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 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?

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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 1

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 2

You can use ethtool:

$ sudo ethtool eth0
Settings for eth0:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Advertised auto-negotiation: Yes
    Speed: 1000Mb/s
    Duplex: Full
    Port: Twisted Pair
    PHYAD: 0
    Transceiver: internal
    Auto-negotiation: on
    Supports Wake-on: umbg
    Wake-on: g
    Current message level: 0x00000007 (7)
    Link detected: yes

To only get the Link status you can use grep:

$ sudo ethtool eth0 | grep Link
    Link detected: yes

Answer 3

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/operstate:up
eth0/carrier:1

_{Side Trick: harvesting all properties at once the easy way:}

grep "" eth0/*

This forms a nice list of key:value pairs.

Answer 4

You could try this:

gw="$(ip route | awk "/^default via/ {print \$3}")"
ping -c 1 "$gw" > /dev/null
result="$?"

as a workaround for the product will be within a LAN using an unknown network configuration and unknown hosts problem.

If there is something else than "0" in the result, than Your gateway is not responding to ICMP pings. Beware, some router admins might block icmp. I haven't tested it on a networkless workstation (it's unusual to have one, these days).

Answer 5

I am having a problem with ethtool. When I call the ethtool from the script inside the cgi-bin it doesnt giving any output. here is my bash shell file

!/bin/bash

char1=$(ethtool eth0 | grep Link | awk '{print $3}')

echo Content-type: text/html echo "" echo $char1 echo "" echo "
" echo "hello" echo ""

but when i try the command in terminal, i can see all the output. then i have changed my script

!/bin/bash

char1=$(ethtool eth0)

echo Content-type: text/html echo "" echo $char1 echo "" echo "
" echo "hello" echo ""

when I check from webpage i am getting the following only

Settings for eth0: No data available

do you know the reason why i am getting like this?