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Linux is a computer operating system kernel first developed by Linus Torvalds in an attempt to provide a free Unix-like operating system that ran on Intel 386 processors. It received help from Minix hackers. Packaged with GNU utilities and other free software, Linux distributions (see GNU/Linux) became popular in the mid to late 1990s as a free alternative to the Microsoft Windows operating systems and MacOS on the desktop and proprietary Unices on Servers.
While Linus Torvalds didn't originally set out to make Linux a portable operating system, it has evolved in that direction. Linux is now in fact one of the most widely ported operating system kernels, running on systems as diverse as the Ipaq (a handheld computer) to the IBM S/390 (IBM's massive, hugely expensive mainframe systems). However, it is important to note that Linus's efforts were also directed successfully at a different sort of portability. Portability, according to Linus, was the ability to easily compile applications from a variety of sources on his system; thus Linux originally became popular in part because it required the least effort to get everybody's favorite GPL'd and other open source applications running.
Today Linux is a hybrid monolithic kernel. Device drivers and kernel extensions typically run in ring 0, with full access to the hardware, although some run in user space. Unlike standard monolithic kernels, device drivers are easily configured as modules, and loaded or unloaded while running the system. Also unlike standard monolithic kernels, device drivers can be pre-empted under certain conditions. This latter feature was added to correctly handle hardware interrupts, and to improve support for [symmetric multiprocessing]?.
The name "Linux" properly refers to the Linux kernel, which presents the hardware abstraction layer, disk and filesystem control, multi-tasking, load-balancing, and security enforcement. A "Linux distribution", for example [Red Hat]?, SuSE, Debian, Caldera?, Mandrake etc. is a collection of free and sometimes non-free software created by individuals, groups and organizations from around the world and having the Linux kernel at their core. The resulting operating system is sometimes called "GNU/Linux" (especially in Debian), to distinguish between the kernel and the broader system borrowing much of the software it contains from the Free Software Foundation's GNU project; more commonly, people refer to the complete system as simply "Linux", and so do we in what follows.
Linux is available under the terms of the GNU General Public License for a multitude of platforms and architectures, and has proven highly popular in the server market, primarily for Web and database servers. Desktop Linux users, who had to install and configure their own system, have traditionally been more technologically oriented than those of Microsoft Windows and MacOS, often revelling in the tag of "hacker" or "geek". With the adoption of Linux by several large PC manufacturers, computers with Linux distributions pre-installed have become available, and Linux has begun to make inroads in the wider desktop market. With [desktop managers]? such as KDE and GNOME, Linux offers a graphical user interface more like MacOS/Windows than the traditional Unix [command line interface]?, and many free (though not always open source) software packages offer the functionality of programs available on the other desktop operating systems. See OS Advocacy.