The dd(1) 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(1) 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, 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.
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.
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 skipped (see the seek operand) the output file is
truncated at that point.
seek=n
Seek n blocks from the beginning of the output before
copying. On non-tape devices, an lseek operation is used. 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, a lseek 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-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. The value ibm is a slightly
different mapping, which is compatible with the AT&T System V
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.
notrunc
Do not truncate the output file. This will preserve any blocks
in the output file not explicitly written by dd(1). 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(1) 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(1) 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(1)
will exit.
The dd(1) utility is usually used for converting data
from one medium (such as tape) to another, but it can also be used
for other conversions. This command strips the first 128 bytes from
a file:
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