How to save changes and installed packages so I can install them in other systems

Question

I want to format my system, but I don’t want to lose my updates and installed packages. I also want the same packages on other systems as well.

Answer 1

Backing up should be:

dpkg --get-selections > ~/Package.list
sudo cp /etc/apt/sources.list ~/sources.list
sudo apt-key exportall > ~/Repo.keys

and restoring should be

sudo apt-key add ~/Repo.keys
sudo cp ~/sources.list /etc/apt/sources.list 
sudo apt-get update
sudo apt-get install dselect
sudo dpkg --set-selections < ~/Package.list
sudo apt-get dselect-upgrade -y

...If I'm not mistaken.