# args

添加只有在构建阶段才能访问的环境变量。

首先在Dockerfile中指定这些参数：

```
 ARG buildno
 ARG gitcommithash
 ​
 RUN echo "Build number: $buildno"
 RUN echo "Based on commit: $gitcommithash"
```

然后在Compose文件中的build字段下面指定这些参数的值：

```
 build:
   context: .
   args:
     buildno: 1
     gitcommithash: cdc3b19
```

```
 build:
   context: .
   args:
     - buildno=1
     - gitcommithash=cdc3b19
```

在指定构建参数时，可以省略该值。如果省略了值，则这些变量的值会从Compose运行的环境中去获取。

```
 args:
   - buildno
   - gitcommithash
```

Note:

使用boolean值时，用双引号包围这些值，以便可以被解析为字符串。


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