ENTRYPOINT

ENTRYPOINT is a Dockerfile instruction that configures the container’s main executable. It establishes the default command that is run when a container starts, effectively defining the container’s primary process. The behavior is that this process runs as PID 1 inside the container, and its proper handling of signals is important for graceful shutdown.

There are two forms of ENTRYPOINT. The exec form uses a JSON array and is preferred: ENTRYPOINT ["executable","param1","param2"]. This form does not invoke a shell and avoids shell string parsing issues. The shell form uses a regular command string: ENTRYPOINT "executable param1 param2". This runs under /bin/sh -c, which can be less predictable and can interfere with signal handling.

ENTRYPOINT is commonly paired with CMD. In this pairing, CMD provides default arguments to the ENTRYPOINT. For example, ENTRYPOINT ["python","server.py"] and CMD ["--port","8080"] result in the container running python server.py --port 8080 by default. If you supply arguments at runtime, those override CMD, while the ENTRYPOINT remains in effect unless you override it with --entrypoint.

Overriding entrypoints is possible with docker run --entrypoint. When used, the original CMD can become arguments to the new entrypoint, or the new entrypoint may implement its own behavior. In practice, ENTRYPOINT is used to enforce a specific executable while CMD provides optional defaults or parameters, enabling reusable images and consistent startup behavior.

Notes include that a well-designed entrypoint should be able to handle signals properly and avoid relying on shell features when using the exec form. Outside Docker, the term “entry point” broadly refers to the starting function of a program.