Yhyjyk

There are several hard disk partitions my system (Linux josDeb 4.9.0-8-amd64 #1 SMP Debian 4.9.144-3.1 (2019-02-19) x86_64 GNU/Linux) is working with:

bejo@josDeb:~$ ls -l /dev/disk/by-uuid

yields:

total 0
lrwxrwxrwx 1 root root 10 Apr 13 16:20 00FB-604A -> ../../sdb1
lrwxrwxrwx 1 root root 10 Apr 13 16:19 4425-7572 -> ../../sda1
lrwxrwxrwx 1 root root 10 Apr 13 16:19 8dc07aba-5729-4525-883f-09c32d1a9e98 -> ../../sda2
lrwxrwxrwx 1 root root 10 Apr 13 16:19 95a8efff-92d2-4e31-8632-bf7a640e100f -> ../../sda3
lrwxrwxrwx 1 root root 10 Apr 13 16:19 f5a05b5e-c3ed-4227-bb62-fe4576b72643 -> ../../sda4

Some partition uuids are long - some are short. I would like to understand why. I thought, uuids always have 16 bytes. How come I have uuids of different sizes?

Actual UUIDs are supposed to be 128-bit long and meant to be unique. Prior to this, various systems provided various serial numbers of various size to be distinguishable. So Linux just takes whatever serial it can find and sticks them in the /dev/by-uuid/ directory even if they aren't matching the UUID definition. That's the case for the FAT32 volume ID:

Sector offset FAT32 EBPB offset Length (bytes) Contents 
0x043 0x38 4 Cf. 0x027 for FAT12/FAT16 (Volume ID) 

Historical description:

Volume ID (serial number)

Typically the serial number "xxxx-xxxx" is created by a 16-bit
addition of both DX values returned by INT 21h/AH=2Ah (get system
date)[nb 7] and INT 21h/AH=2Ch (get system time)[nb 7] for the high
word and another 16-bit addition of both CX values for the low word of
the serial number. Alternatively, some DR-DOS disk utilities provide a
/# option to generate a human-readable time stamp "mmdd-hhmm" build
from BCD-encoded 8-bit values for the month, day, hour and minute
instead of a serial number.

This is a 32 bits value, which can be displayed for example as 4425-7572. Most likely those two partitions are EFI System partitions since they have to be FAT32.

You can get better informations (probably coming from parsing several /dev/disks/by-*/ entries) with the blkid command instead:

# blkid

or limited to those short entries:

# blkid /dev/sda1 /dev/sdb1

The manual suggest to use lsblk instead which doesn't require root. So with the right options that would be lsblk -o +UUID,FSTYPE /dev/sda1 /dev/sdb1.

E.g. here:

$ lsblk -o +UUID,FSTYPE /dev/sda1 /dev/sdb1
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT UUID FSTYPE
sda1 8:1 0 200M 0 part /boot/efi 1234-5678 vfat
sdb1 8:17 1 200M 0 part 9ABC-DEF0 vfat