{ pkgs, ... }: { ... }This is a function. It receives the package collection and returns your config.
Step 0
What even is an OS?
An operating system is the layer that starts your computer, runs programs, talks to hardware, and keeps everything organized. Windows and macOS are operating systems. Linux systems are operating systems too, and NixOS is one of them.
Step 1
What is NixOS?
The OS that never lies about its state.
Think of it this way: most operating systems are like a messy desk. You install things, delete things, update things — and over time the desk gets cluttered with stuff you don't remember putting there. NixOS is different. It's like having a desk that resets to exactly the state you described every single morning. You write down what you want — 'I want Firefox, git, and a running web server' — and NixOS makes exactly that happen. Nothing extra. Nothing hidden. And if you wrote it down wrong, you just go back to yesterday's note.
Step 2
nixpkgs is the source map
A GitHub repo with 80,000+ Nix expressions. Each expression says how software should be built.
The important part: nixpkgs is not a magic binary package server.
It is a git repo full of build recipes. Binary caches are an optimization layered on top.
firefox: 109.0 → 110.0
+ version = "110.0";
+ hash = "sha256-new";
+ patches = [ ./sandbox.patch ];
- version = "109.0";
openssl: patch CVE metadata
+ knownVulnerabilities = [];
+ meta.security = "patched";
- meta.security = "pending";
nginx: enable brotli module
+ modules = [ brotli ];
+ passthru.tests = nginxTests;
- modules = [];
python312Packages: refresh hashes
+ pytestCheckHook
+ sha256 = "sha256-refreshed";
- sha256 = "sha256-old";
Step 3
A derivation is a recipe
It describes inputs and build steps. It is not the package itself.
{
name = "hello-2.12.1";
src = fetchurl {
url = "mirror://gnu/hello/hello-2.12.1.tar.gz";
sha256 = "sha256-...";
};
buildInputs = [ gcc glibc ];
buildPhase = "make";
}name
A human-readable package identity. Still only part of the recipe.
Step 4
The Nix store makes state explicit
Every package lands in a unique immutable path. Dependencies are not hidden.
Step 5
configuration.nix describes truth
You declare the system you want. NixOS builds that state instead of guessing what changed.
Step 6
Generations & Rollback
Every time you change your config and rebuild, NixOS saves the old version. You can jump back to any point like a save file in a game.
Step 7
Flakes make inputs explicit
A flake adds locked inputs and predictable outputs. The lockfile pins the exact nixpkgs revision.
{
description = "demo system";
inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
outputs = { self, nixpkgs }: {
packages.x86_64-linux.default = ...;
devShells.x86_64-linux.default = ...;
};
}inputs.nixpkgs.url
Where nixpkgs comes from. No implicit channel.
outputs
A function from inputs to buildable results.
packages / devShells
Named things other machines can build the same way.
Step 8
Dev shells remove setup drift
nix develop drops you into a shell with every tool your project needs — exact versions, no conflicts, works on any machine.
Before (other OS)
click to run setup
With Nix
click to enter shell
Step 9
The Nix Language
Nix code looks strange first, but most common snippets are small ideas repeated.
with pkgs; [ git firefox ]with is like saying 'look inside pkgs for these names'. The list is your packages.
mkDerivation { ... }mkDerivation is the recipe builder. Give it a name, source, and build steps — it gives you a package.
Step 10
Why it matters
NixOS turns machine state into data you can inspect, rebuild, and roll back.
Reproducible Builds
The recipe lists exact inputs. If inputs match, the build path and result can match too.
Atomic Upgrades
A new system generation is built beside the old one. Switch succeeds as one step or the old system remains.
Instant Rollbacks
Old generations stay available. If an upgrade breaks, boot or switch back to known-good state.
Step 11
Where to go from here
Built as a school project by Made with vanilla HTML, CSS, and JS — no frameworks. Design inspired by Anthropic/Claude.ai. fan detected ✦