Variables

A variable is an input to a Terraform configuration or a Terraform module.

Variables are conventionally declared in a file named variables.tf¹. Declare variables using variable blocks:

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variable "name" {
  description = "The name of a person"
  default     = "Jane Doe"
  type        = string
}

A variable has a type to indicate what type of value Terraform can expect for this variable. There are three basic types:

• string
• bool (true or false)
• number (used for both integers and decimal numbers, e.g. 1, -1242, and 3.1415)

There are also four composite types:

• object()
• map()
• list()
• tuple()

If you need to pass sensitive data (e.g. passwords) into a Terraform configuration or module you should specify that the variable is sensitive:

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variable "password" {
  description = "Database password"
  type        = string
  sensitive   = true
}

If you expect a value for a variable to fulfill certain conditions you can add one or more validation blocks to the variable:

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variable "region" {
  description = "Cloud region name"
  type        = string
  
  validation {
    condition     = contains(["us", "eu"], var.region)
    error_message = "Cloud region is expected to be us or eu."
  }
}

If the value passed to the region variable is not one of us or eu, Terraform will not proceed and will instead print the error_message.

You reference the value of a variable using the var.<name> syntax. An example of what this could look like:

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variable "filename" {
  description = "Filename of the configuration file to create"
  default     = "server.hcl"
  type        = string
}

resource "local_file" "config" {
  filename = var.filename
  content  = <<-EOF
    # omitted ...
  EOF>>
}

Best practices

• Add the default argument to variables if a sensible default value exist.
• Declare sensitive values using the sensitive = true argument.
• Add the description argument with a descriptive value to tell users what this variable is.