getopts	recall getopts syntax etc
# getopts optstring opt [ arg ... ]
#
# Reads an option from arg ..., at position OPTIND; fails at end of arg list.
# arglist stops at single '-' or at first word not starting with '-' (ksh: '+')
# arg ...:    arguments to analyse (defaults to "$@")
# optstring:  option letters - if letter followed by ':', that option requires
#             an argument, to be read into OPTARG. If optstring does not start
#             :optstring - unknown option: opt='?', $OPTARG=<option-letter>
#                        - missing arg:    opt=':', $OPTARG=<option-letter>
#             optstring  - unknown option: opt='?', stderr message, OPTARG unset
#                        - missing arg:    opt='?', stderr message, OPTARG unset
# opt:        variable name will be set to the recognised option letter, or
#             on error '?' or ':' (see optstring).
# <symbols>   OPTIND: index of next argument to read: set it to 1 to restart
#             OPTARG: set to an option argument (if any)
# For ksh: options starting with '+' rather than '-': opt=+<option-letter>
# For bash: OPTERR (initially 1): if not set to 1, no stderr messages on error

# ------------------------------------------------------------------------------
# reset options counter (only set to 1 automatically for new shells)
OPTIND=1

while getopts :s:l opt "$@"; do

    if [[ "$opt" == : ]]; then
        echo "Option -$OPTARG requires an argument" >&2
    fi

    case "$opt" in
        # Flag:
        l)    l_opt=1 ;;
        # Option with argument
        s)    s_opt="$OPTARG" ;;

        # Invalid arguments:
        ':')  echo "Option -$OPTARG requires an argument" && usage && exit 1 ;;
        '?')  echo "Invalid option: -$OPTARG" && usage && exit 1 ;;
        *)    echo "Option error: $opt" && usage && exit 1 ;;
    esac
done

# forget the options in the command line now
shift $(($OPTIND - 1))      # ksh, bash
shift `expr $OPTIND - 1`    # sh

# ------------------------------------------------------------------------------