ck4up is a small command-line utility, writen in ruby, to watch http and ftp sites for updates. ck4up scans through a configuration file, fetches the listed URLs from the web, computes the md5sum of the page, and compares the value with one stored in a gdbm database. If both differ a message will be written to the standard output. To improve performance, ck4up is multi-threaded. Regular expressions can be applied to each page in the event that ck4up produces repetitive diff messages. For example, if daily snapshots and a web counter are on the same page. To simplify maintenance of the configuration file, a kind of macro-expansion can be used.