Use dd to manage disks

Everything is a File

In Linux, everything is treated as a file — not just .txt or .jpg.

Examples:

Hard disk → /dev/sda
Partition → /dev/sda1
CD-ROM → /dev/sr0
Even keyboard or mouse → /dev/input/*

If it can be read or written, it behaves like a file.

And filesystem helps transform the disk from raw bytes under /dev/sda into a structured directory tree like /home/user/documents.

What is `dd`?

dd is a low-level copy tool. It copies data byte-by-byte from one file (or device) to another.

dd if=input_file of=output_file

if = input file/device
of = output file/device

dd doesn't care if it's copying a text file or a disk. It's raw.

Copying a Disk = Copying a File

Since disks are just files in /dev/, you can back up a disk like this:

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

This reads /dev/sda (the whole disk), and writes it to a regular file.

You can later restore it:

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

Data loss risk

dd is powerful but dangerous. A small mistake can wipe out your data. The command above will wipe everything on /dev/sda and replace it with the contents of disk.img. Always double-check your commands!

Special Files: `/dev/zero`, `/dev/null`, `/dev/random`

Linux has “virtual files” that behave like endless data sources or sinks:

/dev/zero → infinite zero bytes (0x00)
/dev/null → a black hole; discards anything written
/dev/random → random bytes (from entropy pool)

Examples:

dd if=/dev/zero of=zero.bin bs=1M count=1

Creates a 1MB file filled with zeros.

dd if=/dev/random of=random.bin bs=512 count=4

Creates 2KB of random binary data.

Files Can Be Virtual Disks

You can treat any file like a disk using dd and loop mounts.

Create a 4GB file:

dd if=/dev/zero of=disk.img bs=1M count=4096

Format it as ext4:

mkfs.ext4 disk.img

Mount it:

sudo mount -o loop disk.img /mnt

Now /mnt behaves like a real mounted disk.

To unmount:

sudo umount /mnt

Use Cases: Real-World `dd` Examples

1. Full Disk Backup

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

Saves your entire disk into one image file.

2. Full Disk Restore

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

Restores the image to the original disk (destructive!).

3. Disk-to-Disk Cloning

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

Copies /dev/sda directly onto /dev/sdb. Useful for migration or duplicaion.

4. Wipe a Disk

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

Wipes /dev/sda by writing zeros, effectively erasing all data.

However, random data is more secure:

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

5. Test file system read/write speed

To test the read/write speed of a folder, you can use dd to write a large file and measure the time taken:

Test write speed:

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

Test read speed:

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

6. Test raw disk read/write speed

To test the raw disk read/write speed, you can use dd directly on the device:

Caution: This will overwrite data on the disk!

Make sure you know what you're doing. Running these commands on device results in data loss.