Variable vs Local in Terraform: Key Differences and Usage

In Terraform, variable defines input values that users or modules can set from outside, while local defines internal values computed within the configuration. Variables are for external configuration, and locals are for simplifying expressions inside the code.

Quick Comparison

This table summarizes the main differences between variable and local in Terraform.

Aspect	Variable	Local
Purpose	Input values set externally	Internal computed values
Definition Location	Declared with variable block	Declared with locals block
Value Source	Provided by user, environment, or defaults	Derived from expressions in config
Changeability	Can be overridden at runtime	Fixed within the configuration
Use Case	Configure modules or root module inputs	Simplify complex expressions or reuse values
Visibility	Accessible outside the module	Only accessible inside the module

Key Differences

Variables in Terraform are designed to accept input from users or calling modules. They allow you to customize your infrastructure by passing values when you run Terraform commands or through environment variables. Variables can have default values but are primarily meant for external configuration.

Locals, on the other hand, are used to define values inside your Terraform configuration that are computed from other values. They help you avoid repeating complex expressions and make your code cleaner. Locals cannot be set or overridden from outside; they are fixed once defined.

In summary, use variables to accept input and locals to simplify and reuse computed values within your Terraform code.

Code Comparison

Here is an example showing how to use a variable to accept an input value for an AWS region.

terraform

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

provider "aws" {
  region = var.region
}

resource "aws_s3_bucket" "example" {
  bucket = "my-bucket-${var.region}"
  acl    = "private"
}

Output

Creates an S3 bucket named 'my-bucket-us-west-2' in the specified AWS region.

Local Equivalent

Here is how to use a local value to compute the bucket name inside the configuration.

terraform

locals {
  region      = "us-west-2"
  bucket_name = "my-bucket-${local.region}"
}

provider "aws" {
  region = local.region
}

resource "aws_s3_bucket" "example" {
  bucket = local.bucket_name
  acl    = "private"
}

Output

Creates an S3 bucket named 'my-bucket-us-west-2' in the fixed AWS region defined in locals.

When to Use Which

Choose variable when you want to allow users or modules to provide input values that can change between runs or environments. This makes your Terraform code flexible and reusable.

Choose local when you want to simplify your configuration by computing values from other inputs or constants inside your code. Locals help keep your code clean and avoid repetition but are not meant for external input.

Key Takeaways

Use variable to accept input values from users or modules.

Use local to define internal computed values within your configuration.

Variables can be overridden at runtime; locals cannot.

Variables are visible outside modules; locals are only inside.

Choose variables for flexibility and locals for code simplicity.