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In the mid-1980s, the two common versions of Unix were BSD, from the University of California, Berkeley, and System V, from AT&T Corporation. Both were derived from the earlier Version 7 Unix, but had diverged considerably. Further, each vendor's version of Unix was different to some degree.
X/Open caught AT&T's attention. To increase the uniformity of Unix, AT&T and leading BSD Unix vendor Sun Microsystems started work in 1987 on a unified system. (The feasibility of this had been demonstrated a few years earlier by the US Army Ballistic Research Laboratory's System V environment for BSD Unix.) This was eventually released as System V Release 4 (SVR4).
While this decision was applauded by customers and the trade press, certain other Unix licensees feared Sun would be unduly advantaged. They formed the Open Software Foundation (OSF) in 1988. The same year, AT&T and another group of licensees responded by forming UNIX International. Technical issues soon took a back seat to vicious and public commercial competition between the two "open" versions of Unix, with X/Open holding the middle ground.
 The move towards open source code
In March 1993 the major participants in UI and OSF formed the Common Open Software Environment (COSE) alliance, effectively marking the end of the most significant era of the Unix wars. In June, AT&T sold its UNIX assets to Novell, and in October Novell transferred the Unix brand to X/Open.
Since then, occasional bursts of Unix factionalism have broken out, such as the HP/SCO "3DA" alliance in 1995, and Project Monterey in 1998, a teaming of IBM, SCO, Sequent and Intel which was followed by litigation (SCO v. IBM) between IBM and the new SCO, formerly Caldera.
The Berkeley Software Distribution emerged as an independent Unix-like operating system, with the purging of code copyrighted by AT&T, in the period 1989-1994. During this time various open-source BSD derivatives took shape, starting with 386BSD, which was soon succeeded by FreeBSD and NetBSD. OpenBSD emerged in 1995 as a fork of NetBSD.
 Rise of GNU and Linux
During BSD's period of legal turmoil, the nearly-complete GNU operating system was made operational by the inclusion of the Linux kernel, and countless operating systems, lumped together under the label "GNU/Linux", emerged that were based on this combination. GNU/Linux was written from scratch to avoid copyright issues, and its derivatives are not compatible enough to qualify for the Unix trademark. On the other hand, BSD systems can at least claim direct ancestry to Version 7 Unix. Or, according to Open Source advocate Eric Raymond, BSD systems can be considered "genetic Unix", if not "trademark Unix". Mac OS X v10.5 is the first operating system with open source BSD code to be certified as fully Unix compliant.
 See also
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- Unix Wars (Living Internet)
- The UNIX Wars (Bell Labs)
- The UNIX System — History and Timeline (The Open Group)
- Unix Standards (Eric S. Raymond, The Art of Unix Programming)
- Chapter 11. OSF and UNIX International (Peter H. Salus, The Daemon, the GNU and the Penguin)