The history of Unix dates back to the mid-1960s, when the Massachusetts Institute of Technology, Bell Labs, and General Electric were jointly developing an experimental time-sharing operating system called Multics for the GE-645 mainframe.

Their last researchers to leave Multics – among them Ken Thompson, Dennis Ritchie, Doug McIlroy, and Joe Ossanna[2] – decided to redo the work, but on a much smaller scale.

[3] In 1979, Ritchie described the group's vision for Unix:[3] What we wanted to preserve was not just a good environment in which to do programming, but a system around which a fellowship could form.

We knew from experience that the essence of communal computing, as supplied by remote-access, time-shared machines, is not just to type programs into a terminal instead of a keypunch, but to encourage close communication.In the late 1960s, Bell Labs was involved in a project with MIT and General Electric to develop a time-sharing system, called Multics, allowing multiple users to access a mainframe simultaneously.

[4] Bell Labs management ultimately withdrew from the project, and it seemed likely the company was going to end their lease on the very expensive GE 645 mainframe.

[5] Ken Thompson, a programmer in the Labs' computing research department, enjoyed the flexibility of Multics and began considering a new operating system that could run on less sophisticated machines.

Program code is rarely modified at runtime, may be placed in a read-only area of memory, and is randomly accessed due to branching.

[5] In 1969, Thompson had also written a video game, Space Travel, under the GECOS operating system on the smaller GE 635 machine.

[6] When he learned that Visual and Acoustics Research[7] department had a small PDP-7 that was largely unused, he began to rewrite the game for this machine.

Bell Labs used this initial text-processing system, consisting of Unix, roff, and the editor, for text processing of patent applications.

The UNIX Programmer's Manual was published on 3 November 1971; commands were documented in the "man page" format that is still used, offering terse reference information about usage as well as bugs in the software, and listing the authors of programs to channel questions to them.

By Version 4 it was widely used within the laboratory and a Unix Support Group was formed, helping the operating system survive by formalizing its distribution.

Thompson and Ritchie were so influential on early Unix that McIlroy estimated that they wrote and debugged about 100,000 lines of code that year, stating that "[their names] may safely be assumed to be attached to almost everything not otherwise attributed".

[12] Although assembly did not disappear from the man pages until Version 8,[12] the migration to C suggested portability of the software, requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms.

[21] Target machines of further Bell Labs ports for research and AT&T-internal use included an Intel 8086-based computer (with custom-built MMU) and the UNIVAC 1100.

Linux 3.2.0 has 380 system calls and FreeBSD 8.0 has over 450.A microprocessor port of Unix, to the LSI-11, was completed in 1978,[24] and an Intel 8086 version was reported to be "in progress" the same year.

[citation needed] This research focus then shifted to the development of Plan 9 from Bell Labs, a new portable distributed operating system.

The newly created competition nearly destroyed the long-term viability of Unix, because it stifled the free exchanging of source code and led to fragmentation and incompatibility.

During this period, many observers expected that UNIX, with its portability, rich capabilities, and support from companies like DEC and IBM, was likely to become an industry-standard operating system for microcomputers.

[37][38] Citing its much smaller software library and installed base than that of MS-DOS and the IBM PC, others expected that customers would prefer personal computers on local area networks to Unix multiuser systems.

[39] Microsoft planned to make Xenix MS-DOS's multiuser successor;[20] by 1983 a Xenix-based Altos 586 with 512 KB RAM and 10 MB hard drive cost US$8,000 (equivalent to $24,473 in 2023).

In 1984, several European computer vendors established the X/Open consortium with the goal of creating an open system specification based on Unix (and eventually the SVID).

[47] Yet another standardization effort was the IEEE's POSIX specification (1988), designed as a compromise API readily implemented on both BSD and System V platforms.

[46] The Unix wars continued into the 1990s, but turned out to be less of a threat than originally thought: AT&T and Sun went their own ways after System V.4, while OSF/1's schedule slipped behind.

In 1991, a group of BSD developers (Donn Seeley, Mike Karels, Bill Jolitz, and Trent Hein) left the University of California to found Berkeley Software Design, Inc. (BSDi), which sold a fully functional commercial version of BSD Unix for the Intel platform, which they advertised as free of AT&T code.

They ran into legal trouble when AT&T's Unix subsidiary sued BSDi for copyright infringement and various other charges in relation to BSD; subsequently, the University of California countersued.

[49] Shortly after it was founded, Bill Jolitz left BSDi to pursue distribution of 386BSD, the free software ancestor of FreeBSD, OpenBSD, and NetBSD.

[52] In 1995, the business of administering and supporting the existing UNIX licenses, plus rights to further develop the System V code base, were sold by Novell to the Santa Cruz Operation.

The deployment of Darwin in Mac OS X makes it, according to a statement made by an Apple employee at a USENIX conference, the most widely used Unix-based system in the desktop computer market.

Of the many commercial variants of Unix that were born in the 1980s, only Solaris, HP-UX, and AIX were still doing relatively well in the market, though SGI's IRIX persisted for quite some time.