Variables

HCL (Hashicorp Configuration language) is NOT the most aesthetically pleasing thing to look at. To make matters worse, there are a couple of layers of indirection as to how variables can be defined and used.

• Layer #1: A top-level variables.tf file

  If only it could have entries like foo=123 or bar="four five six". Alas, NO. It takes 3 poxy lines to declare a variable, thusly:

  variable "my_var_name" {
    default = "its (default) value"
  }

• Layer #2: A locals block, in your main.tf file. eg:

  locals {
    # note string interpolation
    foobar = "foo-${var.my_var_name}-bar"
  
    # also a good place to define a bunch of resuable tags / tag lexemes
    common_tags = {
      Environment = terraform.workspace
      Project     = var.project
      ManagedBy   = "Terraform"
    }
  }

  Besides the terraform.workspace freebie populated by terraform itself, note that var.* refers to a variable defined in the variables.tf file.

• Layer #3: Using values in the rest of your .tf files

  • use var.xxx in the same way shown ealier
  • use local.xxx (note the unexpected singular: local.xxx, NOT locals.xxx) to reference items in main.tf locals {}.

• Layer #4: Use data blocks to fetch info about resources; each resource type:

  • is defined as data <type> <name> {...} block
  • furnishes a bunch of useful data exposed as a list of values.
  • These values are referenced as data.<type>.<name>.<attribute> in whatever resource needs it.

• Layer #5: A bit like the data blocks above, there are also output blocks, which:

  • are defined as output <name> { ... } blocks
  • yield resource attributes after a resource has been created... i.e. attributes like assigned IP addresses and such, which we can have no way of knowing until the resource exists.
• Layer #6: .tfvars files
• Layer #7: TF_VAR_* env files