What is nix?

A Toolchain for Reproducible Systems

a package manager
a build system
a configuration language

The Problem Nix Focuses On

Given the same inputs, we want the same result — every time.

Inputs

source code
dependencies
compilers
build tools
system libraries

Most tools only control some of these. Nix tries to control all of them.

Core Idea

Instead of mutating a system step by step, Nix describes what the system should be, and builds it from explicit inputs.

This is what enables reproducibility.

How Nix Works: Store + Derivations

flowchart TD
  A["Nix expression flakes nix files"]
  B["Derivation build recipe"]
  C["Realization build in sandbox"]
  D["/nix/store/&lt;hash&gt;-name immutable output"]
  E["Profile symlink current environment"]

    A --> B --> C --> D --> E

The Nix Store

All build outputs live in a single place: /nix/store

Each output path includes a hash

The hash is derived from:

inputs
dependencies
build instructions

Example shape:

/nix/store/<hash>-openssl-3.0.12

Why This Matters

Multiple versions can exist at the same time
Nothing is overwritten
No accidental upgrades
Rollbacks are trivial

If the inputs change, the path changes. If the path didn’t change, the inputs didn’t either.

Key Property: The filesystem path is a proof of how something was built.