knowing a device special file major and minor numbers in linux

Question

All files in /dev are special files... they represent devices of the computer. They were created with the mknod syscall. My question is: How can I know the minor and major numbers that were used to create this special file?

Answer 1

The list is called the LANANA Linux Device List, and it is administered by Alan Cox.

You can find the latest copy online (direct link), or in the Linux source. Its filename in the kernel tree is Documentation/devices.txt.

To see the major and minor numbers that created a node in /dev (or any device node for that matter), simply use ls with the -l option:

22:26 jsmith@undertow% ls -l /dev/xvd?
brw-rw---- 1 root disk    202,   0 Nov  1 20:31 /dev/xvda
brw-rw---- 1 root disk    202,  16 Nov  1 20:31 /dev/xvdb
brw-rw---- 1 root disk    202,  32 Nov  1 20:31 /dev/xvdc

In this example, 202 is the three devices' major number, and 0, 16, and 32 are minors. The b at left indicates that the node is a block device. The alternative is c, a character device:

crw-rw-rw- 1 root tty       5,   0 Nov 22 00:29 /dev/tty

Answer 2

$ ls -l /dev/fd0 /dev/null
brw-rw---- 1 root floppy 2, 0 Nov 22 19:48 /dev/fd0
crw-rw-rw- 1 root root   1, 3 Nov 22 19:48 /dev/null
$ stat -c '%n: %F, major %t minor %T' /dev/fd0 /dev/null
/dev/fd0: block special file, major 2 minor 0
/dev/null: character special file, major 1 minor 3

Most device numbers are fixed (i.e. /dev/null will always be character device 1:3) but on Linux, some are dynamically allocated.

$ cat /proc/devices
Character devices:
...
 10 misc
...

Block devices:
...
253 mdp
254 device-mapper
$ cat /proc/misc
...
 57 device-mapper
...

For example, on this system, it just so happens that /dev/mapper/control will be c:10:57 while the rest of /dev/mapper/* will be b:254:*, and this could differ from one boot cycle to another -- or even as modules are loaded/unloaded and devices are added/removed.

You can explore these device registrations further in /sys.

$ readlink /sys/dev/block/2:0
../../devices/platform/floppy.0/block/fd0
$ cat /sys/devices/platform/floppy.0/block/fd0/dev
2:0
$ readlink /sys/dev/char/1:3
../../devices/virtual/mem/null
$ cat /sys/devices/virtual/mem/null/dev
1:3

Answer 3

You can also use stat.

$ stat -c 'major: %t minor: %T' <file>

Answer 4

Especially for block devices:

$ lsblk
NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda       8:0    0   90G  0 disk
├─sda1    8:1    0    4G  0 part [SWAP]
├─sda2    8:2    0    4G  0 part /

Answer 5

Alternative that doesn't depend on stat:

$ cat /sys/class/*/random/dev
1:8