Unix

Emphasizes simplicity and composability. Some of the most important principles:

• Text streams are the universal interface. (see Narrow Waist)
  
• Expect your output to be the input to some unknown or not-yet-existing other program, and be nice to that program. Keep your output simple, predictably structured, and not necessarily interactive.
  
• DOTADIW (Do One Thing And Do It Well): Focus on a single use case. If you have a new use case, make a new program rather than adding new features. (cf Single Responsibility Principle)
  

Despite its apparently compelling philosophy and enormous success, Unix kind of sucks - in particular, Unix UIs (especially CLIs) are notoriously obtuse. What the Unix philosophy produces may not so much be “good” software as software with powerful survival characteristics: see Worse is Better

Originally trademarked at Bell Labs as UNIX, which was consistently used by Bell Labs people, but Unix makes more sense for non-BL
See https://unix.stackexchange.com/questions/532340/how-to-capitalize-the-word-unix