silent mode, and an invalid option is seen, getopts places '?' into
    NAME and unsets OPTARG.  If a required argument is not found, a '?'
    is placed in NAME, OPTARG is unset, and a diagnostic message is
    printed.
    
    If the shell variable OPTERR has the value 0, getopts disables the
    printing of error messages, even if the first character of
    OPTSTRING is not a colon.  OPTERR has the value 1 by default.
    
    Getopts normally parses the positional parameters, but if arguments
    are supplied as ARG values, they are parsed instead.
    
    Exit Status:
    Returns success if an option is found; fails if the end of options is
    encountered or an error occurs.