05. Docker CMD vs ENTRYPOINT

CMD vs ENTRYPOINT

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Prerequisites

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1. Docker CMD vs ENTRYPOINT

This section explains the roles of CMD and ENTRYPOINT, how they work together, and what happens when you override them using --entrypoint.

ENTRYPOINT = fixed executable (program) CMD = default arguments

ENTRYPOINT(Not Replaceable) + CMD(Fully Replaceable) = final command

ENTRYPOINT = program
CMD        = default parameter

1-1. CMD Only

CMD is fully replaceable

FROM ubuntu:22.04
CMD ["echo", "hello"]

✔️ Run

docker run my-image

same as:

docker run my-image echo hello

hello

✔️ Override CMD

docker run my-image ls

same as:

docker run my-image ls

👉 Actual execution:

ls

1-2. ENTRYPOINT Only

ENTRYPOINT is not replaced, arguments are appended

FROM ubuntu:22.04
ENTRYPOINT ["echo", "hello"]

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✔️ Run

docker run my-image world

same as:

docker run my-image echo hello world

👉 Actual execution:

echo hello world

✔️ Override ENTRYPOINT → X

docker run my-image ls

same as:

docker run my-image echo hello ls

👉 Actual execution:

echo hello ls

1-3. ENTRYPOINT + CMD

CMD is fully replaceable

ENTRYPOINT is not replaced, arguments are appended

FROM ubuntu:22.04
ENTRYPOINT ["echo"]
CMD ["hello"]

✔️ Run (default)

docker run my-image

same as:

docker run my-image echo hello

👉 Actual execution:

echo hello

✔️ Run (override argument)

docker run my-image world

same as:

docker run my-image echo world

👉 Actual execution:

echo world

2. Practical Example (Python)

FROM python:3.10
ENTRYPOINT ["python"]
CMD ["app.py"]

✔️ Default

docker run my-image

python app.py

✔️ Change argument

docker run my-image test.py

python test.py

✔️ Invalid argument case

docker run my-image ls

👉 Actual execution:

python ls

👉 Result:

python: can't open file 'ls': No such file or directory

3. Overriding ENTRYPOINT with --entrypoint

CMD provides default arguments, ENTRYPOINT defines the executable, and --entrypoint allows you to override it at runtime.

docker run -it --entrypoint bash my-image

✔️ What happens?

👉 ENTRYPOINT is replaced:

bash

• ENTRYPOINT → replaced
• CMD → ignored

Case	Result
docker run image	ENTRYPOINT + CMD
docker run image arg	ENTRYPOINT + arg
docker run –entrypoint bash image	bash only

3-1. Why Use --entrypoint?

✔️ Debugging

docker run -it --entrypoint bash my-image

• Access container manually
• Inspect files, logs, environment

✔️ Bypass default execution

If container always runs an app:

ENTRYPOINT ["python", "app.py"]

👉 You cannot stop it normally

👉 But:

docker run -it --entrypoint bash my-image

👉 You can enter container without running the app