Nix: What are the values on the left?

What are the values on the left inside Nix expressions? Let's find out!

Create a file named shell.nix.

Paste the following into it:

# shell.nix
{ nixpkgs ? import <nixpkgs> {  } }:

let pkgs = [];
in nixpkgs.stdenv.mkDerivation {
  name = "env";
  buildInputs = pkgs;

  LOCALE_ARCHIVE = "/usr/lib/locale/locale-archive";

  libgpiod_dir = builtins.fetchGit {
    url = "git://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git";
    ref = "master";
  };
}

Next, run nix-shell and observe, what happens to env variable named libgpiod_dir:

➜  ~ nix-shell

[nix-shell:~]$ echo $libgpiod_dir 
/nix/store/macpk6l9c44j84791i27n0d1xc327wlh-source

[nix-shell:~]$ ls $libgpiod_dir
autogen.sh    COPYING      lib          man     tests
bindings      Doxyfile.in  LICENSES     NEWS    TODO
configure.ac  include      Makefile.am  README  tools

Exit the shell inside the Nix environment with [Ctrl] + [D], or by typing exit.

Check if variable libgpiod_dir is still set:

➜  ~ echo $libgpiod_dir

It is not!

In this experiment, the value of libgpiod_dir was propagated from Nix to the shell as an environment variable. It contained the path to the resource we requested creation of on the right - a repo downloaded ^[1] using builtins.fetchGit.