change-hostname.sh

#!/bin/bash
# Change hostname on Ubuntu/Debian system
# Run with: sudo bash change-hostname.sh new-hostname

set -e

# Check if running as root
if [ "$EUID" -ne 0 ]; then 
    echo "Please run with sudo: sudo bash $0 <new-hostname>"
    exit 1
fi

# Check if hostname provided
if [ -z "$1" ]; then
    echo "Usage: sudo bash $0 <new-hostname>"
    echo "Example: sudo bash $0 dk-u-dev-3"
    exit 1
fi

NEW_HOSTNAME="$1"
OLD_HOSTNAME=$(hostname)

# Validate hostname
if [[ ! "$NEW_HOSTNAME" =~ ^[a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?$ ]]; then
    echo "Error: Invalid hostname. Use only letters, numbers, and hyphens."
    echo "Hostname must start with a letter or number."
    exit 1
fi

echo "Changing hostname from '$OLD_HOSTNAME' to '$NEW_HOSTNAME'"

# Update hostname file
echo "$NEW_HOSTNAME" > /etc/hostname

# Update hosts file
if grep -q "127.0.1.1" /etc/hosts; then
    # Replace existing 127.0.1.1 entry
    sed -i "s/127.0.1.1.*/127.0.1.1\t$NEW_HOSTNAME/" /etc/hosts
else
    # Add 127.0.1.1 entry if it doesn't exist
    echo "127.0.1.1\t$NEW_HOSTNAME" >> /etc/hosts
fi

# Also update any occurrence of old hostname in hosts file
sed -i "s/\b$OLD_HOSTNAME\b/$NEW_HOSTNAME/g" /etc/hosts

# Update current session hostname
hostname "$NEW_HOSTNAME"

# Update systemd if available
if command -v hostnamectl &> /dev/null; then
    hostnamectl set-hostname "$NEW_HOSTNAME"
fi

# Update cloud-init if present (for cloud images)
if [ -f /etc/cloud/cloud.cfg ]; then
    if grep -q "preserve_hostname: false" /etc/cloud/cloud.cfg; then
        sed -i 's/preserve_hostname: false/preserve_hostname: true/' /etc/cloud/cloud.cfg
    elif ! grep -q "preserve_hostname:" /etc/cloud/cloud.cfg; then
        echo "preserve_hostname: true" >> /etc/cloud/cloud.cfg
    fi
fi

echo "✓ Hostname changed successfully!"
echo ""
echo "Changes made:"
echo "- /etc/hostname updated"
echo "- /etc/hosts updated"
echo "- Current session hostname updated"
[ -f /etc/cloud/cloud.cfg ] && echo "- Cloud-init configured to preserve hostname"
echo ""
echo "Please reboot for all changes to take effect: sudo reboot"