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

On OpenWRT the only way to reliably do this, at least for me, is by running these commands:

# Get switch name
swconfig list

# assuming switch name is "switch0"
swconfig dev switch0 show | grep link:

# Possible output
root@OpenWrt:~# swconfig dev switch0 show | grep link:
        link: port:0 link:up speed:1000baseT full-duplex txflow rxflow
        link: port:1 link:up speed:1000baseT full-duplex txflow rxflow eee100 eee1000 auto
        link: port:2 link:up speed:1000baseT full-duplex txflow rxflow eee100 eee1000 auto
        link: port:3 link:down
        link: port:4 link:up speed:1000baseT full-duplex eee100 eee1000 auto
        link: port:5 link:down
        link: port:6 link:up speed:1000baseT full-duplex txflow rxflow

This will show either "link:down" or "link:up" on every port of your switch.

Answer 2

Use 'ip monitor' to get REAL TIME link state changes.

Answer 3

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!

Answer 4

tail -f /var/log/syslog | grep -E 'link (up|down)'

or for me faster gets:

tail -f /var/log/syslog | grep 'link \(up\|down\)'

It will listen to the syslog file.

Result (if disconnect and after 4 seconds connect again):

Jan 31 13:21:09 user kernel: [19343.897157] r8169 0000:06:00.0 enp6s0: link down
Jan 31 13:21:13 user kernel: [19347.143506] r8169 0000:06:00.0 enp6s0: link up

Answer 5

Most modern Linux distributions use NetworkManager for this. You could use D-BUS to listen for the events.

If you want a command-line tool to check the status, you can also use mii-tool, given that you have Ethernet in mind.

Answer 6

on arch linux. (im not sure on other distros) you can view the operstate. which shows up if connected or down if not the operstate lives on

/sys/class/net/(interface name here)/operstate
#you can also put watch 
watch -d -n -1 /sys/class/net/(interface name here)/operstate