Advanced Bash−Scripting Guide

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Chapter 7. Tests Every reasonably complete programming language can test for a condition, then act according to the result of the test. Bash has the test command, various bracket and parenthesis operators, and the if/then construct. 7.1. Test Constructs • An if/then construct tests whether the exit status of a list of commands is 0 (since 0 means "success" by UNIX convention), and if so, executes one or more commands. • There exists a dedicated command called [ (left bracket special character). It is a synonym for test, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false). • With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command. Bash sees [[ $a −lt $b ]] as a single element, which returns an exit status. The (( ... )) and let ... constructs also return an exit status of 0 if the arithmetic expressions they evaluate expand to a non−zero value. These arithmetic expansion constructs may therefore be used to perform arithmetic comparisons. let "1
Advanced Bash−Scripting Guide This detailed "if−test" explanation courtesy of Stephane Chazelas. Example 7−1. What is truth? #!/bin/bash echo echo "Testing \"0\"" if [ 0 ] # zero then echo "0 is true." else echo "0 is false." fi # 0 is true. echo echo "Testing \"1\"" if [ 1 ] # one then echo "1 is true." else echo "1 is false." fi # 1 is true. echo echo "Testing \"−1\"" if [ −1 ] # minus one then echo "−1 is true." else echo "−1 is false." fi # −1 is true. echo echo "Testing \"NULL\"" if [ ] # NULL (empty condition) then echo "NULL is true." else echo "NULL is false." fi # NULL is false. echo echo "Testing \"xyz\"" if [ xyz ] # string then echo "Random string is true." else echo "Random string is false." fi # Random string is true. echo echo "Testing \"\$xyz\"" if [ $xyz ] # Tests if $xyz is null, but... Chapter 7. Tests 41
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t=${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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[ ${NETWORKING} = "no" ] && exit 0
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chroot Advanced Bash−Scripting Gu
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Chapter 15. Arithmetic Expansion Ar
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# Saves stdin. Advanced Bash−Scri
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Chapter 17. Here Documents A here d
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Example 17−11. A self−documenti
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Part 4. Advanced Topics At this poi
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"Sed & Awk", by Dougherty and Robbi
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Chapter 20. Subshells Running a she
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# Range: 0 − 200 # It's crude, bu
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# grep PATTERN /etc/passwd | awk
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Advanced Bash−Scripting Guide # Y
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Chapter 25. List Constructs The "an
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# AND LIST executes on true, OR LIS
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echo ${colors[$index]} let "index =
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# Replace all matching wildcarded s
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fi let "t += $i " Primes[t]=$NON_PR
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# −−−−−−−−− # 1)
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Chapter 28. /dev and /proc A Linux
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Advanced Bash−Scripting Guide # A
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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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