This post is a Flake-based rewrite of [Learn Nix the Fun Way on fzakaria.com][0].
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.

<!--more-->

## what-is-my-ip

Let's walk through a single example of a shell script one may write: _what-is-my-ip_

```bash
#!/usr/bin/env bash
curl -s http://httpbin.org/get | \
  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](https://ryantm.github.io/nixpkgs/builders/trivial-builders/)_ (specifically, `writeShellScriptBin`) in a basic Nix flake to turn this into a Nix derivation (i.e. build recipe):

```nix
# 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 {};
    });
  };
}
```

```nix
# what-is-my-ip.nix
{pkgs}:
pkgs.writeShellScriptBin "what-is-my-ip" ''
  ${pkgs.curl}/bin/curl -s http://httpbin.org/get | \
    ${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:

```console
$ nix build --print-out-paths
/nix/store/lr6wlz2652r35rwzc79samg77l6iqmii-what-is-my-ip
```

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

```console
$ ./result/bin/what-is-my-ip 
24.5.113.148
```

Or run it from the Flake directly:

```console
$ nix run
24.5.113.148
```

Now that this is in our Nix store, we've naturally modeled our dependencies
and can do _fun_ things like generate graph diagrams (click the image to view
larger):

```console
$ 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](./what-is-my-ip-deps.png)](./what-is-my-ip-deps.png)

Let's add a _developer environment_ which contains our new tool.
This is a great way to create developer environments with reproducible tools.

```diff
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!"
+        '';
+      };
+    });
   };
 }
```

```console
$ nix develop -c $SHELL
Hello, Nix!

$ which what-is-my-ip
/nix/store/lr6wlz2652r35rwzc79samg77l6iqmii-what-is-my-ip/bin/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.

```console
$ nix copy --to ssh://beefcake $(nix build --print-out-paths)
$ ssh beefcake /nix/store/lr6wlz2652r35rwzc79samg77l6iqmii-what-is-my-ip/bin/what-is-my-ip
98.147.178.19
```

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

```diff
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: {
```

```console
$ docker load < $(nix build .#docker-image --print-out-paths)
Loaded image: what-is-my-ip-docker:c9g6x30invdq1bjfah3w1aw5w52vkdfn
$ docker run -it what-is-my-ip-container:c9g6x30invdq1bjfah3w1aw5w52vkdfn
24.5.113.148
```

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!**

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

```diff
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:

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

# I/O snippet from QEMU
nixos login: alice
Password: 

$ readlink $(which what-is-my-ip)
/nix/store/lr6wlz2652r35rwzc79samg77l6iqmii-what-is-my-ip/bin/what-is-my-ip

$ what-is-my-ip
24.5.113.148
```

💥 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.

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.

```console
# run a flake's package
$ nix run git+https://git.lyte.dev/lytedev/learn-flakes-the-fun-way
24.5.113.148

# enter a flake's development environment
$ nix develop git+https://git.lyte.dev/lytedev/learn-flakes-the-fun-way -c $SHELL
Hello, Nix!
$ what-is-my-ip
24.5.113.148

# load a flake's docker image and run it
$ docker load < $(nix build git+https://git.lyte.dev/lytedev/learn-flakes-the-fun-way#container --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+https://git.lyte.dev/lytedev/learn-flakes-the-fun-way#default
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

# like running a pre-configured version of my workstation's NixOS configuration
# NOTE: this will probably take a good, long time to build and lots of bandwidth
$ nixos-rebuild build-vm --flake git+https://git.lyte.dev/lytedev/nix#dragon
Done.  The virtual machine can be started by running /nix/store/abc-nixos-vm/bin/run-nixos-vm
$ /nix/store/abc-nixos-vm/bin/run-nixos-vm

```

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 Nix the fun way.**

[0]: https://fzakaria.com/2024/07/05/learn-nix-the-fun-way.html