Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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# Chapter 9. Another Look at Variables

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

# 9.1. 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 Example 21-1 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 ...

         1 #!/bin/bash4
         2
         3 echo "\$\$ outside of subshell = $$"                              # 9602
         4 echo "\$BASH_SUBSHELL  outside of subshell = $BASH_SUBSHELL"      # 0
         5 echo "\$BASHPID outside of subshell = $BASHPID"                   # 9602
         6
         7 echo
         8
         9 ( echo "\$\$ inside of subshell = $$"                             # 9602
        10   echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL"      # 1
        11   echo "\$BASHPID inside of subshell = $BASHPID" )                # 9603
        12   # 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.

         1 # Bash version info:
         2
         3 for n in 0 1 2 3 4 5
         4 do
         5   echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
         6 done
         7
         8 # BASH_VERSINFO[0] = 3                      # Major version no.
         9 # BASH_VERSINFO[1] = 00                     # Minor version no.
        10 # BASH_VERSINFO[2] = 14                     # Patch level.
        11 # BASH_VERSINFO[3] = 1                      # Build version.
        12 # BASH_VERSINFO[4] = release                # Release status.
        13 # BASH_VERSINFO[5] = i386-redhat-linux-gnu  # Architecture
        14                                             # (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 [1] (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

         1 xyz23 ()
         2 {
         3   echo "$FUNCNAME now executing."  # xyz23 now executing.
         4 }
         5
         6 xyz23
         7
         8 echo "FUNCNAME = $FUNCNAME"        # FUNCNAME =
         9                                    # Null value outside a function.

  See also Example A-50.

`$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 Example 10-7)

`$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 Example 10-7.

`$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 Example 5-1.

       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.

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

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

  Example 9-1. $IFS and whitespace

         1 #!/bin/bash
         2 # ifs.sh
         3
         4
         5 var1="a+b+c"
         6 var2="d-e-f"
         7 var3="g,h,i"
         8
         9 IFS=+
        10 # The plus sign will be interpreted as a separator.
        11 echo $var1     # a b c
        12 echo $var2     # d-e-f
        13 echo $var3     # g,h,i
        14
        15 echo
        16
        17 IFS="-"
        18 # The plus sign reverts to default interpretation.
        19 # The minus sign will be interpreted as a separator.
        20 echo $var1     # a+b+c
        21 echo $var2     # d e f
        22 echo $var3     # g,h,i
        23
        24 echo
        25
        26 IFS=","
        27 # The comma will be interpreted as a separator.
        28 # The minus sign reverts to default interpretation.
        29 echo $var1     # a+b+c
        30 echo $var2     # d-e-f
        31 echo $var3     # g h i
        32
        33 echo
        34
        35 IFS=" "
        36 # The space character will be interpreted as a separator.
        37 # The comma reverts to default interpretation.
        38 echo $var1     # a+b+c
        39 echo $var2     # d-e-f
        40 echo $var3     # g,h,i
        41
        42 # ======================================================== #
        43
        44 # However ...
        45 # $IFS treats whitespace differently than other characters.
        46
        47 output_args_one_per_line()
        48 {
        49   for arg
        50   do
        51     echo "[$arg]"
        52   done #  ^    ^   Embed within brackets, for your viewing pleasure.
        53 }
        54
        55 echo; echo "IFS=\" \""
        56 echo "-------"
        57
        58 IFS=" "
        59 var=" a  b c   "
        60 #    ^ ^^   ^^^
        61 output_args_one_per_line $var  # output_args_one_per_line `echo " a  b c   "`
        62 # [a]
        63 # [b]
        64 # [c]
        65
        66
        67 echo; echo "IFS=:"
        68 echo "-----"
        69
        70 IFS=:
        71 var=":a::b:c:::"               # Same pattern as above,
        72 #    ^ ^^   ^^^                #+ but substituting ":" for " "  ...
        73 output_args_one_per_line $var
        74 # []
        75 # [a]
        76 # []
        77 # [b]
        78 # [c]
        79 # []
        80 # []
        81
        82 # Note "empty" brackets.
        83 # The same thing happens with the "FS" field separator in awk.
        84
        85
        86 echo
        87
        88 exit

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

  See also Example 16-41, Example 11-8, and Example 19-14 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 `.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.

         1 # *** BEGIN DEBUG BLOCK ***
         2 last_cmd_arg=$_  # Save it.
         3
         4 echo "At line number $LINENO, variable \"v1\" = $v1"
         5 echo "Last command argument processed = $last_cmd_arg"
         6 # *** 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 Appendix H).

       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. [2]

  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 Example 11-30).

`$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`.

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

`$PWD`

  Working directory (directory you are in at the time)

  This is the analog to the pwd builtin command.

         1 #!/bin/bash
         2
         3 E_WRONG_DIRECTORY=85
         4
         5 clear # Clear the screen.
         6
         7 TargetDirectory=/home/bozo/projects/GreatAmericanNovel
         8
         9 cd $TargetDirectory
        10 echo "Deleting stale files in $TargetDirectory."
        11
        12 if [ "$PWD" != "$TargetDirectory" ]
        13 then    # Keep from wiping out wrong directory by accident.
        14   echo "Wrong directory!"
        15   echo "In $PWD, rather than $TargetDirectory!"
        16   echo "Bailing out!"
        17   exit $E_WRONG_DIRECTORY
        18 fi
        19
        20 rm -rf *
        21 rm .[A-Za-z0-9]*    # Delete dotfiles.
        22 # rm -f .[^.]* ..?*   to remove filenames beginning with multiple dots.
        23 # (shopt -s dotglob; rm -f *)   will also work.
        24 # Thanks, S.C. for pointing this out.
        25
        26 #  A filename (`basename`) may contain all characters in the 0 - 255 range,
        27 #+ except "/".
        28 #  Deleting files beginning with weird characters, such as -
        29 #+ is left as an exercise. (Hint: rm ./-weirdname or rm -- -weirdname)
        30 result=$?   # Result of delete operations. If successful = 0.
        31
        32 echo
        33 ls -al              # Any files left?
        34 echo "Done."
        35 echo "Old files deleted in $TargetDirectory."
        36 echo
        37
        38 # Various other operations here, as necessary.
        39
        40 exit $result

`$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.

         1 #!/bin/bash
         2 # reply.sh
         3
         4 # REPLY is the default value for a 'read' command.
         5
         6 echo
         7 echo -n "What is your favorite vegetable? "
         8 read
         9
        10 echo "Your favorite vegetable is $REPLY."
        11 #  REPLY holds the value of last "read" if and only if
        12 #+ no variable supplied.
        13
        14 echo
        15 echo -n "What is your favorite fruit? "
        16 read fruit
        17 echo "Your favorite fruit is $fruit."
        18 echo "but..."
        19 echo "Value of \$REPLY is still $REPLY."
        20 #  $REPLY is still set to its previous value because
        21 #+ the variable $fruit absorbed the new "read" value.
        22
        23 echo
        24
        25 exit 0

`$SECONDS`

  The number of seconds the script has been running.

         1 #!/bin/bash
         2
         3 TIME_LIMIT=10
         4 INTERVAL=1
         5
         6 echo
         7 echo "Hit Control-C to exit before $TIME_LIMIT seconds."
         8 echo
         9
        10 while [ "$SECONDS" -le "$TIME_LIMIT" ]
        11 do   #   $SECONDS is an internal shell variable.
        12   if [ "$SECONDS" -eq 1 ]
        13   then
        14     units=second
        15   else
        16     units=seconds
        17   fi
        18
        19   echo "This script has been running $SECONDS $units."
        20   #  On a slow or overburdened machine, the script may skip a count
        21   #+ every once in a while.
        22   sleep $INTERVAL
        23 done
        24
        25 echo -e "\a"  # Beep!
        26
        27 exit 0

`$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. [3] 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.

         1 # Works in scripts for Bash, versions 2.05b and later.
         2
         3 TMOUT=3    # Prompt times out at three seconds.
         4
         5 echo "What is your favorite song?"
         6 echo "Quickly now, you only have $TMOUT seconds to answer!"
         7 read song
         8
         9 if [ -z "$song" ]
        10 then
        11   song="(no answer)"
        12   # Default response.
        13 fi
        14
        15 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 Example 32-5) the interrupt generated by the timing loop (whew!).

  Example 9-2. Timed Input

         1 #!/bin/bash
         2 # timed-input.sh
         3
         4 # TMOUT=3    Also works, as of newer versions of Bash.
         5
         6 TIMER_INTERRUPT=14
         7 TIMELIMIT=3  # Three seconds in this instance.
         8              # May be set to different value.
         9
        10 PrintAnswer()
        11 {
        12   if [ "$answer" = TIMEOUT ]
        13   then
        14     echo $answer
        15   else       # Don't want to mix up the two instances.
        16     echo "Your favorite veggie is $answer"
        17     kill $!  #  Kills no-longer-needed TimerOn function
        18              #+ running in background.
        19              #  $! is PID of last job running in background.
        20   fi
        21
        22 }
        23
        24
        25 TimerOn()
        26 {
        27   sleep $TIMELIMIT && kill -s 14 $$ &
        28   # Waits 3 seconds, then sends sigalarm to script.
        29 }
        30
        31
        32 Int14Vector()
        33 {
        34   answer="TIMEOUT"
        35   PrintAnswer
        36   exit $TIMER_INTERRUPT
        37 }
        38
        39 trap Int14Vector $TIMER_INTERRUPT
        40 # Timer interrupt (14) subverted for our purposes.
        41
        42 echo "What is your favorite vegetable "
        43 TimerOn
        44 read answer
        45 PrintAnswer
        46
        47
        48 #  Admittedly, this is a kludgy implementation of timed input.
        49 #  However, the "-t" option to "read" simplifies this task.
        50 #  See the "t-out.sh" script.
        51 #  However, what about timing not just single user input,
        52 #+ but an entire script?
        53
        54 #  If you need something really elegant ...
        55 #+ consider writing the application in C or C++,
        56 #+ using appropriate library functions, such as 'alarm' and 'setitimer.'
        57
        58 exit 0

  An alternative is using stty.

  Example 9-3. Once more, timed input

         1 #!/bin/bash
         2 # timeout.sh
         3
         4 #  Written by Stephane Chazelas,
         5 #+ and modified by the document author.
         6
         7 INTERVAL=5                # timeout interval
         8
         9 timedout_read() {
        10   timeout=$1
        11   varname=$2
        12   old_tty_settings=`stty -g`
        13   stty -icanon min 0 time ${timeout}0
        14   eval read $varname      # or just  read $varname
        15   stty "$old_tty_settings"
        16   # See man page for "stty."
        17 }
        18
        19 echo; echo -n "What's your name? Quick! "
        20 timedout_read $INTERVAL your_name
        21
        22 #  This may not work on every terminal type.
        23 #  The maximum timeout depends on the terminal.
        24 #+ (it is often 25.5 seconds).
        25
        26 echo
        27
        28 if [ ! -z "$your_name" ]  # If name input before timeout ...
        29 then
        30   echo "Your name is $your_name."
        31 else
        32   echo "Timed out."
        33 fi
        34
        35 echo
        36
        37 # The behavior of this script differs somewhat from "timed-input.sh."
        38 # At each keystroke, the counter resets.
        39
        40 exit 0

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

  Example 9-4. Timed read

         1 #!/bin/bash
         2 # t-out.sh [time-out]
         3 # Inspired by a suggestion from "syngin seven" (thanks).
         4
         5
         6 TIMELIMIT=4         # 4 seconds
         7
         8 read -t $TIMELIMIT variable <&1
         9 #                           ^^^
        10 #  In this instance, "<&1" is needed for Bash 1.x and 2.x,
        11 #  but unnecessary for Bash 3+.
        12
        13 echo
        14
        15 if [ -z "$variable" ]  # Is null?
        16 then
        17   echo "Timed out, variable still unset."
        18 else
        19   echo "variable = $variable"
        20 fi
        21
        22 exit 0

`$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.

  Example 9-5. Am I root?

         1 #!/bin/bash
         2 # am-i-root.sh:   Am I root or not?
         3
         4 ROOT_UID=0   # Root has $UID 0.
         5
         6 if [ "$UID" -eq "$ROOT_UID" ]  # Will the real "root" please stand up?
         7 then
         8   echo "You are root."
         9 else
        10   echo "You are just an ordinary user (but mom loves you just the same)."
        11 fi
        12
        13 exit 0
        14
        15
        16 # ============================================================= #
        17 # Code below will not execute, because the script already exited.
        18
        19 # An alternate method of getting to the root of matters:
        20
        21 ROOTUSER_NAME=root
        22
        23 username=`id -nu`              # Or...   username=`whoami`
        24 if [ "$username" = "$ROOTUSER_NAME" ]
        25 then
        26   echo "Rooty, toot, toot. You are root."
        27 else
        28   echo "You are just a regular fella."
        29 fi

  See also Example 2-3.

  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 Example 4-5 and Example 15-16)

`$#`

  Number of command-line arguments [4] or positional parameters (see Example 36-2)

`$*`

  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.

  Example 9-6. arglist: Listing arguments with $* and $@

         1 #!/bin/bash
         2 # arglist.sh
         3 # Invoke this script with several arguments, such as "one two three" ...
         4
         5 E_BADARGS=85
         6
         7 if [ ! -n "$1" ]
         8 then
         9   echo "Usage: `basename $0` argument1 argument2 etc."
        10   exit $E_BADARGS
        11 fi
        12
        13 echo
        14
        15 index=1          # Initialize count.
        16
        17 echo "Listing args with \"\$*\":"
        18 for arg in "$*"  # Doesn't work properly if "$*" isn't quoted.
        19 do
        20   echo "Arg #$index = $arg"
        21   let "index+=1"
        22 done             # $* sees all arguments as single word.
        23 echo "Entire arg list seen as single word."
        24
        25 echo
        26
        27 index=1          # Reset count.
        28                  # What happens if you forget to do this?
        29
        30 echo "Listing args with \"\$@\":"
        31 for arg in "$@"
        32 do
        33   echo "Arg #$index = $arg"
        34   let "index+=1"
        35 done             # $@ sees arguments as separate words.
        36 echo "Arg list seen as separate words."
        37
        38 echo
        39
        40 index=1          # Reset count.
        41
        42 echo "Listing args with \$* (unquoted):"
        43 for arg in $*
        44 do
        45   echo "Arg #$index = $arg"
        46   let "index+=1"
        47 done             # Unquoted $* sees arguments as separate words.
        48 echo "Arg list seen as separate words."
        49
        50 exit 0

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

         1 #!/bin/bash
         2 # Invoke with ./scriptname 1 2 3 4 5
         3
         4 echo "$@"    # 1 2 3 4 5
         5 shift
         6 echo "$@"    # 2 3 4 5
         7 shift
         8 echo "$@"    # 3 4 5
         9
        10 # Each "shift" loses parameter $1.
        11 # "$@" 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 Example 16-24 and Example 16-25.

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

  Example 9-7. Inconsistent `$*` and `$@` behavior

         1 #!/bin/bash
         2
         3 #  Erratic behavior of the "$*" and "$@" internal Bash variables,
         4 #+ depending on whether or not they are quoted.
         5 #  Demonstrates inconsistent handling of word splitting and linefeeds.
         6
         7
         8 set -- "First one" "second" "third:one" "" "Fifth: :one"
         9 # Setting the script arguments, $1, $2, $3, etc.
        10
        11 echo
        12
        13 echo 'IFS unchanged, using "$*"'
        14 c=0
        15 for i in "$*"               # quoted
        16 do echo "$((c+=1)): [$i]"   # This line remains the same in every instance.
        17                             # Echo args.
        18 done
        19 echo ---
        20
        21 echo 'IFS unchanged, using $*'
        22 c=0
        23 for i in $*                 # unquoted
        24 do echo "$((c+=1)): [$i]"
        25 done
        26 echo ---
        27
        28 echo 'IFS unchanged, using "$@"'
        29 c=0
        30 for i in "$@"
        31 do echo "$((c+=1)): [$i]"
        32 done
        33 echo ---
        34
        35 echo 'IFS unchanged, using $@'
        36 c=0
        37 for i in $@
        38 do echo "$((c+=1)): [$i]"
        39 done
        40 echo ---
        41
        42 IFS=:
        43 echo 'IFS=":", using "$*"'
        44 c=0
        45 for i in "$*"
        46 do echo "$((c+=1)): [$i]"
        47 done
        48 echo ---
        49
        50 echo 'IFS=":", using $*'
        51 c=0
        52 for i in $*
        53 do echo "$((c+=1)): [$i]"
        54 done
        55 echo ---
        56
        57 var=$*
        58 echo 'IFS=":", using "$var" (var=$*)'
        59 c=0
        60 for i in "$var"
        61 do echo "$((c+=1)): [$i]"
        62 done
        63 echo ---
        64
        65 echo 'IFS=":", using $var (var=$*)'
        66 c=0
        67 for i in $var
        68 do echo "$((c+=1)): [$i]"
        69 done
        70 echo ---
        71
        72 var="$*"
        73 echo 'IFS=":", using $var (var="$*")'
        74 c=0
        75 for i in $var
        76 do echo "$((c+=1)): [$i]"
        77 done
        78 echo ---
        79
        80 echo 'IFS=":", using "$var" (var="$*")'
        81 c=0
        82 for i in "$var"
        83 do echo "$((c+=1)): [$i]"
        84 done
        85 echo ---
        86
        87 echo 'IFS=":", using "$@"'
        88 c=0
        89 for i in "$@"
        90 do echo "$((c+=1)): [$i]"
        91 done
        92 echo ---
        93
        94 echo 'IFS=":", using $@'
        95 c=0
        96 for i in $@
        97 do echo "$((c+=1)): [$i]"
        98 done
        99 echo ---
       100
       101 var=$@
       102 echo 'IFS=":", using $var (var=$@)'
       103 c=0
       104 for i in $var
       105 do echo "$((c+=1)): [$i]"
       106 done
       107 echo ---
       108
       109 echo 'IFS=":", using "$var" (var=$@)'
       110 c=0
       111 for i in "$var"
       112 do echo "$((c+=1)): [$i]"
       113 done
       114 echo ---
       115
       116 var="$@"
       117 echo 'IFS=":", using "$var" (var="$@")'
       118 c=0
       119 for i in "$var"
       120 do echo "$((c+=1)): [$i]"
       121 done
       122 echo ---
       123
       124 echo 'IFS=":", using $var (var="$@")'
       125 c=0
       126 for i in $var
       127 do echo "$((c+=1)): [$i]"
       128 done
       129
       130 echo
       131
       132 # Try this script with ksh or zsh -y.
       133
       134 exit 0
       135
       136 #  This example script written by Stephane Chazelas,
       137 #+ and slightly modified by the document author.

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

  Example 9-8. `$*` and `$@` when `$IFS` is empty

         1 #!/bin/bash
         2
         3 #  If $IFS set, but empty,
         4 #+ then "$*" and "$@" do not echo positional params as expected.
         5
         6 mecho ()       # Echo positional parameters.
         7 {
         8 echo "$1,$2,$3";
         9 }
        10
        11
        12 IFS=""         # Set, but empty.
        13 set a b c      # Positional parameters.
        14
        15 mecho "$*"     # abc,,
        16 #                   ^^
        17 mecho $*       # a,b,c
        18
        19 mecho $@       # a,b,c
        20 mecho "$@"     # a,b,c
        21
        22 #  The behavior of $* and $@ when $IFS is empty depends
        23 #+ on which Bash or sh version being run.
        24 #  It is therefore inadvisable to depend on this "feature" in a script.
        25
        26
        27 # Thanks, Stephane Chazelas.
        28
        29 exit

Other Special Parameters

`$-`

  Flags passed to script (using set). See Example 15-16.

  | 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

         1 LOG=$0.log
         2
         3 COMMAND1="sleep 100"
         4
         5 echo "Logging PIDs background commands for script: $0" >> "$LOG"
         6 # So they can be monitored, and killed as necessary.
         7 echo >> "$LOG"
         8
         9 # Logging commands.
        10
        11 echo -n "PID of \"$COMMAND1\":  " >> "$LOG"
        12 ${COMMAND1} &
        13 echo $! >> "$LOG"
        14 # PID of "sleep 100":  1506
        15
        16 # Thank you, Jacques Lederer, for suggesting this.

  Using `$!` for job control:

         1 possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }
         2 # Forces completion of an ill-behaved program.
         3 # Useful, for example, in init scripts.
         4
         5 # Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.

  Or, alternately:

         1 # This example by Matthew Sage.
         2 # Used with permission.
         3
         4 TIMEOUT=30   # Timeout value in seconds
         5 count=0
         6
         7 possibly_hanging_job & {
         8         while ((count < TIMEOUT )); do
         9                 eval '[ ! -d "/proc/$!" ] && ((count = TIMEOUT))'
        10                 # /proc is where information about running processes is found.
        11                 # "-d" tests whether it exists (whether directory exists).
        12                 # So, we're waiting for the job in question to show up.
        13                 ((count++))
        14                 sleep 1
        15         done
        16         eval '[ -d "/proc/$!" ] && kill -15 $!'
        17         # If the hanging job is running, kill it.
        18 }
        19
        20 #  -------------------------------------------------------------- #
        21
        22 #  However, this may not not work as specified if another process
        23 #+ begins to run after the "hanging_job" . . .
        24 #  In such a case, the wrong job may be killed.
        25 #  Ariel Meragelman suggests the following fix.
        26
        27 TIMEOUT=30
        28 count=0
        29 # Timeout value in seconds
        30 possibly_hanging_job & {
        31
        32 while ((count < TIMEOUT )); do
        33   eval '[ ! -d "/proc/$lastjob" ] && ((count = TIMEOUT))'
        34   lastjob=$!
        35   ((count++))
        36   sleep 1
        37 done
        38 eval '[ -d "/proc/$lastjob" ] && kill -15 $lastjob'
        39
        40 }
        41
        42 exit

`$_`

  Special variable set to final argument of previous command executed.

  Example 9-9. Underscore variable

         1 #!/bin/bash
         2
         3 echo $_              #  /bin/bash
         4                      #  Just called /bin/bash to run the script.
         5                      #  Note that this will vary according to
         6                      #+ how the script is invoked.
         7
         8 du >/dev/null        #  So no output from command.
         9 echo $_              #  du
        10
        11 ls -al >/dev/null    #  So no output from command.
        12 echo $_              #  -al  (last argument)
        13
        14 :
        15 echo $_              #  :

`$?`

  Exit status of a command, function, or the script itself (see Example 24-7)

`$$`

  Process ID (PID) of the script itself. [5] The `$$` variable often finds use in scripts to construct "unique" temp file names (see Example 32-6, Example 16-31, and Example 15-27). This is usually simpler than invoking mktemp.

### Notes

[1] | 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.
[2] | The PID of the currently running script is `$$`, of course.
[3] | Somewhat analogous to recursion, in this context nesting refers to a pattern embedded within a larger pattern. One of the definitions of nest, according to the 1913 edition of Webster's Dictionary, illustrates this beautifully: "A collection of boxes, cases, or the like, of graduated size, each put within the one next larger."
[4] | The words "argument" and "parameter" are often used interchangeably. In the context of this document, they have the same precise meaning: a variable passed to a script or function.
[5] | Within a script, inside a subshell, `$$` returns the PID of the script, not the subshell.

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