Advanced Bash−Scripting Guide

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Advanced Bash−Scripting Guide Elements of a string assigned to a variable may be escaped, but the escape character alone may not be assigned to a variable. variable=\ echo "$variable" # Will not work − gives an error message: # test.sh: : command not found # A "naked" escape cannot safely be assigned to a variable. # # What actually happens here is that the "\" escapes the newline and #+ the effect is variable=echo "$variable" #+ invalid variable assignment variable=\ 23skidoo echo "$variable" # 23skidoo # This works, since the second line #+ is a valid variable assignment. variable=\ # \^ escape followed by space echo "$variable" # space variable=\\ echo "$variable" # \ variable=\\\ echo "$variable" # Will not work − gives an error message: # test.sh: \: command not found # # First escape escapes second one, but the third one is left "naked", #+ with same result as first instance, above. variable=\\\\ echo "$variable" # \\ # Second and fourth escapes escaped. # This is o.k. Escaping a space can prevent word splitting in a command's argument list. file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs−20.7" # List of files as argument(s) to a command. # Add two files to the list, and list all. ls −l /usr/X11R6/bin/xsetroot /sbin/dump $file_list echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−" # What happens if we escape a couple of spaces? ls −l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list # Error: the first three files concatenated into a single argument to 'ls −l' # because the two escaped spaces prevent argument (word) splitting. The escape also provides a means of writing a multi−line command. Normally, each separate line constitutes a different command, but an escape at the end of a line escapes the newline character, and the command sequence continues on to the next line. Chapter 5. Quoting 36
(cd /source/directory && tar cf − . ) | \ (cd /dest/directory && tar xpvf −) # Repeating Alan Cox's directory tree copy command, # but split into two lines for increased legibility. # As an alternative: tar cf − −C /source/directory . | tar xpvf − −C /dest/directory # See note below. # (Thanks, Stephane Chazelas.) Advanced Bash−Scripting Guide If a script line ends with a |, a pipe character, then a \, an escape, is not strictly necessary. It is, however, good programming practice to always escape the end of a line of code that continues to the following line. echo "foo bar" #foo #bar echo echo 'foo bar' # No difference yet. #foo #bar echo echo foo\ bar # Newline escaped. #foobar echo echo "foo\ bar" # Same here, as \ still interpreted as escape within weak quotes. #foobar echo echo 'foo\ bar' # Escape character \ taken literally because of strong quoting. #foor\ #bar # Examples suggested by Stephane Chazelas. Chapter 5. Quoting 37
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Page 7 and 8: Chapter 1. Why Shell Programming? A
Page 9 and 10: Chapter 2. Starting Off With a Sha
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Page 23 and 24: # Sometimes, though, you get: # 11
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Page 37 and 38: The shift command reassigns the pos
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Page 45 and 46: true # the "true" builtin. echo "ex
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echo '${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 Exa
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Advanced Bash−Scripting Guide #!/
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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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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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# 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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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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# 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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Advanced Bash−Scripting Guide Cop
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fi let "t += $i " Primes[t]=$NON_PR
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Advanced Bash−Scripting Guide # =
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# −−−−−−−−− # 1)
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Chapter 28. /dev and /proc A Linux
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Advanced Bash−Scripting Guide net
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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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Advanced Bash−Scripting Guide the
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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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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 # =
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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 /GU
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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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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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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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