Google Startup Scripts

Google provides a set of startup scripts that interact with the virtual machine environment. On boot, the startup script /usr/share/google/onboot queries the instance metadata for a user-provided startup script to run. User-provided startup scripts can be specified in the instance metadata under startup-script or, if the metadata is in a small script or a downloadable file, it can be specified via startup-script-url. You can use gcloud compute or the Google Compute Engine API to specify a startup script.

For more information on how to use startup scripts, read the Using Start Up Scripts documentation.

Below is an example of metadata that indicates a startup script URL and a startup script file was passed to the instance:

{ // instance
  metadata: {
    "kind": "compute#metadata",
    "items": [
    {
      "key": "startup-script-url",
      "value": "http://startup-script-url:
    }
  ]
 }
}
{  // instance
  metadata: {
    "kind": "compute#metadata",
    "items": [
    {
       "key": "startup-script",
       "value": "#! /bin/python\nprint ‘startup’\n"
    }
  ]
 }
}

Google startup scripts also perform the following actions:

• Checks the value of the instance id key

  Startup scripts check the value of the instance ID at:

    http://169.254.169.254/computeMetadata/v1/instance/id
  

  and compares it to the last instance ID the disk booted on.

• Sets the hostname from the metadata server via DHCP exit hooks.

• Updates gsutil authentication.

  Startup scripts run /usr/share/google/boto/boot_setup.py which configures and copies /usr/share/google/boto/boto_plugins/compute_auth.py into the boto plugin directory.

• Provides udev rules to give friendly names to disks.

  Google Compute Engine provides /lib/udev/rules.d/65-gce-disk-naming.rules in our images.

• Safely formats persistent disks via /usr/share/google/safe_format_and_mount.