dd is a utility to create a disk dump by reading every single block on a disk, e.g. your hard drive. However, its architecture is laid out so it can do much more than creating a dump. Here is what dd can do for you:
- manage a disk backup
- create a backup from a disk to a file
- restore a backup from a file to a disk
- clone a harddisk
- create a disk image and transfer it over the network
- create an iso image of a CD
- rescue a file that contains bad blocks
- analyze your disk by displaying selected blocks
- create your own operating system by dumping your bootloader to the boot sector
- benchmark the throughput of your disks
Say we have a harddisk /dev/sda that we want to backup entirely (sector-by-sector) to a USB volume /dev/sdb1, mounted on /mnt/sdb1. We call this a dump or an image of /dev/sda. The dump shall be named backup.img. Here is the dd command:
dd if=/dev/sda of=/mnt/sdb1/backup.img
In this command, if stands for input file and of for output file.
To restore this backup, we boot from a live CD and do the command vice versa. This can overwrite all content on your harddisk, this is the intention.
dd if=/mnt/sdb1/backup.img of=/dev/sda
To clone a disk A to B, both disks need to have the same capacity. It is very convenient for USB disks. Say our USB disk source is called /dev/sdb and the target is called /dev/sdc. Do it like this:
dd if=/dev/sdb of=/dev/sdc
Now if sdc has a bigger capacity, this capacity will be lost because the file system is not aware of it.
To transfer a disk image over the network, use
dd if=/dev/sdb | ssh root@target "(cat >backup.img)"