How to format numeric variable in Terraform

Question

I have a following (simplified) Terraform code:

variable "cluster_id" {
    default = 1
}

resource "aws_instance" "instance" {
    ... some instance properties ...
    tags {
        "Name" = "${format("cluster-%02d", var.cluster_id)}"
    }
}

And when I run terraform apply the plan shows:

tags.Name: "%!d(string=1)"

The cluster_id in format() is not handled as a number so formatting fails. I would expect that I get cluster-01 but that's not the case.

Am I doing something wrong or is it really not possible to use custom variables as numbers in formatting?

Answer 1

Terraform, pre 0.12, only supports string, list and map types as an input variable so despite you providing an integer (or a float or a boolean) it will be cast to a string.

Both Terraform and Go allow you to use the same padding for integers and strings though so you can just use the following to 0 pad the cluster_id:

resource "aws_instance" "instance" {
    # ... some instance properties ...
    tags {
        "Name" = "${format("cluster-%02s", var.cluster_id)}"
    }
}