Exploring Expect

Job Control

Changing spawn_id can be viewed as job control, similar to that performed by a user in the shell when pressing ^Z and using fg and bg. In each case, the user chooses which of several processes with which to interact. After making the choice, the process appears to be the only one present—until the user is ready to switch to interacting with another process.

Shell-style job control, however, cannot be automated in a shell script. It is tied to the idea of a controlling terminal, and without one, job control makes no sense. You cannot embed commands such as fg or bg in a shell script. Shell-style job control is oriented towards keyboard convenience. Jobs are switched with a minimum of keystrokes. Expect’s job control—spawn_id—is not intended for interactive use. By comparison with the shell, Expect’s job control is verbose. But it is quite appropriate for a programming language. In later chapters, I will show an alternative form of job control that is less verbose, plus I will demonstrate how to imitate C-shell job control. For now, though, I will stick with this verbose form.

In a programming language, you can embed the repetitive things inside of procedures. This is the right way to use Expect as well. If you find yourself frequently writing "set spawn_id . . .“, consider defining a procedure to automate these commands.

For example, suppose you have a script that automates an ftp process. As the ftp process runs, it writes status messages to a user via write. In this case, ...