Sams Teach Yourself CGI in 24 Hours, Second Edition

Tracking User Activity

Let’s say you have a service that sends out HTML email to all of the subscribers to a mailing list. (Let’s also assume that you use email ethically and that the emails are only sent to subscribers who have also confirmed their subscriptions, and that your users don’t mind getting HTML email.) One question you might have is how many people actually open the emails that are sent to them, as opposed to just deleting them without looking at them. One way to keep track of this statistic is to include an image tag in the email message that requests an image served by a CGI script.

If the only place that the image-serving CGI is referenced is in your HTML email, or you pass back an identifier for the email with the request, ...