How to see the logs of running application inside docker container?

Question

I have a Node.js application running inside Docker container in /usr/src/app.

I want to run this application using nohup (nohup node index.js &).

What is the best way to see the output appended to nohup.out for general/debugging purposes?

Do I need to use docker cp all the time to copy to host os. Is there a better way to achieve this?

Thanks.

Answer 1

Although using logfiles inside container (without volume mounting) is an anti-pattern, you can easily output those files using exec for example along with cat:

docker exec -it [CONTAINERID] cat /usr/src/app/nohup.out

Better pattern would be to store output into separate volume-mounted folder removing state from inside your container, but would also allow you to access logs directly from host system.

In this specific case, I don't see a need for a separate logfile, and not even for nohup. Just set policy for your container to restart it automatically (--restart always) instead of nohup.

Answer 2

You do output your logs in

/dev/stdout for all normal logs

use /dev/stderr to pipe all the errors you have, if your application can differ between normal logs and error logs.

So instead of defining your log like /var/log/nohup.log you define the logfile to be dev/stdout

Accessing the logs will then be as easy as writing docker logs <containername> or docker-compose logs <servicename> - if you have started your stack using docker-compose up you will see the logs right in front of you anyway - very convenient.

In production grade you want to process the log stream further, look at https://github.com/gliderlabs/logspout

And if you want to go a level deeper, add the ELK stack to it https://logz.io/learn/complete-guide-elk-stack/