NAME
dd
—
convert and copy a file
SYNOPSIS
dd |
[operand ...] |
DESCRIPTION
Thedd
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
andobs
operands. If no conversion values other thannoerror
,notrunc
orsync
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 theseek
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 inoflag
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 ofoseek
,seek
, orconv=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 implyunblock
if the operandcbs
is also specified.) There are two conversion maps for ASCII. The valueascii
specifies the recommended one which is compatible with AT&T System V UNIX. The valueoldascii
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 implyblock
if the operandcbs
is also specified.) There are four conversion maps for EBCDIC. The valueebcdic
specifies the recommended one which is compatible with AT&T System V UNIX. The valueibm
is a slightly different mapping, which is compatible with the AT&T System V UNIXibm
value. The valuesoldebcdic
andoldibm
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 thesync
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
. Thenotrunc
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 withNUL
s. 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 raisingSIGPIPE
. 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.
EXIT STATUS
The dd
utility exits 0 on success,
and >0 if an error occurs.
EXAMPLES
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
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.