umount-oldroot.sh

vagrant@ubuntu-groovy:~$ sudo unshare --mount bash
root@ubuntu-groovy:/home/vagrant# ls /proc
1    115   1324  157  180  21   26   29   373    473  490    556    606    629    66620  682  83  9   97          bus       devices        fb           irq        kpagecgroup  meminfo  net           scsi      sys            uptime
10   116   1325  16   195  22   27   3    387    474  52538  558    617    635    66621  79   84  91  98          cgroups   diskstats      filesystems  kallsyms   kpagecount   misc     pagetypeinfo  self      sysrq-trigger  version
101  12    1398  17   196  229  273  30   39529  475  52557  56868  618    64015  66632  792  85  92  99          cmdline   dma            fs           kcore      kpageflags   modules  partitions    slabinfo  sysvipc        version_signature
102  121   1399  173  197  23   274  31   39530  476  54058  6      619    643    674    80   86  94  acpi        consoles  driver         interrupts   keys       loadavg      mounts   pressure      softirqs  thread-self    vmallocinfo
11   13    14    176  2    24   275  32   4      487  54171  601    621    65694  676    81   87  95  bootconfig  cpuinfo   dynamic_debug  iomem        key-users  locks        mpt      sched_debug   stat      timer_list     vmstat
112  1321  15    18   20   25   28   348  472    489  54490  604    62436  66077  677    82   89  96  buddyinfo   crypto    execdomains    ioports      kmsg       mdstat       mtrr     schedstat     swaps     tty            zoneinfo
root@ubuntu-groovy:/home/vagrant# umount -l /proc
root@ubuntu-groovy:/home/vagrant# ls /proc
root@ubuntu-groovy:/home/vagrant# /bin/mount
mount: failed to read mtab: No such file or directory
root@ubuntu-groovy:/home/vagrant# /bin/mount -rbind rootfs rootfs
/bin/mount: invalid option -- 'b'
Try 'mount --help' for more information.
root@ubuntu-groovy:/home/vagrant# /bin/mount --rbind rootfs rootfs
root@ubuntu-groovy:/home/vagrant# mkdir -p rootfs/oldroot
root@ubuntu-groovy:/home/vagrant# pivot_root rootfs rootfs/oldroot/
root@ubuntu-groovy:/home/vagrant# cd /
root@ubuntu-groovy:/# umount -lR /oldroot
bash: /usr/bin/umount: No such file or directory
root@ubuntu-groovy:/# /bin/umount -lR /oldroot
umount: failed to parse /proc/self/mountinfo: No such file or directory
root@ubuntu-groovy:/# /bin/mount -t proc none /proc
root@ubuntu-groovy:/# /bin/mount
proc on /proc type proc (rw)
proc on /proc type proc (rw)
none on /proc type proc (rw)
root@ubuntu-groovy:/# /bin/umount -a
root@ubuntu-groovy:/# /bin/mount -t proc none /proc
root@ubuntu-groovy:/# /bin/mount
proc on /proc type proc (rw)
proc on /proc type proc (rw)
none on /proc type proc (rw)
none on /proc type proc (rw)
root@ubuntu-groovy:/# /bin/umount /oldroot
umount: /oldroot: target is busy
        (In some cases useful info about processes that
         use the device is found by lsof(8) or fuser(1).)
root@ubuntu-groovy:/# /bin/umount -l /oldroot
root@ubuntu-groovy:/# /bin/mount
proc on /proc type proc (rw)
proc on /proc type proc (rw)
none on /proc type proc (rw)
none on /proc type proc (rw)
root@ubuntu-groovy:/# ls /oldroot
bash: /usr/bin/ls: No such file or directory
root@ubuntu-groovy:/# /bin/ls /oldroot
root@ubuntu-groovy:/# rmdir /oldroot
root@ubuntu-groovy:/# ls
bash: /usr/bin/ls: No such file or directory
root@ubuntu-groovy:/# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin
root@ubuntu-groovy:/# export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin
root@ubuntu-groovy:/# ls
bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var
root@ubuntu-groovy:/# ls /oldroot
ls: cannot access /oldroot: No such file or directory