When troubleshooting cloud-init initialization on cloud instances (AWS EC2, Azure, GCP etc.), the logging system can initially seem fragmented. Unlike traditional services that log to a single file, cloud-init employs a multi-file approach that captures different stages of the boot process.

Cloud-init stores logs across several files in /var/log/:

/var/log/cloud-init.log         # Main execution log (often truncated)
/var/log/cloud-init-output.log  # Full output of user-data scripts
/var/log/boot.log               # System boot messages including early cloud-init stages

For a complete picture of your instance initialization, examine these files in combination:

# Check all cloud-init related logs:
sudo cat /var/log/cloud-init.log
sudo cat /var/log/cloud-init-output.log

# For timing information:
journalctl -u cloud-init

# Debugging modules:
sudo cat /var/lib/cloud/data/result.json

To get more detailed logging, modify your cloud-init configuration:

# /etc/cloud/cloud.cfg.d/05_logging.cfg
debug: True
log_dir: /var/log/cloud
log_file: cloud-init-debug.log

When debugging a failing user-data script, this command sequence helps pinpoint issues:

# Check if cloud-init completed successfully:
cloud-init status --wait

# View all initialization stages:
cloud-init analyze show

# Filter logs for specific modules:
sudo journalctl -u cloud-init | grep "cc_scripts_user"

For persistent logging across reboots in AWS environments:

# EC2 user-data to preserve logs in S3
#!/bin/bash
exec > >(tee /var/log/user-data.log|logger -t user-data -s 2>/dev/console) 2>&1
apt-get update
apt-get install -y awscli
aws s3 cp /var/log/cloud-init-output.log s3://your-bucket/${INSTANCE_ID}-logs/

When debugging instance initialization on cloud platforms like AWS EC2, DigitalOcean, or Azure, cloud-init generates logs in multiple locations:

/var/log/cloud-init.log
/var/log/cloud-init-output.log
/run/cloud-init/
/var/lib/cloud/instance/

The key log files contain different stages of initialization:

• cloud-init.log: Core initialization process and module execution
• cloud-init-output.log: Combined stdout/stderr from user-data scripts
• /var/lib/cloud/instance/: Contains user-data, meta-data, and processed configs

Verify if cloud-init completed successfully:

cloud-init status
# Expected output: status: done

For detailed analysis of execution phases:

cloud-init analyze show

When troubleshooting user-data scripts:

1. Check timestamps in logs for execution duration
2. Look for error markers in cloud-init-output.log
3. Verify cloud-init modules executed as expected

To locate output from your custom user-data script:

grep "user-data" /var/log/cloud-init.log
# Or examine the full output:
cat /var/log/cloud-init-output.log

For verbose logging, create /etc/cloud/cloud.cfg.d/05_logging.cfg:

# Debug level logging
debug: True
log_dir: /var/log/cloud-init
log_file: cloud-init-debug.log

• Log rotation deleting older entries
• Different cloud platforms having varying default log locations
• Permission issues preventing log access