The Debian distribution is dynamic and changes continually. Most of the changes are in the Testing and Unstable versions, but even Stable is updated from time to time, mostly for security-related fixes. Whatever version of Debian a system runs, it is generally a good idea to keep it up to date, so that you can get the benefit of recent evolutions and bug fixes.

While it is of course possible to periodically run a tool to check for available updates and run the upgrades, such a repetitive task is tedious, especially when it needs to be performed on several machines. Fortunately, like many repetitive tasks, it can be partly automated, and a set of tools have already been developed to that effect.

The first of these tools is apticron, in the package of the same name. Its main effect is to run a script daily (via cron). The script updates the list of available packages, and, if some installed packages are not in the latest available version, it sends an email with a list of these packages along with the changes that have been made in the new versions. Obviously, this package mostly targets users of Debian Stable, since the daily emails would be very long for the more mobile versions of Debian. When updates are available, apticron automatically downloads them. It does not install them — the administrator will still do it — but having the packages already downloaded and available locally (in APT's cache) makes the job faster.

Administrators in charge of several computers will no doubt appreciate being informed of pending upgrades, but the upgrades themselves are still as tedious as they used to be, which is where the /etc/cron.daily/apt script (in the apt package) comes in handy. This script is also run daily (and non-interactively) by cron. To control its behavior, use APT configuration variables (which are therefore stored in a file under /etc/apt/apt.conf.d/). The main three variables are:

Other options can allow you to control the cache cleaning behavior with more precision. They are not listed here, but they are described in the /etc/cron.daily/apt script.

These tools work very well for servers, but desktop users generally prefer a more interactive system. That is why the “Graphical desktop environment” task installs update-notifier and update-manager. The former displays an icon in the notification area of desktop environments when updates are available; clicking on this icon then runs update-manager, a simplified interface to perform updates. You can browse through available updates, read the description of the relevant packages and their changelog, and select whether to apply the update or not on a case-by-case basis. Note that these packages contain configuration data for /etc/cron.daily/apt so that it refreshes the lists of available packages and downloads the relevant ones. The update-notifier/update-manager combination is far from having as many features as aptitude and synaptic, since it only handles the upgrades of packages which are already installed; as a consequence, its minimalistic interface leaves little room for mistakes, and therefore little risk of breaking the system.