* Pure numbers in date strings:: 19931219, 1440.
* Seconds since the Epoch:: @@1078100502.
* Specifying time zone rules:: TZ="America/New_York", TZ="UTC0".
-* Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al.
+* Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al.
Controlling the Archive Format
@item --dereference
@itemx -h
-When creating a @command{tar} archive, @command{tar} will archive the
-file that a symbolic link points to, rather than archiving the
-symlink. @xref{dereference}.
+When reading or writing a file to be archived, @command{tar} accesses
+the file that a symbolic link points to, rather than the symlink
+itself. @xref{dereference}.
@opsummary{directory}
@item --directory=@var{dir}
@cindex @samp{Ignoring unknown extended header keyword `%s'}, warning message
@item unknown-keyword
@samp{Ignoring unknown extended header keyword `%s'}
+@kwindex decompress-program
+@item decompress-program
+Controls verbose messages describing execution failures when trying
+alternative decompressor programs (@pxref{alternative decompression
+programs}). This warning is disabled by default (unless
+@option{--verbose} is used). A common example of what you can get
+when using this warning is:
+
+@smallexample
+$ @kbd{tar --warning=decompress-program -x -f archive.Z}
+tar (child): cannot run compress: No such file or directory
+tar (child): trying gzip
+@end smallexample
+
+This means that @command{tar} first tried to decompress
+@file{archive.Z} using @command{compress}, and, when that
+failed, switched to @command{gzip}.
@end table
@subheading Keywords controlling incremental extraction:
@xref{Integrity}, for some of the security-related implications
of using this option.
-@include getdate.texi
+@include parse-datetime.texi
@node Formats
@chapter Controlling the Archive Format
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{}
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.
-@option{--dereference} (@option{-h}) is used with @option{--create} (@option{-c}), and causes
-@command{tar} to archive the files symbolic links point to, instead of
-the links themselves. When this option is used, when @command{tar}
-encounters a symbolic link, it will archive the linked-to file,
-instead of simply recording the presence of a symbolic link.
-
-The name under which the file is stored in the file system is not
-recorded in the archive. To record both the symbolic link name and
-the file name in the system, archive the file under both names. If
-all links were recorded automatically by @command{tar}, an extracted file
-might be linked to a file name that no longer exists in the file
-system.
+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.
-If a linked-to file is encountered again by @command{tar} while creating
-the same archive, an entire second copy of it will be stored. (This
-@emph{might} be considered a bug.)
-
-So, for portable archives, do not archive symbolic links as such,
-and use @option{--dereference} (@option{-h}): many systems do not support
+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.
-The @option{--dereference} option is not secure if an untrusted user
-can modify files during creation or extraction. @xref{Security}.
+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
projects / chaz / tar / blobdiff