getopt handmade alternative in bash, supporting short and long options

script-with-options.sh

#!/usr/bin/env bash

# File name
readonly PROGNAME=$(basename $0)
# File name, without the extension
readonly PROGBASENAME=${PROGNAME%.*}
# File directory
readonly PROGDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
# Arguments
readonly ARGS="$@"
# Arguments number
readonly ARGNUM="$#"

usage() {
	echo "Script description"
	echo
	echo "Usage: $PROGNAME -i <file> -o <file> [options]..."
	echo
	echo "Options:"
	echo
	echo "  -h, --help"
	echo "      This help text."
	echo
	echo "  -i <file>, --input <file>"
	echo "      Input file. If \"-\", stdin will be used instead."
	echo
	echo "  -o <file>, --output <file>"
	echo "      Output file."
	echo
	echo "  --"
	echo "      Do not interpret any more arguments as options."
	echo
}

while [ "$#" -gt 0 ]
do
	case "$1" in
	-h|--help)
		usage
		exit 0
		;;
	-i|--input)
		input="$2"

		# Jump over <file>, in case "-" is a valid input file 
		# (keyword to standard input). Jumping here prevents reaching
		# "-*)" case when parsing <file>
		shift
		;;
	-o|--output)
		output="$2"
		;;
	--)
		break
		;;
	-*)
		echo "Invalid option '$1'. Use --help to see the valid options" >&2
		exit 1
		;;
	# an option argument, continue
	*)	;;
	esac
	shift
done

# script content!