Flood ping. For every ECHO_REQUEST sent a period ``.'' is
printed, while for ever ECHO_REPLY received a backspace is
printed. This provides a rapid display of how many packets are
being dropped. If interval is not given, it sets interval to
zero and outputs packets as fast as they come back or one hun?lt;br /> dred times per second, whichever is more. Only the super-user
may use this option with zero interval.

linux要安装vsftpd，然后启动服务就行了
sudo apt-get install vsftpd
sudo service vsftpd start

man ls有文档，好好看。