Difference between CMD and ENTRYPOINT

In a Dockerfile, both CMD and ENTRYPOINT instructions are used to define the command that should be run when a container is started. However, they serve different purposes and have different behaviors.

CMD

• Purpose: Specifies the default command to run when a container starts.
• Usage: It can provide default arguments for the ENTRYPOINT instruction, or it can be overridden by supplying arguments to docker run.
• Syntax:
  • Shell form: CMD command param1 param2
  • Exec form: CMD ["executable", "param1", "param2"]

Example:

Dockerfile with CMD:

FROM ubuntu:latest
CMD ["echo", "Hello, World!"]

When you build and run this Docker image, it will execute echo Hello, World!.

docker build -t my-echo-image .
docker run my-echo-image

Output:

Hello, World!

If you run the container with a different command, it will override the CMD instruction:

docker run my-echo-image echo "Hi there!"

Output:

Hi there!

ENTRYPOINT

• Purpose: Configures a container to run as an executable.
• Usage: It is not intended to be overridden; it sets the main command that will always be executed.
• Syntax:
  • Shell form: ENTRYPOINT command param1 param2
  • Exec form: ENTRYPOINT ["executable", "param1", "param2"]

Example:

Dockerfile with ENTRYPOINT:

FROM ubuntu:latest
ENTRYPOINT ["echo"]
CMD ["Hello, World!"]

When you build and run this Docker image, it will execute echo Hello, World!.

docker build -t my-entrypoint-image .
docker run my-entrypoint-image

Output:

Hello, World!

If you provide additional arguments to docker run, they will be appended to the ENTRYPOINT instruction:

docker run my-entrypoint-image "Hi there!"

Output:

Hi there!

Combining CMD and ENTRYPOINT

When both ENTRYPOINT and CMD are used together, CMD provides default arguments to the ENTRYPOINT.

Dockerfile with both ENTRYPOINT and CMD:

FROM ubuntu:latest
ENTRYPOINT ["echo"]
CMD ["Hello, World!"]

This means that CMD arguments will be passed to the ENTRYPOINT command by default, but they can still be overridden:

docker run my-entrypoint-image

Output:

Hello, World!

Override the default CMD arguments:

docker run my-entrypoint-image "Hi there!"

Output:

Hi there!

Key Differences

1. Default Behavior:

   • CMD: Provides default command and/or parameters that can be overridden from the command line when running the container.
   • ENTRYPOINT: Sets the command and parameters that are not overridden from the command line.

2. Override Capability:

   • CMD: Can be easily overridden by specifying a different command when running the container.
   • ENTRYPOINT: The command itself is not easily overridden, but arguments can be appended.

3. Use Case:

   • CMD: Suitable for specifying the default command and parameters when there's a need to provide flexibility to override.
   • ENTRYPOINT: Suitable for ensuring a specific command is always run, useful for containers that should always run the same executable (e.g., a web server).