nix-direnv

A fast, persistent use_nix implementation for direnv. 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 loose your build cache of your project if you are in a plane without 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

Storing .direnv outside the project directory

.direnv might interact badly with backups (i.e. Dropbox) or IDEs. Therefore it's possible to override in $HOME/.config/direnv/direnvrc or in own project's .envrc a variable called $direnv_layout_dir. 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 gnugrep and a modern bash version. This might lead to problems on macOS. As a work-around we suggest to install direnv/grep via nix or homebrew.

Why not using lorri instead?

Lorri causes large CPU load when $NIXPKGS is pointed to a directory, i.e. a git checkout. This is because it tries to watch any referenced nix file and re-evaluates if those changes. Nix-direnv comprises between performance and correctness and only reevaluate 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 additional software besides direnv + nix i.e. a daemon and the function could be included into the project itself.