Man

Command Section
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.

     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.

     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.

     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.

              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/acd0 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)

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 11.1-RELEASE-p4          April 2, 2017         FreeBSD 11.1-RELEASE-p4
Command Section