This is a very simple script, but I use it and it works. First go to
FreeDNS' Dynamic DNS Page and copy each of the URLs labeled "Direct URL" for the domains you want to update. The URLs should be somewhat like this...
http://freedns.afraid.org/dynamic/update.php?(hash)
...where
(hash) represents a hash of your username and password as well as domain ID. Paste each URL into a file named something like
update_freedns.sh. In front of the URLs, just put
curl, so it looks like this:
curl http://freedns.afraid.org/dynamic/update.php?(hash)
This works very well, it shows the output for each of the domains, and you can just call it from any shell by typing its name (remember to make it executable first,
chmod a+x udate_freedns.sh).