The parameters getopt is called with can be divided into two parts: options which modify the way getopt will do the parsing (the options and the optstring in the SYNOPSIS), and the parameters which are to be parsed (parameters in the SYNOPSIS). The second part will start at the first non-option parameter that is not an option argument, or after the first occurrence of '--'. If no '-o' or '--options' option is found in the first part, the first parameter of the second part is used as the short options string.
If the environment variable GETOPT_COMPATIBLE is set, or if the first parameter is not an option (does not start with a '-', the first format in the SYNOPSIS), getopt will generate output that is compatible with that of other versions of getopt(1). It will still do parameter shuffling and recognize optional arguments (see section COMPATIBILITY for more information).
Traditional implementations of getopt(1) are unable to cope with whitespace and other (shell-specific) special characters in arguments and non-option parameters. To solve this problem, this implementation can generate quoted output which must once again be interpreted by the shell (usually by using the eval command). This has the effect of preserving those characters, but you must call getopt in a way that is no longer compatible with other versions (the second or third format in the SYNOPSIS). To determine whether this enhanced version of getopt(1) is installed, a special test option (-T) can be used.
The parameters are parsed from left to right. Each parameter is classified as a short option, a long option, an argument to an option, or a non-option parameter.
A simple short option is a '-' followed by a short option character. If the option has a required argument, it may be written directly after the option character or as the next parameter (i.e. separated by whitespace on the command line). If the option has an optional argument, it must be written directly after the option character if present.
It is possible to specify several short options after one '-', as long as all (except possibly the last) do not have required or optional arguments.
A long option normally begins with '--' followed by the long option name. If the option has a required argument, it may be written directly after the long option name, separated by '=', or as the next argument (i.e. separated by whitespace on the command line). If the option has an optional argument, it must be written directly after the long option name, separated by '=', if present (if you add the '=' but nothing behind it, it is interpreted as if no argument was present; this is a slight bug, see the BUGS). Long options may be abbreviated, as long as the abbreviation is not ambiguous.
Each parameter not starting with a '-', and not a required argument of a previous option, is a non-option parameter. Each parameter after a '--' parameter is always interpreted as a non-option parameter. If the environment variable POSIXLY_CORRECT is set, or if the short option string started with a '+', all remaining parameters are interpreted as non-option parameters as soon as the first non-option parameter is found.
If there are problems parsing the parameters, for example because a required argument is not found or an option is not recognized, an error will be reported on stderr, there will be no output for the offending element, and a non-zero error status is returned.
For a short option, a single '-' and the option character are generated as one parameter. If the option has an argument, the next parameter will be the argument. If the option takes an optional argument, but none was found, the next parameter will be generated but be empty in quoting mode, but no second parameter will be generated in unquoted (compatible) mode. Note that many other getopt(1) implementations do not support optional arguments.
If several short options were specified after a single '-', each will be present in the output as a separate parameter.
For a long option, '--' and the full option name are generated as one parameter. This is done regardless whether the option was abbreviated or specified with a single '-' in the input. Arguments are handled as with short options.
Normally, no non-option parameters output is generated until all options and their arguments have been generated. Then '--' is generated as a single parameter, and after it the non-option parameters in the order they were found, each as a separate parameter. Only if the first character of the short options string was a '-', non-option parameter output is generated at the place they are found in the input (this is not supported if the first format of the SYNOPSIS is used; in that case all preceding occurrences of '-' and '+' are ignored).
Quoting is not enabled if the environment variable GETOPT_COMPATIBLE is set, if the first form of the SYNOPSIS is used, or if the option '-u' is found.
Different shells use different quoting conventions. You can use the '-s' option to select the shell you are using. The following shells are currently supported: 'sh', 'bash', 'csh' and 'tcsh'. Actually, only two 'flavors' are distinguished: sh-like quoting conventions and csh-like quoting conventions. Chances are that if you use another shell script language, one of these flavors can still be used.
If the first character is '+', or if the environment variable POSIXLY_CORRECT is set, parsing stops as soon as the first non-option parameter (i.e. a parameter that does not start with a '-') is found that is not an option argument. The remaining parameters are all interpreted as non-option parameters.
If the first character is a '-', non-option parameters are outputted at the place where they are found; in normal operation, they are all collected at the end of output after a '--' parameter has been generated. Note that this '--' parameter is still generated, but it will always be the last parameter in this mode.
If the first character of the first parameter of getopt is not a '-', getopt goes into compatibility mode. It will interpret its first parameter as the string of short options, and all other arguments will be parsed. It will still do parameter shuffling (i.e. all non-option parameters are output at the end), unless the environment variable POSIXLY_CORRECT is set.
The environment variable GETOPT_COMPATIBLE forces getopt into compatibility mode. Setting both this environment variable and POSIXLY_CORRECT offers 100% compatibility for 'difficult' programs. Usually, though, neither is needed.
In compatibility mode, leading '-' and '+' characters in the short options string are ignored.
The syntax if you do not want any short option variables at all is not very intuitive (you have to set them explicitly to the empty string).