Docker ENTRYPOINT and CMD : Differences & Real world Examples

Docker, a powerful containerization platform, provides flexibility in defining how your containers should behave. Two key instructions, CMD and ENTRYPOINT, play crucial roles in determining what happens when you start a Docker container. Let’s delve into the distinctions between these instructions and explore how they impact container behavior.

The Basics

CMD Instruction

The CMD instruction in a Dockerfile sets the default command and its parameters to be executed when a container starts. It can be overridden by providing arguments at runtime. You can specify the command and its arguments as a list or as a string.

FROM busybox:latest
CMD ["echo", "Hello, CMD!"]

In this example, if you run the container without providing a command, it will execute echo Hello, CMD! by default.

docker run my-busybox-container

You can also override the CMD when running the container:

docker run my-ubuntu-container echo "Override CMD"

ENTRYPOINT Instruction

The ENTRYPOINT instruction sets the primary command to run and its default parameters. Unlike CMD, the ENTRYPOINT command and its parameters are not ignored when you provide a command at runtime. As with CMD, you can specify the command and its arguments as a list or as a string.

FROM busybox:latest
ENTRYPOINT ["echo", "Hello, ENTRYPOINT!"]
CMD ["Default CMD argument"]

Here, if you run the container without providing a command, it will execute echo Hello, ENTRYPOINT! Default CMD argument by default.

docker run my-busybox-container

You can still override both ENTRYPOINT and CMD when running the container:

docker run my-busybox-container echo "Override both ENTRYPOINT and CMD"

The provided command is appended to the ENTRYPOINT and overrides the CMD argument.

Key Differences

CMD vs. ENTRYPOINT

1. Overridability:

   • CMD: Can be overridden by providing a command at runtime.
   • ENTRYPOINT: The primary command and its parameters are not ignored even when providing a command at runtime.

2. Command Combination:

   • CMD: If the Dockerfile has multiple CMD instructions, only the last one takes effect.
   • ENTRYPOINT: The CMD arguments are appended to the ENTRYPOINT command if both instructions are present.

Choosing Between CMD and ENTRYPOINT

Use CMD When:

• You want to provide defaults for the command.
• You anticipate users may want to override the command frequently.

Use ENTRYPOINT When:

• You want to define the main command and its default parameters.
• You want to enforce a specific behavior and not allow easy overrides.

Conclusion

Understanding the nuances between CMD and ENTRYPOINT is crucial for effectively configuring your Docker containers. Whether you need flexibility for command overrides or wish to enforce a specific behavior, choosing the right instruction is key. With this knowledge, you can harness the full power of Docker to streamline your containerized applications.