CMD and ENTRYPOINT

Learn to set the container start command with CMD.

CMD and ENTRYPOINT

CMD is the container's default command; it is completely replaced by docker run image other-command. ENTRYPOINT is the fixed program; the arguments given to docker run are appended to it. When used together, CMD becomes the default arguments:

ENTRYPOINT ["ping"]
CMD ["-c", "3", "localhost"]

# docker run pinger              -> ping -c 3 localhost
# docker run pinger -c 1 1.1.1.1 -> ping -c 1 1.1.1.1

Exec form and shell form

CMD ["python", "app.py"] (a JSON array) is the exec form: the program becomes PID 1 directly and receives the SIGTERM sent by docker stop. CMD python app.py is the shell form: the command runs inside /bin/sh -c, the signal goes to the shell and the application may not shut down cleanly.

CMD ["python", "app.py"]     # recommended
CMD python app.py            # signals stay in the shell

Exercise A tool with default arguments

Write a Dockerfile based on alpine:3.20: ENTRYPOINT in exec form is ping; CMD gives the default arguments -c, 3, localhost in exec form.

Show example solution

FROM alpine:3.20
ENTRYPOINT ["ping"]
CMD ["-c", "3", "localhost"]

Test yourself

With ENTRYPOINT ["ping"] and CMD ["localhost"], what does docker run img 8.8.8.8 run?
- ping 8.8.8.8
- ping localhost 8.8.8.8
- 8.8.8.8
- ping localhost
Show answer

ping 8.8.8.8 — The given arguments replace CMD and are appended to ENTRYPOINT.
Write the start command in exec form.
```
CMD ____
```
Show answer
```
CMD ["python", "app.py"]
```
Match.

CMD

Replaceable default
ENTRYPOINT

Fixed program
["a", "b"]

Exec form
CMD a b

Shell form

Next lesson: Summary - Dockerfile

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