Tag: dd

About

dd is a common UNIX program whose primary purpose is the low-level copying and conversion of files. The title of the UNIX sixth edition manual page of May 1975 was dd - convert and copy a file, and some have suggested[citation needed] that since "cc" was already in use as the code for the C Compiler, "dd" was used instead; in any case, DD is an acronym for "data definition" in IBM JCL (to which the command's syntax bears a strong resemblance) and the name and syntax of the command is generally presumed to be a bit of gallows humor.

dd has a different set of command line options than the ordinary cp command (which copies files in their entirety) that allow copying a fixed number of bytes or blocks, performing on-the-fly byte order conversions, as well as more esoteric EBCDIC to ASCII conversions. dd is commonly used to copy regions of raw device files, e.g. backing up the boot sector of a hard disk, or to read fixed amounts of data from special files like /dev/zero or /dev/random. It also works well reading and writing to block based media like magnetic tape. Because dd can copy entire partitions or disks (this is more due to Unix providing access to these devices than the functionality of dd per se), it is used in computer forensics when the contents of a partition need to be preserved in a byte-exact copy. Using cp would not be enough, since data from deleted files that may still be present on a disk are not visible through the file system interface. It is jokingly said to stand for "destroy disk" or "delete data", since, being used for low-level operations on hard disks, a small mistake, such as reversing the if and of parameters, may accidentally render the entire disk unusable.

The command line syntax of dd is unlike that of any other Unix program; a violation of the Unix philosophy of using a common syntax for all command line tools. Generally, dd uses an option=value format, whereas most Unix programs use a -option value format. Also, dd's input is specified using the "if" (input file) option, while most programs simply take the name by itself. It is rumored to have been based on IBM's JCL, and though the syntax may have been a joke, there seems never to have been any effort to write a more Unix-like replacement. The obscure syntax may also have been deliberately designed to prevent careless usage, due to the major damage this utility can cause if misused (see anti-examples).

From en.wikipedia.org/wiki/Dd_(Unix)