replacestart

Unlike Bourne Again Shell (bash), POSIX shell does not have a native find/replace function for replacing a pattern at the start of a given string with another. Below is an implementation written in native POSIX shell that is:
Faster than piping to awk, sed, perl, python, ruby, or other langauge
Compatible with bash and all other POSIX-compliant shells
Optimized to use ${parameter/#pattern[/string]} when bash is detected
1
 1 #!/bin/sh
2
 2 if [ "$BASH_VERSION" ]; then
3
 3     replacestart() # $var_to_get $find $replace [$var_to_set]
4
 4     {
5
 5         eval ${4:-$1}=\"\${$1/#\$2${3:+/\$3}}\"
6
 6     }
7
 7 else
8
 8     # NB: On FreeBSD, sh(1) runs this faster than bash(1) runs the above
9
 9     replacestart() # $var_to_get $find $replace [$var_to_set]
10
10     {
11
11         eval local __string=\"\$1\" || return
12
12         case "$__string" in $2*)
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13             __string="$3${__string#$2}"
14
14         esac
15
15         eval ${4:-$1}=\"\$__string\"
16
16     }
17
17 fi
18
18 eval echo \"before=[\$1]\"
19
19 replacestart "[email protected]"
20
20 eval echo \" after=[\${4:-$1}]\"
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