Stop Letting apt autoremove Surprise You: Practical apt-mark for Debian and Ubuntu

apt autoremove is useful, but a lot of Linux admins treat it a little like a haunted button.

You know it is supposed to remove packages that were installed only as dependencies and are no longer needed. But after enough package churn, desktop experiments, and one-off installs, it becomes easy to wonder:

Why is APT trying to remove that package?

Why is this dependency still hanging around?

How do I keep a package I care about from getting swept up later?

The answer is usually not guesswork.…