Advanced Bash−Scripting Guide

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Advanced Bash−Scripting Guide Directory changes made in a subshell do not carry over to the parent shell. Example 20−2. List User Profiles #!/bin/bash # allprofs.sh: print all user profiles # This script written by Heiner Steven, and modified by the document author. FILE=.bashrc # File containing user profile, #+ was ".profile" in original script. for home in `awk −F: '{print $6}' /etc/passwd` do [ −d "$home" ] || continue # If no home directory, go to next. [ −r "$home" ] || continue # If not readable, go to next. (cd $home; [ −e $FILE ] && less $FILE) done # When script terminates, there is no need to 'cd' back to original directory, #+ because 'cd $home' takes place in a subshell. exit 0 A subshell may be used to set up a "dedicated environment" for a command group. COMMAND1 COMMAND2 COMMAND3 ( IFS=: PATH=/bin unset TERMINFO set −C shift 5 COMMAND4 COMMAND5 exit 3 # Only exits the subshell. ) # The parent shell has not been affected, and the environment is preserved. COMMAND6 COMMAND7 One application of this is testing whether a variable is defined. if (set −u; : $variable) 2> /dev/null then echo "Variable is set." fi # Could also be written [[ ${variable−x} != x || ${variable−y} != y ]] # or [[ ${variable−x} != x$variable ]] # or [[ ${variable+x} = x ]]) Another application is checking for a lock file: if (set −C; : > lock_file) 2> /dev/null then echo "Another user is already running that script." exit 65 Chapter 20. Subshells 268
Advanced Bash−Scripting Guide fi # Thanks, S.C. Processes may execute in parallel within different subshells. This permits breaking a complex task into subcomponents processed concurrently. Example 20−3. Running parallel processes in subshells (cat list1 list2 list3 | sort | uniq > list123) & (cat list4 list5 list6 | sort | uniq > list456) & # Merges and sorts both sets of lists simultaneously. # Running in background ensures parallel execution. # # Same effect as # cat list1 list2 list3 | sort | uniq > list123 & # cat list4 list5 list6 | sort | uniq > list456 & wait # Don't execute the next command until subshells finish. diff list123 list456 Redirecting I/O to a subshell uses the "|" pipe operator, as in ls −al | (command). A command block between curly braces does not launch a subshell. { command1; command2; command3; ... } Chapter 20. Subshells 269
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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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# 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 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 "$@"' c=0 for
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Advanced Bash−Scripting Guide #+
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Advanced Bash−Scripting Guide use
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echo '${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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Advanced Bash−Scripting Guide ena
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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 No
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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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Advanced Bash−Scripting Guide •
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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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Advanced Bash−Scripting Guide # R
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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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Advanced Bash−Scripting Guide #+
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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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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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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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