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CMD and ENTRYPOINT

Updated Mar 11, 2022 ·

Overview​

Containers are designed to run specific applications rather than host full operating systems. For instance, when you launch a Linux image like Ubuntu, its default command (CMD) is bash. If there’s no terminal detected, the container will stop.

CMD​

You can define a default command for the container using the CMD keyword in the Dockerfile. For example, to make the container sleep for 60 seconds:

docker run ubuntu sleep 60

You can also include this command directly in the Dockerfile:

$ cat > Dockerfile 

FROM ubuntu
CMD sleep 60

Command Formats:

  • CMD <command> <parameter1>
  • CMD ["<command>", "<parameter1>"] (JSON format)

ENTRYPOINT​

To allow parameters from the command line, use ENTRYPOINT in the Dockerfile:

FROM ubuntu
ENTRYPOINT ["sleep"]

When running the container, provide the parameter:

docker run ubuntu-sleeper 60

If no parameter is given, you'll encounter an error since ENTRYPOINT requires one.

Default Parameter:

To set a default parameter if none is provided, combine CMD and ENTRYPOINT:

FROM ubuntu
ENTRYPOINT ["sleep"]
CMD ["60"]

Overriding ENTRYPOINT​

You can override the ENTRYPOINT at runtime using the --entrypoint flag:

docker run --entrypoint sleep2.0 ubuntu-sleeper 60