Get the active interface for a given route

active_interface.sh

#!/bin/sh

# Get network interface. We sleep so that the interface has a chance to come up!
# Substitute the 0.0.0.0 address if you have a specific route that you want to test, however this may impact
# portability if a machine spends more time without access to that route than with.

# Give the interface a chance to come up - useful if this is used in any boot up processes
while [ $(route get 0.0.0.0 2>/dev/null | grep -c interface) != 1 ]; do
    sleep 5
done

# Actual interface
interface=$(route get 0.0.0.0 2>/dev/null | awk '/interface: / {print $2}')