How to Detect the Physical Connected State of a Network Cable/connector?

[[email protected] ~]# for i in $( ls /sys/class/net ); do echo -n $i; ethtool $i | grep Link\ d; done
eth0    Link detected: yes
eth1    Link detected: yes
eth2    Link detected: no
eth3    Link detected: no
lo  Link detected: yes
tun0    Link detected: yes
usb0    Link detected: yes

yes: mean cable connected
no: no cable connected