#!/bin/bash

# Generate hosts-file with private IP's in AWS ec2

# Vars
hosts_file="/etc/hosts"
region="eu-west-1"

# AWS call
described_instances=`aws ec2 describe-instances --region ${region}`

# jq
private_ips=`echo $described_instances | jq '.Reservations[] .Instances[] .PrivateIpAddress' | tr -d \"`
hostnames=`echo $described_instances | jq '.Reservations[] .Instances[] .Tags[] | select(.Key == "hostname").Value' | tr -d \"`

# Associative array
declare -A arr

pcounter=0
for p in $private_ips
do
        arr[0,$pcounter]=$p
        let pcounter=pcounter+1
done

hcounter=0
for h in $hostnames
do
        arr[1,$hcounter]=$h
        let hcounter=hcounter+1
done

# Do your thing
counter=0
for i in $private_ips
do
        found=`fgrep -c "${arr[1,${counter}]}" ${hosts_file}`

        if [ $found -eq 0 ]; then
                echo "${arr[0,${counter}]}              ${arr[1,${counter}]}" >> /etc/hosts
        else
                sed -i "/${arr[1,${counter}]}/d" ${hosts_file}
                echo "${arr[0,${counter}]}              ${arr[1,${counter}]}" >> /etc/hosts
        fi

        let counter=counter+1
done