Why provisioners run scripts on resources in Terraform - See It in Action

Why provisioners run scripts on resources

📖 Scenario: You are setting up a virtual machine in the cloud. After creating the machine, you want to install software automatically on it. This helps save time and ensures the machine is ready to use right after creation.

🎯 Goal: Learn how to use Terraform provisioners to run scripts on cloud resources after they are created.

📋 What You'll Learn

Create a virtual machine resource

Add a provisioner to run a script on the virtual machine

Use a local-exec provisioner to run a simple command

Use a remote-exec provisioner to run commands on the machine

💡 Why This Matters

🌍 Real World

Automating software installation and configuration on cloud servers right after they are created saves time and reduces manual errors.

💼 Career

Cloud engineers and DevOps professionals use provisioners to automate resource setup, making deployments faster and more reliable.

1

Create a virtual machine resource

Write Terraform code to create an aws_instance resource named example with the AMI ID ami-0c55b159cbfafe1f0 and instance type t2.micro.

Terraform

# Your code here to create aws_instance example

Hint

Use the resource block with type aws_instance and name example. Set ami and instance_type exactly as given.

2

Add a local-exec provisioner to run a script locally

Add a provisioner block inside the aws_instance.example resource that uses local-exec to run the command echo 'Instance created'.

Terraform

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  # Add local-exec provisioner here
}

Hint

Inside the resource, add provisioner "local-exec" with a command that echoes the text.

3

Add a remote-exec provisioner to run commands on the instance

Add a provisioner block inside aws_instance.example that uses remote-exec to run the command sudo apt-get update. Use connection block with type = "ssh", user = "ubuntu", and private_key = file("~/.ssh/id_rsa").

Terraform

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  provisioner "local-exec" {
    command = "echo 'Instance created'"
  }

  # Add remote-exec provisioner here
}

Hint

Use provisioner "remote-exec" with inline commands and a connection block for SSH details.

4

Complete the resource with both provisioners

Ensure the aws_instance.example resource includes both the local-exec provisioner that runs echo 'Instance created' and the remote-exec provisioner that runs sudo apt-get update with the correct SSH connection settings.

Terraform

resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"

  provisioner "local-exec" {
    command = "echo 'Instance created'"
  }

  provisioner "remote-exec" {
    inline = ["sudo apt-get update"]

    connection {
      type        = "ssh"
      user        = "ubuntu"
      private_key = file("~/.ssh/id_rsa")
    }
  }

  # Finalize the resource here if needed
}

Hint

Check that both provisioners are inside the resource with correct commands and connection details.

Practice

1. Why do Terraform provisioners run scripts on resources?

easy

A. To backup resource data before creation

B. To delete resources automatically

C. To configure resources after they are created

D. To monitor resource usage continuously

Solution

1. Step 1: Understand provisioners' purpose

   Provisioners run scripts to configure resources after Terraform creates them.

2. Step 2: Identify correct use case

   They help install software or run commands Terraform cannot manage directly.

3. Final Answer:

   To configure resources after they are created -> Option C

4. Quick Check:

   Provisioners configure resources post-creation = D [OK]

Hint: Provisioners run scripts after resource creation [OK]

Common Mistakes:

• Thinking provisioners delete resources
• Assuming provisioners monitor resources
• Believing provisioners backup data

2. Which of the following is the correct syntax to use a local-exec provisioner in Terraform?

easy

A. provisioner "local-exec" { command = "echo Hello" }

B. provisioner local_exec { run = "echo Hello" }

C. provisioner "local-exec" (command: "echo Hello")

D. provisioner local-exec { execute = "echo Hello" }

Solution

1. Step 1: Recall correct provisioner block syntax

   Terraform uses provisioner blocks with type in quotes and a command attribute.

2. Step 2: Match syntax to options

   provisioner "local-exec" { command = "echo Hello" } matches the correct syntax: provisioner "local-exec" { command = "echo Hello" }.

3. Final Answer:

   provisioner "local-exec" { command = "echo Hello" } -> Option A

4. Quick Check:

   Correct provisioner syntax uses quotes and command attribute = A [OK]

Hint: Use quotes and command attribute for local-exec provisioner [OK]

Common Mistakes:

• Omitting quotes around provisioner type
• Using wrong attribute names like run or execute
• Using parentheses instead of braces

3. Given this Terraform snippet, what will happen when the resource is created?

resource "null_resource" "example" {
  provisioner "local-exec" {
    command = "echo Resource created"
  }
}

medium

A. Terraform will fail because null_resource cannot have provisioners

B. Terraform will print 'Resource created' after creating the resource

C. Terraform will create the resource but not run the command

D. Terraform will run the command before creating the resource

Solution

1. Step 1: Understand null_resource and local-exec provisioner

   null_resource supports provisioners; local-exec runs commands on the machine running Terraform after resource creation.

2. Step 2: Analyze command execution timing

   The command 'echo Resource created' runs after the resource is created, printing the message.

3. Final Answer:

   Terraform will print 'Resource created' after creating the resource -> Option B

4. Quick Check:

   local-exec runs command post-creation = A [OK]

Hint: local-exec runs commands after resource creation [OK]

Common Mistakes:

• Thinking null_resource disallows provisioners
• Assuming commands run before resource creation
• Believing commands won't run at all

4. You wrote this provisioner block but Terraform shows an error:

provisioner "remote-exec" {
  command = "sudo apt-get update"
}

What is the likely cause of the error?

medium

A. Provisioners cannot run sudo commands

B. The command attribute is invalid for remote-exec

C. remote-exec provisioner requires a local script file

D. Missing connection block to specify how to connect to the resource

Solution

1. Step 1: Check remote-exec requirements

   remote-exec provisioner needs a connection block to connect to the resource (SSH or WinRM).

2. Step 2: Identify missing connection block

   The snippet lacks a connection block, causing Terraform to error because it cannot connect to run the command.

3. Final Answer:

   Missing connection block to specify how to connect to the resource -> Option D

4. Quick Check:

   remote-exec needs connection block = C [OK]

Hint: Always add connection block for remote-exec provisioners [OK]

Common Mistakes:

• Assuming command attribute is invalid
• Thinking sudo commands are disallowed
• Believing remote-exec runs local scripts only

5. You want to install software on a new VM after Terraform creates it. Which approach best uses provisioners to do this reliably?

hard

A. Use a remote-exec provisioner with a connection block to run installation commands after creation

B. Use a local-exec provisioner to run installation commands on your local machine

C. Run installation commands manually after Terraform finishes

D. Use a provisioner without connection block to run commands remotely

Solution

1. Step 1: Identify provisioner type for remote VM configuration

   remote-exec provisioner runs commands on the remote VM after creation using a connection block.

2. Step 2: Evaluate options for reliability

   Using remote-exec with connection block automates installation reliably; local-exec runs locally, manual runs are error-prone, and missing connection causes failure.

3. Final Answer:

   Use a remote-exec provisioner with a connection block to run installation commands after creation -> Option A

4. Quick Check:

   remote-exec + connection block = reliable remote config [OK]

Hint: Use remote-exec with connection block for remote installs [OK]

Common Mistakes:

• Using local-exec for remote installs
• Skipping connection block for remote-exec
• Relying on manual post-creation steps