Advanced Bash−Scripting Guide

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Advanced Bash−Scripting Guide names instead. This makes the script easier to understand and permits making changes and updates without breaking the application. if [ −f /var/log/messages ] then ... fi # A year later, you decide to change the script to check /var/log/syslog. # It is now necessary to manually change the script, instance by instance, # and hope nothing breaks. # A better way: LOGFILE=/var/log/messages # Only line that needs to be changed. if [ −f "$LOGFILE" ] then ... fi • Choose descriptive names for variables and functions. fl=`ls −al $dirname` # Cryptic. file_listing=`ls −al $dirname` # Better. MAXVAL=10 # All caps used for a script constant. while [ "$index" −le "$MAXVAL" ] ... E_NOTFOUND=75 if [ ! −e "$filename" ] then echo "File $filename not found." exit $E_NOTFOUND fi # Uppercase for an errorcode, # and name begins with "E_". MAIL_DIRECTORY=/var/spool/mail/bozo # Uppercase for an environmental variable. export MAIL_DIRECTORY GetAnswer () { prompt=$1 echo −n $prompt read answer return $answer } # Mixed case works well for a function. GetAnswer "What is your favorite number? " favorite_number=$? echo $favorite_number _uservariable=23 # Permissable, but not recommended. # It's better for user−defined variables not to start with an underscore. # Leave that for system variables. • Use exit codes in a systematic and meaningful way. E_WRONG_ARGS=65 ... ... exit $E_WRONG_ARGS Chapter 33. Scripting With Style 334
Advanced Bash−Scripting Guide See also Appendix C. • Break complex scripts into simpler modules. Use functions where appropriate. See Example 35−4. • Don't use a complex construct where a simpler one will do. COMMAND if [ $? −eq 0 ] ... # Redundant and non−intuitive. if COMMAND ... # More concise (if perhaps not quite as legible). ... reading the UNIX source code to the Bourne shell (/bin/sh). I was shocked at how much simple algorithms could be made cryptic, and therefore useless, by a poor choice of code style. I asked myself, "Could someone be proud of this code?" Landon Noll Chapter 33. Scripting With Style 335
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Advanced Bash−Scripting Guide An
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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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Advanced Bash−Scripting Guide Eac
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Chapter 3. Special Characters Speci
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Advanced Bash−Scripting Guide \ /
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Advanced Bash−Scripting Guide * ?
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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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Advanced Bash−Scripting Guide Exa
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Advanced Bash−Scripting Guide MIN
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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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Advanced Bash−Scripting Guide Thi
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Advanced Bash−Scripting Guide if
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if [[ −e $file ]] then echo "Pass
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Advanced Bash−Scripting Guide −
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7.3. Comparison operators (binary)
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Example 7−5. arithmetic and strin
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Chapter 8. Operations and Related T
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Advanced Bash−Scripting Guide ! ^
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Advanced Bash−Scripting Guide let
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Part 3. Beyond the Basics Table of
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$EDITOR the default editor invoked
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Advanced Bash−Scripting Guide (Th
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Advanced Bash−Scripting Guide #!/
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Example 9−2. Timed Input Advanced
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Advanced Bash−Scripting Guide fi
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echo 'IFS=":", using "$@"' c=0 for
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Advanced Bash−Scripting Guide use
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echo '${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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Advanced Bash−Scripting Guide # T
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Advanced Bash−Scripting Guide OUT
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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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Advanced Bash−Scripting Guide ###
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# Accept upper or lowercase input.
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Advanced Bash−Scripting Guide a=`
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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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Advanced Bash−Scripting Guide gre
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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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Advanced Bash−Scripting Guide gzi
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Advanced Bash−Scripting Guide No
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mailto Similar to the mail command,
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[ ${NETWORKING} = "no" ] && exit 0
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chroot Advanced Bash−Scripting Gu
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Advanced Bash−Scripting Guide ' c
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Advanced Bash−Scripting Guide Not
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Chapter 15. Arithmetic Expansion Ar
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Advanced Bash−Scripting Guide # T
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# Saves stdin. Advanced Bash−Scri
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Advanced Bash−Scripting Guide 16.
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Chapter 17. Here Documents A here d
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Advanced Bash−Scripting Guide Fil
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Example 17−11. A self−documenti
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Part 4. Advanced Topics At this poi
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Advanced Bash−Scripting Guide lit
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"Sed & Awk", by Dougherty and Robbi
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Chapter 20. Subshells Running a she
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Advanced Bash−Scripting Guide exe
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Advanced Bash−Scripting Guide f1
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# Range: 0 − 200 # It's crude, bu
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Advanced Bash−Scripting Guide alt
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Page 335 and 336: command1 2> − | command2 # Trying
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Page 369 and 370: Bibliography Edited by Peter Dennin
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next_gen "$Cur" let "generation +=
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[−c localdirectory] [−m filepat
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Advanced Bash−Scripting Guide # =
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Advanced Bash−Scripting Guide } z
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Advanced Bash−Scripting Guide #:d
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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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Advanced Bash−Scripting Guide Tha
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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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Advanced Bash−Scripting Guide } X
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Advanced Bash−Scripting Guide pow
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Advanced Bash−Scripting Guide *)
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Advanced Bash−Scripting Guide com
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Appendix H. Converting DOS Batch Fi
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Advanced Bash−Scripting Guide XCO
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Appendix I. Exercises I.1. Analyzin
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Lottery Numbers One type of lottery
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Advanced Bash−Scripting Guide Usi
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Advanced Bash−Scripting Guide and
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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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