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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.

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Key Terraform Block Attributes

AttributeDescriptionExample
sourceLocation of the Terraform module (Registry, Git repo, or local path)source = "tfr://terraform-aws-modules/vpc/aws/?version=5.8.1"
version / refLocks a Registry module or Git source to a specific version or commit?version=5.8.1 (Registry) / ?ref=v1.2.0 (Git)
extra_argumentsDefines additional CLI flags or variable files to pass to Terraformarguments = ["-var-file=prod.tfvars"]
hooksbefore_hook or after_hook blocks for executing scripts around Terraform commandsbefore_hook "lint" { commands = ["plan"]; execute = ["npm","run","lint"] }
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=.

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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

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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"
}

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1.2 GitHub (HTTPS)

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

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1.3 GitHub (SSH)

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

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1.4 Local Modules

Ideal for local development and testing: 
terraform {
  source = "../modules/vpc-module"
}

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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"]
  }
}
FieldDescription
commandsList of Terraform commands (e.g., plan, apply)
argumentsArray of CLI flags or variable file references

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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 TypePurpose
before_hookRun tasks prior to Terraform execution
after_hookRun cleanup or notifications post Terraform run
Use hooks carefully in production: unexpected script failures can block Terraform runs.
Always test hooks in a development environment first.

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References

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