Filesystem Hierarchy Standard
|Developed by||Linux Foundation|
|Initial release||14 February 1994|
(3 June 2015)
Official website (Historical)
The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix and Unix-like operating systems. It is maintained by the Linux Foundation. The latest version is 3.0, released on 3 June 2015.  Currently it is only used by Linux distributions.
In the FHS all files and directories appear under the root directory
/, even if they are stored on different physical or virtual devices. Note however that some of these directories may or may not be present on a Unix system depending on whether certain subsystems, such as the X Window System, are installed.
The majority of these directories exist in all UNIX operating systems and are generally used in much the same way; however, the descriptions here are those used specifically for the FHS, and are not considered authoritative for platforms other than Linux.
||Primary hierarchy root and root directory of the entire file system hierarchy.|
||Essential command binaries that need to be available in single user mode; for all users, e.g., cat, ls, cp.|
|Boot loader files, e.g., kernels, initrd.|
|Essential devices, e.g.,
||Host-specific system-wide configuration files
There has been controversy over the meaning of the name itself. In early versions of the UNIX Implementation Document from Bell labs, /etc is referred to as the etcetera directory, as this directory historically held everything that did not belong elsewhere (however, the FHS restricts /etc to static configuration files and may not contain binaries). Since the publication of early documentation, the directory name has been re-explained in various ways. Recent interpretations include backronyms such as "Editable Text Configuration" or "Extended Tool Chest".
||Configuration files for add-on packages that are stored in
||Configuration files, such as catalogs, for software that processes SGML.|
||Configuration files for the X Window System, version 11.|
||Configuration files, such as catalogs, for software that processes XML.|
||Users' home directories, containing saved files, personal settings, etc.|
||Libraries essential for the binaries in
||Alternate format essential libraries. Such directories are optional, but if they exist, they have some requirements.|
||Mount points for removable media such as CD-ROMs (appeared in FHS-2.3).|
||Temporarily mounted filesystems.|
||Optional application software packages.|
|Virtual filesystem providing process and kernel information as files. In Linux, corresponds to a procfs mount.|
||Home directory for the root user.|
||Run-time variable data: Information about the running system since last boot, e.g., currently logged-in users and running daemons.|
||Essential system binaries, e.g., fsck, init, route.|
||Site-specific data which are served by the system.|
||Temporary files (see also
||Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications.|
||Non-essential command binaries (not needed in single user mode); for all users.|
||Standard include files.|
||Libraries for the binaries in
||Alternate format libraries (optional).|
||Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g.,
||Non-essential system binaries, e.g., daemons for various network-services.|
||Architecture-independent (shared) data.|
||Source code, e.g., the kernel source code with its header files.|
||X Window System, Version 11, Release 6 (up to FHS-2.3, optional).|
||Variable files—files whose content is expected to continually change during normal operation of the system—such as logs, spool files, and temporary e-mail files.|
||Application cache data. Such data are locally generated as a result of time-consuming I/O or calculation. The application must be able to regenerate or restore the data. The cached files can be deleted without loss of data.|
||State information. Persistent data modified by programs as they run, e.g., databases, packaging system metadata, etc.|
||Lock files. Files keeping track of resources currently in use.|
||Log files. Various logs.|
||Variable data from add-on packages that are stored in
||In FHS-2.3, information about the running system since last boot. In FHS-3.0, this functionality has been moved to
||Spool for tasks waiting to be processed, e.g., print queues and outgoing mail queue.|
||Deprecated location for users' mailboxes.|
||Temporary files to be preserved between reboots.|
Most Linux distributions follow the Filesystem Hierarchy Standard and declare it their own policy to maintain FHS compliance. GoboLinux and NixOS provide examples of intentionally non-compliant filesystem implementations.
Some distributions that generally follow the standard deviate from it in some areas. Common deviations include:
- Modern Linux distributions include a
/sysdirectory as a virtual filesystem (sysfs, comparable to
/proc, which is a procfs), which stores and allows modification of the devices connected to the system, whereas many traditional UNIX and Unix-like operating systems use
/sysas a symbolic link to the kernel source tree.
- Many modern UNIX systems (like FreeBSD via its ports system) install third party packages into
/usr/localwhile keeping locally developed code in
- Some Linux distributions no longer differentiate between
- Some Linux distributions no longer differentiate between
/usr/sbin. They symlink
/usr/sbinmay get symlinked to
Modern Linux distributions include a
/run directory as a temporary filesystem (tmpfs) which stores volatile runtime data, following the FHS version 3.0. According to the FHS version 2.3, such data were stored in
/var/run but this was a problem in some cases because this directory isn't always available at early boot. As a result, these programs have had to resort to trickery, such as using
/dev/.mount directories, even though the device directory isn't intended for such data. Among other advantages, this makes the system easier to use normally with the root filesystem mounted read-only. For example, below are the changes Debian made in its 2013 Wheezy release:
/etc/*(writeable files) →
When the FHS was created, other UNIX and Unix-like operating systems already had their own standards. Notable examples are these: the hier(7) description of file system layout, which has existed since the release of Version 7 Unix (in 1979); the SunOS filesystem(7) and its successor, the Solaris filesystem(5).
|Red||Old Standard/Draft; not supported|
|Yellow||Old Standard; still supported|
|v2.0||1997-10-26||FHS 2.0 is the direct successor for FSSTND 1.2. Name of the standard was changed to Filesystem Hierarchy Standard.|
- Unix directory structure
- Intel Binary Compatibility Standard (iBCS)
- hier(7) - Linux man page : The Linux manual page that describes the hierarchy
- "Filesystem Hierarchy Standard Specifications Archive". Retrieved 3 June 2015.
- J. DeFelicc (1972-03-17). "E.0". Preliminary Release of UNIX Implementation Document (PDF) (D). p. 8. IMO.1-1.
- "/etc : Host-specific system configuration". Filesystem Hierarchy Standard. Retrieved 18 October 2012.
- Define - /etc?, Posted by Cliff, 3 March 2007 - Slashdot
- "/opt : Add-on application software packages". Filesystem Hierarchy Standard. Retrieved 18 October 2012.
- Should be shareable and read-only, cf. http://www.pathname.com/fhs/pub/fhs-2.3.html.
- Historically and strictly according to the standard,
/usr/local/is for data that must be stored on the local host (as opposed to
/usr/, which may be mounted across a network). Most of the time
/usr/local/is used for installing software/data that are not part of the standard operating system distribution (in such case,
/usr/would only contain software/data that are part of the standard operating system distribution). It is possible that the FHS standard may in the future be changed to reflect this de facto convention).
- "File System Standard" (PDF). Linux Foundation. p. 5.11.1.
- Red Hat reference guide on file system structure
- SuSE Linux Enterprise Server Administration, Novell authorized courseware, by Jason W. Eckert, Novell; Course Technology, 2006; ISBN 1-4188-3731-8, ISBN 978-1-4188-3731-0
- Debian policy on FHS compliance
- Ubuntu Linux File system Tree Overview - Community Ubuntu Documentation
- Hisham Muhammad (9 May 2003). "The Unix tree rethought: an introduction to GoboLinux". Retrieved 2008-03-17.
- Dolstra, E. and Löh, A. "NixOS: A Purely Functional Linux Distribution." In ICFP 2008: 13th ACM SIGPLAN International Conference on Functional Programming, pages 367–378, Victoria, British Columbia, Canada. September 2008.
- Allan McRae. "Arch Linux - News: The /lib directory becomes a symlink". archlinux.org. Archived from the original on 9 September 2014. Retrieved 28 December 2014.
- Allan McRae. "Arch Linux - News: Binaries move to /usr/bin requiring update intervention". archlinux.org. Archived from the original on 10 September 2014. Retrieved 28 December 2014.
- FreeBSD Miscellaneous Information Manual –
- SunOS 4.1.3 manual page for filesystem(7), dated 10 January 1988 (from the FreeBSD Man Pages library)
- Solaris 10 Standards, Environments and Macros Reference Manual –
- "filesystem man page - Solaris 10 11/06 Man Pages". Retrieved 2011-10-15.
- "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.0/". Ibiblio.org. Retrieved 2012-10-16.
- "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.1/". Ibiblio.org. Retrieved 2012-10-16.
- "Index of /pub/Linux/docs/fsstnd/old/". Ibiblio.org. Retrieved 2012-10-16.
- "FHS 2.0 Announcement". Pathname.com. Retrieved 2012-10-16.
- Quinlan, Daniel (14 March 2012) , "FHS 2.0 Announcement", BSD, Linux and Unix Information - Research by Kenneth R. Saborio (San Jose, Costa Rica: Kenneth R. Saborio), retrieved 18 October 2012
- "Index of /pub/Linux/docs/fsstnd/". Ibiblio.org. Retrieved 2012-10-16.
- "FHS 2.1 Announcement". Pathname.com. Retrieved 2012-10-16.
- "FHS 2.1 is released". Lists.debian.org. 2000-04-13. Retrieved 2012-10-16.
- Quinlan, Daniel (12 April 2000). "Filesystem Hierarchy Standard — Version 2.1, Filesystem Hierarchy Standard Group" (PDF). Acadia Linux Tutorials. Wolfville, Nova Scotia, Canada: Jodrey School of Computer Science, Acadia University. Retrieved 18 October 2012.
- Russell, Rusty; Quinlan, Daniel, eds. (23 May 2001). "Filesystem Hierarchy Standard — Version 2.2 final Filesystem Hierarchy Standard Group" (PDF). Filesystem Hierarchy Standard. Retrieved 18 October 2012.
- Russell, Rusty; Quinlan, Daniel; Yeoh, Christopher, eds. (28 January 2004). "Filesystem Hierarchy Standard - Filesystem Hierarchy Standard Group" (PDF). Retrieved 2014-11-29.
- Yeoh, Christopher; Russell, Rusty; Quinlan, Daniel, eds. (19 March 2015). "Filesystem Hierarchy Standard" (PDF). The Linux Foundation. Retrieved 2015-05-20.
- Official Home of the Filesystem Hierarchy Standard (FHS) at The Linux Foundation
- objectroot – a proposal for a new filesystem hierarchy, based on object-oriented design principles
- The Dotted Standard Filename Hierarchy, yet another very different hierarchy (used in cLIeNUX) (mirror)
- Linux Filesystem Hierarchy Version 0.65