I am looking for a tool that: + is free (free as defined in https://www.fsf.org/) + preferably shell-based + can run as a background process or as a cron job + watch for changes on a webpage + send a notification on OSX or Linux when a change occurs (not an email)

If you could also add pointers on how you use the tool or how you have configured it, that'd be greatly helpful too.

Thank You!