convert and copy a file
dd utility copies the standard input
to the standard output, applying any specified conversions. 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
The following operands are available:
- Read input from file instead of the standard input.
- Write output to file instead of the standard output.
Any regular output file is truncated unless the
notruncconversion value is specified. If an initial portion of the output file is skipped (see the
seekoperand), the output file is truncated at that point.
- Set the input block size to n bytes instead of the default 512.
- Set the output block size to n bytes instead of the default 512.
- Set both the input and output block size to n bytes,
obsoperands. If no conversion values other than
syncare specified, then each input block is copied to the output as a single block without any aggregation of short blocks.
- Set the conversion record size to n bytes. The conversion record size is required by the record oriented conversion values.
- Copy only n input blocks.
- Copy n input files before terminating. This operand is only applicable when the input device is a tape.
- 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 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.
- The value is one of the symbols from the following
- Do not print the transfer statistics as the last line of status output.
- Do not print the status output. Error messages are shown; informational messages are not.
- Each value is one of the symbols from the following
- The same as the
unblockvalue except that characters are translated from EBCDIC to ASCII before the records are converted. (These values imply
unblockif the operand
cbsis also specified.)
- 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
cbsoperand. 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, is reported to the standard error output at the completion of the copy.
- The same as the
blockvalue except that characters are translated from ASCII to EBCDIC after the records are converted. (These values imply
blockif the operand
cbsis also specified.) There are two conversion maps for EBCDIC. The value
ebcdicspecifies the recommended one which is compatible with AT&T System V UNIX. The value
ibmis a slightly different mapping, which is compatible with the AT&T System V UNIX
- Call fsync(2) on the output file before exiting.
- Transform uppercase characters into lowercase characters.
- 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
syncconversion 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
syncconversion 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).
- Do not truncate the output file. This will preserve any blocks in the
output file not explicitly written by
notruncvalue is not supported for tapes.
- Pad the final output block to the full output block size. This 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 the
bsblock size specification.
- Swap every pair of input bytes. If an input buffer has an odd number of bytes, the last byte will be ignored during swapping.
- 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.
- Transform lowercase characters into uppercase characters.
- 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
cbsoperand. Any trailing space characters are discarded and a newline character is appended.
Where sizes are specified, a decimal number of bytes is expected. If the number ends with a ‘b’, ‘k’ or ‘K’, ‘m’ or ‘M’, ‘g’ or ‘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.
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
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.
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 utility exits 0 on success,
and >0 if an error occurs.
Write an OpenBSD floppy image to a floppy disk:
# dd if=floppy67.img of=/dev/rfd0c bs=32k
Create an ISO-9660 image of a CD-ROM:
# dd if=/dev/rcd0c of=disk.iso bs=32k
cp(1), mt(1), tr(1)
dd utility is compliant with the
IEEE Std 1003.1-2008 (“POSIX.1”)
ibm are marked
by IEEE Std 1003.1-2008 (“POSIX.1”) as
being an X/Open System Interfaces option.
status operands, the
osync conversions, the ‘K’,
‘m’, ‘M’, ‘g’, ‘G’,
and ‘w’ size multipliers and
handling are all extensions to the IEEE Std 1003.1-2008
dd utility appeared in
Version 5 AT&T UNIX.