When launching Ubuntu-based EC2 instances on AWS, the system automatically creates a default user named 'ubuntu' with UID 1000. While convenient for quick starts, this becomes problematic when:

• Company security policies require standardized usernames
• Automated deployment scripts expect specific user configurations
• You need to maintain consistency across hybrid environments

CloudInit provides several approaches to modify the default user configuration through user-data. Here's the most effective method I've found after extensive testing:

#cloud-config
system_info:
  default_user:
    name: myadmin
    groups: [adm, audio, cdrom, dialout, dip, floppy, netdev, plugdev, sudo, video]
    sudo: ["ALL=(ALL) NOPASSWD:ALL"]
    shell: /bin/bash
    lock_passwd: true
    ssh_authorized_keys:
      - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQ... user@example.com

The system_info.default_user directive completely overrides the default user creation. Important notes:

• This must appear before any other user-data directives
• SSH keys must be specified here as the user won't inherit from the AWS keypair
• Groups should mirror the default Ubuntu user's groups for full functionality

For instances already launched with the default user, you can use this bootcmd approach:

#cloud-config
bootcmd:
  - [ sh, -c, "if id ubuntu; then usermod -l myadmin ubuntu && groupmod -n myadmin ubuntu && mv /home/ubuntu /home/myadmin && sed -i 's/ubuntu/myadmin/g' /etc/sudoers.d/90-cloud-init-users; fi" ]
  - [ sh, -c, "echo 'myadmin ALL=(ALL) NOPASSWD:ALL' > /etc/sudoers.d/91-myadmin" ]

Always verify your configuration with:

cloud-init schema --config-file user-data.yml
cloud-init devel schema --config-file user-data.yml

• Avoid mixing users: and system_info.default_user directives
• Remember to update any automation that assumes the 'ubuntu' username
• Test in a non-production environment first - mistakes can lock you out

For more complex environments, you might need platform-specific configuration:

#cloud-config
variant: aws
system_info:
  default_user:
    name: ec2-user
    groups: [ wheel ]

variant: azure
system_info:
  default_user:
    name: azureuser
    groups: [ sudo ]

When launching Ubuntu instances on AWS, you'll notice the system automatically creates a default user ubuntu with UID 1000. While this works for most cases, there are situations where you need a different default username:

• Security policies requiring non-standard usernames
• Enterprise environments with naming conventions
• Automation scripts expecting specific usernames

CloudInit provides the users directive in cloud-config files that lets you override default user creation. Here's the most effective approach:

#cloud-config
system_info:
  default_user:
    name: myadmin
    groups: [adm, audio, cdrom, dialout, dip, floppy, netdev, plugdev, sudo, video]
    sudo: ["ALL=(ALL) NOPASSWD:ALL"]
    shell: /bin/bash
    lock_passwd: true
    ssh_authorized_keys:
      - ssh-rsa AAAAB3NzaC1... user@host

For a production-ready implementation, combine this with other essential settings:

#cloud-config
system_info:
  default_user:
    name: deploy
    groups: sudo
    sudo: ["ALL=(ALL) NOPASSWD:ALL"]
    ssh_authorized_keys:
      - ssh-rsa AAAAB3NzaC1... user@workstation

users:
  - name: backupadmin
    groups: sudo
    ssh-authorized-keys:
      - ssh-rsa AAAAB3NzaC1... admin@backup
    shell: /bin/bash

disable_root: true
ssh_pwauth: false

When applying this configuration:

• The change must be applied during initial instance launch
• Existing instances require manual user migration
• Test in a non-production environment first
• Combine with IAM roles for complete security

If your user configuration isn't applying:

1. Check CloudInit logs: tail -f /var/log/cloud-init-output.log
2. Verify YAML syntax (spaces, not tabs)
3. Ensure the config is properly passed to the instance
4. Confirm the image supports CloudInit