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 Free Software Foundation, Inc.
43 Permission is granted to copy, distribute and/or modify this document
44 under the terms of the GNU Free Documentation License, Version 1.3 or
45 any later version published by the Free Software Foundation; with no
46 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual'',
47 and with the Back-Cover Texts as in (a) below. A copy of the license
48 is included in the section entitled ``GNU Free Documentation
51 (a) The FSF's Back-Cover Text is: ``You have the freedom to
52 copy and modify this GNU manual. Buying copies from the FSF
53 supports it in developing GNU and promoting software freedom.''
57 @dircategory Archiving
59 * Tar: (tar). Making tape (or disk) archives.
62 @dircategory Individual utilities
64 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
67 @shorttitlepage @acronym{GNU} @command{tar}
70 @title @acronym{GNU} tar: an archiver tool
71 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
72 @author John Gilmore, Jay Fenlason et al.
75 @vskip 0pt plus 1filll
81 @top @acronym{GNU} tar: an archiver tool
86 @cindex archiving files
88 The first part of this master menu lists the major nodes in this Info
89 document. The rest of the menu lists all the lower level nodes.
92 @c The master menu goes here.
94 @c NOTE: To update it from within Emacs, make sure mastermenu.el is
95 @c loaded and run texinfo-master-menu.
96 @c To update it from the command line, run
107 * Date input formats::
110 * Reliability and security::
115 * Configuring Help Summary::
116 * Fixing Snapshot Files::
119 * Free Software Needs Free Documentation::
120 * GNU Free Documentation License::
121 * Index of Command Line Options::
125 --- The Detailed Node Listing ---
129 * Book Contents:: What this Book Contains
130 * Definitions:: Some Definitions
131 * What tar Does:: What @command{tar} Does
132 * Naming tar Archives:: How @command{tar} Archives are Named
133 * Authors:: @GNUTAR{} Authors
134 * Reports:: Reporting bugs or suggestions
136 Tutorial Introduction to @command{tar}
139 * stylistic conventions::
140 * basic tar options:: Basic @command{tar} Operations and Options
141 * frequent operations::
142 * Two Frequent Options::
143 * create:: How to Create Archives
144 * list:: How to List Archives
145 * extract:: How to Extract Members from an Archive
148 Two Frequently Used Options
154 How to Create Archives
156 * prepare for examples::
157 * Creating the archive::
166 How to Extract Members from an Archive
168 * extracting archives::
171 * extracting untrusted archives::
177 * using tar options::
187 The Three Option Styles
189 * Long Options:: Long Option Style
190 * Short Options:: Short Option Style
191 * Old Options:: Old Option Style
192 * Mixing:: Mixing Option Styles
194 All @command{tar} Options
196 * Operation Summary::
198 * Short Option Summary::
210 Advanced @GNUTAR{} Operations
219 How to Add Files to Existing Archives: @option{--append}
221 * appending files:: Appending Files to an Archive
228 Options Used by @option{--create}
230 * override:: Overriding File Metadata.
231 * Ignore Failed Read::
233 Options Used by @option{--extract}
235 * Reading:: Options to Help Read Archives
236 * Writing:: Changing How @command{tar} Writes Files
237 * Scarce:: Coping with Scarce Resources
239 Options to Help Read Archives
241 * read full records::
244 Changing How @command{tar} Writes Files
246 * Dealing with Old Files::
247 * Overwrite Old Files::
252 * Data Modification Times::
253 * Setting Access Permissions::
254 * Directory Modification Times and Permissions::
255 * Writing to Standard Output::
256 * Writing to an External Program::
259 Coping with Scarce Resources
264 Performing Backups and Restoring Files
266 * Full Dumps:: Using @command{tar} to Perform Full Dumps
267 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
268 * Backup Levels:: Levels of Backups
269 * Backup Parameters:: Setting Parameters for Backups and Restoration
270 * Scripted Backups:: Using the Backup Scripts
271 * Scripted Restoration:: Using the Restore Script
273 Setting Parameters for Backups and Restoration
275 * General-Purpose Variables::
276 * Magnetic Tape Control::
278 * backup-specs example:: An Example Text of @file{Backup-specs}
280 Choosing Files and Names for @command{tar}
282 * file:: Choosing the Archive's Name
283 * Selecting Archive Members::
284 * files:: Reading Names from a File
285 * exclude:: Excluding Some Files
286 * wildcards:: Wildcards Patterns and Matching
287 * quoting styles:: Ways of Quoting Special Characters in Names
288 * transform:: Modifying File and Member Names
289 * after:: Operating Only on New Files
290 * recurse:: Descending into Directories
291 * one:: Crossing File System Boundaries
293 Reading Names from a File
299 * problems with exclude::
301 Wildcards Patterns and Matching
303 * controlling pattern-matching::
305 Crossing File System Boundaries
307 * directory:: Changing Directory
308 * absolute:: Absolute File Names
312 * General date syntax:: Common rules.
313 * Calendar date items:: 19 Dec 1994.
314 * Time of day items:: 9:20pm.
315 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}.
316 * Day of week items:: Monday and others.
317 * Relative items in date strings:: next tuesday, 2 years ago.
318 * Pure numbers in date strings:: 19931219, 1440.
319 * Seconds since the Epoch:: @@1078100502.
320 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0".
321 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al.
323 Controlling the Archive Format
325 * Compression:: Using Less Space through Compression
326 * Attributes:: Handling File Attributes
327 * Portability:: Making @command{tar} Archives More Portable
328 * cpio:: Comparison of @command{tar} and @command{cpio}
330 Using Less Space through Compression
332 * gzip:: Creating and Reading Compressed Archives
333 * sparse:: Archiving Sparse Files
335 Creating and Reading Compressed Archives
337 * lbzip2:: Using lbzip2 with @GNUTAR{}.
339 Making @command{tar} Archives More Portable
341 * Portable Names:: Portable Names
342 * dereference:: Symbolic Links
343 * hard links:: Hard Links
344 * old:: Old V7 Archives
345 * ustar:: Ustar Archives
346 * gnu:: GNU and old GNU format archives.
347 * posix:: @acronym{POSIX} archives
348 * Checksumming:: Checksumming Problems
349 * Large or Negative Values:: Large files, negative time stamps, etc.
350 * Other Tars:: How to Extract GNU-Specific Data Using
351 Other @command{tar} Implementations
353 @GNUTAR{} and @acronym{POSIX} @command{tar}
355 * PAX keywords:: Controlling Extended Header Keywords.
357 How to Extract GNU-Specific Data Using Other @command{tar} Implementations
359 * Split Recovery:: Members Split Between Volumes
360 * Sparse Recovery:: Sparse Members
362 Tapes and Other Archive Media
364 * Device:: Device selection and switching
365 * Remote Tape Server::
366 * Common Problems and Solutions::
367 * Blocking:: Blocking
368 * Many:: Many archives on one tape
369 * Using Multiple Tapes:: Using Multiple Tapes
370 * label:: Including a Label in the Archive
376 * Format Variations:: Format Variations
377 * Blocking Factor:: The Blocking Factor of an Archive
379 Many Archives on One Tape
381 * Tape Positioning:: Tape Positions and Tape Marks
382 * mt:: The @command{mt} Utility
386 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
387 * Tape Files:: Tape Files
388 * Tarcat:: Concatenate Volumes into a Single Archive
393 * Standard:: Basic Tar Format
394 * Extensions:: @acronym{GNU} Extensions to the Archive Format
395 * Sparse Formats:: Storing Sparse Files
402 * PAX 0:: PAX Format, Versions 0.0 and 0.1
403 * PAX 1:: PAX Format, Version 1.0
407 * Generate Mode:: File Generation Mode.
408 * Status Mode:: File Status Mode.
409 * Exec Mode:: Synchronous Execution mode.
413 * GNU Free Documentation License:: License for copying this manual
419 @chapter Introduction
422 and manipulates @dfn{archives} which are actually collections of
423 many other files; the program provides users with an organized and
424 systematic method for controlling a large amount of data.
425 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
426 archives need not (and these days, typically do not) reside on tapes.
429 * Book Contents:: What this Book Contains
430 * Definitions:: Some Definitions
431 * What tar Does:: What @command{tar} Does
432 * Naming tar Archives:: How @command{tar} Archives are Named
433 * Authors:: @GNUTAR{} Authors
434 * Reports:: Reporting bugs or suggestions
438 @section What this Book Contains
440 The first part of this chapter introduces you to various terms that will
441 recur throughout the book. It also tells you who has worked on @GNUTAR{}
442 and its documentation, and where you should send bug reports
445 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
446 gentle introduction for people who are new to using @command{tar}. It is
447 meant to be self-contained, not requiring any reading from subsequent
448 chapters to make sense. It moves from topic to topic in a logical,
449 progressive order, building on information already explained.
451 Although the tutorial is paced and structured to allow beginners to
452 learn how to use @command{tar}, it is not intended solely for beginners.
453 The tutorial explains how to use the three most frequently used
454 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
455 two frequently used options (@samp{file} and @samp{verbose}). The other
456 chapters do not refer to the tutorial frequently; however, if a section
457 discusses something which is a complex variant of a basic concept, there
458 may be a cross-reference to that basic concept. (The entire book,
459 including the tutorial, assumes that the reader understands some basic
460 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
462 The third chapter presents the remaining five operations, and
463 information about using @command{tar} options and option syntax.
465 The other chapters are meant to be used as a reference. Each chapter
466 presents everything that needs to be said about a specific topic.
468 One of the chapters (@pxref{Date input formats}) exists in its
469 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
470 In addition, one section of this manual (@pxref{Standard}) contains a
471 big quote which is taken directly from @command{tar} sources.
473 In general, we give both long and short (abbreviated) option names
474 at least once in each section where the relevant option is covered, so
475 that novice readers will become familiar with both styles. (A few
476 options have no short versions, and the relevant sections will
480 @section Some Definitions
484 The @command{tar} program is used to create and manipulate @command{tar}
485 archives. An @dfn{archive} is a single file which contains the contents
486 of many files, while still identifying the names of the files, their
487 owner(s), and so forth. (In addition, archives record access
488 permissions, user and group, size in bytes, and data modification time.
489 Some archives also record the file names in each archived directory, as
490 well as other file and directory information.) You can use @command{tar}
491 to @dfn{create} a new archive in a specified directory.
494 @cindex archive member
497 The files inside an archive are called @dfn{members}. Within this
498 manual, we use the term @dfn{file} to refer only to files accessible in
499 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
500 @dfn{member} to refer only to the members of an archive. Similarly, a
501 @dfn{file name} is the name of a file, as it resides in the file system,
502 and a @dfn{member name} is the name of an archive member within the
507 The term @dfn{extraction} refers to the process of copying an archive
508 member (or multiple members) into a file in the file system. Extracting
509 all the members of an archive is often called @dfn{extracting the
510 archive}. The term @dfn{unpack} can also be used to refer to the
511 extraction of many or all the members of an archive. Extracting an
512 archive does not destroy the archive's structure, just as creating an
513 archive does not destroy the copies of the files that exist outside of
514 the archive. You may also @dfn{list} the members in a given archive
515 (this is often thought of as ``printing'' them to the standard output,
516 or the command line), or @dfn{append} members to a pre-existing archive.
517 All of these operations can be performed using @command{tar}.
520 @section What @command{tar} Does
523 The @command{tar} program provides the ability to create @command{tar}
524 archives, as well as various other kinds of manipulation. For example,
525 you can use @command{tar} on previously created archives to extract files,
526 to store additional files, or to update or list files which were already
529 Initially, @command{tar} archives were used to store files conveniently on
530 magnetic tape. The name @command{tar} comes from this use; it stands for
531 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
532 direct its output to available devices, files, or other programs (using
533 pipes). @command{tar} may even access remote devices or files (as archives).
535 You can use @command{tar} archives in many ways. We want to stress a few
536 of them: storage, backup, and transportation.
538 @FIXME{the following table entries need a bit of work.}
541 Often, @command{tar} archives are used to store related files for
542 convenient file transfer over a network. For example, the
543 @acronym{GNU} Project distributes its software bundled into
544 @command{tar} archives, so that all the files relating to a particular
545 program (or set of related programs) can be transferred as a single
548 A magnetic tape can store several files in sequence. However, the tape
549 has no names for these files; it only knows their relative position on
550 the tape. One way to store several files on one tape and retain their
551 names is by creating a @command{tar} archive. Even when the basic transfer
552 mechanism can keep track of names, as FTP can, the nuisance of handling
553 multiple files, directories, and multiple links makes @command{tar}
556 Archive files are also used for long-term storage. You can think of
557 this as transportation from the present into the future. (It is a
558 science-fiction idiom that you can move through time as well as in
559 space; the idea here is that @command{tar} can be used to move archives in
560 all dimensions, even time!)
563 Because the archive created by @command{tar} is capable of preserving
564 file information and directory structure, @command{tar} is commonly
565 used for performing full and incremental backups of disks. A backup
566 puts a collection of files (possibly pertaining to many users and
567 projects) together on a disk or a tape. This guards against
568 accidental destruction of the information in those files.
569 @GNUTAR{} has special features that allow it to be
570 used to make incremental and full dumps of all the files in a
574 You can create an archive on one system, transfer it to another system,
575 and extract the contents there. This allows you to transport a group of
576 files from one system to another.
579 @node Naming tar Archives
580 @section How @command{tar} Archives are Named
582 Conventionally, @command{tar} archives are given names ending with
583 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
584 but this manual follows that convention in order to accustom readers to
585 it and to make examples more clear.
590 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
591 archive members as ``files'' or ``entries''. For people familiar with
592 the operation of @command{tar}, this causes no difficulty. However, in
593 this manual, we consistently refer to ``archives'' and ``archive
594 members'' to make learning to use @command{tar} easier for novice users.
597 @section @GNUTAR{} Authors
599 @GNUTAR{} was originally written by John Gilmore,
600 and modified by many people. The @acronym{GNU} enhancements were
601 written by Jay Fenlason, then Joy Kendall, and the whole package has
602 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
603 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
604 numerous and kind users.
606 We wish to stress that @command{tar} is a collective work, and owes much to
607 all those people who reported problems, offered solutions and other
608 insights, or shared their thoughts and suggestions. An impressive, yet
609 partial list of those contributors can be found in the @file{THANKS}
610 file from the @GNUTAR{} distribution.
612 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
613 sure i want to spell out the history in this detail, at least not for
614 the printed book. i'm just not sure it needs to be said this way.
615 i'll think about it.}
617 @FIXME{History is more important, and surely more interesting, than
618 actual names. Quoting names without history would be meaningless. FP}
620 Jay Fenlason put together a draft of a @GNUTAR{}
621 manual, borrowing notes from the original man page from John Gilmore.
622 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
623 Gorin worked on a tutorial and manual for @GNUTAR{}.
624 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
625 taking information from all these sources and merging them. Melissa
626 Weisshaus finally edited and redesigned the book to create version
627 1.12. The book for versions from 1.14 up to @value{VERSION} were edited
628 by the current maintainer, Sergey Poznyakoff.
630 For version 1.12, Daniel Hagerty contributed a great deal of technical
631 consulting. In particular, he is the primary author of @ref{Backups}.
633 In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org
634 (see @url{http://savannah.gnu.org/projects/tar}), and
635 active development and maintenance work has started
636 again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey
637 Poznyakoff and Jeff Bailey.
639 Support for @acronym{POSIX} archives was added by Sergey Poznyakoff.
642 @section Reporting bugs or suggestions
645 @cindex reporting bugs
646 If you find problems or have suggestions about this program or manual,
647 please report them to @file{bug-tar@@gnu.org}.
649 When reporting a bug, please be sure to include as much detail as
650 possible, in order to reproduce it. @FIXME{Be more specific, I'd
651 like to make this node as detailed as 'Bug reporting' node in Emacs
655 @chapter Tutorial Introduction to @command{tar}
657 This chapter guides you through some basic examples of three @command{tar}
658 operations: @option{--create}, @option{--list}, and @option{--extract}. If
659 you already know how to use some other version of @command{tar}, then you
660 may not need to read this chapter. This chapter omits most complicated
661 details about how @command{tar} works.
665 * stylistic conventions::
666 * basic tar options:: Basic @command{tar} Operations and Options
667 * frequent operations::
668 * Two Frequent Options::
669 * create:: How to Create Archives
670 * list:: How to List Archives
671 * extract:: How to Extract Members from an Archive
676 @section Assumptions this Tutorial Makes
678 This chapter is paced to allow beginners to learn about @command{tar}
679 slowly. At the same time, we will try to cover all the basic aspects of
680 these three operations. In order to accomplish both of these tasks, we
681 have made certain assumptions about your knowledge before reading this
682 manual, and the hardware you will be using:
686 Before you start to work through this tutorial, you should understand
687 what the terms ``archive'' and ``archive member'' mean
688 (@pxref{Definitions}). In addition, you should understand something
689 about how Unix-type operating systems work, and you should know how to
690 use some basic utilities. For example, you should know how to create,
691 list, copy, rename, edit, and delete files and directories; how to
692 change between directories; and how to figure out where you are in the
693 file system. You should have some basic understanding of directory
694 structure and how files are named according to which directory they are
695 in. You should understand concepts such as standard output and standard
696 input, what various definitions of the term @samp{argument} mean, and the
697 differences between relative and absolute file names. @FIXME{and what
701 This manual assumes that you are working from your own home directory
702 (unless we state otherwise). In this tutorial, you will create a
703 directory to practice @command{tar} commands in. When we show file names,
704 we will assume that those names are relative to your home directory.
705 For example, my home directory is @file{/home/fsf/melissa}. All of
706 my examples are in a subdirectory of the directory named by that file
707 name; the subdirectory is called @file{practice}.
710 In general, we show examples of archives which exist on (or can be
711 written to, or worked with from) a directory on a hard disk. In most
712 cases, you could write those archives to, or work with them on any other
713 device, such as a tape drive. However, some of the later examples in
714 the tutorial and next chapter will not work on tape drives.
715 Additionally, working with tapes is much more complicated than working
716 with hard disks. For these reasons, the tutorial does not cover working
717 with tape drives. @xref{Media}, for complete information on using
718 @command{tar} archives with tape drives.
720 @FIXME{this is a cop out. need to add some simple tape drive info.}
723 @node stylistic conventions
724 @section Stylistic Conventions
726 In the examples, @samp{$} represents a typical shell prompt. It
727 precedes lines you should type; to make this more clear, those lines are
728 shown in @kbd{this font}, as opposed to lines which represent the
729 computer's response; those lines are shown in @code{this font}, or
730 sometimes @samp{like this}.
732 @c When we have lines which are too long to be
733 @c displayed in any other way, we will show them like this:
735 @node basic tar options
736 @section Basic @command{tar} Operations and Options
738 @command{tar} can take a wide variety of arguments which specify and define
739 the actions it will have on the particular set of files or the archive.
740 The main types of arguments to @command{tar} fall into one of two classes:
741 operations, and options.
743 Some arguments fall into a class called @dfn{operations}; exactly one of
744 these is both allowed and required for any instance of using @command{tar};
745 you may @emph{not} specify more than one. People sometimes speak of
746 @dfn{operating modes}. You are in a particular operating mode when you
747 have specified the operation which specifies it; there are eight
748 operations in total, and thus there are eight operating modes.
750 The other arguments fall into the class known as @dfn{options}. You are
751 not required to specify any options, and you are allowed to specify more
752 than one at a time (depending on the way you are using @command{tar} at
753 that time). Some options are used so frequently, and are so useful for
754 helping you type commands more carefully that they are effectively
755 ``required''. We will discuss them in this chapter.
757 You can write most of the @command{tar} operations and options in any
758 of three forms: long (mnemonic) form, short form, and old style. Some
759 of the operations and options have no short or ``old'' forms; however,
760 the operations and options which we will cover in this tutorial have
761 corresponding abbreviations. We will indicate those abbreviations
762 appropriately to get you used to seeing them. Note, that the ``old
763 style'' option forms exist in @GNUTAR{} for compatibility with Unix
764 @command{tar}. In this book we present a full discussion of this way
765 of writing options and operations (@pxref{Old Options}), and we discuss
766 the other two styles of writing options (@xref{Long Options}, and
767 @pxref{Short Options}).
769 In the examples and in the text of this tutorial, we usually use the
770 long forms of operations and options; but the ``short'' forms produce
771 the same result and can make typing long @command{tar} commands easier.
772 For example, instead of typing
775 @kbd{tar --create --verbose --file=afiles.tar apple angst aspic}
781 @kbd{tar -c -v -f afiles.tar apple angst aspic}
787 @kbd{tar -cvf afiles.tar apple angst aspic}
791 For more information on option syntax, see @ref{Advanced tar}. In
792 discussions in the text, when we name an option by its long form, we
793 also give the corresponding short option in parentheses.
795 The term, ``option'', can be confusing at times, since ``operations''
796 are often lumped in with the actual, @emph{optional} ``options'' in certain
797 general class statements. For example, we just talked about ``short and
798 long forms of options and operations''. However, experienced @command{tar}
799 users often refer to these by shorthand terms such as, ``short and long
800 options''. This term assumes that the ``operations'' are included, also.
801 Context will help you determine which definition of ``options'' to use.
803 Similarly, the term ``command'' can be confusing, as it is often used in
804 two different ways. People sometimes refer to @command{tar} ``commands''.
805 A @command{tar} @dfn{command} is the entire command line of user input
806 which tells @command{tar} what to do --- including the operation, options,
807 and any arguments (file names, pipes, other commands, etc.). However,
808 you will also sometimes hear the term ``the @command{tar} command''. When
809 the word ``command'' is used specifically like this, a person is usually
810 referring to the @command{tar} @emph{operation}, not the whole line.
811 Again, use context to figure out which of the meanings the speaker
814 @node frequent operations
815 @section The Three Most Frequently Used Operations
817 Here are the three most frequently used operations (both short and long
818 forms), as well as a brief description of their meanings. The rest of
819 this chapter will cover how to use these operations in detail. We will
820 present the rest of the operations in the next chapter.
825 Create a new @command{tar} archive.
828 List the contents of an archive.
831 Extract one or more members from an archive.
834 @node Two Frequent Options
835 @section Two Frequently Used Options
837 To understand how to run @command{tar} in the three operating modes listed
838 previously, you also need to understand how to use two of the options to
839 @command{tar}: @option{--file} (which takes an archive file as an argument)
840 and @option{--verbose}. (You are usually not @emph{required} to specify
841 either of these options when you run @command{tar}, but they can be very
842 useful in making things more clear and helping you avoid errors.)
851 @unnumberedsubsec The @option{--file} Option
854 @xopindex{file, tutorial}
855 @item --file=@var{archive-name}
856 @itemx -f @var{archive-name}
857 Specify the name of an archive file.
860 You can specify an argument for the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option whenever you
861 use @command{tar}; this option determines the name of the archive file
862 that @command{tar} will work on.
865 If you don't specify this argument, then @command{tar} will examine
866 the environment variable @env{TAPE}. If it is set, its value will be
867 used as the archive name. Otherwise, @command{tar} will use the
868 default archive, determined at compile time. Usually it is
869 standard output or some physical tape drive attached to your machine
870 (you can verify what the default is by running @kbd{tar
871 --show-defaults}, @pxref{defaults}). If there is no tape drive
872 attached, or the default is not meaningful, then @command{tar} will
873 print an error message. The error message might look roughly like one
877 tar: can't open /dev/rmt8 : No such device or address
878 tar: can't open /dev/rsmt0 : I/O error
882 To avoid confusion, we recommend that you always specify an archive file
883 name by using @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) when writing your @command{tar} commands.
884 For more information on using the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option, see
887 @node verbose tutorial
888 @unnumberedsubsec The @option{--verbose} Option
891 @xopindex{verbose, introduced}
894 Show the files being worked on as @command{tar} is running.
897 @option{--verbose} (@option{-v}) shows details about the results of running
898 @command{tar}. This can be especially useful when the results might not be
899 obvious. For example, if you want to see the progress of @command{tar} as
900 it writes files into the archive, you can use the @option{--verbose}
901 option. In the beginning, you may find it useful to use
902 @option{--verbose} at all times; when you are more accustomed to
903 @command{tar}, you will likely want to use it at certain times but not at
904 others. We will use @option{--verbose} at times to help make something
905 clear, and we will give many examples both using and not using
906 @option{--verbose} to show the differences.
908 Each instance of @option{--verbose} on the command line increases the
909 verbosity level by one, so if you need more details on the output,
912 When reading archives (@option{--list}, @option{--extract},
913 @option{--diff}), @command{tar} by default prints only the names of
914 the members being extracted. Using @option{--verbose} will show a full,
915 @command{ls} style member listing.
917 In contrast, when writing archives (@option{--create}, @option{--append},
918 @option{--update}), @command{tar} does not print file names by
919 default. So, a single @option{--verbose} option shows the file names
920 being added to the archive, while two @option{--verbose} options
921 enable the full listing.
923 For example, to create an archive in verbose mode:
926 $ @kbd{tar -cvf afiles.tar apple angst aspic}
933 Creating the same archive with the verbosity level 2 could give:
936 $ @kbd{tar -cvvf afiles.tar apple angst aspic}
937 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
938 -rw-r--r-- gray/staff 11481 2006-06-09 12:06 angst
939 -rw-r--r-- gray/staff 23152 2006-06-09 12:06 aspic
943 This works equally well using short or long forms of options. Using
944 long forms, you would simply write out the mnemonic form of the option
948 $ @kbd{tar --create --verbose --verbose @dots{}}
952 Note that you must double the hyphens properly each time.
954 Later in the tutorial, we will give examples using @w{@option{--verbose
957 @anchor{verbose member listing}
958 The full output consists of six fields:
961 @item File type and permissions in symbolic form.
962 These are displayed in the same format as the first column of
963 @command{ls -l} output (@pxref{What information is listed,
964 format=verbose, Verbose listing, fileutils, GNU file utilities}).
966 @item Owner name and group separated by a slash character.
967 If these data are not available (for example, when listing a @samp{v7} format
968 archive), numeric @acronym{ID} values are printed instead.
970 @item Size of the file, in bytes.
972 @item File modification date in ISO 8601 format.
974 @item File modification time.
977 If the name contains any special characters (white space, newlines,
978 etc.) these are displayed in an unambiguous form using so called
979 @dfn{quoting style}. For the detailed discussion of available styles
980 and on how to use them, see @ref{quoting styles}.
982 Depending on the file type, the name can be followed by some
983 additional information, described in the following table:
986 @item -> @var{link-name}
987 The file or archive member is a @dfn{symbolic link} and
988 @var{link-name} is the name of file it links to.
990 @item link to @var{link-name}
991 The file or archive member is a @dfn{hard link} and @var{link-name} is
992 the name of file it links to.
995 The archive member is an old GNU format long link. You will normally
999 The archive member is an old GNU format long name. You will normally
1002 @item --Volume Header--
1003 The archive member is a GNU @dfn{volume header} (@pxref{Tape Files}).
1005 @item --Continued at byte @var{n}--
1006 Encountered only at the beginning of a multi-volume archive
1007 (@pxref{Using Multiple Tapes}). This archive member is a continuation
1008 from the previous volume. The number @var{n} gives the offset where
1009 the original file was split.
1011 @item unknown file type @var{c}
1012 An archive member of unknown type. @var{c} is the type character from
1013 the archive header. If you encounter such a message, it means that
1014 either your archive contains proprietary member types @GNUTAR{} is not
1015 able to handle, or the archive is corrupted.
1020 For example, here is an archive listing containing most of the special
1021 suffixes explained above:
1025 V--------- 0/0 1536 2006-06-09 13:07 MyVolume--Volume Header--
1026 -rw-r--r-- gray/staff 456783 2006-06-09 12:06 aspic--Continued at byte 32456--
1027 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
1028 lrwxrwxrwx gray/staff 0 2006-06-09 13:01 angst -> apple
1029 -rw-r--r-- gray/staff 35793 2006-06-09 12:06 blues
1030 hrw-r--r-- gray/staff 0 2006-06-09 12:06 music link to blues
1038 @unnumberedsubsec Getting Help: Using the @option{--help} Option
1044 The @option{--help} option to @command{tar} prints out a very brief list of
1045 all operations and option available for the current version of
1046 @command{tar} available on your system.
1050 @section How to Create Archives
1053 @cindex Creation of the archive
1054 @cindex Archive, creation of
1055 One of the basic operations of @command{tar} is @option{--create} (@option{-c}), which
1056 you use to create a @command{tar} archive. We will explain
1057 @option{--create} first because, in order to learn about the other
1058 operations, you will find it useful to have an archive available to
1061 To make this easier, in this section you will first create a directory
1062 containing three files. Then, we will show you how to create an
1063 @emph{archive} (inside the new directory). Both the directory, and
1064 the archive are specifically for you to practice on. The rest of this
1065 chapter and the next chapter will show many examples using this
1066 directory and the files you will create: some of those files may be
1067 other directories and other archives.
1069 The three files you will archive in this example are called
1070 @file{blues}, @file{folk}, and @file{jazz}. The archive is called
1071 @file{collection.tar}.
1073 This section will proceed slowly, detailing how to use @option{--create}
1074 in @code{verbose} mode, and showing examples using both short and long
1075 forms. In the rest of the tutorial, and in the examples in the next
1076 chapter, we will proceed at a slightly quicker pace. This section
1077 moves more slowly to allow beginning users to understand how
1078 @command{tar} works.
1081 * prepare for examples::
1082 * Creating the archive::
1088 @node prepare for examples
1089 @subsection Preparing a Practice Directory for Examples
1091 To follow along with this and future examples, create a new directory
1092 called @file{practice} containing files called @file{blues}, @file{folk}
1093 and @file{jazz}. The files can contain any information you like:
1094 ideally, they should contain information which relates to their names,
1095 and be of different lengths. Our examples assume that @file{practice}
1096 is a subdirectory of your home directory.
1098 Now @command{cd} to the directory named @file{practice}; @file{practice}
1099 is now your @dfn{working directory}. (@emph{Please note}: Although
1100 the full file name of this directory is
1101 @file{/@var{homedir}/practice}, in our examples we will refer to
1102 this directory as @file{practice}; the @var{homedir} is presumed.)
1104 In general, you should check that the files to be archived exist where
1105 you think they do (in the working directory) by running @command{ls}.
1106 Because you just created the directory and the files and have changed to
1107 that directory, you probably don't need to do that this time.
1109 It is very important to make sure there isn't already a file in the
1110 working directory with the archive name you intend to use (in this case,
1111 @samp{collection.tar}), or that you don't care about its contents.
1112 Whenever you use @samp{create}, @command{tar} will erase the current
1113 contents of the file named by @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) if it exists. @command{tar}
1114 will not tell you if you are about to overwrite an archive unless you
1115 specify an option which does this (@pxref{backup}, for the
1116 information on how to do so). To add files to an existing archive,
1117 you need to use a different option, such as @option{--append} (@option{-r}); see
1118 @ref{append} for information on how to do this.
1120 @node Creating the archive
1121 @subsection Creating the Archive
1123 @xopindex{create, introduced}
1124 To place the files @file{blues}, @file{folk}, and @file{jazz} into an
1125 archive named @file{collection.tar}, use the following command:
1128 $ @kbd{tar --create --file=collection.tar blues folk jazz}
1131 The order of the arguments is not very important, @emph{when using long
1132 option forms}. You could also say:
1135 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1139 However, you can see that this order is harder to understand; this is
1140 why we will list the arguments in the order that makes the commands
1141 easiest to understand (and we encourage you to do the same when you use
1142 @command{tar}, to avoid errors).
1144 Note that the sequence
1145 @option{--file=@-collection.tar} is considered to be @emph{one} argument.
1146 If you substituted any other string of characters for
1147 @kbd{collection.tar}, then that string would become the name of the
1148 archive file you create.
1150 The order of the options becomes more important when you begin to use
1151 short forms. With short forms, if you type commands in the wrong order
1152 (even if you type them correctly in all other ways), you may end up with
1153 results you don't expect. For this reason, it is a good idea to get
1154 into the habit of typing options in the order that makes inherent sense.
1155 @xref{short create}, for more information on this.
1157 In this example, you type the command as shown above: @option{--create}
1158 is the operation which creates the new archive
1159 (@file{collection.tar}), and @option{--file} is the option which lets
1160 you give it the name you chose. The files, @file{blues}, @file{folk},
1161 and @file{jazz}, are now members of the archive, @file{collection.tar}
1162 (they are @dfn{file name arguments} to the @option{--create} operation.
1163 @xref{Choosing}, for the detailed discussion on these.) Now that they are
1164 in the archive, they are called @emph{archive members}, not files.
1165 (@pxref{Definitions,members}).
1167 When you create an archive, you @emph{must} specify which files you
1168 want placed in the archive. If you do not specify any archive
1169 members, @GNUTAR{} will complain.
1171 If you now list the contents of the working directory (@command{ls}), you will
1172 find the archive file listed as well as the files you saw previously:
1175 blues folk jazz collection.tar
1179 Creating the archive @samp{collection.tar} did not destroy the copies of
1180 the files in the directory.
1182 Keep in mind that if you don't indicate an operation, @command{tar} will not
1183 run and will prompt you for one. If you don't name any files, @command{tar}
1184 will complain. You must have write access to the working directory,
1185 or else you will not be able to create an archive in that directory.
1187 @emph{Caution}: Do not attempt to use @option{--create} (@option{-c}) to add files to
1188 an existing archive; it will delete the archive and write a new one.
1189 Use @option{--append} (@option{-r}) instead. @xref{append}.
1191 @node create verbose
1192 @subsection Running @option{--create} with @option{--verbose}
1194 @xopindex{create, using with @option{--verbose}}
1195 @xopindex{verbose, using with @option{--create}}
1196 If you include the @option{--verbose} (@option{-v}) option on the command line,
1197 @command{tar} will list the files it is acting on as it is working. In
1198 verbose mode, the @code{create} example above would appear as:
1201 $ @kbd{tar --create --verbose --file=collection.tar blues folk jazz}
1207 This example is just like the example we showed which did not use
1208 @option{--verbose}, except that @command{tar} generated the remaining
1210 lines (note the different font styles).
1216 In the rest of the examples in this chapter, we will frequently use
1217 @code{verbose} mode so we can show actions or @command{tar} responses that
1218 you would otherwise not see, and which are important for you to
1222 @subsection Short Forms with @samp{create}
1224 As we said before, the @option{--create} (@option{-c}) operation is one of the most
1225 basic uses of @command{tar}, and you will use it countless times.
1226 Eventually, you will probably want to use abbreviated (or ``short'')
1227 forms of options. A full discussion of the three different forms that
1228 options can take appears in @ref{Styles}; for now, here is what the
1229 previous example (including the @option{--verbose} (@option{-v}) option) looks like
1230 using short option forms:
1233 $ @kbd{tar -cvf collection.tar blues folk jazz}
1240 As you can see, the system responds the same no matter whether you use
1241 long or short option forms.
1243 @FIXME{i don't like how this is worded:} One difference between using
1244 short and long option forms is that, although the exact placement of
1245 arguments following options is no more specific when using short forms,
1246 it is easier to become confused and make a mistake when using short
1247 forms. For example, suppose you attempted the above example in the
1251 $ @kbd{tar -cfv collection.tar blues folk jazz}
1255 In this case, @command{tar} will make an archive file called @file{v},
1256 containing the files @file{blues}, @file{folk}, and @file{jazz}, because
1257 the @samp{v} is the closest ``file name'' to the @option{-f} option, and
1258 is thus taken to be the chosen archive file name. @command{tar} will try
1259 to add a file called @file{collection.tar} to the @file{v} archive file;
1260 if the file @file{collection.tar} did not already exist, @command{tar} will
1261 report an error indicating that this file does not exist. If the file
1262 @file{collection.tar} does already exist (e.g., from a previous command
1263 you may have run), then @command{tar} will add this file to the archive.
1264 Because the @option{-v} option did not get registered, @command{tar} will not
1265 run under @samp{verbose} mode, and will not report its progress.
1267 The end result is that you may be quite confused about what happened,
1268 and possibly overwrite a file. To illustrate this further, we will show
1269 you how an example we showed previously would look using short forms.
1274 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1278 is confusing as it is. When shown using short forms, however, it
1279 becomes much more so:
1282 $ @kbd{tar blues -c folk -f collection.tar jazz}
1286 It would be very easy to put the wrong string of characters
1287 immediately following the @option{-f}, but doing that could sacrifice
1290 For this reason, we recommend that you pay very careful attention to
1291 the order of options and placement of file and archive names,
1292 especially when using short option forms. Not having the option name
1293 written out mnemonically can affect how well you remember which option
1294 does what, and therefore where different names have to be placed.
1297 @subsection Archiving Directories
1299 @cindex Archiving Directories
1300 @cindex Directories, Archiving
1301 You can archive a directory by specifying its directory name as a
1302 file name argument to @command{tar}. The files in the directory will be
1303 archived relative to the working directory, and the directory will be
1304 re-created along with its contents when the archive is extracted.
1306 To archive a directory, first move to its superior directory. If you
1307 have followed the previous instructions in this tutorial, you should
1316 This will put you into the directory which contains @file{practice},
1317 i.e., your home directory. Once in the superior directory, you can
1318 specify the subdirectory, @file{practice}, as a file name argument. To
1319 store @file{practice} in the new archive file @file{music.tar}, type:
1322 $ @kbd{tar --create --verbose --file=music.tar practice}
1326 @command{tar} should output:
1333 practice/collection.tar
1336 Note that the archive thus created is not in the subdirectory
1337 @file{practice}, but rather in the current working directory---the
1338 directory from which @command{tar} was invoked. Before trying to archive a
1339 directory from its superior directory, you should make sure you have
1340 write access to the superior directory itself, not only the directory
1341 you are trying archive with @command{tar}. For example, you will probably
1342 not be able to store your home directory in an archive by invoking
1343 @command{tar} from the root directory; @xref{absolute}. (Note
1344 also that @file{collection.tar}, the original archive file, has itself
1345 been archived. @command{tar} will accept any file as a file to be
1346 archived, regardless of its content. When @file{music.tar} is
1347 extracted, the archive file @file{collection.tar} will be re-written
1348 into the file system).
1350 If you give @command{tar} a command such as
1353 $ @kbd{tar --create --file=foo.tar .}
1357 @command{tar} will report @samp{tar: ./foo.tar is the archive; not
1358 dumped}. This happens because @command{tar} creates the archive
1359 @file{foo.tar} in the current directory before putting any files into
1360 it. Then, when @command{tar} attempts to add all the files in the
1361 directory @file{.} to the archive, it notices that the file
1362 @file{./foo.tar} is the same as the archive @file{foo.tar}, and skips
1363 it. (It makes no sense to put an archive into itself.) @GNUTAR{}
1364 will continue in this case, and create the archive
1365 normally, except for the exclusion of that one file. (@emph{Please
1366 note:} Other implementations of @command{tar} may not be so clever;
1367 they will enter an infinite loop when this happens, so you should not
1368 depend on this behavior unless you are certain you are running
1369 @GNUTAR{}. In general, it is wise to always place the archive outside
1370 of the directory being dumped.)
1373 @section How to List Archives
1376 Frequently, you will find yourself wanting to determine exactly what a
1377 particular archive contains. You can use the @option{--list}
1378 (@option{-t}) operation to get the member names as they currently
1379 appear in the archive, as well as various attributes of the files at
1380 the time they were archived. For example, you can examine the archive
1381 @file{collection.tar} that you created in the last section with the
1385 $ @kbd{tar --list --file=collection.tar}
1389 The output of @command{tar} would then be:
1398 The archive @file{bfiles.tar} would list as follows:
1407 Be sure to use a @option{--file=@var{archive-name}} (@option{-f
1408 @var{archive-name}}) option just as with @option{--create}
1409 (@option{-c}) to specify the name of the archive.
1411 @xopindex{list, using with @option{--verbose}}
1412 @xopindex{verbose, using with @option{--list}}
1413 If you use the @option{--verbose} (@option{-v}) option with
1414 @option{--list}, then @command{tar} will print out a listing
1415 reminiscent of @w{@samp{ls -l}}, showing owner, file size, and so
1416 forth. This output is described in detail in @ref{verbose member listing}.
1418 If you had used @option{--verbose} (@option{-v}) mode, the example
1419 above would look like:
1422 $ @kbd{tar --list --verbose --file=collection.tar folk}
1423 -rw-r--r-- myself/user 62 1990-05-23 10:55 folk
1426 @cindex listing member and file names
1427 @anchor{listing member and file names}
1428 It is important to notice that the output of @kbd{tar --list
1429 --verbose} does not necessarily match that produced by @kbd{tar
1430 --create --verbose} while creating the archive. It is because
1431 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1432 prefixes from file names before storing them in the archive
1433 (@xref{absolute}, for more information). In other
1434 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1435 an archive and @dfn{member names} when listing it. Consider this
1440 $ @kbd{tar --create --verbose --file archive /etc/mail}
1441 tar: Removing leading `/' from member names
1443 /etc/mail/sendmail.cf
1445 $ @kbd{tar --test --file archive}
1447 etc/mail/sendmail.cf
1452 @opindex show-stored-names
1453 This default behavior can sometimes be inconvenient. You can force
1454 @GNUTAR{} show member names when creating archive by supplying
1455 @option{--show-stored-names} option.
1458 @item --show-stored-names
1459 Print member (as opposed to @emph{file}) names when creating the archive.
1462 @cindex File name arguments, using @option{--list} with
1463 @xopindex{list, using with file name arguments}
1464 You can specify one or more individual member names as arguments when
1465 using @samp{list}. In this case, @command{tar} will only list the
1466 names of members you identify. For example, @w{@kbd{tar --list
1467 --file=afiles.tar apple}} would only print @file{apple}.
1469 Because @command{tar} preserves file names, these must be specified as
1470 they appear in the archive (i.e., relative to the directory from which
1471 the archive was created). Therefore, it is essential when specifying
1472 member names to @command{tar} that you give the exact member names.
1473 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1474 error message something like @samp{tar: birds: Not found in archive},
1475 because there is no member named @file{birds}, only one named
1476 @file{./birds}. While the names @file{birds} and @file{./birds} name
1477 the same file, @emph{member} names by default are compared verbatim.
1479 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1480 with @file{baboon}, because this exact member name is in the archive file
1481 @file{bfiles.tar}. If you are not sure of the exact file name,
1482 use @dfn{globbing patterns}, for example:
1485 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1489 will list all members whose name contains @samp{b}. @xref{wildcards},
1490 for a detailed discussion of globbing patterns and related
1491 @command{tar} command line options.
1498 @unnumberedsubsec Listing the Contents of a Stored Directory
1500 To get information about the contents of an archived directory,
1501 use the directory name as a file name argument in conjunction with
1502 @option{--list} (@option{-t}). To find out file attributes, include the
1503 @option{--verbose} (@option{-v}) option.
1505 For example, to find out about files in the directory @file{practice}, in
1506 the archive file @file{music.tar}, type:
1509 $ @kbd{tar --list --verbose --file=music.tar practice}
1512 @command{tar} responds:
1515 drwxrwxrwx myself/user 0 1990-05-31 21:49 practice/
1516 -rw-r--r-- myself/user 42 1990-05-21 13:29 practice/blues
1517 -rw-r--r-- myself/user 62 1990-05-23 10:55 practice/folk
1518 -rw-r--r-- myself/user 40 1990-05-21 13:30 practice/jazz
1519 -rw-r--r-- myself/user 10240 1990-05-31 21:49 practice/collection.tar
1522 When you use a directory name as a file name argument, @command{tar} acts on
1523 all the files (including sub-directories) in that directory.
1526 @section How to Extract Members from an Archive
1528 @cindex Retrieving files from an archive
1529 @cindex Resurrecting files from an archive
1532 Creating an archive is only half the job---there is no point in storing
1533 files in an archive if you can't retrieve them. The act of retrieving
1534 members from an archive so they can be used and manipulated as
1535 unarchived files again is called @dfn{extraction}. To extract files
1536 from an archive, use the @option{--extract} (@option{--get} or
1537 @option{-x}) operation. As with @option{--create}, specify the name
1538 of the archive with @option{--file} (@option{-f}) option. Extracting
1539 an archive does not modify the archive in any way; you can extract it
1540 multiple times if you want or need to.
1542 Using @option{--extract}, you can extract an entire archive, or specific
1543 files. The files can be directories containing other files, or not. As
1544 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1545 long form of the operation without affecting the performance.
1548 * extracting archives::
1549 * extracting files::
1551 * extracting untrusted archives::
1552 * failing commands::
1555 @node extracting archives
1556 @subsection Extracting an Entire Archive
1558 To extract an entire archive, specify the archive file name only, with
1559 no individual file names as arguments. For example,
1562 $ @kbd{tar -xvf collection.tar}
1569 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
1570 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
1571 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
1574 @node extracting files
1575 @subsection Extracting Specific Files
1577 To extract specific archive members, give their exact member names as
1578 arguments, as printed by @option{--list} (@option{-t}). If you had
1579 mistakenly deleted one of the files you had placed in the archive
1580 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1581 from the archive without changing the archive's structure. Its
1582 contents will be identical to the original file @file{blues} that you
1585 First, make sure you are in the @file{practice} directory, and list the
1586 files in the directory. Now, delete the file, @samp{blues}, and list
1587 the files in the directory again.
1589 You can now extract the member @file{blues} from the archive file
1590 @file{collection.tar} like this:
1593 $ @kbd{tar --extract --file=collection.tar blues}
1597 If you list the files in the directory again, you will see that the file
1598 @file{blues} has been restored, with its original permissions, data
1599 modification times, and owner.@footnote{This is only accidentally
1600 true, but not in general. Whereas modification times are always
1601 restored, in most cases, one has to be root for restoring the owner,
1602 and use a special option for restoring permissions. Here, it just
1603 happens that the restoring user is also the owner of the archived
1604 members, and that the current @code{umask} is compatible with original
1605 permissions.} (These parameters will be identical to those which
1606 the file had when you originally placed it in the archive; any changes
1607 you may have made before deleting the file from the file system,
1608 however, will @emph{not} have been made to the archive member.) The
1609 archive file, @samp{collection.tar}, is the same as it was before you
1610 extracted @samp{blues}. You can confirm this by running @command{tar} with
1611 @option{--list} (@option{-t}).
1613 Remember that as with other operations, specifying the exact member
1614 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1615 will fail, because there is no member named @file{birds}. To extract
1616 the member named @file{./birds}, you must specify @w{@kbd{tar
1617 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1618 exact member names, use @option{--list} (@option{-t}) option
1619 (@pxref{list}). You can also extract those members that match a
1620 specific @dfn{globbing pattern}. For example, to extract from
1621 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1622 directory prefix, you could type:
1625 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1629 Here, @option{--wildcards} instructs @command{tar} to treat
1630 command line arguments as globbing patterns and @option{--no-anchored}
1631 informs it that the patterns apply to member names after any @samp{/}
1632 delimiter. The use of globbing patterns is discussed in detail in
1635 You can extract a file to standard output by combining the above options
1636 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1639 If you give the @option{--verbose} option, then @option{--extract}
1640 will print the names of the archive members as it extracts them.
1643 @subsection Extracting Files that are Directories
1645 Extracting directories which are members of an archive is similar to
1646 extracting other files. The main difference to be aware of is that if
1647 the extracted directory has the same name as any directory already in
1648 the working directory, then files in the extracted directory will be
1649 placed into the directory of the same name. Likewise, if there are
1650 files in the pre-existing directory with the same names as the members
1651 which you extract, the files from the extracted archive will replace
1652 the files already in the working directory (and possible
1653 subdirectories). This will happen regardless of whether or not the
1654 files in the working directory were more recent than those extracted
1655 (there exist, however, special options that alter this behavior
1658 However, if a file was stored with a directory name as part of its file
1659 name, and that directory does not exist under the working directory when
1660 the file is extracted, @command{tar} will create the directory.
1662 We can demonstrate how to use @option{--extract} to extract a directory
1663 file with an example. Change to the @file{practice} directory if you
1664 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1665 go back to the parent directory and extract the archive
1666 @file{music.tar}. You may either extract the entire archive, or you may
1667 extract only the files you just deleted. To extract the entire archive,
1668 don't give any file names as arguments after the archive name
1669 @file{music.tar}. To extract only the files you deleted, use the
1673 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1679 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1680 would have displayed more detail about the extracted files, as shown
1681 in the example below:
1684 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1685 -rw-r--r-- me/user 28 1996-10-18 16:31 practice/jazz
1686 -rw-r--r-- me/user 20 1996-09-23 16:44 practice/folk
1690 Because you created the directory with @file{practice} as part of the
1691 file names of each of the files by archiving the @file{practice}
1692 directory as @file{practice}, you must give @file{practice} as part
1693 of the file names when you extract those files from the archive.
1695 @node extracting untrusted archives
1696 @subsection Extracting Archives from Untrusted Sources
1698 Extracting files from archives can overwrite files that already exist.
1699 If you receive an archive from an untrusted source, you should make a
1700 new directory and extract into that directory, so that you don't have
1701 to worry about the extraction overwriting one of your existing files.
1702 For example, if @file{untrusted.tar} came from somewhere else on the
1703 Internet, and you don't necessarily trust its contents, you can
1704 extract it as follows:
1707 $ @kbd{mkdir newdir}
1709 $ @kbd{tar -xvf ../untrusted.tar}
1712 It is also a good practice to examine contents of the archive
1713 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1714 with @option{--verbose} (@option{-v}).
1716 @node failing commands
1717 @subsection Commands That Will Fail
1719 Here are some sample commands you might try which will not work, and why
1722 If you try to use this command,
1725 $ @kbd{tar -xvf music.tar folk jazz}
1729 you will get the following response:
1732 tar: folk: Not found in archive
1733 tar: jazz: Not found in archive
1737 This is because these files were not originally @emph{in} the parent
1738 directory @file{..}, where the archive is located; they were in the
1739 @file{practice} directory, and their file names reflect this:
1742 $ @kbd{tar -tvf music.tar}
1748 @FIXME{make sure the above works when going through the examples in
1752 Likewise, if you try to use this command,
1755 $ @kbd{tar -tvf music.tar folk jazz}
1759 you would get a similar response. Members with those names are not in the
1760 archive. You must use the correct member names, or wildcards, in order
1761 to extract the files from the archive.
1763 If you have forgotten the correct names of the files in the archive,
1764 use @w{@kbd{tar --list --verbose}} to list them correctly.
1766 @FIXME{more examples, here? hag thinks it's a good idea.}
1769 @section Going Further Ahead in this Manual
1772 @FIXME{need to write up a node here about the things that are going to
1773 be in the rest of the manual.}
1775 @node tar invocation
1776 @chapter Invoking @GNUTAR{}
1778 This chapter is about how one invokes the @GNUTAR{}
1779 command, from the command synopsis (@pxref{Synopsis}). There are
1780 numerous options, and many styles for writing them. One mandatory
1781 option specifies the operation @command{tar} should perform
1782 (@pxref{Operation Summary}), other options are meant to detail how
1783 this operation should be performed (@pxref{Option Summary}).
1784 Non-option arguments are not always interpreted the same way,
1785 depending on what the operation is.
1787 You will find in this chapter everything about option styles and rules for
1788 writing them (@pxref{Styles}). On the other hand, operations and options
1789 are fully described elsewhere, in other chapters. Here, you will find
1790 only synthetic descriptions for operations and options, together with
1791 pointers to other parts of the @command{tar} manual.
1793 Some options are so special they are fully described right in this
1794 chapter. They have the effect of inhibiting the normal operation of
1795 @command{tar} or else, they globally alter the amount of feedback the user
1796 receives about what is going on. These are the @option{--help} and
1797 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1798 and @option{--interactive} options (@pxref{interactive}).
1802 * using tar options::
1814 @section General Synopsis of @command{tar}
1816 The @GNUTAR{} program is invoked as either one of:
1819 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1820 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1823 The second form is for when old options are being used.
1825 You can use @command{tar} to store files in an archive, to extract them from
1826 an archive, and to do other types of archive manipulation. The primary
1827 argument to @command{tar}, which is called the @dfn{operation}, specifies
1828 which action to take. The other arguments to @command{tar} are either
1829 @dfn{options}, which change the way @command{tar} performs an operation,
1830 or file names or archive members, which specify the files or members
1831 @command{tar} is to act on.
1833 You can actually type in arguments in any order, even if in this manual
1834 the options always precede the other arguments, to make examples easier
1835 to understand. Further, the option stating the main operation mode
1836 (the @command{tar} main command) is usually given first.
1838 Each @var{name} in the synopsis above is interpreted as an archive member
1839 name when the main command is one of @option{--compare}
1840 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1841 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1842 @option{--update} (@option{-u}). When naming archive members, you
1843 must give the exact name of the member in the archive, as it is
1844 printed by @option{--list}. For @option{--append} (@option{-r}) and
1845 @option{--create} (@option{-c}), these @var{name} arguments specify
1846 the names of either files or directory hierarchies to place in the archive.
1847 These files or hierarchies should already exist in the file system,
1848 prior to the execution of the @command{tar} command.
1850 @command{tar} interprets relative file names as being relative to the
1851 working directory. @command{tar} will make all file names relative
1852 (by removing leading slashes when archiving or restoring files),
1853 unless you specify otherwise (using the @option{--absolute-names}
1854 option). @xref{absolute}, for more information about
1855 @option{--absolute-names}.
1857 If you give the name of a directory as either a file name or a member
1858 name, then @command{tar} acts recursively on all the files and directories
1859 beneath that directory. For example, the name @file{/} identifies all
1860 the files in the file system to @command{tar}.
1862 The distinction between file names and archive member names is especially
1863 important when shell globbing is used, and sometimes a source of confusion
1864 for newcomers. @xref{wildcards}, for more information about globbing.
1865 The problem is that shells may only glob using existing files in the
1866 file system. Only @command{tar} itself may glob on archive members, so when
1867 needed, you must ensure that wildcard characters reach @command{tar} without
1868 being interpreted by the shell first. Using a backslash before @samp{*}
1869 or @samp{?}, or putting the whole argument between quotes, is usually
1870 sufficient for this.
1872 Even if @var{name}s are often specified on the command line, they
1873 can also be read from a text file in the file system, using the
1874 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1876 If you don't use any file name arguments, @option{--append} (@option{-r}),
1877 @option{--delete} and @option{--concatenate} (@option{--catenate},
1878 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1879 will usually yield a diagnostic and inhibit @command{tar} execution.
1880 The other operations of @command{tar} (@option{--list},
1881 @option{--extract}, @option{--compare}, and @option{--update})
1882 will act on the entire contents of the archive.
1885 @cindex return status
1886 Besides successful exits, @GNUTAR{} may fail for
1887 many reasons. Some reasons correspond to bad usage, that is, when the
1888 @command{tar} command line is improperly written. Errors may be
1889 encountered later, while processing the archive or the files. Some
1890 errors are recoverable, in which case the failure is delayed until
1891 @command{tar} has completed all its work. Some errors are such that
1892 it would be not meaningful, or at least risky, to continue processing:
1893 @command{tar} then aborts processing immediately. All abnormal exits,
1894 whether immediate or delayed, should always be clearly diagnosed on
1895 @code{stderr}, after a line stating the nature of the error.
1897 Possible exit codes of @GNUTAR{} are summarized in the following
1902 @samp{Successful termination}.
1905 @samp{Some files differ}. If tar was invoked with @option{--compare}
1906 (@option{--diff}, @option{-d}) command line option, this means that
1907 some files in the archive differ from their disk counterparts
1908 (@pxref{compare}). If tar was given @option{--create},
1909 @option{--append} or @option{--update} option, this exit code means
1910 that some files were changed while being archived and so the resulting
1911 archive does not contain the exact copy of the file set.
1914 @samp{Fatal error}. This means that some fatal, unrecoverable error
1918 If @command{tar} has invoked a subprocess and that subprocess exited with a
1919 nonzero exit code, @command{tar} exits with that code as well.
1920 This can happen, for example, if @command{tar} was given some
1921 compression option (@pxref{gzip}) and the external compressor program
1922 failed. Another example is @command{rmt} failure during backup to the
1923 remote device (@pxref{Remote Tape Server}).
1925 @node using tar options
1926 @section Using @command{tar} Options
1928 @GNUTAR{} has a total of eight operating modes which
1929 allow you to perform a variety of tasks. You are required to choose
1930 one operating mode each time you employ the @command{tar} program by
1931 specifying one, and only one operation as an argument to the
1932 @command{tar} command (the corresponding options may be found
1933 at @ref{frequent operations} and @ref{Operations}). Depending on
1934 circumstances, you may also wish to customize how the chosen operating
1935 mode behaves. For example, you may wish to change the way the output
1936 looks, or the format of the files that you wish to archive may require
1937 you to do something special in order to make the archive look right.
1939 You can customize and control @command{tar}'s performance by running
1940 @command{tar} with one or more options (such as @option{--verbose}
1941 (@option{-v}), which we used in the tutorial). As we said in the
1942 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1943 their name suggests) optional. Depending on the operating mode, you
1944 may specify one or more options. Different options will have different
1945 effects, but in general they all change details of the operation, such
1946 as archive format, archive name, or level of user interaction. Some
1947 options make sense with all operating modes, while others are
1948 meaningful only with particular modes. You will likely use some
1949 options frequently, while you will only use others infrequently, or
1950 not at all. (A full list of options is available in @pxref{All Options}.)
1952 @vrindex TAR_OPTIONS, environment variable
1953 @anchor{TAR_OPTIONS}
1954 The @env{TAR_OPTIONS} environment variable specifies default options to
1955 be placed in front of any explicit options. For example, if
1956 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1957 if the two options @option{-v} and @option{--unlink-first} had been
1958 specified before any explicit options. Option specifications are
1959 separated by whitespace. A backslash escapes the next character, so it
1960 can be used to specify an option containing whitespace or a backslash.
1962 Note that @command{tar} options are case sensitive. For example, the
1963 options @option{-T} and @option{-t} are different; the first requires an
1964 argument for stating the name of a file providing a list of @var{name}s,
1965 while the second does not require an argument and is another way to
1966 write @option{--list} (@option{-t}).
1968 In addition to the eight operations, there are many options to
1969 @command{tar}, and three different styles for writing both: long (mnemonic)
1970 form, short form, and old style. These styles are discussed below.
1971 Both the options and the operations can be written in any of these three
1974 @FIXME{menu at end of this node. need to think of an actual outline
1975 for this chapter; probably do that after stuff from chapter 4 is
1979 @section The Three Option Styles
1981 There are three styles for writing operations and options to the command
1982 line invoking @command{tar}. The different styles were developed at
1983 different times during the history of @command{tar}. These styles will be
1984 presented below, from the most recent to the oldest.
1986 Some options must take an argument@footnote{For example, @option{--file}
1987 (@option{-f}) takes the name of an archive file as an argument. If
1988 you do not supply an archive file name, @command{tar} will use a
1989 default, but this can be confusing; thus, we recommend that you always
1990 supply a specific archive file name.}. Where you @emph{place} the
1991 arguments generally depends on which style of options you choose. We
1992 will detail specific information relevant to each option style in the
1993 sections on the different option styles, below. The differences are
1994 subtle, yet can often be very important; incorrect option placement
1995 can cause you to overwrite a number of important files. We urge you
1996 to note these differences, and only use the option style(s) which
1997 makes the most sense to you until you feel comfortable with the others.
1999 Some options @emph{may} take an argument. Such options may have at
2000 most long and short forms, they do not have old style equivalent. The
2001 rules for specifying an argument for such options are stricter than
2002 those for specifying mandatory arguments. Please, pay special
2006 * Long Options:: Long Option Style
2007 * Short Options:: Short Option Style
2008 * Old Options:: Old Option Style
2009 * Mixing:: Mixing Option Styles
2013 @subsection Long Option Style
2015 @cindex long options
2016 @cindex options, long style
2017 @cindex options, GNU style
2018 @cindex options, mnemonic names
2019 Each option has at least one @dfn{long} (or @dfn{mnemonic}) name starting with two
2020 dashes in a row, e.g., @option{--list}. The long names are more clear than
2021 their corresponding short or old names. It sometimes happens that a
2022 single long option has many different names which are
2023 synonymous, such as @option{--compare} and @option{--diff}. In addition,
2024 long option names can be given unique abbreviations. For example,
2025 @option{--cre} can be used in place of @option{--create} because there is no
2026 other long option which begins with @samp{cre}. (One way to find
2027 this out is by trying it and seeing what happens; if a particular
2028 abbreviation could represent more than one option, @command{tar} will tell
2029 you that that abbreviation is ambiguous and you'll know that that
2030 abbreviation won't work. You may also choose to run @samp{tar --help}
2031 to see a list of options. Be aware that if you run @command{tar} with a
2032 unique abbreviation for the long name of an option you didn't want to
2033 use, you are stuck; @command{tar} will perform the command as ordered.)
2035 Long options are meant to be obvious and easy to remember, and their
2036 meanings are generally easier to discern than those of their
2037 corresponding short options (see below). For example:
2040 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
2044 gives a fairly good set of hints about what the command does, even
2045 for those not fully acquainted with @command{tar}.
2047 @cindex arguments to long options
2048 @cindex long options with mandatory arguments
2049 Long options which require arguments take those arguments
2050 immediately following the option name. There are two ways of
2051 specifying a mandatory argument. It can be separated from the
2052 option name either by an equal sign, or by any amount of
2053 white space characters. For example, the @option{--file} option (which
2054 tells the name of the @command{tar} archive) is given a file such as
2055 @file{archive.tar} as argument by using any of the following notations:
2056 @option{--file=archive.tar} or @option{--file archive.tar}.
2058 @cindex optional arguments to long options
2059 @cindex long options with optional arguments
2060 In contrast, optional arguments must always be introduced using
2061 an equal sign. For example, the @option{--backup} option takes
2062 an optional argument specifying backup type. It must be used
2063 as @option{--backup=@var{backup-type}}.
2066 @subsection Short Option Style
2068 @cindex short options
2069 @cindex options, short style
2070 @cindex options, traditional
2071 Most options also have a @dfn{short option} name. Short options start with
2072 a single dash, and are followed by a single character, e.g., @option{-t}
2073 (which is equivalent to @option{--list}). The forms are absolutely
2074 identical in function; they are interchangeable.
2076 The short option names are faster to type than long option names.
2078 @cindex arguments to short options
2079 @cindex short options with mandatory arguments
2080 Short options which require arguments take their arguments immediately
2081 following the option, usually separated by white space. It is also
2082 possible to stick the argument right after the short option name, using
2083 no intervening space. For example, you might write @w{@option{-f
2084 archive.tar}} or @option{-farchive.tar} instead of using
2085 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
2086 @w{@option{-f @var{archive-name}}} denote the option which indicates a
2087 specific archive, here named @file{archive.tar}.
2089 @cindex optional arguments to short options
2090 @cindex short options with optional arguments
2091 Short options which take optional arguments take their arguments
2092 immediately following the option letter, @emph{without any intervening
2093 white space characters}.
2095 Short options' letters may be clumped together, but you are not
2096 required to do this (as compared to old options; see below). When
2097 short options are clumped as a set, use one (single) dash for them
2098 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
2099 such a set is allowed to have an argument@footnote{Clustering many
2100 options, the last of which has an argument, is a rather opaque way to
2101 write options. Some wonder if @acronym{GNU} @code{getopt} should not
2102 even be made helpful enough for considering such usages as invalid.}.
2104 When the options are separated, the argument for each option which requires
2105 an argument directly follows that option, as is usual for Unix programs.
2109 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
2112 If you reorder short options' locations, be sure to move any arguments
2113 that belong to them. If you do not move the arguments properly, you may
2114 end up overwriting files.
2117 @subsection Old Option Style
2118 @cindex options, old style
2119 @cindex old option style
2120 @cindex option syntax, traditional
2122 As far as we know, all @command{tar} programs, @acronym{GNU} and
2123 non-@acronym{GNU}, support @dfn{old options}: that is, if the first
2124 argument does not start with @samp{-}, it is assumed to specify option
2125 letters. @GNUTAR{} supports old options not only for historical
2126 reasons, but also because many people are used to them. If the first
2127 argument does not start with a dash, you are announcing the old option
2128 style instead of the short option style; old options are decoded
2131 Like short options, old options are single letters. However, old options
2132 must be written together as a single clumped set, without spaces separating
2133 them or dashes preceding them. This set
2134 of letters must be the first to appear on the command line, after the
2135 @command{tar} program name and some white space; old options cannot appear
2136 anywhere else. The letter of an old option is exactly the same letter as
2137 the corresponding short option. For example, the old option @samp{t} is
2138 the same as the short option @option{-t}, and consequently, the same as the
2139 long option @option{--list}. So for example, the command @w{@samp{tar
2140 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
2142 @cindex arguments to old options
2143 @cindex old options with mandatory arguments
2144 When options that need arguments are given together with the command,
2145 all the associated arguments follow, in the same order as the options.
2146 Thus, the example given previously could also be written in the old
2150 $ @kbd{tar cvbf 20 /dev/rmt0}
2154 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
2155 the argument of @option{-f}.
2157 The old style syntax can make it difficult to match
2158 option letters with their corresponding arguments, and is often
2159 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
2160 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
2161 argument for @option{-f}, and @option{-v} does not have a corresponding
2162 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
2163 /dev/rmt0}} is clearer, putting all arguments next to the option they
2166 If you want to reorder the letters in the old option argument, be
2167 sure to reorder any corresponding argument appropriately.
2169 This old way of writing @command{tar} options can surprise even experienced
2170 users. For example, the two commands:
2173 @kbd{tar cfz archive.tar.gz file}
2174 @kbd{tar -cfz archive.tar.gz file}
2178 are quite different. The first example uses @file{archive.tar.gz} as
2179 the value for option @samp{f} and recognizes the option @samp{z}. The
2180 second example, however, uses @file{z} as the value for option
2181 @samp{f} --- probably not what was intended.
2183 This second example could be corrected in many ways, among which the
2184 following are equivalent:
2187 @kbd{tar -czf archive.tar.gz file}
2188 @kbd{tar -cf archive.tar.gz -z file}
2189 @kbd{tar cf archive.tar.gz -z file}
2193 @subsection Mixing Option Styles
2195 @cindex options, mixing different styles
2196 All three styles may be intermixed in a single @command{tar} command,
2197 so long as the rules for each style are fully
2198 respected@footnote{Before @GNUTAR{} version 1.11.6,
2199 a bug prevented intermixing old style options with long options in
2200 some cases.}. Old style options and either of the modern styles of
2201 options may be mixed within a single @command{tar} command. However,
2202 old style options must be introduced as the first arguments only,
2203 following the rule for old options (old options must appear directly
2204 after the @command{tar} command and some white space). Modern options
2205 may be given only after all arguments to the old options have been
2206 collected. If this rule is not respected, a modern option might be
2207 falsely interpreted as the value of the argument to one of the old
2210 For example, all the following commands are wholly equivalent, and
2211 illustrate the many combinations and orderings of option styles.
2214 @kbd{tar --create --file=archive.tar}
2215 @kbd{tar --create -f archive.tar}
2216 @kbd{tar --create -farchive.tar}
2217 @kbd{tar --file=archive.tar --create}
2218 @kbd{tar --file=archive.tar -c}
2219 @kbd{tar -c --file=archive.tar}
2220 @kbd{tar -c -f archive.tar}
2221 @kbd{tar -c -farchive.tar}
2222 @kbd{tar -cf archive.tar}
2223 @kbd{tar -cfarchive.tar}
2224 @kbd{tar -f archive.tar --create}
2225 @kbd{tar -f archive.tar -c}
2226 @kbd{tar -farchive.tar --create}
2227 @kbd{tar -farchive.tar -c}
2228 @kbd{tar c --file=archive.tar}
2229 @kbd{tar c -f archive.tar}
2230 @kbd{tar c -farchive.tar}
2231 @kbd{tar cf archive.tar}
2232 @kbd{tar f archive.tar --create}
2233 @kbd{tar f archive.tar -c}
2234 @kbd{tar fc archive.tar}
2237 On the other hand, the following commands are @emph{not} equivalent to
2241 @kbd{tar -f -c archive.tar}
2242 @kbd{tar -fc archive.tar}
2243 @kbd{tar -fcarchive.tar}
2244 @kbd{tar -farchive.tarc}
2245 @kbd{tar cfarchive.tar}
2249 These last examples mean something completely different from what the
2250 user intended (judging based on the example in the previous set which
2251 uses long options, whose intent is therefore very clear). The first
2252 four specify that the @command{tar} archive would be a file named
2253 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2254 respectively. The first two examples also specify a single non-option,
2255 @var{name} argument having the value @samp{archive.tar}. The last
2256 example contains only old style option letters (repeating option
2257 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2258 @samp{h}, or @samp{i}), with no argument value. @FIXME{not sure i liked
2259 the first sentence of this paragraph..}
2262 @section All @command{tar} Options
2264 The coming manual sections contain an alphabetical listing of all
2265 @command{tar} operations and options, with brief descriptions and
2266 cross-references to more in-depth explanations in the body of the manual.
2267 They also contain an alphabetically arranged table of the short option
2268 forms with their corresponding long option. You can use this table as
2269 a reference for deciphering @command{tar} commands in scripts.
2272 * Operation Summary::
2274 * Short Option Summary::
2277 @node Operation Summary
2278 @subsection Operations
2286 Appends files to the end of the archive. @xref{append}.
2288 @opsummary{catenate}
2292 Same as @option{--concatenate}. @xref{concatenate}.
2298 Compares archive members with their counterparts in the file
2299 system, and reports differences in file size, mode, owner,
2300 modification date and contents. @xref{compare}.
2302 @opsummary{concatenate}
2306 Appends other @command{tar} archives to the end of the archive.
2313 Creates a new @command{tar} archive. @xref{create}.
2318 Deletes members from the archive. Don't try this on an archive on a
2319 tape! @xref{delete}.
2325 Same @option{--compare}. @xref{compare}.
2331 Extracts members from the archive into the file system. @xref{extract}.
2337 Same as @option{--extract}. @xref{extract}.
2343 Lists the members in an archive. @xref{list}.
2349 Adds files to the end of the archive, but only if they are newer than
2350 their counterparts already in the archive, or if they do not already
2351 exist in the archive. @xref{update}.
2355 @node Option Summary
2356 @subsection @command{tar} Options
2360 @opsummary{absolute-names}
2361 @item --absolute-names
2364 Normally when creating an archive, @command{tar} strips an initial
2365 @samp{/} from member names. This option disables that behavior.
2368 @opsummary{after-date}
2371 (See @option{--newer}, @pxref{after})
2373 @opsummary{anchored}
2375 A pattern must match an initial subsequence of the name's components.
2376 @xref{controlling pattern-matching}.
2378 @opsummary{atime-preserve}
2379 @item --atime-preserve
2380 @itemx --atime-preserve=replace
2381 @itemx --atime-preserve=system
2383 Attempt to preserve the access time of files when reading them. This
2384 option currently is effective only on files that you own, unless you
2385 have superuser privileges.
2387 @option{--atime-preserve=replace} remembers the access time of a file
2388 before reading it, and then restores the access time afterwards. This
2389 may cause problems if other programs are reading the file at the same
2390 time, as the times of their accesses will be lost. On most platforms
2391 restoring the access time also requires @command{tar} to restore the
2392 data modification time too, so this option may also cause problems if
2393 other programs are writing the file at the same time (@command{tar} attempts
2394 to detect this situation, but cannot do so reliably due to race
2395 conditions). Worse, on most platforms restoring the access time also
2396 updates the status change time, which means that this option is
2397 incompatible with incremental backups.
2399 @option{--atime-preserve=system} avoids changing time stamps on files,
2400 without interfering with time stamp updates
2401 caused by other programs, so it works better with incremental backups.
2402 However, it requires a special @code{O_NOATIME} option from the
2403 underlying operating and file system implementation, and it also requires
2404 that searching directories does not update their access times. As of
2405 this writing (November 2005) this works only with Linux, and only with
2406 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2407 way to know whether this feature actually works. Sometimes
2408 @command{tar} knows that it does not work, and if you use
2409 @option{--atime-preserve=system} then @command{tar} complains and
2410 exits right away. But other times @command{tar} might think that the
2411 option works when it actually does not.
2413 Currently @option{--atime-preserve} with no operand defaults to
2414 @option{--atime-preserve=replace}, but this may change in the future
2415 as support for @option{--atime-preserve=system} improves.
2417 If your operating or file system does not support
2418 @option{--atime-preserve=@-system}, you might be able to preserve access
2419 times reliably by using the @command{mount} command. For example,
2420 you can mount the file system read-only, or access the file system via
2421 a read-only loopback mount, or use the @samp{noatime} mount option
2422 available on some systems. However, mounting typically requires
2423 superuser privileges and can be a pain to manage.
2425 @opsummary{auto-compress}
2426 @item --auto-compress
2429 During a @option{--create} operation, enables automatic compressed
2430 format recognition based on the archive suffix. The effect of this
2431 option is cancelled by @option{--no-auto-compress}. @xref{gzip}.
2434 @item --backup=@var{backup-type}
2436 Rather than deleting files from the file system, @command{tar} will
2437 back them up using simple or numbered backups, depending upon
2438 @var{backup-type}. @xref{backup}.
2440 @opsummary{block-number}
2441 @item --block-number
2444 With this option present, @command{tar} prints error messages for read errors
2445 with the block number in the archive file. @xref{block-number}.
2447 @opsummary{blocking-factor}
2448 @item --blocking-factor=@var{blocking}
2449 @itemx -b @var{blocking}
2451 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2452 record. @xref{Blocking Factor}.
2458 This option tells @command{tar} to read or write archives through
2459 @code{bzip2}. @xref{gzip}.
2461 @opsummary{check-device}
2462 @item --check-device
2463 Check device numbers when creating a list of modified files for
2464 incremental archiving. This is the default. @xref{device numbers},
2465 for a detailed description.
2467 @opsummary{checkpoint}
2468 @item --checkpoint[=@var{number}]
2470 This option directs @command{tar} to print periodic checkpoint
2471 messages as it reads through the archive. It is intended for when you
2472 want a visual indication that @command{tar} is still running, but
2473 don't want to see @option{--verbose} output. You can also instruct
2474 @command{tar} to execute a list of actions on each checkpoint, see
2475 @option{--checkpoint-action} below. For a detailed description, see
2478 @opsummary{checkpoint-action}
2479 @item --checkpoint-action=@var{action}
2480 Instruct @command{tar} to execute an action upon hitting a
2481 breakpoint. Here we give only a brief outline. @xref{checkpoints},
2482 for a complete description.
2484 The @var{action} argument can be one of the following:
2488 Produce an audible bell on the console.
2492 Print a single dot on the standard listing stream.
2495 Display a textual message on the standard error, with the status and
2496 number of the checkpoint. This is the default.
2498 @item echo=@var{string}
2499 Display @var{string} on the standard error. Before output, the string
2500 is subject to meta-character expansion.
2502 @item exec=@var{command}
2503 Execute the given @var{command}.
2505 @item sleep=@var{time}
2506 Wait for @var{time} seconds.
2508 @item ttyout=@var{string}
2509 Output @var{string} on the current console (@file{/dev/tty}).
2512 Several @option{--checkpoint-action} options can be specified. The
2513 supplied actions will be executed in order of their appearance in the
2516 Using @option{--checkpoint-action} without @option{--checkpoint}
2517 assumes default checkpoint frequency of one checkpoint per 10 records.
2519 @opsummary{check-links}
2522 If this option was given, @command{tar} will check the number of links
2523 dumped for each processed file. If this number does not match the
2524 total number of hard links for the file, a warning message will be
2525 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2526 synonym for @option{--one-file-system}. The current semantics, which
2527 complies to UNIX98, was introduced with version
2528 1.15.91. @xref{Changes}, for more information.}.
2532 @opsummary{compress}
2533 @opsummary{uncompress}
2538 @command{tar} will use the @command{compress} program when reading or
2539 writing the archive. This allows you to directly act on archives
2540 while saving space. @xref{gzip}.
2542 @opsummary{confirmation}
2543 @item --confirmation
2545 (See @option{--interactive}.) @xref{interactive}.
2547 @opsummary{delay-directory-restore}
2548 @item --delay-directory-restore
2550 Delay setting modification times and permissions of extracted
2551 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2553 @opsummary{dereference}
2557 When reading or writing a file to be archived, @command{tar} accesses
2558 the file that a symbolic link points to, rather than the symlink
2559 itself. @xref{dereference}.
2561 @opsummary{directory}
2562 @item --directory=@var{dir}
2565 When this option is specified, @command{tar} will change its current directory
2566 to @var{dir} before performing any operations. When this option is used
2567 during archive creation, it is order sensitive. @xref{directory}.
2570 @item --exclude=@var{pattern}
2572 When performing operations, @command{tar} will skip files that match
2573 @var{pattern}. @xref{exclude}.
2575 @opsummary{exclude-backups}
2576 @item --exclude-backups
2577 Exclude backup and lock files. @xref{exclude,, exclude-backups}.
2579 @opsummary{exclude-from}
2580 @item --exclude-from=@var{file}
2581 @itemx -X @var{file}
2583 Similar to @option{--exclude}, except @command{tar} will use the list of
2584 patterns in the file @var{file}. @xref{exclude}.
2586 @opsummary{exclude-caches}
2587 @item --exclude-caches
2589 Exclude from dump any directory containing a valid cache directory
2590 tag file, but still dump the directory node and the tag file itself.
2592 @xref{exclude,, exclude-caches}.
2594 @opsummary{exclude-caches-under}
2595 @item --exclude-caches-under
2597 Exclude from dump any directory containing a valid cache directory
2598 tag file, but still dump the directory node itself.
2602 @opsummary{exclude-caches-all}
2603 @item --exclude-caches-all
2605 Exclude from dump any directory containing a valid cache directory
2606 tag file. @xref{exclude}.
2608 @opsummary{exclude-tag}
2609 @item --exclude-tag=@var{file}
2611 Exclude from dump any directory containing file named @var{file}, but
2612 dump the directory node and @var{file} itself. @xref{exclude,, exclude-tag}.
2614 @opsummary{exclude-tag-under}
2615 @item --exclude-tag-under=@var{file}
2617 Exclude from dump the contents of any directory containing file
2618 named @var{file}, but dump the directory node itself. @xref{exclude,,
2621 @opsummary{exclude-tag-all}
2622 @item --exclude-tag-all=@var{file}
2624 Exclude from dump any directory containing file named @var{file}.
2625 @xref{exclude,,exclude-tag-all}.
2627 @opsummary{exclude-vcs}
2630 Exclude from dump directories and files, that are internal for some
2631 widely used version control systems.
2633 @xref{exclude,,exclude-vcs}.
2636 @item --file=@var{archive}
2637 @itemx -f @var{archive}
2639 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2640 performs operations on, rather than @command{tar}'s compilation dependent
2641 default. @xref{file tutorial}.
2643 @opsummary{files-from}
2644 @item --files-from=@var{file}
2645 @itemx -T @var{file}
2647 @command{tar} will use the contents of @var{file} as a list of archive members
2648 or files to operate on, in addition to those specified on the
2649 command-line. @xref{files}.
2651 @opsummary{force-local}
2654 Forces @command{tar} to interpret the file name given to @option{--file}
2655 as a local file, even if it looks like a remote tape drive name.
2656 @xref{local and remote archives}.
2659 @item --format=@var{format}
2660 @itemx -H @var{format}
2662 Selects output archive format. @var{Format} may be one of the
2667 Creates an archive that is compatible with Unix V7 @command{tar}.
2670 Creates an archive that is compatible with GNU @command{tar} version
2674 Creates archive in GNU tar 1.13 format. Basically it is the same as
2675 @samp{oldgnu} with the only difference in the way it handles long
2679 Creates a @acronym{POSIX.1-1988} compatible archive.
2682 Creates a @acronym{POSIX.1-2001 archive}.
2686 @xref{Formats}, for a detailed discussion of these formats.
2688 @opsummary{full-time}
2690 This option instructs @command{tar} to print file times to their full
2691 resolution. Usually this means 1-second resolution, but that depends
2692 on the underlying file system. The @option{--full-time} option takes
2693 effect only when detailed output (verbosity level 2 or higher) has
2694 been requested using the @option{--verbose} option, e.g., when listing
2695 or extracting archives:
2698 $ @kbd{tar -t -v --full-time -f archive.tar}
2702 or, when creating an archive:
2705 $ @kbd{tar -c -vv --full-time -f archive.tar .}
2708 Notice, thar when creating the archive you need to specify
2709 @option{--verbose} twice to get a detailed output (@pxref{verbose
2713 @item --group=@var{group}
2715 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
2716 rather than the group from the source file. @var{group} can specify a
2717 symbolic name, or a numeric @acronym{ID}, or both as
2718 @var{name}:@var{id}. @xref{override}.
2720 Also see the comments for the @option{--owner=@var{user}} option.
2730 This option tells @command{tar} to read or write archives through
2731 @command{gzip}, allowing @command{tar} to directly operate on several
2732 kinds of compressed archives transparently. @xref{gzip}.
2734 @opsummary{hard-dereference}
2735 @item --hard-dereference
2736 When creating an archive, dereference hard links and store the files
2737 they refer to, instead of creating usual hard link members.
2745 @command{tar} will print out a short message summarizing the operations and
2746 options to @command{tar} and exit. @xref{help}.
2748 @opsummary{ignore-case}
2750 Ignore case when matching member or file names with
2751 patterns. @xref{controlling pattern-matching}.
2753 @opsummary{ignore-command-error}
2754 @item --ignore-command-error
2755 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2757 @opsummary{ignore-failed-read}
2758 @item --ignore-failed-read
2760 Do not exit unsuccessfully merely because an unreadable file was encountered.
2763 @opsummary{ignore-zeros}
2764 @item --ignore-zeros
2767 With this option, @command{tar} will ignore zeroed blocks in the
2768 archive, which normally signals EOF. @xref{Reading}.
2770 @opsummary{incremental}
2774 Informs @command{tar} that it is working with an old
2775 @acronym{GNU}-format incremental backup archive. It is intended
2776 primarily for backwards compatibility only. @xref{Incremental Dumps},
2777 for a detailed discussion of incremental archives.
2779 @opsummary{index-file}
2780 @item --index-file=@var{file}
2782 Send verbose output to @var{file} instead of to standard output.
2784 @opsummary{info-script}
2785 @opsummary{new-volume-script}
2786 @item --info-script=@var{script-file}
2787 @itemx --new-volume-script=@var{script-file}
2788 @itemx -F @var{script-file}
2790 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2791 at the end of each tape. If @var{script-file} exits with nonzero status,
2792 @command{tar} fails immediately. @xref{info-script}, for a detailed
2793 discussion of @var{script-file}.
2795 @opsummary{interactive}
2797 @itemx --confirmation
2800 Specifies that @command{tar} should ask the user for confirmation before
2801 performing potentially destructive options, such as overwriting files.
2804 @opsummary{keep-newer-files}
2805 @item --keep-newer-files
2807 Do not replace existing files that are newer than their archive copies
2808 when extracting files from an archive.
2810 @opsummary{keep-old-files}
2811 @item --keep-old-files
2814 Do not overwrite existing files when extracting files from an archive.
2815 @xref{Keep Old Files}.
2818 @item --label=@var{name}
2819 @itemx -V @var{name}
2821 When creating an archive, instructs @command{tar} to write @var{name}
2822 as a name record in the archive. When extracting or listing archives,
2823 @command{tar} will only operate on archives that have a label matching
2824 the pattern specified in @var{name}. @xref{Tape Files}.
2827 @item --level=@var{n}
2828 Force incremental backup of level @var{n}. As of @GNUTAR version
2829 @value{VERSION}, the option @option{--level=0} truncates the snapshot
2830 file, thereby forcing the level 0 dump. Other values of @var{n} are
2831 effectively ignored. @xref{--level=0}, for details and examples.
2833 The use of this option is valid only in conjunction with the
2834 @option{--listed-incremental} option. @xref{Incremental Dumps},
2835 for a detailed description.
2837 @opsummary{listed-incremental}
2838 @item --listed-incremental=@var{snapshot-file}
2839 @itemx -g @var{snapshot-file}
2841 During a @option{--create} operation, specifies that the archive that
2842 @command{tar} creates is a new @acronym{GNU}-format incremental
2843 backup, using @var{snapshot-file} to determine which files to backup.
2844 With other operations, informs @command{tar} that the archive is in
2845 incremental format. @xref{Incremental Dumps}.
2850 This option tells @command{tar} to read or write archives through
2851 @command{lzip}. @xref{gzip}.
2856 This option tells @command{tar} to read or write archives through
2857 @command{lzma}. @xref{gzip}.
2861 This option tells @command{tar} to read or write archives through
2862 @command{lzop}. @xref{gzip}.
2865 @item --mode=@var{permissions}
2867 When adding files to an archive, @command{tar} will use
2868 @var{permissions} for the archive members, rather than the permissions
2869 from the files. @var{permissions} can be specified either as an octal
2870 number or as symbolic permissions, like with
2871 @command{chmod}. @xref{override}.
2874 @item --mtime=@var{date}
2876 When adding files to an archive, @command{tar} will use @var{date} as
2877 the modification time of members when creating archives, instead of
2878 their actual modification times. The value of @var{date} can be
2879 either a textual date representation (@pxref{Date input formats}) or a
2880 name of the existing file, starting with @samp{/} or @samp{.}. In the
2881 latter case, the modification time of that file is used. @xref{override}.
2883 @opsummary{multi-volume}
2884 @item --multi-volume
2887 Informs @command{tar} that it should create or otherwise operate on a
2888 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2890 @opsummary{new-volume-script}
2891 @item --new-volume-script
2893 (see @option{--info-script})
2896 @item --newer=@var{date}
2897 @itemx --after-date=@var{date}
2900 When creating an archive, @command{tar} will only add files that have changed
2901 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2902 is taken to be the name of a file whose data modification time specifies
2903 the date. @xref{after}.
2905 @opsummary{newer-mtime}
2906 @item --newer-mtime=@var{date}
2908 Like @option{--newer}, but add only files whose
2909 contents have changed (as opposed to just @option{--newer}, which will
2910 also back up files for which any status information has
2911 changed). @xref{after}.
2913 @opsummary{no-anchored}
2915 An exclude pattern can match any subsequence of the name's components.
2916 @xref{controlling pattern-matching}.
2918 @opsummary{no-auto-compress}
2919 @item --no-auto-compress
2921 Disables automatic compressed format recognition based on the archive
2922 suffix. @xref{--auto-compress}. @xref{gzip}.
2924 @opsummary{no-check-device}
2925 @item --no-check-device
2926 Do not check device numbers when creating a list of modified files
2927 for incremental archiving. @xref{device numbers}, for
2928 a detailed description.
2930 @opsummary{no-delay-directory-restore}
2931 @item --no-delay-directory-restore
2933 Modification times and permissions of extracted
2934 directories are set when all files from this directory have been
2935 extracted. This is the default.
2936 @xref{Directory Modification Times and Permissions}.
2938 @opsummary{no-ignore-case}
2939 @item --no-ignore-case
2940 Use case-sensitive matching.
2941 @xref{controlling pattern-matching}.
2943 @opsummary{no-ignore-command-error}
2944 @item --no-ignore-command-error
2945 Print warnings about subprocesses that terminated with a nonzero exit
2946 code. @xref{Writing to an External Program}.
2951 If the @option{--null} option was given previously, this option
2952 cancels its effect, so that any following @option{--files-from}
2953 options will expect their file lists to be newline-terminated.
2955 @opsummary{no-overwrite-dir}
2956 @item --no-overwrite-dir
2958 Preserve metadata of existing directories when extracting files
2959 from an archive. @xref{Overwrite Old Files}.
2961 @opsummary{no-quote-chars}
2962 @item --no-quote-chars=@var{string}
2963 Remove characters listed in @var{string} from the list of quoted
2964 characters set by the previous @option{--quote-chars} option
2965 (@pxref{quoting styles}).
2967 @opsummary{no-recursion}
2968 @item --no-recursion
2970 With this option, @command{tar} will not recurse into directories.
2973 @opsummary{no-same-owner}
2974 @item --no-same-owner
2977 When extracting an archive, do not attempt to preserve the owner
2978 specified in the @command{tar} archive. This the default behavior
2981 @opsummary{no-same-permissions}
2982 @item --no-same-permissions
2984 When extracting an archive, subtract the user's umask from files from
2985 the permissions specified in the archive. This is the default behavior
2991 The archive media does not support seeks to arbitrary
2992 locations. Usually @command{tar} determines automatically whether
2993 the archive can be seeked or not. Use this option to disable this
2996 @opsummary{no-unquote}
2998 Treat all input file or member names literally, do not interpret
2999 escape sequences. @xref{input name quoting}.
3001 @opsummary{no-wildcards}
3002 @item --no-wildcards
3003 Do not use wildcards.
3004 @xref{controlling pattern-matching}.
3006 @opsummary{no-wildcards-match-slash}
3007 @item --no-wildcards-match-slash
3008 Wildcards do not match @samp{/}.
3009 @xref{controlling pattern-matching}.
3014 When @command{tar} is using the @option{--files-from} option, this option
3015 instructs @command{tar} to expect file names terminated with @acronym{NUL}, so
3016 @command{tar} can correctly work with file names that contain newlines.
3019 @opsummary{numeric-owner}
3020 @item --numeric-owner
3022 This option will notify @command{tar} that it should use numeric user
3023 and group IDs when creating a @command{tar} file, rather than names.
3027 The function of this option depends on the action @command{tar} is
3028 performing. When extracting files, @option{-o} is a synonym for
3029 @option{--no-same-owner}, i.e., it prevents @command{tar} from
3030 restoring ownership of files being extracted.
3032 When creating an archive, it is a synonym for
3033 @option{--old-archive}. This behavior is for compatibility
3034 with previous versions of @GNUTAR{}, and will be
3035 removed in future releases.
3037 @xref{Changes}, for more information.
3039 @opsummary{occurrence}
3040 @item --occurrence[=@var{number}]
3042 This option can be used in conjunction with one of the subcommands
3043 @option{--delete}, @option{--diff}, @option{--extract} or
3044 @option{--list} when a list of files is given either on the command
3045 line or via @option{-T} option.
3047 This option instructs @command{tar} to process only the @var{number}th
3048 occurrence of each named file. @var{Number} defaults to 1, so
3051 tar -x -f archive.tar --occurrence filename
3055 will extract the first occurrence of the member @file{filename} from @file{archive.tar}
3056 and will terminate without scanning to the end of the archive.
3058 @opsummary{old-archive}
3060 Synonym for @option{--format=v7}.
3062 @opsummary{one-file-system}
3063 @item --one-file-system
3064 Used when creating an archive. Prevents @command{tar} from recursing into
3065 directories that are on different file systems from the current
3068 @opsummary{overwrite}
3071 Overwrite existing files and directory metadata when extracting files
3072 from an archive. @xref{Overwrite Old Files}.
3074 @opsummary{overwrite-dir}
3075 @item --overwrite-dir
3077 Overwrite the metadata of existing directories when extracting files
3078 from an archive. @xref{Overwrite Old Files}.
3081 @item --owner=@var{user}
3083 Specifies that @command{tar} should use @var{user} as the owner of members
3084 when creating archives, instead of the user associated with the source
3085 file. @var{user} can specify a symbolic name, or a numeric
3086 @acronym{ID}, or both as @var{name}:@var{id}.
3089 This option does not affect extraction from archives.
3091 @opsummary{pax-option}
3092 @item --pax-option=@var{keyword-list}
3093 This option enables creation of the archive in @acronym{POSIX.1-2001}
3094 format (@pxref{posix}) and modifies the way @command{tar} handles the
3095 extended header keywords. @var{Keyword-list} is a comma-separated
3096 list of keyword options. @xref{PAX keywords}, for a detailed
3099 @opsummary{portability}
3101 @itemx --old-archive
3102 Synonym for @option{--format=v7}.
3106 Same as @option{--format=posix}.
3108 @opsummary{preserve}
3111 Synonymous with specifying both @option{--preserve-permissions} and
3112 @option{--same-order}. @xref{Setting Access Permissions}.
3114 @opsummary{preserve-order}
3115 @item --preserve-order
3117 (See @option{--same-order}; @pxref{Reading}.)
3119 @opsummary{preserve-permissions}
3120 @opsummary{same-permissions}
3121 @item --preserve-permissions
3122 @itemx --same-permissions
3125 When @command{tar} is extracting an archive, it normally subtracts the
3126 users' umask from the permissions specified in the archive and uses
3127 that number as the permissions to create the destination file.
3128 Specifying this option instructs @command{tar} that it should use the
3129 permissions directly from the archive. @xref{Setting Access Permissions}.
3131 @opsummary{quote-chars}
3132 @item --quote-chars=@var{string}
3133 Always quote characters from @var{string}, even if the selected
3134 quoting style would not quote them (@pxref{quoting styles}).
3136 @opsummary{quoting-style}
3137 @item --quoting-style=@var{style}
3138 Set quoting style to use when printing member and file names
3139 (@pxref{quoting styles}). Valid @var{style} values are:
3140 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
3141 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
3142 style is @code{escape}, unless overridden while configuring the
3145 @opsummary{read-full-records}
3146 @item --read-full-records
3149 Specifies that @command{tar} should reblock its input, for reading
3150 from pipes on systems with buggy implementations. @xref{Reading}.
3152 @opsummary{record-size}
3153 @item --record-size=@var{size}[@var{suf}]
3155 Instructs @command{tar} to use @var{size} bytes per record when accessing the
3156 archive. The argument can be suffixed with a @dfn{size suffix}, e.g.
3157 @option{--record-size=10K} for 10 Kilobytes. @xref{size-suffixes},
3158 for a list of valid suffixes. @xref{Blocking Factor}, for a detailed
3159 description of this option.
3161 @opsummary{recursion}
3164 With this option, @command{tar} recurses into directories (default).
3167 @opsummary{recursive-unlink}
3168 @item --recursive-unlink
3171 directory hierarchies before extracting directories of the same name
3172 from the archive. @xref{Recursive Unlink}.
3174 @opsummary{remove-files}
3175 @item --remove-files
3177 Directs @command{tar} to remove the source file from the file system after
3178 appending it to an archive. @xref{remove files}.
3180 @opsummary{restrict}
3183 Disable use of some potentially harmful @command{tar} options.
3184 Currently this option disables shell invocation from multi-volume menu
3185 (@pxref{Using Multiple Tapes}).
3187 @opsummary{rmt-command}
3188 @item --rmt-command=@var{cmd}
3190 Notifies @command{tar} that it should use @var{cmd} instead of
3191 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
3193 @opsummary{rsh-command}
3194 @item --rsh-command=@var{cmd}
3196 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
3197 devices. @xref{Device}.
3199 @opsummary{same-order}
3201 @itemx --preserve-order
3204 This option is an optimization for @command{tar} when running on machines with
3205 small amounts of memory. It informs @command{tar} that the list of file
3206 arguments has already been sorted to match the order of files in the
3207 archive. @xref{Reading}.
3209 @opsummary{same-owner}
3212 When extracting an archive, @command{tar} will attempt to preserve the owner
3213 specified in the @command{tar} archive with this option present.
3214 This is the default behavior for the superuser; this option has an
3215 effect only for ordinary users. @xref{Attributes}.
3217 @opsummary{same-permissions}
3218 @item --same-permissions
3220 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
3226 Assume that the archive media supports seeks to arbitrary
3227 locations. Usually @command{tar} determines automatically whether
3228 the archive can be seeked or not. This option is intended for use
3229 in cases when such recognition fails. It takes effect only if the
3230 archive is open for reading (e.g. with @option{--list} or
3231 @option{--extract} options).
3233 @opsummary{show-defaults}
3234 @item --show-defaults
3236 Displays the default options used by @command{tar} and exits
3237 successfully. This option is intended for use in shell scripts.
3238 Here is an example of what you can see using this option:
3241 $ @kbd{tar --show-defaults}
3242 --format=gnu -f- -b20 --quoting-style=escape
3243 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
3247 Notice, that this option outputs only one line. The example output
3248 above has been split to fit page boundaries.
3250 @opsummary{show-omitted-dirs}
3251 @item --show-omitted-dirs
3253 Instructs @command{tar} to mention the directories it is skipping when
3254 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
3256 @opsummary{show-transformed-names}
3257 @opsummary{show-stored-names}
3258 @item --show-transformed-names
3259 @itemx --show-stored-names
3261 Display file or member names after applying any transformations
3262 (@pxref{transform}). In particular, when used in conjunction with one of
3263 the archive creation operations it instructs @command{tar} to list the
3264 member names stored in the archive, as opposed to the actual file
3265 names. @xref{listing member and file names}.
3271 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3272 sparse files efficiently. @xref{sparse}.
3274 @opsummary{sparse-version}
3275 @item --sparse-version=@var{version}
3277 Specifies the @dfn{format version} to use when archiving sparse
3278 files. Implies @option{--sparse}. @xref{sparse}. For the description
3279 of the supported sparse formats, @xref{Sparse Formats}.
3281 @opsummary{starting-file}
3282 @item --starting-file=@var{name}
3283 @itemx -K @var{name}
3285 This option affects extraction only; @command{tar} will skip extracting
3286 files in the archive until it finds one that matches @var{name}.
3289 @opsummary{strip-components}
3290 @item --strip-components=@var{number}
3291 Strip given @var{number} of leading components from file names before
3292 extraction. For example, if archive @file{archive.tar} contained
3293 @file{/some/file/name}, then running
3296 tar --extract --file archive.tar --strip-components=2
3300 would extract this file to file @file{name}.
3303 @item --suffix=@var{suffix}
3305 Alters the suffix @command{tar} uses when backing up files from the default
3306 @samp{~}. @xref{backup}.
3308 @opsummary{tape-length}
3309 @item --tape-length=@var{num}[@var{suf}]
3310 @itemx -L @var{num}[@var{suf}]
3312 Specifies the length of tapes that @command{tar} is writing as being
3313 @w{@var{num} x 1024} bytes long. If optional @var{suf} is given, it
3314 specifies a multiplicative factor to be used instead of 1024. For
3315 example, @samp{-L2M} means 2 megabytes. @xref{size-suffixes}, for a
3316 list of allowed suffixes. @xref{Using Multiple Tapes}, for a detailed
3317 discussion of this option.
3319 @opsummary{test-label}
3322 Reads the volume label. If an argument is specified, test whether it
3323 matches the volume label. @xref{--test-label option}.
3325 @opsummary{to-command}
3326 @item --to-command=@var{command}
3328 During extraction @command{tar} will pipe extracted files to the
3329 standard input of @var{command}. @xref{Writing to an External Program}.
3331 @opsummary{to-stdout}
3335 During extraction, @command{tar} will extract files to stdout rather
3336 than to the file system. @xref{Writing to Standard Output}.
3339 @item --totals[=@var{signo}]
3341 Displays the total number of bytes transferred when processing an
3342 archive. If an argument is given, these data are displayed on
3343 request, when signal @var{signo} is delivered to @command{tar}.
3350 Sets the data modification time of extracted files to the extraction time,
3351 rather than the data modification time stored in the archive.
3352 @xref{Data Modification Times}.
3354 @opsummary{transform}
3356 @item --transform=@var{sed-expr}
3357 @itemx --xform=@var{sed-expr}
3358 Transform file or member names using @command{sed} replacement expression
3359 @var{sed-expr}. For example,
3362 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
3366 will add to @file{archive} files from the current working directory,
3367 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
3368 discussion, @xref{transform}.
3370 To see transformed member names in verbose listings, use
3371 @option{--show-transformed-names} option
3372 (@pxref{show-transformed-names}).
3374 @opsummary{uncompress}
3377 (See @option{--compress}, @pxref{gzip})
3382 (See @option{--gzip}, @pxref{gzip})
3384 @opsummary{unlink-first}
3385 @item --unlink-first
3388 Directs @command{tar} to remove the corresponding file from the file
3389 system before extracting it from the archive. @xref{Unlink First}.
3393 Enable unquoting input file or member names (default). @xref{input
3396 @opsummary{use-compress-program}
3397 @item --use-compress-program=@var{prog}
3398 @itemx -I=@var{prog}
3400 Instructs @command{tar} to access the archive through @var{prog}, which is
3401 presumed to be a compression program of some sort. @xref{gzip}.
3406 Display file modification dates in @acronym{UTC}. This option implies
3413 Specifies that @command{tar} should be more verbose about the
3414 operations it is performing. This option can be specified multiple
3415 times for some operations to increase the amount of information displayed.
3422 Verifies that the archive was correctly written when creating an
3423 archive. @xref{verify}.
3428 Print information about the program's name, version, origin and legal
3429 status, all on standard output, and then exit successfully.
3432 @opsummary{volno-file}
3433 @item --volno-file=@var{file}
3435 Used in conjunction with @option{--multi-volume}. @command{tar} will
3436 keep track of which volume of a multi-volume archive it is working in
3437 @var{file}. @xref{volno-file}.
3440 @item --warning=@var{keyword}
3442 Enable or disable warning messages identified by @var{keyword}. The
3443 messages are suppressed if @var{keyword} is prefixed with @samp{no-}.
3446 @opsummary{wildcards}
3448 Use wildcards when matching member names with patterns.
3449 @xref{controlling pattern-matching}.
3451 @opsummary{wildcards-match-slash}
3452 @item --wildcards-match-slash
3453 Wildcards match @samp{/}.
3454 @xref{controlling pattern-matching}.
3459 Use @command{xz} for compressing or decompressing the archives. @xref{gzip}.
3463 @node Short Option Summary
3464 @subsection Short Options Cross Reference
3466 Here is an alphabetized list of all of the short option forms, matching
3467 them with the equivalent long option.
3469 @multitable @columnfractions 0.20 0.80
3470 @headitem Short Option @tab Reference
3472 @item -A @tab @ref{--concatenate}.
3474 @item -B @tab @ref{--read-full-records}.
3476 @item -C @tab @ref{--directory}.
3478 @item -F @tab @ref{--info-script}.
3480 @item -G @tab @ref{--incremental}.
3482 @item -J @tab @ref{--xz}.
3484 @item -K @tab @ref{--starting-file}.
3486 @item -L @tab @ref{--tape-length}.
3488 @item -M @tab @ref{--multi-volume}.
3490 @item -N @tab @ref{--newer}.
3492 @item -O @tab @ref{--to-stdout}.
3494 @item -P @tab @ref{--absolute-names}.
3496 @item -R @tab @ref{--block-number}.
3498 @item -S @tab @ref{--sparse}.
3500 @item -T @tab @ref{--files-from}.
3502 @item -U @tab @ref{--unlink-first}.
3504 @item -V @tab @ref{--label}.
3506 @item -W @tab @ref{--verify}.
3508 @item -X @tab @ref{--exclude-from}.
3510 @item -Z @tab @ref{--compress}.
3512 @item -b @tab @ref{--blocking-factor}.
3514 @item -c @tab @ref{--create}.
3516 @item -d @tab @ref{--compare}.
3518 @item -f @tab @ref{--file}.
3520 @item -g @tab @ref{--listed-incremental}.
3522 @item -h @tab @ref{--dereference}.
3524 @item -i @tab @ref{--ignore-zeros}.
3526 @item -j @tab @ref{--bzip2}.
3528 @item -k @tab @ref{--keep-old-files}.
3530 @item -l @tab @ref{--check-links}.
3532 @item -m @tab @ref{--touch}.
3534 @item -o @tab When creating, @ref{--no-same-owner}, when extracting ---
3535 @ref{--portability}.
3537 The latter usage is deprecated. It is retained for compatibility with
3538 the earlier versions of @GNUTAR{}. In future releases
3539 @option{-o} will be equivalent to @option{--no-same-owner} only.
3541 @item -p @tab @ref{--preserve-permissions}.
3543 @item -r @tab @ref{--append}.
3545 @item -s @tab @ref{--same-order}.
3547 @item -t @tab @ref{--list}.
3549 @item -u @tab @ref{--update}.
3551 @item -v @tab @ref{--verbose}.
3553 @item -w @tab @ref{--interactive}.
3555 @item -x @tab @ref{--extract}.
3557 @item -z @tab @ref{--gzip}.
3562 @section @GNUTAR{} documentation
3564 @cindex Getting program version number
3566 @cindex Version of the @command{tar} program
3567 Being careful, the first thing is really checking that you are using
3568 @GNUTAR{}, indeed. The @option{--version} option
3569 causes @command{tar} to print information about its name, version,
3570 origin and legal status, all on standard output, and then exit
3571 successfully. For example, @w{@samp{tar --version}} might print:
3574 tar (GNU tar) @value{VERSION}
3575 Copyright (C) 2010 Free Software Foundation, Inc.
3576 Copyright (C) 2010 Free Software Foundation, Inc.
3577 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
3578 This is free software: you are free to change and redistribute it.
3579 There is NO WARRANTY, to the extent permitted by law.
3581 Written by John Gilmore and Jay Fenlason.
3585 The first occurrence of @samp{tar} in the result above is the program
3586 name in the package (for example, @command{rmt} is another program),
3587 while the second occurrence of @samp{tar} is the name of the package
3588 itself, containing possibly many programs. The package is currently
3589 named @samp{tar}, after the name of the main program it
3590 contains@footnote{There are plans to merge the @command{cpio} and
3591 @command{tar} packages into a single one which would be called
3592 @code{paxutils}. So, who knows if, one of this days, the
3593 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3596 @cindex Obtaining help
3597 @cindex Listing all @command{tar} options
3598 @xopindex{help, introduction}
3599 Another thing you might want to do is checking the spelling or meaning
3600 of some particular @command{tar} option, without resorting to this
3601 manual, for once you have carefully read it. @GNUTAR{}
3602 has a short help feature, triggerable through the
3603 @option{--help} option. By using this option, @command{tar} will
3604 print a usage message listing all available options on standard
3605 output, then exit successfully, without doing anything else and
3606 ignoring all other options. Even if this is only a brief summary, it
3607 may be several screens long. So, if you are not using some kind of
3608 scrollable window, you might prefer to use something like:
3611 $ @kbd{tar --help | less}
3615 presuming, here, that you like using @command{less} for a pager. Other
3616 popular pagers are @command{more} and @command{pg}. If you know about some
3617 @var{keyword} which interests you and do not want to read all the
3618 @option{--help} output, another common idiom is doing:
3621 tar --help | grep @var{keyword}
3625 for getting only the pertinent lines. Notice, however, that some
3626 @command{tar} options have long description lines and the above
3627 command will list only the first of them.
3629 The exact look of the option summary displayed by @kbd{tar --help} is
3630 configurable. @xref{Configuring Help Summary}, for a detailed description.
3633 If you only wish to check the spelling of an option, running @kbd{tar
3634 --usage} may be a better choice. This will display a terse list of
3635 @command{tar} options without accompanying explanations.
3637 The short help output is quite succinct, and you might have to get
3638 back to the full documentation for precise points. If you are reading
3639 this paragraph, you already have the @command{tar} manual in some
3640 form. This manual is available in a variety of forms from
3641 @url{http://www.gnu.org/software/tar/manual}. It may be printed out of the @GNUTAR{}
3642 distribution, provided you have @TeX{} already installed somewhere,
3643 and a laser printer around. Just configure the distribution, execute
3644 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3645 usual way (contact your local guru to know how). If @GNUTAR{}
3646 has been conveniently installed at your place, this
3647 manual is also available in interactive, hypertextual form as an Info
3648 file. Just call @w{@samp{info tar}} or, if you do not have the
3649 @command{info} program handy, use the Info reader provided within
3650 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3652 There is currently no @code{man} page for @GNUTAR{}.
3653 If you observe such a @code{man} page on the system you are running,
3654 either it does not belong to @GNUTAR{}, or it has not
3655 been produced by @acronym{GNU}. Some package maintainers convert
3656 @kbd{tar --help} output to a man page, using @command{help2man}. In
3657 any case, please bear in mind that the authoritative source of
3658 information about @GNUTAR{} is this Texinfo documentation.
3661 @section Obtaining @GNUTAR{} default values
3663 @opindex show-defaults
3664 @GNUTAR{} has some predefined defaults that are used when you do not
3665 explicitly specify another values. To obtain a list of such
3666 defaults, use @option{--show-defaults} option. This will output the
3667 values in the form of @command{tar} command line options:
3671 $ @kbd{tar --show-defaults}
3672 --format=gnu -f- -b20 --quoting-style=escape
3673 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3678 Notice, that this option outputs only one line. The example output above
3679 has been split to fit page boundaries.
3682 The above output shows that this version of @GNUTAR{} defaults to
3683 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3684 output as the archive, if no @option{--file} option has been given
3685 (@pxref{file tutorial}), the default blocking factor is 20
3686 (@pxref{Blocking Factor}). It also shows the default locations where
3687 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3690 @section Checking @command{tar} progress
3692 Typically, @command{tar} performs most operations without reporting any
3693 information to the user except error messages. When using @command{tar}
3694 with many options, particularly ones with complicated or
3695 difficult-to-predict behavior, it is possible to make serious mistakes.
3696 @command{tar} provides several options that make observing @command{tar}
3697 easier. These options cause @command{tar} to print information as it
3698 progresses in its job, and you might want to use them just for being
3699 more careful about what is going on, or merely for entertaining
3700 yourself. If you have encountered a problem when operating on an
3701 archive, however, you may need more information than just an error
3702 message in order to solve the problem. The following options can be
3703 helpful diagnostic tools.
3705 @cindex Verbose operation
3707 Normally, the @option{--list} (@option{-t}) command to list an archive
3708 prints just the file names (one per line) and the other commands are
3709 silent. When used with most operations, the @option{--verbose}
3710 (@option{-v}) option causes @command{tar} to print the name of each
3711 file or archive member as it is processed. This and the other options
3712 which make @command{tar} print status information can be useful in
3713 monitoring @command{tar}.
3715 With @option{--create} or @option{--extract}, @option{--verbose} used
3716 once just prints the names of the files or members as they are processed.
3717 Using it twice causes @command{tar} to print a longer listing
3718 (@xref{verbose member listing}, for the description) for each member.
3719 Since @option{--list} already prints the names of the members,
3720 @option{--verbose} used once with @option{--list} causes @command{tar}
3721 to print an @samp{ls -l} type listing of the files in the archive.
3722 The following examples both extract members with long list output:
3725 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3726 $ @kbd{tar xvvf archive.tar}
3729 Verbose output appears on the standard output except when an archive is
3730 being written to the standard output, as with @samp{tar --create
3731 --file=- --verbose} (@samp{tar cvf -}, or even @samp{tar cv}---if the
3732 installer let standard output be the default archive). In that case
3733 @command{tar} writes verbose output to the standard error stream.
3735 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3736 verbose output to @var{file} rather than to standard output or standard
3740 @cindex Obtaining total status information
3742 The @option{--totals} option causes @command{tar} to print on the
3743 standard error the total amount of bytes transferred when processing
3744 an archive. When creating or appending to an archive, this option
3745 prints the number of bytes written to the archive and the average
3746 speed at which they have been written, e.g.:
3750 $ @kbd{tar -c -f archive.tar --totals /home}
3751 Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
3755 When reading an archive, this option displays the number of bytes
3760 $ @kbd{tar -x -f archive.tar --totals}
3761 Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
3765 Finally, when deleting from an archive, the @option{--totals} option
3766 displays both numbers plus number of bytes removed from the archive:
3770 $ @kbd{tar --delete -f foo.tar --totals --wildcards '*~'}
3771 Total bytes read: 9543680 (9.2MiB, 201MiB/s)
3772 Total bytes written: 3829760 (3.7MiB, 81MiB/s)
3773 Total bytes deleted: 1474048
3777 You can also obtain this information on request. When
3778 @option{--totals} is used with an argument, this argument is
3779 interpreted as a symbolic name of a signal, upon delivery of which the
3780 statistics is to be printed:
3783 @item --totals=@var{signo}
3784 Print statistics upon delivery of signal @var{signo}. Valid arguments
3785 are: @code{SIGHUP}, @code{SIGQUIT}, @code{SIGINT}, @code{SIGUSR1} and
3786 @code{SIGUSR2}. Shortened names without @samp{SIG} prefix are also
3790 Both forms of @option{--totals} option can be used simultaneously.
3791 Thus, @kbd{tar -x --totals --totals=USR1} instructs @command{tar} to
3792 extract all members from its default archive and print statistics
3793 after finishing the extraction, as well as when receiving signal
3796 @anchor{Progress information}
3797 @cindex Progress information
3798 The @option{--checkpoint} option prints an occasional message
3799 as @command{tar} reads or writes the archive. It is designed for
3800 those who don't need the more detailed (and voluminous) output of
3801 @option{--block-number} (@option{-R}), but do want visual confirmation
3802 that @command{tar} is actually making forward progress. By default it
3803 prints a message each 10 records read or written. This can be changed
3804 by giving it a numeric argument after an equal sign:
3807 $ @kbd{tar -c --checkpoint=1000} /var
3808 tar: Write checkpoint 1000
3809 tar: Write checkpoint 2000
3810 tar: Write checkpoint 3000
3813 This example shows the default checkpoint message used by
3814 @command{tar}. If you place a dot immediately after the equal
3815 sign, it will print a @samp{.} at each checkpoint@footnote{This is
3816 actually a shortcut for @option{--checkpoint=@var{n}
3817 --checkpoint-action=dot}. @xref{checkpoints, dot}.}. For example:
3820 $ @kbd{tar -c --checkpoint=.1000} /var
3824 The @option{--checkpoint} option provides a flexible mechanism for
3825 executing arbitrary actions upon hitting checkpoints, see the next
3826 section (@pxref{checkpoints}), for more information on it.
3828 @opindex show-omitted-dirs
3829 @anchor{show-omitted-dirs}
3830 The @option{--show-omitted-dirs} option, when reading an archive---with
3831 @option{--list} or @option{--extract}, for example---causes a message
3832 to be printed for each directory in the archive which is skipped.
3833 This happens regardless of the reason for skipping: the directory might
3834 not have been named on the command line (implicitly or explicitly),
3835 it might be excluded by the use of the
3836 @option{--exclude=@var{pattern}} option, or some other reason.
3838 @opindex block-number
3839 @cindex Block number where error occurred
3840 @anchor{block-number}
3841 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3842 every message it would normally produce, the block number within the
3843 archive where the message was triggered. Also, supplementary messages
3844 are triggered when reading blocks full of NULs, or when hitting end of
3845 file on the archive. As of now, if the archive is properly terminated
3846 with a NUL block, the reading of the file may stop before end of file
3847 is met, so the position of end of file will not usually show when
3848 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3849 drains the archive before exiting when reading the
3850 archive from a pipe.
3852 @cindex Error message, block number of
3853 This option is especially useful when reading damaged archives, since
3854 it helps pinpoint the damaged sections. It can also be used with
3855 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3856 choose among several backup tapes when retrieving a file later, in
3857 favor of the tape where the file appears earliest (closest to the
3858 front of the tape). @xref{backup}.
3861 @section Checkpoints
3862 @cindex checkpoints, defined
3864 @opindex checkpoint-action
3866 A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
3867 the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
3868 from the archive (a @dfn{read checkpoint}). Checkpoints allow to
3869 periodically execute arbitrary actions.
3871 The checkpoint facility is enabled using the following option:
3874 @xopindex{checkpoint, defined}
3875 @item --checkpoint[=@var{n}]
3876 Schedule checkpoints before writing or reading each @var{n}th record.
3877 The default value for @var{n} is 10.
3880 A list of arbitrary @dfn{actions} can be executed at each checkpoint.
3881 These actions include: pausing, displaying textual messages, and
3882 executing arbitrary external programs. Actions are defined using
3883 the @option{--checkpoint-action} option.
3886 @xopindex{checkpoint-action, defined}
3887 @item --checkpoint-action=@var{action}
3888 Execute an @var{action} at each checkpoint.
3891 @cindex @code{echo}, checkpoint action
3892 The simplest value of @var{action} is @samp{echo}. It instructs
3893 @command{tar} to display the default message on the standard error
3894 stream upon arriving at each checkpoint. The default message is (in
3895 @acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
3896 checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
3897 Here, @var{n} represents ordinal number of the checkpoint.
3899 In another locales, translated versions of this message are used.
3901 This is the default action, so running:
3904 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
3911 $ @kbd{tar -c --checkpoint=1000} /var
3914 The @samp{echo} action also allows to supply a customized message.
3915 You do so by placing an equals sign and the message right after it,
3919 --checkpoint-action="echo=Hit %s checkpoint #%u"
3922 The @samp{%s} and @samp{%u} in the above example are
3923 @dfn{meta-characters}. The @samp{%s} meta-character is replaced with
3924 the @dfn{type} of the checkpoint: @samp{write} or
3925 @samp{read} (or a corresponding translated version in locales other
3926 than @acronym{POSIX}). The @samp{%u} meta-character is replaced with
3927 the ordinal number of the checkpoint. Thus, the above example could
3928 produce the following output when used with the @option{--create}
3932 tar: Hit write checkpoint #10
3933 tar: Hit write checkpoint #20
3934 tar: Hit write checkpoint #30
3937 Aside from meta-character expansion, the message string is subject to
3938 @dfn{unquoting}, during which the backslash @dfn{escape sequences} are
3939 replaced with their corresponding @acronym{ASCII} characters
3940 (@pxref{escape sequences}). E.g. the following action will produce an
3941 audible bell and the message described above at each checkpoint:
3944 --checkpoint-action='echo=\aHit %s checkpoint #%u'
3947 @cindex @code{bell}, checkpoint action
3948 There is also a special action which produces an audible signal:
3949 @samp{bell}. It is not equivalent to @samp{echo='\a'}, because
3950 @samp{bell} sends the bell directly to the console (@file{/dev/tty}),
3951 whereas @samp{echo='\a'} sends it to the standard error.
3953 @cindex @code{ttyout}, checkpoint action
3954 The @samp{ttyout=@var{string}} action outputs @var{string} to
3955 @file{/dev/tty}, so it can be used even if the standard output is
3956 redirected elsewhere. The @var{string} is subject to the same
3957 modifications as with @samp{echo} action. In contrast to the latter,
3958 @samp{ttyout} does not prepend @command{tar} executable name to the
3959 string, nor does it output a newline after it. For example, the
3960 following action will print the checkpoint message at the same screen
3961 line, overwriting any previous message:
3964 --checkpoint-action="ttyout=\rHit %s checkpoint #%u"
3967 @cindex @code{dot}, checkpoint action
3968 Another available checkpoint action is @samp{dot} (or @samp{.}). It
3969 instructs @command{tar} to print a single dot on the standard listing
3973 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
3977 For compatibility with previous @GNUTAR{} versions, this action can
3978 be abbreviated by placing a dot in front of the checkpoint frequency,
3979 as shown in the previous section.
3981 @cindex @code{sleep}, checkpoint action
3982 Yet another action, @samp{sleep}, pauses @command{tar} for a specified
3983 amount of seconds. The following example will stop for 30 seconds at each
3987 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
3990 @cindex @code{exec}, checkpoint action
3991 Finally, the @code{exec} action executes a given external program.
3995 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
3998 This program is executed using @command{/bin/sh -c}, with no
3999 additional arguments. Its exit code is ignored. It gets a copy of
4000 @command{tar}'s environment plus the following variables:
4003 @vrindex TAR_VERSION, checkpoint script environment
4005 @GNUTAR{} version number.
4007 @vrindex TAR_ARCHIVE, checkpoint script environment
4009 The name of the archive @command{tar} is processing.
4011 @vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
4012 @item TAR_BLOCKING_FACTOR
4013 Current blocking factor (@pxref{Blocking}).
4015 @vrindex TAR_CHECKPOINT, checkpoint script environment
4016 @item TAR_CHECKPOINT
4017 Number of the checkpoint.
4019 @vrindex TAR_SUBCOMMAND, checkpoint script environment
4020 @item TAR_SUBCOMMAND
4021 A short option describing the operation @command{tar} is executing.
4022 @xref{Operations}, for a complete list of subcommand options.
4024 @vrindex TAR_FORMAT, checkpoint script environment
4026 Format of the archive being processed. @xref{Formats}, for a complete
4027 list of archive format names.
4030 Any number of actions can be defined, by supplying several
4031 @option{--checkpoint-action} options in the command line. For
4032 example, the command below displays two messages, pauses
4033 execution for 30 seconds and executes the @file{/sbin/cpoint} script:
4037 $ @kbd{tar -c -f arc.tar \
4038 --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
4039 --checkpoint-action='echo=Sleeping for 30 seconds' \
4040 --checkpoint-action='sleep=30' \
4041 --checkpoint-action='exec=/sbin/cpoint'}
4045 This example also illustrates the fact that
4046 @option{--checkpoint-action} can be used without
4047 @option{--checkpoint}. In this case, the default checkpoint frequency
4048 (at each 10th record) is assumed.
4051 @section Controlling Warning Messages
4053 Sometimes, while performing the requested task, @GNUTAR{} notices
4054 some conditions that are not exactly errors, but which the user
4055 should be aware of. When this happens, @command{tar} issues a
4056 @dfn{warning message} describing the condition. Warning messages
4057 are output to the standard error and they do not affect the exit
4058 code of @command{tar} command.
4060 @xopindex{warning, explained}
4061 @GNUTAR{} allows the user to suppress some or all of its warning
4065 @item --warning=@var{keyword}
4066 Control display of the warning messages identified by @var{keyword}.
4067 If @var{keyword} starts with the prefix @samp{no-}, such messages are
4068 suppressed. Otherwise, they are enabled.
4070 Multiple @option{--warning} messages accumulate.
4072 The tables below list allowed values for @var{keyword} along with the
4073 warning messages they control.
4076 @subheading Keywords controlling @command{tar} operation
4080 Enable all warning messages. This is the default.
4083 Disable all warning messages.
4084 @kwindex filename-with-nuls
4085 @cindex @samp{file name read contains nul character}, warning message
4086 @item filename-with-nuls
4087 @samp{%s: file name read contains nul character}
4088 @kwindex alone-zero-block
4089 @cindex @samp{A lone zero block at}, warning message
4090 @item alone-zero-block
4091 @samp{A lone zero block at %s}
4094 @subheading Keywords applicable for @command{tar --create}
4097 @cindex @samp{contains a cache directory tag}, warning message
4099 @samp{%s: contains a cache directory tag %s; %s}
4100 @kwindex file-shrank
4101 @cindex @samp{File shrank by %s bytes}, warning message
4103 @samp{%s: File shrank by %s bytes; padding with zeros}
4105 @cindex @samp{file is on a different filesystem}, warning message
4107 @samp{%s: file is on a different filesystem; not dumped}
4108 @kwindex file-ignored
4109 @cindex @samp{Unknown file type; file ignored}, warning message
4110 @cindex @samp{socket ignored}, warning message
4111 @cindex @samp{door ignored}, warning message
4113 @samp{%s: Unknown file type; file ignored}
4114 @*@samp{%s: socket ignored}
4115 @*@samp{%s: door ignored}
4116 @kwindex file-unchanged
4117 @cindex @samp{file is unchanged; not dumped}, warning message
4118 @item file-unchanged
4119 @samp{%s: file is unchanged; not dumped}
4120 @kwindex ignore-archive
4121 @cindex @samp{file is the archive; not dumped}, warning message
4122 @kwindex ignore-archive
4123 @cindex @samp{file is the archive; not dumped}, warning message
4124 @item ignore-archive
4125 @samp{%s: file is the archive; not dumped}
4126 @kwindex file-removed
4127 @cindex @samp{File removed before we read it}, warning message
4129 @samp{%s: File removed before we read it}
4130 @kwindex file-changed
4131 @cindex @samp{file changed as we read it}, warning message
4133 @samp{%s: file changed as we read it}
4136 @subheading Keywords applicable for @command{tar --extract}
4139 @cindex @samp{implausibly old time stamp %s}, warning message
4140 @cindex @samp{time stamp %s is %s s in the future}, warning message
4142 @samp{%s: implausibly old time stamp %s}
4143 @*@samp{%s: time stamp %s is %s s in the future}
4144 @kwindex contiguous-cast
4145 @cindex @samp{Extracting contiguous files as regular files}, warning message
4146 @item contiguous-cast
4147 @samp{Extracting contiguous files as regular files}
4148 @kwindex symlink-cast
4149 @cindex @samp{Attempting extraction of symbolic links as hard links}, warning message
4151 @samp{Attempting extraction of symbolic links as hard links}
4152 @kwindex unknown-cast
4153 @cindex @samp{Unknown file type `%c', extracted as normal file}, warning message
4155 @samp{%s: Unknown file type `%c', extracted as normal file}
4156 @kwindex ignore-newer
4157 @cindex @samp{Current %s is newer or same age}, warning message
4159 @samp{Current %s is newer or same age}
4160 @kwindex unknown-keyword
4161 @cindex @samp{Ignoring unknown extended header keyword `%s'}, warning message
4162 @item unknown-keyword
4163 @samp{Ignoring unknown extended header keyword `%s'}
4164 @kwindex decompress-program
4165 @item decompress-program
4166 Controls verbose description of failures occurring when trying to run
4167 alternative decompressor programs (@pxref{alternative decompression
4168 programs}). This warning is disabled by default (unless
4169 @option{--verbose} is used). A common example of what you can get
4170 when using this warning is:
4173 $ @kbd{tar --warning=decompress-program -x -f archive.Z}
4174 tar (child): cannot run compress: No such file or directory
4175 tar (child): trying gzip
4178 This means that @command{tar} first tried to decompress
4179 @file{archive.Z} using @command{compress}, and, when that
4180 failed, switched to @command{gzip}.
4183 @subheading Keywords controlling incremental extraction:
4185 @kwindex rename-directory
4186 @cindex @samp{%s: Directory has been renamed from %s}, warning message
4187 @cindex @samp{%s: Directory has been renamed}, warning message
4188 @item rename-directory
4189 @samp{%s: Directory has been renamed from %s}
4190 @*@samp{%s: Directory has been renamed}
4191 @kwindex new-directory
4192 @cindex @samp{%s: Directory is new}, warning message
4194 @samp{%s: Directory is new}
4196 @cindex @samp{%s: directory is on a different device: not purging}, warning message
4198 @samp{%s: directory is on a different device: not purging}
4199 @kwindex bad-dumpdir
4200 @cindex @samp{Malformed dumpdir: 'X' never used}, warning message
4202 @samp{Malformed dumpdir: 'X' never used}
4206 @section Asking for Confirmation During Operations
4207 @cindex Interactive operation
4209 Typically, @command{tar} carries out a command without stopping for
4210 further instructions. In some situations however, you may want to
4211 exclude some files and archive members from the operation (for instance
4212 if disk or storage space is tight). You can do this by excluding
4213 certain files automatically (@pxref{Choosing}), or by performing
4214 an operation interactively, using the @option{--interactive} (@option{-w}) option.
4215 @command{tar} also accepts @option{--confirmation} for this option.
4217 @opindex interactive
4218 When the @option{--interactive} (@option{-w}) option is specified, before
4219 reading, writing, or deleting files, @command{tar} first prints a message
4220 for each such file, telling what operation it intends to take, then asks
4221 for confirmation on the terminal. The actions which require
4222 confirmation include adding a file to the archive, extracting a file
4223 from the archive, deleting a file from the archive, and deleting a file
4224 from disk. To confirm the action, you must type a line of input
4225 beginning with @samp{y}. If your input line begins with anything other
4226 than @samp{y}, @command{tar} skips that file.
4228 If @command{tar} is reading the archive from the standard input,
4229 @command{tar} opens the file @file{/dev/tty} to support the interactive
4232 Verbose output is normally sent to standard output, separate from
4233 other error messages. However, if the archive is produced directly
4234 on standard output, then verbose output is mixed with errors on
4235 @code{stderr}. Producing the archive on standard output may be used
4236 as a way to avoid using disk space, when the archive is soon to be
4237 consumed by another process reading it, say. Some people felt the need
4238 of producing an archive on stdout, still willing to segregate between
4239 verbose output and error output. A possible approach would be using a
4240 named pipe to receive the archive, and having the consumer process to
4241 read from that named pipe. This has the advantage of letting standard
4242 output free to receive verbose output, all separate from errors.
4245 @chapter @GNUTAR{} Operations
4258 @section Basic @GNUTAR{} Operations
4260 The basic @command{tar} operations, @option{--create} (@option{-c}),
4261 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
4262 @option{-x}), are currently presented and described in the tutorial
4263 chapter of this manual. This section provides some complementary notes
4264 for these operations.
4267 @xopindex{create, complementary notes}
4271 Creating an empty archive would have some kind of elegance. One can
4272 initialize an empty archive and later use @option{--append}
4273 (@option{-r}) for adding all members. Some applications would not
4274 welcome making an exception in the way of adding the first archive
4275 member. On the other hand, many people reported that it is
4276 dangerously too easy for @command{tar} to destroy a magnetic tape with
4277 an empty archive@footnote{This is well described in @cite{Unix-haters
4278 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
4279 Books, ISBN 1-56884-203-1.}. The two most common errors are:
4283 Mistakingly using @code{create} instead of @code{extract}, when the
4284 intent was to extract the full contents of an archive. This error
4285 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
4286 the QWERTY keyboard. Instead of being unpacked, the archive then
4287 gets wholly destroyed. When users speak about @dfn{exploding} an
4288 archive, they usually mean something else :-).
4291 Forgetting the argument to @code{file}, when the intent was to create
4292 an archive with a single file in it. This error is likely because a
4293 tired user can easily add the @kbd{f} key to the cluster of option
4294 letters, by the mere force of habit, without realizing the full
4295 consequence of doing so. The usual consequence is that the single
4296 file, which was meant to be saved, is rather destroyed.
4299 So, recognizing the likelihood and the catastrophic nature of these
4300 errors, @GNUTAR{} now takes some distance from elegance, and
4301 cowardly refuses to create an archive when @option{--create} option is
4302 given, there are no arguments besides options, and
4303 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
4304 around the cautiousness of @GNUTAR{} and nevertheless create an
4305 archive with nothing in it, one may still use, as the value for the
4306 @option{--files-from} option, a file with no names in it, as shown in
4307 the following commands:
4310 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
4311 @kbd{tar -cf empty-archive.tar -T /dev/null}
4314 @xopindex{extract, complementary notes}
4319 A socket is stored, within a @GNUTAR{} archive, as a pipe.
4321 @item @option{--list} (@option{-t})
4323 @GNUTAR{} now shows dates as @samp{1996-08-30},
4324 while it used to show them as @samp{Aug 30 1996}. Preferably,
4325 people should get used to ISO 8601 dates. Local American dates should
4326 be made available again with full date localization support, once
4327 ready. In the meantime, programs not being localizable for dates
4328 should prefer international dates, that's really the way to go.
4330 Look up @url{http://www.cl.cam.ac.uk/@/~mgk25/@/iso-time.html} if you
4331 are curious, it contains a detailed explanation of the ISO 8601 standard.
4336 @section Advanced @GNUTAR{} Operations
4338 Now that you have learned the basics of using @GNUTAR{}, you may want
4339 to learn about further ways in which @command{tar} can help you.
4341 This chapter presents five, more advanced operations which you probably
4342 won't use on a daily basis, but which serve more specialized functions.
4343 We also explain the different styles of options and why you might want
4344 to use one or another, or a combination of them in your @command{tar}
4345 commands. Additionally, this chapter includes options which allow you to
4346 define the output from @command{tar} more carefully, and provide help and
4347 error correction in special circumstances.
4349 @FIXME{check this after the chapter is actually revised to make sure
4350 it still introduces the info in the chapter correctly : ).}
4362 @subsection The Five Advanced @command{tar} Operations
4364 @cindex basic operations
4365 In the last chapter, you learned about the first three operations to
4366 @command{tar}. This chapter presents the remaining five operations to
4367 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
4368 @option{--delete}, and @option{--compare}.
4370 You are not likely to use these operations as frequently as those
4371 covered in the last chapter; however, since they perform specialized
4372 functions, they are quite useful when you do need to use them. We
4373 will give examples using the same directory and files that you created
4374 in the last chapter. As you may recall, the directory is called
4375 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
4376 and the two archive files you created are
4377 @samp{collection.tar} and @samp{music.tar}.
4379 We will also use the archive files @samp{afiles.tar} and
4380 @samp{bfiles.tar}. The archive @samp{afiles.tar} contains the members @samp{apple},
4381 @samp{angst}, and @samp{aspic}; @samp{bfiles.tar} contains the members
4382 @samp{./birds}, @samp{baboon}, and @samp{./box}.
4384 Unless we state otherwise, all practicing you do and examples you follow
4385 in this chapter will take place in the @file{practice} directory that
4386 you created in the previous chapter; see @ref{prepare for examples}.
4387 (Below in this section, we will remind you of the state of the examples
4388 where the last chapter left them.)
4390 The five operations that we will cover in this chapter are:
4395 Add new entries to an archive that already exists.
4398 Add more recent copies of archive members to the end of an archive, if
4403 Add one or more pre-existing archives to the end of another archive.
4405 Delete items from an archive (does not work on tapes).
4409 Compare archive members to their counterparts in the file system.
4413 @subsection How to Add Files to Existing Archives: @option{--append}
4415 @cindex appending files to existing archive
4417 If you want to add files to an existing archive, you don't need to
4418 create a new archive; you can use @option{--append} (@option{-r}).
4419 The archive must already exist in order to use @option{--append}. (A
4420 related operation is the @option{--update} operation; you can use this
4421 to add newer versions of archive members to an existing archive. To learn how to
4422 do this with @option{--update}, @pxref{update}.)
4424 If you use @option{--append} to add a file that has the same name as an
4425 archive member to an archive containing that archive member, then the
4426 old member is not deleted. What does happen, however, is somewhat
4427 complex. @command{tar} @emph{allows} you to have infinite number of files
4428 with the same name. Some operations treat these same-named members no
4429 differently than any other set of archive members: for example, if you
4430 view an archive with @option{--list} (@option{-t}), you will see all
4431 of those members listed, with their data modification times, owners, etc.
4433 Other operations don't deal with these members as perfectly as you might
4434 prefer; if you were to use @option{--extract} to extract the archive,
4435 only the most recently added copy of a member with the same name as
4436 other members would end up in the working directory. This is because
4437 @option{--extract} extracts an archive in the order the members appeared
4438 in the archive; the most recently archived members will be extracted
4439 last. Additionally, an extracted member will @emph{replace} a file of
4440 the same name which existed in the directory already, and @command{tar}
4441 will not prompt you about this@footnote{Unless you give it
4442 @option{--keep-old-files} option, or the disk copy is newer than
4443 the one in the archive and you invoke @command{tar} with
4444 @option{--keep-newer-files} option.}. Thus, only the most recently archived
4445 member will end up being extracted, as it will replace the one
4446 extracted before it, and so on.
4448 @cindex extracting @var{n}th copy of the file
4449 @xopindex{occurrence, described}
4450 There exists a special option that allows you to get around this
4451 behavior and extract (or list) only a particular copy of the file.
4452 This is @option{--occurrence} option. If you run @command{tar} with
4453 this option, it will extract only the first copy of the file. You
4454 may also give this option an argument specifying the number of
4455 copy to be extracted. Thus, for example if the archive
4456 @file{archive.tar} contained three copies of file @file{myfile}, then
4460 tar --extract --file archive.tar --occurrence=2 myfile
4464 would extract only the second copy. @xref{Option
4465 Summary,---occurrence}, for the description of @option{--occurrence}
4468 @FIXME{ hag -- you might want to incorporate some of the above into the
4469 MMwtSN node; not sure. i didn't know how to make it simpler...
4471 There are a few ways to get around this. Xref to Multiple Members
4472 with the Same Name, maybe.}
4474 @cindex Members, replacing with other members
4475 @cindex Replacing members with other members
4476 @xopindex{delete, using before --append}
4477 If you want to replace an archive member, use @option{--delete} to
4478 delete the member you want to remove from the archive, and then use
4479 @option{--append} to add the member you want to be in the archive. Note
4480 that you can not change the order of the archive; the most recently
4481 added member will still appear last. In this sense, you cannot truly
4482 ``replace'' one member with another. (Replacing one member with another
4483 will not work on certain types of media, such as tapes; see @ref{delete}
4484 and @ref{Media}, for more information.)
4487 * appending files:: Appending Files to an Archive
4491 @node appending files
4492 @subsubsection Appending Files to an Archive
4493 @cindex Adding files to an Archive
4494 @cindex Appending files to an Archive
4495 @cindex Archives, Appending files to
4498 The simplest way to add a file to an already existing archive is the
4499 @option{--append} (@option{-r}) operation, which writes specified
4500 files into the archive whether or not they are already among the
4503 When you use @option{--append}, you @emph{must} specify file name
4504 arguments, as there is no default. If you specify a file that already
4505 exists in the archive, another copy of the file will be added to the
4506 end of the archive. As with other operations, the member names of the
4507 newly added files will be exactly the same as their names given on the
4508 command line. The @option{--verbose} (@option{-v}) option will print
4509 out the names of the files as they are written into the archive.
4511 @option{--append} cannot be performed on some tape drives, unfortunately,
4512 due to deficiencies in the formats those tape drives use. The archive
4513 must be a valid @command{tar} archive, or else the results of using this
4514 operation will be unpredictable. @xref{Media}.
4516 To demonstrate using @option{--append} to add a file to an archive,
4517 create a file called @file{rock} in the @file{practice} directory.
4518 Make sure you are in the @file{practice} directory. Then, run the
4519 following @command{tar} command to add @file{rock} to
4520 @file{collection.tar}:
4523 $ @kbd{tar --append --file=collection.tar rock}
4527 If you now use the @option{--list} (@option{-t}) operation, you will see that
4528 @file{rock} has been added to the archive:
4531 $ @kbd{tar --list --file=collection.tar}
4532 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4533 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4534 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4535 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4539 @subsubsection Multiple Members with the Same Name
4540 @cindex members, multiple
4541 @cindex multiple members
4543 You can use @option{--append} (@option{-r}) to add copies of files
4544 which have been updated since the archive was created. (However, we
4545 do not recommend doing this since there is another @command{tar}
4546 option called @option{--update}; @xref{update}, for more information.
4547 We describe this use of @option{--append} here for the sake of
4548 completeness.) When you extract the archive, the older version will
4549 be effectively lost. This works because files are extracted from an
4550 archive in the order in which they were archived. Thus, when the
4551 archive is extracted, a file archived later in time will replace a
4552 file of the same name which was archived earlier, even though the
4553 older version of the file will remain in the archive unless you delete
4554 all versions of the file.
4556 Supposing you change the file @file{blues} and then append the changed
4557 version to @file{collection.tar}. As you saw above, the original
4558 @file{blues} is in the archive @file{collection.tar}. If you change the
4559 file and append the new version of the file to the archive, there will
4560 be two copies in the archive. When you extract the archive, the older
4561 version of the file will be extracted first, and then replaced by the
4562 newer version when it is extracted.
4564 You can append the new, changed copy of the file @file{blues} to the
4565 archive in this way:
4568 $ @kbd{tar --append --verbose --file=collection.tar blues}
4573 Because you specified the @option{--verbose} option, @command{tar} has
4574 printed the name of the file being appended as it was acted on. Now
4575 list the contents of the archive:
4578 $ @kbd{tar --list --verbose --file=collection.tar}
4579 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4580 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4581 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4582 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4583 -rw-r--r-- me/user 58 1996-10-24 18:30 blues
4587 The newest version of @file{blues} is now at the end of the archive
4588 (note the different creation dates and file sizes). If you extract
4589 the archive, the older version of the file @file{blues} will be
4590 replaced by the newer version. You can confirm this by extracting
4591 the archive and running @samp{ls} on the directory.
4593 If you wish to extract the first occurrence of the file @file{blues}
4594 from the archive, use @option{--occurrence} option, as shown in
4595 the following example:
4598 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
4599 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4602 @xref{Writing}, for more information on @option{--extract} and
4603 see @ref{Option Summary, --occurrence}, for a description of
4604 @option{--occurrence} option.
4607 @subsection Updating an Archive
4608 @cindex Updating an archive
4611 In the previous section, you learned how to use @option{--append} to
4612 add a file to an existing archive. A related operation is
4613 @option{--update} (@option{-u}). The @option{--update} operation
4614 updates a @command{tar} archive by comparing the date of the specified
4615 archive members against the date of the file with the same name. If
4616 the file has been modified more recently than the archive member, then
4617 the newer version of the file is added to the archive (as with
4620 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
4621 The operation will fail.
4623 @FIXME{other examples of media on which --update will fail? need to ask
4624 charles and/or mib/thomas/dave shevett..}
4626 Both @option{--update} and @option{--append} work by adding to the end
4627 of the archive. When you extract a file from the archive, only the
4628 version stored last will wind up in the file system, unless you use
4629 the @option{--backup} option. @xref{multiple}, for a detailed discussion.
4636 @subsubsection How to Update an Archive Using @option{--update}
4639 You must use file name arguments with the @option{--update}
4640 (@option{-u}) operation. If you don't specify any files,
4641 @command{tar} won't act on any files and won't tell you that it didn't
4642 do anything (which may end up confusing you).
4644 @c note: the above parenthetical added because in fact, this
4645 @c behavior just confused the author. :-)
4647 To see the @option{--update} option at work, create a new file,
4648 @file{classical}, in your practice directory, and some extra text to the
4649 file @file{blues}, using any text editor. Then invoke @command{tar} with
4650 the @samp{update} operation and the @option{--verbose} (@option{-v})
4651 option specified, using the names of all the files in the @file{practice}
4652 directory as file name arguments:
4655 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4662 Because we have specified verbose mode, @command{tar} prints out the names
4663 of the files it is working on, which in this case are the names of the
4664 files that needed to be updated. If you run @samp{tar --list} and look
4665 at the archive, you will see @file{blues} and @file{classical} at its
4666 end. There will be a total of two versions of the member @samp{blues};
4667 the one at the end will be newer and larger, since you added text before
4670 The reason @command{tar} does not overwrite the older file when updating
4671 it is because writing to the middle of a section of tape is a difficult
4672 process. Tapes are not designed to go backward. @xref{Media}, for more
4673 information about tapes.
4675 @option{--update} (@option{-u}) is not suitable for performing backups for two
4676 reasons: it does not change directory content entries, and it
4677 lengthens the archive every time it is used. The @GNUTAR{}
4678 options intended specifically for backups are more
4679 efficient. If you need to run backups, please consult @ref{Backups}.
4682 @subsection Combining Archives with @option{--concatenate}
4684 @cindex Adding archives to an archive
4685 @cindex Concatenating Archives
4686 @opindex concatenate
4688 @c @cindex @option{-A} described
4689 Sometimes it may be convenient to add a second archive onto the end of
4690 an archive rather than adding individual files to the archive. To add
4691 one or more archives to the end of another archive, you should use the
4692 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4694 To use @option{--concatenate}, give the first archive with
4695 @option{--file} option and name the rest of archives to be
4696 concatenated on the command line. The members, and their member
4697 names, will be copied verbatim from those archives to the first
4698 one@footnote{This can cause multiple members to have the same name. For
4699 information on how this affects reading the archive, see @ref{multiple}.}.
4700 The new, concatenated archive will be called by the same name as the
4701 one given with the @option{--file} option. As usual, if you omit
4702 @option{--file}, @command{tar} will use the value of the environment
4703 variable @env{TAPE}, or, if this has not been set, the default archive name.
4705 @FIXME{There is no way to specify a new name...}
4707 To demonstrate how @option{--concatenate} works, create two small archives
4708 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4709 files from @file{practice}:
4712 $ @kbd{tar -cvf bluesrock.tar blues rock}
4715 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4721 If you like, You can run @samp{tar --list} to make sure the archives
4722 contain what they are supposed to:
4725 $ @kbd{tar -tvf bluesrock.tar}
4726 -rw-r--r-- melissa/user 105 1997-01-21 19:42 blues
4727 -rw-r--r-- melissa/user 33 1997-01-20 15:34 rock
4728 $ @kbd{tar -tvf jazzfolk.tar}
4729 -rw-r--r-- melissa/user 20 1996-09-23 16:44 folk
4730 -rw-r--r-- melissa/user 65 1997-01-30 14:15 jazz
4733 We can concatenate these two archives with @command{tar}:
4737 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4740 If you now list the contents of the @file{bluesrock.tar}, you will see
4741 that now it also contains the archive members of @file{jazzfolk.tar}:
4744 $ @kbd{tar --list --file=bluesrock.tar}
4751 When you use @option{--concatenate}, the source and target archives must
4752 already exist and must have been created using compatible format
4753 parameters. Notice, that @command{tar} does not check whether the
4754 archives it concatenates have compatible formats, it does not
4755 even check if the files are really tar archives.
4757 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4758 tape drives, due to deficiencies in the formats those tape drives use.
4760 @cindex @code{concatenate} vs @command{cat}
4761 @cindex @command{cat} vs @code{concatenate}
4762 It may seem more intuitive to you to want or try to use @command{cat} to
4763 concatenate two archives instead of using the @option{--concatenate}
4764 operation; after all, @command{cat} is the utility for combining files.
4766 However, @command{tar} archives incorporate an end-of-file marker which
4767 must be removed if the concatenated archives are to be read properly as
4768 one archive. @option{--concatenate} removes the end-of-archive marker
4769 from the target archive before each new archive is appended. If you use
4770 @command{cat} to combine the archives, the result will not be a valid
4771 @command{tar} format archive. If you need to retrieve files from an
4772 archive that was added to using the @command{cat} utility, use the
4773 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4774 information on dealing with archives improperly combined using the
4775 @command{cat} shell utility.
4778 @subsection Removing Archive Members Using @option{--delete}
4779 @cindex Deleting files from an archive
4780 @cindex Removing files from an archive
4783 You can remove members from an archive by using the @option{--delete}
4784 option. Specify the name of the archive with @option{--file}
4785 (@option{-f}) and then specify the names of the members to be deleted;
4786 if you list no member names, nothing will be deleted. The
4787 @option{--verbose} option will cause @command{tar} to print the names
4788 of the members as they are deleted. As with @option{--extract}, you
4789 must give the exact member names when using @samp{tar --delete}.
4790 @option{--delete} will remove all versions of the named file from the
4791 archive. The @option{--delete} operation can run very slowly.
4793 Unlike other operations, @option{--delete} has no short form.
4795 @cindex Tapes, using @option{--delete} and
4796 @cindex Deleting from tape archives
4797 This operation will rewrite the archive. You can only use
4798 @option{--delete} on an archive if the archive device allows you to
4799 write to any point on the media, such as a disk; because of this, it
4800 does not work on magnetic tapes. Do not try to delete an archive member
4801 from a magnetic tape; the action will not succeed, and you will be
4802 likely to scramble the archive and damage your tape. There is no safe
4803 way (except by completely re-writing the archive) to delete files from
4804 most kinds of magnetic tape. @xref{Media}.
4806 To delete all versions of the file @file{blues} from the archive
4807 @file{collection.tar} in the @file{practice} directory, make sure you
4808 are in that directory, and then,
4811 $ @kbd{tar --list --file=collection.tar}
4816 $ @kbd{tar --delete --file=collection.tar blues}
4817 $ @kbd{tar --list --file=collection.tar}
4823 @FIXME{Check if the above listing is actually produced after running
4824 all the examples on collection.tar.}
4826 The @option{--delete} option has been reported to work properly when
4827 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4830 @subsection Comparing Archive Members with the File System
4831 @cindex Verifying the currency of an archive
4834 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4835 specified archive members against files with the same names, and then
4836 reports differences in file size, mode, owner, modification date and
4837 contents. You should @emph{only} specify archive member names, not file
4838 names. If you do not name any members, then @command{tar} will compare the
4839 entire archive. If a file is represented in the archive but does not
4840 exist in the file system, @command{tar} reports a difference.
4842 You have to specify the record size of the archive when modifying an
4843 archive with a non-default record size.
4845 @command{tar} ignores files in the file system that do not have
4846 corresponding members in the archive.
4848 The following example compares the archive members @file{rock},
4849 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4850 files of the same name in the file system. (Note that there is no file,
4851 @file{funk}; @command{tar} will report an error message.)
4854 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4857 tar: funk not found in archive
4860 The spirit behind the @option{--compare} (@option{--diff},
4861 @option{-d}) option is to check whether the archive represents the
4862 current state of files on disk, more than validating the integrity of
4863 the archive media. For this latter goal, see @ref{verify}.
4865 @node create options
4866 @section Options Used by @option{--create}
4868 @xopindex{create, additional options}
4869 The previous chapter described the basics of how to use
4870 @option{--create} (@option{-c}) to create an archive from a set of files.
4871 @xref{create}. This section described advanced options to be used with
4875 * override:: Overriding File Metadata.
4876 * Ignore Failed Read::
4880 @subsection Overriding File Metadata
4882 As described above, a @command{tar} archive keeps, for each member it contains,
4883 its @dfn{metadata}, such as modification time, mode and ownership of
4884 the file. @GNUTAR{} allows to replace these data with other values
4885 when adding files to the archive. The options described in this
4886 section affect creation of archives of any type. For POSIX archives,
4887 see also @ref{PAX keywords}, for additional ways of controlling
4888 metadata, stored in the archive.
4892 @item --mode=@var{permissions}
4894 When adding files to an archive, @command{tar} will use
4895 @var{permissions} for the archive members, rather than the permissions
4896 from the files. @var{permissions} can be specified either as an octal
4897 number or as symbolic permissions, like with
4898 @command{chmod} (@xref{File permissions, Permissions, File
4899 permissions, fileutils, @acronym{GNU} file utilities}. This reference
4900 also has useful information for those not being overly familiar with
4901 the UNIX permission system). Using latter syntax allows for
4902 more flexibility. For example, the value @samp{a+rw} adds read and write
4903 permissions for everybody, while retaining executable bits on directories
4904 or on any other file already marked as executable:
4907 $ @kbd{tar -c -f archive.tar --mode='a+rw' .}
4910 @item --mtime=@var{date}
4913 When adding files to an archive, @command{tar} will use @var{date} as
4914 the modification time of members when creating archives, instead of
4915 their actual modification times. The argument @var{date} can be
4916 either a textual date representation in almost arbitrary format
4917 (@pxref{Date input formats}) or a name of an existing file, starting
4918 with @samp{/} or @samp{.}. In the latter case, the modification time
4919 of that file will be used.
4921 The following example will set the modification date to 00:00:00,
4925 $ @kbd{tar -c -f archive.tar --mtime='1970-01-01' .}
4929 When used with @option{--verbose} (@pxref{verbose tutorial}) @GNUTAR{}
4930 will try to convert the specified date back to its textual
4931 representation and compare it with the one given with
4932 @option{--mtime} options. If the two dates differ, @command{tar} will
4933 print a warning saying what date it will use. This is to help user
4934 ensure he is using the right date.
4939 $ @kbd{tar -c -f archive.tar -v --mtime=yesterday .}
4940 tar: Option --mtime: Treating date `yesterday' as 2006-06-20
4945 @item --owner=@var{user}
4948 Specifies that @command{tar} should use @var{user} as the owner of members
4949 when creating archives, instead of the user associated with the source
4952 If @var{user} contains a colon, it is taken to be of the form
4953 @var{name}:@var{id} where a nonempty @var{name} specifies the user
4954 name and a nonempty @var{id} specifies the decimal numeric user
4955 @acronym{ID}. If @var{user} does not contain a colon, it is taken to
4956 be a user number if it is one or more decimal digits; otherwise it is
4957 taken to be a user name.
4959 If a name is given but no number, the number is inferred from the
4960 current host's user database if possible, and the file's user number
4961 is used otherwise. If a number is given but no name, the name is
4962 inferred from the number if possible, and an empty name is used
4963 otherwise. If both name and number are given, the user database is
4964 not consulted, and the name and number need not be valid on the
4967 There is no value indicating a missing number, and @samp{0} usually means
4968 @code{root}. Some people like to force @samp{0} as the value to offer in
4969 their distributions for the owner of files, because the @code{root} user is
4970 anonymous anyway, so that might as well be the owner of anonymous
4971 archives. For example:
4974 $ @kbd{tar -c -f archive.tar --owner=0 .}
4981 $ @kbd{tar -c -f archive.tar --owner=root .}
4984 @item --group=@var{group}
4987 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
4988 rather than the group from the source file. As with @option{--owner},
4989 the argument @var{group} can be an existing group symbolic name, or a
4990 decimal numeric group @acronym{ID}, or @var{name}:@var{id}.
4993 @node Ignore Failed Read
4994 @subsection Ignore Fail Read
4997 @item --ignore-failed-read
4998 @opindex ignore-failed-read
4999 Do not exit with nonzero on unreadable files or directories.
5002 @node extract options
5003 @section Options Used by @option{--extract}
5004 @cindex options for use with @option{--extract}
5006 @xopindex{extract, additional options}
5007 The previous chapter showed how to use @option{--extract} to extract
5008 an archive into the file system. Various options cause @command{tar} to
5009 extract more information than just file contents, such as the owner,
5010 the permissions, the modification date, and so forth. This section
5011 presents options to be used with @option{--extract} when certain special
5012 considerations arise. You may review the information presented in
5013 @ref{extract} for more basic information about the
5014 @option{--extract} operation.
5017 * Reading:: Options to Help Read Archives
5018 * Writing:: Changing How @command{tar} Writes Files
5019 * Scarce:: Coping with Scarce Resources
5023 @subsection Options to Help Read Archives
5024 @cindex Options when reading archives
5026 @cindex Reading incomplete records
5027 @cindex Records, incomplete
5028 @opindex read-full-records
5029 Normally, @command{tar} will request data in full record increments from
5030 an archive storage device. If the device cannot return a full record,
5031 @command{tar} will report an error. However, some devices do not always
5032 return full records, or do not require the last record of an archive to
5033 be padded out to the next record boundary. To keep reading until you
5034 obtain a full record, or to accept an incomplete record if it contains
5035 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
5036 in conjunction with the @option{--extract} or @option{--list} operations.
5039 The @option{--read-full-records} (@option{-B}) option is turned on by default when
5040 @command{tar} reads an archive from standard input, or from a remote
5041 machine. This is because on @acronym{BSD} Unix systems, attempting to read a
5042 pipe returns however much happens to be in the pipe, even if it is
5043 less than was requested. If this option were not enabled, @command{tar}
5044 would fail as soon as it read an incomplete record from the pipe.
5046 If you're not sure of the blocking factor of an archive, you can
5047 read the archive by specifying @option{--read-full-records} (@option{-B}) and
5048 @option{--blocking-factor=@var{512-size}} (@option{-b
5049 @var{512-size}}), using a blocking factor larger than what the archive
5050 uses. This lets you avoid having to determine the blocking factor
5051 of an archive. @xref{Blocking Factor}.
5054 * read full records::
5058 @node read full records
5059 @unnumberedsubsubsec Reading Full Records
5061 @FIXME{need sentence or so of intro here}
5064 @opindex read-full-records
5065 @item --read-full-records
5067 Use in conjunction with @option{--extract} (@option{--get},
5068 @option{-x}) to read an archive which contains incomplete records, or
5069 one which has a blocking factor less than the one specified.
5073 @unnumberedsubsubsec Ignoring Blocks of Zeros
5075 @cindex End-of-archive blocks, ignoring
5076 @cindex Ignoring end-of-archive blocks
5077 @opindex ignore-zeros
5078 Normally, @command{tar} stops reading when it encounters a block of zeros
5079 between file entries (which usually indicates the end of the archive).
5080 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
5081 completely read an archive which contains a block of zeros before the
5082 end (i.e., a damaged archive, or one that was created by concatenating
5083 several archives together).
5085 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
5086 versions of @command{tar} write garbage after the end-of-archive entry,
5087 since that part of the media is never supposed to be read. @GNUTAR{}
5088 does not write after the end of an archive, but seeks to
5089 maintain compatibility among archiving utilities.
5092 @item --ignore-zeros
5094 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
5095 encountered while reading an archive. Use in conjunction with
5096 @option{--extract} or @option{--list}.
5100 @subsection Changing How @command{tar} Writes Files
5103 @FIXME{Introductory paragraph}
5106 * Dealing with Old Files::
5107 * Overwrite Old Files::
5109 * Keep Newer Files::
5111 * Recursive Unlink::
5112 * Data Modification Times::
5113 * Setting Access Permissions::
5114 * Directory Modification Times and Permissions::
5115 * Writing to Standard Output::
5116 * Writing to an External Program::
5120 @node Dealing with Old Files
5121 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
5123 @xopindex{overwrite-dir, introduced}
5124 When extracting files, if @command{tar} discovers that the extracted
5125 file already exists, it normally replaces the file by removing it before
5126 extracting it, to prevent confusion in the presence of hard or symbolic
5127 links. (If the existing file is a symbolic link, it is removed, not
5128 followed.) However, if a directory cannot be removed because it is
5129 nonempty, @command{tar} normally overwrites its metadata (ownership,
5130 permission, etc.). The @option{--overwrite-dir} option enables this
5131 default behavior. To be more cautious and preserve the metadata of
5132 such a directory, use the @option{--no-overwrite-dir} option.
5134 @cindex Overwriting old files, prevention
5135 @xopindex{keep-old-files, introduced}
5136 To be even more cautious and prevent existing files from being replaced, use
5137 the @option{--keep-old-files} (@option{-k}) option. It causes @command{tar} to refuse
5138 to replace or update a file that already exists, i.e., a file with the
5139 same name as an archive member prevents extraction of that archive
5140 member. Instead, it reports an error.
5142 @xopindex{overwrite, introduced}
5143 To be more aggressive about altering existing files, use the
5144 @option{--overwrite} option. It causes @command{tar} to overwrite
5145 existing files and to follow existing symbolic links when extracting.
5147 @cindex Protecting old files
5148 Some people argue that @GNUTAR{} should not hesitate
5149 to overwrite files with other files when extracting. When extracting
5150 a @command{tar} archive, they expect to see a faithful copy of the
5151 state of the file system when the archive was created. It is debatable
5152 that this would always be a proper behavior. For example, suppose one
5153 has an archive in which @file{usr/local} is a link to
5154 @file{usr/local2}. Since then, maybe the site removed the link and
5155 renamed the whole hierarchy from @file{/usr/local2} to
5156 @file{/usr/local}. Such things happen all the time. I guess it would
5157 not be welcome at all that @GNUTAR{} removes the
5158 whole hierarchy just to make room for the link to be reinstated
5159 (unless it @emph{also} simultaneously restores the full
5160 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
5161 able to remove a whole hierarchy to reestablish a symbolic link, for
5162 example, but @emph{only if} @option{--recursive-unlink} is specified
5163 to allow this behavior. In any case, single files are silently
5166 @xopindex{unlink-first, introduced}
5167 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
5168 some cases by causing @command{tar} to remove files unconditionally
5169 before extracting them.
5171 @node Overwrite Old Files
5172 @unnumberedsubsubsec Overwrite Old Files
5177 Overwrite existing files and directory metadata when extracting files
5180 This causes @command{tar} to write extracted files into the file system without
5181 regard to the files already on the system; i.e., files with the same
5182 names as archive members are overwritten when the archive is extracted.
5183 It also causes @command{tar} to extract the ownership, permissions,
5184 and time stamps onto any preexisting files or directories.
5185 If the name of a corresponding file name is a symbolic link, the file
5186 pointed to by the symbolic link will be overwritten instead of the
5187 symbolic link itself (if this is possible). Moreover, special devices,
5188 empty directories and even symbolic links are automatically removed if
5189 they are in the way of extraction.
5191 Be careful when using the @option{--overwrite} option, particularly when
5192 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
5193 can change the contents, ownership or permissions of any file on your
5194 system. Also, many systems do not take kindly to overwriting files that
5195 are currently being executed.
5197 @opindex overwrite-dir
5198 @item --overwrite-dir
5199 Overwrite the metadata of directories when extracting files from an
5200 archive, but remove other files before extracting.
5203 @node Keep Old Files
5204 @unnumberedsubsubsec Keep Old Files
5207 @opindex keep-old-files
5208 @item --keep-old-files
5210 Do not replace existing files from archive. The
5211 @option{--keep-old-files} (@option{-k}) option prevents @command{tar}
5212 from replacing existing files with files with the same name from the
5213 archive. The @option{--keep-old-files} option is meaningless with
5214 @option{--list} (@option{-t}). Prevents @command{tar} from replacing
5215 files in the file system during extraction.
5218 @node Keep Newer Files
5219 @unnumberedsubsubsec Keep Newer Files
5222 @opindex keep-newer-files
5223 @item --keep-newer-files
5224 Do not replace existing files that are newer than their archive
5225 copies. This option is meaningless with @option{--list} (@option{-t}).
5229 @unnumberedsubsubsec Unlink First
5232 @opindex unlink-first
5233 @item --unlink-first
5235 Remove files before extracting over them.
5236 This can make @command{tar} run a bit faster if you know in advance
5237 that the extracted files all need to be removed. Normally this option
5238 slows @command{tar} down slightly, so it is disabled by default.
5241 @node Recursive Unlink
5242 @unnumberedsubsubsec Recursive Unlink
5245 @opindex recursive-unlink
5246 @item --recursive-unlink
5247 When this option is specified, try removing files and directory hierarchies
5248 before extracting over them. @emph{This is a dangerous option!}
5251 If you specify the @option{--recursive-unlink} option,
5252 @command{tar} removes @emph{anything} that keeps you from extracting a file
5253 as far as current permissions will allow it. This could include removal
5254 of the contents of a full directory hierarchy.
5256 @node Data Modification Times
5257 @unnumberedsubsubsec Setting Data Modification Times
5259 @cindex Data modification times of extracted files
5260 @cindex Modification times of extracted files
5261 Normally, @command{tar} sets the data modification times of extracted
5262 files to the corresponding times recorded for the files in the archive, but
5263 limits the permissions of extracted files by the current @code{umask}
5266 To set the data modification times of extracted files to the time when
5267 the files were extracted, use the @option{--touch} (@option{-m}) option in
5268 conjunction with @option{--extract} (@option{--get}, @option{-x}).
5274 Sets the data modification time of extracted archive members to the time
5275 they were extracted, not the time recorded for them in the archive.
5276 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
5279 @node Setting Access Permissions
5280 @unnumberedsubsubsec Setting Access Permissions
5282 @cindex Permissions of extracted files
5283 @cindex Modes of extracted files
5284 To set the modes (access permissions) of extracted files to those
5285 recorded for those files in the archive, use @option{--same-permissions}
5286 in conjunction with the @option{--extract} (@option{--get},
5287 @option{-x}) operation.
5290 @opindex preserve-permissions
5291 @opindex same-permissions
5292 @item --preserve-permissions
5293 @itemx --same-permissions
5294 @c @itemx --ignore-umask
5296 Set modes of extracted archive members to those recorded in the
5297 archive, instead of current umask settings. Use in conjunction with
5298 @option{--extract} (@option{--get}, @option{-x}).
5301 @node Directory Modification Times and Permissions
5302 @unnumberedsubsubsec Directory Modification Times and Permissions
5304 After successfully extracting a file member, @GNUTAR{} normally
5305 restores its permissions and modification times, as described in the
5306 previous sections. This cannot be done for directories, because
5307 after extracting a directory @command{tar} will almost certainly
5308 extract files into that directory and this will cause the directory
5309 modification time to be updated. Moreover, restoring that directory
5310 permissions may not permit file creation within it. Thus, restoring
5311 directory permissions and modification times must be delayed at least
5312 until all files have been extracted into that directory. @GNUTAR{}
5313 restores directories using the following approach.
5315 The extracted directories are created with the mode specified in the
5316 archive, as modified by the umask of the user, which gives sufficient
5317 permissions to allow file creation. The meta-information about the
5318 directory is recorded in the temporary list of directories. When
5319 preparing to extract next archive member, @GNUTAR{} checks if the
5320 directory prefix of this file contains the remembered directory. If
5321 it does not, the program assumes that all files have been extracted
5322 into that directory, restores its modification time and permissions
5323 and removes its entry from the internal list. This approach allows
5324 to correctly restore directory meta-information in the majority of
5325 cases, while keeping memory requirements sufficiently small. It is
5326 based on the fact, that most @command{tar} archives use the predefined
5327 order of members: first the directory, then all the files and
5328 subdirectories in that directory.
5330 However, this is not always true. The most important exception are
5331 incremental archives (@pxref{Incremental Dumps}). The member order in
5332 an incremental archive is reversed: first all directory members are
5333 stored, followed by other (non-directory) members. So, when extracting
5334 from incremental archives, @GNUTAR{} alters the above procedure. It
5335 remembers all restored directories, and restores their meta-data
5336 only after the entire archive has been processed. Notice, that you do
5337 not need to specify any special options for that, as @GNUTAR{}
5338 automatically detects archives in incremental format.
5340 There may be cases, when such processing is required for normal archives
5341 too. Consider the following example:
5345 $ @kbd{tar --no-recursion -cvf archive \
5346 foo foo/file1 bar bar/file foo/file2}
5355 During the normal operation, after encountering @file{bar}
5356 @GNUTAR{} will assume that all files from the directory @file{foo}
5357 were already extracted and will therefore restore its timestamp and
5358 permission bits. However, after extracting @file{foo/file2} the
5359 directory timestamp will be offset again.
5361 To correctly restore directory meta-information in such cases, use
5362 the @option{--delay-directory-restore} command line option:
5365 @opindex delay-directory-restore
5366 @item --delay-directory-restore
5367 Delays restoring of the modification times and permissions of extracted
5368 directories until the end of extraction. This way, correct
5369 meta-information is restored even if the archive has unusual member
5372 @opindex no-delay-directory-restore
5373 @item --no-delay-directory-restore
5374 Cancel the effect of the previous @option{--delay-directory-restore}.
5375 Use this option if you have used @option{--delay-directory-restore} in
5376 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
5377 temporarily disable it.
5380 @node Writing to Standard Output
5381 @unnumberedsubsubsec Writing to Standard Output
5383 @cindex Writing extracted files to standard output
5384 @cindex Standard output, writing extracted files to
5385 To write the extracted files to the standard output, instead of
5386 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
5387 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
5388 extracting files to send them through a pipe, and do not need to
5389 preserve them in the file system. If you extract multiple members,
5390 they appear on standard output concatenated, in the order they are
5391 found in the archive.
5397 Writes files to the standard output. Use only in conjunction with
5398 @option{--extract} (@option{--get}, @option{-x}). When this option is
5399 used, instead of creating the files specified, @command{tar} writes
5400 the contents of the files extracted to its standard output. This may
5401 be useful if you are only extracting the files in order to send them
5402 through a pipe. This option is meaningless with @option{--list}
5406 This can be useful, for example, if you have a tar archive containing
5407 a big file and don't want to store the file on disk before processing
5408 it. You can use a command like this:
5411 tar -xOzf foo.tgz bigfile | process
5414 or even like this if you want to process the concatenation of the files:
5417 tar -xOzf foo.tgz bigfile1 bigfile2 | process
5420 However, @option{--to-command} may be more convenient for use with
5421 multiple files. See the next section.
5423 @node Writing to an External Program
5424 @unnumberedsubsubsec Writing to an External Program
5426 You can instruct @command{tar} to send the contents of each extracted
5427 file to the standard input of an external program:
5431 @item --to-command=@var{command}
5432 Extract files and pipe their contents to the standard input of
5433 @var{command}. When this option is used, instead of creating the
5434 files specified, @command{tar} invokes @var{command} and pipes the
5435 contents of the files to its standard output. The @var{command} may
5436 contain command line arguments. The program is executed via
5437 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
5438 extracted. Non-regular files (directories, etc.) are ignored when this
5442 The command can obtain the information about the file it processes
5443 from the following environment variables:
5446 @vrindex TAR_FILETYPE, to-command environment
5448 Type of the file. It is a single letter with the following meaning:
5450 @multitable @columnfractions 0.10 0.90
5451 @item f @tab Regular file
5452 @item d @tab Directory
5453 @item l @tab Symbolic link
5454 @item h @tab Hard link
5455 @item b @tab Block device
5456 @item c @tab Character device
5459 Currently only regular files are supported.
5461 @vrindex TAR_MODE, to-command environment
5463 File mode, an octal number.
5465 @vrindex TAR_FILENAME, to-command environment
5467 The name of the file.
5469 @vrindex TAR_REALNAME, to-command environment
5471 Name of the file as stored in the archive.
5473 @vrindex TAR_UNAME, to-command environment
5475 Name of the file owner.
5477 @vrindex TAR_GNAME, to-command environment
5479 Name of the file owner group.
5481 @vrindex TAR_ATIME, to-command environment
5483 Time of last access. It is a decimal number, representing seconds
5484 since the Epoch. If the archive provides times with nanosecond
5485 precision, the nanoseconds are appended to the timestamp after a
5488 @vrindex TAR_MTIME, to-command environment
5490 Time of last modification.
5492 @vrindex TAR_CTIME, to-command environment
5494 Time of last status change.
5496 @vrindex TAR_SIZE, to-command environment
5500 @vrindex TAR_UID, to-command environment
5502 UID of the file owner.
5504 @vrindex TAR_GID, to-command environment
5506 GID of the file owner.
5509 Additionally, the following variables contain information about
5510 tar mode and the archive being processed:
5513 @vrindex TAR_VERSION, to-command environment
5515 @GNUTAR{} version number.
5517 @vrindex TAR_ARCHIVE, to-command environment
5519 The name of the archive @command{tar} is processing.
5521 @vrindex TAR_BLOCKING_FACTOR, to-command environment
5522 @item TAR_BLOCKING_FACTOR
5523 Current blocking factor (@pxref{Blocking}).
5525 @vrindex TAR_VOLUME, to-command environment
5527 Ordinal number of the volume @command{tar} is processing.
5529 @vrindex TAR_FORMAT, to-command environment
5531 Format of the archive being processed. @xref{Formats}, for a complete
5532 list of archive format names.
5535 If @var{command} exits with a non-0 status, @command{tar} will print
5536 an error message similar to the following:
5539 tar: 2345: Child returned status 1
5542 Here, @samp{2345} is the PID of the finished process.
5544 If this behavior is not wanted, use @option{--ignore-command-error}:
5547 @opindex ignore-command-error
5548 @item --ignore-command-error
5549 Ignore exit codes of subprocesses. Notice that if the program
5550 exits on signal or otherwise terminates abnormally, the error message
5551 will be printed even if this option is used.
5553 @opindex no-ignore-command-error
5554 @item --no-ignore-command-error
5555 Cancel the effect of any previous @option{--ignore-command-error}
5556 option. This option is useful if you have set
5557 @option{--ignore-command-error} in @env{TAR_OPTIONS}
5558 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
5562 @unnumberedsubsubsec Removing Files
5564 @FIXME{The section is too terse. Something more to add? An example,
5568 @opindex remove-files
5569 @item --remove-files
5570 Remove files after adding them to the archive.
5574 @subsection Coping with Scarce Resources
5577 @cindex Small memory
5578 @cindex Running out of space
5586 @unnumberedsubsubsec Starting File
5589 @opindex starting-file
5590 @item --starting-file=@var{name}
5591 @itemx -K @var{name}
5592 Starts an operation in the middle of an archive. Use in conjunction
5593 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
5596 @cindex Middle of the archive, starting in the
5597 If a previous attempt to extract files failed due to lack of disk
5598 space, you can use @option{--starting-file=@var{name}} (@option{-K
5599 @var{name}}) to start extracting only after member @var{name} of the
5600 archive. This assumes, of course, that there is now free space, or
5601 that you are now extracting into a different file system. (You could
5602 also choose to suspend @command{tar}, remove unnecessary files from
5603 the file system, and then resume the same @command{tar} operation.
5604 In this case, @option{--starting-file} is not necessary.) See also
5605 @ref{interactive}, and @ref{exclude}.
5608 @unnumberedsubsubsec Same Order
5611 @cindex Large lists of file names on small machines
5613 @opindex preserve-order
5615 @itemx --preserve-order
5617 To process large lists of file names on machines with small amounts of
5618 memory. Use in conjunction with @option{--compare} (@option{--diff},
5619 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
5620 (@option{--get}, @option{-x}).
5623 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
5624 names to be listed or extracted is sorted in the same order as the
5625 files in the archive. This allows a large list of names to be used,
5626 even on a small machine that would not otherwise be able to hold all
5627 the names in memory at the same time. Such a sorted list can easily be
5628 created by running @samp{tar -t} on the archive and editing its output.
5630 This option is probably never needed on modern computer systems.
5633 @section Backup options
5635 @cindex backup options
5637 @GNUTAR{} offers options for making backups of files
5638 before writing new versions. These options control the details of
5639 these backups. They may apply to the archive itself before it is
5640 created or rewritten, as well as individual extracted members. Other
5641 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
5642 and @command{mv}, for example) offer similar options.
5644 Backup options may prove unexpectedly useful when extracting archives
5645 containing many members having identical name, or when extracting archives
5646 on systems having file name limitations, making different members appear
5647 as having similar names through the side-effect of name truncation.
5648 @FIXME{This is true only if we have a good scheme for truncated backup names,
5649 which I'm not sure at all: I suspect work is needed in this area.}
5650 When any existing file is backed up before being overwritten by extraction,
5651 then clashing files are automatically be renamed to be unique, and the
5652 true name is kept for only the last file of a series of clashing files.
5653 By using verbose mode, users may track exactly what happens.
5655 At the detail level, some decisions are still experimental, and may
5656 change in the future, we are waiting comments from our users. So, please
5657 do not learn to depend blindly on the details of the backup features.
5658 For example, currently, directories themselves are never renamed through
5659 using these options, so, extracting a file over a directory still has
5660 good chances to fail. Also, backup options apply to created archives,
5661 not only to extracted members. For created archives, backups will not
5662 be attempted when the archive is a block or character device, or when it
5663 refers to a remote file.
5665 For the sake of simplicity and efficiency, backups are made by renaming old
5666 files prior to creation or extraction, and not by copying. The original
5667 name is restored if the file creation fails. If a failure occurs after a
5668 partial extraction of a file, both the backup and the partially extracted
5672 @item --backup[=@var{method}]
5674 @vindex VERSION_CONTROL
5676 Back up files that are about to be overwritten or removed.
5677 Without this option, the original versions are destroyed.
5679 Use @var{method} to determine the type of backups made.
5680 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
5681 environment variable. And if @env{VERSION_CONTROL} is not set,
5682 use the @samp{existing} method.
5684 @vindex version-control @r{Emacs variable}
5685 This option corresponds to the Emacs variable @samp{version-control};
5686 the same values for @var{method} are accepted as in Emacs. This option
5687 also allows more descriptive names. The valid @var{method}s are:
5692 @cindex numbered @r{backup method}
5693 Always make numbered backups.
5697 @cindex existing @r{backup method}
5698 Make numbered backups of files that already have them, simple backups
5703 @cindex simple @r{backup method}
5704 Always make simple backups.
5708 @item --suffix=@var{suffix}
5710 @cindex backup suffix
5711 @vindex SIMPLE_BACKUP_SUFFIX
5712 Append @var{suffix} to each backup file made with @option{--backup}. If this
5713 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5714 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5715 set, the default is @samp{~}, just as in Emacs.
5720 @section Notable @command{tar} Usages
5723 @FIXME{Using Unix file linking capability to recreate directory
5724 structures---linking files into one subdirectory and then
5725 @command{tar}ring that directory.}
5727 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5730 You can easily use archive files to transport a group of files from
5731 one system to another: put all relevant files into an archive on one
5732 computer system, transfer the archive to another system, and extract
5733 the contents there. The basic transfer medium might be magnetic tape,
5734 Internet FTP, or even electronic mail (though you must encode the
5735 archive with @command{uuencode} in order to transport it properly by
5736 mail). Both machines do not have to use the same operating system, as
5737 long as they both support the @command{tar} program.
5739 For example, here is how you might copy a directory's contents from
5740 one disk to another, while preserving the dates, modes, owners and
5741 link-structure of all the files therein. In this case, the transfer
5742 medium is a @dfn{pipe}:
5745 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5749 You can avoid subshells by using @option{-C} option:
5752 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5756 The command also works using long option forms:
5760 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5761 | (cd targetdir; tar --extract --file=-)}
5770 $ @kbd{tar --directory sourcedir --create --file=- . \
5771 | tar --directory targetdir --extract --file=-}
5776 This is one of the easiest methods to transfer a @command{tar} archive.
5779 @section Looking Ahead: The Rest of this Manual
5781 You have now seen how to use all eight of the operations available to
5782 @command{tar}, and a number of the possible options. The next chapter
5783 explains how to choose and change file and archive names, how to use
5784 files to store names of other files which you can then call as
5785 arguments to @command{tar} (this can help you save time if you expect to
5786 archive the same list of files a number of times), and so forth.
5787 @FIXME{in case it's not obvious, i'm making this up in some sense
5788 based on my limited memory of what the next chapter *really* does. i
5789 just wanted to flesh out this final section a little bit so i'd
5790 remember to stick it in here. :-)}
5792 If there are too many files to conveniently list on the command line,
5793 you can list the names in a file, and @command{tar} will read that file.
5796 There are various ways of causing @command{tar} to skip over some files,
5797 and not archive them. @xref{Choosing}.
5800 @chapter Performing Backups and Restoring Files
5803 @GNUTAR{} is distributed along with the scripts for performing backups
5804 and restores. Even if there is a good chance those scripts may be
5805 satisfying to you, they are not the only scripts or methods available for doing
5806 backups and restore. You may well create your own, or use more
5807 sophisticated packages dedicated to that purpose.
5809 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5810 Automatic Network Disk Archiver), a backup system developed by James
5811 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5812 This is free software, and it is available from @uref{http://www.amanda.org}.
5816 Here is a possible plan for a future documentation about the backuping
5817 scripts which are provided within the @GNUTAR{}
5823 @item what are dumps
5824 @item different levels of dumps
5826 @item full dump = dump everything
5827 @item level 1, level 2 dumps etc
5828 A level @var{n} dump dumps everything changed since the last level
5831 @item how to use scripts for dumps (ie, the concept)
5833 @item scripts to run after editing backup specs (details)
5835 @item Backup Specs, what is it.
5837 @item how to customize
5838 @item actual text of script [/sp/dump/backup-specs]
5842 @item rsh doesn't work
5843 @item rtape isn't installed
5846 @item the @option{--incremental} option of tar
5849 @item write protection
5850 @item types of media, different sizes and types, useful for different things
5851 @item files and tape marks
5852 one tape mark between files, two at end.
5853 @item positioning the tape
5854 MT writes two at end of write,
5855 backspaces over one when writing again.
5861 This chapter documents both the provided shell scripts and @command{tar}
5862 options which are more specific to usage as a backup tool.
5864 To @dfn{back up} a file system means to create archives that contain
5865 all the files in that file system. Those archives can then be used to
5866 restore any or all of those files (for instance if a disk crashes or a
5867 file is accidentally deleted). File system @dfn{backups} are also
5871 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5872 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5873 * Backup Levels:: Levels of Backups
5874 * Backup Parameters:: Setting Parameters for Backups and Restoration
5875 * Scripted Backups:: Using the Backup Scripts
5876 * Scripted Restoration:: Using the Restore Script
5880 @section Using @command{tar} to Perform Full Dumps
5886 @cindex corrupted archives
5887 Full dumps should only be made when no other people or programs
5888 are modifying files in the file system. If files are modified while
5889 @command{tar} is making the backup, they may not be stored properly in
5890 the archive, in which case you won't be able to restore them if you
5891 have to. (Files not being modified are written with no trouble, and do
5892 not corrupt the entire archive.)
5894 You will want to use the @option{--label=@var{archive-label}}
5895 (@option{-V @var{archive-label}}) option to give the archive a
5896 volume label, so you can tell what this archive is even if the label
5897 falls off the tape, or anything like that.
5899 Unless the file system you are dumping is guaranteed to fit on
5900 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5901 Make sure you have enough tapes on hand to complete the backup.
5903 If you want to dump each file system separately you will need to use
5904 the @option{--one-file-system} option to prevent
5905 @command{tar} from crossing file system boundaries when storing
5908 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5909 option is not needed, since this is a complete copy of everything in
5910 the file system, and a full restore from this backup would only be
5911 done onto a completely
5914 Unless you are in a hurry, and trust the @command{tar} program (and your
5915 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5916 option, to make sure your files really made it onto the dump properly.
5917 This will also detect cases where the file was modified while (or just
5918 after) it was being archived. Not all media (notably cartridge tapes)
5919 are capable of being verified, unfortunately.
5921 @node Incremental Dumps
5922 @section Using @command{tar} to Perform Incremental Dumps
5924 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5925 stores additional metadata so that exact state of the file system
5926 can be restored when extracting the archive.
5928 @GNUTAR{} currently offers two options for handling incremental
5929 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5930 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5932 @xopindex{listed-incremental, described}
5933 The option @option{--listed-incremental} instructs tar to operate on
5934 an incremental archive with additional metadata stored in a standalone
5935 file, called a @dfn{snapshot file}. The purpose of this file is to help
5936 determine which files have been changed, added or deleted since the
5937 last backup, so that the next incremental backup will contain only
5938 modified files. The name of the snapshot file is given as an argument
5942 @item --listed-incremental=@var{file}
5943 @itemx -g @var{file}
5944 Handle incremental backups with snapshot data in @var{file}.
5947 To create an incremental backup, you would use
5948 @option{--listed-incremental} together with @option{--create}
5949 (@pxref{create}). For example:
5952 $ @kbd{tar --create \
5953 --file=archive.1.tar \
5954 --listed-incremental=/var/log/usr.snar \
5958 This will create in @file{archive.1.tar} an incremental backup of
5959 the @file{/usr} file system, storing additional metadata in the file
5960 @file{/var/log/usr.snar}. If this file does not exist, it will be
5961 created. The created archive will then be a @dfn{level 0 backup};
5962 please see the next section for more on backup levels.
5964 Otherwise, if the file @file{/var/log/usr.snar} exists, it
5965 determines which files are modified. In this case only these files will be
5966 stored in the archive. Suppose, for example, that after running the
5967 above command, you delete file @file{/usr/doc/old} and create
5968 directory @file{/usr/local/db} with the following contents:
5971 $ @kbd{ls /usr/local/db}
5976 Some time later you create another incremental backup. You will
5980 $ @kbd{tar --create \
5981 --file=archive.2.tar \
5982 --listed-incremental=/var/log/usr.snar \
5984 tar: usr/local/db: Directory is new
5991 The created archive @file{archive.2.tar} will contain only these
5992 three members. This archive is called a @dfn{level 1 backup}. Notice
5993 that @file{/var/log/usr.snar} will be updated with the new data, so if
5994 you plan to create more @samp{level 1} backups, it is necessary to
5995 create a working copy of the snapshot file before running
5996 @command{tar}. The above example will then be modified as follows:
5999 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
6000 $ @kbd{tar --create \
6001 --file=archive.2.tar \
6002 --listed-incremental=/var/log/usr.snar-1 \
6007 @xopindex{level, described}
6008 You can force @samp{level 0} backups either by removing the snapshot
6009 file before running @command{tar}, or by supplying the
6010 @option{--level=0} option, e.g.:
6013 $ @kbd{tar --create \
6014 --file=archive.2.tar \
6015 --listed-incremental=/var/log/usr.snar-0 \
6020 Incremental dumps depend crucially on time stamps, so the results are
6021 unreliable if you modify a file's time stamps during dumping (e.g.,
6022 with the @option{--atime-preserve=replace} option), or if you set the clock
6025 @anchor{device numbers}
6026 @cindex Device numbers, using in incremental backups
6027 Metadata stored in snapshot files include device numbers, which,
6028 obviously are supposed to be non-volatile values. However, it turns
6029 out that @acronym{NFS} devices have undependable values when an automounter
6030 gets in the picture. This can lead to a great deal of spurious
6031 redumping in incremental dumps, so it is somewhat useless to compare
6032 two @acronym{NFS} devices numbers over time. The solution implemented
6033 currently is to consider all @acronym{NFS} devices as being equal
6034 when it comes to comparing directories; this is fairly gross, but
6035 there does not seem to be a better way to go.
6037 Apart from using @acronym{NFS}, there are a number of cases where
6038 relying on device numbers can cause spurious redumping of unmodified
6039 files. For example, this occurs when archiving @acronym{LVM} snapshot
6040 volumes. To avoid this, use @option{--no-check-device} option:
6043 @xopindex{no-check-device, described}
6044 @item --no-check-device
6045 Do not rely on device numbers when preparing a list of changed files
6046 for an incremental dump.
6048 @xopindex{check-device, described}
6049 @item --check-device
6050 Use device numbers when preparing a list of changed files
6051 for an incremental dump. This is the default behavior. The purpose
6052 of this option is to undo the effect of the @option{--no-check-device}
6053 if it was given in @env{TAR_OPTIONS} environment variable
6054 (@pxref{TAR_OPTIONS}).
6057 There is also another way to cope with changing device numbers. It is
6058 described in detail in @ref{Fixing Snapshot Files}.
6060 Note that incremental archives use @command{tar} extensions and may
6061 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
6063 @xopindex{listed-incremental, using with @option{--extract}}
6064 @xopindex{extract, using with @option{--listed-incremental}}
6065 To extract from the incremental dumps, use
6066 @option{--listed-incremental} together with @option{--extract}
6067 option (@pxref{extracting files}). In this case, @command{tar} does
6068 not need to access snapshot file, since all the data necessary for
6069 extraction are stored in the archive itself. So, when extracting, you
6070 can give whatever argument to @option{--listed-incremental}, the usual
6071 practice is to use @option{--listed-incremental=/dev/null}.
6072 Alternatively, you can use @option{--incremental}, which needs no
6073 arguments. In general, @option{--incremental} (@option{-G}) can be
6074 used as a shortcut for @option{--listed-incremental} when listing or
6075 extracting incremental backups (for more information regarding this
6076 option, @pxref{incremental-op}).
6078 When extracting from the incremental backup @GNUTAR{} attempts to
6079 restore the exact state the file system had when the archive was
6080 created. In particular, it will @emph{delete} those files in the file
6081 system that did not exist in their directories when the archive was
6082 created. If you have created several levels of incremental files,
6083 then in order to restore the exact contents the file system had when
6084 the last level was created, you will need to restore from all backups
6085 in turn. Continuing our example, to restore the state of @file{/usr}
6086 file system, one would do@footnote{Notice, that since both archives
6087 were created without @option{-P} option (@pxref{absolute}), these
6088 commands should be run from the root file system.}:
6091 $ @kbd{tar --extract \
6092 --listed-incremental=/dev/null \
6093 --file archive.1.tar}
6094 $ @kbd{tar --extract \
6095 --listed-incremental=/dev/null \
6096 --file archive.2.tar}
6099 To list the contents of an incremental archive, use @option{--list}
6100 (@pxref{list}), as usual. To obtain more information about the
6101 archive, use @option{--listed-incremental} or @option{--incremental}
6102 combined with two @option{--verbose} options@footnote{Two
6103 @option{--verbose} options were selected to avoid breaking usual
6104 verbose listing output (@option{--list --verbose}) when using in
6107 @xopindex{incremental, using with @option{--list}}
6108 @xopindex{listed-incremental, using with @option{--list}}
6109 @xopindex{list, using with @option{--incremental}}
6110 @xopindex{list, using with @option{--listed-incremental}}
6111 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
6112 contents of the DUMPDIR header (with terminating nulls) when
6113 @option{--incremental} or @option{--listed-incremental} option was
6114 given, no matter what the verbosity level. This behavior, and,
6115 especially, the binary output it produced were considered inconvenient
6116 and were changed in version 1.16.}:
6119 @kbd{tar --list --incremental --verbose --verbose archive.tar}
6122 This command will print, for each directory in the archive, the list
6123 of files in that directory at the time the archive was created. This
6124 information is put out in a format which is both human-readable and
6125 unambiguous for a program: each file name is printed as
6132 where @var{x} is a letter describing the status of the file: @samp{Y}
6133 if the file is present in the archive, @samp{N} if the file is not
6134 included in the archive, or a @samp{D} if the file is a directory (and
6135 is included in the archive). @xref{Dumpdir}, for the detailed
6136 description of dumpdirs and status codes. Each such
6137 line is terminated by a newline character. The last line is followed
6138 by an additional newline to indicate the end of the data.
6140 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
6141 gives the same behavior as @option{--listed-incremental} when used
6142 with @option{--list} and @option{--extract} options. When used with
6143 @option{--create} option, it creates an incremental archive without
6144 creating snapshot file. Thus, it is impossible to create several
6145 levels of incremental backups with @option{--incremental} option.
6148 @section Levels of Backups
6150 An archive containing all the files in the file system is called a
6151 @dfn{full backup} or @dfn{full dump}. You could insure your data by
6152 creating a full dump every day. This strategy, however, would waste a
6153 substantial amount of archive media and user time, as unchanged files
6154 are daily re-archived.
6156 It is more efficient to do a full dump only occasionally. To back up
6157 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
6158 one} dump archives all the files that have changed since the last full
6161 A typical dump strategy would be to perform a full dump once a week,
6162 and a level one dump once a day. This means some versions of files
6163 will in fact be archived more than once, but this dump strategy makes
6164 it possible to restore a file system to within one day of accuracy by
6165 only extracting two archives---the last weekly (full) dump and the
6166 last daily (level one) dump. The only information lost would be in
6167 files changed or created since the last daily backup. (Doing dumps
6168 more than once a day is usually not worth the trouble.)
6170 @GNUTAR{} comes with scripts you can use to do full
6171 and level-one (actually, even level-two and so on) dumps. Using
6172 scripts (shell programs) to perform backups and restoration is a
6173 convenient and reliable alternative to typing out file name lists
6174 and @command{tar} commands by hand.
6176 Before you use these scripts, you need to edit the file
6177 @file{backup-specs}, which specifies parameters used by the backup
6178 scripts and by the restore script. This file is usually located
6179 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
6180 detailed description. Once the backup parameters are set, you can
6181 perform backups or restoration by running the appropriate script.
6183 The name of the backup script is @code{backup}. The name of the
6184 restore script is @code{restore}. The following sections describe
6185 their use in detail.
6187 @emph{Please Note:} The backup and restoration scripts are
6188 designed to be used together. While it is possible to restore files by
6189 hand from an archive which was created using a backup script, and to create
6190 an archive by hand which could then be extracted using the restore script,
6191 it is easier to use the scripts. @xref{Incremental Dumps}, before
6192 making such an attempt.
6194 @node Backup Parameters
6195 @section Setting Parameters for Backups and Restoration
6197 The file @file{backup-specs} specifies backup parameters for the
6198 backup and restoration scripts provided with @command{tar}. You must
6199 edit @file{backup-specs} to fit your system configuration and schedule
6200 before using these scripts.
6202 Syntactically, @file{backup-specs} is a shell script, containing
6203 mainly variable assignments. However, any valid shell construct
6204 is allowed in this file. Particularly, you may wish to define
6205 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
6206 For more information about shell script syntax, please refer to
6207 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
6208 g_02, the definition of the Shell Command Language}. See also
6209 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
6211 The shell variables controlling behavior of @code{backup} and
6212 @code{restore} are described in the following subsections.
6215 * General-Purpose Variables::
6216 * Magnetic Tape Control::
6218 * backup-specs example:: An Example Text of @file{Backup-specs}
6221 @node General-Purpose Variables
6222 @subsection General-Purpose Variables
6224 @defvr {Backup variable} ADMINISTRATOR
6225 The user name of the backup administrator. @code{Backup} scripts
6226 sends a backup report to this address.
6229 @defvr {Backup variable} BACKUP_HOUR
6230 The hour at which the backups are done. This can be a number from 0
6231 to 23, or the time specification in form @var{hours}:@var{minutes},
6232 or the string @samp{now}.
6234 This variable is used by @code{backup}. Its value may be overridden
6235 using @option{--time} option (@pxref{Scripted Backups}).
6238 @defvr {Backup variable} TAPE_FILE
6240 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
6241 is a remote archive (@pxref{remote-dev}), backup script will suppose
6242 that your @command{mt} is able to access remote devices. If @var{RSH}
6243 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
6244 invocations of @command{mt}.
6247 @defvr {Backup variable} BLOCKING
6249 The blocking factor @command{tar} will use when writing the dump archive.
6250 @xref{Blocking Factor}.
6253 @defvr {Backup variable} BACKUP_DIRS
6255 A list of file systems to be dumped (for @code{backup}), or restored
6256 (for @code{restore}). You can include any directory
6257 name in the list --- subdirectories on that file system will be
6258 included, regardless of how they may look to other networked machines.
6259 Subdirectories on other file systems will be ignored.
6261 The host name specifies which host to run @command{tar} on, and should
6262 normally be the host that actually contains the file system. However,
6263 the host machine must have @GNUTAR{} installed, and
6264 must be able to access the directory containing the backup scripts and
6265 their support files using the same file name that is used on the
6266 machine where the scripts are run (i.e., what @command{pwd} will print
6267 when in that directory on that machine). If the host that contains
6268 the file system does not have this capability, you can specify another
6269 host as long as it can access the file system through @acronym{NFS}.
6271 If the list of file systems is very long you may wish to put it
6272 in a separate file. This file is usually named
6273 @file{/etc/backup/dirs}, but this name may be overridden in
6274 @file{backup-specs} using @code{DIRLIST} variable.
6277 @defvr {Backup variable} DIRLIST
6279 The name of the file that contains a list of file systems to backup
6280 or restore. By default it is @file{/etc/backup/dirs}.
6283 @defvr {Backup variable} BACKUP_FILES
6285 A list of individual files to be dumped (for @code{backup}), or restored
6286 (for @code{restore}). These should be accessible from the machine on
6287 which the backup script is run.
6289 If the list of individual files is very long you may wish to store it
6290 in a separate file. This file is usually named
6291 @file{/etc/backup/files}, but this name may be overridden in
6292 @file{backup-specs} using @code{FILELIST} variable.
6295 @defvr {Backup variable} FILELIST
6297 The name of the file that contains a list of individual files to backup
6298 or restore. By default it is @file{/etc/backup/files}.
6301 @defvr {Backup variable} MT
6303 Full file name of @command{mt} binary.
6306 @defvr {Backup variable} RSH
6308 Full file name of @command{rsh} binary or its equivalent. You may wish to
6309 set it to @code{ssh}, to improve security. In this case you will have
6310 to use public key authentication.
6313 @defvr {Backup variable} RSH_COMMAND
6315 Full file name of @command{rsh} binary on remote machines. This will
6316 be passed via @option{--rsh-command} option to the remote invocation
6320 @defvr {Backup variable} VOLNO_FILE
6322 Name of temporary file to hold volume numbers. This needs to be accessible
6323 by all the machines which have file systems to be dumped.
6326 @defvr {Backup variable} XLIST
6328 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
6329 located on the remote machine and containing the list of files to
6330 be excluded from the backup. Exclude file lists are searched in
6331 /etc/tar-backup directory. A common use for exclude file lists
6332 is to exclude files containing security-sensitive information
6333 (e.g., @file{/etc/shadow} from backups).
6335 This variable affects only @code{backup}.
6338 @defvr {Backup variable} SLEEP_TIME
6340 Time to sleep between dumps of any two successive file systems
6342 This variable affects only @code{backup}.
6345 @defvr {Backup variable} DUMP_REMIND_SCRIPT
6347 Script to be run when it's time to insert a new tape in for the next
6348 volume. Administrators may want to tailor this script for their site.
6349 If this variable isn't set, @GNUTAR{} will display its built-in
6350 prompt, and will expect confirmation from the console. For the
6351 description of the default prompt, see @ref{change volume prompt}.
6355 @defvr {Backup variable} SLEEP_MESSAGE
6357 Message to display on the terminal while waiting for dump time. Usually
6358 this will just be some literal text.
6361 @defvr {Backup variable} TAR
6363 Full file name of the @GNUTAR{} executable. If this is not set, backup
6364 scripts will search @command{tar} in the current shell path.
6367 @node Magnetic Tape Control
6368 @subsection Magnetic Tape Control
6370 Backup scripts access tape device using special @dfn{hook functions}.
6371 These functions take a single argument --- the name of the tape
6372 device. Their names are kept in the following variables:
6374 @defvr {Backup variable} MT_BEGIN
6375 The name of @dfn{begin} function. This function is called before
6376 accessing the drive. By default it retensions the tape:
6382 mt -f "$1" retension
6387 @defvr {Backup variable} MT_REWIND
6388 The name of @dfn{rewind} function. The default definition is as
6401 @defvr {Backup variable} MT_OFFLINE
6402 The name of the function switching the tape off line. By default
6403 it is defined as follows:
6406 MT_OFFLINE=mt_offline
6414 @defvr {Backup variable} MT_STATUS
6415 The name of the function used to obtain the status of the archive device,
6416 including error count. Default definition:
6428 @subsection User Hooks
6430 @dfn{User hooks} are shell functions executed before and after
6431 each @command{tar} invocation. Thus, there are @dfn{backup
6432 hooks}, which are executed before and after dumping each file
6433 system, and @dfn{restore hooks}, executed before and
6434 after restoring a file system. Each user hook is a shell function
6435 taking four arguments:
6437 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
6442 Current backup or restore level.
6445 Name or IP address of the host machine being dumped or restored.
6448 Full file name of the file system being dumped or restored.
6451 File system name with directory separators replaced with colons. This
6452 is useful, e.g., for creating unique files.
6456 Following variables keep the names of user hook functions:
6458 @defvr {Backup variable} DUMP_BEGIN
6459 Dump begin function. It is executed before dumping the file system.
6462 @defvr {Backup variable} DUMP_END
6463 Executed after dumping the file system.
6466 @defvr {Backup variable} RESTORE_BEGIN
6467 Executed before restoring the file system.
6470 @defvr {Backup variable} RESTORE_END
6471 Executed after restoring the file system.
6474 @node backup-specs example
6475 @subsection An Example Text of @file{Backup-specs}
6477 The following is an example of @file{backup-specs}:
6480 # site-specific parameters for file system backup.
6482 ADMINISTRATOR=friedman
6484 TAPE_FILE=/dev/nrsmt0
6486 # Use @code{ssh} instead of the less secure @code{rsh}
6488 RSH_COMMAND=/usr/bin/ssh
6490 # Override MT_STATUS function:
6496 # Disable MT_OFFLINE function
6513 apple-gunkies:/com/mailer/gnu
6514 apple-gunkies:/com/archive/gnu"
6516 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
6520 @node Scripted Backups
6521 @section Using the Backup Scripts
6523 The syntax for running a backup script is:
6526 backup --level=@var{level} --time=@var{time}
6529 The @option{--level} option requests the dump level. Thus, to produce
6530 a full dump, specify @code{--level=0} (this is the default, so
6531 @option{--level} may be omitted if its value is
6532 @code{0})@footnote{For backward compatibility, the @code{backup} will also
6533 try to deduce the requested dump level from the name of the
6534 script itself. If the name consists of a string @samp{level-}
6535 followed by a single decimal digit, that digit is taken as
6536 the dump level number. Thus, you may create a link from @code{backup}
6537 to @code{level-1} and then run @code{level-1} whenever you need to
6538 create a level one dump.}.
6540 The @option{--time} option determines when should the backup be
6541 run. @var{Time} may take three forms:
6544 @item @var{hh}:@var{mm}
6546 The dump must be run at @var{hh} hours @var{mm} minutes.
6550 The dump must be run at @var{hh} hours.
6554 The dump must be run immediately.
6557 You should start a script with a tape or disk mounted. Once you
6558 start a script, it prompts you for new tapes or disks as it
6559 needs them. Media volumes don't have to correspond to archive
6560 files --- a multi-volume archive can be started in the middle of a
6561 tape that already contains the end of another multi-volume archive.
6562 The @code{restore} script prompts for media by its archive volume,
6563 so to avoid an error message you should keep track of which tape
6564 (or disk) contains which volume of the archive (@pxref{Scripted
6567 The backup scripts write two files on the file system. The first is a
6568 record file in @file{/etc/tar-backup/}, which is used by the scripts
6569 to store and retrieve information about which files were dumped. This
6570 file is not meant to be read by humans, and should not be deleted by
6571 them. @xref{Snapshot Files}, for a more detailed explanation of this
6574 The second file is a log file containing the names of the file systems
6575 and files dumped, what time the backup was made, and any error
6576 messages that were generated, as well as how much space was left in
6577 the media volume after the last volume of the archive was written.
6578 You should check this log file after every backup. The file name is
6579 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
6580 represents current date, and @var{n} represents current dump level number.
6582 The script also prints the name of each system being dumped to the
6585 Following is the full list of options accepted by @code{backup}
6589 @item -l @var{level}
6590 @itemx --level=@var{level}
6591 Do backup level @var{level} (default 0).
6595 Force backup even if today's log file already exists.
6597 @item -v[@var{level}]
6598 @itemx --verbose[=@var{level}]
6599 Set verbosity level. The higher the level is, the more debugging
6600 information will be output during execution. Default @var{level}
6601 is 100, which means the highest debugging level.
6603 @item -t @var{start-time}
6604 @itemx --time=@var{start-time}
6605 Wait till @var{time}, then do backup.
6609 Display short help message and exit.
6613 Display information about the program's name, version, origin and legal
6614 status, all on standard output, and then exit successfully.
6618 @node Scripted Restoration
6619 @section Using the Restore Script
6621 To restore files that were archived using a scripted backup, use the
6622 @code{restore} script. Its usage is quite straightforward. In the
6623 simplest form, invoke @code{restore --all}, it will
6624 then restore all the file systems and files specified in
6625 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
6627 You may select the file systems (and/or files) to restore by
6628 giving @code{restore} a list of @dfn{patterns} in its command
6629 line. For example, running
6636 will restore all file systems on the machine @samp{albert}. A more
6637 complicated example:
6640 restore 'albert:*' '*:/var'
6644 This command will restore all file systems on the machine @samp{albert}
6645 as well as @file{/var} file system on all machines.
6647 By default @code{restore} will start restoring files from the lowest
6648 available dump level (usually zero) and will continue through
6649 all available dump levels. There may be situations where such a
6650 thorough restore is not necessary. For example, you may wish to
6651 restore only files from the recent level one backup. To do so,
6652 use @option{--level} option, as shown in the example below:
6658 The full list of options accepted by @code{restore} follows:
6663 Restore all file systems and files specified in @file{backup-specs}.
6665 @item -l @var{level}
6666 @itemx --level=@var{level}
6667 Start restoring from the given backup level, instead of the default 0.
6669 @item -v[@var{level}]
6670 @itemx --verbose[=@var{level}]
6671 Set verbosity level. The higher the level is, the more debugging
6672 information will be output during execution. Default @var{level}
6673 is 100, which means the highest debugging level.
6677 Display short help message and exit.
6681 Display information about the program's name, version, origin and legal
6682 status, all on standard output, and then exit successfully.
6685 You should start the restore script with the media containing the
6686 first volume of the archive mounted. The script will prompt for other
6687 volumes as they are needed. If the archive is on tape, you don't need
6688 to rewind the tape to to its beginning---if the tape head is
6689 positioned past the beginning of the archive, the script will rewind
6690 the tape as needed. @xref{Tape Positioning}, for a discussion of tape
6694 @strong{Warning:} The script will delete files from the active file
6695 system if they were not in the file system when the archive was made.
6698 @xref{Incremental Dumps}, for an explanation of how the script makes
6702 @chapter Choosing Files and Names for @command{tar}
6704 Certain options to @command{tar} enable you to specify a name for your
6705 archive. Other options let you decide which files to include or exclude
6706 from the archive, based on when or whether files were modified, whether
6707 the file names do or don't match specified patterns, or whether files
6708 are in specified directories.
6710 This chapter discusses these options in detail.
6713 * file:: Choosing the Archive's Name
6714 * Selecting Archive Members::
6715 * files:: Reading Names from a File
6716 * exclude:: Excluding Some Files
6717 * wildcards:: Wildcards Patterns and Matching
6718 * quoting styles:: Ways of Quoting Special Characters in Names
6719 * transform:: Modifying File and Member Names
6720 * after:: Operating Only on New Files
6721 * recurse:: Descending into Directories
6722 * one:: Crossing File System Boundaries
6726 @section Choosing and Naming Archive Files
6728 @cindex Naming an archive
6729 @cindex Archive Name
6730 @cindex Choosing an archive file
6731 @cindex Where is the archive?
6733 By default, @command{tar} uses an archive file name that was compiled when
6734 it was built on the system; usually this name refers to some physical
6735 tape drive on the machine. However, the person who installed @command{tar}
6736 on the system may not have set the default to a meaningful value as far as
6737 most users are concerned. As a result, you will usually want to tell
6738 @command{tar} where to find (or create) the archive. The
6739 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
6740 option allows you to either specify or name a file to use as the archive
6741 instead of the default archive file location.
6744 @xopindex{file, short description}
6745 @item --file=@var{archive-name}
6746 @itemx -f @var{archive-name}
6747 Name the archive to create or operate on. Use in conjunction with
6751 For example, in this @command{tar} command,
6754 $ @kbd{tar -cvf collection.tar blues folk jazz}
6758 @file{collection.tar} is the name of the archive. It must directly
6759 follow the @option{-f} option, since whatever directly follows @option{-f}
6760 @emph{will} end up naming the archive. If you neglect to specify an
6761 archive name, you may end up overwriting a file in the working directory
6762 with the archive you create since @command{tar} will use this file's name
6763 for the archive name.
6765 An archive can be saved as a file in the file system, sent through a
6766 pipe or over a network, or written to an I/O device such as a tape,
6767 floppy disk, or CD write drive.
6769 @cindex Writing new archives
6770 @cindex Archive creation
6771 If you do not name the archive, @command{tar} uses the value of the
6772 environment variable @env{TAPE} as the file name for the archive. If
6773 that is not available, @command{tar} uses a default, compiled-in archive
6774 name, usually that for tape unit zero (i.e., @file{/dev/tu00}).
6776 @cindex Standard input and output
6777 @cindex tar to standard input and output
6778 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
6779 archive from standard input (when listing or extracting files), or
6780 writes it to standard output (when creating an archive). If you use
6781 @file{-} as an @var{archive-name} when modifying an archive,
6782 @command{tar} reads the original archive from its standard input and
6783 writes the entire new archive to its standard output.
6785 The following example is a convenient way of copying directory
6786 hierarchy from @file{sourcedir} to @file{targetdir}.
6789 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
6792 The @option{-C} option allows to avoid using subshells:
6795 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
6798 In both examples above, the leftmost @command{tar} invocation archives
6799 the contents of @file{sourcedir} to the standard output, while the
6800 rightmost one reads this archive from its standard input and
6801 extracts it. The @option{-p} option tells it to restore permissions
6802 of the extracted files.
6804 @cindex Remote devices
6805 @cindex tar to a remote device
6807 To specify an archive file on a device attached to a remote machine,
6811 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
6815 @command{tar} will set up the remote connection, if possible, and
6816 prompt you for a username and password. If you use
6817 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
6818 will attempt to set up the remote connection using your username
6819 as the username on the remote machine.
6821 @cindex Local and remote archives
6822 @anchor{local and remote archives}
6823 If the archive file name includes a colon (@samp{:}), then it is assumed
6824 to be a file on another machine. If the archive file is
6825 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6826 host @var{host}. The remote host is accessed using the @command{rsh}
6827 program, with a username of @var{user}. If the username is omitted
6828 (along with the @samp{@@} sign), then your user name will be used.
6829 (This is the normal @command{rsh} behavior.) It is necessary for the
6830 remote machine, in addition to permitting your @command{rsh} access, to
6831 have the @file{rmt} program installed (this command is included in
6832 the @GNUTAR{} distribution and by default is installed under
6833 @file{@var{prefix}/libexec/rmt}, where @var{prefix} means your
6834 installation prefix). If you need to use a file whose name includes a
6835 colon, then the remote tape drive behavior
6836 can be inhibited by using the @option{--force-local} option.
6838 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6839 tries to minimize input and output operations. The Amanda backup
6840 system, when used with @GNUTAR{}, has an initial sizing pass which
6843 @node Selecting Archive Members
6844 @section Selecting Archive Members
6845 @cindex Specifying files to act on
6846 @cindex Specifying archive members
6848 @dfn{File Name arguments} specify which files in the file system
6849 @command{tar} operates on, when creating or adding to an archive, or which
6850 archive members @command{tar} operates on, when reading or deleting from
6851 an archive. @xref{Operations}.
6853 To specify file names, you can include them as the last arguments on
6854 the command line, as follows:
6856 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6859 If a file name begins with dash (@samp{-}), precede it with
6860 @option{--add-file} option to prevent it from being treated as an
6863 @anchor{input name quoting}
6864 By default @GNUTAR{} attempts to @dfn{unquote} each file or member
6865 name, replacing @dfn{escape sequences} according to the following
6868 @multitable @columnfractions 0.20 0.60
6869 @headitem Escape @tab Replaced with
6870 @item \a @tab Audible bell (@acronym{ASCII} 7)
6871 @item \b @tab Backspace (@acronym{ASCII} 8)
6872 @item \f @tab Form feed (@acronym{ASCII} 12)
6873 @item \n @tab New line (@acronym{ASCII} 10)
6874 @item \r @tab Carriage return (@acronym{ASCII} 13)
6875 @item \t @tab Horizontal tabulation (@acronym{ASCII} 9)
6876 @item \v @tab Vertical tabulation (@acronym{ASCII} 11)
6877 @item \? @tab @acronym{ASCII} 127
6878 @item \@var{n} @tab @acronym{ASCII} @var{n} (@var{n} should be an octal number
6882 A backslash followed by any other symbol is retained.
6884 This default behavior is controlled by the following command line
6890 Enable unquoting input file or member names (default).
6894 Disable unquoting input file or member names.
6897 If you specify a directory name as a file name argument, all the files
6898 in that directory are operated on by @command{tar}.
6900 If you do not specify files, @command{tar} behavior differs depending
6901 on the operation mode as described below:
6903 When @command{tar} is invoked with @option{--create} (@option{-c}),
6904 @command{tar} will stop immediately, reporting the following:
6908 $ @kbd{tar cf a.tar}
6909 tar: Cowardly refusing to create an empty archive
6910 Try `tar --help' or `tar --usage' for more information.
6914 If you specify either @option{--list} (@option{-t}) or
6915 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6916 operates on all the archive members in the archive.
6918 If run with @option{--diff} option, tar will compare the archive with
6919 the contents of the current working directory.
6921 If you specify any other operation, @command{tar} does nothing.
6923 By default, @command{tar} takes file names from the command line. However,
6924 there are other ways to specify file or member names, or to modify the
6925 manner in which @command{tar} selects the files or members upon which to
6926 operate. In general, these methods work both for specifying the names
6927 of files and archive members.
6930 @section Reading Names from a File
6932 @cindex Reading file names from a file
6933 @cindex Lists of file names
6934 @cindex File Name arguments, alternatives
6935 @cindex @command{find}, using with @command{tar}
6936 Instead of giving the names of files or archive members on the command
6937 line, you can put the names into a file, and then use the
6938 @option{--files-from=@var{file-of-names}} (@option{-T
6939 @var{file-of-names}}) option to @command{tar}. Give the name of the
6940 file which contains the list of files to include as the argument to
6941 @option{--files-from}. In the list, the file names should be separated by
6942 newlines. You will frequently use this option when you have generated
6943 the list of files to archive with the @command{find} utility.
6947 @item --files-from=@var{file-name}
6948 @itemx -T @var{file-name}
6949 Get names to extract or create from file @var{file-name}.
6952 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6953 you specify either @code{--files-from=-} or @code{-T -}), then the file
6954 names are read from standard input.
6956 Unless you are running @command{tar} with @option{--create}, you can not use
6957 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6960 Any number of @option{-T} options can be given in the command line.
6962 The following example shows how to use @command{find} to generate a list of
6963 files smaller than 400K in length and put that list into a file
6964 called @file{small-files}. You can then use the @option{-T} option to
6965 @command{tar} to specify the files from that file, @file{small-files}, to
6966 create the archive @file{little.tgz}. (The @option{-z} option to
6967 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
6971 $ @kbd{find . -size -400 -print > small-files}
6972 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
6976 In the file list given by @option{-T} option, any file name beginning
6977 with @samp{-} character is considered a @command{tar} option and is
6978 processed accordingly@footnote{Versions of @GNUTAR{} up to 1.15.1
6979 recognized only @option{-C} option in file lists, and only if the
6980 option and its argument occupied two consecutive lines.}. For example,
6981 the common use of this feature is to change to another directory by
6982 specifying @option{-C} option:
6992 $ @kbd{tar -c -f foo.tar --files-from list}
6997 In this example, @command{tar} will first switch to @file{/etc}
6998 directory and add files @file{passwd} and @file{hosts} to the
6999 archive. Then it will change to @file{/lib} directory and will archive
7000 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
7005 $ @kbd{tar tf foo.tar}
7013 @xopindex{directory, using in @option{--files-from} argument}
7014 Notice that the option parsing algorithm used with @option{-T} is
7015 stricter than the one used by shell. Namely, when specifying option
7016 arguments, you should observe the following rules:
7020 When using short (single-letter) option form, its argument must
7021 immediately follow the option letter, without any intervening
7022 whitespace. For example: @code{-Cdir}.
7025 When using long option form, the option argument must be separated
7026 from the option by a single equal sign. No whitespace is allowed on
7027 any side of the equal sign. For example: @code{--directory=dir}.
7030 For both short and long option forms, the option argument can be given
7031 on the next line after the option name, e.g.:
7052 If you happen to have a file whose name starts with @samp{-},
7053 precede it with @option{--add-file} option to prevent it from
7054 being recognized as an option. For example: @code{--add-file=--my-file}.
7061 @subsection @code{NUL}-Terminated File Names
7063 @cindex File names, terminated by @code{NUL}
7064 @cindex @code{NUL}-terminated file names
7065 The @option{--null} option causes
7066 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
7067 to read file names terminated by a @code{NUL} instead of a newline, so
7068 files whose names contain newlines can be archived using
7069 @option{--files-from}.
7072 @xopindex{null, described}
7074 Only consider @code{NUL}-terminated file names, instead of files that
7075 terminate in a newline.
7077 @xopindex{no-null, described}
7079 Undo the effect of any previous @option{--null} option.
7082 The @option{--null} option is just like the one in @acronym{GNU}
7083 @command{xargs} and @command{cpio}, and is useful with the
7084 @option{-print0} predicate of @acronym{GNU} @command{find}. In
7085 @command{tar}, @option{--null} also disables special handling for
7086 file names that begin with dash.
7088 This example shows how to use @command{find} to generate a list of files
7089 larger than 800K in length and put that list into a file called
7090 @file{long-files}. The @option{-print0} option to @command{find} is just
7091 like @option{-print}, except that it separates files with a @code{NUL}
7092 rather than with a newline. You can then run @command{tar} with both the
7093 @option{--null} and @option{-T} options to specify that @command{tar} gets the
7094 files from that file, @file{long-files}, to create the archive
7095 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
7096 @command{tar} to recognize the @code{NUL} separator between files.
7099 $ @kbd{find . -size +800 -print0 > long-files}
7100 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
7103 The @option{--no-null} option can be used if you need to read both
7104 @code{NUL}-terminated and newline-terminated files on the same command line.
7105 For example, if @file{flist} is a newline-terminated file, then the
7106 following command can be used to combine it with the above command:
7110 $ @kbd{find . -size +800 -print0 |
7111 tar -c -f big.tar --null -T - --no-null -T flist}
7115 This example uses short options for typographic reasons, to avoid
7118 @GNUTAR is able to automatically detect @code{NUL}-terminated file lists, so
7119 it is safe to use them even without the @option{--null} option. In
7120 this case @command{tar} will print a warning and continue reading such
7121 a file as if @option{--null} were actually given:
7125 $ @kbd{find . -size +800 -print0 | tar -c -f big.tar -T -}
7126 tar: -: file name read contains nul character
7130 The null terminator, however, remains in effect only for this
7131 particular file, any following @option{-T} options will assume
7132 newline termination. Of course, the null autodetection applies
7133 to these eventual surplus @option{-T} options as well.
7136 @section Excluding Some Files
7138 @cindex File names, excluding files by
7139 @cindex Excluding files by name and pattern
7140 @cindex Excluding files by file system
7142 @opindex exclude-from
7143 To avoid operating on files whose names match a particular pattern,
7144 use the @option{--exclude} or @option{--exclude-from} options.
7148 @item --exclude=@var{pattern}
7149 Causes @command{tar} to ignore files that match the @var{pattern}.
7153 The @option{--exclude=@var{pattern}} option prevents any file or
7154 member whose name matches the shell wildcard (@var{pattern}) from
7156 For example, to create an archive with all the contents of the directory
7157 @file{src} except for files whose names end in @file{.o}, use the
7158 command @samp{tar -cf src.tar --exclude='*.o' src}.
7160 You may give multiple @option{--exclude} options.
7163 @opindex exclude-from
7164 @item --exclude-from=@var{file}
7165 @itemx -X @var{file}
7166 Causes @command{tar} to ignore files that match the patterns listed in
7170 @findex exclude-from
7171 Use the @option{--exclude-from} option to read a
7172 list of patterns, one per line, from @var{file}; @command{tar} will
7173 ignore files matching those patterns. Thus if @command{tar} is
7174 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
7175 single line @file{*.o}, no files whose names end in @file{.o} will be
7176 added to the archive.
7178 Notice, that lines from @var{file} are read verbatim. One of the
7179 frequent errors is leaving some extra whitespace after a file name,
7180 which is difficult to catch using text editors.
7182 However, empty lines are OK.
7185 @cindex version control system, excluding files
7186 @cindex VCS, excluding files
7187 @cindex SCCS, excluding files
7188 @cindex RCS, excluding files
7189 @cindex CVS, excluding files
7190 @cindex SVN, excluding files
7191 @cindex git, excluding files
7192 @cindex Bazaar, excluding files
7193 @cindex Arch, excluding files
7194 @cindex Mercurial, excluding files
7195 @cindex Darcs, excluding files
7196 @opindex exclude-vcs
7198 Exclude files and directories used by following version control
7199 systems: @samp{CVS}, @samp{RCS}, @samp{SCCS}, @samp{SVN}, @samp{Arch},
7200 @samp{Bazaar}, @samp{Mercurial}, and @samp{Darcs}.
7202 As of version @value{VERSION}, the following files are excluded:
7205 @item @file{CVS/}, and everything under it
7206 @item @file{RCS/}, and everything under it
7207 @item @file{SCCS/}, and everything under it
7208 @item @file{.git/}, and everything under it
7209 @item @file{.gitignore}
7210 @item @file{.cvsignore}
7211 @item @file{.svn/}, and everything under it
7212 @item @file{.arch-ids/}, and everything under it
7213 @item @file{@{arch@}/}, and everything under it
7214 @item @file{=RELEASE-ID}
7215 @item @file{=meta-update}
7216 @item @file{=update}
7218 @item @file{.bzrignore}
7219 @item @file{.bzrtags}
7221 @item @file{.hgignore}
7222 @item @file{.hgrags}
7226 @opindex exclude-backups
7227 @item --exclude-backups
7228 Exclude backup and lock files. This option causes exclusion of files
7229 that match the following shell globbing patterns:
7239 @findex exclude-caches
7240 When creating an archive, the @option{--exclude-caches} option family
7241 causes @command{tar} to exclude all directories that contain a @dfn{cache
7242 directory tag}. A cache directory tag is a short file with the
7243 well-known name @file{CACHEDIR.TAG} and having a standard header
7244 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
7245 Various applications write cache directory tags into directories they
7246 use to hold regenerable, non-precious data, so that such data can be
7247 more easily excluded from backups.
7249 There are three @samp{exclude-caches} options, each providing a different
7250 exclusion semantics:
7253 @opindex exclude-caches
7254 @item --exclude-caches
7255 Do not archive the contents of the directory, but archive the
7256 directory itself and the @file{CACHEDIR.TAG} file.
7258 @opindex exclude-caches-under
7259 @item --exclude-caches-under
7260 Do not archive the contents of the directory, nor the
7261 @file{CACHEDIR.TAG} file, archive only the directory itself.
7263 @opindex exclude-caches-all
7264 @item --exclude-caches-all
7265 Omit directories containing @file{CACHEDIR.TAG} file entirely.
7269 Another option family, @option{--exclude-tag}, provides a generalization of
7270 this concept. It takes a single argument, a file name to look for.
7271 Any directory that contains this file will be excluded from the dump.
7272 Similarly to @samp{exclude-caches}, there are three options in this
7276 @opindex exclude-tag
7277 @item --exclude-tag=@var{file}
7278 Do not dump the contents of the directory, but dump the
7279 directory itself and the @var{file}.
7281 @opindex exclude-tag-under
7282 @item --exclude-tag-under=@var{file}
7283 Do not dump the contents of the directory, nor the
7284 @var{file}, archive only the directory itself.
7286 @opindex exclude-tag-all
7287 @item --exclude-tag-all=@var{file}
7288 Omit directories containing @var{file} file entirely.
7291 Multiple @option{--exclude-tag*} options can be given.
7293 For example, given this directory:
7308 The @option{--exclude-tag} will produce the following:
7311 $ @kbd{tar -cf archive.tar --exclude-tag=tagfile -v dir}
7316 tar: dir/folk/: contains a cache directory tag tagfile;
7321 Both the @file{dir/folk} directory and its tagfile are preserved in
7322 the archive, however the rest of files in this directory are not.
7324 Now, using the @option{--exclude-tag-under} option will exclude
7325 @file{tagfile} from the dump, while still preserving the directory
7326 itself, as shown in this example:
7329 $ @kbd{tar -cf archive.tar --exclude-tag-under=tagfile -v dir}
7334 ./tar: dir/folk/: contains a cache directory tag tagfile;
7338 Finally, using @option{--exclude-tag-all} omits the @file{dir/folk}
7342 $ @kbd{tar -cf archive.tar --exclude-tag-all=tagfile -v dir}
7346 ./tar: dir/folk/: contains a cache directory tag tagfile;
7347 directory not dumped
7351 * problems with exclude::
7354 @node problems with exclude
7355 @unnumberedsubsec Problems with Using the @code{exclude} Options
7357 @xopindex{exclude, potential problems with}
7358 Some users find @samp{exclude} options confusing. Here are some common
7363 The main operating mode of @command{tar} does not act on a file name
7364 explicitly listed on the command line, if one of its file name
7365 components is excluded. In the example above, if
7366 you create an archive and exclude files that end with @samp{*.o}, but
7367 explicitly name the file @samp{dir.o/foo} after all the options have been
7368 listed, @samp{dir.o/foo} will be excluded from the archive.
7371 You can sometimes confuse the meanings of @option{--exclude} and
7372 @option{--exclude-from}. Be careful: use @option{--exclude} when files
7373 to be excluded are given as a pattern on the command line. Use
7374 @option{--exclude-from} to introduce the name of a file which contains
7375 a list of patterns, one per line; each of these patterns can exclude
7376 zero, one, or many files.
7379 When you use @option{--exclude=@var{pattern}}, be sure to quote the
7380 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
7381 like @samp{*}. If you do not do this, the shell might expand the
7382 @samp{*} itself using files at hand, so @command{tar} might receive a
7383 list of files instead of one pattern, or none at all, making the
7384 command somewhat illegal. This might not correspond to what you want.
7389 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
7397 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
7401 You must use use shell syntax, or globbing, rather than @code{regexp}
7402 syntax, when using exclude options in @command{tar}. If you try to use
7403 @code{regexp} syntax to describe files to be excluded, your command
7407 @FIXME{The change in semantics must have occurred before 1.11,
7408 so I doubt if it is worth mentioning at all. Anyway, should at
7409 least specify in which version the semantics changed.}
7410 In earlier versions of @command{tar}, what is now the
7411 @option{--exclude-from} option was called @option{--exclude} instead.
7412 Now, @option{--exclude} applies to patterns listed on the command
7413 line and @option{--exclude-from} applies to patterns listed in a
7419 @section Wildcards Patterns and Matching
7421 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
7422 @samp{*} or @samp{?} for example, are replaced and expanded into all
7423 existing files matching the given pattern. @GNUTAR{} can use wildcard
7424 patterns for matching (or globbing) archive members when extracting
7425 from or listing an archive. Wildcard patterns are also used for
7426 verifying volume labels of @command{tar} archives. This section has the
7427 purpose of explaining wildcard syntax for @command{tar}.
7429 @FIXME{the next few paragraphs need work.}
7431 A @var{pattern} should be written according to shell syntax, using wildcard
7432 characters to effect globbing. Most characters in the pattern stand
7433 for themselves in the matched string, and case is significant: @samp{a}
7434 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
7435 pattern matches any single character in the matched string. The character
7436 @samp{*} in the pattern matches zero, one, or more single characters in
7437 the matched string. The character @samp{\} says to take the following
7438 character of the pattern @emph{literally}; it is useful when one needs to
7439 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
7441 The character @samp{[}, up to the matching @samp{]}, introduces a character
7442 class. A @dfn{character class} is a list of acceptable characters
7443 for the next single character of the matched string. For example,
7444 @samp{[abcde]} would match any of the first five letters of the alphabet.
7445 Note that within a character class, all of the ``special characters''
7446 listed above other than @samp{\} lose their special meaning; for example,
7447 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
7448 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
7449 the characters @samp{-} and @samp{]} must either come @emph{first} or
7450 @emph{last} in a character class.)
7452 @cindex Excluding characters from a character class
7453 @cindex Character class, excluding characters from
7454 If the first character of the class after the opening @samp{[}
7455 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
7456 Rather than listing character to match, it lists those characters which
7457 are @emph{forbidden} as the next single character of the matched string.
7459 Other characters of the class stand for themselves. The special
7460 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
7461 letters, is meant to represent all characters between @var{a} and
7464 @FIXME{need to add a sentence or so here to make this clear for those
7465 who don't have dan around.}
7467 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
7468 special for wildcard matches. However, if a pattern completely matches
7469 a directory prefix of a matched string, then it matches the full matched
7470 string: thus, excluding a directory also excludes all the files beneath it.
7473 * controlling pattern-matching::
7476 @node controlling pattern-matching
7477 @unnumberedsubsec Controlling Pattern-Matching
7479 For the purposes of this section, we call @dfn{exclusion members} all
7480 member names obtained while processing @option{--exclude} and
7481 @option{--exclude-from} options, and @dfn{inclusion members} those
7482 member names that were given in the command line or read from the file
7483 specified with @option{--files-from} option.
7485 These two pairs of member lists are used in the following operations:
7486 @option{--diff}, @option{--extract}, @option{--list},
7489 There are no inclusion members in create mode (@option{--create} and
7490 @option{--append}), since in this mode the names obtained from the
7491 command line refer to @emph{files}, not archive members.
7493 By default, inclusion members are compared with archive members
7494 literally @footnote{Notice that earlier @GNUTAR{} versions used
7495 globbing for inclusion members, which contradicted to UNIX98
7496 specification and was not documented. @xref{Changes}, for more
7497 information on this and other changes.} and exclusion members are
7498 treated as globbing patterns. For example:
7502 $ @kbd{tar tf foo.tar}
7507 # @i{Member names are used verbatim:}
7508 $ @kbd{tar -xf foo.tar -v '[remarks]'}
7510 # @i{Exclude member names are globbed:}
7511 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
7517 This behavior can be altered by using the following options:
7522 Treat all member names as wildcards.
7524 @opindex no-wildcards
7525 @item --no-wildcards
7526 Treat all member names as literal strings.
7529 Thus, to extract files whose names end in @samp{.c}, you can use:
7532 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
7538 Notice quoting of the pattern to prevent the shell from interpreting
7541 The effect of @option{--wildcards} option is canceled by
7542 @option{--no-wildcards}. This can be used to pass part of
7543 the command line arguments verbatim and other part as globbing
7544 patterns. For example, the following invocation:
7547 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
7551 instructs @command{tar} to extract from @file{foo.tar} all files whose
7552 names end in @samp{.txt} and the file named @file{[remarks]}.
7554 Normally, a pattern matches a name if an initial subsequence of the
7555 name's components matches the pattern, where @samp{*}, @samp{?}, and
7556 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
7557 and wildcards can match @samp{/}.
7559 Other than optionally stripping leading @samp{/} from names
7560 (@pxref{absolute}), patterns and names are used as-is. For
7561 example, trailing @samp{/} is not trimmed from a user-specified name
7562 before deciding whether to exclude it.
7564 However, this matching procedure can be altered by the options listed
7565 below. These options accumulate. For example:
7568 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
7572 ignores case when excluding @samp{makefile}, but not when excluding
7577 @opindex no-anchored
7579 @itemx --no-anchored
7580 If anchored, a pattern must match an initial subsequence
7581 of the name's components. Otherwise, the pattern can match any
7582 subsequence. Default is @option{--no-anchored} for exclusion members
7583 and @option{--anchored} inclusion members.
7585 @opindex ignore-case
7586 @opindex no-ignore-case
7588 @itemx --no-ignore-case
7589 When ignoring case, upper-case patterns match lower-case names and vice versa.
7590 When not ignoring case (the default), matching is case-sensitive.
7592 @opindex wildcards-match-slash
7593 @opindex no-wildcards-match-slash
7594 @item --wildcards-match-slash
7595 @itemx --no-wildcards-match-slash
7596 When wildcards match slash (the default for exclusion members), a
7597 wildcard like @samp{*} in the pattern can match a @samp{/} in the
7598 name. Otherwise, @samp{/} is matched only by @samp{/}.
7602 The @option{--recursion} and @option{--no-recursion} options
7603 (@pxref{recurse}) also affect how member patterns are interpreted. If
7604 recursion is in effect, a pattern matches a name if it matches any of
7605 the name's parent directories.
7607 The following table summarizes pattern-matching default values:
7609 @multitable @columnfractions .3 .7
7610 @headitem Members @tab Default settings
7611 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
7612 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
7615 @node quoting styles
7616 @section Quoting Member Names
7618 When displaying member names, @command{tar} takes care to avoid
7619 ambiguities caused by certain characters. This is called @dfn{name
7620 quoting}. The characters in question are:
7623 @item Non-printable control characters:
7624 @anchor{escape sequences}
7625 @multitable @columnfractions 0.20 0.10 0.60
7626 @headitem Character @tab @acronym{ASCII} @tab Character name
7627 @item \a @tab 7 @tab Audible bell
7628 @item \b @tab 8 @tab Backspace
7629 @item \f @tab 12 @tab Form feed
7630 @item \n @tab 10 @tab New line
7631 @item \r @tab 13 @tab Carriage return
7632 @item \t @tab 9 @tab Horizontal tabulation
7633 @item \v @tab 11 @tab Vertical tabulation
7636 @item Space (@acronym{ASCII} 32)
7638 @item Single and double quotes (@samp{'} and @samp{"})
7640 @item Backslash (@samp{\})
7643 The exact way @command{tar} uses to quote these characters depends on
7644 the @dfn{quoting style}. The default quoting style, called
7645 @dfn{escape} (see below), uses backslash notation to represent control
7646 characters, space and backslash. Using this quoting style, control
7647 characters are represented as listed in column @samp{Character} in the
7648 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
7650 @GNUTAR{} offers seven distinct quoting styles, which can be selected
7651 using @option{--quoting-style} option:
7654 @item --quoting-style=@var{style}
7655 @opindex quoting-style
7657 Sets quoting style. Valid values for @var{style} argument are:
7658 literal, shell, shell-always, c, escape, locale, clocale.
7661 These styles are described in detail below. To illustrate their
7662 effect, we will use an imaginary tar archive @file{arch.tar}
7663 containing the following members:
7667 # 1. Contains horizontal tabulation character.
7669 # 2. Contains newline character
7672 # 3. Contains a space
7674 # 4. Contains double quotes
7676 # 5. Contains single quotes
7678 # 6. Contains a backslash character:
7683 Here is how usual @command{ls} command would have listed them, if they
7684 had existed in the current working directory:
7702 No quoting, display each character as is:
7706 $ @kbd{tar tf arch.tar --quoting-style=literal}
7719 Display characters the same way Bourne shell does:
7720 control characters, except @samp{\t} and @samp{\n}, are printed using
7721 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
7722 single quote is printed as @samp{\'}. If a name contains any quoted
7723 characters, it is enclosed in single quotes. In particular, if a name
7724 contains single quotes, it is printed as several single-quoted strings:
7728 $ @kbd{tar tf arch.tar --quoting-style=shell}
7731 './a'\''single'\''quote'
7741 Same as @samp{shell}, but the names are always enclosed in single
7746 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
7749 './a'\''single'\''quote'
7759 Use the notation of the C programming language. All names are
7760 enclosed in double quotes. Control characters are quoted using
7761 backslash notations, double quotes are represented as @samp{\"},
7762 backslash characters are represented as @samp{\\}. Single quotes and
7763 spaces are not quoted:
7767 $ @kbd{tar tf arch.tar --quoting-style=c}
7771 "./a\"double\"quote"
7779 Control characters are printed using backslash notation, a space is
7780 printed as @samp{\ } and a backslash as @samp{\\}. This is the
7781 default quoting style, unless it was changed when configured the
7786 $ @kbd{tar tf arch.tar --quoting-style=escape}
7798 Control characters, single quote and backslash are printed using
7799 backslash notation. All names are quoted using left and right
7800 quotation marks, appropriate to the current locale. If it does not
7801 define quotation marks, use @samp{`} as left and @samp{'} as right
7802 quotation marks. Any occurrences of the right quotation mark in a
7803 name are escaped with @samp{\}, for example:
7809 $ @kbd{tar tf arch.tar --quoting-style=locale}
7812 `./a\'single\'quote'
7821 Same as @samp{locale}, but @samp{"} is used for both left and right
7822 quotation marks, if not provided by the currently selected locale:
7826 $ @kbd{tar tf arch.tar --quoting-style=clocale}
7830 "./a\"double\"quote"
7838 You can specify which characters should be quoted in addition to those
7839 implied by the current quoting style:
7842 @item --quote-chars=@var{string}
7843 Always quote characters from @var{string}, even if the selected
7844 quoting style would not quote them.
7847 For example, using @samp{escape} quoting (compare with the usual
7848 escape listing above):
7852 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
7863 To disable quoting of such additional characters, use the following
7867 @item --no-quote-chars=@var{string}
7868 Remove characters listed in @var{string} from the list of quoted
7869 characters set by the previous @option{--quote-chars} option.
7872 This option is particularly useful if you have added
7873 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
7874 and wish to disable it for the current invocation.
7876 Note, that @option{--no-quote-chars} does @emph{not} disable those
7877 characters that are quoted by default in the selected quoting style.
7880 @section Modifying File and Member Names
7882 @command{Tar} archives contain detailed information about files stored
7883 in them and full file names are part of that information. When
7884 storing a file to an archive, its file name is recorded in it,
7885 along with the actual file contents. When restoring from an archive,
7886 a file is created on disk with exactly the same name as that stored
7887 in the archive. In the majority of cases this is the desired behavior
7888 of a file archiver. However, there are some cases when it is not.
7890 First of all, it is often unsafe to extract archive members with
7891 absolute file names or those that begin with a @file{../}. @GNUTAR{}
7892 takes special precautions when extracting such names and provides a
7893 special option for handling them, which is described in
7896 Secondly, you may wish to extract file names without some leading
7897 directory components, or with otherwise modified names. In other
7898 cases it is desirable to store files under differing names in the
7901 @GNUTAR{} provides several options for these needs.
7904 @opindex strip-components
7905 @item --strip-components=@var{number}
7906 Strip given @var{number} of leading components from file names before
7910 For example, suppose you have archived whole @file{/usr} hierarchy to
7911 a tar archive named @file{usr.tar}. Among other files, this archive
7912 contains @file{usr/include/stdlib.h}, which you wish to extract to
7913 the current working directory. To do so, you type:
7916 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
7919 The option @option{--strip=2} instructs @command{tar} to strip the
7920 two leading components (@file{usr/} and @file{include/}) off the file
7923 If you add the @option{--verbose} (@option{-v}) option to the invocation
7924 above, you will note that the verbose listing still contains the
7925 full file name, with the two removed components still in place. This
7926 can be inconvenient, so @command{tar} provides a special option for
7927 altering this behavior:
7929 @anchor{show-transformed-names}
7931 @opindex show-transformed-names
7932 @item --show-transformed-names
7933 Display file or member names with all requested transformations
7942 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
7943 usr/include/stdlib.h
7944 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
7949 Notice that in both cases the file @file{stdlib.h} is extracted to the
7950 current working directory, @option{--show-transformed-names} affects
7951 only the way its name is displayed.
7953 This option is especially useful for verifying whether the invocation
7954 will have the desired effect. Thus, before running
7957 $ @kbd{tar -x --strip=@var{n}}
7961 it is often advisable to run
7964 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
7968 to make sure the command will produce the intended results.
7970 In case you need to apply more complex modifications to the file name,
7971 @GNUTAR{} provides a general-purpose transformation option:
7976 @item --transform=@var{expression}
7977 @itemx --xform=@var{expression}
7978 Modify file names using supplied @var{expression}.
7982 The @var{expression} is a @command{sed}-like replace expression of the
7986 s/@var{regexp}/@var{replace}/[@var{flags}]
7990 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
7991 replacement for each file name part that matches @var{regexp}. Both
7992 @var{regexp} and @var{replace} are described in detail in
7993 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
7995 Any delimiter can be used in lieu of @samp{/}, the only requirement being
7996 that it be used consistently throughout the expression. For example,
7997 the following two expressions are equivalent:
8006 Changing delimiters is often useful when the @var{regex} contains
8007 slashes. For example, it is more convenient to write @code{s,/,-,} than
8010 As in @command{sed}, you can give several replace expressions,
8011 separated by a semicolon.
8013 Supported @var{flags} are:
8017 Apply the replacement to @emph{all} matches to the @var{regexp}, not
8021 Use case-insensitive matching.
8024 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
8025 regexps, Extended regular expressions, Extended regular expressions,
8029 Only replace the @var{number}th match of the @var{regexp}.
8031 Note: the @acronym{POSIX} standard does not specify what should happen
8032 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
8033 follows the GNU @command{sed} implementation in this regard, so
8034 the interaction is defined to be: ignore matches before the
8035 @var{number}th, and then match and replace all matches from the
8040 In addition, several @dfn{transformation scope} flags are supported,
8041 that control to what files transformations apply. These are:
8045 Apply transformation to regular archive members.
8048 Do not apply transformation to regular archive members.
8051 Apply transformation to symbolic link targets.
8054 Do not apply transformation to symbolic link targets.
8057 Apply transformation to hard link targets.
8060 Do not apply transformation to hard link targets.
8063 Default is @samp{rsh}, which means to apply tranformations to both archive
8064 members and targets of symbolic and hard links.
8066 Default scope flags can also be changed using @samp{flags=} statement
8067 in the transform expression. The flags set this way remain in force
8068 until next @samp{flags=} statement or end of expression, whichever
8069 occurs first. For example:
8072 --transform 'flags=S;s|^|/usr/local/|'
8075 Here are several examples of @option{--transform} usage:
8078 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
8081 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
8084 @item Strip two leading directory components (equivalent to
8085 @option{--strip-components=2}):
8088 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
8091 @item Convert each file name to lower case:
8094 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
8097 @item Prepend @file{/prefix/} to each file name:
8100 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
8103 @item Archive the @file{/lib} directory, prepending @samp{/usr/local}
8104 to each archive member:
8107 $ @kbd{tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib}
8111 Notice the use of flags in the last example. The @file{/lib}
8112 directory often contains many symbolic links to files within it.
8113 It may look, for example, like this:
8117 drwxr-xr-x root/root 0 2008-07-08 16:20 /lib/
8118 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
8119 lrwxrwxrwx root/root 0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
8123 Using the expression @samp{s,^,/usr/local/,} would mean adding
8124 @samp{/usr/local} to both regular archive members and to link
8125 targets. In this case, @file{/lib/libc.so.6} would become:
8128 /usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so
8131 This is definitely not desired. To avoid this, the @samp{S} flag
8132 is used, which excludes symbolic link targets from filename
8133 transformations. The result is:
8136 $ @kbd{tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \
8137 --show-transformed /lib}
8138 drwxr-xr-x root/root 0 2008-07-08 16:20 /usr/local/lib/
8139 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
8140 lrwxrwxrwx root/root 0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
8144 Unlike @option{--strip-components}, @option{--transform} can be used
8145 in any @GNUTAR{} operation mode. For example, the following command
8146 adds files to the archive while replacing the leading @file{usr/}
8147 component with @file{var/}:
8150 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
8153 To test @option{--transform} effect we suggest using
8154 @option{--show-transformed-names} option:
8157 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
8158 --verbose --show-transformed-names /}
8161 If both @option{--strip-components} and @option{--transform} are used
8162 together, then @option{--transform} is applied first, and the required
8163 number of components is then stripped from its result.
8165 You can use as many @option{--transform} options in a single command
8166 line as you want. The specified expressions will then be applied in
8167 order of their appearance. For example, the following two invocations
8171 $ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
8172 --transform='s,/usr/local,/usr/,'}
8173 $ @kbd{tar -cf arch.tar \
8174 --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
8178 @section Operating Only on New Files
8180 @cindex Excluding file by age
8181 @cindex Data Modification time, excluding files by
8182 @cindex Modification time, excluding files by
8183 @cindex Age, excluding files by
8184 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
8185 @option{-N @var{date}}) option causes @command{tar} to only work on
8186 files whose data modification or status change times are newer than
8187 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
8188 it is taken to be a file name; the data modification time of that file
8189 is used as the date. If you use this option when creating or appending
8190 to an archive, the archive will only include new files. If you use
8191 @option{--after-date} when extracting an archive, @command{tar} will
8192 only extract files newer than the @var{date} you specify.
8194 If you only want @command{tar} to make the date comparison based on
8195 modification of the file's data (rather than status
8196 changes), then use the @option{--newer-mtime=@var{date}} option.
8198 @cindex --after-date and --update compared
8199 @cindex --newer-mtime and --update compared
8200 You may use these options with any operation. Note that these options
8201 differ from the @option{--update} (@option{-u}) operation in that they
8202 allow you to specify a particular date against which @command{tar} can
8203 compare when deciding whether or not to archive the files.
8208 @item --after-date=@var{date}
8209 @itemx --newer=@var{date}
8210 @itemx -N @var{date}
8211 Only store files newer than @var{date}.
8213 Acts on files only if their data modification or status change times are
8214 later than @var{date}. Use in conjunction with any operation.
8216 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
8217 name; the data modification time of that file is used as the date.
8219 @opindex newer-mtime
8220 @item --newer-mtime=@var{date}
8221 Acts like @option{--after-date}, but only looks at data modification times.
8224 These options limit @command{tar} to operate only on files which have
8225 been modified after the date specified. A file's status is considered to have
8226 changed if its contents have been modified, or if its owner,
8227 permissions, and so forth, have been changed. (For more information on
8228 how to specify a date, see @ref{Date input formats}; remember that the
8229 entire date argument must be quoted if it contains any spaces.)
8231 Gurus would say that @option{--after-date} tests both the data
8232 modification time (@code{mtime}, the time the contents of the file
8233 were last modified) and the status change time (@code{ctime}, the time
8234 the file's status was last changed: owner, permissions, etc.@:)
8235 fields, while @option{--newer-mtime} tests only the @code{mtime}
8238 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
8239 @code{ctime} and processes the file if either one is more recent than
8240 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
8241 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
8242 contents of the file were looked at).
8244 Date specifiers can have embedded spaces. Because of this, you may need
8245 to quote date arguments to keep the shell from parsing them as separate
8246 arguments. For example, the following command will add to the archive
8247 all the files modified less than two days ago:
8250 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
8253 When any of these options is used with the option @option{--verbose}
8254 (@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
8255 date back to its textual representation and compare that with the
8256 one given with the option. If the two dates differ, @command{tar} will
8257 print a warning saying what date it will use. This is to help user
8258 ensure he is using the right date. For example:
8262 $ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
8263 tar: Option --after-date: Treating date `10 days ago' as 2006-06-11
8269 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
8270 should not be used for incremental backups. @xref{Incremental Dumps},
8271 for proper way of creating incremental backups.
8275 @section Descending into Directories
8276 @cindex Avoiding recursion in directories
8277 @cindex Descending directories, avoiding
8278 @cindex Directories, avoiding recursion
8279 @cindex Recursion in directories, avoiding
8281 Usually, @command{tar} will recursively explore all directories (either
8282 those given on the command line or through the @option{--files-from}
8283 option) for the various files they contain. However, you may not always
8284 want @command{tar} to act this way.
8286 @opindex no-recursion
8287 @cindex @command{find}, using with @command{tar}
8288 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
8289 into specified directories. If you specify @option{--no-recursion}, you can
8290 use the @command{find} (@pxref{Top,, find, find, GNU Find Manual})
8291 utility for hunting through levels of directories to
8292 construct a list of file names which you could then pass to @command{tar}.
8293 @command{find} allows you to be more selective when choosing which files to
8294 archive; see @ref{files}, for more information on using @command{find} with
8298 @item --no-recursion
8299 Prevents @command{tar} from recursively descending directories.
8303 Requires @command{tar} to recursively descend directories.
8304 This is the default.
8307 When you use @option{--no-recursion}, @GNUTAR{} grabs
8308 directory entries themselves, but does not descend on them
8309 recursively. Many people use @command{find} for locating files they
8310 want to back up, and since @command{tar} @emph{usually} recursively
8311 descends on directories, they have to use the @samp{@w{-not -type d}}
8312 test in their @command{find} invocation (@pxref{Type, Type, Type test,
8313 find, Finding Files}), as they usually do not want all the files in a
8314 directory. They then use the @option{--files-from} option to archive
8315 the files located via @command{find}.
8317 The problem when restoring files archived in this manner is that the
8318 directories themselves are not in the archive; so the
8319 @option{--same-permissions} (@option{--preserve-permissions},
8320 @option{-p}) option does not affect them---while users might really
8321 like it to. Specifying @option{--no-recursion} is a way to tell
8322 @command{tar} to grab only the directory entries given to it, adding
8323 no new files on its own. To summarize, if you use @command{find} to
8324 create a list of files to be stored in an archive, use it as follows:
8328 $ @kbd{find @var{dir} @var{tests} | \
8329 tar -cf @var{archive} -T - --no-recursion}
8333 The @option{--no-recursion} option also applies when extracting: it
8334 causes @command{tar} to extract only the matched directory entries, not
8335 the files under those directories.
8337 The @option{--no-recursion} option also affects how globbing patterns
8338 are interpreted (@pxref{controlling pattern-matching}).
8340 The @option{--no-recursion} and @option{--recursion} options apply to
8341 later options and operands, and can be overridden by later occurrences
8342 of @option{--no-recursion} and @option{--recursion}. For example:
8345 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
8349 creates an archive with one entry for @file{grape}, and the recursive
8350 contents of @file{grape/concord}, but no entries under @file{grape}
8351 other than @file{grape/concord}.
8354 @section Crossing File System Boundaries
8355 @cindex File system boundaries, not crossing
8357 @command{tar} will normally automatically cross file system boundaries in
8358 order to archive files which are part of a directory tree. You can
8359 change this behavior by running @command{tar} and specifying
8360 @option{--one-file-system}. This option only affects files that are
8361 archived because they are in a directory that is being archived;
8362 @command{tar} will still archive files explicitly named on the command line
8363 or through @option{--files-from}, regardless of where they reside.
8366 @opindex one-file-system
8367 @item --one-file-system
8368 Prevents @command{tar} from crossing file system boundaries when
8369 archiving. Use in conjunction with any write operation.
8372 The @option{--one-file-system} option causes @command{tar} to modify its
8373 normal behavior in archiving the contents of directories. If a file in
8374 a directory is not on the same file system as the directory itself, then
8375 @command{tar} will not archive that file. If the file is a directory
8376 itself, @command{tar} will not archive anything beneath it; in other words,
8377 @command{tar} will not cross mount points.
8379 This option is useful for making full or incremental archival backups of
8380 a file system. If this option is used in conjunction with
8381 @option{--verbose} (@option{-v}), files that are excluded are
8382 mentioned by name on the standard error.
8385 * directory:: Changing Directory
8386 * absolute:: Absolute File Names
8390 @subsection Changing the Working Directory
8392 @FIXME{need to read over this node now for continuity; i've switched
8393 things around some.}
8395 @cindex Changing directory mid-stream
8396 @cindex Directory, changing mid-stream
8397 @cindex Working directory, specifying
8398 To change the working directory in the middle of a list of file names,
8399 either on the command line or in a file specified using
8400 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
8401 This will change the working directory to the specified directory
8402 after that point in the list.
8406 @item --directory=@var{directory}
8407 @itemx -C @var{directory}
8408 Changes the working directory in the middle of a command line.
8414 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
8418 will place the files @file{grape} and @file{prune} from the current
8419 directory into the archive @file{jams.tar}, followed by the file
8420 @file{cherry} from the directory @file{food}. This option is especially
8421 useful when you have several widely separated files that you want to
8422 store in the same archive.
8424 Note that the file @file{cherry} is recorded in the archive under the
8425 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
8426 archive will contain three files that all appear to have come from the
8427 same directory; if the archive is extracted with plain @samp{tar
8428 --extract}, all three files will be written in the current directory.
8430 Contrast this with the command,
8433 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
8437 which records the third file in the archive under the name
8438 @file{red/cherry} so that, if the archive is extracted using
8439 @samp{tar --extract}, the third file will be written in a subdirectory
8442 You can use the @option{--directory} option to make the archive
8443 independent of the original name of the directory holding the files.
8444 The following command places the files @file{/etc/passwd},
8445 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
8449 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
8453 However, the names of the archive members will be exactly what they were
8454 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
8455 They will not appear to be related by file name to the original
8456 directories where those files were located.
8458 Note that @option{--directory} options are interpreted consecutively. If
8459 @option{--directory} specifies a relative file name, it is interpreted
8460 relative to the then current directory, which might not be the same as
8461 the original current working directory of @command{tar}, due to a previous
8462 @option{--directory} option.
8464 When using @option{--files-from} (@pxref{files}), you can put various
8465 @command{tar} options (including @option{-C}) in the file list. Notice,
8466 however, that in this case the option and its argument may not be
8467 separated by whitespace. If you use short option, its argument must
8468 either follow the option letter immediately, without any intervening
8469 whitespace, or occupy the next line. Otherwise, if you use long
8470 option, separate its argument by an equal sign.
8472 For instance, the file list for the above example will be:
8485 To use it, you would invoke @command{tar} as follows:
8488 $ @kbd{tar -c -f foo.tar --files-from list}
8491 The interpretation of @option{--directory} is disabled by
8492 @option{--null} option.
8495 @subsection Absolute File Names
8496 @cindex absolute file names
8497 @cindex file names, absolute
8499 By default, @GNUTAR{} drops a leading @samp{/} on
8500 input or output, and complains about file names containing a @file{..}
8501 component. There is an option that turns off this behavior:
8504 @opindex absolute-names
8505 @item --absolute-names
8507 Do not strip leading slashes from file names, and permit file names
8508 containing a @file{..} file name component.
8511 When @command{tar} extracts archive members from an archive, it strips any
8512 leading slashes (@samp{/}) from the member name. This causes absolute
8513 member names in the archive to be treated as relative file names. This
8514 allows you to have such members extracted wherever you want, instead of
8515 being restricted to extracting the member in the exact directory named
8516 in the archive. For example, if the archive member has the name
8517 @file{/etc/passwd}, @command{tar} will extract it as if the name were
8518 really @file{etc/passwd}.
8520 File names containing @file{..} can cause problems when extracting, so
8521 @command{tar} normally warns you about such files when creating an
8522 archive, and rejects attempts to extracts such files.
8524 Other @command{tar} programs do not do this. As a result, if you
8525 create an archive whose member names start with a slash, they will be
8526 difficult for other people with a non-@GNUTAR{}
8527 program to use. Therefore, @GNUTAR{} also strips
8528 leading slashes from member names when putting members into the
8529 archive. For example, if you ask @command{tar} to add the file
8530 @file{/bin/ls} to an archive, it will do so, but the member name will
8531 be @file{bin/ls}@footnote{A side effect of this is that when
8532 @option{--create} is used with @option{--verbose} the resulting output
8533 is not, generally speaking, the same as the one you'd get running
8534 @kbd{tar --list} command. This may be important if you use some
8535 scripts for comparing both outputs. @xref{listing member and file names},
8536 for the information on how to handle this case.}.
8538 If you use the @option{--absolute-names} (@option{-P}) option,
8539 @command{tar} will do none of these transformations.
8541 To archive or extract files relative to the root directory, specify
8542 the @option{--absolute-names} (@option{-P}) option.
8544 Normally, @command{tar} acts on files relative to the working
8545 directory---ignoring superior directory names when archiving, and
8546 ignoring leading slashes when extracting.
8548 When you specify @option{--absolute-names} (@option{-P}),
8549 @command{tar} stores file names including all superior directory
8550 names, and preserves leading slashes. If you only invoked
8551 @command{tar} from the root directory you would never need the
8552 @option{--absolute-names} option, but using this option
8553 may be more convenient than switching to root.
8555 @FIXME{Should be an example in the tutorial/wizardry section using this
8556 to transfer files between systems.}
8559 @item --absolute-names
8560 Preserves full file names (including superior directory names) when
8561 archiving files. Preserves leading slash when extracting files.
8565 @command{tar} prints out a message about removing the @samp{/} from
8566 file names. This message appears once per @GNUTAR{}
8567 invocation. It represents something which ought to be told; ignoring
8568 what it means can cause very serious surprises, later.
8570 Some people, nevertheless, do not want to see this message. Wanting to
8571 play really dangerously, one may of course redirect @command{tar} standard
8572 error to the sink. For example, under @command{sh}:
8575 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
8579 Another solution, both nicer and simpler, would be to change to
8580 the @file{/} directory first, and then avoid absolute notation.
8584 $ @kbd{tar -c -f archive.tar -C / home}
8587 @xref{Integrity}, for some of the security-related implications
8588 of using this option.
8590 @include parse-datetime.texi
8593 @chapter Controlling the Archive Format
8595 @cindex Tar archive formats
8596 Due to historical reasons, there are several formats of tar archives.
8597 All of them are based on the same principles, but have some subtle
8598 differences that often make them incompatible with each other.
8600 GNU tar is able to create and handle archives in a variety of formats.
8601 The most frequently used formats are (in alphabetical order):
8605 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
8606 from an early @acronym{POSIX} standard, adding some improvements such as
8607 sparse file handling and incremental archives. Unfortunately these
8608 features were implemented in a way incompatible with other archive
8611 Archives in @samp{gnu} format are able to hold file names of unlimited
8615 Format used by @GNUTAR{} of versions prior to 1.12.
8618 Archive format, compatible with the V7 implementation of tar. This
8619 format imposes a number of limitations. The most important of them
8623 @item The maximum length of a file name is limited to 99 characters.
8624 @item The maximum length of a symbolic link is limited to 99 characters.
8625 @item It is impossible to store special files (block and character
8626 devices, fifos etc.)
8627 @item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
8629 @item V7 archives do not contain symbolic ownership information (user
8630 and group name of the file owner).
8633 This format has traditionally been used by Automake when producing
8634 Makefiles. This practice will change in the future, in the meantime,
8635 however this means that projects containing file names more than 99
8636 characters long will not be able to use @GNUTAR{} @value{VERSION} and
8637 Automake prior to 1.9.
8640 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
8641 symbolic ownership information. It is also able to store
8642 special files. However, it imposes several restrictions as well:
8645 @item The maximum length of a file name is limited to 256 characters,
8646 provided that the file name can be split at a directory separator in
8647 two parts, first of them being at most 155 bytes long. So, in most
8648 cases the maximum file name length will be shorter than 256
8650 @item The maximum length of a symbolic link name is limited to
8652 @item Maximum size of a file the archive is able to accommodate
8654 @item Maximum value of UID/GID is 2097151.
8655 @item Maximum number of bits in device major and minor numbers is 21.
8659 Format used by J@"org Schilling @command{star}
8660 implementation. @GNUTAR{} is able to read @samp{star} archives but
8661 currently does not produce them.
8664 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
8665 most flexible and feature-rich format. It does not impose any
8666 restrictions on file sizes or file name lengths. This format is quite
8667 recent, so not all tar implementations are able to handle it properly.
8668 However, this format is designed in such a way that any tar
8669 implementation able to read @samp{ustar} archives will be able to read
8670 most @samp{posix} archives as well, with the only exception that any
8671 additional information (such as long file names etc.) will in such
8672 case be extracted as plain text files along with the files it refers to.
8674 This archive format will be the default format for future versions
8679 The following table summarizes the limitations of each of these
8682 @multitable @columnfractions .10 .20 .20 .20 .20
8683 @headitem Format @tab UID @tab File Size @tab File Name @tab Devn
8684 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8685 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8686 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
8687 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
8688 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
8691 The default format for @GNUTAR{} is defined at compilation
8692 time. You may check it by running @command{tar --help}, and examining
8693 the last lines of its output. Usually, @GNUTAR{} is configured
8694 to create archives in @samp{gnu} format, however, future version will
8695 switch to @samp{posix}.
8698 * Compression:: Using Less Space through Compression
8699 * Attributes:: Handling File Attributes
8700 * Portability:: Making @command{tar} Archives More Portable
8701 * cpio:: Comparison of @command{tar} and @command{cpio}
8705 @section Using Less Space through Compression
8708 * gzip:: Creating and Reading Compressed Archives
8709 * sparse:: Archiving Sparse Files
8713 @subsection Creating and Reading Compressed Archives
8714 @cindex Compressed archives
8715 @cindex Storing archives in compressed format
8723 @GNUTAR{} is able to create and read compressed archives. It supports
8724 a wide variety of compression programs, namely: @command{gzip},
8725 @command{bzip2}, @command{lzip}, @command{lzma}, @command{lzop},
8726 @command{xz} and traditional @command{compress}. The latter is
8727 supported mostly for backward compatibility, and we recommend
8728 against using it, because it is by far less effective than the other
8729 compression programs@footnote{It also had patent problems in the past.}.
8731 Creating a compressed archive is simple: you just specify a
8732 @dfn{compression option} along with the usual archive creation
8733 commands. The compression option is @option{-z} (@option{--gzip}) to
8734 create a @command{gzip} compressed archive, @option{-j}
8735 (@option{--bzip2}) to create a @command{bzip2} compressed archive,
8736 @option{--lzip} to create an @asis{lzip} compressed archive,
8737 @option{-J} (@option{--xz}) to create an @asis{XZ} archive,
8738 @option{--lzma} to create an @asis{LZMA} compressed
8739 archive, @option{--lzop} to create an @asis{LSOP} archive, and
8740 @option{-Z} (@option{--compress}) to use @command{compress} program.
8744 $ @kbd{tar czf archive.tar.gz .}
8747 You can also let @GNUTAR{} select the compression program based on
8748 the suffix of the archive file name. This is done using
8749 @option{--auto-compress} (@option{-a}) command line option. For
8750 example, the following invocation will use @command{bzip2} for
8754 $ @kbd{tar caf archive.tar.bz2 .}
8758 whereas the following one will use @command{lzma}:
8761 $ @kbd{tar caf archive.tar.lzma .}
8764 For a complete list of file name suffixes recognized by @GNUTAR{},
8765 see @ref{auto-compress}.
8767 Reading compressed archive is even simpler: you don't need to specify
8768 any additional options as @GNUTAR{} recognizes its format
8769 automatically. Thus, the following commands will list and extract the
8770 archive created in previous example:
8773 # List the compressed archive
8774 $ @kbd{tar tf archive.tar.gz}
8775 # Extract the compressed archive
8776 $ @kbd{tar xf archive.tar.gz}
8779 The format recognition algorithm is based on @dfn{signatures}, a
8780 special byte sequences in the beginning of file, that are specific for
8781 certain compression formats. If this approach fails, @command{tar}
8782 falls back to using archive name suffix to determine its format
8783 (@pxref{auto-compress}, for a list of recognized suffixes).
8785 @anchor{alternative decompression programs}
8786 @cindex alternative decompression programs
8787 Some compression programs are able to handle different compression
8788 formats. @GNUTAR{} uses this, if the principal decompressor for the
8789 given format is not available. For example, if @command{compress} is
8790 not installed, @command{tar} will try to use @command{gzip}. As of
8791 version @value{VERSION} the following alternatives are
8792 tried@footnote{To verbosely trace the decompressor selection, use the
8793 @option{--warning=decompress-program} option
8794 (@pxref{warnings,decompress-program}).}:
8796 @multitable @columnfractions 0.3 0.3 0.3
8797 @headitem Format @tab Main decompressor @tab Alternatives
8798 @item compress @tab compress @tab gzip
8799 @item lzma @tab lzma @tab xz
8800 @item bzip2 @tab bzip2 @tab lbzip2
8803 The only case when you have to specify a decompression option while
8804 reading the archive is when reading from a pipe or from a tape drive
8805 that does not support random access. However, in this case @GNUTAR{}
8806 will indicate which option you should use. For example:
8809 $ @kbd{cat archive.tar.gz | tar tf -}
8810 tar: Archive is compressed. Use -z option
8811 tar: Error is not recoverable: exiting now
8814 If you see such diagnostics, just add the suggested option to the
8815 invocation of @GNUTAR{}:
8818 $ @kbd{cat archive.tar.gz | tar tzf -}
8821 Notice also, that there are several restrictions on operations on
8822 compressed archives. First of all, compressed archives cannot be
8823 modified, i.e., you cannot update (@option{--update}, alias @option{-u})
8824 them or delete (@option{--delete}) members from them or
8825 add (@option{--append}, alias @option{-r}) members to them. Likewise, you
8826 cannot append another @command{tar} archive to a compressed archive using
8827 @option{--concatenate} (@option{-A}). Secondly, multi-volume
8828 archives cannot be compressed.
8830 The following options allow to select a particular compressor program:
8838 Filter the archive through @command{gzip}.
8843 Filter the archive through @code{xz}.
8847 Filter the archive through @code{bzip2}.
8851 Filter the archive through @command{lzip}.
8855 Filter the archive through @command{lzma}.
8859 Filter the archive through @command{lzop}.
8866 Filter the archive through @command{compress}.
8869 When any of these options is given, @GNUTAR{} searches the compressor
8870 binary in the current path and invokes it. The name of the compressor
8871 program is specified at compilation time using a corresponding
8872 @option{--with-@var{compname}} option to @command{configure}, e.g.
8873 @option{--with-bzip2} to select a specific @command{bzip2} binary.
8874 @xref{lbzip2}, for a detailed discussion.
8876 The output produced by @command{tar --help} shows the actual
8877 compressor names along with each of these options.
8879 You can use any of these options on physical devices (tape drives,
8880 etc.) and remote files as well as on normal files; data to or from
8881 such devices or remote files is reblocked by another copy of the
8882 @command{tar} program to enforce the specified (or default) record
8883 size. The default compression parameters are used. Most compression
8884 programs allow to override these by setting a program-specific
8885 environment variable. For example, when using @command{gzip} you can
8886 use @env{GZIP} as in the example below:
8889 $ @kbd{GZIP=--best tar czf archive.tar.gz subdir}
8893 Another way would be to use the @option{-I} option instead (see
8897 $ @kbd{tar -cf archive.tar.gz -I 'gzip --best' subdir}
8901 Finally, the third, traditional, way to achieve the same result is to
8905 $ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz}
8908 @cindex corrupted archives
8909 About corrupted compressed archives: compressed files have no
8910 redundancy, for maximum compression. The adaptive nature of the
8911 compression scheme means that the compression tables are implicitly
8912 spread all over the archive. If you lose a few blocks, the dynamic
8913 construction of the compression tables becomes unsynchronized, and there
8914 is little chance that you could recover later in the archive.
8916 Another compression options provide a better control over creating
8917 compressed archives. These are:
8920 @anchor{auto-compress}
8921 @opindex auto-compress
8922 @item --auto-compress
8924 Select a compression program to use by the archive file name
8925 suffix. The following suffixes are recognized:
8927 @multitable @columnfractions 0.3 0.6
8928 @headitem Suffix @tab Compression program
8929 @item @samp{.gz} @tab @command{gzip}
8930 @item @samp{.tgz} @tab @command{gzip}
8931 @item @samp{.taz} @tab @command{gzip}
8932 @item @samp{.Z} @tab @command{compress}
8933 @item @samp{.taZ} @tab @command{compress}
8934 @item @samp{.bz2} @tab @command{bzip2}
8935 @item @samp{.tz2} @tab @command{bzip2}
8936 @item @samp{.tbz2} @tab @command{bzip2}
8937 @item @samp{.tbz} @tab @command{bzip2}
8938 @item @samp{.lz} @tab @command{lzip}
8939 @item @samp{.lzma} @tab @command{lzma}
8940 @item @samp{.tlz} @tab @command{lzma}
8941 @item @samp{.lzo} @tab @command{lzop}
8942 @item @samp{.xz} @tab @command{xz}
8945 @opindex use-compress-program
8946 @item --use-compress-program=@var{prog}
8947 @itemx -I=@var{prog}
8948 Use external compression program @var{prog}. Use this option if you
8949 are not happy with the compression program associated with the suffix
8950 at compile time or if you have a compression program that @GNUTAR{}
8951 does not support. There are two requirements to which @var{prog}
8954 First, when called without options, it should read data from standard
8955 input, compress it and output it on standard output.
8957 Secondly, if called with @option{-d} argument, it should do exactly
8958 the opposite, i.e., read the compressed data from the standard input
8959 and produce uncompressed data on the standard output.
8962 @cindex gpg, using with tar
8963 @cindex gnupg, using with tar
8964 @cindex Using encrypted archives
8965 The @option{--use-compress-program} option, in particular, lets you
8966 implement your own filters, not necessarily dealing with
8967 compression/decompression. For example, suppose you wish to implement
8968 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
8969 gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
8970 Manual}). The following script does that:
8976 -d) gpg --decrypt - | gzip -d -c;;
8977 '') gzip -c | gpg -s;;
8978 *) echo "Unknown option $1">&2; exit 1;;
8983 Suppose you name it @file{gpgz} and save it somewhere in your
8984 @env{PATH}. Then the following command will create a compressed
8985 archive signed with your private key:
8988 $ @kbd{tar -cf foo.tar.gpgz -Igpgz .}
8992 Likewise, the command below will list its contents:
8995 $ @kbd{tar -tf foo.tar.gpgz -Igpgz .}
8999 The above is based on the following discussion:
9001 I have one question, or maybe it's a suggestion if there isn't a way
9002 to do it now. I would like to use @option{--gzip}, but I'd also like
9003 the output to be fed through a program like @acronym{GNU}
9004 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
9005 to use :-)), basically adding ECC protection on top of compression.
9006 It seems as if this should be quite easy to do, but I can't work out
9007 exactly how to go about it. Of course, I can pipe the standard output
9008 of @command{tar} through @command{ecc}, but then I lose (though I
9009 haven't started using it yet, I confess) the ability to have
9010 @command{tar} use @command{rmt} for it's I/O (I think).
9012 I think the most straightforward thing would be to let me specify a
9013 general set of filters outboard of compression (preferably ordered,
9014 so the order can be automatically reversed on input operations, and
9015 with the options they require specifiable), but beggars shouldn't be
9016 choosers and anything you decide on would be fine with me.
9018 By the way, I like @command{ecc} but if (as the comments say) it can't
9019 deal with loss of block sync, I'm tempted to throw some time at adding
9020 that capability. Supposing I were to actually do such a thing and
9021 get it (apparently) working, do you accept contributed changes to
9022 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
9024 Isn't that exactly the role of the
9025 @option{--use-compress-prog=@var{program}} option?
9026 I never tried it myself, but I suspect you may want to write a
9027 @var{prog} script or program able to filter stdin to stdout to
9028 way you want. It should recognize the @option{-d} option, for when
9029 extraction is needed rather than creation.
9031 It has been reported that if one writes compressed data (through the
9032 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
9033 the DLT compression mode, the data will actually get bigger and one will
9034 end up with less space on the tape.
9038 * lbzip2:: Using lbzip2 with @GNUTAR{}.
9042 @subsubsection Using lbzip2 with @GNUTAR{}.
9044 @cindex Laszlo Ersek
9045 @command{Lbzip2} is a multithreaded utility for handling
9046 @samp{bzip2} compression, written by Laszlo Ersek. It makes use of
9047 multiple processors to speed up its operation and in general works
9048 considerably faster than @command{bzip2}. For a detailed description
9049 of @command{lbzip2} see @uref{http://freshmeat.net/@/projects/@/lbzip2} and
9050 @uref{http://www.linuxinsight.com/@/lbzip2-parallel-bzip2-utility.html,
9051 lbzip2: parallel bzip2 utility}.
9053 Recent versions of @command{lbzip2} are mostly command line compatible
9054 with @command{bzip2}, which makes it possible to automatically invoke
9055 it via the @option{--bzip2} @GNUTAR{} command line option. To do so,
9056 @GNUTAR{} must be configured with the @option{--with-bzip2} command
9057 line option, like this:
9060 $ @kbd{./configure --with-bzip2=lbzip2 [@var{other-options}]}
9063 Once configured and compiled this way, @command{tar --help} will show the
9068 $ @kbd{tar --help | grep -- --bzip2}
9069 -j, --bzip2 filter the archive through lbzip2
9074 which means that running @command{tar --bzip2} will invoke @command{lbzip2}.
9077 @subsection Archiving Sparse Files
9078 @cindex Sparse Files
9080 Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
9081 in a file is a section of the file's contents which was never written.
9082 The contents of a hole reads as all zeros. On many operating systems,
9083 actual disk storage is not allocated for holes, but they are counted
9084 in the length of the file. If you archive such a file, @command{tar}
9085 could create an archive longer than the original. To have @command{tar}
9086 attempt to recognize the holes in a file, use @option{--sparse}
9087 (@option{-S}). When you use this option, then, for any file using
9088 less disk space than would be expected from its length, @command{tar}
9089 searches the file for consecutive stretches of zeros. It then records
9090 in the archive for the file where the consecutive stretches of zeros
9091 are, and only archives the ``real contents'' of the file. On
9092 extraction (using @option{--sparse} is not needed on extraction) any
9093 such files have holes created wherever the continuous stretches of zeros
9094 were found. Thus, if you use @option{--sparse}, @command{tar} archives
9095 won't take more space than the original.
9101 This option instructs @command{tar} to test each file for sparseness
9102 before attempting to archive it. If the file is found to be sparse it
9103 is treated specially, thus allowing to decrease the amount of space
9104 used by its image in the archive.
9106 This option is meaningful only when creating or updating archives. It
9107 has no effect on extraction.
9110 Consider using @option{--sparse} when performing file system backups,
9111 to avoid archiving the expanded forms of files stored sparsely in the
9114 Even if your system has no sparse files currently, some may be
9115 created in the future. If you use @option{--sparse} while making file
9116 system backups as a matter of course, you can be assured the archive
9117 will never take more space on the media than the files take on disk
9118 (otherwise, archiving a disk filled with sparse files might take
9119 hundreds of tapes). @xref{Incremental Dumps}.
9121 However, be aware that @option{--sparse} option presents a serious
9122 drawback. Namely, in order to determine if the file is sparse
9123 @command{tar} has to read it before trying to archive it, so in total
9124 the file is read @strong{twice}. So, always bear in mind that the
9125 time needed to process all files with this option is roughly twice
9126 the time needed to archive them without it.
9127 @FIXME{A technical note:
9129 Programs like @command{dump} do not have to read the entire file; by
9130 examining the file system directly, they can determine in advance
9131 exactly where the holes are and thus avoid reading through them. The
9132 only data it need read are the actual allocated data blocks.
9133 @GNUTAR{} uses a more portable and straightforward
9134 archiving approach, it would be fairly difficult that it does
9135 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
9139 What I did say is that you cannot tell the difference between a hole and an
9140 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
9141 best tells you how many holes there are; it doesn't tell you @emph{where}.
9142 Just as programs may, conceivably, care what @code{st_blocks} is (care
9143 to name one that does?), they may also care where the holes are (I have
9144 no examples of this one either, but it's equally imaginable).
9146 I conclude from this that good archivers are not portable. One can
9147 arguably conclude that if you want a portable program, you can in good
9148 conscience restore files with as many holes as possible, since you can't
9153 @cindex sparse formats, defined
9154 When using @samp{POSIX} archive format, @GNUTAR{} is able to store
9155 sparse files using in three distinct ways, called @dfn{sparse
9156 formats}. A sparse format is identified by its @dfn{number},
9157 consisting, as usual of two decimal numbers, delimited by a dot. By
9158 default, format @samp{1.0} is used. If, for some reason, you wish to
9159 use an earlier format, you can select it using
9160 @option{--sparse-version} option.
9163 @opindex sparse-version
9164 @item --sparse-version=@var{version}
9166 Select the format to store sparse files in. Valid @var{version} values
9167 are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
9168 for a detailed description of each format.
9171 Using @option{--sparse-format} option implies @option{--sparse}.
9174 @section Handling File Attributes
9175 @cindex atrributes, files
9176 @cindex file attributes
9178 When @command{tar} reads files, it updates their access times. To
9179 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
9180 reset the access time retroactively or avoid changing it in the first
9184 @opindex atime-preserve
9185 @item --atime-preserve
9186 @itemx --atime-preserve=replace
9187 @itemx --atime-preserve=system
9188 Preserve the access times of files that are read. This works only for
9189 files that you own, unless you have superuser privileges.
9191 @option{--atime-preserve=replace} works on most systems, but it also
9192 restores the data modification time and updates the status change
9193 time. Hence it doesn't interact with incremental dumps nicely
9194 (@pxref{Incremental Dumps}), and it can set access or data modification times
9195 incorrectly if other programs access the file while @command{tar} is
9198 @option{--atime-preserve=system} avoids changing the access time in
9199 the first place, if the operating system supports this.
9200 Unfortunately, this may or may not work on any given operating system
9201 or file system. If @command{tar} knows for sure it won't work, it
9202 complains right away.
9204 Currently @option{--atime-preserve} with no operand defaults to
9205 @option{--atime-preserve=replace}, but this is intended to change to
9206 @option{--atime-preserve=system} when the latter is better-supported.
9211 Do not extract data modification time.
9213 When this option is used, @command{tar} leaves the data modification times
9214 of the files it extracts as the times when the files were extracted,
9215 instead of setting it to the times recorded in the archive.
9217 This option is meaningless with @option{--list} (@option{-t}).
9221 Create extracted files with the same ownership they have in the
9224 This is the default behavior for the superuser,
9225 so this option is meaningful only for non-root users, when @command{tar}
9226 is executed on those systems able to give files away. This is
9227 considered as a security flaw by many people, at least because it
9228 makes quite difficult to correctly account users for the disk space
9229 they occupy. Also, the @code{suid} or @code{sgid} attributes of
9230 files are easily and silently lost when files are given away.
9232 When writing an archive, @command{tar} writes the user @acronym{ID} and user name
9233 separately. If it can't find a user name (because the user @acronym{ID} is not
9234 in @file{/etc/passwd}), then it does not write one. When restoring,
9235 it tries to look the name (if one was written) up in
9236 @file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
9237 the archive instead.
9239 @opindex no-same-owner
9240 @item --no-same-owner
9242 Do not attempt to restore ownership when extracting. This is the
9243 default behavior for ordinary users, so this option has an effect
9244 only for the superuser.
9246 @opindex numeric-owner
9247 @item --numeric-owner
9248 The @option{--numeric-owner} option allows (ANSI) archives to be written
9249 without user/group name information or such information to be ignored
9250 when extracting. It effectively disables the generation and/or use
9251 of user/group name information. This option forces extraction using
9252 the numeric ids from the archive, ignoring the names.
9254 This is useful in certain circumstances, when restoring a backup from
9255 an emergency floppy with different passwd/group files for example.
9256 It is otherwise impossible to extract files with the right ownerships
9257 if the password file in use during the extraction does not match the
9258 one belonging to the file system(s) being extracted. This occurs,
9259 for example, if you are restoring your files after a major crash and
9260 had booted from an emergency floppy with no password file or put your
9261 disk into another machine to do the restore.
9263 The numeric ids are @emph{always} saved into @command{tar} archives.
9264 The identifying names are added at create time when provided by the
9265 system, unless @option{--format=oldgnu} is used. Numeric ids could be
9266 used when moving archives between a collection of machines using
9267 a centralized management for attribution of numeric ids to users
9268 and groups. This is often made through using the NIS capabilities.
9270 When making a @command{tar} file for distribution to other sites, it
9271 is sometimes cleaner to use a single owner for all files in the
9272 distribution, and nicer to specify the write permission bits of the
9273 files as stored in the archive independently of their actual value on
9274 the file system. The way to prepare a clean distribution is usually
9275 to have some Makefile rule creating a directory, copying all needed
9276 files in that directory, then setting ownership and permissions as
9277 wanted (there are a lot of possible schemes), and only then making a
9278 @command{tar} archive out of this directory, before cleaning
9279 everything out. Of course, we could add a lot of options to
9280 @GNUTAR{} for fine tuning permissions and ownership.
9281 This is not the good way, I think. @GNUTAR{} is
9282 already crowded with options and moreover, the approach just explained
9283 gives you a great deal of control already.
9285 @xopindex{same-permissions, short description}
9286 @xopindex{preserve-permissions, short description}
9288 @itemx --same-permissions
9289 @itemx --preserve-permissions
9290 Extract all protection information.
9292 This option causes @command{tar} to set the modes (access permissions) of
9293 extracted files exactly as recorded in the archive. If this option
9294 is not used, the current @code{umask} setting limits the permissions
9295 on extracted files. This option is by default enabled when
9296 @command{tar} is executed by a superuser.
9299 This option is meaningless with @option{--list} (@option{-t}).
9303 Same as both @option{--same-permissions} and @option{--same-order}.
9305 This option is deprecated, and will be removed in @GNUTAR{} version 1.23.
9310 @section Making @command{tar} Archives More Portable
9312 Creating a @command{tar} archive on a particular system that is meant to be
9313 useful later on many other machines and with other versions of @command{tar}
9314 is more challenging than you might think. @command{tar} archive formats
9315 have been evolving since the first versions of Unix. Many such formats
9316 are around, and are not always compatible with each other. This section
9317 discusses a few problems, and gives some advice about making @command{tar}
9318 archives more portable.
9320 One golden rule is simplicity. For example, limit your @command{tar}
9321 archives to contain only regular files and directories, avoiding
9322 other kind of special files. Do not attempt to save sparse files or
9323 contiguous files as such. Let's discuss a few more problems, in turn.
9325 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
9326 archives and archive labels) in GNU and PAX formats.}
9329 * Portable Names:: Portable Names
9330 * dereference:: Symbolic Links
9331 * hard links:: Hard Links
9332 * old:: Old V7 Archives
9333 * ustar:: Ustar Archives
9334 * gnu:: GNU and old GNU format archives.
9335 * posix:: @acronym{POSIX} archives
9336 * Checksumming:: Checksumming Problems
9337 * Large or Negative Values:: Large files, negative time stamps, etc.
9338 * Other Tars:: How to Extract GNU-Specific Data Using
9339 Other @command{tar} Implementations
9342 @node Portable Names
9343 @subsection Portable Names
9345 Use portable file and member names. A name is portable if it contains
9346 only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
9347 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
9348 contain @samp{/-}. Avoid deep directory nesting. For portability to
9349 old Unix hosts, limit your file name components to 14 characters or
9352 If you intend to have your @command{tar} archives to be read under
9353 MSDOS, you should not rely on case distinction for file names, and you
9354 might use the @acronym{GNU} @command{doschk} program for helping you
9355 further diagnosing illegal MSDOS names, which are even more limited
9359 @subsection Symbolic Links
9360 @cindex File names, using symbolic links
9361 @cindex Symbolic link as file name
9363 @opindex dereference
9364 Normally, when @command{tar} archives a symbolic link, it writes a
9365 block to the archive naming the target of the link. In that way, the
9366 @command{tar} archive is a faithful record of the file system contents.
9367 When @option{--dereference} (@option{-h}) is used with
9368 @option{--create} (@option{-c}), @command{tar} archives the files
9369 symbolic links point to, instead of
9370 the links themselves.
9372 When creating portable archives, use @option{--dereference}
9373 (@option{-h}): some systems do not support
9374 symbolic links, and moreover, your distribution might be unusable if
9375 it contains unresolved symbolic links.
9377 When reading from an archive, the @option{--dereference} (@option{-h})
9378 option causes @command{tar} to follow an already-existing symbolic
9379 link when @command{tar} writes or reads a file named in the archive.
9380 Ordinarily, @command{tar} does not follow such a link, though it may
9381 remove the link before writing a new file. @xref{Dealing with Old
9384 The @option{--dereference} option is unsafe if an untrusted user can
9385 modify directories while @command{tar} is running. @xref{Security}.
9388 @subsection Hard Links
9389 @cindex File names, using hard links
9390 @cindex hard links, dereferencing
9391 @cindex dereferencing hard links
9393 Normally, when @command{tar} archives a hard link, it writes a
9394 block to the archive naming the target of the link (a @samp{1} type
9395 block). In that way, the actual file contents is stored in file only
9396 once. For example, consider the following two files:
9401 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
9402 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
9406 Here, @file{jeden} is a link to @file{one}. When archiving this
9407 directory with a verbose level 2, you will get an output similar to
9411 $ tar cvvf ../archive.tar .
9412 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9413 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9414 hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
9417 The last line shows that, instead of storing two copies of the file,
9418 @command{tar} stored it only once, under the name @file{jeden}, and
9419 stored file @file{one} as a hard link to this file.
9421 It may be important to know that all hard links to the given file are
9422 stored in the archive. For example, this may be necessary for exact
9423 reproduction of the file system. The following option does that:
9426 @xopindex{check-links, described}
9429 Check the number of links dumped for each processed file. If this
9430 number does not match the total number of hard links for the file, print
9434 For example, trying to archive only file @file{jeden} with this option
9435 produces the following diagnostics:
9438 $ tar -c -f ../archive.tar -l jeden
9439 tar: Missing links to `jeden'.
9442 Although creating special records for hard links helps keep a faithful
9443 record of the file system contents and makes archives more compact, it
9444 may present some difficulties when extracting individual members from
9445 the archive. For example, trying to extract file @file{one} from the
9446 archive created in previous examples produces, in the absense of file
9450 $ tar xf archive.tar ./one
9451 tar: ./one: Cannot hard link to `./jeden': No such file or directory
9452 tar: Error exit delayed from previous errors
9455 The reason for this behavior is that @command{tar} cannot seek back in
9456 the archive to the previous member (in this case, @file{one}), to
9457 extract it@footnote{There are plans to fix this in future releases.}.
9458 If you wish to avoid such problems at the cost of a bigger archive,
9459 use the following option:
9462 @xopindex{hard-dereference, described}
9463 @item --hard-dereference
9464 Dereference hard links and store the files they refer to.
9467 For example, trying this option on our two sample files, we get two
9468 copies in the archive, each of which can then be extracted
9469 independently of the other:
9473 $ tar -c -vv -f ../archive.tar --hard-dereference .
9474 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9475 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9476 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
9481 @subsection Old V7 Archives
9482 @cindex Format, old style
9483 @cindex Old style format
9484 @cindex Old style archives
9485 @cindex v7 archive format
9487 Certain old versions of @command{tar} cannot handle additional
9488 information recorded by newer @command{tar} programs. To create an
9489 archive in V7 format (not ANSI), which can be read by these old
9490 versions, specify the @option{--format=v7} option in
9491 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
9492 accepts @option{--portability} or @option{--old-archive} for this
9493 option). When you specify it,
9494 @command{tar} leaves out information about directories, pipes, fifos,
9495 contiguous files, and device files, and specifies file ownership by
9496 group and user IDs instead of group and user names.
9498 When updating an archive, do not use @option{--format=v7}
9499 unless the archive was created using this option.
9501 In most cases, a @emph{new} format archive can be read by an @emph{old}
9502 @command{tar} program without serious trouble, so this option should
9503 seldom be needed. On the other hand, most modern @command{tar}s are
9504 able to read old format archives, so it might be safer for you to
9505 always use @option{--format=v7} for your distributions. Notice,
9506 however, that @samp{ustar} format is a better alternative, as it is
9507 free from many of @samp{v7}'s drawbacks.
9510 @subsection Ustar Archive Format
9512 @cindex ustar archive format
9513 Archive format defined by @acronym{POSIX}.1-1988 specification is called
9514 @code{ustar}. Although it is more flexible than the V7 format, it
9515 still has many restrictions (@pxref{Formats,ustar}, for the detailed
9516 description of @code{ustar} format). Along with V7 format,
9517 @code{ustar} format is a good choice for archives intended to be read
9518 with other implementations of @command{tar}.
9520 To create archive in @code{ustar} format, use @option{--format=ustar}
9521 option in conjunction with the @option{--create} (@option{-c}).
9524 @subsection @acronym{GNU} and old @GNUTAR{} format
9526 @cindex GNU archive format
9527 @cindex Old GNU archive format
9528 @GNUTAR{} was based on an early draft of the
9529 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
9530 @command{tar}, such as the support for file names longer than 100
9531 characters, use portions of the @command{tar} header record which were
9532 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
9533 @acronym{POSIX} have allocated the same parts of the header record for
9534 other purposes. As a result, @GNUTAR{} format is
9535 incompatible with the current @acronym{POSIX} specification, and with
9536 @command{tar} programs that follow it.
9538 In the majority of cases, @command{tar} will be configured to create
9539 this format by default. This will change in future releases, since
9540 we plan to make @samp{POSIX} format the default.
9542 To force creation a @GNUTAR{} archive, use option
9543 @option{--format=gnu}.
9546 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
9548 @cindex POSIX archive format
9549 @cindex PAX archive format
9550 Starting from version 1.14 @GNUTAR{} features full support for
9551 @acronym{POSIX.1-2001} archives.
9553 A @acronym{POSIX} conformant archive will be created if @command{tar}
9554 was given @option{--format=posix} (@option{--format=pax}) option. No
9555 special option is required to read and extract from a @acronym{POSIX}
9559 * PAX keywords:: Controlling Extended Header Keywords.
9563 @subsubsection Controlling Extended Header Keywords
9567 @item --pax-option=@var{keyword-list}
9568 Handle keywords in @acronym{PAX} extended headers. This option is
9569 equivalent to @option{-o} option of the @command{pax} utility.
9572 @var{Keyword-list} is a comma-separated
9573 list of keyword options, each keyword option taking one of
9574 the following forms:
9577 @item delete=@var{pattern}
9578 When used with one of archive-creation commands,
9579 this option instructs @command{tar} to omit from extended header records
9580 that it produces any keywords matching the string @var{pattern}.
9582 When used in extract or list mode, this option instructs tar
9583 to ignore any keywords matching the given @var{pattern} in the extended
9584 header records. In both cases, matching is performed using the pattern
9585 matching notation described in @acronym{POSIX 1003.2}, 3.13
9586 (@pxref{wildcards}). For example:
9589 --pax-option delete=security.*
9592 would suppress security-related information.
9594 @item exthdr.name=@var{string}
9596 This keyword allows user control over the name that is written into the
9597 ustar header blocks for the extended headers. The name is obtained
9598 from @var{string} after making the following substitutions:
9600 @multitable @columnfractions .25 .55
9601 @headitem Meta-character @tab Replaced By
9602 @item %d @tab The directory name of the file, equivalent to the
9603 result of the @command{dirname} utility on the translated file name.
9604 @item %f @tab The name of the file with the directory information
9605 stripped, equivalent to the result of the @command{basename} utility
9606 on the translated file name.
9607 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9608 @item %% @tab A @samp{%} character.
9611 Any other @samp{%} characters in @var{string} produce undefined
9614 If no option @samp{exthdr.name=string} is specified, @command{tar}
9615 will use the following default value:
9621 @item exthdr.mtime=@var{value}
9623 This keyword defines the value of the @samp{mtime} field that
9624 is written into the ustar header blocks for the extended headers.
9625 By default, the @samp{mtime} field is set to the modification time
9626 of the archive member described by that extended headers.
9628 @item globexthdr.name=@var{string}
9629 This keyword allows user control over the name that is written into
9630 the ustar header blocks for global extended header records. The name
9631 is obtained from the contents of @var{string}, after making
9632 the following substitutions:
9634 @multitable @columnfractions .25 .55
9635 @headitem Meta-character @tab Replaced By
9636 @item %n @tab An integer that represents the
9637 sequence number of the global extended header record in the archive,
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 results.
9645 If no option @samp{globexthdr.name=string} is specified, @command{tar}
9646 will use the following default value:
9649 $TMPDIR/GlobalHead.%p.%n
9653 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
9654 environment variable. If @var{TMPDIR} is not set, @command{tar}
9657 @item globexthdr.mtime=@var{value}
9659 This keyword defines the value of the @samp{mtime} field that
9660 is written into the ustar header blocks for the global extended headers.
9661 By default, the @samp{mtime} field is set to the time when
9662 @command{tar} was invoked.
9664 @item @var{keyword}=@var{value}
9665 When used with one of archive-creation commands, these keyword/value pairs
9666 will be included at the beginning of the archive in a global extended
9667 header record. When used with one of archive-reading commands,
9668 @command{tar} will behave as if it has encountered these keyword/value
9669 pairs at the beginning of the archive in a global extended header
9672 @item @var{keyword}:=@var{value}
9673 When used with one of archive-creation commands, these keyword/value pairs
9674 will be included as records at the beginning of an extended header for
9675 each file. This is effectively equivalent to @var{keyword}=@var{value}
9676 form except that it creates no global extended header records.
9678 When used with one of archive-reading commands, @command{tar} will
9679 behave as if these keyword/value pairs were included as records at the
9680 end of each extended header; thus, they will override any global or
9681 file-specific extended header record keywords of the same names.
9682 For example, in the command:
9685 tar --format=posix --create \
9686 --file archive --pax-option gname:=user .
9689 the group name will be forced to a new value for all files
9690 stored in the archive.
9693 In any of the forms described above, the @var{value} may be
9694 a string enclosed in curly braces. In that case, the string
9695 between the braces is understood either as a textual time
9696 representation, as described in @ref{Date input formats}, or a name of
9697 the existing file, starting with @samp{/} or @samp{.}. In the latter
9698 case, the modification time of that file is used.
9700 For example, to set all modification times to the current date, you
9701 use the following option:
9704 --pax-option='mtime:=@{now@}'
9707 Note quoting of the option's argument.
9709 @cindex archives, binary equivalent
9710 @cindex binary equivalent archives, creating
9711 As another example, here is the option that ensures that any two
9712 archives created using it, will be binary equivalent if they have the
9716 --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0
9720 @subsection Checksumming Problems
9722 SunOS and HP-UX @command{tar} fail to accept archives created using
9723 @GNUTAR{} and containing non-@acronym{ASCII} file names, that
9724 is, file names having characters with the eight bit set, because they
9725 use signed checksums, while @GNUTAR{} uses unsigned
9726 checksums while creating archives, as per @acronym{POSIX} standards. On
9727 reading, @GNUTAR{} computes both checksums and
9728 accepts any. It is somewhat worrying that a lot of people may go
9729 around doing backup of their files using faulty (or at least
9730 non-standard) software, not learning about it until it's time to
9731 restore their missing files with an incompatible file extractor, or
9734 @GNUTAR{} computes checksums both ways, and accept
9735 any on read, so @acronym{GNU} tar can read Sun tapes even with their
9736 wrong checksums. @GNUTAR{} produces the standard
9737 checksum, however, raising incompatibilities with Sun. That is to
9738 say, @GNUTAR{} has not been modified to
9739 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
9740 I've been told that more recent Sun @command{tar} now read standard
9741 archives, so maybe Sun did a similar patch, after all?
9743 The story seems to be that when Sun first imported @command{tar}
9744 sources on their system, they recompiled it without realizing that
9745 the checksums were computed differently, because of a change in
9746 the default signing of @code{char}'s in their compiler. So they
9747 started computing checksums wrongly. When they later realized their
9748 mistake, they merely decided to stay compatible with it, and with
9749 themselves afterwards. Presumably, but I do not really know, HP-UX
9750 has chosen that their @command{tar} archives to be compatible with Sun's.
9751 The current standards do not favor Sun @command{tar} format. In any
9752 case, it now falls on the shoulders of SunOS and HP-UX users to get
9753 a @command{tar} able to read the good archives they receive.
9755 @node Large or Negative Values
9756 @subsection Large or Negative Values
9757 @cindex large values
9758 @cindex future time stamps
9759 @cindex negative time stamps
9762 The above sections suggest to use @samp{oldest possible} archive
9763 format if in doubt. However, sometimes it is not possible. If you
9764 attempt to archive a file whose metadata cannot be represented using
9765 required format, @GNUTAR{} will print error message and ignore such a
9766 file. You will than have to switch to a format that is able to
9767 handle such values. The format summary table (@pxref{Formats}) will
9770 In particular, when trying to archive files larger than 8GB or with
9771 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
9772 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
9773 @acronym{POSIX} archive formats. When considering which format to
9774 choose, bear in mind that the @acronym{GNU} format uses
9775 two's-complement base-256 notation to store values that do not fit
9776 into standard @acronym{ustar} range. Such archives can generally be
9777 read only by a @GNUTAR{} implementation. Moreover, they sometimes
9778 cannot be correctly restored on another hosts even by @GNUTAR{}. For
9779 example, using two's complement representation for negative time
9780 stamps that assumes a signed 32-bit @code{time_t} generates archives
9781 that are not portable to hosts with differing @code{time_t}
9784 On the other hand, @acronym{POSIX} archives, generally speaking, can
9785 be extracted by any tar implementation that understands older
9786 @acronym{ustar} format. The only exception are files larger than 8GB.
9788 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
9792 @subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations
9794 In previous sections you became acquainted with various quirks
9795 necessary to make your archives portable. Sometimes you may need to
9796 extract archives containing GNU-specific members using some
9797 third-party @command{tar} implementation or an older version of
9798 @GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed,
9799 but if it is for some reason impossible, this section will explain
9800 how to cope without it.
9802 When we speak about @dfn{GNU-specific} members we mean two classes of
9803 them: members split between the volumes of a multi-volume archive and
9804 sparse members. You will be able to always recover such members if
9805 the archive is in PAX format. In addition split members can be
9806 recovered from archives in old GNU format. The following subsections
9807 describe the required procedures in detail.
9810 * Split Recovery:: Members Split Between Volumes
9811 * Sparse Recovery:: Sparse Members
9814 @node Split Recovery
9815 @subsubsection Extracting Members Split Between Volumes
9817 @cindex Mutli-volume archives, extracting using non-GNU tars
9818 If a member is split between several volumes of an old GNU format archive
9819 most third party @command{tar} implementation will fail to extract
9820 it. To extract it, use @command{tarcat} program (@pxref{Tarcat}).
9821 This program is available from
9822 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat.html, @GNUTAR{}
9823 home page}. It concatenates several archive volumes into a single
9824 valid archive. For example, if you have three volumes named from
9825 @file{vol-1.tar} to @file{vol-3.tar}, you can do the following to
9826 extract them using a third-party @command{tar}:
9829 $ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -}
9832 @cindex Mutli-volume archives in PAX format, extracting using non-GNU tars
9833 You could use this approach for most (although not all) PAX
9834 format archives as well. However, extracting split members from a PAX
9835 archive is a much easier task, because PAX volumes are constructed in
9836 such a way that each part of a split member is extracted to a
9837 different file by @command{tar} implementations that are not aware of
9838 GNU extensions. More specifically, the very first part retains its
9839 original name, and all subsequent parts are named using the pattern:
9842 %d/GNUFileParts.%p/%f.%n
9846 where symbols preceeded by @samp{%} are @dfn{macro characters} that
9847 have the following meaning:
9849 @multitable @columnfractions .25 .55
9850 @headitem Meta-character @tab Replaced By
9851 @item %d @tab The directory name of the file, equivalent to the
9852 result of the @command{dirname} utility on its full name.
9853 @item %f @tab The file name of the file, equivalent to the result
9854 of the @command{basename} utility on its full name.
9855 @item %p @tab The process @acronym{ID} of the @command{tar} process that
9856 created the archive.
9857 @item %n @tab Ordinal number of this particular part.
9860 For example, if the file @file{var/longfile} was split during archive
9861 creation between three volumes, and the creator @command{tar} process
9862 had process @acronym{ID} @samp{27962}, then the member names will be:
9866 var/GNUFileParts.27962/longfile.1
9867 var/GNUFileParts.27962/longfile.2
9870 When you extract your archive using a third-party @command{tar}, these
9871 files will be created on your disk, and the only thing you will need
9872 to do to restore your file in its original form is concatenate them in
9873 the proper order, for example:
9878 $ @kbd{cat GNUFileParts.27962/longfile.1 \
9879 GNUFileParts.27962/longfile.2 >> longfile}
9880 $ rm -f GNUFileParts.27962
9884 Notice, that if the @command{tar} implementation you use supports PAX
9885 format archives, it will probably emit warnings about unknown keywords
9886 during extraction. They will look like this:
9891 Unknown extended header keyword 'GNU.volume.filename' ignored.
9892 Unknown extended header keyword 'GNU.volume.size' ignored.
9893 Unknown extended header keyword 'GNU.volume.offset' ignored.
9898 You can safely ignore these warnings.
9900 If your @command{tar} implementation is not PAX-aware, you will get
9901 more warnings and more files generated on your disk, e.g.:
9905 $ @kbd{tar xf vol-1.tar}
9906 var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
9908 Unexpected EOF in archive
9909 $ @kbd{tar xf vol-2.tar}
9910 tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
9911 GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
9912 'x', extracted as normal file
9916 Ignore these warnings. The @file{PaxHeaders.*} directories created
9917 will contain files with @dfn{extended header keywords} describing the
9918 extracted files. You can delete them, unless they describe sparse
9919 members. Read further to learn more about them.
9921 @node Sparse Recovery
9922 @subsubsection Extracting Sparse Members
9924 @cindex sparse files, extracting with non-GNU tars
9925 Any @command{tar} implementation will be able to extract sparse members from a
9926 PAX archive. However, the extracted files will be @dfn{condensed},
9927 i.e., any zero blocks will be removed from them. When we restore such
9928 a condensed file to its original form, by adding zero blocks (or
9929 @dfn{holes}) back to their original locations, we call this process
9930 @dfn{expanding} a compressed sparse file.
9933 To expand a file, you will need a simple auxiliary program called
9934 @command{xsparse}. It is available in source form from
9935 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse.html, @GNUTAR{}
9938 @cindex sparse files v.1.0, extracting with non-GNU tars
9939 Let's begin with archive members in @dfn{sparse format
9940 version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand.
9941 The condensed file will contain both file map and file data, so no
9942 additional data will be needed to restore it. If the original file
9943 name was @file{@var{dir}/@var{name}}, then the condensed file will be
9944 named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where
9945 @var{n} is a decimal number@footnote{Technically speaking, @var{n} is a
9946 @dfn{process @acronym{ID}} of the @command{tar} process which created the
9947 archive (@pxref{PAX keywords}).}.
9949 To expand a version 1.0 file, run @command{xsparse} as follows:
9952 $ @kbd{xsparse @file{cond-file}}
9956 where @file{cond-file} is the name of the condensed file. The utility
9957 will deduce the name for the resulting expanded file using the
9958 following algorithm:
9961 @item If @file{cond-file} does not contain any directories,
9962 @file{../cond-file} will be used;
9964 @item If @file{cond-file} has the form
9965 @file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name}
9966 are simple names, with no @samp{/} characters in them, the output file
9967 name will be @file{@var{dir}/@var{name}}.
9969 @item Otherwise, if @file{cond-file} has the form
9970 @file{@var{dir}/@var{name}}, the output file name will be
9974 In the unlikely case when this algorithm does not suit your needs,
9975 you can explicitly specify output file name as a second argument to
9979 $ @kbd{xsparse @file{cond-file} @file{out-file}}
9982 It is often a good idea to run @command{xsparse} in @dfn{dry run} mode
9983 first. In this mode, the command does not actually expand the file,
9984 but verbosely lists all actions it would be taking to do so. The dry
9985 run mode is enabled by @option{-n} command line argument:
9989 $ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile}
9990 Reading v.1.0 sparse map
9991 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
9992 `/home/gray/sparsefile'
9997 To actually expand the file, you would run:
10000 $ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile}
10004 The program behaves the same way all UNIX utilities do: it will keep
10005 quiet unless it has simething important to tell you (e.g. an error
10006 condition or something). If you wish it to produce verbose output,
10007 similar to that from the dry run mode, use @option{-v} option:
10011 $ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile}
10012 Reading v.1.0 sparse map
10013 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
10014 `/home/gray/sparsefile'
10019 Additionally, if your @command{tar} implementation has extracted the
10020 @dfn{extended headers} for this file, you can instruct @command{xstar}
10021 to use them in order to verify the integrity of the expanded file.
10022 The option @option{-x} sets the name of the extended header file to
10023 use. Continuing our example:
10027 $ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
10028 /home/gray/GNUSparseFile.6058/sparsefile}
10029 Reading extended header file
10030 Found variable GNU.sparse.major = 1
10031 Found variable GNU.sparse.minor = 0
10032 Found variable GNU.sparse.name = sparsefile
10033 Found variable GNU.sparse.realsize = 217481216
10034 Reading v.1.0 sparse map
10035 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
10036 `/home/gray/sparsefile'
10041 @anchor{extracting sparse v.0.x}
10042 @cindex sparse files v.0.1, extracting with non-GNU tars
10043 @cindex sparse files v.0.0, extracting with non-GNU tars
10044 An @dfn{extended header} is a special @command{tar} archive header
10045 that precedes an archive member and contains a set of
10046 @dfn{variables}, describing the member properties that cannot be
10047 stored in the standard @code{ustar} header. While optional for
10048 expanding sparse version 1.0 members, the use of extended headers is
10049 mandatory when expanding sparse members in older sparse formats: v.0.0
10050 and v.0.1 (The sparse formats are described in detail in @ref{Sparse
10051 Formats}.) So, for these formats, the question is: how to obtain
10052 extended headers from the archive?
10054 If you use a @command{tar} implementation that does not support PAX
10055 format, extended headers for each member will be extracted as a
10056 separate file. If we represent the member name as
10057 @file{@var{dir}/@var{name}}, then the extended header file will be
10058 named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where
10059 @var{n} is an integer number.
10061 Things become more difficult if your @command{tar} implementation
10062 does support PAX headers, because in this case you will have to
10063 manually extract the headers. We recommend the following algorithm:
10067 Consult the documentation of your @command{tar} implementation for an
10068 option that prints @dfn{block numbers} along with the archive
10069 listing (analogous to @GNUTAR{}'s @option{-R} option). For example,
10070 @command{star} has @option{-block-number}.
10073 Obtain verbose listing using the @samp{block number} option, and
10074 find block numbers of the sparse member in question and the member
10075 immediately following it. For example, running @command{star} on our
10080 $ @kbd{star -t -v -block-number -f arc.tar}
10082 star: Unknown extended header keyword 'GNU.sparse.size' ignored.
10083 star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
10084 star: Unknown extended header keyword 'GNU.sparse.name' ignored.
10085 star: Unknown extended header keyword 'GNU.sparse.map' ignored.
10086 block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
10087 block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
10093 (as usual, ignore the warnings about unknown keywords.)
10096 Let @var{size} be the size of the sparse member, @var{Bs} be its block number
10097 and @var{Bn} be the block number of the next member.
10101 @var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2
10105 This number gives the size of the extended header part in tar @dfn{blocks}.
10106 In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2
10110 Use @command{dd} to extract the headers:
10113 @kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}}
10117 where @var{archive} is the archive name, @var{hname} is a name of the
10118 file to store the extended header in, @var{Bs} and @var{N} are
10119 computed in previous steps.
10121 In our example, this command will be
10124 $ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7}
10128 Finally, you can expand the condensed file, using the obtained header:
10132 $ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile}
10133 Reading extended header file
10134 Found variable GNU.sparse.size = 217481216
10135 Found variable GNU.sparse.numblocks = 208
10136 Found variable GNU.sparse.name = sparsefile
10137 Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{}
10138 Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile'
10144 @section Comparison of @command{tar} and @command{cpio}
10147 @FIXME{Reorganize the following material}
10149 The @command{cpio} archive formats, like @command{tar}, do have maximum
10150 file name lengths. The binary and old @acronym{ASCII} formats have a maximum file
10151 length of 256, and the new @acronym{ASCII} and @acronym{CRC ASCII} formats have a max
10152 file length of 1024. @acronym{GNU} @command{cpio} can read and write archives
10153 with arbitrary file name lengths, but other @command{cpio} implementations
10154 may crash unexplainedly trying to read them.
10156 @command{tar} handles symbolic links in the form in which it comes in @acronym{BSD};
10157 @command{cpio} doesn't handle symbolic links in the form in which it comes
10158 in System V prior to SVR4, and some vendors may have added symlinks
10159 to their system without enhancing @command{cpio} to know about them.
10160 Others may have enhanced it in a way other than the way I did it
10161 at Sun, and which was adopted by AT&T (and which is, I think, also
10162 present in the @command{cpio} that Berkeley picked up from AT&T and put
10163 into a later @acronym{BSD} release---I think I gave them my changes).
10165 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
10166 can handle @command{tar} format input, and write it on output, and it
10167 probably handles symbolic links. They may not have bothered doing
10168 anything to enhance @command{tar} as a result.)
10170 @command{cpio} handles special files; traditional @command{tar} doesn't.
10172 @command{tar} comes with V7, System III, System V, and @acronym{BSD} source;
10173 @command{cpio} comes only with System III, System V, and later @acronym{BSD}
10174 (4.3-tahoe and later).
10176 @command{tar}'s way of handling multiple hard links to a file can handle
10177 file systems that support 32-bit i-numbers (e.g., the @acronym{BSD} file system);
10178 @command{cpio}s way requires you to play some games (in its ``binary''
10179 format, i-numbers are only 16 bits, and in its ``portable @acronym{ASCII}'' format,
10180 they're 18 bits---it would have to play games with the "file system @acronym{ID}"
10181 field of the header to make sure that the file system @acronym{ID}/i-number pairs
10182 of different files were always different), and I don't know which
10183 @command{cpio}s, if any, play those games. Those that don't might get
10184 confused and think two files are the same file when they're not, and
10185 make hard links between them.
10187 @command{tar}s way of handling multiple hard links to a file places only
10188 one copy of the link on the tape, but the name attached to that copy
10189 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
10190 way puts one copy for every link, but you can retrieve it using any
10194 What type of check sum (if any) is used, and how is this calculated.
10197 See the attached manual pages for @command{tar} and @command{cpio} format.
10198 @command{tar} uses a checksum which is the sum of all the bytes in the
10199 @command{tar} header for a file; @command{cpio} uses no checksum.
10202 If anyone knows why @command{cpio} was made when @command{tar} was present
10206 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
10207 generally-available version of UNIX had @command{tar} at the time. I don't
10208 know whether any version that was generally available @emph{within AT&T}
10209 had @command{tar}, or, if so, whether the people within AT&T who did
10210 @command{cpio} knew about it.
10212 On restore, if there is a corruption on a tape @command{tar} will stop at
10213 that point, while @command{cpio} will skip over it and try to restore the
10216 The main difference is just in the command syntax and header format.
10218 @command{tar} is a little more tape-oriented in that everything is blocked
10219 to start on a record boundary.
10222 Is there any differences between the ability to recover crashed
10223 archives between the two of them. (Is there any chance of recovering
10224 crashed archives at all.)
10227 Theoretically it should be easier under @command{tar} since the blocking
10228 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
10229 However, modern @command{cpio}'s and variations have an option to just
10230 search for the next file header after an error with a reasonable chance
10231 of resyncing. However, lots of tape driver software won't allow you to
10232 continue past a media error which should be the only reason for getting
10233 out of sync unless a file changed sizes while you were writing the
10237 If anyone knows why @command{cpio} was made when @command{tar} was present
10238 at the unix scene, please tell me about this too.
10241 Probably because it is more media efficient (by not blocking everything
10242 and using only the space needed for the headers where @command{tar}
10243 always uses 512 bytes per file header) and it knows how to archive
10246 You might want to look at the freely available alternatives. The
10247 major ones are @command{afio}, @GNUTAR{}, and
10248 @command{pax}, each of which have their own extensions with some
10249 backwards compatibility.
10251 Sparse files were @command{tar}red as sparse files (which you can
10252 easily test, because the resulting archive gets smaller, and
10253 @acronym{GNU} @command{cpio} can no longer read it).
10256 @chapter Tapes and Other Archive Media
10259 A few special cases about tape handling warrant more detailed
10260 description. These special cases are discussed below.
10262 Many complexities surround the use of @command{tar} on tape drives. Since
10263 the creation and manipulation of archives located on magnetic tape was
10264 the original purpose of @command{tar}, it contains many features making
10265 such manipulation easier.
10267 Archives are usually written on dismountable media---tape cartridges,
10268 mag tapes, or floppy disks.
10270 The amount of data a tape or disk holds depends not only on its size,
10271 but also on how it is formatted. A 2400 foot long reel of mag tape
10272 holds 40 megabytes of data when formatted at 1600 bits per inch. The
10273 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
10275 Magnetic media are re-usable---once the archive on a tape is no longer
10276 needed, the archive can be erased and the tape or disk used over.
10277 Media quality does deteriorate with use, however. Most tapes or disks
10278 should be discarded when they begin to produce data errors. EXABYTE
10279 tape cartridges should be discarded when they generate an @dfn{error
10280 count} (number of non-usable bits) of more than 10k.
10282 Magnetic media are written and erased using magnetic fields, and
10283 should be protected from such fields to avoid damage to stored data.
10284 Sticking a floppy disk to a filing cabinet using a magnet is probably
10288 * Device:: Device selection and switching
10289 * Remote Tape Server::
10290 * Common Problems and Solutions::
10291 * Blocking:: Blocking
10292 * Many:: Many archives on one tape
10293 * Using Multiple Tapes:: Using Multiple Tapes
10294 * label:: Including a Label in the Archive
10296 * Write Protection::
10300 @section Device Selection and Switching
10304 @item -f [@var{hostname}:]@var{file}
10305 @itemx --file=[@var{hostname}:]@var{file}
10306 Use archive file or device @var{file} on @var{hostname}.
10309 This option is used to specify the file name of the archive @command{tar}
10312 If the file name is @samp{-}, @command{tar} reads the archive from standard
10313 input (when listing or extracting), or writes it to standard output
10314 (when creating). If the @samp{-} file name is given when updating an
10315 archive, @command{tar} will read the original archive from its standard
10316 input, and will write the entire new archive to its standard output.
10318 If the file name contains a @samp{:}, it is interpreted as
10319 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
10320 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
10321 either case, @command{tar} will invoke the command @command{rsh} (or
10322 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
10323 machine. If you give an alternate login name, it will be given to the
10325 Naturally, the remote machine must have an executable
10326 @command{/usr/libexec/rmt}. This program is free software from the
10327 University of California, and a copy of the source code can be found
10328 with the sources for @command{tar}; it's compiled and installed by default.
10329 The exact path to this utility is determined when configuring the package.
10330 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
10331 your installation prefix. This location may also be overridden at
10332 runtime by using the @option{--rmt-command=@var{command}} option (@xref{Option Summary,
10333 ---rmt-command}, for detailed description of this option. @xref{Remote
10334 Tape Server}, for the description of @command{rmt} command).
10336 If this option is not given, but the environment variable @env{TAPE}
10337 is set, its value is used; otherwise, old versions of @command{tar}
10338 used a default archive name (which was picked when @command{tar} was
10339 compiled). The default is normally set up to be the @dfn{first} tape
10340 drive or other transportable I/O medium on the system.
10342 Starting with version 1.11.5, @GNUTAR{} uses
10343 standard input and standard output as the default device, and I will
10344 not try anymore supporting automatic device detection at installation
10345 time. This was failing really in too many cases, it was hopeless.
10346 This is now completely left to the installer to override standard
10347 input and standard output for default device, if this seems
10348 preferable. Further, I think @emph{most} actual usages of
10349 @command{tar} are done with pipes or disks, not really tapes,
10350 cartridges or diskettes.
10352 Some users think that using standard input and output is running
10353 after trouble. This could lead to a nasty surprise on your screen if
10354 you forget to specify an output file name---especially if you are going
10355 through a network or terminal server capable of buffering large amounts
10356 of output. We had so many bug reports in that area of configuring
10357 default tapes automatically, and so many contradicting requests, that
10358 we finally consider the problem to be portably intractable. We could
10359 of course use something like @samp{/dev/tape} as a default, but this
10360 is @emph{also} running after various kind of trouble, going from hung
10361 processes to accidental destruction of real tapes. After having seen
10362 all this mess, using standard input and output as a default really
10363 sounds like the only clean choice left, and a very useful one too.
10365 @GNUTAR{} reads and writes archive in records, I
10366 suspect this is the main reason why block devices are preferred over
10367 character devices. Most probably, block devices are more efficient
10368 too. The installer could also check for @samp{DEFTAPE} in
10369 @file{<sys/mtio.h>}.
10372 @xopindex{force-local, short description}
10373 @item --force-local
10374 Archive file is local even if it contains a colon.
10376 @opindex rsh-command
10377 @item --rsh-command=@var{command}
10378 Use remote @var{command} instead of @command{rsh}. This option exists
10379 so that people who use something other than the standard @command{rsh}
10380 (e.g., a Kerberized @command{rsh}) can access a remote device.
10382 When this command is not used, the shell command found when
10383 the @command{tar} program was installed is used instead. This is
10384 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
10385 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
10386 The installer may have overridden this by defining the environment
10387 variable @env{RSH} @emph{at installation time}.
10390 Specify drive and density.
10392 @xopindex{multi-volume, short description}
10394 @itemx --multi-volume
10395 Create/list/extract multi-volume archive.
10397 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
10398 that may be larger than will fit on the medium used to hold it.
10399 @xref{Multi-Volume Archives}.
10401 @xopindex{tape-length, short description}
10403 @itemx --tape-length=@var{size}[@var{suf}]
10404 Change tape after writing @var{size} units of data. Unless @var{suf} is
10405 given, @var{size} is treated as kilobytes, i.e. @samp{@var{size} x
10406 1024} bytes. The following suffixes alter this behavior:
10408 @float Table, size-suffixes
10409 @caption{Size Suffixes}
10410 @multitable @columnfractions 0.2 0.3 0.3
10411 @headitem Suffix @tab Units @tab Byte Equivalent
10412 @item b @tab Blocks @tab @var{size} x 512
10413 @item B @tab Kilobytes @tab @var{size} x 1024
10414 @item c @tab Bytes @tab @var{size}
10415 @item G @tab Gigabytes @tab @var{size} x 1024^3
10416 @item K @tab Kilobytes @tab @var{size} x 1024
10417 @item k @tab Kilobytes @tab @var{size} x 1024
10418 @item M @tab Megabytes @tab @var{size} x 1024^2
10419 @item P @tab Petabytes @tab @var{size} x 1024^5
10420 @item T @tab Terabytes @tab @var{size} x 1024^4
10421 @item w @tab Words @tab @var{size} x 2
10425 This option might be useful when your tape drivers do not properly
10426 detect end of physical tapes. By being slightly conservative on the
10427 maximum tape length, you might avoid the problem entirely.
10429 @xopindex{info-script, short description}
10430 @xopindex{new-volume-script, short description}
10431 @item -F @var{file}
10432 @itemx --info-script=@var{file}
10433 @itemx --new-volume-script=@var{file}
10434 Execute @file{file} at end of each tape. This implies
10435 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
10436 description of this option.
10439 @node Remote Tape Server
10440 @section Remote Tape Server
10442 @cindex remote tape drive
10444 In order to access the tape drive on a remote machine, @command{tar}
10445 uses the remote tape server written at the University of California at
10446 Berkeley. The remote tape server must be installed as
10447 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
10448 want to use. @command{tar} calls @command{rmt} by running an
10449 @command{rsh} or @command{remsh} to the remote machine, optionally
10450 using a different login name if one is supplied.
10452 A copy of the source for the remote tape server is provided. It is
10453 Copyright @copyright{} 1983 by the Regents of the University of
10454 California, but can be freely distributed. It is compiled and
10455 installed by default.
10457 @cindex absolute file names
10458 Unless you use the @option{--absolute-names} (@option{-P}) option,
10459 @GNUTAR{} will not allow you to create an archive that contains
10460 absolute file names (a file name beginning with @samp{/}.) If you try,
10461 @command{tar} will automatically remove the leading @samp{/} from the
10462 file names it stores in the archive. It will also type a warning
10463 message telling you what it is doing.
10465 When reading an archive that was created with a different
10466 @command{tar} program, @GNUTAR{} automatically
10467 extracts entries in the archive which have absolute file names as if
10468 the file names were not absolute. This is an important feature. A
10469 visitor here once gave a @command{tar} tape to an operator to restore;
10470 the operator used Sun @command{tar} instead of @GNUTAR{},
10471 and the result was that it replaced large portions of
10472 our @file{/bin} and friends with versions from the tape; needless to
10473 say, we were unhappy about having to recover the file system from
10476 For example, if the archive contained a file @file{/usr/bin/computoy},
10477 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
10478 relative to the current directory. If you want to extract the files in
10479 an archive to the same absolute names that they had when the archive
10480 was created, you should do a @samp{cd /} before extracting the files
10481 from the archive, or you should either use the @option{--absolute-names}
10482 option, or use the command @samp{tar -C / @dots{}}.
10484 @cindex Ultrix 3.1 and write failure
10485 Some versions of Unix (Ultrix 3.1 is known to have this problem),
10486 can claim that a short write near the end of a tape succeeded,
10487 when it actually failed. This will result in the -M option not
10488 working correctly. The best workaround at the moment is to use a
10489 significantly larger blocking factor than the default 20.
10491 In order to update an archive, @command{tar} must be able to backspace the
10492 archive in order to reread or rewrite a record that was just read (or
10493 written). This is currently possible only on two kinds of files: normal
10494 disk files (or any other file that can be backspaced with @samp{lseek}),
10495 and industry-standard 9-track magnetic tape (or any other kind of tape
10496 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}).
10498 This means that the @option{--append}, @option{--concatenate}, and
10499 @option{--delete} commands will not work on any other kind of file.
10500 Some media simply cannot be backspaced, which means these commands and
10501 options will never be able to work on them. These non-backspacing
10502 media include pipes and cartridge tape drives.
10504 Some other media can be backspaced, and @command{tar} will work on them
10505 once @command{tar} is modified to do so.
10507 Archives created with the @option{--multi-volume}, @option{--label}, and
10508 @option{--incremental} (@option{-G}) options may not be readable by other version
10509 of @command{tar}. In particular, restoring a file that was split over
10510 a volume boundary will require some careful work with @command{dd}, if
10511 it can be done at all. Other versions of @command{tar} may also create
10512 an empty file whose name is that of the volume header. Some versions
10513 of @command{tar} may create normal files instead of directories archived
10514 with the @option{--incremental} (@option{-G}) option.
10516 @node Common Problems and Solutions
10517 @section Some Common Problems and their Solutions
10522 errors from system:
10524 no such file or directory
10527 errors from @command{tar}:
10528 directory checksum error
10529 header format error
10531 errors from media/system:
10543 @dfn{Block} and @dfn{record} terminology is rather confused, and it
10544 is also confusing to the expert reader. On the other hand, readers
10545 who are new to the field have a fresh mind, and they may safely skip
10546 the next two paragraphs, as the remainder of this manual uses those
10547 two terms in a quite consistent way.
10549 John Gilmore, the writer of the public domain @command{tar} from which
10550 @GNUTAR{} was originally derived, wrote (June 1995):
10553 The nomenclature of tape drives comes from IBM, where I believe
10554 they were invented for the IBM 650 or so. On IBM mainframes, what
10555 is recorded on tape are tape blocks. The logical organization of
10556 data is into records. There are various ways of putting records into
10557 blocks, including @code{F} (fixed sized records), @code{V} (variable
10558 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
10559 to a block), @code{VB} (variable size records, @var{n} to a block),
10560 @code{VSB} (variable spanned blocked: variable sized records that can
10561 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
10562 parameter specified this to the operating system.
10564 The Unix man page on @command{tar} was totally confused about this.
10565 When I wrote @code{PD TAR}, I used the historically correct terminology
10566 (@command{tar} writes data records, which are grouped into blocks).
10567 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
10568 here), and now Fran@,{c}ois has migrated that terminology back
10569 into the source code too.
10572 The term @dfn{physical block} means the basic transfer chunk from or
10573 to a device, after which reading or writing may stop without anything
10574 being lost. In this manual, the term @dfn{block} usually refers to
10575 a disk physical block, @emph{assuming} that each disk block is 512
10576 bytes in length. It is true that some disk devices have different
10577 physical blocks, but @command{tar} ignore these differences in its own
10578 format, which is meant to be portable, so a @command{tar} block is always
10579 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
10580 The term @dfn{logical block} often represents the basic chunk of
10581 allocation of many disk blocks as a single entity, which the operating
10582 system treats somewhat atomically; this concept is only barely used
10585 The term @dfn{physical record} is another way to speak of a physical
10586 block, those two terms are somewhat interchangeable. In this manual,
10587 the term @dfn{record} usually refers to a tape physical block,
10588 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
10589 It is true that archives may be put on disk or used with pipes,
10590 but nevertheless, @command{tar} tries to read and write the archive one
10591 @dfn{record} at a time, whatever the medium in use. One record is made
10592 up of an integral number of blocks, and this operation of putting many
10593 disk blocks into a single tape block is called @dfn{reblocking}, or
10594 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
10595 the logical organization of many characters into something meaningful
10596 to the application. The term @dfn{unit record} describes a small set
10597 of characters which are transmitted whole to or by the application,
10598 and often refers to a line of text. Those two last terms are unrelated
10599 to what we call a @dfn{record} in @GNUTAR{}.
10601 When writing to tapes, @command{tar} writes the contents of the archive
10602 in chunks known as @dfn{records}. To change the default blocking
10603 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
10604 @var{512-size}}) option. Each record will then be composed of
10605 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
10606 @xref{Standard}.) Each file written to the archive uses at least one
10607 full record. As a result, using a larger record size can result in
10608 more wasted space for small files. On the other hand, a larger record
10609 size can often be read and written much more efficiently.
10611 Further complicating the problem is that some tape drives ignore the
10612 blocking entirely. For these, a larger record size can still improve
10613 performance (because the software layers above the tape drive still
10614 honor the blocking), but not as dramatically as on tape drives that
10617 When reading an archive, @command{tar} can usually figure out the
10618 record size on itself. When this is the case, and a non-standard
10619 record size was used when the archive was created, @command{tar} will
10620 print a message about a non-standard blocking factor, and then operate
10621 normally. On some tape devices, however, @command{tar} cannot figure
10622 out the record size itself. On most of those, you can specify a
10623 blocking factor (with @option{--blocking-factor}) larger than the
10624 actual blocking factor, and then use the @option{--read-full-records}
10625 (@option{-B}) option. (If you specify a blocking factor with
10626 @option{--blocking-factor} and don't use the
10627 @option{--read-full-records} option, then @command{tar} will not
10628 attempt to figure out the recording size itself.) On some devices,
10629 you must always specify the record size exactly with
10630 @option{--blocking-factor} when reading, because @command{tar} cannot
10631 figure it out. In any case, use @option{--list} (@option{-t}) before
10632 doing any extractions to see whether @command{tar} is reading the archive
10635 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
10636 putting them into records is to put a whole number of them (one or
10637 more) into each record. @command{tar} records are all the same size;
10638 at the end of the file there's a block containing all zeros, which
10639 is how you tell that the remainder of the last record(s) are garbage.
10641 In a standard @command{tar} file (no options), the block size is 512
10642 and the record size is 10240, for a blocking factor of 20. What the
10643 @option{--blocking-factor} option does is sets the blocking factor,
10644 changing the record size while leaving the block size at 512 bytes.
10645 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
10646 most tape drives these days prefer much bigger records in order to
10647 stream and not waste tape. When writing tapes for myself, some tend
10648 to use a factor of the order of 2048, say, giving a record size of
10649 around one megabyte.
10651 If you use a blocking factor larger than 20, older @command{tar}
10652 programs might not be able to read the archive, so we recommend this
10653 as a limit to use in practice. @GNUTAR{}, however,
10654 will support arbitrarily large record sizes, limited only by the
10655 amount of virtual memory or the physical characteristics of the tape
10659 * Format Variations:: Format Variations
10660 * Blocking Factor:: The Blocking Factor of an Archive
10663 @node Format Variations
10664 @subsection Format Variations
10665 @cindex Format Parameters
10666 @cindex Format Options
10667 @cindex Options, archive format specifying
10668 @cindex Options, format specifying
10671 Format parameters specify how an archive is written on the archive
10672 media. The best choice of format parameters will vary depending on
10673 the type and number of files being archived, and on the media used to
10676 To specify format parameters when accessing or creating an archive,
10677 you can use the options described in the following sections.
10678 If you do not specify any format parameters, @command{tar} uses
10679 default parameters. You cannot modify a compressed archive.
10680 If you create an archive with the @option{--blocking-factor} option
10681 specified (@pxref{Blocking Factor}), you must specify that
10682 blocking-factor when operating on the archive. @xref{Formats}, for other
10683 examples of format parameter considerations.
10685 @node Blocking Factor
10686 @subsection The Blocking Factor of an Archive
10687 @cindex Blocking Factor
10688 @cindex Record Size
10689 @cindex Number of blocks per record
10690 @cindex Number of bytes per record
10691 @cindex Bytes per record
10692 @cindex Blocks per record
10695 @opindex blocking-factor
10696 The data in an archive is grouped into blocks, which are 512 bytes.
10697 Blocks are read and written in whole number multiples called
10698 @dfn{records}. The number of blocks in a record (i.e., the size of a
10699 record in units of 512 bytes) is called the @dfn{blocking factor}.
10700 The @option{--blocking-factor=@var{512-size}} (@option{-b
10701 @var{512-size}}) option specifies the blocking factor of an archive.
10702 The default blocking factor is typically 20 (i.e., 10240 bytes), but
10703 can be specified at installation. To find out the blocking factor of
10704 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
10705 This may not work on some devices.
10707 Records are separated by gaps, which waste space on the archive media.
10708 If you are archiving on magnetic tape, using a larger blocking factor
10709 (and therefore larger records) provides faster throughput and allows you
10710 to fit more data on a tape (because there are fewer gaps). If you are
10711 archiving on cartridge, a very large blocking factor (say 126 or more)
10712 greatly increases performance. A smaller blocking factor, on the other
10713 hand, may be useful when archiving small files, to avoid archiving lots
10714 of nulls as @command{tar} fills out the archive to the end of the record.
10715 In general, the ideal record size depends on the size of the
10716 inter-record gaps on the tape you are using, and the average size of the
10717 files you are archiving. @xref{create}, for information on
10720 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
10722 Archives with blocking factors larger than 20 cannot be read
10723 by very old versions of @command{tar}, or by some newer versions
10724 of @command{tar} running on old machines with small address spaces.
10725 With @GNUTAR{}, the blocking factor of an archive is limited
10726 only by the maximum record size of the device containing the archive,
10727 or by the amount of available virtual memory.
10729 Also, on some systems, not using adequate blocking factors, as sometimes
10730 imposed by the device drivers, may yield unexpected diagnostics. For
10731 example, this has been reported:
10734 Cannot write to /dev/dlt: Invalid argument
10738 In such cases, it sometimes happen that the @command{tar} bundled by
10739 the system is aware of block size idiosyncrasies, while @GNUTAR{}
10740 requires an explicit specification for the block size,
10741 which it cannot guess. This yields some people to consider
10742 @GNUTAR{} is misbehaving, because by comparison,
10743 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
10744 for example, might resolve the problem.
10746 If you use a non-default blocking factor when you create an archive, you
10747 must specify the same blocking factor when you modify that archive. Some
10748 archive devices will also require you to specify the blocking factor when
10749 reading that archive, however this is not typically the case. Usually, you
10750 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
10751 reports a non-default record size and then lists the archive members as
10752 it would normally. To extract files from an archive with a non-standard
10753 blocking factor (particularly if you're not sure what the blocking factor
10754 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
10755 specifying a blocking factor larger then the blocking factor of the archive
10756 (i.e., @samp{tar --extract --read-full-records --blocking-factor=300}).
10757 @xref{list}, for more information on the @option{--list} (@option{-t})
10758 operation. @xref{Reading}, for a more detailed explanation of that option.
10761 @item --blocking-factor=@var{number}
10762 @itemx -b @var{number}
10763 Specifies the blocking factor of an archive. Can be used with any
10764 operation, but is usually not necessary with @option{--list} (@option{-t}).
10770 @item -b @var{blocks}
10771 @itemx --blocking-factor=@var{blocks}
10772 Set record size to @math{@var{blocks}*512} bytes.
10774 This option is used to specify a @dfn{blocking factor} for the archive.
10775 When reading or writing the archive, @command{tar}, will do reads and writes
10776 of the archive in records of @math{@var{block}*512} bytes. This is true
10777 even when the archive is compressed. Some devices requires that all
10778 write operations be a multiple of a certain size, and so, @command{tar}
10779 pads the archive out to the next record boundary.
10781 The default blocking factor is set when @command{tar} is compiled, and is
10782 typically 20. Blocking factors larger than 20 cannot be read by very
10783 old versions of @command{tar}, or by some newer versions of @command{tar}
10784 running on old machines with small address spaces.
10786 With a magnetic tape, larger records give faster throughput and fit
10787 more data on a tape (because there are fewer inter-record gaps).
10788 If the archive is in a disk file or a pipe, you may want to specify
10789 a smaller blocking factor, since a large one will result in a large
10790 number of null bytes at the end of the archive.
10792 When writing cartridge or other streaming tapes, a much larger
10793 blocking factor (say 126 or more) will greatly increase performance.
10794 However, you must specify the same blocking factor when reading or
10795 updating the archive.
10797 Apparently, Exabyte drives have a physical block size of 8K bytes.
10798 If we choose our blocksize as a multiple of 8k bytes, then the problem
10799 seems to disappear. Id est, we are using block size of 112 right
10800 now, and we haven't had the problem since we switched@dots{}
10802 With @GNUTAR{} the blocking factor is limited only
10803 by the maximum record size of the device containing the archive, or by
10804 the amount of available virtual memory.
10806 However, deblocking or reblocking is virtually avoided in a special
10807 case which often occurs in practice, but which requires all the
10808 following conditions to be simultaneously true:
10811 the archive is subject to a compression option,
10813 the archive is not handled through standard input or output, nor
10814 redirected nor piped,
10816 the archive is directly handled to a local disk, instead of any special
10819 @option{--blocking-factor} is not explicitly specified on the @command{tar}
10823 If the output goes directly to a local disk, and not through
10824 stdout, then the last write is not extended to a full record size.
10825 Otherwise, reblocking occurs. Here are a few other remarks on this
10831 @command{gzip} will complain about trailing garbage if asked to
10832 uncompress a compressed archive on tape, there is an option to turn
10833 the message off, but it breaks the regularity of simply having to use
10834 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
10835 silently ignoring any number of trailing zeros. I'll ask Jean-loup
10836 Gailly, by sending a copy of this message to him.
10839 @command{compress} does not show this problem, but as Jean-loup pointed
10840 out to Michael, @samp{compress -d} silently adds garbage after
10841 the result of decompression, which tar ignores because it already
10842 recognized its end-of-file indicator. So this bug may be safely
10846 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
10847 but will still return an exit status of 2 which tar reports in turn.
10848 @command{tar} might ignore the exit status returned, but I hate doing
10849 that, as it weakens the protection @command{tar} offers users against
10850 other possible problems at decompression time. If @command{gzip} was
10851 silently skipping trailing zeros @emph{and} also avoiding setting the
10852 exit status in this innocuous case, that would solve this situation.
10855 @command{tar} should become more solid at not stopping to read a pipe at
10856 the first null block encountered. This inelegantly breaks the pipe.
10857 @command{tar} should rather drain the pipe out before exiting itself.
10860 @xopindex{ignore-zeros, short description}
10862 @itemx --ignore-zeros
10863 Ignore blocks of zeros in archive (means EOF).
10865 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
10866 of zeros in the archive. Normally a block of zeros indicates the
10867 end of the archive, but when reading a damaged archive, or one which
10868 was created by concatenating several archives together, this option
10869 allows @command{tar} to read the entire archive. This option is not on
10870 by default because many versions of @command{tar} write garbage after
10873 Note that this option causes @command{tar} to read to the end of the
10874 archive file, which may sometimes avoid problems when multiple files
10875 are stored on a single physical tape.
10877 @xopindex{read-full-records, short description}
10879 @itemx --read-full-records
10880 Reblock as we read (for reading 4.2@acronym{BSD} pipes).
10882 If @option{--read-full-records} is used, @command{tar}
10883 will not panic if an attempt to read a record from the archive does
10884 not return a full record. Instead, @command{tar} will keep reading
10885 until it has obtained a full
10888 This option is turned on by default when @command{tar} is reading
10889 an archive from standard input, or from a remote machine. This is
10890 because on @acronym{BSD} Unix systems, a read of a pipe will return however
10891 much happens to be in the pipe, even if it is less than @command{tar}
10892 requested. If this option was not used, @command{tar} would fail as
10893 soon as it read an incomplete record from the pipe.
10895 This option is also useful with the commands for updating an archive.
10901 @FIXME{Appropriate options should be moved here from elsewhere.}
10903 @cindex blocking factor
10904 @cindex tape blocking
10906 When handling various tapes or cartridges, you have to take care of
10907 selecting a proper blocking, that is, the number of disk blocks you
10908 put together as a single tape block on the tape, without intervening
10909 tape gaps. A @dfn{tape gap} is a small landing area on the tape
10910 with no information on it, used for decelerating the tape to a
10911 full stop, and for later regaining the reading or writing speed.
10912 When the tape driver starts reading a record, the record has to
10913 be read whole without stopping, as a tape gap is needed to stop the
10914 tape motion without losing information.
10916 @cindex Exabyte blocking
10917 @cindex DAT blocking
10918 Using higher blocking (putting more disk blocks per tape block) will use
10919 the tape more efficiently as there will be less tape gaps. But reading
10920 such tapes may be more difficult for the system, as more memory will be
10921 required to receive at once the whole record. Further, if there is a
10922 reading error on a huge record, this is less likely that the system will
10923 succeed in recovering the information. So, blocking should not be too
10924 low, nor it should be too high. @command{tar} uses by default a blocking of
10925 20 for historical reasons, and it does not really matter when reading or
10926 writing to disk. Current tape technology would easily accommodate higher
10927 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
10928 We were told that for some DLT drives, the blocking should be a multiple
10929 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
10930 Other manufacturers may use different recommendations for the same tapes.
10931 This might also depends of the buffering techniques used inside modern
10932 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
10933 Others request blocking to be some exponent of two.
10935 So, there is no fixed rule for blocking. But blocking at read time
10936 should ideally be the same as blocking used at write time. At one place
10937 I know, with a wide variety of equipment, they found it best to use a
10938 blocking of 32 to guarantee that their tapes are fully interchangeable.
10940 I was also told that, for recycled tapes, prior erasure (by the same
10941 drive unit that will be used to create the archives) sometimes lowers
10942 the error rates observed at rewriting time.
10944 I might also use @option{--number-blocks} instead of
10945 @option{--block-number}, so @option{--block} will then expand to
10946 @option{--blocking-factor} unambiguously.
10949 @section Many Archives on One Tape
10951 @FIXME{Appropriate options should be moved here from elsewhere.}
10953 @findex ntape @r{device}
10954 Most tape devices have two entries in the @file{/dev} directory, or
10955 entries that come in pairs, which differ only in the minor number for
10956 this device. Let's take for example @file{/dev/tape}, which often
10957 points to the only or usual tape device of a given system. There might
10958 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
10959 name is the @emph{rewinding} version of the device, while the name
10960 having @samp{nr} in it is the @emph{no rewinding} version of the same
10963 A rewinding tape device will bring back the tape to its beginning point
10964 automatically when this device is opened or closed. Since @command{tar}
10965 opens the archive file before using it and closes it afterwards, this
10966 means that a simple:
10969 $ @kbd{tar cf /dev/tape @var{directory}}
10973 will reposition the tape to its beginning both prior and after saving
10974 @var{directory} contents to it, thus erasing prior tape contents and
10975 making it so that any subsequent write operation will destroy what has
10978 @cindex tape positioning
10979 So, a rewinding device is normally meant to hold one and only one file.
10980 If you want to put more than one @command{tar} archive on a given tape, you
10981 will need to avoid using the rewinding version of the tape device. You
10982 will also have to pay special attention to tape positioning. Errors in
10983 positioning may overwrite the valuable data already on your tape. Many
10984 people, burnt by past experiences, will only use rewinding devices and
10985 limit themselves to one file per tape, precisely to avoid the risk of
10986 such errors. Be fully aware that writing at the wrong position on a
10987 tape loses all information past this point and most probably until the
10988 end of the tape, and this destroyed information @emph{cannot} be
10991 To save @var{directory-1} as a first archive at the beginning of a
10992 tape, and leave that tape ready for a second archive, you should use:
10995 $ @kbd{mt -f /dev/nrtape rewind}
10996 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
11000 @dfn{Tape marks} are special magnetic patterns written on the tape
11001 media, which are later recognizable by the reading hardware. These
11002 marks are used after each file, when there are many on a single tape.
11003 An empty file (that is to say, two tape marks in a row) signal the
11004 logical end of the tape, after which no file exist. Usually,
11005 non-rewinding tape device drivers will react to the close request issued
11006 by @command{tar} by first writing two tape marks after your archive, and by
11007 backspacing over one of these. So, if you remove the tape at that time
11008 from the tape drive, it is properly terminated. But if you write
11009 another file at the current position, the second tape mark will be
11010 erased by the new information, leaving only one tape mark between files.
11012 So, you may now save @var{directory-2} as a second archive after the
11013 first on the same tape by issuing the command:
11016 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
11020 and so on for all the archives you want to put on the same tape.
11022 Another usual case is that you do not write all the archives the same
11023 day, and you need to remove and store the tape between two archive
11024 sessions. In general, you must remember how many files are already
11025 saved on your tape. Suppose your tape already has 16 files on it, and
11026 that you are ready to write the 17th. You have to take care of skipping
11027 the first 16 tape marks before saving @var{directory-17}, say, by using
11031 $ @kbd{mt -f /dev/nrtape rewind}
11032 $ @kbd{mt -f /dev/nrtape fsf 16}
11033 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
11036 In all the previous examples, we put aside blocking considerations, but
11037 you should do the proper things for that as well. @xref{Blocking}.
11040 * Tape Positioning:: Tape Positions and Tape Marks
11041 * mt:: The @command{mt} Utility
11044 @node Tape Positioning
11045 @subsection Tape Positions and Tape Marks
11048 Just as archives can store more than one file from the file system,
11049 tapes can store more than one archive file. To keep track of where
11050 archive files (or any other type of file stored on tape) begin and
11051 end, tape archive devices write magnetic @dfn{tape marks} on the
11052 archive media. Tape drives write one tape mark between files,
11053 two at the end of all the file entries.
11055 If you think of data as a series of records "rrrr"'s, and tape marks as
11056 "*"'s, a tape might look like the following:
11059 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
11062 Tape devices read and write tapes using a read/write @dfn{tape
11063 head}---a physical part of the device which can only access one
11064 point on the tape at a time. When you use @command{tar} to read or
11065 write archive data from a tape device, the device will begin reading
11066 or writing from wherever on the tape the tape head happens to be,
11067 regardless of which archive or what part of the archive the tape
11068 head is on. Before writing an archive, you should make sure that no
11069 data on the tape will be overwritten (unless it is no longer needed).
11070 Before reading an archive, you should make sure the tape head is at
11071 the beginning of the archive you want to read. You can do it manually
11072 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
11073 that automatically (@pxref{Scripted Restoration}).
11075 If you want to add new archive file entries to a tape, you should
11076 advance the tape to the end of the existing file entries, backspace
11077 over the last tape mark, and write the new archive file. If you were
11078 to add two archives to the example above, the tape might look like the
11082 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
11086 @subsection The @command{mt} Utility
11089 @FIXME{Is it true that this only works on non-block devices?
11090 should explain the difference, (fixed or variable).}
11091 @xref{Blocking Factor}.
11093 You can use the @command{mt} utility to advance or rewind a tape past a
11094 specified number of archive files on the tape. This will allow you
11095 to move to the beginning of an archive before extracting or reading
11096 it, or to the end of all the archives before writing a new one.
11097 @FIXME{Why isn't there an "advance 'til you find two tape marks
11100 The syntax of the @command{mt} command is:
11103 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
11106 where @var{tapename} is the name of the tape device, @var{number} is
11107 the number of times an operation is performed (with a default of one),
11108 and @var{operation} is one of the following:
11110 @FIXME{is there any use for record operations?}
11115 Writes @var{number} tape marks at the current position on the tape.
11118 Moves tape position forward @var{number} files.
11121 Moves tape position back @var{number} files.
11124 Rewinds the tape. (Ignores @var{number}.)
11128 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}.)
11131 Prints status information about the tape unit.
11135 If you don't specify a @var{tapename}, @command{mt} uses the environment
11136 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
11137 the default device specified in your @file{sys/mtio.h} file
11138 (@code{DEFTAPE} variable). If this is not defined, the program will
11139 display a descriptive error message and exit with code 1.
11141 @command{mt} returns a 0 exit status when the operation(s) were
11142 successful, 1 if the command was unrecognized, and 2 if an operation
11145 @node Using Multiple Tapes
11146 @section Using Multiple Tapes
11148 Often you might want to write a large archive, one larger than will fit
11149 on the actual tape you are using. In such a case, you can run multiple
11150 @command{tar} commands, but this can be inconvenient, particularly if you
11151 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
11152 Therefore, @command{tar} provides a special mode for creating
11153 multi-volume archives.
11155 @dfn{Multi-volume} archive is a single @command{tar} archive, stored
11156 on several media volumes of fixed size. Although in this section we will
11157 often call @samp{volume} a @dfn{tape}, there is absolutely no
11158 requirement for multi-volume archives to be stored on tapes. Instead,
11159 they can use whatever media type the user finds convenient, they can
11160 even be located on files.
11162 When creating a multi-volume archive, @GNUTAR{} continues to fill
11163 current volume until it runs out of space, then it switches to
11164 next volume (usually the operator is queried to replace the tape on
11165 this point), and continues working on the new volume. This operation
11166 continues until all requested files are dumped. If @GNUTAR{} detects
11167 end of media while dumping a file, such a file is archived in split
11168 form. Some very big files can even be split across several volumes.
11170 Each volume is itself a valid @GNUTAR{} archive, so it can be read
11171 without any special options. Consequently any file member residing
11172 entirely on one volume can be extracted or otherwise operated upon
11173 without needing the other volume. Sure enough, to extract a split
11174 member you would need all volumes its parts reside on.
11176 Multi-volume archives suffer from several limitations. In particular,
11177 they cannot be compressed.
11179 @GNUTAR{} is able to create multi-volume archives of two formats
11180 (@pxref{Formats}): @samp{GNU} and @samp{POSIX}.
11183 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
11184 * Tape Files:: Tape Files
11185 * Tarcat:: Concatenate Volumes into a Single Archive
11189 @node Multi-Volume Archives
11190 @subsection Archives Longer than One Tape or Disk
11191 @cindex Multi-volume archives
11193 @opindex multi-volume
11194 To create an archive that is larger than will fit on a single unit of
11195 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
11196 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
11197 archive can be manipulated like any other archive (provided the
11198 @option{--multi-volume} option is specified), but is stored on more
11199 than one tape or file.
11201 When you specify @option{--multi-volume}, @command{tar} does not report an
11202 error when it comes to the end of an archive volume (when reading), or
11203 the end of the media (when writing). Instead, it prompts you to load
11204 a new storage volume. If the archive is on a magnetic tape, you
11205 should change tapes when you see the prompt; if the archive is on a
11206 floppy disk, you should change disks; etc.
11209 @item --multi-volume
11211 Creates a multi-volume archive, when used in conjunction with
11212 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
11213 archive, specify @option{--multi-volume} in conjunction with that
11218 $ @kbd{tar --create --multi-volume --file=/dev/tape @var{files}}
11222 The method @command{tar} uses to detect end of tape is not perfect, and
11223 fails on some operating systems or on some devices. If @command{tar}
11224 cannot detect the end of the tape itself, you can use
11225 @option{--tape-length} option to inform it about the capacity of the
11228 @anchor{tape-length}
11230 @opindex tape-length
11231 @item --tape-length=@var{size}[@var{suf}]
11232 @itemx -L @var{size}[@var{suf}]
11233 Set maximum length of a volume. The @var{suf}, if given, specifies
11234 units in which @var{size} is expressed, e.g. @samp{2M} mean 2
11235 megabytes (@pxref{size-suffixes}, for a list of allowed size
11236 suffixes). Without @var{suf}, units of 1024 bytes (kilobyte) are
11239 This option selects @option{--multi-volume} automatically. For example:
11242 $ @kbd{tar --create --tape-length=41943040 --file=/dev/tape @var{files}}
11246 or, which is equivalent:
11249 $ @kbd{tar --create --tape-length=4G --file=/dev/tape @var{files}}
11253 @anchor{change volume prompt}
11254 When @GNUTAR{} comes to the end of a storage media, it asks you to
11255 change the volume. The built-in prompt for POSIX locale
11256 is@footnote{If you run @GNUTAR{} under a different locale, the
11257 translation to the locale's language will be used.}:
11260 Prepare volume #@var{n} for `@var{archive}' and hit return:
11264 where @var{n} is the ordinal number of the volume to be created and
11265 @var{archive} is archive file or device name.
11267 When prompting for a new tape, @command{tar} accepts any of the following
11272 Request @command{tar} to explain possible responses.
11274 Request @command{tar} to exit immediately.
11275 @item n @var{file-name}
11276 Request @command{tar} to write the next volume on the file @var{file-name}.
11278 Request @command{tar} to run a subshell. This option can be disabled
11279 by giving @option{--restrict} command line option to
11280 @command{tar}@footnote{@xref{--restrict}, for more information about
11283 Request @command{tar} to begin writing the next volume.
11286 (You should only type @samp{y} after you have changed the tape;
11287 otherwise @command{tar} will write over the volume it just finished.)
11289 @cindex Volume number file
11291 @anchor{volno-file}
11292 @opindex volno-file
11293 The volume number used by @command{tar} in its tape-changing prompt
11294 can be changed; if you give the
11295 @option{--volno-file=@var{file-of-number}} option, then
11296 @var{file-of-number} should be an non-existing file to be created, or
11297 else, a file already containing a decimal number. That number will be
11298 used as the volume number of the first volume written. When
11299 @command{tar} is finished, it will rewrite the file with the
11300 now-current volume number. (This does not change the volume number
11301 written on a tape label, as per @ref{label}, it @emph{only} affects
11302 the number used in the prompt.)
11304 @cindex End-of-archive info script
11305 @cindex Info script
11306 @anchor{info-script}
11307 @opindex info-script
11308 @opindex new-volume-script
11309 If you want more elaborate behavior than this, you can write a special
11310 @dfn{new volume script}, that will be responsible for changing the
11311 volume, and instruct @command{tar} to use it instead of its normal
11312 prompting procedure:
11315 @item --info-script=@var{script-name}
11316 @itemx --new-volume-script=@var{script-name}
11317 @itemx -F @var{script-name}
11318 Specify the full name of the volume script to use. The script can be
11319 used to eject cassettes, or to broadcast messages such as
11320 @samp{Someone please come change my tape} when performing unattended
11324 The @var{script-name} is executed without any command line
11325 arguments. It inherits @command{tar}'s shell environment.
11326 Additional data is passed to it via the following
11327 environment variables:
11330 @vrindex TAR_VERSION, info script environment variable
11332 @GNUTAR{} version number.
11334 @vrindex TAR_ARCHIVE, info script environment variable
11336 The name of the archive @command{tar} is processing.
11338 @vrindex TAR_BLOCKING_FACTOR, info script environment variable
11339 @item TAR_BLOCKING_FACTOR
11340 Current blocking factor (@pxref{Blocking}).
11342 @vrindex TAR_VOLUME, info script environment variable
11344 Ordinal number of the volume @command{tar} is about to start.
11346 @vrindex TAR_SUBCOMMAND, info script environment variable
11347 @item TAR_SUBCOMMAND
11348 A short option describing the operation @command{tar} is executing.
11349 @xref{Operations}, for a complete list of subcommand options.
11351 @vrindex TAR_FORMAT, info script environment variable
11353 Format of the archive being processed. @xref{Formats}, for a complete
11354 list of archive format names.
11356 @vrindex TAR_FD, info script environment variable
11358 File descriptor which can be used to communicate the new volume
11359 name to @command{tar}.
11362 The volume script can instruct @command{tar} to use new archive name,
11363 by writing in to file descriptor @env{$TAR_FD} (see below for an example).
11365 If the info script fails, @command{tar} exits; otherwise, it begins
11366 writing the next volume.
11368 If you want @command{tar} to cycle through a series of files or tape
11369 drives, there are three approaches to choose from. First of all, you
11370 can give @command{tar} multiple @option{--file} options. In this case
11371 the specified files will be used, in sequence, as the successive
11372 volumes of the archive. Only when the first one in the sequence needs
11373 to be used again will @command{tar} prompt for a tape change (or run
11374 the info script). For example, suppose someone has two tape drives on
11375 a system named @file{/dev/tape0} and @file{/dev/tape1}. For having
11376 @GNUTAR{} to switch to the second drive when it needs to write the
11377 second tape, and then back to the first tape, etc., just do either of:
11380 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
11381 $ @kbd{tar -cM -f /dev/tape0 -f /dev/tape1 @var{files}}
11384 The second method is to use the @samp{n} response to the tape-change
11387 Finally, the most flexible approach is to use a volume script, that
11388 writes new archive name to the file descriptor @env{$TAR_FD}. For example, the
11389 following volume script will create a series of archive files, named
11390 @file{@var{archive}-@var{vol}}, where @var{archive} is the name of the
11391 archive being created (as given by @option{--file} option) and
11392 @var{vol} is the ordinal number of the archive being created:
11397 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
11399 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
11400 case $TAR_SUBCOMMAND in
11402 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
11407 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&$TAR_FD
11411 The same script can be used while listing, comparing or extracting
11412 from the created archive. For example:
11416 # @r{Create a multi-volume archive:}
11417 $ @kbd{tar -c -L1024 -f archive.tar -F new-volume .}
11418 # @r{Extract from the created archive:}
11419 $ @kbd{tar -x -f archive.tar -F new-volume .}
11424 Notice, that the first command had to use @option{-L} option, since
11425 otherwise @GNUTAR{} will end up writing everything to file
11426 @file{archive.tar}.
11428 You can read each individual volume of a multi-volume archive as if it
11429 were an archive by itself. For example, to list the contents of one
11430 volume, use @option{--list}, without @option{--multi-volume} specified.
11431 To extract an archive member from one volume (assuming it is described
11432 that volume), use @option{--extract}, again without
11433 @option{--multi-volume}.
11435 If an archive member is split across volumes (i.e., its entry begins on
11436 one volume of the media and ends on another), you need to specify
11437 @option{--multi-volume} to extract it successfully. In this case, you
11438 should load the volume where the archive member starts, and use
11439 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
11440 volumes as it needs them. @xref{extracting archives}, for more
11441 information about extracting archives.
11443 Multi-volume archives can be modified like any other archive. To add
11444 files to a multi-volume archive, you need to only mount the last
11445 volume of the archive media (and new volumes, if needed). For all
11446 other operations, you need to use the entire archive.
11448 If a multi-volume archive was labeled using
11449 @option{--label=@var{archive-label}} (@pxref{label}) when it was
11450 created, @command{tar} will not automatically label volumes which are
11451 added later. To label subsequent volumes, specify
11452 @option{--label=@var{archive-label}} again in conjunction with the
11453 @option{--append}, @option{--update} or @option{--concatenate} operation.
11455 Notice that multi-volume support is a GNU extension and the archives
11456 created in this mode should be read only using @GNUTAR{}. If you
11457 absolutely have to process such archives using a third-party @command{tar}
11458 implementation, read @ref{Split Recovery}.
11461 @subsection Tape Files
11462 @cindex labeling archives
11466 To give the archive a name which will be recorded in it, use the
11467 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
11468 option. This will write a special block identifying
11469 @var{volume-label} as the name of the archive to the front of the
11470 archive which will be displayed when the archive is listed with
11471 @option{--list}. If you are creating a multi-volume archive with
11472 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
11473 volume label will have @samp{Volume @var{nnn}} appended to the name
11474 you give, where @var{nnn} is the number of the volume of the archive.
11475 If you use the @option{--label=@var{volume-label}} option when
11476 reading an archive, it checks to make sure the label on the tape
11477 matches the one you gave. @xref{label}.
11479 When @command{tar} writes an archive to tape, it creates a single
11480 tape file. If multiple archives are written to the same tape, one
11481 after the other, they each get written as separate tape files. When
11482 extracting, it is necessary to position the tape at the right place
11483 before running @command{tar}. To do this, use the @command{mt} command.
11484 For more information on the @command{mt} command and on the organization
11485 of tapes into a sequence of tape files, see @ref{mt}.
11487 People seem to often do:
11490 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
11493 or such, for pushing a common date in all volumes or an archive set.
11496 @subsection Concatenate Volumes into a Single Archive
11499 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
11500 archive to a single @command{tar} archive. Simply concatenating all
11501 volumes into one will not work, since each volume carries an additional
11502 information at the beginning. @GNUTAR{} is shipped with the shell
11503 script @command{tarcat} designed for this purpose.
11505 The script takes a list of files comprising a multi-volume archive
11506 and creates the resulting archive at the standard output. For example:
11509 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
11512 The script implements a simple heuristics to determine the format of
11513 the first volume file and to decide how to process the rest of the
11514 files. However, it makes no attempt to verify whether the files are
11515 given in order or even if they are valid @command{tar} archives.
11516 It uses @command{dd} and does not filter its standard error, so you
11517 will usually see lots of spurious messages.
11519 @FIXME{The script is not installed. Should we install it?}
11522 @section Including a Label in the Archive
11523 @cindex Labeling an archive
11524 @cindex Labels on the archive media
11525 @cindex Labeling multi-volume archives
11528 To avoid problems caused by misplaced paper labels on the archive
11529 media, you can include a @dfn{label} entry --- an archive member which
11530 contains the name of the archive --- in the archive itself. Use the
11531 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11532 option@footnote{Until version 1.10, that option was called
11533 @option{--volume}, but is not available under that name anymore.} in
11534 conjunction with the @option{--create} operation to include a label
11535 entry in the archive as it is being created.
11538 @item --label=@var{archive-label}
11539 @itemx -V @var{archive-label}
11540 Includes an @dfn{archive-label} at the beginning of the archive when
11541 the archive is being created, when used in conjunction with the
11542 @option{--create} operation. Checks to make sure the archive label
11543 matches the one specified (when used in conjunction with any other
11547 If you create an archive using both
11548 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11549 and @option{--multi-volume} (@option{-M}), each volume of the archive
11550 will have an archive label of the form @samp{@var{archive-label}
11551 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
11552 next, and so on. @xref{Using Multiple Tapes}, for information on
11553 creating multiple volume archives.
11555 @cindex Volume label, listing
11556 @cindex Listing volume label
11557 The volume label will be displayed by @option{--list} along with
11558 the file contents. If verbose display is requested, it will also be
11559 explicitly marked as in the example below:
11563 $ @kbd{tar --verbose --list --file=iamanarchive}
11564 V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header--
11565 -rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename
11569 @opindex test-label
11570 @anchor{--test-label option}
11571 However, @option{--list} option will cause listing entire
11572 contents of the archive, which may be undesirable (for example, if the
11573 archive is stored on a tape). You can request checking only the volume
11574 label by specifying @option{--test-label} option. This option reads only the
11575 first block of an archive, so it can be used with slow storage
11576 devices. For example:
11580 $ @kbd{tar --test-label --file=iamanarchive}
11585 If @option{--test-label} is used with one or more command line
11586 arguments, @command{tar} compares the volume label with each
11587 argument. It exits with code 0 if a match is found, and with code 1
11588 otherwise@footnote{Note that @GNUTAR{} versions up to 1.23 indicated
11589 mismatch with an exit code 2 and printed a spurious diagnostics on
11590 stderr.}. No output is displayed, unless you also used the
11591 @option{--verbose} option. For example:
11595 $ @kbd{tar --test-label --file=iamanarchive 'iamalabel'}
11597 $ @kbd{tar --test-label --file=iamanarchive 'alabel'}
11602 When used with the @option{--verbose} option, @command{tar}
11603 prints the actual volume label (if any), and a verbose diagnostics in
11604 case of a mismatch:
11608 $ @kbd{tar --test-label --verbose --file=iamanarchive 'iamalabel'}
11611 $ @kbd{tar --test-label --verbose --file=iamanarchive 'alabel'}
11613 tar: Archive label mismatch
11618 If you request any operation, other than @option{--create}, along
11619 with using @option{--label} option, @command{tar} will first check if
11620 the archive label matches the one specified and will refuse to proceed
11621 if it does not. Use this as a safety precaution to avoid accidentally
11622 overwriting existing archives. For example, if you wish to add files
11623 to @file{archive}, presumably labeled with string @samp{My volume},
11628 $ @kbd{tar -rf archive --label 'My volume' .}
11629 tar: Archive not labeled to match `My volume'
11634 in case its label does not match. This will work even if
11635 @file{archive} is not labeled at all.
11637 Similarly, @command{tar} will refuse to list or extract the
11638 archive if its label doesn't match the @var{archive-label}
11639 specified. In those cases, @var{archive-label} argument is interpreted
11640 as a globbing-style pattern which must match the actual magnetic
11641 volume label. @xref{exclude}, for a precise description of how match
11642 is attempted@footnote{Previous versions of @command{tar} used full
11643 regular expression matching, or before that, only exact string
11644 matching, instead of wildcard matchers. We decided for the sake of
11645 simplicity to use a uniform matching device through
11646 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
11647 the volume label matcher will also suffix @var{archive-label} by
11648 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
11649 up. Since the volume numbering is automatically added in labels at
11650 creation time, it sounded logical to equally help the user taking care
11651 of it when the archive is being read.
11653 You can also use @option{--label} to get a common information on
11654 all tapes of a series. For having this information different in each
11655 series created through a single script used on a regular basis, just
11656 manage to get some date string as part of the label. For example:
11660 $ @kbd{tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"}
11661 $ @kbd{tar --create --file=/dev/tape --multi-volume \
11662 --label="Daily backup for `date +%Y-%m-%d`"}
11666 Some more notes about volume labels:
11669 @item Each label has its own date and time, which corresponds
11670 to the time when @GNUTAR{} initially attempted to write it,
11671 often soon after the operator launches @command{tar} or types the
11672 carriage return telling that the next tape is ready.
11674 @item Comparing date labels to get an idea of tape throughput is
11675 unreliable. It gives correct results only if the delays for rewinding
11676 tapes and the operator switching them were negligible, which is
11677 usually not the case.
11681 @section Verifying Data as It is Stored
11682 @cindex Verifying a write operation
11683 @cindex Double-checking a write operation
11688 @opindex verify, short description
11689 Attempt to verify the archive after writing.
11692 This option causes @command{tar} to verify the archive after writing it.
11693 Each volume is checked after it is written, and any discrepancies
11694 are recorded on the standard error output.
11696 Verification requires that the archive be on a back-space-able medium.
11697 This means pipes, some cartridge tape drives, and some other devices
11698 cannot be verified.
11700 You can insure the accuracy of an archive by comparing files in the
11701 system with archive members. @command{tar} can compare an archive to the
11702 file system as the archive is being written, to verify a write
11703 operation, or can compare a previously written archive, to insure that
11706 @xopindex{verify, using with @option{--create}}
11707 @xopindex{create, using with @option{--verify}}
11708 To check for discrepancies in an archive immediately after it is
11709 written, use the @option{--verify} (@option{-W}) option in conjunction with
11710 the @option{--create} operation. When this option is
11711 specified, @command{tar} checks archive members against their counterparts
11712 in the file system, and reports discrepancies on the standard error.
11714 To verify an archive, you must be able to read it from before the end
11715 of the last written entry. This option is useful for detecting data
11716 errors on some tapes. Archives written to pipes, some cartridge tape
11717 drives, and some other devices cannot be verified.
11719 One can explicitly compare an already made archive with the file
11720 system by using the @option{--compare} (@option{--diff}, @option{-d})
11721 option, instead of using the more automatic @option{--verify} option.
11724 Note that these two options have a slightly different intent. The
11725 @option{--compare} option checks how identical are the logical contents of some
11726 archive with what is on your disks, while the @option{--verify} option is
11727 really for checking if the physical contents agree and if the recording
11728 media itself is of dependable quality. So, for the @option{--verify}
11729 operation, @command{tar} tries to defeat all in-memory cache pertaining to
11730 the archive, while it lets the speed optimization undisturbed for the
11731 @option{--compare} option. If you nevertheless use @option{--compare} for
11732 media verification, you may have to defeat the in-memory cache yourself,
11733 maybe by opening and reclosing the door latch of your recording unit,
11734 forcing some doubt in your operating system about the fact this is really
11735 the same volume as the one just written or read.
11737 The @option{--verify} option would not be necessary if drivers were indeed
11738 able to detect dependably all write failures. This sometimes require many
11739 magnetic heads, some able to read after the writes occurred. One would
11740 not say that drivers unable to detect all cases are necessarily flawed,
11741 as long as programming is concerned.
11743 The @option{--verify} (@option{-W}) option will not work in
11744 conjunction with the @option{--multi-volume} (@option{-M}) option or
11745 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
11746 and @option{--delete} operations. @xref{Operations}, for more
11747 information on these operations.
11749 Also, since @command{tar} normally strips leading @samp{/} from file
11750 names (@pxref{absolute}), a command like @samp{tar --verify -cf
11751 /tmp/foo.tar /etc} will work as desired only if the working directory is
11752 @file{/}, as @command{tar} uses the archive's relative member names
11753 (e.g., @file{etc/motd}) when verifying the archive.
11755 @node Write Protection
11756 @section Write Protection
11758 Almost all tapes and diskettes, and in a few rare cases, even disks can
11759 be @dfn{write protected}, to protect data on them from being changed.
11760 Once an archive is written, you should write protect the media to prevent
11761 the archive from being accidentally overwritten or deleted. (This will
11762 protect the archive from being changed with a tape or floppy drive---it
11763 will not protect it from magnet fields or other physical hazards.)
11765 The write protection device itself is usually an integral part of the
11766 physical media, and can be a two position (write enabled/write
11767 disabled) switch, a notch which can be popped out or covered, a ring
11768 which can be removed from the center of a tape reel, or some other
11769 changeable feature.
11771 @node Reliability and security
11772 @chapter Reliability and Security
11774 The @command{tar} command reads and writes files as any other
11775 application does, and is subject to the usual caveats about
11776 reliability and security. This section contains some commonsense
11777 advice on the topic.
11785 @section Reliability
11787 Ideally, when @command{tar} is creating an archive, it reads from a
11788 file system that is not being modified, and encounters no errors or
11789 inconsistencies while reading and writing. If this is the case, the
11790 archive should faithfully reflect what was read. Similarly, when
11791 extracting from an archive, ideally @command{tar} ideally encounters
11792 no errors and the extracted files faithfully reflect what was in the
11795 However, when reading or writing real-world file systems, several
11796 things can go wrong; these include permissions problems, corruption of
11797 data, and race conditions.
11800 * Permissions problems::
11801 * Data corruption and repair::
11802 * Race conditions::
11805 @node Permissions problems
11806 @subsection Permissions Problems
11808 If @command{tar} encounters errors while reading or writing files, it
11809 normally reports an error and exits with nonzero status. The work it
11810 does may therefore be incomplete. For example, when creating an
11811 archive, if @command{tar} cannot read a file then it cannot copy the
11812 file into the archive.
11814 @node Data corruption and repair
11815 @subsection Data Corruption and Repair
11817 If an archive becomes corrupted by an I/O error, this may corrupt the
11818 data in an extracted file. Worse, it may corrupt the file's metadata,
11819 which may cause later parts of the archive to become misinterpreted.
11820 An tar-format archive contains a checksum that most likely will detect
11821 errors in the metadata, but it will not detect errors in the data.
11823 If data corruption is a concern, you can compute and check your own
11824 checksums of an archive by using other programs, such as
11827 When attempting to recover from a read error or data corruption in an
11828 archive, you may need to skip past the questionable data and read the
11829 rest of the archive. This requires some expertise in the archive
11830 format and in other software tools.
11832 @node Race conditions
11833 @subsection Race conditions
11835 If some other process is modifying the file system while @command{tar}
11836 is reading or writing files, the result may well be inconsistent due
11837 to race conditions. For example, if another process creates some
11838 files in a directory while @command{tar} is creating an archive
11839 containing the directory's files, @command{tar} may see some of the
11840 files but not others, or it may see a file that is in the process of
11841 being created. The resulting archive may not be a snapshot of the
11842 file system at any point in time. If an application such as a
11843 database system depends on an accurate snapshot, restoring from the
11844 @command{tar} archive of a live file system may therefore break that
11845 consistency and may break the application. The simplest way to avoid
11846 the consistency issues is to avoid making other changes to the file
11847 system while tar is reading it or writing it.
11849 When creating an archive, several options are available to avoid race
11850 conditions. Some hosts have a way of snapshotting a file system, or
11851 of temporarily suspending all changes to a file system, by (say)
11852 suspending the only virtual machine that can modify a file system; if
11853 you use these facilities and have @command{tar -c} read from a
11854 snapshot when creating an archive, you can avoid inconsistency
11855 problems. More drastically, before starting @command{tar} you could
11856 suspend or shut down all processes other than @command{tar} that have
11857 access to the file system, or you could unmount the file system and
11858 then mount it read-only.
11860 When extracting from an archive, one approach to avoid race conditions
11861 is to create a directory that no other process can write to, and
11867 In some cases @command{tar} may be used in an adversarial situation,
11868 where an untrusted user is attempting to gain information about or
11869 modify otherwise-inaccessible files. Dealing with untrusted data
11870 (that is, data generated by an untrusted user) typically requires
11871 extra care, because even the smallest mistake in the use of
11872 @command{tar} is more likely to be exploited by an adversary than by a
11878 * Live untrusted data::
11879 * Security rules of thumb::
11883 @subsection Privacy
11885 Standard privacy concerns apply when using @command{tar}. For
11886 example, suppose you are archiving your home directory into a file
11887 @file{/archive/myhome.tar}. Any secret information in your home
11888 directory, such as your SSH secret keys, are copied faithfully into
11889 the archive. Therefore, if your home directory contains any file that
11890 should not be read by some other user, the archive itself should be
11891 not be readable by that user. And even if the archive's data are
11892 inaccessible to untrusted users, its metadata (such as size or
11893 last-modified date) may reveal some information about your home
11894 directory; if the metadata are intended to be private, the archive's
11895 parent directory should also be inaccessible to untrusted users.
11897 One precaution is to create @file{/archive} so that it is not
11898 accessible to any user, unless that user also has permission to access
11899 all the files in your home directory.
11901 Similarly, when extracting from an archive, take care that the
11902 permissions of the extracted files are not more generous than what you
11903 want. Even if the archive itself is readable only to you, files
11904 extracted from it have their own permissions that may differ.
11907 @subsection Integrity
11909 When creating archives, take care that they are not writable by a
11910 untrusted user; otherwise, that user could modify the archive, and
11911 when you later extract from the archive you will get incorrect data.
11913 When @command{tar} extracts from an archive, by default it writes into
11914 files relative to the working directory. If the archive was generated
11915 by an untrusted user, that user therefore can write into any file
11916 under the working directory. If the working directory contains a
11917 symbolic link to another directory, the untrusted user can also write
11918 into any file under the referenced directory. When extracting from an
11919 untrusted archive, it is therefore good practice to create an empty
11920 directory and run @command{tar} in that directory.
11922 When extracting from two or more untrusted archives, each one should
11923 be extracted independently, into different empty directories.
11924 Otherwise, the first archive could create a symbolic link into an area
11925 outside the working directory, and the second one could follow the
11926 link and overwrite data that is not under the working directory. For
11927 example, when restoring from a series of incremental dumps, the
11928 archives should have been created by a trusted process, as otherwise
11929 the incremental restores might alter data outside the working
11932 If you use the @option{--absolute-names} (@option{-P}) option when
11933 extracting, @command{tar} respects any file names in the archive, even
11934 file names that begin with @file{/} or contain @file{..}. As this
11935 lets the archive overwrite any file in your system that you can write,
11936 the @option{--absolute-names} (@option{-P}) option should be used only
11937 for trusted archives.
11939 Conversely, with the @option{--keep-old-files} (@option{-k}) option,
11940 @command{tar} refuses to replace existing files when extracting; and
11941 with the @option{--no-overwrite-dir} option, @command{tar} refuses to
11942 replace the permissions or ownership of already-existing directories.
11943 These options may help when extracting from untrusted archives.
11945 @node Live untrusted data
11946 @subsection Dealing with Live Untrusted Data
11948 Extra care is required when creating from or extracting into a file
11949 system that is accessible to untrusted users. For example, superusers
11950 who invoke @command{tar} must be wary about its actions being hijacked
11951 by an adversary who is reading or writing the file system at the same
11952 time that @command{tar} is operating.
11954 When creating an archive from a live file system, @command{tar} is
11955 vulnerable to denial-of-service attacks. For example, an adversarial
11956 user could create the illusion of an indefinitely-deep directory
11957 hierarchy @file{d/e/f/g/...} by creating directories one step ahead of
11958 @command{tar}, or the illusion of an indefinitely-long file by
11959 creating a sparse file but arranging for blocks to be allocated just
11960 before @command{tar} reads them. There is no easy way for
11961 @command{tar} to distinguish these scenarios from legitimate uses, so
11962 you may need to monitor @command{tar}, just as you'd need to monitor
11963 any other system service, to detect such attacks.
11965 While a superuser is extracting from an archive into a live file
11966 system, an untrusted user might replace a directory with a symbolic
11967 link, in hopes that @command{tar} will follow the symbolic link and
11968 extract data into files that the untrusted user does not have access
11969 to. Even if the archive was generated by the superuser, it may
11970 contain a file such as @file{d/etc/passwd} that the untrusted user
11971 earlier created in order to break in; if the untrusted user replaces
11972 the directory @file{d/etc} with a symbolic link to @file{/etc} while
11973 @command{tar} is running, @command{tar} will overwrite
11974 @file{/etc/passwd}. This attack can be prevented by extracting into a
11975 directory that is inaccessible to untrusted users.
11977 Similar attacks via symbolic links are also possible when creating an
11978 archive, if the untrusted user can modify an ancestor of a top-level
11979 argument of @command{tar}. For example, an untrusted user that can
11980 modify @file{/home/eve} can hijack a running instance of @samp{tar -cf
11981 - /home/eve/Documents/yesterday} by replacing
11982 @file{/home/eve/Documents} with a symbolic link to some other
11983 location. Attacks like these can be prevented by making sure that
11984 untrusted users cannot modify any files that are top-level arguments
11985 to @command{tar}, or any ancestor directories of these files.
11987 @node Security rules of thumb
11988 @subsection Security Rules of Thumb
11990 This section briefly summarizes rules of thumb for avoiding security
11996 Protect archives at least as much as you protect any of the files
12000 Extract from an untrusted archive only into an otherwise-empty
12001 directory. This directory and its parent should be accessible only to
12002 trusted users. For example:
12006 $ @kbd{chmod go-rwx .}
12007 $ @kbd{mkdir -m go-rwx dir}
12009 $ @kbd{tar -xvf /archives/got-it-off-the-net.tar.gz}
12013 As a corollary, do not do an incremental restore from an untrusted archive.
12016 Do not let untrusted users access files extracted from untrusted
12017 archives without checking first for problems such as setuid programs.
12020 Do not let untrusted users modify directories that are ancestors of
12021 top-level arguments of @command{tar}. For example, while you are
12022 executing @samp{tar -cf /archive/u-home.tar /u/home}, do not let an
12023 untrusted user modify @file{/}, @file{/archive}, or @file{/u}.
12026 Pay attention to the diagnostics and exit status of @command{tar}.
12029 When archiving live file systems, monitor running instances of
12030 @command{tar} to detect denial-of-service attacks.
12033 Avoid unusual options such as @option{--absolute-names} (@option{-P}),
12034 @option{--dereference} (@option{-h}), @option{--overwrite},
12035 @option{--recursive-unlink}, and @option{--remove-files} unless you
12036 understand their security implications.
12043 This appendix lists some important user-visible changes between
12044 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
12045 version of this document is available at
12046 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
12047 @GNUTAR{} documentation page}.
12050 @item Use of globbing patterns when listing and extracting.
12052 Previous versions of GNU tar assumed shell-style globbing when
12053 extracting from or listing an archive. For example:
12056 $ @kbd{tar xf foo.tar '*.c'}
12059 would extract all files whose names end in @samp{.c}. This behavior
12060 was not documented and was incompatible with traditional tar
12061 implementations. Therefore, starting from version 1.15.91, GNU tar
12062 no longer uses globbing by default. For example, the above invocation
12063 is now interpreted as a request to extract from the archive the file
12066 To facilitate transition to the new behavior for those users who got
12067 used to the previous incorrect one, @command{tar} will print a warning
12068 if it finds out that a requested member was not found in the archive
12069 and its name looks like a globbing pattern. For example:
12072 $ @kbd{tar xf foo.tar '*.c'}
12073 tar: Pattern matching characters used in file names. Please,
12074 tar: use --wildcards to enable pattern matching, or --no-wildcards to
12075 tar: suppress this warning.
12076 tar: *.c: Not found in archive
12077 tar: Error exit delayed from previous errors
12080 To treat member names as globbing patterns, use the @option{--wildcards} option.
12081 If you want to tar to mimic the behavior of versions prior to 1.15.91,
12082 add this option to your @env{TAR_OPTIONS} variable.
12084 @xref{wildcards}, for the detailed discussion of the use of globbing
12085 patterns by @GNUTAR{}.
12087 @item Use of short option @option{-o}.
12089 Earlier versions of @GNUTAR{} understood @option{-o} command line
12090 option as a synonym for @option{--old-archive}.
12092 @GNUTAR{} starting from version 1.13.90 understands this option as
12093 a synonym for @option{--no-same-owner}. This is compatible with
12094 UNIX98 @command{tar} implementations.
12096 However, to facilitate transition, @option{-o} option retains its
12097 old semantics when it is used with one of archive-creation commands.
12098 Users are encouraged to use @option{--format=oldgnu} instead.
12100 It is especially important, since versions of @acronym{GNU} Automake
12101 up to and including 1.8.4 invoke tar with this option to produce
12102 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
12103 of this issue and its implications.
12105 @xref{Options, tar-formats, Changing Automake's Behavior,
12106 automake, GNU Automake}, for a description on how to use various
12107 archive formats with @command{automake}.
12109 Future versions of @GNUTAR{} will understand @option{-o} only as a
12110 synonym for @option{--no-same-owner}.
12112 @item Use of short option @option{-l}
12114 Earlier versions of @GNUTAR{} understood @option{-l} option as a
12115 synonym for @option{--one-file-system}. Since such usage contradicted
12116 to UNIX98 specification and harmed compatibility with other
12117 implementations, it was declared deprecated in version 1.14. However,
12118 to facilitate transition to its new semantics, it was supported by
12119 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
12120 variant of @option{--check-links} was introduced in version 1.15.91.
12122 @item Use of options @option{--portability} and @option{--old-archive}
12124 These options are deprecated. Please use @option{--format=v7} instead.
12126 @item Use of option @option{--posix}
12128 This option is deprecated. Please use @option{--format=posix} instead.
12131 @node Configuring Help Summary
12132 @appendix Configuring Help Summary
12134 Running @kbd{tar --help} displays the short @command{tar} option
12135 summary (@pxref{help}). This summary is organized by @dfn{groups} of
12136 semantically close options. The options within each group are printed
12137 in the following order: a short option, eventually followed by a list
12138 of corresponding long option names, followed by a short description of
12139 the option. For example, here is an excerpt from the actual @kbd{tar
12143 Main operation mode:
12145 -A, --catenate, --concatenate append tar files to an archive
12146 -c, --create create a new archive
12147 -d, --diff, --compare find differences between archive and
12149 --delete delete from the archive
12152 @vrindex ARGP_HELP_FMT, environment variable
12153 The exact visual representation of the help output is configurable via
12154 @env{ARGP_HELP_FMT} environment variable. The value of this variable
12155 is a comma-separated list of @dfn{format variable} assignments. There
12156 are two kinds of format variables. An @dfn{offset variable} keeps the
12157 offset of some part of help output text from the leftmost column on
12158 the screen. A @dfn{boolean} variable is a flag that toggles some
12159 output feature on or off. Depending on the type of the corresponding
12160 variable, there are two kinds of assignments:
12163 @item Offset assignment
12165 The assignment to an offset variable has the following syntax:
12168 @var{variable}=@var{value}
12172 where @var{variable} is the variable name, and @var{value} is a
12173 numeric value to be assigned to the variable.
12175 @item Boolean assignment
12177 To assign @code{true} value to a variable, simply put this variable name. To
12178 assign @code{false} value, prefix the variable name with @samp{no-}. For
12183 # Assign @code{true} value:
12185 # Assign @code{false} value:
12191 Following variables are declared:
12193 @deftypevr {Help Output} boolean dup-args
12194 If true, arguments for an option are shown with both short and long
12195 options, even when a given option has both forms, for example:
12198 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
12201 If false, then if an option has both short and long forms, the
12202 argument is only shown with the long one, for example:
12205 -f, --file=ARCHIVE use archive file or device ARCHIVE
12209 and a message indicating that the argument is applicable to both
12210 forms is printed below the options. This message can be disabled
12211 using @code{dup-args-note} (see below).
12213 The default is false.
12216 @deftypevr {Help Output} boolean dup-args-note
12217 If this variable is true, which is the default, the following notice
12218 is displayed at the end of the help output:
12221 Mandatory or optional arguments to long options are also mandatory or
12222 optional for any corresponding short options.
12225 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
12226 variables @code{dup-args} or @code{dup-args-note} should be set.
12229 @deftypevr {Help Output} offset short-opt-col
12230 Column in which short options start. Default is 2.
12234 $ @kbd{tar --help|grep ARCHIVE}
12235 -f, --file=ARCHIVE use archive file or device ARCHIVE
12236 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
12237 -f, --file=ARCHIVE use archive file or device ARCHIVE
12242 @deftypevr {Help Output} offset long-opt-col
12243 Column in which long options start. Default is 6. For example:
12247 $ @kbd{tar --help|grep ARCHIVE}
12248 -f, --file=ARCHIVE use archive file or device ARCHIVE
12249 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
12250 -f, --file=ARCHIVE use archive file or device ARCHIVE
12255 @deftypevr {Help Output} offset doc-opt-col
12256 Column in which @dfn{doc options} start. A doc option isn't actually
12257 an option, but rather an arbitrary piece of documentation that is
12258 displayed in much the same manner as the options. For example, in
12259 the description of @option{--format} option:
12263 -H, --format=FORMAT create archive of the given format.
12265 FORMAT is one of the following:
12267 gnu GNU tar 1.13.x format
12268 oldgnu GNU format as per tar <= 1.12
12269 pax POSIX 1003.1-2001 (pax) format
12271 ustar POSIX 1003.1-1988 (ustar) format
12272 v7 old V7 tar format
12277 the format names are doc options. Thus, if you set
12278 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
12279 will look as follows:
12283 -H, --format=FORMAT create archive of the given format.
12285 FORMAT is one of the following:
12287 gnu GNU tar 1.13.x format
12288 oldgnu GNU format as per tar <= 1.12
12289 pax POSIX 1003.1-2001 (pax) format
12291 ustar POSIX 1003.1-1988 (ustar) format
12292 v7 old V7 tar format
12297 @deftypevr {Help Output} offset opt-doc-col
12298 Column in which option description starts. Default is 29.
12302 $ @kbd{tar --help|grep ARCHIVE}
12303 -f, --file=ARCHIVE use archive file or device ARCHIVE
12304 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
12305 -f, --file=ARCHIVE use archive file or device ARCHIVE
12306 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
12308 use archive file or device ARCHIVE
12313 Notice, that the description starts on a separate line if
12314 @code{opt-doc-col} value is too small.
12317 @deftypevr {Help Output} offset header-col
12318 Column in which @dfn{group headers} are printed. A group header is a
12319 descriptive text preceding an option group. For example, in the
12323 Main operation mode:
12325 -A, --catenate, --concatenate append tar files to
12327 -c, --create create a new archive
12330 @samp{Main operation mode:} is the group header.
12332 The default value is 1.
12335 @deftypevr {Help Output} offset usage-indent
12336 Indentation of wrapped usage lines. Affects @option{--usage}
12337 output. Default is 12.
12340 @deftypevr {Help Output} offset rmargin
12341 Right margin of the text output. Used for wrapping.
12344 @node Fixing Snapshot Files
12345 @appendix Fixing Snapshot Files
12346 @include tar-snapshot-edit.texi
12348 @node Tar Internals
12349 @appendix Tar Internals
12350 @include intern.texi
12354 @include genfile.texi
12356 @node Free Software Needs Free Documentation
12357 @appendix Free Software Needs Free Documentation
12358 @include freemanuals.texi
12360 @node GNU Free Documentation License
12361 @appendix GNU Free Documentation License
12365 @node Index of Command Line Options
12366 @appendix Index of Command Line Options
12368 This appendix contains an index of all @GNUTAR{} long command line
12369 options. The options are listed without the preceding double-dash.
12370 For a cross-reference of short command line options, see
12371 @ref{Short Option Summary}.
12384 @c Local variables:
12385 @c texinfo-column-for-description: 32
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