Thursday, January 18, 2018

Learn dd command with examples

Beginners guide to learn dd command along with list of examples. Article includes outputs for command examples too.
Learn dd command
Learn dd command

Beginners guide to learn dd command! In this article we will learn about dd (Disk Duplication) command and various usage of it along with examples.
dd command mainly used to convert and copy files in Linux and Unix systems. dd command syntax is
It has very large list of options which can be used as per your requirement. Most of the commonly used options are :
Let me walk you through examples to understand dd command usage.

Backup complete disk using dd

For copying whole disk to another disk, dd is very helpful. You just need to give it disk to read from and disk to write. Check below example –
In above output you can see disk /dev/xvdf is copied to /dev/xvdg. Command will show you how much data and what speed it copied.

Identify disk physically using dd

When there are bunch of disks attached to server and if you want to trace particular disk physically, then dd command might be helpful. You have to run dd command to red from disk and write into void. This will keep the hard disk activity light solid (physical on disk).
Normally all other disk blinking activity LED wheras this one will be having its LED solid. Easy to spot the disk then! Be careful with IF and OF. IF you switch their arguments, you will end up wiping out your hard disk clean.

Create image of hard disk using dd

You can create image of hard disk using dd. Its same as what we seen in first example backup of disk. Here we will use output file OF as a data file on mount point and not the another disk.
In above output, we created image of disk /dev/xvdf into file located in / named xvdf_disk.img
Compressed image can be created as well using gzip along with dd
You can observe output zipped image is very much less in size.

Restore image of hard disk using dd

Yup, next question will be how to restore this hard disk image on another disk? Answer is simple use it as source and destination as another disk.
Make sure your disk image and target disk has same size.
Restore compressed hard disk image using dd along with gzip command as below –

Create ISO from CD or DVD using dd

Another popular use of dd command is creating optical disk image file i.e. ISO file from CD or DVD. You need to first mount CD or DVD on your server then use it as a source device and file on mount point as a destination.
Here, we specified 4096 block size using bs option. Make sure no other application or user is accessing CD or DVD when running this command. You can use fuser command to check if someone is accessing it.
Next question will be how to mount ISO file in Linux? Well we have already article on it here 🙂

Creating file of definite size with zero data using dd

Many times sysadmins or developers needs files with junk data or zero data for testing. Using dd you can create such files with definite size.
Lets say you want to create file of 1GB then you define block size of 1M and count of 1024. So 1M x 1024 = 1024M = 1G.
In above output you can see our math worked perfectly. 1G file is created out of our command.

Changing file uppercase to lowercase using dd

All above examples we seen so far are of data copy using dd command. Now this example is of data convert using dd command. Using dd, you can change file data from all uppercase to lowercase and vice versa.
You can see all data in file is converted to uppercase.
For changing data from uppercase to lowercase use option conv=lcase

If you another interesting use of dd command, let us know in comments down below.

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