How to get IP address and the connected interface name in bash

Question

PC with onboard linux has several network interfaces, e.g. two ethernet cards and one wifi adapter (numbers and types may vary). Each of that interfaces could be connected to a network (and could be offline) and the network could be connected to internet(superuser.com site is reachable), and could be connected to local-only network.

I want to write a bash script to get (local) IP address and name of adapter that can be used to reach internet. If several are connected - it should be the one that would be used by browser that is "asked" to load superuser.com web pages. If no connected script should echo "internet unavailble"

How to write a check condition for such script? How could i even iterate all networking-capable devices under /dev/* in a system?

Answer 1

Maybe you don't need to iterate. In my Debian 9 (IPv4 only, I'm not sure how relevant it is) I can use this:

ip route get "$(host superuser.com | awk 'NR==1 {print $NF}')" \
| grep -o "dev .* src [0-9]*\.[0-9]*\.[0-9]*\.[0-9]*"

where host (from bind9-host package) is to convert superuser.com to its IP address. To handle errors etc. you need to deconstruct the pipe and build a script with some logic around its parts.

Example output:

dev wlan0  src 192.168.1.2

---

To iterate anyway, peek into /sys/class/net:

cd /sys/class/net
ls
for interface in *; do
    printf "Doing something with %s\n" "$interface"
done

These devices are not under /dev though, at least in none of my systems.