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FHS (Filesystem Hierarchy Standard) is a set of guidelines and specifications for organizing the structure and layout of directories in a Unix-like operating system.
The goal of FHS is to create a consistent and predictable directory structure across different Unix and Unix-like systems, making it easier for software developers, system administrators, and users to understand where different types of files and data are located.
Required Directories:
Optional Directories:
Linux Specific Directories:
In addition to the directory structure described in the FHS, the following directory can be found on Debian systems:
Filesystem Hierarchy Standard by LSB Workgroup, The Linux Foundation.
ChatGPT
Contains commands for system administrators and users. These commands must be required required when no other filesystems are mounted (e.g. in single user mode). It may also contain commands which are used indirectly by scripts.
In Debian 12 and Ubuntu 24.04 /bin is a symbolic link to /usr/bin.
There must be no subdirectories in /bin.
Command binaries that are not essential enough to place into /bin must be placed in /usr/bin, instead.
Some example commands: cat, chgrp, chmod, chown, cp, date, dd, df, dmesg, echo, false, hostname, kill, ln, login, ls, mkdir, mknod, more, mount, mv, ps, pwd, rm, rmdir, sed, sh, stty, su, sync, true, umount, uname.
Contains everything required for the boot process except configuration files not needed at boot time and the map installer. 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.
The /dev directory is the location of special or device files.
The devices in the /dev directory are created dynamically during the boot process or when new hardware is detected.
Some examples:
The /etc hierarchy contains configuration files.
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 directory (or symbolic link to directory) is required:
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:
Host-specific configuration files for add-on application software packages must be installed within the directory /etc/opt/
No structure is imposed on the internal arrangement of /etc/opt/
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/
Location for all X11 host-specific configuration. This directory is necessary to allow local control if /usr is mounted read only.
The following files, or symbolic links to files, must be in /etc/X11 if the corresponding subsystem is installed:
Generic configuration files defining high-level parameters of the SGML systems.
Files with names *.conf indicate generic configuration files.
Files with names *.cat are the DTD-specific centralized catalogs, containing references to all other catalogs needed to use the given DTD.
Generic configuration files defining high-level parameters of the XML systems are installed here.
Files with names *.conf indicate generic configuration files.
Home directories of users are stored here.
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.
To find a user's home directory, use a library function such as getpwent, getpwent_r of fgetpwent rather than relying on /etc/passwd because user information may be stored remotely using systems such as NIS.
It is recommended that, apart from autosave and lock files, programs should refrain from creating non dot files or directories in a home directory without user consent.
In Debian 12 and Ubuntu 24.04 /lib is a symbolic link to /usr/lib.
Contains shared library images needed to boot the system and run the commands in /bin and /sbin.
At least one of each of the following filename patterns are required (files or symbolic links):
If a C preprocessor is installed, /lib/cpp must be a reference to it, for historical reasons.
The following directory (or symbolic link to directory) 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.
By default; Debian 12 and Ubuntu 24.04 has the following directories:
Debian 12 also has the following directory:
If one or more of these directories exist, the requirements for their contents are the same as the normal /lib directory, except that /lib*/cpp is not required.
Contains subdirectories which are used as mount points for removable media such as floppy disks, cdroms and zip disks.
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.
System administrators may temporarily mount a filesystem here.
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.
This folder is empty on default Debian 12 and Ubuntu 24.04 installations.
Reserved for the installations of add-on application software packages.
The purpose of the /opt directory is to provide a location where software vendors can install their software and organize it in a way that is separate from the rest of the system's directory structure.
A package in /opt must locate its static files in a separate /opt/package or /opt/provider directory tree. 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.
Package files that are variable (change in normal operation) must be installed in /var/opt.
Host-specific configuration files must be installed in /etc/opt.
Recommended home directory for the root user.
The root account's home directory may be determined by developer or local preference, but this is the recommended default location.
It is not recommended to use the root account for tasks that can be performed by an unprivileged user, and that it be used solely for system administration.
For this reason, it is recommended that subdirectories for mail and other applications not appear in the root account's home 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 may have a subdirectory of /run; this is encouraged for programs that use more than one run-time file.
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.
In Debian 12 and Ubuntu 24.04 /sbin is a symbolic link to /usr/bin.
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.
There must be no subdirectories in /sbin.
If a normal user will ever run a command 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.
The following command (or symbolic link) is required in /sbin:
The following files (or symbolic links to files) must be in /sbin if the corresponding subsystem is installed:
This directory is empty on default installations of Debian 12 and Ubuntu 24.04.
Contains site-specific data that is served by the system. This directory provides a location for data that is served by various services running on the system, separate from other files associated with the operating system.
Unlike directories such as /usr or /var, which may contain data related to installed packages, the /srv directory is not intended for distribution packages. It is for locally administered data associated with specific services.
Provides a location for temporary files that are used by programs and users during the course of their activities.
It is recommended that files and directories located in /tmp be deleted whenever the system is booted.
Many applications and system processes use the /tmp directory for temporary file storage. For example, the system's package manager or various software installers may use /tmp to download and store temporary files before installation.
/tmp directory is typically world-writable, allowing any user on the system to create, modify, or delete files within it. This openness facilitates the sharing of temporary files among users and processes.
Stands for "Unix System Resources".
Contains various subdirectories with user-related programs, libraries, documentation, and other resources.
Typically mounted as a separate partition and may be shared among multiple machines in a networked environment.
The following directories (or symbolic links to directories) are required:
The following directories (or symbolic links to directories) are optional:
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:
All of the system's general-use include files for the C programming language should be placed here.
The following directory (or symbolic link to directory) must be in /usr/include, if the corresponding subsystem is installed:
Includes object files and libraries. On some systems, it may also include internal binaries that are not intended to be executed directly by users or shell scripts.
Applications may use a single subdirectory under /usr/lib. If so; all architecture-dependent data exclusively used by the application must be placed within that subdirectory.
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.
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.
By default; Debian 12 and Ubuntu 24.04 has the following directories:
Debian 12 also has the following directory:
/usr/lib performs the same role as /usr/lib for an alternate binary format, except that the symbolic links /usr/lib/sendmail and /usr/lib*/X11 are not required.
The case where /usr/lib and /usr/lib
For use by the system administrator when installing software locally.
It needs to be safe from being overwritten when the system software is updated.
The following directories (or symbolic links to directories), must be in /usr/local:
If directories /lib or /usr/lib exist, the equivalent directories must also exist in /usr/local.
/usr/local/etc may be a symbolic link to /etc/local.
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. There must be no subdirectories in /usr/sbin.
This hierarchy is for all read-only architecture independent data files.
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.
However, that /usr/share is generally not intended to be shared by different OSes or by different releases of the same OS.
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:
This directory is the home for ICC color management files installed by the system.
The following directory must be in /usr/share/color, if the corresponding subsystem is installed:
The top-level directory /usr/share/color must not contain any files; all files should be in subdirectories.
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.
The reason that only word lists are located here is that they are the only files common to all spell checkers.
The following file, or symbolic link to a file, must be in /usr/share/dict:
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.
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.
Contains miscellaneous architecture-independent files which don't require a separate subdirectory under /usr/share.
The following files, or symbolic links to files, must be in /usr/share/misc if the corresponding subsystem is installed:
Contains PostScript Printer Definition (PPD) files, which are used as descriptions of printer drivers by many print systems.
PPD files may be placed in this directory, or in a subdirectory.
Contains architecture-independent files used by SGML applications, such as ordinary catalogs (not the centralized ones, see /etc/sgml), DTDs, entities, or style sheets.
The following directories, or symbolic links to directories, must be in /usr/share/sgml, if the corresponding subsystem is installed:
Other files that are not specific to a given DTD may reside in their own subdirectory.
Contains architecture-independent files used by XML applications, such as ordinary catalogs (not the centralized ones, see /etc/sgml), DTDs, entities, or style sheets.
The following directories (or symbolic links to directories) must be in /usr/share/xml, if the corresponding subsystem is installed:
Source codes may be placed in this subdirectory, only for reference purposes.
Generally, source should not be built within this hierarchy.
Contains variable data files. This includes spool directories and files, administrative and logging data, and transient and temporary files.
Applications must generally not add directories to the top level of /var.
The following directories (or symbolic links to directories) are required in /var:
Following directories are reserved and must not be used arbitrarily by some new application:
The following directories, or symbolic links to directories, must be in /var, if the corresponding subsystem is installed:
There are two more directory standarts specific to Linux:
The proc filesystem is the de-facto standard Linux method for handling process and system information, rather than /dev/kmem and other similar methods.
/proc is a virtual filesystem, which means that it doesn't exist on a physical storage device like a hard drive. Instead, it is dynamically generated by the kernel and provides a window into the current state of the running kernel and system.
The sys filesystem is the location where information about devices, drivers, and some kernel features are exposed.
The /sys directory exposes information about various kernel parameters and configurations. You can read and modify certain kernel parameters through the files in this directory.
/sys is a virtual filesystem, which means that it doesn't exist on a physical storage device like a hard drive.