It is a long and twisted story that marks the various phases through which the Unix operating system passed. It has been going on for 40 years, and signs show that it won't end anytime soon.

Unix's impact on today's computing experience is extremely significant. It popularized the idea of using higher level programming languages for creating operating systems, simplified the file model, showed the advantages of the hierarchical file system with an arbitrary number of levels, made the command interpreter an ordinary program, with commands being programs themselves, and this list could go on for a long while.

Another important heritage of Unix is that it sparked the efforts to create a free software system to replicate its functionality. Combining this project, called GNU, to the Linux kernel that appeared in 1992 gave us a free, as in speech, system that anyone can use, study, modify and redistribute.

Hoping that the Unix experience will continue to live and provide its users with the concepts of a free, sustainable and secure operating system, we've put together a small subset of that legendary story. Here we go, and we hope you won't fall asleep while reading this recollection of the happenings that led to Unix as we know it:

In was the 1960s, and the Massachusetts Institute of Technology, AT&T Bell Labs, and General Electric were working on an experimental operating system called Multics (Multiplexed Information and Computing Service), designed to run on the GE-645 mainframe computer. It was an interactive operating system with many new features, including enhanced security. When the project became a commercial product, the low sales forced AT&T Bell Labs to end it and left the developers on a hiatus. Ken Thompson, one of these programmers, continued to work on the GE-645, but the mainframe was slow and its use costly, so he looked for an alternative and, with the aid of Dennis Ritchie, settled on the Digital Equipment Corporation's PDP-7 minicomputer.

This experience, combined with the experience gained on the Multics project, led Thompson to start designing an operating system for the PDP-7. Along with Ritchie, he assembled a development team and started working on a multi-tasking operating system and a filesystem to go along with it.

In the 1970s, the project was advanced enough to support two simultaneous users. Brian Kernighan came up with a name for this operating system: Unics, intentionally contrasting to Multics; the spelling was eventually changed to Unix.

When another company became interested in deploying Unix on a larger computer, Thompson and Ritchie seized the chance and received a PDP-11/20 machine and official financing from Bell Labs. The name Unix became official, and until November 1971 the operating system gained text-processing utilities and a programmer's manual.

Another milestone in the operating system's development is 1973, when Unix was rewritten in the C programming language, disproving the idea "that something as complex as an operating system, which must deal with time-critical events, had to be written exclusively in assembly language." The software was now more portable, requiring minimal intervention in order to work on new platforms.

AT&T started licensing Unix to universities and commercial entities, giving them the complete source code along with the operating system. By 1975, Unix reached version 6, gained the concept of pipes, and the modular code base experienced an accelerated growth. This led to many versions of the operating system being created both within and independently of AT&T. The 1970s saw an annotated version of Unix kernel sources that circulated widely, even in the form of a book, making it a valuable educational example.

In 1982, AT&T started licensing UNIX System III for commercial use, but continued to license the older versions as well. This led to confusion among all the versions in existence. To end it, AT&T combined them to create UNIX System V Release 1, which had new features such as vi and curses.

Since the new commercial license being used by AT&T on UNIX wasn't favorable for academic use, the Berkeley researchers developed BSD Unix as an alternative to System III and V. Many contributions to Unix were started on the BSD platform, like the C shell, job control and maybe one of the most important aspects of BSD development, TCP/IP networking support. Later revisions of that networking code were incorporated into AT&T System V UNIX and even in Microsoft's Windows.

AT&T developed numerous enhancements for UNIX System V, such as file locking, streams, interprocess communication, and merged features from Xenix, BSD, SunOS and System V into its UNIX System V Release 4. Shortly after producing it, AT&T sold all its rights on UNIX to Novell, which combined its NetWare operating system with the System V Release 4 code to produce UnixWare. Novell tried to use it as a competitor to Windows NT, but its efforts were unsuccessful.

By 1993, most commercial vendors were basing their Unix variants on System V, with many BSD features added on top. In the meantime, a group of BSD developers created 386BSD, a free, software-operating system that worked on Intel's inexpensive computing platform.

Novell decided to transfer the UNIX trademark and certification rights to the X/Open consortium. In 1996, the Open Group was created by the merger between the Open Software Foundation and X/Open. This group creates the standards that define what is a "UNIX" operating system.

In 1995, Novell sold the administration and support for the existing UNIX licenses along with the rights to the development of the System V code base to the Santa Cruz Operation. This sparked an ongoing litigation for the copyright.

In 1997, the base of the MacOS X operating system was set when Apple acquired NEXTSTEP, an operating system based on BSD and the Mach kernel.

2000 saw the entire UNIX business being sold by SCO to Caldera Systems, which later changed its name to The SCO Group. The SCO Group started legal action various users and vendors of Linux, IBM and other customers of Santa Cruz, citing that it owned the copyright to UNIX and its code. Novell disputed their claims and regarded them as franchise operators, resulting in a SCO v. Novell lawsuit. In 2007, the court ruled that Novell had the copyright to UNIX.

Here we are, in 2009, and the Unix heritage continues to stand strong, through the various operating systems that we deem Unix-like. Happy 40th birthday, Unix!