Fresh

Functions

Back

Manuals

• Get started
• Guides
• Reference

Functions

Copy as Markdown

Open MarkdownAsk Docs AIClaudeOpen in Claude

Table of contents

---

HCL functions are great for when you need to manipulate values in your build configuration in more complex ways than just concatenation or interpolation.

Standard library

Bake ships with built-in support for the standard library functions.

The following example shows the add function:

docker-bake.hcl

variable "TAG" {
  default = "latest"
}

group "default" {
  targets = ["webapp"]
}

target "webapp" {
  args = {
    buildno = "${add(123, 1)}"
  }
}

$ docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "buildno": "124"
      }
    }
  }
}

User-defined functions

You can create user-defined functions that do just what you want, if the built-in standard library functions don't meet your needs.

The following example defines an increment function.

docker-bake.hcl

function "increment" {
  params = [number]
  result = number + 1
}

group "default" {
  targets = ["webapp"]
}

target "webapp" {
  args = {
    buildno = "${increment(123)}"
  }
}

$ docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "args": {
        "buildno": "124"
      }
    }
  }
}

Variables in functions

You can make references to variables and standard library functions inside your functions.

The following example uses a global variable (REPO) in a custom function.

docker-bake.hcl

# docker-bake.hcl
variable "REPO" {
  default = "user/repo"
}

function "tag" {
  params = [tag]
  result = ["${REPO}:${tag}"]
}

target "webapp" {
  tags = tag("v1")
}

Printing the Bake file with the --print flag shows that the tag function uses the value of REPO to set the prefix of the tag.

$ docker buildx bake --print webapp

{
  "group": {
    "default": {
      "targets": ["webapp"]
    }
  },
  "target": {
    "webapp": {
      "context": ".",
      "dockerfile": "Dockerfile",
      "tags": ["user/repo:v1"]
    }
  }
}