Would like help understanding Terragrunt behavior

[nicholasarvelo]

The Terragrunt code I am sharing does what I want it to do. However, I am not 100% certain I understand why it is working. It doesn't align with what I understand from reading Gruntwork.io's documentation. I was hoping someone could help me better understand.

This is my directory structure:

terragrunt/
├── development/
│   ├── ecs/
│   │   ├── ecs_service_name/
│   │   │   ├── data.tf
│   │   │   ├── main.tf
│   │   │   ├── task_definition.tf
│   │   │   ├── terragrunt.hcl
│   │   │   └── variables.tf
│   └── environment_variables.hcl
└── backend_configuration.hcl

The contents of terragrunt.hcl:

include "backend_configuration" {
  path = find_in_parent_folders("backend_configuration.hcl")
}

include "environment_variables" {
  path = find_in_parent_folders("environment_variables.hcl")
}

The contents of environment_variables.hcl:

locals {
  environment = basename(dirname(find_in_parent_folders("environment_variables.hcl")))
}

inputs = {
  environment = local.environment
}

The contents of backend_configuration.hcl:

remote_state {
  backend = "s3"

  generate = {
    path      = "backend.tf"
    if_exists = "overwrite_terragrunt"
  }

  config = {
    bucket         = "terraform-bucket-name"
    key            = "${path_relative_to_include()}/terraform.tfstate"
    dynamodb_table = "dynamodb-table-name"
    region         = "us-east-1"
  }
}

As I already mentioned above, everything is working. Meaning:

• terragrunt.hcl adopts the configuration details from the two parent Terragrunt configuration files.
• backend.tf is dynamically created with the expected key value.
• environment evaluates to development and is exported for Terraform interpolation.

So, this is my question:

Per Gruntwork's documentation:

find_in_parent_folders() searches up the directory tree from the current terragrunt.hcl file and returns the absolute path to the first terragrunt.hcl in a parent folder

The function takes an optional name parameter that allows you to specify a different filename to search for

This would lead me to believe that in order for environment to evaluate to development , I should be setting

environment = basename(dirname(find_in_parent_folders("environment_variables.hcl")))

to

environment = basename(dirname(find_in_parent_folders("backend_configuration.hcl")))

However this is not the resulting behavior. Setting it this way evaluates to terragrunt.

It only generates the desired result if set to:

environment = basename(dirname(find_in_parent_folders("environment_variables.hcl")))

or, interestingly, if set to:

environment = basename(dirname(find_in_parent_folders()))

I would expect to fail with the latter example since there is no terragrunt.hcl.

Can someone help me understand why it is behaving as such?

---

Tracked in ticket #110128

[denis256]

Hi,
since backend_configuration.hcl is located in terragrunt, find_in_parent_folders() will return path to terragrunt directory, not to development

In my tests:

$ tree 
discussion-702/
├── backend_configuration.hcl
└── development
    ├── ecs
    │   └── ecs_service_name
    │       ├── file.txt
    │       ├── main.tf
    │       ├── terraform.tfstate
    │       ├── terraform.tfstate.backup
    │       └── terragrunt.hcl
    └── environment_variables.hcl

# backend_configuration.hcl
find_in_parent_folders("backend_configuration.hcl") = /projects/gruntwork/terragrunt-tests/discussion-702/backend_configuration.hcl
dirname(find_in_parent_folders("backend_configuration.hcl")) = /projects/gruntwork/terragrunt-tests/discussion-702
basename(dirname(find_in_parent_folders("backend_configuration.hcl"))) = discussion-702

# environment_variables.hcl
find_in_parent_folders("environment_variables.hcl") = /projects/gruntwork/terragrunt-tests/discussion-702/development/environment_variables.hcl
dirname(find_in_parent_folders("environment_variables.hcl")) = /projects/gruntwork/terragrunt-tests/discussion-702/development
basename(dirname(find_in_parent_folders("environment_variables.hcl"))) = development

References:
https://developer.hashicorp.com/terraform/language/functions/dirname
https://developer.hashicorp.com/terraform/language/functions/basename
https://github.com/denis256/terragrunt-tests/tree/master/discussion-702

[nicholasarvelo]

Thanks @denis256,

But I don't have environment_variables.hcl looking for backend_configuration.hcl. In my environment_variables.hcl I have find_in_parent_folders("environment_variables.hcl") set, which somehow works. Isn't that command as written essentially telling environment_variables.hcl to look for itself? It doesn't make sense to me.

[denis256]

It depends on where find_in_parent_folders() is executed - in the current case, after includes resolving, find_in_parent_folders("environment_variables.hcl") is executed in development/ecs/ecs_service_name and it is finding environment_variables.hcl