1 \input texinfo @c -*-texinfo-*-
2 @comment %**start of header
5 @settitle GNU tar @value{VERSION}
14 @c 1. Pay attention to @FIXME{}s and @UNREVISED{}s
15 @c 2. Before creating final variant:
16 @c 2.1. Run 'make check-options' to make sure all options are properly
18 @c 2.2. Run 'make master-menu' (see comment before the master menu).
20 @include rendition.texi
26 @c Put everything in one index (arbitrarily chosen to be the concept index).
35 This manual is for @acronym{GNU} @command{tar} (version
36 @value{VERSION}, @value{UPDATED}), which creates and extracts files
39 Copyright @copyright{} 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001,
40 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Free Software
44 Permission is granted to copy, distribute and/or modify this document
45 under the terms of the GNU Free Documentation License, Version 1.3 or
46 any later version published by the Free Software Foundation; with no
47 Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
48 A copy of the license is included in the section entitled ``GNU Free
49 Documentation License''.
53 @dircategory Archiving
55 * Tar: (tar). Making tape (or disk) archives.
58 @dircategory Individual utilities
60 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
63 @shorttitlepage @acronym{GNU} @command{tar}
66 @title @acronym{GNU} tar: an archiver tool
67 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
68 @author John Gilmore, Jay Fenlason et al.
71 @vskip 0pt plus 1filll
77 @top @acronym{GNU} tar: an archiver tool
82 @cindex archiving files
84 The first part of this master menu lists the major nodes in this Info
85 document. The rest of the menu lists all the lower level nodes.
88 @c The master menu goes here.
90 @c NOTE: To update it from within Emacs, make sure mastermenu.el is
91 @c loaded and run texinfo-master-menu.
92 @c To update it from the command line, run
103 * Date input formats::
106 * Reliability and security::
111 * Configuring Help Summary::
112 * Fixing Snapshot Files::
115 * Free Software Needs Free Documentation::
116 * GNU Free Documentation License::
117 * Index of Command Line Options::
121 --- The Detailed Node Listing ---
125 * Book Contents:: What this Book Contains
126 * Definitions:: Some Definitions
127 * What tar Does:: What @command{tar} Does
128 * Naming tar Archives:: How @command{tar} Archives are Named
129 * Authors:: @GNUTAR{} Authors
130 * Reports:: Reporting bugs or suggestions
132 Tutorial Introduction to @command{tar}
135 * stylistic conventions::
136 * basic tar options:: Basic @command{tar} Operations and Options
137 * frequent operations::
138 * Two Frequent Options::
139 * create:: How to Create Archives
140 * list:: How to List Archives
141 * extract:: How to Extract Members from an Archive
144 Two Frequently Used Options
150 How to Create Archives
152 * prepare for examples::
153 * Creating the archive::
162 How to Extract Members from an Archive
164 * extracting archives::
167 * extracting untrusted archives::
173 * using tar options::
183 The Three Option Styles
185 * Long Options:: Long Option Style
186 * Short Options:: Short Option Style
187 * Old Options:: Old Option Style
188 * Mixing:: Mixing Option Styles
190 All @command{tar} Options
192 * Operation Summary::
194 * Short Option Summary::
206 Advanced @GNUTAR{} Operations
215 How to Add Files to Existing Archives: @option{--append}
217 * appending files:: Appending Files to an Archive
224 Options Used by @option{--create}
226 * override:: Overriding File Metadata.
227 * Ignore Failed Read::
229 Options Used by @option{--extract}
231 * Reading:: Options to Help Read Archives
232 * Writing:: Changing How @command{tar} Writes Files
233 * Scarce:: Coping with Scarce Resources
235 Options to Help Read Archives
237 * read full records::
240 Changing How @command{tar} Writes Files
242 * Dealing with Old Files::
243 * Overwrite Old Files::
248 * Data Modification Times::
249 * Setting Access Permissions::
250 * Directory Modification Times and Permissions::
251 * Writing to Standard Output::
252 * Writing to an External Program::
255 Coping with Scarce Resources
260 Performing Backups and Restoring Files
262 * Full Dumps:: Using @command{tar} to Perform Full Dumps
263 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
264 * Backup Levels:: Levels of Backups
265 * Backup Parameters:: Setting Parameters for Backups and Restoration
266 * Scripted Backups:: Using the Backup Scripts
267 * Scripted Restoration:: Using the Restore Script
269 Setting Parameters for Backups and Restoration
271 * General-Purpose Variables::
272 * Magnetic Tape Control::
274 * backup-specs example:: An Example Text of @file{Backup-specs}
276 Choosing Files and Names for @command{tar}
278 * file:: Choosing the Archive's Name
279 * Selecting Archive Members::
280 * files:: Reading Names from a File
281 * exclude:: Excluding Some Files
282 * wildcards:: Wildcards Patterns and Matching
283 * quoting styles:: Ways of Quoting Special Characters in Names
284 * transform:: Modifying File and Member Names
285 * after:: Operating Only on New Files
286 * recurse:: Descending into Directories
287 * one:: Crossing File System Boundaries
289 Reading Names from a File
295 * problems with exclude::
297 Wildcards Patterns and Matching
299 * controlling pattern-matching::
301 Crossing File System Boundaries
303 * directory:: Changing Directory
304 * absolute:: Absolute File Names
308 * General date syntax:: Common rules.
309 * Calendar date items:: 19 Dec 1994.
310 * Time of day items:: 9:20pm.
311 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}.
312 * Day of week items:: Monday and others.
313 * Relative items in date strings:: next tuesday, 2 years ago.
314 * Pure numbers in date strings:: 19931219, 1440.
315 * Seconds since the Epoch:: @@1078100502.
316 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0".
317 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al.
319 Controlling the Archive Format
321 * Compression:: Using Less Space through Compression
322 * Attributes:: Handling File Attributes
323 * Portability:: Making @command{tar} Archives More Portable
324 * cpio:: Comparison of @command{tar} and @command{cpio}
326 Using Less Space through Compression
328 * gzip:: Creating and Reading Compressed Archives
329 * sparse:: Archiving Sparse Files
331 Creating and Reading Compressed Archives
333 * lbzip2:: Using lbzip2 with @GNUTAR{}.
335 Making @command{tar} Archives More Portable
337 * Portable Names:: Portable Names
338 * dereference:: Symbolic Links
339 * hard links:: Hard Links
340 * old:: Old V7 Archives
341 * ustar:: Ustar Archives
342 * gnu:: GNU and old GNU format archives.
343 * posix:: @acronym{POSIX} archives
344 * Checksumming:: Checksumming Problems
345 * Large or Negative Values:: Large files, negative time stamps, etc.
346 * Other Tars:: How to Extract GNU-Specific Data Using
347 Other @command{tar} Implementations
349 @GNUTAR{} and @acronym{POSIX} @command{tar}
351 * PAX keywords:: Controlling Extended Header Keywords.
353 How to Extract GNU-Specific Data Using Other @command{tar} Implementations
355 * Split Recovery:: Members Split Between Volumes
356 * Sparse Recovery:: Sparse Members
358 Tapes and Other Archive Media
360 * Device:: Device selection and switching
361 * Remote Tape Server::
362 * Common Problems and Solutions::
363 * Blocking:: Blocking
364 * Many:: Many archives on one tape
365 * Using Multiple Tapes:: Using Multiple Tapes
366 * label:: Including a Label in the Archive
372 * Format Variations:: Format Variations
373 * Blocking Factor:: The Blocking Factor of an Archive
375 Many Archives on One Tape
377 * Tape Positioning:: Tape Positions and Tape Marks
378 * mt:: The @command{mt} Utility
382 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
383 * Tape Files:: Tape Files
384 * Tarcat:: Concatenate Volumes into a Single Archive
389 * Standard:: Basic Tar Format
390 * Extensions:: @acronym{GNU} Extensions to the Archive Format
391 * Sparse Formats:: Storing Sparse Files
398 * PAX 0:: PAX Format, Versions 0.0 and 0.1
399 * PAX 1:: PAX Format, Version 1.0
403 * Generate Mode:: File Generation Mode.
404 * Status Mode:: File Status Mode.
405 * Exec Mode:: Synchronous Execution mode.
409 * GNU Free Documentation License:: License for copying this manual
415 @chapter Introduction
418 and manipulates @dfn{archives} which are actually collections of
419 many other files; the program provides users with an organized and
420 systematic method for controlling a large amount of data.
421 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
422 archives need not (and these days, typically do not) reside on tapes.
425 * Book Contents:: What this Book Contains
426 * Definitions:: Some Definitions
427 * What tar Does:: What @command{tar} Does
428 * Naming tar Archives:: How @command{tar} Archives are Named
429 * Authors:: @GNUTAR{} Authors
430 * Reports:: Reporting bugs or suggestions
434 @section What this Book Contains
436 The first part of this chapter introduces you to various terms that will
437 recur throughout the book. It also tells you who has worked on @GNUTAR{}
438 and its documentation, and where you should send bug reports
441 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
442 gentle introduction for people who are new to using @command{tar}. It is
443 meant to be self-contained, not requiring any reading from subsequent
444 chapters to make sense. It moves from topic to topic in a logical,
445 progressive order, building on information already explained.
447 Although the tutorial is paced and structured to allow beginners to
448 learn how to use @command{tar}, it is not intended solely for beginners.
449 The tutorial explains how to use the three most frequently used
450 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
451 two frequently used options (@samp{file} and @samp{verbose}). The other
452 chapters do not refer to the tutorial frequently; however, if a section
453 discusses something which is a complex variant of a basic concept, there
454 may be a cross-reference to that basic concept. (The entire book,
455 including the tutorial, assumes that the reader understands some basic
456 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
458 The third chapter presents the remaining five operations, and
459 information about using @command{tar} options and option syntax.
461 The other chapters are meant to be used as a reference. Each chapter
462 presents everything that needs to be said about a specific topic.
464 One of the chapters (@pxref{Date input formats}) exists in its
465 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
466 In addition, one section of this manual (@pxref{Standard}) contains a
467 big quote which is taken directly from @command{tar} sources.
469 In general, we give both long and short (abbreviated) option names
470 at least once in each section where the relevant option is covered, so
471 that novice readers will become familiar with both styles. (A few
472 options have no short versions, and the relevant sections will
476 @section Some Definitions
480 The @command{tar} program is used to create and manipulate @command{tar}
481 archives. An @dfn{archive} is a single file which contains the contents
482 of many files, while still identifying the names of the files, their
483 owner(s), and so forth. (In addition, archives record access
484 permissions, user and group, size in bytes, and data modification time.
485 Some archives also record the file names in each archived directory, as
486 well as other file and directory information.) You can use @command{tar}
487 to @dfn{create} a new archive in a specified directory.
490 @cindex archive member
493 The files inside an archive are called @dfn{members}. Within this
494 manual, we use the term @dfn{file} to refer only to files accessible in
495 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
496 @dfn{member} to refer only to the members of an archive. Similarly, a
497 @dfn{file name} is the name of a file, as it resides in the file system,
498 and a @dfn{member name} is the name of an archive member within the
503 The term @dfn{extraction} refers to the process of copying an archive
504 member (or multiple members) into a file in the file system. Extracting
505 all the members of an archive is often called @dfn{extracting the
506 archive}. The term @dfn{unpack} can also be used to refer to the
507 extraction of many or all the members of an archive. Extracting an
508 archive does not destroy the archive's structure, just as creating an
509 archive does not destroy the copies of the files that exist outside of
510 the archive. You may also @dfn{list} the members in a given archive
511 (this is often thought of as ``printing'' them to the standard output,
512 or the command line), or @dfn{append} members to a pre-existing archive.
513 All of these operations can be performed using @command{tar}.
516 @section What @command{tar} Does
519 The @command{tar} program provides the ability to create @command{tar}
520 archives, as well as various other kinds of manipulation. For example,
521 you can use @command{tar} on previously created archives to extract files,
522 to store additional files, or to update or list files which were already
525 Initially, @command{tar} archives were used to store files conveniently on
526 magnetic tape. The name @command{tar} comes from this use; it stands for
527 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
528 direct its output to available devices, files, or other programs (using
529 pipes). @command{tar} may even access remote devices or files (as archives).
531 You can use @command{tar} archives in many ways. We want to stress a few
532 of them: storage, backup, and transportation.
534 @FIXME{the following table entries need a bit of work.}
537 Often, @command{tar} archives are used to store related files for
538 convenient file transfer over a network. For example, the
539 @acronym{GNU} Project distributes its software bundled into
540 @command{tar} archives, so that all the files relating to a particular
541 program (or set of related programs) can be transferred as a single
544 A magnetic tape can store several files in sequence. However, the tape
545 has no names for these files; it only knows their relative position on
546 the tape. One way to store several files on one tape and retain their
547 names is by creating a @command{tar} archive. Even when the basic transfer
548 mechanism can keep track of names, as FTP can, the nuisance of handling
549 multiple files, directories, and multiple links makes @command{tar}
552 Archive files are also used for long-term storage. You can think of
553 this as transportation from the present into the future. (It is a
554 science-fiction idiom that you can move through time as well as in
555 space; the idea here is that @command{tar} can be used to move archives in
556 all dimensions, even time!)
559 Because the archive created by @command{tar} is capable of preserving
560 file information and directory structure, @command{tar} is commonly
561 used for performing full and incremental backups of disks. A backup
562 puts a collection of files (possibly pertaining to many users and
563 projects) together on a disk or a tape. This guards against
564 accidental destruction of the information in those files.
565 @GNUTAR{} has special features that allow it to be
566 used to make incremental and full dumps of all the files in a
570 You can create an archive on one system, transfer it to another system,
571 and extract the contents there. This allows you to transport a group of
572 files from one system to another.
575 @node Naming tar Archives
576 @section How @command{tar} Archives are Named
578 Conventionally, @command{tar} archives are given names ending with
579 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
580 but this manual follows that convention in order to accustom readers to
581 it and to make examples more clear.
586 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
587 archive members as ``files'' or ``entries''. For people familiar with
588 the operation of @command{tar}, this causes no difficulty. However, in
589 this manual, we consistently refer to ``archives'' and ``archive
590 members'' to make learning to use @command{tar} easier for novice users.
593 @section @GNUTAR{} Authors
595 @GNUTAR{} was originally written by John Gilmore,
596 and modified by many people. The @acronym{GNU} enhancements were
597 written by Jay Fenlason, then Joy Kendall, and the whole package has
598 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
599 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
600 numerous and kind users.
602 We wish to stress that @command{tar} is a collective work, and owes much to
603 all those people who reported problems, offered solutions and other
604 insights, or shared their thoughts and suggestions. An impressive, yet
605 partial list of those contributors can be found in the @file{THANKS}
606 file from the @GNUTAR{} distribution.
608 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
609 sure i want to spell out the history in this detail, at least not for
610 the printed book. i'm just not sure it needs to be said this way.
611 i'll think about it.}
613 @FIXME{History is more important, and surely more interesting, than
614 actual names. Quoting names without history would be meaningless. FP}
616 Jay Fenlason put together a draft of a @GNUTAR{}
617 manual, borrowing notes from the original man page from John Gilmore.
618 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
619 Gorin worked on a tutorial and manual for @GNUTAR{}.
620 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
621 taking information from all these sources and merging them. Melissa
622 Weisshaus finally edited and redesigned the book to create version
623 1.12. The book for versions from 1.14 up to @value{VERSION} were edited
624 by the current maintainer, Sergey Poznyakoff.
626 For version 1.12, Daniel Hagerty contributed a great deal of technical
627 consulting. In particular, he is the primary author of @ref{Backups}.
629 In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org
630 (see @url{http://savannah.gnu.org/projects/tar}), and
631 active development and maintenance work has started
632 again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey
633 Poznyakoff and Jeff Bailey.
635 Support for @acronym{POSIX} archives was added by Sergey Poznyakoff.
638 @section Reporting bugs or suggestions
641 @cindex reporting bugs
642 If you find problems or have suggestions about this program or manual,
643 please report them to @file{bug-tar@@gnu.org}.
645 When reporting a bug, please be sure to include as much detail as
646 possible, in order to reproduce it. @FIXME{Be more specific, I'd
647 like to make this node as detailed as 'Bug reporting' node in Emacs
651 @chapter Tutorial Introduction to @command{tar}
653 This chapter guides you through some basic examples of three @command{tar}
654 operations: @option{--create}, @option{--list}, and @option{--extract}. If
655 you already know how to use some other version of @command{tar}, then you
656 may not need to read this chapter. This chapter omits most complicated
657 details about how @command{tar} works.
661 * stylistic conventions::
662 * basic tar options:: Basic @command{tar} Operations and Options
663 * frequent operations::
664 * Two Frequent Options::
665 * create:: How to Create Archives
666 * list:: How to List Archives
667 * extract:: How to Extract Members from an Archive
672 @section Assumptions this Tutorial Makes
674 This chapter is paced to allow beginners to learn about @command{tar}
675 slowly. At the same time, we will try to cover all the basic aspects of
676 these three operations. In order to accomplish both of these tasks, we
677 have made certain assumptions about your knowledge before reading this
678 manual, and the hardware you will be using:
682 Before you start to work through this tutorial, you should understand
683 what the terms ``archive'' and ``archive member'' mean
684 (@pxref{Definitions}). In addition, you should understand something
685 about how Unix-type operating systems work, and you should know how to
686 use some basic utilities. For example, you should know how to create,
687 list, copy, rename, edit, and delete files and directories; how to
688 change between directories; and how to figure out where you are in the
689 file system. You should have some basic understanding of directory
690 structure and how files are named according to which directory they are
691 in. You should understand concepts such as standard output and standard
692 input, what various definitions of the term @samp{argument} mean, and the
693 differences between relative and absolute file names. @FIXME{and what
697 This manual assumes that you are working from your own home directory
698 (unless we state otherwise). In this tutorial, you will create a
699 directory to practice @command{tar} commands in. When we show file names,
700 we will assume that those names are relative to your home directory.
701 For example, my home directory is @file{/home/fsf/melissa}. All of
702 my examples are in a subdirectory of the directory named by that file
703 name; the subdirectory is called @file{practice}.
706 In general, we show examples of archives which exist on (or can be
707 written to, or worked with from) a directory on a hard disk. In most
708 cases, you could write those archives to, or work with them on any other
709 device, such as a tape drive. However, some of the later examples in
710 the tutorial and next chapter will not work on tape drives.
711 Additionally, working with tapes is much more complicated than working
712 with hard disks. For these reasons, the tutorial does not cover working
713 with tape drives. @xref{Media}, for complete information on using
714 @command{tar} archives with tape drives.
716 @FIXME{this is a cop out. need to add some simple tape drive info.}
719 @node stylistic conventions
720 @section Stylistic Conventions
722 In the examples, @samp{$} represents a typical shell prompt. It
723 precedes lines you should type; to make this more clear, those lines are
724 shown in @kbd{this font}, as opposed to lines which represent the
725 computer's response; those lines are shown in @code{this font}, or
726 sometimes @samp{like this}.
728 @c When we have lines which are too long to be
729 @c displayed in any other way, we will show them like this:
731 @node basic tar options
732 @section Basic @command{tar} Operations and Options
734 @command{tar} can take a wide variety of arguments which specify and define
735 the actions it will have on the particular set of files or the archive.
736 The main types of arguments to @command{tar} fall into one of two classes:
737 operations, and options.
739 Some arguments fall into a class called @dfn{operations}; exactly one of
740 these is both allowed and required for any instance of using @command{tar};
741 you may @emph{not} specify more than one. People sometimes speak of
742 @dfn{operating modes}. You are in a particular operating mode when you
743 have specified the operation which specifies it; there are eight
744 operations in total, and thus there are eight operating modes.
746 The other arguments fall into the class known as @dfn{options}. You are
747 not required to specify any options, and you are allowed to specify more
748 than one at a time (depending on the way you are using @command{tar} at
749 that time). Some options are used so frequently, and are so useful for
750 helping you type commands more carefully that they are effectively
751 ``required''. We will discuss them in this chapter.
753 You can write most of the @command{tar} operations and options in any
754 of three forms: long (mnemonic) form, short form, and old style. Some
755 of the operations and options have no short or ``old'' forms; however,
756 the operations and options which we will cover in this tutorial have
757 corresponding abbreviations. We will indicate those abbreviations
758 appropriately to get you used to seeing them. Note, that the ``old
759 style'' option forms exist in @GNUTAR{} for compatibility with Unix
760 @command{tar}. In this book we present a full discussion of this way
761 of writing options and operations (@pxref{Old Options}), and we discuss
762 the other two styles of writing options (@xref{Long Options}, and
763 @pxref{Short Options}).
765 In the examples and in the text of this tutorial, we usually use the
766 long forms of operations and options; but the ``short'' forms produce
767 the same result and can make typing long @command{tar} commands easier.
768 For example, instead of typing
771 @kbd{tar --create --verbose --file=afiles.tar apple angst aspic}
777 @kbd{tar -c -v -f afiles.tar apple angst aspic}
783 @kbd{tar -cvf afiles.tar apple angst aspic}
787 For more information on option syntax, see @ref{Advanced tar}. In
788 discussions in the text, when we name an option by its long form, we
789 also give the corresponding short option in parentheses.
791 The term, ``option'', can be confusing at times, since ``operations''
792 are often lumped in with the actual, @emph{optional} ``options'' in certain
793 general class statements. For example, we just talked about ``short and
794 long forms of options and operations''. However, experienced @command{tar}
795 users often refer to these by shorthand terms such as, ``short and long
796 options''. This term assumes that the ``operations'' are included, also.
797 Context will help you determine which definition of ``options'' to use.
799 Similarly, the term ``command'' can be confusing, as it is often used in
800 two different ways. People sometimes refer to @command{tar} ``commands''.
801 A @command{tar} @dfn{command} is the entire command line of user input
802 which tells @command{tar} what to do --- including the operation, options,
803 and any arguments (file names, pipes, other commands, etc.). However,
804 you will also sometimes hear the term ``the @command{tar} command''. When
805 the word ``command'' is used specifically like this, a person is usually
806 referring to the @command{tar} @emph{operation}, not the whole line.
807 Again, use context to figure out which of the meanings the speaker
810 @node frequent operations
811 @section The Three Most Frequently Used Operations
813 Here are the three most frequently used operations (both short and long
814 forms), as well as a brief description of their meanings. The rest of
815 this chapter will cover how to use these operations in detail. We will
816 present the rest of the operations in the next chapter.
821 Create a new @command{tar} archive.
824 List the contents of an archive.
827 Extract one or more members from an archive.
830 @node Two Frequent Options
831 @section Two Frequently Used Options
833 To understand how to run @command{tar} in the three operating modes listed
834 previously, you also need to understand how to use two of the options to
835 @command{tar}: @option{--file} (which takes an archive file as an argument)
836 and @option{--verbose}. (You are usually not @emph{required} to specify
837 either of these options when you run @command{tar}, but they can be very
838 useful in making things more clear and helping you avoid errors.)
847 @unnumberedsubsec The @option{--file} Option
850 @xopindex{file, tutorial}
851 @item --file=@var{archive-name}
852 @itemx -f @var{archive-name}
853 Specify the name of an archive file.
856 You can specify an argument for the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option whenever you
857 use @command{tar}; this option determines the name of the archive file
858 that @command{tar} will work on.
861 If you don't specify this argument, then @command{tar} will examine
862 the environment variable @env{TAPE}. If it is set, its value will be
863 used as the archive name. Otherwise, @command{tar} will use the
864 default archive, determined at compile time. Usually it is
865 standard output or some physical tape drive attached to your machine
866 (you can verify what the default is by running @kbd{tar
867 --show-defaults}, @pxref{defaults}). If there is no tape drive
868 attached, or the default is not meaningful, then @command{tar} will
869 print an error message. The error message might look roughly like one
873 tar: can't open /dev/rmt8 : No such device or address
874 tar: can't open /dev/rsmt0 : I/O error
878 To avoid confusion, we recommend that you always specify an archive file
879 name by using @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) when writing your @command{tar} commands.
880 For more information on using the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option, see
883 @node verbose tutorial
884 @unnumberedsubsec The @option{--verbose} Option
887 @xopindex{verbose, introduced}
890 Show the files being worked on as @command{tar} is running.
893 @option{--verbose} (@option{-v}) shows details about the results of running
894 @command{tar}. This can be especially useful when the results might not be
895 obvious. For example, if you want to see the progress of @command{tar} as
896 it writes files into the archive, you can use the @option{--verbose}
897 option. In the beginning, you may find it useful to use
898 @option{--verbose} at all times; when you are more accustomed to
899 @command{tar}, you will likely want to use it at certain times but not at
900 others. We will use @option{--verbose} at times to help make something
901 clear, and we will give many examples both using and not using
902 @option{--verbose} to show the differences.
904 Each instance of @option{--verbose} on the command line increases the
905 verbosity level by one, so if you need more details on the output,
908 When reading archives (@option{--list}, @option{--extract},
909 @option{--diff}), @command{tar} by default prints only the names of
910 the members being extracted. Using @option{--verbose} will show a full,
911 @command{ls} style member listing.
913 In contrast, when writing archives (@option{--create}, @option{--append},
914 @option{--update}), @command{tar} does not print file names by
915 default. So, a single @option{--verbose} option shows the file names
916 being added to the archive, while two @option{--verbose} options
917 enable the full listing.
919 For example, to create an archive in verbose mode:
922 $ @kbd{tar -cvf afiles.tar apple angst aspic}
929 Creating the same archive with the verbosity level 2 could give:
932 $ @kbd{tar -cvvf afiles.tar apple angst aspic}
933 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
934 -rw-r--r-- gray/staff 11481 2006-06-09 12:06 angst
935 -rw-r--r-- gray/staff 23152 2006-06-09 12:06 aspic
939 This works equally well using short or long forms of options. Using
940 long forms, you would simply write out the mnemonic form of the option
944 $ @kbd{tar --create --verbose --verbose @dots{}}
948 Note that you must double the hyphens properly each time.
950 Later in the tutorial, we will give examples using @w{@option{--verbose
953 @anchor{verbose member listing}
954 The full output consists of six fields:
957 @item File type and permissions in symbolic form.
958 These are displayed in the same format as the first column of
959 @command{ls -l} output (@pxref{What information is listed,
960 format=verbose, Verbose listing, fileutils, GNU file utilities}).
962 @item Owner name and group separated by a slash character.
963 If these data are not available (for example, when listing a @samp{v7} format
964 archive), numeric @acronym{ID} values are printed instead.
966 @item Size of the file, in bytes.
968 @item File modification date in ISO 8601 format.
970 @item File modification time.
973 If the name contains any special characters (white space, newlines,
974 etc.) these are displayed in an unambiguous form using so called
975 @dfn{quoting style}. For the detailed discussion of available styles
976 and on how to use them, see @ref{quoting styles}.
978 Depending on the file type, the name can be followed by some
979 additional information, described in the following table:
982 @item -> @var{link-name}
983 The file or archive member is a @dfn{symbolic link} and
984 @var{link-name} is the name of file it links to.
986 @item link to @var{link-name}
987 The file or archive member is a @dfn{hard link} and @var{link-name} is
988 the name of file it links to.
991 The archive member is an old GNU format long link. You will normally
995 The archive member is an old GNU format long name. You will normally
998 @item --Volume Header--
999 The archive member is a GNU @dfn{volume header} (@pxref{Tape Files}).
1001 @item --Continued at byte @var{n}--
1002 Encountered only at the beginning of a multi-volume archive
1003 (@pxref{Using Multiple Tapes}). This archive member is a continuation
1004 from the previous volume. The number @var{n} gives the offset where
1005 the original file was split.
1007 @item unknown file type @var{c}
1008 An archive member of unknown type. @var{c} is the type character from
1009 the archive header. If you encounter such a message, it means that
1010 either your archive contains proprietary member types @GNUTAR{} is not
1011 able to handle, or the archive is corrupted.
1016 For example, here is an archive listing containing most of the special
1017 suffixes explained above:
1021 V--------- 0/0 1536 2006-06-09 13:07 MyVolume--Volume Header--
1022 -rw-r--r-- gray/staff 456783 2006-06-09 12:06 aspic--Continued at byte 32456--
1023 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
1024 lrwxrwxrwx gray/staff 0 2006-06-09 13:01 angst -> apple
1025 -rw-r--r-- gray/staff 35793 2006-06-09 12:06 blues
1026 hrw-r--r-- gray/staff 0 2006-06-09 12:06 music link to blues
1034 @unnumberedsubsec Getting Help: Using the @option{--help} Option
1040 The @option{--help} option to @command{tar} prints out a very brief list of
1041 all operations and option available for the current version of
1042 @command{tar} available on your system.
1046 @section How to Create Archives
1049 @cindex Creation of the archive
1050 @cindex Archive, creation of
1051 One of the basic operations of @command{tar} is @option{--create} (@option{-c}), which
1052 you use to create a @command{tar} archive. We will explain
1053 @option{--create} first because, in order to learn about the other
1054 operations, you will find it useful to have an archive available to
1057 To make this easier, in this section you will first create a directory
1058 containing three files. Then, we will show you how to create an
1059 @emph{archive} (inside the new directory). Both the directory, and
1060 the archive are specifically for you to practice on. The rest of this
1061 chapter and the next chapter will show many examples using this
1062 directory and the files you will create: some of those files may be
1063 other directories and other archives.
1065 The three files you will archive in this example are called
1066 @file{blues}, @file{folk}, and @file{jazz}. The archive is called
1067 @file{collection.tar}.
1069 This section will proceed slowly, detailing how to use @option{--create}
1070 in @code{verbose} mode, and showing examples using both short and long
1071 forms. In the rest of the tutorial, and in the examples in the next
1072 chapter, we will proceed at a slightly quicker pace. This section
1073 moves more slowly to allow beginning users to understand how
1074 @command{tar} works.
1077 * prepare for examples::
1078 * Creating the archive::
1084 @node prepare for examples
1085 @subsection Preparing a Practice Directory for Examples
1087 To follow along with this and future examples, create a new directory
1088 called @file{practice} containing files called @file{blues}, @file{folk}
1089 and @file{jazz}. The files can contain any information you like:
1090 ideally, they should contain information which relates to their names,
1091 and be of different lengths. Our examples assume that @file{practice}
1092 is a subdirectory of your home directory.
1094 Now @command{cd} to the directory named @file{practice}; @file{practice}
1095 is now your @dfn{working directory}. (@emph{Please note}: Although
1096 the full file name of this directory is
1097 @file{/@var{homedir}/practice}, in our examples we will refer to
1098 this directory as @file{practice}; the @var{homedir} is presumed.)
1100 In general, you should check that the files to be archived exist where
1101 you think they do (in the working directory) by running @command{ls}.
1102 Because you just created the directory and the files and have changed to
1103 that directory, you probably don't need to do that this time.
1105 It is very important to make sure there isn't already a file in the
1106 working directory with the archive name you intend to use (in this case,
1107 @samp{collection.tar}), or that you don't care about its contents.
1108 Whenever you use @samp{create}, @command{tar} will erase the current
1109 contents of the file named by @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) if it exists. @command{tar}
1110 will not tell you if you are about to overwrite an archive unless you
1111 specify an option which does this (@pxref{backup}, for the
1112 information on how to do so). To add files to an existing archive,
1113 you need to use a different option, such as @option{--append} (@option{-r}); see
1114 @ref{append} for information on how to do this.
1116 @node Creating the archive
1117 @subsection Creating the Archive
1119 @xopindex{create, introduced}
1120 To place the files @file{blues}, @file{folk}, and @file{jazz} into an
1121 archive named @file{collection.tar}, use the following command:
1124 $ @kbd{tar --create --file=collection.tar blues folk jazz}
1127 The order of the arguments is not very important, @emph{when using long
1128 option forms}. You could also say:
1131 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1135 However, you can see that this order is harder to understand; this is
1136 why we will list the arguments in the order that makes the commands
1137 easiest to understand (and we encourage you to do the same when you use
1138 @command{tar}, to avoid errors).
1140 Note that the sequence
1141 @option{--file=@-collection.tar} is considered to be @emph{one} argument.
1142 If you substituted any other string of characters for
1143 @kbd{collection.tar}, then that string would become the name of the
1144 archive file you create.
1146 The order of the options becomes more important when you begin to use
1147 short forms. With short forms, if you type commands in the wrong order
1148 (even if you type them correctly in all other ways), you may end up with
1149 results you don't expect. For this reason, it is a good idea to get
1150 into the habit of typing options in the order that makes inherent sense.
1151 @xref{short create}, for more information on this.
1153 In this example, you type the command as shown above: @option{--create}
1154 is the operation which creates the new archive
1155 (@file{collection.tar}), and @option{--file} is the option which lets
1156 you give it the name you chose. The files, @file{blues}, @file{folk},
1157 and @file{jazz}, are now members of the archive, @file{collection.tar}
1158 (they are @dfn{file name arguments} to the @option{--create} operation.
1159 @xref{Choosing}, for the detailed discussion on these.) Now that they are
1160 in the archive, they are called @emph{archive members}, not files.
1161 (@pxref{Definitions,members}).
1163 When you create an archive, you @emph{must} specify which files you
1164 want placed in the archive. If you do not specify any archive
1165 members, @GNUTAR{} will complain.
1167 If you now list the contents of the working directory (@command{ls}), you will
1168 find the archive file listed as well as the files you saw previously:
1171 blues folk jazz collection.tar
1175 Creating the archive @samp{collection.tar} did not destroy the copies of
1176 the files in the directory.
1178 Keep in mind that if you don't indicate an operation, @command{tar} will not
1179 run and will prompt you for one. If you don't name any files, @command{tar}
1180 will complain. You must have write access to the working directory,
1181 or else you will not be able to create an archive in that directory.
1183 @emph{Caution}: Do not attempt to use @option{--create} (@option{-c}) to add files to
1184 an existing archive; it will delete the archive and write a new one.
1185 Use @option{--append} (@option{-r}) instead. @xref{append}.
1187 @node create verbose
1188 @subsection Running @option{--create} with @option{--verbose}
1190 @xopindex{create, using with @option{--verbose}}
1191 @xopindex{verbose, using with @option{--create}}
1192 If you include the @option{--verbose} (@option{-v}) option on the command line,
1193 @command{tar} will list the files it is acting on as it is working. In
1194 verbose mode, the @code{create} example above would appear as:
1197 $ @kbd{tar --create --verbose --file=collection.tar blues folk jazz}
1203 This example is just like the example we showed which did not use
1204 @option{--verbose}, except that @command{tar} generated the remaining
1206 lines (note the different font styles).
1212 In the rest of the examples in this chapter, we will frequently use
1213 @code{verbose} mode so we can show actions or @command{tar} responses that
1214 you would otherwise not see, and which are important for you to
1218 @subsection Short Forms with @samp{create}
1220 As we said before, the @option{--create} (@option{-c}) operation is one of the most
1221 basic uses of @command{tar}, and you will use it countless times.
1222 Eventually, you will probably want to use abbreviated (or ``short'')
1223 forms of options. A full discussion of the three different forms that
1224 options can take appears in @ref{Styles}; for now, here is what the
1225 previous example (including the @option{--verbose} (@option{-v}) option) looks like
1226 using short option forms:
1229 $ @kbd{tar -cvf collection.tar blues folk jazz}
1236 As you can see, the system responds the same no matter whether you use
1237 long or short option forms.
1239 @FIXME{i don't like how this is worded:} One difference between using
1240 short and long option forms is that, although the exact placement of
1241 arguments following options is no more specific when using short forms,
1242 it is easier to become confused and make a mistake when using short
1243 forms. For example, suppose you attempted the above example in the
1247 $ @kbd{tar -cfv collection.tar blues folk jazz}
1251 In this case, @command{tar} will make an archive file called @file{v},
1252 containing the files @file{blues}, @file{folk}, and @file{jazz}, because
1253 the @samp{v} is the closest ``file name'' to the @option{-f} option, and
1254 is thus taken to be the chosen archive file name. @command{tar} will try
1255 to add a file called @file{collection.tar} to the @file{v} archive file;
1256 if the file @file{collection.tar} did not already exist, @command{tar} will
1257 report an error indicating that this file does not exist. If the file
1258 @file{collection.tar} does already exist (e.g., from a previous command
1259 you may have run), then @command{tar} will add this file to the archive.
1260 Because the @option{-v} option did not get registered, @command{tar} will not
1261 run under @samp{verbose} mode, and will not report its progress.
1263 The end result is that you may be quite confused about what happened,
1264 and possibly overwrite a file. To illustrate this further, we will show
1265 you how an example we showed previously would look using short forms.
1270 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1274 is confusing as it is. When shown using short forms, however, it
1275 becomes much more so:
1278 $ @kbd{tar blues -c folk -f collection.tar jazz}
1282 It would be very easy to put the wrong string of characters
1283 immediately following the @option{-f}, but doing that could sacrifice
1286 For this reason, we recommend that you pay very careful attention to
1287 the order of options and placement of file and archive names,
1288 especially when using short option forms. Not having the option name
1289 written out mnemonically can affect how well you remember which option
1290 does what, and therefore where different names have to be placed.
1293 @subsection Archiving Directories
1295 @cindex Archiving Directories
1296 @cindex Directories, Archiving
1297 You can archive a directory by specifying its directory name as a
1298 file name argument to @command{tar}. The files in the directory will be
1299 archived relative to the working directory, and the directory will be
1300 re-created along with its contents when the archive is extracted.
1302 To archive a directory, first move to its superior directory. If you
1303 have followed the previous instructions in this tutorial, you should
1312 This will put you into the directory which contains @file{practice},
1313 i.e., your home directory. Once in the superior directory, you can
1314 specify the subdirectory, @file{practice}, as a file name argument. To
1315 store @file{practice} in the new archive file @file{music.tar}, type:
1318 $ @kbd{tar --create --verbose --file=music.tar practice}
1322 @command{tar} should output:
1329 practice/collection.tar
1332 Note that the archive thus created is not in the subdirectory
1333 @file{practice}, but rather in the current working directory---the
1334 directory from which @command{tar} was invoked. Before trying to archive a
1335 directory from its superior directory, you should make sure you have
1336 write access to the superior directory itself, not only the directory
1337 you are trying archive with @command{tar}. For example, you will probably
1338 not be able to store your home directory in an archive by invoking
1339 @command{tar} from the root directory; @xref{absolute}. (Note
1340 also that @file{collection.tar}, the original archive file, has itself
1341 been archived. @command{tar} will accept any file as a file to be
1342 archived, regardless of its content. When @file{music.tar} is
1343 extracted, the archive file @file{collection.tar} will be re-written
1344 into the file system).
1346 If you give @command{tar} a command such as
1349 $ @kbd{tar --create --file=foo.tar .}
1353 @command{tar} will report @samp{tar: ./foo.tar is the archive; not
1354 dumped}. This happens because @command{tar} creates the archive
1355 @file{foo.tar} in the current directory before putting any files into
1356 it. Then, when @command{tar} attempts to add all the files in the
1357 directory @file{.} to the archive, it notices that the file
1358 @file{./foo.tar} is the same as the archive @file{foo.tar}, and skips
1359 it. (It makes no sense to put an archive into itself.) @GNUTAR{}
1360 will continue in this case, and create the archive
1361 normally, except for the exclusion of that one file. (@emph{Please
1362 note:} Other implementations of @command{tar} may not be so clever;
1363 they will enter an infinite loop when this happens, so you should not
1364 depend on this behavior unless you are certain you are running
1365 @GNUTAR{}. In general, it is wise to always place the archive outside
1366 of the directory being dumped.)
1369 @section How to List Archives
1372 Frequently, you will find yourself wanting to determine exactly what a
1373 particular archive contains. You can use the @option{--list}
1374 (@option{-t}) operation to get the member names as they currently
1375 appear in the archive, as well as various attributes of the files at
1376 the time they were archived. For example, you can examine the archive
1377 @file{collection.tar} that you created in the last section with the
1381 $ @kbd{tar --list --file=collection.tar}
1385 The output of @command{tar} would then be:
1394 The archive @file{bfiles.tar} would list as follows:
1403 Be sure to use a @option{--file=@var{archive-name}} (@option{-f
1404 @var{archive-name}}) option just as with @option{--create}
1405 (@option{-c}) to specify the name of the archive.
1407 @xopindex{list, using with @option{--verbose}}
1408 @xopindex{verbose, using with @option{--list}}
1409 If you use the @option{--verbose} (@option{-v}) option with
1410 @option{--list}, then @command{tar} will print out a listing
1411 reminiscent of @w{@samp{ls -l}}, showing owner, file size, and so
1412 forth. This output is described in detail in @ref{verbose member listing}.
1414 If you had used @option{--verbose} (@option{-v}) mode, the example
1415 above would look like:
1418 $ @kbd{tar --list --verbose --file=collection.tar folk}
1419 -rw-r--r-- myself/user 62 1990-05-23 10:55 folk
1422 @cindex listing member and file names
1423 @anchor{listing member and file names}
1424 It is important to notice that the output of @kbd{tar --list
1425 --verbose} does not necessarily match that produced by @kbd{tar
1426 --create --verbose} while creating the archive. It is because
1427 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1428 prefixes from file names before storing them in the archive
1429 (@xref{absolute}, for more information). In other
1430 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1431 an archive and @dfn{member names} when listing it. Consider this
1436 $ @kbd{tar --create --verbose --file archive /etc/mail}
1437 tar: Removing leading '/' from member names
1439 /etc/mail/sendmail.cf
1441 $ @kbd{tar --test --file archive}
1443 etc/mail/sendmail.cf
1448 @opindex show-stored-names
1449 This default behavior can sometimes be inconvenient. You can force
1450 @GNUTAR{} show member names when creating archive by supplying
1451 @option{--show-stored-names} option.
1454 @item --show-stored-names
1455 Print member (as opposed to @emph{file}) names when creating the archive.
1458 @cindex File name arguments, using @option{--list} with
1459 @xopindex{list, using with file name arguments}
1460 You can specify one or more individual member names as arguments when
1461 using @samp{list}. In this case, @command{tar} will only list the
1462 names of members you identify. For example, @w{@kbd{tar --list
1463 --file=afiles.tar apple}} would only print @file{apple}.
1465 Because @command{tar} preserves file names, these must be specified as
1466 they appear in the archive (i.e., relative to the directory from which
1467 the archive was created). Therefore, it is essential when specifying
1468 member names to @command{tar} that you give the exact member names.
1469 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1470 error message something like @samp{tar: birds: Not found in archive},
1471 because there is no member named @file{birds}, only one named
1472 @file{./birds}. While the names @file{birds} and @file{./birds} name
1473 the same file, @emph{member} names by default are compared verbatim.
1475 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1476 with @file{baboon}, because this exact member name is in the archive file
1477 @file{bfiles.tar}. If you are not sure of the exact file name,
1478 use @dfn{globbing patterns}, for example:
1481 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1485 will list all members whose name contains @samp{b}. @xref{wildcards},
1486 for a detailed discussion of globbing patterns and related
1487 @command{tar} command line options.
1494 @unnumberedsubsec Listing the Contents of a Stored Directory
1496 To get information about the contents of an archived directory,
1497 use the directory name as a file name argument in conjunction with
1498 @option{--list} (@option{-t}). To find out file attributes, include the
1499 @option{--verbose} (@option{-v}) option.
1501 For example, to find out about files in the directory @file{practice}, in
1502 the archive file @file{music.tar}, type:
1505 $ @kbd{tar --list --verbose --file=music.tar practice}
1508 @command{tar} responds:
1511 drwxrwxrwx myself/user 0 1990-05-31 21:49 practice/
1512 -rw-r--r-- myself/user 42 1990-05-21 13:29 practice/blues
1513 -rw-r--r-- myself/user 62 1990-05-23 10:55 practice/folk
1514 -rw-r--r-- myself/user 40 1990-05-21 13:30 practice/jazz
1515 -rw-r--r-- myself/user 10240 1990-05-31 21:49 practice/collection.tar
1518 When you use a directory name as a file name argument, @command{tar} acts on
1519 all the files (including sub-directories) in that directory.
1522 @section How to Extract Members from an Archive
1524 @cindex Retrieving files from an archive
1525 @cindex Resurrecting files from an archive
1528 Creating an archive is only half the job---there is no point in storing
1529 files in an archive if you can't retrieve them. The act of retrieving
1530 members from an archive so they can be used and manipulated as
1531 unarchived files again is called @dfn{extraction}. To extract files
1532 from an archive, use the @option{--extract} (@option{--get} or
1533 @option{-x}) operation. As with @option{--create}, specify the name
1534 of the archive with @option{--file} (@option{-f}) option. Extracting
1535 an archive does not modify the archive in any way; you can extract it
1536 multiple times if you want or need to.
1538 Using @option{--extract}, you can extract an entire archive, or specific
1539 files. The files can be directories containing other files, or not. As
1540 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1541 long form of the operation without affecting the performance.
1544 * extracting archives::
1545 * extracting files::
1547 * extracting untrusted archives::
1548 * failing commands::
1551 @node extracting archives
1552 @subsection Extracting an Entire Archive
1554 To extract an entire archive, specify the archive file name only, with
1555 no individual file names as arguments. For example,
1558 $ @kbd{tar -xvf collection.tar}
1565 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
1566 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
1567 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
1570 @node extracting files
1571 @subsection Extracting Specific Files
1573 To extract specific archive members, give their exact member names as
1574 arguments, as printed by @option{--list} (@option{-t}). If you had
1575 mistakenly deleted one of the files you had placed in the archive
1576 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1577 from the archive without changing the archive's structure. Its
1578 contents will be identical to the original file @file{blues} that you
1581 First, make sure you are in the @file{practice} directory, and list the
1582 files in the directory. Now, delete the file, @samp{blues}, and list
1583 the files in the directory again.
1585 You can now extract the member @file{blues} from the archive file
1586 @file{collection.tar} like this:
1589 $ @kbd{tar --extract --file=collection.tar blues}
1593 If you list the files in the directory again, you will see that the file
1594 @file{blues} has been restored, with its original permissions, data
1595 modification times, and owner.@footnote{This is only accidentally
1596 true, but not in general. Whereas modification times are always
1597 restored, in most cases, one has to be root for restoring the owner,
1598 and use a special option for restoring permissions. Here, it just
1599 happens that the restoring user is also the owner of the archived
1600 members, and that the current @code{umask} is compatible with original
1601 permissions.} (These parameters will be identical to those which
1602 the file had when you originally placed it in the archive; any changes
1603 you may have made before deleting the file from the file system,
1604 however, will @emph{not} have been made to the archive member.) The
1605 archive file, @samp{collection.tar}, is the same as it was before you
1606 extracted @samp{blues}. You can confirm this by running @command{tar} with
1607 @option{--list} (@option{-t}).
1609 Remember that as with other operations, specifying the exact member
1610 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1611 will fail, because there is no member named @file{birds}. To extract
1612 the member named @file{./birds}, you must specify @w{@kbd{tar
1613 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1614 exact member names, use @option{--list} (@option{-t}) option
1615 (@pxref{list}). You can also extract those members that match a
1616 specific @dfn{globbing pattern}. For example, to extract from
1617 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1618 directory prefix, you could type:
1621 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1625 Here, @option{--wildcards} instructs @command{tar} to treat
1626 command line arguments as globbing patterns and @option{--no-anchored}
1627 informs it that the patterns apply to member names after any @samp{/}
1628 delimiter. The use of globbing patterns is discussed in detail in
1631 You can extract a file to standard output by combining the above options
1632 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1635 If you give the @option{--verbose} option, then @option{--extract}
1636 will print the names of the archive members as it extracts them.
1639 @subsection Extracting Files that are Directories
1641 Extracting directories which are members of an archive is similar to
1642 extracting other files. The main difference to be aware of is that if
1643 the extracted directory has the same name as any directory already in
1644 the working directory, then files in the extracted directory will be
1645 placed into the directory of the same name. Likewise, if there are
1646 files in the pre-existing directory with the same names as the members
1647 which you extract, the files from the extracted archive will replace
1648 the files already in the working directory (and possible
1649 subdirectories). This will happen regardless of whether or not the
1650 files in the working directory were more recent than those extracted
1651 (there exist, however, special options that alter this behavior
1654 However, if a file was stored with a directory name as part of its file
1655 name, and that directory does not exist under the working directory when
1656 the file is extracted, @command{tar} will create the directory.
1658 We can demonstrate how to use @option{--extract} to extract a directory
1659 file with an example. Change to the @file{practice} directory if you
1660 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1661 go back to the parent directory and extract the archive
1662 @file{music.tar}. You may either extract the entire archive, or you may
1663 extract only the files you just deleted. To extract the entire archive,
1664 don't give any file names as arguments after the archive name
1665 @file{music.tar}. To extract only the files you deleted, use the
1669 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1675 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1676 would have displayed more detail about the extracted files, as shown
1677 in the example below:
1680 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1681 -rw-r--r-- me/user 28 1996-10-18 16:31 practice/jazz
1682 -rw-r--r-- me/user 20 1996-09-23 16:44 practice/folk
1686 Because you created the directory with @file{practice} as part of the
1687 file names of each of the files by archiving the @file{practice}
1688 directory as @file{practice}, you must give @file{practice} as part
1689 of the file names when you extract those files from the archive.
1691 @node extracting untrusted archives
1692 @subsection Extracting Archives from Untrusted Sources
1694 Extracting files from archives can overwrite files that already exist.
1695 If you receive an archive from an untrusted source, you should make a
1696 new directory and extract into that directory, so that you don't have
1697 to worry about the extraction overwriting one of your existing files.
1698 For example, if @file{untrusted.tar} came from somewhere else on the
1699 Internet, and you don't necessarily trust its contents, you can
1700 extract it as follows:
1703 $ @kbd{mkdir newdir}
1705 $ @kbd{tar -xvf ../untrusted.tar}
1708 It is also a good practice to examine contents of the archive
1709 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1710 with @option{--verbose} (@option{-v}).
1712 @node failing commands
1713 @subsection Commands That Will Fail
1715 Here are some sample commands you might try which will not work, and why
1718 If you try to use this command,
1721 $ @kbd{tar -xvf music.tar folk jazz}
1725 you will get the following response:
1728 tar: folk: Not found in archive
1729 tar: jazz: Not found in archive
1733 This is because these files were not originally @emph{in} the parent
1734 directory @file{..}, where the archive is located; they were in the
1735 @file{practice} directory, and their file names reflect this:
1738 $ @kbd{tar -tvf music.tar}
1744 @FIXME{make sure the above works when going through the examples in
1748 Likewise, if you try to use this command,
1751 $ @kbd{tar -tvf music.tar folk jazz}
1755 you would get a similar response. Members with those names are not in the
1756 archive. You must use the correct member names, or wildcards, in order
1757 to extract the files from the archive.
1759 If you have forgotten the correct names of the files in the archive,
1760 use @w{@kbd{tar --list --verbose}} to list them correctly.
1762 @FIXME{more examples, here? hag thinks it's a good idea.}
1765 @section Going Further Ahead in this Manual
1768 @FIXME{need to write up a node here about the things that are going to
1769 be in the rest of the manual.}
1771 @node tar invocation
1772 @chapter Invoking @GNUTAR{}
1774 This chapter is about how one invokes the @GNUTAR{}
1775 command, from the command synopsis (@pxref{Synopsis}). There are
1776 numerous options, and many styles for writing them. One mandatory
1777 option specifies the operation @command{tar} should perform
1778 (@pxref{Operation Summary}), other options are meant to detail how
1779 this operation should be performed (@pxref{Option Summary}).
1780 Non-option arguments are not always interpreted the same way,
1781 depending on what the operation is.
1783 You will find in this chapter everything about option styles and rules for
1784 writing them (@pxref{Styles}). On the other hand, operations and options
1785 are fully described elsewhere, in other chapters. Here, you will find
1786 only synthetic descriptions for operations and options, together with
1787 pointers to other parts of the @command{tar} manual.
1789 Some options are so special they are fully described right in this
1790 chapter. They have the effect of inhibiting the normal operation of
1791 @command{tar} or else, they globally alter the amount of feedback the user
1792 receives about what is going on. These are the @option{--help} and
1793 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1794 and @option{--interactive} options (@pxref{interactive}).
1798 * using tar options::
1810 @section General Synopsis of @command{tar}
1812 The @GNUTAR{} program is invoked as either one of:
1815 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1816 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1819 The second form is for when old options are being used.
1821 You can use @command{tar} to store files in an archive, to extract them from
1822 an archive, and to do other types of archive manipulation. The primary
1823 argument to @command{tar}, which is called the @dfn{operation}, specifies
1824 which action to take. The other arguments to @command{tar} are either
1825 @dfn{options}, which change the way @command{tar} performs an operation,
1826 or file names or archive members, which specify the files or members
1827 @command{tar} is to act on.
1829 You can actually type in arguments in any order, even if in this manual
1830 the options always precede the other arguments, to make examples easier
1831 to understand. Further, the option stating the main operation mode
1832 (the @command{tar} main command) is usually given first.
1834 Each @var{name} in the synopsis above is interpreted as an archive member
1835 name when the main command is one of @option{--compare}
1836 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1837 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1838 @option{--update} (@option{-u}). When naming archive members, you
1839 must give the exact name of the member in the archive, as it is
1840 printed by @option{--list}. For @option{--append} (@option{-r}) and
1841 @option{--create} (@option{-c}), these @var{name} arguments specify
1842 the names of either files or directory hierarchies to place in the archive.
1843 These files or hierarchies should already exist in the file system,
1844 prior to the execution of the @command{tar} command.
1846 @command{tar} interprets relative file names as being relative to the
1847 working directory. @command{tar} will make all file names relative
1848 (by removing leading slashes when archiving or restoring files),
1849 unless you specify otherwise (using the @option{--absolute-names}
1850 option). @xref{absolute}, for more information about
1851 @option{--absolute-names}.
1853 If you give the name of a directory as either a file name or a member
1854 name, then @command{tar} acts recursively on all the files and directories
1855 beneath that directory. For example, the name @file{/} identifies all
1856 the files in the file system to @command{tar}.
1858 The distinction between file names and archive member names is especially
1859 important when shell globbing is used, and sometimes a source of confusion
1860 for newcomers. @xref{wildcards}, for more information about globbing.
1861 The problem is that shells may only glob using existing files in the
1862 file system. Only @command{tar} itself may glob on archive members, so when
1863 needed, you must ensure that wildcard characters reach @command{tar} without
1864 being interpreted by the shell first. Using a backslash before @samp{*}
1865 or @samp{?}, or putting the whole argument between quotes, is usually
1866 sufficient for this.
1868 Even if @var{name}s are often specified on the command line, they
1869 can also be read from a text file in the file system, using the
1870 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1872 If you don't use any file name arguments, @option{--append} (@option{-r}),
1873 @option{--delete} and @option{--concatenate} (@option{--catenate},
1874 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1875 will usually yield a diagnostic and inhibit @command{tar} execution.
1876 The other operations of @command{tar} (@option{--list},
1877 @option{--extract}, @option{--compare}, and @option{--update})
1878 will act on the entire contents of the archive.
1880 @anchor{exit status}
1882 @cindex return status
1883 Besides successful exits, @GNUTAR{} may fail for
1884 many reasons. Some reasons correspond to bad usage, that is, when the
1885 @command{tar} command line is improperly written. Errors may be
1886 encountered later, while processing the archive or the files. Some
1887 errors are recoverable, in which case the failure is delayed until
1888 @command{tar} has completed all its work. Some errors are such that
1889 it would be not meaningful, or at least risky, to continue processing:
1890 @command{tar} then aborts processing immediately. All abnormal exits,
1891 whether immediate or delayed, should always be clearly diagnosed on
1892 @code{stderr}, after a line stating the nature of the error.
1894 Possible exit codes of @GNUTAR{} are summarized in the following
1899 @samp{Successful termination}.
1902 @samp{Some files differ}. If tar was invoked with @option{--compare}
1903 (@option{--diff}, @option{-d}) command line option, this means that
1904 some files in the archive differ from their disk counterparts
1905 (@pxref{compare}). If tar was given @option{--create},
1906 @option{--append} or @option{--update} option, this exit code means
1907 that some files were changed while being archived and so the resulting
1908 archive does not contain the exact copy of the file set.
1911 @samp{Fatal error}. This means that some fatal, unrecoverable error
1915 If @command{tar} has invoked a subprocess and that subprocess exited with a
1916 nonzero exit code, @command{tar} exits with that code as well.
1917 This can happen, for example, if @command{tar} was given some
1918 compression option (@pxref{gzip}) and the external compressor program
1919 failed. Another example is @command{rmt} failure during backup to the
1920 remote device (@pxref{Remote Tape Server}).
1922 @node using tar options
1923 @section Using @command{tar} Options
1925 @GNUTAR{} has a total of eight operating modes which
1926 allow you to perform a variety of tasks. You are required to choose
1927 one operating mode each time you employ the @command{tar} program by
1928 specifying one, and only one operation as an argument to the
1929 @command{tar} command (the corresponding options may be found
1930 at @ref{frequent operations} and @ref{Operations}). Depending on
1931 circumstances, you may also wish to customize how the chosen operating
1932 mode behaves. For example, you may wish to change the way the output
1933 looks, or the format of the files that you wish to archive may require
1934 you to do something special in order to make the archive look right.
1936 You can customize and control @command{tar}'s performance by running
1937 @command{tar} with one or more options (such as @option{--verbose}
1938 (@option{-v}), which we used in the tutorial). As we said in the
1939 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1940 their name suggests) optional. Depending on the operating mode, you
1941 may specify one or more options. Different options will have different
1942 effects, but in general they all change details of the operation, such
1943 as archive format, archive name, or level of user interaction. Some
1944 options make sense with all operating modes, while others are
1945 meaningful only with particular modes. You will likely use some
1946 options frequently, while you will only use others infrequently, or
1947 not at all. (A full list of options is available in @pxref{All Options}.)
1949 @vrindex TAR_OPTIONS, environment variable
1950 @anchor{TAR_OPTIONS}
1951 The @env{TAR_OPTIONS} environment variable specifies default options to
1952 be placed in front of any explicit options. For example, if
1953 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1954 if the two options @option{-v} and @option{--unlink-first} had been
1955 specified before any explicit options. Option specifications are
1956 separated by whitespace. A backslash escapes the next character, so it
1957 can be used to specify an option containing whitespace or a backslash.
1959 Note that @command{tar} options are case sensitive. For example, the
1960 options @option{-T} and @option{-t} are different; the first requires an
1961 argument for stating the name of a file providing a list of @var{name}s,
1962 while the second does not require an argument and is another way to
1963 write @option{--list} (@option{-t}).
1965 In addition to the eight operations, there are many options to
1966 @command{tar}, and three different styles for writing both: long (mnemonic)
1967 form, short form, and old style. These styles are discussed below.
1968 Both the options and the operations can be written in any of these three
1971 @FIXME{menu at end of this node. need to think of an actual outline
1972 for this chapter; probably do that after stuff from chapter 4 is
1976 @section The Three Option Styles
1978 There are three styles for writing operations and options to the command
1979 line invoking @command{tar}. The different styles were developed at
1980 different times during the history of @command{tar}. These styles will be
1981 presented below, from the most recent to the oldest.
1983 Some options must take an argument@footnote{For example, @option{--file}
1984 (@option{-f}) takes the name of an archive file as an argument. If
1985 you do not supply an archive file name, @command{tar} will use a
1986 default, but this can be confusing; thus, we recommend that you always
1987 supply a specific archive file name.}. Where you @emph{place} the
1988 arguments generally depends on which style of options you choose. We
1989 will detail specific information relevant to each option style in the
1990 sections on the different option styles, below. The differences are
1991 subtle, yet can often be very important; incorrect option placement
1992 can cause you to overwrite a number of important files. We urge you
1993 to note these differences, and only use the option style(s) which
1994 makes the most sense to you until you feel comfortable with the others.
1996 Some options @emph{may} take an argument. Such options may have at
1997 most long and short forms, they do not have old style equivalent. The
1998 rules for specifying an argument for such options are stricter than
1999 those for specifying mandatory arguments. Please, pay special
2003 * Long Options:: Long Option Style
2004 * Short Options:: Short Option Style
2005 * Old Options:: Old Option Style
2006 * Mixing:: Mixing Option Styles
2010 @subsection Long Option Style
2012 @cindex long options
2013 @cindex options, long style
2014 @cindex options, GNU style
2015 @cindex options, mnemonic names
2016 Each option has at least one @dfn{long} (or @dfn{mnemonic}) name starting with two
2017 dashes in a row, e.g., @option{--list}. The long names are more clear than
2018 their corresponding short or old names. It sometimes happens that a
2019 single long option has many different names which are
2020 synonymous, such as @option{--compare} and @option{--diff}. In addition,
2021 long option names can be given unique abbreviations. For example,
2022 @option{--cre} can be used in place of @option{--create} because there is no
2023 other long option which begins with @samp{cre}. (One way to find
2024 this out is by trying it and seeing what happens; if a particular
2025 abbreviation could represent more than one option, @command{tar} will tell
2026 you that that abbreviation is ambiguous and you'll know that that
2027 abbreviation won't work. You may also choose to run @samp{tar --help}
2028 to see a list of options. Be aware that if you run @command{tar} with a
2029 unique abbreviation for the long name of an option you didn't want to
2030 use, you are stuck; @command{tar} will perform the command as ordered.)
2032 Long options are meant to be obvious and easy to remember, and their
2033 meanings are generally easier to discern than those of their
2034 corresponding short options (see below). For example:
2037 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
2041 gives a fairly good set of hints about what the command does, even
2042 for those not fully acquainted with @command{tar}.
2044 @cindex arguments to long options
2045 @cindex long options with mandatory arguments
2046 Long options which require arguments take those arguments
2047 immediately following the option name. There are two ways of
2048 specifying a mandatory argument. It can be separated from the
2049 option name either by an equal sign, or by any amount of
2050 white space characters. For example, the @option{--file} option (which
2051 tells the name of the @command{tar} archive) is given a file such as
2052 @file{archive.tar} as argument by using any of the following notations:
2053 @option{--file=archive.tar} or @option{--file archive.tar}.
2055 @cindex optional arguments to long options
2056 @cindex long options with optional arguments
2057 In contrast, optional arguments must always be introduced using
2058 an equal sign. For example, the @option{--backup} option takes
2059 an optional argument specifying backup type. It must be used
2060 as @option{--backup=@var{backup-type}}.
2063 @subsection Short Option Style
2065 @cindex short options
2066 @cindex options, short style
2067 @cindex options, traditional
2068 Most options also have a @dfn{short option} name. Short options start with
2069 a single dash, and are followed by a single character, e.g., @option{-t}
2070 (which is equivalent to @option{--list}). The forms are absolutely
2071 identical in function; they are interchangeable.
2073 The short option names are faster to type than long option names.
2075 @cindex arguments to short options
2076 @cindex short options with mandatory arguments
2077 Short options which require arguments take their arguments immediately
2078 following the option, usually separated by white space. It is also
2079 possible to stick the argument right after the short option name, using
2080 no intervening space. For example, you might write @w{@option{-f
2081 archive.tar}} or @option{-farchive.tar} instead of using
2082 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
2083 @w{@option{-f @var{archive-name}}} denote the option which indicates a
2084 specific archive, here named @file{archive.tar}.
2086 @cindex optional arguments to short options
2087 @cindex short options with optional arguments
2088 Short options which take optional arguments take their arguments
2089 immediately following the option letter, @emph{without any intervening
2090 white space characters}.
2092 Short options' letters may be clumped together, but you are not
2093 required to do this (as compared to old options; see below). When
2094 short options are clumped as a set, use one (single) dash for them
2095 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
2096 such a set is allowed to have an argument@footnote{Clustering many
2097 options, the last of which has an argument, is a rather opaque way to
2098 write options. Some wonder if @acronym{GNU} @code{getopt} should not
2099 even be made helpful enough for considering such usages as invalid.}.
2101 When the options are separated, the argument for each option which requires
2102 an argument directly follows that option, as is usual for Unix programs.
2106 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
2109 If you reorder short options' locations, be sure to move any arguments
2110 that belong to them. If you do not move the arguments properly, you may
2111 end up overwriting files.
2114 @subsection Old Option Style
2115 @cindex options, old style
2116 @cindex old option style
2117 @cindex option syntax, traditional
2119 As far as we know, all @command{tar} programs, @acronym{GNU} and
2120 non-@acronym{GNU}, support @dfn{old options}: that is, if the first
2121 argument does not start with @samp{-}, it is assumed to specify option
2122 letters. @GNUTAR{} supports old options not only for historical
2123 reasons, but also because many people are used to them. If the first
2124 argument does not start with a dash, you are announcing the old option
2125 style instead of the short option style; old options are decoded
2128 Like short options, old options are single letters. However, old options
2129 must be written together as a single clumped set, without spaces separating
2130 them or dashes preceding them. This set
2131 of letters must be the first to appear on the command line, after the
2132 @command{tar} program name and some white space; old options cannot appear
2133 anywhere else. The letter of an old option is exactly the same letter as
2134 the corresponding short option. For example, the old option @samp{t} is
2135 the same as the short option @option{-t}, and consequently, the same as the
2136 long option @option{--list}. So for example, the command @w{@samp{tar
2137 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
2139 @cindex arguments to old options
2140 @cindex old options with mandatory arguments
2141 When options that need arguments are given together with the command,
2142 all the associated arguments follow, in the same order as the options.
2143 Thus, the example given previously could also be written in the old
2147 $ @kbd{tar cvbf 20 /dev/rmt0}
2151 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
2152 the argument of @option{-f}.
2154 The old style syntax can make it difficult to match
2155 option letters with their corresponding arguments, and is often
2156 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
2157 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
2158 argument for @option{-f}, and @option{-v} does not have a corresponding
2159 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
2160 /dev/rmt0}} is clearer, putting all arguments next to the option they
2163 If you want to reorder the letters in the old option argument, be
2164 sure to reorder any corresponding argument appropriately.
2166 This old way of writing @command{tar} options can surprise even experienced
2167 users. For example, the two commands:
2170 @kbd{tar cfz archive.tar.gz file}
2171 @kbd{tar -cfz archive.tar.gz file}
2175 are quite different. The first example uses @file{archive.tar.gz} as
2176 the value for option @samp{f} and recognizes the option @samp{z}. The
2177 second example, however, uses @file{z} as the value for option
2178 @samp{f} --- probably not what was intended.
2180 This second example could be corrected in many ways, among which the
2181 following are equivalent:
2184 @kbd{tar -czf archive.tar.gz file}
2185 @kbd{tar -cf archive.tar.gz -z file}
2186 @kbd{tar cf archive.tar.gz -z file}
2190 @subsection Mixing Option Styles
2192 @cindex options, mixing different styles
2193 All three styles may be intermixed in a single @command{tar} command,
2194 so long as the rules for each style are fully
2195 respected@footnote{Before @GNUTAR{} version 1.11.6,
2196 a bug prevented intermixing old style options with long options in
2197 some cases.}. Old style options and either of the modern styles of
2198 options may be mixed within a single @command{tar} command. However,
2199 old style options must be introduced as the first arguments only,
2200 following the rule for old options (old options must appear directly
2201 after the @command{tar} command and some white space). Modern options
2202 may be given only after all arguments to the old options have been
2203 collected. If this rule is not respected, a modern option might be
2204 falsely interpreted as the value of the argument to one of the old
2207 For example, all the following commands are wholly equivalent, and
2208 illustrate the many combinations and orderings of option styles.
2211 @kbd{tar --create --file=archive.tar}
2212 @kbd{tar --create -f archive.tar}
2213 @kbd{tar --create -farchive.tar}
2214 @kbd{tar --file=archive.tar --create}
2215 @kbd{tar --file=archive.tar -c}
2216 @kbd{tar -c --file=archive.tar}
2217 @kbd{tar -c -f archive.tar}
2218 @kbd{tar -c -farchive.tar}
2219 @kbd{tar -cf archive.tar}
2220 @kbd{tar -cfarchive.tar}
2221 @kbd{tar -f archive.tar --create}
2222 @kbd{tar -f archive.tar -c}
2223 @kbd{tar -farchive.tar --create}
2224 @kbd{tar -farchive.tar -c}
2225 @kbd{tar c --file=archive.tar}
2226 @kbd{tar c -f archive.tar}
2227 @kbd{tar c -farchive.tar}
2228 @kbd{tar cf archive.tar}
2229 @kbd{tar f archive.tar --create}
2230 @kbd{tar f archive.tar -c}
2231 @kbd{tar fc archive.tar}
2234 On the other hand, the following commands are @emph{not} equivalent to
2238 @kbd{tar -f -c archive.tar}
2239 @kbd{tar -fc archive.tar}
2240 @kbd{tar -fcarchive.tar}
2241 @kbd{tar -farchive.tarc}
2242 @kbd{tar cfarchive.tar}
2246 These last examples mean something completely different from what the
2247 user intended (judging based on the example in the previous set which
2248 uses long options, whose intent is therefore very clear). The first
2249 four specify that the @command{tar} archive would be a file named
2250 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2251 respectively. The first two examples also specify a single non-option,
2252 @var{name} argument having the value @samp{archive.tar}. The last
2253 example contains only old style option letters (repeating option
2254 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2255 @samp{h}, or @samp{i}), with no argument value. @FIXME{not sure i liked
2256 the first sentence of this paragraph..}
2259 @section All @command{tar} Options
2261 The coming manual sections contain an alphabetical listing of all
2262 @command{tar} operations and options, with brief descriptions and
2263 cross-references to more in-depth explanations in the body of the manual.
2264 They also contain an alphabetically arranged table of the short option
2265 forms with their corresponding long option. You can use this table as
2266 a reference for deciphering @command{tar} commands in scripts.
2269 * Operation Summary::
2271 * Short Option Summary::
2274 @node Operation Summary
2275 @subsection Operations
2283 Appends files to the end of the archive. @xref{append}.
2285 @opsummary{catenate}
2289 Same as @option{--concatenate}. @xref{concatenate}.
2295 Compares archive members with their counterparts in the file
2296 system, and reports differences in file size, mode, owner,
2297 modification date and contents. @xref{compare}.
2299 @opsummary{concatenate}
2303 Appends other @command{tar} archives to the end of the archive.
2310 Creates a new @command{tar} archive. @xref{create}.
2315 Deletes members from the archive. Don't try this on an archive on a
2316 tape! @xref{delete}.
2322 Same @option{--compare}. @xref{compare}.
2328 Extracts members from the archive into the file system. @xref{extract}.
2334 Same as @option{--extract}. @xref{extract}.
2340 Lists the members in an archive. @xref{list}.
2346 Adds files to the end of the archive, but only if they are newer than
2347 their counterparts already in the archive, or if they do not already
2348 exist in the archive. @xref{update}.
2352 @node Option Summary
2353 @subsection @command{tar} Options
2357 @opsummary{absolute-names}
2358 @item --absolute-names
2361 Normally when creating an archive, @command{tar} strips an initial
2362 @samp{/} from member names, and when extracting from an archive @command{tar}
2363 treats names specially if they have initial @samp{/} or internal
2364 @samp{..}. This option disables that behavior. @xref{absolute}.
2366 @opsummary{after-date}
2369 (See @option{--newer}, @pxref{after})
2371 @opsummary{anchored}
2373 A pattern must match an initial subsequence of the name's components.
2374 @xref{controlling pattern-matching}.
2376 @opsummary{atime-preserve}
2377 @item --atime-preserve
2378 @itemx --atime-preserve=replace
2379 @itemx --atime-preserve=system
2381 Attempt to preserve the access time of files when reading them. This
2382 option currently is effective only on files that you own, unless you
2383 have superuser privileges.
2385 @option{--atime-preserve=replace} remembers the access time of a file
2386 before reading it, and then restores the access time afterwards. This
2387 may cause problems if other programs are reading the file at the same
2388 time, as the times of their accesses will be lost. On most platforms
2389 restoring the access time also requires @command{tar} to restore the
2390 data modification time too, so this option may also cause problems if
2391 other programs are writing the file at the same time (@command{tar} attempts
2392 to detect this situation, but cannot do so reliably due to race
2393 conditions). Worse, on most platforms restoring the access time also
2394 updates the status change time, which means that this option is
2395 incompatible with incremental backups.
2397 @option{--atime-preserve=system} avoids changing time stamps on files,
2398 without interfering with time stamp updates
2399 caused by other programs, so it works better with incremental backups.
2400 However, it requires a special @code{O_NOATIME} option from the
2401 underlying operating and file system implementation, and it also requires
2402 that searching directories does not update their access times. As of
2403 this writing (November 2005) this works only with Linux, and only with
2404 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2405 way to know whether this feature actually works. Sometimes
2406 @command{tar} knows that it does not work, and if you use
2407 @option{--atime-preserve=system} then @command{tar} complains and
2408 exits right away. But other times @command{tar} might think that the
2409 option works when it actually does not.
2411 Currently @option{--atime-preserve} with no operand defaults to
2412 @option{--atime-preserve=replace}, but this may change in the future
2413 as support for @option{--atime-preserve=system} improves.
2415 If your operating or file system does not support
2416 @option{--atime-preserve=@-system}, you might be able to preserve access
2417 times reliably by using the @command{mount} command. For example,
2418 you can mount the file system read-only, or access the file system via
2419 a read-only loopback mount, or use the @samp{noatime} mount option
2420 available on some systems. However, mounting typically requires
2421 superuser privileges and can be a pain to manage.
2423 @opsummary{auto-compress}
2424 @item --auto-compress
2427 During a @option{--create} operation, enables automatic compressed
2428 format recognition based on the archive suffix. The effect of this
2429 option is cancelled by @option{--no-auto-compress}. @xref{gzip}.
2432 @item --backup=@var{backup-type}
2434 Rather than deleting files from the file system, @command{tar} will
2435 back them up using simple or numbered backups, depending upon
2436 @var{backup-type}. @xref{backup}.
2438 @opsummary{block-number}
2439 @item --block-number
2442 With this option present, @command{tar} prints error messages for read errors
2443 with the block number in the archive file. @xref{block-number}.
2445 @opsummary{blocking-factor}
2446 @item --blocking-factor=@var{blocking}
2447 @itemx -b @var{blocking}
2449 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2450 record. @xref{Blocking Factor}.
2456 This option tells @command{tar} to read or write archives through
2457 @code{bzip2}. @xref{gzip}.
2459 @opsummary{check-device}
2460 @item --check-device
2461 Check device numbers when creating a list of modified files for
2462 incremental archiving. This is the default. @xref{device numbers},
2463 for a detailed description.
2465 @opsummary{checkpoint}
2466 @item --checkpoint[=@var{number}]
2468 This option directs @command{tar} to print periodic checkpoint
2469 messages as it reads through the archive. It is intended for when you
2470 want a visual indication that @command{tar} is still running, but
2471 don't want to see @option{--verbose} output. You can also instruct
2472 @command{tar} to execute a list of actions on each checkpoint, see
2473 @option{--checkpoint-action} below. For a detailed description, see
2476 @opsummary{checkpoint-action}
2477 @item --checkpoint-action=@var{action}
2478 Instruct @command{tar} to execute an action upon hitting a
2479 breakpoint. Here we give only a brief outline. @xref{checkpoints},
2480 for a complete description.
2482 The @var{action} argument can be one of the following:
2486 Produce an audible bell on the console.
2490 Print a single dot on the standard listing stream.
2493 Display a textual message on the standard error, with the status and
2494 number of the checkpoint. This is the default.
2496 @item echo=@var{string}
2497 Display @var{string} on the standard error. Before output, the string
2498 is subject to meta-character expansion.
2500 @item exec=@var{command}
2501 Execute the given @var{command}.
2503 @item sleep=@var{time}
2504 Wait for @var{time} seconds.
2506 @item ttyout=@var{string}
2507 Output @var{string} on the current console (@file{/dev/tty}).
2510 Several @option{--checkpoint-action} options can be specified. The
2511 supplied actions will be executed in order of their appearance in the
2514 Using @option{--checkpoint-action} without @option{--checkpoint}
2515 assumes default checkpoint frequency of one checkpoint per 10 records.
2517 @opsummary{check-links}
2520 If this option was given, @command{tar} will check the number of links
2521 dumped for each processed file. If this number does not match the
2522 total number of hard links for the file, a warning message will be
2523 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2524 synonym for @option{--one-file-system}. The current semantics, which
2525 complies to UNIX98, was introduced with version
2526 1.15.91. @xref{Changes}, for more information.}.
2530 @opsummary{compress}
2531 @opsummary{uncompress}
2536 @command{tar} will use the @command{compress} program when reading or
2537 writing the archive. This allows you to directly act on archives
2538 while saving space. @xref{gzip}.
2540 @opsummary{confirmation}
2541 @item --confirmation
2543 (See @option{--interactive}.) @xref{interactive}.
2545 @opsummary{delay-directory-restore}
2546 @item --delay-directory-restore
2548 Delay setting modification times and permissions of extracted
2549 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2551 @opsummary{dereference}
2555 When reading or writing a file to be archived, @command{tar} accesses
2556 the file that a symbolic link points to, rather than the symlink
2557 itself. @xref{dereference}.
2559 @opsummary{directory}
2560 @item --directory=@var{dir}
2563 When this option is specified, @command{tar} will change its current directory
2564 to @var{dir} before performing any operations. When this option is used
2565 during archive creation, it is order sensitive. @xref{directory}.
2568 @item --exclude=@var{pattern}
2570 When performing operations, @command{tar} will skip files that match
2571 @var{pattern}. @xref{exclude}.
2573 @opsummary{exclude-backups}
2574 @item --exclude-backups
2575 Exclude backup and lock files. @xref{exclude,, exclude-backups}.
2577 @opsummary{exclude-from}
2578 @item --exclude-from=@var{file}
2579 @itemx -X @var{file}
2581 Similar to @option{--exclude}, except @command{tar} will use the list of
2582 patterns in the file @var{file}. @xref{exclude}.
2584 @opsummary{exclude-caches}
2585 @item --exclude-caches
2587 Exclude from dump any directory containing a valid cache directory
2588 tag file, but still dump the directory node and the tag file itself.
2590 @xref{exclude,, exclude-caches}.
2592 @opsummary{exclude-caches-under}
2593 @item --exclude-caches-under
2595 Exclude from dump any directory containing a valid cache directory
2596 tag file, but still dump the directory node itself.
2600 @opsummary{exclude-caches-all}
2601 @item --exclude-caches-all
2603 Exclude from dump any directory containing a valid cache directory
2604 tag file. @xref{exclude}.
2606 @opsummary{exclude-tag}
2607 @item --exclude-tag=@var{file}
2609 Exclude from dump any directory containing file named @var{file}, but
2610 dump the directory node and @var{file} itself. @xref{exclude,, exclude-tag}.
2612 @opsummary{exclude-tag-under}
2613 @item --exclude-tag-under=@var{file}
2615 Exclude from dump the contents of any directory containing file
2616 named @var{file}, but dump the directory node itself. @xref{exclude,,
2619 @opsummary{exclude-tag-all}
2620 @item --exclude-tag-all=@var{file}
2622 Exclude from dump any directory containing file named @var{file}.
2623 @xref{exclude,,exclude-tag-all}.
2625 @opsummary{exclude-vcs}
2628 Exclude from dump directories and files, that are internal for some
2629 widely used version control systems.
2631 @xref{exclude,,exclude-vcs}.
2634 @item --file=@var{archive}
2635 @itemx -f @var{archive}
2637 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2638 performs operations on, rather than @command{tar}'s compilation dependent
2639 default. @xref{file tutorial}.
2641 @opsummary{files-from}
2642 @item --files-from=@var{file}
2643 @itemx -T @var{file}
2645 @command{tar} will use the contents of @var{file} as a list of archive members
2646 or files to operate on, in addition to those specified on the
2647 command-line. @xref{files}.
2649 @opsummary{force-local}
2652 Forces @command{tar} to interpret the file name given to @option{--file}
2653 as a local file, even if it looks like a remote tape drive name.
2654 @xref{local and remote archives}.
2657 @item --format=@var{format}
2658 @itemx -H @var{format}
2660 Selects output archive format. @var{Format} may be one of the
2665 Creates an archive that is compatible with Unix V7 @command{tar}.
2668 Creates an archive that is compatible with GNU @command{tar} version
2672 Creates archive in GNU tar 1.13 format. Basically it is the same as
2673 @samp{oldgnu} with the only difference in the way it handles long
2677 Creates a @acronym{POSIX.1-1988} compatible archive.
2680 Creates a @acronym{POSIX.1-2001 archive}.
2684 @xref{Formats}, for a detailed discussion of these formats.
2686 @opsummary{full-time}
2688 This option instructs @command{tar} to print file times to their full
2689 resolution. Usually this means 1-second resolution, but that depends
2690 on the underlying file system. The @option{--full-time} option takes
2691 effect only when detailed output (verbosity level 2 or higher) has
2692 been requested using the @option{--verbose} option, e.g., when listing
2693 or extracting archives:
2696 $ @kbd{tar -t -v --full-time -f archive.tar}
2700 or, when creating an archive:
2703 $ @kbd{tar -c -vv --full-time -f archive.tar .}
2706 Notice, thar when creating the archive you need to specify
2707 @option{--verbose} twice to get a detailed output (@pxref{verbose
2711 @item --group=@var{group}
2713 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
2714 rather than the group from the source file. @var{group} can specify a
2715 symbolic name, or a numeric @acronym{ID}, or both as
2716 @var{name}:@var{id}. @xref{override}.
2718 Also see the comments for the @option{--owner=@var{user}} option.
2728 This option tells @command{tar} to read or write archives through
2729 @command{gzip}, allowing @command{tar} to directly operate on several
2730 kinds of compressed archives transparently. @xref{gzip}.
2732 @opsummary{hard-dereference}
2733 @item --hard-dereference
2734 When creating an archive, dereference hard links and store the files
2735 they refer to, instead of creating usual hard link members.
2743 @command{tar} will print out a short message summarizing the operations and
2744 options to @command{tar} and exit. @xref{help}.
2746 @opsummary{ignore-case}
2748 Ignore case when matching member or file names with
2749 patterns. @xref{controlling pattern-matching}.
2751 @opsummary{ignore-command-error}
2752 @item --ignore-command-error
2753 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2755 @opsummary{ignore-failed-read}
2756 @item --ignore-failed-read
2758 Do not exit unsuccessfully merely because an unreadable file was encountered.
2759 @xref{Ignore Failed Read}.
2761 @opsummary{ignore-zeros}
2762 @item --ignore-zeros
2765 With this option, @command{tar} will ignore zeroed blocks in the
2766 archive, which normally signals EOF. @xref{Reading}.
2768 @opsummary{incremental}
2772 Informs @command{tar} that it is working with an old
2773 @acronym{GNU}-format incremental backup archive. It is intended
2774 primarily for backwards compatibility only. @xref{Incremental Dumps},
2775 for a detailed discussion of incremental archives.
2777 @opsummary{index-file}
2778 @item --index-file=@var{file}
2780 Send verbose output to @var{file} instead of to standard output.
2782 @opsummary{info-script}
2783 @opsummary{new-volume-script}
2784 @item --info-script=@var{script-file}
2785 @itemx --new-volume-script=@var{script-file}
2786 @itemx -F @var{script-file}
2788 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2789 at the end of each tape. If @var{script-file} exits with nonzero status,
2790 @command{tar} fails immediately. @xref{info-script}, for a detailed
2791 discussion of @var{script-file}.
2793 @opsummary{interactive}
2795 @itemx --confirmation
2798 Specifies that @command{tar} should ask the user for confirmation before
2799 performing potentially destructive options, such as overwriting files.
2802 @opsummary{keep-newer-files}
2803 @item --keep-newer-files
2805 Do not replace existing files that are newer than their archive copies
2806 when extracting files from an archive.
2808 @opsummary{keep-old-files}
2809 @item --keep-old-files
2812 Do not overwrite existing files when extracting files from an
2813 archive. Return error if such files exist. See also
2814 @ref{--skip-old-files}.
2816 @xref{Keep Old Files}.
2819 @item --label=@var{name}
2820 @itemx -V @var{name}
2822 When creating an archive, instructs @command{tar} to write @var{name}
2823 as a name record in the archive. When extracting or listing archives,
2824 @command{tar} will only operate on archives that have a label matching
2825 the pattern specified in @var{name}. @xref{Tape Files}.
2828 @item --level=@var{n}
2829 Force incremental backup of level @var{n}. As of @GNUTAR version
2830 @value{VERSION}, the option @option{--level=0} truncates the snapshot
2831 file, thereby forcing the level 0 dump. Other values of @var{n} are
2832 effectively ignored. @xref{--level=0}, for details and examples.
2834 The use of this option is valid only in conjunction with the
2835 @option{--listed-incremental} option. @xref{Incremental Dumps},
2836 for a detailed description.
2838 @opsummary{listed-incremental}
2839 @item --listed-incremental=@var{snapshot-file}
2840 @itemx -g @var{snapshot-file}
2842 During a @option{--create} operation, specifies that the archive that
2843 @command{tar} creates is a new @acronym{GNU}-format incremental
2844 backup, using @var{snapshot-file} to determine which files to backup.
2845 With other operations, informs @command{tar} that the archive is in
2846 incremental format. @xref{Incremental Dumps}.
2851 This option tells @command{tar} to read or write archives through
2852 @command{lzip}. @xref{gzip}.
2857 This option tells @command{tar} to read or write archives through
2858 @command{lzma}. @xref{gzip}.
2862 This option tells @command{tar} to read or write archives through
2863 @command{lzop}. @xref{gzip}.
2866 @item --mode=@var{permissions}
2868 When adding files to an archive, @command{tar} will use
2869 @var{permissions} for the archive members, rather than the permissions
2870 from the files. @var{permissions} can be specified either as an octal
2871 number or as symbolic permissions, like with
2872 @command{chmod}. @xref{override}.
2875 @item --mtime=@var{date}
2877 When adding files to an archive, @command{tar} will use @var{date} as
2878 the modification time of members when creating archives, instead of
2879 their actual modification times. The value of @var{date} can be
2880 either a textual date representation (@pxref{Date input formats}) or a
2881 name of the existing file, starting with @samp{/} or @samp{.}. In the
2882 latter case, the modification time of that file is used. @xref{override}.
2884 @opsummary{multi-volume}
2885 @item --multi-volume
2888 Informs @command{tar} that it should create or otherwise operate on a
2889 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2891 @opsummary{new-volume-script}
2892 @item --new-volume-script
2894 (see @option{--info-script})
2897 @item --newer=@var{date}
2898 @itemx --after-date=@var{date}
2901 When creating an archive, @command{tar} will only add files that have changed
2902 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2903 is taken to be the name of a file whose data modification time specifies
2904 the date. @xref{after}.
2906 @opsummary{newer-mtime}
2907 @item --newer-mtime=@var{date}
2909 Like @option{--newer}, but add only files whose
2910 contents have changed (as opposed to just @option{--newer}, which will
2911 also back up files for which any status information has
2912 changed). @xref{after}.
2914 @opsummary{no-anchored}
2916 An exclude pattern can match any subsequence of the name's components.
2917 @xref{controlling pattern-matching}.
2919 @opsummary{no-auto-compress}
2920 @item --no-auto-compress
2922 Disables automatic compressed format recognition based on the archive
2923 suffix. @xref{--auto-compress}. @xref{gzip}.
2925 @opsummary{no-check-device}
2926 @item --no-check-device
2927 Do not check device numbers when creating a list of modified files
2928 for incremental archiving. @xref{device numbers}, for
2929 a detailed description.
2931 @opsummary{no-delay-directory-restore}
2932 @item --no-delay-directory-restore
2934 Modification times and permissions of extracted
2935 directories are set when all files from this directory have been
2936 extracted. This is the default.
2937 @xref{Directory Modification Times and Permissions}.
2939 @opsummary{no-ignore-case}
2940 @item --no-ignore-case
2941 Use case-sensitive matching.
2942 @xref{controlling pattern-matching}.
2944 @opsummary{no-ignore-command-error}
2945 @item --no-ignore-command-error
2946 Print warnings about subprocesses that terminated with a nonzero exit
2947 code. @xref{Writing to an External Program}.
2952 If the @option{--null} option was given previously, this option
2953 cancels its effect, so that any following @option{--files-from}
2954 options will expect their file lists to be newline-terminated.
2956 @opsummary{no-overwrite-dir}
2957 @item --no-overwrite-dir
2959 Preserve metadata of existing directories when extracting files
2960 from an archive. @xref{Overwrite Old Files}.
2962 @opsummary{no-quote-chars}
2963 @item --no-quote-chars=@var{string}
2964 Remove characters listed in @var{string} from the list of quoted
2965 characters set by the previous @option{--quote-chars} option
2966 (@pxref{quoting styles}).
2968 @opsummary{no-recursion}
2969 @item --no-recursion
2971 With this option, @command{tar} will not recurse into directories.
2974 @opsummary{no-same-owner}
2975 @item --no-same-owner
2978 When extracting an archive, do not attempt to preserve the owner
2979 specified in the @command{tar} archive. This the default behavior
2982 @opsummary{no-same-permissions}
2983 @item --no-same-permissions
2985 When extracting an archive, subtract the user's umask from files from
2986 the permissions specified in the archive. This is the default behavior
2992 The archive media does not support seeks to arbitrary
2993 locations. Usually @command{tar} determines automatically whether
2994 the archive can be seeked or not. Use this option to disable this
2997 @opsummary{no-unquote}
2999 Treat all input file or member names literally, do not interpret
3000 escape sequences. @xref{input name quoting}.
3002 @opsummary{no-wildcards}
3003 @item --no-wildcards
3004 Do not use wildcards.
3005 @xref{controlling pattern-matching}.
3007 @opsummary{no-wildcards-match-slash}
3008 @item --no-wildcards-match-slash
3009 Wildcards do not match @samp{/}.
3010 @xref{controlling pattern-matching}.
3015 When @command{tar} is using the @option{--files-from} option, this option
3016 instructs @command{tar} to expect file names terminated with @acronym{NUL}, so
3017 @command{tar} can correctly work with file names that contain newlines.
3020 @opsummary{numeric-owner}
3021 @item --numeric-owner
3023 This option will notify @command{tar} that it should use numeric user
3024 and group IDs when creating a @command{tar} file, rather than names.
3028 The function of this option depends on the action @command{tar} is
3029 performing. When extracting files, @option{-o} is a synonym for
3030 @option{--no-same-owner}, i.e., it prevents @command{tar} from
3031 restoring ownership of files being extracted.
3033 When creating an archive, it is a synonym for
3034 @option{--old-archive}. This behavior is for compatibility
3035 with previous versions of @GNUTAR{}, and will be
3036 removed in future releases.
3038 @xref{Changes}, for more information.
3040 @opsummary{occurrence}
3041 @item --occurrence[=@var{number}]
3043 This option can be used in conjunction with one of the subcommands
3044 @option{--delete}, @option{--diff}, @option{--extract} or
3045 @option{--list} when a list of files is given either on the command
3046 line or via @option{-T} option.
3048 This option instructs @command{tar} to process only the @var{number}th
3049 occurrence of each named file. @var{Number} defaults to 1, so
3052 tar -x -f archive.tar --occurrence filename
3056 will extract the first occurrence of the member @file{filename} from @file{archive.tar}
3057 and will terminate without scanning to the end of the archive.
3059 @opsummary{old-archive}
3061 Synonym for @option{--format=v7}.
3063 @opsummary{one-file-system}
3064 @item --one-file-system
3065 Used when creating an archive. Prevents @command{tar} from recursing into
3066 directories that are on different file systems from the current
3069 @opsummary{overwrite}
3072 Overwrite existing files and directory metadata when extracting files
3073 from an archive. @xref{Overwrite Old Files}.
3075 @opsummary{overwrite-dir}
3076 @item --overwrite-dir
3078 Overwrite the metadata of existing directories when extracting files
3079 from an archive. @xref{Overwrite Old Files}.
3082 @item --owner=@var{user}
3084 Specifies that @command{tar} should use @var{user} as the owner of members
3085 when creating archives, instead of the user associated with the source
3086 file. @var{user} can specify a symbolic name, or a numeric
3087 @acronym{ID}, or both as @var{name}:@var{id}.
3090 This option does not affect extraction from archives.
3092 @opsummary{pax-option}
3093 @item --pax-option=@var{keyword-list}
3094 This option enables creation of the archive in @acronym{POSIX.1-2001}
3095 format (@pxref{posix}) and modifies the way @command{tar} handles the
3096 extended header keywords. @var{Keyword-list} is a comma-separated
3097 list of keyword options. @xref{PAX keywords}, for a detailed
3100 @opsummary{portability}
3102 @itemx --old-archive
3103 Synonym for @option{--format=v7}.
3107 Same as @option{--format=posix}.
3109 @opsummary{preserve}
3112 Synonymous with specifying both @option{--preserve-permissions} and
3113 @option{--same-order}. @xref{Setting Access Permissions}.
3115 @opsummary{preserve-order}
3116 @item --preserve-order
3118 (See @option{--same-order}; @pxref{Reading}.)
3120 @opsummary{preserve-permissions}
3121 @opsummary{same-permissions}
3122 @item --preserve-permissions
3123 @itemx --same-permissions
3126 When @command{tar} is extracting an archive, it normally subtracts the
3127 users' umask from the permissions specified in the archive and uses
3128 that number as the permissions to create the destination file.
3129 Specifying this option instructs @command{tar} that it should use the
3130 permissions directly from the archive. @xref{Setting Access Permissions}.
3132 @opsummary{quote-chars}
3133 @item --quote-chars=@var{string}
3134 Always quote characters from @var{string}, even if the selected
3135 quoting style would not quote them (@pxref{quoting styles}).
3137 @opsummary{quoting-style}
3138 @item --quoting-style=@var{style}
3139 Set quoting style to use when printing member and file names
3140 (@pxref{quoting styles}). Valid @var{style} values are:
3141 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
3142 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
3143 style is @code{escape}, unless overridden while configuring the
3146 @opsummary{read-full-records}
3147 @item --read-full-records
3150 Specifies that @command{tar} should reblock its input, for reading
3151 from pipes on systems with buggy implementations. @xref{Reading}.
3153 @opsummary{record-size}
3154 @item --record-size=@var{size}[@var{suf}]
3156 Instructs @command{tar} to use @var{size} bytes per record when accessing the
3157 archive. The argument can be suffixed with a @dfn{size suffix}, e.g.
3158 @option{--record-size=10K} for 10 Kilobytes. @xref{size-suffixes},
3159 for a list of valid suffixes. @xref{Blocking Factor}, for a detailed
3160 description of this option.
3162 @opsummary{recursion}
3165 With this option, @command{tar} recurses into directories (default).
3168 @opsummary{recursive-unlink}
3169 @item --recursive-unlink
3172 directory hierarchies before extracting directories of the same name
3173 from the archive. @xref{Recursive Unlink}.
3175 @opsummary{remove-files}
3176 @item --remove-files
3178 Directs @command{tar} to remove the source file from the file system after
3179 appending it to an archive. @xref{remove files}.
3181 @opsummary{restrict}
3184 Disable use of some potentially harmful @command{tar} options.
3185 Currently this option disables shell invocation from multi-volume menu
3186 (@pxref{Using Multiple Tapes}).
3188 @opsummary{rmt-command}
3189 @item --rmt-command=@var{cmd}
3191 Notifies @command{tar} that it should use @var{cmd} instead of
3192 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
3194 @opsummary{rsh-command}
3195 @item --rsh-command=@var{cmd}
3197 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
3198 devices. @xref{Device}.
3200 @opsummary{same-order}
3202 @itemx --preserve-order
3205 This option is an optimization for @command{tar} when running on machines with
3206 small amounts of memory. It informs @command{tar} that the list of file
3207 arguments has already been sorted to match the order of files in the
3208 archive. @xref{Reading}.
3210 @opsummary{same-owner}
3213 When extracting an archive, @command{tar} will attempt to preserve the owner
3214 specified in the @command{tar} archive with this option present.
3215 This is the default behavior for the superuser; this option has an
3216 effect only for ordinary users. @xref{Attributes}.
3218 @opsummary{same-permissions}
3219 @item --same-permissions
3221 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
3227 Assume that the archive media supports seeks to arbitrary
3228 locations. Usually @command{tar} determines automatically whether
3229 the archive can be seeked or not. This option is intended for use
3230 in cases when such recognition fails. It takes effect only if the
3231 archive is open for reading (e.g. with @option{--list} or
3232 @option{--extract} options).
3234 @opsummary{show-defaults}
3235 @item --show-defaults
3237 Displays the default options used by @command{tar} and exits
3238 successfully. This option is intended for use in shell scripts.
3239 Here is an example of what you can see using this option:
3242 $ @kbd{tar --show-defaults}
3243 --format=gnu -f- -b20 --quoting-style=escape
3244 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
3248 Notice, that this option outputs only one line. The example output
3249 above has been split to fit page boundaries.
3251 @opsummary{show-omitted-dirs}
3252 @item --show-omitted-dirs
3254 Instructs @command{tar} to mention the directories it is skipping when
3255 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
3257 @opsummary{show-transformed-names}
3258 @opsummary{show-stored-names}
3259 @item --show-transformed-names
3260 @itemx --show-stored-names
3262 Display file or member names after applying any transformations
3263 (@pxref{transform}). In particular, when used in conjunction with one of
3264 the archive creation operations it instructs @command{tar} to list the
3265 member names stored in the archive, as opposed to the actual file
3266 names. @xref{listing member and file names}.
3268 @opsummary{skip-old-files}
3269 @item --skip-old-files
3271 Do not overwrite existing files when extracting files from an
3272 archive. @xref{Keep Old Files}.
3274 This option differs from @option{--keep-old-files} in that it does not
3275 treat such files as an error, instead it just silently avoids
3278 The @option{--warning=existing-file} option can be used together with
3279 this option to produce warning messages about existing old files
3286 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3287 sparse files efficiently. @xref{sparse}.
3289 @opsummary{sparse-version}
3290 @item --sparse-version=@var{version}
3292 Specifies the @dfn{format version} to use when archiving sparse
3293 files. Implies @option{--sparse}. @xref{sparse}. For the description
3294 of the supported sparse formats, @xref{Sparse Formats}.
3296 @opsummary{starting-file}
3297 @item --starting-file=@var{name}
3298 @itemx -K @var{name}
3300 This option affects extraction only; @command{tar} will skip extracting
3301 files in the archive until it finds one that matches @var{name}.
3304 @opsummary{strip-components}
3305 @item --strip-components=@var{number}
3306 Strip given @var{number} of leading components from file names before
3307 extraction. For example, if archive @file{archive.tar} contained
3308 @file{/some/file/name}, then running
3311 tar --extract --file archive.tar --strip-components=2
3315 would extract this file to file @file{name}.
3318 @item --suffix=@var{suffix}
3320 Alters the suffix @command{tar} uses when backing up files from the default
3321 @samp{~}. @xref{backup}.
3323 @opsummary{tape-length}
3324 @item --tape-length=@var{num}[@var{suf}]
3325 @itemx -L @var{num}[@var{suf}]
3327 Specifies the length of tapes that @command{tar} is writing as being
3328 @w{@var{num} x 1024} bytes long. If optional @var{suf} is given, it
3329 specifies a multiplicative factor to be used instead of 1024. For
3330 example, @samp{-L2M} means 2 megabytes. @xref{size-suffixes}, for a
3331 list of allowed suffixes. @xref{Using Multiple Tapes}, for a detailed
3332 discussion of this option.
3334 @opsummary{test-label}
3337 Reads the volume label. If an argument is specified, test whether it
3338 matches the volume label. @xref{--test-label option}.
3340 @opsummary{to-command}
3341 @item --to-command=@var{command}
3343 During extraction @command{tar} will pipe extracted files to the
3344 standard input of @var{command}. @xref{Writing to an External Program}.
3346 @opsummary{to-stdout}
3350 During extraction, @command{tar} will extract files to stdout rather
3351 than to the file system. @xref{Writing to Standard Output}.
3354 @item --totals[=@var{signo}]
3356 Displays the total number of bytes transferred when processing an
3357 archive. If an argument is given, these data are displayed on
3358 request, when signal @var{signo} is delivered to @command{tar}.
3365 Sets the data modification time of extracted files to the extraction time,
3366 rather than the data modification time stored in the archive.
3367 @xref{Data Modification Times}.
3369 @opsummary{transform}
3371 @item --transform=@var{sed-expr}
3372 @itemx --xform=@var{sed-expr}
3373 Transform file or member names using @command{sed} replacement expression
3374 @var{sed-expr}. For example,
3377 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
3381 will add to @file{archive} files from the current working directory,
3382 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
3383 discussion, @xref{transform}.
3385 To see transformed member names in verbose listings, use
3386 @option{--show-transformed-names} option
3387 (@pxref{show-transformed-names}).
3389 @opsummary{uncompress}
3392 (See @option{--compress}, @pxref{gzip})
3397 (See @option{--gzip}, @pxref{gzip})
3399 @opsummary{unlink-first}
3400 @item --unlink-first
3403 Directs @command{tar} to remove the corresponding file from the file
3404 system before extracting it from the archive. @xref{Unlink First}.
3408 Enable unquoting input file or member names (default). @xref{input
3411 @opsummary{use-compress-program}
3412 @item --use-compress-program=@var{prog}
3413 @itemx -I=@var{prog}
3415 Instructs @command{tar} to access the archive through @var{prog}, which is
3416 presumed to be a compression program of some sort. @xref{gzip}.
3421 Display file modification dates in @acronym{UTC}. This option implies
3428 Specifies that @command{tar} should be more verbose about the
3429 operations it is performing. This option can be specified multiple
3430 times for some operations to increase the amount of information displayed.
3437 Verifies that the archive was correctly written when creating an
3438 archive. @xref{verify}.
3443 Print information about the program's name, version, origin and legal
3444 status, all on standard output, and then exit successfully.
3447 @opsummary{volno-file}
3448 @item --volno-file=@var{file}
3450 Used in conjunction with @option{--multi-volume}. @command{tar} will
3451 keep track of which volume of a multi-volume archive it is working in
3452 @var{file}. @xref{volno-file}.
3455 @item --warning=@var{keyword}
3457 Enable or disable warning messages identified by @var{keyword}. The
3458 messages are suppressed if @var{keyword} is prefixed with @samp{no-}.
3461 @opsummary{wildcards}
3463 Use wildcards when matching member names with patterns.
3464 @xref{controlling pattern-matching}.
3466 @opsummary{wildcards-match-slash}
3467 @item --wildcards-match-slash
3468 Wildcards match @samp{/}.
3469 @xref{controlling pattern-matching}.
3474 Use @command{xz} for compressing or decompressing the archives. @xref{gzip}.
3478 @node Short Option Summary
3479 @subsection Short Options Cross Reference
3481 Here is an alphabetized list of all of the short option forms, matching
3482 them with the equivalent long option.
3484 @multitable @columnfractions 0.20 0.80
3485 @headitem Short Option @tab Reference
3487 @item -A @tab @ref{--concatenate}.
3489 @item -B @tab @ref{--read-full-records}.
3491 @item -C @tab @ref{--directory}.
3493 @item -F @tab @ref{--info-script}.
3495 @item -G @tab @ref{--incremental}.
3497 @item -J @tab @ref{--xz}.
3499 @item -K @tab @ref{--starting-file}.
3501 @item -L @tab @ref{--tape-length}.
3503 @item -M @tab @ref{--multi-volume}.
3505 @item -N @tab @ref{--newer}.
3507 @item -O @tab @ref{--to-stdout}.
3509 @item -P @tab @ref{--absolute-names}.
3511 @item -R @tab @ref{--block-number}.
3513 @item -S @tab @ref{--sparse}.
3515 @item -T @tab @ref{--files-from}.
3517 @item -U @tab @ref{--unlink-first}.
3519 @item -V @tab @ref{--label}.
3521 @item -W @tab @ref{--verify}.
3523 @item -X @tab @ref{--exclude-from}.
3525 @item -Z @tab @ref{--compress}.
3527 @item -b @tab @ref{--blocking-factor}.
3529 @item -c @tab @ref{--create}.
3531 @item -d @tab @ref{--compare}.
3533 @item -f @tab @ref{--file}.
3535 @item -g @tab @ref{--listed-incremental}.
3537 @item -h @tab @ref{--dereference}.
3539 @item -i @tab @ref{--ignore-zeros}.
3541 @item -j @tab @ref{--bzip2}.
3543 @item -k @tab @ref{--keep-old-files}.
3545 @item -l @tab @ref{--check-links}.
3547 @item -m @tab @ref{--touch}.
3549 @item -o @tab When creating, @ref{--no-same-owner}, when extracting ---
3550 @ref{--portability}.
3552 The latter usage is deprecated. It is retained for compatibility with
3553 the earlier versions of @GNUTAR{}. In future releases
3554 @option{-o} will be equivalent to @option{--no-same-owner} only.
3556 @item -p @tab @ref{--preserve-permissions}.
3558 @item -r @tab @ref{--append}.
3560 @item -s @tab @ref{--same-order}.
3562 @item -t @tab @ref{--list}.
3564 @item -u @tab @ref{--update}.
3566 @item -v @tab @ref{--verbose}.
3568 @item -w @tab @ref{--interactive}.
3570 @item -x @tab @ref{--extract}.
3572 @item -z @tab @ref{--gzip}.
3577 @section @GNUTAR{} documentation
3579 @cindex Getting program version number
3581 @cindex Version of the @command{tar} program
3582 Being careful, the first thing is really checking that you are using
3583 @GNUTAR{}, indeed. The @option{--version} option
3584 causes @command{tar} to print information about its name, version,
3585 origin and legal status, all on standard output, and then exit
3586 successfully. For example, @w{@samp{tar --version}} might print:
3589 tar (GNU tar) @value{VERSION}
3590 Copyright (C) 2011 Free Software Foundation, Inc.
3591 Copyright (C) 2011 Free Software Foundation, Inc.
3592 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
3593 This is free software: you are free to change and redistribute it.
3594 There is NO WARRANTY, to the extent permitted by law.
3596 Written by John Gilmore and Jay Fenlason.
3600 The first occurrence of @samp{tar} in the result above is the program
3601 name in the package (for example, @command{rmt} is another program),
3602 while the second occurrence of @samp{tar} is the name of the package
3603 itself, containing possibly many programs. The package is currently
3604 named @samp{tar}, after the name of the main program it
3605 contains@footnote{There are plans to merge the @command{cpio} and
3606 @command{tar} packages into a single one which would be called
3607 @code{paxutils}. So, who knows if, one of this days, the
3608 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3611 @cindex Obtaining help
3612 @cindex Listing all @command{tar} options
3613 @xopindex{help, introduction}
3614 Another thing you might want to do is checking the spelling or meaning
3615 of some particular @command{tar} option, without resorting to this
3616 manual, for once you have carefully read it. @GNUTAR{}
3617 has a short help feature, triggerable through the
3618 @option{--help} option. By using this option, @command{tar} will
3619 print a usage message listing all available options on standard
3620 output, then exit successfully, without doing anything else and
3621 ignoring all other options. Even if this is only a brief summary, it
3622 may be several screens long. So, if you are not using some kind of
3623 scrollable window, you might prefer to use something like:
3626 $ @kbd{tar --help | less}
3630 presuming, here, that you like using @command{less} for a pager. Other
3631 popular pagers are @command{more} and @command{pg}. If you know about some
3632 @var{keyword} which interests you and do not want to read all the
3633 @option{--help} output, another common idiom is doing:
3636 tar --help | grep @var{keyword}
3640 for getting only the pertinent lines. Notice, however, that some
3641 @command{tar} options have long description lines and the above
3642 command will list only the first of them.
3644 The exact look of the option summary displayed by @kbd{tar --help} is
3645 configurable. @xref{Configuring Help Summary}, for a detailed description.
3648 If you only wish to check the spelling of an option, running @kbd{tar
3649 --usage} may be a better choice. This will display a terse list of
3650 @command{tar} options without accompanying explanations.
3652 The short help output is quite succinct, and you might have to get
3653 back to the full documentation for precise points. If you are reading
3654 this paragraph, you already have the @command{tar} manual in some
3655 form. This manual is available in a variety of forms from
3656 @url{http://www.gnu.org/software/tar/manual}. It may be printed out of the @GNUTAR{}
3657 distribution, provided you have @TeX{} already installed somewhere,
3658 and a laser printer around. Just configure the distribution, execute
3659 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3660 usual way (contact your local guru to know how). If @GNUTAR{}
3661 has been conveniently installed at your place, this
3662 manual is also available in interactive, hypertextual form as an Info
3663 file. Just call @w{@samp{info tar}} or, if you do not have the
3664 @command{info} program handy, use the Info reader provided within
3665 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3667 There is currently no @code{man} page for @GNUTAR{}.
3668 If you observe such a @code{man} page on the system you are running,
3669 either it does not belong to @GNUTAR{}, or it has not
3670 been produced by @acronym{GNU}. Some package maintainers convert
3671 @kbd{tar --help} output to a man page, using @command{help2man}. In
3672 any case, please bear in mind that the authoritative source of
3673 information about @GNUTAR{} is this Texinfo documentation.
3676 @section Obtaining @GNUTAR{} default values
3678 @opindex show-defaults
3679 @GNUTAR{} has some predefined defaults that are used when you do not
3680 explicitly specify another values. To obtain a list of such
3681 defaults, use @option{--show-defaults} option. This will output the
3682 values in the form of @command{tar} command line options:
3686 $ @kbd{tar --show-defaults}
3687 --format=gnu -f- -b20 --quoting-style=escape
3688 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3693 Notice, that this option outputs only one line. The example output above
3694 has been split to fit page boundaries.
3697 The above output shows that this version of @GNUTAR{} defaults to
3698 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3699 output as the archive, if no @option{--file} option has been given
3700 (@pxref{file tutorial}), the default blocking factor is 20
3701 (@pxref{Blocking Factor}). It also shows the default locations where
3702 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3705 @section Checking @command{tar} progress
3707 Typically, @command{tar} performs most operations without reporting any
3708 information to the user except error messages. When using @command{tar}
3709 with many options, particularly ones with complicated or
3710 difficult-to-predict behavior, it is possible to make serious mistakes.
3711 @command{tar} provides several options that make observing @command{tar}
3712 easier. These options cause @command{tar} to print information as it
3713 progresses in its job, and you might want to use them just for being
3714 more careful about what is going on, or merely for entertaining
3715 yourself. If you have encountered a problem when operating on an
3716 archive, however, you may need more information than just an error
3717 message in order to solve the problem. The following options can be
3718 helpful diagnostic tools.
3720 @cindex Verbose operation
3722 Normally, the @option{--list} (@option{-t}) command to list an archive
3723 prints just the file names (one per line) and the other commands are
3724 silent. When used with most operations, the @option{--verbose}
3725 (@option{-v}) option causes @command{tar} to print the name of each
3726 file or archive member as it is processed. This and the other options
3727 which make @command{tar} print status information can be useful in
3728 monitoring @command{tar}.
3730 With @option{--create} or @option{--extract}, @option{--verbose} used
3731 once just prints the names of the files or members as they are processed.
3732 Using it twice causes @command{tar} to print a longer listing
3733 (@xref{verbose member listing}, for the description) for each member.
3734 Since @option{--list} already prints the names of the members,
3735 @option{--verbose} used once with @option{--list} causes @command{tar}
3736 to print an @samp{ls -l} type listing of the files in the archive.
3737 The following examples both extract members with long list output:
3740 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3741 $ @kbd{tar xvvf archive.tar}
3744 Verbose output appears on the standard output except when an archive is
3745 being written to the standard output, as with @samp{tar --create
3746 --file=- --verbose} (@samp{tar cvf -}, or even @samp{tar cv}---if the
3747 installer let standard output be the default archive). In that case
3748 @command{tar} writes verbose output to the standard error stream.
3750 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3751 verbose output to @var{file} rather than to standard output or standard
3755 @cindex Obtaining total status information
3757 The @option{--totals} option causes @command{tar} to print on the
3758 standard error the total amount of bytes transferred when processing
3759 an archive. When creating or appending to an archive, this option
3760 prints the number of bytes written to the archive and the average
3761 speed at which they have been written, e.g.:
3765 $ @kbd{tar -c -f archive.tar --totals /home}
3766 Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
3770 When reading an archive, this option displays the number of bytes
3775 $ @kbd{tar -x -f archive.tar --totals}
3776 Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
3780 Finally, when deleting from an archive, the @option{--totals} option
3781 displays both numbers plus number of bytes removed from the archive:
3785 $ @kbd{tar --delete -f foo.tar --totals --wildcards '*~'}
3786 Total bytes read: 9543680 (9.2MiB, 201MiB/s)
3787 Total bytes written: 3829760 (3.7MiB, 81MiB/s)
3788 Total bytes deleted: 1474048
3792 You can also obtain this information on request. When
3793 @option{--totals} is used with an argument, this argument is
3794 interpreted as a symbolic name of a signal, upon delivery of which the
3795 statistics is to be printed:
3798 @item --totals=@var{signo}
3799 Print statistics upon delivery of signal @var{signo}. Valid arguments
3800 are: @code{SIGHUP}, @code{SIGQUIT}, @code{SIGINT}, @code{SIGUSR1} and
3801 @code{SIGUSR2}. Shortened names without @samp{SIG} prefix are also
3805 Both forms of @option{--totals} option can be used simultaneously.
3806 Thus, @kbd{tar -x --totals --totals=USR1} instructs @command{tar} to
3807 extract all members from its default archive and print statistics
3808 after finishing the extraction, as well as when receiving signal
3811 @anchor{Progress information}
3812 @cindex Progress information
3813 The @option{--checkpoint} option prints an occasional message
3814 as @command{tar} reads or writes the archive. It is designed for
3815 those who don't need the more detailed (and voluminous) output of
3816 @option{--block-number} (@option{-R}), but do want visual confirmation
3817 that @command{tar} is actually making forward progress. By default it
3818 prints a message each 10 records read or written. This can be changed
3819 by giving it a numeric argument after an equal sign:
3822 $ @kbd{tar -c --checkpoint=1000} /var
3823 tar: Write checkpoint 1000
3824 tar: Write checkpoint 2000
3825 tar: Write checkpoint 3000
3828 This example shows the default checkpoint message used by
3829 @command{tar}. If you place a dot immediately after the equal
3830 sign, it will print a @samp{.} at each checkpoint@footnote{This is
3831 actually a shortcut for @option{--checkpoint=@var{n}
3832 --checkpoint-action=dot}. @xref{checkpoints, dot}.}. For example:
3835 $ @kbd{tar -c --checkpoint=.1000} /var
3839 The @option{--checkpoint} option provides a flexible mechanism for
3840 executing arbitrary actions upon hitting checkpoints, see the next
3841 section (@pxref{checkpoints}), for more information on it.
3843 @opindex show-omitted-dirs
3844 @anchor{show-omitted-dirs}
3845 The @option{--show-omitted-dirs} option, when reading an archive---with
3846 @option{--list} or @option{--extract}, for example---causes a message
3847 to be printed for each directory in the archive which is skipped.
3848 This happens regardless of the reason for skipping: the directory might
3849 not have been named on the command line (implicitly or explicitly),
3850 it might be excluded by the use of the
3851 @option{--exclude=@var{pattern}} option, or some other reason.
3853 @opindex block-number
3854 @cindex Block number where error occurred
3855 @anchor{block-number}
3856 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3857 every message it would normally produce, the block number within the
3858 archive where the message was triggered. Also, supplementary messages
3859 are triggered when reading blocks full of NULs, or when hitting end of
3860 file on the archive. As of now, if the archive is properly terminated
3861 with a NUL block, the reading of the file may stop before end of file
3862 is met, so the position of end of file will not usually show when
3863 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3864 drains the archive before exiting when reading the
3865 archive from a pipe.
3867 @cindex Error message, block number of
3868 This option is especially useful when reading damaged archives, since
3869 it helps pinpoint the damaged sections. It can also be used with
3870 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3871 choose among several backup tapes when retrieving a file later, in
3872 favor of the tape where the file appears earliest (closest to the
3873 front of the tape). @xref{backup}.
3876 @section Checkpoints
3877 @cindex checkpoints, defined
3879 @opindex checkpoint-action
3881 A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
3882 the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
3883 from the archive (a @dfn{read checkpoint}). Checkpoints allow to
3884 periodically execute arbitrary actions.
3886 The checkpoint facility is enabled using the following option:
3889 @xopindex{checkpoint, defined}
3890 @item --checkpoint[=@var{n}]
3891 Schedule checkpoints before writing or reading each @var{n}th record.
3892 The default value for @var{n} is 10.
3895 A list of arbitrary @dfn{actions} can be executed at each checkpoint.
3896 These actions include: pausing, displaying textual messages, and
3897 executing arbitrary external programs. Actions are defined using
3898 the @option{--checkpoint-action} option.
3901 @xopindex{checkpoint-action, defined}
3902 @item --checkpoint-action=@var{action}
3903 Execute an @var{action} at each checkpoint.
3906 @cindex @code{echo}, checkpoint action
3907 The simplest value of @var{action} is @samp{echo}. It instructs
3908 @command{tar} to display the default message on the standard error
3909 stream upon arriving at each checkpoint. The default message is (in
3910 @acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
3911 checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
3912 Here, @var{n} represents ordinal number of the checkpoint.
3914 In another locales, translated versions of this message are used.
3916 This is the default action, so running:
3919 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
3926 $ @kbd{tar -c --checkpoint=1000} /var
3929 The @samp{echo} action also allows to supply a customized message.
3930 You do so by placing an equals sign and the message right after it,
3934 --checkpoint-action="echo=Hit %s checkpoint #%u"
3937 The @samp{%s} and @samp{%u} in the above example are
3938 @dfn{meta-characters}. The @samp{%s} meta-character is replaced with
3939 the @dfn{type} of the checkpoint: @samp{write} or
3940 @samp{read} (or a corresponding translated version in locales other
3941 than @acronym{POSIX}). The @samp{%u} meta-character is replaced with
3942 the ordinal number of the checkpoint. Thus, the above example could
3943 produce the following output when used with the @option{--create}
3947 tar: Hit write checkpoint #10
3948 tar: Hit write checkpoint #20
3949 tar: Hit write checkpoint #30
3952 Aside from meta-character expansion, the message string is subject to
3953 @dfn{unquoting}, during which the backslash @dfn{escape sequences} are
3954 replaced with their corresponding @acronym{ASCII} characters
3955 (@pxref{escape sequences}). E.g. the following action will produce an
3956 audible bell and the message described above at each checkpoint:
3959 --checkpoint-action='echo=\aHit %s checkpoint #%u'
3962 @cindex @code{bell}, checkpoint action
3963 There is also a special action which produces an audible signal:
3964 @samp{bell}. It is not equivalent to @samp{echo='\a'}, because
3965 @samp{bell} sends the bell directly to the console (@file{/dev/tty}),
3966 whereas @samp{echo='\a'} sends it to the standard error.
3968 @cindex @code{ttyout}, checkpoint action
3969 The @samp{ttyout=@var{string}} action outputs @var{string} to
3970 @file{/dev/tty}, so it can be used even if the standard output is
3971 redirected elsewhere. The @var{string} is subject to the same
3972 modifications as with @samp{echo} action. In contrast to the latter,
3973 @samp{ttyout} does not prepend @command{tar} executable name to the
3974 string, nor does it output a newline after it. For example, the
3975 following action will print the checkpoint message at the same screen
3976 line, overwriting any previous message:
3979 --checkpoint-action="ttyout=\rHit %s checkpoint #%u"
3982 @cindex @code{dot}, checkpoint action
3983 Another available checkpoint action is @samp{dot} (or @samp{.}). It
3984 instructs @command{tar} to print a single dot on the standard listing
3988 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
3992 For compatibility with previous @GNUTAR{} versions, this action can
3993 be abbreviated by placing a dot in front of the checkpoint frequency,
3994 as shown in the previous section.
3996 @cindex @code{sleep}, checkpoint action
3997 Yet another action, @samp{sleep}, pauses @command{tar} for a specified
3998 amount of seconds. The following example will stop for 30 seconds at each
4002 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
4005 @cindex @code{exec}, checkpoint action
4006 Finally, the @code{exec} action executes a given external program.
4010 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
4013 This program is executed using @command{/bin/sh -c}, with no
4014 additional arguments. Its exit code is ignored. It gets a copy of
4015 @command{tar}'s environment plus the following variables:
4018 @vrindex TAR_VERSION, checkpoint script environment
4020 @GNUTAR{} version number.
4022 @vrindex TAR_ARCHIVE, checkpoint script environment
4024 The name of the archive @command{tar} is processing.
4026 @vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
4027 @item TAR_BLOCKING_FACTOR
4028 Current blocking factor (@pxref{Blocking}).
4030 @vrindex TAR_CHECKPOINT, checkpoint script environment
4031 @item TAR_CHECKPOINT
4032 Number of the checkpoint.
4034 @vrindex TAR_SUBCOMMAND, checkpoint script environment
4035 @item TAR_SUBCOMMAND
4036 A short option describing the operation @command{tar} is executing.
4037 @xref{Operations}, for a complete list of subcommand options.
4039 @vrindex TAR_FORMAT, checkpoint script environment
4041 Format of the archive being processed. @xref{Formats}, for a complete
4042 list of archive format names.
4045 Any number of actions can be defined, by supplying several
4046 @option{--checkpoint-action} options in the command line. For
4047 example, the command below displays two messages, pauses
4048 execution for 30 seconds and executes the @file{/sbin/cpoint} script:
4052 $ @kbd{tar -c -f arc.tar \
4053 --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
4054 --checkpoint-action='echo=Sleeping for 30 seconds' \
4055 --checkpoint-action='sleep=30' \
4056 --checkpoint-action='exec=/sbin/cpoint'}
4060 This example also illustrates the fact that
4061 @option{--checkpoint-action} can be used without
4062 @option{--checkpoint}. In this case, the default checkpoint frequency
4063 (at each 10th record) is assumed.
4066 @section Controlling Warning Messages
4068 Sometimes, while performing the requested task, @GNUTAR{} notices
4069 some conditions that are not exactly errors, but which the user
4070 should be aware of. When this happens, @command{tar} issues a
4071 @dfn{warning message} describing the condition. Warning messages
4072 are output to the standard error and they do not affect the exit
4073 code of @command{tar} command.
4075 @xopindex{warning, explained}
4076 @GNUTAR{} allows the user to suppress some or all of its warning
4080 @item --warning=@var{keyword}
4081 Control display of the warning messages identified by @var{keyword}.
4082 If @var{keyword} starts with the prefix @samp{no-}, such messages are
4083 suppressed. Otherwise, they are enabled.
4085 Multiple @option{--warning} messages accumulate.
4087 The tables below list allowed values for @var{keyword} along with the
4088 warning messages they control.
4091 @subheading Keywords controlling @command{tar} operation
4095 Enable all warning messages. This is the default.
4098 Disable all warning messages.
4099 @kwindex filename-with-nuls
4100 @cindex @samp{file name read contains nul character}, warning message
4101 @item filename-with-nuls
4102 @samp{%s: file name read contains nul character}
4103 @kwindex alone-zero-block
4104 @cindex @samp{A lone zero block at}, warning message
4105 @item alone-zero-block
4106 @samp{A lone zero block at %s}
4109 @subheading Keywords applicable for @command{tar --create}
4112 @cindex @samp{contains a cache directory tag}, warning message
4114 @samp{%s: contains a cache directory tag %s; %s}
4115 @kwindex file-shrank
4116 @cindex @samp{File shrank by %s bytes}, warning message
4118 @samp{%s: File shrank by %s bytes; padding with zeros}
4120 @cindex @samp{file is on a different filesystem}, warning message
4122 @samp{%s: file is on a different filesystem; not dumped}
4123 @kwindex file-ignored
4124 @cindex @samp{Unknown file type; file ignored}, warning message
4125 @cindex @samp{socket ignored}, warning message
4126 @cindex @samp{door ignored}, warning message
4128 @samp{%s: Unknown file type; file ignored}
4129 @*@samp{%s: socket ignored}
4130 @*@samp{%s: door ignored}
4131 @kwindex file-unchanged
4132 @cindex @samp{file is unchanged; not dumped}, warning message
4133 @item file-unchanged
4134 @samp{%s: file is unchanged; not dumped}
4135 @kwindex ignore-archive
4136 @cindex @samp{file is the archive; not dumped}, warning message
4137 @kwindex ignore-archive
4138 @cindex @samp{file is the archive; not dumped}, warning message
4139 @item ignore-archive
4140 @samp{%s: file is the archive; not dumped}
4141 @kwindex file-removed
4142 @cindex @samp{File removed before we read it}, warning message
4144 @samp{%s: File removed before we read it}
4145 @kwindex file-changed
4146 @cindex @samp{file changed as we read it}, warning message
4148 @samp{%s: file changed as we read it}
4151 @subheading Keywords applicable for @command{tar --extract}
4154 @cindex @samp{implausibly old time stamp %s}, warning message
4155 @cindex @samp{time stamp %s is %s s in the future}, warning message
4157 @samp{%s: implausibly old time stamp %s}
4158 @*@samp{%s: time stamp %s is %s s in the future}
4159 @kwindex contiguous-cast
4160 @cindex @samp{Extracting contiguous files as regular files}, warning message
4161 @item contiguous-cast
4162 @samp{Extracting contiguous files as regular files}
4163 @kwindex symlink-cast
4164 @cindex @samp{Attempting extraction of symbolic links as hard links}, warning message
4166 @samp{Attempting extraction of symbolic links as hard links}
4167 @kwindex unknown-cast
4168 @cindex @samp{Unknown file type '%c', extracted as normal file}, warning message
4170 @samp{%s: Unknown file type '%c', extracted as normal file}
4171 @kwindex ignore-newer
4172 @cindex @samp{Current %s is newer or same age}, warning message
4174 @samp{Current %s is newer or same age}
4175 @kwindex unknown-keyword
4176 @cindex @samp{Ignoring unknown extended header keyword '%s'}, warning message
4177 @item unknown-keyword
4178 @samp{Ignoring unknown extended header keyword '%s'}
4179 @kwindex decompress-program
4180 @item decompress-program
4181 Controls verbose description of failures occurring when trying to run
4182 alternative decompressor programs (@pxref{alternative decompression
4183 programs}). This warning is disabled by default (unless
4184 @option{--verbose} is used). A common example of what you can get
4185 when using this warning is:
4188 $ @kbd{tar --warning=decompress-program -x -f archive.Z}
4189 tar (child): cannot run compress: No such file or directory
4190 tar (child): trying gzip
4193 This means that @command{tar} first tried to decompress
4194 @file{archive.Z} using @command{compress}, and, when that
4195 failed, switched to @command{gzip}.
4198 @subheading Keywords controlling incremental extraction:
4200 @kwindex rename-directory
4201 @cindex @samp{%s: Directory has been renamed from %s}, warning message
4202 @cindex @samp{%s: Directory has been renamed}, warning message
4203 @item rename-directory
4204 @samp{%s: Directory has been renamed from %s}
4205 @*@samp{%s: Directory has been renamed}
4206 @kwindex new-directory
4207 @cindex @samp{%s: Directory is new}, warning message
4209 @samp{%s: Directory is new}
4211 @cindex @samp{%s: directory is on a different device: not purging}, warning message
4213 @samp{%s: directory is on a different device: not purging}
4214 @kwindex bad-dumpdir
4215 @cindex @samp{Malformed dumpdir: 'X' never used}, warning message
4217 @samp{Malformed dumpdir: 'X' never used}
4221 @section Asking for Confirmation During Operations
4222 @cindex Interactive operation
4224 Typically, @command{tar} carries out a command without stopping for
4225 further instructions. In some situations however, you may want to
4226 exclude some files and archive members from the operation (for instance
4227 if disk or storage space is tight). You can do this by excluding
4228 certain files automatically (@pxref{Choosing}), or by performing
4229 an operation interactively, using the @option{--interactive} (@option{-w}) option.
4230 @command{tar} also accepts @option{--confirmation} for this option.
4232 @opindex interactive
4233 When the @option{--interactive} (@option{-w}) option is specified, before
4234 reading, writing, or deleting files, @command{tar} first prints a message
4235 for each such file, telling what operation it intends to take, then asks
4236 for confirmation on the terminal. The actions which require
4237 confirmation include adding a file to the archive, extracting a file
4238 from the archive, deleting a file from the archive, and deleting a file
4239 from disk. To confirm the action, you must type a line of input
4240 beginning with @samp{y}. If your input line begins with anything other
4241 than @samp{y}, @command{tar} skips that file.
4243 If @command{tar} is reading the archive from the standard input,
4244 @command{tar} opens the file @file{/dev/tty} to support the interactive
4247 Verbose output is normally sent to standard output, separate from
4248 other error messages. However, if the archive is produced directly
4249 on standard output, then verbose output is mixed with errors on
4250 @code{stderr}. Producing the archive on standard output may be used
4251 as a way to avoid using disk space, when the archive is soon to be
4252 consumed by another process reading it, say. Some people felt the need
4253 of producing an archive on stdout, still willing to segregate between
4254 verbose output and error output. A possible approach would be using a
4255 named pipe to receive the archive, and having the consumer process to
4256 read from that named pipe. This has the advantage of letting standard
4257 output free to receive verbose output, all separate from errors.
4260 @chapter @GNUTAR{} Operations
4273 @section Basic @GNUTAR{} Operations
4275 The basic @command{tar} operations, @option{--create} (@option{-c}),
4276 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
4277 @option{-x}), are currently presented and described in the tutorial
4278 chapter of this manual. This section provides some complementary notes
4279 for these operations.
4282 @xopindex{create, complementary notes}
4286 Creating an empty archive would have some kind of elegance. One can
4287 initialize an empty archive and later use @option{--append}
4288 (@option{-r}) for adding all members. Some applications would not
4289 welcome making an exception in the way of adding the first archive
4290 member. On the other hand, many people reported that it is
4291 dangerously too easy for @command{tar} to destroy a magnetic tape with
4292 an empty archive@footnote{This is well described in @cite{Unix-haters
4293 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
4294 Books, ISBN 1-56884-203-1.}. The two most common errors are:
4298 Mistakingly using @code{create} instead of @code{extract}, when the
4299 intent was to extract the full contents of an archive. This error
4300 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
4301 the QWERTY keyboard. Instead of being unpacked, the archive then
4302 gets wholly destroyed. When users speak about @dfn{exploding} an
4303 archive, they usually mean something else :-).
4306 Forgetting the argument to @code{file}, when the intent was to create
4307 an archive with a single file in it. This error is likely because a
4308 tired user can easily add the @kbd{f} key to the cluster of option
4309 letters, by the mere force of habit, without realizing the full
4310 consequence of doing so. The usual consequence is that the single
4311 file, which was meant to be saved, is rather destroyed.
4314 So, recognizing the likelihood and the catastrophic nature of these
4315 errors, @GNUTAR{} now takes some distance from elegance, and
4316 cowardly refuses to create an archive when @option{--create} option is
4317 given, there are no arguments besides options, and
4318 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
4319 around the cautiousness of @GNUTAR{} and nevertheless create an
4320 archive with nothing in it, one may still use, as the value for the
4321 @option{--files-from} option, a file with no names in it, as shown in
4322 the following commands:
4325 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
4326 @kbd{tar -cf empty-archive.tar -T /dev/null}
4329 @xopindex{extract, complementary notes}
4334 A socket is stored, within a @GNUTAR{} archive, as a pipe.
4336 @item @option{--list} (@option{-t})
4338 @GNUTAR{} now shows dates as @samp{1996-08-30},
4339 while it used to show them as @samp{Aug 30 1996}. Preferably,
4340 people should get used to ISO 8601 dates. Local American dates should
4341 be made available again with full date localization support, once
4342 ready. In the meantime, programs not being localizable for dates
4343 should prefer international dates, that's really the way to go.
4345 Look up @url{http://www.cl.cam.ac.uk/@/~mgk25/@/iso-time.html} if you
4346 are curious, it contains a detailed explanation of the ISO 8601 standard.
4351 @section Advanced @GNUTAR{} Operations
4353 Now that you have learned the basics of using @GNUTAR{}, you may want
4354 to learn about further ways in which @command{tar} can help you.
4356 This chapter presents five, more advanced operations which you probably
4357 won't use on a daily basis, but which serve more specialized functions.
4358 We also explain the different styles of options and why you might want
4359 to use one or another, or a combination of them in your @command{tar}
4360 commands. Additionally, this chapter includes options which allow you to
4361 define the output from @command{tar} more carefully, and provide help and
4362 error correction in special circumstances.
4364 @FIXME{check this after the chapter is actually revised to make sure
4365 it still introduces the info in the chapter correctly : ).}
4377 @subsection The Five Advanced @command{tar} Operations
4379 @cindex basic operations
4380 In the last chapter, you learned about the first three operations to
4381 @command{tar}. This chapter presents the remaining five operations to
4382 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
4383 @option{--delete}, and @option{--compare}.
4385 You are not likely to use these operations as frequently as those
4386 covered in the last chapter; however, since they perform specialized
4387 functions, they are quite useful when you do need to use them. We
4388 will give examples using the same directory and files that you created
4389 in the last chapter. As you may recall, the directory is called
4390 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
4391 and the two archive files you created are
4392 @samp{collection.tar} and @samp{music.tar}.
4394 We will also use the archive files @samp{afiles.tar} and
4395 @samp{bfiles.tar}. The archive @samp{afiles.tar} contains the members @samp{apple},
4396 @samp{angst}, and @samp{aspic}; @samp{bfiles.tar} contains the members
4397 @samp{./birds}, @samp{baboon}, and @samp{./box}.
4399 Unless we state otherwise, all practicing you do and examples you follow
4400 in this chapter will take place in the @file{practice} directory that
4401 you created in the previous chapter; see @ref{prepare for examples}.
4402 (Below in this section, we will remind you of the state of the examples
4403 where the last chapter left them.)
4405 The five operations that we will cover in this chapter are:
4410 Add new entries to an archive that already exists.
4413 Add more recent copies of archive members to the end of an archive, if
4418 Add one or more pre-existing archives to the end of another archive.
4420 Delete items from an archive (does not work on tapes).
4424 Compare archive members to their counterparts in the file system.
4428 @subsection How to Add Files to Existing Archives: @option{--append}
4430 @cindex appending files to existing archive
4432 If you want to add files to an existing archive, you don't need to
4433 create a new archive; you can use @option{--append} (@option{-r}).
4434 The archive must already exist in order to use @option{--append}. (A
4435 related operation is the @option{--update} operation; you can use this
4436 to add newer versions of archive members to an existing archive. To learn how to
4437 do this with @option{--update}, @pxref{update}.)
4439 If you use @option{--append} to add a file that has the same name as an
4440 archive member to an archive containing that archive member, then the
4441 old member is not deleted. What does happen, however, is somewhat
4442 complex. @command{tar} @emph{allows} you to have infinite number of files
4443 with the same name. Some operations treat these same-named members no
4444 differently than any other set of archive members: for example, if you
4445 view an archive with @option{--list} (@option{-t}), you will see all
4446 of those members listed, with their data modification times, owners, etc.
4448 Other operations don't deal with these members as perfectly as you might
4449 prefer; if you were to use @option{--extract} to extract the archive,
4450 only the most recently added copy of a member with the same name as
4451 other members would end up in the working directory. This is because
4452 @option{--extract} extracts an archive in the order the members appeared
4453 in the archive; the most recently archived members will be extracted
4454 last. Additionally, an extracted member will @emph{replace} a file of
4455 the same name which existed in the directory already, and @command{tar}
4456 will not prompt you about this@footnote{Unless you give it
4457 @option{--keep-old-files} (or @option{--skip-old-files}) option, or
4458 the disk copy is newer than the one in the archive and you invoke
4459 @command{tar} with @option{--keep-newer-files} option.}. Thus, only
4460 the most recently archived member will end up being extracted, as it
4461 will replace the one extracted before it, and so on.
4463 @cindex extracting @var{n}th copy of the file
4464 @xopindex{occurrence, described}
4465 There exists a special option that allows you to get around this
4466 behavior and extract (or list) only a particular copy of the file.
4467 This is @option{--occurrence} option. If you run @command{tar} with
4468 this option, it will extract only the first copy of the file. You
4469 may also give this option an argument specifying the number of
4470 copy to be extracted. Thus, for example if the archive
4471 @file{archive.tar} contained three copies of file @file{myfile}, then
4475 tar --extract --file archive.tar --occurrence=2 myfile
4479 would extract only the second copy. @xref{Option
4480 Summary,---occurrence}, for the description of @option{--occurrence}
4483 @FIXME{ hag -- you might want to incorporate some of the above into the
4484 MMwtSN node; not sure. i didn't know how to make it simpler...
4486 There are a few ways to get around this. Xref to Multiple Members
4487 with the Same Name, maybe.}
4489 @cindex Members, replacing with other members
4490 @cindex Replacing members with other members
4491 @xopindex{delete, using before --append}
4492 If you want to replace an archive member, use @option{--delete} to
4493 delete the member you want to remove from the archive, and then use
4494 @option{--append} to add the member you want to be in the archive. Note
4495 that you can not change the order of the archive; the most recently
4496 added member will still appear last. In this sense, you cannot truly
4497 ``replace'' one member with another. (Replacing one member with another
4498 will not work on certain types of media, such as tapes; see @ref{delete}
4499 and @ref{Media}, for more information.)
4502 * appending files:: Appending Files to an Archive
4506 @node appending files
4507 @subsubsection Appending Files to an Archive
4508 @cindex Adding files to an Archive
4509 @cindex Appending files to an Archive
4510 @cindex Archives, Appending files to
4513 The simplest way to add a file to an already existing archive is the
4514 @option{--append} (@option{-r}) operation, which writes specified
4515 files into the archive whether or not they are already among the
4518 When you use @option{--append}, you @emph{must} specify file name
4519 arguments, as there is no default. If you specify a file that already
4520 exists in the archive, another copy of the file will be added to the
4521 end of the archive. As with other operations, the member names of the
4522 newly added files will be exactly the same as their names given on the
4523 command line. The @option{--verbose} (@option{-v}) option will print
4524 out the names of the files as they are written into the archive.
4526 @option{--append} cannot be performed on some tape drives, unfortunately,
4527 due to deficiencies in the formats those tape drives use. The archive
4528 must be a valid @command{tar} archive, or else the results of using this
4529 operation will be unpredictable. @xref{Media}.
4531 To demonstrate using @option{--append} to add a file to an archive,
4532 create a file called @file{rock} in the @file{practice} directory.
4533 Make sure you are in the @file{practice} directory. Then, run the
4534 following @command{tar} command to add @file{rock} to
4535 @file{collection.tar}:
4538 $ @kbd{tar --append --file=collection.tar rock}
4542 If you now use the @option{--list} (@option{-t}) operation, you will see that
4543 @file{rock} has been added to the archive:
4546 $ @kbd{tar --list --file=collection.tar}
4547 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4548 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4549 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4550 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4554 @subsubsection Multiple Members with the Same Name
4555 @cindex members, multiple
4556 @cindex multiple members
4558 You can use @option{--append} (@option{-r}) to add copies of files
4559 which have been updated since the archive was created. (However, we
4560 do not recommend doing this since there is another @command{tar}
4561 option called @option{--update}; @xref{update}, for more information.
4562 We describe this use of @option{--append} here for the sake of
4563 completeness.) When you extract the archive, the older version will
4564 be effectively lost. This works because files are extracted from an
4565 archive in the order in which they were archived. Thus, when the
4566 archive is extracted, a file archived later in time will replace a
4567 file of the same name which was archived earlier, even though the
4568 older version of the file will remain in the archive unless you delete
4569 all versions of the file.
4571 Supposing you change the file @file{blues} and then append the changed
4572 version to @file{collection.tar}. As you saw above, the original
4573 @file{blues} is in the archive @file{collection.tar}. If you change the
4574 file and append the new version of the file to the archive, there will
4575 be two copies in the archive. When you extract the archive, the older
4576 version of the file will be extracted first, and then replaced by the
4577 newer version when it is extracted.
4579 You can append the new, changed copy of the file @file{blues} to the
4580 archive in this way:
4583 $ @kbd{tar --append --verbose --file=collection.tar blues}
4588 Because you specified the @option{--verbose} option, @command{tar} has
4589 printed the name of the file being appended as it was acted on. Now
4590 list the contents of the archive:
4593 $ @kbd{tar --list --verbose --file=collection.tar}
4594 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4595 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4596 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4597 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4598 -rw-r--r-- me/user 58 1996-10-24 18:30 blues
4602 The newest version of @file{blues} is now at the end of the archive
4603 (note the different creation dates and file sizes). If you extract
4604 the archive, the older version of the file @file{blues} will be
4605 replaced by the newer version. You can confirm this by extracting
4606 the archive and running @samp{ls} on the directory.
4608 If you wish to extract the first occurrence of the file @file{blues}
4609 from the archive, use @option{--occurrence} option, as shown in
4610 the following example:
4613 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
4614 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4617 @xref{Writing}, for more information on @option{--extract} and
4618 see @ref{Option Summary, --occurrence}, for a description of
4619 @option{--occurrence} option.
4622 @subsection Updating an Archive
4623 @cindex Updating an archive
4626 In the previous section, you learned how to use @option{--append} to
4627 add a file to an existing archive. A related operation is
4628 @option{--update} (@option{-u}). The @option{--update} operation
4629 updates a @command{tar} archive by comparing the date of the specified
4630 archive members against the date of the file with the same name. If
4631 the file has been modified more recently than the archive member, then
4632 the newer version of the file is added to the archive (as with
4635 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
4636 The operation will fail.
4638 @FIXME{other examples of media on which --update will fail? need to ask
4639 charles and/or mib/thomas/dave shevett..}
4641 Both @option{--update} and @option{--append} work by adding to the end
4642 of the archive. When you extract a file from the archive, only the
4643 version stored last will wind up in the file system, unless you use
4644 the @option{--backup} option. @xref{multiple}, for a detailed discussion.
4651 @subsubsection How to Update an Archive Using @option{--update}
4654 You must use file name arguments with the @option{--update}
4655 (@option{-u}) operation. If you don't specify any files,
4656 @command{tar} won't act on any files and won't tell you that it didn't
4657 do anything (which may end up confusing you).
4659 @c note: the above parenthetical added because in fact, this
4660 @c behavior just confused the author. :-)
4662 To see the @option{--update} option at work, create a new file,
4663 @file{classical}, in your practice directory, and some extra text to the
4664 file @file{blues}, using any text editor. Then invoke @command{tar} with
4665 the @samp{update} operation and the @option{--verbose} (@option{-v})
4666 option specified, using the names of all the files in the @file{practice}
4667 directory as file name arguments:
4670 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4677 Because we have specified verbose mode, @command{tar} prints out the names
4678 of the files it is working on, which in this case are the names of the
4679 files that needed to be updated. If you run @samp{tar --list} and look
4680 at the archive, you will see @file{blues} and @file{classical} at its
4681 end. There will be a total of two versions of the member @samp{blues};
4682 the one at the end will be newer and larger, since you added text before
4685 The reason @command{tar} does not overwrite the older file when updating
4686 it is because writing to the middle of a section of tape is a difficult
4687 process. Tapes are not designed to go backward. @xref{Media}, for more
4688 information about tapes.
4690 @option{--update} (@option{-u}) is not suitable for performing backups for two
4691 reasons: it does not change directory content entries, and it
4692 lengthens the archive every time it is used. The @GNUTAR{}
4693 options intended specifically for backups are more
4694 efficient. If you need to run backups, please consult @ref{Backups}.
4697 @subsection Combining Archives with @option{--concatenate}
4699 @cindex Adding archives to an archive
4700 @cindex Concatenating Archives
4701 @opindex concatenate
4703 @c @cindex @option{-A} described
4704 Sometimes it may be convenient to add a second archive onto the end of
4705 an archive rather than adding individual files to the archive. To add
4706 one or more archives to the end of another archive, you should use the
4707 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4709 To use @option{--concatenate}, give the first archive with
4710 @option{--file} option and name the rest of archives to be
4711 concatenated on the command line. The members, and their member
4712 names, will be copied verbatim from those archives to the first
4713 one@footnote{This can cause multiple members to have the same name. For
4714 information on how this affects reading the archive, see @ref{multiple}.}.
4715 The new, concatenated archive will be called by the same name as the
4716 one given with the @option{--file} option. As usual, if you omit
4717 @option{--file}, @command{tar} will use the value of the environment
4718 variable @env{TAPE}, or, if this has not been set, the default archive name.
4720 @FIXME{There is no way to specify a new name...}
4722 To demonstrate how @option{--concatenate} works, create two small archives
4723 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4724 files from @file{practice}:
4727 $ @kbd{tar -cvf bluesrock.tar blues rock}
4730 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4736 If you like, You can run @samp{tar --list} to make sure the archives
4737 contain what they are supposed to:
4740 $ @kbd{tar -tvf bluesrock.tar}
4741 -rw-r--r-- melissa/user 105 1997-01-21 19:42 blues
4742 -rw-r--r-- melissa/user 33 1997-01-20 15:34 rock
4743 $ @kbd{tar -tvf jazzfolk.tar}
4744 -rw-r--r-- melissa/user 20 1996-09-23 16:44 folk
4745 -rw-r--r-- melissa/user 65 1997-01-30 14:15 jazz
4748 We can concatenate these two archives with @command{tar}:
4752 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4755 If you now list the contents of the @file{bluesrock.tar}, you will see
4756 that now it also contains the archive members of @file{jazzfolk.tar}:
4759 $ @kbd{tar --list --file=bluesrock.tar}
4766 When you use @option{--concatenate}, the source and target archives must
4767 already exist and must have been created using compatible format
4768 parameters. Notice, that @command{tar} does not check whether the
4769 archives it concatenates have compatible formats, it does not
4770 even check if the files are really tar archives.
4772 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4773 tape drives, due to deficiencies in the formats those tape drives use.
4775 @cindex @code{concatenate} vs @command{cat}
4776 @cindex @command{cat} vs @code{concatenate}
4777 It may seem more intuitive to you to want or try to use @command{cat} to
4778 concatenate two archives instead of using the @option{--concatenate}
4779 operation; after all, @command{cat} is the utility for combining files.
4781 However, @command{tar} archives incorporate an end-of-file marker which
4782 must be removed if the concatenated archives are to be read properly as
4783 one archive. @option{--concatenate} removes the end-of-archive marker
4784 from the target archive before each new archive is appended. If you use
4785 @command{cat} to combine the archives, the result will not be a valid
4786 @command{tar} format archive. If you need to retrieve files from an
4787 archive that was added to using the @command{cat} utility, use the
4788 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4789 information on dealing with archives improperly combined using the
4790 @command{cat} shell utility.
4793 @subsection Removing Archive Members Using @option{--delete}
4794 @cindex Deleting files from an archive
4795 @cindex Removing files from an archive
4798 You can remove members from an archive by using the @option{--delete}
4799 option. Specify the name of the archive with @option{--file}
4800 (@option{-f}) and then specify the names of the members to be deleted;
4801 if you list no member names, nothing will be deleted. The
4802 @option{--verbose} option will cause @command{tar} to print the names
4803 of the members as they are deleted. As with @option{--extract}, you
4804 must give the exact member names when using @samp{tar --delete}.
4805 @option{--delete} will remove all versions of the named file from the
4806 archive. The @option{--delete} operation can run very slowly.
4808 Unlike other operations, @option{--delete} has no short form.
4810 @cindex Tapes, using @option{--delete} and
4811 @cindex Deleting from tape archives
4812 This operation will rewrite the archive. You can only use
4813 @option{--delete} on an archive if the archive device allows you to
4814 write to any point on the media, such as a disk; because of this, it
4815 does not work on magnetic tapes. Do not try to delete an archive member
4816 from a magnetic tape; the action will not succeed, and you will be
4817 likely to scramble the archive and damage your tape. There is no safe
4818 way (except by completely re-writing the archive) to delete files from
4819 most kinds of magnetic tape. @xref{Media}.
4821 To delete all versions of the file @file{blues} from the archive
4822 @file{collection.tar} in the @file{practice} directory, make sure you
4823 are in that directory, and then,
4826 $ @kbd{tar --list --file=collection.tar}
4831 $ @kbd{tar --delete --file=collection.tar blues}
4832 $ @kbd{tar --list --file=collection.tar}
4838 @FIXME{Check if the above listing is actually produced after running
4839 all the examples on collection.tar.}
4841 The @option{--delete} option has been reported to work properly when
4842 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4845 @subsection Comparing Archive Members with the File System
4846 @cindex Verifying the currency of an archive
4849 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4850 specified archive members against files with the same names, and then
4851 reports differences in file size, mode, owner, modification date and
4852 contents. You should @emph{only} specify archive member names, not file
4853 names. If you do not name any members, then @command{tar} will compare the
4854 entire archive. If a file is represented in the archive but does not
4855 exist in the file system, @command{tar} reports a difference.
4857 You have to specify the record size of the archive when modifying an
4858 archive with a non-default record size.
4860 @command{tar} ignores files in the file system that do not have
4861 corresponding members in the archive.
4863 The following example compares the archive members @file{rock},
4864 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4865 files of the same name in the file system. (Note that there is no file,
4866 @file{funk}; @command{tar} will report an error message.)
4869 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4872 tar: funk not found in archive
4875 The spirit behind the @option{--compare} (@option{--diff},
4876 @option{-d}) option is to check whether the archive represents the
4877 current state of files on disk, more than validating the integrity of
4878 the archive media. For this latter goal, see @ref{verify}.
4880 @node create options
4881 @section Options Used by @option{--create}
4883 @xopindex{create, additional options}
4884 The previous chapter described the basics of how to use
4885 @option{--create} (@option{-c}) to create an archive from a set of files.
4886 @xref{create}. This section described advanced options to be used with
4890 * override:: Overriding File Metadata.
4891 * Ignore Failed Read::
4895 @subsection Overriding File Metadata
4897 As described above, a @command{tar} archive keeps, for each member it contains,
4898 its @dfn{metadata}, such as modification time, mode and ownership of
4899 the file. @GNUTAR{} allows to replace these data with other values
4900 when adding files to the archive. The options described in this
4901 section affect creation of archives of any type. For POSIX archives,
4902 see also @ref{PAX keywords}, for additional ways of controlling
4903 metadata, stored in the archive.
4907 @item --mode=@var{permissions}
4909 When adding files to an archive, @command{tar} will use
4910 @var{permissions} for the archive members, rather than the permissions
4911 from the files. @var{permissions} can be specified either as an octal
4912 number or as symbolic permissions, like with
4913 @command{chmod} (@xref{File permissions, Permissions, File
4914 permissions, fileutils, @acronym{GNU} file utilities}. This reference
4915 also has useful information for those not being overly familiar with
4916 the UNIX permission system). Using latter syntax allows for
4917 more flexibility. For example, the value @samp{a+rw} adds read and write
4918 permissions for everybody, while retaining executable bits on directories
4919 or on any other file already marked as executable:
4922 $ @kbd{tar -c -f archive.tar --mode='a+rw' .}
4925 @item --mtime=@var{date}
4928 When adding files to an archive, @command{tar} will use @var{date} as
4929 the modification time of members when creating archives, instead of
4930 their actual modification times. The argument @var{date} can be
4931 either a textual date representation in almost arbitrary format
4932 (@pxref{Date input formats}) or a name of an existing file, starting
4933 with @samp{/} or @samp{.}. In the latter case, the modification time
4934 of that file will be used.
4936 The following example will set the modification date to 00:00:00,
4940 $ @kbd{tar -c -f archive.tar --mtime='1970-01-01' .}
4944 When used with @option{--verbose} (@pxref{verbose tutorial}) @GNUTAR{}
4945 will try to convert the specified date back to its textual
4946 representation and compare it with the one given with
4947 @option{--mtime} options. If the two dates differ, @command{tar} will
4948 print a warning saying what date it will use. This is to help user
4949 ensure he is using the right date.
4954 $ @kbd{tar -c -f archive.tar -v --mtime=yesterday .}
4955 tar: Option --mtime: Treating date 'yesterday' as 2006-06-20
4960 @item --owner=@var{user}
4963 Specifies that @command{tar} should use @var{user} as the owner of members
4964 when creating archives, instead of the user associated with the source
4967 If @var{user} contains a colon, it is taken to be of the form
4968 @var{name}:@var{id} where a nonempty @var{name} specifies the user
4969 name and a nonempty @var{id} specifies the decimal numeric user
4970 @acronym{ID}. If @var{user} does not contain a colon, it is taken to
4971 be a user number if it is one or more decimal digits; otherwise it is
4972 taken to be a user name.
4974 If a name is given but no number, the number is inferred from the
4975 current host's user database if possible, and the file's user number
4976 is used otherwise. If a number is given but no name, the name is
4977 inferred from the number if possible, and an empty name is used
4978 otherwise. If both name and number are given, the user database is
4979 not consulted, and the name and number need not be valid on the
4982 There is no value indicating a missing number, and @samp{0} usually means
4983 @code{root}. Some people like to force @samp{0} as the value to offer in
4984 their distributions for the owner of files, because the @code{root} user is
4985 anonymous anyway, so that might as well be the owner of anonymous
4986 archives. For example:
4989 $ @kbd{tar -c -f archive.tar --owner=0 .}
4996 $ @kbd{tar -c -f archive.tar --owner=root .}
4999 @item --group=@var{group}
5002 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
5003 rather than the group from the source file. As with @option{--owner},
5004 the argument @var{group} can be an existing group symbolic name, or a
5005 decimal numeric group @acronym{ID}, or @var{name}:@var{id}.
5008 @node Ignore Failed Read
5009 @subsection Ignore Fail Read
5012 @item --ignore-failed-read
5013 @opindex ignore-failed-read
5014 Do not exit with nonzero on unreadable files or directories.
5017 @node extract options
5018 @section Options Used by @option{--extract}
5019 @cindex options for use with @option{--extract}
5021 @xopindex{extract, additional options}
5022 The previous chapter showed how to use @option{--extract} to extract
5023 an archive into the file system. Various options cause @command{tar} to
5024 extract more information than just file contents, such as the owner,
5025 the permissions, the modification date, and so forth. This section
5026 presents options to be used with @option{--extract} when certain special
5027 considerations arise. You may review the information presented in
5028 @ref{extract} for more basic information about the
5029 @option{--extract} operation.
5032 * Reading:: Options to Help Read Archives
5033 * Writing:: Changing How @command{tar} Writes Files
5034 * Scarce:: Coping with Scarce Resources
5038 @subsection Options to Help Read Archives
5039 @cindex Options when reading archives
5041 @cindex Reading incomplete records
5042 @cindex Records, incomplete
5043 @opindex read-full-records
5044 Normally, @command{tar} will request data in full record increments from
5045 an archive storage device. If the device cannot return a full record,
5046 @command{tar} will report an error. However, some devices do not always
5047 return full records, or do not require the last record of an archive to
5048 be padded out to the next record boundary. To keep reading until you
5049 obtain a full record, or to accept an incomplete record if it contains
5050 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
5051 in conjunction with the @option{--extract} or @option{--list} operations.
5054 The @option{--read-full-records} (@option{-B}) option is turned on by default when
5055 @command{tar} reads an archive from standard input, or from a remote
5056 machine. This is because on @acronym{BSD} Unix systems, attempting to read a
5057 pipe returns however much happens to be in the pipe, even if it is
5058 less than was requested. If this option were not enabled, @command{tar}
5059 would fail as soon as it read an incomplete record from the pipe.
5061 If you're not sure of the blocking factor of an archive, you can
5062 read the archive by specifying @option{--read-full-records} (@option{-B}) and
5063 @option{--blocking-factor=@var{512-size}} (@option{-b
5064 @var{512-size}}), using a blocking factor larger than what the archive
5065 uses. This lets you avoid having to determine the blocking factor
5066 of an archive. @xref{Blocking Factor}.
5069 * read full records::
5073 @node read full records
5074 @unnumberedsubsubsec Reading Full Records
5076 @FIXME{need sentence or so of intro here}
5079 @opindex read-full-records
5080 @item --read-full-records
5082 Use in conjunction with @option{--extract} (@option{--get},
5083 @option{-x}) to read an archive which contains incomplete records, or
5084 one which has a blocking factor less than the one specified.
5088 @unnumberedsubsubsec Ignoring Blocks of Zeros
5090 @cindex End-of-archive blocks, ignoring
5091 @cindex Ignoring end-of-archive blocks
5092 @opindex ignore-zeros
5093 Normally, @command{tar} stops reading when it encounters a block of zeros
5094 between file entries (which usually indicates the end of the archive).
5095 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
5096 completely read an archive which contains a block of zeros before the
5097 end (i.e., a damaged archive, or one that was created by concatenating
5098 several archives together).
5100 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
5101 versions of @command{tar} write garbage after the end-of-archive entry,
5102 since that part of the media is never supposed to be read. @GNUTAR{}
5103 does not write after the end of an archive, but seeks to
5104 maintain compatibility among archiving utilities.
5107 @item --ignore-zeros
5109 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
5110 encountered while reading an archive. Use in conjunction with
5111 @option{--extract} or @option{--list}.
5115 @subsection Changing How @command{tar} Writes Files
5118 @FIXME{Introductory paragraph}
5121 * Dealing with Old Files::
5122 * Overwrite Old Files::
5124 * Keep Newer Files::
5126 * Recursive Unlink::
5127 * Data Modification Times::
5128 * Setting Access Permissions::
5129 * Directory Modification Times and Permissions::
5130 * Writing to Standard Output::
5131 * Writing to an External Program::
5135 @node Dealing with Old Files
5136 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
5138 @xopindex{overwrite-dir, introduced}
5139 When extracting files, if @command{tar} discovers that the extracted
5140 file already exists, it normally replaces the file by removing it before
5141 extracting it, to prevent confusion in the presence of hard or symbolic
5142 links. (If the existing file is a symbolic link, it is removed, not
5143 followed.) However, if a directory cannot be removed because it is
5144 nonempty, @command{tar} normally overwrites its metadata (ownership,
5145 permission, etc.). The @option{--overwrite-dir} option enables this
5146 default behavior. To be more cautious and preserve the metadata of
5147 such a directory, use the @option{--no-overwrite-dir} option.
5149 @cindex Overwriting old files, prevention
5150 @xopindex{keep-old-files, introduced}
5151 To be even more cautious and prevent existing files from being replaced, use
5152 the @option{--keep-old-files} (@option{-k}) option. It causes
5153 @command{tar} to refuse to replace or update a file that already
5154 exists, i.e., a file with the same name as an archive member prevents
5155 extraction of that archive member. Instead, it reports an error. For
5161 $ @kbd{tar -x -k -f archive.tar}
5162 tar: blues: Cannot open: File exists
5163 tar: Exiting with failure status due to previous errors
5166 @xopindex{skip-old-files, introduced}
5167 If you wish to preserve old files untouched, but don't want
5168 @command{tar} to treat them as errors, use the
5169 @option{--skip-old-files} option. This option causes @command{tar} to
5170 silently skip extracting over existing files.
5172 @xopindex{overwrite, introduced}
5173 To be more aggressive about altering existing files, use the
5174 @option{--overwrite} option. It causes @command{tar} to overwrite
5175 existing files and to follow existing symbolic links when extracting.
5177 @cindex Protecting old files
5178 Some people argue that @GNUTAR{} should not hesitate
5179 to overwrite files with other files when extracting. When extracting
5180 a @command{tar} archive, they expect to see a faithful copy of the
5181 state of the file system when the archive was created. It is debatable
5182 that this would always be a proper behavior. For example, suppose one
5183 has an archive in which @file{usr/local} is a link to
5184 @file{usr/local2}. Since then, maybe the site removed the link and
5185 renamed the whole hierarchy from @file{/usr/local2} to
5186 @file{/usr/local}. Such things happen all the time. I guess it would
5187 not be welcome at all that @GNUTAR{} removes the
5188 whole hierarchy just to make room for the link to be reinstated
5189 (unless it @emph{also} simultaneously restores the full
5190 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
5191 able to remove a whole hierarchy to reestablish a symbolic link, for
5192 example, but @emph{only if} @option{--recursive-unlink} is specified
5193 to allow this behavior. In any case, single files are silently
5196 @xopindex{unlink-first, introduced}
5197 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
5198 some cases by causing @command{tar} to remove files unconditionally
5199 before extracting them.
5201 @node Overwrite Old Files
5202 @unnumberedsubsubsec Overwrite Old Files
5207 Overwrite existing files and directory metadata when extracting files
5210 This causes @command{tar} to write extracted files into the file system without
5211 regard to the files already on the system; i.e., files with the same
5212 names as archive members are overwritten when the archive is extracted.
5213 It also causes @command{tar} to extract the ownership, permissions,
5214 and time stamps onto any preexisting files or directories.
5215 If the name of a corresponding file name is a symbolic link, the file
5216 pointed to by the symbolic link will be overwritten instead of the
5217 symbolic link itself (if this is possible). Moreover, special devices,
5218 empty directories and even symbolic links are automatically removed if
5219 they are in the way of extraction.
5221 Be careful when using the @option{--overwrite} option, particularly when
5222 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
5223 can change the contents, ownership or permissions of any file on your
5224 system. Also, many systems do not take kindly to overwriting files that
5225 are currently being executed.
5227 @opindex overwrite-dir
5228 @item --overwrite-dir
5229 Overwrite the metadata of directories when extracting files from an
5230 archive, but remove other files before extracting.
5233 @node Keep Old Files
5234 @unnumberedsubsubsec Keep Old Files
5236 @GNUTAR{} provides two options to control its actions in a situation
5237 when it is about to extract a file which already exists on disk.
5240 @opindex keep-old-files
5241 @item --keep-old-files
5243 Do not replace existing files from archive. When such a file is
5244 encountered, @command{tar} issues an error message. Upon end of
5245 extraction, @command{tar} exits with code 2 (@pxref{exit status}).
5247 @item --skip-old-files
5248 Do not replace existing files from archive, but do not treat that
5249 as error. Such files are silently skipped and do not affect
5250 @command{tar} exit status.
5252 Additional verbosity can be obtained using @option{--warning=existing-file}
5253 together with that option (@pxref{warnings}).
5256 @node Keep Newer Files
5257 @unnumberedsubsubsec Keep Newer Files
5260 @opindex keep-newer-files
5261 @item --keep-newer-files
5262 Do not replace existing files that are newer than their archive
5263 copies. This option is meaningless with @option{--list} (@option{-t}).
5267 @unnumberedsubsubsec Unlink First
5270 @opindex unlink-first
5271 @item --unlink-first
5273 Remove files before extracting over them.
5274 This can make @command{tar} run a bit faster if you know in advance
5275 that the extracted files all need to be removed. Normally this option
5276 slows @command{tar} down slightly, so it is disabled by default.
5279 @node Recursive Unlink
5280 @unnumberedsubsubsec Recursive Unlink
5283 @opindex recursive-unlink
5284 @item --recursive-unlink
5285 When this option is specified, try removing files and directory hierarchies
5286 before extracting over them. @emph{This is a dangerous option!}
5289 If you specify the @option{--recursive-unlink} option,
5290 @command{tar} removes @emph{anything} that keeps you from extracting a file
5291 as far as current permissions will allow it. This could include removal
5292 of the contents of a full directory hierarchy.
5294 @node Data Modification Times
5295 @unnumberedsubsubsec Setting Data Modification Times
5297 @cindex Data modification times of extracted files
5298 @cindex Modification times of extracted files
5299 Normally, @command{tar} sets the data modification times of extracted
5300 files to the corresponding times recorded for the files in the archive, but
5301 limits the permissions of extracted files by the current @code{umask}
5304 To set the data modification times of extracted files to the time when
5305 the files were extracted, use the @option{--touch} (@option{-m}) option in
5306 conjunction with @option{--extract} (@option{--get}, @option{-x}).
5312 Sets the data modification time of extracted archive members to the time
5313 they were extracted, not the time recorded for them in the archive.
5314 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
5317 @node Setting Access Permissions
5318 @unnumberedsubsubsec Setting Access Permissions
5320 @cindex Permissions of extracted files
5321 @cindex Modes of extracted files
5322 To set the modes (access permissions) of extracted files to those
5323 recorded for those files in the archive, use @option{--same-permissions}
5324 in conjunction with the @option{--extract} (@option{--get},
5325 @option{-x}) operation.
5328 @opindex preserve-permissions
5329 @opindex same-permissions
5330 @item --preserve-permissions
5331 @itemx --same-permissions
5332 @c @itemx --ignore-umask
5334 Set modes of extracted archive members to those recorded in the
5335 archive, instead of current umask settings. Use in conjunction with
5336 @option{--extract} (@option{--get}, @option{-x}).
5339 @node Directory Modification Times and Permissions
5340 @unnumberedsubsubsec Directory Modification Times and Permissions
5342 After successfully extracting a file member, @GNUTAR{} normally
5343 restores its permissions and modification times, as described in the
5344 previous sections. This cannot be done for directories, because
5345 after extracting a directory @command{tar} will almost certainly
5346 extract files into that directory and this will cause the directory
5347 modification time to be updated. Moreover, restoring that directory
5348 permissions may not permit file creation within it. Thus, restoring
5349 directory permissions and modification times must be delayed at least
5350 until all files have been extracted into that directory. @GNUTAR{}
5351 restores directories using the following approach.
5353 The extracted directories are created with the mode specified in the
5354 archive, as modified by the umask of the user, which gives sufficient
5355 permissions to allow file creation. The meta-information about the
5356 directory is recorded in the temporary list of directories. When
5357 preparing to extract next archive member, @GNUTAR{} checks if the
5358 directory prefix of this file contains the remembered directory. If
5359 it does not, the program assumes that all files have been extracted
5360 into that directory, restores its modification time and permissions
5361 and removes its entry from the internal list. This approach allows
5362 to correctly restore directory meta-information in the majority of
5363 cases, while keeping memory requirements sufficiently small. It is
5364 based on the fact, that most @command{tar} archives use the predefined
5365 order of members: first the directory, then all the files and
5366 subdirectories in that directory.
5368 However, this is not always true. The most important exception are
5369 incremental archives (@pxref{Incremental Dumps}). The member order in
5370 an incremental archive is reversed: first all directory members are
5371 stored, followed by other (non-directory) members. So, when extracting
5372 from incremental archives, @GNUTAR{} alters the above procedure. It
5373 remembers all restored directories, and restores their meta-data
5374 only after the entire archive has been processed. Notice, that you do
5375 not need to specify any special options for that, as @GNUTAR{}
5376 automatically detects archives in incremental format.
5378 There may be cases, when such processing is required for normal archives
5379 too. Consider the following example:
5383 $ @kbd{tar --no-recursion -cvf archive \
5384 foo foo/file1 bar bar/file foo/file2}
5393 During the normal operation, after encountering @file{bar}
5394 @GNUTAR{} will assume that all files from the directory @file{foo}
5395 were already extracted and will therefore restore its timestamp and
5396 permission bits. However, after extracting @file{foo/file2} the
5397 directory timestamp will be offset again.
5399 To correctly restore directory meta-information in such cases, use
5400 the @option{--delay-directory-restore} command line option:
5403 @opindex delay-directory-restore
5404 @item --delay-directory-restore
5405 Delays restoring of the modification times and permissions of extracted
5406 directories until the end of extraction. This way, correct
5407 meta-information is restored even if the archive has unusual member
5410 @opindex no-delay-directory-restore
5411 @item --no-delay-directory-restore
5412 Cancel the effect of the previous @option{--delay-directory-restore}.
5413 Use this option if you have used @option{--delay-directory-restore} in
5414 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
5415 temporarily disable it.
5418 @node Writing to Standard Output
5419 @unnumberedsubsubsec Writing to Standard Output
5421 @cindex Writing extracted files to standard output
5422 @cindex Standard output, writing extracted files to
5423 To write the extracted files to the standard output, instead of
5424 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
5425 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
5426 extracting files to send them through a pipe, and do not need to
5427 preserve them in the file system. If you extract multiple members,
5428 they appear on standard output concatenated, in the order they are
5429 found in the archive.
5435 Writes files to the standard output. Use only in conjunction with
5436 @option{--extract} (@option{--get}, @option{-x}). When this option is
5437 used, instead of creating the files specified, @command{tar} writes
5438 the contents of the files extracted to its standard output. This may
5439 be useful if you are only extracting the files in order to send them
5440 through a pipe. This option is meaningless with @option{--list}
5444 This can be useful, for example, if you have a tar archive containing
5445 a big file and don't want to store the file on disk before processing
5446 it. You can use a command like this:
5449 tar -xOzf foo.tgz bigfile | process
5452 or even like this if you want to process the concatenation of the files:
5455 tar -xOzf foo.tgz bigfile1 bigfile2 | process
5458 However, @option{--to-command} may be more convenient for use with
5459 multiple files. See the next section.
5461 @node Writing to an External Program
5462 @unnumberedsubsubsec Writing to an External Program
5464 You can instruct @command{tar} to send the contents of each extracted
5465 file to the standard input of an external program:
5469 @item --to-command=@var{command}
5470 Extract files and pipe their contents to the standard input of
5471 @var{command}. When this option is used, instead of creating the
5472 files specified, @command{tar} invokes @var{command} and pipes the
5473 contents of the files to its standard output. The @var{command} may
5474 contain command line arguments. The program is executed via
5475 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
5476 extracted. Non-regular files (directories, etc.) are ignored when this
5480 The command can obtain the information about the file it processes
5481 from the following environment variables:
5484 @vrindex TAR_FILETYPE, to-command environment
5486 Type of the file. It is a single letter with the following meaning:
5488 @multitable @columnfractions 0.10 0.90
5489 @item f @tab Regular file
5490 @item d @tab Directory
5491 @item l @tab Symbolic link
5492 @item h @tab Hard link
5493 @item b @tab Block device
5494 @item c @tab Character device
5497 Currently only regular files are supported.
5499 @vrindex TAR_MODE, to-command environment
5501 File mode, an octal number.
5503 @vrindex TAR_FILENAME, to-command environment
5505 The name of the file.
5507 @vrindex TAR_REALNAME, to-command environment
5509 Name of the file as stored in the archive.
5511 @vrindex TAR_UNAME, to-command environment
5513 Name of the file owner.
5515 @vrindex TAR_GNAME, to-command environment
5517 Name of the file owner group.
5519 @vrindex TAR_ATIME, to-command environment
5521 Time of last access. It is a decimal number, representing seconds
5522 since the Epoch. If the archive provides times with nanosecond
5523 precision, the nanoseconds are appended to the timestamp after a
5526 @vrindex TAR_MTIME, to-command environment
5528 Time of last modification.
5530 @vrindex TAR_CTIME, to-command environment
5532 Time of last status change.
5534 @vrindex TAR_SIZE, to-command environment
5538 @vrindex TAR_UID, to-command environment
5540 UID of the file owner.
5542 @vrindex TAR_GID, to-command environment
5544 GID of the file owner.
5547 Additionally, the following variables contain information about
5548 tar mode and the archive being processed:
5551 @vrindex TAR_VERSION, to-command environment
5553 @GNUTAR{} version number.
5555 @vrindex TAR_ARCHIVE, to-command environment
5557 The name of the archive @command{tar} is processing.
5559 @vrindex TAR_BLOCKING_FACTOR, to-command environment
5560 @item TAR_BLOCKING_FACTOR
5561 Current blocking factor (@pxref{Blocking}).
5563 @vrindex TAR_VOLUME, to-command environment
5565 Ordinal number of the volume @command{tar} is processing.
5567 @vrindex TAR_FORMAT, to-command environment
5569 Format of the archive being processed. @xref{Formats}, for a complete
5570 list of archive format names.
5573 If @var{command} exits with a non-0 status, @command{tar} will print
5574 an error message similar to the following:
5577 tar: 2345: Child returned status 1
5580 Here, @samp{2345} is the PID of the finished process.
5582 If this behavior is not wanted, use @option{--ignore-command-error}:
5585 @opindex ignore-command-error
5586 @item --ignore-command-error
5587 Ignore exit codes of subprocesses. Notice that if the program
5588 exits on signal or otherwise terminates abnormally, the error message
5589 will be printed even if this option is used.
5591 @opindex no-ignore-command-error
5592 @item --no-ignore-command-error
5593 Cancel the effect of any previous @option{--ignore-command-error}
5594 option. This option is useful if you have set
5595 @option{--ignore-command-error} in @env{TAR_OPTIONS}
5596 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
5600 @unnumberedsubsubsec Removing Files
5602 @FIXME{The section is too terse. Something more to add? An example,
5606 @opindex remove-files
5607 @item --remove-files
5608 Remove files after adding them to the archive.
5612 @subsection Coping with Scarce Resources
5615 @cindex Small memory
5616 @cindex Running out of space
5624 @unnumberedsubsubsec Starting File
5627 @opindex starting-file
5628 @item --starting-file=@var{name}
5629 @itemx -K @var{name}
5630 Starts an operation in the middle of an archive. Use in conjunction
5631 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
5634 @cindex Middle of the archive, starting in the
5635 If a previous attempt to extract files failed due to lack of disk
5636 space, you can use @option{--starting-file=@var{name}} (@option{-K
5637 @var{name}}) to start extracting only after member @var{name} of the
5638 archive. This assumes, of course, that there is now free space, or
5639 that you are now extracting into a different file system. (You could
5640 also choose to suspend @command{tar}, remove unnecessary files from
5641 the file system, and then resume the same @command{tar} operation.
5642 In this case, @option{--starting-file} is not necessary.) See also
5643 @ref{interactive}, and @ref{exclude}.
5646 @unnumberedsubsubsec Same Order
5649 @cindex Large lists of file names on small machines
5651 @opindex preserve-order
5653 @itemx --preserve-order
5655 To process large lists of file names on machines with small amounts of
5656 memory. Use in conjunction with @option{--compare} (@option{--diff},
5657 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
5658 (@option{--get}, @option{-x}).
5661 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
5662 names to be listed or extracted is sorted in the same order as the
5663 files in the archive. This allows a large list of names to be used,
5664 even on a small machine that would not otherwise be able to hold all
5665 the names in memory at the same time. Such a sorted list can easily be
5666 created by running @samp{tar -t} on the archive and editing its output.
5668 This option is probably never needed on modern computer systems.
5671 @section Backup options
5673 @cindex backup options
5675 @GNUTAR{} offers options for making backups of files
5676 before writing new versions. These options control the details of
5677 these backups. They may apply to the archive itself before it is
5678 created or rewritten, as well as individual extracted members. Other
5679 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
5680 and @command{mv}, for example) offer similar options.
5682 Backup options may prove unexpectedly useful when extracting archives
5683 containing many members having identical name, or when extracting archives
5684 on systems having file name limitations, making different members appear
5685 as having similar names through the side-effect of name truncation.
5686 @FIXME{This is true only if we have a good scheme for truncated backup names,
5687 which I'm not sure at all: I suspect work is needed in this area.}
5688 When any existing file is backed up before being overwritten by extraction,
5689 then clashing files are automatically be renamed to be unique, and the
5690 true name is kept for only the last file of a series of clashing files.
5691 By using verbose mode, users may track exactly what happens.
5693 At the detail level, some decisions are still experimental, and may
5694 change in the future, we are waiting comments from our users. So, please
5695 do not learn to depend blindly on the details of the backup features.
5696 For example, currently, directories themselves are never renamed through
5697 using these options, so, extracting a file over a directory still has
5698 good chances to fail. Also, backup options apply to created archives,
5699 not only to extracted members. For created archives, backups will not
5700 be attempted when the archive is a block or character device, or when it
5701 refers to a remote file.
5703 For the sake of simplicity and efficiency, backups are made by renaming old
5704 files prior to creation or extraction, and not by copying. The original
5705 name is restored if the file creation fails. If a failure occurs after a
5706 partial extraction of a file, both the backup and the partially extracted
5710 @item --backup[=@var{method}]
5712 @vindex VERSION_CONTROL
5714 Back up files that are about to be overwritten or removed.
5715 Without this option, the original versions are destroyed.
5717 Use @var{method} to determine the type of backups made.
5718 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
5719 environment variable. And if @env{VERSION_CONTROL} is not set,
5720 use the @samp{existing} method.
5722 @vindex version-control @r{Emacs variable}
5723 This option corresponds to the Emacs variable @samp{version-control};
5724 the same values for @var{method} are accepted as in Emacs. This option
5725 also allows more descriptive names. The valid @var{method}s are:
5730 @cindex numbered @r{backup method}
5731 Always make numbered backups.
5735 @cindex existing @r{backup method}
5736 Make numbered backups of files that already have them, simple backups
5741 @cindex simple @r{backup method}
5742 Always make simple backups.
5746 @item --suffix=@var{suffix}
5748 @cindex backup suffix
5749 @vindex SIMPLE_BACKUP_SUFFIX
5750 Append @var{suffix} to each backup file made with @option{--backup}. If this
5751 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5752 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5753 set, the default is @samp{~}, just as in Emacs.
5758 @section Notable @command{tar} Usages
5761 @FIXME{Using Unix file linking capability to recreate directory
5762 structures---linking files into one subdirectory and then
5763 @command{tar}ring that directory.}
5765 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5768 You can easily use archive files to transport a group of files from
5769 one system to another: put all relevant files into an archive on one
5770 computer system, transfer the archive to another system, and extract
5771 the contents there. The basic transfer medium might be magnetic tape,
5772 Internet FTP, or even electronic mail (though you must encode the
5773 archive with @command{uuencode} in order to transport it properly by
5774 mail). Both machines do not have to use the same operating system, as
5775 long as they both support the @command{tar} program.
5777 For example, here is how you might copy a directory's contents from
5778 one disk to another, while preserving the dates, modes, owners and
5779 link-structure of all the files therein. In this case, the transfer
5780 medium is a @dfn{pipe}:
5783 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5787 You can avoid subshells by using @option{-C} option:
5790 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5794 The command also works using long option forms:
5798 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5799 | (cd targetdir; tar --extract --file=-)}
5808 $ @kbd{tar --directory sourcedir --create --file=- . \
5809 | tar --directory targetdir --extract --file=-}
5814 This is one of the easiest methods to transfer a @command{tar} archive.
5817 @section Looking Ahead: The Rest of this Manual
5819 You have now seen how to use all eight of the operations available to
5820 @command{tar}, and a number of the possible options. The next chapter
5821 explains how to choose and change file and archive names, how to use
5822 files to store names of other files which you can then call as
5823 arguments to @command{tar} (this can help you save time if you expect to
5824 archive the same list of files a number of times), and so forth.
5825 @FIXME{in case it's not obvious, i'm making this up in some sense
5826 based on my limited memory of what the next chapter *really* does. i
5827 just wanted to flesh out this final section a little bit so i'd
5828 remember to stick it in here. :-)}
5830 If there are too many files to conveniently list on the command line,
5831 you can list the names in a file, and @command{tar} will read that file.
5834 There are various ways of causing @command{tar} to skip over some files,
5835 and not archive them. @xref{Choosing}.
5838 @chapter Performing Backups and Restoring Files
5841 @GNUTAR{} is distributed along with the scripts for performing backups
5842 and restores. Even if there is a good chance those scripts may be
5843 satisfying to you, they are not the only scripts or methods available for doing
5844 backups and restore. You may well create your own, or use more
5845 sophisticated packages dedicated to that purpose.
5847 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5848 Automatic Network Disk Archiver), a backup system developed by James
5849 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5850 This is free software, and it is available from @uref{http://www.amanda.org}.
5854 Here is a possible plan for a future documentation about the backuping
5855 scripts which are provided within the @GNUTAR{}
5861 @item what are dumps
5862 @item different levels of dumps
5864 @item full dump = dump everything
5865 @item level 1, level 2 dumps etc
5866 A level @var{n} dump dumps everything changed since the last level
5869 @item how to use scripts for dumps (ie, the concept)
5871 @item scripts to run after editing backup specs (details)
5873 @item Backup Specs, what is it.
5875 @item how to customize
5876 @item actual text of script [/sp/dump/backup-specs]
5880 @item rsh doesn't work
5881 @item rtape isn't installed
5884 @item the @option{--incremental} option of tar
5887 @item write protection
5888 @item types of media, different sizes and types, useful for different things
5889 @item files and tape marks
5890 one tape mark between files, two at end.
5891 @item positioning the tape
5892 MT writes two at end of write,
5893 backspaces over one when writing again.
5899 This chapter documents both the provided shell scripts and @command{tar}
5900 options which are more specific to usage as a backup tool.
5902 To @dfn{back up} a file system means to create archives that contain
5903 all the files in that file system. Those archives can then be used to
5904 restore any or all of those files (for instance if a disk crashes or a
5905 file is accidentally deleted). File system @dfn{backups} are also
5909 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5910 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5911 * Backup Levels:: Levels of Backups
5912 * Backup Parameters:: Setting Parameters for Backups and Restoration
5913 * Scripted Backups:: Using the Backup Scripts
5914 * Scripted Restoration:: Using the Restore Script
5918 @section Using @command{tar} to Perform Full Dumps
5924 @cindex corrupted archives
5925 Full dumps should only be made when no other people or programs
5926 are modifying files in the file system. If files are modified while
5927 @command{tar} is making the backup, they may not be stored properly in
5928 the archive, in which case you won't be able to restore them if you
5929 have to. (Files not being modified are written with no trouble, and do
5930 not corrupt the entire archive.)
5932 You will want to use the @option{--label=@var{archive-label}}
5933 (@option{-V @var{archive-label}}) option to give the archive a
5934 volume label, so you can tell what this archive is even if the label
5935 falls off the tape, or anything like that.
5937 Unless the file system you are dumping is guaranteed to fit on
5938 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5939 Make sure you have enough tapes on hand to complete the backup.
5941 If you want to dump each file system separately you will need to use
5942 the @option{--one-file-system} option to prevent
5943 @command{tar} from crossing file system boundaries when storing
5946 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5947 option is not needed, since this is a complete copy of everything in
5948 the file system, and a full restore from this backup would only be
5949 done onto a completely
5952 Unless you are in a hurry, and trust the @command{tar} program (and your
5953 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5954 option, to make sure your files really made it onto the dump properly.
5955 This will also detect cases where the file was modified while (or just
5956 after) it was being archived. Not all media (notably cartridge tapes)
5957 are capable of being verified, unfortunately.
5959 @node Incremental Dumps
5960 @section Using @command{tar} to Perform Incremental Dumps
5962 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5963 stores additional metadata so that exact state of the file system
5964 can be restored when extracting the archive.
5966 @GNUTAR{} currently offers two options for handling incremental
5967 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5968 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5970 @xopindex{listed-incremental, described}
5971 The option @option{--listed-incremental} instructs tar to operate on
5972 an incremental archive with additional metadata stored in a standalone
5973 file, called a @dfn{snapshot file}. The purpose of this file is to help
5974 determine which files have been changed, added or deleted since the
5975 last backup, so that the next incremental backup will contain only
5976 modified files. The name of the snapshot file is given as an argument
5980 @item --listed-incremental=@var{file}
5981 @itemx -g @var{file}
5982 Handle incremental backups with snapshot data in @var{file}.
5985 To create an incremental backup, you would use
5986 @option{--listed-incremental} together with @option{--create}
5987 (@pxref{create}). For example:
5990 $ @kbd{tar --create \
5991 --file=archive.1.tar \
5992 --listed-incremental=/var/log/usr.snar \
5996 This will create in @file{archive.1.tar} an incremental backup of
5997 the @file{/usr} file system, storing additional metadata in the file
5998 @file{/var/log/usr.snar}. If this file does not exist, it will be
5999 created. The created archive will then be a @dfn{level 0 backup};
6000 please see the next section for more on backup levels.
6002 Otherwise, if the file @file{/var/log/usr.snar} exists, it
6003 determines which files are modified. In this case only these files will be
6004 stored in the archive. Suppose, for example, that after running the
6005 above command, you delete file @file{/usr/doc/old} and create
6006 directory @file{/usr/local/db} with the following contents:
6009 $ @kbd{ls /usr/local/db}
6014 Some time later you create another incremental backup. You will
6018 $ @kbd{tar --create \
6019 --file=archive.2.tar \
6020 --listed-incremental=/var/log/usr.snar \
6022 tar: usr/local/db: Directory is new
6029 The created archive @file{archive.2.tar} will contain only these
6030 three members. This archive is called a @dfn{level 1 backup}. Notice
6031 that @file{/var/log/usr.snar} will be updated with the new data, so if
6032 you plan to create more @samp{level 1} backups, it is necessary to
6033 create a working copy of the snapshot file before running
6034 @command{tar}. The above example will then be modified as follows:
6037 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
6038 $ @kbd{tar --create \
6039 --file=archive.2.tar \
6040 --listed-incremental=/var/log/usr.snar-1 \
6045 @xopindex{level, described}
6046 You can force @samp{level 0} backups either by removing the snapshot
6047 file before running @command{tar}, or by supplying the
6048 @option{--level=0} option, e.g.:
6051 $ @kbd{tar --create \
6052 --file=archive.2.tar \
6053 --listed-incremental=/var/log/usr.snar-0 \
6058 Incremental dumps depend crucially on time stamps, so the results are
6059 unreliable if you modify a file's time stamps during dumping (e.g.,
6060 with the @option{--atime-preserve=replace} option), or if you set the clock
6063 @anchor{device numbers}
6064 @cindex Device numbers, using in incremental backups
6065 Metadata stored in snapshot files include device numbers, which,
6066 obviously are supposed to be non-volatile values. However, it turns
6067 out that @acronym{NFS} devices have undependable values when an automounter
6068 gets in the picture. This can lead to a great deal of spurious
6069 redumping in incremental dumps, so it is somewhat useless to compare
6070 two @acronym{NFS} devices numbers over time. The solution implemented
6071 currently is to consider all @acronym{NFS} devices as being equal
6072 when it comes to comparing directories; this is fairly gross, but
6073 there does not seem to be a better way to go.
6075 Apart from using @acronym{NFS}, there are a number of cases where
6076 relying on device numbers can cause spurious redumping of unmodified
6077 files. For example, this occurs when archiving @acronym{LVM} snapshot
6078 volumes. To avoid this, use @option{--no-check-device} option:
6081 @xopindex{no-check-device, described}
6082 @item --no-check-device
6083 Do not rely on device numbers when preparing a list of changed files
6084 for an incremental dump.
6086 @xopindex{check-device, described}
6087 @item --check-device
6088 Use device numbers when preparing a list of changed files
6089 for an incremental dump. This is the default behavior. The purpose
6090 of this option is to undo the effect of the @option{--no-check-device}
6091 if it was given in @env{TAR_OPTIONS} environment variable
6092 (@pxref{TAR_OPTIONS}).
6095 There is also another way to cope with changing device numbers. It is
6096 described in detail in @ref{Fixing Snapshot Files}.
6098 Note that incremental archives use @command{tar} extensions and may
6099 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
6101 @xopindex{listed-incremental, using with @option{--extract}}
6102 @xopindex{extract, using with @option{--listed-incremental}}
6103 To extract from the incremental dumps, use
6104 @option{--listed-incremental} together with @option{--extract}
6105 option (@pxref{extracting files}). In this case, @command{tar} does
6106 not need to access snapshot file, since all the data necessary for
6107 extraction are stored in the archive itself. So, when extracting, you
6108 can give whatever argument to @option{--listed-incremental}, the usual
6109 practice is to use @option{--listed-incremental=/dev/null}.
6110 Alternatively, you can use @option{--incremental}, which needs no
6111 arguments. In general, @option{--incremental} (@option{-G}) can be
6112 used as a shortcut for @option{--listed-incremental} when listing or
6113 extracting incremental backups (for more information regarding this
6114 option, @pxref{incremental-op}).
6116 When extracting from the incremental backup @GNUTAR{} attempts to
6117 restore the exact state the file system had when the archive was
6118 created. In particular, it will @emph{delete} those files in the file
6119 system that did not exist in their directories when the archive was
6120 created. If you have created several levels of incremental files,
6121 then in order to restore the exact contents the file system had when
6122 the last level was created, you will need to restore from all backups
6123 in turn. Continuing our example, to restore the state of @file{/usr}
6124 file system, one would do@footnote{Notice, that since both archives
6125 were created without @option{-P} option (@pxref{absolute}), these
6126 commands should be run from the root file system.}:
6129 $ @kbd{tar --extract \
6130 --listed-incremental=/dev/null \
6131 --file archive.1.tar}
6132 $ @kbd{tar --extract \
6133 --listed-incremental=/dev/null \
6134 --file archive.2.tar}
6137 To list the contents of an incremental archive, use @option{--list}
6138 (@pxref{list}), as usual. To obtain more information about the
6139 archive, use @option{--listed-incremental} or @option{--incremental}
6140 combined with two @option{--verbose} options@footnote{Two
6141 @option{--verbose} options were selected to avoid breaking usual
6142 verbose listing output (@option{--list --verbose}) when using in
6145 @xopindex{incremental, using with @option{--list}}
6146 @xopindex{listed-incremental, using with @option{--list}}
6147 @xopindex{list, using with @option{--incremental}}
6148 @xopindex{list, using with @option{--listed-incremental}}
6149 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
6150 contents of the DUMPDIR header (with terminating nulls) when
6151 @option{--incremental} or @option{--listed-incremental} option was
6152 given, no matter what the verbosity level. This behavior, and,
6153 especially, the binary output it produced were considered inconvenient
6154 and were changed in version 1.16.}:
6157 @kbd{tar --list --incremental --verbose --verbose archive.tar}
6160 This command will print, for each directory in the archive, the list
6161 of files in that directory at the time the archive was created. This
6162 information is put out in a format which is both human-readable and
6163 unambiguous for a program: each file name is printed as
6170 where @var{x} is a letter describing the status of the file: @samp{Y}
6171 if the file is present in the archive, @samp{N} if the file is not
6172 included in the archive, or a @samp{D} if the file is a directory (and
6173 is included in the archive). @xref{Dumpdir}, for the detailed
6174 description of dumpdirs and status codes. Each such
6175 line is terminated by a newline character. The last line is followed
6176 by an additional newline to indicate the end of the data.
6178 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
6179 gives the same behavior as @option{--listed-incremental} when used
6180 with @option{--list} and @option{--extract} options. When used with
6181 @option{--create} option, it creates an incremental archive without
6182 creating snapshot file. Thus, it is impossible to create several
6183 levels of incremental backups with @option{--incremental} option.
6186 @section Levels of Backups
6188 An archive containing all the files in the file system is called a
6189 @dfn{full backup} or @dfn{full dump}. You could insure your data by
6190 creating a full dump every day. This strategy, however, would waste a
6191 substantial amount of archive media and user time, as unchanged files
6192 are daily re-archived.
6194 It is more efficient to do a full dump only occasionally. To back up
6195 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
6196 one} dump archives all the files that have changed since the last full
6199 A typical dump strategy would be to perform a full dump once a week,
6200 and a level one dump once a day. This means some versions of files
6201 will in fact be archived more than once, but this dump strategy makes
6202 it possible to restore a file system to within one day of accuracy by
6203 only extracting two archives---the last weekly (full) dump and the
6204 last daily (level one) dump. The only information lost would be in
6205 files changed or created since the last daily backup. (Doing dumps
6206 more than once a day is usually not worth the trouble.)
6208 @GNUTAR{} comes with scripts you can use to do full
6209 and level-one (actually, even level-two and so on) dumps. Using
6210 scripts (shell programs) to perform backups and restoration is a
6211 convenient and reliable alternative to typing out file name lists
6212 and @command{tar} commands by hand.
6214 Before you use these scripts, you need to edit the file
6215 @file{backup-specs}, which specifies parameters used by the backup
6216 scripts and by the restore script. This file is usually located
6217 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
6218 detailed description. Once the backup parameters are set, you can
6219 perform backups or restoration by running the appropriate script.
6221 The name of the backup script is @code{backup}. The name of the
6222 restore script is @code{restore}. The following sections describe
6223 their use in detail.
6225 @emph{Please Note:} The backup and restoration scripts are
6226 designed to be used together. While it is possible to restore files by
6227 hand from an archive which was created using a backup script, and to create
6228 an archive by hand which could then be extracted using the restore script,
6229 it is easier to use the scripts. @xref{Incremental Dumps}, before
6230 making such an attempt.
6232 @node Backup Parameters
6233 @section Setting Parameters for Backups and Restoration
6235 The file @file{backup-specs} specifies backup parameters for the
6236 backup and restoration scripts provided with @command{tar}. You must
6237 edit @file{backup-specs} to fit your system configuration and schedule
6238 before using these scripts.
6240 Syntactically, @file{backup-specs} is a shell script, containing
6241 mainly variable assignments. However, any valid shell construct
6242 is allowed in this file. Particularly, you may wish to define
6243 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
6244 For more information about shell script syntax, please refer to
6245 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
6246 g_02, the definition of the Shell Command Language}. See also
6247 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
6249 The shell variables controlling behavior of @code{backup} and
6250 @code{restore} are described in the following subsections.
6253 * General-Purpose Variables::
6254 * Magnetic Tape Control::
6256 * backup-specs example:: An Example Text of @file{Backup-specs}
6259 @node General-Purpose Variables
6260 @subsection General-Purpose Variables
6262 @defvr {Backup variable} ADMINISTRATOR
6263 The user name of the backup administrator. @code{Backup} scripts
6264 sends a backup report to this address.
6267 @defvr {Backup variable} BACKUP_HOUR
6268 The hour at which the backups are done. This can be a number from 0
6269 to 23, or the time specification in form @var{hours}:@var{minutes},
6270 or the string @samp{now}.
6272 This variable is used by @code{backup}. Its value may be overridden
6273 using @option{--time} option (@pxref{Scripted Backups}).
6276 @defvr {Backup variable} TAPE_FILE
6278 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
6279 is a remote archive (@pxref{remote-dev}), backup script will suppose
6280 that your @command{mt} is able to access remote devices. If @var{RSH}
6281 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
6282 invocations of @command{mt}.
6285 @defvr {Backup variable} BLOCKING
6287 The blocking factor @command{tar} will use when writing the dump archive.
6288 @xref{Blocking Factor}.
6291 @defvr {Backup variable} BACKUP_DIRS
6293 A list of file systems to be dumped (for @code{backup}), or restored
6294 (for @code{restore}). You can include any directory
6295 name in the list --- subdirectories on that file system will be
6296 included, regardless of how they may look to other networked machines.
6297 Subdirectories on other file systems will be ignored.
6299 The host name specifies which host to run @command{tar} on, and should
6300 normally be the host that actually contains the file system. However,
6301 the host machine must have @GNUTAR{} installed, and
6302 must be able to access the directory containing the backup scripts and
6303 their support files using the same file name that is used on the
6304 machine where the scripts are run (i.e., what @command{pwd} will print
6305 when in that directory on that machine). If the host that contains
6306 the file system does not have this capability, you can specify another
6307 host as long as it can access the file system through @acronym{NFS}.
6309 If the list of file systems is very long you may wish to put it
6310 in a separate file. This file is usually named
6311 @file{/etc/backup/dirs}, but this name may be overridden in
6312 @file{backup-specs} using @code{DIRLIST} variable.
6315 @defvr {Backup variable} DIRLIST
6317 The name of the file that contains a list of file systems to backup
6318 or restore. By default it is @file{/etc/backup/dirs}.
6321 @defvr {Backup variable} BACKUP_FILES
6323 A list of individual files to be dumped (for @code{backup}), or restored
6324 (for @code{restore}). These should be accessible from the machine on
6325 which the backup script is run.
6327 If the list of individual files is very long you may wish to store it
6328 in a separate file. This file is usually named
6329 @file{/etc/backup/files}, but this name may be overridden in
6330 @file{backup-specs} using @code{FILELIST} variable.
6333 @defvr {Backup variable} FILELIST
6335 The name of the file that contains a list of individual files to backup
6336 or restore. By default it is @file{/etc/backup/files}.
6339 @defvr {Backup variable} MT
6341 Full file name of @command{mt} binary.
6344 @defvr {Backup variable} RSH
6346 Full file name of @command{rsh} binary or its equivalent. You may wish to
6347 set it to @code{ssh}, to improve security. In this case you will have
6348 to use public key authentication.
6351 @defvr {Backup variable} RSH_COMMAND
6353 Full file name of @command{rsh} binary on remote machines. This will
6354 be passed via @option{--rsh-command} option to the remote invocation
6358 @defvr {Backup variable} VOLNO_FILE
6360 Name of temporary file to hold volume numbers. This needs to be accessible
6361 by all the machines which have file systems to be dumped.
6364 @defvr {Backup variable} XLIST
6366 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
6367 located on the remote machine and containing the list of files to
6368 be excluded from the backup. Exclude file lists are searched in
6369 /etc/tar-backup directory. A common use for exclude file lists
6370 is to exclude files containing security-sensitive information
6371 (e.g., @file{/etc/shadow} from backups).
6373 This variable affects only @code{backup}.
6376 @defvr {Backup variable} SLEEP_TIME
6378 Time to sleep between dumps of any two successive file systems
6380 This variable affects only @code{backup}.
6383 @defvr {Backup variable} DUMP_REMIND_SCRIPT
6385 Script to be run when it's time to insert a new tape in for the next
6386 volume. Administrators may want to tailor this script for their site.
6387 If this variable isn't set, @GNUTAR{} will display its built-in
6388 prompt, and will expect confirmation from the console. For the
6389 description of the default prompt, see @ref{change volume prompt}.
6393 @defvr {Backup variable} SLEEP_MESSAGE
6395 Message to display on the terminal while waiting for dump time. Usually
6396 this will just be some literal text.
6399 @defvr {Backup variable} TAR
6401 Full file name of the @GNUTAR{} executable. If this is not set, backup
6402 scripts will search @command{tar} in the current shell path.
6405 @node Magnetic Tape Control
6406 @subsection Magnetic Tape Control
6408 Backup scripts access tape device using special @dfn{hook functions}.
6409 These functions take a single argument --- the name of the tape
6410 device. Their names are kept in the following variables:
6412 @defvr {Backup variable} MT_BEGIN
6413 The name of @dfn{begin} function. This function is called before
6414 accessing the drive. By default it retensions the tape:
6420 mt -f "$1" retension
6425 @defvr {Backup variable} MT_REWIND
6426 The name of @dfn{rewind} function. The default definition is as
6439 @defvr {Backup variable} MT_OFFLINE
6440 The name of the function switching the tape off line. By default
6441 it is defined as follows:
6444 MT_OFFLINE=mt_offline
6452 @defvr {Backup variable} MT_STATUS
6453 The name of the function used to obtain the status of the archive device,
6454 including error count. Default definition:
6466 @subsection User Hooks
6468 @dfn{User hooks} are shell functions executed before and after
6469 each @command{tar} invocation. Thus, there are @dfn{backup
6470 hooks}, which are executed before and after dumping each file
6471 system, and @dfn{restore hooks}, executed before and
6472 after restoring a file system. Each user hook is a shell function
6473 taking four arguments:
6475 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
6480 Current backup or restore level.
6483 Name or IP address of the host machine being dumped or restored.
6486 Full file name of the file system being dumped or restored.
6489 File system name with directory separators replaced with colons. This
6490 is useful, e.g., for creating unique files.
6494 Following variables keep the names of user hook functions:
6496 @defvr {Backup variable} DUMP_BEGIN
6497 Dump begin function. It is executed before dumping the file system.
6500 @defvr {Backup variable} DUMP_END
6501 Executed after dumping the file system.
6504 @defvr {Backup variable} RESTORE_BEGIN
6505 Executed before restoring the file system.
6508 @defvr {Backup variable} RESTORE_END
6509 Executed after restoring the file system.
6512 @node backup-specs example
6513 @subsection An Example Text of @file{Backup-specs}
6515 The following is an example of @file{backup-specs}:
6518 # site-specific parameters for file system backup.
6520 ADMINISTRATOR=friedman
6522 TAPE_FILE=/dev/nrsmt0
6524 # Use @code{ssh} instead of the less secure @code{rsh}
6526 RSH_COMMAND=/usr/bin/ssh
6528 # Override MT_STATUS function:
6534 # Disable MT_OFFLINE function
6551 apple-gunkies:/com/mailer/gnu
6552 apple-gunkies:/com/archive/gnu"
6554 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
6558 @node Scripted Backups
6559 @section Using the Backup Scripts
6561 The syntax for running a backup script is:
6564 backup --level=@var{level} --time=@var{time}
6567 The @option{--level} option requests the dump level. Thus, to produce
6568 a full dump, specify @code{--level=0} (this is the default, so
6569 @option{--level} may be omitted if its value is
6570 @code{0})@footnote{For backward compatibility, the @code{backup} will also
6571 try to deduce the requested dump level from the name of the
6572 script itself. If the name consists of a string @samp{level-}
6573 followed by a single decimal digit, that digit is taken as
6574 the dump level number. Thus, you may create a link from @code{backup}
6575 to @code{level-1} and then run @code{level-1} whenever you need to
6576 create a level one dump.}.
6578 The @option{--time} option determines when should the backup be
6579 run. @var{Time} may take three forms:
6582 @item @var{hh}:@var{mm}
6584 The dump must be run at @var{hh} hours @var{mm} minutes.
6588 The dump must be run at @var{hh} hours.
6592 The dump must be run immediately.
6595 You should start a script with a tape or disk mounted. Once you
6596 start a script, it prompts you for new tapes or disks as it
6597 needs them. Media volumes don't have to correspond to archive
6598 files --- a multi-volume archive can be started in the middle of a
6599 tape that already contains the end of another multi-volume archive.
6600 The @code{restore} script prompts for media by its archive volume,
6601 so to avoid an error message you should keep track of which tape
6602 (or disk) contains which volume of the archive (@pxref{Scripted
6605 The backup scripts write two files on the file system. The first is a
6606 record file in @file{/etc/tar-backup/}, which is used by the scripts
6607 to store and retrieve information about which files were dumped. This
6608 file is not meant to be read by humans, and should not be deleted by
6609 them. @xref{Snapshot Files}, for a more detailed explanation of this
6612 The second file is a log file containing the names of the file systems
6613 and files dumped, what time the backup was made, and any error
6614 messages that were generated, as well as how much space was left in
6615 the media volume after the last volume of the archive was written.
6616 You should check this log file after every backup. The file name is
6617 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
6618 represents current date, and @var{n} represents current dump level number.
6620 The script also prints the name of each system being dumped to the
6623 Following is the full list of options accepted by @code{backup}
6627 @item -l @var{level}
6628 @itemx --level=@var{level}
6629 Do backup level @var{level} (default 0).
6633 Force backup even if today's log file already exists.
6635 @item -v[@var{level}]
6636 @itemx --verbose[=@var{level}]
6637 Set verbosity level. The higher the level is, the more debugging
6638 information will be output during execution. Default @var{level}
6639 is 100, which means the highest debugging level.
6641 @item -t @var{start-time}
6642 @itemx --time=@var{start-time}
6643 Wait till @var{time}, then do backup.
6647 Display short help message and exit.
6651 Display information about the program's name, version, origin and legal
6652 status, all on standard output, and then exit successfully.
6656 @node Scripted Restoration
6657 @section Using the Restore Script
6659 To restore files that were archived using a scripted backup, use the
6660 @code{restore} script. Its usage is quite straightforward. In the
6661 simplest form, invoke @code{restore --all}, it will
6662 then restore all the file systems and files specified in
6663 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
6665 You may select the file systems (and/or files) to restore by
6666 giving @code{restore} a list of @dfn{patterns} in its command
6667 line. For example, running
6674 will restore all file systems on the machine @samp{albert}. A more
6675 complicated example:
6678 restore 'albert:*' '*:/var'
6682 This command will restore all file systems on the machine @samp{albert}
6683 as well as @file{/var} file system on all machines.
6685 By default @code{restore} will start restoring files from the lowest
6686 available dump level (usually zero) and will continue through
6687 all available dump levels. There may be situations where such a
6688 thorough restore is not necessary. For example, you may wish to
6689 restore only files from the recent level one backup. To do so,
6690 use @option{--level} option, as shown in the example below:
6696 The full list of options accepted by @code{restore} follows:
6701 Restore all file systems and files specified in @file{backup-specs}.
6703 @item -l @var{level}
6704 @itemx --level=@var{level}
6705 Start restoring from the given backup level, instead of the default 0.
6707 @item -v[@var{level}]
6708 @itemx --verbose[=@var{level}]
6709 Set verbosity level. The higher the level is, the more debugging
6710 information will be output during execution. Default @var{level}
6711 is 100, which means the highest debugging level.
6715 Display short help message and exit.
6719 Display information about the program's name, version, origin and legal
6720 status, all on standard output, and then exit successfully.
6723 You should start the restore script with the media containing the
6724 first volume of the archive mounted. The script will prompt for other
6725 volumes as they are needed. If the archive is on tape, you don't need
6726 to rewind the tape to to its beginning---if the tape head is
6727 positioned past the beginning of the archive, the script will rewind
6728 the tape as needed. @xref{Tape Positioning}, for a discussion of tape
6732 @strong{Warning:} The script will delete files from the active file
6733 system if they were not in the file system when the archive was made.
6736 @xref{Incremental Dumps}, for an explanation of how the script makes
6740 @chapter Choosing Files and Names for @command{tar}
6742 Certain options to @command{tar} enable you to specify a name for your
6743 archive. Other options let you decide which files to include or exclude
6744 from the archive, based on when or whether files were modified, whether
6745 the file names do or don't match specified patterns, or whether files
6746 are in specified directories.
6748 This chapter discusses these options in detail.
6751 * file:: Choosing the Archive's Name
6752 * Selecting Archive Members::
6753 * files:: Reading Names from a File
6754 * exclude:: Excluding Some Files
6755 * wildcards:: Wildcards Patterns and Matching
6756 * quoting styles:: Ways of Quoting Special Characters in Names
6757 * transform:: Modifying File and Member Names
6758 * after:: Operating Only on New Files
6759 * recurse:: Descending into Directories
6760 * one:: Crossing File System Boundaries
6764 @section Choosing and Naming Archive Files
6766 @cindex Naming an archive
6767 @cindex Archive Name
6768 @cindex Choosing an archive file
6769 @cindex Where is the archive?
6771 By default, @command{tar} uses an archive file name that was compiled when
6772 it was built on the system; usually this name refers to some physical
6773 tape drive on the machine. However, the person who installed @command{tar}
6774 on the system may not have set the default to a meaningful value as far as
6775 most users are concerned. As a result, you will usually want to tell
6776 @command{tar} where to find (or create) the archive. The
6777 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
6778 option allows you to either specify or name a file to use as the archive
6779 instead of the default archive file location.
6782 @xopindex{file, short description}
6783 @item --file=@var{archive-name}
6784 @itemx -f @var{archive-name}
6785 Name the archive to create or operate on. Use in conjunction with
6789 For example, in this @command{tar} command,
6792 $ @kbd{tar -cvf collection.tar blues folk jazz}
6796 @file{collection.tar} is the name of the archive. It must directly
6797 follow the @option{-f} option, since whatever directly follows @option{-f}
6798 @emph{will} end up naming the archive. If you neglect to specify an
6799 archive name, you may end up overwriting a file in the working directory
6800 with the archive you create since @command{tar} will use this file's name
6801 for the archive name.
6803 An archive can be saved as a file in the file system, sent through a
6804 pipe or over a network, or written to an I/O device such as a tape,
6805 floppy disk, or CD write drive.
6807 @cindex Writing new archives
6808 @cindex Archive creation
6809 If you do not name the archive, @command{tar} uses the value of the
6810 environment variable @env{TAPE} as the file name for the archive. If
6811 that is not available, @command{tar} uses a default, compiled-in archive
6812 name, usually that for tape unit zero (i.e., @file{/dev/tu00}).
6814 @cindex Standard input and output
6815 @cindex tar to standard input and output
6816 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
6817 archive from standard input (when listing or extracting files), or
6818 writes it to standard output (when creating an archive). If you use
6819 @file{-} as an @var{archive-name} when modifying an archive,
6820 @command{tar} reads the original archive from its standard input and
6821 writes the entire new archive to its standard output.
6823 The following example is a convenient way of copying directory
6824 hierarchy from @file{sourcedir} to @file{targetdir}.
6827 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
6830 The @option{-C} option allows to avoid using subshells:
6833 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
6836 In both examples above, the leftmost @command{tar} invocation archives
6837 the contents of @file{sourcedir} to the standard output, while the
6838 rightmost one reads this archive from its standard input and
6839 extracts it. The @option{-p} option tells it to restore permissions
6840 of the extracted files.
6842 @cindex Remote devices
6843 @cindex tar to a remote device
6845 To specify an archive file on a device attached to a remote machine,
6849 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
6853 @command{tar} will set up the remote connection, if possible, and
6854 prompt you for a username and password. If you use
6855 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
6856 will attempt to set up the remote connection using your username
6857 as the username on the remote machine.
6859 @cindex Local and remote archives
6860 @anchor{local and remote archives}
6861 If the archive file name includes a colon (@samp{:}), then it is assumed
6862 to be a file on another machine. If the archive file is
6863 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6864 host @var{host}. The remote host is accessed using the @command{rsh}
6865 program, with a username of @var{user}. If the username is omitted
6866 (along with the @samp{@@} sign), then your user name will be used.
6867 (This is the normal @command{rsh} behavior.) It is necessary for the
6868 remote machine, in addition to permitting your @command{rsh} access, to
6869 have the @file{rmt} program installed (this command is included in
6870 the @GNUTAR{} distribution and by default is installed under
6871 @file{@var{prefix}/libexec/rmt}, where @var{prefix} means your
6872 installation prefix). If you need to use a file whose name includes a
6873 colon, then the remote tape drive behavior
6874 can be inhibited by using the @option{--force-local} option.
6876 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6877 tries to minimize input and output operations. The Amanda backup
6878 system, when used with @GNUTAR{}, has an initial sizing pass which
6881 @node Selecting Archive Members
6882 @section Selecting Archive Members
6883 @cindex Specifying files to act on
6884 @cindex Specifying archive members
6886 @dfn{File Name arguments} specify which files in the file system
6887 @command{tar} operates on, when creating or adding to an archive, or which
6888 archive members @command{tar} operates on, when reading or deleting from
6889 an archive. @xref{Operations}.
6891 To specify file names, you can include them as the last arguments on
6892 the command line, as follows:
6894 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6897 If a file name begins with dash (@samp{-}), precede it with
6898 @option{--add-file} option to prevent it from being treated as an
6901 @anchor{input name quoting}
6902 By default @GNUTAR{} attempts to @dfn{unquote} each file or member
6903 name, replacing @dfn{escape sequences} according to the following
6906 @multitable @columnfractions 0.20 0.60
6907 @headitem Escape @tab Replaced with
6908 @item \a @tab Audible bell (@acronym{ASCII} 7)
6909 @item \b @tab Backspace (@acronym{ASCII} 8)
6910 @item \f @tab Form feed (@acronym{ASCII} 12)
6911 @item \n @tab New line (@acronym{ASCII} 10)
6912 @item \r @tab Carriage return (@acronym{ASCII} 13)
6913 @item \t @tab Horizontal tabulation (@acronym{ASCII} 9)
6914 @item \v @tab Vertical tabulation (@acronym{ASCII} 11)
6915 @item \? @tab @acronym{ASCII} 127
6916 @item \@var{n} @tab @acronym{ASCII} @var{n} (@var{n} should be an octal number
6920 A backslash followed by any other symbol is retained.
6922 This default behavior is controlled by the following command line
6928 Enable unquoting input file or member names (default).
6932 Disable unquoting input file or member names.
6935 If you specify a directory name as a file name argument, all the files
6936 in that directory are operated on by @command{tar}.
6938 If you do not specify files, @command{tar} behavior differs depending
6939 on the operation mode as described below:
6941 When @command{tar} is invoked with @option{--create} (@option{-c}),
6942 @command{tar} will stop immediately, reporting the following:
6946 $ @kbd{tar cf a.tar}
6947 tar: Cowardly refusing to create an empty archive
6948 Try 'tar --help' or 'tar --usage' for more information.
6952 If you specify either @option{--list} (@option{-t}) or
6953 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6954 operates on all the archive members in the archive.
6956 If run with @option{--diff} option, tar will compare the archive with
6957 the contents of the current working directory.
6959 If you specify any other operation, @command{tar} does nothing.
6961 By default, @command{tar} takes file names from the command line. However,
6962 there are other ways to specify file or member names, or to modify the
6963 manner in which @command{tar} selects the files or members upon which to
6964 operate. In general, these methods work both for specifying the names
6965 of files and archive members.
6968 @section Reading Names from a File
6970 @cindex Reading file names from a file
6971 @cindex Lists of file names
6972 @cindex File Name arguments, alternatives
6973 @cindex @command{find}, using with @command{tar}
6974 Instead of giving the names of files or archive members on the command
6975 line, you can put the names into a file, and then use the
6976 @option{--files-from=@var{file-of-names}} (@option{-T
6977 @var{file-of-names}}) option to @command{tar}. Give the name of the
6978 file which contains the list of files to include as the argument to
6979 @option{--files-from}. In the list, the file names should be separated by
6980 newlines. You will frequently use this option when you have generated
6981 the list of files to archive with the @command{find} utility.
6985 @item --files-from=@var{file-name}
6986 @itemx -T @var{file-name}
6987 Get names to extract or create from file @var{file-name}.
6990 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6991 you specify either @code{--files-from=-} or @code{-T -}), then the file
6992 names are read from standard input.
6994 Unless you are running @command{tar} with @option{--create}, you can not use
6995 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6998 Any number of @option{-T} options can be given in the command line.
7000 The following example shows how to use @command{find} to generate a list of
7001 files smaller than 400K in length and put that list into a file
7002 called @file{small-files}. You can then use the @option{-T} option to
7003 @command{tar} to specify the files from that file, @file{small-files}, to
7004 create the archive @file{little.tgz}. (The @option{-z} option to
7005 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
7009 $ @kbd{find . -size -400 -print > small-files}
7010 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
7014 In the file list given by @option{-T} option, any file name beginning
7015 with @samp{-} character is considered a @command{tar} option and is
7016 processed accordingly@footnote{Versions of @GNUTAR{} up to 1.15.1
7017 recognized only @option{-C} option in file lists, and only if the
7018 option and its argument occupied two consecutive lines.}. For example,
7019 the common use of this feature is to change to another directory by
7020 specifying @option{-C} option:
7030 $ @kbd{tar -c -f foo.tar --files-from list}
7035 In this example, @command{tar} will first switch to @file{/etc}
7036 directory and add files @file{passwd} and @file{hosts} to the
7037 archive. Then it will change to @file{/lib} directory and will archive
7038 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
7043 $ @kbd{tar tf foo.tar}
7051 @xopindex{directory, using in @option{--files-from} argument}
7052 Notice that the option parsing algorithm used with @option{-T} is
7053 stricter than the one used by shell. Namely, when specifying option
7054 arguments, you should observe the following rules:
7058 When using short (single-letter) option form, its argument must
7059 immediately follow the option letter, without any intervening
7060 whitespace. For example: @code{-Cdir}.
7063 When using long option form, the option argument must be separated
7064 from the option by a single equal sign. No whitespace is allowed on
7065 any side of the equal sign. For example: @code{--directory=dir}.
7068 For both short and long option forms, the option argument can be given
7069 on the next line after the option name, e.g.:
7090 If you happen to have a file whose name starts with @samp{-},
7091 precede it with @option{--add-file} option to prevent it from
7092 being recognized as an option. For example: @code{--add-file=--my-file}.
7099 @subsection @code{NUL}-Terminated File Names
7101 @cindex File names, terminated by @code{NUL}
7102 @cindex @code{NUL}-terminated file names
7103 The @option{--null} option causes
7104 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
7105 to read file names terminated by a @code{NUL} instead of a newline, so
7106 files whose names contain newlines can be archived using
7107 @option{--files-from}.
7110 @xopindex{null, described}
7112 Only consider @code{NUL}-terminated file names, instead of files that
7113 terminate in a newline.
7115 @xopindex{no-null, described}
7117 Undo the effect of any previous @option{--null} option.
7120 The @option{--null} option is just like the one in @acronym{GNU}
7121 @command{xargs} and @command{cpio}, and is useful with the
7122 @option{-print0} predicate of @acronym{GNU} @command{find}. In
7123 @command{tar}, @option{--null} also disables special handling for
7124 file names that begin with dash.
7126 This example shows how to use @command{find} to generate a list of files
7127 larger than 800K in length and put that list into a file called
7128 @file{long-files}. The @option{-print0} option to @command{find} is just
7129 like @option{-print}, except that it separates files with a @code{NUL}
7130 rather than with a newline. You can then run @command{tar} with both the
7131 @option{--null} and @option{-T} options to specify that @command{tar} gets the
7132 files from that file, @file{long-files}, to create the archive
7133 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
7134 @command{tar} to recognize the @code{NUL} separator between files.
7137 $ @kbd{find . -size +800 -print0 > long-files}
7138 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
7141 The @option{--no-null} option can be used if you need to read both
7142 @code{NUL}-terminated and newline-terminated files on the same command line.
7143 For example, if @file{flist} is a newline-terminated file, then the
7144 following command can be used to combine it with the above command:
7148 $ @kbd{find . -size +800 -print0 |
7149 tar -c -f big.tar --null -T - --no-null -T flist}
7153 This example uses short options for typographic reasons, to avoid
7156 @GNUTAR is able to automatically detect @code{NUL}-terminated file lists, so
7157 it is safe to use them even without the @option{--null} option. In
7158 this case @command{tar} will print a warning and continue reading such
7159 a file as if @option{--null} were actually given:
7163 $ @kbd{find . -size +800 -print0 | tar -c -f big.tar -T -}
7164 tar: -: file name read contains nul character
7168 The null terminator, however, remains in effect only for this
7169 particular file, any following @option{-T} options will assume
7170 newline termination. Of course, the null autodetection applies
7171 to these eventual surplus @option{-T} options as well.
7174 @section Excluding Some Files
7176 @cindex File names, excluding files by
7177 @cindex Excluding files by name and pattern
7178 @cindex Excluding files by file system
7180 @opindex exclude-from
7181 To avoid operating on files whose names match a particular pattern,
7182 use the @option{--exclude} or @option{--exclude-from} options.
7186 @item --exclude=@var{pattern}
7187 Causes @command{tar} to ignore files that match the @var{pattern}.
7191 The @option{--exclude=@var{pattern}} option prevents any file or
7192 member whose name matches the shell wildcard (@var{pattern}) from
7194 For example, to create an archive with all the contents of the directory
7195 @file{src} except for files whose names end in @file{.o}, use the
7196 command @samp{tar -cf src.tar --exclude='*.o' src}.
7198 You may give multiple @option{--exclude} options.
7201 @opindex exclude-from
7202 @item --exclude-from=@var{file}
7203 @itemx -X @var{file}
7204 Causes @command{tar} to ignore files that match the patterns listed in
7208 @findex exclude-from
7209 Use the @option{--exclude-from} option to read a
7210 list of patterns, one per line, from @var{file}; @command{tar} will
7211 ignore files matching those patterns. Thus if @command{tar} is
7212 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
7213 single line @file{*.o}, no files whose names end in @file{.o} will be
7214 added to the archive.
7216 Notice, that lines from @var{file} are read verbatim. One of the
7217 frequent errors is leaving some extra whitespace after a file name,
7218 which is difficult to catch using text editors.
7220 However, empty lines are OK.
7223 @cindex version control system, excluding files
7224 @cindex VCS, excluding files
7225 @cindex SCCS, excluding files
7226 @cindex RCS, excluding files
7227 @cindex CVS, excluding files
7228 @cindex SVN, excluding files
7229 @cindex git, excluding files
7230 @cindex Bazaar, excluding files
7231 @cindex Arch, excluding files
7232 @cindex Mercurial, excluding files
7233 @cindex Darcs, excluding files
7234 @opindex exclude-vcs
7236 Exclude files and directories used by following version control
7237 systems: @samp{CVS}, @samp{RCS}, @samp{SCCS}, @samp{SVN}, @samp{Arch},
7238 @samp{Bazaar}, @samp{Mercurial}, and @samp{Darcs}.
7240 As of version @value{VERSION}, the following files are excluded:
7243 @item @file{CVS/}, and everything under it
7244 @item @file{RCS/}, and everything under it
7245 @item @file{SCCS/}, and everything under it
7246 @item @file{.git/}, and everything under it
7247 @item @file{.gitignore}
7248 @item @file{.cvsignore}
7249 @item @file{.svn/}, and everything under it
7250 @item @file{.arch-ids/}, and everything under it
7251 @item @file{@{arch@}/}, and everything under it
7252 @item @file{=RELEASE-ID}
7253 @item @file{=meta-update}
7254 @item @file{=update}
7256 @item @file{.bzrignore}
7257 @item @file{.bzrtags}
7259 @item @file{.hgignore}
7260 @item @file{.hgrags}
7264 @opindex exclude-backups
7265 @item --exclude-backups
7266 Exclude backup and lock files. This option causes exclusion of files
7267 that match the following shell globbing patterns:
7277 @findex exclude-caches
7278 When creating an archive, the @option{--exclude-caches} option family
7279 causes @command{tar} to exclude all directories that contain a @dfn{cache
7280 directory tag}. A cache directory tag is a short file with the
7281 well-known name @file{CACHEDIR.TAG} and having a standard header
7282 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
7283 Various applications write cache directory tags into directories they
7284 use to hold regenerable, non-precious data, so that such data can be
7285 more easily excluded from backups.
7287 There are three @samp{exclude-caches} options, each providing a different
7288 exclusion semantics:
7291 @opindex exclude-caches
7292 @item --exclude-caches
7293 Do not archive the contents of the directory, but archive the
7294 directory itself and the @file{CACHEDIR.TAG} file.
7296 @opindex exclude-caches-under
7297 @item --exclude-caches-under
7298 Do not archive the contents of the directory, nor the
7299 @file{CACHEDIR.TAG} file, archive only the directory itself.
7301 @opindex exclude-caches-all
7302 @item --exclude-caches-all
7303 Omit directories containing @file{CACHEDIR.TAG} file entirely.
7307 Another option family, @option{--exclude-tag}, provides a generalization of
7308 this concept. It takes a single argument, a file name to look for.
7309 Any directory that contains this file will be excluded from the dump.
7310 Similarly to @samp{exclude-caches}, there are three options in this
7314 @opindex exclude-tag
7315 @item --exclude-tag=@var{file}
7316 Do not dump the contents of the directory, but dump the
7317 directory itself and the @var{file}.
7319 @opindex exclude-tag-under
7320 @item --exclude-tag-under=@var{file}
7321 Do not dump the contents of the directory, nor the
7322 @var{file}, archive only the directory itself.
7324 @opindex exclude-tag-all
7325 @item --exclude-tag-all=@var{file}
7326 Omit directories containing @var{file} file entirely.
7329 Multiple @option{--exclude-tag*} options can be given.
7331 For example, given this directory:
7346 The @option{--exclude-tag} will produce the following:
7349 $ @kbd{tar -cf archive.tar --exclude-tag=tagfile -v dir}
7354 tar: dir/folk/: contains a cache directory tag tagfile;
7359 Both the @file{dir/folk} directory and its tagfile are preserved in
7360 the archive, however the rest of files in this directory are not.
7362 Now, using the @option{--exclude-tag-under} option will exclude
7363 @file{tagfile} from the dump, while still preserving the directory
7364 itself, as shown in this example:
7367 $ @kbd{tar -cf archive.tar --exclude-tag-under=tagfile -v dir}
7372 ./tar: dir/folk/: contains a cache directory tag tagfile;
7376 Finally, using @option{--exclude-tag-all} omits the @file{dir/folk}
7380 $ @kbd{tar -cf archive.tar --exclude-tag-all=tagfile -v dir}
7384 ./tar: dir/folk/: contains a cache directory tag tagfile;
7385 directory not dumped
7389 * problems with exclude::
7392 @node problems with exclude
7393 @unnumberedsubsec Problems with Using the @code{exclude} Options
7395 @xopindex{exclude, potential problems with}
7396 Some users find @samp{exclude} options confusing. Here are some common
7401 The main operating mode of @command{tar} does not act on a file name
7402 explicitly listed on the command line, if one of its file name
7403 components is excluded. In the example above, if
7404 you create an archive and exclude files that end with @samp{*.o}, but
7405 explicitly name the file @samp{dir.o/foo} after all the options have been
7406 listed, @samp{dir.o/foo} will be excluded from the archive.
7409 You can sometimes confuse the meanings of @option{--exclude} and
7410 @option{--exclude-from}. Be careful: use @option{--exclude} when files
7411 to be excluded are given as a pattern on the command line. Use
7412 @option{--exclude-from} to introduce the name of a file which contains
7413 a list of patterns, one per line; each of these patterns can exclude
7414 zero, one, or many files.
7417 When you use @option{--exclude=@var{pattern}}, be sure to quote the
7418 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
7419 like @samp{*}. If you do not do this, the shell might expand the
7420 @samp{*} itself using files at hand, so @command{tar} might receive a
7421 list of files instead of one pattern, or none at all, making the
7422 command somewhat illegal. This might not correspond to what you want.
7427 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
7435 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
7439 You must use use shell syntax, or globbing, rather than @code{regexp}
7440 syntax, when using exclude options in @command{tar}. If you try to use
7441 @code{regexp} syntax to describe files to be excluded, your command
7445 @FIXME{The change in semantics must have occurred before 1.11,
7446 so I doubt if it is worth mentioning at all. Anyway, should at
7447 least specify in which version the semantics changed.}
7448 In earlier versions of @command{tar}, what is now the
7449 @option{--exclude-from} option was called @option{--exclude} instead.
7450 Now, @option{--exclude} applies to patterns listed on the command
7451 line and @option{--exclude-from} applies to patterns listed in a
7457 @section Wildcards Patterns and Matching
7459 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
7460 @samp{*} or @samp{?} for example, are replaced and expanded into all
7461 existing files matching the given pattern. @GNUTAR{} can use wildcard
7462 patterns for matching (or globbing) archive members when extracting
7463 from or listing an archive. Wildcard patterns are also used for
7464 verifying volume labels of @command{tar} archives. This section has the
7465 purpose of explaining wildcard syntax for @command{tar}.
7467 @FIXME{the next few paragraphs need work.}
7469 A @var{pattern} should be written according to shell syntax, using wildcard
7470 characters to effect globbing. Most characters in the pattern stand
7471 for themselves in the matched string, and case is significant: @samp{a}
7472 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
7473 pattern matches any single character in the matched string. The character
7474 @samp{*} in the pattern matches zero, one, or more single characters in
7475 the matched string. The character @samp{\} says to take the following
7476 character of the pattern @emph{literally}; it is useful when one needs to
7477 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
7479 The character @samp{[}, up to the matching @samp{]}, introduces a character
7480 class. A @dfn{character class} is a list of acceptable characters
7481 for the next single character of the matched string. For example,
7482 @samp{[abcde]} would match any of the first five letters of the alphabet.
7483 Note that within a character class, all of the ``special characters''
7484 listed above other than @samp{\} lose their special meaning; for example,
7485 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
7486 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
7487 the characters @samp{-} and @samp{]} must either come @emph{first} or
7488 @emph{last} in a character class.)
7490 @cindex Excluding characters from a character class
7491 @cindex Character class, excluding characters from
7492 If the first character of the class after the opening @samp{[}
7493 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
7494 Rather than listing character to match, it lists those characters which
7495 are @emph{forbidden} as the next single character of the matched string.
7497 Other characters of the class stand for themselves. The special
7498 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
7499 letters, is meant to represent all characters between @var{a} and
7502 @FIXME{need to add a sentence or so here to make this clear for those
7503 who don't have dan around.}
7505 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
7506 special for wildcard matches. However, if a pattern completely matches
7507 a directory prefix of a matched string, then it matches the full matched
7508 string: thus, excluding a directory also excludes all the files beneath it.
7511 * controlling pattern-matching::
7514 @node controlling pattern-matching
7515 @unnumberedsubsec Controlling Pattern-Matching
7517 For the purposes of this section, we call @dfn{exclusion members} all
7518 member names obtained while processing @option{--exclude} and
7519 @option{--exclude-from} options, and @dfn{inclusion members} those
7520 member names that were given in the command line or read from the file
7521 specified with @option{--files-from} option.
7523 These two pairs of member lists are used in the following operations:
7524 @option{--diff}, @option{--extract}, @option{--list},
7527 There are no inclusion members in create mode (@option{--create} and
7528 @option{--append}), since in this mode the names obtained from the
7529 command line refer to @emph{files}, not archive members.
7531 By default, inclusion members are compared with archive members
7532 literally @footnote{Notice that earlier @GNUTAR{} versions used
7533 globbing for inclusion members, which contradicted to UNIX98
7534 specification and was not documented. @xref{Changes}, for more
7535 information on this and other changes.} and exclusion members are
7536 treated as globbing patterns. For example:
7540 $ @kbd{tar tf foo.tar}
7545 # @i{Member names are used verbatim:}
7546 $ @kbd{tar -xf foo.tar -v '[remarks]'}
7548 # @i{Exclude member names are globbed:}
7549 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
7555 This behavior can be altered by using the following options:
7560 Treat all member names as wildcards.
7562 @opindex no-wildcards
7563 @item --no-wildcards
7564 Treat all member names as literal strings.
7567 Thus, to extract files whose names end in @samp{.c}, you can use:
7570 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
7576 Notice quoting of the pattern to prevent the shell from interpreting
7579 The effect of @option{--wildcards} option is canceled by
7580 @option{--no-wildcards}. This can be used to pass part of
7581 the command line arguments verbatim and other part as globbing
7582 patterns. For example, the following invocation:
7585 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
7589 instructs @command{tar} to extract from @file{foo.tar} all files whose
7590 names end in @samp{.txt} and the file named @file{[remarks]}.
7592 Normally, a pattern matches a name if an initial subsequence of the
7593 name's components matches the pattern, where @samp{*}, @samp{?}, and
7594 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
7595 and wildcards can match @samp{/}.
7597 Other than optionally stripping leading @samp{/} from names
7598 (@pxref{absolute}), patterns and names are used as-is. For
7599 example, trailing @samp{/} is not trimmed from a user-specified name
7600 before deciding whether to exclude it.
7602 However, this matching procedure can be altered by the options listed
7603 below. These options accumulate. For example:
7606 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
7610 ignores case when excluding @samp{makefile}, but not when excluding
7615 @opindex no-anchored
7617 @itemx --no-anchored
7618 If anchored, a pattern must match an initial subsequence
7619 of the name's components. Otherwise, the pattern can match any
7620 subsequence. Default is @option{--no-anchored} for exclusion members
7621 and @option{--anchored} inclusion members.
7623 @opindex ignore-case
7624 @opindex no-ignore-case
7626 @itemx --no-ignore-case
7627 When ignoring case, upper-case patterns match lower-case names and vice versa.
7628 When not ignoring case (the default), matching is case-sensitive.
7630 @opindex wildcards-match-slash
7631 @opindex no-wildcards-match-slash
7632 @item --wildcards-match-slash
7633 @itemx --no-wildcards-match-slash
7634 When wildcards match slash (the default for exclusion members), a
7635 wildcard like @samp{*} in the pattern can match a @samp{/} in the
7636 name. Otherwise, @samp{/} is matched only by @samp{/}.
7640 The @option{--recursion} and @option{--no-recursion} options
7641 (@pxref{recurse}) also affect how member patterns are interpreted. If
7642 recursion is in effect, a pattern matches a name if it matches any of
7643 the name's parent directories.
7645 The following table summarizes pattern-matching default values:
7647 @multitable @columnfractions .3 .7
7648 @headitem Members @tab Default settings
7649 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
7650 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
7653 @node quoting styles
7654 @section Quoting Member Names
7656 When displaying member names, @command{tar} takes care to avoid
7657 ambiguities caused by certain characters. This is called @dfn{name
7658 quoting}. The characters in question are:
7661 @item Non-printable control characters:
7662 @anchor{escape sequences}
7663 @multitable @columnfractions 0.20 0.10 0.60
7664 @headitem Character @tab @acronym{ASCII} @tab Character name
7665 @item \a @tab 7 @tab Audible bell
7666 @item \b @tab 8 @tab Backspace
7667 @item \f @tab 12 @tab Form feed
7668 @item \n @tab 10 @tab New line
7669 @item \r @tab 13 @tab Carriage return
7670 @item \t @tab 9 @tab Horizontal tabulation
7671 @item \v @tab 11 @tab Vertical tabulation
7674 @item Space (@acronym{ASCII} 32)
7676 @item Single and double quotes (@samp{'} and @samp{"})
7678 @item Backslash (@samp{\})
7681 The exact way @command{tar} uses to quote these characters depends on
7682 the @dfn{quoting style}. The default quoting style, called
7683 @dfn{escape} (see below), uses backslash notation to represent control
7684 characters, space and backslash. Using this quoting style, control
7685 characters are represented as listed in column @samp{Character} in the
7686 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
7688 @GNUTAR{} offers seven distinct quoting styles, which can be selected
7689 using @option{--quoting-style} option:
7692 @item --quoting-style=@var{style}
7693 @opindex quoting-style
7695 Sets quoting style. Valid values for @var{style} argument are:
7696 literal, shell, shell-always, c, escape, locale, clocale.
7699 These styles are described in detail below. To illustrate their
7700 effect, we will use an imaginary tar archive @file{arch.tar}
7701 containing the following members:
7705 # 1. Contains horizontal tabulation character.
7707 # 2. Contains newline character
7710 # 3. Contains a space
7712 # 4. Contains double quotes
7714 # 5. Contains single quotes
7716 # 6. Contains a backslash character:
7721 Here is how usual @command{ls} command would have listed them, if they
7722 had existed in the current working directory:
7740 No quoting, display each character as is:
7744 $ @kbd{tar tf arch.tar --quoting-style=literal}
7757 Display characters the same way Bourne shell does:
7758 control characters, except @samp{\t} and @samp{\n}, are printed using
7759 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
7760 single quote is printed as @samp{\'}. If a name contains any quoted
7761 characters, it is enclosed in single quotes. In particular, if a name
7762 contains single quotes, it is printed as several single-quoted strings:
7766 $ @kbd{tar tf arch.tar --quoting-style=shell}
7769 './a'\''single'\''quote'
7779 Same as @samp{shell}, but the names are always enclosed in single
7784 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
7787 './a'\''single'\''quote'
7797 Use the notation of the C programming language. All names are
7798 enclosed in double quotes. Control characters are quoted using
7799 backslash notations, double quotes are represented as @samp{\"},
7800 backslash characters are represented as @samp{\\}. Single quotes and
7801 spaces are not quoted:
7805 $ @kbd{tar tf arch.tar --quoting-style=c}
7809 "./a\"double\"quote"
7817 Control characters are printed using backslash notation, a space is
7818 printed as @samp{\ } and a backslash as @samp{\\}. This is the
7819 default quoting style, unless it was changed when configured the
7824 $ @kbd{tar tf arch.tar --quoting-style=escape}
7836 Control characters, single quote and backslash are printed using
7837 backslash notation. All names are quoted using left and right
7838 quotation marks, appropriate to the current locale. If it does not
7839 define quotation marks, use @samp{'} as left and as right
7840 quotation marks. Any occurrences of the right quotation mark in a
7841 name are escaped with @samp{\}, for example:
7847 $ @kbd{tar tf arch.tar --quoting-style=locale}
7850 './a\'single\'quote'
7859 Same as @samp{locale}, but @samp{"} is used for both left and right
7860 quotation marks, if not provided by the currently selected locale:
7864 $ @kbd{tar tf arch.tar --quoting-style=clocale}
7868 "./a\"double\"quote"
7876 You can specify which characters should be quoted in addition to those
7877 implied by the current quoting style:
7880 @item --quote-chars=@var{string}
7881 Always quote characters from @var{string}, even if the selected
7882 quoting style would not quote them.
7885 For example, using @samp{escape} quoting (compare with the usual
7886 escape listing above):
7890 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
7901 To disable quoting of such additional characters, use the following
7905 @item --no-quote-chars=@var{string}
7906 Remove characters listed in @var{string} from the list of quoted
7907 characters set by the previous @option{--quote-chars} option.
7910 This option is particularly useful if you have added
7911 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
7912 and wish to disable it for the current invocation.
7914 Note, that @option{--no-quote-chars} does @emph{not} disable those
7915 characters that are quoted by default in the selected quoting style.
7918 @section Modifying File and Member Names
7920 @command{Tar} archives contain detailed information about files stored
7921 in them and full file names are part of that information. When
7922 storing a file to an archive, its file name is recorded in it,
7923 along with the actual file contents. When restoring from an archive,
7924 a file is created on disk with exactly the same name as that stored
7925 in the archive. In the majority of cases this is the desired behavior
7926 of a file archiver. However, there are some cases when it is not.
7928 First of all, it is often unsafe to extract archive members with
7929 absolute file names or those that begin with a @file{../}. @GNUTAR{}
7930 takes special precautions when extracting such names and provides a
7931 special option for handling them, which is described in
7934 Secondly, you may wish to extract file names without some leading
7935 directory components, or with otherwise modified names. In other
7936 cases it is desirable to store files under differing names in the
7939 @GNUTAR{} provides several options for these needs.
7942 @opindex strip-components
7943 @item --strip-components=@var{number}
7944 Strip given @var{number} of leading components from file names before
7948 For example, suppose you have archived whole @file{/usr} hierarchy to
7949 a tar archive named @file{usr.tar}. Among other files, this archive
7950 contains @file{usr/include/stdlib.h}, which you wish to extract to
7951 the current working directory. To do so, you type:
7954 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
7957 The option @option{--strip=2} instructs @command{tar} to strip the
7958 two leading components (@file{usr/} and @file{include/}) off the file
7961 If you add the @option{--verbose} (@option{-v}) option to the invocation
7962 above, you will note that the verbose listing still contains the
7963 full file name, with the two removed components still in place. This
7964 can be inconvenient, so @command{tar} provides a special option for
7965 altering this behavior:
7967 @anchor{show-transformed-names}
7969 @opindex show-transformed-names
7970 @item --show-transformed-names
7971 Display file or member names with all requested transformations
7980 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
7981 usr/include/stdlib.h
7982 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
7987 Notice that in both cases the file @file{stdlib.h} is extracted to the
7988 current working directory, @option{--show-transformed-names} affects
7989 only the way its name is displayed.
7991 This option is especially useful for verifying whether the invocation
7992 will have the desired effect. Thus, before running
7995 $ @kbd{tar -x --strip=@var{n}}
7999 it is often advisable to run
8002 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
8006 to make sure the command will produce the intended results.
8008 In case you need to apply more complex modifications to the file name,
8009 @GNUTAR{} provides a general-purpose transformation option:
8014 @item --transform=@var{expression}
8015 @itemx --xform=@var{expression}
8016 Modify file names using supplied @var{expression}.
8020 The @var{expression} is a @command{sed}-like replace expression of the
8024 s/@var{regexp}/@var{replace}/[@var{flags}]
8028 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
8029 replacement for each file name part that matches @var{regexp}. Both
8030 @var{regexp} and @var{replace} are described in detail in
8031 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
8033 Any delimiter can be used in lieu of @samp{/}, the only requirement being
8034 that it be used consistently throughout the expression. For example,
8035 the following two expressions are equivalent:
8044 Changing delimiters is often useful when the @var{regex} contains
8045 slashes. For example, it is more convenient to write @code{s,/,-,} than
8048 As in @command{sed}, you can give several replace expressions,
8049 separated by a semicolon.
8051 Supported @var{flags} are:
8055 Apply the replacement to @emph{all} matches to the @var{regexp}, not
8059 Use case-insensitive matching.
8062 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
8063 regexps, Extended regular expressions, Extended regular expressions,
8067 Only replace the @var{number}th match of the @var{regexp}.
8069 Note: the @acronym{POSIX} standard does not specify what should happen
8070 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
8071 follows the GNU @command{sed} implementation in this regard, so
8072 the interaction is defined to be: ignore matches before the
8073 @var{number}th, and then match and replace all matches from the
8078 In addition, several @dfn{transformation scope} flags are supported,
8079 that control to what files transformations apply. These are:
8083 Apply transformation to regular archive members.
8086 Do not apply transformation to regular archive members.
8089 Apply transformation to symbolic link targets.
8092 Do not apply transformation to symbolic link targets.
8095 Apply transformation to hard link targets.
8098 Do not apply transformation to hard link targets.
8101 Default is @samp{rsh}, which means to apply tranformations to both archive
8102 members and targets of symbolic and hard links.
8104 Default scope flags can also be changed using @samp{flags=} statement
8105 in the transform expression. The flags set this way remain in force
8106 until next @samp{flags=} statement or end of expression, whichever
8107 occurs first. For example:
8110 --transform 'flags=S;s|^|/usr/local/|'
8113 Here are several examples of @option{--transform} usage:
8116 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
8119 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
8122 @item Strip two leading directory components (equivalent to
8123 @option{--strip-components=2}):
8126 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
8129 @item Convert each file name to lower case:
8132 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
8135 @item Prepend @file{/prefix/} to each file name:
8138 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
8141 @item Archive the @file{/lib} directory, prepending @samp{/usr/local}
8142 to each archive member:
8145 $ @kbd{tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib}
8149 Notice the use of flags in the last example. The @file{/lib}
8150 directory often contains many symbolic links to files within it.
8151 It may look, for example, like this:
8155 drwxr-xr-x root/root 0 2008-07-08 16:20 /lib/
8156 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
8157 lrwxrwxrwx root/root 0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
8161 Using the expression @samp{s,^,/usr/local/,} would mean adding
8162 @samp{/usr/local} to both regular archive members and to link
8163 targets. In this case, @file{/lib/libc.so.6} would become:
8166 /usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so
8169 This is definitely not desired. To avoid this, the @samp{S} flag
8170 is used, which excludes symbolic link targets from filename
8171 transformations. The result is:
8174 $ @kbd{tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \
8175 --show-transformed /lib}
8176 drwxr-xr-x root/root 0 2008-07-08 16:20 /usr/local/lib/
8177 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
8178 lrwxrwxrwx root/root 0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
8182 Unlike @option{--strip-components}, @option{--transform} can be used
8183 in any @GNUTAR{} operation mode. For example, the following command
8184 adds files to the archive while replacing the leading @file{usr/}
8185 component with @file{var/}:
8188 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
8191 To test @option{--transform} effect we suggest using
8192 @option{--show-transformed-names} option:
8195 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
8196 --verbose --show-transformed-names /}
8199 If both @option{--strip-components} and @option{--transform} are used
8200 together, then @option{--transform} is applied first, and the required
8201 number of components is then stripped from its result.
8203 You can use as many @option{--transform} options in a single command
8204 line as you want. The specified expressions will then be applied in
8205 order of their appearance. For example, the following two invocations
8209 $ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
8210 --transform='s,/usr/local,/usr/,'}
8211 $ @kbd{tar -cf arch.tar \
8212 --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
8216 @section Operating Only on New Files
8218 @cindex Excluding file by age
8219 @cindex Data Modification time, excluding files by
8220 @cindex Modification time, excluding files by
8221 @cindex Age, excluding files by
8222 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
8223 @option{-N @var{date}}) option causes @command{tar} to only work on
8224 files whose data modification or status change times are newer than
8225 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
8226 it is taken to be a file name; the data modification time of that file
8227 is used as the date. If you use this option when creating or appending
8228 to an archive, the archive will only include new files. If you use
8229 @option{--after-date} when extracting an archive, @command{tar} will
8230 only extract files newer than the @var{date} you specify.
8232 If you only want @command{tar} to make the date comparison based on
8233 modification of the file's data (rather than status
8234 changes), then use the @option{--newer-mtime=@var{date}} option.
8236 @cindex --after-date and --update compared
8237 @cindex --newer-mtime and --update compared
8238 You may use these options with any operation. Note that these options
8239 differ from the @option{--update} (@option{-u}) operation in that they
8240 allow you to specify a particular date against which @command{tar} can
8241 compare when deciding whether or not to archive the files.
8246 @item --after-date=@var{date}
8247 @itemx --newer=@var{date}
8248 @itemx -N @var{date}
8249 Only store files newer than @var{date}.
8251 Acts on files only if their data modification or status change times are
8252 later than @var{date}. Use in conjunction with any operation.
8254 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
8255 name; the data modification time of that file is used as the date.
8257 @opindex newer-mtime
8258 @item --newer-mtime=@var{date}
8259 Acts like @option{--after-date}, but only looks at data modification times.
8262 These options limit @command{tar} to operate only on files which have
8263 been modified after the date specified. A file's status is considered to have
8264 changed if its contents have been modified, or if its owner,
8265 permissions, and so forth, have been changed. (For more information on
8266 how to specify a date, see @ref{Date input formats}; remember that the
8267 entire date argument must be quoted if it contains any spaces.)
8269 Gurus would say that @option{--after-date} tests both the data
8270 modification time (@code{mtime}, the time the contents of the file
8271 were last modified) and the status change time (@code{ctime}, the time
8272 the file's status was last changed: owner, permissions, etc.@:)
8273 fields, while @option{--newer-mtime} tests only the @code{mtime}
8276 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
8277 @code{ctime} and processes the file if either one is more recent than
8278 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
8279 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
8280 contents of the file were looked at).
8282 Date specifiers can have embedded spaces. Because of this, you may need
8283 to quote date arguments to keep the shell from parsing them as separate
8284 arguments. For example, the following command will add to the archive
8285 all the files modified less than two days ago:
8288 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
8291 When any of these options is used with the option @option{--verbose}
8292 (@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
8293 date back to its textual representation and compare that with the
8294 one given with the option. If the two dates differ, @command{tar} will
8295 print a warning saying what date it will use. This is to help user
8296 ensure he is using the right date. For example:
8300 $ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
8301 tar: Option --after-date: Treating date '10 days ago' as 2006-06-11
8307 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
8308 should not be used for incremental backups. @xref{Incremental Dumps},
8309 for proper way of creating incremental backups.
8313 @section Descending into Directories
8314 @cindex Avoiding recursion in directories
8315 @cindex Descending directories, avoiding
8316 @cindex Directories, avoiding recursion
8317 @cindex Recursion in directories, avoiding
8319 Usually, @command{tar} will recursively explore all directories (either
8320 those given on the command line or through the @option{--files-from}
8321 option) for the various files they contain. However, you may not always
8322 want @command{tar} to act this way.
8324 @opindex no-recursion
8325 @cindex @command{find}, using with @command{tar}
8326 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
8327 into specified directories. If you specify @option{--no-recursion}, you can
8328 use the @command{find} (@pxref{Top,, find, find, GNU Find Manual})
8329 utility for hunting through levels of directories to
8330 construct a list of file names which you could then pass to @command{tar}.
8331 @command{find} allows you to be more selective when choosing which files to
8332 archive; see @ref{files}, for more information on using @command{find} with
8336 @item --no-recursion
8337 Prevents @command{tar} from recursively descending directories.
8341 Requires @command{tar} to recursively descend directories.
8342 This is the default.
8345 When you use @option{--no-recursion}, @GNUTAR{} grabs
8346 directory entries themselves, but does not descend on them
8347 recursively. Many people use @command{find} for locating files they
8348 want to back up, and since @command{tar} @emph{usually} recursively
8349 descends on directories, they have to use the @samp{@w{-not -type d}}
8350 test in their @command{find} invocation (@pxref{Type, Type, Type test,
8351 find, Finding Files}), as they usually do not want all the files in a
8352 directory. They then use the @option{--files-from} option to archive
8353 the files located via @command{find}.
8355 The problem when restoring files archived in this manner is that the
8356 directories themselves are not in the archive; so the
8357 @option{--same-permissions} (@option{--preserve-permissions},
8358 @option{-p}) option does not affect them---while users might really
8359 like it to. Specifying @option{--no-recursion} is a way to tell
8360 @command{tar} to grab only the directory entries given to it, adding
8361 no new files on its own. To summarize, if you use @command{find} to
8362 create a list of files to be stored in an archive, use it as follows:
8366 $ @kbd{find @var{dir} @var{tests} | \
8367 tar -cf @var{archive} -T - --no-recursion}
8371 The @option{--no-recursion} option also applies when extracting: it
8372 causes @command{tar} to extract only the matched directory entries, not
8373 the files under those directories.
8375 The @option{--no-recursion} option also affects how globbing patterns
8376 are interpreted (@pxref{controlling pattern-matching}).
8378 The @option{--no-recursion} and @option{--recursion} options apply to
8379 later options and operands, and can be overridden by later occurrences
8380 of @option{--no-recursion} and @option{--recursion}. For example:
8383 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
8387 creates an archive with one entry for @file{grape}, and the recursive
8388 contents of @file{grape/concord}, but no entries under @file{grape}
8389 other than @file{grape/concord}.
8392 @section Crossing File System Boundaries
8393 @cindex File system boundaries, not crossing
8395 @command{tar} will normally automatically cross file system boundaries in
8396 order to archive files which are part of a directory tree. You can
8397 change this behavior by running @command{tar} and specifying
8398 @option{--one-file-system}. This option only affects files that are
8399 archived because they are in a directory that is being archived;
8400 @command{tar} will still archive files explicitly named on the command line
8401 or through @option{--files-from}, regardless of where they reside.
8404 @opindex one-file-system
8405 @item --one-file-system
8406 Prevents @command{tar} from crossing file system boundaries when
8407 archiving. Use in conjunction with any write operation.
8410 The @option{--one-file-system} option causes @command{tar} to modify its
8411 normal behavior in archiving the contents of directories. If a file in
8412 a directory is not on the same file system as the directory itself, then
8413 @command{tar} will not archive that file. If the file is a directory
8414 itself, @command{tar} will not archive anything beneath it; in other words,
8415 @command{tar} will not cross mount points.
8417 This option is useful for making full or incremental archival backups of
8418 a file system. If this option is used in conjunction with
8419 @option{--verbose} (@option{-v}), files that are excluded are
8420 mentioned by name on the standard error.
8423 * directory:: Changing Directory
8424 * absolute:: Absolute File Names
8428 @subsection Changing the Working Directory
8430 @FIXME{need to read over this node now for continuity; i've switched
8431 things around some.}
8433 @cindex Changing directory mid-stream
8434 @cindex Directory, changing mid-stream
8435 @cindex Working directory, specifying
8436 To change the working directory in the middle of a list of file names,
8437 either on the command line or in a file specified using
8438 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
8439 This will change the working directory to the specified directory
8440 after that point in the list.
8444 @item --directory=@var{directory}
8445 @itemx -C @var{directory}
8446 Changes the working directory in the middle of a command line.
8452 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
8456 will place the files @file{grape} and @file{prune} from the current
8457 directory into the archive @file{jams.tar}, followed by the file
8458 @file{cherry} from the directory @file{food}. This option is especially
8459 useful when you have several widely separated files that you want to
8460 store in the same archive.
8462 Note that the file @file{cherry} is recorded in the archive under the
8463 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
8464 archive will contain three files that all appear to have come from the
8465 same directory; if the archive is extracted with plain @samp{tar
8466 --extract}, all three files will be written in the current directory.
8468 Contrast this with the command,
8471 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
8475 which records the third file in the archive under the name
8476 @file{red/cherry} so that, if the archive is extracted using
8477 @samp{tar --extract}, the third file will be written in a subdirectory
8480 You can use the @option{--directory} option to make the archive
8481 independent of the original name of the directory holding the files.
8482 The following command places the files @file{/etc/passwd},
8483 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
8487 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
8491 However, the names of the archive members will be exactly what they were
8492 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
8493 They will not appear to be related by file name to the original
8494 directories where those files were located.
8496 Note that @option{--directory} options are interpreted consecutively. If
8497 @option{--directory} specifies a relative file name, it is interpreted
8498 relative to the then current directory, which might not be the same as
8499 the original current working directory of @command{tar}, due to a previous
8500 @option{--directory} option.
8502 When using @option{--files-from} (@pxref{files}), you can put various
8503 @command{tar} options (including @option{-C}) in the file list. Notice,
8504 however, that in this case the option and its argument may not be
8505 separated by whitespace. If you use short option, its argument must
8506 either follow the option letter immediately, without any intervening
8507 whitespace, or occupy the next line. Otherwise, if you use long
8508 option, separate its argument by an equal sign.
8510 For instance, the file list for the above example will be:
8523 To use it, you would invoke @command{tar} as follows:
8526 $ @kbd{tar -c -f foo.tar --files-from list}
8529 The interpretation of @option{--directory} is disabled by
8530 @option{--null} option.
8533 @subsection Absolute File Names
8534 @cindex absolute file names
8535 @cindex file names, absolute
8537 By default, @GNUTAR{} drops a leading @samp{/} on
8538 input or output, and complains about file names containing a @file{..}
8539 component. There is an option that turns off this behavior:
8542 @opindex absolute-names
8543 @item --absolute-names
8545 Do not strip leading slashes from file names, and permit file names
8546 containing a @file{..} file name component.
8549 When @command{tar} extracts archive members from an archive, it strips any
8550 leading slashes (@samp{/}) from the member name. This causes absolute
8551 member names in the archive to be treated as relative file names. This
8552 allows you to have such members extracted wherever you want, instead of
8553 being restricted to extracting the member in the exact directory named
8554 in the archive. For example, if the archive member has the name
8555 @file{/etc/passwd}, @command{tar} will extract it as if the name were
8556 really @file{etc/passwd}.
8558 File names containing @file{..} can cause problems when extracting, so
8559 @command{tar} normally warns you about such files when creating an
8560 archive, and rejects attempts to extracts such files.
8562 Other @command{tar} programs do not do this. As a result, if you
8563 create an archive whose member names start with a slash, they will be
8564 difficult for other people with a non-@GNUTAR{}
8565 program to use. Therefore, @GNUTAR{} also strips
8566 leading slashes from member names when putting members into the
8567 archive. For example, if you ask @command{tar} to add the file
8568 @file{/bin/ls} to an archive, it will do so, but the member name will
8569 be @file{bin/ls}@footnote{A side effect of this is that when
8570 @option{--create} is used with @option{--verbose} the resulting output
8571 is not, generally speaking, the same as the one you'd get running
8572 @kbd{tar --list} command. This may be important if you use some
8573 scripts for comparing both outputs. @xref{listing member and file names},
8574 for the information on how to handle this case.}.
8576 Symbolic links containing @file{..} or leading @samp{/} can also cause
8577 problems when extracting, so @command{tar} normally extracts them last;
8578 it may create empty files as placeholders during extraction.
8580 If you use the @option{--absolute-names} (@option{-P}) option,
8581 @command{tar} will do none of these transformations.
8583 To archive or extract files relative to the root directory, specify
8584 the @option{--absolute-names} (@option{-P}) option.
8586 Normally, @command{tar} acts on files relative to the working
8587 directory---ignoring superior directory names when archiving, and
8588 ignoring leading slashes when extracting.
8590 When you specify @option{--absolute-names} (@option{-P}),
8591 @command{tar} stores file names including all superior directory
8592 names, and preserves leading slashes. If you only invoked
8593 @command{tar} from the root directory you would never need the
8594 @option{--absolute-names} option, but using this option
8595 may be more convenient than switching to root.
8597 @FIXME{Should be an example in the tutorial/wizardry section using this
8598 to transfer files between systems.}
8601 @item --absolute-names
8602 Preserves full file names (including superior directory names) when
8603 archiving and extracting files.
8607 @command{tar} prints out a message about removing the @samp{/} from
8608 file names. This message appears once per @GNUTAR{}
8609 invocation. It represents something which ought to be told; ignoring
8610 what it means can cause very serious surprises, later.
8612 Some people, nevertheless, do not want to see this message. Wanting to
8613 play really dangerously, one may of course redirect @command{tar} standard
8614 error to the sink. For example, under @command{sh}:
8617 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
8621 Another solution, both nicer and simpler, would be to change to
8622 the @file{/} directory first, and then avoid absolute notation.
8626 $ @kbd{tar -c -f archive.tar -C / home}
8629 @xref{Integrity}, for some of the security-related implications
8630 of using this option.
8632 @include parse-datetime.texi
8635 @chapter Controlling the Archive Format
8637 @cindex Tar archive formats
8638 Due to historical reasons, there are several formats of tar archives.
8639 All of them are based on the same principles, but have some subtle
8640 differences that often make them incompatible with each other.
8642 GNU tar is able to create and handle archives in a variety of formats.
8643 The most frequently used formats are (in alphabetical order):
8647 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
8648 from an early @acronym{POSIX} standard, adding some improvements such as
8649 sparse file handling and incremental archives. Unfortunately these
8650 features were implemented in a way incompatible with other archive
8653 Archives in @samp{gnu} format are able to hold file names of unlimited
8657 Format used by @GNUTAR{} of versions prior to 1.12.
8660 Archive format, compatible with the V7 implementation of tar. This
8661 format imposes a number of limitations. The most important of them
8665 @item The maximum length of a file name is limited to 99 characters.
8666 @item The maximum length of a symbolic link is limited to 99 characters.
8667 @item It is impossible to store special files (block and character
8668 devices, fifos etc.)
8669 @item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
8671 @item V7 archives do not contain symbolic ownership information (user
8672 and group name of the file owner).
8675 This format has traditionally been used by Automake when producing
8676 Makefiles. This practice will change in the future, in the meantime,
8677 however this means that projects containing file names more than 99
8678 characters long will not be able to use @GNUTAR{} @value{VERSION} and
8679 Automake prior to 1.9.
8682 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
8683 symbolic ownership information. It is also able to store
8684 special files. However, it imposes several restrictions as well:
8687 @item The maximum length of a file name is limited to 256 characters,
8688 provided that the file name can be split at a directory separator in
8689 two parts, first of them being at most 155 bytes long. So, in most
8690 cases the maximum file name length will be shorter than 256
8692 @item The maximum length of a symbolic link name is limited to
8694 @item Maximum size of a file the archive is able to accommodate
8696 @item Maximum value of UID/GID is 2097151.
8697 @item Maximum number of bits in device major and minor numbers is 21.
8701 Format used by J@"org Schilling @command{star}
8702 implementation. @GNUTAR{} is able to read @samp{star} archives but
8703 currently does not produce them.
8706 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
8707 most flexible and feature-rich format. It does not impose any
8708 restrictions on file sizes or file name lengths. This format is quite
8709 recent, so not all tar implementations are able to handle it properly.
8710 However, this format is designed in such a way that any tar
8711 implementation able to read @samp{ustar} archives will be able to read
8712 most @samp{posix} archives as well, with the only exception that any
8713 additional information (such as long file names etc.) will in such
8714 case be extracted as plain text files along with the files it refers to.
8716 This archive format will be the default format for future versions
8721 The following table summarizes the limitations of each of these
8724 @multitable @columnfractions .10 .20 .20 .20 .20
8725 @headitem Format @tab UID @tab File Size @tab File Name @tab Devn
8726 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8727 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8728 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
8729 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
8730 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
8733 The default format for @GNUTAR{} is defined at compilation
8734 time. You may check it by running @command{tar --help}, and examining
8735 the last lines of its output. Usually, @GNUTAR{} is configured
8736 to create archives in @samp{gnu} format, however, future version will
8737 switch to @samp{posix}.
8740 * Compression:: Using Less Space through Compression
8741 * Attributes:: Handling File Attributes
8742 * Portability:: Making @command{tar} Archives More Portable
8743 * cpio:: Comparison of @command{tar} and @command{cpio}
8747 @section Using Less Space through Compression
8750 * gzip:: Creating and Reading Compressed Archives
8751 * sparse:: Archiving Sparse Files
8755 @subsection Creating and Reading Compressed Archives
8756 @cindex Compressed archives
8757 @cindex Storing archives in compressed format
8765 @GNUTAR{} is able to create and read compressed archives. It supports
8766 a wide variety of compression programs, namely: @command{gzip},
8767 @command{bzip2}, @command{lzip}, @command{lzma}, @command{lzop},
8768 @command{xz} and traditional @command{compress}. The latter is
8769 supported mostly for backward compatibility, and we recommend
8770 against using it, because it is by far less effective than the other
8771 compression programs@footnote{It also had patent problems in the past.}.
8773 Creating a compressed archive is simple: you just specify a
8774 @dfn{compression option} along with the usual archive creation
8775 commands. The compression option is @option{-z} (@option{--gzip}) to
8776 create a @command{gzip} compressed archive, @option{-j}
8777 (@option{--bzip2}) to create a @command{bzip2} compressed archive,
8778 @option{--lzip} to create an @asis{lzip} compressed archive,
8779 @option{-J} (@option{--xz}) to create an @asis{XZ} archive,
8780 @option{--lzma} to create an @asis{LZMA} compressed
8781 archive, @option{--lzop} to create an @asis{LSOP} archive, and
8782 @option{-Z} (@option{--compress}) to use @command{compress} program.
8786 $ @kbd{tar czf archive.tar.gz .}
8789 You can also let @GNUTAR{} select the compression program based on
8790 the suffix of the archive file name. This is done using
8791 @option{--auto-compress} (@option{-a}) command line option. For
8792 example, the following invocation will use @command{bzip2} for
8796 $ @kbd{tar caf archive.tar.bz2 .}
8800 whereas the following one will use @command{lzma}:
8803 $ @kbd{tar caf archive.tar.lzma .}
8806 For a complete list of file name suffixes recognized by @GNUTAR{},
8807 see @ref{auto-compress}.
8809 Reading compressed archive is even simpler: you don't need to specify
8810 any additional options as @GNUTAR{} recognizes its format
8811 automatically. Thus, the following commands will list and extract the
8812 archive created in previous example:
8815 # List the compressed archive
8816 $ @kbd{tar tf archive.tar.gz}
8817 # Extract the compressed archive
8818 $ @kbd{tar xf archive.tar.gz}
8821 The format recognition algorithm is based on @dfn{signatures}, a
8822 special byte sequences in the beginning of file, that are specific for
8823 certain compression formats. If this approach fails, @command{tar}
8824 falls back to using archive name suffix to determine its format
8825 (@pxref{auto-compress}, for a list of recognized suffixes).
8827 @anchor{alternative decompression programs}
8828 @cindex alternative decompression programs
8829 Some compression programs are able to handle different compression
8830 formats. @GNUTAR{} uses this, if the principal decompressor for the
8831 given format is not available. For example, if @command{compress} is
8832 not installed, @command{tar} will try to use @command{gzip}. As of
8833 version @value{VERSION} the following alternatives are
8834 tried@footnote{To verbosely trace the decompressor selection, use the
8835 @option{--warning=decompress-program} option
8836 (@pxref{warnings,decompress-program}).}:
8838 @multitable @columnfractions 0.3 0.3 0.3
8839 @headitem Format @tab Main decompressor @tab Alternatives
8840 @item compress @tab compress @tab gzip
8841 @item lzma @tab lzma @tab xz
8842 @item bzip2 @tab bzip2 @tab lbzip2
8845 The only case when you have to specify a decompression option while
8846 reading the archive is when reading from a pipe or from a tape drive
8847 that does not support random access. However, in this case @GNUTAR{}
8848 will indicate which option you should use. For example:
8851 $ @kbd{cat archive.tar.gz | tar tf -}
8852 tar: Archive is compressed. Use -z option
8853 tar: Error is not recoverable: exiting now
8856 If you see such diagnostics, just add the suggested option to the
8857 invocation of @GNUTAR{}:
8860 $ @kbd{cat archive.tar.gz | tar tzf -}
8863 Notice also, that there are several restrictions on operations on
8864 compressed archives. First of all, compressed archives cannot be
8865 modified, i.e., you cannot update (@option{--update}, alias @option{-u})
8866 them or delete (@option{--delete}) members from them or
8867 add (@option{--append}, alias @option{-r}) members to them. Likewise, you
8868 cannot append another @command{tar} archive to a compressed archive using
8869 @option{--concatenate} (@option{-A}). Secondly, multi-volume
8870 archives cannot be compressed.
8872 The following options allow to select a particular compressor program:
8880 Filter the archive through @command{gzip}.
8885 Filter the archive through @code{xz}.
8889 Filter the archive through @code{bzip2}.
8893 Filter the archive through @command{lzip}.
8897 Filter the archive through @command{lzma}.
8901 Filter the archive through @command{lzop}.
8908 Filter the archive through @command{compress}.
8911 When any of these options is given, @GNUTAR{} searches the compressor
8912 binary in the current path and invokes it. The name of the compressor
8913 program is specified at compilation time using a corresponding
8914 @option{--with-@var{compname}} option to @command{configure}, e.g.
8915 @option{--with-bzip2} to select a specific @command{bzip2} binary.
8916 @xref{lbzip2}, for a detailed discussion.
8918 The output produced by @command{tar --help} shows the actual
8919 compressor names along with each of these options.
8921 You can use any of these options on physical devices (tape drives,
8922 etc.) and remote files as well as on normal files; data to or from
8923 such devices or remote files is reblocked by another copy of the
8924 @command{tar} program to enforce the specified (or default) record
8925 size. The default compression parameters are used. Most compression
8926 programs let you override these by setting a program-specific
8927 environment variable. For example, with @command{gzip} you can set
8931 $ @kbd{GZIP='-9 -n' tar czf archive.tar.gz subdir}
8935 The traditional way to do this is to use a pipe:
8938 $ @kbd{tar cf - subdir | gzip -9 -n > archive.tar.gz}
8941 @cindex corrupted archives
8942 Compressed archives are easily corrupted, because compressed files
8943 have little redundancy. The adaptive nature of the
8944 compression scheme means that the compression tables are implicitly
8945 spread all over the archive. If you lose a few blocks, the dynamic
8946 construction of the compression tables becomes unsynchronized, and there
8947 is little chance that you could recover later in the archive.
8949 Other compression options provide better control over creating
8950 compressed archives. These are:
8953 @anchor{auto-compress}
8954 @opindex auto-compress
8955 @item --auto-compress
8957 Select a compression program to use by the archive file name
8958 suffix. The following suffixes are recognized:
8960 @multitable @columnfractions 0.3 0.6
8961 @headitem Suffix @tab Compression program
8962 @item @samp{.gz} @tab @command{gzip}
8963 @item @samp{.tgz} @tab @command{gzip}
8964 @item @samp{.taz} @tab @command{gzip}
8965 @item @samp{.Z} @tab @command{compress}
8966 @item @samp{.taZ} @tab @command{compress}
8967 @item @samp{.bz2} @tab @command{bzip2}
8968 @item @samp{.tz2} @tab @command{bzip2}
8969 @item @samp{.tbz2} @tab @command{bzip2}
8970 @item @samp{.tbz} @tab @command{bzip2}
8971 @item @samp{.lz} @tab @command{lzip}
8972 @item @samp{.lzma} @tab @command{lzma}
8973 @item @samp{.tlz} @tab @command{lzma}
8974 @item @samp{.lzo} @tab @command{lzop}
8975 @item @samp{.xz} @tab @command{xz}
8978 @opindex use-compress-program
8979 @item --use-compress-program=@var{prog}
8980 @itemx -I=@var{prog}
8981 Use external compression program @var{prog}. Use this option if you
8982 are not happy with the compression program associated with the suffix
8983 at compile time or if you have a compression program that @GNUTAR{}
8984 does not support. The program should follow two conventions:
8986 First, when invoked without options, it should read data from standard
8987 input, compress it and output it on standard output.
8989 Secondly, if invoked with the @option{-d} option, it should do exactly
8990 the opposite, i.e., read the compressed data from the standard input
8991 and produce uncompressed data on the standard output.
8994 @cindex gpg, using with tar
8995 @cindex gnupg, using with tar
8996 @cindex Using encrypted archives
8997 The @option{--use-compress-program} option, in particular, lets you
8998 implement your own filters, not necessarily dealing with
8999 compression/decompression. For example, suppose you wish to implement
9000 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
9001 gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
9002 Manual}). The following script does that:
9008 -d) gpg --decrypt - | gzip -d -c;;
9009 '') gzip -c | gpg -s;;
9010 *) echo "Unknown option $1">&2; exit 1;;
9015 Suppose you name it @file{gpgz} and save it somewhere in your
9016 @env{PATH}. Then the following command will create a compressed
9017 archive signed with your private key:
9020 $ @kbd{tar -cf foo.tar.gpgz -Igpgz .}
9024 Likewise, the command below will list its contents:
9027 $ @kbd{tar -tf foo.tar.gpgz -Igpgz .}
9031 The above is based on the following discussion:
9033 I have one question, or maybe it's a suggestion if there isn't a way
9034 to do it now. I would like to use @option{--gzip}, but I'd also like
9035 the output to be fed through a program like @acronym{GNU}
9036 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
9037 to use :-)), basically adding ECC protection on top of compression.
9038 It seems as if this should be quite easy to do, but I can't work out
9039 exactly how to go about it. Of course, I can pipe the standard output
9040 of @command{tar} through @command{ecc}, but then I lose (though I
9041 haven't started using it yet, I confess) the ability to have
9042 @command{tar} use @command{rmt} for it's I/O (I think).
9044 I think the most straightforward thing would be to let me specify a
9045 general set of filters outboard of compression (preferably ordered,
9046 so the order can be automatically reversed on input operations, and
9047 with the options they require specifiable), but beggars shouldn't be
9048 choosers and anything you decide on would be fine with me.
9050 By the way, I like @command{ecc} but if (as the comments say) it can't
9051 deal with loss of block sync, I'm tempted to throw some time at adding
9052 that capability. Supposing I were to actually do such a thing and
9053 get it (apparently) working, do you accept contributed changes to
9054 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
9056 Isn't that exactly the role of the
9057 @option{--use-compress-prog=@var{program}} option?
9058 I never tried it myself, but I suspect you may want to write a
9059 @var{prog} script or program able to filter stdin to stdout to
9060 way you want. It should recognize the @option{-d} option, for when
9061 extraction is needed rather than creation.
9063 It has been reported that if one writes compressed data (through the
9064 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
9065 the DLT compression mode, the data will actually get bigger and one will
9066 end up with less space on the tape.
9070 * lbzip2:: Using lbzip2 with @GNUTAR{}.
9074 @subsubsection Using lbzip2 with @GNUTAR{}.
9076 @cindex Laszlo Ersek
9077 @command{Lbzip2} is a multithreaded utility for handling
9078 @samp{bzip2} compression, written by Laszlo Ersek. It makes use of
9079 multiple processors to speed up its operation and in general works
9080 considerably faster than @command{bzip2}. For a detailed description
9081 of @command{lbzip2} see @uref{http://freshmeat.net/@/projects/@/lbzip2} and
9082 @uref{http://www.linuxinsight.com/@/lbzip2-parallel-bzip2-utility.html,
9083 lbzip2: parallel bzip2 utility}.
9085 Recent versions of @command{lbzip2} are mostly command line compatible
9086 with @command{bzip2}, which makes it possible to automatically invoke
9087 it via the @option{--bzip2} @GNUTAR{} command line option. To do so,
9088 @GNUTAR{} must be configured with the @option{--with-bzip2} command
9089 line option, like this:
9092 $ @kbd{./configure --with-bzip2=lbzip2 [@var{other-options}]}
9095 Once configured and compiled this way, @command{tar --help} will show the
9100 $ @kbd{tar --help | grep -- --bzip2}
9101 -j, --bzip2 filter the archive through lbzip2
9106 which means that running @command{tar --bzip2} will invoke @command{lbzip2}.
9109 @subsection Archiving Sparse Files
9110 @cindex Sparse Files
9112 Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
9113 in a file is a section of the file's contents which was never written.
9114 The contents of a hole reads as all zeros. On many operating systems,
9115 actual disk storage is not allocated for holes, but they are counted
9116 in the length of the file. If you archive such a file, @command{tar}
9117 could create an archive longer than the original. To have @command{tar}
9118 attempt to recognize the holes in a file, use @option{--sparse}
9119 (@option{-S}). When you use this option, then, for any file using
9120 less disk space than would be expected from its length, @command{tar}
9121 searches the file for consecutive stretches of zeros. It then records
9122 in the archive for the file where the consecutive stretches of zeros
9123 are, and only archives the ``real contents'' of the file. On
9124 extraction (using @option{--sparse} is not needed on extraction) any
9125 such files have holes created wherever the continuous stretches of zeros
9126 were found. Thus, if you use @option{--sparse}, @command{tar} archives
9127 won't take more space than the original.
9133 This option instructs @command{tar} to test each file for sparseness
9134 before attempting to archive it. If the file is found to be sparse it
9135 is treated specially, thus allowing to decrease the amount of space
9136 used by its image in the archive.
9138 This option is meaningful only when creating or updating archives. It
9139 has no effect on extraction.
9142 Consider using @option{--sparse} when performing file system backups,
9143 to avoid archiving the expanded forms of files stored sparsely in the
9146 Even if your system has no sparse files currently, some may be
9147 created in the future. If you use @option{--sparse} while making file
9148 system backups as a matter of course, you can be assured the archive
9149 will never take more space on the media than the files take on disk
9150 (otherwise, archiving a disk filled with sparse files might take
9151 hundreds of tapes). @xref{Incremental Dumps}.
9153 However, be aware that @option{--sparse} option presents a serious
9154 drawback. Namely, in order to determine if the file is sparse
9155 @command{tar} has to read it before trying to archive it, so in total
9156 the file is read @strong{twice}. So, always bear in mind that the
9157 time needed to process all files with this option is roughly twice
9158 the time needed to archive them without it.
9159 @FIXME{A technical note:
9161 Programs like @command{dump} do not have to read the entire file; by
9162 examining the file system directly, they can determine in advance
9163 exactly where the holes are and thus avoid reading through them. The
9164 only data it need read are the actual allocated data blocks.
9165 @GNUTAR{} uses a more portable and straightforward
9166 archiving approach, it would be fairly difficult that it does
9167 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
9171 What I did say is that you cannot tell the difference between a hole and an
9172 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
9173 best tells you how many holes there are; it doesn't tell you @emph{where}.
9174 Just as programs may, conceivably, care what @code{st_blocks} is (care
9175 to name one that does?), they may also care where the holes are (I have
9176 no examples of this one either, but it's equally imaginable).
9178 I conclude from this that good archivers are not portable. One can
9179 arguably conclude that if you want a portable program, you can in good
9180 conscience restore files with as many holes as possible, since you can't
9185 @cindex sparse formats, defined
9186 When using @samp{POSIX} archive format, @GNUTAR{} is able to store
9187 sparse files using in three distinct ways, called @dfn{sparse
9188 formats}. A sparse format is identified by its @dfn{number},
9189 consisting, as usual of two decimal numbers, delimited by a dot. By
9190 default, format @samp{1.0} is used. If, for some reason, you wish to
9191 use an earlier format, you can select it using
9192 @option{--sparse-version} option.
9195 @opindex sparse-version
9196 @item --sparse-version=@var{version}
9198 Select the format to store sparse files in. Valid @var{version} values
9199 are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
9200 for a detailed description of each format.
9203 Using @option{--sparse-format} option implies @option{--sparse}.
9206 @section Handling File Attributes
9207 @cindex atrributes, files
9208 @cindex file attributes
9210 When @command{tar} reads files, it updates their access times. To
9211 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
9212 reset the access time retroactively or avoid changing it in the first
9216 @opindex atime-preserve
9217 @item --atime-preserve
9218 @itemx --atime-preserve=replace
9219 @itemx --atime-preserve=system
9220 Preserve the access times of files that are read. This works only for
9221 files that you own, unless you have superuser privileges.
9223 @option{--atime-preserve=replace} works on most systems, but it also
9224 restores the data modification time and updates the status change
9225 time. Hence it doesn't interact with incremental dumps nicely
9226 (@pxref{Incremental Dumps}), and it can set access or data modification times
9227 incorrectly if other programs access the file while @command{tar} is
9230 @option{--atime-preserve=system} avoids changing the access time in
9231 the first place, if the operating system supports this.
9232 Unfortunately, this may or may not work on any given operating system
9233 or file system. If @command{tar} knows for sure it won't work, it
9234 complains right away.
9236 Currently @option{--atime-preserve} with no operand defaults to
9237 @option{--atime-preserve=replace}, but this is intended to change to
9238 @option{--atime-preserve=system} when the latter is better-supported.
9243 Do not extract data modification time.
9245 When this option is used, @command{tar} leaves the data modification times
9246 of the files it extracts as the times when the files were extracted,
9247 instead of setting it to the times recorded in the archive.
9249 This option is meaningless with @option{--list} (@option{-t}).
9253 Create extracted files with the same ownership they have in the
9256 This is the default behavior for the superuser,
9257 so this option is meaningful only for non-root users, when @command{tar}
9258 is executed on those systems able to give files away. This is
9259 considered as a security flaw by many people, at least because it
9260 makes quite difficult to correctly account users for the disk space
9261 they occupy. Also, the @code{suid} or @code{sgid} attributes of
9262 files are easily and silently lost when files are given away.
9264 When writing an archive, @command{tar} writes the user @acronym{ID} and user name
9265 separately. If it can't find a user name (because the user @acronym{ID} is not
9266 in @file{/etc/passwd}), then it does not write one. When restoring,
9267 it tries to look the name (if one was written) up in
9268 @file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
9269 the archive instead.
9271 @opindex no-same-owner
9272 @item --no-same-owner
9274 Do not attempt to restore ownership when extracting. This is the
9275 default behavior for ordinary users, so this option has an effect
9276 only for the superuser.
9278 @opindex numeric-owner
9279 @item --numeric-owner
9280 The @option{--numeric-owner} option allows (ANSI) archives to be written
9281 without user/group name information or such information to be ignored
9282 when extracting. It effectively disables the generation and/or use
9283 of user/group name information. This option forces extraction using
9284 the numeric ids from the archive, ignoring the names.
9286 This is useful in certain circumstances, when restoring a backup from
9287 an emergency floppy with different passwd/group files for example.
9288 It is otherwise impossible to extract files with the right ownerships
9289 if the password file in use during the extraction does not match the
9290 one belonging to the file system(s) being extracted. This occurs,
9291 for example, if you are restoring your files after a major crash and
9292 had booted from an emergency floppy with no password file or put your
9293 disk into another machine to do the restore.
9295 The numeric ids are @emph{always} saved into @command{tar} archives.
9296 The identifying names are added at create time when provided by the
9297 system, unless @option{--format=oldgnu} is used. Numeric ids could be
9298 used when moving archives between a collection of machines using
9299 a centralized management for attribution of numeric ids to users
9300 and groups. This is often made through using the NIS capabilities.
9302 When making a @command{tar} file for distribution to other sites, it
9303 is sometimes cleaner to use a single owner for all files in the
9304 distribution, and nicer to specify the write permission bits of the
9305 files as stored in the archive independently of their actual value on
9306 the file system. The way to prepare a clean distribution is usually
9307 to have some Makefile rule creating a directory, copying all needed
9308 files in that directory, then setting ownership and permissions as
9309 wanted (there are a lot of possible schemes), and only then making a
9310 @command{tar} archive out of this directory, before cleaning
9311 everything out. Of course, we could add a lot of options to
9312 @GNUTAR{} for fine tuning permissions and ownership.
9313 This is not the good way, I think. @GNUTAR{} is
9314 already crowded with options and moreover, the approach just explained
9315 gives you a great deal of control already.
9317 @xopindex{same-permissions, short description}
9318 @xopindex{preserve-permissions, short description}
9320 @itemx --same-permissions
9321 @itemx --preserve-permissions
9322 Extract all protection information.
9324 This option causes @command{tar} to set the modes (access permissions) of
9325 extracted files exactly as recorded in the archive. If this option
9326 is not used, the current @code{umask} setting limits the permissions
9327 on extracted files. This option is by default enabled when
9328 @command{tar} is executed by a superuser.
9331 This option is meaningless with @option{--list} (@option{-t}).
9335 Same as both @option{--same-permissions} and @option{--same-order}.
9337 This option is deprecated, and will be removed in @GNUTAR{} version 1.23.
9342 @section Making @command{tar} Archives More Portable
9344 Creating a @command{tar} archive on a particular system that is meant to be
9345 useful later on many other machines and with other versions of @command{tar}
9346 is more challenging than you might think. @command{tar} archive formats
9347 have been evolving since the first versions of Unix. Many such formats
9348 are around, and are not always compatible with each other. This section
9349 discusses a few problems, and gives some advice about making @command{tar}
9350 archives more portable.
9352 One golden rule is simplicity. For example, limit your @command{tar}
9353 archives to contain only regular files and directories, avoiding
9354 other kind of special files. Do not attempt to save sparse files or
9355 contiguous files as such. Let's discuss a few more problems, in turn.
9357 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
9358 archives and archive labels) in GNU and PAX formats.}
9361 * Portable Names:: Portable Names
9362 * dereference:: Symbolic Links
9363 * hard links:: Hard Links
9364 * old:: Old V7 Archives
9365 * ustar:: Ustar Archives
9366 * gnu:: GNU and old GNU format archives.
9367 * posix:: @acronym{POSIX} archives
9368 * Checksumming:: Checksumming Problems
9369 * Large or Negative Values:: Large files, negative time stamps, etc.
9370 * Other Tars:: How to Extract GNU-Specific Data Using
9371 Other @command{tar} Implementations
9374 @node Portable Names
9375 @subsection Portable Names
9377 Use portable file and member names. A name is portable if it contains
9378 only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
9379 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
9380 contain @samp{/-}. Avoid deep directory nesting. For portability to
9381 old Unix hosts, limit your file name components to 14 characters or
9384 If you intend to have your @command{tar} archives to be read under
9385 MSDOS, you should not rely on case distinction for file names, and you
9386 might use the @acronym{GNU} @command{doschk} program for helping you
9387 further diagnosing illegal MSDOS names, which are even more limited
9391 @subsection Symbolic Links
9392 @cindex File names, using symbolic links
9393 @cindex Symbolic link as file name
9395 @opindex dereference
9396 Normally, when @command{tar} archives a symbolic link, it writes a
9397 block to the archive naming the target of the link. In that way, the
9398 @command{tar} archive is a faithful record of the file system contents.
9399 When @option{--dereference} (@option{-h}) is used with
9400 @option{--create} (@option{-c}), @command{tar} archives the files
9401 symbolic links point to, instead of
9402 the links themselves.
9404 When creating portable archives, use @option{--dereference}
9405 (@option{-h}): some systems do not support
9406 symbolic links, and moreover, your distribution might be unusable if
9407 it contains unresolved symbolic links.
9409 When reading from an archive, the @option{--dereference} (@option{-h})
9410 option causes @command{tar} to follow an already-existing symbolic
9411 link when @command{tar} writes or reads a file named in the archive.
9412 Ordinarily, @command{tar} does not follow such a link, though it may
9413 remove the link before writing a new file. @xref{Dealing with Old
9416 The @option{--dereference} option is unsafe if an untrusted user can
9417 modify directories while @command{tar} is running. @xref{Security}.
9420 @subsection Hard Links
9421 @cindex File names, using hard links
9422 @cindex hard links, dereferencing
9423 @cindex dereferencing hard links
9425 Normally, when @command{tar} archives a hard link, it writes a
9426 block to the archive naming the target of the link (a @samp{1} type
9427 block). In that way, the actual file contents is stored in file only
9428 once. For example, consider the following two files:
9433 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
9434 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
9438 Here, @file{jeden} is a link to @file{one}. When archiving this
9439 directory with a verbose level 2, you will get an output similar to
9443 $ tar cvvf ../archive.tar .
9444 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9445 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9446 hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
9449 The last line shows that, instead of storing two copies of the file,
9450 @command{tar} stored it only once, under the name @file{jeden}, and
9451 stored file @file{one} as a hard link to this file.
9453 It may be important to know that all hard links to the given file are
9454 stored in the archive. For example, this may be necessary for exact
9455 reproduction of the file system. The following option does that:
9458 @xopindex{check-links, described}
9461 Check the number of links dumped for each processed file. If this
9462 number does not match the total number of hard links for the file, print
9466 For example, trying to archive only file @file{jeden} with this option
9467 produces the following diagnostics:
9470 $ tar -c -f ../archive.tar -l jeden
9471 tar: Missing links to 'jeden'.
9474 Although creating special records for hard links helps keep a faithful
9475 record of the file system contents and makes archives more compact, it
9476 may present some difficulties when extracting individual members from
9477 the archive. For example, trying to extract file @file{one} from the
9478 archive created in previous examples produces, in the absense of file
9482 $ tar xf archive.tar ./one
9483 tar: ./one: Cannot hard link to './jeden': No such file or directory
9484 tar: Error exit delayed from previous errors
9487 The reason for this behavior is that @command{tar} cannot seek back in
9488 the archive to the previous member (in this case, @file{one}), to
9489 extract it@footnote{There are plans to fix this in future releases.}.
9490 If you wish to avoid such problems at the cost of a bigger archive,
9491 use the following option:
9494 @xopindex{hard-dereference, described}
9495 @item --hard-dereference
9496 Dereference hard links and store the files they refer to.
9499 For example, trying this option on our two sample files, we get two
9500 copies in the archive, each of which can then be extracted
9501 independently of the other:
9505 $ tar -c -vv -f ../archive.tar --hard-dereference .
9506 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9507 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9508 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
9513 @subsection Old V7 Archives
9514 @cindex Format, old style
9515 @cindex Old style format
9516 @cindex Old style archives
9517 @cindex v7 archive format
9519 Certain old versions of @command{tar} cannot handle additional
9520 information recorded by newer @command{tar} programs. To create an
9521 archive in V7 format (not ANSI), which can be read by these old
9522 versions, specify the @option{--format=v7} option in
9523 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
9524 accepts @option{--portability} or @option{--old-archive} for this
9525 option). When you specify it,
9526 @command{tar} leaves out information about directories, pipes, fifos,
9527 contiguous files, and device files, and specifies file ownership by
9528 group and user IDs instead of group and user names.
9530 When updating an archive, do not use @option{--format=v7}
9531 unless the archive was created using this option.
9533 In most cases, a @emph{new} format archive can be read by an @emph{old}
9534 @command{tar} program without serious trouble, so this option should
9535 seldom be needed. On the other hand, most modern @command{tar}s are
9536 able to read old format archives, so it might be safer for you to
9537 always use @option{--format=v7} for your distributions. Notice,
9538 however, that @samp{ustar} format is a better alternative, as it is
9539 free from many of @samp{v7}'s drawbacks.
9542 @subsection Ustar Archive Format
9544 @cindex ustar archive format
9545 Archive format defined by @acronym{POSIX}.1-1988 specification is called
9546 @code{ustar}. Although it is more flexible than the V7 format, it
9547 still has many restrictions (@pxref{Formats,ustar}, for the detailed
9548 description of @code{ustar} format). Along with V7 format,
9549 @code{ustar} format is a good choice for archives intended to be read
9550 with other implementations of @command{tar}.
9552 To create archive in @code{ustar} format, use @option{--format=ustar}
9553 option in conjunction with the @option{--create} (@option{-c}).
9556 @subsection @acronym{GNU} and old @GNUTAR{} format
9558 @cindex GNU archive format
9559 @cindex Old GNU archive format
9560 @GNUTAR{} was based on an early draft of the
9561 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
9562 @command{tar}, such as the support for file names longer than 100
9563 characters, use portions of the @command{tar} header record which were
9564 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
9565 @acronym{POSIX} have allocated the same parts of the header record for
9566 other purposes. As a result, @GNUTAR{} format is
9567 incompatible with the current @acronym{POSIX} specification, and with
9568 @command{tar} programs that follow it.
9570 In the majority of cases, @command{tar} will be configured to create
9571 this format by default. This will change in future releases, since
9572 we plan to make @samp{POSIX} format the default.
9574 To force creation a @GNUTAR{} archive, use option
9575 @option{--format=gnu}.
9578 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
9580 @cindex POSIX archive format
9581 @cindex PAX archive format
9582 Starting from version 1.14 @GNUTAR{} features full support for
9583 @acronym{POSIX.1-2001} archives.
9585 A @acronym{POSIX} conformant archive will be created if @command{tar}
9586 was given @option{--format=posix} (@option{--format=pax}) option. No
9587 special option is required to read and extract from a @acronym{POSIX}
9591 * PAX keywords:: Controlling Extended Header Keywords.
9595 @subsubsection Controlling Extended Header Keywords
9599 @item --pax-option=@var{keyword-list}
9600 Handle keywords in @acronym{PAX} extended headers. This option is
9601 equivalent to @option{-o} option of the @command{pax} utility.
9604 @var{Keyword-list} is a comma-separated
9605 list of keyword options, each keyword option taking one of
9606 the following forms:
9609 @item delete=@var{pattern}
9610 When used with one of archive-creation commands,
9611 this option instructs @command{tar} to omit from extended header records
9612 that it produces any keywords matching the string @var{pattern}.
9614 When used in extract or list mode, this option instructs tar
9615 to ignore any keywords matching the given @var{pattern} in the extended
9616 header records. In both cases, matching is performed using the pattern
9617 matching notation described in @acronym{POSIX 1003.2}, 3.13
9618 (@pxref{wildcards}). For example:
9621 --pax-option delete=security.*
9624 would suppress security-related information.
9626 @item exthdr.name=@var{string}
9628 This keyword allows user control over the name that is written into the
9629 ustar header blocks for the extended headers. The name is obtained
9630 from @var{string} after making the following substitutions:
9632 @multitable @columnfractions .25 .55
9633 @headitem Meta-character @tab Replaced By
9634 @item %d @tab The directory name of the file, equivalent to the
9635 result of the @command{dirname} utility on the translated file name.
9636 @item %f @tab The name of the file with the directory information
9637 stripped, equivalent to the result of the @command{basename} utility
9638 on the translated file name.
9639 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9640 @item %% @tab A @samp{%} character.
9643 Any other @samp{%} characters in @var{string} produce undefined
9646 If no option @samp{exthdr.name=string} is specified, @command{tar}
9647 will use the following default value:
9653 @item exthdr.mtime=@var{value}
9655 This keyword defines the value of the @samp{mtime} field that
9656 is written into the ustar header blocks for the extended headers.
9657 By default, the @samp{mtime} field is set to the modification time
9658 of the archive member described by that extended headers.
9660 @item globexthdr.name=@var{string}
9661 This keyword allows user control over the name that is written into
9662 the ustar header blocks for global extended header records. The name
9663 is obtained from the contents of @var{string}, after making
9664 the following substitutions:
9666 @multitable @columnfractions .25 .55
9667 @headitem Meta-character @tab Replaced By
9668 @item %n @tab An integer that represents the
9669 sequence number of the global extended header record in the archive,
9671 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9672 @item %% @tab A @samp{%} character.
9675 Any other @samp{%} characters in @var{string} produce undefined results.
9677 If no option @samp{globexthdr.name=string} is specified, @command{tar}
9678 will use the following default value:
9681 $TMPDIR/GlobalHead.%p.%n
9685 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
9686 environment variable. If @var{TMPDIR} is not set, @command{tar}
9689 @item globexthdr.mtime=@var{value}
9691 This keyword defines the value of the @samp{mtime} field that
9692 is written into the ustar header blocks for the global extended headers.
9693 By default, the @samp{mtime} field is set to the time when
9694 @command{tar} was invoked.
9696 @item @var{keyword}=@var{value}
9697 When used with one of archive-creation commands, these keyword/value pairs
9698 will be included at the beginning of the archive in a global extended
9699 header record. When used with one of archive-reading commands,
9700 @command{tar} will behave as if it has encountered these keyword/value
9701 pairs at the beginning of the archive in a global extended header
9704 @item @var{keyword}:=@var{value}
9705 When used with one of archive-creation commands, these keyword/value pairs
9706 will be included as records at the beginning of an extended header for
9707 each file. This is effectively equivalent to @var{keyword}=@var{value}
9708 form except that it creates no global extended header records.
9710 When used with one of archive-reading commands, @command{tar} will
9711 behave as if these keyword/value pairs were included as records at the
9712 end of each extended header; thus, they will override any global or
9713 file-specific extended header record keywords of the same names.
9714 For example, in the command:
9717 tar --format=posix --create \
9718 --file archive --pax-option gname:=user .
9721 the group name will be forced to a new value for all files
9722 stored in the archive.
9725 In any of the forms described above, the @var{value} may be
9726 a string enclosed in curly braces. In that case, the string
9727 between the braces is understood either as a textual time
9728 representation, as described in @ref{Date input formats}, or a name of
9729 the existing file, starting with @samp{/} or @samp{.}. In the latter
9730 case, the modification time of that file is used.
9732 For example, to set all modification times to the current date, you
9733 use the following option:
9736 --pax-option='mtime:=@{now@}'
9739 Note quoting of the option's argument.
9741 @cindex archives, binary equivalent
9742 @cindex binary equivalent archives, creating
9743 As another example, here is the option that ensures that any two
9744 archives created using it, will be binary equivalent if they have the
9748 --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0
9752 @subsection Checksumming Problems
9754 SunOS and HP-UX @command{tar} fail to accept archives created using
9755 @GNUTAR{} and containing non-@acronym{ASCII} file names, that
9756 is, file names having characters with the eight bit set, because they
9757 use signed checksums, while @GNUTAR{} uses unsigned
9758 checksums while creating archives, as per @acronym{POSIX} standards. On
9759 reading, @GNUTAR{} computes both checksums and
9760 accepts any. It is somewhat worrying that a lot of people may go
9761 around doing backup of their files using faulty (or at least
9762 non-standard) software, not learning about it until it's time to
9763 restore their missing files with an incompatible file extractor, or
9766 @GNUTAR{} computes checksums both ways, and accept
9767 any on read, so @acronym{GNU} tar can read Sun tapes even with their
9768 wrong checksums. @GNUTAR{} produces the standard
9769 checksum, however, raising incompatibilities with Sun. That is to
9770 say, @GNUTAR{} has not been modified to
9771 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
9772 I've been told that more recent Sun @command{tar} now read standard
9773 archives, so maybe Sun did a similar patch, after all?
9775 The story seems to be that when Sun first imported @command{tar}
9776 sources on their system, they recompiled it without realizing that
9777 the checksums were computed differently, because of a change in
9778 the default signing of @code{char}'s in their compiler. So they
9779 started computing checksums wrongly. When they later realized their
9780 mistake, they merely decided to stay compatible with it, and with
9781 themselves afterwards. Presumably, but I do not really know, HP-UX
9782 has chosen that their @command{tar} archives to be compatible with Sun's.
9783 The current standards do not favor Sun @command{tar} format. In any
9784 case, it now falls on the shoulders of SunOS and HP-UX users to get
9785 a @command{tar} able to read the good archives they receive.
9787 @node Large or Negative Values
9788 @subsection Large or Negative Values
9789 @cindex large values
9790 @cindex future time stamps
9791 @cindex negative time stamps
9794 The above sections suggest to use @samp{oldest possible} archive
9795 format if in doubt. However, sometimes it is not possible. If you
9796 attempt to archive a file whose metadata cannot be represented using
9797 required format, @GNUTAR{} will print error message and ignore such a
9798 file. You will than have to switch to a format that is able to
9799 handle such values. The format summary table (@pxref{Formats}) will
9802 In particular, when trying to archive files larger than 8GB or with
9803 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
9804 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
9805 @acronym{POSIX} archive formats. When considering which format to
9806 choose, bear in mind that the @acronym{GNU} format uses
9807 two's-complement base-256 notation to store values that do not fit
9808 into standard @acronym{ustar} range. Such archives can generally be
9809 read only by a @GNUTAR{} implementation. Moreover, they sometimes
9810 cannot be correctly restored on another hosts even by @GNUTAR{}. For
9811 example, using two's complement representation for negative time
9812 stamps that assumes a signed 32-bit @code{time_t} generates archives
9813 that are not portable to hosts with differing @code{time_t}
9816 On the other hand, @acronym{POSIX} archives, generally speaking, can
9817 be extracted by any tar implementation that understands older
9818 @acronym{ustar} format. The only exception are files larger than 8GB.
9820 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
9824 @subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations
9826 In previous sections you became acquainted with various quirks
9827 necessary to make your archives portable. Sometimes you may need to
9828 extract archives containing GNU-specific members using some
9829 third-party @command{tar} implementation or an older version of
9830 @GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed,
9831 but if it is for some reason impossible, this section will explain
9832 how to cope without it.
9834 When we speak about @dfn{GNU-specific} members we mean two classes of
9835 them: members split between the volumes of a multi-volume archive and
9836 sparse members. You will be able to always recover such members if
9837 the archive is in PAX format. In addition split members can be
9838 recovered from archives in old GNU format. The following subsections
9839 describe the required procedures in detail.
9842 * Split Recovery:: Members Split Between Volumes
9843 * Sparse Recovery:: Sparse Members
9846 @node Split Recovery
9847 @subsubsection Extracting Members Split Between Volumes
9849 @cindex Mutli-volume archives, extracting using non-GNU tars
9850 If a member is split between several volumes of an old GNU format archive
9851 most third party @command{tar} implementation will fail to extract
9852 it. To extract it, use @command{tarcat} program (@pxref{Tarcat}).
9853 This program is available from
9854 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat.html, @GNUTAR{}
9855 home page}. It concatenates several archive volumes into a single
9856 valid archive. For example, if you have three volumes named from
9857 @file{vol-1.tar} to @file{vol-3.tar}, you can do the following to
9858 extract them using a third-party @command{tar}:
9861 $ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -}
9864 @cindex Mutli-volume archives in PAX format, extracting using non-GNU tars
9865 You could use this approach for most (although not all) PAX
9866 format archives as well. However, extracting split members from a PAX
9867 archive is a much easier task, because PAX volumes are constructed in
9868 such a way that each part of a split member is extracted to a
9869 different file by @command{tar} implementations that are not aware of
9870 GNU extensions. More specifically, the very first part retains its
9871 original name, and all subsequent parts are named using the pattern:
9874 %d/GNUFileParts.%p/%f.%n
9878 where symbols preceeded by @samp{%} are @dfn{macro characters} that
9879 have the following meaning:
9881 @multitable @columnfractions .25 .55
9882 @headitem Meta-character @tab Replaced By
9883 @item %d @tab The directory name of the file, equivalent to the
9884 result of the @command{dirname} utility on its full name.
9885 @item %f @tab The file name of the file, equivalent to the result
9886 of the @command{basename} utility on its full name.
9887 @item %p @tab The process @acronym{ID} of the @command{tar} process that
9888 created the archive.
9889 @item %n @tab Ordinal number of this particular part.
9892 For example, if the file @file{var/longfile} was split during archive
9893 creation between three volumes, and the creator @command{tar} process
9894 had process @acronym{ID} @samp{27962}, then the member names will be:
9898 var/GNUFileParts.27962/longfile.1
9899 var/GNUFileParts.27962/longfile.2
9902 When you extract your archive using a third-party @command{tar}, these
9903 files will be created on your disk, and the only thing you will need
9904 to do to restore your file in its original form is concatenate them in
9905 the proper order, for example:
9910 $ @kbd{cat GNUFileParts.27962/longfile.1 \
9911 GNUFileParts.27962/longfile.2 >> longfile}
9912 $ rm -f GNUFileParts.27962
9916 Notice, that if the @command{tar} implementation you use supports PAX
9917 format archives, it will probably emit warnings about unknown keywords
9918 during extraction. They will look like this:
9923 Unknown extended header keyword 'GNU.volume.filename' ignored.
9924 Unknown extended header keyword 'GNU.volume.size' ignored.
9925 Unknown extended header keyword 'GNU.volume.offset' ignored.
9930 You can safely ignore these warnings.
9932 If your @command{tar} implementation is not PAX-aware, you will get
9933 more warnings and more files generated on your disk, e.g.:
9937 $ @kbd{tar xf vol-1.tar}
9938 var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
9940 Unexpected EOF in archive
9941 $ @kbd{tar xf vol-2.tar}
9942 tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
9943 GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
9944 'x', extracted as normal file
9948 Ignore these warnings. The @file{PaxHeaders.*} directories created
9949 will contain files with @dfn{extended header keywords} describing the
9950 extracted files. You can delete them, unless they describe sparse
9951 members. Read further to learn more about them.
9953 @node Sparse Recovery
9954 @subsubsection Extracting Sparse Members
9956 @cindex sparse files, extracting with non-GNU tars
9957 Any @command{tar} implementation will be able to extract sparse members from a
9958 PAX archive. However, the extracted files will be @dfn{condensed},
9959 i.e., any zero blocks will be removed from them. When we restore such
9960 a condensed file to its original form, by adding zero blocks (or
9961 @dfn{holes}) back to their original locations, we call this process
9962 @dfn{expanding} a compressed sparse file.
9965 To expand a file, you will need a simple auxiliary program called
9966 @command{xsparse}. It is available in source form from
9967 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse.html, @GNUTAR{}
9970 @cindex sparse files v.1.0, extracting with non-GNU tars
9971 Let's begin with archive members in @dfn{sparse format
9972 version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand.
9973 The condensed file will contain both file map and file data, so no
9974 additional data will be needed to restore it. If the original file
9975 name was @file{@var{dir}/@var{name}}, then the condensed file will be
9976 named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where
9977 @var{n} is a decimal number@footnote{Technically speaking, @var{n} is a
9978 @dfn{process @acronym{ID}} of the @command{tar} process which created the
9979 archive (@pxref{PAX keywords}).}.
9981 To expand a version 1.0 file, run @command{xsparse} as follows:
9984 $ @kbd{xsparse @file{cond-file}}
9988 where @file{cond-file} is the name of the condensed file. The utility
9989 will deduce the name for the resulting expanded file using the
9990 following algorithm:
9993 @item If @file{cond-file} does not contain any directories,
9994 @file{../cond-file} will be used;
9996 @item If @file{cond-file} has the form
9997 @file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name}
9998 are simple names, with no @samp{/} characters in them, the output file
9999 name will be @file{@var{dir}/@var{name}}.
10001 @item Otherwise, if @file{cond-file} has the form
10002 @file{@var{dir}/@var{name}}, the output file name will be
10006 In the unlikely case when this algorithm does not suit your needs,
10007 you can explicitly specify output file name as a second argument to
10011 $ @kbd{xsparse @file{cond-file} @file{out-file}}
10014 It is often a good idea to run @command{xsparse} in @dfn{dry run} mode
10015 first. In this mode, the command does not actually expand the file,
10016 but verbosely lists all actions it would be taking to do so. The dry
10017 run mode is enabled by @option{-n} command line argument:
10021 $ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile}
10022 Reading v.1.0 sparse map
10023 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10024 '/home/gray/sparsefile'
10029 To actually expand the file, you would run:
10032 $ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile}
10036 The program behaves the same way all UNIX utilities do: it will keep
10037 quiet unless it has simething important to tell you (e.g. an error
10038 condition or something). If you wish it to produce verbose output,
10039 similar to that from the dry run mode, use @option{-v} option:
10043 $ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile}
10044 Reading v.1.0 sparse map
10045 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10046 '/home/gray/sparsefile'
10051 Additionally, if your @command{tar} implementation has extracted the
10052 @dfn{extended headers} for this file, you can instruct @command{xstar}
10053 to use them in order to verify the integrity of the expanded file.
10054 The option @option{-x} sets the name of the extended header file to
10055 use. Continuing our example:
10059 $ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
10060 /home/gray/GNUSparseFile.6058/sparsefile}
10061 Reading extended header file
10062 Found variable GNU.sparse.major = 1
10063 Found variable GNU.sparse.minor = 0
10064 Found variable GNU.sparse.name = sparsefile
10065 Found variable GNU.sparse.realsize = 217481216
10066 Reading v.1.0 sparse map
10067 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10068 '/home/gray/sparsefile'
10073 @anchor{extracting sparse v.0.x}
10074 @cindex sparse files v.0.1, extracting with non-GNU tars
10075 @cindex sparse files v.0.0, extracting with non-GNU tars
10076 An @dfn{extended header} is a special @command{tar} archive header
10077 that precedes an archive member and contains a set of
10078 @dfn{variables}, describing the member properties that cannot be
10079 stored in the standard @code{ustar} header. While optional for
10080 expanding sparse version 1.0 members, the use of extended headers is
10081 mandatory when expanding sparse members in older sparse formats: v.0.0
10082 and v.0.1 (The sparse formats are described in detail in @ref{Sparse
10083 Formats}.) So, for these formats, the question is: how to obtain
10084 extended headers from the archive?
10086 If you use a @command{tar} implementation that does not support PAX
10087 format, extended headers for each member will be extracted as a
10088 separate file. If we represent the member name as
10089 @file{@var{dir}/@var{name}}, then the extended header file will be
10090 named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where
10091 @var{n} is an integer number.
10093 Things become more difficult if your @command{tar} implementation
10094 does support PAX headers, because in this case you will have to
10095 manually extract the headers. We recommend the following algorithm:
10099 Consult the documentation of your @command{tar} implementation for an
10100 option that prints @dfn{block numbers} along with the archive
10101 listing (analogous to @GNUTAR{}'s @option{-R} option). For example,
10102 @command{star} has @option{-block-number}.
10105 Obtain verbose listing using the @samp{block number} option, and
10106 find block numbers of the sparse member in question and the member
10107 immediately following it. For example, running @command{star} on our
10112 $ @kbd{star -t -v -block-number -f arc.tar}
10114 star: Unknown extended header keyword 'GNU.sparse.size' ignored.
10115 star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
10116 star: Unknown extended header keyword 'GNU.sparse.name' ignored.
10117 star: Unknown extended header keyword 'GNU.sparse.map' ignored.
10118 block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
10119 block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
10125 (as usual, ignore the warnings about unknown keywords.)
10128 Let @var{size} be the size of the sparse member, @var{Bs} be its block number
10129 and @var{Bn} be the block number of the next member.
10133 @var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2
10137 This number gives the size of the extended header part in tar @dfn{blocks}.
10138 In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2
10142 Use @command{dd} to extract the headers:
10145 @kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}}
10149 where @var{archive} is the archive name, @var{hname} is a name of the
10150 file to store the extended header in, @var{Bs} and @var{N} are
10151 computed in previous steps.
10153 In our example, this command will be
10156 $ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7}
10160 Finally, you can expand the condensed file, using the obtained header:
10164 $ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile}
10165 Reading extended header file
10166 Found variable GNU.sparse.size = 217481216
10167 Found variable GNU.sparse.numblocks = 208
10168 Found variable GNU.sparse.name = sparsefile
10169 Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{}
10170 Expanding file 'GNUSparseFile.28124/sparsefile' to 'sparsefile'
10176 @section Comparison of @command{tar} and @command{cpio}
10179 @FIXME{Reorganize the following material}
10181 The @command{cpio} archive formats, like @command{tar}, do have maximum
10182 file name lengths. The binary and old @acronym{ASCII} formats have a maximum file
10183 length of 256, and the new @acronym{ASCII} and @acronym{CRC ASCII} formats have a max
10184 file length of 1024. @acronym{GNU} @command{cpio} can read and write archives
10185 with arbitrary file name lengths, but other @command{cpio} implementations
10186 may crash unexplainedly trying to read them.
10188 @command{tar} handles symbolic links in the form in which it comes in @acronym{BSD};
10189 @command{cpio} doesn't handle symbolic links in the form in which it comes
10190 in System V prior to SVR4, and some vendors may have added symlinks
10191 to their system without enhancing @command{cpio} to know about them.
10192 Others may have enhanced it in a way other than the way I did it
10193 at Sun, and which was adopted by AT&T (and which is, I think, also
10194 present in the @command{cpio} that Berkeley picked up from AT&T and put
10195 into a later @acronym{BSD} release---I think I gave them my changes).
10197 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
10198 can handle @command{tar} format input, and write it on output, and it
10199 probably handles symbolic links. They may not have bothered doing
10200 anything to enhance @command{tar} as a result.)
10202 @command{cpio} handles special files; traditional @command{tar} doesn't.
10204 @command{tar} comes with V7, System III, System V, and @acronym{BSD} source;
10205 @command{cpio} comes only with System III, System V, and later @acronym{BSD}
10206 (4.3-tahoe and later).
10208 @command{tar}'s way of handling multiple hard links to a file can handle
10209 file systems that support 32-bit i-numbers (e.g., the @acronym{BSD} file system);
10210 @command{cpio}s way requires you to play some games (in its ``binary''
10211 format, i-numbers are only 16 bits, and in its ``portable @acronym{ASCII}'' format,
10212 they're 18 bits---it would have to play games with the "file system @acronym{ID}"
10213 field of the header to make sure that the file system @acronym{ID}/i-number pairs
10214 of different files were always different), and I don't know which
10215 @command{cpio}s, if any, play those games. Those that don't might get
10216 confused and think two files are the same file when they're not, and
10217 make hard links between them.
10219 @command{tar}s way of handling multiple hard links to a file places only
10220 one copy of the link on the tape, but the name attached to that copy
10221 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
10222 way puts one copy for every link, but you can retrieve it using any
10226 What type of check sum (if any) is used, and how is this calculated.
10229 See the attached manual pages for @command{tar} and @command{cpio} format.
10230 @command{tar} uses a checksum which is the sum of all the bytes in the
10231 @command{tar} header for a file; @command{cpio} uses no checksum.
10234 If anyone knows why @command{cpio} was made when @command{tar} was present
10238 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
10239 generally-available version of UNIX had @command{tar} at the time. I don't
10240 know whether any version that was generally available @emph{within AT&T}
10241 had @command{tar}, or, if so, whether the people within AT&T who did
10242 @command{cpio} knew about it.
10244 On restore, if there is a corruption on a tape @command{tar} will stop at
10245 that point, while @command{cpio} will skip over it and try to restore the
10248 The main difference is just in the command syntax and header format.
10250 @command{tar} is a little more tape-oriented in that everything is blocked
10251 to start on a record boundary.
10254 Is there any differences between the ability to recover crashed
10255 archives between the two of them. (Is there any chance of recovering
10256 crashed archives at all.)
10259 Theoretically it should be easier under @command{tar} since the blocking
10260 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
10261 However, modern @command{cpio}'s and variations have an option to just
10262 search for the next file header after an error with a reasonable chance
10263 of resyncing. However, lots of tape driver software won't allow you to
10264 continue past a media error which should be the only reason for getting
10265 out of sync unless a file changed sizes while you were writing the
10269 If anyone knows why @command{cpio} was made when @command{tar} was present
10270 at the unix scene, please tell me about this too.
10273 Probably because it is more media efficient (by not blocking everything
10274 and using only the space needed for the headers where @command{tar}
10275 always uses 512 bytes per file header) and it knows how to archive
10278 You might want to look at the freely available alternatives. The
10279 major ones are @command{afio}, @GNUTAR{}, and
10280 @command{pax}, each of which have their own extensions with some
10281 backwards compatibility.
10283 Sparse files were @command{tar}red as sparse files (which you can
10284 easily test, because the resulting archive gets smaller, and
10285 @acronym{GNU} @command{cpio} can no longer read it).
10288 @chapter Tapes and Other Archive Media
10291 A few special cases about tape handling warrant more detailed
10292 description. These special cases are discussed below.
10294 Many complexities surround the use of @command{tar} on tape drives. Since
10295 the creation and manipulation of archives located on magnetic tape was
10296 the original purpose of @command{tar}, it contains many features making
10297 such manipulation easier.
10299 Archives are usually written on dismountable media---tape cartridges,
10300 mag tapes, or floppy disks.
10302 The amount of data a tape or disk holds depends not only on its size,
10303 but also on how it is formatted. A 2400 foot long reel of mag tape
10304 holds 40 megabytes of data when formatted at 1600 bits per inch. The
10305 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
10307 Magnetic media are re-usable---once the archive on a tape is no longer
10308 needed, the archive can be erased and the tape or disk used over.
10309 Media quality does deteriorate with use, however. Most tapes or disks
10310 should be discarded when they begin to produce data errors. EXABYTE
10311 tape cartridges should be discarded when they generate an @dfn{error
10312 count} (number of non-usable bits) of more than 10k.
10314 Magnetic media are written and erased using magnetic fields, and
10315 should be protected from such fields to avoid damage to stored data.
10316 Sticking a floppy disk to a filing cabinet using a magnet is probably
10320 * Device:: Device selection and switching
10321 * Remote Tape Server::
10322 * Common Problems and Solutions::
10323 * Blocking:: Blocking
10324 * Many:: Many archives on one tape
10325 * Using Multiple Tapes:: Using Multiple Tapes
10326 * label:: Including a Label in the Archive
10328 * Write Protection::
10332 @section Device Selection and Switching
10336 @item -f [@var{hostname}:]@var{file}
10337 @itemx --file=[@var{hostname}:]@var{file}
10338 Use archive file or device @var{file} on @var{hostname}.
10341 This option is used to specify the file name of the archive @command{tar}
10344 If the file name is @samp{-}, @command{tar} reads the archive from standard
10345 input (when listing or extracting), or writes it to standard output
10346 (when creating). If the @samp{-} file name is given when updating an
10347 archive, @command{tar} will read the original archive from its standard
10348 input, and will write the entire new archive to its standard output.
10350 If the file name contains a @samp{:}, it is interpreted as
10351 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
10352 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
10353 either case, @command{tar} will invoke the command @command{rsh} (or
10354 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
10355 machine. If you give an alternate login name, it will be given to the
10357 Naturally, the remote machine must have an executable
10358 @command{/usr/libexec/rmt}. This program is free software from the
10359 University of California, and a copy of the source code can be found
10360 with the sources for @command{tar}; it's compiled and installed by default.
10361 The exact path to this utility is determined when configuring the package.
10362 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
10363 your installation prefix. This location may also be overridden at
10364 runtime by using the @option{--rmt-command=@var{command}} option (@xref{Option Summary,
10365 ---rmt-command}, for detailed description of this option. @xref{Remote
10366 Tape Server}, for the description of @command{rmt} command).
10368 If this option is not given, but the environment variable @env{TAPE}
10369 is set, its value is used; otherwise, old versions of @command{tar}
10370 used a default archive name (which was picked when @command{tar} was
10371 compiled). The default is normally set up to be the @dfn{first} tape
10372 drive or other transportable I/O medium on the system.
10374 Starting with version 1.11.5, @GNUTAR{} uses
10375 standard input and standard output as the default device, and I will
10376 not try anymore supporting automatic device detection at installation
10377 time. This was failing really in too many cases, it was hopeless.
10378 This is now completely left to the installer to override standard
10379 input and standard output for default device, if this seems
10380 preferable. Further, I think @emph{most} actual usages of
10381 @command{tar} are done with pipes or disks, not really tapes,
10382 cartridges or diskettes.
10384 Some users think that using standard input and output is running
10385 after trouble. This could lead to a nasty surprise on your screen if
10386 you forget to specify an output file name---especially if you are going
10387 through a network or terminal server capable of buffering large amounts
10388 of output. We had so many bug reports in that area of configuring
10389 default tapes automatically, and so many contradicting requests, that
10390 we finally consider the problem to be portably intractable. We could
10391 of course use something like @samp{/dev/tape} as a default, but this
10392 is @emph{also} running after various kind of trouble, going from hung
10393 processes to accidental destruction of real tapes. After having seen
10394 all this mess, using standard input and output as a default really
10395 sounds like the only clean choice left, and a very useful one too.
10397 @GNUTAR{} reads and writes archive in records, I
10398 suspect this is the main reason why block devices are preferred over
10399 character devices. Most probably, block devices are more efficient
10400 too. The installer could also check for @samp{DEFTAPE} in
10401 @file{<sys/mtio.h>}.
10404 @xopindex{force-local, short description}
10405 @item --force-local
10406 Archive file is local even if it contains a colon.
10408 @opindex rsh-command
10409 @item --rsh-command=@var{command}
10410 Use remote @var{command} instead of @command{rsh}. This option exists
10411 so that people who use something other than the standard @command{rsh}
10412 (e.g., a Kerberized @command{rsh}) can access a remote device.
10414 When this command is not used, the shell command found when
10415 the @command{tar} program was installed is used instead. This is
10416 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
10417 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
10418 The installer may have overridden this by defining the environment
10419 variable @env{RSH} @emph{at installation time}.
10422 Specify drive and density.
10424 @xopindex{multi-volume, short description}
10426 @itemx --multi-volume
10427 Create/list/extract multi-volume archive.
10429 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
10430 that may be larger than will fit on the medium used to hold it.
10431 @xref{Multi-Volume Archives}.
10433 @xopindex{tape-length, short description}
10435 @itemx --tape-length=@var{size}[@var{suf}]
10436 Change tape after writing @var{size} units of data. Unless @var{suf} is
10437 given, @var{size} is treated as kilobytes, i.e. @samp{@var{size} x
10438 1024} bytes. The following suffixes alter this behavior:
10440 @float Table, size-suffixes
10441 @caption{Size Suffixes}
10442 @multitable @columnfractions 0.2 0.3 0.3
10443 @headitem Suffix @tab Units @tab Byte Equivalent
10444 @item b @tab Blocks @tab @var{size} x 512
10445 @item B @tab Kilobytes @tab @var{size} x 1024
10446 @item c @tab Bytes @tab @var{size}
10447 @item G @tab Gigabytes @tab @var{size} x 1024^3
10448 @item K @tab Kilobytes @tab @var{size} x 1024
10449 @item k @tab Kilobytes @tab @var{size} x 1024
10450 @item M @tab Megabytes @tab @var{size} x 1024^2
10451 @item P @tab Petabytes @tab @var{size} x 1024^5
10452 @item T @tab Terabytes @tab @var{size} x 1024^4
10453 @item w @tab Words @tab @var{size} x 2
10457 This option might be useful when your tape drivers do not properly
10458 detect end of physical tapes. By being slightly conservative on the
10459 maximum tape length, you might avoid the problem entirely.
10461 @xopindex{info-script, short description}
10462 @xopindex{new-volume-script, short description}
10463 @item -F @var{file}
10464 @itemx --info-script=@var{file}
10465 @itemx --new-volume-script=@var{file}
10466 Execute @file{file} at end of each tape. This implies
10467 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
10468 description of this option.
10471 @node Remote Tape Server
10472 @section Remote Tape Server
10474 @cindex remote tape drive
10476 In order to access the tape drive on a remote machine, @command{tar}
10477 uses the remote tape server written at the University of California at
10478 Berkeley. The remote tape server must be installed as
10479 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
10480 want to use. @command{tar} calls @command{rmt} by running an
10481 @command{rsh} or @command{remsh} to the remote machine, optionally
10482 using a different login name if one is supplied.
10484 A copy of the source for the remote tape server is provided. It is
10485 Copyright @copyright{} 1983 by the Regents of the University of
10486 California, but can be freely distributed. It is compiled and
10487 installed by default.
10489 @cindex absolute file names
10490 Unless you use the @option{--absolute-names} (@option{-P}) option,
10491 @GNUTAR{} will not allow you to create an archive that contains
10492 absolute file names (a file name beginning with @samp{/}.) If you try,
10493 @command{tar} will automatically remove the leading @samp{/} from the
10494 file names it stores in the archive. It will also type a warning
10495 message telling you what it is doing.
10497 When reading an archive that was created with a different
10498 @command{tar} program, @GNUTAR{} automatically
10499 extracts entries in the archive which have absolute file names as if
10500 the file names were not absolute. This is an important feature. A
10501 visitor here once gave a @command{tar} tape to an operator to restore;
10502 the operator used Sun @command{tar} instead of @GNUTAR{},
10503 and the result was that it replaced large portions of
10504 our @file{/bin} and friends with versions from the tape; needless to
10505 say, we were unhappy about having to recover the file system from
10508 For example, if the archive contained a file @file{/usr/bin/computoy},
10509 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
10510 relative to the current directory. If you want to extract the files in
10511 an archive to the same absolute names that they had when the archive
10512 was created, you should do a @samp{cd /} before extracting the files
10513 from the archive, or you should either use the @option{--absolute-names}
10514 option, or use the command @samp{tar -C / @dots{}}.
10516 @cindex Ultrix 3.1 and write failure
10517 Some versions of Unix (Ultrix 3.1 is known to have this problem),
10518 can claim that a short write near the end of a tape succeeded,
10519 when it actually failed. This will result in the -M option not
10520 working correctly. The best workaround at the moment is to use a
10521 significantly larger blocking factor than the default 20.
10523 In order to update an archive, @command{tar} must be able to backspace the
10524 archive in order to reread or rewrite a record that was just read (or
10525 written). This is currently possible only on two kinds of files: normal
10526 disk files (or any other file that can be backspaced with @samp{lseek}),
10527 and industry-standard 9-track magnetic tape (or any other kind of tape
10528 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}).
10530 This means that the @option{--append}, @option{--concatenate}, and
10531 @option{--delete} commands will not work on any other kind of file.
10532 Some media simply cannot be backspaced, which means these commands and
10533 options will never be able to work on them. These non-backspacing
10534 media include pipes and cartridge tape drives.
10536 Some other media can be backspaced, and @command{tar} will work on them
10537 once @command{tar} is modified to do so.
10539 Archives created with the @option{--multi-volume}, @option{--label}, and
10540 @option{--incremental} (@option{-G}) options may not be readable by other version
10541 of @command{tar}. In particular, restoring a file that was split over
10542 a volume boundary will require some careful work with @command{dd}, if
10543 it can be done at all. Other versions of @command{tar} may also create
10544 an empty file whose name is that of the volume header. Some versions
10545 of @command{tar} may create normal files instead of directories archived
10546 with the @option{--incremental} (@option{-G}) option.
10548 @node Common Problems and Solutions
10549 @section Some Common Problems and their Solutions
10554 errors from system:
10556 no such file or directory
10559 errors from @command{tar}:
10560 directory checksum error
10561 header format error
10563 errors from media/system:
10575 @dfn{Block} and @dfn{record} terminology is rather confused, and it
10576 is also confusing to the expert reader. On the other hand, readers
10577 who are new to the field have a fresh mind, and they may safely skip
10578 the next two paragraphs, as the remainder of this manual uses those
10579 two terms in a quite consistent way.
10581 John Gilmore, the writer of the public domain @command{tar} from which
10582 @GNUTAR{} was originally derived, wrote (June 1995):
10585 The nomenclature of tape drives comes from IBM, where I believe
10586 they were invented for the IBM 650 or so. On IBM mainframes, what
10587 is recorded on tape are tape blocks. The logical organization of
10588 data is into records. There are various ways of putting records into
10589 blocks, including @code{F} (fixed sized records), @code{V} (variable
10590 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
10591 to a block), @code{VB} (variable size records, @var{n} to a block),
10592 @code{VSB} (variable spanned blocked: variable sized records that can
10593 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
10594 parameter specified this to the operating system.
10596 The Unix man page on @command{tar} was totally confused about this.
10597 When I wrote @code{PD TAR}, I used the historically correct terminology
10598 (@command{tar} writes data records, which are grouped into blocks).
10599 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
10600 here), and now Fran@,{c}ois has migrated that terminology back
10601 into the source code too.
10604 The term @dfn{physical block} means the basic transfer chunk from or
10605 to a device, after which reading or writing may stop without anything
10606 being lost. In this manual, the term @dfn{block} usually refers to
10607 a disk physical block, @emph{assuming} that each disk block is 512
10608 bytes in length. It is true that some disk devices have different
10609 physical blocks, but @command{tar} ignore these differences in its own
10610 format, which is meant to be portable, so a @command{tar} block is always
10611 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
10612 The term @dfn{logical block} often represents the basic chunk of
10613 allocation of many disk blocks as a single entity, which the operating
10614 system treats somewhat atomically; this concept is only barely used
10617 The term @dfn{physical record} is another way to speak of a physical
10618 block, those two terms are somewhat interchangeable. In this manual,
10619 the term @dfn{record} usually refers to a tape physical block,
10620 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
10621 It is true that archives may be put on disk or used with pipes,
10622 but nevertheless, @command{tar} tries to read and write the archive one
10623 @dfn{record} at a time, whatever the medium in use. One record is made
10624 up of an integral number of blocks, and this operation of putting many
10625 disk blocks into a single tape block is called @dfn{reblocking}, or
10626 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
10627 the logical organization of many characters into something meaningful
10628 to the application. The term @dfn{unit record} describes a small set
10629 of characters which are transmitted whole to or by the application,
10630 and often refers to a line of text. Those two last terms are unrelated
10631 to what we call a @dfn{record} in @GNUTAR{}.
10633 When writing to tapes, @command{tar} writes the contents of the archive
10634 in chunks known as @dfn{records}. To change the default blocking
10635 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
10636 @var{512-size}}) option. Each record will then be composed of
10637 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
10638 @xref{Standard}.) Each file written to the archive uses at least one
10639 full record. As a result, using a larger record size can result in
10640 more wasted space for small files. On the other hand, a larger record
10641 size can often be read and written much more efficiently.
10643 Further complicating the problem is that some tape drives ignore the
10644 blocking entirely. For these, a larger record size can still improve
10645 performance (because the software layers above the tape drive still
10646 honor the blocking), but not as dramatically as on tape drives that
10649 When reading an archive, @command{tar} can usually figure out the
10650 record size on itself. When this is the case, and a non-standard
10651 record size was used when the archive was created, @command{tar} will
10652 print a message about a non-standard blocking factor, and then operate
10653 normally. On some tape devices, however, @command{tar} cannot figure
10654 out the record size itself. On most of those, you can specify a
10655 blocking factor (with @option{--blocking-factor}) larger than the
10656 actual blocking factor, and then use the @option{--read-full-records}
10657 (@option{-B}) option. (If you specify a blocking factor with
10658 @option{--blocking-factor} and don't use the
10659 @option{--read-full-records} option, then @command{tar} will not
10660 attempt to figure out the recording size itself.) On some devices,
10661 you must always specify the record size exactly with
10662 @option{--blocking-factor} when reading, because @command{tar} cannot
10663 figure it out. In any case, use @option{--list} (@option{-t}) before
10664 doing any extractions to see whether @command{tar} is reading the archive
10667 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
10668 putting them into records is to put a whole number of them (one or
10669 more) into each record. @command{tar} records are all the same size;
10670 at the end of the file there's a block containing all zeros, which
10671 is how you tell that the remainder of the last record(s) are garbage.
10673 In a standard @command{tar} file (no options), the block size is 512
10674 and the record size is 10240, for a blocking factor of 20. What the
10675 @option{--blocking-factor} option does is sets the blocking factor,
10676 changing the record size while leaving the block size at 512 bytes.
10677 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
10678 most tape drives these days prefer much bigger records in order to
10679 stream and not waste tape. When writing tapes for myself, some tend
10680 to use a factor of the order of 2048, say, giving a record size of
10681 around one megabyte.
10683 If you use a blocking factor larger than 20, older @command{tar}
10684 programs might not be able to read the archive, so we recommend this
10685 as a limit to use in practice. @GNUTAR{}, however,
10686 will support arbitrarily large record sizes, limited only by the
10687 amount of virtual memory or the physical characteristics of the tape
10691 * Format Variations:: Format Variations
10692 * Blocking Factor:: The Blocking Factor of an Archive
10695 @node Format Variations
10696 @subsection Format Variations
10697 @cindex Format Parameters
10698 @cindex Format Options
10699 @cindex Options, archive format specifying
10700 @cindex Options, format specifying
10703 Format parameters specify how an archive is written on the archive
10704 media. The best choice of format parameters will vary depending on
10705 the type and number of files being archived, and on the media used to
10708 To specify format parameters when accessing or creating an archive,
10709 you can use the options described in the following sections.
10710 If you do not specify any format parameters, @command{tar} uses
10711 default parameters. You cannot modify a compressed archive.
10712 If you create an archive with the @option{--blocking-factor} option
10713 specified (@pxref{Blocking Factor}), you must specify that
10714 blocking-factor when operating on the archive. @xref{Formats}, for other
10715 examples of format parameter considerations.
10717 @node Blocking Factor
10718 @subsection The Blocking Factor of an Archive
10719 @cindex Blocking Factor
10720 @cindex Record Size
10721 @cindex Number of blocks per record
10722 @cindex Number of bytes per record
10723 @cindex Bytes per record
10724 @cindex Blocks per record
10727 @opindex blocking-factor
10728 The data in an archive is grouped into blocks, which are 512 bytes.
10729 Blocks are read and written in whole number multiples called
10730 @dfn{records}. The number of blocks in a record (i.e., the size of a
10731 record in units of 512 bytes) is called the @dfn{blocking factor}.
10732 The @option{--blocking-factor=@var{512-size}} (@option{-b
10733 @var{512-size}}) option specifies the blocking factor of an archive.
10734 The default blocking factor is typically 20 (i.e., 10240 bytes), but
10735 can be specified at installation. To find out the blocking factor of
10736 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
10737 This may not work on some devices.
10739 Records are separated by gaps, which waste space on the archive media.
10740 If you are archiving on magnetic tape, using a larger blocking factor
10741 (and therefore larger records) provides faster throughput and allows you
10742 to fit more data on a tape (because there are fewer gaps). If you are
10743 archiving on cartridge, a very large blocking factor (say 126 or more)
10744 greatly increases performance. A smaller blocking factor, on the other
10745 hand, may be useful when archiving small files, to avoid archiving lots
10746 of nulls as @command{tar} fills out the archive to the end of the record.
10747 In general, the ideal record size depends on the size of the
10748 inter-record gaps on the tape you are using, and the average size of the
10749 files you are archiving. @xref{create}, for information on
10752 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
10754 Archives with blocking factors larger than 20 cannot be read
10755 by very old versions of @command{tar}, or by some newer versions
10756 of @command{tar} running on old machines with small address spaces.
10757 With @GNUTAR{}, the blocking factor of an archive is limited
10758 only by the maximum record size of the device containing the archive,
10759 or by the amount of available virtual memory.
10761 Also, on some systems, not using adequate blocking factors, as sometimes
10762 imposed by the device drivers, may yield unexpected diagnostics. For
10763 example, this has been reported:
10766 Cannot write to /dev/dlt: Invalid argument
10770 In such cases, it sometimes happen that the @command{tar} bundled by
10771 the system is aware of block size idiosyncrasies, while @GNUTAR{}
10772 requires an explicit specification for the block size,
10773 which it cannot guess. This yields some people to consider
10774 @GNUTAR{} is misbehaving, because by comparison,
10775 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
10776 for example, might resolve the problem.
10778 If you use a non-default blocking factor when you create an archive, you
10779 must specify the same blocking factor when you modify that archive. Some
10780 archive devices will also require you to specify the blocking factor when
10781 reading that archive, however this is not typically the case. Usually, you
10782 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
10783 reports a non-default record size and then lists the archive members as
10784 it would normally. To extract files from an archive with a non-standard
10785 blocking factor (particularly if you're not sure what the blocking factor
10786 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
10787 specifying a blocking factor larger then the blocking factor of the archive
10788 (i.e., @samp{tar --extract --read-full-records --blocking-factor=300}).
10789 @xref{list}, for more information on the @option{--list} (@option{-t})
10790 operation. @xref{Reading}, for a more detailed explanation of that option.
10793 @item --blocking-factor=@var{number}
10794 @itemx -b @var{number}
10795 Specifies the blocking factor of an archive. Can be used with any
10796 operation, but is usually not necessary with @option{--list} (@option{-t}).
10802 @item -b @var{blocks}
10803 @itemx --blocking-factor=@var{blocks}
10804 Set record size to @math{@var{blocks}*512} bytes.
10806 This option is used to specify a @dfn{blocking factor} for the archive.
10807 When reading or writing the archive, @command{tar}, will do reads and writes
10808 of the archive in records of @math{@var{block}*512} bytes. This is true
10809 even when the archive is compressed. Some devices requires that all
10810 write operations be a multiple of a certain size, and so, @command{tar}
10811 pads the archive out to the next record boundary.
10813 The default blocking factor is set when @command{tar} is compiled, and is
10814 typically 20. Blocking factors larger than 20 cannot be read by very
10815 old versions of @command{tar}, or by some newer versions of @command{tar}
10816 running on old machines with small address spaces.
10818 With a magnetic tape, larger records give faster throughput and fit
10819 more data on a tape (because there are fewer inter-record gaps).
10820 If the archive is in a disk file or a pipe, you may want to specify
10821 a smaller blocking factor, since a large one will result in a large
10822 number of null bytes at the end of the archive.
10824 When writing cartridge or other streaming tapes, a much larger
10825 blocking factor (say 126 or more) will greatly increase performance.
10826 However, you must specify the same blocking factor when reading or
10827 updating the archive.
10829 Apparently, Exabyte drives have a physical block size of 8K bytes.
10830 If we choose our blocksize as a multiple of 8k bytes, then the problem
10831 seems to disappear. Id est, we are using block size of 112 right
10832 now, and we haven't had the problem since we switched@dots{}
10834 With @GNUTAR{} the blocking factor is limited only
10835 by the maximum record size of the device containing the archive, or by
10836 the amount of available virtual memory.
10838 However, deblocking or reblocking is virtually avoided in a special
10839 case which often occurs in practice, but which requires all the
10840 following conditions to be simultaneously true:
10843 the archive is subject to a compression option,
10845 the archive is not handled through standard input or output, nor
10846 redirected nor piped,
10848 the archive is directly handled to a local disk, instead of any special
10851 @option{--blocking-factor} is not explicitly specified on the @command{tar}
10855 If the output goes directly to a local disk, and not through
10856 stdout, then the last write is not extended to a full record size.
10857 Otherwise, reblocking occurs. Here are a few other remarks on this
10863 @command{gzip} will complain about trailing garbage if asked to
10864 uncompress a compressed archive on tape, there is an option to turn
10865 the message off, but it breaks the regularity of simply having to use
10866 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
10867 silently ignoring any number of trailing zeros. I'll ask Jean-loup
10868 Gailly, by sending a copy of this message to him.
10871 @command{compress} does not show this problem, but as Jean-loup pointed
10872 out to Michael, @samp{compress -d} silently adds garbage after
10873 the result of decompression, which tar ignores because it already
10874 recognized its end-of-file indicator. So this bug may be safely
10878 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
10879 but will still return an exit status of 2 which tar reports in turn.
10880 @command{tar} might ignore the exit status returned, but I hate doing
10881 that, as it weakens the protection @command{tar} offers users against
10882 other possible problems at decompression time. If @command{gzip} was
10883 silently skipping trailing zeros @emph{and} also avoiding setting the
10884 exit status in this innocuous case, that would solve this situation.
10887 @command{tar} should become more solid at not stopping to read a pipe at
10888 the first null block encountered. This inelegantly breaks the pipe.
10889 @command{tar} should rather drain the pipe out before exiting itself.
10892 @xopindex{ignore-zeros, short description}
10894 @itemx --ignore-zeros
10895 Ignore blocks of zeros in archive (means EOF).
10897 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
10898 of zeros in the archive. Normally a block of zeros indicates the
10899 end of the archive, but when reading a damaged archive, or one which
10900 was created by concatenating several archives together, this option
10901 allows @command{tar} to read the entire archive. This option is not on
10902 by default because many versions of @command{tar} write garbage after
10905 Note that this option causes @command{tar} to read to the end of the
10906 archive file, which may sometimes avoid problems when multiple files
10907 are stored on a single physical tape.
10909 @xopindex{read-full-records, short description}
10911 @itemx --read-full-records
10912 Reblock as we read (for reading 4.2@acronym{BSD} pipes).
10914 If @option{--read-full-records} is used, @command{tar}
10915 will not panic if an attempt to read a record from the archive does
10916 not return a full record. Instead, @command{tar} will keep reading
10917 until it has obtained a full
10920 This option is turned on by default when @command{tar} is reading
10921 an archive from standard input, or from a remote machine. This is
10922 because on @acronym{BSD} Unix systems, a read of a pipe will return however
10923 much happens to be in the pipe, even if it is less than @command{tar}
10924 requested. If this option was not used, @command{tar} would fail as
10925 soon as it read an incomplete record from the pipe.
10927 This option is also useful with the commands for updating an archive.
10933 @FIXME{Appropriate options should be moved here from elsewhere.}
10935 @cindex blocking factor
10936 @cindex tape blocking
10938 When handling various tapes or cartridges, you have to take care of
10939 selecting a proper blocking, that is, the number of disk blocks you
10940 put together as a single tape block on the tape, without intervening
10941 tape gaps. A @dfn{tape gap} is a small landing area on the tape
10942 with no information on it, used for decelerating the tape to a
10943 full stop, and for later regaining the reading or writing speed.
10944 When the tape driver starts reading a record, the record has to
10945 be read whole without stopping, as a tape gap is needed to stop the
10946 tape motion without losing information.
10948 @cindex Exabyte blocking
10949 @cindex DAT blocking
10950 Using higher blocking (putting more disk blocks per tape block) will use
10951 the tape more efficiently as there will be less tape gaps. But reading
10952 such tapes may be more difficult for the system, as more memory will be
10953 required to receive at once the whole record. Further, if there is a
10954 reading error on a huge record, this is less likely that the system will
10955 succeed in recovering the information. So, blocking should not be too
10956 low, nor it should be too high. @command{tar} uses by default a blocking of
10957 20 for historical reasons, and it does not really matter when reading or
10958 writing to disk. Current tape technology would easily accommodate higher
10959 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
10960 We were told that for some DLT drives, the blocking should be a multiple
10961 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
10962 Other manufacturers may use different recommendations for the same tapes.
10963 This might also depends of the buffering techniques used inside modern
10964 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
10965 Others request blocking to be some exponent of two.
10967 So, there is no fixed rule for blocking. But blocking at read time
10968 should ideally be the same as blocking used at write time. At one place
10969 I know, with a wide variety of equipment, they found it best to use a
10970 blocking of 32 to guarantee that their tapes are fully interchangeable.
10972 I was also told that, for recycled tapes, prior erasure (by the same
10973 drive unit that will be used to create the archives) sometimes lowers
10974 the error rates observed at rewriting time.
10976 I might also use @option{--number-blocks} instead of
10977 @option{--block-number}, so @option{--block} will then expand to
10978 @option{--blocking-factor} unambiguously.
10981 @section Many Archives on One Tape
10983 @FIXME{Appropriate options should be moved here from elsewhere.}
10985 @findex ntape @r{device}
10986 Most tape devices have two entries in the @file{/dev} directory, or
10987 entries that come in pairs, which differ only in the minor number for
10988 this device. Let's take for example @file{/dev/tape}, which often
10989 points to the only or usual tape device of a given system. There might
10990 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
10991 name is the @emph{rewinding} version of the device, while the name
10992 having @samp{nr} in it is the @emph{no rewinding} version of the same
10995 A rewinding tape device will bring back the tape to its beginning point
10996 automatically when this device is opened or closed. Since @command{tar}
10997 opens the archive file before using it and closes it afterwards, this
10998 means that a simple:
11001 $ @kbd{tar cf /dev/tape @var{directory}}
11005 will reposition the tape to its beginning both prior and after saving
11006 @var{directory} contents to it, thus erasing prior tape contents and
11007 making it so that any subsequent write operation will destroy what has
11010 @cindex tape positioning
11011 So, a rewinding device is normally meant to hold one and only one file.
11012 If you want to put more than one @command{tar} archive on a given tape, you
11013 will need to avoid using the rewinding version of the tape device. You
11014 will also have to pay special attention to tape positioning. Errors in
11015 positioning may overwrite the valuable data already on your tape. Many
11016 people, burnt by past experiences, will only use rewinding devices and
11017 limit themselves to one file per tape, precisely to avoid the risk of
11018 such errors. Be fully aware that writing at the wrong position on a
11019 tape loses all information past this point and most probably until the
11020 end of the tape, and this destroyed information @emph{cannot} be
11023 To save @var{directory-1} as a first archive at the beginning of a
11024 tape, and leave that tape ready for a second archive, you should use:
11027 $ @kbd{mt -f /dev/nrtape rewind}
11028 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
11032 @dfn{Tape marks} are special magnetic patterns written on the tape
11033 media, which are later recognizable by the reading hardware. These
11034 marks are used after each file, when there are many on a single tape.
11035 An empty file (that is to say, two tape marks in a row) signal the
11036 logical end of the tape, after which no file exist. Usually,
11037 non-rewinding tape device drivers will react to the close request issued
11038 by @command{tar} by first writing two tape marks after your archive, and by
11039 backspacing over one of these. So, if you remove the tape at that time
11040 from the tape drive, it is properly terminated. But if you write
11041 another file at the current position, the second tape mark will be
11042 erased by the new information, leaving only one tape mark between files.
11044 So, you may now save @var{directory-2} as a second archive after the
11045 first on the same tape by issuing the command:
11048 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
11052 and so on for all the archives you want to put on the same tape.
11054 Another usual case is that you do not write all the archives the same
11055 day, and you need to remove and store the tape between two archive
11056 sessions. In general, you must remember how many files are already
11057 saved on your tape. Suppose your tape already has 16 files on it, and
11058 that you are ready to write the 17th. You have to take care of skipping
11059 the first 16 tape marks before saving @var{directory-17}, say, by using
11063 $ @kbd{mt -f /dev/nrtape rewind}
11064 $ @kbd{mt -f /dev/nrtape fsf 16}
11065 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
11068 In all the previous examples, we put aside blocking considerations, but
11069 you should do the proper things for that as well. @xref{Blocking}.
11072 * Tape Positioning:: Tape Positions and Tape Marks
11073 * mt:: The @command{mt} Utility
11076 @node Tape Positioning
11077 @subsection Tape Positions and Tape Marks
11080 Just as archives can store more than one file from the file system,
11081 tapes can store more than one archive file. To keep track of where
11082 archive files (or any other type of file stored on tape) begin and
11083 end, tape archive devices write magnetic @dfn{tape marks} on the
11084 archive media. Tape drives write one tape mark between files,
11085 two at the end of all the file entries.
11087 If you think of data as a series of records "rrrr"'s, and tape marks as
11088 "*"'s, a tape might look like the following:
11091 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
11094 Tape devices read and write tapes using a read/write @dfn{tape
11095 head}---a physical part of the device which can only access one
11096 point on the tape at a time. When you use @command{tar} to read or
11097 write archive data from a tape device, the device will begin reading
11098 or writing from wherever on the tape the tape head happens to be,
11099 regardless of which archive or what part of the archive the tape
11100 head is on. Before writing an archive, you should make sure that no
11101 data on the tape will be overwritten (unless it is no longer needed).
11102 Before reading an archive, you should make sure the tape head is at
11103 the beginning of the archive you want to read. You can do it manually
11104 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
11105 that automatically (@pxref{Scripted Restoration}).
11107 If you want to add new archive file entries to a tape, you should
11108 advance the tape to the end of the existing file entries, backspace
11109 over the last tape mark, and write the new archive file. If you were
11110 to add two archives to the example above, the tape might look like the
11114 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
11118 @subsection The @command{mt} Utility
11121 @FIXME{Is it true that this only works on non-block devices?
11122 should explain the difference, (fixed or variable).}
11123 @xref{Blocking Factor}.
11125 You can use the @command{mt} utility to advance or rewind a tape past a
11126 specified number of archive files on the tape. This will allow you
11127 to move to the beginning of an archive before extracting or reading
11128 it, or to the end of all the archives before writing a new one.
11129 @FIXME{Why isn't there an "advance 'til you find two tape marks
11132 The syntax of the @command{mt} command is:
11135 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
11138 where @var{tapename} is the name of the tape device, @var{number} is
11139 the number of times an operation is performed (with a default of one),
11140 and @var{operation} is one of the following:
11142 @FIXME{is there any use for record operations?}
11147 Writes @var{number} tape marks at the current position on the tape.
11150 Moves tape position forward @var{number} files.
11153 Moves tape position back @var{number} files.
11156 Rewinds the tape. (Ignores @var{number}.)
11160 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}.)
11163 Prints status information about the tape unit.
11167 If you don't specify a @var{tapename}, @command{mt} uses the environment
11168 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
11169 the default device specified in your @file{sys/mtio.h} file
11170 (@code{DEFTAPE} variable). If this is not defined, the program will
11171 display a descriptive error message and exit with code 1.
11173 @command{mt} returns a 0 exit status when the operation(s) were
11174 successful, 1 if the command was unrecognized, and 2 if an operation
11177 @node Using Multiple Tapes
11178 @section Using Multiple Tapes
11180 Often you might want to write a large archive, one larger than will fit
11181 on the actual tape you are using. In such a case, you can run multiple
11182 @command{tar} commands, but this can be inconvenient, particularly if you
11183 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
11184 Therefore, @command{tar} provides a special mode for creating
11185 multi-volume archives.
11187 @dfn{Multi-volume} archive is a single @command{tar} archive, stored
11188 on several media volumes of fixed size. Although in this section we will
11189 often call @samp{volume} a @dfn{tape}, there is absolutely no
11190 requirement for multi-volume archives to be stored on tapes. Instead,
11191 they can use whatever media type the user finds convenient, they can
11192 even be located on files.
11194 When creating a multi-volume archive, @GNUTAR{} continues to fill
11195 current volume until it runs out of space, then it switches to
11196 next volume (usually the operator is queried to replace the tape on
11197 this point), and continues working on the new volume. This operation
11198 continues until all requested files are dumped. If @GNUTAR{} detects
11199 end of media while dumping a file, such a file is archived in split
11200 form. Some very big files can even be split across several volumes.
11202 Each volume is itself a valid @GNUTAR{} archive, so it can be read
11203 without any special options. Consequently any file member residing
11204 entirely on one volume can be extracted or otherwise operated upon
11205 without needing the other volume. Sure enough, to extract a split
11206 member you would need all volumes its parts reside on.
11208 Multi-volume archives suffer from several limitations. In particular,
11209 they cannot be compressed.
11211 @GNUTAR{} is able to create multi-volume archives of two formats
11212 (@pxref{Formats}): @samp{GNU} and @samp{POSIX}.
11215 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
11216 * Tape Files:: Tape Files
11217 * Tarcat:: Concatenate Volumes into a Single Archive
11221 @node Multi-Volume Archives
11222 @subsection Archives Longer than One Tape or Disk
11223 @cindex Multi-volume archives
11225 @opindex multi-volume
11226 To create an archive that is larger than will fit on a single unit of
11227 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
11228 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
11229 archive can be manipulated like any other archive (provided the
11230 @option{--multi-volume} option is specified), but is stored on more
11231 than one tape or file.
11233 When you specify @option{--multi-volume}, @command{tar} does not report an
11234 error when it comes to the end of an archive volume (when reading), or
11235 the end of the media (when writing). Instead, it prompts you to load
11236 a new storage volume. If the archive is on a magnetic tape, you
11237 should change tapes when you see the prompt; if the archive is on a
11238 floppy disk, you should change disks; etc.
11241 @item --multi-volume
11243 Creates a multi-volume archive, when used in conjunction with
11244 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
11245 archive, specify @option{--multi-volume} in conjunction with that
11250 $ @kbd{tar --create --multi-volume --file=/dev/tape @var{files}}
11254 The method @command{tar} uses to detect end of tape is not perfect, and
11255 fails on some operating systems or on some devices. If @command{tar}
11256 cannot detect the end of the tape itself, you can use
11257 @option{--tape-length} option to inform it about the capacity of the
11260 @anchor{tape-length}
11262 @opindex tape-length
11263 @item --tape-length=@var{size}[@var{suf}]
11264 @itemx -L @var{size}[@var{suf}]
11265 Set maximum length of a volume. The @var{suf}, if given, specifies
11266 units in which @var{size} is expressed, e.g. @samp{2M} mean 2
11267 megabytes (@pxref{size-suffixes}, for a list of allowed size
11268 suffixes). Without @var{suf}, units of 1024 bytes (kilobyte) are
11271 This option selects @option{--multi-volume} automatically. For example:
11274 $ @kbd{tar --create --tape-length=41943040 --file=/dev/tape @var{files}}
11278 or, which is equivalent:
11281 $ @kbd{tar --create --tape-length=4G --file=/dev/tape @var{files}}
11285 @anchor{change volume prompt}
11286 When @GNUTAR{} comes to the end of a storage media, it asks you to
11287 change the volume. The built-in prompt for POSIX locale
11288 is@footnote{If you run @GNUTAR{} under a different locale, the
11289 translation to the locale's language will be used.}:
11292 Prepare volume #@var{n} for '@var{archive}' and hit return:
11296 where @var{n} is the ordinal number of the volume to be created and
11297 @var{archive} is archive file or device name.
11299 When prompting for a new tape, @command{tar} accepts any of the following
11304 Request @command{tar} to explain possible responses.
11306 Request @command{tar} to exit immediately.
11307 @item n @var{file-name}
11308 Request @command{tar} to write the next volume on the file @var{file-name}.
11310 Request @command{tar} to run a subshell. This option can be disabled
11311 by giving @option{--restrict} command line option to
11312 @command{tar}@footnote{@xref{--restrict}, for more information about
11315 Request @command{tar} to begin writing the next volume.
11318 (You should only type @samp{y} after you have changed the tape;
11319 otherwise @command{tar} will write over the volume it just finished.)
11321 @cindex Volume number file
11323 @anchor{volno-file}
11324 @opindex volno-file
11325 The volume number used by @command{tar} in its tape-changing prompt
11326 can be changed; if you give the
11327 @option{--volno-file=@var{file-of-number}} option, then
11328 @var{file-of-number} should be an non-existing file to be created, or
11329 else, a file already containing a decimal number. That number will be
11330 used as the volume number of the first volume written. When
11331 @command{tar} is finished, it will rewrite the file with the
11332 now-current volume number. (This does not change the volume number
11333 written on a tape label, as per @ref{label}, it @emph{only} affects
11334 the number used in the prompt.)
11336 @cindex End-of-archive info script
11337 @cindex Info script
11338 @anchor{info-script}
11339 @opindex info-script
11340 @opindex new-volume-script
11341 If you want more elaborate behavior than this, you can write a special
11342 @dfn{new volume script}, that will be responsible for changing the
11343 volume, and instruct @command{tar} to use it instead of its normal
11344 prompting procedure:
11347 @item --info-script=@var{script-name}
11348 @itemx --new-volume-script=@var{script-name}
11349 @itemx -F @var{script-name}
11350 Specify the full name of the volume script to use. The script can be
11351 used to eject cassettes, or to broadcast messages such as
11352 @samp{Someone please come change my tape} when performing unattended
11356 The @var{script-name} is executed without any command line
11357 arguments. It inherits @command{tar}'s shell environment.
11358 Additional data is passed to it via the following
11359 environment variables:
11362 @vrindex TAR_VERSION, info script environment variable
11364 @GNUTAR{} version number.
11366 @vrindex TAR_ARCHIVE, info script environment variable
11368 The name of the archive @command{tar} is processing.
11370 @vrindex TAR_BLOCKING_FACTOR, info script environment variable
11371 @item TAR_BLOCKING_FACTOR
11372 Current blocking factor (@pxref{Blocking}).
11374 @vrindex TAR_VOLUME, info script environment variable
11376 Ordinal number of the volume @command{tar} is about to start.
11378 @vrindex TAR_SUBCOMMAND, info script environment variable
11379 @item TAR_SUBCOMMAND
11380 A short option describing the operation @command{tar} is executing.
11381 @xref{Operations}, for a complete list of subcommand options.
11383 @vrindex TAR_FORMAT, info script environment variable
11385 Format of the archive being processed. @xref{Formats}, for a complete
11386 list of archive format names.
11388 @vrindex TAR_FD, info script environment variable
11390 File descriptor which can be used to communicate the new volume
11391 name to @command{tar}.
11394 The volume script can instruct @command{tar} to use new archive name,
11395 by writing in to file descriptor @env{$TAR_FD} (see below for an example).
11397 If the info script fails, @command{tar} exits; otherwise, it begins
11398 writing the next volume.
11400 If you want @command{tar} to cycle through a series of files or tape
11401 drives, there are three approaches to choose from. First of all, you
11402 can give @command{tar} multiple @option{--file} options. In this case
11403 the specified files will be used, in sequence, as the successive
11404 volumes of the archive. Only when the first one in the sequence needs
11405 to be used again will @command{tar} prompt for a tape change (or run
11406 the info script). For example, suppose someone has two tape drives on
11407 a system named @file{/dev/tape0} and @file{/dev/tape1}. For having
11408 @GNUTAR{} to switch to the second drive when it needs to write the
11409 second tape, and then back to the first tape, etc., just do either of:
11412 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
11413 $ @kbd{tar -cM -f /dev/tape0 -f /dev/tape1 @var{files}}
11416 The second method is to use the @samp{n} response to the tape-change
11419 Finally, the most flexible approach is to use a volume script, that
11420 writes new archive name to the file descriptor @env{$TAR_FD}. For example, the
11421 following volume script will create a series of archive files, named
11422 @file{@var{archive}-@var{vol}}, where @var{archive} is the name of the
11423 archive being created (as given by @option{--file} option) and
11424 @var{vol} is the ordinal number of the archive being created:
11429 # For this script it's advisable to use a shell, such as Bash,
11430 # that supports a TAR_FD value greater than 9.
11432 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
11434 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
11435 case $TAR_SUBCOMMAND in
11437 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
11442 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&$TAR_FD
11446 The same script can be used while listing, comparing or extracting
11447 from the created archive. For example:
11451 # @r{Create a multi-volume archive:}
11452 $ @kbd{tar -c -L1024 -f archive.tar -F new-volume .}
11453 # @r{Extract from the created archive:}
11454 $ @kbd{tar -x -f archive.tar -F new-volume .}
11459 Notice, that the first command had to use @option{-L} option, since
11460 otherwise @GNUTAR{} will end up writing everything to file
11461 @file{archive.tar}.
11463 You can read each individual volume of a multi-volume archive as if it
11464 were an archive by itself. For example, to list the contents of one
11465 volume, use @option{--list}, without @option{--multi-volume} specified.
11466 To extract an archive member from one volume (assuming it is described
11467 that volume), use @option{--extract}, again without
11468 @option{--multi-volume}.
11470 If an archive member is split across volumes (i.e., its entry begins on
11471 one volume of the media and ends on another), you need to specify
11472 @option{--multi-volume} to extract it successfully. In this case, you
11473 should load the volume where the archive member starts, and use
11474 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
11475 volumes as it needs them. @xref{extracting archives}, for more
11476 information about extracting archives.
11478 Multi-volume archives can be modified like any other archive. To add
11479 files to a multi-volume archive, you need to only mount the last
11480 volume of the archive media (and new volumes, if needed). For all
11481 other operations, you need to use the entire archive.
11483 If a multi-volume archive was labeled using
11484 @option{--label=@var{archive-label}} (@pxref{label}) when it was
11485 created, @command{tar} will not automatically label volumes which are
11486 added later. To label subsequent volumes, specify
11487 @option{--label=@var{archive-label}} again in conjunction with the
11488 @option{--append}, @option{--update} or @option{--concatenate} operation.
11490 Notice that multi-volume support is a GNU extension and the archives
11491 created in this mode should be read only using @GNUTAR{}. If you
11492 absolutely have to process such archives using a third-party @command{tar}
11493 implementation, read @ref{Split Recovery}.
11496 @subsection Tape Files
11497 @cindex labeling archives
11501 To give the archive a name which will be recorded in it, use the
11502 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
11503 option. This will write a special block identifying
11504 @var{volume-label} as the name of the archive to the front of the
11505 archive which will be displayed when the archive is listed with
11506 @option{--list}. If you are creating a multi-volume archive with
11507 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
11508 volume label will have @samp{Volume @var{nnn}} appended to the name
11509 you give, where @var{nnn} is the number of the volume of the archive.
11510 If you use the @option{--label=@var{volume-label}} option when
11511 reading an archive, it checks to make sure the label on the tape
11512 matches the one you gave. @xref{label}.
11514 When @command{tar} writes an archive to tape, it creates a single
11515 tape file. If multiple archives are written to the same tape, one
11516 after the other, they each get written as separate tape files. When
11517 extracting, it is necessary to position the tape at the right place
11518 before running @command{tar}. To do this, use the @command{mt} command.
11519 For more information on the @command{mt} command and on the organization
11520 of tapes into a sequence of tape files, see @ref{mt}.
11522 People seem to often do:
11525 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
11528 or such, for pushing a common date in all volumes or an archive set.
11531 @subsection Concatenate Volumes into a Single Archive
11534 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
11535 archive to a single @command{tar} archive. Simply concatenating all
11536 volumes into one will not work, since each volume carries an additional
11537 information at the beginning. @GNUTAR{} is shipped with the shell
11538 script @command{tarcat} designed for this purpose.
11540 The script takes a list of files comprising a multi-volume archive
11541 and creates the resulting archive at the standard output. For example:
11544 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
11547 The script implements a simple heuristics to determine the format of
11548 the first volume file and to decide how to process the rest of the
11549 files. However, it makes no attempt to verify whether the files are
11550 given in order or even if they are valid @command{tar} archives.
11551 It uses @command{dd} and does not filter its standard error, so you
11552 will usually see lots of spurious messages.
11554 @FIXME{The script is not installed. Should we install it?}
11557 @section Including a Label in the Archive
11558 @cindex Labeling an archive
11559 @cindex Labels on the archive media
11560 @cindex Labeling multi-volume archives
11563 To avoid problems caused by misplaced paper labels on the archive
11564 media, you can include a @dfn{label} entry --- an archive member which
11565 contains the name of the archive --- in the archive itself. Use the
11566 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11567 option@footnote{Until version 1.10, that option was called
11568 @option{--volume}, but is not available under that name anymore.} in
11569 conjunction with the @option{--create} operation to include a label
11570 entry in the archive as it is being created.
11573 @item --label=@var{archive-label}
11574 @itemx -V @var{archive-label}
11575 Includes an @dfn{archive-label} at the beginning of the archive when
11576 the archive is being created, when used in conjunction with the
11577 @option{--create} operation. Checks to make sure the archive label
11578 matches the one specified (when used in conjunction with any other
11582 If you create an archive using both
11583 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11584 and @option{--multi-volume} (@option{-M}), each volume of the archive
11585 will have an archive label of the form @samp{@var{archive-label}
11586 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
11587 next, and so on. @xref{Using Multiple Tapes}, for information on
11588 creating multiple volume archives.
11590 @cindex Volume label, listing
11591 @cindex Listing volume label
11592 The volume label will be displayed by @option{--list} along with
11593 the file contents. If verbose display is requested, it will also be
11594 explicitly marked as in the example below:
11598 $ @kbd{tar --verbose --list --file=iamanarchive}
11599 V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header--
11600 -rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename
11604 @opindex test-label
11605 @anchor{--test-label option}
11606 However, @option{--list} option will cause listing entire
11607 contents of the archive, which may be undesirable (for example, if the
11608 archive is stored on a tape). You can request checking only the volume
11609 label by specifying @option{--test-label} option. This option reads only the
11610 first block of an archive, so it can be used with slow storage
11611 devices. For example:
11615 $ @kbd{tar --test-label --file=iamanarchive}
11620 If @option{--test-label} is used with one or more command line
11621 arguments, @command{tar} compares the volume label with each
11622 argument. It exits with code 0 if a match is found, and with code 1
11623 otherwise@footnote{Note that @GNUTAR{} versions up to 1.23 indicated
11624 mismatch with an exit code 2 and printed a spurious diagnostics on
11625 stderr.}. No output is displayed, unless you also used the
11626 @option{--verbose} option. For example:
11630 $ @kbd{tar --test-label --file=iamanarchive 'iamalabel'}
11632 $ @kbd{tar --test-label --file=iamanarchive 'alabel'}
11637 When used with the @option{--verbose} option, @command{tar}
11638 prints the actual volume label (if any), and a verbose diagnostics in
11639 case of a mismatch:
11643 $ @kbd{tar --test-label --verbose --file=iamanarchive 'iamalabel'}
11646 $ @kbd{tar --test-label --verbose --file=iamanarchive 'alabel'}
11648 tar: Archive label mismatch
11653 If you request any operation, other than @option{--create}, along
11654 with using @option{--label} option, @command{tar} will first check if
11655 the archive label matches the one specified and will refuse to proceed
11656 if it does not. Use this as a safety precaution to avoid accidentally
11657 overwriting existing archives. For example, if you wish to add files
11658 to @file{archive}, presumably labeled with string @samp{My volume},
11663 $ @kbd{tar -rf archive --label 'My volume' .}
11664 tar: Archive not labeled to match 'My volume'
11669 in case its label does not match. This will work even if
11670 @file{archive} is not labeled at all.
11672 Similarly, @command{tar} will refuse to list or extract the
11673 archive if its label doesn't match the @var{archive-label}
11674 specified. In those cases, @var{archive-label} argument is interpreted
11675 as a globbing-style pattern which must match the actual magnetic
11676 volume label. @xref{exclude}, for a precise description of how match
11677 is attempted@footnote{Previous versions of @command{tar} used full
11678 regular expression matching, or before that, only exact string
11679 matching, instead of wildcard matchers. We decided for the sake of
11680 simplicity to use a uniform matching device through
11681 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
11682 the volume label matcher will also suffix @var{archive-label} by
11683 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
11684 up. Since the volume numbering is automatically added in labels at
11685 creation time, it sounded logical to equally help the user taking care
11686 of it when the archive is being read.
11688 You can also use @option{--label} to get a common information on
11689 all tapes of a series. For having this information different in each
11690 series created through a single script used on a regular basis, just
11691 manage to get some date string as part of the label. For example:
11695 $ @kbd{tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"}
11696 $ @kbd{tar --create --file=/dev/tape --multi-volume \
11697 --label="Daily backup for `date +%Y-%m-%d`"}
11701 Some more notes about volume labels:
11704 @item Each label has its own date and time, which corresponds
11705 to the time when @GNUTAR{} initially attempted to write it,
11706 often soon after the operator launches @command{tar} or types the
11707 carriage return telling that the next tape is ready.
11709 @item Comparing date labels to get an idea of tape throughput is
11710 unreliable. It gives correct results only if the delays for rewinding
11711 tapes and the operator switching them were negligible, which is
11712 usually not the case.
11716 @section Verifying Data as It is Stored
11717 @cindex Verifying a write operation
11718 @cindex Double-checking a write operation
11723 @opindex verify, short description
11724 Attempt to verify the archive after writing.
11727 This option causes @command{tar} to verify the archive after writing it.
11728 Each volume is checked after it is written, and any discrepancies
11729 are recorded on the standard error output.
11731 Verification requires that the archive be on a back-space-able medium.
11732 This means pipes, some cartridge tape drives, and some other devices
11733 cannot be verified.
11735 You can insure the accuracy of an archive by comparing files in the
11736 system with archive members. @command{tar} can compare an archive to the
11737 file system as the archive is being written, to verify a write
11738 operation, or can compare a previously written archive, to insure that
11741 @xopindex{verify, using with @option{--create}}
11742 @xopindex{create, using with @option{--verify}}
11743 To check for discrepancies in an archive immediately after it is
11744 written, use the @option{--verify} (@option{-W}) option in conjunction with
11745 the @option{--create} operation. When this option is
11746 specified, @command{tar} checks archive members against their counterparts
11747 in the file system, and reports discrepancies on the standard error.
11749 To verify an archive, you must be able to read it from before the end
11750 of the last written entry. This option is useful for detecting data
11751 errors on some tapes. Archives written to pipes, some cartridge tape
11752 drives, and some other devices cannot be verified.
11754 One can explicitly compare an already made archive with the file
11755 system by using the @option{--compare} (@option{--diff}, @option{-d})
11756 option, instead of using the more automatic @option{--verify} option.
11759 Note that these two options have a slightly different intent. The
11760 @option{--compare} option checks how identical are the logical contents of some
11761 archive with what is on your disks, while the @option{--verify} option is
11762 really for checking if the physical contents agree and if the recording
11763 media itself is of dependable quality. So, for the @option{--verify}
11764 operation, @command{tar} tries to defeat all in-memory cache pertaining to
11765 the archive, while it lets the speed optimization undisturbed for the
11766 @option{--compare} option. If you nevertheless use @option{--compare} for
11767 media verification, you may have to defeat the in-memory cache yourself,
11768 maybe by opening and reclosing the door latch of your recording unit,
11769 forcing some doubt in your operating system about the fact this is really
11770 the same volume as the one just written or read.
11772 The @option{--verify} option would not be necessary if drivers were indeed
11773 able to detect dependably all write failures. This sometimes require many
11774 magnetic heads, some able to read after the writes occurred. One would
11775 not say that drivers unable to detect all cases are necessarily flawed,
11776 as long as programming is concerned.
11778 The @option{--verify} (@option{-W}) option will not work in
11779 conjunction with the @option{--multi-volume} (@option{-M}) option or
11780 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
11781 and @option{--delete} operations. @xref{Operations}, for more
11782 information on these operations.
11784 Also, since @command{tar} normally strips leading @samp{/} from file
11785 names (@pxref{absolute}), a command like @samp{tar --verify -cf
11786 /tmp/foo.tar /etc} will work as desired only if the working directory is
11787 @file{/}, as @command{tar} uses the archive's relative member names
11788 (e.g., @file{etc/motd}) when verifying the archive.
11790 @node Write Protection
11791 @section Write Protection
11793 Almost all tapes and diskettes, and in a few rare cases, even disks can
11794 be @dfn{write protected}, to protect data on them from being changed.
11795 Once an archive is written, you should write protect the media to prevent
11796 the archive from being accidentally overwritten or deleted. (This will
11797 protect the archive from being changed with a tape or floppy drive---it
11798 will not protect it from magnet fields or other physical hazards.)
11800 The write protection device itself is usually an integral part of the
11801 physical media, and can be a two position (write enabled/write
11802 disabled) switch, a notch which can be popped out or covered, a ring
11803 which can be removed from the center of a tape reel, or some other
11804 changeable feature.
11806 @node Reliability and security
11807 @chapter Reliability and Security
11809 The @command{tar} command reads and writes files as any other
11810 application does, and is subject to the usual caveats about
11811 reliability and security. This section contains some commonsense
11812 advice on the topic.
11820 @section Reliability
11822 Ideally, when @command{tar} is creating an archive, it reads from a
11823 file system that is not being modified, and encounters no errors or
11824 inconsistencies while reading and writing. If this is the case, the
11825 archive should faithfully reflect what was read. Similarly, when
11826 extracting from an archive, ideally @command{tar} ideally encounters
11827 no errors and the extracted files faithfully reflect what was in the
11830 However, when reading or writing real-world file systems, several
11831 things can go wrong; these include permissions problems, corruption of
11832 data, and race conditions.
11835 * Permissions problems::
11836 * Data corruption and repair::
11837 * Race conditions::
11840 @node Permissions problems
11841 @subsection Permissions Problems
11843 If @command{tar} encounters errors while reading or writing files, it
11844 normally reports an error and exits with nonzero status. The work it
11845 does may therefore be incomplete. For example, when creating an
11846 archive, if @command{tar} cannot read a file then it cannot copy the
11847 file into the archive.
11849 @node Data corruption and repair
11850 @subsection Data Corruption and Repair
11852 If an archive becomes corrupted by an I/O error, this may corrupt the
11853 data in an extracted file. Worse, it may corrupt the file's metadata,
11854 which may cause later parts of the archive to become misinterpreted.
11855 An tar-format archive contains a checksum that most likely will detect
11856 errors in the metadata, but it will not detect errors in the data.
11858 If data corruption is a concern, you can compute and check your own
11859 checksums of an archive by using other programs, such as
11862 When attempting to recover from a read error or data corruption in an
11863 archive, you may need to skip past the questionable data and read the
11864 rest of the archive. This requires some expertise in the archive
11865 format and in other software tools.
11867 @node Race conditions
11868 @subsection Race conditions
11870 If some other process is modifying the file system while @command{tar}
11871 is reading or writing files, the result may well be inconsistent due
11872 to race conditions. For example, if another process creates some
11873 files in a directory while @command{tar} is creating an archive
11874 containing the directory's files, @command{tar} may see some of the
11875 files but not others, or it may see a file that is in the process of
11876 being created. The resulting archive may not be a snapshot of the
11877 file system at any point in time. If an application such as a
11878 database system depends on an accurate snapshot, restoring from the
11879 @command{tar} archive of a live file system may therefore break that
11880 consistency and may break the application. The simplest way to avoid
11881 the consistency issues is to avoid making other changes to the file
11882 system while tar is reading it or writing it.
11884 When creating an archive, several options are available to avoid race
11885 conditions. Some hosts have a way of snapshotting a file system, or
11886 of temporarily suspending all changes to a file system, by (say)
11887 suspending the only virtual machine that can modify a file system; if
11888 you use these facilities and have @command{tar -c} read from a
11889 snapshot when creating an archive, you can avoid inconsistency
11890 problems. More drastically, before starting @command{tar} you could
11891 suspend or shut down all processes other than @command{tar} that have
11892 access to the file system, or you could unmount the file system and
11893 then mount it read-only.
11895 When extracting from an archive, one approach to avoid race conditions
11896 is to create a directory that no other process can write to, and
11902 In some cases @command{tar} may be used in an adversarial situation,
11903 where an untrusted user is attempting to gain information about or
11904 modify otherwise-inaccessible files. Dealing with untrusted data
11905 (that is, data generated by an untrusted user) typically requires
11906 extra care, because even the smallest mistake in the use of
11907 @command{tar} is more likely to be exploited by an adversary than by a
11913 * Live untrusted data::
11914 * Security rules of thumb::
11918 @subsection Privacy
11920 Standard privacy concerns apply when using @command{tar}. For
11921 example, suppose you are archiving your home directory into a file
11922 @file{/archive/myhome.tar}. Any secret information in your home
11923 directory, such as your SSH secret keys, are copied faithfully into
11924 the archive. Therefore, if your home directory contains any file that
11925 should not be read by some other user, the archive itself should be
11926 not be readable by that user. And even if the archive's data are
11927 inaccessible to untrusted users, its metadata (such as size or
11928 last-modified date) may reveal some information about your home
11929 directory; if the metadata are intended to be private, the archive's
11930 parent directory should also be inaccessible to untrusted users.
11932 One precaution is to create @file{/archive} so that it is not
11933 accessible to any user, unless that user also has permission to access
11934 all the files in your home directory.
11936 Similarly, when extracting from an archive, take care that the
11937 permissions of the extracted files are not more generous than what you
11938 want. Even if the archive itself is readable only to you, files
11939 extracted from it have their own permissions that may differ.
11942 @subsection Integrity
11944 When creating archives, take care that they are not writable by a
11945 untrusted user; otherwise, that user could modify the archive, and
11946 when you later extract from the archive you will get incorrect data.
11948 When @command{tar} extracts from an archive, by default it writes into
11949 files relative to the working directory. If the archive was generated
11950 by an untrusted user, that user therefore can write into any file
11951 under the working directory. If the working directory contains a
11952 symbolic link to another directory, the untrusted user can also write
11953 into any file under the referenced directory. When extracting from an
11954 untrusted archive, it is therefore good practice to create an empty
11955 directory and run @command{tar} in that directory.
11957 When extracting from two or more untrusted archives, each one should
11958 be extracted independently, into different empty directories.
11959 Otherwise, the first archive could create a symbolic link into an area
11960 outside the working directory, and the second one could follow the
11961 link and overwrite data that is not under the working directory. For
11962 example, when restoring from a series of incremental dumps, the
11963 archives should have been created by a trusted process, as otherwise
11964 the incremental restores might alter data outside the working
11967 If you use the @option{--absolute-names} (@option{-P}) option when
11968 extracting, @command{tar} respects any file names in the archive, even
11969 file names that begin with @file{/} or contain @file{..}. As this
11970 lets the archive overwrite any file in your system that you can write,
11971 the @option{--absolute-names} (@option{-P}) option should be used only
11972 for trusted archives.
11974 Conversely, with the @option{--keep-old-files} (@option{-k}) and
11975 @option{--skip-old-files} options, @command{tar} refuses to replace
11976 existing files when extracting. The difference between the two
11977 options is that the former treats existing files as errors whereas the
11978 latter just silently ignores them.
11980 Finally, with the @option{--no-overwrite-dir} option, @command{tar}
11981 refuses to replace the permissions or ownership of already-existing
11982 directories. These options may help when extracting from untrusted
11985 @node Live untrusted data
11986 @subsection Dealing with Live Untrusted Data
11988 Extra care is required when creating from or extracting into a file
11989 system that is accessible to untrusted users. For example, superusers
11990 who invoke @command{tar} must be wary about its actions being hijacked
11991 by an adversary who is reading or writing the file system at the same
11992 time that @command{tar} is operating.
11994 When creating an archive from a live file system, @command{tar} is
11995 vulnerable to denial-of-service attacks. For example, an adversarial
11996 user could create the illusion of an indefinitely-deep directory
11997 hierarchy @file{d/e/f/g/...} by creating directories one step ahead of
11998 @command{tar}, or the illusion of an indefinitely-long file by
11999 creating a sparse file but arranging for blocks to be allocated just
12000 before @command{tar} reads them. There is no easy way for
12001 @command{tar} to distinguish these scenarios from legitimate uses, so
12002 you may need to monitor @command{tar}, just as you'd need to monitor
12003 any other system service, to detect such attacks.
12005 While a superuser is extracting from an archive into a live file
12006 system, an untrusted user might replace a directory with a symbolic
12007 link, in hopes that @command{tar} will follow the symbolic link and
12008 extract data into files that the untrusted user does not have access
12009 to. Even if the archive was generated by the superuser, it may
12010 contain a file such as @file{d/etc/passwd} that the untrusted user
12011 earlier created in order to break in; if the untrusted user replaces
12012 the directory @file{d/etc} with a symbolic link to @file{/etc} while
12013 @command{tar} is running, @command{tar} will overwrite
12014 @file{/etc/passwd}. This attack can be prevented by extracting into a
12015 directory that is inaccessible to untrusted users.
12017 Similar attacks via symbolic links are also possible when creating an
12018 archive, if the untrusted user can modify an ancestor of a top-level
12019 argument of @command{tar}. For example, an untrusted user that can
12020 modify @file{/home/eve} can hijack a running instance of @samp{tar -cf
12021 - /home/eve/Documents/yesterday} by replacing
12022 @file{/home/eve/Documents} with a symbolic link to some other
12023 location. Attacks like these can be prevented by making sure that
12024 untrusted users cannot modify any files that are top-level arguments
12025 to @command{tar}, or any ancestor directories of these files.
12027 @node Security rules of thumb
12028 @subsection Security Rules of Thumb
12030 This section briefly summarizes rules of thumb for avoiding security
12036 Protect archives at least as much as you protect any of the files
12040 Extract from an untrusted archive only into an otherwise-empty
12041 directory. This directory and its parent should be accessible only to
12042 trusted users. For example:
12046 $ @kbd{chmod go-rwx .}
12047 $ @kbd{mkdir -m go-rwx dir}
12049 $ @kbd{tar -xvf /archives/got-it-off-the-net.tar.gz}
12053 As a corollary, do not do an incremental restore from an untrusted archive.
12056 Do not let untrusted users access files extracted from untrusted
12057 archives without checking first for problems such as setuid programs.
12060 Do not let untrusted users modify directories that are ancestors of
12061 top-level arguments of @command{tar}. For example, while you are
12062 executing @samp{tar -cf /archive/u-home.tar /u/home}, do not let an
12063 untrusted user modify @file{/}, @file{/archive}, or @file{/u}.
12066 Pay attention to the diagnostics and exit status of @command{tar}.
12069 When archiving live file systems, monitor running instances of
12070 @command{tar} to detect denial-of-service attacks.
12073 Avoid unusual options such as @option{--absolute-names} (@option{-P}),
12074 @option{--dereference} (@option{-h}), @option{--overwrite},
12075 @option{--recursive-unlink}, and @option{--remove-files} unless you
12076 understand their security implications.
12083 This appendix lists some important user-visible changes between
12084 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
12085 version of this document is available at
12086 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
12087 @GNUTAR{} documentation page}.
12090 @item Use of globbing patterns when listing and extracting.
12092 Previous versions of GNU tar assumed shell-style globbing when
12093 extracting from or listing an archive. For example:
12096 $ @kbd{tar xf foo.tar '*.c'}
12099 would extract all files whose names end in @samp{.c}. This behavior
12100 was not documented and was incompatible with traditional tar
12101 implementations. Therefore, starting from version 1.15.91, GNU tar
12102 no longer uses globbing by default. For example, the above invocation
12103 is now interpreted as a request to extract from the archive the file
12106 To facilitate transition to the new behavior for those users who got
12107 used to the previous incorrect one, @command{tar} will print a warning
12108 if it finds out that a requested member was not found in the archive
12109 and its name looks like a globbing pattern. For example:
12112 $ @kbd{tar xf foo.tar '*.c'}
12113 tar: Pattern matching characters used in file names. Please,
12114 tar: use --wildcards to enable pattern matching, or --no-wildcards to
12115 tar: suppress this warning.
12116 tar: *.c: Not found in archive
12117 tar: Error exit delayed from previous errors
12120 To treat member names as globbing patterns, use the @option{--wildcards} option.
12121 If you want to tar to mimic the behavior of versions prior to 1.15.91,
12122 add this option to your @env{TAR_OPTIONS} variable.
12124 @xref{wildcards}, for the detailed discussion of the use of globbing
12125 patterns by @GNUTAR{}.
12127 @item Use of short option @option{-o}.
12129 Earlier versions of @GNUTAR{} understood @option{-o} command line
12130 option as a synonym for @option{--old-archive}.
12132 @GNUTAR{} starting from version 1.13.90 understands this option as
12133 a synonym for @option{--no-same-owner}. This is compatible with
12134 UNIX98 @command{tar} implementations.
12136 However, to facilitate transition, @option{-o} option retains its
12137 old semantics when it is used with one of archive-creation commands.
12138 Users are encouraged to use @option{--format=oldgnu} instead.
12140 It is especially important, since versions of @acronym{GNU} Automake
12141 up to and including 1.8.4 invoke tar with this option to produce
12142 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
12143 of this issue and its implications.
12145 @xref{Options, tar-formats, Changing Automake's Behavior,
12146 automake, GNU Automake}, for a description on how to use various
12147 archive formats with @command{automake}.
12149 Future versions of @GNUTAR{} will understand @option{-o} only as a
12150 synonym for @option{--no-same-owner}.
12152 @item Use of short option @option{-l}
12154 Earlier versions of @GNUTAR{} understood @option{-l} option as a
12155 synonym for @option{--one-file-system}. Since such usage contradicted
12156 to UNIX98 specification and harmed compatibility with other
12157 implementations, it was declared deprecated in version 1.14. However,
12158 to facilitate transition to its new semantics, it was supported by
12159 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
12160 variant of @option{--check-links} was introduced in version 1.15.91.
12162 @item Use of options @option{--portability} and @option{--old-archive}
12164 These options are deprecated. Please use @option{--format=v7} instead.
12166 @item Use of option @option{--posix}
12168 This option is deprecated. Please use @option{--format=posix} instead.
12171 @node Configuring Help Summary
12172 @appendix Configuring Help Summary
12174 Running @kbd{tar --help} displays the short @command{tar} option
12175 summary (@pxref{help}). This summary is organized by @dfn{groups} of
12176 semantically close options. The options within each group are printed
12177 in the following order: a short option, eventually followed by a list
12178 of corresponding long option names, followed by a short description of
12179 the option. For example, here is an excerpt from the actual @kbd{tar
12183 Main operation mode:
12185 -A, --catenate, --concatenate append tar files to an archive
12186 -c, --create create a new archive
12187 -d, --diff, --compare find differences between archive and
12189 --delete delete from the archive
12192 @vrindex ARGP_HELP_FMT, environment variable
12193 The exact visual representation of the help output is configurable via
12194 @env{ARGP_HELP_FMT} environment variable. The value of this variable
12195 is a comma-separated list of @dfn{format variable} assignments. There
12196 are two kinds of format variables. An @dfn{offset variable} keeps the
12197 offset of some part of help output text from the leftmost column on
12198 the screen. A @dfn{boolean} variable is a flag that toggles some
12199 output feature on or off. Depending on the type of the corresponding
12200 variable, there are two kinds of assignments:
12203 @item Offset assignment
12205 The assignment to an offset variable has the following syntax:
12208 @var{variable}=@var{value}
12212 where @var{variable} is the variable name, and @var{value} is a
12213 numeric value to be assigned to the variable.
12215 @item Boolean assignment
12217 To assign @code{true} value to a variable, simply put this variable name. To
12218 assign @code{false} value, prefix the variable name with @samp{no-}. For
12223 # Assign @code{true} value:
12225 # Assign @code{false} value:
12231 Following variables are declared:
12233 @deftypevr {Help Output} boolean dup-args
12234 If true, arguments for an option are shown with both short and long
12235 options, even when a given option has both forms, for example:
12238 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
12241 If false, then if an option has both short and long forms, the
12242 argument is only shown with the long one, for example:
12245 -f, --file=ARCHIVE use archive file or device ARCHIVE
12249 and a message indicating that the argument is applicable to both
12250 forms is printed below the options. This message can be disabled
12251 using @code{dup-args-note} (see below).
12253 The default is false.
12256 @deftypevr {Help Output} boolean dup-args-note
12257 If this variable is true, which is the default, the following notice
12258 is displayed at the end of the help output:
12261 Mandatory or optional arguments to long options are also mandatory or
12262 optional for any corresponding short options.
12265 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
12266 variables @code{dup-args} or @code{dup-args-note} should be set.
12269 @deftypevr {Help Output} offset short-opt-col
12270 Column in which short options start. Default is 2.
12274 $ @kbd{tar --help|grep ARCHIVE}
12275 -f, --file=ARCHIVE use archive file or device ARCHIVE
12276 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
12277 -f, --file=ARCHIVE use archive file or device ARCHIVE
12282 @deftypevr {Help Output} offset long-opt-col
12283 Column in which long options start. Default is 6. For example:
12287 $ @kbd{tar --help|grep ARCHIVE}
12288 -f, --file=ARCHIVE use archive file or device ARCHIVE
12289 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
12290 -f, --file=ARCHIVE use archive file or device ARCHIVE
12295 @deftypevr {Help Output} offset doc-opt-col
12296 Column in which @dfn{doc options} start. A doc option isn't actually
12297 an option, but rather an arbitrary piece of documentation that is
12298 displayed in much the same manner as the options. For example, in
12299 the description of @option{--format} option:
12303 -H, --format=FORMAT create archive of the given format.
12305 FORMAT is one of the following:
12307 gnu GNU tar 1.13.x format
12308 oldgnu GNU format as per tar <= 1.12
12309 pax POSIX 1003.1-2001 (pax) format
12311 ustar POSIX 1003.1-1988 (ustar) format
12312 v7 old V7 tar format
12317 the format names are doc options. Thus, if you set
12318 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
12319 will look as follows:
12323 -H, --format=FORMAT create archive of the given format.
12325 FORMAT is one of the following:
12327 gnu GNU tar 1.13.x format
12328 oldgnu GNU format as per tar <= 1.12
12329 pax POSIX 1003.1-2001 (pax) format
12331 ustar POSIX 1003.1-1988 (ustar) format
12332 v7 old V7 tar format
12337 @deftypevr {Help Output} offset opt-doc-col
12338 Column in which option description starts. Default is 29.
12342 $ @kbd{tar --help|grep ARCHIVE}
12343 -f, --file=ARCHIVE use archive file or device ARCHIVE
12344 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
12345 -f, --file=ARCHIVE use archive file or device ARCHIVE
12346 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
12348 use archive file or device ARCHIVE
12353 Notice, that the description starts on a separate line if
12354 @code{opt-doc-col} value is too small.
12357 @deftypevr {Help Output} offset header-col
12358 Column in which @dfn{group headers} are printed. A group header is a
12359 descriptive text preceding an option group. For example, in the
12363 Main operation mode:
12365 -A, --catenate, --concatenate append tar files to
12367 -c, --create create a new archive
12370 @samp{Main operation mode:} is the group header.
12372 The default value is 1.
12375 @deftypevr {Help Output} offset usage-indent
12376 Indentation of wrapped usage lines. Affects @option{--usage}
12377 output. Default is 12.
12380 @deftypevr {Help Output} offset rmargin
12381 Right margin of the text output. Used for wrapping.
12384 @node Fixing Snapshot Files
12385 @appendix Fixing Snapshot Files
12386 @include tar-snapshot-edit.texi
12388 @node Tar Internals
12389 @appendix Tar Internals
12390 @include intern.texi
12394 @include genfile.texi
12396 @node Free Software Needs Free Documentation
12397 @appendix Free Software Needs Free Documentation
12398 @include freemanuals.texi
12400 @node GNU Free Documentation License
12401 @appendix GNU Free Documentation License
12405 @node Index of Command Line Options
12406 @appendix Index of Command Line Options
12408 This appendix contains an index of all @GNUTAR{} long command line
12409 options. The options are listed without the preceding double-dash.
12410 For a cross-reference of short command line options, see
12411 @ref{Short Option Summary}.
12424 @c Local variables:
12425 @c texinfo-column-for-description: 32
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