GNU/Linux Debian Stretch Non-free Repository Add Step-by-step

Adding Repo

2. 2. Adding Non-free Repo

   Then to Add Non Free Repository in Debian Stretch
   Simply run:

   sudo apt-add-repository non-free

   Again you may need enabled also the Contrib one, so play:

   sudo apt-add-repository contrib

   Last, Refresh the Apt Sources with:

   sudo apt update

3. 3. Preserving Debian Free System

   (Optional) This if you want to Preserve the Valuable Free Nature of Debian System.
   To do so you have to strictly Allowing only the Installation of Some Non-free Package with an Apt Pinning.
   First, Disable All Non free Software with this content:

   Explanation: Disable packages from non-free tree by default
   Package: *
   Pin: release o=Debian,a=stable,l=Debian,c=non-free
   Pin-Priority: -1
   

   To Make it at once execute:

   sudo echo -e "Explanation: Disable packages from non-free tree by default
   Package: *
   Pin: release o=Debian,a=stable,l=Debian,c=non-free
   Pin-Priority: -1" > /etc/apt/preferences.d/non-free_policy

   And then Enable the Software of Choice, E.g. firmware-iwlwifi:

   Explanation: Enable package firmware-iwlwifi from non-free tree
   Package: firmware-iwlwifi
   Pin: release o=Debian,a=stable,l=Debian,c=non-free
   Pin-Priority: 90
   

   Again with a simple echo:)

   sudo echo -e "Explanation: Enable package firmware-iwlwifi from non-free tree
   Package: firmware-iwlwifi
   Pin: release o=Debian,a=stable,l=Debian,c=non-free
   Pin-Priority: 90" > /etc/apt/preferences.d/firmware-iwlwifi_non-free

   But to enable Multiple packages at once proceed like that:

   Explanation: Enable package firmware-iwlwifi nvidia-driver from non-free tree
   Package: firmware-iwlwifi nvidia-driver
   Pin: release o=Debian,a=stable,l=Debian,c=non-free
   Pin-Priority: 90
   

   Again translated in a Command like:

   sudo echo -e "Explanation: Enable package firmware-iwlwifi nvidia-driver from non-free tree
   Package: firmware-iwlwifi nvidia-driver
   Pin: release o=Debian,a=stable,l=Debian,c=non-free
   Pin-Priority: 90" > /etc/apt/preferences.d/enabled-packages_non-free

   The only drawback of the procedure is that you have to repeat it every time you need some new Non-free software…
   For further instructions see: Advanced Apt Pinning Techniques.