Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
exit 0
Variable expansion / Substring replacement
These constructs have been adopted from ksh.
${var:pos}
Variable var expanded, starting from offset pos.
${var:pos:len}
Expansion to a max of len characters of variable var, from offset pos. See Example A−15 for an
example of the creative use of this operator.
${var/Pattern/Replacement}
First match of Pattern, within var replaced with Replacement.
If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is,
deleted.
${var//Pattern/Replacement}
Global replacement. All matches of Pattern, within var replaced with Replacement.
As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing,
that is, deleted.
Example 9−18. Using pattern matching to parse arbitrary strings
#!/bin/bash
var1=abcd−1234−defg
echo "var1 = $var1"
t=${var1#*−*}
echo "var1 (with everything, up to and including first − stripped out) = $t"
# t=${var1#*−} works just the same,
#+ since # matches the shortest string,
#+ and * matches everything preceding, including an empty string.
# (Thanks, S. C. for pointing this out.)
t=${var1##*−*}
echo "If var1 contains a \"−\", returns empty string...
var1 = $t"
t=${var1%*−*}
echo "var1 (with everything from the last − on stripped out) = $t"
echo
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
path_name=/home/bozo/ideas/thoughts.for.today
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
echo "path_name = $path_name"
t=${path_name##/*/}
echo "path_name, stripped of prefixes = $t"
# Same effect as t=`basename $path_name` in this particular case.
# t=${path_name%/}; t=${t##*/} is a more general solution,
#+ but still fails sometimes.
# If $path_name ends with a newline, then `basename $path_name` will not work,
#+ but the above expression will.
# (Thanks, S.C.)
Chapter 9. Variables Revisited 90