+@opindex block-number
+@cindex Block number where error occurred
+@anchor{block-number}
+If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
+every message it would normally produce, the block number within the
+archive where the message was triggered. Also, supplementary messages
+are triggered when reading blocks full of NULs, or when hitting end of
+file on the archive. As of now, if the archive is properly terminated
+with a NUL block, the reading of the file may stop before end of file
+is met, so the position of end of file will not usually show when
+@option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
+drains the archive before exiting when reading the
+archive from a pipe.
+
+@cindex Error message, block number of
+This option is especially useful when reading damaged archives, since
+it helps pinpoint the damaged sections. It can also be used with
+@option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
+choose among several backup tapes when retrieving a file later, in
+favor of the tape where the file appears earliest (closest to the
+front of the tape). @xref{backup}.
+
+@node checkpoints
+@section Checkpoints
+@cindex checkpoints, defined
+@opindex checkpoint
+@opindex checkpoint-action
+
+A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
+the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
+from the archive (a @dfn{read checkpoint}). Checkpoints allow to
+periodically execute arbitrary actions.
+
+The checkpoint facility is enabled using the following option:
+
+@table @option
+@xopindex{checkpoint, defined}
+@item --checkpoint[=@var{n}]
+Schedule checkpoints before writing or reading each @var{n}th record.
+The default value for @var{n} is 10.
+@end table
+
+A list of arbitrary @dfn{actions} can be executed at each checkpoint.
+These actions include: pausing, displaying textual messages, and
+executing arbitrary external programs. Actions are defined using
+the @option{--checkpoint-action} option.
+
+@table @option
+@xopindex{checkpoint-action, defined}
+@item --checkpoint-action=@var{action}
+Execute an @var{action} at each checkpoint.
+@end table
+
+@cindex @code{echo}, checkpoint action
+The simplest value of @var{action} is @samp{echo}. It instructs
+@command{tar} to display the default message on the standard error
+stream upon arriving at each checkpoint. The default message is (in
+@acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
+checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
+Here, @var{n} represents ordinal number of the checkpoint.
+
+In another locales, translated versions of this message are used.
+
+This is the default action, so running:
+
+@smallexample
+$ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
+@end smallexample
+
+@noindent
+is equivalent to:
+
+@smallexample
+$ @kbd{tar -c --checkpoint=1000} /var
+@end smallexample
+
+The @samp{echo} action also allows to supply a customized message.
+You do so by placing an equals sign and the message right after it,
+e.g.:
+
+@smallexample
+--checkpoint-action="echo=Hit %s checkpoint #%u"
+@end smallexample
+
+The @samp{%s} and @samp{%u} in the above example are
+@dfn{format specifiers}. The @samp{%s} specifier is replaced with
+the @dfn{type} of the checkpoint: @samp{write} or
+@samp{read} (or a corresponding translated version in locales other
+than @acronym{POSIX}). The @samp{%u} specifier is replaced with
+the ordinal number of the checkpoint. Thus, the above example could
+produce the following output when used with the @option{--create}
+option:
+
+@smallexample
+tar: Hit write checkpoint #10
+tar: Hit write checkpoint #20
+tar: Hit write checkpoint #30
+@end smallexample
+
+The complete list of available format specifiers follows. Some of
+them can take optional arguments. These arguments, if given, are
+supplied in curly braces between the percent sign and the specifier
+letter.
+
+@table @samp
+@item %s
+Print type of the checkpoint (@samp{write} or @samp{read}).
+
+@item %u
+Print number of the checkpoint.
+
+@item %@{r,w,d@}T
+Print number of bytes transferred so far and approximate transfer
+speed. Optional arguments supply prefixes to be used before number
+of bytes read, written and deleted, correspondingly. If absent,
+they default to @samp{R}. @samp{W}, @samp{D}. Any or all of them can
+be omitted, so, that e.g. @samp{%@{@}T} means to print corresponding
+statistics without any prefixes. Any surplus arguments, if present,
+are silently ignored.
+
+@example
+$ @kbd{tar --delete -f f.tar --checkpoint-action=echo="#%u: %T" main.c}
+tar: #1: R: 0 (0B, 0B/s),W: 0 (0B, 0B/s),D: 0
+tar: #2: R: 10240 (10KiB, 19MiB/s),W: 0 (0B, 0B/s),D: 10240
+@end example
+
+@noindent
+See also the @samp{totals} action, described below.
+
+@item %@{@var{fmt}@}t
+Output current local time using @var{fmt} as format for @command{strftime}
+(@pxref{strftime, strftime,,strftime(3), strftime(3) man page}). The
+@samp{@{@var{fmt}@}} part is optional. If not present, the default
+format is @samp{%c}, i.e. the preferred date and time representation
+for the current locale.
+
+@item %@{@var{n}@}*
+Pad output with spaces to the @var{n}th column. If the
+@samp{@{@var{n}@}} part is omitted, the current screen width
+is assumed.
+
+@item %c
+This is a shortcut for @samp{%@{%Y-%m-%d %H:%M:%S@}t: %ds, %@{read,wrote@}T%*\r},
+intended mainly for use with @samp{ttyout} action (see below).
+@end table
+
+Aside from format expansion, the message string is subject to
+@dfn{unquoting}, during which the backslash @dfn{escape sequences} are
+replaced with their corresponding @acronym{ASCII} characters
+(@pxref{escape sequences}). E.g. the following action will produce an
+audible bell and the message described above at each checkpoint:
+
+@smallexample
+--checkpoint-action='echo=\aHit %s checkpoint #%u'
+@end smallexample
+
+@cindex @code{bell}, checkpoint action
+There is also a special action which produces an audible signal:
+@samp{bell}. It is not equivalent to @samp{echo='\a'}, because
+@samp{bell} sends the bell directly to the console (@file{/dev/tty}),
+whereas @samp{echo='\a'} sends it to the standard error.
+
+@cindex @code{ttyout}, checkpoint action
+The @samp{ttyout=@var{string}} action outputs @var{string} to
+@file{/dev/tty}, so it can be used even if the standard output is
+redirected elsewhere. The @var{string} is subject to the same
+modifications as with @samp{echo} action. In contrast to the latter,
+@samp{ttyout} does not prepend @command{tar} executable name to the
+string, nor does it output a newline after it. For example, the
+following action will print the checkpoint message at the same screen
+line, overwriting any previous message:
+
+@smallexample
+--checkpoint-action="ttyout=Hit %s checkpoint #%u%*\r"
+@end smallexample
+
+@noindent
+Notice the use of @samp{%*} specifier to clear out any eventual
+remains of the prior output line. As as more complex example,
+consider this:
+
+@smallexample
+--checkpoint-action=ttyout='%@{%Y-%m-%d %H:%M:%S@}t (%d sec): #%u, %T%*\r'
+@end smallexample
+
+@noindent
+This prints the current local time, number of seconds expired since
+tar was started, the checkpoint ordinal number, transferred bytes and
+average computed I/O speed.
+
+@cindex @code{dot}, checkpoint action
+Another available checkpoint action is @samp{dot} (or @samp{.}). It
+instructs @command{tar} to print a single dot on the standard listing
+stream, e.g.:
+
+@smallexample
+$ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
+...
+@end smallexample
+
+For compatibility with previous @GNUTAR{} versions, this action can
+be abbreviated by placing a dot in front of the checkpoint frequency,
+as shown in the previous section.
+
+@cindex @code{totals}, checkpoint action
+The @samp{totals} action prints the total number of bytes transferred
+so far. The format of the data is the same as for the
+@option{--totals} option (@pxref{totals}). See also @samp{%T} format
+specifier of the @samp{echo} or @samp{ttyout} action.
+
+@cindex @code{sleep}, checkpoint action
+Yet another action, @samp{sleep}, pauses @command{tar} for a specified
+amount of seconds. The following example will stop for 30 seconds at each
+checkpoint:
+
+@smallexample
+$ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
+@end smallexample
+
+@anchor{checkpoint exec}
+@cindex @code{exec}, checkpoint action
+Finally, the @code{exec} action executes a given external command.
+For example:
+
+@smallexample
+$ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
+@end smallexample
+
+The supplied command can be any valid command invocation, with or
+without additional command line arguments. If it does contain
+arguments, don't forget to quote it to prevent it from being split by
+the shell. @xref{external, Running External Commands}, for more detail.
+
+The command gets a copy of @command{tar}'s environment plus the
+following variables:
+
+@table @env
+@vrindex TAR_VERSION, checkpoint script environment
+@item TAR_VERSION
+@GNUTAR{} version number.
+
+@vrindex TAR_ARCHIVE, checkpoint script environment
+@item TAR_ARCHIVE
+The name of the archive @command{tar} is processing.
+
+@vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
+@item TAR_BLOCKING_FACTOR
+Current blocking factor (@pxref{Blocking}).
+
+@vrindex TAR_CHECKPOINT, checkpoint script environment
+@item TAR_CHECKPOINT
+Number of the checkpoint.
+
+@vrindex TAR_SUBCOMMAND, checkpoint script environment
+@item TAR_SUBCOMMAND
+A short option describing the operation @command{tar} is executing.
+@xref{Operations}, for a complete list of subcommand options.
+
+@vrindex TAR_FORMAT, checkpoint script environment
+@item TAR_FORMAT
+Format of the archive being processed. @xref{Formats}, for a complete
+list of archive format names.
+@end table
+
+These environment variables can also be passed as arguments to the
+command, provided that they are properly escaped, for example:
+
+@smallexample
+@kbd{tar -c -f arc.tar \
+ --checkpoint-action='exec=/sbin/cpoint $TAR_FILENAME'}
+@end smallexample
+
+@noindent
+Notice single quotes to prevent variable names from being expanded by
+the shell when invoking @command{tar}.
+
+Any number of actions can be defined, by supplying several
+@option{--checkpoint-action} options in the command line. For
+example, the command below displays two messages, pauses
+execution for 30 seconds and executes the @file{/sbin/cpoint} script:
+
+@example
+@group
+$ @kbd{tar -c -f arc.tar \
+ --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
+ --checkpoint-action='echo=Sleeping for 30 seconds' \
+ --checkpoint-action='sleep=30' \
+ --checkpoint-action='exec=/sbin/cpoint'}
+@end group
+@end example
+
+This example also illustrates the fact that
+@option{--checkpoint-action} can be used without
+@option{--checkpoint}. In this case, the default checkpoint frequency
+(at each 10th record) is assumed.
+
+@node warnings
+@section Controlling Warning Messages
+
+Sometimes, while performing the requested task, @GNUTAR{} notices
+some conditions that are not exactly errors, but which the user
+should be aware of. When this happens, @command{tar} issues a
+@dfn{warning message} describing the condition. Warning messages
+are output to the standard error and they do not affect the exit
+code of @command{tar} command.
+
+@xopindex{warning, explained}
+@GNUTAR{} allows the user to suppress some or all of its warning
+messages:
+
+@table @option
+@item --warning=@var{keyword}
+Control display of the warning messages identified by @var{keyword}.
+If @var{keyword} starts with the prefix @samp{no-}, such messages are
+suppressed. Otherwise, they are enabled.
+
+Multiple @option{--warning} messages accumulate.
+
+The tables below list allowed values for @var{keyword} along with the
+warning messages they control.
+@end table
+
+@subheading Keywords controlling @command{tar} operation
+@table @asis
+@kwindex all
+@item all
+Enable all warning messages. This is the default.
+@kwindex none
+@item none
+Disable all warning messages.
+@kwindex filename-with-nuls
+@cindex @samp{file name read contains nul character}, warning message
+@item filename-with-nuls
+@samp{%s: file name read contains nul character}
+@kwindex alone-zero-block
+@cindex @samp{A lone zero block at}, warning message
+@item alone-zero-block
+@samp{A lone zero block at %s}
+@end table