---
page_title: env function reference
description: The `env` function retrieves environment values for input variables. Learn how to use the `env` function in Packer templates.
---

# `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. 

```hcl
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](/packer/docs/templates/hcl_templates/variables#variable-definition-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](/packer/docs/templates/hcl_templates/variables#custom-validation-rules) to error in that case
to make sure it is set, for example:

```hcl
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
  }
}
```
