Comma runs software without installing it.
Basically it just wraps together nix shell -c and nix-index. You stick a , in front of a command to
run it from whatever location it happens to occupy in nixpkgs without really thinking about it.
comma is in nixpkgs so you can install it just like any other package.
either install it in your nix environment
nix-env -f '<nixpkgs>' -iA commaor add this snippet to your NixOS configuration.
environment.systemPackages = with pkgs; [ comma ];, cowsay neato