Thompson and Ritchie were pulled out of the original MULTICS collaboration after the failure of 1966/7 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 . The name UNIX was chosen as a pun on MULTICS, which implies an expansion along the lines of "UNified Information and Computing Service." When Thompson and Ritchie gained access to a PDP-1120, 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, since an OS had no intrinsic appeal to what was then the telephone monopoly. In 1973/4 the Unix team managed to offer tapes of the new OS to universities for research purposes up through Seventh Edition. University of California, Berkeley was notable for making it the center of its Computer Science OS research and porting it to all of their mini-computers including 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), and close to the same time Bell Labs began packaging AT&T's UNIX system as 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 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 which was used by only one vendor, 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.
Not long after Sun had formalized SVR4, AT&T sold it 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. In 1991 this standard provided an opportunistic target via the GNU project for Linus Torvalds who wanted to have an operating system which would run on standard Intel 386 PC hardware. 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.
The UNIX system had a great impact on the surrounding community. 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 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 JCL for example). The file system hierarchy contained machine services and devices (such as printers, terminals, disk drives), providing a uniform and convenient way for applications to access features of the hardware. 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).
"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 system 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, and Tru64. In practice, the term (especially when written as "UN*X" or "*NIX") 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.