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
25 @c Put everything in one index (arbitrarily chosen to be the concept index).
33 This manual is for @acronym{GNU} @command{tar} (version
34 @value{VERSION}, @value{UPDATED}), which creates and extracts files
37 Copyright @copyright{} 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001,
38 2003, 2004, 2005, 2006, 2007, 2008 Free Software Foundation, Inc.
41 Permission is granted to copy, distribute and/or modify this document
42 under the terms of the GNU Free Documentation License, Version 1.1 or
43 any later version published by the Free Software Foundation; with no
44 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
45 and with the Back-Cover Texts as in (a) below. A copy of the license
46 is included in the section entitled "GNU Free Documentation License".
48 (a) The FSF's Back-Cover Text is: ``You have the freedom to
49 copy and modify this GNU manual. Buying copies from the FSF
50 supports it in developing GNU and promoting software freedom.''
54 @dircategory Archiving
56 * Tar: (tar). Making tape (or disk) archives.
59 @dircategory Individual utilities
61 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
64 @shorttitlepage @acronym{GNU} @command{tar}
67 @title @acronym{GNU} tar: an archiver tool
68 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
69 @author John Gilmore, Jay Fenlason et al.
72 @vskip 0pt plus 1filll
78 @top @acronym{GNU} tar: an archiver tool
83 @cindex archiving files
85 The first part of this master menu lists the major nodes in this Info
86 document. The rest of the menu lists all the lower level nodes.
89 @c The master menu goes here.
91 @c NOTE: To update it from within Emacs, make sure mastermenu.el is
92 @c loaded and run texinfo-master-menu.
93 @c To update it from the command line, run
104 * Date input formats::
111 * Configuring Help Summary::
112 * Fixing Snapshot Files::
115 * Free Software Needs Free Documentation::
116 * Copying This Manual::
117 * Index of Command Line Options::
121 --- The Detailed Node Listing ---
125 * Book Contents:: What this Book Contains
126 * Definitions:: Some Definitions
127 * What tar Does:: What @command{tar} Does
128 * Naming tar Archives:: How @command{tar} Archives are Named
129 * Authors:: @GNUTAR{} Authors
130 * Reports:: Reporting bugs or suggestions
132 Tutorial Introduction to @command{tar}
135 * stylistic conventions::
136 * basic tar options:: Basic @command{tar} Operations and Options
137 * frequent operations::
138 * Two Frequent Options::
139 * create:: How to Create Archives
140 * list:: How to List Archives
141 * extract:: How to Extract Members from an Archive
144 Two Frequently Used Options
150 How to Create Archives
152 * prepare for examples::
153 * Creating the archive::
162 How to Extract Members from an Archive
164 * extracting archives::
167 * extracting untrusted archives::
173 * using tar options::
182 The Three Option Styles
184 * Long Options:: Long Option Style
185 * Short Options:: Short Option Style
186 * Old Options:: Old Option Style
187 * Mixing:: Mixing Option Styles
189 All @command{tar} Options
191 * Operation Summary::
193 * Short Option Summary::
205 Advanced @GNUTAR{} Operations
214 How to Add Files to Existing Archives: @option{--append}
216 * appending files:: Appending Files to an Archive
223 Options Used by @option{--create}
225 * override:: Overriding File Metadata.
226 * Ignore Failed Read::
228 Options Used by @option{--extract}
230 * Reading:: Options to Help Read Archives
231 * Writing:: Changing How @command{tar} Writes Files
232 * Scarce:: Coping with Scarce Resources
234 Options to Help Read Archives
236 * read full records::
239 Changing How @command{tar} Writes Files
241 * Dealing with Old Files::
242 * Overwrite Old Files::
247 * Data Modification Times::
248 * Setting Access Permissions::
249 * Directory Modification Times and Permissions::
250 * Writing to Standard Output::
251 * Writing to an External Program::
254 Coping with Scarce Resources
259 Performing Backups and Restoring Files
261 * Full Dumps:: Using @command{tar} to Perform Full Dumps
262 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
263 * Backup Levels:: Levels of Backups
264 * Backup Parameters:: Setting Parameters for Backups and Restoration
265 * Scripted Backups:: Using the Backup Scripts
266 * Scripted Restoration:: Using the Restore Script
268 Setting Parameters for Backups and Restoration
270 * General-Purpose Variables::
271 * Magnetic Tape Control::
273 * backup-specs example:: An Example Text of @file{Backup-specs}
275 Choosing Files and Names for @command{tar}
277 * file:: Choosing the Archive's Name
278 * Selecting Archive Members::
279 * files:: Reading Names from a File
280 * exclude:: Excluding Some Files
281 * wildcards:: Wildcards Patterns and Matching
282 * quoting styles:: Ways of Quoting Special Characters in Names
283 * transform:: Modifying File and Member Names
284 * after:: Operating Only on New Files
285 * recurse:: Descending into Directories
286 * one:: Crossing File System Boundaries
288 Reading Names from a File
294 * problems with exclude::
296 Wildcards Patterns and Matching
298 * controlling pattern-matching::
300 Crossing File System Boundaries
302 * directory:: Changing Directory
303 * absolute:: Absolute File Names
307 * General date syntax:: Common rules.
308 * Calendar date items:: 19 Dec 1994.
309 * Time of day items:: 9:20pm.
310 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}.
311 * Day of week items:: Monday and others.
312 * Relative items in date strings:: next tuesday, 2 years ago.
313 * Pure numbers in date strings:: 19931219, 1440.
314 * Seconds since the Epoch:: @@1078100502.
315 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0".
316 * Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al.
318 Controlling the Archive Format
320 * Compression:: Using Less Space through Compression
321 * Attributes:: Handling File Attributes
322 * Portability:: Making @command{tar} Archives More Portable
323 * cpio:: Comparison of @command{tar} and @command{cpio}
325 Using Less Space through Compression
327 * gzip:: Creating and Reading Compressed Archives
328 * sparse:: Archiving Sparse Files
330 Making @command{tar} Archives More Portable
332 * Portable Names:: Portable Names
333 * dereference:: Symbolic Links
334 * hard links:: Hard Links
335 * old:: Old V7 Archives
336 * ustar:: Ustar Archives
337 * gnu:: GNU and old GNU format archives.
338 * posix:: @acronym{POSIX} archives
339 * Checksumming:: Checksumming Problems
340 * Large or Negative Values:: Large files, negative time stamps, etc.
341 * Other Tars:: How to Extract GNU-Specific Data Using
342 Other @command{tar} Implementations
344 @GNUTAR{} and @acronym{POSIX} @command{tar}
346 * PAX keywords:: Controlling Extended Header Keywords.
348 How to Extract GNU-Specific Data Using Other @command{tar} Implementations
350 * Split Recovery:: Members Split Between Volumes
351 * Sparse Recovery:: Sparse Members
353 Tapes and Other Archive Media
355 * Device:: Device selection and switching
356 * Remote Tape Server::
357 * Common Problems and Solutions::
358 * Blocking:: Blocking
359 * Many:: Many archives on one tape
360 * Using Multiple Tapes:: Using Multiple Tapes
361 * label:: Including a Label in the Archive
367 * Format Variations:: Format Variations
368 * Blocking Factor:: The Blocking Factor of an Archive
370 Many Archives on One Tape
372 * Tape Positioning:: Tape Positions and Tape Marks
373 * mt:: The @command{mt} Utility
377 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
378 * Tape Files:: Tape Files
379 * Tarcat:: Concatenate Volumes into a Single Archive
384 * Standard:: Basic Tar Format
385 * Extensions:: @acronym{GNU} Extensions to the Archive Format
386 * Sparse Formats:: Storing Sparse Files
393 * PAX 0:: PAX Format, Versions 0.0 and 0.1
394 * PAX 1:: PAX Format, Version 1.0
398 * Generate Mode:: File Generation Mode.
399 * Status Mode:: File Status Mode.
400 * Exec Mode:: Synchronous Execution mode.
404 * GNU Free Documentation License:: License for copying this manual
410 @chapter Introduction
413 and manipulates @dfn{archives} which are actually collections of
414 many other files; the program provides users with an organized and
415 systematic method for controlling a large amount of data.
416 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
417 archives need not (and these days, typically do not) reside on tapes.
420 * Book Contents:: What this Book Contains
421 * Definitions:: Some Definitions
422 * What tar Does:: What @command{tar} Does
423 * Naming tar Archives:: How @command{tar} Archives are Named
424 * Authors:: @GNUTAR{} Authors
425 * Reports:: Reporting bugs or suggestions
429 @section What this Book Contains
431 The first part of this chapter introduces you to various terms that will
432 recur throughout the book. It also tells you who has worked on @GNUTAR{}
433 and its documentation, and where you should send bug reports
436 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
437 gentle introduction for people who are new to using @command{tar}. It is
438 meant to be self contained, not requiring any reading from subsequent
439 chapters to make sense. It moves from topic to topic in a logical,
440 progressive order, building on information already explained.
442 Although the tutorial is paced and structured to allow beginners to
443 learn how to use @command{tar}, it is not intended solely for beginners.
444 The tutorial explains how to use the three most frequently used
445 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
446 two frequently used options (@samp{file} and @samp{verbose}). The other
447 chapters do not refer to the tutorial frequently; however, if a section
448 discusses something which is a complex variant of a basic concept, there
449 may be a cross reference to that basic concept. (The entire book,
450 including the tutorial, assumes that the reader understands some basic
451 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
453 The third chapter presents the remaining five operations, and
454 information about using @command{tar} options and option syntax.
456 The other chapters are meant to be used as a reference. Each chapter
457 presents everything that needs to be said about a specific topic.
459 One of the chapters (@pxref{Date input formats}) exists in its
460 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
461 In addition, one section of this manual (@pxref{Standard}) contains a
462 big quote which is taken directly from @command{tar} sources.
464 In general, we give both long and short (abbreviated) option names
465 at least once in each section where the relevant option is covered, so
466 that novice readers will become familiar with both styles. (A few
467 options have no short versions, and the relevant sections will
471 @section Some Definitions
475 The @command{tar} program is used to create and manipulate @command{tar}
476 archives. An @dfn{archive} is a single file which contains the contents
477 of many files, while still identifying the names of the files, their
478 owner(s), and so forth. (In addition, archives record access
479 permissions, user and group, size in bytes, and data modification time.
480 Some archives also record the file names in each archived directory, as
481 well as other file and directory information.) You can use @command{tar}
482 to @dfn{create} a new archive in a specified directory.
485 @cindex archive member
488 The files inside an archive are called @dfn{members}. Within this
489 manual, we use the term @dfn{file} to refer only to files accessible in
490 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
491 @dfn{member} to refer only to the members of an archive. Similarly, a
492 @dfn{file name} is the name of a file, as it resides in the file system,
493 and a @dfn{member name} is the name of an archive member within the
498 The term @dfn{extraction} refers to the process of copying an archive
499 member (or multiple members) into a file in the file system. Extracting
500 all the members of an archive is often called @dfn{extracting the
501 archive}. The term @dfn{unpack} can also be used to refer to the
502 extraction of many or all the members of an archive. Extracting an
503 archive does not destroy the archive's structure, just as creating an
504 archive does not destroy the copies of the files that exist outside of
505 the archive. You may also @dfn{list} the members in a given archive
506 (this is often thought of as ``printing'' them to the standard output,
507 or the command line), or @dfn{append} members to a pre-existing archive.
508 All of these operations can be performed using @command{tar}.
511 @section What @command{tar} Does
514 The @command{tar} program provides the ability to create @command{tar}
515 archives, as well as various other kinds of manipulation. For example,
516 you can use @command{tar} on previously created archives to extract files,
517 to store additional files, or to update or list files which were already
520 Initially, @command{tar} archives were used to store files conveniently on
521 magnetic tape. The name @command{tar} comes from this use; it stands for
522 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
523 direct its output to available devices, files, or other programs (using
524 pipes). @command{tar} may even access remote devices or files (as archives).
526 You can use @command{tar} archives in many ways. We want to stress a few
527 of them: storage, backup, and transportation.
529 @FIXME{the following table entries need a bit of work.}
532 Often, @command{tar} archives are used to store related files for
533 convenient file transfer over a network. For example, the
534 @acronym{GNU} Project distributes its software bundled into
535 @command{tar} archives, so that all the files relating to a particular
536 program (or set of related programs) can be transferred as a single
539 A magnetic tape can store several files in sequence. However, the tape
540 has no names for these files; it only knows their relative position on
541 the tape. One way to store several files on one tape and retain their
542 names is by creating a @command{tar} archive. Even when the basic transfer
543 mechanism can keep track of names, as FTP can, the nuisance of handling
544 multiple files, directories, and multiple links makes @command{tar}
547 Archive files are also used for long-term storage. You can think of
548 this as transportation from the present into the future. (It is a
549 science-fiction idiom that you can move through time as well as in
550 space; the idea here is that @command{tar} can be used to move archives in
551 all dimensions, even time!)
554 Because the archive created by @command{tar} is capable of preserving
555 file information and directory structure, @command{tar} is commonly
556 used for performing full and incremental backups of disks. A backup
557 puts a collection of files (possibly pertaining to many users and
558 projects) together on a disk or a tape. This guards against
559 accidental destruction of the information in those files.
560 @GNUTAR{} has special features that allow it to be
561 used to make incremental and full dumps of all the files in a
565 You can create an archive on one system, transfer it to another system,
566 and extract the contents there. This allows you to transport a group of
567 files from one system to another.
570 @node Naming tar Archives
571 @section How @command{tar} Archives are Named
573 Conventionally, @command{tar} archives are given names ending with
574 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
575 but this manual follows that convention in order to accustom readers to
576 it and to make examples more clear.
581 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
582 archive members as ``files'' or ``entries''. For people familiar with
583 the operation of @command{tar}, this causes no difficulty. However, in
584 this manual, we consistently refer to ``archives'' and ``archive
585 members'' to make learning to use @command{tar} easier for novice users.
588 @section @GNUTAR{} Authors
590 @GNUTAR{} was originally written by John Gilmore,
591 and modified by many people. The @acronym{GNU} enhancements were
592 written by Jay Fenlason, then Joy Kendall, and the whole package has
593 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
594 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
595 numerous and kind users.
597 We wish to stress that @command{tar} is a collective work, and owes much to
598 all those people who reported problems, offered solutions and other
599 insights, or shared their thoughts and suggestions. An impressive, yet
600 partial list of those contributors can be found in the @file{THANKS}
601 file from the @GNUTAR{} distribution.
603 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
604 sure i want to spell out the history in this detail, at least not for
605 the printed book. i'm just not sure it needs to be said this way.
606 i'll think about it.}
608 @FIXME{History is more important, and surely more interesting, than
609 actual names. Quoting names without history would be meaningless. FP}
611 Jay Fenlason put together a draft of a @GNUTAR{}
612 manual, borrowing notes from the original man page from John Gilmore.
613 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
614 Gorin worked on a tutorial and manual for @GNUTAR{}.
615 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
616 taking information from all these sources and merging them. Melissa
617 Weisshaus finally edited and redesigned the book to create version
618 1.12. The book for versions from 1.14 up to @value{VERSION} were edited
619 by the current maintainer, Sergey Poznyakoff.
621 For version 1.12, Daniel Hagerty contributed a great deal of technical
622 consulting. In particular, he is the primary author of @ref{Backups}.
624 In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org
625 (see @url{http://savannah.gnu.org/projects/tar}), and
626 active development and maintenance work has started
627 again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey
628 Poznyakoff and Jeff Bailey.
630 Support for @acronym{POSIX} archives was added by Sergey Poznyakoff.
633 @section Reporting bugs or suggestions
636 @cindex reporting bugs
637 If you find problems or have suggestions about this program or manual,
638 please report them to @file{bug-tar@@gnu.org}.
640 When reporting a bug, please be sure to include as much detail as
641 possible, in order to reproduce it. @FIXME{Be more specific, I'd
642 like to make this node as detailed as 'Bug reporting' node in Emacs
646 @chapter Tutorial Introduction to @command{tar}
648 This chapter guides you through some basic examples of three @command{tar}
649 operations: @option{--create}, @option{--list}, and @option{--extract}. If
650 you already know how to use some other version of @command{tar}, then you
651 may not need to read this chapter. This chapter omits most complicated
652 details about how @command{tar} works.
656 * stylistic conventions::
657 * basic tar options:: Basic @command{tar} Operations and Options
658 * frequent operations::
659 * Two Frequent Options::
660 * create:: How to Create Archives
661 * list:: How to List Archives
662 * extract:: How to Extract Members from an Archive
667 @section Assumptions this Tutorial Makes
669 This chapter is paced to allow beginners to learn about @command{tar}
670 slowly. At the same time, we will try to cover all the basic aspects of
671 these three operations. In order to accomplish both of these tasks, we
672 have made certain assumptions about your knowledge before reading this
673 manual, and the hardware you will be using:
677 Before you start to work through this tutorial, you should understand
678 what the terms ``archive'' and ``archive member'' mean
679 (@pxref{Definitions}). In addition, you should understand something
680 about how Unix-type operating systems work, and you should know how to
681 use some basic utilities. For example, you should know how to create,
682 list, copy, rename, edit, and delete files and directories; how to
683 change between directories; and how to figure out where you are in the
684 file system. You should have some basic understanding of directory
685 structure and how files are named according to which directory they are
686 in. You should understand concepts such as standard output and standard
687 input, what various definitions of the term @samp{argument} mean, and the
688 differences between relative and absolute file names. @FIXME{and what
692 This manual assumes that you are working from your own home directory
693 (unless we state otherwise). In this tutorial, you will create a
694 directory to practice @command{tar} commands in. When we show file names,
695 we will assume that those names are relative to your home directory.
696 For example, my home directory is @file{/home/fsf/melissa}. All of
697 my examples are in a subdirectory of the directory named by that file
698 name; the subdirectory is called @file{practice}.
701 In general, we show examples of archives which exist on (or can be
702 written to, or worked with from) a directory on a hard disk. In most
703 cases, you could write those archives to, or work with them on any other
704 device, such as a tape drive. However, some of the later examples in
705 the tutorial and next chapter will not work on tape drives.
706 Additionally, working with tapes is much more complicated than working
707 with hard disks. For these reasons, the tutorial does not cover working
708 with tape drives. @xref{Media}, for complete information on using
709 @command{tar} archives with tape drives.
711 @FIXME{this is a cop out. need to add some simple tape drive info.}
714 @node stylistic conventions
715 @section Stylistic Conventions
717 In the examples, @samp{$} represents a typical shell prompt. It
718 precedes lines you should type; to make this more clear, those lines are
719 shown in @kbd{this font}, as opposed to lines which represent the
720 computer's response; those lines are shown in @code{this font}, or
721 sometimes @samp{like this}.
723 @c When we have lines which are too long to be
724 @c displayed in any other way, we will show them like this:
726 @node basic tar options
727 @section Basic @command{tar} Operations and Options
729 @command{tar} can take a wide variety of arguments which specify and define
730 the actions it will have on the particular set of files or the archive.
731 The main types of arguments to @command{tar} fall into one of two classes:
732 operations, and options.
734 Some arguments fall into a class called @dfn{operations}; exactly one of
735 these is both allowed and required for any instance of using @command{tar};
736 you may @emph{not} specify more than one. People sometimes speak of
737 @dfn{operating modes}. You are in a particular operating mode when you
738 have specified the operation which specifies it; there are eight
739 operations in total, and thus there are eight operating modes.
741 The other arguments fall into the class known as @dfn{options}. You are
742 not required to specify any options, and you are allowed to specify more
743 than one at a time (depending on the way you are using @command{tar} at
744 that time). Some options are used so frequently, and are so useful for
745 helping you type commands more carefully that they are effectively
746 ``required''. We will discuss them in this chapter.
748 You can write most of the @command{tar} operations and options in any
749 of three forms: long (mnemonic) form, short form, and old style. Some
750 of the operations and options have no short or ``old'' forms; however,
751 the operations and options which we will cover in this tutorial have
752 corresponding abbreviations. We will indicate those abbreviations
753 appropriately to get you used to seeing them. (Note that the ``old
754 style'' option forms exist in @GNUTAR{} for compatibility with Unix
755 @command{tar}. In this book we present a full discussion of this way
756 of writing options and operations (@pxref{Old Options}), and we discuss
757 the other two styles of writing options (@xref{Long Options}, and
758 @pxref{Short Options}).
760 In the examples and in the text of this tutorial, we usually use the
761 long forms of operations and options; but the ``short'' forms produce
762 the same result and can make typing long @command{tar} commands easier.
763 For example, instead of typing
766 @kbd{tar --create --verbose --file=afiles.tar apple angst aspic}
772 @kbd{tar -c -v -f afiles.tar apple angst aspic}
778 @kbd{tar -cvf afiles.tar apple angst aspic}
782 For more information on option syntax, see @ref{Advanced tar}. In
783 discussions in the text, when we name an option by its long form, we
784 also give the corresponding short option in parentheses.
786 The term, ``option'', can be confusing at times, since ``operations''
787 are often lumped in with the actual, @emph{optional} ``options'' in certain
788 general class statements. For example, we just talked about ``short and
789 long forms of options and operations''. However, experienced @command{tar}
790 users often refer to these by shorthand terms such as, ``short and long
791 options''. This term assumes that the ``operations'' are included, also.
792 Context will help you determine which definition of ``options'' to use.
794 Similarly, the term ``command'' can be confusing, as it is often used in
795 two different ways. People sometimes refer to @command{tar} ``commands''.
796 A @command{tar} @dfn{command} is the entire command line of user input
797 which tells @command{tar} what to do --- including the operation, options,
798 and any arguments (file names, pipes, other commands, etc.). However,
799 you will also sometimes hear the term ``the @command{tar} command''. When
800 the word ``command'' is used specifically like this, a person is usually
801 referring to the @command{tar} @emph{operation}, not the whole line.
802 Again, use context to figure out which of the meanings the speaker
805 @node frequent operations
806 @section The Three Most Frequently Used Operations
808 Here are the three most frequently used operations (both short and long
809 forms), as well as a brief description of their meanings. The rest of
810 this chapter will cover how to use these operations in detail. We will
811 present the rest of the operations in the next chapter.
816 Create a new @command{tar} archive.
819 List the contents of an archive.
822 Extract one or more members from an archive.
825 @node Two Frequent Options
826 @section Two Frequently Used Options
828 To understand how to run @command{tar} in the three operating modes listed
829 previously, you also need to understand how to use two of the options to
830 @command{tar}: @option{--file} (which takes an archive file as an argument)
831 and @option{--verbose}. (You are usually not @emph{required} to specify
832 either of these options when you run @command{tar}, but they can be very
833 useful in making things more clear and helping you avoid errors.)
842 @unnumberedsubsec The @option{--file} Option
845 @xopindex{file, tutorial}
846 @item --file=@var{archive-name}
847 @itemx -f @var{archive-name}
848 Specify the name of an archive file.
851 You can specify an argument for the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option whenever you
852 use @command{tar}; this option determines the name of the archive file
853 that @command{tar} will work on.
856 If you don't specify this argument, then @command{tar} will examine
857 the environment variable @env{TAPE}. If it is set, its value will be
858 used as the archive name. Otherwise, @command{tar} will use the
859 default archive, determined at the compile time. Usually it is
860 standard output or some physical tape drive attached to your machine
861 (you can verify what the default is by running @kbd{tar
862 --show-defaults}, @pxref{defaults}). If there is no tape drive
863 attached, or the default is not meaningful, then @command{tar} will
864 print an error message. The error message might look roughly like one
868 tar: can't open /dev/rmt8 : No such device or address
869 tar: can't open /dev/rsmt0 : I/O error
873 To avoid confusion, we recommend that you always specify an archive file
874 name by using @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) when writing your @command{tar} commands.
875 For more information on using the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option, see
878 @node verbose tutorial
879 @unnumberedsubsec The @option{--verbose} Option
882 @xopindex{verbose, introduced}
885 Show the files being worked on as @command{tar} is running.
888 @option{--verbose} (@option{-v}) shows details about the results of running
889 @command{tar}. This can be especially useful when the results might not be
890 obvious. For example, if you want to see the progress of @command{tar} as
891 it writes files into the archive, you can use the @option{--verbose}
892 option. In the beginning, you may find it useful to use
893 @option{--verbose} at all times; when you are more accustomed to
894 @command{tar}, you will likely want to use it at certain times but not at
895 others. We will use @option{--verbose} at times to help make something
896 clear, and we will give many examples both using and not using
897 @option{--verbose} to show the differences.
899 Each instance of @option{--verbose} on the command line increases the
900 verbosity level by one, so if you need more details on the output,
903 When reading archives (@option{--list}, @option{--extract},
904 @option{--diff}), @command{tar} by default prints only the names of
905 the members being extracted. Using @option{--verbose} will show a full,
906 @command{ls} style member listing.
908 In contrast, when writing archives (@option{--create}, @option{--append},
909 @option{--update}), @command{tar} does not print file names by
910 default. So, a single @option{--verbose} option shows the file names
911 being added to the archive, while two @option{--verbose} options
912 enable the full listing.
914 For example, to create an archive in verbose mode:
917 $ @kbd{tar -cvf afiles.tar apple angst aspic}
924 Creating the same archive with the verbosity level 2 could give:
927 $ @kbd{tar -cvvf afiles.tar apple angst aspic}
928 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
929 -rw-r--r-- gray/staff 11481 2006-06-09 12:06 angst
930 -rw-r--r-- gray/staff 23152 2006-06-09 12:06 aspic
934 This works equally well using short or long forms of options. Using
935 long forms, you would simply write out the mnemonic form of the option
939 $ @kbd{tar --create --verbose --verbose @dots{}}
943 Note that you must double the hyphens properly each time.
945 Later in the tutorial, we will give examples using @w{@option{--verbose
948 @anchor{verbose member listing}
949 The full output consists of six fields:
952 @item File type and permissions in symbolic form.
953 These are displayed in the same format as the first column of
954 @command{ls -l} output (@pxref{What information is listed,
955 format=verbose, Verbose listing, fileutils, GNU file utilities}).
957 @item Owner name and group separated by a slash character.
958 If these data are not available (for example, when listing a @samp{v7} format
959 archive), numeric @acronym{ID} values are printed instead.
961 @item Size of the file, in bytes.
963 @item File modification date in ISO 8601 format.
965 @item File modification time.
968 If the name contains any special characters (white space, newlines,
969 etc.) these are displayed in an unambiguous form using so called
970 @dfn{quoting style}. For the detailed discussion of available styles
971 and on how to use them, see @ref{quoting styles}.
973 Depending on the file type, the name can be followed by some
974 additional information, described in the following table:
977 @item -> @var{link-name}
978 The file or archive member is a @dfn{symbolic link} and
979 @var{link-name} is the name of file it links to.
981 @item link to @var{link-name}
982 The file or archive member is a @dfn{hard link} and @var{link-name} is
983 the name of file it links to.
986 The archive member is an old GNU format long link. You will normally
990 The archive member is an old GNU format long name. You will normally
993 @item --Volume Header--
994 The archive member is a GNU @dfn{volume header} (@pxref{Tape Files}).
996 @item --Continued at byte @var{n}--
997 Encountered only at the beginning of a multi-volume archive
998 (@pxref{Using Multiple Tapes}). This archive member is a continuation
999 from the previous volume. The number @var{n} gives the offset where
1000 the original file was split.
1002 @item unknown file type @var{c}
1003 An archive member of unknown type. @var{c} is the type character from
1004 the archive header. If you encounter such a message, it means that
1005 either your archive contains proprietary member types @GNUTAR{} is not
1006 able to handle, or the archive is corrupted.
1011 For example, here is an archive listing containing most of the special
1012 suffixes explained above:
1016 V--------- 0/0 1536 2006-06-09 13:07 MyVolume--Volume Header--
1017 -rw-r--r-- gray/staff 456783 2006-06-09 12:06 aspic--Continued at
1019 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
1020 lrwxrwxrwx gray/staff 0 2006-06-09 13:01 angst -> apple
1021 -rw-r--r-- gray/staff 35793 2006-06-09 12:06 blues
1022 hrw-r--r-- gray/staff 0 2006-06-09 12:06 music link to blues
1030 @unnumberedsubsec Getting Help: Using the @option{--help} Option
1036 The @option{--help} option to @command{tar} prints out a very brief list of
1037 all operations and option available for the current version of
1038 @command{tar} available on your system.
1042 @section How to Create Archives
1045 @cindex Creation of the archive
1046 @cindex Archive, creation of
1047 One of the basic operations of @command{tar} is @option{--create} (@option{-c}), which
1048 you use to create a @command{tar} archive. We will explain
1049 @option{--create} first because, in order to learn about the other
1050 operations, you will find it useful to have an archive available to
1053 To make this easier, in this section you will first create a directory
1054 containing three files. Then, we will show you how to create an
1055 @emph{archive} (inside the new directory). Both the directory, and
1056 the archive are specifically for you to practice on. The rest of this
1057 chapter and the next chapter will show many examples using this
1058 directory and the files you will create: some of those files may be
1059 other directories and other archives.
1061 The three files you will archive in this example are called
1062 @file{blues}, @file{folk}, and @file{jazz}. The archive is called
1063 @file{collection.tar}.
1065 This section will proceed slowly, detailing how to use @option{--create}
1066 in @code{verbose} mode, and showing examples using both short and long
1067 forms. In the rest of the tutorial, and in the examples in the next
1068 chapter, we will proceed at a slightly quicker pace. This section
1069 moves more slowly to allow beginning users to understand how
1070 @command{tar} works.
1073 * prepare for examples::
1074 * Creating the archive::
1080 @node prepare for examples
1081 @subsection Preparing a Practice Directory for Examples
1083 To follow along with this and future examples, create a new directory
1084 called @file{practice} containing files called @file{blues}, @file{folk}
1085 and @file{jazz}. The files can contain any information you like:
1086 ideally, they should contain information which relates to their names,
1087 and be of different lengths. Our examples assume that @file{practice}
1088 is a subdirectory of your home directory.
1090 Now @command{cd} to the directory named @file{practice}; @file{practice}
1091 is now your @dfn{working directory}. (@emph{Please note}: Although
1092 the full file name of this directory is
1093 @file{/@var{homedir}/practice}, in our examples we will refer to
1094 this directory as @file{practice}; the @var{homedir} is presumed.
1096 In general, you should check that the files to be archived exist where
1097 you think they do (in the working directory) by running @command{ls}.
1098 Because you just created the directory and the files and have changed to
1099 that directory, you probably don't need to do that this time.
1101 It is very important to make sure there isn't already a file in the
1102 working directory with the archive name you intend to use (in this case,
1103 @samp{collection.tar}), or that you don't care about its contents.
1104 Whenever you use @samp{create}, @command{tar} will erase the current
1105 contents of the file named by @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) if it exists. @command{tar}
1106 will not tell you if you are about to overwrite an archive unless you
1107 specify an option which does this (@pxref{backup}, for the
1108 information on how to do so). To add files to an existing archive,
1109 you need to use a different option, such as @option{--append} (@option{-r}); see
1110 @ref{append} for information on how to do this.
1112 @node Creating the archive
1113 @subsection Creating the Archive
1115 @xopindex{create, introduced}
1116 To place the files @file{blues}, @file{folk}, and @file{jazz} into an
1117 archive named @file{collection.tar}, use the following command:
1120 $ @kbd{tar --create --file=collection.tar blues folk jazz}
1123 The order of the arguments is not very important, @emph{when using long
1124 option forms}. You could also say:
1127 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1131 However, you can see that this order is harder to understand; this is
1132 why we will list the arguments in the order that makes the commands
1133 easiest to understand (and we encourage you to do the same when you use
1134 @command{tar}, to avoid errors).
1136 Note that the sequence
1137 @option{--file=@-collection.tar} is considered to be @emph{one} argument.
1138 If you substituted any other string of characters for
1139 @kbd{collection.tar}, then that string would become the name of the
1140 archive file you create.
1142 The order of the options becomes more important when you begin to use
1143 short forms. With short forms, if you type commands in the wrong order
1144 (even if you type them correctly in all other ways), you may end up with
1145 results you don't expect. For this reason, it is a good idea to get
1146 into the habit of typing options in the order that makes inherent sense.
1147 @xref{short create}, for more information on this.
1149 In this example, you type the command as shown above: @option{--create}
1150 is the operation which creates the new archive
1151 (@file{collection.tar}), and @option{--file} is the option which lets
1152 you give it the name you chose. The files, @file{blues}, @file{folk},
1153 and @file{jazz}, are now members of the archive, @file{collection.tar}
1154 (they are @dfn{file name arguments} to the @option{--create} operation.
1155 @xref{Choosing}, for the detailed discussion on these.) Now that they are
1156 in the archive, they are called @emph{archive members}, not files.
1157 (@pxref{Definitions,members}).
1159 When you create an archive, you @emph{must} specify which files you
1160 want placed in the archive. If you do not specify any archive
1161 members, @GNUTAR{} will complain.
1163 If you now list the contents of the working directory (@command{ls}), you will
1164 find the archive file listed as well as the files you saw previously:
1167 blues folk jazz collection.tar
1171 Creating the archive @samp{collection.tar} did not destroy the copies of
1172 the files in the directory.
1174 Keep in mind that if you don't indicate an operation, @command{tar} will not
1175 run and will prompt you for one. If you don't name any files, @command{tar}
1176 will complain. You must have write access to the working directory,
1177 or else you will not be able to create an archive in that directory.
1179 @emph{Caution}: Do not attempt to use @option{--create} (@option{-c}) to add files to
1180 an existing archive; it will delete the archive and write a new one.
1181 Use @option{--append} (@option{-r}) instead. @xref{append}.
1183 @node create verbose
1184 @subsection Running @option{--create} with @option{--verbose}
1186 @xopindex{create, using with @option{--verbose}}
1187 @xopindex{verbose, using with @option{--create}}
1188 If you include the @option{--verbose} (@option{-v}) option on the command line,
1189 @command{tar} will list the files it is acting on as it is working. In
1190 verbose mode, the @code{create} example above would appear as:
1193 $ @kbd{tar --create --verbose --file=collection.tar blues folk jazz}
1199 This example is just like the example we showed which did not use
1200 @option{--verbose}, except that @command{tar} generated the remaining lines
1202 (note the different font styles).
1208 In the rest of the examples in this chapter, we will frequently use
1209 @code{verbose} mode so we can show actions or @command{tar} responses that
1210 you would otherwise not see, and which are important for you to
1214 @subsection Short Forms with @samp{create}
1216 As we said before, the @option{--create} (@option{-c}) operation is one of the most
1217 basic uses of @command{tar}, and you will use it countless times.
1218 Eventually, you will probably want to use abbreviated (or ``short'')
1219 forms of options. A full discussion of the three different forms that
1220 options can take appears in @ref{Styles}; for now, here is what the
1221 previous example (including the @option{--verbose} (@option{-v}) option) looks like
1222 using short option forms:
1225 $ @kbd{tar -cvf collection.tar blues folk jazz}
1232 As you can see, the system responds the same no matter whether you use
1233 long or short option forms.
1235 @FIXME{i don't like how this is worded:} One difference between using
1236 short and long option forms is that, although the exact placement of
1237 arguments following options is no more specific when using short forms,
1238 it is easier to become confused and make a mistake when using short
1239 forms. For example, suppose you attempted the above example in the
1243 $ @kbd{tar -cfv collection.tar blues folk jazz}
1247 In this case, @command{tar} will make an archive file called @file{v},
1248 containing the files @file{blues}, @file{folk}, and @file{jazz}, because
1249 the @samp{v} is the closest ``file name'' to the @option{-f} option, and
1250 is thus taken to be the chosen archive file name. @command{tar} will try
1251 to add a file called @file{collection.tar} to the @file{v} archive file;
1252 if the file @file{collection.tar} did not already exist, @command{tar} will
1253 report an error indicating that this file does not exist. If the file
1254 @file{collection.tar} does already exist (e.g., from a previous command
1255 you may have run), then @command{tar} will add this file to the archive.
1256 Because the @option{-v} option did not get registered, @command{tar} will not
1257 run under @samp{verbose} mode, and will not report its progress.
1259 The end result is that you may be quite confused about what happened,
1260 and possibly overwrite a file. To illustrate this further, we will show
1261 you how an example we showed previously would look using short forms.
1266 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1270 is confusing as it is. When shown using short forms, however, it
1271 becomes much more so:
1274 $ @kbd{tar blues -c folk -f collection.tar jazz}
1278 It would be very easy to put the wrong string of characters
1279 immediately following the @option{-f}, but doing that could sacrifice
1282 For this reason, we recommend that you pay very careful attention to
1283 the order of options and placement of file and archive names,
1284 especially when using short option forms. Not having the option name
1285 written out mnemonically can affect how well you remember which option
1286 does what, and therefore where different names have to be placed.
1289 @subsection Archiving Directories
1291 @cindex Archiving Directories
1292 @cindex Directories, Archiving
1293 You can archive a directory by specifying its directory name as a
1294 file name argument to @command{tar}. The files in the directory will be
1295 archived relative to the working directory, and the directory will be
1296 re-created along with its contents when the archive is extracted.
1298 To archive a directory, first move to its superior directory. If you
1299 have followed the previous instructions in this tutorial, you should
1308 This will put you into the directory which contains @file{practice},
1309 i.e., your home directory. Once in the superior directory, you can
1310 specify the subdirectory, @file{practice}, as a file name argument. To
1311 store @file{practice} in the new archive file @file{music.tar}, type:
1314 $ @kbd{tar --create --verbose --file=music.tar practice}
1318 @command{tar} should output:
1325 practice/collection.tar
1328 Note that the archive thus created is not in the subdirectory
1329 @file{practice}, but rather in the current working directory---the
1330 directory from which @command{tar} was invoked. Before trying to archive a
1331 directory from its superior directory, you should make sure you have
1332 write access to the superior directory itself, not only the directory
1333 you are trying archive with @command{tar}. For example, you will probably
1334 not be able to store your home directory in an archive by invoking
1335 @command{tar} from the root directory; @xref{absolute}. (Note
1336 also that @file{collection.tar}, the original archive file, has itself
1337 been archived. @command{tar} will accept any file as a file to be
1338 archived, regardless of its content. When @file{music.tar} is
1339 extracted, the archive file @file{collection.tar} will be re-written
1340 into the file system).
1342 If you give @command{tar} a command such as
1345 $ @kbd{tar --create --file=foo.tar .}
1349 @command{tar} will report @samp{tar: ./foo.tar is the archive; not
1350 dumped}. This happens because @command{tar} creates the archive
1351 @file{foo.tar} in the current directory before putting any files into
1352 it. Then, when @command{tar} attempts to add all the files in the
1353 directory @file{.} to the archive, it notices that the file
1354 @file{./foo.tar} is the same as the archive @file{foo.tar}, and skips
1355 it. (It makes no sense to put an archive into itself.) @GNUTAR{}
1356 will continue in this case, and create the archive
1357 normally, except for the exclusion of that one file. (@emph{Please
1358 note:} Other implementations of @command{tar} may not be so clever;
1359 they will enter an infinite loop when this happens, so you should not
1360 depend on this behavior unless you are certain you are running
1361 @GNUTAR{}. In general, it is wise to always place the archive outside
1362 of the directory being dumped.
1365 @section How to List Archives
1368 Frequently, you will find yourself wanting to determine exactly what a
1369 particular archive contains. You can use the @option{--list}
1370 (@option{-t}) operation to get the member names as they currently
1371 appear in the archive, as well as various attributes of the files at
1372 the time they were archived. For example, you can examine the archive
1373 @file{collection.tar} that you created in the last section with the
1377 $ @kbd{tar --list --file=collection.tar}
1381 The output of @command{tar} would then be:
1390 The archive @file{bfiles.tar} would list as follows:
1399 Be sure to use a @option{--file=@var{archive-name}} (@option{-f
1400 @var{archive-name}}) option just as with @option{--create}
1401 (@option{-c}) to specify the name of the archive.
1403 @xopindex{list, using with @option{--verbose}}
1404 @xopindex{verbose, using with @option{--list}}
1405 If you use the @option{--verbose} (@option{-v}) option with
1406 @option{--list}, then @command{tar} will print out a listing
1407 reminiscent of @w{@samp{ls -l}}, showing owner, file size, and so
1408 forth. This output is described in detail in @ref{verbose member listing}.
1410 If you had used @option{--verbose} (@option{-v}) mode, the example
1411 above would look like:
1414 $ @kbd{tar --list --verbose --file=collection.tar folk}
1415 -rw-r--r-- myself user 62 1990-05-23 10:55 folk
1418 @cindex listing member and file names
1419 @anchor{listing member and file names}
1420 It is important to notice that the output of @kbd{tar --list
1421 --verbose} does not necessarily match that produced by @kbd{tar
1422 --create --verbose} while creating the archive. It is because
1423 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1424 prefixes from file names before storing them in the archive
1425 (@xref{absolute}, for more information). In other
1426 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1427 an archive and @dfn{member names} when listing it. Consider this
1432 $ @kbd{tar cfv archive /etc/mail}
1433 tar: Removing leading `/' from member names
1435 /etc/mail/sendmail.cf
1437 $ @kbd{tar tf archive}
1439 etc/mail/sendmail.cf
1444 @opindex show-stored-names
1445 This default behavior can sometimes be inconvenient. You can force
1446 @GNUTAR{} show member names when creating archive by supplying
1447 @option{--show-stored-names} option.
1450 @item --show-stored-names
1451 Print member (as opposed to @emph{file}) names when creating the archive.
1454 @cindex File name arguments, using @option{--list} with
1455 @xopindex{list, using with file name arguments}
1456 You can specify one or more individual member names as arguments when
1457 using @samp{list}. In this case, @command{tar} will only list the
1458 names of members you identify. For example, @w{@kbd{tar --list
1459 --file=afiles.tar apple}} would only print @file{apple}.
1461 Because @command{tar} preserves file names, these must be specified as
1462 they appear in the archive (i.e., relative to the directory from which
1463 the archive was created). Therefore, it is essential when specifying
1464 member names to @command{tar} that you give the exact member names.
1465 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1466 error message something like @samp{tar: birds: Not found in archive},
1467 because there is no member named @file{birds}, only one named
1468 @file{./birds}. While the names @file{birds} and @file{./birds} name
1469 the same file, @emph{member} names by default are compared verbatim.
1471 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1472 with @file{baboon}, because this exact member name is in the archive file
1473 @file{bfiles.tar}. If you are not sure of the exact file name,
1474 use @dfn{globbing patterns}, for example:
1477 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1481 will list all members whose name contains @samp{b}. @xref{wildcards},
1482 for a detailed discussion of globbing patterns and related
1483 @command{tar} command line options.
1490 @unnumberedsubsec Listing the Contents of a Stored Directory
1492 To get information about the contents of an archived directory,
1493 use the directory name as a file name argument in conjunction with
1494 @option{--list} (@option{-t}). To find out file attributes, include the
1495 @option{--verbose} (@option{-v}) option.
1497 For example, to find out about files in the directory @file{practice}, in
1498 the archive file @file{music.tar}, type:
1501 $ @kbd{tar --list --verbose --file=music.tar practice}
1504 @command{tar} responds:
1507 drwxrwxrwx myself user 0 1990-05-31 21:49 practice/
1508 -rw-r--r-- myself user 42 1990-05-21 13:29 practice/blues
1509 -rw-r--r-- myself user 62 1990-05-23 10:55 practice/folk
1510 -rw-r--r-- myself user 40 1990-05-21 13:30 practice/jazz
1511 -rw-r--r-- myself user 10240 1990-05-31 21:49 practice/collection.tar
1514 When you use a directory name as a file name argument, @command{tar} acts on
1515 all the files (including sub-directories) in that directory.
1518 @section How to Extract Members from an Archive
1521 @cindex Retrieving files from an archive
1522 @cindex Resurrecting files from an archive
1525 Creating an archive is only half the job---there is no point in storing
1526 files in an archive if you can't retrieve them. The act of retrieving
1527 members from an archive so they can be used and manipulated as
1528 unarchived files again is called @dfn{extraction}. To extract files
1529 from an archive, use the @option{--extract} (@option{--get} or
1530 @option{-x}) operation. As with @option{--create}, specify the name
1531 of the archive with @option{--file} (@option{-f}) option. Extracting
1532 an archive does not modify the archive in any way; you can extract it
1533 multiple times if you want or need to.
1535 Using @option{--extract}, you can extract an entire archive, or specific
1536 files. The files can be directories containing other files, or not. As
1537 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1538 long form of the operation without affecting the performance.
1541 * extracting archives::
1542 * extracting files::
1544 * extracting untrusted archives::
1545 * failing commands::
1548 @node extracting archives
1549 @subsection Extracting an Entire Archive
1551 To extract an entire archive, specify the archive file name only, with
1552 no individual file names as arguments. For example,
1555 $ @kbd{tar -xvf collection.tar}
1562 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
1563 -rw-r--r-- me user 21 1996-09-23 16:44 blues
1564 -rw-r--r-- me user 20 1996-09-23 16:44 folk
1567 @node extracting files
1568 @subsection Extracting Specific Files
1570 To extract specific archive members, give their exact member names as
1571 arguments, as printed by @option{--list} (@option{-t}). If you had
1572 mistakenly deleted one of the files you had placed in the archive
1573 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1574 from the archive without changing the archive's structure. Its
1575 contents will be identical to the original file @file{blues} that you
1578 First, make sure you are in the @file{practice} directory, and list the
1579 files in the directory. Now, delete the file, @samp{blues}, and list
1580 the files in the directory again.
1582 You can now extract the member @file{blues} from the archive file
1583 @file{collection.tar} like this:
1586 $ @kbd{tar --extract --file=collection.tar blues}
1590 If you list the files in the directory again, you will see that the file
1591 @file{blues} has been restored, with its original permissions, data
1592 modification times, and owner.@footnote{This is only accidentally
1593 true, but not in general. Whereas modification times are always
1594 restored, in most cases, one has to be root for restoring the owner,
1595 and use a special option for restoring permissions. Here, it just
1596 happens that the restoring user is also the owner of the archived
1597 members, and that the current @code{umask} is compatible with original
1598 permissions.} (These parameters will be identical to those which
1599 the file had when you originally placed it in the archive; any changes
1600 you may have made before deleting the file from the file system,
1601 however, will @emph{not} have been made to the archive member.) The
1602 archive file, @samp{collection.tar}, is the same as it was before you
1603 extracted @samp{blues}. You can confirm this by running @command{tar} with
1604 @option{--list} (@option{-t}).
1606 Remember that as with other operations, specifying the exact member
1607 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1608 will fail, because there is no member named @file{birds}. To extract
1609 the member named @file{./birds}, you must specify @w{@kbd{tar
1610 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1611 exact member names, use @option{--list} (@option{-t}) option
1612 (@pxref{list}). You can also extract those members that match a
1613 specific @dfn{globbing pattern}. For example, to extract from
1614 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1615 directory prefix, you could type:
1618 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1622 Here, @option{--wildcards} instructs @command{tar} to treat
1623 command line arguments as globbing patterns and @option{--no-anchored}
1624 informs it that the patterns apply to member names after any @samp{/}
1625 delimiter. The use of globbing patterns is discussed in detail in
1628 You can extract a file to standard output by combining the above options
1629 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1632 If you give the @option{--verbose} option, then @option{--extract}
1633 will print the names of the archive members as it extracts them.
1636 @subsection Extracting Files that are Directories
1638 Extracting directories which are members of an archive is similar to
1639 extracting other files. The main difference to be aware of is that if
1640 the extracted directory has the same name as any directory already in
1641 the working directory, then files in the extracted directory will be
1642 placed into the directory of the same name. Likewise, if there are
1643 files in the pre-existing directory with the same names as the members
1644 which you extract, the files from the extracted archive will replace
1645 the files already in the working directory (and possible
1646 subdirectories). This will happen regardless of whether or not the
1647 files in the working directory were more recent than those extracted
1648 (there exist, however, special options that alter this behavior
1651 However, if a file was stored with a directory name as part of its file
1652 name, and that directory does not exist under the working directory when
1653 the file is extracted, @command{tar} will create the directory.
1655 We can demonstrate how to use @option{--extract} to extract a directory
1656 file with an example. Change to the @file{practice} directory if you
1657 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1658 go back to the parent directory and extract the archive
1659 @file{music.tar}. You may either extract the entire archive, or you may
1660 extract only the files you just deleted. To extract the entire archive,
1661 don't give any file names as arguments after the archive name
1662 @file{music.tar}. To extract only the files you deleted, use the
1666 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1672 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1673 would have displayed more detail about the extracted files, as shown
1674 in the example below:
1677 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1678 -rw-r--r-- me user 28 1996-10-18 16:31 practice/jazz
1679 -rw-r--r-- me user 20 1996-09-23 16:44 practice/folk
1683 Because you created the directory with @file{practice} as part of the
1684 file names of each of the files by archiving the @file{practice}
1685 directory as @file{practice}, you must give @file{practice} as part
1686 of the file names when you extract those files from the archive.
1688 @node extracting untrusted archives
1689 @subsection Extracting Archives from Untrusted Sources
1691 Extracting files from archives can overwrite files that already exist.
1692 If you receive an archive from an untrusted source, you should make a
1693 new directory and extract into that directory, so that you don't have
1694 to worry about the extraction overwriting one of your existing files.
1695 For example, if @file{untrusted.tar} came from somewhere else on the
1696 Internet, and you don't necessarily trust its contents, you can
1697 extract it as follows:
1700 $ @kbd{mkdir newdir}
1702 $ @kbd{tar -xvf ../untrusted.tar}
1705 It is also a good practice to examine contents of the archive
1706 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1707 with @option{--verbose} (@option{-v}).
1709 @node failing commands
1710 @subsection Commands That Will Fail
1712 Here are some sample commands you might try which will not work, and why
1715 If you try to use this command,
1718 $ @kbd{tar -xvf music.tar folk jazz}
1722 you will get the following response:
1725 tar: folk: Not found in archive
1726 tar: jazz: Not found in archive
1731 This is because these files were not originally @emph{in} the parent
1732 directory @file{..}, where the archive is located; they were in the
1733 @file{practice} directory, and their file names reflect this:
1736 $ @kbd{tar -tvf music.tar}
1742 @FIXME{make sure the above works when going through the examples in
1746 Likewise, if you try to use this command,
1749 $ @kbd{tar -tvf music.tar folk jazz}
1753 you would get a similar response. Members with those names are not in the
1754 archive. You must use the correct member names, or wildcards, in order
1755 to extract the files from the archive.
1757 If you have forgotten the correct names of the files in the archive,
1758 use @w{@kbd{tar --list --verbose}} to list them correctly.
1760 @FIXME{more examples, here? hag thinks it's a good idea.}
1763 @section Going Further Ahead in this Manual
1766 @FIXME{need to write up a node here about the things that are going to
1767 be in the rest of the manual.}
1769 @node tar invocation
1770 @chapter Invoking @GNUTAR{}
1773 This chapter is about how one invokes the @GNUTAR{}
1774 command, from the command synopsis (@pxref{Synopsis}). There are
1775 numerous options, and many styles for writing them. One mandatory
1776 option specifies the operation @command{tar} should perform
1777 (@pxref{Operation Summary}), other options are meant to detail how
1778 this operation should be performed (@pxref{Option Summary}).
1779 Non-option arguments are not always interpreted the same way,
1780 depending on what the operation is.
1782 You will find in this chapter everything about option styles and rules for
1783 writing them (@pxref{Styles}). On the other hand, operations and options
1784 are fully described elsewhere, in other chapters. Here, you will find
1785 only synthetic descriptions for operations and options, together with
1786 pointers to other parts of the @command{tar} manual.
1788 Some options are so special they are fully described right in this
1789 chapter. They have the effect of inhibiting the normal operation of
1790 @command{tar} or else, they globally alter the amount of feedback the user
1791 receives about what is going on. These are the @option{--help} and
1792 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1793 and @option{--interactive} options (@pxref{interactive}).
1797 * using tar options::
1808 @section General Synopsis of @command{tar}
1810 The @GNUTAR{} program is invoked as either one of:
1813 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1814 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1817 The second form is for when old options are being used.
1819 You can use @command{tar} to store files in an archive, to extract them from
1820 an archive, and to do other types of archive manipulation. The primary
1821 argument to @command{tar}, which is called the @dfn{operation}, specifies
1822 which action to take. The other arguments to @command{tar} are either
1823 @dfn{options}, which change the way @command{tar} performs an operation,
1824 or file names or archive members, which specify the files or members
1825 @command{tar} is to act on.
1827 You can actually type in arguments in any order, even if in this manual
1828 the options always precede the other arguments, to make examples easier
1829 to understand. Further, the option stating the main operation mode
1830 (the @command{tar} main command) is usually given first.
1832 Each @var{name} in the synopsis above is interpreted as an archive member
1833 name when the main command is one of @option{--compare}
1834 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1835 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1836 @option{--update} (@option{-u}). When naming archive members, you
1837 must give the exact name of the member in the archive, as it is
1838 printed by @option{--list}. For @option{--append} (@option{-r}) and
1839 @option{--create} (@option{-c}), these @var{name} arguments specify
1840 the names of either files or directory hierarchies to place in the archive.
1841 These files or hierarchies should already exist in the file system,
1842 prior to the execution of the @command{tar} command.
1844 @command{tar} interprets relative file names as being relative to the
1845 working directory. @command{tar} will make all file names relative
1846 (by removing leading slashes when archiving or restoring files),
1847 unless you specify otherwise (using the @option{--absolute-names}
1848 option). @xref{absolute}, for more information about
1849 @option{--absolute-names}.
1851 If you give the name of a directory as either a file name or a member
1852 name, then @command{tar} acts recursively on all the files and directories
1853 beneath that directory. For example, the name @file{/} identifies all
1854 the files in the file system to @command{tar}.
1856 The distinction between file names and archive member names is especially
1857 important when shell globbing is used, and sometimes a source of confusion
1858 for newcomers. @xref{wildcards}, for more information about globbing.
1859 The problem is that shells may only glob using existing files in the
1860 file system. Only @command{tar} itself may glob on archive members, so when
1861 needed, you must ensure that wildcard characters reach @command{tar} without
1862 being interpreted by the shell first. Using a backslash before @samp{*}
1863 or @samp{?}, or putting the whole argument between quotes, is usually
1864 sufficient for this.
1866 Even if @var{name}s are often specified on the command line, they
1867 can also be read from a text file in the file system, using the
1868 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1870 If you don't use any file name arguments, @option{--append} (@option{-r}),
1871 @option{--delete} and @option{--concatenate} (@option{--catenate},
1872 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1873 will usually yield a diagnostic and inhibit @command{tar} execution.
1874 The other operations of @command{tar} (@option{--list},
1875 @option{--extract}, @option{--compare}, and @option{--update})
1876 will act on the entire contents of the archive.
1879 @cindex return status
1880 Besides successful exits, @GNUTAR{} may fail for
1881 many reasons. Some reasons correspond to bad usage, that is, when the
1882 @command{tar} command is improperly written. Errors may be
1883 encountered later, while encountering an error processing the archive
1884 or the files. Some errors are recoverable, in which case the failure
1885 is delayed until @command{tar} has completed all its work. Some
1886 errors are such that it would not meaningful, or at least risky, to
1887 continue processing: @command{tar} then aborts processing immediately.
1888 All abnormal exits, whether immediate or delayed, should always be
1889 clearly diagnosed on @code{stderr}, after a line stating the nature of
1892 Possible exit codes of @GNUTAR{} are summarized in the following
1897 @samp{Successful termination}.
1900 @samp{Some files differ}. If tar was invoked with @option{--compare}
1901 (@option{--diff}, @option{-d}) command line option, this means that
1902 some files in the archive differ from their disk counterparts
1903 (@pxref{compare}). If tar was given @option{--create},
1904 @option{--append} or @option{--update} option, this exit code means
1905 that some files were changed while being archived and so the resulting
1906 archive does not contain the exact copy of the file set.
1909 @samp{Fatal error}. This means that some fatal, unrecoverable error
1913 If @command{tar} has invoked a subprocess and that subprocess exited with a
1914 nonzero exit code, @command{tar} exits with that code as well.
1915 This can happen, for example, if @command{tar} was given some
1916 compression option (@pxref{gzip}) and the external compressor program
1917 failed. Another example is @command{rmt} failure during backup to the
1918 remote device (@pxref{Remote Tape Server}).
1920 @node using tar options
1921 @section Using @command{tar} Options
1923 @GNUTAR{} has a total of eight operating modes which
1924 allow you to perform a variety of tasks. You are required to choose
1925 one operating mode each time you employ the @command{tar} program by
1926 specifying one, and only one operation as an argument to the
1927 @command{tar} command (two lists of four operations each may be found
1928 at @ref{frequent operations} and @ref{Operations}). Depending on
1929 circumstances, you may also wish to customize how the chosen operating
1930 mode behaves. For example, you may wish to change the way the output
1931 looks, or the format of the files that you wish to archive may require
1932 you to do something special in order to make the archive look right.
1934 You can customize and control @command{tar}'s performance by running
1935 @command{tar} with one or more options (such as @option{--verbose}
1936 (@option{-v}), which we used in the tutorial). As we said in the
1937 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1938 their name suggests) optional. Depending on the operating mode, you
1939 may specify one or more options. Different options will have different
1940 effects, but in general they all change details of the operation, such
1941 as archive format, archive name, or level of user interaction. Some
1942 options make sense with all operating modes, while others are
1943 meaningful only with particular modes. You will likely use some
1944 options frequently, while you will only use others infrequently, or
1945 not at all. (A full list of options is available in @pxref{All Options}.)
1947 @vrindex TAR_OPTIONS, environment variable
1948 @anchor{TAR_OPTIONS}
1949 The @env{TAR_OPTIONS} environment variable specifies default options to
1950 be placed in front of any explicit options. For example, if
1951 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1952 if the two options @option{-v} and @option{--unlink-first} had been
1953 specified before any explicit options. Option specifications are
1954 separated by whitespace. A backslash escapes the next character, so it
1955 can be used to specify an option containing whitespace or a backslash.
1957 Note that @command{tar} options are case sensitive. For example, the
1958 options @option{-T} and @option{-t} are different; the first requires an
1959 argument for stating the name of a file providing a list of @var{name}s,
1960 while the second does not require an argument and is another way to
1961 write @option{--list} (@option{-t}).
1963 In addition to the eight operations, there are many options to
1964 @command{tar}, and three different styles for writing both: long (mnemonic)
1965 form, short form, and old style. These styles are discussed below.
1966 Both the options and the operations can be written in any of these three
1969 @FIXME{menu at end of this node. need to think of an actual outline
1970 for this chapter; probably do that after stuff from chapter 4 is
1974 @section The Three Option Styles
1976 There are three styles for writing operations and options to the command
1977 line invoking @command{tar}. The different styles were developed at
1978 different times during the history of @command{tar}. These styles will be
1979 presented below, from the most recent to the oldest.
1981 Some options must take an argument. (For example, @option{--file}
1982 (@option{-f})) takes the name of an archive file as an argument. If
1983 you do not supply an archive file name, @command{tar} will use a
1984 default, but this can be confusing; thus, we recommend that you always
1985 supply a specific archive file name.) Where you @emph{place} the
1986 arguments generally depends on which style of options you choose. We
1987 will detail specific information relevant to each option style in the
1988 sections on the different option styles, below. The differences are
1989 subtle, yet can often be very important; incorrect option placement
1990 can cause you to overwrite a number of important files. We urge you
1991 to note these differences, and only use the option style(s) which
1992 makes the most sense to you until you feel comfortable with the others.
1994 Some options @emph{may} take an argument. Such options may have at
1995 most long and short forms, they do not have old style equivalent. The
1996 rules for specifying an argument for such options are stricter than
1997 those for specifying mandatory arguments. Please, pay special
2001 * Long Options:: Long Option Style
2002 * Short Options:: Short Option Style
2003 * Old Options:: Old Option Style
2004 * Mixing:: Mixing Option Styles
2008 @subsection Long Option Style
2010 Each option has at least one @dfn{long} (or @dfn{mnemonic}) name starting with two
2011 dashes in a row, e.g., @option{--list}. The long names are more clear than
2012 their corresponding short or old names. It sometimes happens that a
2013 single long option has many different names which are
2014 synonymous, such as @option{--compare} and @option{--diff}. In addition,
2015 long option names can be given unique abbreviations. For example,
2016 @option{--cre} can be used in place of @option{--create} because there is no
2017 other long option which begins with @samp{cre}. (One way to find
2018 this out is by trying it and seeing what happens; if a particular
2019 abbreviation could represent more than one option, @command{tar} will tell
2020 you that that abbreviation is ambiguous and you'll know that that
2021 abbreviation won't work. You may also choose to run @samp{tar --help}
2022 to see a list of options. Be aware that if you run @command{tar} with a
2023 unique abbreviation for the long name of an option you didn't want to
2024 use, you are stuck; @command{tar} will perform the command as ordered.)
2026 Long options are meant to be obvious and easy to remember, and their
2027 meanings are generally easier to discern than those of their
2028 corresponding short options (see below). For example:
2031 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
2035 gives a fairly good set of hints about what the command does, even
2036 for those not fully acquainted with @command{tar}.
2038 Long options which require arguments take those arguments
2039 immediately following the option name. There are two ways of
2040 specifying a mandatory argument. It can be separated from the
2041 option name either by an equal sign, or by any amount of
2042 white space characters. For example, the @option{--file} option (which
2043 tells the name of the @command{tar} archive) is given a file such as
2044 @file{archive.tar} as argument by using any of the following notations:
2045 @option{--file=archive.tar} or @option{--file archive.tar}.
2047 In contrast, optional arguments must always be introduced using
2048 an equal sign. For example, the @option{--backup} option takes
2049 an optional argument specifying backup type. It must be used
2050 as @option{--backup=@var{backup-type}}.
2053 @subsection Short Option Style
2055 Most options also have a @dfn{short option} name. Short options start with
2056 a single dash, and are followed by a single character, e.g., @option{-t}
2057 (which is equivalent to @option{--list}). The forms are absolutely
2058 identical in function; they are interchangeable.
2060 The short option names are faster to type than long option names.
2062 Short options which require arguments take their arguments immediately
2063 following the option, usually separated by white space. It is also
2064 possible to stick the argument right after the short option name, using
2065 no intervening space. For example, you might write @w{@option{-f
2066 archive.tar}} or @option{-farchive.tar} instead of using
2067 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
2068 @w{@option{-f @var{archive-name}}} denote the option which indicates a
2069 specific archive, here named @file{archive.tar}.
2071 Short options which take optional arguments take their arguments
2072 immediately following the option letter, @emph{without any intervening
2073 white space characters}.
2075 Short options' letters may be clumped together, but you are not
2076 required to do this (as compared to old options; see below). When
2077 short options are clumped as a set, use one (single) dash for them
2078 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
2079 such a set is allowed to have an argument@footnote{Clustering many
2080 options, the last of which has an argument, is a rather opaque way to
2081 write options. Some wonder if @acronym{GNU} @code{getopt} should not
2082 even be made helpful enough for considering such usages as invalid.}.
2084 When the options are separated, the argument for each option which requires
2085 an argument directly follows that option, as is usual for Unix programs.
2089 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
2092 If you reorder short options' locations, be sure to move any arguments
2093 that belong to them. If you do not move the arguments properly, you may
2094 end up overwriting files.
2097 @subsection Old Option Style
2100 Like short options, @dfn{old options} are single letters. However, old options
2101 must be written together as a single clumped set, without spaces separating
2102 them or dashes preceding them@footnote{Beware that if you precede options
2103 with a dash, you are announcing the short option style instead of the
2104 old option style; short options are decoded differently.}. This set
2105 of letters must be the first to appear on the command line, after the
2106 @command{tar} program name and some white space; old options cannot appear
2107 anywhere else. The letter of an old option is exactly the same letter as
2108 the corresponding short option. For example, the old option @samp{t} is
2109 the same as the short option @option{-t}, and consequently, the same as the
2110 long option @option{--list}. So for example, the command @w{@samp{tar
2111 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
2113 When options that need arguments are given together with the command,
2114 all the associated arguments follow, in the same order as the options.
2115 Thus, the example given previously could also be written in the old
2119 $ @kbd{tar cvbf 20 /dev/rmt0}
2123 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
2124 the argument of @option{-f}.
2126 On the other hand, this old style syntax makes it difficult to match
2127 option letters with their corresponding arguments, and is often
2128 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
2129 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
2130 argument for @option{-f}, and @option{-v} does not have a corresponding
2131 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
2132 /dev/rmt0}} is clearer, putting all arguments next to the option they
2135 If you want to reorder the letters in the old option argument, be
2136 sure to reorder any corresponding argument appropriately.
2138 This old way of writing @command{tar} options can surprise even experienced
2139 users. For example, the two commands:
2142 @kbd{tar cfz archive.tar.gz file}
2143 @kbd{tar -cfz archive.tar.gz file}
2147 are quite different. The first example uses @file{archive.tar.gz} as
2148 the value for option @samp{f} and recognizes the option @samp{z}. The
2149 second example, however, uses @file{z} as the value for option
2150 @samp{f} --- probably not what was intended.
2152 Old options are kept for compatibility with old versions of @command{tar}.
2154 This second example could be corrected in many ways, among which the
2155 following are equivalent:
2158 @kbd{tar -czf archive.tar.gz file}
2159 @kbd{tar -cf archive.tar.gz -z file}
2160 @kbd{tar cf archive.tar.gz -z file}
2163 @cindex option syntax, traditional
2164 As far as we know, all @command{tar} programs, @acronym{GNU} and
2165 non-@acronym{GNU}, support old options. @GNUTAR{}
2166 supports them not only for historical reasons, but also because many
2167 people are used to them. For compatibility with Unix @command{tar},
2168 the first argument is always treated as containing command and option
2169 letters even if it doesn't start with @samp{-}. Thus, @samp{tar c} is
2170 equivalent to @w{@samp{tar -c}:} both of them specify the
2171 @option{--create} (@option{-c}) command to create an archive.
2174 @subsection Mixing Option Styles
2176 All three styles may be intermixed in a single @command{tar} command,
2177 so long as the rules for each style are fully
2178 respected@footnote{Before @GNUTAR{} version 1.11.6,
2179 a bug prevented intermixing old style options with long options in
2180 some cases.}. Old style options and either of the modern styles of
2181 options may be mixed within a single @command{tar} command. However,
2182 old style options must be introduced as the first arguments only,
2183 following the rule for old options (old options must appear directly
2184 after the @command{tar} command and some white space). Modern options
2185 may be given only after all arguments to the old options have been
2186 collected. If this rule is not respected, a modern option might be
2187 falsely interpreted as the value of the argument to one of the old
2190 For example, all the following commands are wholly equivalent, and
2191 illustrate the many combinations and orderings of option styles.
2194 @kbd{tar --create --file=archive.tar}
2195 @kbd{tar --create -f archive.tar}
2196 @kbd{tar --create -farchive.tar}
2197 @kbd{tar --file=archive.tar --create}
2198 @kbd{tar --file=archive.tar -c}
2199 @kbd{tar -c --file=archive.tar}
2200 @kbd{tar -c -f archive.tar}
2201 @kbd{tar -c -farchive.tar}
2202 @kbd{tar -cf archive.tar}
2203 @kbd{tar -cfarchive.tar}
2204 @kbd{tar -f archive.tar --create}
2205 @kbd{tar -f archive.tar -c}
2206 @kbd{tar -farchive.tar --create}
2207 @kbd{tar -farchive.tar -c}
2208 @kbd{tar c --file=archive.tar}
2209 @kbd{tar c -f archive.tar}
2210 @kbd{tar c -farchive.tar}
2211 @kbd{tar cf archive.tar}
2212 @kbd{tar f archive.tar --create}
2213 @kbd{tar f archive.tar -c}
2214 @kbd{tar fc archive.tar}
2217 On the other hand, the following commands are @emph{not} equivalent to
2221 @kbd{tar -f -c archive.tar}
2222 @kbd{tar -fc archive.tar}
2223 @kbd{tar -fcarchive.tar}
2224 @kbd{tar -farchive.tarc}
2225 @kbd{tar cfarchive.tar}
2229 These last examples mean something completely different from what the
2230 user intended (judging based on the example in the previous set which
2231 uses long options, whose intent is therefore very clear). The first
2232 four specify that the @command{tar} archive would be a file named
2233 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2234 respectively. The first two examples also specify a single non-option,
2235 @var{name} argument having the value @samp{archive.tar}. The last
2236 example contains only old style option letters (repeating option
2237 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2238 @samp{h}, or @samp{i}), with no argument value. @FIXME{not sure i liked
2239 the first sentence of this paragraph..}
2242 @section All @command{tar} Options
2244 The coming manual sections contain an alphabetical listing of all
2245 @command{tar} operations and options, with brief descriptions and cross
2246 references to more in-depth explanations in the body of the manual.
2247 They also contain an alphabetically arranged table of the short option
2248 forms with their corresponding long option. You can use this table as
2249 a reference for deciphering @command{tar} commands in scripts.
2252 * Operation Summary::
2254 * Short Option Summary::
2257 @node Operation Summary
2258 @subsection Operations
2266 Appends files to the end of the archive. @xref{append}.
2268 @opsummary{catenate}
2272 Same as @option{--concatenate}. @xref{concatenate}.
2278 Compares archive members with their counterparts in the file
2279 system, and reports differences in file size, mode, owner,
2280 modification date and contents. @xref{compare}.
2282 @opsummary{concatenate}
2286 Appends other @command{tar} archives to the end of the archive.
2293 Creates a new @command{tar} archive. @xref{create}.
2298 Deletes members from the archive. Don't try this on a archive on a
2299 tape! @xref{delete}.
2305 Same @option{--compare}. @xref{compare}.
2311 Extracts members from the archive into the file system. @xref{extract}.
2317 Same as @option{--extract}. @xref{extract}.
2323 Lists the members in an archive. @xref{list}.
2329 Adds files to the end of the archive, but only if they are newer than
2330 their counterparts already in the archive, or if they do not already
2331 exist in the archive. @xref{update}.
2335 @node Option Summary
2336 @subsection @command{tar} Options
2340 @opsummary{absolute-names}
2341 @item --absolute-names
2344 Normally when creating an archive, @command{tar} strips an initial
2345 @samp{/} from member names. This option disables that behavior.
2348 @opsummary{after-date}
2351 (See @option{--newer}, @pxref{after})
2353 @opsummary{anchored}
2355 A pattern must match an initial subsequence of the name's components.
2356 @xref{controlling pattern-matching}.
2358 @opsummary{atime-preserve}
2359 @item --atime-preserve
2360 @itemx --atime-preserve=replace
2361 @itemx --atime-preserve=system
2363 Attempt to preserve the access time of files when reading them. This
2364 option currently is effective only on files that you own, unless you
2365 have superuser privileges.
2367 @option{--atime-preserve=replace} remembers the access time of a file
2368 before reading it, and then restores the access time afterwards. This
2369 may cause problems if other programs are reading the file at the same
2370 time, as the times of their accesses will be lost. On most platforms
2371 restoring the access time also requires @command{tar} to restore the
2372 data modification time too, so this option may also cause problems if
2373 other programs are writing the file at the same time. (Tar attempts
2374 to detect this situation, but cannot do so reliably due to race
2375 conditions.) Worse, on most platforms restoring the access time also
2376 updates the status change time, which means that this option is
2377 incompatible with incremental backups.
2379 @option{--atime-preserve=system} avoids changing time stamps on files,
2380 without interfering with time stamp updates
2381 caused by other programs, so it works better with incremental backups.
2382 However, it requires a special @code{O_NOATIME} option from the
2383 underlying operating and file system implementation, and it also requires
2384 that searching directories does not update their access times. As of
2385 this writing (November 2005) this works only with Linux, and only with
2386 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2387 way to know whether this feature actually works. Sometimes
2388 @command{tar} knows that it does not work, and if you use
2389 @option{--atime-preserve=system} then @command{tar} complains and
2390 exits right away. But other times @command{tar} might think that the
2391 option works when it actually does not.
2393 Currently @option{--atime-preserve} with no operand defaults to
2394 @option{--atime-preserve=replace}, but this may change in the future
2395 as support for @option{--atime-preserve=system} improves.
2397 If your operating system does not support
2398 @option{--atime-preserve=@-system}, you might be able to preserve access
2399 times reliably by by using the @command{mount} command. For example,
2400 you can mount the file system read-only, or access the file system via
2401 a read-only loopback mount, or use the @samp{noatime} mount option
2402 available on some systems. However, mounting typically requires
2403 superuser privileges and can be a pain to manage.
2405 @opsummary{auto-compress}
2406 @item --auto-compress
2409 During a @option{--create} operation, enables automatic compressed
2410 format recognition based on the archive suffix. The effect of this
2411 option is cancelled by @option{--no-auto-compress}. @xref{gzip}.
2414 @item --backup=@var{backup-type}
2416 Rather than deleting files from the file system, @command{tar} will
2417 back them up using simple or numbered backups, depending upon
2418 @var{backup-type}. @xref{backup}.
2420 @opsummary{block-number}
2421 @item --block-number
2424 With this option present, @command{tar} prints error messages for read errors
2425 with the block number in the archive file. @xref{block-number}.
2427 @opsummary{blocking-factor}
2428 @item --blocking-factor=@var{blocking}
2429 @itemx -b @var{blocking}
2431 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2432 record. @xref{Blocking Factor}.
2438 This option tells @command{tar} to read or write archives through
2439 @code{bzip2}. @xref{gzip}.
2441 @opsummary{check-device}
2442 @item --check-device
2443 Check device numbers when creating a list of modified files for
2444 incremental archiving. This is the default. @xref{device numbers},
2445 for a detailed description.
2447 @opsummary{checkpoint}
2448 @item --checkpoint[=@var{number}]
2450 This option directs @command{tar} to print periodic checkpoint
2451 messages as it reads through the archive. It is intended for when you
2452 want a visual indication that @command{tar} is still running, but
2453 don't want to see @option{--verbose} output. You can also instruct
2454 @command{tar} to execute a list of actions on each checkpoint, see
2455 @option{--checklist-action} below. For a detailed description, see
2458 @opsummary{checkpoint-action}
2459 @item --checkpoint-action=@var{action}
2460 Instruct @command{tar} to execute an action upon hitting a
2461 breakpoint. Here we give only a brief outline. @xref{checkpoints},
2462 for a complete description.
2464 The @var{action} argument can be one of the following:
2468 Produce an audible bell on the console.
2472 Print a single dot on the standard listing stream.
2475 Display a textual message on the standard error, with the status and
2476 number of the checkpoint. This is the default.
2478 @item echo=@var{string}
2479 Display @var{string} on the standard error. Before output, the string
2480 is subject to meta-character expansion.
2482 @item exec=@var{command}
2483 Execute the given @var{command}.
2485 @item sleep=@var{time}
2486 Wait for @var{time} seconds.
2488 @item ttyout=@var{string}
2489 Output @var{string} on the current console (@file{/dev/tty}).
2492 Several @option{--checkpoint-action} options can be specified. The
2493 supplied actions will be executed in order of their appearance in the
2496 Using @option{--checkpoint-action} without @option{--checkpoint}
2497 assumes default checkpoint frequency of one checkpoint per 10 records.
2499 @opsummary{check-links}
2502 If this option was given, @command{tar} will check the number of links
2503 dumped for each processed file. If this number does not match the
2504 total number of hard links for the file, a warning message will be
2505 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2506 synonym for @option{--one-file-system}. The current semantics, which
2507 complies to UNIX98, was introduced with version
2508 1.15.91. @xref{Changes}, for more information.}.
2512 @opsummary{compress}
2513 @opsummary{uncompress}
2518 @command{tar} will use the @command{compress} program when reading or
2519 writing the archive. This allows you to directly act on archives
2520 while saving space. @xref{gzip}.
2522 @opsummary{confirmation}
2523 @item --confirmation
2525 (See @option{--interactive}.) @xref{interactive}.
2527 @opsummary{delay-directory-restore}
2528 @item --delay-directory-restore
2530 Delay setting modification times and permissions of extracted
2531 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2533 @opsummary{dereference}
2537 When creating a @command{tar} archive, @command{tar} will archive the
2538 file that a symbolic link points to, rather than archiving the
2539 symlink. @xref{dereference}.
2541 @opsummary{directory}
2542 @item --directory=@var{dir}
2545 When this option is specified, @command{tar} will change its current directory
2546 to @var{dir} before performing any operations. When this option is used
2547 during archive creation, it is order sensitive. @xref{directory}.
2550 @item --exclude=@var{pattern}
2552 When performing operations, @command{tar} will skip files that match
2553 @var{pattern}. @xref{exclude}.
2555 @opsummary{exclude-from}
2556 @item --exclude-from=@var{file}
2557 @itemx -X @var{file}
2559 Similar to @option{--exclude}, except @command{tar} will use the list of
2560 patterns in the file @var{file}. @xref{exclude}.
2562 @opsummary{exclude-caches}
2563 @item --exclude-caches
2565 Exclude from dump any directory containing a valid cache directory
2566 tag file, but still dump the directory node and the tag file itself.
2570 @opsummary{exclude-caches-under}
2571 @item --exclude-caches-under
2573 Exclude from dump any directory containing a valid cache directory
2574 tag file, but still dump the directory node itself.
2578 @opsummary{exclude-caches-all}
2579 @item --exclude-caches-all
2581 Exclude from dump any directory containing a valid cache directory
2582 tag file. @xref{exclude}.
2584 @opsummary{exclude-tag}
2585 @item --exclude-tag=@var{file}
2587 Exclude from dump any directory containing file named @var{file}, but
2588 dump the directory node and @var{file} itself. @xref{exclude}.
2590 @opsummary{exclude-tag-under}
2591 @item --exclude-tag-under=@var{file}
2593 Exclude from dump the contents of any directory containing file
2594 named @var{file}, but dump the directory node itself. @xref{exclude}.
2596 @opsummary{exclude-tag-all}
2597 @item --exclude-tag-all=@var{file}
2599 Exclude from dump any directory containing file named @var{file}.
2602 @opsummary{exclude-vcs}
2605 Exclude from dump directories and files, that are internal for some
2606 widely used version control systems.
2611 @item --file=@var{archive}
2612 @itemx -f @var{archive}
2614 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2615 performs operations on, rather than @command{tar}'s compilation dependent
2616 default. @xref{file tutorial}.
2618 @opsummary{files-from}
2619 @item --files-from=@var{file}
2620 @itemx -T @var{file}
2622 @command{tar} will use the contents of @var{file} as a list of archive members
2623 or files to operate on, in addition to those specified on the
2624 command-line. @xref{files}.
2626 @opsummary{force-local}
2629 Forces @command{tar} to interpret the file name given to @option{--file}
2630 as a local file, even if it looks like a remote tape drive name.
2631 @xref{local and remote archives}.
2634 @item --format=@var{format}
2635 @itemx -H @var{format}
2637 Selects output archive format. @var{Format} may be one of the
2642 Creates an archive that is compatible with Unix V7 @command{tar}.
2645 Creates an archive that is compatible with GNU @command{tar} version
2649 Creates archive in GNU tar 1.13 format. Basically it is the same as
2650 @samp{oldgnu} with the only difference in the way it handles long
2654 Creates a @acronym{POSIX.1-1988} compatible archive.
2657 Creates a @acronym{POSIX.1-2001 archive}.
2661 @xref{Formats}, for a detailed discussion of these formats.
2664 @item --group=@var{group}
2666 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
2667 rather than the group from the source file. @var{group} is first decoded
2668 as a group symbolic name, but if this interpretation fails, it has to be
2669 a decimal numeric group @acronym{ID}. @xref{override}.
2671 Also see the comments for the @option{--owner=@var{user}} option.
2681 This option tells @command{tar} to read or write archives through
2682 @command{gzip}, allowing @command{tar} to directly operate on several
2683 kinds of compressed archives transparently. @xref{gzip}.
2685 @opsummary{hard-dereference}
2686 @item --hard-dereference
2687 When creating an archive, dereference hard links and store the files
2688 they refer to, instead of creating usual hard link members.
2696 @command{tar} will print out a short message summarizing the operations and
2697 options to @command{tar} and exit. @xref{help}.
2699 @opsummary{ignore-case}
2701 Ignore case when matching member or file names with
2702 patterns. @xref{controlling pattern-matching}.
2704 @opsummary{ignore-command-error}
2705 @item --ignore-command-error
2706 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2708 @opsummary{ignore-failed-read}
2709 @item --ignore-failed-read
2711 Do not exit unsuccessfully merely because an unreadable file was encountered.
2714 @opsummary{ignore-zeros}
2715 @item --ignore-zeros
2718 With this option, @command{tar} will ignore zeroed blocks in the
2719 archive, which normally signals EOF. @xref{Reading}.
2721 @opsummary{incremental}
2725 Informs @command{tar} that it is working with an old
2726 @acronym{GNU}-format incremental backup archive. It is intended
2727 primarily for backwards compatibility only. @xref{Incremental Dumps},
2728 for a detailed discussion of incremental archives.
2730 @opsummary{index-file}
2731 @item --index-file=@var{file}
2733 Send verbose output to @var{file} instead of to standard output.
2735 @opsummary{info-script}
2736 @opsummary{new-volume-script}
2737 @item --info-script=@var{script-file}
2738 @itemx --new-volume-script=@var{script-file}
2739 @itemx -F @var{script-file}
2741 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2742 at the end of each tape. If @var{script-file} exits with nonzero status,
2743 @command{tar} fails immediately. @xref{info-script}, for a detailed
2744 discussion of @var{script-file}.
2746 @opsummary{interactive}
2748 @itemx --confirmation
2751 Specifies that @command{tar} should ask the user for confirmation before
2752 performing potentially destructive options, such as overwriting files.
2755 @opsummary{keep-newer-files}
2756 @item --keep-newer-files
2758 Do not replace existing files that are newer than their archive copies
2759 when extracting files from an archive.
2761 @opsummary{keep-old-files}
2762 @item --keep-old-files
2765 Do not overwrite existing files when extracting files from an archive.
2766 @xref{Keep Old Files}.
2769 @item --label=@var{name}
2770 @itemx -V @var{name}
2772 When creating an archive, instructs @command{tar} to write @var{name}
2773 as a name record in the archive. When extracting or listing archives,
2774 @command{tar} will only operate on archives that have a label matching
2775 the pattern specified in @var{name}. @xref{Tape Files}.
2777 @opsummary{listed-incremental}
2778 @item --listed-incremental=@var{snapshot-file}
2779 @itemx -g @var{snapshot-file}
2781 During a @option{--create} operation, specifies that the archive that
2782 @command{tar} creates is a new @acronym{GNU}-format incremental
2783 backup, using @var{snapshot-file} to determine which files to backup.
2784 With other operations, informs @command{tar} that the archive is in
2785 incremental format. @xref{Incremental Dumps}.
2791 This option tells @command{tar} to read or write archives through
2792 @command{lzma}. @xref{gzip}.
2796 This option tells @command{tar} to read or write archives through
2797 @command{lzop}. @xref{gzip}.
2800 @item --mode=@var{permissions}
2802 When adding files to an archive, @command{tar} will use
2803 @var{permissions} for the archive members, rather than the permissions
2804 from the files. @var{permissions} can be specified either as an octal
2805 number or as symbolic permissions, like with
2806 @command{chmod}. @xref{override}.
2809 @item --mtime=@var{date}
2811 When adding files to an archive, @command{tar} will use @var{date} as
2812 the modification time of members when creating archives, instead of
2813 their actual modification times. The value of @var{date} can be
2814 either a textual date representation (@pxref{Date input formats}) or a
2815 name of the existing file, starting with @samp{/} or @samp{.}. In the
2816 latter case, the modification time of that file is used. @xref{override}.
2818 @opsummary{multi-volume}
2819 @item --multi-volume
2822 Informs @command{tar} that it should create or otherwise operate on a
2823 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2825 @opsummary{new-volume-script}
2826 @item --new-volume-script
2831 @item --newer=@var{date}
2832 @itemx --after-date=@var{date}
2835 When creating an archive, @command{tar} will only add files that have changed
2836 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2837 is taken to be the name of a file whose data modification time specifies
2838 the date. @xref{after}.
2840 @opsummary{newer-mtime}
2841 @item --newer-mtime=@var{date}
2843 Like @option{--newer}, but add only files whose
2844 contents have changed (as opposed to just @option{--newer}, which will
2845 also back up files for which any status information has
2846 changed). @xref{after}.
2848 @opsummary{no-anchored}
2850 An exclude pattern can match any subsequence of the name's components.
2851 @xref{controlling pattern-matching}.
2853 @opsummary{no-auto-compress}
2854 @item --no-auto-compress
2856 Disables automatic compressed format recognition based on the archive
2857 suffix. @xref{--auto-compress}. @xref{gzip}.
2859 @opsummary{no-check-device}
2860 @item --no-check-device
2861 Do not check device numbers when creating a list of modified files
2862 for incremental archiving. @xref{device numbers}, for
2863 a detailed description.
2865 @opsummary{no-delay-directory-restore}
2866 @item --no-delay-directory-restore
2868 Modification times and permissions of extracted
2869 directories are set when all files from this directory have been
2870 extracted. This is the default.
2871 @xref{Directory Modification Times and Permissions}.
2873 @opsummary{no-ignore-case}
2874 @item --no-ignore-case
2875 Use case-sensitive matching.
2876 @xref{controlling pattern-matching}.
2878 @opsummary{no-ignore-command-error}
2879 @item --no-ignore-command-error
2880 Print warnings about subprocesses that terminated with a nonzero exit
2881 code. @xref{Writing to an External Program}.
2883 @opsummary{no-overwrite-dir}
2884 @item --no-overwrite-dir
2886 Preserve metadata of existing directories when extracting files
2887 from an archive. @xref{Overwrite Old Files}.
2889 @opsummary{no-quote-chars}
2890 @item --no-quote-chars=@var{string}
2891 Remove characters listed in @var{string} from the list of quoted
2892 characters set by the previous @option{--quote-chars} option
2893 (@pxref{quoting styles}).
2895 @opsummary{no-recursion}
2896 @item --no-recursion
2898 With this option, @command{tar} will not recurse into directories.
2901 @opsummary{no-same-owner}
2902 @item --no-same-owner
2905 When extracting an archive, do not attempt to preserve the owner
2906 specified in the @command{tar} archive. This the default behavior
2909 @opsummary{no-same-permissions}
2910 @item --no-same-permissions
2912 When extracting an archive, subtract the user's umask from files from
2913 the permissions specified in the archive. This is the default behavior
2916 @opsummary{no-unquote}
2918 Treat all input file or member names literally, do not interpret
2919 escape sequences. @xref{input name quoting}.
2921 @opsummary{no-wildcards}
2922 @item --no-wildcards
2923 Do not use wildcards.
2924 @xref{controlling pattern-matching}.
2926 @opsummary{no-wildcards-match-slash}
2927 @item --no-wildcards-match-slash
2928 Wildcards do not match @samp{/}.
2929 @xref{controlling pattern-matching}.
2934 When @command{tar} is using the @option{--files-from} option, this option
2935 instructs @command{tar} to expect file names terminated with @acronym{NUL}, so
2936 @command{tar} can correctly work with file names that contain newlines.
2939 @opsummary{numeric-owner}
2940 @item --numeric-owner
2942 This option will notify @command{tar} that it should use numeric user
2943 and group IDs when creating a @command{tar} file, rather than names.
2947 The function of this option depends on the action @command{tar} is
2948 performing. When extracting files, @option{-o} is a synonym for
2949 @option{--no-same-owner}, i.e., it prevents @command{tar} from
2950 restoring ownership of files being extracted.
2952 When creating an archive, it is a synonym for
2953 @option{--old-archive}. This behavior is for compatibility
2954 with previous versions of @GNUTAR{}, and will be
2955 removed in future releases.
2957 @xref{Changes}, for more information.
2959 @opsummary{occurrence}
2960 @item --occurrence[=@var{number}]
2962 This option can be used in conjunction with one of the subcommands
2963 @option{--delete}, @option{--diff}, @option{--extract} or
2964 @option{--list} when a list of files is given either on the command
2965 line or via @option{-T} option.
2967 This option instructs @command{tar} to process only the @var{number}th
2968 occurrence of each named file. @var{Number} defaults to 1, so
2971 tar -x -f archive.tar --occurrence filename
2975 will extract the first occurrence of the member @file{filename} from @file{archive.tar}
2976 and will terminate without scanning to the end of the archive.
2978 @opsummary{old-archive}
2980 Synonym for @option{--format=v7}.
2982 @opsummary{one-file-system}
2983 @item --one-file-system
2984 Used when creating an archive. Prevents @command{tar} from recursing into
2985 directories that are on different file systems from the current
2988 @opsummary{overwrite}
2991 Overwrite existing files and directory metadata when extracting files
2992 from an archive. @xref{Overwrite Old Files}.
2994 @opsummary{overwrite-dir}
2995 @item --overwrite-dir
2997 Overwrite the metadata of existing directories when extracting files
2998 from an archive. @xref{Overwrite Old Files}.
3001 @item --owner=@var{user}
3003 Specifies that @command{tar} should use @var{user} as the owner of members
3004 when creating archives, instead of the user associated with the source
3005 file. @var{user} is first decoded as a user symbolic name, but if
3006 this interpretation fails, it has to be a decimal numeric user @acronym{ID}.
3009 This option does not affect extraction from archives.
3011 @opsummary{pax-option}
3012 @item --pax-option=@var{keyword-list}
3013 This option is meaningful only with @acronym{POSIX.1-2001} archives
3014 (@pxref{posix}). It modifies the way @command{tar} handles the
3015 extended header keywords. @var{Keyword-list} is a comma-separated
3016 list of keyword options. @xref{PAX keywords}, for a detailed
3019 @opsummary{portability}
3021 @itemx --old-archive
3022 Synonym for @option{--format=v7}.
3026 Same as @option{--format=posix}.
3028 @opsummary{preserve}
3031 Synonymous with specifying both @option{--preserve-permissions} and
3032 @option{--same-order}. @xref{Setting Access Permissions}.
3034 @opsummary{preserve-order}
3035 @item --preserve-order
3037 (See @option{--same-order}; @pxref{Reading}.)
3039 @opsummary{preserve-permissions}
3040 @opsummary{same-permissions}
3041 @item --preserve-permissions
3042 @itemx --same-permissions
3045 When @command{tar} is extracting an archive, it normally subtracts the
3046 users' umask from the permissions specified in the archive and uses
3047 that number as the permissions to create the destination file.
3048 Specifying this option instructs @command{tar} that it should use the
3049 permissions directly from the archive. @xref{Setting Access Permissions}.
3051 @opsummary{quote-chars}
3052 @item --quote-chars=@var{string}
3053 Always quote characters from @var{string}, even if the selected
3054 quoting style would not quote them (@pxref{quoting styles}).
3056 @opsummary{quoting-style}
3057 @item --quoting-style=@var{style}
3058 Set quoting style to use when printing member and file names
3059 (@pxref{quoting styles}). Valid @var{style} values are:
3060 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
3061 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
3062 style is @code{escape}, unless overridden while configuring the
3065 @opsummary{read-full-records}
3066 @item --read-full-records
3069 Specifies that @command{tar} should reblock its input, for reading
3070 from pipes on systems with buggy implementations. @xref{Reading}.
3072 @opsummary{record-size}
3073 @item --record-size=@var{size}
3075 Instructs @command{tar} to use @var{size} bytes per record when accessing the
3076 archive. @xref{Blocking Factor}.
3078 @opsummary{recursion}
3081 With this option, @command{tar} recurses into directories (default).
3084 @opsummary{recursive-unlink}
3085 @item --recursive-unlink
3088 directory hierarchies before extracting directories of the same name
3089 from the archive. @xref{Recursive Unlink}.
3091 @opsummary{remove-files}
3092 @item --remove-files
3094 Directs @command{tar} to remove the source file from the file system after
3095 appending it to an archive. @xref{remove files}.
3097 @opsummary{restrict}
3100 Disable use of some potentially harmful @command{tar} options.
3101 Currently this option disables shell invocation from multi-volume menu
3102 (@pxref{Using Multiple Tapes}).
3104 @opsummary{rmt-command}
3105 @item --rmt-command=@var{cmd}
3107 Notifies @command{tar} that it should use @var{cmd} instead of
3108 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
3110 @opsummary{rsh-command}
3111 @item --rsh-command=@var{cmd}
3113 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
3114 devices. @xref{Device}.
3116 @opsummary{same-order}
3118 @itemx --preserve-order
3121 This option is an optimization for @command{tar} when running on machines with
3122 small amounts of memory. It informs @command{tar} that the list of file
3123 arguments has already been sorted to match the order of files in the
3124 archive. @xref{Reading}.
3126 @opsummary{same-owner}
3129 When extracting an archive, @command{tar} will attempt to preserve the owner
3130 specified in the @command{tar} archive with this option present.
3131 This is the default behavior for the superuser; this option has an
3132 effect only for ordinary users. @xref{Attributes}.
3134 @opsummary{same-permissions}
3135 @item --same-permissions
3137 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
3143 Assume that the archive media supports seeks to arbitrary
3144 locations. Usually @command{tar} determines automatically whether
3145 the archive can be seeked or not. This option is intended for use
3146 in cases when such recognition fails.
3148 @opsummary{show-defaults}
3149 @item --show-defaults
3151 Displays the default options used by @command{tar} and exits
3152 successfully. This option is intended for use in shell scripts.
3153 Here is an example of what you can see using this option:
3156 $ tar --show-defaults
3157 --format=gnu -f- -b20 --quoting-style=escape \
3158 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
3161 @opsummary{show-omitted-dirs}
3162 @item --show-omitted-dirs
3164 Instructs @command{tar} to mention the directories it is skipping when
3165 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
3167 @opsummary{show-transformed-names}
3168 @opsummary{show-stored-names}
3169 @item --show-transformed-names
3170 @itemx --show-stored-names
3172 Display file or member names after applying any transformations
3173 (@pxref{transform}). In particular, when used in conjunction with one of
3174 the archive creation operations it instructs @command{tar} to list the
3175 member names stored in the archive, as opposed to the actual file
3176 names. @xref{listing member and file names}.
3182 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3183 sparse files efficiently. @xref{sparse}.
3185 @opsummary{sparse-version}
3186 @item --sparse-version=@var{version}
3188 Specifies the @dfn{format version} to use when archiving sparse
3189 files. Implies @option{--sparse}. @xref{sparse}. For the description
3190 of the supported sparse formats, @xref{Sparse Formats}.
3192 @opsummary{starting-file}
3193 @item --starting-file=@var{name}
3194 @itemx -K @var{name}
3196 This option affects extraction only; @command{tar} will skip extracting
3197 files in the archive until it finds one that matches @var{name}.
3200 @opsummary{strip-components}
3201 @item --strip-components=@var{number}
3202 Strip given @var{number} of leading components from file names before
3203 extraction. For example, if archive @file{archive.tar} contained
3204 @file{/some/file/name}, then running
3207 tar --extract --file archive.tar --strip-components=2
3211 would extract this file to file @file{name}.
3213 @opsummary{suffix}, summary
3214 @item --suffix=@var{suffix}
3216 Alters the suffix @command{tar} uses when backing up files from the default
3217 @samp{~}. @xref{backup}.
3219 @opsummary{tape-length}
3220 @item --tape-length=@var{num}
3223 Specifies the length of tapes that @command{tar} is writing as being
3224 @w{@var{num} x 1024} bytes long. @xref{Using Multiple Tapes}.
3226 @opsummary{test-label}
3229 Reads the volume label. If an argument is specified, test whether it
3230 matches the volume label. @xref{--test-label option}.
3232 @opsummary{to-command}
3233 @item --to-command=@var{command}
3235 During extraction @command{tar} will pipe extracted files to the
3236 standard input of @var{command}. @xref{Writing to an External Program}.
3238 @opsummary{to-stdout}
3242 During extraction, @command{tar} will extract files to stdout rather
3243 than to the file system. @xref{Writing to Standard Output}.
3246 @item --totals[=@var{signo}]
3248 Displays the total number of bytes transferred when processing an
3249 archive. If an argument is given, these data are displayed on
3250 request, when signal @var{signo} is delivered to @command{tar}.
3257 Sets the data modification time of extracted files to the extraction time,
3258 rather than the data modification time stored in the archive.
3259 @xref{Data Modification Times}.
3261 @opsummary{transform}
3262 @item --transform=@var{sed-expr}
3264 Transform file or member names using @command{sed} replacement expression
3265 @var{sed-expr}. For example,
3268 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
3272 will add to @file{archive} files from the current working directory,
3273 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
3274 discussion, @xref{transform}.
3276 To see transformed member names in verbose listings, use
3277 @option{--show-transformed-names} option
3278 (@pxref{show-transformed-names}).
3280 @opsummary{uncompress}
3283 (See @option{--compress}. @pxref{gzip})
3288 (See @option{--gzip}. @pxref{gzip})
3290 @opsummary{unlink-first}
3291 @item --unlink-first
3294 Directs @command{tar} to remove the corresponding file from the file
3295 system before extracting it from the archive. @xref{Unlink First}.
3299 Enable unquoting input file or member names (default). @xref{input
3302 @opsummary{use-compress-program}
3303 @item --use-compress-program=@var{prog}
3305 Instructs @command{tar} to access the archive through @var{prog}, which is
3306 presumed to be a compression program of some sort. @xref{gzip}.
3311 Display file modification dates in @acronym{UTC}. This option implies
3318 Specifies that @command{tar} should be more verbose about the
3319 operations it is performing. This option can be specified multiple
3320 times for some operations to increase the amount of information displayed.
3327 Verifies that the archive was correctly written when creating an
3328 archive. @xref{verify}.
3333 Print information about the program's name, version, origin and legal
3334 status, all on standard output, and then exit successfully.
3337 @opsummary{volno-file}
3338 @item --volno-file=@var{file}
3340 Used in conjunction with @option{--multi-volume}. @command{tar} will
3341 keep track of which volume of a multi-volume archive it is working in
3342 @var{file}. @xref{volno-file}.
3344 @opsummary{wildcards}
3346 Use wildcards when matching member names with patterns.
3347 @xref{controlling pattern-matching}.
3349 @opsummary{wildcards-match-slash}
3350 @item --wildcards-match-slash
3351 Wildcards match @samp{/}.
3352 @xref{controlling pattern-matching}.
3355 @node Short Option Summary
3356 @subsection Short Options Cross Reference
3358 Here is an alphabetized list of all of the short option forms, matching
3359 them with the equivalent long option.
3361 @multitable @columnfractions 0.20 0.80
3362 @headitem Short Option @tab Reference
3364 @item -A @tab @ref{--concatenate}.
3366 @item -B @tab @ref{--read-full-records}.
3368 @item -C @tab @ref{--directory}.
3370 @item -F @tab @ref{--info-script}.
3372 @item -G @tab @ref{--incremental}.
3374 @item -J @tab @ref{--lzma}.
3376 @item -K @tab @ref{--starting-file}.
3378 @item -L @tab @ref{--tape-length}.
3380 @item -M @tab @ref{--multi-volume}.
3382 @item -N @tab @ref{--newer}.
3384 @item -O @tab @ref{--to-stdout}.
3386 @item -P @tab @ref{--absolute-names}.
3388 @item -R @tab @ref{--block-number}.
3390 @item -S @tab @ref{--sparse}.
3392 @item -T @tab @ref{--files-from}.
3394 @item -U @tab @ref{--unlink-first}.
3396 @item -V @tab @ref{--label}.
3398 @item -W @tab @ref{--verify}.
3400 @item -X @tab @ref{--exclude-from}.
3402 @item -Z @tab @ref{--compress}.
3404 @item -b @tab @ref{--blocking-factor}.
3406 @item -c @tab @ref{--create}.
3408 @item -d @tab @ref{--compare}.
3410 @item -f @tab @ref{--file}.
3412 @item -g @tab @ref{--listed-incremental}.
3414 @item -h @tab @ref{--dereference}.
3416 @item -i @tab @ref{--ignore-zeros}.
3418 @item -j @tab @ref{--bzip2}.
3420 @item -k @tab @ref{--keep-old-files}.
3422 @item -l @tab @ref{--check-links}.
3424 @item -m @tab @ref{--touch}.
3426 @item -o @tab When creating, @ref{--no-same-owner}, when extracting ---
3427 @ref{--portability}.
3429 The latter usage is deprecated. It is retained for compatibility with
3430 the earlier versions of @GNUTAR{}. In future releases
3431 @option{-o} will be equivalent to @option{--no-same-owner} only.
3433 @item -p @tab @ref{--preserve-permissions}.
3435 @item -r @tab @ref{--append}.
3437 @item -s @tab @ref{--same-order}.
3439 @item -t @tab @ref{--list}.
3441 @item -u @tab @ref{--update}.
3443 @item -v @tab @ref{--verbose}.
3445 @item -w @tab @ref{--interactive}.
3447 @item -x @tab @ref{--extract}.
3449 @item -z @tab @ref{--gzip}.
3454 @section @GNUTAR{} documentation
3456 @cindex Getting program version number
3458 @cindex Version of the @command{tar} program
3459 Being careful, the first thing is really checking that you are using
3460 @GNUTAR{}, indeed. The @option{--version} option
3461 causes @command{tar} to print information about its name, version,
3462 origin and legal status, all on standard output, and then exit
3463 successfully. For example, @w{@samp{tar --version}} might print:
3466 tar (GNU tar) @value{VERSION}
3467 Copyright (C) 2008 Free Software Foundation, Inc.
3468 This is free software. You may redistribute copies of it under the terms
3469 of the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
3470 There is NO WARRANTY, to the extent permitted by law.
3472 Written by John Gilmore and Jay Fenlason.
3476 The first occurrence of @samp{tar} in the result above is the program
3477 name in the package (for example, @command{rmt} is another program),
3478 while the second occurrence of @samp{tar} is the name of the package
3479 itself, containing possibly many programs. The package is currently
3480 named @samp{tar}, after the name of the main program it
3481 contains@footnote{There are plans to merge the @command{cpio} and
3482 @command{tar} packages into a single one which would be called
3483 @code{paxutils}. So, who knows if, one of this days, the
3484 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3487 @cindex Obtaining help
3488 @cindex Listing all @command{tar} options
3489 @xopindex{help, introduction}
3490 Another thing you might want to do is checking the spelling or meaning
3491 of some particular @command{tar} option, without resorting to this
3492 manual, for once you have carefully read it. @GNUTAR{}
3493 has a short help feature, triggerable through the
3494 @option{--help} option. By using this option, @command{tar} will
3495 print a usage message listing all available options on standard
3496 output, then exit successfully, without doing anything else and
3497 ignoring all other options. Even if this is only a brief summary, it
3498 may be several screens long. So, if you are not using some kind of
3499 scrollable window, you might prefer to use something like:
3502 $ @kbd{tar --help | less}
3506 presuming, here, that you like using @command{less} for a pager. Other
3507 popular pagers are @command{more} and @command{pg}. If you know about some
3508 @var{keyword} which interests you and do not want to read all the
3509 @option{--help} output, another common idiom is doing:
3512 tar --help | grep @var{keyword}
3516 for getting only the pertinent lines. Notice, however, that some
3517 @command{tar} options have long description lines and the above
3518 command will list only the first of them.
3520 The exact look of the option summary displayed by @kbd{tar --help} is
3521 configurable. @xref{Configuring Help Summary}, for a detailed description.
3524 If you only wish to check the spelling of an option, running @kbd{tar
3525 --usage} may be a better choice. This will display a terse list of
3526 @command{tar} option without accompanying explanations.
3528 The short help output is quite succinct, and you might have to get
3529 back to the full documentation for precise points. If you are reading
3530 this paragraph, you already have the @command{tar} manual in some
3531 form. This manual is available in a variety of forms from
3532 @url{http://www.gnu.org/software/tar/manual}. It may be printed out of the @GNUTAR{}
3533 distribution, provided you have @TeX{} already installed somewhere,
3534 and a laser printer around. Just configure the distribution, execute
3535 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3536 usual way (contact your local guru to know how). If @GNUTAR{}
3537 has been conveniently installed at your place, this
3538 manual is also available in interactive, hypertextual form as an Info
3539 file. Just call @w{@samp{info tar}} or, if you do not have the
3540 @command{info} program handy, use the Info reader provided within
3541 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3543 There is currently no @code{man} page for @GNUTAR{}.
3544 If you observe such a @code{man} page on the system you are running,
3545 either it does not belong to @GNUTAR{}, or it has not
3546 been produced by @acronym{GNU}. Some package maintainers convert
3547 @kbd{tar --help} output to a man page, using @command{help2man}. In
3548 any case, please bear in mind that the authoritative source of
3549 information about @GNUTAR{} is this Texinfo documentation.
3552 @section Obtaining @GNUTAR{} default values
3554 @opindex show-defaults
3555 @GNUTAR{} has some predefined defaults that are used when you do not
3556 explicitly specify another values. To obtain a list of such
3557 defaults, use @option{--show-defaults} option. This will output the
3558 values in the form of @command{tar} command line options:
3562 @kbd{tar --show-defaults}
3563 --format=gnu -f- -b20 --quoting-style=escape
3564 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3569 Notice, that this option outputs only one line. The example output above
3570 has been split to fit page boundaries.
3573 The above output shows that this version of @GNUTAR{} defaults to
3574 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3575 output as the archive, if no @option{--file} option has been given
3576 (@pxref{file tutorial}), the default blocking factor is 20
3577 (@pxref{Blocking Factor}). It also shows the default locations where
3578 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3581 @section Checking @command{tar} progress
3583 Typically, @command{tar} performs most operations without reporting any
3584 information to the user except error messages. When using @command{tar}
3585 with many options, particularly ones with complicated or
3586 difficult-to-predict behavior, it is possible to make serious mistakes.
3587 @command{tar} provides several options that make observing @command{tar}
3588 easier. These options cause @command{tar} to print information as it
3589 progresses in its job, and you might want to use them just for being
3590 more careful about what is going on, or merely for entertaining
3591 yourself. If you have encountered a problem when operating on an
3592 archive, however, you may need more information than just an error
3593 message in order to solve the problem. The following options can be
3594 helpful diagnostic tools.
3596 @cindex Verbose operation
3598 Normally, the @option{--list} (@option{-t}) command to list an archive
3599 prints just the file names (one per line) and the other commands are
3600 silent. When used with most operations, the @option{--verbose}
3601 (@option{-v}) option causes @command{tar} to print the name of each
3602 file or archive member as it is processed. This and the other options
3603 which make @command{tar} print status information can be useful in
3604 monitoring @command{tar}.
3606 With @option{--create} or @option{--extract}, @option{--verbose} used
3607 once just prints the names of the files or members as they are processed.
3608 Using it twice causes @command{tar} to print a longer listing
3609 (@xref{verbose member listing}, for the description) for each member.
3610 Since @option{--list} already prints the names of the members,
3611 @option{--verbose} used once with @option{--list} causes @command{tar}
3612 to print an @samp{ls -l} type listing of the files in the archive.
3613 The following examples both extract members with long list output:
3616 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3617 $ @kbd{tar xvvf archive.tar}
3620 Verbose output appears on the standard output except when an archive is
3621 being written to the standard output, as with @samp{tar --create
3622 --file=- --verbose} (@samp{tar cfv -}, or even @samp{tar cv}---if the
3623 installer let standard output be the default archive). In that case
3624 @command{tar} writes verbose output to the standard error stream.
3626 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3627 verbose output to @var{file} rather than to standard output or standard
3631 @cindex Obtaining total status information
3633 The @option{--totals} option causes @command{tar} to print on the
3634 standard error the total amount of bytes transferred when processing
3635 an archive. When creating or appending to an archive, this option
3636 prints the number of bytes written to the archive and the average
3637 speed at which they have been written, e.g.:
3641 $ @kbd{tar -c -f archive.tar --totals /home}
3642 Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
3646 When reading an archive, this option displays the number of bytes
3651 $ @kbd{tar -x -f archive.tar --totals}
3652 Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
3656 Finally, when deleting from an archive, the @option{--totals} option
3657 displays both numbers plus number of bytes removed from the archive:
3661 $ @kbd{tar --delete -f foo.tar --totals --wildcards '*~'}
3662 Total bytes read: 9543680 (9.2MiB, 201MiB/s)
3663 Total bytes written: 3829760 (3.7MiB, 81MiB/s)
3664 Total bytes deleted: 1474048
3668 You can also obtain this information on request. When
3669 @option{--totals} is used with an argument, this argument is
3670 interpreted as a symbolic name of a signal, upon delivery of which the
3671 statistics is to be printed:
3674 @item --totals=@var{signo}
3675 Print statistics upon delivery of signal @var{signo}. Valid arguments
3676 are: @code{SIGHUP}, @code{SIGQUIT}, @code{SIGINT}, @code{SIGUSR1} and
3677 @code{SIGUSR2}. Shortened names without @samp{SIG} prefix are also
3681 Both forms of @option{--totals} option can be used simultaneously.
3682 Thus, @kbd{tar -x --totals --totals=USR1} instructs @command{tar} to
3683 extract all members from its default archive and print statistics
3684 after finishing the extraction, as well as when receiving signal
3687 @anchor{Progress information}
3688 @cindex Progress information
3689 The @option{--checkpoint} option prints an occasional message
3690 as @command{tar} reads or writes the archive. It is designed for
3691 those who don't need the more detailed (and voluminous) output of
3692 @option{--block-number} (@option{-R}), but do want visual confirmation
3693 that @command{tar} is actually making forward progress. By default it
3694 prints a message each 10 records read or written. This can be changed
3695 by giving it a numeric argument after an equal sign:
3698 $ @kbd{tar -c --checkpoint=1000} /var
3699 tar: Write checkpoint 1000
3700 tar: Write checkpoint 2000
3701 tar: Write checkpoint 3000
3704 This example shows the default checkpoint message used by
3705 @command{tar}. If you place a dot immediately after the equal
3706 sign, it will print a @samp{.} at each checkpoint@footnote{This is
3707 actually a shortcut for @option{--checkpoint=@var{n}
3708 --checkpoint-action=dot}. @xref{checkpoints, dot}.}. For example:
3711 $ @kbd{tar -c --checkpoint=.1000} /var
3715 The @option{--checkpoint} option provides a flexible mechanism for
3716 executing arbitrary actions upon hitting checkpoints, see the next
3717 section (@pxref{checkpoints}), for more information on it.
3719 @opindex show-omitted-dirs
3720 @anchor{show-omitted-dirs}
3721 The @option{--show-omitted-dirs} option, when reading an archive---with
3722 @option{--list} or @option{--extract}, for example---causes a message
3723 to be printed for each directory in the archive which is skipped.
3724 This happens regardless of the reason for skipping: the directory might
3725 not have been named on the command line (implicitly or explicitly),
3726 it might be excluded by the use of the
3727 @option{--exclude=@var{pattern}} option, or some other reason.
3729 @opindex block-number
3730 @cindex Block number where error occurred
3731 @anchor{block-number}
3732 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3733 every message it would normally produce, the block number within the
3734 archive where the message was triggered. Also, supplementary messages
3735 are triggered when reading blocks full of NULs, or when hitting end of
3736 file on the archive. As of now, if the archive if properly terminated
3737 with a NUL block, the reading of the file may stop before end of file
3738 is met, so the position of end of file will not usually show when
3739 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3740 drains the archive before exiting when reading the
3741 archive from a pipe.
3743 @cindex Error message, block number of
3744 This option is especially useful when reading damaged archives, since
3745 it helps pinpoint the damaged sections. It can also be used with
3746 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3747 choose among several backup tapes when retrieving a file later, in
3748 favor of the tape where the file appears earliest (closest to the
3749 front of the tape). @xref{backup}.
3752 @section Checkpoints
3753 @cindex checkpoints, defined
3755 @opindex checkpoint-action
3757 A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
3758 the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
3759 from the archive (a @dfn{read checkpoint}). Checkpoints allow to
3760 periodically execute arbitrary actions.
3762 The checkpoint facility is enabled using the following option:
3765 @xopindex{checkpoint, defined}
3766 @item --checkpoint[=@var{n}]
3767 Schedule checkpoints before writing or reading each @var{n}th record.
3768 The default value for @var{n} is 10.
3771 A list of arbitrary @dfn{actions} can be executed at each checkpoint.
3772 These actions include: pausing, displaying textual messages, and
3773 executing arbitrary external programs. Actions are defined using
3774 the @option{--checkpoint-action} option.
3777 @xopindex{checkpoint-action, defined}
3778 @item --checkpoint-action=@var{action}
3779 Execute an @var{action} at each checkpoint.
3782 @cindex @code{echo}, checkpoint action
3783 The simplest value of @var{action} is @samp{echo}. It instructs
3784 @command{tar} to display the default message on the standard error
3785 stream upon arriving at each checkpoint. The default message is (in
3786 @acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
3787 checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
3788 Here, @var{n} represents ordinal number of the checkpoint.
3790 In another locales, translated versions of this message are used.
3792 This is the default action, so running:
3795 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
3802 $ @kbd{tar -c --checkpoint=1000} /var
3805 The @samp{echo} action also allows to supply a customized message.
3806 You do so by placing an equals sign and the message right after it,
3810 --checkpoint-action="echo=Hit %s checkpoint #%u"
3813 The @samp{%s} and @samp{%u} in the above example are
3814 @dfn{meta-characters}. The @samp{%s} meta-character is replaced with
3815 the @dfn{type} of the checkpoint: @samp{write} or
3816 @samp{read} (or a corresponding translated version in locales other
3817 than @acronym{POSIX}). The @samp{%u} meta-character is replaced with
3818 the ordinal number of the checkpoint. Thus, the above example could
3819 produce the following output when used with the @option{--create}
3823 tar: Hit write checkpoint #10
3824 tar: Hit write checkpoint #20
3825 tar: Hit write checkpoint #30
3828 Aside from meta-character expansion, the message string is subject to
3829 @dfn{unquoting}, during which the backslash @dfn{escape sequences} are
3830 replaced with their corresponding @acronym{ASCII} characters
3831 (@pxref{escape sequences}). E.g. the following action will produce an
3832 audible bell and the message described above at each checkpoint:
3835 --checkpoint-action='echo=\aHit %s checkpoint #%u'
3838 @cindex @code{bell}, checkpoint action
3839 There is also a special action which produces an audible signal:
3840 @samp{bell}. It is not equivalent to @samp{echo='\a'}, because
3841 @samp{bell} sends the bell directly to the console (@file{/dev/tty}),
3842 whereas @samp{echo='\a'} sends it to the standard error.
3844 @cindex @code{ttyout}, checkpoint action
3845 The @samp{ttyout=@var{string}} action outputs @var{string} to
3846 @file{/dev/tty}, so it can be used even if the standard output is
3847 redirected elsewhere. The @var{string} is subject to the same
3848 modifications as with @samp{echo} action. In contrast to the latter,
3849 @samp{ttyout} does not prepend @command{tar} executable name to the
3850 string, nor does it output a newline after it. For example, the
3851 following action will print the checkpoint message at the same screen
3852 line, overwriting any previous message:
3855 --checkpoint-action="ttyout=\rHit %s checkpoint #%u"
3858 @cindex @code{dot}, checkpoint action
3859 Another available checkpoint action is @samp{dot} (or @samp{.}). It
3860 instructs @command{tar} to print a single dot on the standard listing
3864 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
3868 For compatibility with previous @GNUTAR{} versions, this action can
3869 be abbreviated by placing a dot in front of the checkpoint frequency,
3870 as shown in the previous section.
3872 @cindex @code{sleep}, checkpoint action
3873 Yet another action, @samp{sleep}, pauses @command{tar} for a specified
3874 amount of seconds. The following example will stop for 30 seconds at each
3878 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
3881 @cindex @code{exec}, checkpoint action
3882 Finally, the @code{exec} action executes a given external program.
3886 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
3889 This program is executed using @command{/bin/sh -c}, with no
3890 additional arguments. Its exit code is ignored. It gets a copy of
3891 @command{tar}'s environment plus the following variables:
3894 @vrindex TAR_VERSION, checkpoint script environment
3896 @GNUTAR{} version number.
3898 @vrindex TAR_ARCHIVE, checkpoint script environment
3900 The name of the archive @command{tar} is processing.
3902 @vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
3903 @item TAR_BLOCKING_FACTOR
3904 Current blocking factor (@pxref{Blocking}.
3906 @vrindex TAR_CHECKPOINT, checkpoint script environment
3907 @item TAR_CHECKPOINT
3908 The checkpoint number.
3910 @vrindex TAR_SUBCOMMAND, checkpoint script environment
3911 @item TAR_SUBCOMMAND
3912 A short option describing the operation @command{tar} is executing
3913 @xref{Operations}, for a complete list of subcommand options.
3915 @vrindex TAR_FORMAT, checkpoint script environment
3917 Format of the archive being processed. @xref{Formats}, for a complete
3918 list of archive format names.
3921 Any number of actions can be defined, by supplying several
3922 @option{--checkpoint-action} options in the command line. For
3923 example, the command below displays two messages, pauses
3924 execution for 30 seconds and executes the @file{/sbin/cpoint} script:
3928 $ @kbd{tar -c -f arc.tar \
3929 --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
3930 --checkpoint-action='echo=Sleeping for 30 seconds' \
3931 --checkpoint-action='sleep=30' \
3932 --checkpoint-action='exec=/sbin/cpoint'}
3936 This example also illustrates the fact that
3937 @option{--checkpoint-action} can be used without
3938 @option{--checkpoint}. In this case, the default checkpoint frequency
3939 (at each 10th record) is assumed.
3942 @section Asking for Confirmation During Operations
3943 @cindex Interactive operation
3945 Typically, @command{tar} carries out a command without stopping for
3946 further instructions. In some situations however, you may want to
3947 exclude some files and archive members from the operation (for instance
3948 if disk or storage space is tight). You can do this by excluding
3949 certain files automatically (@pxref{Choosing}), or by performing
3950 an operation interactively, using the @option{--interactive} (@option{-w}) option.
3951 @command{tar} also accepts @option{--confirmation} for this option.
3953 @opindex interactive
3954 When the @option{--interactive} (@option{-w}) option is specified, before
3955 reading, writing, or deleting files, @command{tar} first prints a message
3956 for each such file, telling what operation it intends to take, then asks
3957 for confirmation on the terminal. The actions which require
3958 confirmation include adding a file to the archive, extracting a file
3959 from the archive, deleting a file from the archive, and deleting a file
3960 from disk. To confirm the action, you must type a line of input
3961 beginning with @samp{y}. If your input line begins with anything other
3962 than @samp{y}, @command{tar} skips that file.
3964 If @command{tar} is reading the archive from the standard input,
3965 @command{tar} opens the file @file{/dev/tty} to support the interactive
3968 Verbose output is normally sent to standard output, separate from
3969 other error messages. However, if the archive is produced directly
3970 on standard output, then verbose output is mixed with errors on
3971 @code{stderr}. Producing the archive on standard output may be used
3972 as a way to avoid using disk space, when the archive is soon to be
3973 consumed by another process reading it, say. Some people felt the need
3974 of producing an archive on stdout, still willing to segregate between
3975 verbose output and error output. A possible approach would be using a
3976 named pipe to receive the archive, and having the consumer process to
3977 read from that named pipe. This has the advantage of letting standard
3978 output free to receive verbose output, all separate from errors.
3981 @chapter @GNUTAR{} Operations
3994 @section Basic @GNUTAR{} Operations
3996 The basic @command{tar} operations, @option{--create} (@option{-c}),
3997 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
3998 @option{-x}), are currently presented and described in the tutorial
3999 chapter of this manual. This section provides some complementary notes
4000 for these operations.
4003 @xopindex{create, complementary notes}
4007 Creating an empty archive would have some kind of elegance. One can
4008 initialize an empty archive and later use @option{--append}
4009 (@option{-r}) for adding all members. Some applications would not
4010 welcome making an exception in the way of adding the first archive
4011 member. On the other hand, many people reported that it is
4012 dangerously too easy for @command{tar} to destroy a magnetic tape with
4013 an empty archive@footnote{This is well described in @cite{Unix-haters
4014 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
4015 Books, ISBN 1-56884-203-1.}. The two most common errors are:
4019 Mistakingly using @code{create} instead of @code{extract}, when the
4020 intent was to extract the full contents of an archive. This error
4021 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
4022 the QWERTY keyboard. Instead of being unpacked, the archive then
4023 gets wholly destroyed. When users speak about @dfn{exploding} an
4024 archive, they usually mean something else :-).
4027 Forgetting the argument to @code{file}, when the intent was to create
4028 an archive with a single file in it. This error is likely because a
4029 tired user can easily add the @kbd{f} key to the cluster of option
4030 letters, by the mere force of habit, without realizing the full
4031 consequence of doing so. The usual consequence is that the single
4032 file, which was meant to be saved, is rather destroyed.
4035 So, recognizing the likelihood and the catastrophic nature of these
4036 errors, @GNUTAR{} now takes some distance from elegance, and
4037 cowardly refuses to create an archive when @option{--create} option is
4038 given, there are no arguments besides options, and
4039 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
4040 around the cautiousness of @GNUTAR{} and nevertheless create an
4041 archive with nothing in it, one may still use, as the value for the
4042 @option{--files-from} option, a file with no names in it, as shown in
4043 the following commands:
4046 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
4047 @kbd{tar cfT empty-archive.tar /dev/null}
4050 @xopindex{extract, complementary notes}
4055 A socket is stored, within a @GNUTAR{} archive, as a pipe.
4057 @item @option{--list} (@option{-t})
4059 @GNUTAR{} now shows dates as @samp{1996-08-30},
4060 while it used to show them as @samp{Aug 30 1996}. Preferably,
4061 people should get used to ISO 8601 dates. Local American dates should
4062 be made available again with full date localization support, once
4063 ready. In the meantime, programs not being localizable for dates
4064 should prefer international dates, that's really the way to go.
4066 Look up @url{http://www.cl.cam.ac.uk/@/~mgk25/@/iso-time.html} if you
4067 are curious, it contains a detailed explanation of the ISO 8601 standard.
4072 @section Advanced @GNUTAR{} Operations
4074 Now that you have learned the basics of using @GNUTAR{}, you may want
4075 to learn about further ways in which @command{tar} can help you.
4077 This chapter presents five, more advanced operations which you probably
4078 won't use on a daily basis, but which serve more specialized functions.
4079 We also explain the different styles of options and why you might want
4080 to use one or another, or a combination of them in your @command{tar}
4081 commands. Additionally, this chapter includes options which allow you to
4082 define the output from @command{tar} more carefully, and provide help and
4083 error correction in special circumstances.
4085 @FIXME{check this after the chapter is actually revised to make sure
4086 it still introduces the info in the chapter correctly : ).}
4098 @subsection The Five Advanced @command{tar} Operations
4101 In the last chapter, you learned about the first three operations to
4102 @command{tar}. This chapter presents the remaining five operations to
4103 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
4104 @option{--delete}, and @option{--compare}.
4106 You are not likely to use these operations as frequently as those
4107 covered in the last chapter; however, since they perform specialized
4108 functions, they are quite useful when you do need to use them. We
4109 will give examples using the same directory and files that you created
4110 in the last chapter. As you may recall, the directory is called
4111 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
4112 @samp{rock}, and the two archive files you created are
4113 @samp{collection.tar} and @samp{music.tar}.
4115 We will also use the archive files @samp{afiles.tar} and
4116 @samp{bfiles.tar}. The archive @samp{afiles.tar} contains the members @samp{apple},
4117 @samp{angst}, and @samp{aspic}; @samp{bfiles.tar} contains the members
4118 @samp{./birds}, @samp{baboon}, and @samp{./box}.
4120 Unless we state otherwise, all practicing you do and examples you follow
4121 in this chapter will take place in the @file{practice} directory that
4122 you created in the previous chapter; see @ref{prepare for examples}.
4123 (Below in this section, we will remind you of the state of the examples
4124 where the last chapter left them.)
4126 The five operations that we will cover in this chapter are:
4131 Add new entries to an archive that already exists.
4134 Add more recent copies of archive members to the end of an archive, if
4139 Add one or more pre-existing archives to the end of another archive.
4141 Delete items from an archive (does not work on tapes).
4145 Compare archive members to their counterparts in the file system.
4149 @subsection How to Add Files to Existing Archives: @option{--append}
4153 If you want to add files to an existing archive, you don't need to
4154 create a new archive; you can use @option{--append} (@option{-r}).
4155 The archive must already exist in order to use @option{--append}. (A
4156 related operation is the @option{--update} operation; you can use this
4157 to add newer versions of archive members to an existing archive. To learn how to
4158 do this with @option{--update}, @pxref{update}.)
4160 If you use @option{--append} to add a file that has the same name as an
4161 archive member to an archive containing that archive member, then the
4162 old member is not deleted. What does happen, however, is somewhat
4163 complex. @command{tar} @emph{allows} you to have infinite number of files
4164 with the same name. Some operations treat these same-named members no
4165 differently than any other set of archive members: for example, if you
4166 view an archive with @option{--list} (@option{-t}), you will see all
4167 of those members listed, with their data modification times, owners, etc.
4169 Other operations don't deal with these members as perfectly as you might
4170 prefer; if you were to use @option{--extract} to extract the archive,
4171 only the most recently added copy of a member with the same name as four
4172 other members would end up in the working directory. This is because
4173 @option{--extract} extracts an archive in the order the members appeared
4174 in the archive; the most recently archived members will be extracted
4175 last. Additionally, an extracted member will @emph{replace} a file of
4176 the same name which existed in the directory already, and @command{tar}
4177 will not prompt you about this@footnote{Unless you give it
4178 @option{--keep-old-files} option, or the disk copy is newer than the
4179 the one in the archive and you invoke @command{tar} with
4180 @option{--keep-newer-files} option}. Thus, only the most recently archived
4181 member will end up being extracted, as it will replace the one
4182 extracted before it, and so on.
4184 There exists a special option that allows you to get around this
4185 behavior and extract (or list) only a particular copy of the file.
4186 This is @option{--occurrence} option. If you run @command{tar} with
4187 this option, it will extract only the first copy of the file. You
4188 may also give this option an argument specifying the number of
4189 copy to be extracted. Thus, for example if the archive
4190 @file{archive.tar} contained three copies of file @file{myfile}, then
4194 tar --extract --file archive.tar --occurrence=2 myfile
4198 would extract only the second copy. @xref{Option
4199 Summary,---occurrence}, for the description of @option{--occurrence}
4202 @FIXME{ hag -- you might want to incorporate some of the above into the
4203 MMwtSN node; not sure. i didn't know how to make it simpler...
4205 There are a few ways to get around this. (maybe xref Multiple Members
4206 with the Same Name.}
4208 @cindex Members, replacing with other members
4209 @cindex Replacing members with other members
4210 If you want to replace an archive member, use @option{--delete} to
4211 delete the member you want to remove from the archive, , and then use
4212 @option{--append} to add the member you want to be in the archive. Note
4213 that you can not change the order of the archive; the most recently
4214 added member will still appear last. In this sense, you cannot truly
4215 ``replace'' one member with another. (Replacing one member with another
4216 will not work on certain types of media, such as tapes; see @ref{delete}
4217 and @ref{Media}, for more information.)
4220 * appending files:: Appending Files to an Archive
4224 @node appending files
4225 @subsubsection Appending Files to an Archive
4227 @cindex Adding files to an Archive
4228 @cindex Appending files to an Archive
4229 @cindex Archives, Appending files to
4231 The simplest way to add a file to an already existing archive is the
4232 @option{--append} (@option{-r}) operation, which writes specified
4233 files into the archive whether or not they are already among the
4236 When you use @option{--append}, you @emph{must} specify file name
4237 arguments, as there is no default. If you specify a file that already
4238 exists in the archive, another copy of the file will be added to the
4239 end of the archive. As with other operations, the member names of the
4240 newly added files will be exactly the same as their names given on the
4241 command line. The @option{--verbose} (@option{-v}) option will print
4242 out the names of the files as they are written into the archive.
4244 @option{--append} cannot be performed on some tape drives, unfortunately,
4245 due to deficiencies in the formats those tape drives use. The archive
4246 must be a valid @command{tar} archive, or else the results of using this
4247 operation will be unpredictable. @xref{Media}.
4249 To demonstrate using @option{--append} to add a file to an archive,
4250 create a file called @file{rock} in the @file{practice} directory.
4251 Make sure you are in the @file{practice} directory. Then, run the
4252 following @command{tar} command to add @file{rock} to
4253 @file{collection.tar}:
4256 $ @kbd{tar --append --file=collection.tar rock}
4260 If you now use the @option{--list} (@option{-t}) operation, you will see that
4261 @file{rock} has been added to the archive:
4264 $ @kbd{tar --list --file=collection.tar}
4265 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
4266 -rw-r--r-- me user 21 1996-09-23 16:44 blues
4267 -rw-r--r-- me user 20 1996-09-23 16:44 folk
4268 -rw-r--r-- me user 20 1996-09-23 16:44 rock
4272 @subsubsection Multiple Members with the Same Name
4274 You can use @option{--append} (@option{-r}) to add copies of files
4275 which have been updated since the archive was created. (However, we
4276 do not recommend doing this since there is another @command{tar}
4277 option called @option{--update}; @xref{update}, for more information.
4278 We describe this use of @option{--append} here for the sake of
4279 completeness.) When you extract the archive, the older version will
4280 be effectively lost. This works because files are extracted from an
4281 archive in the order in which they were archived. Thus, when the
4282 archive is extracted, a file archived later in time will replace a
4283 file of the same name which was archived earlier, even though the
4284 older version of the file will remain in the archive unless you delete
4285 all versions of the file.
4287 Supposing you change the file @file{blues} and then append the changed
4288 version to @file{collection.tar}. As you saw above, the original
4289 @file{blues} is in the archive @file{collection.tar}. If you change the
4290 file and append the new version of the file to the archive, there will
4291 be two copies in the archive. When you extract the archive, the older
4292 version of the file will be extracted first, and then replaced by the
4293 newer version when it is extracted.
4295 You can append the new, changed copy of the file @file{blues} to the
4296 archive in this way:
4299 $ @kbd{tar --append --verbose --file=collection.tar blues}
4304 Because you specified the @option{--verbose} option, @command{tar} has
4305 printed the name of the file being appended as it was acted on. Now
4306 list the contents of the archive:
4309 $ @kbd{tar --list --verbose --file=collection.tar}
4310 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
4311 -rw-r--r-- me user 21 1996-09-23 16:44 blues
4312 -rw-r--r-- me user 20 1996-09-23 16:44 folk
4313 -rw-r--r-- me user 20 1996-09-23 16:44 rock
4314 -rw-r--r-- me user 58 1996-10-24 18:30 blues
4318 The newest version of @file{blues} is now at the end of the archive
4319 (note the different creation dates and file sizes). If you extract
4320 the archive, the older version of the file @file{blues} will be
4321 replaced by the newer version. You can confirm this by extracting
4322 the archive and running @samp{ls} on the directory.
4324 If you wish to extract the first occurrence of the file @file{blues}
4325 from the archive, use @option{--occurrence} option, as shown in
4326 the following example:
4329 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
4330 -rw-r--r-- me user 21 1996-09-23 16:44 blues
4333 @xref{Writing}, for more information on @option{--extract} and
4334 @xref{Option Summary, --occurrence}, for the description of
4335 @option{--occurrence} option.
4338 @subsection Updating an Archive
4340 @cindex Updating an archive
4343 In the previous section, you learned how to use @option{--append} to
4344 add a file to an existing archive. A related operation is
4345 @option{--update} (@option{-u}). The @option{--update} operation
4346 updates a @command{tar} archive by comparing the date of the specified
4347 archive members against the date of the file with the same name. If
4348 the file has been modified more recently than the archive member, then
4349 the newer version of the file is added to the archive (as with
4352 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
4353 The operation will fail.
4355 @FIXME{other examples of media on which --update will fail? need to ask
4356 charles and/or mib/thomas/dave shevett..}
4358 Both @option{--update} and @option{--append} work by adding to the end
4359 of the archive. When you extract a file from the archive, only the
4360 version stored last will wind up in the file system, unless you use
4361 the @option{--backup} option. @xref{multiple}, for a detailed discussion.
4368 @subsubsection How to Update an Archive Using @option{--update}
4370 You must use file name arguments with the @option{--update}
4371 (@option{-u}) operation. If you don't specify any files,
4372 @command{tar} won't act on any files and won't tell you that it didn't
4373 do anything (which may end up confusing you).
4375 @c note: the above parenthetical added because in fact, this
4376 @c behavior just confused the author. :-)
4378 To see the @option{--update} option at work, create a new file,
4379 @file{classical}, in your practice directory, and some extra text to the
4380 file @file{blues}, using any text editor. Then invoke @command{tar} with
4381 the @samp{update} operation and the @option{--verbose} (@option{-v})
4382 option specified, using the names of all the files in the practice
4383 directory as file name arguments:
4386 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4393 Because we have specified verbose mode, @command{tar} prints out the names
4394 of the files it is working on, which in this case are the names of the
4395 files that needed to be updated. If you run @samp{tar --list} and look
4396 at the archive, you will see @file{blues} and @file{classical} at its
4397 end. There will be a total of two versions of the member @samp{blues};
4398 the one at the end will be newer and larger, since you added text before
4401 (The reason @command{tar} does not overwrite the older file when updating
4402 it is because writing to the middle of a section of tape is a difficult
4403 process. Tapes are not designed to go backward. @xref{Media}, for more
4404 information about tapes.
4406 @option{--update} (@option{-u}) is not suitable for performing backups for two
4407 reasons: it does not change directory content entries, and it
4408 lengthens the archive every time it is used. The @GNUTAR{}
4409 options intended specifically for backups are more
4410 efficient. If you need to run backups, please consult @ref{Backups}.
4413 @subsection Combining Archives with @option{--concatenate}
4415 @cindex Adding archives to an archive
4416 @cindex Concatenating Archives
4417 @opindex concatenate
4419 @c @cindex @option{-A} described
4420 Sometimes it may be convenient to add a second archive onto the end of
4421 an archive rather than adding individual files to the archive. To add
4422 one or more archives to the end of another archive, you should use the
4423 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4425 To use @option{--concatenate}, give the first archive with
4426 @option{--file} option and name the rest of archives to be
4427 concatenated on the command line. The members, and their member
4428 names, will be copied verbatim from those archives to the first one.
4429 @footnote{This can cause multiple members to have the same name, for
4430 information on how this affects reading the archive, @ref{multiple}.}
4431 The new, concatenated archive will be called by the same name as the
4432 one given with the @option{--file} option. As usual, if you omit
4433 @option{--file}, @command{tar} will use the value of the environment
4434 variable @env{TAPE}, or, if this has not been set, the default archive name.
4436 @FIXME{There is no way to specify a new name...}
4438 To demonstrate how @option{--concatenate} works, create two small archives
4439 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4440 files from @file{practice}:
4443 $ @kbd{tar -cvf bluesrock.tar blues rock}
4446 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4452 If you like, You can run @samp{tar --list} to make sure the archives
4453 contain what they are supposed to:
4456 $ @kbd{tar -tvf bluesrock.tar}
4457 -rw-r--r-- melissa user 105 1997-01-21 19:42 blues
4458 -rw-r--r-- melissa user 33 1997-01-20 15:34 rock
4459 $ @kbd{tar -tvf jazzfolk.tar}
4460 -rw-r--r-- melissa user 20 1996-09-23 16:44 folk
4461 -rw-r--r-- melissa user 65 1997-01-30 14:15 jazz
4464 We can concatenate these two archives with @command{tar}:
4468 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4471 If you now list the contents of the @file{bluesrock.tar}, you will see
4472 that now it also contains the archive members of @file{jazzfolk.tar}:
4475 $ @kbd{tar --list --file=bluesrock.tar}
4482 When you use @option{--concatenate}, the source and target archives must
4483 already exist and must have been created using compatible format
4484 parameters. Notice, that @command{tar} does not check whether the
4485 archives it concatenates have compatible formats, it does not
4486 even check if the files are really tar archives.
4488 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4489 tape drives, due to deficiencies in the formats those tape drives use.
4491 @cindex @code{concatenate} vs @command{cat}
4492 @cindex @command{cat} vs @code{concatenate}
4493 It may seem more intuitive to you to want or try to use @command{cat} to
4494 concatenate two archives instead of using the @option{--concatenate}
4495 operation; after all, @command{cat} is the utility for combining files.
4497 However, @command{tar} archives incorporate an end-of-file marker which
4498 must be removed if the concatenated archives are to be read properly as
4499 one archive. @option{--concatenate} removes the end-of-archive marker
4500 from the target archive before each new archive is appended. If you use
4501 @command{cat} to combine the archives, the result will not be a valid
4502 @command{tar} format archive. If you need to retrieve files from an
4503 archive that was added to using the @command{cat} utility, use the
4504 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4505 information on dealing with archives improperly combined using the
4506 @command{cat} shell utility.
4509 @subsection Removing Archive Members Using @option{--delete}
4511 @cindex Deleting files from an archive
4512 @cindex Removing files from an archive
4515 You can remove members from an archive by using the @option{--delete}
4516 option. Specify the name of the archive with @option{--file}
4517 (@option{-f}) and then specify the names of the members to be deleted;
4518 if you list no member names, nothing will be deleted. The
4519 @option{--verbose} option will cause @command{tar} to print the names
4520 of the members as they are deleted. As with @option{--extract}, you
4521 must give the exact member names when using @samp{tar --delete}.
4522 @option{--delete} will remove all versions of the named file from the
4523 archive. The @option{--delete} operation can run very slowly.
4525 Unlike other operations, @option{--delete} has no short form.
4527 @cindex Tapes, using @option{--delete} and
4528 @cindex Deleting from tape archives
4529 This operation will rewrite the archive. You can only use
4530 @option{--delete} on an archive if the archive device allows you to
4531 write to any point on the media, such as a disk; because of this, it
4532 does not work on magnetic tapes. Do not try to delete an archive member
4533 from a magnetic tape; the action will not succeed, and you will be
4534 likely to scramble the archive and damage your tape. There is no safe
4535 way (except by completely re-writing the archive) to delete files from
4536 most kinds of magnetic tape. @xref{Media}.
4538 To delete all versions of the file @file{blues} from the archive
4539 @file{collection.tar} in the @file{practice} directory, make sure you
4540 are in that directory, and then,
4543 $ @kbd{tar --list --file=collection.tar}
4548 $ @kbd{tar --delete --file=collection.tar blues}
4549 $ @kbd{tar --list --file=collection.tar}
4556 @FIXME{Check if the above listing is actually produced after running
4557 all the examples on collection.tar.}
4559 The @option{--delete} option has been reported to work properly when
4560 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4563 @subsection Comparing Archive Members with the File System
4564 @cindex Verifying the currency of an archive
4568 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4569 specified archive members against files with the same names, and then
4570 reports differences in file size, mode, owner, modification date and
4571 contents. You should @emph{only} specify archive member names, not file
4572 names. If you do not name any members, then @command{tar} will compare the
4573 entire archive. If a file is represented in the archive but does not
4574 exist in the file system, @command{tar} reports a difference.
4576 You have to specify the record size of the archive when modifying an
4577 archive with a non-default record size.
4579 @command{tar} ignores files in the file system that do not have
4580 corresponding members in the archive.
4582 The following example compares the archive members @file{rock},
4583 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4584 files of the same name in the file system. (Note that there is no file,
4585 @file{funk}; @command{tar} will report an error message.)
4588 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4591 tar: funk not found in archive
4594 The spirit behind the @option{--compare} (@option{--diff},
4595 @option{-d}) option is to check whether the archive represents the
4596 current state of files on disk, more than validating the integrity of
4597 the archive media. For this later goal, @xref{verify}.
4599 @node create options
4600 @section Options Used by @option{--create}
4602 @xopindex{create, additional options}
4603 The previous chapter described the basics of how to use
4604 @option{--create} (@option{-c}) to create an archive from a set of files.
4605 @xref{create}. This section described advanced options to be used with
4609 * override:: Overriding File Metadata.
4610 * Ignore Failed Read::
4614 @subsection Overriding File Metadata
4616 As described above, a @command{tar} archive keeps, for each member it contains,
4617 its @dfn{metadata}, such as modification time, mode and ownership of
4618 the file. @GNUTAR{} allows to replace these data with other values
4619 when adding files to the archive. The options described in this
4620 section affect creation of archives of any type. For POSIX archives,
4621 see also @ref{PAX keywords}, for additional ways of controlling
4622 metadata, stored in the archive.
4626 @item --mode=@var{permissions}
4628 When adding files to an archive, @command{tar} will use
4629 @var{permissions} for the archive members, rather than the permissions
4630 from the files. @var{permissions} can be specified either as an octal
4631 number or as symbolic permissions, like with
4632 @command{chmod} (@xref{File permissions, Permissions, File
4633 permissions, fileutils, @acronym{GNU} file utilities}. This reference
4634 also has useful information for those not being overly familiar with
4635 the UNIX permission system). Using latter syntax allows for
4636 more flexibility. For example, the value @samp{a+rw} adds read and write
4637 permissions for everybody, while retaining executable bits on directories
4638 or on any other file already marked as executable:
4641 $ @kbd{tar -c -f archive.tar --mode='a+rw' .}
4644 @item --mtime=@var{date}
4647 When adding files to an archive, @command{tar} will use @var{date} as
4648 the modification time of members when creating archives, instead of
4649 their actual modification times. The argument @var{date} can be
4650 either a textual date representation in almost arbitrary format
4651 (@pxref{Date input formats}) or a name of the existing file, starting
4652 with @samp{/} or @samp{.}. In the latter case, the modification time
4653 of that file will be used.
4655 The following example will set the modification date to 00:00:00 UTC,
4659 $ @kbd{tar -c -f archive.tar --mtime='1970-01-01' .}
4663 When used with @option{--verbose} (@pxref{verbose tutorial}) @GNUTAR{}
4664 will try to convert the specified date back to its textual
4665 representation and compare it with the one given with
4666 @option{--mtime} options. If the two dates differ, @command{tar} will
4667 print a warning saying what date it will use. This is to help user
4668 ensure he is using the right date.
4673 $ @kbd{tar -c -f archive.tar -v --mtime=yesterday .}
4674 tar: Option --mtime: Treating date `yesterday' as 2006-06-20
4679 @item --owner=@var{user}
4682 Specifies that @command{tar} should use @var{user} as the owner of members
4683 when creating archives, instead of the user associated with the source
4684 file. The argument @var{user} can be either an existing user symbolic
4685 name, or a decimal numeric user @acronym{ID}.
4687 There is no value indicating a missing number, and @samp{0} usually means
4688 @code{root}. Some people like to force @samp{0} as the value to offer in
4689 their distributions for the owner of files, because the @code{root} user is
4690 anonymous anyway, so that might as well be the owner of anonymous
4691 archives. For example:
4695 $ @kbd{tar -c -f archive.tar --owner=0 .}
4697 $ @kbd{tar -c -f archive.tar --owner=root .}
4701 @item --group=@var{group}
4704 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
4705 rather than the group from the source file. The argument @var{group}
4706 can be either an existing group symbolic name, or a decimal numeric group @acronym{ID}.
4709 @node Ignore Failed Read
4710 @subsection Ignore Fail Read
4713 @item --ignore-failed-read
4714 @opindex ignore-failed-read
4715 Do not exit with nonzero on unreadable files or directories.
4718 @node extract options
4719 @section Options Used by @option{--extract}
4722 @xopindex{extract, additional options}
4723 The previous chapter showed how to use @option{--extract} to extract
4724 an archive into the file system. Various options cause @command{tar} to
4725 extract more information than just file contents, such as the owner,
4726 the permissions, the modification date, and so forth. This section
4727 presents options to be used with @option{--extract} when certain special
4728 considerations arise. You may review the information presented in
4729 @ref{extract} for more basic information about the
4730 @option{--extract} operation.
4733 * Reading:: Options to Help Read Archives
4734 * Writing:: Changing How @command{tar} Writes Files
4735 * Scarce:: Coping with Scarce Resources
4739 @subsection Options to Help Read Archives
4740 @cindex Options when reading archives
4743 @cindex Reading incomplete records
4744 @cindex Records, incomplete
4745 @opindex read-full-records
4746 Normally, @command{tar} will request data in full record increments from
4747 an archive storage device. If the device cannot return a full record,
4748 @command{tar} will report an error. However, some devices do not always
4749 return full records, or do not require the last record of an archive to
4750 be padded out to the next record boundary. To keep reading until you
4751 obtain a full record, or to accept an incomplete record if it contains
4752 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
4753 in conjunction with the @option{--extract} or @option{--list} operations.
4756 The @option{--read-full-records} (@option{-B}) option is turned on by default when
4757 @command{tar} reads an archive from standard input, or from a remote
4758 machine. This is because on @acronym{BSD} Unix systems, attempting to read a
4759 pipe returns however much happens to be in the pipe, even if it is
4760 less than was requested. If this option were not enabled, @command{tar}
4761 would fail as soon as it read an incomplete record from the pipe.
4763 If you're not sure of the blocking factor of an archive, you can
4764 read the archive by specifying @option{--read-full-records} (@option{-B}) and
4765 @option{--blocking-factor=@var{512-size}} (@option{-b
4766 @var{512-size}}), using a blocking factor larger than what the archive
4767 uses. This lets you avoid having to determine the blocking factor
4768 of an archive. @xref{Blocking Factor}.
4771 * read full records::
4775 @node read full records
4776 @unnumberedsubsubsec Reading Full Records
4778 @FIXME{need sentence or so of intro here}
4781 @opindex read-full-records
4782 @item --read-full-records
4784 Use in conjunction with @option{--extract} (@option{--get},
4785 @option{-x}) to read an archive which contains incomplete records, or
4786 one which has a blocking factor less than the one specified.
4790 @unnumberedsubsubsec Ignoring Blocks of Zeros
4792 @cindex End-of-archive blocks, ignoring
4793 @cindex Ignoring end-of-archive blocks
4794 @opindex ignore-zeros
4795 Normally, @command{tar} stops reading when it encounters a block of zeros
4796 between file entries (which usually indicates the end of the archive).
4797 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
4798 completely read an archive which contains a block of zeros before the
4799 end (i.e., a damaged archive, or one that was created by concatenating
4800 several archives together).
4802 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
4803 versions of @command{tar} write garbage after the end-of-archive entry,
4804 since that part of the media is never supposed to be read. @GNUTAR{}
4805 does not write after the end of an archive, but seeks to
4806 maintain compatibility among archiving utilities.
4809 @item --ignore-zeros
4811 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
4812 encountered while reading an archive. Use in conjunction with
4813 @option{--extract} or @option{--list}.
4817 @subsection Changing How @command{tar} Writes Files
4820 @FIXME{Introductory paragraph}
4823 * Dealing with Old Files::
4824 * Overwrite Old Files::
4826 * Keep Newer Files::
4828 * Recursive Unlink::
4829 * Data Modification Times::
4830 * Setting Access Permissions::
4831 * Directory Modification Times and Permissions::
4832 * Writing to Standard Output::
4833 * Writing to an External Program::
4837 @node Dealing with Old Files
4838 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
4840 @xopindex{overwrite-dir, introduced}
4841 When extracting files, if @command{tar} discovers that the extracted
4842 file already exists, it normally replaces the file by removing it before
4843 extracting it, to prevent confusion in the presence of hard or symbolic
4844 links. (If the existing file is a symbolic link, it is removed, not
4845 followed.) However, if a directory cannot be removed because it is
4846 nonempty, @command{tar} normally overwrites its metadata (ownership,
4847 permission, etc.). The @option{--overwrite-dir} option enables this
4848 default behavior. To be more cautious and preserve the metadata of
4849 such a directory, use the @option{--no-overwrite-dir} option.
4851 @cindex Overwriting old files, prevention
4852 @xopindex{keep-old-files, introduced}
4853 To be even more cautious and prevent existing files from being replaced, use
4854 the @option{--keep-old-files} (@option{-k}) option. It causes @command{tar} to refuse
4855 to replace or update a file that already exists, i.e., a file with the
4856 same name as an archive member prevents extraction of that archive
4857 member. Instead, it reports an error.
4859 @xopindex{overwrite, introduced}
4860 To be more aggressive about altering existing files, use the
4861 @option{--overwrite} option. It causes @command{tar} to overwrite
4862 existing files and to follow existing symbolic links when extracting.
4864 @cindex Protecting old files
4865 Some people argue that @GNUTAR{} should not hesitate
4866 to overwrite files with other files when extracting. When extracting
4867 a @command{tar} archive, they expect to see a faithful copy of the
4868 state of the file system when the archive was created. It is debatable
4869 that this would always be a proper behavior. For example, suppose one
4870 has an archive in which @file{usr/local} is a link to
4871 @file{usr/local2}. Since then, maybe the site removed the link and
4872 renamed the whole hierarchy from @file{/usr/local2} to
4873 @file{/usr/local}. Such things happen all the time. I guess it would
4874 not be welcome at all that @GNUTAR{} removes the
4875 whole hierarchy just to make room for the link to be reinstated
4876 (unless it @emph{also} simultaneously restores the full
4877 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
4878 able to remove a whole hierarchy to reestablish a symbolic link, for
4879 example, but @emph{only if} @option{--recursive-unlink} is specified
4880 to allow this behavior. In any case, single files are silently
4883 @xopindex{unlink-first, introduced}
4884 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
4885 some cases by causing @command{tar} to remove files unconditionally
4886 before extracting them.
4888 @node Overwrite Old Files
4889 @unnumberedsubsubsec Overwrite Old Files
4894 Overwrite existing files and directory metadata when extracting files
4897 This causes @command{tar} to write extracted files into the file system without
4898 regard to the files already on the system; i.e., files with the same
4899 names as archive members are overwritten when the archive is extracted.
4900 It also causes @command{tar} to extract the ownership, permissions,
4901 and time stamps onto any preexisting files or directories.
4902 If the name of a corresponding file name is a symbolic link, the file
4903 pointed to by the symbolic link will be overwritten instead of the
4904 symbolic link itself (if this is possible). Moreover, special devices,
4905 empty directories and even symbolic links are automatically removed if
4906 they are in the way of extraction.
4908 Be careful when using the @option{--overwrite} option, particularly when
4909 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
4910 can change the contents, ownership or permissions of any file on your
4911 system. Also, many systems do not take kindly to overwriting files that
4912 are currently being executed.
4914 @opindex overwrite-dir
4915 @item --overwrite-dir
4916 Overwrite the metadata of directories when extracting files from an
4917 archive, but remove other files before extracting.
4920 @node Keep Old Files
4921 @unnumberedsubsubsec Keep Old Files
4924 @opindex keep-old-files
4925 @item --keep-old-files
4927 Do not replace existing files from archive. The
4928 @option{--keep-old-files} (@option{-k}) option prevents @command{tar}
4929 from replacing existing files with files with the same name from the
4930 archive. The @option{--keep-old-files} option is meaningless with
4931 @option{--list} (@option{-t}). Prevents @command{tar} from replacing
4932 files in the file system during extraction.
4935 @node Keep Newer Files
4936 @unnumberedsubsubsec Keep Newer Files
4939 @opindex keep-newer-files
4940 @item --keep-newer-files
4941 Do not replace existing files that are newer than their archive
4942 copies. This option is meaningless with @option{--list} (@option{-t}).
4946 @unnumberedsubsubsec Unlink First
4949 @opindex unlink-first
4950 @item --unlink-first
4952 Remove files before extracting over them.
4953 This can make @command{tar} run a bit faster if you know in advance
4954 that the extracted files all need to be removed. Normally this option
4955 slows @command{tar} down slightly, so it is disabled by default.
4958 @node Recursive Unlink
4959 @unnumberedsubsubsec Recursive Unlink
4962 @opindex recursive-unlink
4963 @item --recursive-unlink
4964 When this option is specified, try removing files and directory hierarchies
4965 before extracting over them. @emph{This is a dangerous option!}
4968 If you specify the @option{--recursive-unlink} option,
4969 @command{tar} removes @emph{anything} that keeps you from extracting a file
4970 as far as current permissions will allow it. This could include removal
4971 of the contents of a full directory hierarchy.
4973 @node Data Modification Times
4974 @unnumberedsubsubsec Setting Data Modification Times
4976 @cindex Data modification times of extracted files
4977 @cindex Modification times of extracted files
4978 Normally, @command{tar} sets the data modification times of extracted
4979 files to the corresponding times recorded for the files in the archive, but
4980 limits the permissions of extracted files by the current @code{umask}
4983 To set the data modification times of extracted files to the time when
4984 the files were extracted, use the @option{--touch} (@option{-m}) option in
4985 conjunction with @option{--extract} (@option{--get}, @option{-x}).
4991 Sets the data modification time of extracted archive members to the time
4992 they were extracted, not the time recorded for them in the archive.
4993 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
4996 @node Setting Access Permissions
4997 @unnumberedsubsubsec Setting Access Permissions
4999 @cindex Permissions of extracted files
5000 @cindex Modes of extracted files
5001 To set the modes (access permissions) of extracted files to those
5002 recorded for those files in the archive, use @option{--same-permissions}
5003 in conjunction with the @option{--extract} (@option{--get},
5004 @option{-x}) operation.
5007 @opindex preserve-permissions
5008 @opindex same-permissions
5009 @item --preserve-permissions
5010 @itemx --same-permissions
5011 @c @itemx --ignore-umask
5013 Set modes of extracted archive members to those recorded in the
5014 archive, instead of current umask settings. Use in conjunction with
5015 @option{--extract} (@option{--get}, @option{-x}).
5018 @node Directory Modification Times and Permissions
5019 @unnumberedsubsubsec Directory Modification Times and Permissions
5021 After successfully extracting a file member, @GNUTAR{} normally
5022 restores its permissions and modification times, as described in the
5023 previous sections. This cannot be done for directories, because
5024 after extracting a directory @command{tar} will almost certainly
5025 extract files into that directory and this will cause the directory
5026 modification time to be updated. Moreover, restoring that directory
5027 permissions may not permit file creation within it. Thus, restoring
5028 directory permissions and modification times must be delayed at least
5029 until all files have been extracted into that directory. @GNUTAR{}
5030 restores directories using the following approach.
5032 The extracted directories are created with the mode specified in the
5033 archive, as modified by the umask of the user, which gives sufficient
5034 permissions to allow file creation. The meta-information about the
5035 directory is recorded in the temporary list of directories. When
5036 preparing to extract next archive member, @GNUTAR{} checks if the
5037 directory prefix of this file contains the remembered directory. If
5038 it does not, the program assumes that all files have been extracted
5039 into that directory, restores its modification time and permissions
5040 and removes its entry from the internal list. This approach allows
5041 to correctly restore directory meta-information in the majority of
5042 cases, while keeping memory requirements sufficiently small. It is
5043 based on the fact, that most @command{tar} archives use the predefined
5044 order of members: first the directory, then all the files and
5045 subdirectories in that directory.
5047 However, this is not always true. The most important exception are
5048 incremental archives (@pxref{Incremental Dumps}). The member order in
5049 an incremental archive is reversed: first all directory members are
5050 stored, followed by other (non-directory) members. So, when extracting
5051 from incremental archives, @GNUTAR{} alters the above procedure. It
5052 remembers all restored directories, and restores their meta-data
5053 only after the entire archive has been processed. Notice, that you do
5054 not need to specify any special options for that, as @GNUTAR{}
5055 automatically detects archives in incremental format.
5057 There may be cases, when such processing is required for normal archives
5058 too. Consider the following example:
5062 $ @kbd{tar --no-recursion -cvf archive \
5063 foo foo/file1 bar bar/file foo/file2}
5072 During the normal operation, after encountering @file{bar}
5073 @GNUTAR{} will assume that all files from the directory @file{foo}
5074 were already extracted and will therefore restore its timestamp and
5075 permission bits. However, after extracting @file{foo/file2} the
5076 directory timestamp will be offset again.
5078 To correctly restore directory meta-information in such cases, use
5079 @option{delay-directory-restore} command line option:
5082 @opindex delay-directory-restore
5083 @item --delay-directory-restore
5084 Delays restoring of the modification times and permissions of extracted
5085 directories until the end of extraction. This way, correct
5086 meta-information is restored even if the archive has unusual member
5089 @opindex no-delay-directory-restore
5090 @item --no-delay-directory-restore
5091 Cancel the effect of the previous @option{--delay-directory-restore}.
5092 Use this option if you have used @option{--delay-directory-restore} in
5093 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
5094 temporarily disable it.
5097 @node Writing to Standard Output
5098 @unnumberedsubsubsec Writing to Standard Output
5100 @cindex Writing extracted files to standard output
5101 @cindex Standard output, writing extracted files to
5102 To write the extracted files to the standard output, instead of
5103 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
5104 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
5105 extracting files to send them through a pipe, and do not need to
5106 preserve them in the file system. If you extract multiple members,
5107 they appear on standard output concatenated, in the order they are
5108 found in the archive.
5114 Writes files to the standard output. Use only in conjunction with
5115 @option{--extract} (@option{--get}, @option{-x}). When this option is
5116 used, instead of creating the files specified, @command{tar} writes
5117 the contents of the files extracted to its standard output. This may
5118 be useful if you are only extracting the files in order to send them
5119 through a pipe. This option is meaningless with @option{--list}
5123 This can be useful, for example, if you have a tar archive containing
5124 a big file and don't want to store the file on disk before processing
5125 it. You can use a command like this:
5128 tar -xOzf foo.tgz bigfile | process
5131 or even like this if you want to process the concatenation of the files:
5134 tar -xOzf foo.tgz bigfile1 bigfile2 | process
5137 However, @option{--to-command} may be more convenient for use with
5138 multiple files. See the next section.
5140 @node Writing to an External Program
5141 @unnumberedsubsubsec Writing to an External Program
5143 You can instruct @command{tar} to send the contents of each extracted
5144 file to the standard input of an external program:
5148 @item --to-command=@var{command}
5149 Extract files and pipe their contents to the standard input of
5150 @var{command}. When this option is used, instead of creating the
5151 files specified, @command{tar} invokes @var{command} and pipes the
5152 contents of the files to its standard output. @var{Command} may
5153 contain command line arguments. The program is executed via
5154 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
5155 extracted. Non-regular files (directories, etc.) are ignored when this
5159 The command can obtain the information about the file it processes
5160 from the following environment variables:
5163 @vrindex TAR_FILETYPE, to-command environment
5165 Type of the file. It is a single letter with the following meaning:
5167 @multitable @columnfractions 0.10 0.90
5168 @item f @tab Regular file
5169 @item d @tab Directory
5170 @item l @tab Symbolic link
5171 @item h @tab Hard link
5172 @item b @tab Block device
5173 @item c @tab Character device
5176 Currently only regular files are supported.
5178 @vrindex TAR_MODE, to-command environment
5180 File mode, an octal number.
5182 @vrindex TAR_FILENAME, to-command environment
5184 The name of the file.
5186 @vrindex TAR_REALNAME, to-command environment
5188 Name of the file as stored in the archive.
5190 @vrindex TAR_UNAME, to-command environment
5192 Name of the file owner.
5194 @vrindex TAR_GNAME, to-command environment
5196 Name of the file owner group.
5198 @vrindex TAR_ATIME, to-command environment
5200 Time of last access. It is a decimal number, representing seconds
5201 since the epoch. If the archive provides times with nanosecond
5202 precision, the nanoseconds are appended to the timestamp after a
5205 @vrindex TAR_MTIME, to-command environment
5207 Time of last modification.
5209 @vrindex TAR_CTIME, to-command environment
5211 Time of last status change.
5213 @vrindex TAR_SIZE, to-command environment
5217 @vrindex TAR_UID, to-command environment
5219 UID of the file owner.
5221 @vrindex TAR_GID, to-command environment
5223 GID of the file owner.
5226 In addition to these variables, @env{TAR_VERSION} contains the
5227 @GNUTAR{} version number.
5229 If @var{command} exits with a non-0 status, @command{tar} will print
5230 an error message similar to the following:
5233 tar: 2345: Child returned status 1
5236 Here, @samp{2345} is the PID of the finished process.
5238 If this behavior is not wanted, use @option{--ignore-command-error}:
5241 @opindex ignore-command-error
5242 @item --ignore-command-error
5243 Ignore exit codes of subprocesses. Notice that if the program
5244 exits on signal or otherwise terminates abnormally, the error message
5245 will be printed even if this option is used.
5247 @opindex no-ignore-command-error
5248 @item --no-ignore-command-error
5249 Cancel the effect of any previous @option{--ignore-command-error}
5250 option. This option is useful if you have set
5251 @option{--ignore-command-error} in @env{TAR_OPTIONS}
5252 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
5256 @unnumberedsubsubsec Removing Files
5258 @FIXME{The section is too terse. Something more to add? An example,
5262 @opindex remove-files
5263 @item --remove-files
5264 Remove files after adding them to the archive.
5268 @subsection Coping with Scarce Resources
5271 @cindex Small memory
5272 @cindex Running out of space
5280 @unnumberedsubsubsec Starting File
5283 @opindex starting-file
5284 @item --starting-file=@var{name}
5285 @itemx -K @var{name}
5286 Starts an operation in the middle of an archive. Use in conjunction
5287 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
5290 @cindex Middle of the archive, starting in the
5291 If a previous attempt to extract files failed due to lack of disk
5292 space, you can use @option{--starting-file=@var{name}} (@option{-K
5293 @var{name}}) to start extracting only after member @var{name} of the
5294 archive. This assumes, of course, that there is now free space, or
5295 that you are now extracting into a different file system. (You could
5296 also choose to suspend @command{tar}, remove unnecessary files from
5297 the file system, and then restart the same @command{tar} operation.
5298 In this case, @option{--starting-file} is not necessary.
5299 @xref{Incremental Dumps}, @xref{interactive}, and @ref{exclude}.)
5302 @unnumberedsubsubsec Same Order
5305 @cindex Large lists of file names on small machines
5307 @opindex preserve-order
5309 @itemx --preserve-order
5311 To process large lists of file names on machines with small amounts of
5312 memory. Use in conjunction with @option{--compare} (@option{--diff},
5313 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
5314 (@option{--get}, @option{-x}).
5317 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
5318 names to be listed or extracted is sorted in the same order as the
5319 files in the archive. This allows a large list of names to be used,
5320 even on a small machine that would not otherwise be able to hold all
5321 the names in memory at the same time. Such a sorted list can easily be
5322 created by running @samp{tar -t} on the archive and editing its output.
5324 This option is probably never needed on modern computer systems.
5327 @section Backup options
5329 @cindex backup options
5331 @GNUTAR{} offers options for making backups of files
5332 before writing new versions. These options control the details of
5333 these backups. They may apply to the archive itself before it is
5334 created or rewritten, as well as individual extracted members. Other
5335 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
5336 and @command{mv}, for example) offer similar options.
5338 Backup options may prove unexpectedly useful when extracting archives
5339 containing many members having identical name, or when extracting archives
5340 on systems having file name limitations, making different members appear
5341 has having similar names through the side-effect of name truncation.
5342 (This is true only if we have a good scheme for truncated backup names,
5343 which I'm not sure at all: I suspect work is needed in this area.)
5344 When any existing file is backed up before being overwritten by extraction,
5345 then clashing files are automatically be renamed to be unique, and the
5346 true name is kept for only the last file of a series of clashing files.
5347 By using verbose mode, users may track exactly what happens.
5349 At the detail level, some decisions are still experimental, and may
5350 change in the future, we are waiting comments from our users. So, please
5351 do not learn to depend blindly on the details of the backup features.
5352 For example, currently, directories themselves are never renamed through
5353 using these options, so, extracting a file over a directory still has
5354 good chances to fail. Also, backup options apply to created archives,
5355 not only to extracted members. For created archives, backups will not
5356 be attempted when the archive is a block or character device, or when it
5357 refers to a remote file.
5359 For the sake of simplicity and efficiency, backups are made by renaming old
5360 files prior to creation or extraction, and not by copying. The original
5361 name is restored if the file creation fails. If a failure occurs after a
5362 partial extraction of a file, both the backup and the partially extracted
5366 @item --backup[=@var{method}]
5368 @vindex VERSION_CONTROL
5370 Back up files that are about to be overwritten or removed.
5371 Without this option, the original versions are destroyed.
5373 Use @var{method} to determine the type of backups made.
5374 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
5375 environment variable. And if @env{VERSION_CONTROL} is not set,
5376 use the @samp{existing} method.
5378 @vindex version-control @r{Emacs variable}
5379 This option corresponds to the Emacs variable @samp{version-control};
5380 the same values for @var{method} are accepted as in Emacs. This option
5381 also allows more descriptive names. The valid @var{method}s are:
5386 @cindex numbered @r{backup method}
5387 Always make numbered backups.
5391 @cindex existing @r{backup method}
5392 Make numbered backups of files that already have them, simple backups
5397 @cindex simple @r{backup method}
5398 Always make simple backups.
5402 @item --suffix=@var{suffix}
5404 @cindex backup suffix
5405 @vindex SIMPLE_BACKUP_SUFFIX
5406 Append @var{suffix} to each backup file made with @option{--backup}. If this
5407 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5408 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5409 set, the default is @samp{~}, just as in Emacs.
5414 @section Notable @command{tar} Usages
5417 @FIXME{Using Unix file linking capability to recreate directory
5418 structures---linking files into one subdirectory and then
5419 @command{tar}ring that directory.}
5421 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5424 You can easily use archive files to transport a group of files from
5425 one system to another: put all relevant files into an archive on one
5426 computer system, transfer the archive to another system, and extract
5427 the contents there. The basic transfer medium might be magnetic tape,
5428 Internet FTP, or even electronic mail (though you must encode the
5429 archive with @command{uuencode} in order to transport it properly by
5430 mail). Both machines do not have to use the same operating system, as
5431 long as they both support the @command{tar} program.
5433 For example, here is how you might copy a directory's contents from
5434 one disk to another, while preserving the dates, modes, owners and
5435 link-structure of all the files therein. In this case, the transfer
5436 medium is a @dfn{pipe}, which is one a Unix redirection mechanism:
5439 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5443 You can avoid subshells by using @option{-C} option:
5446 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5450 The command also works using short option forms:
5453 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5454 | (cd targetdir; tar --extract --file=-)}
5456 $ @kbd{tar --directory sourcedir --create --file=- . ) \
5457 | tar --directory targetdir --extract --file=-}
5461 This is one of the easiest methods to transfer a @command{tar} archive.
5464 @section Looking Ahead: The Rest of this Manual
5466 You have now seen how to use all eight of the operations available to
5467 @command{tar}, and a number of the possible options. The next chapter
5468 explains how to choose and change file and archive names, how to use
5469 files to store names of other files which you can then call as
5470 arguments to @command{tar} (this can help you save time if you expect to
5471 archive the same list of files a number of times), and so forth.
5472 @FIXME{in case it's not obvious, i'm making this up in some sense
5473 based on my limited memory of what the next chapter *really* does. i
5474 just wanted to flesh out this final section a little bit so i'd
5475 remember to stick it in here. :-)}
5477 If there are too many files to conveniently list on the command line,
5478 you can list the names in a file, and @command{tar} will read that file.
5481 There are various ways of causing @command{tar} to skip over some files,
5482 and not archive them. @xref{Choosing}.
5485 @chapter Performing Backups and Restoring Files
5488 @GNUTAR{} is distributed along with the scripts
5489 which the Free Software Foundation uses for performing backups. There
5490 is no corresponding scripts available yet for doing restoration of
5491 files. Even if there is a good chance those scripts may be satisfying
5492 to you, they are not the only scripts or methods available for doing
5493 backups and restore. You may well create your own, or use more
5494 sophisticated packages dedicated to that purpose.
5496 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5497 Automatic Network Disk Archiver), a backup system developed by James
5498 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5499 This is free software, and it is available at these places:
5502 http://www.cs.umd.edu/projects/amanda/amanda.html
5503 ftp://ftp.cs.umd.edu/pub/amanda
5508 Here is a possible plan for a future documentation about the backuping
5509 scripts which are provided within the @GNUTAR{}
5515 @item what are dumps
5516 @item different levels of dumps
5518 @item full dump = dump everything
5519 @item level 1, level 2 dumps etc
5520 A level @var{n} dump dumps everything changed since the last level
5523 @item how to use scripts for dumps (ie, the concept)
5525 @item scripts to run after editing backup specs (details)
5527 @item Backup Specs, what is it.
5529 @item how to customize
5530 @item actual text of script [/sp/dump/backup-specs]
5534 @item rsh doesn't work
5535 @item rtape isn't installed
5538 @item the @option{--incremental} option of tar
5541 @item write protection
5542 @item types of media, different sizes and types, useful for different things
5543 @item files and tape marks
5544 one tape mark between files, two at end.
5545 @item positioning the tape
5546 MT writes two at end of write,
5547 backspaces over one when writing again.
5553 This chapter documents both the provided shell scripts and @command{tar}
5554 options which are more specific to usage as a backup tool.
5556 To @dfn{back up} a file system means to create archives that contain
5557 all the files in that file system. Those archives can then be used to
5558 restore any or all of those files (for instance if a disk crashes or a
5559 file is accidentally deleted). File system @dfn{backups} are also
5563 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5564 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5565 * Backup Levels:: Levels of Backups
5566 * Backup Parameters:: Setting Parameters for Backups and Restoration
5567 * Scripted Backups:: Using the Backup Scripts
5568 * Scripted Restoration:: Using the Restore Script
5572 @section Using @command{tar} to Perform Full Dumps
5578 @cindex corrupted archives
5579 Full dumps should only be made when no other people or programs
5580 are modifying files in the file system. If files are modified while
5581 @command{tar} is making the backup, they may not be stored properly in
5582 the archive, in which case you won't be able to restore them if you
5583 have to. (Files not being modified are written with no trouble, and do
5584 not corrupt the entire archive.)
5586 You will want to use the @option{--label=@var{archive-label}}
5587 (@option{-V @var{archive-label}}) option to give the archive a
5588 volume label, so you can tell what this archive is even if the label
5589 falls off the tape, or anything like that.
5591 Unless the file system you are dumping is guaranteed to fit on
5592 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5593 Make sure you have enough tapes on hand to complete the backup.
5595 If you want to dump each file system separately you will need to use
5596 the @option{--one-file-system} option to prevent
5597 @command{tar} from crossing file system boundaries when storing
5600 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5601 option is not needed, since this is a complete copy of everything in
5602 the file system, and a full restore from this backup would only be
5603 done onto a completely
5606 Unless you are in a hurry, and trust the @command{tar} program (and your
5607 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5608 option, to make sure your files really made it onto the dump properly.
5609 This will also detect cases where the file was modified while (or just
5610 after) it was being archived. Not all media (notably cartridge tapes)
5611 are capable of being verified, unfortunately.
5613 @node Incremental Dumps
5614 @section Using @command{tar} to Perform Incremental Dumps
5616 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5617 stores additional metadata so that exact state of the file system
5618 can be restored when extracting the archive.
5620 @GNUTAR{} currently offers two options for handling incremental
5621 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5622 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5624 @opindex listed-incremental
5625 The option @option{--listed-incremental} instructs tar to operate on
5626 an incremental archive with additional metadata stored in a standalone
5627 file, called a @dfn{snapshot file}. The purpose of this file is to help
5628 determine which files have been changed, added or deleted since the
5629 last backup, so that the next incremental backup will contain only
5630 modified files. The name of the snapshot file is given as an argument
5634 @item --listed-incremental=@var{file}
5635 @itemx -g @var{file}
5636 Handle incremental backups with snapshot data in @var{file}.
5639 To create an incremental backup, you would use
5640 @option{--listed-incremental} together with @option{--create}
5641 (@pxref{create}). For example:
5644 $ @kbd{tar --create \
5645 --file=archive.1.tar \
5646 --listed-incremental=/var/log/usr.snar \
5650 This will create in @file{archive.1.tar} an incremental backup of
5651 the @file{/usr} file system, storing additional metadata in the file
5652 @file{/var/log/usr.snar}. If this file does not exist, it will be
5653 created. The created archive will then be a @dfn{level 0 backup};
5654 please see the next section for more on backup levels.
5656 Otherwise, if the file @file{/var/log/usr.snar} exists, it
5657 determines which files are modified. In this case only these files will be
5658 stored in the archive. Suppose, for example, that after running the
5659 above command, you delete file @file{/usr/doc/old} and create
5660 directory @file{/usr/local/db} with the following contents:
5663 $ @kbd{ls /usr/local/db}
5668 Some time later you create another incremental backup. You will
5672 $ @kbd{tar --create \
5673 --file=archive.2.tar \
5674 --listed-incremental=/var/log/usr.snar \
5676 tar: usr/local/db: Directory is new
5683 The created archive @file{archive.2.tar} will contain only these
5684 three members. This archive is called a @dfn{level 1 backup}. Notice
5685 that @file{/var/log/usr.snar} will be updated with the new data, so if
5686 you plan to create more @samp{level 1} backups, it is necessary to
5687 create a working copy of the snapshot file before running
5688 @command{tar}. The above example will then be modified as follows:
5691 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
5692 $ @kbd{tar --create \
5693 --file=archive.2.tar \
5694 --listed-incremental=/var/log/usr.snar-1 \
5698 Incremental dumps depend crucially on time stamps, so the results are
5699 unreliable if you modify a file's time stamps during dumping (e.g.,
5700 with the @option{--atime-preserve=replace} option), or if you set the clock
5703 @anchor{device numbers}
5704 @cindex Device numbers, using in incremental backups
5705 Metadata stored in snapshot files include device numbers, which,
5706 obviously are supposed to be a non-volatile values. However, it turns
5707 out that @acronym{NFS} devices have undependable values when an automounter
5708 gets in the picture. This can lead to a great deal of spurious
5709 redumping in incremental dumps, so it is somewhat useless to compare
5710 two @acronym{NFS} devices numbers over time. The solution implemented
5711 currently is to considers all @acronym{NFS} devices as being equal
5712 when it comes to comparing directories; this is fairly gross, but
5713 there does not seem to be a better way to go.
5715 Apart from using @acronym{NFS}, there are a number of cases where
5716 relying on device numbers can cause spurious redumping of unmodified
5717 files. For example, this occurs when archiving @acronym{LVM} snapshot
5718 volumes. To avoid this, use @option{--no-check-device} option:
5721 @xopindex{no-check-device, described}
5722 @item --no-check-device
5723 Do not rely on device numbers when preparing a list of changed files
5724 for an incremental dump.
5726 @xopindex{check-device, described}
5727 @item --check-device
5728 Use device numbers when preparing a list of changed files
5729 for an incremental dump. This is the default behavior. The purpose
5730 of this option is to undo the effect of the @option{--no-check-device}
5731 if it was given in @env{TAR_OPTIONS} environment variable
5732 (@pxref{TAR_OPTIONS}).
5735 There is also another way to cope with changing device numbers. It is
5736 described in detail in @ref{Fixing Snapshot Files}.
5738 Note that incremental archives use @command{tar} extensions and may
5739 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
5741 @xopindex{listed-incremental, using with @option{--extract}}
5742 @xopindex{extract, using with @option{--listed-incremental}}
5743 To extract from the incremental dumps, use
5744 @option{--listed-incremental} together with @option{--extract}
5745 option (@pxref{extracting files}). In this case, @command{tar} does
5746 not need to access snapshot file, since all the data necessary for
5747 extraction are stored in the archive itself. So, when extracting, you
5748 can give whatever argument to @option{--listed-incremental}, the usual
5749 practice is to use @option{--listed-incremental=/dev/null}.
5750 Alternatively, you can use @option{--incremental}, which needs no
5751 arguments. In general, @option{--incremental} (@option{-G}) can be
5752 used as a shortcut for @option{--listed-incremental} when listing or
5753 extracting incremental backups (for more information, regarding this
5754 option, @pxref{incremental-op}).
5756 When extracting from the incremental backup @GNUTAR{} attempts to
5757 restore the exact state the file system had when the archive was
5758 created. In particular, it will @emph{delete} those files in the file
5759 system that did not exist in their directories when the archive was
5760 created. If you have created several levels of incremental files,
5761 then in order to restore the exact contents the file system had when
5762 the last level was created, you will need to restore from all backups
5763 in turn. Continuing our example, to restore the state of @file{/usr}
5764 file system, one would do@footnote{Notice, that since both archives
5765 were created without @option{-P} option (@pxref{absolute}), these
5766 commands should be run from the root file system.}:
5769 $ @kbd{tar --extract \
5770 --listed-incremental=/dev/null \
5771 --file archive.1.tar}
5772 $ @kbd{tar --extract \
5773 --listed-incremental=/dev/null \
5774 --file archive.2.tar}
5777 To list the contents of an incremental archive, use @option{--list}
5778 (@pxref{list}), as usual. To obtain more information about the
5779 archive, use @option{--listed-incremental} or @option{--incremental}
5780 combined with two @option{--verbose} options@footnote{Two
5781 @option{--verbose} options were selected to avoid breaking usual
5782 verbose listing output (@option{--list --verbose}) when using in
5785 @xopindex{incremental, using with @option{--list}}
5786 @xopindex{listed-incremental, using with @option{--list}}
5787 @xopindex{list, using with @option{--incremental}}
5788 @xopindex{list, using with @option{--listed-incremental}}
5789 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
5790 contents of the DUMPDIR header (with terminating nulls) when
5791 @option{--incremental} or @option{--listed-incremental} option was
5792 given, no matter what the verbosity level. This behavior, and,
5793 especially, the binary output it produced were considered inconvenient
5794 and were changed in version 1.16}:
5797 @kbd{tar --list --incremental --verbose --verbose archive.tar}
5800 This command will print, for each directory in the archive, the list
5801 of files in that directory at the time the archive was created. This
5802 information is put out in a format which is both human-readable and
5803 unambiguous for a program: each file name is printed as
5810 where @var{x} is a letter describing the status of the file: @samp{Y}
5811 if the file is present in the archive, @samp{N} if the file is not
5812 included in the archive, or a @samp{D} if the file is a directory (and
5813 is included in the archive). @xref{Dumpdir}, for the detailed
5814 description of dumpdirs and status codes. Each such
5815 line is terminated by a newline character. The last line is followed
5816 by an additional newline to indicate the end of the data.
5818 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
5819 gives the same behavior as @option{--listed-incremental} when used
5820 with @option{--list} and @option{--extract} options. When used with
5821 @option{--create} option, it creates an incremental archive without
5822 creating snapshot file. Thus, it is impossible to create several
5823 levels of incremental backups with @option{--incremental} option.
5826 @section Levels of Backups
5828 An archive containing all the files in the file system is called a
5829 @dfn{full backup} or @dfn{full dump}. You could insure your data by
5830 creating a full dump every day. This strategy, however, would waste a
5831 substantial amount of archive media and user time, as unchanged files
5832 are daily re-archived.
5834 It is more efficient to do a full dump only occasionally. To back up
5835 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
5836 one} dump archives all the files that have changed since the last full
5839 A typical dump strategy would be to perform a full dump once a week,
5840 and a level one dump once a day. This means some versions of files
5841 will in fact be archived more than once, but this dump strategy makes
5842 it possible to restore a file system to within one day of accuracy by
5843 only extracting two archives---the last weekly (full) dump and the
5844 last daily (level one) dump. The only information lost would be in
5845 files changed or created since the last daily backup. (Doing dumps
5846 more than once a day is usually not worth the trouble).
5848 @GNUTAR{} comes with scripts you can use to do full
5849 and level-one (actually, even level-two and so on) dumps. Using
5850 scripts (shell programs) to perform backups and restoration is a
5851 convenient and reliable alternative to typing out file name lists
5852 and @command{tar} commands by hand.
5854 Before you use these scripts, you need to edit the file
5855 @file{backup-specs}, which specifies parameters used by the backup
5856 scripts and by the restore script. This file is usually located
5857 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
5858 detailed description. Once the backup parameters are set, you can
5859 perform backups or restoration by running the appropriate script.
5861 The name of the backup script is @code{backup}. The name of the
5862 restore script is @code{restore}. The following sections describe
5863 their use in detail.
5865 @emph{Please Note:} The backup and restoration scripts are
5866 designed to be used together. While it is possible to restore files by
5867 hand from an archive which was created using a backup script, and to create
5868 an archive by hand which could then be extracted using the restore script,
5869 it is easier to use the scripts. @xref{Incremental Dumps}, before
5870 making such an attempt.
5872 @node Backup Parameters
5873 @section Setting Parameters for Backups and Restoration
5875 The file @file{backup-specs} specifies backup parameters for the
5876 backup and restoration scripts provided with @command{tar}. You must
5877 edit @file{backup-specs} to fit your system configuration and schedule
5878 before using these scripts.
5880 Syntactically, @file{backup-specs} is a shell script, containing
5881 mainly variable assignments. However, any valid shell construct
5882 is allowed in this file. Particularly, you may wish to define
5883 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
5884 For more information about shell script syntax, please refer to
5885 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
5886 g_02, the definition of the Shell Command Language}. See also
5887 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
5889 The shell variables controlling behavior of @code{backup} and
5890 @code{restore} are described in the following subsections.
5893 * General-Purpose Variables::
5894 * Magnetic Tape Control::
5896 * backup-specs example:: An Example Text of @file{Backup-specs}
5899 @node General-Purpose Variables
5900 @subsection General-Purpose Variables
5902 @defvr {Backup variable} ADMINISTRATOR
5903 The user name of the backup administrator. @code{Backup} scripts
5904 sends a backup report to this address.
5907 @defvr {Backup variable} BACKUP_HOUR
5908 The hour at which the backups are done. This can be a number from 0
5909 to 23, or the time specification in form @var{hours}:@var{minutes},
5910 or the string @samp{now}.
5912 This variable is used by @code{backup}. Its value may be overridden
5913 using @option{--time} option (@pxref{Scripted Backups}).
5916 @defvr {Backup variable} TAPE_FILE
5918 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
5919 is a remote archive (@pxref{remote-dev}), backup script will suppose
5920 that your @command{mt} is able to access remote devices. If @var{RSH}
5921 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
5922 invocations of @command{mt}.
5925 @defvr {Backup variable} BLOCKING
5927 The blocking factor @command{tar} will use when writing the dump archive.
5928 @xref{Blocking Factor}.
5931 @defvr {Backup variable} BACKUP_DIRS
5933 A list of file systems to be dumped (for @code{backup}), or restored
5934 (for @code{restore}). You can include any directory
5935 name in the list --- subdirectories on that file system will be
5936 included, regardless of how they may look to other networked machines.
5937 Subdirectories on other file systems will be ignored.
5939 The host name specifies which host to run @command{tar} on, and should
5940 normally be the host that actually contains the file system. However,
5941 the host machine must have @GNUTAR{} installed, and
5942 must be able to access the directory containing the backup scripts and
5943 their support files using the same file name that is used on the
5944 machine where the scripts are run (i.e., what @command{pwd} will print
5945 when in that directory on that machine). If the host that contains
5946 the file system does not have this capability, you can specify another
5947 host as long as it can access the file system through @acronym{NFS}.
5949 If the list of file systems is very long you may wish to put it
5950 in a separate file. This file is usually named
5951 @file{/etc/backup/dirs}, but this name may be overridden in
5952 @file{backup-specs} using @code{DIRLIST} variable.
5955 @defvr {Backup variable} DIRLIST
5957 The name of the file that contains a list of file systems to backup
5958 or restore. By default it is @file{/etc/backup/dirs}.
5961 @defvr {Backup variable} BACKUP_FILES
5963 A list of individual files to be dumped (for @code{backup}), or restored
5964 (for @code{restore}). These should be accessible from the machine on
5965 which the backup script is run.
5967 If the list of file systems is very long you may wish to store it
5968 in a separate file. This file is usually named
5969 @file{/etc/backup/files}, but this name may be overridden in
5970 @file{backup-specs} using @code{FILELIST} variable.
5973 @defvr {Backup variable} FILELIST
5975 The name of the file that contains a list of individual files to backup
5976 or restore. By default it is @file{/etc/backup/files}.
5979 @defvr {Backup variable} MT
5981 Full file name of @command{mt} binary.
5984 @defvr {Backup variable} RSH
5986 Full file name of @command{rsh} binary or its equivalent. You may wish to
5987 set it to @code{ssh}, to improve security. In this case you will have
5988 to use public key authentication.
5991 @defvr {Backup variable} RSH_COMMAND
5993 Full file name of @command{rsh} binary on remote machines. This will
5994 be passed via @option{--rsh-command} option to the remote invocation
5998 @defvr {Backup variable} VOLNO_FILE
6000 Name of temporary file to hold volume numbers. This needs to be accessible
6001 by all the machines which have file systems to be dumped.
6004 @defvr {Backup variable} XLIST
6006 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
6007 located on the remote machine and containing the list of files to
6008 be excluded from the backup. Exclude file lists are searched in
6009 /etc/tar-backup directory. A common use for exclude file lists
6010 is to exclude files containing security-sensitive information
6011 (e.g., @file{/etc/shadow} from backups).
6013 This variable affects only @code{backup}.
6016 @defvr {Backup variable} SLEEP_TIME
6018 Time to sleep between dumps of any two successive file systems
6020 This variable affects only @code{backup}.
6023 @defvr {Backup variable} DUMP_REMIND_SCRIPT
6025 Script to be run when it's time to insert a new tape in for the next
6026 volume. Administrators may want to tailor this script for their site.
6027 If this variable isn't set, @GNUTAR{} will display its built-in
6028 prompt, and will expect confirmation from the console. For the
6029 description of the default prompt, see @ref{change volume prompt}.
6033 @defvr {Backup variable} SLEEP_MESSAGE
6035 Message to display on the terminal while waiting for dump time. Usually
6036 this will just be some literal text.
6039 @defvr {Backup variable} TAR
6041 Full file name of the @GNUTAR{} executable. If this is not set, backup
6042 scripts will search @command{tar} in the current shell path.
6045 @node Magnetic Tape Control
6046 @subsection Magnetic Tape Control
6048 Backup scripts access tape device using special @dfn{hook functions}.
6049 These functions take a single argument -- the name of the tape
6050 device. Their names are kept in the following variables:
6052 @defvr {Backup variable} MT_BEGIN
6053 The name of @dfn{begin} function. This function is called before
6054 accessing the drive. By default it retensions the tape:
6060 mt -f "$1" retension
6065 @defvr {Backup variable} MT_REWIND
6066 The name of @dfn{rewind} function. The default definition is as
6079 @defvr {Backup variable} MT_OFFLINE
6080 The name of the function switching the tape off line. By default
6081 it is defined as follows:
6084 MT_OFFLINE=mt_offline
6092 @defvr {Backup variable} MT_STATUS
6093 The name of the function used to obtain the status of the archive device,
6094 including error count. Default definition:
6106 @subsection User Hooks
6108 @dfn{User hooks} are shell functions executed before and after
6109 each @command{tar} invocation. Thus, there are @dfn{backup
6110 hooks}, which are executed before and after dumping each file
6111 system, and @dfn{restore hooks}, executed before and
6112 after restoring a file system. Each user hook is a shell function
6113 taking four arguments:
6115 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
6120 Current backup or restore level.
6123 Name or IP address of the host machine being dumped or restored.
6126 Full file name of the file system being dumped or restored.
6129 File system name with directory separators replaced with colons. This
6130 is useful, e.g., for creating unique files.
6134 Following variables keep the names of user hook functions
6136 @defvr {Backup variable} DUMP_BEGIN
6137 Dump begin function. It is executed before dumping the file system.
6140 @defvr {Backup variable} DUMP_END
6141 Executed after dumping the file system.
6144 @defvr {Backup variable} RESTORE_BEGIN
6145 Executed before restoring the file system.
6148 @defvr {Backup variable} RESTORE_END
6149 Executed after restoring the file system.
6152 @node backup-specs example
6153 @subsection An Example Text of @file{Backup-specs}
6155 The following is an example of @file{backup-specs}:
6158 # site-specific parameters for file system backup.
6160 ADMINISTRATOR=friedman
6162 TAPE_FILE=/dev/nrsmt0
6164 # Use @code{ssh} instead of the less secure @code{rsh}
6166 RSH_COMMAND=/usr/bin/ssh
6168 # Override MT_STATUS function:
6174 # Disable MT_OFFLINE function
6191 apple-gunkies:/com/mailer/gnu
6192 apple-gunkies:/com/archive/gnu"
6194 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
6198 @node Scripted Backups
6199 @section Using the Backup Scripts
6201 The syntax for running a backup script is:
6204 backup --level=@var{level} --time=@var{time}
6207 The @option{level} option requests the dump level. Thus, to produce
6208 a full dump, specify @code{--level=0} (this is the default, so
6209 @option{--level} may be omitted if its value is @code{0}).
6210 @footnote{For backward compatibility, the @code{backup} will also
6211 try to deduce the requested dump level from the name of the
6212 script itself. If the name consists of a string @samp{level-}
6213 followed by a single decimal digit, that digit is taken as
6214 the dump level number. Thus, you may create a link from @code{backup}
6215 to @code{level-1} and then run @code{level-1} whenever you need to
6216 create a level one dump.}
6218 The @option{--time} option determines when should the backup be
6219 run. @var{Time} may take three forms:
6222 @item @var{hh}:@var{mm}
6224 The dump must be run at @var{hh} hours @var{mm} minutes.
6228 The dump must be run at @var{hh} hours
6232 The dump must be run immediately.
6235 You should start a script with a tape or disk mounted. Once you
6236 start a script, it prompts you for new tapes or disks as it
6237 needs them. Media volumes don't have to correspond to archive
6238 files --- a multi-volume archive can be started in the middle of a
6239 tape that already contains the end of another multi-volume archive.
6240 The @code{restore} script prompts for media by its archive volume,
6241 so to avoid an error message you should keep track of which tape
6242 (or disk) contains which volume of the archive (@pxref{Scripted
6245 The backup scripts write two files on the file system. The first is a
6246 record file in @file{/etc/tar-backup/}, which is used by the scripts
6247 to store and retrieve information about which files were dumped. This
6248 file is not meant to be read by humans, and should not be deleted by
6249 them. @xref{Snapshot Files}, for a more detailed explanation of this
6252 The second file is a log file containing the names of the file systems
6253 and files dumped, what time the backup was made, and any error
6254 messages that were generated, as well as how much space was left in
6255 the media volume after the last volume of the archive was written.
6256 You should check this log file after every backup. The file name is
6257 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
6258 represents current date, and @var{n} represents current dump level number.
6260 The script also prints the name of each system being dumped to the
6263 Following is the full list of options accepted by @code{backup}
6267 @item -l @var{level}
6268 @itemx --level=@var{level}
6269 Do backup level @var{level} (default 0).
6273 Force backup even if today's log file already exists.
6275 @item -v[@var{level}]
6276 @itemx --verbose[=@var{level}]
6277 Set verbosity level. The higher the level is, the more debugging
6278 information will be output during execution. Default @var{level}
6279 is 100, which means the highest debugging level.
6281 @item -t @var{start-time}
6282 @itemx --time=@var{start-time}
6283 Wait till @var{time}, then do backup.
6287 Display short help message and exit.
6291 Display information about the program's name, version, origin and legal
6292 status, all on standard output, and then exit successfully.
6296 @node Scripted Restoration
6297 @section Using the Restore Script
6299 To restore files that were archived using a scripted backup, use the
6300 @code{restore} script. Its usage is quite straightforward. In the
6301 simplest form, invoke @code{restore --all}, it will
6302 then restore all the file systems and files specified in
6303 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
6305 You may select the file systems (and/or files) to restore by
6306 giving @code{restore} list of @dfn{patterns} in its command
6307 line. For example, running
6314 will restore all file systems on the machine @samp{albert}. A more
6315 complicated example:
6318 restore 'albert:*' '*:/var'
6322 This command will restore all file systems on the machine @samp{albert}
6323 as well as @file{/var} file system on all machines.
6325 By default @code{restore} will start restoring files from the lowest
6326 available dump level (usually zero) and will continue through
6327 all available dump levels. There may be situations where such a
6328 thorough restore is not necessary. For example, you may wish to
6329 restore only files from the recent level one backup. To do so,
6330 use @option{--level} option, as shown in the example below:
6336 The full list of options accepted by @code{restore} follows:
6341 Restore all file systems and files specified in @file{backup-specs}
6343 @item -l @var{level}
6344 @itemx --level=@var{level}
6345 Start restoring from the given backup level, instead of the default 0.
6347 @item -v[@var{level}]
6348 @itemx --verbose[=@var{level}]
6349 Set verbosity level. The higher the level is, the more debugging
6350 information will be output during execution. Default @var{level}
6351 is 100, which means the highest debugging level.
6355 Display short help message and exit.
6359 Display information about the program's name, version, origin and legal
6360 status, all on standard output, and then exit successfully.
6363 You should start the restore script with the media containing the
6364 first volume of the archive mounted. The script will prompt for other
6365 volumes as they are needed. If the archive is on tape, you don't need
6366 to rewind the tape to to its beginning---if the tape head is
6367 positioned past the beginning of the archive, the script will rewind
6368 the tape as needed. @xref{Tape Positioning}, for a discussion of tape
6372 @strong{Warning:} The script will delete files from the active file
6373 system if they were not in the file system when the archive was made.
6376 @xref{Incremental Dumps}, for an explanation of how the script makes
6380 @chapter Choosing Files and Names for @command{tar}
6383 Certain options to @command{tar} enable you to specify a name for your
6384 archive. Other options let you decide which files to include or exclude
6385 from the archive, based on when or whether files were modified, whether
6386 the file names do or don't match specified patterns, or whether files
6387 are in specified directories.
6389 This chapter discusses these options in detail.
6392 * file:: Choosing the Archive's Name
6393 * Selecting Archive Members::
6394 * files:: Reading Names from a File
6395 * exclude:: Excluding Some Files
6396 * wildcards:: Wildcards Patterns and Matching
6397 * quoting styles:: Ways of Quoting Special Characters in Names
6398 * transform:: Modifying File and Member Names
6399 * after:: Operating Only on New Files
6400 * recurse:: Descending into Directories
6401 * one:: Crossing File System Boundaries
6405 @section Choosing and Naming Archive Files
6408 @cindex Naming an archive
6409 @cindex Archive Name
6410 @cindex Choosing an archive file
6411 @cindex Where is the archive?
6412 By default, @command{tar} uses an archive file name that was compiled when
6413 it was built on the system; usually this name refers to some physical
6414 tape drive on the machine. However, the person who installed @command{tar}
6415 on the system may not have set the default to a meaningful value as far as
6416 most users are concerned. As a result, you will usually want to tell
6417 @command{tar} where to find (or create) the archive. The
6418 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
6419 option allows you to either specify or name a file to use as the archive
6420 instead of the default archive file location.
6423 @xopindex{file, short description}
6424 @item --file=@var{archive-name}
6425 @itemx -f @var{archive-name}
6426 Name the archive to create or operate on. Use in conjunction with
6430 For example, in this @command{tar} command,
6433 $ @kbd{tar -cvf collection.tar blues folk jazz}
6437 @file{collection.tar} is the name of the archive. It must directly
6438 follow the @option{-f} option, since whatever directly follows @option{-f}
6439 @emph{will} end up naming the archive. If you neglect to specify an
6440 archive name, you may end up overwriting a file in the working directory
6441 with the archive you create since @command{tar} will use this file's name
6442 for the archive name.
6444 An archive can be saved as a file in the file system, sent through a
6445 pipe or over a network, or written to an I/O device such as a tape,
6446 floppy disk, or CD write drive.
6448 @cindex Writing new archives
6449 @cindex Archive creation
6450 If you do not name the archive, @command{tar} uses the value of the
6451 environment variable @env{TAPE} as the file name for the archive. If
6452 that is not available, @command{tar} uses a default, compiled-in archive
6453 name, usually that for tape unit zero (i.e., @file{/dev/tu00}).
6455 @cindex Standard input and output
6456 @cindex tar to standard input and output
6457 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
6458 archive from standard input (when listing or extracting files), or
6459 writes it to standard output (when creating an archive). If you use
6460 @file{-} as an @var{archive-name} when modifying an archive,
6461 @command{tar} reads the original archive from its standard input and
6462 writes the entire new archive to its standard output.
6464 The following example is a convenient way of copying directory
6465 hierarchy from @file{sourcedir} to @file{targetdir}.
6468 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
6471 The @option{-C} option allows to avoid using subshells:
6474 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
6477 In both examples above, the leftmost @command{tar} invocation archives
6478 the contents of @file{sourcedir} to the standard output, while the
6479 rightmost one reads this archive from its standard input and
6480 extracts it. The @option{-p} option tells it to restore permissions
6481 of the extracted files.
6483 @cindex Remote devices
6484 @cindex tar to a remote device
6486 To specify an archive file on a device attached to a remote machine,
6490 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
6494 @command{tar} will complete the remote connection, if possible, and
6495 prompt you for a username and password. If you use
6496 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
6497 will complete the remote connection, if possible, using your username
6498 as the username on the remote machine.
6500 @cindex Local and remote archives
6501 @anchor{local and remote archives}
6502 If the archive file name includes a colon (@samp{:}), then it is assumed
6503 to be a file on another machine. If the archive file is
6504 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6505 host @var{host}. The remote host is accessed using the @command{rsh}
6506 program, with a username of @var{user}. If the username is omitted
6507 (along with the @samp{@@} sign), then your user name will be used.
6508 (This is the normal @command{rsh} behavior.) It is necessary for the
6509 remote machine, in addition to permitting your @command{rsh} access, to
6510 have the @file{rmt} program installed (This command is included in
6511 the @GNUTAR{} distribution and by default is installed under
6512 @file{@var{prefix}/libexec/rmt}, were @var{prefix} means your
6513 installation prefix). If you need to use a file whose name includes a
6514 colon, then the remote tape drive behavior
6515 can be inhibited by using the @option{--force-local} option.
6517 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6518 tries to minimize input and output operations. The Amanda backup
6519 system, when used with @GNUTAR{}, has an initial sizing pass which
6522 @node Selecting Archive Members
6523 @section Selecting Archive Members
6524 @cindex Specifying files to act on
6525 @cindex Specifying archive members
6527 @dfn{File Name arguments} specify which files in the file system
6528 @command{tar} operates on, when creating or adding to an archive, or which
6529 archive members @command{tar} operates on, when reading or deleting from
6530 an archive. @xref{Operations}.
6532 To specify file names, you can include them as the last arguments on
6533 the command line, as follows:
6535 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6538 If a file name begins with dash (@samp{-}), precede it with
6539 @option{--add-file} option to prevent it from being treated as an
6542 @anchor{input name quoting}
6543 By default @GNUTAR{} attempts to @dfn{unquote} each file or member
6544 name, replacing @dfn{escape sequences} according to the following
6547 @multitable @columnfractions 0.20 0.60
6548 @headitem Escape @tab Replaced with
6549 @item \a @tab Audible bell (@acronym{ASCII} 7)
6550 @item \b @tab Backspace (@acronym{ASCII} 8)
6551 @item \f @tab Form feed (@acronym{ASCII} 12)
6552 @item \n @tab New line (@acronym{ASCII} 10)
6553 @item \r @tab Carriage return (@acronym{ASCII} 13)
6554 @item \t @tab Horizontal tabulation (@acronym{ASCII} 9)
6555 @item \v @tab Vertical tabulation (@acronym{ASCII} 11)
6556 @item \? @tab @acronym{ASCII} 127
6557 @item \@var{n} @tab @acronym{ASCII} @var{n} (@var{n} should be an octal number
6561 A backslash followed by any other symbol is retained.
6563 This default behavior is controlled by the following command line
6569 Enable unquoting input file or member names (default).
6573 Disable unquoting input file or member names.
6576 If you specify a directory name as a file name argument, all the files
6577 in that directory are operated on by @command{tar}.
6579 If you do not specify files, @command{tar} behavior differs depending
6580 on the operation mode as described below:
6582 When @command{tar} is invoked with @option{--create} (@option{-c}),
6583 @command{tar} will stop immediately, reporting the following:
6587 $ @kbd{tar cf a.tar}
6588 tar: Cowardly refusing to create an empty archive
6589 Try `tar --help' or `tar --usage' for more information.
6593 If you specify either @option{--list} (@option{-t}) or
6594 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6595 operates on all the archive members in the archive.
6597 If run with @option{--diff} option, tar will compare the archive with
6598 the contents of the current working directory.
6600 If you specify any other operation, @command{tar} does nothing.
6602 By default, @command{tar} takes file names from the command line. However,
6603 there are other ways to specify file or member names, or to modify the
6604 manner in which @command{tar} selects the files or members upon which to
6605 operate. In general, these methods work both for specifying the names
6606 of files and archive members.
6609 @section Reading Names from a File
6611 @cindex Reading file names from a file
6612 @cindex Lists of file names
6613 @cindex File Name arguments, alternatives
6614 Instead of giving the names of files or archive members on the command
6615 line, you can put the names into a file, and then use the
6616 @option{--files-from=@var{file-of-names}} (@option{-T
6617 @var{file-of-names}}) option to @command{tar}. Give the name of the
6618 file which contains the list of files to include as the argument to
6619 @option{--files-from}. In the list, the file names should be separated by
6620 newlines. You will frequently use this option when you have generated
6621 the list of files to archive with the @command{find} utility.
6625 @item --files-from=@var{file-name}
6626 @itemx -T @var{file-name}
6627 Get names to extract or create from file @var{file-name}.
6630 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6631 you specify either @code{--files-from=-} or @code{-T -}), then the file
6632 names are read from standard input.
6634 Unless you are running @command{tar} with @option{--create}, you can not use
6635 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6638 Any number of @option{-T} options can be given in the command line.
6640 The following example shows how to use @command{find} to generate a list of
6641 files smaller than 400K in length and put that list into a file
6642 called @file{small-files}. You can then use the @option{-T} option to
6643 @command{tar} to specify the files from that file, @file{small-files}, to
6644 create the archive @file{little.tgz}. (The @option{-z} option to
6645 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
6649 $ @kbd{find . -size -400 -print > small-files}
6650 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
6654 In the file list given by @option{-T} option, any file name beginning
6655 with @samp{-} character is considered a @command{tar} option and is
6656 processed accordingly.@footnote{Versions of @GNUTAR{} up to 1.15.1
6657 recognized only @option{-C} option in file lists, and only if the
6658 option and its argument occupied two consecutive lines.} For example,
6659 the common use of this feature is to change to another directory by
6660 specifying @option{-C} option:
6670 $ @kbd{tar -c -f foo.tar --files-from list}
6675 In this example, @command{tar} will first switch to @file{/etc}
6676 directory and add files @file{passwd} and @file{hosts} to the
6677 archive. Then it will change to @file{/lib} directory and will archive
6678 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
6683 $ @kbd{tar tf foo.tar}
6691 @xopindex{directory, using in @option{--files-from} argument}
6692 Notice that the option parsing algorithm used with @option{-T} is
6693 stricter than the one used by shell. Namely, when specifying option
6694 arguments, you should observe the following rules:
6698 When using short (single-letter) option form, its argument must
6699 immediately follow the option letter, without any intervening
6700 whitespace. For example: @code{-Cdir}.
6703 When using long option form, the option argument must be separated
6704 from the option by a single equal sign. No whitespace is allowed on
6705 any side of the equal sign. For example: @code{--directory=dir}.
6708 For both short and long option forms, the option argument can be given
6709 on the next line after the option name, e.g.:
6730 If you happen to have a file whose name starts with @samp{-},
6731 precede it with @option{--add-file} option to prevent it from
6732 being recognized as an option. For example: @code{--add-file=--my-file}.
6739 @subsection @code{NUL} Terminated File Names
6741 @cindex File names, terminated by @code{NUL}
6742 @cindex @code{NUL} terminated file names
6743 The @option{--null} option causes
6744 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
6745 to read file names terminated by a @code{NUL} instead of a newline, so
6746 files whose names contain newlines can be archived using
6747 @option{--files-from}.
6752 Only consider @code{NUL} terminated file names, instead of files that
6753 terminate in a newline.
6756 The @option{--null} option is just like the one in @acronym{GNU}
6757 @command{xargs} and @command{cpio}, and is useful with the
6758 @option{-print0} predicate of @acronym{GNU} @command{find}. In
6759 @command{tar}, @option{--null} also disables special handling for
6760 file names that begin with dash.
6762 This example shows how to use @command{find} to generate a list of files
6763 larger than 800K in length and put that list into a file called
6764 @file{long-files}. The @option{-print0} option to @command{find} is just
6765 like @option{-print}, except that it separates files with a @code{NUL}
6766 rather than with a newline. You can then run @command{tar} with both the
6767 @option{--null} and @option{-T} options to specify that @command{tar} get the
6768 files from that file, @file{long-files}, to create the archive
6769 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
6770 @command{tar} to recognize the @code{NUL} separator between files.
6773 $ @kbd{find . -size +800 -print0 > long-files}
6774 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
6777 @FIXME{say anything else here to conclude the section?}
6780 @section Excluding Some Files
6783 @cindex File names, excluding files by
6784 @cindex Excluding files by name and pattern
6785 @cindex Excluding files by file system
6786 To avoid operating on files whose names match a particular pattern,
6787 use the @option{--exclude} or @option{--exclude-from} options.
6791 @item --exclude=@var{pattern}
6792 Causes @command{tar} to ignore files that match the @var{pattern}.
6796 The @option{--exclude=@var{pattern}} option prevents any file or
6797 member whose name matches the shell wildcard (@var{pattern}) from
6799 For example, to create an archive with all the contents of the directory
6800 @file{src} except for files whose names end in @file{.o}, use the
6801 command @samp{tar -cf src.tar --exclude='*.o' src}.
6803 You may give multiple @option{--exclude} options.
6806 @opindex exclude-from
6807 @item --exclude-from=@var{file}
6808 @itemx -X @var{file}
6809 Causes @command{tar} to ignore files that match the patterns listed in
6813 @findex exclude-from
6814 Use the @option{--exclude-from} option to read a
6815 list of patterns, one per line, from @var{file}; @command{tar} will
6816 ignore files matching those patterns. Thus if @command{tar} is
6817 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
6818 single line @file{*.o}, no files whose names end in @file{.o} will be
6819 added to the archive.
6821 Notice, that lines from @var{file} are read verbatim. One of the
6822 frequent errors is leaving some extra whitespace after a file name,
6823 which is difficult to catch using text editors.
6825 However, empty lines are OK.
6827 @cindex version control system, excluding files
6828 @cindex VCS, excluding files
6829 @cindex SCCS, excluding files
6830 @cindex RCS, excluding files
6831 @cindex CVS, excluding files
6832 @cindex SVN, excluding files
6833 @cindex git, excluding files
6834 @cindex Bazaar, excluding files
6835 @cindex Arch, excluding files
6836 @cindex Mercurial, excluding files
6837 @cindex Darcs, excluding files
6839 @opindex exclude-vcs
6841 Exclude files and directories used by following version control
6842 systems: @samp{CVS}, @samp{RCS}, @samp{SCCS}, @samp{SVN}, @samp{Arch},
6843 @samp{Bazaar}, @samp{Mercurial}, and @samp{Darcs}.
6846 As of version @value{VERSION}, the following files are excluded:
6849 @item @file{CVS/}, and everything under it
6850 @item @file{RCS/}, and everything under it
6851 @item @file{SCCS/}, and everything under it
6852 @item @file{.git/}, and everything under it
6853 @item @file{.gitignore}
6854 @item @file{.cvsignore}
6855 @item @file{.svn/}, and everything under it
6856 @item @file{.arch-ids/}, and everything under it
6857 @item @file{@{arch@}/}, and everything under it
6858 @item @file{=RELEASE-ID}
6859 @item @file{=meta-update}
6860 @item @file{=update}
6862 @item @file{.bzrignore}
6863 @item @file{.bzrtags}
6865 @item @file{.hgignore}
6866 @item @file{.hgrags}
6870 @findex exclude-caches
6871 When creating an archive, the @option{--exclude-caches} option family
6872 causes @command{tar} to exclude all directories that contain a @dfn{cache
6873 directory tag}. A cache directory tag is a short file with the
6874 well-known name @file{CACHEDIR.TAG} and having a standard header
6875 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
6876 Various applications write cache directory tags into directories they
6877 use to hold regenerable, non-precious data, so that such data can be
6878 more easily excluded from backups.
6880 There are three @samp{exclude-caches} options, each providing a different
6881 exclusion semantics:
6884 @opindex exclude-caches
6885 @item --exclude-caches
6886 Do not archive the contents of the directory, but archive the
6887 directory itself and the @file{CACHEDIR.TAG} file.
6889 @opindex exclude-caches-under
6890 @item --exclude-caches-under
6891 Do not archive the contents of the directory, nor the
6892 @file{CACHEDIR.TAG} file, archive only the directory itself.
6894 @opindex exclude-caches-all
6895 @item --exclude-caches-all
6896 Omit directories containing @file{CACHEDIR.TAG} file entirely.
6900 Another option family, @option{--exclude-tag}, provides a generalization of
6901 this concept. It takes a single argument, a file name to look for.
6902 Any directory that contains this file will be excluded from the dump.
6903 Similarly to @samp{exclude-caches}, there are three options in this
6907 @opindex exclude-tag
6908 @item --exclude-tag=@var{file}
6909 Do not dump the contents of the directory, but dump the
6910 directory itself and the @var{file}.
6912 @opindex exclude-tag-under
6913 @item --exclude-tag-under=@var{file}
6914 Do not dump the contents of the directory, nor the
6915 @var{file}, archive only the directory itself.
6917 @opindex exclude-tag-all
6918 @item --exclude-tag-all=@var{file}
6919 Omit directories containing @var{file} file entirely.
6922 Multiple @option{--exclude-tag*} options can be given.
6924 For example, given this directory:
6939 The @option{--exclude-tag} will produce the following:
6942 $ @kbd{tar -cf archive.tar --exclude-tag=tagfile -v dir}
6947 tar: dir/folk/: contains a cache directory tag tagfile;
6952 Both the @file{dir/folk} directory and its tagfile are preserved in
6953 the archive, however the rest of files in this directory are not.
6955 Now, using the @option{--exclude-tag-under} option will exclude
6956 @file{tagfile} from the dump, while still preserving the directory
6957 itself, as shown in this example:
6960 $ @kbd{tar -cf archive.tar --exclude-tag-under=tagfile -v dir}
6965 ./tar: dir/folk/: contains a cache directory tag tagfile;
6969 Finally, using @option{--exclude-tag-all} omits the @file{dir/folk}
6973 $ @kbd{tar -cf archive.tar --exclude-tag-all=tagfile -v dir}
6977 ./tar: dir/folk/: contains a cache directory tag tagfile;
6978 directory not dumped
6982 * problems with exclude::
6985 @node problems with exclude
6986 @unnumberedsubsec Problems with Using the @code{exclude} Options
6988 @xopindex{exclude, potential problems with}
6989 Some users find @samp{exclude} options confusing. Here are some common
6994 The main operating mode of @command{tar} does not act on a file name
6995 explicitly listed on the command line, if one of its file name
6996 components is excluded. In the example above, if
6997 you create an archive and exclude files that end with @samp{*.o}, but
6998 explicitly name the file @samp{dir.o/foo} after all the options have been
6999 listed, @samp{dir.o/foo} will be excluded from the archive.
7002 You can sometimes confuse the meanings of @option{--exclude} and
7003 @option{--exclude-from}. Be careful: use @option{--exclude} when files
7004 to be excluded are given as a pattern on the command line. Use
7005 @option{--exclude-from} to introduce the name of a file which contains
7006 a list of patterns, one per line; each of these patterns can exclude
7007 zero, one, or many files.
7010 When you use @option{--exclude=@var{pattern}}, be sure to quote the
7011 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
7012 like @samp{*}. If you do not do this, the shell might expand the
7013 @samp{*} itself using files at hand, so @command{tar} might receive a
7014 list of files instead of one pattern, or none at all, making the
7015 command somewhat illegal. This might not correspond to what you want.
7020 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
7028 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
7032 You must use use shell syntax, or globbing, rather than @code{regexp}
7033 syntax, when using exclude options in @command{tar}. If you try to use
7034 @code{regexp} syntax to describe files to be excluded, your command
7038 @FIXME{The change in semantics must have occurred before 1.11,
7039 so I doubt if it is worth mentioning at all. Anyway, should at
7040 least specify in which version the semantics changed.}
7041 In earlier versions of @command{tar}, what is now the
7042 @option{--exclude-from} option was called @option{--exclude} instead.
7043 Now, @option{--exclude} applies to patterns listed on the command
7044 line and @option{--exclude-from} applies to patterns listed in a
7050 @section Wildcards Patterns and Matching
7052 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
7053 @samp{*} or @samp{?} for example, are replaced and expanded into all
7054 existing files matching the given pattern. @GNUTAR{} can use wildcard
7055 patterns for matching (or globbing) archive members when extracting
7056 from or listing an archive. Wildcard patterns are also used for
7057 verifying volume labels of @command{tar} archives. This section has the
7058 purpose of explaining wildcard syntax for @command{tar}.
7060 @FIXME{the next few paragraphs need work.}
7062 A @var{pattern} should be written according to shell syntax, using wildcard
7063 characters to effect globbing. Most characters in the pattern stand
7064 for themselves in the matched string, and case is significant: @samp{a}
7065 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
7066 pattern matches any single character in the matched string. The character
7067 @samp{*} in the pattern matches zero, one, or more single characters in
7068 the matched string. The character @samp{\} says to take the following
7069 character of the pattern @emph{literally}; it is useful when one needs to
7070 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
7072 The character @samp{[}, up to the matching @samp{]}, introduces a character
7073 class. A @dfn{character class} is a list of acceptable characters
7074 for the next single character of the matched string. For example,
7075 @samp{[abcde]} would match any of the first five letters of the alphabet.
7076 Note that within a character class, all of the ``special characters''
7077 listed above other than @samp{\} lose their special meaning; for example,
7078 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
7079 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
7080 the characters @samp{-} and @samp{]} must either come @emph{first} or
7081 @emph{last} in a character class.)
7083 @cindex Excluding characters from a character class
7084 @cindex Character class, excluding characters from
7085 If the first character of the class after the opening @samp{[}
7086 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
7087 Rather than listing character to match, it lists those characters which
7088 are @emph{forbidden} as the next single character of the matched string.
7090 Other characters of the class stand for themselves. The special
7091 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
7092 letters, is meant to represent all characters between @var{a} and
7095 @FIXME{need to add a sentence or so here to make this clear for those
7096 who don't have dan around.}
7098 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
7099 special for wildcard matches. However, if a pattern completely matches
7100 a directory prefix of a matched string, then it matches the full matched
7101 string: thus, excluding a directory also excludes all the files beneath it.
7104 * controlling pattern-matching::
7107 @node controlling pattern-matching
7108 @unnumberedsubsec Controlling Pattern-Matching
7110 For the purposes of this section, we call @dfn{exclusion members} all
7111 member names obtained while processing @option{--exclude} and
7112 @option{--exclude-from} options, and @dfn{inclusion members} those
7113 member names that were given in the command line or read from the file
7114 specified with @option{--files-from} option.
7116 These two pairs of member lists are used in the following operations:
7117 @option{--diff}, @option{--extract}, @option{--list},
7120 There are no inclusion members in create mode (@option{--create} and
7121 @option{--append}), since in this mode the names obtained from the
7122 command line refer to @emph{files}, not archive members.
7124 By default, inclusion members are compared with archive members
7125 literally @footnote{Notice that earlier @GNUTAR{} versions used
7126 globbing for inclusion members, which contradicted to UNIX98
7127 specification and was not documented. @xref{Changes}, for more
7128 information on this and other changes.} and exclusion members are
7129 treated as globbing patterns. For example:
7133 $ @kbd{tar tf foo.tar}
7138 # @i{Member names are used verbatim:}
7139 $ @kbd{tar -xf foo.tar -v '[remarks]'}
7141 # @i{Exclude member names are globbed:}
7142 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
7148 This behavior can be altered by using the following options:
7153 Treat all member names as wildcards.
7155 @opindex no-wildcards
7156 @item --no-wildcards
7157 Treat all member names as literal strings.
7160 Thus, to extract files whose names end in @samp{.c}, you can use:
7163 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
7169 Notice quoting of the pattern to prevent the shell from interpreting
7172 The effect of @option{--wildcards} option is canceled by
7173 @option{--no-wildcards}. This can be used to pass part of
7174 the command line arguments verbatim and other part as globbing
7175 patterns. For example, the following invocation:
7178 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
7182 instructs @command{tar} to extract from @file{foo.tar} all files whose
7183 names end in @samp{.txt} and the file named @file{[remarks]}.
7185 Normally, a pattern matches a name if an initial subsequence of the
7186 name's components matches the pattern, where @samp{*}, @samp{?}, and
7187 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
7188 and wildcards can match @samp{/}.
7190 Other than optionally stripping leading @samp{/} from names
7191 (@pxref{absolute}), patterns and names are used as-is. For
7192 example, trailing @samp{/} is not trimmed from a user-specified name
7193 before deciding whether to exclude it.
7195 However, this matching procedure can be altered by the options listed
7196 below. These options accumulate. For example:
7199 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
7203 ignores case when excluding @samp{makefile}, but not when excluding
7208 @opindex no-anchored
7210 @itemx --no-anchored
7211 If anchored, a pattern must match an initial subsequence
7212 of the name's components. Otherwise, the pattern can match any
7213 subsequence. Default is @option{--no-anchored} for exclusion members
7214 and @option{--anchored} inclusion members.
7216 @opindex ignore-case
7217 @opindex no-ignore-case
7219 @itemx --no-ignore-case
7220 When ignoring case, upper-case patterns match lower-case names and vice versa.
7221 When not ignoring case (the default), matching is case-sensitive.
7223 @opindex wildcards-match-slash
7224 @opindex no-wildcards-match-slash
7225 @item --wildcards-match-slash
7226 @itemx --no-wildcards-match-slash
7227 When wildcards match slash (the default for exclusion members), a
7228 wildcard like @samp{*} in the pattern can match a @samp{/} in the
7229 name. Otherwise, @samp{/} is matched only by @samp{/}.
7233 The @option{--recursion} and @option{--no-recursion} options
7234 (@pxref{recurse}) also affect how member patterns are interpreted. If
7235 recursion is in effect, a pattern matches a name if it matches any of
7236 the name's parent directories.
7238 The following table summarizes pattern-matching default values:
7240 @multitable @columnfractions .3 .7
7241 @headitem Members @tab Default settings
7242 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
7243 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
7246 @node quoting styles
7247 @section Quoting Member Names
7249 When displaying member names, @command{tar} takes care to avoid
7250 ambiguities caused by certain characters. This is called @dfn{name
7251 quoting}. The characters in question are:
7254 @item Non-printable control characters:
7255 @anchor{escape sequences}
7256 @multitable @columnfractions 0.20 0.10 0.60
7257 @headitem Character @tab @acronym{ASCII} @tab Character name
7258 @item \a @tab 7 @tab Audible bell
7259 @item \b @tab 8 @tab Backspace
7260 @item \f @tab 12 @tab Form feed
7261 @item \n @tab 10 @tab New line
7262 @item \r @tab 13 @tab Carriage return
7263 @item \t @tab 9 @tab Horizontal tabulation
7264 @item \v @tab 11 @tab Vertical tabulation
7267 @item Space (@acronym{ASCII} 32)
7269 @item Single and double quotes (@samp{'} and @samp{"})
7271 @item Backslash (@samp{\})
7274 The exact way @command{tar} uses to quote these characters depends on
7275 the @dfn{quoting style}. The default quoting style, called
7276 @dfn{escape} (see below), uses backslash notation to represent control
7277 characters, space and backslash. Using this quoting style, control
7278 characters are represented as listed in column @samp{Character} in the
7279 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
7281 @GNUTAR{} offers seven distinct quoting styles, which can be selected
7282 using @option{--quoting-style} option:
7285 @item --quoting-style=@var{style}
7286 @opindex quoting-style
7288 Sets quoting style. Valid values for @var{style} argument are:
7289 literal, shell, shell-always, c, escape, locale, clocale.
7292 These styles are described in detail below. To illustrate their
7293 effect, we will use an imaginary tar archive @file{arch.tar}
7294 containing the following members:
7298 # 1. Contains horizontal tabulation character.
7300 # 2. Contains newline character
7303 # 3. Contains a space
7305 # 4. Contains double quotes
7307 # 5. Contains single quotes
7309 # 6. Contains a backslash character:
7314 Here is how usual @command{ls} command would have listed them, if they
7315 had existed in the current working directory:
7333 No quoting, display each character as is:
7337 $ @kbd{tar tf arch.tar --quoting-style=literal}
7350 Display characters the same way Bourne shell does:
7351 control characters, except @samp{\t} and @samp{\n}, are printed using
7352 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
7353 single quote is printed as @samp{\'}. If a name contains any quoted
7354 characters, it is enclosed in single quotes. In particular, if a name
7355 contains single quotes, it is printed as several single-quoted strings:
7359 $ @kbd{tar tf arch.tar --quoting-style=shell}
7362 './a'\''single'\''quote'
7372 Same as @samp{shell}, but the names are always enclosed in single
7377 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
7380 './a'\''single'\''quote'
7390 Use the notation of the C programming language. All names are
7391 enclosed in double quotes. Control characters are quoted using
7392 backslash notations, double quotes are represented as @samp{\"},
7393 backslash characters are represented as @samp{\\}. Single quotes and
7394 spaces are not quoted:
7398 $ @kbd{tar tf arch.tar --quoting-style=c}
7402 "./a\"double\"quote"
7410 Control characters are printed using backslash notation, a space is
7411 printed as @samp{\ } and a backslash as @samp{\\}. This is the
7412 default quoting style, unless it was changed when configured the
7417 $ @kbd{tar tf arch.tar --quoting-style=escape}
7429 Control characters, single quote and backslash are printed using
7430 backslash notation. All names are quoted using left and right
7431 quotation marks, appropriate to the current locale. If it does not
7432 define quotation marks, use @samp{`} as left and @samp{'} as right
7433 quotation marks. Any occurrences of the right quotation mark in a
7434 name are escaped with @samp{\}, for example:
7440 $ @kbd{tar tf arch.tar --quoting-style=locale}
7443 `./a\'single\'quote'
7452 Same as @samp{locale}, but @samp{"} is used for both left and right
7453 quotation marks, if not provided by the currently selected locale:
7457 $ @kbd{tar tf arch.tar --quoting-style=clocale}
7461 "./a\"double\"quote"
7469 You can specify which characters should be quoted in addition to those
7470 implied by the current quoting style:
7473 @item --quote-chars=@var{string}
7474 Always quote characters from @var{string}, even if the selected
7475 quoting style would not quote them.
7478 For example, using @samp{escape} quoting (compare with the usual
7479 escape listing above):
7483 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
7494 To disable quoting of such additional characters, use the following
7498 @item --no-quote-chars=@var{string}
7499 Remove characters listed in @var{string} from the list of quoted
7500 characters set by the previous @option{--quote-chars} option.
7503 This option is particularly useful if you have added
7504 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
7505 and wish to disable it for the current invocation.
7507 Note, that @option{--no-quote-chars} does @emph{not} disable those
7508 characters that are quoted by default in the selected quoting style.
7511 @section Modifying File and Member Names
7513 @command{Tar} archives contain detailed information about files stored
7514 in them and full file names are part of that information. When
7515 storing file to an archive, its file name is recorded in the archive
7516 along with the actual file contents. When restoring from an archive,
7517 a file is created on disk with exactly the same name as that stored
7518 in the archive. In the majority of cases this is the desired behavior
7519 of a file archiver. However, there are some cases when it is not.
7521 First of all, it is often unsafe to extract archive members with
7522 absolute file names or those that begin with a @file{../}. @GNUTAR{}
7523 takes special precautions when extracting such names and provides a
7524 special option for handling them, which is described in
7527 Secondly, you may wish to extract file names without some leading
7528 directory components, or with otherwise modified names. In other
7529 cases it is desirable to store files under differing names in the
7532 @GNUTAR{} provides two options for these needs.
7535 @opindex strip-components
7536 @item --strip-components=@var{number}
7537 Strip given @var{number} of leading components from file names before
7541 For example, suppose you have archived whole @file{/usr} hierarchy to
7542 a tar archive named @file{usr.tar}. Among other files, this archive
7543 contains @file{usr/include/stdlib.h}, which you wish to extract to
7544 the current working directory. To do so, you type:
7547 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
7550 The option @option{--strip=2} instructs @command{tar} to strip the
7551 two leading components (@file{usr/} and @file{include/}) off the file
7554 If you add to the above invocation @option{--verbose} (@option{-v})
7555 option, you will note that the verbose listing still contains the
7556 full file name, with the two removed components still in place. This
7557 can be inconvenient, so @command{tar} provides a special option for
7558 altering this behavior:
7560 @anchor{show-transformed-names}
7562 @opindex show-transformed-names
7563 @item --show-transformed-names
7564 Display file or member names with all requested transformations
7573 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
7574 usr/include/stdlib.h
7575 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
7580 Notice that in both cases the file is @file{stdlib.h} extracted to the
7581 current working directory, @option{--show-transformed-names} affects
7582 only the way its name is displayed.
7584 This option is especially useful for verifying whether the invocation
7585 will have the desired effect. Thus, before running
7588 $ @kbd{tar -x --strip=@var{n}}
7592 it is often advisable to run
7595 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
7599 to make sure the command will produce the intended results.
7601 In case you need to apply more complex modifications to the file name,
7602 @GNUTAR{} provides a general-purpose transformation option:
7606 @item --transform=@var{expression}
7607 Modify file names using supplied @var{expression}.
7611 The @var{expression} is a @command{sed}-like replace expression of the
7615 s/@var{regexp}/@var{replace}/[@var{flags}]
7619 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
7620 replacement for each file name part that matches @var{regexp}. Both
7621 @var{regexp} and @var{replace} are described in detail in
7622 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
7624 As in @command{sed}, you can give several replace expressions,
7625 separated by a semicolon.
7627 Supported @var{flags} are:
7631 Apply the replacement to @emph{all} matches to the @var{regexp}, not
7635 Use case-insensitive matching
7638 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
7639 regexps, Extended regular expressions, Extended regular expressions,
7643 Only replace the @var{number}th match of the @var{regexp}.
7645 Note: the @var{posix} standard does not specify what should happen
7646 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
7647 follows the GNU @command{sed} implementation in this regard, so
7648 the interaction is defined to be: ignore matches before the
7649 @var{number}th, and then match and replace all matches from the
7654 Any delimiter can be used in lieue of @samp{/}, the only requirement being
7655 that it be used consistently throughout the expression. For example,
7656 the following two expressions are equivalent:
7665 Changing delimiters is often useful when the @var{regex} contains
7666 slashes. For example, it is more convenient to write @code{s,/,-,} than
7669 Here are several examples of @option{--transform} usage:
7672 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
7675 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
7678 @item Strip two leading directory components (equivalent to
7679 @option{--strip-components=2}):
7682 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
7685 @item Prepend @file{/prefix/} to each file name:
7688 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
7691 @item Convert each file name to lower case:
7694 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
7699 Unlike @option{--strip-components}, @option{--transform} can be used
7700 in any @GNUTAR{} operation mode. For example, the following command
7701 adds files to the archive while replacing the leading @file{usr/}
7702 component with @file{var/}:
7705 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
7708 To test @option{--transform} effect we suggest using
7709 @option{--show-transformed-names} option:
7712 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
7713 --verbose --show-transformed-names /}
7716 If both @option{--strip-components} and @option{--transform} are used
7717 together, then @option{--transform} is applied first, and the required
7718 number of components is then stripped from its result.
7720 You can use as many @option{--transform} options in a single command
7721 line as you want. The specified expressions will then be applied in
7722 order of their appearance. For example, the following two invocations
7726 $ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
7727 --transform='s,/usr/local,/usr/,'}
7728 $ @kbd{tar -cf arch.tar \
7729 --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
7733 @section Operating Only on New Files
7736 @cindex Excluding file by age
7737 @cindex Data Modification time, excluding files by
7738 @cindex Modification time, excluding files by
7739 @cindex Age, excluding files by
7740 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
7741 @option{-N @var{date}}) option causes @command{tar} to only work on
7742 files whose data modification or status change times are newer than
7743 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
7744 it is taken to be a file name; the data modification time of that file
7745 is used as the date. If you use this option when creating or appending
7746 to an archive, the archive will only include new files. If you use
7747 @option{--after-date} when extracting an archive, @command{tar} will
7748 only extract files newer than the @var{date} you specify.
7750 If you only want @command{tar} to make the date comparison based on
7751 modification of the file's data (rather than status
7752 changes), then use the @option{--newer-mtime=@var{date}} option.
7754 You may use these options with any operation. Note that these options
7755 differ from the @option{--update} (@option{-u}) operation in that they
7756 allow you to specify a particular date against which @command{tar} can
7757 compare when deciding whether or not to archive the files.
7762 @item --after-date=@var{date}
7763 @itemx --newer=@var{date}
7764 @itemx -N @var{date}
7765 Only store files newer than @var{date}.
7767 Acts on files only if their data modification or status change times are
7768 later than @var{date}. Use in conjunction with any operation.
7770 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
7771 name; the data modification time of that file is used as the date.
7773 @opindex newer-mtime
7774 @item --newer-mtime=@var{date}
7775 Acts like @option{--after-date}, but only looks at data modification times.
7778 These options limit @command{tar} to operate only on files which have
7779 been modified after the date specified. A file's status is considered to have
7780 changed if its contents have been modified, or if its owner,
7781 permissions, and so forth, have been changed. (For more information on
7782 how to specify a date, see @ref{Date input formats}; remember that the
7783 entire date argument must be quoted if it contains any spaces.)
7785 Gurus would say that @option{--after-date} tests both the data
7786 modification time (@code{mtime}, the time the contents of the file
7787 were last modified) and the status change time (@code{ctime}, the time
7788 the file's status was last changed: owner, permissions, etc.@:)
7789 fields, while @option{--newer-mtime} tests only the @code{mtime}
7792 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
7793 @code{ctime} and processes the file if either one is more recent than
7794 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
7795 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
7796 contents of the file were looked at).
7798 Date specifiers can have embedded spaces. Because of this, you may need
7799 to quote date arguments to keep the shell from parsing them as separate
7800 arguments. For example, the following command will add to the archive
7801 all the files modified less than two days ago:
7804 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
7807 When any of these options is used with the option @option{--verbose}
7808 (@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
7809 date back to its textual representation and compare that with the
7810 one given with the option. If the two dates differ, @command{tar} will
7811 print a warning saying what date it will use. This is to help user
7812 ensure he is using the right date. For example:
7816 $ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
7817 tar: Option --after-date: Treating date `10 days ago' as 2006-06-11
7823 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
7824 should not be used for incremental backups. @xref{Incremental Dumps},
7825 for proper way of creating incremental backups.
7829 @section Descending into Directories
7831 @cindex Avoiding recursion in directories
7832 @cindex Descending directories, avoiding
7833 @cindex Directories, avoiding recursion
7834 @cindex Recursion in directories, avoiding
7836 @FIXME{arrggh! this is still somewhat confusing to me. :-< }
7838 Usually, @command{tar} will recursively explore all directories (either
7839 those given on the command line or through the @option{--files-from}
7840 option) for the various files they contain. However, you may not always
7841 want @command{tar} to act this way.
7843 @opindex no-recursion
7844 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
7845 into specified directories. If you specify @option{--no-recursion}, you can
7846 use the @command{find} utility for hunting through levels of directories to
7847 construct a list of file names which you could then pass to @command{tar}.
7848 @command{find} allows you to be more selective when choosing which files to
7849 archive; see @ref{files}, for more information on using @command{find} with
7850 @command{tar}, or look.
7853 @item --no-recursion
7854 Prevents @command{tar} from recursively descending directories.
7858 Requires @command{tar} to recursively descend directories.
7859 This is the default.
7862 When you use @option{--no-recursion}, @GNUTAR{} grabs
7863 directory entries themselves, but does not descend on them
7864 recursively. Many people use @command{find} for locating files they
7865 want to back up, and since @command{tar} @emph{usually} recursively
7866 descends on directories, they have to use the @samp{@w{-not -type d}}
7867 test in their @command{find} invocation (@pxref{Type, Type, Type test,
7868 find, Finding Files}), as they usually do not want all the files in a
7869 directory. They then use the @option{--files-from} option to archive
7870 the files located via @command{find}.
7872 The problem when restoring files archived in this manner is that the
7873 directories themselves are not in the archive; so the
7874 @option{--same-permissions} (@option{--preserve-permissions},
7875 @option{-p}) option does not affect them---while users might really
7876 like it to. Specifying @option{--no-recursion} is a way to tell
7877 @command{tar} to grab only the directory entries given to it, adding
7878 no new files on its own. To summarize, if you use @command{find} to
7879 create a list of files to be stored in an archive, use it as follows:
7883 $ @kbd{find @var{dir} @var{tests} | \
7884 tar -cf @var{archive} -T - --no-recursion}
7888 The @option{--no-recursion} option also applies when extracting: it
7889 causes @command{tar} to extract only the matched directory entries, not
7890 the files under those directories.
7892 The @option{--no-recursion} option also affects how globbing patterns
7893 are interpreted (@pxref{controlling pattern-matching}).
7895 The @option{--no-recursion} and @option{--recursion} options apply to
7896 later options and operands, and can be overridden by later occurrences
7897 of @option{--no-recursion} and @option{--recursion}. For example:
7900 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
7904 creates an archive with one entry for @file{grape}, and the recursive
7905 contents of @file{grape/concord}, but no entries under @file{grape}
7906 other than @file{grape/concord}.
7909 @section Crossing File System Boundaries
7910 @cindex File system boundaries, not crossing
7913 @command{tar} will normally automatically cross file system boundaries in
7914 order to archive files which are part of a directory tree. You can
7915 change this behavior by running @command{tar} and specifying
7916 @option{--one-file-system}. This option only affects files that are
7917 archived because they are in a directory that is being archived;
7918 @command{tar} will still archive files explicitly named on the command line
7919 or through @option{--files-from}, regardless of where they reside.
7922 @opindex one-file-system
7923 @item --one-file-system
7924 Prevents @command{tar} from crossing file system boundaries when
7925 archiving. Use in conjunction with any write operation.
7928 The @option{--one-file-system} option causes @command{tar} to modify its
7929 normal behavior in archiving the contents of directories. If a file in
7930 a directory is not on the same file system as the directory itself, then
7931 @command{tar} will not archive that file. If the file is a directory
7932 itself, @command{tar} will not archive anything beneath it; in other words,
7933 @command{tar} will not cross mount points.
7935 This option is useful for making full or incremental archival backups of
7936 a file system. If this option is used in conjunction with
7937 @option{--verbose} (@option{-v}), files that are excluded are
7938 mentioned by name on the standard error.
7941 * directory:: Changing Directory
7942 * absolute:: Absolute File Names
7946 @subsection Changing the Working Directory
7948 @FIXME{need to read over this node now for continuity; i've switched
7949 things around some.}
7951 @cindex Changing directory mid-stream
7952 @cindex Directory, changing mid-stream
7953 @cindex Working directory, specifying
7954 To change the working directory in the middle of a list of file names,
7955 either on the command line or in a file specified using
7956 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
7957 This will change the working directory to the specified directory
7958 after that point in the list.
7962 @item --directory=@var{directory}
7963 @itemx -C @var{directory}
7964 Changes the working directory in the middle of a command line.
7970 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
7974 will place the files @file{grape} and @file{prune} from the current
7975 directory into the archive @file{jams.tar}, followed by the file
7976 @file{cherry} from the directory @file{food}. This option is especially
7977 useful when you have several widely separated files that you want to
7978 store in the same archive.
7980 Note that the file @file{cherry} is recorded in the archive under the
7981 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
7982 archive will contain three files that all appear to have come from the
7983 same directory; if the archive is extracted with plain @samp{tar
7984 --extract}, all three files will be written in the current directory.
7986 Contrast this with the command,
7989 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
7993 which records the third file in the archive under the name
7994 @file{red/cherry} so that, if the archive is extracted using
7995 @samp{tar --extract}, the third file will be written in a subdirectory
7996 named @file{orange-colored}.
7998 You can use the @option{--directory} option to make the archive
7999 independent of the original name of the directory holding the files.
8000 The following command places the files @file{/etc/passwd},
8001 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
8005 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
8009 However, the names of the archive members will be exactly what they were
8010 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
8011 They will not appear to be related by file name to the original
8012 directories where those files were located.
8014 Note that @option{--directory} options are interpreted consecutively. If
8015 @option{--directory} specifies a relative file name, it is interpreted
8016 relative to the then current directory, which might not be the same as
8017 the original current working directory of @command{tar}, due to a previous
8018 @option{--directory} option.
8020 When using @option{--files-from} (@pxref{files}), you can put various
8021 @command{tar} options (including @option{-C}) in the file list. Notice,
8022 however, that in this case the option and its argument may not be
8023 separated by whitespace. If you use short option, its argument must
8024 either follow the option letter immediately, without any intervening
8025 whitespace, or occupy the next line. Otherwise, if you use long
8026 option, separate its argument by an equal sign.
8028 For instance, the file list for the above example will be:
8041 To use it, you would invoke @command{tar} as follows:
8044 $ @kbd{tar -c -f foo.tar --files-from list}
8047 The interpretation of @option{--directory} is disabled by
8048 @option{--null} option.
8051 @subsection Absolute File Names
8055 @opindex absolute-names
8056 @item --absolute-names
8058 Do not strip leading slashes from file names, and permit file names
8059 containing a @file{..} file name component.
8062 By default, @GNUTAR{} drops a leading @samp{/} on
8063 input or output, and complains about file names containing a @file{..}
8064 component. This option turns off this behavior.
8066 When @command{tar} extracts archive members from an archive, it strips any
8067 leading slashes (@samp{/}) from the member name. This causes absolute
8068 member names in the archive to be treated as relative file names. This
8069 allows you to have such members extracted wherever you want, instead of
8070 being restricted to extracting the member in the exact directory named
8071 in the archive. For example, if the archive member has the name
8072 @file{/etc/passwd}, @command{tar} will extract it as if the name were
8073 really @file{etc/passwd}.
8075 File names containing @file{..} can cause problems when extracting, so
8076 @command{tar} normally warns you about such files when creating an
8077 archive, and rejects attempts to extracts such files.
8079 Other @command{tar} programs do not do this. As a result, if you
8080 create an archive whose member names start with a slash, they will be
8081 difficult for other people with a non-@GNUTAR{}
8082 program to use. Therefore, @GNUTAR{} also strips
8083 leading slashes from member names when putting members into the
8084 archive. For example, if you ask @command{tar} to add the file
8085 @file{/bin/ls} to an archive, it will do so, but the member name will
8086 be @file{bin/ls}.@footnote{A side effect of this is that when
8087 @option{--create} is used with @option{--verbose} the resulting output
8088 is not, generally speaking, the same as the one you'd get running
8089 @kbd{tar --list} command. This may be important if you use some
8090 scripts for comparing both outputs. @xref{listing member and file names},
8091 for the information on how to handle this case.}
8093 If you use the @option{--absolute-names} (@option{-P}) option,
8094 @command{tar} will do none of these transformations.
8096 To archive or extract files relative to the root directory, specify
8097 the @option{--absolute-names} (@option{-P}) option.
8099 Normally, @command{tar} acts on files relative to the working
8100 directory---ignoring superior directory names when archiving, and
8101 ignoring leading slashes when extracting.
8103 When you specify @option{--absolute-names} (@option{-P}),
8104 @command{tar} stores file names including all superior directory
8105 names, and preserves leading slashes. If you only invoked
8106 @command{tar} from the root directory you would never need the
8107 @option{--absolute-names} option, but using this option
8108 may be more convenient than switching to root.
8110 @FIXME{Should be an example in the tutorial/wizardry section using this
8111 to transfer files between systems.}
8113 @FIXME{Is write access an issue?}
8116 @item --absolute-names
8117 Preserves full file names (including superior directory names) when
8118 archiving files. Preserves leading slash when extracting files.
8122 @FIXME{this is still horrible; need to talk with dan on monday.}
8124 @command{tar} prints out a message about removing the @samp{/} from
8125 file names. This message appears once per @GNUTAR{}
8126 invocation. It represents something which ought to be told; ignoring
8127 what it means can cause very serious surprises, later.
8129 Some people, nevertheless, do not want to see this message. Wanting to
8130 play really dangerously, one may of course redirect @command{tar} standard
8131 error to the sink. For example, under @command{sh}:
8134 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
8138 Another solution, both nicer and simpler, would be to change to
8139 the @file{/} directory first, and then avoid absolute notation.
8143 $ @kbd{tar -c -f archive.tar -C / home}
8146 @include getdate.texi
8149 @chapter Controlling the Archive Format
8151 @cindex Tar archive formats
8152 Due to historical reasons, there are several formats of tar archives.
8153 All of them are based on the same principles, but have some subtle
8154 differences that often make them incompatible with each other.
8156 GNU tar is able to create and handle archives in a variety of formats.
8157 The most frequently used formats are (in alphabetical order):
8161 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
8162 from an early @acronym{POSIX} standard, adding some improvements such as
8163 sparse file handling and incremental archives. Unfortunately these
8164 features were implemented in a way incompatible with other archive
8167 Archives in @samp{gnu} format are able to hold file names of unlimited
8171 Format used by @GNUTAR{} of versions prior to 1.12.
8174 Archive format, compatible with the V7 implementation of tar. This
8175 format imposes a number of limitations. The most important of them
8179 @item The maximum length of a file name is limited to 99 characters.
8180 @item The maximum length of a symbolic link is limited to 99 characters.
8181 @item It is impossible to store special files (block and character
8182 devices, fifos etc.)
8183 @item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
8185 @item V7 archives do not contain symbolic ownership information (user
8186 and group name of the file owner).
8189 This format has traditionally been used by Automake when producing
8190 Makefiles. This practice will change in the future, in the meantime,
8191 however this means that projects containing file names more than 99
8192 characters long will not be able to use @GNUTAR{} @value{VERSION} and
8193 Automake prior to 1.9.
8196 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
8197 symbolic ownership information. It is also able to store
8198 special files. However, it imposes several restrictions as well:
8201 @item The maximum length of a file name is limited to 256 characters,
8202 provided that the file name can be split at a directory separator in
8203 two parts, first of them being at most 155 bytes long. So, in most
8204 cases the maximum file name length will be shorter than 256
8206 @item The maximum length of a symbolic link name is limited to
8208 @item Maximum size of a file the archive is able to accommodate
8210 @item Maximum value of UID/GID is 2097151.
8211 @item Maximum number of bits in device major and minor numbers is 21.
8215 Format used by J@"org Schilling @command{star}
8216 implementation. @GNUTAR{} is able to read @samp{star} archives but
8217 currently does not produce them.
8220 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
8221 most flexible and feature-rich format. It does not impose any
8222 restrictions on file sizes or file name lengths. This format is quite
8223 recent, so not all tar implementations are able to handle it properly.
8224 However, this format is designed in such a way that any tar
8225 implementation able to read @samp{ustar} archives will be able to read
8226 most @samp{posix} archives as well, with the only exception that any
8227 additional information (such as long file names etc.) will in such
8228 case be extracted as plain text files along with the files it refers to.
8230 This archive format will be the default format for future versions
8235 The following table summarizes the limitations of each of these
8238 @multitable @columnfractions .10 .20 .20 .20 .20
8239 @headitem Format @tab UID @tab File Size @tab File Name @tab Devn
8240 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8241 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8242 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
8243 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
8244 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
8247 The default format for @GNUTAR{} is defined at compilation
8248 time. You may check it by running @command{tar --help}, and examining
8249 the last lines of its output. Usually, @GNUTAR{} is configured
8250 to create archives in @samp{gnu} format, however, future version will
8251 switch to @samp{posix}.
8254 * Compression:: Using Less Space through Compression
8255 * Attributes:: Handling File Attributes
8256 * Portability:: Making @command{tar} Archives More Portable
8257 * cpio:: Comparison of @command{tar} and @command{cpio}
8261 @section Using Less Space through Compression
8264 * gzip:: Creating and Reading Compressed Archives
8265 * sparse:: Archiving Sparse Files
8269 @subsection Creating and Reading Compressed Archives
8270 @cindex Compressed archives
8271 @cindex Storing archives in compressed format
8278 @GNUTAR{} is able to create and read compressed archives. It supports
8279 @command{gzip}, @command{bzip2}, @command{lzma} and @command{lzop} compression
8280 programs. For backward compatibility, it also supports
8281 @command{compress} command, although we strongly recommend against
8282 using it, because it is by far less effective than other compression
8283 programs@footnote{It also had patent problems in the past.}.
8285 Creating a compressed archive is simple: you just specify a
8286 @dfn{compression option} along with the usual archive creation
8287 commands. The compression option is @option{-z} (@option{--gzip}) to
8288 create a @command{gzip} compressed archive, @option{-j}
8289 (@option{--bzip2}) to create a @command{bzip2} compressed archive,
8290 @option{-J} (@option{--lzma}) to create an @asis{LZMA} compressed
8291 archive, @option{--lzop} to create an @asis{LSOP} archive, and
8292 @option{-Z} (@option{--compress}) to use @command{compress} program.
8296 $ @kbd{tar cfz archive.tar.gz .}
8299 You can also let @GNUTAR{} select the compression program basing on
8300 the suffix of the archive file name. This is done using
8301 @option{--auto-compress} (@option{-a}) command line option. For
8302 example, the following invocation will use @command{bzip2} for
8306 $ @kbd{tar cfa archive.tar.bz2 .}
8310 whereas the following one will use @command{lzma}:
8313 $ @kbd{tar cfa archive.tar.lzma .}
8316 For a complete list of file name suffixes recognized by @GNUTAR{},
8317 @ref{auto-compress}.
8319 Reading compressed archive is even simpler: you don't need to specify
8320 any additional options as @GNUTAR{} recognizes its format
8321 automatically. Thus, the following commands will list and extract the
8322 archive created in previous example:
8325 # List the compressed archive
8326 $ @kbd{tar tf archive.tar.gz}
8327 # Extract the compressed archive
8328 $ @kbd{tar xf archive.tar.gz}
8331 The format recognition algorithm is based on @dfn{signatures}, a
8332 special byte sequences in the beginning of file, that are specific for
8333 certain compression formats. If this approach fails, @command{tar}
8334 falls back to using archive name suffix to determine its format
8335 (@xref{auto-compress}, for a list of recognized suffixes).
8337 The only case when you have to specify a decompression option while
8338 reading the archive is when reading from a pipe or from a tape drive
8339 that does not support random access. However, in this case @GNUTAR{}
8340 will indicate which option you should use. For example:
8343 $ @kbd{cat archive.tar.gz | tar tf -}
8344 tar: Archive is compressed. Use -z option
8345 tar: Error is not recoverable: exiting now
8348 If you see such diagnostics, just add the suggested option to the
8349 invocation of @GNUTAR{}:
8352 $ @kbd{cat archive.tar.gz | tar tfz -}
8355 Notice also, that there are several restrictions on operations on
8356 compressed archives. First of all, compressed archives cannot be
8357 modified, i.e., you cannot update (@option{--update} (@option{-u}))
8358 them or delete (@option{--delete}) members from them or
8359 add (@option{--append} (@option{-r})) members to them. Likewise, you
8360 cannot append another @command{tar} archive to a compressed archive using
8361 @option{--concatenate} (@option{-A})). Secondly, multi-volume
8362 archives cannot be compressed.
8364 The following table summarizes compression options used by @GNUTAR{}.
8367 @anchor{auto-compress}
8368 @opindex auto-compress
8369 @item --auto-compress
8371 Select a compression program to use by the archive file name
8372 suffix. The following suffixes are recognized:
8374 @multitable @columnfractions 0.3 0.6
8375 @headitem Suffix @tab Compression program
8376 @item @samp{.gz} @tab @command{gzip}
8377 @item @samp{.tgz} @tab @command{gzip}
8378 @item @samp{.taz} @tab @command{gzip}
8379 @item @samp{.Z} @tab @command{compress}
8380 @item @samp{.taZ} @tab @command{compress}
8381 @item @samp{.bz2} @tab @command{bzip2}
8382 @item @samp{.tz2} @tab @command{bzip2}
8383 @item @samp{.tbz2} @tab @command{bzip2}
8384 @item @samp{.tbz} @tab @command{bzip2}
8385 @item @samp{.lzma} @tab @command{lzma}
8386 @item @samp{.tlz} @tab @command{lzma}
8387 @item @samp{.lzo} @tab @command{lzop}
8395 Filter the archive through @command{gzip}.
8397 You can use @option{--gzip} and @option{--gunzip} on physical devices
8398 (tape drives, etc.) and remote files as well as on normal files; data
8399 to or from such devices or remote files is reblocked by another copy
8400 of the @command{tar} program to enforce the specified (or default) record
8401 size. The default compression parameters are used; if you need to
8402 override them, set @env{GZIP} environment variable, e.g.:
8405 $ @kbd{GZIP=--best tar cfz archive.tar.gz subdir}
8409 Another way would be to avoid the @option{--gzip} (@option{--gunzip}, @option{--ungzip}, @option{-z}) option and run
8410 @command{gzip} explicitly:
8413 $ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz}
8416 @cindex corrupted archives
8417 About corrupted compressed archives: @command{gzip}'ed files have no
8418 redundancy, for maximum compression. The adaptive nature of the
8419 compression scheme means that the compression tables are implicitly
8420 spread all over the archive. If you lose a few blocks, the dynamic
8421 construction of the compression tables becomes unsynchronized, and there
8422 is little chance that you could recover later in the archive.
8424 There are pending suggestions for having a per-volume or per-file
8425 compression in @GNUTAR{}. This would allow for viewing the
8426 contents without decompression, and for resynchronizing decompression at
8427 every volume or file, in case of corrupted archives. Doing so, we might
8428 lose some compressibility. But this would have make recovering easier.
8429 So, there are pros and cons. We'll see!
8434 Filter the archive through @code{bzip2}. Otherwise like @option{--gzip}.
8439 Filter the archive through @command{lzma}. Otherwise like @option{--gzip}.
8443 Filter the archive through @command{lzop}. Otherwise like
8451 Filter the archive through @command{compress}. Otherwise like @option{--gzip}.
8453 @opindex use-compress-program
8454 @item --use-compress-program=@var{prog}
8455 Use external compression program @var{prog}. Use this option if you
8456 have a compression program that @GNUTAR{} does not support. There
8457 are two requirements to which @var{prog} should comply:
8459 First, when called without options, it should read data from standard
8460 input, compress it and output it on standard output.
8462 Secondly, if called with @option{-d} argument, it should do exactly
8463 the opposite, i.e., read the compressed data from the standard input
8464 and produce uncompressed data on the standard output.
8467 @cindex gpg, using with tar
8468 @cindex gnupg, using with tar
8469 @cindex Using encrypted archives
8470 The @option{--use-compress-program} option, in particular, lets you
8471 implement your own filters, not necessarily dealing with
8472 compression/decompression. For example, suppose you wish to implement
8473 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
8474 gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
8475 Manual}). The following script does that:
8481 -d) gpg --decrypt - | gzip -d -c;;
8482 '') gzip -c | gpg -s ;;
8483 *) echo "Unknown option $1">&2; exit 1;;
8488 Suppose you name it @file{gpgz} and save it somewhere in your
8489 @env{PATH}. Then the following command will create a compressed
8490 archive signed with your private key:
8493 $ @kbd{tar -cf foo.tar.gpgz --use-compress=gpgz .}
8497 Likewise, the following command will list its contents:
8500 $ @kbd{tar -tf foo.tar.gpgz --use-compress=gpgz .}
8504 The above is based on the following discussion:
8506 I have one question, or maybe it's a suggestion if there isn't a way
8507 to do it now. I would like to use @option{--gzip}, but I'd also like
8508 the output to be fed through a program like @acronym{GNU}
8509 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
8510 to use :-)), basically adding ECC protection on top of compression.
8511 It seems as if this should be quite easy to do, but I can't work out
8512 exactly how to go about it. Of course, I can pipe the standard output
8513 of @command{tar} through @command{ecc}, but then I lose (though I
8514 haven't started using it yet, I confess) the ability to have
8515 @command{tar} use @command{rmt} for it's I/O (I think).
8517 I think the most straightforward thing would be to let me specify a
8518 general set of filters outboard of compression (preferably ordered,
8519 so the order can be automatically reversed on input operations, and
8520 with the options they require specifiable), but beggars shouldn't be
8521 choosers and anything you decide on would be fine with me.
8523 By the way, I like @command{ecc} but if (as the comments say) it can't
8524 deal with loss of block sync, I'm tempted to throw some time at adding
8525 that capability. Supposing I were to actually do such a thing and
8526 get it (apparently) working, do you accept contributed changes to
8527 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
8529 Isn't that exactly the role of the
8530 @option{--use-compress-prog=@var{program}} option?
8531 I never tried it myself, but I suspect you may want to write a
8532 @var{prog} script or program able to filter stdin to stdout to
8533 way you want. It should recognize the @option{-d} option, for when
8534 extraction is needed rather than creation.
8536 It has been reported that if one writes compressed data (through the
8537 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
8538 the DLT compression mode, the data will actually get bigger and one will
8539 end up with less space on the tape.
8543 @subsection Archiving Sparse Files
8544 @cindex Sparse Files
8546 Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
8547 in a file is a section of the file's contents which was never written.
8548 The contents of a hole reads as all zeros. On many operating systems,
8549 actual disk storage is not allocated for holes, but they are counted
8550 in the length of the file. If you archive such a file, @command{tar}
8551 could create an archive longer than the original. To have @command{tar}
8552 attempt to recognize the holes in a file, use @option{--sparse}
8553 (@option{-S}). When you use this option, then, for any file using
8554 less disk space than would be expected from its length, @command{tar}
8555 searches the file for consecutive stretches of zeros. It then records
8556 in the archive for the file where the consecutive stretches of zeros
8557 are, and only archives the ``real contents'' of the file. On
8558 extraction (using @option{--sparse} is not needed on extraction) any
8559 such files have holes created wherever the continuous stretches of zeros
8560 were found. Thus, if you use @option{--sparse}, @command{tar} archives
8561 won't take more space than the original.
8567 This option instructs @command{tar} to test each file for sparseness
8568 before attempting to archive it. If the file is found to be sparse it
8569 is treated specially, thus allowing to decrease the amount of space
8570 used by its image in the archive.
8572 This option is meaningful only when creating or updating archives. It
8573 has no effect on extraction.
8576 Consider using @option{--sparse} when performing file system backups,
8577 to avoid archiving the expanded forms of files stored sparsely in the
8580 Even if your system has no sparse files currently, some may be
8581 created in the future. If you use @option{--sparse} while making file
8582 system backups as a matter of course, you can be assured the archive
8583 will never take more space on the media than the files take on disk
8584 (otherwise, archiving a disk filled with sparse files might take
8585 hundreds of tapes). @xref{Incremental Dumps}.
8587 However, be aware that @option{--sparse} option presents a serious
8588 drawback. Namely, in order to determine if the file is sparse
8589 @command{tar} has to read it before trying to archive it, so in total
8590 the file is read @strong{twice}. So, always bear in mind that the
8591 time needed to process all files with this option is roughly twice
8592 the time needed to archive them without it.
8593 @FIXME{A technical note:
8595 Programs like @command{dump} do not have to read the entire file; by
8596 examining the file system directly, they can determine in advance
8597 exactly where the holes are and thus avoid reading through them. The
8598 only data it need read are the actual allocated data blocks.
8599 @GNUTAR{} uses a more portable and straightforward
8600 archiving approach, it would be fairly difficult that it does
8601 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
8605 What I did say is that you cannot tell the difference between a hole and an
8606 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
8607 best tells you how many holes there are; it doesn't tell you @emph{where}.
8608 Just as programs may, conceivably, care what @code{st_blocks} is (care
8609 to name one that does?), they may also care where the holes are (I have
8610 no examples of this one either, but it's equally imaginable).
8612 I conclude from this that good archivers are not portable. One can
8613 arguably conclude that if you want a portable program, you can in good
8614 conscience restore files with as many holes as possible, since you can't
8619 @cindex sparse formats, defined
8620 When using @samp{POSIX} archive format, @GNUTAR{} is able to store
8621 sparse files using in three distinct ways, called @dfn{sparse
8622 formats}. A sparse format is identified by its @dfn{number},
8623 consisting, as usual of two decimal numbers, delimited by a dot. By
8624 default, format @samp{1.0} is used. If, for some reason, you wish to
8625 use an earlier format, you can select it using
8626 @option{--sparse-version} option.
8629 @opindex sparse-version
8630 @item --sparse-version=@var{version}
8632 Select the format to store sparse files in. Valid @var{version} values
8633 are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
8634 for a detailed description of each format.
8637 Using @option{--sparse-format} option implies @option{--sparse}.
8640 @section Handling File Attributes
8643 When @command{tar} reads files, it updates their access times. To
8644 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
8645 reset the access time retroactively or avoid changing it in the first
8648 Handling of file attributes
8651 @opindex atime-preserve
8652 @item --atime-preserve
8653 @itemx --atime-preserve=replace
8654 @itemx --atime-preserve=system
8655 Preserve the access times of files that are read. This works only for
8656 files that you own, unless you have superuser privileges.
8658 @option{--atime-preserve=replace} works on most systems, but it also
8659 restores the data modification time and updates the status change
8660 time. Hence it doesn't interact with incremental dumps nicely
8661 (@pxref{Incremental Dumps}), and it can set access or data modification times
8662 incorrectly if other programs access the file while @command{tar} is
8665 @option{--atime-preserve=system} avoids changing the access time in
8666 the first place, if the operating system supports this.
8667 Unfortunately, this may or may not work on any given operating system
8668 or file system. If @command{tar} knows for sure it won't work, it
8669 complains right away.
8671 Currently @option{--atime-preserve} with no operand defaults to
8672 @option{--atime-preserve=replace}, but this is intended to change to
8673 @option{--atime-preserve=system} when the latter is better-supported.
8678 Do not extract data modification time.
8680 When this option is used, @command{tar} leaves the data modification times
8681 of the files it extracts as the times when the files were extracted,
8682 instead of setting it to the times recorded in the archive.
8684 This option is meaningless with @option{--list} (@option{-t}).
8688 Create extracted files with the same ownership they have in the
8691 This is the default behavior for the superuser,
8692 so this option is meaningful only for non-root users, when @command{tar}
8693 is executed on those systems able to give files away. This is
8694 considered as a security flaw by many people, at least because it
8695 makes quite difficult to correctly account users for the disk space
8696 they occupy. Also, the @code{suid} or @code{sgid} attributes of
8697 files are easily and silently lost when files are given away.
8699 When writing an archive, @command{tar} writes the user @acronym{ID} and user name
8700 separately. If it can't find a user name (because the user @acronym{ID} is not
8701 in @file{/etc/passwd}), then it does not write one. When restoring,
8702 it tries to look the name (if one was written) up in
8703 @file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
8704 the archive instead.
8706 @opindex no-same-owner
8707 @item --no-same-owner
8709 Do not attempt to restore ownership when extracting. This is the
8710 default behavior for ordinary users, so this option has an effect
8711 only for the superuser.
8713 @opindex numeric-owner
8714 @item --numeric-owner
8715 The @option{--numeric-owner} option allows (ANSI) archives to be written
8716 without user/group name information or such information to be ignored
8717 when extracting. It effectively disables the generation and/or use
8718 of user/group name information. This option forces extraction using
8719 the numeric ids from the archive, ignoring the names.
8721 This is useful in certain circumstances, when restoring a backup from
8722 an emergency floppy with different passwd/group files for example.
8723 It is otherwise impossible to extract files with the right ownerships
8724 if the password file in use during the extraction does not match the
8725 one belonging to the file system(s) being extracted. This occurs,
8726 for example, if you are restoring your files after a major crash and
8727 had booted from an emergency floppy with no password file or put your
8728 disk into another machine to do the restore.
8730 The numeric ids are @emph{always} saved into @command{tar} archives.
8731 The identifying names are added at create time when provided by the
8732 system, unless @option{--old-archive} (@option{-o}) is used. Numeric ids could be
8733 used when moving archives between a collection of machines using
8734 a centralized management for attribution of numeric ids to users
8735 and groups. This is often made through using the NIS capabilities.
8737 When making a @command{tar} file for distribution to other sites, it
8738 is sometimes cleaner to use a single owner for all files in the
8739 distribution, and nicer to specify the write permission bits of the
8740 files as stored in the archive independently of their actual value on
8741 the file system. The way to prepare a clean distribution is usually
8742 to have some Makefile rule creating a directory, copying all needed
8743 files in that directory, then setting ownership and permissions as
8744 wanted (there are a lot of possible schemes), and only then making a
8745 @command{tar} archive out of this directory, before cleaning
8746 everything out. Of course, we could add a lot of options to
8747 @GNUTAR{} for fine tuning permissions and ownership.
8748 This is not the good way, I think. @GNUTAR{} is
8749 already crowded with options and moreover, the approach just explained
8750 gives you a great deal of control already.
8752 @xopindex{same-permissions, short description}
8753 @xopindex{preserve-permissions, short description}
8755 @itemx --same-permissions
8756 @itemx --preserve-permissions
8757 Extract all protection information.
8759 This option causes @command{tar} to set the modes (access permissions) of
8760 extracted files exactly as recorded in the archive. If this option
8761 is not used, the current @code{umask} setting limits the permissions
8762 on extracted files. This option is by default enabled when
8763 @command{tar} is executed by a superuser.
8766 This option is meaningless with @option{--list} (@option{-t}).
8770 Same as both @option{--same-permissions} and @option{--same-order}.
8772 The @option{--preserve} option has no equivalent short option name.
8773 It is equivalent to @option{--same-permissions} plus @option{--same-order}.
8775 @FIXME{I do not see the purpose of such an option. (Neither I. FP.)
8776 Neither do I. --Sergey}
8781 @section Making @command{tar} Archives More Portable
8783 Creating a @command{tar} archive on a particular system that is meant to be
8784 useful later on many other machines and with other versions of @command{tar}
8785 is more challenging than you might think. @command{tar} archive formats
8786 have been evolving since the first versions of Unix. Many such formats
8787 are around, and are not always compatible with each other. This section
8788 discusses a few problems, and gives some advice about making @command{tar}
8789 archives more portable.
8791 One golden rule is simplicity. For example, limit your @command{tar}
8792 archives to contain only regular files and directories, avoiding
8793 other kind of special files. Do not attempt to save sparse files or
8794 contiguous files as such. Let's discuss a few more problems, in turn.
8796 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
8797 archives and archive labels) in GNU and PAX formats.}
8800 * Portable Names:: Portable Names
8801 * dereference:: Symbolic Links
8802 * hard links:: Hard Links
8803 * old:: Old V7 Archives
8804 * ustar:: Ustar Archives
8805 * gnu:: GNU and old GNU format archives.
8806 * posix:: @acronym{POSIX} archives
8807 * Checksumming:: Checksumming Problems
8808 * Large or Negative Values:: Large files, negative time stamps, etc.
8809 * Other Tars:: How to Extract GNU-Specific Data Using
8810 Other @command{tar} Implementations
8813 @node Portable Names
8814 @subsection Portable Names
8816 Use portable file and member names. A name is portable if it contains
8817 only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
8818 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
8819 contain @samp{/-}. Avoid deep directory nesting. For portability to
8820 old Unix hosts, limit your file name components to 14 characters or
8823 If you intend to have your @command{tar} archives to be read under
8824 MSDOS, you should not rely on case distinction for file names, and you
8825 might use the @acronym{GNU} @command{doschk} program for helping you
8826 further diagnosing illegal MSDOS names, which are even more limited
8830 @subsection Symbolic Links
8831 @cindex File names, using symbolic links
8832 @cindex Symbolic link as file name
8834 @opindex dereference
8835 Normally, when @command{tar} archives a symbolic link, it writes a
8836 block to the archive naming the target of the link. In that way, the
8837 @command{tar} archive is a faithful record of the file system contents.
8838 @option{--dereference} (@option{-h}) is used with @option{--create} (@option{-c}), and causes
8839 @command{tar} to archive the files symbolic links point to, instead of
8840 the links themselves. When this option is used, when @command{tar}
8841 encounters a symbolic link, it will archive the linked-to file,
8842 instead of simply recording the presence of a symbolic link.
8844 The name under which the file is stored in the file system is not
8845 recorded in the archive. To record both the symbolic link name and
8846 the file name in the system, archive the file under both names. If
8847 all links were recorded automatically by @command{tar}, an extracted file
8848 might be linked to a file name that no longer exists in the file
8851 If a linked-to file is encountered again by @command{tar} while creating
8852 the same archive, an entire second copy of it will be stored. (This
8853 @emph{might} be considered a bug.)
8855 So, for portable archives, do not archive symbolic links as such,
8856 and use @option{--dereference} (@option{-h}): many systems do not support
8857 symbolic links, and moreover, your distribution might be unusable if
8858 it contains unresolved symbolic links.
8861 @subsection Hard Links
8863 @cindex File names, using hard links
8864 @cindex hard links, dereferencing
8865 @cindex dereferencing hard links
8867 Normally, when @command{tar} archives a hard link, it writes a
8868 block to the archive naming the target of the link (a @samp{1} type
8869 block). In that way, the actual file contents is stored in file only
8870 once. For example, consider the following two files:
8875 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
8876 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
8880 Here, @file{jeden} is a link to @file{one}. When archiving this
8881 directory with a verbose level 2, you will get an output similar to
8885 $ tar cfvv ../archive.tar .
8886 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
8887 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
8888 hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
8891 The last line shows that, instead of storing two copies of the file,
8892 @command{tar} stored it only once, under the name @file{jeden}, and
8893 stored file @file{one} as a hard link to this file.
8895 It may be important to know that all hard links to the given file are
8896 stored in the archive. For example, this may be necessary for exact
8897 reproduction of the file system. The following option does that:
8900 @xopindex{check-links, described}
8903 Check the number of links dumped for each processed file. If this
8904 number does not match the total number of hard links for the file, print
8908 For example, trying to archive only file @file{jeden} with this option
8909 produces the following diagnostics:
8912 $ tar -c -f ../archive.tar jeden
8913 tar: Missing links to `jeden'.
8916 Although creating special records for hard links helps keep a faithful
8917 record of the file system contents and makes archives more compact, it
8918 may present some difficulties when extracting individual members from
8919 the archive. For example, trying to extract file @file{one} from the
8920 archive created in previous examples produces, in the absense of file
8924 $ tar xf archive.tar ./one
8925 tar: ./one: Cannot hard link to `./jeden': No such file or directory
8926 tar: Error exit delayed from previous errors
8929 The reason for this behavior is that @command{tar} cannot seek back in
8930 the archive to the previous member (in this case, @file{one}), to
8931 extract it@footnote{There are plans to fix this in future releases.}.
8932 If you wish to avoid such problems at the cost of a bigger archive,
8933 use the following option:
8936 @xopindex{hard-dereference, described}
8937 @item --hard-dereference
8938 Dereference hard links and store the files they refer to.
8941 For example, trying this option on our two sample files, we get two
8942 copies in the archive, each of which can then be extracted
8943 independently of the other:
8947 $ tar -c -vv -f ../archive.tar --hard-dereference .
8948 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
8949 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
8950 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
8955 @subsection Old V7 Archives
8956 @cindex Format, old style
8957 @cindex Old style format
8958 @cindex Old style archives
8959 @cindex v7 archive format
8961 Certain old versions of @command{tar} cannot handle additional
8962 information recorded by newer @command{tar} programs. To create an
8963 archive in V7 format (not ANSI), which can be read by these old
8964 versions, specify the @option{--format=v7} option in
8965 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
8966 accepts @option{--portability} or @option{--old-archive} for this
8967 option). When you specify it,
8968 @command{tar} leaves out information about directories, pipes, fifos,
8969 contiguous files, and device files, and specifies file ownership by
8970 group and user IDs instead of group and user names.
8972 When updating an archive, do not use @option{--format=v7}
8973 unless the archive was created using this option.
8975 In most cases, a @emph{new} format archive can be read by an @emph{old}
8976 @command{tar} program without serious trouble, so this option should
8977 seldom be needed. On the other hand, most modern @command{tar}s are
8978 able to read old format archives, so it might be safer for you to
8979 always use @option{--format=v7} for your distributions. Notice,
8980 however, that @samp{ustar} format is a better alternative, as it is
8981 free from many of @samp{v7}'s drawbacks.
8984 @subsection Ustar Archive Format
8986 @cindex ustar archive format
8987 Archive format defined by @acronym{POSIX}.1-1988 specification is called
8988 @code{ustar}. Although it is more flexible than the V7 format, it
8989 still has many restrictions (@xref{Formats,ustar}, for the detailed
8990 description of @code{ustar} format). Along with V7 format,
8991 @code{ustar} format is a good choice for archives intended to be read
8992 with other implementations of @command{tar}.
8994 To create archive in @code{ustar} format, use @option{--format=ustar}
8995 option in conjunction with the @option{--create} (@option{-c}).
8998 @subsection @acronym{GNU} and old @GNUTAR{} format
9000 @cindex GNU archive format
9001 @cindex Old GNU archive format
9002 @GNUTAR{} was based on an early draft of the
9003 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
9004 @command{tar}, such as the support for file names longer than 100
9005 characters, use portions of the @command{tar} header record which were
9006 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
9007 @acronym{POSIX} have allocated the same parts of the header record for
9008 other purposes. As a result, @GNUTAR{} format is
9009 incompatible with the current @acronym{POSIX} specification, and with
9010 @command{tar} programs that follow it.
9012 In the majority of cases, @command{tar} will be configured to create
9013 this format by default. This will change in future releases, since
9014 we plan to make @samp{POSIX} format the default.
9016 To force creation a @GNUTAR{} archive, use option
9017 @option{--format=gnu}.
9020 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
9022 @cindex POSIX archive format
9023 @cindex PAX archive format
9024 Starting from version 1.14 @GNUTAR{} features full support for
9025 @acronym{POSIX.1-2001} archives.
9027 A @acronym{POSIX} conformant archive will be created if @command{tar}
9028 was given @option{--format=posix} (@option{--format=pax}) option. No
9029 special option is required to read and extract from a @acronym{POSIX}
9033 * PAX keywords:: Controlling Extended Header Keywords.
9037 @subsubsection Controlling Extended Header Keywords
9041 @item --pax-option=@var{keyword-list}
9042 Handle keywords in @acronym{PAX} extended headers. This option is
9043 equivalent to @option{-o} option of the @command{pax} utility.
9046 @var{Keyword-list} is a comma-separated
9047 list of keyword options, each keyword option taking one of
9048 the following forms:
9051 @item delete=@var{pattern}
9052 When used with one of archive-creation commands,
9053 this option instructs @command{tar} to omit from extended header records
9054 that it produces any keywords matching the string @var{pattern}.
9056 When used in extract or list mode, this option instructs tar
9057 to ignore any keywords matching the given @var{pattern} in the extended
9058 header records. In both cases, matching is performed using the pattern
9059 matching notation described in @acronym{POSIX 1003.2}, 3.13
9060 (@pxref{wildcards}). For example:
9063 --pax-option delete=security.*
9066 would suppress security-related information.
9068 @item exthdr.name=@var{string}
9070 This keyword allows user control over the name that is written into the
9071 ustar header blocks for the extended headers. The name is obtained
9072 from @var{string} after making the following substitutions:
9074 @multitable @columnfractions .25 .55
9075 @headitem Meta-character @tab Replaced By
9076 @item %d @tab The directory name of the file, equivalent to the
9077 result of the @command{dirname} utility on the translated file name.
9078 @item %f @tab The name of the file with the directory information
9079 stripped, equivalent to the result of the @command{basename} utility
9080 on the translated file name.
9081 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9082 @item %% @tab A @samp{%} character.
9085 Any other @samp{%} characters in @var{string} produce undefined
9088 If no option @samp{exthdr.name=string} is specified, @command{tar}
9089 will use the following default value:
9095 @item globexthdr.name=@var{string}
9096 This keyword allows user control over the name that is written into
9097 the ustar header blocks for global extended header records. The name
9098 is obtained from the contents of @var{string}, after making
9099 the following substitutions:
9101 @multitable @columnfractions .25 .55
9102 @headitem Meta-character @tab Replaced By
9103 @item %n @tab An integer that represents the
9104 sequence number of the global extended header record in the archive,
9106 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9107 @item %% @tab A @samp{%} character.
9110 Any other @samp{%} characters in @var{string} produce undefined results.
9112 If no option @samp{globexthdr.name=string} is specified, @command{tar}
9113 will use the following default value:
9116 $TMPDIR/GlobalHead.%p.%n
9120 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
9121 environment variable. If @var{TMPDIR} is not set, @command{tar}
9124 @item @var{keyword}=@var{value}
9125 When used with one of archive-creation commands, these keyword/value pairs
9126 will be included at the beginning of the archive in a global extended
9127 header record. When used with one of archive-reading commands,
9128 @command{tar} will behave as if it has encountered these keyword/value
9129 pairs at the beginning of the archive in a global extended header
9132 @item @var{keyword}:=@var{value}
9133 When used with one of archive-creation commands, these keyword/value pairs
9134 will be included as records at the beginning of an extended header for
9135 each file. This is effectively equivalent to @var{keyword}=@var{value}
9136 form except that it creates no global extended header records.
9138 When used with one of archive-reading commands, @command{tar} will
9139 behave as if these keyword/value pairs were included as records at the
9140 end of each extended header; thus, they will override any global or
9141 file-specific extended header record keywords of the same names.
9142 For example, in the command:
9145 tar --format=posix --create \
9146 --file archive --pax-option gname:=user .
9149 the group name will be forced to a new value for all files
9150 stored in the archive.
9154 @subsection Checksumming Problems
9156 SunOS and HP-UX @command{tar} fail to accept archives created using
9157 @GNUTAR{} and containing non-@acronym{ASCII} file names, that
9158 is, file names having characters with the eight bit set, because they
9159 use signed checksums, while @GNUTAR{} uses unsigned
9160 checksums while creating archives, as per @acronym{POSIX} standards. On
9161 reading, @GNUTAR{} computes both checksums and
9162 accept any. It is somewhat worrying that a lot of people may go
9163 around doing backup of their files using faulty (or at least
9164 non-standard) software, not learning about it until it's time to
9165 restore their missing files with an incompatible file extractor, or
9168 @GNUTAR{} compute checksums both ways, and accept
9169 any on read, so @acronym{GNU} tar can read Sun tapes even with their
9170 wrong checksums. @GNUTAR{} produces the standard
9171 checksum, however, raising incompatibilities with Sun. That is to
9172 say, @GNUTAR{} has not been modified to
9173 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
9174 I've been told that more recent Sun @command{tar} now read standard
9175 archives, so maybe Sun did a similar patch, after all?
9177 The story seems to be that when Sun first imported @command{tar}
9178 sources on their system, they recompiled it without realizing that
9179 the checksums were computed differently, because of a change in
9180 the default signing of @code{char}'s in their compiler. So they
9181 started computing checksums wrongly. When they later realized their
9182 mistake, they merely decided to stay compatible with it, and with
9183 themselves afterwards. Presumably, but I do not really know, HP-UX
9184 has chosen that their @command{tar} archives to be compatible with Sun's.
9185 The current standards do not favor Sun @command{tar} format. In any
9186 case, it now falls on the shoulders of SunOS and HP-UX users to get
9187 a @command{tar} able to read the good archives they receive.
9189 @node Large or Negative Values
9190 @subsection Large or Negative Values
9191 @cindex large values
9192 @cindex future time stamps
9193 @cindex negative time stamps
9196 The above sections suggest to use @samp{oldest possible} archive
9197 format if in doubt. However, sometimes it is not possible. If you
9198 attempt to archive a file whose metadata cannot be represented using
9199 required format, @GNUTAR{} will print error message and ignore such a
9200 file. You will than have to switch to a format that is able to
9201 handle such values. The format summary table (@pxref{Formats}) will
9204 In particular, when trying to archive files larger than 8GB or with
9205 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
9206 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
9207 @acronym{POSIX} archive formats. When considering which format to
9208 choose, bear in mind that the @acronym{GNU} format uses
9209 two's-complement base-256 notation to store values that do not fit
9210 into standard @acronym{ustar} range. Such archives can generally be
9211 read only by a @GNUTAR{} implementation. Moreover, they sometimes
9212 cannot be correctly restored on another hosts even by @GNUTAR{}. For
9213 example, using two's complement representation for negative time
9214 stamps that assumes a signed 32-bit @code{time_t} generates archives
9215 that are not portable to hosts with differing @code{time_t}
9218 On the other hand, @acronym{POSIX} archives, generally speaking, can
9219 be extracted by any tar implementation that understands older
9220 @acronym{ustar} format. The only exception are files larger than 8GB.
9222 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
9226 @subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations
9228 In previous sections you became acquainted with various quirks
9229 necessary to make your archives portable. Sometimes you may need to
9230 extract archives containing GNU-specific members using some
9231 third-party @command{tar} implementation or an older version of
9232 @GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed,
9233 but if it is for some reason impossible, this section will explain
9234 how to cope without it.
9236 When we speak about @dfn{GNU-specific} members we mean two classes of
9237 them: members split between the volumes of a multi-volume archive and
9238 sparse members. You will be able to always recover such members if
9239 the archive is in PAX format. In addition split members can be
9240 recovered from archives in old GNU format. The following subsections
9241 describe the required procedures in detail.
9244 * Split Recovery:: Members Split Between Volumes
9245 * Sparse Recovery:: Sparse Members
9248 @node Split Recovery
9249 @subsubsection Extracting Members Split Between Volumes
9251 @cindex Mutli-volume archives, extracting using non-GNU tars
9252 If a member is split between several volumes of an old GNU format archive
9253 most third party @command{tar} implementation will fail to extract
9254 it. To extract it, use @command{tarcat} program (@pxref{Tarcat}).
9255 This program is available from
9256 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat.html, @GNUTAR{}
9257 home page}. It concatenates several archive volumes into a single
9258 valid archive. For example, if you have three volumes named from
9259 @file{vol-1.tar} to @file{vol-3.tar}, you can do the following to
9260 extract them using a third-party @command{tar}:
9263 $ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -}
9266 @cindex Mutli-volume archives in PAX format, extracting using non-GNU tars
9267 You could use this approach for most (although not all) PAX
9268 format archives as well. However, extracting split members from a PAX
9269 archive is a much easier task, because PAX volumes are constructed in
9270 such a way that each part of a split member is extracted to a
9271 different file by @command{tar} implementations that are not aware of
9272 GNU extensions. More specifically, the very first part retains its
9273 original name, and all subsequent parts are named using the pattern:
9276 %d/GNUFileParts.%p/%f.%n
9280 where symbols preceeded by @samp{%} are @dfn{macro characters} that
9281 have the following meaning:
9283 @multitable @columnfractions .25 .55
9284 @headitem Meta-character @tab Replaced By
9285 @item %d @tab The directory name of the file, equivalent to the
9286 result of the @command{dirname} utility on its full name.
9287 @item %f @tab The file name of the file, equivalent to the result
9288 of the @command{basename} utility on its full name.
9289 @item %p @tab The process @acronym{ID} of the @command{tar} process that
9290 created the archive.
9291 @item %n @tab Ordinal number of this particular part.
9294 For example, if the file @file{var/longfile} was split during archive
9295 creation between three volumes, and the creator @command{tar} process
9296 had process @acronym{ID} @samp{27962}, then the member names will be:
9300 var/GNUFileParts.27962/longfile.1
9301 var/GNUFileParts.27962/longfile.2
9304 When you extract your archive using a third-party @command{tar}, these
9305 files will be created on your disk, and the only thing you will need
9306 to do to restore your file in its original form is concatenate them in
9307 the proper order, for example:
9312 $ @kbd{cat GNUFileParts.27962/longfile.1 \
9313 GNUFileParts.27962/longfile.2 >> longfile}
9314 $ rm -f GNUFileParts.27962
9318 Notice, that if the @command{tar} implementation you use supports PAX
9319 format archives, it will probably emit warnings about unknown keywords
9320 during extraction. They will look like this:
9325 Unknown extended header keyword 'GNU.volume.filename' ignored.
9326 Unknown extended header keyword 'GNU.volume.size' ignored.
9327 Unknown extended header keyword 'GNU.volume.offset' ignored.
9332 You can safely ignore these warnings.
9334 If your @command{tar} implementation is not PAX-aware, you will get
9335 more warnings and more files generated on your disk, e.g.:
9339 $ @kbd{tar xf vol-1.tar}
9340 var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
9342 Unexpected EOF in archive
9343 $ @kbd{tar xf vol-2.tar}
9344 tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
9345 GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
9346 'x', extracted as normal file
9350 Ignore these warnings. The @file{PaxHeaders.*} directories created
9351 will contain files with @dfn{extended header keywords} describing the
9352 extracted files. You can delete them, unless they describe sparse
9353 members. Read further to learn more about them.
9355 @node Sparse Recovery
9356 @subsubsection Extracting Sparse Members
9358 @cindex sparse files, extracting with non-GNU tars
9359 Any @command{tar} implementation will be able to extract sparse members from a
9360 PAX archive. However, the extracted files will be @dfn{condensed},
9361 i.e., any zero blocks will be removed from them. When we restore such
9362 a condensed file to its original form, by adding zero blocks (or
9363 @dfn{holes}) back to their original locations, we call this process
9364 @dfn{expanding} a compressed sparse file.
9367 To expand a file, you will need a simple auxiliary program called
9368 @command{xsparse}. It is available in source form from
9369 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse.html, @GNUTAR{}
9372 @cindex sparse files v.1.0, extracting with non-GNU tars
9373 Let's begin with archive members in @dfn{sparse format
9374 version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand.
9375 The condensed file will contain both file map and file data, so no
9376 additional data will be needed to restore it. If the original file
9377 name was @file{@var{dir}/@var{name}}, then the condensed file will be
9378 named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where
9379 @var{n} is a decimal number@footnote{technically speaking, @var{n} is a
9380 @dfn{process @acronym{ID}} of the @command{tar} process which created the
9381 archive (@pxref{PAX keywords}).}.
9383 To expand a version 1.0 file, run @command{xsparse} as follows:
9386 $ @kbd{xsparse @file{cond-file}}
9390 where @file{cond-file} is the name of the condensed file. The utility
9391 will deduce the name for the resulting expanded file using the
9392 following algorithm:
9395 @item If @file{cond-file} does not contain any directories,
9396 @file{../cond-file} will be used;
9398 @item If @file{cond-file} has the form
9399 @file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name}
9400 are simple names, with no @samp{/} characters in them, the output file
9401 name will be @file{@var{dir}/@var{name}}.
9403 @item Otherwise, if @file{cond-file} has the form
9404 @file{@var{dir}/@var{name}}, the output file name will be
9408 In the unlikely case when this algorithm does not suit your needs,
9409 you can explicitly specify output file name as a second argument to
9413 $ @kbd{xsparse @file{cond-file} @file{out-file}}
9416 It is often a good idea to run @command{xsparse} in @dfn{dry run} mode
9417 first. In this mode, the command does not actually expand the file,
9418 but verbosely lists all actions it would be taking to do so. The dry
9419 run mode is enabled by @option{-n} command line argument:
9423 $ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile}
9424 Reading v.1.0 sparse map
9425 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
9426 `/home/gray/sparsefile'
9431 To actually expand the file, you would run:
9434 $ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile}
9438 The program behaves the same way all UNIX utilities do: it will keep
9439 quiet unless it has simething important to tell you (e.g. an error
9440 condition or something). If you wish it to produce verbose output,
9441 similar to that from the dry run mode, use @option{-v} option:
9445 $ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile}
9446 Reading v.1.0 sparse map
9447 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
9448 `/home/gray/sparsefile'
9453 Additionally, if your @command{tar} implementation has extracted the
9454 @dfn{extended headers} for this file, you can instruct @command{xstar}
9455 to use them in order to verify the integrity of the expanded file.
9456 The option @option{-x} sets the name of the extended header file to
9457 use. Continuing our example:
9461 $ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
9462 /home/gray/GNUSparseFile.6058/sparsefile}
9463 Reading extended header file
9464 Found variable GNU.sparse.major = 1
9465 Found variable GNU.sparse.minor = 0
9466 Found variable GNU.sparse.name = sparsefile
9467 Found variable GNU.sparse.realsize = 217481216
9468 Reading v.1.0 sparse map
9469 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
9470 `/home/gray/sparsefile'
9475 @anchor{extracting sparse v.0.x}
9476 @cindex sparse files v.0.1, extracting with non-GNU tars
9477 @cindex sparse files v.0.0, extracting with non-GNU tars
9478 An @dfn{extended header} is a special @command{tar} archive header
9479 that precedes an archive member and contains a set of
9480 @dfn{variables}, describing the member properties that cannot be
9481 stored in the standard @code{ustar} header. While optional for
9482 expanding sparse version 1.0 members, the use of extended headers is
9483 mandatory when expanding sparse members in older sparse formats: v.0.0
9484 and v.0.1 (The sparse formats are described in detail in @ref{Sparse
9485 Formats}.) So, for these formats, the question is: how to obtain
9486 extended headers from the archive?
9488 If you use a @command{tar} implementation that does not support PAX
9489 format, extended headers for each member will be extracted as a
9490 separate file. If we represent the member name as
9491 @file{@var{dir}/@var{name}}, then the extended header file will be
9492 named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where
9493 @var{n} is an integer number.
9495 Things become more difficult if your @command{tar} implementation
9496 does support PAX headers, because in this case you will have to
9497 manually extract the headers. We recommend the following algorithm:
9501 Consult the documentation of your @command{tar} implementation for an
9502 option that prints @dfn{block numbers} along with the archive
9503 listing (analogous to @GNUTAR{}'s @option{-R} option). For example,
9504 @command{star} has @option{-block-number}.
9507 Obtain verbose listing using the @samp{block number} option, and
9508 find block numbers of the sparse member in question and the member
9509 immediately following it. For example, running @command{star} on our
9514 $ @kbd{star -t -v -block-number -f arc.tar}
9516 star: Unknown extended header keyword 'GNU.sparse.size' ignored.
9517 star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
9518 star: Unknown extended header keyword 'GNU.sparse.name' ignored.
9519 star: Unknown extended header keyword 'GNU.sparse.map' ignored.
9520 block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
9521 block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
9527 (as usual, ignore the warnings about unknown keywords.)
9530 Let @var{size} be the size of the sparse member, @var{Bs} be its block number
9531 and @var{Bn} be the block number of the next member.
9535 @var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2
9539 This number gives the size of the extended header part in tar @dfn{blocks}.
9540 In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2
9544 Use @command{dd} to extract the headers:
9547 @kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}}
9551 where @var{archive} is the archive name, @var{hname} is a name of the
9552 file to store the extended header in, @var{Bs} and @var{N} are
9553 computed in previous steps.
9555 In our example, this command will be
9558 $ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7}
9562 Finally, you can expand the condensed file, using the obtained header:
9566 $ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile}
9567 Reading extended header file
9568 Found variable GNU.sparse.size = 217481216
9569 Found variable GNU.sparse.numblocks = 208
9570 Found variable GNU.sparse.name = sparsefile
9571 Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{}
9572 Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile'
9578 @section Comparison of @command{tar} and @command{cpio}
9581 @FIXME{Reorganize the following material}
9583 The @command{cpio} archive formats, like @command{tar}, do have maximum
9584 file name lengths. The binary and old @acronym{ASCII} formats have a maximum file
9585 length of 256, and the new @acronym{ASCII} and @acronym{CRC ASCII} formats have a max
9586 file length of 1024. @acronym{GNU} @command{cpio} can read and write archives
9587 with arbitrary file name lengths, but other @command{cpio} implementations
9588 may crash unexplainedly trying to read them.
9590 @command{tar} handles symbolic links in the form in which it comes in @acronym{BSD};
9591 @command{cpio} doesn't handle symbolic links in the form in which it comes
9592 in System V prior to SVR4, and some vendors may have added symlinks
9593 to their system without enhancing @command{cpio} to know about them.
9594 Others may have enhanced it in a way other than the way I did it
9595 at Sun, and which was adopted by AT&T (and which is, I think, also
9596 present in the @command{cpio} that Berkeley picked up from AT&T and put
9597 into a later @acronym{BSD} release---I think I gave them my changes).
9599 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
9600 can handle @command{tar} format input, and write it on output, and it
9601 probably handles symbolic links. They may not have bothered doing
9602 anything to enhance @command{tar} as a result.)
9604 @command{cpio} handles special files; traditional @command{tar} doesn't.
9606 @command{tar} comes with V7, System III, System V, and @acronym{BSD} source;
9607 @command{cpio} comes only with System III, System V, and later @acronym{BSD}
9608 (4.3-tahoe and later).
9610 @command{tar}'s way of handling multiple hard links to a file can handle
9611 file systems that support 32-bit inumbers (e.g., the @acronym{BSD} file system);
9612 @command{cpio}s way requires you to play some games (in its ``binary''
9613 format, i-numbers are only 16 bits, and in its ``portable @acronym{ASCII}'' format,
9614 they're 18 bits---it would have to play games with the "file system @acronym{ID}"
9615 field of the header to make sure that the file system @acronym{ID}/i-number pairs
9616 of different files were always different), and I don't know which
9617 @command{cpio}s, if any, play those games. Those that don't might get
9618 confused and think two files are the same file when they're not, and
9619 make hard links between them.
9621 @command{tar}s way of handling multiple hard links to a file places only
9622 one copy of the link on the tape, but the name attached to that copy
9623 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
9624 way puts one copy for every link, but you can retrieve it using any
9628 What type of check sum (if any) is used, and how is this calculated.
9631 See the attached manual pages for @command{tar} and @command{cpio} format.
9632 @command{tar} uses a checksum which is the sum of all the bytes in the
9633 @command{tar} header for a file; @command{cpio} uses no checksum.
9636 If anyone knows why @command{cpio} was made when @command{tar} was present
9640 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
9641 generally-available version of UNIX had @command{tar} at the time. I don't
9642 know whether any version that was generally available @emph{within AT&T}
9643 had @command{tar}, or, if so, whether the people within AT&T who did
9644 @command{cpio} knew about it.
9646 On restore, if there is a corruption on a tape @command{tar} will stop at
9647 that point, while @command{cpio} will skip over it and try to restore the
9650 The main difference is just in the command syntax and header format.
9652 @command{tar} is a little more tape-oriented in that everything is blocked
9653 to start on a record boundary.
9656 Is there any differences between the ability to recover crashed
9657 archives between the two of them. (Is there any chance of recovering
9658 crashed archives at all.)
9661 Theoretically it should be easier under @command{tar} since the blocking
9662 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
9663 However, modern @command{cpio}'s and variations have an option to just
9664 search for the next file header after an error with a reasonable chance
9665 of resyncing. However, lots of tape driver software won't allow you to
9666 continue past a media error which should be the only reason for getting
9667 out of sync unless a file changed sizes while you were writing the
9671 If anyone knows why @command{cpio} was made when @command{tar} was present
9672 at the unix scene, please tell me about this too.
9675 Probably because it is more media efficient (by not blocking everything
9676 and using only the space needed for the headers where @command{tar}
9677 always uses 512 bytes per file header) and it knows how to archive
9680 You might want to look at the freely available alternatives. The
9681 major ones are @command{afio}, @GNUTAR{}, and
9682 @command{pax}, each of which have their own extensions with some
9683 backwards compatibility.
9685 Sparse files were @command{tar}red as sparse files (which you can
9686 easily test, because the resulting archive gets smaller, and
9687 @acronym{GNU} @command{cpio} can no longer read it).
9690 @chapter Tapes and Other Archive Media
9693 A few special cases about tape handling warrant more detailed
9694 description. These special cases are discussed below.
9696 Many complexities surround the use of @command{tar} on tape drives. Since
9697 the creation and manipulation of archives located on magnetic tape was
9698 the original purpose of @command{tar}, it contains many features making
9699 such manipulation easier.
9701 Archives are usually written on dismountable media---tape cartridges,
9702 mag tapes, or floppy disks.
9704 The amount of data a tape or disk holds depends not only on its size,
9705 but also on how it is formatted. A 2400 foot long reel of mag tape
9706 holds 40 megabytes of data when formatted at 1600 bits per inch. The
9707 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
9709 Magnetic media are re-usable---once the archive on a tape is no longer
9710 needed, the archive can be erased and the tape or disk used over.
9711 Media quality does deteriorate with use, however. Most tapes or disks
9712 should be discarded when they begin to produce data errors. EXABYTE
9713 tape cartridges should be discarded when they generate an @dfn{error
9714 count} (number of non-usable bits) of more than 10k.
9716 Magnetic media are written and erased using magnetic fields, and
9717 should be protected from such fields to avoid damage to stored data.
9718 Sticking a floppy disk to a filing cabinet using a magnet is probably
9722 * Device:: Device selection and switching
9723 * Remote Tape Server::
9724 * Common Problems and Solutions::
9725 * Blocking:: Blocking
9726 * Many:: Many archives on one tape
9727 * Using Multiple Tapes:: Using Multiple Tapes
9728 * label:: Including a Label in the Archive
9730 * Write Protection::
9734 @section Device Selection and Switching
9738 @item -f [@var{hostname}:]@var{file}
9739 @itemx --file=[@var{hostname}:]@var{file}
9740 Use archive file or device @var{file} on @var{hostname}.
9743 This option is used to specify the file name of the archive @command{tar}
9746 If the file name is @samp{-}, @command{tar} reads the archive from standard
9747 input (when listing or extracting), or writes it to standard output
9748 (when creating). If the @samp{-} file name is given when updating an
9749 archive, @command{tar} will read the original archive from its standard
9750 input, and will write the entire new archive to its standard output.
9752 If the file name contains a @samp{:}, it is interpreted as
9753 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
9754 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
9755 either case, @command{tar} will invoke the command @command{rsh} (or
9756 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
9757 machine. If you give an alternate login name, it will be given to the
9759 Naturally, the remote machine must have an executable
9760 @command{/usr/libexec/rmt}. This program is free software from the
9761 University of California, and a copy of the source code can be found
9762 with the sources for @command{tar}; it's compiled and installed by default.
9763 The exact path to this utility is determined when configuring the package.
9764 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
9765 your installation prefix. This location may also be overridden at
9766 runtime by using @option{rmt-command=@var{command}} option (@xref{Option Summary,
9767 ---rmt-command}, for detailed description of this option. @xref{Remote
9768 Tape Server}, for the description of @command{rmt} command).
9770 If this option is not given, but the environment variable @env{TAPE}
9771 is set, its value is used; otherwise, old versions of @command{tar}
9772 used a default archive name (which was picked when @command{tar} was
9773 compiled). The default is normally set up to be the @dfn{first} tape
9774 drive or other transportable I/O medium on the system.
9776 Starting with version 1.11.5, @GNUTAR{} uses
9777 standard input and standard output as the default device, and I will
9778 not try anymore supporting automatic device detection at installation
9779 time. This was failing really in too many cases, it was hopeless.
9780 This is now completely left to the installer to override standard
9781 input and standard output for default device, if this seems
9782 preferable. Further, I think @emph{most} actual usages of
9783 @command{tar} are done with pipes or disks, not really tapes,
9784 cartridges or diskettes.
9786 Some users think that using standard input and output is running
9787 after trouble. This could lead to a nasty surprise on your screen if
9788 you forget to specify an output file name---especially if you are going
9789 through a network or terminal server capable of buffering large amounts
9790 of output. We had so many bug reports in that area of configuring
9791 default tapes automatically, and so many contradicting requests, that
9792 we finally consider the problem to be portably intractable. We could
9793 of course use something like @samp{/dev/tape} as a default, but this
9794 is @emph{also} running after various kind of trouble, going from hung
9795 processes to accidental destruction of real tapes. After having seen
9796 all this mess, using standard input and output as a default really
9797 sounds like the only clean choice left, and a very useful one too.
9799 @GNUTAR{} reads and writes archive in records, I
9800 suspect this is the main reason why block devices are preferred over
9801 character devices. Most probably, block devices are more efficient
9802 too. The installer could also check for @samp{DEFTAPE} in
9803 @file{<sys/mtio.h>}.
9806 @xopindex{force-local, short description}
9808 Archive file is local even if it contains a colon.
9810 @opindex rsh-command
9811 @item --rsh-command=@var{command}
9812 Use remote @var{command} instead of @command{rsh}. This option exists
9813 so that people who use something other than the standard @command{rsh}
9814 (e.g., a Kerberized @command{rsh}) can access a remote device.
9816 When this command is not used, the shell command found when
9817 the @command{tar} program was installed is used instead. This is
9818 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
9819 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
9820 The installer may have overridden this by defining the environment
9821 variable @env{RSH} @emph{at installation time}.
9824 Specify drive and density.
9826 @xopindex{multi-volume, short description}
9828 @itemx --multi-volume
9829 Create/list/extract multi-volume archive.
9831 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
9832 that may be larger than will fit on the medium used to hold it.
9833 @xref{Multi-Volume Archives}.
9835 @xopindex{tape-length, short description}
9837 @itemx --tape-length=@var{num}
9838 Change tape after writing @var{num} x 1024 bytes.
9840 This option might be useful when your tape drivers do not properly
9841 detect end of physical tapes. By being slightly conservative on the
9842 maximum tape length, you might avoid the problem entirely.
9844 @xopindex{info-script, short description}
9845 @xopindex{new-volume-script, short description}
9847 @itemx --info-script=@var{file}
9848 @itemx --new-volume-script=@var{file}
9849 Execute @file{file} at end of each tape. This implies
9850 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
9851 description of this option.
9854 @node Remote Tape Server
9855 @section The Remote Tape Server
9857 @cindex remote tape drive
9859 In order to access the tape drive on a remote machine, @command{tar}
9860 uses the remote tape server written at the University of California at
9861 Berkeley. The remote tape server must be installed as
9862 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
9863 want to use. @command{tar} calls @command{rmt} by running an
9864 @command{rsh} or @command{remsh} to the remote machine, optionally
9865 using a different login name if one is supplied.
9867 A copy of the source for the remote tape server is provided. It is
9868 Copyright @copyright{} 1983 by the Regents of the University of
9869 California, but can be freely distributed. It is compiled and
9870 installed by default.
9872 @cindex absolute file names
9873 Unless you use the @option{--absolute-names} (@option{-P}) option,
9874 @GNUTAR{} will not allow you to create an archive that contains
9875 absolute file names (a file name beginning with @samp{/}.) If you try,
9876 @command{tar} will automatically remove the leading @samp{/} from the
9877 file names it stores in the archive. It will also type a warning
9878 message telling you what it is doing.
9880 When reading an archive that was created with a different
9881 @command{tar} program, @GNUTAR{} automatically
9882 extracts entries in the archive which have absolute file names as if
9883 the file names were not absolute. This is an important feature. A
9884 visitor here once gave a @command{tar} tape to an operator to restore;
9885 the operator used Sun @command{tar} instead of @GNUTAR{},
9886 and the result was that it replaced large portions of
9887 our @file{/bin} and friends with versions from the tape; needless to
9888 say, we were unhappy about having to recover the file system from
9891 For example, if the archive contained a file @file{/usr/bin/computoy},
9892 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
9893 relative to the current directory. If you want to extract the files in
9894 an archive to the same absolute names that they had when the archive
9895 was created, you should do a @samp{cd /} before extracting the files
9896 from the archive, or you should either use the @option{--absolute-names}
9897 option, or use the command @samp{tar -C / @dots{}}.
9899 @cindex Ultrix 3.1 and write failure
9900 Some versions of Unix (Ultrix 3.1 is known to have this problem),
9901 can claim that a short write near the end of a tape succeeded,
9902 when it actually failed. This will result in the -M option not
9903 working correctly. The best workaround at the moment is to use a
9904 significantly larger blocking factor than the default 20.
9906 In order to update an archive, @command{tar} must be able to backspace the
9907 archive in order to reread or rewrite a record that was just read (or
9908 written). This is currently possible only on two kinds of files: normal
9909 disk files (or any other file that can be backspaced with @samp{lseek}),
9910 and industry-standard 9-track magnetic tape (or any other kind of tape
9911 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}.
9913 This means that the @option{--append}, @option{--concatenate}, and
9914 @option{--delete} commands will not work on any other kind of file.
9915 Some media simply cannot be backspaced, which means these commands and
9916 options will never be able to work on them. These non-backspacing
9917 media include pipes and cartridge tape drives.
9919 Some other media can be backspaced, and @command{tar} will work on them
9920 once @command{tar} is modified to do so.
9922 Archives created with the @option{--multi-volume}, @option{--label}, and
9923 @option{--incremental} (@option{-G}) options may not be readable by other version
9924 of @command{tar}. In particular, restoring a file that was split over
9925 a volume boundary will require some careful work with @command{dd}, if
9926 it can be done at all. Other versions of @command{tar} may also create
9927 an empty file whose name is that of the volume header. Some versions
9928 of @command{tar} may create normal files instead of directories archived
9929 with the @option{--incremental} (@option{-G}) option.
9931 @node Common Problems and Solutions
9932 @section Some Common Problems and their Solutions
9939 no such file or directory
9942 errors from @command{tar}:
9943 directory checksum error
9946 errors from media/system:
9957 @dfn{Block} and @dfn{record} terminology is rather confused, and it
9958 is also confusing to the expert reader. On the other hand, readers
9959 who are new to the field have a fresh mind, and they may safely skip
9960 the next two paragraphs, as the remainder of this manual uses those
9961 two terms in a quite consistent way.
9963 John Gilmore, the writer of the public domain @command{tar} from which
9964 @GNUTAR{} was originally derived, wrote (June 1995):
9967 The nomenclature of tape drives comes from IBM, where I believe
9968 they were invented for the IBM 650 or so. On IBM mainframes, what
9969 is recorded on tape are tape blocks. The logical organization of
9970 data is into records. There are various ways of putting records into
9971 blocks, including @code{F} (fixed sized records), @code{V} (variable
9972 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
9973 to a block), @code{VB} (variable size records, @var{n} to a block),
9974 @code{VSB} (variable spanned blocked: variable sized records that can
9975 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
9976 parameter specified this to the operating system.
9978 The Unix man page on @command{tar} was totally confused about this.
9979 When I wrote @code{PD TAR}, I used the historically correct terminology
9980 (@command{tar} writes data records, which are grouped into blocks).
9981 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
9982 here), and now Fran@,{c}ois has migrated that terminology back
9983 into the source code too.
9986 The term @dfn{physical block} means the basic transfer chunk from or
9987 to a device, after which reading or writing may stop without anything
9988 being lost. In this manual, the term @dfn{block} usually refers to
9989 a disk physical block, @emph{assuming} that each disk block is 512
9990 bytes in length. It is true that some disk devices have different
9991 physical blocks, but @command{tar} ignore these differences in its own
9992 format, which is meant to be portable, so a @command{tar} block is always
9993 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
9994 The term @dfn{logical block} often represents the basic chunk of
9995 allocation of many disk blocks as a single entity, which the operating
9996 system treats somewhat atomically; this concept is only barely used
9999 The term @dfn{physical record} is another way to speak of a physical
10000 block, those two terms are somewhat interchangeable. In this manual,
10001 the term @dfn{record} usually refers to a tape physical block,
10002 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
10003 It is true that archives may be put on disk or used with pipes,
10004 but nevertheless, @command{tar} tries to read and write the archive one
10005 @dfn{record} at a time, whatever the medium in use. One record is made
10006 up of an integral number of blocks, and this operation of putting many
10007 disk blocks into a single tape block is called @dfn{reblocking}, or
10008 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
10009 the logical organization of many characters into something meaningful
10010 to the application. The term @dfn{unit record} describes a small set
10011 of characters which are transmitted whole to or by the application,
10012 and often refers to a line of text. Those two last terms are unrelated
10013 to what we call a @dfn{record} in @GNUTAR{}.
10015 When writing to tapes, @command{tar} writes the contents of the archive
10016 in chunks known as @dfn{records}. To change the default blocking
10017 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
10018 @var{512-size}}) option. Each record will then be composed of
10019 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
10020 @xref{Standard}.) Each file written to the archive uses at least one
10021 full record. As a result, using a larger record size can result in
10022 more wasted space for small files. On the other hand, a larger record
10023 size can often be read and written much more efficiently.
10025 Further complicating the problem is that some tape drives ignore the
10026 blocking entirely. For these, a larger record size can still improve
10027 performance (because the software layers above the tape drive still
10028 honor the blocking), but not as dramatically as on tape drives that
10031 When reading an archive, @command{tar} can usually figure out the
10032 record size on itself. When this is the case, and a non-standard
10033 record size was used when the archive was created, @command{tar} will
10034 print a message about a non-standard blocking factor, and then operate
10035 normally. On some tape devices, however, @command{tar} cannot figure
10036 out the record size itself. On most of those, you can specify a
10037 blocking factor (with @option{--blocking-factor}) larger than the
10038 actual blocking factor, and then use the @option{--read-full-records}
10039 (@option{-B}) option. (If you specify a blocking factor with
10040 @option{--blocking-factor} and don't use the
10041 @option{--read-full-records} option, then @command{tar} will not
10042 attempt to figure out the recording size itself.) On some devices,
10043 you must always specify the record size exactly with
10044 @option{--blocking-factor} when reading, because @command{tar} cannot
10045 figure it out. In any case, use @option{--list} (@option{-t}) before
10046 doing any extractions to see whether @command{tar} is reading the archive
10049 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
10050 putting them into records is to put a whole number of them (one or
10051 more) into each record. @command{tar} records are all the same size;
10052 at the end of the file there's a block containing all zeros, which
10053 is how you tell that the remainder of the last record(s) are garbage.
10055 In a standard @command{tar} file (no options), the block size is 512
10056 and the record size is 10240, for a blocking factor of 20. What the
10057 @option{--blocking-factor} option does is sets the blocking factor,
10058 changing the record size while leaving the block size at 512 bytes.
10059 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
10060 most tape drives these days prefer much bigger records in order to
10061 stream and not waste tape. When writing tapes for myself, some tend
10062 to use a factor of the order of 2048, say, giving a record size of
10063 around one megabyte.
10065 If you use a blocking factor larger than 20, older @command{tar}
10066 programs might not be able to read the archive, so we recommend this
10067 as a limit to use in practice. @GNUTAR{}, however,
10068 will support arbitrarily large record sizes, limited only by the
10069 amount of virtual memory or the physical characteristics of the tape
10073 * Format Variations:: Format Variations
10074 * Blocking Factor:: The Blocking Factor of an Archive
10077 @node Format Variations
10078 @subsection Format Variations
10079 @cindex Format Parameters
10080 @cindex Format Options
10081 @cindex Options, archive format specifying
10082 @cindex Options, format specifying
10085 Format parameters specify how an archive is written on the archive
10086 media. The best choice of format parameters will vary depending on
10087 the type and number of files being archived, and on the media used to
10090 To specify format parameters when accessing or creating an archive,
10091 you can use the options described in the following sections.
10092 If you do not specify any format parameters, @command{tar} uses
10093 default parameters. You cannot modify a compressed archive.
10094 If you create an archive with the @option{--blocking-factor} option
10095 specified (@pxref{Blocking Factor}), you must specify that
10096 blocking-factor when operating on the archive. @xref{Formats}, for other
10097 examples of format parameter considerations.
10099 @node Blocking Factor
10100 @subsection The Blocking Factor of an Archive
10101 @cindex Blocking Factor
10102 @cindex Record Size
10103 @cindex Number of blocks per record
10104 @cindex Number of bytes per record
10105 @cindex Bytes per record
10106 @cindex Blocks per record
10109 @opindex blocking-factor
10110 The data in an archive is grouped into blocks, which are 512 bytes.
10111 Blocks are read and written in whole number multiples called
10112 @dfn{records}. The number of blocks in a record (i.e., the size of a
10113 record in units of 512 bytes) is called the @dfn{blocking factor}.
10114 The @option{--blocking-factor=@var{512-size}} (@option{-b
10115 @var{512-size}}) option specifies the blocking factor of an archive.
10116 The default blocking factor is typically 20 (i.e., 10240 bytes), but
10117 can be specified at installation. To find out the blocking factor of
10118 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
10119 This may not work on some devices.
10121 Records are separated by gaps, which waste space on the archive media.
10122 If you are archiving on magnetic tape, using a larger blocking factor
10123 (and therefore larger records) provides faster throughput and allows you
10124 to fit more data on a tape (because there are fewer gaps). If you are
10125 archiving on cartridge, a very large blocking factor (say 126 or more)
10126 greatly increases performance. A smaller blocking factor, on the other
10127 hand, may be useful when archiving small files, to avoid archiving lots
10128 of nulls as @command{tar} fills out the archive to the end of the record.
10129 In general, the ideal record size depends on the size of the
10130 inter-record gaps on the tape you are using, and the average size of the
10131 files you are archiving. @xref{create}, for information on
10134 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
10136 Archives with blocking factors larger than 20 cannot be read
10137 by very old versions of @command{tar}, or by some newer versions
10138 of @command{tar} running on old machines with small address spaces.
10139 With @GNUTAR{}, the blocking factor of an archive is limited
10140 only by the maximum record size of the device containing the archive,
10141 or by the amount of available virtual memory.
10143 Also, on some systems, not using adequate blocking factors, as sometimes
10144 imposed by the device drivers, may yield unexpected diagnostics. For
10145 example, this has been reported:
10148 Cannot write to /dev/dlt: Invalid argument
10152 In such cases, it sometimes happen that the @command{tar} bundled by
10153 the system is aware of block size idiosyncrasies, while @GNUTAR{}
10154 requires an explicit specification for the block size,
10155 which it cannot guess. This yields some people to consider
10156 @GNUTAR{} is misbehaving, because by comparison,
10157 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
10158 for example, might resolve the problem.
10160 If you use a non-default blocking factor when you create an archive, you
10161 must specify the same blocking factor when you modify that archive. Some
10162 archive devices will also require you to specify the blocking factor when
10163 reading that archive, however this is not typically the case. Usually, you
10164 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
10165 reports a non-default record size and then lists the archive members as
10166 it would normally. To extract files from an archive with a non-standard
10167 blocking factor (particularly if you're not sure what the blocking factor
10168 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
10169 specifying a blocking factor larger then the blocking factor of the archive
10170 (i.e., @samp{tar --extract --read-full-records --blocking-factor=300}.
10171 @xref{list}, for more information on the @option{--list} (@option{-t})
10172 operation. @xref{Reading}, for a more detailed explanation of that option.
10175 @item --blocking-factor=@var{number}
10176 @itemx -b @var{number}
10177 Specifies the blocking factor of an archive. Can be used with any
10178 operation, but is usually not necessary with @option{--list} (@option{-t}).
10184 @item -b @var{blocks}
10185 @itemx --blocking-factor=@var{blocks}
10186 Set record size to @math{@var{blocks} * 512} bytes.
10188 This option is used to specify a @dfn{blocking factor} for the archive.
10189 When reading or writing the archive, @command{tar}, will do reads and writes
10190 of the archive in records of @math{@var{block}*512} bytes. This is true
10191 even when the archive is compressed. Some devices requires that all
10192 write operations be a multiple of a certain size, and so, @command{tar}
10193 pads the archive out to the next record boundary.
10195 The default blocking factor is set when @command{tar} is compiled, and is
10196 typically 20. Blocking factors larger than 20 cannot be read by very
10197 old versions of @command{tar}, or by some newer versions of @command{tar}
10198 running on old machines with small address spaces.
10200 With a magnetic tape, larger records give faster throughput and fit
10201 more data on a tape (because there are fewer inter-record gaps).
10202 If the archive is in a disk file or a pipe, you may want to specify
10203 a smaller blocking factor, since a large one will result in a large
10204 number of null bytes at the end of the archive.
10206 When writing cartridge or other streaming tapes, a much larger
10207 blocking factor (say 126 or more) will greatly increase performance.
10208 However, you must specify the same blocking factor when reading or
10209 updating the archive.
10211 Apparently, Exabyte drives have a physical block size of 8K bytes.
10212 If we choose our blocksize as a multiple of 8k bytes, then the problem
10213 seems to disappear. Id est, we are using block size of 112 right
10214 now, and we haven't had the problem since we switched@dots{}
10216 With @GNUTAR{} the blocking factor is limited only
10217 by the maximum record size of the device containing the archive, or by
10218 the amount of available virtual memory.
10220 However, deblocking or reblocking is virtually avoided in a special
10221 case which often occurs in practice, but which requires all the
10222 following conditions to be simultaneously true:
10225 the archive is subject to a compression option,
10227 the archive is not handled through standard input or output, nor
10228 redirected nor piped,
10230 the archive is directly handled to a local disk, instead of any special
10233 @option{--blocking-factor} is not explicitly specified on the @command{tar}
10237 If the output goes directly to a local disk, and not through
10238 stdout, then the last write is not extended to a full record size.
10239 Otherwise, reblocking occurs. Here are a few other remarks on this
10245 @command{gzip} will complain about trailing garbage if asked to
10246 uncompress a compressed archive on tape, there is an option to turn
10247 the message off, but it breaks the regularity of simply having to use
10248 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
10249 silently ignoring any number of trailing zeros. I'll ask Jean-loup
10250 Gailly, by sending a copy of this message to him.
10253 @command{compress} does not show this problem, but as Jean-loup pointed
10254 out to Michael, @samp{compress -d} silently adds garbage after
10255 the result of decompression, which tar ignores because it already
10256 recognized its end-of-file indicator. So this bug may be safely
10260 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
10261 but will still return an exit status of 2 which tar reports in turn.
10262 @command{tar} might ignore the exit status returned, but I hate doing
10263 that, as it weakens the protection @command{tar} offers users against
10264 other possible problems at decompression time. If @command{gzip} was
10265 silently skipping trailing zeros @emph{and} also avoiding setting the
10266 exit status in this innocuous case, that would solve this situation.
10269 @command{tar} should become more solid at not stopping to read a pipe at
10270 the first null block encountered. This inelegantly breaks the pipe.
10271 @command{tar} should rather drain the pipe out before exiting itself.
10274 @xopindex{ignore-zeros, short description}
10276 @itemx --ignore-zeros
10277 Ignore blocks of zeros in archive (means EOF).
10279 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
10280 of zeros in the archive. Normally a block of zeros indicates the
10281 end of the archive, but when reading a damaged archive, or one which
10282 was created by concatenating several archives together, this option
10283 allows @command{tar} to read the entire archive. This option is not on
10284 by default because many versions of @command{tar} write garbage after
10287 Note that this option causes @command{tar} to read to the end of the
10288 archive file, which may sometimes avoid problems when multiple files
10289 are stored on a single physical tape.
10291 @xopindex{read-full-records, short description}
10293 @itemx --read-full-records
10294 Reblock as we read (for reading 4.2@acronym{BSD} pipes).
10296 If @option{--read-full-records} is used, @command{tar}
10297 will not panic if an attempt to read a record from the archive does
10298 not return a full record. Instead, @command{tar} will keep reading
10299 until it has obtained a full
10302 This option is turned on by default when @command{tar} is reading
10303 an archive from standard input, or from a remote machine. This is
10304 because on @acronym{BSD} Unix systems, a read of a pipe will return however
10305 much happens to be in the pipe, even if it is less than @command{tar}
10306 requested. If this option was not used, @command{tar} would fail as
10307 soon as it read an incomplete record from the pipe.
10309 This option is also useful with the commands for updating an archive.
10315 @FIXME{Appropriate options should be moved here from elsewhere.}
10317 @cindex blocking factor
10318 @cindex tape blocking
10320 When handling various tapes or cartridges, you have to take care of
10321 selecting a proper blocking, that is, the number of disk blocks you
10322 put together as a single tape block on the tape, without intervening
10323 tape gaps. A @dfn{tape gap} is a small landing area on the tape
10324 with no information on it, used for decelerating the tape to a
10325 full stop, and for later regaining the reading or writing speed.
10326 When the tape driver starts reading a record, the record has to
10327 be read whole without stopping, as a tape gap is needed to stop the
10328 tape motion without loosing information.
10330 @cindex Exabyte blocking
10331 @cindex DAT blocking
10332 Using higher blocking (putting more disk blocks per tape block) will use
10333 the tape more efficiently as there will be less tape gaps. But reading
10334 such tapes may be more difficult for the system, as more memory will be
10335 required to receive at once the whole record. Further, if there is a
10336 reading error on a huge record, this is less likely that the system will
10337 succeed in recovering the information. So, blocking should not be too
10338 low, nor it should be too high. @command{tar} uses by default a blocking of
10339 20 for historical reasons, and it does not really matter when reading or
10340 writing to disk. Current tape technology would easily accommodate higher
10341 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
10342 We were told that for some DLT drives, the blocking should be a multiple
10343 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
10344 Other manufacturers may use different recommendations for the same tapes.
10345 This might also depends of the buffering techniques used inside modern
10346 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
10347 Others request blocking to be some exponent of two.
10349 So, there is no fixed rule for blocking. But blocking at read time
10350 should ideally be the same as blocking used at write time. At one place
10351 I know, with a wide variety of equipment, they found it best to use a
10352 blocking of 32 to guarantee that their tapes are fully interchangeable.
10354 I was also told that, for recycled tapes, prior erasure (by the same
10355 drive unit that will be used to create the archives) sometimes lowers
10356 the error rates observed at rewriting time.
10358 I might also use @option{--number-blocks} instead of
10359 @option{--block-number}, so @option{--block} will then expand to
10360 @option{--blocking-factor} unambiguously.
10363 @section Many Archives on One Tape
10365 @FIXME{Appropriate options should be moved here from elsewhere.}
10367 @findex ntape @r{device}
10368 Most tape devices have two entries in the @file{/dev} directory, or
10369 entries that come in pairs, which differ only in the minor number for
10370 this device. Let's take for example @file{/dev/tape}, which often
10371 points to the only or usual tape device of a given system. There might
10372 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
10373 name is the @emph{rewinding} version of the device, while the name
10374 having @samp{nr} in it is the @emph{no rewinding} version of the same
10377 A rewinding tape device will bring back the tape to its beginning point
10378 automatically when this device is opened or closed. Since @command{tar}
10379 opens the archive file before using it and closes it afterwards, this
10380 means that a simple:
10383 $ @kbd{tar cf /dev/tape @var{directory}}
10387 will reposition the tape to its beginning both prior and after saving
10388 @var{directory} contents to it, thus erasing prior tape contents and
10389 making it so that any subsequent write operation will destroy what has
10392 @cindex tape positioning
10393 So, a rewinding device is normally meant to hold one and only one file.
10394 If you want to put more than one @command{tar} archive on a given tape, you
10395 will need to avoid using the rewinding version of the tape device. You
10396 will also have to pay special attention to tape positioning. Errors in
10397 positioning may overwrite the valuable data already on your tape. Many
10398 people, burnt by past experiences, will only use rewinding devices and
10399 limit themselves to one file per tape, precisely to avoid the risk of
10400 such errors. Be fully aware that writing at the wrong position on a
10401 tape loses all information past this point and most probably until the
10402 end of the tape, and this destroyed information @emph{cannot} be
10405 To save @var{directory-1} as a first archive at the beginning of a
10406 tape, and leave that tape ready for a second archive, you should use:
10409 $ @kbd{mt -f /dev/nrtape rewind}
10410 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
10414 @dfn{Tape marks} are special magnetic patterns written on the tape
10415 media, which are later recognizable by the reading hardware. These
10416 marks are used after each file, when there are many on a single tape.
10417 An empty file (that is to say, two tape marks in a row) signal the
10418 logical end of the tape, after which no file exist. Usually,
10419 non-rewinding tape device drivers will react to the close request issued
10420 by @command{tar} by first writing two tape marks after your archive, and by
10421 backspacing over one of these. So, if you remove the tape at that time
10422 from the tape drive, it is properly terminated. But if you write
10423 another file at the current position, the second tape mark will be
10424 erased by the new information, leaving only one tape mark between files.
10426 So, you may now save @var{directory-2} as a second archive after the
10427 first on the same tape by issuing the command:
10430 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
10434 and so on for all the archives you want to put on the same tape.
10436 Another usual case is that you do not write all the archives the same
10437 day, and you need to remove and store the tape between two archive
10438 sessions. In general, you must remember how many files are already
10439 saved on your tape. Suppose your tape already has 16 files on it, and
10440 that you are ready to write the 17th. You have to take care of skipping
10441 the first 16 tape marks before saving @var{directory-17}, say, by using
10445 $ @kbd{mt -f /dev/nrtape rewind}
10446 $ @kbd{mt -f /dev/nrtape fsf 16}
10447 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
10450 In all the previous examples, we put aside blocking considerations, but
10451 you should do the proper things for that as well. @xref{Blocking}.
10454 * Tape Positioning:: Tape Positions and Tape Marks
10455 * mt:: The @command{mt} Utility
10458 @node Tape Positioning
10459 @subsection Tape Positions and Tape Marks
10462 Just as archives can store more than one file from the file system,
10463 tapes can store more than one archive file. To keep track of where
10464 archive files (or any other type of file stored on tape) begin and
10465 end, tape archive devices write magnetic @dfn{tape marks} on the
10466 archive media. Tape drives write one tape mark between files,
10467 two at the end of all the file entries.
10469 If you think of data as a series of records "rrrr"'s, and tape marks as
10470 "*"'s, a tape might look like the following:
10473 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
10476 Tape devices read and write tapes using a read/write @dfn{tape
10477 head}---a physical part of the device which can only access one
10478 point on the tape at a time. When you use @command{tar} to read or
10479 write archive data from a tape device, the device will begin reading
10480 or writing from wherever on the tape the tape head happens to be,
10481 regardless of which archive or what part of the archive the tape
10482 head is on. Before writing an archive, you should make sure that no
10483 data on the tape will be overwritten (unless it is no longer needed).
10484 Before reading an archive, you should make sure the tape head is at
10485 the beginning of the archive you want to read. You can do it manually
10486 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
10487 that automatically (@pxref{Scripted Restoration}).
10489 If you want to add new archive file entries to a tape, you should
10490 advance the tape to the end of the existing file entries, backspace
10491 over the last tape mark, and write the new archive file. If you were
10492 to add two archives to the example above, the tape might look like the
10496 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
10500 @subsection The @command{mt} Utility
10503 @FIXME{Is it true that this only works on non-block devices?
10504 should explain the difference, (fixed or variable).}
10505 @xref{Blocking Factor}.
10507 You can use the @command{mt} utility to advance or rewind a tape past a
10508 specified number of archive files on the tape. This will allow you
10509 to move to the beginning of an archive before extracting or reading
10510 it, or to the end of all the archives before writing a new one.
10511 @FIXME{Why isn't there an "advance 'til you find two tape marks
10514 The syntax of the @command{mt} command is:
10517 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
10520 where @var{tapename} is the name of the tape device, @var{number} is
10521 the number of times an operation is performed (with a default of one),
10522 and @var{operation} is one of the following:
10524 @FIXME{is there any use for record operations?}
10529 Writes @var{number} tape marks at the current position on the tape.
10532 Moves tape position forward @var{number} files.
10535 Moves tape position back @var{number} files.
10538 Rewinds the tape. (Ignores @var{number}).
10542 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}).
10545 Prints status information about the tape unit.
10549 @FIXME{Is there a better way to frob the spacing on the list?}
10551 If you don't specify a @var{tapename}, @command{mt} uses the environment
10552 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
10553 the default device specified in your @file{sys/mtio.h} file
10554 (@code{DEFTAPE} variable). If this is not defined, the program will
10555 display a descriptive error message and exit with code 1.
10557 @command{mt} returns a 0 exit status when the operation(s) were
10558 successful, 1 if the command was unrecognized, and 2 if an operation
10561 @node Using Multiple Tapes
10562 @section Using Multiple Tapes
10564 Often you might want to write a large archive, one larger than will fit
10565 on the actual tape you are using. In such a case, you can run multiple
10566 @command{tar} commands, but this can be inconvenient, particularly if you
10567 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
10568 Therefore, @command{tar} provides a special mode for creating
10569 multi-volume archives.
10571 @dfn{Multi-volume} archive is a single @command{tar} archive, stored
10572 on several media volumes of fixed size. Although in this section we will
10573 often call @samp{volume} a @dfn{tape}, there is absolutely no
10574 requirement for multi-volume archives to be stored on tapes. Instead,
10575 they can use whatever media type the user finds convenient, they can
10576 even be located on files.
10578 When creating a multi-volume archive, @GNUTAR{} continues to fill
10579 current volume until it runs out of space, then it switches to
10580 next volume (usually the operator is queried to replace the tape on
10581 this point), and continues working on the new volume. This operation
10582 continues until all requested files are dumped. If @GNUTAR{} detects
10583 end of media while dumping a file, such a file is archived in split
10584 form. Some very big files can even be split across several volumes.
10586 Each volume is itself a valid @GNUTAR{} archive, so it can be read
10587 without any special options. Consequently any file member residing
10588 entirely on one volume can be extracted or otherwise operated upon
10589 without needing the other volume. Sure enough, to extract a split
10590 member you would need all volumes its parts reside on.
10592 Multi-volume archives suffer from several limitations. In particular,
10593 they cannot be compressed.
10595 @GNUTAR{} is able to create multi-volume archives of two formats
10596 (@pxref{Formats}): @samp{GNU} and @samp{POSIX}.
10599 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
10600 * Tape Files:: Tape Files
10601 * Tarcat:: Concatenate Volumes into a Single Archive
10605 @node Multi-Volume Archives
10606 @subsection Archives Longer than One Tape or Disk
10607 @cindex Multi-volume archives
10609 @opindex multi-volume
10610 To create an archive that is larger than will fit on a single unit of
10611 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
10612 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
10613 archive can be manipulated like any other archive (provided the
10614 @option{--multi-volume} option is specified), but is stored on more
10615 than one tape or disk.
10617 When you specify @option{--multi-volume}, @command{tar} does not report an
10618 error when it comes to the end of an archive volume (when reading), or
10619 the end of the media (when writing). Instead, it prompts you to load
10620 a new storage volume. If the archive is on a magnetic tape, you
10621 should change tapes when you see the prompt; if the archive is on a
10622 floppy disk, you should change disks; etc.
10625 @item --multi-volume
10627 Creates a multi-volume archive, when used in conjunction with
10628 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
10629 archive, specify @option{--multi-volume} in conjunction with that
10634 $ @kbd{tar --create --multi-volume --file=/dev/tape @var{files}}
10638 The method @command{tar} uses to detect end of tape is not perfect, and
10639 fails on some operating systems or on some devices. If @command{tar}
10640 cannot detect the end of the tape itself, you can use
10641 @option{--tape-length} option to inform it about the capacity of the
10644 @anchor{tape-length}
10646 @opindex tape-length
10647 @item --tape-length=@var{size}
10648 @itemx -L @var{size}
10649 Set maximum length of a volume. The @var{size} argument should then
10650 be the usable size of the tape in units of 1024 bytes. This option
10651 selects @option{--multi-volume} automatically. For example:
10654 $ @kbd{tar --create --tape-length=41943040 --file=/dev/tape @var{files}}
10658 @anchor{change volume prompt}
10659 When @GNUTAR{} comes to the end of a storage media, it asks you to
10660 change the volume. The built-in prompt for POSIX locale
10661 is@footnote{If you run @GNUTAR{} under a different locale, the
10662 translation to the locale's language will be used.}:
10665 Prepare volume #@var{n} for `@var{archive}' and hit return:
10669 where @var{n} is the ordinal number of the volume to be created and
10670 @var{archive} is archive file or device name.
10672 When prompting for a new tape, @command{tar} accepts any of the following
10677 Request @command{tar} to explain possible responses
10679 Request @command{tar} to exit immediately.
10680 @item n @var{file-name}
10681 Request @command{tar} to write the next volume on the file @var{file-name}.
10683 Request @command{tar} to run a subshell. This option can be disabled
10684 by giving @option{--restrict} command line option to
10685 @command{tar}@footnote{@xref{--restrict}, for more information about
10688 Request @command{tar} to begin writing the next volume.
10691 (You should only type @samp{y} after you have changed the tape;
10692 otherwise @command{tar} will write over the volume it just finished.)
10694 @cindex Volume number file
10696 @anchor{volno-file}
10697 @opindex volno-file
10698 The volume number used by @command{tar} in its tape-changing prompt
10699 can be changed; if you give the
10700 @option{--volno-file=@var{file-of-number}} option, then
10701 @var{file-of-number} should be an non-existing file to be created, or
10702 else, a file already containing a decimal number. That number will be
10703 used as the volume number of the first volume written. When
10704 @command{tar} is finished, it will rewrite the file with the
10705 now-current volume number. (This does not change the volume number
10706 written on a tape label, as per @ref{label}, it @emph{only} affects
10707 the number used in the prompt.)
10709 @cindex End-of-archive info script
10710 @cindex Info script
10711 @anchor{info-script}
10712 @opindex info-script
10713 @opindex new-volume-script
10714 If you want more elaborate behavior than this, you can write a special
10715 @dfn{new volume script}, that will be responsible for changing the
10716 volume, and instruct @command{tar} to use it instead of its normal
10717 prompting procedure:
10720 @item --info-script=@var{script-name}
10721 @itemx --new-volume-script=@var{script-name}
10722 @itemx -F @var{script-name}
10723 Specify the full name of the volume script to use. The script can be
10724 used to eject cassettes, or to broadcast messages such as
10725 @samp{Someone please come change my tape} when performing unattended
10729 The @var{script-name} is executed without any command line
10730 arguments. It inherits @command{tar}'s shell environment.
10731 Additional data is passed to it via the following
10732 environment variables:
10735 @vrindex TAR_VERSION, info script environment variable
10737 @GNUTAR{} version number.
10739 @vrindex TAR_ARCHIVE, info script environment variable
10741 The name of the archive @command{tar} is processing.
10743 @vrindex TAR_BLOCKING_FACTOR, info script environment variable
10744 @item TAR_BLOCKING_FACTOR
10745 Current blocking factor (@pxref{Blocking}.
10747 @vrindex TAR_VOLUME, info script environment variable
10749 Ordinal number of the volume @command{tar} is about to start.
10751 @vrindex TAR_SUBCOMMAND, info script environment variable
10752 @item TAR_SUBCOMMAND
10753 A short option describing the operation @command{tar} is executing
10754 @xref{Operations}, for a complete list of subcommand options.
10756 @vrindex TAR_FORMAT, info script environment variable
10758 Format of the archive being processed. @xref{Formats}, for a complete
10759 list of archive format names.
10761 @vrindex TAR_FD, info script environment variable
10763 File descriptor which can be used to communicate the new volume
10764 name to @command{tar}.
10767 The volume script can instruct @command{tar} to use new archive name,
10768 by writing in to file descriptor @env{$TAR_FD} (see below for an example).
10770 If the info script fails, @command{tar} exits; otherwise, it begins
10771 writing the next volume.
10773 If you want @command{tar} to cycle through a series of files or tape
10774 drives, there are three approaches to choose from. First of all, you
10775 can give @command{tar} multiple @option{--file} options. In this case
10776 the specified files will be used, in sequence, as the successive
10777 volumes of the archive. Only when the first one in the sequence needs
10778 to be used again will @command{tar} prompt for a tape change (or run
10779 the info script). For example, suppose someone has two tape drives on
10780 a system named @file{/dev/tape0} and @file{/dev/tape1}. For having
10781 @GNUTAR{} to switch to the second drive when it needs to write the
10782 second tape, and then back to the first tape, etc., just do either of:
10785 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
10786 $ @kbd{tar cMff /dev/tape0 /dev/tape1 @var{files}}
10789 The second method is to use the @samp{n} response to the tape-change
10792 Finally, the most flexible approach is to use a volume script, that
10793 writes new archive name to the file descriptor @env{$TAR_FD}. For example, the
10794 following volume script will create a series of archive files, named
10795 @file{@var{archive}-@var{vol}}, where @var{archive} is the name of the
10796 archive being created (as given by @option{--file} option) and
10797 @var{vol} is the ordinal number of the archive being created:
10802 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
10804 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
10805 case $TAR_SUBCOMMAND in
10807 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
10812 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&$TAR_FD
10816 The same script can be used while listing, comparing or extracting
10817 from the created archive. For example:
10821 # @r{Create a multi-volume archive:}
10822 $ @kbd{tar -c -L1024 -f archive.tar -F new-volume .}
10823 # @r{Extract from the created archive:}
10824 $ @kbd{tar -x -f archive.tar -F new-volume .}
10829 Notice, that the first command had to use @option{-L} option, since
10830 otherwise @GNUTAR{} will end up writing everything to file
10831 @file{archive.tar}.
10833 You can read each individual volume of a multi-volume archive as if it
10834 were an archive by itself. For example, to list the contents of one
10835 volume, use @option{--list}, without @option{--multi-volume} specified.
10836 To extract an archive member from one volume (assuming it is described
10837 that volume), use @option{--extract}, again without
10838 @option{--multi-volume}.
10840 If an archive member is split across volumes (i.e., its entry begins on
10841 one volume of the media and ends on another), you need to specify
10842 @option{--multi-volume} to extract it successfully. In this case, you
10843 should load the volume where the archive member starts, and use
10844 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
10845 volumes as it needs them. @xref{extracting archives}, for more
10846 information about extracting archives.
10848 Multi-volume archives can be modified like any other archive. To add
10849 files to a multi-volume archive, you need to only mount the last
10850 volume of the archive media (and new volumes, if needed). For all
10851 other operations, you need to use the entire archive.
10853 If a multi-volume archive was labeled using
10854 @option{--label=@var{archive-label}} (@pxref{label}) when it was
10855 created, @command{tar} will not automatically label volumes which are
10856 added later. To label subsequent volumes, specify
10857 @option{--label=@var{archive-label}} again in conjunction with the
10858 @option{--append}, @option{--update} or @option{--concatenate} operation.
10860 Notice that multi-volume support is a GNU extension and the archives
10861 created in this mode should be read only using @GNUTAR{}. If you
10862 absolutely have to process such archives using a third-party @command{tar}
10863 implementation, read @ref{Split Recovery}.
10866 @subsection Tape Files
10869 To give the archive a name which will be recorded in it, use the
10870 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
10871 option. This will write a special block identifying
10872 @var{volume-label} as the name of the archive to the front of the
10873 archive which will be displayed when the archive is listed with
10874 @option{--list}. If you are creating a multi-volume archive with
10875 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
10876 volume label will have @samp{Volume @var{nnn}} appended to the name
10877 you give, where @var{nnn} is the number of the volume of the archive.
10878 (If you use the @option{--label=@var{volume-label}}) option when
10879 reading an archive, it checks to make sure the label on the tape
10880 matches the one you give. @xref{label}.
10882 When @command{tar} writes an archive to tape, it creates a single
10883 tape file. If multiple archives are written to the same tape, one
10884 after the other, they each get written as separate tape files. When
10885 extracting, it is necessary to position the tape at the right place
10886 before running @command{tar}. To do this, use the @command{mt} command.
10887 For more information on the @command{mt} command and on the organization
10888 of tapes into a sequence of tape files, see @ref{mt}.
10890 People seem to often do:
10893 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
10896 or such, for pushing a common date in all volumes or an archive set.
10899 @subsection Concatenate Volumes into a Single Archive
10902 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
10903 archive to a single @command{tar} archive. Simply concatenating all
10904 volumes into one will not work, since each volume carries an additional
10905 information at the beginning. @GNUTAR{} is shipped with the shell
10906 script @command{tarcat} designed for this purpose.
10908 The script takes a list of files comprising a multi-volume archive
10909 and creates the resulting archive at the standard output. For example:
10912 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
10915 The script implements a simple heuristics to determine the format of
10916 the first volume file and to decide how to process the rest of the
10917 files. However, it makes no attempt to verify whether the files are
10918 given in order or even if they are valid @command{tar} archives.
10919 It uses @command{dd} and does not filter its standard error, so you
10920 will usually see lots of spurious messages.
10922 @FIXME{The script is not installed. Should we install it?}
10925 @section Including a Label in the Archive
10926 @cindex Labeling an archive
10927 @cindex Labels on the archive media
10928 @cindex Labeling multi-volume archives
10932 To avoid problems caused by misplaced paper labels on the archive
10933 media, you can include a @dfn{label} entry---an archive member which
10934 contains the name of the archive---in the archive itself. Use the
10935 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
10936 option in conjunction with the @option{--create} operation to include
10937 a label entry in the archive as it is being created.
10940 @item --label=@var{archive-label}
10941 @itemx -V @var{archive-label}
10942 Includes an @dfn{archive-label} at the beginning of the archive when
10943 the archive is being created, when used in conjunction with the
10944 @option{--create} operation. Checks to make sure the archive label
10945 matches the one specified (when used in conjunction with any other
10949 If you create an archive using both
10950 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
10951 and @option{--multi-volume} (@option{-M}), each volume of the archive
10952 will have an archive label of the form @samp{@var{archive-label}
10953 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
10954 next, and so on. @xref{Using Multiple Tapes}, for information on
10955 creating multiple volume archives.
10957 @cindex Volume label, listing
10958 @cindex Listing volume label
10959 The volume label will be displayed by @option{--list} along with
10960 the file contents. If verbose display is requested, it will also be
10961 explicitly marked as in the example below:
10965 $ @kbd{tar --verbose --list --file=iamanarchive}
10966 V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header--
10967 -rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename
10971 @opindex test-label
10972 @anchor{--test-label option}
10973 However, @option{--list} option will cause listing entire
10974 contents of the archive, which may be undesirable (for example, if the
10975 archive is stored on a tape). You can request checking only the volume
10976 by specifying @option{--test-label} option. This option reads only the
10977 first block of an archive, so it can be used with slow storage
10978 devices. For example:
10982 $ @kbd{tar --test-label --file=iamanarchive}
10987 If @option{--test-label} is used with a single command line
10988 argument, @command{tar} compares the volume label with the
10989 argument. It exits with code 0 if the two strings match, and with code
10990 2 otherwise. In this case no output is displayed. For example:
10994 $ @kbd{tar --test-label --file=iamanarchive 'iamalable'}
10996 $ @kbd{tar --test-label --file=iamanarchive 'iamalable' alabel}
11001 If you request any operation, other than @option{--create}, along
11002 with using @option{--label} option, @command{tar} will first check if
11003 the archive label matches the one specified and will refuse to proceed
11004 if it does not. Use this as a safety precaution to avoid accidentally
11005 overwriting existing archives. For example, if you wish to add files
11006 to @file{archive}, presumably labeled with string @samp{My volume},
11011 $ @kbd{tar -rf archive --label 'My volume' .}
11012 tar: Archive not labeled to match `My volume'
11017 in case its label does not match. This will work even if
11018 @file{archive} is not labeled at all.
11020 Similarly, @command{tar} will refuse to list or extract the
11021 archive if its label doesn't match the @var{archive-label}
11022 specified. In those cases, @var{archive-label} argument is interpreted
11023 as a globbing-style pattern which must match the actual magnetic
11024 volume label. @xref{exclude}, for a precise description of how match
11025 is attempted@footnote{Previous versions of @command{tar} used full
11026 regular expression matching, or before that, only exact string
11027 matching, instead of wildcard matchers. We decided for the sake of
11028 simplicity to use a uniform matching device through
11029 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
11030 the volume label matcher will also suffix @var{archive-label} by
11031 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
11032 up. Since the volume numbering is automatically added in labels at
11033 creation time, it sounded logical to equally help the user taking care
11034 of it when the archive is being read.
11036 The @option{--label} was once called @option{--volume}, but is not
11037 available under that name anymore.
11039 You can also use @option{--label} to get a common information on
11040 all tapes of a series. For having this information different in each
11041 series created through a single script used on a regular basis, just
11042 manage to get some date string as part of the label. For example:
11046 $ @kbd{tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"}
11047 $ @kbd{tar --create --file=/dev/tape --multi-volume \
11048 --volume="Daily backup for `date +%Y-%m-%d`"}
11052 Also note that each label has its own date and time, which corresponds
11053 to when @GNUTAR{} initially attempted to write it,
11054 often soon after the operator launches @command{tar} or types the
11055 carriage return telling that the next tape is ready. Comparing date
11056 labels does give an idea of tape throughput only if the delays for
11057 rewinding tapes and the operator switching them were negligible, which
11058 is usually not the case.
11061 @section Verifying Data as It is Stored
11062 @cindex Verifying a write operation
11063 @cindex Double-checking a write operation
11068 @opindex verify, short description
11069 Attempt to verify the archive after writing.
11072 This option causes @command{tar} to verify the archive after writing it.
11073 Each volume is checked after it is written, and any discrepancies
11074 are recorded on the standard error output.
11076 Verification requires that the archive be on a back-space-able medium.
11077 This means pipes, some cartridge tape drives, and some other devices
11078 cannot be verified.
11080 You can insure the accuracy of an archive by comparing files in the
11081 system with archive members. @command{tar} can compare an archive to the
11082 file system as the archive is being written, to verify a write
11083 operation, or can compare a previously written archive, to insure that
11086 @xopindex{verify, using with @option{--create}}
11087 @xopindex{create, using with @option{--verify}}
11088 To check for discrepancies in an archive immediately after it is
11089 written, use the @option{--verify} (@option{-W}) option in conjunction with
11090 the @option{--create} operation. When this option is
11091 specified, @command{tar} checks archive members against their counterparts
11092 in the file system, and reports discrepancies on the standard error.
11094 To verify an archive, you must be able to read it from before the end
11095 of the last written entry. This option is useful for detecting data
11096 errors on some tapes. Archives written to pipes, some cartridge tape
11097 drives, and some other devices cannot be verified.
11099 One can explicitly compare an already made archive with the file
11100 system by using the @option{--compare} (@option{--diff}, @option{-d})
11101 option, instead of using the more automatic @option{--verify} option.
11104 Note that these two options have a slightly different intent. The
11105 @option{--compare} option checks how identical are the logical contents of some
11106 archive with what is on your disks, while the @option{--verify} option is
11107 really for checking if the physical contents agree and if the recording
11108 media itself is of dependable quality. So, for the @option{--verify}
11109 operation, @command{tar} tries to defeat all in-memory cache pertaining to
11110 the archive, while it lets the speed optimization undisturbed for the
11111 @option{--compare} option. If you nevertheless use @option{--compare} for
11112 media verification, you may have to defeat the in-memory cache yourself,
11113 maybe by opening and reclosing the door latch of your recording unit,
11114 forcing some doubt in your operating system about the fact this is really
11115 the same volume as the one just written or read.
11117 The @option{--verify} option would not be necessary if drivers were indeed
11118 able to detect dependably all write failures. This sometimes require many
11119 magnetic heads, some able to read after the writes occurred. One would
11120 not say that drivers unable to detect all cases are necessarily flawed,
11121 as long as programming is concerned.
11123 The @option{--verify} (@option{-W}) option will not work in
11124 conjunction with the @option{--multi-volume} (@option{-M}) option or
11125 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
11126 and @option{--delete} operations. @xref{Operations}, for more
11127 information on these operations.
11129 Also, since @command{tar} normally strips leading @samp{/} from file
11130 names (@pxref{absolute}), a command like @samp{tar --verify -cf
11131 /tmp/foo.tar /etc} will work as desired only if the working directory is
11132 @file{/}, as @command{tar} uses the archive's relative member names
11133 (e.g., @file{etc/motd}) when verifying the archive.
11135 @node Write Protection
11136 @section Write Protection
11138 Almost all tapes and diskettes, and in a few rare cases, even disks can
11139 be @dfn{write protected}, to protect data on them from being changed.
11140 Once an archive is written, you should write protect the media to prevent
11141 the archive from being accidentally overwritten or deleted. (This will
11142 protect the archive from being changed with a tape or floppy drive---it
11143 will not protect it from magnet fields or other physical hazards).
11145 The write protection device itself is usually an integral part of the
11146 physical media, and can be a two position (write enabled/write
11147 disabled) switch, a notch which can be popped out or covered, a ring
11148 which can be removed from the center of a tape reel, or some other
11149 changeable feature.
11154 This appendix lists some important user-visible changes between
11155 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
11156 version of this document is available at
11157 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
11158 @GNUTAR{} documentation page}.
11161 @item Use of globbing patterns when listing and extracting.
11163 Previous versions of GNU tar assumed shell-style globbing when
11164 extracting from or listing an archive. For example:
11167 $ @kbd{tar xf foo.tar '*.c'}
11170 would extract all files whose names end in @samp{.c}. This behavior
11171 was not documented and was incompatible with traditional tar
11172 implementations. Therefore, starting from version 1.15.91, GNU tar
11173 no longer uses globbing by default. For example, the above invocation
11174 is now interpreted as a request to extract from the archive the file
11177 To facilitate transition to the new behavior for those users who got
11178 used to the previous incorrect one, @command{tar} will print a warning
11179 if it finds out that a requested member was not found in the archive
11180 and its name looks like a globbing pattern. For example:
11183 $ @kbd{tar xf foo.tar '*.c'}
11184 tar: Pattern matching characters used in file names. Please,
11185 tar: use --wildcards to enable pattern matching, or --no-wildcards to
11186 tar: suppress this warning.
11187 tar: *.c: Not found in archive
11188 tar: Error exit delayed from previous errors
11191 To treat member names as globbing patterns, use --wildcards option.
11192 If you want to tar to mimic the behavior of versions prior to 1.15.91,
11193 add this option to your @env{TAR_OPTIONS} variable.
11195 @xref{wildcards}, for the detailed discussion of the use of globbing
11196 patterns by @GNUTAR{}.
11198 @item Use of short option @option{-o}.
11200 Earlier versions of @GNUTAR{} understood @option{-o} command line
11201 option as a synonym for @option{--old-archive}.
11203 @GNUTAR{} starting from version 1.13.90 understands this option as
11204 a synonym for @option{--no-same-owner}. This is compatible with
11205 UNIX98 @command{tar} implementations.
11207 However, to facilitate transition, @option{-o} option retains its
11208 old semantics when it is used with one of archive-creation commands.
11209 Users are encouraged to use @option{--format=oldgnu} instead.
11211 It is especially important, since versions of @acronym{GNU} Automake
11212 up to and including 1.8.4 invoke tar with this option to produce
11213 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
11214 of this issue and its implications.
11216 @FIXME{Change the first argument to tar-formats when the new Automake is
11217 out. The proposition to add @anchor{} to the appropriate place of its
11218 docs was accepted by Automake people --Sergey 2006-05-25}.
11219 @xref{Options, tar-v7, Changing Automake's Behavior,
11220 automake, GNU Automake}, for a description on how to use various
11221 archive formats with @command{automake}.
11223 Future versions of @GNUTAR{} will understand @option{-o} only as a
11224 synonym for @option{--no-same-owner}.
11226 @item Use of short option @option{-l}
11228 Earlier versions of @GNUTAR{} understood @option{-l} option as a
11229 synonym for @option{--one-file-system}. Since such usage contradicted
11230 to UNIX98 specification and harmed compatibility with other
11231 implementation, it was declared deprecated in version 1.14. However,
11232 to facilitate transition to its new semantics, it was supported by
11233 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
11234 variant of @option{--check-links} was introduced in version 1.15.91.
11236 @item Use of options @option{--portability} and @option{--old-archive}
11238 These options are deprecated. Please use @option{--format=v7} instead.
11240 @item Use of option @option{--posix}
11242 This option is deprecated. Please use @option{--format=posix} instead.
11245 @node Configuring Help Summary
11246 @appendix Configuring Help Summary
11248 Running @kbd{tar --help} displays the short @command{tar} option
11249 summary (@pxref{help}). This summary is organized by @dfn{groups} of
11250 semantically close options. The options within each group are printed
11251 in the following order: a short option, eventually followed by a list
11252 of corresponding long option names, followed by a short description of
11253 the option. For example, here is an excerpt from the actual @kbd{tar
11257 Main operation mode:
11259 -A, --catenate, --concatenate append tar files to an archive
11260 -c, --create create a new archive
11261 -d, --diff, --compare find differences between archive and
11263 --delete delete from the archive
11266 @vrindex ARGP_HELP_FMT, environment variable
11267 The exact visual representation of the help output is configurable via
11268 @env{ARGP_HELP_FMT} environment variable. The value of this variable
11269 is a comma-separated list of @dfn{format variable} assignments. There
11270 are two kinds of format variables. An @dfn{offset variable} keeps the
11271 offset of some part of help output text from the leftmost column on
11272 the screen. A @dfn{boolean} variable is a flag that toggles some
11273 output feature on or off. Depending on the type of the corresponding
11274 variable, there are two kinds of assignments:
11277 @item Offset assignment
11279 The assignment to an offset variable has the following syntax:
11282 @var{variable}=@var{value}
11286 where @var{variable} is the variable name, and @var{value} is a
11287 numeric value to be assigned to the variable.
11289 @item Boolean assignment
11291 To assign @code{true} value to a variable, simply put this variable name. To
11292 assign @code{false} value, prefix the variable name with @samp{no-}. For
11297 # Assign @code{true} value:
11299 # Assign @code{false} value:
11305 Following variables are declared:
11307 @deftypevr {Help Output} boolean dup-args
11308 If true, arguments for an option are shown with both short and long
11309 options, even when a given option has both forms, for example:
11312 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
11315 If false, then if an option has both short and long forms, the
11316 argument is only shown with the long one, for example:
11319 -f, --file=ARCHIVE use archive file or device ARCHIVE
11323 and a message indicating that the argument is applicable to both
11324 forms is printed below the options. This message can be disabled
11325 using @code{dup-args-note} (see below).
11327 The default is false.
11330 @deftypevr {Help Output} boolean dup-args-note
11331 If this variable is true, which is the default, the following notice
11332 is displayed at the end of the help output:
11335 Mandatory or optional arguments to long options are also mandatory or
11336 optional for any corresponding short options.
11339 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
11340 variables @code{dup-args} or @code{dup-args-note} should be set.
11343 @deftypevr {Help Output} offset short-opt-col
11344 Column in which short options start. Default is 2.
11348 $ @kbd{tar --help|grep ARCHIVE}
11349 -f, --file=ARCHIVE use archive file or device ARCHIVE
11350 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
11351 -f, --file=ARCHIVE use archive file or device ARCHIVE
11356 @deftypevr {Help Output} offset long-opt-col
11357 Column in which long options start. Default is 6. For example:
11361 $ @kbd{tar --help|grep ARCHIVE}
11362 -f, --file=ARCHIVE use archive file or device ARCHIVE
11363 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
11364 -f, --file=ARCHIVE use archive file or device ARCHIVE
11369 @deftypevr {Help Output} offset doc-opt-col
11370 Column in which @dfn{doc options} start. A doc option isn't actually
11371 an option, but rather an arbitrary piece of documentation that is
11372 displayed in much the same manner as the options. For example, in
11373 the description of @option{--format} option:
11377 -H, --format=FORMAT create archive of the given format.
11379 FORMAT is one of the following:
11381 gnu GNU tar 1.13.x format
11382 oldgnu GNU format as per tar <= 1.12
11383 pax POSIX 1003.1-2001 (pax) format
11385 ustar POSIX 1003.1-1988 (ustar) format
11386 v7 old V7 tar format
11391 the format names are doc options. Thus, if you set
11392 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
11393 will look as follows:
11397 -H, --format=FORMAT create archive of the given format.
11399 FORMAT is one of the following:
11401 gnu GNU tar 1.13.x format
11402 oldgnu GNU format as per tar <= 1.12
11403 pax POSIX 1003.1-2001 (pax) format
11405 ustar POSIX 1003.1-1988 (ustar) format
11406 v7 old V7 tar format
11411 @deftypevr {Help Output} offset opt-doc-col
11412 Column in which option description starts. Default is 29.
11416 $ @kbd{tar --help|grep ARCHIVE}
11417 -f, --file=ARCHIVE use archive file or device ARCHIVE
11418 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
11419 -f, --file=ARCHIVE use archive file or device ARCHIVE
11420 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
11422 use archive file or device ARCHIVE
11427 Notice, that the description starts on a separate line if
11428 @code{opt-doc-col} value is too small.
11431 @deftypevr {Help Output} offset header-col
11432 Column in which @dfn{group headers} are printed. A group header is a
11433 descriptive text preceding an option group. For example, in the
11437 Main operation mode:
11439 -A, --catenate, --concatenate append tar files to
11441 -c, --create create a new archive
11444 @samp{Main operation mode:} is the group header.
11446 The default value is 1.
11449 @deftypevr {Help Output} offset usage-indent
11450 Indentation of wrapped usage lines. Affects @option{--usage}
11451 output. Default is 12.
11454 @deftypevr {Help Output} offset rmargin
11455 Right margin of the text output. Used for wrapping.
11458 @node Fixing Snapshot Files
11459 @appendix Fixing Snapshot Files
11460 @include tar-snapshot-edit.texi
11462 @node Tar Internals
11463 @appendix Tar Internals
11464 @include intern.texi
11468 @include genfile.texi
11470 @node Free Software Needs Free Documentation
11471 @appendix Free Software Needs Free Documentation
11472 @include freemanuals.texi
11474 @node Copying This Manual
11475 @appendix Copying This Manual
11478 * GNU Free Documentation License:: License for copying this manual
11483 @node Index of Command Line Options
11484 @appendix Index of Command Line Options
11486 This appendix contains an index of all @GNUTAR{} long command line
11487 options. The options are listed without the preceding double-dash.
11488 For a cross-reference of short command line options, @ref{Short Option Summary}.
11501 @c Local variables:
11502 @c texinfo-column-for-description: 32
projects / chaz / tar / blob