NixOS and the case for declarative configuration


So far, so good. If you like, you can now use the Nix package manager on any Unix-like system. However, things start to get really interesting with NixOS 1, a separate Linux distribution completely based on Nix.

The complete system is assembled and managed with Nix on the lines described previously. This means, in particular, that you can no longer mess around with individual configuration files to suit your mood; instead, you make changes in a working system that are transformed along a defined path into another working system.

System Description

The Nix mindset starts with the installation of NixOS. Apart from the usual gymnastics at boot time, and for mounting the root partition, the essential step consist of writing the declarative specification of the system.

The boot loader, any kernel modules you need to load, the disks, filesystems, the network, and the services are determined right down to the desktop environment level. The installer determines the required packages and then generates the system.

If you have made a mistake in the configuration, the installer lets you know. If an error occurs during the installation, you can restart the process without further ado. A list of the required steps is shown in Listing 1.

Listing 1

NixOS installation


One Step Back

Once NixOS is installed, you can install and remove packages, as described in the section on Nix. The Nix -e option removes a package. For a rollback, which resets the system to its state before the last package installation, Nix provides the --rollback option. In general, you can also jump directly to any previously existing system state (known as a generation) by typing

nix-env --switch-generation Generation

NixOS keeps the old versions of the packages; otherwise, this kind of switch would be impossible. However, you can certainly also delete the old generations; after all, at some point you might not want to go back to Generation 1, and disk space is not infinite. Typing:

nix-env --delete-generations old

deletes all the old generations from the profile, while the same call with numbers instead of old wipes out specific generations. After deletion from the profile, a call to nix-store --gc removes the old files. If you want to take a look at what will happen first, type nix-store --gc --print-dead instead.

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