Concept
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 s (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. 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.