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The UNIX philosophy is documented by Doug McIlroy[1] in the The Bell System Technical Journal from 1978:[2]
Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new "features".
Expect the output of every program to become the input to another, as yet unknown, program. Don't clutter output with extraneous information. Avoid stringently columnar or binary input formats. Don't insist on interactive input.
Design and build software, even operating systems, to be tried early, ideally within weeks. Don't hesitate to throw away the clumsy parts and rebuild them.
Use tools in preference to unskilled help to lighten a programming task, even if you have to detour to build the tools and expect to throw some of them out after you've finished using them.
Later summarized by Peter H. Salus in A Quarter-Century of Unix (1994):[1] This is the Unix philosophy:
Write programs that do one thing and do it well.
Write programs to work together.
Write programs to handle text streams, because that is a universal interface.
In the book The Pragmatic Programmer: From Journeyman to Master the authors mention the philosophy of combining "small, sharp tools" and the use of "common underlying format—the line-oriented, plain text file"[3] to accomplish larger tasks.
The whole philosophy of UNIX seems to stay out of assembler.
— Joseph Henry Condon[4]
The development of pipes in 1973 formalized the existing principle of stdin-stdout into a philosophy in Version 3 Unix, with older software rewritten to comply. Previously visible in early utilities such as wc, cat, and uniq, McIlroy cites Thompson's grep as what "ingrained the tools outlook irrevocably" in the operating system, with later tools like tr, m4, and sed imitating how grep transforms the input stream.[5]
"The truth about Unix: The user interface is horrid"[6] was a 1981 criticism of the design philosophy published in Datamation. It was written by Don Norman, who had a background in cognitive science and was the key proponent of the then-current philosophy of cognitive engineering,[4] apparently focused on how engineers comprehend and form a personal cognitive model of a system.
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