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Packer - locally validate template for EC2 that expects an environment variable

I was working on creating EC2 images using Packer. For that, there's the amazon-ebs Packer Builder to use.

After checking that there's nothing wrong with the syntax, thanks to packer validate --syntax-only /path/to/packter-template.pkr.hcr, I ran packer validate /path/to/packter-template.pkr.hcr and got the error below:

packer validate --syntax-only /path/to/packter-template.pkr.hcr
Syntax-only check passed. Everything looks okay.
packer validate /path/to/packter-template.pkr.hcr
Error: 2 error(s) occurred:

* A source_ami or source_ami_filter must be specified
* For security reasons, your source AMI filter must declare an owner.

  on/path/to/packter-template.pkr.hcr line 42:
  (source code not available)

The question became

is source_ami set ? If so, why isn't packer finding it?

The source_ami is set but expects a value from an environment variable.

. . .
variable "source_ami" {
  type    = string
  default = env("SOURCE_AMI")
}
. . .
source "amazon-ebs" "ami_name" {
. . .
  source_ami   = var.source_ami
}

Because packer uses the env function, the next step is to set SOURCE_AMI as an environment variable.

In the CI part with AWS CodeBuild, SOURCE_AMI is set by Cloudformation. To validate packer locally means that I need to set SOURCE_AMI to some value.

Setting SOURCE_AMI as a "normal" variable, even after taking into account the precedence of how packer evaluates variables, doesn't work. E.g., using the -var option when ivoking packer: packer validate -var SOURCE_AMI=1234 Packer/template.pkr.hcl

In my Powershell console, I set the environment variable with $env:SOURCE_AMI = '1234' and invoke packer validate --syntax-only /path/to/packter-template.pkr.hcr