Updated: 2022/Sep/29
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DD(1) General Commands Manual DD(1)
NAME
dd - convert and copy a file
SYNOPSIS
dd [operand ...]
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, 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.
iflag=flags
Use comma-separated flags when calling open(2) for the input
file. See the INPUT AND OUTPUT FLAGS section for details.
Default value is rdonly.
iseek=n Seek on the input file n blocks. This is synonymous with
skip=n.
msgfmt=fmt
Specify the message format fmt to be used when writing
information to standard output. Possible values are:
quiet turns off information summary report except for
errors and progress.
posix default information summary report as specified by
POSIX.
human default information summary report extended with
human-readable values.
When fmt does not correspond to any value given above, it
contains a string that will be used as format specifier for the
information summary output. Each conversion specification is
introduced by the character %. The following ones are
available:
b total number of bytes transferred
B total number of bytes transferred in
humanize_number(3) format
e speed transfer
E speed transfer in humanize_number(3) format
i number of partial input block(s)
I number of full input block(s)
o number of partial output block(s)
O number of full output block(s)
s time elapsed since the beginning in "seconds.ms"
format
p number of sparse output blocks
t number of truncated blocks
w number of odd-length swab blocks
P singular/plural of "block" depending on number of
sparse blocks
T singular/plural of "block" depending on number of
truncated blocks
W singular/plural of "block" depending on number of swab
blocks
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.
oflag=flags
Same as iflag but for the call to open(2) on the output file.
The default value is creat, which must be explicitly added in
oflag if this option is used in order to output to a nonexistent
file. The default or specified value is or'ed with rdwr for a
first open(2) attempt, then on failure with wronly on a second
attempt. In both cases, trunc is automatically added if none of
oseek, seek, or conv=notrunc operands are used. See the INPUT
AND OUTPUT FLAGS section for details.
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.
progress=n
Switch on display of progress if n is set to any non-zero value.
This will cause a "." to be printed (to the standard error
output) for every n full or partial blocks written to the output
file.
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 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 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 non-final output blocks would consist
solely of NUL bytes, try to seek the output file by the
required space instead of filling them with NULs. This
results in a sparse file on some file systems.
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. Two or
more numbers may be separated by an "x" to indicate a product. Each
number may have one of the following optional suffixes:
b Block; multiply by 512
k Kibi; multiply by 1024 (1 KiB)
m Mebi; multiply by 1048576 (1 MiB)
g Gibi; multiply by 1073741824 (1 GiB)
t Tebi; multiply by 1099511627776 (1 TiB)
w Word; multiply by the number of bytes in an integer
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 signal (see the status argument for stty(1)),
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.
INPUT AND OUTPUT FLAGS
There are flags valid for input only, for output only, or for either.
The flags that apply to both input and output are:
alt_io Use Alternative I/O.
async Use SIGIO signaling for I/O.
cloexec Set the close-on-exec flag.
direct Directly access the data, skipping any caches.
directory Not available for dd.
exlock Atomically obtain an exclusive lock.
noctty Do not consider the file as a potential controlling tty.
nofollow Do not follow symbolic links.
nonblock Do not block on open or I/O requests.
nosigpipe Return EPIPE instead of raising SIGPIPE.
shlock Atomically obtain a shared lock.
sync All I/O will be performed with full synchronization.
The flags that apply to only input are:
rdonly Set the read-only flag.
rdwr Set the read and write flags.
rsync Enable read synchronization, if the sync option is also set.
The flags that apply to only output are:
append Append to the output by default.
creat Create the output file.
dsync Wait for all data to be synchronously written.
excl Ensure that output is to a new file.
trunc Truncate the output file before writing.
wronly Set the write-only flag.
See open(2) and ioctl(2) for more details.
EXIT STATUS
The dd utility exits 0 on success, and >0 if an error occurs.
EXAMPLES
To write a gzip(1) compressed NetBSD image to a removable drive, with
progress(1) output:
zcat NetBSD-9.2-amd64-install.img.gz | \
progress dd of=/dev/rsd0 bs=1m
To print summary information in human-readable form:
dd if=/dev/zero of=/dev/null count=1 msgfmt=human
To customize the information summary output and print it through
unvis(3):
dd if=/dev/zero of=/dev/null count=1 \
msgfmt='speed:%E, in %s seconds\n' 2>&1 | unvis
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 and msgfmt operands and the ascii,
ebcdic, ibm, oldascii, oldebcdic and oldibm values are extensions to the
POSIX standard.
HISTORY
A dd utility appeared in Version 5 AT&T UNIX.
NetBSD 10.99 January 14, 2022 NetBSD 10.99