How to Define Variables in Terraform: Syntax and Examples

In Terraform, you define a variable using the variable block with a name and optional attributes like type and default. This lets you reuse values and customize your infrastructure code easily.

Syntax

The variable block declares a variable by name. Inside, you can specify:

• type: the kind of value (string, number, bool, list, map)
• default: a fallback value if none is provided
• description: a helpful note about the variable

This structure helps Terraform know what input to expect.

terraform

variable "example_variable" {
  type        = string
  default     = "default value"
  description = "This is an example variable"
}

Example

This example shows how to define a variable and use it in a resource. The variable instance_count controls how many instances to create.

terraform

variable "instance_count" {
  type    = number
  default = 2
}

resource "aws_instance" "example" {
  count         = var.instance_count
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
}

Output

Terraform will create 2 AWS EC2 instances using the specified AMI and instance type.

Common Pitfalls

Common mistakes include:

• Not specifying a type when the variable is complex, causing errors.
• Forgetting to provide a value or default, leading to prompts during apply.
• Using incorrect variable references like variable.instance_count instead of var.instance_count.

terraform

/* Wrong: missing type and default, causes prompt */
variable "region" {}

/* Right: specify type and default to avoid prompts */
variable "region" {
  type    = string
  default = "us-west-2"
}

Quick Reference

Attribute	Description	Example
type	Defines the variable data type	type = string
default	Sets a default value	default = "value"
description	Explains the variable purpose	description = "Used for region"
sensitive	Hides value in logs	sensitive = true

Key Takeaways

Define variables in Terraform using the variable block with a name and optional attributes.

Always specify the variable type for clarity and error prevention.

Provide default values to avoid prompts during terraform apply.

Use variables in code with the var. syntax.

Avoid common mistakes like missing types or wrong variable references.