Driving the Command Line with Expect

Expect has been copied many times, and bindings to libexpect exist for many other programming languages. But none of them achieves the same deep integration with the language, the ease of use and the versatility of Expect for Tcl.

Caius simplifies working with Expect by wrapping the most commonly used functionality in an object-oriented programming interface.

Spawning a Process

In order to spawn a process and interact with it through the standard channels, create a CliDriver::Spawn instance like this:

package require CliDriver

set p [CliDriver::Spawn #auto telnet localhost]

You can then invoke send and expect on the object p as you would with classic expect:

set count 0
set timeout 5

$p expect {
    "*assword*:*" {
        $p send "password\n"
        exp_continue
    }
    "*ogin*:*" {
        $p send "username\n"
        exp_continue
    }
    "*user@host*" {
        # done
    }
    timeout {
        incr count

        if {$count > 3} {
            error "Timed out during login procedure."
        }

        exp_continue
    }
}

itcl::delete object $p

The advantages of working with objects become more apparent when you start spawning multiple processes. With classic expect you need to keep track of the spawn ids and set the global spawn_id variable each time you switch to another process. Caius does this for you implicitly.

If you need to, you can still access the spawn id like this

set spawn_id [$p spawn_id]

and use classic expect calls, for example to wait on multiple channels at the same time. Note also how we used timeout and exp_continue just as if we were working with Expect directly.

When you delete a Spawn object while the process it represents is still active, then the process will be terminated. If the process has ended, it will be waited on.

Connecting via SSH and Telnet

Since Expect is often used to log into remote machines using SSH or Telnet, the convenience classes CliDriver::Telnet and CliDriver:Ssh are available. They are used in the same way as Spawn but implicitely invoke the telnet or ssh command installed in the system.

package require CliDriver

set ssh [CliDriver::Ssh #auto user@host]

$ssh expect "*Password:*"
...

Using a Serial Line

Last but not least, there is the CliDriver::Stty class. As the name implies, this one is for running Expect on a serial connection. The constructor takes various optional parameters, such as the baud rate, that are used to configure the terminal device, as shown in the following example:

package require CliDriver

set serial [CliDriver::Stty #auto -baud 56000 \
    -parity n -data_bits 8 -stop_bits 1 /dev/ttyS0]

The settings in the example above are actually the default settings. So in case they match your requirements, it is sufficient to specify the port.

Expecting from Multiple Sources

If you need to expect from multiple processes simultaneously, you currently have to harvest the spawn ids from your CliDriver objects and use a traditional expect call. A more user-friendly way of doing this may be implemented in future versions of Caius.

Important Notes

Caius calls log_user and log_file to ensure that Expect logs all output through a Tcl channel. Be aware that modifying either of those settings may break logging in your test scripts in unexpected ways.