早期的unix系统把帮助文档的8个部分都集中在一起，随着页数的增多，现在按照主题放在不同的手册中。
1 Commands
Those commands
that can be executed by the user from
within a shell.1P Posix Commands
2 System calls
Those functions which must be performed by the kernel.
3 Library calls
Most of the libc functions, such as qsort(3).3P Posix
Functions4 Special files
Files found in /dev.
5 File formats and conventions
The format for
/etc/passwd and other human-readable
files.
6 Games
7 Conventions and miscellaneous
A description of the standard file system
layout, net-
work protocols, ASCII and other character codes, this
man
page, and other things.
8 System management commands
Commands like mount(8),many of which only
root can
execute.
9 Kernel routines
This is an obsolete manual section. Once it
was
thought a good idea to document the Linux kernel here,
but in fact
very little has been documented, and the
documentation that exists is
outdated already. There
are better sources of information for kernel
developers.
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