Bash- Determine what interface is online

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# 1  
Old 09-12-2013
Bash- Determine what interface is online

Hey guys, I want to use a a quick bash script/command to determine what network interface is connected to the internet so I can pipe it out to become a variable, in order so the user does not have to manually type it in each time or have to 'hardcode' the variable into the script.

I know about checking ifconfig etc of course, just looking for another way.

Many thanks and apologies for the kinda double post!
# 2  
Old 09-12-2013
For LINUX, if you don't have to know the IP addresses of the interfaces, but just to enquire the status, mii-tool / ethtool also work. These commands might require root privilege to execute.
# 3  
Old 09-12-2013
ip route get via dev eth0  src

This tests what route is used to connect to a public IP, like this Google DNS IP

gawk version (rs limitation)
ip route get | awk 'NR==2 {print $1}' RS="dev"

You need to find the word after dev, and this can vary on the line, depending if computer is behind a firewall/router, or connected directly on the net.

EDIT: a more portable version
ip route get | awk -F"dev " 'NR==1 {split($2,a," ");print a[1]}'

# 4  
Old 09-12-2013
Which interface is connected to the internet is a routing thing:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface UH    0      0        0 tun0   U     0      0        0 lan     UG    0      0        0 tun0       U     0      0        0 lo         UG    0      0        0 lan

So, to always grab the catch-all default gateway:

$ /sbin/route -n | awk '/0\.0\.0\.0/ && /UG/ { print $NF ; exit }'



# 5  
Old 09-12-2013
This does not work on my VPS server.
It gives:
route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface         U     0      0        0 venet0

This works fine
ip route get dev venet0  src
    cache  ipid 0x2868 mtu 1500 advmss 1460 hoplimit 64

ip route get | awk -F"dev " 'NR==1 {split($2,a," ");print a[1]}'

# 6  
Old 09-12-2013
A default route can work without UG? Must be bridged... That makes my program even simpler though!

$ /sbin/route -n | awk '($1 == "") { print $NF ; exit }'



# 7  
Old 09-12-2013
Spot on, problem solved, cheers guys.
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