The /usr Hierarchy.

/usr is shareable, read-only data. ie. You should be able to mount /usr read-only on multiple machines. Any information that is host-specific or varies with time is stored elsewhere. No large package should use a direct subdirectory under /usr. An exception is made for the X Window System because of 'considerable precedent'.

/usr contains

As well as this, to maintain compatibility with older systems there are often the following symbolic links.

A brief introduction to the subdirectories.

X11R6
X Window System, version 11 release 6, and related files. There are also symbolic links to subdirectories under X11R6. Configuration information should be in /etc/X11.
X386
X Window System, version 11 release 5, and related files. Details the same as for /usr/X11R6.
bin
The primary directory for user commands.
include
Where all of the system's general-use include files for C/C++ should be placed. There is a whole ream of subdirectories under here, but you only need to worry about them if you are a programmer.
lib
/usr/lib is intended for object files and libraries. Historically it has also contained compiler binaries and static data.
Miscellaneous application-specific static files and subdirectories should be place in /usr/share, and application specific binaries in /usr/libexec (for internal binaries), /usr/bin (for user commands), or /usr/sbin (for system commands). However distributions may choose to place certain application-specific static data in subdirectories under /usr/lib. (ie emacs, mf, smail, uucp, etc).
libexec
/usr/libexec is intended for programs to be run by other programs, not by users. It is directly drawn from the Net/2 distribution of BSD, the idea being to remove executables non intended for execution directly be users from the standard execution path.
local
/usr/local is for use by the sysop when installing software locally. It will be empty after first installing a FHS compliant system. The intention being to allow upgrading of the system without overwriting local software installations. It is vary similar to /opt except that it is not normally shareable.
sbin
Contains any non-essential binaries used exclusively by the system administrator. See the discussion of /usr/bin, /bin and /sbin.
share
Another extensive directory hierarchy. Important subdirectories to note: See the standard for a specific description of each of these, and the other subdirectories under /usr/share.
/usr/share is for architecture independent data. Also note that the /usr/share/man directory hierarchy is very extensive, although on most machines it can be simplified to the common man/man{1,2,3,...}.
src
Non-local source code. /usr/src/linux should be a symbolic link to the current kernel source tree. The exact layout of this directory is architecture dependent.

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Andrae Muys
Version: $Id: usr.html,v 2.2 2000/01/06 13:21:13 clinton Exp $