Full-featured argument parsing in non-forking POSIX shell

example.sh

#!/bin/sh

source parse_args.inc.sh

arg_alias() {
    case "$1" in
    h) printf help;;
    m) printf monitor;;
    w) printf wait;;
    r) printf rate; false;;
    esac
}

arg_callback() {
    case "$1" in
    monitor) printf 'mode=monitor';;
    wait)    printf 'mode=wait';;
    rate)    printf 'packet_rate=%q' "$2"; false;;
    esac
}

read -r -d '' usage <<USAGE
Usage: $(basename "$0") [-h|--help] [-m|--monitor] [-w|--wait] [-r RATE|--rate RATE] HOST...
  --help     Print this message.
  --monitor  Report changes to host status indefinitely.
  --wait     Exit when the host status changes.
  --rate     Specify how frequently to poll for host status.
USAGE

parse_args arg_alias arg_callback "$usage" "$@"

mode=${mode:-monitor}
packet_rate="${packet_rate:-1}"

case "$packet_rate" in
''|*[!0-9.]*)
    printf '%s: invalid value for --rate: %s\n' "$(basename "$0")" "$packet_rate" >&2
    exit 1
    ;;
esac

parse_args.inc.sh

parse_args_program="$0"
parse_args_program_name="$(basename "$0")"
parse_args() {
    if [ "$parse_args_args_already_parsed" ]; then
        export -n parse_args_args_already_parsed
        export -n parse_args_variable_names
        parse_args_args="$(printf '%q ' "$@")"
        eval "set -- $parse_arge_variable_name"
        for parse_args_variable in "$@"; do
            export -n $parse_args_variable
        done
        eval "set -- $parse_args_args"
        return 0
    fi

    parse_args_arg_alias="$1"
    shift
    parse_args_arg_callback="$1"
    shift
    parse_args_usage="$1"
    shift

    parse_args_normalized_args=""
    while [ $# -gt 0 ]; do
        case "$1" in
        --?*=*) parse_args_normalized_args="$parse_args_normalized_args $(printf '%q %q' "${1%%=*}" "${1#*=}")";;
        --*) parse_args_normalized_args="$parse_args_normalized_args $(printf '%q' "$1")";;
        -?*)
            parse_args_short_args="${1:1}"
            while [ ${#parse_args_short_args} -gt 0 ]; do
                parse_args_short_arg="${parse_args_short_args:0:1}"
                parse_args_short_args="${parse_args_short_args:1}"

                parse_args_expanded="$("$parse_args_arg_alias" "$parse_args_short_arg")"
                parse_args_exit_code=$?

                case "$parse_args_expanded" in
                '')
                    printf '%s: unknown option: -%s\n' "$parse_args_program_name" "$parse_args_short_arg" >&2
                    printf '%s\n' "$parse_args_usage" >&2
                    exit 1
                    ;;
                *[!A-Za-z_-]*)
                    printf 'parse_args: invalid alias expansion: %s\n' "$parse_args_expanded" >&2
                    exit 1
                    ;;
                *)
                    parse_args_normalized_args="$parse_args_normalized_args --$parse_args_expanded"
                    [ 0 -ne $parse_args_exit_code ] && break
                    ;;
                esac
            done
            if [ ${#parse_args_short_args} -gt 0 ]; then
                parse_args_normalized_args="$parse_args_normalized_args $(printf '%q' "$parse_args_short_args")"
            fi
            ;;
        *) parse_args_normalized_args="$parse_args_normalized_args $(printf '%q' "$1")";;
        esac
        shift
    done
    eval "set -- $parse_args_normalized_args"

    parse_args_variables=""
    parse_args_positional_args=""
    while [ $# -gt 0 ]; do
        case "$1" in
        --)
            shift
            parse_args_positional_args="$parse_args_positional_args $(printf '%q ' "$@")"
            break
            ;;
        --help)
            printf '%s\n' "$parse_args_usage"
            exit 0
            ;;
        --*)
            parse_args_arg="$1"
            shift
            parse_args_callback_result="$("$parse_args_arg_callback" "${parse_args_arg:2}" "$@")"
            parse_args_exit_code=$?

            case "$parse_args_callback_result" in
            '')
                printf '%s: unknown option: %s\n' "$parse_args_program_name" "$parse_args_arg" >&2
                printf '%s\n' "$parse_args_usage" >&2
                exit 1
                ;;
            *)
                if [ $# -lt $parse_args_exit_code ]; then
                    if [ 1 = $parse_args_exit_code ]; then
                        printf '%s: expected 1 parameter for option %s\n' "$parse_args_program_name" "$parse_args_arg" >&2
                    else
                        printf '%s: expected %s parameter for option %s\n' "$parse_args_program_name" "$parse_args_exit_code" "$parse_args_arg" >&2
                    fi
                    printf '%s\n' "$usage" >&2
                    exit 1
                fi
                parse_args_variables="$parse_args_variables $parse_args_callback_result"
                shift $parse_args_exit_code
                ;;
            esac
            ;;
        *)
            parse_args_positional_args="$parse_args_positional_args $(printf '%q' "$1")";
            shift
            ;;
        esac
    done

    parse_args_variable_names=""
    eval "set -- $parse_args_variables"
    for parse_args_variable in "$@"; do
        parse_args_variable_names="$parse_args_variable_names ${parse_args_variable%%=*}"
    done

    eval "parse_args_args_already_parsed=1 parse_args_variable_names=$(printf '%q' "$parse_args_variable_names") $parse_args_variables exec $(printf '%q' "$parse_args_program") $parse_args_positional_args"
}