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 omitted.
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 aggregated
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.
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 output 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-seekable 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 toupper
to the corresponding uppercase character.
Characters for which no mapping is specified shall not be modified
by this conversion.
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
utility. (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 supply 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)
separated 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 conversion (second table). In both tables,
the ASCII values are the row and column headers and the
EBCDIC values are found at their intersections. For example, ASCII
0012 (LF) is the second row, third column, yielding 0045 in
EBCDIC. The inverted tables (for EBCDIC to ASCII conversion) 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.
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 Variables
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
.
For SIGINT, the dd utility shall interrupt its current processing,
write status information to standard error, and exit
as though terminated by SIGINT. It shall take the standard action
for all other signals; see the ASYNCHRONOUS EVENTS section in Utility
Description Defaults .
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.
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 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.
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 repetitively. 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 values. 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 implementations 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 historical 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.
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 operands 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
practice 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 submitted for the ISO POSIX-2:1993 standard.
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
DD (P)
2003
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