Identifying a Process's Cgroup by PID

Read /proc/<PID>/cgroup to find which cgroup (and therefore which container) a process belongs to.

/proc/PID/cgroup

cat /proc/<PID>/cgroup

Shows all cgroup controllers the process belongs to:

12:blkio:/user.slice
11:memory:/user.slice/user-1000.slice
...
0::/user.slice/user-1000.slice/session-1.scope

On cgroup v2, the path after :: is the cgroup path under /sys/fs/cgroup/.

# ps format options
ps -o pid,cgroup -p <PID>

# systemd systems
systemd-cgls --unit <unit-name>
systemd-cgls  # whole tree

cat /proc/self/cgroup  # current shell
cat /proc/$$/cgroup    # also current shell
cat /proc/1234/cgroup  # specific PID