The dup function causes a previously unallocated descriptor to
    refer to its argument, while the dup2 function causes its second
    argument to refer to the same file as its first argument.

    The argument file_desc1 is invalid if it does not describe an
    open file; file_desc2 is invalid if the new file descriptor
    cannot be allocated. If file_desc2 is connected to an open file,
    that file is closed.