diff --git a/content/en/docs/tasks/inject-data-application/define-interdependent-environment-variables.md b/content/en/docs/tasks/inject-data-application/define-interdependent-environment-variables.md new file mode 100644 index 0000000000..74c5c245db --- /dev/null +++ b/content/en/docs/tasks/inject-data-application/define-interdependent-environment-variables.md @@ -0,0 +1,78 @@ +--- +title: Define Dependent Environment Variables +content_type: task +weight: 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 $(VAR_NAME) in the `value` of `env` in the configuration file. + +In this exercise, you create a Pod that runs one container. The configuration +file for the Pod defines an dependent environment variable with common usage defined. Here is the configuration manifest for the +Pod: + +{{< codenew file="pods/inject/dependent-envars.yaml" >}} + +1. Create a Pod based on that manifest: + + ```shell + kubectl apply -f https://k8s.io/examples/pods/inject/dependent-envars.yaml + ``` + ``` + pod/dependent-envars-demo created + ``` + +2. List the running Pods: + + ```shell + kubectl get pods dependent-envars-demo + ``` + ``` + NAME READY STATUS RESTARTS AGE + dependent-envars-demo 1/1 Running 0 9s + ``` + +3. Check the logs for the container running in your Pod: + + ```shell + kubectl logs pod/dependent-envars-demo + ``` + ``` + + UNCHANGED_REFERENCE=$(PROTOCOL)://172.17.0.1:80 + SERVICE_ADDRESS=https://172.17.0.1:80 + ESCAPED_REFERENCE=$(PROTOCOL)://172.17.0.1:80 + ``` + +As shown above, you have defined the correct dependency reference of `SERVICE_ADDRESS`, bad dependency reference of `UNCHANGED_REFERENCE` and skip dependent references of `ESCAPED_REFERENCE`. + +When an environment variable is already defined when being referenced, +the reference can be correctly resolved, such as in the `SERVICE_ADDRESS` case. + +When the environment variable is undefined or only includes some variables, the undefined environment variable is treated as a normal string, such as `UNCHANGED_REFERENCE`. Note that incorrectly parsed environment variables, in general, will not block the container from starting. + +The `$(VAR_NAME)` syntax can be escaped with a double `$`, ie: `$$(VAR_NAME)`. +Escaped references are never expanded, regardless of whether the referenced variable +is defined or not. This can be seen from the `ESCAPED_REFERENCE` case above. + +## {{% heading "whatsnext" %}} + + +* Learn more about [environment variables](/docs/tasks/inject-data-application/environment-variable-expose-pod-information/). +* See [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core). + diff --git a/content/en/examples/pods/inject/dependent-envars.yaml b/content/en/examples/pods/inject/dependent-envars.yaml new file mode 100644 index 0000000000..2509c6f47b --- /dev/null +++ b/content/en/examples/pods/inject/dependent-envars.yaml @@ -0,0 +1,26 @@ +apiVersion: v1 +kind: Pod +metadata: + name: dependent-envars-demo +spec: + containers: + - name: dependent-envars-demo + args: + - while true; do echo -en '\n'; printf UNCHANGED_REFERENCE=$UNCHANGED_REFERENCE'\n'; printf SERVICE_ADDRESS=$SERVICE_ADDRESS'\n';printf ESCAPED_REFERENCE=$ESCAPED_REFERENCE'\n'; sleep 30; done; + command: + - sh + - -c + image: busybox + env: + - name: SERVICE_PORT + value: "80" + - name: SERVICE_IP + value: "172.17.0.1" + - name: UNCHANGED_REFERENCE + value: "$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)" + - name: PROTOCOL + value: "https" + - name: SERVICE_ADDRESS + value: "$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)" + - name: ESCAPED_REFERENCE + value: "$$(PROTOCOL)://$(SERVICE_IP):$(SERVICE_PORT)"