From: Sergey Poznyakoff Date: Sun, 25 Jun 2006 12:45:03 +0000 (+0000) Subject: (Other Tars): New node describing how to extract X-Git-Url: https://git.brokenzipper.com/gitweb?a=commitdiff_plain;h=39e5d9182c02b0a5204d406794640ef6e71bdcb8;p=chaz%2Ftar (Other Tars): New node describing how to extract GNU-specific member formats using third-party tars. --- diff --git a/doc/tar.texi b/doc/tar.texi index d39c2de..1d1131d 100644 --- a/doc/tar.texi +++ b/doc/tar.texi @@ -109,8 +109,8 @@ Appendices * Changes:: * Configuring Help Summary:: -* Genfile:: * Tar Internals:: +* Genfile:: * Free Software Needs Free Documentation:: * Copying This Manual:: * Index of Command Line Options:: @@ -330,11 +330,18 @@ Making @command{tar} Archives More Portable * 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 @GNUTAR{} and @acronym{POSIX} @command{tar} * PAX keywords:: Controlling Extended Header Keywords. +How to Extract GNU-Specific Data Using Other @command{tar} Implementations + +* Split Recovery:: Members Split Between Volumes +* Sparse Recovery:: Sparse Members + Using Less Space through Compression * gzip:: Creating and Reading Compressed Archives @@ -369,12 +376,6 @@ Using Multiple Tapes * Tarcat:: Concatenate Volumes into a Single Archive -Genfile - -* Generate Mode:: File Generation Mode. -* Status Mode:: File Status Mode. -* Exec Mode:: Synchronous Execution mode. - Tar Internals * Standard:: Basic Tar Format @@ -389,6 +390,12 @@ Storing Sparse Files * PAX 0:: PAX Format, Versions 0.0 and 0.1 * PAX 1:: PAX Format, Version 1.0 +Genfile + +* Generate Mode:: File Generation Mode. +* Status Mode:: File Status Mode. +* Exec Mode:: Synchronous Execution mode. + Copying This Manual * GNU Free Documentation License:: License for copying this manual @@ -7753,6 +7760,8 @@ archives and archive labels) in GNU and PAX formats.} * 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 @@ -8070,6 +8079,350 @@ be extracted by any tar implementation that understands older @FIXME{Describe how @acronym{POSIX} archives are extracted by non POSIX-aware tars.} +@node Other Tars +@subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations + +In previous sections you became acquainted with various quircks +necessary to make your archives portable. Sometimes you may need to +extract archives containing GNU-specific members using some +third-party @command{tar} implementation or an older version of +@GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed, +but if it is for some reason impossible, this section will explain +how to cope without it. + +When we speak about @dfn{GNU-specific} members we mean two classes of +them: members split between the volumes of a multi-volume archive and +sparse members. You will be able to always recover such members if +the archive is in PAX format. In addition split members can be +recovered from archives in old GNU format. The following subsections +describe the required procedures in detail. + +@menu +* Split Recovery:: Members Split Between Volumes +* Sparse Recovery:: Sparse Members +@end menu + +@node Split Recovery +@subsubsection Extracting Members Split Between Volumes + +If a member is split between several volumes of an old GNU format archive +most third party @command{tar} implementation will fail to extract +it. To extract it, use @command{tarcat} program (@pxref{Tarcat}). +This program is available from +@uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat, @GNUTAR{} +home page}. It concatenates several archive volumes into a single +valid archive. For example, if you have three volumes named from +@file{vol-1.tar} to @file{vol-2.tar}, you can do the following to +extract them using a third-party @command{tar}: + +@smallexample +$ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -} +@end smallexample + +You could use this approach for many (although not all) PAX +format archives as well. However, extracting split members from a PAX +archive is a much easier task, because PAX volumes are constructed in +such a way that each part of a split member is extracted as a +different file by @command{tar} implementations that are not aware of +GNU extensions. More specifically, the very first part retains its +original name, and all subsequent parts are named using the pattern: + +@smallexample +%d/GNUFileParts.%p/%f.%n +@end smallexample + +@noindent +where symbols preceeded by @samp{%} are @dfn{macro characters} that +have the following meaning: + +@multitable @columnfractions .25 .55 +@headitem Meta-character @tab Replaced By +@item %d @tab The directory name of the file, equivalent to the +result of the @command{dirname} utility on its full name. +@item %f @tab The file name of the file, equivalent to the result +of the @command{basename} utility on its full name. +@item %p @tab The process ID of the @command{tar} process that +created the archive. +@item %n @tab Ordinal number of this particular part. +@end multitable + +For example, if, a file @file{var/longfile} was split during archive +creation between three volumes, and the creator @command{tar} process +had process ID @samp{27962}, then the member names will be: + +@smallexample +var/longfile +var/GNUFileParts.27962/longfile.1 +var/GNUFileParts.27962/longfile.2 +@end smallexample + +When you extract your archive using a third-party @command{tar}, these +files will be created on your disk, and the only thing you will need +to do to restore your file in its original form is concatenate them in +the proper order, for example: + +@smallexample +@group +$ @kbd{cd var} +$ @kbd{cat GNUFileParts.27962/longfile.1 \ + GNUFileParts.27962/longfile.2 >> longfile} +$ rm -f GNUFileParts.27962 +@end group +@end smallexample + +Notice, that if the @command{tar} implementation you use supports PAX +format archives, it will probably emit warnings about unknown keywords +during extraction. They will lool like this: + +@smallexample +@group +Tar file too small +Unknown extended header keyword 'GNU.volume.filename' ignored. +Unknown extended header keyword 'GNU.volume.size' ignored. +Unknown extended header keyword 'GNU.volume.offset' ignored. +@end group +@end smallexample + +@noindent +You can safely ignore these warnings. + +If your @command{tar} implementation is not PAX-aware, you will get +more warnigns and more files generated on your disk, e.g.: + +@smallexample +@group +$ @kbd{tar xf vol-1.tar} +var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as +normal file +Unexpected EOF in archive +$ @kbd{tar xf vol-2.tar} +tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file +GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type +'x', extracted as normal file +@end group +@end smallexample + +Ignore these warnings. The @file{PaxHeaders.*} directories created +will contain files with @dfn{extended header keywords} describing the +extracted files. You can delete them, unless they describe sparse +members. Read further to learn more about them. + +@node Sparse Recovery +@subsubsection Extracting Sparse Members + +Any @command{tar} implementation will be able to extract sparse members from a +PAX archive. However, the extracted files will be @dfn{condensed}, +i.e. any zero blocks will be removed from them. When we restore such +a condensed file to its original form, by adding zero bloks (or +@dfn{holes}) back to their original locations, we call this process +@dfn{expanding} a compressed sparse file. + +To expand a file, you will need a simple auxiliary program called +@command{xsparse}. It is available in source form from +@uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse, @GNUTAR{} +home page}. + +Let's begin with archive members in @dfn{sparse format +version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand. +The condensed file will contain both file map and file data, so no +additional data will be needed to restore it. If the original file +name was @file{@var{dir}/@var{name}}, then the condensed file will be +named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where +@var{n} is a decimal number@footnote{technically speaking, @var{n} is a +@dfn{process ID} of the @command{tar} process which created the +archive (@pxref{PAX keywords}).}. + +To expand a version 1.0 file, run @command{xsparse} as follows: + +@smallexample +$ @kbd{xsparse @file{cond-file}} +@end smallexample + +@noindent +where @file{cond-file} is the name of the condensed file. The utility +will deduce the name for the resulting expanded file using the +following algorithm: + +@enumerate 1 +@item If @file{cond-file} does not contain any directories, +@file{../cond-file} will be used; + +@item If @file{cond-file} has the form +@file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name} +are simple names, with no @samp{/} characters in them, the output file +name will be @file{@var{dir}/@var{name}}. + +@item Otherwise, if @file{cond-file} has the form +@file{@var{dir}/@var{name}}, the output file name will be +@file{@var{name}}. +@end enumerate + +In the unlikely case when this algorithm does not suite your needs, +you can explicitely specify output file name as a second argument to +the command: + +@smallexample +$ @kbd{xsparse @file{cond-file}} +@end smallexample + +It is often a good idea to run @command{xsparse} in @dfn{dry run} mode +first. In this mode, the command does not actually expand the file, +but verbosely lists all actions it would be taking to do so. The dry +run mode is enabled by @option{-n} command line argument: + +@smallexample +@group +$ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile} +Reading v.1.0 sparse map +Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to +`/home/gray/sparsefile' +Finished dry run +@end group +@end smallexample + +To actually expand the file, you would run: + +@smallexample +$ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile} +@end smallexample + +@noindent +The program behaves the same way all UNIX utilities do: it will keep +quiet unless it has simething important to tell you (e.g. an error +condition or something). If you wish it to produce verbose output, +similar to that from the dry run mode, give it @option{-v} option: + +@smallexample +@group +$ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile} +Reading v.1.0 sparse map +Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to +`/home/gray/sparsefile' +Done +@end group +@end smallexample + +Additionally, if your @command{tar} implementation has extracted the +@dfn{extended headers} for this file, you can instruct @command{xstar} +to use them in order to verify the integrity of the expanded file. +The option @option{-x} sets the name of the extended header file to +use. Continuing our example: + +@smallexample +@group +$ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \ + /home/gray/GNUSparseFile.6058/sparsefile} +Reading extended header file +Found variable GNU.sparse.major = 1 +Found variable GNU.sparse.minor = 0 +Found variable GNU.sparse.name = sparsefile +Found variable GNU.sparse.realsize = 217481216 +Reading v.1.0 sparse map +Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to +`/home/gray/sparsefile' +Done +@end group +@end smallexample + +An @dfn{extended header} is a special @command{tar} archive header +that precedes an archive member and contains a set of +@dfn{variables}, describing the member properties that cannot be +stored in the standard @code{ustar} header. While optional for +expanding sparse version 1.0 members, use of extended headers is +mandatory when expanding sparse members in older sparse formats: v.0.0 +and v.0.1 (The sparse formats are described in detail in @pxref{Sparse +Formats}). So, for this format, the question is: how to obtain +extended headers from the archive? + +If you use a @command{tar} implementation that does not support PAX +format, extended headers for each member will be extracted as a +separate file. If we represent the member name as +@file{@var{dir}/@var{name}}, then the extended header file will be +named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where +@var{n} is an integer number. + +Things become more difficult if your @command{tar} implementation +does support PAX headers, because in this case you will have to +manually extract the headers. We recommend the following algorithm: + +@enumerate 1 +@item +Consult the documentation for your @command{tar} implementation for an +option that will print @dfn{block numbers} along with the archive +listing (analogous to @GNUTAR{}'s @option{-R} option). For example, +@command{star} has @option{-block-number}. + +@item +Obtain the verbose listing using the @samp{block number} option, and +find the position of the sparse member in question and the member +immediately following it. For example, running @command{star} on our +archive we obtain: + +@smallexample +@group +$ @kbd{star -t -v -block-number -f arc.tar} +@dots{} +star: Unknown extended header keyword 'GNU.sparse.size' ignored. +star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored. +star: Unknown extended header keyword 'GNU.sparse.name' ignored. +star: Unknown extended header keyword 'GNU.sparse.map' ignored. +block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile +block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README +@dots{} +@end group +@end smallexample + +@noindent +(as usual, ignore the warnings about unknown keywords.) + +@item +Let the size of the sparse member be @var{size}, its block number be +@var{Bs} and the block number of the next member be @var{Bn}. +Compute: + +@smallexample +@var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2 +@end smallexample + +@noindent +This number gives the size of the extended header part in tar @dfn{blocks}. +In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2 += 7}. + +@item +Use @command{dd} to extract the headers: + +@smallexample +@kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}} +@end smallexample + +@noindent +where @var{archive} is the archive name, @var{hname} is a name of the +file to store the extended header in, @var{Bs} and @var{N} are +computed in previous steps. + +In our example, this command will be + +@smallexample +$ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7} +@end smallexample +@end enumerate + +Finally, you can expand the condensed file, using the obtained header: + +@smallexample +@group +$ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile} +Reading extended header file +Found variable GNU.sparse.size = 217481216 +Found variable GNU.sparse.numblocks = 208 +Found variable GNU.sparse.name = sparsefile +Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{} +Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile' +Done +@end group +@end smallexample + @node Compression @section Using Less Space through Compression @@ -10432,14 +10785,14 @@ output. Default is 12. Right margin of the text output. Used for wrapping. @end deftypevr -@node Genfile -@appendix Genfile -@include genfile.texi - @node Tar Internals @appendix Tar Internals @include intern.texi +@node Genfile +@appendix Genfile +@include genfile.texi + @node Free Software Needs Free Documentation @appendix Free Software Needs Free Documentation @include freemanuals.texi