Exploring Expect

Sending Humanly

With the -h flag, the send command sends characters in a way that resembles a human typing. Unlike "send -s“, "send -h" enables variability and randomness. The time periods between every character can be different from one character to the next.

A varying sending speed is quite useful when trying to simulate the effect of interactive loads. For example, suppose you are testing a computer to see if it can handle a particular mix of CPU-intensive background processes and some interactive processes. Processing human interaction usually requires only a tiny fraction of the CPU. But by default, Expect scripts skip all the delays that real humans produce. Such scripts produce a very inaccurate simulation of interactive performance since the scheduler handles Expect processes as if they were just CPU-intensive processes.

The algorithm used to time character output is based upon a Weibull distribution, a common statistical tool for generating pseudo-random inter-arrival times. A few modifications have been made to suit its use in Expect. The output is controlled by the value of the send_human variable which takes a five-element list. The first two elements are average interarrival times of characters in seconds. The first is used by default. The second is used at word endings, to simulate the subtle pauses that occasionally occur at such transitions. The third parameter (the shape) is a measure of variability where .1 is quite variable, 1 is reasonably variable, and 10 is ...