dd is a command on Unix and Unix-like operating systems whose primary purpose is to convert and copy a file.
On Unix, device drivers for hardware (such as hard disks) and special device files (such as /dev/zero and /dev/random) appear in the file system just like normal files; dd can also read and/or write from/to these files, provided that function is implemented in their respective driver. As a result, dd can be used for tasks such as backing up the boot sector of a hard drive, and obtaining fixed amount of random data. The dd 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.
The name dd may be an allusion to the DD statement found in IBM's Job Control Language (JCL), where the initialism stands for "Data Description." The command's syntax resembles the JCL statement more than it does other Unix commands, so the syntax may have been a joke. Another explanation for the command's name is that "cc" (for "convert and copy", as in the command's description) was already taken by the C compiler.
The dd command is specified by IEEE Std 1003.1-2008, which is part of the Single UNIX Specification.
The command line syntax of dd differs from many other Unix programs, in that it uses the syntax option=value for its command line options, rather than the more-standard --option value or -option=value formats. By default, dd reads from STDIN and writes to STDOUT, but these can be changed by using the if (input file) and of (output file) options.
Usage varies across different operating systems. Also, certain features of dd will depend on the computer system capabilities, such as dd's ability to implement an option for direct memory access. Sending a SIGINFO signal (or a USR1 signal on Linux) to a running dd process makes it print I/O statistics to standard error once and then continue copying (note that signals may terminate the process on OS*X). dd can read standard input from the keyboard. When end-of-file (EOF) is reached, dd 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).
In spirit with the Unix philosophy, dd does one thing (and may be considered to do it "well" ). Unlike a sophisticated and highly abstracted utility, dd has no algorithm other than in the low-level decisions of the user concerning how to vary the run options. Often, the options are changed for each run of dd in a multi-step process to solve a computer problem.
The GNU variant of dd as supplied with coreutils on Linux does not describe the format of the messages displayed on standard output on completion. However, these are described by other implementations, e.g. that with BSD.
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.
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 (ibs) compared to output/writing (obs), though the block size (bs) option will override both ibs and obs. 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 count option for copying is measured in blocks, as are both the skip count for reading and seek count for writing. Conversion operations are also affected by the "conversion block size" (cbs).
For some uses of the dd command, block size may have an effect on performance. For example, when recovering data from a hard disk, a small block size will generally cause the most bytes to be recovered. Issuing many small reads is an overhead and may be non-beneficial to execution performance. For greater speed during copy operations, a larger block size may be used. However, because the amount of bytes to copy is given by bs
