DD(1) 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 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 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 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 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 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.
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
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, 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 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), 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.
macOS 12.1 January 13, 1994 macOS 12.1