RUN vs CMD in Dockerfile

TIL the difference between RUN and CMD when defining an image via a dockerfile.

RUN

RUN is used when writing the buildscript for an image via dockerfile, to define steps that will be executed when you build an image from the file using docker build -t imageName:tagName . A dockerfile can have many RUN steps defined.

RUN steps can be used to modify the state of the image being constructed. For example: if I have node and npm available in my docker image, and have copied a package.json into the image, I can use RUN npm install to make the dependencies defined by the package.json available in my container.

CMD

CMD is used do define a command that will be executed when the image is run in a container. There can only be one CMD defined as part of your dockerfile.

If your image contains application code for an express.js server, the CMD step in your dockerfile would be CMD node server.js or whatever file is necessary to kick off your server.

CMD can be overridden from the command line by passing a new command to be run as an option. For example, if the default CMD defined in the dockerfile is CMD node server.js, but instead you just want to exec into the container, try docker run imageName:tagName /bin/sh/