What is the point of WORKDIR on Dockerfile

Question

I m learning Docker  For many times I ve seen that Dockerfile has WORKDIR command   FROM node latest RUN mkdir -p  usr src app WORKDIR  usr src app COPY package json  usr src app  RUN npm install COPY    usr src app EXPOSE 3000 CMD      npm        start         Can t I just omit WORKDIR and Copy and just have my Dockerfile at the root of my project  What are the downsides of using this approach

User · Answer

Before applying WORKDIR  Here the WORKDIR is at the wrong place and is not used wisely  FROM microsoft aspnetcore 2 COPY --from build-env  publish  publish WORKDIR  publish ENTRYPOINT   quot dotnet quot    quot  publish api dll quot    We corrected the above code to put WORKDIR at the right location and optimised the following statements by removing  Publish FROM microsoft aspnetcore 2 WORKDIR  publish COPY --from build-env  publish   ENTRYPOINT   quot dotnet quot    quot api dll quot    So it acts like a cd and sets the tone for the upcoming statements

User · Answer

According to the documentation   The WORKDIR instruction sets the working directory for any RUN  CMD  ENTRYPOINT  COPY and ADD instructions that follow it in the Dockerfile  If the WORKDIR doesn   t exist  it will be created even if it   s not used in any subsequent Dockerfile instruction   Also  in the Docker best practices it recommends you to use it       you should use WORKDIR instead of proliferating instructions like RUN cd      amp  amp  do-something  which are hard to read  troubleshoot  and maintain   I would suggest to keep it  I think you can refactor your Dockerfile to something like  FROM node latest WORKDIR  usr src app COPY package json   RUN npm install COPY      EXPOSE 3000 CMD      npm        start

User · Answer

You can think of WORKDIR like a cd inside the container  it affects commands that come later in the Dockerfile  like the RUN command   If you removed WORKDIR in your example above  RUN npm install wouldn t work because you would not be in the  usr src app directory inside your container   I don t see how this would be related to where you put your Dockerfile  since your Dockerfile location on the host machine has nothing to do with the pwd inside the container   You can put the Dockerfile wherever you d like in your project  However  the first argument to COPY is a relative path  so if you move your Dockerfile you may need to update those COPY commands

User · Answer

You dont have to   RUN mkdir -p  usr src app  This will be created automatically when you specifiy your WORKDIR  FROM node latest WORKDIR  usr src app COPY package json   RUN npm install COPY      EXPOSE 3000 CMD      npm        start

User · Answer

Beware of using vars as the target directory name for WORKDIR - doing that appears to result in a  cannot normalize nothing  fatal error  IMO  it s also worth pointing out that WORKDIR behaves in the same way as mkdir -p  lt path gt  i e  all elements of the path are created if they don t exist already   UPDATE  I encountered the variable related problem  mentioned above  whilst running a multi-stage build - it now appears that using a variable is fine - if it  the variable  is  in scope  e g  in the following  the 2nd WORKDIR reference fails       FROM  lt some image gt  ENV varname varval WORKDIR  varname  FROM  lt some other image gt  WORKDIR  varname   whereas  it succeeds in this      FROM  lt some image gt  ENV varname varval WORKDIR  varname  FROM  lt some other image gt  ENV varname varval WORKDIR  varname    oO Maybe it s in the docs  amp  I ve missed it

User · Answer

Be careful where you set WORKDIR because it can affect the continuous integration flow  For example  setting it to  home circleci project will cause error something like  ssh or whatever is the remote circleci is doing at setup time

[docker] What is the point of WORKDIR on Dockerfile?

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