dd is a
command-line utility for
Unix,
Plan 9 Plan 9 or Plan Nine may refer to:
Music
* Plan 9 (band), a psychedelic rock band from Rhode Island
* ''Plan 9'', an album by Big Guitars From Memphis with Rick Lindy
* "Plan 9", a song on the 1993 album ''Gorgeous'' by electronica band 808 Stat ...
,
Inferno, and
Unix-like operating systems and beyond, the primary purpose of which is to convert and copy files.
On Unix, device drivers for hardware (such as
hard disk drives) and special
device files (such as
/dev/zero and
/dev/random
In Unix-like operating systems, and are special files that serve as cryptographically secure pseudorandom number generators. They allow access to environmental noise collected from device drivers and other sources. typically blocked if there ...
) appear in the file system just like normal files; can also read and/or write from/to these files, provided that function is implemented in their respective driver. As a result, can be used for tasks such as backing up the
boot sector of a hard drive, and obtaining a fixed amount of random data. The program can also perform conversions on the data as it is copied, including
byte order swapping and conversion to and from the
ASCII and
EBCDIC text encodings.
History
The name is an allusion to the
DD statement found in
IBM's
Job Control Language (JCL),
in which it is an abbreviation for "Data Definition". The command's syntax resembles a JCL statement more than other Unix commands do, so much that
Eric S. Raymond
Eric Steven Raymond (born December 4, 1957), often referred to as ESR, is an American software developer, open-source software advocate, and author of the 1997 essay and 1999 book ''The Cathedral and the Bazaar''. He wrote a guidebook for the ...
says "the interface design was clearly a prank".
The interface is redesigned in
Plan 9 Plan 9 or Plan Nine may refer to:
Music
* Plan 9 (band), a psychedelic rock band from Rhode Island
* ''Plan 9'', an album by Big Guitars From Memphis with Rick Lindy
* "Plan 9", a song on the 1993 album ''Gorgeous'' by electronica band 808 Stat ...
's dd command to use a command-line option style.
dd
is sometimes humorously called "Disk Destroyer", due to its drive-erasing capabilities.
Originally intended to convert between
ASCII and
EBCDIC, first appeared in
Version 5 Unix. The command is specified since the
X/Open Portability Guide issue 2 of 1987. This is inherited by
IEEE Std 1003.1-2008 (
POSIX), which is part of the
Single UNIX Specification.
The version of
dd
bundled in
GNU coreutils was written by Paul Rubin, David MacKenzie, and Stuart Kemp.
The command is available as a separate package for
Microsoft Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
as part of the
UnxUtils collection of
native Win32 ports of common GNU Unix-like utilities.
Usage
The
command line syntax of differs from many other Unix programs. It uses the syntax for its
command-line option
A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invoking executables and pro ...
s rather than the more standard or formats. By default, reads from
stdin and writes to
stdout, but these can be changed by using the (input file) and (output file) options.
[
Certain features of will depend on the computer system capabilities, such as 's ability to implement an option for direct memory access. Sending a ]SIGINFO
Signals are standardized messages sent to a running program to trigger specific behavior, such as quitting or error handling. They are a limited form of inter-process communication (IPC), typically used in Unix, Unix-like, and other POSIX-compli ...
signal (or a USR1 signal on Linux) to a running process makes it print I/O statistics to standard error once and then continue copying. can read standard input from the keyboard. When end-of-file (EOF) is reached, will exit. Signals and EOF are determined by the software. For example, Unix tools ported to Windows vary as to the EOF: Cygwin
Cygwin ( ) is a POSIX-compatible programming and runtime environment that runs natively on Microsoft Windows. Under Cygwin, source code designed for Unix-like operating systems may be compiled with minimal modification and executed.
The Cygwin in ...
uses (the usual Unix EOF) and MKS Toolkit uses (the usual Windows EOF).
The non-standardized parts of dd invocation vary among implementations.
Output messages
On completion, prints to the stderr
In computer programming, standard streams are interconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin ...
stream about statistics of the data transfer. The format is standardized in POSIX.[ The manual page for GNU dd does not describe this format, but the BSD manuals do.
Each of the "Records in" and "Records out" lines shows the number of complete blocks transferred + the number of partial blocks, e.g. because the physical medium ended before a complete block was read, or a physical error prevented reading the complete block.
]
Block size
A block is a unit measuring the number of bytes that are read, written, or converted at one time. Command-line options can specify a different block size for input/reading () compared to output/writing (), though the block size () option will override both and . The default value for both input and output block sizes is 512 bytes (the traditional block size of disks, and POSIX-mandated size of "a block"). The option for copying is measured in blocks, as are both the count for reading and count for writing. Conversion operations are also affected by the "conversion block size" ().[
The value provided for block size options is interpreted as a decimal (base 10) integer number of bytes. It can also contain suffixes to indicate that the block size is an integer number of larger units than bytes. POSIX only specifies the suffixes (blocks) for 512 and ( kibibytes) for 1024.][ Implementation differ on the additional suffixes they support: (Free) BSD uses lowercase ( mebibytes), ( gibibytes), and so on for tebibytes, exbibytes, pebibytes, zebibytes, and yobibytes,] while GNU uses and for the same units, with , , and used for their SI unit counterparts ( kilobytes).[ For example, for GNU , indicates a blocksize of 16 mebibytes (16777216 bytes) and specifies 3000 bytes.
Additionally, some implementations understand the character as a multiplication operator for both block size and count parameters. For example, is interpreted as 2 × 80 × 18 × 512 = , the exact size of a 1440 KiB ]floppy disk
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
. This is required in POSIX, but GNU does not seem to support it.[ As a result, it is more portable to use the POSIX shell arithmetic syntax of bs=$((2*80*18))b.
Block size has an effect on the performance of copying commands. Doing many small reads or writes is often slower than doing fewer large ones. Using large blocks requires more RAM and can complicate error recovery. When is used with variable-block-size devices such as tape drives or networks, the block size may determine the tape record size or packet size, depending on the network protocol used.
]
Uses
The command can be used for a variety of purposes. For plain-copying commands it tends to be slower than the domain-specific alternatives, but it excels at its unique ability to "overwrite or truncate a file at any point or seek in a file", a fairly low-level interface to the Unix file API.
The examples below assume the use of GNU dd, mainly in the block size argument. To make them portable, replace e.g. with the shell arithmetic expression or (written equivalently with a bit shift).
Data transfer
can duplicate data across files, devices, partitions and volumes. The data may be input or output to and from any of these; but there are important differences concerning the output when going to a partition. Also, during the transfer, the data can be modified using the options to suit the medium. (For this purpose, however, is slower than .)
The option means to keep going if there is an error, while the option causes output blocks to be padded.
In-place modification
can modify data in place. For example, this overwrites the first 512 bytes of a file with null bytes:
The 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, would create an output file 512 bytes long.
Master boot record backup and restore
The example above can also be used to back up and restore any region of a device to a file, such as a master boot record
A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBR ...
.
To duplicate the first two sectors of a floppy disk:
Disk wipe
For security reasons, it is sometimes necessary to have a disk wipe
Data erasure (sometimes referred to as data clearing, data wiping, or data destruction) is a software-based method of overwriting the data that aims to completely destroy all electronic data residing on a hard disk drive or other digital media b ...
of a discarded device. This can be achieved by a "data transfer" from the Unix special files.
* To write zeros to a disk, use dd if= /dev/zero of=/dev/sda
In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow an ...
bs=16M
.
* To write random data to a disk, use dd if=/dev/urandom
In Unix-like operating systems, and are special files that serve as cryptographically secure pseudorandom number generators. They allow access to environmental noise collected from device drivers and other sources. typically blocked if there ...
of=/dev/sda
In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow an ...
bs=16M
.
When compared to the data modification example above, conversion option is not required as it has no effect when the 's output file is a block device.
The option makes dd read and write 16 mebibytes at a time. For modern systems, an even greater block size may be faster. Note that filling the drive with random data may take longer than zeroing the drive, because the random data must be created by the CPU, while creating zeroes is very fast. On modern hard-disk drives, zeroing the drive will render most data it contains permanently irrecoverable. However, with other kinds of drives such as flash memories, much data may still be recoverable by data remanence.
Modern hard disk drives contain a Secure Erase
Data sanitization involves the secure and permanent erasure of sensitive data from datasets and media to guarantee that no residual data can be recovered even through extensive forensic analysis. Data sanitization has a wide range of applications b ...
command designed to permanently and securely erase every accessible and inaccessible portion of a drive. It may also work for some solid-state drives (flash drives). As of 2017, it does not work on USB flash drives nor on Secure Digital flash memories. When available, this is both faster than using dd, and more secure. On Linux machines it is accessible via the hdparm command's option.
The shred program offers multiple overwrites, as well as more secure deletion of individual files.
Data recovery
Data recovery involves reading from a drive with some parts potentially inaccessible. is a good fit with this job with its flexible skipping () and other low-level settings. The vanilla , however, is clumsy to use as the user has to read the error messages and manually calculate the regions that can be read. The single block size also limits the granuarity of the recovery, as a trade-off has to be made: either use a small one for more data recovered or use a large one for speed.
A C program called was written in October 1999. It did away with the conversion functionality of , and supports two block sizes to deal with the dilemma. If a read using a large size fails, it falls back to the smaller size to gather as much as data possible. It can also run backwards. In 2003, a script was written to automate the process of using , keeping track of what areas have been read on its own.
In 2004, GNU wrote a separate utility, unrelated to , called . It has a more sophisticated dynamic block-size algorithm and keeps track of what has been read internally. The authors of both and consider it superior to their implementation. To help distinguish the newer GNU program from the older script, alternate names are sometimes used for GNU's , including (the name on freecode.com and freshmeat.net), (Debian
Debian (), also known as Debian GNU/Linux, is a Linux distribution composed of free and open-source software, developed by the community-supported Debian Project, which was established by Ian Murdock on August 16, 1993. The first version of D ...
package name), and ( openSUSE package name).
Another open-source program called uses a sophisticated algorithm, but it also requires the installation of its own programming-language interpreter.
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:
* Write performance: dd if= /dev/zero bs=1024 count=1000000 of=1GB_file_to_write
* Read performance: dd if=1GB_file_to_read of=/dev/null
In some operating systems, the null device is a device file that discards all data written to it but reports that the write operation succeeded. This device is called /dev/null on Unix and Unix-like systems, NUL: (see TOPS-20) or NUL on CP/M an ...
bs=1024
Generating a file with random data
To make a file of 100 random bytes using the kernel random driver:
Converting a file to upper case
To convert a file to uppercase:
Progress indicator
Being a program mainly designed as a filter, normally does not provide any progress indication. This can be overcome by sending an signal to the running GNU process ( on BSD systems), resulting in printing the current number of transferred blocks.
The following one-liner results in continuous output of progress every 10 seconds until the transfer is finished, when is replaced by the process-id of :
Newer versions of GNU support the option, which enables periodic printing of transfer statistics to stderr.
Forks
dcfldd
' is a fork of GNU that is an enhanced version developed by Nick Harbour, who at the time was working for the United States' Department of Defense Computer Forensics Lab. Compared to , allows more than one output file, supports simultaneous multiple checksum calculations, provides a verification mode for file matching, and can display the percentage progress of an operation. The last release was in 2021.
dc3dd
is another enhanced GNU from the United States Department of Defense Cyber Crime Center
The Department of Defense Cyber Crime Center (DC3) is designated as a Federal Cyber Center by National Security Presidential Directive 54/Homeland Security Presidential Directive 23, as a Department of Defense (DoD) Center Of Excellence for Digit ...
(DC3). It can be seen as a continuation of the dcfldd, with a stated aim of updating whenever the GNU upstream is updated. Its last release was in 2018.
See also
* Backup
In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "back up", w ...
* Disk cloning
* Disk Copy
Disk Copy was the default utility for handling logical volume images in System 7 through Mac OS X 10.2 (usable in System Software 6 as well). In later versions of macOS it has been replaced by DiskImageMounter for mounting the images and Disk ...
* Disk image
* .img img or IMG is an abbreviation for image.
img or IMG may also refer to:
* IMG (company), global sports and media business headquartered in New York City but with its main offices in Cleveland, originally known as the "International Management Group ...
(filename extension)
* List of Unix commands
* ddrescue a GNU version that copies data from corrupted files
References
External links
*
*
*
dd
manual page from the GNU Core Utilities.
*
dd for Windows
– save a potentially damaged harddisk partition
Softpanorama dd page
DD at Linux Questions Wiki
Forensics (DD) Dcfldd
– a variant specialized in files that are block devices
– Linux specialized variant for devices that use the SCSI command set
{{Backup software
Data recovery software
Disk cloning
Hard disk software
Standard Unix programs
Unix SUS2008 utilities
Plan 9 commands
Inferno (operating system) commands
Data erasure software