What are Directory Tree and Filesystem Hierarchy in Linux


What are Directory Tree and Filesystem Hierarchy in Linux

Recipe ID: hsts-r57


Recipe Overview

This article describes the important parts of a standard Linux directory tree, based on the Filesystem Hierarchy Standard . It outlines the normal way of breaking the directory tree into separate filesystems with different purposes and gives the motivation behind this particular split. Not all Linux distributions follow this standard slavishly, but it is generic enough to give you an overview.

 

1. Background
This article is loosely based on the Filesystems Hierarchy Standard (FHS). version 2.1, which attempts to set a standard for how the directory tree in a Linux system is organized. Such a standard has the advantage that it will be easier to write or port software for Linux, and to administer Linux machines, since everything should be in standardized places. There is no authority behind the standard that forces anyone to comply with it, but it has gained the support of many Linux distributions. It is not a good idea to break with the FHS without very compelling reasons. The FHS attempts to follow Unix tradition and current trends, making Linux systems familiar to those with experience with other Unix systems, and vice versa.
This article is not as detailed as the FHS. A system administrator should also read the full FHS for a complete understanding.
This article does not explain all files in detail. The intention is not to describe every file, but to give an overview of the system from a filesystem point of view. Further information on each file is available elsewhere in this manual or in the Linux manual pages.
The full directory tree is intended to be breakable into smaller parts, each capable of being on its own disk or partition, to accommodate to disk size limits and to ease backup and other system administration tasks. The major parts are the root (/ ), /usr , /var , and /home filesystems (see Figure 1). Each part has a different purpose. The directory tree has been designed so that it works well in a network of Linux machines which may share some parts of the filesystems over a read-only device (e.g., a CD-ROM), or over the network with NFS.

Figure 1. Parts of a Unix directory tree. Dashed lines indicate partition limits.
Linux guide for system admins
The roles of the different parts of the directory tree are described below.

Although the different parts have been called filesystems above, there is no requirement that they actually be on separate filesystems. They could easily be kept in a single one if the system is a small single-user system and the user wants to keep things simple. The directory tree might also be divided into filesystems differently, depending on how large the disks are, and how space is allocated for various purposes. The important part, though, is that all the standard names work; even if, say, /var and /usr are actually on the same partition, the names /usr/lib/libc.a and /var/log/messages must work, for example by moving files below /var into /usr/var, and making /var a symlink to /usr/var.
The Unix filesystem structure groups files according to purpose, i.e., all commands are in one place, all data files in another, documentation in a third, and so on. An alternative would be to group files files according to the program they belong to, i.e., all Emacs files would be in one directory, all TeX in another, and so on. The problem with the latter approach is that it makes it difficult to share files (the program directory often contains both static and sharable and changing and non-sharable files), and sometimes to even find the files (e.g., manual pages in a huge number of places, and making the manual page programs find all of them is a maintenance nightmare).


2. The root filesystem
The root filesystem should generally be small, since it contains very critical files and a small, infrequently modified filesystem has a better chance of not getting corrupted. A corrupted root filesystem will generally mean that the system becomes unbootable except with special measures (e.g., from a floppy), so you don't want to risk it.
The root directory generally doesn't contain any files, except perhaps on older systems where the standard boot image for the system, usually called /vmlinuz was kept there. (Most distributions have moved those files the the /boot directory. Otherwise, all files are kept in subdirectories under the root filesystem:

 

 The /etc directory
The /etc maintains a lot of files. Some of them are described below. For others, you should determine which program they belong to and read the manual page for that program. Many networking configuration files are in /etc as well, and are described in the Networking Administrators' Guide.

 

4. The /dev directory
The /dev directory contains the special device files for all the devices. The device files are created during installation, and later with the /dev/MAKEDEV script. The /dev/MAKEDEV.local is a script written by the system administrator that creates local-only device files or links (i.e. those that are not part of the standard MAKEDEV, such as device files for some non-standard device driver).
This list which follows is by no means exhaustive or as detailed as it could be. Many of these device files will need support compiled into your kernel for the hardware. Read the kernel documentation to find details of any particular device.
If you think there are other devices which should be included here but aren't then let me know. I will try to include them in the next revision.

 

5. The /usr filesystem.
The /usr filesystem is often large, since all programs are installed there. All files in /usr usually come from a Linux distribution; locally installed programs and other stuff goes below /usr/local. This makes it possible to update the system from a new version of the distribution, or even a completely new distribution, without having to install all programs again. Some of the subdirectories of /usr are listed below (some of the less important directories have been dropped; see the FSSTND for more information).

 

6. The /var filesystem
The /var contains data that is changed when the system is running normally. It is specific for each system, i.e., not shared over the network with other computers.

 

7. The /proc filesystem
The /proc filesystem contains a illusionary filesystem. It does not exist on a disk. Instead, the kernel creates it in memory. It is used to provide information about the system (originally about processes, hence the name). Some of the more important files and directories are explained below. The /proc filesystem is described in more detail in the proc manual page.

Note that while the above files tend to be easily readable text files, they can sometimes be formatted in a way that is not easily digestible. There are many commands that do little more than read the above files and format them for easier understanding. For example, the freeprogram reads /proc/meminfo converts the amounts given in bytes to kilobytes (and adds a little more information, as well).

Additional Linux Resources

Here is a list of resources for learning Linux:
Resources for System Administrators

Resources for Linux Kernel Programmers

Linux File System Dictionary
Comprehensive Review of How Linux File and Directory System Works

Hands-on Linux classes

Linux Operating System Distributions

 


Private and Custom Tutoring

We provide private tutoring classes online and offline (at our DC site or your preferred location) with custom curriculum for almost all of our classes for $50 per hour online or $75 per hour in DC. Give us a call or submit our private tutoring registration form to discuss your needs.


View Other Classes!