Exploring Expect

Tcl—A Little History

Underneath Expect is Tcl, a small and simple language designed to be embedded in applications. Much of the reason Expect appears so coherent and well thought out is actually due to Tcl.

Tcl is a tour de force. It is powerful yet elegant, drawing a fine line between primitives and extensibility, and between simplicity and overkill. Tcl allowed me to concentrate on the application requirements of Expect. Tcl will allow you to call and mix the Expect primitives in all sorts of interesting ways to control your applications.

The intent behind Tcl matches Expect’s philosophy perfectly. Expect doesn’t need a specialized language. Any generic but extensible control language would have sufficed. However, at the time that I was thinking about writing Expect, no such extensible control language existed. I was irritated at the thought of having to create a language specifically for such a simple application. There was simply no justification for designing yet another language.

I had been thinking about writing an Expect-like program after helping Scott Paisley write a program to automate the initial login and command in a telnet session. The program understood only a few simple commands. For instance, find waited for a single fixed string to arrive. Every session ended in a permanent sort of interact with a single telnet process. There were no variables and no flow control commands such as if. And the program used pipes instead of ptys. The program solved our immediate problem, ...