POSIX compliant argument parsing

parse_args.sh

#!/bin/sh

expand_single_args() {
	var="${1#-}" # cut off first, and only dash
	while [ "$var" ]; do
		next="${var#?}"
		first_char="${var%$next}"
		echo "-$first_char"
		var="$next" # next
	done
}

# POSIX compliant argument parser.
# This function will parse and return a separated argument list. It handles
# long arguments and checks for missing or extra values.
# it handles both whitespace and '=' separated values
# it also treats quoted parameters as one
# It does NOT check for unknown variables as there is no list of allowed args
# but those are easy to handle later
parse_args() {
	# first parameter is a single string, an IFS separated list of arguments
	# that should have a single value
	with_parameters="$1"
	shift
	while [ "$1" ]; do
		single_cut_with_equalparam="${1##-}"
		single_cut="${single_cut_with_equalparam%%=*}" # = value cut pff
		double_cut_with_equalparam="${1##--}"
		double_cut="${double_cut_with_equalparam%%=*}" # = value cut pff
		equalparam=${1##*=}
		if [ "$equalparam" = "$1" ]; then
			equalparam=''
		fi
		# starts with one dash but not two
		if ! [ "$single_cut_with_equalparam" = "$1" ] && [ "$double_cut_with_equalparam" = "$1" ]; then
			split_args=$(expand_single_args "$single_cut")
			shift
			if [ -n "$equalparam" ]; then
				set -- "$equalparam" "$@"
			fi
			# shellcheck disable=SC2086
			set -- $split_args "$@"
		# two dash
		elif ! [ "$double_cut_with_equalparam" = "$1" ]; then
			shift
			if [ -n "$equalparam" ]; then
				set -- "$equalparam" "$@"
			fi
			set -- "--$double_cut" "$@"
		fi

		has_parameter=''
		for a in $with_parameters; do
			if [ "$a" = "$1" ]; then
				has_parameter='1'
				break
			fi
		done

		if [ -n "$has_parameter" ]; then
			if [ -z "$2" ] || ! [ "${2##-}" = "$2" ]; then
				echo "$1 is missing it's parameter!" >&2
				exit 1
			fi
		else
			if [ -n "$2" ] && [ "${2##-}" = "$2" ]; then
				echo "$1 has a parameter ($2) when it shouldn't have one!" >&2
				exit 1
			fi
		fi

		echo "$1"
		shift
	done
}

handle_args() {
	while [ "$1" ]; do
		case "$1" in
			-h | --help) echo 'help' ;;
			-v | --version) echo 'version' ;;
			-e | --echo) echo "echo: $2"; shift ;;
			* ) echo 'unknown arg' ;;
		esac
		shift
	done
}

args_with_parameters='-e
--echo'

# shellcheck disable=SC2046
handle_args $(parse_args "$args_with_parameters" "$@")