Difference between RUN and CMD in a Dockerfile

Question

I m confused about when should I use CMD vs RUN  For example  to execute bash shell commands  i e  ls -la  I would always use CMD or is there a situation where I would use RUN  Trying to understand the best practices about these two similar Dockerfile directives

User · Answer

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

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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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RUN - command triggers while we build the docker image   CMD - command triggers while we launch the created docker image

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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 starts an image

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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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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  --- gt  be5888e67be6 Step 2 2   RUN echo  Just echo while you work   --- gt  Running in ed37d558c505 Just echo while you work Removing intermediate container ed37d558c505  --- gt  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  --- gt  be5888e67be6 Step 2 2   RUN echo  Just echo while you work   --- gt  Using cache  --- gt  6f46f7a393d8 Successfully built 6f46f7a393d8 Successfully tagged example run-echo latest

User · Answer

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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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

[docker] Difference between RUN and CMD in a Dockerfile

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