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-shellenvironment - 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)
There are different ways to install nix-direnv, pick your favourite:
- via home-manager (recommended)
- via configuration.nix in NixOS
- with nix-env
- from source
- with direnv source_url
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
In /etc/nixos/configuration.nix:
{ 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"
];
}
Then source the direnvrc from this repository in your own $HOME/.direnvrc
# put this in ~/.direnvrc
source /run/current-system/sw/share/nix-direnv/direnvrcAs non-root user do the following:
nix-env -f '<nixpkgs>' -iA nix-direnvThen add nix-direnv to $HOME/.direnvrc:
source $HOME/.nix-profile/share/nix-direnv/direnvrc
You also need to set keep-outputs and keep-derivations to nix.conf as described in the installation
via home-manager section.
Clone the repository to some directory
$ git clone https://github.com/nix-community/nix-direnv $HOME/nix-direnvThen source the direnvrc from this repository in your own .direnvrc
# put this in ~/.direnvrc
source $HOME/nix-direnv/direnvrcYou also need to set keep-outputs and keep-derivations to nix.conf as described in the installation
via home-manager section.
Put the following line in your .envrc
source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/1.2.3/direnvrc" "sha256-/aHqL/6nLpHcZJcB5/7/5+mO338l28uFbq88DMfWJn4="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 allownix-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.
Likewise use_nix the use_flake implementation will prevent garbage
collection of downloaded packages and also for flake inputs.
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 allowA .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 function 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}
declare -A direnv_layout_dirs
direnv_layout_dir() {
echo "${direnv_layout_dirs[$PWD]:=$(
echo -n "$XDG_CACHE_HOME"/direnv/layouts/
echo -n "$PWD" | shasum | cut -d ' ' -f 1
)}"
}During direnv setup direnv_layout_dir can be called multiple times and with different values of $PWD
(when other .envrc files are included). Therefore cache its results in dictionary direnv_layout_dirs.
In some case nix-direnv does not detect if imported file has changed and still
provides the old cached values. An evaluation can be triggered by updating your
default.nix, shell.nix or flake.nix, depending on what is used:
# choose one
$ touch default.nix
$ touch shell.nix
$ touch flake.nixAt 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.
- nix-direnv has flakes support.
- High CPU load/resource usage in some cases: When nixpkgs in
NIX_PATHis pointed to a directory, i.e. a git checkout, Lorri will try to evaluate nixpkgs everytime something changes causing high cpu load. Nix-direnv compromises between performance and correctness, and only re-evaluates direnv if either the project-specificdefault.nix/shell.nixchanges, or if there is a new commit added tonixpkgs. A re-evaluation can be also triggered by usingtouch .envrcin the same project. A different problem is that it might trigger mass-rebuilds when the same nixpkgs checkout is pointed to something like staging. - No additional daemon or services required: The codesize is small enough that it can be vendored into a project itself.