check if interface eth0 is up (configured)

Question

I need to check with a script, whether eth0 is configured. If so, the script will do nothing. Otherwise it will start wlan0. (I don't want both eth0 and wlan0 to be up at the same time).

What would be the easiest way to check, whether eth0 is already up?

I am using Debian Wheezy

CLARIFICATION:

I would like to check not only that the cable in eth0 is plugged in, but rather that the interface is configured (i.e. it has either static IP set, or it has received a DHCP IP). If cable is plugged in, but eth0 is not configured correctly, I want to start wlan0

Answer 1

You can do it many ways. Here an example:

$ cat /sys/class/net/eth0/operstate
up

Answer 2

ip a show ethX up

If nothing displayed then your interface is down

Answer 3

ip a | grep -Eq ': eth0:.*state UP' || _do_your_thing 

So here we grep the ubiquitous ip tool's stdout for a line which contains both our interface of interest and the phrase "state UP" ( thanks to @Lekensteyn for pointing out the need for a little more specificity than just UP). We use the argument a as the short form for address and that should be enough to get a listing of all configured network cards in system.

One advantage to using ip could be that it really should be available everywhere - it is how I commonly configure my Android phone's network devices, for instance.

The :colons are used to avoid partial matches - in this way we guarantee a match for eth0 as opposed to an otherwise possible someothereth0 or eth007.

Thanks @RaphaelAhrens for nudging me toward correctness and explaining my solution.

EDIT:

To handle the current requirements you could:

ip a | sed -rn '/: '"$if"':.*state UP/{N;N;s/.*inet (\S*).*/\1/p}'

The above will only print a CIDR ip address if your target $if is UP, plugged in, and has one. For ipv6 the solution is just as simple with only minor modification.

If you don't like sed you could achieve similar results with another |pipe ... grep and adding a -A context option to the first grep - but I like sed.

Answer 4

The other methods do not work, but this one does (on Ubuntu at least):

while true; do
    ip route | grep "linkdown"
    if [ $? -eq 0 ]; then
        sleep 1  # network not yet up
    else
        break   # network up
    fi
done