Linux bash script to extract IP address

Question

I want to make big script on my Debian 7.3 ( something like translated and much more new user friendly enviroment ). I have a problem. I want to use only some of the informa

Answer 1

Just a note, since I just spent some time trouble-shooting a botched upgrade on a server. Turned out, that (years ago) I had implemented a test to see if dynamically added interfaces (e.g. eth0:1) were present, and if so, I would bind certain proggis to the 'main' IP on eth0. Basically it was a variation on the 'ifconfig|grep...|sed... ' solution (plus checking for 'eth0:' presence).
The upgrade brought new net-tools, and with it the output has changed slightly:

old ifconfig:

eth0      Link encap:Ethernet  HWaddr 42:01:0A:F0:B0:1D
          inet addr:10.240.176.29  Bcast:10.240.176.29  Mask:255.255.255.255
          UP BROADCAST RUNNING MULTICAST  MTU:1460  Metric:1
          ...

whereas the new version will display this:

eth0: flags=4163  mtu 1460
      inet 10.240.212.165  netmask 255.255.255.255  broadcast 10.240.212.165
      ...

rendering the hunt for 'eth0:' as well as 'inet addr:' search busted (never mind interfaces called 'em0','br0' or 'wlan0'...). Sure you could check for 'inet ' (or 'inet6'), and make the addr: part optional, but looking closer, you'll see that more or less everything has changed, 'Mask' is now 'netmask',...

The 'ip route ...' suggestion's pretty nifty - so maybe:

_MyIP="$( ip route get 8.8.8.8 | awk 'NR==1 {print $NF}' )"
if [ "A$_MyIP" == "A" ]
then
    _MyIPs="$( hostname -I )"
    for _MyIP in "$_MyIPs"
    do
        echo "Found IP: \"$_MyIP\""
    done
else
    echo "Found IP: $_MyIP"
fi

Well, something of that sort anyway. Since all proposed solutions seem to have circumstances where they fail, check for possible edge cases - no eth, multiple eth's & lo's, when would 'hostname -i' fail,... and then decide on best solution, check it worked, otherwise 2nd best.

Cheers 'n' beers!