Over my past blog posts, I've made it clear that I enjoy how much I like setting up all my projects in the declarative way of Nix. In this article, I will go more in detail on how exactly I have direnv set up to automatically load my tools when I hop into a project.

In my last article I talked about how I don't have most of my programs installed globally, but instead, I have it set up so that whenever I jump into one of my project directories it automatically loads the correct packages and tools into my terminal environment. Since I discovered direnv 4 months ago I've been using it for all my projects.

The Idea

The basic idea of how I have everything set up is that globally I barely have anything installed. Only tools I use a lot or don't want to install anew every time I wanna use them. This includes applications like Firefox, Neovim, Kitty, etc. Everything else related to programming or tools I only need for a one-time use I install differently.

Tools that I only need for one-time use, say I want to quickly run dig to figure out what IP a domain points to or I quickly try something out in GIMP, I open up my terminal and run nix-shell -p dig (or gimp). Then I run the use tool and when I'm done I'll simply close the terminal and let the Nix garbage collector clean up anything I installed. The garbage collector runs automatically on a daily basis.

The Execution

To get everything to work I use lorri. It extends direnv to add some features specific to Nix.

direnv

direnv is a tool that allows you to automatically load and unload environment variables depending on the directory you're in. Simply add direnv to your configuration.nix, home manager or nix-env packages:

On NixOS:

environment.systemPackages = [
    pkgs.direnv
];

On non-NixOS:

nix-env -iA nixpkgs.direnv

Now enter a directory, create a file called .envrc and add some export.

# .envrc
export HELLO=123

direnv initially blocks a .envrc file upon its creation or upon being changed. You first need to allow it:

direnv allow

direnv now automatically loads the export (which can be confirmed with echo $HELLO). How cool is that? When you exit the directory the export will automatically be unloaded again.

shell.nix

It can quickly become tedious to always install tools using the above nix shell if need to install a lot of things. To make it easier you can create a shell.nix file and specify what packages should be installed and what commands should be executed in the shell.

A barebones shell.nix is as follows:

{ pkgs ? import <nixpkgs> {} }:
with pkgs;
mkShell {
  buildInputs = [
    python311
    poetry
    postgresql_15
  ];
}

To get into this shell simply execute nix-shell. If the file is in another directory you can also specify the file with the -f flag. This will install all the listed programs and set you into a new environment.

But we want this automated; by adding use nix to the .envrc file direnv will now automatically load this shell.nix file whenever you enter the directory, automatically giving you access to Python 3.11, Poetry and Postgresql. When you exit the directory they will all be unloaded from your shell environment.

lorri

Now you might notice that jumping into the directory can start to get really slow once your shell.nix file grows to a lot of packages. Especially after you run the Nix garbage collector, because then everything has to be downloaded anew.

That is what lorri solves. Lorri adds two game-changing changes:

1. shell.nix environments are cached. That makes loading into big projects very quick. You don't even notice that something is happening anymore (ignoring the big message appearing in the terminal).
2. All installed packages are marked to not be garbage collected. Now you can garbage collect without having to worry about deleting all the packages you need for a project.

This is all handled completely automatically. All you need to do is initialize your .envrc file by running lorri init, so that lorri is correctly ran upon entering the directory.

Additionally, when you delete a project or shell.nix, remember to also run lorri gc to remove all the links so that the Nix garbage collector will delete the unused packages.

The Truth

What motivated me to write this blog post was that I actually started to finally get into what Nix flakes are and how they work. Explained very roughly, they are a new thing from Nix that upgrades the reproducibility for everything. It takes inspiration from other project-based package managers like npm or Poetry which generate a lock file to make everything you do extremely reproducible. Have a look at this introduction to flakes blog post or a nice article that talks about some of the new commands.

As of now I'm "flaking" my projects and moving them from shell.nix projects to proper flake projects. After 4 months of using lorri I've now moved on to another tool because lorri doesn't support flakes. The exact benefits, why I moved over and how you can set it up will be a future blog post once I got some more experience with flakes.

A small teaser; flakes allow you to simply use nix develop github:markbeep/markbeep.github.io to get all the tools I use for working on this blog (basically just hugo) without even having to clone the project.