Unix turns 40: The past, present and future of the OS

So What Is ‘Unix,’ Anyway?

A lot of others got hooked as well. University researchers adopted Unix in droves because it was relatively simple and easily modified, it was undemanding in its resource requirements, and the source code was essentially free. Start-ups like Sun Microsystems Inc. and a host of now-defunct companies that specialized in scientific computing, such as Multiflow Computer, made it their operating system of choice for the same reasons.

Unix grew up as a nonproprietary system because in 1956, AT&T had been enjoined by a federal consent decree from straying from its mission to provide telephone service. It was OK to develop software, and even to license it for a "reasonable" fee, but the company was barred from getting into the computer business.

Unix, which was developed with no encouragement from management, was first viewed at AT&T as something between a curiosity and a legal headache.

Then, in the late 1970s, AT&T realized it had something of commercial importance on its hands. Its lawyers began adopting a more favorable interpretation of the 1956 consent decree as they looked for ways to protect Unix as a trade secret. Beginning in 1979, with the release of Version 7, Unix licenses prohibited universities from using the Unix source code for study in their courses.

No problem, said computer science professor Andrew Tanenbaum, who had been using Unix v6 at Vrije Universiteit in Amsterdam. In 1987, he wrote a Unix clone for use in his classrooms, creating the open-source Minix operating system to run on the Intel 80286 microprocessor.

"Minix incorporated all the ideas of Unix, and it was a brilliant job," Salus says. "Only a major programmer, someone who deeply understood the internals of an operating system, could do that." Minix would become the starting point for Linus Torvalds’ 1991 creation of Linux — if not exactly a Unix clone, certainly a Unix look-alike.

Stepping back a decade or so, Bill Joy, who was a graduate student and programmer at UC Berkeley in the ’70s, got his hands on a copy of Unix from Bell Labs, and he saw it as a good platform for his own work on a Pascal compiler and text editor.

Modifications and extensions that he and others at Berkeley made resulted in the second major branch of Unix, called Berkeley Software Distribution (BSD) Unix. In March 1978, Joy sent out copies of 1BSD priced at $50.

So by 1980, there were two major lines of Unix — one from Berkeley and one from AT&T — and the stage was set for what would become known as the Unix Wars. The good news was that software developers anywhere could get the Unix source code and tailor it to their needs and whims. The bad news was they did just that. Unix proliferated, and the variants diverged.

In 1982, Joy co-founded Sun Microsystems and offered a workstation, the Sun-1, running a version of BSD called SunOS. (Solaris would come about a decade later.) The following year, AT&T released the first version of Unix System V, an enormously influential operating system that would become the basis for IBM’s AIX and Hewlett-Packard’s HP-UX.

In the mid-’80s, users, including the federal government, complained that while Unix was in theory a single, portable operating system, in fact it was anything but. Vendors paid lip service to the complaint but worked night and day to lock in customers with custom Unix features and APIs.

In 1987, Unix System Laboratories, a part of Bell Labs at the time, began working with Sun on a system that would unify the two major Unix branches. The product of their collaboration, called Unix System V Release 4.0, became available two years later and combined features from System V Release 3, BSD, SunOS and Microsoft Corp.’s Xenix.

Other Unix vendors feared the AT&T/Sun alliance. The various parties formed competing "standards" bodies with names like X/Open; Unix International; Corporation for Open Systems; and the Open Software Foundation, which included IBM, HP, DEC and others allied against the AT&T/Sun partnership. The arguments, counterarguments and accomplishments of these groups would fill a book, but they all claimed to be taking the high road to a unified Unix while firing potshots at one another.

In an unpublished paper written in 1988 for the Defense Advanced Research Projects Agency, the noted minicomputer pioneer Gordon Bell said this of the just-formed Open Software Foundation: "OSF is a way for the Unix have-nots to get into the evolving market, while maintaining their high-margin code museums.’"

The Unix Wars failed to settle differences or set a true standard for the operating system. But in 1993, the Unix community received a wake-up call from Microsoft in the form of Windows NT, an enterprise-class, 32-bit multiprocessing operating system. The proprietary NT was aimed squarely at Unix and was intended to extend Microsoft’s desktop hegemony to the data center and other places dominated by the likes of Sun servers.

Microsoft users applauded. Unix vendors panicked. The major Unix rivals united in an initiative called the Common Open Software Environment and the following year more or less laid down their arms by merging the AT&T/Sun-backed Unix International group with the Open Software Foundation. That coalition evolved into The Open Group, the certifier of Unix systems and owner of the Single Unix Specification, which is now the official definition of Unix.

As a practical matter, these developments may have "standardized" Unix about as much as possible, given the competitive habits of vendors. But they may have come too late to stem a flood tide called Linux, the open-source operating system that grew out of Tanenbaum’s Minix.

Unix, most people would say, is an operating system written decades ago at AT&T’s Bell Labs, and its descendents. Today’s major versions of Unix branched off a tree with two trunks: one emanating directly from AT&T and one from AT&T via the University of California, Berkeley. The stoutest branches today are AIX from IBM, HP-UX from Hewlett-Packard and Solaris from Sun Microsystems.

However, The Open Group, which owns the Unix trademark, defines Unix as any operating system it has certified as conforming to the Single Unix Specification (SUS). This includes operating systems that are usually not thought of as Unix, such as Mac OS X Leopard (which descended from BSD Unix) and IBM’s z/OS (which descended from the mainframe operating system MVS), because they conform to the SUS and support SUS APIs. The basic idea is that it is Unix if it acts like Unix, regardless of the underlying code.

A still broader definition of Unix would include Unix-like operating systems—sometimes called Unix "clones" or "look-alikes"—that copied many ideas from Unix but didn’t directly incorporate code from Unix. The leading one of these is Linux.

Finally, although it’s reasonable to call Unix an "operating system," as a practical matter it is more. In addition to an OS kernel, Unix implementations typically include utilities such as command-line editors, APIs, development environments, libraries and documentation.