How to find the root device?

20 thoughts on “How to find the root device?”

1. Useful info there, thanks – though n.b. it doesn’t work with root mounted over NFS because there’s no entry in sys/block in this case (which is maybe why BusyBox walks /dev).

   Reply

2. On some embedded systems the rdev command will fail simply because busybox has been compiled without it. As long as you have a writable device this is not a problem. Simply load a later busybox for your processor from the busybox.net website to that device with a different name and call the command with, for example, /dev/sda/busybox-mipsel rdev.

   Reply

3. Hi,

   following your hints I came out with this, maybe simplistic, approach:
   
find_linux_root_device() {
RDEV=$(mountpoint -d /)

   for file in $(find /dev);
   do
   if [ $(stat --printf="%t:%T" "$file") = $RDEV ];
   then
   ROOTDEVICE="$file"
   break;
   fi
   done

   echo "$ROOTDEVICE"
}


   What do you think about it?

   Thanks,
   Antonio

   Reply

   1. In the code above I forgot to consider that ‘stat’ prints the major and minor number in hex, while ‘mountpoint’ outputs them in decimal.

      Something like this should be used:

      
...
CURRENT_DEVICE=$(printf "%d:%d" $(stat --printf="0x%t 0x%T" "$devnode"))
if [ $CURRENT_DEVICE = $RDEV ];
...


      Regards,
      Antonio

      Reply

4. #/bin/bash
   find_linux_root_device() {
   RDEV=$(mountpoint -d /)

   for file in /dev/* ; do
   CURRENT_DEVICE=$(stat -c “%t:%T” $file)
   if [ $CURRENT_DEVICE = $RDEV ]; then
   ROOTDEVICE=”$file”
   break;
   fi
   done

   echo “$ROOTDEVICE”
   }

   Reply

5. This seems to work for a lot of systems:

   #/bin/bash
   find_linux_root_device() {
   local PDEVICE=`stat -c %04D /`

   for file in $(find /dev -type b 2>/dev/null) ; do
   local CURRENT_DEVICE=$(stat -c “%02t%02T” $file)
   if [ $CURRENT_DEVICE = $PDEVICE ]; then
   ROOTDEVICE=”$file”
   break;
   fi
   done

   echo “$ROOTDEVICE”
   }

   Reply

6. Hello Team,

   cat /proc/cmdline
   console=ttyS0,115200 root=/dev/nfs rw nfsroot=192.168.1.100:/home/nfs/IntelCE,nolock ip=dhcp mem=exactmap memmap=1M@0 memmap=639M@1M

   # rdev
   0x000c /

   How can i increase the size of my root partiotion , it is shown as /dev/root.
   Thanks,
   Rashmi

   Reply

7. Assuming you have no duplicate partitions mounted nor any single partition mounted twice:

   df -a | grep "`df -a | grep rootfs | cut -d -f2- | sed 's/^ *//g'`" | cut -d -f1 | tail -1

   Basically, run df -a to get stats on what it lists as rootfs and then use that info to scan for what other listing has those stats.

   Reply

8. on my system, /dev/root is a symlink to /dev/sda1, which is where the system was booted from.

   Reply

   1. Interesting. What kind of system is this? What distribution is it, or what was used to generate the root filesystem?

      As least, this doesn’t seem to be a standard feature. Ubuntu 13.04 with Linux 3.8 doesn’t have such a link.

      Thanks!

      Michael.

      Reply

9. #! /bin/bash

   MAJOR_ROOTFS=$(mountpoint -d / | cut -f 1 -d “:”)
   MINOR_ROOTFS=$(mountpoint -d / | cut -f 2 -d “:”)

   DEV_ROOTFS=$(cat /proc/partitions | awk {‘if ($1 == “‘${MAJOR_ROOTFS}'” && $2 == “‘${MINOR_ROOTFS}'”) print $4 ‘})

   echo /dev/$DEV_ROOTFS

   However, this script doesn’t work when rootfs is an UBI file system. It fails because “mountpoint” returns wrong major/minor numbers. How can this be solved in a more generic way?

   Reply

10. I was just investigating a solution to this and found this command that seems useful:
    findmnt

    
user@someplace:~# findmnt
TARGET SOURCE FSTYPE OPTIONS
/ /dev/mmcblk0p2 ext3 rw,relatime,errors=continue,user_x
|-/dev devtmpfs devtmpfs rw,relatime,size=383868k,nr_inodes
| `-/dev/pts devpts devpts rw,relatime,gid=5,mode=620
|-/proc proc proc rw,relatime
|-/sys sysfs sysfs rw,relatime
| `-/sys/kernel/debug debugfs debugfs rw,relatime
|-/run tmpfs tmpfs rw,nosuid,nodev,mode=755
|-/var/volatile tmpfs tmpfs rw,relatime


    Reply

    1. Single command:

       user@someplace:~# findmnt -n -o SOURCE /
       /dev/mmcblk0p2

       Reply

    2. Great, that’s exactly what I’ve been looking for. It solved the problem for me on a Raspberry Pi with Raspian Jessie.

       Reply

11. Author, can you kindly let me know how to properly read the file system type?

    For instance, I’d like to know my internal and external sdcard file system, but some of the Market apps show types different from what I expect…

    Reply

    1. That’s a slightly out of topic for Android, but I’m glad to help.

       You can get the excellent “Terminal Emulator for Android” app and then run the mount command. It will tell you what filesystems are in use on your system.

       Cheers,

       Michael.

       Reply

12. Couldn’t you just look at /etc/fstab?

    Reply

    1. Sure, that’s the easiest way. However, I was in the perspective of a small embedded Linux system that may not even have a /etc/fstab file and would be mounting its mount points through a simple script…

       Reply

13. what is my rootfs device?

    root@ibm#findmnt
    TARGET SOURCE FSTYPE OPTIONS
    / unionfs rw,relatime,dirs=/host/root_rw/image-a=rw:/host/root_ro=ro
    |-/sys sysfs sysfs rw,nosuid,nodev,noexec,relatime
    | |-/sys/fs/cgroup tmpfs rw,relatime,size=4k,mode=755
    | |-/sys/fs/fuse/connections fusectl rw,relatime
    | |-/sys/kernel/debug debugfs rw,relatime
    | `-/sys/kernel/security securityfs rw,relatime
    |-/proc proc proc rw,nosuid,nodev,noexec,relatime
    |-/dev udev devtmpfs rw,relatime,size=122448k,nr_inodes=30612,mode=755
    | `-/dev/pts devpts devpts rw,nosuid,noexec,relatime,gid=5,mode=620
    |-/run tmpfs tmpfs rw,nosuid,relatime,size=101952k,mode=755
    | |-/run/lock tmpfs rw,nosuid,nodev,noexec,relatime,size=5120k
    | |-/run/shm tmpfs rw,nosuid,nodev,relatime
    | `-/run/user tmpfs rw,nosuid,nodev,noexec,relatime,size=102400k,mode=755
    |-/mnt/ro /dev/mmcblk0p3 ext4 ro,relatime,data=ordered
    `-/mnt/rw /dev/mmcblk0p4 ext4 rw,relatime,data=ordered

    Reply

    1. Very nice utility indeed. Thanks!

       Reply