UNIX, a powerful multi-users and multitasks system. It supports various CPU frameworks. According the classification of operating system, it belongs to time-sharing operating system.

Introduction
It is firstly developed by Ken Thompson, Dennis Ritchie and Douglas McIlroy is AT&T Bell Labs in 1969. Through years' development and improvement, it has become a popular operating system and it has formed a products family based on this operating system techniques. As UNIX has the feature of mature technology, simple structure, high reliability, great portability, high operability, excellent functions of network and data base, outstanding flexibility and openness, it can satisfy various requirements of any situations and has become the main workstation platform and important enterprises operation platform. It is mainly installed in the supercomputer and large computer but it also can be used in PC and embedded system. It has been the best choice of server system and occupied the most of market shares. But recently it has failed in the competition with Windows Server and Linux. Ken and Dennis have firstly developed UNIX in the Bell Labs and the following 10 years, UNIX has been widely used by the academic institutions and large enterprises. At that time the UNIX owner, AT&T authorized the UNIX source code to academic institution for researching or teaching in a very low price. Many institutions have extended and improved the source code, forming a kind of UNIX Variations. Those variations have promoted the development of UNIX. Among the variations the most famous one is the BSD products developed by University of California, Berkeley.
After that, AT&T has relized the commercial value of UNIX, and stopped the authorization of UNIX source code and declared the copyright of UNIX and its variations. The BSD UNIX has a strong effect in the UNIX history and is used by many manufacturers, becoming the base of commercial UNIX. BSD is identified in a way of major version pluses minor version, such as 4.2BSD, 4.3BSD. There is derivative version on the base of original version. Generally these versions have their own names such as 4.3BSD-Net/1, 4.3BSD-Net/2, etc. The growing influence of BSD has finally caused the attention of AT&T, so a lasting copyright lawsuit has started. This lawsuit has made the AT&T sell its UNIX system labs and the Novell has replaced AT&T. The Novell has taken an enlightened way to solve the problem that it allows Berkeley releasing BSD free in the premise of deleting all the code of T&T. Therefore 4.4 BSD Lite has been released. This version has no copyright problems and has become the basic version of modern BSD system. Even later on the non-commercial version UNIX has been through a lot of evolutions, it is at last on the base of BSD version (except Linux). So 4.4BSD is the base of all the Free Version UNIX.
BSD has three main parts in the development: FreeBSD, OpenBSD and NetBSD.
The following several decades, UNIX is still changing. The copyright owner is also changing and the number of lease granters is growing, too. UNIX copyright once is own by AT&T, after that Novell, and SCO (there is still disputability between those two). Some large firms developed their own UNIC products after getting the UNIX authorization, such as AIX of IBM, HPUX of HP, Solaris of SUN, and IRIX of SGI. The design idea of UNIX is advanced. Many present popular techniques and methods like micro kernel technology, process communication, TCP/IP protocol, and client/server mode, etc, are all from the UNIX. It has almost affected all the present operating systems. Because of its high security and high efficiency it is widely used in the server field. Before GNU/Linux gets popular UNIX has been the main stream of science computing, large and super computers.

Differences and Relations between UNIX and LINUX
The biggest difference between Linux and UNIX is the former is the free software with open source code and the latter is the commercial software with copyright protection. The difference reflects in that users have more free space to manage Linxu but have to adapt to UNIX. It also reflects in that Linux is developed in an open environment but UNIX is developed secretly.
The other two differences:
1. UNIX supports more kinds of hardwares than Linux. Both of them run stably.
2. Both of them are multi-users, multitasks operating systems. They can both be used as network operating system.

Existing UNIX System
After the UNIX lawsuit, Novell takes over UNIX source code. The Novell has taken an enlightened way to solve the problem that it allows Berkeley releasing BSD free in the premise of deleting all the code of T&T. Therefore 4.4 BSD Lite has been released. This version has no copyright problems and has become the basic version of modern BSD system. Some time later UNIX Version6 has been released and UNIX has been divided into BSD and UNIX Version7. UNIX Version7 has developed into System III, System V.2, and System V.3.
After Novell gets UNIX source code, it hopes to compete with WindowsNT with UNIX, but the core market has been damaged, at last Novell sells SVR4 ro X/OPEN Consortiun, which has defined UNIX standard. After the merge of X/OPEN and OSF/1, they create Open Group, which defines the standard of UNIX.
The UNIX source code has been sold to Santa Cruz Operation. This firm sells it to Caldera Systems. Caldera used to selling Linx system. After that, the new caompany is named SCO Group. Some large firms developed their own UNIC products after getting the UNIX authorization, such as AIX of IBM, HPUX of HP, Solaris of SUN, and IRIX of SGI
AIX, HP-UX, Solaris, IRIX, BSD, etc, are all the branches of UNIX. We still call them UNIX even though there are some differences. Now there are no any UNIX systems any more. The UNIX 1127 department of AT&T has dismissed in 2005. Linux and Minix can noly be called UNIX-like system but not UNIX.