Nix

• Building Nix packages nix-build involves two steps:
  • evaluation/instantiation (nix-instantiate): goes from nix expression (.nix) to a derivation (.drv)
  • realization (nix-store --realize): goes from a derivation to a store path
• build-loop is how Nix builds software

  flowchart LR
      A[Substitutors] --> B{remainingOutput?}
      B --No--> C[BuildInputs]
      C --> D[Build]
      D --> E[RegisterOutputPaths]
      B --Yes--> Stop

• Nix package is unique across: source, dependencies, build steps, architecture, platform
  • Nix can determine the derivation and the final output path ahead of time (needed for caching)
• derivation is platform-specific way to build a package
  • output of derivation depends only on the inputs and the build script, and each package has a unique hash
• NixOS boot process:
  1. Bootloader loads kernel and initrd
  2. initrd mounts partitions
  3. initrd runs /run/current-system/activate
  4. if first time, it populates POSIX directories (/bin, /etc, /run, /usr, /var)
• Nix binary cache
  • NAR file is a serialized file format for packages used by the Nix package manager (v/s tar files) because they don't contain any non-deterministic information
  • .narinfo files contain metadata info about each package, such as dependencies
• Use of mkIf within module system requires all options to be valid

Useful commands

• Find all GC roots

  sudo -i nix-store --gc --print-roots | egrep -v '^(/nix/var|/run/current-system|/run/booted-system|/proc|{memory|{censored)'
  

• Roots of a dependency

  nix-store --query --roots /nix/store/...
  

• Find package versions and their commits
• Find references, referrer, closures

  nix-store -q --references $(which hello)
  nix-store -q --referrers $(which hello)
  nix-store -qR $(which hello)
  nix-store -q --tree $(which hello)
  

Flakes

• Allow unfree

  let pkgs = import nixpkgs { system = "x86_64-linux"; config.allowUnfree = true; };
  

• Make nixpkgs refer to the same one in your flake enabled nixos

  nix.registry.nixpkgs.flake = inputs.nixpkgs;