We've already discussed the command line - that is, commands you type after the shell prompt. This section describes the structure of more complicated command lines.
A minimal command line contains just a command name, such as whoami. But other things are possible. For example, you might type: man whoami. This command requests the online manual for the whoami program (you may have to press the space bar to scroll through the documentation or press q to quit). A more complicated example is man -k PostScript. This command line has three parts. It begins with the command name, man. Then it has an option or switch, -k, followed by an argument, PostScript. Some people refer to everything except the command name as the parameters of the command. So, options and arguments are both parameters.
Options change the behavior of a command, switching on particular features or functionality. They usually have a - before them. The GNU utilities also have ``long forms'' for the options; the long form of -k is -apropos. You can enter man -h or man -help to get a full list of options for the man command. Every command will have its own set of options, though most have -help and -version options. Some commands, such as tar, do not require the ``-'' before their options for historical reasons.
Anything that isn't an option and isn't the command name is an argument (in this case, PostScript). Arguments can serve many purposes; most commonly, they are filenames that the command should operate on. In this case, PostScript is the word you want man to search for. In the case of man whoami, the argument was the command you wanted information about.
Here's a breakdown of the man -k PostScript command line:
Go ahead and type man -k PostScript, and you will see a list of all the manual pages on your system that have something to do with PostScript. If you haven't installed much software, you might see the message PostScript: nothing appropriate instead.
Note: You can skip this section if you want to move on.
There's a traditional, concise way of describing command syntax. Syntax means the correct ways to combine various options and arguments. For example, if you type man man to get the manual page about man, you'll see several syntax descriptions beginning with the command name man. One of them will look like this: man -k [-M path] keyword ...
Anything in brackets () is an optional unit. In this case you don't have to use the -M option, but if you do, you must use a path argument. You must use the -k option and the keyword argument. The ... means that you could have more of whatever came before it, so you could look up several keywords.
Let's look at one of the more complex descriptions from the man manual page:
[-m system[,...]] [-L locale] [-p string]
[-M path] [-P pager] [-r prompt] [-S list]
[-e extension] [[section] page ...] ...
First, clusters of options usually mean you can use one or more of them in different combinations, so -adhu7V means you can also use -h. However, you can't always use all combinations; this description doesn't make that clear. For example, -h is incompatible with other options, but you could do man -du. Unfortunately, the description's format does not make this clear.
Second, the | symbol means ``or.'' So you can use the -c, the -w, or the -tZT option, followed by a device argument.
Third, notice that you can nest the brackets, because they indicate optional units. So if you have a section, you must also have a page, because e page is not optional within the [[section] page] unit.
There's no need to memorize any of this, just refer to this section as you read documentation.