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. @xref{gzip}.
2413 @item --backup=@var{backup-type}
2415 Rather than deleting files from the file system, @command{tar} will
2416 back them up using simple or numbered backups, depending upon
2417 @var{backup-type}. @xref{backup}.
2419 @opsummary{block-number}
2420 @item --block-number
2423 With this option present, @command{tar} prints error messages for read errors
2424 with the block number in the archive file. @xref{block-number}.
2426 @opsummary{blocking-factor}
2427 @item --blocking-factor=@var{blocking}
2428 @itemx -b @var{blocking}
2430 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2431 record. @xref{Blocking Factor}.
2437 This option tells @command{tar} to read or write archives through
2438 @code{bzip2}. @xref{gzip}.
2440 @opsummary{check-device}
2441 @item --check-device
2442 Check device numbers when creating a list of modified files for
2443 incremental archiving. This is the default. @xref{device numbers},
2444 for a detailed description.
2446 @opsummary{checkpoint}
2447 @item --checkpoint[=@var{number}]
2449 This option directs @command{tar} to print periodic checkpoint
2450 messages as it reads through the archive. It is intended for when you
2451 want a visual indication that @command{tar} is still running, but
2452 don't want to see @option{--verbose} output. You can also instruct
2453 @command{tar} to execute a list of actions on each checkpoint, see
2454 @option{--checklist-action} below. For a detailed description, see
2457 @opsummary{checkpoint-action}
2458 @item --checkpoint-action=@var{action}
2459 Instruct @command{tar} to execute an action upon hitting a
2460 breakpoint. Here we give only a brief outline. @xref{checkpoints},
2461 for a complete description.
2463 The @var{action} argument can be one of the following:
2467 Produce an audible bell on the console.
2471 Print a single dot on the standard listing stream.
2474 Display a textual message on the standard error, with the status and
2475 number of the checkpoint. This is the default.
2477 @item echo=@var{string}
2478 Display @var{string} on the standard error. Before output, the string
2479 is subject to meta-character expansion.
2481 @item exec=@var{command}
2482 Execute the given @var{command}.
2484 @item sleep=@var{time}
2485 Wait for @var{time} seconds.
2487 @item ttyout=@var{string}
2488 Output @var{string} on the current console (@file{/dev/tty}).
2491 Several @option{--checkpoint-action} options can be specified. The
2492 supplied actions will be executed in order of their appearance in the
2495 Using @option{--checkpoint-action} without @option{--checkpoint}
2496 assumes default checkpoint frequency of one checkpoint per 10 records.
2498 @opsummary{check-links}
2501 If this option was given, @command{tar} will check the number of links
2502 dumped for each processed file. If this number does not match the
2503 total number of hard links for the file, a warning message will be
2504 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2505 synonym for @option{--one-file-system}. The current semantics, which
2506 complies to UNIX98, was introduced with version
2507 1.15.91. @xref{Changes}, for more information.}.
2511 @opsummary{compress}
2512 @opsummary{uncompress}
2517 @command{tar} will use the @command{compress} program when reading or
2518 writing the archive. This allows you to directly act on archives
2519 while saving space. @xref{gzip}.
2521 @opsummary{confirmation}
2522 @item --confirmation
2524 (See @option{--interactive}.) @xref{interactive}.
2526 @opsummary{delay-directory-restore}
2527 @item --delay-directory-restore
2529 Delay setting modification times and permissions of extracted
2530 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2532 @opsummary{dereference}
2536 When creating a @command{tar} archive, @command{tar} will archive the
2537 file that a symbolic link points to, rather than archiving the
2538 symlink. @xref{dereference}.
2540 @opsummary{directory}
2541 @item --directory=@var{dir}
2544 When this option is specified, @command{tar} will change its current directory
2545 to @var{dir} before performing any operations. When this option is used
2546 during archive creation, it is order sensitive. @xref{directory}.
2549 @item --exclude=@var{pattern}
2551 When performing operations, @command{tar} will skip files that match
2552 @var{pattern}. @xref{exclude}.
2554 @opsummary{exclude-from}
2555 @item --exclude-from=@var{file}
2556 @itemx -X @var{file}
2558 Similar to @option{--exclude}, except @command{tar} will use the list of
2559 patterns in the file @var{file}. @xref{exclude}.
2561 @opsummary{exclude-caches}
2562 @item --exclude-caches
2564 Exclude from dump any directory containing a valid cache directory
2565 tag file, but still dump the directory node and the tag file itself.
2569 @opsummary{exclude-caches-under}
2570 @item --exclude-caches-under
2572 Exclude from dump any directory containing a valid cache directory
2573 tag file, but still dump the directory node itself.
2577 @opsummary{exclude-caches-all}
2578 @item --exclude-caches-all
2580 Exclude from dump any directory containing a valid cache directory
2581 tag file. @xref{exclude}.
2583 @opsummary{exclude-tag}
2584 @item --exclude-tag=@var{file}
2586 Exclude from dump any directory containing file named @var{file}, but
2587 dump the directory node and @var{file} itself. @xref{exclude}.
2589 @opsummary{exclude-tag-under}
2590 @item --exclude-tag-under=@var{file}
2592 Exclude from dump the contents of any directory containing file
2593 named @var{file}, but dump the directory node itself. @xref{exclude}.
2595 @opsummary{exclude-tag-all}
2596 @item --exclude-tag-all=@var{file}
2598 Exclude from dump any directory containing file named @var{file}.
2601 @opsummary{exclude-vcs}
2604 Exclude from dump directories and files, that are internal for some
2605 widely used version control systems.
2610 @item --file=@var{archive}
2611 @itemx -f @var{archive}
2613 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2614 performs operations on, rather than @command{tar}'s compilation dependent
2615 default. @xref{file tutorial}.
2617 @opsummary{files-from}
2618 @item --files-from=@var{file}
2619 @itemx -T @var{file}
2621 @command{tar} will use the contents of @var{file} as a list of archive members
2622 or files to operate on, in addition to those specified on the
2623 command-line. @xref{files}.
2625 @opsummary{force-local}
2628 Forces @command{tar} to interpret the file name given to @option{--file}
2629 as a local file, even if it looks like a remote tape drive name.
2630 @xref{local and remote archives}.
2633 @item --format=@var{format}
2634 @itemx -H @var{format}
2636 Selects output archive format. @var{Format} may be one of the
2641 Creates an archive that is compatible with Unix V7 @command{tar}.
2644 Creates an archive that is compatible with GNU @command{tar} version
2648 Creates archive in GNU tar 1.13 format. Basically it is the same as
2649 @samp{oldgnu} with the only difference in the way it handles long
2653 Creates a @acronym{POSIX.1-1988} compatible archive.
2656 Creates a @acronym{POSIX.1-2001 archive}.
2660 @xref{Formats}, for a detailed discussion of these formats.
2663 @item --group=@var{group}
2665 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
2666 rather than the group from the source file. @var{group} is first decoded
2667 as a group symbolic name, but if this interpretation fails, it has to be
2668 a decimal numeric group @acronym{ID}. @xref{override}.
2670 Also see the comments for the @option{--owner=@var{user}} option.
2680 This option tells @command{tar} to read or write archives through
2681 @command{gzip}, allowing @command{tar} to directly operate on several
2682 kinds of compressed archives transparently. @xref{gzip}.
2684 @opsummary{hard-dereference}
2685 @item --hard-dereference
2686 When creating an archive, dereference hard links and store the files
2687 they refer to, instead of creating usual hard link members.
2695 @command{tar} will print out a short message summarizing the operations and
2696 options to @command{tar} and exit. @xref{help}.
2698 @opsummary{ignore-case}
2700 Ignore case when matching member or file names with
2701 patterns. @xref{controlling pattern-matching}.
2703 @opsummary{ignore-command-error}
2704 @item --ignore-command-error
2705 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2707 @opsummary{ignore-failed-read}
2708 @item --ignore-failed-read
2710 Do not exit unsuccessfully merely because an unreadable file was encountered.
2713 @opsummary{ignore-zeros}
2714 @item --ignore-zeros
2717 With this option, @command{tar} will ignore zeroed blocks in the
2718 archive, which normally signals EOF. @xref{Reading}.
2720 @opsummary{incremental}
2724 Informs @command{tar} that it is working with an old
2725 @acronym{GNU}-format incremental backup archive. It is intended
2726 primarily for backwards compatibility only. @xref{Incremental Dumps},
2727 for a detailed discussion of incremental archives.
2729 @opsummary{index-file}
2730 @item --index-file=@var{file}
2732 Send verbose output to @var{file} instead of to standard output.
2734 @opsummary{info-script}
2735 @opsummary{new-volume-script}
2736 @item --info-script=@var{script-file}
2737 @itemx --new-volume-script=@var{script-file}
2738 @itemx -F @var{script-file}
2740 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2741 at the end of each tape. If @var{script-file} exits with nonzero status,
2742 @command{tar} fails immediately. @xref{info-script}, for a detailed
2743 discussion of @var{script-file}.
2745 @opsummary{interactive}
2747 @itemx --confirmation
2750 Specifies that @command{tar} should ask the user for confirmation before
2751 performing potentially destructive options, such as overwriting files.
2754 @opsummary{keep-newer-files}
2755 @item --keep-newer-files
2757 Do not replace existing files that are newer than their archive copies
2758 when extracting files from an archive.
2760 @opsummary{keep-old-files}
2761 @item --keep-old-files
2764 Do not overwrite existing files when extracting files from an archive.
2765 @xref{Keep Old Files}.
2768 @item --label=@var{name}
2769 @itemx -V @var{name}
2771 When creating an archive, instructs @command{tar} to write @var{name}
2772 as a name record in the archive. When extracting or listing archives,
2773 @command{tar} will only operate on archives that have a label matching
2774 the pattern specified in @var{name}. @xref{Tape Files}.
2776 @opsummary{listed-incremental}
2777 @item --listed-incremental=@var{snapshot-file}
2778 @itemx -g @var{snapshot-file}
2780 During a @option{--create} operation, specifies that the archive that
2781 @command{tar} creates is a new @acronym{GNU}-format incremental
2782 backup, using @var{snapshot-file} to determine which files to backup.
2783 With other operations, informs @command{tar} that the archive is in
2784 incremental format. @xref{Incremental Dumps}.
2789 This option tells @command{tar} to read or write archives through
2790 @command{lzma}. @xref{gzip}.
2793 @item --mode=@var{permissions}
2795 When adding files to an archive, @command{tar} will use
2796 @var{permissions} for the archive members, rather than the permissions
2797 from the files. @var{permissions} can be specified either as an octal
2798 number or as symbolic permissions, like with
2799 @command{chmod}. @xref{override}.
2802 @item --mtime=@var{date}
2804 When adding files to an archive, @command{tar} will use @var{date} as
2805 the modification time of members when creating archives, instead of
2806 their actual modification times. The value of @var{date} can be
2807 either a textual date representation (@pxref{Date input formats}) or a
2808 name of the existing file, starting with @samp{/} or @samp{.}. In the
2809 latter case, the modification time of that file is used. @xref{override}.
2811 @opsummary{multi-volume}
2812 @item --multi-volume
2815 Informs @command{tar} that it should create or otherwise operate on a
2816 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2818 @opsummary{new-volume-script}
2819 @item --new-volume-script
2824 @item --newer=@var{date}
2825 @itemx --after-date=@var{date}
2828 When creating an archive, @command{tar} will only add files that have changed
2829 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2830 is taken to be the name of a file whose data modification time specifies
2831 the date. @xref{after}.
2833 @opsummary{newer-mtime}
2834 @item --newer-mtime=@var{date}
2836 Like @option{--newer}, but add only files whose
2837 contents have changed (as opposed to just @option{--newer}, which will
2838 also back up files for which any status information has
2839 changed). @xref{after}.
2841 @opsummary{no-anchored}
2843 An exclude pattern can match any subsequence of the name's components.
2844 @xref{controlling pattern-matching}.
2846 @opsummary{no-check-device}
2847 @item --no-check-device
2848 Do not check device numbers when creating a list of modified files
2849 for incremental archiving. @xref{device numbers}, for
2850 a detailed description.
2852 @opsummary{no-delay-directory-restore}
2853 @item --no-delay-directory-restore
2855 Modification times and permissions of extracted
2856 directories are set when all files from this directory have been
2857 extracted. This is the default.
2858 @xref{Directory Modification Times and Permissions}.
2860 @opsummary{no-ignore-case}
2861 @item --no-ignore-case
2862 Use case-sensitive matching.
2863 @xref{controlling pattern-matching}.
2865 @opsummary{no-ignore-command-error}
2866 @item --no-ignore-command-error
2867 Print warnings about subprocesses that terminated with a nonzero exit
2868 code. @xref{Writing to an External Program}.
2870 @opsummary{no-overwrite-dir}
2871 @item --no-overwrite-dir
2873 Preserve metadata of existing directories when extracting files
2874 from an archive. @xref{Overwrite Old Files}.
2876 @opsummary{no-quote-chars}
2877 @item --no-quote-chars=@var{string}
2878 Remove characters listed in @var{string} from the list of quoted
2879 characters set by the previous @option{--quote-chars} option
2880 (@pxref{quoting styles}).
2882 @opsummary{no-recursion}
2883 @item --no-recursion
2885 With this option, @command{tar} will not recurse into directories.
2888 @opsummary{no-same-owner}
2889 @item --no-same-owner
2892 When extracting an archive, do not attempt to preserve the owner
2893 specified in the @command{tar} archive. This the default behavior
2896 @opsummary{no-same-permissions}
2897 @item --no-same-permissions
2899 When extracting an archive, subtract the user's umask from files from
2900 the permissions specified in the archive. This is the default behavior
2903 @opsummary{no-unquote}
2905 Treat all input file or member names literally, do not interpret
2906 escape sequences. @xref{input name quoting}.
2908 @opsummary{no-wildcards}
2909 @item --no-wildcards
2910 Do not use wildcards.
2911 @xref{controlling pattern-matching}.
2913 @opsummary{no-wildcards-match-slash}
2914 @item --no-wildcards-match-slash
2915 Wildcards do not match @samp{/}.
2916 @xref{controlling pattern-matching}.
2921 When @command{tar} is using the @option{--files-from} option, this option
2922 instructs @command{tar} to expect file names terminated with @acronym{NUL}, so
2923 @command{tar} can correctly work with file names that contain newlines.
2926 @opsummary{numeric-owner}
2927 @item --numeric-owner
2929 This option will notify @command{tar} that it should use numeric user
2930 and group IDs when creating a @command{tar} file, rather than names.
2934 The function of this option depends on the action @command{tar} is
2935 performing. When extracting files, @option{-o} is a synonym for
2936 @option{--no-same-owner}, i.e., it prevents @command{tar} from
2937 restoring ownership of files being extracted.
2939 When creating an archive, it is a synonym for
2940 @option{--old-archive}. This behavior is for compatibility
2941 with previous versions of @GNUTAR{}, and will be
2942 removed in future releases.
2944 @xref{Changes}, for more information.
2946 @opsummary{occurrence}
2947 @item --occurrence[=@var{number}]
2949 This option can be used in conjunction with one of the subcommands
2950 @option{--delete}, @option{--diff}, @option{--extract} or
2951 @option{--list} when a list of files is given either on the command
2952 line or via @option{-T} option.
2954 This option instructs @command{tar} to process only the @var{number}th
2955 occurrence of each named file. @var{Number} defaults to 1, so
2958 tar -x -f archive.tar --occurrence filename
2962 will extract the first occurrence of the member @file{filename} from @file{archive.tar}
2963 and will terminate without scanning to the end of the archive.
2965 @opsummary{old-archive}
2967 Synonym for @option{--format=v7}.
2969 @opsummary{one-file-system}
2970 @item --one-file-system
2971 Used when creating an archive. Prevents @command{tar} from recursing into
2972 directories that are on different file systems from the current
2975 @opsummary{overwrite}
2978 Overwrite existing files and directory metadata when extracting files
2979 from an archive. @xref{Overwrite Old Files}.
2981 @opsummary{overwrite-dir}
2982 @item --overwrite-dir
2984 Overwrite the metadata of existing directories when extracting files
2985 from an archive. @xref{Overwrite Old Files}.
2988 @item --owner=@var{user}
2990 Specifies that @command{tar} should use @var{user} as the owner of members
2991 when creating archives, instead of the user associated with the source
2992 file. @var{user} is first decoded as a user symbolic name, but if
2993 this interpretation fails, it has to be a decimal numeric user @acronym{ID}.
2996 This option does not affect extraction from archives.
2998 @opsummary{pax-option}
2999 @item --pax-option=@var{keyword-list}
3000 This option is meaningful only with @acronym{POSIX.1-2001} archives
3001 (@pxref{posix}). It modifies the way @command{tar} handles the
3002 extended header keywords. @var{Keyword-list} is a comma-separated
3003 list of keyword options. @xref{PAX keywords}, for a detailed
3006 @opsummary{portability}
3008 @itemx --old-archive
3009 Synonym for @option{--format=v7}.
3013 Same as @option{--format=posix}.
3015 @opsummary{preserve}
3018 Synonymous with specifying both @option{--preserve-permissions} and
3019 @option{--same-order}. @xref{Setting Access Permissions}.
3021 @opsummary{preserve-order}
3022 @item --preserve-order
3024 (See @option{--same-order}; @pxref{Reading}.)
3026 @opsummary{preserve-permissions}
3027 @opsummary{same-permissions}
3028 @item --preserve-permissions
3029 @itemx --same-permissions
3032 When @command{tar} is extracting an archive, it normally subtracts the
3033 users' umask from the permissions specified in the archive and uses
3034 that number as the permissions to create the destination file.
3035 Specifying this option instructs @command{tar} that it should use the
3036 permissions directly from the archive. @xref{Setting Access Permissions}.
3038 @opsummary{quote-chars}
3039 @item --quote-chars=@var{string}
3040 Always quote characters from @var{string}, even if the selected
3041 quoting style would not quote them (@pxref{quoting styles}).
3043 @opsummary{quoting-style}
3044 @item --quoting-style=@var{style}
3045 Set quoting style to use when printing member and file names
3046 (@pxref{quoting styles}). Valid @var{style} values are:
3047 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
3048 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
3049 style is @code{escape}, unless overridden while configuring the
3052 @opsummary{read-full-records}
3053 @item --read-full-records
3056 Specifies that @command{tar} should reblock its input, for reading
3057 from pipes on systems with buggy implementations. @xref{Reading}.
3059 @opsummary{record-size}
3060 @item --record-size=@var{size}
3062 Instructs @command{tar} to use @var{size} bytes per record when accessing the
3063 archive. @xref{Blocking Factor}.
3065 @opsummary{recursion}
3068 With this option, @command{tar} recurses into directories (default).
3071 @opsummary{recursive-unlink}
3072 @item --recursive-unlink
3075 directory hierarchies before extracting directories of the same name
3076 from the archive. @xref{Recursive Unlink}.
3078 @opsummary{remove-files}
3079 @item --remove-files
3081 Directs @command{tar} to remove the source file from the file system after
3082 appending it to an archive. @xref{remove files}.
3084 @opsummary{restrict}
3087 Disable use of some potentially harmful @command{tar} options.
3088 Currently this option disables shell invocation from multi-volume menu
3089 (@pxref{Using Multiple Tapes}).
3091 @opsummary{rmt-command}
3092 @item --rmt-command=@var{cmd}
3094 Notifies @command{tar} that it should use @var{cmd} instead of
3095 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
3097 @opsummary{rsh-command}
3098 @item --rsh-command=@var{cmd}
3100 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
3101 devices. @xref{Device}.
3103 @opsummary{same-order}
3105 @itemx --preserve-order
3108 This option is an optimization for @command{tar} when running on machines with
3109 small amounts of memory. It informs @command{tar} that the list of file
3110 arguments has already been sorted to match the order of files in the
3111 archive. @xref{Reading}.
3113 @opsummary{same-owner}
3116 When extracting an archive, @command{tar} will attempt to preserve the owner
3117 specified in the @command{tar} archive with this option present.
3118 This is the default behavior for the superuser; this option has an
3119 effect only for ordinary users. @xref{Attributes}.
3121 @opsummary{same-permissions}
3122 @item --same-permissions
3124 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
3130 Assume that the archive media supports seeks to arbitrary
3131 locations. Usually @command{tar} determines automatically whether
3132 the archive can be seeked or not. This option is intended for use
3133 in cases when such recognition fails.
3135 @opsummary{show-defaults}
3136 @item --show-defaults
3138 Displays the default options used by @command{tar} and exits
3139 successfully. This option is intended for use in shell scripts.
3140 Here is an example of what you can see using this option:
3143 $ tar --show-defaults
3144 --format=gnu -f- -b20 --quoting-style=escape \
3145 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
3148 @opsummary{show-omitted-dirs}
3149 @item --show-omitted-dirs
3151 Instructs @command{tar} to mention the directories it is skipping when
3152 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
3154 @opsummary{show-transformed-names}
3155 @opsummary{show-stored-names}
3156 @item --show-transformed-names
3157 @itemx --show-stored-names
3159 Display file or member names after applying any transformations
3160 (@pxref{transform}). In particular, when used in conjunction with one of
3161 the archive creation operations it instructs @command{tar} to list the
3162 member names stored in the archive, as opposed to the actual file
3163 names. @xref{listing member and file names}.
3169 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3170 sparse files efficiently. @xref{sparse}.
3172 @opsummary{sparse-version}
3173 @item --sparse-version=@var{version}
3175 Specifies the @dfn{format version} to use when archiving sparse
3176 files. Implies @option{--sparse}. @xref{sparse}. For the description
3177 of the supported sparse formats, @xref{Sparse Formats}.
3179 @opsummary{starting-file}
3180 @item --starting-file=@var{name}
3181 @itemx -K @var{name}
3183 This option affects extraction only; @command{tar} will skip extracting
3184 files in the archive until it finds one that matches @var{name}.
3187 @opsummary{strip-components}
3188 @item --strip-components=@var{number}
3189 Strip given @var{number} of leading components from file names before
3190 extraction. For example, if archive @file{archive.tar} contained
3191 @file{/some/file/name}, then running
3194 tar --extract --file archive.tar --strip-components=2
3198 would extract this file to file @file{name}.
3200 @opsummary{suffix}, summary
3201 @item --suffix=@var{suffix}
3203 Alters the suffix @command{tar} uses when backing up files from the default
3204 @samp{~}. @xref{backup}.
3206 @opsummary{tape-length}
3207 @item --tape-length=@var{num}
3210 Specifies the length of tapes that @command{tar} is writing as being
3211 @w{@var{num} x 1024} bytes long. @xref{Using Multiple Tapes}.
3213 @opsummary{test-label}
3216 Reads the volume label. If an argument is specified, test whether it
3217 matches the volume label. @xref{--test-label option}.
3219 @opsummary{to-command}
3220 @item --to-command=@var{command}
3222 During extraction @command{tar} will pipe extracted files to the
3223 standard input of @var{command}. @xref{Writing to an External Program}.
3225 @opsummary{to-stdout}
3229 During extraction, @command{tar} will extract files to stdout rather
3230 than to the file system. @xref{Writing to Standard Output}.
3233 @item --totals[=@var{signo}]
3235 Displays the total number of bytes transferred when processing an
3236 archive. If an argument is given, these data are displayed on
3237 request, when signal @var{signo} is delivered to @command{tar}.
3244 Sets the data modification time of extracted files to the extraction time,
3245 rather than the data modification time stored in the archive.
3246 @xref{Data Modification Times}.
3248 @opsummary{transform}
3249 @item --transform=@var{sed-expr}
3251 Transform file or member names using @command{sed} replacement expression
3252 @var{sed-expr}. For example,
3255 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
3259 will add to @file{archive} files from the current working directory,
3260 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
3261 discussion, @xref{transform}.
3263 To see transformed member names in verbose listings, use
3264 @option{--show-transformed-names} option
3265 (@pxref{show-transformed-names}).
3267 @opsummary{uncompress}
3270 (See @option{--compress}. @pxref{gzip})
3275 (See @option{--gzip}. @pxref{gzip})
3277 @opsummary{unlink-first}
3278 @item --unlink-first
3281 Directs @command{tar} to remove the corresponding file from the file
3282 system before extracting it from the archive. @xref{Unlink First}.
3286 Enable unquoting input file or member names (default). @xref{input
3289 @opsummary{use-compress-program}
3290 @item --use-compress-program=@var{prog}
3292 Instructs @command{tar} to access the archive through @var{prog}, which is
3293 presumed to be a compression program of some sort. @xref{gzip}.
3298 Display file modification dates in @acronym{UTC}. This option implies
3305 Specifies that @command{tar} should be more verbose about the
3306 operations it is performing. This option can be specified multiple
3307 times for some operations to increase the amount of information displayed.
3314 Verifies that the archive was correctly written when creating an
3315 archive. @xref{verify}.
3320 Print information about the program's name, version, origin and legal
3321 status, all on standard output, and then exit successfully.
3324 @opsummary{volno-file}
3325 @item --volno-file=@var{file}
3327 Used in conjunction with @option{--multi-volume}. @command{tar} will
3328 keep track of which volume of a multi-volume archive it is working in
3329 @var{file}. @xref{volno-file}.
3331 @opsummary{wildcards}
3333 Use wildcards when matching member names with patterns.
3334 @xref{controlling pattern-matching}.
3336 @opsummary{wildcards-match-slash}
3337 @item --wildcards-match-slash
3338 Wildcards match @samp{/}.
3339 @xref{controlling pattern-matching}.
3342 @node Short Option Summary
3343 @subsection Short Options Cross Reference
3345 Here is an alphabetized list of all of the short option forms, matching
3346 them with the equivalent long option.
3348 @multitable @columnfractions 0.20 0.80
3349 @headitem Short Option @tab Reference
3351 @item -A @tab @ref{--concatenate}.
3353 @item -B @tab @ref{--read-full-records}.
3355 @item -C @tab @ref{--directory}.
3357 @item -F @tab @ref{--info-script}.
3359 @item -G @tab @ref{--incremental}.
3361 @item -K @tab @ref{--starting-file}.
3363 @item -L @tab @ref{--tape-length}.
3365 @item -M @tab @ref{--multi-volume}.
3367 @item -N @tab @ref{--newer}.
3369 @item -O @tab @ref{--to-stdout}.
3371 @item -P @tab @ref{--absolute-names}.
3373 @item -R @tab @ref{--block-number}.
3375 @item -S @tab @ref{--sparse}.
3377 @item -T @tab @ref{--files-from}.
3379 @item -U @tab @ref{--unlink-first}.
3381 @item -V @tab @ref{--label}.
3383 @item -W @tab @ref{--verify}.
3385 @item -X @tab @ref{--exclude-from}.
3387 @item -Z @tab @ref{--compress}.
3389 @item -b @tab @ref{--blocking-factor}.
3391 @item -c @tab @ref{--create}.
3393 @item -d @tab @ref{--compare}.
3395 @item -f @tab @ref{--file}.
3397 @item -g @tab @ref{--listed-incremental}.
3399 @item -h @tab @ref{--dereference}.
3401 @item -i @tab @ref{--ignore-zeros}.
3403 @item -j @tab @ref{--bzip2}.
3405 @item -k @tab @ref{--keep-old-files}.
3407 @item -l @tab @ref{--check-links}.
3409 @item -m @tab @ref{--touch}.
3411 @item -o @tab When creating, @ref{--no-same-owner}, when extracting ---
3412 @ref{--portability}.
3414 The later usage is deprecated. It is retained for compatibility with
3415 the earlier versions of @GNUTAR{}. In future releases
3416 @option{-o} will be equivalent to @option{--no-same-owner} only.
3418 @item -p @tab @ref{--preserve-permissions}.
3420 @item -r @tab @ref{--append}.
3422 @item -s @tab @ref{--same-order}.
3424 @item -t @tab @ref{--list}.
3426 @item -u @tab @ref{--update}.
3428 @item -v @tab @ref{--verbose}.
3430 @item -w @tab @ref{--interactive}.
3432 @item -x @tab @ref{--extract}.
3434 @item -z @tab @ref{--gzip}.
3439 @section @GNUTAR{} documentation
3441 @cindex Getting program version number
3443 @cindex Version of the @command{tar} program
3444 Being careful, the first thing is really checking that you are using
3445 @GNUTAR{}, indeed. The @option{--version} option
3446 causes @command{tar} to print information about its name, version,
3447 origin and legal status, all on standard output, and then exit
3448 successfully. For example, @w{@samp{tar --version}} might print:
3451 tar (GNU tar) @value{VERSION}
3452 Copyright (C) 2008 Free Software Foundation, Inc.
3453 This is free software. You may redistribute copies of it under the terms
3454 of the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
3455 There is NO WARRANTY, to the extent permitted by law.
3457 Written by John Gilmore and Jay Fenlason.
3461 The first occurrence of @samp{tar} in the result above is the program
3462 name in the package (for example, @command{rmt} is another program),
3463 while the second occurrence of @samp{tar} is the name of the package
3464 itself, containing possibly many programs. The package is currently
3465 named @samp{tar}, after the name of the main program it
3466 contains@footnote{There are plans to merge the @command{cpio} and
3467 @command{tar} packages into a single one which would be called
3468 @code{paxutils}. So, who knows if, one of this days, the
3469 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3472 @cindex Obtaining help
3473 @cindex Listing all @command{tar} options
3474 @xopindex{help, introduction}
3475 Another thing you might want to do is checking the spelling or meaning
3476 of some particular @command{tar} option, without resorting to this
3477 manual, for once you have carefully read it. @GNUTAR{}
3478 has a short help feature, triggerable through the
3479 @option{--help} option. By using this option, @command{tar} will
3480 print a usage message listing all available options on standard
3481 output, then exit successfully, without doing anything else and
3482 ignoring all other options. Even if this is only a brief summary, it
3483 may be several screens long. So, if you are not using some kind of
3484 scrollable window, you might prefer to use something like:
3487 $ @kbd{tar --help | less}
3491 presuming, here, that you like using @command{less} for a pager. Other
3492 popular pagers are @command{more} and @command{pg}. If you know about some
3493 @var{keyword} which interests you and do not want to read all the
3494 @option{--help} output, another common idiom is doing:
3497 tar --help | grep @var{keyword}
3501 for getting only the pertinent lines. Notice, however, that some
3502 @command{tar} options have long description lines and the above
3503 command will list only the first of them.
3505 The exact look of the option summary displayed by @kbd{tar --help} is
3506 configurable. @xref{Configuring Help Summary}, for a detailed description.
3509 If you only wish to check the spelling of an option, running @kbd{tar
3510 --usage} may be a better choice. This will display a terse list of
3511 @command{tar} option without accompanying explanations.
3513 The short help output is quite succinct, and you might have to get
3514 back to the full documentation for precise points. If you are reading
3515 this paragraph, you already have the @command{tar} manual in some
3516 form. This manual is available in a variety of forms from
3517 @url{http://www.gnu.org/software/tar/manual}. It may be printed out of the @GNUTAR{}
3518 distribution, provided you have @TeX{} already installed somewhere,
3519 and a laser printer around. Just configure the distribution, execute
3520 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3521 usual way (contact your local guru to know how). If @GNUTAR{}
3522 has been conveniently installed at your place, this
3523 manual is also available in interactive, hypertextual form as an Info
3524 file. Just call @w{@samp{info tar}} or, if you do not have the
3525 @command{info} program handy, use the Info reader provided within
3526 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3528 There is currently no @code{man} page for @GNUTAR{}.
3529 If you observe such a @code{man} page on the system you are running,
3530 either it does not belong to @GNUTAR{}, or it has not
3531 been produced by @acronym{GNU}. Some package maintainers convert
3532 @kbd{tar --help} output to a man page, using @command{help2man}. In
3533 any case, please bear in mind that the authoritative source of
3534 information about @GNUTAR{} is this Texinfo documentation.
3537 @section Obtaining @GNUTAR{} default values
3539 @opindex show-defaults
3540 @GNUTAR{} has some predefined defaults that are used when you do not
3541 explicitly specify another values. To obtain a list of such
3542 defaults, use @option{--show-defaults} option. This will output the
3543 values in the form of @command{tar} command line options:
3547 @kbd{tar --show-defaults}
3548 --format=gnu -f- -b20 --quoting-style=escape
3549 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3554 Notice, that this option outputs only one line. The example output above
3555 has been split to fit page boundaries.
3558 The above output shows that this version of @GNUTAR{} defaults to
3559 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3560 output as the archive, if no @option{--file} option has been given
3561 (@pxref{file tutorial}), the default blocking factor is 20
3562 (@pxref{Blocking Factor}). It also shows the default locations where
3563 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3566 @section Checking @command{tar} progress
3568 Typically, @command{tar} performs most operations without reporting any
3569 information to the user except error messages. When using @command{tar}
3570 with many options, particularly ones with complicated or
3571 difficult-to-predict behavior, it is possible to make serious mistakes.
3572 @command{tar} provides several options that make observing @command{tar}
3573 easier. These options cause @command{tar} to print information as it
3574 progresses in its job, and you might want to use them just for being
3575 more careful about what is going on, or merely for entertaining
3576 yourself. If you have encountered a problem when operating on an
3577 archive, however, you may need more information than just an error
3578 message in order to solve the problem. The following options can be
3579 helpful diagnostic tools.
3581 @cindex Verbose operation
3583 Normally, the @option{--list} (@option{-t}) command to list an archive
3584 prints just the file names (one per line) and the other commands are
3585 silent. When used with most operations, the @option{--verbose}
3586 (@option{-v}) option causes @command{tar} to print the name of each
3587 file or archive member as it is processed. This and the other options
3588 which make @command{tar} print status information can be useful in
3589 monitoring @command{tar}.
3591 With @option{--create} or @option{--extract}, @option{--verbose} used
3592 once just prints the names of the files or members as they are processed.
3593 Using it twice causes @command{tar} to print a longer listing
3594 (@xref{verbose member listing}, for the description) for each member.
3595 Since @option{--list} already prints the names of the members,
3596 @option{--verbose} used once with @option{--list} causes @command{tar}
3597 to print an @samp{ls -l} type listing of the files in the archive.
3598 The following examples both extract members with long list output:
3601 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3602 $ @kbd{tar xvvf archive.tar}
3605 Verbose output appears on the standard output except when an archive is
3606 being written to the standard output, as with @samp{tar --create
3607 --file=- --verbose} (@samp{tar cfv -}, or even @samp{tar cv}---if the
3608 installer let standard output be the default archive). In that case
3609 @command{tar} writes verbose output to the standard error stream.
3611 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3612 verbose output to @var{file} rather than to standard output or standard
3616 @cindex Obtaining total status information
3618 The @option{--totals} option causes @command{tar} to print on the
3619 standard error the total amount of bytes transferred when processing
3620 an archive. When creating or appending to an archive, this option
3621 prints the number of bytes written to the archive and the average
3622 speed at which they have been written, e.g.:
3626 $ @kbd{tar -c -f archive.tar --totals /home}
3627 Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
3631 When reading an archive, this option displays the number of bytes
3636 $ @kbd{tar -x -f archive.tar --totals}
3637 Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
3641 Finally, when deleting from an archive, the @option{--totals} option
3642 displays both numbers plus number of bytes removed from the archive:
3646 $ @kbd{tar --delete -f foo.tar --totals --wildcards '*~'}
3647 Total bytes read: 9543680 (9.2MiB, 201MiB/s)
3648 Total bytes written: 3829760 (3.7MiB, 81MiB/s)
3649 Total bytes deleted: 1474048
3653 You can also obtain this information on request. When
3654 @option{--totals} is used with an argument, this argument is
3655 interpreted as a symbolic name of a signal, upon delivery of which the
3656 statistics is to be printed:
3659 @item --totals=@var{signo}
3660 Print statistics upon delivery of signal @var{signo}. Valid arguments
3661 are: @code{SIGHUP}, @code{SIGQUIT}, @code{SIGINT}, @code{SIGUSR1} and
3662 @code{SIGUSR2}. Shortened names without @samp{SIG} prefix are also
3666 Both forms of @option{--totals} option can be used simultaneously.
3667 Thus, @kbd{tar -x --totals --totals=USR1} instructs @command{tar} to
3668 extract all members from its default archive and print statistics
3669 after finishing the extraction, as well as when receiving signal
3672 @anchor{Progress information}
3673 @cindex Progress information
3674 The @option{--checkpoint} option prints an occasional message
3675 as @command{tar} reads or writes the archive. It is designed for
3676 those who don't need the more detailed (and voluminous) output of
3677 @option{--block-number} (@option{-R}), but do want visual confirmation
3678 that @command{tar} is actually making forward progress. By default it
3679 prints a message each 10 records read or written. This can be changed
3680 by giving it a numeric argument after an equal sign:
3683 $ @kbd{tar -c --checkpoint=1000} /var
3684 tar: Write checkpoint 1000
3685 tar: Write checkpoint 2000
3686 tar: Write checkpoint 3000
3689 This example shows the default checkpoint message used by
3690 @command{tar}. If you place a dot immediately after the equal
3691 sign, it will print a @samp{.} at each checkpoint@footnote{This is
3692 actually a shortcut for @option{--checkpoint=@var{n}
3693 --checkpoint-action=dot}. @xref{checkpoints, dot}.}. For example:
3696 $ @kbd{tar -c --checkpoint=.1000} /var
3700 The @option{--checkpoint} option provides a flexible mechanism for
3701 executing arbitrary actions upon hitting checkpoints, see the next
3702 section (@pxref{checkpoints}), for more information on it.
3704 @opindex show-omitted-dirs
3705 @anchor{show-omitted-dirs}
3706 The @option{--show-omitted-dirs} option, when reading an archive---with
3707 @option{--list} or @option{--extract}, for example---causes a message
3708 to be printed for each directory in the archive which is skipped.
3709 This happens regardless of the reason for skipping: the directory might
3710 not have been named on the command line (implicitly or explicitly),
3711 it might be excluded by the use of the
3712 @option{--exclude=@var{pattern}} option, or some other reason.
3714 @opindex block-number
3715 @cindex Block number where error occurred
3716 @anchor{block-number}
3717 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3718 every message it would normally produce, the block number within the
3719 archive where the message was triggered. Also, supplementary messages
3720 are triggered when reading blocks full of NULs, or when hitting end of
3721 file on the archive. As of now, if the archive if properly terminated
3722 with a NUL block, the reading of the file may stop before end of file
3723 is met, so the position of end of file will not usually show when
3724 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3725 drains the archive before exiting when reading the
3726 archive from a pipe.
3728 @cindex Error message, block number of
3729 This option is especially useful when reading damaged archives, since
3730 it helps pinpoint the damaged sections. It can also be used with
3731 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3732 choose among several backup tapes when retrieving a file later, in
3733 favor of the tape where the file appears earliest (closest to the
3734 front of the tape). @xref{backup}.
3737 @section Checkpoints
3738 @cindex checkpoints, defined
3740 @opindex checkpoint-action
3742 A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
3743 the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
3744 from the archive (a @dfn{read checkpoint}). Checkpoints allow to
3745 periodically execute arbitrary actions.
3747 The checkpoint facility is enabled using the following option:
3750 @xopindex{checkpoint, defined}
3751 @item --checkpoint[=@var{n}]
3752 Schedule checkpoints before writing or reading each @var{n}th record.
3753 The default value for @var{n} is 10.
3756 A list of arbitrary @dfn{actions} can be executed at each checkpoint.
3757 These actions include: pausing, displaying textual messages, and
3758 executing arbitrary external programs. Actions are defined using
3759 the @option{--checkpoint-action} option.
3762 @xopindex{checkpoint-action, defined}
3763 @item --checkpoint-action=@var{action}
3764 Execute an @var{action} at each checkpoint.
3767 @cindex @code{echo}, checkpoint action
3768 The simplest value of @var{action} is @samp{echo}. It instructs
3769 @command{tar} to display the default message on the standard error
3770 stream upon arriving at each checkpoint. The default message is (in
3771 @acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
3772 checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
3773 Here, @var{n} represents ordinal number of the checkpoint.
3775 In another locales, translated versions of this message are used.
3777 This is the default action, so running:
3780 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
3787 $ @kbd{tar -c --checkpoint=1000} /var
3790 The @samp{echo} action also allows to supply a customized message.
3791 You do so by placing an equals sign and the message right after it,
3795 --checkpoint-action="echo=Hit %s checkpoint #%u"
3798 The @samp{%s} and @samp{%u} in the above example are
3799 @dfn{meta-characters}. The @samp{%s} meta-character is replaced with
3800 the @dfn{type} of the checkpoint: @samp{write} or
3801 @samp{read} (or a corresponding translated version in locales other
3802 than @acronym{POSIX}). The @samp{%u} meta-character is replaced with
3803 the ordinal number of the checkpoint. Thus, the above example could
3804 produce the following output when used with the @option{--create}
3808 tar: Hit write checkpoint #10
3809 tar: Hit write checkpoint #20
3810 tar: Hit write checkpoint #30
3813 Aside from meta-character expansion, the message string is subject to
3814 @dfn{unquoting}, during which the backslash @dfn{escape sequences} are
3815 replaced with their corresponding @acronym{ASCII} characters
3816 (@pxref{escape sequences}). E.g. the following action will produce an
3817 audible bell and the message described above at each checkpoint:
3820 --checkpoint-action='echo=\aHit %s checkpoint #%u'
3823 @cindex @code{bell}, checkpoint action
3824 There is also a special action which produces an audible signal:
3825 @samp{bell}. It is not equivalent to @samp{echo='\a'}, because
3826 @samp{bell} sends the bell directly to the console (@file{/dev/tty}),
3827 whereas @samp{echo='\a'} sends it to the standard error.
3829 @cindex @code{ttyout}, checkpoint action
3830 The @samp{ttyout=@var{string}} action outputs @var{string} to
3831 @file{/dev/tty}, so it can be used even if the standard output is
3832 redirected elsewhere. The @var{string} is subject to the same
3833 modifications as with @samp{echo} action. In contrast to the latter,
3834 @samp{ttyout} does not prepend @command{tar} executable name to the
3835 string, nor does it output a newline after it. For example, the
3836 following action will print the checkpoint message at the same screen
3837 line, overwriting any previous message:
3840 --checkpoint-action="ttyout=\rHit %s checkpoint #%u"
3843 @cindex @code{dot}, checkpoint action
3844 Another available checkpoint action is @samp{dot} (or @samp{.}). It
3845 instructs @command{tar} to print a single dot on the standard listing
3849 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
3853 For compatibility with previous @GNUTAR{} versions, this action can
3854 be abbreviated by placing a dot in front of the checkpoint frequency,
3855 as shown in the previous section.
3857 @cindex @code{sleep}, checkpoint action
3858 Yet another action, @samp{sleep}, pauses @command{tar} for a specified
3859 amount of seconds. The following example will stop for 30 seconds at each
3863 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
3866 @cindex @code{exec}, checkpoint action
3867 Finally, the @code{exec} action executes a given external program.
3871 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
3874 This program is executed using @command{/bin/sh -c}, with no
3875 additional arguments. Its exit code is ignored. It gets a copy of
3876 @command{tar}'s environment plus the following variables:
3879 @vrindex TAR_VERSION, checkpoint script environment
3881 @GNUTAR{} version number.
3883 @vrindex TAR_ARCHIVE, checkpoint script environment
3885 The name of the archive @command{tar} is processing.
3887 @vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
3888 @item TAR_BLOCKING_FACTOR
3889 Current blocking factor (@pxref{Blocking}.
3891 @vrindex TAR_CHECKPOINT, checkpoint script environment
3892 @item TAR_CHECKPOINT
3893 The checkpoint number.
3895 @vrindex TAR_SUBCOMMAND, checkpoint script environment
3896 @item TAR_SUBCOMMAND
3897 A short option describing the operation @command{tar} is executing
3898 @xref{Operations}, for a complete list of subcommand options.
3900 @vrindex TAR_FORMAT, checkpoint script environment
3902 Format of the archive being processed. @xref{Formats}, for a complete
3903 list of archive format names.
3906 Any number of actions can be defined, by supplying several
3907 @option{--checkpoint-action} options in the command line. For
3908 example, the command below displays two messages, pauses
3909 execution for 30 seconds and executes the @file{/sbin/cpoint} script:
3913 $ @kbd{tar -c -f arc.tar \
3914 --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
3915 --checkpoint-action='echo=Sleeping for 30 seconds' \
3916 --checkpoint-action='sleep=30' \
3917 --checkpoint-action='exec=/sbin/cpoint'}
3921 This example also illustrates the fact that
3922 @option{--checkpoint-action} can be used without
3923 @option{--checkpoint}. In this case, the default checkpoint frequency
3924 (at each 10th record) is assumed.
3927 @section Asking for Confirmation During Operations
3928 @cindex Interactive operation
3930 Typically, @command{tar} carries out a command without stopping for
3931 further instructions. In some situations however, you may want to
3932 exclude some files and archive members from the operation (for instance
3933 if disk or storage space is tight). You can do this by excluding
3934 certain files automatically (@pxref{Choosing}), or by performing
3935 an operation interactively, using the @option{--interactive} (@option{-w}) option.
3936 @command{tar} also accepts @option{--confirmation} for this option.
3938 @opindex interactive
3939 When the @option{--interactive} (@option{-w}) option is specified, before
3940 reading, writing, or deleting files, @command{tar} first prints a message
3941 for each such file, telling what operation it intends to take, then asks
3942 for confirmation on the terminal. The actions which require
3943 confirmation include adding a file to the archive, extracting a file
3944 from the archive, deleting a file from the archive, and deleting a file
3945 from disk. To confirm the action, you must type a line of input
3946 beginning with @samp{y}. If your input line begins with anything other
3947 than @samp{y}, @command{tar} skips that file.
3949 If @command{tar} is reading the archive from the standard input,
3950 @command{tar} opens the file @file{/dev/tty} to support the interactive
3953 Verbose output is normally sent to standard output, separate from
3954 other error messages. However, if the archive is produced directly
3955 on standard output, then verbose output is mixed with errors on
3956 @code{stderr}. Producing the archive on standard output may be used
3957 as a way to avoid using disk space, when the archive is soon to be
3958 consumed by another process reading it, say. Some people felt the need
3959 of producing an archive on stdout, still willing to segregate between
3960 verbose output and error output. A possible approach would be using a
3961 named pipe to receive the archive, and having the consumer process to
3962 read from that named pipe. This has the advantage of letting standard
3963 output free to receive verbose output, all separate from errors.
3966 @chapter @GNUTAR{} Operations
3979 @section Basic @GNUTAR{} Operations
3981 The basic @command{tar} operations, @option{--create} (@option{-c}),
3982 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
3983 @option{-x}), are currently presented and described in the tutorial
3984 chapter of this manual. This section provides some complementary notes
3985 for these operations.
3988 @xopindex{create, complementary notes}
3992 Creating an empty archive would have some kind of elegance. One can
3993 initialize an empty archive and later use @option{--append}
3994 (@option{-r}) for adding all members. Some applications would not
3995 welcome making an exception in the way of adding the first archive
3996 member. On the other hand, many people reported that it is
3997 dangerously too easy for @command{tar} to destroy a magnetic tape with
3998 an empty archive@footnote{This is well described in @cite{Unix-haters
3999 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
4000 Books, ISBN 1-56884-203-1.}. The two most common errors are:
4004 Mistakingly using @code{create} instead of @code{extract}, when the
4005 intent was to extract the full contents of an archive. This error
4006 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
4007 the QWERTY keyboard. Instead of being unpacked, the archive then
4008 gets wholly destroyed. When users speak about @dfn{exploding} an
4009 archive, they usually mean something else :-).
4012 Forgetting the argument to @code{file}, when the intent was to create
4013 an archive with a single file in it. This error is likely because a
4014 tired user can easily add the @kbd{f} key to the cluster of option
4015 letters, by the mere force of habit, without realizing the full
4016 consequence of doing so. The usual consequence is that the single
4017 file, which was meant to be saved, is rather destroyed.
4020 So, recognizing the likelihood and the catastrophic nature of these
4021 errors, @GNUTAR{} now takes some distance from elegance, and
4022 cowardly refuses to create an archive when @option{--create} option is
4023 given, there are no arguments besides options, and
4024 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
4025 around the cautiousness of @GNUTAR{} and nevertheless create an
4026 archive with nothing in it, one may still use, as the value for the
4027 @option{--files-from} option, a file with no names in it, as shown in
4028 the following commands:
4031 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
4032 @kbd{tar cfT empty-archive.tar /dev/null}
4035 @xopindex{extract, complementary notes}
4040 A socket is stored, within a @GNUTAR{} archive, as a pipe.
4042 @item @option{--list} (@option{-t})
4044 @GNUTAR{} now shows dates as @samp{1996-08-30},
4045 while it used to show them as @samp{Aug 30 1996}. Preferably,
4046 people should get used to ISO 8601 dates. Local American dates should
4047 be made available again with full date localization support, once
4048 ready. In the meantime, programs not being localizable for dates
4049 should prefer international dates, that's really the way to go.
4051 Look up @url{http://www.cl.cam.ac.uk/@/~mgk25/@/iso-time.html} if you
4052 are curious, it contains a detailed explanation of the ISO 8601 standard.
4057 @section Advanced @GNUTAR{} Operations
4059 Now that you have learned the basics of using @GNUTAR{}, you may want
4060 to learn about further ways in which @command{tar} can help you.
4062 This chapter presents five, more advanced operations which you probably
4063 won't use on a daily basis, but which serve more specialized functions.
4064 We also explain the different styles of options and why you might want
4065 to use one or another, or a combination of them in your @command{tar}
4066 commands. Additionally, this chapter includes options which allow you to
4067 define the output from @command{tar} more carefully, and provide help and
4068 error correction in special circumstances.
4070 @FIXME{check this after the chapter is actually revised to make sure
4071 it still introduces the info in the chapter correctly : ).}
4083 @subsection The Five Advanced @command{tar} Operations
4086 In the last chapter, you learned about the first three operations to
4087 @command{tar}. This chapter presents the remaining five operations to
4088 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
4089 @option{--delete}, and @option{--compare}.
4091 You are not likely to use these operations as frequently as those
4092 covered in the last chapter; however, since they perform specialized
4093 functions, they are quite useful when you do need to use them. We
4094 will give examples using the same directory and files that you created
4095 in the last chapter. As you may recall, the directory is called
4096 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
4097 @samp{rock}, and the two archive files you created are
4098 @samp{collection.tar} and @samp{music.tar}.
4100 We will also use the archive files @samp{afiles.tar} and
4101 @samp{bfiles.tar}. The archive @samp{afiles.tar} contains the members @samp{apple},
4102 @samp{angst}, and @samp{aspic}; @samp{bfiles.tar} contains the members
4103 @samp{./birds}, @samp{baboon}, and @samp{./box}.
4105 Unless we state otherwise, all practicing you do and examples you follow
4106 in this chapter will take place in the @file{practice} directory that
4107 you created in the previous chapter; see @ref{prepare for examples}.
4108 (Below in this section, we will remind you of the state of the examples
4109 where the last chapter left them.)
4111 The five operations that we will cover in this chapter are:
4116 Add new entries to an archive that already exists.
4119 Add more recent copies of archive members to the end of an archive, if
4124 Add one or more pre-existing archives to the end of another archive.
4126 Delete items from an archive (does not work on tapes).
4130 Compare archive members to their counterparts in the file system.
4134 @subsection How to Add Files to Existing Archives: @option{--append}
4138 If you want to add files to an existing archive, you don't need to
4139 create a new archive; you can use @option{--append} (@option{-r}).
4140 The archive must already exist in order to use @option{--append}. (A
4141 related operation is the @option{--update} operation; you can use this
4142 to add newer versions of archive members to an existing archive. To learn how to
4143 do this with @option{--update}, @pxref{update}.)
4145 If you use @option{--append} to add a file that has the same name as an
4146 archive member to an archive containing that archive member, then the
4147 old member is not deleted. What does happen, however, is somewhat
4148 complex. @command{tar} @emph{allows} you to have infinite number of files
4149 with the same name. Some operations treat these same-named members no
4150 differently than any other set of archive members: for example, if you
4151 view an archive with @option{--list} (@option{-t}), you will see all
4152 of those members listed, with their data modification times, owners, etc.
4154 Other operations don't deal with these members as perfectly as you might
4155 prefer; if you were to use @option{--extract} to extract the archive,
4156 only the most recently added copy of a member with the same name as four
4157 other members would end up in the working directory. This is because
4158 @option{--extract} extracts an archive in the order the members appeared
4159 in the archive; the most recently archived members will be extracted
4160 last. Additionally, an extracted member will @emph{replace} a file of
4161 the same name which existed in the directory already, and @command{tar}
4162 will not prompt you about this@footnote{Unless you give it
4163 @option{--keep-old-files} option, or the disk copy is newer than the
4164 the one in the archive and you invoke @command{tar} with
4165 @option{--keep-newer-files} option}. Thus, only the most recently archived
4166 member will end up being extracted, as it will replace the one
4167 extracted before it, and so on.
4169 There exists a special option that allows you to get around this
4170 behavior and extract (or list) only a particular copy of the file.
4171 This is @option{--occurrence} option. If you run @command{tar} with
4172 this option, it will extract only the first copy of the file. You
4173 may also give this option an argument specifying the number of
4174 copy to be extracted. Thus, for example if the archive
4175 @file{archive.tar} contained three copies of file @file{myfile}, then
4179 tar --extract --file archive.tar --occurrence=2 myfile
4183 would extract only the second copy. @xref{Option
4184 Summary,---occurrence}, for the description of @option{--occurrence}
4187 @FIXME{ hag -- you might want to incorporate some of the above into the
4188 MMwtSN node; not sure. i didn't know how to make it simpler...
4190 There are a few ways to get around this. (maybe xref Multiple Members
4191 with the Same Name.}
4193 @cindex Members, replacing with other members
4194 @cindex Replacing members with other members
4195 If you want to replace an archive member, use @option{--delete} to
4196 delete the member you want to remove from the archive, , and then use
4197 @option{--append} to add the member you want to be in the archive. Note
4198 that you can not change the order of the archive; the most recently
4199 added member will still appear last. In this sense, you cannot truly
4200 ``replace'' one member with another. (Replacing one member with another
4201 will not work on certain types of media, such as tapes; see @ref{delete}
4202 and @ref{Media}, for more information.)
4205 * appending files:: Appending Files to an Archive
4209 @node appending files
4210 @subsubsection Appending Files to an Archive
4212 @cindex Adding files to an Archive
4213 @cindex Appending files to an Archive
4214 @cindex Archives, Appending files to
4216 The simplest way to add a file to an already existing archive is the
4217 @option{--append} (@option{-r}) operation, which writes specified
4218 files into the archive whether or not they are already among the
4221 When you use @option{--append}, you @emph{must} specify file name
4222 arguments, as there is no default. If you specify a file that already
4223 exists in the archive, another copy of the file will be added to the
4224 end of the archive. As with other operations, the member names of the
4225 newly added files will be exactly the same as their names given on the
4226 command line. The @option{--verbose} (@option{-v}) option will print
4227 out the names of the files as they are written into the archive.
4229 @option{--append} cannot be performed on some tape drives, unfortunately,
4230 due to deficiencies in the formats those tape drives use. The archive
4231 must be a valid @command{tar} archive, or else the results of using this
4232 operation will be unpredictable. @xref{Media}.
4234 To demonstrate using @option{--append} to add a file to an archive,
4235 create a file called @file{rock} in the @file{practice} directory.
4236 Make sure you are in the @file{practice} directory. Then, run the
4237 following @command{tar} command to add @file{rock} to
4238 @file{collection.tar}:
4241 $ @kbd{tar --append --file=collection.tar rock}
4245 If you now use the @option{--list} (@option{-t}) operation, you will see that
4246 @file{rock} has been added to the archive:
4249 $ @kbd{tar --list --file=collection.tar}
4250 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
4251 -rw-r--r-- me user 21 1996-09-23 16:44 blues
4252 -rw-r--r-- me user 20 1996-09-23 16:44 folk
4253 -rw-r--r-- me user 20 1996-09-23 16:44 rock
4257 @subsubsection Multiple Members with the Same Name
4259 You can use @option{--append} (@option{-r}) to add copies of files
4260 which have been updated since the archive was created. (However, we
4261 do not recommend doing this since there is another @command{tar}
4262 option called @option{--update}; @xref{update}, for more information.
4263 We describe this use of @option{--append} here for the sake of
4264 completeness.) When you extract the archive, the older version will
4265 be effectively lost. This works because files are extracted from an
4266 archive in the order in which they were archived. Thus, when the
4267 archive is extracted, a file archived later in time will replace a
4268 file of the same name which was archived earlier, even though the
4269 older version of the file will remain in the archive unless you delete
4270 all versions of the file.
4272 Supposing you change the file @file{blues} and then append the changed
4273 version to @file{collection.tar}. As you saw above, the original
4274 @file{blues} is in the archive @file{collection.tar}. If you change the
4275 file and append the new version of the file to the archive, there will
4276 be two copies in the archive. When you extract the archive, the older
4277 version of the file will be extracted first, and then replaced by the
4278 newer version when it is extracted.
4280 You can append the new, changed copy of the file @file{blues} to the
4281 archive in this way:
4284 $ @kbd{tar --append --verbose --file=collection.tar blues}
4289 Because you specified the @option{--verbose} option, @command{tar} has
4290 printed the name of the file being appended as it was acted on. Now
4291 list the contents of the archive:
4294 $ @kbd{tar --list --verbose --file=collection.tar}
4295 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
4296 -rw-r--r-- me user 21 1996-09-23 16:44 blues
4297 -rw-r--r-- me user 20 1996-09-23 16:44 folk
4298 -rw-r--r-- me user 20 1996-09-23 16:44 rock
4299 -rw-r--r-- me user 58 1996-10-24 18:30 blues
4303 The newest version of @file{blues} is now at the end of the archive
4304 (note the different creation dates and file sizes). If you extract
4305 the archive, the older version of the file @file{blues} will be
4306 replaced by the newer version. You can confirm this by extracting
4307 the archive and running @samp{ls} on the directory.
4309 If you wish to extract the first occurrence of the file @file{blues}
4310 from the archive, use @option{--occurrence} option, as shown in
4311 the following example:
4314 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
4315 -rw-r--r-- me user 21 1996-09-23 16:44 blues
4318 @xref{Writing}, for more information on @option{--extract} and
4319 @xref{Option Summary, --occurrence}, for the description of
4320 @option{--occurrence} option.
4323 @subsection Updating an Archive
4325 @cindex Updating an archive
4328 In the previous section, you learned how to use @option{--append} to
4329 add a file to an existing archive. A related operation is
4330 @option{--update} (@option{-u}). The @option{--update} operation
4331 updates a @command{tar} archive by comparing the date of the specified
4332 archive members against the date of the file with the same name. If
4333 the file has been modified more recently than the archive member, then
4334 the newer version of the file is added to the archive (as with
4337 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
4338 The operation will fail.
4340 @FIXME{other examples of media on which --update will fail? need to ask
4341 charles and/or mib/thomas/dave shevett..}
4343 Both @option{--update} and @option{--append} work by adding to the end
4344 of the archive. When you extract a file from the archive, only the
4345 version stored last will wind up in the file system, unless you use
4346 the @option{--backup} option. @xref{multiple}, for a detailed discussion.
4353 @subsubsection How to Update an Archive Using @option{--update}
4355 You must use file name arguments with the @option{--update}
4356 (@option{-u}) operation. If you don't specify any files,
4357 @command{tar} won't act on any files and won't tell you that it didn't
4358 do anything (which may end up confusing you).
4360 @c note: the above parenthetical added because in fact, this
4361 @c behavior just confused the author. :-)
4363 To see the @option{--update} option at work, create a new file,
4364 @file{classical}, in your practice directory, and some extra text to the
4365 file @file{blues}, using any text editor. Then invoke @command{tar} with
4366 the @samp{update} operation and the @option{--verbose} (@option{-v})
4367 option specified, using the names of all the files in the practice
4368 directory as file name arguments:
4371 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4378 Because we have specified verbose mode, @command{tar} prints out the names
4379 of the files it is working on, which in this case are the names of the
4380 files that needed to be updated. If you run @samp{tar --list} and look
4381 at the archive, you will see @file{blues} and @file{classical} at its
4382 end. There will be a total of two versions of the member @samp{blues};
4383 the one at the end will be newer and larger, since you added text before
4386 (The reason @command{tar} does not overwrite the older file when updating
4387 it is because writing to the middle of a section of tape is a difficult
4388 process. Tapes are not designed to go backward. @xref{Media}, for more
4389 information about tapes.
4391 @option{--update} (@option{-u}) is not suitable for performing backups for two
4392 reasons: it does not change directory content entries, and it
4393 lengthens the archive every time it is used. The @GNUTAR{}
4394 options intended specifically for backups are more
4395 efficient. If you need to run backups, please consult @ref{Backups}.
4398 @subsection Combining Archives with @option{--concatenate}
4400 @cindex Adding archives to an archive
4401 @cindex Concatenating Archives
4402 @opindex concatenate
4404 @c @cindex @option{-A} described
4405 Sometimes it may be convenient to add a second archive onto the end of
4406 an archive rather than adding individual files to the archive. To add
4407 one or more archives to the end of another archive, you should use the
4408 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4410 To use @option{--concatenate}, give the first archive with
4411 @option{--file} option and name the rest of archives to be
4412 concatenated on the command line. The members, and their member
4413 names, will be copied verbatim from those archives to the first one.
4414 @footnote{This can cause multiple members to have the same name, for
4415 information on how this affects reading the archive, @ref{multiple}.}
4416 The new, concatenated archive will be called by the same name as the
4417 one given with the @option{--file} option. As usual, if you omit
4418 @option{--file}, @command{tar} will use the value of the environment
4419 variable @env{TAPE}, or, if this has not been set, the default archive name.
4421 @FIXME{There is no way to specify a new name...}
4423 To demonstrate how @option{--concatenate} works, create two small archives
4424 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4425 files from @file{practice}:
4428 $ @kbd{tar -cvf bluesrock.tar blues rock}
4431 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4437 If you like, You can run @samp{tar --list} to make sure the archives
4438 contain what they are supposed to:
4441 $ @kbd{tar -tvf bluesrock.tar}
4442 -rw-r--r-- melissa user 105 1997-01-21 19:42 blues
4443 -rw-r--r-- melissa user 33 1997-01-20 15:34 rock
4444 $ @kbd{tar -tvf jazzfolk.tar}
4445 -rw-r--r-- melissa user 20 1996-09-23 16:44 folk
4446 -rw-r--r-- melissa user 65 1997-01-30 14:15 jazz
4449 We can concatenate these two archives with @command{tar}:
4453 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4456 If you now list the contents of the @file{bluesrock.tar}, you will see
4457 that now it also contains the archive members of @file{jazzfolk.tar}:
4460 $ @kbd{tar --list --file=bluesrock.tar}
4467 When you use @option{--concatenate}, the source and target archives must
4468 already exist and must have been created using compatible format
4469 parameters. Notice, that @command{tar} does not check whether the
4470 archives it concatenates have compatible formats, it does not
4471 even check if the files are really tar archives.
4473 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4474 tape drives, due to deficiencies in the formats those tape drives use.
4476 @cindex @code{concatenate} vs @command{cat}
4477 @cindex @command{cat} vs @code{concatenate}
4478 It may seem more intuitive to you to want or try to use @command{cat} to
4479 concatenate two archives instead of using the @option{--concatenate}
4480 operation; after all, @command{cat} is the utility for combining files.
4482 However, @command{tar} archives incorporate an end-of-file marker which
4483 must be removed if the concatenated archives are to be read properly as
4484 one archive. @option{--concatenate} removes the end-of-archive marker
4485 from the target archive before each new archive is appended. If you use
4486 @command{cat} to combine the archives, the result will not be a valid
4487 @command{tar} format archive. If you need to retrieve files from an
4488 archive that was added to using the @command{cat} utility, use the
4489 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4490 information on dealing with archives improperly combined using the
4491 @command{cat} shell utility.
4494 @subsection Removing Archive Members Using @option{--delete}
4496 @cindex Deleting files from an archive
4497 @cindex Removing files from an archive
4500 You can remove members from an archive by using the @option{--delete}
4501 option. Specify the name of the archive with @option{--file}
4502 (@option{-f}) and then specify the names of the members to be deleted;
4503 if you list no member names, nothing will be deleted. The
4504 @option{--verbose} option will cause @command{tar} to print the names
4505 of the members as they are deleted. As with @option{--extract}, you
4506 must give the exact member names when using @samp{tar --delete}.
4507 @option{--delete} will remove all versions of the named file from the
4508 archive. The @option{--delete} operation can run very slowly.
4510 Unlike other operations, @option{--delete} has no short form.
4512 @cindex Tapes, using @option{--delete} and
4513 @cindex Deleting from tape archives
4514 This operation will rewrite the archive. You can only use
4515 @option{--delete} on an archive if the archive device allows you to
4516 write to any point on the media, such as a disk; because of this, it
4517 does not work on magnetic tapes. Do not try to delete an archive member
4518 from a magnetic tape; the action will not succeed, and you will be
4519 likely to scramble the archive and damage your tape. There is no safe
4520 way (except by completely re-writing the archive) to delete files from
4521 most kinds of magnetic tape. @xref{Media}.
4523 To delete all versions of the file @file{blues} from the archive
4524 @file{collection.tar} in the @file{practice} directory, make sure you
4525 are in that directory, and then,
4528 $ @kbd{tar --list --file=collection.tar}
4533 $ @kbd{tar --delete --file=collection.tar blues}
4534 $ @kbd{tar --list --file=collection.tar}
4541 @FIXME{Check if the above listing is actually produced after running
4542 all the examples on collection.tar.}
4544 The @option{--delete} option has been reported to work properly when
4545 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4548 @subsection Comparing Archive Members with the File System
4549 @cindex Verifying the currency of an archive
4553 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4554 specified archive members against files with the same names, and then
4555 reports differences in file size, mode, owner, modification date and
4556 contents. You should @emph{only} specify archive member names, not file
4557 names. If you do not name any members, then @command{tar} will compare the
4558 entire archive. If a file is represented in the archive but does not
4559 exist in the file system, @command{tar} reports a difference.
4561 You have to specify the record size of the archive when modifying an
4562 archive with a non-default record size.
4564 @command{tar} ignores files in the file system that do not have
4565 corresponding members in the archive.
4567 The following example compares the archive members @file{rock},
4568 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4569 files of the same name in the file system. (Note that there is no file,
4570 @file{funk}; @command{tar} will report an error message.)
4573 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4576 tar: funk not found in archive
4579 The spirit behind the @option{--compare} (@option{--diff},
4580 @option{-d}) option is to check whether the archive represents the
4581 current state of files on disk, more than validating the integrity of
4582 the archive media. For this later goal, @xref{verify}.
4584 @node create options
4585 @section Options Used by @option{--create}
4587 @xopindex{create, additional options}
4588 The previous chapter described the basics of how to use
4589 @option{--create} (@option{-c}) to create an archive from a set of files.
4590 @xref{create}. This section described advanced options to be used with
4594 * override:: Overriding File Metadata.
4595 * Ignore Failed Read::
4599 @subsection Overriding File Metadata
4601 As described above, a @command{tar} archive keeps, for each member it contains,
4602 its @dfn{metadata}, such as modification time, mode and ownership of
4603 the file. @GNUTAR{} allows to replace these data with other values
4604 when adding files to the archive. The options described in this
4605 section affect creation of archives of any type. For POSIX archives,
4606 see also @ref{PAX keywords}, for additional ways of controlling
4607 metadata, stored in the archive.
4611 @item --mode=@var{permissions}
4613 When adding files to an archive, @command{tar} will use
4614 @var{permissions} for the archive members, rather than the permissions
4615 from the files. @var{permissions} can be specified either as an octal
4616 number or as symbolic permissions, like with
4617 @command{chmod} (@xref{File permissions, Permissions, File
4618 permissions, fileutils, @acronym{GNU} file utilities}. This reference
4619 also has useful information for those not being overly familiar with
4620 the UNIX permission system). Using latter syntax allows for
4621 more flexibility. For example, the value @samp{a+rw} adds read and write
4622 permissions for everybody, while retaining executable bits on directories
4623 or on any other file already marked as executable:
4626 $ @kbd{tar -c -f archive.tar --mode='a+rw' .}
4629 @item --mtime=@var{date}
4632 When adding files to an archive, @command{tar} will use @var{date} as
4633 the modification time of members when creating archives, instead of
4634 their actual modification times. The argument @var{date} can be
4635 either a textual date representation in almost arbitrary format
4636 (@pxref{Date input formats}) or a name of the existing file, starting
4637 with @samp{/} or @samp{.}. In the latter case, the modification time
4638 of that file will be used.
4640 The following example will set the modification date to 00:00:00 UTC,
4644 $ @kbd{tar -c -f archive.tar --mtime='1970-01-01' .}
4648 When used with @option{--verbose} (@pxref{verbose tutorial}) @GNUTAR{}
4649 will try to convert the specified date back to its textual
4650 representation and compare it with the one given with
4651 @option{--mtime} options. If the two dates differ, @command{tar} will
4652 print a warning saying what date it will use. This is to help user
4653 ensure he is using the right date.
4658 $ @kbd{tar -c -f archive.tar -v --mtime=yesterday .}
4659 tar: Option --mtime: Treating date `yesterday' as 2006-06-20
4664 @item --owner=@var{user}
4667 Specifies that @command{tar} should use @var{user} as the owner of members
4668 when creating archives, instead of the user associated with the source
4669 file. The argument @var{user} can be either an existing user symbolic
4670 name, or a decimal numeric user @acronym{ID}.
4672 There is no value indicating a missing number, and @samp{0} usually means
4673 @code{root}. Some people like to force @samp{0} as the value to offer in
4674 their distributions for the owner of files, because the @code{root} user is
4675 anonymous anyway, so that might as well be the owner of anonymous
4676 archives. For example:
4680 $ @kbd{tar -c -f archive.tar --owner=0 .}
4682 $ @kbd{tar -c -f archive.tar --owner=root .}
4686 @item --group=@var{group}
4689 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
4690 rather than the group from the source file. The argument @var{group}
4691 can be either an existing group symbolic name, or a decimal numeric group @acronym{ID}.
4694 @node Ignore Failed Read
4695 @subsection Ignore Fail Read
4698 @item --ignore-failed-read
4699 @opindex ignore-failed-read
4700 Do not exit with nonzero on unreadable files or directories.
4703 @node extract options
4704 @section Options Used by @option{--extract}
4707 @xopindex{extract, additional options}
4708 The previous chapter showed how to use @option{--extract} to extract
4709 an archive into the file system. Various options cause @command{tar} to
4710 extract more information than just file contents, such as the owner,
4711 the permissions, the modification date, and so forth. This section
4712 presents options to be used with @option{--extract} when certain special
4713 considerations arise. You may review the information presented in
4714 @ref{extract} for more basic information about the
4715 @option{--extract} operation.
4718 * Reading:: Options to Help Read Archives
4719 * Writing:: Changing How @command{tar} Writes Files
4720 * Scarce:: Coping with Scarce Resources
4724 @subsection Options to Help Read Archives
4725 @cindex Options when reading archives
4728 @cindex Reading incomplete records
4729 @cindex Records, incomplete
4730 @opindex read-full-records
4731 Normally, @command{tar} will request data in full record increments from
4732 an archive storage device. If the device cannot return a full record,
4733 @command{tar} will report an error. However, some devices do not always
4734 return full records, or do not require the last record of an archive to
4735 be padded out to the next record boundary. To keep reading until you
4736 obtain a full record, or to accept an incomplete record if it contains
4737 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
4738 in conjunction with the @option{--extract} or @option{--list} operations.
4741 The @option{--read-full-records} (@option{-B}) option is turned on by default when
4742 @command{tar} reads an archive from standard input, or from a remote
4743 machine. This is because on @acronym{BSD} Unix systems, attempting to read a
4744 pipe returns however much happens to be in the pipe, even if it is
4745 less than was requested. If this option were not enabled, @command{tar}
4746 would fail as soon as it read an incomplete record from the pipe.
4748 If you're not sure of the blocking factor of an archive, you can
4749 read the archive by specifying @option{--read-full-records} (@option{-B}) and
4750 @option{--blocking-factor=@var{512-size}} (@option{-b
4751 @var{512-size}}), using a blocking factor larger than what the archive
4752 uses. This lets you avoid having to determine the blocking factor
4753 of an archive. @xref{Blocking Factor}.
4756 * read full records::
4760 @node read full records
4761 @unnumberedsubsubsec Reading Full Records
4763 @FIXME{need sentence or so of intro here}
4766 @opindex read-full-records
4767 @item --read-full-records
4769 Use in conjunction with @option{--extract} (@option{--get},
4770 @option{-x}) to read an archive which contains incomplete records, or
4771 one which has a blocking factor less than the one specified.
4775 @unnumberedsubsubsec Ignoring Blocks of Zeros
4777 @cindex End-of-archive blocks, ignoring
4778 @cindex Ignoring end-of-archive blocks
4779 @opindex ignore-zeros
4780 Normally, @command{tar} stops reading when it encounters a block of zeros
4781 between file entries (which usually indicates the end of the archive).
4782 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
4783 completely read an archive which contains a block of zeros before the
4784 end (i.e., a damaged archive, or one that was created by concatenating
4785 several archives together).
4787 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
4788 versions of @command{tar} write garbage after the end-of-archive entry,
4789 since that part of the media is never supposed to be read. @GNUTAR{}
4790 does not write after the end of an archive, but seeks to
4791 maintain compatibility among archiving utilities.
4794 @item --ignore-zeros
4796 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
4797 encountered while reading an archive. Use in conjunction with
4798 @option{--extract} or @option{--list}.
4802 @subsection Changing How @command{tar} Writes Files
4805 @FIXME{Introductory paragraph}
4808 * Dealing with Old Files::
4809 * Overwrite Old Files::
4811 * Keep Newer Files::
4813 * Recursive Unlink::
4814 * Data Modification Times::
4815 * Setting Access Permissions::
4816 * Directory Modification Times and Permissions::
4817 * Writing to Standard Output::
4818 * Writing to an External Program::
4822 @node Dealing with Old Files
4823 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
4825 @xopindex{overwrite-dir, introduced}
4826 When extracting files, if @command{tar} discovers that the extracted
4827 file already exists, it normally replaces the file by removing it before
4828 extracting it, to prevent confusion in the presence of hard or symbolic
4829 links. (If the existing file is a symbolic link, it is removed, not
4830 followed.) However, if a directory cannot be removed because it is
4831 nonempty, @command{tar} normally overwrites its metadata (ownership,
4832 permission, etc.). The @option{--overwrite-dir} option enables this
4833 default behavior. To be more cautious and preserve the metadata of
4834 such a directory, use the @option{--no-overwrite-dir} option.
4836 @cindex Overwriting old files, prevention
4837 @xopindex{keep-old-files, introduced}
4838 To be even more cautious and prevent existing files from being replaced, use
4839 the @option{--keep-old-files} (@option{-k}) option. It causes @command{tar} to refuse
4840 to replace or update a file that already exists, i.e., a file with the
4841 same name as an archive member prevents extraction of that archive
4842 member. Instead, it reports an error.
4844 @xopindex{overwrite, introduced}
4845 To be more aggressive about altering existing files, use the
4846 @option{--overwrite} option. It causes @command{tar} to overwrite
4847 existing files and to follow existing symbolic links when extracting.
4849 @cindex Protecting old files
4850 Some people argue that @GNUTAR{} should not hesitate
4851 to overwrite files with other files when extracting. When extracting
4852 a @command{tar} archive, they expect to see a faithful copy of the
4853 state of the file system when the archive was created. It is debatable
4854 that this would always be a proper behavior. For example, suppose one
4855 has an archive in which @file{usr/local} is a link to
4856 @file{usr/local2}. Since then, maybe the site removed the link and
4857 renamed the whole hierarchy from @file{/usr/local2} to
4858 @file{/usr/local}. Such things happen all the time. I guess it would
4859 not be welcome at all that @GNUTAR{} removes the
4860 whole hierarchy just to make room for the link to be reinstated
4861 (unless it @emph{also} simultaneously restores the full
4862 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
4863 able to remove a whole hierarchy to reestablish a symbolic link, for
4864 example, but @emph{only if} @option{--recursive-unlink} is specified
4865 to allow this behavior. In any case, single files are silently
4868 @xopindex{unlink-first, introduced}
4869 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
4870 some cases by causing @command{tar} to remove files unconditionally
4871 before extracting them.
4873 @node Overwrite Old Files
4874 @unnumberedsubsubsec Overwrite Old Files
4879 Overwrite existing files and directory metadata when extracting files
4882 This causes @command{tar} to write extracted files into the file system without
4883 regard to the files already on the system; i.e., files with the same
4884 names as archive members are overwritten when the archive is extracted.
4885 It also causes @command{tar} to extract the ownership, permissions,
4886 and time stamps onto any preexisting files or directories.
4887 If the name of a corresponding file name is a symbolic link, the file
4888 pointed to by the symbolic link will be overwritten instead of the
4889 symbolic link itself (if this is possible). Moreover, special devices,
4890 empty directories and even symbolic links are automatically removed if
4891 they are in the way of extraction.
4893 Be careful when using the @option{--overwrite} option, particularly when
4894 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
4895 can change the contents, ownership or permissions of any file on your
4896 system. Also, many systems do not take kindly to overwriting files that
4897 are currently being executed.
4899 @opindex overwrite-dir
4900 @item --overwrite-dir
4901 Overwrite the metadata of directories when extracting files from an
4902 archive, but remove other files before extracting.
4905 @node Keep Old Files
4906 @unnumberedsubsubsec Keep Old Files
4909 @opindex keep-old-files
4910 @item --keep-old-files
4912 Do not replace existing files from archive. The
4913 @option{--keep-old-files} (@option{-k}) option prevents @command{tar}
4914 from replacing existing files with files with the same name from the
4915 archive. The @option{--keep-old-files} option is meaningless with
4916 @option{--list} (@option{-t}). Prevents @command{tar} from replacing
4917 files in the file system during extraction.
4920 @node Keep Newer Files
4921 @unnumberedsubsubsec Keep Newer Files
4924 @opindex keep-newer-files
4925 @item --keep-newer-files
4926 Do not replace existing files that are newer than their archive
4927 copies. This option is meaningless with @option{--list} (@option{-t}).
4931 @unnumberedsubsubsec Unlink First
4934 @opindex unlink-first
4935 @item --unlink-first
4937 Remove files before extracting over them.
4938 This can make @command{tar} run a bit faster if you know in advance
4939 that the extracted files all need to be removed. Normally this option
4940 slows @command{tar} down slightly, so it is disabled by default.
4943 @node Recursive Unlink
4944 @unnumberedsubsubsec Recursive Unlink
4947 @opindex recursive-unlink
4948 @item --recursive-unlink
4949 When this option is specified, try removing files and directory hierarchies
4950 before extracting over them. @emph{This is a dangerous option!}
4953 If you specify the @option{--recursive-unlink} option,
4954 @command{tar} removes @emph{anything} that keeps you from extracting a file
4955 as far as current permissions will allow it. This could include removal
4956 of the contents of a full directory hierarchy.
4958 @node Data Modification Times
4959 @unnumberedsubsubsec Setting Data Modification Times
4961 @cindex Data modification times of extracted files
4962 @cindex Modification times of extracted files
4963 Normally, @command{tar} sets the data modification times of extracted
4964 files to the corresponding times recorded for the files in the archive, but
4965 limits the permissions of extracted files by the current @code{umask}
4968 To set the data modification times of extracted files to the time when
4969 the files were extracted, use the @option{--touch} (@option{-m}) option in
4970 conjunction with @option{--extract} (@option{--get}, @option{-x}).
4976 Sets the data modification time of extracted archive members to the time
4977 they were extracted, not the time recorded for them in the archive.
4978 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
4981 @node Setting Access Permissions
4982 @unnumberedsubsubsec Setting Access Permissions
4984 @cindex Permissions of extracted files
4985 @cindex Modes of extracted files
4986 To set the modes (access permissions) of extracted files to those
4987 recorded for those files in the archive, use @option{--same-permissions}
4988 in conjunction with the @option{--extract} (@option{--get},
4989 @option{-x}) operation.
4992 @opindex preserve-permissions
4993 @opindex same-permissions
4994 @item --preserve-permissions
4995 @itemx --same-permissions
4996 @c @itemx --ignore-umask
4998 Set modes of extracted archive members to those recorded in the
4999 archive, instead of current umask settings. Use in conjunction with
5000 @option{--extract} (@option{--get}, @option{-x}).
5003 @node Directory Modification Times and Permissions
5004 @unnumberedsubsubsec Directory Modification Times and Permissions
5006 After successfully extracting a file member, @GNUTAR{} normally
5007 restores its permissions and modification times, as described in the
5008 previous sections. This cannot be done for directories, because
5009 after extracting a directory @command{tar} will almost certainly
5010 extract files into that directory and this will cause the directory
5011 modification time to be updated. Moreover, restoring that directory
5012 permissions may not permit file creation within it. Thus, restoring
5013 directory permissions and modification times must be delayed at least
5014 until all files have been extracted into that directory. @GNUTAR{}
5015 restores directories using the following approach.
5017 The extracted directories are created with the mode specified in the
5018 archive, as modified by the umask of the user, which gives sufficient
5019 permissions to allow file creation. The meta-information about the
5020 directory is recorded in the temporary list of directories. When
5021 preparing to extract next archive member, @GNUTAR{} checks if the
5022 directory prefix of this file contains the remembered directory. If
5023 it does not, the program assumes that all files have been extracted
5024 into that directory, restores its modification time and permissions
5025 and removes its entry from the internal list. This approach allows
5026 to correctly restore directory meta-information in the majority of
5027 cases, while keeping memory requirements sufficiently small. It is
5028 based on the fact, that most @command{tar} archives use the predefined
5029 order of members: first the directory, then all the files and
5030 subdirectories in that directory.
5032 However, this is not always true. The most important exception are
5033 incremental archives (@pxref{Incremental Dumps}). The member order in
5034 an incremental archive is reversed: first all directory members are
5035 stored, followed by other (non-directory) members. So, when extracting
5036 from incremental archives, @GNUTAR{} alters the above procedure. It
5037 remembers all restored directories, and restores their meta-data
5038 only after the entire archive has been processed. Notice, that you do
5039 not need to specify any special options for that, as @GNUTAR{}
5040 automatically detects archives in incremental format.
5042 There may be cases, when such processing is required for normal archives
5043 too. Consider the following example:
5047 $ @kbd{tar --no-recursion -cvf archive \
5048 foo foo/file1 bar bar/file foo/file2}
5057 During the normal operation, after encountering @file{bar}
5058 @GNUTAR{} will assume that all files from the directory @file{foo}
5059 were already extracted and will therefore restore its timestamp and
5060 permission bits. However, after extracting @file{foo/file2} the
5061 directory timestamp will be offset again.
5063 To correctly restore directory meta-information in such cases, use
5064 @option{delay-directory-restore} command line option:
5067 @opindex delay-directory-restore
5068 @item --delay-directory-restore
5069 Delays restoring of the modification times and permissions of extracted
5070 directories until the end of extraction. This way, correct
5071 meta-information is restored even if the archive has unusual member
5074 @opindex no-delay-directory-restore
5075 @item --no-delay-directory-restore
5076 Cancel the effect of the previous @option{--delay-directory-restore}.
5077 Use this option if you have used @option{--delay-directory-restore} in
5078 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
5079 temporarily disable it.
5082 @node Writing to Standard Output
5083 @unnumberedsubsubsec Writing to Standard Output
5085 @cindex Writing extracted files to standard output
5086 @cindex Standard output, writing extracted files to
5087 To write the extracted files to the standard output, instead of
5088 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
5089 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
5090 extracting files to send them through a pipe, and do not need to
5091 preserve them in the file system. If you extract multiple members,
5092 they appear on standard output concatenated, in the order they are
5093 found in the archive.
5099 Writes files to the standard output. Use only in conjunction with
5100 @option{--extract} (@option{--get}, @option{-x}). When this option is
5101 used, instead of creating the files specified, @command{tar} writes
5102 the contents of the files extracted to its standard output. This may
5103 be useful if you are only extracting the files in order to send them
5104 through a pipe. This option is meaningless with @option{--list}
5108 This can be useful, for example, if you have a tar archive containing
5109 a big file and don't want to store the file on disk before processing
5110 it. You can use a command like this:
5113 tar -xOzf foo.tgz bigfile | process
5116 or even like this if you want to process the concatenation of the files:
5119 tar -xOzf foo.tgz bigfile1 bigfile2 | process
5122 However, @option{--to-command} may be more convenient for use with
5123 multiple files. See the next section.
5125 @node Writing to an External Program
5126 @unnumberedsubsubsec Writing to an External Program
5128 You can instruct @command{tar} to send the contents of each extracted
5129 file to the standard input of an external program:
5133 @item --to-command=@var{command}
5134 Extract files and pipe their contents to the standard input of
5135 @var{command}. When this option is used, instead of creating the
5136 files specified, @command{tar} invokes @var{command} and pipes the
5137 contents of the files to its standard output. @var{Command} may
5138 contain command line arguments. The program is executed via
5139 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
5140 extracted. Non-regular files (directories, etc.) are ignored when this
5144 The command can obtain the information about the file it processes
5145 from the following environment variables:
5148 @vrindex TAR_FILETYPE, to-command environment
5150 Type of the file. It is a single letter with the following meaning:
5152 @multitable @columnfractions 0.10 0.90
5153 @item f @tab Regular file
5154 @item d @tab Directory
5155 @item l @tab Symbolic link
5156 @item h @tab Hard link
5157 @item b @tab Block device
5158 @item c @tab Character device
5161 Currently only regular files are supported.
5163 @vrindex TAR_MODE, to-command environment
5165 File mode, an octal number.
5167 @vrindex TAR_FILENAME, to-command environment
5169 The name of the file.
5171 @vrindex TAR_REALNAME, to-command environment
5173 Name of the file as stored in the archive.
5175 @vrindex TAR_UNAME, to-command environment
5177 Name of the file owner.
5179 @vrindex TAR_GNAME, to-command environment
5181 Name of the file owner group.
5183 @vrindex TAR_ATIME, to-command environment
5185 Time of last access. It is a decimal number, representing seconds
5186 since the epoch. If the archive provides times with nanosecond
5187 precision, the nanoseconds are appended to the timestamp after a
5190 @vrindex TAR_MTIME, to-command environment
5192 Time of last modification.
5194 @vrindex TAR_CTIME, to-command environment
5196 Time of last status change.
5198 @vrindex TAR_SIZE, to-command environment
5202 @vrindex TAR_UID, to-command environment
5204 UID of the file owner.
5206 @vrindex TAR_GID, to-command environment
5208 GID of the file owner.
5211 In addition to these variables, @env{TAR_VERSION} contains the
5212 @GNUTAR{} version number.
5214 If @var{command} exits with a non-0 status, @command{tar} will print
5215 an error message similar to the following:
5218 tar: 2345: Child returned status 1
5221 Here, @samp{2345} is the PID of the finished process.
5223 If this behavior is not wanted, use @option{--ignore-command-error}:
5226 @opindex ignore-command-error
5227 @item --ignore-command-error
5228 Ignore exit codes of subprocesses. Notice that if the program
5229 exits on signal or otherwise terminates abnormally, the error message
5230 will be printed even if this option is used.
5232 @opindex no-ignore-command-error
5233 @item --no-ignore-command-error
5234 Cancel the effect of any previous @option{--ignore-command-error}
5235 option. This option is useful if you have set
5236 @option{--ignore-command-error} in @env{TAR_OPTIONS}
5237 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
5241 @unnumberedsubsubsec Removing Files
5243 @FIXME{The section is too terse. Something more to add? An example,
5247 @opindex remove-files
5248 @item --remove-files
5249 Remove files after adding them to the archive.
5253 @subsection Coping with Scarce Resources
5256 @cindex Small memory
5257 @cindex Running out of space
5265 @unnumberedsubsubsec Starting File
5268 @opindex starting-file
5269 @item --starting-file=@var{name}
5270 @itemx -K @var{name}
5271 Starts an operation in the middle of an archive. Use in conjunction
5272 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
5275 @cindex Middle of the archive, starting in the
5276 If a previous attempt to extract files failed due to lack of disk
5277 space, you can use @option{--starting-file=@var{name}} (@option{-K
5278 @var{name}}) to start extracting only after member @var{name} of the
5279 archive. This assumes, of course, that there is now free space, or
5280 that you are now extracting into a different file system. (You could
5281 also choose to suspend @command{tar}, remove unnecessary files from
5282 the file system, and then restart the same @command{tar} operation.
5283 In this case, @option{--starting-file} is not necessary.
5284 @xref{Incremental Dumps}, @xref{interactive}, and @ref{exclude}.)
5287 @unnumberedsubsubsec Same Order
5290 @cindex Large lists of file names on small machines
5292 @opindex preserve-order
5294 @itemx --preserve-order
5296 To process large lists of file names on machines with small amounts of
5297 memory. Use in conjunction with @option{--compare} (@option{--diff},
5298 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
5299 (@option{--get}, @option{-x}).
5302 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
5303 names to be listed or extracted is sorted in the same order as the
5304 files in the archive. This allows a large list of names to be used,
5305 even on a small machine that would not otherwise be able to hold all
5306 the names in memory at the same time. Such a sorted list can easily be
5307 created by running @samp{tar -t} on the archive and editing its output.
5309 This option is probably never needed on modern computer systems.
5312 @section Backup options
5314 @cindex backup options
5316 @GNUTAR{} offers options for making backups of files
5317 before writing new versions. These options control the details of
5318 these backups. They may apply to the archive itself before it is
5319 created or rewritten, as well as individual extracted members. Other
5320 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
5321 and @command{mv}, for example) offer similar options.
5323 Backup options may prove unexpectedly useful when extracting archives
5324 containing many members having identical name, or when extracting archives
5325 on systems having file name limitations, making different members appear
5326 has having similar names through the side-effect of name truncation.
5327 (This is true only if we have a good scheme for truncated backup names,
5328 which I'm not sure at all: I suspect work is needed in this area.)
5329 When any existing file is backed up before being overwritten by extraction,
5330 then clashing files are automatically be renamed to be unique, and the
5331 true name is kept for only the last file of a series of clashing files.
5332 By using verbose mode, users may track exactly what happens.
5334 At the detail level, some decisions are still experimental, and may
5335 change in the future, we are waiting comments from our users. So, please
5336 do not learn to depend blindly on the details of the backup features.
5337 For example, currently, directories themselves are never renamed through
5338 using these options, so, extracting a file over a directory still has
5339 good chances to fail. Also, backup options apply to created archives,
5340 not only to extracted members. For created archives, backups will not
5341 be attempted when the archive is a block or character device, or when it
5342 refers to a remote file.
5344 For the sake of simplicity and efficiency, backups are made by renaming old
5345 files prior to creation or extraction, and not by copying. The original
5346 name is restored if the file creation fails. If a failure occurs after a
5347 partial extraction of a file, both the backup and the partially extracted
5351 @item --backup[=@var{method}]
5353 @vindex VERSION_CONTROL
5355 Back up files that are about to be overwritten or removed.
5356 Without this option, the original versions are destroyed.
5358 Use @var{method} to determine the type of backups made.
5359 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
5360 environment variable. And if @env{VERSION_CONTROL} is not set,
5361 use the @samp{existing} method.
5363 @vindex version-control @r{Emacs variable}
5364 This option corresponds to the Emacs variable @samp{version-control};
5365 the same values for @var{method} are accepted as in Emacs. This option
5366 also allows more descriptive names. The valid @var{method}s are:
5371 @cindex numbered @r{backup method}
5372 Always make numbered backups.
5376 @cindex existing @r{backup method}
5377 Make numbered backups of files that already have them, simple backups
5382 @cindex simple @r{backup method}
5383 Always make simple backups.
5387 @item --suffix=@var{suffix}
5389 @cindex backup suffix
5390 @vindex SIMPLE_BACKUP_SUFFIX
5391 Append @var{suffix} to each backup file made with @option{--backup}. If this
5392 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5393 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5394 set, the default is @samp{~}, just as in Emacs.
5399 @section Notable @command{tar} Usages
5402 @FIXME{Using Unix file linking capability to recreate directory
5403 structures---linking files into one subdirectory and then
5404 @command{tar}ring that directory.}
5406 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5409 You can easily use archive files to transport a group of files from
5410 one system to another: put all relevant files into an archive on one
5411 computer system, transfer the archive to another system, and extract
5412 the contents there. The basic transfer medium might be magnetic tape,
5413 Internet FTP, or even electronic mail (though you must encode the
5414 archive with @command{uuencode} in order to transport it properly by
5415 mail). Both machines do not have to use the same operating system, as
5416 long as they both support the @command{tar} program.
5418 For example, here is how you might copy a directory's contents from
5419 one disk to another, while preserving the dates, modes, owners and
5420 link-structure of all the files therein. In this case, the transfer
5421 medium is a @dfn{pipe}, which is one a Unix redirection mechanism:
5424 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5428 You can avoid subshells by using @option{-C} option:
5431 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5435 The command also works using short option forms:
5438 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5439 | (cd targetdir; tar --extract --file=-)}
5441 $ @kbd{tar --directory sourcedir --create --file=- . ) \
5442 | tar --directory targetdir --extract --file=-}
5446 This is one of the easiest methods to transfer a @command{tar} archive.
5449 @section Looking Ahead: The Rest of this Manual
5451 You have now seen how to use all eight of the operations available to
5452 @command{tar}, and a number of the possible options. The next chapter
5453 explains how to choose and change file and archive names, how to use
5454 files to store names of other files which you can then call as
5455 arguments to @command{tar} (this can help you save time if you expect to
5456 archive the same list of files a number of times), and so forth.
5457 @FIXME{in case it's not obvious, i'm making this up in some sense
5458 based on my limited memory of what the next chapter *really* does. i
5459 just wanted to flesh out this final section a little bit so i'd
5460 remember to stick it in here. :-)}
5462 If there are too many files to conveniently list on the command line,
5463 you can list the names in a file, and @command{tar} will read that file.
5466 There are various ways of causing @command{tar} to skip over some files,
5467 and not archive them. @xref{Choosing}.
5470 @chapter Performing Backups and Restoring Files
5473 @GNUTAR{} is distributed along with the scripts
5474 which the Free Software Foundation uses for performing backups. There
5475 is no corresponding scripts available yet for doing restoration of
5476 files. Even if there is a good chance those scripts may be satisfying
5477 to you, they are not the only scripts or methods available for doing
5478 backups and restore. You may well create your own, or use more
5479 sophisticated packages dedicated to that purpose.
5481 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5482 Automatic Network Disk Archiver), a backup system developed by James
5483 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5484 This is free software, and it is available at these places:
5487 http://www.cs.umd.edu/projects/amanda/amanda.html
5488 ftp://ftp.cs.umd.edu/pub/amanda
5493 Here is a possible plan for a future documentation about the backuping
5494 scripts which are provided within the @GNUTAR{}
5500 @item what are dumps
5501 @item different levels of dumps
5503 @item full dump = dump everything
5504 @item level 1, level 2 dumps etc
5505 A level @var{n} dump dumps everything changed since the last level
5508 @item how to use scripts for dumps (ie, the concept)
5510 @item scripts to run after editing backup specs (details)
5512 @item Backup Specs, what is it.
5514 @item how to customize
5515 @item actual text of script [/sp/dump/backup-specs]
5519 @item rsh doesn't work
5520 @item rtape isn't installed
5523 @item the @option{--incremental} option of tar
5526 @item write protection
5527 @item types of media, different sizes and types, useful for different things
5528 @item files and tape marks
5529 one tape mark between files, two at end.
5530 @item positioning the tape
5531 MT writes two at end of write,
5532 backspaces over one when writing again.
5538 This chapter documents both the provided shell scripts and @command{tar}
5539 options which are more specific to usage as a backup tool.
5541 To @dfn{back up} a file system means to create archives that contain
5542 all the files in that file system. Those archives can then be used to
5543 restore any or all of those files (for instance if a disk crashes or a
5544 file is accidentally deleted). File system @dfn{backups} are also
5548 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5549 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5550 * Backup Levels:: Levels of Backups
5551 * Backup Parameters:: Setting Parameters for Backups and Restoration
5552 * Scripted Backups:: Using the Backup Scripts
5553 * Scripted Restoration:: Using the Restore Script
5557 @section Using @command{tar} to Perform Full Dumps
5563 @cindex corrupted archives
5564 Full dumps should only be made when no other people or programs
5565 are modifying files in the file system. If files are modified while
5566 @command{tar} is making the backup, they may not be stored properly in
5567 the archive, in which case you won't be able to restore them if you
5568 have to. (Files not being modified are written with no trouble, and do
5569 not corrupt the entire archive.)
5571 You will want to use the @option{--label=@var{archive-label}}
5572 (@option{-V @var{archive-label}}) option to give the archive a
5573 volume label, so you can tell what this archive is even if the label
5574 falls off the tape, or anything like that.
5576 Unless the file system you are dumping is guaranteed to fit on
5577 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5578 Make sure you have enough tapes on hand to complete the backup.
5580 If you want to dump each file system separately you will need to use
5581 the @option{--one-file-system} option to prevent
5582 @command{tar} from crossing file system boundaries when storing
5585 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5586 option is not needed, since this is a complete copy of everything in
5587 the file system, and a full restore from this backup would only be
5588 done onto a completely
5591 Unless you are in a hurry, and trust the @command{tar} program (and your
5592 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5593 option, to make sure your files really made it onto the dump properly.
5594 This will also detect cases where the file was modified while (or just
5595 after) it was being archived. Not all media (notably cartridge tapes)
5596 are capable of being verified, unfortunately.
5598 @node Incremental Dumps
5599 @section Using @command{tar} to Perform Incremental Dumps
5601 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5602 stores additional metadata so that exact state of the file system
5603 can be restored when extracting the archive.
5605 @GNUTAR{} currently offers two options for handling incremental
5606 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5607 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5609 @opindex listed-incremental
5610 The option @option{--listed-incremental} instructs tar to operate on
5611 an incremental archive with additional metadata stored in a standalone
5612 file, called a @dfn{snapshot file}. The purpose of this file is to help
5613 determine which files have been changed, added or deleted since the
5614 last backup, so that the next incremental backup will contain only
5615 modified files. The name of the snapshot file is given as an argument
5619 @item --listed-incremental=@var{file}
5620 @itemx -g @var{file}
5621 Handle incremental backups with snapshot data in @var{file}.
5624 To create an incremental backup, you would use
5625 @option{--listed-incremental} together with @option{--create}
5626 (@pxref{create}). For example:
5629 $ @kbd{tar --create \
5630 --file=archive.1.tar \
5631 --listed-incremental=/var/log/usr.snar \
5635 This will create in @file{archive.1.tar} an incremental backup of
5636 the @file{/usr} file system, storing additional metadata in the file
5637 @file{/var/log/usr.snar}. If this file does not exist, it will be
5638 created. The created archive will then be a @dfn{level 0 backup};
5639 please see the next section for more on backup levels.
5641 Otherwise, if the file @file{/var/log/usr.snar} exists, it
5642 determines which files are modified. In this case only these files will be
5643 stored in the archive. Suppose, for example, that after running the
5644 above command, you delete file @file{/usr/doc/old} and create
5645 directory @file{/usr/local/db} with the following contents:
5648 $ @kbd{ls /usr/local/db}
5653 Some time later you create another incremental backup. You will
5657 $ @kbd{tar --create \
5658 --file=archive.2.tar \
5659 --listed-incremental=/var/log/usr.snar \
5661 tar: usr/local/db: Directory is new
5668 The created archive @file{archive.2.tar} will contain only these
5669 three members. This archive is called a @dfn{level 1 backup}. Notice
5670 that @file{/var/log/usr.snar} will be updated with the new data, so if
5671 you plan to create more @samp{level 1} backups, it is necessary to
5672 create a working copy of the snapshot file before running
5673 @command{tar}. The above example will then be modified as follows:
5676 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
5677 $ @kbd{tar --create \
5678 --file=archive.2.tar \
5679 --listed-incremental=/var/log/usr.snar-1 \
5683 Incremental dumps depend crucially on time stamps, so the results are
5684 unreliable if you modify a file's time stamps during dumping (e.g.,
5685 with the @option{--atime-preserve=replace} option), or if you set the clock
5688 @anchor{device numbers}
5689 @cindex Device numbers, using in incremental backups
5690 Metadata stored in snapshot files include device numbers, which,
5691 obviously are supposed to be a non-volatile values. However, it turns
5692 out that @acronym{NFS} devices have undependable values when an automounter
5693 gets in the picture. This can lead to a great deal of spurious
5694 redumping in incremental dumps, so it is somewhat useless to compare
5695 two @acronym{NFS} devices numbers over time. The solution implemented
5696 currently is to considers all @acronym{NFS} devices as being equal
5697 when it comes to comparing directories; this is fairly gross, but
5698 there does not seem to be a better way to go.
5700 Apart from using @acronym{NFS}, there are a number of cases where
5701 relying on device numbers can cause spurious redumping of unmodified
5702 files. For example, this occurs when archiving @acronym{LVM} snapshot
5703 volumes. To avoid this, use @option{--no-check-device} option:
5706 @xopindex{no-check-device, described}
5707 @item --no-check-device
5708 Do not rely on device numbers when preparing a list of changed files
5709 for an incremental dump.
5711 @xopindex{check-device, described}
5712 @item --check-device
5713 Use device numbers when preparing a list of changed files
5714 for an incremental dump. This is the default behavior. The purpose
5715 of this option is to undo the effect of the @option{--no-check-device}
5716 if it was given in @env{TAR_OPTIONS} environment variable
5717 (@pxref{TAR_OPTIONS}).
5720 There is also another way to cope with changing device numbers. It is
5721 described in detail in @ref{Fixing Snapshot Files}.
5723 Note that incremental archives use @command{tar} extensions and may
5724 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
5726 @xopindex{listed-incremental, using with @option{--extract}}
5727 @xopindex{extract, using with @option{--listed-incremental}}
5728 To extract from the incremental dumps, use
5729 @option{--listed-incremental} together with @option{--extract}
5730 option (@pxref{extracting files}). In this case, @command{tar} does
5731 not need to access snapshot file, since all the data necessary for
5732 extraction are stored in the archive itself. So, when extracting, you
5733 can give whatever argument to @option{--listed-incremental}, the usual
5734 practice is to use @option{--listed-incremental=/dev/null}.
5735 Alternatively, you can use @option{--incremental}, which needs no
5736 arguments. In general, @option{--incremental} (@option{-G}) can be
5737 used as a shortcut for @option{--listed-incremental} when listing or
5738 extracting incremental backups (for more information, regarding this
5739 option, @pxref{incremental-op}).
5741 When extracting from the incremental backup @GNUTAR{} attempts to
5742 restore the exact state the file system had when the archive was
5743 created. In particular, it will @emph{delete} those files in the file
5744 system that did not exist in their directories when the archive was
5745 created. If you have created several levels of incremental files,
5746 then in order to restore the exact contents the file system had when
5747 the last level was created, you will need to restore from all backups
5748 in turn. Continuing our example, to restore the state of @file{/usr}
5749 file system, one would do@footnote{Notice, that since both archives
5750 were created without @option{-P} option (@pxref{absolute}), these
5751 commands should be run from the root file system.}:
5754 $ @kbd{tar --extract \
5755 --listed-incremental=/dev/null \
5756 --file archive.1.tar}
5757 $ @kbd{tar --extract \
5758 --listed-incremental=/dev/null \
5759 --file archive.2.tar}
5762 To list the contents of an incremental archive, use @option{--list}
5763 (@pxref{list}), as usual. To obtain more information about the
5764 archive, use @option{--listed-incremental} or @option{--incremental}
5765 combined with two @option{--verbose} options@footnote{Two
5766 @option{--verbose} options were selected to avoid breaking usual
5767 verbose listing output (@option{--list --verbose}) when using in
5770 @xopindex{incremental, using with @option{--list}}
5771 @xopindex{listed-incremental, using with @option{--list}}
5772 @xopindex{list, using with @option{--incremental}}
5773 @xopindex{list, using with @option{--listed-incremental}}
5774 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
5775 contents of the DUMPDIR header (with terminating nulls) when
5776 @option{--incremental} or @option{--listed-incremental} option was
5777 given, no matter what the verbosity level. This behavior, and,
5778 especially, the binary output it produced were considered inconvenient
5779 and were changed in version 1.16}:
5782 @kbd{tar --list --incremental --verbose --verbose archive.tar}
5785 This command will print, for each directory in the archive, the list
5786 of files in that directory at the time the archive was created. This
5787 information is put out in a format which is both human-readable and
5788 unambiguous for a program: each file name is printed as
5795 where @var{x} is a letter describing the status of the file: @samp{Y}
5796 if the file is present in the archive, @samp{N} if the file is not
5797 included in the archive, or a @samp{D} if the file is a directory (and
5798 is included in the archive). @xref{Dumpdir}, for the detailed
5799 description of dumpdirs and status codes. Each such
5800 line is terminated by a newline character. The last line is followed
5801 by an additional newline to indicate the end of the data.
5803 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
5804 gives the same behavior as @option{--listed-incremental} when used
5805 with @option{--list} and @option{--extract} options. When used with
5806 @option{--create} option, it creates an incremental archive without
5807 creating snapshot file. Thus, it is impossible to create several
5808 levels of incremental backups with @option{--incremental} option.
5811 @section Levels of Backups
5813 An archive containing all the files in the file system is called a
5814 @dfn{full backup} or @dfn{full dump}. You could insure your data by
5815 creating a full dump every day. This strategy, however, would waste a
5816 substantial amount of archive media and user time, as unchanged files
5817 are daily re-archived.
5819 It is more efficient to do a full dump only occasionally. To back up
5820 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
5821 one} dump archives all the files that have changed since the last full
5824 A typical dump strategy would be to perform a full dump once a week,
5825 and a level one dump once a day. This means some versions of files
5826 will in fact be archived more than once, but this dump strategy makes
5827 it possible to restore a file system to within one day of accuracy by
5828 only extracting two archives---the last weekly (full) dump and the
5829 last daily (level one) dump. The only information lost would be in
5830 files changed or created since the last daily backup. (Doing dumps
5831 more than once a day is usually not worth the trouble).
5833 @GNUTAR{} comes with scripts you can use to do full
5834 and level-one (actually, even level-two and so on) dumps. Using
5835 scripts (shell programs) to perform backups and restoration is a
5836 convenient and reliable alternative to typing out file name lists
5837 and @command{tar} commands by hand.
5839 Before you use these scripts, you need to edit the file
5840 @file{backup-specs}, which specifies parameters used by the backup
5841 scripts and by the restore script. This file is usually located
5842 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
5843 detailed description. Once the backup parameters are set, you can
5844 perform backups or restoration by running the appropriate script.
5846 The name of the backup script is @code{backup}. The name of the
5847 restore script is @code{restore}. The following sections describe
5848 their use in detail.
5850 @emph{Please Note:} The backup and restoration scripts are
5851 designed to be used together. While it is possible to restore files by
5852 hand from an archive which was created using a backup script, and to create
5853 an archive by hand which could then be extracted using the restore script,
5854 it is easier to use the scripts. @xref{Incremental Dumps}, before
5855 making such an attempt.
5857 @node Backup Parameters
5858 @section Setting Parameters for Backups and Restoration
5860 The file @file{backup-specs} specifies backup parameters for the
5861 backup and restoration scripts provided with @command{tar}. You must
5862 edit @file{backup-specs} to fit your system configuration and schedule
5863 before using these scripts.
5865 Syntactically, @file{backup-specs} is a shell script, containing
5866 mainly variable assignments. However, any valid shell construct
5867 is allowed in this file. Particularly, you may wish to define
5868 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
5869 For more information about shell script syntax, please refer to
5870 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
5871 g_02, the definition of the Shell Command Language}. See also
5872 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
5874 The shell variables controlling behavior of @code{backup} and
5875 @code{restore} are described in the following subsections.
5878 * General-Purpose Variables::
5879 * Magnetic Tape Control::
5881 * backup-specs example:: An Example Text of @file{Backup-specs}
5884 @node General-Purpose Variables
5885 @subsection General-Purpose Variables
5887 @defvr {Backup variable} ADMINISTRATOR
5888 The user name of the backup administrator. @code{Backup} scripts
5889 sends a backup report to this address.
5892 @defvr {Backup variable} BACKUP_HOUR
5893 The hour at which the backups are done. This can be a number from 0
5894 to 23, or the time specification in form @var{hours}:@var{minutes},
5895 or the string @samp{now}.
5897 This variable is used by @code{backup}. Its value may be overridden
5898 using @option{--time} option (@pxref{Scripted Backups}).
5901 @defvr {Backup variable} TAPE_FILE
5903 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
5904 is a remote archive (@pxref{remote-dev}), backup script will suppose
5905 that your @command{mt} is able to access remote devices. If @var{RSH}
5906 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
5907 invocations of @command{mt}.
5910 @defvr {Backup variable} BLOCKING
5912 The blocking factor @command{tar} will use when writing the dump archive.
5913 @xref{Blocking Factor}.
5916 @defvr {Backup variable} BACKUP_DIRS
5918 A list of file systems to be dumped (for @code{backup}), or restored
5919 (for @code{restore}). You can include any directory
5920 name in the list --- subdirectories on that file system will be
5921 included, regardless of how they may look to other networked machines.
5922 Subdirectories on other file systems will be ignored.
5924 The host name specifies which host to run @command{tar} on, and should
5925 normally be the host that actually contains the file system. However,
5926 the host machine must have @GNUTAR{} installed, and
5927 must be able to access the directory containing the backup scripts and
5928 their support files using the same file name that is used on the
5929 machine where the scripts are run (i.e., what @command{pwd} will print
5930 when in that directory on that machine). If the host that contains
5931 the file system does not have this capability, you can specify another
5932 host as long as it can access the file system through @acronym{NFS}.
5934 If the list of file systems is very long you may wish to put it
5935 in a separate file. This file is usually named
5936 @file{/etc/backup/dirs}, but this name may be overridden in
5937 @file{backup-specs} using @code{DIRLIST} variable.
5940 @defvr {Backup variable} DIRLIST
5942 The name of the file that contains a list of file systems to backup
5943 or restore. By default it is @file{/etc/backup/dirs}.
5946 @defvr {Backup variable} BACKUP_FILES
5948 A list of individual files to be dumped (for @code{backup}), or restored
5949 (for @code{restore}). These should be accessible from the machine on
5950 which the backup script is run.
5952 If the list of file systems is very long you may wish to store it
5953 in a separate file. This file is usually named
5954 @file{/etc/backup/files}, but this name may be overridden in
5955 @file{backup-specs} using @code{FILELIST} variable.
5958 @defvr {Backup variable} FILELIST
5960 The name of the file that contains a list of individual files to backup
5961 or restore. By default it is @file{/etc/backup/files}.
5964 @defvr {Backup variable} MT
5966 Full file name of @command{mt} binary.
5969 @defvr {Backup variable} RSH
5971 Full file name of @command{rsh} binary or its equivalent. You may wish to
5972 set it to @code{ssh}, to improve security. In this case you will have
5973 to use public key authentication.
5976 @defvr {Backup variable} RSH_COMMAND
5978 Full file name of @command{rsh} binary on remote machines. This will
5979 be passed via @option{--rsh-command} option to the remote invocation
5983 @defvr {Backup variable} VOLNO_FILE
5985 Name of temporary file to hold volume numbers. This needs to be accessible
5986 by all the machines which have file systems to be dumped.
5989 @defvr {Backup variable} XLIST
5991 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
5992 located on the remote machine and containing the list of files to
5993 be excluded from the backup. Exclude file lists are searched in
5994 /etc/tar-backup directory. A common use for exclude file lists
5995 is to exclude files containing security-sensitive information
5996 (e.g., @file{/etc/shadow} from backups).
5998 This variable affects only @code{backup}.
6001 @defvr {Backup variable} SLEEP_TIME
6003 Time to sleep between dumps of any two successive file systems
6005 This variable affects only @code{backup}.
6008 @defvr {Backup variable} DUMP_REMIND_SCRIPT
6010 Script to be run when it's time to insert a new tape in for the next
6011 volume. Administrators may want to tailor this script for their site.
6012 If this variable isn't set, @GNUTAR{} will display its built-in
6013 prompt, and will expect confirmation from the console. For the
6014 description of the default prompt, see @ref{change volume prompt}.
6018 @defvr {Backup variable} SLEEP_MESSAGE
6020 Message to display on the terminal while waiting for dump time. Usually
6021 this will just be some literal text.
6024 @defvr {Backup variable} TAR
6026 Full file name of the @GNUTAR{} executable. If this is not set, backup
6027 scripts will search @command{tar} in the current shell path.
6030 @node Magnetic Tape Control
6031 @subsection Magnetic Tape Control
6033 Backup scripts access tape device using special @dfn{hook functions}.
6034 These functions take a single argument -- the name of the tape
6035 device. Their names are kept in the following variables:
6037 @defvr {Backup variable} MT_BEGIN
6038 The name of @dfn{begin} function. This function is called before
6039 accessing the drive. By default it retensions the tape:
6045 mt -f "$1" retension
6050 @defvr {Backup variable} MT_REWIND
6051 The name of @dfn{rewind} function. The default definition is as
6064 @defvr {Backup variable} MT_OFFLINE
6065 The name of the function switching the tape off line. By default
6066 it is defined as follows:
6069 MT_OFFLINE=mt_offline
6077 @defvr {Backup variable} MT_STATUS
6078 The name of the function used to obtain the status of the archive device,
6079 including error count. Default definition:
6091 @subsection User Hooks
6093 @dfn{User hooks} are shell functions executed before and after
6094 each @command{tar} invocation. Thus, there are @dfn{backup
6095 hooks}, which are executed before and after dumping each file
6096 system, and @dfn{restore hooks}, executed before and
6097 after restoring a file system. Each user hook is a shell function
6098 taking four arguments:
6100 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
6105 Current backup or restore level.
6108 Name or IP address of the host machine being dumped or restored.
6111 Full file name of the file system being dumped or restored.
6114 File system name with directory separators replaced with colons. This
6115 is useful, e.g., for creating unique files.
6119 Following variables keep the names of user hook functions
6121 @defvr {Backup variable} DUMP_BEGIN
6122 Dump begin function. It is executed before dumping the file system.
6125 @defvr {Backup variable} DUMP_END
6126 Executed after dumping the file system.
6129 @defvr {Backup variable} RESTORE_BEGIN
6130 Executed before restoring the file system.
6133 @defvr {Backup variable} RESTORE_END
6134 Executed after restoring the file system.
6137 @node backup-specs example
6138 @subsection An Example Text of @file{Backup-specs}
6140 The following is an example of @file{backup-specs}:
6143 # site-specific parameters for file system backup.
6145 ADMINISTRATOR=friedman
6147 TAPE_FILE=/dev/nrsmt0
6149 # Use @code{ssh} instead of the less secure @code{rsh}
6151 RSH_COMMAND=/usr/bin/ssh
6153 # Override MT_STATUS function:
6159 # Disable MT_OFFLINE function
6176 apple-gunkies:/com/mailer/gnu
6177 apple-gunkies:/com/archive/gnu"
6179 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
6183 @node Scripted Backups
6184 @section Using the Backup Scripts
6186 The syntax for running a backup script is:
6189 backup --level=@var{level} --time=@var{time}
6192 The @option{level} option requests the dump level. Thus, to produce
6193 a full dump, specify @code{--level=0} (this is the default, so
6194 @option{--level} may be omitted if its value is @code{0}).
6195 @footnote{For backward compatibility, the @code{backup} will also
6196 try to deduce the requested dump level from the name of the
6197 script itself. If the name consists of a string @samp{level-}
6198 followed by a single decimal digit, that digit is taken as
6199 the dump level number. Thus, you may create a link from @code{backup}
6200 to @code{level-1} and then run @code{level-1} whenever you need to
6201 create a level one dump.}
6203 The @option{--time} option determines when should the backup be
6204 run. @var{Time} may take three forms:
6207 @item @var{hh}:@var{mm}
6209 The dump must be run at @var{hh} hours @var{mm} minutes.
6213 The dump must be run at @var{hh} hours
6217 The dump must be run immediately.
6220 You should start a script with a tape or disk mounted. Once you
6221 start a script, it prompts you for new tapes or disks as it
6222 needs them. Media volumes don't have to correspond to archive
6223 files --- a multi-volume archive can be started in the middle of a
6224 tape that already contains the end of another multi-volume archive.
6225 The @code{restore} script prompts for media by its archive volume,
6226 so to avoid an error message you should keep track of which tape
6227 (or disk) contains which volume of the archive (@pxref{Scripted
6230 The backup scripts write two files on the file system. The first is a
6231 record file in @file{/etc/tar-backup/}, which is used by the scripts
6232 to store and retrieve information about which files were dumped. This
6233 file is not meant to be read by humans, and should not be deleted by
6234 them. @xref{Snapshot Files}, for a more detailed explanation of this
6237 The second file is a log file containing the names of the file systems
6238 and files dumped, what time the backup was made, and any error
6239 messages that were generated, as well as how much space was left in
6240 the media volume after the last volume of the archive was written.
6241 You should check this log file after every backup. The file name is
6242 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
6243 represents current date, and @var{n} represents current dump level number.
6245 The script also prints the name of each system being dumped to the
6248 Following is the full list of options accepted by @code{backup}
6252 @item -l @var{level}
6253 @itemx --level=@var{level}
6254 Do backup level @var{level} (default 0).
6258 Force backup even if today's log file already exists.
6260 @item -v[@var{level}]
6261 @itemx --verbose[=@var{level}]
6262 Set verbosity level. The higher the level is, the more debugging
6263 information will be output during execution. Default @var{level}
6264 is 100, which means the highest debugging level.
6266 @item -t @var{start-time}
6267 @itemx --time=@var{start-time}
6268 Wait till @var{time}, then do backup.
6272 Display short help message and exit.
6276 Display information about the program's name, version, origin and legal
6277 status, all on standard output, and then exit successfully.
6281 @node Scripted Restoration
6282 @section Using the Restore Script
6284 To restore files that were archived using a scripted backup, use the
6285 @code{restore} script. Its usage is quite straightforward. In the
6286 simplest form, invoke @code{restore --all}, it will
6287 then restore all the file systems and files specified in
6288 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
6290 You may select the file systems (and/or files) to restore by
6291 giving @code{restore} list of @dfn{patterns} in its command
6292 line. For example, running
6299 will restore all file systems on the machine @samp{albert}. A more
6300 complicated example:
6303 restore 'albert:*' '*:/var'
6307 This command will restore all file systems on the machine @samp{albert}
6308 as well as @file{/var} file system on all machines.
6310 By default @code{restore} will start restoring files from the lowest
6311 available dump level (usually zero) and will continue through
6312 all available dump levels. There may be situations where such a
6313 thorough restore is not necessary. For example, you may wish to
6314 restore only files from the recent level one backup. To do so,
6315 use @option{--level} option, as shown in the example below:
6321 The full list of options accepted by @code{restore} follows:
6326 Restore all file systems and files specified in @file{backup-specs}
6328 @item -l @var{level}
6329 @itemx --level=@var{level}
6330 Start restoring from the given backup level, instead of the default 0.
6332 @item -v[@var{level}]
6333 @itemx --verbose[=@var{level}]
6334 Set verbosity level. The higher the level is, the more debugging
6335 information will be output during execution. Default @var{level}
6336 is 100, which means the highest debugging level.
6340 Display short help message and exit.
6344 Display information about the program's name, version, origin and legal
6345 status, all on standard output, and then exit successfully.
6348 You should start the restore script with the media containing the
6349 first volume of the archive mounted. The script will prompt for other
6350 volumes as they are needed. If the archive is on tape, you don't need
6351 to rewind the tape to to its beginning---if the tape head is
6352 positioned past the beginning of the archive, the script will rewind
6353 the tape as needed. @xref{Tape Positioning}, for a discussion of tape
6357 @strong{Warning:} The script will delete files from the active file
6358 system if they were not in the file system when the archive was made.
6361 @xref{Incremental Dumps}, for an explanation of how the script makes
6365 @chapter Choosing Files and Names for @command{tar}
6368 Certain options to @command{tar} enable you to specify a name for your
6369 archive. Other options let you decide which files to include or exclude
6370 from the archive, based on when or whether files were modified, whether
6371 the file names do or don't match specified patterns, or whether files
6372 are in specified directories.
6374 This chapter discusses these options in detail.
6377 * file:: Choosing the Archive's Name
6378 * Selecting Archive Members::
6379 * files:: Reading Names from a File
6380 * exclude:: Excluding Some Files
6381 * wildcards:: Wildcards Patterns and Matching
6382 * quoting styles:: Ways of Quoting Special Characters in Names
6383 * transform:: Modifying File and Member Names
6384 * after:: Operating Only on New Files
6385 * recurse:: Descending into Directories
6386 * one:: Crossing File System Boundaries
6390 @section Choosing and Naming Archive Files
6393 @cindex Naming an archive
6394 @cindex Archive Name
6395 @cindex Choosing an archive file
6396 @cindex Where is the archive?
6397 By default, @command{tar} uses an archive file name that was compiled when
6398 it was built on the system; usually this name refers to some physical
6399 tape drive on the machine. However, the person who installed @command{tar}
6400 on the system may not have set the default to a meaningful value as far as
6401 most users are concerned. As a result, you will usually want to tell
6402 @command{tar} where to find (or create) the archive. The
6403 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
6404 option allows you to either specify or name a file to use as the archive
6405 instead of the default archive file location.
6408 @xopindex{file, short description}
6409 @item --file=@var{archive-name}
6410 @itemx -f @var{archive-name}
6411 Name the archive to create or operate on. Use in conjunction with
6415 For example, in this @command{tar} command,
6418 $ @kbd{tar -cvf collection.tar blues folk jazz}
6422 @file{collection.tar} is the name of the archive. It must directly
6423 follow the @option{-f} option, since whatever directly follows @option{-f}
6424 @emph{will} end up naming the archive. If you neglect to specify an
6425 archive name, you may end up overwriting a file in the working directory
6426 with the archive you create since @command{tar} will use this file's name
6427 for the archive name.
6429 An archive can be saved as a file in the file system, sent through a
6430 pipe or over a network, or written to an I/O device such as a tape,
6431 floppy disk, or CD write drive.
6433 @cindex Writing new archives
6434 @cindex Archive creation
6435 If you do not name the archive, @command{tar} uses the value of the
6436 environment variable @env{TAPE} as the file name for the archive. If
6437 that is not available, @command{tar} uses a default, compiled-in archive
6438 name, usually that for tape unit zero (i.e., @file{/dev/tu00}).
6440 @cindex Standard input and output
6441 @cindex tar to standard input and output
6442 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
6443 archive from standard input (when listing or extracting files), or
6444 writes it to standard output (when creating an archive). If you use
6445 @file{-} as an @var{archive-name} when modifying an archive,
6446 @command{tar} reads the original archive from its standard input and
6447 writes the entire new archive to its standard output.
6449 The following example is a convenient way of copying directory
6450 hierarchy from @file{sourcedir} to @file{targetdir}.
6453 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
6456 The @option{-C} option allows to avoid using subshells:
6459 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
6462 In both examples above, the leftmost @command{tar} invocation archives
6463 the contents of @file{sourcedir} to the standard output, while the
6464 rightmost one reads this archive from its standard input and
6465 extracts it. The @option{-p} option tells it to restore permissions
6466 of the extracted files.
6468 @cindex Remote devices
6469 @cindex tar to a remote device
6471 To specify an archive file on a device attached to a remote machine,
6475 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
6479 @command{tar} will complete the remote connection, if possible, and
6480 prompt you for a username and password. If you use
6481 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
6482 will complete the remote connection, if possible, using your username
6483 as the username on the remote machine.
6485 @cindex Local and remote archives
6486 @anchor{local and remote archives}
6487 If the archive file name includes a colon (@samp{:}), then it is assumed
6488 to be a file on another machine. If the archive file is
6489 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6490 host @var{host}. The remote host is accessed using the @command{rsh}
6491 program, with a username of @var{user}. If the username is omitted
6492 (along with the @samp{@@} sign), then your user name will be used.
6493 (This is the normal @command{rsh} behavior.) It is necessary for the
6494 remote machine, in addition to permitting your @command{rsh} access, to
6495 have the @file{rmt} program installed (This command is included in
6496 the @GNUTAR{} distribution and by default is installed under
6497 @file{@var{prefix}/libexec/rmt}, were @var{prefix} means your
6498 installation prefix). If you need to use a file whose name includes a
6499 colon, then the remote tape drive behavior
6500 can be inhibited by using the @option{--force-local} option.
6502 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6503 tries to minimize input and output operations. The Amanda backup
6504 system, when used with @GNUTAR{}, has an initial sizing pass which
6507 @node Selecting Archive Members
6508 @section Selecting Archive Members
6509 @cindex Specifying files to act on
6510 @cindex Specifying archive members
6512 @dfn{File Name arguments} specify which files in the file system
6513 @command{tar} operates on, when creating or adding to an archive, or which
6514 archive members @command{tar} operates on, when reading or deleting from
6515 an archive. @xref{Operations}.
6517 To specify file names, you can include them as the last arguments on
6518 the command line, as follows:
6520 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6523 If a file name begins with dash (@samp{-}), precede it with
6524 @option{--add-file} option to prevent it from being treated as an
6527 @anchor{input name quoting}
6528 By default @GNUTAR{} attempts to @dfn{unquote} each file or member
6529 name, replacing @dfn{escape sequences} according to the following
6532 @multitable @columnfractions 0.20 0.60
6533 @headitem Escape @tab Replaced with
6534 @item \a @tab Audible bell (@acronym{ASCII} 7)
6535 @item \b @tab Backspace (@acronym{ASCII} 8)
6536 @item \f @tab Form feed (@acronym{ASCII} 12)
6537 @item \n @tab New line (@acronym{ASCII} 10)
6538 @item \r @tab Carriage return (@acronym{ASCII} 13)
6539 @item \t @tab Horizontal tabulation (@acronym{ASCII} 9)
6540 @item \v @tab Vertical tabulation (@acronym{ASCII} 11)
6541 @item \? @tab @acronym{ASCII} 127
6542 @item \@var{n} @tab @acronym{ASCII} @var{n} (@var{n} should be an octal number
6546 A backslash followed by any other symbol is retained.
6548 This default behavior is controlled by the following command line
6554 Enable unquoting input file or member names (default).
6558 Disable unquoting input file or member names.
6561 If you specify a directory name as a file name argument, all the files
6562 in that directory are operated on by @command{tar}.
6564 If you do not specify files, @command{tar} behavior differs depending
6565 on the operation mode as described below:
6567 When @command{tar} is invoked with @option{--create} (@option{-c}),
6568 @command{tar} will stop immediately, reporting the following:
6572 $ @kbd{tar cf a.tar}
6573 tar: Cowardly refusing to create an empty archive
6574 Try `tar --help' or `tar --usage' for more information.
6578 If you specify either @option{--list} (@option{-t}) or
6579 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6580 operates on all the archive members in the archive.
6582 If run with @option{--diff} option, tar will compare the archive with
6583 the contents of the current working directory.
6585 If you specify any other operation, @command{tar} does nothing.
6587 By default, @command{tar} takes file names from the command line. However,
6588 there are other ways to specify file or member names, or to modify the
6589 manner in which @command{tar} selects the files or members upon which to
6590 operate. In general, these methods work both for specifying the names
6591 of files and archive members.
6594 @section Reading Names from a File
6596 @cindex Reading file names from a file
6597 @cindex Lists of file names
6598 @cindex File Name arguments, alternatives
6599 Instead of giving the names of files or archive members on the command
6600 line, you can put the names into a file, and then use the
6601 @option{--files-from=@var{file-of-names}} (@option{-T
6602 @var{file-of-names}}) option to @command{tar}. Give the name of the
6603 file which contains the list of files to include as the argument to
6604 @option{--files-from}. In the list, the file names should be separated by
6605 newlines. You will frequently use this option when you have generated
6606 the list of files to archive with the @command{find} utility.
6610 @item --files-from=@var{file-name}
6611 @itemx -T @var{file-name}
6612 Get names to extract or create from file @var{file-name}.
6615 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6616 you specify either @code{--files-from=-} or @code{-T -}), then the file
6617 names are read from standard input.
6619 Unless you are running @command{tar} with @option{--create}, you can not use
6620 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6623 Any number of @option{-T} options can be given in the command line.
6625 The following example shows how to use @command{find} to generate a list of
6626 files smaller than 400K in length and put that list into a file
6627 called @file{small-files}. You can then use the @option{-T} option to
6628 @command{tar} to specify the files from that file, @file{small-files}, to
6629 create the archive @file{little.tgz}. (The @option{-z} option to
6630 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
6634 $ @kbd{find . -size -400 -print > small-files}
6635 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
6639 In the file list given by @option{-T} option, any file name beginning
6640 with @samp{-} character is considered a @command{tar} option and is
6641 processed accordingly.@footnote{Versions of @GNUTAR{} up to 1.15.1
6642 recognized only @option{-C} option in file lists, and only if the
6643 option and its argument occupied two consecutive lines.} For example,
6644 the common use of this feature is to change to another directory by
6645 specifying @option{-C} option:
6655 $ @kbd{tar -c -f foo.tar --files-from list}
6660 In this example, @command{tar} will first switch to @file{/etc}
6661 directory and add files @file{passwd} and @file{hosts} to the
6662 archive. Then it will change to @file{/lib} directory and will archive
6663 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
6668 $ @kbd{tar tf foo.tar}
6676 @xopindex{directory, using in @option{--files-from} argument}
6677 Notice that the option parsing algorithm used with @option{-T} is
6678 stricter than the one used by shell. Namely, when specifying option
6679 arguments, you should observe the following rules:
6683 When using short (single-letter) option form, its argument must
6684 immediately follow the option letter, without any intervening
6685 whitespace. For example: @code{-Cdir}.
6688 When using long option form, the option argument must be separated
6689 from the option by a single equal sign. No whitespace is allowed on
6690 any side of the equal sign. For example: @code{--directory=dir}.
6693 For both short and long option forms, the option argument can be given
6694 on the next line after the option name, e.g.:
6715 If you happen to have a file whose name starts with @samp{-},
6716 precede it with @option{--add-file} option to prevent it from
6717 being recognized as an option. For example: @code{--add-file=--my-file}.
6724 @subsection @code{NUL} Terminated File Names
6726 @cindex File names, terminated by @code{NUL}
6727 @cindex @code{NUL} terminated file names
6728 The @option{--null} option causes
6729 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
6730 to read file names terminated by a @code{NUL} instead of a newline, so
6731 files whose names contain newlines can be archived using
6732 @option{--files-from}.
6737 Only consider @code{NUL} terminated file names, instead of files that
6738 terminate in a newline.
6741 The @option{--null} option is just like the one in @acronym{GNU}
6742 @command{xargs} and @command{cpio}, and is useful with the
6743 @option{-print0} predicate of @acronym{GNU} @command{find}. In
6744 @command{tar}, @option{--null} also disables special handling for
6745 file names that begin with dash.
6747 This example shows how to use @command{find} to generate a list of files
6748 larger than 800K in length and put that list into a file called
6749 @file{long-files}. The @option{-print0} option to @command{find} is just
6750 like @option{-print}, except that it separates files with a @code{NUL}
6751 rather than with a newline. You can then run @command{tar} with both the
6752 @option{--null} and @option{-T} options to specify that @command{tar} get the
6753 files from that file, @file{long-files}, to create the archive
6754 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
6755 @command{tar} to recognize the @code{NUL} separator between files.
6758 $ @kbd{find . -size +800 -print0 > long-files}
6759 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
6762 @FIXME{say anything else here to conclude the section?}
6765 @section Excluding Some Files
6768 @cindex File names, excluding files by
6769 @cindex Excluding files by name and pattern
6770 @cindex Excluding files by file system
6771 To avoid operating on files whose names match a particular pattern,
6772 use the @option{--exclude} or @option{--exclude-from} options.
6776 @item --exclude=@var{pattern}
6777 Causes @command{tar} to ignore files that match the @var{pattern}.
6781 The @option{--exclude=@var{pattern}} option prevents any file or
6782 member whose name matches the shell wildcard (@var{pattern}) from
6784 For example, to create an archive with all the contents of the directory
6785 @file{src} except for files whose names end in @file{.o}, use the
6786 command @samp{tar -cf src.tar --exclude='*.o' src}.
6788 You may give multiple @option{--exclude} options.
6791 @opindex exclude-from
6792 @item --exclude-from=@var{file}
6793 @itemx -X @var{file}
6794 Causes @command{tar} to ignore files that match the patterns listed in
6798 @findex exclude-from
6799 Use the @option{--exclude-from} option to read a
6800 list of patterns, one per line, from @var{file}; @command{tar} will
6801 ignore files matching those patterns. Thus if @command{tar} is
6802 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
6803 single line @file{*.o}, no files whose names end in @file{.o} will be
6804 added to the archive.
6806 Notice, that lines from @var{file} are read verbatim. One of the
6807 frequent errors is leaving some extra whitespace after a file name,
6808 which is difficult to catch using text editors.
6810 However, empty lines are OK.
6812 @cindex version control system, excluding files
6813 @cindex VCS, excluding files
6814 @cindex SCCS, excluding files
6815 @cindex RCS, excluding files
6816 @cindex CVS, excluding files
6817 @cindex SVN, excluding files
6818 @cindex git, excluding files
6820 @opindex exclude-vcs
6822 Exclude files and directories used by some version control systems.
6825 As of version @value{VERSION}, the following files are excluded:
6828 @item @file{CVS/}, and everything under it
6829 @item @file{RCS/}, and everything under it
6830 @item @file{SCCS/}, and everything under it
6831 @item @file{.git/}, and everything under it
6832 @item @file{.gitignore}
6833 @item @file{.cvsignore}
6834 @item @file{.svn/}, and everything under it
6835 @item @file{.arch-ids/}, and everything under it
6836 @item @file{@{arch@}/}, and everything under it
6837 @item @file{=RELEASE-ID}
6838 @item @file{=meta-update}
6839 @item @file{=update}
6842 @findex exclude-caches
6843 When creating an archive, the @option{--exclude-caches} option family
6844 causes @command{tar} to exclude all directories that contain a @dfn{cache
6845 directory tag}. A cache directory tag is a short file with the
6846 well-known name @file{CACHEDIR.TAG} and having a standard header
6847 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
6848 Various applications write cache directory tags into directories they
6849 use to hold regenerable, non-precious data, so that such data can be
6850 more easily excluded from backups.
6852 There are three @samp{exclude-caches} options, each providing a different
6853 exclusion semantics:
6856 @opindex exclude-caches
6857 @item --exclude-caches
6858 Do not archive the contents of the directory, but archive the
6859 directory itself and the @file{CACHEDIR.TAG} file.
6861 @opindex exclude-caches-under
6862 @item --exclude-caches-under
6863 Do not archive the contents of the directory, nor the
6864 @file{CACHEDIR.TAG} file, archive only the directory itself.
6866 @opindex exclude-caches-all
6867 @item --exclude-caches-all
6868 Omit directories containing @file{CACHEDIR.TAG} file entirely.
6872 Another option family, @option{--exclude-tag}, provides a generalization of
6873 this concept. It takes a single argument, a file name to look for.
6874 Any directory that contains this file will be excluded from the dump.
6875 Similarly to @samp{exclude-caches}, there are three options in this
6879 @opindex exclude-tag
6880 @item --exclude-tag=@var{file}
6881 Do not dump the contents of the directory, but dump the
6882 directory itself and the @var{file}.
6884 @opindex exclude-tag-under
6885 @item --exclude-tag-under=@var{file}
6886 Do not dump the contents of the directory, nor the
6887 @var{file}, archive only the directory itself.
6889 @opindex exclude-tag-all
6890 @item --exclude-tag-all=@var{file}
6891 Omit directories containing @var{file} file entirely.
6894 Multiple @option{--exclude-tag*} options can be given.
6896 For example, given this directory:
6911 The @option{--exclude-tag} will produce the following:
6914 $ @kbd{tar -cf archive.tar --exclude-tag=tagfile -v dir}
6919 tar: dir/folk/: contains a cache directory tag tagfile;
6924 Both the @file{dir/folk} directory and its tagfile are preserved in
6925 the archive, however the rest of files in this directory are not.
6927 Now, using the @option{--exclude-tag-under} option will exclude
6928 @file{tagfile} from the dump, while still preserving the directory
6929 itself, as shown in this example:
6932 $ @kbd{tar -cf archive.tar --exclude-tag-under=tagfile -v dir}
6937 ./tar: dir/folk/: contains a cache directory tag tagfile;
6941 Finally, using @option{--exclude-tag-all} omits the @file{dir/folk}
6945 $ @kbd{tar -cf archive.tar --exclude-tag-all=tagfile -v dir}
6949 ./tar: dir/folk/: contains a cache directory tag tagfile;
6950 directory not dumped
6954 * problems with exclude::
6957 @node problems with exclude
6958 @unnumberedsubsec Problems with Using the @code{exclude} Options
6960 @xopindex{exclude, potential problems with}
6961 Some users find @samp{exclude} options confusing. Here are some common
6966 The main operating mode of @command{tar} does not act on a file name
6967 explicitly listed on the command line, if one of its file name
6968 components is excluded. In the example above, if
6969 you create an archive and exclude files that end with @samp{*.o}, but
6970 explicitly name the file @samp{dir.o/foo} after all the options have been
6971 listed, @samp{dir.o/foo} will be excluded from the archive.
6974 You can sometimes confuse the meanings of @option{--exclude} and
6975 @option{--exclude-from}. Be careful: use @option{--exclude} when files
6976 to be excluded are given as a pattern on the command line. Use
6977 @option{--exclude-from} to introduce the name of a file which contains
6978 a list of patterns, one per line; each of these patterns can exclude
6979 zero, one, or many files.
6982 When you use @option{--exclude=@var{pattern}}, be sure to quote the
6983 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
6984 like @samp{*}. If you do not do this, the shell might expand the
6985 @samp{*} itself using files at hand, so @command{tar} might receive a
6986 list of files instead of one pattern, or none at all, making the
6987 command somewhat illegal. This might not correspond to what you want.
6992 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
7000 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
7004 You must use use shell syntax, or globbing, rather than @code{regexp}
7005 syntax, when using exclude options in @command{tar}. If you try to use
7006 @code{regexp} syntax to describe files to be excluded, your command
7010 @FIXME{The change in semantics must have occurred before 1.11,
7011 so I doubt if it is worth mentioning at all. Anyway, should at
7012 least specify in which version the semantics changed.}
7013 In earlier versions of @command{tar}, what is now the
7014 @option{--exclude-from} option was called @option{--exclude} instead.
7015 Now, @option{--exclude} applies to patterns listed on the command
7016 line and @option{--exclude-from} applies to patterns listed in a
7022 @section Wildcards Patterns and Matching
7024 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
7025 @samp{*} or @samp{?} for example, are replaced and expanded into all
7026 existing files matching the given pattern. @GNUTAR{} can use wildcard
7027 patterns for matching (or globbing) archive members when extracting
7028 from or listing an archive. Wildcard patterns are also used for
7029 verifying volume labels of @command{tar} archives. This section has the
7030 purpose of explaining wildcard syntax for @command{tar}.
7032 @FIXME{the next few paragraphs need work.}
7034 A @var{pattern} should be written according to shell syntax, using wildcard
7035 characters to effect globbing. Most characters in the pattern stand
7036 for themselves in the matched string, and case is significant: @samp{a}
7037 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
7038 pattern matches any single character in the matched string. The character
7039 @samp{*} in the pattern matches zero, one, or more single characters in
7040 the matched string. The character @samp{\} says to take the following
7041 character of the pattern @emph{literally}; it is useful when one needs to
7042 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
7044 The character @samp{[}, up to the matching @samp{]}, introduces a character
7045 class. A @dfn{character class} is a list of acceptable characters
7046 for the next single character of the matched string. For example,
7047 @samp{[abcde]} would match any of the first five letters of the alphabet.
7048 Note that within a character class, all of the ``special characters''
7049 listed above other than @samp{\} lose their special meaning; for example,
7050 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
7051 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
7052 the characters @samp{-} and @samp{]} must either come @emph{first} or
7053 @emph{last} in a character class.)
7055 @cindex Excluding characters from a character class
7056 @cindex Character class, excluding characters from
7057 If the first character of the class after the opening @samp{[}
7058 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
7059 Rather than listing character to match, it lists those characters which
7060 are @emph{forbidden} as the next single character of the matched string.
7062 Other characters of the class stand for themselves. The special
7063 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
7064 letters, is meant to represent all characters between @var{a} and
7067 @FIXME{need to add a sentence or so here to make this clear for those
7068 who don't have dan around.}
7070 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
7071 special for wildcard matches. However, if a pattern completely matches
7072 a directory prefix of a matched string, then it matches the full matched
7073 string: thus, excluding a directory also excludes all the files beneath it.
7076 * controlling pattern-matching::
7079 @node controlling pattern-matching
7080 @unnumberedsubsec Controlling Pattern-Matching
7082 For the purposes of this section, we call @dfn{exclusion members} all
7083 member names obtained while processing @option{--exclude} and
7084 @option{--exclude-from} options, and @dfn{inclusion members} those
7085 member names that were given in the command line or read from the file
7086 specified with @option{--files-from} option.
7088 These two pairs of member lists are used in the following operations:
7089 @option{--diff}, @option{--extract}, @option{--list},
7092 There are no inclusion members in create mode (@option{--create} and
7093 @option{--append}), since in this mode the names obtained from the
7094 command line refer to @emph{files}, not archive members.
7096 By default, inclusion members are compared with archive members
7097 literally @footnote{Notice that earlier @GNUTAR{} versions used
7098 globbing for inclusion members, which contradicted to UNIX98
7099 specification and was not documented. @xref{Changes}, for more
7100 information on this and other changes.} and exclusion members are
7101 treated as globbing patterns. For example:
7105 $ @kbd{tar tf foo.tar}
7110 # @i{Member names are used verbatim:}
7111 $ @kbd{tar -xf foo.tar -v '[remarks]'}
7113 # @i{Exclude member names are globbed:}
7114 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
7120 This behavior can be altered by using the following options:
7125 Treat all member names as wildcards.
7127 @opindex no-wildcards
7128 @item --no-wildcards
7129 Treat all member names as literal strings.
7132 Thus, to extract files whose names end in @samp{.c}, you can use:
7135 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
7141 Notice quoting of the pattern to prevent the shell from interpreting
7144 The effect of @option{--wildcards} option is canceled by
7145 @option{--no-wildcards}. This can be used to pass part of
7146 the command line arguments verbatim and other part as globbing
7147 patterns. For example, the following invocation:
7150 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
7154 instructs @command{tar} to extract from @file{foo.tar} all files whose
7155 names end in @samp{.txt} and the file named @file{[remarks]}.
7157 Normally, a pattern matches a name if an initial subsequence of the
7158 name's components matches the pattern, where @samp{*}, @samp{?}, and
7159 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
7160 and wildcards can match @samp{/}.
7162 Other than optionally stripping leading @samp{/} from names
7163 (@pxref{absolute}), patterns and names are used as-is. For
7164 example, trailing @samp{/} is not trimmed from a user-specified name
7165 before deciding whether to exclude it.
7167 However, this matching procedure can be altered by the options listed
7168 below. These options accumulate. For example:
7171 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
7175 ignores case when excluding @samp{makefile}, but not when excluding
7180 @opindex no-anchored
7182 @itemx --no-anchored
7183 If anchored, a pattern must match an initial subsequence
7184 of the name's components. Otherwise, the pattern can match any
7185 subsequence. Default is @option{--no-anchored} for exclusion members
7186 and @option{--anchored} inclusion members.
7188 @opindex ignore-case
7189 @opindex no-ignore-case
7191 @itemx --no-ignore-case
7192 When ignoring case, upper-case patterns match lower-case names and vice versa.
7193 When not ignoring case (the default), matching is case-sensitive.
7195 @opindex wildcards-match-slash
7196 @opindex no-wildcards-match-slash
7197 @item --wildcards-match-slash
7198 @itemx --no-wildcards-match-slash
7199 When wildcards match slash (the default for exclusion members), a
7200 wildcard like @samp{*} in the pattern can match a @samp{/} in the
7201 name. Otherwise, @samp{/} is matched only by @samp{/}.
7205 The @option{--recursion} and @option{--no-recursion} options
7206 (@pxref{recurse}) also affect how member patterns are interpreted. If
7207 recursion is in effect, a pattern matches a name if it matches any of
7208 the name's parent directories.
7210 The following table summarizes pattern-matching default values:
7212 @multitable @columnfractions .3 .7
7213 @headitem Members @tab Default settings
7214 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
7215 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
7218 @node quoting styles
7219 @section Quoting Member Names
7221 When displaying member names, @command{tar} takes care to avoid
7222 ambiguities caused by certain characters. This is called @dfn{name
7223 quoting}. The characters in question are:
7226 @item Non-printable control characters:
7227 @anchor{escape sequences}
7228 @multitable @columnfractions 0.20 0.10 0.60
7229 @headitem Character @tab @acronym{ASCII} @tab Character name
7230 @item \a @tab 7 @tab Audible bell
7231 @item \b @tab 8 @tab Backspace
7232 @item \f @tab 12 @tab Form feed
7233 @item \n @tab 10 @tab New line
7234 @item \r @tab 13 @tab Carriage return
7235 @item \t @tab 9 @tab Horizontal tabulation
7236 @item \v @tab 11 @tab Vertical tabulation
7239 @item Space (@acronym{ASCII} 32)
7241 @item Single and double quotes (@samp{'} and @samp{"})
7243 @item Backslash (@samp{\})
7246 The exact way @command{tar} uses to quote these characters depends on
7247 the @dfn{quoting style}. The default quoting style, called
7248 @dfn{escape} (see below), uses backslash notation to represent control
7249 characters, space and backslash. Using this quoting style, control
7250 characters are represented as listed in column @samp{Character} in the
7251 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
7253 @GNUTAR{} offers seven distinct quoting styles, which can be selected
7254 using @option{--quoting-style} option:
7257 @item --quoting-style=@var{style}
7258 @opindex quoting-style
7260 Sets quoting style. Valid values for @var{style} argument are:
7261 literal, shell, shell-always, c, escape, locale, clocale.
7264 These styles are described in detail below. To illustrate their
7265 effect, we will use an imaginary tar archive @file{arch.tar}
7266 containing the following members:
7270 # 1. Contains horizontal tabulation character.
7272 # 2. Contains newline character
7275 # 3. Contains a space
7277 # 4. Contains double quotes
7279 # 5. Contains single quotes
7281 # 6. Contains a backslash character:
7286 Here is how usual @command{ls} command would have listed them, if they
7287 had existed in the current working directory:
7305 No quoting, display each character as is:
7309 $ @kbd{tar tf arch.tar --quoting-style=literal}
7322 Display characters the same way Bourne shell does:
7323 control characters, except @samp{\t} and @samp{\n}, are printed using
7324 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
7325 single quote is printed as @samp{\'}. If a name contains any quoted
7326 characters, it is enclosed in single quotes. In particular, if a name
7327 contains single quotes, it is printed as several single-quoted strings:
7331 $ @kbd{tar tf arch.tar --quoting-style=shell}
7334 './a'\''single'\''quote'
7344 Same as @samp{shell}, but the names are always enclosed in single
7349 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
7352 './a'\''single'\''quote'
7362 Use the notation of the C programming language. All names are
7363 enclosed in double quotes. Control characters are quoted using
7364 backslash notations, double quotes are represented as @samp{\"},
7365 backslash characters are represented as @samp{\\}. Single quotes and
7366 spaces are not quoted:
7370 $ @kbd{tar tf arch.tar --quoting-style=c}
7374 "./a\"double\"quote"
7382 Control characters are printed using backslash notation, a space is
7383 printed as @samp{\ } and a backslash as @samp{\\}. This is the
7384 default quoting style, unless it was changed when configured the
7389 $ @kbd{tar tf arch.tar --quoting-style=escape}
7401 Control characters, single quote and backslash are printed using
7402 backslash notation. All names are quoted using left and right
7403 quotation marks, appropriate to the current locale. If it does not
7404 define quotation marks, use @samp{`} as left and @samp{'} as right
7405 quotation marks. Any occurrences of the right quotation mark in a
7406 name are escaped with @samp{\}, for example:
7412 $ @kbd{tar tf arch.tar --quoting-style=locale}
7415 `./a\'single\'quote'
7424 Same as @samp{locale}, but @samp{"} is used for both left and right
7425 quotation marks, if not provided by the currently selected locale:
7429 $ @kbd{tar tf arch.tar --quoting-style=clocale}
7433 "./a\"double\"quote"
7441 You can specify which characters should be quoted in addition to those
7442 implied by the current quoting style:
7445 @item --quote-chars=@var{string}
7446 Always quote characters from @var{string}, even if the selected
7447 quoting style would not quote them.
7450 For example, using @samp{escape} quoting (compare with the usual
7451 escape listing above):
7455 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
7466 To disable quoting of such additional characters, use the following
7470 @item --no-quote-chars=@var{string}
7471 Remove characters listed in @var{string} from the list of quoted
7472 characters set by the previous @option{--quote-chars} option.
7475 This option is particularly useful if you have added
7476 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
7477 and wish to disable it for the current invocation.
7479 Note, that @option{--no-quote-chars} does @emph{not} disable those
7480 characters that are quoted by default in the selected quoting style.
7483 @section Modifying File and Member Names
7485 @command{Tar} archives contain detailed information about files stored
7486 in them and full file names are part of that information. When
7487 storing file to an archive, its file name is recorded in the archive
7488 along with the actual file contents. When restoring from an archive,
7489 a file is created on disk with exactly the same name as that stored
7490 in the archive. In the majority of cases this is the desired behavior
7491 of a file archiver. However, there are some cases when it is not.
7493 First of all, it is often unsafe to extract archive members with
7494 absolute file names or those that begin with a @file{../}. @GNUTAR{}
7495 takes special precautions when extracting such names and provides a
7496 special option for handling them, which is described in
7499 Secondly, you may wish to extract file names without some leading
7500 directory components, or with otherwise modified names. In other
7501 cases it is desirable to store files under differing names in the
7504 @GNUTAR{} provides two options for these needs.
7507 @opindex strip-components
7508 @item --strip-components=@var{number}
7509 Strip given @var{number} of leading components from file names before
7513 For example, suppose you have archived whole @file{/usr} hierarchy to
7514 a tar archive named @file{usr.tar}. Among other files, this archive
7515 contains @file{usr/include/stdlib.h}, which you wish to extract to
7516 the current working directory. To do so, you type:
7519 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
7522 The option @option{--strip=2} instructs @command{tar} to strip the
7523 two leading components (@file{usr/} and @file{include/}) off the file
7526 If you add to the above invocation @option{--verbose} (@option{-v})
7527 option, you will note that the verbose listing still contains the
7528 full file name, with the two removed components still in place. This
7529 can be inconvenient, so @command{tar} provides a special option for
7530 altering this behavior:
7532 @anchor{show-transformed-names}
7534 @opindex show-transformed-names
7535 @item --show-transformed-names
7536 Display file or member names with all requested transformations
7545 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
7546 usr/include/stdlib.h
7547 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
7552 Notice that in both cases the file is @file{stdlib.h} extracted to the
7553 current working directory, @option{--show-transformed-names} affects
7554 only the way its name is displayed.
7556 This option is especially useful for verifying whether the invocation
7557 will have the desired effect. Thus, before running
7560 $ @kbd{tar -x --strip=@var{n}}
7564 it is often advisable to run
7567 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
7571 to make sure the command will produce the intended results.
7573 In case you need to apply more complex modifications to the file name,
7574 @GNUTAR{} provides a general-purpose transformation option:
7578 @item --transform=@var{expression}
7579 Modify file names using supplied @var{expression}.
7583 The @var{expression} is a @command{sed}-like replace expression of the
7587 s/@var{regexp}/@var{replace}/[@var{flags}]
7591 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
7592 replacement for each file name part that matches @var{regexp}. Both
7593 @var{regexp} and @var{replace} are described in detail in
7594 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
7596 As in @command{sed}, you can give several replace expressions,
7597 separated by a semicolon.
7599 Supported @var{flags} are:
7603 Apply the replacement to @emph{all} matches to the @var{regexp}, not
7607 Use case-insensitive matching
7610 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
7611 regexps, Extended regular expressions, Extended regular expressions,
7615 Only replace the @var{number}th match of the @var{regexp}.
7617 Note: the @var{posix} standard does not specify what should happen
7618 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
7619 follows the GNU @command{sed} implementation in this regard, so
7620 the interaction is defined to be: ignore matches before the
7621 @var{number}th, and then match and replace all matches from the
7626 Any delimiter can be used in lieue of @samp{/}, the only requirement being
7627 that it be used consistently throughout the expression. For example,
7628 the following two expressions are equivalent:
7637 Changing delimiters is often useful when the @var{regex} contains
7638 slashes. For example, it is more convenient to write @code{s,/,-,} than
7641 Here are several examples of @option{--transform} usage:
7644 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
7647 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
7650 @item Strip two leading directory components (equivalent to
7651 @option{--strip-components=2}):
7654 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
7657 @item Prepend @file{/prefix/} to each file name:
7660 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
7663 @item Convert each file name to lower case:
7666 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
7671 Unlike @option{--strip-components}, @option{--transform} can be used
7672 in any @GNUTAR{} operation mode. For example, the following command
7673 adds files to the archive while replacing the leading @file{usr/}
7674 component with @file{var/}:
7677 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
7680 To test @option{--transform} effect we suggest using
7681 @option{--show-transformed-names} option:
7684 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
7685 --verbose --show-transformed-names /}
7688 If both @option{--strip-components} and @option{--transform} are used
7689 together, then @option{--transform} is applied first, and the required
7690 number of components is then stripped from its result.
7692 You can use as many @option{--transform} options in a single command
7693 line as you want. The specified expressions will then be applied in
7694 order of their appearance. For example, the following two invocations
7698 $ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
7699 --transform='s,/usr/local,/usr/,'}
7700 $ @kbd{tar -cf arch.tar \
7701 --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
7705 @section Operating Only on New Files
7708 @cindex Excluding file by age
7709 @cindex Data Modification time, excluding files by
7710 @cindex Modification time, excluding files by
7711 @cindex Age, excluding files by
7712 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
7713 @option{-N @var{date}}) option causes @command{tar} to only work on
7714 files whose data modification or status change times are newer than
7715 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
7716 it is taken to be a file name; the data modification time of that file
7717 is used as the date. If you use this option when creating or appending
7718 to an archive, the archive will only include new files. If you use
7719 @option{--after-date} when extracting an archive, @command{tar} will
7720 only extract files newer than the @var{date} you specify.
7722 If you only want @command{tar} to make the date comparison based on
7723 modification of the file's data (rather than status
7724 changes), then use the @option{--newer-mtime=@var{date}} option.
7726 You may use these options with any operation. Note that these options
7727 differ from the @option{--update} (@option{-u}) operation in that they
7728 allow you to specify a particular date against which @command{tar} can
7729 compare when deciding whether or not to archive the files.
7734 @item --after-date=@var{date}
7735 @itemx --newer=@var{date}
7736 @itemx -N @var{date}
7737 Only store files newer than @var{date}.
7739 Acts on files only if their data modification or status change times are
7740 later than @var{date}. Use in conjunction with any operation.
7742 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
7743 name; the data modification time of that file is used as the date.
7745 @opindex newer-mtime
7746 @item --newer-mtime=@var{date}
7747 Acts like @option{--after-date}, but only looks at data modification times.
7750 These options limit @command{tar} to operate only on files which have
7751 been modified after the date specified. A file's status is considered to have
7752 changed if its contents have been modified, or if its owner,
7753 permissions, and so forth, have been changed. (For more information on
7754 how to specify a date, see @ref{Date input formats}; remember that the
7755 entire date argument must be quoted if it contains any spaces.)
7757 Gurus would say that @option{--after-date} tests both the data
7758 modification time (@code{mtime}, the time the contents of the file
7759 were last modified) and the status change time (@code{ctime}, the time
7760 the file's status was last changed: owner, permissions, etc.@:)
7761 fields, while @option{--newer-mtime} tests only the @code{mtime}
7764 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
7765 @code{ctime} and processes the file if either one is more recent than
7766 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
7767 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
7768 contents of the file were looked at).
7770 Date specifiers can have embedded spaces. Because of this, you may need
7771 to quote date arguments to keep the shell from parsing them as separate
7772 arguments. For example, the following command will add to the archive
7773 all the files modified less than two days ago:
7776 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
7779 When any of these options is used with the option @option{--verbose}
7780 (@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
7781 date back to its textual representation and compare that with the
7782 one given with the option. If the two dates differ, @command{tar} will
7783 print a warning saying what date it will use. This is to help user
7784 ensure he is using the right date. For example:
7788 $ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
7789 tar: Option --after-date: Treating date `10 days ago' as 2006-06-11
7795 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
7796 should not be used for incremental backups. @xref{Incremental Dumps},
7797 for proper way of creating incremental backups.
7801 @section Descending into Directories
7803 @cindex Avoiding recursion in directories
7804 @cindex Descending directories, avoiding
7805 @cindex Directories, avoiding recursion
7806 @cindex Recursion in directories, avoiding
7808 @FIXME{arrggh! this is still somewhat confusing to me. :-< }
7810 Usually, @command{tar} will recursively explore all directories (either
7811 those given on the command line or through the @option{--files-from}
7812 option) for the various files they contain. However, you may not always
7813 want @command{tar} to act this way.
7815 @opindex no-recursion
7816 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
7817 into specified directories. If you specify @option{--no-recursion}, you can
7818 use the @command{find} utility for hunting through levels of directories to
7819 construct a list of file names which you could then pass to @command{tar}.
7820 @command{find} allows you to be more selective when choosing which files to
7821 archive; see @ref{files}, for more information on using @command{find} with
7822 @command{tar}, or look.
7825 @item --no-recursion
7826 Prevents @command{tar} from recursively descending directories.
7830 Requires @command{tar} to recursively descend directories.
7831 This is the default.
7834 When you use @option{--no-recursion}, @GNUTAR{} grabs
7835 directory entries themselves, but does not descend on them
7836 recursively. Many people use @command{find} for locating files they
7837 want to back up, and since @command{tar} @emph{usually} recursively
7838 descends on directories, they have to use the @samp{@w{-not -type d}}
7839 test in their @command{find} invocation (@pxref{Type, Type, Type test,
7840 find, Finding Files}), as they usually do not want all the files in a
7841 directory. They then use the @option{--files-from} option to archive
7842 the files located via @command{find}.
7844 The problem when restoring files archived in this manner is that the
7845 directories themselves are not in the archive; so the
7846 @option{--same-permissions} (@option{--preserve-permissions},
7847 @option{-p}) option does not affect them---while users might really
7848 like it to. Specifying @option{--no-recursion} is a way to tell
7849 @command{tar} to grab only the directory entries given to it, adding
7850 no new files on its own. To summarize, if you use @command{find} to
7851 create a list of files to be stored in an archive, use it as follows:
7855 $ @kbd{find @var{dir} @var{tests} | \
7856 tar -cf @var{archive} -T - --no-recursion}
7860 The @option{--no-recursion} option also applies when extracting: it
7861 causes @command{tar} to extract only the matched directory entries, not
7862 the files under those directories.
7864 The @option{--no-recursion} option also affects how globbing patterns
7865 are interpreted (@pxref{controlling pattern-matching}).
7867 The @option{--no-recursion} and @option{--recursion} options apply to
7868 later options and operands, and can be overridden by later occurrences
7869 of @option{--no-recursion} and @option{--recursion}. For example:
7872 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
7876 creates an archive with one entry for @file{grape}, and the recursive
7877 contents of @file{grape/concord}, but no entries under @file{grape}
7878 other than @file{grape/concord}.
7881 @section Crossing File System Boundaries
7882 @cindex File system boundaries, not crossing
7885 @command{tar} will normally automatically cross file system boundaries in
7886 order to archive files which are part of a directory tree. You can
7887 change this behavior by running @command{tar} and specifying
7888 @option{--one-file-system}. This option only affects files that are
7889 archived because they are in a directory that is being archived;
7890 @command{tar} will still archive files explicitly named on the command line
7891 or through @option{--files-from}, regardless of where they reside.
7894 @opindex one-file-system
7895 @item --one-file-system
7896 Prevents @command{tar} from crossing file system boundaries when
7897 archiving. Use in conjunction with any write operation.
7900 The @option{--one-file-system} option causes @command{tar} to modify its
7901 normal behavior in archiving the contents of directories. If a file in
7902 a directory is not on the same file system as the directory itself, then
7903 @command{tar} will not archive that file. If the file is a directory
7904 itself, @command{tar} will not archive anything beneath it; in other words,
7905 @command{tar} will not cross mount points.
7907 This option is useful for making full or incremental archival backups of
7908 a file system. If this option is used in conjunction with
7909 @option{--verbose} (@option{-v}), files that are excluded are
7910 mentioned by name on the standard error.
7913 * directory:: Changing Directory
7914 * absolute:: Absolute File Names
7918 @subsection Changing the Working Directory
7920 @FIXME{need to read over this node now for continuity; i've switched
7921 things around some.}
7923 @cindex Changing directory mid-stream
7924 @cindex Directory, changing mid-stream
7925 @cindex Working directory, specifying
7926 To change the working directory in the middle of a list of file names,
7927 either on the command line or in a file specified using
7928 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
7929 This will change the working directory to the specified directory
7930 after that point in the list.
7934 @item --directory=@var{directory}
7935 @itemx -C @var{directory}
7936 Changes the working directory in the middle of a command line.
7942 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
7946 will place the files @file{grape} and @file{prune} from the current
7947 directory into the archive @file{jams.tar}, followed by the file
7948 @file{cherry} from the directory @file{food}. This option is especially
7949 useful when you have several widely separated files that you want to
7950 store in the same archive.
7952 Note that the file @file{cherry} is recorded in the archive under the
7953 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
7954 archive will contain three files that all appear to have come from the
7955 same directory; if the archive is extracted with plain @samp{tar
7956 --extract}, all three files will be written in the current directory.
7958 Contrast this with the command,
7961 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
7965 which records the third file in the archive under the name
7966 @file{red/cherry} so that, if the archive is extracted using
7967 @samp{tar --extract}, the third file will be written in a subdirectory
7968 named @file{orange-colored}.
7970 You can use the @option{--directory} option to make the archive
7971 independent of the original name of the directory holding the files.
7972 The following command places the files @file{/etc/passwd},
7973 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
7977 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
7981 However, the names of the archive members will be exactly what they were
7982 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
7983 They will not appear to be related by file name to the original
7984 directories where those files were located.
7986 Note that @option{--directory} options are interpreted consecutively. If
7987 @option{--directory} specifies a relative file name, it is interpreted
7988 relative to the then current directory, which might not be the same as
7989 the original current working directory of @command{tar}, due to a previous
7990 @option{--directory} option.
7992 When using @option{--files-from} (@pxref{files}), you can put various
7993 @command{tar} options (including @option{-C}) in the file list. Notice,
7994 however, that in this case the option and its argument may not be
7995 separated by whitespace. If you use short option, its argument must
7996 either follow the option letter immediately, without any intervening
7997 whitespace, or occupy the next line. Otherwise, if you use long
7998 option, separate its argument by an equal sign.
8000 For instance, the file list for the above example will be:
8013 To use it, you would invoke @command{tar} as follows:
8016 $ @kbd{tar -c -f foo.tar --files-from list}
8019 The interpretation of @option{--directory} is disabled by
8020 @option{--null} option.
8023 @subsection Absolute File Names
8027 @opindex absolute-names
8028 @item --absolute-names
8030 Do not strip leading slashes from file names, and permit file names
8031 containing a @file{..} file name component.
8034 By default, @GNUTAR{} drops a leading @samp{/} on
8035 input or output, and complains about file names containing a @file{..}
8036 component. This option turns off this behavior.
8038 When @command{tar} extracts archive members from an archive, it strips any
8039 leading slashes (@samp{/}) from the member name. This causes absolute
8040 member names in the archive to be treated as relative file names. This
8041 allows you to have such members extracted wherever you want, instead of
8042 being restricted to extracting the member in the exact directory named
8043 in the archive. For example, if the archive member has the name
8044 @file{/etc/passwd}, @command{tar} will extract it as if the name were
8045 really @file{etc/passwd}.
8047 File names containing @file{..} can cause problems when extracting, so
8048 @command{tar} normally warns you about such files when creating an
8049 archive, and rejects attempts to extracts such files.
8051 Other @command{tar} programs do not do this. As a result, if you
8052 create an archive whose member names start with a slash, they will be
8053 difficult for other people with a non-@GNUTAR{}
8054 program to use. Therefore, @GNUTAR{} also strips
8055 leading slashes from member names when putting members into the
8056 archive. For example, if you ask @command{tar} to add the file
8057 @file{/bin/ls} to an archive, it will do so, but the member name will
8058 be @file{bin/ls}.@footnote{A side effect of this is that when
8059 @option{--create} is used with @option{--verbose} the resulting output
8060 is not, generally speaking, the same as the one you'd get running
8061 @kbd{tar --list} command. This may be important if you use some
8062 scripts for comparing both outputs. @xref{listing member and file names},
8063 for the information on how to handle this case.}
8065 If you use the @option{--absolute-names} (@option{-P}) option,
8066 @command{tar} will do none of these transformations.
8068 To archive or extract files relative to the root directory, specify
8069 the @option{--absolute-names} (@option{-P}) option.
8071 Normally, @command{tar} acts on files relative to the working
8072 directory---ignoring superior directory names when archiving, and
8073 ignoring leading slashes when extracting.
8075 When you specify @option{--absolute-names} (@option{-P}),
8076 @command{tar} stores file names including all superior directory
8077 names, and preserves leading slashes. If you only invoked
8078 @command{tar} from the root directory you would never need the
8079 @option{--absolute-names} option, but using this option
8080 may be more convenient than switching to root.
8082 @FIXME{Should be an example in the tutorial/wizardry section using this
8083 to transfer files between systems.}
8085 @FIXME{Is write access an issue?}
8088 @item --absolute-names
8089 Preserves full file names (including superior directory names) when
8090 archiving files. Preserves leading slash when extracting files.
8094 @FIXME{this is still horrible; need to talk with dan on monday.}
8096 @command{tar} prints out a message about removing the @samp{/} from
8097 file names. This message appears once per @GNUTAR{}
8098 invocation. It represents something which ought to be told; ignoring
8099 what it means can cause very serious surprises, later.
8101 Some people, nevertheless, do not want to see this message. Wanting to
8102 play really dangerously, one may of course redirect @command{tar} standard
8103 error to the sink. For example, under @command{sh}:
8106 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
8110 Another solution, both nicer and simpler, would be to change to
8111 the @file{/} directory first, and then avoid absolute notation.
8115 $ @kbd{(cd / && tar -c -f archive.tar home)}
8117 $ @kbd{tar -c -f archive.tar -C / home}
8120 @include getdate.texi
8123 @chapter Controlling the Archive Format
8125 @cindex Tar archive formats
8126 Due to historical reasons, there are several formats of tar archives.
8127 All of them are based on the same principles, but have some subtle
8128 differences that often make them incompatible with each other.
8130 GNU tar is able to create and handle archives in a variety of formats.
8131 The most frequently used formats are (in alphabetical order):
8135 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
8136 from an early @acronym{POSIX} standard, adding some improvements such as
8137 sparse file handling and incremental archives. Unfortunately these
8138 features were implemented in a way incompatible with other archive
8141 Archives in @samp{gnu} format are able to hold file names of unlimited
8145 Format used by @GNUTAR{} of versions prior to 1.12.
8148 Archive format, compatible with the V7 implementation of tar. This
8149 format imposes a number of limitations. The most important of them
8153 @item The maximum length of a file name is limited to 99 characters.
8154 @item The maximum length of a symbolic link is limited to 99 characters.
8155 @item It is impossible to store special files (block and character
8156 devices, fifos etc.)
8157 @item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
8159 @item V7 archives do not contain symbolic ownership information (user
8160 and group name of the file owner).
8163 This format has traditionally been used by Automake when producing
8164 Makefiles. This practice will change in the future, in the meantime,
8165 however this means that projects containing file names more than 99
8166 characters long will not be able to use @GNUTAR{} @value{VERSION} and
8167 Automake prior to 1.9.
8170 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
8171 symbolic ownership information. It is also able to store
8172 special files. However, it imposes several restrictions as well:
8175 @item The maximum length of a file name is limited to 256 characters,
8176 provided that the file name can be split at a directory separator in
8177 two parts, first of them being at most 155 bytes long. So, in most
8178 cases the maximum file name length will be shorter than 256
8180 @item The maximum length of a symbolic link name is limited to
8182 @item Maximum size of a file the archive is able to accommodate
8184 @item Maximum value of UID/GID is 2097151.
8185 @item Maximum number of bits in device major and minor numbers is 21.
8189 Format used by J@"org Schilling @command{star}
8190 implementation. @GNUTAR{} is able to read @samp{star} archives but
8191 currently does not produce them.
8194 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
8195 most flexible and feature-rich format. It does not impose any
8196 restrictions on file sizes or file name lengths. This format is quite
8197 recent, so not all tar implementations are able to handle it properly.
8198 However, this format is designed in such a way that any tar
8199 implementation able to read @samp{ustar} archives will be able to read
8200 most @samp{posix} archives as well, with the only exception that any
8201 additional information (such as long file names etc.) will in such
8202 case be extracted as plain text files along with the files it refers to.
8204 This archive format will be the default format for future versions
8209 The following table summarizes the limitations of each of these
8212 @multitable @columnfractions .10 .20 .20 .20 .20
8213 @headitem Format @tab UID @tab File Size @tab File Name @tab Devn
8214 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8215 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8216 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
8217 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
8218 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
8221 The default format for @GNUTAR{} is defined at compilation
8222 time. You may check it by running @command{tar --help}, and examining
8223 the last lines of its output. Usually, @GNUTAR{} is configured
8224 to create archives in @samp{gnu} format, however, future version will
8225 switch to @samp{posix}.
8228 * Compression:: Using Less Space through Compression
8229 * Attributes:: Handling File Attributes
8230 * Portability:: Making @command{tar} Archives More Portable
8231 * cpio:: Comparison of @command{tar} and @command{cpio}
8235 @section Using Less Space through Compression
8238 * gzip:: Creating and Reading Compressed Archives
8239 * sparse:: Archiving Sparse Files
8243 @subsection Creating and Reading Compressed Archives
8244 @cindex Compressed archives
8245 @cindex Storing archives in compressed format
8247 @GNUTAR{} is able to create and read compressed archives. It supports
8248 @command{gzip}, @command{bzip2} and @command{lzma} compression
8249 programs. For backward compatibility, it also supports
8250 @command{compress} command, although we strongly recommend against
8251 using it, because it is by far less effective than other compression
8252 programs@footnote{It also had patent problems in the past.}.
8254 Creating a compressed archive is simple: you just specify a
8255 @dfn{compression option} along with the usual archive creation
8256 commands. The compression option is @option{-z} (@option{--gzip}) to
8257 create a @command{gzip} compressed archive, @option{-j}
8258 (@option{--bzip2}) to create a @command{bzip2} compressed archive,
8259 @option{--lzma} to create an @asis{LZMA} compressed archive and
8260 @option{-Z} (@option{--compress}) to use @command{compress} program.
8264 $ @kbd{tar cfz archive.tar.gz .}
8267 You can also let @GNUTAR{} select the compression program basing on
8268 the suffix of the archive file name. This is done using
8269 @option{--auto-compress} (@option{-a}) command line option. For
8270 example, the following invocation will use @command{bzip2} for
8274 $ @kbd{tar cfa archive.tar.bz2 .}
8278 whereas the following one will use @command{lzma}:
8281 $ @kbd{tar cfa archive.tar.lzma .}
8284 For a complete list of file name suffixes recognized by @GNUTAR{},
8285 @ref{auto-compress}.
8287 Reading compressed archive is even simpler: you don't need to specify
8288 any additional options as @GNUTAR{} recognizes its format
8289 automatically. Thus, the following commands will list and extract the
8290 archive created in previous example:
8293 # List the compressed archive
8294 $ @kbd{tar tf archive.tar.gz}
8295 # Extract the compressed archive
8296 $ @kbd{tar xf archive.tar.gz}
8299 The only case when you have to specify a decompression option while
8300 reading the archive is when reading from a pipe or from a tape drive
8301 that does not support random access. However, in this case @GNUTAR{}
8302 will indicate which option you should use. For example:
8305 $ @kbd{cat archive.tar.gz | tar tf -}
8306 tar: Archive is compressed. Use -z option
8307 tar: Error is not recoverable: exiting now
8310 If you see such diagnostics, just add the suggested option to the
8311 invocation of @GNUTAR{}:
8314 $ @kbd{cat archive.tar.gz | tar tfz -}
8317 Notice also, that there are several restrictions on operations on
8318 compressed archives. First of all, compressed archives cannot be
8319 modified, i.e., you cannot update (@option{--update} (@option{-u}))
8320 them or delete (@option{--delete}) members from them or
8321 add (@option{--append} (@option{-r})) members to them. Likewise, you
8322 cannot append another @command{tar} archive to a compressed archive using
8323 @option{--concatenate} (@option{-A})). Secondly, multi-volume
8324 archives cannot be compressed.
8326 The following table summarizes compression options used by @GNUTAR{}.
8329 @anchor{auto-compress}
8330 @opindex auto-compress
8331 @item --auto-compress
8333 Select a compression program to use by the archive file name
8334 suffix. The following suffixes are recognized:
8336 @multitable @columnfractions 0.3 0.6
8337 @headitem Suffix @tab Compression program
8338 @item @samp{.gz} @tab @command{gzip}
8339 @item @samp{.tgz} @tab @command{gzip}
8340 @item @samp{.taz} @tab @command{gzip}
8341 @item @samp{.Z} @tab @command{compress}
8342 @item @samp{.taZ} @tab @command{compress}
8343 @item @samp{.bz2} @tab @command{bzip2}
8344 @item @samp{.tz2} @tab @command{bzip2}
8345 @item @samp{.tbz2} @tab @command{bzip2}
8346 @item @samp{.tbz} @tab @command{bzip2}
8347 @item @samp{.lzma} @tab @command{lzma}
8348 @item @samp{.tlz} @tab @command{lzma}
8356 Filter the archive through @command{gzip}.
8358 You can use @option{--gzip} and @option{--gunzip} on physical devices
8359 (tape drives, etc.) and remote files as well as on normal files; data
8360 to or from such devices or remote files is reblocked by another copy
8361 of the @command{tar} program to enforce the specified (or default) record
8362 size. The default compression parameters are used; if you need to
8363 override them, set @env{GZIP} environment variable, e.g.:
8366 $ @kbd{GZIP=--best tar cfz archive.tar.gz subdir}
8370 Another way would be to avoid the @option{--gzip} (@option{--gunzip}, @option{--ungzip}, @option{-z}) option and run
8371 @command{gzip} explicitly:
8374 $ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz}
8377 @cindex corrupted archives
8378 About corrupted compressed archives: @command{gzip}'ed files have no
8379 redundancy, for maximum compression. The adaptive nature of the
8380 compression scheme means that the compression tables are implicitly
8381 spread all over the archive. If you lose a few blocks, the dynamic
8382 construction of the compression tables becomes unsynchronized, and there
8383 is little chance that you could recover later in the archive.
8385 There are pending suggestions for having a per-volume or per-file
8386 compression in @GNUTAR{}. This would allow for viewing the
8387 contents without decompression, and for resynchronizing decompression at
8388 every volume or file, in case of corrupted archives. Doing so, we might
8389 lose some compressibility. But this would have make recovering easier.
8390 So, there are pros and cons. We'll see!
8395 Filter the archive through @code{bzip2}. Otherwise like @option{--gzip}.
8399 Filter the archive through @command{lzma}. Otherwise like @option{--gzip}.
8406 Filter the archive through @command{compress}. Otherwise like @option{--gzip}.
8408 @opindex use-compress-program
8409 @item --use-compress-program=@var{prog}
8410 Use external compression program @var{prog}. Use this option if you
8411 have a compression program that @GNUTAR{} does not support. There
8412 are two requirements to which @var{prog} should comply:
8414 First, when called without options, it should read data from standard
8415 input, compress it and output it on standard output.
8417 Secondly, if called with @option{-d} argument, it should do exactly
8418 the opposite, i.e., read the compressed data from the standard input
8419 and produce uncompressed data on the standard output.
8422 @cindex gpg, using with tar
8423 @cindex gnupg, using with tar
8424 @cindex Using encrypted archives
8425 The @option{--use-compress-program} option, in particular, lets you
8426 implement your own filters, not necessarily dealing with
8427 compression/decompression. For example, suppose you wish to implement
8428 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
8429 gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
8430 Manual}). The following script does that:
8436 -d) gpg --decrypt - | gzip -d -c;;
8437 '') gzip -c | gpg -s ;;
8438 *) echo "Unknown option $1">&2; exit 1;;
8443 Suppose you name it @file{gpgz} and save it somewhere in your
8444 @env{PATH}. Then the following command will create a compressed
8445 archive signed with your private key:
8448 $ @kbd{tar -cf foo.tar.gpgz --use-compress=gpgz .}
8452 Likewise, the following command will list its contents:
8455 $ @kbd{tar -tf foo.tar.gpgz --use-compress=gpgz .}
8459 The above is based on the following discussion:
8461 I have one question, or maybe it's a suggestion if there isn't a way
8462 to do it now. I would like to use @option{--gzip}, but I'd also like
8463 the output to be fed through a program like @acronym{GNU}
8464 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
8465 to use :-)), basically adding ECC protection on top of compression.
8466 It seems as if this should be quite easy to do, but I can't work out
8467 exactly how to go about it. Of course, I can pipe the standard output
8468 of @command{tar} through @command{ecc}, but then I lose (though I
8469 haven't started using it yet, I confess) the ability to have
8470 @command{tar} use @command{rmt} for it's I/O (I think).
8472 I think the most straightforward thing would be to let me specify a
8473 general set of filters outboard of compression (preferably ordered,
8474 so the order can be automatically reversed on input operations, and
8475 with the options they require specifiable), but beggars shouldn't be
8476 choosers and anything you decide on would be fine with me.
8478 By the way, I like @command{ecc} but if (as the comments say) it can't
8479 deal with loss of block sync, I'm tempted to throw some time at adding
8480 that capability. Supposing I were to actually do such a thing and
8481 get it (apparently) working, do you accept contributed changes to
8482 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
8484 Isn't that exactly the role of the
8485 @option{--use-compress-prog=@var{program}} option?
8486 I never tried it myself, but I suspect you may want to write a
8487 @var{prog} script or program able to filter stdin to stdout to
8488 way you want. It should recognize the @option{-d} option, for when
8489 extraction is needed rather than creation.
8491 It has been reported that if one writes compressed data (through the
8492 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
8493 the DLT compression mode, the data will actually get bigger and one will
8494 end up with less space on the tape.
8498 @subsection Archiving Sparse Files
8499 @cindex Sparse Files
8501 Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
8502 in a file is a section of the file's contents which was never written.
8503 The contents of a hole reads as all zeros. On many operating systems,
8504 actual disk storage is not allocated for holes, but they are counted
8505 in the length of the file. If you archive such a file, @command{tar}
8506 could create an archive longer than the original. To have @command{tar}
8507 attempt to recognize the holes in a file, use @option{--sparse}
8508 (@option{-S}). When you use this option, then, for any file using
8509 less disk space than would be expected from its length, @command{tar}
8510 searches the file for consecutive stretches of zeros. It then records
8511 in the archive for the file where the consecutive stretches of zeros
8512 are, and only archives the ``real contents'' of the file. On
8513 extraction (using @option{--sparse} is not needed on extraction) any
8514 such files have holes created wherever the continuous stretches of zeros
8515 were found. Thus, if you use @option{--sparse}, @command{tar} archives
8516 won't take more space than the original.
8522 This option instructs @command{tar} to test each file for sparseness
8523 before attempting to archive it. If the file is found to be sparse it
8524 is treated specially, thus allowing to decrease the amount of space
8525 used by its image in the archive.
8527 This option is meaningful only when creating or updating archives. It
8528 has no effect on extraction.
8531 Consider using @option{--sparse} when performing file system backups,
8532 to avoid archiving the expanded forms of files stored sparsely in the
8535 Even if your system has no sparse files currently, some may be
8536 created in the future. If you use @option{--sparse} while making file
8537 system backups as a matter of course, you can be assured the archive
8538 will never take more space on the media than the files take on disk
8539 (otherwise, archiving a disk filled with sparse files might take
8540 hundreds of tapes). @xref{Incremental Dumps}.
8542 However, be aware that @option{--sparse} option presents a serious
8543 drawback. Namely, in order to determine if the file is sparse
8544 @command{tar} has to read it before trying to archive it, so in total
8545 the file is read @strong{twice}. So, always bear in mind that the
8546 time needed to process all files with this option is roughly twice
8547 the time needed to archive them without it.
8548 @FIXME{A technical note:
8550 Programs like @command{dump} do not have to read the entire file; by
8551 examining the file system directly, they can determine in advance
8552 exactly where the holes are and thus avoid reading through them. The
8553 only data it need read are the actual allocated data blocks.
8554 @GNUTAR{} uses a more portable and straightforward
8555 archiving approach, it would be fairly difficult that it does
8556 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
8560 What I did say is that you cannot tell the difference between a hole and an
8561 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
8562 best tells you how many holes there are; it doesn't tell you @emph{where}.
8563 Just as programs may, conceivably, care what @code{st_blocks} is (care
8564 to name one that does?), they may also care where the holes are (I have
8565 no examples of this one either, but it's equally imaginable).
8567 I conclude from this that good archivers are not portable. One can
8568 arguably conclude that if you want a portable program, you can in good
8569 conscience restore files with as many holes as possible, since you can't
8574 @cindex sparse formats, defined
8575 When using @samp{POSIX} archive format, @GNUTAR{} is able to store
8576 sparse files using in three distinct ways, called @dfn{sparse
8577 formats}. A sparse format is identified by its @dfn{number},
8578 consisting, as usual of two decimal numbers, delimited by a dot. By
8579 default, format @samp{1.0} is used. If, for some reason, you wish to
8580 use an earlier format, you can select it using
8581 @option{--sparse-version} option.
8584 @opindex sparse-version
8585 @item --sparse-version=@var{version}
8587 Select the format to store sparse files in. Valid @var{version} values
8588 are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
8589 for a detailed description of each format.
8592 Using @option{--sparse-format} option implies @option{--sparse}.
8595 @section Handling File Attributes
8598 When @command{tar} reads files, it updates their access times. To
8599 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
8600 reset the access time retroactively or avoid changing it in the first
8603 Handling of file attributes
8606 @opindex atime-preserve
8607 @item --atime-preserve
8608 @itemx --atime-preserve=replace
8609 @itemx --atime-preserve=system
8610 Preserve the access times of files that are read. This works only for
8611 files that you own, unless you have superuser privileges.
8613 @option{--atime-preserve=replace} works on most systems, but it also
8614 restores the data modification time and updates the status change
8615 time. Hence it doesn't interact with incremental dumps nicely
8616 (@pxref{Incremental Dumps}), and it can set access or data modification times
8617 incorrectly if other programs access the file while @command{tar} is
8620 @option{--atime-preserve=system} avoids changing the access time in
8621 the first place, if the operating system supports this.
8622 Unfortunately, this may or may not work on any given operating system
8623 or file system. If @command{tar} knows for sure it won't work, it
8624 complains right away.
8626 Currently @option{--atime-preserve} with no operand defaults to
8627 @option{--atime-preserve=replace}, but this is intended to change to
8628 @option{--atime-preserve=system} when the latter is better-supported.
8633 Do not extract data modification time.
8635 When this option is used, @command{tar} leaves the data modification times
8636 of the files it extracts as the times when the files were extracted,
8637 instead of setting it to the times recorded in the archive.
8639 This option is meaningless with @option{--list} (@option{-t}).
8643 Create extracted files with the same ownership they have in the
8646 This is the default behavior for the superuser,
8647 so this option is meaningful only for non-root users, when @command{tar}
8648 is executed on those systems able to give files away. This is
8649 considered as a security flaw by many people, at least because it
8650 makes quite difficult to correctly account users for the disk space
8651 they occupy. Also, the @code{suid} or @code{sgid} attributes of
8652 files are easily and silently lost when files are given away.
8654 When writing an archive, @command{tar} writes the user @acronym{ID} and user name
8655 separately. If it can't find a user name (because the user @acronym{ID} is not
8656 in @file{/etc/passwd}), then it does not write one. When restoring,
8657 it tries to look the name (if one was written) up in
8658 @file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
8659 the archive instead.
8661 @opindex no-same-owner
8662 @item --no-same-owner
8664 Do not attempt to restore ownership when extracting. This is the
8665 default behavior for ordinary users, so this option has an effect
8666 only for the superuser.
8668 @opindex numeric-owner
8669 @item --numeric-owner
8670 The @option{--numeric-owner} option allows (ANSI) archives to be written
8671 without user/group name information or such information to be ignored
8672 when extracting. It effectively disables the generation and/or use
8673 of user/group name information. This option forces extraction using
8674 the numeric ids from the archive, ignoring the names.
8676 This is useful in certain circumstances, when restoring a backup from
8677 an emergency floppy with different passwd/group files for example.
8678 It is otherwise impossible to extract files with the right ownerships
8679 if the password file in use during the extraction does not match the
8680 one belonging to the file system(s) being extracted. This occurs,
8681 for example, if you are restoring your files after a major crash and
8682 had booted from an emergency floppy with no password file or put your
8683 disk into another machine to do the restore.
8685 The numeric ids are @emph{always} saved into @command{tar} archives.
8686 The identifying names are added at create time when provided by the
8687 system, unless @option{--old-archive} (@option{-o}) is used. Numeric ids could be
8688 used when moving archives between a collection of machines using
8689 a centralized management for attribution of numeric ids to users
8690 and groups. This is often made through using the NIS capabilities.
8692 When making a @command{tar} file for distribution to other sites, it
8693 is sometimes cleaner to use a single owner for all files in the
8694 distribution, and nicer to specify the write permission bits of the
8695 files as stored in the archive independently of their actual value on
8696 the file system. The way to prepare a clean distribution is usually
8697 to have some Makefile rule creating a directory, copying all needed
8698 files in that directory, then setting ownership and permissions as
8699 wanted (there are a lot of possible schemes), and only then making a
8700 @command{tar} archive out of this directory, before cleaning
8701 everything out. Of course, we could add a lot of options to
8702 @GNUTAR{} for fine tuning permissions and ownership.
8703 This is not the good way, I think. @GNUTAR{} is
8704 already crowded with options and moreover, the approach just explained
8705 gives you a great deal of control already.
8707 @xopindex{same-permissions, short description}
8708 @xopindex{preserve-permissions, short description}
8710 @itemx --same-permissions
8711 @itemx --preserve-permissions
8712 Extract all protection information.
8714 This option causes @command{tar} to set the modes (access permissions) of
8715 extracted files exactly as recorded in the archive. If this option
8716 is not used, the current @code{umask} setting limits the permissions
8717 on extracted files. This option is by default enabled when
8718 @command{tar} is executed by a superuser.
8721 This option is meaningless with @option{--list} (@option{-t}).
8725 Same as both @option{--same-permissions} and @option{--same-order}.
8727 The @option{--preserve} option has no equivalent short option name.
8728 It is equivalent to @option{--same-permissions} plus @option{--same-order}.
8730 @FIXME{I do not see the purpose of such an option. (Neither I. FP.)
8731 Neither do I. --Sergey}
8736 @section Making @command{tar} Archives More Portable
8738 Creating a @command{tar} archive on a particular system that is meant to be
8739 useful later on many other machines and with other versions of @command{tar}
8740 is more challenging than you might think. @command{tar} archive formats
8741 have been evolving since the first versions of Unix. Many such formats
8742 are around, and are not always compatible with each other. This section
8743 discusses a few problems, and gives some advice about making @command{tar}
8744 archives more portable.
8746 One golden rule is simplicity. For example, limit your @command{tar}
8747 archives to contain only regular files and directories, avoiding
8748 other kind of special files. Do not attempt to save sparse files or
8749 contiguous files as such. Let's discuss a few more problems, in turn.
8751 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
8752 archives and archive labels) in GNU and PAX formats.}
8755 * Portable Names:: Portable Names
8756 * dereference:: Symbolic Links
8757 * hard links:: Hard Links
8758 * old:: Old V7 Archives
8759 * ustar:: Ustar Archives
8760 * gnu:: GNU and old GNU format archives.
8761 * posix:: @acronym{POSIX} archives
8762 * Checksumming:: Checksumming Problems
8763 * Large or Negative Values:: Large files, negative time stamps, etc.
8764 * Other Tars:: How to Extract GNU-Specific Data Using
8765 Other @command{tar} Implementations
8768 @node Portable Names
8769 @subsection Portable Names
8771 Use portable file and member names. A name is portable if it contains
8772 only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
8773 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
8774 contain @samp{/-}. Avoid deep directory nesting. For portability to
8775 old Unix hosts, limit your file name components to 14 characters or
8778 If you intend to have your @command{tar} archives to be read under
8779 MSDOS, you should not rely on case distinction for file names, and you
8780 might use the @acronym{GNU} @command{doschk} program for helping you
8781 further diagnosing illegal MSDOS names, which are even more limited
8785 @subsection Symbolic Links
8786 @cindex File names, using symbolic links
8787 @cindex Symbolic link as file name
8789 @opindex dereference
8790 Normally, when @command{tar} archives a symbolic link, it writes a
8791 block to the archive naming the target of the link. In that way, the
8792 @command{tar} archive is a faithful record of the file system contents.
8793 @option{--dereference} (@option{-h}) is used with @option{--create} (@option{-c}), and causes
8794 @command{tar} to archive the files symbolic links point to, instead of
8795 the links themselves. When this option is used, when @command{tar}
8796 encounters a symbolic link, it will archive the linked-to file,
8797 instead of simply recording the presence of a symbolic link.
8799 The name under which the file is stored in the file system is not
8800 recorded in the archive. To record both the symbolic link name and
8801 the file name in the system, archive the file under both names. If
8802 all links were recorded automatically by @command{tar}, an extracted file
8803 might be linked to a file name that no longer exists in the file
8806 If a linked-to file is encountered again by @command{tar} while creating
8807 the same archive, an entire second copy of it will be stored. (This
8808 @emph{might} be considered a bug.)
8810 So, for portable archives, do not archive symbolic links as such,
8811 and use @option{--dereference} (@option{-h}): many systems do not support
8812 symbolic links, and moreover, your distribution might be unusable if
8813 it contains unresolved symbolic links.
8816 @subsection Hard Links
8818 @cindex File names, using hard links
8819 @cindex hard links, dereferencing
8820 @cindex dereferencing hard links
8822 Normally, when @command{tar} archives a hard link, it writes a
8823 block to the archive naming the target of the link (a @samp{1} type
8824 block). In that way, the actual file contents is stored in file only
8825 once. For example, consider the following two files:
8830 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
8831 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
8835 Here, @file{jeden} is a link to @file{one}. When archiving this
8836 directory with a verbose level 2, you will get an output similar to
8840 $ tar cfvv ../archive.tar .
8841 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
8842 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
8843 hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
8846 The last line shows that, instead of storing two copies of the file,
8847 @command{tar} stored it only once, under the name @file{jeden}, and
8848 stored file @file{one} as a hard link to this file.
8850 It may be important to know that all hard links to the given file are
8851 stored in the archive. For example, this may be necessary for exact
8852 reproduction of the file system. The following option does that:
8855 @xopindex{check-links, described}
8858 Check the number of links dumped for each processed file. If this
8859 number does not match the total number of hard links for the file, print
8863 For example, trying to archive only file @file{jeden} with this option
8864 produces the following diagnostics:
8867 $ tar -c -f ../archive.tar jeden
8868 tar: Missing links to `jeden'.
8871 Although creating special records for hard links helps keep a faithful
8872 record of the file system contents and makes archives more compact, it
8873 may present some difficulties when extracting individual members from
8874 the archive. For example, trying to extract file @file{one} from the
8875 archive created in previous examples produces, in the absense of file
8879 $ tar xf archive.tar ./one
8880 tar: ./one: Cannot hard link to `./jeden': No such file or directory
8881 tar: Error exit delayed from previous errors
8884 The reason for this behavior is that @command{tar} cannot seek back in
8885 the archive to the previous member (in this case, @file{one}), to
8886 extract it@footnote{There are plans to fix this in future releases.}.
8887 If you wish to avoid such problems at the cost of a bigger archive,
8888 use the following option:
8891 @xopindex{hard-dereference, described}
8892 @item --hard-dereference
8893 Dereference hard links and store the files they refer to.
8896 For example, trying this option on our two sample files, we get two
8897 copies in the archive, each of which can then be extracted
8898 independently of the other:
8902 $ tar -c -vv -f ../archive.tar --hard-dereference .
8903 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
8904 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
8905 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
8910 @subsection Old V7 Archives
8911 @cindex Format, old style
8912 @cindex Old style format
8913 @cindex Old style archives
8914 @cindex v7 archive format
8916 Certain old versions of @command{tar} cannot handle additional
8917 information recorded by newer @command{tar} programs. To create an
8918 archive in V7 format (not ANSI), which can be read by these old
8919 versions, specify the @option{--format=v7} option in
8920 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
8921 accepts @option{--portability} or @option{--old-archive} for this
8922 option). When you specify it,
8923 @command{tar} leaves out information about directories, pipes, fifos,
8924 contiguous files, and device files, and specifies file ownership by
8925 group and user IDs instead of group and user names.
8927 When updating an archive, do not use @option{--format=v7}
8928 unless the archive was created using this option.
8930 In most cases, a @emph{new} format archive can be read by an @emph{old}
8931 @command{tar} program without serious trouble, so this option should
8932 seldom be needed. On the other hand, most modern @command{tar}s are
8933 able to read old format archives, so it might be safer for you to
8934 always use @option{--format=v7} for your distributions. Notice,
8935 however, that @samp{ustar} format is a better alternative, as it is
8936 free from many of @samp{v7}'s drawbacks.
8939 @subsection Ustar Archive Format
8941 @cindex ustar archive format
8942 Archive format defined by @acronym{POSIX}.1-1988 specification is called
8943 @code{ustar}. Although it is more flexible than the V7 format, it
8944 still has many restrictions (@xref{Formats,ustar}, for the detailed
8945 description of @code{ustar} format). Along with V7 format,
8946 @code{ustar} format is a good choice for archives intended to be read
8947 with other implementations of @command{tar}.
8949 To create archive in @code{ustar} format, use @option{--format=ustar}
8950 option in conjunction with the @option{--create} (@option{-c}).
8953 @subsection @acronym{GNU} and old @GNUTAR{} format
8955 @cindex GNU archive format
8956 @cindex Old GNU archive format
8957 @GNUTAR{} was based on an early draft of the
8958 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
8959 @command{tar}, such as the support for file names longer than 100
8960 characters, use portions of the @command{tar} header record which were
8961 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
8962 @acronym{POSIX} have allocated the same parts of the header record for
8963 other purposes. As a result, @GNUTAR{} format is
8964 incompatible with the current @acronym{POSIX} specification, and with
8965 @command{tar} programs that follow it.
8967 In the majority of cases, @command{tar} will be configured to create
8968 this format by default. This will change in future releases, since
8969 we plan to make @samp{POSIX} format the default.
8971 To force creation a @GNUTAR{} archive, use option
8972 @option{--format=gnu}.
8975 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
8977 @cindex POSIX archive format
8978 @cindex PAX archive format
8979 Starting from version 1.14 @GNUTAR{} features full support for
8980 @acronym{POSIX.1-2001} archives.
8982 A @acronym{POSIX} conformant archive will be created if @command{tar}
8983 was given @option{--format=posix} (@option{--format=pax}) option. No
8984 special option is required to read and extract from a @acronym{POSIX}
8988 * PAX keywords:: Controlling Extended Header Keywords.
8992 @subsubsection Controlling Extended Header Keywords
8996 @item --pax-option=@var{keyword-list}
8997 Handle keywords in @acronym{PAX} extended headers. This option is
8998 equivalent to @option{-o} option of the @command{pax} utility.
9001 @var{Keyword-list} is a comma-separated
9002 list of keyword options, each keyword option taking one of
9003 the following forms:
9006 @item delete=@var{pattern}
9007 When used with one of archive-creation commands,
9008 this option instructs @command{tar} to omit from extended header records
9009 that it produces any keywords matching the string @var{pattern}.
9011 When used in extract or list mode, this option instructs tar
9012 to ignore any keywords matching the given @var{pattern} in the extended
9013 header records. In both cases, matching is performed using the pattern
9014 matching notation described in @acronym{POSIX 1003.2}, 3.13
9015 (@pxref{wildcards}). For example:
9018 --pax-option delete=security.*
9021 would suppress security-related information.
9023 @item exthdr.name=@var{string}
9025 This keyword allows user control over the name that is written into the
9026 ustar header blocks for the extended headers. The name is obtained
9027 from @var{string} after making the following substitutions:
9029 @multitable @columnfractions .25 .55
9030 @headitem Meta-character @tab Replaced By
9031 @item %d @tab The directory name of the file, equivalent to the
9032 result of the @command{dirname} utility on the translated file name.
9033 @item %f @tab The name of the file with the directory information
9034 stripped, equivalent to the result of the @command{basename} utility
9035 on the translated file name.
9036 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9037 @item %% @tab A @samp{%} character.
9040 Any other @samp{%} characters in @var{string} produce undefined
9043 If no option @samp{exthdr.name=string} is specified, @command{tar}
9044 will use the following default value:
9050 @item globexthdr.name=@var{string}
9051 This keyword allows user control over the name that is written into
9052 the ustar header blocks for global extended header records. The name
9053 is obtained from the contents of @var{string}, after making
9054 the following substitutions:
9056 @multitable @columnfractions .25 .55
9057 @headitem Meta-character @tab Replaced By
9058 @item %n @tab An integer that represents the
9059 sequence number of the global extended header record in the archive,
9061 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9062 @item %% @tab A @samp{%} character.
9065 Any other @samp{%} characters in @var{string} produce undefined results.
9067 If no option @samp{globexthdr.name=string} is specified, @command{tar}
9068 will use the following default value:
9071 $TMPDIR/GlobalHead.%p.%n
9075 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
9076 environment variable. If @var{TMPDIR} is not set, @command{tar}
9079 @item @var{keyword}=@var{value}
9080 When used with one of archive-creation commands, these keyword/value pairs
9081 will be included at the beginning of the archive in a global extended
9082 header record. When used with one of archive-reading commands,
9083 @command{tar} will behave as if it has encountered these keyword/value
9084 pairs at the beginning of the archive in a global extended header
9087 @item @var{keyword}:=@var{value}
9088 When used with one of archive-creation commands, these keyword/value pairs
9089 will be included as records at the beginning of an extended header for
9090 each file. This is effectively equivalent to @var{keyword}=@var{value}
9091 form except that it creates no global extended header records.
9093 When used with one of archive-reading commands, @command{tar} will
9094 behave as if these keyword/value pairs were included as records at the
9095 end of each extended header; thus, they will override any global or
9096 file-specific extended header record keywords of the same names.
9097 For example, in the command:
9100 tar --format=posix --create \
9101 --file archive --pax-option gname:=user .
9104 the group name will be forced to a new value for all files
9105 stored in the archive.
9109 @subsection Checksumming Problems
9111 SunOS and HP-UX @command{tar} fail to accept archives created using
9112 @GNUTAR{} and containing non-@acronym{ASCII} file names, that
9113 is, file names having characters with the eight bit set, because they
9114 use signed checksums, while @GNUTAR{} uses unsigned
9115 checksums while creating archives, as per @acronym{POSIX} standards. On
9116 reading, @GNUTAR{} computes both checksums and
9117 accept any. It is somewhat worrying that a lot of people may go
9118 around doing backup of their files using faulty (or at least
9119 non-standard) software, not learning about it until it's time to
9120 restore their missing files with an incompatible file extractor, or
9123 @GNUTAR{} compute checksums both ways, and accept
9124 any on read, so @acronym{GNU} tar can read Sun tapes even with their
9125 wrong checksums. @GNUTAR{} produces the standard
9126 checksum, however, raising incompatibilities with Sun. That is to
9127 say, @GNUTAR{} has not been modified to
9128 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
9129 I've been told that more recent Sun @command{tar} now read standard
9130 archives, so maybe Sun did a similar patch, after all?
9132 The story seems to be that when Sun first imported @command{tar}
9133 sources on their system, they recompiled it without realizing that
9134 the checksums were computed differently, because of a change in
9135 the default signing of @code{char}'s in their compiler. So they
9136 started computing checksums wrongly. When they later realized their
9137 mistake, they merely decided to stay compatible with it, and with
9138 themselves afterwards. Presumably, but I do not really know, HP-UX
9139 has chosen that their @command{tar} archives to be compatible with Sun's.
9140 The current standards do not favor Sun @command{tar} format. In any
9141 case, it now falls on the shoulders of SunOS and HP-UX users to get
9142 a @command{tar} able to read the good archives they receive.
9144 @node Large or Negative Values
9145 @subsection Large or Negative Values
9146 @cindex large values
9147 @cindex future time stamps
9148 @cindex negative time stamps
9151 The above sections suggest to use @samp{oldest possible} archive
9152 format if in doubt. However, sometimes it is not possible. If you
9153 attempt to archive a file whose metadata cannot be represented using
9154 required format, @GNUTAR{} will print error message and ignore such a
9155 file. You will than have to switch to a format that is able to
9156 handle such values. The format summary table (@pxref{Formats}) will
9159 In particular, when trying to archive files larger than 8GB or with
9160 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
9161 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
9162 @acronym{POSIX} archive formats. When considering which format to
9163 choose, bear in mind that the @acronym{GNU} format uses
9164 two's-complement base-256 notation to store values that do not fit
9165 into standard @acronym{ustar} range. Such archives can generally be
9166 read only by a @GNUTAR{} implementation. Moreover, they sometimes
9167 cannot be correctly restored on another hosts even by @GNUTAR{}. For
9168 example, using two's complement representation for negative time
9169 stamps that assumes a signed 32-bit @code{time_t} generates archives
9170 that are not portable to hosts with differing @code{time_t}
9173 On the other hand, @acronym{POSIX} archives, generally speaking, can
9174 be extracted by any tar implementation that understands older
9175 @acronym{ustar} format. The only exception are files larger than 8GB.
9177 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
9181 @subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations
9183 In previous sections you became acquainted with various quirks
9184 necessary to make your archives portable. Sometimes you may need to
9185 extract archives containing GNU-specific members using some
9186 third-party @command{tar} implementation or an older version of
9187 @GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed,
9188 but if it is for some reason impossible, this section will explain
9189 how to cope without it.
9191 When we speak about @dfn{GNU-specific} members we mean two classes of
9192 them: members split between the volumes of a multi-volume archive and
9193 sparse members. You will be able to always recover such members if
9194 the archive is in PAX format. In addition split members can be
9195 recovered from archives in old GNU format. The following subsections
9196 describe the required procedures in detail.
9199 * Split Recovery:: Members Split Between Volumes
9200 * Sparse Recovery:: Sparse Members
9203 @node Split Recovery
9204 @subsubsection Extracting Members Split Between Volumes
9206 @cindex Mutli-volume archives, extracting using non-GNU tars
9207 If a member is split between several volumes of an old GNU format archive
9208 most third party @command{tar} implementation will fail to extract
9209 it. To extract it, use @command{tarcat} program (@pxref{Tarcat}).
9210 This program is available from
9211 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat.html, @GNUTAR{}
9212 home page}. It concatenates several archive volumes into a single
9213 valid archive. For example, if you have three volumes named from
9214 @file{vol-1.tar} to @file{vol-3.tar}, you can do the following to
9215 extract them using a third-party @command{tar}:
9218 $ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -}
9221 @cindex Mutli-volume archives in PAX format, extracting using non-GNU tars
9222 You could use this approach for most (although not all) PAX
9223 format archives as well. However, extracting split members from a PAX
9224 archive is a much easier task, because PAX volumes are constructed in
9225 such a way that each part of a split member is extracted to a
9226 different file by @command{tar} implementations that are not aware of
9227 GNU extensions. More specifically, the very first part retains its
9228 original name, and all subsequent parts are named using the pattern:
9231 %d/GNUFileParts.%p/%f.%n
9235 where symbols preceeded by @samp{%} are @dfn{macro characters} that
9236 have the following meaning:
9238 @multitable @columnfractions .25 .55
9239 @headitem Meta-character @tab Replaced By
9240 @item %d @tab The directory name of the file, equivalent to the
9241 result of the @command{dirname} utility on its full name.
9242 @item %f @tab The file name of the file, equivalent to the result
9243 of the @command{basename} utility on its full name.
9244 @item %p @tab The process @acronym{ID} of the @command{tar} process that
9245 created the archive.
9246 @item %n @tab Ordinal number of this particular part.
9249 For example, if the file @file{var/longfile} was split during archive
9250 creation between three volumes, and the creator @command{tar} process
9251 had process @acronym{ID} @samp{27962}, then the member names will be:
9255 var/GNUFileParts.27962/longfile.1
9256 var/GNUFileParts.27962/longfile.2
9259 When you extract your archive using a third-party @command{tar}, these
9260 files will be created on your disk, and the only thing you will need
9261 to do to restore your file in its original form is concatenate them in
9262 the proper order, for example:
9267 $ @kbd{cat GNUFileParts.27962/longfile.1 \
9268 GNUFileParts.27962/longfile.2 >> longfile}
9269 $ rm -f GNUFileParts.27962
9273 Notice, that if the @command{tar} implementation you use supports PAX
9274 format archives, it will probably emit warnings about unknown keywords
9275 during extraction. They will look like this:
9280 Unknown extended header keyword 'GNU.volume.filename' ignored.
9281 Unknown extended header keyword 'GNU.volume.size' ignored.
9282 Unknown extended header keyword 'GNU.volume.offset' ignored.
9287 You can safely ignore these warnings.
9289 If your @command{tar} implementation is not PAX-aware, you will get
9290 more warnings and more files generated on your disk, e.g.:
9294 $ @kbd{tar xf vol-1.tar}
9295 var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
9297 Unexpected EOF in archive
9298 $ @kbd{tar xf vol-2.tar}
9299 tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
9300 GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
9301 'x', extracted as normal file
9305 Ignore these warnings. The @file{PaxHeaders.*} directories created
9306 will contain files with @dfn{extended header keywords} describing the
9307 extracted files. You can delete them, unless they describe sparse
9308 members. Read further to learn more about them.
9310 @node Sparse Recovery
9311 @subsubsection Extracting Sparse Members
9313 @cindex sparse files, extracting with non-GNU tars
9314 Any @command{tar} implementation will be able to extract sparse members from a
9315 PAX archive. However, the extracted files will be @dfn{condensed},
9316 i.e., any zero blocks will be removed from them. When we restore such
9317 a condensed file to its original form, by adding zero blocks (or
9318 @dfn{holes}) back to their original locations, we call this process
9319 @dfn{expanding} a compressed sparse file.
9322 To expand a file, you will need a simple auxiliary program called
9323 @command{xsparse}. It is available in source form from
9324 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse.html, @GNUTAR{}
9327 @cindex sparse files v.1.0, extracting with non-GNU tars
9328 Let's begin with archive members in @dfn{sparse format
9329 version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand.
9330 The condensed file will contain both file map and file data, so no
9331 additional data will be needed to restore it. If the original file
9332 name was @file{@var{dir}/@var{name}}, then the condensed file will be
9333 named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where
9334 @var{n} is a decimal number@footnote{technically speaking, @var{n} is a
9335 @dfn{process @acronym{ID}} of the @command{tar} process which created the
9336 archive (@pxref{PAX keywords}).}.
9338 To expand a version 1.0 file, run @command{xsparse} as follows:
9341 $ @kbd{xsparse @file{cond-file}}
9345 where @file{cond-file} is the name of the condensed file. The utility
9346 will deduce the name for the resulting expanded file using the
9347 following algorithm:
9350 @item If @file{cond-file} does not contain any directories,
9351 @file{../cond-file} will be used;
9353 @item If @file{cond-file} has the form
9354 @file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name}
9355 are simple names, with no @samp{/} characters in them, the output file
9356 name will be @file{@var{dir}/@var{name}}.
9358 @item Otherwise, if @file{cond-file} has the form
9359 @file{@var{dir}/@var{name}}, the output file name will be
9363 In the unlikely case when this algorithm does not suit your needs,
9364 you can explicitly specify output file name as a second argument to
9368 $ @kbd{xsparse @file{cond-file} @file{out-file}}
9371 It is often a good idea to run @command{xsparse} in @dfn{dry run} mode
9372 first. In this mode, the command does not actually expand the file,
9373 but verbosely lists all actions it would be taking to do so. The dry
9374 run mode is enabled by @option{-n} command line argument:
9378 $ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile}
9379 Reading v.1.0 sparse map
9380 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
9381 `/home/gray/sparsefile'
9386 To actually expand the file, you would run:
9389 $ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile}
9393 The program behaves the same way all UNIX utilities do: it will keep
9394 quiet unless it has simething important to tell you (e.g. an error
9395 condition or something). If you wish it to produce verbose output,
9396 similar to that from the dry run mode, use @option{-v} option:
9400 $ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile}
9401 Reading v.1.0 sparse map
9402 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
9403 `/home/gray/sparsefile'
9408 Additionally, if your @command{tar} implementation has extracted the
9409 @dfn{extended headers} for this file, you can instruct @command{xstar}
9410 to use them in order to verify the integrity of the expanded file.
9411 The option @option{-x} sets the name of the extended header file to
9412 use. Continuing our example:
9416 $ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
9417 /home/gray/GNUSparseFile.6058/sparsefile}
9418 Reading extended header file
9419 Found variable GNU.sparse.major = 1
9420 Found variable GNU.sparse.minor = 0
9421 Found variable GNU.sparse.name = sparsefile
9422 Found variable GNU.sparse.realsize = 217481216
9423 Reading v.1.0 sparse map
9424 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
9425 `/home/gray/sparsefile'
9430 @anchor{extracting sparse v.0.x}
9431 @cindex sparse files v.0.1, extracting with non-GNU tars
9432 @cindex sparse files v.0.0, extracting with non-GNU tars
9433 An @dfn{extended header} is a special @command{tar} archive header
9434 that precedes an archive member and contains a set of
9435 @dfn{variables}, describing the member properties that cannot be
9436 stored in the standard @code{ustar} header. While optional for
9437 expanding sparse version 1.0 members, the use of extended headers is
9438 mandatory when expanding sparse members in older sparse formats: v.0.0
9439 and v.0.1 (The sparse formats are described in detail in @ref{Sparse
9440 Formats}.) So, for these formats, the question is: how to obtain
9441 extended headers from the archive?
9443 If you use a @command{tar} implementation that does not support PAX
9444 format, extended headers for each member will be extracted as a
9445 separate file. If we represent the member name as
9446 @file{@var{dir}/@var{name}}, then the extended header file will be
9447 named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where
9448 @var{n} is an integer number.
9450 Things become more difficult if your @command{tar} implementation
9451 does support PAX headers, because in this case you will have to
9452 manually extract the headers. We recommend the following algorithm:
9456 Consult the documentation of your @command{tar} implementation for an
9457 option that prints @dfn{block numbers} along with the archive
9458 listing (analogous to @GNUTAR{}'s @option{-R} option). For example,
9459 @command{star} has @option{-block-number}.
9462 Obtain verbose listing using the @samp{block number} option, and
9463 find block numbers of the sparse member in question and the member
9464 immediately following it. For example, running @command{star} on our
9469 $ @kbd{star -t -v -block-number -f arc.tar}
9471 star: Unknown extended header keyword 'GNU.sparse.size' ignored.
9472 star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
9473 star: Unknown extended header keyword 'GNU.sparse.name' ignored.
9474 star: Unknown extended header keyword 'GNU.sparse.map' ignored.
9475 block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
9476 block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
9482 (as usual, ignore the warnings about unknown keywords.)
9485 Let @var{size} be the size of the sparse member, @var{Bs} be its block number
9486 and @var{Bn} be the block number of the next member.
9490 @var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2
9494 This number gives the size of the extended header part in tar @dfn{blocks}.
9495 In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2
9499 Use @command{dd} to extract the headers:
9502 @kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}}
9506 where @var{archive} is the archive name, @var{hname} is a name of the
9507 file to store the extended header in, @var{Bs} and @var{N} are
9508 computed in previous steps.
9510 In our example, this command will be
9513 $ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7}
9517 Finally, you can expand the condensed file, using the obtained header:
9521 $ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile}
9522 Reading extended header file
9523 Found variable GNU.sparse.size = 217481216
9524 Found variable GNU.sparse.numblocks = 208
9525 Found variable GNU.sparse.name = sparsefile
9526 Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{}
9527 Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile'
9533 @section Comparison of @command{tar} and @command{cpio}
9536 @FIXME{Reorganize the following material}
9538 The @command{cpio} archive formats, like @command{tar}, do have maximum
9539 file name lengths. The binary and old @acronym{ASCII} formats have a maximum file
9540 length of 256, and the new @acronym{ASCII} and @acronym{CRC ASCII} formats have a max
9541 file length of 1024. @acronym{GNU} @command{cpio} can read and write archives
9542 with arbitrary file name lengths, but other @command{cpio} implementations
9543 may crash unexplainedly trying to read them.
9545 @command{tar} handles symbolic links in the form in which it comes in @acronym{BSD};
9546 @command{cpio} doesn't handle symbolic links in the form in which it comes
9547 in System V prior to SVR4, and some vendors may have added symlinks
9548 to their system without enhancing @command{cpio} to know about them.
9549 Others may have enhanced it in a way other than the way I did it
9550 at Sun, and which was adopted by AT&T (and which is, I think, also
9551 present in the @command{cpio} that Berkeley picked up from AT&T and put
9552 into a later @acronym{BSD} release---I think I gave them my changes).
9554 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
9555 can handle @command{tar} format input, and write it on output, and it
9556 probably handles symbolic links. They may not have bothered doing
9557 anything to enhance @command{tar} as a result.)
9559 @command{cpio} handles special files; traditional @command{tar} doesn't.
9561 @command{tar} comes with V7, System III, System V, and @acronym{BSD} source;
9562 @command{cpio} comes only with System III, System V, and later @acronym{BSD}
9563 (4.3-tahoe and later).
9565 @command{tar}'s way of handling multiple hard links to a file can handle
9566 file systems that support 32-bit inumbers (e.g., the @acronym{BSD} file system);
9567 @command{cpio}s way requires you to play some games (in its ``binary''
9568 format, i-numbers are only 16 bits, and in its ``portable @acronym{ASCII}'' format,
9569 they're 18 bits---it would have to play games with the "file system @acronym{ID}"
9570 field of the header to make sure that the file system @acronym{ID}/i-number pairs
9571 of different files were always different), and I don't know which
9572 @command{cpio}s, if any, play those games. Those that don't might get
9573 confused and think two files are the same file when they're not, and
9574 make hard links between them.
9576 @command{tar}s way of handling multiple hard links to a file places only
9577 one copy of the link on the tape, but the name attached to that copy
9578 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
9579 way puts one copy for every link, but you can retrieve it using any
9583 What type of check sum (if any) is used, and how is this calculated.
9586 See the attached manual pages for @command{tar} and @command{cpio} format.
9587 @command{tar} uses a checksum which is the sum of all the bytes in the
9588 @command{tar} header for a file; @command{cpio} uses no checksum.
9591 If anyone knows why @command{cpio} was made when @command{tar} was present
9595 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
9596 generally-available version of UNIX had @command{tar} at the time. I don't
9597 know whether any version that was generally available @emph{within AT&T}
9598 had @command{tar}, or, if so, whether the people within AT&T who did
9599 @command{cpio} knew about it.
9601 On restore, if there is a corruption on a tape @command{tar} will stop at
9602 that point, while @command{cpio} will skip over it and try to restore the
9605 The main difference is just in the command syntax and header format.
9607 @command{tar} is a little more tape-oriented in that everything is blocked
9608 to start on a record boundary.
9611 Is there any differences between the ability to recover crashed
9612 archives between the two of them. (Is there any chance of recovering
9613 crashed archives at all.)
9616 Theoretically it should be easier under @command{tar} since the blocking
9617 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
9618 However, modern @command{cpio}'s and variations have an option to just
9619 search for the next file header after an error with a reasonable chance
9620 of resyncing. However, lots of tape driver software won't allow you to
9621 continue past a media error which should be the only reason for getting
9622 out of sync unless a file changed sizes while you were writing the
9626 If anyone knows why @command{cpio} was made when @command{tar} was present
9627 at the unix scene, please tell me about this too.
9630 Probably because it is more media efficient (by not blocking everything
9631 and using only the space needed for the headers where @command{tar}
9632 always uses 512 bytes per file header) and it knows how to archive
9635 You might want to look at the freely available alternatives. The
9636 major ones are @command{afio}, @GNUTAR{}, and
9637 @command{pax}, each of which have their own extensions with some
9638 backwards compatibility.
9640 Sparse files were @command{tar}red as sparse files (which you can
9641 easily test, because the resulting archive gets smaller, and
9642 @acronym{GNU} @command{cpio} can no longer read it).
9645 @chapter Tapes and Other Archive Media
9648 A few special cases about tape handling warrant more detailed
9649 description. These special cases are discussed below.
9651 Many complexities surround the use of @command{tar} on tape drives. Since
9652 the creation and manipulation of archives located on magnetic tape was
9653 the original purpose of @command{tar}, it contains many features making
9654 such manipulation easier.
9656 Archives are usually written on dismountable media---tape cartridges,
9657 mag tapes, or floppy disks.
9659 The amount of data a tape or disk holds depends not only on its size,
9660 but also on how it is formatted. A 2400 foot long reel of mag tape
9661 holds 40 megabytes of data when formatted at 1600 bits per inch. The
9662 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
9664 Magnetic media are re-usable---once the archive on a tape is no longer
9665 needed, the archive can be erased and the tape or disk used over.
9666 Media quality does deteriorate with use, however. Most tapes or disks
9667 should be discarded when they begin to produce data errors. EXABYTE
9668 tape cartridges should be discarded when they generate an @dfn{error
9669 count} (number of non-usable bits) of more than 10k.
9671 Magnetic media are written and erased using magnetic fields, and
9672 should be protected from such fields to avoid damage to stored data.
9673 Sticking a floppy disk to a filing cabinet using a magnet is probably
9677 * Device:: Device selection and switching
9678 * Remote Tape Server::
9679 * Common Problems and Solutions::
9680 * Blocking:: Blocking
9681 * Many:: Many archives on one tape
9682 * Using Multiple Tapes:: Using Multiple Tapes
9683 * label:: Including a Label in the Archive
9685 * Write Protection::
9689 @section Device Selection and Switching
9693 @item -f [@var{hostname}:]@var{file}
9694 @itemx --file=[@var{hostname}:]@var{file}
9695 Use archive file or device @var{file} on @var{hostname}.
9698 This option is used to specify the file name of the archive @command{tar}
9701 If the file name is @samp{-}, @command{tar} reads the archive from standard
9702 input (when listing or extracting), or writes it to standard output
9703 (when creating). If the @samp{-} file name is given when updating an
9704 archive, @command{tar} will read the original archive from its standard
9705 input, and will write the entire new archive to its standard output.
9707 If the file name contains a @samp{:}, it is interpreted as
9708 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
9709 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
9710 either case, @command{tar} will invoke the command @command{rsh} (or
9711 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
9712 machine. If you give an alternate login name, it will be given to the
9714 Naturally, the remote machine must have an executable
9715 @command{/usr/libexec/rmt}. This program is free software from the
9716 University of California, and a copy of the source code can be found
9717 with the sources for @command{tar}; it's compiled and installed by default.
9718 The exact path to this utility is determined when configuring the package.
9719 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
9720 your installation prefix. This location may also be overridden at
9721 runtime by using @option{rmt-command=@var{command}} option (@xref{Option Summary,
9722 ---rmt-command}, for detailed description of this option. @xref{Remote
9723 Tape Server}, for the description of @command{rmt} command).
9725 If this option is not given, but the environment variable @env{TAPE}
9726 is set, its value is used; otherwise, old versions of @command{tar}
9727 used a default archive name (which was picked when @command{tar} was
9728 compiled). The default is normally set up to be the @dfn{first} tape
9729 drive or other transportable I/O medium on the system.
9731 Starting with version 1.11.5, @GNUTAR{} uses
9732 standard input and standard output as the default device, and I will
9733 not try anymore supporting automatic device detection at installation
9734 time. This was failing really in too many cases, it was hopeless.
9735 This is now completely left to the installer to override standard
9736 input and standard output for default device, if this seems
9737 preferable. Further, I think @emph{most} actual usages of
9738 @command{tar} are done with pipes or disks, not really tapes,
9739 cartridges or diskettes.
9741 Some users think that using standard input and output is running
9742 after trouble. This could lead to a nasty surprise on your screen if
9743 you forget to specify an output file name---especially if you are going
9744 through a network or terminal server capable of buffering large amounts
9745 of output. We had so many bug reports in that area of configuring
9746 default tapes automatically, and so many contradicting requests, that
9747 we finally consider the problem to be portably intractable. We could
9748 of course use something like @samp{/dev/tape} as a default, but this
9749 is @emph{also} running after various kind of trouble, going from hung
9750 processes to accidental destruction of real tapes. After having seen
9751 all this mess, using standard input and output as a default really
9752 sounds like the only clean choice left, and a very useful one too.
9754 @GNUTAR{} reads and writes archive in records, I
9755 suspect this is the main reason why block devices are preferred over
9756 character devices. Most probably, block devices are more efficient
9757 too. The installer could also check for @samp{DEFTAPE} in
9758 @file{<sys/mtio.h>}.
9761 @xopindex{force-local, short description}
9763 Archive file is local even if it contains a colon.
9765 @opindex rsh-command
9766 @item --rsh-command=@var{command}
9767 Use remote @var{command} instead of @command{rsh}. This option exists
9768 so that people who use something other than the standard @command{rsh}
9769 (e.g., a Kerberized @command{rsh}) can access a remote device.
9771 When this command is not used, the shell command found when
9772 the @command{tar} program was installed is used instead. This is
9773 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
9774 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
9775 The installer may have overridden this by defining the environment
9776 variable @env{RSH} @emph{at installation time}.
9779 Specify drive and density.
9781 @xopindex{multi-volume, short description}
9783 @itemx --multi-volume
9784 Create/list/extract multi-volume archive.
9786 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
9787 that may be larger than will fit on the medium used to hold it.
9788 @xref{Multi-Volume Archives}.
9790 @xopindex{tape-length, short description}
9792 @itemx --tape-length=@var{num}
9793 Change tape after writing @var{num} x 1024 bytes.
9795 This option might be useful when your tape drivers do not properly
9796 detect end of physical tapes. By being slightly conservative on the
9797 maximum tape length, you might avoid the problem entirely.
9799 @xopindex{info-script, short description}
9800 @xopindex{new-volume-script, short description}
9802 @itemx --info-script=@var{file}
9803 @itemx --new-volume-script=@var{file}
9804 Execute @file{file} at end of each tape. This implies
9805 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
9806 description of this option.
9809 @node Remote Tape Server
9810 @section The Remote Tape Server
9812 @cindex remote tape drive
9814 In order to access the tape drive on a remote machine, @command{tar}
9815 uses the remote tape server written at the University of California at
9816 Berkeley. The remote tape server must be installed as
9817 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
9818 want to use. @command{tar} calls @command{rmt} by running an
9819 @command{rsh} or @command{remsh} to the remote machine, optionally
9820 using a different login name if one is supplied.
9822 A copy of the source for the remote tape server is provided. It is
9823 Copyright @copyright{} 1983 by the Regents of the University of
9824 California, but can be freely distributed. It is compiled and
9825 installed by default.
9827 @cindex absolute file names
9828 Unless you use the @option{--absolute-names} (@option{-P}) option,
9829 @GNUTAR{} will not allow you to create an archive that contains
9830 absolute file names (a file name beginning with @samp{/}.) If you try,
9831 @command{tar} will automatically remove the leading @samp{/} from the
9832 file names it stores in the archive. It will also type a warning
9833 message telling you what it is doing.
9835 When reading an archive that was created with a different
9836 @command{tar} program, @GNUTAR{} automatically
9837 extracts entries in the archive which have absolute file names as if
9838 the file names were not absolute. This is an important feature. A
9839 visitor here once gave a @command{tar} tape to an operator to restore;
9840 the operator used Sun @command{tar} instead of @GNUTAR{},
9841 and the result was that it replaced large portions of
9842 our @file{/bin} and friends with versions from the tape; needless to
9843 say, we were unhappy about having to recover the file system from
9846 For example, if the archive contained a file @file{/usr/bin/computoy},
9847 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
9848 relative to the current directory. If you want to extract the files in
9849 an archive to the same absolute names that they had when the archive
9850 was created, you should do a @samp{cd /} before extracting the files
9851 from the archive, or you should either use the @option{--absolute-names}
9852 option, or use the command @samp{tar -C / @dots{}}.
9854 @cindex Ultrix 3.1 and write failure
9855 Some versions of Unix (Ultrix 3.1 is known to have this problem),
9856 can claim that a short write near the end of a tape succeeded,
9857 when it actually failed. This will result in the -M option not
9858 working correctly. The best workaround at the moment is to use a
9859 significantly larger blocking factor than the default 20.
9861 In order to update an archive, @command{tar} must be able to backspace the
9862 archive in order to reread or rewrite a record that was just read (or
9863 written). This is currently possible only on two kinds of files: normal
9864 disk files (or any other file that can be backspaced with @samp{lseek}),
9865 and industry-standard 9-track magnetic tape (or any other kind of tape
9866 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}.
9868 This means that the @option{--append}, @option{--concatenate}, and
9869 @option{--delete} commands will not work on any other kind of file.
9870 Some media simply cannot be backspaced, which means these commands and
9871 options will never be able to work on them. These non-backspacing
9872 media include pipes and cartridge tape drives.
9874 Some other media can be backspaced, and @command{tar} will work on them
9875 once @command{tar} is modified to do so.
9877 Archives created with the @option{--multi-volume}, @option{--label}, and
9878 @option{--incremental} (@option{-G}) options may not be readable by other version
9879 of @command{tar}. In particular, restoring a file that was split over
9880 a volume boundary will require some careful work with @command{dd}, if
9881 it can be done at all. Other versions of @command{tar} may also create
9882 an empty file whose name is that of the volume header. Some versions
9883 of @command{tar} may create normal files instead of directories archived
9884 with the @option{--incremental} (@option{-G}) option.
9886 @node Common Problems and Solutions
9887 @section Some Common Problems and their Solutions
9894 no such file or directory
9897 errors from @command{tar}:
9898 directory checksum error
9901 errors from media/system:
9912 @dfn{Block} and @dfn{record} terminology is rather confused, and it
9913 is also confusing to the expert reader. On the other hand, readers
9914 who are new to the field have a fresh mind, and they may safely skip
9915 the next two paragraphs, as the remainder of this manual uses those
9916 two terms in a quite consistent way.
9918 John Gilmore, the writer of the public domain @command{tar} from which
9919 @GNUTAR{} was originally derived, wrote (June 1995):
9922 The nomenclature of tape drives comes from IBM, where I believe
9923 they were invented for the IBM 650 or so. On IBM mainframes, what
9924 is recorded on tape are tape blocks. The logical organization of
9925 data is into records. There are various ways of putting records into
9926 blocks, including @code{F} (fixed sized records), @code{V} (variable
9927 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
9928 to a block), @code{VB} (variable size records, @var{n} to a block),
9929 @code{VSB} (variable spanned blocked: variable sized records that can
9930 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
9931 parameter specified this to the operating system.
9933 The Unix man page on @command{tar} was totally confused about this.
9934 When I wrote @code{PD TAR}, I used the historically correct terminology
9935 (@command{tar} writes data records, which are grouped into blocks).
9936 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
9937 here), and now Fran@,{c}ois has migrated that terminology back
9938 into the source code too.
9941 The term @dfn{physical block} means the basic transfer chunk from or
9942 to a device, after which reading or writing may stop without anything
9943 being lost. In this manual, the term @dfn{block} usually refers to
9944 a disk physical block, @emph{assuming} that each disk block is 512
9945 bytes in length. It is true that some disk devices have different
9946 physical blocks, but @command{tar} ignore these differences in its own
9947 format, which is meant to be portable, so a @command{tar} block is always
9948 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
9949 The term @dfn{logical block} often represents the basic chunk of
9950 allocation of many disk blocks as a single entity, which the operating
9951 system treats somewhat atomically; this concept is only barely used
9954 The term @dfn{physical record} is another way to speak of a physical
9955 block, those two terms are somewhat interchangeable. In this manual,
9956 the term @dfn{record} usually refers to a tape physical block,
9957 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
9958 It is true that archives may be put on disk or used with pipes,
9959 but nevertheless, @command{tar} tries to read and write the archive one
9960 @dfn{record} at a time, whatever the medium in use. One record is made
9961 up of an integral number of blocks, and this operation of putting many
9962 disk blocks into a single tape block is called @dfn{reblocking}, or
9963 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
9964 the logical organization of many characters into something meaningful
9965 to the application. The term @dfn{unit record} describes a small set
9966 of characters which are transmitted whole to or by the application,
9967 and often refers to a line of text. Those two last terms are unrelated
9968 to what we call a @dfn{record} in @GNUTAR{}.
9970 When writing to tapes, @command{tar} writes the contents of the archive
9971 in chunks known as @dfn{records}. To change the default blocking
9972 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
9973 @var{512-size}}) option. Each record will then be composed of
9974 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
9975 @xref{Standard}.) Each file written to the archive uses at least one
9976 full record. As a result, using a larger record size can result in
9977 more wasted space for small files. On the other hand, a larger record
9978 size can often be read and written much more efficiently.
9980 Further complicating the problem is that some tape drives ignore the
9981 blocking entirely. For these, a larger record size can still improve
9982 performance (because the software layers above the tape drive still
9983 honor the blocking), but not as dramatically as on tape drives that
9986 When reading an archive, @command{tar} can usually figure out the
9987 record size on itself. When this is the case, and a non-standard
9988 record size was used when the archive was created, @command{tar} will
9989 print a message about a non-standard blocking factor, and then operate
9990 normally. On some tape devices, however, @command{tar} cannot figure
9991 out the record size itself. On most of those, you can specify a
9992 blocking factor (with @option{--blocking-factor}) larger than the
9993 actual blocking factor, and then use the @option{--read-full-records}
9994 (@option{-B}) option. (If you specify a blocking factor with
9995 @option{--blocking-factor} and don't use the
9996 @option{--read-full-records} option, then @command{tar} will not
9997 attempt to figure out the recording size itself.) On some devices,
9998 you must always specify the record size exactly with
9999 @option{--blocking-factor} when reading, because @command{tar} cannot
10000 figure it out. In any case, use @option{--list} (@option{-t}) before
10001 doing any extractions to see whether @command{tar} is reading the archive
10004 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
10005 putting them into records is to put a whole number of them (one or
10006 more) into each record. @command{tar} records are all the same size;
10007 at the end of the file there's a block containing all zeros, which
10008 is how you tell that the remainder of the last record(s) are garbage.
10010 In a standard @command{tar} file (no options), the block size is 512
10011 and the record size is 10240, for a blocking factor of 20. What the
10012 @option{--blocking-factor} option does is sets the blocking factor,
10013 changing the record size while leaving the block size at 512 bytes.
10014 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
10015 most tape drives these days prefer much bigger records in order to
10016 stream and not waste tape. When writing tapes for myself, some tend
10017 to use a factor of the order of 2048, say, giving a record size of
10018 around one megabyte.
10020 If you use a blocking factor larger than 20, older @command{tar}
10021 programs might not be able to read the archive, so we recommend this
10022 as a limit to use in practice. @GNUTAR{}, however,
10023 will support arbitrarily large record sizes, limited only by the
10024 amount of virtual memory or the physical characteristics of the tape
10028 * Format Variations:: Format Variations
10029 * Blocking Factor:: The Blocking Factor of an Archive
10032 @node Format Variations
10033 @subsection Format Variations
10034 @cindex Format Parameters
10035 @cindex Format Options
10036 @cindex Options, archive format specifying
10037 @cindex Options, format specifying
10040 Format parameters specify how an archive is written on the archive
10041 media. The best choice of format parameters will vary depending on
10042 the type and number of files being archived, and on the media used to
10045 To specify format parameters when accessing or creating an archive,
10046 you can use the options described in the following sections.
10047 If you do not specify any format parameters, @command{tar} uses
10048 default parameters. You cannot modify a compressed archive.
10049 If you create an archive with the @option{--blocking-factor} option
10050 specified (@pxref{Blocking Factor}), you must specify that
10051 blocking-factor when operating on the archive. @xref{Formats}, for other
10052 examples of format parameter considerations.
10054 @node Blocking Factor
10055 @subsection The Blocking Factor of an Archive
10056 @cindex Blocking Factor
10057 @cindex Record Size
10058 @cindex Number of blocks per record
10059 @cindex Number of bytes per record
10060 @cindex Bytes per record
10061 @cindex Blocks per record
10064 @opindex blocking-factor
10065 The data in an archive is grouped into blocks, which are 512 bytes.
10066 Blocks are read and written in whole number multiples called
10067 @dfn{records}. The number of blocks in a record (i.e., the size of a
10068 record in units of 512 bytes) is called the @dfn{blocking factor}.
10069 The @option{--blocking-factor=@var{512-size}} (@option{-b
10070 @var{512-size}}) option specifies the blocking factor of an archive.
10071 The default blocking factor is typically 20 (i.e., 10240 bytes), but
10072 can be specified at installation. To find out the blocking factor of
10073 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
10074 This may not work on some devices.
10076 Records are separated by gaps, which waste space on the archive media.
10077 If you are archiving on magnetic tape, using a larger blocking factor
10078 (and therefore larger records) provides faster throughput and allows you
10079 to fit more data on a tape (because there are fewer gaps). If you are
10080 archiving on cartridge, a very large blocking factor (say 126 or more)
10081 greatly increases performance. A smaller blocking factor, on the other
10082 hand, may be useful when archiving small files, to avoid archiving lots
10083 of nulls as @command{tar} fills out the archive to the end of the record.
10084 In general, the ideal record size depends on the size of the
10085 inter-record gaps on the tape you are using, and the average size of the
10086 files you are archiving. @xref{create}, for information on
10089 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
10091 Archives with blocking factors larger than 20 cannot be read
10092 by very old versions of @command{tar}, or by some newer versions
10093 of @command{tar} running on old machines with small address spaces.
10094 With @GNUTAR{}, the blocking factor of an archive is limited
10095 only by the maximum record size of the device containing the archive,
10096 or by the amount of available virtual memory.
10098 Also, on some systems, not using adequate blocking factors, as sometimes
10099 imposed by the device drivers, may yield unexpected diagnostics. For
10100 example, this has been reported:
10103 Cannot write to /dev/dlt: Invalid argument
10107 In such cases, it sometimes happen that the @command{tar} bundled by
10108 the system is aware of block size idiosyncrasies, while @GNUTAR{}
10109 requires an explicit specification for the block size,
10110 which it cannot guess. This yields some people to consider
10111 @GNUTAR{} is misbehaving, because by comparison,
10112 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
10113 for example, might resolve the problem.
10115 If you use a non-default blocking factor when you create an archive, you
10116 must specify the same blocking factor when you modify that archive. Some
10117 archive devices will also require you to specify the blocking factor when
10118 reading that archive, however this is not typically the case. Usually, you
10119 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
10120 reports a non-default record size and then lists the archive members as
10121 it would normally. To extract files from an archive with a non-standard
10122 blocking factor (particularly if you're not sure what the blocking factor
10123 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
10124 specifying a blocking factor larger then the blocking factor of the archive
10125 (i.e., @samp{tar --extract --read-full-records --blocking-factor=300}.
10126 @xref{list}, for more information on the @option{--list} (@option{-t})
10127 operation. @xref{Reading}, for a more detailed explanation of that option.
10130 @item --blocking-factor=@var{number}
10131 @itemx -b @var{number}
10132 Specifies the blocking factor of an archive. Can be used with any
10133 operation, but is usually not necessary with @option{--list} (@option{-t}).
10139 @item -b @var{blocks}
10140 @itemx --blocking-factor=@var{blocks}
10141 Set record size to @math{@var{blocks} * 512} bytes.
10143 This option is used to specify a @dfn{blocking factor} for the archive.
10144 When reading or writing the archive, @command{tar}, will do reads and writes
10145 of the archive in records of @math{@var{block}*512} bytes. This is true
10146 even when the archive is compressed. Some devices requires that all
10147 write operations be a multiple of a certain size, and so, @command{tar}
10148 pads the archive out to the next record boundary.
10150 The default blocking factor is set when @command{tar} is compiled, and is
10151 typically 20. Blocking factors larger than 20 cannot be read by very
10152 old versions of @command{tar}, or by some newer versions of @command{tar}
10153 running on old machines with small address spaces.
10155 With a magnetic tape, larger records give faster throughput and fit
10156 more data on a tape (because there are fewer inter-record gaps).
10157 If the archive is in a disk file or a pipe, you may want to specify
10158 a smaller blocking factor, since a large one will result in a large
10159 number of null bytes at the end of the archive.
10161 When writing cartridge or other streaming tapes, a much larger
10162 blocking factor (say 126 or more) will greatly increase performance.
10163 However, you must specify the same blocking factor when reading or
10164 updating the archive.
10166 Apparently, Exabyte drives have a physical block size of 8K bytes.
10167 If we choose our blocksize as a multiple of 8k bytes, then the problem
10168 seems to disappear. Id est, we are using block size of 112 right
10169 now, and we haven't had the problem since we switched@dots{}
10171 With @GNUTAR{} the blocking factor is limited only
10172 by the maximum record size of the device containing the archive, or by
10173 the amount of available virtual memory.
10175 However, deblocking or reblocking is virtually avoided in a special
10176 case which often occurs in practice, but which requires all the
10177 following conditions to be simultaneously true:
10180 the archive is subject to a compression option,
10182 the archive is not handled through standard input or output, nor
10183 redirected nor piped,
10185 the archive is directly handled to a local disk, instead of any special
10188 @option{--blocking-factor} is not explicitly specified on the @command{tar}
10192 If the output goes directly to a local disk, and not through
10193 stdout, then the last write is not extended to a full record size.
10194 Otherwise, reblocking occurs. Here are a few other remarks on this
10200 @command{gzip} will complain about trailing garbage if asked to
10201 uncompress a compressed archive on tape, there is an option to turn
10202 the message off, but it breaks the regularity of simply having to use
10203 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
10204 silently ignoring any number of trailing zeros. I'll ask Jean-loup
10205 Gailly, by sending a copy of this message to him.
10208 @command{compress} does not show this problem, but as Jean-loup pointed
10209 out to Michael, @samp{compress -d} silently adds garbage after
10210 the result of decompression, which tar ignores because it already
10211 recognized its end-of-file indicator. So this bug may be safely
10215 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
10216 but will still return an exit status of 2 which tar reports in turn.
10217 @command{tar} might ignore the exit status returned, but I hate doing
10218 that, as it weakens the protection @command{tar} offers users against
10219 other possible problems at decompression time. If @command{gzip} was
10220 silently skipping trailing zeros @emph{and} also avoiding setting the
10221 exit status in this innocuous case, that would solve this situation.
10224 @command{tar} should become more solid at not stopping to read a pipe at
10225 the first null block encountered. This inelegantly breaks the pipe.
10226 @command{tar} should rather drain the pipe out before exiting itself.
10229 @xopindex{ignore-zeros, short description}
10231 @itemx --ignore-zeros
10232 Ignore blocks of zeros in archive (means EOF).
10234 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
10235 of zeros in the archive. Normally a block of zeros indicates the
10236 end of the archive, but when reading a damaged archive, or one which
10237 was created by concatenating several archives together, this option
10238 allows @command{tar} to read the entire archive. This option is not on
10239 by default because many versions of @command{tar} write garbage after
10242 Note that this option causes @command{tar} to read to the end of the
10243 archive file, which may sometimes avoid problems when multiple files
10244 are stored on a single physical tape.
10246 @xopindex{read-full-records, short description}
10248 @itemx --read-full-records
10249 Reblock as we read (for reading 4.2@acronym{BSD} pipes).
10251 If @option{--read-full-records} is used, @command{tar}
10252 will not panic if an attempt to read a record from the archive does
10253 not return a full record. Instead, @command{tar} will keep reading
10254 until it has obtained a full
10257 This option is turned on by default when @command{tar} is reading
10258 an archive from standard input, or from a remote machine. This is
10259 because on @acronym{BSD} Unix systems, a read of a pipe will return however
10260 much happens to be in the pipe, even if it is less than @command{tar}
10261 requested. If this option was not used, @command{tar} would fail as
10262 soon as it read an incomplete record from the pipe.
10264 This option is also useful with the commands for updating an archive.
10270 @FIXME{Appropriate options should be moved here from elsewhere.}
10272 @cindex blocking factor
10273 @cindex tape blocking
10275 When handling various tapes or cartridges, you have to take care of
10276 selecting a proper blocking, that is, the number of disk blocks you
10277 put together as a single tape block on the tape, without intervening
10278 tape gaps. A @dfn{tape gap} is a small landing area on the tape
10279 with no information on it, used for decelerating the tape to a
10280 full stop, and for later regaining the reading or writing speed.
10281 When the tape driver starts reading a record, the record has to
10282 be read whole without stopping, as a tape gap is needed to stop the
10283 tape motion without loosing information.
10285 @cindex Exabyte blocking
10286 @cindex DAT blocking
10287 Using higher blocking (putting more disk blocks per tape block) will use
10288 the tape more efficiently as there will be less tape gaps. But reading
10289 such tapes may be more difficult for the system, as more memory will be
10290 required to receive at once the whole record. Further, if there is a
10291 reading error on a huge record, this is less likely that the system will
10292 succeed in recovering the information. So, blocking should not be too
10293 low, nor it should be too high. @command{tar} uses by default a blocking of
10294 20 for historical reasons, and it does not really matter when reading or
10295 writing to disk. Current tape technology would easily accommodate higher
10296 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
10297 We were told that for some DLT drives, the blocking should be a multiple
10298 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
10299 Other manufacturers may use different recommendations for the same tapes.
10300 This might also depends of the buffering techniques used inside modern
10301 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
10302 Others request blocking to be some exponent of two.
10304 So, there is no fixed rule for blocking. But blocking at read time
10305 should ideally be the same as blocking used at write time. At one place
10306 I know, with a wide variety of equipment, they found it best to use a
10307 blocking of 32 to guarantee that their tapes are fully interchangeable.
10309 I was also told that, for recycled tapes, prior erasure (by the same
10310 drive unit that will be used to create the archives) sometimes lowers
10311 the error rates observed at rewriting time.
10313 I might also use @option{--number-blocks} instead of
10314 @option{--block-number}, so @option{--block} will then expand to
10315 @option{--blocking-factor} unambiguously.
10318 @section Many Archives on One Tape
10320 @FIXME{Appropriate options should be moved here from elsewhere.}
10322 @findex ntape @r{device}
10323 Most tape devices have two entries in the @file{/dev} directory, or
10324 entries that come in pairs, which differ only in the minor number for
10325 this device. Let's take for example @file{/dev/tape}, which often
10326 points to the only or usual tape device of a given system. There might
10327 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
10328 name is the @emph{rewinding} version of the device, while the name
10329 having @samp{nr} in it is the @emph{no rewinding} version of the same
10332 A rewinding tape device will bring back the tape to its beginning point
10333 automatically when this device is opened or closed. Since @command{tar}
10334 opens the archive file before using it and closes it afterwards, this
10335 means that a simple:
10338 $ @kbd{tar cf /dev/tape @var{directory}}
10342 will reposition the tape to its beginning both prior and after saving
10343 @var{directory} contents to it, thus erasing prior tape contents and
10344 making it so that any subsequent write operation will destroy what has
10347 @cindex tape positioning
10348 So, a rewinding device is normally meant to hold one and only one file.
10349 If you want to put more than one @command{tar} archive on a given tape, you
10350 will need to avoid using the rewinding version of the tape device. You
10351 will also have to pay special attention to tape positioning. Errors in
10352 positioning may overwrite the valuable data already on your tape. Many
10353 people, burnt by past experiences, will only use rewinding devices and
10354 limit themselves to one file per tape, precisely to avoid the risk of
10355 such errors. Be fully aware that writing at the wrong position on a
10356 tape loses all information past this point and most probably until the
10357 end of the tape, and this destroyed information @emph{cannot} be
10360 To save @var{directory-1} as a first archive at the beginning of a
10361 tape, and leave that tape ready for a second archive, you should use:
10364 $ @kbd{mt -f /dev/nrtape rewind}
10365 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
10369 @dfn{Tape marks} are special magnetic patterns written on the tape
10370 media, which are later recognizable by the reading hardware. These
10371 marks are used after each file, when there are many on a single tape.
10372 An empty file (that is to say, two tape marks in a row) signal the
10373 logical end of the tape, after which no file exist. Usually,
10374 non-rewinding tape device drivers will react to the close request issued
10375 by @command{tar} by first writing two tape marks after your archive, and by
10376 backspacing over one of these. So, if you remove the tape at that time
10377 from the tape drive, it is properly terminated. But if you write
10378 another file at the current position, the second tape mark will be
10379 erased by the new information, leaving only one tape mark between files.
10381 So, you may now save @var{directory-2} as a second archive after the
10382 first on the same tape by issuing the command:
10385 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
10389 and so on for all the archives you want to put on the same tape.
10391 Another usual case is that you do not write all the archives the same
10392 day, and you need to remove and store the tape between two archive
10393 sessions. In general, you must remember how many files are already
10394 saved on your tape. Suppose your tape already has 16 files on it, and
10395 that you are ready to write the 17th. You have to take care of skipping
10396 the first 16 tape marks before saving @var{directory-17}, say, by using
10400 $ @kbd{mt -f /dev/nrtape rewind}
10401 $ @kbd{mt -f /dev/nrtape fsf 16}
10402 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
10405 In all the previous examples, we put aside blocking considerations, but
10406 you should do the proper things for that as well. @xref{Blocking}.
10409 * Tape Positioning:: Tape Positions and Tape Marks
10410 * mt:: The @command{mt} Utility
10413 @node Tape Positioning
10414 @subsection Tape Positions and Tape Marks
10417 Just as archives can store more than one file from the file system,
10418 tapes can store more than one archive file. To keep track of where
10419 archive files (or any other type of file stored on tape) begin and
10420 end, tape archive devices write magnetic @dfn{tape marks} on the
10421 archive media. Tape drives write one tape mark between files,
10422 two at the end of all the file entries.
10424 If you think of data as a series of records "rrrr"'s, and tape marks as
10425 "*"'s, a tape might look like the following:
10428 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
10431 Tape devices read and write tapes using a read/write @dfn{tape
10432 head}---a physical part of the device which can only access one
10433 point on the tape at a time. When you use @command{tar} to read or
10434 write archive data from a tape device, the device will begin reading
10435 or writing from wherever on the tape the tape head happens to be,
10436 regardless of which archive or what part of the archive the tape
10437 head is on. Before writing an archive, you should make sure that no
10438 data on the tape will be overwritten (unless it is no longer needed).
10439 Before reading an archive, you should make sure the tape head is at
10440 the beginning of the archive you want to read. You can do it manually
10441 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
10442 that automatically (@pxref{Scripted Restoration}).
10444 If you want to add new archive file entries to a tape, you should
10445 advance the tape to the end of the existing file entries, backspace
10446 over the last tape mark, and write the new archive file. If you were
10447 to add two archives to the example above, the tape might look like the
10451 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
10455 @subsection The @command{mt} Utility
10458 @FIXME{Is it true that this only works on non-block devices?
10459 should explain the difference, (fixed or variable).}
10460 @xref{Blocking Factor}.
10462 You can use the @command{mt} utility to advance or rewind a tape past a
10463 specified number of archive files on the tape. This will allow you
10464 to move to the beginning of an archive before extracting or reading
10465 it, or to the end of all the archives before writing a new one.
10466 @FIXME{Why isn't there an "advance 'til you find two tape marks
10469 The syntax of the @command{mt} command is:
10472 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
10475 where @var{tapename} is the name of the tape device, @var{number} is
10476 the number of times an operation is performed (with a default of one),
10477 and @var{operation} is one of the following:
10479 @FIXME{is there any use for record operations?}
10484 Writes @var{number} tape marks at the current position on the tape.
10487 Moves tape position forward @var{number} files.
10490 Moves tape position back @var{number} files.
10493 Rewinds the tape. (Ignores @var{number}).
10497 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}).
10500 Prints status information about the tape unit.
10504 @FIXME{Is there a better way to frob the spacing on the list?}
10506 If you don't specify a @var{tapename}, @command{mt} uses the environment
10507 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
10508 the default device specified in your @file{sys/mtio.h} file
10509 (@code{DEFTAPE} variable). If this is not defined, the program will
10510 display a descriptive error message and exit with code 1.
10512 @command{mt} returns a 0 exit status when the operation(s) were
10513 successful, 1 if the command was unrecognized, and 2 if an operation
10516 @node Using Multiple Tapes
10517 @section Using Multiple Tapes
10519 Often you might want to write a large archive, one larger than will fit
10520 on the actual tape you are using. In such a case, you can run multiple
10521 @command{tar} commands, but this can be inconvenient, particularly if you
10522 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
10523 Therefore, @command{tar} provides a special mode for creating
10524 multi-volume archives.
10526 @dfn{Multi-volume} archive is a single @command{tar} archive, stored
10527 on several media volumes of fixed size. Although in this section we will
10528 often call @samp{volume} a @dfn{tape}, there is absolutely no
10529 requirement for multi-volume archives to be stored on tapes. Instead,
10530 they can use whatever media type the user finds convenient, they can
10531 even be located on files.
10533 When creating a multi-volume archive, @GNUTAR{} continues to fill
10534 current volume until it runs out of space, then it switches to
10535 next volume (usually the operator is queried to replace the tape on
10536 this point), and continues working on the new volume. This operation
10537 continues until all requested files are dumped. If @GNUTAR{} detects
10538 end of media while dumping a file, such a file is archived in split
10539 form. Some very big files can even be split across several volumes.
10541 Each volume is itself a valid @GNUTAR{} archive, so it can be read
10542 without any special options. Consequently any file member residing
10543 entirely on one volume can be extracted or otherwise operated upon
10544 without needing the other volume. Sure enough, to extract a split
10545 member you would need all volumes its parts reside on.
10547 Multi-volume archives suffer from several limitations. In particular,
10548 they cannot be compressed.
10550 @GNUTAR{} is able to create multi-volume archives of two formats
10551 (@pxref{Formats}): @samp{GNU} and @samp{POSIX}.
10554 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
10555 * Tape Files:: Tape Files
10556 * Tarcat:: Concatenate Volumes into a Single Archive
10560 @node Multi-Volume Archives
10561 @subsection Archives Longer than One Tape or Disk
10562 @cindex Multi-volume archives
10564 @opindex multi-volume
10565 To create an archive that is larger than will fit on a single unit of
10566 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
10567 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
10568 archive can be manipulated like any other archive (provided the
10569 @option{--multi-volume} option is specified), but is stored on more
10570 than one tape or disk.
10572 When you specify @option{--multi-volume}, @command{tar} does not report an
10573 error when it comes to the end of an archive volume (when reading), or
10574 the end of the media (when writing). Instead, it prompts you to load
10575 a new storage volume. If the archive is on a magnetic tape, you
10576 should change tapes when you see the prompt; if the archive is on a
10577 floppy disk, you should change disks; etc.
10580 @item --multi-volume
10582 Creates a multi-volume archive, when used in conjunction with
10583 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
10584 archive, specify @option{--multi-volume} in conjunction with that
10589 $ @kbd{tar --create --multi-volume --file=/dev/tape @var{files}}
10593 The method @command{tar} uses to detect end of tape is not perfect, and
10594 fails on some operating systems or on some devices. If @command{tar}
10595 cannot detect the end of the tape itself, you can use
10596 @option{--tape-length} option to inform it about the capacity of the
10599 @anchor{tape-length}
10601 @opindex tape-length
10602 @item --tape-length=@var{size}
10603 @itemx -L @var{size}
10604 Set maximum length of a volume. The @var{size} argument should then
10605 be the usable size of the tape in units of 1024 bytes. This option
10606 selects @option{--multi-volume} automatically. For example:
10609 $ @kbd{tar --create --tape-length=41943040 --file=/dev/tape @var{files}}
10613 @anchor{change volume prompt}
10614 When @GNUTAR{} comes to the end of a storage media, it asks you to
10615 change the volume. The built-in prompt for POSIX locale
10616 is@footnote{If you run @GNUTAR{} under a different locale, the
10617 translation to the locale's language will be used.}:
10620 Prepare volume #@var{n} for `@var{archive}' and hit return:
10624 where @var{n} is the ordinal number of the volume to be created and
10625 @var{archive} is archive file or device name.
10627 When prompting for a new tape, @command{tar} accepts any of the following
10632 Request @command{tar} to explain possible responses
10634 Request @command{tar} to exit immediately.
10635 @item n @var{file-name}
10636 Request @command{tar} to write the next volume on the file @var{file-name}.
10638 Request @command{tar} to run a subshell. This option can be disabled
10639 by giving @option{--restrict} command line option to
10640 @command{tar}@footnote{@xref{--restrict}, for more information about
10643 Request @command{tar} to begin writing the next volume.
10646 (You should only type @samp{y} after you have changed the tape;
10647 otherwise @command{tar} will write over the volume it just finished.)
10649 @cindex Volume number file
10651 @anchor{volno-file}
10652 @opindex volno-file
10653 The volume number used by @command{tar} in its tape-changing prompt
10654 can be changed; if you give the
10655 @option{--volno-file=@var{file-of-number}} option, then
10656 @var{file-of-number} should be an non-existing file to be created, or
10657 else, a file already containing a decimal number. That number will be
10658 used as the volume number of the first volume written. When
10659 @command{tar} is finished, it will rewrite the file with the
10660 now-current volume number. (This does not change the volume number
10661 written on a tape label, as per @ref{label}, it @emph{only} affects
10662 the number used in the prompt.)
10664 @cindex End-of-archive info script
10665 @cindex Info script
10666 @anchor{info-script}
10667 @opindex info-script
10668 @opindex new-volume-script
10669 If you want more elaborate behavior than this, you can write a special
10670 @dfn{new volume script}, that will be responsible for changing the
10671 volume, and instruct @command{tar} to use it instead of its normal
10672 prompting procedure:
10675 @item --info-script=@var{script-name}
10676 @itemx --new-volume-script=@var{script-name}
10677 @itemx -F @var{script-name}
10678 Specify the full name of the volume script to use. The script can be
10679 used to eject cassettes, or to broadcast messages such as
10680 @samp{Someone please come change my tape} when performing unattended
10684 The @var{script-name} is executed without any command line
10685 arguments. It inherits @command{tar}'s shell environment.
10686 Additional data is passed to it via the following
10687 environment variables:
10690 @vrindex TAR_VERSION, info script environment variable
10692 @GNUTAR{} version number.
10694 @vrindex TAR_ARCHIVE, info script environment variable
10696 The name of the archive @command{tar} is processing.
10698 @vrindex TAR_BLOCKING_FACTOR, info script environment variable
10699 @item TAR_BLOCKING_FACTOR
10700 Current blocking factor (@pxref{Blocking}.
10702 @vrindex TAR_VOLUME, info script environment variable
10704 Ordinal number of the volume @command{tar} is about to start.
10706 @vrindex TAR_SUBCOMMAND, info script environment variable
10707 @item TAR_SUBCOMMAND
10708 A short option describing the operation @command{tar} is executing
10709 @xref{Operations}, for a complete list of subcommand options.
10711 @vrindex TAR_FORMAT, info script environment variable
10713 Format of the archive being processed. @xref{Formats}, for a complete
10714 list of archive format names.
10716 @vrindex TAR_FD, info script environment variable
10718 File descriptor which can be used to communicate the new volume
10719 name to @command{tar}.
10722 The volume script can instruct @command{tar} to use new archive name,
10723 by writing in to file descriptor @env{$TAR_FD} (see below for an example).
10725 If the info script fails, @command{tar} exits; otherwise, it begins
10726 writing the next volume.
10728 If you want @command{tar} to cycle through a series of files or tape
10729 drives, there are three approaches to choose from. First of all, you
10730 can give @command{tar} multiple @option{--file} options. In this case
10731 the specified files will be used, in sequence, as the successive
10732 volumes of the archive. Only when the first one in the sequence needs
10733 to be used again will @command{tar} prompt for a tape change (or run
10734 the info script). For example, suppose someone has two tape drives on
10735 a system named @file{/dev/tape0} and @file{/dev/tape1}. For having
10736 @GNUTAR{} to switch to the second drive when it needs to write the
10737 second tape, and then back to the first tape, etc., just do either of:
10740 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
10741 $ @kbd{tar cMff /dev/tape0 /dev/tape1 @var{files}}
10744 The second method is to use the @samp{n} response to the tape-change
10747 Finally, the most flexible approach is to use a volume script, that
10748 writes new archive name to the file descriptor @env{$TAR_FD}. For example, the
10749 following volume script will create a series of archive files, named
10750 @file{@var{archive}-@var{vol}}, where @var{archive} is the name of the
10751 archive being created (as given by @option{--file} option) and
10752 @var{vol} is the ordinal number of the archive being created:
10757 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
10759 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
10760 case $TAR_SUBCOMMAND in
10762 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
10767 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&$TAR_FD
10771 The same script can be used while listing, comparing or extracting
10772 from the created archive. For example:
10776 # @r{Create a multi-volume archive:}
10777 $ @kbd{tar -c -L1024 -f archive.tar -F new-volume .}
10778 # @r{Extract from the created archive:}
10779 $ @kbd{tar -x -f archive.tar -F new-volume .}
10784 Notice, that the first command had to use @option{-L} option, since
10785 otherwise @GNUTAR{} will end up writing everything to file
10786 @file{archive.tar}.
10788 You can read each individual volume of a multi-volume archive as if it
10789 were an archive by itself. For example, to list the contents of one
10790 volume, use @option{--list}, without @option{--multi-volume} specified.
10791 To extract an archive member from one volume (assuming it is described
10792 that volume), use @option{--extract}, again without
10793 @option{--multi-volume}.
10795 If an archive member is split across volumes (i.e., its entry begins on
10796 one volume of the media and ends on another), you need to specify
10797 @option{--multi-volume} to extract it successfully. In this case, you
10798 should load the volume where the archive member starts, and use
10799 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
10800 volumes as it needs them. @xref{extracting archives}, for more
10801 information about extracting archives.
10803 Multi-volume archives can be modified like any other archive. To add
10804 files to a multi-volume archive, you need to only mount the last
10805 volume of the archive media (and new volumes, if needed). For all
10806 other operations, you need to use the entire archive.
10808 If a multi-volume archive was labeled using
10809 @option{--label=@var{archive-label}} (@pxref{label}) when it was
10810 created, @command{tar} will not automatically label volumes which are
10811 added later. To label subsequent volumes, specify
10812 @option{--label=@var{archive-label}} again in conjunction with the
10813 @option{--append}, @option{--update} or @option{--concatenate} operation.
10815 Notice that multi-volume support is a GNU extension and the archives
10816 created in this mode should be read only using @GNUTAR{}. If you
10817 absolutely have to process such archives using a third-party @command{tar}
10818 implementation, read @ref{Split Recovery}.
10821 @subsection Tape Files
10824 To give the archive a name which will be recorded in it, use the
10825 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
10826 option. This will write a special block identifying
10827 @var{volume-label} as the name of the archive to the front of the
10828 archive which will be displayed when the archive is listed with
10829 @option{--list}. If you are creating a multi-volume archive with
10830 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
10831 volume label will have @samp{Volume @var{nnn}} appended to the name
10832 you give, where @var{nnn} is the number of the volume of the archive.
10833 (If you use the @option{--label=@var{volume-label}}) option when
10834 reading an archive, it checks to make sure the label on the tape
10835 matches the one you give. @xref{label}.
10837 When @command{tar} writes an archive to tape, it creates a single
10838 tape file. If multiple archives are written to the same tape, one
10839 after the other, they each get written as separate tape files. When
10840 extracting, it is necessary to position the tape at the right place
10841 before running @command{tar}. To do this, use the @command{mt} command.
10842 For more information on the @command{mt} command and on the organization
10843 of tapes into a sequence of tape files, see @ref{mt}.
10845 People seem to often do:
10848 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
10851 or such, for pushing a common date in all volumes or an archive set.
10854 @subsection Concatenate Volumes into a Single Archive
10857 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
10858 archive to a single @command{tar} archive. Simply concatenating all
10859 volumes into one will not work, since each volume carries an additional
10860 information at the beginning. @GNUTAR{} is shipped with the shell
10861 script @command{tarcat} designed for this purpose.
10863 The script takes a list of files comprising a multi-volume archive
10864 and creates the resulting archive at the standard output. For example:
10867 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
10870 The script implements a simple heuristics to determine the format of
10871 the first volume file and to decide how to process the rest of the
10872 files. However, it makes no attempt to verify whether the files are
10873 given in order or even if they are valid @command{tar} archives.
10874 It uses @command{dd} and does not filter its standard error, so you
10875 will usually see lots of spurious messages.
10877 @FIXME{The script is not installed. Should we install it?}
10880 @section Including a Label in the Archive
10881 @cindex Labeling an archive
10882 @cindex Labels on the archive media
10883 @cindex Labeling multi-volume archives
10887 To avoid problems caused by misplaced paper labels on the archive
10888 media, you can include a @dfn{label} entry---an archive member which
10889 contains the name of the archive---in the archive itself. Use the
10890 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
10891 option in conjunction with the @option{--create} operation to include
10892 a label entry in the archive as it is being created.
10895 @item --label=@var{archive-label}
10896 @itemx -V @var{archive-label}
10897 Includes an @dfn{archive-label} at the beginning of the archive when
10898 the archive is being created, when used in conjunction with the
10899 @option{--create} operation. Checks to make sure the archive label
10900 matches the one specified (when used in conjunction with any other
10904 If you create an archive using both
10905 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
10906 and @option{--multi-volume} (@option{-M}), each volume of the archive
10907 will have an archive label of the form @samp{@var{archive-label}
10908 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
10909 next, and so on. @xref{Using Multiple Tapes}, for information on
10910 creating multiple volume archives.
10912 @cindex Volume label, listing
10913 @cindex Listing volume label
10914 The volume label will be displayed by @option{--list} along with
10915 the file contents. If verbose display is requested, it will also be
10916 explicitly marked as in the example below:
10920 $ @kbd{tar --verbose --list --file=iamanarchive}
10921 V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header--
10922 -rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename
10926 @opindex test-label
10927 @anchor{--test-label option}
10928 However, @option{--list} option will cause listing entire
10929 contents of the archive, which may be undesirable (for example, if the
10930 archive is stored on a tape). You can request checking only the volume
10931 by specifying @option{--test-label} option. This option reads only the
10932 first block of an archive, so it can be used with slow storage
10933 devices. For example:
10937 $ @kbd{tar --test-label --file=iamanarchive}
10942 If @option{--test-label} is used with a single command line
10943 argument, @command{tar} compares the volume label with the
10944 argument. It exits with code 0 if the two strings match, and with code
10945 2 otherwise. In this case no output is displayed. For example:
10949 $ @kbd{tar --test-label --file=iamanarchive 'iamalable'}
10951 $ @kbd{tar --test-label --file=iamanarchive 'iamalable' alabel}
10956 If you request any operation, other than @option{--create}, along
10957 with using @option{--label} option, @command{tar} will first check if
10958 the archive label matches the one specified and will refuse to proceed
10959 if it does not. Use this as a safety precaution to avoid accidentally
10960 overwriting existing archives. For example, if you wish to add files
10961 to @file{archive}, presumably labeled with string @samp{My volume},
10966 $ @kbd{tar -rf archive --label 'My volume' .}
10967 tar: Archive not labeled to match `My volume'
10972 in case its label does not match. This will work even if
10973 @file{archive} is not labeled at all.
10975 Similarly, @command{tar} will refuse to list or extract the
10976 archive if its label doesn't match the @var{archive-label}
10977 specified. In those cases, @var{archive-label} argument is interpreted
10978 as a globbing-style pattern which must match the actual magnetic
10979 volume label. @xref{exclude}, for a precise description of how match
10980 is attempted@footnote{Previous versions of @command{tar} used full
10981 regular expression matching, or before that, only exact string
10982 matching, instead of wildcard matchers. We decided for the sake of
10983 simplicity to use a uniform matching device through
10984 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
10985 the volume label matcher will also suffix @var{archive-label} by
10986 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
10987 up. Since the volume numbering is automatically added in labels at
10988 creation time, it sounded logical to equally help the user taking care
10989 of it when the archive is being read.
10991 The @option{--label} was once called @option{--volume}, but is not
10992 available under that name anymore.
10994 You can also use @option{--label} to get a common information on
10995 all tapes of a series. For having this information different in each
10996 series created through a single script used on a regular basis, just
10997 manage to get some date string as part of the label. For example:
11001 $ @kbd{tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"}
11002 $ @kbd{tar --create --file=/dev/tape --multi-volume \
11003 --volume="Daily backup for `date +%Y-%m-%d`"}
11007 Also note that each label has its own date and time, which corresponds
11008 to when @GNUTAR{} initially attempted to write it,
11009 often soon after the operator launches @command{tar} or types the
11010 carriage return telling that the next tape is ready. Comparing date
11011 labels does give an idea of tape throughput only if the delays for
11012 rewinding tapes and the operator switching them were negligible, which
11013 is usually not the case.
11016 @section Verifying Data as It is Stored
11017 @cindex Verifying a write operation
11018 @cindex Double-checking a write operation
11023 @opindex verify, short description
11024 Attempt to verify the archive after writing.
11027 This option causes @command{tar} to verify the archive after writing it.
11028 Each volume is checked after it is written, and any discrepancies
11029 are recorded on the standard error output.
11031 Verification requires that the archive be on a back-space-able medium.
11032 This means pipes, some cartridge tape drives, and some other devices
11033 cannot be verified.
11035 You can insure the accuracy of an archive by comparing files in the
11036 system with archive members. @command{tar} can compare an archive to the
11037 file system as the archive is being written, to verify a write
11038 operation, or can compare a previously written archive, to insure that
11041 @xopindex{verify, using with @option{--create}}
11042 @xopindex{create, using with @option{--verify}}
11043 To check for discrepancies in an archive immediately after it is
11044 written, use the @option{--verify} (@option{-W}) option in conjunction with
11045 the @option{--create} operation. When this option is
11046 specified, @command{tar} checks archive members against their counterparts
11047 in the file system, and reports discrepancies on the standard error.
11049 To verify an archive, you must be able to read it from before the end
11050 of the last written entry. This option is useful for detecting data
11051 errors on some tapes. Archives written to pipes, some cartridge tape
11052 drives, and some other devices cannot be verified.
11054 One can explicitly compare an already made archive with the file
11055 system by using the @option{--compare} (@option{--diff}, @option{-d})
11056 option, instead of using the more automatic @option{--verify} option.
11059 Note that these two options have a slightly different intent. The
11060 @option{--compare} option checks how identical are the logical contents of some
11061 archive with what is on your disks, while the @option{--verify} option is
11062 really for checking if the physical contents agree and if the recording
11063 media itself is of dependable quality. So, for the @option{--verify}
11064 operation, @command{tar} tries to defeat all in-memory cache pertaining to
11065 the archive, while it lets the speed optimization undisturbed for the
11066 @option{--compare} option. If you nevertheless use @option{--compare} for
11067 media verification, you may have to defeat the in-memory cache yourself,
11068 maybe by opening and reclosing the door latch of your recording unit,
11069 forcing some doubt in your operating system about the fact this is really
11070 the same volume as the one just written or read.
11072 The @option{--verify} option would not be necessary if drivers were indeed
11073 able to detect dependably all write failures. This sometimes require many
11074 magnetic heads, some able to read after the writes occurred. One would
11075 not say that drivers unable to detect all cases are necessarily flawed,
11076 as long as programming is concerned.
11078 The @option{--verify} (@option{-W}) option will not work in
11079 conjunction with the @option{--multi-volume} (@option{-M}) option or
11080 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
11081 and @option{--delete} operations. @xref{Operations}, for more
11082 information on these operations.
11084 Also, since @command{tar} normally strips leading @samp{/} from file
11085 names (@pxref{absolute}), a command like @samp{tar --verify -cf
11086 /tmp/foo.tar /etc} will work as desired only if the working directory is
11087 @file{/}, as @command{tar} uses the archive's relative member names
11088 (e.g., @file{etc/motd}) when verifying the archive.
11090 @node Write Protection
11091 @section Write Protection
11093 Almost all tapes and diskettes, and in a few rare cases, even disks can
11094 be @dfn{write protected}, to protect data on them from being changed.
11095 Once an archive is written, you should write protect the media to prevent
11096 the archive from being accidentally overwritten or deleted. (This will
11097 protect the archive from being changed with a tape or floppy drive---it
11098 will not protect it from magnet fields or other physical hazards).
11100 The write protection device itself is usually an integral part of the
11101 physical media, and can be a two position (write enabled/write
11102 disabled) switch, a notch which can be popped out or covered, a ring
11103 which can be removed from the center of a tape reel, or some other
11104 changeable feature.
11109 This appendix lists some important user-visible changes between
11110 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
11111 version of this document is available at
11112 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
11113 @GNUTAR{} documentation page}.
11116 @item Use of globbing patterns when listing and extracting.
11118 Previous versions of GNU tar assumed shell-style globbing when
11119 extracting from or listing an archive. For example:
11122 $ @kbd{tar xf foo.tar '*.c'}
11125 would extract all files whose names end in @samp{.c}. This behavior
11126 was not documented and was incompatible with traditional tar
11127 implementations. Therefore, starting from version 1.15.91, GNU tar
11128 no longer uses globbing by default. For example, the above invocation
11129 is now interpreted as a request to extract from the archive the file
11132 To facilitate transition to the new behavior for those users who got
11133 used to the previous incorrect one, @command{tar} will print a warning
11134 if it finds out that a requested member was not found in the archive
11135 and its name looks like a globbing pattern. For example:
11138 $ @kbd{tar xf foo.tar '*.c'}
11139 tar: Pattern matching characters used in file names. Please,
11140 tar: use --wildcards to enable pattern matching, or --no-wildcards to
11141 tar: suppress this warning.
11142 tar: *.c: Not found in archive
11143 tar: Error exit delayed from previous errors
11146 To treat member names as globbing patterns, use --wildcards option.
11147 If you want to tar to mimic the behavior of versions prior to 1.15.91,
11148 add this option to your @env{TAR_OPTIONS} variable.
11150 @xref{wildcards}, for the detailed discussion of the use of globbing
11151 patterns by @GNUTAR{}.
11153 @item Use of short option @option{-o}.
11155 Earlier versions of @GNUTAR{} understood @option{-o} command line
11156 option as a synonym for @option{--old-archive}.
11158 @GNUTAR{} starting from version 1.13.90 understands this option as
11159 a synonym for @option{--no-same-owner}. This is compatible with
11160 UNIX98 @command{tar} implementations.
11162 However, to facilitate transition, @option{-o} option retains its
11163 old semantics when it is used with one of archive-creation commands.
11164 Users are encouraged to use @option{--format=oldgnu} instead.
11166 It is especially important, since versions of @acronym{GNU} Automake
11167 up to and including 1.8.4 invoke tar with this option to produce
11168 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
11169 of this issue and its implications.
11171 @FIXME{Change the first argument to tar-formats when the new Automake is
11172 out. The proposition to add @anchor{} to the appropriate place of its
11173 docs was accepted by Automake people --Sergey 2006-05-25}.
11174 @xref{Options, tar-v7, Changing Automake's Behavior,
11175 automake, GNU Automake}, for a description on how to use various
11176 archive formats with @command{automake}.
11178 Future versions of @GNUTAR{} will understand @option{-o} only as a
11179 synonym for @option{--no-same-owner}.
11181 @item Use of short option @option{-l}
11183 Earlier versions of @GNUTAR{} understood @option{-l} option as a
11184 synonym for @option{--one-file-system}. Since such usage contradicted
11185 to UNIX98 specification and harmed compatibility with other
11186 implementation, it was declared deprecated in version 1.14. However,
11187 to facilitate transition to its new semantics, it was supported by
11188 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
11189 variant of @option{--check-links} was introduced in version 1.15.91.
11191 @item Use of options @option{--portability} and @option{--old-archive}
11193 These options are deprecated. Please use @option{--format=v7} instead.
11195 @item Use of option @option{--posix}
11197 This option is deprecated. Please use @option{--format=posix} instead.
11200 @node Configuring Help Summary
11201 @appendix Configuring Help Summary
11203 Running @kbd{tar --help} displays the short @command{tar} option
11204 summary (@pxref{help}). This summary is organized by @dfn{groups} of
11205 semantically close options. The options within each group are printed
11206 in the following order: a short option, eventually followed by a list
11207 of corresponding long option names, followed by a short description of
11208 the option. For example, here is an excerpt from the actual @kbd{tar
11212 Main operation mode:
11214 -A, --catenate, --concatenate append tar files to an archive
11215 -c, --create create a new archive
11216 -d, --diff, --compare find differences between archive and
11218 --delete delete from the archive
11221 @vrindex ARGP_HELP_FMT, environment variable
11222 The exact visual representation of the help output is configurable via
11223 @env{ARGP_HELP_FMT} environment variable. The value of this variable
11224 is a comma-separated list of @dfn{format variable} assignments. There
11225 are two kinds of format variables. An @dfn{offset variable} keeps the
11226 offset of some part of help output text from the leftmost column on
11227 the screen. A @dfn{boolean} variable is a flag that toggles some
11228 output feature on or off. Depending on the type of the corresponding
11229 variable, there are two kinds of assignments:
11232 @item Offset assignment
11234 The assignment to an offset variable has the following syntax:
11237 @var{variable}=@var{value}
11241 where @var{variable} is the variable name, and @var{value} is a
11242 numeric value to be assigned to the variable.
11244 @item Boolean assignment
11246 To assign @code{true} value to a variable, simply put this variable name. To
11247 assign @code{false} value, prefix the variable name with @samp{no-}. For
11252 # Assign @code{true} value:
11254 # Assign @code{false} value:
11260 Following variables are declared:
11262 @deftypevr {Help Output} boolean dup-args
11263 If true, arguments for an option are shown with both short and long
11264 options, even when a given option has both forms, for example:
11267 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
11270 If false, then if an option has both short and long forms, the
11271 argument is only shown with the long one, for example:
11274 -f, --file=ARCHIVE use archive file or device ARCHIVE
11278 and a message indicating that the argument is applicable to both
11279 forms is printed below the options. This message can be disabled
11280 using @code{dup-args-note} (see below).
11282 The default is false.
11285 @deftypevr {Help Output} boolean dup-args-note
11286 If this variable is true, which is the default, the following notice
11287 is displayed at the end of the help output:
11290 Mandatory or optional arguments to long options are also mandatory or
11291 optional for any corresponding short options.
11294 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
11295 variables @code{dup-args} or @code{dup-args-note} should be set.
11298 @deftypevr {Help Output} offset short-opt-col
11299 Column in which short options start. Default is 2.
11303 $ @kbd{tar --help|grep ARCHIVE}
11304 -f, --file=ARCHIVE use archive file or device ARCHIVE
11305 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
11306 -f, --file=ARCHIVE use archive file or device ARCHIVE
11311 @deftypevr {Help Output} offset long-opt-col
11312 Column in which long options start. Default is 6. For example:
11316 $ @kbd{tar --help|grep ARCHIVE}
11317 -f, --file=ARCHIVE use archive file or device ARCHIVE
11318 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
11319 -f, --file=ARCHIVE use archive file or device ARCHIVE
11324 @deftypevr {Help Output} offset doc-opt-col
11325 Column in which @dfn{doc options} start. A doc option isn't actually
11326 an option, but rather an arbitrary piece of documentation that is
11327 displayed in much the same manner as the options. For example, in
11328 the description of @option{--format} option:
11332 -H, --format=FORMAT create archive of the given format.
11334 FORMAT is one of the following:
11336 gnu GNU tar 1.13.x format
11337 oldgnu GNU format as per tar <= 1.12
11338 pax POSIX 1003.1-2001 (pax) format
11340 ustar POSIX 1003.1-1988 (ustar) format
11341 v7 old V7 tar format
11346 the format names are doc options. Thus, if you set
11347 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
11348 will look as follows:
11352 -H, --format=FORMAT create archive of the given format.
11354 FORMAT is one of the following:
11356 gnu GNU tar 1.13.x format
11357 oldgnu GNU format as per tar <= 1.12
11358 pax POSIX 1003.1-2001 (pax) format
11360 ustar POSIX 1003.1-1988 (ustar) format
11361 v7 old V7 tar format
11366 @deftypevr {Help Output} offset opt-doc-col
11367 Column in which option description starts. Default is 29.
11371 $ @kbd{tar --help|grep ARCHIVE}
11372 -f, --file=ARCHIVE use archive file or device ARCHIVE
11373 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
11374 -f, --file=ARCHIVE use archive file or device ARCHIVE
11375 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
11377 use archive file or device ARCHIVE
11382 Notice, that the description starts on a separate line if
11383 @code{opt-doc-col} value is too small.
11386 @deftypevr {Help Output} offset header-col
11387 Column in which @dfn{group headers} are printed. A group header is a
11388 descriptive text preceding an option group. For example, in the
11392 Main operation mode:
11394 -A, --catenate, --concatenate append tar files to
11396 -c, --create create a new archive
11399 @samp{Main operation mode:} is the group header.
11401 The default value is 1.
11404 @deftypevr {Help Output} offset usage-indent
11405 Indentation of wrapped usage lines. Affects @option{--usage}
11406 output. Default is 12.
11409 @deftypevr {Help Output} offset rmargin
11410 Right margin of the text output. Used for wrapping.
11413 @node Fixing Snapshot Files
11414 @appendix Fixing Snapshot Files
11415 @include tar-snapshot-edit.texi
11417 @node Tar Internals
11418 @appendix Tar Internals
11419 @include intern.texi
11423 @include genfile.texi
11425 @node Free Software Needs Free Documentation
11426 @appendix Free Software Needs Free Documentation
11427 @include freemanuals.texi
11429 @node Copying This Manual
11430 @appendix Copying This Manual
11433 * GNU Free Documentation License:: License for copying this manual
11438 @node Index of Command Line Options
11439 @appendix Index of Command Line Options
11441 This appendix contains an index of all @GNUTAR{} long command line
11442 options. The options are listed without the preceding double-dash.
11443 For a cross-reference of short command line options, @ref{Short Option Summary}.
11456 @c Local variables:
11457 @c texinfo-column-for-description: 32
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