How to Pass the Real Azure VM Name to Your Linux App—Even When $HOSTNAME Fails

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How to Pass the Real Azure VM Name to Your Linux App—Even When $HOSTNAME Fails

When deploying Linux VMs in the cloud, it’s common to need the actual VM name inside your application’s configuration. You might think, “Easy! Just use the $HOSTNAME environment variable.” But if you’ve ever tried to reference $HOSTNAME in a shell script that’s run by systemctl as a service, you’ve probably discovered it doesn’t always work as expected. Why? Because systemd services don’t always inherit the environment you expect, and sometimes $HOSTNAME is not set or is set incorrectly.

The Problem

Let’s say you have a shell script that configures your application, and you want to inject the real VM name into a config file. If you try something like this in your script:

echo "vm_name=$HOSTNAME" > /etc/myapp/config.ini

It might work when you run the script manually, but when systemd runs it as a service, $HOSTNAME could be empty or wrong. That’s a headache, especially in automated cloud deployments.

The Solution: Let Terraform Do the Heavy Lifting

Instead of relying on the Linux environment, you can use your Infrastructure-as-Code tool—Terraform—to inject the real VM name at deployment time. Here’s how we solved it:

1. Terraform Knows the VM Name

When you define your Azure Linux VM in Terraform, you control the name:

resource "azurerm_linux_virtual_machine" "app" {
  count = var.vm-instance-count
  name  = format("%s-%d", var.app_vm_name, count.index)
  # ...other config...
}

2. Pass the Name to the VM Extension

Azure’s Custom Script Extension lets you run scripts on your VM after provisioning. You can pass the VM name as a parameter:

resource "azurerm_virtual_machine_extension" "custom_script" {
  count              = var.vm-instance-count
  name               = "BuildAndUpgradeScripts"
  virtual_machine_id = azurerm_linux_virtual_machine.app[count.index].id
  publisher          = "Microsoft.Azure.Extensions"
  type               = "CustomScript"
  type_handler_version = "2.1"

  settings = jsonencode({
    "fileUris": [],
    "commandToExecute": <<-EOT
      #!/bin/bash
      /bin/bash /usr/local/bin/build_server.sh "app" "${azurerm_linux_virtual_machine.app[count.index].name}" >> /var/log/custom-script.log 2>&1
    EOT
  })
}

Notice how we use "${azurerm_linux_virtual_machine.app[count.index].name}"—this is the real, provisioned VM name.

3. Use the Name in Your Script

Your shell script can now accept the VM name as a parameter:

#!/bin/bash
APP_NAME=$1
VM_NAME=$2

echo "vm_name=$VM_NAME" > /etc/myapp/config.ini

And when the extension runs, it calls:

/bin/bash /usr/local/bin/build_server.sh "app" "real-vm-name"

No more guessing, no more broken $HOSTNAME references.

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Why This Works

• Terraform knows the VM name at deploy time.
• Azure Custom Script Extension lets you pass parameters directly.
• Your script gets the real VM name, every time, regardless of how or when it’s run.

Privacy Note

All company and environment-specific names have been masked for privacy. This pattern works for any cloud, any app, and any organization.

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Takeaway:
If you need the real VM name inside your Linux app, don’t trust $HOSTNAME—let your IaC tool (like Terraform) pass it as a parameter. It’s reliable, repeatable, and cloud-native.

Happy automating!