1DD(P) POSIX Programmer's Manual DD(P)
2
6 dd - convert and copy a file
7
9 dd [operand ...]
10
12 The dd utility shall copy the specified input file to the specified
13 output file with possible conversions using specific input and output
14 block sizes. It shall read the input one block at a time, using the
15 specified input block size; it shall then process the block of data
16 actually returned, which could be smaller than the requested block
17 size. It shall apply any conversions that have been specified and write
18 the resulting data to the output in blocks of the specified output
19 block size. If the bs= expr operand is specified and no conversions
20 other than sync, noerror, or notrunc are requested, the data returned
21 from each input block shall be written as a separate output block; if
22 the read returns less than a full block and the sync conversion is not
23 specified, the resulting output block shall be the same size as the
24 input block. If the bs= expr operand is not specified, or a conversion
25 other than sync, noerror, or notrunc is requested, the input shall be
26 processed and collected into full-sized output blocks until the end of
27 the input is reached.
28 The processing order shall be as follows:
30 1. An input block is read.
32 2. If the input block is shorter than the specified input block size
34 and the sync conversion is specified, null bytes shall be appended
35 to the input data up to the specified size. (If either block or
36 unblock is also specified, <space>s shall be appended instead of
37 null bytes.) The remaining conversions and output shall include the
38 pad characters as if they had been read from the input.
39 3. If the bs= expr operand is specified and no conversion other than
41 sync or noerror is requested, the resulting data shall be written
42 to the output as a single block, and the remaining steps are omit‐
43 ted.
44 4. If the swab conversion is specified, each pair of input data bytes
46 shall be swapped. If there is an odd number of bytes in the input
47 block, the last byte in the input record shall not be swapped.
48 5. Any remaining conversions ( block, unblock, lcase, and ucase) shall
50 be performed. These conversions shall operate on the input data
51 independently of the input blocking; an input or output fixed-
52 length record may span block boundaries.
53 6. The data resulting from input or conversion or both shall be aggre‐
55 gated into output blocks of the specified size. After the end of
56 input is reached, any remaining output shall be written as a block
57 without padding if conv= sync is not specified; thus, the final
58 output block may be shorter than the output block size.
59
61 None.
62
64 All of the operands shall be processed before any input is read. The
65 following operands shall be supported:
66 if=file
68 Specify the input pathname; the default is standard input.
69 of=file
71 Specify the output pathname; the default is standard output. If
72 the seek= expr conversion is not also specified, the output file
73 shall be truncated before the copy begins if an explicit of=
74 file operand is specified, unless conv= notrunc is specified. If
75 seek= expr is specified, but conv= notrunc is not, the effect of
76 the copy shall be to preserve the blocks in the output file over
77 which dd seeks, but no other portion of the output file shall be
78 preserved. (If the size of the seek plus the size of the input
79 file is less than the previous size of the output file, the out‐
80 put file shall be shortened by the copy.)
81 ibs=expr
83 Specify the input block size, in bytes, by expr (default is
84 512).
85 obs=expr
87 Specify the output block size, in bytes, by expr (default is
88 512).
89 bs=expr
91 Set both input and output block sizes to expr bytes, superseding
92 ibs= and obs=. If no conversion other than sync, noerror, and
93 notrunc is specified, each input block shall be copied to the
94 output as a single block without aggregating short blocks.
95 cbs=expr
97 Specify the conversion block size for block and unblock in bytes
98 by expr (default is zero). If cbs= is omitted or given a value
99 of zero, using block or unblock produces unspecified results.
100 The application shall ensure that this operand is also specified if the
102 conv= operand is specified with a value of ascii, ebcdic, or ibm. For a
103 conv= operand with an ascii value, the input is handled as described
104 for the unblock value, except that characters are converted to ASCII
105 before any trailing <space>s are deleted. For conv= operands with
106 ebcdic or ibm values, the input is handled as described for the block
107 value except that the characters are converted to EBCDIC or IBM EBCDIC,
108 respectively, after any trailing <space>s are added.
109 skip=n Skip n input blocks (using the specified input block size)
111 before starting to copy. On seekable files, the implementation
112 shall read the blocks or seek past them; on non-seekable files,
113 the blocks shall be read and the data shall be discarded.
114 seek=n Skip n blocks (using the specified output block size) from the
116 beginning of the output file before copying. On non-seekable
117 files, existing blocks shall be read and space from the current
118 end-of-file to the specified offset, if any, filled with null
119 bytes; on seekable files, the implementation shall seek to the
120 specified offset or read the blocks as described for non-seek‐
121 able files.
122 count=n
124 Copy only n input blocks.
125 conv=value[,value ...]
127 Where values are comma-separated symbols from the following
129 list:
130 ascii
132 Convert EBCDIC to ASCII; see ASCII to EBCDIC Conversion .
133 ebcdic
135 Convert ASCII to EBCDIC; see ASCII to EBCDIC Conversion .
136 ibm
138 Convert ASCII to a different EBCDIC set; see ASCII to IBM EBCDIC
139 Conversion .
140 The ascii, ebcdic, and ibm values are mutually-exclusive.
143 block
145 Treat the input as a sequence of <newline>-terminated or end-of-
146 file-terminated variable-length records independent of the input
147 block boundaries. Each record shall be converted to a record
148 with a fixed length specified by the conversion block size. Any
149 <newline> shall be removed from the input line; <space>s shall
150 be appended to lines that are shorter than their conversion
151 block size to fill the block. Lines that are longer than the
152 conversion block size shall be truncated to the largest number
153 of characters that fit into that size; the number of truncated
154 lines shall be reported (see the STDERR section).
155 The block and unblock values are mutually-exclusive.
157 unblock
159 Convert fixed-length records to variable length. Read a number
160 of bytes equal to the conversion block size (or the number of
161 bytes remaining in the input, if less than the conversion block
162 size), delete all trailing <space>s, and append a <newline>.
163 lcase
165 Map uppercase characters specified by the LC_CTYPE keyword
166 tolower to the corresponding lowercase character. Characters
167 for which no mapping is specified shall not be modified by this
168 conversion.
169 The lcase and ucase symbols are mutually-exclusive.
171 ucase
173 Map lowercase characters specified by the LC_CTYPE keyword toup‐
174 per to the corresponding uppercase character. Characters for
175 which no mapping is specified shall not be modified by this con‐
176 version.
177 swab
179 Swap every pair of input bytes.
180 noerror
182 Do not stop processing on an input error. When an input error
183 occurs, a diagnostic message shall be written on standard error,
184 followed by the current input and output block counts in the
185 same format as used at completion (see the STDERR section). If
186 the sync conversion is specified, the missing input shall be
187 replaced with null bytes and processed normally; otherwise, the
188 input block shall be omitted from the output.
189 notrunc
191 Do not truncate the output file. Preserve blocks in the output
192 file not explicitly written by this invocation of the dd util‐
193 ity. (See also the preceding of= file operand.)
194 sync
196 Pad every input block to the size of the ibs= buffer, appending
197 null bytes. (If either block or unblock is also specified,
198 append <space>s, rather than null bytes.)
199 The behavior is unspecified if operands other than conv= are specified
203 more than once.
204 For the bs=, cbs=, ibs=, and obs= operands, the application shall sup‐
206 ply an expression specifying a size in bytes. The expression, expr, can
207 be:
208 1. A positive decimal number
210 2. A positive decimal number followed by k, specifying multiplication
212 by 1024
213 3. A positive decimal number followed by b, specifying multiplication
215 by 512
216 4. Two or more positive decimal numbers (with or without k or b) sepa‐
218 rated by x, specifying the product of the indicated values
219 All of the operands are processed before any input is read.
221 The following two tables display the octal number character values used
223 for the ascii and ebcdic conversions (first table) and for the ibm con‐
224 version (second table). In both tables, the ASCII values are the row
225 and column headers and the EBCDIC values are found at their intersec‐
226 tions. For example, ASCII 0012 (LF) is the second row, third column,
227 yielding 0045 in EBCDIC. The inverted tables (for EBCDIC to ASCII con‐
228 version) are not shown, but are in one-to-one correspondence with these
229 tables. The differences between the two tables are highlighted by
230 small boxes drawn around five entries.
231 Table: ASCII to EBCDIC Conversion
233 Table: ASCII to IBM EBCDIC Conversion
236
238 If no if= operand is specified, the standard input shall be used. See
239 the INPUT FILES section.
240
242 The input file can be any file type.
243
245 The following environment variables shall affect the execution of dd:
246 LANG Provide a default value for the internationalization variables
248 that are unset or null. (See the Base Definitions volume of
249 IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
250 ables for the precedence of internationalization variables used
251 to determine the values of locale categories.)
252 LC_ALL If set to a non-empty string value, override the values of all
254 the other internationalization variables.
255 LC_CTYPE
257 Determine the locale for the interpretation of sequences of
258 bytes of text data as characters (for example, single-byte as
259 opposed to multi-byte characters in arguments and input files),
260 the classification of characters as uppercase or lowercase, and
261 the mapping of characters from one case to the other.
262 LC_MESSAGES
264 Determine the locale that should be used to affect the format
265 and contents of diagnostic messages written to standard error
266 and informative messages written to standard output.
267 NLSPATH
269 Determine the location of message catalogs for the processing of
270 LC_MESSAGES .
271
274 For SIGINT, the dd utility shall interrupt its current processing,
275 write status information to standard error, and exit as though termi‐
276 nated by SIGINT. It shall take the standard action for all other sig‐
277 nals; see the ASYNCHRONOUS EVENTS section in Utility Description
278 Defaults .
279
281 If no of= operand is specified, the standard output shall be used. The
282 nature of the output depends on the operands selected.
283
285 On completion, dd shall write the number of input and output blocks to
286 standard error. In the POSIX locale the following formats shall be
287 used:
288 "%u+%u records in\n", <number of whole input blocks>,
291 <number of partial input blocks>
292 "%u+%u records out\n", <number of whole output blocks>,
295 <number of partial output blocks>
296 A partial input block is one for which read() returned less than the
298 input block size. A partial output block is one that was written with
299 fewer bytes than specified by the output block size.
300 In addition, when there is at least one truncated block, the number of
302 truncated blocks shall be written to standard error. In the POSIX
303 locale, the format shall be:
304 "%u truncated %s\n", <number of truncated blocks>, "record" (if
307 <number of truncated blocks> is one) "records" (otherwise)
308 Diagnostic messages may also be written to standard error.
310
312 If the of= operand is used, the output shall be the same as described
313 in the STDOUT section.
314
316 None.
317
319 The following exit values shall be returned:
320 0 The input file was copied successfully.
322 >0 An error occurred.
324
327 If an input error is detected and the noerror conversion has not been
328 specified, any partial output block shall be written to the output
329 file, a diagnostic message shall be written, and the copy operation
330 shall be discontinued. If some other error is detected, a diagnostic
331 message shall be written and the copy operation shall be discontinued.
332 The following sections are informative.
334
336 The input and output block size can be specified to take advantage of
337 raw physical I/O.
338 There are many different versions of the EBCDIC codesets. The ASCII and
340 EBCDIC conversions specified for the dd utility perform conversions for
341 the version specified by the tables.
342
344 The following command:
345 dd if=/dev/rmt0h of=/dev/rmt1h
348 copies from tape drive 0 to tape drive 1, using a common historical
350 device naming convention.
351 The following command:
353 dd ibs=10 skip=1
356 strips the first 10 bytes from standard input.
358 This example reads an EBCDIC tape blocked ten 80-byte EBCDIC card
360 images per block into the ASCII file x:
361 dd if=/dev/tape of=x ibs=800 cbs=80 conv=ascii,lcase
364
366 The OPTIONS section is listed as "None" because there are no options
367 recognized by historical dd utilities. Certainly, many of the operands
368 could have been designed to use the Utility Syntax Guidelines, which
369 would have resulted in the classic hyphenated option letters. In this
370 version of this volume of IEEE Std 1003.1-2001, dd retains its curious
371 JCL-like syntax due to the large number of applications that depend on
372 the historical implementation.
373 A suggested implementation technique for conv= noerror, sync is to zero
375 (or <space>-fill, if blocking or unblocking) the input buffer before
376 each read and to write the contents of the input buffer to the output
377 even after an error. In this manner, any data transferred to the input
378 buffer before the error was detected is preserved. Another point is
379 that a failed read on a regular file or a disk generally does not
380 increment the file offset, and dd must then seek past the block on
381 which the error occurred; otherwise, the input error occurs repeti‐
382 tively. When the input is a magnetic tape, however, the tape normally
383 has passed the block containing the error when the error is reported,
384 and thus no seek is necessary.
385 The default ibs= and obs= sizes are specified as 512 bytes because
387 there are historical (largely portable) scripts that assume these val‐
388 ues. If they were left unspecified, unusual results could occur if an
389 implementation chose an odd block size.
390 Historical implementations of dd used creat() when processing of= file.
392 This makes the seek= operand unusable except on special files. The
393 conv= notrunc feature was added because more recent BSD-based implemen‐
394 tations use open() (without O_TRUNC) instead of creat(), but they fail
395 to delete output file contents after the data copied.
396 The w multiplier (historically meaning word), is used in System V to
398 mean 2 and in 4.2 BSD to mean 4. Since word is inherently non-portable,
399 its use is not supported by this volume of IEEE Std 1003.1-2001.
400 Standard EBCDIC does not have the characters '[' and ']' . The values
402 used in the table are taken from a common print train that does contain
403 them. Other than those characters, the print train values are not
404 filled in, but appear to provide some of the motivation for the histor‐
405 ical choice of translations reflected here.
406 The Standard EBCDIC table provides a 1:1 translation for all 256 bytes.
408 The IBM EBCDIC table does not provide such a translation. The marked
410 cells in the tables differ in such a way that:
411 1. EBCDIC 0112 ( 'cent' ) and 0152 (broken pipe) do not appear in the
413 table.
414 2. EBCDIC 0137 ( 'not' ) translates to/from ASCII 0236 ( '^' ). In the
416 standard table, EBCDIC 0232 (no graphic) is used.
417 3. EBCDIC 0241 ( '~' ) translates to/from ASCII 0176 ( '~' ). In the
419 standard table, EBCDIC 0137 ( 'not' ) is used.
420 4. 0255 ( '[' ) and 0275 ( ']' ) appear twice, once in the same place
422 as for the standard table and once in place of 0112 ( 'cent' ) and
423 0241 ( '~' ).
424 In net result: EBCDIC 0275 ( ']' ) displaced EBCDIC 0241 ( '~' ) in
426 cell 0345.
427 That displaced EBCDIC 0137 ( 'not' ) in cell 0176.
429 That displaced EBCDIC 0232 (no graphic) in cell 0136.
431 That replaced EBCDIC 0152 (broken pipe) in cell 0313.
433 EBCDIC 0255 ( '[' ) replaced EBCDIC 0112 ( 'cent' ).
435 This translation, however, reflects historical practice that (ASCII)
437 '~' and 'not' were often mapped to each other, as were '[' and 'cent' ;
438 and ']' and (EBCDIC) '~' .
439 The cbs operand is required if any of the ascii, ebcdic, or ibm oper‐
441 ands are specified. For the ascii operand, the input is handled as
442 described for the unblock operand except that characters are converted
443 to ASCII before the trailing <space>s are deleted. For the ebcdic and
444 ibm operands, the input is handled as described for the block operand
445 except that the characters are converted to EBCDIC or IBM EBCDIC after
446 the trailing <space>s are added.
447 The block and unblock keywords are from historical BSD practice.
449 The consistent use of the word record in standard error messages
451 matches most historical practice. An earlier version of System V used
452 block, but this has been updated in more recent releases.
453 Early proposals only allowed two numbers separated by x to be used in a
455 product when specifying bs=, cbs=, ibs=, and obs= sizes. This was
456 changed to reflect the historical practice of allowing multiple numbers
457 in the product as provided by Version 7 and all releases of System V
458 and BSD.
459 A change to the swab conversion is required to match historical prac‐
461 tice and is the result of IEEE PASC Interpretations 1003.2 #03 and #04,
462 submitted for the ISO POSIX-2:1993 standard.
463 A change to the handling of SIGINT is required to match historical
465 practice and is the result of IEEE PASC Interpretation 1003.2 #06 sub‐
466 mitted for the ISO POSIX-2:1993 standard.
467
469 None.
470
472 Utility Description Defaults , sed , tr
473
475 Portions of this text are reprinted and reproduced in electronic form
476 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
477 -- Portable Operating System Interface (POSIX), The Open Group Base
478 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
479 Electrical and Electronics Engineers, Inc and The Open Group. In the
480 event of any discrepancy between this version and the original IEEE and
481 The Open Group Standard, the original IEEE and The Open Group Standard
482 is the referee document. The original Standard can be obtained online
483 at http://www.opengroup.org/unix/online.html .
484IEEE/The Open Group 2003 DD(P)