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