Input variables

In Terraform, input variables allow you to parameterize your configurations. This makes your Terraform configurations more flexible and reusable by allowing users to define values externally, without hardcoding them in the configuration files.

Here’s a detailed explanation of how to use input variables in Terraform, including examples:

Defining Input Variables

Creating a variables.tf File

You define input variables in a separate file, commonly named variables.tf. Each variable is defined using the variable block.

 variable "vpc_cidr" {
   description = "The CIDR block for the VPC"
   type        = string
   default     = "10.0.0.0/16"
 }

 variable "public_subnet_cidr" {
   description = "The CIDR block for the public subnet"
   type        = string
 }

 variable "private_subnet_cidr" {
   description = "The CIDR block for the private subnet"
   type        = string
 }

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

Using Input Variables

Referencing Variables in Configuration Files

Use the defined variables in your resource configurations by referencing them with var.<variable_name>.

 provider "aws" {
   region = var.region
 }

 resource "aws_vpc" "main" {
   cidr_block = var.vpc_cidr

   tags = {
     Name = "main-vpc"
   }
 }

 resource "aws_subnet" "public" {
   vpc_id     = aws_vpc.main.id
   cidr_block = var.public_subnet_cidr

   tags = {
     Name = "public-subnet"
   }
 }

 resource "aws_subnet" "private" {
   vpc_id     = aws_vpc.main.id
   cidr_block = var.private_subnet_cidr

   tags = {
     Name = "private-subnet"
   }
 }

Providing Values for Variables

Using a terraform.tfvars File

Create a terraform.tfvars file to assign values to your variables.

 vpc_cidr           = "10.0.0.0/16"
 public_subnet_cidr = "10.0.1.0/24"
 private_subnet_cidr = "10.0.2.0/24"
 region             = "us-west-2"

Using Command-Line Flags

You can also specify variable values directly when running the terraform apply command.

 terraform apply -var="vpc_cidr=10.0.0.0/16" -var="public_subnet_cidr=10.0.1.0/24" -var="private_subnet_cidr=10.0.2.0/24"

Using Environment Variables

Set environment variables using the TF_VAR_ prefix.

 export TF_VAR_vpc_cidr="10.0.0.0/16"
 export TF_VAR_public_subnet_cidr="10.0.1.0/24"
 export TF_VAR_private_subnet_cidr="10.0.2.0/24"
 terraform apply

Example

Here’s a complete example demonstrating the use of input variables in a Terraform configuration:

variables.tf

 variable "vpc_cidr" {
   description = "The CIDR block for the VPC"
   type        = string
   default     = "10.0.0.0/16"
 }

 variable "public_subnet_cidr" {
   description = "The CIDR block for the public subnet"
   type        = string
 }

 variable "private_subnet_cidr" {
   description = "The CIDR block for the private subnet"
   type        = string
 }

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

main.tf

 provider "aws" {
   region = var.region
 }

 resource "aws_vpc" "main" {
   cidr_block = var.vpc_cidr

   tags = {
     Name = "main-vpc"
   }
 }

 resource "aws_subnet" "public" {
   vpc_id     = aws_vpc.main.id
   cidr_block = var.public_subnet_cidr

   tags = {
     Name = "public-subnet"
   }
 }

 resource "aws_subnet" "private" {
   vpc_id     = aws_vpc.main.id
   cidr_block = var.private_subnet_cidr

   tags = {
     Name = "private-subnet"
   }
 }

terraform.tfvars

 public_subnet_cidr = "10.0.1.0/24"
 private_subnet_cidr = "10.0.2.0/24"

Running Terraform

terraform init
terraform apply

This setup ensures that you can easily manage your Terraform configurations by changing variable values without modifying the configuration files themselves, promoting better modularity and reusability.

Output variables

In Terraform, output variables are used to display information about your infrastructure after it's been applied. These outputs can be useful for retrieving values like instance IDs, IP addresses, or other resource attributes that might be needed for further automation or integration.

Here’s an overview of how to define and use output variables in Terraform:

How to Define an Output Variable

You define an output variable in a separate file (typically outputs.tf) or at the end of your main.tf file. Here's the general syntax:

output "output_name" {
  value = resource_attribute_or_expression
}

Example of Output Variables

Let's add some output variables to the project we've been working on. We’ll output the VPC ID, public subnet ID, private subnet ID, and the public and private EC2 instance IDs.

`outputs.tf`

output "vpc_id" {
  description = "The ID of the VPC"
  value       = aws_vpc.dev_vpc.id
}

output "public_subnet_id" {
  description = "The ID of the public subnet"
  value       = aws_subnet.public_subnet.id
}

output "private_subnet_id" {
  description = "The ID of the private subnet"
  value       = aws_subnet.private_subnet.id
}

output "public_instance_id" {
  description = "The ID of the public EC2 instance"
  value       = aws_instance.public_instance.id
}

output "private_instance_id" {
  description = "The ID of the private EC2 instance"
  value       = aws_instance.private_instance.id
}

output "public_instance_ip" {
  description = "The public IP address of the public EC2 instance"
  value       = aws_instance.public_instance.public_ip
}

output "private_instance_ip" {
  description = "The private IP address of the private EC2 instance"
  value       = aws_instance.private_instance.private_ip
}

Explanation of the Outputs

VPC ID:

 output "vpc_id" {
   description = "The ID of the VPC"
   value       = aws_vpc.dev_vpc.id
 }

Outputs the ID of the created VPC.

Public Subnet ID:

 output "public_subnet_id" {
   description = "The ID of the public subnet"
   value       = aws_subnet.public_subnet.id
 }

Outputs the ID of the public subnet.

Private Subnet ID:

 output "private_subnet_id" {
   description = "The ID of the private subnet"
   value       = aws_subnet.private_subnet.id
 }

Outputs the ID of the private subnet.

Public Instance ID:

 output "public_instance_id" {
   description = "The ID of the public EC2 instance"
   value       = aws_instance.public_instance.id
 }

Outputs the ID of the public EC2 instance.

Private Instance ID:

 output "private_instance_id" {
   description = "The ID of the private EC2 instance"
   value       = aws_instance.private_instance.id
 }

Outputs the ID of the private EC2 instance.

Public Instance IP:

 output "public_instance_ip" {
   description = "The public IP address of the public EC2 instance"
   value       = aws_instance.public_instance.public_ip
 }

Outputs the public IP address of the public EC2 instance.

Private Instance IP:

 output "private_instance_ip" {
   description = "The private IP address of the private EC2 instance"
   value       = aws_instance.private_instance.private_ip
 }

Outputs the private IP address of the private EC2 instance.

Usage

After you apply your Terraform configuration with terraform apply, Terraform will output the specified values. This can be very helpful for debugging, integration with other scripts, or just verifying that your infrastructure was created as expected.

Example Output

When you run terraform apply, the output might look something like this:

Apply complete! Resources: X added, Y changed, Z destroyed.

Outputs:

private_instance_id = "i-0a1b2c3d4e5f6g7h8"
private_instance_ip = "10.0.20.123"
private_subnet_id = "subnet-0a1b2c3d4e5f6g7h8"
public_instance_id = "i-1a2b3c4d5e6f7g8h9"
public_instance_ip = "54.210.123.456"
public_subnet_id = "subnet-1a2b3c4d5e6f7g8h9"
vpc_id = "vpc-0a1b2c3d4e5f6g7h8"

This makes it easy to see the IDs and IP addresses of your resources without having to manually look them up in the AWS Management Console.

Local Variables

Local variables in Terraform provide a way to simplify and manage your configuration by creating reusable values that can be referenced throughout your Terraform files. They help to make your configuration more modular, readable, and maintainable.

Defining Local Variables

Local variables are defined using the locals block. Here’s a simple example:

locals {
  instance_type = "t2.micro"
  environment   = "dev"
  ami_id        = "ami-0c55b159cbfafe1f0"
}

Using Local Variables

Once defined, you can reference local variables using the local keyword followed by the variable name. Here’s how you can use the local variables in your Terraform configuration:

# Using local variables in an aws_instance resource
resource "aws_instance" "example" {
  ami           = local.ami_id
  instance_type = local.instance_type

  tags = {
    Name = "Example-${local.environment}"
  }
}

This example showcasing the use of local variables in a different Terraform configuration. This example will set up an S3 bucket with versioning and server-side encryption enabled, and a DynamoDB table.

Configuration with Local Variables

locals.tf

locals {
  region           = "us-west-2"
  bucket_name      = "my-unique-bucket-name"
  dynamodb_table   = "my-dynamodb-table"
  environment      = "production"
  versioning       = true
  sse_algorithm    = "AES256"
  read_capacity    = 5
  write_capacity   = 5
  tags = {
    Environment = local.environment
    ManagedBy   = "Terraform"
  }
}

provider.tf

provider "aws" {
  region = local.region
}

s3.tf

# Create S3 bucket
resource "aws_s3_bucket" "bucket" {
  bucket = local.bucket_name
  tags   = local.tags
}

# Enable versioning on the S3 bucket
resource "aws_s3_bucket_versioning" "versioning" {
  bucket = aws_s3_bucket.bucket.id

  versioning_configuration {
    status = local.versioning ? "Enabled" : "Suspended"
  }
}

# Enable server-side encryption on the S3 bucket
resource "aws_s3_bucket_server_side_encryption_configuration" "sse" {
  bucket = aws_s3_bucket.bucket.bucket

  rule {
    apply_server_side_encryption_by_default {
      sse_algorithm = local.sse_algorithm
    }
  }
}

dynamodb.tf

# Create DynamoDB table
resource "aws_dynamodb_table" "table" {
  name           = local.dynamodb_table
  billing_mode   = "PROVISIONED"
  read_capacity  = local.read_capacity
  write_capacity = local.write_capacity
  hash_key       = "ID"

  attribute {
    name = "ID"
    type = "S"
  }

  tags = local.tags
}

Explanation

locals.tf:
- Defines various local variables for region, bucket name, DynamoDB table name, environment, versioning, server-side encryption algorithm, read and write capacities, and common tags.
provider.tf:
- Configures the AWS provider to use the specified region from local variables.
s3.tf:
- Uses local variables to create an S3 bucket with versioning and server-side encryption enabled.
- aws_s3_bucket: Creates the S3 bucket with a name from local variables and tags.
- aws_s3_bucket_versioning: Enables or disables versioning on the bucket based on the local variable.
- aws_s3_bucket_server_side_encryption_configuration: Sets up server-side encryption using the specified algorithm.
dynamodb.tf:
- Uses local variables to create a DynamoDB table with the specified read and write capacities, and tags.
- aws_dynamodb_table: Defines the table with attributes and capacities based on local variables.

Benefits of Using Local Variables

Consistency: Ensures consistent values across your configuration.
Ease of Updates: Changing a value in one place updates it everywhere it is used.
Readability: Makes your configuration easier to read and understand by abstracting frequently used values.

Local variables are a powerful feature in Terraform that help to keep your configuration DRY (Don't Repeat Yourself), making it more manageable and less error-prone.

Terraform Variables

Unlocking Terraform's Power: Harnessing the Magic of Input, Output & Local Variables.

Table of contents