Terraform: use a variable's value to define another variable

I was looking for a way to set a default terraform variable value from another variable but also have a way to override. Terraform does not allow this natively:

variable nickname {
  default = var.fullname
}

variable fullname {
  default = "richard"
}

output name {
  value = var.nickname
}

$ terraform apply

Error: Variables not allowed

  on var-to-var.tf line 2, in variable "nickname":
   2:   default = var.fullname

Variables may not be used here.

But I was able to get this working, see below!

Disclaimer: This does not technically default the value from one variable to another. However, it provides the desired output; at least the one I was looking for.

variable fullname {
  default = "richard"
}

variable nickname {
  default = ""
}

output name {
  value = var.nickname != "" ? var.nickname : var.fullname
}

$ terraform apply
Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
Outputs:
name = richard

$ terraform apply -var nickname=drew
Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
Outputs:
name = drew

Comments

[David J Eddy]

The reason this works is due to Terraform variable values (and providers) do not support interpolation. The TF engine is not yet running when the values are assigned.

outputs on the other hand are evaluated near the end of a TF life cycle. Thus the engine is running and interpolation is supported.

Another way to to this is use a null object and apply the value = "${var.nickname != "" ? var.nickname : var.fullname}" logic to it.

Side note; might want to check out upgrading the TF 0.12.x when you get a chance. The resource/var/output sytax changes a bit.

[Colin Johnson]

This is brilliant -> thank you!!!

[Alexey B.]

I like it! realy useful while HCL does not not support interpolation in this way

[Sergey Podalov]

Thanks, but this works only for outputs.

[drewmullen]

worked for for me with an ssm parameter value:

resource "aws_ssm_parameter" "foo" {
  name  = "foo"
  type  = "String"
  value = var.nickname != "" ? var.nickname : var.fullname
}

$ tf apply

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # aws_ssm_parameter.foo will be created
  + resource "aws_ssm_parameter" "foo" {
      + arn       = (known after apply)
      + data_type = (known after apply)
      + id        = (known after apply)
      + key_id    = (known after apply)
      + name      = "foo"
      + tier      = "Standard"
      + type      = "String"
      + value     = (sensitive value)
      + version   = (known after apply)
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

aws_ssm_parameter.foo: Creating...
aws_ssm_parameter.foo: Creation complete after 4s [id=foo]

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

[Sergey Podalov]

It woks in a resource section and in a output section but it does not work if try to set an input variable this way.

[drewmullen]

correct. this is a work around for similar functionality