Run the man command

The man command displays an online manual page, or manpage, for a given program. For example, to get the documentation on listing files with ls, run:

$ man ls

To search for manpages by keyword for a particular topic, use the -k option followed by the keyword:

$ man -k database 

Run the info command

The info command is an extended, hypertext (webpage) help system covering many Linux programs.

$ info ls 

If no documentation is found on a given program, info displays the program's manpage. For a listing of available documentation, type info by itself. To learn how to navigate the info system, type info info.

Use the --help option (if any)

Many Linux commands respond to the option --help by printing a short help message. Try:

$ ls --help

Examine the directory /usr/share/doc

This directory contains supporting documents for many programs, usually organized by program name and version. For example, files for the text editor Emacs, Version 21.3, are found in /usr/share/doc/emacs-21.3

GNOME and KDE Help

For help with GNOME or KDE, choose the Help item in the main menu.

Distribution Specific Help

For help with a specific distribution you should start by going to its website. See below:

CentOS - http://www.centos.org/

Debian - http://www.debian.org/

Fedora - http://fedoraproject.org/

openSUSE - http://www.opensuse.org/

Ubuntu - http://www.ubuntu.com/