add define interdependent environment variables page

Signed-off-by: Weiping Cai <weiping.cai@daocloud.io>

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

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+---
+title: Define Dependent Environment Variables
+content_type: task
+weight: 20
+---
+
+<!-- overview -->
+
+This page shows how to define dependent environment variables for a container
+in a Kubernetes Pod.
+
+
+
+
+## {{% heading "prerequisites" %}}
+
+
+{{< include "task-tutorial-prereqs.md" >}}
+
+
+
+
+<!-- steps -->
+
+## 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 an 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:
+
+    ```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. Get log to the container running in your Pod:
+
+    ```shell
+    kubectl logs pod/dependent-envars-demo
+    ```
+    ```
+    SERVICE_ADDRESS
+    https://172.17.0.1:80
+    UNCHANGE_REFERENCE
+    $(PROTOCOL)://172.17.0.1:80
+    ESCAPED_REFERENCE
+    $(PROTOCOL)://172.17.0.1:80
+    ```
+
+As shown above, we have defined the correct dependency reference of `SERVICE_ADDRESS`, bad dependency reference of `UNCHANGE_REFERENCE` and skip dependent references of `ESCAPED_REFERENCE`.
+
+When the environment variable is defined, you can use it directly. You can use it after the definition is completed, such as `SERVICE_ADDRESS`.
+
+When the environment variable is undefined or only includes some variables, the undefined environment variable is treated as a normal string, such as `UNCHANGE_REFERENCE`. A bad reference does not interfere with the operation of the container.
+
+The $(VAR_NAME) syntax can be escaped with a double $$, ie: $$(VAR_NAME).Escaped references will never be expanded, regardless of whether the variable exists or not.
+
+## {{% heading "whatsnext" %}}
+
+
+* Learn more about [environment variables](/docs/tasks/inject-data-application/environment-variable-expose-pod-information/).
+* See [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core).
+
+

content/en/examples/pods/inject/dependent-envars.yaml

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+apiVersion: v1
+kind: Pod
+metadata:
+  name: dependent-envars-demo
+spec:
+  containers:
+  - name: dependent-envars-demo
+    args:
+    - while true; do echo -en '\n'; echo SERVICE_ADDRESS;printenv SERVICE_ADDRESS;echo UNCHANGE_REFERENCE;printenv UNCHANGE_REFERENCE;echo ESCAPED_REFERENCE;printenv ESCAPED_REFERENCE; sleep 30; done;
+    command:
+        - sh
+        - -c
+    image: busybox
+    env:
+    - name: SERVICE_PORT
+      value: "80"
+    - name: SERVICE_IP
+      value: "172.17.0.1"
+    - name: UNCHANGE_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)"