> It also turns out that getting a shell correct is a lot more experimentation and a lot less following what the textbooks say. In practice, things like ‘stdout’ and ‘stderr’ are used in a variety of ways, as are the exit codes output by programs. Our initial designs started with a stricter view of these ideas, and softened over time as we saw that the real world is a much fuzzier place than we realized.

Good point. Exit codes can be difficult. A non-zero error value may not be an error; it could indicate a type of success or be a warning or a result of a calculation or any number of things depending on the application.