Exploring Expect

Example—Connecting Together Two Users To An Application

The following fragment is one possible way of writing the kernel of kibitz, an Expect script that connects together two users and an application (such as a shell). Both users can type to the application, and both users see the results. This is very useful for consulting or group editing.

Here, app is the spawned process shared between the users. One user is connected to the standard input and the other user is referred to by the spawn id user2.

expect {
    −i "$user_spawn_id $user2" -re ".+" {
        send −i $app $expect_out(buffer)
        exp_continue
    }
    −i $app -re ".+" {
        send_user -raw $expect_out(buffer)
        send −i $user1 $expect_out(buffer)
        exp_continue
    }
}

The script waits for input from both users and the application, all at the same time. The .+ pattern in each case allows the script to process as many characters as arrive. Actual processing is simple. Characters from either user are sent to the application. Characters from the application are sent to both users.

No code is necessary to send the keystrokes of one user to the other for echoing purposes. The application takes care of all the echoing automatically. For example, a program such as a shell normally echos typed characters back to the user. In this case, the script ends up sending them to both users. So both users see what each other types.

Each action ends by executing exp_continue. This is more verbose than wrapping the entire expect in a while loop, but it is more efficient. By remaining ...