nix-playgraound/01-devshell/flake.nix

{
  description = "A minimal dev shell: your first flake";

  # Inputs: other flakes this one depends on.
  # `nixpkgs` is the main package set. `follows` isn't used here, but you'll
  # see it a lot when composing flakes (e.g. `inputs.foo.inputs.nixpkgs.follows = "nixpkgs";`).
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  };

  # Outputs: a function that receives all inputs and returns an attrset.
  # The attrset's structure is what `nix` CLI commands look for (e.g. `devShells.<system>.default`).
  outputs =
    { nixpkgs, ... }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs { inherit system; };
    in
    {
      # `nix develop` picks this up by default.
      devShells.${system}.default = pkgs.mkShell {
        packages = with pkgs; [
          cowsay
          jq
          ripgrep
        ];

        # Runs once when you enter the shell.
        shellHook = ''
          echo "welcome to your first nix dev shell"
          cowsay "hello, flakes"
        '';
      };
    };
}