Debian の安定版は次のリリースまでの間にかなり長い時間をかけて大きく進化するので、アップグレードの前にリリースノートを読むべきです。

In this section, we will focus on upgrading a Buster system to Bullseye. This is a major operation on a system; as such, it is never 100% risk-free, and should not be attempted before all important data has been backed up.

Another good habit which makes the upgrade easier (and shorter) is to tidy your installed packages and keep only the ones that are really needed. Helpful tools to do that include aptitude, deborphan, debfoster, and apt-show-versions (see 第 6.2.7 節「自動的にインストールされたパッケージの追跡」). For example, you can use the following command, and then use aptitude's interactive mode to double check and fine-tune the scheduled removals:

# deborphan | xargs aptitude --schedule-only remove

Now for the upgrading itself. First, you need to change the /etc/apt/sources.list file to tell APT to get its packages from Bullseye instead of Buster. If the file only contains references to Stable rather than explicit codenames, the change isn't even required, since Stable always refers to the latest released version of Debian. In both cases, the database of available packages must be refreshed with the apt update command or the refresh button in synaptic (第 6.2.1 節「初期設定」).

Once these new package sources are registered, you should first do a minimal upgrade with apt upgrade et al. as described in 第 6.2.3 節「システムのアップグレード」. By doing the upgrade in two steps, we ease the job of the package management tools and often ensure that we have the latest versions of those, which might have accumulated bugfixes and improvements required to complete the full distribution upgrade.

Once this first upgrade is done, it is time to handle the upgrade itself, either with apt full-upgrade, aptitude, or synaptic (第 6.7 節「安定版から次のディストリビューションへのアップグレード」). You should carefully check the suggested actions before applying them: you might want to add suggested packages or deselect packages which are only recommended and known not to be useful. In any case, the frontend should come up with a scenario ending in a coherent and up-to-date Bullseye system. Then, all you need is to do is wait while the required packages are downloaded, answer the debconf questions and possibly those about locally modified configuration files, and sit back while APT does its magic.

Debian メンテナの最大限の努力にも関わらず、システムのメジャーアップグレードは常に望んだ通りに順調に進むわけではありません。新しいソフトウェアバージョンは古いものと互換性がないかもしれません (たとえば、デフォルト動作やデータフォーマットが変わっているかもしれません)。また、いくつかのバグは Debian リリース前のテスト段階を経たにも関わらず見過ごされているかもしれません。

いくつかの問題を未然に防ぐために、apt-listchanges パッケージをインストールすることが可能です。apt-listchanges は、あるパッケージのアップグレード作業の開始時に、このパッケージのアップグレードを行うことで生じる可能性のある問題の情報を表示します。この情報はパッケージメンテナがまとめたもので、ユーザが内容を確認できるように /usr/share/doc/package/NEWS.Debian ファイルに含まれています。これらのファイルを (場合によっては apt-listchanges を通じて) 読めば、予期しない状況に驚かされることがなくなるでしょう。

You might sometimes find that the new version of a software doesn't work at all. This generally happens if the application isn't particularly popular and hasn't been tested enough; a last-minute update can also introduce regressions which are only found after the stable release. In both cases, the first thing to do is to have a look at the bug tracking system at https://bugs.debian.org/package, and check whether the problem has already been reported. If this is case, it will be also listed before the upgrade begins if you have apt-listbugs installed. If it hasn't, you should report it yourself with reportbug. If it is already known, the bug report and the associated messages are usually an excellent source of information related to the bug:

Depending on the severity of the bug, a new version of the package may be prepared specifically for a new revision of the stable release. When this happens, the fixed package is made available in the proposed-updates section of the Debian mirrors (see 第 6.1.2.3 節「提案された更新」). The corresponding entry can then be temporarily added to the sources.list file, and updated packages can be installed with apt or aptitude.

Sometimes the fixed package isn't available in this section yet because it is pending a validation by the Stable Release Managers. You can verify if that is the case on their web page. Packages listed there aren't available yet, but at least you know that the publication process is ongoing.

APT usually ensures a clean upgrade, pulling in new and updated dependencies, or removing conflicting packages. But even being such a great tool, it cannot cover all tasks users and administrators will face after an upgrade, because they require a human decision.

$ apt-show-versions | grep "No available version"

$ dpkg -l | grep ^ii | grep -i -E "(transition|dummy)"