文件描述符

    1. Any specified file descriptor has to support reading.
    2. If a file descriptor is specified as the path, it will not be closed automatically.
    3. The reading will begin at the current position. For example, if the file already had 'Hello World' and six bytes are read with the file descriptor, the call to fs.readFile() with the same file descriptor, would give 'World', rather than 'Hello World'.