Must Be Set

A variable value must be known:

Take the following variable for example:

variable "foo" {
  type = string
}

Here foo must have a known value but you can default it to null to make this behavior optional :

1: Null is a valid value. Packer will only error when the receiving field needs a value, example:

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

source "example" "foo" {
  arg = var.example
}

In the above case, as long as "arg" is optional for an "example" source, there is no error and arg won’t be set.