In Docker, RUN, CMD, and ENTRYPOINT are three distinct instructions used in Dockerfiles, each serving a different purpose in the lifecycle of a Docker container. Here’s a detailed explanation of the differences between them:
RUN
Purpose: The RUN instruction is used to execute commands in a new layer on top of the current image and commit the results. This is typically used for installing software packages or making other modifications to the image.
Usage: It is executed during the build process of the Docker image, not at runtime.
Forms:
Shell form: RUN <command>
Exec form: RUN ["executable", "param1", "param2"]
Example:
RUN apt-get update && apt-get install -y python3
CMD
Purpose: The CMD instruction provides default arguments for the ENTRYPOINT instruction or specifies the default command to run when a container is started. It is overridden if any command is provided at runtime.
Usage: It is executed at runtime, but only if no other command is specified when the container is started.
Forms:
Shell form: CMD <command>
Exec form: CMD ["executable", "param1", "param2"]
Parameter form: CMD ["param1", "param2"] (used to provide default parameters to the ENTRYPOINT)
Example:
CMD ["python3", "app.py"]
ENTRYPOINT
Purpose: The ENTRYPOINT instruction configures a container to run as an executable. It specifies a command that will always be executed when the container starts. Unlike CMD, ENTRYPOINT is not overridden by the command line arguments provided at runtime; instead, those arguments are passed as parameters to the ENTRYPOINT.
Usage: It is executed at runtime and is intended to be the...
