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

---
title: Expose Pod Information to Containers Through Environment Variables
content_type: task
weight: 30
---

<!-- overview -->

This page shows how a Pod can use environment variables to expose information
about itself to containers running in the Pod, using the _downward API_.
You can use environment variables to expose Pod fields, container fields, or both.

In Kubernetes, there are two ways to expose Pod and container fields to a running container:

* _Environment variables_, as explained in this task
* [Volume files](/docs/tasks/inject-data-application/downward-api-volume-expose-pod-information/)

Together, these two ways of exposing Pod and container fields are called the
downward API.

## {{% heading "prerequisites" %}}

{{< include "task-tutorial-prereqs.md" >}}

<!-- steps -->

## Use Pod fields as values for environment variables

In this part of exercise, you create a Pod that has one container, and you
project Pod-level fields into the running container as environment variables.

{{< codenew file="pods/inject/dapi-envars-pod.yaml" >}}

In that manifest, you can see five environment variables. The `env`
field is an array of
environment variable definitions.
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.
{{< /note >}}

Create the Pod:

```shell
kubectl apply -f https://k8s.io/examples/pods/inject/dapi-envars-pod.yaml
```

Verify that the container in the Pod is running:

```shell
# If the new Pod isn't yet healthy, rerun this command a few times.
kubectl get pods
```

View the container's logs:

```shell
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:

```shell
kubectl exec -it dapi-envars-fieldref -- sh
```

In your shell, view the environment variables:

```shell
# Run this in a shell inside the container
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
```

## Use container fields as values for environment variables

In the preceding exercise, you used information from Pod-level fields as the values
for environment variables.
In this next exercise, you are going to pass fields that are part of the Pod
definition, but taken from the specific
[container](/docs/reference/kubernetes-api/workload-resources/pod-v1/#Container)
rather than from the Pod overall.

Here is a manifest for another Pod that again has just one container:

{{< codenew file="pods/inject/dapi-envars-container.yaml" >}}

In this manifest, you can see four environment variables. The `env`
field is an array of
environment variable definitions.
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 fields that are specific to this container.

Create the Pod:

```shell
kubectl apply -f https://k8s.io/examples/pods/inject/dapi-envars-container.yaml
```

Verify that the container in the Pod is running:

```shell
# If the new Pod isn't yet healthy, rerun this command a few times.
kubectl get pods
```

View the container's logs:

```shell
kubectl logs dapi-envars-resourcefieldref
```

The output shows the values of selected environment variables:

```
1
1
33554432
67108864
```

## {{% heading "whatsnext" %}}


* Read [Defining Environment Variables for a Container](/docs/tasks/inject-data-application/define-environment-variable-container/)
* Read the [`spec`](/docs/reference/kubernetes-api/workload-resources/pod-v1/#PodSpec)
  API definition for Pod. This includes the definition of Container (part of Pod).
* Read the list of [available fields](/docs/concepts/workloads/pods/downward-api/#available-fields) that you
  can expose using the downward API.

Read about Pods, containers and environment variables in the legacy API reference:

* [PodSpec](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#podspec-v1-core)
* [Container](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#container-v1-core)
* [EnvVar](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvar-v1-core)
* [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core)
* [ObjectFieldSelector](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#objectfieldselector-v1-core)
* [ResourceFieldSelector](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#resourcefieldselector-v1-core)