Very basic version of a shell.

An interactive program.
Repeatedly:
  print a prompt
  read whole line from user, it may be quite long.
  split the line into its components (assuming it is a unix-like command)
     no need to process quotes or stars or shell variables etc yet.
  if the first component is "echo" or "exit", obey it
  otherwise use fork and exec to make it happen
     wait for subprocess to terminate before continuing.
     if exit code is not zero, print a warning "exit code NNN"