Exploring Expect

A Very Brief Overview

Expect programs can be written in C or C++, but are almost always written using Tcl.^[4] Tcl is an interpreted language that is widely used in many other applications. If you already use a Tcl-based application, you will not have to learn a new language for Expect.

Tcl is a very typical-looking shell-like language. There are commands to set variables (set), control flow (if, while, foreach, etc.), and perform the usual math and string operations. Of course, UNIX programs can be called (exec). I will provide a quick introduction to the language in Chapter 2 (p. 23).

Expect is integrated on top of Tcl and provides additional commands for interacting with programs. Expect is named after the specific command that waits for output from a program. The expect command is the heart of the Expect program. The expect command describes a list of patterns to watch for. Each pattern is followed by an action. If the pattern is found, the action is executed.

For example, the following fragment is from a script that involves a login. When executed, the script waits for the strings "welcome“, "failed“, or "busy“, and then it evaluates one of the corresponding actions. The action associated with busy shows how multiple commands can be evaluated. The timeout keyword is a special pattern that matches if no other pattern matches in a certain amount of time.

expect {
    "welcome"            break
    "failed"            abort
    timeout            abort
    "busy"        {
        puts "busy"
        continue
    }
}

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^[4] Tcl is pronounced “tickle”. Some people are uncomfortable using this particular word in a formal setting. In that case, I recommend either saying the letters (i.e., “tee cee ell”) or coming up with a name suitable to your audience (e.g., “macho stud language”). Whatever works for you.