# ARG

```
ARG <name>[=<default value>]
```

**ARG指令**定义了一个用户能够在构建时通过**docker build --build-arg \<key>=\<value>**&#x4F20;递给构建器的变量。如果用户指定了一个没有在**Dockerfile**中定义的构建参数，构建将输出一个警告。

```
[Warning] One or more build-args [foo] were not consumed.
```

**Dockerfile**可以包含多个**ARG指令**，例如下面就是一个有效的**Dockerfile**：

```
FROM busybox
ARG user1
ARG buildno
# ...
```

**Warning：**

不推荐使用构建时（**build-time**）变量传递私密信息（**secrets**）**,** 如**github**密钥、用户凭据等。任何一个镜像的用户使用**docker history**命令都可以查看到镜像的构建时（**build-time**）变量值。

参考使用[**BuildKit构建镜像**](https://docs.docker.com/develop/develop-images/build_enhancements/#new-docker-build-secret-information)的部分，了解在构建镜像时使用**secrets**的安全方法 。


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