DD(P) POSIX Programmer's Manual DD(P)NAMEdd - convert and copy a file
SYNOPSISdd [operand ...]
DESCRIPTION
The dd utility shall copy the specified input file to the specified
output file with possible conversions using specific input and output
block sizes. It shall read the input one block at a time, using the
specified input block size; it shall then process the block of data
actually returned, which could be smaller than the requested block
size. It shall apply any conversions that have been specified and write
the resulting data to the output in blocks of the specified output
block size. If the bs= expr operand is specified and no conversions
other than sync, noerror, or notrunc are requested, the data returned
from each input block shall be written as a separate output block; if
the read returns less than a full block and the sync conversion is not
specified, the resulting output block shall be the same size as the
input block. If the bs= expr operand is not specified, or a conversion
other than sync, noerror, or notrunc is requested, the input shall be
processed and collected into full-sized output blocks until the end of
the input is reached.
The processing order shall be as follows:
1. An input block is read.
2. If the input block is shorter than the specified input block size
and the sync conversion is specified, null bytes shall be appended
to the input data up to the specified size. (If either block or
unblock is also specified, <space>s shall be appended instead of
null bytes.) The remaining conversions and output shall include the
pad characters as if they had been read from the input.
3. If the bs= expr operand is specified and no conversion other than
sync or noerror is requested, the resulting data shall be written
to the output as a single block, and the remaining steps are omit‐
ted.
4. If the swab conversion is specified, each pair of input data bytes
shall be swapped. If there is an odd number of bytes in the input
block, the last byte in the input record shall not be swapped.
5. Any remaining conversions ( block, unblock, lcase, and ucase) shall
be performed. These conversions shall operate on the input data
independently of the input blocking; an input or output fixed-
length record may span block boundaries.
6. The data resulting from input or conversion or both shall be aggre‐
gated into output blocks of the specified size. After the end of
input is reached, any remaining output shall be written as a block
without padding if conv= sync is not specified; thus, the final
output block may be shorter than the output block size.
OPTIONS
None.
OPERANDS
All of the operands shall be processed before any input is read. The
following operands shall be supported:
if=file
Specify the input pathname; the default is standard input.
of=file
Specify the output pathname; the default is standard output. If
the seek= expr conversion is not also specified, the output file
shall be truncated before the copy begins if an explicit of=
file operand is specified, unless conv= notrunc is specified. If
seek= expr is specified, but conv= notrunc is not, the effect of
the copy shall be to preserve the blocks in the output file over
which dd seeks, but no other portion of the output file shall be
preserved. (If the size of the seek plus the size of the input
file is less than the previous size of the output file, the out‐
put file shall be shortened by the copy.)
ibs=expr
Specify the input block size, in bytes, by expr (default is
512).
obs=expr
Specify the output block size, in bytes, by expr (default is
512).
bs=expr
Set both input and output block sizes to expr bytes, superseding
ibs= and obs=. If no conversion other than sync, noerror, and
notrunc is specified, each input block shall be copied to the
output as a single block without aggregating short blocks.
cbs=expr
Specify the conversion block size for block and unblock in bytes
by expr (default is zero). If cbs= is omitted or given a value
of zero, using block or unblock produces unspecified results.
The application shall ensure that this operand is also specified if the
conv= operand is specified with a value of ascii, ebcdic, or ibm. For a
conv= operand with an ascii value, the input is handled as described
for the unblock value, except that characters are converted to ASCII
before any trailing <space>s are deleted. For conv= operands with
ebcdic or ibm values, the input is handled as described for the block
value except that the characters are converted to EBCDIC or IBM EBCDIC,
respectively, after any trailing <space>s are added.
skip=n Skip n input blocks (using the specified input block size)
before starting to copy. On seekable files, the implementation
shall read the blocks or seek past them; on non-seekable files,
the blocks shall be read and the data shall be discarded.
seek=n Skip n blocks (using the specified output block size) from the
beginning of the output file before copying. On non-seekable
files, existing blocks shall be read and space from the current
end-of-file to the specified offset, if any, filled with null
bytes; on seekable files, the implementation shall seek to the
specified offset or read the blocks as described for non-seek‐
able files.
count=n
Copy only n input blocks.
conv=value[,value ...]
Where values are comma-separated symbols from the following
list:
ascii
Convert EBCDIC to ASCII; see ASCII to EBCDIC Conversion .
ebcdic
Convert ASCII to EBCDIC; see ASCII to EBCDIC Conversion .
ibm
Convert ASCII to a different EBCDIC set; see ASCII to IBM EBCDIC
Conversion .
The ascii, ebcdic, and ibm values are mutually-exclusive.
block
Treat the input as a sequence of <newline>-terminated or end-of-
file-terminated variable-length records independent of the input
block boundaries. Each record shall be converted to a record
with a fixed length specified by the conversion block size. Any
<newline> shall be removed from the input line; <space>s shall
be appended to lines that are shorter than their conversion
block size to fill the block. Lines that are longer than the
conversion block size shall be truncated to the largest number
of characters that fit into that size; the number of truncated
lines shall be reported (see the STDERR section).
The block and unblock values are mutually-exclusive.
unblock
Convert fixed-length records to variable length. Read a number
of bytes equal to the conversion block size (or the number of
bytes remaining in the input, if less than the conversion block
size), delete all trailing <space>s, and append a <newline>.
lcase
Map uppercase characters specified by the LC_CTYPE keyword
tolower to the corresponding lowercase character. Characters
for which no mapping is specified shall not be modified by this
conversion.
The lcase and ucase symbols are mutually-exclusive.
ucase
Map lowercase characters specified by the LC_CTYPE keyword toup‐
per to the corresponding uppercase character. Characters for
which no mapping is specified shall not be modified by this con‐
version.
swab
Swap every pair of input bytes.
noerror
Do not stop processing on an input error. When an input error
occurs, a diagnostic message shall be written on standard error,
followed by the current input and output block counts in the
same format as used at completion (see the STDERR section). If
the sync conversion is specified, the missing input shall be
replaced with null bytes and processed normally; otherwise, the
input block shall be omitted from the output.
notrunc
Do not truncate the output file. Preserve blocks in the output
file not explicitly written by this invocation of the dd util‐
ity. (See also the preceding of= file operand.)
sync
Pad every input block to the size of the ibs= buffer, appending
null bytes. (If either block or unblock is also specified,
append <space>s, rather than null bytes.)
The behavior is unspecified if operands other than conv= are specified
more than once.
For the bs=, cbs=, ibs=, and obs= operands, the application shall sup‐
ply an expression specifying a size in bytes. The expression, expr, can
be:
1. A positive decimal number
2. A positive decimal number followed by k, specifying multiplication
by 1024
3. A positive decimal number followed by b, specifying multiplication
by 512
4. Two or more positive decimal numbers (with or without k or b) sepa‐
rated by x, specifying the product of the indicated values
All of the operands are processed before any input is read.
The following two tables display the octal number character values used
for the ascii and ebcdic conversions (first table) and for the ibm con‐
version (second table). In both tables, the ASCII values are the row
and column headers and the EBCDIC values are found at their intersec‐
tions. For example, ASCII 0012 (LF) is the second row, third column,
yielding 0045 in EBCDIC. The inverted tables (for EBCDIC to ASCII con‐
version) are not shown, but are in one-to-one correspondence with these
tables. The differences between the two tables are highlighted by
small boxes drawn around five entries.
Table: ASCII to EBCDIC Conversion
Table: ASCII to IBM EBCDIC Conversion
STDIN
If no if= operand is specified, the standard input shall be used. See
the INPUT FILES section.
INPUT FILES
The input file can be any file type.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of dd:
LANG Provide a default value for the internationalization variables
that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
ables for the precedence of internationalization variables used
to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all
the other internationalization variables.
LC_CTYPE
Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments and input files),
the classification of characters as uppercase or lowercase, and
the mapping of characters from one case to the other.
LC_MESSAGES
Determine the locale that should be used to affect the format
and contents of diagnostic messages written to standard error
and informative messages written to standard output.
NLSPATH
Determine the location of message catalogs for the processing of
LC_MESSAGES .
ASYNCHRONOUS EVENTS
For SIGINT, the dd utility shall interrupt its current processing,
write status information to standard error, and exit as though termi‐
nated by SIGINT. It shall take the standard action for all other sig‐
nals; see the ASYNCHRONOUS EVENTS section in Utility Description
Defaults .
STDOUT
If no of= operand is specified, the standard output shall be used. The
nature of the output depends on the operands selected.
STDERR
On completion, dd shall write the number of input and output blocks to
standard error. In the POSIX locale the following formats shall be
used:
"%u+%u records in\n", <number of whole input blocks>,
<number of partial input blocks>
"%u+%u records out\n", <number of whole output blocks>,
<number of partial output blocks>
A partial input block is one for which read() returned less than the
input block size. A partial output block is one that was written with
fewer bytes than specified by the output block size.
In addition, when there is at least one truncated block, the number of
truncated blocks shall be written to standard error. In the POSIX
locale, the format shall be:
"%u truncated %s\n", <number of truncated blocks>, "record" (if
<number of truncated blocks> is one) "records" (otherwise)
Diagnostic messages may also be written to standard error.
OUTPUT FILES
If the of= operand is used, the output shall be the same as described
in the STDOUT section.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 The input file was copied successfully.
>0 An error occurred.
CONSEQUENCES OF ERRORS
If an input error is detected and the noerror conversion has not been
specified, any partial output block shall be written to the output
file, a diagnostic message shall be written, and the copy operation
shall be discontinued. If some other error is detected, a diagnostic
message shall be written and the copy operation shall be discontinued.
The following sections are informative.
APPLICATION USAGE
The input and output block size can be specified to take advantage of
raw physical I/O.
There are many different versions of the EBCDIC codesets. The ASCII and
EBCDIC conversions specified for the dd utility perform conversions for
the version specified by the tables.
EXAMPLES
The following command:
dd if=/dev/rmt0h of=/dev/rmt1h
copies from tape drive 0 to tape drive 1, using a common historical
device naming convention.
The following command:
dd ibs=10 skip=1
strips the first 10 bytes from standard input.
This example reads an EBCDIC tape blocked ten 80-byte EBCDIC card
images per block into the ASCII file x:
dd if=/dev/tape of=x ibs=800 cbs=80 conv=ascii,lcase
RATIONALE
The OPTIONS section is listed as "None" because there are no options
recognized by historical dd utilities. Certainly, many of the operands
could have been designed to use the Utility Syntax Guidelines, which
would have resulted in the classic hyphenated option letters. In this
version of this volume of IEEE Std 1003.1-2001, dd retains its curious
JCL-like syntax due to the large number of applications that depend on
the historical implementation.
A suggested implementation technique for conv= noerror, sync is to zero
(or <space>-fill, if blocking or unblocking) the input buffer before
each read and to write the contents of the input buffer to the output
even after an error. In this manner, any data transferred to the input
buffer before the error was detected is preserved. Another point is
that a failed read on a regular file or a disk generally does not
increment the file offset, and dd must then seek past the block on
which the error occurred; otherwise, the input error occurs repeti‐
tively. When the input is a magnetic tape, however, the tape normally
has passed the block containing the error when the error is reported,
and thus no seek is necessary.
The default ibs= and obs= sizes are specified as 512 bytes because
there are historical (largely portable) scripts that assume these val‐
ues. If they were left unspecified, unusual results could occur if an
implementation chose an odd block size.
Historical implementations of dd used creat() when processing of= file.
This makes the seek= operand unusable except on special files. The
conv= notrunc feature was added because more recent BSD-based implemen‐
tations use open() (without O_TRUNC) instead of creat(), but they fail
to delete output file contents after the data copied.
The w multiplier (historically meaning word), is used in System V to
mean 2 and in 4.2 BSD to mean 4. Since word is inherently non-portable,
its use is not supported by this volume of IEEE Std 1003.1-2001.
Standard EBCDIC does not have the characters '[' and ']' . The values
used in the table are taken from a common print train that does contain
them. Other than those characters, the print train values are not
filled in, but appear to provide some of the motivation for the histor‐
ical choice of translations reflected here.
The Standard EBCDIC table provides a 1:1 translation for all 256 bytes.
The IBM EBCDIC table does not provide such a translation. The marked
cells in the tables differ in such a way that:
1. EBCDIC 0112 ( 'cent' ) and 0152 (broken pipe) do not appear in the
table.
2. EBCDIC 0137 ( 'not' ) translates to/from ASCII 0236 ( '^' ). In the
standard table, EBCDIC 0232 (no graphic) is used.
3. EBCDIC 0241 ( '~' ) translates to/from ASCII 0176 ( '~' ). In the
standard table, EBCDIC 0137 ( 'not' ) is used.
4. 0255 ( '[' ) and 0275 ( ']' ) appear twice, once in the same place
as for the standard table and once in place of 0112 ( 'cent' ) and
0241 ( '~' ).
In net result: EBCDIC 0275 ( ']' ) displaced EBCDIC 0241 ( '~' ) in
cell 0345.
That displaced EBCDIC 0137 ( 'not' ) in cell 0176.
That displaced EBCDIC 0232 (no graphic) in cell 0136.
That replaced EBCDIC 0152 (broken pipe) in cell 0313.
EBCDIC 0255 ( '[' ) replaced EBCDIC 0112 ( 'cent' ).
This translation, however, reflects historical practice that (ASCII)
'~' and 'not' were often mapped to each other, as were '[' and 'cent' ;
and ']' and (EBCDIC) '~' .
The cbs operand is required if any of the ascii, ebcdic, or ibm oper‐
ands are specified. For the ascii operand, the input is handled as
described for the unblock operand except that characters are converted
to ASCII before the trailing <space>s are deleted. For the ebcdic and
ibm operands, the input is handled as described for the block operand
except that the characters are converted to EBCDIC or IBM EBCDIC after
the trailing <space>s are added.
The block and unblock keywords are from historical BSD practice.
The consistent use of the word record in standard error messages
matches most historical practice. An earlier version of System V used
block, but this has been updated in more recent releases.
Early proposals only allowed two numbers separated by x to be used in a
product when specifying bs=, cbs=, ibs=, and obs= sizes. This was
changed to reflect the historical practice of allowing multiple numbers
in the product as provided by Version 7 and all releases of System V
and BSD.
A change to the swab conversion is required to match historical prac‐
tice and is the result of IEEE PASC Interpretations 1003.2 #03 and #04,
submitted for the ISO POSIX-2:1993 standard.
A change to the handling of SIGINT is required to match historical
practice and is the result of IEEE PASC Interpretation 1003.2 #06 sub‐
mitted for the ISO POSIX-2:1993 standard.
FUTURE DIRECTIONS
None.
SEE ALSO
Utility Description Defaults , sed , tr
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 DD(P)
