Explaining the Difference between Input and Local Variables in Terraform

2 min readSep 14, 2023

The aim of this page📝 is to explain how to use variables in Terraform based on the particular example of assigning values to input variables from outside of the configuration files.

Terraform has three types of variables: input variables, local values (locals), and outputs
Let’s focus on the difference between the first two, inputs versus locals
Input variables are parameters of a Terraform module.
You can set them directly in the configuration file or pass them as command-line arguments when running Terraform.
Local values are named expressions that you can use to avoid repeating the same values or expressions multiple times in a configuration.
Unlike input variables, you cannot directly assign values to locals from outside the configuration.
NOTE: this does not mean that locals have to be set in the very same file, the files are collected when a plan is executed, i.e. they can be located in e.g. locals.tf file
Instead, locals are defined within the configuration, and their values are computed based on other values or expressions within the same configuration.
Hashicorp docs say

Unlike input variables, locals are not set directly by users of your configuration.

— https://developer.hashicorp.com/terraform/tutorials/configuration-language/locals

“Users of your configuration” refers to anyone who is using your Terraform module or configuration.
I am for example a member of the Infra support team and I am setting input variables by manipulating values in the Consul KV Store
I and other users can provide input variables when they use modules or configuration, but we cannot directly assign values to local variables unless we manipulate config files directly
We can therefore assign values to input variables in several ways:
Command Line Flags
From a File
Environment Variables
Variable Defaults
Module Blocks
As said, we can also use Consul and other databases to store and retrieve variable values in Terraform.

CODE

Here is a particular example of a terraform file

variable "image_id" {
  description = "The id of the machine image (AMI) to use for the server."
  type        = string
}

locals {
  instance_type = "t2.micro"
}

resource "aws_instance" "example" {
  ami           = var.image_id
  instance_type = local.instance_type

  tags = {
    Name = "example-instance"
  }
}

In this example, image_id is an input variable and instance_type is a local value. The aws_instance resource uses these variables.

Explaining the Difference between Input and Local Variables in Terraform

CODE

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Written by Pavol Kutaj