Getting a docker container's stdout logs into a variable on Linux

First posted on: 2019/03/11

Last modified: 2019/07/11,6a40e30

Categories: infrastructure

docker logs by default shows the container’s stdout and stderr logs. However, what I discovered was that the stderr logs from the container are output to the host system’s stderr as well. I was expecting everything from the container to be on the host’s stdout.

Let’s see a demo. Consider the Dockerfile:

FROM alpine:3.7

CMD echo "I echoed to stdout" && >&2 echo "I echoed to stderr"

Let’s build it and run it:

$ docker build -t amit/test .

$ sudo docker run --name test amit/test
I echoed to stdout
I echoed to stderr

$ sudo docker logs test
I echoed to stdout
I echoed to stderr

$ sudo docker logs test 2> /dev/null
I echoed to stdout

In the second docker logs command, we redirect the host’s stderr to /dev/null. So, if you are looking to get only the output that was written stdout inside the container, we will need to make sure, we pipe the stderr to /dev/null on the host.

Assigning the output of docker logs

Coming back to the primary use case which triggered this post, if we wanted just the standard output of the container to be assigned to a variable in bash, here’s what we should do:

data="$(sudo docker logs test 2> /dev/null)"

If we don’t do the above stderr redirection, we will still see that container’s stderr output on the host system. That may leave you scratching your head, as it did to me, since we think we are assigning all the output of docker logs to a variable.