examples: improve Flakes-based VM

- Show login message
- Poweroff on shell exit
- Move to top of examples/README

examples/README.md

@@ -1,8 +1,22 @@
-Examples
----
+## Examples
 
 The easiest way to try out nix-bitcoin is to use one of the provided examples.
 
+### Flakes-based quick start
+
+If you use a Flakes-enabled version of Nix, run the following command to start a minimal
+nix-bitcoin QEMU VM:
+```bash
+nix run github:fort-nix/nix-bitcoin/release
+```
+The VM (defined in [flake.nix](../flake.nix)) runs in the terminal and has `bitcoind`
+and `clightning` installed.\
+It leaves no traces (outside of `/nix/store`) on the host system.
+
+
+### More examples
+
+Clone this repo and enter the examples shell:
 ```bash
 git clone https://github.com/fort-nix/nix-bitcoin
 cd nix-bitcoin/examples/
@@ -79,7 +93,3 @@ The commands in `shell.nix` allow you to locally run the node in a VM or contain
 
 Flakes make it easy to include `nix-bitcoin` in an existing NixOS config.
 The [flakes example](./flakes/flake.nix) shows how to use `nix-bitcoin` as an input to a system flake.
-
-Run `nix run` or `nix run .#vm` from the nix-bitcoin root directory to start an example
-nix-bitcoin node VM.
-This command is defined by the nix-bitcoin flake (in [flake.nix](../flake.nix)).

examples/qemu-vm/minimal-vm.nix

@@ -13,7 +13,7 @@ rec {
 
   vm = (import "${nixpkgs}/nixos" {
     inherit system;
-    configuration = {
+    configuration = { lib, ... }: {
       imports = [
         nix-bitcoin.nixosModules.default
         "${nix-bitcoin}/modules/presets/secure-node.nix"
@@ -28,6 +28,20 @@ rec {
       virtualisation.graphics = false;
       services.getty.autologinUser = "root";
       nix.nixPath = [ "nixpkgs=${nixpkgs}" ];
+
+      services.getty.helpLine = lib.mkAfter ''
+
+        Welcome to nix-bitcoin!
+        To explore running services, try the following commands:
+        - nodeinfo
+        - systemctl status bitcoind
+        - systemctl status clightning
+      '';
+
+      # Power off VM when the user exits the shell
+      systemd.services."serial-getty@".preStop = ''
+        echo o >/proc/sysrq-trigger
+      '';
     };
   }).vm;