Variables in Bake
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.
variable "TAG" {
default = "docker.io/username/webapp:latest"
}
The following example shows how to use the TAG
variable in a target.
target "default" {
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
.
variable "TAG" {
default = "latest"
}
To interpolate the TAG
variable into the value of an attribute, use the
${TAG}
syntax.
target "default" {
context = "."
dockerfile = "Dockerfile"
tags = ["docker.io/username/webapp:${TAG}"]
}
Printing the Bake file with the --print
flag shows the interpolated value in
the resolved build configuration.
$ docker buildx bake --print
{
"group": {
"default": {
"targets": ["webapp"]
}
},
"target": {
"webapp": {
"context": ".",
"dockerfile": "Dockerfile",
"tags": ["docker.io/username/webapp:latest"]
}
}
}
Using variables in variables across files
When multiple files are specified, one file can use variables defined in
another file. In the following example, the vars.hcl
file defines a
BASE_IMAGE
variable with a default value of docker.io/library/alpine
.
variable "BASE_IMAGE" {
default = "docker.io/library/alpine"
}
The following docker-bake.hcl
file defines a BASE_LATEST
variable that
references the BASE_IMAGE
variable.
variable "BASE_LATEST" {
default = "${BASE_IMAGE}:latest"
}
target "default" {
contexts = {
base = BASE_LATEST
}
}
When you print the resolved build configuration, using the -f
flag to specify
the vars.hcl
and docker-bake.hcl
files, you see that the BASE_LATEST
variable is resolved to docker.io/library/alpine:latest
.
$ docker buildx bake -f vars.hcl -f docker-bake.hcl --print app
{
"target": {
"default": {
"context": ".",
"contexts": {
"base": "docker.io/library/alpine:latest"
},
"dockerfile": "Dockerfile"
}
}
}
Additional resources
Here are some additional resources that show how you can use variables in Bake:
- You can override
variable
values using environment variables. See Overriding configurations for more information. - You can refer to and use global variables in functions. See HCL functions
- You can use variable values when evaluating expressions. See Expression evaluation