nix-direnv/README.md

# nix-direnv

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

A faster, persistent implementation of `direnv`'s `use_nix` and `use_flake`, 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](https://github.com/nix-community/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. MacOS ships with bash 3.2
> from 2007. As a work-around we suggest that macOS users install `direnv` via
> Nix or Homebrew. There are different ways to install nix-direnv, pick your
> favourite:

<details>
  <summary> Via home-manager (Recommended)</summary>

### Via home-manager

Note that while the home-manager integration is recommended, some use cases
require the use of features only present in some versions of nix-direnv. It is
much harder to control the version of nix-direnv installed with this method. If
you require such specific control, please use another method of installing
nix-direnv.

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

```Nix
{
  # ...other config, other config...

  programs = {
    direnv = {
      enable = true;
      enableBashIntegration = true; # see note on other shells below
      nix-direnv.enable = true;
    };

    bash.enable = true; # see note on other shells below
  };
}
```

Check the current
[Home Manager Options](https://mipmip.github.io/home-manager-option-search/?query=direnv)
for integration with shells other than Bash. Be sure to also allow
`home-manager` to manage your shell with `programs.<your_shell>.enable = true`.

</details>
<details>
  <summary>Direnv's source_url</summary>

### Direnv source_url

Put the following lines in your `.envrc`:

```bash
if ! has nix_direnv_version || ! nix_direnv_version 3.0.4; then
  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/3.0.4/direnvrc" "sha256-DzlYZ33mWF/Gs8DDeyjr8mnVmQGx7ASYqA5WlxwvBG4="
fi
```

</details>

<details>
  <summary>Via system configuration on NixOS</summary>

### Via system configuration on NixOS

For NixOS 23.05+ all that's required is

```Nix
{
  programs.direnv.enable = true;
}
```

other available options are:

```Nix
{ pkgs, ... }: {
  #set to default values
  programs.direnv = {
    package = pkgs.direnv;
    silent = false;
    loadInNixShell = true;
    direnvrcExtra = "";
    nix-direnv = {
      enable = true;
      package = pkgs.nix-direnv;
    };
  }
```

</details>

<details>
  <summary>With `nix profile`</summary>

### With `nix profile`

As **non-root** user do the following:

```shell
nix profile install nixpkgs#nix-direnv
```

Then add nix-direnv to `$HOME/.config/direnv/direnvrc`:

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

</details>

<details>
  <summary>From source</summary>

### From source

Clone the repository to some directory and then source the direnvrc from this
repository in your own `~/.config/direnv/direnvrc`:

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

</details>

## Usage example

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

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

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

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

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

If you haven't used direnv before, make sure to
[hook it into your shell](https://direnv.net/docs/hook.html) 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.:

```console
$ 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
cannot guarantee stability after a 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](https://github.com/nix-community/nix-direnv/tree/master/templates/flake).
which provides a basic flake with devShell integration and a basic `.envrc`.

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

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

### Integrating with a existing flake

```console
$ 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:

```bash
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:

```console
$ 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 interpreted or passed along.

#### Manual reload of the nix environment

To avoid delays and time consuming rebuilds at unexpected times, you can use
nix-direnv in the "manual reload" mode. nix-direnv will then tell you when the
nix environment is no longer up to date. You can then decide yourself when you
want to reload the nix environment.

To activate manual mode, use `nix_direnv_manual_reload` in your `.envrc` like
this:

```shell
nix_direnv_manual_reload
use nix # or use flake
```

To reload your nix environment, use the `nix-direnv-reload` command:

```console
$ nix-direnv-reload
```

##### 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

As a convenience, `nix-direnv` adds common files to direnv's watched file list
automatically.

The list of additionally tracked files is as follows:

- for `use nix`:
  - `~/.direnvrc`
  - `~/.config/direnv/direnvrc`
  - `.envrc`,
  - A single nix file. In order of preference:
    - The file argument to `use nix`
    - `default.nix` if it exists
    - `shell.nix` if it exists

- for `use flake`:
  - `~/.direnvrc`
  - `~/.config/direnv/direnvrc`
  - `.envrc`
  - `flake.nix`
  - `flake.lock`
  - `devshell.toml` if it exists

Users are free to use direnv's builtin `watch_file` function to track additional
files. `watch_file` must be invoked before either `use flake` or `use nix` to
take effect.

#### Environment Variables

nix-direnv sets the following environment variables for user consumption. All
other environment variables are either a product of the underlying nix
invocation or are purely incidental and should not be relied upon.

- `NIX_DIRENV_DID_FALLBACK`: Set when the current revision of your nix shell or
  flake's devShell are invalid and nix-direnv has loaded the last known working
  shell.

## General direnv tips

- [Changing where direnv stores its cache][cache_location]
- [Quickly setting up direnv in a new nix project][new_project]
- [Disable the diff notice (requires direnv 2.34+)][hide_diff_notice]: Note that
  this goes into direnv's TOML configuration!

[cache_location]: https://github.com/direnv/direnv/wiki/Customizing-cache-location
[new_project]: https://github.com/nix-community/nix-direnv/wiki/Shell-integration
[hide_diff_notice]: https://direnv.net/man/direnv.toml.1.html#codehideenvdiffcode

## Other projects in the field

- [lorri](https://github.com/nix-community/lorri)
- [sorri](https://github.com/nmattia/sorri)
- [nixify](https://github.com/kalbasit/nur-packages/blob/master/pkgs/nixify/envrc)
- [lorelei](https://github.com/shajra/direnv-nix-lorelei)