#!/bin/bash
# Copyright (C) 2016 Ian Kelling

# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at

#     http://www.apache.org/licenses/LICENSE-2.0

# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

set -eE -o pipefail
trap 'echo "$0:$LINENO:error: \"$BASH_COMMAND\" returned $?" >&2' ERR

case $1 in
    -h|--help)
        cat <<'EOF'
Usage: hostmac HOST
Get a host's mac address

If it's not in ip a, then ping it and look in the arp cache.
EOF
        exit 0
        ;;
esac
host=$1

ip=$(host $host | sed -rn 's/^\S+ has address //p')

# first sed joins ip a paragraphs into a single line.
if ip a | sed -r '1{x;d};/^[0-9]/{: p;x;s/\n//g;b};$b p;H;d' \
        | sed -rn "/${ip//./\\.}/{s#^.*link/ether\s(\S+)\s.*#\1#p;q1}"; then
    ping -c1 $ip &>/dev/null
    arp -n $ip | tail -n+2 | awk '{print $3}'
fi