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

Question

In a Linux environment, I need to detect the physical connected or disconnected state of an RJ45 connector to its socket. Preferably using BASH scripting only.

The

Answer 1

I was using my OpenWRT enhanced device as a repeater (which adds virtual ethernet and wireless lan capabilities) and found that the /sys/class/net/eth0 carrier and opstate values were unreliable. I played around with /sys/class/net/eth0.1 and /sys/class/net/eth0.2 as well with (at least to my finding) no reliable way to detect that something was physically plugged in and talking on any of the ethernet ports. I figured out a bit crude but seemingly reliable way to detect if anything had been plugged in since the last reboot/poweron state at least (which worked exactly as I needed it to in my case).

ifconfig eth0 | grep -o 'RX packets:[0-9]*' | grep -o '[0-9]*'

You'll get a 0 if nothing has been plugged in and something > 0 if anything has been plugged in (even if it was plugged in and since removed) since the last power on or reboot cycle.

Hope this helps somebody out at least!