What does the “(8)” in fsck(8) mean?

Question

*nix commands (and functions?) have a number with them, like fsck(8), killall(1), etc.

What does the number mean?

Answer 1

The character explicitly specifies the section that the manual page is part of. On most Unices, the section definitions are as follows:

1. General/user commands
2. System calls
3. Library functions
4. Special files and drivers
5. File formats
6. Games and screensavers
7. Miscellanea and conventions
8. System administration commands, priveleged commands, and daemons
9. Kernel routines

SysV has a similar, but not identical structure:

1. General commands

   (M) System administration commands and daemons

2. System calls
3. C library functions
4. File formats and conventions
5. Miscellanea
6. Games and screensavers
7. Special files and drivers

On some systems, the following sections also exist:

• 0 - C library headers
• L - Math library functions
• N - TCL functions/keywords
• X - X-Windows documentation
• P - POSIX specifications

Answer 2

In addition to the answers already provided. Keep in mind that some Unix don't have the 8 number in their man's categories, but have the 1M instead.

Unix System V uses a similar numbering scheme, except in a different order:
Section     Description
1   General commands
1M  System administration commands and daemons
2   System calls
3   C library functions
4   File formats and conventions
5   Miscellanea
6   Games and screensavers
7   Special files (usually devices, those found in /dev) and drivers

from Wikipedia.

Answer 3

The number is the manpage it refers to. Each manpage has its own purpose, like beeing the manpage for the POSIX definition or specific linux definition (for programmers) or the manpage for the tools usage itself, if you can invoke it from the commandline.

the purpose of the specific manpage is covered in the first section

man 8 fsck

will show you the specific page

man -a fsck

will show you page after page after you quit with q