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Lab #14: Create an image with SHELL instruction

Pre-requisite:

Tested Infrastructure

Platform Number of Instance Reading Time
Play with Docker 1 5 min

Pre-requisite

  • Create an account with DockerHub
  • Open PWD Platform on your browser
  • Click on Add New Instance on the left side of the screen to bring up Alpine OS instance on the right side

1- Install Docker latest version.
2- Shell instruction syntax 

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.

  • 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.

  • The SHELL instruction can appear multiple times. Each SHELL instruction overrides all previous SHELL instructions, and affects all subsequent instructions.

3- Create the Dockerfile with instruction. 

FROM 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.

The following example is a common pattern found on Windows which can be streamlined by using the SHELL instruction:


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

The command invoked by docker will be:


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.

  • 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:


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:

# escape=`

FROM windowsservercore
SHELL ["powershell","-command"]
RUN New-Item -ItemType Directory C:\Example
ADD Execute-MyCmdlet.ps1 c:\example\
RUN c:\example\Execute-MyCmdlet -sample 'hello world'
  1. output 
PS E:\docker\build\shell> docker build -t shell .
Sending build context to Docker daemon 3.584 kB
Step 1 : FROM windowsservercore
 ---> 5bc36a335344
Step 2 : SHELL powershell -command
 ---> Running in 87d7a64c9751
 ---> 4327358436c1
Removing intermediate container 87d7a64c9751
Step 3 : RUN New-Item -ItemType Directory C:\Example
 ---> Running in 3e6ba16b8df9


Directory: C:\


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
d-----         6/2/2016   2:59 PM                Example


 ---> 1f1dfdcec085
Removing intermediate container 3e6ba16b8df9
Step 4 : ADD Execute-MyCmdlet.ps1 c:\example\
 ---> 6770b4c17f29
Removing intermediate container b139e34291dc
Step 5 : RUN c:\example\Execute-MyCmdlet -sample 'hello world'
 ---> Running in abdcf50dfd1f
Hello from Execute-MyCmdlet.ps1 - passed hello world
 ---> ba0e25255fda
Removing intermediate container abdcf50dfd1f
Successfully built ba0e25255fda
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.
  • The SHELL instruction can also be used on Linux should an alternate shell be required such zsh, csh, tcsh and others.
  • The SHELL feature was added in Docker 1.12.

Next >> Lab #16: Entrypoint Vs RUN