Automatically set a structured, formatted hostname containing the AWS EC2 Instance ID for an EC2 virtual machine running RHEL/CentOS 6.x.

autoset-hostname.sh

#!/usr/bin/env bash

# Enable strict error handling
set -u -o pipefail

# Define our hostname formats
readonly HOSTNAME_FORMAT="%s.my-service-name.environment-name"
readonly FQDN_HOSTNAME_FORMAT="%s.service.example.com"

# Get the current hostname
readonly OLD_HOSTNAME="$(hostname --fqdn)"

# Get the AWS "instance-ID" associated with the current (v) machine
instance_id=""
for _ in {1..5}; do
    instance_id="$(curl --fail --silent http://169.254.169.254/latest/meta-data/instance-id)"

    [ 0 == "$?" ] && break

    sleep 2
done;

# If our current hostname already contains our instance ID
if [[ "${OLD_HOSTNAME}" == *"${instance_id}"* ]]; then
    # No need to change anything
    echo "Hostname unchanged: '$(hostname)' '${OLD_HOSTNAME}'"
    exit 0
fi

# Create our new hostname from our format and instance id
new_hostname="$(printf -- "$HOSTNAME_FORMAT" "$instance_id")"
fqdn_new_hostname="$(printf -- "$FQDN_HOSTNAME_FORMAT" "$new_hostname")"

# Replace our hostname as defined in our network config
sed -i "s/HOSTNAME=.*/HOSTNAME=${new_hostname}/g" /etc/sysconfig/network

# Set the hostname through the CLI for temporary resolution
hostname "${new_hostname}"

# Add a hosts file DNS definition
sed -i "s/\(\(127\.0\.0\.1\|::1\)\s\+\)\(.*\)/\1 ${fqdn_new_hostname} ${new_hostname} \3/g" /etc/hosts

# Restart our networking service
service network restart

# Report our success
echo "New hostname: '${new_hostname}' '${fqdn_new_hostname}'"
exit 0