get-opt-and-optarg-gnu.bash

function get_opt_and_optarg_gnu {
	local optional=false

	if [[ $1 == @optional ]]; then
		optional=true
		shift
	fi

	OPT=$1 OPTARG= OPTSHIFT=0

	if [[ $1 == -[!-]* && ${optional} == true ]]; then
		OPT=${1:0:2} OPTARG=${1:2}
		[[ ${OPTARG} ]] || return 1
	elif [[ $1 == -[!-]?* ]]; then
		OPT=${1:0:2} OPTARG=${1:2}
	elif [[ $1 == --*=* ]]; then
		OPT=${1%%=*} OPTARG=${1#*=}
	elif [[ ${2+.} && ${optional} == false ]]; then
		OPTARG=$2 OPTSHIFT=1
	elif [[ $1 == --* && ${optional} == true ]]; then
		return 1
	else
		echo "No argument specified for '$1'."
		exit 2
	fi

	return 0
}