future is not run even if get() is called

I want to run some system calls in parallel, but even if I call f1.get() the coping in the lines below does not take place. What is wrong with this code?

auto f1 = std::async( system, string("cp a b").c_str() );
f1.get(); // there is still no file "b".

Note: I wrote string(...).c_str() because in my real code I'm using putting the argument together from different strings.