dd is a common UNIX program whose primary purpose is the low-level copying and conversion of raw data. dd is an abbreviation for "data definition" in IBM JCL, and the command's syntax is meant to be reminiscent of this.

dd is used to copy a specified number of bytes or blocks, performing on-the-fly byte order conversions, as well as more esoteric EBCDIC to ASCII conversions.

dd can also be 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 can also be used in computer forensics when the contents of a disk need to be preserved as a byte-exact copy. Using cp would not be sufficient because data from deleted files still physically present on a disk are not visible through the file system interface.

It is jokingly said to stand for "data destroyer" 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.

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