What is Unix?
Unix was created at AT&T Bell labs in 1969 following the more ambitious, but less successful MULTICS project. The popularity of the system grew quickly, especially at universities, because it was cheap, portable, and because the entire source code was provided to users. During the 1980s and 1990s several commercial implementations of Unix were created, particularly for the domain of high-end scientific workstations.
The proliferation of many different versions of Unix prompted various standardisation efforts, most notably the Portable Operating Systems Interface (POSIX).
The Open Source movement
Increasingly restrictive software licences and legal battles over Unix prompted the creation of the GNU project (GNU’s Not Unix) to make an operating system based on the philosophy of free and open software. In the early 1990s an open source kernel, called Linux, was created for IBM PC compatible computers. The combination of this kernel with the tools created by the GNU project led to the GNU/Linux operating system, which is now one of the most widely used Unix-like systems in the world. In recent times GNU/Linux has been ported to a wide variety of systems; everything from wristwatches to supercomputers.
Other popular implementations of Unix
It is also worth noting that Unix is the foundation of the Apple Mac OS X operating system, and thanks to POSIX, the standard suite of Unix tools is also available on that platform.
Unix remains popular after 40 years, especially in scientific computing, because of its robustness and flexibility. From the very beginning it was designed to support multiple users running multiple tasks concurrently, which means that security and stability are key parts of its design. The flexibility of Unix comes from a philosophy of building complex systems as compositions of smaller and simpler tools. Unix has accumulated a large suite of small programs which do one thing and do it well, and which adhere to a common interface. This enables users to build their own custom tools by simply plugging existing ones together. It is this inherent programmability which makes Unix adaptable to a wide range of computing tasks, and it is the reason that Unix users continue to favor the command line interface, even when graphical user interfaces are commonplace on modern computers.