subst

#!/usr/bin/env bash

#
# INSTALLATION
#
# ln -s ${PWD}/subst $(brew --prefix)/bin/
# sudo ln -s ${PWD}/subst /usr/local/bin/
#
# ALTERNATIVE IMPLEMENTATION
#
# perl -p -i -e 's/${1?}/${2?}/g' file1.txt file2.txt ...
#
# NOTE: perl - behind the scenes - will create backup
# files, unlike the following ed-based implementation
# which carries out the substitution 'in situ'...  as
# an additional bonus, it doesn't touch the timestamp
# of the files it processes if no substitution occurs
#

FROM="$1" ; shift ; # exclude ${FROM} from [<file>...] list
TO="$2"   ; shift ; # exclude ${TO} from [<file>...] list

[ -z "${FROM}" ] || [ -z "${TO}" ] && { # check command-line options
	>&2 echo 'Usage: subst <from> <to> [<file>...]' ;
	exit 1 ;
}

DELIM=$'\a' ; # use 'BEL' as delimiter in ed addresses and commands

MATCH="g${DELIM}${FROM}${DELIM}" ;               # g/<from>/
SUBST="s${DELIM}${FROM}${DELIM}${TO}${DELIM}g" ; # s/<from>/<to>/g

for FILE ; do
	# this grep is required to prevent subst from
	# changing the timestamp of files that DO NOT
	# contain the string we want to substitute!!!
	if grep --quiet -- "${FROM}" "${FILE}" ; then
		# now do the actual substitution using ed
		# but only if the ${FROM} string actually
		# occurs on the line: suppress ed warning
		# when saving the subst file back to disk
		echo "${MATCH}${SUBST}
		w
		q" | ed -s "${FILE}" ;
	fi
done

# That's all Folks!