What is the Purpose of a null_resource in Terraform?

A null_resource is a special Terraform resource that does not manage any real cloud infrastructure. Instead, it acts as a placeholder to trigger side effects — like running scripts or commands — at the right point in the Terraform lifecycle.

When to Use null_resource

Use it when you need to:

Run a local or remote script after certain infrastructure is ready.
Execute a command that does not map to a Terraform resource.
Trigger actions based on changes to other resources.

Basic Example: Run a Local Script

resource "null_resource" "run_setup_script" {
  provisioner "local-exec" {
    command = "bash ./scripts/setup.sh"
  }

  depends_on = [aws_instance.web]
}

This runs setup.sh on your local machine after the EC2 instance is created.

Using triggers to Re-run on Change

By default, a null_resource only runs once. Use triggers to re-execute it when a value changes:

resource "null_resource" "redeploy_app" {
  triggers = {
    app_version = var.app_version
    instance_id = aws_instance.web.id
  }

  provisioner "local-exec" {
    command = "ansible-playbook -i '${aws_instance.web.public_ip},' deploy.yml"
  }
}

Every time app_version or the instance ID changes, this resource will be destroyed and recreated, triggering the provisioner again.

Remote Execution with remote-exec

resource "null_resource" "configure_server" {
  connection {
    type        = "ssh"
    user        = "ubuntu"
    private_key = file("~/.ssh/id_rsa")
    host        = aws_instance.web.public_ip
  }

  provisioner "remote-exec" {
    inline = [
      "sudo apt-get update -y",
      "sudo apt-get install -y nginx",
      "sudo systemctl start nginx"
    ]
  }

  depends_on = [aws_instance.web]
}

Modern Alternative: terraform_data (Terraform 1.4+)

# null_resource is being replaced by terraform_data in newer versions
resource "terraform_data" "run_script" {
  triggers_replace = [var.app_version]

  provisioner "local-exec" {
    command = "echo Deploying version ${var.app_version}"
  }
}

Key Takeaway

null_resource is your escape hatch for actions Terraform cannot model as a resource. Use it sparingly with provisioners for bootstrapping or triggering scripts. For Terraform 1.4 and above, prefer the built-in terraform_data resource instead.

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What is the Purpose of a null_resource in Terraform?