dd Command Tutorial for Sysadmins

I think I only used the dd command less than ten times in my Linux sysadmin life. So, I decided to put together a simple dd Command Tutorial for Sysadmins. But, first I had to brew my favorite Caturra coffee variety and check out their Specialty Coffee news section.

What is dd?

dd is a low-level copying tool in Linux/UNIX. Unlike cp, it works at the raw byte level, meaning it can copy anything: files, partitions, disks, even memory.
This makes it invaluable for backups, recovery, cloning, and troubleshooting.

Basic Syntax

dd if=<input> of=<output> [options]

if= → input file/device (source)
of= → output file/device (destination)
bs= → block size (how much data to read/write at once)
count= → how many blocks to copy
status=progress → show progress bar (modern feature)

Real-Life Use Cases for Sysadmins

Backup the first 512 bytes (bootloader + partition table):

sudo dd if=/dev/sda of=mbr_backup.img bs=512 count=1

Restore it if corrupted:

sudo dd if=mbr_backup.img of=/dev/sda bs=512 count=1

Clone a Full Disk

sudo dd if=/dev/sda of=/dev/sdb bs=4M status=progress

Makes /dev/sdb a bootable copy of /dev/sda.

Useful for migrations or duplicating lab servers.

Create a Disk Image

sudo dd if=/dev/sda of=disk_backup.img bs=4M status=progress

Creates an image file (disk_backup.img) you can compress or transfer.

Write an ISO to USB

sudo dd if=ubuntu.iso of=/dev/sdb bs=4M status=progress oflag=sync

Common method for creating bootable USB drives.

Wipe a Disk Securely

Overwrite a disk with zeros:

sudo dd if=/dev/zero of=/dev/sdb bs=1M status=progress

Overwrite with random data:

sudo dd if=/dev/urandom of=/dev/sdb bs=1M status=progress

Careful: This destroys all data on the target disk.

Performance Testing (Disk Speed)

Write test:

dd if=/dev/zero of=testfile bs=1G count=1 oflag=dsync

Read test:

dd if=testfile of=/dev/null bs=1G count=1

Partial File Copy (Skip & Limit)

Copy the first 100MB of a file:

dd if=bigfile.iso of=sample.iso bs=1M count=100

Skip first 50MB and copy the next 100MB:

dd if=bigfile.iso of=part.iso bs=1M skip=50 count=100

Create Swap File

sudo dd if=/dev/zero of=/swapfile bs=1M count=2048
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

Makes a 2GB swap file.

Safety Tips

Always double-check if= and of= before pressing Enter.

Use lsblk or fdisk -l to confirm disk names.

Practice with small files before using real disks.

Quick Reference Cheatsheet

• Backup MBR: dd if=/dev/sda of=mbr.img bs=512 count=1
• Clone disk: dd if=/dev/sda of=/dev/sdb bs=4M status=progress
• ISO → USB: dd if=linux.iso of=/dev/sdb bs=4M status=progress
• Wipe disk: dd if=/dev/zero of=/dev/sdb bs=1M status=progress
• Create swap: dd if=/dev/zero of=/swapfile bs=1M count=2048

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