advanced package tool

Favoured by Debian-based distros

Installing packages

apt-get install <package> - just install a package

Updating

apt-get update - re-fetch package index files

apt-get upgrade - upgrade all currently installed packages. No packages will be removed, and no new packages will be installed

Querying installed packages

apt list --installed - list all installed packages

apt-cache policy - list all package repos

apt-cache policy <package> - show where an installed package was installed from, or whether a package is installed at all (useful resource)

Querying not-installed packages

apt-cache show <package> show information about a package that isn't installed

Installing additional repos

Add a line such as the following to /etc/apt/sources.list.d/:

deb [<http://archive.raspbian.org/raspbian>](<http://archive.raspbian.org/raspbian>) testing main

You probably then want to adjust the priority of that repo. For a Docker repo you want to increase the priority so it installs that Docker package; for a testing repo you want to decrease it so it doesn't update all your existing packages!

Update /etc/apt/preferences.d/limit-testing with a line such as Pin: release a=testing\\nPin-Priority: 50

Then view your repos with apt-cache policy, as above.

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