Filesystem Hierarchy Standard

Filesystem Hierarchy Standard

Developed by	Linux Foundation
Initial release	14 February 1994 (1994-02-14)
Latest release	2.3 / 29 January 2004 (2004-01-29)
Website	www.linuxfoundation.org/collaborate/workgroups/lsb/fhs www.pathname.com/fhs/ (Historical)

The Filesystem Hierarchy Standard (FHS) defines the directory structure and directory contents in Linux operating systems. For the most part, it is a formalization and extension of the traditional BSD filesystem hierarchy.

The FHS is maintained by the Linux Foundation. The current version is 2.3, announced on 29 January 2004.

FHS compliance

Most Linux distributions follow the FHS and declare it their own policy to maintain FHS compliance.

Some distributions which 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. According to the FHS 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 such trickery as using /dev/.udev, /dev/.mdadm, /dev/.systemd or /dev/.mount directories, even though the device directory isn't intended for such data.
• Some Linux distributions no longer differentiate between /lib versus /usr/lib and have /lib symlinked to /usr/lib.
• 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.

History

When the FHS was created, other UNIX and Unix-like operating systems already had their own standards, notably the hier(7) description of file system layout that has existed since the release of Version 7 Unix (in 1979), or the SunOS filesystem(7), later Solaris filesystem(5).

Release history

	Meaning
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
v1.1	1994-10-09	FSSTND
v1.2	1995-03-28	FSSTND
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.
v2.1	2000-04-12	FHS
v2.2	2001-05-23	FHS
v2.3	2004-01-29	FHS
v3.0	under development	FHS

Directory structure

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.
/bin	Essential command binaries that need to be available in single user mode; for all users, e.g., cat, ls, cp.
/boot	Boot loader files, e.g., kernels, initrd.
/dev	Essential devices, e.g., /dev/null.
/etc	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, 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). Since the publication of early documentation, the directory name has been re-designated in various ways. Recent interpretations include backronyms such as "Editable Text Configuration" or "Extended Tool Chest".
/etc/opt	Configuration files for /opt/.
/etc/sgml	Configuration files for SGML.
/etc/X11	Configuration files for the X Window System, version 11.
/etc/xml	Configuration files for XML.
/home	Users' home directories, containing saved files, personal settings, etc.
/lib	Libraries essential for the binaries in /bin/ and /sbin/.
/media	Mount points for removable media such as CD-ROMs (appeared in FHS-2.3).
/mnt	Temporarily mounted filesystems.
/opt	Optional application software packages.
/proc	Virtual filesystem providing information about processes and kernel information as files. In Linux, corresponds to a procfs mount.
/root	Home directory for the root user.
/sbin	Essential system binaries, e.g., init, ip, mount.
/srv	Site-specific data which are served by the system.
/tmp	Temporary files (see also /var/tmp). Often not preserved between system reboots.
/usr	Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications.
/usr/bin	Non-essential command binaries (not needed in single user mode); for all users.
/usr/include	Standard include files.
/usr/lib	Libraries for the binaries in /usr/bin/ and /usr/sbin/.
/usr/local	Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g., bin/, lib/, share/.
/usr/sbin	Non-essential system binaries, e.g., daemons for various network-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.
/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	Users' mailboxes.
/var/run	Information about the running system since last boot, e.g., currently logged-in users and running daemons.
/var/spool	Spool for tasks waiting to be processed, e.g., print queues and unread mail.
/var/spool/mail	Deprecated location for users' mailboxes.
/var/tmp	Temporary files to be preserved between reboots.

See also

• Unix directory structure

External links

• Official Home of the Filesystem Hierarchy Standard (FHS), The Linux Foundation
  • New homepage for specs
• objectroot – a proposal for a new filesystem hierarchy, based on object-oriented design principles
• Fortune Cookies through the /proc Filesystem
• The Dotted Standard Filename Hierarchy, yet another very different hierarchy (used in cLIeNUX) (mirror)
• Linux Filesystem Hierarchy Version 0.65