Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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# 16.4. 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 Example 11-10, Example 11-11, and Example A-8.

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.

         1 cat list-1 list-2 list-3 | sort | uniq > final.list
         2 # Concatenates the list files,
         3 # sorts them,
         4 # removes duplicate lines,
         5 # 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.

  Example 16-12. Word Frequency Analysis

         1 #!/bin/bash
         2 # wf.sh: Crude word frequency analysis on a text file.
         3 # This is a more efficient version of the "wf2.sh" script.
         4
         5
         6 # Check for input file on command-line.
         7 ARGS=1
         8 E_BADARGS=85
         9 E_NOFILE=86
        10
        11 if [ $# -ne "$ARGS" ]  # Correct number of arguments passed to script?
        12 then
        13   echo "Usage: `basename $0` filename"
        14   exit $E_BADARGS
        15 fi
        16
        17 if [ ! -f "$1" ]       # Check if file exists.
        18 then
        19   echo "File \"$1\" does not exist."
        20   exit $E_NOFILE
        21 fi
        22
        23
        24
        25 ########################################################
        26 # main ()
        27 sed -e 's/\.//g'  -e 's/\,//g' -e 's/ /\
        28 /g' "$1" | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr
        29 #                           =========================
        30 #                            Frequency of occurrence
        31
        32 #  Filter out periods and commas, and
        33 #+ change space between words to linefeed,
        34 #+ then shift characters to lowercase, and
        35 #+ finally prefix occurrence count and sort numerically.
        36
        37 #  Arun Giridhar suggests modifying the above to:
        38 #  . . . | sort | uniq -c | sort +1 [-f] | sort +0 -nr
        39 #  This adds a secondary sort key, so instances of
        40 #+ equal occurrence are sorted alphabetically.
        41 #  As he explains it:
        42 #  "This is effectively a radix sort, first on the
        43 #+ least significant column
        44 #+ (word or string, optionally case-insensitive)
        45 #+ and last on the most significant column (frequency)."
        46 #
        47 #  As Frank Wang explains, the above is equivalent to
        48 #+       . . . | sort | uniq -c | sort +0 -nr
        49 #+ and the following also works:
        50 #+       . . . | sort | uniq -c | sort -k1nr -k
        51 ########################################################
        52
        53 exit 0
        54
        55 # Exercises:
        56 # ---------
        57 # 1) Add 'sed' commands to filter out other punctuation,
        58 #+   such as semicolons.
        59 # 2) Modify the script to also filter out multiple spaces and
        60 #+   other whitespace.

       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:

         1 cut -d ' ' -f1,2 /etc/mtab

  Using cut to list the OS and kernel version:

         1 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:

         1 # List all the users in /etc/passwd.
         2
         3 FILENAME=/etc/passwd
         4
         5 for user in $(cut -d: -f1 $FILENAME)
         6 do
         7   echo $user
         8 done
         9
        10 # 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 Example 16-48.

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.

         1 File: 1.data
         2
         3 100 Shoes
         4 200 Laces
         5 300 Socks

         1 File: 2.data
         2
         3 100 $40.00
         4 200 $1.00
         5 300 $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.

  Example 16-13. Which files are scripts?

         1 #!/bin/bash
         2 # script-detector.sh: Detects scripts within a directory.
         3
         4 TESTCHARS=2    # Test first 2 characters.
         5 SHABANG='#!'   # Scripts begin with a "sha-bang."
         6
         7 for file in *  # Traverse all the files in current directory.
         8 do
         9   if [[ `head -c$TESTCHARS "$file"` = "$SHABANG" ]]
        10   #      head -c2                      #!
        11   #  The '-c' option to "head" outputs a specified
        12   #+ number of characters, rather than lines (the default).
        13   then
        14     echo "File \"$file\" is a script."
        15   else
        16     echo "File \"$file\" is *not* a script."
        17   fi
        18 done
        19
        20 exit 0
        21
        22 #  Exercises:
        23 #  ---------
        24 #  1) Modify this script to take as an optional argument
        25 #+    the directory to scan for scripts
        26 #+    (rather than just the current working directory).
        27 #
        28 #  2) As it stands, this script gives "false positives" for
        29 #+    Perl, awk, and other scripting language scripts.
        30 #     Correct this.

  Example 16-14. Generating 10-digit random numbers

         1 #!/bin/bash
         2 # rnd.sh: Outputs a 10-digit random number
         3
         4 # Script by Stephane Chazelas.
         5
         6 head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
         7
         8
         9 # =================================================================== #
        10
        11 # Analysis
        12 # --------
        13
        14 # head:
        15 # -c4 option takes first 4 bytes.
        16
        17 # od:
        18 # -N4 option limits output to 4 bytes.
        19 # -tu4 option selects unsigned decimal format for output.
        20
        21 # sed:
        22 # -n option, in combination with "p" flag to the "s" command,
        23 # outputs only matched lines.
        24
        25
        26
        27 # The author of this script explains the action of 'sed', as follows.
        28
        29 # head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
        30 # ----------------------------------> |
        31
        32 # Assume output up to "sed" --------> |
        33 # is 0000000 1198195154\n
        34
        35 #  sed begins reading characters: 0000000 1198195154\n.
        36 #  Here it finds a newline character,
        37 #+ so it is ready to process the first line (0000000 1198195154).
        38 #  It looks at its <range><action>s. The first and only one is
        39
        40 #   range     action
        41 #   1         s/.* //p
        42
        43 #  The line number is in the range, so it executes the action:
        44 #+ tries to substitute the longest string ending with a space in the line
        45 #  ("0000000 ") with nothing (//), and if it succeeds, prints the result
        46 #  ("p" is a flag to the "s" command here, this is different
        47 #+ from the "p" command).
        48
        49 #  sed is now ready to continue reading its input. (Note that before
        50 #+ continuing, if -n option had not been passed, sed would have printed
        51 #+ the line once again).
        52
        53 #  Now, sed reads the remainder of the characters, and finds the
        54 #+ end of the file.
        55 #  It is now ready to process its 2nd line (which is also numbered '$' as
        56 #+ it's the last one).
        57 #  It sees it is not matched by any <range>, so its job is done.
        58
        59 #  In few word this sed commmand means:
        60 #  "On the first line only, remove any character up to the right-most space,
        61 #+ then print it."
        62
        63 # A better way to do this would have been:
        64 #           sed -e 's/.* //;q'
        65
        66 # Here, two <range><action>s (could have been written
        67 #           sed -e 's/.* //' -e q):
        68
        69 #   range                    action
        70 #   nothing (matches line)   s/.* //
        71 #   nothing (matches line)   q (quit)
        72
        73 #  Here, sed only reads its first line of input.
        74 #  It performs both actions, and prints the line (substituted) before
        75 #+ quitting (because of the "q" action) since the "-n" option is not passed.
        76
        77 # =================================================================== #
        78
        79 # An even simpler altenative to the above one-line script would be:
        80 #           head -c4 /dev/urandom| od -An -tu4
        81
        82 exit

  See also Example 16-39.
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.

  Example 16-15. Using tail to monitor the system log

         1 #!/bin/bash
         2
         3 filename=sys.log
         4
         5 cat /dev/null > $filename; echo "Creating / cleaning out file."
         6 #  Creates the file if it does not already exist,
         7 #+ and truncates it to zero length if it does.
         8 #  : > filename   and   > filename also work.
         9
        10 tail /var/log/messages > $filename
        11 # /var/log/messages must have world read permission for this to work.
        12
        13 echo "$filename contains tail end of system log."
        14
        15 exit 0

  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:

         1 var=$(head -n $m $filename | tail -n $n)
         2
         3 # filename = name of file
         4 # m = from beginning of file, number of lines to end of block
         5 # 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 Example 16-5, Example 16-39 and Example 32-6.

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.

         1 grep pattern1 *.txt | grep -v pattern2
         2
         3 # Matches all lines in "*.txt" files containing "pattern1",
         4 # but ***not*** "pattern2".

  The `-c` (`--count`) option gives a numerical count of matches, rather than actually listing the matches.

         1 grep -c txt *.sgml   # (number of occurrences of "txt" in "*.sgml" files)
         2
         3
         4 #   grep -cz .
         5 #            ^ dot
         6 # means count (-c) zero-separated (-z) items matching "."
         7 # that is, non-empty ones (containing at least 1 character).
         8 #
         9 printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz .     # 3
        10 printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$'   # 5
        11 printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^'   # 5
        12 #
        13 printf 'a b\nc  d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$'    # 9
        14 # By default, newline chars (\n) separate items to match.
        15
        16 # Note that the -z option is GNU "grep" specific.
        17
        18
        19 # 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).

  Example 16-16. Printing out the From lines in stored e-mail messages

         1 #!/bin/bash
         2 # from.sh
         3
         4 #  Emulates the useful 'from' utility in Solaris, BSD, etc.
         5 #  Echoes the "From" header line in all messages
         6 #+ in your e-mail directory.
         7
         8
         9 MAILDIR=~/mail/*               #  No quoting of variable. Why?
        10 # Maybe check if-exists $MAILDIR:   if [ -d $MAILDIR ] . . .
        11 GREP_OPTS="-H -A 5 --color"    #  Show file, plus extra context lines
        12                                #+ and display "From" in color.
        13 TARGETSTR="^From"              # "From" at beginning of line.
        14
        15 for file in $MAILDIR           #  No quoting of variable.
        16 do
        17   grep $GREP_OPTS "$TARGETSTR" "$file"
        18   #    ^^^^^^^^^^              #  Again, do not quote this variable.
        19   echo
        20 done
        21
        22 exit $?
        23
        24 #  You might wish to pipe the output of this script to 'more'
        25 #+ or redirect it to a file . . .

  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.

         1 SUCCESS=0                      # if grep lookup succeeds
         2 word=Linux
         3 filename=data.file
         4
         5 grep -q "$word" "$filename"    #  The "-q" option
         6                                #+ causes nothing to echo to stdout.
         7 if [ $? -eq $SUCCESS ]
         8 # if grep -q "$word" "$filename"   can replace lines 5 - 7.
         9 then
        10   echo "$word found in $filename"
        11 else
        12   echo "$word not found in $filename"
        13 fi

  Example 32-6 demonstrates how to use grep to search for a word pattern in a system logfile.

  Example 16-17. Emulating grep in a script

         1 #!/bin/bash
         2 # grp.sh: Rudimentary reimplementation of grep.
         3
         4 E_BADARGS=85
         5
         6 if [ -z "$1" ]    # Check for argument to script.
         7 then
         8   echo "Usage: `basename $0` pattern"
         9   exit $E_BADARGS
        10 fi
        11
        12 echo
        13
        14 for file in *     # Traverse all files in $PWD.
        15 do
        16   output=$(sed -n /"$1"/p $file)  # Command substitution.
        17
        18   if [ ! -z "$output" ]           # What happens if "$output" is not quoted?
        19   then
        20     echo -n "$file: "
        21     echo "$output"
        22   fi              #  sed -ne "/$1/s|^|${file}: |p"  is equivalent to above.
        23
        24   echo
        25 done
        26
        27 echo
        28
        29 exit 0
        30
        31 # Exercises:
        32 # ---------
        33 # 1) Add newlines to output, if more than one match in any given file.
        34 # 2) Add features.

  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:

         1 # Filename: tstfile
         2
         3 This is a sample file.
         4 This is an ordinary text file.
         5 This file does not contain any unusual text.
         6 This file is not unusual.
         7 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 . . .

  Example 16-18. Crossword puzzle solver

         1 #!/bin/bash
         2 # cw-solver.sh
         3 # This is actually a wrapper around a one-liner (line 46).
         4
         5 #  Crossword puzzle and anagramming word game solver.
         6 #  You know *some* of the letters in the word you're looking for,
         7 #+ so you need a list of all valid words
         8 #+ with the known letters in given positions.
         9 #  For example: w...i....n
        10 #               1???5????10
        11 # w in position 1, 3 unknowns, i in the 5th, 4 unknowns, n at the end.
        12 # (See comments at end of script.)
        13
        14
        15 E_NOPATT=71
        16 DICT=/usr/share/dict/word.lst
        17 #                    ^^^^^^^^   Looks for word list here.
        18 #  ASCII word list, one word per line.
        19 #  If you happen to need an appropriate list,
        20 #+ download the author's "yawl" word list package.
        21 #  http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
        22 #  or
        23 #  http://bash.deta.in/yawl-0.3.2.tar.gz
        24
        25
        26 if [ -z "$1" ]   #  If no word pattern specified
        27 then             #+ as a command-line argument . . .
        28   echo           #+ . . . then . . .
        29   echo "Usage:"  #+ Usage message.
        30   echo
        31   echo ""$0" \"pattern,\""
        32   echo "where \"pattern\" is in the form"
        33   echo "xxx..x.x..."
        34   echo
        35   echo "The x's represent known letters,"
        36   echo "and the periods are unknown letters (blanks)."
        37   echo "Letters and periods can be in any position."
        38   echo "For example, try:   sh cw-solver.sh w...i....n"
        39   echo
        40   exit $E_NOPATT
        41 fi
        42
        43 echo
        44 # ===============================================
        45 # This is where all the work gets done.
        46 grep ^"$1"$ "$DICT"   # Yes, only one line!
        47 #    |    |
        48 # ^ is start-of-word regex anchor.
        49 # $ is end-of-word regex anchor.
        50
        51 #  From _Stupid Grep Tricks_, vol. 1,
        52 #+ a book the ABS Guide author may yet get around
        53 #+ to writing . . . one of these days . . .
        54 # ===============================================
        55 echo
        56
        57
        58 exit $?  # Script terminates here.
        59 #  If there are too many words generated,
        60 #+ redirect the output to a file.
        61
        62 $ sh cw-solver.sh w...i....n
        63
        64 wellington
        65 workingman
        66 workingmen

  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.

  Example 16-19. Looking up definitions in Webster's 1913 Dictionary

         1 #!/bin/bash
         2 # dict-lookup.sh
         3
         4 #  This script looks up definitions in the 1913 Webster's Dictionary.
         5 #  This Public Domain dictionary is available for download
         6 #+ from various sites, including
         7 #+ Project Gutenberg (http://www.gutenberg.org/etext/247).
         8 #
         9 #  Convert it from DOS to UNIX format (with only LF at end of line)
        10 #+ before using it with this script.
        11 #  Store the file in plain, uncompressed ASCII text.
        12 #  Set DEFAULT_DICTFILE variable below to path/filename.
        13
        14
        15 E_BADARGS=85
        16 MAXCONTEXTLINES=50                        # Maximum number of lines to show.
        17 DEFAULT_DICTFILE="/usr/share/dict/webster1913-dict.txt"
        18                                           # Default dictionary file pathname.
        19                                           # Change this as necessary.
        20 #  Note:
        21 #  ----
        22 #  This particular edition of the 1913 Webster's
        23 #+ begins each entry with an uppercase letter
        24 #+ (lowercase for the remaining characters).
        25 #  Only the *very first line* of an entry begins this way,
        26 #+ and that's why the search algorithm below works.
        27
        28
        29
        30 if [[ -z $(echo "$1" | sed -n '/^[A-Z]/p') ]]
        31 #  Must at least specify word to look up, and
        32 #+ it must start with an uppercase letter.
        33 then
        34   echo "Usage: `basename $0` Word-to-define [dictionary-file]"
        35   echo
        36   echo "Note: Word to look up must start with capital letter,"
        37   echo "with the rest of the word in lowercase."
        38   echo "--------------------------------------------"
        39   echo "Examples: Abandon, Dictionary, Marking, etc."
        40   exit $E_BADARGS
        41 fi
        42
        43
        44 if [ -z "$2" ]                            #  May specify different dictionary
        45                                           #+ as an argument to this script.
        46 then
        47   dictfile=$DEFAULT_DICTFILE
        48 else
        49   dictfile="$2"
        50 fi
        51
        52 # ---------------------------------------------------------
        53 Definition=$(fgrep -A $MAXCONTEXTLINES "$1 \\" "$dictfile")
        54 #                  Definitions in form "Word \..."
        55 #
        56 #  And, yes, "fgrep" is fast enough
        57 #+ to search even a very large text file.
        58
        59
        60 # Now, snip out just the definition block.
        61
        62 echo "$Definition" |
        63 sed -n '1,/^[A-Z]/p' |
        64 #  Print from first line of output
        65 #+ to the first line of the next entry.
        66 sed '$d' | sed '$d'
        67 #  Delete last two lines of output
        68 #+ (blank line and first line of next entry).
        69 # ---------------------------------------------------------
        70
        71 exit $?
        72
        73 # Exercises:
        74 # ---------
        75 # 1)  Modify the script to accept any type of alphabetic input
        76 #   + (uppercase, lowercase, mixed case), and convert it
        77 #   + to an acceptable format for processing.
        78 #
        79 # 2)  Convert the script to a GUI application,
        80 #   + using something like 'gdialog' or 'zenity' . . .
        81 #     The script will then no longer take its argument(s)
        82 #   + from the command-line.
        83 #
        84 # 3)  Modify the script to parse one of the other available
        85 #   + Public Domain Dictionaries, such as the U.S. Census Bureau Gazetteer.

  | See also Example A-41 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.

  Example 16-20. Checking words in a list for validity

         1 #!/bin/bash
         2 # lookup: Does a dictionary lookup on each word in a data file.
         3
         4 file=words.data  # Data file from which to read words to test.
         5
         6 echo
         7 echo "Testing file $file"
         8 echo
         9
        10 while [ "$word" != end ]  # Last word in data file.
        11 do               # ^^^
        12   read word      # From data file, because of redirection at end of loop.
        13   look $word > /dev/null  # Don't want to display lines in dictionary file.
        14   #  Searches for words in the file /usr/share/dict/words
        15   #+ (usually a link to linux.words).
        16   lookup=$?      # Exit status of 'look' command.
        17
        18   if [ "$lookup" -eq 0 ]
        19   then
        20     echo "\"$word\" is valid."
        21   else
        22     echo "\"$word\" is invalid."
        23   fi
        24
        25 done <"$file"    # Redirects stdin to $file, so "reads" come from there.
        26
        27 echo
        28
        29 exit 0
        30
        31 # ----------------------------------------------------------------
        32 # Code below line will not execute because of "exit" command above.
        33
        34
        35 # Stephane Chazelas proposes the following, more concise alternative:
        36
        37 while read word && [[ $word != end ]]
        38 do if look "$word" > /dev/null
        39    then echo "\"$word\" is valid."
        40    else echo "\"$word\" is invalid."
        41    fi
        42 done <"$file"
        43
        44 exit 0

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:

         1 $ ls *.txt | wc -l
         2 #  Will work as long as none of the "*.txt" files
         3 #+ have a linefeed embedded in their name.
         4
         5 #  Alternative ways of doing this are:
         6 #      find . -maxdepth 1 -name \*.txt -print0 | grep -cz .
         7 #      (shopt -s nullglob; set -- *.txt; echo $#)
         8
         9 #  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 Example 16-39 and Example 20-8.

  Certain commands include some of the functionality of wc as options.

         1 ... | grep foo | wc -l
         2 # This frequently used construct can be more concisely rendered.
         3
         4 ... | grep -c foo
         5 # Just use the "-c" (or "--count") option of grep.
         6
         7 # 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.

         1 echo "abcdef"                 # abcdef
         2 echo "abcdef" | tr -d b-d     # aef
         3
         4
         5 tr -d 0-9 <filename
         6 # 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. [1]

       bash$ echo "abcd2ef1" | tr '[:alpha:]' -
       ----2--1

  Example 16-21. toupper: Transforms a file to all uppercase.

         1 #!/bin/bash
         2 # Changes a file to all uppercase.
         3
         4 E_BADARGS=85
         5
         6 if [ -z "$1" ]  # Standard check for command-line arg.
         7 then
         8   echo "Usage: `basename $0` filename"
         9   exit $E_BADARGS
        10 fi
        11
        12 tr a-z A-Z <"$1"
        13
        14 # Same effect as above, but using POSIX character set notation:
        15 #        tr '[:lower:]' '[:upper:]' <"$1"
        16 # Thanks, S.C.
        17
        18 #     Or even . . .
        19 #     cat "$1" | tr a-z A-Z
        20 #     Or dozens of other ways . . .
        21
        22 exit 0
        23
        24 #  Exercise:
        25 #  Rewrite this script to give the option of changing a file
        26 #+ to *either* upper or lowercase.
        27 #  Hint: Use either the "case" or "select" command.

  Example 16-22. lowercase: Changes all filenames in working directory to lowercase.

         1 #!/bin/bash
         2 #
         3 #  Changes every filename in working directory to all lowercase.
         4 #
         5 #  Inspired by a script of John Dubois,
         6 #+ which was translated into Bash by Chet Ramey,
         7 #+ and considerably simplified by the author of the ABS Guide.
         8
         9
        10 for filename in *                # Traverse all files in directory.
        11 do
        12    fname=`basename $filename`
        13    n=`echo $fname | tr A-Z a-z`  # Change name to lowercase.
        14    if [ "$fname" != "$n" ]       # Rename only files not already lowercase.
        15    then
        16      mv $fname $n
        17    fi
        18 done
        19
        20 exit $?
        21
        22
        23 # Code below this line will not execute because of "exit".
        24 #--------------------------------------------------------#
        25 # To run it, delete script above line.
        26
        27 # The above script will not work on filenames containing blanks or newlines.
        28 # Stephane Chazelas therefore suggests the following alternative:
        29
        30
        31 for filename in *    # Not necessary to use basename,
        32                      # since "*" won't return any file containing "/".
        33 do n=`echo "$filename/" | tr '[:upper:]' '[:lower:]'`
        34 #                             POSIX char set notation.
        35 #                    Slash added so that trailing newlines are not
        36 #                    removed by command substitution.
        37    # Variable substitution:
        38    n=${n%/}          # Removes trailing slash, added above, from filename.
        39    [[ $filename == $n ]] || mv "$filename" "$n"
        40                      # Checks if filename already lowercase.
        41 done
        42
        43 exit $?

  Example 16-23. du: DOS to UNIX text file conversion.

         1 #!/bin/bash
         2 # Du.sh: DOS to UNIX text file converter.
         3
         4 E_WRONGARGS=85
         5
         6 if [ -z "$1" ]
         7 then
         8   echo "Usage: `basename $0` filename-to-convert"
         9   exit $E_WRONGARGS
        10 fi
        11
        12 NEWFILENAME=$1.unx
        13
        14 CR='\015'  # Carriage return.
        15            # 015 is octal ASCII code for CR.
        16            # Lines in a DOS text file end in CR-LF.
        17            # Lines in a UNIX text file end in LF only.
        18
        19 tr -d $CR < $1 > $NEWFILENAME
        20 # Delete CR's and write to new file.
        21
        22 echo "Original DOS text file is \"$1\"."
        23 echo "Converted UNIX text file is \"$NEWFILENAME\"."
        24
        25 exit 0
        26
        27 # Exercise:
        28 # --------
        29 # Change the above script to convert from UNIX to DOS.

  Example 16-24. rot13: ultra-weak encryption.

         1 #!/bin/bash
         2 # rot13.sh: Classic rot13 algorithm,
         3 #           encryption that might fool a 3-year old
         4 #           for about 10 minutes.
         5
         6 # Usage: ./rot13.sh filename
         7 # or     ./rot13.sh <filename
         8 # or     ./rot13.sh and supply keyboard input (stdin)
         9
        10 cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M'   # "a" goes to "n", "b" to "o" ...
        11 #  The   cat "$@"   construct
        12 #+ permits input either from stdin or from files.
        13
        14 exit 0

  Example 16-25. Generating "Crypto-Quote" Puzzles

         1 #!/bin/bash
         2 # crypto-quote.sh: Encrypt quotes
         3
         4 #  Will encrypt famous quotes in a simple monoalphabetic substitution.
         5 #  The result is similar to the "Crypto Quote" puzzles
         6 #+ seen in the Op Ed pages of the Sunday paper.
         7
         8
         9 key=ETAOINSHRDLUBCFGJMQPVWZYXK
        10 # The "key" is nothing more than a scrambled alphabet.
        11 # Changing the "key" changes the encryption.
        12
        13 # The 'cat "$@"' construction gets input either from stdin or from files.
        14 # If using stdin, terminate input with a Control-D.
        15 # Otherwise, specify filename as command-line parameter.
        16
        17 cat "$@" | tr "a-z" "A-Z" | tr "A-Z" "$key"
        18 #        |  to uppercase  |     encrypt
        19 # Will work on lowercase, uppercase, or mixed-case quotes.
        20 # Passes non-alphabetic characters through unchanged.
        21
        22
        23 # Try this script with something like:
        24 # "Nothing so needs reforming as other people's habits."
        25 # --Mark Twain
        26 #
        27 # Output is:
        28 # "CFPHRCS QF CIIOQ MINFMBRCS EQ FPHIM GIFGUI'Q HETRPQ."
        29 # --BEML PZERC
        30
        31 # To reverse the encryption:
        32 # cat "$@" | tr "$key" "A-Z"
        33
        34
        35 #  This simple-minded cipher can be broken by an average 12-year old
        36 #+ using only pencil and paper.
        37
        38 exit 0
        39
        40 #  Exercise:
        41 #  --------
        42 #  Modify the script so that it will either encrypt or decrypt,
        43 #+ depending on command-line argument(s).

  Of course, tr lends itself to code obfuscation.

         1 #!/bin/bash
         2 # jabh.sh
         3
         4 x="wftedskaebjgdBstbdbsmnjgz"
         5 echo $x | tr "a-z" 'oh, turtleneck Phrase Jar!'
         6
         7 # Based on the Wikipedia "Just another Perl hacker" article.

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 Example 16-26 and Example A-1).

fmt

  Simple-minded file formatter, used as a filter in a pipe to "wrap" long lines of text output.

  Example 16-26. Formatted file listing.

         1 #!/bin/bash
         2
         3 WIDTH=40                    # 40 columns wide.
         4
         5 b=`ls /usr/local/bin`       # Get a file listing...
         6
         7 echo $b | fmt -w $WIDTH
         8
         9 # Could also have been done by
        10 #    echo $b | fold - -s -w $WIDTH
        11
        12 exit 0

  See also Example 16-5.

  | 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.

  Example 16-27. Using column to format a directory listing

         1 #!/bin/bash
         2 # colms.sh
         3 # A minor modification of the example file in the "column" man page.
         4
         5
         6 (printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \
         7 ; ls -l | sed 1d) | column -t
         8 #         ^^^^^^           ^^
         9
        10 #  The "sed 1d" in the pipe deletes the first line of output,
        11 #+ which would be "total        N",
        12 #+ where "N" is the total number of files found by "ls -l".
        13
        14 # The -t option to "column" pretty-prints a table.
        15
        16 exit 0

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.

  Example 16-28. nl: A self-numbering script.

         1 #!/bin/bash
         2 # line-number.sh
         3
         4 # This script echoes itself twice to stdout with its lines numbered.
         5
         6 echo "     line number = $LINENO" # 'nl' sees this as line 4
         7 #                                   (nl does not number blank lines).
         8 #                                   'cat -n' sees it correctly as line #6.
         9
        10 nl `basename $0`
        11
        12 echo; echo  # Now, let's try it with 'cat -n'
        13
        14 cat -n `basename $0`
        15 # The difference is that 'cat -n' numbers the blank lines.
        16 # Note that 'nl -ba' will also do so.
        17
        18 exit 0
        19 # -----------------------------------------------------------------

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.

         1 # Convert a string from UTF-8 to UTF-16 and print to the BookList
         2 function write_utf8_string {
         3     STRING=$1
         4     BOOKLIST=$2
         5     echo -n "$STRING" | iconv -f UTF8 -t UTF16 | \
         6     cut -b 3- | tr -d \\n >> "$BOOKLIST"
         7 }
         8
         9 #  From Peter Knowles' "booklistgen.sh" script
        10 #+ for converting files to Sony Librie/PRS-50X format.
        11 #  (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.

         1 texexec --pdfarrange --result=Concatenated.pdf *pdf
         2
         3 #  Concatenates all the pdf files in the current working directory
         4 #+ into the merged file, Concatenated.pdf . . .
         5 #  (The --pdfarrange option repaginates a pdf file. See also --pdfcombine.)
         6 #  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.

  Example 16-29. manview: Viewing formatted manpages

         1 #!/bin/bash
         2 # manview.sh: Formats the source of a man page for viewing.
         3
         4 #  This script is useful when writing man page source.
         5 #  It lets you look at the intermediate results on the fly
         6 #+ while working on it.
         7
         8 E_WRONGARGS=85
         9
        10 if [ -z "$1" ]
        11 then
        12   echo "Usage: `basename $0` filename"
        13   exit $E_WRONGARGS
        14 fi
        15
        16 # ---------------------------
        17 groff -Tascii -man $1 | less
        18 # From the man page for groff.
        19 # ---------------------------
        20
        21 #  If the man page includes tables and/or equations,
        22 #+ then the above code will barf.
        23 #  The following line can handle such cases.
        24 #
        25 #   gtbl < "$1" | geqn -Tlatin1 | groff -Tlatin1 -mtty-char -man
        26 #
        27 #   Thanks, S.C.
        28
        29 exit $?   # See also the "maned.sh" script.

  See also Example A-39.

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.

### Notes

[1] | This is only true of the GNU version of tr, not the generic version often found on commercial UNIX systems.

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