bash script to sync/merge a local `hosts` file with the one on remote machine

sync_hosts.sh

#!/bin/bash

# title       : sync_host.sh
# description : sync/merge a local `hosts` file with one on a remote server
# author      : Daniel Wang // @ddwang // ZING
# usage       : bash sync_host.sh <host_file> <user@remote_addr> <ssh_key_file>
# notes       : the remote user (user@remote_addr) must have `sudo` privilege
# 
# Copyright (c) 2017 ZING
# 
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:

# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

if [ "$3" = "" ]
then
  echo "Usage: $0 <host_file> <user@remote_addr> <ssh_key_file>"
  exit
fi

HOSTS_FILE="$1"
REMOTE_CREDENTIALS="$2"
SSH_KEY_FILE="$3"

function func_max_host_entry_len() {
	local __RET=$1
	while read -r line || [[ -n "$line" ]]; do
		read -a array <<< "$line"
		local __IP="${array[0]}"
		local __HOST="${array[1]}"
		local __LEN=$((${#__IP} + ${#__HOST} + 4))
		eval $__RET=$(( $2 > $__LEN ? $2 : $__LEN ))
	done < "$HOSTS_FILE"
}

function func_pretty_host_entry() {
	local __RET=$1
	local __FORTY_SPACES="                                        "
	local __GAP_LEN=$(($2 - ${#3} - ${#4}))
	eval $__RET="\"$3${__FORTY_SPACES:0:$__GAP_LEN}$4\""
}

# calculate the max len of a host entry
func_max_host_entry_len MAX_HOST_ENTRY_LEN 0 "$HOSTS_FILE"

# re-read the host file into an array
IFS=$'\n' read -d '' -r -a lines < "$HOSTS_FILE"
for line in "${lines[@]}"
do
	read -a array <<< "$line"
	IP="${array[0]}"
	if [ "$IP" = "" ]
	then
		echo "invalid line in $HOSTS_FILE: '$line'"
		exit -1
	fi
	HOST="${array[1]}"
	if [ "$HOST" = "" ]
	then
		echo "invalid line in $HOSTS_FILE: '$line'"
		exit -1
	fi

	func_pretty_host_entry HOST_ENTRY $MAX_HOST_ENTRY_LEN $IP $HOST

	ssh -i $SSH_KEY_FILE $REMOTE_CREDENTIALS "grep -q '$HOST' /etc/hosts && exit 42"
	rc=$?; if [ $rc -eq 42 ]; then
		echo "[INFO]  host entry for [$HOST] already exist, patching ..."

		ssh -i $SSH_KEY_FILE $REMOTE_CREDENTIALS "sudo sed -i 's/^.*$HOST\$/$HOST_ENTRY/' /etc/hosts || exit -1"
		rc=$?; if [ $rc -ne 0 ]; then
			echo "[ERROR] patching /etc/hosts failed $rc"
			exit $rc
		fi
	else
		echo "[INFO]  appending host entry for [$HOST] ..."

		ssh -i $SSH_KEY_FILE $REMOTE_CREDENTIALS "echo '$HOST_ENTRY' | sudo tee -a /etc/hosts || exit -1"
		rc=$?; if [ $rc -ne 0 ]; then
			echo "[ERROR] appending to /etc/hosts failed $rc"
			exit $rc
		fi
	fi
done

echo "[INFO]  done."
echo
echo "~~~ /etc/hosts ~~~"
echo

ssh -i $SSH_KEY_FILE $REMOTE_CREDENTIALS "cat /etc/hosts"