Leveraging null_resource in Terraform for Complex Operations

Terraform Resources 101 (Quick Recap)#

Terraform’s core mechanic is the resource block — which tells the provider, “Hey, make this thing exist.”

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resource "azurerm_windows_function_app" "app" {
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  name     = "example-function-app"
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  location = "East US"
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}

That’s your bread-and-butter declaration: define the desired state, let Terraform do the heavy lifting.

But sometimes, you don’t want to create anything in the cloud. You just want Terraform to do something — run a script, call a webhook, poke Jenkins with a stick. That’s where null_resource comes in.

What the Hell is null_resource?#

null_resource is exactly what it sounds like: a Terraform resource that manages nothing. No infrastructure, no API objects — just logic.

But here’s the trick: it still behaves like a real resource. It supports lifecycle actions (create, destroy, etc.), can depend on other resources, and — most importantly — supports provisioners and triggers.

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resource "null_resource" "example" {
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  triggers = {
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    always_run = timestamp()
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  }
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  provisioner "local-exec" {
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    command = "echo 'Triggering follow-up actions'"
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  }
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}

This block runs every time because the timestamp() changes on every plan. Perfect for when you need to kick off external processes after infra changes.

Triggers: The Secret Sauce#

Triggers are the magic behind null_resource. They control when it gets re-executed — not based on resource state, but on changes to arbitrary data.

Example:

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triggers = {
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  hash = filemd5("config.json")
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}

Now your null_resource only re-runs when config.json changes. This is gold in CI/CD setups where you’re tracking file changes, API responses, or even environment variables.

Real-World Scenarios (Where null_resource Actually Earns Its Keep)#

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resource "null_resource" "notify" {
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  triggers = {
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    infra_version = var.release_tag
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  }
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  provisioner "local-exec" {
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    command = "curl -X POST https://hooks.slack.com/services/XXX -d 'Terraform apply complete: ${var.release_tag}'"
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  }
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}

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data "azurerm_storage_account" "example" {
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  name                = "examplestorageaccount"
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  resource_group_name = "my-rg"
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}
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resource "null_resource" "trigger_on_key_change" {
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  triggers = {
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    key = data.azurerm_storage_account.example.primary_access_key
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  }
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  provisioner "local-exec" {
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    command = "echo 'Access key has changed, executing...'"
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  }
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}

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resource "null_resource" "smoke_test" {
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  depends_on = [azurerm_function_app.example]
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  provisioner "local-exec" {
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    command = "curl -sf http://example-app.azurewebsites.net/health || exit 1"
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  }
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}

The Cleaner Alternative: terraform_data (1.4+)#

Starting in Terraform 1.4, HashiCorp added a new built-in: terraform_data. It does what null_resource does — just with less baggage and without relying on a provider plugin.

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resource "terraform_data" "run_command" {
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  provisioner "local-exec" {
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    command = "echo 'Still works!'"
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  }
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}

You lose triggers (for now), but it’s first-party and cleaner for one-off tasks. Ideal for scripting or injecting data during apply without pretending to be a cloud resource.

null_resource vs terraform_data: When to Use What#

TL;DR: If you need triggers, stick with null_resource. Otherwise, migrate to terraform_data as the cleaner, future-proof alternative.

Final Thoughts#

If Terraform were a programming language, null_resource would be its eval() — powerful, dangerous, and misused 99% of the time.

But in the hands of someone who knows what they’re doing? It bridges the gap between infrastructure and orchestration — especially in CI/CD pipelines, edge cases, or situations where Terraform alone can’t model reality.

Just don’t go overboard. If you find yourself writing a bash script inside a null_resource that runs a Python script that generates more HCL… maybe reconsider your life choices.

Pro Tip: Pair null_resource with depends_on and triggers for controlled chaos. Or use terraform_data if you want to sleep at night.

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