Setting environment variables in nix via a shebang

Question

Is it possible to set environment variables while using nix-shell which gets invoked via a shebang? For example, I'm using

#! /usr/bin/env nix-shell
#! nix-shell -i runghc --pure

and would like to set an environment variable like FOO=bar. Note, FOO is not necessarily defined by the surrounding environment. Thus --keep FOO is not an option.

Answer 1

Looks like you're using an accompanying shell.nix file. You could simply define your environment variable as a derivation attribute there.

shell.nix

pkgs.mkShell {
  buildInputs = [ pkgs.ghc ];
  shellHook = ''
    VIA_SHELL_HOOK=it\ works   # no it doesn't
  '';
  VIA_DRV_ATTR = "working";
}

Example executable script

#!/usr/bin/env nix-shell
#!nix-shell -i runghc --pure

import System.Environment(getEnv)

main = do
  getEnv "VIA_DRV_ATTR" >>= print
  getEnv "VIA_SHELL_HOOK" >>= print

Output:

"working"
hello: VIA_SHELL_HOOK: getEnv: does not exist (no environment variable)

(Nix 2.3.4, Nixpkgs 20.03)