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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

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4 Answers 4

59

You can do it many ways. Here an example:

$ cat /sys/class/net/eth0/operstate
up
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  • 5
    Unfortunately, this doesn't guarantee that the interface is fully configured. You can cat /sys/class/net/eth0/carrier, to check for a signal, but it still may not have a DHCP lease yet, etc.
    – orodbhen
    Jul 29, 2016 at 19:36
  • In addition to this comment, several other answers here have gotten comments to the effect: This doesn't work. While it's useful knowing that this answer may not work in all instances, it would be much more helpful if the commentor: 1.) gave a reference, cited an example or explained how to correct the exception, or 2.) submitted an answer that covers the exception.
    – Seamus
    Dec 21, 2020 at 7:15
9
ip a show ethX up

If nothing displayed then your interface is down

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    This method: "ip a show ethX up" and check for output, is not sufficient Feb 2, 2018 at 22:43
  • @JoshuaClayton It would be nice if you could elaborate, because for me it works.
    – x-yuri
    Feb 26, 2021 at 14:26
  • I can't remember for the world what didn't work about this for me back then. It does seem to always print something, UP or DOWN, but thats easily parsed by human or machine. Feb 26, 2021 at 17:43
  • I see no different in output with or without up (e.g. ip a show wlan0 and ip a show wlan0 up produce identical output on my machine). The other answers all grep for state UP; is there something extra needed to make the up argument work?
    – dimo414
    Feb 20 at 19:03
5
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.

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    You should explain what you are doing. So that everyone can benefit from your answer. Mar 26, 2014 at 9:37
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    This is insufficient. The output also contains UP if the device is up, but no cable is connected: 2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000. You can also specify the device to avoid false positives: ip addr show dev eth0.
    – Lekensteyn
    Mar 26, 2014 at 9:37
  • On an embedded linux 2.6 I get state UNKNOWN for all operable interfaces Jun 6, 2020 at 21:06
0

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
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    There is no "linkdown" to grep with ip route on Debian Stretch.
    – Ingo
    Apr 3, 2019 at 22:50

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