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. See the table of contents:
Create a backup
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.
Restore a backup
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
Clone a harddisk
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.
Transfer a disk image
To transfer a disk image over the network to a computer named target, use
dd if=/dev/sdb | ssh root@target "(cat >backup.img)"
create an iso image of a CD
To create an iso image of a CD, read it block-by-block and save the blocks to a file:
dd if=/dev/cdrom of=cdimage.iso
rescue a file that contains bad blocks
If your favorite movie or song cannot be played any longer because the file is corrupt, you can use dd to ignore the corrupt part:
dd if=movie.avi of=rescued_movie.avi conv=noerror
analyze your disk
DD is great to learn about your system. To analyze your disk by displaying selected blocks, in this case block 1001 of /dev/sdc1 use:
dd if=/dev/sdc1 count=1 skip=1000
To see the first 40 bytes of your first harddisk as a hexdump use
Here, bs stands for blocksize.
Create your own bootloader
To create your own operating system by dumping your bootloader to the boot sector of a bootable disk image use
dd conv=notrunc if=bootloader of=qemu.img
benchmark the throughput of your disks
# dd if=/dev/sdg1 of=/dev/null bs=512 count=1000000 1000000+0 records in 1000000+0 records out 512000000 bytes (512 MB) copied, 4.25186 s, 120 MB/s # dd if=/dev/sdg1 of=/dev/null bs=4096 count=1000000 1000000+0 records in 1000000+0 records out 4096000000 bytes (4.1 GB) copied, 29.8747 s, 137 MB/s
However, make sure you have read Background:How caching works first otherwise you will be surprised by a mysterious accelleration like this:
# dd if=/dev/sdg1 of=/dev/null bs=512 count=1000000 1000000+0 records in 1000000+0 records out 512000000 bytes (512 MB) copied, 4.25186 s, 120 MB/s # dd if=/dev/sdg1 of=/dev/null bs=512 count=1000000 1000000+0 records in 1000000+0 records out 512000000 bytes (512 MB) copied, 0.417317 s, 1.2 GB/s
It is best to circumvent the file system cache completely using direct I/O:
# dd iflag=direct if=/dev/sdg1 of=/dev/null bs=512 count=100000 100000+0 records in 100000+0 records out 51200000 bytes (51 MB) copied, 5.01053 s, 10.2 MB/s
The WinDos pendant of dd is rawrite.
- Blanking a hard drive
- vmStat - find out how many reads per time occur in your system
- pv - for viewing progress in a pipeline
- Provider's man page
- Another dd man page
- Tutorial at LinuxQuestions
- How to monitor progress of a long copy