Filesystem Hierarchy Standard

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.^[1]

FHS compliance

Most Linux distributions follow the FHS and declare it their own policy to maintain FHS compliance.^[2][3][4][5]

Deviations:

• 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. ^[6]
• 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.
• Some Linux systems such as GoboLinux and Syllable Server use a completely different approach from the FHS.^{[clarification needed]}

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^[7] that has existed since the release of Version 7 Unix (in 1979), or the SunOS filesystem(7),^[8] later Solaris filesystem(5).^[9][10]

The process of developing a standard filesystem hierarchy began in August 1993 with an effort to restructure the file and directory structure of Linux-based operating systems. The FSSTND (Filesystem Standard), a filesystem hierarchy standard specific to the Linux operating system, was released on 14 February 1994. Subsequent revisions were released on 9 October 1994 and 28 March 1995.^[11]

In early 1996, the goal of developing a more comprehensive version of FSSTND to address not only Linux, but other Unix-like systems was adopted with the help of members of the BSD development community. As a result, a concerted effort was made to focus on issues that were general to Unix-like systems. In recognition of this widening of scope, the name of the standard was changed to Filesystem Hierarchy Standard.

Release history

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.

See also

• Unix directory structure

References

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)