Variables and Outputs

Variables make your Terraform configuration flexible and reusable. Outputs expose values from your infrastructure for use by other configurations or scripts. This lesson covers input variables, output values, local values, and variable files.

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Input Variables

An input variable is declared with the variable block:

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

variable "instance_count" {
  description = "Number of instances to create"
  type        = number
  default     = 1
}

variable "enable_monitoring" {
  description = "Whether to enable detailed monitoring"
  type        = bool
  default     = false
}

Variable Arguments

Argument	Required	Purpose
description	No (recommended)	Documents the variable's purpose
type	No (recommended)	Constrains the variable type
default	No	Default value if none is provided
validation	No	Custom validation rules
sensitive	No	Hides the value in plan output
nullable	No	Whether the variable can be null

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Variable Types

Primitive Types

variable "name"      { type = string }
variable "count"     { type = number }
variable "enabled"   { type = bool }

Collection Types

variable "availability_zones" {
  type    = list(string)
  default = ["eu-west-2a", "eu-west-2b"]
}

variable "tags" {
  type = map(string)
  default = {
    Environment = "development"
    Team        = "platform"
  }
}

variable "ports" {
  type    = set(number)
  default = [80, 443]
}

Structural Types

variable "instance_config" {
  type = object({
    instance_type = string
    ami           = string
    monitoring    = bool
  })
  default = {
    instance_type = "t3.micro"
    ami           = "ami-0c55b159cbfafe1f0"
    monitoring    = false
  }
}

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Setting Variable Values

Variables can be set in multiple ways, listed by precedence (highest to lowest):

Method	Example
Command-line flag	terraform apply -var="region=us-east-1"
Variable file flag	terraform apply -var-file="prod.tfvars"
terraform.tfvars	Automatically loaded
*.auto.tfvars	Automatically loaded (alphabetical order)
Environment variable	export TF_VAR_region=us-east-1
Default value	Defined in the variable block
Interactive prompt	Terraform asks if no value is provided

terraform.tfvars

region          = "eu-west-2"
instance_count  = 3
enable_monitoring = true

Environment Variables

export TF_VAR_region="eu-west-2"
export TF_VAR_instance_count=3

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Variable Validation

Add custom validation rules to catch errors early:

variable "environment" {
  type        = string
  description = "Deployment environment"