Bake files in the HCL format support expression evaluation, which lets you perform arithmetic operations, conditionally set values, and more.

Arithmetic operations#

You can perform arithmetic operations in expressions. The following example shows how to multiply two numbers.

```hcl {title=docker-bake.hcl} sum = 7*6

target "default" { args = { answer = sum } }

Printing the Bake file with the `--print` flag shows the evaluated value for
the `answer` build argument.

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

{
  "target": {
    "default": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "answer": "42"
      }
    }
  }
}

Ternary operators#

You can use ternary operators to conditionally register a value.

The following example adds a tag only when a variable is not empty, using the built-in notequal function.

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

target "default" { context="." dockerfile="Dockerfile" tags = [ "my-image:latest", notequal("",TAG) ? "my-image:${TAG}": "", ] }

In this case, `TAG` is an empty string, so the resulting build configuration
only contains the hard-coded `my-image:latest` tag.

```console
$ docker buildx bake --print

{
  "group": {
    "default": {
      "targets": ["default"]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": ["my-image:latest"]
    }
  }
}

Expressions with variables#

You can use expressions with variables to conditionally set values, or to perform arithmetic operations.

The following example uses expressions to set values based on the value of variables. The v1 build argument is set to "higher" if the variable FOO is greater than 5, otherwise it is set to "lower". The v2 build argument is set to "yes" if the IS_FOO variable is true, otherwise it is set to "no".

```hcl {title=docker-bake.hcl} variable "FOO" { default = 3 }

variable "IS_FOO" { default = true }

target "app" { args = { v1 = FOO > 5 ? "higher" : "lower" v2 = IS_FOO ? "yes" : "no" } }

Printing the Bake file with the `--print` flag shows the evaluated values for
the `v1` and `v2` build arguments.

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

{
  "group": {
    "default": {
      "targets": ["app"]
    }
  },
  "target": {
    "app": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "v1": "lower",
        "v2": "yes"
      }
    }
  }
}