Define Dependent Environment Variables [page]deterministic
This page shows how to define dependent environment variables for a container in a Kubernetes Pod.
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## 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 $(VAR_NAME) in the `value` of `env` in the configuration file.
In this exercise, you create a Pod that runs one container. The configuration file for the Pod defines a dependent environment variable with common usage defined. Here is the configuration manifest for the Pod:
1. Create a Pod based on that manifest:
```shell kubectl apply -f https://k8s.io/examples/pods/inject/dependent-envars.yaml ``` ``` pod/dependent-envars-demo created ```
2. List the running Pods:
```shell kubectl get pods dependent-envars-demo ``` ``` NAME READY STATUS RESTARTS AGE dependent-envars-demo 1/1 Running 0 9s ```
3. Check the logs for the container running in your Pod:
```shell kubectl logs pod/dependent-envars-demo ``` ```
UNCHANGED_REFERENCE=$(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.
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* Learn more about [environment variables](/docs/tasks/inject-data-application/environment-variable-expose-pod-information/). * See [EnvVarSource](/docs/reference/generated/kubernetes-api//#envvarsource-v1-core).