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

In August 1969, Ken Thompson, a programmer at AT&T Bell Laboratories, saw the monthlong absence of his wife and young son as an opportunity to put his ideas for a new operating system into practice

Forty years ago this summer, a programmer sat down and knocked out in one month what would become one of the most important pieces of software ever created.

In August 1969, Ken Thompson, a programmer at AT&T Bell Laboratories, saw the monthlong absence of his wife and young son as an opportunity to put his ideas for a new operating system into practice. He wrote the first version of Unix in assembly language for a wimpy Digital Equipment Corp. PDP-7 minicomputer, spending one week each on the operating system, a shell, an editor and an assembler.

Thompson and a colleague, Dennis Ritchie , had been feeling adrift since Bell Labs had withdrawn earlier in the year from a troubled project to develop a time-sharing system called Multics, short for Multiplexed Information and Computing Service. They had no desire to stick with any of the batch operating systems that predominated at the time, nor did they want to reinvent Multics, which they saw as grotesque and unwieldy.

After batting around some ideas for a new system, Thompson wrote the first version of Unix, which the pair would continue to develop over the next several years with the help of colleagues Doug McIlroy, Joe Ossanna and Rudd Canaday. Some of the principles of Multics were carried over into their new operating system, but the beauty of Unix then (if not now) lay in its "less is more" philosophy.

"A powerful operating system for interactive use need not be expensive either in equipment or in human effort," Ritchie and Thompson would write five years later in the Communications of the ACM (CACM), the journal of the Association for Computing Machinery. "[We hope that] users of Unix will find that the most important characteristics of the system are its simplicity, elegance, and ease of use."

So What Is 'Unix,' Anyway?

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.

Apparently, they did. Unix would go on to become a cornerstone of IT, widely deployed to run servers and workstations in universities, government facilities and corporations. And its influence spread even further than its actual deployments, as the ACM noted in 1983 when it gave Thompson and Ritchie its top prize, the A.M. Turing Award for contributions to IT: "The model of the Unix system has led a generation of software designers to new ways of thinking about programming."

Of course, Unix's success didn't happen all at once. In 1971, it was ported to the PDP-11 minicomputer, a more powerful platform than the PDP-7. Text-formatting and text-editing programs were added, and it was rolled out to a few typists in the Bell Labs patent department, its first users outside the development team.

In 1972, Ritchie wrote the high-level C programming language (based on Thompson's earlier B language); subsequently, Thompson rewrote Unix in C, greatly increasing the operating system's portability across computing environments. Along the way, it picked up the name Unics (Uniplexed Information and Computing Service), a play on Multics; the spelling soon morphed into Unix.

It was time to spread the word. Ritchie and Thompson's July 1974 CACM article, "The UNIX Time-Sharing System," took the IT world by storm. Until then, Unix had been confined to a handful of users at Bell Labs. But now, with the Association for Computing Machinery behind it -- an editor called it "elegant" -- Unix was at a tipping point.

"The CACM article had a dramatic impact," IT historian Peter Salus wrote in his book The Daemon, the Gnu and the Penguin (Reed Media Services, 2008). "Soon, Ken was awash in requests for Unix."

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Gary Anthes

Computerworld (US)

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