/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. ^[1]

There must be no subdirectories in /bin.

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

If /bin/sh is not the POSIX compatible shell command itself, 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.

Tip: Rationale

Various shells behave differently when called as sh, so as to preserve POSIX compatibility while allowing changes or extensions to POSIX when desired.

The requirement for the [ and test commands to be included as binaries (even if implemented internally by the shell) is shared with the POSIX.1-2008 standard.

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

/bin/csh may be a symbolic link to /bin/tcsh or /usr/bin/tcsh.

Tip: 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.

This directory contains everything required for the boot process except configuration files not needed at boot time 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 and sector map files.

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 that are not required at boot time must be placed in /etc.

The operating system kernel must be located in either / or /boot.

Certain architectures may have other requirements for /boot related to limitations or expectations specific to that architecture. These requirements are not enumerated here; distributions are allowed to add requirements as needed to enable system startup on these architectures.

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

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 distribution installs.

The /etc hierarchy contains configuration files. A "configuration file" is a local file used to control the operation of a program; it must be static and cannot be an executable binary. ^[2]

It is recommended that files be stored in subdirectories of /etc rather than directly in /etc.

No binaries may be located under /etc.

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

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

The following files, or symbolic links to files, must be in /etc if the corresponding subsystem is installed: ^[3]

mtab does not fit the static nature of /etc: it is excepted for historical reasons. ^[4]

/home is a fairly standard concept, but it is clearly a site-specific filesystem. ^[6] The setup will differ from host to host. Therefore, no program should assume any specific location for a home directory, rather it should query for it. ^[7]

User specific configuration files for applications are stored in the user's home directory in a file that starts with the '.' character (a "dot file"). If an application needs to create more than one dot file then they should be placed in a subdirectory with a name starting with a '.' character, (a "dot directory"). In this case the configuration files should not start with the '.' character. ^[8]

A number of efforts have been made in the past to standardize the layout of home directories, including the XDG Base Directories specification ^[9] and the GLib conventions on user directory contents. ^[10] Additional efforts in this direction are possible in the future. To accomodate software which makes use of these specifications and conventions, distributions may create directory hierarchies which follow the specifications and conventions. Those directory hierarchies may be located underneath home directories.

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. ^[11]

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

If a C preprocessor is installed, /lib/cpp must be a reference to it, for historical reasons. ^[12]

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

There may be one or more variants of the /lib directory on systems which support more than one binary format requiring separate libraries. ^[13]

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. ^[14]

This directory contains subdirectories which are used as mount points for removable media such as floppy disks, cdroms and zip disks.

Tip: Rationale

Historically there have been a number of other different places used to mount removable media such as /cdrom, /mnt or /mnt/cdrom. Placing the mount points for all removable media directly in the root directory would potentially result in a large number of extra directories in /. Although the use of subdirectories in /mnt as a mount point has recently been common, it conflicts with a much older tradition of using /mnt directly as a temporary mount point.

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

On systems where more than one device exists for mounting a certain type of media, mount directories can be created by appending a digit to the name of those available above starting with '0', but the unqualified name must also exist. ^[15]

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.

/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> or /opt/<provider> directory tree, where <package> is a name that describes the software package and <provider> is the provider's LANANA registered name.

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 or under the /opt/<provider> hierarchy. If the package includes UNIX manual pages, they must be located in /opt/<package>/share/man or under the /opt/<provider> hierarchy, 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 and otherwise manage software in /opt under an appropriately registered subdirectory.

Tip: 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.

The structure of the directories below /opt/<provider> is left up to the packager of the software, though it is recommended that packages are installed in /opt/<provider>/<package> and follow a similar structure to the guidelines for /opt/<package>. A valid reason for diverging from this structure is for support packages which may have files installed in /opt/<provider>/lib or /opt/<provider>/bin.

The root account's home directory may be determined by developer or local preference, but this is the recommended default location. ^[16]

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.

The purposes of this directory were once served by /var/run. In general, programs may continue to use /var/run to fulfill the requirements set out for /run for the purposes of backwards compatibility. Programs which have migrated to use /run should cease their usage of /var/run, except as noted in the section on /var/run.

Programs may have a subdirectory of /run; this is encouraged for programs that use more than one run-time file. Users may also have a subdirectory of /run, although care must be taken to appropriately limit access rights to prevent unauthorized use of /run itself and other subdirectories. ^[17]

Process identifier (PID) files, which were originally placed in /etc, must be placed in /run. The naming convention for PID files is <program-name>.pid. For example, the crond PID file is named /run/crond.pid.

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, /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.

System programs that maintain transient UNIX-domain sockets must place them in this directory or an appropriate subdirectory as outlined above.

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. ^[18] 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. ^[19]

There must be no subdirectories in /sbin.

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

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

/srv contains site-specific data which is served by this system.

Tip: Rationale

This main purpose of specifying this is so that users may find the location of the data files for a particular service, and so that services which require a single tree for readonly data, writable data and scripts (such as cgi scripts) can be reasonably placed. Data that is only of interest to a specific user should go in that users' home directory. If the directory and file structure of the data is not exposed to consumers, it should go in /var/lib.

The methodology used to name subdirectories of /srv is unspecified as there is currently no consensus on how this should be done. One method for structuring data under /srv is by protocol, eg. ftp, rsync, www, and cvs. On large systems it can be useful to structure /srv by administrative context, such as /srv/physics/www, /srv/compsci/cvs, etc. This setup will differ from host to host. Therefore, no program should rely on a specific subdirectory structure of /srv existing or data necessarily being stored in /srv. However /srv should always exist on FHS compliant systems and should be used as the default location for such data.

Distributions must take care not to remove locally placed files in these directories without administrator permission. ^[20]

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.

Tip: Rationale

IEEE standard POSIX.1-2008 lists requirements 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.

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

There must be no subdirectories in /usr/bin.

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

Tip: Rationale

In many executable scripts, the interpreter to be invoked to execute the script is specified using #!path_to_interpreter on the first line of a script. To make such scripts portable among different systems, it is advantageous to standardize the interpreter locations. The shell interpreter is already fixed in /bin by this specification, but interpreters for Perl, Python, Tcl and expect may be installed in various places. The locations specified here may be implemented as symbolic links to the physical location of the interpreters.

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

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

/usr/lib includes object files and libraries. ^[21] On some systems, it may also include internal binaries that are not intended to be executed directly by users or shell scripts. ^[22]

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. ^[23]

For historical reasons, /usr/lib/sendmail must be a symbolic link which resolves to the sendmail-compatible command provided by the system's mail transfer agent, if the latter exists. ^{[24] [25]}

/usr/libexec includes internal binaries that are not intended to be executed directly by users or shell scripts. Applications may use a single subdirectory under /usr/libexec.

Applications which use /usr/libexec in this way must not also use /usr/lib to store internal binaries, though they may use /usr/lib for the other purposes documented here.

Tip: Rationale

Some previous versions of this document did not support /usr/libexec, despite it being standard practice in a number of environments. ^[26] To accomodate this restriction, it became common practice to use /usr/lib instead. Either practice is now acceptable, but each application must choose one way or the other to organize itself.

/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. ^[27]

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. ^[28]

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

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

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

/usr/local/etc may be a symbolic link to /etc/local.

Tip: Rationale

The consistency of /usr/local/etc is beneficial to installers, and is already used in other systems. As all of /usr/local needs to be backed up to reproduce a system, it introduces no additional maintenance overhead, but a symlink to /etc/local is suitable if systems want all their configuration under one hierarchy.

Note that /usr/etc is still not allowed: programs in /usr should place configuration files in /etc.

If the directory /usr/share/color exists as specified in this document, then the directory /usr/local/share/color must also exist, governed by the same rules as /usr/share/color.

Tip: Rationale

This usage allows the sysadmin a place to install color profiles manually when necessary.

The requirements for the contents of this directory are the same as for /usr/share.

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. ^[29]

There must be no subdirectories in /usr/sbin.

The /usr/share hierarchy is for all read-only architecture independent data files. ^[30]

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. Applications using a single file may use /usr/share/misc.

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.

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

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

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.

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

Source code may be placed in this subdirectory, only for reference purposes. ^[37]

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).

/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.

Tip: 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.

This directory holds system crash dumps. As of the date of this release of the standard, system crash dumps were not supported under Linux but may be supported by other systems which may comply with the FHS.

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.

Tip: Rationale

/var/games has been given a hierarchy of its own, rather than leaving it underneath /var/lib as in release 1.2 of this standard. 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.

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, and the specific file hierarchy used to store the data must not be exposed to regular users. ^[40]

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. 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. ^[41]

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

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

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

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. For example, to lock /dev/ttyS0 the file "LCK..ttyS0" would be created. ^[43]

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.

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

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

The mail spool must be accessible through /var/mail and the mail spool files must take the form <username>. ^[44]

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

Tip: Rationale

The logical location for this directory was changed from /var/spool/mail in order to bring FHS in-line with nearly every UNIX distribution. This change is important for inter-operability since a single /var/mail is often shared between multiple hosts and multiple UNIX distribution (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.

Variable data of the packages in /opt must be installed in /var/opt/<subdir>, where <subdir> 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/<subdir>.

Tip: Rationale

Refer to the rationale for /opt.

This directory was once intended for system information data describing the system since it was booted. These functions have been moved to /run; this directory exists to ensure compatibility with systems and software using an older version of this specification.

In general, the requirements for /run shall also apply to /var/run. It is valid to implement /var/run as a symlink to /run.

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

Programs should not access both /var/run and /run directly, except to access /var/run/utmp. ^[45]

/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. ^[46]

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

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.

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

Tip: Rationale

/var/yp is the standard directory for NIS (YP) data and is almost exclusively used in NIS documentation and systems. ^[47]

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.

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

The following devices must exist under /dev.

Tip: Rationale

Previous versions of the FHS had stricter requirements for /dev. Other devices may also exist in /dev. Device names may exist as symbolic links to other device nodes located in /dev or subdirectories of /dev. There is no requirement concerning major/minor number values.

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

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.

Linux systems place commands relating to filesystem maintenance and boot loader management into /sbin.

Optional files for /sbin:

The sys filesystem is the location where information about devices, drivers, and some kernel features is exposed. Its underlying structure is determined by the particular Linux kernel being used at the moment, and is otherwise unspecified.

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

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: Note

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

Tip: 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.

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