freebsd/bin/dd/dd.1

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.\"     @(#)dd.1	8.2 (Berkeley) 1/13/94
.\" $FreeBSD$
.\"
.Dd January 23, 2009
.Dt DD 1
.Os
.Sh NAME
.Nm dd
.Nd convert and copy a file
.Sh SYNOPSIS
.Nm
.Op Ar operands ...
.Sh DESCRIPTION
The
.Nm
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,
.Nm
displays the number of complete and partial input and output blocks
and truncated input records to the standard error output.
.Pp
The following operands are available:
.Bl -tag -width ".Cm of Ns = Ns Ar file"
.It Cm bs Ns = Ns Ar n
Set both input and output block size to
.Ar n
bytes, superseding the
.Cm ibs
and
.Cm obs
operands.
If no conversion values other than
.Cm noerror ,
.Cm notrunc
or
.Cm sync
are specified, then each input block is copied to the output as a
single block without any aggregation of short blocks.
.It Cm cbs Ns = Ns Ar n
Set the conversion record size to
.Ar n
bytes.
The conversion record size is required by the record oriented conversion
values.
.It Cm count Ns = Ns Ar n
Copy only
.Ar n
input blocks.
.It Cm files Ns = Ns Ar n
Copy
.Ar n
input files before terminating.
This operand is only applicable when the input device is a tape.
.It Cm fillchar Ns = Ns Ar c
When padding a block in conversion mode or due to use of
.Cm noerror
and
.Cm sync
modes, fill with the specified
.Tn ASCII
character, rather than using a space or
.Dv NUL .
.It Cm ibs Ns = Ns Ar n
Set the input block size to
.Ar n
bytes instead of the default 512.
.It Cm if Ns = Ns Ar file
Read input from
.Ar file
instead of the standard input.
.It Cm iseek Ns = Ns Ar n
Seek on the input file
.Ar n
blocks.
This is synonymous with
.Cm skip Ns = Ns Ar n .
.It Cm obs Ns = Ns Ar n
Set the output block size to
.Ar n
bytes instead of the default 512.
.It Cm of Ns = Ns Ar file
Write output to
.Ar file
instead of the standard output.
Any regular output file is truncated unless the
.Cm notrunc
conversion value is specified.
If an initial portion of the output file is seeked past (see the
.Cm oseek
operand),
the output file is truncated at that point.
.It Cm oseek Ns = Ns Ar n
Seek on the output file
.Ar n
blocks.
This is synonymous with
.Cm seek Ns = Ns Ar n .
.It Cm seek Ns = Ns Ar n
Seek
.Ar n
blocks from the beginning of the output before copying.
On non-tape devices, an
.Xr 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
.Xr 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
.Dv NUL
bytes.
.It Cm skip Ns = Ns Ar n
Skip
.Ar n
blocks from the beginning of the input before copying.
On input which supports seeks, an
.Xr 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.
.It Cm conv Ns = Ns Ar value Ns Op , Ns Ar value ...
Where
.Cm value
is one of the symbols from the following list.
.Bl -tag -width ".Cm unblock"
.It Cm ascii , oldascii
The same as the
.Cm unblock
value except that characters are translated from
.Tn EBCDIC
to
.Tn ASCII
before the
records are converted.
(These values imply
.Cm unblock
if the operand
.Cm cbs
is also specified.)
There are two conversion maps for
.Tn ASCII .
The value
.Cm ascii
specifies the recommended one which is compatible with
.At V .
The value
.Cm oldascii
specifies the one used in historic
.At
and
.No pre- Ns Bx 4.3 reno
systems.
.It Cm 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
.Cm 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.
.It Cm ebcdic , ibm , oldebcdic , oldibm
The same as the
.Cm block
value except that characters are translated from
.Tn ASCII
to
.Tn EBCDIC
after the
records are converted.
(These values imply
.Cm block
if the operand
.Cm cbs
is also specified.)
There are four conversion maps for
.Tn EBCDIC .
The value
.Cm ebcdic
specifies the recommended one which is compatible with
.At V .
The value
.Cm ibm
is a slightly different mapping, which is compatible with the
.At V
.Cm ibm
value.
The values
.Cm oldebcdic
and
.Cm oldibm
are maps used in historic
.At
and
.No pre- Ns Bx 4.3 reno
systems.
.It Cm lcase
Transform uppercase characters into lowercase characters.
.It Cm pareven , parnone , parodd , parset
Output data with the specified parity.
The parity bit on input is stripped unless
.Tn EBCDIC
to
.Tn ASCII
conversions is also specified.
.It Cm 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
.Cm sync
conversion is also specified, any missing input data will be replaced
with
.Dv NUL
bytes (or with spaces if a block oriented conversion value was
specified) and processed as a normal input buffer.
If the
.Cm fillchar
option is specified, the fill character provided on the command line
will override
the automatic selection of the fill character.
If the
.Cm 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
.Xr lseek 2 .
.It Cm notrunc
Do not truncate the output file.
This will preserve any blocks in the output file not explicitly written
by
.Nm .
The
.Cm notrunc
value is not supported for tapes.
.It Cm 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
.Cm bs Ns = Ns Ar n
block size specification.
.It Cm sparse
If one or more output blocks would consist solely of
.Dv NUL
bytes, try to seek the output file by the required space instead of
filling them with
.Dv NUL Ns s ,
resulting in a sparse file.
.It Cm 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.
.It Cm 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
.Dv NUL
bytes are used.
.It Cm ucase
Transform lowercase characters into uppercase characters.
.It Cm 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
.Cm cbs
operand.
Any trailing space characters are discarded and a newline character is
appended.
.El
.El
.Pp
Where sizes are specified, a decimal, octal, or hexadecimal number of
bytes is expected.
If the number ends with a
.Dq Li b ,
.Dq Li k ,
.Dq Li m ,
.Dq Li g ,
or
.Dq Li 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
.Dq Li x
to indicate a product.
.Pp
When finished,
.Nm
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.
.Pp
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.
.Pp
If
.Nm
receives a
.Dv SIGINFO
(see the
.Cm status
argument for
.Xr 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
.Nm
receives a
.Dv 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
.Nm
will exit.
.Sh EXIT STATUS
.Ex -std
.Sh EXAMPLES
Check that a disk drive contains no bad blocks:
.Pp
.Dl "dd if=/dev/ad0 of=/dev/null bs=1m"
.Pp
Do a refresh of a disk drive, in order to prevent presently
recoverable read errors from progressing into unrecoverable read errors:
.Pp
.Dl "dd if=/dev/ad0 of=/dev/ad0 bs=1m"
.Pp
Remove parity bit from a file:
.Pp
.Dl "dd if=file conv=parnone of=file.txt"
.Pp
Check for (even) parity errors on a file:
.Pp
.Dl "dd if=file conv=pareven | cmp -x - file"
.Pp
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:
.Pp
.Dl "dd if=/dev/acd0 of=filename.iso bs=2048"
.Sh SEE ALSO
.Xr cp 1 ,
.Xr mt 1 ,
.Xr tr 1 ,
.Xr geom 4
.Sh STANDARDS
The
.Nm
utility is expected to be a superset of the
.St -p1003.2
standard.
The
.Cm files
operand and the
.Cm ascii ,
.Cm ebcdic ,
.Cm ibm ,
.Cm oldascii ,
.Cm oldebcdic
and
.Cm oldibm
values are extensions to the
.Tn POSIX
standard.
.Sh BUGS
Protection mechanisms in the
.Xr 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
.Xr geom 4
manpage.