packer-cn/website/pages/guides/hcl/variables.mdx

---
layout: guides
page_title: Input and Local Variables guide
sidebar_title: 'Variables'
description: |-
  This page introduces input variables and local variables as a way to
  parameterize a configuration.
---

# Input Variables and local variables

@include 'guides/hcl2-beta-note.mdx'

This page introduces input variables and local variables as a way to
parameterize a configuration. Once defined, input variables are settable from a
default value, environment, special var files, and command line arguments.
Local variables can be a compound of input variables and local variables.

## Defining Variables and locals

Let's create a file `variables.pkr.hcl` with the following contents.

-> **Note**: that the file can be named anything, since Packer loads all
files ending in `.pkr.hcl` in a directory. If you split your configuration
across multiple files, use `packer build <source directory>` to initiate
a build.

```hcl
# variables.pkr.hcl

variable "access_key" {}
variable "secret_key" {}
variable "region" {
  default = "us-east-1"
}

locals {
  debian_ami_name = "${var.image_id}-debian"
  foo             = "bar"
}
```

This defines three variables within your Packer configuration. The first
two have empty blocks `{}`. The third sets a default. If a default value is
set, the variable is optional. Otherwise, the variable is required.
This also defines two locals: `debian_ami_name` and `foo`.

-> **Note**: that it is _not_ possible to use variables in a variable definition
but it _is_ possible to use locals and variables in a local definition.

## Using Variables and locals in Configuration

Next, you can define a source using the variables :

```hcl
# source.pkr.hcl

source "amazon-ebs" "debian" {
  ami_name   = local.debian_ami_name
  access_key = var.aws_access_key
  secret_key = "${var.aws_secret_key}"
  region     = "${var.aws_region}"
}
```

This uses more interpolations, this time prefixed with `var.` and `.local`.
This tells Packer that you're accessing variables. This configures the builder
with the given variables.

## Assigning Variables

There are multiple ways to assign variables. Below is also the order
in which variable values are chosen. The following is the descending order
of precedence in which variables are considered. So first defined; first used.

#### Command-line flags

You can set variables directly on the command-line with the
`-var` flag.:

```shell
$ packer build \
  -var 'access_key=foo' \
  -var 'secret_key=bar'
# ...
```

Once again, setting variables this way will not save them, and they'll
have to be input repeatedly as commands are executed.

#### From a file

To persist variable values, create a file and assign variables within
this file. Create a file named `variables.pkrvars.hcl` with the following
contents:

```hcl
access_key = "foo"
secret_key = "bar"
```

For all files which match `*.auto.pkrvars.hcl` present in the current
directory, Packer automatically loads them to populate variables. If the file
is named something else, you can use the `-var-file` flag directly to specify a
file. These files are the same syntax as Packer configuration files. And like
Packer configuration files, these files can also be JSON.

We don't recommend saving usernames and password to version control, but you
can create a local secret variables file and use `-var-file` to load it.

You can use multiple `-var-file` arguments in a single command, with some
checked in to version control and others not checked in. For example:

```shell
$ packer build \
  -var-file="secret.pkrvars.hcl" \
  -var-file="production.pkrvars.hcl"
```

#### From environment variables

Packer will read environment variables in the form of `PKR_VAR_name` to find
the value for a variable. For example, the `PKR_VAR_access_key` variable can be
set to set the `access_key` variable.

#### Variable Defaults

If no value is assigned to a variable via any of these methods and the
variable has a `default` key in its declaration, that value will be used
for the variable.

#### Unspecified values fails

If you execute `packer build` with certain variables unspecified and those are
used somewhere, Packer will error.

## Lists

Lists are defined either explicitly or implicitly

```hcl
# implicitly by using brackets [...]
variable "cidrs" { default = [] }

# explicitly
variable "cidrs" { type = "list" }
```

You can specify lists in a `variables.pkrvars.hcl` file:

```hcl
cidrs = [ "10.0.0.0/16", "10.1.0.0/16" ]
```

## Maps

Maps are a way to create variables that are lookup tables. An example
will show this best. Let's extract our AMIs into a map and add
support for the `us-west-2` region as well:

```hcl
variable "amis" {
  type = "map"
  default = {
    "us-east-1" = "ami-b374d5a5"
    "us-west-2" = "ami-4b32be2b"
  }
}
```

A variable can have a `map` type assigned explicitly, or it can be implicitly
declared as a map by specifying a default value that is a map. The above
demonstrates both.

Then we can `lookup` in maps like in the following:

```hcl
source "amazon-ebs" "example" {
  source_ami    = "${lookup(var.amis, var.region)}"
  instance_type = "t2.micro"
}
```

This introduces a new type of interpolation: a function call. The
`lookup` function does a dynamic lookup in a map for a key. The
key is `var.region`, which specifies that the value of the region
variables is the key.

You can also do a static lookup of a map directly with
`${var.amis["us-east-1"]}`.

## Assigning Maps

We set defaults above, but maps can also be set using the `-var` and
`-var-file` values. For example:

```shell
$ packer build -var 'amis={ us-east-1 = "foo", us-west-2 = "bar" }'
# ...
```

Here is an example of setting a map's keys from a file. Starting with these
variable definitions:

```hcl
variable "region" {}
variable "amis" {
  type = "map"
}
```

You can specify keys in a `variables.pkrvars.hcl` file:

```hcl
amis = {
  "us-east-1" = "ami-abc123"
  "us-west-2" = "ami-def456"
}
```

And access them via `lookup()`:

```hcl
output "ami" {
  value = "${lookup(var.amis, var.region)}"
}
```

Like so:

```shell
$ packer build -var region=us-west-2
```