Perl Cookbook

Controlling Another Program with Expect

Problem

You want to automate interaction with a full-screen program that expects to have a terminal behind STDIN and STDOUT.

Solution

Use the Expect module from CPAN:

use Expect;

$command = Expect->spawn("program to run")
    or die "Couldn't start program: $!\n";

# prevent the program's output from being shown on our STDOUT
$command->log_stdout(0);

# wait 10 seconds for "Password:" to appear
unless ($command->expect(10, "Password")) {
    # timed out
}

# wait 20 seconds for something that matches /[lL]ogin: ?/
unless ($command->expect(20, -re => '[lL]ogin: ?')) {
    # timed out
}

# wait forever for "invalid" to appear
unless ($command->expect(undef, "invalid")) {
    # error occurred; the program probably went away
}

# send "Hello, world" and a carriage return to the program
print $command "Hello, world\r";

# if the program will terminate by itself, finish up with
$command->soft_close();
    
# if the program must be explicitly killed, finish up with
$command->hard_close();

Discussion

This module requires two other modules from CPAN: IO::Pty and IO::Stty. It sets up a pseudo-terminal to interact with programs that insist on using talking to the terminal device driver. People often use this for talking to passwd to change passwords. telnet (Net::Telnet, described in Section 18.6, is probably more suitable and portable) and ftp are also programs that expect a real tty.

Start the program you want to run with Expect->spawn, passing a program name and arguments either ...