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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
DD(1P) POSIX Programmer's Manual DD(1P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
dd — convert and copy a file
dd [operand...]
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> characters 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.
None.
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. If the input file is
empty and either the size of the seek is greater than the
previous size of the output file or the output file did not
previously exist, the size of the output file shall be set
to the file offset after the seek.)
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> characters 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> characters 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 Table 4-7, ASCII to
EBCDIC Conversion.
ebcdic Convert ASCII to EBCDIC; see Table 4-7, ASCII to
EBCDIC Conversion.
ibm Convert ASCII to a different EBCDIC set; see Table
4-8, 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> characters
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> characters, 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>
characters, 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.
Table 4-7: ASCII to EBCDIC Conversion
Table 4-8: ASCII to IBM EBCDIC Conversion
If no if= operand is specified, the standard input shall be used. See
the INPUT FILES section.
The input file can be any file type.
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 POSIX.1‐2008, 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 Section 1.4, Utility
Description Defaults.
If no of= operand is specified, the standard output shall be used.
The nature of the output depends on the operands selected.
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 the of= operand is used, the output shall be the same as described
in the STDOUT section.
None.
The following exit values shall be returned:
0 The input file was copied successfully.
>0 An error occurred.
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.
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 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
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 POSIX.1‐2008, 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 POSIX.1‐2008.
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 ('¢') and 0152 (broken pipe) do not appear in the
table.
2. EBCDIC 0137 ('¬') 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 ('¬') is used.
4. 0255 ('[') and 0275 (']') appear twice, once in the same place as
for the standard table and once in place of 0112 ('¢') and 0241
('~').
In net result:
EBCDIC 0275 (']') displaced EBCDIC 0241 ('~') in cell 0345.
That displaced EBCDIC 0137 ('¬') 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 ('¢').
This translation, however, reflects historical practice that (ASCII)
'~' and '¬' were often mapped to each other, as were '[' and '¢'; 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> characters 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>
characters 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.
None.
Section 1.4, Utility Description Defaults, sed(1p), tr(1p)
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 DD(1P)
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