website/content/en/docs/tasks/inject-data-application/define-interdependent-environment-variables.md

title	content_type	weight
Define Dependent Environment Variables	task	20

This page shows how to define dependent environment variables for a container in a Kubernetes Pod.

{{% heading "prerequisites" %}}

{{< include "task-tutorial-prereqs.md" >}}

Define an environment dependent variable for a container

When you create a Pod, you can set dependent environment variables for the containers that run in the Pod. To set dependent environment variables, you can use $(VAR_NAME) in the value of env in the configuration file.

In this exercise, you create a Pod that runs one container. The configuration file for the Pod defines a dependent environment variable with common usage defined. Here is the configuration manifest for the Pod:

{{< codenew file="pods/inject/dependent-envars.yaml" >}}

1. Create a Pod based on that manifest:

   kubectl apply -f https://k8s.io/examples/pods/inject/dependent-envars.yaml
   

   pod/dependent-envars-demo created
   

2. List the running Pods:

   kubectl get pods dependent-envars-demo
   

   NAME                      READY     STATUS    RESTARTS   AGE
   dependent-envars-demo     1/1       Running   0          9s
   

3. Check the logs for the container running in your Pod:

   kubectl logs pod/dependent-envars-demo
   

   
   UNCHANGED_REFERENCE=$(PROTOCOL)://172.17.0.1:80
   SERVICE_ADDRESS=https://172.17.0.1:80
   ESCAPED_REFERENCE=$(PROTOCOL)://172.17.0.1:80
   

As shown above, you have defined the correct dependency reference of SERVICE_ADDRESS, bad dependency reference of UNCHANGED_REFERENCE and skip dependent references of ESCAPED_REFERENCE.

When an environment variable is already defined when being referenced, the reference can be correctly resolved, such as in the SERVICE_ADDRESS case.

Note that order matters in the env list. An environment variable is not considered "defined" if it is specified further down the list. That is why UNCHANGED_REFERENCE fails to resolve $(PROTOCOL) in the example above.

When the environment variable is undefined or only includes some variables, the undefined environment variable is treated as a normal string, such as UNCHANGED_REFERENCE. Note that incorrectly parsed environment variables, in general, will not block the container from starting.

The $(VAR_NAME) syntax can be escaped with a double $, ie: $$(VAR_NAME). Escaped references are never expanded, regardless of whether the referenced variable is defined or not. This can be seen from the ESCAPED_REFERENCE case above.

{{% heading "whatsnext" %}}

• Learn more about environment variables.
• See [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core).