printroot - print the name of the root device on standard output
printroot
-r Print only the root device, not a full mtab line
printroot # Print the name of the root device
Printroot is useful for initializing the /etc/mtab when the system is booted. It figures out what the root device is by searching /dev until it finds a block special file with the right major/minor device numbers.
fstab(5), boot(8).