PROGNAME(x) references in manpages -- What does the digit in ()s mean? [duplicate]

If you run man man you will see the following information in the man page:

1   Executable programs or shell commands2   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/passwd6   Games7   Miscellaneous (including macro  packages  and  conven‐    tions), e.g. man(7), groff(7)8   System administration commands (usually only for root)9   Kernel routines [Non standard]

Some names are associated with multiple entries, eg on my system 'sleep' has an entry in section 1 and an entry in section 3. You can specify the one you want with e.g.

man 3 sleep

Sometimes I just guess with

man -a sleep

which displays each entry associated with sleep in turn. I just go through them until I find the one I want. You can also try

man -k sleep

to get a slightly bigger listing of pages involving the term 'sleep'

The number indicates which section the manpage is in. For your examples:

grep(3)

To get the documentation, type

man 3 grep

More commonly, if there is no grep(2) or grep(1), you can get away with

man grep

However, I should note that grep is in section 1. Section 3 is generally reserved for C functions. An example is getopt: getopt(1) refers to the command-line utility getopt, but getopt(3) refers to the C function getopt. Likewise, read(1) is a program that reads from standard input, but read(2) is a POSIX system call for use in programs - it is one of the lowest-level forms of input you can get on most Linux (and other Unix) systems.

It's to tell you what man page section help is in... 8 is typically the location of Administrative related utilities (/sbin, /usr/sbin, etc.)

So help for GREP(3) is in man page section 3, and you could type man 3 grep to get the help for grep(3) directly.