Skip to Content

about blog contact

Learn Flakes the Fun Way

Posted on Jul 9 2024

This post is a Flake-based rewrite of Learn Nix the Fun Way on I really enjoyed the content of the post and wanted to write it as a Nix user who is just using and prefers Flakes. It does add a few extra steps and complexity, but I think it’s still valuable and perhaps reveals a bit more about Nix and why it’s pretty fantastic. I also include a few neat things that are more Flake-specific at the end.

But yes, it’s basically plagiarism.


Let’s walk through a single example of a shell script one may write, what-is-my-ip:

#!/usr/bin/env bash
curl -s | jq --raw-output .origin

Sure, it’s sort of portable, if you tell the person running it to have curl and jq. What if you relied on a specific version of either, though?

Nix guarantees portability.

We might leverage Nixpkgs’ trivial builders (specifically, writeShellScriptBin) in a basic Nix Flake to turn this into a Nix derivation (i.e. build recipe):

# flake.nix
  inputs.nixpkgs.url = "nixpkgs/nixos-24.05";
  outputs = {nixpkgs, ...}: let
    systems = ["aarch64-linux" "aarch64-darwin" "x86_64-darwin" "x86_64-linux"];
    pkgsFor = func: (nixpkgs.lib.genAttrs systems (system: (func (import nixpkgs {inherit system;}))));
  in {
    packages = pkgsFor (pkgs: {
      default = pkgs.callPackage ./what-is-my-ip.nix {};
# what-is-my-ip.nix
pkgs.writeShellScriptBin "what-is-my-ip" ''
  ${pkgs.curl}/bin/curl -s | ${pkgs.jq}/bin/jq --raw-output .origin

😬 Avoid over-focusing on the fact I just introduced a new language and a good chunk of boilerplate. Just come along for the ride.

Here we are pinning our package to dependencies which come from NixOS/Nixpkgs release branch 24.05.

We can build our package and find out the Nix store path (which contains the hash) like so:

$ nix build --print-out-paths

And, of course, we can run our built result:

$ ./result/bin/what-is-my-ip

Or run it from the Flake directly:

$ nix run

Graph Dependencies

Now that this is in our Nix store, we’ve naturally modeled our dependencies and can do fun things like generate graph diagrams:

$ nix-store --query --graph $(readlink result) | nix shell nixpkgs#graphviz -c dot -Tpng -o what-is-my-ip-deps.png

Image of what-is-my-ip dependencies as a graph

Developer Environments

Let’s add a developer environment which contains our new tool. This is a great way to create developer environments with reproducible tools. Generally, though, devShells are for working on the product of the Flake and do not usually include the product itself.

diff --git a/flake.nix b/flake.nix
index 2a99357..ab32421 100644
--- a/flake.nix
+++ b/flake.nix
@@ -1,11 +1,24 @@
   inputs.nixpkgs.url = "nixpkgs/nixos-24.05";
-  outputs = {nixpkgs, ...}: let
+  outputs = {
+    self, # we will need to reference our own outputs to pull in the package we've declared
+    nixpkgs,
+    ...
+  }: let
     systems = ["aarch64-linux" "aarch64-darwin" "x86_64-darwin" "x86_64-linux"];
     pkgsFor = func: (nixpkgs.lib.genAttrs systems (system: (func (import nixpkgs {inherit system;}))));
   in {
     packages = pkgsFor (pkgs: {
       default = pkgs.callPackage ./what-is-my-ip.nix {};
+    devShells = pkgsFor (pkgs: {
+      default = pkgs.mkShell {
+        packages = [self.outputs.packages.${pkgs.system}.default];
+        shellHook = ''
+          echo "Hello, Nix!"
+        '';
+      };
+    });
$ nix develop -c $SHELL
Hello, Nix!

$ which what-is-my-ip

🕵️ Notice that the hash lr6wlz2652r35rwzc79samg77l6iqmii is exactly the same which we built earlier.


We can now do binary or source deployments 🚀🛠️📦 since we know the full dependency closure of our tool. We simply copy the necessary /nix/store paths to another machine with Nix installed.

$ nix copy --to ssh://beefcake $(nix build --print-out-paths)
$ ssh beefcake /nix/store/lr6wlz2652r35rwzc79samg77l6iqmii-what-is-my-ip/bin/what-is-my-ip

With Containers

Maybe though you are stuck with Kubernetes or Docker. Let’s use Nix to create an OCI-compatible image with our tool:

diff --git a/flake.nix b/flake.nix
index 99d6d52..81e98c9 100644
--- a/flake.nix
+++ b/flake.nix
@@ -10,6 +10,12 @@
   in {
     packages = pkgsFor (pkgs: {
       default = pkgs.callPackage ./what-is-my-ip.nix {};
+      container = pkgs.dockerTools.buildImage {
+        name = "what-is-my-ip-container";
+        config = {
+          Cmd = ["${self.outputs.packages.${pkgs.system}.default}/bin/what-is-my-ip"];
+        };
+      };
     devShells = pkgsFor (pkgs: {
$ docker load < $(nix build .#container --print-out-paths)
Loaded image: what-is-my-ip-docker:c9g6x30invdq1bjfah3w1aw5w52vkdfn
$ docker run -it what-is-my-ip-container:c9g6x30invdq1bjfah3w1aw5w52vkdfn

Cool! Nix + Docker integration perfectly. The image produced has only the files exactly necessary to run the tool provided, effectively distroless. You may also note that if you are following along, your image digest is exactly the same. Reproducibility!

In Virtual Machines

Finally, let’s take the last step and create a reproducible operating system using NixOS to contain only the programs we want:

diff --git a/flake.nix b/flake.nix
index 99d6d52..81e98c9 100644
--- a/flake.nix
+++ b/flake.nix
@@ -20,5 +26,27 @@
+    nixosConfigurations = let
+      system = "x86_64-linux";
+    in {
+      default = nixpkgs.lib.nixosSystem {
+        inherit system;
+        modules = [
+          {
+            users.users.alice = {
+              isNormalUser = true;
+              # enable sudo
+              extraGroups = ["wheel"];
+              packages = [
+                self.outputs.packages.${system}.default
+              ];
+              initialPassword = "swordfish";
+            };
+            system.stateVersion = "24.05";
+          }
+        ];
+      };
+    };

Now we can run this NixOS configuration as a reproducible virtual machine:

$ nixos-rebuild build-vm --flake .#default
$ ./result/bin/run-nixos-vm

# I/O snippet from QEMU
nixos login: alice

$ readlink $(which what-is-my-ip)

$ what-is-my-ip

💥 Hash lr6wlz2652r35rwzc79samg77l6iqmii present again!

We took a relatively simple script through a variety of applications in the Nix ecosystem: build recipe, shell, docker image, and finally NixOS VM.

Flakes as All Kinds of Package Management

One of the super neat part about Flakes is that anywhere you find a Flake, you can make use of it. Try it out now!

NOTE: The following obviously runs code from the internet. Be wary of doing this in general.

Run a Flake’s package:

$ nix run git+

Enter a Flake’s development environment:

$ nix develop git+ -c $SHELL
Hello, Nix!
$ what-is-my-ip

# want to hack on the Helix text editor?
$ nix develop github:helix-editor/helix
# yeah, seriously! that's it!

Load a Flake’s docker image and run it:

$ docker load < $(nix build git+ --print-out-paths)
Loaded image: what-is-my-ip-container:wg0z43v4sc1qhq7rsqg02w80vsfk9dl0
$ docker run -it what-is-my-ip-container:c9g6x30invdq1bjfah3w1aw5w52vkdfn

Run a Flake’s NixOS configuration as a virtual machine:

$ nixos-rebuild build-vm --flake git+
Done.  The virtual machine can be started by running /nix/store/xswwdly9m5bwhcz9ajd6km5hx9vdmfzw-nixos-vm/bin/run-nixos-vm
$ /nix/store/xswwdly9m5bwhcz9ajd6km5hx9vdmfzw-nixos-vm/bin/run-nixos-vm

Run an exact replica my workstation in a virtual machine (as configured via Nix – meaning it will not have my passwords on it 😉):

# NOTE: this will probably take a good, long time to build and lots of bandwidth
# NOTE: you probably won't even be able to login
$ nixos-rebuild build-vm --flake git+
Done.  The virtual machine can be started by running /nix/store/abc-nixos-vm/bin/run-dragon-vm
$ /nix/store/abc-nixos-vm/bin/run-dragon-vm

Flakes as Inputs

Finally, Flakes can take any other Flake as input and use them for their own outputs. This means that they compose wonderfully. Any Flake can use anything from any other Flake. They’re basically big ol’ distributed functions (if you squint just right). nixpkgs itself is “simply” a huge Flake.

You can generate your own Flake however you like; Flakes provide templating facilities. Here, you can use my very own template:

$ mkdir -p ~/my-fun-flake
$ cd ~/my-fun-flake
$ nix flake init --template git+
$ git init
$ git add -A
$ git commit -am 'initial commit'
$ nix develop -c $SHELL

And now you have everything you need to work on a Nix flake as if you were me!

To add the package from this tutorial, we can simply do this:

diff --git a/flake.nix b/flake.nix
index 408d4f2..c5d6883 100644
--- a/flake.nix
+++ b/flake.nix
@@ -2,16 +2,20 @@
   inputs.pre-commit-hooks.url = "github:cachix/pre-commit-hooks.nix";
   inputs.pre-commit-hooks.inputs.nixpkgs.follows = "nixpkgs";
+  inputs.learn-flakes-the-fun-way.url = "git+";
+  inputs.learn-flakes-the-fun-way.inputs.nixpkgs.follows = "nixpkgs";
   outputs = {
+    learn-flakes-the-fun-way,
   }: let
     systems = ["aarch64-linux" "aarch64-darwin" "x86_64-darwin" "x86_64-linux"];
     forSystems = nixpkgs.lib.genAttrs systems;
   in {
     formatter = genPkgs (pkgs: pkgs.alejandra);
@@ -26,7 +30,7 @@
     devShells = genPkgs (pkgs: {
       nix = pkgs.mkShell {
-        packages = with pkgs; [nil alejandra];
+        packages = with pkgs; [nil alejandra learn-flakes-the-fun-way.packages.${pkgs.system}.default];
         inherit (self.outputs.checks.${pkgs.system}.pre-commit-check) shellHook;

Afterwards you can re-enter your development shell and run our script:

$ nix develop -c $SHELL
$ what-is-my-ip

Other Flakey (but in a good way) Tools

On top of all this, many very awesome tools can exist. You can leverage a direnv-style setup such that whenever you enter a directory you enter its development shell and are completely ready to work within that project with all the necessary tools with all the correct versions reproducibly.

Flakes have checks which are run when you run nix flake check. You can use this for CI to ensure that your Flake is always able to build whatever packages it needs or even check that code is formatted correctly.

All of this in one place makes for a pretty incredible developer experience.


Hopefully, seeing the fun things you can do with Nix might inspire you to push through the hard parts.

There is a golden pot 💰 at the end of this rainbow 🌈 awaiting you.

Learn Flakes the fun way.