Thompson and Ritchie were pulled out of the original MULTICS collaboration after the failure of 1966-67 era hardware to support its implementation. Inspired by his work on the MULTICS (MULTiplexed Information and Computing Service) project in 1969 Ken Thompson wrote the first UNIX system in assembly language for the Digital PDP-7, mostly to do text processing and playing games.
The name UNIX was chosen as a pun on MULTICS, which implies an expansion along the lines of "UNIplexed Information and Computing Service" (since early versions, on the DEC PDP-7 minicomputer, were limited to two users at a time). UNIX is also a homophone of "eunuchs", which inspired the joke that "UNIX is a castrated MULTICS". When Thompson and Ritchie gained access to a PDP-11/20, they rewrote the UNIX system in PDP-11 assembly language. After these initial versions drew the attention of other Bell Labs scientists, they teamed up in 1972 (?) to reimplement the operating system using Dennis Ritchie's new C programming language. Interestingly enough, AT&T funded the initial C language version development as an electronic documentation system, despite the fact that as an OS it had no intrinsic appeal to what was then the telephone monopoly.
In 1973-74 the Unix team managed to offer tapes of the new OS to universities for research purposes up through Seventh Edition (UNIX versions were originally named after the versions of the accompanying man pages, or manual).
The University of California, Berkeley was notable for making UNIX the center of its Computer Science OS research and porting it to all of their mini-computers including the VAX, the successor to the DEC PDP-11. Though AT&T stopped distributing the source for the UNIX system after Seventh Edition, UCB continued to develop what became known as "Berkeley Unix" often matching and sometimes exceeding the capabilities being developed by the team at Bell Labs. Once Berkeley Unix became sufficiently mature it was packaged as the Berkeley Software Distribution (BSD), close to the same time Bell Labs began packaging AT&T's UNIX system as System III (three), the precursor to System V (five).
Starting in about 1980, various companies started marketing commercial distributions of the UNIX system for their mini-computers and workstations. While some companies chose to base their distributions on AT&T System V, many of these companies started out using the Berkeley BSD version because of its interesting enhancements.
In about 1987 the AT&T team decided to standardize the UNIX system by developing, in conjunction with Sun Microsystems, System V Release 4 (SVR4), which incorporated and normalized the technologies used by all of the different distributions and vendors. Despite a fracture in the standardization effort resulting in the creation of OSF/1, - a competitor to the AT&T/Sun Microsystems alliance, which was used by only one vendor, (ironically, DIGITAL) - the AT&T team released SVR4 in 1989 with Data General being the first vendor to use the new operating system on their AViiON[?] server machines in 1990.
By 1993, most of the original commercial BSD vendors including Sun and SGI moved to SVR4 as the foundation of their operating systems with vendor and BSD enhancements layered on top of the new standard foundation. The first published description of the system's inner workings was a book by an Australian university teacher, the so-called Lions book, which described Version 7[?].
Not long after Sun had formalized SVR4, AT&T sold its remaining interest in the UNIX system together with the Unix Labs to Novell. At the time, Novell was becoming increasingly frightened by Microsoft's entry into the networked computers business; company leaders saw the UNIX system as a chance to offset their progress. The UNIX system at Novell languished in the financial difficulties of the company as their sales growth was bounded. Eventually Novell was forced to sell off many of its assets including the UNIX system to concentrate on its core software.
UNIX was split into three parts upon leaving Novell. The UNIX trademark was transferred to an industry consortium, the X/Open group, (now just The Open Group) for certifying UNIX implementations as standard. The source base was sold to Santa Cruz Operation (SCO) who sold SVR4 on Intel hardware alongside their traditional Xenix-like SCO-UNIX offering. Hewlett-Packard got the operating system laboratory, which became mired in internal politics when asked to design the eventual replacement for HP's UNIX, HP-UX. Eventually the remaining employees were absorbed into the broader HP Corporation.
Later an open operating system standardization effort known as POSIX provided a common baseline for all operating systems; IEEE based POSIX around the structure of the UNIX system.
Torvalds produced a kernel called Linux to which he and others ported the Free Software Foundation's GNU libraries and tools. The GNU tools, when combined with the Linux kernel, provided the foundation for a POSIX conformant operating system known as GNU/Linux or just Linux.
Several UNIX system vendors agreed on the modern ELF format as standard for binary and object code files.
Linux, and other free UNIX-like operating systems such as FreeBSD, OpenBSD and NetBSD are now rapidly occupying the market traditionally occupied by proprietary UNIX operating systems, as well as expanding into new markets such as the consumer desktop and mobile and embedded devices. However, BSD is, at present, still one of the most widely-used versions of UNIX for the desktop market, in the guise of MacOS X ("ten"), a UNIX with extensions (such as Finder[?]) based on the original MacOS (versions through MacOS 9).
The UNIX system had a great impact on the surrounding community. Some consider it the most influential operating system in changing other proprietary operating systems, leading Unix to be called "the most important operating system you may never use".
It led in the way in operating systems that were written in high level language as opposed to assembler (assembler was vogue at the time).
It had a drastically simplified file model compared to many contemporary operating systems. The file system hierarchy contained machine services and devices (such as printers, terminals, or disk drives), providing a uniform and convenient way for applications to access features of the hardware.
The recursive file system with the ability to create arbitrarily-nested subdirectories was a major innovation, first implemented by Multics. Other common operating systems of the era had ways to divide a storage device into multiple directories or sections, but they were a fixed number of levels and often only one level. The major proprietary operating systems all added recursive subdirectory capabilities patterned after Unix. DEC's RSTS[?] programmer/project hierarchy evolved into VMS directories, CP/M's volumes evolved into MS-DOS 2.0+ subdirectories, and HP's MPE[?] group.account hierarchy and IBM's System 36 and OS/400 library systems were folded into broader POSIX file systems.
The command prompt with which users interacted was just an ordinary user-level process, a Unix shell. The shell itself was novel in that the same language was used for interactive commands and for scripting the system (there was no separate job control language, like IBM's JCL[?] for example). Also, the fact that, unlike on other early systems, the shell and OS commands were "just another program", enabled each user to choose his/her own shell (and even to write his or her own, if the user in question were able to program). Finally, new commands could be added without recompiling[?] the shell.
It popularised a syntax for regular expressions that found much wider use. The UNIX programming interface became the basis for a standard operating system interface (POSIX, see above).
The C programming language, now ubiquitous in systems and applications programming, originated under Unix. Early Unix developers were important in bringing the theory of software modularity and re-use into engineering practice.
Over time, the leading developers of Unix (and programs that ran on it) developed a set of cultural norms for developing software, norms which became as important and influential as the technology of Unix itself. See Unix philosophy for more information.
"UNIX" is a trademark of The Open Group and, like all trademarks, should be used as an adjective followed by a generic term such as "system." The term refers more to a class of operating systems than to a specific implementation of an operating system; those operating systems which meet The Open Group's Single UNIX Specification should be able to bear the "UNIX" and UNIX98 trademarks today. UNIX systems include AIX, HP-UX, Irix, SCO UNIX, Solaris, Tru64, A/UX and a part of z/OS. In practice, the term, especially when written as "UN*X", "*NIX", or "*N?X" is applied to a number of other multiuser POSIX-based systems such as GNU/Linux, Mac OS X, FreeBSD, NetBSD, OpenBSD that do not seek UNIX branding because the royalties would be too expensive for a product marketed to consumers or freely available over the Internet.
The term "Unix" is also used, and in fact was the original capitalisation, but the name UNIX stuck because, in the words of Dennis Ritchie "[when presenting the original Unix paper to the third Operating Systems Symposium[?] of the American Association for Computing Machinery], we had just acquired a new typesetter and were intoxicated by being able to produce small caps" (quoted from The Jargon File[?], version 4.3.3, 20 September 2002).