// returns file with program output (STDOUT+STDERR)
static File backtickToReceiver(S cmd, fO outputReceiver) {
  File outputFile = createTempFile(cmd, ".out");
  makeEmptyFile(outputFile); // unnecessary?
  TailFile tail = tailFile2(outputFile, 100, voidfunc(S s) {
    pcallF(outputReceiver, s)
  });
  
  // make a dependent child process that is exited when we exit
  print("Running: " + cmd);
  pcall {
    backtickToFile(cmd, outputFile);
  }
  tail.stop();
  ret outputFile;
}