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NixOS #75: Nix Tips and Tricks

These are some tips 'n tricks related to Nix that I've learned over time.

Companion to video at ....

This text script available via link in the video description.

See the other videos in this series by visiting the playlist at https://www.youtube.com/playlist?list=PLa01scHy0YEmg8trm421aYq4OtPD8u1SN

See a Summary of Changes After nixos-rebuild

Stolen from https://chattingdarkly.org/@lhf@fosstodon.org/110661879831891580

system.activationScripts.diff = {
  supportsDryActivation = true;
  text = ''
    ${pkgs.nvd}/bin/nvd --nix-bin-dir=${pkgs.nix}/bin diff \
         /run/current-system "$systemConfig"
  '';
};

Then:

nixos-rebuild switch

Will show (e.g.):

[R-]  #1  sops  3.8.1
Closure size: 4195 -> 4194 (12 paths added, 13 paths removed, delta -1, disk usage -21.4MiB).

Use nix-tree to See What Depends on What

nix-tree (https://github.com/utdemir/nix-tree) is a TUI tool that allows you to see the dependency graph of any derivation :

nix-tree /nix/store/yb5k3n56ywpng5d41bd02yfwwppbdyjw-python3-3.11.7

Type "/" to search.

Use nix-du to See What's Consuming Disk Space

nix-du is a tool that produces a Graphviz file showing disk space consumption of derivation realizations (https://github.com/symphorien/nix-du).

Ensure you have graphviz and nix-du in your environment.systemPackages, then:

nix-du -s=500MB | dot -Tpng > store.png

See Which Derivation Supplies a File

nix-index is a tool that indexes your system and tells you which derivations provide a particular file or pattern of files. You run nix-index and then a subsequent run of nix-locate finds the derivation:

$ nix-index
$ nix-locate libc.so.6

Requires nix-index in environment.systemPackages.

Pin nixpkgs in Flakes Registry to the Lockfile Version

Add an item for nixpkgs to the flakes registry that matches the one in your NixOS configuration (via https://dataswamp.org/~solene/2022-07-20-nixos-flakes-command-sync-with-system.html).

Without:

$ nix registry list|grep nixpkgs
global flake:nixpkgs github:NixOS/nixpkgs/nixpkgs-unstable

After adding this bit to your NixOS configuration ("inputs" should come from your flake.nix in specialArgs):

{ inputs, ...}:
{
nix.registry = {
  nixpkgs.flake = inputs.nixpkgs;
};
}

With:

$ nix registry list|grep nixpkgs
global flake:nixpkgs github:NixOS/nixpkgs/nixpkgs-unstable
system flake:nixpkgs path:/nix/store/52yawfmb2rz0sm07px5zcrgv3y78v27v-source?lastModified=1708831307&narHash=sha256-0iL/DuGjiUeck1zEaL%2BaIe2WvA3/cVhp/SlmTcOZXH4%3D&rev=5bf1cadb72ab4e77cb0b700dab76bcdaf88f706b

Adds the system flake:nixpkgs What does system mean? From nix registry --help:

There are multiple registries. These are, in order from lowest to highest
precedence:

· The global registry, which is a file downloaded from the URL specified by
  the setting flake-registry. It is cached locally and updated
  automatically when it's older than tarball-ttl seconds. The default
  global registry is kept in a GitHub repository.

· The system registry, which is shared by all users. The default location
  is /etc/nix/registry.json. On NixOS, the system registry can be
  specified using the NixOS option nix.registry.

Also: won't fetch nixpkgs-unstable for every nix shell / nix run, it'll just use the version of nixpkgs you've already downloaded.

This will be the default soon in Nix. See https://chattingdarkly.org/@picnoir@social.alternativebit.fr/112002571368237940 and NixOS/nixpkgs#254405.

Sync the nixpkgs Input Between nix-build / nix build and nix-shell/ nix shell

After adding this bit to your NixOS configuration ("inputs" should come from your flake.nix in specialArgs):

{inputs, ...}:
{
nix.nixPath = [ "nixpkgs=${inputs.nixpkgs}" ];
}

nix-shell will use the same nixpkgs version as nix shell and nix-build will use the same nixpkgs version as nix build.

The PR mentioned in the last section also handles this; it will be the default soon enough.

Use Flakes in the Nix REPL

Use the repl-flake experimental feature

{
nix.settings.experimental-features = "nix-command flakes repl-flake";
}

Now you can consult a flake when starting nix-repl.:

$ cd /etc/nixos
$ nix repl ".#"
Welcome to Nix 2.18.1. Type :? for help.

warning: Git tree '/etc/nixos' is dirty
Loading installable 'git+file:///etc/nixos#'...
Added 1 variables.
nix-repl> :lf .
warning: Git tree '/etc/nixos' is dirty
Added 12 variables.

E.g. nixosConfigurations.optinix.config.hardware.cpu.intel.updateMicrocode.

Before this, the way I loaded a flake was:

f = builtins.getFlake "git+file://${builtins.toString ./.}"

This will be the default soon enough: NixOS/nix#10103

nixos-repl

I usually mostly want to use the REPL to inspect the pkgs namespace. It's convenient to just have that loaded right off the rip:

nixos-repl = pkgs.writeScriptBin "nixos-repl" ''
  #!/usr/bin/env ${pkgs.expect}/bin/expect
  set timeout 120
  spawn -noecho nix --extra-experimental-features repl-flake repl nixpkgs
  expect "nix-repl> " {
    send ":a builtins\n"
    send "pkgs = legacyPackages.${system}\n"
    interact
  }
'';

Now:

$ nixos-repl
Welcome to Nix 2.18.1. Type :? for help.

Loading installable 'flake:nixpkgs#'...
Added 5 variables.
nix-repl> :a builtins
Added 115 variables.

nix-repl> pkgs = legacyPackages.x86_64-linux

nix-repl> pkgs.linux<TAB>