Docker: Sending log files as non-root user to /dev/stdout

Question

I'm trying to start a docker container, which has 2 services. One of those services needs to be run as a non-root User, otherwise he won't start. The other must run as root.

Now I want to link the non_root_service.log file to either /dev/tty or /dev/stdout, so that the logs could be catched by docker (check with docker logs $CONTAINER):

ln -s /dev/stdout non_root_service.log

The Problem is by doing so, that the non-root User has no rights to write to either /dev/tty or /dev/stdout resulting in the following error on startup of the non-root-service:

cannot open "non_root_service.log": Permission denied

But if I don't create the link, I could not catch the logs with docker logs $CONTAINER. Does anyone has an idea how that could be fixed, so that I could run the user as non-root AND link the log files to /dev/{tty,stdout}?

P.S.: Does not matter if I use as shell script CMD ["starter.sh"] or supervisor CMD ["supervisord", "-n", "-c", "/app/supervisord.conf"] to start the 2 services, I get with both the same error.

Answer 1

Maybe not the cleanest solution, but could you try this:

tail -f non_root_service.log &

To simply tail the file to stdout and background the tail process? You might also try tailing straight to the stdout p:

ln -s ln -fs /proc/self/fd/1 non_root_service.log

However, are you sure that you have rights to the non_root_service.log file? What do you see if you run stat non_root_service.log and whoami?