miaou-bash/tools/append_or_replace

#!/bin/bash

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

[[ $# -ne 3 ]] && usage && exit 2

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