# Define Dependent Environment Variables LLMS index: [llms.txt](/llms.txt) --- 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:

## 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: ```shell kubectl apply -f https://k8s.io/examples/pods/inject/dependent-envars.yaml ``` ``` pod/dependent-envars-demo created ``` 2. List the running Pods: ```shell 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: ```shell 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](/docs/tasks/inject-data-application/environment-variable-expose-pod-information/). * See [EnvVarSource](/docs/reference/generated/kubernetes-api/v1.36/#envvarsource-v1-core).