(officially trademarked as UNIX) is a multitasking, multi-user computer operating system that exists in many variants. The original Unix was developed at AT&T’s Bell Labs research center by Ken Thompson, Dennis Ritchie, and others.From the power user’s or programmer’s perspective, Unix systems are characterized by a modular design that is sometimes called the “Unix philosophy,” meaning the OS provides a set of simple tools that each perform a limited, well-defined function, with a unified filesystem as the main means of communication and a shellscripting and command language to combine the tools to perform complex workflows.

The C programming language was designed by Dennis Ritchie as a systems programming language for Unix, allowing for portability beyond the initial PDP-11 development platform and the use of Unix on a plethora of computing platforms.

During the late 1970s and 1980s, Unix developed into a standard operating system for academia. AT&T tried to commercialize it by licensing the OS to third-party vendors, leading to a variety of both academic (e.g., BSD) and commercial variants of Unix (such as Xenix) and eventually to the “Unix wars” between groups of vendors. AT&T finally sold its rights in Unix to Novell in the early 1990s.

The Open Group, an industry standards consortium, now owns the UNIX trademark and allows its use for certified operating systems compliant with its standard, the Single UNIX Specification. Other operating systems that emulate Unix to some extent may be called Unix-like, although the Open Group disapproves of this term. The term Unix is also often used informally to denote any operating system that closely resembles the trademarked system. The most common version of Unix (bearing certification) is Apple’s OS X, while Linux is the most popular non-certified workalike.

The history of Unix dates back to the mid-1960s when the Massachusetts Institute of Technology, AT&T Bell Labs, and General Electric were developing an experimental time sharing operating system called Multics for the GE-645 mainframe. Multics introduced many innovations, but had many problems. Bell Labs, frustrated by the size and complexity of Multics but not the aims, slowly pulled out of the project. Their last researchers to leave Multics, Ken Thompson, Dennis Ritchie, M. D. McIlroy, and J. F. Ossanna, decided to redo the work on a much smaller scale.

The first production instance of Unix was installed in early 1972 at New York Telephone Co. Systems Development Center, under the direction of Dan Gielan. In 1972, Unix was rewritten in the C programming language. The migration from assembly to the higher-level language C, resulted in much more portable software, requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms. Bell Labs produced several versions of Unix that are collectively referred to as Research Unix.

During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (BSD and System V) by commercial startups, some of the most notable of which are Sequent, HP-UX, Solaris, AIX, and Xenix. In the late 1980s, System V Release 4 (SVR4) was developed by AT&T Unix System Laboratories and Sun Microsystems. SVR4 was subsequently adopted by many commercial Unix vendors.

In the 1990s, Unix-like systems grew in popularity as Linux and BSD distributions were developed through collaboration by a worldwide network of programmers. Later, Apple also released Darwin, which became the core of the OS X operating system.
sources from : http://en.wikipedia.org/wiki/Unix

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