local bEnd = false;

print( "press <SPACE> to stop attacking." )

parallel.waitForAny(
function()
  while not bEnd do
    local event, key = os.pullEvent("key");
    if key ~= 1 then
      bEnd = true;
      print("Aborting...");
    end
  end
end,
function()
  while not bEnd do
    turtle.attack()
    sleep(0.25)
  end
end
)