# nix-direnv

![Test](https://github.com/nix-community/nix-direnv/workflows/Test/badge.svg)

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`

```bash
# 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:

```console
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

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

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

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

## Usage example

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

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

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

Then add the line `use nix` to your envrc:
```console
$ 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`:

``` 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`:

```console
$ 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/`:

```bash
# $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](https://github.com/nix-community/nix-direnv/issues/3) 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.