Doug McIlroy (Bell Labs, 1978): (i) Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new features. (ii) 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. (iii) 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. (iv) 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.
From Mike Gancarz in 1994:
Small is beautiful. Make each program do one thing well. Build a prototype as soon as possible. Choose portability over efficiency. Store data in flat text files. Use software leverage to your advantage. Use shell scripts to increase leverage and portability. Avoid captive user interfaces. Make every program a filter.