The Unix philosophy is documented by Doug McIlroy in the Bell System Technical Journal from 1978:

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.

It was later summarized by Peter H. Salus in A Quarter-Century of Unix (1994):

• 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 their award-winning Unix paper of 1974, Ritchie and Thompson quote the following design considerations:

• Make it easy to write, test, and run programs.
• Interactive use instead of batch processing.
• Economy and elegance of design due to size constraints ("salvation through suffering").
• Self-supporting system: all Unix software is maintained under Unix.

The whole philosophy of UNIX seems to stay out of assembler.

— Michael Sean Mahoney