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Terraform Variables

Hard‑coding values in Terraform configurations makes them inflexible. Input variables allow you to parameterize your infrastructure, making configurations reusable across environments (dev, stage, prod).

Declaring a Variable

Use the variable block. At minimum, provide a name. Optionally add a description, type, and default value.
variable "instance_type" {
  description = "EC2 instance type"
  type        = string
  default     = "t2.micro"
}

Using Variables

Reference a variable with var.<name>:
resource "aws_instance" "web" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = var.instance_type
}

Variable Types

Common types: string, number, bool, list(string), map(string), object, tuple.
variable "subnet_ids" {
  type    = list(string)
  default = ["subnet-abc", "subnet-def"]
}

variable "tags" {
  type    = map(string)
  default = {
    Environment = "dev"
    Project     = "demo"
  }
}

Setting Variable Values

Several ways (from lowest to highest precedence):
  • Default value in variable block
  • Environment variables (TF_VAR_name)
  • terraform.tfvars or *.auto.tfvars files
  • Command line -var or -var-file
Example terraform.tfvars:
instance_type = "t3.small"
subnet_ids    = ["subnet-123", "subnet-456"]

Sensitive Variables

Mark a variable as sensitive to hide its value in logs:
variable "db_password" {
  type      = string
  sensitive = true
}


Two Minute Drill
  • Variables parameterize configurations.
  • Declare with variable block, set type and optional default.
  • Reference with var.name.
  • Set values via tfvars files, env vars, or CLI.

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