How to write Docker logs to a file in real time (à la `tail -f`)

Question

I docker that output logs stdout stderr, which can be viewed using:

docker logs -f $LOGS_CONTAINER_ID

I also added 'sed', which puts the container id on each line:

docker logs -f $LOGS_CONTAINER_ID | sed "s/^/$LOGS_CONTAINER_ID /"

If I run it, I get something like:

container112 error 10:20:10 problem
container112 info 10:20:09 not problem
container112 error 10:20:01 problem

where "container112" is $LOGS_CONTAINER_ID.

SO FAR SO GOOD. Now I want to output the above command to a file (log.out), so I wrote the following command:

docker logs -f $LOGS_CONTAINER_ID | sed "s/^/$LOGS_CONTAINER_ID /" >> log.out

What happens is that it writes the logs to log.out, but it doesn't get new logs (if I open a new session and run tail -f log.out, I don't get output). So I also tried:

tail -f $(docker logs -f $LOGS_CONTAINER_ID | sed "s/^/$LOGS_CONTAINER_ID /") >> log.out

But it also didn't work. What is the problem?

Answer 1

Would this help?

docker logs $CONTAINERID 2>&1 | cat > logfile.log