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Variables and Outputs

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In this section, we will delve into the concepts of variables and outputs in Terraform. These are fundamental components that help in making your Terraform configurations more dynamic and reusable.

What are Variables?

Variables in Terraform allow you to parameterize your configurations. This means you can define values that can be reused across your configuration files, making them more flexible and easier to manage.

Types of Variables

Terraform supports three types of variables:

  1. String: A single line of text.
  2. Number: A numeric value.
  3. Bool: A boolean value (true or false).

Defining Variables

Variables are defined using the variable block. Here is an example of defining a string variable:

variable "region" {
  description = "The AWS region to deploy resources in"
  type        = string
  default     = "us-west-2"
}

Using Variables

Once defined, you can use variables in your Terraform configuration by referencing them with the var keyword:

provider "aws" {
  region = var.region
}

Variable Files

Variables can also be defined in separate files, typically with a .tfvars extension. This is useful for managing different sets of variables for different environments (e.g., development, staging, production).

Example variables.tf:

variable "region" {
  description = "The AWS region to deploy resources in"
  type        = string
}

Example terraform.tfvars:

region = "us-east-1"

What are Outputs?

Outputs in Terraform are used to extract information from your Terraform state files and make it available for other configurations or for external use.

Defining Outputs

Outputs are defined using the output block. Here is an example of defining an output:

output "instance_id" {
  description = "The ID of the EC2 instance"
  value       = aws_instance.my_instance.id
}

Using Outputs

Outputs can be accessed after running terraform apply by using the terraform output command:

$ terraform output instance_id

Output Sensitivity

You can mark outputs as sensitive to prevent them from being displayed in the CLI output or logs:

output "db_password" {
  description = "The database password"
  value       = aws_db_instance.my_db.password
  sensitive   = true
}

Practical Example

Let's create a simple Terraform configuration that uses variables and outputs.

Step 1: Define Variables

Create a file named variables.tf:

variable "region" {
  description = "The AWS region to deploy resources in"
  type        = string
  default     = "us-west-2"
}

variable "instance_type" {
  description = "The type of EC2 instance"
  type        = string
  default     = "t2.micro"
}

Step 2: Use Variables in Configuration

Create a file named main.tf:

provider "aws" {
  region = var.region
}

resource "aws_instance" "my_instance" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = var.instance_type
}

Step 3: Define Outputs

Add the following to main.tf:

output "instance_id" {
  description = "The ID of the EC2 instance"
  value       = aws_instance.my_instance.id
}

Step 4: Apply Configuration

Run the following commands to apply the configuration:

$ terraform init
$ terraform apply

After the apply is complete, you can view the output:

$ terraform output instance_id

Exercises

Exercise 1: Define and Use a Variable

  1. Define a variable for the EC2 instance's key name in variables.tf.
  2. Use this variable in the aws_instance resource in main.tf.

Solution:

Add to variables.tf:

variable "key_name" {
  description = "The key name for the EC2 instance"
  type        = string
  default     = "my-key"
}

Modify main.tf:

resource "aws_instance" "my_instance" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = var.instance_type
  key_name      = var.key_name
}

Exercise 2: Define and Use an Output

  1. Define an output for the public IP of the EC2 instance in main.tf.

Solution:

Add to main.tf:

output "instance_public_ip" {
  description = "The public IP of the EC2 instance"
  value       = aws_instance.my_instance.public_ip
}

Summary

In this section, we covered the basics of variables and outputs in Terraform. We learned how to define and use variables to make our configurations more dynamic and reusable. We also explored how to define outputs to extract useful information from our Terraform state. These concepts are essential for writing efficient and maintainable Terraform configurations.

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