Tomcat, Docker, Logging, and STDOUT?

Question

I'm running tomcat in docker, but I can't see the logs. They are written to various log files under tomcat/logs, but I can't see them when tomcat is running in a docker container.

Here is my Dockerfile

FROM tomcat:7-jre8
COPY target/MYAPP.war /usr/local/tomcat/webapps/MYAPP.war
RUN ["/usr/local/tomcat/bin/catalina.sh", "start"]

This is how I build image & start container from it:

docker build -t MYAPP .
docker run -it --rm -p 8080:8080 --name MYAPP MYAPP

My app creates log file: /var/log/MYAPP.log after tomcat deploys MYAPP.war

How should I amend Dockerfile and which command should I use to run it ("docker run ...") so that right after starting the container MYAPP using the oneliner "docker run -it --rm -p 8080:8080 --name MYAPP MYAPP" the contents of /var/log/MYAPP.log would be printed to stdout?

I tried to add to Dockerfile the command below but it didn't help.

CMD tail -f /usr/local/MYAPP.log

Answer 1

You seem to be confused about the different between RUN and CMD.

The RUN directive is used to run commands during the build process. It is never executed in a container. When you write...

RUN ["/usr/local/tomcat/bin/catalina.sh", "start"]

...this means that during the docker build process, Docker will start tomcat, but will immediately kill it and continue to build your image.

Only the CMD and ENTRYPOINT directives define commands that will be run when you boot an image with docker run. So possibly you want something like:

CMD /usr/local/tomcat/bin/catalina.sh start && tail -f /usr/local/MYAPP.log