How to Get Docker Container Id from Within the Container with Cgroup V2

How to get docker container ID from within the container with cgroup v2

The --cgroupns host fix is effective, but not available if you don't control the container's creation. Further, this docker run option is not available in the API or docker compose (https://github.com/compose-spec/compose-spec/issues/148).

But... good news - the container ID is still exposed via /proc/self/mountinfo:

678 655 254:1 /docker/containers/7a0144cee1256c539fab790199527b7051aff1b603ebcf7ed3fd436440ef3b3a/resolv.conf /etc/resolv.conf rw,relatime - ext4 /dev/vda1 rw
679 655 254:1 /docker/containers/7a0144cee1256c539fab790199527b7051aff1b603ebcf7ed3fd436440ef3b3a/hostname /etc/hostname rw,relatime - ext4 /dev/vda1 rw
680 655 254:1 /docker/containers/7a0144cee1256c539fab790199527b7051aff1b603ebcf7ed3fd436440ef3b3a/hosts /etc/hosts rw,relatime - ext4 /dev/vda1 rw

Here's a Python snippet that'll parse it:

with open( '/proc/self/mountinfo' ) as file:
    line = file.readline().strip()    
    while line:
        if '/docker/containers/' in line:
            containerID = line.split('/docker/containers/')[-1]     # Take only text to the right
            containerID = containerID.split('/')[0]                 # Take only text to the left
            break
        line = file.readline().strip()

Credit goes to richgriswold: https://community.toradex.com/t/python-nullresource-error-when-running-torizoncore-builder-build/15240/4

How can I get Docker Linux container information from within the container itself?

I've found out that the container id can be found in /proc/self/cgroup

So you can get the id with :

cat /proc/self/cgroup | grep -o  -e "docker-.*.scope" | head -n 1 | sed "s/docker-\(.*\).scope/\\1/"

CoreOS - get docker container name by PID?

Something like this?

$ docker ps -q | xargs docker inspect --format '{{.State.Pid}}, {{.ID}}' | grep "^${PID},"

[EDIT]

Disclaimer This is for "normal" linux. I don't know anything useful about CoreOS, so this may or may not work there.

Docker jenkins ssh-agent not running inside container on Ubuntu 22.04

It turned out that the problem was related to cgroup v2 after all. It seems that when using v2 the cgroup namespace is private by default when you create a container, in my case the Jenkins agents, which caused the container id to not be available in /proc/self/cgroup.

The easy solution is to run the docker container with --cgroupns host as suggested in another question here. When I did that Jenkins could once again detect the container it's running inside.

An update was probably released for Ubuntu 21.10 switching to cgroup v2, just as I posted the question, since I could later reproduce the issue there as well.

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