‘dd’ Command in Linux: Explained

The dd command in Linux is a powerful utility for low-level data copying and conversion, primarily used for disk cloning, creating disk images, partition backups, and writing ISO files to USB drives. Mastering the dd command is essential for Linux system administrators, as it enables precise control over data manipulation, backup processes, and disk recovery.

This guide will demonstrate how to use the dd command for disk cloning, data backup, and creating bootable media. You’ll also learn to handle common options like bs and conv to customize your data copying process effectively.

What is the dd Command in Linux

The dd command in Linux is a command-line tool used for copying and converting data at a low level. It can be used to duplicate entire disks, create disk images, back up partitions, and even write files like ISO images to USB drives. It works with raw data and is highly flexible, which makes it one of the most versatile tools for system administrators.

The dd command operates by copying data byte-by-byte, giving you the ability to control the copying process down to the smallest level. This feature makes it ideal for tasks like creating exact backups of your disk partitions and cloning entire disks.

dd Command Syntax:

Below is the syntax for “dd command” in Linux:

dd if=[input file] of=[output file] [options]

Here,

• if: Specifies the input file or device
• of: Specifies the output file or device
• options: is an additional flags and parameters to modify the operation

Key Options for the dd Command

The dd command` has several options that can be used to modify the data copy process. Here are some important options:

How the dd Command Works

The command line syntax of dd differs from many other Unix programs, in that it uses the syntax option=value for its command line options, rather than the more standard -option value or –option=value formats. By default, dd reads from stdin and writes to stdout, but these can be changed by using the if (input file) and of (output file) options.

Practical Use Cases for the dd command

The dd command is versatile and can be used for several key tasks in Linux:

1. To Backup The Entire Hard Disk

To backup an entire copy of a hard disk to another hard disk connected to the same system, execute the dd command as shown. In this dd command example, the UNIX device name of the source hard disk is /dev/hda, and device name of the target hard disk is /dev/hdb.

# dd if=/dev/sda of=/dev/sdb

Explanation:

• if stands for “input file,” and of stands for “output file.” In this example, it copies all data from /dev/sda to /dev/sdb.
• The command creates an exact copy of /dev/sda on /dev/sdb.

Handling Errors:

• If there are any errors, the above command will fail. If you give the parameter “conv=noerror” then it will continue to copy if there are read errors.
• Input file and output file should be mentioned very carefully. Just in case, you mention source device in the target and vice versa, you might loss all your data.
• To copy, hard drive to hard drive using dd command given below, sync option allows you to copy everything using synchronized I/O.

# dd if=/dev/sda of=/dev/sdb conv=noerror, sync

2. To Backup a Partition

You can use the device name of a partition in the input file, and in the output either you can specify your target path or image file as shown in the dd command.

# dd if=/dev/hda1 of=~/partition.img

3. To Create An Image of a Hard Disk

Instead of taking a backup of the hard disk, you can create an image file of the hard disk and save it in other storage devices. There are many advantages of backing up your data to a disk image, one being the ease of use.

This method is typically faster than other types of backups, enabling you to quickly restore data following an unexpected catastrophe.It creates the image of a hard disk /dev/hda.

# dd if=/dev/hda of=~/hdadisk.img

4. To Restore Using the Hard Disk Image

To restore a hard disk with the image file of an another hard disk, the following dd command can be used

# dd if=hdadisk.img of=/dev/hdb

The image file hdadisk.img file, is the image of a /dev/hda, so the above command will restore the image of /dev/hda to /dev/hdb.

5. To Create CDROM Backup

dd command allows you to create an iso file from a source file. So we can insert the CD and enter dd command to create an iso file of a CD content.

# dd if=/dev/cdrom of=tgsservice.iso bs=2048

dd command reads one block of input and process it and writes it into an output file. You can specify the block size for input and output file. In the above dd command example, the parameter “bs” specifies the block size for the both the input and output file. So dd uses 2048bytes as a block size in the above command.

Conclusion

The dd command in Linux is a powerful utility for disk cloning, data backup, creating bootable USB drives, and many more advanced operations. By understanding its syntax and options, you can easily manage your system’s data and create disk images, partition backups, and restore systems. Although dd is a simple command, it must be used carefully to avoid data loss, especially when dealing with low-level system components like hard drives and partitions.