If it's a one off, you just use something like "nix shell" to add it to your path for running the script.

For non one-off sorts of things, you would substitute in the nix expression "${gnugrep}/bin/grep" the "${gnugrep}" will expand to "/nix/store/grep-hash" and also make a dependency on the gnugrep package, so that the grep install won't get garbage-collected as long as your package is still around.

Here's an example[1] from a package expression for e-mail client I use, which will shell out to base64 and file. Upstream relies on these two programs being in $PATH, but this replaces the string used for shelling out with the absolute path in the nix store.

For shell scripts, I'll just do something like this near the top:

   GREP="${GNU_GREP:-$(command -v grep)}"

Then I use "$GREP" in the script itself, and develop with grep in my path, but it's trivial to prepend all of my dependencies when I bundle it up for nix.

1: https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/no...