6. Variables

Contents

• Variable Block
• Variable Precedence

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Use the variable block to parameterize your configuration so that module consumers can pass custom values into the configuration at runtime. Input variables let you customize Terraform modules without altering their source code. Variables serve as parameters for modules, making them composable and reusable.

You can define variables in root modules, or in child modules:

• In the root modules, you can set variable values using CLI options, environment variables, variable definition files, or through an HCP Terraform workspace.
• In child modules, the parent module passes values to child modules as arguments to the module block.

---

Variable Block

variable "<LABEL>" {
  type        = <TYPE>
  default     = <DEFAULT_VALUE>
  description = "<DESCRIPTION>"
  sensitive   = <true|false>
  nullable    = <true|false>
  ephemeral   = <true|false>
 
  validation {
    condition     = <EXPRESSION>
    error_message = "<ERROR_MESSAGE>"
  }
}

The following arguments are supported in a variable block:

Argument	Description	Type	Required?
type	Specify a type constraint for the value argument of this variable.	Type constraint	Optional
default	Sets the default value for this variable. Variables without a default value are required to define a value argument.	Expression	Optional
description	A description of the variable’s purpose and the value it expects.	String	Optional
validation	Specify a rule the value of this variable must meet, in addition to any type constraints.	Block	Optional
sensitive	Specifies if Terraform hides this value in CLI output.	Boolean	Optional
nullable	Specifies if the value of a variable can be null.	Boolean	Optional
ephemeral	Specifies whether to prevent storing this value in state or plan files.	Boolean	Optional

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Variable Precedence

If the root module receives multiple values for the same variable name from different sources, Terraform uses the following order of precedence:

1. Any -var and -var-file options on the command line in the order provided and variables from HCP Terraform
2. Any *.auto.tfvars or *.auto.tfvars.json files in lexical order
3. The terraform.tfvars.json file
4. The terraform.tfvars file
5. Environment variables
6. The default argument of the variable block