Man page

I found much useful by using this man page.
Man pages (short for "manual pages") are the extensive documentation that comes preinstalled with almost all substantial Unix and Unix-like operating systems. The Unix command used to display them is man. Each page is a self-contained document.   

To read a manual page for a Unix command, one can use
 man

Ubuntu Manpage Repository 

 http://manpages.ubuntu.com/

This site contains hundreds of thousands of dynamically generated manuals, extracted from every package of every supported version of Ubuntu, and updated on a daily basis