Inputs, includes & locals

Share config across units.

Advanced12 min · lesson 6 of 12

Beyond state, Terragrunt keeps inputs and settings DRY through includes and locals. An include pulls configuration from a parent file (the same mechanism as the shared backend), and Terragrunt merges parent and child — so common inputs (region, tags, account id) live in a parent and each unit adds only its specifics. locals define reusable values within a file, and you can read shared data (like a region config) from a parent locals file.

THE INCLUDE / MERGE HIERARCHY

Repo root (root.hcl)

backend

remote_state, one definition

provider

generated once

Account level (account.hcl)

assume-role

per-account credentials

account id

Environment level (env.hcl)

aws_region

e.g. us-east-1

common_tags

environment, owner

Unit (terragrunt.hcl)

module source

pinned ref

specific inputs

e.g. instance_type

Terragrunt deep-merges each level top-down; a unit inherits everything above it unless it overrides the value.

live/prod/env.hcl

locals {
  environment = "prod"
  aws_region  = "us-east-1"
  common_tags = { environment = "prod", owner = "platform" }
}

live/prod/app/terragrunt.hcl

include "root" { path = find_in_parent_folders("root.hcl") }
locals {
  env = read_terragrunt_config(find_in_parent_folders("env.hcl")).locals
}
inputs = {
  environment = local.env.environment    # shared value, defined once
  tags        = local.env.common_tags
  instance_type = "m5.large"             # this unit’s specific input
}

Merge behavior and hierarchy

The pattern is a hierarchy of hcl files — a repo root (backend, provider), an account level, an environment level (region, tags), and the unit (its module and specific inputs) — each including the ones above and contributing its layer. Terragrunt deep-merges them, so a value set at the environment level applies to every unit under it unless a unit overrides it. This is how a change like “add a cost-center tag everywhere” is one edit at the right level.

Deep merge means know where a value comes from

With inputs assembled from several included files, a surprising value usually means it was set at a higher level than you were looking. Learn the include hierarchy for your repo, use terragrunt render-json (or terragrunt terragrunt-info) to see the effective merged config, and set each value at the level where it logically belongs so overrides stay predictable.

DRY remote stateDependencies & outputs