nix-instantiate '<nixpkgs>' -A hello
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos' does not exist, ignoring
warning: you did not specify '--add-root'; the result might be removed by the garbage collector
/nix/store/r4vd7wq0j9py6vrzsv7r4ggpcg95jkys-hello-2.10.drv