UNIX® and Linux® are two types of operating systems which can be quite similar from a user's point of view, but are very different internally. The operating system (OS) kernels are structured differently and require different device drivers. Linux® operating systems are often entirely open-source software while many UNIX® implementations are not. UNIX® and Linux® are frequently used by businesses, governments and students for a large variety of computing tasks. They are also used in many hand-held devices such as cell phones, Personal Digital Assistants (PDAs) and netbook computers.
The original UNIX® OS was developed at American Telephone and Telegraph Company (AT&T) in 1969. During the 1970s, AT&T licensed the OS and its source code to many commercial firms as well as the US government and educational institutions. Many variations have been developed outside of AT&T, in part due to reactions to the company's increasing control and licensing fees. Some of these variants adhere more closely to the informal UNIX® standards of the 1980s and 1990s than others. UNIX® is a fairly broad term and is often attached to similar, but not standardized, versions of the OS.
The Berkeley Software Distribution (BSD) is a competing version of UNIX® developed at the University of California, Berkeley. A free open-source variation called 386BSD was first released in 1992, one year after Linus Torvalds began writing the first Linux® kernel. Most UNIX® implementations are not open-source, which is a major difference between UNIX® and Linux®. UNIX® is often sold with a quite restrictive license which includes a large fee assessed per user or per site. In contrast, Linux® systems are usually distributed freely with source code under the liberal terms of the GNU General Public License (GPL).
The Linux® OS was developed long after UNIX® had matured. UNIX® and Linux® share many of the same concepts and the software tools included with each provide similar functionality. The design and implementation of the Linux® kernel and other OS software is different from UNIX®, however. Many of the utilities, compilers and editors developed by the GNU Project are distributed with the Linux® OS. The goal of GNU is to produce and maintain a free software system compatible with UNIX®, including a kernel.
The Linux® kernel created by Torvalds, distributed under the terms of the GNU GPL, is the foundation of all Linux® systems. It is generally smaller and more efficient than most UNIX® kernels, designed from scratch to be very similar to UNIX®. Device drivers, file systems and other OS internals are quite different, however. UNIX® and Linux® systems, therefore, require separate drivers for each device, as they are not compatible in that respect.
User interfaces are frequently different between UNIX® and Linux® systems as well. UNIX® has traditionally used a text-based command-line interface which is also available with Linux®. Many developers have created full-featured graphical user interface (GUI) systems for Linux®, some of which have also been ported to UNIX®.
Linux® kernels, GNU utilities and related free software are continually supported and enhanced by a large open-source development community. In contrast, each version of UNIX® tends to have its own small, specialized group of developers, most of whom work for a particular UNIX® vendor. UNIX® support is often available only through a paid support arrangement with the vendor or a third party.