1DD(1P) POSIX Programmer's Manual DD(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 dd — convert and copy a file
13
15 dd [operand...]
16
18 The dd utility shall copy the specified input file to the specified
19 output file with possible conversions using specific input and output
20 block sizes. It shall read the input one block at a time, using the
21 specified input block size; it shall then process the block of data
22 actually returned, which could be smaller than the requested block
23 size. It shall apply any conversions that have been specified and write
24 the resulting data to the output in blocks of the specified output
25 block size. If the bs=expr operand is specified and no conversions
26 other than sync, noerror, or notrunc are requested, the data returned
27 from each input block shall be written as a separate output block; if
28 the read returns less than a full block and the sync conversion is not
29 specified, the resulting output block shall be the same size as the
30 input block. If the bs=expr operand is not specified, or a conversion
31 other than sync, noerror, or notrunc is requested, the input shall be
32 processed and collected into full-sized output blocks until the end of
33 the input is reached.
34 The processing order shall be as follows:
36 1. An input block is read.
38 2. If the input block is shorter than the specified input block size
40 and the sync conversion is specified, null bytes shall be appended
41 to the input data up to the specified size. (If either block or
42 unblock is also specified, <space> characters shall be appended
43 instead of null bytes.) The remaining conversions and output shall
44 include the pad characters as if they had been read from the input.
45 3. If the bs=expr operand is specified and no conversion other than
47 sync or noerror is requested, the resulting data shall be written
48 to the output as a single block, and the remaining steps are omit‐
49 ted.
50 4. If the swab conversion is specified, each pair of input data bytes
52 shall be swapped. If there is an odd number of bytes in the input
53 block, the last byte in the input record shall not be swapped.
54 5. Any remaining conversions (block, unblock, lcase, and ucase) shall
56 be performed. These conversions shall operate on the input data
57 independently of the input blocking; an input or output fixed-
58 length record may span block boundaries.
59 6. The data resulting from input or conversion or both shall be aggre‐
61 gated into output blocks of the specified size. After the end of
62 input is reached, any remaining output shall be written as a block
63 without padding if conv=sync is not specified; thus, the final out‐
64 put block may be shorter than the output block size.
65
67 None.
68
70 All of the operands shall be processed before any input is read. The
71 following operands shall be supported:
72 if=file Specify the input pathname; the default is standard input.
74 of=file Specify the output pathname; the default is standard output.
76 If the seek=expr conversion is not also specified, the output
77 file shall be truncated before the copy begins if an explicit
78 of=file operand is specified, unless conv=notrunc is speci‐
79 fied. If seek=expr is specified, but conv=notrunc is not, the
80 effect of the copy shall be to preserve the blocks in the
81 output file over which dd seeks, but no other portion of the
82 output file shall be preserved. (If the size of the seek plus
83 the size of the input file is less than the previous size of
84 the output file, the output file shall be shortened by the
85 copy. If the input file is empty and either the size of the
86 seek is greater than the previous size of the output file or
87 the output file did not previously exist, the size of the
88 output file shall be set to the file offset after the seek.)
89 ibs=expr Specify the input block size, in bytes, by expr (default is
91 512).
92 obs=expr Specify the output block size, in bytes, by expr (default is
94 512).
95 bs=expr Set both input and output block sizes to expr bytes, super‐
97 seding ibs= and obs=. If no conversion other than sync,
98 noerror, and notrunc is specified, each input block shall be
99 copied to the output as a single block without aggregating
100 short blocks.
101 cbs=expr Specify the conversion block size for block and unblock in
103 bytes by expr (default is zero). If cbs= is omitted or given
104 a value of zero, using block or unblock produces unspecified
105 results.
106 The application shall ensure that this operand is also speci‐
108 fied if the conv= operand is specified with a value of ascii,
109 ebcdic, or ibm. For a conv= operand with an ascii value, the
110 input is handled as described for the unblock value, except
111 that characters are converted to ASCII before any trailing
112 <space> characters are deleted. For conv= operands with
113 ebcdic or ibm values, the input is handled as described for
114 the block value except that the characters are converted to
115 EBCDIC or IBM EBCDIC, respectively, after any trailing
116 <space> characters are added.
117 skip=n Skip n input blocks (using the specified input block size)
119 before starting to copy. On seekable files, the implementa‐
120 tion shall read the blocks or seek past them; on non-seekable
121 files, the blocks shall be read and the data shall be dis‐
122 carded.
123 seek=n Skip n blocks (using the specified output block size) from
125 the beginning of the output file before copying. On non-seek‐
126 able files, existing blocks shall be read and space from the
127 current end-of-file to the specified offset, if any, filled
128 with null bytes; on seekable files, the implementation shall
129 seek to the specified offset or read the blocks as described
130 for non-seekable files.
131 count=n Copy only n input blocks. If n is zero, it is unspecified
133 whether no blocks or all blocks are copied.
134 conv=value[,value ...]
136 Where values are <comma>-separated symbols from the following
137 list:
138 ascii Convert EBCDIC to ASCII; see Table 4-7, ASCII to
140 EBCDIC Conversion.
141 ebcdic Convert ASCII to EBCDIC; see Table 4-7, ASCII to
143 EBCDIC Conversion.
144 ibm Convert ASCII to a different EBCDIC set; see Table
146 4-8, ASCII to IBM EBCDIC Conversion.
147 The ascii, ebcdic, and ibm values are mutually-exclusive.
149 block Treat the input as a sequence of <newline>-termi‐
151 nated or end-of-file-terminated variable-length
152 records independent of the input block boundaries.
153 Each record shall be converted to a record with a
154 fixed length specified by the conversion block size.
155 Any <newline> shall be removed from the input line;
156 <space> characters shall be appended to lines that
157 are shorter than their conversion block size to fill
158 the block. Lines that are longer than the conversion
159 block size shall be truncated to the largest number
160 of characters that fit into that size; the number of
161 truncated lines shall be reported (see the STDERR
162 section).
163 The block and unblock values are mutually-exclusive.
165 unblock Convert fixed-length records to variable length.
167 Read a number of bytes equal to the conversion block
168 size (or the number of bytes remaining in the input,
169 if less than the conversion block size), delete all
170 trailing <space> characters, and append a <newline>.
171 lcase Map uppercase characters specified by the LC_CTYPE
173 keyword tolower to the corresponding lowercase char‐
174 acter. Characters for which no mapping is specified
175 shall not be modified by this conversion.
176 The lcase and ucase symbols are mutually-exclusive.
178 ucase Map lowercase characters specified by the LC_CTYPE
180 keyword toupper to the corresponding uppercase char‐
181 acter. Characters for which no mapping is specified
182 shall not be modified by this conversion.
183 swab Swap every pair of input bytes.
185 noerror Do not stop processing on an input error. When an
187 input error occurs, a diagnostic message shall be
188 written on standard error, followed by the current
189 input and output block counts in the same format as
190 used at completion (see the STDERR section). If the
191 sync conversion is specified, the missing input
192 shall be replaced with null bytes and processed nor‐
193 mally; otherwise, the input block shall be omitted
194 from the output.
195 notrunc Do not truncate the output file. Preserve blocks in
197 the output file not explicitly written by this invo‐
198 cation of the dd utility. (See also the preceding
199 of=file operand.)
200 sync Pad every input block to the size of the ibs= buf‐
202 fer, appending null bytes. (If either block or
203 unblock is also specified, append <space> charac‐
204 ters, rather than null bytes.)
205 The behavior is unspecified if operands other than conv= are specified
207 more than once.
208 For the bs=, cbs=, ibs=, and obs= operands, the application shall sup‐
210 ply an expression specifying a size in bytes. The expression, expr, can
211 be:
212 1. A positive decimal number
214 2. A positive decimal number followed by k, specifying multiplication
216 by 1024
217 3. A positive decimal number followed by b, specifying multiplication
219 by 512
220 4. Two or more positive decimal numbers (with or without k or b) sepa‐
222 rated by x, specifying the product of the indicated values
223 All of the operands are processed before any input is read.
225 The following two tables display the octal number character values used
227 for the ascii and ebcdic conversions (first table) and for the ibm con‐
228 version (second table). In both tables, the ASCII values are the row
229 and column headers and the EBCDIC values are found at their intersec‐
230 tions. For example, ASCII 0012 (LF) is the second row, third column,
231 yielding 0045 in EBCDIC. The inverted tables (for EBCDIC to ASCII con‐
232 version) are not shown, but are in one-to-one correspondence with these
233 tables. The differences between the two tables are highlighted by small
234 boxes drawn around five entries.
235 Table 4-7: ASCII to EBCDIC Conversion
237 Table 4-8: ASCII to IBM EBCDIC Conversion
239
241 If no if= operand is specified, the standard input shall be used. See
242 the INPUT FILES section.
243
245 The input file can be any file type.
246
248 The following environment variables shall affect the execution of dd:
249 LANG Provide a default value for the internationalization vari‐
251 ables that are unset or null. (See the Base Definitions vol‐
252 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
253 ables for the precedence of internationalization variables
254 used to determine the values of locale categories.)
255 LC_ALL If set to a non-empty string value, override the values of
257 all the other internationalization variables.
258 LC_CTYPE Determine the locale for the interpretation of sequences of
260 bytes of text data as characters (for example, single-byte as
261 opposed to multi-byte characters in arguments and input
262 files), the classification of characters as uppercase or low‐
263 ercase, and the mapping of characters from one case to the
264 other.
265 LC_MESSAGES
267 Determine the locale that should be used to affect the format
268 and contents of diagnostic messages written to standard error
269 and informative messages written to standard output.
270 NLSPATH Determine the location of message catalogs for the processing
272 of LC_MESSAGES.
273
275 For SIGINT, the dd utility shall interrupt its current processing,
276 write status information to standard error, and exit as though termi‐
277 nated by SIGINT. It shall take the standard action for all other sig‐
278 nals; see the ASYNCHRONOUS EVENTS section in Section 1.4, Utility
279 Description Defaults.
280
282 If no of= operand is specified, the standard output shall be used. The
283 nature of the output depends on the operands selected.
284
286 On completion, dd shall write the number of input and output blocks to
287 standard error. In the POSIX locale the following formats shall be
288 used:
289 "%u+%u records in\n", <number of whole input blocks>,
292 <number of partial input blocks>
293 "%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
326 If an input error is detected and the noerror conversion has not been
327 specified, any partial output block shall be written to the output
328 file, a diagnostic message shall be written, and the copy operation
329 shall be discontinued. If some other error is detected, a diagnostic
330 message shall be written and the copy operation shall be discontinued.
331 The following sections are informative.
333
335 The input and output block size can be specified to take advantage of
336 raw physical I/O.
337 There are many different versions of the EBCDIC codesets. The ASCII and
339 EBCDIC conversions specified for the dd utility perform conversions for
340 the version specified by the tables.
341
343 The following command:
344 dd if=/dev/rmt0h of=/dev/rmt1h
347 copies from tape drive 0 to tape drive 1, using a common historical
349 device naming convention.
350 The following command:
352 dd ibs=10 skip=1
355 strips the first 10 bytes from standard input.
357 This example reads an EBCDIC tape blocked ten 80-byte EBCDIC card
359 images per block into the ASCII file x:
360 dd if=/dev/tape of=x ibs=800 cbs=80 conv=ascii,lcase
363
365 The OPTIONS section is listed as ``None'' because there are no options
366 recognized by historical dd utilities. Certainly, many of the operands
367 could have been designed to use the Utility Syntax Guidelines, which
368 would have resulted in the classic hyphenated option letters. In this
369 version of this volume of POSIX.1‐2017, dd retains its curious JCL-like
370 syntax due to the large number of applications that depend on the his‐
371 torical implementation.
372 A suggested implementation technique for conv=noerror,sync is to zero
374 (or <space>-fill, if blocking or unblocking) the input buffer before
375 each read and to write the contents of the input buffer to the output
376 even after an error. In this manner, any data transferred to the input
377 buffer before the error was detected is preserved. Another point is
378 that a failed read on a regular file or a disk generally does not
379 increment the file offset, and dd must then seek past the block on
380 which the error occurred; otherwise, the input error occurs repeti‐
381 tively. When the input is a magnetic tape, however, the tape normally
382 has passed the block containing the error when the error is reported,
383 and thus no seek is necessary.
384 The default ibs= and obs= sizes are specified as 512 bytes because
386 there are historical (largely portable) scripts that assume these val‐
387 ues. If they were left unspecified, unusual results could occur if an
388 implementation chose an odd block size.
389 Historical implementations of dd used creat() when processing of=file.
391 This makes the seek= operand unusable except on special files. The
392 conv=notrunc feature was added because more recent BSD-based implemen‐
393 tations use open() (without O_TRUNC) instead of creat(), but they fail
394 to delete output file contents after the data copied.
395 The w multiplier (historically meaning word), is used in System V to
397 mean 2 and in 4.2 BSD to mean 4. Since word is inherently non-portable,
398 its use is not supported by this volume of POSIX.1‐2017.
399 Standard EBCDIC does not have the characters '[' and ']'. The values
401 used in the table are taken from a common print train that does contain
402 them. Other than those characters, the print train values are not
403 filled in, but appear to provide some of the motivation for the histor‐
404 ical choice of translations reflected here.
405 The Standard EBCDIC table provides a 1:1 translation for all 256 bytes.
407 The IBM EBCDIC table does not provide such a translation. The marked
409 cells in the tables differ in such a way that:
410 1. EBCDIC 0112 ('¢') and 0152 (broken pipe) do not appear in the ta‐
412 ble.
413 2. EBCDIC 0137 ('¬') translates to/from ASCII 0236 ('^'). In the
415 standard table, EBCDIC 0232 (no graphic) is used.
416 3. EBCDIC 0241 ('~') translates to/from ASCII 0176 ('~'). In the
418 standard table, EBCDIC 0137 ('¬') is used.
419 4. 0255 ('[') and 0275 (']') appear twice, once in the same place as
421 for the standard table and once in place of 0112 ('¢') and 0241
422 ('~').
423 In net result:
425 EBCDIC 0275 (']') displaced EBCDIC 0241 ('~') in cell 0345.
427 That displaced EBCDIC 0137 ('¬') 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 ('¢').
435 This translation, however, reflects historical practice that (ASCII)
437 '~' and '¬' were often mapped to each other, as were '[' and '¢'; and
438 ']' 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> characters are deleted. For the
444 ebcdic and ibm operands, the input is handled as described for the
445 block operand except that the characters are converted to EBCDIC or IBM
446 EBCDIC after the trailing <space> characters 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 Section 1.4, Utility Description Defaults, sed, tr
473 The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
475 Variables
476
478 Portions of this text are reprinted and reproduced in electronic form
479 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
480 table Operating System Interface (POSIX), The Open Group Base Specifi‐
481 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
482 Electrical and Electronics Engineers, Inc and The Open Group. In the
483 event of any discrepancy between this version and the original IEEE and
484 The Open Group Standard, the original IEEE and The Open Group Standard
485 is the referee document. The original Standard can be obtained online
486 at http://www.opengroup.org/unix/online.html .
487 Any typographical or formatting errors that appear in this page are
489 most likely to have been introduced during the conversion of the source
490 files to man page format. To report such errors, see https://www.ker‐
491 nel.org/doc/man-pages/reporting_bugs.html .
492IEEE/The Open Group 2017 DD(1P)