Exploring Expect

A Little More About Tcl

Expect’s language is general-purpose. It has control structures and data structures so that you can do just about anything. This can make up for the lack of programmability in other programs. For example, most debuggers do not provide very sophisticated methods of programmed control. Typically, you cannot declare variables and are restricted to simple loops.

The core of Expect’s language facilities are provided by Tcl, a language used by many other tools. “Tcl” stands for Tool Command Language. It comes as a library intended to be embedded in applications, providing a generic language facility to any application. Tcl solves a long-standing problem in designing application languages.

Actually, most tools do not have very sophisticated internal control languages. The shell represents the extreme—it has a very rich language and can even call upon other programs. Start-up scripts such as .cshrc and .profile can be very complex. Few programs have such flexibility. And that is just as well. Such programs each have their own language and it is daunting to master each new one.

In the middle of the spectrum are programs such as ftp that have a limited ad hoc language—such as permitted in the file .netrc—not very flexible but better than nothing. A lot of programs are designed like ftp—their languages are limited, not extensible, and different from one tool to the next.

At the far end are programs such as telnet which have no control language whatsoever. The bulk of all ...