Cheatsheet & Examples: dd

The dd command is a powerful utility for copying files, converting data, and performing low-level disk operations. It is often used for cloning disks, creating backups, and transferring data between devices.

Copy a Disk to an Image File

Example Usage:
dd if=/dev/sdX of=image.img

What it does:
Creates a disk image of the source device (/dev/sdX) and saves it to image.img.

Command-line Arguments Explained:

• if: Specifies the input file or device (e.g., /dev/sdX is the source disk).
• of: Specifies the output file or device (e.g., image.img is the destination file).

Create a Bootable USB Drive

Example Usage:
dd if=image.iso of=/dev/sdY bs=4M status=progress

What it does:
Writes a bootable ISO file (image.iso) to a USB drive (/dev/sdY) for creating a bootable device.

Command-line Arguments Explained:

• if: Input file (e.g., the ISO image to write to the USB).
• of: Output device (e.g., the USB drive, typically /dev/sdY).
• bs: Block size (e.g., 4M improves performance by reading/writing 4MB chunks).
• status=progress: Displays real-time progress of the copy operation.

Copy Files with Custom Block Size

Example Usage:
dd if=source.txt of=destination.txt bs=1M

What it does:
Copies source.txt to destination.txt using a block size of 1MB for faster transfers.

Command-line Arguments Explained:

• if: Source file (e.g., source.txt).
• of: Destination file (e.g., destination.txt).
• bs: Block size (e.g., 1M sets 1MB blocks for efficiency).

Monitor Copy Progress

Example Usage:
dd if=largefile of=backupfile bs=1G status=progress

What it does:
Copies a large file (largefile) to backupfile and shows real-time progress updates.

Command-line Arguments Explained:

• if: Input file (e.g., largefile).
• of: Output file (e.g., backupfile).
• bs: Block size (e.g., 1G for 1GB chunks).
• status=progress: Displays progress (bytes copied, time elapsed, speed).

Skip or Seek Specific Blocks

Example Usage:
dd if=image.img of=part.img skip=512 count=100 bs=512

What it does:
Extracts a portion of a disk image (image.img) starting from block 512, copying 100 blocks to part.img.

Command-line Arguments Explained:

• if: Input file (e.g., image.img).
• of: Output file (e.g., part.img).
• skip: Number of input blocks to skip before starting (e.g., 512 blocks).
• count: Number of blocks to copy (e.g., 100 blocks).
• bs: Block size (e.g., 512 for sector-sized blocks).

Convert File Case or Process Data

Example Usage:
dd if=input.txt of=output.txt conv=ucase

What it does:
Converts all lowercase letters in input.txt to uppercase and saves the result to output.txt.

Command-line Arguments Explained:

• if: Input file (e.g., input.txt).
• of: Output file (e.g., output.txt).
• conv=ucase: Converts input text to uppercase (other options like lcases exist).

Transfer Data Over a Network

Example Usage:
dd if=/dev/sda | ssh user@remote 'dd of=/backup/sda.img'

What it does:
Sends data from /dev/sda to a remote server's /backup/sda.img via SSH.

Command-line Arguments Explained:

• if: Input device (e.g., /dev/sda).
• of: Output path on the remote system (e.g., /backup/sda.img).
• ssh: Executes the dd command on the remote machine.

Create a Zero-Filled File

Example Usage:
dd if=/dev/zero of=file.img bs=1M count=1024

What it does:
Generates a 1GB file (file.img) filled with zeros, useful for testing or allocating space.

Command-line Arguments Explained:

• if: Input source (e.g., /dev/zero provides null bytes).
• of: Output file (e.g., file.img).
• bs: Block size (e.g., 1M for 1MB blocks).
• count: Number of blocks to copy (e.g., 1024 × 1MB = 1GB).

Clone One Disk to Another

Example Usage:
dd if=/dev/sda of=/dev/sdb

What it does:
Clones the entire contents of /dev/sda to /dev/sdb, creating an exact copy of the disk.

Command-line Arguments Explained:

• if: Source disk (/dev/sda).
• of: Destination disk (/dev/sdb).

Verify Data Integrity

Example Usage:
dd if=/dev/sdX of=/dev/null bs=1M conv=noerror,sync

What it does:
Reads data from /dev/sdX and discards it (/dev/null), skipping errors and padding blocks to ensure data integrity checks.

Command-line Arguments Explained:

• if: Input source (/dev/sdX).
• of: Output destination (/dev/null to discard data).
• bs: Block size (e.g., 1M for efficiency).
• conv=noerror,sync: noerror skips read errors, sync pads blocks with nulls to handle bad sectors.

Compress Data on the Fly

Example Usage:
dd if=image.img | gzip > image.gz

What it does:
Streams a disk image (image.img) through gzip for compression, saving the result as image.gz.

Command-line Arguments Explained:

• if: Input file (image.img).
• | gzip: Pipes the output to gzip for compression (not a dd argument but part of the command chain).

Benchmark Disk Performance

Example Usage:
dd if=/dev/zero of=tempfile bs=1G count=1 oflag=direct

What it does:
Tests write performance by writing 1GB of zeros to tempfile without using system cache (oflag=direct).

Command-line Arguments Explained:

• if: Input source (/dev/zero for zeros).
• of: Output file (tempfile).
• bs: Block size (1G for large chunks).
• count: Number of blocks to copy (1 block of 1GB).
• oflag=direct: Bypasses the system cache for raw I/O.

Copy File Segments

Example Usage:
dd if=bigfile of=partfile skip=100 seek=500 bs=1M

What it does:
Copies data from bigfile and writes it to partfile, skipping 100 blocks and seeking 500 blocks.

Command-line Arguments Explained:

• if: Input file (bigfile).
• of: Output file (partfile).
• skip: Skips the first 100 input blocks.
• seek: Starts writing at the 500th output block.
• bs: Block size (e.g., 1M for 1MB blocks).