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 of dd command
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=<source> –This is a source from where you want to copy data and ‘if’ stands for input-file.
of=<destination> –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.
Input(source file name) and Output(target file name) in syntax are disks, partitions, files and devices to which you want to write and read data from. There are many options which we will discuss in examples.
Learn Linux dd command with examples
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
Ok, that is fine for basic use of dd command. But the beauty of dd command lies in advanced usages like wiping of disks, complete wiping of disks, MBR backups etc.
Advanced usage of Linux dd command
From here you should be careful when using this command and you should first try these commands on a test machine before getting familiarity.
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 /<your data> 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;doneThis 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.
N and BYTES may be followed by the following multiplicative suffixes: c =1, w =2, b =512,
kB =1000, K =1024,
Mb =1000*1000, MB=1024*1024,
Gb =1000*1000*1000, GB =1024*1024*1024,
and so on for T, P, E, Z, Y.
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.
Example 11: Data recovery using ddrescue command.
ddrescue tool is used for cloning and recovering data. this is not installed by default in many Linux machines. Below are the commands to install for respective distributions.
Installing ddrescue in Debian based machines
apt-get install gddrescue
Installing ddrescue in Redhat based machines
yum install gddrescue
It can copy data from one file/block device(hard disk or CD-ROM) to another while trying to rescue the data i.e read error to maximizing the recovered data. To recover data ddrescue command needs to run in two steps followed by running fsck command.
Step 1: It copies each and every block without reading errors and logging these errors in log file.
Step 2: It will copy only bad block and try to read 3 times to source before it gives up. Than you can run command to check file system for corruption.
Step 3: Running fsck command.
ddrescue –f –n /dev/sdX /dev/sdY rescue.log ddrescue –d –f –r3 /dev/sdX /dev/sdY rescue.log fsck –f /dev/sdY
Example 12: Computer forensics using dcfldd
dcfldd is the enhanced version of dd having useful features used for computer forensics and security as well. It takes dd’s parameters. You can use dcfldd command when you need to know that a copy and subsequent copies are identical to the original. dcfldd has some additional features like;
- Hashing on-the-fly
- Status output
- Image/wipe verify
- Split output
- hashing
Again We have to install this command as well, if that is not present.
Installing dcfldd in Debian based machines
apt-get install dcfldd
Installing dcfldd in Redhat based machines
yum install dcfldd
Example
dcfldd if=/dev/source hash=md5,sha512 hashwindow=1G md5log=md5.txt sha512log=sha512.txt \hashconv=after bs=512 conv=noerror,sync split=1G splitformat=aa of=image.dd
The above command will read one Gb from the source drive and write to a file called image.dd.aa. it will also calculate the MD5 hash and the sha512 has of each Gigbyte read.
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- 12 Linux dd command examples - March 8, 2016
