dd

Understanding dd’s Functionality

At its core, dd reads data from an input source (specified by if=) and writes it to an output destination (of=). It operates at a low level, dealing directly with raw bytes. This makes it suitable for creating bitwise identical copies, but also means errors can be catastrophic.

Basic Syntax:

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

• if=<input>: Specifies the input source. This could be a file, a block device (like a hard drive /dev/sda), or a character device.
• of=<output>: Specifies the output destination. This is typically a file, or another block device.
• [options]: Numerous options control the copying process, including block size (bs=), count (count=), and conversion (conv=).

Backing Up a Partition with dd

Let’s say you want to create a full backup of your /dev/sda1 partition to a file named partition_backup.img. Proceed with extreme caution! A single typo can lead to data loss.

Command:

sudo dd if=/dev/sda1 of=/path/to/backup/partition_backup.img bs=4M status=progress

• sudo: Requires root privileges.
• if=/dev/sda1: Specifies the input partition. Double-check this!
• of=/path/to/backup/partition_backup.img: Specifies the output file. Ensure sufficient space on the target drive.
• bs=4M: Sets the block size to 4MB, speeding up the process. Experiment with different sizes to find the optimal value for your system.
• status=progress: Shows a progress bar, indicating the copying progress.

Important Considerations:

• Space: The output file will be the same size as the input partition. Ensure you have enough free space.
• Target Device: Never specify a block device as the output (of=) unless you are absolutely certain. Mistakes here can lead to irreversible data loss.

Restoring from a dd Backup

Restoring a partition from a dd image requires similar care. Let’s assume you want to restore partition_backup.img to /dev/sdb1. Again, proceed with extreme caution. Verify all device names meticulously. It’s highly recommended to test this process on a non-critical system first.

Command:

sudo dd if=/path/to/backup/partition_backup.img of=/dev/sdb1 bs=4M status=progress conv=sync

• conv=sync: This option pads the output with zeros if the input is smaller than the output. This is generally recommended for partition restoration.

Backing Up an Entire Disk

Backing up an entire disk is similar, but requires even greater attention to detail. Consider the following example, backing up /dev/sda to /path/to/backup/disk_backup.img

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

Remember to replace /dev/sda and /path/to/backup/disk_backup.img with your correct device and file path. This process takes a considerable amount of time, especially for larger disks.

Using dd with Other Options

dd offers numerous other options, allowing for more fine-grained control. Some useful options include:

• count=N: Copies only N blocks. Useful for testing or partial backups.
• skip=N: Skips N blocks at the beginning of the input.
• seek=N: Seeks to N blocks at the beginning of the output.

Disclaimer: The dd command is a powerful tool capable of causing significant data loss if used incorrectly. Always double-check your commands before execution and test your backup/restore procedures on a non-critical system. Consider using more backup solutions like rsync or dedicated backup software for routine backups. dd is best suited for specific scenarios requiring a bit-level copy.