---
title: Variables in Bake
linkTitle: Variables
weight: 40
description: 
keywords: build, buildx, bake, buildkit, hcl, variables
---

You can define and use variables in a Bake file to set attribute values,
interpolate them into other values, and perform arithmetic operations.
Variables can be defined with default values, and can be overridden with
environment variables.

## Using variables as attribute values

Use the `variable` block to define a variable.

```hcl {title=docker-bake.hcl}
variable "TAG" {
  default = "docker.io/username/webapp:latest"
}
```

The following example shows how to use the `TAG` variable in a target.

```hcl {title=docker-bake.hcl}
target "webapp" {
  context = "."
  dockerfile = "Dockerfile"
  tags = [ TAG ]
}
```

## Interpolate variables into values

Bake supports string interpolation of variables into values. You can use the
`${}` syntax to interpolate a variable into a value. The following example
defines a `TAG` variable with a value of `latest`.

```hcl {title=docker-bake.hcl}
variable "TAG" {
  default = "latest"
}
```

To interpolate the `TAG` variable into the value of an attribute, use the
`${TAG}` syntax.

```hcl {title=docker-bake.hcl}
group "default" {
  targets = [ "webapp" ]
}

variable "TAG" {
  default = "latest"
}

target "webapp" {
  context = "."
  dockerfile = "Dockerfile"
  tags = ["docker.io/username/webapp:${TAG}"]
}
```

Printing the Bake file with the `--print` flag shows the interpolated value in
the resolved build configuration.

```console
$ docker buildx bake --print
```

```json
{
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": ["docker.io/username/webapp:latest"]
    }
  }
}
```

## Validating variables

To verify that the value of a variable conforms to an expected type, value
range, or other condition, you can define custom validation rules using the
`validation` block.

In the following example, validation is used to enforce a numeric constraint on
a variable value; the `PORT` variable must be 1024 or higher.

```hcl {title=docker-bake.hcl}
# Define a variable `PORT` with a default value and a validation rule
variable "PORT" {
  default = 3000  # Default value assigned to `PORT`

  # Validation block to ensure `PORT` is a valid number within the acceptable range
  validation {
    condition = PORT >= 1024  # Ensure `PORT` is at least 1024
    error_message = "The variable 'PORT' must be 1024 or higher."  # Error message for invalid values
  }
}
```

If the `condition` expression evaluates to `false`, the variable value is
considered invalid, whereby the build invocation fails and `error_message` is
emitted. For example, if `PORT=443`, the condition evaluates to `false`, and
the error is raised.

Values are coerced into the expected type before the validation is set. This
ensures that any overrides set with environment variables work as expected.

### Validate multiple conditions

To evaluate more than one condition, define multiple `validation` blocks for
the variable. All conditions must be `true`.

Here’s an example:

```hcl {title=docker-bake.hcl}
# Define a variable `VAR` with multiple validation rules
variable "VAR" {
  # First validation block: Ensure the variable is not empty
  validation {
    condition = VAR != ""
    error_message = "The variable 'VAR' must not be empty."
  }

  # Second validation block: Ensure the value contains only alphanumeric characters