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

---
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

In the legacy JSON packer templates, any variables we hadn't already defined in
the "variables" stanza of our json template could simply be passed in via the
command line or a var-file; if a variable was never defined it would generally
be interpolated to an empty string.

_In the HCL2 packer templates, we must always pre-define our variables in the
HCL equivalent of the "variables" stanza._

Another difference between JSON and HCL packer templates is that in JSON packer
templates, the "variables" stanza, if used, was always in the same .json file
as the builds and builders. In HCL, it can exist in its own file if you want it
to.

To demonstrate, 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 <command line flags> <source directory>` to initiate a build.

```hcl
//  variables.pkr.hcl

// For those variables that you don't provide a default for, you must
// set them from the command line, a var-file, or the environment.

variable "weekday" {}

variable "sudo_password" {
  type =  string
  default = "mypassword"
  // Sensitive vars are hidden from output as of Packer v1.6.5
  sensitive = true
}

variable "flavor" {
  type = string
  default = "strawberry"
}

variable "exit_codes" {
  type = list(number)
  default = [0]
}

locals {
  ice_cream_flavor = "${var.flavor}-ice-cream"
  foo             = "bar"
}
```

This defines several variables within your Packer configuration, each showing
off a different way to set them. The first variable, "weekday", is an empty
block `{}`, without a type or a default.

However, it's generally best to provide the type in your variable definition,
as you can see in variable "flavor", which we have given a type of "string",
and variable "exit_codes", which we have given a type of "list(number)",
meaning it is a list/array of numbers.

When a variable is passed from the cli or environment and the variable's type
is not set, Packer will expect it to be a string. But if it is passed from a
var-file where Packer can interpret HCL properly it can be a slice or any
supported type.

In addition to setting the type, the "flavor" and "exit_codes" variables also
provide a default. If you set a default value, then you don't need to set the
variable at run time. Packer will use a provided command-line var,
var-file, or environment var if it exists, but if not Packer will fall back to
this default value.

If you do not set a default value, Packer will fail immediately when you try to
run a `build` if you have not provided the missing variable via the
command-line, a var-file, or the environment. The `validate`, `inspect` and
`console` commands will work, but variables with unknown values will be
`<unknown>`.

This also defines two locals: `ice_cream_flavor` and `foo`.

-> **Note**: that it is _not_ possible to reference a variable in the
definition of another variable. But it _is_ possible to use locals and
variables in the definition of a local, as shown in the ice_cream_flavor
definition.

## Using Variables and locals in Configuration

For simplicity's sake, we're going to put a null source in the same file as
we are putting the build configuration. This file demonstrates how to use the
variables we previously defined.

```hcl
// null_example.pkr.hcl

source "null" "example" {
    communicator = "none"
}

build {
  sources = [
    "source.null.example"
  ]
  provisioner "shell-local" {
    // Note that for options that are documented as template engines,
    // we still have to use the golang template engine syntax rather than our
    // specialized HCL2 variable syntax. This example shows a combination of
    // an HCL2 variable and the golang template engines built into the
    // execute_command option
    execute_command  = ["/bin/sh", "-c", "echo ${var.sudo_password}| {{.Vars}} {{.Script}}"]
    environment_vars = ["HELLO_USER=packeruser", "UUID=${build.PackerRunUUID}"]
    inline           = ["echo the Packer run uuid is $UUID"]
  }
  provisioner "shell-local" {
    inline           = ["echo var.flavor is: ${var.flavor}",
                        "echo local.ice_cream_flavor is: ${local.ice_cream_flavor}"]
    valid_exit_codes = var.exit_codes
  }
}
```

As you can see in the example, you can access your variables directly by
giving them the `var.` or `local.` prefix. If you want to embed the variables
in a string, you can do so with the `${}` HCL interpolation syntax. If you are
using an option that is a template engine, you still need to use the golang
templating engine syntax `{{ .OPTION }}` for those engines.

## 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-session
$ packer build \
  -var 'weekday=Sunday' \
  -var 'flavor=chocolate' \
  -var 'sudo_password=hunter42' .
```

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

If you plan to assign variables via the command line, we strongly recommend that
you at least set a default type instead of using empty blocks; this helps the
HCL parser understand what is being set. Otherwise it will interpret all of your
command line variables as strings.

#### 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
sudo_password = "partyparrot"
weekday       = "Sunday"
```

You tell Packer to use this var file using the `-var-file` command line flag.

```shell-session
$ packer build -var-file="variables.pkrvars.hcl" .
```

Packer will automatically load any var file that matches the name
`*.auto.pkrvars.hcl`, without the need to pass the file via the command line.
if we rename the above var-file from variables.pkrvars.hcl to
variables.auto.pkrvars.hcl, then we can run our build simply by calling

```shell-session
$ packer build .
```

You may provide as many -var-file flags as you would like:

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

These files can also be JSON:

variables.json:

```json
{
  "weekday": "sunday",
  "flavor": "mint"
}
```

```shell-session
$ packer build -var-file=variables.json
```

We don't recommend saving sensitive information 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-session
$ 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.

```shell-session
$ export PKR_VAR_weekday=Monday
$ packer build .
```

#### 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.

If all of your variables have defaults, then you can call a packer build using:

```shell-session
$ packer build .
```

You can make this work for yourself using the variable example file above by
commenting out or removing the "weekday" variable declaration, since it is not
actually used in the example build.

If your variable definitions are stored in the same file as your source and
build, you can call the build against that specific file:

```shell-session
$ packer build self_contained_example.pkr.hcl
```

#### Unspecified Values Fail

If you call `packer build` with any variables defined but not set, Packer will
error.

## Variable Type Reference

### 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-session
$ 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-session
$ packer build -var region=us-west-2
```