DD(1) BSD General Commands Manual DD(1)
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 trun-cated truncated
cated 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 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.
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.
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 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 char-acter character
acter 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 conver-sion conversion
sion 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 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 pro-cessed processed
cessed 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.
sparse If one or more output blocks would consist solely of NUL bytes, try to seek the out-put output
put 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 out-put output
put block boundaries. The length of the input records is specified by the cbs oper-and. operand.
and. 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 out-put output
put block size was written. Partial output blocks to tape devices are considered fatal errors. Other-wise, Otherwise,
wise, 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.
DIAGNOSTICS
The dd utility exits 0 on success, and >0 if an error occurs.
SEE ALSO
cp(1), mt(1), tr(1)
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
standard.
BSD January 13, 1994 BSD
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