Passing Custom Data to Terraform Azure RM Compute Module

If you use Azure RM Compute Module from Terraform Registry you were probably thinking of ways to configure VM during provisioning. And there’s a bunch of, one of which is to use Azure Compute module’s custom data to pass configuration script:

 custom_data string

Description: The custom data to supply to the machine. This can be used as a
cloud-init for Linux systems.

Default: ""

As you can see, documentation is really sparse, and if you’ll try to pass couple of commands, they’ll not be executed.

Trying to pass script also doesn’t work:

custom_data         = file("scripts/k8s-manager.sh")

After spending some time, I was able to clarify why this doesn’t work:

• As per documentation of linux virtual machine

custom_data - (Optional) The Base64-Encoded Custom Data which should be used
for this Virtual Machine. Changing this forces a new resource to be created.

• For example, you can set up your script in local variable and pass it encoded:

locals {
  custom_data = <<CUSTOM_DATA
  #!/bin/bash
  echo "Execute your super awesome commands here!"
  CUSTOM_DATA
  }

# Encode and pass you script
custom_data = base64encode(local.custom_data)

• You can also pass your script as file on local filesystem:

# Using filebase64 to encode script
custom_data = filebase64("scripts/k8s-node.sh")

• Make sure that your script(for Linux) starts with shebang #!/bin/sh, otherwise it won’t be processed.

• Your script will be executed as cloud-init, so all rules from cloud-init overview applies.

• You can review cloud-init logs at /var/log/cloud-init.log. It will contain all outputs from your script.

• Finally, for deep troubleshooting, follow cloud-init-troubleshooting guide