Manage your software sources in Debian, Ubuntu and Mint

Introduction
The following information concentrates on the latest long-term versions of Debian, Ubuntu and Mint, namely Debian 9 (Stretch), Ubuntu 18.04 (Bionic) and Mint 19.x (Tara, Tessa, Tina, ...), but should be applicable, with modifications, to any Debian-derived distro.

Note that the instructions don't cover the addition or deletion of repositories to/from your software sources. It is intended rather as a diagnostic tool when apt update is generating errors that appear to relate to an incorrect software source setup.

A distro's software sources consist of files that specify the location of the repositories that it will use to obtain its packages. The distro usually keeps its main software sources in /etc/apt/sources.list while other software sources (third-party software etc.) are kept in .list files in the /etc/apt/sources.list.d directory. Mint differs in that its main software sources are kept in /etc/apt/sources.list.d/official-package-repositories.list.

One practice that is important is to not mix distro versions in your software sources. This can lead to what Debian users, for example, call a FrankenDebian. This can cause significant problems in your system and may even necessitate a re-install if the situation is too far advanced. So, for example, if you're a Debian user, don't mix Stretch and Jessie or Sid; if you're an Ubuntu user, don't mix Bionic and Xenial; if you're a Mint user, don't mix tessa and Xenial.

Listing your software sources
You can obtain a list of your software sources by running

If you don't have the system info tool inxi installed, then it can be installed from the repositories or downloaded from here.

As an alternative, if for some reason you can't install or run inxi, use the following command:

Note that some lines start with deb-src rather than deb - these lines indicate where your distro will obtain the source code for packages, rather than the usual binaries. Also, any line can be commented out by placing a # at the start of the line. For example, if you don't want to download any source code at all then you can comment out all the deb-src lines. Duplicate lines will be ignored, but it best to delete these where possible.

Editing your software sources
You can modify your software sources by either editing them directly (as root) or using a GUI program available in your distro.

For example (assuming you use sudo to gain root privileges; replace gedit or xed with your favourite test editor),

In Ubuntu, you can use the Software & Updates program to edit your repositories. In Mint, you can use Software Sources. In all three distros, you can use Synaptic.

Main software sources - Debian (reference)

For Debian Stretch, /etc/apt/sources.list should look like this (miss out the contrib and non-free keywords if you don't want those repositories):

In addition, if you need to use the backports repository, add:

Main software sources - Ubuntu

In Ubuntu, /etc/apt/sources.list comes in a more cluttered form than the Debian equivalent. However, for Ubuntu 18.04 (Bionic), the key elements it contains should be the following (miss out the universe and multiverse keywords if you don't want those repositories):

Main software sources - Mint

For Mint, /etc/apt/sources.list.d/official-package-repositories.list should be as follows (deb-src lines have been left out for simplicity; replace tessa (Mint 19.1) with the codename of your version of Mint 19.x):

Other software sources

Additional third-party repositories (including Ubuntu's Personal Package Archives (PPAs)) can be added to your software sources by different means. It is important to remember only to add such repositories if they apply to the version of your distro (for Mint 19.x users, you can also include Ubuntu Bionic repositories). Do not mix Debian and Ubuntu repositories. Ubuntu PPAs can cause problems if installed in Debian and doing so is not advised.

Following are some examples of third-party repositories (deb-src lines have been missed out to keep things simple):

Chrome (/etc/apt/sources.list.d/google-chrome.list)
Etcher (/etc/apt/sources.list.d/balena-etcher.list)
Git (/etc/apt/sources.list.d/git-core-ppa-bionic.list)
Skype (/etc/apt/sources.list.d/skype-stable.list)
VirtualBox (/etc/apt/sources.list.d/virtualbox.org.list)
Wine (/etc/apt/sources.list.d/additional-repositories.list )
Please provide any feedback on these instructions in the comments so that they can be improved if necessary.