Exploring Expect

Character Graphics

The output produced by rogue in the previous section contains explicit cursor positioning character sequences. This can potentially cause the screen to be drawn in such a way that patterns fail to match the visible output. For example, imagine a score of 1000 being updated to 2000. To make the screen reflect this change, the program need only position the cursor appropriately and then overwrite the 1 with a 2. Needless to say, this will not match the string 2000 because the 2 arrived after the 000.

This particular problem does not arise in the rogue example because the screen is being drawn from scratch. This idea can be used to provide a general solution. To read the screen as if it were printed from top to bottom, force the spawned program to redraw the screen from scratch. Typically, sending a ^L suffices.

Alas, redrawing the screen does not solve other problems. For instance, there is still no way to tell where the cursor is. This may be critical if you are testing, for example, a menu-application to make sure that the cursor moves correctly from one entry to the next.

In Chapter 19 (p. 453), I will describe a way to handle this and other related problems more directly by maintaining an explicit representation of the terminal screen.