dd

Data Duplicator, 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.

Syntax
     dd [Options]

Key
   if=FILE
      Input file : Read from FILE instead of standard input.

   of=FILE
      Output 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=BYTES
      Read BYTES bytes at a time.

   obs=BYTES
      Write BYTES bytes at a time.

   bs=BYTES
      Block size, both read and write BYTES bytes at a time.  This overrides 'ibs'
      and 'obs'.

   cbs=BYTES
      Convert BYTES bytes at a time.

   skip=BLOCKS
      Skip BLOCKS 'ibs'-byte blocks in the input file before copying.

   seek=BLOCKS
      Skip BLOCKS 'obs'-byte blocks in the output file before copying.

   count=BLOCKS
      Copy BLOCKS 'ibs'-byte blocks from the input file, instead of
      everything until the end of the file.

   conv=CONVERSION[,CONVERSION]...
      Convert the file as specified by the CONVERSION argument(s).  
      (No spaces around any comma)

     Conversions:
    '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
               necessary.
    '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 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.

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.

Examples

Clone the drive sda onto drive sdb:
$ dd if=/dev/sda of=/dev/sdb

Clone the drive hda onto an image file:
$ dd if=/dev/hda of=/image.img

Copy a CD or DVD disc to a .iso image file, first unmounting the disc:
sudo umount /dev/dvd-device
dd if=/dev/dvd-device of=dvd.iso bs=2048 conv=sync,notrunc
# dvd-device will typically be dvd for a dvd disc or cdrom for a cdrom disc.

Clone a hard drive to a zipped image file in 100Mb blocks:
$ dd if=/dev/hda bs=100M | gzip -c > /image.img

Create a 10 KB file filled with random data (10 x 1K blocks):
$ dd if=/dev/random of=random.bin bs=1024 count=10

Create an 8 GB empty file (also known as "sparse file") called spacer.img
$ dd if=/dev/zero of=spacer.img bs=1 count=0 seek=8G

Completely wipe the hard drive hdz by overwriting it with random data:
$ dd if=/dev/urandom of=/dev/hdz

Create a boot floppy:
$ dd if=boot.img of=/dev/fd0 bs=1440

“All of my servers have an 8GB empty spacer.img file that does absolutely nothing except take up space. That way in a moment of full-disk crisis I can simply delete it and buy myself some critical time” ~ Brian Schrader

Related linux commands

CodeCoffee.com - dd examples.
cp - Copy one or more files to another location.
cpio - Copy files to and from archives.
ddrescue - Data recovery tool.
install - Copy files and set attributes.
mtools - Manipulate MS-DOS files.
sum - Print a checksum for a file.
GNOME Disks - Disk Management Utility for GNOME.
Equivalent Windows command: FSUTIL file setzerodata, or bbcopy (not secure)