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

環境

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

環境變數（用 ENV語句 宣告）也可以在某些指令中用作由 Dockerfile 解釋的變數。轉義也可以通過將類似變數的語法實際包含在語句中來進行處理。

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

環境變數在 Dockerfile 中用 $variable_name 或 ${variable_name} 表示。它們被同等對待，並且大括號語法通常用於解決變數名稱沒有空格的問題，例如 ${foo}_bar。

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

${variable_name} 語法還支援一些標準的 bash 修飾符，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} 表示如果設定了 variable 則結果將是該值。如果未設定 variable，那麼將是 word。

• ${variable:+word}表示如果設定了 variable 則將以 word 作為結果，否則結果為空字串。

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

在所有情況下，word 可以是任何字串，包括其他環境變數。

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

可以通過在變數前新增一個\來進行轉義：例如，\$foo 或 \${foo} 將分別轉換為 $foo 和 ${foo}。

Example (parsed representation is displayed after the #):

示例（解析的表示形式顯示在 ＃ 之後）：

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援環境變數：

• 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:

在整個指令中，環境變數替換將對每個變數使用相同的值。換句話說，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.

將導致 def 的值為 hello，而不是 bye。但是，ghi 將具有 bye 的值，因為它不是將 abc 設定為 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]#

總結

介紹了 Dockerfile 中的環境變數和 ENV 指令的使用。