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Linux (commonly pronounced IPA: /ˈlɪnəks/ in English; variants exist[1]) is a generic term referring to Unix-like computer operating systems based on the Linux kernel. Their development is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed by anyone under the terms of the GNU GPL[2] and other free licenses.
Linux distributions are predominantly known for their use in servers, although they are installed on a wide variety of computer hardware, ranging from embedded devices and mobile phones to supercomputers,[3] and their popularity as a desktop/laptop operating system has been growing lately due to the rise of netbooks and the Ubuntu distribution of the operating system.[4][5]
The name "Linux" comes from the Linux kernel, originally written in 1991 by Linus Torvalds. The rest of the system, including utilities and libraries, usually comes from the GNU operating system announced in 1983 by Richard Stallman. The GNU contribution is the basis for the alternative name GNU/Linux.[6]
History
 |  | |
| Richard Stallman, left, founder of the GNU project, and Linus Torvalds, right, creator of the Linux kernel | ||
- See also: History of Linux
The Unix operating system was conceived and implemented in the 1960s and first released in 1970. Its wide availability and portability meant that it was widely adopted, copied and modified by academic institutions and businesses, with its design being influential on authors of other systems.
The GNU Project, started in 1984 by Richard Stallman, had the goal of creating a "complete Unix-compatible software system"[7] made entirely of free software. The next year Stallman created the Free Software Foundation and wrote the GNU General Public License (GNU GPL) in 1989. By the early 1990s, many of the programs required in an operating system (such as libraries, compilers, text editors, a Unix shell, and a windowing system) were completed, although low-level elements such as device drivers, daemons, and the kernel were stalled and incomplete.[8] Linus Torvalds has said that if the GNU kernel had been available at the time (1991), he would not have decided to write his own.[9]
MINIX
- Further information: Tanenbaum-Torvalds debate
MINIX, a Unix-like system intended for academic use, was released by Andrew S. Tanenbaum in 1987. While source code for the system was available, modification and redistribution were restricted (that is not the case today). In addition, MINIX's 16-bit design was not well adapted to the 32-bit design of the increasingly cheap and popular Intel 386 architecture for personal computers.
In 1991 while attending the University of Helsinki, Torvalds began to work on a non-commercial replacement for MINIX [10], which would eventually become the Linux kernel. In 1992, Tanenbaum posted an article on Usenet claiming Linux was obsolete. In the article, he criticized the operating system as being monolithic in design and being tied closely to the x86 architecture and thus not portable, as he described "a fundamental error."[11] Tanenbaum suggested that those who wanted a modern operating system should look into one based on the microkernel model. The posting elicited the response of Torvalds, which resulted in a well known debate over the microkernel and monolithic kernel designs.[11]
Linux was dependent on the MINIX user space at first. With code from the GNU system freely available, it was advantageous if this could be used with the fledgling OS. Code licensed under the GNU GPL can be used in other projects, so long as they also are released under the same or a compatible license. In order to make the Linux kernel compatible with the components from the GNU Project, Torvalds initiated a switch from his original license (which prohibited commercial redistribution) to the GNU GPL.[12] Developers worked to integrate GNU components with Linux to make a fully functional and free operating system.[8]
Commercial and popular uptake
Today Linux distributions are used in numerous domains, from embedded systems to supercomputers,[13][14] and have secured a place in server installations with the popular LAMP application stack.[15] Use of Linux distributions in home and enterprise desktops has been rapidly expanding and now claims a significant share of the desktop market.[16][17][18][19][20][21][22]
Linux distributions have also become popular with the newly founded netbook market, with many devices such as the ASUS Eee PC and Acer Aspire One shipping with customized Linux distributions pre-installed.
Current development
Torvalds continues to direct the development of the kernel. Stallman heads the Free Software Foundation, which in turn supports the GNU components. Finally, individuals and corporations develop third-party non-GNU components. These third-party components comprise a vast body of work and may include both kernel modules and user applications and libraries. Linux vendors and communities combine and distribute the kernel, GNU components, and non-GNU components, with additional package management software in the form of Linux distributions.
Design
A Linux-based system is a modular Unix-like operating system. It derives much of its basic design from principles established in Unix during the 1970s and 1980s. Such a system uses a monolithic kernel, the Linux kernel, which handles process control, networking, and peripheral and file system access. Device drivers are integrated directly with the kernel.
Separate projects that interface with the kernel provide much of the system's higher-level functionality. The GNU userland is an important part of most Linux-based systems, providing the most common implementation of the C library, a popular shell, and many of the common Unix tools which carry out many basic operating system tasks. The graphical user interface on most Linux systems is based on the X Window System.
User interface
- See also: User interface
A Linux-based system can be controlled by one or more of a text-based command line interface (CLI), graphical user interface (GUI) (usually the default for desktop), or through controls on the device itself (common on embedded machines).
On desktop machines, KDE, GNOME and Xfce are the most popular user interfaces,[23] though a variety of other user interfaces exist. Most popular user interfaces run on top of the X Window System (X), which provides network transparency, enabling a graphical application running on one machine to be displayed and controlled from another.
Other GUIs include X window managers such as FVWM, Enlightenment and Window Maker. The window manager provides a means to control the placement and appearance of individual application windows, and interacts with the X window system.
A Linux system typically provides a CLI of some sort through a shell, which is the traditional way of interacting with a Unix system. A Linux distribution specialized for servers may use the CLI as its only interface. A “headless system” run without even a monitor can be controlled by the command line via a remote-control protocol such as SSH or telnet.
Most low-level Linux components, including the GNU Userland, use the CLI exclusively. The CLI is particularly suited for automation of repetitive or delayed tasks, and provides very simple inter-process communication. A graphical terminal emulator program is often used to access the CLI from a Linux desktop.
Development
The primary difference between Linux and many other popular contemporary operating systems is that the Linux kernel and other components are free and open source software. Linux is not the only such operating system, although it is the best-known and most widely used. Some free and open source software licences are based on the principle of copyleft, a kind of reciprocity: any work derived from a copyleft piece of software must also be copyleft itself. The most common free software license, the GNU GPL, is a form of copyleft, and is used for the Linux kernel and many of the components from the GNU project.
As an operating system underdog competing with mainstream operating systems, Linux based distributions cannot rely on a monopoly advantage; in order for distributions to be convenient for users, developers aim for interoperability with other operating systems and established computing standards. Linux systems adhere to POSIX,[24] SUS,[25] ISO and ANSI standards where possible, although to date only one Linux distribution has been POSIX.1 certified, Linux-FT.[26]
Free software projects, although developed in a collaborative fashion, are often produced independently of each other. The fact that the software licenses explicitly permit redistribution, however, provides a basis for larger scale projects that collect the software produced by stand-alone projects and make it available all at once in the form of a Linux distribution.
A Linux distribution, commonly called a “distro”, is a project that manages a remote collection of system software and application software packages available for download and installation through a network connection. This allows the user to adapt the operating system to his/her specific needs. Distributions are maintained by individuals, loose-knit teams, volunteer organizations, and commercial entities. A distribution can be installed using a CD that contains distribution-specific software for initial system installation and configuration. A package manager such as Synaptic allows later package upgrades and installs. A distribution is responsible for the default configuration of the installed Linux kernel, general system security, and more generally integration of the different software packages into a coherent whole.
Community
- See also: Free software community
A distribution is largely driven by its developer and user communities. Some vendors develop and fund their distributions on a volunteer basis, Debian being a well-known example. Others maintain a community version of their commercial distributions, as Red Hat does with Fedora.
In many cities and regions, local associations known as Linux Users Groups (LUGs) seek to promote their preferred distribution and by extension free software. They hold meetings and provide free demonstrations, training, technical support, and operating system installation to new users. Many Internet communities also provide support to Linux users and developers. Most distributions and free software / open source projects have IRC chatrooms or newsgroups. Online forums are another means for support, with notable examples being LinuxQuestions.org and the Gentoo forums. Linux distributions host mailing lists; commonly there will be a specific topic such as usage or development for a given list.
There are several technology websites with a Linux focus. Print magazines on Linux often include cover disks including software or even complete Linux distributions.[27][28]
Although Linux distributions are generally available without charge, several large corporations sell, support, and contribute to the development of the components of the system and of free software. These include Dell, IBM, HP, Oracle, Sun Microsystems, Novell, Nokia. A number of corporations, notably Red Hat, have built their entire business around Linux distributions.
The free software licenses, on which the various software packages of a distribution built on the Linux kernel are based, explicitly accommodate and encourage commercialization; the relationship between a Linux distribution as a whole and individual vendors may be seen as symbiotic. One common business model of commercial suppliers is charging for support, especially for business users. A number of companies also offer a specialized business version of their distribution, which adds proprietary support packages and tools to administer higher numbers of installations or to simplify administrative tasks. Another business model is to give away the software in order to sell hardware.
Programming on Linux
Most Linux distributions support dozens of programming languages. The most common collection of utilities for building both Linux applications and operating system programs is found within the GNU toolchain, which includes the GNU Compiler Collection (GCC) and the GNU build system. Amongst others, GCC provides compilers for Ada, C, C++, Java, and Fortran. The Linux kernel itself is written to be compiled with GCC. Proprietary compilers for Linux include the Intel C++ Compiler and IBM XL C/C++ Compiler.
Most distributions also include support for Perl, Ruby, Python and other dynamic languages. Examples of languages that are less common, but still well-supported, are C# via the Mono project, sponsored by Novell, and Scheme. A number of Java Virtual Machines and development kits run on Linux, including the original Sun Microsystems JVM (HotSpot), and IBM's J2SE RE, as well as many open-source projects like Kaffe.
The two main frameworks for developing graphical applications are those of GNOME and KDE. These projects are based on the GTK+ and Qt widget toolkits, respectively, which can also be used independently of the larger framework. Both support a wide variety of languages. There are a number of Integrated development environments available including Anjuta, Code::Blocks, Eclipse, KDevelop, Lazarus, MonoDevelop, NetBeans, and Omnis Studio while the long-established editors Vim and Emacs remain popular.[29]
Uses
As well as those designed for general purpose use on desktops and servers, distributions may be specialized for different purposes including: computer architecture support, embedded systems, stability, security, localization to a specific region or language, targeting of specific user groups, support for real-time applications, or commitment to a given desktop environment. Furthermore, some distributions deliberately include only free software. Currently, over three hundred distributions are actively developed, with about a dozen distributions being most popular for general-purpose use.[31]
Linux is a widely ported operating system kernel. The Linux kernel runs on the most diverse range of computer architectures: in the hand-held ARM-based iPAQ and the mainframe IBM System z9, in devices ranging from mobile phones to supercomputers.[32] Specialized distributions exist for less mainstream architectures. The ELKS kernel fork can run on Intel 8086 or Intel 80286 16-bit microprocessors, while the µClinux kernel fork may run on systems without a memory management unit. The kernel also runs on architectures that were only ever intended to use a manufacturer-created operating system, such as Macintosh computers (with both PowerPC and Intel processors), PDAs, video game consoles, portable music players, and mobile phones.
Desktop
Although there is a lack of Linux ports for some Mac OS X and Microsoft Windows programs in domains such as desktop publishing[33] and professional audio,[34][35][36] support for common applications roughly equivalent to those available for Mac and Windows is available for Linux.[37]
Most Linux distributions provide a program for browsing a list of thousands of free software applications that have already been tested and configured for a specific distribution. These free programs can be downloaded and installed with one mouse click. A digital signature guarantees that no one has added a virus or a spyware to these programs.
The two main frameworks for developing graphical applications are those of GNOME and KDE. These projects are based on the GTK+ and Qt widget toolkits, respectively, which can also be used independently of the larger framework. Both support a wide variety of languages.
Many free software titles that are popular on Windows, such as Pidgin, Mozilla Firefox, OpenOffice.org, and GIMP, are also available in versions that run on Linux. A growing amount of proprietary desktop software is also supported,[38] see List of proprietary software for Linux. In the field of animation and visual effects, most high end software, such as AutoDesk Maya, Softimage XSI and Apple Shake, is available for Linux, Windows and/or Mac OS X. CrossOver is a proprietary solution based on the open source Wine project that supports running Windows versions of Microsoft Office, Intuit applications such as Quicken and QuickBooks, Adobe Photoshop versions through CS2, and many popular games such as World of Warcraft and Team Fortress 2.
Besides the free Windows compatibility layer Wine, most distributions offer dual boot and x86 virtualization for running both Linux and Windows on the same computer. According to the Wine developers, "Wine is still under development, and it is not yet suitable for general use."[39]
The collaborative nature of free software development allows distributed teams to localize Linux distributions for use in locales where localizing proprietary systems would not be cost-effective. For example the Sinhalese language version of the Knoppix distribution was available for a long time before Microsoft Windows XP was translated to Sinhalese. In this case the Lanka Linux User Group played a major part in developing the localized system by combining the knowledge of university professors, linguists, and local developers.
The performance of Linux on the desktop has been a controversial topic; for example, Con Kolivas accused the Linux community of favoring performance on servers. He quit Linux kernel development because he was frustrated with this lack of focus on the desktop, and then gave a "tell all" interview on the topic.[40]
- See also: Linux gaming
Servers and supercomputers
Historically, Linux distributions have mainly been used as server operating systems, and have risen to prominence in that area; Netcraft reported in September 2006 that eight of the ten most reliable internet hosting companies ran Linux distributions on their web servers.[41] (As of June 2008, Linux distributions represented five of ten, FreeBSD three of ten, and Microsoft two of ten.[42]) This is due to its relative stability and long uptime, and the fact that desktop software with a graphical user interface for servers is often unneeded. Enterprise and non-enterprise Linux distributions may be found running on servers. Linux distributions are the cornerstone of the LAMP server-software combination (Linux, Apache, MySQL, Perl/PHP/Python) which has achieved popularity among developers, and which is one of the more common platforms for website hosting. Linux distributions are commonly used as operating systems for supercomputers. As of August 2008, out of the top 500 systems, 423 (84.6%) run a Linux distribution.[43]
Embedded devices
- See also: Embedded Linux and Linux devices
Due to its low cost and ability to be easily modified, an embedded Linux is often used in embedded systems. Linux has become a major competitor to the proprietary Symbian OS found in the majority of smartphones—16.7% of smartphones sold worldwide during 2006 were using Linux[44]—and it is an alternative to the proprietary Windows CE and Palm OS operating systems on mobile devices. Cell phones or PDAs running on Linux and built on open source platform became a trend from 2007, like Nokia N810, Openmoko's Neo1973, Motorola RAZR2 v8, Motorola ROKR E8, Motorola MING series, Motorola ZINE and the on-going Google Android. The popular TiVo digital video recorder uses a customized version of Linux.[45] Several network firewall and router standalone products, including several from Linksys, use Linux internally, using its advanced firewall and routing capabilities. The Korg OASYS and the Yamaha Motif XS music workstations also run Linux.[46] Furthermore, Linux is used in the leading stage lighting control system, FlyingPig/HighEnd WholeHogIII Console.[47]
Market share and uptake
Many quantitative studies of free / open source software focus on topics including market share and reliability, with numerous studies specifically examining Linux.[48] The Linux market is growing rapidly, and the revenue of servers, desktops, and packaged software running Linux is expected to exceed $35.7 billion by 2008.[49]
IDC's report for Q1 2007 says that Linux now holds 12.7% of the overall server market.[50] This estimate was based on the number of Linux servers sold by various companies. Although, with web servers that do not belong to companies, i.e. personal web servers and blog sites, the percentage of overall market share is higher than that of the Microsoft web server.[citation needed]
Estimates for the desktop market share of Linux range from less than one percent to almost two percent. In comparison, Microsoft operating systems hold more than 90%.[51][52][53][54][55][56][57]
The frictional cost of switching operating systems and lack of support for certain hardware and application programs designed for Microsoft Windows have been two factors that have inhibited adoption. Proponents and analysts attribute the relative success of Linux to its security, reliability,[58] low cost, and freedom from vendor lock-in.[59]
Also most recently Google has begun to fund Wine, which acts as a compatibility layer, allowing users to run some Windows programs under Linux.[60][61]
The XO laptop project of One Laptop Per Child is creating a new and potentially much larger Linux community, planned to reach several hundred million schoolchildren and their families and communities in developing countries.[62] Six countries have ordered a million or more units each for delivery in 2007 to distribute to schoolchildren at no charge. Google, Red Hat, and eBay are major supporters of the project.[63] While the XO will also have a Windows option, it will be primarily deployed using Red Hat Enterprise Linux.
Copyright and naming
The Linux kernel and most GNU software are licensed under the GNU General Public License (GPL). The GPL requires that anyone who distributes the Linux kernel must make the source code (and any modifications) available to the recipient under the same terms. In 1997, Linus Torvalds stated, “Making Linux GPL'd was definitely the best thing I ever did.”[64] Other key components of a Linux system may use other licenses; many libraries use the GNU Lesser General Public License (LGPL), a more permissive variant of the GPL, and the X Window System uses the MIT License.
Torvalds has publicly stated that he would not move the Linux kernel (currently licensed under GPL version 2) to version 3 of the GPL, released in mid-2007, specifically citing some provisions in the new license which prohibit the use of the software in digital rights management.[65][66]
A 2001 study of Red Hat Linux 7.1 found that this distribution contained 30 million source lines of code. Using the Constructive Cost Model, the study estimated that this distribution required about eight thousand man-years of development time. According to the study, if all this software had been developed by conventional proprietary means, it would have cost about 1.08 billion dollars (year 2000 U.S. dollars) to develop in the United States.[67]
Most of the code (71%) was written in the C programming language, but many other languages were used, including C++, assembly language, Perl, Python, Fortran, and various shell scripting languages. Slightly over half of all lines of code were licensed under the GPL. The Linux kernel itself was 2.4 million lines of code, or 8% of the total.[67]
In a later study, the same analysis was performed for Debian GNU/Linux version 4.0.[68] This distribution contained over 283 million source lines of code, and the study estimated that it would have cost 5.4 billion euros to develop by conventional means.
In the United States, the name Linux is a trademark registered to Linus Torvalds.[69] Initially, nobody registered it, but on 15 August 1994, William R. Della Croce, Jr. filed for the trademark Linux, and then demanded royalties from Linux distributors. In 1996, Torvalds and some affected organizations sued him to have the trademark assigned to Torvalds, and in 1997 the case was settled.[70] The licensing of the trademark has since been handled by the Linux Mark Institute. Torvalds has stated that he trademarked the name only to prevent someone else from using it, but was bound in 2005 by United States trademark law to take active measures to enforce the trademark. As a result, the LMI sent letters to distribution vendors requesting that a fee be paid for the use of the name, and a number of companies have complied.[71]
- See also: SCO-Linux controversies
GNU/Linux
The Free Software Foundation views Linux distributions which use GNU software as GNU variants and they ask that such operating systems be referred to as GNU/Linux or a Linux-based GNU system.[72] The media and common usage, however, refers to this family of operating systems simply as Linux, as do many large Linux distributions (e.g. Ubuntu and SuSE Linux). Some distributions use GNU/Linux (particularly notable is Debian GNU/Linux), but the term's use outside of the enthusiast community is limited. The naming issue remains a source of confusion to many newcomers, and the naming remains controversial. Linus Torvalds is against the GNU/Linux naming, stating that Linux is not a GNU project. [73
