How to define a variable in a Dockerfile

Question

In my Dockerfile  I would like to define variables that I can use later in the Dockerfile   I am aware of the ENV instruction  but I do no want these variables to be environment variables   Is there a way to declare variables at Dockerfile scope

User · Answer

You can use ARG variable defaultValue and during the run command you can even update this value using --build-arg variable value  To use these variables in the docker file you can refer them as  variable in run command   Note  These variables would be available for Linux commands like RUN echo  variable and they wouldn t persist in the image

User · Answer

If the variable is re-used within the same RUN instruction  one could simply set a shell variable  I really like how they approached this with the official Ruby Dockerfile

User · Answer

You can use ARG - see https   docs docker com engine reference builder  arg     The ARG instruction defines a variable that users can pass at   build-time to the builder with the docker build command using the   --build-arg  lt varname gt   lt value gt  flag  If a user specifies a build   argument that was not defined in the Dockerfile  the build outputs an   error

User · Answer

To my knowledge  only ENV allows that  as mentioned in  Environment replacement      Environment variables  declared with the ENV statement  can also be used in certain instructions as variables to be interpreted by the Dockerfile     They have to be environment variables in order to be redeclared in each new containers created for each line of the Dockerfile by docker build   In other words  those variables aren t interpreted directly in a Dockerfile  but in a container created for a Dockerfile line  hence the use of environment variable     This day  I use both ARG  docker 1 10   and docker build --build-arg var value  and ENV  Using ARG alone means your variable is visible at build time  not at runtime     My Dockerfile usually has   ARG var ENV var   var    In your case  ARG is enough  I use it typically for setting http proxy variable  that docker build needs for accessing internet at build time

User · Answer

To answer your question      In my Dockerfile  I would like to define variables that I can use later in the Dockerfile    You can define a variable with   ARG myvalue 3   Spaces around the equal character are not allowed   And use it later with   RUN echo  myvalue  gt   test

User · Answer

Late to the party  but if you don t want to expose environment variables  I guess it s easier to do something like this  RUN echo 1  gt   tmp   var 1 RUN echo  cat  tmp   var 1  RUN rm -f  tmp   var 1  I ended up doing it because we host private npm packages in aws codeartifact  RUN aws codeartifact get-authorization-token --output text  gt   tmp codeartifact token RUN npm config set   company-123456 d codeartifact us-east-2 amazonaws com npm internal   authToken  cat  tmp codeartifact token  RUN rm -f  tmp codeartifact token  And here ARG cannot work and i don t want to use ENV because i don t want to expose this token to anything else

[variables] How to define a variable in a Dockerfile?

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