getopts optstring name [ arg ... ]
       Checks the args for legal options.  If the args are omitted,  use
       the positional parameters.  A valid option argument begins with a
       `+'  or a `-'.  An argument not beginning with a `+' or a `-', or
       the argument `--', ends the options.  Note that a single  `-'  is
       not  considered  a valid option argument.  optstring contains the
       letters that getopts recognizes.  If a letter is  followed  by  a
       `:',  that option requires an argument.  The options can be sepa-
       rated from the argument by blanks.

       Each time it is invoked, getopts  places  the  option  letter  it
       finds  in the shell parameter name, prepended with a `+' when arg
       begins with a `+'.  The index  of  the  next  arg  is  stored  in
       OPTIND.  The option argument, if any, is stored in OPTARG.

       The  first option to be examined may be changed by explicitly as-
       signing to OPTIND.  OPTIND has an initial value of 1, and is nor-
       mally set to 1 upon entry to a shell function and  restored  upon
       exit.  (The POSIX_BUILTINS option disables this, and also changes
       the  way  the value is calculated to match other shells.)  OPTARG
       is not reset and retains its value from the most recent  call  to
       getopts.   If  either of OPTIND or OPTARG is explicitly unset, it
       remains unset, and the index or option argument  is  not  stored.
       The option itself is still stored in name in this case.

       A  leading `:' in optstring causes getopts to store the letter of
       any invalid option in OPTARG, and to set name to `?' for  an  un-
       known  option  and  to  `:'  when a required argument is missing.
       Otherwise, getopts sets name to `?' and prints an  error  message
       when an option is invalid.  The exit status is nonzero when there
       are no more options.