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Upgrade Debian 10 (Buster) to Debian 11 (Bullseye)

Upgrading your operating system can seem daunting, but moving from Debian 10 (Buster) to Debian 11 (Bullseye) is a straightforward process if you follow these steps. Before proceeding, ensure you have a backup of your data, a stable internet connection, and root or sudo privileges for your user account.

Prerequisites​

  1. Superuser Privileges: You must perform the upgrade with superuser privileges. Log in as root or a user with sudo privileges.
  2. Data Backup: Back up your data before starting the upgrade. If you're using a virtual machine, consider taking a complete system snapshot.

Update Current Packages​

  1. Check for Held Back Packages: Run sudo apt-mark showhold to check for any packages that are held back, as they can cause issues during the upgrade. Unhold them if necessary.
  2. Update Installed Packages: Refresh your package index and upgrade all installed packages using the following commands: 
    sudo apt update
    sudo apt upgrade
  3. Perform a Full Upgrade: Use sudo apt full-upgrade to update your packages to the latest versions. This command may also remove unnecessary packages.
  4. Clean Up: After the full upgrade, remove any automatically installed dependencies that are no longer needed with sudo apt autoremove.

Modify APT’s Source-List Files​

  1. Reconfigure APT Sources: Open /etc/apt/sources.list and replace each occurrence of buster with bullseye. If you have other source files under /etc/apt/sources.list.d, update those as well.
  2. Using sed Command: Alternatively, execute the following sed commands to update your sources list: 
    sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list
    sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list.d/*.list
    sudo sed -i 's#/debian-security bullseye/updates# bullseye-security#g' /etc/apt/sources.list
    deb http://deb.debian.org/debian bullseye main contrib non-free
    deb http://deb.debian.org/debian bullseye-updates main contrib non-free
    deb http://deb.debian.org/debian-security bullseye-security/updates main
  3. Set Terminal Output to English: This helps to avoid language-specific issues during the upgrade. Use export LC_ALL=C to set the language to English.
  4. Update Packages Index Again: Run sudo apt update to refresh the package index with the new sources.

Perform the System Upgrade​

  1. Upgrade Installed Packages: Begin the system upgrade with sudo apt upgrade. This step upgrades packages without requiring additional packages to be installed or removed.
  2. Full System Upgrade: Execute sudo apt full-upgrade to perform a complete system upgrade. This command resolves dependency changes and upgrades packages that were not updated in the previous step.
  3. Clean Up and Reboot: After completing the full upgrade, clean up unnecessary packages again with sudo apt autoremove. Then, reboot your machine to activate the new kernel using sudo systemctl reboot.

Confirm the Upgrade​

After rebooting, confirm that your system has been successfully upgraded to Debian 11 (Bullseye) by running lsb_release -a. The output should indicate Debian GNU/Linux 11 (Bullseye) as the distribution.