Introduction

Terraform, renowned for its infrastructure as code (IaC) capabilities, provides flexibility in managing configurations through variables. While default values and input files (like terraform.tfvars) are common ways to set variable values, another powerful method is through command-line assignments. In this comprehensive guide, we'll unravel the intricacies of passing variable values using Terraform commands, exploring various scenarios and shedding light on best practices.

1. Understanding Command-Line Variable Assignment:

Terraform allows you to set variable values directly from the command line using the -var option. This dynamic approach proves invaluable when you need to override or set variables on the fly without modifying configuration files.

Syntax:

terraform apply -var="variable_name=value"

2. Basic Variable Assignment:

Let's begin with a simple scenario where we have a Terraform configuration with a variable for the AWS region.

variable "aws_region" {
  type        = string
  description = "The AWS region for resource deployment"
}

To assign a value to this variable using a Terraform command:

terraform apply -var="aws_region=us-west-2"

This overrides the default value or any value set in a variable file with the specified value for the aws_region variable during the terraform apply operation.

3. Multiple Variables Assignment:

In scenarios with multiple variables, you can use the -var option for each variable, providing a concise and readable command. For instance:

terraform apply -var="aws_region=us-west-2" -var="instance_type=t2.micro"

Here, both the aws_region and instance_type variables receive new values through the command line.

4. Assigning List and Map Variables:

Dealing with list or map variables? The command-line approach remains seamless. Consider the following example:

variable "tags" {
  type        = map(string)
  default     = {
    environment = "dev"
    owner       = "team-terraform"
  }
  description = "Map of tags for resources"
}

To assign values to the tags variable:

terraform apply -var="tags={environment='prod', owner='admin'}"

Here, the map variable receives a new set of values directly from the command line.

5. Overriding Default Values:

It's crucial to note that when using command-line assignments, you're essentially overriding default values and any values from variable files. This dynamic behavior is powerful but should be used judiciously to avoid unexpected configurations.

6. Variable Interpolation:

Variable interpolation can also be utilized in command-line assignments. Let's say we have the following variable:

variable "instance_name" {
  type        = string
  description = "The name of the instance"
}

You can interpolate the value of an environment variable in the command line:

export INSTANCE_NAME="web-server"
terraform apply -var="instance_name=${var.INSTANCE_NAME}"

This demonstrates how to leverage environment variables in variable assignments.

7. Best Practices:

7.1. Encapsulate Complex Values:

For complex data types like maps or lists, encapsulate values using quotes to ensure proper parsing.

terraform apply -var="tags={key='value'}"

7.2. Be Explicit:

Clearly specify variable assignments, even if the values seem obvious. This enhances readability and avoids confusion.

7.3. Document Commands:

Include comments or documentation in your scripts to explain the purpose and usage of command-line variable assignments.

8. Conclusion:

Command-line variable assignment in Terraform adds a dynamic dimension to configuration management. Whether you need to swiftly override defaults, experiment with different values, or integrate with external systems, this method empowers you with real-time control. However, wield this power responsibly, adhering to best practices and maintaining transparency in your Terraform workflows. As you embark on your Terraform journey, may the command line be your ally in crafting dynamic and responsive infrastructures. Happy Terraforming! 🌐🛠️