My (expanding) list of usage notes for Nix, NixOS and NixOps

Manual page sources

When it comes to Nix, there are some different manuals which overlaps in terms of , however, they don’t contain the same amount of information. In order to find all information available for a certain topic you might look for, prepare to look through all of the following sources:

• https://nixos.org/nix/manual/
  • Language constructs
• https://nixos.org/nixos/manual/
• https://nixos.org/nixpkgs/manual/
  • Library functions

Package management

Search for package

$ nix search packagename

Use variable as map/object key

let
  customUser = "stigok";
in {
  users.extraUsers = {
    ${customUser} = {
      systemUser = true;
      description = "This is a user called ${customUser}";
    };
  };
}

Check a file for syntax errors

$ nix-instantiate --parse <file>
$ nix-instantiate --parse myfile.nix

Pass function arguments with nix-instantiate

$ nix-instantiate --arg config <val1> --arg nixpkgs <val2> myfile.nix
$ nix-instantiate --arg config '{}' --arg pkgs '<nixpkgs>' --eval postgresql.nix

Reference: slack1256 #nixos @ Libera.chat

Inject config and services variables to the nix repl

This has to be run on a machine running NixOS itself

$ nix repl '<nixpkgs/nixos>'
> config
> config.services

Reference: energizer bqv[m] clever #nixos @ Libera.chat

Install a package for current user

$ nix-env -iA nixos.thunderbird

This will make the package available for the current logged in user only, except when logged in as root, which will make it available for everyone.

Reference: https://nixos.org/nixos/manual/index.html#sec-ad-hoc-packages

Fetch git repository from private server over SSH

pkgs.fetchgit does not use the local ssh_config. Use builtins.fetchGit instead.

src = builtins.fetchGit {
  url = "ssh://git@git.stigok.com/stigok/utils.git";
  ref = "master";
  rev = "f8bdc053406ad28ef4b6cbb29e418ce69f31f05f";
};

Reference: https://nixos.org/nix/manual/#ssec-builtins

Change package source to subdirectory after fetchGit

I have a monorepo with multiple utility programs inside. I have to change root to a subdirectory, using sourceRoot, after I’ve downloaded its sources.

{ pkgs ? import <nixpkgs> {} }:
  let
    package = (import ./default.nix { inherit pkgs system; }).package;
    newSrc  = builtins.fetchGit {
      url = "ssh://git@git.stigok.com/stigok/utils.git";
      ref = "master";
      rev = "f8bdc053406ad28ef4b6cbb29e418ce69f31f05f";
    };
  in
    package.overrideAttrs (old: {
      src = newSrc;
      sourceRoot = "${newSrc.outPath}/{package.packageName}";
    })

The original package derivation resides in ./default.nix and the package tree that fetchGit downloads looks like this:

README.md
my-package-src-in-a-subdir/
my-package-src-in-a-subdir/package.json
my-package-src-in-a-subdir/package-lock.json
my-package-src-in-a-subdir/index.js

I can now build the overridden package using nix-build override.nix.

References:

• https://github.com/tfc/node2nix_bootstrap/blob/master/override.nix
• infinisil #nixos @ Libera.chat

Create a tar archive of a fetchGit

The title could have been “create a tar archive”, but using it straight after builtins.fetchGit is what I needed to know.

gitSrc = builtins.fetchGit {
  url = "ssh://git@git.stigok.com/stigok/utils.git";
  ref = "master";
  rev = "c3c4be782efd79518ddbac006a7fcb3532c35f7e";
};
tarSrc = runCommand "tar-src" {} "${gnutar}/bin/tar -cf $out ${gitSrc}";

tarSrc now contains a path to the tar file.

References:

• euank #nixos @ Libera.chat
• https://nixos.org/nixpkgs/manual/#chap-trivial-builders

Determine NixOS machine architecture

To figure out what architecture your box is running, you can run nix-info or for example nix-eval:

$ nix-info
system: "x86_64-linux", multi-user?: yes, version: nix-env (Nix) 2.3.3, channels(username): "", channels(root): "nixos-19.09.2213.71c6a1c4a83", nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

$ nix eval nixpkgs.system
"x86_64-linux"

NixOps: switch-to-configuration throws error deployment fails

[...]
trivial> /nix/var/nix/profiles/system/bin/switch-to-configuration: line 3: use: command not found
[...]

I was getting erros while attempting to deploy to a specific machine in my NixOps deployment. It’s a 32-bit Jetway box, and so it needs 32-bit packages. When deploying from a different architecture, like in my case, a 64-bit intel, nixops needs to know what system it is targetting explicitly.

{
  network.enableRollback = true;
  network.description = "private infra";
  nix =
    { resources, ... }:
    {
      imports = [
        ./servers/nix/configuration.nix
      ];
      deployment.targetHost = "192.168.0.2";
      nixpkgs.system = "i686-linux";
    };
}

Reference: https://github.com/NixOS/nixops/issues/864

Use a specific channel for nixos-rebuild

Add the desired channel/version

$ NIXOS_VERSION=21.11
$ nix-channel --add https://nixos.org/channels/nixos-${NIXOS_VERSION} nixos-${NIXOS_VERSION}
$ nix-channel --update

Then reference the channel by path in ~/.nix-defexpr/channels/<channel>/nixpkgs

$ nixos-rebuild build -I ~/.nix-defexpr/channels/nixos-${NIXOS_VERSION}/nixpkgs

Reference: clever #nixos @ Libera.chat

Use a different configuration.nix file for nixos-rebuild

$ nixos-rebuild -I nixos-config=./configuration-21.11.nix

References

• https://nixery.dev/nix-1p.html