The /usr filesystem

The Linux System Administrators' Guide
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# 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.

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