SHELL

SHELL ["executable", "parameters"]

SHELL指令被用于覆盖shell形式的命令的默认shell。在Linux上的默认shell是["/bin/sh", "-c"], 在windows上是["cmd", "/S", "/C"]。在Dockerfile中SHELL指令必须是JSON形式。

SHELL指令在Windows上特别有用，windows有两个常用的但是完全不同的原生shell：cmd和powershell，以及包括一个备用的sh。

SHELL指令可以出现多次。每一个SHELL指令可以覆盖先前的SHELL指令，并影响后续的指令。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

当RUN，CMD和ENTRYPOINT指令使用shell形式的命令时，都会受到SHELL指令的影响。

以下示例是Windows上常见的查找模式，可以通过使用SHELL指令进行简化：

RUN powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"

被docker调用的命令执行时将是下面这个样子：

cmd /S /C powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"

上面这种方式并不高效，原因有两个：

这里有一个不必要的cmd.exe命令处理器（aka shell）被调用。
每一个shell形式的RUN指令需要一个额外的powershell -command作为命令前缀。

为了提高效率，可以采用两种机制之一。一种是使用RUN命令的JSON形式，如：

RUN ["powershell", "-command", "Execute-MyCmdlet", "-param1 \"c:\\foo.txt\""]

虽然JSON形式比较清晰，不会使用不必要的cmd.exe，但通过双引号和转义使得写法非常啰嗦。替代方案是使用SHELL指令和shell形式，为Windows用户创建一种更自然的语法，特别与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'

运行结果：

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>

SHELL指令也可以被用于修改shell的操作。例如，在windows上使用SHELL CMD /S /C /V:ON|OFF可以修改延迟环境变量扩展语义。

SHELL特性在Docker1.12被添加。

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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

# 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'

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>