nix

name

nix

description

Use this skill for any Nix, NixOS, Flakes, Home Manager, or nix-darwin task. Also trigger when the user mentions flake.nix, devShells, mkShell, nixos-rebuild, darwin-rebuild, home-manager switch, nix develop, overlays, derivations, or anything involving declarative system/package configuration on Linux or macOS — even if they don't explicitly say 'Nix'. NOT for writing packages from scratch with stdenv.mkDerivation or language-specific builders (see nix-packaging). NOT for the dendritic flake-parts pattern — if the query mentions import-tree, `flake.modules.<class>.<aspect>`, aspect-oriented config, generic module class, or feature-based module sharing across NixOS/darwin, see nix-dendritic instead.

Nix Ecosystem Guide

Core Philosophy

1. Declarative over Imperative - Describe desired state, not steps to reach it
2. Reproducibility - Lock files (flake.lock) pin exact versions
3. Immutability - Nix Store is read-only; same inputs = same outputs
4. Rollback - Every generation preserved; instant recovery via boot menu (NixOS) or --rollback (all platforms)

Flake Structure

{
  description = "My Nix configuration";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
    home-manager = {
      url = "github:nix-community/home-manager/release-25.11";
      inputs.nixpkgs.follows = "nixpkgs";  # Avoid duplicate nixpkgs evaluations
    };
    # macOS support
    nix-darwin = {
      url = "github:nix-darwin/nix-darwin/nix-darwin-25.11";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs = { self, nixpkgs, home-manager, nix-darwin, ... }@inputs: {
    # NixOS configurations
    nixosConfigurations.hostname = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [ ./configuration.nix ];
    };

    # macOS configurations
    darwinConfigurations.hostname = nix-darwin.lib.darwinSystem {
      modules = [
        ./darwin.nix
        { nixpkgs.hostPlatform = "aarch64-darwin"; }  # or x86_64-darwin for Intel
      ];
    };

    # Development shells
    devShells.x86_64-linux.default = nixpkgs.legacyPackages.x86_64-linux.mkShell {
      packages = [ /* ... */ ];
    };
  };
}

Essential Patterns

Input Management

inputs = {
  # Add an unstable channel alongside stable (shown in Flake Structure above)
  unstable.url = "github:NixOS/nixpkgs/nixos-unstable";

  # Non-flake input (config files, plain repos, etc.)
  private-config = {
    url = "git+ssh://[e-mail]/user/config.git";
    flake = false;
  };
};

Module System

# Every module has three parts: imports, options, config
{ config, pkgs, lib, ... }: {
  imports = [ ./hardware.nix ./services.nix ];

  options.myOption = lib.mkOption {
    type = lib.types.bool;
    default = false;
  };

  config = lib.mkIf config.myOption {
    # Only applied when myOption is true
  };
}

Priority Control

The module system merges options from multiple modules. When two modules set the same option, the lower priority number wins (overrides the others).

{
  # lib.mkDefault (priority 1000) - loses to almost everything
  # Use in shared/common modules so host-specific configs can override
  services.nginx.enable = lib.mkDefault true;

  # Direct assignment (priority 100) - normal, overrides mkDefault
  services.nginx.enable = true;

  # lib.mkForce (priority 50) - wins over everything
  # Use when another module sets a value you can't change any other way
  services.nginx.enable = lib.mkForce false;
}

Package Customization

Three levels of customization, from least to most invasive:

{
  # override: change function arguments (what goes INTO a package build)
  # Use when a package accepts configuration parameters (e.g., enabling optional features)
  pkgs.fcitx5-rime.override { rimeDataPkgs = [ ./custom-rime ]; }

  # overrideAttrs: change derivation attributes (the build itself)
  # Use when you need to patch source, change build flags, or pin a version
  pkgs.hello.overrideAttrs (old: { doCheck = false; })

  # Overlays: modify packages globally across your entire config
  # Use when you want the change to apply everywhere (all dependents use the modified version)
  # Warning: overlays invalidate binary cache for affected packages — they'll build from source
  nixpkgs.overlays = [
    (final: prev: {
      myPackage = prev.myPackage.override { /* ... */ };
    })
  ];
}

Rule of thumb: override for feature flags → overrideAttrs for build changes → overlays for global changes. See references/nixpkgs-advanced.md for full details including callPackage and trivial builders.

Platform-Specific

NixOS

sudo nixos-rebuild switch --flake .#hostname
sudo nixos-rebuild boot --flake .#hostname    # apply on next boot only
sudo nixos-rebuild test --flake .#hostname    # test without creating boot entry

nix-darwin (macOS)

nix-darwin now requires sudo and system.primaryUser in your config:

# In darwin.nix — this is required:
system.primaryUser = "username";

sudo darwin-rebuild switch --flake .#hostname

Home Manager

# As NixOS/Darwin module (recommended):
home-manager.useGlobalPkgs = true;
home-manager.useUserPackages = true;
home-manager.users.username = import ./home.nix;

# Standalone:
home-manager switch --flake .#username@hostname

Commands Reference

Task	Command
Rebuild NixOS	sudo nixos-rebuild switch --flake .#hostname
Rebuild Darwin	sudo darwin-rebuild switch --flake .#hostname
Dev shell	nix develop
Temp package	nix shell nixpkgs#package
Run without install	nix run nixpkgs#package
Update all inputs	nix flake update
Update single input	nix flake update nixpkgs
GC old generations	sudo nix-collect-garbage -d
List generations	nix profile history --profile /nix/var/nix/profiles/system
Debug build	sudo nixos-rebuild switch --show-trace -L -v
REPL	nix repl then :lf . to load flake
Search packages	nix search nixpkgs#<term>
Explore flake outputs	nix flake show or nix flake show github:owner/repo

Common Gotchas

1. Untracked files invisible to flakes - Run git add before any flake command. Nix only copies git-tracked files into the store for evaluation (staged but uncommitted is fine).
2. allowUnfree breaks in devShells - nixpkgs.config.allowUnfree doesn't flow into standalone nix develop because devShells bypass the module system. Use nixpkgs-unfree (recommended), --impure, or ~/.config/nixpkgs/config.nix. See references/nixpkgs-advanced.md for details.
3. Duplicate nixpkgs downloads - Use inputs.nixpkgs.follows = "nixpkgs" on all inputs that depend on nixpkgs, or each input downloads its own copy.
4. Python pip install fails - Nix's sandbox blocks network and filesystem writes. Use venv inside mkShell, poetry2nix, or containers. See references/dev-environments.md.
5. Downloaded binaries crash - Pre-built binaries expect FHS paths (/lib, /usr) that don't exist on NixOS. Use pkgs.buildFHSEnv or nix-ld. See references/dev-environments.md.
6. String interpolation in multi-line strings - Use ''$ to escape ${ inside ''...'' strings. See references/nix-language.md for the full escaping table.
7. Build from source unexpectedly - Overlays change derivation hashes, invalidating binary cache. Consider a separate nixpkgs instance for overlayed packages. See references/nixpkgs-advanced.md.
8. legacyPackages is not legacy - When using nix search or consuming Nixpkgs outputs directly, packages appear under legacyPackages not packages. The name is misleading — it's how Nixpkgs exposes its deep nested package set through the flake interface. Use nixpkgs.legacyPackages.${system} to access them programmatically.

References

Consult these based on what you're working on:

Reference	When to read it
references/nix-language.md	Writing or debugging Nix expressions, syntax, builtins, lib functions, string escaping
references/flakes.md	Flake input types, outputs schema, lock files, flake-parts, flake-compat
references/home-manager.md	User dotfiles, program modules, file management, mkOutOfStoreSymlink, activation scripts
references/nix-darwin.md	macOS system config, Homebrew integration, system defaults (Dock/Finder/Keyboard), launchd, TouchID
references/nixpkgs-advanced.md	Custom packages, callPackage, overlays, overrides, unfree packages, trivial builders, fetchers
references/dev-environments.md	Dev shells, mkShell, direnv, devenv, FHS compat, nix-ld, language-specific setups (Python, Rust, Node, etc.)
references/best-practices.md	Project structure, debugging (nix repl, --show-trace), deployment (Colmena, deploy-rs), secrets, CI/CD
references/templates.md	Copy-paste flake.nix starting points for NixOS, Darwin, Home Manager, dev shells, Docker images