Storing command output in a variable?

Hi, I am trying to wrap an AppImage, without success. The bambu.nix file I have now is: { pkgs, lib, fetchurl, appimageTools, runCommandWith ? import <nixpkgs> {} }: let pname = "bambu"; version = "2.09.01.70"; fullname = lib.readFile "${runCommandWith { name="flnm"; derivationArgs.nativebuildInputs = [ pkgs.curl pkgs.gnugrep ]; } "${pkgs.curl}/bin/curl -Ls https://github.com/bambulab/BambuStudio/releases/expanded_assets/v${version} | ${pkgs.gnugrep}/bin/grep -Eom1 'v${version}-[0-9]+' > ...