Install & the terragrunt.hcl model

Wrapping a Terraform module.

Terragrunt is a single binary you install alongside Terraform (or OpenTofu); it invokes whichever binary you point it at. The unit of work is a terragrunt.hcl file in a directory — the wrapper for one Terraform module. At its simplest, a terragrunt.hcl declares a source (the Terraform module to run) and the inputs to pass it, and running terragrunt behaves like running terraform in that generated module.

$ terragrunt --version
terragrunt version v0.58.x
$ terraform --version   # (or tofu) — Terragrunt calls this under the hood
Terraform v1.9.x
 
$ cd live/prod/vpc
$ terragrunt plan       # behaves like terraform plan, with Terragrunt’s glue applied

terraform {
  source = "git::git@github.com:acme/modules.git//vpc?ref=v1.4.0"
}
 
inputs = {
  cidr_block  = "10.20.0.0/16"
  environment = "prod"
}

source and the module cache

The source can be a local path or a remote module reference pinned to a version (a Git ref, a registry version). Terragrunt downloads it into a local .terragrunt-cache and runs Terraform there, injecting the generated backend and inputs. Pinning the source ref is essential — it is exactly the module-versioning discipline from Terraform, and it makes each unit reproducible.