Move to Injecting section: Environment Variables. (#3801)

docs/tasks/inject-data-application/environment-variable-expose-pod-information.md

@@ -0,0 +1,180 @@
+---
+title: Exposing Pod Information to Containers Through Environment Variables
+redirect_from:
+- "/docs/user-guide/environment-guide/"
+- "/docs/user-guide/environment-guide/index.html"
+- "/docs/tasks/configure-pod-container/environment-variable-expose-pod-information/"
+- "/docs/tasks/configure-pod-container/environment-variable-expose-pod-information.html"
+---
+
+{% capture overview %}
+
+This page shows how a Pod can use environment variables to expose information
+about itself to Containers running in the Pod. Environment variables can expose
+Pod fields and Container fields.
+
+There are two ways to expose Pod and Container fields to a running Container:
+environment variables and
+[DownwardAPIVolumeFiles](/docs/resources-reference/v1.6/#downwardapivolumefile-v1-core).
+Together, these two ways of exposing Pod and Container fields are called the
+*Downward API*.
+
+{% endcapture %}
+
+
+{% capture prerequisites %}
+
+{% include task-tutorial-prereqs.md %}
+
+{% endcapture %}
+
+
+{% capture steps %}
+
+## The Downward API
+
+There are two ways to expose Pod and Container fields to a running Container:
+
+* Environment variables
+* [DownwardAPIVolumeFiles](/docs/resources-reference/v1.6/#downwardapivolumefile-v1-core)
+
+Together, these two ways of exposing Pod and Container fields are called the
+*Downward API*.
+
+
+## Using Pod fields as values for environment variables
+
+In this exercise, you create a Pod that has one Container. Here is the
+configuration file for the Pod:
+
+{% include code.html language="yaml" file="dapi-envars-pod.yaml" ghlink="/docs/tasks/inject-data-application/dapi-envars-pod.yaml" %}
+
+In the configuration file, you can see five environment variables. The `env`
+field is an array of
+[EnvVars](/docs/resources-reference/v1.6/#envvar-v1-core).
+The first element in the array specifies that the `MY_NODE_NAME` environment
+variable gets its value from the Pod's `spec.nodeName` field. Similarly, the
+other environment variables get their names from Pod fields.
+
+**Note**: The fields in this example are Pod fields. They are not fields of the
+Container in the Pod.
+
+Create the Pod:
+
+```shell
+kubectl create -f http://k8s.io/docs/tasks/inject-data-application/dapi-envars-pod.yaml
+```
+
+Verify that the Container in the Pod is running:
+
+```
+kubectl get pods
+```
+
+View the Container's logs:
+
+```
+kubectl logs dapi-envars-fieldref
+```
+
+The output shows the values of selected environment variables:
+
+```
+minikube
+dapi-envars-fieldref
+default
+172.17.0.4
+default
+```
+
+To see why these values are in the log, look at the `command` and `args` fields
+in the configuration file. When the Container starts, it writes the values of
+five environment variables to stdout. It repeats this every ten seconds.
+
+Next, get a shell into the Container that is running in your Pod:
+
+```
+kubectl exec -it dapi-envars-fieldref -- sh
+```
+
+In your shell, view the environment variables:
+
+```
+/# printenv
+```
+
+The output shows that certain environment variables have been assigned the
+values of Pod fields:
+
+```
+MY_POD_SERVICE_ACCOUNT=default
+...
+MY_POD_NAMESPACE=default
+MY_POD_IP=172.17.0.4
+...
+MY_NODE_NAME=minikube
+...
+MY_POD_NAME=dapi-envars-fieldref
+```
+
+## Using Container fields as values for environment variables
+
+In the preceding exercise, you used Pod fields as the values for environment
+variables. In this next exercise, you use Container fields as the values for
+environment variables. Here is the configuration file for a Pod that has one
+container:
+
+{% include code.html language="yaml" file="dapi-envars-container.yaml" ghlink="/docs/tasks/inject-data-application/dapi-envars-container.yaml" %}
+
+In the configuration file, you can see four environment variables. The `env`
+field is an array of
+[EnvVars](/docs/resources-reference/v1.6/#envvar-v1-core).
+The first element in the array specifies that the `MY_CPU_REQUEST` environment
+variable gets its value from the `requests.cpu` field of a Container named
+`test-container`. Similarly, the other environment variables get their values
+from Container fields.
+
+Create the Pod:
+
+```shell
+kubectl create -f http://k8s.io/docs/tasks/inject-data-application/dapi-envars-container.yaml
+```
+
+Verify that the Container in the Pod is running:
+
+```
+kubectl get pods
+```
+
+View the Container's logs:
+
+```
+kubectl logs dapi-envars-resourcefieldref
+```
+
+The output shows the values of selected environment variables:
+
+```
+1
+1
+33554432
+67108864
+```
+
+{% endcapture %}
+
+{% capture whatsnext %}
+
+* [Defining Environment Variables for a Container](/docs/tasks/configure-pod-container/define-environment-variable-container/)
+* [PodSpec](/docs/resources-reference/v1.6/#podspec-v1-core)
+* [Container](/docs/resources-reference/v1.6/#container-v1-core)
+* [EnvVar](/docs/resources-reference/v1.6/#envvar-v1-core)
+* [EnvVarSource](/docs/resources-reference/v1.6/#envvarsource-v1-core)
+* [ObjectFieldSelector](/docs/resources-reference/v1.6/#objectfieldselector-v1-core)
+* [ResourceFieldSelector](/docs/resources-reference/v1.6/#resourcefieldselector-v1-core)
+
+{% endcapture %}
+
+
+{% include templates/task.md %}
+