【Docker】Dockerfile 之 CMD

参考教程：https://docs.docker.com/engine/reference/builder/

环境

1. virtual box 6.1
2. centos 7.8
3. docker 19.03

CMD

The CMD instruction has three forms:

CMD 指令具有三种形式：

• CMD ["executable","param1","param2"] (exec form, this is the preferred form)

• CMD ["param1","param2"] (as default parameters to ENTRYPOINT)

• CMD command param1 param2 (shell form)

• CMD ["executable","param1","param2"]（exec 形式，这是首选形式）

• CMD [“ param1”，“ param2”]（作为 ENTRYPOINT 的默认参数）

• CMD command param1 param2（shell 形式）

There can only be one CMD instruction in a Dockerfile. If you list more than one CMD then only the last CMD will take effect.

在 Dockerfile 中只能有一个 CMD 指令。如果您列出多个 CMD，那么只有最后一个 CMD 才会生效。

The main purpose of a CMD is to provide defaults for an executing container. These defaults can include an executable, or they can omit the executable, in which case you must specify an ENTRYPOINT instruction as well.

CMD 的主要目的是为执行中的容器提供默认值。这些默认值可以包含可执行文件，也可以省略可执行文件，在这种情况下，您还必须指定一条 ENTRYPOINT 指令。

If CMD is used to provide default arguments for the ENTRYPOINT instruction, both the CMD and ENTRYPOINT instructions should be specified with the JSON array format.

如果使用 CMD 为 ENTRYPOINT 指令提供默认参数，则 CMD 和 ENTRYPOINT 指令均应使用 JSON 数组格式指定。

Note

The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').

注意

exec 形式被解析为 JSON 数组，这意味着您必须使用双引号（"）而非单引号（'）包围单词。

Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo $HOME" ]. When using the exec form and executing a shell directly, as in the case for the shell form, it is the shell that is doing the environment variable expansion, not docker.

与 shell 格式不同，exec 格式不调用命令外壳程序。这意味着不会进行常规的外壳处理。例如，CMD [ "echo", "$HOME" ] 不会在 $HOME 上进行变量替换。如果要进行 shell 处理，则可以使用 shell 形式或直接执行 shell，例如： CMD [ "sh", "-c", "echo $HOME" ]。当使用 exec 格式并直接执行 shell 时，是由 shell 进行环境变量扩展，而不是 docker。

When used in the shell or exec formats, the CMD instruction sets the command to be executed when running the image.

当以 shell 或 exec 格式使用时， CMD 指令设置运行镜像时要执行的命令。

If you use the shell form of the CMD, then the <command> will execute in /bin/sh -c:

如果使用 CMD 的 shell 形式，则 <command> 将在 /bin/sh -c 中执行：

FROM ubuntu
CMD echo "This is a test." | wc -

If you want to run your <command> without a shell then you must express the command as a JSON array and give the full path to the executable. This array form is the preferred format of CMD. Any additional parameters must be individually expressed as strings in the array:

如果要在没有外壳的情况下运行您的 <command>，则必须将该命令表示为 JSON 数组，并提供可执行文件的完整路径。 数组形式是 CMD 的首选格式。 任何其他参数必须在数组中分别表示为字符串：

FROM ubuntu
CMD ["/usr/bin/wc","--help"]

If you would like your container to run the same executable every time, then you should consider using ENTRYPOINT in combination with CMD. See ENTRYPOINT.

如果您希望容器每次都运行相同的可执行文件，则应考虑将 ENTRYPOINT 与 CMD 结合使用。请参阅ENTRYPOINT。

If the user specifies arguments to docker run then they will override the default specified in CMD.

如果用户为 docker run 指定了参数，则它们将覆盖 CMD 中指定的默认值。

Note

Do not confuse RUN with CMD. RUN actually runs a command and commits the result; CMD does not execute anything at build time, but specifies the intended command for the image.

注意

不要将 RUN 和 CMD 混淆。RUN实际上运行命令并提交结果。CMD 在生成时不执行任何操作，但是指定了镜像的预期命令。

示例

Dockerfile

FROM busybox
CMD echo jiangbo

结果

[root@master env]# docker build -t jiangbo:0.0.1 .
Sending build context to Docker daemon  3.584kB
Step 1/2 : FROM busybox
 ---> dc3bacd8b5ea
Step 2/2 : CMD echo jiangbo
 ---> Running in 8eb219d5669f
Removing intermediate container 8eb219d5669f
 ---> c1f4d3207e37
Successfully built c1f4d3207e37
Successfully tagged jiangbo:0.0.1
[root@master env]# docker run -it jiangbo:0.0.1
jiangbo

总结

介绍了 Dockerfile 中 RUN 指令的使用。