Quick Intro To Terraform Variables

Terraform effectively has 3 variable types as depicted in the digram below. You will note that the Output and Local are refered to a "Values", this is because - in part - their declaration syntax is different.

In summary:

• Input Variables serve as parameters for a Terraform module, so users can customize behavior without editing the source.
• Output Values are like return values for a Terraform module.
• Local Values are a convenience feature for assigning a short name to an expression.

Input Variable Example

Input Variables can have the following data types:

• string
• number
• bool

The type constructors allow you to specify complex types such as collections:

• list()
• set()
• map()
• object({ = , ... })
• tuple([, ...])

Assume that we want to store a Virtual Network Name such as:

virtual_network_name = "vnet-trans-p-001"

The syntax for declaring an input variable is as follows:

variable "virtual_network_name"{
     type        = string
    description = "The name of the virtual network."

  validation {
    condition     = length(var.virtual_network_name) > 5 && substr(var.virtual_network_name, 0, 5) == "vnet-"
    error_message = "The virtual_network_name value must be a valid vNet Name, starting with \"vnet-\"."
  }
}

If we now run this with some incorrect values we get:

• Prefix incorrect:

var.virtual_network_name is "vn-trans-p-001"
│
│ The virtual_network_name value must be a valid vNet Name, starting with "vnet-".
│
│ This was checked by the validation rule at variables.tf:5,3-13.

• Input too short:

var.virtual_network_name is "vnet-"
│
│ The virtual_network_name value must be a valid vNet Name, starting with "vnet-".
│
│ This was checked by the validation rule at variables.tf:5,3-13.