Filesystem Hierarchy Standard

Last updated

Filesystem Hierarchy Standard
AbbreviationFHS
StatusPublished
Year started14 February 1994;30 years ago (1994-02-14)
Latest version3.0
3 June 2015;9 years ago (2015-06-03)
Organization Linux Foundation
Domain Directory structure
Website Official website
Official website (Historical)

The Filesystem Hierarchy Standard (FHS) is a reference describing the conventions used for the layout of Unix-like systems. It has been made popular by its use in Linux distributions, but it is used by other Unix-like systems as well. [1] It is maintained by the Linux Foundation. The latest version is 3.0, released on 3 June 2015. [2]

Contents

Directory structure

Typical Ubuntu filesystem hierarchy Ubuntu Filesystem Hierarchy.png
Typical Ubuntu filesystem hierarchy

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 in 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.

DirectoryDescription
/Primary hierarchy root and root directory of the entire file system hierarchy.
/binEssential command binaries that need to be available in single-user mode, including to bring up the system or repair it, [3] for all users (e.g., cat, ls, cp).
/boot Boot loader files (e.g., kernels, initrd).
/dev Device files (e.g., /dev/null , /dev/disk0, /dev/sda1, /dev/tty, /dev/random ).
/etcHost-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, [4] 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). [5] 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". [6]

/etc/optConfiguration files for add-on packages stored in /opt.
/etc/sgmlConfiguration files, such as catalogs, for software that processes SGML.
/etc/X11Configuration files for the X Window System, version 11.
/etc/xmlConfiguration files, such as catalogs, for software that processes XML.
/homeUsers' home directories, containing saved files, personal settings, etc.
/lib Libraries essential for the binaries in /bin and /sbin.
/lib<qual>Alternate format essential libraries. These are typically used on systems that support more than one executable code format, such as systems supporting 32-bit and 64-bit versions of an instruction set. Such directories are optional, but if they exist, they have some requirements.
/mediaMount points for removable media such as CD-ROMs (appeared in FHS-2.3 in 2004).
/mntTemporarily mounted filesystems.
/optAdd-on application software packages. [7]
/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.
/runRun-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) (appeared in FHS-3.0 in 2015).
/sbinEssential system binaries (e.g., fsck, init, route).
/srvSite-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).
/sysContains information about devices, drivers, and some kernel features. [8]
/tmp Directory for temporary files (see also /var/tmp). Often not preserved between system reboots and may be severely size-restricted.
/usrSecondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications. Should be shareable and read-only. [9] [10]
/usr/binNon-essential command binaries (not needed in single-user mode); for all users.
/usr/includeStandard include files.
/usr/lib Libraries for the binaries in /usr/bin and /usr/sbin.
/usr/libexecBinaries run by other programs that are not intended to be executed directly by users or shell scripts (optional).
/usr/lib<qual>Alternative-format libraries (e.g., /usr/lib32 for 32-bit libraries on a 64-bit machine (optional)).
/usr/localTertiary hierarchy for local data, specific to this host. Typically has further subdirectories (e.g., bin, lib, share). [NB 1]
/usr/sbinNon-essential system binaries (e.g., daemons for various network services).
/usr/shareArchitecture-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).
/varVariable 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/cacheApplication 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/libState information. Persistent data modified by programs as they run (e.g., databases, packaging system metadata, etc.).
/var/lockLock files. Files keeping track of resources currently in use.
/var/logLog files. Various logs.
/var/mailMailbox files. In some distributions, these files may be located in the deprecated /var/spool/mail.
/var/optVariable data from add-on packages that are stored in /opt.
/var/runRun-time variable data. This directory contains system information data describing the system since it was booted. [11]

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. [12]

/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. [13]
/var/tmpTemporary 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. [14] [15] [16] [17] GoboLinux [18] and NixOS [19] provide examples of intentionally non-compliant filesystem implementations.

Some distributions generally follow the standard but deviate from it in some areas. The FHS is a "trailing standard", and so documents common practices at a point in time. Of course, times change, and distribution goals and needs call for experimentation. Some 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 is not intended for such data. [24] 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: [25]

History

The name of usr

/usr originally stood for "user". [26] This was an artifact of early Unix programming. Specifically, when Ken Thompson and Dennis Ritchie were migrating Unix to a PDP-11, the contents of the /bin and /lib directories, which were to be the first directories mounted on startup and to contain all essentials for the OS to function, became too large to fit on an RK05 disk drive. So they put some of those files on a second RK05, making sure that the first drive contained everything required for loading the second one. The rest of the files were put into the /usr directory. [27] When they got a third drive, users' files were moved to a new directory named /home. [28]

FHS was created as the FSSTND (short for "Filesystem Standard" [29] ), largely based on similar standards for other Unix-like operating systems. Notable examples are these: the hier(7) description of file system layout, [30] which has existed since the release of Version 7 Unix (in 1979); [31] the SunOS filesystem(7) [32] and its successor, the Solaris filesystem(7). [33] [34]

Release history

VersionRelease dateNotes
Old version, no longer maintained: 1.01994-02-14FSSTND [35]
Old version, no longer maintained: 1.11994-10-09FSSTND [36]
Old version, no longer maintained: 1.21995-03-28FSSTND [37]
Old version, no longer maintained: 2.01997-10-26FHS 2.0 is the direct successor for FSSTND 1.2. Name of the standard was changed to Filesystem Hierarchy Standard. [38] [39] [40]
Old version, no longer maintained: 2.12000-04-12FHS [41] [42] [43]
Old version, no longer maintained: 2.22001-05-23FHS [44]
Old version, no longer maintained: 2.32004-01-29FHS [45]
Current stable version:3.02015-05-18FHS [46]
Legend:
Old version
Old version, still maintained
Latest version
Latest preview version
Future release

See also

Notes

  1. ↑ 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.

Related Research Articles

<span class="mw-page-title-main">GNU Hurd</span> Operating system kernel designed as a replacement for Unix

GNU Hurd is a collection of microkernel servers written as part of GNU, for the GNU Mach microkernel. It has been under development since 1990 by the GNU Project of the Free Software Foundation, designed as a replacement for the Unix kernel, and released as free software under the GNU General Public License. When the Linux kernel proved to be a viable solution, development of GNU Hurd slowed, at times alternating between stasis and renewed activity and interest.

fsck System tool for checking the consistency of a file system

The system utility fsck is a tool for checking the consistency of a file system in Unix and Unix-like operating systems, such as Linux, macOS, and FreeBSD. The equivalent programs on MS-DOS and Microsoft Windows are CHKDSK, SFC, and SCANDISK.

Almquist shell is a lightweight Unix shell originally written by Kenneth Almquist in the late 1980s. Initially a clone of the System V.4 variant of the Bourne shell, it replaced the original Bourne shell in the BSD versions of Unix released in the early 1990s.

In computing, a symbolic link is a file whose purpose is to point to a file or directory by specifying a path thereto.

<span class="mw-page-title-main">GoboLinux</span> Linux distribution

GoboLinux is a Linux distribution whose most prominent feature is a reorganization of the traditional Linux file system. Rather than following the Filesystem Hierarchy Standard like most Unix-like systems, each program in a GoboLinux system has its own subdirectory tree, where all of its files may be found. Thus, a program "Foo" has all of its specific files and libraries in /Programs/Foo, under the corresponding version of this program at hand. For example, the commonly known GCC compiler suite version 8.1.0, would reside under the directory /Programs/GCC/8.1.0.

A home directory is a file system directory on a multi-user operating system containing files for a given user of the system. The specifics of the home directory are defined by the operating system involved; for example, Linux / BSD (FHS) systems use /home/⟨username⟩ or /usr/home/⟨username⟩ and Windows systems since Windows Vista use \Users\⟨username⟩.

fortune is a program that displays a pseudorandom message from a database of quotations. Early versions of the program appeared in Version 7 Unix in 1979. The most common version on modern systems is the BSD fortune, originally written by Ken Arnold. Distributions of fortune are usually bundled with a collection of themed files, containing sayings like those found on fortune cookies, quotations from famous people, jokes, or poetry.

chroot Operation that changes the apparent root directory in Unix-like systems

chroot is an operation on Unix and Unix-like operating systems that changes the apparent root directory for the current running process and its children. A program that is run in such a modified environment cannot name files outside the designated directory tree. The term "chroot" may refer to the chroot(2) system call or the chroot(8) wrapper program. The modified environment is called a chroot jail.

df (Unix) Standard Unix command

df is a standard Unix command used to display the amount of available disk space for file systems on which the invoking user has appropriate read access. df is typically implemented using the statfs or statvfs system calls.

vmlinux Executable file containing the Linux kernel

vmlinux is a statically linked executable file that contains the Linux kernel in one of the object file formats supported by Linux, which includes Executable and Linkable Format (ELF) and Common Object File Format (COFF). The vmlinux file might be required for kernel debugging, symbol table generation or other operations, but must be made bootable before being used as an operating system kernel by adding a multiboot header, bootsector and setup routines.

file (command) Standard Unix program

The file command is a standard program of Unix and Unix-like operating systems for recognizing the type of data contained in a computer file.

The proc filesystem (procfs) is a special filesystem in Unix-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized method for dynamically accessing process data held in the kernel than traditional tracing methods or direct access to kernel memory. Typically, it is mapped to a mount point named /proc at boot time. The proc file system acts as an interface to internal data structures about running processes in the kernel. In Linux, it can also be used to obtain information about the kernel and to change certain kernel parameters at runtime (sysctl).

In computing, the sticky bit is a user ownership access right flag that can be assigned to files and directories on Unix-like systems.

mount (Unix) Unix command to mount a filesystem

In computing, mount is a command in various operating systems. Before a user can access a file on a Unix-like machine, the file system on the device which contains the file needs to be mounted with the mount command. Frequently mount is used for SD card, USB storage, DVD and other removable storage devices. The command is also available in the EFI shell.

In Unix-like operating systems, a device file, device node, or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow an application program to interact with a device by using its device driver via standard input/output system calls. Using standard system calls simplifies many programming tasks, and leads to consistent user-space I/O mechanisms regardless of device features and functions.

sar (Unix) Unix command to collect, report or save system activity information

System Activity Report (sar) is a Unix System V-derived system monitor command used to report on various system loads, including CPU activity, memory/paging, interrupts, device load, network and swap space utilization. Sar uses /proc filesystem for gathering information.

<span class="mw-page-title-main">Unix</span> Family of computer operating systems

Unix is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors including University of California, Berkeley (BSD), Microsoft (Xenix), Sun Microsystems (SunOS/Solaris), HP/HPE (HP-UX), and IBM (AIX).

<span class="mw-page-title-main">Unix filesystem</span> Directory structure used by a Unix-like operating system

In Unix and operating systems inspired by it, the file system is considered a central component of the operating system. It was also one of the first parts of the system to be designed and implemented by Ken Thompson in the first experimental version of Unix, dated 1969.

"Everything is a file" is an approach to interface design in Unix derivatives. While this turn of phrase does not as such figure as a Unix design principle or philosophy, it is a common way to analyse designs, and informs the design of new interfaces in a way that prefers, in rough order of import:

  1. representing objects as file descriptors in favour of alternatives like abstract handles or names,
  2. operating on the objects with standard input/output operations returning byte streams to be interpreted by applications, and
  3. allowing the usage or creation of objects by opening or creating files in the global filesystem name space.

doas is a program to execute commands as another user. The system administrator can configure it to give specified users privileges to execute specified commands. It is free and open-source under the ISC license and available in Unix and Unix-like operating systems.

References

  1. ↑ "FHS". The Linux Foundation Wiki. Retrieved 4 January 2022.
  2. ↑ "lsb:fhs-30 [Wiki]". wiki.linuxfoundation.org. Retrieved 22 February 2023.
  3. ↑ "hier(7) - Linux manual page". man7.org. Retrieved 6 January 2021.
  4. ↑ J. DeFelicc (17 March 1972). "E.0". Preliminary Release of UNIX Implementation Document (PDF). p. 8. IMO.1-1. Retrieved 6 June 2024.
  5. ↑ "/etc : Host-specific system configuration". Filesystem Hierarchy Standard 2.3. Retrieved 18 February 2016.
  6. ↑ Define - /etc?, Posted by Cliff, 3 March 2007 - Slashdot.
  7. ↑ "/opt : Add-on application software packages". Filesystem Hierarchy Standard 2.3. Retrieved 18 February 2016.
  8. ↑ "/sys : Kernel and system information virtual filesystem". Filesystem Hierarchy Standard 3.0. Retrieved 4 June 2017.
  9. ↑ "Chapter 4. The /usr Hierarchy". Filesystem Hierarchy Standard 2.3.
  10. ↑ "Chapter 4. The /usr Hierarchy, Section 4.1 Purpose". Filesystem Hierarchy Standard 3.0.
  11. ↑ "/var/run : Run-time variable data". Filesystem Hierarchy Standard 2.3.
  12. ↑ "5.13. /var/run : Run-time variable data". Filesystem Hierarchy Standard 3.0.
  13. ↑ "File System Standard" (PDF). Linux Foundation. p. 5.11.1.
  14. ↑ Red Hat reference guide on file system structure.
  15. ↑ 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.
  16. ↑ Debian policy on FHS compliance.
  17. ↑ Ubuntu Linux File system Tree Overview  – Community Ubuntu Documentation.
  18. ↑ Hisham Muhammad (9 May 2003). "The Unix tree rethought: an introduction to GoboLinux" . Retrieved 4 October 2016.
  19. ↑ Dolstra, E.; Löh, A. (September 2008). NixOS: A Purely Functional Linux Distribution (PDF). ICFP 2008: 13th ACM SIGPLAN International Conference on Functional Programming. Victoria, British Columbia, Canada. pp. 367–378.
  20. ↑ "5.3 About the /sys Virtual File System". docs.oracle.com. Oracle. Archived from the original on 8 July 2016. Retrieved 8 July 2016.
  21. ↑ Lehey, Greg (May 2003). The Complete FreeBSD: Documentation from the Source (Fourth ed.). O'Reilly Media, Incorporated. pp. 188, 609. ISBN   9780596005160.
  22. ↑ Allan McRae. "Arch Linux – News: The /lib directory becomes a symlink". archlinux.org. Archived from the original on 9 September 2014. Retrieved 14 December 2019.
  23. ↑ Allan McRae. "Arch Linux – News: Binaries move to /usr/bin requiring update intervention". archlinux.org. Archived from the original on 10 September 2014. Retrieved 15 December 2019.
  24. ↑ Lennart Poettering (30 March 2011). "What's this /run directory doing on my system and where does it come from?". devel@lists.fedoraproject.org (Mailing list).
  25. ↑ "ReleaseGoalsRunDirectory". Debian Wiki.
  26. ↑ "/usr". tldp.org. Retrieved 10 December 2023.
  27. ↑ M. D. McIlroy (1987). "A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986" (PDF). Bell Labs. CSTR 139.
  28. ↑ Landley, Rob (9 December 2010). "Understanding the bin, sbin, usr/bin , usr/sbin split". busybox (Mailing list).
  29. ↑ "FSSTND FAQ page" . Retrieved 10 May 2016.
  30. ↑ hier(7)  –  FreeBSD Miscellaneous Information Manual.
  31. ↑ hier(7)  –  Version 7 Unix Programmer's Manual
  32. ↑ SunOS 4.1.3 manual page for filesystem(7), dated 10 January 1988 (from the FreeBSD Man Pages library).
  33. ↑ filesystem(7)  –  Solaris 11.4 Standards, Environments, Macros, Character Sets, and Miscellany Reference Manual.
  34. ↑ "filesystem man page – Solaris 10 11/06 Man Pages". Archived from the original on 4 July 2007. Retrieved 15 October 2011.
  35. ↑ "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.0/". Ibiblio.org. Retrieved 16 October 2012.
  36. ↑ "Index of /pub/Linux/docs/fsstnd/old/fsstnd-1.1/". Ibiblio.org. Retrieved 16 October 2012.
  37. ↑ "Index of /pub/Linux/docs/fsstnd/old/". Ibiblio.org. Retrieved 16 October 2012.
  38. ↑ "FHS 2.0 Announcement". Pathname.com. Retrieved 16 October 2012.
  39. ↑ 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, archived from the original on 5 March 2016, retrieved 18 February 2016.
  40. ↑ "Index of /pub/Linux/docs/fsstnd/". Ibiblio.org. Retrieved 16 October 2012.
  41. ↑ "FHS 2.1 Announcement". Pathname.com. Retrieved 16 October 2012.
  42. ↑ "FHS 2.1 is released". Lists.debian.org. 13 April 2000. Retrieved 16 October 2012.
  43. ↑ 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. Archived from the original (PDF) on 27 March 2012. Retrieved 18 October 2012.
  44. ↑ 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.
  45. ↑ Russell, Rusty; Quinlan, Daniel; Yeoh, Christopher, eds. (28 January 2004). "Filesystem Hierarchy Standard - Filesystem Hierarchy Standard Group" (PDF). Retrieved 29 November 2014.
  46. ↑ Yeoh, Christopher; Russell, Rusty; Quinlan, Daniel, eds. (19 March 2015). "Filesystem Hierarchy Standard" (PDF). The Linux Foundation . Retrieved 20 May 2015.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.