Terraform variables within variables

Question

First off - apologies - I’m extremely new (3 hours in!) to using terraform.

I am looking to try and use the value of a variable inside the declaration of another variable.

Below is my code - what am I doing wrong?

variables.tf:

variable "EnvironmentName" {     type = "string" } variable "tags" {     type = "map"     default = {         Environment = "${var.EnvironmentName}"         CostCentre = "C1234"         Project = "TerraformTest"         Department = "Systems"     } } 

Variables-dev.tfvars:

EnvShortName = "Dev" EnvironmentName = "Development1" #Location Location = "westeurope" 

main.tf:

resource “azurerm_resource_group” “TestAppRG” {     name = “EUW-RGs-${var.EnvShortName}”     location = “${var.Location}”     tags = “${var.tags}” } 

I am getting the following error:

Error: Variables not allowed on variables.tf line 18, in variable
“tags”: 18: Environment = “${var.EnvironmentName}”
Variables may not be used here.

I understand that the error message is fairly self explanatory and it is probably my approach that is wrong - but how do I use a variable in the definition of another variable map? is this even possible?

I will be standing up multiple resources - so want the tags to be built as a map and be passed into each resource - but I also want to recycle the map with other tfvars files to deploy multiple instances for different teams to work on.

Answer 1

Terraform does not support variables inside a variable. If you want to generate a value based on two or more variables then you can try Terraform locals (https://www.terraform.io/docs/configuration/locals.html).

Locals should help you here to achieve goal.

something like

locals { tags = {         Environment = "${var.EnvironmentName}"         CostCentre = "C1234"         Project = "TerraformTest"         Department = "Systems"        } } 

And then you can use as local.tags

resource “azurerm_resource_group” “TestAppRG” {     name = “EUW-RGs-${var.EnvShortName}”     location = “${var.Location}”     tags = “${local.tags}” } 

Hope this helps