Despite its dull-sounding description -- "Convert and copy a file" -- dd is one of the most powerful tools in the Linux toolbox. That's because in Linux, everything is a file! I should warn you however that dd can also be d-d-dangerous if you get your ifs and ofs mixed up. But more on that in a moment.

To get a feel for what it can do, imagine you have a USB drive you want to temporarily use for something else. Running df -h shows it's mounted on /dev/sdc. The commmand...

dd if=/dev/sdc of=/path/to/image bs=4096

...will do just that. To write the image back to the drive you'd use...

dd if=/path/to/image of=/dev/sdc bs=4096

Notice how similar those commands are? We've just swapped the if= and the of=. That's what makes dd dangerous. If you mixed them up, in this example you'd write the new contents of your USB disk over the old image file, essentially nuking it. Be careful!

(By convention, if= comes first, but there's nothing to stop you using dd of=/dev/sdc if=/path/to/image bs=4096 for the restore step.)

The bs=4096, by the way, is optional. dd reads and writes 512 bytes at a time by default. Increasing this will speed up writes to the USB drive.

Since we're talking about syntax, here's a brief overview. (For more details type dd --help.)

if=	Input file.
of=	Output file.
bs=	Bytes to read and write at a time. The default is 512.
count=	Copy this number of blocks from the input file (instead  of everything until the end of the file is reached).
conv=	Convert the file as specified in the following comma-separated arguments.
noerror	Continue even if read errors are encountered. (dd will normall stop.)
notrunc	Don't truncate the output file.

Create an ISO image of a CD or DVD

dd if=/dev/cdrom of=/path/to/image.iso bs=2048 conv=sync,notrunc

The ISO image can then be used in a regular CD/DVD burner, or you can mount and access it locally with these commands...

mkdir /mnt/temp mount -o loop /path/to/image.iso /mnt/temp/ cd /mnt/temp

MBR Tricks
Backup the Master Boot Record (MBR) including the partition table:

dd if=/dev/sdx of=/path/to/image count=1 bs=512

Backup the MBR without the partition table:
(Partition tables are unique to each disk so unless you're cloning a whole drive you shouldn't overwrite them.)

dd if=/dev/sdx of=/path/to/image count=1 bs=446

Restore the MBR:

dd if=/path/to/image of=/dev/sdx

Backing Up Drives
dd makes cloning drives easy, but you shouldn't do it with either drive mounted! Boot from a Linux startup CD and run the command from there.

Backup an existing drive:

dd if=/dev/sdx of=/dev/sdy

Backup a hard disk to an image file:

dd if=/dev/sdx of=/path/to/image

Backup a hard disk to a compressed image file:

dd if=/dev/sdx | gzip > /path/to/image.gz

Restore a backup image:

dd if=/path/to/image of=/dev/sdx

Restore a compressed backup image:

gzip -dc /path/to/image.gz | dd of=/dev/sdx

Backup a partition to a different partition on another drive:

dd if=/dev/sdx2 of=/dev/sdy5 bs=4096 conv=notrunc,noerror

Nuking a Disk
You don't have to write meaningful data to a disk, of course. You can use dd to wipe a disk too.

Wipe an entire disk by filling it with random data:

dd if=/dev/urandom of=/dev/sdx conv=notrunc

Wipe an entire disk by filling it with zeros:

dd if=/dev/zero of=/dev/sdx conv=notrunc

There are some even more cunning tricks you can do with dd. More next time!