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[!IMPORTANT]
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env Function

The env function gets the value for an environment variable inside input variables. This is the only function that is callable from a variable block. You can only use the env function in the default input.

variable "aws_region" {
  default = env("AWS_DEFAULT_REGION")
}

In the previous example, the value of aws_region will be what's stored in the AWS_DEFAULT_REGION env var, unless aws_region is also set in a manner that takes precedence.

-> Why can't I use environment variables elsewhere? User variables are the single source of configurable input. We felt that having environment variables used anywhere in a configuration would confuse the user about the possible inputs to a template. By allowing environment variables only within default values for input variables, input variables remain as the single source of input to a template that a user can easily discover using packer inspect.

When the environment variable is not set at all -- not even with the empty string -- the value returned by env will be an empty string. It will still be possible to set it using other means but you could use custom validation rules to error in that case to make sure it is set, for example:

variable "aws_region" {
  default = env("AWS_DEFAULT_REGION")

  validation {
    condition     = length(var.aws_region) > 0
    error_message = <<EOF
The aws_region var is not set: make sure to at least set the AWS_DEFAULT_REGION env var.
To fix this you could also set the aws_region variable from the arguments, for example:
$ packer build -var=aws_region=us-something-1...
EOF
  }
}