The /usr Hierarchy
/usr is the second major section of the filesystem. /usr is shareable, read-only data. That means that /usr should be shareable between various FHS-compliant hosts and must not be written to. Any information that is host-specific or varies with time is stored elsewhere.
Large software packages must not use a direct subdirectory under the /usr hierarchy.
The following directories, or symbolic links to directories, are required in /usr.
|bin||Most user commands|
|include||Header files included by C programs|
|local||Local hierarchy (empty after main installation)|
|sbin||Non-vital system binaries|
Directory Description X11R6 XWindow System, version 11 release 6 (optional) games Games and educational binaries (optional) lib<qual> Alternate Format Libraries (optional) src Source code (optional)
An exception is made for the X Window System because of considerable precedent and widely-accepted practice.
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.
/usr/X11R6 : X Window System, Version 11 Release 6 (optional)
This hierarchy is reserved for the X Window System, version 11 release 6, and related files.
To simplify matters and make XFree86 more compatible with the X Window System on other systems, the following symbolic links must be present if /usr/X11R6 exists:
/usr/bin/X11 -> /usr/X11R6/bin /usr/lib/X11 -> /usr/X11R6/lib/X11 /usr/include/X11 -> /usr/X11R6/include/X11
In general, software must not be installed or managed via the above symbolic links. They are intended for utilization by users only. The difficulty is related to the release version of the X Window System — in transitional periods, it is impossible to know what release of X11 is in use.
Host-specific data in /usr/X11R6/lib/X11 should be interpreted as a demonstration file. Applications requiring information about the current host must reference a configuration file in /etc/X11, which may be linked to a file in /usr/X11R6/lib. 
/usr/bin : Most user commands
This is the primary directory of executable commands on the system.
The following files, or symbolic links to files, must be in /usr/bin, if the corresponding subsystem is installed:
- The Practical Extraction and Report Language (stage3)
- The Python interpreted language (stage3)
- Simple shell containing Tcl interpreter (optional)
- Simple Tcl/Tk windowing shell (optional)
- Program for interactive dialog (optional)
- Ruby (optional)
Rationale: Because shell script interpreters (invoked with #!<path> on the first line of a shell script) cannot rely on a path, it is advantageous to standardize their locations. The Bourne shell and C-shell interpreters are already fixed in /bin, but Perl, Python, and Tcl are often found in many different places. They may be symlinks to the physical location of the shell interpreters.
/usr/include : Directory for standard include files.
This is where all of the system's general-use include files for the C programming language should be placed. Core include files are installed here as part of the sys-libs/glibc and sys-kernel/linux-headers packages, although other ebuilds can install their header files in this directory as well.
The /usr/include/X11 stores X-related include files.
/usr/lib : Libraries for programming and packages
/usr/lib includes object files, libraries, and internal binaries that are not intended to be executed directly by users or shell scripts.  On 64-bit multilib systems, /usr/lib is a symbolic link (see below.)
/usr/lib64: 64-bit libraries (optional)
On 64-bit multilib systems, /usr/lib64 contains 64-bit libraries, and /usr/lib is a symlink pointing to "lib64".
/usr/lib32 : 32-bit libraries (optional)
On 64-bit mulitlib systems, /usr/lib32 contains 32-bit libraries.
Applications may use a single subdirectory under /usr/lib. If an application uses a subdirectory, all architecture-dependent data exclusively used by the application must be placed within that subdirectory. 
The /usr/lib/locale directory contains runtime locale data that is used directly by programs, and is generated by the locale-gen command. This command uses settings in /etc/locale.gen to determine what runtime locale data to generate. The data in /usr/lib/locale can always be regenerated using locale-gen, assuming that the source files in /usr/share/locale still exist. Therefore, it's recommended that future versions of Funtoo Linux reconfigure glibc to move this data to /var/lib/locale or /var/cache/locale instead.
This directory contains support libraries for the glibc iconv() text encoding/character set conversion function. The "iconvconfig" can be used to generate an optional cache file that is also stored in this directory.
For historical reasons, /usr/lib/sendmail must be a symbolic link to /usr/sbin/sendmail if the latter exists. 
/usr/local : Local hierarchy
The /usr/local hierarchy is for use by the system administrator when installing software manually, without using Portage. It may be used for programs and data that are shareable amongst a group of hosts, but not found in /usr. Typically, /etc is used to store configuration files for these programs, as it is generally not necessary to have an independent /usr/local/etc.
Locally installed software must be placed within /usr/local rather than /usr unless it is being installed to replace or upgrade software in /usr.
The following directories, or symbolic links to directories, are typically found in /usr/local:
- Local binaries
- Local game binaries
- Local C header files
- Local libraries
- Local online manuals
- Local system binaries
- Local architecture-independent hierarchy
- Local source code
/usr/sbin : Non-essential standard system binaries
This directory contains any non-essential binaries used exclusively by the system administrator. System administration programs that are required for system repair, system recovery, mounting /usr, or other essential functions must be placed in /sbin instead. 
The /usr/share hierarchy is for all read-only architecture independent data files. 
This hierarchy is intended to be shareable among all architecture platforms of a given OS; thus, for example, a site with i386, Alpha, and PPC platforms might maintain a single /usr/share directory that is centrally-mounted. Note, however, that /usr/share is generally not intended to be shared by different OSes or by different releases of the same OS.
Any program or package which contains or requires data that doesn't need to be modified should store that data in /usr/share (or /usr/local/share, if installed locally). It is recommended that a subdirectory be used in /usr/share for this purpose.
Game data stored in /usr/share/games must be purely static data. Any modifiable files, such as score files, game play logs, and so forth, should be placed in /var/games.
The /usr/share/locale hierarchy stores GNU message catalog (.mo) files that contain application messages in alternate locales/languages. Applications may install .mo files into this directory tree as well.
The /usr/share/zoneinfo contains time zone data, and is populated by the sys-libs/timezone-data ebuild.
The following directories, or symbolic links to directories, must be in /usr/share
Directory Description man Online manuals misc Miscellaneous architecture-independent data
The following directories, or symbolic links to directories, must be in /usr/share, if the corresponding subsystem is installed:
Directory Description dict Word lists (optional) doc Miscellaneous documentation (optional) games Static data files for /usr/games (optional) nls Message catalogs for Native language support (optional) sgml SGML data (optional) terminfo Directories for terminfo database (optional) tmac troff macros not distributed with groff (optional) xml XML data (optional)
It is recommended that application-specific, architecture-independent directories be placed here. Such directories include groff, perl, ghostscript, texmf, and kbd (Linux) or syscons (BSD). They may, however, be placed in /usr/lib for backwards compatibility, at the distributor's discretion. Similarly, a /usr/lib/games hierarchy may be used in addition to the /usr/share/games hierarchy if the distributor wishes to place some game data there.
This directory is the home for word lists on the system; Traditionally this directory contains only the English words file, which is used by look(1) and various spelling programs. words may use either American or British spelling.
The reason that only word lists are located here is that they are the only files common to all spell checkers.
The following files, or symbolic links to files, must be in /usr/share/dict, if the corresponding subsystem is installed:
File Description words List of English words (optional)
Sites that require both American and British spelling may link words to /usr/share/dict/american-english or /usr/share/dict/british-english.
Word lists for other languages may be added using the English name for that language, e.g., /usr/share/dict/french, /usr/share/dict/danish, etc. These should, if possible, use an ISO 8859 character set which is appropriate for the language in question; if possible the Latin1 (ISO 8859-1) character set should be used (this is often not possible).
Other word lists must be included here, if present.
This section details the organization for manual pages throughout the system, including /usr/share/man. Also refer to the section on /var/cache/man.
The primary <mandir> of the system is /usr/share/man. /usr/share/man contains manual information for commands and data under the / and /usr filesystems. 
Manual pages are stored in <mandir>/<locale>/man<section>/<arch>. An explanation of <mandir>, <locale>, <section>, and <arch> is given below.
A description of each section follows:
- man1: User programs Manual pages that describe publicly accessible commands are contained in this chapter. Most program documentation that a user will need to use is located here.
- man2: System calls This section describes all of the system calls (requests for the kernel to perform operations).
- man3: Library functions and subroutines Section 3 describes program library routines that are not direct calls to kernel services. This and chapter 2 are only really of interest to programmers.
- man4: Special files Section 4 describes the special files, related driver functions, and networking support available in the system. Typically, this includes the device files found in /dev and the kernel interface to networking protocol support.
- man5: File formats The formats for many data files are documented in the section 5. This includes various include files, program output files, and system files.
- man6: Games This chapter documents games, demos, and generally trivial programs. Different people have various notions about how essential this is.
- man7: Miscellaneous Manual pages that are difficult to classify are designated as being section 7. The troff and other text processing macro packages are found here.
- man8: System administration Programs used by system administrators for system operation and maintenance are documented here. Some of these programs are also occasionally useful for normal users.
The following directories, or symbolic links to directories, must be in /usr/share/<mandir>/<locale>, unless they are empty: 
Directory Description man1 User programs (optional) man2 System calls (optional) man3 Library calls (optional) man4 Special files (optional) man5 File formats (optional) man6 Games (optional) man7 Miscellaneous (optional) man8 System administration (optional)
The component <section> describes the manual section.
Provisions must be made in the structure of /usr/share/man to support manual pages which are written in different (or multiple) languages. These provisions must take into account the storage and reference of these manual pages. Relevant factors include language (including geographical-based differences), and character code set.
This naming of language subdirectories of /usr/share/man is based on Appendix E of the POSIX 1003.1 standard which describes the locale identification string — the most well-accepted method to describe a cultural environment. The <locale> string is:
The <language> field must be taken from ISO 639 (a code for the representation of names of languages). It must be two characters wide and specified with lowercase letters only.
The <territory> field must be the two-letter code of ISO 3166 (a specification of representations of countries), if possible. (Most people are familiar with the two-letter codes used for the country codes in email addresses.) It must be two characters wide and specified with uppercase letters only. 
The <character-set> field must represent the standard describing the character set. If the <character-set> field is just a numeric specification, the number represents the number of the international standard describing the character set. It is recommended that this be a numeric representation if possible (ISO standards, especially), not include additional punctuation symbols, and that any letters be in lowercase.
A parameter specifying a <version> of the profile may be placed after the <character-set> field, delimited by a comma. This may be used to discriminate between different cultural needs; for instance, dictionary order versus a more systems-oriented collating order. This standard recommends not using the <version> field, unless it is necessary.
Systems which use a unique language and code set for all manual pages may omit the <locale> substring and store all manual pages in <mandir>. For example, systems which only have English manual pages coded with ASCII, may store manual pages (the man<section> directories) directly in /usr/share/man. (That is the traditional circumstance and arrangement, in fact.)
Countries for which there is a well-accepted standard character code set may omit the <character-set> field, but it is strongly recommended that it be included, especially for countries with several competing standards.
Language Territory Character Set Directory English — ASCII /usr/share/man/en English United Kingdom ISO 8859-15 /usr/share/man/en_GB English United States ASCII /usr/share/man/en_US French Canada ISO 8859-1 /usr/share/man/fr_CA French France ISO 8859-1 /usr/share/man/fr_FR German Germany ISO 646 /usr/share/man/de_DE.646 German Germany ISO 6937 /usr/share/man/de_DE.6937 German Germany ISO 8859-1 /usr/share/man/de_DE.88591 German Switzerland ISO 646 /usr/share/man/de_CH.646 Japanese Japan JIS /usr/share/man/ja_JP.jis Japanese Japan SJIS /usr/share/man/ja_JP.sjis Japanese Japan UJIS (or EUC-J) /usr/share/man/ja_JP.ujis
Similarly, provision must be made for manual pages which are architecture-dependent, such as documentation on device-drivers or low-level system administration commands. These must be placed under an <arch> directory in the appropriate man<section> directory; for example, a man page for the i386 ctrlaltdel(8) command might be placed in /usr/share/man/<locale>/man8/i386/ctrlaltdel.8.
Manual pages for commands and data under /usr/local are stored in /usr/local/man. Manual pages for X11R6 are stored in /usr/X11R6/man. It follows that all manual page hierarchies in the system must have the same structure as /usr/share/man.
The cat page sections (cat<section>) containing formatted manual page entries are also found within subdirectories of <mandir>/<locale>, but are not required nor may they be distributed in lieu of nroff source manual pages.
The numbered sections "1" through "8" are traditionally defined. In general, the file name for manual pages located within a particular section end with .<section>.
In addition, some large sets of application-specific manual pages have an additional suffix appended to the manual page filename. For example, the MH mail handling system manual pages must have mh appended to all MH manuals. All X Window System manual pages must have an x appended to the filename.
The practice of placing various language manual pages in appropriate subdirectories of /usr/share/man also applies to the other manual page hierarchies, such as /usr/local/man and /usr/X11R6/man. (This portion of the standard also applies later in the section on the optional /var/cache/man structure.)
This directory contains miscellaneous architecture-independent files which don't require a separate subdirectory under /usr/share.
The following files, or symbolic links to files, must be in /usr/share/misc, if the corresponding subsystem is installed:
File Description ascii ASCII character set table (optional) magic Default list of magic numbers for the file command (optional) termcap Terminal capability database (optional) termcap.db Terminal capability database (optional)
Other (application-specific) files may appear here, but a distributor may place them in /usr/lib at their discretion. 
/usr/share/sgml contains architecture-independent files used by SGML applications, such as ordinary catalogs (not the centralized ones, see /etc/sgml), DTDs, entities, or style sheets.
The following directories, or symbolic links to directories, must be in /usr/share/sgml, if the corresponding subsystem is installed:
Directory Description docbook docbook DTD (optional) tei tei DTD (optional) html html DTD (optional) mathml mathml DTD (optional)
Other files that are not specific to a given DTD may reside in their own subdirectory.
/usr/share/xml contains architecture-independent files used by XML applications, such as ordinary catalogs (not the centralized ones, see /etc/sgml), DTDs, entities, or style sheets.
The following directories, or symbolic links to directories, must be in /usr/share/xml, if the corresponding subsystem is installed:
Directory Description docbook docbook XML DTD (optional) xhtml XHTML DTD (optional) mathml MathML DTD (optional)
/usr/src : Source code
In Funtoo and Gentoo Linux, kernel source code is stored in /usr/src. /usr/src/linux is a symbolic link that points to the current version of the Linux kernel that third-party (externally packaged) kernel module ebuilds should be compiled against.
The eselect command can be used to inspect or change the setting of the /usr/src/linux symlink:
ninja1 portage-funtoo # eselect kernel list Available kernel symlink targets:  linux-2.6.18-openvz-028.068.9 *
installsources feature is enabled, Portage will put the source code in
/usr/src/debug in a subdirectory based on the package atom.
- Examples of such configuration files include Xconfig, XF86Config, or system.twmrc)
- Miscellaneous architecture-independent application-specific static files and subdirectories must be placed in /usr/share.
- For example, the perl5 subdirectory for Perl 5 modules and libraries.
- Some executable commands such as makewhatis and sendmail have also been traditionally placed in /usr/lib. makewhatis is an internal binary and must be placed in a binary directory; users access only catman. Newer sendmail binaries are now placed by default in /usr/sbin. Additionally, systems using a sendmail-compatible mail transfer agent must provide /usr/sbin/sendmail as a symbolic link to the appropriate executable.
- Locally installed system administration programs should be placed in /usr/local/sbin.
- Much of this data originally lived in /usr (man, doc) or /usr/lib (dict, terminfo, zoneinfo).
- Obviously, there are no manual pages in / because they are not required at boot time nor are they required in emergencies. Really.
- For example, if /usr/local/man has no manual pages in section 4 (Devices), then /usr/local/man/man4 may be omitted.
- A major exception to this rule is the United Kingdom, which is `GB' in the ISO 3166, but `UK' for most email addresses.
- Some such files include: airport, birthtoken, eqnchar, getopt, gprof.callg, gprof.flat, inter.phone, ipfw.samp.filters, ipfw.samp.scripts, keycap.pcvt, mail.help, mail.tildehelp, man.template, map3270, mdoc.template, more.help, na.phone, nslookup.help, operator, scsi_modes, sendmail.hf, style, units.lib, vgrindefs, vgrindefs.db, zipcodes