719d3b44b7
UPDATE_COPYRIGHT_USE_INTERVALS=1 \
gnulib/build-aux/update-copyright \
$(git ls-files | sed -e '/^gnulib$/d
/^paxutils$/d
/^COPYING$/d
/\/fdl.texi$/d')
133 lines
3.7 KiB
Plaintext
133 lines
3.7 KiB
Plaintext
@c This is part of the paxutils manual.
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@c Copyright (C) 2006--2023 Free Software Foundation, Inc.
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@c Written by Sergey Poznyakoff
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@c This file is distributed under GFDL 1.1 or any later version
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@c published by the Free Software Foundation.
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Incremental archives keep information about contents of each
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dumped directory in special data blocks called @dfn{dumpdirs}.
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Dumpdir is a sequence of entries of the following form:
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@smallexample
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@var{C} @var{filename} \0
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@end smallexample
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@noindent
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where @var{C} is one of the @dfn{control codes} described below,
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@var{filename} is the name of the file @var{C} operates upon, and
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@samp{\0} represents a nul character (ASCII 0). The white space
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characters were added for readability, real dumpdirs do not contain
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them.
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Each dumpdir ends with a single nul character.
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The following table describes control codes and their meanings:
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@table @samp
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@item Y
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@var{filename} is contained in the archive.
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@item N
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@var{filename} was present in the directory at the time the archive
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was made, yet it was not dumped to the archive, because it had not
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changed since the last backup.
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@item D
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@var{filename} is a directory.
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@item R
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This code requests renaming of the @var{filename} to the name
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specified with the @samp{T} command, that immediately follows it.
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@item T
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Specify target file name for @samp{R} command (see below).
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@item X
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Specify @dfn{temporary directory} name for a rename operation (see below).
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@end table
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Codes @samp{Y}, @samp{N} and @samp{D} require @var{filename} argument
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to be a relative file name to the directory this dumpdir describes,
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whereas codes @samp{R}, @samp{T} and @samp{X} require their argument
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to be an absolute file name.
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The three codes @samp{R}, @samp{T} and @samp{X} specify a
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@dfn{renaming operation}. In the simplest case it is:
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@smallexample
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R@file{source}\0T@file{dest}\0
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@end smallexample
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@noindent
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which means ``rename file @file{source} to file @file{dest}''.
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However, there are cases that require using a @dfn{temporary
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directory}. For example, consider the following scenario:
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@enumerate 1
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@item
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Previous run dumped a directory @file{foo} which contained the
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following three directories:
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@smallexample
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a
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b
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c
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@end smallexample
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@item
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They were renamed @emph{cyclically}, so that:
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@example
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@file{a} became @file{b}
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@file{b} became @file{c}
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@file{c} became @file{a}
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@end example
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@item
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New incremental dump was made.
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@end enumerate
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This case cannot be handled by three successive renames, since
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renaming @file{a} to @file{b} will destroy the existing directory.
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To correctly process it, @GNUTAR{} needs a temporary directory, so
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it creates the following dumpdir (newlines have been added for
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readability):
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@smallexample
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@group
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Xfoo\0
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Rfoo/a\0T\0
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Rfoo/b\0Tfoo/c\0
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Rfoo/c\0Tfoo/a\0
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R\0Tfoo/a\0
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@end group
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@end smallexample
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The first command, @samp{Xfoo\0}, instructs the extractor to create a
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temporary directory in the directory @file{foo}. Second command,
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@samp{Rfoo/aT\0}, says ``rename file @file{foo/a} to the temporary
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directory that has just been created'' (empty file name after a
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command means use temporary directory). Third and fourth commands
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work as usual, and, finally, the last command, @samp{R\0Tfoo/a\0}
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tells tar to rename the temporary directory to @file{foo/a}.
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The exact placement of a dumpdir in the archive depends on the
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archive format (@pxref{Formats}):
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@itemize
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@item PAX archives
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In PAX archives, dumpdir is stored in the extended header of the
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corresponding directory, in variable @code{GNU.dumpdir}.
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@item GNU and old GNU archives
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These formats implement special header type @samp{D}, which is similar
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to ustar header @samp{5} (directory), except that it precedes a data
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block containing the dumpdir.
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@end itemize
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@c End of dumpdir.texi
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