Advanced Bash−Scripting Guide

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Chapter 19. Regular Expressions To fully utilize the power of shell scripting, you need to master Regular Expressions. Certain commands and utilities commonly used in scripts, such as expr, sed and awk interpret and use REs. 19.1. A Brief Introduction to Regular Expressions An expression is a string of characters. Those characters that have 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 for the symbols that follow. Regular Expressions are sets of characters and/or metacharacters that UNIX endows with special features. [47] 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 substring or an entire string). • The asterisk −− * −− matches any number of repeats of the character string or RE preceding it, including zero. "1133*" matches 11 + one or more 3's + possibly other characters: 113, 1133, 111312, and so forth. • The dot −− . −− matches any one character, except a newline. [48] "13." matches 13 + at least one of any character (including a space): 1133, 11333, but not 13 (additional character missing). • 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. "^$" matches blank lines. • Brackets −− [...] −− enclose a set of characters to match in a single RE. "[xyz]" matches the characters x, y, or z. "[c−n]" matches any of the characters in the range c to n. "[B−Pk−y]" matches any of the characters in the ranges B to P and k to y. "[a−z0−9]" matches any lowercase letter or any digit. "[^b−d]" matches all characters except those in the range b to d. This is an instance of ^ negating or inverting the meaning of the following RE (taking on a role similar to ! in a different context). Combined sequences of bracketed characters match common word patterns. "[Yy][Ee][Ss]" matches yes, Yes, YES, yEs, and so forth. "[0−9][0−9][0−9]−[0−9][0−9]−[0−9][0−9][0−9][0−9]" matches any Social Security number. • The backslash −− \ −− escapes a special character, which means that character gets interpreted Chapter 19. Regular Expressions 262
Advanced Bash−Scripting Guide literally. A "\$" reverts back to its literal meaning of "$", 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. "\" 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 '\' textfile This is the only instance of line 2. • Extended REs. 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, gawk has the −−re−interval option that permits them (without being escaped). Chapter 19. Regular Expressions 263
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Chapter 1. Why Shell Programming? A
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Chapter 2. Starting Off With a Sha
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Chapter 3. Special Characters Speci
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# Sometimes, though, you get: # 11
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The "−" can be used to pipe stdou
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change the behavior of the terminal
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Chapter 4. Introduction to Variable
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The shift command reassigns the pos
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Example 5−1. Echoing Weird Variab
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(cd /source/directory && tar cf −
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true # the "true" builtin. echo "ex
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if [[ −e $file ]] then echo "Pass
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7.3. Comparison operators (binary)
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Example 7−5. arithmetic and strin
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Chapter 8. Operations and Related T
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Part 3. Beyond the Basics Table of
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$EDITOR the default editor invoked
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Example 9−2. Timed Input Advanced
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echo 'IFS=":", using "$@"' c=0 for
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echo '${var1#$pattern1} =' "${var1#
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t=${path_name%/*.*} # Same effect a
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Chapter 10. Loops and Branches Oper
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As is the case with for/in loops, p
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This construct tests for a conditio
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# Accept upper or lowercase input.
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# May need to also press ENTER if a
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cat, tac # Burn the CDR. cdrecord
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Advanced Bash−Scripting Guide 12.
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exit $E_CHANGED_MIND ;; *) echo "De
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The multiplication operator must be
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echo Number of digits at the beginn
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Outputs very verbose timing statist
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This line occurs three times. This
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# List all the users in /etc/passwd
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Advanced Bash−Scripting Guide # r
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The −d option deletes a range of
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# Delete CR and write to new file.
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mailto Similar to the mail command,
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Chapter 30. Debugging The Bash shel
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4. Advanced Bash−Scripting Guide
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Chapter 31. Options Options are set
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Chapter 32. Gotchas Turandot: Gli e
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command1 2> − | command2 # Trying
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echo "one two three" | read a b c #
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Chapter 33. Scripting With Style Ge
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#!/bin/bash # color−echo.sh: Echo
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34.7. Assorted Tips Advanced Bash
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# agram.sh: Playing games with anag
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Chapter 35. Bash, version 2 The cur
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1−56592−580−7). This is the s
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Bibliography Edited by Peter Dennin
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Advanced Bash−Scripting Guide * A
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Mark Komarinski's Printing−Usage
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let "row = $r" # Count center row o
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next_gen "$Cur" let "generation +=
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[−c localdirectory] [−m filepat
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# ListDirectory "FileGlob" "Field
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Advanced Bash−Scripting Guide # I
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The md5sum program is documented as
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Note: 'mount −l' does return the
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Advanced Bash−Scripting Guide dec
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Appendix B. A Sed and Awk Micro−P
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Appendix D. A Detailed Introduction
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Appendix E. Localization Localizati
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Appendix F. History Commands The Ba
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Appendix H. Converting DOS Batch Fi
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Appendix I. Exercises I.1. Analyzin
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Lottery Numbers One type of lottery
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[33] This is actually a script adap
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# Thanks to Stephane Chazelas for p
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