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 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.

If we `nix build` and `readlink result` we get:

  **/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 Nix and we've modeled our dependencies, we can do _fun_ things like generate graph diagrams to view them (click the image to view larger).

```bash
$ 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](/assets/images/what-is-my-ip-deps.png)](/assets/images/what-is-my-ip-deps.png)

Let's create a _developer environment_ and bring in 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,
+    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: {
       what-is-my-ip = pkgs.callPackage ./what-is-my-ip.nix {};
     });
+
+    devShells = pkgsFor (pkgs: {
+      default = pkgs.mkShell {
+        packages = [self.outputs.packages.${pkgs.system}.what-is-my-ip];
+        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

[fmzakari@nixie:~]$ /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.

```nix
let
  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/nixos-24.05.tar.gz") {};
  what-is-my-ip = import ./what-is-my-ip.nix {inherit pkgs;};
in
  pkgs.dockerTools.buildImage {
    name = "what-is-my-ip-docker";
    config = {
      Cmd = ["${what-is-my-ip}/bin/what-is-my-ip"];
    };
  }
```

```bash
❯ docker load < $(nix-build what-is-my-ip-docker.nix)
Loaded image: what-is-my-ip-docker:c9g6x30invdq1bjfah3w1aw5w52vkdfn

❯ docker run -it what-is-my-ip-docker: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**.

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

```nix
let
  nixpkgs = fetchTarball "https://github.com/NixOS/nixpkgs/archive/nixos-24.05.tar.gz";
  pkgs = import nixpkgs {};
  what-is-my-ip = import ./what-is-my-ip.nix {inherit pkgs;};
  nixos = import "${nixpkgs}/nixos" {
    configuration = {
      users.users.alice = {
        isNormalUser = true;
        # enable sudo
        extraGroups = ["wheel"];
        packages = [
          what-is-my-ip
        ];
        initialPassword = "swordfish";
      };

      system.stateVersion = "24.05";
    };
  };
in
  nixos.vm
```

```console
❯ nix-build what-is-my-ip-vm.nix

❯ QEMU_KERNEL_PARAMS=console=ttyS0 ./result/bin/run-nixos-vm -nographic; reset

<<< Welcome to NixOS 24.05pre-git (x86_64) - ttyS0 >>>

Run 'nixos-help' for the NixOS manual.

nixos login: alice
Password: 

[alice@nixos:~]$ which what-is-my-ip
/etc/profiles/per-user/alice/bin/what-is-my-ip

[alice@nixos:~]$ readlink $(which what-is-my-ip)
/nix/store/lr6wlz2652r35rwzc79samg77l6iqmii-what-is-my-ip/bin/what-is-my-ip

[alice@nixos:~]$ 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.

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