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This article is about Linux-based operating systems, GNU/Linux, and related topics. See Linux kernel for technical details of the Linux kernel.

Linux strictly refers to the Linux kernel, but is commonly used to describe the entire free Unix-like operating system, also known as GNU/Linux, that is formed by combining the Linux kernel with the GNU libraries and tools. The term "Linux" is even applied to whole Linux distributions, which typically bundle large quantities of software, from web servers like Apache to graphical environments like GNOME to word processors like OpenOffice.org, with the core operating system.

The official logo of Linux is Tux the penguin (see Linux kernel). Local Linux User Groups exist as forums for users of Linux-based operating systems in most areas. The Linux trademark (SN: 1916230) is owned by Linus Torvalds, and is defined as "Computer operating system software to facilitate computer use and operation."

There are many Linux distributions (distros), assembled by individuals, corporations, and other organizations, and each may include any number of additional system software and application programs, as well as a program to install the whole system on a new computer. The core of each distribution includes the Linux kernel, but also various software packages from the GNU project including a shell and utilities such as libraries, compilers and editors. Because these facilities (without which the system would not resemble Unix from a user perspective) stem from a longstanding free operating-system project that pre-dates the Linux kernel itself, Richard Stallman of GNU asks users to refer to the entire system as GNU/Linux. Some people do; most simply call the system "Linux".

Most systems also include tools and utilities from BSD backgrounds and typically use XFree86 to provide the underpinnings of a GUI interface.

Linux users, who traditionally had to install and configure their own system, have been stereotypically more technologically oriented than those of Microsoft Windows and Mac OS, often revelling in the tag of "hacker" or "geek". With the adoption of Linux by several large PC manufacturers, however, computers with Linux distributions pre-installed have become available and Linux has begun to make slow inroads in the wider desktop market. Linux is also a cornerstone of the LAMP server-software combination that has achieved widespread popularity among web developers.

Linux is also being used as an embedded operating system. The relatively low cost of Linux makes it possible to use it in devices such as the Simputer, a low-cost computer aimed especially at low-income populations in developing nations.

With desktop environments such as KDE and GNOME, Linux offers a graphical user interface more like Mac OS or Windows than the traditional Unix command line interface, and many no-cost (though not always open source) software packages offer the functionality of programs available on the other desktop operating systems.

One study of the Red Hat Linux 7.1 distribution found that this particular distribution contained 30 million physical source lines of code (SLOC). Using the COCOMO cost model, it could be estimated that this distribution required about 8,000 person-years of development time. Had it been developed by conventional proprietary means, it would have cost over $1.08 billion (1,000 million) to develop in the U.S. (in year 2000 dollars). The majority of its code (71%) was in C, but many other languages were used including C++, shell, Lisp, assembly, Perl, Fortran, and Python. Slightly over half of all its code (counting by line) was licensed under the GPL. The Linux kernel contained 2.4 million lines of code, or 8% of the total, showing that the vast majority of a Linux operating system is not contained in the Linux kernel.

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GNU/Linux is the term used by the GNU project and its supporters, promoted in particular by its founder and main activist Richard Stallman, to refer to the Linux operating system.

The name "GNU/Linux" was first used by Debian in 1994 as the name of their OS distribution based on the Linux kernel and GNU programs. (In 1992, The Yggrasil distribution was called Linux/GNU/X).

In GNU's 1994-June Bulletin, Linux is referred to as a "free UNIX clone" (with many GNU utilities and libraries). In the 1995-January edition, the references to Linux were changed to "GNU/Linux".

In May of 1996, Richard Stallman released Emacs 19.31, changing the system target "Linux" to "Lignux". He argued that to give rightful credit to GNU, it is proper to use the terms "Linux-based GNU system", "GNU/Linux system", or "Lignux" to refer to the combination of the Linux kernel and the GNU system. Stallman later stopped using the term "Lignux" and used "GNU/Linux" exclusively.

The requests to call the system "GNU/Linux" have met with mixed success at best. Only a few distributions have followed the lead of Debian in calling their systems "GNU/Linux". The corporate world, including most media outlets, do not. Amongst the users and developers in the free software and open source movements, some have followed this request; many others have ignored or opposed it.

Some consider the term "operating system" to refer to only the kernel, while the rest are simply utilities (regardless of the practical necessity and volume of such utilities). In this sense, the operating system is called Linux, and a Linux distribution is based on Linux with the addition of the GNU tools. On the other hand, both the name GNU and the name Linux are intentionally parallel to the name Unix, and Unix has always referred to the C library and userland tools as well as the kernel.

Some of the reasons people refer to the system as "Linux" instead of "GNU/Linux" are because the former is shorter and thus easier to say, because kernel-author Torvalds has called the combined system Linux since its 1991 release, and because Stallman only began asking people to call the system "GNU/Linux" in the mid 1990s after the system had become popular.

One practical problem with the use of the word "Linux" to refer to both the kernel and the distributions as a whole is that it has often led to confusion in the popular media (and hence among the general public). Thus, media sources frequently make erroneous statements such as claims that the entire Linux operating system (in the popular sense) was written from scratch by Torvalds in 1991, that Torvalds directs the development of other components such as graphical interfaces, or that new releases of the kernel involve a similar degree of user-visible change to new major versions of proprietary operating systems such as Windows.

History The history of Linux is heavily tied to that of GNU project, a prominent free-software project led by Richard Stallman. The GNU project was begun in 1984 to develop a complete Unix-like operating system composed entirely of free software. By 1991, when Linux was written, the GNU project had produced nearly all of the components of this system, including a shell, a C library[?], and a C compiler. The kernel of the system was incomplete, however, because the GNU kernel (called the Hurd) was so ambitious that it proved unexpectedly difficult.

The Linux kernel was initially written as a hobby by a young Finnish university student, Linus Torvalds, who was attending the University of Helsinki. But, subsequently, thousands of volunteers of computer programmers thoughout the world have participated in the project. Torvalds and other early Linux developers adapted the GNU components to work with the Linux kernel, creating a completely functional operating system.

Linux, thus, filled that final gap in the GNU operating system. Although the Linux kernel is licensed under the GNU General Public License, it is not part of the GNU project.

Early in 2003, SCO filed a lawsuit against IBM claiming that IBM had included portions of SCO's intellectual property into the Linux kernel. Additionally, SCO reportedly sent letters to many companies warning them that Linux may be a liability.
See SCO v. IBM Linux lawsuit for details.

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