What is the difference between CMD and ENTRYPOINT in a Dockerfile?

In Docker, CMD and ENTRYPOINT are Dockerfile instructions that can both be used to specify the command executed when the container starts. However, there are key differences between them, primarily in how they handle commands and arguments, and how they influence the container's execution behavior.

1. Default Behavior

• CMD: The CMD instruction provides the default command for the container. If no command is specified at startup, the command and arguments defined in CMD are executed. If a command is specified during startup, the CMD command is overridden.
• ENTRYPOINT: The ENTRYPOINT instruction sets the command executed when the container starts, making the container run centered around a specific program or service. Unlike CMD, even if other commands are specified during startup, the ENTRYPOINT command is still executed, and the startup command is passed as arguments to the ENTRYPOINT.

2. Usage Scenarios

• Using CMD:

  dockerfile
  FROM python:3.8
  COPY script.py /script.py
  CMD ["python", "/script.py"]

  In this case, if no command is specified at startup, it executes python /script.py. If a command is specified, such as docker run <image> bash, the CMD command is overridden with bash.
• Using ENTRYPOINT:

  dockerfile
  FROM python:3.8
  COPY script.py /script.py
  ENTRYPOINT ["python", "/script.py"]

  In this case, regardless of whether a command is specified at startup, the ENTRYPOINT is executed, and the startup command is passed as arguments to python /script.py. For example, if you run docker run <image> arg1 arg2, the actual command executed is python /script.py arg1 arg2.

3. Combined Usage

CMD and ENTRYPOINT can be used together, where the content of CMD is passed as arguments to the ENTRYPOINT. For example:

dockerfile
FROM python:3.8
COPY script.py /script.py
ENTRYPOINT ["python"]
CMD ["/script.py"]

In this example, if no startup command is specified, the container executes python /script.py by default. If arguments are specified, such as arg1, the command becomes python /script.py arg1.

By understanding and combining these, you can more flexibly control the startup behavior and argument handling of Docker containers.