Malcolm Douglas McIlroy headed the Computing Sciences Research Center at Bell Labs, the group where Unix was built. He is credited with proposing the idea of connecting programs through pipes, so that the output of one program could become the input of the next without temporary files or special glue code.
In the foreword to the July-August 1978 Bell System Technical Journal issue devoted to Unix, McIlroy set out the design ideas that became known as the Unix philosophy. He wrote that a maker of Unix software should “make each program do one thing well,” and should “expect the output of every program to become the input to another, as yet unknown, program.” Those two sentences capture the pipe-and-filter model that shaped Unix and much software after it.
He also urged builders to design and build software, “even operating systems,” to be tried early, and not to hesitate to throw away the clumsy parts and rebuild them. The same foreword recommended using tools in preference to unskilled help, “even if you have to detour to build the tools.”
McIlroy wrote or inspired many of the small utilities that made the philosophy concrete, including text-processing tools that pipe cleanly together. His insistence on small programs that do one job and combine through plain text streams remains a defining trait of Unix-derived systems.