User Data (Startup Script) Documentation For VM on Spot

Overview

User Data allows you to pass a startup script when creating a virtual machine (VM). This script runs automatically during the first boot of the instance and is typically used to:

• Install packages and dependencies
• Configure applications and services
• Set up environments (e.g., web servers, agents)
• Perform initial system provisioning

This helps automate instance setup and reduces manual configuration.

How It Works

• User Data is provided at the time of VM creation.
• The script runs once during the initial boot.
• It executes with root privileges.
• The script is non-interactive, so it must not require user input.

Supported Format

Only shell scripts are supported.

Your script must start with a shebang:

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#!/bin/bash

Example: Basic User Data Script For Ubuntu

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#!/bin/bash

# Update system packages

apt-get update -y

# Install Apache web server

apt-get install -y apache2

# Start and enable Apache

systemctl enable apache2

systemctl start apache2

# Create a simple web page

echo "Hello from Spot VM" > /var/www/html/index.html

# Log execution

mkdir -p /opt/rackspace-userdata-test

echo "Userdata execution confirmed at: $(date)" > /opt/rackspace-userdata-test/success.log

# Append a login banner to the system profile

echo "echo '======================================================'" >> /etc/profile

echo "echo '  USERDATA SCRIPT RAN SUCCESSFULLY      !!!!!!!!!     '" >> /etc/profile

echo "echo '======================================================'" >> /etc/profile

Example: Basic User Data Script For Rocky

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#!/bin/bash

dnf install -y httpd

systemctl enable httpd

systemctl start httpd

Execution Behaviour

• Runs automatically when the VM boots for the first time
• Runs as root user at boot time
• Does not run again on reboot

Best Practices:

1. Handle non-critical failures safely:

Ensure your script can run safely without breaking if partially executed. Use || true only when failure is acceptable.

Example:

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rm file.txt || true

2. Avoid Interactive Commands:

Do not use commands that require user input.

Example:

✔ Correct:

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apt-get install -y nginx

✘ Incorrect:

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apt-get install nginx

3. Add Logging for Debugging:

Redirect output to a log file:

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#!/bin/bash

exec > /var/log/user-data.log 2>&1

set -x

4. Handle Failures Gracefully:

Use error handling where needed:

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if !systemctl start apache2; then

 echo "Failed to start apache2" >> /var/log/user-data.log

fi

5. Keep Script Lightweight:

Large scripts increase boot time. For complex setups:

• Download scripts from a remote source
• Or use configuration management tools

Limitations

• Script size may be limited (platform-dependent)
• Runs only once during initial boot
• No interactive input supported
• Execution time impacts instance startup time

Advanced Usage Download and Execute External Script

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#!/bin/bash

set -e

curl -f -o setup.sh https://example.com/setup.sh

chmod +x setup.sh

./setup.sh

Install and Configure Docker

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#!/bin/bash

set -e

apt-get update -y

apt-get install -y docker.io

systemctl start docker

systemctl enable docker

usermod -aG docker ubuntu

Common Issues

Security Considerations

• Do not hardcode secrets (API keys, passwords) in user data
• Scripts are visible via instance metadata depending on platform configuration

Summary

User Data is a simple and powerful way to automate VM initialization. By using shell scripts, you can ensure consistent, repeatable, and fast provisioning of infrastructure.