Creating a Filesystem

A raw disk is empty

Attaching a disk doesn't make it usable. The blocks are still blank: there's no structure that says where files begin and end, what they're called, or who owns them. That structure is a filesystem, and you create it with mkfs (make filesystem). This is called formatting.

Linux supports many filesystem types. You pick one when you format:

Formatting a disk: mkfs.ext4

Attach the disk, then format it as ext4. -L gives it a label, a friendly name. mkfs prints what it builds — the number of blocks and inodes:

sudo losetup -fP --show disk.img
sudo mkfs.ext4 -L mydata /dev/loop0

In the terminal

student@vulntech:~$ sudo losetup -fP --show disk.img
[sudo] password for student:
/dev/loop0
student@vulntech:~$ sudo mkfs.ext4 -L mydata /dev/loop0
mke2fs 1.47.0 (5-Feb-2023)
Creating filesystem with 65536 1k blocks and 16384 inodes
Filesystem UUID: 0526eec9-f28f-4061-88c5-dfce4d3b5bcd
Superblock backups stored on blocks:
	8193, 24577, 40961, 57345

Allocating group tables: 0/8   done
Writing inode tables: 0/8   done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: 0/8   done

What's on the disk? blkid

blkid reads the filesystem's identity: its type, its label, and its UUID — a long unique id that never changes, even if the disk is renamed or moved. lsblk -f shows the same thing in the tree:

sudo blkid /dev/loop0
lsblk -f /dev/loop0

In the terminal

student@vulntech:~$ sudo blkid /dev/loop0
[sudo] password for student:
/dev/loop0: LABEL="mydata" UUID="33350c7a-835f-4921-bb10-c51d0f96bd9f" BLOCK_SIZE="1024" TYPE="ext4"
student@vulntech:~$ lsblk -f /dev/loop0
NAME FSTYPE FSVER LABEL UUID                                 FSAVAIL FSUSE% MOUNTPOINTS
loop0
     ext4   1.0   mydata
                        33350c7a-835f-4921-bb10-c51d0f96bd9f

A different filesystem: a FAT stick

To prepare a USB stick that Windows and macOS can also read, format it as vfat (FAT). Same idea, different tool; -n sets the label:

sudo losetup -fP --show usb.img
sudo mkfs.vfat -n MYUSB /dev/loop0
sudo blkid /dev/loop0

In the terminal

student@vulntech:~$ sudo losetup -fP --show usb.img
[sudo] password for student:
/dev/loop0
student@vulntech:~$ sudo mkfs.vfat -n MYUSB /dev/loop0
mkfs.fat 4.2 (2021-01-31)
student@vulntech:~$ sudo blkid /dev/loop0
/dev/loop0: SEC_TYPE="msdos" LABEL_FATBOOT="MYUSB" LABEL="MYUSB" UUID="4231-1FCF" BLOCK_SIZE="512" TYPE="vfat"

Common mistake: mkfs erases everything on the target. There is no undo. Always check the device name (lsblk) before you format — formatting the wrong disk destroys it.

Remember: mkfs.ext4 DEVICE puts an ext4 filesystem on a disk (mkfs.vfat for FAT). -L/-n sets a label. blkid and lsblk -f show a filesystem's type, label and UUID.