140 lines
4.7 KiB
Markdown
140 lines
4.7 KiB
Markdown
---
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title: Define Environment Variables for a Container
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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 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 variable for a container
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When you create a Pod, you can set environment variables for the containers
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that run in the Pod. To set environment variables, include the `env` or
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`envFrom` field in the configuration file.
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The `env` and `envFrom` fields have different effects.
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`env`
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: allows you to set environment variables for a container, specifying a value directly for each variable that you name.
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`envFrom`
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: allows you to set environment variables for a container by referencing either a ConfigMap or a Secret.
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When you use `envFrom`, all the key-value pairs in the referenced ConfigMap or Secret
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are set as environment variables for the container.
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You can also specify a common prefix string.
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You can read more about [ConfigMap](/docs/tasks/configure-pod-container/configure-pod-configmap/#configure-all-key-value-pairs-in-a-configmap-as-container-environment-variables)
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and [Secret](/docs/tasks/inject-data-application/distribute-credentials-secure/#configure-all-key-value-pairs-in-a-secret-as-container-environment-variables).
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This page explains how to use `env`.
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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 environment variable with name `DEMO_GREETING` and
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value `"Hello from the environment"`. Here is the configuration manifest for the
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Pod:
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{{% code_sample file="pods/inject/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/envars.yaml
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```
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1. List the running Pods:
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```shell
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kubectl get pods -l purpose=demonstrate-envars
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```
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The output is similar to:
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```
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NAME READY STATUS RESTARTS AGE
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envar-demo 1/1 Running 0 9s
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```
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1. List the Pod's container environment variables:
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```shell
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kubectl exec envar-demo -- printenv
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```
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The output is similar to this:
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```
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NODE_VERSION=4.4.2
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EXAMPLE_SERVICE_PORT_8080_TCP_ADDR=10.3.245.237
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HOSTNAME=envar-demo
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...
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DEMO_GREETING=Hello from the environment
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DEMO_FAREWELL=Such a sweet sorrow
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```
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{{< note >}}
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The environment variables set using the `env` or `envFrom` field
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override any environment variables specified in the container image.
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{{< /note >}}
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{{< note >}}
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Environment variables may reference each other, however ordering is important.
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Variables making use of others defined in the same context must come later in
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the list. Similarly, avoid circular references.
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{{< /note >}}
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## Using environment variables inside of your config
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Environment variables that you define in a Pod's configuration under
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`.spec.containers[*].env[*]` can be used elsewhere in the configuration, for
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example in commands and arguments that you set for the Pod's containers.
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In the example configuration below, the `GREETING`, `HONORIFIC`, and
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`NAME` environment variables are set to `Warm greetings to`, `The Most
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Honorable`, and `Kubernetes`, respectively. The environment variable
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`MESSAGE` combines the set of all these environment variables and then uses it
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as a CLI argument passed to the `env-print-demo` container.
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Environment variable names consist of letters, numbers, underscores,
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dots, or hyphens, but the first character cannot be a digit.
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If the `RelaxedEnvironmentVariableValidation` [feature gate](/docs/reference/command-line-tools-reference/feature-gates/) is enabled,
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all [printable ASCII characters](https://www.ascii-code.com/characters/printable-characters) except "=" may be used for environment variable names.
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```yaml
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apiVersion: v1
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kind: Pod
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metadata:
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name: print-greeting
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spec:
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containers:
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- name: env-print-demo
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image: bash
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env:
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- name: GREETING
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value: "Warm greetings to"
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- name: HONORIFIC
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value: "The Most Honorable"
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- name: NAME
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value: "Kubernetes"
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- name: MESSAGE
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value: "$(GREETING) $(HONORIFIC) $(NAME)"
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command: ["echo"]
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args: ["$(MESSAGE)"]
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```
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Upon creation, the command `echo Warm greetings to The Most Honorable Kubernetes` is run on the container.
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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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* Learn about [using secrets as environment variables](/docs/concepts/configuration/secret/#using-secrets-as-environment-variables).
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* See [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core).
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