CMD Vs ENTRYPOINT Basics

CMD and ENTRYPOINT both affect what happens when a container starts.

They look similar, but they are used for slightly different purposes. Understanding the difference helps you write Dockerfiles that behave predictably.

The simple difference

Use this mental model:

Most beginner application Dockerfiles work well with CMD.

ENTRYPOINT becomes useful when the image is meant to behave like a specific command-line tool or service.

CMD example

A simple Java Dockerfile may use:

CMD ["java", "-jar", "app.jar"]

When you run:

docker run my-app

Docker starts:

java -jar app.jar

But CMD can be overridden from the command line:

docker run my-app echo "hello"

In this case, Docker runs the new command instead of the default CMD.

ENTRYPOINT example

With ENTRYPOINT, you define the main executable:

ENTRYPOINT ["java", "-jar", "app.jar"]

Now the container is strongly tied to running that Java application.

When someone runs the image, Docker uses the entrypoint as the main command.

CMD and ENTRYPOINT together

They can also work together.

ENTRYPOINT ["java", "-jar", "app.jar"]
CMD ["--server.port=8080"]

Here:

• ENTRYPOINT defines the fixed command
• CMD provides default arguments

If you run:

docker run my-app

Docker uses the default port argument.

If you run:

docker run my-app --server.port=9090

The new argument replaces the CMD part.

Shell form and exec form

Prefer exec form:

CMD ["java", "-jar", "app.jar"]
ENTRYPOINT ["java", "-jar", "app.jar"]

Avoid shell form for most production-style examples:

CMD java -jar app.jar

Exec form handles process signals more cleanly, which matters when containers stop or restart.

Which one should beginners use

For normal web applications, start with CMD.

CMD ["java", "-jar", "app.jar"]

Use ENTRYPOINT when:

• The image should always run one main executable
• You want command-line arguments to be appended to that executable
• You are building a utility-style image
• You want stronger control over startup behaviour

Common mistake

Do not use both without understanding how they combine.

This can confuse beginners:

ENTRYPOINT ["java"]
CMD ["-jar", "app.jar"]

It works, but it is less obvious than:

CMD ["java", "-jar", "app.jar"]

For course examples, clarity matters more than cleverness.

Written By: Shiva Srivastava