After graduation, Ritchie and Ken Thompson joined the Bell Laboratories Computing Sciences Research Center in Murray Hill, NJ.  Since 1964, members of the group had been participating in the design and development of the Multics system, along with developers from MIT's Project MAC and General Electric.

Ritchie worked with others in many different software projects associated with the Multics operating systems or tools. For example, in 1967, Ritchie, Robert Morris and Rudd Canaday ported the programming language BCPL from CTSS to the Multics and GECOS systems.

In 1969 Bell Labs withdrew from the Multics project. The Computing Sciences Research group members searched for other projects, and in particular for a computing environment with an on-line community that avoided the "big system mentality." Unix was to provide such an environment. Ritchie wrote:

"It began in 1969 when Ken Thompson discovered a little-used PDP-7 computer and set out to fashion a computing environment that he liked. Thompson wrote the first version of Unix for a Digital Equipment Corporation PDP-7 in a month, using a cross-assembler that ran on GECOS. The PDP-7 he used had 4K of 18-bit words. His work soon attracted me; I joined in the enterprise, though most of the ideas, and most of the work for that matter, were his."

The resulting Unix system provided users with interactive remote terminal computing and a shared file system. Source code was provided with the system, and the community of users could share ideas and programs directly and informally. Because Unix ran on a relatively inexpensive minicomputer, small research groups could experiment with it without dealing with computation center bureaucracies.

In 1971, the Bell Laboratories Computing Sciences Research group ported Unix to a Digital Equipment Corporation PDP-11 to support text processing for the Bell Laboratories Patents Office. By 1972, there were 10 installations of Unix at AT&T.

Ken Thompson also created an interpretive language called B, based on BCPL, which he used to re-implement the non-kernel parts of Unix. Ritchie added types to the B language, and later created a compiler for the C language. Thompson and Ritchie rewrote most of Unix in C in 1973, which made further development and porting to other platforms much easier.

The second ACM Symposium on Operating Systems Principles was held in Elmsford, NY in 1973, and Thompson and Ritchie presented a clear and well-written paper describing Unix. The Unix system presented in the paper was elegant and simple, providing a useful and extensible multi-user programming environment on an affordable machine. The file system and libraries included with the system made it easy to build and share application programs and to augment the system's functions. By the end of 1973, there were over 20 Unix systems running.

Thompson and Ritchie, along with other Computing Sciences Research group members, continued the development of Unix and C at Bell Laboratories, and Unix use spread further within AT&T. The Sixth Edition, released in 1975, began the spread of Unix to university, commercial, and government users of the popular DEC PDP-11 computers. AT&T, forbidden by court decree from selling Unix, licensed it for the cost of media. Enthusiastic users had the source code available, and fed improvements to Unix back to the Bell Labs developers. A 1977 retrospective paper by Ritchie said that there were more than 300 Unix installations running on configurations from a single-user DEC LSI-11 to a 48-user PDP-11/70. By 1978, there were over 600 Unix installations, and Unix had begun to be ported to other minicomputers.

In the late 1970s, John Lions of the University of New South Wales circulated a book on Unix that included the source code and commentaries on it. This book was used to teach Unix in operating systems courses around the world, and created a generation of computer scientists familiar with Unix internals.

In 1983, Thompson and Ritchie received the ACM A. M. Turing Award. The Turing Award selection committee wrote:

Ritchie retired as head of Lucent Technologies' System Software Research Department in 2007, and died in early October, 2011.

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