Linux · disks & space

fdisk

read and edit the partition table

fdisk works on the partition table — the map at the start of a disk that says where each partition begins and ends. With -l it only lists; opened on a disk it is an editor in which nothing is changed until you write with w.

fdisk -l is safe to run any time. Everything else here can erase a disk.

List

fdisk -l

reads only

Edit

fdisk /dev/sdb

interactive

Save

q quits without saving

Needs root

Yes

always

At the terminal

Read the session first, then the notes: each numbered line in the output is answered by the note with the same number.

root@netflow — bashutil-linux

# fdisk -l /dev/sdb

1Disk /dev/sdb: 14.91 GiB, 16008609792 bytes, 31266816 sectors

Disk model: Flash Drive

Units: sectors of 1 * 512 = 512 bytes

2Disklabel type: dos

3Device Boot Start End Sectors Size Id Type

/dev/sdb1 2048 31266815 31264768 14.9G c W95 FAT32 (LBA)

4# fdisk /dev/sdb

Command (m for help):

Illustrative session · names, sizes and dates are examples, and every address is from a documentation or private range

Reading the output

What the numbered lines are telling you

The disk itself

Its size and model. Check these against what you expect before going any further.

The kind of partition table

dos is the older MBR scheme; gpt is the modern one, needed for disks over 2 TB.

One line per partition

Where it starts and ends, its size, and the type recorded for it — here a FAT32 partition filling the stick.

The interactive editor

p prints the table, n makes a new partition, d deletes one. Changes stay in memory until w writes them; q leaves without saving.

Switches

A switch changes how the command behaves. These are the ones used day to day; man fdisk lists the rest. Single-letter switches can usually be joined, so -l -a may be written -la.

-l: List partition tables and exit. Changes nothing.
-x: Like -l, with more detail.
-u: Show sizes in sectors.
-w auto: Wipe old filesystem signatures when creating a table.

Variations worth knowing

The same tool, pointed at a narrower question. These four cover most of what it is reached for day to day.

sudo fdisk -l

Lists the partition tables of every disk.

sudo parted -l

The same information from parted, which some prefer for GPT disks.

sudo mkfs.ext4 /dev/sdb1

Creates a filesystem on a new partition — the step after partitioning.

sudo partprobe /dev/sdb

Asks the kernel to re-read a partition table after it has changed.

In cybersecurity

Editing a partition table is the quickest way to lose everything on a disk.

Check the device, then check again

fdisk /dev/sda and fdisk /dev/sdb differ by one letter. Compare the size and model shown with lsblk before pressing w.

Deleting a partition does not erase data

It removes only the map. The contents remain on the disk and can be recovered — by you, or by whoever gets the disk next.

Wiping before disposal

To make data unrecoverable a disk must be overwritten or, better, have been encrypted from the start. Repartitioning is not wiping.

Filed under

Disks & space

Read alongside

lsblkLists the disks and partitions attached to the machine, as a tree, with where each is mounted.mountAttaches a filesystem — a partition, USB stick or network share — to a directory in the tree.