Updating NixOS

Introduction

NixOS stands out due to its declarative configuration and atomic updates, which ensure that system updates are predictable, reversible, and don’t risk breaking the setup. This approach guarantees consistency across versions, allowing any changes to be easily rolled back. NixOS also offers flexibility, multi-version support, and advanced dependency management, making it an excellent choice for developers and system administrators.

As part of this process, only repository channels are updated or removed during updates. The system requires an internet connection to download the latest changes, and users cannot directly modify the system. For optimal stability, security, and access to new features, regular updates — ideally once a week — are recommended.

Rebuilding the system after editing configuration.nix file

To apply the configuration changes made in /etc/nixos/configuration.nix without updating the channels, Nixpkgs and package versions. This is typically used when you've edited the system configuration, and you just want to apply those changes:

# nixos-rebuild switch

For non-flake configurations

Updating NixOS channels

# nix-channel --update

For more information on channels, see the main Channel branches page.

Rebuilding the system after updating channels

If you want to not only apply your configuration changes but also update the packages and system environment to the latest versions available from the Nixpkgs repository. This is typically used when you want to ensure you are using the latest versions of your software and system services:

# nixos-rebuild switch

To apply configuration changes and new package updates only after rebooting the system, use the following command instead:

# nixos-rebuild boot

Changing Nixpkgs version

To see what is the latest channel available, see https://channels.nixos.org

# nix-channel --add https://channels.nixos.org/nixos-<version> nixos

Deleting old generations

# nix-collect-garbage -d

Example of a system update

# nix-channel --update && nixos-rebuild switch && reboot

For flake based configurations

Because Flakes do not use channels and instead rely on explicitly defined inputs, updating a configuration involves modifying the system’s flake.nix to reference the desired versions of inputs. For example:

❄︎ flake.nix

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-25.05"; # update version
    ...
  };
  ...
}

Once the input URLs have been updated, refresh the flake lockfile with:

# nix flake update

Finally, rebuild the system configuration to apply the changes:

# nixos-rebuild switch

Tips and tricks

Limiting the maximum number of running jobs

Sometimes, the update process may hang when the system CPU has a high number of cores. You can limit the maximum number of running jobs:

# nixos-rebuild switch --option max-jobs 8

To make the change permanent, add the following to your configuration.nix:

nix = {
  settings = {
    max-jobs = 8;
  };
};