Assignment 2.

Create a very basic shell.

It should use your command line processing library from assignment 1, along with
the fork/exec/wait family of functions.

It should print a prompt when ready for a command, process the command typed by
the user, and execute the appropriate executable with the given command line.

It should also accept a few built-in commands which it executes itself. The
built-ins should include at least echo and logout/exit.

Do it properly. Good clear programming. You are going to work further on it
in the future.