DD(1) BSD General Commands Manual DD(1)
dd -- convert and copy a file
dd [operands ...]
The dd utility copies the standard input to the standard output. Input data is read and written in 512-byte blocks. If input reads are
short, input from multiple reads are aggregated to form the output block. When finished, dd displays the number of complete and partial
input and output blocks and truncated input records to the standard error output.
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
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.
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.
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
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. Other-
wise, 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.
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.
Where value is one of the symbols from the following list.
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 System V UNIX. The value oldascii specifies the one used
in historic AT&T UNIX and pre-4.3BSD-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 System V UNIX. The value ibm is a slightly different
mapping, which is compatible with the AT&T System V UNIX ibm value. The values oldebcdic and oldibm are maps used in his-
toric AT&T UNIX and pre-4.3BSD-Reno systems.
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
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 con-
version 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 specifica-
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 swap-
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
Where sizes are specified, a decimal, octal, or hexadecimal number of bytes is expected. If the number ends with a ``b'', ``k'', ``m'',
``g'', or ``w'', the number is multiplied by 512, 1024 (1K), 1048576 (1M), 1073741824 (1G) or the number of bytes in an integer, respec-
tively. 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.
If dd receives a SIGINFO (see the status argument for stty(1)) signal, the current input and output block counts will be written to the stan-
dard error output in the same format as the standard completion message. If dd receives a SIGINT signal, the current input and output block
counts will be written to the standard error output in the same format as the standard completion message and dd will exit.
The dd utility exits 0 on success, and >0 if an error occurs.
Check that a disk drive contains no bad blocks:
dd if=/dev/ada0 of=/dev/null bs=1m
Do a refresh of a disk drive, in order to prevent presently recoverable read errors from progressing into unrecoverable read errors:
dd if=/dev/ada0 of=/dev/ada0 bs=1m
Remove parity bit from a file:
dd if=file conv=parnone of=file.txt
Check for (even) parity errors on a file:
dd if=file conv=pareven | cmp -x - file
To create an image of a Mode-1 CD-ROM, which is a commonly used format for data CD-ROM disks, use a block size of 2048 bytes:
dd if=/dev/acd0 of=filename.iso bs=2048
Write a filesystem image to a memory stick, padding the end with zeros, if necessary, to a 1MiB boundary:
dd if=memstick.img of=/dev/da0 bs=1m conv=noerror,sync
cp(1), mt(1), tr(1), geom(4)
The dd utility is expected to be a superset of the IEEE Std 1003.2 (``POSIX.2'') standard. The files and status operands and the ascii,
ebcdic, ibm, oldascii, oldebcdic and oldibm values are extensions to the POSIX standard.
Protection mechanisms in the geom(4) subsystem might prevent the super-user from writing blocks to a disk. Instructions for temporarily dis-
abling these protection mechanisms can be found in the geom(4) manpage.
August 28, 2014 BSD