Exploring Expect

The log_user Command

In Expect scripts, it is very useful to hide the underlying dialogue, perhaps substituting new output. Sometimes it is useful just to show parts of the dialogue. You can do this with the log_user command. "log_user 0" stops the output of the spawned process from appearing in the standard output of Expect. "log_user 1" restores it.

The following script verifies email destinations at remote hosts by exercising the vrfy command supported by SMTP, a mail transfer protocol commonly used on the Internet. The script takes an email address as an argument.

#!/usr/local/bin/expect --

regexp (.*)@(.*) $argv ignore user host
spawn telnet $host smtp
set timeout −1
expect -re "220.*\r\n"
send "vrfy $user\r"
expect -re "(250|550).*\r\n"

The first command in the script extracts the user and host names from the original argument. This is done using the very same regexp command that I used as an example in Chapter 6 (p. 135). The remainder of the script performs the interaction with the remote host.

The script is named vrfy. When I run it, here is what appears:

% vrfy [e-mail]
spawn telnet next.com smtp
Trying 129.18.1.2 ...
Connected to next.com.
Escape character is '^]'.
220 NeXT.COM Sendmail NX5.67d/NeXT0.5-Aleph-amm ($Revision: 1.4 $ $State: Exp $)
ready at Thu, 1 Apr 93 21:32:19 -0800
vrfy jobs
250 <jobs>

By adding "log_user 0" to the script and giving the final expect command an explicit action, the output is simplified dramatically. Here is the new version using log_user ...