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Subsections

35.1 Introduction
- 35.1.1 Purpose
- 35.1.2 Conventions
35.2 The Filesystem
35.3 The Root Filesystem
35.4 The /usr Hierarchy
35.5 The /var Hierarchy
35.6 Operating System Specific Annex
- 35.6.1 Linux
35.7 Appendix

35. The LINUX File System Standard

This chapter reproduces the Filesystem Hierarchy Standard, translated into L^ATEX with some minor formatting changes and the addition of this book's chapter number to all the section headers. An original can be obtained from the FHS home page <http://www.pathname.com/fhs/>.

If you have ever asked the questions ``Where in my file system does file xxx go?'' or ``What is directory yyy for?'', then consult this document. It can be considered to provide the final word on such matters. Although this is mostly a reference for people creating new LINUX distributions, all administrators can benefit from an understanding of the rulings and explanations provided here.

Filesystem Hierarchy Standard -- Version 2.2 final

Filesystem Hierarchy Standard Group

edited by Rusty Russell and Daniel Quinlan

ABSTRACT

This standard consists of a set of requirements and guidelines for file and directory placement under UNIX-like operating systems. The guidelines are intended to support interoperability of applications, system administration tools, development tools, and scripts as well as greater uniformity of documentation for these systems.

May 23, 2001

All trademarks and copyrights are owned by their owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark.

Permission is granted to make and distribute verbatim copies of this standard provided the copyright and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this standard under the conditions for verbatim copying, provided also that the title page is labeled as modified including a reference to the original standard, provided that information on retrieving the original standard is included, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this standard into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the copyright holder.

35.1 Introduction

35.1.1 Purpose

This standard enables

Software to predict the location of installed files and directories, and
Users to predict the location of installed files and directories.

We do this by

Specifying guiding principles for each area of the filesystem,
Specifying the minimum files and directories required,
Enumerating exceptions to the principles, and
Enumerating specific cases where there has been historical conflict.

The FHS document is used by

Independent software suppliers to create applications which are FHS compliant, and work with distributions which are FHS complaint,
OS creators to provide systems which are FHS compliant, and
Users to understand and maintain the FHS compliance of a system.

35.1.2 Conventions

A constant-width font is used for displaying the names of files and directories.

Components of filenames that vary are represented by a description of the contents enclosed in "<" and ">" characters, <thus>. Electronic mail addresses are also enclosed in "<" and ">" but are shown in the usual typeface.

Optional components of filenames are enclosed in "[" and "]" characters and may be combined with the "<" and ">" convention. For example, if a filename is allowed to occur either with or without an extension, it might be represented by <filename>[.<extension>].

Variable substrings of directory names and filenames are indicated by "*".

35.2 The Filesystem

This standard assumes that the operating system underlying an FHS-compliant file system supports the same basic security features found in most UNIX filesystems.

It is possible to define two independent categories of files: shareable vs. unshareable and variable vs. static. There should be a simple and easily understandable mapping from directories to the type of data they contain: directories may be mount points for other filesystems with different characteristics from the filesystem on which they are mounted.

Shareable data is that which can be shared between several different hosts; unshareable is that which must be specific to a particular host. For example, user home directories are shareable data, but device lock files are not.

Static data includes binaries, libraries, documentation, and anything that does not change without system administrator intervention; variable data is anything else that does change without system administrator intervention.

BEGIN RATIONALE
The distinction between shareable and unshareable data is needed for several reasons:
- In a networked environment (i.e., more than one host at a site), there is a good deal of data that can be shared between different hosts to save space and ease the task of maintenance.
- In a networked environment, certain files contain information specific to a single host. Therefore these filesystems cannot be shared (without taking special measures).
- Historical implementations of UNIX-like filesystems interspersed shareable and unshareable data in the same hierarchy, making it difficult to share large portions of the filesystem.
The "shareable" distinction can be used to support, for example:
- A /usr partition (or components of /usr) mounted (read-only) through the network (using NFS).
- A /usr partition (or components of /usr) mounted from read-only media. A CD-ROM is one copy of many identical ones distributed to other users by the postal mail system and other methods. It can thus be regarded as a read-only filesystem shared with other FHS-compliant systems by some kind of "network".
The "static" versus "variable" distinction affects the filesystem in two major ways:
- Since / contains both variable and static data, it needs to be mounted read-write.
- Since the traditional /usr contains both variable and static data, and since we may want to mount it read-only (see above), it is necessary to provide a method to have /usr mounted read-only. This is done through the creation of a /var hierarchy that is mounted read-write (or is a part of another read-write partition, such as /), taking over much of the /usr partition's traditional functionality.
Here is a summarizing chart. This chart is only an example for a common FHS-compliant system, other chart layouts are possible within FHS-compliance.
$\begin{longtable}[c]{\vert l \vert l \vert l \vert} \hline & shareable & unshar... .../var/mail & /var/run\\ & /var/spool/news & /var/lock\\ \hline \end{longtable}$
END RATIONALE

35.3 The Root Filesystem

35.3.1 Purpose

The contents of the root filesystem must be adequate to boot, restore, recover, and/or repair the system.

To boot a system, enough must be present on the root partition to mount other filesystems. This includes utilities, configuration, boot loader information, and other essential start-up data. /usr, /opt, and /var are designed such that they may be located on other partitions or filesystems.
To enable recovery and/or repair of a system, those utilities needed by an experienced maintainer to diagnose and reconstruct a damaged system must be present on the root filesystem.
To restore a system, those utilities needed to restore from system backups (on floppy, tape, etc.) must be present on the root filesystem.

BEGIN RATIONALE
The primary concern used to balance these considerations, which favor placing many things on the root filesystem, is the goal of keeping root as small as reasonably possible. For several reasons, it is desirable to keep the root filesystem small:
- It is occasionally mounted from very small media.
- The root filesystem contains many system-specific configuration files. Possible examples include a kernel that is specific to the system, a specific hostname, etc. This means that the root filesystem isn't always shareable between networked systems. Keeping it small on servers in networked systems minimizes the amount of lost space for areas of unshareable files. It also allows workstations with smaller local hard drives.
- While you may have the root filesystem on a large partition, and may be able to fill it to your heart's content, there will be people with smaller partitions. If you have more files installed, you may find incompatibilities with other systems using root filesystems on smaller partitions. If you are a developer then you may be turning your assumption into a problem for a large number of users.
- Disk errors that corrupt data on the root filesystem are a greater problem than errors on any other partition. A small root filesystem is less prone to corruption as the result of a system crash.
Software must never create or require special files or subdirectories in the root directory. Other locations in the FHS hierarchy provide more than enough flexibility for any package.
There are several reasons why introducing a new subdirectory of the root filesystem is prohibited:
- It demands space on a root partition which the system administrator may want kept small and simple for either performance or security reasons.
- It evades whatever discipline the system administrator may have set up for distributing standard file hierarchies across mountable volumes.
END RATIONALE

35.3.2 Requirements

The following directories, or symbolic links to directories, are required in /.

$\begin{longtable}[l]{r l l} \par {\tt {}/} & \multicolumn{2}{l}{--------- the ro... ...rarchy \\ ~ & \speciallcorner{}{\tt {}~var} & Variable data \\ \end{longtable}$

Each directory listed above is specified in detail in separate subsections below. /usr and /var each have a complete section in this document due to the complexity of those directories.

35.3.3 Specific Options

The following directories, or symbolic links to directories, must be in /, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/} & \multicolumn{2}{l}{--------- the ro... ...}{\tt {}~root} & Home directory for the root user (optional) \\ \end{longtable}$

Each directory listed above is specified in detail in separate subsections below.

35.3.4 /bin : Essential user command binaries (for use by all users)

35.3.4.1 Purpose

/bin contains commands that may be used by both the system administrator and by users, but which are required when no other filesystems are mounted (e.g. in single user mode). It may also contain commands which are used indirectly by scripts.(footnote follows) [Command binaries that are not essential enough to place into /bin must be placed in /usr/bin, instead. Items that are required only by non-root users (the X Window System, chsh, etc.) are generally not essential enough to be placed into the root partition. ]

35.3.4.2 Requirements

There must be no subdirectories in /bin.

The following commands, or symbolic links to commands, are required in /bin.

$\begin{longtable}[l]{l l} {\tt {}cat} & Utility to concatenate files to standard... ...tems \\ {\tt {}uname} & Utility to print system information \\ \end{longtable}$

If /bin/sh is not a true Bourne shell, it must be a hard or symbolic link to the real shell command.

The [ and test commands must be placed together in either /bin or /usr/bin.

BEGIN RATIONALE
For example bash behaves differently when called as sh or bash. The use of a symbolic link also allows users to easily see that /bin/sh is not a true Bourne shell.
The requirement for the [ and test commands to be included as binaries (even if implemented internally by the shell) is shared with the POSIX.2 standard.
END RATIONALE

35.3.4.3 Specific Options

The following programs, or symbolic links to programs, must be in /bin if the corresponding subsystem is installed:

$\begin{longtable}[l]{l l} {\tt {}csh} & The C shell (optional) \\ {\tt {}ed} & ... ... \\ {\tt {}ping} & The ICMP network test utility (optional) \\ \end{longtable}$

If the gunzip and zcat programs exist, they must be symbolic or hard links to gzip. /bin/csh may be a symbolic link to /bin/tcsh or /usr/bin/tcsh.

BEGIN RATIONALE
The tar, gzip and cpio commands have been added to make restoration of a system possible (provided that / is intact).
Conversely, if no restoration from the root partition is ever expected, then these binaries might be omitted (e.g., a ROM chip root, mounting /usr through NFS). If restoration of a system is planned through the network, then ftp or tftp (along with everything necessary to get an ftp connection) must be available on the root partition.
END RATIONALE

35.3.5 /boot : Static files of the boot loader

35.3.5.1 Purpose

This directory contains everything required for the boot process except configuration files and the map installer. Thus /boot stores data that is used before the kernel begins executing user-mode programs. This may include saved master boot sectors, sector map files, and other data that is not directly edited by hand.(footnote follows) [Programs necessary to arrange for the boot loader to be able to boot a file must be placed in /sbin. Configuration files for boot loaders must be placed in /etc. ]

35.3.5.2 Specific Options

The operating system kernel must be located in either / or /boot.(footnote follows) [On some i386 machines, it may be necessary for /boot to be located on a separate partition located completely below cylinder 1024 of the boot device due to hardware constraints.

Certain MIPS systems require a /boot partition that is a mounted MS-DOS filesystem or whatever other filesystem type is accessible for the firmware. This may result in restrictions with respect to usable filenames within /boot (only for affected systems). ]

35.3.6 /dev : Device files

35.3.6.1 Purpose

The /dev directory is the location of special or device files.

35.3.6.2 Specific Options

If it is possible that devices in /dev will need to be manually created, /dev must contain a command named MAKEDEV, which can create devices as needed. It may also contain a MAKEDEV.local for any local devices.

If required, MAKEDEV must have provisions for creating any device that may be found on the system, not just those that a particular implementation installs.

35.3.7 /etc : Host-specific system configuration

35.3.7.1 Purpose

/etc contains configuration files and directories that are specific to the current system.(footnote follows) [The setup of command scripts invoked at boot time may resemble System V, BSD or other models. Further specification in this area may be added to a future version of this standard. ]

35.3.7.2 Requirements

No binaries may be located under /etc.

The following directories, or symbolic links to directories are required in /etc:

$\begin{longtable}[l]{r l l} \par {\tt {}/etc} & \multicolumn{2}{l}{--------- Hos... ... ~ & \speciallcorner{}{\tt {}~opt} & Configuration for /opt \\ \end{longtable}$

35.3.7.3 Specific Options

The following directories, or symbolic links to directories must be in /etc, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/etc} & \multicolumn{2}{l}{--------- Hos... ...r{}{\tt {}~sgml} & Configuration for SGML and XML (optional) \\ \end{longtable}$

The following files, or symbolic links to files, must be in /etc if the corresponding subsystem is installed:(footnote follows)

$\begin{longtable}[l]{l l} {\tt {}csh.login} & Systemwide initialization file for... ...t {}syslog.conf} & Configuration file for syslogd (optional) \\ \end{longtable}$

[Systems that use the shadow password suite will have additional configuration files in /etc (/etc/shadow and others) and programs in /usr/sbin (useradd, usermod, and others). ]

mtab does not fit the static nature of /etc: it is excepted for historical reasons.(footnote follows) [On some Linux systems, this may be a symbolic link to /proc/mounts, in which case this exception is not required. ]

35.3.7.4 /etc/opt : Configuration files for /opt

35.3.7.4.1 Purpose

Host-specific configuration files for add-on application software packages must be installed within the directory /etc/opt/<package>, where <package> is the name of the subtree in /opt where the static data from that package is stored.

35.3.7.4.2 Requirements

No structure is imposed on the internal arrangement of /etc/opt/<package>.

If a configuration file must reside in a different location in order for the package or system to function properly, it may be placed in a location other than /etc/opt/<package>.

BEGIN RATIONALE
Refer to the rationale for /opt.
END RATIONALE

35.3.7.5 /etc/X11 : Configuration for the X Window System (optional)

35.3.7.5.1 Purpose

/etc/X11 is the location for all X11 host-specific configuration. This directory is necessary to allow local control if /usr is mounted read only.

35.3.7.5.2 Specific Options

The following files, or symbolic links to files, must be in /etc/X11 if the corresponding subsystem is installed:

$\begin{longtable}[l]{l l} {\tt {}Xconfig} & The configuration file for early ver... ...}Xmodmap} & Global X11 keyboard modification file (optional) \\ \end{longtable}$

Subdirectories of /etc/X11 may include those for xdm and for any other programs (some window managers, for example) that need them.(footnote follows) [/etc/X11/xdm holds the configuration files for xdm. These are most of the files previously found in /usr/lib/X11/xdm. Some local variable data for xdm is stored in /var/lib/xdm. ]We recommend that window managers with only one configuration file which is a default .*wmrc file must name it system.*wmrc (unless there is a widely-accepted alternative name) and not use a subdirectory. Any window manager subdirectories must be identically named to the actual window manager binary.

35.3.7.6 /etc/sgml : Configuration files for SGML and XML (optional)

35.3.7.6.1 Purpose

Generic configuration files defining high-level parameters of the SGML or XML systems are installed here. Files with names *.conf indicate generic configuration files. File with names *.cat are the DTD-specific centralized catalogs, containing references to all other catalogs needed to use the given DTD. The super catalog file catalog references all the centralized catalogs.

35.3.8 /home : User home directories (optional)

35.3.8.1 Purpose

/home is a fairly standard concept, but it is clearly a site-specific filesystem.(footnote follows) [Different people prefer to place user accounts in a variety of places. This section describes only a suggested placement for user home directories; nevertheless we recommend that all FHS-compliant distributions use this as the default location for home directories.

On small systems, each user's directory is typically one of the many subdirectories of /home such as /home/smith, /home/torvalds, /home/operator, etc. On large systems (especially when the /home directories are shared amongst many hosts using NFS) it is useful to subdivide user home directories. Subdivision may be accomplished by using subdirectories such as /home/staff, /home/guests, /home/students, etc. ]The setup will differ from host to host. Therefore, no program should rely on this location.(footnote follows) [If you want to find out a user's home directory, you should use the getpwent(3) library function rather than relying on /etc/passwd because user information may be stored remotely using systems such as NIS. ]

35.3.9 /lib : Essential shared libraries and kernel modules

35.3.9.1 Purpose

The /lib directory contains those shared library images needed to boot the system and run the commands in the root filesystem, ie. by binaries in /bin and /sbin.(footnote follows) [Shared libraries that are only necessary for binaries in /usr (such as any X Window binaries) must not be in /lib. Only the shared libraries required to run binaries in /bin and /sbin may be here. In particular, the library libm.so.* may also be placed in /usr/lib if it is not required by anything in /bin or /sbin. ]

35.3.9.2 Requirements

At least one of each of the following filename patterns are required (they may be files, or symbolic links):

$\begin{longtable}[l]{l l} {\tt {}libc.so.*} & The dynamically-linked C library (... ...\ {\tt {}ld*} & The execution time linker/loader (optional) \\ \end{longtable}$

If a C preprocessor is installed, /lib/cpp must be a reference to it, for historical reasons.(footnote follows) [The usual placement of this binary is /usr/lib/gcc-lib/<target>/<version>/cpp. /lib/cpp can either point at this binary, or at any other reference to this binary which exists in the filesystem. (For example, /usr/bin/cpp is also often used.) ]

35.3.9.3 Specific Options

The following directories, or symbolic links to directories, must be in /lib, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/lib} & \multicolumn{2}{l}{--------- ess... ...orner{}{\tt {}~modules} & Loadable kernel modules (optional) \\ \end{longtable}$

35.3.10 /lib<qual> : Alternate format essential shared libraries (optional)

35.3.10.1 Purpose

There may be one or more variants of the /lib directory on systems which support more than one binary format requiring separate libraries.(footnote follows) [This is commonly used for 64-bit or 32-bit support on systems which support multiple binary formats, but require libraries of the same name. In this case, /lib32 and /lib64 might be the library directories, and /lib a symlink to one of them. ]

35.3.10.2 Requirements

If one or more of these directories exist, the requirements for their contents are the same as the normal /lib directory, except that /lib<qual>/cpp is not required.(footnote follows) [/lib<qual>/cpp is still permitted: this allows the case where /lib and /lib<qual> are the same (one is a symbolic link to the other). ]

35.3.11 /mnt : Mount point for a temporarily mounted filesystem

35.3.11.1 Purpose

This directory is provided so that the system administrator may temporarily mount a filesystem as needed. The content of this directory is a local issue and should not affect the manner in which any program is run.

This directory must not be used by installation programs: a suitable temporary directory not in use by the system must be used instead.

35.3.12 /opt : Add-on application software packages

35.3.12.1 Purpose

/opt is reserved for the installation of add-on application software packages.

A package to be installed in /opt must locate its static files in a separate /opt/<package> directory tree, where <package> is a name that describes the software package.

35.3.12.2 Requirements

$\begin{longtable}[l]{r l l} \par {\tt {}/opt} & \multicolumn{2}{l}{--------- Add... ...}~\textless{}package\textgreater{}} & Static package objects \\ \end{longtable}$

The directories /opt/bin, /opt/doc, /opt/include, /opt/info, /opt/lib, and /opt/man are reserved for local system administrator use. Packages may provide "front-end" files intended to be placed in (by linking or copying) these reserved directories by the local system administrator, but must function normally in the absence of these reserved directories.

Programs to be invoked by users must be located in the directory /opt/<package>/bin. If the package includes UNIX manual pages, they must be located in /opt/<package>/man and the same substructure as /usr/share/man must be used.

Package files that are variable (change in normal operation) must be installed in /var/opt. See the section on /var/opt for more information.

Host-specific configuration files must be installed in /etc/opt. See the section on /etc for more information.

No other package files may exist outside the /opt, /var/opt, and /etc/opt hierarchies except for those package files that must reside in specific locations within the filesystem tree in order to function properly. For example, device lock files must be placed in /var/lock and devices must be located in /dev.

Distributions may install software in /opt, but must not modify or delete software installed by the local system administrator without the assent of the local system administrator.

BEGIN RATIONALE
The use of /opt for add-on software is a well-established practice in the UNIX community. The System V Application Binary Interface [AT&T 1990], based on the System V Interface Definition (Third Edition), provides for an /opt structure very similar to the one defined here.
The Intel Binary Compatibility Standard v. 2 (iBCS2) also provides a similar structure for /opt.
Generally, all data required to support a package on a system must be present within /opt/<package>, including files intended to be copied into /etc/opt/<package> and /var/opt/<package> as well as reserved directories in /opt.
The minor restrictions on distributions using /opt are necessary because conflicts are possible between distribution-installed and locally-installed software, especially in the case of fixed pathnames found in some binary software.
END RATIONALE

35.3.13 /root : Home directory for the root user (optional)

35.3.13.1 Purpose

The root account's home directory may be determined by developer or local preference, but this is the recommended default location.(footnote follows) [If the home directory of the root account is not stored on the root partition it will be necessary to make certain it will default to / if it can not be located.

We recommend against using the root account for tasks that can be performed as an unprivileged user, and that it be used solely for system administration. For this reason, we recommend that subdirectories for mail and other applications not appear in the root account's home directory, and that mail for administration roles such as root, postmaster, and webmaster be forwarded to an appropriate user. ]

35.3.14 /sbin : System binaries

35.3.14.1 Purpose

Utilities used for system administration (and other root-only commands) are stored in /sbin, /usr/sbin, and /usr/local/sbin. /sbin contains binaries essential for booting, restoring, recovering, and/or repairing the system in addition to the binaries in /bin.(footnote follows) [Originally, /sbin binaries were kept in /etc. ]Programs executed after /usr is known to be mounted (when there are no problems) are generally placed into /usr/sbin. Locally-installed system administration programs should be placed into /usr/local/sbin.(footnote follows) [Deciding what things go into "sbin" directories is simple: if a normal (not a system administrator) user will ever run it directly, then it must be placed in one of the "bin" directories. Ordinary users should not have to place any of the sbin directories in their path.

For example, files such as chfn which users only occasionally use must still be placed in /usr/bin. ping, although it is absolutely necessary for root (network recovery and diagnosis) is often used by users and must live in /bin for that reason.

We recommend that users have read and execute permission for everything in /sbin except, perhaps, certain setuid and setgid programs. The division between /bin and /sbin was not created for security reasons or to prevent users from seeing the operating system, but to provide a good partition between binaries that everyone uses and ones that are primarily used for administration tasks. There is no inherent security advantage in making /sbin off-limits for users. ]

35.3.14.2 Requirements

The following commands, or symbolic links to commands, are required in /sbin.

$\begin{longtable}[l]{l l} {\tt {}shutdown} & Command to bring the system down. \\ \end{longtable}$

35.3.14.3 Specific Options

The following files, or symbolic links to files, must be in /sbin if the corresponding subsystem is installed:

$\begin{longtable}[l]{l l} {\tt {}fastboot} & Reboot the system without checking ... ...& Daemon to periodically flush filesystem buffers (optional) \\ \end{longtable}$

35.3.15 /tmp : Temporary files

35.3.15.1 Purpose

The /tmp directory must be made available for programs that require temporary files.

Programs must not assume that any files or directories in /tmp are preserved between invocations of the program.

BEGIN RATIONALE
IEEE standard P1003.2 (POSIX, part 2) makes requirements that are similar to the above section.
Although data stored in /tmp may be deleted in a site-specific manner, it is recommended that files and directories located in /tmp be deleted whenever the system is booted.
FHS added this recommendation on the basis of historical precedent and common practice, but did not make it a requirement because system administration is not within the scope of this standard.
END RATIONALE

35.4 The /usr Hierarchy

35.4.1 Purpose

/usr is the second major section of the filesystem. /usr is shareable, read-only data. That means that /usr should be shareable between various FHS-compliant hosts and must not be written to. Any information that is host-specific or varies with time is stored elsewhere.

Large software packages must not use a direct subdirectory under the /usr hierarchy.

35.4.2 Requirements

The following directories, or symbolic links to directories, are required in /usr.

$\begin{longtable}[l]{r l l} \par {\tt {}/usr} & \multicolumn{2}{l}{--------- Sec... ...eciallcorner{}{\tt {}~share} & Architecture-independent data \\ \end{longtable}$

35.4.3 Specific Options

$\begin{longtable}[l]{r l l} \par {\tt {}/usr} & \multicolumn{2}{l}{--------- Sec... ... ~ & \speciallcorner{}{\tt {}~src} & Source code (optional) \\ \end{longtable}$

An exception is made for the X Window System because of considerable precedent and widely-accepted practice.

The following symbolic links to directories may be present. This possibility is based on the need to preserve compatibility with older systems until all implementations can be assumed to use the /var hierarchy.

    /usr/spool -> /var/spool
    /usr/tmp -> /var/tmp
    /usr/spool/locks -> /var/lock

Once a system no longer requires any one of the above symbolic links, the link may be removed, if desired.

35.4.4 /usr/X11R6 : X Window System, Version 11 Release 6 (optional)

35.4.4.1 Purpose

This hierarchy is reserved for the X Window System, version 11 release 6, and related files.

To simplify matters and make XFree86 more compatible with the X Window System on other systems, the following symbolic links must be present if /usr/X11R6 exists:

    /usr/bin/X11 -> /usr/X11R6/bin
    /usr/lib/X11 -> /usr/X11R6/lib/X11
    /usr/include/X11 -> /usr/X11R6/include/X11

In general, software must not be installed or managed via the above symbolic links. They are intended for utilization by users only. The difficulty is related to the release version of the X Window System -- in transitional periods, it is impossible to know what release of X11 is in use.

35.4.4.2 Specific Options

Host-specific data in /usr/X11R6/lib/X11 should be interpreted as a demonstration file. Applications requiring information about the current host must reference a configuration file in /etc/X11, which may be linked to a file in /usr/X11R6/lib.(footnote follows) [Examples of such configuration files include Xconfig, XF86Config, or system.twmrc) ]

35.4.5 /usr/bin : Most user commands

35.4.5.1 Purpose

This is the primary directory of executable commands on the system.

35.4.5.2 Specific Options

The following directories, or symbolic links to directories, must be in /usr/bin, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/usr/bin} & \multicolumn{2}{l}{---------... ...{}~mh} & Commands for the MH mail handling system (optional) \\ \end{longtable}$

/usr/bin/X11 must be a symlink to /usr/X11R6/bin if the latter exists.

The following files, or symbolic links to files, must be in /usr/bin, if the corresponding subsystem is installed:

$\begin{longtable}[l]{l l} {\tt {}perl} & The Practical Extraction and Report Lan... ... {\tt {}expect} & Program for interactive dialog (optional) \\ \end{longtable}$

BEGIN RATIONALE
Because shell script interpreters (invoked with #!<path> on the first line of a shell script) cannot rely on a path, it is advantageous to standardize their locations. The Bourne shell and C-shell interpreters are already fixed in /bin, but Perl, Python, and Tcl are often found in many different places. They may be symlinks to the physical location of the shell interpreters.
END RATIONALE

35.4.6 /usr/include : Directory for standard include files.

35.4.6.1 Purpose

This is where all of the system's general-use include files for the C programming language should be placed.

35.4.6.2 Specific Options

The following directories, or symbolic links to directories, must be in /usr/include, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/usr/include} & \multicolumn{2}{l}{-----... ...r{}{\tt {}~bsd} & BSD compatibility include files (optional) \\ \end{longtable}$

The symbolic link /usr/include/X11 must link to /usr/X11R6/include/X11 if the latter exists.

35.4.7 /usr/lib : Libraries for programming and packages

35.4.7.1 Purpose

/usr/lib includes object files, libraries, and internal binaries that are not intended to be executed directly by users or shell scripts.(footnote follows) [Miscellaneous architecture-independent application-specific static files and subdirectories must be placed in /usr/share. ]

Applications may use a single subdirectory under /usr/lib. If an application uses a subdirectory, all architecture-dependent data exclusively used by the application must be placed within that subdirectory.(footnote follows) [For example, the perl5 subdirectory for Perl 5 modules and libraries. ]

35.4.7.2 Specific Options

For historical reasons, /usr/lib/sendmail must be a symbolic link to /usr/sbin/sendmail if the latter exists.(footnote follows) [Some executable commands such as makewhatis and sendmail have also been traditionally placed in /usr/lib. makewhatis is an internal binary and must be placed in a binary directory; users access only catman. Newer sendmail binaries are now placed by default in /usr/sbin. Additionally, systems using a sendmail-compatible mail transfer agent must provide /usr/sbin/sendmail as a symbolic link to the appropriate executable. ]

If /lib/X11 exists, /usr/lib/X11 must be a symbolic link to /lib/X11, or to whatever /lib/X11 is a symbolic link to.(footnote follows) [Host-specific data for the X Window System must not be stored in /usr/lib/X11. Host-specific configuration files such as Xconfig or XF86Config must be stored in /etc/X11. This includes configuration data such as system.twmrc even if it is only made a symbolic link to a more global configuration file (probably in /usr/X11R6/lib/X11). ]

35.4.8 /usr/lib<qual> : Alternate format libraries (optional)

35.4.8.1 Purpose

/usr/lib<qual> performs the same role as /usr/lib for an alternate binary format, except that the symbolic links /usr/lib<qual>/sendmail and /usr/lib<qual>/X11 are not required.(footnote follows) [The case where /usr/lib and /usr/lib<qual> are the same (one is a symbolic link to the other) these files and the per-application subdirectories will exist. ]

35.4.9 /usr/local : Local hierarchy

35.4.9.1 Purpose

The /usr/local hierarchy is for use by the system administrator when installing software locally. It needs to be safe from being overwritten when the system software is updated. It may be used for programs and data that are shareable amongst a group of hosts, but not found in /usr.

Locally installed software must be placed within /usr/local rather than /usr unless it is being installed to replace or upgrade software in /usr.(footnote follows) [Software placed in / or /usr may be overwritten by system upgrades (though we recommend that distributions do not overwrite data in /etc under these circumstances). For this reason, local software must not be placed outside of /usr/local without good reason. ]

35.4.9.2 Requirements

The following directories, or symbolic links to directories, must be in /usr/local

$\begin{longtable}[l]{r l l} \par {\tt {}/usr/local} & \multicolumn{2}{l}{-------... ...hy \\ ~ & \speciallcorner{}{\tt {}~src} & Local source code \\ \end{longtable}$

No other directories, except those listed below, may be in /usr/local after first installing a FHS-compliant system.

35.4.9.3 Specific Options

If directories /lib<qual> or /usr/lib<qual> exist, the equivalent directories must also exist in /usr/local.

35.4.10 /usr/sbin : Non-essential standard system binaries

35.4.10.1 Purpose

This directory contains any non-essential binaries used exclusively by the system administrator. System administration programs that are required for system repair, system recovery, mounting /usr, or other essential functions must be placed in /sbin instead.(footnote follows) [Locally installed system administration programs should be placed in /usr/local/sbin. ]

35.4.11 /usr/share : Architecture-independent data

35.4.11.1 Purpose

The /usr/share hierarchy is for all read-only architecture independent data files.(footnote follows) [Much of this data originally lived in /usr (man, doc) or /usr/lib (dict, terminfo, zoneinfo). ]

This hierarchy is intended to be shareable among all architecture platforms of a given OS; thus, for example, a site with i386, Alpha, and PPC platforms might maintain a single /usr/share directory that is centrally-mounted. Note, however, that /usr/share is generally not intended to be shared by different OSes or by different releases of the same OS.

Any program or package which contains or requires data that doesn't need to be modified should store that data in /usr/share (or /usr/local/share, if installed locally). It is recommended that a subdirectory be used in /usr/share for this purpose.

Game data stored in /usr/share/games must be purely static data. Any modifiable files, such as score files, game play logs, and so forth, should be placed in /var/games.

35.4.11.2 Requirements

The following directories, or symbolic links to directories, must be in /usr/share

$\begin{longtable}[l]{r l l} \par {\tt {}/usr/share} & \multicolumn{2}{l}{-------... ...}{\tt {}~misc} & Miscellaneous architecture-independent data \\ \end{longtable}$

35.4.11.3 Specific Options

The following directories, or symbolic links to directories, must be in /usr/share, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/usr/share} & \multicolumn{2}{l}{-------... ...oneinfo} & Timezone information and configuration (optional) \\ \end{longtable}$

It is recommended that application-specific, architecture-independent directories be placed here. Such directories include groff, perl, ghostscript, texmf, and kbd (Linux) or syscons (BSD). They may, however, be placed in /usr/lib for backwards compatibility, at the distributor's discretion. Similarly, a /usr/lib/games hierarchy may be used in addition to the /usr/share/games hierarchy if the distributor wishes to place some game data there.

35.4.11.4 /usr/share/dict : Word lists (optional)

35.4.11.4.1 Purpose

This directory is the home for word lists on the system; Traditionally this directory contains only the English words file, which is used by look(1) and various spelling programs. words may use either American or British spelling.

BEGIN RATIONALE
The reason that only word lists are located here is that they are the only files common to all spell checkers.
END RATIONALE

35.4.11.4.2 Specific Options

The following files, or symbolic links to files, must be in /usr/share/dict, if the corresponding subsystem is installed:

$\begin{longtable}[l]{l l} {\tt {}words} & List of English words (optional) \\ \end{longtable}$

Sites that require both American and British spelling may link words to /usr/share/dict/american-english or /usr/share/dict/british-english.

Word lists for other languages may be added using the English name for that language, e.g., /usr/share/dict/french, /usr/share/dict/danish, etc. These should, if possible, use an ISO 8859 character set which is appropriate for the language in question; if possible the Latin1 (ISO 8859-1) character set should be used (this is often not possible).

Other word lists must be included here, if present.

35.4.11.5 /usr/share/man : Manual pages

35.4.11.5.1 Purpose

This section details the organization for manual pages throughout the system, including /usr/share/man. Also refer to the section on /var/cache/man.

The primary <mandir> of the system is /usr/share/man. /usr/share/man contains manual information for commands and data under the / and /usr filesystems.(footnote follows) [Obviously, there are no manual pages in / because they are not required at boot time nor are they required in emergencies.(footnote follows) ] [Really. ]

Manual pages are stored in <mandir>/<locale>/man<section>/<arch>. An explanation of <mandir>, <locale>, <section>, and <arch> is given below.

A description of each section follows:

man1: User programs
Manual pages that describe publicly accessible commands are contained in this chapter. Most program documentation that a user will need to use is located here.
man2: System calls
This section describes all of the system calls (requests for the kernel to perform operations).
man3: Library functions and subroutines
Section 3 describes program library routines that are not direct calls to kernel services. This and chapter 2 are only really of interest to programmers.
man4: Special files
Section 4 describes the special files, related driver functions, and networking support available in the system. Typically, this includes the device files found in /dev and the kernel interface to networking protocol support.
man5: File formats
The formats for many data files are documented in the section 5. This includes various include files, program output files, and system files.
man6: Games
This chapter documents games, demos, and generally trivial programs. Different people have various notions about how essential this is.
man7: Miscellaneous
Manual pages that are difficult to classify are designated as being section 7. The troff and other text processing macro packages are found here.
man8: System administration
Programs used by system administrators for system operation and maintenance are documented here. Some of these programs are also occasionally useful for normal users.

35.4.11.5.2 Specific Options

The following directories, or symbolic links to directories, must be in /usr/share/<mandir>/<locale>, unless they are empty:(footnote follows) [For example, if /usr/local/man has no manual pages in section 4 (Devices), then /usr/local/man/man4 may be omitted. ]

$\begin{longtable}[l]{r l l} \par {\tt {}\textless{}mandir\textgreater{}/\textles... ...iallcorner{}{\tt {}~man8} & System administration (optional) \\ \end{longtable}$

The component <section> describes the manual section.

Provisions must be made in the structure of /usr/share/man to support manual pages which are written in different (or multiple) languages. These provisions must take into account the storage and reference of these manual pages. Relevant factors include language (including geographical-based differences), and character code set.

This naming of language subdirectories of /usr/share/man is based on Appendix E of the POSIX 1003.1 standard which describes the locale identification string -- the most well-accepted method to describe a cultural environment. The <locale> string is:

<language>[_<territory>][.<character-set>][,<version>]

The <language> field must be taken from ISO 639 (a code for the representation of names of languages). It must be two characters wide and specified with lowercase letters only.

The <territory> field must be the two-letter code of ISO 3166 (a specification of representations of countries), if possible. (Most people are familiar with the two-letter codes used for the country codes in email addresses.(footnote follows)) It must be two characters wide and specified with uppercase letters only. [A major exception to this rule is the United Kingdom, which is `GB' in the ISO 3166, but `UK' for most email addresses. ]

The <character-set> field must represent the standard describing the character set. If the <character-set> field is just a numeric specification, the number represents the number of the international standard describing the character set. It is recommended that this be a numeric representation if possible (ISO standards, especially), not include additional punctuation symbols, and that any letters be in lowercase.

A parameter specifying a <version> of the profile may be placed after the <character-set> field, delimited by a comma. This may be used to discriminate between different cultural needs; for instance, dictionary order versus a more systems-oriented collating order. This standard recommends not using the <version> field, unless it is necessary.

Systems which use a unique language and code set for all manual pages may omit the <locale> substring and store all manual pages in <mandir>. For example, systems which only have English manual pages coded with ASCII, may store manual pages (the man<section> directories) directly in /usr/share/man. (That is the traditional circumstance and arrangement, in fact.)

Countries for which there is a well-accepted standard character code set may omit the <character-set> field, but it is strongly recommended that it be included, especially for countries with several competing standards.

Various examples:

$\begin{longtable}[l]{l l l l} Language & Territory & Character Set & {\tt {}Dire... ...Japan & UJIS (or EUC-J) & {\tt {}/usr/share/man/ja\_JP.ujis} \\ \end{longtable}$

Similarly, provision must be made for manual pages which are architecture-dependent, such as documentation on device-drivers or low-level system administration commands. These must be placed under an <arch> directory in the appropriate man<section> directory; for example, a man page for the i386 ctrlaltdel(8) command might be placed in /usr/share/man/<locale>/man8/i386/ctrlaltdel.8.

Manual pages for commands and data under /usr/local are stored in /usr/local/man. Manual pages for X11R6 are stored in /usr/X11R6/man. It follows that all manual page hierarchies in the system must have the same structure as /usr/share/man.

The cat page sections (cat<section>) containing formatted manual page entries are also found within subdirectories of <mandir>/<locale>, but are not required nor may they be distributed in lieu of nroff source manual pages.

The numbered sections "1" through "8" are traditionally defined. In general, the file name for manual pages located within a particular section end with .<section>.

In addition, some large sets of application-specific manual pages have an additional suffix appended to the manual page filename. For example, the MH mail handling system manual pages must have mh appended to all MH manuals. All X Window System manual pages must have an x appended to the filename.

The practice of placing various language manual pages in appropriate subdirectories of /usr/share/man also applies to the other manual page hierarchies, such as /usr/local/man and /usr/X11R6/man. (This portion of the standard also applies later in the section on the optional /var/cache/man structure.)

35.4.11.6 /usr/share/misc : Miscellaneous architecture-independent data

This directory contains miscellaneous architecture-independent files which don't require a separate subdirectory under /usr/share.

35.4.11.6.1 Specific Options

The following files, or symbolic links to files, must be in /usr/share/misc, if the corresponding subsystem is installed:

$\begin{longtable}[l]{l l} {\tt {}ascii} & ASCII character set table (optional) \... ...{\tt {}termcap.db} & Terminal capability database (optional) \\ \end{longtable}$

Other (application-specific) files may appear here,(footnote follows) but a distributor may place them in /usr/lib at their discretion.

[Some such files include: { airport, birthtoken, eqnchar, getopt, gprof.callg, gprof.flat,inter.phone, ipfw.samp.filters, ipfw.samp.scripts, keycap.pcvt, mail.help,mail.tildehelp, man.template, map3270, mdoc.template, more.help, na.phone,nslookup.help, operator, scsi_modes, sendmail.hf, style, units.lib,vgrindefs, vgrindefs.db, zipcodes } ]

35.4.11.7 /usr/share/sgml : SGML and XML data (optional)

35.4.11.7.1 Purpose

/usr/share/sgml contains architecture-independent files used by SGML or XML applications, such as ordinary catalogs (not the centralized ones, see /etc/sgml), DTDs, entities, or style sheets.

35.4.11.7.2 Specific Options

The following directories, or symbolic links to directories, must be in /usr/share/sgml, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/usr/share/sgml} & \multicolumn{2}{l}{--... ...~ & \speciallcorner{}{\tt {}~mathml} & mathml DTD (optional) \\ \end{longtable}$

Other files that are not specific to a given DTD may reside in their own subdirectory.

35.4.12 /usr/src : Source code (optional)

35.4.12.1 Purpose

Any non-local source code should be placed in this subdirectory.

35.5 The /var Hierarchy

35.5.1 Purpose

/var contains variable data files. This includes spool directories and files, administrative and logging data, and transient and temporary files.

Some portions of /var are not shareable between different systems. For instance, /var/log, /var/lock, and /var/run. Other portions may be shared, notably /var/mail, /var/cache/man, /var/cache/fonts, and /var/spool/news.

/var is specified here in order to make it possible to mount /usr read-only. Everything that once went into /usr that is written to during system operation (as opposed to installation and software maintenance) must be in /var.

If /var cannot be made a separate partition, it is often preferable to move /var out of the root partition and into the /usr partition. (This is sometimes done to reduce the size of the root partition or when space runs low in the root partition.) However, /var must not be linked to /usr because this makes separation of /usr and /var more difficult and is likely to create a naming conflict. Instead, link /var to /usr/var.

Applications must generally not add directories to the top level of /var. Such directories should only be added if they have some system-wide implication, and in consultation with the FHS mailing list.

35.5.2 Requirements

The following directories, or symbolic links to directories, are required in /var.

$\begin{longtable}[l]{r l l} \par {\tt {}/var} & \multicolumn{2}{l}{--------- Var... ...t {}~tmp} & Temporary files preserved between system reboots \\ \end{longtable}$

Several directories are `reserved' in the sense that they must not be used arbitrarily by some new application, since they would conflict with historical and/or local practice. They are:

    /var/backups
    /var/cron
    /var/msgs
    /var/preserve

35.5.3 Specific Options

The following directories, or symbolic links to directories, must be in /var, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/var} & \multicolumn{2}{l}{--------- Var... ... Network Information Service (NIS) database files (optional) \\ \end{longtable}$

35.5.4 /var/account : Process accounting logs (optional)

35.5.4.1 Purpose

This directory holds the current active process accounting log and the composite process usage data (as used in some UNIX-like systems by lastcomm and sa).

35.5.5 /var/cache : Application cache data

35.5.5.1 Purpose

/var/cache is intended for cached data from applications. Such data is locally generated as a result of time-consuming I/O or calculation. The application must be able to regenerate or restore the data. Unlike /var/spool, the cached files can be deleted without data loss. The data must remain valid between invocations of the application and rebooting the system.

Files located under /var/cache may be expired in an application specific manner, by the system administrator, or both. The application must always be able to recover from manual deletion of these files (generally because of a disk space shortage). No other requirements are made on the data format of the cache directories.

BEGIN RATIONALE
The existence of a separate directory for cached data allows system administrators to set different disk and backup policies from other directories in /var.
END RATIONALE

35.5.5.2 Specific Options

$\begin{longtable}[l]{r l l} \par {\tt {}/var/cache} & \multicolumn{2}{l}{-------... ...kage\textgreater{}} & Package specific cache data (optional) \\ \end{longtable}$

35.5.5.3 /var/cache/fonts : Locally-generated fonts (optional)

35.5.5.3.1 Purpose

The directory /var/cache/fonts should be used to store any dynamically-created fonts. In particular, all of the fonts which are automatically generated by mktexpk must be located in appropriately-named subdirectories of /var/cache/fonts.(footnote follows) [This standard does not currently incorporate the TEX Directory Structure (a document that describes the layout TEX files and directories), but it may be useful reading. It is located at ftp://ctan.tug.org/tex/. ]

35.5.5.3.2 Specific Options

Other dynamically created fonts may also be placed in this tree, under appropriately-named subdirectories of /var/cache/fonts.

35.5.5.4 /var/cache/man : Locally-formatted manual pages (optional)

35.5.5.4.1 Purpose

This directory provides a standard location for sites that provide a read-only /usr partition, but wish to allow caching of locally-formatted man pages. Sites that mount /usr as writable (e.g., single-user installations) may choose not to use /var/cache/man and may write formatted man pages into the cat<section> directories in /usr/share/man directly. We recommend that most sites use one of the following options instead:

Preformat all manual pages alongside the unformatted versions.
Allow no caching of formatted man pages, and require formatting to be done each time a man page is brought up.
Allow local caching of formatted man pages in /var/cache/man.

The structure of /var/cache/man needs to reflect both the fact of multiple man page hierarchies and the possibility of multiple language support.

Given an unformatted manual page that normally appears in <path>/man/<locale>/man<section>, the directory to place formatted man pages in is /var/cache/man/<catpath>/<locale>/cat<section>, where <catpath> is derived from <path> by removing any leading usr and/or trailing share pathname components.(footnote follows) (Note that the <locale> component may be missing.) [For example, /usr/share/man/man1/ls.1 is formatted into /var/cache/man/cat1/ls.1, and /usr/X11R6/man/<locale>/man3/XtClass.3x into /var/cache/man/X11R6/<locale>/cat3/XtClass.3x. ]

Man pages written to /var/cache/man may eventually be transferred to the appropriate preformatted directories in the source man hierarchy or expired; likewise formatted man pages in the source man hierarchy may be expired if they are not accessed for a period of time.

If preformatted manual pages come with a system on read-only media (a CD-ROM, for instance), they must be installed in the source man hierarchy (e.g. /usr/share/man/cat<section>). /var/cache/man is reserved as a writable cache for formatted manual pages.

BEGIN RATIONALE
Release 1.2 of the standard specified /var/catman for this hierarchy. The path has been moved under /var/cache to better reflect the dynamic nature of the formatted man pages. The directory name has been changed to man to allow for enhancing the hierarchy to include post-processed formats other than "cat", such as PostScript, HTML, or DVI.
END RATIONALE

35.5.6 /var/crash : System crash dumps (optional)

35.5.6.1 Purpose

This directory holds system crash dumps. As of the date of this release of the standard, system crash dumps were not supported under Linux.

35.5.7 /var/games : Variable game data (optional)

35.5.7.1 Purpose

Any variable data relating to games in /usr should be placed here. /var/games should hold the variable data previously found in /usr; static data, such as help text, level descriptions, and so on, must remain elsewhere, such as /usr/share/games.

BEGIN RATIONALE
/var/games has been given a hierarchy of its own, rather than leaving it merged in with the old /var/lib as in release 1.2. The separation allows local control of backup strategies, permissions, and disk usage, as well as allowing inter-host sharing and reducing clutter in /var/lib. Additionally, /var/games is the path traditionally used by BSD.
END RATIONALE

35.5.8 /var/lib : Variable state information

35.5.8.1 Purpose

This hierarchy holds state information pertaining to an application or the system. State information is data that programs modify while they run, and that pertains to one specific host. Users must never need to modify files in /var/lib to configure a package's operation.

State information is generally used to preserve the condition of an application (or a group of inter-related applications) between invocations and between different instances of the same application. State information should generally remain valid after a reboot, should not be logging output, and should not be spooled data.

An application (or a group of inter-related applications) must use a subdirectory of /var/lib for its data.(footnote follows) There is one required subdirectory, /var/lib/misc, which is intended for state files that don't need a subdirectory; the other subdirectories should only be present if the application in question is included in the distribution. [An important difference between this version of this standard and previous ones is that applications are now required to use a subdirectory of /var/lib. ]

/var/lib/<name> is the location that must be used for all distribution packaging support. Different distributions may use different names, of course.

35.5.8.2 Requirements

The following directories, or symbolic links to directories, are required in /var/lib:

$\begin{longtable}[l]{r l l} \par {\tt {}/var/lib} & \multicolumn{2}{l}{---------... ... & \speciallcorner{}{\tt {}~misc} & Miscellaneous state data \\ \end{longtable}$

35.5.8.3 Specific Options

The following directories, or symbolic links to directories, must be in /var/lib, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/var/lib} & \multicolumn{2}{l}{---------... ...r{}{\tt {}~xdm} & X display manager variable data (optional) \\ \end{longtable}$

35.5.8.4 /var/lib/<editor> : Editor backup files and state (optional)

35.5.8.4.1 Purpose

These directories contain saved files generated by any unexpected termination of an editor (e.g., elvis, jove, nvi).

Other editors may not require a directory for crash-recovery files, but may require a well-defined place to store other information while the editor is running. This information should be stored in a subdirectory under /var/lib (for example, GNU Emacs would place lock files in /var/lib/emacs/lock).

Future editors may require additional state information beyond crash-recovery files and lock files -- this information should also be placed under /var/lib/<editor>.

BEGIN RATIONALE
Previous Linux releases, as well as all commercial vendors, use /var/preserve for vi or its clones. However, each editor uses its own format for these crash-recovery files, so a separate directory is needed for each editor.
Editor-specific lock files are usually quite different from the device or resource lock files that are stored in /var/lock and, hence, are stored under /var/lib.
END RATIONALE

35.5.8.5 /var/lib/hwclock : State directory for hwclock (optional)

35.5.8.5.1 Purpose

This directory contains the file /var/lib/hwclock/adjtime.

BEGIN RATIONALE
In FHS 2.1, this file was /etc/adjtime, but as hwclock updates it, that was obviously incorrect.
END RATIONALE

35.5.8.6 /var/lib/misc : Miscellaneous variable data

35.5.8.6.1 Purpose

This directory contains variable data not placed in a subdirectory in /var/lib. An attempt should be made to use relatively unique names in this directory to avoid namespace conflicts.(footnote follows) [This hierarchy should contain files stored in /var/db in current BSD releases. These include locate.database and mountdtab, and the kernel symbol database(s). ]

35.5.9 /var/lock : Lock files

35.5.9.1 Purpose

Lock files should be stored within the /var/lock directory structure.

Lock files for devices and other resources shared by multiple applications, such as the serial device lock files that were originally found in either /usr/spool/locks or /usr/spool/uucp, must now be stored in /var/lock. The naming convention which must be used is LCK.. followed by the base name of the device file. For example, to lock /dev/ttyS0 the file LCK..ttyS0 would be created.

(footnote follows) [Then, anything wishing to use /dev/ttyS0 can read the lock file and act accordingly (all locks in /var/lock should be world-readable). ]

The format used for the contents of such lock files must be the HDB UUCP lock file format. The HDB format is to store the process identifier (PID) as a ten byte ASCII decimal number, with a trailing newline. For example, if process 1230 holds a lock file, it would contain the eleven characters: space, space, space, space, space, space, one, two, three, zero, and newline.

35.5.10 /var/log : Log files and directories

35.5.10.1 Purpose

This directory contains miscellaneous log files. Most logs must be written to this directory or an appropriate subdirectory.

35.5.10.2 Specific Options

The following files, or symbolic links to files, must be in /var/log, if the corresponding subsystem is installed:

$\begin{longtable}[l]{l l} {\tt {}lastlog} & record of last login of each user \\... ...syslogd} \\ {\tt {}wtmp} & record of all logins and logouts \\ \end{longtable}$

35.5.11 /var/mail : User mailbox files (optional)

35.5.11.1 Purpose

The mail spool must be accessible through /var/mail and the mail spool files must take the form <username>.(footnote follows) [Note that /var/mail may be a symbolic link to another directory. ]

User mailbox files in this location must be stored in the standard UNIX mailbox format.

BEGIN RATIONALE
The logical location for this directory was changed from /var/spool/mail in order to bring FHS in-line with nearly every UNIX implementation. This change is important for inter-operability since a single /var/mail is often shared between multiple hosts and multiple UNIX implementations (despite NFS locking issues).
It is important to note that there is no requirement to physically move the mail spool to this location. However, programs and header files must be changed to use /var/mail.
END RATIONALE

35.5.12 /var/opt : Variable data for /opt

35.5.12.1 Purpose

Variable data of the packages in /opt must be installed in /var/opt/<package>, where <package> is the name of the subtree in /opt where the static data from an add-on software package is stored, except where superseded by another file in /etc. No structure is imposed on the internal arrangement of /var/opt/<package>.

BEGIN RATIONALE
Refer to the rationale for /opt.
END RATIONALE

35.5.13 /var/run : Run-time variable data

35.5.13.1 Purpose

This directory contains system information data describing the system since it was booted. Files under this directory must be cleared (removed or truncated as appropriate) at the beginning of the boot process. Programs may have a subdirectory of /var/run; this is encouraged for programs that use more than one run-time file.(footnote follows) [/var/run should be unwritable for unprivileged users (root or users running daemons); it is a major security problem if any user can write in this directory. ]Process identifier (PID) files, which were originally placed in /etc, must be placed in /var/run. The naming convention for PID files is <program-name>.pid. For example, the crond PID file is named /var/run/crond.pid.

35.5.13.2 Requirements

The internal format of PID files remains unchanged. The file must consist of the process identifier in ASCII-encoded decimal, followed by a newline character. For example, if crond was process number 25, /var/run/crond.pid would contain three characters: two, five, and newline.

Programs that read PID files should be somewhat flexible in what they accept; i.e., they should ignore extra whitespace, leading zeroes, absence of the trailing newline, or additional lines in the PID file. Programs that create PID files should use the simple specification located in the above paragraph.

The utmp file, which stores information about who is currently using the system, is located in this directory.

Programs that maintain transient UNIX-domain sockets must place them in this directory.

35.5.14 /var/spool : Application spool data

35.5.14.1 Purpose

/var/spool contains data which is awaiting some kind of later processing. Data in /var/spool represents work to be done in the future (by a program, user, or administrator); often data is deleted after it has been processed.(footnote follows)

[UUCP lock files must be placed in /var/lock. See the above section on /var/lock. ]

35.5.14.2 Specific Options

The following directories, or symbolic links to directories, must be in /var/spool, if the corresponding subsystem is installed:

$\begin{longtable}[l]{r l l} \par {\tt {}/var/spool} & \multicolumn{2}{l}{-------... ...lcorner{}{\tt {}~uucp} & Spool directory for UUCP (optional) \\ \end{longtable}$

35.5.14.3 /var/spool/lpd : Line-printer daemon print queues (optional)

35.5.14.3.1 Purpose

The lock file for lpd, lpd.lock, must be placed in /var/spool/lpd. It is suggested that the lock file for each printer be placed in the spool directory for that specific printer and named lock.

35.5.14.3.2 Specific Options

$\begin{longtable}[l]{r l l} \par {\tt {}/var/spool/lpd} & \multicolumn{2}{l}{---... ...er\textgreater{}} & Spools for a specific printer (optional) \\ \end{longtable}$

35.5.14.4 /var/spool/rwho : Rwhod files (optional)

35.5.14.4.1 Purpose

This directory holds the rwhod information for other systems on the local net.

BEGIN RATIONALE
Some BSD releases use /var/rwho for this data; given its historical location in /var/spool on other systems and its approximate fit to the definition of `spooled' data, this location was deemed more appropriate.
END RATIONALE

35.5.15 /var/tmp : Temporary files preserved between system reboots

35.5.15.1 Purpose

The /var/tmp directory is made available for programs that require temporary files or directories that are preserved between system reboots. Therefore, data stored in /var/tmp is more persistent than data in /tmp.

Files and directories located in /var/tmp must not be deleted when the system is booted. Although data stored in /var/tmp is typically deleted in a site-specific manner, it is recommended that deletions occur at a less frequent interval than /tmp.

35.5.16 /var/yp : Network Information Service (NIS) database files (optional)

35.5.16.1 Purpose

Variable data for the Network Information Service (NIS), formerly known as the Sun Yellow Pages (YP), must be placed in this directory.

BEGIN RATIONALE
/var/yp is the standard directory for NIS (YP) data and is almost exclusively used in NIS documentation and systems.(footnote follows)
END RATIONALE

[NIS should not be confused with Sun NIS+, which uses a different directory, /var/nis. ]

35.6 Operating System Specific Annex

This section is for additional requirements and recommendations that only apply to a specific operating system. The material in this section should never conflict with the base standard.

35.6.1 Linux

This is the annex for the Linux operating system.

35.6.1.1 / : Root directory

On Linux systems, if the kernel is located in /, we recommend using the names vmlinux or vmlinuz, which have been used in recent Linux kernel source packages.

35.6.1.2 /bin : Essential user command binaries (for use by all users)

Linux systems which require them place these additional files into /bin.

{: setserial }

35.6.1.3 /dev : Devices and special files

All devices and special files in /dev should adhere to the Linux Allocated Devices document, which is available with the Linux kernel source. It is maintained by H. Peter Anvin <hpa@zytor.com>.

Symbolic links in /dev should not be distributed with Linux systems except as provided in the Linux Allocated Devices document.

BEGIN RATIONALE
The requirement not to make symlinks promiscuously is made because local setups will often differ from that on the distributor's development machine. Also, if a distribution install script configures the symbolic links at install time, these symlinks will often not get updated if local changes are made in hardware. When used responsibly at a local level, however, they can be put to good use.
END RATIONALE

35.6.1.4 /etc : Host-specific system configuration

Linux systems which require them place these additional files into /etc.

{: lilo.conf }

35.6.1.5 /proc : Kernel and process information virtual filesystem

The proc filesystem is the de-facto standard Linux method for handling process and system information, rather than /dev/kmem and other similar methods. We strongly encourage this for the storage and retrieval of process information as well as other kernel and memory information.

35.6.1.6 /sbin : Essential system binaries

Linux systems place these additional files into /sbin.

$o$: Second extended filesystem commands (optional):
{: badblocks, dumpe2fs, e2fsck, mke2fs, mklost+found, tune2fs }

$o$: Boot-loader map installer (optional):
{: lilo }

Optional files for /sbin:

$o$: Static binaries:
{: ldconfig, sln, ssync }

Static ln (sln) and static sync (ssync) are useful when things go wrong. The primary use of sln (to repair incorrect symlinks in /lib after a poorly orchestrated upgrade) is no longer a major concern now that the ldconfig program (usually located in /usr/sbin) exists and can act as a guiding hand in upgrading the dynamic libraries. Static sync is useful in some emergency situations. Note that these need not be statically linked versions of the standard ln and sync, but may be.

The ldconfig binary is optional for /sbin since a site may choose to run ldconfig at boot time, rather than only when upgrading the shared libraries. (It's not clear whether or not it is advantageous to run ldconfig on each boot.) Even so, some people like ldconfig around for the following (all too common) situation:

I've just removed /lib/<file>.
I can't find out the name of the library because ls is dynamically linked, I'm using a shell that doesn't have ls built-in, and I don't know about using "echo *" as a replacement.
I have a static sln, but I don't know what to call the link.

$o$: Miscellaneous:
{: ctrlaltdel, kbdrate }

So as to cope with the fact that some keyboards come up with such a high repeat rate as to be unusable, kbdrate may be installed in /sbin on some systems.

Since the default action in the kernel for the Ctrl-Alt-Del key combination is an instant hard reboot, it is generally advisable to disable the behavior before mounting the root filesystem in read-write mode. Some init suites are able to disable Ctrl-Alt-Del, but others may require the ctrlaltdel program, which may be installed in /sbin on those systems.

35.6.1.7 /usr/include : Header files included by C programs

These symbolic links are required if a C or C++ compiler is installed and only for systems not based on glibc.

    /usr/include/asm -> /usr/src/linux/include/asm-<arch>
    /usr/include/linux -> /usr/src/linux/include/linux

35.6.1.8 /usr/src : Source code

For systems based on glibc, there are no specific guidelines for this directory. For systems based on Linux libc revisions prior to glibc, the following guidelines and rationale apply:

The only source code that should be placed in a specific location is the Linux kernel source code. It is located in /usr/src/linux.

If a C or C++ compiler is installed, but the complete Linux kernel source code is not installed, then the include files from the kernel source code must be located in these directories:

    /usr/src/linux/include/asm-<arch>
    /usr/src/linux/include/linux

<arch> is the name of the system architecture.

Note: /usr/src/linux may be a symbolic link to a kernel source code tree.

BEGIN RATIONALE
It is important that the kernel include files be located in /usr/src/linux and not in /usr/include so there are no problems when system administrators upgrade their kernel version for the first time.
END RATIONALE

35.6.1.9 /var/spool/cron : cron and at jobs

This directory contains the variable data for the cron and at programs.

35.7 Appendix

35.7.1 The FHS mailing list

The FHS mailing list is located at <fhs-discuss@ucsd.edu>. To subscribe to the list send mail to <listserv@ucsd.edu> with body "ADD fhs-discuss".

Thanks to Network Operations at the University of California at San Diego who allowed us to use their excellent mailing list server.

As noted in the introduction, please do not send mail to the mailing list without first contacting the FHS editor or a listed contributor.

35.7.2 Background of the FHS

The process of developing a standard filesystem hierarchy began in August 1993 with an effort to restructure the file and directory structure of Linux. The FSSTND, a filesystem hierarchy standard specific to the Linux operating system, was released on February 14, 1994. Subsequent revisions were released on October 9, 1994 and March 28, 1995.

In early 1995, 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 or FHS for short.

Volunteers who have contributed extensively to this standard are listed at the end of this document. This standard represents a consensus view of those and other contributors.

35.7.3 General Guidelines

Here are some of the guidelines that have been used in the development of this standard:

Solve technical problems while limiting transitional difficulties.
Make the specification reasonably stable.
Gain the approval of distributors, developers, and other decision-makers in relevant development groups and encourage their participation.
Provide a standard that is attractive to the implementors of different UNIX-like systems.

35.7.4 Scope

This document specifies a standard filesystem hierarchy for FHS filesystems by specifying the location of files and directories, and the contents of some system files.

This standard has been designed to be used by system integrators, package developers, and system administrators in the construction and maintenance of FHS compliant filesystems. It is primarily intended to be a reference and is not a tutorial on how to manage a conforming filesystem hierarchy.

The FHS grew out of earlier work on FSSTND, a filesystem organization standard for the Linux operating system. It builds on FSSTND to address interoperability issues not just in the Linux community but in a wider arena including 4.4BSD-based operating systems. It incorporates lessons learned in the BSD world and elsewhere about multi-architecture support and the demands of heterogeneous networking.

Although this standard is more comprehensive than previous attempts at filesystem hierarchy standardization, periodic updates may become necessary as requirements change in relation to emerging technology. It is also possible that better solutions to the problems addressed here will be discovered so that our solutions will no longer be the best possible solutions. Supplementary drafts may be released in addition to periodic updates to this document. However, a specific goal is backwards compatibility from one release of this document to the next.

Comments related to this standard are welcome. Any comments or suggestions for changes may be directed to the FHS editor (Daniel Quinlan <quinlan@pathname.com>) or the FHS mailing list. Typographical or grammatical comments should be directed to the FHS editor.

Before sending mail to the mailing list it is requested that you first contact the FHS editor in order to avoid excessive re-discussion of old topics.

Questions about how to interpret items in this document may occasionally arise. If you have need for a clarification, please contact the FHS editor. Since this standard represents a consensus of many participants, it is important to make certain that any interpretation also represents their collective opinion. For this reason it may not be possible to provide an immediate response unless the inquiry has been the subject of previous discussion.

35.7.5 Acknowledgments

The developers of the FHS wish to thank the developers, system administrators, and users whose input was essential to this standard. We wish to thank each of the contributors who helped to write, compile, and compose this standard.

The FHS Group also wishes to thank those Linux developers who supported the FSSTND, the predecessor to this standard. If they hadn't demonstrated that the FSSTND was beneficial, the FHS could never have evolved.

35.7.6 Contributors

$\begin{longtable}[l]{l l} Brandon S. Allbery & {\tt {}\textless{}bsa@kf8nh.waria... ...nd Warken & {\tt {}\textless{}bwarken@mayn.de\textgreater{}} \\ \end{longtable}$

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