The dd command of linux

The dd command stands for “data duplicator” and used for copying and converting data. It is very powerful low level utility of Linux which can do much more like;

• Backup and restore the entire hard disk or partition.
• Backup of MBR (Master Boot Record)
• It can copy and convert magnetic tape format, convert between ASCII and EBCDIC formats, swap bytes and can also convert lower case to upper case.
• It can also be used by Linux kernel make files to make boot images.

Only superuser can run this command because you can face a big data loss due to it’s improper usage, so you should be very careful while working with this utility. At that moment data loss can convert the dd utility as a “data destroyer” for you. That’s why it is recommended that beginners should not use this command on a production machine until they get familiarity on this. You must make sure that target location must have sufficient space while running this command.

SYNTAX

Before we start with some practical work we need to talk about it’s syntax.

dd if=<source file name> of=<target file name> [Options]

We normally do not explain about syntax but this command syntax require some explanation. The syntax is totally different when compared to many Linux commands we know. In this syntax dd is followed by two things

if= –This is a source from where you want to copy data and ‘if’ stands for input-file.

of= –This is a source from where you want to write/paste data and ‘of’ stands for output-file.

[options] –These options include, how fast data should be written, what format etc.

if =输入文件（或设备名称）。

of =输出文件（或设备名称）。

ibs = bytes 一次读取bytes字节，即读入缓冲区的字节数。

skip = blocks 跳过读入缓冲区开头的ibs*blocks块。

obs = bytes 一次写入bytes字节，即写 入缓冲区的字节数。

bs = bytes 同时设置读/写缓冲区的字节数（等于设置obs和obs）。

cbs = bytes 一次转换bytes字节。

count = blocks 只拷贝输入的blocks块。

conv = ASCII 把EBCDIC码转换为ASCII码。

conv = ebcdic 把ASCII码转换为EBCDIC码。

conv = ibm 把ASCII码转换为alternate EBCDIC码。

conv = blick 把变动位转换成固定字符。

conv = ublock 把固定们转换成变动位

conv = ucase 把字母由小写变为大写。

conv = lcase 把字母由大写变为小写。

conv = notrunc 不截短输出文件。

conv = swab 交换每一对输入字节。

conv = noerror 出错时不停止处理。

conv = sync 把每个输入记录的大小都调到ibs的大小（用ibs填充）。

fdformat 命令,低级格式化软盘。

EXAMPLE

Example 1: Clone one hard disk to another hard disk. This is useful when we are building many machines with same configuration. We no need to install OS on all the machines. Just install OS and required software on machine then clone with below example.

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

Example 2: We can take backup of a partition/complete HDD for future restoration.
Backing up a partition to a file(to my home directory as hdadisk.img)

dd if =/dev/sda2 of=~/hdadisk.img

Restoring this image file in to other machine

dd if=hdadisk.img of=/dev/sdb3

Example 3: Do you feel hdadisk.img is bit big? Use gzip or bzip2 to compress when creating image.

dd if =/dev/sda2 | bzip2 hdadisk.img.bz2

Example 4: Do you know dd command can be used as file copier as well? Yes, if you don’t have cp command use dd command to copy a file from one location to other.

dd if=/home/imran/abc.txt of=/mnt/abc.txt

Example 5: Wipe/delete content of a disk so that it will be empty for some one to use it.

dd if=/dev/zero of=/dev/sdb

This will wipe out your second hard disk and every bit is written with zero. you may be interested in learning /dev/null and /dev/zero files which do similar stuff but there is a bit of difference.

HOW WRITING OF A FILE ON HARD DISK WITH DATA HAPPEN?

Normally what ever you write on to a disk at the block level it will write combination of zeros and ones. Hope you know this and what we are doing here is that we are just writing zeros which will clear all 1’s from the hard disk. This eventually makes your disk empty.

Example 6: What to hide your ass by deleting your personal data. Many people think if we do rm -rf / will do the needful. But we can recover those deletion by using disk recovery tools like Photorec or some forensic tools. But if you want some not to recover your data you have to write random data on your partition where you data resides.

dd if=/dev/random of=/dev/sdb

Do above command multiple times so that it is real hard to recover data. If I am in your place, I will write below shell script to do that.

for i in {1..10};do dd if=/dev/random of=/dev/sdb;done
This will execute dd command 10 times in a row one after the other.

Example 7: We can create virtual file system with dd command which can be used as swap. To know more about you should know on how to create virtual swap space in Linux .

dd if=/dev/zero of=/swapfile bs=1024 count=200000

where bs stands for block size and count is nothing but number of such blocks used to crate this swap file.

Make sure you use block sizes in multiples of 1024 bytes which is equal to 1KB. Ff you do not specify block size, dd use a default block size of 512 bytes. Below conventions will work for block sizes.

Example 8: We can even create ISO files from a CD-ROM or DVD-ROM using dd command.

dd if=/dev/dvd of=/opt/my_linux_image.iso

or with more

dd if=/dev/sr0 of=/home/$user/mycd_image.iso bs=2048 conv=sync

Some other examples:

dd if=/dev/sda1 of=/dev/sdb1 bs=4096 conv=noerror,sync

This will make clone of one partition sda1 to other sdb1 partition, also used sync option to synchronize the partition

dd if=/dev/sdx of=/dev/sdy bs=64k conv=noerror,sync

This will clone the entire drive, including MBR, all partitions and data where noerrr instructs dd to ignore all read errors while continuing operations. The snyc data offsets stay in sync And bs=sets block size which is set to 64k.

Example 9: We can even check disk quota using dd command by creating huge files which eats up HDD in no time.

dd if=/dev/zero of=/usr/disk-img/disk-quota.ext3 count=40960

This will create 20MB file (disk image) at said path.

Example 10: We can even create bootable USB’s using dd command.

dd if=/home/$user/bootimage.img of=/dev/sdc

This will create boot-able USB drive where /dev/sdc is an USB drive.