KodeKloud Notes

The Terraform block in Terragrunt defines how Terragrunt locates and invokes your Terraform configuration. It centralizes settings such as module source, version locking, extra CLI arguments, and lifecycle hooks. By configuring this block once, you ensure consistent Terraform behavior across all environments and modules.

Key Terraform Block Attributes

Attribute	Description	Example
`source`	Location of the Terraform module (Registry, Git repo, or local path)	`source = "tfr://terraform-aws-modules/vpc/aws/?version=5.8.1"`
`version` / `ref`	Locks a Registry module or Git source to a specific version or commit	`?version=5.8.1` (Registry) / `?ref=v1.2.0` (Git)
`extra_arguments`	Defines additional CLI flags or variable files to pass to Terraform	`arguments = ["-var-file=prod.tfvars"]`
`hooks`	`before_hook` or `after_hook` blocks for executing scripts around Terraform commands	`before_hook "lint" { commands = ["plan"]; execute = ["npm","run","lint"] }`

Note

Always lock your modules to a specific version or ref to avoid unintended upgrades.
For Registry modules, append ?version=, and for Git sources, use ?ref=.

1. Defining the Source

The source attribute tells Terragrunt where to fetch Terraform code. You can reference:

Terraform Registry Modules
Git Repositories (HTTPS or SSH)
Local Directories

1.1 Terraform Registry

terraform {
  source = "tfr://terraform-aws-modules/vpc/aws/?version=5.8.1"
}

Include subdirectories by adding extra slashes:

terraform {
  source = "tfr://terraform-aws-modules/vpc/aws//modules/subnet?version=5.8.1"
}

1.2 GitHub (HTTPS)

terraform {
  source = "github.com/terraform-aws-modules/terraform-aws-vpc?ref=v5.8.1"
}

1.3 GitHub (SSH)

terraform {
  source = "[email protected]:terraform-aws-modules/terraform-aws-vpc.git?ref=v5.8.1"
}

1.4 Local Modules

Ideal for local development and testing:

terraform {
  source = "../modules/vpc-module"
}

2. Passing Extra Arguments

Use extra_arguments to pass flags, variable files, or arbitrary flags to Terraform:

terraform {
  extra_arguments "prod_vars" {
    commands  = get_terraform_commands_that_need_vars()
    arguments = ["-var-file=prod.tfvars"]
  }
}

Field	Description
`commands`	List of Terraform commands (e.g., `plan`, `apply`)
`arguments`	Array of CLI flags or variable file references

3. Hooks and Custom Commands

Hooks allow you to run scripts before or after Terraform commands:

terraform {
  before_hook "validate_schema" {
    commands = ["plan", "apply"]
    execute  = ["python", "scripts/validate.py"]
  }

  after_hook "notify" {
    commands = ["apply"]
    execute  = ["bash", "scripts/notify.sh"]
  }
}

Hook Type	Purpose
`before_hook`	Run tasks prior to Terraform execution
`after_hook`	Run cleanup or notifications post Terraform run

Warning

Use hooks carefully in production: unexpected script failures can block Terraform runs.
Always test hooks in a development environment first.

References

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