How to define a variable in a Dockerfile?
VariablesDockerDockerfileVariables Problem Overview
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?
Variables Solutions
Solution 1 - Variables
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 <varname>=<value> flag. If a user specifies a build
> argument that was not defined in the Dockerfile, the build outputs an
> error.
>
>
> Can be useful with COPY during build time (e.g. copying tag specific content like specific folders) For example:
ARG MODEL_TO_COPY
COPY application ./application
COPY $MODEL_TO_COPY ./application/$MODEL_TO_COPY
>While building the container:
docker build --build-arg MODEL_TO_COPY=model_name -t <container>:<model_name specific tag> .
Solution 2 - Variables
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 $myvalue > /test
Solution 3 - Variables
To my knowledge, only ENV allows that, as mentioned in "Environment replacement"
> Environment variables (declared with the ENV statement) can also be used in certain instructions as variables to be interpreted by the Dockerfile.
They have to be environment variables in order to be redeclared in each new containers created for each line of the Dockerfile by docker build.
In other words, those variables aren't interpreted directly in a Dockerfile, but in a container created for a Dockerfile line, hence the use of environment variable.
This day, I use both ARG (docker 1.10+, and docker build --build-arg var=value) and ENV.
Using ARG alone means your variable is visible at build time, not at runtime.
My Dockerfile usually has:
ARG var
ENV var=${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.
Solution 4 - Variables
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.
Solution 5 - Variables
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.
Solution 6 - Variables
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 > /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 > /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