dd is a command-line utility for Unix, Plan 9, 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) 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
In 1974, the command appeared as part of Version 5 Unix. According to Dennis Ritchie, 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". According to Douglas McIlroy, was "originally intended for converting files between the ASCII, little-endian, byte-stream world of DEC computers and the EBCDIC, big-endian, blocked world of IBM"; thus, explaining the cultural context of its syntax. Eric S. Raymond believes "the interface design was clearly a prank", due to the command's syntax resembling a JCL statement more than other Unix commands do.
In 1987, the command is specified in 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.
In 1990, David MacKenzie announces GNU fileutils (now part of coreutils) which includes the dd command; it was written by Paul Rubin, David MacKenzie, and Stuart Kemp. Since 1991, Jim Meyering is its maintainer.
In 1995, Plan 9 2nd edition is released; its command interface is redesigned to use a traditional command-line option style instead of a JCL statement style.
Since at least 1999, there's UnxUtils, a native Win32 port for Microsoft Windows using GNU fileutils.
dd is sometimes humorously called "Disk Destroyer", due to its drive-erasing capabilities involving typos.
Usage
The command line syntax of differs from many other Unix programs. It uses the syntax for its command-line options 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 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 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 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" ().