mirror of https://github.com/docker/cli.git
Builder default shell
Signed-off-by: John Howard <jhoward@microsoft.com>
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@ -497,7 +497,8 @@ generated images.
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RUN has 2 forms:
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- `RUN <command>` (*shell* form, the command is run in a shell - `/bin/sh -c`)
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- `RUN <command>` (*shell* form, the command is run in a shell, which by
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default is `/bin/sh -c` on Linux or `cmd /S /C` on Windows)
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- `RUN ["executable", "param1", "param2"]` (*exec* form)
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The `RUN` instruction will execute any commands in a new layer on top of the
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@ -509,7 +510,10 @@ concepts of Docker where commits are cheap and containers can be created from
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any point in an image's history, much like source control.
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The *exec* form makes it possible to avoid shell string munging, and to `RUN`
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commands using a base image that does not contain `/bin/sh`.
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commands using a base image that does not contain the specified shell executable.
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The default shell for the *shell* form can be changed using the `SHELL`
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command.
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In the *shell* form you can use a `\` (backslash) to continue a single
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RUN instruction onto the next line. For example, consider these two lines:
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@ -1469,7 +1473,7 @@ For example you might add something like this:
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## STOPSIGNAL
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STOPSIGNAL signal
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STOPSIGNAL signal
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The `STOPSIGNAL` instruction sets the system call signal that will be sent to the container to exit.
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This signal can be a valid unsigned number that matches a position in the kernel's syscall table, for instance 9,
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@ -1541,6 +1545,120 @@ generated with the new status.
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The `HEALTHCHECK` feature was added in Docker 1.12.
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## SHELL
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SHELL ["executable", "parameters"]
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The `SHELL` instruction allows the default shell used for the *shell* form of
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commands to be overridden. The default shell on Linux is `["/bin/sh", "-c"]`, and on
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Windows is `["cmd", "/S", "/C"]`. The `SHELL` instruction *must* be written in JSON
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form in a Dockerfile.
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The `SHELL` instruction is particularly useful on Windows where there are
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two commonly used and quite different native shells: `cmd` and `powershell`, as
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well as alternate shells available including `sh`.
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The `SHELL` instruction can appear multiple times. Each `SHELL` instruction overrides
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all previous `SHELL` instructions, and affects all subsequent instructions. For example:
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FROM windowsservercore
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# Executed as cmd /S /C echo default
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RUN echo default
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# Executed as cmd /S /C powershell -command Write-Host default
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RUN powershell -command Write-Host default
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# Executed as powershell -command Write-Host hello
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SHELL ["powershell", "-command"]
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RUN Write-Host hello
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# Executed as cmd /S /C echo hello
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SHELL ["cmd", "/S"", "/C"]
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RUN echo hello
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The following instructions can be affected by the `SHELL` instruction when the
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*shell* form of them is used in a Dockerfile: `RUN`, `CMD` and `ENTRYPOINT`.
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The following example is a common pattern found on Windows which can be
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streamlined by using the `SHELL` instruction:
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...
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RUN powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
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...
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The command invoked by docker will be:
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cmd /S /C powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
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This is inefficient for two reasons. First, there is an un-necessary cmd.exe command
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processor (aka shell) being invoked. Second, each `RUN` instruction in the *shell*
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form requires an extra `powershell -command` prefixing the command.
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To make this more efficient, one of two mechanisms can be employed. One is to
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use the JSON form of the RUN command such as:
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...
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RUN ["powershell", "-command", "Execute-MyCmdlet", "-param1 \"c:\\foo.txt\""]
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...
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While the JSON form is unambiguous and does not use the un-necessary cmd.exe,
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it does require more verbosity through double-quoting and escaping. The alternate
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mechanism is to use the `SHELL` instruction and the *shell* form,
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making a more natural syntax for Windows users, especially when combined with
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the `escape` parser directive:
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# escape=`
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FROM windowsservercore
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SHELL ["powershell","-command"]
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RUN New-Item -ItemType Directory C:\Example
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ADD Execute-MyCmdlet.ps1 c:\example\
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RUN c:\example\Execute-MyCmdlet -sample 'hello world'
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Resulting in:
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PS E:\docker\build\shell> docker build -t shell .
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Sending build context to Docker daemon 3.584 kB
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Step 1 : FROM windowsservercore
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---> 5bc36a335344
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Step 2 : SHELL powershell -command
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---> Running in 87d7a64c9751
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---> 4327358436c1
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Removing intermediate container 87d7a64c9751
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Step 3 : RUN New-Item -ItemType Directory C:\Example
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---> Running in 3e6ba16b8df9
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Directory: C:\
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Mode LastWriteTime Length Name
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---- ------------- ------ ----
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d----- 6/2/2016 2:59 PM Example
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---> 1f1dfdcec085
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Removing intermediate container 3e6ba16b8df9
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Step 4 : ADD Execute-MyCmdlet.ps1 c:\example\
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---> 6770b4c17f29
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Removing intermediate container b139e34291dc
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Step 5 : RUN c:\example\Execute-MyCmdlet -sample 'hello world'
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---> Running in abdcf50dfd1f
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Hello from Execute-MyCmdlet.ps1 - passed hello world
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---> ba0e25255fda
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Removing intermediate container abdcf50dfd1f
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Successfully built ba0e25255fda
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PS E:\docker\build\shell>
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The `SHELL` instruction could also be used to modify the way in which
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a shell operates. For example, using `SHELL cmd /S /C /V:ON|OFF` on Windows, delayed
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environment variable expansion semantics could be modified.
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The `SHELL` instruction can also be used on Linux should an alternate shell be
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required such `zsh`, `csh`, `tcsh` and others.
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The `SHELL` feature was added in Docker 1.12.
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## Dockerfile examples
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