Bash- Determine what interface is online

Shell Programming and Scripting
1

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

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 
ip route get 8.8.8.8
8.8.8.8 via 192.168.10.1 dev eth0  src 192.168.10.33
    cache

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

gawk version (rs limitation)

ip route get 8.8.8.8 | awk 'NR==2 {print $1}' RS="dev"
eth0

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 8.8.8.8 | awk -F"dev " 'NR==1 {split($2,a," ");print a[1]}'
4

Which interface is connected to the internet is a routing thing:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
172.16.0.2      0.0.0.0         255.255.255.255 UH    0      0        0 tun0
192.168.0.0     0.0.0.0         255.255.255.0   U     0      0        0 lan
172.16.0.0      172.16.0.2      255.255.0.0     UG    0      0        0 tun0
127.0.0.0       0.0.0.0         255.0.0.0       U     0      0        0 lo
0.0.0.0         192.168.0.1     0.0.0.0         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 }'

lan

$
5

This does not work on my VPS server.
It gives:

route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         0.0.0.0         0.0.0.0         U     0      0        0 venet0

This works fine

ip route get 8.8.8.8
8.8.8.8 dev venet0  src 32.244.0.10
    cache  ipid 0x2868 mtu 1500 advmss 1460 hoplimit 64

ip route get 8.8.8.8 | awk -F"dev " 'NR==1 {split($2,a," ");print a[1]}'
venet0
6

A default route can work without UG? Must be bridged... That makes my program even simpler though!

$ /sbin/route -n | awk '($1 == "0.0.0.0") { print $NF ; exit }'

lan

$
7

Spot on, problem solved, cheers guys.