Ytdyklly

file /dev/* prints the description of a bunch of files as "block special (M/N)" and "character special (M/N)", where M and N are numbers. For example:

$ file /dev/null

/dev/null: character special (1/3)

man file doesn't seem to document these, but refers vaguely to man stat, which seems to refer to these as major and minor device types. apropos 'device type' finds nothing. So what do these numbers mean?

Devices on Unix have a type (e.g. character or block), a major number (which typically refers to a driver), and a minor number (which typically refers to an instance).

So, for example:

% ls -l /dev/vda

brw-rw---- 1 root disk 253, 0 Feb  3 09:09 /dev/vda

This is a block device, major 253, minor 0.

If we look at /proc/devices we see it ends with something similar to

Block devices:

  2 fd

259 blkext

  9 md

253 virtblk

254 mdp

So we can see that 253 is "virtblk". Which makes sense, since this is a virtual machine with virtual disks!

The minor number, for this driver, refers to the block device and partition in the device

% ls -l /dev/vd*

brw-rw---- 1 root disk 253,  0 Feb  3 09:09 /dev/vda

brw-rw---- 1 root disk 253,  1 Feb  3 09:09 /dev/vda1

brw-rw---- 1 root disk 253,  2 Feb  3 09:09 /dev/vda2

brw-rw---- 1 root disk 253,  3 Feb  3 09:09 /dev/vda3

brw-rw---- 1 root disk 253, 16 Feb  3 09:09 /dev/vdb

brw-rw---- 1 root disk 253, 32 Feb  3 09:09 /dev/vdc

brw-rw---- 1 root disk 253, 33 Feb  3 09:09 /dev/vdc1

There are some special drivers which don't refer to "real" hardware. eg

% ls -l /dev/null

crw-rw-rw- 1 root root 1, 3 Feb  3 09:09 /dev/null

This is a character device, major 1, minor 3. /proc/devices tells us driver 1

  1 mem

We can see this "mem" driver handles a few other devices as well

% ls -l /dev | grep ' 1, '

crw-rw-rw- 1 root root      1,   7 Feb  3 09:09 full

crw-r--r-- 1 root root      1,  11 Feb  3 09:09 kmsg

crw-r----- 1 root kmem      1,   1 Feb  3 09:09 mem

crw-rw-rw- 1 root root      1,   3 Feb  3 09:09 null

crw------- 1 root root      1,  12 Feb  3 09:09 oldmem

crw-r----- 1 root kmem      1,   4 Feb  3 09:09 port

crw-rw-rw- 1 root root      1,   8 Feb  3 09:09 random

crw-rw-rw- 1 root root      1,   9 Feb  3 09:09 urandom

crw-rw-rw- 1 root root      1,   5 Feb  3 09:09 zero

Each device node’s type (block or character) and numbers (known as the major and minor number) serve as identifiers for the kernel.

On Linux, the canonical list of devices, with a brief explanation of their function, is maintained in the kernel. You can see there that a major number typically identifies a driver (e.g. the TTY device driver) and a minor number identifies a device managed by that driver.