#!/bin/sh
# This script uses the p utility to monitor all the interfaces for any
# link level change. Any change that occurs will trigger an echo to
# stdout or to the file received as argument, if there is any.

if [ $# -gt 1 ]; then
  echo "Usage: $("basename $0") [output_filename]"
  exit 1
fi

ip -oneline monitor link |
while read line; do
  if [ $# = 1 ]; then
    echo >"$1"
  else
    echo "link monitor triggered"
  fi
done