Variable Interpolation


Using rancher up, environment variables from the machine running rancher can be used within the docker-compose.yml and rancher-compose.yml files. This is only supported in rancher commands and not in the Rancher UI.

How to use it

With the docker-compose.yml and rancher-compose.yml files, you can reference the environment variables on your machine. If there are no environment variables on the machine, it will replace the variable with a blank string. Rancher will provide a warning on which environment variables are not set. If using environment variables for image tags, please note that rancher will not strip the : from the image to fetch the latest image. Since the image name, i.e. <imagename>: is an invalid image name, no container will be deployed. It’s up to the user to ensure that all environment variables are present and valid on the machine.

Example

On our machine running rancher, we have an environment variable, IMAGE_TAG=14.04.

# Image tag is set as environment variable
$ env | grep IMAGE
IMAGE_TAG=14.04
# Run rancher
$ rancher up

Example docker-compose.yml

version: '2'
services:
  ubuntu:
    tty: true
    image: ubuntu:$IMAGE_TAG
    stdin_open: true


In Rancher, an ubuntu service will be deployed with an ubuntu:14.04 image.

Variable Interpolation Formats

Rancher supports the same formats as docker-compose.

version: '2'
services:
  web:
    # unbracketed name
    image: "$IMAGE"

    # bracketed name
    command: "${COMMAND}"

    # array element
    ports:
    - "${HOST_PORT}:8000"

    # dictionary item value
    labels:
      mylabel: "${LABEL_VALUE}"

    # unset value - this will expand to "host-"
    hostname: "host-${UNSET_VALUE}"

    # escaped interpolation - this will expand to "${ESCAPED}"
    command: "$${ESCAPED}"

Templating

Inside the docker-compose.yml, Rancher supports the ability to use conditional logic by using the Go template system.

Templating can be used with the Rancher CLI or combined with Rancher Catalog allow you to be able to configure your catalog templates to answer questions and change your files based on the answers.

Note: Currently we only support evaluating string comparisons.

Example

If you wanted to have the ability to produce a service that publishes ports externally versus internally, you would be able to set conditional logic to support this. In the example, port 8000 will be published under ports if the public variable is true. Otherwise, the ports will be published under expose. Using the catalog ability to answer questions is how the user will answer. In our example, the default value is true.

docker-compose.yml

version: '2'
services:
  web:
    image: nginx
    {{- if eq .Values.PUBLIC "true" }}
    ports:
    - 8000
    {{- else }}
    expose:
    - 8000
    {{- end }}

rancher-compose.yml

version: '2'
catalog:
  name: Nginx Application
  version: v0.0.1
  questions:
  - variable: PUBLIC
    label: Publish Ports?
    required: true
    default: true
    type: boolean

config.yml

name: "Nginx Application"
version: v0.0.1