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PROLOG
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.
NAME
dd — convert and copy a file
SYNOPSIS
dd [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> 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.
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 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
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 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.
ASYNCHRONOUS EVENTS
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.
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
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.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 1.4, Utility Description Defaults, sed, tr
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment Variables
COPYRIGHT
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 .
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