/usr is the second major section of the filesystem. /usr is shareable, read-only data. That means that /usr should be shareable between various hosts running FHS-compliant and should not be written to. Any information that is host-specific or varies with time is stored elsewhere.

No large software packages should use a direct subdirectory under the /usr hierarchy. An exception is made for the X Window System because of considerable precedent and widely-accepted practice. This section of the standard specifies the location for most such packages.

"/usr" X11R6 bin games include lib local sbin share src

"Secondary Hierarchy" X Window System, version 11 release 6 Most user commands Games and educational binaries Header files included by C programs Libraries Local hierarchy (empty after main installation) Non-vital system binaries Architecture-independent data Source code

Tree 4.1

The following symbolic links to directories may be present. This possibility is based on the need to preserve compatibility with older systems until all implementations can be assumed to use the /var hierarchy.

    /usr/spool -> /var/spool
    /usr/tmp -> /var/tmp
    /usr/spool/locks -> /var/lock

Once a system no longer requires any one of the above symbolic links, the link may be removed, if desired.