How to Add Debian Testing Repository on Debian-Based Systems

Setting Up Repo

2. 2. Adding Debian Testing Apt Repo

   Then to Add Debian Testing Repository
   Simply Copy and Execute this Command:

   (And in case of “su: Authentication failure” try instead with “sudo su“)

   su -c "echo 'deb http://ftp.fr.debian.org/debian testing main' >> /etc/apt/sources.list.d/testing.list"

   Finally, to Enable also the Contrib and Non-free Repos Instead:
   (And in case of “su: Authentication failure” try instead with “sudo su“)

   su -c "echo 'deb http://ftp.fr.debian.org/debian testing main contrib non-free' >> /etc/apt/sources.list.d/testing.list"

   Last, it should be useful to Add also the Kernel and Security ones with:

   su -c "echo 'deb http://security.debian.org/debian-security testing-security main' >> /etc/apt/sources.list.d/bullseye.list"

   Possibly Change the Debian Mirror in base of your Location

   Debian Ftp Mirror List

   Debian Servers Worldwide List

   Finally, Ctrl+x to Save & Exit from nano Editor :)

3. 3. Setting Up Default

   Then to Set the Default Release
   And this is Indispensable to Preserve the Stability of the System in Case of Updates/Upgrades!
   Make or Edit the Apt Config File:

   sudo nano /etc/apt/apt.conf

   And Depending on your Release Append
   For an OldStable Based OS:

   APT::Default-Release "oldstable";

   Or to stay for the good you can also Replace it with the Release Codename like:

   APT::Default-Release "stretch";

   When instead for the Stable ones, and so also for Deepin:

   APT::Default-Release "/^stable(|-security|-updates)$/";

   Or respectively:

   APT::Default-Release "/^buster(|-security|-updates)$/";

   Last, for Kali:

   APT::Default-Release "kali-rolling";

   Possibly if needed to Find out What’s your actual Repo Base
   First, try to look up with a canonical:

   sudo apt update

   Or again use the cat Command like:

   cat /etc/apt/sources.list

   Thanks to dracae

4. 4. Enabling Repo

   Finally, to Re-Load Repositories
   Do:

   sudo apt update

   Authenticate with the User Admin Pass.
   If Got “User is Not in Sudoers file” then see: How to Enable sudo.

5. 5. Installing Single Package

   Finally, to Install One Package from Testing Repo
   Execute:

   sudo apt -t testing install [MYCOOLPACKAGE]

   So for instance:

   sudo apt -t testing install firefox