RUN and CMD instructions in Docker

In Docker, the RUN and CMD instructions serve different purposes, primarily relating to when and how commands are executed within the container lifecycle.

RUN Instruction:

• Purpose: The RUN command is used to build the image. It executes the specified command(s) and commits the result to the image, so the changes become part of the image layers.

Execution Time: During the build phase of the Docker image.
Use Case: Commonly used for installing software, setting up files, or configuring the environment.

RUN apt-get update && apt-get install -y nginx

Here, RUN installs nginx in the image. When the image is built, the installed software is included in the image.

CMD Instruction:

• Purpose: The CMD command provides default instructions for the container. It defines what should be executed when a container starts.

Execution Time: At container runtime (when the container is launched from the image).
Use Case: Typically, used to specify the main application or command to run in the container.

CMD ["nginx", "-g", "daemon off;"]

Key Differences

• Layering: RUN adds layers to the image (since it's for building), while CMD does not.
• Multiple Commands: You can use multiple RUN instructions to set up the image, but only one CMD. If there are multiple CMD instructions, only the last one takes effect.
• Overriding: The CMD command can be overridden when starting a container with docker run, whereas RUN cannot.

RUN is used to execute commands during the build process of a Docker image, while CMD is used to specify the default command to run when a Docker container is started from the image.