Cloud Alchemist
~/blog/terraform $ cat terraform-azurerm-compute-custom-data.md
title: Passing Custom Data to Terraform Azure RM Compute Module
date:
categories: terraform
tags: terraform , azure
reading: 2 min

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:

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

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