#!/bin/sh
set -eu

usage() {
	cat <<-'EOF'
		Usage:
		  assert-arg -- STRING MESSAGE
		  assert-arg -h
	EOF
}

help() {
	cat <<-'EOF'

		Options:
		  -h, --help    show this message

		  STRING        the string to check if is empty
		  MESSAGE       the message to print when STRING is empty


		Examples:

		  Assert that $1 contains an argument, named FILENAME:

		    $ eval "$(assert-arg -- "${1:-}" 'FILENAME')"
	EOF
}


for flag in "$@"; do
	case "$flag" in
		--)
			break
			;;
		--help)
			usage
			help
			exit
			;;
		*)
			;;
	esac
done

while getopts 'h' flag; do
	case "$flag" in
		h)
			usage
			help
			exit
			;;
		*)
			usage >&2
			exit 2
			;;
	esac
done
shift $((OPTIND - 1))


STRING="${1:-}"
MESSAGE="${2:-}"

if [ -z "$MESSAGE" ]; then
	printf 'Missing MESSAGE, an argument to assert-arg.\n\n' >&2
	usage >&2
	exit 2
fi


if [ -z "$STRING" ]; then
	printf 'Missing %s.\n\n' "$MESSAGE" >&2
	cat <<-'EOF'
		usage >&2
		exit 2
	EOF
fi