Distributions

Technically speaking, there is no such thing as a "Linux system". Indeed, there only exists a Linux kernel. Systems that are said to be running the "Linux operating system", are in fact Unix-like systems, incorporating the Linux kernel, and running GNU utilities and BSD software. For this reason, a Linux systems is sometimes more correctly referred to as a "GNU/Linux" system, indicating its reliance on system software developed by the Free Software Foundation's GNU project.

Many Linux systems operate the X Window system for windowing and graphics. A free X Window implementation, called XFree86, exists and conforms to the X/Open standard. Therefore, most existing X based programs run under Linux without any modification.

It is possible to build a complete Linux system from scratch, by building and installing the kernel together with a set of utilities (LinuxFromScratch). This is however a monumental task, which is best left to hardcore hackers. For this reason, Linux distributions exist.

A Linux distribution is the collection of a Linux kernel, some operating system-level utilities and user-level applications, (usually) a desktop environment, together with an installation program. A common set of basic tools is present in all distributions. The distributions differ in the number, the types and the version numbers of extra tools, as well as in the boot-loader and the installation program that are included. Most distributions furthermore provide a packaging system, which bundles all files that comprise a logical unit (e.g. a web-server or a mail client) into a single package that can be easily added, removed or updated. Such a packaging system can also verify, before installing a given package, whether the other packages on which this package relies, have already been installed.

Depending on the application, a different distribution will be the most appropriate one. Indeed, the packagers of the different distributions have different primary goals, such as ease of installation, ease of upgrades, software development, security, etc. Many distributions are available freely via ftp. Most can also be bought on CD-ROM.

Three broad categories of distributions exist: standard general-purpose distributions (e.g. Slackware, Red Hat, SuSE, ...), distributions designed for specific purposes (e.g. a secure web-server, a router, ...), and minimalist distributions (for instance fitting on a single floppy). In the remainder of this section, some general-purpose distributions will be discussed.

The first well-known distribution was "Soft Landing Systems" (SLS) in 1992, which was later improved and renamed Slackware. The first versions of this distribution were hard to install; one had to install different "disk-sets" with 10 to 20 floppy disks each. They required manually editing a large number of configuration files after installing the software packages. A software package was simply a compressed tar file, with no information on how its files interrelate, no dependencies on other packages, and no path for upgrades.

Since then, installing Slackware has become a lot easier. Furthermore, a huge number of new distributions were created, such as Red Hat and SuSE. The first Red Hat distribution was released in 1994, based on the version 1.0 kernel.

The "Linux Standard Base" project aims at developing a set of standards that will increase compatibility among Linux distributions and enable software applications to run on any compliant Linux system. The companies behind most of the major distributions are a member of the group behind this project. This group is called "The Free Standards Group". An affiliated project is the "Filesystem Hierarchy Standard".