nix-shell: how to specify a custom environment variable? - nixos

Question

I'm learning about nixos and nix expressions. In a project folder I created a shell.nix and I when I run nix-shell I want it to preset an environment variable for me.
For example to set the PGDATA env var.
I know there are several ways to write nix expression files (I'm not yet used to most of them). Here is my sample:
shell.nix
let
pkgs = import <nixpkgs> {};
name = "test";
in pkgs.myEnvFun {
buildInputs = [
pkgs.python
pkgs.libxml2
];
inherit name;
extraCmds = ''
export TEST="ABC"
'';
}

Answer 1

Use buildPythonPackage function (that uses mkDerivation). Passing anything to it will set env variables in bash shell:
with import <nixpkgs> {};
buildPythonPackage {
name = "test";
buildInputs = [ pkgs.python pkgs.libxml2 ];
src = null;
PGDATA = "...";
}

Answer 2

You may also use pkgs.stdenv.mkDerivation.shellHook.
let
pkgs = import <nixpkgs> {};
name = "test";
in pkgs.stdenv.mkDerivation {
buildInputs = [
pkgs.python
pkgs.libxml2
];
inherit name;
shellHook = ''
export TEST="ABC"
'';
}

Answer 3

To set environment variable for a nix-shell without creating a new package, shellHook option can be used. As shown in the example from the manual:
shellHook =
''
echo "Hello shell"
export SOME_API_TOKEN="$(cat ~/.config/some-app/api-token)"
'';
A full shell.nix example based on my use-case - with go of version 1.18 from unstable channel:
let
pkgs = import <nixpkgs> {};
unstable = import <nixos-unstable> { config = { allowUnfree = true; }; };
in pkgs.mkShell rec {
name = "go-1.18";
buildInputs = with pkgs; [
unstable.go_1_18
];
shellHook = ''
export PATH="$HOME/go/bin:$PATH"
'';
}
The script also sets name option, which is then shown in the shell prompt (works nicely with Starship shell prompt).