The /usr filesystem

The /usr  filesystem is often large, since all programs are installed there. All files in /usr  usually come from a Linux distribution; locally installed programs and other stuff goes below /usr/local . This makes it possible to update the system from a new version of the distribution, or even a completely new distribution, without having to install all programs again. Some of the subdirectories of /usr  are listed below (some of the less important directories have been dropped; see the FSSTND for more information).

/usr/X11R6 

The X Window System, all files. To simplify the development and installation of X, the X files have not been integrated into the rest of the system. There is a directory tree below /usr/X11R6  similar to that below /usr  itself.

/usr/X386 

Similar to /usr/X11R6 , but for X11 Release 5.

/usr/bin 

Almost all user commands. Some commands are in /bin  or in /usr/local/bin .

/usr/sbin 

System administration commands that are not needed on the root filesystem, e.g., most server programs.

/usr/man , /usr/info , /usr/doc 

Manual pages, GNU Info documents, and miscellaneous other documentation files, respectively.

/usr/include 

Header files for the C programming language. This should actually be below /usr/lib  for consistency, but the tradition is overwhelmingly in support for this name.

/usr/lib 

Unchanging data files for programs and subsystems, including some site-wide configuration files. The name lib comes from library; originally libraries of programming subroutines were stored in /usr/lib .

/usr/local 

The place for locally installed software and other files.