Installation guide

Chapter 5.
29
File System Structure
5.1. Why Share a Common Structure?
The file system structure is the most basic level of organization in an operating system. Almost all of
the ways an operating system interacts with its users, applications, and security model are dependent
on how the operating system organizes files on storage devices. Providing a common file system
structure ensures users and programs can access and write files.
File systems break files down into two logical categories:
• Shareable vs. unsharable files
• Variable vs. static files
Shareable files can be accessed locally and by remote hosts; unsharable files are only available
locally. Variable files, such as documents, can be changed at any time; static files, such as binaries, do
not change without an action from the system administrator.
Categorizing files in this manner helps correlate the function of each file with the permissions
assigned to the directories which hold them. How the operating system and its users interact with a
file determines the directory in which it is placed, whether that directory is mounted with read-only
or read/write permissions, and the level of of access each user has to that file. The top level of this
organization is crucial; access to the underlying directories can be restricted, otherwise security
problems could arise if, from the top level down, access rules do not adhere to a rigid structure.
5.2. Overview of File System Hierarchy Standard (FHS)
Red Hat Enterprise Linux uses the Filesystem Hierarchy Standard (FHS) file system structure, which
defines the names, locations, and permissions for many file types and directories.
The FHS document is the authoritative reference to any FHS-compliant file system, but the standard
leaves many areas undefined or extensible. This section is an overview of the standard and a
description of the parts of the file system not covered by the standard.
The two most important elements of FHS compliance are:
• Compatibility with other FHS-compliant systems
• The ability to mount a /usr/ partition as read-only. This is especially crucial, since /usr/ contains
common executables and should not be changed by users. In addition, since /usr/ is mounted as
read-only, it should be mountable from the CD-ROM drive or from another machine via a read-only
NFS mount.
5.2.1. FHS Organization
The directories and files noted here are a small subset of those specified by the FHS document. Refer
to the latest FHS document for the most complete information at http://www.pathname.com/fhs/.
5.2.1.1. Gathering File System Information
The df command reports the system's disk space usage. Its output looks similar to the following: