【Docker】Dockerfile 之 SHELL
参考教程：https://docs.docker.com/engine/reference/builder/
环境
- virtual box 6.1
- centos 7.8
- docker 19.03
SHELL
SHELL ["executable", "parameters"]
The SHELL instruction allows the default shell used for the shell form of commands to be overridden. The default shell on Linux is ["/bin/sh", "-c"], and on Windows is ["cmd", "/S", "/C"]. The SHELL instruction must be written in JSON form in a Dockerfile.
SHELL 指令允许覆盖用于 shell 形式的命令的默认 shell。在 Linux 上，默认 shell 是 ["/bin/sh", "-c"]，而在 Windows 上，默认 shell 是 ["cmd", "/S", "/C"]。 SHELL 指令必须以 JSON 格式写入 Dockerfile 中。
The SHELL instruction is particularly useful on Windows where there are two commonly used and quite different native shells: cmd and powershell, as well as alternate shells available including sh.
SHELL 指令在 Windows 上特别有用，在 Windows 上有两个常用且完全不同的本机shell：cmd 和 powershell，以及可用的替代 shell，包括 sh。
The SHELL instruction can appear multiple times. Each SHELL instruction overrides all previous SHELL instructions, and affects all subsequent instructions. For example:
SHELL 指令可以出现多次。每条 SHELL 指令都将覆盖所有先前的 SHELL 指令，并影响所有后续指令。例如：
FROM microsoft/windowsservercore
# Executed as cmd /S /C echo default
RUN echo default
# Executed as cmd /S /C powershell -command Write-Host default
RUN powershell -command Write-Host default
# Executed as powershell -command Write-Host hello
SHELL ["powershell", "-command"]
RUN Write-Host hello
# Executed as cmd /S /C echo hello
SHELL ["cmd", "/S", "/C"]
RUN echo hello
The following instructions can be affected by the SHELL instruction when the shell form of them is used in a Dockerfile: RUN, CMD and ENTRYPOINT.
当它们的 shell 形式在 Dockerfile 中使用时，以下指令可能会受到 SHELL 指令的影响：RUN，CMD 和 ENTRYPOINT。
The following example is a common pattern found on Windows which can be streamlined by using the SHELL instruction:
以下示例是 Windows 上常见的模式，可以通过使用 SHELL 指令进行精简：
RUN powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
The command invoked by docker will be:
docker 调用的命令将是：
cmd /S /C powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
This is inefficient for two reasons. First, there is an un-necessary cmd.exe command processor (aka shell) being invoked. Second, each RUN instruction in the shell form requires an extra powershell -command prefixing the command.
这效率低下有两个原因。首先，有一个不必要的 cmd.exe 命令处理器（也称为 shell）被调用。其次，shell 格式的每条 RUN 指令都需要在命令前面加上一个额外的 powershell -command。
To make this more efficient, one of two mechanisms can be employed. One is to use the JSON form of the RUN command such as:
为了使其更有效，可以采用两种机制之一。一种是使用 RUN 命令的 JSON 形式，例如：
RUN ["powershell", "-command", "Execute-MyCmdlet", "-param1 \"c:\\foo.txt\""]
While the JSON form is unambiguous and does not use the un-necessary cmd.exe, it does require more verbosity through double-quoting and escaping. The alternate mechanism is to use the SHELL instruction and the shell form, making a more natural syntax for Windows users, especially when combined with the escape parser directive:
尽管 JSON 形式是明确的，并且不使用不必要的 cmd.exe，但它确实需要通过双引号和转义来实现更多的详细信息。另一种机制是使用 SHELL 指令和 shell 形式，使 Windows 用户的语法更自然，尤其是与 escape 解析器指令结合使用时：
# escape=`
FROM microsoft/nanoserver
SHELL ["powershell","-command"]
RUN New-Item -ItemType Directory C:\Example
ADD Execute-MyCmdlet.ps1 c:\example\
RUN c:\example\Execute-MyCmdlet -sample 'hello world'
Resulting in:
PS E:\docker\build\shell> docker build -t shell .
Sending build context to Docker daemon 4.096 kB
Step 1/5 : FROM microsoft/nanoserver
---> 22738ff49c6d
Step 2/5 : SHELL powershell -command
---> Running in 6fcdb6855ae2
---> 6331462d4300
Removing intermediate container 6fcdb6855ae2
Step 3/5 : RUN New-Item -ItemType Directory C:\Example
---> Running in d0eef8386e97
Directory: C:\
Mode LastWriteTime Length Name
---- ------------- ------ ----
d----- 10/28/2016 11:26 AM Example
---> 3f2fbf1395d9
Removing intermediate container d0eef8386e97
Step 4/5 : ADD Execute-MyCmdlet.ps1 c:\example\
---> a955b2621c31
Removing intermediate container b825593d39fc
Step 5/5 : RUN c:\example\Execute-MyCmdlet 'hello world'
---> Running in be6d8e63fe75
hello world
---> 8e559e9bf424
Removing intermediate container be6d8e63fe75
Successfully built 8e559e9bf424
PS E:\docker\build\shell>
The SHELL instruction could also be used to modify the way in which a shell operates. For example, using SHELL cmd /S /C /V:ON|OFF on Windows, delayed environment variable expansion semantics could be modified.
SHELL 指令还可用于修改外壳的运行方式。例如，在 Windows 上使用 SHELL cmd /S /C /V:ON|OFF，可以修改延迟的环境变量扩展语义。
The SHELL instruction can also be used on Linux should an alternate shell be required such as zsh, csh, tcsh and others.
如果需要备用外壳，例如 zsh，csh，tcsh等，也可以在 Linux 上使用 SHELL 指令。
总结
介绍了 Dockerfile 中 SHELL 指令的用法和注意事项。
