Manual Pages  — DD

The following operands are available:

bs=n	Set both input and output block size to n bytes, superseding the ibs and obs operands. If no conversion values other than noerror, notrunc or sync are specified, then each input block is copied to the output as a single block without any aggregation of short blocks.
cbs=n	Set the conversion record size to n bytes. The conversion record size is required by the record oriented conversion values.
count=n
	Copy only n input blocks.
files=n
	Copy n input files before terminating. This operand is only applicable when the input device is a tape.
fillchar=c
	When padding a block in conversion mode or due to use of noerror and sync modes, fill with the specified ASCII character, rather than using a space or NUL.
ibs=n	Set the input block size to n bytes instead of the default 512.
if=file
	Read input from file instead of the standard input.
iflag=value[,value ...]
	Where value is one of the symbols from the following list.

fullblock
	Reading from the input file may not obtain a full block. When a read returns short, continue reading to fill the block. Without this flag, count limits the number of times read(2) is called on the input rather than the number of blocks copied in full. May not be combined with conv=sync.
direct	Set the O_DIRECT flag on the input file to make reads bypass any local caching.

iseek=n	Seek on the input file n blocks. This is synonymous with skip=n.
obs=n	Set the output block size to n bytes instead of the default 512.
of=file	Write output to file instead of the standard output. Any regular output file is truncated unless the notrunc conversion value is specified. If an initial portion of the output file is seeked past (see the oseek operand), the output file is truncated at that point.
oflag=value[,value ...]
	Where value is one of the symbols from the following list.

fsync	Set the O_FSYNC flag on the output file to make writes synchronous.
sync	Set the O_SYNC flag on the output file to make writes synchronous. This is synonymous with the fsync value.
direct
	Set the O_DIRECT flag on the output file to make writes bypass any local caching.

oseek=n
	Seek on the output file n blocks. This is synonymous with seek=n.
seek=n
	Seek n blocks from the beginning of the output before copying. On non-tape devices, an lseek(2) operation is used. Otherwise, existing blocks are read and the data discarded. If the user does not have read permission for the tape, it is positioned using the tape ioctl(2) function calls. If the seek operation is past the end of file, space from the current end of file to the specified offset is filled with blocks of NUL bytes.
skip=n
	Skip n blocks from the beginning of the input before copying. On input which supports seeks, an lseek(2) operation is used. Otherwise, input data is read and discarded. For pipes, the correct number of bytes is read. For all other devices, the correct number of blocks is read without distinguishing between a partial or complete block being read.
speed=n
	Limit the copying speed to n bytes per second.
status=value
	Where value is one of the symbols from the following list.

noxfer	Do not print the transfer statistics as the last line of status output.
none	Do not print the status output. Error messages are shown; informational messages are not.
progress
	Print basic transfer statistics once per second.

conv=value[,value ...]
	Where value is one of the symbols from the following list.

ascii, oldascii
	The same as the unblock value except that characters are translated from EBCDIC to ASCII before the records are converted. (These values imply unblock if the operand cbs is also specified.) There are two conversion maps for ASCII. The value ascii specifies the recommended one which is compatible with AT&T V . The value oldascii specifies the one used in historic AT&T and pre-Bx 4.3 reno systems.
block	Treats the input as a sequence of newline or end-of-file terminated variable length records independent of input and output block boundaries. Any trailing newline character is discarded. Each input record is converted to a fixed length output record where the length is specified by the cbs operand. Input records shorter than the conversion record size are padded with spaces. Input records longer than the conversion record size are truncated. The number of truncated input records, if any, are reported to the standard error output at the completion of the copy.
ebcdic, ibm, oldebcdic, oldibm
	The same as the block value except that characters are translated from ASCII to EBCDIC after the records are converted. (These values imply block if the operand cbs is also specified.) There are four conversion maps for EBCDIC. The value ebcdic specifies the recommended one which is compatible with AT&T V . The value ibm is a slightly different mapping, which is compatible with the AT&T V ibm value. The values oldebcdic and oldibm are maps used in historic AT&T and pre-Bx 4.3 reno systems.
fdatasync
	Perform an fdatasync(2) on the output file before closing it.
fsync	Perform an fsync(2) on the output file before closing it.
lcase	Transform uppercase characters into lowercase characters.
pareven, parnone, parodd, parset
	Output data with the specified parity. The parity bit on input is stripped unless EBCDIC to ASCII conversions is also specified.
noerror
	Do not stop processing on an input error. When an input error occurs, a diagnostic message followed by the current input and output block counts will be written to the standard error output in the same format as the standard completion message. If the sync conversion is also specified, any missing input data will be replaced with NUL bytes (or with spaces if a block oriented conversion value was specified) and processed as a normal input buffer. If the fillchar option is specified, the fill character provided on the command line will override the automatic selection of the fill character. If the sync conversion is not specified, the input block is omitted from the output. On input files which are not tapes or pipes, the file offset will be positioned past the block in which the error occurred using lseek(2).
notrunc
	Do not truncate the output file. This will preserve any blocks in the output file not explicitly written by dd. The notrunc value is not supported for tapes.
osync	Pad the final output block to the full output block size. If the input file is not a multiple of the output block size after conversion, this conversion forces the final output block to be the same size as preceding blocks for use on devices that require regularly sized blocks to be written. This option is incompatible with use of the bs=n block size specification.
sparse	If one or more output blocks would consist solely of NUL bytes, try to seek the output file by the required space instead of filling them with NULs, resulting in a sparse file.
swab	Swap every pair of input bytes. If an input buffer has an odd number of bytes, the last byte will be ignored during swapping.
sync	Pad every input block to the input buffer size. Spaces are used for pad bytes if a block oriented conversion value is specified, otherwise NUL bytes are used.
ucase	Transform lowercase characters into uppercase characters.
unblock
	Treats the input as a sequence of fixed length records independent of input and output block boundaries. The length of the input records is specified by the cbs operand. Any trailing space characters are discarded and a newline character is appended.

Where sizes or speed are specified, a decimal, octal, or hexadecimal number of bytes is expected. If the number ends with a "b", "k", "m", "g", "t", "p", or "w", the number is multiplied by 512, 1024 (1K), 1048576 (1M), 1073741824 (1G), 1099511627776 (1T), 1125899906842624 (1P) or the number of bytes in an integer, respectively. Two or more numbers may be separated by an "x" to indicate a product.

When finished, dd displays the number of complete and partial input and output blocks, truncated input records and odd-length byte-swapping blocks to the standard error output. A partial input block is one where less than the input block size was read. A partial output block is one where less than the output block size was written. Partial output blocks to tape devices are considered fatal errors. Otherwise, the rest of the block will be written. Partial output blocks to character devices will produce a warning message. A truncated input block is one where a variable length record oriented conversion value was specified and the input line was too long to fit in the conversion record or was not newline terminated.

Normally, data resulting from input or conversion or both are aggregated into output blocks of the specified size. After the end of input is reached, any remaining output is written as a block. This means that the final output block may be shorter than the output block size.