Advanced Bash−Scripting Guide

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Advanced Bash−Scripting Guide 11.1. Job Control Commands Certain of the following job control commands take a "job identifier" as an argument. See the table at end of the chapter. jobs Lists the jobs running in the background, giving the job number. Not as useful as ps. It is all too easy to confuse jobs and processes. Certain builtins, such as kill, disown, and wait accept either a job number or a process number as an argument. The fg, bg and jobs commands accept only a job number. bash$ sleep 100 & [1] 1384 bash $ jobs [1]+ Running sleep 100 & "1" is the job number (jobs are maintained by the current shell), and "1384" is the process number (processes are maintained by the system). To kill this job/process, either a kill %1 or a kill 1384 works. Thanks, S.C. disown Remove job(s) from the shell's table of active jobs. fg, bg The fg command switches a job running in the background into the foreground. The bg command restarts a suspended job, and runs it in the background. If no job number is specified, then the fg or bg command acts upon the currently running job. wait Stop script execution until all jobs running in background have terminated, or until the job number or process id specified as an option terminates. Returns the exit status of waited−for command. You may use the wait command to prevent a script from exiting before a background job finishes executing (this would create a dreaded orphan process). Example 11−22. Waiting for a process to finish before proceeding #!/bin/bash ROOT_UID=0 # Only users with $UID 0 have root privileges. E_NOTROOT=65 E_NOPARAMS=66 if [ "$UID" −ne "$ROOT_UID" ] then echo "Must be root to run this script." # "Run along kid, it's past your bedtime." exit $E_NOTROOT fi Chapter 11. Internal Commands and Builtins 144
Advanced Bash−Scripting Guide if [ −z "$1" ] then echo "Usage: `basename $0` find−string" exit $E_NOPARAMS fi echo "Updating 'locate' database..." echo "This may take a while." updatedb /usr & # Must be run as root. wait # Don't run the rest of the script until 'updatedb' finished. # You want the the database updated before looking up the file name. locate $1 # Without the wait command, in the worse case scenario, # the script would exit while 'updatedb' was still running, # leaving it as an orphan process. exit 0 Optionally, wait can take a job identifier as an argument, for example, wait%1 or wait $PPID. See the job id table. Within a script, running a command in the background with an ampersand (&) may cause the script to hang until ENTER is hit. This seems to occur with commands that write to stdout. It can be a major annoyance. #!/bin/bash # test.sh ls −l & echo "Done." bash$ ./test.sh Done. [bozo@localhost test−scripts]$ total 1 −rwxr−xr−x 1 bozo bozo 34 Oct 11 15:09 test.sh _ Placing a wait after the background command seems to remedy this. #!/bin/bash # test.sh ls −l & echo "Done." wait bash$ ./test.sh Done. [bozo@localhost test−scripts]$ total 1 −rwxr−xr−x 1 bozo bozo 34 Oct 11 15:09 test.sh Chapter 11. Internal Commands and Builtins 145
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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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Page 155 and 156: cat, tac # Burn the CDR. cdrecord
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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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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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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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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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