Bash-Beginners-Guide

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[bob in testdir] export TESTVAR=present [bob in testdir] ./vartest.sh present TESTVAR is set, we can proceed. Using "+" instead of the exclamation mark sets the variable to the expansion of WORD; if it does not exist, nothing happens. 10.3.3.2. Removing substrings To strip a number of characters, equal to OFFSET, from a variable, use this syntax: ${VAR:OFFSET:LENGTH} Bash Guide for Beginners The LENGTH parameter defines how many characters to keep, starting from the first character after the offset point. If LENGTH is omitted, the remainder of the variable content is taken: [bob in ~] export STRING="thisisaverylongname" [bob in ~] echo ${STRING:4} isaverylongname [bob in ~] echo ${STRING:6:5} avery ${VAR#WORD} and ${VAR##WORD} These constructs are used for deleting the pattern matching the expansion of WORD in VAR. WORD is expanded to produce a pattern just as in file name expansion. If the pattern matches the beginning of the expanded value of VAR, then the result of the expansion is the expanded value of VAR with the shortest matching pattern ("#") or the longest matching pattern (indicated with "##"). If VAR is * or @, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If VAR is an array variable subscribed with "*" or "@", the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. This is shown in the examples below: [bob in ~] echo ${ARRAY[*]} one two one three one four [bob in ~] echo ${ARRAY[*]#one} two three four [bob in ~] echo ${ARRAY[*]#t} one wo one hree one four [bob in ~] echo ${ARRAY[*]#t*} one wo one hree one four [bob in ~] echo ${ARRAY[*]##t*} one one one four Chapter 10. More on variables 128
Bash Guide for Beginners The opposite effect is obtained using "%" and "%%", as in this example below. WORD should match a trailing portion of string: [bob in ~] echo $STRING thisisaverylongname [bob in ~] echo ${STRING%name} thisisaverylong 10.3.3.3. Replacing parts of variable names This is done using the ${VAR/PATTERN/STRING} or ${VAR//PATTERN/STRING} syntax. The first form replaces only the first match, the second replaces all matches of PATTERN with STRING: [bob in ~] echo ${STRING/name/string} thisisaverylongstring More information can be found in the Bash info pages. 10.4. Summary Normally, a variable can hold any type of data, unless variables are declared explicitly. Constant variables are set using the readonly built-in command. An array holds a set of variables. If a type of data is declared, then all elements in the array will be set to hold only this type of data. Bash features allow for substitution and transformation of variables "on the fly". Standard operations include calculating the length of a variable, arithmetic on variables, substituting variable content and substituting part of the content. 10.5. Exercises Here are some brain crackers: 1. Write a script that does the following: ♦ Display the name of the script being executed. ♦ Display the first, third and tenth argument given to the script. ♦ Display the total number of arguments passed to the script. ♦ If there were more than three positional parameters, use shift to move all the values 3 places to the left. ♦ Print all the values of the remaining arguments. Chapter 10. More on variables 129
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Bash Guide for Beginners Machtelt G
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Bash Guide for Beginners Table of C
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Bash Guide for Beginners Table of C
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Table of Contents Glossary C.......
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Introduction 1. Why this guide? The
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Revision 1.6 2005-03-01 Revised by:
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Bash Guide for Beginners See Chapte
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Bash Guide for Beginners mia:~> cat
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Bash Guide for Beginners When invok
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More information about the file des
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Built-in commands are necessary to
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• Brace expansion • Tilde expan
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A number of methods can be used; on
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Chapter 2. Writing and debugging sc
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the documentation of your favorite
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Bash Guide for Beginners Comments a
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Chapter 3. The Bash environment In
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Bash Guide for Beginners local pid
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MOZILLA_FIVE_HOME=/usr/lib/mozilla
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Bash Guide for Beginners franky ~>
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the GNU Awk programming language is
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Bash Guide for Beginners 3.2.4. Res
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LINES MACHTYPE MAILCHECK OLDPWD OPT
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two is the second positional parame
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Bash Guide for Beginners 3.3.1. Why
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A correctly-formed brace expansion
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Bash Guide for Beginners If the sub
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of the whitespace characters "space
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Bash Guide for Beginners /dev/hda5
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Bash Guide for Beginners 3.7. Summa
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Two regular expressions may be conc
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LABEL=/ / ext3 defaults 1 1 If this
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cathy ~> ls -ld [[:digit:]]* drwxrw
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Bash Guide for Beginners p Print te
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sandy ~> sed -n '/a text/,/This/p'
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Bash Guide for Beginners # This is
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Chapter 6. The GNU awk programming
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Bash Guide for Beginners 121script.
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Page 159 and 160: Send and receive mail. man Read man
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