how to make a docker application write to stdout

Some applications do not redirect their logs to the stdout but rather to some files. Hence, when putting in a Docker image, the output of the docker logs doesn’t display the logs of the application.

In other cases, a docker image contains background processes and they are not attached to docker log collector, i.e. performing docker log <container_name> does not display their content.

Having their logs displayed directly from the docker logs can be useful for various reasons:

• a simple docker logs will read the logs, instead of reading the log file in the container
• some tools leverages the docker logs to collect and aggregate into a logging system (e.g. ELK)

If the application is running as PID 1, we can create a symbolic link to /dev/stdout from the log files that the application is writing to. The result will then not write into a file but go to stdout and stderr.

Example of a Dockerfile that add this “hack”:

FROM debian
 
# forward request and error logs to docker log collector
# assuming the app is producing logs into access.log and error.log
RUN ln -sf /dev/stdout /var/log/access.log \
  ln -sf /dev/stderr /var/log/error.log
 
CMD ["/app", "--not-logging-to-stdout"]

If the application is not running as PID 1, instead of /dev/stdout, we can use /proc/1/fd/1 instead.

Note: we could directly use /proc/1/fd/1 for both cases, but I prefer the /dev/stdout syntax as it’s more understandable.

src: https://serverfault.com/questions/599103/make-a-docker-application-write-to-stdout