Difference between revisions of "Dd (command)"

Revision as of 09:40, 8 January 2008

The correct title of this article is dd. The initial letter is capitalized due to technical restrictions.

dd is a command-line utility whose primary purpose is the low-level copying and conversion of files.

see: wikipedia:dd (Unix) for more details.

Usage

dd [options]

operands

if=file 

Input File: Read from file instead from standard input.

of=file 

Output File: Write to file instead to standard output. See also the keyword notrunc.

ibs=bytes 

Input Block Size: Read bytes bytes at once.

obs=bytes 

Output Block Size: Write bytes bytes at once.

bs=bytes 

Block Size: A shortcut for ibs=bytes obs=bytes. If the user does not provide a block size, 512 bytes is used.^[1] One can determine the block size of a storage medium by assuming a block size, writing one block and checking for an error message. For most modern drives, the optimum value for this operand for maximum read/write performance is 4k (i.e. bs=4k)^[2]

count=blocks 

Count: copy only this many blocks from the input to the output, then stop.

skip=blocks 

When starting to read from input, skip blocks number of blocks of size ibs.

seek=blocks 

When starting to write to output, skip blocks number of blocks of size obs.

conv=keywords 

Convert the file according to a comma-separated list of keywords.

cbs=bytes 

Convert Block Size: Convert bytes bytes at once.

conv

When specifying conv as parameter the following keywords may be used:

ascii 

Convert from EBCDIC to ASCII.

ebcdic 

Convert from ASCII to EBCDIC.

ibm 

Convert from ASCII to an alternative EBCDIC.

block 

Fill datasets which are terminated by a newline-character with space-characters to fit size of cbs.

unblock 

Replace trailing space-characters in datasets of size cbs with newline-characters.

lcase 

Change uppercase characters to lowercase.

ucase 

Change lowercase characters to uppercase.

notrunc 

Do not truncate output file to zero bytes before writing to it. If the existing output file is shorter than the amount of data to be written to it, this will cause the written data to overwrite the initial portion, leaving the remainder intact.

swab 

Swap every pair of input bytes.

noerror 

Ignore reading errors and continue.

sync 

Pad every input block with null bytes if it is shorter than the size specified. If used with block or unblock, pad with space characters instead.

Notes and units

On various systems the option --version is supported. dd will then output its version number and quit.

file may be any real file or any block-device file.

On certain systems bytes may be specified with multiplicative units. This units may then be:

c 

Character: 1

w 

Word: 2

b 

Block: 512

kB 

Decimal kilobytes: 1,000 bytes

k 

Binary kilobytes (kibibytes): 1,024 bytes

MB 

Decimal megabytes: 1,000,000 bytes

M 

Binary megabytes (mebibytes): 1,048,576 bytes (1,024×1,024)

This may be carried on similarly with G (gigabyte), T (terabyte), P (petabyte), E (exabyte), Z (zettabyte), Y (yottabyte). The standard "IEEE Std 1003.1" only requires that the 'b' and 'k' multipliers be supported, and does not specify the meaning of any other multipliers. Also, multiple numbers may be provided separated by 'x'. These numbers are multiplied together.

Examples

To create an ISO image file from a CD. Insert the source cd and unmount it first if auto CD mount is enabled

dd if=/dev/cdrom of=/tmp/image.iso

Note: The device might also be called /dev/dvd or /dev/scd0 (for a SCSI-drive).

To create an image file named floppy.img of a floppy disk in the drive whose block-device name is /dev/fd0 (as the first floppy device is on Linux), one may invoke dd in the following way:

dd if=/dev/fd0 of=floppy.img

Or to copy the img file back to a floppy:

dd if=floppy.img of=/dev/fd0 bs=16k

To create a file with name reallylargefile with the size of 1 GB, filled with random data, do this (1G = 1073741824, 1073741824 / 512 = 2097152 (the default block size is 512)):

dd if=/dev/random of=reallylargefile count=2097152

To fill the file with NULL characters, use /dev/zero instead of /dev/random. This can be used to mount a filesystem if you have a FAT32 device, but need the flexibility of EXT2 - as follows to create a 10GB image:

dd if=/dev/zero of=my.new.virtual.harddisk bs=1k count=10MB
mke2fs my.new.virtual.harddisk  # (reply yes when it says it's not a block device)
mkdir /mnt/virtual
mount -o loop my.new.virtual.harddisk /mnt/virtual/

To create a 10GB sparse file which doesn't allocate any actual space (if the filesystem supports this feature):

dd if=/dev/zero of=sparsefile.img bs=1 seek=10G count=0

To increase writing performance obs can be increased:

dd if=/dev/random of=reallylargefile obs=4096

This will write the first partition of the first harddisk to the file mywindowspartition.image.

dd if=/dev/hda1 of=mywindowspartition.image

These examples are Linux-centric: on other platforms the device names may be different.

View this article for many more examples of the Linux dd command.

Anti-examples

WARNING: Do NOT try these examples; they will destroy data!

The following examples are provided to warn about the dangers of dd, if used incorrectly. Trying any of these commands with the proper privileges will almost certainly result in major data loss, and may make the system unusable.

This overwrites the complete first hard disk with null bytes, erasing it (do this if you want to sell a hard drive that contains company secrets):

dd if=/dev/zero of=/dev/hda
# ~OR ~
dd if=/dev/zero of=/dev/hda bs=10MB conv=noerror,sync

This overwrites the first few blocks of the first hard disk with the file, resulting in a loss of the partition table:

dd if=funnysong.mp3 of=/dev/hda

This will completely corrupt an entire hard disk (/dev/dsp is the sound player/recorder):

dd if=/dev/dsp of=/dev/hda

This will overwrite an entire disk with pseudorandom data, making its initial contents relatively irrecoverable

dd if=/dev/urandom of=/dev/hda

These examples are Linux-centric. On other platforms the device names may be different.

References

1. ↑ The Open Group Base Specifications Issue 6. IEEE Std 1003.1, 2004 Edition (accessed: 2007-02-12).
2. ↑ Learn The DD Command Revised. LinuxQuestions.org. 2005-01 (accessed: 2007-02-12).

See also

• ISO Images

External links

• wikipedia:dd (Unix)
• The Open Group Base Specifications Issue 6 of the dd utility
• User group post on using DD
• Wonders of 'dd' and 'netcat' :: Cloning Operating Systems - Several examples of use
• Cloning Hard Drives with GNU/Linux
• ddrescue: a version of dd that ignores errors (useful for recovering data off corrupt hard drives)
• GNU dd_rescue: similar to ddrescue, but automates the recovery process much more
• The Great Zero Challenge
• dd(1):convert and copy a file — Linux man page on usr-share-man.org.