docs: add colmena eval

NixOS.md

@@ -28,6 +28,7 @@ This document contains general notes about NixOS that are independent of my NixO
   * [NixOS configuration debugging](#nixos-configuration-debugging)
   * [Show Nix configuration](#show-nix-configuration)
   * [Evaluate NixOS configuration to JSON](#evaluate-nixos-configuration-to-json)
+  * [Evaluate expressions in the context of a NixOS configuration managed with Colmena](#evaluate-expressions-in-the-context-of-a-nixos-configuration-managed-with-colmena)
   * [Evaluate expressions](#evaluate-expressions)
   * [Escape strings](#escape-strings)
   * [Run AppImages](#run-appimages)
@@ -674,6 +675,30 @@ import ./hosts/yodaTab/configuration.nix  {
 
 Then open `evaluated-config.json`.
 
+## Evaluate expressions in the context of a NixOS configuration managed with Colmena
+
+https://colmena.cli.rs/unstable/reference/cli.html#colmena-eval
+
+Either specify a file containing the expression or add it as argument with `-E`.
+
+```shell
+colmena eval -E '{ nodes, lib, pkgs, ... }: nodes.yodaTux.config.networking.hostName'
+```
+
+```shell
+nix-shell -p jq
+colmena eval -E '{ nodes, lib, pkgs, ... }: nodes.yodaTux.config.environment.systemPackages' | jq
+```
+
+```shell
+colmena eval -E '{ nodes, lib, pkgs, ... }: pkgs.linux' 
+#=> "/nix/store/6l9b3dly45iv2rp17hk5c3wvawaq0viy-linux-6.6.46"
+```
+
+```shell
+colmena eval -E '{ nodes, lib, pkgs, ... }: { imports = [ ./modules/nur-and-unstable.nix ]; pkgs.linux }'
+```
+
 ## Evaluate expressions
 
 * https://discourse.nixos.org/t/eval-nix-expression-from-the-command-line/8993/6