UNIX Online Documentation
The apropos Command
apropos on Systems Without apropos
whatis: One-Line Command Summaries
whereis: Finding Where a Command Is Located
Searching Online Manual Pages
How UNIX Systems Remember Their Name
Which Version Am I Using?
Reading a Permuted Index
Make Your Own Man Pages Without Learning troff
Writing a Simple Man Page with the -man Macros
Common UNIX Error Messages
The UNIX operating system was one of the first to include online documentation. It's not the best in the world - most users who haven't internalized the manual set curse it once a week - but it has proven surprisingly resilient. What's particularly interesting about UNIX's online documentation is that, unlike other early help systems, it isn't an adjunct to another set of printed documentation that contains the "real" truth. The online manual is complete, authoritative, and usually more current than any printed documentation.
where topic is usually the name of some command; but it can also be the name of a system call, a library routine, an I/O device, or an administrative file (or file type). The output from man is usually sent to a pager like, which allows you to page through.
The manual pages are grouped into a number of categories. Unfortunately, there are three slightly different groupings: one for Berkeley-based systems, one for System V, and one for systems derived for XENIX. Table 50.1 lists these categories.
|I/O and special files||4||7||p_man|
|Games||6||6 or 1||u_man|
|Maintenance commands||8||1M||u_man or a_man|
|Local commands||l (letter)||*|
|Obsolete commands||o (letter)||*|
 As you can see, System V makes a strange distinction between "administration" and "maintenance"; if you can figure out what they really mean, please let us know! To a poor Berkeley soul, this has never made sense. Some of these categories are subdivided further; for example, you may see section 3S (the standard I/O library), 3M (the mathematics library), 1G (Berkeley graphics), 1V (commands derived from System V), and so on.
 System V and Xenix make no provision for "local" or "obsolete" commands in their man system.
If you want to refer to a manual entry in a specific section of the manual, you can give a command like one of these:
Some systems require the -s "section" option. For example, if you want to read documentation about the /etc/passwd file (rather than the passwd command) on a System V machine, give the command:
man -s 4 passwd
This is an easy way to distinguish between topics with the same name, but in different groups.