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4.1: Variables and Outputs (Theory)

Terraform Variables and Outputs Theory

Terraform variables and outputs enable more flexible and modular code by breaking out hard-coded values.

Types of Variables

  1. Input Variables: Input variables are like input parameters or arguments for a function. They are referenced using To declare an input variable, use the following syntax:
variable "instance_type" {
  type = string
  default = "t2.micro"
  1. Local Variables: Local variables are like temporary variables within the scope of a function. They are referenced using, and declared with locals (plural). For example:
locals {
  service_name = "example-service"
  owner = "your_name"
  1. Output Variables: Output variables are like the return values of a function. They allow bundling multiple Terraform configurations together. To declare an output variable, use the following syntax:
  output "instance_ip" {
  value = aws_instance.example.public_ip

Setting Input Variables

Input variables can be set in several ways, ranked in order of precedence from lowest to highest:

  1. Terraform CLI prompts: If you don't specify a variable anywhere and there's no default value, the Terraform CLI will prompt you for a value.
  2. Default value in the block: You can specify a default value when declaring the variable.
  3. Environment variables: Use the prefix TF_VAR_ followed by the variable name.
  4. Terraform .tfvars files: Store values in .tfvars files.
  5. Auto-applied .auto.tfvars files: These files will be applied over the .tfvars files.
  6. -var or -var-file options: Pass values when issuing the terraform plan or terraform apply commands.

Variable Value Types

Variables can hold different value types:

  • Primitive types: string, number, or boolean.
  • Complex types: lists, sets, maps, etc.

Type checking happens automatically in Terraform. You can also write your own validation rules.

Handling Sensitive Data

When using sensitive data in variables, like a database password, add the sensitive = true attribute when defining the variable. This will cause those data to be masked in the Terraform plan output to prevent leaking credentials.

Also, avoid storing sensitive data in files, and consider using these options for passing in those data:

  • environment Variables
  • -var command
  • external secret stores like AWS Secrets Manager or HashiCorp Vault.