Exploring Expect

Which Spawn Id Matched

Except for timeout, patterns are always associated with a particular spawn id. You have already seen cases where actions are completely different. A common technique is to pass the spawn id itself to each action as a parameter, reusing the same action.

The following fragment runs a command on the fastest of three systems.

spawn rlogin $host1; set spawn_id1 $spawn_id
spawn rlogin $host2; set spawn_id2 $spawn_id
spawn rlogin $host3; set spawn_id3 $spawn_id
expect {
    -i $spawn_id1 $prompt {work $host1}
    -i $spawn_id2 $prompt {work $host2}
    -i $spawn_id3 $prompt {work $host3}
}

There is no explicit comparison to find which is fastest. Rather, it is implicitly derived just by virtue of it being the first to complete the rlogin sequence and return a prompt.

When the first of the three systems returns the prompt, the winning spawn id is passed to the procedure work which does the actual interacting. The remaining spawn ids are simply ignored.

The technique demonstrated here can certainly be refined. For example, the systems could all have their load average checked, memory usage checked, etc., but the general idea extends to other applications involving multiple processes being controlled. As an aside, the simplistic use of rlogin as a test for speed is in many cases quite accurate. Both the initial network connection and login sequences themselves are fairly high-overhead operations, and serve as good indicators of subsequent response time for many interactive programs. ...