Exploring Expect

Chapter 7. Debugging Patterns And Controlling Output

In this chapter, I will discuss the generation and suppression of certain types of output, including normal and diagnostic output. Diagnostic output includes information helpful for debugging pattern matching problems. I will discuss debugging of script control flow and scripts as a whole in Chapter 18 (p. 399).

Pattern Debugging

In the last couple of chapters, I described how to write patterns. Clearly there are some tricky issues of which you have to be aware. Writing effective patterns is a challenging art for several reasons.

First, you have to know the rules for constructing patterns. Second, you have to understand the rules for expressing them in Tcl. Third, you have to know what characters are in the string you expect. Misunderstanding any one of these steps can cause you to write patterns that do not match.

When patterns do not match as intended, a common symptom is that the script executes very slowly. For example, the following is a fragment of a script to log in. It ought to execute quickly.

expect "Login: "
send "don\r"
expect "Password: "
send "swordfish\r"

However, on a typical system this fragment takes 20 seconds to execute instead of one or two seconds. There are two problems. The first is with the patterns. The first pattern says to expect "Login:" but on a typical UNIX system the prompt is "login:“.

Instead of matching, the script waits for more input for another 10 seconds (because that is the default timeout). After ...