Is it possible to read messages passed over stdout from within a Docker container? (without `docker logs`)

Question

Motivation: To run a basic health-check on a docker container by counting that a certain number of messages flow across stdout over a certain time horizon

Immediate goal: From within a shell started by docker exec, read data that is being piped to stdout from the main process (PID 1)

I am not even sure if what I want is possible. If that is the case, an explanation as to why not would be much appreciated -- and would help advance my knowledge.

Steps to reproduce:

1. Start the container -- container1 docker run -it --name container1 ubuntu bash -c -i "COUNT=0; while true; do echo Keep the dance floor beat going; ((COUNT++)); sleep 1; echo \"Count is: \${COUNT}\"; done;"

2. In another terminal window, docker exec to start another process in the same container docker exec -it container1 bash

Can I somehow tail/print/read the messages being passed over stdout by PID 1?

I understand that there are work arounds -- for example piping through tee or otherwise writing to disk -- but was hoping for a magic bullet.

Answer 1

If you are OK with strace then try this:

docker exec -it container1 bash -c -i "\
    apt-get update && apt-get install strace && \
    strace -ff -e trace=write -e write=1 -p 1"

• -p 1 is the PID
• -e write=1 is there to narrow the output to STDOUT (-e write=1,2 would show both STDOUT and STDERR)

Depending on Docker version you might need to loosen up Docker's syscall security policy, e.g. by disabling it completely by adding --security-opt seccomp:unconfined to docker run when starting the container:

docker run --security-opt seccomp:unconfined -it --name container1 ubuntu bash -c -i "COUNT=0; while true; do echo Keep the dance floor beat going; ((COUNT++)); sleep 5; ech o \"Count is: \${COUNT}\"; done;"

Read more about the Docker's seccomp profiles here (>1.10).

Tested with:

• Windows 8.1
• Docker version 1.10.2, build c3959b1
• Docker-machine version 0.6.0, build e27fb87