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)

PROLOG         top

       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         top

       dd — convert and copy a file

SYNOPSIS         top

       dd [operand...]

DESCRIPTION         top

       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         top

       None.

OPERANDS         top

       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         top

       If no if= operand is specified, the standard input shall be used. See
       the INPUT FILES section.

INPUT FILES         top

       The input file can be any file type.

ENVIRONMENT VARIABLES         top

       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         top

       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         top

       If no of= operand is specified, the standard output shall be used.
       The nature of the output depends on the operands selected.

STDERR         top

       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         top

       If the of= operand is used, the output shall be the same as described
       in the STDOUT section.

EXTENDED DESCRIPTION         top

       None.

EXIT STATUS         top

       The following exit values shall be returned:

        0    The input file was copied successfully.

       >0    An error occurred.

CONSEQUENCES OF ERRORS         top

       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         top

       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         top

       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         top

       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         top

       None.

SEE ALSO         top

       Section 1.4, Utility Description Defaults, sed(1p), tr(1p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
       Variables

COPYRIGHT         top

       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)