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 Linux distributions.[1] It is maintained by the Linux Foundation. The latest version is 3.0, released on 3 June 2015.[2]

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-like 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>
 Alternative 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 in 2004).
/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. Generally automatically generated and populated by the system, on the fly.
/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. Files under this directory must be either removed or truncated at the beginning of the boot process; but this is not necessary on systems that provide this directory as a temporary filesystem (tmpfs).
/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 (appeared in FHS-2.3 in 2004).
/sys
 Contains information about devices, drivers, and some kernel features.[7]
/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.[8]
/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>
 Alternative format libraries, e.g. /usr/lib32 for 32-bit libraries on a 64-bit machine (optional).
/usr/local
 Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g., bin, lib, share.[9]
/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
 Mailbox files. In some distributions, these files may be located in the deprecated /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.[10]

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.[11]

/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.[12]
/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.[13][14][15][16] GoboLinux[17] and NixOS[18] provide examples of intentionally non-compliant filesystem implementations.

Some distributions generally follow the standard but deviate from it in some areas. Common deviations include:

  • Modern Linux distributions include a /sys directory as a virtual filesystem (sysfs, comparable to /proc, which is a procfs), which stores and allows modification of the devices connected to the system,[19] whereas many traditional Unix-like operating systems use /sys as a symbolic link to the kernel source tree.[20]
  • Many modern Unix-like systems (like FreeBSD via its ports system) install third party packages into /usr/local while keeping code considered part of the operating system in /usr.
  • Some Linux distributions no longer differentiate between /lib versus /usr/lib and have /lib symlinked to /usr/lib.[21]
  • Some Linux distributions no longer differentiate between /bin versus /usr/bin and /sbin versus /usr/sbin. They may symlink /bin to /usr/bin and /sbin to /usr/sbin. Other distributions choose to consolidate them all, symlinking them to /usr/bin.[22]

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.[23] 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:[24]

  • /dev/.* → /run/*
  • /dev/shm → /run/shm
  • /dev/shm/* → /run/*
  • /etc/* (writeable files) → /run/*
  • /lib/init/rw → /run
  • /var/lock → /run/lock
  • /var/run → /run
  • /tmp → /run/tmp

History

FHS was created as the FSSTND (short for "Filesystem Standard"[25]), largely based on similar standards for other Unix-like operating systems. Notable examples are these: the hier(7) description of file system layout,[26] which has existed since the release of Version 7 Unix (in 1979); the SunOS filesystem(7)[27] and its successor, the Solaris filesystem(5).[28][29]

Release history

Version Release Date Notes
Old version, no longer supported: 1.0 1994-02-14 FSSTND[30]
Old version, no longer supported: 1.1 1994-10-09 FSSTND[31]
Old version, no longer supported: 1.2 1995-03-28 FSSTND[32]
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.[33][34][35]
Old version, no longer supported: 2.1 2000-04-12 FHS[36][37][38]
Old version, no longer supported: 2.2 2001-05-23 FHS[39]
Older version, yet still supported: 2.3 2004-01-29 FHS[40]
Current stable version: 3.0 2015-05-18 FHS[41]
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. ↑ "/sys : Kernel and system information virtual filesystem". Filesystem Hierarchy Standard. Retrieved 4 June 2017.
  8. ↑ Should be shareable and read-only, cf. http://www.pathname.com/fhs/pub/fhs-2.3.html.
  9. ↑ 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.
  10. ↑ "Filesystem Hierarchy Standard". FHS 2.3. Section /var/run : Run-time variable data.
  11. ↑ "5.13. /var/run : Run-time variable data". FHS 3.0.
  12. ↑ "File System Standard" (PDF). Linux Foundation. p. 5.11.1.
  13. ↑ Red Hat reference guide on file system structure
  14. ↑ 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
  15. ↑ Debian policy on FHS compliance
  16. ↑ Ubuntu Linux File system Tree Overview - Community Ubuntu Documentation
  17. ↑ Hisham Muhammad (9 May 2003). "The Unix tree rethought: an introduction to GoboLinux". Retrieved 2016-10-04.
  18. ↑ 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.
  19. ↑ "5.3 About the /sys Virtual File System". docs.oracle.com. Oracle. Archived from the original on 8 July 2016. Retrieved 8 July 2016.
  20. ↑ 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.
  21. ↑ 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.
  22. ↑ 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.
  23. ↑ https://lwn.net/Articles/436012/
  24. ↑ http://wiki.debian.org/ReleaseGoals/RunDirectory
  25. ↑ "FSSTND FAQ page". Retrieved 2016-05-10.
  26. ↑ hier(7) – FreeBSD Miscellaneous Information Manual
  27. ↑ SunOS 4.1.3 manual page for filesystem(7), dated 10 January 1988 (from the FreeBSD Man Pages library)
  28. ↑ filesystem(5) – Solaris 10 Standards, Environments and Macros Reference Manual
  29. ↑ "filesystem man page - Solaris 10 11/06 Man Pages". Retrieved 2011-10-15.
  30. ↑ "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.0/". Ibiblio.org. Retrieved 2012-10-16.
  31. ↑ "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.1/". Ibiblio.org. Retrieved 2012-10-16.
  32. ↑ "Index of /pub/Linux/docs/fsstnd/old/". Ibiblio.org. Retrieved 2012-10-16.
  33. ↑ "FHS 2.0 Announcement". Pathname.com. Retrieved 2012-10-16.
  34. ↑ 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
  35. ↑ "Index of /pub/Linux/docs/fsstnd/". Ibiblio.org. Retrieved 2012-10-16.
  36. ↑ "FHS 2.1 Announcement". Pathname.com. Retrieved 2012-10-16.
  37. ↑ "FHS 2.1 is released". Lists.debian.org. 2000-04-13. Retrieved 2012-10-16.
  38. ↑ 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.
  39. ↑ 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.
  40. ↑ Russell, Rusty; Quinlan, Daniel; Yeoh, Christopher, eds. (28 January 2004). "Filesystem Hierarchy Standard - Filesystem Hierarchy Standard Group" (PDF). Retrieved 2014-11-29.
  41. ↑ 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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