# 理解CMD和ENTRYPOINT是如何交互的

**CMD**和**ENTRYPOINT指令**定义了容器运行时应该执行什么命令。有一些规则描述了它们彼此之间的协作。

1. **Dockerfile**中至少应该指定一个**CMD**或者**ENTRYPOINT**命令
2. 当将容器作为可执行命令运行时，需要一个**ENTRYPOINT**
3. **CMD**应该作为定义**ENTRYPOINT**默认参数的一种方式或者在容器中作为一个**ad-hoc**命令。
4. 在运行带有替代参数的容器时，**CMD**将被覆盖。

下面的表列出了针对不同的**ENTRYPOINT/CMD组合**所执行的命令：

|                                   | **No ENTRYPOINT**                | **ENTRYPOINT exec\_entry p1\_entry** | **ENTRYPOINT \[“exec\_entry”, “p1\_entry”]**           |
| --------------------------------- | -------------------------------- | ------------------------------------ | ------------------------------------------------------ |
| **No CMD**                        | **error, not allowed**           | **/bin/sh -c exec\_entry p1\_entry** | **exec\_entry p1\_entry**                              |
| **CMD \[“exec\_cmd”, “p1\_cmd”]** | **exec\_cmd p1\_cmd**            | **/bin/sh -c exec\_entry p1\_entry** | **exec\_entry p1\_entry exec\_cmd p1\_cmd**            |
| **CMD \[“p1\_cmd”, “p2\_cmd”]**   | **p1\_cmd p2\_cmd**              | **/bin/sh -c exec\_entry p1\_entry** | **exec\_entry p1\_entry p1\_cmd p2\_cmd**              |
| **CMD exec\_cmd p1\_cmd**         | **/bin/sh -c exec\_cmd p1\_cmd** | **/bin/sh -c exec\_entry p1\_entry** | **exec\_entry p1\_entry /bin/sh -c exec\_cmd p1\_cmd** |

**NOTE:**

如果**CMD**是在基础镜像中定义的，设置**ENTRYPOINT**将重置**CMD**为空值。在这种情形下，**CMD**在当前的镜像中必须拥有一个值。


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