Exploring Expect

Prompting For A Password On Behalf Of A Program

Reading passwords is the most frequent reason to use "stty -echo“, and it comes up in many places and in many ways. Here is an excerpt from a script that logs in to another host. If the second host demands a password, the script turns around and asks the user. The script does not print the original prompt but instead manufactures a new prompt including the username and host so that the user understands exactly which password is expected, even though the user has seen no other dialogue (and may not even know what the script is doing).

expect {
    assword: {
        stty -echo
        send_user "password (for [exec whoami]) on $host:"
        set old_timeout $timeout; set timeout −1
        expect_user -re "(.*)\n"
        send_user "\n"
        set timeout $old_timeout
        send "$expect_out(1,string)\r"
        exp_continue
    } "incorrect" {
        send_user "invalid password or account\n"
        exit
    } timeout {
        send_user "connection to $host timed out\n"
        exit
    } eof {
        send_user "connection to host failed: "
        send_user "$expect_out(buffer)"
        exit
    } -re $prompt
}

The first expect looks for a password prompt, a shell prompt, and various failure conditions all at the same time. If no password is required, the final pattern matches and the script goes on to the next command. If the remote computer does prompt for a password, the user is requested to supply the password. The current timeout is saved in old_timeout and restored later. This is analogous to setting it as a local variable in a procedure.

Once the user has supplied ...