Exploring Expect

Using The Terminal Emulator For Testing And Automation

This book describes a version of Expect that does not provide built-in support for understanding character graphics. Nonetheless, it is possible to use the terminal emulator in the previous section to partially or fully automate character-graphic applications.

For instance, each expect-like operation could be a loop that repeatedly performs various tests of interest on the text widget contents. In the following code, the entrance to the loop is protected by "tkwait var test_pats“. This blocks the loop from proceeding until the test_pats variable is changed. The variable is changed by the term_chars_changed procedure, invoked whenever the screen changes. Using this idea, the following code waits for a % prompt anywhere on the first line:

proc term_chars_changed {} {
    uplevel #0 set test_pats 1
}

while 1 {
    if {!$test_pats} {tkwait var test_pats}
    set test_pats 0
    if {[regexp "%" [$term get 1.0 1.end]]} break
}

Writing a substantial script this way would be clumsy. Furthermore, it prevents the use of control flow commands in the actions. One solution is to create a procedure that does all of the work handling the semaphore and hiding the while loop.

Based on a procedure (shown later) called term_expect, the rogue script in Chapter 6 (p. 139) can be rewritten with the following code. This code is similar to the earlier version except that instead of patterns, tests are composed of explicit statements. Any nonzero result causes term_expect ...