Exploring Expect

An Extended Example—tkpasswd

tkpasswd is an Expectk script that creates a GUI for changing passwords conveniently. You might wonder how there could be any value in a GUI for such a trivial program; however, people who change passwords frequently (such as system administrators) will find many benefits. For instance, the GUI provides the same interface whether you are changing local passwords (/etc/passwd) or remote passwords (NIS). The GUI can show accounts in different ways, such as sorted by name or uid. The GUI also highlights accounts that have no passwords—a potential security problem. Lastly, the GUI can reject passwords that are inappropriate. Naturally, all of this is done without modifying the passwd program itself.

Even with these and other features, the script is only 300 lines (of which 60 are empty or comment lines). About 100 lines are related to laying out the graphics. Only about 10 of the lines are directly related to driving the passwd program, but it is worthwhile to examine other parts of the program to see how, for example, the press of a button is translated into a behavior change in the passwd interaction. The script comes with the Expect distribution as an example.

When run, the script displays the image shown on page 431. At the top are several radio buttons which control the script. In the middle is a user browser. Below this are several more buttons and an entry widget in which passwords can be entered.

The script begins with the usual incantation and commentary ...