Question

I see these numbers everywhere. For example, on this page: http://linux.die.net/man/1/tar

What is the meaning of the number – 1 in tar(1) ? I have seen 2, 5, etc also.

Answer 1

It tells you what group its manpage is in or, more generally, which group the item itself belongs to. Here’s a list of the sections and their contents:

   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]

See the manpage of ‘man’ for more details. Or have a look here:
http://linux.die.net/man/

Sometimes items from different groups can have the same name and this is the way to distinguish between them. For example, there is a manpage for printf(1) which is an executable, callable from a shell, as well as a manpage for printf(3) which is a C function definded in stdio.h.

Using the man binary from a bash you can call for the distinct manpages by:

man printf       # displays printf(1)
man 1 printf     # displays printf(1)
man 3 prinft     # displays printf(3)
man -a printf    # displays all manpages matching printf

Depending on what manpages are installed on the system you sometimes get pages from different manuals for the same item. For example printf(3) from the Linux Programmer’s Manual could have a printf(3p) counterpart from the Posix Programmer’s manual.