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dd: add count_bytes, skip_bytes and seek_bytes flags
dd now accepts the count_bytes and skip_bytes input flag and the seek_bytes output flag, to more easily allow processing portions of a file. * src/dd.c (scanargs): Compute skip_records and skip_bytes when 'skip_bytes' iflag is used. Compute max_records and max_bytes when 'count_bytes' iflag is used. Compute seek_records and seek_bytes when 'seek_bytes' oflag is used. (skip_via_lseek): Use new 'bytes' parameter and handle potential 'records' equals to zero. Update the bytes parameter when called with 'fdesc' equal to STDOUT_FILENO. Update the header comments. (dd_copy): Skip accordingly to skip_records AND skip_bytes. Count accordingly to max_records AND max_bytes. Seek on output accordingly to seek_records AND seek_bytes. * NEWS (New features): Mention it. * doc/coreutils.texi (dd invocation): Detail new flags and behaviors. * tests/dd/bytes: New file. Tests for these new flags. * tests/Makefile.am (TESTS): Add it.
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‎NEWS‎

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@@ -2,6 +2,11 @@ GNU coreutils NEWS -*- outline -*-
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* Noteworthy changes in release ?.? (????-??-??) [?]
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** New features
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dd now accepts the count_bytes, skip_bytes iflags and the count_bytes
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oflag, to more easily allow processing portions of a file.
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** Bug fixes
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mv now lets you move a symlink onto a same-inode destination file that

‎doc/coreutils.texi‎

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@@ -8057,18 +8057,24 @@ When converting variable-length records to fixed-length ones
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(@option{conv=block}) or the reverse (@option{conv=unblock}),
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use @var{bytes} as the fixed record length.
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@item skip=@var{blocks}
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@item skip=@var{n}
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@opindex skip
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Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
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Skip @var{n} @samp{ibs}-byte blocks in the input file before copying.
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If @samp{iflag=skip_bytes} is specified, @var{n} is interpreted
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as a byte count rather than a block count.
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@item seek=@var{blocks}
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@item seek=@var{n}
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@opindex seek
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Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
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Skip @var{n} @samp{obs}-byte blocks in the output file before copying.
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if @samp{oflag=seek_bytes} is specified, @var{n} is interpreted
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as a byte count rather than a block count.
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@item count=@var{blocks}
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@item count=@var{n}
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@opindex count
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Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
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Copy @var{n} @samp{ibs}-byte blocks from the input file, instead
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of everything until the end of the file.
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if @samp{iflag=count_bytes} is specified, @var{n} is interpreted
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as a byte count rather than a block count.
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@item status=noxfer
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@opindex status
@@ -8321,6 +8327,27 @@ When that happens, continue calling @code{read} to fill the remainder
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of the block.
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This flag can be used only with @code{iflag}.
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@item count_bytes
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@opindex count_bytes
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Interpret the @samp{count=} operand as a byte count,
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rather than a block count, which allows specifying
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a length that is not a multiple of the I/O block size.
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This flag can be used only with @code{iflag}.
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@item skip_bytes
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@opindex skip_bytes
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Interpret the @samp{skip=} operand as a byte count,
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rather than a block count, which allows specifying
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an offset that is not a multiple of the I/O block size.
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This flag can be used only with @code{iflag}.
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@item seek_bytes
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@opindex seek_bytes
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Interpret the @samp{seek=} operand as a byte count,
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rather than a block count, which allows specifying
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an offset that is not a multiple of the I/O block size.
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This flag can be used only with @code{oflag}.
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@end table
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These flags are not supported on all systems, and @samp{dd} rejects
@@ -8343,10 +8370,13 @@ should not be too large---values larger than a few megabytes
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are generally wasteful or (as in the gigabyte..exabyte case) downright
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counterproductive or error-inducing.
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Use different @command{dd} invocations to use different block sizes for
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skipping and I/O@. For example, the following shell commands copy data
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in 512 KiB blocks between a disk and a tape, but do not save or restore a
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4 KiB label at the start of the disk:
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To process data that is at an offset or size that is not a
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multiple of the I/O@ block size, you can use the @samp{skip_bytes},
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@samp{seek_bytes} and @samp{count_bytes} flags. Alternatively
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the traditional method of separate @command{dd} invocations can be used.
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For example, the following shell commands copy data
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in 512 KiB blocks between a disk and a tape, but do not save
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or restore a 4 KiB label at the start of the disk:
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@example
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disk=/dev/rdsk/c0t1d0s2

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