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]
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.
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, including to bring up the system or repair it,[3] for all users (e.g., cat, ls, cp).
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/opt
Configuration files for add-on packages stored in /opt.
/etc/sgml
Configuration files, such as catalogs, for software that processes SGML.
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.
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.
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) (appeared in FHS-3.0 in 2015).
/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.[8]
/tmp
Directory for 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. Should be shareable and read-only.[9][10]
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.[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).
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 /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,[20] whereas many traditional Unix-like operating systems use /sys as a symbolic link to the kernel source tree.[21]
Many modern Unix-like systems (such as FreeBSD and OpenBSD) via their ports systems 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 and /usr/lib and have /lib symlinked to /usr/lib.[22]
Some Linux distributions no longer differentiate between /bin and /usr/bin and between /sbin and /usr/sbin. They may symlink /bin to /usr/bin and /sbin to /usr/sbin. Other distributions choose to consolidate all four, symlinking them to /usr/bin.[23]
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]
/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
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 /bin, /lib... directories, which were to be the first directories to be mounted on startup, and must contain all essentials for the OS to function, became bigger than the RK05 disk drive. So they put some of those in the first RK05, including those that are required for loading the second RK05 drive. The rest 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 SunOSfilesystem(7)[32] and its successor, the Solarisfilesystem(7).[33][34]
^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.
References
^"FHS". The Linux Foundation Wiki. Retrieved 4 January 2022.
^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.
