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

linux networking connection detection
Jeach picture Jeach · Apr 30, 2009 · Viewed 301.8k times · Source

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 following solutions which have been proposed on other sites do NOT work for this purpose:

  1. Using 'ifconfig' - since a network cable may be connected but the network not properly configured or not currently up.
  2. Ping a host - since the product will be within a LAN using an unknown network configuration and unknown hosts.

Isn't there some state which can be used in the /proc file system (everything else is in there)?

How is the Linux world suppose to have their own version of the Windows bubble that pop up from the icon tray indicating that you've just unplugged the network cable?

Kent Fredric and lothar, both of your answers satisfy my need... thanks a lot! Which one I'll use... I still don't know.

I guess I can't put you both down as the correct answer? And its probably fair for you that I do choose one. Flip a coin I guess? Again, thanks!

Answer

Kent Fredric picture Kent Fredric · Apr 30, 2009

You want to look at the nodes in 

/sys/class/net/

I experimented with mine:

Wire Plugged in: 

eth0/carrier:1
eth0/operstate:unknown

Wire Removed: 

eth0/carrier:0
eth0/operstate:down

Wire Plugged in Again:

eth0/carrier:1
eth0/operstate:up

Side Trick: harvesting all properties at once the easy way: 

grep "" eth0/*

This forms a nice list of key:value pairs.

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