How Do I Avoid Repeating A Variable In Terraform? - terraform

Question

Terraform doesn't allow you to interpolate variables within the variables file otherwise you get the error:
Error: Variables not allowed
on variables.tf line 9, in variable "resource_group_name": 9:
default = "${var.prefix}-terraform-dev_rg"
Variables may not be used here.
This then means I end up duplicating the value of the prefix in my variables.tf file when I try to create the name for the resource group.
Is there a nice way around this to avoid duplicating the value of the variable?
variables.tf
variable "prefix" {
description = "The prefix used for all resources in this plan"
default = "terraform-dev"
}
variable resource_group_name {
type = "string"
default = "terraform-dev_rg"
}
variable resource_group_location {
type = "string"
default = "eastus"
}
main.tf
# Configure the Microsoft Azure Provider
provider "azurerm" {
version = "=1.28.0"
}
# Create a resource group
resource "azurerm_resource_group" "resource-group" {
name = var.resource_group_name
location = var.resource_group_location
}
#Create an application gateway with web app firewall
module "firewall" {
source = "./firewall"
resource_group_name = var.resource_group_name
resource_group_location = var.resource_group_location
}
./firewall/variables.tf
#Passed down from the root variables.tf
variable "prefix" {}
variable "resource_group_name" {}
variable "resource_group_location" {}
./firewall/main.tf
# Create a virtual network for the firewall
resource "azurerm_virtual_network" "firewall-vnet" {
name = "${var.prefix}-waf-vnet"
address_space = ["10.0.0.0/16"]
resource_group_name = var.resource_group_name
location = var.resource_group_location
}

Answer 1

Try to use local values,
https://www.terraform.io/docs/configuration/locals.html
variable "prefix" {
description = "The prefix used for all resources in this plan"
default = "terraform-dev"
}
variable resource_group_location {
type = "string"
default = "eastus"
}
locals {
resource_group_name = "${var.prefix}_rg"
}
resource "azurerm_resource_group" "resource-group" {
name = local.resource_group_name
location = var.resource_group_location
}

Answer 2

Terraform does not support variables inside a variable.
If you want to generate a value based on two or more variables then you can try Terraform locals (https://www.terraform.io/docs/configuration/locals.html).
Locals should help you here to achieve goal.
something like
variables.tf
variable "prefix" {
description = "The prefix used for all resources in this plan"
default = "terraform-dev"
}
variable resource_group_location {
type = "string"
default = "eastus"
}
main.tf
locals {
resource_group_name = "${var.prefix}_rg"
}
# Configure the Microsoft Azure Provider
provider "azurerm" {
version = "=1.28.0"
}
# Create a resource group
resource "azurerm_resource_group" "resource-group" {
name = local.resource_group_name
location = var.resource_group_location
}
Hope this helps.
Please read similar discussion here -https://stackoverflow.com/questions/58841060/terraform-variables-within-variables/58841360?noredirect=1#comment129460631_58841360