Basics

Given a hello world example, click on the end of each line to get an explanation:

devenv.nix

{ pkgs, ... }: # (1)!

{ # (2)!
  env.UNICORNS = "yes"; # (3)!

  enterShell = ''
    echo hello
    ${pkgs.jq}/bin/jq --version
  ''; # (4)!
}

devenv.nix is a function with inputs. We use a special input ... at the end as a catch-all to avoid enumuerating all of the inputs.
Our function is returning an attribute set, similar to an object in JSON.
Attributes can be nested and have a similar values as in JSON.
Values can refer to the inputs. See Inputs how to define inputs.

enterShell allows you to execute bash code once the shell activates, while env allows you to set environment variables:

$ devenv shell
Building shell ...
Entering shell ...

hello
jq-1.6

(devenv) $ echo $UNICORNS
yes

See Nix language tutorial for a 1-2 hours deep dive that will allow you to read any Nix file.

Note