GNU (pronunciation ) is a computer operating system composed entirely of free software. Its name is a recursive acronym for GNU's Not Unix; it was chosen because its design is Unix-like, but differs from Unix by being free software and containing no Unix code. Development of GNU was initiated by Richard Stallman and was the original focus of the Free Software Foundation (FSF).
GNU is developed by the GNU Project, and programs released under the auspices of the project are called GNU packages or GNU programs. The system's basic components include the GNU Compiler Collection (GCC), the GNU Binary Utilities (binutils), the bash shell, the GNU C library (glibc), and GNU Core Utilities (coreutils).
GNU is in active development. Although nearly all components have been completed long ago and have been in production use for a decade or more, its official kernel, GNU Hurd, is incomplete and not all GNU components work with it. Thus, the third-party Linux kernel is most commonly used instead. While this kernel has not been officially adopted by the GNU project, some third-party software is included, such as the X.Org release of the X Window System and the TeX typesetting system. Many GNU programs have also been ported to numerous other operating systems such as Microsoft Windows, BSD variants, Solaris and Mac OS.
The goal was to bring a wholly-free software operating system into existence. Stallman wanted computer users to be free, as most were in the 1960s and 1970s — free to study the source code of the software they use, free to share the software with other people, free to modify the behaviour of the software, and free to publish their modified versions of the software. This philosophy was later published as the GNU Manifesto in March 1985.
Richard Stallman's experience with the Incompatible Timesharing System (ITS), an early operating system written in assembly language that became obsolete due to discontinuation of PDP-10, the computer architecture for which ITS was written, led to a decision that a portable system was necessary. It was thus decided that GNU would be mostly compatible with Unix. At the time, Unix was already a popular proprietary operating system. The design of Unix had proven to be solid, and it was modular, so it could be reimplemented piece by piece.
Much of the needed software had to be written from scratch, but existing compatible free software components were also used such as the TeX typesetting system, and the X Window System. Most of GNU has been written by volunteers; some in their spare time, some paid by companies, educational institutions, and other non-profit organizations. In October 1985, Stallman set up the Free Software Foundation (FSF). In the late 1980s and 1990s, the FSF hired software developers to write the software needed for GNU.
As GNU gained prominence, interested businesses began contributing to development or selling GNU software and technical support. The most prominent and successful of these was Cygnus Solutions, now part of Red Hat.
The initial plan for GNU was to be mostly Unix-compatible, while adding enhancements where they were useful. By 1990, the GNU system had an extensible text editor (Emacs), a very successful optimizing compiler (GCC), and most of the core libraries and utilities of a standard Unix distribution. As the goal was to make a whole free operating system exist - rather than necessarily to write a whole free operating system - Stallman tried to use existing free software when possible. In the 1980s there was not much free software, but there was the X Window System for graphical display, the TeX typesetting system, and the Mach microkernel. These components were integrated into GNU.
In the GNU Manifesto, Stallman had mentioned that "an initial kernel exists but many more features are needed to emulate Unix." He was referring to TRIX, a remote procedure call kernel developed at Massachusetts Institute of Technology, whose authors had decided to distribute it as free software, and was compatible with Version 7 Unix. In December 1986, work had started on modifying this kernel. However, the developers eventually decided it was unusable as a starting point, primarily because it only ran on "an obscure, expensive 68000 box" and would therefore have to be ported to other architectures before it could be used.
The GNU Project's early plan was to adapt the BSD 4.4-Lite kernel for GNU. However, due to a lack of cooperation from the Berkeley programmers, by 1988 Stallman decided instead to use the Mach kernel being developed at Carnegie Mellon University, although its release as free software was delayed until 1990 while its developers worked to remove code copyrighted to AT&T. Thomas Bushnell, the initial Hurd architect, said in hindsight that the decision to start a new kernel rather than adapt the BSD work set the project back considerably, and that the project should have used the BSD kernel for this reason.
The design of the kernel was to be GNU's largest departure from "traditional" Unix. GNU's kernel was to be a multi-server microkernel, and was to consist of a set of programs called servers that offers the same functionality as the traditional Unix kernel. Since the Mach microkernel, by design, provided just the low-level kernel functionality, the GNU Project had to develop the higher-level parts of the kernel, as a collection of user programs. Initially, this collection was to be called Alix, but developer Thomas Bushnell later preferred the name Hurd, so the Alix name was moved to a subsystem and eventually dropped completely. Eventually, development progress of the Hurd became very slow due to ongoing technical issues.
Despite an optimistic announcement by Stallman in 2002 predicting a release of GNU/Hurd, further development and design are still required. The latest release of the Hurd is version 0.2. It is fairly stable, suitable for use in non-critical applications. As of 2005, Hurd is in slow development, and is now the official kernel of the GNU system. There are also projects working on porting the GNU system to the kernels of FreeBSD, NetBSD, and OpenSolaris.
After the Linux kernel became usable and was switched to a free software license, it became the most common host for GNU software. The GNU project coined the term GNU/Linux for such systems.
The GNU Project suggests contributors assign the copyright for GNU packages to the Free Software Foundation although this is not required.
Copyright law grants the copyright-holder significant control over the copying and distributing of a work, but FSF wrote a license for the GNU software which grant recipients permission to copy and redistribute the software under highly permissive terms. For most of the 80s, each GNU package had its own license - the Emacs General Public License, the GCC General Public License, etc. In 1989, FSF published a single license they could use for all their software, and which could be used by non-GNU projects: the GNU General Public License (GPL).
This license is now used by most GNU programs, as well as a large number of free software programs that are not part of the GNU project; it is the most commonly used free software license. It gives all recipients of a program the right to run, copy, modify and distribute it, while forbidding them from imposing further restrictions on any copies they distribute. This idea is often referred to as copyleft.
In 1991, the GNU Lesser General Public License (LGPL) was written for certain libraries. 1991 also saw the release of version 2 of the GNU GPL. The GNU Free Documentation License (FDL), for documentation, followed in 2000. The GPL and LGPL were revised to version 3 in 2007, improving their international applicability, and adding protection for users whose hardware restricts software changes.
Most GNU software is distributed under the GPL. A minority is distributed under the LGPL, and a handful of packages are distributed under permissive free software licences.
Many GNU programs have been ported to a multitude of other operating systems, including various proprietary platforms such as Microsoft Windows and Mac OS X. They are often installed on proprietary UNIX systems as a replacement for proprietary utilities, however, this is often a hot topic among enthusiasts, as the motive for developing these programs was to replace those systems with free software, not to enhance them. These GNU programs have in contested cases been tested to show as being more reliable than their proprietary Unix counterparts.
As of 2007, there are a total of 319 GNU packages hosted on the official GNU development site.
Other GNU variants which do not use the Hurd as a kernel include Debian GNU/kFreeBSD and Debian GNU/NetBSD from Debian, Nexenta OS (GNU plus the kernel of OpenSolaris) and GNU-Darwin. GNU itself is distributed as Debian GNU/Hurd by the Debian project, and a Live CD is also available from Superunprivileged.org