Difference between RUN and CMD in a Dockerfile

RUN is an image build step, the state of the container after a RUN command will be committed to the container image. A Dockerfile can have many RUN steps that layer on top of one another to build the image.

CMD is the command the container executes by default when you launch the built image. A Dockerfile will only use the final CMD defined. The CMD can be overridden when starting a container with docker run $image $other_command.

ENTRYPOINT is also closely related to CMD and can modify the way a container is started from an image.

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RUN - command triggers while we build the docker image.

CMD - command triggers while we launch the created docker image.

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I found this article very helpful to understand the difference between them:

RUN - RUN instruction allows you to install your application and packages required for it. It executes any commands on top of the current image and creates a new layer by committing the results. Often you will find multiple RUN instructions in a Dockerfile.

CMD - CMD instruction allows you to set a default command, which will be executed only when you run container without specifying a command. If Docker container runs with a command, the default command will be ignored. If Dockerfile has more than one CMD instruction, all but last CMD instructions are ignored.

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The existing answers cover most of what anyone looking at this question would need. So I'll just cover some niche areas for CMD and RUN.

CMD: Duplicates are Allowed but Wasteful

GingerBeer makes an important point: you won't get any errors if you put in more than one CMD - but it's wasteful to do so. I'd like to elaborate with an example:

FROM busybox
CMD echo "Executing CMD"
CMD echo "Executing CMD 2"

If you build this into an image and run a container in this image, then as GingerBeer states, only the last CMD will be heeded. So the output of that container will be:

Executing CMD 2

The way I think of it is that "CMD" is setting a single global variable for the entire image that is being built, so successive "CMD" statements simply overwrite any previous writes to that global variable, and in the final image that's built the last one to write wins. Since a Dockerfile executes in order from top to bottom, we know that the bottom-most CMD is the one gets this final "write" (metaphorically speaking).

RUN: Commands May not Execute if Images are Cached

A subtle point to notice about RUN is that it's treated as a pure function even if there are side-effects, and is thus cached. What this means is that if RUN had some side effects that don't change the resultant image, and that image has already been cached, the RUN won't be executed again and so the side effects won't happen on subsequent builds. For example, take this Dockerfile:

FROM busybox
RUN echo "Just echo while you work"

First time you run it, you'll get output such as this, with different alphanumeric IDs:

docker build -t example/run-echo .
Sending build context to Docker daemon  9.216kB
Step 1/2 : FROM busybox
 ---> be5888e67be6
Step 2/2 : RUN echo "Just echo while you work"
 ---> Running in ed37d558c505
Just echo while you work
Removing intermediate container ed37d558c505
 ---> 6f46f7a393d8
Successfully built 6f46f7a393d8
Successfully tagged example/run-echo:latest

Notice that the echo statement was executed in the above. Second time you run it, it uses the cache, and you won't see any echo in the output of the build:

docker build -t example/run-echo .
Sending build context to Docker daemon  9.216kB
Step 1/2 : FROM busybox
 ---> be5888e67be6
Step 2/2 : RUN echo "Just echo while you work"
 ---> Using cache
 ---> 6f46f7a393d8
Successfully built 6f46f7a393d8
Successfully tagged example/run-echo:latest

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RUN - Install Python , your container now has python burnt into its image CMD - python hello.py , run your favourite script

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Note: Don’t confuse RUN with CMD. RUN actually runs a command and commits the result; CMD does not execute anything at build time, but specifies the intended command for the image.

from docker file reference

https://docs.docker.com/engine/reference/builder/#cmd

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RUN: Can be many, and it is used in build process, e.g. install multiple libraries

CMD: Can only have 1, which is your execute start point (e.g. ["npm", "start"], ["node", "app.js"])

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RUN Command: RUN command will basically, execute the default command, when we are building the image. It also will commit the image changes for next step.

There can be more than 1 RUN command, to aid in process of building a new image.

CMD Command: CMD commands will just set the default command for the new container. This will not be executed at build time.

If a docker file has more than 1 CMD commands then all of them are ignored except the last one. As this command will not execute anything but just set the default command.

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There has been enough answers on RUN and CMD. I just want to add a few words on ENTRYPOINT. CMD arguments can be overwritten by command line arguments, while ENTRYPOINT arguments are always used.

This article is a good source of information.

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