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50 lines
1.9 KiB
Plaintext
50 lines
1.9 KiB
Plaintext
---
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page_title: env - Functions - Configuration Language
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description: The env function retrieves environment values for input variables.
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---
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# `env` Function
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```hcl
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variable "aws_region" {
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default = env("AWS_DEFAULT_REGION")
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}
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```
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`env` allows you to get the value for an environment variable inside input
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variables _only_. This is the only function that is callable from a variable
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block and it can only be used in the default input. `env` cannot be called from
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other places.
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In the previous example, the value of `aws_region` will be what's stored in the
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`AWS_DEFAULT_REGION` env var, unless aws_region is also set in a [manner that takes
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precedence](/docs/templates/hcl_templates/variables#variable-definition-precedence).
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-> **Why can't I use environment variables elsewhere?** User variables are the
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single source of configurable input. We felt that having environment variables
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used _anywhere_ in a configuration would confuse the user about the possible inputs
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to a template. By allowing environment variables only within default values for
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input variables, input variables remain as the single source of input to a
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template that a user can easily discover using `packer inspect`.
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When the environment variable is not set at all -- not even with the empty
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string -- the value returned by `env` will be an an empty string. It will still
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be possible to set it using other means but you could use [custom validation
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rules](/docs/templates/hcl_templates/variables#custom-validation-rules) to error in that case
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to make sure it is set, for example:
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```hcl
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variable "aws_region" {
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default = env("AWS_DEFAULT_REGION")
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validation {
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condition = length(var.aws_region) > 0
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error_message = <<EOF
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The aws_region var is not set: make sure to at least set the AWS_DEFAULT_REGION env var.
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To fix this you could also set the aws_region variable from the arguments, for example:
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$ packer build -var=aws_region=us-something-1...
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EOF
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}
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}
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```
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