NAME
dd -- convert and copy a file
SYNOPSIS
dd [operands ...]
DESCRIPTION
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 blocks.
cbs=n Set the conversion record size to n bytes. The conversion
record size is required by the record oriented conversion val-
ues.
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.
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 regu-
lar 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 trun-
cated at that point.
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.
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 op-
erand cbs is also specified.) There are two conversion
maps for ASCII. The value ascii specifies the recom-
mended 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 new-
line character is discarded. Each input record is con-
verted 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 historic
AT&T UNIX and pre-4.3BSD-Reno systems.
lcase Transform uppercase characters into lowercase charac-
ters.
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 writ-
ten to the standard error output in the same format as
the standard completion message. If the sync conver-
sion is also specified, any missing input data will be
replaced with NUL bytes (or with spaces if a block ori-
ented conversion value was specified) and processed as
a normal input buffer. If the fillchar option is spec-
ified, the fill character provided on the command line
will override the automatic selection of the fill char-
acter. 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 out-
put 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 charac-
ters.
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 op-
erand. Any trailing space characters are discarded and
a newline character is appended.
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, 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 vari-
able length record oriented conversion value was specified and the input
line was too long to fit in the conversion record or was not newline ter-
minated.
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 standard
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.
EXIT STATUS
The dd utility exits 0 on success, and >0 if an error occurs.
EXAMPLES
Check that a disk drive contains no bad blocks:
dd if=/dev/ad0 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/ad0 of=/dev/ad0 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
SEE ALSO
cp(1), mt(1), tr(1), geom(4)
STANDARDS
The dd utility is expected to be a superset of the IEEE Std 1003.2
(``POSIX.2'') standard. The files operand and the ascii, ebcdic, ibm,
oldascii, oldebcdic and oldibm values are extensions to the POSIX stan-
dard.
BUGS
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.
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