方法：
[root@RG-PowerCache-X ~]# ethtool eth0
Settings for eth0:
Supported ports: [ FIBRE ]
Supported link modes: 1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 1000baseT/Full
Advertised pause frame use: No
Advertised auto-negotiation: Yes
Speed: Unknown!
Duplex: Unknown! (255)
Port: FIBRE
PHYAD: 0
Transceiver: external
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: d
Current message level: 0x00000001 (1)
Link detected: no
如上所示，“Link detected”为no时，表示网线没有插上；反之为yes时，表示网线插上了。