Elegant commandline argument parsing on the shell

Parsing command line arguments on the shell is often done in an ad-hoc fashion, growing unwieldy as time goes by, but there are tools to make that elegant. Here’s a complete example.

I use this in the conf [1] project (easy setup of autotools projects). It builds on the great solution by Adam Katz [2].

# outer loop to allow processing option arguments at the end
while test ! $# -eq 0; do
    # getopts loop, here you define the short options: 
    # h for -h, l: for -l <lang>. -: provides support for long-options.
    while getopts -- hl:-: arg "$@"; do
        case $arg in
            h ) ARG_HELP=true ;;
            l ) ARG_LANG="$OPTARG" ;;
            - ) LONG_OPTARG="${OPTARG#*=}"
                case "$OPTARG" in
                    help    ) ARG_HELP=true;;
                    lang=?* ) ARG_LANG="$LONG_OPTARG" ;;
                    # FIXME: using the same option twice (either both
                    # after the argument or both before it) gives the
                    # first, not the second value
                    lang*   ) ARG_LANG="${@:$OPTIND:1}" ; OPTIND=$(($OPTIND + 1));;
                    vcs=?*  ) ARG_VCS="$LONG_OPTARG" ;;
                    vcs*    ) ARG_VCS="${@:$OPTIND:1}" ; OPTIND=$(($OPTIND + 1));;
                    '' )      break ;; # "--" terminates argument
                                       # processing to allow giving
                                       # options for autogen.sh after
                                       # --
                    * )       echo "Illegal option --$OPTARG" >&2; exit 2;;
                esac;;
            \? ) exit 2 ;;  # getopts already reported the illegal
                            # option
        esac
    done
    shift $((OPTIND-1)) # remove parsed options and args from $@ list
    # reinitialize OPTIND to allow parsing again
    OPTIND=1
    # provide help output.
    if test x"${ARG_HELP}" = x"true"; then
        echo "${PROG} new [-h | --help] [-l | --lang <LANGUAGE>] [--vcs <VCS>] PROJECT_NAME"
        exit 0
    fi
    # get the argument
    if test x"${1}" = x"--"; then
        if test x"${PROJ}" = x""; then
            echo "Missing project name." >&2; exit 2
        else
            # nothing more to parse.
            # Remove -- from the remaining arguments
            shift 1
            break
        fi
    fi
    if test ! x"${1}" = x""; then
        PROJ="${1%/}" # without trailing slash
    fi
    # remove the argument, then continue the loop to allow putting
    # the options after the argument
    shift 1
done

Additional explanation for this is available from Adam Katz (2015) [3]. I’m allowed to include it here, because every answer on Stackoverflow is licensed under creativecommons attribution sharealike (cc by-sa) [4] and because cc by-sa is upwards compatible [5] to GPLv3 [6].

# From Adam Katz, 2015: http://stackoverflow.com/users/519360/adam-katz
# Available at http://stackoverflow.com/a/28466267/7666
# License: cc by-sa: https://creativecommons.org/licenses/by-sa/3.0/
while getopts ab:c-: arg; do
  case $arg in
    a )  ARG_A=true ;;
    b )  ARG_B="$OPTARG" ;;
    c )  ARG_C=true ;;
    - )  LONG_OPTARG="${OPTARG#*=}"
         case $OPTARG in
           alpha    )  ARG_A=true ;;
           bravo=?* )  ARG_B="$LONG_OPTARG" ;;
           bravo*   )  echo "No arg for --$OPTARG option" >&2; exit 2 ;;
           charlie  )  ARG_C=true ;;
           alpha* | charlie* )
                       echo "No arg allowed for --$OPTARG option" >&2; exit 2 ;;
           '' )        break ;; # "--" terminates argument processing
           * )         echo "Illegal option --$OPTARG" >&2; exit 2 ;;
         esac ;;
    \? ) exit 2 ;;  # getopts already reported the illegal option
  esac
done
shift $((OPTIND-1)) # remove parsed options and args from $@ list

With this and with the practical usage at the top you should be able to implement clean commandline parsing with ease.

Happy Hacking!