docs: add documentation on nix-channel

Author: Mic92

docs/SUMMARY.md

@@ -20,5 +20,6 @@ Refer to the following documentation for more information.
 - [Secrets and full disk encryption](./howtos/secrets.md)
 - [Use without flakes](./howtos/use-without-flakes.md)
 - [Terraform](./howtos/terraform.md)
+- [Nix-channels / `NIX_PATH`](./howtos/nix-path.md)
 
 [Reference](./reference.md): Reference Guide

docs/howtos/INDEX.md

@@ -17,3 +17,5 @@
 [Use without flakes](./use-without-flakes.md)
 
 [Terraform](./terraform.md)
+
+[Nix-channels / `NIX_PATH`](./nix-path.md)

docs/howtos/nix-path.md

@@ -0,0 +1,57 @@
+# Nix-channels / `NIX_PATH`
+
+nixos-anywhere does not install channels onto the new system by default to save
+time and disk space. This for example results in errors like:
+
+```
+(stack trace truncated; use '--show-trace' to show the full trace)
+
+error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)
+
+at «none»:0: (source not available)
+```
+
+when using tools like nix-shell/nix-env that rely on `NIX_PATH` beeing set.
+
+# Solution 1: Set the `NIX_PATH` via nixos configuration (recommended)
+
+Instead of stateful channels, one can also populate the `NIX_PATH` using nixos
+configuration instead:
+
+```nix
+{
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  # ... other inputs
+
+  outputs = { nixpkgs, ... }:
+    {
+      nixosConfigurations.yoursystem = nixpkgs.lib.nixosSystem {
+        system = "x86_64-linux"; # adapt to your actual system
+        modules = [
+          # This line will populate NIX_PATH
+          { nix.nixPath = [ "nixpkgs=${inputs.nixpkgs}" ]; }
+          # ... other modules and your configuration.nix
+        ];
+      };
+    };
+}
+```
+
+Advantage: This solution will be automatically kept up-to-date everytime the
+flake is updated.
+
+In your shell you will see something in your `$NIX_PATH`:
+
+```shellSession
+$ echo $NIX_PATH
+/root/.nix-defexpr/channels:nixpkgs=/nix/store/8b61j28rpy11dg8hanbs2x710d8w3v0d-source
+```
+
+# Solution 2: Manually add the channel
+
+On the installed machine, run:
+
+```shellSession
+$ nix-channel --add https://nixos.org/channels/nixos-unstable nixos
+$ nix-channel --update
+```