Similarly to cp, by default dd makes a bit-to-bit copy of the file, but with lower-level I/O flow control features.
For more information see full documentation.or the
status=progressoption to the command.
dd is part of the GNU Core utilities.. For other utilities in the package, please refer to
Disk cloning and restore
The dd command is a simple, yet versatile and powerful tool. It can be used to copy from source to destination, block-by-block, regardless of their filesystem types or operating systems. A convenient method is to use dd from a live environment, as in a Live CD.
if=) and output file (
of=) and do not reverse them! Always ensure that the destination drive or partition (
of=) is of equal or greater size than the source (
Cloning a partition
From physical disk
/dev/sda, partition 1, to physical disk
/dev/sdb, partition 1:
# dd if=/dev/sda1 of=/dev/sdb1 bs=64K conv=noerror,sync status=progress
sdb1in the example) does not exist, dd will create a file with this name and will start filling up your root file system.
Cloning an entire hard disk
From physical disk
/dev/sda to physical disk
# dd if=/dev/sda of=/dev/sdb bs=64K conv=noerror,sync status=progress
This will clone the entire drive, including the MBR (and therefore bootloader), all partitions, UUIDs, and data.
bs=sets the block size. Defaults to 512 bytes, which is the "classic" block size for hard drives since the early 1980s, but is not the most convenient. Use a bigger value, 64K or 128K. Also, please read the warning below, because there is more to this than just "block sizes" -it also influences how read errors propagate. See  and  for details and to figure out the best bs value for your use case.
noerrorinstructs dd to continue operation, ignoring all read errors. Default behavior for dd is to halt at any error.
syncfills input blocks with zeroes if there were any read errors, so data offsets stay in sync.
status=progressshows periodic transfer statistics which can be used to estimate when the operation may be complete.
The dd utility technically has an "input block size" (IBS) and an "output block size" (OBS). When you set
bs, you effectively set both IBS and OBS. Normally, if your block size is, say, 1 MiB, dd will read 1024×1024 bytes and write as many bytes. But if a read error occurs, things will go wrong. Many people seem to think that dd will "fill up read errors with zeroes" if you use the
noerror,sync options, but this is not what happens. dd will, according to documentation, fill up the OBS to IBS size after completing its read, which means adding zeroes at the end of the block. This means, for a disk, that effectively the whole 1 MiB would become messed up because of a single 512 byte read error in the beginning of the read: 12ERROR89 would become 128900000 instead of 120000089.
If you are positive that your disk does not contain any errors, you could proceed using a larger block size, which will increase the speed of your copying several fold. For example, changing bs from 512 to 64K changed copying speed from 35 MB/s to 120 MB/s on a simple Celeron 2.7 GHz system. But keep in mind that read errors on the source disk will end up as block errors on the destination disk, i.e. a single 512-byte read error will mess up the whole 64 KiB output block.
status=progressoption. See for details.
- To regain unique UUIDs of an ext2/3/4 filesystem, use
tune2fs /dev/sdXY -U randomon every partition. For swap partitions, use
- Partition table changes from dd are not registered by the kernel. To notify of changes without rebooting, use a utility like partprobe (part of GNU Parted).
Backing up the partition table
Create disk image
Boot from a live medium and make sure no partitions are mounted from the source hard drive.
Then mount the external hard drive and backup the drive:
# dd if=/dev/sda conv=sync,noerror bs=64K | gzip -c > /path/to/backup.img.gz
If necessary (e.g. when the resulting files will be stored on a FAT32 file system) split the disk image into multiple parts (see also ):
# dd if=/dev/sda conv=sync,noerror bs=64K | gzip -c | split -a3 -b2G - /path/to/backup.img.gz
If there is not enough disk space locally, you may send the image through ssh:
# dd if=/dev/sda conv=sync,noerror bs=64K | gzip -c | ssh user@local dd of=backup.img.gz
Finally, save extra information about the drive geometry necessary in order to interpret the partition table stored within the image. The most important of which is the cylinder size.
# fdisk -l /dev/sda > /path/to/list_fdisk.info
bs=) that is equal to the amount of cache on the HD you are backing up. For example,
bs=8192Kworks for an 8 MiB cache. The 64 KiB mentioned in this article is better than the default
bs=512bytes, but it will run faster with a larger
gzip -ccommand above with
pigz -c. For large disks, this can potentially save hours.
To restore your system:
# gunzip -c /path/to/backup.img.gz | dd of=/dev/sda
When the image has been split, use the following instead:
# cat /path/to/backup.img.gz* | gunzip -c | dd of=/dev/sda
Binary file patching
If one wants to replace offset
0x123AB of a file with the
FF C0 14 hexadecimal sequence, this can be done with the command line:
# printf '\xff\xc0\x14' | dd seek=$((0x123AB)) conv=notrunc bs=1 of=/path/to/file
Backup and restore MBR
Before making changes to a disk, you may want to backup the partition table and partition scheme of the drive. You can also use a backup to copy the same partition layout to numerous drives.
The MBR is stored in the the first 512 bytes of the disk. It consists of 4 parts:
- The first 440 bytes contain the bootstrap code (boot loader).
- The next 6 bytes contain the disk signature.
- The next 64 bytes contain the partition table (4 entries of 16 bytes each, one entry for each primary partition).
- The last 2 bytes contain a boot signature.
To save the MBR as
# dd if=/dev/sdX of=/path/to/mbr_file.img bs=512 count=1
You can also extract the MBR from a full dd disk image:
# dd if=/path/to/disk.img of=/path/to/mbr_file.img bs=512 count=1
To restore (be careful, this destroys the existing partition table and with it access to all data on the disk):
# dd if=/path/to/mbr_file.img of=/dev/sdX bs=512 count=1
If you only want to restore the boot loader, but not the primary partition table entries, just restore the first 440 bytes of the MBR:
# dd if=/path/to/mbr_file.img of=/dev/sdX bs=440 count=1
To restore only the partition table, one must use:
# dd if=/path/to/mbr_file.img of=/dev/sdX bs=1 skip=446 count=64
To erase the MBR bootstrap code (may be useful if you have to do a full reinstall of another operating system) only the first 440 bytes need to be zeroed:
# dd if=/dev/zero of=/dev/sdX bs=440 count=1
Files created with dd can end up with a smaller size than requested if a full input block is not available and the
/dev/random and there is not enough entropy, or from
/dev/urandom when reading more than 32 MiB.
It is possible, but not guaranteed, that dd will warn you about the issue:
dd: warning: partial read (X bytes); suggest iflag=fullblock
The solution is to do as the warning says and add
iflag=fullblock to the dd command. For example:
$ dd if=/dev/random of=bigsecret.img bs=1K count=1 iflag=fullblock $ dd if=/dev/urandom of=bigsecret.img bs=40M count=1 iflag=fullblock
iflag=fullblockoption to the dd command when the input file is
An alternative for
/dev/urandom is to specify a block size smaller than 32 MiB, but a larger copy count. For example:
$ dd if=/dev/urandom of=bigsecret.img bs=1M count=40
When reading from a pipe, an alternative to
iflag=fullblock is to limit
bs to the
PIPE_BUF constant value, defined in
/usr/include/linux/limits.h . For example:
$ cat input.img | dd of=output.img bs=4k count=100