Chapter 3 : Organization of the file tree

These days, a Unix system is big, very big, and that is particularly true with Linux: the profusion of software available would make it an unmanageable system if there were not guidelines for the location of files in the tree structure.

The acknowledged standard in this respect is FHS (Filesystem Hierarchy Standard), which is on version 2.0 at the time of writing of this manual. The document which describes the standard is available on the Internet in different formats at URL http://www.pathname.com/fhs/. This chapter gives only a brief summary, but it should be enough to teach you in what directory to look for (or place) a given file.

Shareable and unshareable, static and variable data

Data on a Unix system can be classified according to these two criteria. You will have guessed what they both mean: shareable data is data that can be common to several machines in a network, while unshareable data cannot be. Static data must not been modified in normal use, while variable data can be. As we explore the tree structure, we will classify the different directories into each of these categories.

Note that these classifications are only recommended. You are not obliged to follow them, but adopting these guidelines will greatly help you manage your system. Note, too, that the static/variable distinction only applies to the use of the system and not its configuration. If you install a program, you will obviously have to modify "normally" static directories, e.g. /usr.

The root directory: /

The root directory contains the whole system hierarchy. It cannot be classified since its subdirectories may or may not be static or shareable. Here is a list of the main directories and subdirectories:

• /bin: essential binary files. This directory contains the basic commands which will be used by all users and are necessary to the operation of the system: ls, cp, login, etc. Static, unshareable.
• /boot: contains the files required by the Linux boot manager (LILO for Intel platforms). This may or may not contain the kernel: if it is not here, it must be located in the root directory. Static, unshareable.
• /dev: system device files (dev for DEVices). Static, unshareable.
• /etc: this directory contains all the configuration files specific to the machine.
• /home: contains all the personal directories of users of the system. This directory may or may not be shareable (some large networks make it shareable by NFS), and it is obviously variable.
• /lib: this directory contains the libraries essential to the system and the kernel modules, in /lib/modules. All the libraries required by the binaries in directories /bin and /sbin must be located here, together with the linker ld.so. Static, unshareable.
• /mnt: directory containing the mounting points for the temporary file systems. Variable, unshareable.
• /opt: this directory contains packages not required for system operation. It is recommended to place static files (binaries, libraries, manual pages, etc.) for such packets in /opt/package_name and their specific configuration files for the machine in /etc/opt.
• /root: personal directory of the Almighty. Variable, unshareable.
• /usr: see next section. Static, shareable.
• /sbin: contains the system binaries essential to system startup, operable by root only. A normal user can also run them but will not get very far. Static, unshareable.
• /tmp: directory intended to contain temporary files which certain programs may create. Variable, unshareable .
• /var: location for data which may be modified in real time by programs (e.g. the e-mail server, audit programs, the print server, etc.). All of /var is variable, but its different subdirectories may be shareable or unshareable.

/usr: the big one

The /usr directory is the main application storage directory. All the binary files in this directory must not be required for the system startup or maintenance, since the /usr hierarchy is very often a separate filesystem. Given its often large size, /usr has its own hierarchy of subdirectories. We will mention just a few:

• /usr/X11R6: the whole X Window System hierarchy. All the binaries required for the operation of X (obviously including the X servers) and all the necessary libraries must be located in it. The /usr/X11R6/lib/X11 directory contains all aspects of the configuration of X which do not vary from one machine to another. Specific configuration for each machine is in /etc/X11.
• /usr/bin: this directory contains the large majority of the system's binary programs. Any binary program which is not necessary to the maintenance of the system and is not a system administration program must be located in this directory, apart from programs you install yourselves, which must be in /usr/local.
• /usr/lib: this directory contains all the libraries necessary to run programs located in /usr/bin and /usr/sbin. There is also a symbolic link /usr/lib/X11 pointing to the directory which contains the X Window System libraries, /usr/X11R6/lib (if X Window System is installed, of course).
• /usr/local: this is the directory where you should install your personal applications. The installation program will have created the necessary hierarchy: lib/, man/, etc.
• /usr/share: this directory contains all the data required by the applications in /usr/, and all the independent architecture data. Among other things, you will find in it zone and location information (zoneinfo and locale).

There are also /usr/doc and /usr/man directories, which respectively contain application documentation and the system manual pages, but the standard recommends they be eventually moved to /usr/share.

/var: data modifiable during use

The /var directory contains all the operating data for programs running on the system. Unlike the working data in /tmp, these data must be kept intact on the event of a reboot. There are many subdirectories, and some are very useful:

• /var/log: contains the system log files;
• /var/spool: contains the system daemon working files. For example, /var/spool/lpd contains the print server working files and /var/spool/mail e-mail server working files (i.e. all mail arriving on and leaving your system).
• /var/run: this directory is used to keep track of all processes being used by the system, so that you can act on these processes in the event of a change of system runlevel (see a previous chapter).

/etc: configuration files

The /etc directory is one of the essential directories in any Unix system. It contains all the basic system configuration files. Please do not delete it to save space! Likewise, if you want to extend your tree structure over several partitions, remember that /etc must not be put on a separate partition: it is needed for system initialization.

Some important files are:

• passwd and shadow: these two files are text files which contains all the system users and their passwords (encrypted :)). shadow is only there are if you use shadow passwords, but it is the default installation option;
• inittab: is the configuration file for the program init, which plays a fundamental role when starting up the system, as we will see later on;
• services: this file contains a list of existing network services;
• profile: this is the shell configuration file, although certain shells use others. For example, Bash uses bashrc;
• crontab: configuration file of cron, the program responsible for periodic execution of commands.

There are also certain subdirectories for programs which require large numbers of files for configuration. This applies to X Window System, for example, which has the whole /etc/X11 directory.