How to check programmatically if the Ethernet cable is plugged in?

Question

As you know, in Windows when we plug in the network cable, the network symbol will change to another status.

How can I know whether the cable is plugged in or not via the command prompt in Linux?

Solution 1

The 2 methods I've seen used predominately are to use ethtool or to manually parse the contents of /sys.

ethtool

For example if your interface is eth0 you can query it using ethtool and then parse for the line, "Link detected".

Example

$ sudo ethtool eth0
Settings for eth0:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
    Advertised auto-negotiation: Yes
    Speed: 100Mb/s
    Duplex: Full
    Port: Twisted Pair
    PHYAD: 1
    Transceiver: internal
    Auto-negotiation: on
    Supports Wake-on: pumbag
    Wake-on: g
    Current message level: 0x00000001 (1)
    Link detected: yes

Specifically this command:

$ ethtool eth0 | grep "Link"
Link detected: yes

If it were down it would say no.

Using /sys

Again assuming we're interested in eth0, you can manually parse the contents of /sys/class/net/ and then eth0 for your device. There are 2 files under this directory that will tell you the status of whether the link is active or not, carrier and operstate:

When the wire is connected these 2 files will present as follows:

$ cat /sys/class/net/eth0/{carrier,operstate}
1
up

When the wire is disconnected these 2 files will present as follows:

$ cat /sys/class/net/eth0/{carrier,operstate}
0
down

References

• How to detect the physical connected state of a network cable/connector?

Solution 2

Use mii-tool (man page):

# mii-tool em1
em1: negotiated 100baseTx-FD flow-control, link ok

There is also nmcli from NetworkManager:

$ nmcli -f capabilities.carrier-detect,capabilities.speed device show em1
CAPABILITIES.CARRIER-DETECT:            yes
CAPABILITIES.SPEED:                     100 Mb/s

^{* device can be shorten to d}

Comments

[Anthony Collins]

Better solution, but not all interfaces support MII inquiries.