# Predefined ARGS（预先定义好的ARG变量）

**Docker**有一组预先定义**ARG**变量，我们可以在**Dockerfile**中不声明**ARG**变量的情况下使用这些变量。

* **HTTP\_PROXY**
* **http\_proxy**
* **HTTPS\_PROXY**
* **https\_proxy**
* **FTP\_PROXY**
* **ftp\_proxy**
* **NO\_PROXY**
* **no\_proxy**

为了使用这些变量，我们可以通过命令行标志传递值。

```bash
--build-arg <varname>=<value>
```

默认情况下，这些预先定义的变量被排除在**docker history**的输出之外。排除它们可降低在**HTTP\_PROXY**变量中意外泄露敏感身份验证信息的风险。

思考下面的例子：**Dockerfile**使用&#x4E86;**--build-arg HTTP\_PROXY=**[**http://user:pass@proxy.lon.example.com**](http://user:pass@proxy.lon.example.com)

```
FROM ubuntu
RUN echo "Hello World"
```

在这种情况下，**HTTP\_PROXY**变量的值在**docker history**中不可用，并且未缓存。如果想要更改位置，将代理服务器更改为[http://user:pass@proxy.sfo.example.com，则后续的构建不会导致缓存丢失。](http://user:pass@proxy.sfo.example.xn--com,-jb5fke78ejzk32e24pexfh5bp3rn84ay90che2awsar82b)

如果需要覆盖此行为，则可以在**Dockerfile**中添加**ARG**声明，如下所示：

```
FROM ubuntu
ARG HTTP_PROXY
RUN echo "Hello World"
```

当构建这个**Dockerfile**时，**HTTP\_PROXY**会保留在**docker history**中，修改该变量的值会导致缓存失效。


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