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)

Installation via Nix

Since 20.03 you can install nix-direnv via nix:

NixOS

In /etc/nixos/configuration.nix:

{ pkgs, ... }: {
  environment.systemPackages = with pkgs; [ nix-direnv ];
  # nix options for derivations to persist garbage collection
  nix.extraOptions = ''
    keep-outputs = true
    keep-derivations = true
  '';
  environment.pathsToLink = [
    "/share/nix-direnv"
  ];
}

Then source the direnvrc from this repository in your own .direnvrc

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

if [ -f /run/current-system/sw/share/nix-direnv/direnvrc ]; then
  source /run/current-system/sw/share/nix-direnv/direnvrc
fi

Home-manager

In $HOME/.config/nixpkgs/home.nix add

{ pkgs, ... }:

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

  programs.direnv.enable = true;
  programs.direnv.enableNixDirenvIntegration = true;
}

Optional: To protect your nix-shell against garbage collection you also need to add these options to your Nix configuration.

If you are on NixOS also add the following lines to your /etc/nixos/configuration.nix:

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

On other systems with Nix add the following configuration to your /etc/nix/nix.conf:

keep-derivations = true
keep-outputs = true

Installation via nix-env

As non-root user do the following:

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

Than follow the home-manager installation except for the $HOME/.config/nixpkgs/home.nix changes.

Installation from source

Clone the repository to some directory

$ git clone https://github.com/nix-community/nix-direnv $HOME/nix-direnv

Then source the direnvrc from this repository in your own .direnvrc

# put this in ~/.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 ? with import <nixpkgs> {}}:

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

Then add the line use nix to your envrc:

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

Experimental flakes support

nix-direnv also comes with a flake alternative. The code is tested and works however since future nix versions might change their api regarding this feature we cannot guarantee stability after an nix upgrade.

Save this file as flake.nix:

{
  description = "A very basic flake";
  # Provides abstraction to boiler-code when specifying multi-platform outputs.
  inputs.flake-utils.url = "github:numtide/flake-utils";
  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system: let
      pkgs = nixpkgs.legacyPackages.${system};
    in {
      devShell = pkgs.mkShell {
        nativeBuildInputs = [ pkgs.hello ];
      };
    });
}

Then add use flake to your .envrc:

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

Storing .direnv outside the project directory

A .direnv directory will be created in each use_nix project, which might interact badly with backups (e.g. Dropbox) or IDEs.

Therefore it's possible to override a variable called $direnv_layout_dir in $HOME/.config/direnv/direnvrc or in each project's .envrc.

The following example will create a unique directory name per project in $HOME/.cache/direnv/layouts/:

# $HOME/.config/direnv/direnvrc
: ${XDG_CACHE_HOME:=$HOME/.cache}
pwd_hash=$(echo -n $PWD | shasum | cut -d ' ' -f 1)
direnv_layout_dir=$XDG_CACHE_HOME/direnv/layouts/$pwd_hash

Known Bugs

At the moment nix-direnv depends on GNU Grep and a modern Bash version. This might lead to problems on macOS. As a work-around we suggest that macOS users install direnv/grep via Nix or Homebrew.

Why not use lorri instead?

Lorri causes large CPU load when $NIXPKGS is pointed to a directory, e.g. a git checkout. This is because it tries to watch all referenced Nix files and re-evaluate them when they change. Nix-direnv compromises between performance and correctness, and only re-evaluates direnv if either the project-specific default.nix / shell.nix changes, or if there is a new commit added to nixpkgs. A re-evaluation can be also triggered by using touch shell.nix in the same project. Also nix-direnv does not require an additional daemon, so it can be included into the project itself and enabled without further user effort.