user-data

The user-data file is a core component of cloud-init, used to customize and configure cloud instances during their first boot. It allows users to inject scripts, configuration, or even full YAML-formatted cloud-init directives into cloud instances to automate provisioning tasks.

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Purpose of user-data

user-data enables automation of initial configuration such as:

• Installing software packages

• Creating users and setting SSH keys

• Configuring network interfaces

• Mounting volumes

• Running custom shell scripts or configuration management tools (e.g., Ansible, Puppet)

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Supported Formats

cloud-init supports various formats for user-data. The most common include:

1. Shell Scripts

#!/bin/bash
echo "Hello from user-data script" > /home/ubuntu/welcome.txt

Must start with a shebang (#!/bin/bash).

2. Cloud-Config (YAML)

#cloud-config
users:
  - name: devuser
    groups: sudo
    shell: /bin/bash
    sudo: ['ALL=(ALL) NOPASSWD:ALL']
    ssh-authorized-keys:
      - ssh-rsa AAAAB3...

packages:
  - nginx
  - git

runcmd:
  - [ systemctl, start, nginx ]

Starts with #cloud-config.

3. Multi-Part MIME Archives

For combining shell scripts, cloud-config, and other formats.

Content-Type: multipart/mixed; boundary="===============123=="
MIME-Version: 1.0

--===============123==
Content-Type: text/cloud-config

#cloud-config
hostname: myserver

--===============123==
Content-Type: text/x-shellscript

#!/bin/bash
echo "Configuring server" > /tmp/setup.log

--===============123==--

Generated using tools like cloud-init devel make-mime.

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How to Provide user-data

• AWS EC2: Via --user-data parameter in CLI or through the console.

• OpenStack: Through Horizon UI or nova boot --user-data.

• Vagrant: By placing it in the Vagrantfile under config.vm.provision.

Example with Vagrant:

config.vm.provision "file", source: "user-data", destination: "/tmp/user-data"

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Viewing user-data on a Running Instance

cat /var/lib/cloud/instances/$(cloud-init query instance_id)/user-data.txt

Or for EC2:

curl http://169.254.169.254/latest/user-data

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Tips for Writing user-data

• Always validate cloud-config YAML using cloud-init schema --config-file <file>.

• Use cloud-init single --file <file> --name <module> to test modules.

• Avoid putting sensitive data in plain text user-data.

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Conclusion

The user-data file is a powerful and flexible way to automate cloud instance initialization, supporting everything from basic scripting to full-blown configuration management. When properly used, it enables reproducible, hands-free infrastructure provisioning in cloud-native environments.