[mongol]

Good point. I wonder if there was some criteria that determined if a utility were to be a command or a builtin, originally. I thought a shell first and foremost was about working with processes, and a little less about working with files. But both are obviously needed, one way or another, so will be interesting to see what becomes of this.

[forgotpwd16]

Based on list seems builtin functionality is what is used to work with processes (nohup, kill, etc), environment (where, which, etc), and for shell as programming language ([, while, etc).

[mongol]

Yes. As an example, the source builtin would not be possible to implement as a command as a child process can't change its parent's environment. So those types of concerns play in I think.