Nix

NixOS is a Linux distribution centered around Nix, the purely-functional programming language.

Finding what you need

Nix's documentation is vast, but shallow. Finding what you need can be tough.

The Nix language

Path literals

In non-interactive use, path literals always resolve to a path copied in the Nix store. This may be unexpected at times, since they don't do that at the repl! 

  # CWD: /persist/dots
  ./file
  # At the REPL:
  #   => "/persist/dots/file"
  # Anywhere else:
  #   => "/nix/store/…-source/file"

No, this cannot be escaped by using builtins.toString.

This can be rather frustrating at times, as it often leaves you no choice but to hard-code you wish to use mutably.

Debugging

The REPL

  • Debugging Home-manager: browse nixosConfigurations.«host».config.home-manager.users.«user».

Derivations

Add pkgs.breakpointHook to your nativeBuildInputs. Upon build failure, a message like 

build for nbb-1.3.200 failed in buildPhase with exit code 1
To attach, run the following command:
    sudo /nix/store/m2mjhykymxr9d4caiwkwccp9332ksim7-attach/bin/attach 9501261

will be printed. Leave the build process running, and run the printed command to get a Bash shell in the build environment at the point of failure.Debug a failed derivation with breakpointHook and cntr

Cookbook

Run latest version of a package

$ nix run nixpkgs#«package» --override-flake nixpkgs 'github:NixOS/nixpkgs/nixpkgs-unstable'

Errors

"error: collision between X and Y"

This is most often seen as a harmless warning. As an error, it usually means e.g. a command name appears on $PATH twice.

Dev shells