How to enable contrib and non-free repos in Debian

Debian-apt-get-contrib-non-free.md

As root you need to edit /etc/apt/sources.lst

Then add contrib and non-free at the end of each line that begins with deb and deb-src just like the example:

deb http://http.us.debian.org/debian jessie main contrib non-free

deb http://security.debian.org jessie/updates main contrib non-free

Save the file, and run ‘apt-get update‘ and optionally ‘apt-get upgrade‘ to activate the changes.

If you have software-properties-common installed you can use apt-add-repository contrib and apt-add-repository non-free to do it conditionally and be a little more robust to potentially differently modified sources lists.

thanks @tfoote !!

Thank you a lot @tfoote, your solution is gust perfect.
I perform this task on all my machines now:
sudo apt-get install software-properties-common -y && sudo apt-add-repository contrib -y # https://dragonserw.ru/wiki:debian

Unfortunately, this does not work currently with the new sources format, see https://stackoverflow.com/a/78720328.