get_long_opts.sh

#!/bin/sh

# Check this answer for more advanced stuff: https://stackoverflow.com/a/28466267/2502746

while getopts "*-:" arg; do
    case $arg in
        - )  LONG_OPTARG="${OPTARG#*=}"
            case $OPTARG in
                only-foo ) ONLY_FOO=1 ;;
                only-bar ) ONLY_BAR=1 ;;
                # "--" terminates argument processing
                '' ) break ;;
                * ) echo "Illegal option --$OPTARG" >&2; exit 2 ;;
            esac ;;
        # getopts already reported the illegal option
        \? ) exit 2 ;;
    esac
done

[ $ONLY_FOO ] && echo "only foo"
[ $ONLY_BAR ] && echo "only bar"