Variables, Expressions, and Functions

Updated Jan 24, 2021 ·

Overview

Variables keep Terraform code reusable. Expressions and functions let configuration select values, calculate network ranges, and render small dynamic strings without duplicating resources.

Variable Files

Variables are declared in the configuration and assigned through defaults, .tfvars files, CLI flags, environment variables, or module inputs.

For the basic root-module file layout, see Configurations.

File	Purpose
variables.tf	Declares accepted input variables and their types.
terraform.tfvars	Assigns values to declared variables.
outputs.tf	Prints useful values after Terraform applies.

Example variable declaration:

variable "instance_type" {
  description = "EC2 instance type"
  type        = string
  default     = "t3.micro"
}

Example assignment:

instance_type = "t3.micro"

Useful Types

Terraform variables can use simple and complex types.

Type	Example	Use case
string	"ap-southeast-1"	Region names and resource names.
number	2	Counts and sizes.
bool	true	Feature toggles.
list	["a", "b"]	Availability zones or names.
map	{ linux = "ami-1234567890" }	Lookup tables.
object	{ size = 20, encrypted = true }	Grouped settings.

lookup

The lookup function selects a value from a map. It is useful when a variable controls which AMI, CIDR block, or setting should be used.

ami = lookup(var.ami_ids, var.os_type, null)

cidrsubnet

The cidrsubnet function creates smaller subnets from a larger CIDR block.

cidr_block = cidrsubnet(var.cidr_block, 8, 1)

This keeps VPC subnet math in code and reduces hardcoded subnet values.