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Docker Tip #36: The Difference between RUN and CMD in a Dockerfile

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RUN lets you run commands but CMD also lets you run a command, so which one should you use, and when?

While you can run commands with both instructions, they are very different.

RUN happens at build time. When you build your Docker image, Docker will read in your RUN command and build it into your image as a separate image layer.

CMD happens at run time. This will likely be calling some type of process, such as nginx, bash or whatever process your Docker image runs. This does not create a separate image layer.

Your Dockerfile may have many RUN instructions but will have at most one CMD.

Which one should you use and when?

Are you building dependencies into your image? Use RUN.
Are you launching / running your Dockerized process? Use CMD.

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