The traditional method of getting online help was through the use of the man pages. Some of the variations of the man command:
$ man passwd specifically for the "passwd" command $ man -k password keyword search for "password"-related entries $ apropos password same as above
Historically, UNIX commands were very terse and supported only single-letter options, as in
$ ls -a show hidden files
Given the obvious limitations of this approach, when the Free Software Foundation designed their own versions of the standard UNIX commands that became the basis for Linux commands, they extended the traditional option format to include the "long" or "double-hyphen" form, as in:
$ ls -a old, short form $ ls --all newer, GNU long formas you may already have noticed from viewing the Linux man pages.
In addition, many of the GNU commands have the really, handy long option --help, as in:
$ cat --help $ ls --helpwhich typically displays a very terse version of the corresponding man page.