Understanding ENTRYPOINT in a Dockerfile: the command that starts your container

If you’re building containers and want them to behave predictably the moment they start, ENTRYPOINT is the little switch that matters. It’s one of those Dockerfile instructions you’ll see a lot in real-world projects, and it’s a topic that shows up in the Docker certification material too. Let’s untangle what ENTRYPOINT does, why it matters, and how to use it without getting tangled yourself.

What is ENTRYPOINT, exactly?

Let me explain with a simple picture. When you create an image, you’re packaging a program (or a set of programs) that should run inside a container. ENTRYPOINT defines the main command that runs when the container starts. In other words, it’s the default “what should this container do when it wakes up?” instruction.

Think of ENTRYPOINT as setting the container’s mission, the core task it’s built to perform. If your image is meant to run a web server, the ENTRYPOINT would be the command that launches that server. If it’s a script that processes data, the ENTRYPOINT would kick off that script. The container’s behavior is guided by this default command, which makes startup consistent across runs.

ENTRYPOINT vs CMD: how the two work together

A lot of confusion comes from how ENTRYPOINT and CMD interact. Here’s the practical way to see it:

• ENTRYPOINT is the primary command. It must run when the container starts.

• CMD provides default arguments to that command. You can think of CMD as “the default knobs and options” you pass to the ENTRYPOINT.

What does that look like in real Dockerfiles?

• Exec form (preferred for most use cases):

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

In this form, the command is executed directly, without going through a shell. That means signals (like Ctrl+C) are more reliably delivered to your process, which is important for clean shutdowns.

• Shell form (less recommended for most production-like containers):

ENTRYPOINT "python /app/server.py"

This runs through /bin/sh -c, which can be convenient for simple commands but can complicate signal handling and process management.

Now, add CMD to offer defaults:

• CMD ["--port", "8080"]

With the above, running the container as docker run yourimage will execute:

• ENTRYPOINT: python /app/server.py

• CMD: --port 8080

If you want to override those defaults at run time, you can supply your own arguments:

• docker run yourimage --port 9090

And if you really want to override the entry point itself, you can use the --entrypoint flag in docker run:

• docker run --entrypoint /bin/bash yourimage

This replaces ENTRYPOINT for that particular run, which can be useful for debugging.

A quick mental model

Think of ENTRYPOINT as the container’s built-in function, like the main button on a coffee machine. CMD is the default setting you see next to it—how strong the coffee is, what size, etc. If you want a different coffee strength, you press different options you’ve already set in CMD. If you want a completely different coffee machine function, you’d swap out the whole button—your entrypoint—with --entrypoint.

Where ENTRYPOINT shines in real-world apps

• Predictable startup: If your container’s purpose is a single, long-running process, ENTRYPOINT helps ensure that process starts reliably every time. This is especially valuable for microservices, where one container often maps to one service.

• Wrapping or adapting commands: You can wrap a core executable with a small script that performs setup tasks (like copying a config file, adjusting permissions, or loading environment-specific values) and then launches the real application. The ENTRYPOINT would start that wrapper script.

• Extending with arguments: By combining ENTRYPOINT with CMD, you can provide sensible defaults while still letting operators tweak behavior at runtime. It’s a nice balance between prescripted behavior and flexibility.

Common pitfalls to watch for

• Overusing CMD alone: If you define CMD without an ENTRYPOINT, the container will still run the command at startup, but you miss the clarity of a dedicated primary process. ENTRYPOINT clarifies the container’s mission.

• Forgetting to handle signals: If you use a shell script as your ENTRYPOINT and you don’t forward signals properly, your container can’t shut down gracefully. Prefer the exec form so your process receives signals cleanly.

• Mixing form types without care: Mixing shell-form ENTRYPOINT with CMD can produce confusing behavior, especially when overriding at runtime. When in doubt, stick to the exec form for ENTRYPOINT and use CMD for default arguments.

• Not thinking about user context: If your container runs as root by default, you might want to switch to a non-root user with the USER instruction. ENTRYPOINT won’t fix security issues by itself; pairing it with proper user permissions helps keep things safer.

A concrete starter example

Suppose you’re building a tiny web service in Python. You might write something like this:

• FROM python:3.11-slim

• WORKDIR /app

• COPY . /app

• RUN pip install -r requirements.txt

• ENTRYPOINT ["python", "server.py"]

• CMD ["--host", "0.0.0.0", "--port", "8000"]

Here’s what happens:

• When the container starts, Python runs server.py.

• If you don’t supply extra arguments, server.py receives --host 0.0.0.0 --port 8000.

• If you want to run the server on a different port, you can do docker run image --port 8080, and that argument goes straight into server.py.

If you prefer a tiny wrapper, you could switch to an ENTRYPOINT that points to a script, like ENTRYPOINT ["/app/start.sh"], and have start.sh set up environment specifics before launching the main app. The script approach is powerful, but you still want to ensure the script itself exits correctly and forwards signals to the main process.

Why this matters for the Docker certification track

Understanding ENTRYPOINT isn’t just about memorizing a line in a Dockerfile. It’s about recognizing how containers are meant to behave in production—starting reliably, handling inputs, and playing nicely with orchestration tools. When you design an image, you’re answering: What is this container’s core job? How can I let operators influence its behavior without breaking its startup? That clarity translates into fewer surprises when the container runs in staging or production, and that’s exactly the kind of insight the Docker topics aim to build.

A few friendly reminders for daily work

• Prefer exec form for ENTRYPOINT to get clean signal handling and predictable behavior.

• Use CMD to offer defaults, not to repeat what ENTRYPOINT already defines.

• If you need to override the entry point itself, remember the docker run --entrypoint option exists—use it sparingly and only when debugging.

• Don’t forget about security basics: pick a non-root USER for the container where possible, and keep the base image lean to minimize risk.

A small digression that connects the dots

If you’ve ever deployed something using containers on a tiny cloud instance, you’ve probably marveled at how containers feel almost magical—until you realize reliability is born from small, deliberate choices. ENTRYPOINT is one of those choices that makes a container’s life predictable. When the service inside starts the same way every time, you save yourself headaches during scaling, testing, and maintenance. And yes, these are the kind of nuances you’ll encounter again and again as you deepen your Docker knowledge.

Closing thoughts

ENTRYPOINT may seem like a single line in a Dockerfile, but its impact runs deeper. It defines the container’s reason for being, sets expectations for how the application starts, and works in harmony with CMD to offer both consistency and flexibility. By embracing the exec form, planning for runtime arguments, and keeping an eye on graceful shutdowns, you’re building containers that behave well in any environment.

If you’re exploring Docker and mapping out the core concepts you’ll see in the certification materials, keep ENTRYPOINT near the top of your mental checklist. It’s a practical, workhorse-like instruction that reveals a lot about how a container should operate. And when you pair it with real-world examples—the kind you run across in actual projects—it starts to click in a way that’s not just theoretical, but genuinely useful for real software work.