---
title: 定义相互依赖的环境变量
content_type: task
weight: 20
---
<!-- 
title: Define Dependent Environment Variables
-->

<!-- overview -->

<!-- 
This page shows how to define dependent environment variables for a container
in a Kubernetes Pod.
-->
本页展示了如何为 Kubernetes Pod 中的容器定义相互依赖的环境变量。

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

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

<!-- steps -->

<!-- 
## Define an environment dependent variable for a container

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.

In this exercise, you create a Pod that runs one container. The configuration
file for the Pod defines a dependent environment variable with common usage defined. Here is the configuration manifest for the
Pod:
-->
## 为容器定义相互依赖的环境变量   {#define-an-environment-dependent-variable-for-a-container}

当创建一个 Pod 时，你可以为运行在 Pod 中的容器设置相互依赖的环境变量。
若要设置相互依赖的环境变量，你可以在配置清单文件的 `env` 的 `value` 中使用 $(VAR_NAME)。

在本练习中，你会创建一个单容器的 Pod。
此 Pod 的配置文件定义了一个已定义常用用法的相互依赖的环境变量。
下面是此 Pod 的配置清单：

{{% code_sample file="pods/inject/dependent-envars.yaml" %}}

<!--
1. Create a Pod based on that manifest:
-->
1. 依据清单创建 Pod：

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

<!--
2. List the running Pods:
-->
2. 列出运行的 Pod：

   ```shell
   kubectl get pods dependent-envars-demo
   ```
   ```
   NAME                      READY     STATUS    RESTARTS   AGE
   dependent-envars-demo     1/1       Running   0          9s
   ```

<!--
3. Check the logs for the container running in your Pod:
-->
3. 检查 Pod 中运行容器的日志：

   ```shell
   kubectl logs pod/dependent-envars-demo
   ```
   ```

   UNCHANGED_REFERENCE=$(PROTOCOL)://172.17.0.1:80
   SERVICE_ADDRESS=https://172.17.0.1:80
   ESCAPED_REFERENCE=$(PROTOCOL)://172.17.0.1:80
   ```

<!-- 
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`.

When an environment variable is already defined when being referenced,
the reference can be correctly resolved, such as in the `SERVICE_ADDRESS` case.
-->
如上所示，你已经定义了 `SERVICE_ADDRESS` 的正确依赖引用，
`UNCHANGED_REFERENCE` 的错误依赖引用，
并跳过了 `ESCAPED_REFERENCE` 的依赖引用。

如果环境变量被引用时已事先定义，则引用可以正确解析，
比如 `SERVICE_ADDRESS` 的例子。

<!--
Note that order matters in the `env` list. An environment variable is not considered
"defined" if it is specified further down the list. That is why `UNCHANGED_REFERENCE`
fails to resolve `$(PROTOCOL)` in the example above.
-->
请注意，`env` 列表中的顺序很重要。如果某环境变量定义出现在列表的尾部，
则在解析列表前部环境变量时不会视其为“已被定义”。
这就是为什么 `UNCHANGED_REFERENCE` 在上面的示例中解析 `$(PROTOCOL)` 失败的原因。

<!-- 
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.

The `$(VAR_NAME)` syntax can be escaped with a double `$`, ie: `$$(VAR_NAME)`.
Escaped references are never expanded, regardless of whether the referenced variable
is defined or not. This can be seen from the `ESCAPED_REFERENCE` case above.
-->
当环境变量未定义或仅包含部分变量时，未定义的变量会被当做普通字符串对待，
比如 `UNCHANGED_REFERENCE` 的例子。
注意，解析不正确的环境变量通常不会阻止容器启动。

`$(VAR_NAME)` 这样的语法可以用两个 `$` 转义，即：`$$(VAR_NAME)`。
无论引用的变量是否定义，转义的引用永远不会展开。
这一点可以从上面 `ESCAPED_REFERENCE` 的例子得到印证。

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

<!--
* Learn more about [environment variables](/docs/tasks/inject-data-application/environment-variable-expose-pod-information/).
* See [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core).
-->
* 进一步了解[环境变量](/zh-cn/docs/tasks/inject-data-application/environment-variable-expose-pod-information/)。
* 参阅 [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core)。