Filesystem Hierarchy Standard

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Filesystem Hierarchy Standard
Developed by Linux Foundation
Initial release 14 February 1994; 21 years ago (1994-02-14)
Latest release
(29 January 2004; 11 years ago (2004-01-29))
Website Official website
Official website (Historical)

The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Unix[citation needed] and Unix-like operating systems, maintained by the Linux Foundation. The current version is 2.3, announced on 29 January 2004.[1]

Directory structure[edit]

Filesystem in Ubuntu 14.04.

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.

Directory Description
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., /dev/null.
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,[2] 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).[3] 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".[4]

Configuration files for add-on packages that are stored in /opt/.
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 /bin/ and /sbin/.
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.[5]
Virtual filesystem providing process and kernel information as files. In Linux, corresponds to a procfs mount.
Home directory for the root user.
Essential system binaries, e.g., init, ip, mount.
Site-specific data which are served by the system.
Temporary files (see also /var/tmp). Often not preserved between system reboots, and may be severely size restricted.
Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications.[6]
Non-essential command binaries (not needed in single user mode); for all users.
Standard include files.
Libraries for the binaries in /usr/bin/ and /usr/sbin/.
Alternate format libraries (optional).
Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g., bin/, lib/, share/.[7]
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.
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.
Users' mailboxes.
Variable data from add-on packages that are stored in /opt/.
Information about the running system since last boot, e.g., currently logged-in users and running daemons.
Spool for tasks waiting to be processed, e.g., print queues and outgoing mail queue.
Deprecated location for users' mailboxes.[8]
Temporary files to be preserved between reboots.

FHS compliance[edit]

Most Linux distributions follow the Filesystem Hierarchy Standard and declare it their own policy to maintain FHS compliance.[9][10][11][12] GoboLinux is an example of an intentionally non-compliant filesystem implementation.[13]

Some distributions that generally follow the standard 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, whereas many traditional UNIX and Unix-like operating systems use /sys as a symbolic link to the kernel source tree.[citation needed]
  • Modern Linux distributions include a /run directory as a temporary filesystem (tmpfs) which stores volatile runtime data, and which is being considered for the next version of the FHS.[14] According to the FHS version 2.3, this data should be 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/.udev, /dev/.mdadm, /dev/.systemd or /dev/.mount directories, even though the device directory isn't intended for such data.[15] Among other advantages, this makes the system easier to use normally with the root filesystem mounted read-only.
    • This is a detailed example from Debian:[16]
      • /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
  • Many modern UNIX systems (like FreeBSD via its ports system) install third party packages into /usr/local while keeping locally developed code in /usr.
  • Some Linux distributions no longer differentiate between /lib versus /usr/lib and have /lib symlinked to /usr/lib.[17]
  • Some Linux distributions no longer differentiate between /bin versus /usr/bin and /sbin versus /usr/sbin. They symlink /bin to /usr/bin and /sbin to /usr/sbin. And /usr/sbin may get symlinked to /usr/bin.[18]


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,[19] which has existed since the release of Version 7 Unix (in 1979); the SunOS filesystem(7)[20] and its successor, the Solaris filesystem(5).[21][22]

Release history[edit]

Red Old Standard/Draft; not supported
Yellow Old Standard; still supported
Green Current Standard
Blue Future Draft
Version Release Date Notes
v1.0 1994-02-14 FSSTND[23]
v1.1 1994-10-09 FSSTND[24]
v1.2 1995-03-28 FSSTND[25]
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.[26][27][28]
v2.1 2000-04-12 FHS[29][30][31]
v2.2 2001-05-23 FHS[32]
v2.3 2004-01-29 FHS[33]
v3.0 under

See also[edit]


  1. ^ (ANNOUNCE) FHS 2.3 Released, From: Christopher Yeoh - 2004-01-29, Email Archive: freestandards-fhs-discuss (read-only), Free Standards Group,
  2. ^ J. DeFelicc (1972-03-17). "E.0". Preliminary Release of UNIX Implementation Document (D). p. 8. IMO.1-1. 
  3. ^ "/etc : Host-specific system configuration". Filesystem Hierarchy Standard. Retrieved 18 October 2012. 
  4. ^ Define - /etc?, Posted by Cliff, 3 March 2007 - Slashdot
  5. ^ "/opt : Add-on application software packages". Filesystem Hierarchy Standard. Retrieved 18 October 2012. 
  6. ^ Should be shareable and read-only, cf.
  7. ^ 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).
  8. ^ "File System Standard". Linux Foundation. p. 5.11.1. 
  9. ^ Red Hat reference guide on file system structure
  10. ^ 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
  11. ^ Debian policy on FHS compliance
  12. ^ Ubuntu Linux File system Tree Overview - Community Ubuntu Documentation
  13. ^ Hisham Muhammad (9 May 2003). "The Unix tree rethought: an introduction to GoboLinux". Retrieved 2008-03-17. 
  14. ^
  15. ^
  16. ^
  17. ^ Allan McRae. "Arch Linux - News: The /lib directory becomes a symlink". Archived from the original on 9 September 2014. Retrieved 28 December 2014. 
  18. ^ Allan McRae. "Arch Linux - News: Binaries move to /usr/bin requiring update intervention". Archived from the original on 10 September 2014. Retrieved 28 December 2014. 
  19. ^ hier(7) – FreeBSD Miscellaneous Information Manual
  20. ^ SunOS 4.1.3 manual page for filesystem(7), dated 10 January 1988 (from the FreeBSD Man Pages library)
  21. ^ filesystem(5) – Solaris 10 Standards, Environments and Macros Reference Manual
  22. ^ "filesystem man page - Solaris 10 11/06 Man Pages". Retrieved 2011-10-15. 
  23. ^ "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.0/". Retrieved 2012-10-16. 
  24. ^ "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.1/". Retrieved 2012-10-16. 
  25. ^ "Index of /pub/Linux/docs/fsstnd/old/". Retrieved 2012-10-16. 
  26. ^ "FHS 2.0 Announcement". Retrieved 2012-10-16. 
  27. ^ Quinlan, Daniel (14 March 2012) [1997], "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 
  28. ^ "Index of /pub/Linux/docs/fsstnd/". Retrieved 2012-10-16. 
  29. ^ "FHS 2.1 Announcement". Retrieved 2012-10-16. 
  30. ^ "FHS 2.1 is released". 2000-04-13. Retrieved 2012-10-16. 
  31. ^ Quinlan, Daniel (12 April 2000). "Filesystem Hierarchy Standard — Version 2.1, Filesystem Hierarchy Standard Group". Acadia Linux Tutorials. Wolfville, Nova Scotia, Canada: Jodrey School of Computer Science, Acadia University. Retrieved 18 October 2012. 
  32. ^ Russell, Rusty; Quinlan, Daniel, eds. (23 May 2001). "Filesystem Hierarchy Standard — Version 2.2 final Filesystem Hierarchy Standard Group". Filesystem Hierarchy Standard. Retrieved 18 October 2012. 
  33. ^ Russell, Rusty; Quinlan, Daniel; Yeoh, Christopher, eds. (28 January 2004). "Filesystem Hierarchy Standard - Filesystem Hierarchy Standard Group". Retrieved 2014-11-29. 
  34. ^ Licquia, Jeff (27 April 2011). "FHS". The Linux Foundation. Retrieved 2012-10-16. 

External links[edit]