Expression evaluation in Bake

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.

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.

$ 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.

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.

$ 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".

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.

$ docker buildx bake --print app

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