly, at specified time intervals.

  The default is two-second intervals, but this may be changed with the `-n` option.

      watch -n 5 tail /var/log/messages
      # Shows tail end of system log, /var/log/messages, every five seconds.

  Unfortunately, piping the output of `watch command` to grep does not work.

`strip`

  Remove the debugging symbolic references from an executable binary. This decreases its size, but makes debugging it impossible.

  This command often occurs in a Makefile, but rarely in a shell script.

`nm`

  List symbols in an unstripped compiled binary.

`xrandr`

  Command-line tool for manipulating the root window of the screen.

  #### Backlight: changes the brightness of the (laptop) screen backlight

      &backlight;

`rdist`

  Remote distribution client: synchronizes, clones, or backs up a file system on a remote server.

#### Analyzing a System Script

Using our knowledge of administrative commands, let us examine a system script. One of the shortest and simplest to understand scripts is killall, The killall system script should not be confused with the killall command in `/usr/bin`. used to suspend running processes at system shutdown.

##### killall, from `/etc/rc.d/init.d`

    &ex55;

That wasn't so bad. Aside from a little fancy footwork with variable matching, there is no new material there.

##### Exercise 1

In `/etc/rc.d/init.d`, analyze the `halt` script. It is a bit longer than `killall`, but similar in concept. Make a copy of this script somewhere in your home directory and experiment with it (do not run it as root). Do a simulated run with the `-vn` flags (`sh -vn scriptname`). Add extensive comments. Change the commands to echos.

##### Exercise 2

Look at some of the more complex scripts in `/etc/rc.d/init.d`. Try to understand at least portions of them. Follow the above procedure to analyze them. For some additional insight, you might also examine the file `sysvinitfiles` in `/usr/share/doc/initscripts-?.??`, which is part of the initscripts documentation.

## Advanced Topics

At this point, we are ready to delve into certain of the difficult and unusual aspects of scripting. Along the way, we will attempt to push the envelope in various ways and examine boundary conditions (what happens when we move into uncharted territory?).

### Regular Expressions

. . . the intellectual activity associated with software development is largely one of gaining insight.

--Stowe Boyd

To fully utilize the power of shell scripting, you need to master Regular Expressions. Certain commands and utilities commonly used in scripts, such as grep, expr, sed and awk, interpret and use REs. As of version 3, Bash has acquired its own RE-match operator: `=~`.

#### A Brief Introduction to Regular Expressions

An expression is a string of characters. Those characters having an interpretation above and beyond their literal meaning are called metacharacters. A quote symbol, for example, may denote speech by a person, ditto, or a meta-meaning

A meta-meaning is the meaning of a term or expression on a higher level of abstraction. For example, the literal meaning of regular expression is an ordinary expression that conforms to accepted usage. The meta-meaning is drastically different, as discussed at length in this chapter.

for the symbols that follow. Regular Expressions are sets of characters and/or metacharacters that match (or specify) patterns.

A Regular Expression contains one or more of the following:

- A character set. These are the characters retaining their literal meaning. The simplest type of Regular Expression consists only of a character set, with no metacharacters.

- An anchor. These designate (anchor) the position in the line of text that the RE is to match. For example, ^, and $ are anchors.

- Modifiers. These expand or narrow (modify) the range of text the RE is to match. Modifiers include the asterisk, brackets, and the backslash.

The main uses for Regular Expressions (REs) are text searches and string manipulation. An RE matches a single character or a set of characters -- a string or a part of a string.

- The asterisk -- * -- matches any number of repeats of the character string or RE preceding it, including zero instances.

  1133* matches 11 + one or more 3's: 113, 1133, 1133333, and so forth.

- The dot -- . -- matches any one character, except a newline. Since sed, awk, and grep process single lines, there will usually not be a newline to match. In those cases where there is a newline in a multiple line expression, the dot will match the newline.

      #!/bin/bash

      sed -e 'N;s/.*/[&]/' << EOF   # Here Document
      line1
      line2
      EOF
      # OUTPUT:
      # [line1
      # line2]

      echo

      awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF
      line 1
      line 2
      EOF
      # OUTPUT:
      # line
      # 1

      # Thanks, S.C.

      exit 0

  13. matches 13 + at least one of any character (including a space): 1133, 11333, but not 13 (additional character missing).

  See for a demonstration of dot single-character matching.

- The caret -- ^ -- matches the beginning of a line, but sometimes, depending on context, negates the meaning of a set of characters in an RE.

- The dollar sign -- $ -- at the end of an RE matches the end of a line.

  XXX$ matches XXX at the end of a line.

  ^$ matches blank lines.

- Brackets -- [...] -- enclose a set of characters to match in a single RE.

  [xyz] matches any one of the characters x, y, or z.

  [c-n] matches any one of the characters in the range c to n.

  [B-Pk-y] matches any one of the characters in the ranges B to P and k to y.

  [a-z0-9] matches any single lowercase letter or any digit.

  [^b-d] matches any character except those in the range b to d. This is an instance of ^ negating or inverting the meaning of the following RE (taking on a role similar to ! in a different context).

  Combined sequences of bracketed characters match common word patterns. [Yy][Ee][Ss] matches yes, Yes, YES, yEs, and so forth. [0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9] matches any Social Security number.

- The backslash -- \ -- escapes a special character, which means that character gets interpreted literally (and is therefore no longer special).

  A \$ reverts back to its literal meaning of $, rather than its RE meaning of end-of-line. Likewise a \\ has the literal meaning of \.

- Escaped angle brackets -- \<...\> -- mark word boundaries.

  The angle brackets must be escaped, since otherwise they have only their literal character meaning.

  \<the\> matches the word the, but not the words them, there, other, etc.

      bash$ cat textfile
      This is line 1, of which there is only one instance.
       This is the only instance of line 2.
       This is line 3, another line.
       This is line 4.

      bash$ grep 'the' textfile
      This is line 1, of which there is only one instance.
       This is the only instance of line 2.
       This is line 3, another line.

      bash$ grep '\<the\>' textfile
      This is the only instance of line 2.

The only way to be certain that a particular RE works is to test it.

    TEST FILE: tstfile                          # No match.
                                                # No match.
    Run   grep "1133*"  on this file.           # Match.
                                                # No match.
                                                # No match.
    This line contains the number 113.          # Match.
    This line contains the number 13.           # No match.
    This line contains the number 133.          # No match.
    This line contains the number 1133.         # Match.
    This line contains the number 113312.       # Match.
    This line contains the number 1112.         # No match.
    This line contains the number 113312312.    # Match.
    This line contains no numbers at all.       # No match.

    bash$ grep "1133*" tstfile
    Run   grep "1133*"  on this file.           # Match.
     This line contains the number 113.          # Match.
     This line contains the number 1133.         # Match.
     This line contains the number 113312.       # Match.
     This line contains the number 113312312.    # Match.

- ##### Extended REs

  Additional metacharacters added to the basic set. Used in egrep, awk, and Perl.

- The question mark -- ? -- matches zero or one of the previous RE. It is generally used for matching single characters.

- The plus -- + -- matches one or more of the previous RE. It serves a role similar to the *, but does not match zero occurrences.

      # GNU versions of sed and awk can use "+",
      # but it needs to be escaped.

      echo a111b | sed -ne '/a1\+b/p'
      echo a111b | grep 'a1\+b'
      echo a111b | gawk '/a1+b/'
      # All of above are equivalent.

      # Thanks, S.C.

- Escaped curly brackets -- \{ \} -- indicate the number of occurrences of a preceding RE to match.

  It is necessary to escape the curly brackets since they have only their literal character meaning otherwise. This usage is technically not part of the basic RE set.

  [0-9]\{5\} matches exactly five digits (characters in the range of 0 to 9).

  Curly brackets are not available as an RE in the classic (non-POSIX compliant) version of awk. However, the GNU extended version of awk, `gawk`, has the `--re-interval` option that permits them (without being escaped).

      bash$ echo 2222 | gawk --re-interval '/2{3}/'
      2222

  `Perl` and some `egrep` versions do not require escaping the curly brackets.

- Parentheses -- `( )` -- enclose a group of REs. They are useful with the following | operator and in substring extraction using expr.

- The -- `|` -- or RE operator matches any of a set of alternate characters.

      bash$ egrep 're(a|e)d' misc.txt
      People who read seem to be better informed than those who do not.
       The clarinet produces sound by the vibration of its reed.

Some versions of `sed`, `ed`, and `ex` support escaped versions of the extended Regular Expressions described above, as do the GNU utilities.

- ##### POSIX Character Classes

  `[:class:]`

  This is an alternate method of specifying a range of characters to match.

- `[:alnum:]` matches alphabetic or numeric characters. This is equivalent to `A-Za-z0-9`.

- `[:alpha:]` matches alphabetic characters. This is equivalent to `A-Za-z`.

- `[:blank:]` matches a space or a tab.

- `[:cntrl:]` matches control characters.

- `[:digit:]` matches (decimal) digits. This is equivalent to `0-9`.

- `[:graph:]` (graphic printable characters). Matches characters in the range of ASCII 33 - 126. This is the same as `[:print:]`, below, but excluding the space character.

- `[:lower:]` matches lowercase alphabetic characters. This is equivalent to `a-z`.

- `[:print:]` (printable characters). Matches characters in the range of ASCII 32 - 126. This is the same as `[:graph:]`, above, but adding the space character.

- `[:space:]` matches whitespace characters (space and horizontal tab).

- `[:upper:]` matches uppercase alphabetic characters. This is equivalent to `A-Z`.

- `[:xdigit:]` matches hexadecimal digits. This is equivalent to `0-9A-Fa-f`.

  POSIX character classes generally require quoting or double brackets ([[ ]]).

      bash$ grep [[:digit:]] test.file
      abc=723

      # ...
      if [[ $arow =~ [[:digit:]] ]]   #  Numerical input?
      then       #  POSIX char class
        if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal!
      # ...
      # From ktour.sh example script.

  These character classes may even be used with globbing, to a limited extent.

      bash$ ls -l ?[[:digit:]][[:digit:]]?
      -rw-rw-r--    1 bozo  bozo         0 Aug 21 14:47 a33b

  POSIX character classes are used in and .

Sed, awk, and Perl, used as filters in scripts, take REs as arguments when "sifting" or transforming files or I/O streams. See and for illustrations of this.

The standard reference on this complex topic is Friedl's Mastering Regular Expressions. Sed & Awk, by Dougherty and Robbins, also gives a very lucid treatment of REs. See the for more information on these books.

#### Globbing

Bash itself cannot recognize Regular Expressions. Inside scripts, it is commands and utilities -- such as sed and awk -- that interpret RE's.

Bash does carry out filename expansion

Filename expansion means expanding filename patterns or templates containing special characters. For example, `example.???` might expand to `example.001` and/or `example.txt`.

-- a process known as globbing -- but this does not use the standard RE set. Instead, globbing recognizes and expands wild cards. Globbing interprets the standard wild card characters A wild card character, analogous to a wild card in poker, can represent (almost) any other character. -- * and ?, character lists in square brackets, and certain other special characters (such as ^ for negating the sense of a match). There are important limitations on wild card characters in globbing, however. Strings containing * will not match filenames that start with a dot, as, for example, `.bashrc`. Filename expansion can match dotfiles, but only if the pattern explicitly includes the dot as a literal character.

    ~/[.]bashrc    #  Will not expand to ~/.bashrc
    ~/?bashrc      #  Neither will this.
                   #  Wild cards and metacharacters will NOT
                   #+ expand to a dot in globbing.

    ~/.[b]ashrc    #  Will expand to ~/.bashrc
    ~/.ba?hrc      #  Likewise.
    ~/.bashr*      #  Likewise.

    # Setting the "dotglob" option turns this off.

    # Thanks, S.C.

Likewise, the ? has a different meaning in globbing than as part of an RE.

    bash$ ls -l
    total 2
     -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 a.1
     -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 b.1
     -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 c.1
     -rw-rw-r--    1 bozo  bozo       466 Aug  6 17:48 t2.sh
     -rw-rw-r--    1 bozo  bozo       758 Jul 30 09:02 test1.txt

    bash$ ls -l t?.sh
    -rw-rw-r--    1 bozo  bozo       466 Aug  6 17:48 t2.sh

    bash$ ls -l [ab]*
    -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 a.1
     -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 b.1

    bash$ ls -l [a-c]*
    -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 a.1
     -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 b.1
     -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 c.1

    bash$ ls -l [^ab]*
    -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 c.1
     -rw-rw-r--    1 bozo  bozo       466 Aug  6 17:48 t2.sh
     -rw-rw-r--    1 bozo  bozo       758 Jul 30 09:02 test1.txt

    bash$ ls -l {b*,c*,*est*}
    -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 b.1
     -rw-rw-r--    1 bozo  bozo         0 Aug  6 18:42 c.1
     -rw-rw-r--    1 bozo  bozo       758 Jul 30 09:02 test1.txt

Bash performs filename expansion on unquoted command-line arguments. The echo command demonstrates this.

    bash$ echo *
    a.1 b.1 c.1 t2.sh test1.txt

    bash$ echo t*
    t2.sh test1.txt

    bash$ echo t?.sh
    t2.sh

It is possible to modify the way Bash interprets special characters in globbing. A `set -f` command disables globbing, and the `nocaseglob` and `nullglob` options to shopt change globbing behavior.

See also .

Filenames with embedded whitespace can cause globbing to choke. David Wheeler shows how to avoid many such pitfalls.

    IFS="$(printf '\n\t')"   # Remove space.

    #  Correct glob use:
    #  Always use for-loop, prefix glob, check if exists file.
    for file in ./* ; do         # Use ./* ... NEVER bare *
      if [ -e "$file" ] ; then   # Check whether file exists.
         COMMAND ... "$file" ...
      fi
    done

    # This example taken from David Wheeler's site, with permission.

### Here Documents

Here and now, boys.

--Aldous Huxley, Island

A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, or the ex text editor.

    COMMAND <<InputComesFromHERE
    ...
    ...
    ...
    InputComesFromHERE

A limit string delineates (frames) the command list. The special symbol << precedes the limit string. This has the effect of redirecting the output of a command block into the `stdin` of the program or command. It is similar to `interactive-program < command-file`, where `command-file` contains

    command #1
    command #2
    ...

The here document equivalent looks like this:

    interactive-program <<LimitString
    command #1
    command #2
    ...
    LimitString

Choose a limit string sufficiently unusual that it will not occur anywhere in the command list and confuse matters.

Note that here documents may sometimes be used to good effect with non-interactive utilities and commands, such as, for example, wall.

#### broadcast: Sends message to everyone logged in

    &ex70;

Even such unlikely candidates as the vi text editor lend themselves to here documents.

#### dummyfile: Creates a 2-line dummy file

    &ex69;

The above script could just as effectively have been implemented with `ex`, rather than `vi`. Here documents containing a list of `ex` commands are common enough to form their own category, known as ex scripts.

    #!/bin/bash
    #  Replace all instances of "Smith" with "Jones"
    #+ in files with a ".txt" filename suffix.

    ORIGINAL=Smith
    REPLACEMENT=Jones

    for word in $(fgrep -l $ORIGINAL *.txt)
    do
      # -------------------------------------
      ex $word <<EOF
      :%s/$ORIGINAL/$REPLACEMENT/g
      :wq
    EOF
      # :%s is the "ex" substitution command.
      # :wq is write-and-quit.
      # -------------------------------------
    done

Analogous to ex scripts are cat scripts.

#### Multi-line message using cat

    &ex71;

The `-` option to mark a here document limit string (`<<-LimitString`) suppresses leading tabs (but not spaces) in the output. This may be useful in making a script more readable.

#### Multi-line message, with tabs suppressed

    &ex71a;

A here document supports parameter and command substitution. It is therefore possible to pass different parameters to the body of the here document, changing its output accordingly.

#### Here document with replaceable parameters

    &ex71b;

This is a useful script containing a here document with parameter substitution.

#### Upload a file pair to Sunsite incoming directory

    &ex72;

Quoting or escaping the limit string at the head of a here document disables parameter substitution within its body. The reason for this is that quoting/escaping the limit string effectively escapes the $, `, and \ special characters, and causes them to be interpreted literally. (Thank you, Allen Halsey, for pointing this out.)

#### Parameter substitution turned off

    &ex71c;

Disabling parameter substitution permits outputting literal text. Generating scripts or even program code is one use for this.

#### A script that generates another script

    &generatescript;

It is possible to set a variable from the output of a here document. This is actually a devious form of command substitution.

    variable=$(cat <<SETVAR
    This variable
    runs over multiple lines.
    SETVAR
    )

    echo "$variable"

A here document can supply input to a function in the same script.

#### Here documents and functions

    &hf;

It is possible to use : as a dummy command accepting output from a here document. This, in effect, creates an anonymous here document.

#### Anonymous Here Document

    #!/bin/bash

    : <<TESTVARIABLES
    ${HOSTNAME?}${USER?}${MAIL?}  # Print error message if one of the variables not set.
    TESTVARIABLES

    exit $?

A variation of the above technique permits commenting out blocks of code.

#### Commenting out a block of code

    &commentblock;

Yet another twist of this nifty trick makes self-documenting scripts possible.

#### A self-documenting script

    &selfdocument;

Using a cat script is an alternate way of accomplishing this.

    DOC_REQUEST=70

    if [ "$1" = "-h"  -o "$1" = "--help" ]     # Request help.
    then                                       # Use a "cat script" . . .
      cat <<DOCUMENTATIONXX
    List the statistics of a specified directory in tabular format.
    ---------------------------------------------------------------
    The command-line parameter gives the directory to be listed.
    If no directory specified or directory specified cannot be read,
    then list the current working directory.

    DOCUMENTATIONXX
    exit $DOC_REQUEST
    fi

See also , , , and for more examples of self-documenting scripts.

Here documents create temporary files, but these files are deleted after opening and are not accessible to any other process.

    bash$ bash -c 'lsof -a -p $$ -d0' << EOF
    > EOF
    lsof    1213 bozo    0r   REG    3,5    0 30386 /tmp/t1213-0-sh (deleted)

Some utilities will not work inside a here document.

The closing limit string, on the final line of a here document, must start in the first character position. There can be no leading whitespace. Trailing whitespace after the limit string likewise causes unexpected behavior. The whitespace prevents the limit string from being recognized.

Except, as Dennis Benzinger points out, if using `<<-` to suppress tabs.

    #!/bin/bash

    echo "----------------------------------------------------------------------"

    cat <<LimitString
    echo "This is line 1 of the message inside the here document."
    echo "This is line 2 of the message inside the here document."
    echo "This is the final line of the message inside the here document."
         LimitString
    #^^^^Indented limit string. Error! This script will not behave as expected.

    echo "----------------------------------------------------------------------"

    #  These comments are outside the 'here document',
    #+ and should not echo.

    echo "Outside the here document."

    exit 0

    echo "This line had better not echo."  # Follows an 'exit' command.

Some people very cleverly use a single ! as a limit string. But, that's not necessarily a good idea.

    # This works.
    cat <<!
    Hello!
    ! Three more exclamations !!!
    !

    # But . . .
    cat <<!
    Hello!
    Single exclamation point follows!
    !
    !
    # Crashes with an error message.

    # However, the following will work.
    cat <<EOF
    Hello!
    Single exclamation point follows!
    !
    EOF
    # It's safer to use a multi-character limit string.

For those tasks too complex for a here document, consider using the expect scripting language, which was specifically designed for feeding input into interactive programs.

#### Here Strings

>     A here string can be considered as a stripped-down form of a here document.
>     It consists of nothing more than COMMAND <<< $WORD,
>     where $WORD is expanded and fed to the stdin of COMMAND.

As a simple example, consider this alternative to the echo-grep construction.

    # Instead of:
    if echo "$VAR" | grep -q txt   # if [[ $VAR = *txt* ]]
    # etc.

    # Try:
    if grep -q "txt" <<< "$VAR"
    then   #         ^^^
       echo "$VAR contains the substring sequence \"txt\""
    fi
    # Thank you, Sebastian Kaminski, for the suggestion.

Or, in combination with read:

    String="This is a string of words."

    read -r -a Words <<< "$String"
    #  The -a option to "read"
    #+ assigns the resulting values to successive members of an array.

    echo "First word in String is:    ${Words[0]}"   # This
    echo "Second word in String is:   ${Words[1]}"   # is
    echo "Third word in String is:    ${Words[2]}"   # a
    echo "Fourth word in String is:   ${Words[3]}"   # string
    echo "Fifth word in String is:    ${Words[4]}"   # of
    echo "Sixth word in String is:    ${Words[5]}"   # words.
    echo "Seventh word in String is:  ${Words[6]}"   # (null)
                                                     # Past end of $String.

    # Thank you, Francisco Lobo, for the suggestion.

It is, of course, possible to feed the output of a here string into the `stdin` of a loop.

    # As Seamus points out . . .

    ArrayVar=( element0 element1 element2 {A..D} )

    while read element ; do
      echo "$element" 1>&2
    done <<< $(echo ${ArrayVar[*]})

    # element0 element1 element2 A B C D

##### Prepending a line to a file

    &prependex;

##### Parsing a mailbox

    &mailboxgrep;

Exercise: Find other uses for here strings, such as, for example, feeding input to dc.

### I/O Redirection

There are always three default files By convention in UNIX and Linux, data streams and peripherals (device files) are treated as files, in a fashion analogous to ordinary files. open, `stdin` (the keyboard), `stdout` (the screen), and `stderr` (error messages output to the screen). These, and any other open files, can be redirected. Redirection simply means capturing output from a file, command, program, script, or even code block within a script (see and ) and sending it as input to another file, command, program, or script.

Each open file gets assigned a file descriptor. A file descriptor is simply a number that the operating system assigns to an open file to keep track of it. Consider it a simplified type of file pointer. It is analogous to a file handle in `C`. The file descriptors for `stdin`, `stdout`, and `stderr` are 0, 1, and 2, respectively. For opening additional files, there remain descriptors 3 to 9. It is sometimes useful to assign one of these additional file descriptors to `stdin`, `stdout`, or `stderr` as a temporary duplicate link. Using file descriptor 5 might cause problems. When Bash creates a child process, as with exec, the child inherits fd 5 (see Chet Ramey's archived e-mail, SUBJECT: RE: File descriptor 5 is held open). Best leave this particular fd alone. This simplifies restoration to normal after complex redirection and reshuffling (see ).

       COMMAND_OUTPUT >
          # Redirect stdout to a file.
          # Creates the file if not present, otherwise overwrites it.

          ls -lR > dir-tree.list
          # Creates a file containing a listing of the directory tree.

       : > filename
          # The > truncates file "filename" to zero length.
          # If file not present, creates zero-length file (same effect as 'touch').
          # The : serves as a dummy placeholder, producing no output.

       > filename
          # The > truncates file "filename" to zero length.
          # If file not present, creates zero-length file (same effect as 'touch').
          # (Same result as ": >", above, but this does not work with some shells.)

       COMMAND_OUTPUT >>
          # Redirect stdout to a file.
          # Creates the file if not present, otherwise appends to it.

          # Single-line redirection commands (affect only the line they are on):
          # --------------------------------------------------------------------

       1>filename
          # Redirect stdout to file "filename."
       1>>filename
          # Redirect and append stdout to file "filename."
       2>filename
          # Redirect stderr to file "filename."
       2>>filename
          # Redirect and append stderr to file "filename."
       &>filename
          # Redirect both stdout and stderr to file "filename."
          # This operator is now functional, as of Bash 4, final release.

       M>N
         # "M" is a file descriptor, which defaults to 1, if not explicitly set.
         # "N" is a filename.
         # File descriptor "M" is redirect to file "N."
       M>&N
         # "M" is a file descriptor, which defaults to 1, if not set.
         # "N" is another file descriptor.

          #==============================================================================

          # Redirecting stdout, one line at a time.
          LOGFILE=script.log

          echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE
          echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE
          echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE
          echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"."
          # These redirection commands automatically "reset" after each line.

          # Redirecting stderr, one line at a time.
          ERRORFILE=script.errors

          bad_command1 2>$ERRORFILE       #  Error message sent to $ERRORFILE.
          bad_command2 2>>$ERRORFILE      #  Error message appended to $ERRORFILE.
          bad_command3                    #  Error message echoed to stderr,
                                          #+ and does not appear in $ERRORFILE.
          # These redirection commands also automatically "reset" after each line.
          #=======================================================================

       2>&1
          # Redirects stderr to stdout.
          # Error messages get sent to same place as standard output.
            >>filename 2>&1
                bad_command >>filename 2>&1
                # Appends both stdout and stderr to the file "filename" ...
            2>&1 | [command(s)]
                bad_command 2>&1 | awk '{print $5}'   # found
                # Sends stderr through a pipe.
                # |&amp was added to Bash 4 as an abbreviation for 2>&1 |.

       i>&j
          # Redirects file descriptor i to j.
          # All output of file pointed to by i gets sent to file pointed to by j.

       >&j
          # Redirects, by default, file descriptor 1 (stdout) to j.
          # All stdout gets sent to file pointed to by j.

       0< FILENAME
        < FILENAME
          # Accept input from a file.
          # Companion command to >, and often used in combination with it.
          #
          # grep search-word <filename

       [j]<>filename
          #  Open file "filename" for reading and writing,
          #+ and assign file descriptor "j" to it.
          #  If "filename" does not exist, create it.
          #  If file descriptor "j" is not specified, default to fd 0, stdin.
          #
          #  An application of this is writing at a specified place in a file.
          echo 1234567890 > File    # Write string to "File".
          exec 3<> File             # Open "File" and assign fd 3 to it.
          read -n 4 <&3             # Read only 4 characters.
          echo -n . >&3             # Write a decimal point there.
          exec 3>&-                 # Close fd 3.
          cat File                  # ==> 1234.67890
          #  Random access, by golly.

       |
          # Pipe.
          # General purpose process and command chaining tool.
          # Similar to >, but more general in effect.
          # Useful for chaining commands, scripts, files, and programs together.
          cat *.txt | sort | uniq > result-file
          # Sorts the output of all the .txt files and deletes duplicate lines,
          # finally saves results to result-file.

Multiple instances of input and output redirection and/or pipes can be combined in a single command line.

    command < input-file > output-file
    # Or the equivalent:
    < input-file command > output-file   # Although this is non-standard.

    command1 | command2 | command3 > output-file

See and .

Multiple output streams may be redirected to one file.

    ls -yz >> command.log 2>&1
    #  Capture result of illegal options "yz" in file "command.log."
    #  Because stderr is redirected to the file,
    #+ any error messages will also be there.

    #  Note, however, that the following does *not* give the same result.
    ls -yz 2>&1 >> command.log
    #  Outputs an error message, but does not write to file.
    #  More precisely, the command output (in this case, null)
    #+ writes to the file, but the error message goes only to stdout.

    #  If redirecting both stdout and stderr,
    #+ the order of the commands makes a difference.

#### Closing File Descriptors

n<&-

  Close input file descriptor n.

0<&-
<&-

  Close `stdin`.

n>&-

  Close output file descriptor n.

1>&-
>&-

  Close `stdout`.

Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited, close it.

    # Redirecting only stderr to a pipe.

    exec 3>&1                              # Save current "value" of stdout.
    ls -l 2>&1 >&3 3>&- | grep bad 3>&-    # Close fd 3 for 'grep' (but not 'ls').
    #              ^^^^   ^^^^
    exec 3>&-                              # Now close it for the remainder of the script.

    # Thanks, S.C.

For a more detailed introduction to I/O redirection see .

#### Using exec

An `exec <filename` command redirects `stdin` to a file. From that point on, all `stdin` comes from that file, rather than its normal source (usually keyboard input). This provides a method of reading a file line by line and possibly parsing each line of input using sed and/or awk.

##### Redirecting `stdin` using exec

    &redir1;

Similarly, an `exec >filename` command redirects `stdout` to a designated file. This sends all command output that would normally go to `stdout` to that file.

`exec N > filename` affects the entire script or current shell. Redirection in the PID of the script or shell from that point on has changed. However . . .

`N > filename` affects only the newly-forked process, not the entire script or shell.

Thank you, Ahmed Darwish, for pointing this out.

##### Redirecting `stdout` using exec

    &reassignstdout;

##### Redirecting both `stdin` and `stdout` in the same script with exec

    &upperconv;

I/O redirection is a clever way of avoiding the dreaded inaccessible variables within a subshell problem.

##### Avoiding a subshell

    &avoidsubshell;

#### Redirecting Code Blocks

Blocks of code, such as while, until, and for loops, even if/then test blocks can also incorporate redirection of `stdin`. Even a function may use this form of redirection (see ). The < operator at the end of the code block accomplishes this.

##### Redirected while loop

    &redir2;

##### Alternate form of redirected while loop

    &redir2a;

##### Redirected until loop

    &redir3;

##### Redirected for loop

    &redir4;

We can modify the previous example to also redirect the output of the loop.

##### Redirected for loop (both `stdin` and `stdout` redirected)

    &redir4a;

##### Redirected if/then test

    &redir5;

##### Data file names.data for above examples

    &namesdata;

Redirecting the `stdout` of a code block has the effect of saving its output to a file. See .

Here documents are a special case of redirected code blocks. That being the case, it should be possible to feed the output of a here document into the `stdin` for a while loop.

    # This example by Albert Siersema
    # Used with permission (thanks!).

    function doesOutput()
     # Could be an external command too, of course.
     # Here we show you can use a function as well.
    {
      ls -al *.jpg | awk '{print $5,$9}'
    }

    nr=0          #  We want the while loop to be able to manipulate these and
    totalSize=0   #+ to be able to see the changes after the 'while' finished.

    while read fileSize fileName ; do
      echo "$fileName is $fileSize bytes"
      let nr++
      totalSize=$((totalSize+fileSize))   # Or: "let totalSize+=fileSize"
    done<<EOF
    $(doesOutput)
    EOF

    echo "$nr files totaling $totalSize bytes"

#### Applications

Clever use of I/O redirection permits parsing and stitching together snippets of command output (see ). This permits generating report and log files.

##### Logging events

    &logevents;

### Subshells

Running a shell script launches a new process, a subshell.

`Definition:` A subshell is a child process launched by a shell (or shell script).

A subshell is a separate instance of the command processor -- the shell that gives you the prompt at the console or in an xterm window. Just as your commands are interpreted at the command-line prompt, similarly does a script batch-process a list of commands. Each shell script running is, in effect, a subprocess (child process) of the parent shell.

A shell script can itself launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.

    #!/bin/bash
    # subshell-test.sh

    (
    # Inside parentheses, and therefore a subshell . . .
    while [ 1 ]   # Endless loop.
    do
      echo "Subshell running . . ."
    done
    )

    #  Script will run forever,
    #+ or at least until terminated by a Ctl-C.

    exit $?  # End of script (but will never get here).

    Now, run the script:
    sh subshell-test.sh

    And, while the script is running, from a different xterm:
    ps -ef | grep subshell-test.sh

    UID       PID   PPID  C STIME TTY      TIME     CMD
    500       2698  2502  0 14:26 pts/4    00:00:00 sh subshell-test.sh
    500       2699  2698 21 14:26 pts/4    00:00:24 sh subshell-test.sh

              ^^^^

    Analysis:
    PID 2698, the script, launched PID 2699, the subshell.

    Note: The "UID ..." line would be filtered out by the "grep" command,
    but is shown here for illustrative purposes.

In general, an external command in a script forks off a subprocess, An external command invoked with an exec does not (usually) fork off a subprocess / subshell. whereas a Bash builtin does not. For this reason, builtins execute more quickly and use fewer system resources than their external command equivalents.

#### Command List within Parentheses

( command1; command2; command3; ... )

  A command list embedded between parentheses runs as a subshell.

Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible to the parent process, to the shell that launched the subshell. These are, in effect, variables local to the child process.

#### Variable scope in a subshell

    &subshell;

See also $BASHPID and .

`Definition:` The scope of a variable is the context in which it has meaning, in which it has a value that can be referenced. For example, the scope of a local variable lies only within the function, block of code, or subshell within which it is defined, while the scope of a global variable is the entire script in which it appears.

While the $BASH_SUBSHELL internal variable indicates the nesting level of a subshell, the $SHLVL variable shows no change within a subshell.

    echo " \$BASH_SUBSHELL outside subshell       = $BASH_SUBSHELL"           # 0
      ( echo " \$BASH_SUBSHELL inside subshell        = $BASH_SUBSHELL" )     # 1
      ( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2
    # ^ ^                           *** nested ***                        ^ ^

    echo

    echo " \$SHLVL outside subshell = $SHLVL"       # 3
    ( echo " \$SHLVL inside subshell  = $SHLVL" )   # 3 (No change!)

Directory changes made in a subshell do not carry over to the parent shell.

#### List User Profiles

    &allprofs;

A subshell may be used to set up a dedicated environment for a command group.

    COMMAND1
    COMMAND2
    COMMAND3
    (
      IFS=:
      PATH=/bin
      unset TERMINFO
      set -C
      shift 5
      COMMAND4
      COMMAND5
      exit 3 # Only exits the subshell!
    )
    # The parent shell has not been affected, and the environment is preserved.
    COMMAND6
    COMMAND7

As seen here, the exit command only terminates the subshell in which it is running, not the parent shell or script.

One application of such a dedicated environment is testing whether a variable is defined.

    if (set -u; : $variable) 2> /dev/null
    then
      echo "Variable is set."
    fi     #  Variable has been set in current script,
           #+ or is an an internal Bash variable,
           #+ or is present in environment (has been exported).

    # Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]
    # or                    [[ ${variable-x} != x$variable ]]
    # or                    [[ ${variable+x} = x ]]
    # or                    [[ ${variable-x} != x ]]

Another application is checking for a lock file:

    if (set -C; : > lock_file) 2> /dev/null
    then
      :   # lock_file didn't exist: no user running the script
    else
      echo "Another user is already running that script."
    exit 65
    fi

    #  Code snippet by Stéphane Chazelas,
    #+ with modifications by Paulo Marcel Coelho Aragao.

+

Processes may execute in parallel within different subshells. This permits breaking a complex task into subcomponents processed concurrently.

#### Running parallel processes in subshells

    	(cat list1 list2 list3 | sort | uniq > list123) &
    	(cat list4 list5 list6 | sort | uniq > list456) &
    	# Merges and sorts both sets of lists simultaneously.
    	# Running in background ensures parallel execution.
    	#
    	# Same effect as
    	#   cat list1 list2 list3 | sort | uniq > list123 &
    	#   cat list4 list5 list6 | sort | uniq > list456 &

    	wait   # Don't execute the next command until subshells finish.

    	diff list123 list456

Redirecting I/O to a subshell uses the | pipe operator, as in `ls -al | (command)`.

A code block between curly brackets does not launch a subshell.

{ command1; command2; command3; . . . commandN; }

    var1=23
    echo "$var1"   # 23

    { var1=76; }
    echo "$var1"   # 76

### Restricted Shells

#### Disabled commands in restricted shells

  ####

  Running a script or portion of a script in restricted mode disables certain commands that would otherwise be available. This is a security measure intended to limit the privileges of the script user and to minimize possible damage from running the script.

The following commands and actions are disabled:

- Using cd to change the working directory.

- Changing the values of the $PATH, $SHELL, $BASH_ENV, or $ENV environmental variables.

- Reading or changing the $SHELLOPTS, shell environmental options.

- Output redirection.

- Invoking commands containing one or more /'s.

- Invoking exec to substitute a different process for the shell.

- Various other commands that would enable monkeying with or attempting to subvert the script for an unintended purpose.

- Getting out of restricted mode within the script.

#### Running a script in restricted mode

    &restricted;

### Process Substitution

Piping the `stdout` of a command into the `stdin` of another is a powerful technique. But, what if you need to pipe the `stdout` of multiple commands? This is where process substitution comes in.

Process substitution feeds the output of a process (or processes) into the `stdin` of another process.

#### Template

Command list enclosed within parentheses

  `>(command_list)`

  `<(command_list)`

  Process substitution uses `/dev/fd/<n>` files to send the results of the process(es) within parentheses to another process. This has the same effect as a named pipe (temp file), and, in fact, named pipes were at one time used in process substitution.

  There is no space between the the < or > and the parentheses. Space there would give an error message.

    bash$ echo >(true)
    /dev/fd/63

    bash$ echo <(true)
    /dev/fd/63

    bash$ echo >(true) <(true)
    /dev/fd/63 /dev/fd/62

    bash$ wc <(cat /usr/share/dict/linux.words)
     483523  483523 4992010 /dev/fd/63

    bash$ grep script /usr/share/dict/linux.words | wc
        262     262    3601

    bash$ wc <(grep script /usr/share/dict/linux.words)
        262     262    3601 /dev/fd/63

Bash creates a pipe with two file descriptors, `--fIn` and `fOut--`. The `stdin` of true connects to `fOut` (dup2(fOut, 0)), then Bash passes a `/dev/fd/fIn` argument to `echo`. On systems lacking `/dev/fd/<n>` files, Bash may use temporary files. (Thanks, S.C.)

Process substitution can compare the output of two different commands, or even the output of different options to the same command.

    bash$ comm <(ls -l) <(ls -al)
    total 12
    -rw-rw-r--    1 bozo bozo       78 Mar 10 12:58 File0
    -rw-rw-r--    1 bozo bozo       42 Mar 10 12:58 File2
    -rw-rw-r--    1 bozo bozo      103 Mar 10 12:58 t2.sh
            total 20
            drwxrwxrwx    2 bozo bozo     4096 Mar 10 18:10 .
            drwx------   72 bozo bozo     4096 Mar 10 17:58 ..
            -rw-rw-r--    1 bozo bozo       78 Mar 10 12:58 File0
            -rw-rw-r--    1 bozo bozo       42 Mar 10 12:58 File2
            -rw-rw-r--    1 bozo bozo      103 Mar 10 12:58 t2.sh

Process substitution can compare the contents of two directories -- to see which filenames are in one, but not the other.

    diff <(ls $first_directory) <(ls $second_directory)

Some other usages and uses of process substitution:

    read -a list < <( od -Ad -w24 -t u2 /dev/urandom )
    #  Read a list of random numbers from /dev/urandom,
    #+ process with "od"
    #+ and feed into stdin of "read" . . .

    #  From "insertion-sort.bash" example script.
    #  Courtesy of JuanJo Ciarlante.

    PORT=6881   # bittorrent

    # Scan the port to make sure nothing nefarious is going on.
    netcat -l $PORT | tee>(md5sum ->mydata-orig.md5) |
    gzip | tee>(md5sum - | sed 's/-$/mydata.lz2/'>mydata-gz.md5)>mydata.gz

    # Check the decompression:
      gzip -d<mydata.gz | md5sum -c mydata-orig.md5)
    # The MD5sum of the original checks stdin and detects compression issues.

    #  Bill Davidsen contributed this example
    #+ (with light edits by the ABS Guide author).

    cat <(ls -l)
    # Same as     ls -l | cat

    sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin)
    # Lists all the files in the 3 main 'bin' directories, and sorts by filename.
    # Note that three (count 'em) distinct commands are fed to 'sort'.

    diff <(command1) <(command2)    # Gives difference in command output.

    tar cf >(bzip2 -c > file.tar.bz2) $directory_name
    # Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2".
    #
    # Because of the /dev/fd/<n> system feature,
    # the pipe between both commands does not need to be named.
    #
    # This can be emulated.
    #
    bzip2 -c < pipe > file.tar.bz2&
    tar cf pipe $directory_name
    rm pipe
    #        or
    exec 3>&1
    tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&-
    exec 3>&-

    # Thanks, Stéphane Chazelas

Here is a method of circumventing the problem of an echo piped to a while-read loop running in a subshell.

#### Code block redirection without forking

    &wrps;

This is a similar example.

#### Redirecting the output of process substitution into a loop.

    &psubp;

A reader sent in the following interesting example of process substitution.

    # Script fragment taken from SuSE distribution:

    # --------------------------------------------------------------#
    while read  des what mask iface; do
    # Some commands ...
    done < <(route -n)
    #    ^ ^  First < is redirection, second is process substitution.

    # To test it, let's make it do something.
    while read  des what mask iface; do
      echo $des $what $mask $iface
    done < <(route -n)

    # Output:
    # Kernel IP routing table
    # Destination Gateway Genmask Flags Metric Ref Use Iface
    # 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
    # --------------------------------------------------------------#

    #  As Stéphane Chazelas points out,
    #+ an easier-to-understand equivalent is:
    route -n |
      while read des what mask iface; do   # Variables set from output of pipe.
        echo $des $what $mask $iface
      done  #  This yields the same output as above.
            #  However, as Ulrich Gayer points out . . .
            #+ this simplified equivalent uses a subshell for the while loop,
            #+ and therefore the variables disappear when the pipe terminates.

    # --------------------------------------------------------------#

    #  However, Filip Moritz comments that there is a subtle difference
    #+ between the above two examples, as the following shows.

    (
    route -n | while read x; do ((y++)); done
    echo $y # $y is still unset

    while read x; do ((y++)); done < <(route -n)
    echo $y # $y has the number of lines of output of route -n
    )

    More generally spoken
    (
    : | x=x
    # seems to start a subshell like
    : | ( x=x )
    # while
    x=x < <(:)
    # does not
    )

    # This is useful, when parsing csv and the like.
    # That is, in effect, what the original SuSE code fragment does.

### Functions

Like real programming languages, Bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a black box that performs a specified task. Wherever there is repetitive code, when a task repeats with only slight variations in procedure, then consider using a function.

    	  function
    	  function_name
    	  {
    	  command
    	  }

or

    	  function_name
    	  ()
    	  {
    	  command
    	  }

This second form will cheer the hearts of C programmers (and is more portable).

As in C, the function's opening bracket may optionally appear on the second line.

    	  function_name
    	  ()
    	  {
    	  command
    	  }

A function may be compacted into a single line.

    fun () { echo "This is a function"; echo; }
    #                                 ^     ^

In this case, however, a semicolon must follow the final command in the function.

    fun () { echo "This is a function"; echo } # Error!
    #                                       ^

    fun2 () { echo "Even a single-command function? Yes!"; }
    #                                                    ^

Functions are called, triggered, simply by invoking their names. A function call is equivalent to a command.

#### Simple functions

    &ex59;

The function definition must precede the first call to it. There is no method of declaring the function, as, for example, in C.

    f1
    # Will give an error message, since function "f1" not yet defined.

    declare -f f1      # This doesn't help either.
    f1                 # Still an error message.

    # However...

    f1 ()
    {
      echo "Calling function \"f2\" from within function \"f1\"."
      f2
    }

    f2 ()
    {
      echo "Function \"f2\"."
    }

    f1  #  Function "f2" is not actually called until this point,
        #+ although it is referenced before its definition.
        #  This is permissible.

        # Thanks, S.C.

Functions may not be empty!

    #!/bin/bash
    # empty-function.sh

    empty ()
    {
    }

    exit 0  # Will not exit here!

    # $ sh empty-function.sh
    # empty-function.sh: line 6: syntax error near unexpected token `}'
    # empty-function.sh: line 6: `}'

    # $ echo $?
    # 2

    # Note that a function containing only comments is empty.

    func ()
    {
      # Comment 1.
      # Comment 2.
      # This is still an empty function.
      # Thank you, Mark Bova, for pointing this out.
    }
    # Results in same error message as above.

    # However ...

    not_quite_empty ()
    {
      illegal_command
    } #  A script containing this function will *not* bomb
      #+ as long as the function is not called.

    not_empty ()
    {
      :
    } # Contains a : (null command), and this is okay.

    # Thank you, Dominick Geyer and Thiemo Kellner.

It is even possible to nest a function within another function, although this is not very useful.

    f1 ()
    {

      f2 () # nested
      {
        echo "Function \"f2\", inside \"f1\"."
      }

    }

    f2  #  Gives an error message.
        #  Even a preceding "declare -f f2" wouldn't help.

    echo

    f1  #  Does nothing, since calling "f1" does not automatically call "f2".
    f2  #  Now, it's all right to call "f2",
        #+ since its definition has been made visible by calling "f1".

        # Thanks, S.C.

Function declarations can appear in unlikely places, even where a command would otherwise go.

    ls -l | foo() { echo "foo"; }  # Permissible, but useless.

    if [ "$USER" = bozo ]
    then
      bozo_greet ()   # Function definition embedded in an if/then construct.
      {
        echo "Hello, Bozo."
      }
    fi

    bozo_greet        # Works only for Bozo, and other users get an error.

    # Something like this might be useful in some contexts.
    NO_EXIT=1   # Will enable function definition below.

    [[ $NO_EXIT -eq 1 ]] && exit() { true; }     # Function definition in an "and-list".
    # If $NO_EXIT is 1, declares "exit ()".
    # This disables the "exit" builtin by aliasing it to "true".

    exit  # Invokes "exit ()" function, not "exit" builtin.

    # Or, similarly:
    filename=file1

    [ -f "$filename" ] &&
    foo () { rm -f "$filename"; echo "File "$filename" deleted."; } ||
    foo () { echo "File "$filename" not found."; touch bar; }

    foo

    # Thanks, S.C. and Christopher Head

Function names can take strange forms.

      _(){ for i in {1..10}; do echo -n "$FUNCNAME"; done; echo; }
    # ^^^         No space between function name and parentheses.
    #             This doesn't always work. Why not?

    # Now, let's invoke the function.
      _         # __________
    #             ^^^^^^^^^^   10 underscores (10 x function name)!
    # A "naked" underscore is an acceptable function name.

    # In fact, a colon is likewise an acceptable function name.

    :(){ echo ":"; }; :

    # Of what use is this?
    # It's a devious way to obfuscate the code in a script.

See also

What happens when different versions of the same function appear in a script?

    #  As Yan Chen points out,
    #  when a function is defined multiple times,
    #  the final version is what is invoked.
    #  This is not, however, particularly useful.

    func ()
    {
      echo "First version of func ()."
    }

    func ()
    {
      echo "Second version of func ()."
    }

    func   # Second version of func ().

    exit $?

    #  It is even possible to use functions to override
    #+ or preempt system commands.
    #  Of course, this is *not* advisable.

#### Complex Functions and Function Complexities

Functions may process arguments passed to them and return an exit status to the script for further processing.

    function_name $arg1 $arg2

The function refers to the passed arguments by position (as if they were positional parameters), that is, `$1`, `$2`, and so forth.

##### Function Taking Parameters

    &ex60;

The shift command works on arguments passed to functions (see ).

But, what about command-line arguments passed to the script? Does a function see them? Well, let's clear up the confusion.

##### Functions and command-line args passed to the script

    &funccmdlinearg;

In contrast to certain other programming languages, shell scripts normally pass only value parameters to functions. Variable names (which are actually pointers), if passed as parameters to functions, will be treated as string literals. Functions interpret their arguments literally.

Indirect variable references (see ) provide a clumsy sort of mechanism for passing variable pointers to functions.

##### Passing an indirect reference to a function

    &indfunc;

The next logical question is whether parameters can be dereferenced after being passed to a function.

##### Dereferencing a parameter passed to a function

    &dereferencecl;

##### Again, dereferencing a parameter passed to a function

    &refparams;

##### Exit and Return

`exit status`

  Functions return a value, called an exit status. This is analogous to the exit status returned by a command. The exit status may be explicitly specified by a `return` statement, otherwise it is the exit status of the last command in the function (0 if successful, and a non-zero error code if not). This exit status may be used in the script by referencing it as $?. This mechanism effectively permits script functions to have a return value similar to C functions.

`return`

  Terminates a function. A `return` command The `return` command is a Bash builtin. optionally takes an integer argument, which is returned to the calling script as the exit status of the function, and this exit status is assigned to the variable $?.

  ##### Maximum of two numbers

      &max;

  For a function to return a string or array, use a dedicated variable.

      count_lines_in_etc_passwd()
      {
        [[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd))
        #  If /etc/passwd is readable, set REPLY to line count.
        #  Returns both a parameter value and status information.
        #  The 'echo' seems unnecessary, but . . .
        #+ it removes excess whitespace from the output.
      }

      if count_lines_in_etc_passwd
      then
        echo "There are $REPLY lines in /etc/passwd."
      else
        echo "Cannot count lines in /etc/passwd."
      fi

      # Thanks, S.C.

  ##### Converting numbers to Roman numerals

      &ex61;

  See also .

  The largest positive integer a function can return is 255. The `return` command is closely tied to the concept of exit status, which accounts for this particular limitation. Fortunately, there are various workarounds for those situations requiring a large integer return value from a function.

  ##### Testing large return values in a function

      &returntest;

  A workaround for obtaining large integer return values is to simply assign the return value to a global variable.

      Return_Val=   # Global variable to hold oversize return value of function.

      alt_return_test ()
      {
        fvar=$1
        Return_Val=$fvar
        return   # Returns 0 (success).
      }

      alt_return_test 1
      echo $?                              # 0
      echo "return value = $Return_Val"    # 1

      alt_return_test 256
      echo "return value = $Return_Val"    # 256

      alt_return_test 257
      echo "return value = $Return_Val"    # 257

      alt_return_test 25701
      echo "return value = $Return_Val"    #25701

  A more elegant method is to have the function `echo` its return value to `stdout`, and then capture it by command substitution. See the discussion of this in .

  ##### Comparing two large integers

      &max2;

  Here is another example of capturing a function return value. Understanding it requires some knowledge of awk.

      month_length ()  # Takes month number as an argument.
      {                # Returns number of days in month.
      monthD="31 28 31 30 31 30 31 31 30 31 30 31"  # Declare as local?
      echo "$monthD" | awk '{ print $'"${1}"' }'    # Tricky.
      #                             ^^^^^^^^^
      # Parameter passed to function  ($1 -- month number), then to awk.
      # Awk sees this as "print $1 . . . print $12" (depending on month number)
      # Template for passing a parameter to embedded awk script:
      #                                 $'"${script_parameter}"'

      #    Here's a slightly simpler awk construct:
      #    echo $monthD | awk -v month=$1 '{print $(month)}'
      #    Uses the -v awk option, which assigns a variable value
      #+   prior to execution of the awk program block.
      #    Thank you, Rich.

      #  Needs error checking for correct parameter range (1-12)
      #+ and for February in leap year.
      }

      # ----------------------------------------------
      # Usage example:
      month=4        # April, for example (4th month).
      days_in=$(month_length $month)
      echo $days_in  # 30
      # ----------------------------------------------

  See also and .

  `Exercise:` Using what we have just learned, extend the previous Roman numerals example to accept arbitrarily large input.

##### Redirection

Redirecting the stdin of a function

  A function is essentially a code block, which means its `stdin` can be redirected (as in ).

  ##### Real name from username

      &realname;

  There is an alternate, and perhaps less confusing method of redirecting a function's `stdin`. This involves redirecting the `stdin` to an embedded bracketed code block within the function.

      # Instead of:
      Function ()
      {
       ...
       } < file

      # Try this:
      Function ()
      {
        {
          ...
         } < file
      }

      # Similarly,

      Function ()  # This works.
      {
        {
         echo $*
        } | tr a b
      }

      Function ()  # This doesn't work.
      {
        echo $*
      } | tr a b   # A nested code block is mandatory here.

      # Thanks, S.C.

  Emmanuel Rouat's sample `bashrc` file contains some instructive examples of functions.

#### Local Variables

##### What makes a variable local?

local variables

  A variable declared as local is one that is visible only within the block of code in which it appears. It has local scope. In a function, a local variable has meaning only within that function block.

  However, as Thomas Braunberger points out, a local variable declared in a function is also visible to functions called by the parent function.

      #!/bin/bash

      function1 ()
      {
        local func1var=20

        echo "Within function1, \$func1var = $func1var."

        function2
      }

      function2 ()
      {
        echo "Within function2, \$func1var = $func1var."
      }

      function1

      exit 0

      # Output of the script:

      # Within function1, $func1var = 20.
      # Within function2, $func1var = 20.

  This is documented in the Bash manual:

  Local can only be used within a function; it makes the variable name have a visible scope restricted to that function and its children. [emphasis added] The ABS Guide author considers this behavior to be a bug.

  ##### Local variable visibility

      &ex62;

  Before a function is called, all variables declared within the function are invisible outside the body of the function, not just those explicitly declared as local.

      #!/bin/bash

      func ()
      {
      global_var=37    #  Visible only within the function block
                       #+ before the function has been called.
      }                #  END OF FUNCTION

      echo "global_var = $global_var"  # global_var =
                                       #  Function "func" has not yet been called,
                                       #+ so $global_var is not visible here.

      func
      echo "global_var = $global_var"  # global_var = 37
                                       # Has been set by function call.

  As Evgeniy Ivanov points out, when declaring and setting a local variable in a single command, apparently the order of operations is to first set the variable, and only afterwards restrict it to local scope. This is reflected in the return value.

      #!/bin/bash

      echo "==OUTSIDE Function (global)=="
      t=$(exit 1)
      echo $?      # 1
                   # As expected.
      echo

      function0 ()
      {

      echo "==INSIDE Function=="
      echo "Global"
      t0=$(exit 1)
      echo $?      # 1
                   # As expected.

      echo
      echo "Local declared & assigned in same command."
      local t1=$(exit 1)
      echo $?      # 0
                   # Unexpected!
      #  Apparently, the variable assignment takes place before
      #+ the local declaration.
      #+ The return value is for the latter.

      echo
      echo "Local declared, then assigned (separate commands)."
      local t2
      t2=$(exit 1)
      echo $?      # 1
                   # As expected.

      }

      function0

##### Local variables and recursion.

Recursion is an interesting and sometimes useful form of self-reference. Herbert Mayer defines it as . . . expressing an algorithm by using a simpler version of that same algorithm . . .

Consider a definition defined in terms of itself, Otherwise known as redundancy. an expression implicit in its own expression, Otherwise known as tautology. a snake swallowing its own tail, Otherwise known as a metaphor. or . . . a function that calls itself. Otherwise known as a recursive function.

###### Demonstration of a simple recursive function

    &recursiondemo;

###### Another simple demonstration

    &recursiondemo2;

Local variables are a useful tool for writing recursive code, but this practice generally involves a great deal of computational overhead and is definitely not recommended in a shell script.

Too many levels of recursion may crash a script with a segfault.

    #!/bin/bash

    #  Warning: Running this script could possibly lock up your system!
    #  If you're lucky, it will segfault before using up all available memory.

    recursive_function ()
    {
    echo "$1"     # Makes the function do something, and hastens the segfault.
    (( $1 < $2 )) && recursive_function $(( $1 + 1 )) $2;
    #  As long as 1st parameter is less than 2nd,
    #+ increment 1st and recurse.
    }

    recursive_function 1 50000  # Recurse 50,000 levels!
    #  Most likely segfaults (depending on stack size, set by ulimit -m).

    #  Recursion this deep might cause even a C program to segfault,
    #+ by using up all the memory allotted to the stack.

    echo "This will probably not print."
    exit 0  # This script will not exit normally.

    #  Thanks, Stéphane Chazelas.

###### Recursion, using a local variable

    &ex63;

Also see for an example of recursion in a script. Be aware that recursion is resource-intensive and executes slowly, and is therefore generally not appropriate in a script.

#### Recursion Without Local Variables

A function may recursively call itself even without use of local variables.

##### The Fibonacci Sequence

    &fibo;

##### The Towers of Hanoi

    &hanoi;

### Aliases

A Bash alias is essentially nothing more than a keyboard shortcut, an abbreviation, a means of avoiding typing a long command sequence. If, for example, we include `alias lm="ls -l | more"` in the `~/.bashrc` file, then each `lm` ... as the first word of a command string. Obviously, an alias is only meaningful at the beginning of a command. typed at the command-line will automatically be replaced by a `ls -l | more`. This can save a great deal of typing at the command-line and avoid having to remember complex combinations of commands and options. Setting `alias rm="rm -i"` (interactive mode delete) may save a good deal of grief, since it can prevent inadvertently deleting important files.

In a script, aliases have very limited usefulness. It would be nice if aliases could assume some of the functionality of the `C` preprocessor, such as macro expansion, but unfortunately Bash does not expand arguments within the alias body. However, aliases do seem to expand positional parameters. Moreover, a script fails to expand an alias itself within compound constructs, such as if/then statements, loops, and functions. An added limitation is that an alias will not expand recursively. Almost invariably, whatever we would like an alias to do could be accomplished much more effectively with a function.

#### Aliases within a script

    &al;

The `unalias` command removes a previously set alias.

#### unalias: Setting and unsetting an alias

    &unal;

    bash$ ./unalias.sh

    total 6
    drwxrwxr-x    2 bozo     bozo         3072 Feb  6 14:04 .
    drwxr-xr-x   40 bozo     bozo         2048 Feb  6 14:04 ..
    -rwxr-xr-x    1 bozo     bozo          199 Feb  6 14:04 unalias.sh

    ./unalias.sh: llm: command not found

### List Constructs

The and list and or list constructs provide a means of processing a number of commands consecutively. These can effectively replace complex nested if/then or even case statements.

#### Chaining together commands

and list

      command-1 && command-2 && command-3 && ... command-n

  Each command executes in turn, provided that the previous command has given a return value of true (zero). At the first false (non-zero) return, the command chain terminates (the first command returning false is the last one to execute).

  An interesting use of a two-condition and list from an early version of YongYe's Tetris game script:

      equation()

      {  # core algorithm used for doubling and halving the coordinates
         [[ ${cdx} ]] && ((y=cy+(ccy-cdy)${2}2))
         eval ${1}+=\"${x} ${y} \"
      }

  #### Using an and list to test for command-line arguments

      &ex64;

  #### Another command-line arg test using an and list

      &andlist2;

  Of course, an and list can also set variables to a default value.

      arg1=$@ && [ -z "$arg1" ] && arg1=DEFAULT

                    # Set $arg1 to command-line arguments, if any.
                    # But . . . set to DEFAULT if not specified on command-line.

or list

      command-1 || command-2 || command-3 || ... command-n

  Each command executes in turn for as long as the previous command returns false. At the first true return, the command chain terminates (the first command returning true is the last one to execute). This is obviously the inverse of the and list.

  #### Using or lists in combination with an and list

      &ex65;

  If the first command in an or list returns true, it will execute.

    # ==> The following snippets from the /etc/rc.d/init.d/single
    #+==> script by Miquel van Smoorenburg
    #+==> illustrate use of "and" and "or" lists.
    # ==> "Arrowed" comments added by document author.

    [ -x /usr/bin/clear ] && /usr/bin/clear
      # ==> If /usr/bin/clear exists, then invoke it.
      # ==> Checking for the existence of a command before calling it
      #+==> avoids error messages and other awkward consequences.

      # ==> . . .

    # If they want to run something in single user mode, might as well run it...
    for i in /etc/rc1.d/S[0-9][0-9]* ; do
            # Check if the script is there.
            [ -x "$i" ] || continue
      # ==> If corresponding file in $PWD *not* found,
      #+==> then "continue" by jumping to the top of the loop.

            # Reject backup files and files generated by rpm.
            case "$1" in
                    *.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig)
                            continue;;
            esac
            [ "$i" = "/etc/rc1.d/S00single" ] && continue
      # ==> Set script name, but don't execute it yet.
            $i start
    done

      # ==> . . .

The exit status of an `and list` or an `or list` is the exit status of the last command executed.

Clever combinations of and and or lists are possible, but the logic may easily become convoluted and require close attention to operator precedence rules, and possibly extensive debugging.

    false && true || echo false         # false

    # Same result as
    ( false && true ) || echo false     # false
    # But NOT
    false && ( true || echo false )     # (nothing echoed)

    #  Note left-to-right grouping and evaluation of statements.

    #  It's usually best to avoid such complexities.

    #  Thanks, S.C.

See and for illustrations of using `and / or list` constructs to test variables.

### Arrays

Newer versions of Bash support one-dimensional arrays. Array elements may be initialized with the `variable[xx]` notation. Alternatively, a script may introduce the entire array by an explicit `declare -a variable` statement. To dereference (retrieve the contents of) an array element, use curly bracket notation, that is, `${element[xx]}`.

#### Simple array usage

    &ex66;

As we have seen, a convenient way of initializing an entire array is the `array=( element1 element2 ... elementN )` notation.

    base64_charset=( {A..Z} {a..z} {0..9} + / = )
                   #  Using extended brace expansion
                   #+ to initialize the elements of the array.
                   #  Excerpted from vladz's "base64.sh" script
                   #+ in the "Contributed Scripts" appendix.

Bash permits array operations on variables, even if the variables are not explicitly declared as arrays.

    string=abcABC123ABCabc
    echo ${string[@]}               # abcABC123ABCabc
    echo ${string[*]}               # abcABC123ABCabc
    echo ${string[0]}               # abcABC123ABCabc
    echo ${string[1]}               # No output!
                                    # Why?
    echo ${#string[@]}              # 1
                                    # One element in the array.
                                    # The string itself.

    # Thank you, Michael Zick, for pointing this out.

Once again this demonstrates that Bash variables are untyped.

#### Formatting a poem

    &poem;

Array variables have a syntax all their own, and even standard Bash commands and operators have special options adapted for array use.

#### Various array operations

    &arrayops;

Many of the standard string operations work on arrays.

#### String operations on arrays

    &arraystrops;

Command substitution can construct the individual elements of an array.

#### Loading the contents of a script into an array

    &scriptarray;

In an array context, some Bash builtins have a slightly altered meaning. For example, unset deletes array elements, or even an entire array.

#### Some special properties of arrays

    &ex67;

As seen in the previous example, either `${array_name[@]}` or `${array_name[*]}` refers to all the elements of the array. Similarly, to get a count of the number of elements in an array, use either `${#array_name[@]}` or `${#array_name[*]}`. `${#array_name}` is the length (number of characters) of `${array_name[0]}`, the first element of the array.

#### Of empty arrays and empty elements

    &emptyarray;

The relationship of `${array_name[@]}` and `${array_name[*]}` is analogous to that between $@ and $*. This powerful array notation has a number of uses.

    # Copying an array.
    array2=( "${array1[@]}" )
    # or
    array2="${array1[@]}"
    #
    #  However, this fails with "sparse" arrays,
    #+ arrays with holes (missing elements) in them,
    #+ as Jochen DeSmet points out.
    # ------------------------------------------
      array1[0]=0
    # array1[1] not assigned
      array1[2]=2
      array2=( "${array1[@]}" )       # Copy it?

    echo ${array2[0]}      # 0
    echo ${array2[2]}      # (null), should be 2
    # ------------------------------------------

    # Adding an element to an array.
    array=( "${array[@]}" "new element" )
    # or
    array[${#array[*]}]="new element"

    # Thanks, S.C.

The `array=( element1 element2 ... elementN )` initialization operation, with the help of command substitution, makes it possible to load the contents of a text file into an array.

    #!/bin/bash

    filename=sample_file

    #            cat sample_file
    #
    #            1 a b c
    #            2 d e fg

    declare -a array1

    array1=( `cat "$filename"`)                #  Loads contents
    #         List file to stdout              #+ of $filename into array1.
    #
    #  array1=( `cat "$filename" | tr '\n' ' '`)
    #                            change linefeeds in file to spaces.
    #  Not necessary because Bash does word splitting,
    #+ changing linefeeds to spaces.

    echo ${array1[@]}            # List the array.
    #                              1 a b c 2 d e fg
    #
    #  Each whitespace-separated "word" in the file
    #+ has been assigned to an element of the array.

    element_count=${#array1[*]}
    echo $element_count          # 8

Clever scripting makes it possible to add array operations.

#### Initializing arrays

    &arrayassign;

Adding a superfluous `declare -a` statement to an array declaration may speed up execution of subsequent operations on the array.

#### Copying and concatenating arrays

    &copyarray;

#### More on concatenating arrays

    &arrayappend;

--

Arrays permit deploying old familiar algorithms as shell scripts. Whether this is necessarily a good idea is left for the reader to decide.

#### The Bubble Sort

    &bubble;

--

Is it possible to nest arrays within arrays?

    #!/bin/bash
    # "Nested" array.

    #  Michael Zick provided this example,
    #+ with corrections and clarifications by William Park.

    AnArray=( $(ls --inode --ignore-backups --almost-all \
    	--directory --full-time --color=none --time=status \
    	--sort=time -l ${PWD} ) )  # Commands and options.

    # Spaces are significant . . . and don't quote anything in the above.

    SubArray=( ${AnArray[@]:11:1}  ${AnArray[@]:6:5} )
    #  This array has six elements:
    #+     SubArray=( [0]=${AnArray[11]} [1]=${AnArray[6]} [2]=${AnArray[7]}
    #      [3]=${AnArray[8]} [4]=${AnArray[9]} [5]=${AnArray[10]} )
    #
    #  Arrays in Bash are (circularly) linked lists
    #+ of type string (char *).
    #  So, this isn't actually a nested array,
    #+ but it's functionally similar.

    echo "Current directory and date of last status change:"
    echo "${SubArray[@]}"

    exit 0

--

Embedded arrays in combination with indirect references create some fascinating possibilities

#### Embedded arrays and indirect references

    &embarr;

--

Arrays enable implementing a shell script version of the Sieve of Eratosthenes. Of course, a resource-intensive application of this nature should really be written in a compiled language, such as C. It runs excruciatingly slowly as a script.

#### The Sieve of Eratosthenes

    &ex68;

#### The Sieve of Eratosthenes, Optimized

    &ex68a;

Compare these array-based prime number generators with alternatives that do not use arrays, , and .

--

Arrays lend themselves, to some extent, to emulating data structures for which Bash has no native support.

#### Emulating a push-down stack

    &stackex;

--

Fancy manipulation of array subscripts may require intermediate variables. For projects involving this, again consider using a more powerful programming language, such as Perl or C.

#### Complex array application: Exploring a weird mathematical series

    &qfunction;

--

Bash supports only one-dimensional arrays, though a little trickery permits simulating multi-dimensional ones.

#### Simulating a two-dimensional array, then tilting it

    &twodim;

A two-dimensional array is essentially equivalent to a one-dimensional one, but with additional addressing modes for referencing and manipulating the individual elements by row and column position.

For an even more elaborate example of simulating a two-dimensional array, see .

--

For more interesting scripts using arrays, see:

### Indirect References

We have seen that referencing a variable, `$var`, fetches its value. But, what about the value of a value? What about `$$var`?

The actual notation is \$$var, usually preceded by an eval (and sometimes an echo). This is called an indirect reference.

#### Indirect Variable References

    &indref;

Indirect referencing in Bash is a multi-step process. First, take the name of a variable: `varname`. Then, reference it: `$varname`. Then, reference the reference: `$$varname`. Then, escape the first $: `\$$varname`. Finally, force a reevaluation of the expression and assign it: `eval newvar=\$$varname`.

Of what practical use is indirect referencing of variables? It gives Bash a little of the functionality of pointers in C, for instance, in table lookup. And, it also has some other very interesting applications. . . .

Nils Radtke shows how to build dynamic variable names and evaluate their contents. This can be useful when sourcing configuration files.

    #!/bin/bash

    # ---------------------------------------------
    # This could be "sourced" from a separate file.
    isdnMyProviderRemoteNet=172.16.0.100
    isdnYourProviderRemoteNet=10.0.0.10
    isdnOnlineService="MyProvider"
    # ---------------------------------------------

    remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
    remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
    remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")
    remoteNet=$(eval "echo $isdnMyProviderRemoteNet")

    echo "$remoteNet"    # 172.16.0.100

    # ================================================================

    #  And, it gets even better.

    #  Consider the following snippet given a variable named getSparc,
    #+ but no such variable getIa64:

    chkMirrorArchs () {
      arch="$1";
      if [ "$(eval "echo \${$(echo get$(echo -ne $arch |
           sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |
           sed 's/^.\(.*\)/\1/g')):-false}")" = true ]
      then
         return 0;
      else
         return 1;
      fi;
    }

    getSparc="true"
    unset getIa64
    chkMirrorArchs sparc
    echo $?        # 0
                   # True

    chkMirrorArchs Ia64
    echo $?        # 1
                   # False

    # Notes:
    # -----
    # Even the to-be-substituted variable name part is built explicitly.
    # The parameters to the chkMirrorArchs calls are all lower case.
    # The variable name is composed of two parts: "get" and "Sparc" . . .

#### Passing an indirect reference to awk

    &coltotaler2;

This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the first order variable must be properly dereferenced (as in the above example). Fortunately, the ${!variable} notation introduced with version 2 of Bash (see and ) makes indirect referencing more intuitive.

Bash does not support pointer arithmetic, and this severely limits the usefulness of indirect referencing. In fact, indirect referencing in a scripting language is, at best, something of an afterthought.

### `/dev` and `/proc`

A Linux or UNIX filesystem typically has the `/dev` and `/proc` special-purpose directories.

#### `/dev`

The `/dev` directory contains entries for the physical devices that may or may not be present in the hardware. The entries in `/dev` provide mount points for physical and virtual devices. These entries use very little drive space. Some devices, such as `/dev/null`, `/dev/zero`, and `/dev/urandom` are virtual. They are not actual physical devices and exist only in software. Appropriately enough, these are called device files. As an example, the hard drive partitions containing the mounted filesystem(s) have entries in `/dev`, as df shows.

    bash$ df
    Filesystem           1k-blocks      Used Available Use%
     Mounted on
     /dev/hda6               495876    222748    247527  48% /
     /dev/hda1                50755      3887     44248   9% /boot
     /dev/hda8               367013     13262    334803   4% /home
     /dev/hda5              1714416   1123624    503704  70% /usr

Among other things, the `/dev` directory contains loopback devices, such as `/dev/loop0`. A loopback device is a gimmick that allows an ordinary file to be accessed as if it were a block device. A block device reads and/or writes data in chunks, or blocks, in contrast to a character device, which acesses data in character units. Examples of block devices are hard drives, CDROM drives, and flash drives. Examples of character devices are keyboards, modems, sound cards. This permits mounting an entire filesystem within a single large file. See and .

A few of the pseudo-devices in `/dev` have other specialized uses, such as `/dev/null`, `/dev/zero`, `/dev/urandom`, `/dev/sda1` (hard drive partition), `/dev/udp` (User Datagram Packet port), and `/dev/tcp`.

For instance:

To manually mount a USB flash drive, append the following line to `/etc/fstab`.

Of course, the mount point `/mnt/flashdrive` must exist. If not, then, as root, `mkdir /mnt/flashdrive`. To actually mount the drive, use the following command: `mount /mnt/flashdrive` Newer Linux distros automount flash drives in the `/media` directory without user intervention.

    /dev/sda1    /mnt/flashdrive    auto    noauto,user,noatime    0 0

(See also .)

Checking whether a disk is in the CD-burner (soft-linked to `/dev/hdc`):

    head -1 /dev/hdc

    #  head: cannot open '/dev/hdc' for reading: No medium found
    #  (No disc in the drive.)

    #  head: error reading '/dev/hdc': Input/output error
    #  (There is a disk in the drive, but it can't be read;
    #+  possibly it's an unrecorded CDR blank.)

    #  Stream of characters and assorted gibberish
    #  (There is a pre-recorded disk in the drive,
    #+ and this is raw output -- a stream of ASCII and binary data.)
    #  Here we see the wisdom of using 'head' to limit the output
    #+ to manageable proportions, rather than 'cat' or something similar.

    #  Now, it's just a matter of checking/parsing the output and taking
    #+ appropriate action.

When executing a command on a `/dev/tcp/$host/$port` pseudo-device file, Bash opens a TCP connection to the associated socket.

A socket is a communications node associated with a specific I/O port. (This is analogous to a hardware socket, or receptacle, for a connecting cable.) It permits data transfer between hardware devices on the same machine, between machines on the same network, between machines across different networks, and, of course, between machines at different locations on the Internet.

The following examples assume an active Internet connection.

Getting the time from `nist.gov`:

    bash$ cat </dev/tcp/time.nist.gov/13
    53082 04-03-18 04:26:54 68 0 0 502.3 UTC(NIST) *

[Mark contributed this example.]

Generalizing the above into a script:

    #!/bin/bash
    # This script must run with root permissions.

    URL="time.nist.gov/13"

    Time=$(cat </dev/tcp/"$URL")
    UTC=$(echo "$Time" | awk '{print$3}')   # Third field is UTC (GMT) time.
    # Exercise: modify this for different time zones.

    echo "UTC Time = "$UTC""

Downloading a URL:

    bash$ exec 5<>/dev/tcp/www.net.cn/80
    bash$ echo -e "GET / HTTP/1.0\n" >&5
    bash$ cat <&5

[Thanks, Mark and Mihai Maties.]

##### Using `/dev/tcp` for troubleshooting

    &devtcp;

##### Playing music

    &musicscr;

#### `/proc`

The `/proc` directory is actually a pseudo-filesystem. The files in `/proc` mirror currently running system and kernel processes and contain information and statistics about them.

    bash$ cat /proc/devices
    Character devices:
       1 mem
       2 pty
       3 ttyp
       4 ttyS
       5 cua
       7 vcs
      10 misc
      14 sound
      29 fb
      36 netlink
     128 ptm
     136 pts
     162 raw
     254 pcmcia

     Block devices:
       1 ramdisk
       2 fd
       3 ide0
       9 md

    bash$ cat /proc/interrupts
               CPU0
       0:      84505          XT-PIC  timer
       1:       3375          XT-PIC  keyboard
       2:          0          XT-PIC  cascade
       5:          1          XT-PIC  soundblaster
       8:          1          XT-PIC  rtc
      12:       4231          XT-PIC  PS/2 Mouse
      14:     109373          XT-PIC  ide0
     NMI:          0
     ERR:          0

    bash$ cat /proc/partitions
    major minor  #blocks  name     rio rmerge rsect ruse wio wmerge wsect wuse running use aveq

        3     0    3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030
        3     1      52416 hda1 27 395 844 960 4 2 14 180 0 800 1140
        3     2          1 hda2 0 0 0 0 0 0 0 0 0 0 0
        3     4     165280 hda4 10 0 20 210 0 0 0 0 0 210 210
        ...

    bash$ cat /proc/loadavg
    0.13 0.42 0.27 2/44 1119

    bash$ cat /proc/apm
    1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ?

    bash$ cat /proc/acpi/battery/BAT0/info
    present:                 yes
     design capacity:         43200 mWh
     last full capacity:      36640 mWh
     battery technology:      rechargeable
     design voltage:          10800 mV
     design capacity warning: 1832 mWh
     design capacity low:     200 mWh
     capacity granularity 1:  1 mWh
     capacity granularity 2:  1 mWh
     model number:            IBM-02K6897
     serial number:            1133
     battery type:            LION
     OEM info:                Panasonic

    bash$ fgrep Mem /proc/meminfo
    MemTotal:       515216 kB
     MemFree:        266248 kB

Shell scripts may extract data from certain of the files in `/proc`.

Certain system commands, such as procinfo, free, vmstat, lsdev, and uptime do this as well.

    FS=iso                       # ISO filesystem support in kernel?

    grep $FS /proc/filesystems   # iso9660

    kernel_version=$( awk '{ print $3 }' /proc/version )

    CPU=$( awk '/model name/ {print $5}' < /proc/cpuinfo )

    if [ "$CPU" = "Pentium(R)" ]
    then
      run_some_commands
      ...
    else
      run_other_commands
      ...
    fi

    cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' )
    #  Current operating speed (in MHz) of the cpu on your machine.
    #  On a laptop this may vary, depending on use of battery
    #+ or AC power.

    #!/bin/bash
    # get-commandline.sh
    # Get the command-line parameters of a process.

    OPTION=cmdline

    # Identify PID.
    pid=$( echo $(pidof "$1") | awk '{ print $1 }' )
    # Get only first            ^^^^^^^^^^^^^^^^^^ of multiple instances.

    echo
    echo "Process ID of (first instance of) "$1" = $pid"
    echo -n "Command-line arguments: "
    cat /proc/"$pid"/"$OPTION" | xargs -0 echo
    #   Formats output:        ^^^^^^^^^^^^^^^
    #   (Thanks, Han Holl, for the fixup!)

    echo; echo

    # For example:
    # sh get-commandline.sh xterm

+

    devfile="/proc/bus/usb/devices"
    text="Spd"
    USB1="Spd=12"
    USB2="Spd=480"

    bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}')
    #                 ^^^^ Stop after first match.

    if [ "$bus_speed" = "$USB1" ]
    then
      echo "USB 1.1 port found."
      # Do something appropriate for USB 1.1.
    fi

It is even possible to control certain peripherals with commands sent to the `/proc` directory.

    	  root# echo on > /proc/acpi/ibm/light

This turns on the Thinklight in certain models of IBM/Lenovo Thinkpads. (May not work on all Linux distros.)

Of course, caution is advised when writing to `/proc`.

The `/proc` directory contains subdirectories with unusual numerical names. Every one of these names maps to the process ID of a currently running process. Within each of these subdirectories, there are a number of files that hold useful information about the corresponding process. The `stat` and `status` files keep running statistics on the process, the `cmdline` file holds the command-line arguments the process was invoked with, and the `exe` file is a symbolic link to the complete path name of the invoking process. There are a few more such files, but these seem to be the most interesting from a scripting standpoint.

##### Finding the process associated with a PID

    &pidid;

##### On-line connect status

    &constat;

In general, it is dangerous to write to the files in `/proc`, as this can corrupt the filesystem or crash the machine.

### Network Programming

The Net's a cross between an elephant and a white elephant sale: it never forgets, and it's always crap.

--Nemo

A Linux system has quite a number of tools for accessing, manipulating, and troubleshooting network connections. We can incorporate some of these tools into scripts -- scripts that expand our knowledge of networking, useful scripts that can facilitate the administration of a network.

Here is a simple CGI script that demonstrates connecting to a remote server.

#### Print the server environment

    &testcgi;

For security purposes, it may be helpful to identify the IP addresses a computer is accessing.

#### IP addresses

    &ipaddresses;

More examples of network programming:

1. Getting the time from nist.gov

2. Downloading a URL

3. A GRE tunnel

4. Checking if an Internet server is up

See also the networking commands in the System and Administrative Commands chapter and the communications commands in the External Filters, Programs and Commands chapter.

### Of Zeros and Nulls

Faultily faultless, icily regular, splendidly null

Dead perfection; no more.

--Alfred Lord Tennyson

#### `/dev/zero` ... `/dev/null`

Uses of `/dev/null`

  Think of `/dev/null` as a black hole. It is essentially the equivalent of a write-only file. Everything written to it disappears. Attempts to read or output from it result in nothing. All the same, `/dev/null` can be quite useful from both the command-line and in scripts.

  Suppressing `stdout`.

      cat $filename >/dev/null
      # Contents of the file will not list to stdout.

  Suppressing `stderr` (from ).

      rm $badname 2>/dev/null
      #           So error messages [stderr] deep-sixed.

  Suppressing output from both `stdout` and `stderr`.

      cat $filename 2>/dev/null >/dev/null
      # If "$filename" does not exist, there will be no error message output.
      # If "$filename" does exist, the contents of the file will not list to stdout.
      # Therefore, no output at all will result from the above line of code.
      #
      #  This can be useful in situations where the return code from a command
      #+ needs to be tested, but no output is desired.
      #
      # cat $filename &>/dev/null
      #     also works, as Baris Cicek points out.

  Deleting contents of a file, but preserving the file itself, with all attendant permissions (from and ):

      cat /dev/null > /var/log/messages
      #  : > /var/log/messages   has same effect, but does not spawn a new process.

      cat /dev/null > /var/log/wtmp

  Automatically emptying the contents of a logfile (especially good for dealing with those nasty cookies sent by commercial Web sites):

  #### Hiding the cookie jar

      # Obsolete Netscape browser.
      # Same principle applies to newer browsers.

      if [ -f ~/.netscape/cookies ]  # Remove, if exists.
      then
        rm -f ~/.netscape/cookies
      fi

      ln -s /dev/null ~/.netscape/cookies
      # All cookies now get sent to a black hole, rather than saved to disk.

Uses of `/dev/zero`

  Like `/dev/null`, `/dev/zero` is a pseudo-device file, but it actually produces a stream of nulls (binary zeros, not the ASCII kind). Output written to `/dev/zero` disappears, and it is fairly difficult to actually read the nulls emitted there, though it can be done with od or a hex editor. The chief use of `/dev/zero` is creating an initialized dummy file of predetermined length intended as a temporary swap file.

  #### Setting up a swapfile using `/dev/zero`

      &ex73;

  Another application of `/dev/zero` is to zero out a file of a designated size for a special purpose, such as mounting a filesystem on a loopback device (see ) or securely deleting a file (see ).

  #### Creating a ramdisk

      &ramdisk;

  In addition to all the above, `/dev/zero` is needed by ELF (Executable and Linking Format) UNIX/Linux binaries.

### Debugging

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.

--Brian Kernighan

The Bash shell contains no built-in debugger, and only bare-bones debugging-specific commands and constructs. Syntax errors or outright typos in the script generate cryptic error messages that are often of no help in debugging a non-functional script.

#### A buggy script

    &ex74;

Output from script:

    ./ex74.sh: [37: command not found

What's wrong with the above script? Hint: after the if.

#### Missing keyword

    &missingkeyword;

Output from script:

    missing-keyword.sh: line 10: syntax error: unexpected end of file

Note that the error message does not necessarily reference the line in which the error occurs, but the line where the Bash interpreter finally becomes aware of the error.

Error messages may disregard comment lines in a script when reporting the line number of a syntax error.

What if the script executes, but does not work as expected? This is the all too familiar logic error.

#### test24: another buggy script

    &ex75;

Try to find out what's wrong with by uncommenting the `echo "$badname"` line. Echo statements are useful for seeing whether what you expect is actually what you get.

In this particular case, `rm "$badname"` will not give the desired results because `$badname` should not be quoted. Placing it in quotes ensures that `rm` has only one argument (it will match only one filename). A partial fix is to remove to quotes from `$badname` and to reset `$IFS` to contain only a newline, `IFS=$'\n'`. However, there are simpler ways of going about it.

    # Correct methods of deleting filenames containing spaces.
    rm *\ *
    rm *" "*
    rm *' '*
    # Thank you. S.C.

Summarizing the symptoms of a buggy script,

1. It bombs with a syntax error message, or

2. It runs, but does not work as expected (logic error).

3. It runs, works as expected, but has nasty side effects (logic bomb).

Tools for debugging non-working scripts include

1. Inserting echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on.

   Even better is an `echo` that echoes only when debug is on.

       ### debecho (debug-echo), by Stefano Falsetto ###
       ### Will echo passed parameters only if DEBUG is set to a value. ###
       debecho () {
         if [ ! -z "$DEBUG" ]; then
            echo "$1" >&2
            #         ^^^ to stderr
         fi
       }

       DEBUG=on
       Whatever=whatnot
       debecho $Whatever   # whatnot

       DEBUG=
       Whatever=notwhat
       debecho $Whatever   # (Will not echo.)

2. Using the tee filter to check processes or data flows at critical points.

3. Setting option flags `-n -v -x`

   `sh -n scriptname` checks for syntax errors without actually running the script. This is the equivalent of inserting `set -n` or `set -o noexec` into the script. Note that certain types of syntax errors can slip past this check.

   `sh -v scriptname` echoes each command before executing it. This is the equivalent of inserting `set -v` or `set -o verbose` in the script.

   The `-n` and `-v` flags work well together. `sh -nv scriptname` gives a verbose syntax check.

   `sh -x scriptname` echoes the result each command, but in an abbreviated manner. This is the equivalent of inserting `set -x` or `set -o xtrace` in the script.

   Inserting `set -u` or `set -o nounset` in the script runs it, but gives an unbound variable error message and aborts the script.

       set -u   # Or   set -o nounset

       # Setting a variable to null will not trigger the error/abort.
       # unset_var=

       echo $unset_var   # Unset (and undeclared) variable.

       echo "Should not echo!"

       # sh t2.sh
       # t2.sh: line 6: unset_var: unbound variable

4. Using an assert function to test a variable or condition at critical points in a script. (This is an idea borrowed from C.)

   #### Testing a condition with an assert

       &assert;

5. Using the $LINENO variable and the caller builtin.

6. Trapping at exit.

   The exit command in a script triggers a signal 0, terminating the process, that is, the script itself.

   By convention, signal 0 is assigned to exit.

   It is often useful to trap the exit, forcing a printout of variables, for example. The trap must be the first command in the script.

#### Trapping signals

`trap`

  Specifies an action on receipt of a signal; also useful for debugging.

  A signal is a message sent to a process, either by the kernel or another process, telling it to take some specified action (usually to terminate). For example, hitting a Control-C sends a user interrupt, an INT signal, to a running program.

  A simple instance:

      trap '' 2
      # Ignore interrupt 2 (Control-C), with no action specified.

      trap 'echo "Control-C disabled."' 2
      # Message when Control-C pressed.

#### Trapping at exit

    &ex76;

#### Cleaning up after Control-C

    &online;

#### A Simple Implementation of a Progress Bar

    &progressbar2;

The `DEBUG` argument to `trap` causes a specified action to execute after every command in a script. This permits tracing variables, for example.

#### Tracing a variable

    &vartrace;

Of course, the `trap` command has other uses aside from debugging, such as disabling certain keystrokes within a script (see ).

#### Running multiple processes (on an SMP box)

    &multipleproc;

`trap '' SIGNAL` (two adjacent apostrophes) disables SIGNAL for the remainder of the script. `trap SIGNAL` restores the functioning of SIGNAL once more. This is useful to protect a critical portion of a script from an undesirable interrupt.

    	trap '' 2  # Signal 2 is Control-C, now disabled.
    	command
    	command
    	command
    	trap 2     # Reenables Control-C

Version 3 of Bash adds the following internal variables for use by the debugger.

1. `$BASH_ARGC`

   Number of command-line arguments passed to script, similar to `$#`.

2. `$BASH_ARGV`

   Final command-line parameter passed to script, equivalent `${!#}`.

3. `$BASH_COMMAND`

   Command currently executing.

4. `$BASH_EXECUTION_STRING`

   The option string following the `-c` option to Bash.

5. `$BASH_LINENO`

   In a function, indicates the line number of the function call.

6. `$BASH_REMATCH`

   Array variable associated with `=~` conditional regex matching.

7. `$BASH_SOURCE`

   This is the name of the script, usually the same as $0.

8. `$BASH_SUBSHELL`

### Options

Options are settings that change shell and/or script behavior.

The set command enables options within a script. At the point in the script where you want the options to take effect, use `set -o option-name` or, in short form, `set -option-abbrev`. These two forms are equivalent.

          #!/bin/bash

          set -o verbose
          # Echoes all commands before executing.

          #!/bin/bash

          set -v
          # Exact same effect as above.

To disable an option within a script, use `set +o option-name` or `set +option-abbrev`.

          #!/bin/bash

          set -o verbose
          # Command echoing on.
          command
          ...
          command

          set +o verbose
          # Command echoing off.
          command
          # Not echoed.

          set -v
          # Command echoing on.
          command
          ...
          command

          set +v
          # Command echoing off.
          command

          exit 0

An alternate method of enabling options in a script is to specify them immediately following the #! script header.

          #!/bin/bash -x
          #
          # Body of script follows.

It is also possible to enable script options from the command line. Some options that will not work with `set` are available this way. Among these are -i, force script to run interactive.

`bash -v script-name`

`bash -o verbose script-name`

The following is a listing of some useful options. They may be specified in either abbreviated form (preceded by a single dash) or by complete name (preceded by a double dash or by `-o`).

#### Bash options

Abbreviation | Name | Effect
--- | --- | ---
`-B` | brace expansion | Enable brace expansion (default setting = on)
`+B` | brace expansion | Disable brace expansion
`-C` | noclobber | Prevent overwriting of files by redirection (may be overridden by >|)
`-D` | (none) | List double-quoted strings prefixed by $, but do not execute commands in script
`-a` | allexport | Export all defined variables
`-b` | notify | Notify when jobs running in background terminate (not of much use in a script)
`-c ...` | (none) | Read commands from `...`
`checkjobs` |  | Informs user of any open jobs upon shell exit. Introduced in version 4 of Bash, and still experimental. Usage: shopt -s checkjobs (Caution: may hang!)
`-e` | errexit | Abort script at first error, when a command exits with non-zero status (except in until or while loops, if-tests, list constructs)
`-f` | noglob | Filename expansion (globbing) disabled
`globstar` | globbing star-match | Enables the ** globbing operator (version 4+ of Bash). Usage: shopt -s globstar
`-i` | interactive | Script runs in interactive mode
`-n` | noexec | Read commands in script, but do not execute them (syntax check)
`-o Option-Name` | (none) | Invoke the Option-Name option
`-o posix` | POSIX | Change the behavior of Bash, or invoked script, to conform to POSIX standard.
`-o pipefail` | pipe failure | Causes a pipeline to return the exit status of the last command in the pipe that returned a non-zero return value.
`-p` | privileged | Script runs as suid (caution!)
`-r` | restricted | Script runs in restricted mode (see ).
`-s` | stdin | Read commands from `stdin`
`-t` | (none) | Exit after first command
`-u` | nounset | Attempt to use undefined variable outputs error message, and forces an exit
`-v` | verbose | Print each command to `stdout` before executing it
`-x` | xtrace | Similar to `-v`, but expands commands
`-` | (none) | End of options flag. All other arguments are positional parameters.
`--` | (none) | Unset positional parameters. If arguments given (`-- arg1 arg2`), positional parameters set to arguments.

### Gotchas

Turandot: Gli enigmi sono tre, la morte una!

Caleph: No, no! Gli enigmi sono tre, una la vita!

--Puccini

Here are some (non-recommended!) scripting practices that will bring excitement into an otherwise dull life.

- Assigning reserved words or characters to variable names.

      case=value0       # Causes problems.
      23skidoo=value1   # Also problems.
      # Variable names starting with a digit are reserved by the shell.
      # Try _23skidoo=value1. Starting variables with an underscore is okay.

      # However . . .   using just an underscore will not work.
      _=25
      echo $_           # $_ is a special variable set to last arg of last command.
      # But . . .       _ is a valid function name!

      xyz((!*=value2    # Causes severe problems.
      # As of version 3 of Bash, periods are not allowed within variable names.

- Using a hyphen or other reserved characters in a variable name (or function name).

      var-1=23
      # Use 'var_1' instead.

      function-whatever ()   # Error
      # Use 'function_whatever ()' instead.

      # As of version 3 of Bash, periods are not allowed within function names.
      function.whatever ()   # Error
      # Use 'functionWhatever ()' instead.

- Using the same name for a variable and a function. This can make a script difficult to understand.

      do_something ()
      {
        echo "This function does something with \"$1\"."
      }

      do_something=do_something

      do_something do_something

      # All this is legal, but highly confusing.

- Using whitespace inappropriately. In contrast to other programming languages, Bash can be quite finicky about whitespace.

      var1 = 23   # 'var1=23' is correct.
      # On line above, Bash attempts to execute command "var1"
      # with the arguments "=" and "23".

      let c = $a - $b   # Instead:   let c=$a-$b   or   let "c = $a - $b"

      if [ $a -le 5]    # if [ $a -le 5 ]   is correct.
      #           ^^      if [ "$a" -le 5 ]   is even better.
                        # [[ $a -le 5 ]] also works.

- Not terminating with a semicolon the final command in a code block within curly brackets.

      { ls -l; df; echo "Done." }
      # bash: syntax error: unexpected end of file

      { ls -l; df; echo "Done."; }
      #                        ^     ### Final command needs semicolon.

- Assuming uninitialized variables (variables before a value is assigned to them) are zeroed out. An uninitialized variable has a value of null, not zero.

      #!/bin/bash

      echo "uninitialized_var = $uninitialized_var"
      # uninitialized_var =

      # However . . .
      # if $BASH_VERSION ≥ 4.2; then

      if [[ ! -v uninitialized_var ]]
      then
        uninitialized_var=0   # Initialize it to zero!
      fi

- Mixing up = and -eq in a test. Remember, = is for comparing literal variables and -eq for integers.

      if [ "$a" = 273 ]      # Is $a an integer or string?
      if [ "$a" -eq 273 ]    # If $a is an integer.

      # Sometimes you can interchange -eq and = without adverse consequences.
      # However . . .

      a=273.0   # Not an integer.

      if [ "$a" = 273 ]
      then
        echo "Comparison works."
      else
        echo "Comparison does not work."
      fi    # Comparison does not work.

      # Same with   a=" 273"  and a="0273".

      # Likewise, problems trying to use "-eq" with non-integer values.

      if [ "$a" -eq 273.0 ]
      then
        echo "a = $a"
      fi  # Aborts with an error message.
      # test.sh: [: 273.0: integer expression expected

- Misusing string comparison operators.

  #### Numerical and string comparison are not equivalent

      &badop;

- Attempting to use let to set string variables.

      let "a = hello, you"
      echo "$a"   # 0

- Sometimes variables within test brackets ([ ]) need to be quoted (double quotes). Failure to do so may cause unexpected behavior. See , , and .

- Quoting a variable containing whitespace prevents splitting. Sometimes this produces unintended consequences.

- Commands issued from a script may fail to execute because the script owner lacks execute permission for them. If a user cannot invoke a command from the command-line, then putting it into a script will likewise fail. Try changing the attributes of the command in question, perhaps even setting the suid bit (as root, of course).

- Attempting to use `-` as a redirection operator (which it is not) will usually result in an unpleasant surprise.

      command1 2> - | command2
      # Trying to redirect error output of command1 into a pipe . . .
      # . . . will not work.

      command1 2>& - | command2  # Also futile.

      Thanks, S.C.

- Using Bash version 2+ functionality may cause a bailout with error messages. Older Linux machines may have version 1.XX of Bash as the default installation.

      #!/bin/bash

      minimum_version=2
      # Since Chet Ramey is constantly adding features to Bash,
      # you may set $minimum_version to 2.XX, 3.XX, or whatever is appropriate.
      E_BAD_VERSION=80

      if [ "$BASH_VERSION" \< "$minimum_version" ]
      then
        echo "This script works only with Bash, version $minimum or greater."
        echo "Upgrade strongly recommended."
        exit $E_BAD_VERSION
      fi

      ...

- Using Bash-specific functionality in a Bourne shell script (`#!/bin/sh`) on a non-Linux machine may cause unexpected behavior. A Linux system usually aliases `sh` to `bash`, but this does not necessarily hold true for a generic UNIX machine.

- Using undocumented features in Bash turns out to be a dangerous practice. In previous releases of this book there were several scripts that depended on the feature that, although the maximum value of an exit or return value was 255, that limit did not apply to negative integers. Unfortunately, in version 2.05b and later, that loophole disappeared. See .

- In certain contexts, a misleading exit status may be returned. This may occur when setting a local variable within a function or when assigning an arithmetic value to a variable.

- The exit status of an arithmetic expression is not equivalent to an error code.

      var=1 && ((--var)) && echo $var
      #        ^^^^^^^^^ Here the and-list terminates with exit status 1.
      #                     $var doesn't echo!
      echo $?   # 1

- A script with DOS-type newlines (\r\n) will fail to execute, since `#!/bin/bash\r\n` is not recognized, not the same as the expected `#!/bin/bash\n`. The fix is to convert the script to UNIX-style newlines.

      #!/bin/bash

      echo "Here"

      unix2dos $0    # Script changes itself to DOS format.
      chmod 755 $0   # Change back to execute permission.
                     # The 'unix2dos' command removes execute permission.

      ./$0           # Script tries to run itself again.
                     # But it won't work as a DOS file.

      echo "There"

      exit 0

- A shell script headed by `#!/bin/sh` will not run in full Bash-compatibility mode. Some Bash-specific functions might be disabled. Scripts that need complete access to all the Bash-specific extensions should start with `#!/bin/bash`.

- Putting whitespace in front of the terminating limit string of a here document will cause unexpected behavior in a script.

- Putting more than one echo statement in a function whose output is captured.

      add2 ()
      {
        echo "Whatever ... "   # Delete this line!
        let "retval = $1 + $2"
          echo $retval
          }

          num1=12
          num2=43
          echo "Sum of $num1 and $num2 = $(add2 $num1 $num2)"

      #   Sum of 12 and 43 = Whatever ...
      #   55

      #        The "echoes" concatenate.

  This will not work.

- A script may not `export` variables back to its parent process, the shell, or to the environment. Just as we learned in biology, a child process can inherit from a parent, but not vice versa.

      WHATEVER=/home/bozo
      export WHATEVER
      exit 0

      bash$ echo $WHATEVER

      bash$

  Sure enough, back at the command prompt, $WHATEVER remains unset.

- Setting and manipulating variables in a subshell, then attempting to use those same variables outside the scope of the subshell will result an unpleasant surprise.

  #### Subshell Pitfalls

      &subpit;

- Piping `echo` output to a read may produce unexpected results. In this scenario, the `read` acts as if it were running in a subshell. Instead, use the set command (as in ).

  #### Piping the output of echo to a read

      &badread;

  In fact, as Anthony Richardson points out, piping to any loop can cause a similar problem.

      # Loop piping troubles.
      #  This example by Anthony Richardson,
      #+ with addendum by Wilbert Berendsen.

      foundone=false
      find $HOME -type f -atime +30 -size 100k |
      while true
      do
         read f
         echo "$f is over 100KB and has not been accessed in over 30 days"
         echo "Consider moving the file to archives."
         foundone=true
         # ------------------------------------
           echo "Subshell level = $BASH_SUBSHELL"
         # Subshell level = 1
         # Yes, we're inside a subshell.
         # ------------------------------------
      done

      #  foundone will always be false here since it is
      #+ set to true inside a subshell
      if [ $foundone = false ]
      then
         echo "No files need archiving."
      fi

      # =====================Now, here is the correct way:=================

      foundone=false
      for f in $(find $HOME -type f -atime +30 -size 100k)  # No pipe here.
      do
         echo "$f is over 100KB and has not been accessed in over 30 days"
         echo "Consider moving the file to archives."
         foundone=true
      done

      if [ $foundone = false ]
      then
         echo "No files need archiving."
      fi

      # ==================And here is another alternative==================

      #  Places the part of the script that reads the variables
      #+ within a code block, so they share the same subshell.
      #  Thank you, W.B.

      find $HOME -type f -atime +30 -size 100k | {
           foundone=false
           while read f
           do
             echo "$f is over 100KB and has not been accessed in over 30 days"
             echo "Consider moving the file to archives."
             foundone=true
           done

           if ! $foundone
           then
             echo "No files need archiving."
           fi
      }

  A lookalike problem occurs when trying to write the `stdout` of a `tail -f` piped to grep.

      tail -f /var/log/messages | grep "$ERROR_MSG" >> error.log
      #  The "error.log" file will not have anything written to it.
      #  As Samuli Kaipiainen points out, this results from grep
      #+ buffering its output.
      #  The fix is to add the "--line-buffered" parameter to grep.

- Using suid commands within scripts is risky, as it may compromise system security. Setting the suid permission on the script itself has no effect in Linux and most other UNIX flavors.

- Using shell scripts for CGI programming may be problematic. Shell script variables are not typesafe, and this can cause undesirable behavior as far as CGI is concerned. Moreover, it is difficult to cracker-proof shell scripts.

- Bash does not handle the double slash (//) string correctly.

- Bash scripts written for Linux or BSD systems may need fixups to run on a commercial UNIX machine. Such scripts often employ the GNU set of commands and filters, which have greater functionality than their generic UNIX counterparts. This is particularly true of such text processing utilites as tr.

- Sadly, updates to Bash itself have broken older scripts that used to work perfectly fine. Let us recall how risky it is to use undocumented Bash features.

Danger is near thee --

Beware, beware, beware, beware.

Many brave hearts are asleep in the deep.

So beware --

Beware.

--A.J. Lamb and H.W. Petrie

### Scripting With Style

Get into the habit of writing shell scripts in a structured and systematic manner. Even on-the-fly and written on the back of an envelope scripts will benefit if you take a few minutes to plan and organize your thoughts before sitting down and coding.

Herewith are a few stylistic guidelines. This is not (necessarily) intended as an Official Shell Scripting Stylesheet.

#### Unofficial Shell Scripting Stylesheet

- Comment your code. This makes it easier for others to understand (and appreciate), and easier for you to maintain.

      PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"
      #  It made perfect sense when you wrote it last year,
      #+ but now it's a complete mystery.
      #  (From Antek Sawicki's "pw.sh" script.)

  Add descriptive headers to your scripts and functions.

      #!/bin/bash

      #************************************************#
      #                   xyz.sh                       #
      #           written by Bozo Bozeman              #
      #                July 05, 2001                   #
      #                                                #
      #           Clean up project files.              #
      #************************************************#

      E_BADDIR=85                       # No such directory.
      projectdir=/home/bozo/projects    # Directory to clean up.

      # --------------------------------------------------------- #
      # cleanup_pfiles ()                                         #
      # Removes all files in designated directory.                #
      # Parameter: $target_directory                              #
      # Returns: 0 on success, $E_BADDIR if something went wrong. #
      # --------------------------------------------------------- #
      cleanup_pfiles ()
      {
        if [ ! -d "$1" ]  # Test if target directory exists.
        then
          echo "$1 is not a directory."
          return $E_BADDIR
        fi

        rm -f "$1"/*
        return 0   # Success.
      }

      cleanup_pfiles $projectdir

      exit $?

- Avoid using magic numbers,

  In this context, magic numbers have an entirely different meaning than the magic numbers used to designate file types.

  that is, hard-wired literal constants. Use meaningful variable names instead. This makes the script easier to understand and permits making changes and updates without breaking the application.

      if [ -f /var/log/messages ]
      then
        ...
      fi
      #  A year later, you decide to change the script to check /var/log/syslog.
      #  It is now necessary to manually change the script, instance by instance,
      #+ and hope nothing breaks.

      # A better way:
      LOGFILE=/var/log/messages  # Only line that needs to be changed.
      if [ -f "$LOGFILE" ]
      then
        ...
      fi

- Choose descriptive names for variables and functions.

      fl=`ls -al $dirname`                 # Cryptic.
      file_listing=`ls -al $dirname`       # Better.

      MAXVAL=10   # All caps used for a script constant.
      while [ "$index" -le "$MAXVAL" ]
      ...

      E_NOTFOUND=95                        #  Uppercase for an errorcode,
                                           #+ and name prefixed with E_.
      if [ ! -e "$filename" ]
      then
        echo "File $filename not found."
        exit $E_NOTFOUND
      fi

      MAIL_DIRECTORY=/var/spool/mail/bozo  #  Uppercase for an environmental
      export MAIL_DIRECTORY                #+ variable.

      GetAnswer ()                         #  Mixed case works well for a
      {                                    #+ function name, especially
        prompt=$1                          #+ when it improves legibility.
        echo -n $prompt
        read answer
        return $answer
      }

      GetAnswer "What is your favorite number? "
      favorite_number=$?
      echo $favorite_number

      _uservariable=23                     # Permissible, but not recommended.
      # It's better for user-defined variables not to start with an underscore.
      # Leave that for system variables.

- Use exit codes in a systematic and meaningful way.

      E_WRONG_ARGS=95
      ...
      ...
      exit $E_WRONG_ARGS

  See also .

  Ender suggests using the exit codes in `/usr/include/sysexits.h` in shell scripts, though these are primarily intended for C and C++ programming.

- Use standardized parameter flags for script invocation. Ender proposes the following set of flags.

      -a      All: Return all information (including hidden file info).
      -b      Brief: Short version, usually for other scripts.
      -c      Copy, concatenate, etc.
      -d      Daily: Use information from the whole day, and not merely
              information for a specific instance/user.
      -e      Extended/Elaborate: (often does not include hidden file info).
      -h      Help: Verbose usage w/descs, aux info, discussion, help.
              See also -V.
      -l      Log output of script.
      -m      Manual: Launch man-page for base command.
      -n      Numbers: Numerical data only.
      -r      Recursive: All files in a directory (and/or all sub-dirs).
      -s      Setup & File Maintenance: Config files for this script.
      -u      Usage: List of invocation flags for the script.
      -v      Verbose: Human readable output, more or less formatted.
      -V      Version / License / Copy(right|left) / Contribs (email too).

  See also .

- Break complex scripts into simpler modules. Use functions where appropriate. See .

- Don't use a complex construct where a simpler one will do.

      COMMAND
      if [ $? -eq 0 ]
      ...
      # Redundant and non-intuitive.

      if COMMAND
      ...
      # More concise (if perhaps not quite as legible).

... reading the UNIX source code to the Bourne shell (/bin/sh). I was shocked at how much simple algorithms could be made cryptic, and therefore useless, by a poor choice of code style. I asked myself, Could someone be proud of this code?

--Landon Noll

### Miscellany

Nobody really knows what the Bourne shell's grammar is. Even examination of the source code is little help.

--Tom Duff

#### Interactive and non-interactive shells and scripts

An interactive shell reads commands from user input on a `tty`. Among other things, such a shell reads startup files on activation, displays a prompt, and enables job control by default. The user can interact with the shell.

A shell running a script is always a non-interactive shell. All the same, the script can still access its `tty`. It is even possible to emulate an interactive shell in a script.

    #!/bin/bash
    MY_PROMPT='$ '
    while :
    do
      echo -n "$MY_PROMPT"
      read line
      eval "$line"
      done

    exit 0

    # This example script, and much of the above explanation supplied by
    # Stéphane Chazelas (thanks again).

Let us consider an interactive script to be one that requires input from the user, usually with read statements (see ). Real life is actually a bit messier than that. For now, assume an interactive script is bound to a tty, a script that a user has invoked from the console or an xterm.

Init and startup scripts are necessarily non-interactive, since they must run without human intervention. Many administrative and system maintenance scripts are likewise non-interactive. Unvarying repetitive tasks cry out for automation by non-interactive scripts.

Non-interactive scripts can run in the background, but interactive ones hang, waiting for input that never comes. Handle that difficulty by having an `expect` script or embedded here document feed input to an interactive script running as a background job. In the simplest case, redirect a file to supply input to a `read` statement (`read variable <file`). These particular workarounds make possible general purpose scripts that run in either interactive or non-interactive modes.

If a script needs to test whether it is running in an interactive shell, it is simply a matter of finding whether the prompt variable, $PS1 is set. (If the user is being prompted for input, then the script needs to display a prompt.)

    if [ -z $PS1 ] # no prompt?
    ### if [ -v PS1 ]   # On Bash 4.2+ ...
    then
      # non-interactive
      ...
    else
      # interactive
      ...
    fi

Alternatively, the script can test for the presence of option i in the $- flag.

    case $- in
    *i*)    # interactive shell
    ;;
    *)      # non-interactive shell
    ;;
    # (Courtesy of "UNIX F.A.Q.," 1993)

However, John Lange describes an alternative method, using the -t test operator.

    # Test for a terminal!

    fd=0   # stdin

    #  As we recall, the -t test option checks whether the stdin, [ -t 0 ],
    #+ or stdout, [ -t 1 ], in a given script is running in a terminal.
    if [ -t "$fd" ]
    then
      echo interactive
    else
      echo non-interactive
    fi

    #  But, as John points out:
    #    if [ -t 0 ] works ... when you're logged in locally
    #    but fails when you invoke the command remotely via ssh.
    #    So for a true test you also have to test for a socket.

    if [[ -t "$fd" || -p /dev/stdin ]]
    then
      echo interactive
    else
      echo non-interactive
    fi

Scripts may be forced to run in interactive mode with the -i option or with a `#!/bin/bash -i` header. Be aware that this can cause erratic script behavior or show error messages even when no error is present.

#### Shell Wrappers

A wrapper is a shell script that embeds a system command or utility, that accepts and passes a set of parameters to that command.

Quite a number of Linux utilities are, in fact, shell wrappers. Some examples are `/usr/bin/pdf2ps`, `/usr/bin/batch`, and `/usr/bin/xmkmf`.

Wrapping a script around a complex command-line simplifies invoking it. This is expecially useful with sed and awk.

A `
sed` or `
awk` script would normally be invoked from the command-line by a `sed -e 'commands'` or `awk 'commands'`. Embedding such a script in a Bash script permits calling it more simply, and makes it reusable. This also enables combining the functionality of sed and awk, for example piping the output of a set of sed commands to awk. As a saved executable file, you can then repeatedly invoke it in its original form or modified, without the inconvenience of retyping it on the command-line.

##### shell wrapper

    &ex3;

##### A slightly more complex shell wrapper

    &ex4;

##### A generic shell wrapper that writes to a logfile

    &loggingwrapper;

##### A shell wrapper around an awk script

    &prasc;

##### A shell wrapper around another awk script

    &coltotaler;

For those scripts needing a single do-it-all tool, a Swiss army knife, there is Perl. Perl combines the capabilities of sed and awk, and throws in a large subset of `C`, to boot. It is modular and contains support for everything ranging from object-oriented programming up to and including the kitchen sink. Short Perl scripts lend themselves to embedding within shell scripts, and there may be some substance to the claim that Perl can totally replace shell scripting (though the author of the ABS Guide remains skeptical).

##### Perl embedded in a Bash script

    &ex56;

It is even possible to combine a Bash script and Perl script within the same file. Depending on how the script is invoked, either the Bash part or the Perl part will execute.

##### Bash and Perl scripts combined

    &bashandperl;

    bash$ bash bashandperl.sh
    Greetings from the Bash part of the script.

    bash$ perl -x bashandperl.sh
    Greetings from the Perl part of the script.

It is, of course, possible to embed even more exotic scripting languages within shell wrappers. Python, for example ...

##### Python embedded in a Bash script

    &ex56py;

Wrapping a script around mplayer and the Google's translation server, you can create something that talks back to you.

##### A script that speaks

    &speech0;

One interesting example of a complex shell wrapper is Martin Matusiak's undvd script, which provides an easy-to-use command-line interface to the complex mencoder utility. Another example is Itzchak Rehberg's Ext3Undel, a set of scripts to recover deleted file on an ext3 filesystem.

#### Tests and Comparisons: Alternatives

For tests, the [[ ]] construct may be more appropriate than `[ ]`. Likewise, arithmetic comparisons might benefit from the (( )) construct.

    a=8

    # All of the comparisons below are equivalent.
    test "$a" -lt 16 && echo "yes, $a < 16"         # "and list"
    /bin/test "$a" -lt 16 && echo "yes, $a < 16"
    [ "$a" -lt 16 ] && echo "yes, $a < 16"
    [[ $a -lt 16 ]] && echo "yes, $a < 16"          # Quoting variables within
    (( a < 16 )) && echo "yes, $a < 16"             # [[ ]] and (( )) not necessary.

    city="New York"
    # Again, all of the comparisons below are equivalent.
    test "$city" \< Paris && echo "Yes, Paris is greater than $city"
                                      # Greater ASCII order.
    /bin/test "$city" \< Paris && echo "Yes, Paris is greater than $city"
    [ "$city" \< Paris ] && echo "Yes, Paris is greater than $city"
    [[ $city < Paris ]] && echo "Yes, Paris is greater than $city"
                                      # Need not quote $city.

    # Thank you, S.C.

#### Recursion: a script calling itself

Can a script recursively call itself? Indeed.

##### A (useless) script that recursively calls itself

    &recurse;

##### A (useful) script that recursively calls itself

    &pbook;

##### Another (useful) script that recursively calls itself

    &usrmnt;

Too many levels of recursion can exhaust the script's stack space, causing a segfault.

#### Colorizing Scripts

The ANSI ANSI is, of course, the acronym for the American National Standards Institute. This august body establishes and maintains various technical and industrial standards. escape sequences set screen attributes, such as bold text, and color of foreground and background. DOS batch files commonly used ANSI escape codes for color output, and so can Bash scripts.

##### A colorized address database

    &ex30a;

##### Drawing a box

    &drawbox;

The simplest, and perhaps most useful ANSI escape sequence is bold text, `\033[1m ... \033[0m`. The \033 represents an escape, the [1 turns on the bold attribute, while the [0 switches it off. The m terminates each term of the escape sequence.

    bash$ echo -e "\033[1mThis is bold text.\033[0m"

A similar escape sequence switches on the underline attribute (on an rxvt and an aterm).

    bash$ echo -e "\033[4mThis is underlined text.\033[0m"

With an `echo`, the `-e` option enables the escape sequences.

Other escape sequences change the text and/or background color.

    bash$ echo -e '\E[34;47mThis prints in blue.'; tput sgr0

    bash$ echo -e '\E[33;44m'"yellow text on blue background"; tput sgr0

    bash$ echo -e '\E[1;33;44m'"BOLD yellow text on blue background"; tput sgr0

It's usually advisable to set the bold attribute for light-colored foreground text.

The `tput sgr0` restores the terminal settings to normal. Omitting this lets all subsequent output from that particular terminal remain blue.

Since `tput sgr0` fails to restore terminal settings under certain circumstances, `echo -ne \E[0m` may be a better choice.

Use the following template for writing colored text on a colored background.

`echo -e '\E[COLOR1;COLOR2mSome text goes here.'`

The \E[ begins the escape sequence. The semicolon-separated numbers COLOR1 and COLOR2 specify a foreground and a background color, according to the table below. (The order of the numbers does not matter, since the foreground and background numbers fall in non-overlapping ranges.) The m terminates the escape sequence, and the text begins immediately after that.

Note also that single quotes enclose the remainder of the command sequence following the `echo -e`.

The numbers in the following table work for an rxvt terminal. Results may vary for other terminal emulators.

##### Numbers representing colors in Escape Sequences

Color | Foreground | Background
--- | --- | ---
`black` | 30 | 40
`red` | 31 | 41
`green` | 32 | 42
`yellow` | 33 | 43
`blue` | 34 | 44
`magenta` | 35 | 45
`cyan` | 36 | 46
`white` | 37 | 47

##### Echoing colored text

    &colorecho;

##### A horserace game

    &horserace;

See also , , , and .

There is, however, a major problem with all this. ANSI escape sequences are emphatically non-portable. What works fine on some terminal emulators (or the console) may work differently, or not at all, on others. A colorized script that looks stunning on the script author's machine may produce unreadable output on someone else's. This somewhat compromises the usefulness of colorizing scripts, and possibly relegates this technique to the status of a gimmick. Colorized scripts are probably inappropriate in a commercial setting, i.e., your supervisor might disapprove.

Alister's ansi-color utility (based on Moshe Jacobson's color utility considerably simplifies using ANSI escape sequences. It substitutes a clean and logical syntax for the clumsy constructs just discussed.

Henry/teikedvl has likewise created a utility (http://scriptechocolor.sourceforge.net/) to simplify creation of colorized scripts.

#### Optimizations

Most shell scripts are quick 'n dirty solutions to non-complex problems. As such, optimizing them for speed is not much of an issue. Consider the case, though, where a script carries out an important task, does it well, but runs too slowly. Rewriting it in a compiled language may not be a palatable option. The simplest fix would be to rewrite the parts of the script that slow it down. Is it possible to apply principles of code optimization even to a lowly shell script?

Check the loops in the script. Time consumed by repetitive operations adds up quickly. If at all possible, remove time-consuming operations from within loops.

Use builtin commands in preference to system commands. Builtins execute faster and usually do not launch a subshell when invoked.

Avoid unnecessary commands, particularly in a pipe.

    cat "$file" | grep "$word"

    grep "$word" "$file"

    #  The above command-lines have an identical effect,
    #+ but the second runs faster since it launches one fewer subprocess.

The cat command seems especially prone to overuse in scripts.

Disabling certain Bash options can speed up scripts.

As Erik Brandsberg points out:

If you don't need Unicode support, you can get potentially a 2x or more improvement in speed by simply setting the `LC_ALL` variable.

       export LC_ALL=C

       [specifies the locale as ANSI C,
       thereby disabling Unicode support]

    [In an example script ...]

    Without [Unicode support]:
    erik@erik-desktop:~/capture$ time ./cap-ngrep.sh
    live2.pcap > out.txt

      real        0m20.483s
      user        1m34.470s
      sys         0m12.869s

    With [Unicode support]:
    erik@erik-desktop:~/capture$ time ./cap-ngrep.sh
    live2.pcap > out.txt

      real        0m50.232s
      user        3m51.118s
      sys         0m11.221s

    A large part of the overhead that is optimized is, I believe,
    regex match using [[ string =~ REGEX ]],
    but it may help with other portions of the code as well.
    I hadn't [seen it] mentioned that this optimization helped
    with Bash, but I had seen it helped with "grep,"
    so why not try?

Certain operators, notably expr, are very inefficient and might be replaced by double parentheses arithmetic expansion. See .

    Math tests

    math via $(( ))
    real          0m0.294s
    user          0m0.288s
    sys           0m0.008s

    math via expr:
    real          1m17.879s   # Much slower!
    user          0m3.600s
    sys           0m8.765s

    math via let:
    real          0m0.364s
    user          0m0.372s
    sys           0m0.000s

Condition testing constructs in scripts deserve close scrutiny. Substitute case for if-then constructs and combine tests when possible, to minimize script execution time. Again, refer to .

    Test using "case" construct:
    real          0m0.329s
    user          0m0.320s
    sys           0m0.000s

    Test with if [], no quotes:
    real          0m0.438s
    user          0m0.432s
    sys           0m0.008s

    Test with if [], quotes:
    real          0m0.476s
    user          0m0.452s
    sys           0m0.024s

    Test with if [], using -eq:
    real          0m0.457s
    user          0m0.456s
    sys           0m0.000s

Erik Brandsberg recommends using associative arrays in preference to conventional numeric-indexed arrays in most cases. When overwriting values in a numeric array, there is a significant performance penalty vs. associative arrays. Running a test script confirms this. See .

    Assignment tests

    Assigning a simple variable
    real          0m0.418s
    user          0m0.416s
    sys           0m0.004s

    Assigning a numeric index array entry
    real          0m0.582s
    user          0m0.564s
    sys           0m0.016s

    Overwriting a numeric index array entry
    real          0m21.931s
    user          0m21.913s
    sys           0m0.016s

    Linear reading of numeric index array
    real          0m0.422s
    user          0m0.416s
    sys           0m0.004s

    Assigning an associative array entry
    real          0m1.800s
    user          0m1.796s
    sys           0m0.004s

    Overwriting an associative array entry
    real          0m1.798s
    user          0m1.784s
    sys           0m0.012s

    Linear reading an associative array entry
    real          0m0.420s
    user          0m0.420s
    sys           0m0.000s

    Assigning a random number to a simple variable
    real          0m0.402s
    user          0m0.388s
    sys           0m0.016s

    Assigning a sparse numeric index array entry randomly into 64k cells
    real          0m12.678s
    user          0m12.649s
    sys           0m0.028s

    Reading sparse numeric index array entry
    real          0m0.087s
    user          0m0.084s
    sys           0m0.000s

    Assigning a sparse associative array entry randomly into 64k cells
    real          0m0.698s
    user          0m0.696s
    sys           0m0.004s

    Reading sparse associative index array entry
    real          0m0.083s
    user          0m0.084s
    sys           0m0.000s

Use the time and times tools to profile computation-intensive commands. Consider rewriting time-critical code sections in C, or even in assembler.

Try to minimize file I/O. Bash is not particularly efficient at handling files, so consider using more appropriate tools for this within the script, such as awk or Perl.

Write your scripts in a modular and coherent form, This usually means liberal use of functions. so they can be reorganized and tightened up as necessary. Some of the optimization techniques applicable to high-level languages may work for scripts, but others, such as loop unrolling, are mostly irrelevant. Above all, use common sense.

For an excellent demonstration of how optimization can dramatically reduce the execution time of a script, see .

#### Assorted Tips

##### Ideas for more powerful scripts

- You have a problem that you want to solve by writing a Bash script. Unfortunately, you don't know quite where to start. One method is to plunge right in and code those parts of the script that come easily, and write the hard parts as pseudo-code.

      #!/bin/bash

      ARGCOUNT=1                     # Need name as argument.
      E_WRONGARGS=65

      if [ number-of-arguments is-not-equal-to "$ARGCOUNT" ]
      #    ^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^
      #  Can't figure out how to code this . . .
      #+ . . . so write it in pseudo-code.

      then
        echo "Usage: name-of-script name"
        #            ^^^^^^^^^^^^^^     More pseudo-code.
        exit $E_WRONGARGS
      fi

      . . .

      exit 0

      # Later on, substitute working code for the pseudo-code.

      # Line 6 becomes:
      if [ $# -ne "$ARGCOUNT" ]

      # Line 12 becomes:
        echo "Usage: `basename $0` name"

  For an example of using pseudo-code, see the Square Root exercise.

- To keep a record of which user scripts have run during a particular session or over a number of sessions, add the following lines to each script you want to keep track of. This will keep a continuing file record of the script names and invocation times.

      # Append (>>) following to end of each script tracked.

      whoami>> $SAVE_FILE    # User invoking the script.
      echo $0>> $SAVE_FILE   # Script name.
      date>> $SAVE_FILE      # Date and time.
      echo>> $SAVE_FILE      # Blank line as separator.

      #  Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc
      #+ (something like ~/.scripts-run)

- The >> operator appends lines to a file. What if you wish to prepend a line to an existing file, that is, to paste it in at the beginning?

      file=data.txt
      title="***This is the title line of data text file***"

      echo $title | cat - $file >$file.new
      # "cat -" concatenates stdout to $file.
      #  End result is
      #+ to write a new file with $title appended at *beginning*.

  This is a simplified variant of the script given earlier. And, of course, sed can also do this.

- A shell script may act as an embedded command inside another shell script, a Tcl or wish script, or even a Makefile. It can be invoked as an external shell command in a C program using the system() call, i.e., system("script_name");.

- Setting a variable to the contents of an embedded sed or awk script increases the readability of the surrounding shell wrapper. See and .

- Put together files containing your favorite and most useful definitions and functions. As necessary, include one or more of these library files in scripts with either the dot (`.`) or source command.

      # SCRIPT LIBRARY
      # ------ -------

      # Note:
      # No "#!" here.
      # No "live code" either.

      # Useful variable definitions

      ROOT_UID=0             # Root has $UID 0.
      E_NOTROOT=101          # Not root user error.
      MAXRETVAL=255          # Maximum (positive) return value of a function.
      SUCCESS=0
      FAILURE=-1

      # Functions

      Usage ()               # "Usage:" message.
      {
        if [ -z "$1" ]       # No arg passed.
        then
          msg=filename
        else
          msg=$@
        fi

        echo "Usage: `basename $0` "$msg""
      }

      Check_if_root ()       # Check if root running script.
      {                      # From "ex39.sh" example.
        if [ "$UID" -ne "$ROOT_UID" ]
        then
          echo "Must be root to run this script."
          exit $E_NOTROOT
        fi
      }

      CreateTempfileName ()  # Creates a "unique" temp filename.
      {                      # From "ex51.sh" example.
        prefix=temp
        suffix=`eval date +%s`
        Tempfilename=$prefix.$suffix
      }

      isalpha2 ()            # Tests whether *entire string* is alphabetic.
      {                      # From "isalpha.sh" example.
        [ $# -eq 1 ] || return $FAILURE

        case $1 in
        *[!a-zA-Z]*|"") return $FAILURE;;
        *) return $SUCCESS;;
        esac                 # Thanks, S.C.
      }

      abs ()                           # Absolute value.
      {                                # Caution: Max return value = 255.
        E_ARGERR=-999999

        if [ -z "$1" ]                 # Need arg passed.
        then
          return $E_ARGERR             # Obvious error value returned.
        fi

        if [ "$1" -ge 0 ]              # If non-negative,
        then                           #
          absval=$1                    # stays as-is.
        else                           # Otherwise,
          let "absval = (( 0 - $1 ))"  # change sign.
        fi

        return $absval
      }

      tolower ()             #  Converts string(s) passed as argument(s)
      {                      #+ to lowercase.

        if [ -z "$1" ]       #  If no argument(s) passed,
        then                 #+ send error message
          echo "(null)"      #+ (C-style void-pointer error message)
          return             #+ and return from function.
        fi

        echo "$@" | tr A-Z a-z
        # Translate all passed arguments ($@).

        return

      # Use command substitution to set a variable to function output.
      # For example:
      #    oldvar="A seT of miXed-caSe LEtTerS"
      #    newvar=`tolower "$oldvar"`
      #    echo "$newvar"    # a set of mixed-case letters
      #
      # Exercise: Rewrite this function to change lowercase passed argument(s)
      #           to uppercase ... toupper()  [easy].
      }

- Use special-purpose comment headers to increase clarity and legibility in scripts.

      ## Caution.
      rm -rf *.zzy   ##  The "-rf" options to "rm" are very dangerous,
                     ##+ especially with wild cards.

      #+ Line continuation.
      #  This is line 1
      #+ of a multi-line comment,
      #+ and this is the final line.

      #* Note.

      #o List item.

      #> Another point of view.
      while [ "$var1" != "end" ]    #> while test "$var1" != "end"

- Dotan Barak contributes template code for a progress bar in a script.

  ###### A Progress Bar

      &progressbar;

- A particularly clever use of if-test constructs is for comment blocks.

      #!/bin/bash

      COMMENT_BLOCK=
      #  Try setting the above variable to some value
      #+ for an unpleasant surprise.

      if [ $COMMENT_BLOCK ]; then

      Comment block --
      =================================
      This is a comment line.
      This is another comment line.
      This is yet another comment line.
      =================================

      echo "This will not echo."

      Comment blocks are error-free! Whee!

      fi

      echo "No more comments, please."

      exit 0

  Compare this with using here documents to comment out code blocks.

- Using the $? exit status variable, a script may test if a parameter contains only digits, so it can be treated as an integer.

      #!/bin/bash

      SUCCESS=0
      E_BADINPUT=85

      test "$1" -ne 0 -o "$1" -eq 0 2>/dev/null
      # An integer is either equal to 0 or not equal to 0.
      # 2>/dev/null suppresses error message.

      if [ $? -ne "$SUCCESS" ]
      then
        echo "Usage: `basename $0` integer-input"
        exit $E_BADINPUT
      fi

      let "sum = $1 + 25"             # Would give error if $1 not integer.
      echo "Sum = $sum"

      # Any variable, not just a command-line parameter, can be tested this way.

      exit 0

- The 0 - 255 range for function return values is a severe limitation. Global variables and other workarounds are often problematic. An alternative method for a function to communicate a value back to the main body of the script is to have the function write to `stdout` (usually with echo) the return value, and assign this to a variable. This is actually a variant of command substitution.

  ###### Return value trickery

      &multiplication;

  The same technique also works for alphanumeric strings. This means that a function can return a non-numeric value.

      capitalize_ichar ()          #  Capitalizes initial character
      {                            #+ of argument string(s) passed.

        string0="$@"               # Accepts multiple arguments.

        firstchar=${string0:0:1}   # First character.
        string1=${string0:1}       # Rest of string(s).

        FirstChar=`echo "$firstchar" | tr a-z A-Z`
                                   # Capitalize first character.

        echo "$FirstChar$string1"  # Output to stdout.

      }

      newstring=`capitalize_ichar "every sentence should start with a capital letter."`
      echo "$newstring"          # Every sentence should start with a capital letter.

  It is even possible for a function to return multiple values with this method.

  ###### Even more return value trickery

      &sumproduct;

  There can be only `one` echo statement in the function for this to work. If you alter the previous example:

      sum_and_product ()
      {
        echo "This is the sum_and_product function." # This messes things up!
        echo $(( $1 + $2 )) $(( $1 * $2 ))
      }
      ...
      retval=`sum_and_product $first $second`      # Assigns output of function.
      # Now, this will not work correctly.

- Next in our bag of tricks are techniques for passing an array to a function, then returning an array back to the main body of the script.

  Passing an array involves loading the space-separated elements of the array into a variable with command substitution. Getting an array back as the return value from a function uses the previously mentioned strategem of echoing the array in the function, then invoking command substitution and the `( ... )` operator to assign it to an array.

  ###### Passing and returning arrays

      &arrfunc;

  For a more elaborate example of passing arrays to functions, see .

- Using the double-parentheses construct, it is possible to use C-style syntax for setting and incrementing/decrementing variables and in for and while loops. See and .

- Setting the path and umask at the beginning of a script makes it more portable -- more likely to run on a foreign machine whose user may have bollixed up the `$PATH` and `umask`.

      #!/bin/bash
      PATH=/bin:/usr/bin:/usr/local/bin ; export PATH
      umask 022   # Files that the script creates will have 755 permission.

      # Thanks to Ian D. Allen, for this tip.

- A useful scripting technique is to repeatedly feed the output of a filter (by piping) back to the same filter, but with a different set of arguments and/or options. Especially suitable for this are tr and grep.

      # From "wstrings.sh" example.

      wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
      tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`

  ###### Fun with anagrams

      &agram;

  See also , , and .

- Use anonymous here documents to comment out blocks of code, to save having to individually comment out each line with a #. See .

- Running a script on a machine that relies on a command that might not be installed is dangerous. Use whatis to avoid potential problems with this.

      CMD=command1                 # First choice.
      PlanB=command2               # Fallback option.

      command_test=$(whatis "$CMD" | grep 'nothing appropriate')
      #  If 'command1' not found on system , 'whatis' will return
      #+ "command1: nothing appropriate."
      #
      #  A safer alternative is:
      #     command_test=$(whereis "$CMD" | grep \/)
      #  But then the sense of the following test would have to be reversed,
      #+ since the $command_test variable holds content only if
      #+ the $CMD exists on the system.
      #     (Thanks, bojster.)

      if [[ -z "$command_test" ]]  # Check whether command present.
      then
        $CMD option1 option2       #  Run command1 with options.
      else                         #  Otherwise,
        $PlanB                     #+ run command2.
      fi

- An if-grep test may not return expected results in an error case, when text is output to `stderr`, rather that `stdout`.

      if ls -l nonexistent_filename | grep -q 'No such file or directory'
        then echo "File \"nonexistent_filename\" does not exist."
      fi

  Redirecting `stderr` to `stdout` fixes this.

      if ls -l nonexistent_filename 2>&1 | grep -q 'No such file or directory'
      #                             ^^^^
        then echo "File \"nonexistent_filename\" does not exist."
      fi

      # Thanks, Chris Martin, for pointing this out.

- If you absolutely must access a subshell variable outside the subshell, here's a way to do it.

      TMPFILE=tmpfile                  # Create a temp file to store the variable.

      (   # Inside the subshell ...
      inner_variable=Inner
      echo $inner_variable
      echo $inner_variable >>$TMPFILE  # Append to temp file.
      )

          # Outside the subshell ...

      echo; echo "-----"; echo
      echo $inner_variable             # Null, as expected.
      echo "-----"; echo

      # Now ...
      read inner_variable <$TMPFILE    # Read back shell variable.
      rm -f "$TMPFILE"                 # Get rid of temp file.
      echo "$inner_variable"           # It's an ugly kludge, but it works.

- The run-parts command is handy for running a set of command scripts in a particular sequence, especially in combination with cron or at.

- For doing multiple revisions on a complex script, use the rcs Revision Control System package.

  Among other benefits of this is automatically updated ID header tags. The `co` command in rcs does a parameter replacement of certain reserved key words, for example, replacing `# $Id$` in a script with something like:

      # $Id: hello-world.sh,v 1.1 2004/10/16 02:43:05 bozo Exp $

##### Widgets

It would be nice to be able to invoke X-Windows widgets from a shell script. There happen to exist several packages that purport to do so, namely Xscript, Xmenu, and widtools. The first two of these no longer seem to be maintained. Fortunately, it is still possible to obtain widtools here.

The widtools (widget tools) package requires the XForms library to be installed. Additionally, the Makefile needs some judicious editing before the package will build on a typical Linux system. Finally, three of the six widgets offered do not work (and, in fact, segfault).

The dialog family of tools offers a method of calling dialog widgets from a shell script. The original dialog utility works in a text console, but its successors, gdialog, Xdialog, and kdialog use X-Windows-based widget sets.

###### Widgets invoked from a shell script

    &dialog;

The xmessage command is a simple method of popping up a message/query window. For example:

    xmessage Fatal error in script! -button exit

The latest entry in the widget sweepstakes is zenity. This utility pops up GTK+ dialog widgets-and-windows, and it works very nicely within a script.

    get_info ()
    {
      zenity --entry       #  Pops up query window . . .
                           #+ and prints user entry to stdout.

                           #  Also try the --calendar and --scale options.
    }

    answer=$( get_info )   #  Capture stdout in $answer variable.

    echo "User entered: "$answer""

For other methods of scripting with widgets, try Tk or wish (Tcl derivatives), PerlTk (Perl with Tk extensions), tksh (ksh with Tk extensions), XForms4Perl (Perl with XForms extensions), Gtk-Perl (Perl with Gtk extensions), or PyQt (Python with Qt extensions).

#### Security Issues

##### Infected Shell Scripts

A brief warning about script security is indicated. A shell script may contain a worm, trojan, or even a virus. For that reason, never run as root a script (or permit it to be inserted into the system startup scripts in `/etc/rc.d`) unless you have obtained said script from a trusted source or you have carefully analyzed it to make certain it does nothing harmful.

Various researchers at Bell Labs and other sites, including M. Douglas McIlroy, Tom Duff, and Fred Cohen have investigated the implications of shell script viruses. They conclude that it is all too easy for even a novice, a script kiddie, to write one. See Marius van Oers' article, Unix Shell Scripting Malware, and also the Denning reference in the bibliography.

Here is yet another reason to learn scripting. Being able to look at and understand scripts may protect your system from being compromised by a rogue script.

##### Hiding Shell Script Source

For security purposes, it may be necessary to render a script unreadable. If only there were a utility to create a stripped binary executable from a script. Francisco Rosales' shc -- generic shell script compiler does exactly that.

Unfortunately, according to an article in the October, 2005 Linux Journal, the binary can, in at least some cases, be decrypted to recover the original script source. Still, this could be a useful method of keeping scripts secure from all but the most skilled hackers.

##### Writing Secure Shell Scripts

Dan Stromberg suggests the following guidelines for writing (relatively) secure shell scripts.

- Don't put secret data in environment variables.

- Don't pass secret data in an external command's arguments (pass them in via a pipe or redirection instead).

- Set your $PATH carefully. Don't just trust whatever path you inherit from the caller if your script is running as root. In fact, whenever you use an environment variable inherited from the caller, think about what could happen if the caller put something misleading in the variable, e.g., if the caller set $HOME to `/etc`.

#### Portability Issues

It is easier to port a shell than a shell script.

--Larry Wall

This book deals specifically with Bash scripting on a GNU/Linux system. All the same, users of `sh` and `ksh` will find much of value here.

As it happens, many of the various shells and scripting languages seem to be converging toward the POSIX 1003.2 standard. Invoking Bash with the `--posix` option or inserting a `set -o posix` at the head of a script causes Bash to conform very closely to this standard. Another alternative is to use a #!/bin/sh sha-bang header in the script, rather than #!/bin/bash. Or, better yet, #!/bin/env sh. Note that `/bin/sh` is a link to `/bin/bash` in Linux and certain other flavors of UNIX, and a script invoked this way disables extended Bash functionality.

Most Bash scripts will run as-is under `ksh`, and vice-versa, since Chet Ramey has been busily porting `ksh` features to the latest versions of Bash.

On a commercial UNIX machine, scripts using GNU-specific features of standard commands may not work. This has become less of a problem in the last few years, as the GNU utilities have pretty much displaced their proprietary counterparts even on big-iron UNIX. Caldera's release of the source to many of the original UNIX utilities has accelerated the trend.

Bash has certain features that the traditional Bourne shell lacks. Among these are:

- Certain extended invocation options

- Command substitution using `$( )` notation

- Brace expansion

- Certain array operations, and associative arrays

- The double brackets extended test construct

- The double-parentheses arithmetic-evaluation construct

- Certain string manipulation operations

- Process substitution

- A Regular Expression matching operator

- Bash-specific builtins

- Coprocesses

See the Bash F.A.Q. for a complete listing.

##### A Test Suite

Let us illustrate some of the incompatibilities between Bash and the classic Bourne shell. Download and install the Heirloom Bourne Shell and run the following script, first using Bash, then the classic sh.

###### Test Suite

    &testsuite;

#### Shell Scripting Under Windows

Even users running that other OS can run UNIX-like shell scripts, and therefore benefit from many of the lessons of this book. The Cygwin package from Cygnus and the MKS utilities from Mortice Kern Associates add shell scripting capabilities to Windows.

Another alternative is UWIN, written by David Korn of AT&T, of Korn Shell fame.

In 2006, Microsoft released the Windows Powershell, which contains limited Bash-like command-line scripting capabilities.

### Bash, versions 2, 3, and 4

#### Bash, version 2

The current version of Bash, the one you have running on your machine, is most likely version 2.xx.yy, 3.xx.yy, or 4.xx.yy.

    bash$ echo $BASH_VERSION
    3.2.25(1)-release

The version 2 update of the classic Bash scripting language added array variables, string and parameter expansion, and a better method of indirect variable references, among other features.

##### String expansion

    &ex77;

##### Indirect variable references - the new way

    &ex78;

##### Simple database application, using indirect variable referencing

    &resistor;

##### Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards

    &cards;

#### Bash, version 3

On July 27, 2004, Chet Ramey released version 3 of Bash. This update fixed quite a number of bugs and added new features.

Some of the more important added features:

- A new, more generalized `{a..z}` brace expansion operator.

      #!/bin/bash

      for i in {1..10}
      #  Simpler and more straightforward than
      #+ for i in $(seq 10)
      do
        echo -n "$i "
      done

      echo

      # 1 2 3 4 5 6 7 8 9 10

      # Or just . . .

      echo {a..z}    #  a b c d e f g h i j k l m n o p q r s t u v w x y z
      echo {e..m}    #  e f g h i j k l m
      echo {z..a}    #  z y x w v u t s r q p o n m l k j i h g f e d c b a
                     #  Works backwards, too.
      echo {25..30}  #  25 26 27 28 29 30
      echo {3..-2}   #  3 2 1 0 -1 -2
      echo {X..d}    #  X Y Z [  ] ^ _ ` a b c d
                     #  Shows (some of) the ASCII characters between Z and a,
                     #+ but don't rely on this type of behavior because . . .
      echo {]..a}    #  {]..a}
                     #  Why?

      # You can tack on prefixes and suffixes.
      echo "Number #"{1..4}, "..."
           # Number #1, Number #2, Number #3, Number #4, ...

      # You can concatenate brace-expansion sets.
      echo {1..3}{x..z}" +" "..."
           # 1x + 1y + 1z + 2x + 2y + 2z + 3x + 3y + 3z + ...
           # Generates an algebraic expression.
           # This could be used to find permutations.

      # You can nest brace-expansion sets.
      echo {{a..c},{1..3}}
           # a b c 1 2 3
           # The "comma operator" splices together strings.

      # ########## ######### ############ ########### ######### ###############
      # Unfortunately, brace expansion does not lend itself to parameterization.
      var1=1
      var2=5
      echo {$var1..$var2}   # {1..5}

      # Yet, as Emiliano G. points out, using "eval" overcomes this limitation.

      start=0
      end=10
      for index in $(eval echo {$start..$end})
      do
        echo -n "$index "   # 0 1 2 3 4 5 6 7 8 9 10
      done

      echo

- The `${!array[@]}` operator, which expands to all the indices of a given array.

      #!/bin/bash

      Array=(element-zero element-one element-two element-three)

      echo ${Array[0]}   # element-zero
                         # First element of array.

      echo ${!Array[@]}  # 0 1 2 3
                         # All the indices of Array.

      for i in ${!Array[@]}
      do
        echo ${Array[i]} # element-zero
                         # element-one
                         # element-two
                         # element-three
                         #
                         # All the elements in Array.
      done

- The `=~` Regular Expression matching operator within a double brackets test expression. (Perl has a similar operator.)

      #!/bin/bash

      variable="This is a fine mess."

      echo "$variable"

      # Regex matching with =~ operator within [[ double brackets ]].
      if [[ "$variable" =~ T.........fin*es* ]]
      # NOTE: As of version 3.2 of Bash, expression to match no longer quoted.
      then
        echo "match found"
            # match found
      fi

  Or, more usefully:

      #!/bin/bash

      input=$1

      if [[ "$input" =~ "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" ]]
      #                 ^ NOTE: Quoting not necessary, as of version 3.2 of Bash.
      # NNN-NN-NNNN (where each N is a digit).
      then
        echo "Social Security number."
        # Process SSN.
      else
        echo "Not a Social Security number!"
        # Or, ask for corrected input.
      fi

  For additional examples of using the `=~` operator, see , , , and .

- The new `set -o pipefail` option is useful for debugging pipes. If this option is set, then the exit status of a pipe is the exit status of the last command in the pipe to fail (return a non-zero value), rather than the actual final command in the pipe.

  See .

The update to version 3 of Bash breaks a few scripts that worked under earlier versions. Test critical legacy scripts to make sure they still work!

As it happens, a couple of the scripts in the Advanced Bash Scripting Guide had to be fixed up (see , for instance).

##### Bash, version 3.1

The version 3.1 update of Bash introduces a number of bugfixes and a few minor changes.

- The += operator is now permitted in in places where previously only the = assignment operator was recognized.

      a=1
      echo $a        # 1

      a+=5           # Won't work under versions of Bash earlier than 3.1.
      echo $a        # 15

      a+=Hello
      echo $a        # 15Hello

  Here, += functions as a string concatenation operator. Note that its behavior in this particular context is different than within a let construct.

      a=1
      echo $a        # 1

      let a+=5       # Integer arithmetic, rather than string concatenation.
      echo $a        # 6

      let a+=Hello   # Doesn't "add" anything to a.
      echo $a        # 6

  Jeffrey Haemer points out that this concatenation operator can be quite useful. In this instance, we append a directory to the `$PATH`.

      bash$ echo $PATH
      /usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games

      bash$ PATH+=:/opt/bin

      bash$ echo $PATH
      /usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games:/opt/bin

##### Bash, version 3.2

This is pretty much a bugfix update.

- In global parameter substitutions, the pattern no longer anchors at the start of the string.

- The `--wordexp` option disables process substitution.

- The `=~` Regular Expression match operator no longer requires quoting of the pattern within [[ ... ]].

  In fact, quoting in this context is not advisable as it may cause regex evaluation to fail. Chet Ramey states in the Bash FAQ that quoting explicitly disables regex evaluation. See also the Ubuntu Bug List and Wikinerds on Bash syntax.

  Setting shopt -s compat31 in a script causes reversion to the original behavior.

#### Bash, version 4

Chet Ramey announced Version 4 of Bash on the 20th of February, 2009. This release has a number of significant new features, as well as some important bugfixes.

Among the new goodies:

- Associative arrays. To be more specific, Bash 4+ has limited support for associative arrays. It's a bare-bones implementation, and it lacks the much of the functionality of such arrays in other programming languages. Note, however, that associative arrays in Bash seem to execute faster and more efficiently than numerically-indexed arrays.

  An associative array can be thought of as a set of two linked arrays -- one holding the data, and the other the keys that index the individual elements of the data array.

  ##### A simple address database

      &fetchaddress;

  ##### A somewhat more elaborate address database

      &fetchaddress2;

  See for an interesting usage of an associative array.

  Elements of the index array may include embedded space characters, or even leading and/or trailing space characters. However, index array elements containing only whitespace are not permitted.

      address[   ]="Blank"   # Error!

- Enhancements to the case construct: the ;;& and ;& terminators.

  ##### Testing characters

      &case4;

- The new `coproc` builtin enables two parallel processes to communicate and interact. As Chet Ramey states in the Bash FAQ Copyright 1995-2009 by Chester Ramey. , ver. 4.01:

  >         There is a new 'coproc' reserved word that specifies a coprocess:
  >         an asynchronous command run with two pipes connected to the creating
  >         shell. Coprocs can be named. The input and output file descriptors
  >         and the PID of the coprocess are available to the calling shell in
  >         variables with coproc-specific names.

  >         George Dimitriu explains,
  >         "... coproc ... is a feature used in Bash process substitution,
  >         which now is made publicly available."
  >         This means it can be explicitly invoked in a script, rather than
  >         just being a behind-the-scenes mechanism used by Bash.

  Coprocesses use file descriptors. File descriptors enable processes and pipes to communicate.

      #!/bin/bash4
      # A coprocess communicates with a while-read loop.

      coproc { cat mx_data.txt; sleep 2; }
      #                         ^^^^^^^
      # Try running this without "sleep 2" and see what happens.

      while read -u ${COPROC[0]} line    #  ${COPROC[0]} is the
      do                                 #+ file descriptor of the coprocess.
        echo "$line" | sed -e 's/line/NOT-ORIGINAL-TEXT/'
      done

      kill $COPROC_PID                   #  No longer need the coprocess,
                                         #+ so kill its PID.

  But, be careful!

      #!/bin/bash4

      echo; echo
      a=aaa
      b=bbb
      c=ccc

      coproc echo "one two three"
      while read -u ${COPROC[0]} a b c;  #  Note that this loop
      do                                 #+ runs in a subshell.
        echo "Inside while-read loop: ";
        echo "a = $a"; echo "b = $b"; echo "c = $c"
        echo "coproc file descriptor: ${COPROC[0]}"
      done

      # a = one
      # b = two
      # c = three
      # So far, so good, but ...

      echo "-----------------"
      echo "Outside while-read loop: "
      echo "a = $a"  # a =
      echo "b = $b"  # b =
      echo "c = $c"  # c =
      echo "coproc file descriptor: ${COPROC[0]}"
      echo
      #  The coproc is still running, but ...
      #+ it still doesn't enable the parent process
      #+ to "inherit" variables from the child process, the while-read loop.

      #  Compare this to the "badread.sh" script.

  The coprocess is asynchronous, and this might cause a problem. It may terminate before another process has finished communicating with it.

      #!/bin/bash4

      coproc cpname { for i in {0..10}; do echo "index = $i"; done; }
      #      ^^^^^^ This is a *named* coprocess.
      read -u ${cpname[0]}
      echo $REPLY         #  index = 0
      echo ${COPROC[0]}   #+ No output ... the coprocess timed out
      #  after the first loop iteration.

      # However, George Dimitriu has a partial fix.

      coproc cpname { for i in {0..10}; do echo "index = $i"; done; sleep 1;
      echo hi > myo; cat - >> myo; }
      #       ^^^^^ This is a *named* coprocess.

      echo "I am main"$'\04' >&${cpname[1]}
      myfd=${cpname[0]}
      echo myfd=$myfd

      ### while read -u $myfd
      ### do
      ###   echo $REPLY;
      ### done

      echo $cpname_PID

      #  Run this with and without the commented-out while-loop, and it is
      #+ apparent that each process, the executing shell and the coprocess,
      #+ waits for the other to finish writing in its own write-enabled pipe.

- The new `mapfile` builtin makes it possible to load an array with the contents of a text file without using a loop or command substitution.

      #!/bin/bash4

      mapfile Arr1 < $0
      # Same result as     Arr1=( $(cat $0) )
      echo "${Arr1[@]}"  # Copies this entire script out to stdout.

      echo "--"; echo

      # But, not the same as   read -a   !!!
      read -a Arr2 < $0
      echo "${Arr2[@]}"  # Reads only first line of script into the array.

      exit

- The read builtin got a minor facelift. The `-t` timeout option now accepts (decimal) fractional values This only works with pipes and certain other special files. and the `-i` option permits preloading the edit buffer. But only in conjunction with readline, i.e., from the command-line. Unfortunately, these enhancements are still a work in progress and not (yet) usable in scripts.

- Parameter substitution gets case-modification operators.

      #!/bin/bash4

      var=veryMixedUpVariable
      echo ${var}            # veryMixedUpVariable
      echo ${var^}           # VeryMixedUpVariable
      #         *              First char --> uppercase.
      echo ${var^^}          # VERYMIXEDUPVARIABLE
      #         **             All chars  --> uppercase.
      echo ${var,}           # veryMixedUpVariable
      #         *              First char --> lowercase.
      echo ${var,,}          # verymixedupvariable
      #         **             All chars  --> lowercase.

- The declare builtin now accepts the `-l` lowercase and `-c` capitalize options.

      #!/bin/bash4

      declare -l var1            # Will change to lowercase
      var1=MixedCaseVARIABLE
      echo "$var1"               # mixedcasevariable
      # Same effect as             echo $var1 | tr A-Z a-z

      declare -c var2            # Changes only initial char to uppercase.
      var2=originally_lowercase
      echo "$var2"               # Originally_lowercase
      # NOT the same effect as     echo $var2 | tr a-z A-Z

- Brace expansion has more options.

  Increment/decrement, specified in the final term within braces.

      #!/bin/bash4

      echo {40..60..2}
      # 40 42 44 46 48 50 52 54 56 58 60
      # All the even numbers, between 40 and 60.

      echo {60..40..2}
      # 60 58 56 54 52 50 48 46 44 42 40
      # All the even numbers, between 40 and 60, counting backwards.
      # In effect, a decrement.
      echo {60..40..-2}
      # The same output. The minus sign is not necessary.

      # But, what about letters and symbols?
      echo {X..d}
      # X Y Z [  ] ^ _ ` a b c d
      # Does not echo the \ which escapes a space.

  Zero-padding, specified in the first term within braces, prefixes each term in the output with the same number of zeroes.

      bash4$ echo {010..15}
      010 011 012 013 014 015

      bash4$ echo {000..10}
      000 001 002 003 004 005 006 007 008 009 010

- Substring extraction on positional parameters now starts with $0 as the zero-index. (This corrects an inconsistency in the treatment of positional parameters.)

      #!/bin/bash
      # show-params.bash
      # Requires version 4+ of Bash.

      # Invoke this scripts with at least one positional parameter.

      E_BADPARAMS=99

      if [ -z "$1" ]
      then
        echo "Usage $0 param1 ..."
        exit $E_BADPARAMS
      fi

      echo ${@:0}

      # bash3 show-params.bash4 one two three
      # one two three

      # bash4 show-params.bash4 one two three
      # show-params.bash4 one two three

      # $0                $1  $2  $3

- The new ** globbing operator matches filenames and directories recursively.

      #!/bin/bash4
      # filelist.bash4

      shopt -s globstar  # Must enable globstar, otherwise ** doesn't work.
                         # The globstar shell option is new to version 4 of Bash.

      echo "Using *"; echo
      for filename in *
      do
        echo "$filename"
      done   # Lists only files in current directory ($PWD).

      echo; echo "--------------"; echo

      echo "Using **"
      for filename in **
      do
        echo "$filename"
      done   # Lists complete file tree, recursively.

      exit

      Using *

      allmyfiles
      filelist.bash4

      --------------

      Using **

      allmyfiles
      allmyfiles/file.index.txt
      allmyfiles/my_music
      allmyfiles/my_music/me-singing-60s-folksongs.ogg
      allmyfiles/my_music/me-singing-opera.ogg
      allmyfiles/my_music/piano-lesson.1.ogg
      allmyfiles/my_pictures
      allmyfiles/my_pictures/at-beach-with-Jade.png
      allmyfiles/my_pictures/picnic-with-Melissa.png
      filelist.bash4

- The new $BASHPID internal variable.

- There is a new builtin error-handling function named `command_not_found_handle`.

      #!/bin/bash4

      command_not_found_handle ()
      { # Accepts implicit parameters.
        echo "The following command is not valid: \""$1\"""
        echo "With the following argument(s): \""$2\"" \""$3\"""   # $4, $5 ...
      } # $1, $2, etc. are not explicitly passed to the function.

      bad_command arg1 arg2

      # The following command is not valid: "bad_command"
      # With the following argument(s): "arg1" "arg2"

Editorial comment

Associative arrays? Coprocesses? Whatever happened to the lean and mean Bash we have come to know and love? Could it be suffering from (horrors!) feature creep? Or perhaps even Korn shell envy?

Note to Chet Ramey: Please add only essential features in future Bash releases -- perhaps for-each loops and support for multi-dimensional arrays. And while you're at it, consider fixing the notorious piped read problem. Most Bash users won't need, won't use, and likely won't greatly appreciate complex features like built-in debuggers, Perl interfaces, and bolt-on rocket boosters.

##### Bash, version 4.1

Version 4.1 of Bash, released in May, 2010, was primarily a bugfix update.

- The printf command now accepts a `-v` option for setting array indices.

- Within double brackets, the `>` and `<` string comparison operators now conform to the locale. Since the locale setting may affect the sorting order of string expressions, this has side-effects on comparison tests within [[ ... ]] expressions.

- The read builtin now takes a `-N` option (read -N chars), which causes the read to terminate after chars characters.

  ###### Reading N characters

      &readn;

- Here documents embedded in `$( ... )` command substitution constructs may terminate with a simple `)`.

  ###### Using a here document to set a variable

      &herecommsub;

##### Bash, version 4.2

Version 4.2 of Bash, released in February, 2011, contains a number of new features and enhancements, in addition to bugfixes.

- Bash now supports the the \u and \U Unicode escape.

  Unicode is a cross-platform standard for encoding into numerical values letters and graphic symbols. This permits representing and displaying characters in foreign alphabets and unusual fonts.

      echo -e '\u2630'   # Horizontal triple bar character.
      # Equivalent to the more roundabout:
      echo -e "\xE2\x98\xB0"
                         # Recognized by earlier Bash versions.

      echo -e '\u220F'   # PI (Greek letter and mathematical symbol)
      echo -e '\u0416'   # Capital "ZHE" (Cyrillic letter)
      echo -e '\u2708'   # Airplane (Dingbat font) symbol
      echo -e '\u2622'   # Radioactivity trefoil

      echo -e "The amplifier circuit requires a 100 \u2126 pull-up resistor."

      unicode_var='\u2640'
      echo -e $unicode_var      # Female symbol
      printf "$unicode_var \n"  # Female symbol, with newline

      #  And for something a bit more elaborate . . .

      #  We can store Unicode symbols in an associative array,
      #+ then retrieve them by name.
      #  Run this in a gnome-terminal or a terminal with a large, bold font
      #+ for better legibility.

      declare -A symbol  # Associative array.

      symbol[script_E]='\u2130'
      symbol[script_F]='\u2131'
      symbol[script_J]='\u2110'
      symbol[script_M]='\u2133'
      symbol[Rx]='\u211E'
      symbol[TEL]='\u2121'
      symbol[FAX]='\u213B'
      symbol[care_of]='\u2105'
      symbol[account]='\u2100'
      symbol[trademark]='\u2122'

      echo -ne "${symbol[script_E]}   "
      echo -ne "${symbol[script_F]}   "
      echo -ne "${symbol[script_J]}   "
      echo -ne "${symbol[script_M]}   "
      echo -ne "${symbol[Rx]}   "
      echo -ne "${symbol[TEL]}   "
      echo -ne "${symbol[FAX]}   "
      echo -ne "${symbol[care_of]}   "
      echo -ne "${symbol[account]}   "
      echo -ne "${symbol[trademark]}   "
      echo

  The above example uses the `$' ... '` string-expansion construct.

- When the lastpipe shell option is set, the last command in a pipe doesn't run in a subshell.

  ###### Piping input to a read

      &lastpipeopt;

  This option offers possible fixups for these example scripts: and .

- Negative array indices permit counting backwards from the end of an array.

  ###### Negative array indices

      &negarray;

- Substring extraction uses a negative length parameter to specify an offset from the end of the target string.

  ###### Negative parameter in string-extraction construct

      &negoffset;

## Endnotes

### Author's Note

doce ut discas

(Teach, that you yourself may learn.)

How did I come to write a scripting book? It's a strange tale. It seems that a few years back I needed to learn shell scripting -- and what better way to do that than to read a good book on the subject? I was looking to buy a tutorial and reference covering all aspects of the subject. I was looking for a book that would take difficult concepts, turn them inside out, and explain them in excruciating detail, with well-commented examples. This is the notorious flog it to death technique that works so well with slow learners, eccentrics, odd ducks, fools and geniuses. In fact, I was looking for this very book, or something very much like it. Unfortunately, it didn't exist, and if I wanted it, I'd have to write it. And so, here we are, folks.

That reminds me of the apocryphal story about a mad professor. Crazy as a loon, the fellow was. At the sight of a book, any book -- at the library, at a bookstore, anywhere -- he would become totally obsessed with the idea that he could have written it, should have written it -- and done a better job of it to boot. He would thereupon rush home and proceed to do just that, write a book with the very same title. When he died some years later, he allegedly had several thousand books to his credit, probably putting even Asimov to shame. The books might not have been any good, who knows, but does that really matter? Here's a fellow who lived his dream, even if he was obsessed by it, driven by it . . . and somehow I can't help admiring the old coot.

### About the Author

Who is this guy anyhow?

The author claims no credentials or special qualifications, In fact, he has no credentials or special qualifications. He's a school dropout with no formal credentials or professional experience whatsoever. None. Zero. Nada. Aside from the ABS Guide, his major claim to fame is a First Place in the sack race at the Colfax Elementary School Field Day in June, 1958. other than a compulsion to write. Those who can, do. Those who can't . . . get an MCSE.

This book is somewhat of a departure from his other major work, HOW-2 Meet Women: The Shy Man's Guide to Relationships. He has also written the Software-Building HOWTO. Of late, he has been trying his (heavy) hand at fiction: Dave Dawson Over Berlin (First Installment) Dave Dawson Over Berlin (Second Installment) and Dave Dawson Over Berlin (Third Installment)

. He also has a few Instructables (here, here, here, here, here,

here, and here to his (dis)credit.

A Linux user since 1995 (Slackware 2.2, kernel 1.2.1), the author has emitted a few software truffles, including the cruft one-time pad encryption utility, the mcalc mortgage calculator, the judge Scrabble® adjudicator, the yawl word gaming list package, and the Quacky anagramming gaming package. He got off to a rather shaky start in the computer game -- programming FORTRAN IV on a CDC 3800 (on paper coding pads, with occasional forays on a keypunch machine and a Friden Flexowriter) -- and is not the least bit nostalgic for those days.

Living in an out-of-the-way community with wife and orange tabby, he cherishes human frailty, especially his own. Sometimes it seems as if he has spent his entire life flouting conventional wisdom and defying the sonorous Voice of Authority: Hey, you can't do that!

### Where to Go For Help

The author is no longer supporting or updating this document. He will not answer questions about this book or about general scripting topics.

If you need assistance with a schoolwork assignment, read the pertinent sections of this and other reference works. Do your best to solve the problem using your own wits and resources. Please do not waste the author's time. You will get neither help nor sympathy. Well, if you absolutely insist, you can try modifying to suit your purposes.

Likewise, kindly refrain from annoying the author with solicitations, offers of employment, or business opportunities. He is doing just fine, and requires neither help nor sympathy, thank you.

Please note that the author will not answer scripting questions for Sun/Solaris/Oracle or Apple systems. The endarkened execs and the arachnoid corporate attorneys of those particular outfits have been using litigation in a predatory manner and/or as a weapon against the Open Source Community. Any Solaris or Apple users needing scripting help will therefore kindly direct their concerns to corporate customer service.

... sophisticated in mechanism but possibly agile operating under noises being extremely suppressed ...

--CI-300 printer manual

### Tools Used to Produce This Book

#### Hardware

A used IBM Thinkpad, model 760XL laptop (P166, 104 meg RAM) running Red Hat 7.1/7.3. Sure, it's slow and has a funky keyboard, but it beats the heck out of a No. 2 pencil and a Big Chief tablet.

Update: upgraded to a 770Z Thinkpad (P2-366, 192 meg RAM) running FC3. Anyone feel like donating a later-model laptop to a starving writer <g>?

Update: upgraded to a T61 Thinkpad running Mandriva 2011. No longer starving <g>, but not too proud to accept donations.

#### Software and Printware

1. Bram Moolenaar's powerful SGML-aware vim text editor.

2. OpenJade, a DSSSL rendering engine for converting SGML documents into other formats.

3. Norman Walsh's DSSSL stylesheets.

4. DocBook, The Definitive Guide, by Norman Walsh and Leonard Muellner (O'Reilly, ISBN 1-56592-580-7). This is still the standard reference for anyone attempting to write a document in Docbook SGML format.

### Credits

Community participation made this project possible. The author gratefully acknowledges that writing this book would have been unthinkable without help and feedback from all you people out there.

Philippe Martin translated the first version (0.1) of this document into DocBook/SGML. While not on the job at a small French company as a software developer, he enjoys working on GNU/Linux documentation and software, reading literature, playing music, and, for his peace of mind, making merry with friends. You may run across him somewhere in France or in the Basque Country, or you can email him at feloy@free.fr.

Philippe Martin also pointed out that positional parameters past $9 are possible using {bracket} notation. (See ).

Stéphane Chazelas sent a long list of corrections, additions, and example scripts. More than a contributor, he had, in effect, for a while taken on the role of co-editor for this document. Merci beaucoup!

Paulo Marcel Coelho Aragao offered many corrections, both major and minor, and contributed quite a number of helpful suggestions.

I would like to especially thank Patrick Callahan, Mike Novak, and Pal Domokos for catching bugs, pointing out ambiguities, and for suggesting clarifications and changes in the preliminary version (0.1) of this document. Their lively discussion of shell scripting and general documentation issues inspired me to try to make this document more readable.

I'm grateful to Jim Van Zandt for pointing out errors and omissions in version 0.2 of this document. He also contributed an instructive example script.

Many thanks to Jordi Sanfeliu for giving permission to use his fine tree script (), and to Rick Boivie for revising it.

Likewise, thanks to Michel Charpentier for permission to use his dc factoring script ().

Kudos to Noah Friedman for permission to use his string function script ().

Emmanuel Rouat suggested corrections and additions on command substitution, aliases, and path management. He also contributed a very nice sample `.bashrc` file ().

Heiner Steven kindly gave permission to use his base conversion script, . He also made a number of corrections and many helpful suggestions. Special thanks.

Rick Boivie contributed the delightfully recursive pb.sh script (), revised the tree.sh script (), and suggested performance improvements for the monthlypmt.sh script ().

Florian Wisser enlightened me on some of the fine points of testing strings (see ), and on other matters.

Oleg Philon sent suggestions concerning cut and pidof.

Michael Zick extended the empty array example to demonstrate some surprising array properties. He also contributed the isspammer scripts ( and ).

Marc-Jano Knopp sent corrections and clarifications on DOS batch files.

Hyun Jin Cha found several typos in the document in the process of doing a Korean translation. Thanks for pointing these out.

Andreas Abraham sent in a long list of typographical errors and other corrections. Special thanks!

Others contributing scripts, making helpful suggestions, and pointing out errors were Gabor Kiss, Leopold Toetsch, Peter Tillier, Marcus Berglof, Tony Richardson, Nick Drage (script ideas!), Rich Bartell, Jess Thrysoee, Adam Lazur, Bram Moolenaar, Baris Cicek, Greg Keraunen, Keith Matthews, Sandro Magi, Albert Reiner, Dim Segebart, Rory Winston, Lee Bigelow, Wayne Pollock, jipe, bojster, nyal, Hobbit, Ender, Little Monster (Alexis), Mark, Patsie, vladz, Peggy Russell, Emilio Conti, Ian. D. Allen, Hans-Joerg Diers, Arun Giridhar, Dennis Leeuw, Dan Jacobson, Aurelio Marinho Jargas, Edward Scholtz, Jean Helou, Chris Martin, Lee Maschmeyer, Bruno Haible, Wilbert Berendsen, Sebastien Godard, Bjön Eriksson, John MacDonald, John Lange, Joshua Tschida, Troy Engel, Manfred Schwarb, Amit Singh, Bill Gradwohl, E. Choroba, David Lombard, Jason Parker, Steve Parker, Bruce W. Clare, William Park, Vernia Damiano, Mihai Maties, Mark Alexander, Jeremy Impson, Ken Fuchs, Jared Martin, Frank Wang, Sylvain Fourmanoit, Matthew Sage, Matthew Walker, Kenny Stauffer, Filip Moritz, Andrzej Stefanski, Daniel Albers, Jeffrey Haemer, Stefano Palmeri, Nils Radtke, Sigurd Solaas, Serghey Rodin, Jeroen Domburg, Alfredo Pironti, Phil Braham, Bruno de Oliveira Schneider, Stefano Falsetto, Chris Morgan, Walter Dnes, Linc Fessenden, Michael Iatrou, Pharis Monalo, Jesse Gough, Fabian Kreutz, Mark Norman, Harald Koenig, Dan Stromberg, Peter Knowles, Francisco Lobo, Mariusz Gniazdowski, Sebastian Arming, Chetankumar Phulpagare, Benno Schulenberg, Tedman Eng, Jochen DeSmet, Juan Nicolas Ruiz, Oliver Beckstein, Achmed Darwish, Dotan Barak, Richard Neill, Albert Siersema, Omair Eshkenazi, Geoff Lee, Graham Ewart, JuanJo Ciarlante, Cliff Bamford, Nathan Coulter, Ramses Rodriguez Martinez, Evgeniy Ivanov, Craig Barnes, George Dimitriu, Kevin LeBlanc, Antonio Macchi, Tomas Pospisek, David Wheeler, Erik Brandsberg, YongYe, Andreas Kühne, Pádraig Brady, Joseph Steinhauser, and David Lawyer (himself an author of four HOWTOs).

My gratitude to Chet Ramey and Brian Fox for writing Bash, and building into it elegant and powerful scripting capabilities rivaling those of ksh.

Very special thanks to the hard-working volunteers at the Linux Documentation Project. The LDP hosts a repository of Linux knowledge and lore, and has, to a great extent, enabled the publication of this book.

Thanks and appreciation to IBM, Red Hat, Google, the Free Software Foundation, and all the good people fighting the good fight to keep Open Source software free and open.

Belated thanks to my fourth grade teacher, Miss Spencer, for emotional support and for convincing me that maybe, just maybe I wasn't a total loss.

Thanks most of all to my wife, Anita, for her encouragement, inspiration, and emotional support.

### Disclaimer

(This is a variant of the standard LDP disclaimer.)

No liability for the contents of this document can be accepted. Use the concepts, examples and information at your own risk. There may be errors, omissions, and inaccuracies that could cause you to lose data, harm your system, or induce involuntary electrocution, so proceed with appropriate caution. The author takes no responsibility for any damages, incidental or otherwise.

As it happens, it is highly unlikely that either you or your system will suffer ill effects, aside from uncontrollable hiccups. In fact, the raison d'etre of this book is to enable its readers to analyze shell scripts and determine whether they have unanticipated consequences.

Those who do not understand UNIX are condemned to reinvent it, poorly.

--Henry Spencer

Peter
Denning

### Computers Under Attack: Intruders, Worms, and Viruses

ACM Press
1990
0-201-53067-8

This compendium contains a couple of articles on shell script viruses.

*

Ken
Burtch

### Linux Shell Scripting with Bash

1st edition
Sams Publishing (Pearson)
2004
0672326426

Covers much of the same material as the ABS Guide, though in a different style.

*

Daniel
Goldman

### Definitive Guide to Sed

1st edition
2013

This ebook is an excellent introduction to sed. Rather than being a conversion from a printed volume, it was specifically designed and formatted for viewing on an ebook reader. Well-written, informative, and useful as a reference as well as a tutorial. Highly recommended.

*

Dale
Dougherty
Arnold
Robbins

### Sed and Awk

2nd edition
O'Reilly and Associates
1997
1-156592-225-5

Unfolding the full power of shell scripting requires at least a passing familiarity with sed and awk. This is the classic tutorial. It includes an excellent introduction to Regular Expressions. Recommended.

*

Jeffrey
Friedl

### Mastering Regular Expressions

O'Reilly and Associates
2002
0-596-00289-0

Still the best all-around reference on Regular Expressions.

*

Aeleen
Frisch

### Essential System Administration

3rd edition
O'Reilly and Associates
2002
0-596-00343-9

This excellent manual provides a decent introduction to shell scripting from a sys admin point of view. It includes comprehensive explanations of the startup and initialization scripts in a UNIX system.

*

Stephen
Kochan
Patrick
Wood

### Unix Shell Programming

Hayden
1990
067248448X

Still considered a standard reference, though somewhat dated, and a bit wooden stylistically speaking. It was hard to resist the obvious pun. No slight intended, since the book is a pretty decent introduction to the basic concepts of shell scripting. In fact, this book was the ABS Guide author's first exposure to UNIX shell scripting, lo these many years ago.

*

Neil
Matthew
Richard
Stones

### Beginning Linux Programming

Wrox Press
1996
1874416680

Surprisingly good in-depth coverage of various programming languages available for Linux, including a fairly strong chapter on shell scripting.

*

Herbert
Mayer

### Advanced C Programming on the IBM PC

Windcrest Books
1989
0830693637

Excellent coverage of algorithms and general programming practices. Highly recommended, but unfortunately out of print.

*

David
Medinets

### Unix Shell Programming Tools

McGraw-Hill
1999
0070397333

Pretty good treatment of shell scripting, with examples, and a short intro to Tcl and Perl.

*

Cameron
Newham
Bill
Rosenblatt

### Learning the Bash Shell

2nd edition
O'Reilly and Associates
1998
1-56592-347-2

This is a valiant effort at a decent shell primer, but sadly deficient in its coverage of writing scripts and lacking sufficient examples.

*

Anatole
Olczak

### Bourne Shell Quick Reference Guide

ASP, Inc.
1991
093573922X

A very handy pocket reference, despite lacking coverage of Bash-specific features.

*

Jerry
Peek
Tim
O'Reilly
Mike
Loukides

### Unix Power Tools

3rd edition
O'Reilly and Associates
Random House
2002
0-596-00330-7

Contains a couple of sections of very informative in-depth articles on shell programming, but falls short of being a self-teaching manual. It reproduces much of the Regular Expressions tutorial from the Dougherty and Robbins book, above. The comprehensive coverage of UNIX commands makes this book worthy of a place on your bookshelf.

*

Clifford
Pickover

### Computers, Pattern, Chaos, and Beauty

St. Martin's Press
1990
0-312-04123-3

A treasure trove of ideas and recipes for computer-based exploration of mathematical oddities.

*

George
Polya

### How To Solve It

Princeton University Press
1973
0-691-02356-5

The classic tutorial on problem-solving methods (algorithms), with special emphasis on how to teach them.

*

Chet
Ramey
Brian
Fox

### The GNU Bash Reference Manual

Network Theory Ltd
2003
0-9541617-7-7

This manual is the definitive reference for GNU Bash. The authors of this manual, Chet Ramey and Brian Fox, are the original developers of GNU Bash. For each copy sold, the publisher donates $1 to the Free Software Foundation.

*

Arnold
Robbins

### Bash Reference Card

SSC
1998
1-58731-010-5

Excellent Bash pocket reference (don't leave home without it, especially if you're a sysadmin). A bargain at $4.95, but unfortunately no longer available for free download.

*

Arnold
Robbins

### Effective Awk Programming

Free Software Foundation / O'Reilly and Associates
2000
1-882114-26-4

The absolute best awk tutorial and reference. The free electronic version of this book is part of the awk documentation, and printed copies of the latest version are available from O'Reilly and Associates.

This book served as an inspiration for the author of the ABS Guide.

*

Bill
Rosenblatt

### Learning the Korn Shell

O'Reilly and Associates
1993
1-56592-054-6

This well-written book contains some excellent pointers on shell scripting in general.

*

Paul
Sheer

### LINUX: Rute User's Tutorial and Exposition

1st edition
2002
0-13-033351-4

Very detailed and readable introduction to Linux system administration.

The book is available in print, or on-line.

*

Ellen
Siever
the staff of O'Reilly and Associates

### Linux in a Nutshell

2nd edition
O'Reilly and Associates
1999
1-56592-585-8

The all-around best Linux command reference. It even has a Bash section.

*

Dave
Taylor

### Wicked Cool Shell Scripts: 101 Scripts for Linux, Mac OS X, and Unix Systems

1st edition
No Starch Press
2004
1-59327-012-7

Pretty much what the title promises . . .

*

### The UNIX CD Bookshelf

3rd edition
O'Reilly and Associates
2003
0-596-00392-7

An array of seven UNIX books on CD ROM, including UNIX Power Tools, Sed and Awk, and Learning the Korn Shell. A complete set of all the UNIX references and tutorials you would ever need at about $130. Buy this one, even if it means going into debt and not paying the rent.

Update: Seems to have somehow fallen out of print. Ah, well. You can still buy the dead-tree editions of these books.

*

The O'Reilly books on Perl. (Actually, any O'Reilly books.)

* * *

`Other Resources`

Fioretti, Marco, Scripting for X Productivity, Linux Journal, Issue 113, September, 2003, pp. 86-9.

Ben Okopnik's well-written introductory Bash scripting articles in issues 53, 54, 55, 57, and 59 of the Linux Gazette, and his explanation of The Deep, Dark Secrets of Bash in issue 56.

Chet Ramey's Bash - The GNU Shell, a two-part series published in issues 3 and 4 of the Linux Journal, July-August 1994.

Mike G's Bash-Programming-Intro HOWTO.

Richard's Unix Scripting Universe.

Chet Ramey's Bash FAQ.

Greg's WIKI: Bash FAQ.

Example shell scripts at Lucc's Shell Scripts .

Example shell scripts at SHELLdorado .

Example shell scripts at Noah Friedman's script site.

Examples from the The Bash Scripting Cookbook, by Albing, Vossen, and Newham.

Example shell scripts at zazzybob.

Steve Parker's Shell Programming Stuff. In fact, all of his shell scripting books are highly recommended. See also Steve's Arcade Games written in a shell script.

An excellent collection of Bash scripting tips, tricks, and resources at the Bash Hackers Wiki.

Giles Orr's Bash-Prompt HOWTO.

The Pixelbeat command-line reference.

Very nice `sed`, `awk`, and regular expression tutorials at The UNIX Grymoire.

The GNU sed and gawk manuals. As you recall, gawk is the enhanced GNU version of `awk`.

Many interesting sed scripts at the seder's grab bag.

Tips and tricks at Linux Reviews.

Trent Fisher's groff tutorial.

David Wheeler's Filenames in Shell essay.

Shelltris and shellitaire at Shell Script Games.

YongYe's wonderfully complex Tetris game script.

Mark Komarinski's Printing-Usage HOWTO.

The Linux USB subsystem (helpful in writing scripts affecting USB peripherals).

There is some nice material on I/O redirection in chapter 10 of the textutils documentation at the University of Alberta site.

Rick Hohensee has written the osimpa i386 assembler entirely as Bash scripts.

dgatwood has a very nice shell script games site, featuring a Tetris® clone and solitaire.

Aurelio Marinho Jargas has written a Regular expression wizard. He has also written an informative book on Regular Expressions, in Portuguese.

Ben Tomkins has created the Bash Navigator directory management tool.

William Park has been working on a project to incorporate certain Awk and Python features into Bash. Among these is a gdbm interface. He has released bashdiff on Freshmeat.net. He has an article in the November, 2004 issue of the Linux Gazette on adding string functions to Bash, with a followup article in the December issue, and yet another in the January, 2005 issue.

Peter Knowles has written an elaborate Bash script that generates a book list on the Sony Librie e-book reader. This useful tool facilitates loading non-DRM user content on the Librie (and the newer PRS-xxx-series devices).

Tim Waugh's xmlto is an elaborate Bash script for converting Docbook XML documents to other formats.

Philip Patterson's logforbash logging/debugging script.

AuctionGallery, an application for eBay power sellers coded in Bash.

Of historical interest are Colin Needham's original International Movie Database (IMDB) reader polling scripts, which nicely illustrate the use of awk for string parsing. Unfortunately, the URL link is broken.

---

Fritz Mehner has written a bash-support plugin for the vim text editor. He has also also come up with his own stylesheet for Bash. Compare it with the ABS Guide Unofficial Stylesheet.

---

Penguin Pete has quite a number of shell scripting tips and hints on his superb site. Highly recommended.

The excellent Bash Reference Manual, by Chet Ramey and Brian Fox, distributed as part of the bash-2-doc package (available as an rpm). See especially the instructive example scripts in this package.

John Lion's classic, A Commentary on the Sixth Edition UNIX Operating System.

The comp.os.unix.shell newsgroup.

The dd thread on Linux Questions.

The comp.os.unix.shell FAQ.

Assorted comp.os.unix FAQs.

The Wikipedia article covering dc.

The manpages for `bash` and `bash2`, `date`, `expect`, `expr`, `find`, `grep`, `gzip`, `ln`, `patch`, `tar`, `tr`, `bc`, `xargs`. The texinfo documentation on `bash`, `dd`, `m4`, `gawk`, and `sed`.

## Contributed Scripts

These scripts, while not fitting into the text of this document, do illustrate some interesting shell programming techniques. Some are useful, too. Have fun analyzing and running them.

### mailformat: Formatting an e-mail message

    &mailformat;

### rn: A simple-minded file renaming utility

This script is a modification of .

    &rn;

### blank-rename: Renames filenames containing blanks

This is an even simpler-minded version of previous script.

    &blankrename;

### encryptedpw: Uploading to an ftp site, using a locally encrypted password

    &encryptedpw;

### copy-cd: Copying a data CD

    &copycd;

### Collatz series

    &collatz;

### days-between: Days between two dates

    &daysbetween;

### Making a dictionary

    &makedict;

### Soundex conversion

    &soundex;

### Game of Life

    &lifeslow;

### Data file for Game of Life

    &gen0data;

+++

The following script is by Mark Moraes of the University of Toronto. See the file `Moraes-COPYRIGHT` for permissions and restrictions. This file is included in the combined HTML/source tarball of the ABS Guide.

### behead: Removing mail and news message headers

    &behead;

+

Antek Sawicki contributed the following script, which makes very clever use of the parameter substitution operators discussed in .

### password: Generating random 8-character passwords

    &pw;

+

James R. Van Zandt contributed this script which uses named pipes and, in his words, really exercises quoting and escaping.

### fifo: Making daily backups, using named pipes

    &fifo;

+

Stéphane Chazelas used the following script to demonstrate generating prime numbers without arrays.

### Generating prime numbers using the modulo operator

    &primes;

+

Rick Boivie's revision of Jordi Sanfeliu's tree script.

### tree: Displaying a directory tree

    &tree;

Patsie's version of a directory tree script.

### tree2: Alternate directory tree script

    &tree2;

Noah Friedman permitted use of his string function script. It essentially reproduces some of the C-library string manipulation functions.

### string functions: C-style string functions

    &string;

Michael Zick's complex array example uses the md5sum check sum command to encode directory information.

### Directory information

    &directoryinfo;

Stéphane Chazelas demonstrates object-oriented programming in a Bash script.

Mariusz Gniazdowski contributed a hash library for use in scripts.

### Library of hash functions

    &hashlib;

Here is an example script using the foregoing hash library.

### Colorizing text using hash functions

    &hashexample;

An example illustrating the mechanics of hashing, but from a different point of view.

### More on hash functions

    &hashex2;

Now for a script that installs and mounts those cute USB keychain solid-state hard drives.

### Mounting USB keychain storage devices

    &usbinst;

Converting a text file to HTML format.

### Converting to HTML

    &tohtml;

Here is something to warm the hearts of webmasters and mistresses: a script that saves weblogs.

### Preserving weblogs

    &archiveweblogs;

How to keep the shell from expanding and reinterpreting text strings.

### Protecting literal strings

    &protectliteral;

But, what if you want the shell to expand and reinterpret strings?

### Unprotecting literal strings

    &unprotectliteral;

This interesting script helps hunt down spammers.

### Spammer Identification

    &isspammer2;

Another anti-spam script.

### Spammer Hunt

    &whx;

Little Monster's front end to wget.

### Making wget easier to use

    &wgetter2;

### A podcasting script

    &bashpodder;

### Nightly backup to a firewire HD

    &nightlybackup;

### An expanded cd command

    &cdll;

### A soundcard setup script

    &soundcardon;

### Locating split paragraphs in a text file

    &findsplit;

### Insertion sort

    &insertionsort;

### Standard Deviation

    &stddev;

### A pad file generator for shareware authors

    &padsw;

### A man page editor

    &maned;

### Petals Around the Rose

    &petals;

### Quacky: a Perquackey-type word game

    &qky;

### Nim

    &nim;

### A command-line stopwatch

    &stopwatch;

### An all-purpose shell scripting homework assignment solution

    &homework;

### The Knight's Tour

    &ktour;

### Magic Squares

    &msquare;

### Fifteen Puzzle

    &fifteen;

### The Towers of Hanoi, graphic version

    &hanoi2;

### The Towers of Hanoi, alternate graphic version

    &hanoi2a;

### An alternate version of the getopt-simple.sh script

    &usegetopt;

### The version of the UseGetOpt.sh example used in the Tab Expansion appendix

    &usegetopt2;

### Cycling through all the possible color backgrounds

    &showallc;

### Morse Code Practice

    &samorse;

### Base64 encoding/decoding

    &base64;

### Inserting text in a file using sed

    &sedappend;

### The Gronsfeld Cipher

    &gronsfeld;

### Bingo Number Generator

    &bingo;

To end this section, a review of the basics . . . and more.

### Basics Reviewed

    &basicsreviewed;

### Testing execution times of various commands

    &testexectime;

### Associative arrays vs. conventional arrays (execution times)

    &assocarrtest;

## Reference Cards

The following reference cards provide a useful summary of certain scripting concepts. The foregoing text treats these matters in more depth, as well as giving usage examples.

### Special Shell Variables

Variable | Meaning
--- | ---
`$0` | Filename of script
`$1` | Positional parameter #1
`$2 - $9` | Positional parameters #2 - #9
`${10}` | Positional parameter #10
`$#` | Number of positional parameters
`"$*"` | All the positional parameters (as a single word) *
`"$@"` | All the positional parameters (as separate strings)
`${#*}` | Number of positional parameters
`${#@}` | Number of positional parameters
`$?` | Return value
`$$` | Process ID (PID) of script
`$-` | Flags passed to script (using set)
`$_` | Last argument of previous command
`$!` | Process ID (PID) of last job run in background

`*` Must be quoted, otherwise it defaults to `$@`.

### TEST Operators: Binary Comparison

Operator | Meaning | ----- | Operator | Meaning
--- | --- | --- | --- | ---
Arithmetic Comparison |  |  | String Comparison |
`-eq` | Equal to |  | `=` | Equal to
|  |  | `==` | Equal to
`-ne` | Not equal to |  | `!=` | Not equal to
`-lt` | Less than |  | `\<` | Less than (ASCII) *
`-le` | Less than or equal to |  |
`-gt` | Greater than |  | `\>` | Greater than (ASCII) *
`-ge` | Greater than or equal to |  |
|  |  | `-z` | String is empty
|  |  | `-n` | String is not empty
Arithmetic Comparison | within double parentheses (( ... ))
`>` | Greater than
`>=` | Greater than or equal to
`<` | Less than
`<=` | Less than or equal to

`*` If within a double-bracket [[ ... ]] test construct, then no escape \ is needed.

### TEST Operators: Files

Operator | Tests Whether | ----- | Operator | Tests Whether
--- | --- | --- | --- | ---
`-e` | File exists |  | `-s` | File is not zero size
`-f` | File is a regular file
`-d` | File is a directory |  | `-r` | File has read permission
`-h` | File is a symbolic link |  | `-w` | File has write permission
`-L` | File is a symbolic link |  | `-x` | File has execute permission
`-b` | File is a block device
`-c` | File is a character device |  | `-g` | sgid flag set
`-p` | File is a pipe |  | `-u` | suid flag set
`-S` | File is a socket |  | `-k` | sticky bit set
`-t` | File is associated with a terminal
`-N` | File modified since it was last read |  | `F1 -nt F2` | File F1 is newer than F2 *
`-O` | You own the file |  | `F1 -ot F2` | File F1 is older than F2 *
`-G` | Group id of file same as yours |  | `F1 -ef F2` | Files F1 and F2 are hard links to the same file *
`!` | NOT (inverts sense of above tests)

`*` Binary operator (requires two operands).

### Parameter Substitution and Expansion

Expression | Meaning
--- | ---
`${var}` | Value of `var` (same as `$var`)
`${var-$DEFAULT}` | If `var` not set, evaluate expression as `$DEFAULT` *
`${var:-$DEFAULT}` | If `var` not set or is empty, evaluate expression as `$DEFAULT` *
`${var=$DEFAULT}` | If `var` not set, evaluate expression as `$DEFAULT` *
`${var:=$DEFAULT}` | If `var` not set or is empty, evaluate expression as `$DEFAULT` *
`${var+$OTHER}` | If `var` set, evaluate expression as `$OTHER`, otherwise as null string
`${var:+$OTHER}` | If `var` set, evaluate expression as `$OTHER`, otherwise as null string
`${var?$ERR_MSG}` | If `var` not set, print `$ERR_MSG` and abort script with an exit status of 1.*
`${var:?$ERR_MSG}` | If `var` not set, print `$ERR_MSG` and abort script with an exit status of 1.*
`${!varprefix*}` | Matches all previously declared variables beginning with `varprefix`
`${!varprefix@}` | Matches all previously declared variables beginning with `varprefix`

`*` If `var` is set, evaluate the expression as `$var` with no side-effects.

`# Note` that some of the above behavior of operators has changed from earlier versions of Bash.

### String Operations

Expression | Meaning
--- | ---
`${#string}` | Length of `$string`
`${string:position}` | Extract substring from `$string` at `$position`
`${string:position:length}` | Extract `$length` characters substring from `$string` at `$position` [zero-indexed, first character is at position 0]
`${string#substring}` | Strip shortest match of `$substring` from front of `$string`
`${string##substring}` | Strip longest match of `$substring` from front of `$string`
`${string%substring}` | Strip shortest match of `$substring` from back of `$string`
`${string%%substring}` | Strip longest match of `$substring` from back of `$string`
`${string/substring/replacement}` | Replace first match of `$substring` with `$replacement`
`${string//substring/replacement}` | Replace all matches of `$substring` with `$replacement`
`${string/#substring/replacement}` | If `$substring` matches front end of `$string`, substitute `$replacement` for `$substring`
`${string/%substring/replacement}` | If `$substring` matches back end of `$string`, substitute `$replacement` for `$substring`
`expr match "$string" '$substring'` | Length of matching `$substring`* at beginning of `$string`
`expr "$string" : '$substring'` | Length of matching `$substring`* at beginning of `$string`
`expr index "$string" $substring` | Numerical position in `$string` of first character in `$substring`* that matches [0 if no match, first character counts as position 1]
`expr substr $string $position $length` | Extract `$length` characters from `$string` starting at `$position` [0 if no match, first character counts as position 1]
`expr match "$string" '\($substring\)'` | Extract `$substring`*, searching from beginning of `$string`
`expr "$string" : '\($substring\)'` | Extract `$substring`* , searching from beginning of `$string`
`expr match "$string" '.*\($substring\)'` | Extract `$substring`*, searching from end of `$string`
`expr "$string" : '.*\($substring\)'` | Extract `$substring`*, searching from end of `$string`

`*` Where `$substring` is a Regular Expression.

### Miscellaneous Constructs

Expression | Interpretation
--- | ---
Brackets |
`if [ CONDITION ]` | Test construct
`if [[ CONDITION ]]` | Extended test construct
`Array[1]=element1` | Array initialization
`[a-z]` | Range of characters within a Regular Expression
Curly Brackets |
`${variable}` | Parameter substitution
`${!variable}` | Indirect variable reference
`{ command1; command2; . . . commandN; }` | Block of code
`{string1,string2,string3,...}` | Brace expansion
`{a..z}` | Extended brace expansion
`{}` | Text replacement, after find and xargs
Parentheses |
`( command1; command2 )` | Command group executed within a subshell
`Array=(element1 element2 element3)` | Array initialization
`result=$(COMMAND)` | Command substitution, new style
`>(COMMAND)` | Process substitution
`<(COMMAND)` | Process substitution
Double Parentheses |
`(( var = 78 ))` | Integer arithmetic
`var=$(( 20 + 5 ))` | Integer arithmetic, with variable assignment
`(( var++ ))` | C-style variable increment
`(( var-- ))` | C-style variable decrement
`(( var0 = var1<98?9:21 ))` | C-style ternary operation
Quoting |
`"$variable"` | "Weak" quoting
`'string'` | 'Strong' quoting
Back Quotes |
`result=`COMMAND`` | Command substitution, classic style

## A Sed and Awk Micro-Primer

This is a very brief introduction to the `sed` and `awk` text processing utilities. We will deal with only a few basic commands here, but that will suffice for understanding simple sed and awk constructs within shell scripts.

`sed`: a non-interactive text file editor

`awk`: a field-oriented pattern processing language with a `C`-style syntax

For all their differences, the two utilities share a similar invocation syntax, use regular expressions , read input by default from `stdin`, and output to `stdout`. These are well-behaved UNIX tools, and they work together well. The output from one can be piped to the other, and their combined capabilities give shell scripts some of the power of Perl.

One important difference between the utilities is that while shell scripts can easily pass arguments to sed, it is more cumbersome for awk (see and ).

### Sed

Sed is a non-interactive Sed executes without user intervention. `s`tream `ed`itor. It receives text input, whether from `stdin` or from a file, performs certain operations on specified lines of the input, one line at a time, then outputs the result to `stdout` or to a file. Within a shell script, sed is usually one of several tool components in a pipe.

Sed determines which lines of its input that it will operate on from the address range passed to it. If no address range is specified, the default is all lines. Specify this address range either by line number or by a pattern to match. For example, 3d signals sed to delete line 3 of the input, and /Windows/d tells sed that you want every line of the input containing a match to Windows deleted.

Of all the operations in the sed toolkit, we will focus primarily on the three most commonly used ones. These are `p`rinting (to `stdout`), `d`eletion, and `s`ubstitution.

#### Basic sed operators

Operator | Name | Effect
--- | --- | ---
`[address-range]/p` | print | Print [specified address range]
`[address-range]/d` | delete | Delete [specified address range]
`s/pattern1/pattern2/` | substitute | Substitute pattern2 for first instance of pattern1 in a line
`[address-range]/s/pattern1/pattern2/` | substitute | Substitute pattern2 for first instance of pattern1 in a line, over address-range
`[address-range]/y/pattern1/pattern2/` | transform | replace any character in pattern1 with the corresponding character in pattern2, over address-range (equivalent of `tr`)
`[address] i pattern Filename` | insert | Insert pattern at address indicated in file Filename. Usually used with `-i` in-place option.
`g` | global | Operate on every pattern match within each matched line of input

Unless the `g` (global) operator is appended to a substitute command, the substitution operates only on the first instance of a pattern match within each line.

From the command-line and in a shell script, a sed operation may require quoting and certain options.

    sed -e '/^$/d' $filename
    # The -e option causes the next string to be interpreted as an editing instruction.
    #  (If passing only a single instruction to sed, the "-e" is optional.)
    #  The "strong" quotes ('') protect the RE characters in the instruction
    #+ from reinterpretation as special characters by the body of the script.
    # (This reserves RE expansion of the instruction for sed.)
    #
    # Operates on the text contained in file $filename.

In certain cases, a sed editing command will not work with single quotes.

    filename=file1.txt
    pattern=BEGIN

      sed "/^$pattern/d" "$filename"  # Works as specified.
    # sed '/^$pattern/d' "$filename"    has unexpected results.
    #        In this instance, with strong quoting (' ... '),
    #+      "$pattern" will not expand to "BEGIN".

Sed uses the `-e` option to specify that the following string is an instruction or set of instructions. If there is only a single instruction contained in the string, then this may be omitted.

    sed -n '/xzy/p' $filename
    # The -n option tells sed to print only those lines matching the pattern.
    # Otherwise all input lines would print.
    # The -e option not necessary here since there is only a single editing instruction.

#### Examples of sed operators

Notation | Effect
--- | ---
`8d` | Delete 8th line of input.
`/^$/d` | Delete all blank lines.
`1,/^$/d` | Delete from beginning of input up to, and including first blank line.
`/Jones/p` | Print only lines containing Jones (with -n option).
`s/Windows/Linux/` | Substitute Linux for first instance of Windows found in each input line.
`s/BSOD/stability/g` | Substitute stability for every instance of BSOD found in each input line.
`s/ *$//` | Delete all spaces at the end of every line.
`s/00*/0/g` | Compress all consecutive sequences of zeroes into a single zero.
`echo "Working on it." | sed -e '1i How far are you along?'` | Prints "How far are you along?" as first line, "Working on it" as second.
`5i 'Linux is great.' file.txt` | Inserts 'Linux is great.' at line 5 of the file file.txt.
`/GUI/d` | Delete all lines containing GUI.
`s/GUI//g` | Delete all instances of GUI, leaving the remainder of each line intact.

Substituting a zero-length string for another is equivalent to deleting that string within a line of input. This leaves the remainder of the line intact. Applying `s/GUI//` to the line

    The most important parts of any application are its GUI and sound effects

results in

    The most important parts of any application are its  and sound effects

A backslash forces the `sed` replacement command to continue on to the next line. This has the effect of using the newline at the end of the first line as the replacement string.

    s/^  */\
    /g

This substitution replaces line-beginning spaces with a newline. The net result is to replace paragraph indents with a blank line between paragraphs.

An address range followed by one or more operations may require open and closed curly brackets, with appropriate newlines.

    /[0-9A-Za-z]/,/^$/{
    /^$/d
    }

This deletes only the first of each set of consecutive blank lines. That might be useful for single-spacing a text file, but retaining the blank line(s) between paragraphs.

The usual delimiter that sed uses is /. However, sed allows other delimiters, such as %. This is useful when / is part of a replacement string, as in a file pathname. See and .

A quick way to double-space a text file is `sed G filename`.

For illustrative examples of sed within shell scripts, see:

For a more extensive treatment of sed, refer to the pertinent references in the .

### Awk

Awk Its name derives from the initials of its authors, `A`ho, `W`einberg, and `K`ernighan. is a full-featured text processing language with a syntax reminiscent of C. While it possesses an extensive set of operators and capabilities, we will cover only a few of these here - the ones most useful in shell scripts.

Awk breaks each line of input passed to it into fields. By default, a field is a string of consecutive characters delimited by whitespace, though there are options for changing this. Awk parses and operates on each separate field. This makes it ideal for handling structured text files -- especially tables -- data organized into consistent chunks, such as rows and columns.

Strong quoting and curly brackets enclose blocks of awk code within a shell script.

    # $1 is field #1, $2 is field #2, etc.

    echo one two | awk '{print $1}'
    # one

    echo one two | awk '{print $2}'
    # two

    # But what is field #0 ($0)?
    echo one two | awk '{print $0}'
    # one two
    # All the fields!

    awk '{print $3}' $filename
    # Prints field #3 of file $filename to stdout.

    awk '{print $1 $5 $6}' $filename
    # Prints fields #1, #5, and #6 of file $filename.

    awk '{print $0}' $filename
    # Prints the entire file!
    # Same effect as:   cat $filename . . . or . . . sed '' $filename

We have just seen the awk print command in action. The only other feature of awk we need to deal with here is variables. Awk handles variables similarly to shell scripts, though a bit more flexibly.

    { total += ${column_number} }

This adds the value of `column_number` to the running total of `total`>. Finally, to print total, there is an `END` command block, executed after the script has processed all its input.

    END { print total }

Corresponding to the `END`, there is a `BEGIN`, for a code block to be performed before awk starts processing its input.

The following example illustrates how `awk` can add text-parsing tools to a shell script.

#### Counting Letter Occurrences

    &lettercount2;

For simpler examples of awk within shell scripts, see:

That's all the awk we'll cover here, folks, but there's lots more to learn. See the appropriate references in the .

## Parsing and Managing Pathnames

Emmanual Rouat contributed the following example of parsing and transforming filenames and, in particular, pathnames. It draws heavily on the functionality of sed.

    #!/usr/bin/env bash
    #-----------------------------------------------------------
    # Management of PATH, LD_LIBRARY_PATH, MANPATH variables...
    # By Emmanuel Rouat <no-email>
    # (Inspired by the bash documentation 'pathfuncs' and on
    # discussions found on stackoverflow:
    # http://stackoverflow.com/questions/370047/
    # http://stackoverflow.com/questions/273909/#346860 )
    # Last modified: Sat Sep 22 12:01:55 CEST 2012
    #
    # The following functions handle spaces correctly.
    # These functions belong in .bash_profile rather than in
    # .bashrc, I guess.
    #
    # The modular aspect of these functions should make it easy
    # to expand them to handle path substitutions instead
    # of path removal etc....
    #
    # See http://www.catonmat.net/blog/awk-one-liners-explained-part-two/
    # (item 43) for an explanation of the 'duplicate-entries' removal
    # (it's a nice trick!)
    #-----------------------------------------------------------

    # Show $@ (usually PATH) as list.
    function p_show() { local p="$@" && for p; do [[ ${!p} ]] &&
    echo -e ${!p//:/\\n}; done }

    # Filter out empty lines, multiple/trailing slashes, and duplicate entries.
    function p_filter()
    { awk '/^[ \t]*$/ {next} {sub(/\/+$/, "");gsub(/\/+/, "/")}!x[$0]++' ;}

    # Rebuild list of items into ':' separated word (PATH-like).
    function p_build() { paste -sd: ;}

    # Clean $1 (typically PATH) and rebuild it
    function p_clean()
    { local p=${1} && eval ${p}='$(p_show ${p} | p_filter | p_build)' ;}

    # Remove $1 from $2 (found on stackoverflow, with modifications).
    function p_rm()
    { local d=$(echo $1 | p_filter) p=${2} &&
      eval ${p}='$(p_show ${p} | p_filter | grep -xv "${d}" | p_build)' ;}

    #  Same as previous, but filters on a pattern (dangerous...
    #+ don't use 'bin' or '/' as pattern!).
    function p_rmpat()
    { local d=$(echo $1 | p_filter) p=${2} && eval ${p}='$(p_show ${p} |
      p_filter | grep -v "${d}" | p_build)' ;}

    # Delete $1 from $2 and append it cleanly.
    function p_append()
    { local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&
      eval ${p}='$(p_show ${p} d | p_build)' ;}

    # Delete $1 from $2 and prepend it cleanly.
    function p_prepend()
    { local d=$(echo $1 | p_filter) p=${2} && p_rm "${d}" ${p} &&
      eval ${p}='$(p_show d ${p} | p_build)' ;}

    # Some tests:
    echo
    MYPATH="/bin:/usr/bin/:/bin://bin/"
    p_append "/project//my project/bin" MYPATH
    echo "Append '/project//my project/bin' to '/bin:/usr/bin/:/bin://bin/'"
    echo "(result should be: /bin:/usr/bin:/project/my project/bin)"
    echo $MYPATH

    echo
    MYOTHERPATH="/bin:/usr/bin/:/bin:/project//my project/bin"
    p_prepend "/project//my project/bin" MYOTHERPATH
    echo "Prepend '/project//my project/bin' \
    to '/bin:/usr/bin/:/bin:/project//my project/bin/'"
    echo "(result should be: /project/my project/bin:/bin:/usr/bin)"
    echo $MYOTHERPATH

    echo
    p_prepend "/project//my project/bin" FOOPATH  # FOOPATH doesn't exist.
    echo "Prepend '/project//my project/bin' to an unset variable"
    echo "(result should be: /project/my project/bin)"
    echo $FOOPATH

    echo
    BARPATH="/a:/b/://b c://a:/my local pub"
    p_clean BARPATH
    echo "Clean BARPATH='/a:/b/://b c://a:/my local pub'"
    echo "(result should be: /a:/b:/b c:/my local pub)"
    echo $BARPATH

***

David Wheeler kindly permitted me to use his instructive examples.

    Doing it correctly: A quick summary
    by David Wheeler
    http://www.dwheeler.com/essays/filenames-in-shell.html

    So, how can you process filenames correctly in shell? Here's a quick
    summary about how to do it correctly, for the impatient who "just want the
    answer". In short: Double-quote to use "$variable" instead of $variable,
    set IFS to just newline and tab, prefix all globs/filenames so they cannot
    begin with "-" when expanded, and use one of a few templates that work
    correctly. Here are some of those templates that work correctly:

     IFS="$(printf '\n\t')"
     # Remove SPACE, so filenames with spaces work well.

     #  Correct glob use:
     #+ always use "for" loop, prefix glob, check for existence:
     for file in ./* ; do          # Use "./*" ... NEVER bare "*" ...
       if [ -e "$file" ] ; then    # Make sure it isn't an empty match.
         COMMAND ... "$file" ...
       fi
     done

     # Correct glob use, but requires nonstandard bash extension.
     shopt -s nullglob  #  Bash extension,
                        #+ so that empty glob matches will work.
     for file in ./* ; do        # Use "./*", NEVER bare "*"
       COMMAND ... "$file" ...
     done

     #  These handle all filenames correctly;
     #+ can be unwieldy if COMMAND is large:
     find ... -exec COMMAND... {} \;
     find ... -exec COMMAND... {} \+ # If multiple files are okay for COMMAND.

     #  This skips filenames with control characters
     #+ (including tab and newline).
     IFS="$(printf '\n\t')"
     controlchars="$(printf '*[\001-\037\177]*')"
     for file in $(find . ! -name "$controlchars"') ; do
       COMMAND "$file" ...
     done

     #  Okay if filenames can't contain tabs or newlines --
     #+ beware the assumption.
     IFS="$(printf '\n\t')"
     for file in $(find .) ; do
       COMMAND "$file" ...
     done

     # Requires nonstandard but common extensions in find and xargs:
     find . -print0 | xargs -0 COMMAND

     # Requires nonstandard extensions to find and to shell (bash works).
     # variables might not stay set once the loop ends:
     find . -print0 | while IFS="" read -r -d "" file ; do ...
       COMMAND "$file" # Use quoted "$file", not $file, everywhere.
     done

     #  Requires nonstandard extensions to find and to shell (bash works).
     #  Underlying system must include named pipes (FIFOs)
     #+ or the /dev/fd mechanism.
     #  In this version, variables *do* stay set after the loop ends,
     #  and you can read from stdin.
     #+ (Change the 4 to another number if fd 4 is needed.)

     while IFS="" read -r -d "" file <&4 ; do
       COMMAND "$file"   # Use quoted "$file" -- not $file, everywhere.
     done 4< <(find . -print0)

     #  Named pipe version.
     #  Requires nonstandard extensions to find and to shell's read (bash ok).
     #  Underlying system must include named pipes (FIFOs).
     #  Again, in this version, variables *do* stay set after the loop ends,
     #  and you can read from stdin.
     # (Change the 4 to something else if fd 4 needed).

     mkfifo mypipe

     find . -print0 > mypipe &
     while IFS="" read -r -d "" file <&4 ; do
       COMMAND "$file" # Use quoted "$file", not $file, everywhere.
     done 4< mypipe

## Exit Codes With Special Meanings

### Reserved Exit Codes

Exit Code Number | Meaning | Example | Comments
--- | --- | --- | ---
`1` | Catchall for general errors | let "var1 = 1/0" | Miscellaneous errors, such as divide by zero and other impermissible operations
`2` | Misuse of shell builtins (according to Bash documentation) | empty_function() {} | Missing keyword or command, or permission problem (and diff return code on a failed binary file comparison).
`126` | Command invoked cannot execute | /dev/null | Permission problem or command is not an executable
`127` | command not found | illegal_command | Possible problem with `$PATH` or a typo
`128` | Invalid argument to exit | exit 3.14159 | `exit` takes only integer args in the range 0 - 255 (see first footnote)
`128+n` | Fatal error signal n | kill -9 `$PPID` of script | `$?` returns 137 (128 + 9)
`130` | Script terminated by Control-C | Ctl-C | Control-C is fatal error signal 2, (130 = 128 + 2, see above)
`255*` | Exit status out of range | exit -1 | `exit` takes only integer args in the range 0 - 255

According to the above table, exit codes 1 - 2, 126 - 165, and 255

Out of range exit values can result in unexpected exit codes. An exit value greater than 255 returns an exit code modulo 256. For example, exit 3809 gives an exit code of 225 (3809 % 256 = 225).

have special meanings, and should therefore be avoided for user-specified exit parameters. Ending a script with exit 127 would certainly cause confusion when troubleshooting (is the error code a command not found or a user-defined one?). However, many scripts use an exit 1 as a general bailout-upon-error. Since exit code 1 signifies so many possible errors, it is not particularly useful in debugging.

There has been an attempt to systematize exit status numbers (see `/usr/include/sysexits.h`), but this is intended for C and C++ programmers. A similar standard for scripting might be appropriate. The author of this document proposes restricting user-defined exit codes to the range 64 - 113 (in addition to 0, for success), to conform with the C/C++ standard. This would allot 50 valid codes, and make troubleshooting scripts more straightforward. An update of `/usr/include/sysexits.h` allocates previously unused exit codes from 64 - 78. It may be anticipated that the range of unallotted exit codes will be further restricted in the future. The author of this document will not do fixups on the scripting examples to conform to the changing standard. This should not cause any problems, since there is no overlap or conflict in usage of exit codes between compiled C/C++ binaries and shell scripts. All user-defined exit codes in the accompanying examples to this document conform to this standard, except where overriding circumstances exist, as in .

Issuing a $? from the command-line after a shell script exits gives results consistent with the table above only from the Bash or sh prompt. Running the C-shell or tcsh may give different values in some cases.

## A Detailed Introduction to I/O and I/O Redirection

written by Stéphane Chazelas, and revised by the document author

A command expects the first three file descriptors to be available. The first, fd 0 (standard input, `stdin`), is for reading. The other two (fd 1, `stdout` and fd 2, `stderr`) are for writing.

There is a `stdin`, `stdout`, and a `stderr` associated with each command. `ls 2>&1` means temporarily connecting the `stderr` of the `ls` command to the same resource as the shell's `stdout`.

By convention, a command reads its input from fd 0 (`stdin`), prints normal output to fd 1 (`stdout`), and error ouput to fd 2 (`stderr`). If one of those three fd's is not open, you may encounter problems:

    bash$ cat /etc/passwd >&-
    cat: standard output: Bad file descriptor

For example, when `xterm` runs, it first initializes itself. Before running the user's shell, `xterm` opens the terminal device (/dev/pts/<n> or something similar) three times.

At this point, Bash inherits these three file descriptors, and each command (child process) run by Bash inherits them in turn, except when you redirect the command. Redirection means reassigning one of the file descriptors to another file (or a pipe, or anything permissible). File descriptors may be reassigned locally (for a command, a command group, a subshell, a while or if or case or for loop...), or globally, for the remainder of the shell (using exec).

`ls > /dev/null` means running `ls` with its fd 1 connected to `/dev/null`.

    bash$ lsof -a -p $$ -d0,1,2
    COMMAND PID     USER   FD   TYPE DEVICE SIZE NODE NAME
     bash    363 bozo        0u   CHR  136,1         3 /dev/pts/1
     bash    363 bozo        1u   CHR  136,1         3 /dev/pts/1
     bash    363 bozo        2u   CHR  136,1         3 /dev/pts/1

    bash$ exec 2> /dev/null
    bash$ lsof -a -p $$ -d0,1,2
    COMMAND PID     USER   FD   TYPE DEVICE SIZE NODE NAME
     bash    371 bozo        0u   CHR  136,1         3 /dev/pts/1
     bash    371 bozo        1u   CHR  136,1         3 /dev/pts/1
     bash    371 bozo        2w   CHR    1,3       120 /dev/null

    bash$ bash -c 'lsof -a -p $$ -d0,1,2' | cat
    COMMAND PID USER   FD   TYPE DEVICE SIZE NODE NAME
     lsof    379 root    0u   CHR  136,1         3 /dev/pts/1
     lsof    379 root    1w  FIFO    0,0      7118 pipe
     lsof    379 root    2u   CHR  136,1         3 /dev/pts/1

    bash$ echo "$(bash -c 'lsof -a -p $$ -d0,1,2' 2>&1)"
    COMMAND PID USER   FD   TYPE DEVICE SIZE NODE NAME
     lsof    426 root    0u   CHR  136,1         3 /dev/pts/1
     lsof    426 root    1w  FIFO    0,0      7520 pipe
     lsof    426 root    2w  FIFO    0,0      7520 pipe

This works for different types of redirection.

`Exercise:` Analyze the following script.

    #! /usr/bin/env bash

    mkfifo /tmp/fifo1 /tmp/fifo2
    while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1
    exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7)

    exec 3>&1
    (
     (
      (
       while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr \
       | tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2

       echo 1st, to stdout
       sleep 1
       echo 2nd, to stderr >&2
       sleep 1
       echo 3rd, to fd 3 >&3
       sleep 1
       echo 4th, to fd 4 >&4
       sleep 1
       echo 5th, to fd 5 >&5
       sleep 1
       echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5
       sleep 1
       echo 7th, to fd 6 >&6
       sleep 1
       echo 8th, to fd 7 >&7
       sleep 1
       echo 9th, to fd 8 >&8

      ) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&-
     ) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&-
    ) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&-

    rm -f /tmp/fifo1 /tmp/fifo2

    # For each command and subshell, figure out which fd points to what.
    # Good luck!

    exit 0

## Command-Line Options

Many executables, whether binaries or script files, accept options to modify their run-time behavior. For example: from the command-line, typing `command -o` would invoke command, with option `o`.

### Standard Command-Line Options

Over time, there has evolved a loose standard for the meanings of command-line option flags. The GNU utilities conform more closely to this standard than older UNIX utilities.

Traditionally, UNIX command-line options consist of a dash, followed by one or more lowercase letters. The GNU utilities added a double-dash, followed by a complete word or compound word.

The two most widely-accepted options are:

- `-h`

  `--help`

  Help: Give usage message and exit.

- `-v`

  `--version`

  Version: Show program version and exit.

Other common options are:

- `-a`

  `--all`

  All: show all information or operate on all arguments.

- `-l`

  `--list`

  List: list files or arguments without taking other action.

- `-o`

  Output filename

- `-q`

  `--quiet`

  Quiet: suppress `stdout`.

- `-r`

  `-R`

  `--recursive`

  Recursive: Operate recursively (down directory tree).

- `-v`

  `--verbose`

  Verbose: output additional information to `stdout` or `stderr`.

- `-z`

  `--compress`

  Compress: apply compression (usually gzip).

However:

- In `tar` and `gawk`:

  `-f`

  `--file`

  File: filename follows.

- In `cp`, `mv`, `rm`:

  `-f`

  `--force`

  Force: force overwrite of target file(s).

Many UNIX and Linux utilities deviate from this standard, so it is dangerous to assume that a given option will behave in a standard way. Always check the man page for the command in question when in doubt.

A complete table of recommended options for the GNU utilities is available at the GNU standards page.

### Bash Command-Line Options

Bash itself has a number of command-line options. Here are some of the more useful ones.

- `-c`

  Read commands from the following string and assign any arguments to the positional parameters.

      bash$ bash -c 'set a b c d; IFS="+-;"; echo "$*"'
      a+b+c+d

- `-r`

  `--restricted`

  Runs the shell, or a script, in restricted mode.

- `--posix`

  Forces Bash to conform to POSIX mode.

- `--version`

  Display Bash version information and exit.

- `--`

  End of options. Anything further on the command line is an argument, not an option.

## Important Files

### startup files

  These files contain the aliases and environmental variables made available to Bash running as a user shell and to all Bash scripts invoked after system initialization.

`/etc/profile`

  Systemwide defaults, mostly setting the environment (all Bourne-type shells, not just Bash This does not apply to `csh`, `tcsh`, and other shells not related to or descended from the classic Bourne shell (`sh`).)

`/etc/bashrc`

  systemwide functions and aliases for Bash

``$HOME`/.bash_profile`

  user-specific Bash environmental default settings, found in each user's home directory (the local counterpart to `/etc/profile`)

``$HOME`/.bashrc`

  user-specific Bash init file, found in each user's home directory (the local counterpart to `/etc/bashrc`). Only interactive shells and user scripts read this file. See for a sample `.bashrc` file.

### logout file

``$HOME`/.bash_logout`

  user-specific instruction file, found in each user's home directory. Upon exit from a login (Bash) shell, the commands in this file execute.

### data files

`/etc/passwd`

  A listing of all the user accounts on the system, their identities, their home directories, the groups they belong to, and their default shell. Note that the user passwords are not stored in this file, In older versions of UNIX, passwords were stored in `/etc/passwd`, and that explains the name of the file. but in `/etc/shadow` in encrypted form.

### system configuration files

`/etc/sysconfig/hwconf`

  Listing and description of attached hardware devices. This information is in text form and can be extracted and parsed.

      bash$ grep -A 5 AUDIO /etc/sysconfig/hwconf
      class: AUDIO
       bus: PCI
       detached: 0
       driver: snd-intel8x0
       desc: "Intel Corporation 82801CA/CAM AC'97 Audio Controller"
       vendorId: 8086

  This file is present on Red Hat and Fedora Core installations, but may be missing from other distros.

## Important System Directories

Sysadmins and anyone else writing administrative scripts should be intimately familiar with the following system directories.

- `/bin`

  Binaries (executables). Basic system programs and utilities (such as `bash`).

- `/usr/bin`

  Some early UNIX systems had a fast, small-capacity fixed disk (containing `/`, the root partition), and a second drive which was larger, but slower (containing `/usr` and other partitions). The most frequently used programs and utilities therefore resided on the small-but-fast drive, in `/bin`, and the others on the slower drive, in `/usr/bin`. This likewise accounts for the split between `/sbin` and `/usr/sbin`, `/lib` and `/usr/lib`, etc.

  More system binaries.

- `/usr/local/bin`

  Miscellaneous binaries local to the particular machine.

- `/sbin`

  System binaries. Basic system administrative programs and utilities (such as `fsck`).

- `/usr/sbin`

  More system administrative programs and utilities.

- `/etc`

  Et cetera. Systemwide configuration scripts.

  Of particular interest are the `/etc/fstab` (filesystem table), `/etc/mtab` (mounted filesystem table), and the `/etc/inittab` files.

- `/etc/rc.d`

  Boot scripts, on Red Hat and derivative distributions of Linux.

- `/usr/share/doc`

  Documentation for installed packages.

- `/usr/man`

  The systemwide manpages.

- `/dev`

  Device directory. Entries (but not mount points) for physical and virtual devices. See .

- `/proc`

  Process directory. Contains information and statistics about running processes and kernel parameters. See .

- `/sys`

  Systemwide device directory. Contains information and statistics about device and device names. This is newly added to Linux with the 2.6.X kernels.

- `/mnt`

  Mount. Directory for mounting hard drive partitions, such as `/mnt/dos`, and physical devices. In newer Linux distros, the `/media` directory has taken over as the preferred mount point for I/O devices.

- `/media`

  In newer Linux distros, the preferred mount point for I/O devices, such as CD/DVD drives or USB flash drives.

- `/var`

  Variable (changeable) system files. This is a catchall scratchpad directory for data generated while a Linux/UNIX machine is running.

- `/var/log`

  Systemwide log files.

- `/var/spool/mail`

  User mail spool.

- `/lib`

  Systemwide library files.

- `/usr/lib`

  More systemwide library files.

- `/tmp`

  System temporary files.

- `/boot`

  System boot directory. The kernel, module links, system map, and boot manager reside here.

  Altering files in this directory may result in an unbootable system.

&TABEXP;

## Localization

Localization is an undocumented Bash feature.

A localized shell script echoes its text output in the language defined as the system's locale. A Linux user in Berlin, Germany, would get script output in German, whereas his cousin in Berlin, Maryland, would get output from the same script in English.

To create a localized script, use the following template to write all messages to the user (error messages, prompts, etc.).

    #!/bin/bash
    # localized.sh
    #  Script by Stéphane Chazelas,
    #+ modified by Bruno Haible, bugfixed by Alfredo Pironti.

    . gettext.sh

    E_CDERROR=65

    error()
    {
      printf "$@" >&2
      exit $E_CDERROR
    }

    cd $var || error "`eval_gettext \"Can\'t cd to \\\$var.\"`"
    #  The triple backslashes (escapes) in front of $var needed
    #+ "because eval_gettext expects a string
    #+ where the variable values have not yet been substituted."
    #    -- per Bruno Haible
    read -p "`gettext \"Enter the value: \"`" var
    #  ...

    #  ------------------------------------------------------------------
    #  Alfredo Pironti comments:

    #  This script has been modified to not use the $"..." syntax in
    #+ favor of the "`gettext \"...\"`" syntax.
    #  This is ok, but with the new localized.sh program, the commands
    #+ "bash -D filename" and "bash --dump-po-string filename"
    #+ will produce no output
    #+ (because those command are only searching for the $"..." strings)!
    #  The ONLY way to extract strings from the new file is to use the
    # 'xgettext' program. However, the xgettext program is buggy.

    # Note that 'xgettext' has another bug.
    #
    # The shell fragment:
    #    gettext -s "I like Bash"
    # will be correctly extracted, but . . .
    #    xgettext -s "I like Bash"
    # . . . fails!
    #  'xgettext' will extract "-s" because
    #+ the command only extracts the
    #+ very first argument after the 'gettext' word.

    #  Escape characters:
    #
    #  To localize a sentence like
    #     echo -e "Hello\tworld!"
    #+ you must use
    #     echo -e "`gettext \"Hello\\tworld\"`"
    #  The "double escape character" before the `t' is needed because
    #+ 'gettext' will search for a string like: 'Hello\tworld'
    #  This is because gettext will read one literal `\')
    #+ and will output a string like "Bonjour\tmonde",
    #+ so the 'echo' command will display the message correctly.
    #
    #  You may not use
    #     echo "`gettext -e \"Hello\tworld\"`"
    #+ due to the xgettext bug explained above.

    # Let's localize the following shell fragment:
    #     echo "-h display help and exit"
    #
    # First, one could do this:
    #     echo "`gettext \"-h display help and exit\"`"
    #  This way 'xgettext' will work ok,
    #+ but the 'gettext' program will read "-h" as an option!
    #
    # One solution could be
    #     echo "`gettext -- \"-h display help and exit\"`"
    #  This way 'gettext' will work,
    #+ but 'xgettext' will extract "--", as referred to above.
    #
    # The workaround you may use to get this string localized is
    #     echo -e "`gettext \"\\0-h display help and exit\"`"
    #  We have added a \0 (NULL) at the beginning of the sentence.
    #  This way 'gettext' works correctly, as does 'xgettext.'
    #  Moreover, the NULL character won't change the behavior
    #+ of the 'echo' command.
    #  ------------------------------------------------------------------

    bash$ bash -D localized.sh
    "Can't cd to %s."
     "Enter the value: "

This lists all the localized text. (The `-D` option lists double-quoted strings prefixed by a $, without executing the script.)

    bash$ bash --dump-po-strings localized.sh
    #: a:6
     msgid "Can't cd to %s."
     msgstr ""
     #: a:7
     msgid "Enter the value: "
     msgstr ""

The `--dump-po-strings` option to Bash resembles the `-D` option, but uses gettext po format.

Bruno Haible points out:

Starting with gettext-0.12.2, `xgettext -o - localized.sh` is recommended instead of `bash --dump-po-strings localized.sh`, because `xgettext` . . .

1. understands the gettext and eval_gettext commands (whereas bash --dump-po-strings understands only its deprecated $"..." syntax)

2. can extract comments placed by the programmer, intended to be read by the translator.

This shell code is then not specific to Bash any more; it works the same way with Bash 1.x and other /bin/sh implementations.

Now, build a `language.po` file for each language that the script will be translated into, specifying the msgstr. Alfredo Pironti gives the following example:

fr.po:

    #: a:6
    msgid "Can't cd to $var."
    msgstr "Impossible de se positionner dans le repertoire $var."
    #: a:7
    msgid "Enter the value: "
    msgstr "Entrez la valeur : "

    #  The string are dumped with the variable names, not with the %s syntax,
    #+ similar to C programs.
    #+ This is a very cool feature if the programmer uses
    #+ variable names that make sense!

Then, run msgfmt.

`msgfmt -o localized.sh.mo fr.po`

Place the resulting `localized.sh.mo` file in the `/usr/local/share/locale/fr/LC_MESSAGES` directory, and at the beginning of the script, insert the lines:

    TEXTDOMAINDIR=/usr/local/share/locale
    TEXTDOMAIN=localized.sh

If a user on a French system runs the script, she will get French messages.

With older versions of Bash or other shells, localization requires gettext, using the `-s` option. In this case, the script becomes:

    #!/bin/bash
    # localized.sh

    E_CDERROR=65

    error() {
      local format=$1
      shift
      printf "$(gettext -s "$format")" "$@" >&2
      exit $E_CDERROR
    }
    cd $var || error "Can't cd to %s." "$var"
    read -p "$(gettext -s "Enter the value: ")" var
    # ...

The `TEXTDOMAIN` and `TEXTDOMAINDIR` variables need to be set and exported to the environment. This should be done within the script itself.

---

This appendix written by Stéphane Chazelas, with modifications suggested by Alfredo Pironti, and by Bruno Haible, maintainer of GNU gettext.

## History Commands

The Bash shell provides command-line tools for editing and manipulating a user's command history. This is primarily a convenience, a means of saving keystrokes.

Bash history commands:

1. `history`

2. `fc`

    bash$ history
       1  mount /mnt/cdrom
        2  cd /mnt/cdrom
        3  ls
         ...

Internal variables associated with Bash history commands:

1. $HISTCMD

2. $HISTCONTROL

3. $HISTIGNORE

4. $HISTFILE

5. $HISTFILESIZE

6. $HISTSIZE

7. $HISTTIMEFORMAT (Bash, ver. 3.0 or later)

8. !!

9. !$

10. !#

11. !N

12. !-N

13. !STRING

14. !?STRING?

15. ^STRING^string^

Unfortunately, the Bash history tools find no use in scripting.

    #!/bin/bash
    # history.sh
    # A (vain) attempt to use the 'history' command in a script.

    history                      # No output.

    var=$(history); echo "$var"  # $var is empty.

    #  History commands are, by default, disabled within a script.
    #  However, as dhw points out,
    #+ set -o history
    #+ enables the history mechanism.

    set -o history
    var=$(history); echo "$var"   # 1  var=$(history)

    bash$ ./history.sh
    (no output)

The Advancing in the Bash Shell site gives a good introduction to the use of history commands in Bash.

## Sample `.bashrc` and `.bash_profile` Files

The `~/.bashrc` file determines the behavior of interactive shells. A good look at this file can lead to a better understanding of Bash.

Emmanuel Rouat contributed the following very elaborate `.bashrc` file, written for a Linux system. He welcomes reader feedback on it.

Study the file carefully, and feel free to reuse code snippets and functions from it in your own `.bashrc` file or even in your scripts.

### Sample `.bashrc` file

    &bashrc;

And, here is a snippet from Andrzej Szelachowski's instructive `.bash_profile` file.

### `.bash_profile` file

    &bashprof;

## Converting DOS Batch Files to Shell Scripts

Quite a number of programmers learned scripting on a PC running DOS. Even the crippled DOS batch file language allowed writing some fairly powerful scripts and applications, though they often required extensive kludges and workarounds. Occasionally, the need still arises to convert an old DOS batch file to a UNIX shell script. This is generally not difficult, as DOS batch file operators are only a limited subset of the equivalent shell scripting ones.

### Batch file keywords / variables / operators, and their shell equivalents

Batch File Operator | Shell Script Equivalent | Meaning
--- | --- | ---
`%` | $ | command-line parameter prefix
`/` | - | command option flag
`\` | / | directory path separator
`==` | = | (equal-to) string comparison test
`!==!` | != | (not equal-to) string comparison test
`|` | | | pipe
`@` | set `+v` | do not echo current command
`*` | * | filename wild card
`>` | > | file redirection (overwrite)
`>>` | >> | file redirection (append)
`<` | < | redirect `stdin`
`%VAR%` | $VAR | environmental variable
`REM` | # | comment
`NOT` | ! | negate following test
`NUL` | `/dev/null` | black hole for burying command output
`ECHO` | echo | echo (many more option in Bash)
`ECHO.` | echo | echo blank line
`ECHO OFF` | set `+v` | do not echo command(s) following
`FOR %%VAR IN (LIST) DO` | for var in [list]; do | for loop
`:LABEL` | none (unnecessary) | label
`GOTO` | none (use a function) | jump to another location in the script
`PAUSE` | sleep | pause or wait an interval
`CHOICE` | case or select | menu choice
`IF` | if | if-test
`IF EXIST FILENAME` | if [ -e filename ] | test if file exists
`IF !%N==!` | if [ -z "$N" ] | if replaceable parameter N not present
`CALL` | source or . (dot operator) | include another script
`COMMAND /C` | source or . (dot operator) | include another script (same as CALL)
`SET` | export | set an environmental variable
`SHIFT` | shift | left shift command-line argument list
`SGN` | -lt or -gt | sign (of integer)
`ERRORLEVEL` | $? | exit status
`CON` | `stdin` | console (`stdin`)
`PRN` | `/dev/lp0` | (generic) printer device
`LPT1` | `/dev/lp0` | first printer device
`COM1` | `/dev/ttyS0` | first serial port

Batch files usually contain DOS commands. These must be translated into their UNIX equivalents in order to convert a batch file into a shell script.

### DOS commands and their UNIX equivalents

DOS Command | UNIX Equivalent | Effect
--- | --- | ---
`ASSIGN` | ln | link file or directory
`ATTRIB` | chmod | change file permissions
`CD` | cd | change directory
`CHDIR` | cd | change directory
`CLS` | clear | clear screen
`COMP` | diff, comm, cmp | file compare
`COPY` | cp | file copy
`Ctl-C` | Ctl-C | break (signal)
`Ctl-Z` | Ctl-D | EOF (end-of-file)
`DEL` | rm | delete file(s)
`DELTREE` | rm -rf | delete directory recursively
`DIR` | ls -l | directory listing
`ERASE` | rm | delete file(s)
`EXIT` | exit | exit current process
`FC` | comm, cmp | file compare
`FIND` | grep | find strings in files
`MD` | mkdir | make directory
`MKDIR` | mkdir | make directory
`MORE` | more | text file paging filter
`MOVE` | mv | move
`PATH` | $PATH | path to executables
`REN` | mv | rename (move)
`RENAME` | mv | rename (move)
`RD` | rmdir | remove directory
`RMDIR` | rmdir | remove directory
`SORT` | sort | sort file
`TIME` | date | display system time
`TYPE` | cat | output file to `stdout`
`XCOPY` | cp | (extended) file copy

Virtually all UNIX and shell operators and commands have many more options and enhancements than their DOS and batch file counterparts. Many DOS batch files rely on auxiliary utilities, such as `ask.com`, a crippled counterpart to read.

DOS supports only a very limited and incompatible subset of filename wild-card expansion, recognizing just the * and ? characters.

Converting a DOS batch file into a shell script is generally straightforward, and the result ofttimes reads better than the original.

### VIEWDATA.BAT: DOS Batch File

    &VIEWDAT;

The script conversion is somewhat of an improvement. Various readers have suggested modifications of the above batch file to prettify it and make it more compact and efficient. In the opinion of the ABS Guide author, this is wasted effort. A Bash script can access a DOS filesystem, or even an NTFS partition (with the help of ntfs-3g) to do batch or scripted operations.

### viewdata.sh: Shell Script Conversion of VIEWDATA.BAT

    &viewdata;

Ted Davis' Shell Scripts on the PC site had a set of comprehensive tutorials on the old-fashioned art of batch file programming. Unfortunately the page has vanished without a trace.

## Exercises

The exercises that follow test and extend your knowledge of scripting. Think of them as a challenge, as an entertaining way to take you further along the stony path toward UNIX wizardry.

          On a dingy side street in a run-down section of Hoboken, New Jersey,
          there sits a nondescript squat two-story brick building with an inscription
          incised on a marble plate in its wall:

                           Bash Scripting Hall of Fame.

          Inside, among various dusty uninteresting exhibits is a corroding,
          cobweb-festooned brass plaque inscribed with a short, very short
          list of those few persons who have successfully mastered the material
          in the Advanced Bash Scripting Guide, as evidenced by their performance
          on the following Exercise sections.

          (Alas, the author of the ABS Guide is not represented among the exhibits.
          This is possibly due to malicious rumors about lack of credentials and
          deficient scripting skills.)

### Analyzing Scripts

Examine the following script. Run it, then explain what it does. Annotate the script and rewrite it in a more compact and elegant manner.

    #!/bin/bash

    MAX=10000

      for((nr=1; nr<$MAX; nr++))
      do

        let "t1 = nr % 5"
        if [ "$t1" -ne 3 ]
        then
          continue
        fi

        let "t2 = nr % 7"
        if [ "$t2" -ne 4 ]
        then
          continue
        fi

        let "t3 = nr % 9"
        if [ "$t3" -ne 5 ]
        then
          continue
        fi

      break   # What happens when you comment out this line? Why?

      done

      echo "Number = $nr"

    exit 0

---

Explain what the following script does. It is really just a parameterized command-line pipe.

    #!/bin/bash

    DIRNAME=/usr/bin
    FILETYPE="shell script"
    LOGFILE=logfile

    file "$DIRNAME"/* | fgrep "$FILETYPE" | tee $LOGFILE | wc -l

    exit 0

---

Examine and explain the following script. For hints, you might refer to the listings for find and stat.

    #!/bin/bash

    # Author:  Nathan Coulter
    # This code is released to the public domain.
    # The author gave permission to use this code snippet in the ABS Guide.

    find -maxdepth 1 -type f -printf '%f\000'  | {
       while read -d $'\000'; do
          mv "$REPLY" "$(date -d "$(stat -c '%y' "$REPLY") " '+%Y%m%d%H%M%S'
          )-$REPLY"
       done
    }

    # Warning: Test-drive this script in a "scratch" directory.
    # It will somehow affect all the files there.

---

A reader sent in the following code snippet.

    while read LINE
    do
      echo $LINE
    done < `tail -f /var/log/messages`

He wished to write a script tracking changes to the system log file, `/var/log/messages`. Unfortunately, the above code block hangs and does nothing useful. Why? Fix this so it does work. (Hint: rather than redirecting the `stdin` of the loop, try a pipe.)

---

Analyze the following one-liner (here split into two lines for clarity) contributed by Rory Winston:

    export SUM=0; for f in $(find src -name "*.java");
    do export SUM=$(($SUM + $(wc -l $f | awk '{ print $1 }'))); done; echo $SUM

Hint: First, break the script up into bite-sized sections. Then, carefully examine its use of double-parentheses arithmetic, the export command, the find command, the wc command, and awk.

---

Analyze , and reorganize it in a simplified and more logical style. See how many of the variables can be eliminated, and try to optimize the script to speed up its execution time.

Alter the script so that it accepts any ordinary ASCII text file as input for its initial generation. The script will read the first `$ROW*$COL` characters, and set the occurrences of vowels as living cells. Hint: be sure to translate the spaces in the input file to underscore characters.

### Writing Scripts

Write a script to carry out each of the following tasks.

#### EASY

`Self-reproducing Script`

  Write a script that backs itself up, that is, copies itself to a file named `backup.sh`.

  Hint: Use the cat command and the appropriate positional parameter.

`Home Directory Listing`

  Perform a recursive directory listing on the user's home directory and save the information to a file. Compress the file, have the script prompt the user to insert a USB flash drive, then press ENTER. Finally, save the file to the flash drive after making certain the flash drive has properly mounted by parsing the output of df. Note that the flash drive must be unmounted before it is removed.

`Converting for loops to while and until loops`

  Convert the for loops in to while loops. Hint: store the data in an array and step through the array elements.

  Having already done the heavy lifting, now convert the loops in the example to until loops.

`Changing the line spacing of a text file`

  Write a script that reads each line of a target file, then writes the line back to `stdout`, but with an extra blank line following. This has the effect of double-spacing the file.

  Include all necessary code to check whether the script gets the necessary command-line argument (a filename), and whether the specified file exists.

  When the script runs correctly, modify it to triple-space the target file.

  Finally, write a script to remove all blank lines from the target file, single-spacing it.

`Backwards Listing`

  Write a script that echoes itself to `stdout`, but backwards.

`Automatically Decompressing Files`

  Given a list of filenames as input, this script queries each target file (parsing the output of the file command) for the type of compression used on it. Then the script automatically invokes the appropriate decompression command (`gunzip`, `bunzip2`, `unzip`, `uncompress`, or whatever). If a target file is not compressed, the script emits a warning message, but takes no other action on that particular file.

`Unique System ID`

  Generate a unique 6-digit hexadecimal identifier for your computer. Do not use the flawed hostid command. Hint: `md5sum `/etc/passwd``, then select the first 6 digits of output.

`Backup`

  Archive as a tarball (`*.tar.gz` file) all the files in your home directory tree (`/home/your-name`) that have been modified in the last 24 hours. Hint: use find.

  Optional: you may use this as the basis of a backup script.

`Checking whether a process is still running`

  Given a process ID (PID) as an argument, this script will check, at user-specified intervals, whether the given process is still running. You may use the ps and sleep commands.

`Primes`

  Print (to `stdout`) all prime numbers between 60000 and 63000. The output should be nicely formatted in columns (hint: use printf).

`Lottery Numbers`

  One type of lottery involves picking five different numbers, in the range of 1 - 50. Write a script that generates five pseudorandom numbers in this range, with no duplicates. The script will give the option of echoing the numbers to `stdout` or saving them to a file, along with the date and time the particular number set was generated. (If your script consistently generates winning lottery numbers, then you can retire on the proceeds and leave shell scripting to those of us who have to work for a living.)

#### INTERMEDIATE

`Integer or String`

  Write a script function that determines if an argument passed to it is an integer or a string. The function will return TRUE (0) if passed an integer, and FALSE (1) if passed a string.

  Hint: What does the following expression return when `$1` is not an integer?

  `expr $1 + 0`

`ASCII to Integer`

  The atoi function in `C` converts a string character to an integer. Write a shell script function that performs the same operation. Likewise, write a shell script function that does the inverse, mirroring the `C` itoa function which converts an integer into an ASCII character.

`Managing Disk Space`

  List, one at a time, all files larger than 100K in the `/home/username` directory tree. Give the user the option to delete or compress the file, then proceed to show the next one. Write to a logfile the names of all deleted files and the deletion times.

`Banner`

  Simulate the functionality of the deprecated banner command in a script.

`Removing Inactive Accounts`

  Inactive accounts on a network server waste disk space and may become a security risk. Write an administrative script (to be invoked by root or the cron daemon) that checks for and deletes user accounts that have not been accessed within the last 90 days.

`Enforcing Disk Quotas`

  Write a script for a multi-user system that checks users' disk usage. If a user surpasses a preset limit (500 MB, for example) in her `/home/username` directory, then the script automatically sends her a pigout warning e-mail.

  The script will use the du and mail commands. As an option, it will allow setting and enforcing quotas using the quota and setquota commands.

`Logged in User Information`

  For all logged in users, show their real names and the time and date of their last login.

  Hint: use who, lastlog, and parse `/etc/passwd`.

`Safe Delete`

  Implement, as a script, a safe delete command, `sdel.sh`. Filenames passed as command-line arguments to this script are not deleted, but instead gzipped if not already compressed (use file to check), then moved to a `~/TRASH` directory. Upon invocation, the script checks the `~/TRASH` directory for files older than 48 hours and permanently deletes them. (An better alternative might be to have a second script handle this, periodically invoked by the cron daemon.)

  Extra credit: Write the script so it can handle files and directories recursively. This would give it the capability of safely deleting entire directory structures.

`Making Change`

  What is the most efficient way to make change for $1.68, using only coins in common circulations (up to 25c)? It's 6 quarters, 1 dime, a nickel, and three cents.

  Given any arbitrary command-line input in dollars and cents ($*.??), calculate the change, using the minimum number of coins. If your home country is not the United States, you may use your local currency units instead. The script will need to parse the command-line input, then change it to multiples of the smallest monetary unit (cents or whatever). Hint: look at .

`Quadratic Equations`

  Solve a quadratic equation of the form `Ax^2 + Bx + C = 0`. Have a script take as arguments the coefficients, `A`, `B`, and `C`, and return the solutions to five decimal places.

  Hint: pipe the coefficients to bc, using the well-known formula, `x = ( -B +/- sqrt( B^2 - 4AC ) ) / 2A`.

`Table of Logarithms`

  Using the bc and printf commands, print out a nicely-formatted table of eight-place natural logarithms in the interval between 0.00 and 100.00, in steps of .01.

  Hint: bc requires the `-l` option to load the math library.

`Unicode Table`

  Using as a template, write a script that prints to a file a complete Unicode table.

  Hint: Use the `-e` option to echo: `echo -e '\uXXXX'`, where XXXX is the Unicode numerical character designation. This requires version 4.2 or later of Bash.

`Sum of Matching Numbers`

  Find the sum of all five-digit numbers (in the range 10000 - 99999) containing exactly two out of the following set of digits: { 4, 5, 6 }. These may repeat within the same number, and if so, they count once for each occurrence.

  Some examples of matching numbers are 42057, 74638, and 89515.

`Lucky Numbers`

  A lucky number is one whose individual digits add up to 7, in successive additions. For example, 62431 is a lucky number (6 + 2 + 4 + 3 + 1 = 16, 1 + 6 = 7). Find all the lucky numbers between 1000 and 10000.

`Craps`

  Borrowing the ASCII graphics from , write a script that plays the well-known gambling game of craps. The script will accept bets from one or more players, roll the dice, and keep track of wins and losses, as well as of each player's bankroll.

`Tic-tac-toe`

  Write a script that plays the child's game of tic-tac-toe against a human player. The script will let the human choose whether to take the first move. The script will follow an optimal strategy, and therefore never lose. To simplify matters, you may use ASCII graphics:

         o | x |
         ----------
           | x |
         ----------
           | o |

         Your move, human (row, column)?

`Alphabetizing a String`

  Alphabetize (in ASCII order) an arbitrary string read from the command-line.

`Parsing`

  Parse `/etc/passwd`, and output its contents in nice, easy-to-read tabular form.

`Logging Logins`

  Parse `/var/log/messages` to produce a nicely formatted file of user logins and login times. The script may need to run as root. (Hint: Search for the string LOGIN.)

`Pretty-Printing a Data File`

  Certain database and spreadsheet packages use save-files with the fields separated by commas, commonly referred to as comma-separated values or CSVs. Other applications often need to parse these files.

  Given a data file with comma-separated fields, of the form:

      Jones,Bill,235 S. Williams St.,Denver,CO,80221,(303) 244-7989
      Smith,Tom,404 Polk Ave.,Los Angeles,CA,90003,(213) 879-5612
      ...

  Reformat the data and print it out to `stdout` in labeled, evenly-spaced columns.
`Justification`

  Given ASCII text input either from `stdin` or a file, adjust the word spacing to right-justify each line to a user-specified line-width, then send the output to `stdout`.

`Mailing List`

  Using the mail command, write a script that manages a simple mailing list. The script automatically e-mails the monthly company newsletter, read from a specified text file, and sends it to all the addresses on the mailing list, which the script reads from another specified file.

`Generating Passwords`

  Generate pseudorandom 8-character passwords, using characters in the ranges [0-9], [A-Z], [a-z]. Each password must contain at least two digits.

`Monitoring a User`

  You suspect that one particular user on the network has been abusing her privileges and possibly attempting to hack the system. Write a script to automatically monitor and log her activities when she's signed on. The log file will save entries for the previous week, and delete those entries more than seven days old.

  You may use last, lastlog, and lastcomm to aid your surveillance of the suspected fiend.

`Checking for Broken Links`

  Using lynx with the `-traversal` option, write a script that checks a Web site for broken links.

#### DIFFICULT

`Testing Passwords`

  Write a script to check and validate passwords. The object is to flag weak or easily guessed password candidates.

  A trial password will be input to the script as a command-line parameter. To be considered acceptable, a password must meet the following minimum qualifications:

  - Minimum length of 8 characters

  - Must contain at least one numeric character

  - Must contain at least one of the following non-alphabetic characters: @, #, $, %, &, *, +, -, =

  Optional:

  - Do a dictionary check on every sequence of at least four consecutive alphabetic characters in the password under test. This will eliminate passwords containing embedded words found in a standard dictionary.

  - Enable the script to check all the passwords on your system. These do not reside in `/etc/passwd`.

  This exercise tests mastery of Regular Expressions.

`Cross Reference`

  Write a script that generates a cross-reference (concordance) on a target file. The output will be a listing of all word occurrences in the target file, along with the line numbers in which each word occurs. Traditionally, linked list constructs would be used in such applications. Therefore, you should investigate arrays in the course of this exercise. is probably not a good place to start.

`Square Root`

  Write a script to calculate square roots of numbers using Newton's Method.

  The algorithm for this, expressed as a snippet of Bash pseudo-code is:

      #  (Isaac) Newton's Method for speedy extraction
      #+ of square roots.

      guess = $argument
      #  $argument is the number to find the square root of.
      #  $guess is each successive calculated "guess" -- or trial solution --
      #+ of the square root.
      #  Our first "guess" at a square root is the argument itself.

      oldguess = 0
      # $oldguess is the previous $guess.

      tolerance = .000001
      # To how close a tolerance we wish to calculate.

      loopcnt = 0
      # Let's keep track of how many times through the loop.
      # Some arguments will require more loop iterations than others.

      while [ ABS( $guess $oldguess ) -gt $tolerance ]
      #       ^^^^^^^^^^^^^^^^^^^^^^^ Fix up syntax, of course.

      #      "ABS" is a (floating point) function to find the absolute value
      #+      of the difference between the two terms.
      #             So, as long as difference between current and previous
      #+            trial solution (guess) exceeds the tolerance, keep looping.

      do
         oldguess = $guess  # Update $oldguess to previous $guess.

      #  =======================================================
         guess = ( $oldguess + ( $argument / $oldguess ) ) / 2.0
      #        = 1/2 ( ($oldguess **2 + $argument) / $oldguess )
      #  equivalent to:
      #        = 1/2 ( $oldguess + $argument / $oldguess )
      #  that is, "averaging out" the trial solution and
      #+ the proportion of argument deviation
      #+ (in effect, splitting the error in half).
      #  This converges on an accurate solution
      #+ with surprisingly few loop iterations . . .
      #+ for arguments > $tolerance, of course.
      #  =======================================================

         (( loopcnt++ ))     # Update loop counter.
      done

  It's a simple enough recipe, and seems at first glance easy enough to convert into a working Bash script. The problem, though, is that Bash has no native support for floating point numbers. So, the script writer needs to use bc or possibly awk to convert the numbers and do the calculations. It could get rather messy . . .

`Logging File Accesses`

  Log all accesses to the files in `/etc` during the course of a single day. This information should include the filename, user name, and access time. If any alterations to the files take place, that will be flagged. Write this data as tabular (tab-separated) formatted records in a logfile.

`Monitoring Processes`

  Write a script to continually monitor all running processes and to keep track of how many child processes each parent spawns. If a process spawns more than five children, then the script sends an e-mail to the system administrator (or root) with all relevant information, including the time, PID of the parent, PIDs of the children, etc. The script appends a report to a log file every ten minutes.

`Strip Comments`

  Strip all comments from a shell script whose name is specified on the command-line. Note that the initial #! line must not be stripped out.

`Strip HTML Tags`

  Strip all the HTML tags from a specified HTML file, then reformat it into lines between 60 and 75 characters in length. Reset paragraph and block spacing, as appropriate, and convert HTML tables to their approximate text equivalent.

`XML Conversion`

  Convert an XML file to both HTML and text format.

  Optional: A script that converts Docbook/SGML to XML.

`Chasing Spammers`

  Write a script that analyzes a spam e-mail by doing DNS lookups on the IP addresses in the headers to identify the relay hosts as well as the originating ISP. The script will forward the unaltered spam message to the responsible ISPs. Of course, it will be necessary to filter out your own ISP's IP address, so you don't end up complaining about yourself.

  As necessary, use the appropriate network analysis commands.

  For some ideas, see and .

  Optional: Write a script that searches through a list of e-mail messages and deletes the spam according to specified filters.

`Creating man pages`

  Write a script that automates the process of creating man pages.

  Given a text file which contains information to be formatted into a man page, the script will read the file, then invoke the appropriate groff commands to output the corresponding man page to `stdout`. The text file contains blocks of information under the standard man page headings, i.e., NAME, SYNOPSIS, DESCRIPTION, etc.

  is an instructive first step.

`Hex Dump`

  Do a hex(adecimal) dump on a binary file specified as an argument to the script. The output should be in neat tabular fields, with the first field showing the address, each of the next 8 fields a 4-byte hex number, and the final field the ASCII equivalent of the previous 8 fields.

  The obvious followup to this is to extend the hex dump script into a disassembler. Using a lookup table, or some other clever gimmick, convert the hex values into 80x86 op codes.

`Emulating a Shift Register`

  Using as an inspiration, write a script that emulates a 64-bit shift register as an array. Implement functions to load the register, shift left, shift right, and rotate it. Finally, write a function that interprets the register contents as eight 8-bit ASCII characters.

`Calculating Determinants`

  Write a script that calculates determinants For all you clever types who failed intermediate algebra, a determinant is a numerical value associated with a multidimensional matrix (array of numbers).

      For the simple case of a 2 x 2 determinant:

        |a  b|
        |b  a|

      The solution is a*a - b*b, where "a" and "b" represent numbers.

  by recursively expanding the minors. Use a 4 x 4 determinant as a test case.
`Hidden Words`

  Write a word-find puzzle generator, a script that hides 10 input words in a 10 x 10 array of random letters. The words may be hidden across, down, or diagonally.

  Optional: Write a script that solves word-find puzzles. To keep this from becoming too difficult, the solution script will find only horizontal and vertical words. (Hint: Treat each row and column as a string, and search for substrings.)

`Anagramming`

  Anagram 4-letter input. For example, the anagrams of word are: do or rod row word. You may use `/usr/share/dict/linux.words` as the reference list.

`Word Ladders`

  A word ladder is a sequence of words, with each successive word in the sequence differing from the previous one by a single letter.

  For example, to ladder from mark to vase:

      mark --> park --> part --> past --> vast --> vase
               ^           ^       ^      ^           ^

  Write a script that solves word ladder puzzles. Given a starting and an ending word, the script will list all intermediate steps in the ladder. Note that all words in the sequence must be legitimate dictionary words.

`Fog Index`

  The fog index of a passage of text estimates its reading difficulty, as a number corresponding roughly to a school grade level. For example, a passage with a fog index of 12 should be comprehensible to anyone with 12 years of schooling.

  The Gunning version of the fog index uses the following algorithm.

  1. Choose a section of the text at least 100 words in length.

  2. Count the number of sentences (a portion of a sentence truncated by the boundary of the text section counts as one).

  3. Find the average number of words per sentence.

     AVE_WDS_SEN = TOTAL_WORDS / SENTENCES

  4. Count the number of difficult words in the segment -- those containing at least 3 syllables. Divide this quantity by total words to get the proportion of difficult words.

     PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS

  5. The Gunning fog index is the sum of the above two quantities, multiplied by 0.4, then rounded to the nearest integer.

     G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN + PRO_DIFF_WORDS ) )

  Step 4 is by far the most difficult portion of the exercise. There exist various algorithms for estimating the syllable count of a word. A rule-of-thumb formula might consider the number of letters in a word and the vowel-consonant mix.

  A strict interpretation of the Gunning fog index does not count compound words and proper nouns as difficult words, but this would enormously complicate the script.

`Calculating PI using Buffon's Needle`

  The Eighteenth Century French mathematician de Buffon came up with a novel experiment. Repeatedly drop a needle of length n onto a wooden floor composed of long and narrow parallel boards. The cracks separating the equal-width floorboards are a fixed distance d apart. Keep track of the total drops and the number of times the needle intersects a crack on the floor. The ratio of these two quantities turns out to be a fractional multiple of PI.

  In the spirit of , write a script that runs a Monte Carlo simulation of Buffon's Needle. To simplify matters, set the needle length equal to the distance between the cracks, `n = d`.

  Hint: there are actually two critical variables: the distance from the center of the needle to the nearest crack, and the inclination angle of the needle to that crack. You may use bc to handle the calculations.

`Playfair Cipher`

  Implement the Playfair (Wheatstone) Cipher in a script.

  The Playfair Cipher encrypts text by substitution of digrams (2-letter groupings). It is traditional to use a 5 x 5 letter scrambled-alphabet key square for the encryption and decryption.

         C O D E S
         A B F G H
         I K L M N
         P Q R T U
         V W X Y Z

      Each letter of the alphabet appears once, except "I" also represents
      "J". The arbitrarily chosen key word, "CODES" comes first, then all
      the rest of the alphabet, in order from left to right, skipping letters
      already used.

      To encrypt, separate the plaintext message into digrams (2-letter
      groups). If a group has two identical letters, delete the second, and
      form a new group. If there is a single letter left over at the end,
      insert a "null" character, typically an "X."

      THIS IS A TOP SECRET MESSAGE

      TH IS IS AT OP SE CR ET ME SA GE

      For each digram, there are three possibilities.
      -----------------------------------------------

      1) Both letters will be on the same row of the key square:
         For each letter, substitute the one immediately to the right, in that
         row. If necessary, wrap around left to the beginning of the row.

      or

      2) Both letters will be in the same column of the key square:
         For each letter, substitute the one immediately below it, in that
         row. If necessary, wrap around to the top of the column.

      or

      3) Both letters will form the corners of a rectangle within the key square:
         For each letter, substitute the one on the other corner the rectangle
         which lies on the same row.

      The "TH" digram falls under case #3.
      G H
      M N
      T U           (Rectangle with "T" and "H" at corners)

      T --> U
      H --> G

      The "SE" digram falls under case #1.
      C O D E S     (Row containing "S" and "E")

      S --> C  (wraps around left to beginning of row)
      E --> S

      =========================================================================

      To decrypt encrypted text, reverse the above procedure under cases #1
      and #2 (move in opposite direction for substitution). Under case #3,
      just take the remaining two corners of the rectangle.

      Helen Fouche Gaines' classic work, ELEMENTARY CRYPTANALYSIS (1939), gives a
      fairly detailed description of the Playfair Cipher and its solution methods.

  This script will have three main sections

  1. Generating the key square, based on a user-input keyword.

  2. Encrypting a plaintext message.

  3. Decrypting encrypted text.

  The script will make extensive use of arrays and functions. You may use as an inspiration.

--

Please do not send the author your solutions to these exercises. There are more appropriate ways to impress him with your cleverness, such as submitting bugfixes and suggestions for improving the book.

## Revision History

          This document first appeared as a 60-page HOWTO in the late spring
          of 2000. Since then, it has gone through quite a number of updates
          and revisions. This book could not have been written without the
          assistance of the Linux community, and especially of the volunteers
          of the Linux Documentation Project.

Here is the e-mail to the LDP requesting permission to submit version 0.1.

    From thegrendel@theriver.com Sat Jun 10 09:05:33 2000 -0700
    Date: Sat, 10 Jun 2000 09:05:28 -0700 (MST)
    From: "M. Leo Cooper" <thegrendel@theriver.com>
    X-Sender: thegrendel@localhost
    To: ldp-discuss@lists.linuxdoc.org
    Subject: Permission to submit HOWTO

    Dear HOWTO Coordinator,

    I am working on and would like to submit to the LDP a HOWTO on the subject
    of "Bash Scripting" (shell scripting, using 'bash'). As it happens,
    I have been writing this document, off and on, for about the last eight
    months or so, and I could produce a first draft in ASCII text format in
    a matter of just a few more days.

    I began writing this out of frustration at being unable to find a
    decent book on shell scripting. I managed to locate some pretty good
    articles on various aspects of scripting, but nothing like a complete,
    beginning-to-end tutorial.  Well, in keeping with my philosophy, if all
    else fails, do it yourself.

    As it stands, this proposed "Bash-Scripting HOWTO" would serve as a
    combination tutorial and reference, with the heavier emphasis on the
    tutorial. It assumes Linux experience, but only a very basic level
    of programming skills. Interspersed with the text are 79 illustrative
    example scripts of varying complexity, all liberally commented. There
    are even exercises for the reader.

    At this stage, I'm up to 18,000+ words (124k), and that's over 50 pages of
    text (whew!).

    I haven't mentioned that I've previously authored an LDP HOWTO, the
    "Software-Building HOWTO", which I wrote in Linuxdoc/SGML. I don't know
    if I could handle Docbook/SGML, and I'm glad you have volunteers to do
    the conversion. You people seem to have gotten on a more organized basis
    these last few months. Working with Greg Hankins and Tim Bynum was nice,
    but a professional team is even nicer.

    Anyhow, please advise.

    Mendel Cooper
    thegrendel@theriver.com

### Revision History

Release | Date | Comments
--- | --- | ---
0.1 | 14 Jun 2000 | Initial release.
`0.2` | 30 Oct 2000 | Bugs fixed, plus much additional material and more example scripts.
`0.3` | 12 Feb 2001 | Major update.
`0.4` | 08 Jul 2001 | Complete revision and expansion of the book.
`0.5` | 03 Sep 2001 | Major update: Bugfixes, material added, sections reorganized.
`1.0` | 14 Oct 2001 | Stable release: Bugfixes, reorganization, material added.
`1.1` | 06 Jan 2002 | Bugfixes, material and scripts added.
`1.2` | 31 Mar 2002 | Bugfixes, material and scripts added.
`1.3` | 02 Jun 2002 | TANGERINE release: A few bugfixes, much more material and scripts added.
`1.4` | 16 Jun 2002 | MANGO release: A number of typos fixed, more material and scripts.
`1.5` | 13 Jul 2002 | PAPAYA release: A few bugfixes, much more material and scripts added.
`1.6` | 29 Sep 2002 | POMEGRANATE release: Bugfixes, more material, one more script.
`1.7` | 05 Jan 2003 | COCONUT release: A couple of bugfixes, more material, one more script.
`1.8` | 10 May 2003 | BREADFRUIT release: A number of bugfixes, more scripts and material.
`1.9` | 21 Jun 2003 | PERSIMMON release: Bugfixes, and more material.
`2.0` | 24 Aug 2003 | GOOSEBERRY release: Major update.
`2.1` | 14 Sep 2003 | HUCKLEBERRY release: Bugfixes, and more material.
`2.2` | 31 Oct 2003 | CRANBERRY release: Major update.
`2.3` | 03 Jan 2004 | STRAWBERRY release: Bugfixes and more material.
`2.4` | 25 Jan 2004 | MUSKMELON release: Bugfixes.
`2.5` | 15 Feb 2004 | STARFRUIT release: Bugfixes and more material.
`2.6` | 15 Mar 2004 | SALAL release: Minor update.
`2.7` | 18 Apr 2004 | MULBERRY release: Minor update.
`2.8` | 11 Jul 2004 | ELDERBERRY release: Minor update.
`3.0` | 03 Oct 2004 | LOGANBERRY release: Major update.
`3.1` | 14 Nov 2004 | BAYBERRY release: Bugfix update.
`3.2` | 06 Feb 2005 | BLUEBERRY release: Minor update.
`3.3` | 20 Mar 2005 | RASPBERRY release: Bugfixes, much material added.
`3.4` | 08 May 2005 | TEABERRY release: Bugfixes, stylistic revisions.
`3.5` | 05 Jun 2005 | BOXBERRY release: Bugfixes, some material added.
`3.6` | 28 Aug 2005 | POKEBERRY release: Bugfixes, some material added.
`3.7` | 23 Oct 2005 | WHORTLEBERRY release: Bugfixes, some material added.
`3.8` | 26 Feb 2006 | BLAEBERRY release: Bugfixes, some material added.
`3.9` | 15 May 2006 | SPICEBERRY release: Bugfixes, some material added.
`4.0` | 18 Jun 2006 | WINTERBERRY release: Major reorganization.
`4.1` | 08 Oct 2006 | WAXBERRY release: Minor update.
`4.2` | 10 Dec 2006 | SPARKLEBERRY release: Important update.
`4.3` | 29 Apr 2007 | INKBERRY release: Bugfixes, material added.
`5.0` | 24 Jun 2007 | SERVICEBERRY release: Major update.
`5.1` | 10 Nov 2007 | LINGONBERRY release: Minor update.
`5.2` | 16 Mar 2008 | SILVERBERRY release: Important update.
`5.3` | 11 May 2008 | GOLDENBERRY release: Minor update.
`5.4` | 21 Jul 2008 | ANGLEBERRY release: Major update.
`5.5` | 23 Nov 2008 | FARKLEBERRY release: Minor update.
`5.6` | 26 Jan 2009 | WORCESTERBERRY release: Minor update.
`6.0` | 23 Mar 2009 | THIMBLEBERRY release: Major update.
`6.1` | 30 Sep 2009 | BUFFALOBERRY release: Minor update.
`6.2` | 17 Mar 2010 | ROWANBERRY release: Minor update.
`6.3` | 30 Apr 2011 | SWOZZLEBERRY release: Major update.
`6.4` | 30 Aug 2011 | VORTEXBERRY release: Minor update.
`6.5` | 05 Apr 2012 | TUNGSTENBERRY release: Minor update.
`6.6` | 27 Nov 2012 | YTTERBIUMBERRY release: Minor update.
`10` | 10 Mar 2014 | YTTERBIUMBERRY release: License change.

## Download and Mirror Sites

The latest update of this document, as an archived, bzip2-ed tarball including both the SGML source and rendered HTML, may be downloaded from the author's home site).

A pdf version is also available (mirror site). There is likewise an epub version, courtesy of Craig Barnes and Michael Satke.

The change log gives a detailed revision history. The ABS Guide even has its own `freshmeat.net/freecode` page to keep track of major updates, user comments, and popularity ratings for the project.

The legacy hosting site for this document is the Linux Documentation Project, which maintains many other Guides and HOWTOs as well.

Many thanks to Ronny Bangsund for donating server space to host this project.

## To Do List

- A comprehensive survey of incompatibilities between Bash and the classic Bourne shell.

- Same as above, but for the Korn shell (ksh).

## Copyright

The Advanced Bash Scripting Guide is herewith granted to the PUBLIC DOMAIN. This has the following implications and consequences.

    A.  All previous releases of the Advanced Bash Scripting Guide
        are as well granted to the Public Domain.

    A1. All printed editions, whether authorized by the author or not,
        are as well granted to the Public Domain. This legally overrides
        any stated intention or wishes of the publishers. Any statement
        of copyright is void and invalid.
        THERE ARE NO EXCEPTIONS TO THIS.

    A2. Any release of the Advanced Bash Scripting Guide, whether in
        electronic or print form is granted to the Public Domain by the
        express directive of the author and previous copyright holder, Mendel
        Cooper. No other person(s) or entities have ever held a valid copyright.

    B.  As a Public Domain document, unlimited copying and distribution rights
        are granted. There can be NO restrictions. If anyone has published or will
        in the future publish an original or modified version of this document,
        then only additional original material may be copyrighted. The core
        work will remain in the Public Domain.

By law, distributors and publishers (including on-line publishers) are prohibited from imposing any conditions, strictures, or provisions on this document, any previous versions, or any derivative versions. The author asserts that he has not entered into any contractual obligations that would alter the foregoing declarations.

Essentially, you may freely distribute this book or any derivative thereof in electronic or printed form. If you have previously purchased or are in possession of a printed copy of a current or previous edition, you have the LEGAL RIGHT to copy and/or redistribute it, regardless of any copyright notice. Any copyright notice is void.

Additionally, the author wishes to state his intention that:

    If you copy or distribute this book, kindly DO NOT
    use the materials within, or any portion thereof, in a patent or copyright
    lawsuit against the Open Source community, its developers, its
    distributors, or against any of its associated software or documentation
    including, but not limited to, the Linux kernel, Open Office, Samba,
    and Wine. Kindly DO NOT use any of the materials within
    this book in testimony or depositions as a plaintiff's "expert witness" in
    any lawsuit against the Open Source community, any of its developers, its
    distributors, or any of its associated software or documentation.

A Public Domain license essentially does not restrict ANY legitimate distribution or use of this book. The author especially encourages its (royalty-free!) use for classroom and instructional purposes.

To date, limited print rights (Lulu edition) have been granted to one individual and to no one else. Neither that individual nor Lulu holds or ever has held a valid copyright.

It has come to the attention of the author that unauthorized electronic and print editions of this book are being sold commercially on itunes, amazon.com and elsewhere. These are illegal and pirated editions produced without the author's permission, and readers of this book are strongly urged not to purchase them. In fact, these pirated editions are now legal, but necessarily fall into the Public Domain, and any copyright notices contained within them are invalid and void.

The author produced this book in a manner consistent with the spirit of the LDP Manifesto.

Linux is a trademark registered to Linus Torvalds.

Fedora is a trademark registered to Red Hat.

Unix and UNIX are trademarks registered to the Open Group.

MS Windows is a trademark registered to the Microsoft Corp.

Solaris is a trademark registered to Oracle, Inc.

OSX is a trademark registered to Apple, Inc.

Yahoo is a trademark registered to Yahoo, Inc.

Pentium is a trademark registered to Intel, Inc.

Thinkpad is a trademark registered to Lenovo, Inc.

Scrabble is a trademark registered to Hasbro, Inc.

Librie, PRS-500, and PRS-505 are trademarks registered to Sony, Inc.

All other commercial trademarks mentioned in the body of this work are registered to their respective owners.

Hyun Jin Cha has done a Korean translation of version 1.0.11 of this book. Spanish, Portuguese, French, German, Italian, Russian, Czech, Chinese, Indonesian, Dutch, Romanian, Bulgarian, and Turkish translations are also available or in progress. If you wish to translate this document into another language, please feel free to do so, subject to the terms stated above. The author wishes to be notified of such efforts.

Those generous readers desiring to make a donation to the author may contribute a small amount via Paypal to my e-mail address, thegrendel.abs@gmail.com. (An `Honor Roll of Supporters` is given at the beginning of the Change Log.) This is not a requirement. The ABS Guide is a free and freely distributed document for the use and enjoyment of the Linux community. However, in these difficult times, showing support for voluntary projects and especially to authors of limited means is more critically important than ever.

## ASCII Table

Traditionally, a book of this sort has an ASCII Table appendix. This book does not. Instead, here are several short scripts, each of which generates a complete ASCII table.

### A script that generates an ASCII table

    &asciish;

### Another ASCII table script

    &ascii2sh;

### A third ASCII table script, using awk

    &ascii3sh;

&INDEX00;