Chapter 10), the shell looks at it carefully. The shell knows how to do a few commands all by itself, including cd and exit . If the command isn’t one that the shell can do by itself, the shell looks around for a program stored in a file by the same name.
Old DOS users may recognize the way this process works — commands DOS can do itself are called internal commands, and commands that require running another program are called external commands. Internal commands are also called built-in commands.