Exploring Expect

Example—Answerback

At login time, some systems attempt to figure out the type of terminal being used. This is done by a technique called answerback. The system queries the terminal by sending it a special escape sequence. Instead of printing the escape sequence on the screen, the terminal responds by returning an identification code describing what type of terminal it is.

The fragment below telnets to a host, logs in, and then begins interacting with a program. What happens if the systems asks the terminal to identify itself?

spawn telnet hostname
expect "name: " {send "$username\r"}
expect "word: " {send "$password\r"}

# possible answerback occurs here

expect $prompt {send "$program\n"}
. . .

After the password, the script waits for the prompt. Instead, the system sends the request for identification sequence. Expect handles this just like any other output from the process—it echoes the request to the standard output and continues looking for the prompt. Now the terminal intercepts the escape sequence and responds with its identification code. The identification code appears to Expect on the standard input, just as if the user had typed it. Unfortunately, the Expect script is not watching the standard input. So the script continues to wait, the system also waits, and the user fumes.

Here is a better version:

spawn telnet hostname expect "name: " {send "$username\r"} expect "word: " {send "$password\r"} # handle possible answerback here stty raw -echo expect { "$prompt" {send $program\r} ...