Linux Command Tutorial: fdisk

The Linux Command Tutorial series provides rigorous, upstream-verified references for essential system commands across Linux distributions and UNIX-like environments. Each article focuses on a single executable, combining exhaustive option documentation, verified real-world examples, security boundaries, and best practices directly derived from official source documentation and POSIX standards.

1. Introduction

Upstream: util-linux (util-linux 2.40) | POSIX: Linux-Specific (util-linux extension) | Safety Tier: privileged-system-destructive | Scope: partition-table-manipulation

fdisk is a dialog-driven and scriptable partition table manipulator for block devices. It inspects, creates, alters, and deletes partitions on disks formatted with GUID Partition Table (GPT), Master Boot Record (MBR/DOS), Sun, or SGI partition schemes.

Upstream Project & Provenance: Maintained within util-linux under the fdisks subsystem, built upon libfdisk.
Portability & Standards Baseline: Linux-specific system management tool; not standardized in IEEE Std 1003.1-2024 (POSIX.1-2024).
Target Research Implementation: Audited against util-linux 2.40 (fdisk(8)).
Applicability & Lifecycle: The foundational low-level tool for disk partitioning, supporting both traditional MBR and modern GPT disks.

2. Syntax and Command Model

2.1 Canonical Synopsis

bash

fdisk [options] device
fdisk -l [device...]

2.2 Execution & In-Memory Buffer Model

fdisk operates on a staging buffer in user-space memory:

In-Memory Modification: All partition creation (n), deletion (d), type changes (t), and label initialization (g, o) occur strictly in memory.
Commit Boundary (w): Disk structures are only modified when the user explicitly issues the w (write) command.
Abort Boundary (q): Exiting with q (quit) terminates the session without committing any changes to the physical disk.
Kernel Notification: Upon writing (w), fdisk calls the BLKRRPART ioctl to instruct the kernel to re-read the partition table.

2.3 Interactive Command Summary

Command	Action
`m`	Print the interactive command menu and help text.
`p`	Print the current in-memory partition table.
`n`	Add a new partition.
`d`	Delete an existing partition.
`t`	Change a partition type / partition type GUID.
`g`	Create a new, empty GPT partition table.
`o`	Create a new, empty DOS/MBR partition table.
`v`	Verify the partition table integrity and detect overlapping sectors.
`w`	Write partition table to disk and exit (commits all modifications).
`q`	Quit without saving changes.

3. Options

3.1 Command-Line Options

Option	Long Option	Description	Default
`-l`	`--list`	List partition tables for specified devices (or all devices if omitted) and exit.	Interactive mode
`-b size`	`--sector-size size`	Explicitly specify physical sector size (512, 1024, 2048, 4096).	Hardware probed
`-B`	`--protect-boot`	Do not wipe bootloader boot code when creating a new disk label.	Normal label creation
`-c[=mode]`	`--compatibility[=mode]`	Set compatibility mode (`dos` or `nondos`).	nondos
`-L[=when]`	`--color[=when]`	Colorize output (`auto`, `always`, `never`).	auto
`-o list`	`--output list`	Specify comma-separated output columns for `--list` display.	Default columns
`-t type`	`--type type`	Restrict partition type listing to specified label type (e.g. `gpt`, `dos`).	All types
`-u[=unit]`	`--units[=unit]`	Display units (`sectors` or `cylinders`).	sectors
`-w when`	`--wipe when`	Wipe filesystem, RAID, and partition-table signatures (`auto`, `never`, `always`).	auto
`-W when`	`--wipe-partitions when`	Wipe signatures from newly created partitions (`auto`, `never`, `always`).	auto

4. Basic Usage

4.1 Quick-Reference Cheatsheet Card

Operation	Command Pattern	Copyable One-Liner	Notes
List all partition tables	`fdisk -l`	`sudo fdisk -l`	Read-only scan of all detected block devices
Inspect specific disk	`fdisk -l [device]`	`sudo fdisk -l /dev/sda`	Target single drive without flooding terminal
Interactive partitioner	`fdisk [device]`	`sudo fdisk /dev/sdb`	Opens in-memory staging session (commit with `w`)
Backup partition table	`sfdisk -d [device]`	`sudo sfdisk -d /dev/sda > sda_backup.dump`	Generates textual, restorable partition layout
Restore partition backup	`sfdisk [device] < [file]`	`sudo sfdisk /dev/sda < sda_backup.dump`	Restores partition table from sfdisk dump
Force kernel re-read	`partx -u [partition] [device]`	`sudo partx -u /dev/sdc1 /dev/sdc`	Informs kernel of partition changes without reboot

4.2 Listing All Partition Tables

Query all system block storage devices in read-only mode using -l:

bash

sudo fdisk -l

Output:

console

Disk /dev/sda: 476.94 GiB, 512110190592 bytes, 1000215216 sectors
Disk model: Samsung SSD 980 500GB
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 7E5A2A11-9F3C-4328-98C3-324F159C2056

Device         Start        End   Sectors   Size Type
/dev/sda1       2048    1050623   1048576   512M EFI System
/dev/sda2    1050624    3147775   2097152     1G Linux filesystem
/dev/sda3    3147776 1000214527 997066752 475.4G Linux LVM

4.3 Querying a Specific Block Device

Target a single drive without listing unrelated devices:

bash

sudo fdisk -l /dev/sdb

Output:

console

Disk /dev/sdb: 1.82 TiB, 2000398934016 bytes, 3907029168 sectors
Disk model: WDC WD20EZAZ-00L
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: A91B24E3-5572-4B61-912A-6A792E5B1012

Device     Start        End    Sectors  Size Type
/dev/sdb1   2048 3907028991 3907026944  1.8T Linux filesystem

5. Practical Operations

5.1 Interactive Partitioning Workflow (Creating a GPT Partition)

To initialize a raw storage disk /dev/sdc with a GPT label and a single partition, open the interactive staging session:

bash

sudo fdisk /dev/sdc

Interactive dialogue sequence:

console

Welcome to fdisk (util-linux 2.40).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): g
Created a new GPT disklabel (GUID: B8D3E421-1F3B-4D2A-98F1-44E2319A5B87).

Command (m for help): n
Partition number (1-128, default 1): 1
First sector (2048-209715166, default 2048): 2048
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-209715166, default 209715166): +50G

Created a new partition 1 of type 'Linux filesystem' and of size 50 GiB.

Command (m for help): p
Disk /dev/sdc: 100 GiB, 107374182400 bytes, 209715200 sectors
Units: sectors of 1 * 512 = 512 bytes
Disklabel type: gpt

Device        Start       End   Sectors Size Type
/dev/sdc1      2048 104859647 104857600  50G Linux filesystem

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

5.2 Changing a Partition Type (e.g. Linux Swap or EFI)

Change the type identifier of partition 1 to Linux swap:

bash

sudo fdisk /dev/sdc

Interactive session dialogue:

console

Command (m for help): t
Partition number (1, default 1): 1
Partition type or alias (type L to list all): swap

Changed type of partition 'Linux filesystem' to 'Linux swap'.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

5.3 Deleting an Existing Partition

Remove an unwanted partition from the in-memory staging table:

bash

sudo fdisk /dev/sdc

Interactive session dialogue:

console

Command (m for help): d
Partition number (1, default 1): 1
Partition 1 has been deleted.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

6. Advanced Usage

6.1 Non-Interactive Scripting via Heredocs

Automated provisioning scripts can pipe commands into fdisk using here-documents:

bash

# Non-interactively create a GPT label and a 10G partition on /dev/sdc
sudo fdisk /dev/sdc <<EOF
g
n
1
2048
+10G
w
EOF

Note: For robust scripted partitioning without interactive prompts, upstream util-linux recommends sfdisk (sfdisk(8)), which is designed specifically for automation.

6.2 Handling Busy Kernel Partition Tables

When writing a partition table to a disk where other partitions are actively mounted, the kernel ioctl(BLKRRPART) call may report that the device is busy:

text

Re-reading the partition table failed.: Device or resource busy
The kernel still uses the old table. The new table will be used at the next reboot or you can run partx(8) or kpartx(8).

Force the kernel to update partition table mappings for a specific partition without rebooting:

bash

# Inform kernel of changes to partition 1 on /dev/sdc
sudo partx -u /dev/sdc1 /dev/sdc

# Or trigger a full partition rescan via partprobe
sudo partprobe /dev/sdc

6.3 Sector Alignment Verification

Modern block devices use 4096-byte (4K) physical sectors. If partitions do not start on sector boundaries divisible by 8 (for 512-byte logical emulation), write amplification degrades performance. fdisk automatically enforces 2048-sector (1 MiB) alignment:

bash

# Check starting sector divisibility by 2048
sudo fdisk -l /dev/sda | awk '$1 ~ /\/dev\// {print $1, $2, ($2 % 2048 == 0 ? "ALIGNED" : "MISALIGNED")}'

7. Exit Status, Environment, and Configuration

7.1 Exit Status

Exit Code	Meaning
`0`	Success: command completed, listing printed, or partition table committed.
`1`	Failure: invalid arguments, hardware I/O error, or permission denied.

7.2 Environment Variables

Variable	Description
`LIBFDISK_DEBUG`	Enables debugging output for `libfdisk` backend routines (`all` or comma-separated masks).
`FDISK_DEBUG`	Traces interactive UI events and dialog parsing.

7.3 Relevant Kernel Interfaces

Interface	Purpose
`BLKRRPART`	Kernel `ioctl` invoked by `fdisk` upon `w` to re-read partition tables.
`/proc/partitions`	Kernel view of registered block devices and active partition boundaries.

8. Safety, Security, and Portability

8.1 Destructive Impact and Execution Safety

CAUTION

Destructive Partition Table Alterations: fdisk operates directly on low-level disk structures. Specifying the wrong target drive (e.g. /dev/nvme0n1 or /dev/sda instead of /dev/sdb) permanently overwrites partition headers, filesystem superblocks, and boot records upon issuing w. Always verify target device paths using lsblk before committing changes.

8.2 Privilege Boundaries

Modifying partition tables requires CAP_SYS_ADMIN capability, typically achieved via sudo or the root user account. Unprivileged users cannot open raw block device nodes for writing.

8.3 Signature Wiping Safeguards

NOTE

Modern fdisk automatically detects existing filesystem or RAID signatures on sectors assigned to new partitions. It prompts before wiping existing signatures to prevent accidental data destruction:

text

Created a new partition 1 of type 'Linux filesystem' and of size 50 GiB.
Partition #1 contains a ext4 signature.
Do you want to remove the signature? [Y]es/[N]o:

9. Best Practices

9.1 Always Back Up Partition Tables Before Modification

TIP

Always Backup Partition Tables: Always dump the existing partition table prior to running fdisk to enable instantaneous rollback if an error occurs.

Upstream Rationale: fdisk(8) and sfdisk(8) manuals emphasize that partition table changes can result in unbootable systems or lost partition boundaries. Always dump the existing partition table prior to running fdisk:

bash

sudo sfdisk -d /dev/sda > sda_partition_backup.dump

To restore the backup if an error occurs:

bash

sudo sfdisk /dev/sda < sda_partition_backup.dump

9.2 Prefer GPT Over MBR for Modern Deployments

Upstream Rationale: MBR (DOS disklabel) is limited to 2 TiB disk capacities, a maximum of four primary partitions, and lacks redundant partition headers. GPT supports disks up to 8 ZiB, provides 128 partition slots by default, and maintains secondary backup headers at the end of the physical disk.

9.3 Maintain 1 MiB (2048-Sector) Alignment

Upstream Rationale: Modern Advanced Format hard drives (4Kn / 512e) and solid-state drives (SSDs) organize physical storage into 4096-byte pages and multi-megabyte erase blocks. Starting partitions at sector 2048 (1 MiB offset) ensures perfect alignment across all physical page boundaries.

References

fdisk(8) — Linux man page, util-linux project: https://man7.org/linux/man-pages/man8/fdisk.8.html
util-linux source repository (fdisks/fdisk.8.adoc): https://github.com/util-linux/util-linux/blob/master/fdisks/fdisk.8.adoc
UEFI Forum, Unified Extensible Firmware Interface (UEFI) Specification (GPT Partitioning): https://uefi.org/specifications

1. Introduction ​

2. Syntax and Command Model ​

2.1 Canonical Synopsis ​

2.2 Execution & In-Memory Buffer Model ​

2.3 Interactive Command Summary ​

3. Options ​

3.1 Command-Line Options ​

4. Basic Usage ​

4.1 Quick-Reference Cheatsheet Card ​

4.2 Listing All Partition Tables ​

4.3 Querying a Specific Block Device ​

5. Practical Operations ​

5.1 Interactive Partitioning Workflow (Creating a GPT Partition) ​

5.2 Changing a Partition Type (e.g. Linux Swap or EFI) ​

5.3 Deleting an Existing Partition ​

6. Advanced Usage ​

6.1 Non-Interactive Scripting via Heredocs ​

6.2 Handling Busy Kernel Partition Tables ​

6.3 Sector Alignment Verification ​

7. Exit Status, Environment, and Configuration ​

7.1 Exit Status ​

7.2 Environment Variables ​

7.3 Relevant Kernel Interfaces ​

8. Safety, Security, and Portability ​

8.1 Destructive Impact and Execution Safety ​

8.2 Privilege Boundaries ​

8.3 Signature Wiping Safeguards ​

9. Best Practices ​

9.1 Always Back Up Partition Tables Before Modification ​

9.2 Prefer GPT Over MBR for Modern Deployments ​

9.3 Maintain 1 MiB (2048-Sector) Alignment ​

References ​

1. Introduction

2. Syntax and Command Model

2.1 Canonical Synopsis

2.2 Execution & In-Memory Buffer Model

2.3 Interactive Command Summary

3. Options

3.1 Command-Line Options

4. Basic Usage

4.1 Quick-Reference Cheatsheet Card

4.2 Listing All Partition Tables

4.3 Querying a Specific Block Device

5. Practical Operations

5.1 Interactive Partitioning Workflow (Creating a GPT Partition)

5.2 Changing a Partition Type (e.g. Linux Swap or EFI)

5.3 Deleting an Existing Partition

6. Advanced Usage

6.1 Non-Interactive Scripting via Heredocs

6.2 Handling Busy Kernel Partition Tables

6.3 Sector Alignment Verification

7. Exit Status, Environment, and Configuration

7.1 Exit Status

7.2 Environment Variables

7.3 Relevant Kernel Interfaces

8. Safety, Security, and Portability

8.1 Destructive Impact and Execution Safety

8.2 Privilege Boundaries

8.3 Signature Wiping Safeguards

9. Best Practices

9.1 Always Back Up Partition Tables Before Modification

9.2 Prefer GPT Over MBR for Modern Deployments

9.3 Maintain 1 MiB (2048-Sector) Alignment

References