Linux – man page command: man 3 vs. man

linuxmanunix

I'm wondering what's the difference between "man 3 command" vs "man command"?

I read in wiki (http://en.wikipedia.org/wiki/Man_page) that man 3 is used in Linux in section 3 of the man pages. And section 3 refers to Library functions, covering in particular the C standard library.

I tried "man 3 ls" and got this:

No entry for ls in section 3 of the manual

I wonder when to use man 3 and when to use man and do they serve difference purposes?

Thanks.

Best Answer

man 3 ls will only show you a man page for ls in section 3 of the manual. Section 3 covers library functions; since there's (probably) no library function named ls, it won't find anything.

The man command without a section number searches the sections in a predefined order that I don't remember, but it's likely to be close to numerical order starting at 1. So man ls will find the ls man page in section 1, which covers user commands.

The sections (on my Ubuntu system) are:

   1   Executable programs or shell commands
   2   System calls (functions provided by the kernel)
   3   Library calls (functions within program libraries)
   4   Special files (usually found in /dev)
   5   File formats and conventions eg /etc/passwd
   6   Games
   7   Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7)
   8   System administration commands (usually only for root)
   9   Kernel routines [Non standard]

Specifying the section can be useful for things that exist with the same name in more than one section. For example, man printf will show you the man page for the printf user command in section 1; to see the man page for the printf function, use man 3 printf. You'll often see these man pages referred to as printf(1) and printf(3), respectively.

Stealing Borrowing from abernert's answer, it's common to see a user command (section 1) that's a wrapper for a system call (section 2) or library call (section 3) with the same name; chown and chmod are good examples of this.

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