【Docker】解析器指令之 escape

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

环境

1. virtual box 6.1
2. centos 7.8
3. docker 19.03

escape

# escape=\ (backslash)

Or

# escape=` (backtick)

The escape directive sets the character used to escape characters in a Dockerfile. If not specified, the default escape character is \.

escape 指令设置用于转义 Dockerfile 中的字符的字符。如果未指定，则默认转义字符为 \。

The escape character is used both to escape characters in a line, and to escape a newline. This allows a Dockerfile instruction to span multiple lines. Note that regardless of whether the escape parser directive is included in a Dockerfile, escaping is not performed in a RUN command, except at the end of a line.

转义字符用于转义一行中的字符和转义换行符。这允许 Dockerfile 指令跨越多行。请注意，无论 Dockerfile 中是否包含 escape 解析器指令，都不会在 RUN 命令中执行转义，除非在行末。

Setting the escape character to ` is especially useful on Windows, where \ is the directory path separator. ` is consistent with Windows PowerShell.

在 Windows 上将转义符设置为 ` 特别有用，其中 \ 是目录路径分隔符。 ` 与 Windows PowerShell 一致。

Consider the following example which would fail in a non-obvious way on Windows. The second \ at the end of the second line would be interpreted as an escape for the newline, instead of a target of the escape from the first \. Similarly, the \ at the end of the third line would, assuming it was actually handled as an instruction, cause it be treated as a line continuation. The result of this dockerfile is that second and third lines are considered a single instruction:

考虑以下示例，该示例将在 Windows 上以非明显的方式失败。第二行末尾的第二个 \ 将被解释为换行符的转义，而不是第一个 \ 的转义目标。类似地，假设实际将其作为指令处理，则第三行末尾的 \ 会导致将其视为行的延续。该 dockerfile 的结果是第二和第三行被视为一条指令：

FROM microsoft/nanoserver
COPY testfile.txt c:\\
RUN dir c:\

Results in:

PS C:\John> docker build -t cmd .
Sending build context to Docker daemon 3.072 kB
Step 1/2 : FROM microsoft/nanoserver
 ---> 22738ff49c6d
Step 2/2 : COPY testfile.txt c:\RUN dir c:
GetFileAttributesEx c:RUN: The system cannot find the file specified.
PS C:\John>

One solution to the above would be to use / as the target of both the COPY instruction, and dir. However, this syntax is, at best, confusing as it is not natural for paths on Windows, and at worst, error prone as not all commands on Windows support / as the path separator.

上面情况的一种解决方案是将 / 用作 COPY 指令和 dir 的目标。但是，这种语法充其量是令人困惑的，因为 Windows 上的路径并不自然，更糟糕的是容易出错，因为并非 Windows 上的所有命令都支持 / 作为路径分隔符。

By adding the escape parser directive, the following Dockerfile succeeds as expected with the use of natural platform semantics for file paths on Windows:

通过添加 escape 解析器指令，以下 Dockerfile 可以通过在 Windows 上为文件路径使用自然平台语义而成功完成：

# escape=`

FROM microsoft/nanoserver
COPY testfile.txt c:\
RUN dir c:\

Results in:

PS C:\John> docker build -t succeeds --no-cache=true .
Sending build context to Docker daemon 3.072 kB
Step 1/3 : FROM microsoft/nanoserver
 ---> 22738ff49c6d
Step 2/3 : COPY testfile.txt c:\
 ---> 96655de338de
Removing intermediate container 4db9acbb1682
Step 3/3 : RUN dir c:\
 ---> Running in a2c157f842f5
 Volume in drive C has no label.
 Volume Serial Number is 7E6D-E0F7

 Directory of c:\

10/05/2016  05:04 PM             1,894 License.txt
10/05/2016  02:22 PM    <DIR>          Program Files
10/05/2016  02:14 PM    <DIR>          Program Files (x86)
10/28/2016  11:18 AM                62 testfile.txt
10/28/2016  11:20 AM    <DIR>          Users
10/28/2016  11:20 AM    <DIR>          Windows
           2 File(s)          1,956 bytes
           4 Dir(s)  21,259,096,064 bytes free
 ---> 01c7f3bef04f
Removing intermediate container a2c157f842f5
Successfully built 01c7f3bef04f
PS C:\John>

总结

介绍了 Dockerfile 指令解析器的 escape 用法。