How to delete old NixOS boot configurations

24 March 2017

NixOS is a Linux distribution with declarative configuration management. Your system configuration is specified in a set of files, and can run a command to update your system to the current specification. A consequence of this is that your entire system configuration can be versioned.

Every time you rebuild your NixOS configuration, a new entry is added to the bootloader. This is helpful if you ever make a configuration change that breaks on your machine because you can reboot into the last known working state and try something different.

If you don’t need to have access to all your old configurations, you can delete them:

  1. Delete the old (excludes the current) package configurations for the NixOS system sudo nix-env -p /nix/var/nix/profiles/system --delete-generations old

  2. Collect garbage nix-collect-garbage -d

  3. View the remaining generation nix-env -p /nix/var/nix/profiles/system --list-generations. Take note of this for the next step.

  4. Remove unnecessary boot loader entries. I use systemd-boot, so all my entries are located in /boot/loader/entries. To remove all the old entries, run sudo bash -c "cd /boot/loader/entries; ls | grep -v <current-generation-name> | xargs rm" (you might want to back up the entries somewhere to be safe)