CMD sets an image’s default command or default arguments for when a container starts; it does not run a command during the image build. For build-time work—such as installing packages or creating files—use RUN. At startup, Docker combines the image’s CMD with its ENTRYPOINT, if present, and command-line arguments can replace the defaults.
What does CMD do in a Dockerfile?
CMD records the default action or arguments in the image configuration. Building the image stores that configuration; the configured default takes effect when you start a container. Docker’s Dockerfile reference puts it plainly: “CMD doesn’t execute anything at build time, but specifies the intended command for the image.”
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By contrast, RUN executes during the build, and its result becomes part of an image layer. For example:
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CMD ["curl", "--version"]The package installation happens while the image is built. If you start a container from that image without supplying a replacement command, Docker uses curl --version as the default. See Docker’s Dockerfile overview for the distinction between build instructions and the command used when a container starts.
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Which CMD syntax should you use?
Docker documents three forms:
CMD ["executable", "param1", "param2"]
CMD ["param1", "param2"]
CMD command param1 param2- Exec form with an executable: The first form specifies the default command and its arguments.
- Exec form with arguments only: The second form provides default arguments for an
ENTRYPOINT; it is not a complete command by itself. - Shell form: The third form is written as a command line rather than a JSON array. Docker processes it through a shell.
Only the last CMD instruction in a Dockerfile takes effect. If you intend CMD to provide default arguments to an ENTRYPOINT, Docker recommends using exec form for both instructions.
How CMD and ENTRYPOINT work together
A Dockerfile should specify at least one of CMD or ENTRYPOINT. Use ENTRYPOINT when the image should behave like a particular executable; use CMD for a default command or for default arguments to that executable.
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CMD supplies a replaceable default command
Without an ENTRYPOINT, CMD supplies the default command. Arguments provided after the image name in docker run replace that default command and its arguments.
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Exec-form ENTRYPOINT keeps the executable fixed
ENTRYPOINT ["python", "app.py"]
CMD ["--port", "8000"]With this combination, the default startup runs python app.py --port 8000. If you run docker run my-image --port 9000, Docker keeps the exec-form entrypoint and replaces the CMD arguments with --port 9000. This is useful when the executable should stay fixed but users need to change its options.
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Shell-form ENTRYPOINT does not pass arguments the same way
Docker’s reference says a shell-form ENTRYPOINT ignores both CMD and command-line arguments supplied to docker run. If you expect runtime arguments to reach an entrypoint, do not assume shell form behaves like the exec-form pattern above.
How to override CMD when starting a container
Docker’s running containers guide documents the command shape as docker run [OPTIONS] IMAGE [COMMAND] [ARG...]. The command is optional when the image already has a default.
# Use the image's CMD default
docker run my-image
# Replace CMD with another command and its arguments
docker run my-image echo hello
# Replace ENTRYPOINT; this also clears the image's default CMD
docker run --entrypoint /bin/sh my-image- To use the default: Run the image name without a command after it. Docker uses the image’s startup configuration.
- To replace CMD: Put the replacement command and any arguments after the image name. If there is no
ENTRYPOINT, these replaceCMD; with an exec-formENTRYPOINT, they become its arguments and replace theCMDarguments. - To replace ENTRYPOINT: Use
--entrypointbefore the image name, followed by the image and any arguments for the new entrypoint. Docker clears the image’s defaultCMDwhen this option is used.
These are choices made for a container startup. They do not change the Dockerfile or rebuild the image.
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Choosing between CMD and ENTRYPOINT
| Pattern | Use it when | Runtime behavior |
|---|---|---|
CMD alone |
The image needs a default command that users may replace wholesale. | Arguments after the image name replace the default command and arguments. |
Exec-form ENTRYPOINT plus exec-form CMD |
The executable should remain fixed while users can adjust its default arguments. | Arguments after the image name replace the CMD arguments and are passed to the entrypoint. |
Shell-form ENTRYPOINT |
Use only when its shell-command behavior is intended. | Docker’s reference says it ignores CMD and docker run command-line arguments. |
Common CMD mistakes
- Expecting a build-time command: Use
RUNfor actions that must happen while building.CMDsets startup defaults. - Assuming CMD always names an executable: With an
ENTRYPOINT, exec-formCMDcan contain only default arguments. - Expecting runtime arguments to be added to CMD: Arguments after the image name replace the
CMDdefaults. With exec-formENTRYPOINT, they replace the default arguments passed to that entrypoint. - Using shell-form ENTRYPOINT but expecting normal argument passing: Docker documents that shell-form
ENTRYPOINTignoresCMDanddocker runarguments. - Adding several CMD instructions: Only the last one applies.
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