79 lines
2.7 KiB
Markdown
79 lines
2.7 KiB
Markdown
---
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title: Define Dependent Environment Variables
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content_type: task
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weight: 20
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---
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<!-- overview -->
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This page shows how to define dependent environment variables for a container
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in a Kubernetes Pod.
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## {{% heading "prerequisites" %}}
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{{< include "task-tutorial-prereqs.md" >}}
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<!-- steps -->
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## Define an environment dependent variable for a container
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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.
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In this exercise, you create a Pod that runs one container. The configuration
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file for the Pod defines an dependent environment variable with common usage defined. Here is the configuration manifest for the
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Pod:
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{{< codenew file="pods/inject/dependent-envars.yaml" >}}
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1. Create a Pod based on that manifest:
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```shell
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kubectl apply -f https://k8s.io/examples/pods/inject/dependent-envars.yaml
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```
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```
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pod/dependent-envars-demo created
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```
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2. List the running Pods:
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```shell
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kubectl get pods dependent-envars-demo
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```
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```
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NAME READY STATUS RESTARTS AGE
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dependent-envars-demo 1/1 Running 0 9s
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```
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3. Check the logs for the container running in your Pod:
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```shell
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kubectl logs pod/dependent-envars-demo
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```
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```
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UNCHANGED_REFERENCE=$(PROTOCOL)://172.17.0.1:80
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SERVICE_ADDRESS=https://172.17.0.1:80
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ESCAPED_REFERENCE=$(PROTOCOL)://172.17.0.1:80
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```
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As shown above, you have defined the correct dependency reference of `SERVICE_ADDRESS`, bad dependency reference of `UNCHANGED_REFERENCE` and skip dependent references of `ESCAPED_REFERENCE`.
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When an environment variable is already defined when being referenced,
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the reference can be correctly resolved, such as in the `SERVICE_ADDRESS` case.
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When the environment variable is undefined or only includes some variables, the undefined environment variable is treated as a normal string, such as `UNCHANGED_REFERENCE`. Note that incorrectly parsed environment variables, in general, will not block the container from starting.
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The `$(VAR_NAME)` syntax can be escaped with a double `$`, ie: `$$(VAR_NAME)`.
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Escaped references are never expanded, regardless of whether the referenced variable
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is defined or not. This can be seen from the `ESCAPED_REFERENCE` case above.
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## {{% heading "whatsnext" %}}
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* Learn more about [environment variables](/docs/tasks/inject-data-application/environment-variable-expose-pod-information/).
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* See [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core).
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