docker logs filebeat > file.log don't work

Question

With Docker version 18.09.5, build e8ff056 and filebeat:7.1.1 (by elasticsearch) when I type:

$ docker logs filebeat > filebeat.log

I see the logs but the filebeat.log is empty.

If I try:

$ docker logs logstash > logstash.log

I don't see the log to console but the file il full with the logs lines.

The used OS is: Ubuntu 19.04

Answer 1

The > redirects STDOUT, but you can also have STDERR output from containers. To redirect that, you can use:

 docker logs filebeat > filebeat.log 2> filebeat.err

or send both to the same file:

 docker logs filebeat > filebeat.log 2>&1

Answer 2

You should run the container with the -t flag which will allocate a pseudo-tty for the container. Example:

docker run -td --rm --name test store/elastic/filebeat:7.1.1

docker logs test > test.txt

This will store the output to the file. When running without the -t flag it will simply dump the logs in your terminal