Into the Nix

In this post I want to show a very brief introduction to Nix, the language and package manager, and how it can help you to achieve a fully reproducible development environment.

What can Nix do for me? #

One of the features of Nix is that it is designed to allow for fully reproducible software development. Let’s say you have a project, for which you want your coworkers, for yourself in the future or anybody else to be able to get the same results as you. There will be many variables that will be taken into account, such as the programs available in the host, the versions, configurations, etc; that will affect your results (for example, a built binary or a developing environment). Nix allows you not only to specify which package versions your project depends, but also the specific build which will be used. So, given the exact same inputs, Nix will produce identical results, be it in your machine, your coworkers or in a remote server.

So what is Nix exactly? #

Nix is a purely functional programming language, dynamically typed, with a syntax very similar to JSON. Currently, the main purpose of this language is not being general purpose, but rather being the main tool of the Nix package manager.

with { a = 1; b = 2; };
{
  x = a + b
};
# { x = 3; }

Nix is in a strange situation, where many tutorials will try to teach you the basics semantics of the language, using arithmetics operations, handling strings, etc; but the real usage of the language is to build packages, in which these snippets might not be very useful.

pkgs.stdenv.mkDerivation {
  name = "viperML-home";

  src = ./.;

  buildPhase = ''
    mkdir -p themes
    ln -s ${inputs.bookworm} themes/bookworm
    ${pkgs.hugo}/bin/hugo --minify
  '';

  installPhase = ''
    cp -r public $out
  '';

  meta = with pkgs.lib; {
    description = "My awesome webpage";
    license = licenses.cc-by-nc-sa-40;
    platforms = platforms.all;
  };

This snippet of nix code may be more representative of how the Nix language can be used. It defines a derivation, which is how to build a package. While you could say that this package is called viperML-home (see name = ), the derivation name will be named differently if anything changes.

mkDerivation is a function that creates the derivation, and takes as an argument an set of attributes, such as { foo = "bar"; foo2 = [ 1 2 ] }.
name, meta define basic information about the package.
buildPhase, installPhase are bash snippets, and correspond to how Nix will build the derivation. The derivation will just be a directory in /nix, with path $out, so we can just point our installers to put the files in there, and our derivation will be built.

Now the nix cli utilty, will be used to call this function, and install the files into /nix/store/<hash>-<package>-<version>. Because two packages with the same version and name can have different build or installation methods, a hash is computed and put into the name. This way, any number of “editions” of this package can coexist in the same system. Using one or another will be just a task of using the correct path.

Now, every dependency of our package will be substituted by their exact derivation path. In the example snippet before, ${pkgs.hugo}/bin/hugo will be substituted by /nix/store/7jnn3g8871yqih4m61ggbjs1dif6hksa-hugo-0.91.2/bin/hugo and this will be the same across any system that calls the expression. If the derivation for hugo changes, their hashes will be different, so my viperML-home will be built different, and produce a different hash in the store. So, going backwards, if we use the same inputs for our build process, Nix will make sure to produce the same exact result; and if we know a derivation, we know how it was built exactly.

But how do we make sure that Nix gets always the same inputs to build the same results?

When calling that previous Nix expression, it was in a context with the pkgs variable defined. We can define this by referencing the commit of nixpkgs, the git repo from which we pull or packages. This can also be done with the upcoming feature, flakes, will will be discussed in a future post.

How do I start using Nix? #

Nix can be used in Linux, macOS or Windows via WSL2. Link to official documentation.

After installing (and rebooting) your computer, you can start using Nix with:

nix-shell --packages python3

This pull the python package, according to the nixpkgs revision configured in your channels, and put it into your path

python --version
# Python 3.9.9
which python
# /nix/store/rppr9s436950i1dlzknbmz40m2xqqnxc-python3-3.9.9/bin/python

What can Nix do for me? #

So what is Nix exactly? #

How do I start using Nix? #

Further reading #