terraform - set a variable from commandline for module variable

Question

I have following structure

main.tf
modules
--moduleA
----worker.tf
----variables.tf

Content of main.tf:

module "moduleA" {
  source = "./modules/moduleA"
}

Content of variables.tf:

variable "num_of_workers" {
  type        = number
  description = "This is number of workers"
  default     = 1

I want co call terraform apply var="num_of_workers=12" I am getting an error:

Error: Value for undeclared variable
│ A variable named "num_of_workers" was assigned on the command line, but the root module does not declare a variable of that name. To use this value, add a "variable" block to the configuration.

Is there any way to set variables in variables.tf in module and set them from commandline? What I am missing here?

Eugen Konkov · Accepted Answer · 2024-10-18 18:29:34Z

7

You need to also declare the variable at the parent level. Then you would pass the parent level value to the module like this:

variable "num_of_workers" {
  type = number
}

module "moduleA" {
  source = "./modules/moduleA"
  num_of_workers = var.num_of_workers
}

Then you would set the parent-level value at the command line like this:

terraform apply -var num_of_workers=2

Documentation: https://developer.hashicorp.com/terraform/language/values/variables

edited Oct 18, 2024 at 18:29

Eugen Konkov

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answered Dec 28, 2021 at 18:50

Mark B

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9 Comments

Wojtas.Zet Over a year ago

Yes, I got to this answer by trial and error myself. I hoped there could be a more elegant solution to this - I would like to keep everything in the moduleA itself. So basically now I have a dummy variable placeholder defined in module A: variable "num_of_workers" {type = number} and a whole variable with validation defined in root level - more code, and the glue num_of_workers = var.num_of_workers. It is not elegant, but works.

Wojtas.Zet Over a year ago

whats more, somehow it does not work with targeting. Any idea why? terraform apply -target=modules.moduleA="num_of_workers=4" does not use the variable value, and says no changes.

Mark B Over a year ago

I find it very elegant that Terraform modules are entirely self contained and don't leak their variables into the parent module. The way you are saying it "should" work would actually break the ability to use a module multiple times in the parent module.

Mark B Over a year ago

As for the targeting, please take a look at the syntax. terraform apply -target=modules.moduleA="num_of_workers=4" You are missing the -var argument, and have added unnecessary quotes. It should be something like this: terraform apply -target=modules.moduleA -var num_of_workers=2

Mark B Over a year ago

@EugenKonkov there's an example in the official documentation: developer.hashicorp.com/terraform/language/values/variables terraform apply -var='image_id_list=["ami-abc123","ami-def456"]' -var="instance_type=t2.micro"

|

Ervin Szilagyi · Accepted Answer · 2021-12-28 16:00:26Z

1

You can assign a variable from the command line as follows:

terraform apply -var num_of_workers=2

Or like this:

terraform apply -var 'num_of_workers=2'

Reference: Terraform docs

edited Dec 28, 2021 at 16:00

answered Dec 28, 2021 at 15:53

Ervin Szilagyi

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3 Comments

Wojtas.Zet Over a year ago

yes, that's actually how I am calling it. The issue is different though - terraform requires variable to be defined twice in order to work, and only allows to pass variable defined in the top level root directory which is a shame - see answer above.

Mark B Over a year ago

@Wojtas.Zet Saying "Terraform requires variables to be defined twice in order to work" is not really correct. Terraform requires variables to be defined at the top level, if you want to override them at the command line. Overriding variables at the command line is not the only reason to use variables in Terraform, and they "work" just fine for other purposes without "defining them twice".

Wojtas.Zet Over a year ago

I suppose it is the way it is. For my use case it would be nicer to be able to pass them just to the module without another definition in the root module. I am trying to have several independent modules for different teams to use.

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