[Docker] RUN/CMD/ENTRYPOINT

Rex Chiang

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Mar 11, 2022

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Forms of Dockerfile

• Shell Form

RUN pip3 install -r requirements.txt
CMD echo Hello world
ENTRYPOINT echo Hello world

# In image generation, will transfer to exec form as below

RUN /bin/sh -c pip3 install -r requirements.txt
CMD ["/bin/sh" "-c" "echo Hello world"]
ENTRYPOINT ["/bin/sh" "-c" "echo Hello world"]

• Exec Form

RUN ["pip3", "install", "-r", "requirements.txt"]
CMD ["echo", "Hello world"]
ENTRYPOINT ["echo", "Hello world"]

# In image generation, will transfer to exec form as below

RUN pip3 install -r requirements.txt
CMD ["echo" "Hello world"]
ENTRYPOINT ["echo" "Hello world"]

RUN

• Execute commands in a new layer and commit to create a new image.
• Execute commands during image generation, instead of container execution.
• Will use the the result in cache when you try to rebuild the image, but you can avoid this by using — no-cache in argument.
• Usually used for installing software packages or fixed updating.

CMD

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
CMD ["echo", "Hello Word"]< Terminal >% docker image build -t hello .
% docker run -it hello
% Hello Word

• Set a default command when container runs.
• If there are many CMD in Dockerfile, only the last command is valid.
• Usually used for unfixed command.

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
CMD ["echo", "Hello Word"]< Terminal >% docker image build -t hello .
% docker run -it hello echo Hello Rex
% Hello Rex

• If container runs with a command, the default CMD will be replaced.

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
ENTRYPOINT ["echo", "Hello"]
CMD ["Rex"]< Terminal >% docker image build -t hello .
% docker run -it hello
% Hello Rex

• If there are no executable element, then it will be the parameter of ENTRYPOINT.

ENTRYPOINT

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
ENTRYPOINT ["echo", "Hello Word"]< Terminal >% docker image build -t hello .
% docker run -it hello
% Hello Word

• Configure the container to executable, and raise error in container execution if it wasn’t set.
• Set command when container runs.
• If there are many commands in Dockerfile, only the last command is valid.
• Usually used for fixed command.

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
ENTRYPOINT ["echo", "Hello"]< Terminal >% docker image build -t hello .
% docker run -it hello Rex
% Hello Rex

• If container runs with argument, then it will be the parameter of ENTRYPOINT.

Advanced Usage

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
ENTRYPOINT ["echo", "Hello"]
CMD ["Word"]< Terminal >% docker image build -t hello .
% docker run -it hello Rex
% Hello Rex

• If container runs with argument, then the default CMD will be replaced, and it will be the parameter of ENTRYPOINT.

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
ENTRYPOINT echo Hello
CMD ["Word"]< Terminal >% docker image build -t hello .
% docker run -it hello Rex
% Hello

• If use shell form in ENTRYPOINT, then the CMD will not be the parameter of ENTRYPOINT.

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
ENV name Rex
ENTRYPOINT echo Hello $name< Terminal >% docker image build -t hello .
% docker run -it hello Rex
% Hello Rex

• If use shell form, it can work with variable substituted.

< Dockerfile >FROM ubuntu
LABEL maintainer = "Rex"
ENV name Rex
ENTRYPOINT ["echo", "Hello $name"]< Terminal >% docker image build -t hello .
% docker run -it hello Rex
% Hello $name

• If use exec form, it can’t work with variable substituted.