【Docker】Dockerfile 环境变量替换

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

环境

virtual box 6.1
centos 7.8
docker 19.03

Environment replacement

Environment variables (declared with the ENV statement) can also be used in certain instructions as variables to be interpreted by the Dockerfile. Escapes are also handled for including variable-like syntax into a statement literally.

环境变量（用 ENV语句声明）也可以在某些指令中用作由 Dockerfile 解释的变量。转义也可以通过将类似变量的语法实际包含在语句中来进行处理。

Environment variables are notated in the Dockerfile either with $variable_name or ${variable_name}. They are treated equivalently and the brace syntax is typically used to address issues with variable names with no whitespace, like ${foo}_bar.

环境变量在 Dockerfile 中用 $variable_name 或 ${variable_name} 表示。它们被同等对待，并且大括号语法通常用于解决变量名称没有空格的问题，例如 ${foo}_bar。

The ${variable_name} syntax also supports a few of the standard bash modifiers as specified below:

${variable_name} 语法还支持一些标准的 bash 修饰符，如下所示：

${variable:-word} indicates that if variable is set then the result will be that value. If variable is not set then word will be the result.
${variable:+word} indicates that if variable is set then word will be the result, otherwise the result is the empty string.
${variable:-word} 表示如果设置了 variable 则结果将是该值。如果未设置 variable，那么将是 word。
${variable:+word}表示如果设置了 variable 则将以 word 作为结果，否则结果为空字符串。

In all cases, word can be any string, including additional environment variables.

在所有情况下，word 可以是任何字符串，包括其他环境变量。

Escaping is possible by adding a before the variable: $foo or ${foo}, for example, will translate to $foo and ${foo} literals respectively.

可以通过在变量前添加一个来进行转义：例如，$foo 或 ${foo} 将分别转换为 $foo 和 ${foo}。

Example (parsed representation is displayed after the #):

示例（解析的表示形式显示在 ＃ 之后）：

FROM busybox
ENV FOO=/bar
WORKDIR ${FOO}   # WORKDIR /bar
ADD . $FOO       # ADD . /bar
COPY $FOO /quux # COPY $FOO /quux

Environment variables are supported by the following list of instructions in the Dockerfile:

Dockerfile 中的以下指令列表支持环境变量：

ADD
COPY
ENV
EXPOSE
FROM
LABEL
STOPSIGNAL
USER
VOLUME
WORKDIR
ONBUILD (when combined with one of the supported instructions above)

Environment variable substitution will use the same value for each variable throughout the entire instruction. In other words, in this example:

在整个指令中，环境变量替换将对每个变量使用相同的值。换句话说，在此示例中：

ENV abc=hello
ENV abc=bye def=$abc
ENV ghi=$abc

will result in def having a value of hello, not bye. However, ghi will have a value of bye because it is not part of the same instruction that set abc to bye.

将导致 def 的值为 hello，而不是 bye。但是，ghi 将具有 bye 的值，因为它不是将 abc 设置为 bye 的同一指令的一部分。

[root@master env]# docker build .
Sending build context to Docker daemon  2.048kB
Step 1/6 : FROM busybox
 ---> dc3bacd8b5ea
Step 2/6 : ENV abc=hello
 ---> Using cache
 ---> 2c674628ad0d
Step 3/6 : ENV abc=bye def=$abc
 ---> Using cache
 ---> 27401f4e57f7
Step 4/6 : ENV ghi=$abc
 ---> Using cache
 ---> ef6aa3e1c3ea
Step 5/6 : RUN echo $def
 ---> Running in 4cf63c33a97d
hello
Removing intermediate container 4cf63c33a97d
 ---> 72c1cae07a7c
Step 6/6 : RUN echo $ghi
 ---> Running in 3cba9330d595
bye
Removing intermediate container 3cba9330d595
 ---> a43f85633ec2
Successfully built a43f85633ec2
[root@master env]# cat Dockerfile
FROM busybox
ENV abc=hello
ENV abc=bye def=$abc
ENV ghi=$abc
RUN echo $def
RUN echo $ghi
[root@master env]#

总结

介绍了 Dockerfile 中的环境变量和 ENV 指令的使用。