ifplugd ifupdown action script (see https://linux.die.net/man/8/ifplugd)

ifupdown.sh

#!/bin/sh                                       
set -e
set -o errexit
set -o nounset

if [ -z "$1" ] || [ -z "$2" ] ; then
    echo "Wrong arguments" > /dev/stderr
    exit 1
fi

case "$2" in
    up)
	/usr/bin/ip link set $1 up
	[ "$2" = "up" ] && exec /usr/sbin/dhclient $1
	;;
    down)
	if [ "$2" = "down" ]; then
	    /usr/sbin/dhclient -x
	    /usr/bin/ip link set $1 down
	fi
	;;
esac
	
exit 1