memcached -u www-data -m 2048 -d
After I run the above,I've no idea which port it's using ...
you can use netstat -lp
-l is short form of --listening and -p is short form of --program
For example, to list programs listening on TCP ports (-t or --tcp) and show ports and IPs numerically (-n or --numeric):
$ netstat -nltp
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 127.0.0.1:3306 0.0.0.0:* LISTEN 995/mysqld
tcp 0 0 127.0.0.1:11211 0.0.0.0:* LISTEN 1350/memcached