Essentially, I'm looking at the
.list files in
/etc/apt/sources.list.d/. Each of them has a
deb entry; I'm curious to know if there's a way to see if any new distribution exists for any of the entries.
sudo apt-get update; sudo apt-get upgrade what you're looking for?
Look at section 2.4 of this intro: http://newbiedoc.sourceforge.net/system/apt-get-intro.html
You tagged you're question as Ubuntu and Debian which is a bit confusing. Your comment to rescdsk's answer seems to indicate you're really looking for an answer for Debian.
However here's a solution for Ubuntu I'm not sure if it will work for Debian or not.
Install update-manager-core if it's not already on the system then you can see if there's a new release using
Correct me if I am wrong, but if you want to do a dist upgrade with apt-get you would issue
apt-get update; apt-get -u dist-upgrade
This should prompt you for what will be upgraded. You may have to do this a few times if you are behind. If you are currently on Lenny than a simple apt-get update; apt-get -u upgrade will prompt you on what you need to install to update the system. The -u is important as it will show you what will happen before you hit Y (yes).
Now the above may not be what you are looking for but that is typical Debian/Ubuntu maintenance.
If you are trying to update programs that would be considered outside of the Lenny stable environment, you can add those 'repositories' to /etc/apt/sources.list. Once the appropriate lines are entered there you should enter the apt-get update command and then either apt-cache search NAME (to search for a specific program) command or you could do a apt-get -u dist-upgrade as above but be wary where you are pulling packages from.
If you actually want to scan the /etc/apt/sources.list.d/ files, you can do this but I do not see the point as apt-get will take care of all issues as far as stable debs are concerned. If you need a solution to check each deb, versus the update tool, I can definitely help. Just need more info on what you are actually trying to accomplish as the tools do the general admin for you.
Hope this helps
If you are looking to do centralized management on DEB package updates for multiple systems, someone wrote a thesis on this. The project added DEB support to Spacewalk, which is an open source update manager for RPM-based systems.