The variable `FOO` can *only* be overridden using CSV because `FOO_JSON`, which would typically used for a JSON override,
is already a defined variable.
Since `FOO_JSON` is an actual variable, setting that environment variable would be expected to a CSV value.
A JSON override *is* possible for this variable, using environment variable `FOO_JSON_JSON`.

```Console
# These three are all equivalent, setting variable FOO=bar
$ FOO=bar docker buildx bake <...>
$ FOO='bar' docker buildx bake <...>
$ FOO="bar" docker buildx bake <...>

# Sets *only* variable FOO_JSON; FOO is untouched
$ FOO_JSON=bar docker buildx bake <...>

# This also sets FOO_JSON, but will fail due to not being valid JSON
$ FOO_JSON_JSON=bar docker buildx bake <...>

# These are all equivalent
$ cat data.json
"bar"
$ FOO_JSON_JSON=$(< data.json) docker buildx bake <...>
$ FOO_JSON_JSON='"bar"' docker buildx bake <...>
$ FOO_JSON=bar docker buildx bake <...>

# This results in setting two different variables, both specified as CSV (FOO=bar and FOO_JSON="baz")
$ FOO=bar FOO_JSON='"baz"' docker buildx bake <...>

# These refer to the same variable with FOO_JSON_JSON having precedence and read as JSON (FOO_JSON=baz)
$ FOO_JSON=bar FOO_JSON_JSON='"baz"' docker buildx bake <...>
```

### Built-in variables

The following variables are built-ins that you can use with Bake without having
to define them.

| Variable              | Description                                                                         |
| --------------------- | ----------------------------------------------------------------------------------- |
| `BAKE_CMD_CONTEXT`    | Holds the main context when building using a remote Bake file.                      |
| `BAKE_LOCAL_PLATFORM` | Returns the current platform’s default platform specification (e.g. `linux/amd64`). |

### Use environment variable as default

You can set a Bake variable to use the value of an environment variable as a default value:

```hcl
variable "HOME" {
  default = "$HOME"
}
```

### Interpolate variables into attributes

To interpolate a variable into an attribute string value,
you must use curly brackets.
The following doesn't work:

```hcl
variable "HOME" {
  default = "$HOME"
}

target "default" {
  ssh = ["default=$HOME/.ssh/id_rsa"]
}
```

Wrap the variable in curly brackets where you want to insert it:

```diff
  variable "HOME" {
    default = "$HOME"
  }

  target "default" {
-   ssh = ["default=$HOME/.ssh/id_rsa"]
+   ssh = ["default=${HOME}/.ssh/id_rsa"]
  }
```

Before you can interpolate a variable into an attribute,
first you must declare it in the bake file,
as demonstrated in the following example.

```console
$ cat docker-bake.hcl
target "default" {
  dockerfile-inline = "FROM ${BASE_IMAGE}"
}
$ docker buildx bake
[+] Building 0.0s (0/0)
docker-bake.hcl:2
--------------------
   1 |     target "default" {
   2 | >>>   dockerfile-inline = "FROM ${BASE_IMAGE}"
   3 |     }
   4 |
--------------------
ERROR: docker-bake.hcl:2,31-41: Unknown variable; There is no variable named "BASE_IMAGE"., and 1 other diagnostic(s)
$ cat >> docker-bake.hcl

variable "BASE_IMAGE" {
  default = "alpine"
}

$ docker buildx bake
[+] Building 0.6s (5/5) FINISHED
```

## Function

A [set of general-purpose functions][bake_stdlib]
provided by [go-cty][go-cty]
are available for use in HCL files:

```hcl
# docker-bake.hcl
target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
  args = {
    buildno = "${add(123, 1)}"
  }
}
```

In addition, [user defined functions][userfunc]
are also supported:

```hcl
# docker-bake.hcl
function "increment" {
  params = [number]
  result = number + 1
}

target "webapp-dev" {
  dockerfile = "Dockerfile.webapp"
  tags = ["docker.io/username/webapp:latest"]
  args = {
    buildno = "${increment(123)}"
  }
}
```

> [!NOTE]
> See [User defined HCL functions][hcl-funcs] page for more details.

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