Filesystem Hierarchy Standard

3 The Root Directory

This section describes the root directory structure. The contents of the root filesystem should 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 filesystems.
• To enable recovery and/or repair of a system, those utilities needed by an experienced maintainer to diagnose and reconstruct a damaged system should be present on the root filesystem.
• To restore a system, those utilities needed to restore from system backups (on floppy, tape, etc.) should be present on the root filesystem.

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 has many system-specific configuration files in it. Possible examples include a kernel that is specific to the system, a different hostname, etc. This means that the root filesystem isn't always shareable between networked systems. Keeping it small on networked systems minimizes the amount of space lost on servers to 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 should 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.

BEGIN RATIONALE

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

Tree 3.1

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.

The operating system kernel image should be located in either / or /boot. Additional information on kernel placement can be found in the section regarding /boot, below.