Using the dd command

The dd (Dataset Definition) command is a simple, useful, and surprisingly easy to use tool; With this tool you can do the same, on devices: disks and partitions, as with commercial programs like Norton Ghost, on Windows or free programs like CloneZilla, on Linux, with just a short command line.

Basic Syntax:

The most basic syntax for using the dd command would be this:

sudo dd if=origen of=destino

Where if means "input file = input file“, That is, what you want to copy and of means "output file = output file“, That is, the destination file (where the data will be copied);origin and The destination can be devices (CD or DVD reader, hard disk, floppy disk, pendrive, partition, etc.), backup file or disk image, etc., but not folders or subfolders.

For the smooth use of this command, the first thing is always to be clear about what partitions / hard drives are called in Linux (/ dev / sda1 for example; / dev derives from device = device, in English). to find out the source and destination disk / partition, something we can easily find out with the command sudo fdisk -lo with some graphical partition program like gparted. All the information about the dd command can be consulted with the command man dd and info dd,

This command should be used with caution, and always checking the order and name of the disks / partitions, because the same as a disk is cloned, it deletes it in a jiffy.

Syntax with the pv command: Using the dd command with the previous syntax has a small drawback, since it is a very reserved command - it does not give information - because when it is executed, the terminal prompt remains immobile, so we do not know what is what is happening and how much time is left for it to finish executing. This small inconvenience can be solved by adding the command pv, (*) - which acts as a terminal pipe that measures the data that passes through it - to the syntax of the dd command, so that now the syntax would be:

dd if=origen |pv|dd of=destino

As a result, we would obtain a kind of progress bar in the terminal, the information on bytes transferred, the time it has been running and the transfer rate, all in real time.

(alf) - (~) └──┤ dd if = / devmmcblk0p1 | pv | dd of = / dev / mmcblk0p2 1,630GB 0:21:30 [1,12MB / s] [

Also when finished it shows us statistics of the transfer rate, the total bytes transferred and the total time it took to transfer all the data.

(alf) - (~) └──┤ dd if = / devmmcblk0p1 | pv | dd of = / dev / mmcblk0p2 10530816 + 0 records read <=> 10530816 + 0 records written 5391777792 bytes (5.4 GB) copied, 3873,48 s, 1,4 MB / s 5,02GB 1:04:33 [1,33MB / s] [<=> 10530816 + 0 records read <=> 10530816 + 0 records written 5391777792 bytes (5.4GB) copied, 3873,48, 1,4 s, XNUMX MB / s (alf) - (~) └──┤

(*) Check before executing any command line with this second syntax, that we have the pv package installed on the system, or install it if we don't have it: both with Synaptic.

Let's see some practical examples and options of this command (in its version with the trick explained above with the pv command):

A) About hard drives and partitions:

= Clone a hard drive:

With this we would clone the hda disk in hdb. (IDE disks):

sudo dd if=/dev/hda |pv|dd of=/dev/hdb bs=1M

With this we would clone the sda ​​disk in sdb. (SATA disks):

sudo dd if=/dev/sda |pv|dd of=/dev/sdb bs=1M

With the option bs = 1M, it is achieved that both reading and writing are done in 1 megabyte blocks, (less, it would be slower but more secure, and with more we risk losing data along the way).

Keep in mind that this way the disk is recorded "as is", MBR, partition table, empty space, etc ..., so you can only record on a disk of the same or larger size.

= Write only the first partition (hda1) of the source disk to the destination disk (hdb):

sudo dd if=/dev/hda1 |pv|dd of=/dev/hdb bs=1M

= Burn the entire disk (hda) to the first partition (hdb1) of the destination disk:

sudo dd if=/dev/hda |pv|dd of=/dev/hdb1 bs=1M

= Create an image - can be bin or iso - from the hard disk (hda), in the / home directory:

sudo dd if=/dev/hda |pv|dd of=/home/hda.bin

= Completely erase information on a disc: to do this, fill the disc with random characters, five times. There will be no trace of information on the disk:

for n in {1..5}; do dd if=/dev/urandom |pv|dd of=/dev/hda bs=8b conv=notrunc;

= Erase any partition and entire disk from any device:

sudo dd if=/dev/zero |pv|dd of=/dev/sdx (Full disk erase)

sudo dd if=/dev/zero |pv|dd of=/dev/sdxa (Disk partition erase)

where: x is the disk to erase, a is the partition to erase

This operation is very useful to completely erase any partition, in addition the data that will be erased cannot be recovered, therefore it is a low-level and very safe erasure, very useful for example when the pendrives get infected with viruses with our dear windows.

B) On a CD / DVD

= Create / Mount an .iso image from a CD (or DVD)

To create the .iso image of a CD in the / home directory:

sudo dd if=/dev/cdrom |pv|dd of=/home/imagendeCD.iso

To mount the .iso image from the CD:

sudo mount -o loop imagedeCD.iso /mnt/home

= Recover a scratched DVD (This does not recover the entire DVD, in this case only the readable sectors). It is also suitable for defective hard drives:

sudo dd if=/dev/cdrom |pv|dd of=/home/dvd_recuperado.iso conv=noerror,sync

The noerror option is used to avoid reading errors in any situation.

C) About MBR and VBS:

= Copy / Restore Master Boot Record (MBR):

To copy the MBR:

sudo dd if=/dev/hda |pv|dd of=mbr count=1 bs=512

To restore the MBR:

sudo dd if=mbr |pv|dd of=/dev/hda

= Clean our MBR and partition table:

sudo dd if=/dev/zero |pv|dd of=/dev/hda bs=512 count=1

= Clears the MBR but does not touch the partition table, (very useful to erase the GRUB without losing data on the partitions):

sudo dd if=/dev/zero |pv|dd of=/dev/hda bs=446 count=1

= Copy / Restore Volume Boot Sector (VBS):

To copy the VBS:

sudo dd if=/dev/hda |pv|dd of=/home/sector_arranque_hda count=1 bs=512

To restore the VBS:

sudo dd if=/home/sector_arranque_hda |pv|dd of=/dev/hda

D) Others:

= Burn a disk image to our / home directory, skipping disk errors (very useful for disks that are dying):

sudo dd conv=noerror if=/dev/hda |pv|dd of=~/home/imagen_disco_con_errores.iso

= Create an empty 1 Mb file:

sudo dd if=/dev/zero |pv|dd of=archivo_nuevo_vacio bs=1024 count=1024

= Create a 2Gb swap file:

sudo dd if=/dev/zero |pv|dd of=/swapspace bs=4k count=2048M
mkswap /swapspace
swapon /swapspace

= Convert all letters to uppercase:

sudo dd if=miarchivo |pv|dd of=miarchivo conv=ucase

Use the dd command with a graphical environment program: Gdiskdump, is a graphical environment for this dd command that facilitates the task of clone partitions or disks, quickly and easily. It can be downloaded from the page https://launchpad.net/gdiskdump/ Once we open it, with root permissions - sudo gdiskdump -, we see that the program is very easy to use, since we only have to tell it the partition or disk to clone (Input Format) and its destination (Output Format).