How to use null with a different value in place of default value

Question

I know that null can be used like this to set default behavior if var not specified:

variable "override_private_ip" {
  type    = string
  default = null
}

resource "aws_instance" "example" {
  # ... (other aws_instance arguments) ...
  private_ip = var.override_private_ip
}

But I want to set my own default behavior if it's not specified.

I'm doing this:

#### if user sets an id use that but if not get id from data source
resource "aws_instance" "myserver" {
  ami = var.ami_id != null ? var.ami_id : data.aws_ami.getami.id

This seems to work but is this the correct way? I want to make sure I'm not missing a feature for this. I tried just var.ami_id ? var.ami_id : data.aws_ami.getami.id but null is not converted to a bool so did not work.

Answer 1

A conditional expression like the one you showed is indeed the right way to express this in Terraform. As you've seen, Terraform does not consider null to be a boolean false.

---

It doesn't seem like it would be necessary for this particular situation, but if you have input variables that are used in many places where all uses would need the same normalization/preparation logic then you can factor out the expression into a local value to use it many times:

variable "ami_id" {
  type    = string
  default = null
}

data "aws_ami" "example" {
  count = var.ami_id == null ? 1 : 0

  # ...
}

locals {
  ami_id = var.ami_id != null ? var.ami_id : data.aws_ami.example[0].id
}

resource "aws_instance" "example" {
  # ... (other aws_instance arguments) ...
  ami = local.ami_id
}

You could then use local.ami_id many times in the module without duplicating the logic that handles the default value lookup.