Day 23: Mastering Docker CMD and ENTRYPOINT

The way containers start is crucial when building Docker images. This can be achieved by using CMD and ENTRYPOINT commands provided by Docker. While both define a default executable for a container, they behave differently and serve different purposes.

In this blog, we will explore the differences between CMD and ENTRYPOINT, their best practices, and when to use each. By the end, you will have a clear understanding of how to control container execution effectively.

CMD Instruction

The CMD instruction in a Dockerfile specifies the default command to run when a container starts. However, it can be overridden if a command is provided at runtime.

Syntax:

Used with ENTRYPOINT:

Key Points:

• Only one CMD instruction is allowed in a Dockerfile. If multiple CMD instructions exist, only the last one is used.
• Can be overridden by providing a command when running the container (docker run <image> <command>).
• Works well for default arguments when used with ENTRYPOINT.

Example CMD Instruction:
(As example, my build image docker build -t ubuntu_cmd:v1 ubuntu_cmd\)

When the container runs, it will execute:

Overriding CMD at Runtime:

This replaces the default CMD instruction and runs "Overriding CMD!" instead.

ENTRYPOINT Instruction

The ENTRYPOINT instruction in a Dockerfile defines the main command that always executes when a container starts. Unlike CMD, it cannot be easily overridden unless specifically allowed.

Syntax:

Key Points:

• Ensures the container runs a specific application every time it starts.
• Cannot be easily overridden, but additional parameters can be passed at runtime.
• Works well with CMD to define default arguments.
• Best practice for containers that serve a single purpose, like a web server or database.

Example ENTRYPOINT Instruction:
(As example, my build image docker build -t ubuntu_entrypoint:v1 ubuntu_entrypoint\)

If we run:

The container will execute:

Overriding ENTRYPOINT at Runtime:
To override the default entrypoint, use the --entrypoint flag:

This replaces echo Hello, World! with ls -l.

Combining CMD & ENTRYPOINT

CMD and ENTRYPOINT can be used together in a Dockerfile to define a default executable (ENTRYPOINT) and provide default arguments (CMD). This combination ensures flexibility while maintaining control over the container's execution behavior.

Key Points:

• ENTRYPOINT sets the main command that always runs.
• CMD provides default arguements for ENTRYPOINT, but these can be overridden at runtime.
• This approach is useful when the container should always run a specific application, but with customizable parameters.

Example Combining CMD & ENTRYPOINT:
(As example, my build image docker build -t ubuntu_ce:v1 ubuntu_ce\)

• ENTRYPOINT ensure echo is always executed.
• CMD provides default arguments (Hello, Docker!).

Running the Container:

Executes:

Overriding CMD Arguments at Runtime:
If a user wants to different message:

Difference Between CMD & ENTRYPOINT

While both CMD and ENTRYPOINT define how a container runs, they serve different purposes. Below is a comparison:

Conclusion

When building Docker images, it is crucial to understand how containers start. With Docker, we can use the CMD and ENTRYPOINT instructions. When both are combined, control over container execution can be maintained while flexibility can be achieved.

In the next blog, we will learn about Docker Compose, a powerful tool for defining and managing multi-container applications. 🚀