EC2 User Data

A script passed to an instance at launch and executed by cloud-init on first boot, as root. Used for bootstrapping — install packages, fetch config, register with a fleet.

• Runs once by default (first boot only). Re-run on every boot with a cloud-init directive or [scripts-user, always].
• Size limit 16 KB (before base64 encoding) — for anything bigger, bootstrap a fetch from S3.
• Output/logs land in /var/log/cloud-init-output.log — first place to look when a launch “did nothing”.
• Retrieve at runtime from IMDS: http://169.254.169.254/latest/user-data (prefer IMDSv2, token-based).

Don't hand-bake AMIs

For repeatable images use EC2 Image Builder or Packer; keep user data for last-mile, environment-specific config.

EC2 Instance SSH

ssh -i "x.pem" ec2-user@ip

User Data Script Example

#!/bin/bash
yum update -y
yum install httpd php php-mysql -y
chkconfig httpd on
service httpd start
echo "<?php phpinfo();?>" > /var/www/html/index.php
cd /var/www/html
wget https://s3.amazonaws.com/acloudguru-production/connect.php

# Inspect / re-fetch user data from inside the instance (IMDSv2)
TOKEN=$(curl -sX PUT "http://169.254.169.254/latest/api/token" \
  -H "X-aws-ec2-metadata-token-ttl-seconds: 600")
curl -s -H "X-aws-ec2-metadata-token: $TOKEN" \
  http://169.254.169.254/latest/user-data