That appropriate action may be something performed internally to the command processor, or the command processor may load another program (e.g. a compiler), or it may cause the creation of a whole new process which is then set to execute some program.
In early operating systems, such as CTSS and TOPS-10, the command processor was built into the OS itself. It soon became apparent (from experience on CTSS) that it could and should be a separate program, run in the user's top-level process, and Multics took this approach. Other time-sharing systems (such as TENEX and ITS) followed this example.
The most well-known command interpreter at this point is the shell of UNIX, which is also based on the similar program of Multics.
- Neal Stephenson, In the Beginning ... was the Command Line, Avon, New York, 1999