the-magic

What does terragrunt actually do?

⚠️ This isn't strictly the exact process that terragrunt follows. What's important is that you understand what each piece of terragrunt config means in relation to vanilla terraform.

The magic

So what happens when you run terragrunt plan/apply in a deployment directory e.g. dev/vpc?

The below will walk you thorough the steps, and link to relevant files in a folder (vpc) where I've manually carried out the steps, so you can see what the end result looks like.

1. In a staging directory (.terragrunt-cache) terragrunt pulls down the module source either from git or a relative path. In this example you can see it's just git cloned: https://github.com/terraform-aws-modules/terraform-aws-vpc
2. Creates a backend.tf based on your remote state config defined in your root terragrunt.hcl, and figures out the relative path and uses that as the key for your state file
3. Creates a provider.tf based on your provider config in your root terragrunt.hcl
4. Creates a terraform.tfvars based on all the values declared in your root terragrunt.hcl, env.hcl, and "inputs" specified in the terragrunt.hcl deployment file
5. Runs terraform init for you
6. Runs terraform plan/apply (or whatever other terragrunt command you invoked e.g. terragrunt destroy)