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9.7 String Editing in ksh and bash

The C shell's string editing operators ( 9.6 ) can be used with shell variables and, in some cases, with command history. Those operators also work with bash history. But the Korn shell and bash have a different way to edit shell variables. Table 9.1 shows them:

Table 9.1: ksh and bash String Editing Operators
Operator Explanation
${ variable # pattern }

Delete the shortest part of pattern that matches the beginning of variable 's value. Return the rest.

${ variable ## pattern }

Delete the longest part of pattern that matches the beginning of variable 's value. Return the rest.

${ variable % pattern }

Delete the shortest part of pattern that matches the end of variable 's value.Return the rest.

${ variable %% pattern }

Delete the longest part of pattern that matches the end of variable 's value.Return the rest.

The pattern s can be filename wildcard characters: * , ? , and [] ; with string editing operators, wildcards match strings in the same way they match filenames. (These are not sed -like regular expressions.) The first two operators, with # , edit variables from the front. The other two, with % , edit from the end. Here's a system for remembering which does what: you put a number sign ( # ) at the front of a number and a percent sign ( % ) at the end of a number.

Time for some examples. The variable var contains /a/b/c/d/e.f.g :


Expression
   
Result


${var}   /a/b/c/d/e.f.g
${var#/*/}   b/c/d/e.f.g
${var##/*/}   e.f.g
${var%.*}   /a/b/c/d/e.f
${var%%.*}   /a/b/c/d/e
${var%%/*/}   /a/b/c/d/e.f.g
${var%%/*}
${var%/b*}   /a
${var%%/b*}   /a

How about a practical example? The PATH variable ( 6.4 ) is a string separated by colons ( : ). Let's say you want to remove the last directory from the system path and add $HOME/bin in place of the last directory. You'd type this command, or put a line like this in your .profile :

PATH=${PATH%:*}:$HOME/bin

Because the ${PATH%:*} has a single % , that operator removes the least it can: just the last colon plus the directory name after it. After string editing, the rest of the PATH has :$HOME/bin appended to it. The new value is saved as the new PATH .

The Bourne shell's parameter substitution operators ( 45.12 ) look similar, but they're mostly useful for shell programming.

- JP


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