How to evaluate a nix value from a shell script?

claes · December 29, 2022, 4:29pm

Hi,

I have written some bash scripts that helps me to manage my nix configuration. Part of what the scripts do is to help me ssh into the hosts that are managed by nix. In my nix config I have specified the static IP addresses of some hosts. What would be a good way for my script to “know” what IP address the nix config specifies?

To illustrate, I chose a random example from Github. In this file there is an IP address specified

github.com

redbrick/nix-configs/blob/96223b1e3197be09077d0a997de79cb1b3abd3e4/hosts/butlerxvm/configuration.nix

{
  imports = [
    ./hardware-configuration.nix
    ../../common/sysconfig.nix
    ../../services/ssh.nix
    ../../services/prometheus.nix
    ../../services/promtail.nix
    ../../services/ldap
  ];

  # This value determines the NixOS release with which your system is to be
  # compatible, in order to avoid breaking some software such as database
  # servers. You should change this only after NixOS release notes say you
  # should.
  system.stateVersion = "20.03";

  # Use the GRUB 2 boot loader.
  boot.loader.grub.enable = true;
  boot.loader.grub.version = 2;
  boot.loader.grub.device = "/dev/vda";

This file has been truncated. show original

What would be an accurate and clean way to parse that from a shell script? I could parse it with grep and regex but perhaps I can somehow evaluate the nix config to return it?

tejing · December 29, 2022, 4:38pm

You can. nix-instantiate --eval or nix eval can do this, though how depends on if you’re using a flake or not.

No flake (I think… been a while since I did this):

nix-instantiate --eval '<nixos/nixos>' -A config.path.to.option

Flake:

nix eval .#nixosConfigurations.hostname.config.path.to.option

claes · December 29, 2022, 4:51pm

Wow, that was super cool. So for the record, the following provides me with what I will use

nix eval .#nixosConfigurations.mediaclient.config.networking.interfaces.enp1s0.ipv4.addresses --json