As Docker supports cgroup v2 since engine version 20.10, it will automatically use it on distributions that have cgroups v2 enabled. The known solutions to get the unique container ID from within the container, do not work anymore.
/ # cat /proc/self/cgroup
0::/
/ # cat /proc/1/cpuset
/
Tried with docker v20.10.8 on Debian 11 with alpine:latest.
Working solutions for cgroup v1: How can I get Docker Linux container information from within the container itself?
As stated in the docker reference, with cgroup v2, the container id is still visible in the filesystem at the following places, but those aren't accessible from the container itself.
/sys/fs/cgroup/memory/docker/<longid>/ on cgroup v1, cgroupfs driver
/sys/fs/cgroup/memory/system.slice/docker-<longid>.scope/ on cgroup v1, systemd driver
/sys/fs/cgroup/docker/<longid/> on cgroup v2, cgroupfs driver
/sys/fs/cgroup/system.slice/docker-<longid>.scope/ on cgroup v2, systemd driver
https://docs.docker.com/config/containers/runmetrics/#find-the-cgroup-for-a-given-container
Edit 1/2021-09-01:
One Workaround is to run the container with the option --cgroupns host. But that requires control over the creation of the container.
$ docker run -it --cgroupns host alpine cat /proc/self/cgroup
0::/system.slice/docker-09ec67119d38768dbf7994d81c325e2267214428a3c2e581c81557e3650863d8.scope
$ docker run -it alpine cat /proc/self/cgroup
0::/
Question:
Is there any way, to get the unique container id from within? (without relying on the container hostname or having to use the docker api to fetch the id)
I have encountered the same problem while trying to fetch the container identification from cgroup.
But there is another way to achieve the same goal. Docker containers are rely on OverlayFS storage driver, each container will be assigned an unqiue directory which is mapped to a virtual filesystem where the container files are written.
root@container:~# cat /proc/self/mountinfo | grep -i overlay
767 553 0:187 / / rw,relatime master:167 - overlay overlay rw,lowerdir=/var/lib/docker/overlay2/l/ZTHS22AHCPD2HJEY6UIKIO3BHY:/var/lib/docker/overlay2/l/4P6QELQ6532G5362S5VVTA7Y7K,upperdir=/var/lib/docker/overlay2/76c8877e95fa589df1fb97bf831ec221df130fdfb8f1f1cb8166bd99bebf51de/diff,workdir=/var/lib/docker/overlay2/76c8877e95fa589df1fb97bf831ec221df130fdfb8f1f1cb8166bd99bebf51de/work
as the command result has shown above, upperdir=/var/lib/docker/overlay2/76c8877e95fa589df1fb97bf831ec221df130fdfb8f1f1cb8166bd99bebf51de/diff locates in host machine and it serves only one container that it is allocated to.
Notice that OverlayFS could be replace to any driver that implement the speicification of storage driver, but upperdir information remains the same structure.
Make sure that the decision must take inside the container since the host machine would show up OverlayFS mount information if LXC is delopyed on the host machine.
This seems to work without the need to query to cgroup and without using the value of hostname, which sometimes is not set to the value of container id, for example in gitlab runners with docker executors.
OVERLAY_ID=`cat /proc/self/mountinfo | grep -i overlay | sed -n "s/.\+upperdir\\=\\(.\+\\)\\/diff.\+/\1/p"`
CONTAINER_ID=`docker inspect -f $'{{.ID}}\t{{.Name}}\t{{.GraphDriver.Data.MergedDir}}' $(docker ps -aq) | grep $OVERLAY_ID | sed -n "s/\t\+.\+//p"`
echo $CONTAINER_ID
Thanks @soxfmr and to this How do I identify which container owns which overlay directory?