Filesystem Hierarchy Standard (FHS) 2.3 defines where files and directories belong in a UNIX-like filesystem layout. Released on January 29, 2004, it is a historical edition: the current official FHS publication page identifies version 3.0, published September 23, 2026. FHS 2.3 remains useful for understanding the traditional roles of paths such as /usr, /var, and /etc, but it should not be mistaken for the latest edition or a guarantee that every modern distribution follows every rule literally.
What FHS 2.3 is for
FHS 2.3 is a reference standard describing the organization of files and directories in UNIX-like systems. Its goals include improving interoperability among applications, administration and development tools, and scripts, as well as making system documentation more uniform. The specification is aimed primarily at system integrators, package developers, and system administrators; it is not a step-by-step guide to managing a system.
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The Linux Foundation archive identifies January 29, 2004 as the release date of the final 2.3-series specification and offers digital editions: FHS archive. The current official publication page lists FHS 3.0, published September 23, 2026: current FHS publication.
How the standard organizes the hierarchy
FHS 2.3 classifies filesystem areas using two distinctions:
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- Shareable or unshareable: Shareable files can be stored on one host and used by others. Host-specific configuration, for example, is unshareable.
- Static or variable: Static files do not change without intervention by the system administrator. Variable data changes during normal operation, such as logs and spool files.
These distinctions help explain why directories with different characteristics might reside on separate filesystems. The specification gives /usr and /opt as shareable, static areas; /etc and /boot as unshareable, static areas; and parts of /var as variable. This is an example of a compliant arrangement, not the only possible layout.
What the main directories mean in FHS 2.3
/: the root filesystem
The root filesystem must contain enough to boot the system and to restore, recover, or repair it. FHS 2.3 is designed so that /usr, /opt, and /var can be placed on other partitions or filesystems. Separating them does not remove the root filesystem’s essential boot and recovery role.
/etc: host-specific configuration
/etc holds system configuration specific to the host. In the standard’s example of filesystem characteristics, it is static and unshareable: it describes how this particular system is configured rather than being general software data intended for sharing across hosts.
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/usr: shareable, read-only data
The FHS 2.3 specification states: “/usr is the second major section of the filesystem. /usr is shareable, read-only data.” Most user commands belong in /usr/bin, while architecture-independent data belongs in /usr/share. Large software packages must not create a direct subdirectory under /usr.
/usr/local is the administrator’s local software hierarchy. Its purpose is to keep locally installed software safe from being overwritten by updates to system software.
/var: operational data that changes
/var holds variable data, including spool files, administrative data, logs, transient files, and temporary files. Keeping this changing data separate supports arrangements in which /usr is mounted read-only.
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/tmp and /var/tmp are both used for temporary data, but FHS 2.3 distinguishes their retention expectations: /var/tmp is for temporary files intended to be preserved across system reboots, unlike temporary data that need not persist that way. They are not interchangeable under the standard’s definitions.
/opt: add-on application packages
/opt is for add-on application software packages. The standard assigns package-specific variable data to /var/opt and host-specific configuration to /etc/opt.
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Using the distinctions to compare paths
| Path | Role in FHS 2.3 | Data characteristics in the standard’s example | Placement significance |
|---|---|---|---|
/ |
Boot, recovery, and repair essentials | Contains the essentials needed for those tasks | /usr, /opt, and /var may be on separate filesystems |
/etc |
Host-specific system configuration | Static and unshareable | Belongs with configuration tied to the individual host |
/usr |
Commands and other system software data | Shareable and read-only | Designed as a major hierarchy that may be separated from / |
/var |
Logs, spool, administrative, transient, and temporary data | Variable | Separating it accommodates data that changes during operation |
/opt |
Add-on application packages | Shareable and static in the standard’s example | Related variable data and configuration go in /var/opt and /etc/opt |
The table describes roles in FHS 2.3, not a promise about a particular current operating system. Individual distributions can have their own policies, and the current FHS edition is later than 2.3.
Where FHS 2.3 fits today
FHS 2.3 is useful when interpreting the historical rationale for familiar directory names, reading documentation that targets that edition, or understanding older conformance requirements. For example, the Linux Standard Base 2.0 specification required conforming implementations to adhere to FHS 2.3 and conforming applications to follow it: Linux Standard Base 2.0 specification. That relationship applies to that historical standard; it does not establish a blanket requirement for every current Linux distribution.
When applying the standard to a real system, distinguish the intended directory role from the distribution’s actual implementation. FHS 2.3 explains the conventions and their rationale; it does not by itself establish how a particular present-day system handles every path.
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