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Chapter 3.
File System Structure
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 upon the way it organizes files on storage devices. Providing a common file system
structure ensures users and programs are able to access and write files.
File systems break files down into two logical categories:
• Shareable vs. unsharable files
• Variable vs. static files
Shareable files are those that 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.
The reason for looking at files in this manner is to help correlate the function of the file with the
permissions assigned to the directories which hold them. The way in which the operating
system and its users interact with a given 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
access each user has to that file. The top level of this organization is crucial. Access to the
underlying directories can be restricted or security problems could manifest themselves if, from
the top level down, it does not adhere to a rigid structure.
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.
Compliance with the standard means many things, but the two most important are compatibility
with other compliant systems and the ability to mount a
partition as read-only. This
/usr/
second point is important because the directory contains common executables and should not
be changed by users. Also, since the
directory is mounted as read-only, it can be
/usr/
mounted from the CD-ROM or from another machine via a read-only NFS mount.
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.
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