#!/bin/sh
set -eu

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


if [ "${1:-}" = '--' ]; then
	shift
fi

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

STRING="$1"
MESSAGE="$2"

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