dd - Unix, Linux Command



dd - Convert and copy a file, write disk headers, boot records, create a boot floppy. dd can make an exact clone of an (unmounted) disk, this will include all blank space so the output destination must be at least as large as the input.


dd [Options]


The numeric-valued options (BYTES and BLOCKS) can be followed by a multiplier: 'b'=512, 'c'=1, 'w'=2, 'xM'=M, or any of the standard block size suffixes like 'k'=1024. The name dd can be an allusion to the DD statement found in IBM's Job Control Language (JCL), where the acronym stands for "Data Description". dd can copy a smaller drive to a larger one, but can’t copy a larger drive to a smaller one. When cloning a disk drive, its a good idea to list the partitions before and afterwards using sudo fdisk –l. An empty/unformatted drive will appear as "Disk dev/sdn doesn't contain a vaild partition table". If dd receives a USR1 signal, the current input and output block counts will be written to the standard error output in the same format as the standard completion message. e.g. kill -USR1 1234 where 1234 is the process id.


if=FILEInput file : Read from FILE instead of standard input.
of=FILEOutput file : Write to FILE instead of standard output. Unless 'conv=notrunc' is given, 'dd' truncates FILE to zero bytes (or the size specified with 'seek=').
ibs=BYTESRead BYTES bytes at a time.
obs=BYTESWrite BYTES bytes at a time.
bs=BYTESBlock size, both read and write BYTES bytes at a time. This overrides 'ibs' and 'obs'.
cbs=BYTESConvert BYTES bytes at a time.
skip=BLOCKSSkip BLOCKS 'ibs'-byte blocks in the input file before copying.
seek=BLOCKSSkip BLOCKS 'obs'-byte blocks in the output file before copying.
count=BLOCKSCopy BLOCKS 'ibs'-byte blocks from the input file, instead of everything until the end of the file.

Convert the file as specified by the CONVERSION argument(s). (No spaces around any comma)


'ascii'    Convert EBCDIC to ASCII.
'ebcdic'   Convert ASCII to EBCDIC.
'ibm'      Convert ASCII to alternate EBCDIC.
'block'    For each line in the input, output 'cbs' bytes, replacing the
input newline with a space and padding with spaces as
'unblock'  Replace trailing spaces in each 'cbs'-sized input block with a newline.
'lcase'    Change uppercase letters to lowercase.
'ucase'    Change lowercase letters to uppercase.
'swab'     Swap every pair of input bytes.  GNU 'dd', unlike others,
works when an odd number of bytes are read--the last byte is
simply copied (since there is nothing to swap it with).
'noerror'  Continue after read errors.
'notrunc'  Do not truncate the output file.
'sync'     Pad every input block to size of 'ibs' with trailing zero bytes.


To clone the drive sda onto drive sdb.

$ dd if=/dev/sda of=/dev/sdb

To clone the drive hda onto an image file.

$ dd if=/dev/hda of=/image.img

To copy a CD or DVD disc to a .iso image file, first unmounting the disc. dvd-device will typically be dvd for a dvd disc or cdrom for a cdrom disc.

$ sudo umount /dev/dvd-device
$ dd if=/dev/dvd-device of=dvd.iso bs=2048 conv=sync,notrunc

To clone a hard drive to a zipped image file in 100Mb blocks.

$ dd if=/dev/hda bs=100M | gzip -c > /image.img

To create a 10 KB file filled with random data (10 x 1K blocks).

$ dd if=/dev/random of=random.bin bs=1024 count=10

To completely wipe the hard drive hdz by overwriting it with random data.

$ dd if=/dev/urandom of=/dev/hdz

To create a boot floppy.

$ dd if=boot.img of=/dev/fd0 bs=1440