Modules

Generally any nix file is a module, contains either a function or a attribute set.

Module Identifier

relative path to a nix file
keys in NIX_PATH: any key in NIX_PATH is a valid identifier of nix module.

sh

$ echo $NIX_PATH

nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels

nix

{
    import = [
        <nixpkgs>
    ];
}

# or

let
    pkgs = import <nixpkgs>;
in {

}