Define Dependent Environment Variables

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

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

• Killercoda
• Play with Kubernetes

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:

pods/inject/dependent-envars.yaml

apiVersion: v1 kind: Pod metadata:  name: dependent-envars-demo spec:  containers:  - name: dependent-envars-demo  args:  - while true; do echo -en '\n'; printf UNCHANGED_REFERENCE=$UNCHANGED_REFERENCE'\n'; printf SERVICE_ADDRESS=$SERVICE_ADDRESS'\n';printf ESCAPED_REFERENCE=$ESCAPED_REFERENCE'\n'; sleep 30; done;  command:  - sh  - -c  image: busybox:1.28  env:  - name: SERVICE_PORT  value: "80"  - name: SERVICE_IP  value: "172.17.0.1"  - name: UNCHANGED_REFERENCE  value: "$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)"  - name: PROTOCOL  value: "https"  - name: SERVICE_ADDRESS  value: "$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)"  - name: ESCAPED_REFERENCE  value: "$$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)" 

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.

What's next

• Learn more about environment variables.
• See EnvVarSource.