Setting environment variable derived in container at runtime within a shell

Question

I have custom entrypoint where a environment variable is exported. The value of the environment variable is constructed using two other variables provided at runtime.

Snippet from Dockerfile CMD ["bash", "/opt/softwareag/laas-api-server/entrypoint.sh"]. Snippet from entrypoint.sh

export URL="$SCHEME://$HOST:$PORT"
echo "URL:$URL"

Command docker run -e HOST="localhost" -e PORT="443" mycentos prints URL:localhost:443 as expected but the same variable appears to have lost the value when the following command is executed.

docker exec -ti <that-running-container-from-myimage> bash 
container-prompt> echo $URL
<empty-line>

Why would the exported variable appear to have lost the value of URL? What is getting lost here?

Adiii · Accepted Answer · 2020-07-02T06:42:00.433

The environment variable will not persist across all bash session. when the container run it will only available in that entrypoint session but later it will not available if it set using export.

docker ENV vs RUN export

If you want to use across all session you should set them in Dockerfile.

ENV SCHEME=http
ENV HOST=example.com
ENV PORT=3000

And in the application side, you can use them together.also it will be available for all session.

curl "${SCHEME}://${HOST}:${PORT}
#
Step 8/9 : RUN echo "${SCHEME}://${HOST}:${PORT}"
 ---> Running in afab41115019
http://example.com:3000

Now if we look into the way you are using, it will not work because

export URL="$SCHEME://$HOST:$PORT"
# only in this session
echo "URL:$URL"
# will be available for node process too but for this session only
node app.js

For example look into this Dockerfile

FROM node:alpine
RUN echo $'#!/bin/sh \n\ 
     export URL=example.com  \n\
     echo "${URL}" \n\
     node -e \'console.log("ENV URL value inside nodejs", process.env.URL)\' \n\
     exec "$@" \n\
     ' >> /bin/entrypoint.sh
RUN chmod +x /bin/entrypoint.sh
entrypoint ["entrypoint.sh"]

So you when you Run docker container for the first time you will able to see the expected response.

docker run -it --rm myapp
example.com
ENV URL value inside nodejs example.com

Now we want to check for later session.

docker run -it --rm abc  tail -f /dev/null
example.com
ENV URL value inside nodejs example.com

so the container is up during this time, we can verify for another session

docker exec -it myapp sh -c "node -e 'console.log(\"ENV URL value inside nodejs\", process.env.URL)'"
ENV URL value inside nodejs undefined

As we can same script but different behaviour because of docker, so the variable is only available in that session, you can write them to file if you are interested in later use.

Is there a standard way to set a variable for all sessions? The value of such a variable can only be determined when the container starts and not be determined during docker builds — nashter, Jul 02 '20 at 10:56
In dokcer not, you can write to file or dot env, then read it from dot env the only way to keep it persistent. — Adiii, Jul 02 '20 at 10:58

Setting environment variable derived in container at runtime within a shell

1 Answers1