dd - convert and copy a file
dd [operands ...]
The dd utility copies the standard input to the standard
output, applying
any specified conversions. 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:
if=file Read input from file instead of the standard input.
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
skipped (see the seek operand), the output file is
truncated at
that point.
ibs=n Set the input block size to n bytes instead of the
default 512.
obs=n Set the output block size to n bytes instead of the
default 512.
bs=n Set both the 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.
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
operand cbs is also specified.) There are
two conversion
maps for ASCII. The value ascii
specifies the
recommended one which is compatible with
System V. The
value oldascii specifies the one used in
historic AT&T
and pre-4.3BSD-reno systems.
block Treats the input as a sequence of newline
or end-offile
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, is 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 and pre-4.3BSD-reno systems.
lcase Transform uppercase characters into lowercase characters.
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 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.
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 are specified, a decimal number of bytes is expected. If the
number ends with a `b', `k', `m', or `w', the number is multiplied by
512, 1024 (1K), 1048576 (1M), 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 byteswapping
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
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.
The dd utility exits 0 on success or >0 if an error occurred.
# dd if=floppy34.fs of=/dev/rfd0c bs=32k
Write an OpenBSD floppy image to a floppy disk.
cp(1), mt(1), tr(1)
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 standard.
OpenBSD 3.6 January 13, 1994
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