How stable are Unix shell "stdin/stdout APIs"?

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http://unix.stackexchange.com – grepping, awking, sedding, and piping are day-to-day routine of a user of any Unix-like operating system, may it be on the command line or inside a shell script (collectively called filters from now on). At their essence, when working with "standard" Unix CLI programs and shell builtins (collectively called commands from now on), filters need a precise expected format for stdin, stdout, and stderr in each filter step in order to work correctly. I call this precise expected format of some command an API of this command in the following. As someone with web development background, I compare th (HowTos)