Computerworld — 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Thompson and a colleague, Dennis Ritchie , had been feeling adrift since Bell Labs had withdrawn earlier in the year from a troubled project to develop a time-sharing system called Multics, short for Multiplexed Information and Computing Service.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.

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