Filesystem Hierarchy Standard

Filesystem Hierarchy Standard
Developed by Linux Foundation
Initial release 14 February 1994 (1994-02-14)
Latest release
3.0
(3 June 2015 (2015-06-03))
Website Official website
Official website (Historical)

The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix-like operating systems.[1] It is maintained by the Linux Foundation. The latest version is 3.0, released on 3 June 2015.[2] Currently it is only used by Linux distributions.

Directory structure

In the FHS, all files and directories appear under the root directory /, even if they are stored on different physical or virtual devices. Some of these directories only exist on a particular system if certain subsystems, such as the X Window System, are installed.

Most 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.

Directory Description
/
 Primary hierarchy root and root directory of the entire file system hierarchy.
/bin
 Essential command binaries that need to be available in single user mode; for all users, e.g., cat, ls, cp.
/boot
 Boot loader files, e.g., kernels, initrd.
/dev
 Essential device files, e.g., /dev/null.
/etc
 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,[3] 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).[4] 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".[5]

/etc/opt
 Configuration files for add-on packages that are stored in /opt/.
/etc/sgml
 Configuration files, such as catalogs, for software that processes SGML.
/etc/X11
 Configuration files for the X Window System, version 11.
/etc/xml
 Configuration files, such as catalogs, for software that processes XML.
/home
 Users' home directories, containing saved files, personal settings, etc.
/lib
 Libraries essential for the binaries in /bin/ and /sbin/.
/lib<qual>
 Alternate format essential libraries. Such directories are optional, but if they exist, they have some requirements.
/media
 Mount points for removable media such as CD-ROMs (appeared in FHS-2.3).
/mnt
 Temporarily mounted filesystems.
/opt
 Optional application software packages.[6]
/proc
 Virtual filesystem providing process and kernel information as files. In Linux, corresponds to a procfs mount.
/root
 Home directory for the root user.
/run
 Run-time variable data: Information about the running system since last boot, e.g., currently logged-in users and running daemons.
/sbin
 Essential system binaries, e.g., fsck, init, route.
/srv
 Site-specific data served by this system, such as data and scripts for web servers, data offered by FTP servers, and repositories for version control systems.
/sys
 Contains information about the devices connected to the computer.
/tmp
 Temporary files (see also /var/tmp). Often not preserved between system reboots, and may be severely size restricted.
/usr
 Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications.[7]
/usr/bin
 Non-essential command binaries (not needed in single user mode); for all users.
/usr/include
 Standard include files.
/usr/lib
 Libraries for the binaries in /usr/bin/ and /usr/sbin/.
/usr/lib<qual>
 Alternate format libraries (optional).
/usr/local
 Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g., bin/, lib/, share/.[8]
/usr/sbin
 Non-essential system binaries, e.g., daemons for various network-services.
/usr/share
 Architecture-independent (shared) data.
/usr/src
 Source code, e.g., the kernel source code with its header files.
/usr/X11R6
 X Window System, Version 11, Release 6 (up to FHS-2.3, optional).
/var
 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.
/var/cache
 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.
/var/lib
 State information. Persistent data modified by programs as they run, e.g., databases, packaging system metadata, etc.
/var/lock
 Lock files. Files keeping track of resources currently in use.
/var/log
 Log files. Various logs.
/var/mail
 This is the location defined by the FHS (File System Hierarchy Standard) to store users' mailbox files. Depending on the extent to which your distribution complies with the FHS, these files may be located in /var/spool/mail.
/var/opt
 Variable data from add-on packages that are stored in /opt/.
/var/run
 Run-time variable data. This directory contains system information data describing the system since it was booted.[9]

In FHS 3.0, /var/run is replaced by /run; a system should either continue to provide a /var/run directory, or provide a symbolic link from /var/run to /run, for backwards compatibility.[10]

/var/spool
 Spool for tasks waiting to be processed, e.g., print queues and outgoing mail queue.
/var/spool/mail
 Deprecated location for users' mailboxes.[11]
/var/tmp
 Temporary files to be preserved between reboots.

FHS compliance

Most Linux distributions follow the Filesystem Hierarchy Standard and declare it their own policy to maintain FHS compliance.[12][13][14][15] GoboLinux[16] and NixOS[17] 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 /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 is not always available at early boot. As a result, these programs have had to resort to trickery, such as using /dev/.udev, /dev/.mdadm, /dev/.systemd or /dev/.mount directories, even though the device directory isn't intended for such data.[22] 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:[23]

History

When the FHS was created as the FSSTND (short for "Filesystem Standard"[24]), other UNIX and Unix-like operating systems already had their own standards. Notable examples are these: the hier(7) description of file system layout,[25] which has existed since the release of Version 7 Unix (in 1979); the SunOS filesystem(7)[26] and its successor, the Solaris filesystem(5).[27][28]

Release history

Version Release Date Notes
Old version, no longer supported: 1.0 1994-02-14 FSSTND[29]
Old version, no longer supported: 1.1 1994-10-09 FSSTND[30]
Old version, no longer supported: 1.2 1995-03-28 FSSTND[31]
Old version, no longer supported: 2.0 1997-10-26 FHS 2.0 is the direct successor for FSSTND 1.2. Name of the standard was changed to Filesystem Hierarchy Standard.[32][33][34]
Old version, no longer supported: 2.1 2000-04-12 FHS[35][36][37]
Old version, no longer supported: 2.2 2001-05-23 FHS[38]
Older version, yet still supported: 2.3 2004-01-29 FHS[39]
Current stable version: 3.0 2015-05-18 FHS[40]
Legend:
Old version
Older version, still supported
Latest version
Latest preview version
Future release

See also

References

  1. ↑ "FilesystemHierarchyStandard - Debian Wiki". wiki.debian.org. Retrieved 2016-07-06.
  2. ↑ "FHS 3.0 Released". June 3, 2015.
  3. ↑ J. DeFelicc (1972-03-17). "E.0". Preliminary Release of UNIX Implementation Document (PDF). p. 8. IMO.1-1.
  4. ↑ "/etc : Host-specific system configuration". Filesystem Hierarchy Standard. Retrieved 18 February 2016.
  5. ↑ Define - /etc?, Posted by Cliff, 3 March 2007 - Slashdot
  6. ↑ "/opt : Add-on application software packages". Filesystem Hierarchy Standard. Retrieved 18 February 2016.
  7. ↑ Should be shareable and read-only, cf. http://www.pathname.com/fhs/pub/fhs-2.3.html.
  8. ↑ 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.
  9. ↑ "Filesystem Hierarchy Standard". FHS 2.3. Section /var/run : Run-time variable data.
  10. ↑ "5.13. /var/run : Run-time variable data". FHS 3.0.
  11. ↑ "File System Standard" (PDF). Linux Foundation. p. 5.11.1.
  12. ↑ Red Hat reference guide on file system structure
  13. ↑ 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
  14. ↑ Debian policy on FHS compliance
  15. ↑ Ubuntu Linux File system Tree Overview - Community Ubuntu Documentation
  16. ↑ Hisham Muhammad (9 May 2003). "The Unix tree rethought: an introduction to GoboLinux". Retrieved 2016-10-04.
  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.
  18. ↑ "5.3 About the /sys Virtual File System". docs.oracle.com. Oracle. Archived from the original on 8 July 2016. Retrieved 8 July 2016.
  19. ↑ Lehey, Greg (May 2003). The Complete FreeBSD: Documentation from the Source (Fourth ed.). O'Reilly Media, Incorporated. pp. 188, 609. ISBN 9780596005160. Retrieved 8 July 2016.
  20. ↑ 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.
  21. ↑ 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.
  22. ↑ https://lwn.net/Articles/436012/
  23. ↑ http://wiki.debian.org/ReleaseGoals/RunDirectory
  24. ↑ "FSSTND FAQ page". Retrieved 2016-05-10.
  25. ↑ hier(7) – FreeBSD Miscellaneous Information Manual
  26. ↑ SunOS 4.1.3 manual page for filesystem(7), dated 10 January 1988 (from the FreeBSD Man Pages library)
  27. ↑ filesystem(5) – Solaris 10 Standards, Environments and Macros Reference Manual
  28. ↑ "filesystem man page - Solaris 10 11/06 Man Pages". Retrieved 2011-10-15.
  29. ↑ "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.0/". Ibiblio.org. Retrieved 2012-10-16.
  30. ↑ "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.1/". Ibiblio.org. Retrieved 2012-10-16.
  31. ↑ "Index of /pub/Linux/docs/fsstnd/old/". Ibiblio.org. Retrieved 2012-10-16.
  32. ↑ "FHS 2.0 Announcement". Pathname.com. Retrieved 2012-10-16.
  33. ↑ Quinlan, Daniel (14 March 2012) [1997], "FHS 2.0 Announcement", BSD, Linux, Unix and The Internet - Research by Kenneth R. Saborio, San Jose, Costa Rica: Kenneth R. Saborio, retrieved 18 February 2016
  34. ↑ "Index of /pub/Linux/docs/fsstnd/". Ibiblio.org. Retrieved 2012-10-16.
  35. ↑ "FHS 2.1 Announcement". Pathname.com. Retrieved 2012-10-16.
  36. ↑ "FHS 2.1 is released". Lists.debian.org. 2000-04-13. Retrieved 2012-10-16.
  37. ↑ 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.
  38. ↑ 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 February 2016.
  39. ↑ Russell, Rusty; Quinlan, Daniel; Yeoh, Christopher, eds. (28 January 2004). "Filesystem Hierarchy Standard - Filesystem Hierarchy Standard Group" (PDF). Retrieved 2014-11-29.
  40. ↑ Yeoh, Christopher; Russell, Rusty; Quinlan, Daniel, eds. (19 March 2015). "Filesystem Hierarchy Standard" (PDF). The Linux Foundation. Retrieved 2015-05-20.

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