## A variable value must be known:

Take the following variable for example:

```hcl
variable "foo" {
  type = string
```

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

|                               |          no default          | `default = null` | `default = "xy"` |
| :---------------------------: | :--------------------------: | :--------------: | :--------------: |
|          foo unused           | error, "foo needs to be set" |        -         |        -         |
|            var.foo            | error, "foo needs to be set" |      null¹       |        xy        |
| `PKR_VAR_foo=yz`<br />var.foo |              yz              |        yz        |        yz        |
|  `-var foo=yz`<br />var.foo   |              yz              |        yz        |        yz        |

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

```hcl
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.