Provided by: manpages-posix_2017a-2_all 
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. If n is zero, it is unspecified whether no blocks or
all blocks are copied.
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.1‐2017, 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.1‐2017, 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.1‐2017.
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.1‐2017, Chapter 8, Environment Variables
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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 .