POSIX shell: support long options by converting them to short options

longopts2getopts.sh

# a refinement of https://stackoverflow.com/a/5255468/519360
# see also my non-translating version at https://stackoverflow.com/a/28466267/519360

# translate long options to short
reset=true stopped=""
for opt in "$@"; do
    if [ -n "$reset" ]; then
        unset reset
        set --          # reset the "$@" array so we can rebuild it
    fi
    case "$opt" in      # --option=argument -> opt='--option' optarg='argument'
        --?*=* )        optarg="${opt#*=}" opt="${opt%%=*}" ;;
        * )             unset optarg ;;
    esac
    case "$stopped$opt" in
        -- )            stopped=true; set -- "$@" -- ;;
        --help )        set -- "$@" -h ;;
        --verbose )     set -- "$@" -v ;;
        --config )      set -- "$@" -c ${optarg+"$optarg"} ;;
        --long-only )   DEMO_LONG_ONLY_FLAG=true ;;
        # pass anything else through, including spaced arguments
        * )             set -- "$@" "$opt" ;;
    esac
done

# now we can process with getopt
while getopts ":hvc:" opt; do
    case $opt in
        h )     usage ;;
        v )     VERBOSE=true ;;
        c )     source $OPTARG ;;
        \? )    usage ;;
        : )
                echo "option -$OPTARG requires an argument"
                usage
        ;;
    esac
done
shift $((OPTIND-1))

Aside from needing a preprocessing loop, this approach has a flaw in that it loses the long option name; if you trigger that : clause (meaning you've forgotten an option's argument), the complaint uses $opt (which getopts has converted to $OPTARG), e.g. -c in place of --config.

Working around that is only a little ugly: Add "--$opt" after each set -- "$@" in the second case of the for loop excluding the * clause. Before that final clause, add a new -* ) set -- "$@" "--$opt" "$opt" ;; clause. Add -: to the getopts optstring. Add - ) param="$OPTARG" ;; to the getopts loop's case stanza, and then refer to $param instead of -$OPTARG.