| Abbreviation | FHS |
|---|---|
| Status | Published |
| Year started | 14 February 1994; 31 years ago (1994-02-14) |
| Latest version | 3.0 3 June 2015; 10 years ago (2015-06-03) |
| Organization | Linux Foundation |
| Domain | Directory structure |
| Website | Official website Official website (Historical) |
TheFilesystem Hierarchy Standard (FHS) is a reference describing the conventions used for the layout ofUnix-like systems. It has been made popular by its use inLinux distributions, but it is used by other Unix-like systems as well.[1] It is maintained by theLinux Foundation. The latest version is 3.0, released on 3 June 2015.[2]
In the FHS, all files anddirectories appear under theroot 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 theX Window System, are installed.
Most of these directories exist in allUnix-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 androot directory of the entire file system hierarchy. |
/bin | Essential commandbinaries that need to be available insingle-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). |
/etc | Host-specific system-wideconfiguration files. There has been controversy over the meaning of the name itself. In early versions of the UNIX Implementation Document from Bell Labs, |
/etc/opt | Configuration files for add-on packages stored in/opt. |
/etc/sgml | Configuration files, such as catalogs, for software that processesSGML. |
/etc/X11 | Configuration files for theX Window System, version 11. |
/etc/xml | Configuration files, such as catalogs, for software that processesXML. |
/home | Users'home directories, containing saved files, personal settings, etc. |
/lib | Libraries essential for thebinaries 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 supporting32-bit and64-bit versions of aninstruction set. Such directories are optional, but if they exist, they have some requirements. |
/media | Mount points forremovable media such asCD-ROMs (appeared in FHS-2.3 in 2004). |
/mnt | Temporarilymounted filesystems. |
/opt | Add-onapplication softwarepackages.[7] |
/proc | Virtualfilesystem providingprocess andkernel information as files. In Linux, corresponds to aprocfs mount. Generally, automatically generated and populated by the system, on the fly. |
/root | Home directory for theroot user. |
/run | Run-time variable data: Information about the running system since last boot, e.g., currently logged-in users and runningdaemons. 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 atemporary 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 byFTP servers, and repositories forversion 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] |
/usr/bin | Non-essential commandbinaries (not needed insingle-user mode); for all users. |
/usr/include | Standardinclude files. |
/usr/lib | Libraries for thebinaries in/usr/bin and/usr/sbin. |
/usr/libexec | Binaries 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/local | Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories (e.g.,bin,lib,share).[NB 1] |
/usr/sbin | Non-essential system binaries (e.g.,daemons for variousnetwork 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.[11] In FHS 3.0, |
/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/tmp | Temporary files to be preserved between reboots. |
MostLinux distributions follow the Filesystem Hierarchy Standard and declare it their own policy to maintain FHS compliance.[14][15][16][17]GoboLinux[18] andNixOS[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:
/sys directory as avirtual filesystem (sysfs, comparable to/proc, which is aprocfs), which stores and allows modification of the devices connected to the system,[20] whereas many traditionalUnix-like operating systems use/sys as asymbolic link to thekernel source tree.[21]/usr/local, while keeping code considered part of the operating system in/usr./lib and/usr/lib and have/lib symlinked to/usr/lib.[22]/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 atemporary 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 changesDebian 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/tmpFHS was created as the FSSTND (short for "Filesystem Standard"[26]), largely based on similar standards for otherUnix-like operating systems. Notable examples are these: thehier(7) description of file system layout,[27] which has existed since the release ofVersion 7 Unix (in 1979);[28] theSunOSfilesystem(7)[29] and its successor, theSolarisfilesystem(7).[30][31]
usr/usr originally stood for "user".[32] This was an artifact of early Unix programming. Specifically, whenKen Thompson andDennis 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 anRK05 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.[33] When they got a third drive, users' files were moved to a new directory named/home.[34]
| Version | Release date | Notes |
|---|---|---|
| Unsupported: 1.0 | 1994-02-14 | FSSTND[35] |
| Unsupported: 1.1 | 1994-10-09 | FSSTND[36] |
| Unsupported: 1.2 | 1995-03-28 | FSSTND[37] |
| Unsupported: 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.[38][39][40] |
| Unsupported: 2.1 | 2000-04-12 | FHS[41][42][43] |
| Unsupported: 2.2 | 2001-05-23 | FHS[44] |
| Unsupported: 2.3 | 2004-01-29 | FHS[45] |
| Latest version:3.0 | 2015-05-18 | FHS[46] |
Legend: Unsupported Supported Latest version Preview version Future version | ||
/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 arenot part of the standard operating system distribution (in such case,/usr would only contain software/data thatare 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.hier(7) – FreeBSD Miscellaneous InformationManual.hier(7) – Version 7 Unix Programmer'sManualfilesystem(7) – Solaris 11.4 Standards, Environments, Macros, Character Sets, and Miscellany ReferenceManual.hier(7) – Linux Programmer'sManual – Overview, Conventions and Miscellanea| Linux kernel | |
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