#!/bin/sh
set -eu


usage() {
	cat <<-'EOF'
		Usage:
		  free-port
		  free-port -h
	EOF
}

help() {
	cat <<-'EOF'


		Options:
		  -h, --help    show this message


		Returns a free TCP port.


		Examples:

		  Get a port for a command:

		    $ cmd -p `free-port`
	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))


PORT=1024

while true; do
	if ! lsof -s TCP:LISTEN -i ":$PORT" >/dev/null; then
		break
	fi
	PORT=$((PORT + 1))
done

echo "$PORT"