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 ${parameter/#pattern[/string]} when bash is detected
#!/bin/sh
if [ "$BASH_VERSION" ]; then
   replacestart() # $var_to_get $find $replace [$var_to_set]
   {
       eval ${4:-$1}=\"\${$1/#\$2${3:+/\$3}}\"
   }
else
   # NB: On FreeBSD, sh(1) runs this faster than bash(1) runs the above
   replacestart() # $var_to_get $find $replace [$var_to_set]
   {
       eval local __string=\"\$1\" || return
       case "$__string" in $2*)
           __string="$3${__string#$2}"
       esac
       eval ${4:-$1}=\"\$__string\"
   }
fi
eval echo \"before=[\$1]\"
replacestart "[email protected]"
eval echo \" after=[\${4:-$1}]\"

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