debian-bash/tools/append_or_replace

#!/bin/bash

usage() {
	echo "Usage: $(basename "$0") <REGEX> <STRING> <FILE>"
}

if [[ $# -ne 3 ]]; then
	usage
	exit 2
fi

REGEX=$1
STRING=$2
FILE=$3

if ! grep -Eq "$REGEX" "$FILE"; then
	printf "%s\n" "$STRING" >>$FILE
	echo appended
else
	sed -Ei "s|$REGEX|$STRING|g" $FILE
	echo replaced
fi