Exploring Expect

Disabling Normal Program Output

In the previous section, I showed how to generate, control, and suppress diagnostics produced by Expect. All output produced by spawned programs, however, appears on Expect’s standard output. It is possible to redirect or suppress this as well, but it is done in a different way.

Figure 7-1. Inside of Expect, the standard output and standard error of a spawned process are joined together and ultimately appear as the standard output of Expect

The standard error and standard output of a spawned process are joined together inside of Expect. This is done before pattern matching occurs so that patterns must account for both streams appearing simultaneously. The reason this is done is partly philosophical and partly practical.

Philosophically speaking, interactive programs always have the outputs merged together anyway—on the screen. It is impossible for users to visually distinguish between characters appearing on the standard output versus the standard error. Their minds make up for it by scanning the output, both for normal output and errors at the same time. If humans can do it, then so can scripts. This is the philosophy that Expect takes.

Practically speaking, few interactive programs send things to the standard error. What is the point? A user is watching the standard output anyway. The user sees the error whether it appears on one stream or the other. ...