GNU Mailutils
	General-Purpose Mail Package	Official GNU Software

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3.1.1 Basic Notions About Command Line Options

Many command line options have two forms, called short and long forms. Both forms are absolutely identical in function; they are interchangeable.

The short form is a traditional form for UNIX utilities. In this form, the option consists of a single dash, followed by a single letter, e.g. -c.

Short options which require arguments take their arguments immediately following the option letter, optionally separated by white space. For example, you might write -f name, or -fname. Here, -f is the option, and name is its argument.

Short options which allow optional arguments take their arguments immediately following the option letter, without any intervening white space characters. This is important, so that the command line parser might discern that the text following option is its argument, not the next command line parameter. For example, if option -d took an optional argument, then -dname would mean the option with its argument (name in this case), and -d name would mean the -d option without any argument, followed by command line argument name.

Short options’ letters may be clumped together, but you are not required to do this. When short options are clumped as a set, use one (single) dash for them all, e.g. -cvl is equivalent to -c -v -l. However, only options that do not take arguments may be clustered this way. If an option takes an argument, it can only be the last option in such a cluster, otherwise it would be impossible to specify the argument for it. Anyway, it is much more readable to specify such options separated.

The long option names are probably easier to memorize than their short counterparts. They consist of two dashes, followed by a multi-letter option name, which is usually selected to be a mnemonics for the operation it requests. For example, --verbose is a long option that increases the verbosity of a utility. In addition, long option names can abbreviated, provided that such an abbreviation is unique among the options understood by a given utility. For example, if a utility takes options --foreground and --forward, then the shortest possible abbreviations for these options are --fore and --forw, correspondingly. If you try to use --for, the utility will abort and inform you that the abbreviation you use is ambiguous, so it is not clear which of the options you intended to use.

Long options which require arguments take those arguments following the option name. There are two ways of specifying a mandatory argument. It can be separated from the option name either by an equal sign, or by any amount of white space characters. For example, if the --file option requires an argument, and you wish to supply name as its argument, then you can do so using any of the following notations: --file=name or --file name.

In contrast, optional arguments must always be introduced using an equal sign.