# Docker Entrypoint and CMD

**ENTRYPOINT** is the command that runs when a container starts. **CMD** provides default arguments. The Dockerfile CMD is overridden by arguments to `docker run`. Use ENTRYPOINT for the main command and CMD for common default arguments. Both can be exec form (JSON array) or shell form (string).

```bash
# Dockerfile
ENTRYPOINT ["python", "app.py"]
CMD ["--port", "8000"]

# At runtime
$ docker run myapp                    # uses default: python app.py --port 8000
$ docker run myapp --port 9000        # override CMD: python app.py --port 9000
$ docker run --entrypoint sh myapp    # override ENTRYPOINT
```

If you specify only CMD in the Dockerfile, `docker run` arguments replace it entirely. If you specify ENTRYPOINT, `docker run` arguments append to it. Exec form (`["cmd", "arg"]`) avoids shell interpretation; shell form (`"cmd arg"`) runs through `/bin/sh -c`. Use exec form for consistency and to avoid shell quirks.