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File: coreutils.info,  Node: dd invocation,  Next: install invocation,  Prev: cp invocation,  Up: Basic operations

11.2 'dd': Convert and copy a file

'dd' copies a file (from standard input to standard output, by default)
with a changeable I/O block size, while optionally performing
conversions on it.  Synopses:

     dd [OPERAND]...
     dd OPTION

   The only options are '--help' and '--version'.  *Note Common
options::.  'dd' accepts the following operands, whose syntax was
inspired by the DD (data definition) statement of OS/360 JCL.

     Read from FILE instead of standard input.

     Write to FILE instead of standard output.  Unless 'conv=notrunc' is
     given, 'dd' truncates FILE to zero bytes (or the size specified
     with 'seek=').

     Set the input block size to BYTES.  This makes 'dd' read BYTES per
     block.  The default is 512 bytes.

     Set the output block size to BYTES.  This makes 'dd' write BYTES
     per block.  The default is 512 bytes.

     Set both input and output block sizes to BYTES.  This makes 'dd'
     read and write BYTES per block, overriding any 'ibs' and 'obs'
     settings.  In addition, if no data-transforming 'conv' option is
     specified, input is copied to the output as soon as it's read, even
     if it is smaller than the block size.

     Set the conversion block size to BYTES.  When converting
     variable-length records to fixed-length ones ('conv=block') or the
     reverse ('conv=unblock'), use BYTES as the fixed record length.

     Skip N 'ibs'-byte blocks in the input file before copying.  If
     'iflag=skip_bytes' is specified, N is interpreted as a byte count
     rather than a block count.

     Skip N 'obs'-byte blocks in the output file before copying.  if
     'oflag=seek_bytes' is specified, N is interpreted as a byte count
     rather than a block count.

     Copy N 'ibs'-byte blocks from the input file, instead of everything
     until the end of the file.  if 'iflag=count_bytes' is specified, N
     is interpreted as a byte count rather than a block count.  Note if
     the input may return short reads as could be the case when reading
     from a pipe for example, 'iflag=fullblock' will ensure that
     'count=' corresponds to complete input blocks rather than the
     traditional POSIX specified behavior of counting input read

     Transfer information is normally output to stderr upon receipt of
     the 'INFO' signal or when 'dd' exits.  Specifying LEVEL will adjust
     the amount of information printed, with the last LEVEL specified
     taking precedence.

          Do not print any informational or warning messages to stderr.
          Error messages are output as normal.

          Do not print the final transfer rate and volume statistics
          that normally make up the last status line.

          Print the transfer rate and volume statistics on stderr, when
          processing each input block.  Statistics are output on a
          single line at most once every second, but updates can be
          delayed when waiting on I/O.

     Convert the file as specified by the CONVERSION argument(s).  (No
     spaces around any comma(s).)


          Convert EBCDIC to ASCII, using the conversion table specified
          by POSIX.  This provides a 1:1 translation for all 256 bytes.
          This option implies 'conv=unblock'; input is converted to
          ASCII before trailing spaces are deleted.

          Convert ASCII to EBCDIC.  This is the inverse of the 'ascii'
          conversion.  This option implies 'conv=block'; trailing spaces
          are added before being converted to EBCDIC.

          This acts like 'conv=ebcdic', except it uses the alternate
          conversion table specified by POSIX.  This is not a 1:1
          translation, but reflects common historical practice for '~',
          '[', and ']'.

          The 'ascii', 'ebcdic', and 'ibm' conversions are mutually
          exclusive.  If you use any of these options, you should also
          use the 'cbs=' option.

          For each line in the input, output 'cbs' bytes, replacing the
          input newline with a space and padding with spaces as

          Remove any trailing spaces in each 'cbs'-sized input block,
          and append a newline.

          The 'block' and 'unblock' conversions are mutually exclusive.

          Change uppercase letters to lowercase.

          Change lowercase letters to uppercase.

          The 'lcase' and 'ucase' conversions are mutually exclusive.

          Try to seek rather than write NUL output blocks.  On a file
          system that supports sparse files, this will create sparse
          output when extending the output file.  Be careful when using
          this option in conjunction with 'conv=notrunc' or
          'oflag=append'.  With 'conv=notrunc', existing data in the
          output file corresponding to NUL blocks from the input, will
          be untouched.  With 'oflag=append' the seeks performed will be
          ineffective.  Similarly, when the output is a device rather
          than a file, NUL input blocks are not copied, and therefore
          this option is most useful with virtual or pre zeroed devices.

          Swap every pair of input bytes.  GNU 'dd', unlike others,
          works when an odd number of bytes are read--the last byte is
          simply copied (since there is nothing to swap it with).

          Pad every input block to size of 'ibs' with trailing zero
          bytes.  When used with 'block' or 'unblock', pad with spaces
          instead of zero bytes.

     The following "conversions" are really file flags and don't affect
     internal processing:

          Fail if the output file already exists; 'dd' must create the
          output file itself.

          Do not create the output file; the output file must already

          The 'excl' and 'nocreat' conversions are mutually exclusive.

          Do not truncate the output file.

          Continue after read errors.

          Synchronize output data just before finishing.  This forces a
          physical write of output data.

          Synchronize output data and metadata just before finishing.
          This forces a physical write of output data and metadata.

     Access the input file using the flags specified by the FLAG
     argument(s).  (No spaces around any comma(s).)

     Access the output file using the flags specified by the FLAG
     argument(s).  (No spaces around any comma(s).)

     Here are the flags.  Not every flag is supported on every operating

          Write in append mode, so that even if some other process is
          writing to this file, every 'dd' write will append to the
          current contents of the file.  This flag makes sense only for
          output.  If you combine this flag with the 'of=FILE' operand,
          you should also specify 'conv=notrunc' unless you want the
          output file to be truncated before being appended to.

          Use concurrent I/O mode for data.  This mode performs direct
          I/O and drops the POSIX requirement to serialize all I/O to
          the same file.  A file cannot be opened in CIO mode and with a
          standard open at the same time.

          Use direct I/O for data, avoiding the buffer cache.  Note that
          the kernel may impose restrictions on read or write buffer
          sizes.  For example, with an ext4 destination file system and
          a Linux-based kernel, using 'oflag=direct' will cause writes
          to fail with 'EINVAL' if the output buffer size is not a
          multiple of 512.


          Fail unless the file is a directory.  Most operating systems
          do not allow I/O to a directory, so this flag has limited

          Use synchronized I/O for data.  For the output file, this
          forces a physical write of output data on each write.  For the
          input file, this flag can matter when reading from a remote
          file that has been written to synchronously by some other
          process.  Metadata (e.g., last-access and last-modified time)
          is not necessarily synchronized.

          Use synchronized I/O for both data and metadata.

          Request to discard the system data cache for a file.  When
          count=0 all cached data for the file is specified, otherwise
          the cache is dropped for the processed portion of the file.
          Also when count=0, failure to discard the cache is diagnosed
          and reflected in the exit status.

          Note data that is not already persisted to storage will not be
          discarded from cache, so note the use of the "sync" options in
          the examples below, which are used to maximize the
          effectiveness of the 'nocache' flag.

          Here are some usage examples:

               # Advise to drop cache for whole file
               dd if=ifile iflag=nocache count=0

               # Ensure drop cache for the whole file
               dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0

               # Drop cache for part of file
               dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null

               # Stream data using just the read-ahead cache.
               # See also the 'direct' flag.
               dd if=ifile of=ofile iflag=nocache oflag=nocache,sync

          Use non-blocking I/O.

          Do not update the file's access timestamp.  *Note File
          timestamps::.  Some older file systems silently ignore this
          flag, so it is a good idea to test it on your files before
          relying on it.

          Do not assign the file to be a controlling terminal for 'dd'.
          This has no effect when the file is not a terminal.  On many
          hosts (e.g., GNU/Linux hosts), this option has no effect at

          Do not follow symbolic links.

          Fail if the file has multiple hard links.

          Use binary I/O.  This option has an effect only on nonstandard
          platforms that distinguish binary from text I/O.

          Use text I/O.  Like 'binary', this option has no effect on
          standard platforms.

          Accumulate full blocks from input.  The 'read' system call may
          return early if a full block is not available.  When that
          happens, continue calling 'read' to fill the remainder of the
          block.  This flag can be used only with 'iflag'.  This flag is
          useful with pipes for example as they may return short reads.
          In that case, this flag is needed to ensure that a 'count='
          argument is interpreted as a block count rather than a count
          of read operations.

          Interpret the 'count=' operand as a byte count, rather than a
          block count, which allows specifying a length that is not a
          multiple of the I/O block size.  This flag can be used only
          with 'iflag'.

          Interpret the 'skip=' operand as a byte count, rather than a
          block count, which allows specifying an offset that is not a
          multiple of the I/O block size.  This flag can be used only
          with 'iflag'.

          Interpret the 'seek=' operand as a byte count, rather than a
          block count, which allows specifying an offset that is not a
          multiple of the I/O block size.  This flag can be used only
          with 'oflag'.

     These flags are not supported on all systems, and 'dd' rejects
     attempts to use them when they are not supported.  When reading
     from standard input or writing to standard output, the 'nofollow'
     and 'noctty' flags should not be specified, and the other flags
     (e.g., 'nonblock') can affect how other processes behave with the
     affected file descriptors, even after 'dd' exits.

   The numeric-valued strings above (N and BYTES) can be followed by a
multiplier: 'b'=512, 'c'=1, 'w'=2, 'xM'=M, or any of the standard block
size suffixes like 'k'=1024 (*note Block size::).

   Any block size you specify via 'bs=', 'ibs=', 'obs=', 'cbs=' should
not be too large--values larger than a few megabytes are generally
wasteful or (as in the gigabyte..exabyte case) downright
counterproductive or error-inducing.

   To process data that is at an offset or size that is not a multiple
of the I/O block size, you can use the 'skip_bytes', 'seek_bytes' and
'count_bytes' flags.  Alternatively the traditional method of separate
'dd' invocations can be used.  For example, the following shell commands
copy data in 512 KiB blocks between a disk and a tape, but do not save
or restore a 4 KiB label at the start of the disk:


     # Copy all but the label from disk to tape.
     (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape

     # Copy from tape back to disk, but leave the disk label alone.
     (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk

   For failing disks, other tools come with a great variety of extra
functionality to ease the saving of as much data as possible before the
disk finally dies, e.g.  GNU 'ddrescue'
(https://www.gnu.org/software/ddrescue/).  However, in some cases such a
tool is not available or the administrator feels more comfortable with
the handling of 'dd'.  As a simple rescue method, call 'dd' as shown in
the following example: the options 'conv=noerror,sync' are used to
continue after read errors and to pad out bad reads with NULs, while
'iflag=fullblock' caters for short reads (which traditionally never
occur on disk based devices):

     # Rescue data from an (unmounted!) partition of a failing disk.
     dd conv=noerror,sync iflag=fullblock </dev/sda1 > /mnt/rescue.img

   Sending an 'INFO' signal (or 'USR1' signal where that is unavailable)
to a running 'dd' process makes it print I/O statistics to standard
error and then resume copying.  In the example below, 'dd' is run in the
background to copy 5GB of data.  The 'kill' command makes it output
intermediate I/O statistics, and when 'dd' completes normally or is
killed by the 'SIGINT' signal, it outputs the final statistics.

     # Ignore the signal so we never inadvertently terminate the dd child.
     # Note this is not needed when SIGINFO is available.
     trap '' USR1

     # Run dd with the fullblock iflag to avoid short reads
     # which can be triggered by reception of signals.
     dd iflag=fullblock if=/dev/zero of=/dev/null count=5000000 bs=1000 & pid=$!

     # Output stats every second.
     while kill -s USR1 $pid 2>/dev/null; do sleep 1; done

   The above script will output in the following format:

     3441325+0 records in
     3441325+0 records out
     3441325000 bytes (3.4 GB, 3.2 GiB) copied, 1.00036 s, 3.4 GB/s
     5000000+0 records in
     5000000+0 records out
     5000000000 bytes (5.0 GB, 4.7 GiB) copied, 1.44433 s, 3.5 GB/s

   The 'status=progress' option periodically updates the last line of
the transfer statistics above.

   On systems lacking the 'INFO' signal 'dd' responds to the 'USR1'
signal instead, unless the 'POSIXLY_CORRECT' environment variable is

   An exit status of zero indicates success, and a nonzero value
indicates failure.

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