How to get the current nixos version (for example 23.05) in a nixos config?

Is there a method to retrieve the current NixOS version within a Nixos configuration.nix? I'm in the process of upgrading from NixOS 20.03 to 23.05 and I'd like to maintain a single configuration that is compatible with both versions.

Due to some package versions being exclusive to either version, I require conditional logic, like an if or switch construct, based on the NixOS version to allow me to specify which version of a package to use in a given Nixos version/system.

I guess a workaround is to just run nixos-version > /etc/nixos/nixos-version and read this as a normal file. But if there is a way to do this natively in the config that would be ideal.

Solution

I've used this pattern:

{
  fonts =
    if (config.system.nixos.release == "23.05")
    then {
      fonts = [
        pkgs.nerdfonts
        pkgs.noto-fonts
      ];
    }
    else {
      packages = [
        pkgs.nerdfonts
        pkgs.noto-fonts
      ];
    };
}