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Computerworld - Programmers Ken Thompson and Dennis Ritchie are most often credited with the invention of Unix at Bell Labs in 1969 and the early 1970s. That's entirely fair, but as with most important technologies, it's the people who follow the pioneers who often make the difference between a fabulous lab prototype and a technology that really transforms the landscape.

Here's a brief look at three people, among thousands, who have made a difference in the Unix world.

David Korn, builder of tools

Now an AT&T Fellow, Korn came to Bell Labs in 1976, just when Unix was beginning to move into the outside world in a serious way. Korn, an application software developer with a background in aerodynamics, was assigned to two of the "first real Unix projects at Bell Labs," he says -- one to establish a centralized, mainframe-based database for internal systems, and the other to create a way to update electronic communication switches.

Looking for a better and easier-to-use Unix command language, Korn in the early 1980s wrote what was to become the ubiquitous Korn shell. Borrowing ideas from the original Unix shell written by Ken Thompson, the Bourne shell written by Steve Bourne at Bell Labs and the C shell written by Bill Joy at Berkeley, Korn added his own ideas and turned them into a more general scripting language. The result was a high-level programming language that became the de facto standard for Unix and Unix-like systems.

Then, about 10 years ago, Korn wrote Uwin (Unix for Windows), a Posix-based interface for Windows that allows AT&T's Unix-based code to run on Windows computers. Microsoft wrote Windows NT to allow multiple operating systems to run on a Windows machine as subsystems, but before Korn wrote Uwin, a programmer couldn't mix Unix and Windows calls in one integrated application.

Like so many things associated with Unix at Bell Labs and AT&T, the open-source Uwin software has propagated far and wide. "Hundreds of thousands of users have downloaded it already," Korn says.

The Unix pioneer confesses to a certain nostalgia for his early days with the operating system. "An early hallmark of Unix was its simplicity," Korn says. "Now, much of that is gone. Unix has a lot of things that a lot of people would say they wish weren't there."

Rick Rashid, mucking with Unix

Now head of Microsoft Research, Rashid was a computer science professor at Carnegie Mellon University in 1983 when he began work on Mach, a Unix-based message-passing operating system for multiprocessing applications. Mach was built on a BSD version of Unix; it was a "microkernel" that replaced the BSD kernel. Mach was a follow-on to CMU's Accent system, with support for advanced features such as multiple simultaneous computations within a process.

The Mach research project at CMU ended 10 years later, but it pioneered concepts still in use today. For example, it employed a machine-independent memory management system so it could be targeted at many different types of computers and computing -- uniprocessors, multiprocessors and distributed processors. "And the underlying idea of a microkernel became quite popular," Rashid says. "It spawned a variety of other efforts where operating systems were effectively layered on top of smaller, simpler systems."