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.