main.tf

variable "field_A" {
  type = string
  default = "default value A"
}

output "field_A" {
  value = var.field_A
}

variable "field_B" {
  type = string
  default = "default value B"
}

output "field_B" {
  value = var.field_B
}

terragrunt.hcl

locals {
  # read values from variables.yaml
  global = yamldecode(file("variables.yaml"))
  # map yaml schema to module input variables
  # default to null if not defined
  inputs = {
    field_A = try(local.global.brownfield.field, null)
    field_B = try(local.global.brownfield.field_B, null)
  }
  # filter out values that were not provided by user
  final = {
    for field, value in local.inputs:
    field => value
    if value != null
  }
}

# pass the filtered map of inputs to the module
inputs = local.final

test.sh

terragrunt apply

variables.yaml

brownfield:
  field: "custom field A"
  field_B: "my custom field B"