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. [1]

Requirements

There must be no subdirectories in /bin.

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

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.

	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.

Specific Options

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

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.

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.

Purpose

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. [2]

Specific Options

The operating system kernel must be located in either / or /boot. [3]

Purpose

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

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.

Purpose

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. [4]

Requirements

No binaries may be located under /etc. [5]

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

Specific Options

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: [6]

mtab does not fit the static nature of /etc: it is excepted for historical reasons. [7]

/etc/opt : Configuration files for /opt

---

Requirements

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

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/<subdir>.

	Rationale
	Refer to the rationale for /opt.

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

/etc/sgml : Configuration files for SGML (optional)

/etc/xml : Configuration files for XML (optional)

Purpose

/home is a fairly standard concept, but it is clearly a site-specific filesystem. [9] The setup will differ from host to host. Therefore, no program should rely on this location. [10]

Requirements

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. [11]

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. [12]

Requirements

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. [13]

Specific Options

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

Purpose

There may be one or more variants of the /lib directory on systems which support more than one binary format requiring separate libraries. [14]

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. [15]

Purpose

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

Rationale

Historically there have been a number of other different places used to mount removeable media such as /cdrom, /mnt or /mnt/cdrom. Placing the mount points for all removeable 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.

Specific Options

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. [16]

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.

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

Requirements

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

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.

Purpose

The root account's home directory may be determined by developer or local preference, but this is the recommended default location. [17]

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. [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]

Requirements

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

Specific Options

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

Purpose

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

Rationale

This main purpose of specifying this is so that users may find the location of the data files for 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. 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]

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.

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.

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.

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. [21]

Purpose

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

Specific Options

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

/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:

	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.

Purpose

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

Specific Options

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

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

Purpose

/usr/lib includes object files, libraries, and 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]

Specific Options

For historical reasons, /usr/lib/sendmail must be a symbolic link to /usr/sbin/sendmail if the latter exists. [24]

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. [25]

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. [26]

/usr/local : Local hierarchy

---

Specific Options

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.

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

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. [29]

Purpose

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.

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.

Requirements

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

Specific Options

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.

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

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.

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

/usr/share/man : Manual pages

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

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

/usr/share/sgml : SGML data (optional)

/usr/share/xml : XML data (optional)

Purpose

Source code may be place placed in this subdirectory, only for reference purposes. [35]

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).

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.

	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.

Specific Options

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

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

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. (Note that the <locale> component may be missing.) [37]

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.

	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.

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 but may be supported by other systems which may comply with the FHS.

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.

	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.

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. 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. [38]

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

Requirements

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

Specific Options

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

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

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>.

	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.

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

Purpose

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

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

/var/lib/misc : Miscellaneous variable data

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. For example, to lock /dev/ttyS0 the file "LCK..ttyS0" would be created. [40]

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.

Purpose

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

Specific Options

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

Purpose

The mail spool must be accessible through /var/mail and the mail spool files must take the form <username>. [41]

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

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.

Purpose

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>.

	Rationale
	Refer to the rationale for /opt.

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. [42] 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.

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.

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

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. [43]

Specific Options

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

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

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

Purpose

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

	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.

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.

Purpose

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

	Rationale
	/var/yp is the standard directory for NIS (YP) data and is almost exclusively used in NIS documentation and systems. [44]

/ : 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.

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

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

• setserial

/dev : Devices and special files

The following devices must exist under /dev.

	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.

/etc : Host-specific system configuration

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

• lilo.conf

/lib64 and /lib32 : 64/32-bit libraries (architecture dependent)

The 64-bit architectures PPC64, s390x, sparc64 and AMD64 must place 64-bit libraries in /lib64, and 32-bit (or 31-bit on s390) libraries in /lib.

The 64-bit architecture IA64 must place 64-bit libraries in /lib.

Rationale

This is a refinement of the general rules for /lib<qual> and /usr/lib<qual>. The architectures PPC64, s390x, sparc64 and AMD64 support support both 32-bit (for s390 more precise 31-bit) and 64-bit programs. Using lib for 32-bit binaries allows existing binaries from the 32-bit systems to work without any changes: such binaries are expected to be numerous. IA-64 uses a different scheme, reflecting the deprecation of 32-bit binaries (and hence libraries) on that architecture.

/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.

/sbin : Essential system binaries

Linux systems place these additional files into /sbin.

• Second extended filesystem commands (optional):

  • badblocks

  • dumpe2fs

  • e2fsck

  • mke2fs

  • mklost+found

  • tune2fs

• Boot-loader map installer (optional):

  • lilo

Optional files for /sbin:

• 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:

  1. I've just removed /lib/<file>.

  2. 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.

  3. I have a static sln, but I don't know what to call the link.

• 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.

/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

/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.

	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.

/var/spool/cron : cron and at jobs

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