Just as you personally can interact with multiple programs at the same time, so can Expect. Analogous to the way you can say fg and bg to switch between processes in a shell, Expect can also switch its attention. Of course, Expect does it a lot more quickly than you can. The result is that Expect can act as “glue” for programs that were never designed to operate with other programs. An amusing example is the original chess program distributed with V7 and BSD UNIX written by Ken Thompson. It was designed to interact with a user—it prompts for moves and echoes user input in algebraic notation. Unfortunately, it does not accept its own output as input. Even if it did, there is no way to pipe both inputs and outputs between two processes simultaneously from the shell.
With a few lines of Expect, it is possible to make one chess process play another, including both the communication and the massaging of the output so that it is acceptable as input. I will show this in Chapter 10 (p. 229), along with some more serious uses for these techniques.
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