Chapter 3. Overview of the Directory Tree
" Two days later, there was Pooh, sitting on his branch, dangling his legs, and there, beside him, were four pots of honey..." (A.A. Milne)
This chapter describes the important parts of a standard Linux directory tree, based on the FSSTND filesystem 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. Some alternative ways of splitting are also described.
Background
This chapter is loosely based on the Linux filesystem standard, FSSTND, version 1.2 (see the bibliography), 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 will be in their usual places. There is no authority behind the standard that forces anyone to comply with it, but it has got the support of most, if not all, Linux distributions. It is not a good idea to break with the FSSTND without very compelling reasons. The FSSTND attempts to follow Unix tradition and current trends, making Linux systems familiar to those with experience with other Unix systems, and vice versa.
This chapter is not as detailed as the FSSTND. A system administrator should also read the FSSTND for a complete understanding.
This chapter 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 the manual pages.
The full directory tree is intended to be breakable into smaller parts, each on its own disk or partition, to accomodate to disk size limits and to ease backup and other system administration. The major parts are the root, /usr, /var, and /home filesystems (see Figure 3-1). 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.
The roles of the different parts of the directory tree are described below.
The root filesystem is specific for each machine (it is generally stored on a local disk, although it could be a ramdisk or network drive as well) and contains the files that are necessary for booting the system up, and to bring it up to such a state that the other filesystems may be mounted. The contents of the root filesystem will therefore be sufficient for the single user state. It will also contain tools for fixing a broken system, and for recovering lost files from backups.
The /usr filesystem contains all commands, libraries, manual pages, and other unchanging files needed during normal operation. No files in /usr should be specific for any given machine, nor should they be modified during normal use. This allows the files to be shared over the network, which can be cost-effective since it saves disk space (there can easily be hundreds of megabytes in /usr), and can make administration easier (only the master /usr needs to be changed when updating an application, not each machine separately). Even if the filesystem is on a local disk, it could be mounted read-only, to lessen the chance of filesystem corruption during a crash.
The /var filesystem contains files that change, such as spool directories (for mail, news, printers, etc), log files, formatted manual pages, and temporary files. Traditionally everything in /var has been somewhere below /usr, but that made it impossible to mount /usr read-only.
The /home filesystem contains the users' home directories, i.e., all the real data on the system. Separating home directories to their own directory tree or filesystem makes backups easier; the other parts often do not have to be backed up, or at least not as often (they seldom change). A big /home might have to be broken on several filesystems, which requires adding an extra naming level below /home, e.g., /home/students and /home/staff.
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 shareable and changing and non-shareable 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).