The Linux man-pages project documents the Linux kernel and C library interfaces that are employed by user-space programs. With respect to the C library, the primary focus is the GNU C library (glibc), although, where known, documentation of variations on other C libraries available for Linux is also included. The project provides manual pages in the following sections:
• 1: User commands; man-pages includes a very few Section 1 pages that document programs supplied by the GNU C library.
• 2: System calls documents the system calls provided by the Linux kernel.
• 3: Library functions documents the functions provided by the standard C library.
• 4: Devices documents details of various devices, most of which reside in /dev.
• 5: Files describes various file formats, and includes proc(5), which documents the /proc file system.
• 7: Overviews, conventions, and miscellaneous.
• 8: Superuser and system administration commands; man-pages includes a very few Section 8 pages that document programs supplied by the GNU C library.
https://www.kernel.org/doc/man-pages
http://man7.org/linux/man-pages/index.html
实用命令：
man man
man -f COMMAND
whatis COMMAND
man 1 CMMAND
man 3 system