+To test @option{--transform} effect we suggest using
+@option{--show-transformed-names} option:
+
+@smallexample
+$ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
+ --verbose --show-transformed-names /}
+@end smallexample
+
+If both @option{--strip-components} and @option{--transform} are used
+together, then @option{--transform} is applied first, and the required
+number of components is then stripped from its result.
+
+You can use as many @option{--transform} options in a single command
+line as you want. The specified expressions will then be applied in
+order of their appearance. For example, the following two invocations
+are equivalent:
+
+@smallexample
+$ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
+ --transform='s,/usr/local,/usr/,'}
+$ @kbd{tar -cf arch.tar \
+ --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
+@end smallexample
+
+@node after
+@section Operating Only on New Files
+
+@cindex Excluding file by age
+@cindex Data Modification time, excluding files by
+@cindex Modification time, excluding files by
+@cindex Age, excluding files by
+The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
+@option{-N @var{date}}) option causes @command{tar} to only work on
+files whose data modification or status change times are newer than
+the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
+it is taken to be a file name; the data modification time of that file
+is used as the date. If you use this option when creating or appending
+to an archive, the archive will only include new files. If you use
+@option{--after-date} when extracting an archive, @command{tar} will
+only extract files newer than the @var{date} you specify.
+
+If you only want @command{tar} to make the date comparison based on
+modification of the file's data (rather than status
+changes), then use the @option{--newer-mtime=@var{date}} option.
+
+@cindex --after-date and --update compared
+@cindex --newer-mtime and --update compared
+You may use these options with any operation. Note that these options
+differ from the @option{--update} (@option{-u}) operation in that they
+allow you to specify a particular date against which @command{tar} can
+compare when deciding whether or not to archive the files.
+
+@table @option
+@opindex after-date
+@opindex newer
+@item --after-date=@var{date}
+@itemx --newer=@var{date}
+@itemx -N @var{date}
+Only store files newer than @var{date}.
+
+Acts on files only if their data modification or status change times are
+later than @var{date}. Use in conjunction with any operation.
+
+If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
+name; the data modification time of that file is used as the date.
+
+@opindex newer-mtime
+@item --newer-mtime=@var{date}
+Acts like @option{--after-date}, but only looks at data modification times.
+@end table
+
+These options limit @command{tar} to operate only on files which have
+been modified after the date specified. A file's status is considered to have
+changed if its contents have been modified, or if its owner,
+permissions, and so forth, have been changed. (For more information on
+how to specify a date, see @ref{Date input formats}; remember that the
+entire date argument must be quoted if it contains any spaces.)
+
+Gurus would say that @option{--after-date} tests both the data
+modification time (@code{mtime}, the time the contents of the file
+were last modified) and the status change time (@code{ctime}, the time
+the file's status was last changed: owner, permissions, etc.@:)
+fields, while @option{--newer-mtime} tests only the @code{mtime}
+field.
+
+To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
+@code{ctime} and processes the file if either one is more recent than
+@var{date}, while @option{--newer-mtime} only checks @code{mtime} and
+disregards @code{ctime}. Neither does it use @code{atime} (the last time the
+contents of the file were looked at).
+
+Date specifiers can have embedded spaces. Because of this, you may need
+to quote date arguments to keep the shell from parsing them as separate
+arguments. For example, the following command will add to the archive
+all the files modified less than two days ago:
+
+@smallexample
+$ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
+@end smallexample
+
+When any of these options is used with the option @option{--verbose}
+(@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
+date back to its textual representation and compare that with the
+one given with the option. If the two dates differ, @command{tar} will
+print a warning saying what date it will use. This is to help user
+ensure he is using the right date. For example:
+
+@smallexample
+@group
+$ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
+tar: Option --after-date: Treating date '10 days ago' as 2006-06-11
+13:19:37.232434
+@end group
+@end smallexample
+
+@quotation
+@strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
+should not be used for incremental backups. @xref{Incremental Dumps},
+for proper way of creating incremental backups.
+@end quotation
+
+@node recurse
+@section Descending into Directories
+@cindex Avoiding recursion in directories
+@cindex Descending directories, avoiding
+@cindex Directories, avoiding recursion
+@cindex Recursion in directories, avoiding
+
+Usually, @command{tar} will recursively explore all directories (either
+those given on the command line or through the @option{--files-from}
+option) for the various files they contain. However, you may not always
+want @command{tar} to act this way.
+
+@opindex no-recursion
+@cindex @command{find}, using with @command{tar}
+The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
+into specified directories. If you specify @option{--no-recursion}, you can
+use the @command{find} (@pxref{Top,, find, find, GNU Find Manual})
+utility for hunting through levels of directories to
+construct a list of file names which you could then pass to @command{tar}.
+@command{find} allows you to be more selective when choosing which files to
+archive; see @ref{files}, for more information on using @command{find} with
+@command{tar}.
+
+@table @option
+@item --no-recursion
+Prevents @command{tar} from recursively descending directories.
+
+@opindex recursion
+@item --recursion
+Requires @command{tar} to recursively descend directories.
+This is the default.
+@end table
+
+When you use @option{--no-recursion}, @GNUTAR{} grabs
+directory entries themselves, but does not descend on them
+recursively. Many people use @command{find} for locating files they
+want to back up, and since @command{tar} @emph{usually} recursively
+descends on directories, they have to use the @samp{@w{-not -type d}}
+test in their @command{find} invocation (@pxref{Type, Type, Type test,
+find, Finding Files}), as they usually do not want all the files in a
+directory. They then use the @option{--files-from} option to archive
+the files located via @command{find}.
+
+The problem when restoring files archived in this manner is that the
+directories themselves are not in the archive; so the
+@option{--same-permissions} (@option{--preserve-permissions},
+@option{-p}) option does not affect them---while users might really
+like it to. Specifying @option{--no-recursion} is a way to tell
+@command{tar} to grab only the directory entries given to it, adding
+no new files on its own. To summarize, if you use @command{find} to
+create a list of files to be stored in an archive, use it as follows:
+
+@smallexample
+@group
+$ @kbd{find @var{dir} @var{tests} | \
+ tar -cf @var{archive} -T - --no-recursion}
+@end group
+@end smallexample
+
+The @option{--no-recursion} option also applies when extracting: it
+causes @command{tar} to extract only the matched directory entries, not
+the files under those directories.
+
+The @option{--no-recursion} option also affects how globbing patterns
+are interpreted (@pxref{controlling pattern-matching}).
+
+The @option{--no-recursion} and @option{--recursion} options apply to
+later options and operands, and can be overridden by later occurrences
+of @option{--no-recursion} and @option{--recursion}. For example:
+
+@smallexample
+$ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
+@end smallexample
+
+@noindent
+creates an archive with one entry for @file{grape}, and the recursive
+contents of @file{grape/concord}, but no entries under @file{grape}
+other than @file{grape/concord}.
+
+@node one
+@section Crossing File System Boundaries
+@cindex File system boundaries, not crossing
+
+@command{tar} will normally automatically cross file system boundaries in
+order to archive files which are part of a directory tree. You can
+change this behavior by running @command{tar} and specifying
+@option{--one-file-system}. This option only affects files that are
+archived because they are in a directory that is being archived;
+@command{tar} will still archive files explicitly named on the command line
+or through @option{--files-from}, regardless of where they reside.
+
+@table @option
+@opindex one-file-system
+@item --one-file-system
+Prevents @command{tar} from crossing file system boundaries when
+archiving. Use in conjunction with any write operation.
+@end table
+
+The @option{--one-file-system} option causes @command{tar} to modify its
+normal behavior in archiving the contents of directories. If a file in
+a directory is not on the same file system as the directory itself, then
+@command{tar} will not archive that file. If the file is a directory
+itself, @command{tar} will not archive anything beneath it; in other words,
+@command{tar} will not cross mount points.
+
+This option is useful for making full or incremental archival backups of
+a file system. If this option is used in conjunction with
+@option{--verbose} (@option{-v}), files that are excluded are
+mentioned by name on the standard error.
+
+@menu
+* directory:: Changing Directory
+* absolute:: Absolute File Names
+@end menu
+
+@node directory
+@subsection Changing the Working Directory
+
+@FIXME{need to read over this node now for continuity; i've switched
+things around some.}
+
+@cindex Changing directory mid-stream
+@cindex Directory, changing mid-stream
+@cindex Working directory, specifying
+To change the working directory in the middle of a list of file names,
+either on the command line or in a file specified using
+@option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
+This will change the working directory to the specified directory
+after that point in the list.
+
+@table @option
+@opindex directory
+@item --directory=@var{directory}
+@itemx -C @var{directory}
+Changes the working directory in the middle of a command line.
+@end table
+
+For example,
+
+@smallexample
+$ @kbd{tar -c -f jams.tar grape prune -C food cherry}
+@end smallexample
+
+@noindent
+will place the files @file{grape} and @file{prune} from the current
+directory into the archive @file{jams.tar}, followed by the file
+@file{cherry} from the directory @file{food}. This option is especially
+useful when you have several widely separated files that you want to
+store in the same archive.
+
+Note that the file @file{cherry} is recorded in the archive under the
+precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
+archive will contain three files that all appear to have come from the
+same directory; if the archive is extracted with plain @samp{tar
+--extract}, all three files will be written in the current directory.
+
+Contrast this with the command,
+
+@smallexample
+$ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
+@end smallexample
+
+@noindent
+which records the third file in the archive under the name
+@file{red/cherry} so that, if the archive is extracted using
+@samp{tar --extract}, the third file will be written in a subdirectory
+named @file{red}.
+
+You can use the @option{--directory} option to make the archive
+independent of the original name of the directory holding the files.
+The following command places the files @file{/etc/passwd},
+@file{/etc/hosts}, and @file{/lib/libc.a} into the archive
+@file{foo.tar}:
+
+@smallexample
+$ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
+@end smallexample
+
+@noindent
+However, the names of the archive members will be exactly what they were
+on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
+They will not appear to be related by file name to the original
+directories where those files were located.
+
+Note that @option{--directory} options are interpreted consecutively. If
+@option{--directory} specifies a relative file name, it is interpreted
+relative to the then current directory, which might not be the same as
+the original current working directory of @command{tar}, due to a previous
+@option{--directory} option.
+
+When using @option{--files-from} (@pxref{files}), you can put various
+@command{tar} options (including @option{-C}) in the file list. Notice,
+however, that in this case the option and its argument may not be
+separated by whitespace. If you use short option, its argument must
+either follow the option letter immediately, without any intervening
+whitespace, or occupy the next line. Otherwise, if you use long
+option, separate its argument by an equal sign.
+
+For instance, the file list for the above example will be:
+
+@smallexample
+@group
+-C/etc
+passwd
+hosts
+--directory=/lib
+libc.a
+@end group
+@end smallexample
+
+@noindent
+To use it, you would invoke @command{tar} as follows:
+
+@smallexample
+$ @kbd{tar -c -f foo.tar --files-from list}
+@end smallexample
+
+The interpretation of @option{--directory} is disabled by
+@option{--null} option.
+
+@node absolute
+@subsection Absolute File Names
+@cindex absolute file names
+@cindex file names, absolute
+
+By default, @GNUTAR{} drops a leading @samp{/} on
+input or output, and complains about file names containing a @file{..}
+component. There is an option that turns off this behavior:
+
+@table @option
+@opindex absolute-names
+@item --absolute-names
+@itemx -P
+Do not strip leading slashes from file names, and permit file names
+containing a @file{..} file name component.
+@end table
+
+When @command{tar} extracts archive members from an archive, it strips any
+leading slashes (@samp{/}) from the member name. This causes absolute
+member names in the archive to be treated as relative file names. This
+allows you to have such members extracted wherever you want, instead of
+being restricted to extracting the member in the exact directory named
+in the archive. For example, if the archive member has the name
+@file{/etc/passwd}, @command{tar} will extract it as if the name were
+really @file{etc/passwd}.
+
+File names containing @file{..} can cause problems when extracting, so
+@command{tar} normally warns you about such files when creating an
+archive, and rejects attempts to extracts such files.
+
+Other @command{tar} programs do not do this. As a result, if you
+create an archive whose member names start with a slash, they will be
+difficult for other people with a non-@GNUTAR{}
+program to use. Therefore, @GNUTAR{} also strips
+leading slashes from member names when putting members into the
+archive. For example, if you ask @command{tar} to add the file
+@file{/bin/ls} to an archive, it will do so, but the member name will
+be @file{bin/ls}@footnote{A side effect of this is that when
+@option{--create} is used with @option{--verbose} the resulting output
+is not, generally speaking, the same as the one you'd get running
+@kbd{tar --list} command. This may be important if you use some
+scripts for comparing both outputs. @xref{listing member and file names},
+for the information on how to handle this case.}.
+
+Symbolic links containing @file{..} or leading @samp{/} can also cause
+problems when extracting, so @command{tar} normally extracts them last;
+it may create empty files as placeholders during extraction.
+
+If you use the @option{--absolute-names} (@option{-P}) option,
+@command{tar} will do none of these transformations.
+
+To archive or extract files relative to the root directory, specify
+the @option{--absolute-names} (@option{-P}) option.
+
+Normally, @command{tar} acts on files relative to the working
+directory---ignoring superior directory names when archiving, and
+ignoring leading slashes when extracting.
+
+When you specify @option{--absolute-names} (@option{-P}),
+@command{tar} stores file names including all superior directory
+names, and preserves leading slashes. If you only invoked
+@command{tar} from the root directory you would never need the
+@option{--absolute-names} option, but using this option
+may be more convenient than switching to root.
+
+@FIXME{Should be an example in the tutorial/wizardry section using this
+to transfer files between systems.}
+
+@table @option
+@item --absolute-names
+Preserves full file names (including superior directory names) when
+archiving and extracting files.
+
+@end table
+
+@command{tar} prints out a message about removing the @samp{/} from
+file names. This message appears once per @GNUTAR{}
+invocation. It represents something which ought to be told; ignoring
+what it means can cause very serious surprises, later.
+
+Some people, nevertheless, do not want to see this message. Wanting to
+play really dangerously, one may of course redirect @command{tar} standard
+error to the sink. For example, under @command{sh}:
+
+@smallexample
+$ @kbd{tar -c -f archive.tar /home 2> /dev/null}
+@end smallexample
+
+@noindent
+Another solution, both nicer and simpler, would be to change to
+the @file{/} directory first, and then avoid absolute notation.
+For example:
+
+@smallexample
+$ @kbd{tar -c -f archive.tar -C / home}
+@end smallexample
+
+@xref{Integrity}, for some of the security-related implications
+of using this option.
+
+@include parse-datetime.texi
+
+@node Formats
+@chapter Controlling the Archive Format
+
+@cindex Tar archive formats
+Due to historical reasons, there are several formats of tar archives.
+All of them are based on the same principles, but have some subtle
+differences that often make them incompatible with each other.
+
+GNU tar is able to create and handle archives in a variety of formats.
+The most frequently used formats are (in alphabetical order):
+
+@table @asis
+@item gnu
+Format used by @GNUTAR{} versions up to 1.13.25. This format derived
+from an early @acronym{POSIX} standard, adding some improvements such as
+sparse file handling and incremental archives. Unfortunately these
+features were implemented in a way incompatible with other archive
+formats.
+
+Archives in @samp{gnu} format are able to hold file names of unlimited
+length.
+
+@item oldgnu
+Format used by @GNUTAR{} of versions prior to 1.12.
+
+@item v7
+Archive format, compatible with the V7 implementation of tar. This
+format imposes a number of limitations. The most important of them
+are:
+
+@enumerate
+@item The maximum length of a file name is limited to 99 characters.
+@item The maximum length of a symbolic link is limited to 99 characters.
+@item It is impossible to store special files (block and character
+devices, fifos etc.)
+@item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
+octal)
+@item V7 archives do not contain symbolic ownership information (user
+and group name of the file owner).
+@end enumerate
+
+This format has traditionally been used by Automake when producing
+Makefiles. This practice will change in the future, in the meantime,
+however this means that projects containing file names more than 99
+characters long will not be able to use @GNUTAR{} @value{VERSION} and
+Automake prior to 1.9.
+
+@item ustar
+Archive format defined by @acronym{POSIX.1-1988} specification. It stores
+symbolic ownership information. It is also able to store
+special files. However, it imposes several restrictions as well:
+
+@enumerate
+@item The maximum length of a file name is limited to 256 characters,
+provided that the file name can be split at a directory separator in
+two parts, first of them being at most 155 bytes long. So, in most
+cases the maximum file name length will be shorter than 256
+characters.
+@item The maximum length of a symbolic link name is limited to
+100 characters.
+@item Maximum size of a file the archive is able to accommodate
+is 8GB
+@item Maximum value of UID/GID is 2097151.
+@item Maximum number of bits in device major and minor numbers is 21.
+@end enumerate
+
+@item star
+Format used by J@"org Schilling @command{star}
+implementation. @GNUTAR{} is able to read @samp{star} archives but
+currently does not produce them.
+
+@item posix
+Archive format defined by @acronym{POSIX.1-2001} specification. This is the
+most flexible and feature-rich format. It does not impose any
+restrictions on file sizes or file name lengths. This format is quite
+recent, so not all tar implementations are able to handle it properly.
+However, this format is designed in such a way that any tar
+implementation able to read @samp{ustar} archives will be able to read
+most @samp{posix} archives as well, with the only exception that any
+additional information (such as long file names etc.) will in such
+case be extracted as plain text files along with the files it refers to.
+
+This archive format will be the default format for future versions
+of @GNUTAR{}.
+
+@end table
+
+The following table summarizes the limitations of each of these
+formats:
+
+@multitable @columnfractions .10 .20 .20 .20 .20
+@headitem Format @tab UID @tab File Size @tab File Name @tab Devn
+@item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
+@item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
+@item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
+@item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
+@item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
+@end multitable
+
+The default format for @GNUTAR{} is defined at compilation
+time. You may check it by running @command{tar --help}, and examining
+the last lines of its output. Usually, @GNUTAR{} is configured
+to create archives in @samp{gnu} format, however, future version will
+switch to @samp{posix}.
+
+@menu
+* Compression:: Using Less Space through Compression
+* Attributes:: Handling File Attributes
+* Portability:: Making @command{tar} Archives More Portable
+* cpio:: Comparison of @command{tar} and @command{cpio}
+@end menu
+
+@node Compression
+@section Using Less Space through Compression
+
+@menu
+* gzip:: Creating and Reading Compressed Archives
+* sparse:: Archiving Sparse Files
+@end menu
+
+@node gzip
+@subsection Creating and Reading Compressed Archives
+@cindex Compressed archives
+@cindex Storing archives in compressed format
+
+@cindex gzip
+@cindex bzip2
+@cindex lzip
+@cindex lzma
+@cindex lzop
+@cindex compress
+@GNUTAR{} is able to create and read compressed archives. It supports
+a wide variety of compression programs, namely: @command{gzip},
+@command{bzip2}, @command{lzip}, @command{lzma}, @command{lzop},
+@command{xz} and traditional @command{compress}. The latter is
+supported mostly for backward compatibility, and we recommend
+against using it, because it is by far less effective than the other
+compression programs@footnote{It also had patent problems in the past.}.
+
+Creating a compressed archive is simple: you just specify a
+@dfn{compression option} along with the usual archive creation
+commands. The compression option is @option{-z} (@option{--gzip}) to
+create a @command{gzip} compressed archive, @option{-j}
+(@option{--bzip2}) to create a @command{bzip2} compressed archive,
+@option{--lzip} to create an @asis{lzip} compressed archive,
+@option{-J} (@option{--xz}) to create an @asis{XZ} archive,
+@option{--lzma} to create an @asis{LZMA} compressed
+archive, @option{--lzop} to create an @asis{LSOP} archive, and
+@option{-Z} (@option{--compress}) to use @command{compress} program.
+For example:
+
+@smallexample
+$ @kbd{tar czf archive.tar.gz .}
+@end smallexample
+
+You can also let @GNUTAR{} select the compression program based on
+the suffix of the archive file name. This is done using
+@option{--auto-compress} (@option{-a}) command line option. For
+example, the following invocation will use @command{bzip2} for
+compression:
+
+@smallexample
+$ @kbd{tar caf archive.tar.bz2 .}
+@end smallexample
+
+@noindent
+whereas the following one will use @command{lzma}:
+
+@smallexample
+$ @kbd{tar caf archive.tar.lzma .}
+@end smallexample
+
+For a complete list of file name suffixes recognized by @GNUTAR{},
+see @ref{auto-compress}.
+
+Reading compressed archive is even simpler: you don't need to specify
+any additional options as @GNUTAR{} recognizes its format
+automatically. Thus, the following commands will list and extract the
+archive created in previous example:
+
+@smallexample
+# List the compressed archive
+$ @kbd{tar tf archive.tar.gz}
+# Extract the compressed archive
+$ @kbd{tar xf archive.tar.gz}
+@end smallexample
+
+The format recognition algorithm is based on @dfn{signatures}, a
+special byte sequences in the beginning of file, that are specific for
+certain compression formats. If this approach fails, @command{tar}
+falls back to using archive name suffix to determine its format
+(@pxref{auto-compress}, for a list of recognized suffixes).
+
+@anchor{alternative decompression programs}
+@cindex alternative decompression programs
+Some compression programs are able to handle different compression
+formats. @GNUTAR{} uses this, if the principal decompressor for the
+given format is not available. For example, if @command{compress} is
+not installed, @command{tar} will try to use @command{gzip}. As of
+version @value{VERSION} the following alternatives are
+tried@footnote{To verbosely trace the decompressor selection, use the
+@option{--warning=decompress-program} option
+(@pxref{warnings,decompress-program}).}:
+
+@multitable @columnfractions 0.3 0.3 0.3
+@headitem Format @tab Main decompressor @tab Alternatives
+@item compress @tab compress @tab gzip
+@item lzma @tab lzma @tab xz
+@item bzip2 @tab bzip2 @tab lbzip2
+@end multitable
+
+The only case when you have to specify a decompression option while
+reading the archive is when reading from a pipe or from a tape drive
+that does not support random access. However, in this case @GNUTAR{}
+will indicate which option you should use. For example:
+
+@smallexample
+$ @kbd{cat archive.tar.gz | tar tf -}
+tar: Archive is compressed. Use -z option
+tar: Error is not recoverable: exiting now
+@end smallexample
+
+If you see such diagnostics, just add the suggested option to the
+invocation of @GNUTAR{}:
+
+@smallexample
+$ @kbd{cat archive.tar.gz | tar tzf -}
+@end smallexample
+
+Notice also, that there are several restrictions on operations on
+compressed archives. First of all, compressed archives cannot be
+modified, i.e., you cannot update (@option{--update}, alias @option{-u})
+them or delete (@option{--delete}) members from them or
+add (@option{--append}, alias @option{-r}) members to them. Likewise, you
+cannot append another @command{tar} archive to a compressed archive using
+@option{--concatenate} (@option{-A}). Secondly, multi-volume
+archives cannot be compressed.
+
+The following options allow to select a particular compressor program:
+
+@table @option
+@opindex gzip
+@opindex ungzip
+@item -z
+@itemx --gzip
+@itemx --ungzip
+Filter the archive through @command{gzip}.
+
+@opindex xz
+@item -J
+@itemx --xz
+Filter the archive through @code{xz}.
+
+@item -j
+@itemx --bzip2
+Filter the archive through @code{bzip2}.
+
+@opindex lzip
+@item --lzip
+Filter the archive through @command{lzip}.
+
+@opindex lzma
+@item --lzma
+Filter the archive through @command{lzma}.
+
+@opindex lzop
+@item --lzop
+Filter the archive through @command{lzop}.
+
+@opindex compress
+@opindex uncompress
+@item -Z
+@itemx --compress
+@itemx --uncompress
+Filter the archive through @command{compress}.
+@end table
+
+When any of these options is given, @GNUTAR{} searches the compressor
+binary in the current path and invokes it. The name of the compressor
+program is specified at compilation time using a corresponding
+@option{--with-@var{compname}} option to @command{configure}, e.g.
+@option{--with-bzip2} to select a specific @command{bzip2} binary.
+@xref{lbzip2}, for a detailed discussion.
+
+The output produced by @command{tar --help} shows the actual
+compressor names along with each of these options.
+
+You can use any of these options on physical devices (tape drives,
+etc.) and remote files as well as on normal files; data to or from
+such devices or remote files is reblocked by another copy of the
+@command{tar} program to enforce the specified (or default) record
+size. The default compression parameters are used. Most compression
+programs let you override these by setting a program-specific
+environment variable. For example, with @command{gzip} you can set
+@env{GZIP}:
+
+@smallexample
+$ @kbd{GZIP='-9 -n' tar czf archive.tar.gz subdir}
+@end smallexample
+Another way would be to use the @option{-I} option instead (see
+below), e.g.:
+
+@smallexample
+$ @kbd{tar -cf archive.tar.gz -I 'gzip -9 -n' subdir}
+@end smallexample
+
+@noindent
+Finally, the third, traditional, way to do this is to use a pipe:
+
+@smallexample
+$ @kbd{tar cf - subdir | gzip -9 -n > archive.tar.gz}
+@end smallexample
+
+@cindex corrupted archives
+Compressed archives are easily corrupted, because compressed files
+have little redundancy. The adaptive nature of the
+compression scheme means that the compression tables are implicitly
+spread all over the archive. If you lose a few blocks, the dynamic
+construction of the compression tables becomes unsynchronized, and there
+is little chance that you could recover later in the archive.
+
+Other compression options provide better control over creating
+compressed archives. These are:
+
+@table @option
+@anchor{auto-compress}
+@opindex auto-compress
+@item --auto-compress
+@itemx -a
+Select a compression program to use by the archive file name
+suffix. The following suffixes are recognized:
+
+@multitable @columnfractions 0.3 0.6
+@headitem Suffix @tab Compression program
+@item @samp{.gz} @tab @command{gzip}
+@item @samp{.tgz} @tab @command{gzip}
+@item @samp{.taz} @tab @command{gzip}
+@item @samp{.Z} @tab @command{compress}
+@item @samp{.taZ} @tab @command{compress}
+@item @samp{.bz2} @tab @command{bzip2}
+@item @samp{.tz2} @tab @command{bzip2}
+@item @samp{.tbz2} @tab @command{bzip2}
+@item @samp{.tbz} @tab @command{bzip2}
+@item @samp{.lz} @tab @command{lzip}
+@item @samp{.lzma} @tab @command{lzma}
+@item @samp{.tlz} @tab @command{lzma}
+@item @samp{.lzo} @tab @command{lzop}
+@item @samp{.xz} @tab @command{xz}
+@end multitable
+
+@anchor{use-compress-program}
+@opindex use-compress-program
+@item --use-compress-program=@var{command}
+@itemx -I=@var{command}
+Use external compression program @var{command}. Use this option if you
+are not happy with the compression program associated with the suffix
+at compile time or if you have a compression program that @GNUTAR{}
+does not support. The @var{command} argument is a valid command
+invocation, as you would type it at the command line prompt, with any
+additional options as needed. Enclose it in quotes if it contains
+white space (see @ref{external, Running External Commands}, for more detail).
+
+The @var{command} should follow two conventions:
+
+First, when invoked without additional options, it should read data
+from standard input, compress it and output it on standard output.
+
+Secondly, if invoked with the additional @option{-d} option, it should
+do exactly the opposite, i.e., read the compressed data from the
+standard input and produce uncompressed data on the standard output.
+
+The latter requirement means that you must not use the @option{-d}
+option as a part of the @var{command} itself.
+@end table
+
+@cindex gpg, using with tar
+@cindex gnupg, using with tar
+@cindex Using encrypted archives
+The @option{--use-compress-program} option, in particular, lets you
+implement your own filters, not necessarily dealing with
+compression/decompression. For example, suppose you wish to implement
+PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
+gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
+Manual}). The following script does that:
+
+@smallexample
+@group
+#! /bin/sh
+case $1 in
+-d) gpg --decrypt - | gzip -d -c;;
+'') gzip -c | gpg -s;;
+*) echo "Unknown option $1">&2; exit 1;;
+esac
+@end group
+@end smallexample
+
+Suppose you name it @file{gpgz} and save it somewhere in your
+@env{PATH}. Then the following command will create a compressed
+archive signed with your private key:
+
+@smallexample
+$ @kbd{tar -cf foo.tar.gpgz -Igpgz .}
+@end smallexample
+
+@noindent
+Likewise, the command below will list its contents:
+
+@smallexample
+$ @kbd{tar -tf foo.tar.gpgz -Igpgz .}
+@end smallexample
+
+@ignore
+The above is based on the following discussion:
+
+ I have one question, or maybe it's a suggestion if there isn't a way
+ to do it now. I would like to use @option{--gzip}, but I'd also like
+ the output to be fed through a program like @acronym{GNU}
+ @command{ecc} (actually, right now that's @samp{exactly} what I'd like
+ to use :-)), basically adding ECC protection on top of compression.
+ It seems as if this should be quite easy to do, but I can't work out
+ exactly how to go about it. Of course, I can pipe the standard output
+ of @command{tar} through @command{ecc}, but then I lose (though I
+ haven't started using it yet, I confess) the ability to have
+ @command{tar} use @command{rmt} for it's I/O (I think).
+
+ I think the most straightforward thing would be to let me specify a
+ general set of filters outboard of compression (preferably ordered,
+ so the order can be automatically reversed on input operations, and
+ with the options they require specifiable), but beggars shouldn't be
+ choosers and anything you decide on would be fine with me.
+
+ By the way, I like @command{ecc} but if (as the comments say) it can't
+ deal with loss of block sync, I'm tempted to throw some time at adding
+ that capability. Supposing I were to actually do such a thing and
+ get it (apparently) working, do you accept contributed changes to
+ utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
+
+ Isn't that exactly the role of the
+ @option{--use-compress-prog=@var{program}} option?
+ I never tried it myself, but I suspect you may want to write a
+ @var{prog} script or program able to filter stdin to stdout to
+ way you want. It should recognize the @option{-d} option, for when
+ extraction is needed rather than creation.
+
+ It has been reported that if one writes compressed data (through the
+ @option{--gzip} or @option{--compress} options) to a DLT and tries to use
+ the DLT compression mode, the data will actually get bigger and one will
+ end up with less space on the tape.
+@end ignore
+
+@menu
+* lbzip2:: Using lbzip2 with @GNUTAR{}.
+@end menu
+
+@node lbzip2
+@subsubsection Using lbzip2 with @GNUTAR{}.
+@cindex lbzip2
+@cindex Laszlo Ersek
+ @command{Lbzip2} is a multithreaded utility for handling
+@samp{bzip2} compression, written by Laszlo Ersek. It makes use of
+multiple processors to speed up its operation and in general works
+considerably faster than @command{bzip2}. For a detailed description
+of @command{lbzip2} see @uref{http://freshmeat.net/@/projects/@/lbzip2} and
+@uref{http://www.linuxinsight.com/@/lbzip2-parallel-bzip2-utility.html,
+lbzip2: parallel bzip2 utility}.
+
+ Recent versions of @command{lbzip2} are mostly command line compatible
+with @command{bzip2}, which makes it possible to automatically invoke
+it via the @option{--bzip2} @GNUTAR{} command line option. To do so,
+@GNUTAR{} must be configured with the @option{--with-bzip2} command
+line option, like this:
+
+@smallexample
+$ @kbd{./configure --with-bzip2=lbzip2 [@var{other-options}]}
+@end smallexample
+
+ Once configured and compiled this way, @command{tar --help} will show the
+following:
+
+@smallexample
+@group
+$ @kbd{tar --help | grep -- --bzip2}
+ -j, --bzip2 filter the archive through lbzip2
+@end group
+@end smallexample
+
+@noindent
+which means that running @command{tar --bzip2} will invoke @command{lbzip2}.
+
+@node sparse
+@subsection Archiving Sparse Files
+@cindex Sparse Files
+
+Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
+in a file is a section of the file's contents which was never written.
+The contents of a hole reads as all zeros. On many operating systems,
+actual disk storage is not allocated for holes, but they are counted
+in the length of the file. If you archive such a file, @command{tar}
+could create an archive longer than the original. To have @command{tar}
+attempt to recognize the holes in a file, use @option{--sparse}
+(@option{-S}). When you use this option, then, for any file using
+less disk space than would be expected from its length, @command{tar}
+searches the file for consecutive stretches of zeros. It then records
+in the archive for the file where the consecutive stretches of zeros
+are, and only archives the ``real contents'' of the file. On
+extraction (using @option{--sparse} is not needed on extraction) any
+such files have holes created wherever the continuous stretches of zeros
+were found. Thus, if you use @option{--sparse}, @command{tar} archives
+won't take more space than the original.
+
+@table @option
+@opindex sparse
+@item -S
+@itemx --sparse
+This option instructs @command{tar} to test each file for sparseness
+before attempting to archive it. If the file is found to be sparse it
+is treated specially, thus allowing to decrease the amount of space
+used by its image in the archive.
+
+This option is meaningful only when creating or updating archives. It
+has no effect on extraction.
+@end table
+
+Consider using @option{--sparse} when performing file system backups,
+to avoid archiving the expanded forms of files stored sparsely in the
+system.
+
+Even if your system has no sparse files currently, some may be
+created in the future. If you use @option{--sparse} while making file
+system backups as a matter of course, you can be assured the archive
+will never take more space on the media than the files take on disk
+(otherwise, archiving a disk filled with sparse files might take
+hundreds of tapes). @xref{Incremental Dumps}.
+
+However, be aware that @option{--sparse} option presents a serious
+drawback. Namely, in order to determine if the file is sparse
+@command{tar} has to read it before trying to archive it, so in total
+the file is read @strong{twice}. So, always bear in mind that the
+time needed to process all files with this option is roughly twice
+the time needed to archive them without it.
+@FIXME{A technical note:
+
+Programs like @command{dump} do not have to read the entire file; by
+examining the file system directly, they can determine in advance
+exactly where the holes are and thus avoid reading through them. The
+only data it need read are the actual allocated data blocks.
+@GNUTAR{} uses a more portable and straightforward
+archiving approach, it would be fairly difficult that it does
+otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
+1990-12-10:
+
+@quotation
+What I did say is that you cannot tell the difference between a hole and an
+equivalent number of nulls without reading raw blocks. @code{st_blocks} at
+best tells you how many holes there are; it doesn't tell you @emph{where}.
+Just as programs may, conceivably, care what @code{st_blocks} is (care
+to name one that does?), they may also care where the holes are (I have
+no examples of this one either, but it's equally imaginable).
+
+I conclude from this that good archivers are not portable. One can
+arguably conclude that if you want a portable program, you can in good
+conscience restore files with as many holes as possible, since you can't
+get it right.
+@end quotation
+}
+
+@cindex sparse formats, defined
+When using @samp{POSIX} archive format, @GNUTAR{} is able to store
+sparse files using in three distinct ways, called @dfn{sparse
+formats}. A sparse format is identified by its @dfn{number},
+consisting, as usual of two decimal numbers, delimited by a dot. By
+default, format @samp{1.0} is used. If, for some reason, you wish to
+use an earlier format, you can select it using
+@option{--sparse-version} option.
+
+@table @option
+@opindex sparse-version
+@item --sparse-version=@var{version}
+
+Select the format to store sparse files in. Valid @var{version} values
+are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
+for a detailed description of each format.
+@end table
+
+Using @option{--sparse-format} option implies @option{--sparse}.
+
+@node Attributes
+@section Handling File Attributes
+@cindex atrributes, files
+@cindex file attributes
+
+When @command{tar} reads files, it updates their access times. To
+avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
+reset the access time retroactively or avoid changing it in the first
+place.
+
+@table @option
+@opindex atime-preserve
+@item --atime-preserve
+@itemx --atime-preserve=replace
+@itemx --atime-preserve=system
+Preserve the access times of files that are read. This works only for
+files that you own, unless you have superuser privileges.
+
+@option{--atime-preserve=replace} works on most systems, but it also
+restores the data modification time and updates the status change
+time. Hence it doesn't interact with incremental dumps nicely
+(@pxref{Incremental Dumps}), and it can set access or data modification times
+incorrectly if other programs access the file while @command{tar} is
+running.
+
+@option{--atime-preserve=system} avoids changing the access time in
+the first place, if the operating system supports this.
+Unfortunately, this may or may not work on any given operating system
+or file system. If @command{tar} knows for sure it won't work, it
+complains right away.
+
+Currently @option{--atime-preserve} with no operand defaults to
+@option{--atime-preserve=replace}, but this is intended to change to
+@option{--atime-preserve=system} when the latter is better-supported.
+
+@opindex touch
+@item -m
+@itemx --touch
+Do not extract data modification time.
+
+When this option is used, @command{tar} leaves the data modification times
+of the files it extracts as the times when the files were extracted,
+instead of setting it to the times recorded in the archive.
+
+This option is meaningless with @option{--list} (@option{-t}).
+
+@opindex same-owner
+@item --same-owner
+Create extracted files with the same ownership they have in the
+archive.
+
+This is the default behavior for the superuser,
+so this option is meaningful only for non-root users, when @command{tar}
+is executed on those systems able to give files away. This is
+considered as a security flaw by many people, at least because it
+makes quite difficult to correctly account users for the disk space
+they occupy. Also, the @code{suid} or @code{sgid} attributes of
+files are easily and silently lost when files are given away.
+
+When writing an archive, @command{tar} writes the user @acronym{ID} and user name
+separately. If it can't find a user name (because the user @acronym{ID} is not
+in @file{/etc/passwd}), then it does not write one. When restoring,
+it tries to look the name (if one was written) up in
+@file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
+the archive instead.
+
+@opindex no-same-owner
+@item --no-same-owner
+@itemx -o
+Do not attempt to restore ownership when extracting. This is the
+default behavior for ordinary users, so this option has an effect
+only for the superuser.
+
+@opindex numeric-owner
+@item --numeric-owner
+The @option{--numeric-owner} option allows (ANSI) archives to be written
+without user/group name information or such information to be ignored
+when extracting. It effectively disables the generation and/or use
+of user/group name information. This option forces extraction using
+the numeric ids from the archive, ignoring the names.
+
+This is useful in certain circumstances, when restoring a backup from
+an emergency floppy with different passwd/group files for example.
+It is otherwise impossible to extract files with the right ownerships
+if the password file in use during the extraction does not match the
+one belonging to the file system(s) being extracted. This occurs,
+for example, if you are restoring your files after a major crash and
+had booted from an emergency floppy with no password file or put your
+disk into another machine to do the restore.
+
+The numeric ids are @emph{always} saved into @command{tar} archives.
+The identifying names are added at create time when provided by the
+system, unless @option{--format=oldgnu} is used. Numeric ids could be
+used when moving archives between a collection of machines using
+a centralized management for attribution of numeric ids to users
+and groups. This is often made through using the NIS capabilities.
+
+When making a @command{tar} file for distribution to other sites, it
+is sometimes cleaner to use a single owner for all files in the
+distribution, and nicer to specify the write permission bits of the
+files as stored in the archive independently of their actual value on
+the file system. The way to prepare a clean distribution is usually
+to have some Makefile rule creating a directory, copying all needed
+files in that directory, then setting ownership and permissions as
+wanted (there are a lot of possible schemes), and only then making a
+@command{tar} archive out of this directory, before cleaning
+everything out. Of course, we could add a lot of options to
+@GNUTAR{} for fine tuning permissions and ownership.
+This is not the good way, I think. @GNUTAR{} is
+already crowded with options and moreover, the approach just explained
+gives you a great deal of control already.
+
+@xopindex{same-permissions, short description}
+@xopindex{preserve-permissions, short description}
+@item -p
+@itemx --same-permissions
+@itemx --preserve-permissions
+Extract all protection information.
+
+This option causes @command{tar} to set the modes (access permissions) of
+extracted files exactly as recorded in the archive. If this option
+is not used, the current @code{umask} setting limits the permissions
+on extracted files. This option is by default enabled when
+@command{tar} is executed by a superuser.
+
+
+This option is meaningless with @option{--list} (@option{-t}).
+
+@opindex preserve
+@item --preserve
+Same as both @option{--same-permissions} and @option{--same-order}.
+
+This option is deprecated, and will be removed in @GNUTAR{} version 1.23.
+
+@end table
+
+@node Portability
+@section Making @command{tar} Archives More Portable
+
+Creating a @command{tar} archive on a particular system that is meant to be
+useful later on many other machines and with other versions of @command{tar}
+is more challenging than you might think. @command{tar} archive formats
+have been evolving since the first versions of Unix. Many such formats
+are around, and are not always compatible with each other. This section
+discusses a few problems, and gives some advice about making @command{tar}
+archives more portable.
+
+One golden rule is simplicity. For example, limit your @command{tar}
+archives to contain only regular files and directories, avoiding
+other kind of special files. Do not attempt to save sparse files or
+contiguous files as such. Let's discuss a few more problems, in turn.
+
+@FIXME{Discuss GNU extensions (incremental backups, multi-volume
+archives and archive labels) in GNU and PAX formats.}
+
+@menu
+* Portable Names:: Portable Names
+* dereference:: Symbolic Links
+* hard links:: Hard Links
+* old:: Old V7 Archives
+* ustar:: Ustar Archives
+* gnu:: GNU and old GNU format archives.
+* posix:: @acronym{POSIX} archives
+* Checksumming:: Checksumming Problems
+* Large or Negative Values:: Large files, negative time stamps, etc.
+* Other Tars:: How to Extract GNU-Specific Data Using
+ Other @command{tar} Implementations
+@end menu
+
+@node Portable Names
+@subsection Portable Names
+
+Use portable file and member names. A name is portable if it contains
+only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
+@samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
+contain @samp{/-}. Avoid deep directory nesting. For portability to
+old Unix hosts, limit your file name components to 14 characters or
+less.
+
+If you intend to have your @command{tar} archives to be read under
+MSDOS, you should not rely on case distinction for file names, and you
+might use the @acronym{GNU} @command{doschk} program for helping you
+further diagnosing illegal MSDOS names, which are even more limited
+than System V's.
+
+@node dereference
+@subsection Symbolic Links
+@cindex File names, using symbolic links
+@cindex Symbolic link as file name
+
+@opindex dereference
+Normally, when @command{tar} archives a symbolic link, it writes a
+block to the archive naming the target of the link. In that way, the
+@command{tar} archive is a faithful record of the file system contents.
+When @option{--dereference} (@option{-h}) is used with
+@option{--create} (@option{-c}), @command{tar} archives the files
+symbolic links point to, instead of
+the links themselves.
+
+When creating portable archives, use @option{--dereference}
+(@option{-h}): some systems do not support
+symbolic links, and moreover, your distribution might be unusable if
+it contains unresolved symbolic links.
+
+When reading from an archive, the @option{--dereference} (@option{-h})
+option causes @command{tar} to follow an already-existing symbolic
+link when @command{tar} writes or reads a file named in the archive.
+Ordinarily, @command{tar} does not follow such a link, though it may
+remove the link before writing a new file. @xref{Dealing with Old
+Files}.
+
+The @option{--dereference} option is unsafe if an untrusted user can
+modify directories while @command{tar} is running. @xref{Security}.
+
+@node hard links
+@subsection Hard Links
+@cindex File names, using hard links
+@cindex hard links, dereferencing
+@cindex dereferencing hard links
+
+Normally, when @command{tar} archives a hard link, it writes a
+block to the archive naming the target of the link (a @samp{1} type
+block). In that way, the actual file contents is stored in file only
+once. For example, consider the following two files:
+
+@smallexample
+@group
+$ ls -l
+-rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
+-rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
+@end group
+@end smallexample
+
+Here, @file{jeden} is a link to @file{one}. When archiving this
+directory with a verbose level 2, you will get an output similar to
+the following:
+
+@smallexample
+$ tar cvvf ../archive.tar .
+drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
+-rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
+hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
+@end smallexample
+
+The last line shows that, instead of storing two copies of the file,
+@command{tar} stored it only once, under the name @file{jeden}, and
+stored file @file{one} as a hard link to this file.
+
+It may be important to know that all hard links to the given file are
+stored in the archive. For example, this may be necessary for exact
+reproduction of the file system. The following option does that:
+
+@table @option
+@xopindex{check-links, described}
+@item --check-links
+@itemx -l
+Check the number of links dumped for each processed file. If this
+number does not match the total number of hard links for the file, print
+a warning message.
+@end table
+
+For example, trying to archive only file @file{jeden} with this option
+produces the following diagnostics:
+
+@smallexample
+$ tar -c -f ../archive.tar -l jeden
+tar: Missing links to 'jeden'.
+@end smallexample
+
+Although creating special records for hard links helps keep a faithful
+record of the file system contents and makes archives more compact, it
+may present some difficulties when extracting individual members from
+the archive. For example, trying to extract file @file{one} from the
+archive created in previous examples produces, in the absense of file
+@file{jeden}:
+
+@smallexample
+$ tar xf archive.tar ./one
+tar: ./one: Cannot hard link to './jeden': No such file or directory
+tar: Error exit delayed from previous errors
+@end smallexample
+
+The reason for this behavior is that @command{tar} cannot seek back in
+the archive to the previous member (in this case, @file{one}), to
+extract it@footnote{There are plans to fix this in future releases.}.
+If you wish to avoid such problems at the cost of a bigger archive,
+use the following option:
+
+@table @option
+@xopindex{hard-dereference, described}
+@item --hard-dereference
+Dereference hard links and store the files they refer to.
+@end table
+
+For example, trying this option on our two sample files, we get two
+copies in the archive, each of which can then be extracted
+independently of the other:
+
+@smallexample
+@group
+$ tar -c -vv -f ../archive.tar --hard-dereference .
+drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
+-rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
+-rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
+@end group
+@end smallexample
+
+@node old
+@subsection Old V7 Archives
+@cindex Format, old style
+@cindex Old style format
+@cindex Old style archives
+@cindex v7 archive format
+
+Certain old versions of @command{tar} cannot handle additional
+information recorded by newer @command{tar} programs. To create an
+archive in V7 format (not ANSI), which can be read by these old
+versions, specify the @option{--format=v7} option in
+conjunction with the @option{--create} (@option{-c}) (@command{tar} also
+accepts @option{--portability} or @option{--old-archive} for this
+option). When you specify it,
+@command{tar} leaves out information about directories, pipes, fifos,
+contiguous files, and device files, and specifies file ownership by
+group and user IDs instead of group and user names.
+
+When updating an archive, do not use @option{--format=v7}
+unless the archive was created using this option.
+
+In most cases, a @emph{new} format archive can be read by an @emph{old}
+@command{tar} program without serious trouble, so this option should
+seldom be needed. On the other hand, most modern @command{tar}s are
+able to read old format archives, so it might be safer for you to
+always use @option{--format=v7} for your distributions. Notice,
+however, that @samp{ustar} format is a better alternative, as it is
+free from many of @samp{v7}'s drawbacks.
+
+@node ustar
+@subsection Ustar Archive Format
+
+@cindex ustar archive format
+Archive format defined by @acronym{POSIX}.1-1988 specification is called
+@code{ustar}. Although it is more flexible than the V7 format, it
+still has many restrictions (@pxref{Formats,ustar}, for the detailed
+description of @code{ustar} format). Along with V7 format,
+@code{ustar} format is a good choice for archives intended to be read
+with other implementations of @command{tar}.
+
+To create archive in @code{ustar} format, use @option{--format=ustar}
+option in conjunction with the @option{--create} (@option{-c}).
+
+@node gnu
+@subsection @acronym{GNU} and old @GNUTAR{} format