Worth noting that the Unix shell itself is not a "built-in". The kernel is agnostic about the user interface, which is left entirely to userspace programs to implement. (That was one of Unix's major innovations.)
> Applications should note that the standard PATH to the shell cannot be assumed to be either /bin/sh or /usr/bin/sh, and should be determined by interrogation of the PATH returned by getconf PATH, ensuring that the returned pathname is an absolute pathname and not a shell built-in.
