nix-direnv

A faster, persistent implementation of direnv's use_nix, to replace the built-in one.

Prominent features:

• significantly faster after the first run by caching the nix-shell environment
• prevents garbage collection of build dependencies by symlinking the resulting shell derivation in the user's gcroots (Life is too short to lose your project's build cache if you are on a flight with no internet connection)

Why not use lorri instead?

Compared to lorri, nix-direnv is simpler (and requires no external daemon) and supports flakes. Additionally, lorri can sometimes re-evaluate the entirety of nixpkgs on every change (leading to perpetual high CPU load).

Installation

Heads up: nix-direnv requires a modern Bash and GNU Grep. MacOS ships with outdated or non-GNU versions of these tools, As a work-around we suggest that macOS users install direnv/grep via Nix or Homebrew. Discussion of these problems can be found here.

There are different ways to install nix-direnv, pick your favourite:

{ pkgs, ... }:

{
  # ...other config, other config...

  programs.direnv.enable = true;
  programs.direnv.nix-direnv.enable = true;
  # optional for nix flakes support in home-manager 21.11, not required in home-manager unstable or 22.05
  programs.direnv.nix-direnv.enableFlakes = true;

  programs.bash.enable = true;
  # OR
  programs.zsh.enable = true;
  # Or any other shell you're using.
}

{ pkgs, ... }: {
  nix.extraOptions = ''
    keep-outputs = true
    keep-derivations = true
  '';
}

keep-derivations = true
keep-outputs = true

if ! has nix_direnv_version || ! nix_direnv_version 2.1.1; then
  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/2.1.1/direnvrc" "sha256-b6qJ4r34rbE23yWjMqbmu3ia2z4b2wIlZUksBke/ol0="
fi

{ pkgs, ... }: {
  environment.systemPackages = with pkgs; [ direnv nix-direnv ];
  # nix options for derivations to persist garbage collection
  nix.extraOptions = ''
    keep-outputs = true
    keep-derivations = true
  '';
  environment.pathsToLink = [
    "/share/nix-direnv"
  ];
  # if you also want support for flakes 
  nixpkgs.overlays = [
    (self: super: { nix-direnv = super.nix-direnv.override { enableFlakes = true; }; } )
  ];
}

# put this in ~/.direnvrc
source /run/current-system/sw/share/nix-direnv/direnvrc

nix-env -f '<nixpkgs>' -iA nix-direnv

source $HOME/.nix-profile/share/nix-direnv/direnvrc

# put this in ~/.direnvrc or ~/.config/direnv/direnvrc
source $HOME/nix-direnv/direnvrc

Usage example

Either add shell.nix or a default.nix to the same directory:

# save this as shell.nix
{ pkgs ? import <nixpkgs> {}}:

pkgs.mkShell {
  nativeBuildInputs = [ pkgs.hello ];
}

Then add the line use nix to your envrc:

$ echo "use nix" >> .envrc
$ direnv allow

If you haven't used direnv before, make sure to hook it into your shell first.

Using a non-standard file name

You may use a different file name than shell.nix or default.nix by passing the file name in .envrc, e.g.:

$ echo "use nix foo.nix" >> .envrc

Flakes support

nix-direnv also comes with an alternative use_flake implementation. The code is tested and does work but the upstream flake api is not finalized, so we we cannot guarantee stability after an nix upgrade.

Like use_nix, our use_flake will prevent garbage collection of downloaded packages, including flake inputs.

Creating a new flake-native project

This repository ships with a flake template. which provides a basic flake with devShell integration and a basic .envrc.

To make use of this template, you may issue the following command:

$ nix flake new -t github:nix-community/nix-direnv <desired output path>

Integrating with a existing flake

$ echo "use flake" >> .envrc && direnv allow

The use flake line also takes an additional arbitrary flake parameter, so you can point at external flakes as follows:

use flake ~/myflakes#project

Advanced usage

use flake

Under the covers, use_flake calls nix print-dev-env. The first argument to the use_flake function is the flake expression to use, and all other arguments are proxied along to the call to print-dev-env. You may make use of this fact for some more arcane invocations.

For instance, if you have a flake that needs to be called impurely under some conditions, you may wish to pass --impure to the print-dev-env invocation so that the environment of the calling shell is passed in.

You can do that as follows:

$ echo "use flake . --impure" > .envrc
$ direnv allow

use nix

Like use flake, use nix now uses nix print-dev-env. Due to historical reasons, the argument parsing emulates nix shell.

This leads to some limitations in what we can reasonably parse.

Currently, all single-word arguments and some well-known double arguments will be interpeted or passed along.

Known arguments

• -p: Starts a list of packages to install; consumes all remaining arguments
• --include / -I: Add the following path to the list of lookup locations for <...> file names
• --attr / -A: Specify the output attribute to utilize

--command, --run, --exclude, --pure, -i, and --keep are explicitly ignored.

All single word arguments (-j4, --impure etc) are passed to the underlying nix invocation.

Tracked files

nix-direnv makes a performance tradeoff only considers changes in a limited number of files when deciding to update its cache.

• for use nix this is ~/.direnvrc, ~/.config/direnv/direnvrc, .envrc, default.nix and shell.nix
• for use flake this is ~/.direnvrc, ~/.config/direnv/direnvrc, .envrc, flake.nix, flake.lock and devshell.toml

To add more files to be checked use nix_direnv_watch_file like this: nix_direnv_watch_file your-file.nix

General direnv tips

• Changing where direnv stores its cache
• Quickly setting up direnv in a new nix project

Other projects in the field

• lorri
• sorri
• nixify
• lorelei