#!/bin/sh
# Simple shell script to parse one command line argument at a time.
#
#   Usage: arg <flag> args...
#
# If the flag is present among the given arguments, its value (if any) will be printed.
# Otherwise, the command will fail (subject to the special values below).
# Positional arguments are those that either don't begin with a '-', or appear
# after a lone "--" argument.
#
#   Special Values:
#     @                 Print all positional arguments (i.e. excluding flags).
#     <num>             Print the Nth positional arg, if present, otherwise fail.
#     <flag>?           Pipe output to /dev/null, but still use success status
#                       to indicate if flag is present.
#     <flag>=<default>  If flag is not present, use <default> as the value.
#                       Always succeeds.
#     <flag1>|<flag2>   If flag1 is not present, check flag2 instead.
#
#   Examples:
#     arg '-v|--verbose?' "$@" && verbose=yes || verbose=no
#     if logfile="$(arg --log "$@")"; then ...; fi
#     path="$(arg '-p|--path=/tmp/default' "$@")"
#     for f in $(arg @ "$@"); do ...; done
#     case "$(arg 1 "$@")" in ... esac
#     arg verboten.txt'?' "$@" && echo "You can't use that file!" && exit 1
#
# Note: space-separated flag/value pairs are not supported, e.g. `ls -I \*.txt`
# or `ls --ignore \*.txt`. In these cases, please use the following call
# syntax instead: `ls -I\*.txt` or `ls -I=\*.txt` and `ls --ignore=\*.txt`.
# Supporting space-separated flag/value pairs would make positional argument
# functionality impossible because there would be no way to know whether x or y
# is the first positional arg in these examples: `arg 1 -f x y`, `arg 1 --foo x y`
# (At least, not without telling `arg` whether or not each flag expects a value)
#
# Note 2: If you expect arguments to contain "\x1E", you may want to run `arg`
# with a custom record separator in the environment variable `RS`. Or, if you want
# to customize the output of `arg @ ...` to use a custom output record separator,
# you can use the environment variable `ORS`.
#
[ $# -eq 0 ] && echo "arg: a single-flag argument parser. Usage: arg <flag> args..." && exit
target="$1"; shift
RS=${RS-$'\x1E'}
ORS=${ORS-$'\n'}
case "$target" in
    @) printf "%s$RS" "$@" | awk -v RS="$RS" -v ORS="$ORS" 'raw || !/^-/; /^--$/ {raw=1}' ;;
    --) ! printf "%s$RS" "$@" | awk -v RS="$RS" "/^--$/ {if (++count>1) exit 1}" ;;
    -[^-]\?) ! printf "%s$RS" "$@" | awk -v RS="$RS" "/^-[^-]*${target#-}/ {exit 1}" ;;
    *\?) $0 "${target%\?}" "$@" >/dev/null ;;
    *=*) default="${target#*=}"; $0 "${target%%=*}" "$@" || echo "$default" ;;
    *\|*) $0 "${target%%\|*}" "$@" || $0 "${target#*\|}" "$@" ;;
    [1-9]*) n="$target"; [ $# -ge "$n" ] && ORS="$RS" $0 @ "$@" | awk -v RS="$RS" "NR==$n" ;;
    -[^-]) ! printf "%s$RS" "$@" | awk -v RS="$RS" "
        /^--$/ {exit 0}
        /^-[^-]*${target#-}/ {sub(/^-[^-]*${target#-}=?/,\"\"); print; exit 1}" ;;
    *) ! printf "%s$RS" "$@" | awk -v RS="$RS" "
        /^--$/ {exit 0}
        /^$target$/ {exit 1}
        /^$target=/ {sub(/^$target=/,\"\"); print; exit 1}" ;;
esac