RUN has 2 forms:
RUN <command> (shell form, the command is run in a shell, which by default is /bin/sh -c on Linux or cmd /S /C on Windows) RUN ["executable", "param1", "param2"] (exec form)
RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting committed image will be used for the next step in the
RUN instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image’s history, much like source control.
The exec form makes it possible to avoid shell string munging, and to
RUN commands using a base image that does not contain the specified shell executable.
The default shell for the shell form can be changed using the
In the shell form you can use a
\ (backslash) to continue a single
RUN instruction onto the next line. For example, consider these two lines:
RUN /bin/bash -c 'source $HOME/.bashrc ;\ echo $HOME'
Together they are equivalent to this single line:
RUN /bin/bash -c 'source $HOME/.bashrc ; echo $HOME'
Note: To use a different shell, other than ‘/bin/sh’, use the exec form passing in the desired shell. For example,
RUN ["/bin/bash", "-c", "echo hello"]
Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (
“) around words not single-quotes (
Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example,
RUN [ "echo", "$HOME" ] will not do variable substitution on
$HOME. If you want shell processing then either use the shell form or execute a shell directly, for example:
RUN [ "sh", "-c", "echo $HOME" ].
Note: In the JSON form, it is necessary to escape backslashes. This is particularly relevant on Windows where the backslash is the path separator. The following line would otherwise be treated as shell form due to not being valid JSON, and fail in an unexpected way:
The correct syntax for this example is:
The cache for
RUN instructions isn’t invalidated automatically during the next build. The cache for an instruction like
RUN apt-get dist-upgrade -y will be reused during the next build. The cache for
RUN instructions can be invalidated by using the --no-cache flag, for example docker build --no-cache.
See the Dockerfile Best Practices guide for more information.
The cache for
RUN instructions can be invalidated by
ADD instructions. See below for details.