Dd (command) - Revision history

Christoph: /* Examples */

2018-05-07T01:46:58Z

‎Examples

Christoph: /* See also */

2014-07-11T17:36:22Z

‎See also

Christoph: /* External links */

2008-01-08T09:40:34Z

‎External links

Christoph: /* Anti-examples */

2008-01-08T09:39:55Z

‎Anti-examples

Christoph: /* Anti-examples */

2007-04-04T05:51:15Z

‎Anti-examples

Christoph at 02:20, 13 February 2007

2007-02-13T02:20:14Z

New page

{{lowercase|title=dd}}
'''dd''' is a [[:Category:Linux Command Line Tools|command-line]] utility whose primary purpose is the low-level copying and conversion of files.

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

==Usage==
<tt>dd [options]</tt>

===operands===
; '''if'''=''file'' : Input File: Read from ''file'' instead from [[Redirection (Linux)|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.<ref>[http://www.opengroup.org/onlinepubs/009695399/utilities/dd.html The Open Group Base Specifications Issue 6]. IEEE Std 1003.1, 2004 Edition (accessed: 2007-02-12).</ref> 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)<ref>[http://www.linuxquestions.org/questions/showthread.php?p=1848009#post1848009 Learn The DD Command Revised]. LinuxQuestions.org. 2005-01 (accessed: 2007-02-12).</ref>
; '''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 <code>G</code> (gigabyte), <code>T</code> (terabyte), <code>P</code> (petabyte), <code>E</code> (exabyte), <code>Z</code> (zettabyte), <code>Y</code> (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 Images|ISO image]] file from a CD. Insert the source cd and '''unmount''' it first if auto CD mount is enabled
{{prg_name|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:

{{prg_name|dd}} if=/dev/fd0 of=floppy.img

Or to copy the img file back to a floppy:

{{prg_name|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)):

{{prg_name|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:

{{prg_name|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 point|mount]] -o loop my.new.virtual.harddisk /mnt/virtual/

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

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

To increase writing performance '''obs''' can be increased:

{{prg_name|dd}} if=/dev/random of=reallylargefile obs=4096

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

{{prg_name|dd}} if=/dev/hda1 of=mywindowspartition.image

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

[http://www.linuxquestions.org/questions/showthread.php?t=362506 View this article] for many more examples of the Linux dd command.

== Anti-examples ==

<font color="red">'''WARNING:''' Do NOT try these examples; they will destroy data!</font>

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, though this is not really secure):

{{prg_name|dd}} if=/dev/zero of=/dev/hda

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

{{prg_name|dd}} if=funnysong.mp3 of=/dev/hda

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

{{prg_name|dd}} if=/dev/dsp of=/dev/hda

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

{{prg_name|dd}} if=/dev/urandom of=/dev/hda

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

==References==
<references/>
==See also==
*[[ISO Images]]

==External links==
*[[wikipedia:dd (Unix)]]
*[http://www.opengroup.org/onlinepubs/009695399/utilities/dd.html The Open Group Base Specifications Issue 6 of the dd utility]
*[http://www.linuxquestions.org/questions/showthread.php?p=1848006#post1848006 User group post on using DD]
*[http://www.rajeevnet.com/hacks_hints/os_clone/os_cloning.html Wonders of 'dd' and 'netcat' :: Cloning Operating Systems] - Several examples of use
*[http://www.netadmintools.com/part165.html Cloning Hard Drives with GNU/Linux]
*[http://www.garloff.de/kurt/linux/ddrescue/ ddrescue]: a version of dd that ignores errors (useful for recovering data off corrupt hard drives)
*[http://www.gnu.org/software/ddrescue/ddrescue.html GNU dd_rescue]: similar to <tt>ddrescue</tt>, but automates the recovery process much more
*{{man|1|dd|convert and copy a file}}

[[Category:Linux Command Line Tools]]

← Older revision		Revision as of 01:46, 7 May 2018
Line 94:		Line 94:

	[http://www.linuxquestions.org/questions/showthread.php?t=362506 View this article] for many more examples of the Linux dd command.		[http://www.linuxquestions.org/questions/showthread.php?t=362506 View this article] for many more examples of the Linux dd command.
		+
		+	;Master boot record backup and restore
		+	It is possible to repair a master boot record. It can be transferred to and from a repair file.
		+
		+	* To duplicate the first two sectors of a floppy drive:
		+
		+	$ dd if=/dev/fd0 of=MBRboot.img bs=512 count=2
		+
		+	* To create an image of the entire x86 master boot record (including a MS-DOS partition table and MBR magic bytes):
		+
		+	$ dd if=/dev/sda of=MBR.img bs=512 count=1
		+
		+	* To create an image of only the boot code of the master boot record (without the partition table and without the magic bytes required for booting):
		+
		+	$ dd if=/dev/sda of=MBR_boot.img bs=446 count=1
		+
		+	;Data modification
		+	dd can modify data in place.
		+
		+	* For example, this overwrites the first 512 bytes of a file with null bytes:
		+
		+	$ dd if=/dev/zero of=path/to/file bs=512 count=1 conv=notrunc
		+
		+	The notrunc conversion option means do not truncate the output file — that is, if the output file already exists, just replace the specified bytes and leave the rest of the output file alone. Without this option, dd would create an output file 512 bytes long.
		+
		+	* To duplicate a disk partition as a disk image file on a different partition:
		+
		+	$ dd if=/dev/sdb2 of=partition.image bs=4096 conv=noerror
		+
		+	; Disk wipe
		+	Main article: Data erasure
		+	For security reasons, it is sometimes necessary to have a disk wipe of a discarded device.
		+
		+	* To wipe a disk by writing zeros to it, dd can be used this way:
		+
		+	$ dd if=/dev/zero of=/dev/sda bs=16M
		+
		+	* Another approach could be to wipe a disk by writing random data to it:
		+
		+	$ dd if=/dev/urandom of=/dev/sda bs=16M
		+
		+	; Benchmarking drive performance
		+
		+	* To make drive benchmark test and analyze the sequential (and usually single-threaded) system read and write performance for 1024-byte blocks:
		+
		+	$ dd if=/dev/zero bs=1024 count=1000000 of=file_1GB
		+	$ dd if=file_1GB of=/dev/null bs=1024
		+
		+	; Generating a file with random data
		+
		+	* To make a file of 100 random bytes using the kernel random driver:
		+
		+	$ dd if=/dev/urandom of=myrandom bs=100 count=1
		+
		+	; Converting a file to upper case
		+
		+	* To convert a file to uppercase:
		+
		+	$ dd if=filename of=filename1 conv=ucase,notrunc

	==Anti-examples==		==Anti-examples==

@@ Line 126: / Line 126: @@
 <references/>
 ==See also==
 *[[ISO Images]]

@@ Line 135: / Line 135: @@
 *[http://www.netadmintools.com/part165.html Cloning Hard Drives with GNU/Linux]
 *[http://www.garloff.de/kurt/linux/ddrescue/ ddrescue]: a version of dd that ignores errors (useful for recovering data off corrupt hard drives)
 *[http://www.gnu.org/software/ddrescue/ddrescue.html GNU dd_rescue]: similar to <tt>ddrescue</tt>, but automates the recovery process much more
 *{{man|1|dd|convert and copy a file}}
 [[Category:Linux Command Line Tools]]

@@ Line 103: / Line 103: @@
 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, though this is not really secure):
+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):
   {{prg_name|dd}} if=/dev/zero of=/dev/hda
 This overwrites the first few blocks of the first hard disk with the file, resulting in a loss of the partition table:

@@ Line 95: / Line 95: @@
 [http://www.linuxquestions.org/questions/showthread.php?t=362506 View this article] for many more examples of the Linux dd command.
-== Anti-examples ==
+==Anti-examples==
 <!--{{Warning|'''WARNING:''' Do NOT try these examples; they will destroy data!}}-->
 <font color="red">'''WARNING:''' Do NOT try these examples; they will destroy data!</font>