The Filesystem Hierarchy Standard (FHS) is a reference describing the conventions used for the layout of a UNIX system. It has been made popular by its use in Linux distributions, but it is used by other UNIX variants as well. It is maintained by the
Linux Foundation. The latest version is 3.0, released on 3 June 2015.
Directory structure
In the FHS, all files and
directories appear under the
root directory
In a computer file system, and primarily used in the Unix and Unix-like operating systems, the root directory is the first or top-most directory in a hierarchy. It can be likened to the trunk of a tree, as the starting point where all branche ...
/
, 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
The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems.
X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting wi ...
, are installed.
Most of these directories exist in all
Unix-like
A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
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.
FHS compliance
Most
Linux distributions follow the Filesystem Hierarchy Standard and declare it their own policy to maintain FHS compliance.
GoboLinux
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 h ...
and
NixOS 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, whereas many traditional
Unix-like
A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
operating systems use
/sys
as a
symbolic link to the
kernel source tree.
* Many modern Unix-like systems (like
FreeBSD
FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
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
and
/usr/lib
and have
/lib
symlinked to
/usr/lib
.
* 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
.
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. 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:
*
/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"), largely based on similar standards for other
Unix-like
A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
operating systems. Notable examples are these: the description of file system layout, which has existed since the release of
Version 7 Unix (in 1979); the
SunOS
SunOS is a Unix-branded operating system developed by Sun Microsystems for their workstation and server computer systems. The ''SunOS'' name is usually only used to refer to versions 1.0 to 4.1.4, which were based on BSD, while versions 5.0 an ...
and its successor, the
Solaris .
Release history
See also
*
Unix directory structure
*
XDG Base Directory Specification
Notes
References
External links
*
*
Full specification textsobjectroot– a proposal for a new filesystem hierarchy, based on object-oriented design principles
* The Dotted Standard Filename Hierarchy, yet another very different hierarchy (used in cLIeNUX)
*
{{Linux
Computer standards
Linux
System administration
Unix file system technology