Exploring Expect

Chapter 17. Background Processing

It is useful to run scripts in the background when they are totally automated. Then your terminal is not tied up and you can work on other things. In this chapter, I will describe some of the subtle points of background processing.

As an example, imagine a script that is supposedly automated but prompts for a password anyway or demands interactive attention for some other reason. This type of problem can be handled with Expect. In this chapter, I will discuss several techniques for getting information to and from background processes in a convenient manner.

I will also describe how to build a telnet daemon in Expect that can be run from inetd. While rewriting a telnet daemon is of little value for its own sake, with a few customizations such a daemon can be used to solve a wide variety of problems.

Putting Expect In The Background

You can have Expect run in the background in several ways. You can explicitly start it asynchronously by appending & to the command line. You can start Expect and then press ^Z and enter bg. Or you can run Expect from cron or at. Some systems have a third interface called batch.^[55] Expect can also put itself into the background using the fork and disconnect commands.

The definition of background is not precisely defined. However, a background process usually means one that cannot read from the terminal. The word terminal is a historic term referring to the keyboard on which you are typing and the screen showing your output. ...