DD(1) FreeBSD 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.
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.
iflag=value[,value ...]
Where value is one of the symbols from the following list.
fullblock Reading from the input file may not obtain a full
block. When a read returns short, continue reading
to fill the block. Without this flag, count limits
the number of times read(2) is called on the input
rather than the number of blocks copied in full. May
not be combined with conv=sync.
direct Set the O_DIRECT flag on the input file to make reads
bypass any local caching.
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.
oflag=value[,value ...]
Where value is one of the symbols from the following list.
fsync Set the O_FSYNC flag on the output file to make writes
synchronous.
sync Set the O_SYNC flag on the output file to make writes
synchronous. This is synonymous with the fsync value.
direct Set the O_DIRECT flag on the output file to make writes
bypass any local caching.
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.
speed=n Limit the copying speed to n bytes per second.
status=value
Where value is one of the symbols from the following list.
noxfer Do not print the transfer statistics as the last line
of status output.
none Do not print the status output. Error messages are
shown; informational messages are not.
progress Print basic transfer statistics once per second.
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.
fdatasync
Perform an fdatasync(2) on the output file before
closing it.
fsync Perform an fsync(2) on the output file before closing
it.
lcase Transform uppercase characters into lowercase
characters.
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
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 fillchar
option is specified, the fill character provided on the
command line will override the automatic selection of
the fill character. 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 or speed are specified, a decimal, octal, or hexadecimal
number of bytes is expected. If the number ends with a "b", "k", "m",
"g", "t", "p", or "w", the number is multiplied by 512, 1024 (1K),
1048576 (1M), 1073741824 (1G), 1099511627776 (1T), 1125899906842624 (1P)
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.
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/ada0 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/ada0 of=/dev/ada0 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
To create an image of a Mode-1 CD-ROM, which is a commonly used format
for data CD-ROM disks, use a block size of 2048 bytes:
dd if=/dev/cd0 of=filename.iso bs=2048
Write a filesystem image to a memory stick, padding the end with zeros,
if necessary, to a 1MiB boundary:
dd if=memstick.img of=/dev/da0 bs=1m conv=noerror,sync
SEE ALSO
cp(1), mt(1), recoverdisk(1), tr(1), geom(4), trim(8)
STANDARDS
The dd utility is expected to be a superset of the IEEE Std 1003.2
("POSIX.2") standard. The files and status operands and the ascii,
ebcdic, ibm, oldascii, oldebcdic and oldibm values are extensions to the
POSIX standard.
HISTORY
A dd command appeared in Version 5 AT&T UNIX.
BUGS
Protection mechanisms in the geom(4) subsystem might prevent the super-
user from writing blocks to a disk. Instructions for temporarily
disabling these protection mechanisms can be found in the geom(4) man
page.
FreeBSD 13.1-RELEASE-p6 June 4, 2020 FreeBSD 13.1-RELEASE-p6
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