Docker image run with one command in CMD vs run as container

In Docker, there is a fundamental difference between running a command as part of the Docker image's CMD instruction and running a container from that image with a specific command.

CMD Instruction in Dockerfile:

When you define a CMD instruction in your Dockerfile, you are specifying the default command that should be executed when a container is started from that image. The CMD instruction is typically used to specify the primary process that the container should run when it starts. For example:

Dockerfile

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FROM ubuntu:latest

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

In the above example, the CMD instruction specifies that the container should run the Nginx web server as its primary process when it starts.

When you build an image with this Dockerfile and run a container from it, the default command specified in CMD is executed unless you override it by providing a different command when running the container.

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docker run -d my-nginx-image        # Runs the default CMD command (Nginx)

docker run -d my-nginx-image bash   # Overrides CMD with a different command (bash)

Running a Container with a Command:

When you run a Docker container with a specific command, you are telling Docker to start a container from an image and immediately execute the specified command within the container. This command is temporary and does not change the image's default CMD. For example:

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docker run -d my-nginx-image nginx -g "daemon off;"   # Runs Nginx with the specified command

In this case, the container is started with the specified command (nginx -g "daemon off;") instead of the default CMD instruction from the image.

Key Differences:

The CMD instruction in the Dockerfile sets the default command for the container, which is executed when the container is started without specifying a command.

Running a container with a specific command overrides the default CMD and runs the specified command instead.

The Docker image remains unchanged when you run a container with a command. The image's CMD instruction is still present and can be used if no command is specified when running the container.

In summary, the choice between using CMD in the Dockerfile and specifying a command when running a container depends on whether you want to define a default behavior for the container or if you need to run a specific command just for that instance of the container.

references:

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