Chapter 1.

File System Structure

1.1. Why Share a Common Structure?

An operating system's file system structure is its most basic level of organization. Almost all of the

ways an operating system interacts with its users, applications, and security model are dependent upon

the way it stores its files on a storage device. It is crucial for a variety of reasons that users, as well as

programs, be able to refer to a common guideline to know where to read and write files.

A file system can be seen in terms of two different logical categories of files:

Shareable vs. unshareable files

Variable vs. static files

Shareable files are those that can be accessed by various hosts; unshareable files are not available to

any other hosts. Variable files can change at any time without any intervention; static files, such as

read only documentation and binaries, do not change without an action from the system administrator

or an agent that the system administrator has placed in motion to accomplish that task.

The reason for looking at files in this way is to help you understand the type of permissions given to

the directory that holds them. The way in which the operating system and its users need to use the files

determines the directory where those files should be placed, whether the directory is mounted read 

only or read write, and the level of access allowed on each file. The top level of this organization is

crucial, as the access to the underlying directories can be restricted or security problems may manifest

themselves if the top level is left disorganized or without a widely used structure.

However, simply having a structure does not mean very much unless it is a standard. Competing

structures can actually cause more problems than they fix. Because of this, Red Hat has chosen the

the most widely used file system structure and extended it only slightly to accommodate special files

used within Red Hat Linux.

1.2. Overview of File System Hierarchy Standard (FHS)

Red Hat is committed to the Filesystem Hierarchy Standard (FHS), a collaborative document that

defines the names and locations of many files and directories. We will continue to track and follow

the standard to keep Red Hat Linux FHS compliant.

The current FHS document is the authoritative reference to any FHS compliant file system, but the

standard leaves many areas undefined or extensible. In this section, we provide an overview of the

standard and a description of the parts of the file system not covered by the standard.

The complete standard is available at:

http://www.pathname.com/fhs

Compliance with the standard means many things, but the two most important are compatibility with

other compliant systems and the ability to mount a

/usr

partition as read only because it contains

common executables and is not meant to be changed by users. Since the

/usr

directory is mounted

read only, it an be mounted from the CD ROM or from another machine via a read only NFS mount.