direnv: manage dev environments

direnv (along with nix-direnv) allows one to persist 1 nix development shell environments and share them seamlessly with text editors and IDEs. It obviates having to run nix develop manually every time you open a new terminal. The moment you cd into your project directory, the devshell is automatically activated, thanks to direnv.


It is recommended to use starship along with nix-direnv, because it gives a visual indication of the current environment. For example, if you are in a Nix Shell, your terminal prompt automatically changes to something like this:

srid on nixos haskell-template on  master [!] via λ 9.2.6 via ❄️  impure (ghc-shell-for-haskell-template-


If you use home-manager, both nix-direnv and starship can be installed using the following configuration:

# home.nix
programs.direnv = {
  enable = true;
  nix-direnv.enable = true;
programs.starship = {
  enable = true;
Newcomer Tip

If you have never used home-manager before, we recommend that you set it up 1 by following the instrutions at https://github.com/juspay/nix-dev-home.

The “nix-dev-home” template uses nixos-flake, and works on both Linux or macOS.

Text Editor configuration


For VSCode, use Martin Kühl’s direnv extension.

Doom Emacs

Doom Emacs has the :tools direnv module to automatically load the devshell environment when you open the project directory.

Add a .envrc

To enable direnv on Flake-based projects, add the following to your .envrc:

use flake

Now run direnv allow to authorize the current .envrc file. You can now cd into the project directory in a terminal and the devshell will be automatically activated.

Reload automatically when some files change

Haskell - when .cabal files change

Since both nixpkgs and haskell-flake use Nix expressions that read the .cabal file to get dependency information, you will want the devshell be recreated every time a .cabal file changes. This can be achieved using the watch_file function. Modify your .envrc to contain:

watch_file *.cabal
use flake

As a result of this whenever you change a .cabal file, direnv will reload the environment. If you are using VSCode, you will see a notification that the environment has changed, prompting you to restart it (see example).

nix-direnv prevents garbage collection of the devshell, so you do not have to re-download things again. direnv also enables activating the devshell in your current shell, without needing to use a customized bash.
The “nix-dev-home” template uses nixos-flake, and works on both Linux or macOS.
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