Share variable definitions across multiple folders in Terraform

Question

I have my project structured like this:

.
├── modules/
│   ├── application/
│   │   ├── main.tf
│   │   └── variables.tf
│   ├── db/
│   │   ├── main.tf
│   │   └── variables.tf
│   └── cdn/
│       ├── main.tf
│       └── variables.tf
└── proj/
    ├── website_1/
    │   ├── main.tf
    │   ├── variables.tf
    │   ├── dev.tfvars
    │   └── prod.tfvars
    └── website_2/
        ├── main.tf
        ├── variables.tf
        ├── dev.tfvars
        └── prod.tfvars

----


### application/main.tf

resource "my_resource_type" "application" {
  description = var.app_description
  name        = var.app_name
  env         = var.env_name
}

# lots more resources....

-----

### application/variables.tf

variable "app_name" {
    type = string
    description = "Name of the application."
}

variable "app_description" {
    type = string
    description = "Description for the application."
}

variable "env_name" {
    type = string
    description = "Name of the environment."
}

# lots more variable definitions...

db/main.tf and cdn/main.tf follow similar structure.

Then I have the files within my proj/ folder for the actual resources I want to apply.

### proj/website_1/main.tf

# shared resource configuration

module "application" {
  source = "../../modules/application"
  app_description = var.app_description
  app_name        = var.app_name
  env_name        = var.env_name
}

module "db" {
  source = "../../modules/db"
  # paramaters
}

module "cdn" {
  source = "../../modules/cdn"
  # paramaters
}

# unique website_1 config...

--------

### proj/website_2/main.tf

# shared resource configuration

module "application" {
  source = "../../modules/application"
  app_description = var.app_description
  app_name        = var.app_name
  env_name        = var.env_name
}


module "db" {
  source = "../../modules/db"
  # paramaters
}

module "cdn" {
  source = "../../modules/cdn"
  # paramaters
}

# unique website_2 config...

Website 1 and Website 2 combine multiple AWS resources in a reusable way, hence the separate modules. The problem is having to go inside project/website_1/ and project/website_2/ and retype the same variable definitions I used across my modules.

I understand this is a common problem in Terraform, but still, I'd like to avoid repeating my variable definitions if I can. It seems like symlinking a common variables.tf file is a bad practice, so what is the "correct"/best practice way (if any) to achieve what I'm trying to achieve within Terraform (without using a separate tool such as Terragrunt)? I'm also open to changing my folder and file structure.

Answer 1

If they all follow the same structure, I would create a single source of truth variables_template.tf file in a directory called templates and in the main directory or in a sub-directory I would create a shell script to cp or rsync the variables_template.tf

Then anytime you need to make changes to all the variables, just update the template and run the update-variables.sh script.

I would also .gitignore the variables in the subdirectories, as running the script once will populate them.

#!/bin/bash
DIRECTORIES=("a" "b" "c" "d" "e" "f")
TEMPLATE="./template/variables_template.tf"

for i in "${DIRECTORIES[@]}"
do
    cp $TEMPLATE "$i"/variables.tf
done