How to know if eth0 is up?

Discussion in 'Linux Networking' started by Guest, Mar 30, 2012.

  1. Guest

    Guest Guest


    I'd like to know if there is a command line instruction or a /proc/
    entry that tells reliably if a cable is hooked in an ethernet port
    (typically, to know at boot if I must use my dhcp client on eth0, or
    look for a WIFI cell).

    I have found /proc/net/dev but it shows eth0 even though no cable is
    plugged in.

    Guest, Mar 30, 2012
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  2. Guest

    Chris Davies Guest

    Either mii-tool or ethtool.
    Chris Davies, Mar 30, 2012
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  3. Guest

    Bit Twister Guest

    Hi. You know if you were to read
    it suggest giving meaningful information pertinent to your setup.

    At a minimum I would suggest which linux distribution, release, and
    Ethernet card. That kind of information might get you some exact answers.

    NIC may dictate which tool (ethtool, or mii-tool).

    Distribution might get you what net function can be used to make you code
    much more maintainable as time goes by.

    On my distribution, carrier status is found over in /sys/class/net and
    the network-functions script has a check_link_down function to do
    all the dirty work.

    Based on the lack of any detailed information, I can only suggest
    trying something like parsing the output from something like

    LC_ALL=C ip link show dev eth0
    Bit Twister, Mar 30, 2012
  4. Guest

    J G Miller Guest

    Which is quite a different thing to your "subject" which could be
    answered perhaps with

    ifconfig eth0 | egrep 'UP' | egrep 'RUNNING'


    As others have suggested perhaps

    ethtool eth0 | egrep "Link detected"

    Link detected: yes
    J G Miller, Mar 30, 2012
  5. Guest

    Jorgen Grahn Guest


    A general answer is more enlightening for the rest of us, though.


    .... like this one. I know kernel interfaces change now and then, but
    I'd like to see one modern distribution which didn't enable that one.
    (Thanks, by the way!)

    Jorgen Grahn, Mar 31, 2012
  6. Guest

    Mark Evans Guest

    Have you looked at ifplugd?
    Mark Evans, Apr 5, 2012
  7. Guest


    May 7, 2012
    Likes Received:
    'ethtool' shows 'Link Detected: yes' when the ethernet cable was plugged in and network was up and running. Then I removed the cable and issued 'ethtool'. It still showed 'Link Detected: yes'.
    So, I feel this tools is not dynamic enough to give us to most recent information. Probably, it may be showing up cached information.
    Is there a way/options in this tool to get the most recent status?

    sreeramabs, May 8, 2012
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