1 \input texinfo @c -*-texinfo-*-
2 @comment %**start of header
5 @settitle GNU tar @value{VERSION}
14 @c 1. Pay attention to @FIXME{}s and @UNREVISED{}s
15 @c 2. Before creating final variant:
16 @c 2.1. Run 'make check-options' to make sure all options are properly
18 @c 2.2. Run 'make master-menu' (see comment before the master menu).
20 @include rendition.texi
26 @c Put everything in one index (arbitrarily chosen to be the concept index).
35 This manual is for @acronym{GNU} @command{tar} (version
36 @value{VERSION}, @value{UPDATED}), which creates and extracts files
39 Copyright @copyright{} 1992, 1994--1997, 1999--2001, 2003--2013 Free
40 Software Foundation, Inc.
43 Permission is granted to copy, distribute and/or modify this document
44 under the terms of the GNU Free Documentation License, Version 1.3 or
45 any later version published by the Free Software Foundation; with the
46 Invariant Sections being ``GNU General Public License'', with the
47 Front-Cover Texts being ``A GNU Manual'', and with the Back-Cover Texts
48 as in (a) below. A copy of the license is included in the section
49 entitled ``GNU Free Documentation License''.
51 (a) The FSF's Back-Cover Text is: ``You have the freedom to
52 copy and modify this GNU manual.''
56 @dircategory Archiving
58 * Tar: (tar). Making tape (or disk) archives.
61 @dircategory Individual utilities
63 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
66 @shorttitlepage @acronym{GNU} @command{tar}
69 @title @acronym{GNU} tar: an archiver tool
70 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
71 @author John Gilmore, Jay Fenlason et al.
74 @vskip 0pt plus 1filll
80 @top @acronym{GNU} tar: an archiver tool
85 @cindex archiving files
87 The first part of this master menu lists the major nodes in this Info
88 document. The rest of the menu lists all the lower level nodes.
91 @c The master menu goes here.
93 @c NOTE: To update it from within Emacs, make sure mastermenu.el is
94 @c loaded and run texinfo-master-menu.
95 @c To update it from the command line, run
106 * Date input formats::
109 * Reliability and security::
114 * Configuring Help Summary::
115 * Fixing Snapshot Files::
118 * Free Software Needs Free Documentation::
119 * GNU Free Documentation License::
120 * Index of Command Line Options::
124 --- The Detailed Node Listing ---
128 * Book Contents:: What this Book Contains
129 * Definitions:: Some Definitions
130 * What tar Does:: What @command{tar} Does
131 * Naming tar Archives:: How @command{tar} Archives are Named
132 * Authors:: @GNUTAR{} Authors
133 * Reports:: Reporting bugs or suggestions
135 Tutorial Introduction to @command{tar}
138 * stylistic conventions::
139 * basic tar options:: Basic @command{tar} Operations and Options
140 * frequent operations::
141 * Two Frequent Options::
142 * create:: How to Create Archives
143 * list:: How to List Archives
144 * extract:: How to Extract Members from an Archive
147 Two Frequently Used Options
153 How to Create Archives
155 * prepare for examples::
156 * Creating the archive::
165 How to Extract Members from an Archive
167 * extracting archives::
170 * extracting untrusted archives::
176 * using tar options::
186 The Three Option Styles
188 * Long Options:: Long Option Style
189 * Short Options:: Short Option Style
190 * Old Options:: Old Option Style
191 * Mixing:: Mixing Option Styles
193 All @command{tar} Options
195 * Operation Summary::
197 * Short Option Summary::
209 Advanced @GNUTAR{} Operations
218 How to Add Files to Existing Archives: @option{--append}
220 * appending files:: Appending Files to an Archive
227 Options Used by @option{--create}
229 * override:: Overriding File Metadata.
230 * Ignore Failed Read::
232 Options Used by @option{--extract}
234 * Reading:: Options to Help Read Archives
235 * Writing:: Changing How @command{tar} Writes Files
236 * Scarce:: Coping with Scarce Resources
238 Options to Help Read Archives
240 * read full records::
243 Changing How @command{tar} Writes Files
245 * Dealing with Old Files::
246 * Overwrite Old Files::
251 * Data Modification Times::
252 * Setting Access Permissions::
253 * Directory Modification Times and Permissions::
254 * Writing to Standard Output::
255 * Writing to an External Program::
258 Coping with Scarce Resources
263 Performing Backups and Restoring Files
265 * Full Dumps:: Using @command{tar} to Perform Full Dumps
266 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
267 * Backup Levels:: Levels of Backups
268 * Backup Parameters:: Setting Parameters for Backups and Restoration
269 * Scripted Backups:: Using the Backup Scripts
270 * Scripted Restoration:: Using the Restore Script
272 Setting Parameters for Backups and Restoration
274 * General-Purpose Variables::
275 * Magnetic Tape Control::
277 * backup-specs example:: An Example Text of @file{Backup-specs}
279 Choosing Files and Names for @command{tar}
281 * file:: Choosing the Archive's Name
282 * Selecting Archive Members::
283 * files:: Reading Names from a File
284 * exclude:: Excluding Some Files
285 * wildcards:: Wildcards Patterns and Matching
286 * quoting styles:: Ways of Quoting Special Characters in Names
287 * transform:: Modifying File and Member Names
288 * after:: Operating Only on New Files
289 * recurse:: Descending into Directories
290 * one:: Crossing File System Boundaries
292 Reading Names from a File
298 * problems with exclude::
300 Wildcards Patterns and Matching
302 * controlling pattern-matching::
304 Crossing File System Boundaries
306 * directory:: Changing Directory
307 * absolute:: Absolute File Names
311 * General date syntax:: Common rules.
312 * Calendar date items:: 19 Dec 1994.
313 * Time of day items:: 9:20pm.
314 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}.
315 * Day of week items:: Monday and others.
316 * Relative items in date strings:: next tuesday, 2 years ago.
317 * Pure numbers in date strings:: 19931219, 1440.
318 * Seconds since the Epoch:: @@1078100502.
319 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0".
320 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al.
322 Controlling the Archive Format
324 * Compression:: Using Less Space through Compression
325 * Attributes:: Handling File Attributes
326 * Portability:: Making @command{tar} Archives More Portable
327 * cpio:: Comparison of @command{tar} and @command{cpio}
329 Using Less Space through Compression
331 * gzip:: Creating and Reading Compressed Archives
332 * sparse:: Archiving Sparse Files
334 Creating and Reading Compressed Archives
336 * lbzip2:: Using lbzip2 with @GNUTAR{}.
338 Making @command{tar} Archives More Portable
340 * Portable Names:: Portable Names
341 * dereference:: Symbolic Links
342 * hard links:: Hard Links
343 * old:: Old V7 Archives
344 * ustar:: Ustar Archives
345 * gnu:: GNU and old GNU format archives.
346 * posix:: @acronym{POSIX} archives
347 * Checksumming:: Checksumming Problems
348 * Large or Negative Values:: Large files, negative time stamps, etc.
349 * Other Tars:: How to Extract GNU-Specific Data Using
350 Other @command{tar} Implementations
352 @GNUTAR{} and @acronym{POSIX} @command{tar}
354 * PAX keywords:: Controlling Extended Header Keywords.
356 How to Extract GNU-Specific Data Using Other @command{tar} Implementations
358 * Split Recovery:: Members Split Between Volumes
359 * Sparse Recovery:: Sparse Members
361 Tapes and Other Archive Media
363 * Device:: Device selection and switching
364 * Remote Tape Server::
365 * Common Problems and Solutions::
366 * Blocking:: Blocking
367 * Many:: Many archives on one tape
368 * Using Multiple Tapes:: Using Multiple Tapes
369 * label:: Including a Label in the Archive
375 * Format Variations:: Format Variations
376 * Blocking Factor:: The Blocking Factor of an Archive
378 Many Archives on One Tape
380 * Tape Positioning:: Tape Positions and Tape Marks
381 * mt:: The @command{mt} Utility
385 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
386 * Tape Files:: Tape Files
387 * Tarcat:: Concatenate Volumes into a Single Archive
392 * Standard:: Basic Tar Format
393 * Extensions:: @acronym{GNU} Extensions to the Archive Format
394 * Sparse Formats:: Storing Sparse Files
401 * PAX 0:: PAX Format, Versions 0.0 and 0.1
402 * PAX 1:: PAX Format, Version 1.0
406 * Generate Mode:: File Generation Mode.
407 * Status Mode:: File Status Mode.
408 * Exec Mode:: Synchronous Execution mode.
412 * GNU Free Documentation License:: License for copying this manual
418 @chapter Introduction
421 and manipulates @dfn{archives} which are actually collections of
422 many other files; the program provides users with an organized and
423 systematic method for controlling a large amount of data.
424 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
425 archives need not (and these days, typically do not) reside on tapes.
428 * Book Contents:: What this Book Contains
429 * Definitions:: Some Definitions
430 * What tar Does:: What @command{tar} Does
431 * Naming tar Archives:: How @command{tar} Archives are Named
432 * Authors:: @GNUTAR{} Authors
433 * Reports:: Reporting bugs or suggestions
437 @section What this Book Contains
439 The first part of this chapter introduces you to various terms that will
440 recur throughout the book. It also tells you who has worked on @GNUTAR{}
441 and its documentation, and where you should send bug reports
444 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
445 gentle introduction for people who are new to using @command{tar}. It is
446 meant to be self-contained, not requiring any reading from subsequent
447 chapters to make sense. It moves from topic to topic in a logical,
448 progressive order, building on information already explained.
450 Although the tutorial is paced and structured to allow beginners to
451 learn how to use @command{tar}, it is not intended solely for beginners.
452 The tutorial explains how to use the three most frequently used
453 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
454 two frequently used options (@samp{file} and @samp{verbose}). The other
455 chapters do not refer to the tutorial frequently; however, if a section
456 discusses something which is a complex variant of a basic concept, there
457 may be a cross-reference to that basic concept. (The entire book,
458 including the tutorial, assumes that the reader understands some basic
459 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
461 The third chapter presents the remaining five operations, and
462 information about using @command{tar} options and option syntax.
464 The other chapters are meant to be used as a reference. Each chapter
465 presents everything that needs to be said about a specific topic.
467 One of the chapters (@pxref{Date input formats}) exists in its
468 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
469 In addition, one section of this manual (@pxref{Standard}) contains a
470 big quote which is taken directly from @command{tar} sources.
472 In general, we give both long and short (abbreviated) option names
473 at least once in each section where the relevant option is covered, so
474 that novice readers will become familiar with both styles. (A few
475 options have no short versions, and the relevant sections will
479 @section Some Definitions
483 The @command{tar} program is used to create and manipulate @command{tar}
484 archives. An @dfn{archive} is a single file which contains the contents
485 of many files, while still identifying the names of the files, their
486 owner(s), and so forth. (In addition, archives record access
487 permissions, user and group, size in bytes, and data modification time.
488 Some archives also record the file names in each archived directory, as
489 well as other file and directory information.) You can use @command{tar}
490 to @dfn{create} a new archive in a specified directory.
493 @cindex archive member
496 The files inside an archive are called @dfn{members}. Within this
497 manual, we use the term @dfn{file} to refer only to files accessible in
498 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
499 @dfn{member} to refer only to the members of an archive. Similarly, a
500 @dfn{file name} is the name of a file, as it resides in the file system,
501 and a @dfn{member name} is the name of an archive member within the
506 The term @dfn{extraction} refers to the process of copying an archive
507 member (or multiple members) into a file in the file system. Extracting
508 all the members of an archive is often called @dfn{extracting the
509 archive}. The term @dfn{unpack} can also be used to refer to the
510 extraction of many or all the members of an archive. Extracting an
511 archive does not destroy the archive's structure, just as creating an
512 archive does not destroy the copies of the files that exist outside of
513 the archive. You may also @dfn{list} the members in a given archive
514 (this is often thought of as ``printing'' them to the standard output,
515 or the command line), or @dfn{append} members to a pre-existing archive.
516 All of these operations can be performed using @command{tar}.
519 @section What @command{tar} Does
522 The @command{tar} program provides the ability to create @command{tar}
523 archives, as well as various other kinds of manipulation. For example,
524 you can use @command{tar} on previously created archives to extract files,
525 to store additional files, or to update or list files which were already
528 Initially, @command{tar} archives were used to store files conveniently on
529 magnetic tape. The name @command{tar} comes from this use; it stands for
530 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
531 direct its output to available devices, files, or other programs (using
532 pipes). @command{tar} may even access remote devices or files (as archives).
534 You can use @command{tar} archives in many ways. We want to stress a few
535 of them: storage, backup, and transportation.
537 @FIXME{the following table entries need a bit of work.}
540 Often, @command{tar} archives are used to store related files for
541 convenient file transfer over a network. For example, the
542 @acronym{GNU} Project distributes its software bundled into
543 @command{tar} archives, so that all the files relating to a particular
544 program (or set of related programs) can be transferred as a single
547 A magnetic tape can store several files in sequence. However, the tape
548 has no names for these files; it only knows their relative position on
549 the tape. One way to store several files on one tape and retain their
550 names is by creating a @command{tar} archive. Even when the basic transfer
551 mechanism can keep track of names, as FTP can, the nuisance of handling
552 multiple files, directories, and multiple links makes @command{tar}
555 Archive files are also used for long-term storage. You can think of
556 this as transportation from the present into the future. (It is a
557 science-fiction idiom that you can move through time as well as in
558 space; the idea here is that @command{tar} can be used to move archives in
559 all dimensions, even time!)
562 Because the archive created by @command{tar} is capable of preserving
563 file information and directory structure, @command{tar} is commonly
564 used for performing full and incremental backups of disks. A backup
565 puts a collection of files (possibly pertaining to many users and
566 projects) together on a disk or a tape. This guards against
567 accidental destruction of the information in those files.
568 @GNUTAR{} has special features that allow it to be
569 used to make incremental and full dumps of all the files in a
573 You can create an archive on one system, transfer it to another system,
574 and extract the contents there. This allows you to transport a group of
575 files from one system to another.
578 @node Naming tar Archives
579 @section How @command{tar} Archives are Named
581 Conventionally, @command{tar} archives are given names ending with
582 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
583 but this manual follows that convention in order to accustom readers to
584 it and to make examples more clear.
589 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
590 archive members as ``files'' or ``entries''. For people familiar with
591 the operation of @command{tar}, this causes no difficulty. However, in
592 this manual, we consistently refer to ``archives'' and ``archive
593 members'' to make learning to use @command{tar} easier for novice users.
596 @section @GNUTAR{} Authors
598 @GNUTAR{} was originally written by John Gilmore,
599 and modified by many people. The @acronym{GNU} enhancements were
600 written by Jay Fenlason, then Joy Kendall, and the whole package has
601 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
602 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
603 numerous and kind users.
605 We wish to stress that @command{tar} is a collective work, and owes much to
606 all those people who reported problems, offered solutions and other
607 insights, or shared their thoughts and suggestions. An impressive, yet
608 partial list of those contributors can be found in the @file{THANKS}
609 file from the @GNUTAR{} distribution.
611 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
612 sure i want to spell out the history in this detail, at least not for
613 the printed book. i'm just not sure it needs to be said this way.
614 i'll think about it.}
616 @FIXME{History is more important, and surely more interesting, than
617 actual names. Quoting names without history would be meaningless. FP}
619 Jay Fenlason put together a draft of a @GNUTAR{}
620 manual, borrowing notes from the original man page from John Gilmore.
621 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
622 Gorin worked on a tutorial and manual for @GNUTAR{}.
623 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
624 taking information from all these sources and merging them. Melissa
625 Weisshaus finally edited and redesigned the book to create version
626 1.12. The book for versions from 1.14 up to @value{VERSION} were edited
627 by the current maintainer, Sergey Poznyakoff.
629 For version 1.12, Daniel Hagerty contributed a great deal of technical
630 consulting. In particular, he is the primary author of @ref{Backups}.
632 In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org
633 (see @url{http://savannah.gnu.org/projects/tar}), and
634 active development and maintenance work has started
635 again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey
636 Poznyakoff and Jeff Bailey.
638 Support for @acronym{POSIX} archives was added by Sergey Poznyakoff.
641 @section Reporting bugs or suggestions
644 @cindex reporting bugs
645 If you find problems or have suggestions about this program or manual,
646 please report them to @file{bug-tar@@gnu.org}.
648 When reporting a bug, please be sure to include as much detail as
649 possible, in order to reproduce it.
650 @FIXME{Be more specific, I'd like to make this node as detailed as
651 'Bug reporting' node in Emacs manual.}
654 @chapter Tutorial Introduction to @command{tar}
656 This chapter guides you through some basic examples of three @command{tar}
657 operations: @option{--create}, @option{--list}, and @option{--extract}. If
658 you already know how to use some other version of @command{tar}, then you
659 may not need to read this chapter. This chapter omits most complicated
660 details about how @command{tar} works.
664 * stylistic conventions::
665 * basic tar options:: Basic @command{tar} Operations and Options
666 * frequent operations::
667 * Two Frequent Options::
668 * create:: How to Create Archives
669 * list:: How to List Archives
670 * extract:: How to Extract Members from an Archive
675 @section Assumptions this Tutorial Makes
677 This chapter is paced to allow beginners to learn about @command{tar}
678 slowly. At the same time, we will try to cover all the basic aspects of
679 these three operations. In order to accomplish both of these tasks, we
680 have made certain assumptions about your knowledge before reading this
681 manual, and the hardware you will be using:
685 Before you start to work through this tutorial, you should understand
686 what the terms ``archive'' and ``archive member'' mean
687 (@pxref{Definitions}). In addition, you should understand something
688 about how Unix-type operating systems work, and you should know how to
689 use some basic utilities. For example, you should know how to create,
690 list, copy, rename, edit, and delete files and directories; how to
691 change between directories; and how to figure out where you are in the
692 file system. You should have some basic understanding of directory
693 structure and how files are named according to which directory they are
694 in. You should understand concepts such as standard output and standard
695 input, what various definitions of the term @samp{argument} mean, and the
696 differences between relative and absolute file names.
697 @FIXME{and what else?}
700 This manual assumes that you are working from your own home directory
701 (unless we state otherwise). In this tutorial, you will create a
702 directory to practice @command{tar} commands in. When we show file names,
703 we will assume that those names are relative to your home directory.
704 For example, my home directory is @file{/home/fsf/melissa}. All of
705 my examples are in a subdirectory of the directory named by that file
706 name; the subdirectory is called @file{practice}.
709 In general, we show examples of archives which exist on (or can be
710 written to, or worked with from) a directory on a hard disk. In most
711 cases, you could write those archives to, or work with them on any other
712 device, such as a tape drive. However, some of the later examples in
713 the tutorial and next chapter will not work on tape drives.
714 Additionally, working with tapes is much more complicated than working
715 with hard disks. For these reasons, the tutorial does not cover working
716 with tape drives. @xref{Media}, for complete information on using
717 @command{tar} archives with tape drives.
719 @FIXME{this is a cop out. need to add some simple tape drive info.}
722 @node stylistic conventions
723 @section Stylistic Conventions
725 In the examples, @samp{$} represents a typical shell prompt. It
726 precedes lines you should type; to make this more clear, those lines are
727 shown in @kbd{this font}, as opposed to lines which represent the
728 computer's response; those lines are shown in @code{this font}, or
729 sometimes @samp{like this}.
731 @c When we have lines which are too long to be
732 @c displayed in any other way, we will show them like this:
734 @node basic tar options
735 @section Basic @command{tar} Operations and Options
737 @command{tar} can take a wide variety of arguments which specify and define
738 the actions it will have on the particular set of files or the archive.
739 The main types of arguments to @command{tar} fall into one of two classes:
740 operations, and options.
742 Some arguments fall into a class called @dfn{operations}; exactly one of
743 these is both allowed and required for any instance of using @command{tar};
744 you may @emph{not} specify more than one. People sometimes speak of
745 @dfn{operating modes}. You are in a particular operating mode when you
746 have specified the operation which specifies it; there are eight
747 operations in total, and thus there are eight operating modes.
749 The other arguments fall into the class known as @dfn{options}. You are
750 not required to specify any options, and you are allowed to specify more
751 than one at a time (depending on the way you are using @command{tar} at
752 that time). Some options are used so frequently, and are so useful for
753 helping you type commands more carefully that they are effectively
754 ``required''. We will discuss them in this chapter.
756 You can write most of the @command{tar} operations and options in any
757 of three forms: long (mnemonic) form, short form, and old style. Some
758 of the operations and options have no short or ``old'' forms; however,
759 the operations and options which we will cover in this tutorial have
760 corresponding abbreviations. We will indicate those abbreviations
761 appropriately to get you used to seeing them. Note, that the ``old
762 style'' option forms exist in @GNUTAR{} for compatibility with Unix
763 @command{tar}. In this book we present a full discussion of this way
764 of writing options and operations (@pxref{Old Options}), and we discuss
765 the other two styles of writing options (@xref{Long Options}, and
766 @pxref{Short Options}).
768 In the examples and in the text of this tutorial, we usually use the
769 long forms of operations and options; but the ``short'' forms produce
770 the same result and can make typing long @command{tar} commands easier.
771 For example, instead of typing
774 @kbd{tar --create --verbose --file=afiles.tar apple angst aspic}
780 @kbd{tar -c -v -f afiles.tar apple angst aspic}
786 @kbd{tar -cvf afiles.tar apple angst aspic}
790 For more information on option syntax, see @ref{Advanced tar}. In
791 discussions in the text, when we name an option by its long form, we
792 also give the corresponding short option in parentheses.
794 The term, ``option'', can be confusing at times, since ``operations''
795 are often lumped in with the actual, @emph{optional} ``options'' in certain
796 general class statements. For example, we just talked about ``short and
797 long forms of options and operations''. However, experienced @command{tar}
798 users often refer to these by shorthand terms such as, ``short and long
799 options''. This term assumes that the ``operations'' are included, also.
800 Context will help you determine which definition of ``options'' to use.
802 Similarly, the term ``command'' can be confusing, as it is often used in
803 two different ways. People sometimes refer to @command{tar} ``commands''.
804 A @command{tar} @dfn{command} is the entire command line of user input
805 which tells @command{tar} what to do --- including the operation, options,
806 and any arguments (file names, pipes, other commands, etc.). However,
807 you will also sometimes hear the term ``the @command{tar} command''. When
808 the word ``command'' is used specifically like this, a person is usually
809 referring to the @command{tar} @emph{operation}, not the whole line.
810 Again, use context to figure out which of the meanings the speaker
813 @node frequent operations
814 @section The Three Most Frequently Used Operations
816 Here are the three most frequently used operations (both short and long
817 forms), as well as a brief description of their meanings. The rest of
818 this chapter will cover how to use these operations in detail. We will
819 present the rest of the operations in the next chapter.
824 Create a new @command{tar} archive.
827 List the contents of an archive.
830 Extract one or more members from an archive.
833 @node Two Frequent Options
834 @section Two Frequently Used Options
836 To understand how to run @command{tar} in the three operating modes listed
837 previously, you also need to understand how to use two of the options to
838 @command{tar}: @option{--file} (which takes an archive file as an argument)
839 and @option{--verbose}. (You are usually not @emph{required} to specify
840 either of these options when you run @command{tar}, but they can be very
841 useful in making things more clear and helping you avoid errors.)
850 @unnumberedsubsec The @option{--file} Option
853 @xopindex{file, tutorial}
854 @item --file=@var{archive-name}
855 @itemx -f @var{archive-name}
856 Specify the name of an archive file.
859 You can specify an argument for the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option whenever you
860 use @command{tar}; this option determines the name of the archive file
861 that @command{tar} will work on.
864 If you don't specify this argument, then @command{tar} will examine
865 the environment variable @env{TAPE}. If it is set, its value will be
866 used as the archive name. Otherwise, @command{tar} will use the
867 default archive, determined at compile time. Usually it is
868 standard output or some physical tape drive attached to your machine
869 (you can verify what the default is by running @kbd{tar
870 --show-defaults}, @pxref{defaults}). If there is no tape drive
871 attached, or the default is not meaningful, then @command{tar} will
872 print an error message. The error message might look roughly like one
876 tar: can't open /dev/rmt8 : No such device or address
877 tar: can't open /dev/rsmt0 : I/O error
881 To avoid confusion, we recommend that you always specify an archive file
882 name by using @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) when writing your @command{tar} commands.
883 For more information on using the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option, see
886 @node verbose tutorial
887 @unnumberedsubsec The @option{--verbose} Option
890 @xopindex{verbose, introduced}
893 Show the files being worked on as @command{tar} is running.
896 @option{--verbose} (@option{-v}) shows details about the results of running
897 @command{tar}. This can be especially useful when the results might not be
898 obvious. For example, if you want to see the progress of @command{tar} as
899 it writes files into the archive, you can use the @option{--verbose}
900 option. In the beginning, you may find it useful to use
901 @option{--verbose} at all times; when you are more accustomed to
902 @command{tar}, you will likely want to use it at certain times but not at
903 others. We will use @option{--verbose} at times to help make something
904 clear, and we will give many examples both using and not using
905 @option{--verbose} to show the differences.
907 Each instance of @option{--verbose} on the command line increases the
908 verbosity level by one, so if you need more details on the output,
911 When reading archives (@option{--list}, @option{--extract},
912 @option{--diff}), @command{tar} by default prints only the names of
913 the members being extracted. Using @option{--verbose} will show a full,
914 @command{ls} style member listing.
916 In contrast, when writing archives (@option{--create}, @option{--append},
917 @option{--update}), @command{tar} does not print file names by
918 default. So, a single @option{--verbose} option shows the file names
919 being added to the archive, while two @option{--verbose} options
920 enable the full listing.
922 For example, to create an archive in verbose mode:
925 $ @kbd{tar -cvf afiles.tar apple angst aspic}
932 Creating the same archive with the verbosity level 2 could give:
935 $ @kbd{tar -cvvf afiles.tar apple angst aspic}
936 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
937 -rw-r--r-- gray/staff 11481 2006-06-09 12:06 angst
938 -rw-r--r-- gray/staff 23152 2006-06-09 12:06 aspic
942 This works equally well using short or long forms of options. Using
943 long forms, you would simply write out the mnemonic form of the option
947 $ @kbd{tar --create --verbose --verbose @dots{}}
951 Note that you must double the hyphens properly each time.
953 Later in the tutorial, we will give examples using @w{@option{--verbose
956 @anchor{verbose member listing}
957 The full output consists of six fields:
960 @item File type and permissions in symbolic form.
961 These are displayed in the same format as the first column of
962 @command{ls -l} output (@pxref{What information is listed,
963 format=verbose, Verbose listing, fileutils, GNU file utilities}).
965 @item Owner name and group separated by a slash character.
966 If these data are not available (for example, when listing a @samp{v7} format
967 archive), numeric @acronym{ID} values are printed instead.
969 @item Size of the file, in bytes.
971 @item File modification date in ISO 8601 format.
973 @item File modification time.
976 If the name contains any special characters (white space, newlines,
977 etc.) these are displayed in an unambiguous form using so called
978 @dfn{quoting style}. For the detailed discussion of available styles
979 and on how to use them, see @ref{quoting styles}.
981 Depending on the file type, the name can be followed by some
982 additional information, described in the following table:
985 @item -> @var{link-name}
986 The file or archive member is a @dfn{symbolic link} and
987 @var{link-name} is the name of file it links to.
989 @item link to @var{link-name}
990 The file or archive member is a @dfn{hard link} and @var{link-name} is
991 the name of file it links to.
994 The archive member is an old GNU format long link. You will normally
998 The archive member is an old GNU format long name. You will normally
1001 @item --Volume Header--
1002 The archive member is a GNU @dfn{volume header} (@pxref{Tape Files}).
1004 @item --Continued at byte @var{n}--
1005 Encountered only at the beginning of a multi-volume archive
1006 (@pxref{Using Multiple Tapes}). This archive member is a continuation
1007 from the previous volume. The number @var{n} gives the offset where
1008 the original file was split.
1010 @item unknown file type @var{c}
1011 An archive member of unknown type. @var{c} is the type character from
1012 the archive header. If you encounter such a message, it means that
1013 either your archive contains proprietary member types @GNUTAR{} is not
1014 able to handle, or the archive is corrupted.
1019 For example, here is an archive listing containing most of the special
1020 suffixes explained above:
1024 V--------- 0/0 1536 2006-06-09 13:07 MyVolume--Volume Header--
1025 -rw-r--r-- gray/staff 456783 2006-06-09 12:06 aspic--Continued at byte 32456--
1026 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
1027 lrwxrwxrwx gray/staff 0 2006-06-09 13:01 angst -> apple
1028 -rw-r--r-- gray/staff 35793 2006-06-09 12:06 blues
1029 hrw-r--r-- gray/staff 0 2006-06-09 12:06 music link to blues
1037 @unnumberedsubsec Getting Help: Using the @option{--help} Option
1043 The @option{--help} option to @command{tar} prints out a very brief list of
1044 all operations and option available for the current version of
1045 @command{tar} available on your system.
1049 @section How to Create Archives
1052 @cindex Creation of the archive
1053 @cindex Archive, creation of
1054 One of the basic operations of @command{tar} is @option{--create} (@option{-c}), which
1055 you use to create a @command{tar} archive. We will explain
1056 @option{--create} first because, in order to learn about the other
1057 operations, you will find it useful to have an archive available to
1060 To make this easier, in this section you will first create a directory
1061 containing three files. Then, we will show you how to create an
1062 @emph{archive} (inside the new directory). Both the directory, and
1063 the archive are specifically for you to practice on. The rest of this
1064 chapter and the next chapter will show many examples using this
1065 directory and the files you will create: some of those files may be
1066 other directories and other archives.
1068 The three files you will archive in this example are called
1069 @file{blues}, @file{folk}, and @file{jazz}. The archive is called
1070 @file{collection.tar}.
1072 This section will proceed slowly, detailing how to use @option{--create}
1073 in @code{verbose} mode, and showing examples using both short and long
1074 forms. In the rest of the tutorial, and in the examples in the next
1075 chapter, we will proceed at a slightly quicker pace. This section
1076 moves more slowly to allow beginning users to understand how
1077 @command{tar} works.
1080 * prepare for examples::
1081 * Creating the archive::
1087 @node prepare for examples
1088 @subsection Preparing a Practice Directory for Examples
1090 To follow along with this and future examples, create a new directory
1091 called @file{practice} containing files called @file{blues}, @file{folk}
1092 and @file{jazz}. The files can contain any information you like:
1093 ideally, they should contain information which relates to their names,
1094 and be of different lengths. Our examples assume that @file{practice}
1095 is a subdirectory of your home directory.
1097 Now @command{cd} to the directory named @file{practice}; @file{practice}
1098 is now your @dfn{working directory}. (@emph{Please note}: Although
1099 the full file name of this directory is
1100 @file{/@var{homedir}/practice}, in our examples we will refer to
1101 this directory as @file{practice}; the @var{homedir} is presumed.)
1103 In general, you should check that the files to be archived exist where
1104 you think they do (in the working directory) by running @command{ls}.
1105 Because you just created the directory and the files and have changed to
1106 that directory, you probably don't need to do that this time.
1108 It is very important to make sure there isn't already a file in the
1109 working directory with the archive name you intend to use (in this case,
1110 @samp{collection.tar}), or that you don't care about its contents.
1111 Whenever you use @samp{create}, @command{tar} will erase the current
1112 contents of the file named by @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) if it exists. @command{tar}
1113 will not tell you if you are about to overwrite an archive unless you
1114 specify an option which does this (@pxref{backup}, for the
1115 information on how to do so). To add files to an existing archive,
1116 you need to use a different option, such as @option{--append} (@option{-r}); see
1117 @ref{append} for information on how to do this.
1119 @node Creating the archive
1120 @subsection Creating the Archive
1122 @xopindex{create, introduced}
1123 To place the files @file{blues}, @file{folk}, and @file{jazz} into an
1124 archive named @file{collection.tar}, use the following command:
1127 $ @kbd{tar --create --file=collection.tar blues folk jazz}
1130 The order of the arguments is not very important, @emph{when using long
1131 option forms}. You could also say:
1134 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1138 However, you can see that this order is harder to understand; this is
1139 why we will list the arguments in the order that makes the commands
1140 easiest to understand (and we encourage you to do the same when you use
1141 @command{tar}, to avoid errors).
1143 Note that the sequence
1144 @option{--file=@-collection.tar} is considered to be @emph{one} argument.
1145 If you substituted any other string of characters for
1146 @kbd{collection.tar}, then that string would become the name of the
1147 archive file you create.
1149 The order of the options becomes more important when you begin to use
1150 short forms. With short forms, if you type commands in the wrong order
1151 (even if you type them correctly in all other ways), you may end up with
1152 results you don't expect. For this reason, it is a good idea to get
1153 into the habit of typing options in the order that makes inherent sense.
1154 @xref{short create}, for more information on this.
1156 In this example, you type the command as shown above: @option{--create}
1157 is the operation which creates the new archive
1158 (@file{collection.tar}), and @option{--file} is the option which lets
1159 you give it the name you chose. The files, @file{blues}, @file{folk},
1160 and @file{jazz}, are now members of the archive, @file{collection.tar}
1161 (they are @dfn{file name arguments} to the @option{--create} operation.
1162 @xref{Choosing}, for the detailed discussion on these.) Now that they are
1163 in the archive, they are called @emph{archive members}, not files.
1164 (@pxref{Definitions,members}).
1166 When you create an archive, you @emph{must} specify which files you
1167 want placed in the archive. If you do not specify any archive
1168 members, @GNUTAR{} will complain.
1170 If you now list the contents of the working directory (@command{ls}), you will
1171 find the archive file listed as well as the files you saw previously:
1174 blues folk jazz collection.tar
1178 Creating the archive @samp{collection.tar} did not destroy the copies of
1179 the files in the directory.
1181 Keep in mind that if you don't indicate an operation, @command{tar} will not
1182 run and will prompt you for one. If you don't name any files, @command{tar}
1183 will complain. You must have write access to the working directory,
1184 or else you will not be able to create an archive in that directory.
1186 @emph{Caution}: Do not attempt to use @option{--create} (@option{-c}) to add files to
1187 an existing archive; it will delete the archive and write a new one.
1188 Use @option{--append} (@option{-r}) instead. @xref{append}.
1190 @node create verbose
1191 @subsection Running @option{--create} with @option{--verbose}
1193 @xopindex{create, using with @option{--verbose}}
1194 @xopindex{verbose, using with @option{--create}}
1195 If you include the @option{--verbose} (@option{-v}) option on the command line,
1196 @command{tar} will list the files it is acting on as it is working. In
1197 verbose mode, the @code{create} example above would appear as:
1200 $ @kbd{tar --create --verbose --file=collection.tar blues folk jazz}
1206 This example is just like the example we showed which did not use
1207 @option{--verbose}, except that @command{tar} generated the remaining
1209 lines (note the different font styles).
1215 In the rest of the examples in this chapter, we will frequently use
1216 @code{verbose} mode so we can show actions or @command{tar} responses that
1217 you would otherwise not see, and which are important for you to
1221 @subsection Short Forms with @samp{create}
1223 As we said before, the @option{--create} (@option{-c}) operation is one of the most
1224 basic uses of @command{tar}, and you will use it countless times.
1225 Eventually, you will probably want to use abbreviated (or ``short'')
1226 forms of options. A full discussion of the three different forms that
1227 options can take appears in @ref{Styles}; for now, here is what the
1228 previous example (including the @option{--verbose} (@option{-v}) option) looks like
1229 using short option forms:
1232 $ @kbd{tar -cvf collection.tar blues folk jazz}
1239 As you can see, the system responds the same no matter whether you use
1240 long or short option forms.
1242 @FIXME{i don't like how this is worded:} One difference between using
1243 short and long option forms is that, although the exact placement of
1244 arguments following options is no more specific when using short forms,
1245 it is easier to become confused and make a mistake when using short
1246 forms. For example, suppose you attempted the above example in the
1250 $ @kbd{tar -cfv collection.tar blues folk jazz}
1254 In this case, @command{tar} will make an archive file called @file{v},
1255 containing the files @file{blues}, @file{folk}, and @file{jazz}, because
1256 the @samp{v} is the closest ``file name'' to the @option{-f} option, and
1257 is thus taken to be the chosen archive file name. @command{tar} will try
1258 to add a file called @file{collection.tar} to the @file{v} archive file;
1259 if the file @file{collection.tar} did not already exist, @command{tar} will
1260 report an error indicating that this file does not exist. If the file
1261 @file{collection.tar} does already exist (e.g., from a previous command
1262 you may have run), then @command{tar} will add this file to the archive.
1263 Because the @option{-v} option did not get registered, @command{tar} will not
1264 run under @samp{verbose} mode, and will not report its progress.
1266 The end result is that you may be quite confused about what happened,
1267 and possibly overwrite a file. To illustrate this further, we will show
1268 you how an example we showed previously would look using short forms.
1273 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1277 is confusing as it is. When shown using short forms, however, it
1278 becomes much more so:
1281 $ @kbd{tar blues -c folk -f collection.tar jazz}
1285 It would be very easy to put the wrong string of characters
1286 immediately following the @option{-f}, but doing that could sacrifice
1289 For this reason, we recommend that you pay very careful attention to
1290 the order of options and placement of file and archive names,
1291 especially when using short option forms. Not having the option name
1292 written out mnemonically can affect how well you remember which option
1293 does what, and therefore where different names have to be placed.
1296 @subsection Archiving Directories
1298 @cindex Archiving Directories
1299 @cindex Directories, Archiving
1300 You can archive a directory by specifying its directory name as a
1301 file name argument to @command{tar}. The files in the directory will be
1302 archived relative to the working directory, and the directory will be
1303 re-created along with its contents when the archive is extracted.
1305 To archive a directory, first move to its superior directory. If you
1306 have followed the previous instructions in this tutorial, you should
1315 This will put you into the directory which contains @file{practice},
1316 i.e., your home directory. Once in the superior directory, you can
1317 specify the subdirectory, @file{practice}, as a file name argument. To
1318 store @file{practice} in the new archive file @file{music.tar}, type:
1321 $ @kbd{tar --create --verbose --file=music.tar practice}
1325 @command{tar} should output:
1332 practice/collection.tar
1335 Note that the archive thus created is not in the subdirectory
1336 @file{practice}, but rather in the current working directory---the
1337 directory from which @command{tar} was invoked. Before trying to archive a
1338 directory from its superior directory, you should make sure you have
1339 write access to the superior directory itself, not only the directory
1340 you are trying archive with @command{tar}. For example, you will probably
1341 not be able to store your home directory in an archive by invoking
1342 @command{tar} from the root directory; @xref{absolute}. (Note
1343 also that @file{collection.tar}, the original archive file, has itself
1344 been archived. @command{tar} will accept any file as a file to be
1345 archived, regardless of its content. When @file{music.tar} is
1346 extracted, the archive file @file{collection.tar} will be re-written
1347 into the file system).
1349 If you give @command{tar} a command such as
1352 $ @kbd{tar --create --file=foo.tar .}
1356 @command{tar} will report @samp{tar: ./foo.tar is the archive; not
1357 dumped}. This happens because @command{tar} creates the archive
1358 @file{foo.tar} in the current directory before putting any files into
1359 it. Then, when @command{tar} attempts to add all the files in the
1360 directory @file{.} to the archive, it notices that the file
1361 @file{./foo.tar} is the same as the archive @file{foo.tar}, and skips
1362 it. (It makes no sense to put an archive into itself.) @GNUTAR{}
1363 will continue in this case, and create the archive
1364 normally, except for the exclusion of that one file. (@emph{Please
1365 note:} Other implementations of @command{tar} may not be so clever;
1366 they will enter an infinite loop when this happens, so you should not
1367 depend on this behavior unless you are certain you are running
1368 @GNUTAR{}. In general, it is wise to always place the archive outside
1369 of the directory being dumped.)
1372 @section How to List Archives
1375 Frequently, you will find yourself wanting to determine exactly what a
1376 particular archive contains. You can use the @option{--list}
1377 (@option{-t}) operation to get the member names as they currently
1378 appear in the archive, as well as various attributes of the files at
1379 the time they were archived. For example, you can examine the archive
1380 @file{collection.tar} that you created in the last section with the
1384 $ @kbd{tar --list --file=collection.tar}
1388 The output of @command{tar} would then be:
1397 The archive @file{bfiles.tar} would list as follows:
1406 Be sure to use a @option{--file=@var{archive-name}} (@option{-f
1407 @var{archive-name}}) option just as with @option{--create}
1408 (@option{-c}) to specify the name of the archive.
1410 @xopindex{list, using with @option{--verbose}}
1411 @xopindex{verbose, using with @option{--list}}
1412 If you use the @option{--verbose} (@option{-v}) option with
1413 @option{--list}, then @command{tar} will print out a listing
1414 reminiscent of @w{@samp{ls -l}}, showing owner, file size, and so
1415 forth. This output is described in detail in @ref{verbose member listing}.
1417 If you had used @option{--verbose} (@option{-v}) mode, the example
1418 above would look like:
1421 $ @kbd{tar --list --verbose --file=collection.tar folk}
1422 -rw-r--r-- myself/user 62 1990-05-23 10:55 folk
1425 @cindex listing member and file names
1426 @anchor{listing member and file names}
1427 It is important to notice that the output of @kbd{tar --list
1428 --verbose} does not necessarily match that produced by @kbd{tar
1429 --create --verbose} while creating the archive. It is because
1430 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1431 prefixes from file names before storing them in the archive
1432 (@xref{absolute}, for more information). In other
1433 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1434 an archive and @dfn{member names} when listing it. Consider this
1439 $ @kbd{tar --create --verbose --file archive /etc/mail}
1440 tar: Removing leading '/' from member names
1442 /etc/mail/sendmail.cf
1444 $ @kbd{tar --test --file archive}
1446 etc/mail/sendmail.cf
1451 @opindex show-stored-names
1452 This default behavior can sometimes be inconvenient. You can force
1453 @GNUTAR{} show member names when creating archive by supplying
1454 @option{--show-stored-names} option.
1457 @item --show-stored-names
1458 Print member (as opposed to @emph{file}) names when creating the archive.
1461 @cindex File name arguments, using @option{--list} with
1462 @xopindex{list, using with file name arguments}
1463 You can specify one or more individual member names as arguments when
1464 using @samp{list}. In this case, @command{tar} will only list the
1465 names of members you identify. For example, @w{@kbd{tar --list
1466 --file=afiles.tar apple}} would only print @file{apple}.
1468 Because @command{tar} preserves file names, these must be specified as
1469 they appear in the archive (i.e., relative to the directory from which
1470 the archive was created). Therefore, it is essential when specifying
1471 member names to @command{tar} that you give the exact member names.
1472 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1473 error message something like @samp{tar: birds: Not found in archive},
1474 because there is no member named @file{birds}, only one named
1475 @file{./birds}. While the names @file{birds} and @file{./birds} name
1476 the same file, @emph{member} names by default are compared verbatim.
1478 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1479 with @file{baboon}, because this exact member name is in the archive file
1480 @file{bfiles.tar}. If you are not sure of the exact file name,
1481 use @dfn{globbing patterns}, for example:
1484 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1488 will list all members whose name contains @samp{b}. @xref{wildcards},
1489 for a detailed discussion of globbing patterns and related
1490 @command{tar} command line options.
1497 @unnumberedsubsec Listing the Contents of a Stored Directory
1499 To get information about the contents of an archived directory,
1500 use the directory name as a file name argument in conjunction with
1501 @option{--list} (@option{-t}). To find out file attributes, include the
1502 @option{--verbose} (@option{-v}) option.
1504 For example, to find out about files in the directory @file{practice}, in
1505 the archive file @file{music.tar}, type:
1508 $ @kbd{tar --list --verbose --file=music.tar practice}
1511 @command{tar} responds:
1514 drwxrwxrwx myself/user 0 1990-05-31 21:49 practice/
1515 -rw-r--r-- myself/user 42 1990-05-21 13:29 practice/blues
1516 -rw-r--r-- myself/user 62 1990-05-23 10:55 practice/folk
1517 -rw-r--r-- myself/user 40 1990-05-21 13:30 practice/jazz
1518 -rw-r--r-- myself/user 10240 1990-05-31 21:49 practice/collection.tar
1521 When you use a directory name as a file name argument, @command{tar} acts on
1522 all the files (including sub-directories) in that directory.
1525 @section How to Extract Members from an Archive
1527 @cindex Retrieving files from an archive
1528 @cindex Resurrecting files from an archive
1531 Creating an archive is only half the job---there is no point in storing
1532 files in an archive if you can't retrieve them. The act of retrieving
1533 members from an archive so they can be used and manipulated as
1534 unarchived files again is called @dfn{extraction}. To extract files
1535 from an archive, use the @option{--extract} (@option{--get} or
1536 @option{-x}) operation. As with @option{--create}, specify the name
1537 of the archive with @option{--file} (@option{-f}) option. Extracting
1538 an archive does not modify the archive in any way; you can extract it
1539 multiple times if you want or need to.
1541 Using @option{--extract}, you can extract an entire archive, or specific
1542 files. The files can be directories containing other files, or not. As
1543 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1544 long form of the operation without affecting the performance.
1547 * extracting archives::
1548 * extracting files::
1550 * extracting untrusted archives::
1551 * failing commands::
1554 @node extracting archives
1555 @subsection Extracting an Entire Archive
1557 To extract an entire archive, specify the archive file name only, with
1558 no individual file names as arguments. For example,
1561 $ @kbd{tar -xvf collection.tar}
1568 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
1569 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
1570 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
1573 @node extracting files
1574 @subsection Extracting Specific Files
1576 To extract specific archive members, give their exact member names as
1577 arguments, as printed by @option{--list} (@option{-t}). If you had
1578 mistakenly deleted one of the files you had placed in the archive
1579 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1580 from the archive without changing the archive's structure. Its
1581 contents will be identical to the original file @file{blues} that you
1584 First, make sure you are in the @file{practice} directory, and list the
1585 files in the directory. Now, delete the file, @samp{blues}, and list
1586 the files in the directory again.
1588 You can now extract the member @file{blues} from the archive file
1589 @file{collection.tar} like this:
1592 $ @kbd{tar --extract --file=collection.tar blues}
1596 If you list the files in the directory again, you will see that the file
1597 @file{blues} has been restored, with its original permissions, data
1598 modification times, and owner.@footnote{This is only accidentally
1599 true, but not in general. Whereas modification times are always
1600 restored, in most cases, one has to be root for restoring the owner,
1601 and use a special option for restoring permissions. Here, it just
1602 happens that the restoring user is also the owner of the archived
1603 members, and that the current @code{umask} is compatible with original
1604 permissions.} (These parameters will be identical to those which
1605 the file had when you originally placed it in the archive; any changes
1606 you may have made before deleting the file from the file system,
1607 however, will @emph{not} have been made to the archive member.) The
1608 archive file, @samp{collection.tar}, is the same as it was before you
1609 extracted @samp{blues}. You can confirm this by running @command{tar} with
1610 @option{--list} (@option{-t}).
1612 Remember that as with other operations, specifying the exact member
1613 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1614 will fail, because there is no member named @file{birds}. To extract
1615 the member named @file{./birds}, you must specify @w{@kbd{tar
1616 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1617 exact member names, use @option{--list} (@option{-t}) option
1618 (@pxref{list}). You can also extract those members that match a
1619 specific @dfn{globbing pattern}. For example, to extract from
1620 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1621 directory prefix, you could type:
1624 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1628 Here, @option{--wildcards} instructs @command{tar} to treat
1629 command line arguments as globbing patterns and @option{--no-anchored}
1630 informs it that the patterns apply to member names after any @samp{/}
1631 delimiter. The use of globbing patterns is discussed in detail in
1634 You can extract a file to standard output by combining the above options
1635 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1638 If you give the @option{--verbose} option, then @option{--extract}
1639 will print the names of the archive members as it extracts them.
1642 @subsection Extracting Files that are Directories
1644 Extracting directories which are members of an archive is similar to
1645 extracting other files. The main difference to be aware of is that if
1646 the extracted directory has the same name as any directory already in
1647 the working directory, then files in the extracted directory will be
1648 placed into the directory of the same name. Likewise, if there are
1649 files in the pre-existing directory with the same names as the members
1650 which you extract, the files from the extracted archive will replace
1651 the files already in the working directory (and possible
1652 subdirectories). This will happen regardless of whether or not the
1653 files in the working directory were more recent than those extracted
1654 (there exist, however, special options that alter this behavior
1657 However, if a file was stored with a directory name as part of its file
1658 name, and that directory does not exist under the working directory when
1659 the file is extracted, @command{tar} will create the directory.
1661 We can demonstrate how to use @option{--extract} to extract a directory
1662 file with an example. Change to the @file{practice} directory if you
1663 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1664 go back to the parent directory and extract the archive
1665 @file{music.tar}. You may either extract the entire archive, or you may
1666 extract only the files you just deleted. To extract the entire archive,
1667 don't give any file names as arguments after the archive name
1668 @file{music.tar}. To extract only the files you deleted, use the
1672 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1678 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1679 would have displayed more detail about the extracted files, as shown
1680 in the example below:
1683 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1684 -rw-r--r-- me/user 28 1996-10-18 16:31 practice/jazz
1685 -rw-r--r-- me/user 20 1996-09-23 16:44 practice/folk
1689 Because you created the directory with @file{practice} as part of the
1690 file names of each of the files by archiving the @file{practice}
1691 directory as @file{practice}, you must give @file{practice} as part
1692 of the file names when you extract those files from the archive.
1694 @node extracting untrusted archives
1695 @subsection Extracting Archives from Untrusted Sources
1697 Extracting files from archives can overwrite files that already exist.
1698 If you receive an archive from an untrusted source, you should make a
1699 new directory and extract into that directory, so that you don't have
1700 to worry about the extraction overwriting one of your existing files.
1701 For example, if @file{untrusted.tar} came from somewhere else on the
1702 Internet, and you don't necessarily trust its contents, you can
1703 extract it as follows:
1706 $ @kbd{mkdir newdir}
1708 $ @kbd{tar -xvf ../untrusted.tar}
1711 It is also a good practice to examine contents of the archive
1712 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1713 with @option{--verbose} (@option{-v}).
1715 @node failing commands
1716 @subsection Commands That Will Fail
1718 Here are some sample commands you might try which will not work, and why
1721 If you try to use this command,
1724 $ @kbd{tar -xvf music.tar folk jazz}
1728 you will get the following response:
1731 tar: folk: Not found in archive
1732 tar: jazz: Not found in archive
1736 This is because these files were not originally @emph{in} the parent
1737 directory @file{..}, where the archive is located; they were in the
1738 @file{practice} directory, and their file names reflect this:
1741 $ @kbd{tar -tvf music.tar}
1747 @FIXME{make sure the above works when going through the examples in
1751 Likewise, if you try to use this command,
1754 $ @kbd{tar -tvf music.tar folk jazz}
1758 you would get a similar response. Members with those names are not in the
1759 archive. You must use the correct member names, or wildcards, in order
1760 to extract the files from the archive.
1762 If you have forgotten the correct names of the files in the archive,
1763 use @w{@kbd{tar --list --verbose}} to list them correctly.
1765 @FIXME{more examples, here? hag thinks it's a good idea.}
1768 @section Going Further Ahead in this Manual
1771 @FIXME{need to write up a node here about the things that are going to
1772 be in the rest of the manual.}
1774 @node tar invocation
1775 @chapter Invoking @GNUTAR{}
1777 This chapter is about how one invokes the @GNUTAR{}
1778 command, from the command synopsis (@pxref{Synopsis}). There are
1779 numerous options, and many styles for writing them. One mandatory
1780 option specifies the operation @command{tar} should perform
1781 (@pxref{Operation Summary}), other options are meant to detail how
1782 this operation should be performed (@pxref{Option Summary}).
1783 Non-option arguments are not always interpreted the same way,
1784 depending on what the operation is.
1786 You will find in this chapter everything about option styles and rules for
1787 writing them (@pxref{Styles}). On the other hand, operations and options
1788 are fully described elsewhere, in other chapters. Here, you will find
1789 only synthetic descriptions for operations and options, together with
1790 pointers to other parts of the @command{tar} manual.
1792 Some options are so special they are fully described right in this
1793 chapter. They have the effect of inhibiting the normal operation of
1794 @command{tar} or else, they globally alter the amount of feedback the user
1795 receives about what is going on. These are the @option{--help} and
1796 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1797 and @option{--interactive} options (@pxref{interactive}).
1801 * using tar options::
1803 * All Options:: All @command{tar} Options.
1804 * help:: Where to Get Help.
1805 * defaults:: What are the Default Values.
1806 * verbose:: Checking @command{tar} progress.
1807 * checkpoints:: Checkpoints.
1808 * warnings:: Controlling Warning Messages.
1809 * interactive:: Asking for Confirmation During Operations.
1810 * external:: Running External Commands.
1814 @section General Synopsis of @command{tar}
1816 The @GNUTAR{} program is invoked as either one of:
1819 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1820 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1823 The second form is for when old options are being used.
1825 You can use @command{tar} to store files in an archive, to extract them from
1826 an archive, and to do other types of archive manipulation. The primary
1827 argument to @command{tar}, which is called the @dfn{operation}, specifies
1828 which action to take. The other arguments to @command{tar} are either
1829 @dfn{options}, which change the way @command{tar} performs an operation,
1830 or file names or archive members, which specify the files or members
1831 @command{tar} is to act on.
1833 You can actually type in arguments in any order, even if in this manual
1834 the options always precede the other arguments, to make examples easier
1835 to understand. Further, the option stating the main operation mode
1836 (the @command{tar} main command) is usually given first.
1838 Each @var{name} in the synopsis above is interpreted as an archive member
1839 name when the main command is one of @option{--compare}
1840 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1841 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1842 @option{--update} (@option{-u}). When naming archive members, you
1843 must give the exact name of the member in the archive, as it is
1844 printed by @option{--list}. For @option{--append} (@option{-r}) and
1845 @option{--create} (@option{-c}), these @var{name} arguments specify
1846 the names of either files or directory hierarchies to place in the archive.
1847 These files or hierarchies should already exist in the file system,
1848 prior to the execution of the @command{tar} command.
1850 @command{tar} interprets relative file names as being relative to the
1851 working directory. @command{tar} will make all file names relative
1852 (by removing leading slashes when archiving or restoring files),
1853 unless you specify otherwise (using the @option{--absolute-names}
1854 option). @xref{absolute}, for more information about
1855 @option{--absolute-names}.
1857 If you give the name of a directory as either a file name or a member
1858 name, then @command{tar} acts recursively on all the files and directories
1859 beneath that directory. For example, the name @file{/} identifies all
1860 the files in the file system to @command{tar}.
1862 The distinction between file names and archive member names is especially
1863 important when shell globbing is used, and sometimes a source of confusion
1864 for newcomers. @xref{wildcards}, for more information about globbing.
1865 The problem is that shells may only glob using existing files in the
1866 file system. Only @command{tar} itself may glob on archive members, so when
1867 needed, you must ensure that wildcard characters reach @command{tar} without
1868 being interpreted by the shell first. Using a backslash before @samp{*}
1869 or @samp{?}, or putting the whole argument between quotes, is usually
1870 sufficient for this.
1872 Even if @var{name}s are often specified on the command line, they
1873 can also be read from a text file in the file system, using the
1874 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1876 If you don't use any file name arguments, @option{--append} (@option{-r}),
1877 @option{--delete} and @option{--concatenate} (@option{--catenate},
1878 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1879 will usually yield a diagnostic and inhibit @command{tar} execution.
1880 The other operations of @command{tar} (@option{--list},
1881 @option{--extract}, @option{--compare}, and @option{--update})
1882 will act on the entire contents of the archive.
1884 @anchor{exit status}
1886 @cindex return status
1887 Besides successful exits, @GNUTAR{} may fail for
1888 many reasons. Some reasons correspond to bad usage, that is, when the
1889 @command{tar} command line is improperly written. Errors may be
1890 encountered later, while processing the archive or the files. Some
1891 errors are recoverable, in which case the failure is delayed until
1892 @command{tar} has completed all its work. Some errors are such that
1893 it would be not meaningful, or at least risky, to continue processing:
1894 @command{tar} then aborts processing immediately. All abnormal exits,
1895 whether immediate or delayed, should always be clearly diagnosed on
1896 @code{stderr}, after a line stating the nature of the error.
1898 Possible exit codes of @GNUTAR{} are summarized in the following
1903 @samp{Successful termination}.
1906 @samp{Some files differ}. If tar was invoked with @option{--compare}
1907 (@option{--diff}, @option{-d}) command line option, this means that
1908 some files in the archive differ from their disk counterparts
1909 (@pxref{compare}). If tar was given @option{--create},
1910 @option{--append} or @option{--update} option, this exit code means
1911 that some files were changed while being archived and so the resulting
1912 archive does not contain the exact copy of the file set.
1915 @samp{Fatal error}. This means that some fatal, unrecoverable error
1919 If @command{tar} has invoked a subprocess and that subprocess exited with a
1920 nonzero exit code, @command{tar} exits with that code as well.
1921 This can happen, for example, if @command{tar} was given some
1922 compression option (@pxref{gzip}) and the external compressor program
1923 failed. Another example is @command{rmt} failure during backup to the
1924 remote device (@pxref{Remote Tape Server}).
1926 @node using tar options
1927 @section Using @command{tar} Options
1929 @GNUTAR{} has a total of eight operating modes which
1930 allow you to perform a variety of tasks. You are required to choose
1931 one operating mode each time you employ the @command{tar} program by
1932 specifying one, and only one operation as an argument to the
1933 @command{tar} command (the corresponding options may be found
1934 at @ref{frequent operations} and @ref{Operations}). Depending on
1935 circumstances, you may also wish to customize how the chosen operating
1936 mode behaves. For example, you may wish to change the way the output
1937 looks, or the format of the files that you wish to archive may require
1938 you to do something special in order to make the archive look right.
1940 You can customize and control @command{tar}'s performance by running
1941 @command{tar} with one or more options (such as @option{--verbose}
1942 (@option{-v}), which we used in the tutorial). As we said in the
1943 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1944 their name suggests) optional. Depending on the operating mode, you
1945 may specify one or more options. Different options will have different
1946 effects, but in general they all change details of the operation, such
1947 as archive format, archive name, or level of user interaction. Some
1948 options make sense with all operating modes, while others are
1949 meaningful only with particular modes. You will likely use some
1950 options frequently, while you will only use others infrequently, or
1951 not at all. (A full list of options is available in @pxref{All Options}.)
1953 @vrindex TAR_OPTIONS, environment variable
1954 @anchor{TAR_OPTIONS}
1955 The @env{TAR_OPTIONS} environment variable specifies default options to
1956 be placed in front of any explicit options. For example, if
1957 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1958 if the two options @option{-v} and @option{--unlink-first} had been
1959 specified before any explicit options. Option specifications are
1960 separated by whitespace. A backslash escapes the next character, so it
1961 can be used to specify an option containing whitespace or a backslash.
1963 Note that @command{tar} options are case sensitive. For example, the
1964 options @option{-T} and @option{-t} are different; the first requires an
1965 argument for stating the name of a file providing a list of @var{name}s,
1966 while the second does not require an argument and is another way to
1967 write @option{--list} (@option{-t}).
1969 In addition to the eight operations, there are many options to
1970 @command{tar}, and three different styles for writing both: long (mnemonic)
1971 form, short form, and old style. These styles are discussed below.
1972 Both the options and the operations can be written in any of these three
1975 @FIXME{menu at end of this node. need to think of an actual outline
1976 for this chapter; probably do that after stuff from chapter 4 is
1980 @section The Three Option Styles
1982 There are three styles for writing operations and options to the command
1983 line invoking @command{tar}. The different styles were developed at
1984 different times during the history of @command{tar}. These styles will be
1985 presented below, from the most recent to the oldest.
1987 Some options must take an argument@footnote{For example, @option{--file}
1988 (@option{-f}) takes the name of an archive file as an argument. If
1989 you do not supply an archive file name, @command{tar} will use a
1990 default, but this can be confusing; thus, we recommend that you always
1991 supply a specific archive file name.}. Where you @emph{place} the
1992 arguments generally depends on which style of options you choose. We
1993 will detail specific information relevant to each option style in the
1994 sections on the different option styles, below. The differences are
1995 subtle, yet can often be very important; incorrect option placement
1996 can cause you to overwrite a number of important files. We urge you
1997 to note these differences, and only use the option style(s) which
1998 makes the most sense to you until you feel comfortable with the others.
2000 Some options @emph{may} take an argument. Such options may have at
2001 most long and short forms, they do not have old style equivalent. The
2002 rules for specifying an argument for such options are stricter than
2003 those for specifying mandatory arguments. Please, pay special
2007 * Long Options:: Long Option Style
2008 * Short Options:: Short Option Style
2009 * Old Options:: Old Option Style
2010 * Mixing:: Mixing Option Styles
2014 @subsection Long Option Style
2016 @cindex long options
2017 @cindex options, long style
2018 @cindex options, GNU style
2019 @cindex options, mnemonic names
2020 Each option has at least one @dfn{long} (or @dfn{mnemonic}) name starting with two
2021 dashes in a row, e.g., @option{--list}. The long names are more clear than
2022 their corresponding short or old names. It sometimes happens that a
2023 single long option has many different names which are
2024 synonymous, such as @option{--compare} and @option{--diff}. In addition,
2025 long option names can be given unique abbreviations. For example,
2026 @option{--cre} can be used in place of @option{--create} because there is no
2027 other long option which begins with @samp{cre}. (One way to find
2028 this out is by trying it and seeing what happens; if a particular
2029 abbreviation could represent more than one option, @command{tar} will tell
2030 you that that abbreviation is ambiguous and you'll know that that
2031 abbreviation won't work. You may also choose to run @samp{tar --help}
2032 to see a list of options. Be aware that if you run @command{tar} with a
2033 unique abbreviation for the long name of an option you didn't want to
2034 use, you are stuck; @command{tar} will perform the command as ordered.)
2036 Long options are meant to be obvious and easy to remember, and their
2037 meanings are generally easier to discern than those of their
2038 corresponding short options (see below). For example:
2041 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
2045 gives a fairly good set of hints about what the command does, even
2046 for those not fully acquainted with @command{tar}.
2048 @cindex arguments to long options
2049 @cindex long options with mandatory arguments
2050 Long options which require arguments take those arguments
2051 immediately following the option name. There are two ways of
2052 specifying a mandatory argument. It can be separated from the
2053 option name either by an equal sign, or by any amount of
2054 white space characters. For example, the @option{--file} option (which
2055 tells the name of the @command{tar} archive) is given a file such as
2056 @file{archive.tar} as argument by using any of the following notations:
2057 @option{--file=archive.tar} or @option{--file archive.tar}.
2059 @cindex optional arguments to long options
2060 @cindex long options with optional arguments
2061 In contrast, optional arguments must always be introduced using
2062 an equal sign. For example, the @option{--backup} option takes
2063 an optional argument specifying backup type. It must be used
2064 as @option{--backup=@var{backup-type}}.
2067 @subsection Short Option Style
2069 @cindex short options
2070 @cindex options, short style
2071 @cindex options, traditional
2072 Most options also have a @dfn{short option} name. Short options start with
2073 a single dash, and are followed by a single character, e.g., @option{-t}
2074 (which is equivalent to @option{--list}). The forms are absolutely
2075 identical in function; they are interchangeable.
2077 The short option names are faster to type than long option names.
2079 @cindex arguments to short options
2080 @cindex short options with mandatory arguments
2081 Short options which require arguments take their arguments immediately
2082 following the option, usually separated by white space. It is also
2083 possible to stick the argument right after the short option name, using
2084 no intervening space. For example, you might write @w{@option{-f
2085 archive.tar}} or @option{-farchive.tar} instead of using
2086 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
2087 @w{@option{-f @var{archive-name}}} denote the option which indicates a
2088 specific archive, here named @file{archive.tar}.
2090 @cindex optional arguments to short options
2091 @cindex short options with optional arguments
2092 Short options which take optional arguments take their arguments
2093 immediately following the option letter, @emph{without any intervening
2094 white space characters}.
2096 Short options' letters may be clumped together, but you are not
2097 required to do this (as compared to old options; see below). When
2098 short options are clumped as a set, use one (single) dash for them
2099 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
2100 such a set is allowed to have an argument@footnote{Clustering many
2101 options, the last of which has an argument, is a rather opaque way to
2102 write options. Some wonder if @acronym{GNU} @code{getopt} should not
2103 even be made helpful enough for considering such usages as invalid.}.
2105 When the options are separated, the argument for each option which requires
2106 an argument directly follows that option, as is usual for Unix programs.
2110 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
2113 If you reorder short options' locations, be sure to move any arguments
2114 that belong to them. If you do not move the arguments properly, you may
2115 end up overwriting files.
2118 @subsection Old Option Style
2119 @cindex options, old style
2120 @cindex old option style
2121 @cindex option syntax, traditional
2123 As far as we know, all @command{tar} programs, @acronym{GNU} and
2124 non-@acronym{GNU}, support @dfn{old options}: that is, if the first
2125 argument does not start with @samp{-}, it is assumed to specify option
2126 letters. @GNUTAR{} supports old options not only for historical
2127 reasons, but also because many people are used to them. If the first
2128 argument does not start with a dash, you are announcing the old option
2129 style instead of the short option style; old options are decoded
2132 Like short options, old options are single letters. However, old options
2133 must be written together as a single clumped set, without spaces separating
2134 them or dashes preceding them. This set
2135 of letters must be the first to appear on the command line, after the
2136 @command{tar} program name and some white space; old options cannot appear
2137 anywhere else. The letter of an old option is exactly the same letter as
2138 the corresponding short option. For example, the old option @samp{t} is
2139 the same as the short option @option{-t}, and consequently, the same as the
2140 long option @option{--list}. So for example, the command @w{@samp{tar
2141 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
2143 @cindex arguments to old options
2144 @cindex old options with mandatory arguments
2145 When options that need arguments are given together with the command,
2146 all the associated arguments follow, in the same order as the options.
2147 Thus, the example given previously could also be written in the old
2151 $ @kbd{tar cvbf 20 /dev/rmt0}
2155 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
2156 the argument of @option{-f}.
2158 The old style syntax can make it difficult to match
2159 option letters with their corresponding arguments, and is often
2160 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
2161 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
2162 argument for @option{-f}, and @option{-v} does not have a corresponding
2163 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
2164 /dev/rmt0}} is clearer, putting all arguments next to the option they
2167 If you want to reorder the letters in the old option argument, be
2168 sure to reorder any corresponding argument appropriately.
2170 This old way of writing @command{tar} options can surprise even experienced
2171 users. For example, the two commands:
2174 @kbd{tar cfz archive.tar.gz file}
2175 @kbd{tar -cfz archive.tar.gz file}
2179 are quite different. The first example uses @file{archive.tar.gz} as
2180 the value for option @samp{f} and recognizes the option @samp{z}. The
2181 second example, however, uses @file{z} as the value for option
2182 @samp{f} --- probably not what was intended.
2184 This second example could be corrected in many ways, among which the
2185 following are equivalent:
2188 @kbd{tar -czf archive.tar.gz file}
2189 @kbd{tar -cf archive.tar.gz -z file}
2190 @kbd{tar cf archive.tar.gz -z file}
2194 @subsection Mixing Option Styles
2196 @cindex options, mixing different styles
2197 All three styles may be intermixed in a single @command{tar} command,
2198 so long as the rules for each style are fully
2199 respected@footnote{Before @GNUTAR{} version 1.11.6,
2200 a bug prevented intermixing old style options with long options in
2201 some cases.}. Old style options and either of the modern styles of
2202 options may be mixed within a single @command{tar} command. However,
2203 old style options must be introduced as the first arguments only,
2204 following the rule for old options (old options must appear directly
2205 after the @command{tar} command and some white space). Modern options
2206 may be given only after all arguments to the old options have been
2207 collected. If this rule is not respected, a modern option might be
2208 falsely interpreted as the value of the argument to one of the old
2211 For example, all the following commands are wholly equivalent, and
2212 illustrate the many combinations and orderings of option styles.
2215 @kbd{tar --create --file=archive.tar}
2216 @kbd{tar --create -f archive.tar}
2217 @kbd{tar --create -farchive.tar}
2218 @kbd{tar --file=archive.tar --create}
2219 @kbd{tar --file=archive.tar -c}
2220 @kbd{tar -c --file=archive.tar}
2221 @kbd{tar -c -f archive.tar}
2222 @kbd{tar -c -farchive.tar}
2223 @kbd{tar -cf archive.tar}
2224 @kbd{tar -cfarchive.tar}
2225 @kbd{tar -f archive.tar --create}
2226 @kbd{tar -f archive.tar -c}
2227 @kbd{tar -farchive.tar --create}
2228 @kbd{tar -farchive.tar -c}
2229 @kbd{tar c --file=archive.tar}
2230 @kbd{tar c -f archive.tar}
2231 @kbd{tar c -farchive.tar}
2232 @kbd{tar cf archive.tar}
2233 @kbd{tar f archive.tar --create}
2234 @kbd{tar f archive.tar -c}
2235 @kbd{tar fc archive.tar}
2238 On the other hand, the following commands are @emph{not} equivalent to
2242 @kbd{tar -f -c archive.tar}
2243 @kbd{tar -fc archive.tar}
2244 @kbd{tar -fcarchive.tar}
2245 @kbd{tar -farchive.tarc}
2246 @kbd{tar cfarchive.tar}
2250 These last examples mean something completely different from what the
2251 user intended (judging based on the example in the previous set which
2252 uses long options, whose intent is therefore very clear). The first
2253 four specify that the @command{tar} archive would be a file named
2254 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2255 respectively. The first two examples also specify a single non-option,
2256 @var{name} argument having the value @samp{archive.tar}. The last
2257 example contains only old style option letters (repeating option
2258 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2259 @samp{h}, or @samp{i}), with no argument value.
2260 @FIXME{not sure i liked
2261 the first sentence of this paragraph..}
2264 @section All @command{tar} Options
2266 The coming manual sections contain an alphabetical listing of all
2267 @command{tar} operations and options, with brief descriptions and
2268 cross-references to more in-depth explanations in the body of the manual.
2269 They also contain an alphabetically arranged table of the short option
2270 forms with their corresponding long option. You can use this table as
2271 a reference for deciphering @command{tar} commands in scripts.
2274 * Operation Summary::
2276 * Short Option Summary::
2279 @node Operation Summary
2280 @subsection Operations
2288 Appends files to the end of the archive. @xref{append}.
2290 @opsummary{catenate}
2294 Same as @option{--concatenate}. @xref{concatenate}.
2300 Compares archive members with their counterparts in the file
2301 system, and reports differences in file size, mode, owner,
2302 modification date and contents. @xref{compare}.
2304 @opsummary{concatenate}
2308 Appends other @command{tar} archives to the end of the archive.
2315 Creates a new @command{tar} archive. @xref{create}.
2320 Deletes members from the archive. Don't try this on an archive on a
2321 tape! @xref{delete}.
2327 Same @option{--compare}. @xref{compare}.
2333 Extracts members from the archive into the file system. @xref{extract}.
2339 Same as @option{--extract}. @xref{extract}.
2345 Lists the members in an archive. @xref{list}.
2351 Adds files to the end of the archive, but only if they are newer than
2352 their counterparts already in the archive, or if they do not already
2353 exist in the archive. @xref{update}.
2357 @node Option Summary
2358 @subsection @command{tar} Options
2362 @opsummary{absolute-names}
2363 @item --absolute-names
2366 Normally when creating an archive, @command{tar} strips an initial
2367 @samp{/} from member names, and when extracting from an archive @command{tar}
2368 treats names specially if they have initial @samp{/} or internal
2369 @samp{..}. This option disables that behavior. @xref{absolute}.
2371 @opsummary{after-date}
2374 (See @option{--newer}, @pxref{after})
2376 @opsummary{anchored}
2378 A pattern must match an initial subsequence of the name's components.
2379 @xref{controlling pattern-matching}.
2381 @opsummary{atime-preserve}
2382 @item --atime-preserve
2383 @itemx --atime-preserve=replace
2384 @itemx --atime-preserve=system
2386 Attempt to preserve the access time of files when reading them. This
2387 option currently is effective only on files that you own, unless you
2388 have superuser privileges.
2390 @option{--atime-preserve=replace} remembers the access time of a file
2391 before reading it, and then restores the access time afterwards. This
2392 may cause problems if other programs are reading the file at the same
2393 time, as the times of their accesses will be lost. On most platforms
2394 restoring the access time also requires @command{tar} to restore the
2395 data modification time too, so this option may also cause problems if
2396 other programs are writing the file at the same time (@command{tar} attempts
2397 to detect this situation, but cannot do so reliably due to race
2398 conditions). Worse, on most platforms restoring the access time also
2399 updates the status change time, which means that this option is
2400 incompatible with incremental backups.
2402 @option{--atime-preserve=system} avoids changing time stamps on files,
2403 without interfering with time stamp updates
2404 caused by other programs, so it works better with incremental backups.
2405 However, it requires a special @code{O_NOATIME} option from the
2406 underlying operating and file system implementation, and it also requires
2407 that searching directories does not update their access times. As of
2408 this writing (November 2005) this works only with Linux, and only with
2409 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2410 way to know whether this feature actually works. Sometimes
2411 @command{tar} knows that it does not work, and if you use
2412 @option{--atime-preserve=system} then @command{tar} complains and
2413 exits right away. But other times @command{tar} might think that the
2414 option works when it actually does not.
2416 Currently @option{--atime-preserve} with no operand defaults to
2417 @option{--atime-preserve=replace}, but this may change in the future
2418 as support for @option{--atime-preserve=system} improves.
2420 If your operating or file system does not support
2421 @option{--atime-preserve=@-system}, you might be able to preserve access
2422 times reliably by using the @command{mount} command. For example,
2423 you can mount the file system read-only, or access the file system via
2424 a read-only loopback mount, or use the @samp{noatime} mount option
2425 available on some systems. However, mounting typically requires
2426 superuser privileges and can be a pain to manage.
2428 @opsummary{auto-compress}
2429 @item --auto-compress
2432 During a @option{--create} operation, enables automatic compressed
2433 format recognition based on the archive suffix. The effect of this
2434 option is cancelled by @option{--no-auto-compress}. @xref{gzip}.
2437 @item --backup=@var{backup-type}
2439 Rather than deleting files from the file system, @command{tar} will
2440 back them up using simple or numbered backups, depending upon
2441 @var{backup-type}. @xref{backup}.
2443 @opsummary{block-number}
2444 @item --block-number
2447 With this option present, @command{tar} prints error messages for read errors
2448 with the block number in the archive file. @xref{block-number}.
2450 @opsummary{blocking-factor}
2451 @item --blocking-factor=@var{blocking}
2452 @itemx -b @var{blocking}
2454 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2455 record. @xref{Blocking Factor}.
2461 This option tells @command{tar} to read or write archives through
2462 @code{bzip2}. @xref{gzip}.
2464 @opsummary{check-device}
2465 @item --check-device
2466 Check device numbers when creating a list of modified files for
2467 incremental archiving. This is the default. @xref{device numbers},
2468 for a detailed description.
2470 @opsummary{checkpoint}
2471 @item --checkpoint[=@var{number}]
2473 This option directs @command{tar} to print periodic checkpoint
2474 messages as it reads through the archive. It is intended for when you
2475 want a visual indication that @command{tar} is still running, but
2476 don't want to see @option{--verbose} output. You can also instruct
2477 @command{tar} to execute a list of actions on each checkpoint, see
2478 @option{--checkpoint-action} below. For a detailed description, see
2481 @opsummary{checkpoint-action}
2482 @item --checkpoint-action=@var{action}
2483 Instruct @command{tar} to execute an action upon hitting a
2484 breakpoint. Here we give only a brief outline. @xref{checkpoints},
2485 for a complete description.
2487 The @var{action} argument can be one of the following:
2491 Produce an audible bell on the console.
2495 Print a single dot on the standard listing stream.
2498 Display a textual message on the standard error, with the status and
2499 number of the checkpoint. This is the default.
2501 @item echo=@var{string}
2502 Display @var{string} on the standard error. Before output, the string
2503 is subject to meta-character expansion.
2505 @item exec=@var{command}
2506 Execute the given @var{command}.
2508 @item sleep=@var{time}
2509 Wait for @var{time} seconds.
2511 @item ttyout=@var{string}
2512 Output @var{string} on the current console (@file{/dev/tty}).
2515 Several @option{--checkpoint-action} options can be specified. The
2516 supplied actions will be executed in order of their appearance in the
2519 Using @option{--checkpoint-action} without @option{--checkpoint}
2520 assumes default checkpoint frequency of one checkpoint per 10 records.
2522 @opsummary{check-links}
2525 If this option was given, @command{tar} will check the number of links
2526 dumped for each processed file. If this number does not match the
2527 total number of hard links for the file, a warning message will be
2528 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2529 synonym for @option{--one-file-system}. The current semantics, which
2530 complies to UNIX98, was introduced with version
2531 1.15.91. @xref{Changes}, for more information.}.
2535 @opsummary{compress}
2536 @opsummary{uncompress}
2541 @command{tar} will use the @command{compress} program when reading or
2542 writing the archive. This allows you to directly act on archives
2543 while saving space. @xref{gzip}.
2545 @opsummary{confirmation}
2546 @item --confirmation
2548 (See @option{--interactive}.) @xref{interactive}.
2550 @opsummary{delay-directory-restore}
2551 @item --delay-directory-restore
2553 Delay setting modification times and permissions of extracted
2554 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2556 @opsummary{dereference}
2560 When reading or writing a file to be archived, @command{tar} accesses
2561 the file that a symbolic link points to, rather than the symlink
2562 itself. @xref{dereference}.
2564 @opsummary{directory}
2565 @item --directory=@var{dir}
2568 When this option is specified, @command{tar} will change its current directory
2569 to @var{dir} before performing any operations. When this option is used
2570 during archive creation, it is order sensitive. @xref{directory}.
2573 @item --exclude=@var{pattern}
2575 When performing operations, @command{tar} will skip files that match
2576 @var{pattern}. @xref{exclude}.
2578 @opsummary{exclude-backups}
2579 @item --exclude-backups
2580 Exclude backup and lock files. @xref{exclude,, exclude-backups}.
2582 @opsummary{exclude-from}
2583 @item --exclude-from=@var{file}
2584 @itemx -X @var{file}
2586 Similar to @option{--exclude}, except @command{tar} will use the list of
2587 patterns in the file @var{file}. @xref{exclude}.
2589 @opsummary{exclude-caches}
2590 @item --exclude-caches
2592 Exclude from dump any directory containing a valid cache directory
2593 tag file, but still dump the directory node and the tag file itself.
2595 @xref{exclude,, exclude-caches}.
2597 @opsummary{exclude-caches-under}
2598 @item --exclude-caches-under
2600 Exclude from dump any directory containing a valid cache directory
2601 tag file, but still dump the directory node itself.
2605 @opsummary{exclude-caches-all}
2606 @item --exclude-caches-all
2608 Exclude from dump any directory containing a valid cache directory
2609 tag file. @xref{exclude}.
2611 @opsummary{exclude-tag}
2612 @item --exclude-tag=@var{file}
2614 Exclude from dump any directory containing file named @var{file}, but
2615 dump the directory node and @var{file} itself. @xref{exclude,, exclude-tag}.
2617 @opsummary{exclude-tag-under}
2618 @item --exclude-tag-under=@var{file}
2620 Exclude from dump the contents of any directory containing file
2621 named @var{file}, but dump the directory node itself. @xref{exclude,,
2624 @opsummary{exclude-tag-all}
2625 @item --exclude-tag-all=@var{file}
2627 Exclude from dump any directory containing file named @var{file}.
2628 @xref{exclude,,exclude-tag-all}.
2630 @opsummary{exclude-vcs}
2633 Exclude from dump directories and files, that are internal for some
2634 widely used version control systems.
2636 @xref{exclude,,exclude-vcs}.
2639 @item --file=@var{archive}
2640 @itemx -f @var{archive}
2642 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2643 performs operations on, rather than @command{tar}'s compilation dependent
2644 default. @xref{file tutorial}.
2646 @opsummary{files-from}
2647 @item --files-from=@var{file}
2648 @itemx -T @var{file}
2650 @command{tar} will use the contents of @var{file} as a list of archive members
2651 or files to operate on, in addition to those specified on the
2652 command-line. @xref{files}.
2654 @opsummary{force-local}
2657 Forces @command{tar} to interpret the file name given to @option{--file}
2658 as a local file, even if it looks like a remote tape drive name.
2659 @xref{local and remote archives}.
2662 @item --format=@var{format}
2663 @itemx -H @var{format}
2665 Selects output archive format. @var{Format} may be one of the
2670 Creates an archive that is compatible with Unix V7 @command{tar}.
2673 Creates an archive that is compatible with GNU @command{tar} version
2677 Creates archive in GNU tar 1.13 format. Basically it is the same as
2678 @samp{oldgnu} with the only difference in the way it handles long
2682 Creates a @acronym{POSIX.1-1988} compatible archive.
2685 Creates a @acronym{POSIX.1-2001 archive}.
2689 @xref{Formats}, for a detailed discussion of these formats.
2691 @opsummary{full-time}
2693 This option instructs @command{tar} to print file times to their full
2694 resolution. Usually this means 1-second resolution, but that depends
2695 on the underlying file system. The @option{--full-time} option takes
2696 effect only when detailed output (verbosity level 2 or higher) has
2697 been requested using the @option{--verbose} option, e.g., when listing
2698 or extracting archives:
2701 $ @kbd{tar -t -v --full-time -f archive.tar}
2705 or, when creating an archive:
2708 $ @kbd{tar -c -vv --full-time -f archive.tar .}
2711 Notice, thar when creating the archive you need to specify
2712 @option{--verbose} twice to get a detailed output (@pxref{verbose
2716 @item --group=@var{group}
2718 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
2719 rather than the group from the source file. @var{group} can specify a
2720 symbolic name, or a numeric @acronym{ID}, or both as
2721 @var{name}:@var{id}. @xref{override}.
2723 Also see the comments for the @option{--owner=@var{user}} option.
2733 This option tells @command{tar} to read or write archives through
2734 @command{gzip}, allowing @command{tar} to directly operate on several
2735 kinds of compressed archives transparently. @xref{gzip}.
2737 @opsummary{hard-dereference}
2738 @item --hard-dereference
2739 When creating an archive, dereference hard links and store the files
2740 they refer to, instead of creating usual hard link members.
2748 @command{tar} will print out a short message summarizing the operations and
2749 options to @command{tar} and exit. @xref{help}.
2751 @opsummary{ignore-case}
2753 Ignore case when matching member or file names with
2754 patterns. @xref{controlling pattern-matching}.
2756 @opsummary{ignore-command-error}
2757 @item --ignore-command-error
2758 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2760 @opsummary{ignore-failed-read}
2761 @item --ignore-failed-read
2763 Do not exit unsuccessfully merely because an unreadable file was encountered.
2764 @xref{Ignore Failed Read}.
2766 @opsummary{ignore-zeros}
2767 @item --ignore-zeros
2770 With this option, @command{tar} will ignore zeroed blocks in the
2771 archive, which normally signals EOF. @xref{Reading}.
2773 @opsummary{incremental}
2777 Informs @command{tar} that it is working with an old
2778 @acronym{GNU}-format incremental backup archive. It is intended
2779 primarily for backwards compatibility only. @xref{Incremental Dumps},
2780 for a detailed discussion of incremental archives.
2782 @opsummary{index-file}
2783 @item --index-file=@var{file}
2785 Send verbose output to @var{file} instead of to standard output.
2787 @opsummary{info-script}
2788 @opsummary{new-volume-script}
2789 @item --info-script=@var{command}
2790 @itemx --new-volume-script=@var{command}
2791 @itemx -F @var{command}
2793 When @command{tar} is performing multi-tape backups, @var{command} is run
2794 at the end of each tape. If it exits with nonzero status,
2795 @command{tar} fails immediately. @xref{info-script}, for a detailed
2796 discussion of this feature.
2798 @opsummary{interactive}
2800 @itemx --confirmation
2803 Specifies that @command{tar} should ask the user for confirmation before
2804 performing potentially destructive options, such as overwriting files.
2807 @opsummary{--keep-directory-symlink}
2808 @item --keep-directory-symlink
2810 This option changes the behavior of tar when it encounters a symlink
2811 with the same name as the directory that it is about to extract. By
2812 default, in this case tar would first remove the symlink and then
2813 proceed extracting the directory.
2815 The @option{--keep-directory-symlink} option disables this behavior
2816 and instructs tar to follow symlinks to directories when extracting
2819 It is mainly intended to provide compatibility with the Slackware
2820 installation scripts.
2822 @opsummary{keep-newer-files}
2823 @item --keep-newer-files
2825 Do not replace existing files that are newer than their archive copies
2826 when extracting files from an archive.
2828 @opsummary{keep-old-files}
2829 @item --keep-old-files
2832 Do not overwrite existing files when extracting files from an
2833 archive. Return error if such files exist. See also
2834 @ref{--skip-old-files}.
2836 @xref{Keep Old Files}.
2839 @item --label=@var{name}
2840 @itemx -V @var{name}
2842 When creating an archive, instructs @command{tar} to write @var{name}
2843 as a name record in the archive. When extracting or listing archives,
2844 @command{tar} will only operate on archives that have a label matching
2845 the pattern specified in @var{name}. @xref{Tape Files}.
2848 @item --level=@var{n}
2849 Force incremental backup of level @var{n}. As of @GNUTAR version
2850 @value{VERSION}, the option @option{--level=0} truncates the snapshot
2851 file, thereby forcing the level 0 dump. Other values of @var{n} are
2852 effectively ignored. @xref{--level=0}, for details and examples.
2854 The use of this option is valid only in conjunction with the
2855 @option{--listed-incremental} option. @xref{Incremental Dumps},
2856 for a detailed description.
2858 @opsummary{listed-incremental}
2859 @item --listed-incremental=@var{snapshot-file}
2860 @itemx -g @var{snapshot-file}
2862 During a @option{--create} operation, specifies that the archive that
2863 @command{tar} creates is a new @acronym{GNU}-format incremental
2864 backup, using @var{snapshot-file} to determine which files to backup.
2865 With other operations, informs @command{tar} that the archive is in
2866 incremental format. @xref{Incremental Dumps}.
2871 This option tells @command{tar} to read or write archives through
2872 @command{lzip}. @xref{gzip}.
2877 This option tells @command{tar} to read or write archives through
2878 @command{lzma}. @xref{gzip}.
2882 This option tells @command{tar} to read or write archives through
2883 @command{lzop}. @xref{gzip}.
2886 @item --mode=@var{permissions}
2888 When adding files to an archive, @command{tar} will use
2889 @var{permissions} for the archive members, rather than the permissions
2890 from the files. @var{permissions} can be specified either as an octal
2891 number or as symbolic permissions, like with
2892 @command{chmod}. @xref{override}.
2895 @item --mtime=@var{date}
2897 When adding files to an archive, @command{tar} will use @var{date} as
2898 the modification time of members when creating archives, instead of
2899 their actual modification times. The value of @var{date} can be
2900 either a textual date representation (@pxref{Date input formats}) or a
2901 name of the existing file, starting with @samp{/} or @samp{.}. In the
2902 latter case, the modification time of that file is used. @xref{override}.
2904 @opsummary{multi-volume}
2905 @item --multi-volume
2908 Informs @command{tar} that it should create or otherwise operate on a
2909 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2911 @opsummary{new-volume-script}
2912 @item --new-volume-script
2914 (see @option{--info-script})
2917 @item --newer=@var{date}
2918 @itemx --after-date=@var{date}
2921 When creating an archive, @command{tar} will only add files that have changed
2922 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2923 is taken to be the name of a file whose data modification time specifies
2924 the date. @xref{after}.
2926 @opsummary{newer-mtime}
2927 @item --newer-mtime=@var{date}
2929 Like @option{--newer}, but add only files whose
2930 contents have changed (as opposed to just @option{--newer}, which will
2931 also back up files for which any status information has
2932 changed). @xref{after}.
2934 @opsummary{no-anchored}
2936 An exclude pattern can match any subsequence of the name's components.
2937 @xref{controlling pattern-matching}.
2939 @opsummary{no-auto-compress}
2940 @item --no-auto-compress
2942 Disables automatic compressed format recognition based on the archive
2943 suffix. @xref{--auto-compress}. @xref{gzip}.
2945 @opsummary{no-check-device}
2946 @item --no-check-device
2947 Do not check device numbers when creating a list of modified files
2948 for incremental archiving. @xref{device numbers}, for
2949 a detailed description.
2951 @opsummary{no-delay-directory-restore}
2952 @item --no-delay-directory-restore
2954 Modification times and permissions of extracted
2955 directories are set when all files from this directory have been
2956 extracted. This is the default.
2957 @xref{Directory Modification Times and Permissions}.
2959 @opsummary{no-ignore-case}
2960 @item --no-ignore-case
2961 Use case-sensitive matching.
2962 @xref{controlling pattern-matching}.
2964 @opsummary{no-ignore-command-error}
2965 @item --no-ignore-command-error
2966 Print warnings about subprocesses that terminated with a nonzero exit
2967 code. @xref{Writing to an External Program}.
2972 If the @option{--null} option was given previously, this option
2973 cancels its effect, so that any following @option{--files-from}
2974 options will expect their file lists to be newline-terminated.
2976 @opsummary{no-overwrite-dir}
2977 @item --no-overwrite-dir
2979 Preserve metadata of existing directories when extracting files
2980 from an archive. @xref{Overwrite Old Files}.
2982 @opsummary{no-quote-chars}
2983 @item --no-quote-chars=@var{string}
2984 Remove characters listed in @var{string} from the list of quoted
2985 characters set by the previous @option{--quote-chars} option
2986 (@pxref{quoting styles}).
2988 @opsummary{no-recursion}
2989 @item --no-recursion
2991 With this option, @command{tar} will not recurse into directories.
2994 @opsummary{no-same-owner}
2995 @item --no-same-owner
2998 When extracting an archive, do not attempt to preserve the owner
2999 specified in the @command{tar} archive. This the default behavior
3002 @opsummary{no-same-permissions}
3003 @item --no-same-permissions
3005 When extracting an archive, subtract the user's umask from files from
3006 the permissions specified in the archive. This is the default behavior
3012 The archive media does not support seeks to arbitrary
3013 locations. Usually @command{tar} determines automatically whether
3014 the archive can be seeked or not. Use this option to disable this
3017 @opsummary{no-unquote}
3019 Treat all input file or member names literally, do not interpret
3020 escape sequences. @xref{input name quoting}.
3022 @opsummary{no-wildcards}
3023 @item --no-wildcards
3024 Do not use wildcards.
3025 @xref{controlling pattern-matching}.
3027 @opsummary{no-wildcards-match-slash}
3028 @item --no-wildcards-match-slash
3029 Wildcards do not match @samp{/}.
3030 @xref{controlling pattern-matching}.
3035 When @command{tar} is using the @option{--files-from} option, this option
3036 instructs @command{tar} to expect file names terminated with @acronym{NUL}, so
3037 @command{tar} can correctly work with file names that contain newlines.
3040 @opsummary{numeric-owner}
3041 @item --numeric-owner
3043 This option will notify @command{tar} that it should use numeric user
3044 and group IDs when creating a @command{tar} file, rather than names.
3048 The function of this option depends on the action @command{tar} is
3049 performing. When extracting files, @option{-o} is a synonym for
3050 @option{--no-same-owner}, i.e., it prevents @command{tar} from
3051 restoring ownership of files being extracted.
3053 When creating an archive, it is a synonym for
3054 @option{--old-archive}. This behavior is for compatibility
3055 with previous versions of @GNUTAR{}, and will be
3056 removed in future releases.
3058 @xref{Changes}, for more information.
3060 @opsummary{occurrence}
3061 @item --occurrence[=@var{number}]
3063 This option can be used in conjunction with one of the subcommands
3064 @option{--delete}, @option{--diff}, @option{--extract} or
3065 @option{--list} when a list of files is given either on the command
3066 line or via @option{-T} option.
3068 This option instructs @command{tar} to process only the @var{number}th
3069 occurrence of each named file. @var{Number} defaults to 1, so
3072 tar -x -f archive.tar --occurrence filename
3076 will extract the first occurrence of the member @file{filename} from @file{archive.tar}
3077 and will terminate without scanning to the end of the archive.
3079 @opsummary{old-archive}
3081 Synonym for @option{--format=v7}.
3083 @opsummary{one-file-system}
3084 @item --one-file-system
3085 Used when creating an archive. Prevents @command{tar} from recursing into
3086 directories that are on different file systems from the current
3089 @opsummary{overwrite}
3092 Overwrite existing files and directory metadata when extracting files
3093 from an archive. @xref{Overwrite Old Files}.
3095 @opsummary{overwrite-dir}
3096 @item --overwrite-dir
3098 Overwrite the metadata of existing directories when extracting files
3099 from an archive. @xref{Overwrite Old Files}.
3102 @item --owner=@var{user}
3104 Specifies that @command{tar} should use @var{user} as the owner of members
3105 when creating archives, instead of the user associated with the source
3106 file. @var{user} can specify a symbolic name, or a numeric
3107 @acronym{ID}, or both as @var{name}:@var{id}.
3110 This option does not affect extraction from archives.
3112 @opsummary{pax-option}
3113 @item --pax-option=@var{keyword-list}
3114 This option enables creation of the archive in @acronym{POSIX.1-2001}
3115 format (@pxref{posix}) and modifies the way @command{tar} handles the
3116 extended header keywords. @var{Keyword-list} is a comma-separated
3117 list of keyword options. @xref{PAX keywords}, for a detailed
3120 @opsummary{portability}
3122 @itemx --old-archive
3123 Synonym for @option{--format=v7}.
3127 Same as @option{--format=posix}.
3129 @opsummary{preserve}
3132 Synonymous with specifying both @option{--preserve-permissions} and
3133 @option{--same-order}. @xref{Setting Access Permissions}.
3135 @opsummary{preserve-order}
3136 @item --preserve-order
3138 (See @option{--same-order}; @pxref{Reading}.)
3140 @opsummary{preserve-permissions}
3141 @opsummary{same-permissions}
3142 @item --preserve-permissions
3143 @itemx --same-permissions
3146 When @command{tar} is extracting an archive, it normally subtracts the
3147 users' umask from the permissions specified in the archive and uses
3148 that number as the permissions to create the destination file.
3149 Specifying this option instructs @command{tar} that it should use the
3150 permissions directly from the archive. @xref{Setting Access Permissions}.
3152 @opsummary{quote-chars}
3153 @item --quote-chars=@var{string}
3154 Always quote characters from @var{string}, even if the selected
3155 quoting style would not quote them (@pxref{quoting styles}).
3157 @opsummary{quoting-style}
3158 @item --quoting-style=@var{style}
3159 Set quoting style to use when printing member and file names
3160 (@pxref{quoting styles}). Valid @var{style} values are:
3161 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
3162 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
3163 style is @code{escape}, unless overridden while configuring the
3166 @opsummary{read-full-records}
3167 @item --read-full-records
3170 Specifies that @command{tar} should reblock its input, for reading
3171 from pipes on systems with buggy implementations. @xref{Reading}.
3173 @opsummary{record-size}
3174 @item --record-size=@var{size}[@var{suf}]
3176 Instructs @command{tar} to use @var{size} bytes per record when accessing the
3177 archive. The argument can be suffixed with a @dfn{size suffix}, e.g.
3178 @option{--record-size=10K} for 10 Kilobytes. @xref{size-suffixes},
3179 for a list of valid suffixes. @xref{Blocking Factor}, for a detailed
3180 description of this option.
3182 @opsummary{recursion}
3185 With this option, @command{tar} recurses into directories (default).
3188 @opsummary{recursive-unlink}
3189 @item --recursive-unlink
3192 directory hierarchies before extracting directories of the same name
3193 from the archive. @xref{Recursive Unlink}.
3195 @opsummary{remove-files}
3196 @item --remove-files
3198 Directs @command{tar} to remove the source file from the file system after
3199 appending it to an archive. @xref{remove files}.
3201 @opsummary{restrict}
3204 Disable use of some potentially harmful @command{tar} options.
3205 Currently this option disables shell invocation from multi-volume menu
3206 (@pxref{Using Multiple Tapes}).
3208 @opsummary{rmt-command}
3209 @item --rmt-command=@var{cmd}
3211 Notifies @command{tar} that it should use @var{cmd} instead of
3212 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
3214 @opsummary{rsh-command}
3215 @item --rsh-command=@var{cmd}
3217 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
3218 devices. @xref{Device}.
3220 @opsummary{same-order}
3222 @itemx --preserve-order
3225 This option is an optimization for @command{tar} when running on machines with
3226 small amounts of memory. It informs @command{tar} that the list of file
3227 arguments has already been sorted to match the order of files in the
3228 archive. @xref{Reading}.
3230 @opsummary{same-owner}
3233 When extracting an archive, @command{tar} will attempt to preserve the owner
3234 specified in the @command{tar} archive with this option present.
3235 This is the default behavior for the superuser; this option has an
3236 effect only for ordinary users. @xref{Attributes}.
3238 @opsummary{same-permissions}
3239 @item --same-permissions
3241 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
3247 Assume that the archive media supports seeks to arbitrary
3248 locations. Usually @command{tar} determines automatically whether
3249 the archive can be seeked or not. This option is intended for use
3250 in cases when such recognition fails. It takes effect only if the
3251 archive is open for reading (e.g. with @option{--list} or
3252 @option{--extract} options).
3254 @opsummary{show-defaults}
3255 @item --show-defaults
3257 Displays the default options used by @command{tar} and exits
3258 successfully. This option is intended for use in shell scripts.
3259 Here is an example of what you can see using this option:
3262 $ @kbd{tar --show-defaults}
3263 --format=gnu -f- -b20 --quoting-style=escape
3264 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
3268 Notice, that this option outputs only one line. The example output
3269 above has been split to fit page boundaries. @xref{defaults}.
3271 @opsummary{show-omitted-dirs}
3272 @item --show-omitted-dirs
3274 Instructs @command{tar} to mention the directories it is skipping when
3275 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
3277 @opsummary{show-snapshot-field-ranges}
3278 @item --show-snapshot-field-ranges
3280 Displays the range of values allowed by this version of @command{tar}
3281 for each field in the snapshot file, then exits successfully.
3282 @xref{Snapshot Files}.
3284 @opsummary{show-transformed-names}
3285 @opsummary{show-stored-names}
3286 @item --show-transformed-names
3287 @itemx --show-stored-names
3289 Display file or member names after applying any transformations
3290 (@pxref{transform}). In particular, when used in conjunction with one of
3291 the archive creation operations it instructs @command{tar} to list the
3292 member names stored in the archive, as opposed to the actual file
3293 names. @xref{listing member and file names}.
3295 @opsummary{skip-old-files}
3296 @item --skip-old-files
3298 Do not overwrite existing files when extracting files from an
3299 archive. @xref{Keep Old Files}.
3301 This option differs from @option{--keep-old-files} in that it does not
3302 treat such files as an error, instead it just silently avoids
3305 The @option{--warning=existing-file} option can be used together with
3306 this option to produce warning messages about existing old files
3313 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3314 sparse files efficiently. @xref{sparse}.
3316 @opsummary{sparse-version}
3317 @item --sparse-version=@var{version}
3319 Specifies the @dfn{format version} to use when archiving sparse
3320 files. Implies @option{--sparse}. @xref{sparse}. For the description
3321 of the supported sparse formats, @xref{Sparse Formats}.
3323 @opsummary{starting-file}
3324 @item --starting-file=@var{name}
3325 @itemx -K @var{name}
3327 This option affects extraction only; @command{tar} will skip extracting
3328 files in the archive until it finds one that matches @var{name}.
3331 @opsummary{strip-components}
3332 @item --strip-components=@var{number}
3333 Strip given @var{number} of leading components from file names before
3334 extraction. For example, if archive @file{archive.tar} contained
3335 @file{/some/file/name}, then running
3338 tar --extract --file archive.tar --strip-components=2
3342 would extract this file to file @file{name}.
3345 @item --suffix=@var{suffix}
3347 Alters the suffix @command{tar} uses when backing up files from the default
3348 @samp{~}. @xref{backup}.
3350 @opsummary{tape-length}
3351 @item --tape-length=@var{num}[@var{suf}]
3352 @itemx -L @var{num}[@var{suf}]
3354 Specifies the length of tapes that @command{tar} is writing as being
3355 @w{@var{num} x 1024} bytes long. If optional @var{suf} is given, it
3356 specifies a multiplicative factor to be used instead of 1024. For
3357 example, @samp{-L2M} means 2 megabytes. @xref{size-suffixes}, for a
3358 list of allowed suffixes. @xref{Using Multiple Tapes}, for a detailed
3359 discussion of this option.
3361 @opsummary{test-label}
3364 Reads the volume label. If an argument is specified, test whether it
3365 matches the volume label. @xref{--test-label option}.
3367 @opsummary{to-command}
3368 @item --to-command=@var{command}
3370 During extraction @command{tar} will pipe extracted files to the
3371 standard input of @var{command}. @xref{Writing to an External Program}.
3373 @opsummary{to-stdout}
3377 During extraction, @command{tar} will extract files to stdout rather
3378 than to the file system. @xref{Writing to Standard Output}.
3381 @item --totals[=@var{signo}]
3383 Displays the total number of bytes transferred when processing an
3384 archive. If an argument is given, these data are displayed on
3385 request, when signal @var{signo} is delivered to @command{tar}.
3392 Sets the data modification time of extracted files to the extraction time,
3393 rather than the data modification time stored in the archive.
3394 @xref{Data Modification Times}.
3396 @opsummary{transform}
3398 @item --transform=@var{sed-expr}
3399 @itemx --xform=@var{sed-expr}
3400 Transform file or member names using @command{sed} replacement expression
3401 @var{sed-expr}. For example,
3404 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
3408 will add to @file{archive} files from the current working directory,
3409 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
3410 discussion, @xref{transform}.
3412 To see transformed member names in verbose listings, use
3413 @option{--show-transformed-names} option
3414 (@pxref{show-transformed-names}).
3416 @opsummary{uncompress}
3419 (See @option{--compress}, @pxref{gzip})
3424 (See @option{--gzip}, @pxref{gzip})
3426 @opsummary{unlink-first}
3427 @item --unlink-first
3430 Directs @command{tar} to remove the corresponding file from the file
3431 system before extracting it from the archive. @xref{Unlink First}.
3435 Enable unquoting input file or member names (default). @xref{input
3438 @opsummary{use-compress-program}
3439 @item --use-compress-program=@var{prog}
3440 @itemx -I=@var{prog}
3442 Instructs @command{tar} to access the archive through @var{prog}, which is
3443 presumed to be a compression program of some sort. @xref{gzip}.
3448 Display file modification dates in @acronym{UTC}. This option implies
3455 Specifies that @command{tar} should be more verbose about the
3456 operations it is performing. This option can be specified multiple
3457 times for some operations to increase the amount of information displayed.
3464 Verifies that the archive was correctly written when creating an
3465 archive. @xref{verify}.
3470 Print information about the program's name, version, origin and legal
3471 status, all on standard output, and then exit successfully.
3474 @opsummary{volno-file}
3475 @item --volno-file=@var{file}
3477 Used in conjunction with @option{--multi-volume}. @command{tar} will
3478 keep track of which volume of a multi-volume archive it is working in
3479 @var{file}. @xref{volno-file}.
3482 @item --warning=@var{keyword}
3484 Enable or disable warning messages identified by @var{keyword}. The
3485 messages are suppressed if @var{keyword} is prefixed with @samp{no-}.
3488 @opsummary{wildcards}
3490 Use wildcards when matching member names with patterns.
3491 @xref{controlling pattern-matching}.
3493 @opsummary{wildcards-match-slash}
3494 @item --wildcards-match-slash
3495 Wildcards match @samp{/}.
3496 @xref{controlling pattern-matching}.
3501 Use @command{xz} for compressing or decompressing the archives. @xref{gzip}.
3505 @node Short Option Summary
3506 @subsection Short Options Cross Reference
3508 Here is an alphabetized list of all of the short option forms, matching
3509 them with the equivalent long option.
3511 @multitable @columnfractions 0.20 0.80
3512 @headitem Short Option @tab Reference
3514 @item -A @tab @ref{--concatenate}.
3516 @item -B @tab @ref{--read-full-records}.
3518 @item -C @tab @ref{--directory}.
3520 @item -F @tab @ref{--info-script}.
3522 @item -G @tab @ref{--incremental}.
3524 @item -J @tab @ref{--xz}.
3526 @item -K @tab @ref{--starting-file}.
3528 @item -L @tab @ref{--tape-length}.
3530 @item -M @tab @ref{--multi-volume}.
3532 @item -N @tab @ref{--newer}.
3534 @item -O @tab @ref{--to-stdout}.
3536 @item -P @tab @ref{--absolute-names}.
3538 @item -R @tab @ref{--block-number}.
3540 @item -S @tab @ref{--sparse}.
3542 @item -T @tab @ref{--files-from}.
3544 @item -U @tab @ref{--unlink-first}.
3546 @item -V @tab @ref{--label}.
3548 @item -W @tab @ref{--verify}.
3550 @item -X @tab @ref{--exclude-from}.
3552 @item -Z @tab @ref{--compress}.
3554 @item -b @tab @ref{--blocking-factor}.
3556 @item -c @tab @ref{--create}.
3558 @item -d @tab @ref{--compare}.
3560 @item -f @tab @ref{--file}.
3562 @item -g @tab @ref{--listed-incremental}.
3564 @item -h @tab @ref{--dereference}.
3566 @item -i @tab @ref{--ignore-zeros}.
3568 @item -j @tab @ref{--bzip2}.
3570 @item -k @tab @ref{--keep-old-files}.
3572 @item -l @tab @ref{--check-links}.
3574 @item -m @tab @ref{--touch}.
3576 @item -o @tab When creating, @ref{--no-same-owner}, when extracting ---
3577 @ref{--portability}.
3579 The latter usage is deprecated. It is retained for compatibility with
3580 the earlier versions of @GNUTAR{}. In future releases
3581 @option{-o} will be equivalent to @option{--no-same-owner} only.
3583 @item -p @tab @ref{--preserve-permissions}.
3585 @item -r @tab @ref{--append}.
3587 @item -s @tab @ref{--same-order}.
3589 @item -t @tab @ref{--list}.
3591 @item -u @tab @ref{--update}.
3593 @item -v @tab @ref{--verbose}.
3595 @item -w @tab @ref{--interactive}.
3597 @item -x @tab @ref{--extract}.
3599 @item -z @tab @ref{--gzip}.
3604 @section @GNUTAR{} documentation
3606 @cindex Getting program version number
3608 @cindex Version of the @command{tar} program
3609 Being careful, the first thing is really checking that you are using
3610 @GNUTAR{}, indeed. The @option{--version} option
3611 causes @command{tar} to print information about its name, version,
3612 origin and legal status, all on standard output, and then exit
3613 successfully. For example, @w{@samp{tar --version}} might print:
3616 tar (GNU tar) @value{VERSION}
3617 Copyright (C) 2013 Free Software Foundation, Inc.
3618 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
3619 This is free software: you are free to change and redistribute it.
3620 There is NO WARRANTY, to the extent permitted by law.
3622 Written by John Gilmore and Jay Fenlason.
3626 The first occurrence of @samp{tar} in the result above is the program
3627 name in the package (for example, @command{rmt} is another program),
3628 while the second occurrence of @samp{tar} is the name of the package
3629 itself, containing possibly many programs. The package is currently
3630 named @samp{tar}, after the name of the main program it
3631 contains@footnote{There are plans to merge the @command{cpio} and
3632 @command{tar} packages into a single one which would be called
3633 @code{paxutils}. So, who knows if, one of this days, the
3634 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3637 @cindex Obtaining help
3638 @cindex Listing all @command{tar} options
3639 @xopindex{help, introduction}
3640 Another thing you might want to do is checking the spelling or meaning
3641 of some particular @command{tar} option, without resorting to this
3642 manual, for once you have carefully read it. @GNUTAR{}
3643 has a short help feature, triggerable through the
3644 @option{--help} option. By using this option, @command{tar} will
3645 print a usage message listing all available options on standard
3646 output, then exit successfully, without doing anything else and
3647 ignoring all other options. Even if this is only a brief summary, it
3648 may be several screens long. So, if you are not using some kind of
3649 scrollable window, you might prefer to use something like:
3652 $ @kbd{tar --help | less}
3656 presuming, here, that you like using @command{less} for a pager. Other
3657 popular pagers are @command{more} and @command{pg}. If you know about some
3658 @var{keyword} which interests you and do not want to read all the
3659 @option{--help} output, another common idiom is doing:
3662 tar --help | grep @var{keyword}
3666 for getting only the pertinent lines. Notice, however, that some
3667 @command{tar} options have long description lines and the above
3668 command will list only the first of them.
3670 The exact look of the option summary displayed by @kbd{tar --help} is
3671 configurable. @xref{Configuring Help Summary}, for a detailed description.
3674 If you only wish to check the spelling of an option, running @kbd{tar
3675 --usage} may be a better choice. This will display a terse list of
3676 @command{tar} options without accompanying explanations.
3678 The short help output is quite succinct, and you might have to get
3679 back to the full documentation for precise points. If you are reading
3680 this paragraph, you already have the @command{tar} manual in some
3681 form. This manual is available in a variety of forms from
3682 @url{http://www.gnu.org/software/tar/manual}. It may be printed out of the @GNUTAR{}
3683 distribution, provided you have @TeX{} already installed somewhere,
3684 and a laser printer around. Just configure the distribution, execute
3685 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3686 usual way (contact your local guru to know how). If @GNUTAR{}
3687 has been conveniently installed at your place, this
3688 manual is also available in interactive, hypertextual form as an Info
3689 file. Just call @w{@samp{info tar}} or, if you do not have the
3690 @command{info} program handy, use the Info reader provided within
3691 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3693 There is currently no @code{man} page for @GNUTAR{}.
3694 If you observe such a @code{man} page on the system you are running,
3695 either it does not belong to @GNUTAR{}, or it has not
3696 been produced by @acronym{GNU}. Some package maintainers convert
3697 @kbd{tar --help} output to a man page, using @command{help2man}. In
3698 any case, please bear in mind that the authoritative source of
3699 information about @GNUTAR{} is this Texinfo documentation.
3702 @section Obtaining @GNUTAR{} default values
3704 @opindex show-defaults
3705 @GNUTAR{} has some predefined defaults that are used when you do not
3706 explicitly specify another values. To obtain a list of such
3707 defaults, use @option{--show-defaults} option. This will output the
3708 values in the form of @command{tar} command line options:
3712 $ @kbd{tar --show-defaults}
3713 --format=gnu -f- -b20 --quoting-style=escape
3714 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3719 Notice, that this option outputs only one line. The example output above
3720 has been split to fit page boundaries.
3723 The above output shows that this version of @GNUTAR{} defaults to
3724 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3725 output as the archive, if no @option{--file} option has been given
3726 (@pxref{file tutorial}), the default blocking factor is 20
3727 (@pxref{Blocking Factor}). It also shows the default locations where
3728 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3731 @section Checking @command{tar} progress
3733 Typically, @command{tar} performs most operations without reporting any
3734 information to the user except error messages. When using @command{tar}
3735 with many options, particularly ones with complicated or
3736 difficult-to-predict behavior, it is possible to make serious mistakes.
3737 @command{tar} provides several options that make observing @command{tar}
3738 easier. These options cause @command{tar} to print information as it
3739 progresses in its job, and you might want to use them just for being
3740 more careful about what is going on, or merely for entertaining
3741 yourself. If you have encountered a problem when operating on an
3742 archive, however, you may need more information than just an error
3743 message in order to solve the problem. The following options can be
3744 helpful diagnostic tools.
3746 @cindex Verbose operation
3748 Normally, the @option{--list} (@option{-t}) command to list an archive
3749 prints just the file names (one per line) and the other commands are
3750 silent. When used with most operations, the @option{--verbose}
3751 (@option{-v}) option causes @command{tar} to print the name of each
3752 file or archive member as it is processed. This and the other options
3753 which make @command{tar} print status information can be useful in
3754 monitoring @command{tar}.
3756 With @option{--create} or @option{--extract}, @option{--verbose} used
3757 once just prints the names of the files or members as they are processed.
3758 Using it twice causes @command{tar} to print a longer listing
3759 (@xref{verbose member listing}, for the description) for each member.
3760 Since @option{--list} already prints the names of the members,
3761 @option{--verbose} used once with @option{--list} causes @command{tar}
3762 to print an @samp{ls -l} type listing of the files in the archive.
3763 The following examples both extract members with long list output:
3766 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3767 $ @kbd{tar xvvf archive.tar}
3770 Verbose output appears on the standard output except when an archive is
3771 being written to the standard output, as with @samp{tar --create
3772 --file=- --verbose} (@samp{tar cvf -}, or even @samp{tar cv}---if the
3773 installer let standard output be the default archive). In that case
3774 @command{tar} writes verbose output to the standard error stream.
3776 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3777 verbose output to @var{file} rather than to standard output or standard
3781 @cindex Obtaining total status information
3783 The @option{--totals} option causes @command{tar} to print on the
3784 standard error the total amount of bytes transferred when processing
3785 an archive. When creating or appending to an archive, this option
3786 prints the number of bytes written to the archive and the average
3787 speed at which they have been written, e.g.:
3791 $ @kbd{tar -c -f archive.tar --totals /home}
3792 Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
3796 When reading an archive, this option displays the number of bytes
3801 $ @kbd{tar -x -f archive.tar --totals}
3802 Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
3806 Finally, when deleting from an archive, the @option{--totals} option
3807 displays both numbers plus number of bytes removed from the archive:
3811 $ @kbd{tar --delete -f foo.tar --totals --wildcards '*~'}
3812 Total bytes read: 9543680 (9.2MiB, 201MiB/s)
3813 Total bytes written: 3829760 (3.7MiB, 81MiB/s)
3814 Total bytes deleted: 1474048
3818 You can also obtain this information on request. When
3819 @option{--totals} is used with an argument, this argument is
3820 interpreted as a symbolic name of a signal, upon delivery of which the
3821 statistics is to be printed:
3824 @item --totals=@var{signo}
3825 Print statistics upon delivery of signal @var{signo}. Valid arguments
3826 are: @code{SIGHUP}, @code{SIGQUIT}, @code{SIGINT}, @code{SIGUSR1} and
3827 @code{SIGUSR2}. Shortened names without @samp{SIG} prefix are also
3831 Both forms of @option{--totals} option can be used simultaneously.
3832 Thus, @kbd{tar -x --totals --totals=USR1} instructs @command{tar} to
3833 extract all members from its default archive and print statistics
3834 after finishing the extraction, as well as when receiving signal
3837 @anchor{Progress information}
3838 @cindex Progress information
3839 The @option{--checkpoint} option prints an occasional message
3840 as @command{tar} reads or writes the archive. It is designed for
3841 those who don't need the more detailed (and voluminous) output of
3842 @option{--block-number} (@option{-R}), but do want visual confirmation
3843 that @command{tar} is actually making forward progress. By default it
3844 prints a message each 10 records read or written. This can be changed
3845 by giving it a numeric argument after an equal sign:
3848 $ @kbd{tar -c --checkpoint=1000} /var
3849 tar: Write checkpoint 1000
3850 tar: Write checkpoint 2000
3851 tar: Write checkpoint 3000
3854 This example shows the default checkpoint message used by
3855 @command{tar}. If you place a dot immediately after the equal
3856 sign, it will print a @samp{.} at each checkpoint@footnote{This is
3857 actually a shortcut for @option{--checkpoint=@var{n}
3858 --checkpoint-action=dot}. @xref{checkpoints, dot}.}. For example:
3861 $ @kbd{tar -c --checkpoint=.1000} /var
3865 The @option{--checkpoint} option provides a flexible mechanism for
3866 executing arbitrary actions upon hitting checkpoints, see the next
3867 section (@pxref{checkpoints}), for more information on it.
3869 @opindex show-omitted-dirs
3870 @anchor{show-omitted-dirs}
3871 The @option{--show-omitted-dirs} option, when reading an archive---with
3872 @option{--list} or @option{--extract}, for example---causes a message
3873 to be printed for each directory in the archive which is skipped.
3874 This happens regardless of the reason for skipping: the directory might
3875 not have been named on the command line (implicitly or explicitly),
3876 it might be excluded by the use of the
3877 @option{--exclude=@var{pattern}} option, or some other reason.
3879 @opindex block-number
3880 @cindex Block number where error occurred
3881 @anchor{block-number}
3882 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3883 every message it would normally produce, the block number within the
3884 archive where the message was triggered. Also, supplementary messages
3885 are triggered when reading blocks full of NULs, or when hitting end of
3886 file on the archive. As of now, if the archive is properly terminated
3887 with a NUL block, the reading of the file may stop before end of file
3888 is met, so the position of end of file will not usually show when
3889 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3890 drains the archive before exiting when reading the
3891 archive from a pipe.
3893 @cindex Error message, block number of
3894 This option is especially useful when reading damaged archives, since
3895 it helps pinpoint the damaged sections. It can also be used with
3896 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3897 choose among several backup tapes when retrieving a file later, in
3898 favor of the tape where the file appears earliest (closest to the
3899 front of the tape). @xref{backup}.
3902 @section Checkpoints
3903 @cindex checkpoints, defined
3905 @opindex checkpoint-action
3907 A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
3908 the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
3909 from the archive (a @dfn{read checkpoint}). Checkpoints allow to
3910 periodically execute arbitrary actions.
3912 The checkpoint facility is enabled using the following option:
3915 @xopindex{checkpoint, defined}
3916 @item --checkpoint[=@var{n}]
3917 Schedule checkpoints before writing or reading each @var{n}th record.
3918 The default value for @var{n} is 10.
3921 A list of arbitrary @dfn{actions} can be executed at each checkpoint.
3922 These actions include: pausing, displaying textual messages, and
3923 executing arbitrary external programs. Actions are defined using
3924 the @option{--checkpoint-action} option.
3927 @xopindex{checkpoint-action, defined}
3928 @item --checkpoint-action=@var{action}
3929 Execute an @var{action} at each checkpoint.
3932 @cindex @code{echo}, checkpoint action
3933 The simplest value of @var{action} is @samp{echo}. It instructs
3934 @command{tar} to display the default message on the standard error
3935 stream upon arriving at each checkpoint. The default message is (in
3936 @acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
3937 checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
3938 Here, @var{n} represents ordinal number of the checkpoint.
3940 In another locales, translated versions of this message are used.
3942 This is the default action, so running:
3945 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
3952 $ @kbd{tar -c --checkpoint=1000} /var
3955 The @samp{echo} action also allows to supply a customized message.
3956 You do so by placing an equals sign and the message right after it,
3960 --checkpoint-action="echo=Hit %s checkpoint #%u"
3963 The @samp{%s} and @samp{%u} in the above example are
3964 @dfn{format specifiers}. The @samp{%s} specifier is replaced with
3965 the @dfn{type} of the checkpoint: @samp{write} or
3966 @samp{read} (or a corresponding translated version in locales other
3967 than @acronym{POSIX}). The @samp{%u} specifier is replaced with
3968 the ordinal number of the checkpoint. Thus, the above example could
3969 produce the following output when used with the @option{--create}
3973 tar: Hit write checkpoint #10
3974 tar: Hit write checkpoint #20
3975 tar: Hit write checkpoint #30
3978 The complete list of available format specifiers follows. Some of
3979 them can take optional arguments. These arguments, if given, are
3980 supplied in curly braces between the percent sign and the specifier
3985 Print type of the checkpoint (@samp{write} or @samp{read}).
3988 Print number of the checkpoint.
3991 Print number of bytes transferred so far and approximate transfer
3992 speed. Optional arguments supply prefixes to be used before number
3993 of bytes read, written and deleted, correspondingly. If absent,
3994 they default to @samp{R}. @samp{W}, @samp{D}. Any or all of them can
3995 be omitted, so, that e.g. @samp{%@{@}T} means to print corresponding
3996 statistics without any prefixes. Any surplus arguments, if present,
3997 are silently ignored.
4000 $ @kbd{tar --delete -f f.tar --checkpoint-action=echo="#%u: %T" main.c}
4001 tar: #1: R: 0 (0B, 0B/s),W: 0 (0B, 0B/s),D: 0
4002 tar: #2: R: 10240 (10KiB, 19MiB/s),W: 0 (0B, 0B/s),D: 10240
4006 See also the @samp{totals} action, described below.
4008 @item %@{@var{fmt}@}t
4009 Output current local time using @var{fmt} as format for @command{strftime}
4010 (@pxref{strftime, strftime,,strftime(3), strftime(3) man page}). The
4011 @samp{@{@var{fmt}@}} part is optional. If not present, the default
4012 format is @samp{%c}, i.e. the preferred date and time representation
4013 for the current locale.
4016 Pad output with spaces to the @var{n}th column. If the
4017 @samp{@{@var{n}@}} part is omitted, the current screen width
4021 This is a shortcut for @samp{%@{%Y-%m-%d %H:%M:%S@}t: %ds, %@{read,wrote@}T%*\r},
4022 intended mainly for use with @samp{ttyout} action (see below).
4025 Aside from format expansion, the message string is subject to
4026 @dfn{unquoting}, during which the backslash @dfn{escape sequences} are
4027 replaced with their corresponding @acronym{ASCII} characters
4028 (@pxref{escape sequences}). E.g. the following action will produce an
4029 audible bell and the message described above at each checkpoint:
4032 --checkpoint-action='echo=\aHit %s checkpoint #%u'
4035 @cindex @code{bell}, checkpoint action
4036 There is also a special action which produces an audible signal:
4037 @samp{bell}. It is not equivalent to @samp{echo='\a'}, because
4038 @samp{bell} sends the bell directly to the console (@file{/dev/tty}),
4039 whereas @samp{echo='\a'} sends it to the standard error.
4041 @cindex @code{ttyout}, checkpoint action
4042 The @samp{ttyout=@var{string}} action outputs @var{string} to
4043 @file{/dev/tty}, so it can be used even if the standard output is
4044 redirected elsewhere. The @var{string} is subject to the same
4045 modifications as with @samp{echo} action. In contrast to the latter,
4046 @samp{ttyout} does not prepend @command{tar} executable name to the
4047 string, nor does it output a newline after it. For example, the
4048 following action will print the checkpoint message at the same screen
4049 line, overwriting any previous message:
4052 --checkpoint-action="ttyout=Hit %s checkpoint #%u%*\r"
4056 Notice the use of @samp{%*} specifier to clear out any eventual
4057 remains of the prior output line. As as more complex example,
4061 --checkpoint-action=ttyout='%@{%Y-%m-%d %H:%M:%S@}t (%d sec): #%u, %T%*\r'
4065 This prints the current local time, number of seconds expired since
4066 tar was started, the checkpoint ordinal number, transferred bytes and
4067 average computed I/O speed.
4069 @cindex @code{dot}, checkpoint action
4070 Another available checkpoint action is @samp{dot} (or @samp{.}). It
4071 instructs @command{tar} to print a single dot on the standard listing
4075 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
4079 For compatibility with previous @GNUTAR{} versions, this action can
4080 be abbreviated by placing a dot in front of the checkpoint frequency,
4081 as shown in the previous section.
4083 @cindex @code{totals}, checkpoint action
4084 The @samp{totals} action prints the total number of bytes transferred
4085 so far. The format of the data is the same as for the
4086 @option{--totals} option (@pxref{totals}). See also @samp{%T} format
4087 specifier of the @samp{echo} or @samp{ttyout} action.
4089 @cindex @code{sleep}, checkpoint action
4090 Yet another action, @samp{sleep}, pauses @command{tar} for a specified
4091 amount of seconds. The following example will stop for 30 seconds at each
4095 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
4098 @anchor{checkpoint exec}
4099 @cindex @code{exec}, checkpoint action
4100 Finally, the @code{exec} action executes a given external command.
4104 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
4107 The supplied command can be any valid command invocation, with or
4108 without additional command line arguments. If it does contain
4109 arguments, don't forget to quote it to prevent it from being split by
4110 the shell. @xref{external, Running External Commands}, for more detail.
4112 The command gets a copy of @command{tar}'s environment plus the
4113 following variables:
4116 @vrindex TAR_VERSION, checkpoint script environment
4118 @GNUTAR{} version number.
4120 @vrindex TAR_ARCHIVE, checkpoint script environment
4122 The name of the archive @command{tar} is processing.
4124 @vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
4125 @item TAR_BLOCKING_FACTOR
4126 Current blocking factor (@pxref{Blocking}).
4128 @vrindex TAR_CHECKPOINT, checkpoint script environment
4129 @item TAR_CHECKPOINT
4130 Number of the checkpoint.
4132 @vrindex TAR_SUBCOMMAND, checkpoint script environment
4133 @item TAR_SUBCOMMAND
4134 A short option describing the operation @command{tar} is executing.
4135 @xref{Operations}, for a complete list of subcommand options.
4137 @vrindex TAR_FORMAT, checkpoint script environment
4139 Format of the archive being processed. @xref{Formats}, for a complete
4140 list of archive format names.
4143 These environment variables can also be passed as arguments to the
4144 command, provided that they are properly escaped, for example:
4147 @kbd{tar -c -f arc.tar \
4148 --checkpoint-action='exec=/sbin/cpoint $TAR_FILENAME'}
4152 Notice single quotes to prevent variable names from being expanded by
4153 the shell when invoking @command{tar}.
4155 Any number of actions can be defined, by supplying several
4156 @option{--checkpoint-action} options in the command line. For
4157 example, the command below displays two messages, pauses
4158 execution for 30 seconds and executes the @file{/sbin/cpoint} script:
4162 $ @kbd{tar -c -f arc.tar \
4163 --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
4164 --checkpoint-action='echo=Sleeping for 30 seconds' \
4165 --checkpoint-action='sleep=30' \
4166 --checkpoint-action='exec=/sbin/cpoint'}
4170 This example also illustrates the fact that
4171 @option{--checkpoint-action} can be used without
4172 @option{--checkpoint}. In this case, the default checkpoint frequency
4173 (at each 10th record) is assumed.
4176 @section Controlling Warning Messages
4178 Sometimes, while performing the requested task, @GNUTAR{} notices
4179 some conditions that are not exactly errors, but which the user
4180 should be aware of. When this happens, @command{tar} issues a
4181 @dfn{warning message} describing the condition. Warning messages
4182 are output to the standard error and they do not affect the exit
4183 code of @command{tar} command.
4185 @xopindex{warning, explained}
4186 @GNUTAR{} allows the user to suppress some or all of its warning
4190 @item --warning=@var{keyword}
4191 Control display of the warning messages identified by @var{keyword}.
4192 If @var{keyword} starts with the prefix @samp{no-}, such messages are
4193 suppressed. Otherwise, they are enabled.
4195 Multiple @option{--warning} messages accumulate.
4197 The tables below list allowed values for @var{keyword} along with the
4198 warning messages they control.
4201 @subheading Keywords controlling @command{tar} operation
4205 Enable all warning messages. This is the default.
4208 Disable all warning messages.
4209 @kwindex filename-with-nuls
4210 @cindex @samp{file name read contains nul character}, warning message
4211 @item filename-with-nuls
4212 @samp{%s: file name read contains nul character}
4213 @kwindex alone-zero-block
4214 @cindex @samp{A lone zero block at}, warning message
4215 @item alone-zero-block
4216 @samp{A lone zero block at %s}
4219 @subheading Keywords applicable for @command{tar --create}
4222 @cindex @samp{contains a cache directory tag}, warning message
4224 @samp{%s: contains a cache directory tag %s; %s}
4225 @kwindex file-shrank
4226 @cindex @samp{File shrank by %s bytes}, warning message
4228 @samp{%s: File shrank by %s bytes; padding with zeros}
4230 @cindex @samp{file is on a different filesystem}, warning message
4232 @samp{%s: file is on a different filesystem; not dumped}
4233 @kwindex file-ignored
4234 @cindex @samp{Unknown file type; file ignored}, warning message
4235 @cindex @samp{socket ignored}, warning message
4236 @cindex @samp{door ignored}, warning message
4238 @samp{%s: Unknown file type; file ignored}
4239 @*@samp{%s: socket ignored}
4240 @*@samp{%s: door ignored}
4241 @kwindex file-unchanged
4242 @cindex @samp{file is unchanged; not dumped}, warning message
4243 @item file-unchanged
4244 @samp{%s: file is unchanged; not dumped}
4245 @kwindex ignore-archive
4246 @cindex @samp{file is the archive; not dumped}, warning message
4247 @kwindex ignore-archive
4248 @cindex @samp{file is the archive; not dumped}, warning message
4249 @item ignore-archive
4250 @samp{%s: file is the archive; not dumped}
4251 @kwindex file-removed
4252 @cindex @samp{File removed before we read it}, warning message
4254 @samp{%s: File removed before we read it}
4255 @kwindex file-changed
4256 @cindex @samp{file changed as we read it}, warning message
4258 @samp{%s: file changed as we read it}
4261 @subheading Keywords applicable for @command{tar --extract}
4264 @cindex @samp{implausibly old time stamp %s}, warning message
4265 @cindex @samp{time stamp %s is %s s in the future}, warning message
4267 @samp{%s: implausibly old time stamp %s}
4268 @*@samp{%s: time stamp %s is %s s in the future}
4269 @kwindex contiguous-cast
4270 @cindex @samp{Extracting contiguous files as regular files}, warning message
4271 @item contiguous-cast
4272 @samp{Extracting contiguous files as regular files}
4273 @kwindex symlink-cast
4274 @cindex @samp{Attempting extraction of symbolic links as hard links}, warning message
4276 @samp{Attempting extraction of symbolic links as hard links}
4277 @kwindex unknown-cast
4278 @cindex @samp{Unknown file type '%c', extracted as normal file}, warning message
4280 @samp{%s: Unknown file type '%c', extracted as normal file}
4281 @kwindex ignore-newer
4282 @cindex @samp{Current %s is newer or same age}, warning message
4284 @samp{Current %s is newer or same age}
4285 @kwindex unknown-keyword
4286 @cindex @samp{Ignoring unknown extended header keyword '%s'}, warning message
4287 @item unknown-keyword
4288 @samp{Ignoring unknown extended header keyword '%s'}
4289 @kwindex decompress-program
4290 @item decompress-program
4291 Controls verbose description of failures occurring when trying to run
4292 alternative decompressor programs (@pxref{alternative decompression
4293 programs}). This warning is disabled by default (unless
4294 @option{--verbose} is used). A common example of what you can get
4295 when using this warning is:
4298 $ @kbd{tar --warning=decompress-program -x -f archive.Z}
4299 tar (child): cannot run compress: No such file or directory
4300 tar (child): trying gzip
4303 This means that @command{tar} first tried to decompress
4304 @file{archive.Z} using @command{compress}, and, when that
4305 failed, switched to @command{gzip}.
4306 @kwindex record-size
4307 @cindex @samp{Record size = %lu blocks}, warning message
4309 @samp{Record size = %lu blocks}
4312 @subheading Keywords controlling incremental extraction:
4314 @kwindex rename-directory
4315 @cindex @samp{%s: Directory has been renamed from %s}, warning message
4316 @cindex @samp{%s: Directory has been renamed}, warning message
4317 @item rename-directory
4318 @samp{%s: Directory has been renamed from %s}
4319 @*@samp{%s: Directory has been renamed}
4320 @kwindex new-directory
4321 @cindex @samp{%s: Directory is new}, warning message
4323 @samp{%s: Directory is new}
4325 @cindex @samp{%s: directory is on a different device: not purging}, warning message
4327 @samp{%s: directory is on a different device: not purging}
4328 @kwindex bad-dumpdir
4329 @cindex @samp{Malformed dumpdir: 'X' never used}, warning message
4331 @samp{Malformed dumpdir: 'X' never used}
4335 @section Asking for Confirmation During Operations
4336 @cindex Interactive operation
4338 Typically, @command{tar} carries out a command without stopping for
4339 further instructions. In some situations however, you may want to
4340 exclude some files and archive members from the operation (for instance
4341 if disk or storage space is tight). You can do this by excluding
4342 certain files automatically (@pxref{Choosing}), or by performing
4343 an operation interactively, using the @option{--interactive} (@option{-w}) option.
4344 @command{tar} also accepts @option{--confirmation} for this option.
4346 @opindex interactive
4347 When the @option{--interactive} (@option{-w}) option is specified, before
4348 reading, writing, or deleting files, @command{tar} first prints a message
4349 for each such file, telling what operation it intends to take, then asks
4350 for confirmation on the terminal. The actions which require
4351 confirmation include adding a file to the archive, extracting a file
4352 from the archive, deleting a file from the archive, and deleting a file
4353 from disk. To confirm the action, you must type a line of input
4354 beginning with @samp{y}. If your input line begins with anything other
4355 than @samp{y}, @command{tar} skips that file.
4357 If @command{tar} is reading the archive from the standard input,
4358 @command{tar} opens the file @file{/dev/tty} to support the interactive
4361 Verbose output is normally sent to standard output, separate from
4362 other error messages. However, if the archive is produced directly
4363 on standard output, then verbose output is mixed with errors on
4364 @code{stderr}. Producing the archive on standard output may be used
4365 as a way to avoid using disk space, when the archive is soon to be
4366 consumed by another process reading it, say. Some people felt the need
4367 of producing an archive on stdout, still willing to segregate between
4368 verbose output and error output. A possible approach would be using a
4369 named pipe to receive the archive, and having the consumer process to
4370 read from that named pipe. This has the advantage of letting standard
4371 output free to receive verbose output, all separate from errors.
4374 @section Running External Commands
4376 Certain @GNUTAR{} operations imply running external commands that you
4377 supply on the command line. One of such operations is checkpointing,
4378 described above (@pxref{checkpoint exec}). Another example of this
4379 feature is the @option{-I} option, which allows you to supply the
4380 program to use for compressing or decompressing the archive
4381 (@pxref{use-compress-program}).
4383 Whenever such operation is requested, @command{tar} first splits the
4384 supplied command into words much like the shell does. It then treats
4385 the first word as the name of the program or the shell script to execute
4386 and the rest of words as its command line arguments. The program,
4387 unless given as an absolute file name, is searched in the shell's
4390 Any additional information is normally supplied to external commands
4391 in environment variables, specific to each particular operation. For
4392 example, the @option{--checkpoint-action=exec} option, defines the
4393 @env{TAR_ARCHIVE} variable to the name of the archive being worked
4394 upon. You can, should the need be, use these variables in the
4395 command line of the external command. For example:
4398 $ @kbd{tar -x -f archive.tar \
4399 --checkpoint=exec='printf "%04d in %32s\r" $TAR_CHECKPOINT $TAR_ARCHIVE'}
4403 This command prints for each checkpoint its number and the name of the
4404 archive, using the same output line on the screen.
4406 Notice the use of single quotes to prevent variable names from being
4407 expanded by the shell when invoking @command{tar}.
4410 @chapter @GNUTAR{} Operations
4423 @section Basic @GNUTAR{} Operations
4425 The basic @command{tar} operations, @option{--create} (@option{-c}),
4426 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
4427 @option{-x}), are currently presented and described in the tutorial
4428 chapter of this manual. This section provides some complementary notes
4429 for these operations.
4432 @xopindex{create, complementary notes}
4436 Creating an empty archive would have some kind of elegance. One can
4437 initialize an empty archive and later use @option{--append}
4438 (@option{-r}) for adding all members. Some applications would not
4439 welcome making an exception in the way of adding the first archive
4440 member. On the other hand, many people reported that it is
4441 dangerously too easy for @command{tar} to destroy a magnetic tape with
4442 an empty archive@footnote{This is well described in @cite{Unix-haters
4443 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
4444 Books, ISBN 1-56884-203-1.}. The two most common errors are:
4448 Mistakingly using @code{create} instead of @code{extract}, when the
4449 intent was to extract the full contents of an archive. This error
4450 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
4451 the QWERTY keyboard. Instead of being unpacked, the archive then
4452 gets wholly destroyed. When users speak about @dfn{exploding} an
4453 archive, they usually mean something else :-).
4456 Forgetting the argument to @code{file}, when the intent was to create
4457 an archive with a single file in it. This error is likely because a
4458 tired user can easily add the @kbd{f} key to the cluster of option
4459 letters, by the mere force of habit, without realizing the full
4460 consequence of doing so. The usual consequence is that the single
4461 file, which was meant to be saved, is rather destroyed.
4464 So, recognizing the likelihood and the catastrophic nature of these
4465 errors, @GNUTAR{} now takes some distance from elegance, and
4466 cowardly refuses to create an archive when @option{--create} option is
4467 given, there are no arguments besides options, and
4468 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
4469 around the cautiousness of @GNUTAR{} and nevertheless create an
4470 archive with nothing in it, one may still use, as the value for the
4471 @option{--files-from} option, a file with no names in it, as shown in
4472 the following commands:
4475 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
4476 @kbd{tar -cf empty-archive.tar -T /dev/null}
4479 @xopindex{extract, complementary notes}
4484 A socket is stored, within a @GNUTAR{} archive, as a pipe.
4486 @item @option{--list} (@option{-t})
4488 @GNUTAR{} now shows dates as @samp{1996-08-30},
4489 while it used to show them as @samp{Aug 30 1996}. Preferably,
4490 people should get used to ISO 8601 dates. Local American dates should
4491 be made available again with full date localization support, once
4492 ready. In the meantime, programs not being localizable for dates
4493 should prefer international dates, that's really the way to go.
4495 Look up @url{http://www.cl.cam.ac.uk/@/~mgk25/@/iso-time.html} if you
4496 are curious, it contains a detailed explanation of the ISO 8601 standard.
4501 @section Advanced @GNUTAR{} Operations
4503 Now that you have learned the basics of using @GNUTAR{}, you may want
4504 to learn about further ways in which @command{tar} can help you.
4506 This chapter presents five, more advanced operations which you probably
4507 won't use on a daily basis, but which serve more specialized functions.
4508 We also explain the different styles of options and why you might want
4509 to use one or another, or a combination of them in your @command{tar}
4510 commands. Additionally, this chapter includes options which allow you to
4511 define the output from @command{tar} more carefully, and provide help and
4512 error correction in special circumstances.
4514 @FIXME{check this after the chapter is actually revised to make sure
4515 it still introduces the info in the chapter correctly : ).}
4527 @subsection The Five Advanced @command{tar} Operations
4529 @cindex basic operations
4530 In the last chapter, you learned about the first three operations to
4531 @command{tar}. This chapter presents the remaining five operations to
4532 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
4533 @option{--delete}, and @option{--compare}.
4535 You are not likely to use these operations as frequently as those
4536 covered in the last chapter; however, since they perform specialized
4537 functions, they are quite useful when you do need to use them. We
4538 will give examples using the same directory and files that you created
4539 in the last chapter. As you may recall, the directory is called
4540 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
4541 and the two archive files you created are
4542 @samp{collection.tar} and @samp{music.tar}.
4544 We will also use the archive files @samp{afiles.tar} and
4545 @samp{bfiles.tar}. The archive @samp{afiles.tar} contains the members @samp{apple},
4546 @samp{angst}, and @samp{aspic}; @samp{bfiles.tar} contains the members
4547 @samp{./birds}, @samp{baboon}, and @samp{./box}.
4549 Unless we state otherwise, all practicing you do and examples you follow
4550 in this chapter will take place in the @file{practice} directory that
4551 you created in the previous chapter; see @ref{prepare for examples}.
4552 (Below in this section, we will remind you of the state of the examples
4553 where the last chapter left them.)
4555 The five operations that we will cover in this chapter are:
4560 Add new entries to an archive that already exists.
4563 Add more recent copies of archive members to the end of an archive, if
4568 Add one or more pre-existing archives to the end of another archive.
4570 Delete items from an archive (does not work on tapes).
4574 Compare archive members to their counterparts in the file system.
4578 @subsection How to Add Files to Existing Archives: @option{--append}
4580 @cindex appending files to existing archive
4582 If you want to add files to an existing archive, you don't need to
4583 create a new archive; you can use @option{--append} (@option{-r}).
4584 The archive must already exist in order to use @option{--append}. (A
4585 related operation is the @option{--update} operation; you can use this
4586 to add newer versions of archive members to an existing archive. To learn how to
4587 do this with @option{--update}, @pxref{update}.)
4589 If you use @option{--append} to add a file that has the same name as an
4590 archive member to an archive containing that archive member, then the
4591 old member is not deleted. What does happen, however, is somewhat
4592 complex. @command{tar} @emph{allows} you to have infinite number of files
4593 with the same name. Some operations treat these same-named members no
4594 differently than any other set of archive members: for example, if you
4595 view an archive with @option{--list} (@option{-t}), you will see all
4596 of those members listed, with their data modification times, owners, etc.
4598 Other operations don't deal with these members as perfectly as you might
4599 prefer; if you were to use @option{--extract} to extract the archive,
4600 only the most recently added copy of a member with the same name as
4601 other members would end up in the working directory. This is because
4602 @option{--extract} extracts an archive in the order the members appeared
4603 in the archive; the most recently archived members will be extracted
4604 last. Additionally, an extracted member will @emph{replace} a file of
4605 the same name which existed in the directory already, and @command{tar}
4606 will not prompt you about this@footnote{Unless you give it
4607 @option{--keep-old-files} (or @option{--skip-old-files}) option, or
4608 the disk copy is newer than the one in the archive and you invoke
4609 @command{tar} with @option{--keep-newer-files} option.}. Thus, only
4610 the most recently archived member will end up being extracted, as it
4611 will replace the one extracted before it, and so on.
4613 @cindex extracting @var{n}th copy of the file
4614 @xopindex{occurrence, described}
4615 There exists a special option that allows you to get around this
4616 behavior and extract (or list) only a particular copy of the file.
4617 This is @option{--occurrence} option. If you run @command{tar} with
4618 this option, it will extract only the first copy of the file. You
4619 may also give this option an argument specifying the number of
4620 copy to be extracted. Thus, for example if the archive
4621 @file{archive.tar} contained three copies of file @file{myfile}, then
4625 tar --extract --file archive.tar --occurrence=2 myfile
4629 would extract only the second copy. @xref{Option
4630 Summary,---occurrence}, for the description of @option{--occurrence}
4633 @FIXME{ hag -- you might want to incorporate some of the above into the
4634 MMwtSN node; not sure. i didn't know how to make it simpler...
4636 There are a few ways to get around this. Xref to Multiple Members
4637 with the Same Name, maybe.}
4639 @cindex Members, replacing with other members
4640 @cindex Replacing members with other members
4641 @xopindex{delete, using before --append}
4642 If you want to replace an archive member, use @option{--delete} to
4643 delete the member you want to remove from the archive, and then use
4644 @option{--append} to add the member you want to be in the archive. Note
4645 that you can not change the order of the archive; the most recently
4646 added member will still appear last. In this sense, you cannot truly
4647 ``replace'' one member with another. (Replacing one member with another
4648 will not work on certain types of media, such as tapes; see @ref{delete}
4649 and @ref{Media}, for more information.)
4652 * appending files:: Appending Files to an Archive
4656 @node appending files
4657 @subsubsection Appending Files to an Archive
4658 @cindex Adding files to an Archive
4659 @cindex Appending files to an Archive
4660 @cindex Archives, Appending files to
4663 The simplest way to add a file to an already existing archive is the
4664 @option{--append} (@option{-r}) operation, which writes specified
4665 files into the archive whether or not they are already among the
4668 When you use @option{--append}, you @emph{must} specify file name
4669 arguments, as there is no default. If you specify a file that already
4670 exists in the archive, another copy of the file will be added to the
4671 end of the archive. As with other operations, the member names of the
4672 newly added files will be exactly the same as their names given on the
4673 command line. The @option{--verbose} (@option{-v}) option will print
4674 out the names of the files as they are written into the archive.
4676 @option{--append} cannot be performed on some tape drives, unfortunately,
4677 due to deficiencies in the formats those tape drives use. The archive
4678 must be a valid @command{tar} archive, or else the results of using this
4679 operation will be unpredictable. @xref{Media}.
4681 To demonstrate using @option{--append} to add a file to an archive,
4682 create a file called @file{rock} in the @file{practice} directory.
4683 Make sure you are in the @file{practice} directory. Then, run the
4684 following @command{tar} command to add @file{rock} to
4685 @file{collection.tar}:
4688 $ @kbd{tar --append --file=collection.tar rock}
4692 If you now use the @option{--list} (@option{-t}) operation, you will see that
4693 @file{rock} has been added to the archive:
4696 $ @kbd{tar --list --file=collection.tar}
4697 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4698 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4699 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4700 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4704 @subsubsection Multiple Members with the Same Name
4705 @cindex members, multiple
4706 @cindex multiple members
4708 You can use @option{--append} (@option{-r}) to add copies of files
4709 which have been updated since the archive was created. (However, we
4710 do not recommend doing this since there is another @command{tar}
4711 option called @option{--update}; @xref{update}, for more information.
4712 We describe this use of @option{--append} here for the sake of
4713 completeness.) When you extract the archive, the older version will
4714 be effectively lost. This works because files are extracted from an
4715 archive in the order in which they were archived. Thus, when the
4716 archive is extracted, a file archived later in time will replace a
4717 file of the same name which was archived earlier, even though the
4718 older version of the file will remain in the archive unless you delete
4719 all versions of the file.
4721 Supposing you change the file @file{blues} and then append the changed
4722 version to @file{collection.tar}. As you saw above, the original
4723 @file{blues} is in the archive @file{collection.tar}. If you change the
4724 file and append the new version of the file to the archive, there will
4725 be two copies in the archive. When you extract the archive, the older
4726 version of the file will be extracted first, and then replaced by the
4727 newer version when it is extracted.
4729 You can append the new, changed copy of the file @file{blues} to the
4730 archive in this way:
4733 $ @kbd{tar --append --verbose --file=collection.tar blues}
4738 Because you specified the @option{--verbose} option, @command{tar} has
4739 printed the name of the file being appended as it was acted on. Now
4740 list the contents of the archive:
4743 $ @kbd{tar --list --verbose --file=collection.tar}
4744 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4745 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4746 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4747 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4748 -rw-r--r-- me/user 58 1996-10-24 18:30 blues
4752 The newest version of @file{blues} is now at the end of the archive
4753 (note the different creation dates and file sizes). If you extract
4754 the archive, the older version of the file @file{blues} will be
4755 replaced by the newer version. You can confirm this by extracting
4756 the archive and running @samp{ls} on the directory.
4758 If you wish to extract the first occurrence of the file @file{blues}
4759 from the archive, use @option{--occurrence} option, as shown in
4760 the following example:
4763 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
4764 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4767 @xref{Writing}, for more information on @option{--extract} and
4768 see @ref{Option Summary, --occurrence}, for a description of
4769 @option{--occurrence} option.
4772 @subsection Updating an Archive
4773 @cindex Updating an archive
4776 In the previous section, you learned how to use @option{--append} to
4777 add a file to an existing archive. A related operation is
4778 @option{--update} (@option{-u}). The @option{--update} operation
4779 updates a @command{tar} archive by comparing the date of the specified
4780 archive members against the date of the file with the same name. If
4781 the file has been modified more recently than the archive member, then
4782 the newer version of the file is added to the archive (as with
4785 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
4786 The operation will fail.
4788 @FIXME{other examples of media on which --update will fail? need to ask
4789 charles and/or mib/thomas/dave shevett..}
4791 Both @option{--update} and @option{--append} work by adding to the end
4792 of the archive. When you extract a file from the archive, only the
4793 version stored last will wind up in the file system, unless you use
4794 the @option{--backup} option. @xref{multiple}, for a detailed discussion.
4801 @subsubsection How to Update an Archive Using @option{--update}
4804 You must use file name arguments with the @option{--update}
4805 (@option{-u}) operation. If you don't specify any files,
4806 @command{tar} won't act on any files and won't tell you that it didn't
4807 do anything (which may end up confusing you).
4809 @c note: the above parenthetical added because in fact, this
4810 @c behavior just confused the author. :-)
4812 To see the @option{--update} option at work, create a new file,
4813 @file{classical}, in your practice directory, and some extra text to the
4814 file @file{blues}, using any text editor. Then invoke @command{tar} with
4815 the @samp{update} operation and the @option{--verbose} (@option{-v})
4816 option specified, using the names of all the files in the @file{practice}
4817 directory as file name arguments:
4820 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4827 Because we have specified verbose mode, @command{tar} prints out the names
4828 of the files it is working on, which in this case are the names of the
4829 files that needed to be updated. If you run @samp{tar --list} and look
4830 at the archive, you will see @file{blues} and @file{classical} at its
4831 end. There will be a total of two versions of the member @samp{blues};
4832 the one at the end will be newer and larger, since you added text before
4835 The reason @command{tar} does not overwrite the older file when updating
4836 it is because writing to the middle of a section of tape is a difficult
4837 process. Tapes are not designed to go backward. @xref{Media}, for more
4838 information about tapes.
4840 @option{--update} (@option{-u}) is not suitable for performing backups for two
4841 reasons: it does not change directory content entries, and it
4842 lengthens the archive every time it is used. The @GNUTAR{}
4843 options intended specifically for backups are more
4844 efficient. If you need to run backups, please consult @ref{Backups}.
4847 @subsection Combining Archives with @option{--concatenate}
4849 @cindex Adding archives to an archive
4850 @cindex Concatenating Archives
4851 @opindex concatenate
4853 @c @cindex @option{-A} described
4854 Sometimes it may be convenient to add a second archive onto the end of
4855 an archive rather than adding individual files to the archive. To add
4856 one or more archives to the end of another archive, you should use the
4857 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4859 To use @option{--concatenate}, give the first archive with
4860 @option{--file} option and name the rest of archives to be
4861 concatenated on the command line. The members, and their member
4862 names, will be copied verbatim from those archives to the first
4863 one@footnote{This can cause multiple members to have the same name. For
4864 information on how this affects reading the archive, see @ref{multiple}.}.
4865 The new, concatenated archive will be called by the same name as the
4866 one given with the @option{--file} option. As usual, if you omit
4867 @option{--file}, @command{tar} will use the value of the environment
4868 variable @env{TAPE}, or, if this has not been set, the default archive name.
4870 @FIXME{There is no way to specify a new name...}
4872 To demonstrate how @option{--concatenate} works, create two small archives
4873 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4874 files from @file{practice}:
4877 $ @kbd{tar -cvf bluesrock.tar blues rock}
4880 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4886 If you like, You can run @samp{tar --list} to make sure the archives
4887 contain what they are supposed to:
4890 $ @kbd{tar -tvf bluesrock.tar}
4891 -rw-r--r-- melissa/user 105 1997-01-21 19:42 blues
4892 -rw-r--r-- melissa/user 33 1997-01-20 15:34 rock
4893 $ @kbd{tar -tvf jazzfolk.tar}
4894 -rw-r--r-- melissa/user 20 1996-09-23 16:44 folk
4895 -rw-r--r-- melissa/user 65 1997-01-30 14:15 jazz
4898 We can concatenate these two archives with @command{tar}:
4902 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4905 If you now list the contents of the @file{bluesrock.tar}, you will see
4906 that now it also contains the archive members of @file{jazzfolk.tar}:
4909 $ @kbd{tar --list --file=bluesrock.tar}
4916 When you use @option{--concatenate}, the source and target archives must
4917 already exist and must have been created using compatible format
4918 parameters. Notice, that @command{tar} does not check whether the
4919 archives it concatenates have compatible formats, it does not
4920 even check if the files are really tar archives.
4922 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4923 tape drives, due to deficiencies in the formats those tape drives use.
4925 @cindex @code{concatenate} vs @command{cat}
4926 @cindex @command{cat} vs @code{concatenate}
4927 It may seem more intuitive to you to want or try to use @command{cat} to
4928 concatenate two archives instead of using the @option{--concatenate}
4929 operation; after all, @command{cat} is the utility for combining files.
4931 However, @command{tar} archives incorporate an end-of-file marker which
4932 must be removed if the concatenated archives are to be read properly as
4933 one archive. @option{--concatenate} removes the end-of-archive marker
4934 from the target archive before each new archive is appended. If you use
4935 @command{cat} to combine the archives, the result will not be a valid
4936 @command{tar} format archive. If you need to retrieve files from an
4937 archive that was added to using the @command{cat} utility, use the
4938 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4939 information on dealing with archives improperly combined using the
4940 @command{cat} shell utility.
4943 @subsection Removing Archive Members Using @option{--delete}
4944 @cindex Deleting files from an archive
4945 @cindex Removing files from an archive
4948 You can remove members from an archive by using the @option{--delete}
4949 option. Specify the name of the archive with @option{--file}
4950 (@option{-f}) and then specify the names of the members to be deleted;
4951 if you list no member names, nothing will be deleted. The
4952 @option{--verbose} option will cause @command{tar} to print the names
4953 of the members as they are deleted. As with @option{--extract}, you
4954 must give the exact member names when using @samp{tar --delete}.
4955 @option{--delete} will remove all versions of the named file from the
4956 archive. The @option{--delete} operation can run very slowly.
4958 Unlike other operations, @option{--delete} has no short form.
4960 @cindex Tapes, using @option{--delete} and
4961 @cindex Deleting from tape archives
4962 This operation will rewrite the archive. You can only use
4963 @option{--delete} on an archive if the archive device allows you to
4964 write to any point on the media, such as a disk; because of this, it
4965 does not work on magnetic tapes. Do not try to delete an archive member
4966 from a magnetic tape; the action will not succeed, and you will be
4967 likely to scramble the archive and damage your tape. There is no safe
4968 way (except by completely re-writing the archive) to delete files from
4969 most kinds of magnetic tape. @xref{Media}.
4971 To delete all versions of the file @file{blues} from the archive
4972 @file{collection.tar} in the @file{practice} directory, make sure you
4973 are in that directory, and then,
4976 $ @kbd{tar --list --file=collection.tar}
4981 $ @kbd{tar --delete --file=collection.tar blues}
4982 $ @kbd{tar --list --file=collection.tar}
4988 @FIXME{Check if the above listing is actually produced after running
4989 all the examples on collection.tar.}
4991 The @option{--delete} option has been reported to work properly when
4992 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4995 @subsection Comparing Archive Members with the File System
4996 @cindex Verifying the currency of an archive
4999 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
5000 specified archive members against files with the same names, and then
5001 reports differences in file size, mode, owner, modification date and
5002 contents. You should @emph{only} specify archive member names, not file
5003 names. If you do not name any members, then @command{tar} will compare the
5004 entire archive. If a file is represented in the archive but does not
5005 exist in the file system, @command{tar} reports a difference.
5007 You have to specify the record size of the archive when modifying an
5008 archive with a non-default record size.
5010 @command{tar} ignores files in the file system that do not have
5011 corresponding members in the archive.
5013 The following example compares the archive members @file{rock},
5014 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
5015 files of the same name in the file system. (Note that there is no file,
5016 @file{funk}; @command{tar} will report an error message.)
5019 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
5022 tar: funk not found in archive
5025 The spirit behind the @option{--compare} (@option{--diff},
5026 @option{-d}) option is to check whether the archive represents the
5027 current state of files on disk, more than validating the integrity of
5028 the archive media. For this latter goal, see @ref{verify}.
5030 @node create options
5031 @section Options Used by @option{--create}
5033 @xopindex{create, additional options}
5034 The previous chapter described the basics of how to use
5035 @option{--create} (@option{-c}) to create an archive from a set of files.
5036 @xref{create}. This section described advanced options to be used with
5040 * override:: Overriding File Metadata.
5041 * Ignore Failed Read::
5045 @subsection Overriding File Metadata
5047 As described above, a @command{tar} archive keeps, for each member it contains,
5048 its @dfn{metadata}, such as modification time, mode and ownership of
5049 the file. @GNUTAR{} allows to replace these data with other values
5050 when adding files to the archive. The options described in this
5051 section affect creation of archives of any type. For POSIX archives,
5052 see also @ref{PAX keywords}, for additional ways of controlling
5053 metadata, stored in the archive.
5057 @item --mode=@var{permissions}
5059 When adding files to an archive, @command{tar} will use
5060 @var{permissions} for the archive members, rather than the permissions
5061 from the files. @var{permissions} can be specified either as an octal
5062 number or as symbolic permissions, like with
5063 @command{chmod} (@xref{File permissions, Permissions, File
5064 permissions, fileutils, @acronym{GNU} file utilities}. This reference
5065 also has useful information for those not being overly familiar with
5066 the UNIX permission system). Using latter syntax allows for
5067 more flexibility. For example, the value @samp{a+rw} adds read and write
5068 permissions for everybody, while retaining executable bits on directories
5069 or on any other file already marked as executable:
5072 $ @kbd{tar -c -f archive.tar --mode='a+rw' .}
5075 @item --mtime=@var{date}
5078 When adding files to an archive, @command{tar} will use @var{date} as
5079 the modification time of members when creating archives, instead of
5080 their actual modification times. The argument @var{date} can be
5081 either a textual date representation in almost arbitrary format
5082 (@pxref{Date input formats}) or a name of an existing file, starting
5083 with @samp{/} or @samp{.}. In the latter case, the modification time
5084 of that file will be used.
5086 The following example will set the modification date to 00:00:00,
5090 $ @kbd{tar -c -f archive.tar --mtime='1970-01-01' .}
5094 When used with @option{--verbose} (@pxref{verbose tutorial}) @GNUTAR{}
5095 will try to convert the specified date back to its textual
5096 representation and compare it with the one given with
5097 @option{--mtime} options. If the two dates differ, @command{tar} will
5098 print a warning saying what date it will use. This is to help user
5099 ensure he is using the right date.
5104 $ @kbd{tar -c -f archive.tar -v --mtime=yesterday .}
5105 tar: Option --mtime: Treating date 'yesterday' as 2006-06-20
5110 @item --owner=@var{user}
5113 Specifies that @command{tar} should use @var{user} as the owner of members
5114 when creating archives, instead of the user associated with the source
5117 If @var{user} contains a colon, it is taken to be of the form
5118 @var{name}:@var{id} where a nonempty @var{name} specifies the user
5119 name and a nonempty @var{id} specifies the decimal numeric user
5120 @acronym{ID}. If @var{user} does not contain a colon, it is taken to
5121 be a user number if it is one or more decimal digits; otherwise it is
5122 taken to be a user name.
5124 If a name is given but no number, the number is inferred from the
5125 current host's user database if possible, and the file's user number
5126 is used otherwise. If a number is given but no name, the name is
5127 inferred from the number if possible, and an empty name is used
5128 otherwise. If both name and number are given, the user database is
5129 not consulted, and the name and number need not be valid on the
5132 There is no value indicating a missing number, and @samp{0} usually means
5133 @code{root}. Some people like to force @samp{0} as the value to offer in
5134 their distributions for the owner of files, because the @code{root} user is
5135 anonymous anyway, so that might as well be the owner of anonymous
5136 archives. For example:
5139 $ @kbd{tar -c -f archive.tar --owner=0 .}
5146 $ @kbd{tar -c -f archive.tar --owner=root .}
5149 @item --group=@var{group}
5152 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
5153 rather than the group from the source file. As with @option{--owner},
5154 the argument @var{group} can be an existing group symbolic name, or a
5155 decimal numeric group @acronym{ID}, or @var{name}:@var{id}.
5158 @node Ignore Failed Read
5159 @subsection Ignore Fail Read
5162 @item --ignore-failed-read
5163 @opindex ignore-failed-read
5164 Do not exit with nonzero on unreadable files or directories.
5167 @node extract options
5168 @section Options Used by @option{--extract}
5169 @cindex options for use with @option{--extract}
5171 @xopindex{extract, additional options}
5172 The previous chapter showed how to use @option{--extract} to extract
5173 an archive into the file system. Various options cause @command{tar} to
5174 extract more information than just file contents, such as the owner,
5175 the permissions, the modification date, and so forth. This section
5176 presents options to be used with @option{--extract} when certain special
5177 considerations arise. You may review the information presented in
5178 @ref{extract} for more basic information about the
5179 @option{--extract} operation.
5182 * Reading:: Options to Help Read Archives
5183 * Writing:: Changing How @command{tar} Writes Files
5184 * Scarce:: Coping with Scarce Resources
5188 @subsection Options to Help Read Archives
5189 @cindex Options when reading archives
5191 @cindex Reading incomplete records
5192 @cindex Records, incomplete
5193 @opindex read-full-records
5194 Normally, @command{tar} will request data in full record increments from
5195 an archive storage device. If the device cannot return a full record,
5196 @command{tar} will report an error. However, some devices do not always
5197 return full records, or do not require the last record of an archive to
5198 be padded out to the next record boundary. To keep reading until you
5199 obtain a full record, or to accept an incomplete record if it contains
5200 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
5201 in conjunction with the @option{--extract} or @option{--list} operations.
5204 The @option{--read-full-records} (@option{-B}) option is turned on by default when
5205 @command{tar} reads an archive from standard input, or from a remote
5206 machine. This is because on @acronym{BSD} Unix systems, attempting to read a
5207 pipe returns however much happens to be in the pipe, even if it is
5208 less than was requested. If this option were not enabled, @command{tar}
5209 would fail as soon as it read an incomplete record from the pipe.
5211 If you're not sure of the blocking factor of an archive, you can
5212 read the archive by specifying @option{--read-full-records} (@option{-B}) and
5213 @option{--blocking-factor=@var{512-size}} (@option{-b
5214 @var{512-size}}), using a blocking factor larger than what the archive
5215 uses. This lets you avoid having to determine the blocking factor
5216 of an archive. @xref{Blocking Factor}.
5219 * read full records::
5223 @node read full records
5224 @unnumberedsubsubsec Reading Full Records
5226 @FIXME{need sentence or so of intro here}
5229 @opindex read-full-records
5230 @item --read-full-records
5232 Use in conjunction with @option{--extract} (@option{--get},
5233 @option{-x}) to read an archive which contains incomplete records, or
5234 one which has a blocking factor less than the one specified.
5238 @unnumberedsubsubsec Ignoring Blocks of Zeros
5240 @cindex End-of-archive blocks, ignoring
5241 @cindex Ignoring end-of-archive blocks
5242 @opindex ignore-zeros
5243 Normally, @command{tar} stops reading when it encounters a block of zeros
5244 between file entries (which usually indicates the end of the archive).
5245 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
5246 completely read an archive which contains a block of zeros before the
5247 end (i.e., a damaged archive, or one that was created by concatenating
5248 several archives together).
5250 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
5251 versions of @command{tar} write garbage after the end-of-archive entry,
5252 since that part of the media is never supposed to be read. @GNUTAR{}
5253 does not write after the end of an archive, but seeks to
5254 maintain compatibility among archiving utilities.
5257 @item --ignore-zeros
5259 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
5260 encountered while reading an archive. Use in conjunction with
5261 @option{--extract} or @option{--list}.
5265 @subsection Changing How @command{tar} Writes Files
5268 @FIXME{Introductory paragraph}
5271 * Dealing with Old Files::
5272 * Overwrite Old Files::
5274 * Keep Newer Files::
5276 * Recursive Unlink::
5277 * Data Modification Times::
5278 * Setting Access Permissions::
5279 * Directory Modification Times and Permissions::
5280 * Writing to Standard Output::
5281 * Writing to an External Program::
5285 @node Dealing with Old Files
5286 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
5288 @xopindex{overwrite-dir, introduced}
5289 When extracting files, if @command{tar} discovers that the extracted
5290 file already exists, it normally replaces the file by removing it before
5291 extracting it, to prevent confusion in the presence of hard or symbolic
5292 links. (If the existing file is a symbolic link, it is removed, not
5293 followed.) However, if a directory cannot be removed because it is
5294 nonempty, @command{tar} normally overwrites its metadata (ownership,
5295 permission, etc.). The @option{--overwrite-dir} option enables this
5296 default behavior. To be more cautious and preserve the metadata of
5297 such a directory, use the @option{--no-overwrite-dir} option.
5299 @cindex Overwriting old files, prevention
5300 @xopindex{keep-old-files, introduced}
5301 To be even more cautious and prevent existing files from being replaced, use
5302 the @option{--keep-old-files} (@option{-k}) option. It causes
5303 @command{tar} to refuse to replace or update a file that already
5304 exists, i.e., a file with the same name as an archive member prevents
5305 extraction of that archive member. Instead, it reports an error. For
5311 $ @kbd{tar -x -k -f archive.tar}
5312 tar: blues: Cannot open: File exists
5313 tar: Exiting with failure status due to previous errors
5316 @xopindex{skip-old-files, introduced}
5317 If you wish to preserve old files untouched, but don't want
5318 @command{tar} to treat them as errors, use the
5319 @option{--skip-old-files} option. This option causes @command{tar} to
5320 silently skip extracting over existing files.
5322 @xopindex{overwrite, introduced}
5323 To be more aggressive about altering existing files, use the
5324 @option{--overwrite} option. It causes @command{tar} to overwrite
5325 existing files and to follow existing symbolic links when extracting.
5327 @cindex Protecting old files
5328 Some people argue that @GNUTAR{} should not hesitate
5329 to overwrite files with other files when extracting. When extracting
5330 a @command{tar} archive, they expect to see a faithful copy of the
5331 state of the file system when the archive was created. It is debatable
5332 that this would always be a proper behavior. For example, suppose one
5333 has an archive in which @file{usr/local} is a link to
5334 @file{usr/local2}. Since then, maybe the site removed the link and
5335 renamed the whole hierarchy from @file{/usr/local2} to
5336 @file{/usr/local}. Such things happen all the time. I guess it would
5337 not be welcome at all that @GNUTAR{} removes the
5338 whole hierarchy just to make room for the link to be reinstated
5339 (unless it @emph{also} simultaneously restores the full
5340 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
5341 able to remove a whole hierarchy to reestablish a symbolic link, for
5342 example, but @emph{only if} @option{--recursive-unlink} is specified
5343 to allow this behavior. In any case, single files are silently
5346 @xopindex{unlink-first, introduced}
5347 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
5348 some cases by causing @command{tar} to remove files unconditionally
5349 before extracting them.
5351 @node Overwrite Old Files
5352 @unnumberedsubsubsec Overwrite Old Files
5357 Overwrite existing files and directory metadata when extracting files
5360 This causes @command{tar} to write extracted files into the file system without
5361 regard to the files already on the system; i.e., files with the same
5362 names as archive members are overwritten when the archive is extracted.
5363 It also causes @command{tar} to extract the ownership, permissions,
5364 and time stamps onto any preexisting files or directories.
5365 If the name of a corresponding file name is a symbolic link, the file
5366 pointed to by the symbolic link will be overwritten instead of the
5367 symbolic link itself (if this is possible). Moreover, special devices,
5368 empty directories and even symbolic links are automatically removed if
5369 they are in the way of extraction.
5371 Be careful when using the @option{--overwrite} option, particularly when
5372 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
5373 can change the contents, ownership or permissions of any file on your
5374 system. Also, many systems do not take kindly to overwriting files that
5375 are currently being executed.
5377 @opindex overwrite-dir
5378 @item --overwrite-dir
5379 Overwrite the metadata of directories when extracting files from an
5380 archive, but remove other files before extracting.
5383 @node Keep Old Files
5384 @unnumberedsubsubsec Keep Old Files
5386 @GNUTAR{} provides two options to control its actions in a situation
5387 when it is about to extract a file which already exists on disk.
5390 @opindex keep-old-files
5391 @item --keep-old-files
5393 Do not replace existing files from archive. When such a file is
5394 encountered, @command{tar} issues an error message. Upon end of
5395 extraction, @command{tar} exits with code 2 (@pxref{exit status}).
5397 @item --skip-old-files
5398 Do not replace existing files from archive, but do not treat that
5399 as error. Such files are silently skipped and do not affect
5400 @command{tar} exit status.
5402 Additional verbosity can be obtained using @option{--warning=existing-file}
5403 together with that option (@pxref{warnings}).
5406 @node Keep Newer Files
5407 @unnumberedsubsubsec Keep Newer Files
5410 @opindex keep-newer-files
5411 @item --keep-newer-files
5412 Do not replace existing files that are newer than their archive
5413 copies. This option is meaningless with @option{--list} (@option{-t}).
5417 @unnumberedsubsubsec Unlink First
5420 @opindex unlink-first
5421 @item --unlink-first
5423 Remove files before extracting over them.
5424 This can make @command{tar} run a bit faster if you know in advance
5425 that the extracted files all need to be removed. Normally this option
5426 slows @command{tar} down slightly, so it is disabled by default.
5429 @node Recursive Unlink
5430 @unnumberedsubsubsec Recursive Unlink
5433 @opindex recursive-unlink
5434 @item --recursive-unlink
5435 When this option is specified, try removing files and directory hierarchies
5436 before extracting over them. @emph{This is a dangerous option!}
5439 If you specify the @option{--recursive-unlink} option,
5440 @command{tar} removes @emph{anything} that keeps you from extracting a file
5441 as far as current permissions will allow it. This could include removal
5442 of the contents of a full directory hierarchy.
5444 @node Data Modification Times
5445 @unnumberedsubsubsec Setting Data Modification Times
5447 @cindex Data modification times of extracted files
5448 @cindex Modification times of extracted files
5449 Normally, @command{tar} sets the data modification times of extracted
5450 files to the corresponding times recorded for the files in the archive, but
5451 limits the permissions of extracted files by the current @code{umask}
5454 To set the data modification times of extracted files to the time when
5455 the files were extracted, use the @option{--touch} (@option{-m}) option in
5456 conjunction with @option{--extract} (@option{--get}, @option{-x}).
5462 Sets the data modification time of extracted archive members to the time
5463 they were extracted, not the time recorded for them in the archive.
5464 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
5467 @node Setting Access Permissions
5468 @unnumberedsubsubsec Setting Access Permissions
5470 @cindex Permissions of extracted files
5471 @cindex Modes of extracted files
5472 To set the modes (access permissions) of extracted files to those
5473 recorded for those files in the archive, use @option{--same-permissions}
5474 in conjunction with the @option{--extract} (@option{--get},
5475 @option{-x}) operation.
5478 @opindex preserve-permissions
5479 @opindex same-permissions
5480 @item --preserve-permissions
5481 @itemx --same-permissions
5482 @c @itemx --ignore-umask
5484 Set modes of extracted archive members to those recorded in the
5485 archive, instead of current umask settings. Use in conjunction with
5486 @option{--extract} (@option{--get}, @option{-x}).
5489 @node Directory Modification Times and Permissions
5490 @unnumberedsubsubsec Directory Modification Times and Permissions
5492 After successfully extracting a file member, @GNUTAR{} normally
5493 restores its permissions and modification times, as described in the
5494 previous sections. This cannot be done for directories, because
5495 after extracting a directory @command{tar} will almost certainly
5496 extract files into that directory and this will cause the directory
5497 modification time to be updated. Moreover, restoring that directory
5498 permissions may not permit file creation within it. Thus, restoring
5499 directory permissions and modification times must be delayed at least
5500 until all files have been extracted into that directory. @GNUTAR{}
5501 restores directories using the following approach.
5503 The extracted directories are created with the mode specified in the
5504 archive, as modified by the umask of the user, which gives sufficient
5505 permissions to allow file creation. The meta-information about the
5506 directory is recorded in the temporary list of directories. When
5507 preparing to extract next archive member, @GNUTAR{} checks if the
5508 directory prefix of this file contains the remembered directory. If
5509 it does not, the program assumes that all files have been extracted
5510 into that directory, restores its modification time and permissions
5511 and removes its entry from the internal list. This approach allows
5512 to correctly restore directory meta-information in the majority of
5513 cases, while keeping memory requirements sufficiently small. It is
5514 based on the fact, that most @command{tar} archives use the predefined
5515 order of members: first the directory, then all the files and
5516 subdirectories in that directory.
5518 However, this is not always true. The most important exception are
5519 incremental archives (@pxref{Incremental Dumps}). The member order in
5520 an incremental archive is reversed: first all directory members are
5521 stored, followed by other (non-directory) members. So, when extracting
5522 from incremental archives, @GNUTAR{} alters the above procedure. It
5523 remembers all restored directories, and restores their meta-data
5524 only after the entire archive has been processed. Notice, that you do
5525 not need to specify any special options for that, as @GNUTAR{}
5526 automatically detects archives in incremental format.
5528 There may be cases, when such processing is required for normal archives
5529 too. Consider the following example:
5533 $ @kbd{tar --no-recursion -cvf archive \
5534 foo foo/file1 bar bar/file foo/file2}
5543 During the normal operation, after encountering @file{bar}
5544 @GNUTAR{} will assume that all files from the directory @file{foo}
5545 were already extracted and will therefore restore its timestamp and
5546 permission bits. However, after extracting @file{foo/file2} the
5547 directory timestamp will be offset again.
5549 To correctly restore directory meta-information in such cases, use
5550 the @option{--delay-directory-restore} command line option:
5553 @opindex delay-directory-restore
5554 @item --delay-directory-restore
5555 Delays restoring of the modification times and permissions of extracted
5556 directories until the end of extraction. This way, correct
5557 meta-information is restored even if the archive has unusual member
5560 @opindex no-delay-directory-restore
5561 @item --no-delay-directory-restore
5562 Cancel the effect of the previous @option{--delay-directory-restore}.
5563 Use this option if you have used @option{--delay-directory-restore} in
5564 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
5565 temporarily disable it.
5568 @node Writing to Standard Output
5569 @unnumberedsubsubsec Writing to Standard Output
5571 @cindex Writing extracted files to standard output
5572 @cindex Standard output, writing extracted files to
5573 To write the extracted files to the standard output, instead of
5574 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
5575 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
5576 extracting files to send them through a pipe, and do not need to
5577 preserve them in the file system. If you extract multiple members,
5578 they appear on standard output concatenated, in the order they are
5579 found in the archive.
5585 Writes files to the standard output. Use only in conjunction with
5586 @option{--extract} (@option{--get}, @option{-x}). When this option is
5587 used, instead of creating the files specified, @command{tar} writes
5588 the contents of the files extracted to its standard output. This may
5589 be useful if you are only extracting the files in order to send them
5590 through a pipe. This option is meaningless with @option{--list}
5594 This can be useful, for example, if you have a tar archive containing
5595 a big file and don't want to store the file on disk before processing
5596 it. You can use a command like this:
5599 tar -xOzf foo.tgz bigfile | process
5602 or even like this if you want to process the concatenation of the files:
5605 tar -xOzf foo.tgz bigfile1 bigfile2 | process
5608 However, @option{--to-command} may be more convenient for use with
5609 multiple files. See the next section.
5611 @node Writing to an External Program
5612 @unnumberedsubsubsec Writing to an External Program
5614 You can instruct @command{tar} to send the contents of each extracted
5615 file to the standard input of an external program:
5619 @item --to-command=@var{command}
5620 Extract files and pipe their contents to the standard input of
5621 @var{command}. When this option is used, instead of creating the
5622 files specified, @command{tar} invokes @var{command} and pipes the
5623 contents of the files to its standard output. The @var{command} may
5624 contain command line arguments (see @ref{external, Running External Commands},
5627 Notice, that @var{command} is executed once for each regular file
5628 extracted. Non-regular files (directories, etc.) are ignored when this
5632 The command can obtain the information about the file it processes
5633 from the following environment variables:
5636 @vrindex TAR_FILETYPE, to-command environment
5638 Type of the file. It is a single letter with the following meaning:
5640 @multitable @columnfractions 0.10 0.90
5641 @item f @tab Regular file
5642 @item d @tab Directory
5643 @item l @tab Symbolic link
5644 @item h @tab Hard link
5645 @item b @tab Block device
5646 @item c @tab Character device
5649 Currently only regular files are supported.
5651 @vrindex TAR_MODE, to-command environment
5653 File mode, an octal number.
5655 @vrindex TAR_FILENAME, to-command environment
5657 The name of the file.
5659 @vrindex TAR_REALNAME, to-command environment
5661 Name of the file as stored in the archive.
5663 @vrindex TAR_UNAME, to-command environment
5665 Name of the file owner.
5667 @vrindex TAR_GNAME, to-command environment
5669 Name of the file owner group.
5671 @vrindex TAR_ATIME, to-command environment
5673 Time of last access. It is a decimal number, representing seconds
5674 since the Epoch. If the archive provides times with nanosecond
5675 precision, the nanoseconds are appended to the timestamp after a
5678 @vrindex TAR_MTIME, to-command environment
5680 Time of last modification.
5682 @vrindex TAR_CTIME, to-command environment
5684 Time of last status change.
5686 @vrindex TAR_SIZE, to-command environment
5690 @vrindex TAR_UID, to-command environment
5692 UID of the file owner.
5694 @vrindex TAR_GID, to-command environment
5696 GID of the file owner.
5699 Additionally, the following variables contain information about
5700 tar mode and the archive being processed:
5703 @vrindex TAR_VERSION, to-command environment
5705 @GNUTAR{} version number.
5707 @vrindex TAR_ARCHIVE, to-command environment
5709 The name of the archive @command{tar} is processing.
5711 @vrindex TAR_BLOCKING_FACTOR, to-command environment
5712 @item TAR_BLOCKING_FACTOR
5713 Current blocking factor (@pxref{Blocking}).
5715 @vrindex TAR_VOLUME, to-command environment
5717 Ordinal number of the volume @command{tar} is processing.
5719 @vrindex TAR_FORMAT, to-command environment
5721 Format of the archive being processed. @xref{Formats}, for a complete
5722 list of archive format names.
5725 These variables are defined prior to executing the command, so you can
5726 pass them as arguments, if you prefer. For example, if the command
5727 @var{proc} takes the member name and size as its arguments, then you
5731 $ @kbd{tar -x -f archive.tar \
5732 --to-command='proc $TAR_FILENAME $TAR_SIZE'}
5736 Notice single quotes to prevent variable names from being expanded by
5737 the shell when invoking @command{tar}.
5739 If @var{command} exits with a non-0 status, @command{tar} will print
5740 an error message similar to the following:
5743 tar: 2345: Child returned status 1
5746 Here, @samp{2345} is the PID of the finished process.
5748 If this behavior is not wanted, use @option{--ignore-command-error}:
5751 @opindex ignore-command-error
5752 @item --ignore-command-error
5753 Ignore exit codes of subprocesses. Notice that if the program
5754 exits on signal or otherwise terminates abnormally, the error message
5755 will be printed even if this option is used.
5757 @opindex no-ignore-command-error
5758 @item --no-ignore-command-error
5759 Cancel the effect of any previous @option{--ignore-command-error}
5760 option. This option is useful if you have set
5761 @option{--ignore-command-error} in @env{TAR_OPTIONS}
5762 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
5766 @unnumberedsubsubsec Removing Files
5768 @FIXME{The section is too terse. Something more to add? An example,
5772 @opindex remove-files
5773 @item --remove-files
5774 Remove files after adding them to the archive.
5778 @subsection Coping with Scarce Resources
5781 @cindex Small memory
5782 @cindex Running out of space
5790 @unnumberedsubsubsec Starting File
5793 @opindex starting-file
5794 @item --starting-file=@var{name}
5795 @itemx -K @var{name}
5796 Starts an operation in the middle of an archive. Use in conjunction
5797 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
5800 @cindex Middle of the archive, starting in the
5801 If a previous attempt to extract files failed due to lack of disk
5802 space, you can use @option{--starting-file=@var{name}} (@option{-K
5803 @var{name}}) to start extracting only after member @var{name} of the
5804 archive. This assumes, of course, that there is now free space, or
5805 that you are now extracting into a different file system. (You could
5806 also choose to suspend @command{tar}, remove unnecessary files from
5807 the file system, and then resume the same @command{tar} operation.
5808 In this case, @option{--starting-file} is not necessary.) See also
5809 @ref{interactive}, and @ref{exclude}.
5812 @unnumberedsubsubsec Same Order
5815 @cindex Large lists of file names on small machines
5817 @opindex preserve-order
5819 @itemx --preserve-order
5821 To process large lists of file names on machines with small amounts of
5822 memory. Use in conjunction with @option{--compare} (@option{--diff},
5823 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
5824 (@option{--get}, @option{-x}).
5827 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
5828 names to be listed or extracted is sorted in the same order as the
5829 files in the archive. This allows a large list of names to be used,
5830 even on a small machine that would not otherwise be able to hold all
5831 the names in memory at the same time. Such a sorted list can easily be
5832 created by running @samp{tar -t} on the archive and editing its output.
5834 This option is probably never needed on modern computer systems.
5837 @section Backup options
5839 @cindex backup options
5841 @GNUTAR{} offers options for making backups of files
5842 before writing new versions. These options control the details of
5843 these backups. They may apply to the archive itself before it is
5844 created or rewritten, as well as individual extracted members. Other
5845 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
5846 and @command{mv}, for example) offer similar options.
5848 Backup options may prove unexpectedly useful when extracting archives
5849 containing many members having identical name, or when extracting archives
5850 on systems having file name limitations, making different members appear
5851 as having similar names through the side-effect of name truncation.
5852 @FIXME{This is true only if we have a good scheme for truncated backup names,
5853 which I'm not sure at all: I suspect work is needed in this area.}
5854 When any existing file is backed up before being overwritten by extraction,
5855 then clashing files are automatically be renamed to be unique, and the
5856 true name is kept for only the last file of a series of clashing files.
5857 By using verbose mode, users may track exactly what happens.
5859 At the detail level, some decisions are still experimental, and may
5860 change in the future, we are waiting comments from our users. So, please
5861 do not learn to depend blindly on the details of the backup features.
5862 For example, currently, directories themselves are never renamed through
5863 using these options, so, extracting a file over a directory still has
5864 good chances to fail. Also, backup options apply to created archives,
5865 not only to extracted members. For created archives, backups will not
5866 be attempted when the archive is a block or character device, or when it
5867 refers to a remote file.
5869 For the sake of simplicity and efficiency, backups are made by renaming old
5870 files prior to creation or extraction, and not by copying. The original
5871 name is restored if the file creation fails. If a failure occurs after a
5872 partial extraction of a file, both the backup and the partially extracted
5876 @item --backup[=@var{method}]
5878 @vindex VERSION_CONTROL
5880 Back up files that are about to be overwritten or removed.
5881 Without this option, the original versions are destroyed.
5883 Use @var{method} to determine the type of backups made.
5884 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
5885 environment variable. And if @env{VERSION_CONTROL} is not set,
5886 use the @samp{existing} method.
5888 @vindex version-control @r{Emacs variable}
5889 This option corresponds to the Emacs variable @samp{version-control};
5890 the same values for @var{method} are accepted as in Emacs. This option
5891 also allows more descriptive names. The valid @var{method}s are:
5896 @cindex numbered @r{backup method}
5897 Always make numbered backups.
5901 @cindex existing @r{backup method}
5902 Make numbered backups of files that already have them, simple backups
5907 @cindex simple @r{backup method}
5908 Always make simple backups.
5912 @item --suffix=@var{suffix}
5914 @cindex backup suffix
5915 @vindex SIMPLE_BACKUP_SUFFIX
5916 Append @var{suffix} to each backup file made with @option{--backup}. If this
5917 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5918 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5919 set, the default is @samp{~}, just as in Emacs.
5924 @section Notable @command{tar} Usages
5927 @FIXME{Using Unix file linking capability to recreate directory
5928 structures---linking files into one subdirectory and then
5929 @command{tar}ring that directory.}
5931 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5934 You can easily use archive files to transport a group of files from
5935 one system to another: put all relevant files into an archive on one
5936 computer system, transfer the archive to another system, and extract
5937 the contents there. The basic transfer medium might be magnetic tape,
5938 Internet FTP, or even electronic mail (though you must encode the
5939 archive with @command{uuencode} in order to transport it properly by
5940 mail). Both machines do not have to use the same operating system, as
5941 long as they both support the @command{tar} program.
5943 For example, here is how you might copy a directory's contents from
5944 one disk to another, while preserving the dates, modes, owners and
5945 link-structure of all the files therein. In this case, the transfer
5946 medium is a @dfn{pipe}:
5949 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5953 You can avoid subshells by using @option{-C} option:
5956 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5960 The command also works using long option forms:
5964 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5965 | (cd targetdir; tar --extract --file=-)}
5974 $ @kbd{tar --directory sourcedir --create --file=- . \
5975 | tar --directory targetdir --extract --file=-}
5980 This is one of the easiest methods to transfer a @command{tar} archive.
5983 @section Looking Ahead: The Rest of this Manual
5985 You have now seen how to use all eight of the operations available to
5986 @command{tar}, and a number of the possible options. The next chapter
5987 explains how to choose and change file and archive names, how to use
5988 files to store names of other files which you can then call as
5989 arguments to @command{tar} (this can help you save time if you expect to
5990 archive the same list of files a number of times), and so forth.
5991 @FIXME{in case it's not obvious, i'm making this up in some sense
5992 based on my limited memory of what the next chapter *really* does. i
5993 just wanted to flesh out this final section a little bit so i'd
5994 remember to stick it in here. :-)}
5996 If there are too many files to conveniently list on the command line,
5997 you can list the names in a file, and @command{tar} will read that file.
6000 There are various ways of causing @command{tar} to skip over some files,
6001 and not archive them. @xref{Choosing}.
6004 @chapter Performing Backups and Restoring Files
6007 @GNUTAR{} is distributed along with the scripts for performing backups
6008 and restores. Even if there is a good chance those scripts may be
6009 satisfying to you, they are not the only scripts or methods available for doing
6010 backups and restore. You may well create your own, or use more
6011 sophisticated packages dedicated to that purpose.
6013 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
6014 Automatic Network Disk Archiver), a backup system developed by James
6015 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
6016 This is free software, and it is available from @uref{http://www.amanda.org}.
6020 Here is a possible plan for a future documentation about the backuping
6021 scripts which are provided within the @GNUTAR{}
6027 @item what are dumps
6028 @item different levels of dumps
6030 @item full dump = dump everything
6031 @item level 1, level 2 dumps etc
6032 A level @var{n} dump dumps everything changed since the last level
6035 @item how to use scripts for dumps (ie, the concept)
6037 @item scripts to run after editing backup specs (details)
6039 @item Backup Specs, what is it.
6041 @item how to customize
6042 @item actual text of script [/sp/dump/backup-specs]
6046 @item rsh doesn't work
6047 @item rtape isn't installed
6050 @item the @option{--incremental} option of tar
6053 @item write protection
6054 @item types of media, different sizes and types, useful for different things
6055 @item files and tape marks
6056 one tape mark between files, two at end.
6057 @item positioning the tape
6058 MT writes two at end of write,
6059 backspaces over one when writing again.
6065 This chapter documents both the provided shell scripts and @command{tar}
6066 options which are more specific to usage as a backup tool.
6068 To @dfn{back up} a file system means to create archives that contain
6069 all the files in that file system. Those archives can then be used to
6070 restore any or all of those files (for instance if a disk crashes or a
6071 file is accidentally deleted). File system @dfn{backups} are also
6075 * Full Dumps:: Using @command{tar} to Perform Full Dumps
6076 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
6077 * Backup Levels:: Levels of Backups
6078 * Backup Parameters:: Setting Parameters for Backups and Restoration
6079 * Scripted Backups:: Using the Backup Scripts
6080 * Scripted Restoration:: Using the Restore Script
6084 @section Using @command{tar} to Perform Full Dumps
6090 @cindex corrupted archives
6091 Full dumps should only be made when no other people or programs
6092 are modifying files in the file system. If files are modified while
6093 @command{tar} is making the backup, they may not be stored properly in
6094 the archive, in which case you won't be able to restore them if you
6095 have to. (Files not being modified are written with no trouble, and do
6096 not corrupt the entire archive.)
6098 You will want to use the @option{--label=@var{archive-label}}
6099 (@option{-V @var{archive-label}}) option to give the archive a
6100 volume label, so you can tell what this archive is even if the label
6101 falls off the tape, or anything like that.
6103 Unless the file system you are dumping is guaranteed to fit on
6104 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
6105 Make sure you have enough tapes on hand to complete the backup.
6107 If you want to dump each file system separately you will need to use
6108 the @option{--one-file-system} option to prevent
6109 @command{tar} from crossing file system boundaries when storing
6112 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
6113 option is not needed, since this is a complete copy of everything in
6114 the file system, and a full restore from this backup would only be
6115 done onto a completely
6118 Unless you are in a hurry, and trust the @command{tar} program (and your
6119 tapes), it is a good idea to use the @option{--verify} (@option{-W})
6120 option, to make sure your files really made it onto the dump properly.
6121 This will also detect cases where the file was modified while (or just
6122 after) it was being archived. Not all media (notably cartridge tapes)
6123 are capable of being verified, unfortunately.
6125 @node Incremental Dumps
6126 @section Using @command{tar} to Perform Incremental Dumps
6128 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
6129 stores additional metadata so that exact state of the file system
6130 can be restored when extracting the archive.
6132 @GNUTAR{} currently offers two options for handling incremental
6133 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
6134 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
6136 @xopindex{listed-incremental, described}
6137 The option @option{--listed-incremental} instructs tar to operate on
6138 an incremental archive with additional metadata stored in a standalone
6139 file, called a @dfn{snapshot file}. The purpose of this file is to help
6140 determine which files have been changed, added or deleted since the
6141 last backup, so that the next incremental backup will contain only
6142 modified files. The name of the snapshot file is given as an argument
6146 @item --listed-incremental=@var{file}
6147 @itemx -g @var{file}
6148 Handle incremental backups with snapshot data in @var{file}.
6151 To create an incremental backup, you would use
6152 @option{--listed-incremental} together with @option{--create}
6153 (@pxref{create}). For example:
6156 $ @kbd{tar --create \
6157 --file=archive.1.tar \
6158 --listed-incremental=/var/log/usr.snar \
6162 This will create in @file{archive.1.tar} an incremental backup of
6163 the @file{/usr} file system, storing additional metadata in the file
6164 @file{/var/log/usr.snar}. If this file does not exist, it will be
6165 created. The created archive will then be a @dfn{level 0 backup};
6166 please see the next section for more on backup levels.
6168 Otherwise, if the file @file{/var/log/usr.snar} exists, it
6169 determines which files are modified. In this case only these files will be
6170 stored in the archive. Suppose, for example, that after running the
6171 above command, you delete file @file{/usr/doc/old} and create
6172 directory @file{/usr/local/db} with the following contents:
6175 $ @kbd{ls /usr/local/db}
6180 Some time later you create another incremental backup. You will
6184 $ @kbd{tar --create \
6185 --file=archive.2.tar \
6186 --listed-incremental=/var/log/usr.snar \
6188 tar: usr/local/db: Directory is new
6195 The created archive @file{archive.2.tar} will contain only these
6196 three members. This archive is called a @dfn{level 1 backup}. Notice
6197 that @file{/var/log/usr.snar} will be updated with the new data, so if
6198 you plan to create more @samp{level 1} backups, it is necessary to
6199 create a working copy of the snapshot file before running
6200 @command{tar}. The above example will then be modified as follows:
6203 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
6204 $ @kbd{tar --create \
6205 --file=archive.2.tar \
6206 --listed-incremental=/var/log/usr.snar-1 \
6211 @xopindex{level, described}
6212 You can force @samp{level 0} backups either by removing the snapshot
6213 file before running @command{tar}, or by supplying the
6214 @option{--level=0} option, e.g.:
6217 $ @kbd{tar --create \
6218 --file=archive.2.tar \
6219 --listed-incremental=/var/log/usr.snar-0 \
6224 Incremental dumps depend crucially on time stamps, so the results are
6225 unreliable if you modify a file's time stamps during dumping (e.g.,
6226 with the @option{--atime-preserve=replace} option), or if you set the clock
6229 @anchor{device numbers}
6230 @cindex Device numbers, using in incremental backups
6231 Metadata stored in snapshot files include device numbers, which,
6232 obviously are supposed to be non-volatile values. However, it turns
6233 out that @acronym{NFS} devices have undependable values when an automounter
6234 gets in the picture. This can lead to a great deal of spurious
6235 redumping in incremental dumps, so it is somewhat useless to compare
6236 two @acronym{NFS} devices numbers over time. The solution implemented
6237 currently is to consider all @acronym{NFS} devices as being equal
6238 when it comes to comparing directories; this is fairly gross, but
6239 there does not seem to be a better way to go.
6241 Apart from using @acronym{NFS}, there are a number of cases where
6242 relying on device numbers can cause spurious redumping of unmodified
6243 files. For example, this occurs when archiving @acronym{LVM} snapshot
6244 volumes. To avoid this, use @option{--no-check-device} option:
6247 @xopindex{no-check-device, described}
6248 @item --no-check-device
6249 Do not rely on device numbers when preparing a list of changed files
6250 for an incremental dump.
6252 @xopindex{check-device, described}
6253 @item --check-device
6254 Use device numbers when preparing a list of changed files
6255 for an incremental dump. This is the default behavior. The purpose
6256 of this option is to undo the effect of the @option{--no-check-device}
6257 if it was given in @env{TAR_OPTIONS} environment variable
6258 (@pxref{TAR_OPTIONS}).
6261 There is also another way to cope with changing device numbers. It is
6262 described in detail in @ref{Fixing Snapshot Files}.
6264 Note that incremental archives use @command{tar} extensions and may
6265 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
6267 @xopindex{listed-incremental, using with @option{--extract}}
6268 @xopindex{extract, using with @option{--listed-incremental}}
6269 To extract from the incremental dumps, use
6270 @option{--listed-incremental} together with @option{--extract}
6271 option (@pxref{extracting files}). In this case, @command{tar} does
6272 not need to access snapshot file, since all the data necessary for
6273 extraction are stored in the archive itself. So, when extracting, you
6274 can give whatever argument to @option{--listed-incremental}, the usual
6275 practice is to use @option{--listed-incremental=/dev/null}.
6276 Alternatively, you can use @option{--incremental}, which needs no
6277 arguments. In general, @option{--incremental} (@option{-G}) can be
6278 used as a shortcut for @option{--listed-incremental} when listing or
6279 extracting incremental backups (for more information regarding this
6280 option, @pxref{incremental-op}).
6282 When extracting from the incremental backup @GNUTAR{} attempts to
6283 restore the exact state the file system had when the archive was
6284 created. In particular, it will @emph{delete} those files in the file
6285 system that did not exist in their directories when the archive was
6286 created. If you have created several levels of incremental files,
6287 then in order to restore the exact contents the file system had when
6288 the last level was created, you will need to restore from all backups
6289 in turn. Continuing our example, to restore the state of @file{/usr}
6290 file system, one would do@footnote{Notice, that since both archives
6291 were created without @option{-P} option (@pxref{absolute}), these
6292 commands should be run from the root file system.}:
6295 $ @kbd{tar --extract \
6296 --listed-incremental=/dev/null \
6297 --file archive.1.tar}
6298 $ @kbd{tar --extract \
6299 --listed-incremental=/dev/null \
6300 --file archive.2.tar}
6303 To list the contents of an incremental archive, use @option{--list}
6304 (@pxref{list}), as usual. To obtain more information about the
6305 archive, use @option{--listed-incremental} or @option{--incremental}
6306 combined with two @option{--verbose} options@footnote{Two
6307 @option{--verbose} options were selected to avoid breaking usual
6308 verbose listing output (@option{--list --verbose}) when using in
6311 @xopindex{incremental, using with @option{--list}}
6312 @xopindex{listed-incremental, using with @option{--list}}
6313 @xopindex{list, using with @option{--incremental}}
6314 @xopindex{list, using with @option{--listed-incremental}}
6315 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
6316 contents of the DUMPDIR header (with terminating nulls) when
6317 @option{--incremental} or @option{--listed-incremental} option was
6318 given, no matter what the verbosity level. This behavior, and,
6319 especially, the binary output it produced were considered inconvenient
6320 and were changed in version 1.16.}:
6323 @kbd{tar --list --incremental --verbose --verbose archive.tar}
6326 This command will print, for each directory in the archive, the list
6327 of files in that directory at the time the archive was created. This
6328 information is put out in a format which is both human-readable and
6329 unambiguous for a program: each file name is printed as
6336 where @var{x} is a letter describing the status of the file: @samp{Y}
6337 if the file is present in the archive, @samp{N} if the file is not
6338 included in the archive, or a @samp{D} if the file is a directory (and
6339 is included in the archive). @xref{Dumpdir}, for the detailed
6340 description of dumpdirs and status codes. Each such
6341 line is terminated by a newline character. The last line is followed
6342 by an additional newline to indicate the end of the data.
6344 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
6345 gives the same behavior as @option{--listed-incremental} when used
6346 with @option{--list} and @option{--extract} options. When used with
6347 @option{--create} option, it creates an incremental archive without
6348 creating snapshot file. Thus, it is impossible to create several
6349 levels of incremental backups with @option{--incremental} option.
6352 @section Levels of Backups
6354 An archive containing all the files in the file system is called a
6355 @dfn{full backup} or @dfn{full dump}. You could insure your data by
6356 creating a full dump every day. This strategy, however, would waste a
6357 substantial amount of archive media and user time, as unchanged files
6358 are daily re-archived.
6360 It is more efficient to do a full dump only occasionally. To back up
6361 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
6362 one} dump archives all the files that have changed since the last full
6365 A typical dump strategy would be to perform a full dump once a week,
6366 and a level one dump once a day. This means some versions of files
6367 will in fact be archived more than once, but this dump strategy makes
6368 it possible to restore a file system to within one day of accuracy by
6369 only extracting two archives---the last weekly (full) dump and the
6370 last daily (level one) dump. The only information lost would be in
6371 files changed or created since the last daily backup. (Doing dumps
6372 more than once a day is usually not worth the trouble.)
6374 @GNUTAR{} comes with scripts you can use to do full
6375 and level-one (actually, even level-two and so on) dumps. Using
6376 scripts (shell programs) to perform backups and restoration is a
6377 convenient and reliable alternative to typing out file name lists
6378 and @command{tar} commands by hand.
6380 Before you use these scripts, you need to edit the file
6381 @file{backup-specs}, which specifies parameters used by the backup
6382 scripts and by the restore script. This file is usually located
6383 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
6384 detailed description. Once the backup parameters are set, you can
6385 perform backups or restoration by running the appropriate script.
6387 The name of the backup script is @code{backup}. The name of the
6388 restore script is @code{restore}. The following sections describe
6389 their use in detail.
6391 @emph{Please Note:} The backup and restoration scripts are
6392 designed to be used together. While it is possible to restore files by
6393 hand from an archive which was created using a backup script, and to create
6394 an archive by hand which could then be extracted using the restore script,
6395 it is easier to use the scripts. @xref{Incremental Dumps}, before
6396 making such an attempt.
6398 @node Backup Parameters
6399 @section Setting Parameters for Backups and Restoration
6401 The file @file{backup-specs} specifies backup parameters for the
6402 backup and restoration scripts provided with @command{tar}. You must
6403 edit @file{backup-specs} to fit your system configuration and schedule
6404 before using these scripts.
6406 Syntactically, @file{backup-specs} is a shell script, containing
6407 mainly variable assignments. However, any valid shell construct
6408 is allowed in this file. Particularly, you may wish to define
6409 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
6410 For more information about shell script syntax, please refer to
6411 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
6412 g_02, the definition of the Shell Command Language}. See also
6413 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
6415 The shell variables controlling behavior of @code{backup} and
6416 @code{restore} are described in the following subsections.
6419 * General-Purpose Variables::
6420 * Magnetic Tape Control::
6422 * backup-specs example:: An Example Text of @file{Backup-specs}
6425 @node General-Purpose Variables
6426 @subsection General-Purpose Variables
6428 @defvr {Backup variable} ADMINISTRATOR
6429 The user name of the backup administrator. @code{Backup} scripts
6430 sends a backup report to this address.
6433 @defvr {Backup variable} BACKUP_HOUR
6434 The hour at which the backups are done. This can be a number from 0
6435 to 23, or the time specification in form @var{hours}:@var{minutes},
6436 or the string @samp{now}.
6438 This variable is used by @code{backup}. Its value may be overridden
6439 using @option{--time} option (@pxref{Scripted Backups}).
6442 @defvr {Backup variable} TAPE_FILE
6444 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
6445 is a remote archive (@pxref{remote-dev}), backup script will suppose
6446 that your @command{mt} is able to access remote devices. If @var{RSH}
6447 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
6448 invocations of @command{mt}.
6451 @defvr {Backup variable} BLOCKING
6453 The blocking factor @command{tar} will use when writing the dump archive.
6454 @xref{Blocking Factor}.
6457 @defvr {Backup variable} BACKUP_DIRS
6459 A list of file systems to be dumped (for @code{backup}), or restored
6460 (for @code{restore}). You can include any directory
6461 name in the list --- subdirectories on that file system will be
6462 included, regardless of how they may look to other networked machines.
6463 Subdirectories on other file systems will be ignored.
6465 The host name specifies which host to run @command{tar} on, and should
6466 normally be the host that actually contains the file system. However,
6467 the host machine must have @GNUTAR{} installed, and
6468 must be able to access the directory containing the backup scripts and
6469 their support files using the same file name that is used on the
6470 machine where the scripts are run (i.e., what @command{pwd} will print
6471 when in that directory on that machine). If the host that contains
6472 the file system does not have this capability, you can specify another
6473 host as long as it can access the file system through @acronym{NFS}.
6475 If the list of file systems is very long you may wish to put it
6476 in a separate file. This file is usually named
6477 @file{/etc/backup/dirs}, but this name may be overridden in
6478 @file{backup-specs} using @code{DIRLIST} variable.
6481 @defvr {Backup variable} DIRLIST
6483 The name of the file that contains a list of file systems to backup
6484 or restore. By default it is @file{/etc/backup/dirs}.
6487 @defvr {Backup variable} BACKUP_FILES
6489 A list of individual files to be dumped (for @code{backup}), or restored
6490 (for @code{restore}). These should be accessible from the machine on
6491 which the backup script is run.
6493 If the list of individual files is very long you may wish to store it
6494 in a separate file. This file is usually named
6495 @file{/etc/backup/files}, but this name may be overridden in
6496 @file{backup-specs} using @code{FILELIST} variable.
6499 @defvr {Backup variable} FILELIST
6501 The name of the file that contains a list of individual files to backup
6502 or restore. By default it is @file{/etc/backup/files}.
6505 @defvr {Backup variable} MT
6507 Full file name of @command{mt} binary.
6510 @defvr {Backup variable} RSH
6512 Full file name of @command{rsh} binary or its equivalent. You may wish to
6513 set it to @code{ssh}, to improve security. In this case you will have
6514 to use public key authentication.
6517 @defvr {Backup variable} RSH_COMMAND
6519 Full file name of @command{rsh} binary on remote machines. This will
6520 be passed via @option{--rsh-command} option to the remote invocation
6524 @defvr {Backup variable} VOLNO_FILE
6526 Name of temporary file to hold volume numbers. This needs to be accessible
6527 by all the machines which have file systems to be dumped.
6530 @defvr {Backup variable} XLIST
6532 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
6533 located on the remote machine and containing the list of files to
6534 be excluded from the backup. Exclude file lists are searched in
6535 /etc/tar-backup directory. A common use for exclude file lists
6536 is to exclude files containing security-sensitive information
6537 (e.g., @file{/etc/shadow} from backups).
6539 This variable affects only @code{backup}.
6542 @defvr {Backup variable} SLEEP_TIME
6544 Time to sleep between dumps of any two successive file systems
6546 This variable affects only @code{backup}.
6549 @defvr {Backup variable} DUMP_REMIND_SCRIPT
6551 Script to be run when it's time to insert a new tape in for the next
6552 volume. Administrators may want to tailor this script for their site.
6553 If this variable isn't set, @GNUTAR{} will display its built-in
6554 prompt, and will expect confirmation from the console. For the
6555 description of the default prompt, see @ref{change volume prompt}.
6559 @defvr {Backup variable} SLEEP_MESSAGE
6561 Message to display on the terminal while waiting for dump time. Usually
6562 this will just be some literal text.
6565 @defvr {Backup variable} TAR
6567 Full file name of the @GNUTAR{} executable. If this is not set, backup
6568 scripts will search @command{tar} in the current shell path.
6571 @node Magnetic Tape Control
6572 @subsection Magnetic Tape Control
6574 Backup scripts access tape device using special @dfn{hook functions}.
6575 These functions take a single argument --- the name of the tape
6576 device. Their names are kept in the following variables:
6578 @defvr {Backup variable} MT_BEGIN
6579 The name of @dfn{begin} function. This function is called before
6580 accessing the drive. By default it retensions the tape:
6586 mt -f "$1" retension
6591 @defvr {Backup variable} MT_REWIND
6592 The name of @dfn{rewind} function. The default definition is as
6605 @defvr {Backup variable} MT_OFFLINE
6606 The name of the function switching the tape off line. By default
6607 it is defined as follows:
6610 MT_OFFLINE=mt_offline
6618 @defvr {Backup variable} MT_STATUS
6619 The name of the function used to obtain the status of the archive device,
6620 including error count. Default definition:
6632 @subsection User Hooks
6634 @dfn{User hooks} are shell functions executed before and after
6635 each @command{tar} invocation. Thus, there are @dfn{backup
6636 hooks}, which are executed before and after dumping each file
6637 system, and @dfn{restore hooks}, executed before and
6638 after restoring a file system. Each user hook is a shell function
6639 taking four arguments:
6641 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
6646 Current backup or restore level.
6649 Name or IP address of the host machine being dumped or restored.
6652 Full file name of the file system being dumped or restored.
6655 File system name with directory separators replaced with colons. This
6656 is useful, e.g., for creating unique files.
6660 Following variables keep the names of user hook functions:
6662 @defvr {Backup variable} DUMP_BEGIN
6663 Dump begin function. It is executed before dumping the file system.
6666 @defvr {Backup variable} DUMP_END
6667 Executed after dumping the file system.
6670 @defvr {Backup variable} RESTORE_BEGIN
6671 Executed before restoring the file system.
6674 @defvr {Backup variable} RESTORE_END
6675 Executed after restoring the file system.
6678 @node backup-specs example
6679 @subsection An Example Text of @file{Backup-specs}
6681 The following is an example of @file{backup-specs}:
6684 # site-specific parameters for file system backup.
6686 ADMINISTRATOR=friedman
6688 TAPE_FILE=/dev/nrsmt0
6690 # Use @code{ssh} instead of the less secure @code{rsh}
6692 RSH_COMMAND=/usr/bin/ssh
6694 # Override MT_STATUS function:
6700 # Disable MT_OFFLINE function
6717 apple-gunkies:/com/mailer/gnu
6718 apple-gunkies:/com/archive/gnu"
6720 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
6724 @node Scripted Backups
6725 @section Using the Backup Scripts
6727 The syntax for running a backup script is:
6730 backup --level=@var{level} --time=@var{time}
6733 The @option{--level} option requests the dump level. Thus, to produce
6734 a full dump, specify @code{--level=0} (this is the default, so
6735 @option{--level} may be omitted if its value is
6736 @code{0})@footnote{For backward compatibility, the @code{backup} will also
6737 try to deduce the requested dump level from the name of the
6738 script itself. If the name consists of a string @samp{level-}
6739 followed by a single decimal digit, that digit is taken as
6740 the dump level number. Thus, you may create a link from @code{backup}
6741 to @code{level-1} and then run @code{level-1} whenever you need to
6742 create a level one dump.}.
6744 The @option{--time} option determines when should the backup be
6745 run. @var{Time} may take three forms:
6748 @item @var{hh}:@var{mm}
6750 The dump must be run at @var{hh} hours @var{mm} minutes.
6754 The dump must be run at @var{hh} hours.
6758 The dump must be run immediately.
6761 You should start a script with a tape or disk mounted. Once you
6762 start a script, it prompts you for new tapes or disks as it
6763 needs them. Media volumes don't have to correspond to archive
6764 files --- a multi-volume archive can be started in the middle of a
6765 tape that already contains the end of another multi-volume archive.
6766 The @code{restore} script prompts for media by its archive volume,
6767 so to avoid an error message you should keep track of which tape
6768 (or disk) contains which volume of the archive (@pxref{Scripted
6771 The backup scripts write two files on the file system. The first is a
6772 record file in @file{/etc/tar-backup/}, which is used by the scripts
6773 to store and retrieve information about which files were dumped. This
6774 file is not meant to be read by humans, and should not be deleted by
6775 them. @xref{Snapshot Files}, for a more detailed explanation of this
6778 The second file is a log file containing the names of the file systems
6779 and files dumped, what time the backup was made, and any error
6780 messages that were generated, as well as how much space was left in
6781 the media volume after the last volume of the archive was written.
6782 You should check this log file after every backup. The file name is
6783 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
6784 represents current date, and @var{n} represents current dump level number.
6786 The script also prints the name of each system being dumped to the
6789 Following is the full list of options accepted by @code{backup}
6793 @item -l @var{level}
6794 @itemx --level=@var{level}
6795 Do backup level @var{level} (default 0).
6799 Force backup even if today's log file already exists.
6801 @item -v[@var{level}]
6802 @itemx --verbose[=@var{level}]
6803 Set verbosity level. The higher the level is, the more debugging
6804 information will be output during execution. Default @var{level}
6805 is 100, which means the highest debugging level.
6807 @item -t @var{start-time}
6808 @itemx --time=@var{start-time}
6809 Wait till @var{time}, then do backup.
6813 Display short help message and exit.
6817 Display information about the program's name, version, origin and legal
6818 status, all on standard output, and then exit successfully.
6822 @node Scripted Restoration
6823 @section Using the Restore Script
6825 To restore files that were archived using a scripted backup, use the
6826 @code{restore} script. Its usage is quite straightforward. In the
6827 simplest form, invoke @code{restore --all}, it will
6828 then restore all the file systems and files specified in
6829 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
6831 You may select the file systems (and/or files) to restore by
6832 giving @code{restore} a list of @dfn{patterns} in its command
6833 line. For example, running
6840 will restore all file systems on the machine @samp{albert}. A more
6841 complicated example:
6844 restore 'albert:*' '*:/var'
6848 This command will restore all file systems on the machine @samp{albert}
6849 as well as @file{/var} file system on all machines.
6851 By default @code{restore} will start restoring files from the lowest
6852 available dump level (usually zero) and will continue through
6853 all available dump levels. There may be situations where such a
6854 thorough restore is not necessary. For example, you may wish to
6855 restore only files from the recent level one backup. To do so,
6856 use @option{--level} option, as shown in the example below:
6862 The full list of options accepted by @code{restore} follows:
6867 Restore all file systems and files specified in @file{backup-specs}.
6869 @item -l @var{level}
6870 @itemx --level=@var{level}
6871 Start restoring from the given backup level, instead of the default 0.
6873 @item -v[@var{level}]
6874 @itemx --verbose[=@var{level}]
6875 Set verbosity level. The higher the level is, the more debugging
6876 information will be output during execution. Default @var{level}
6877 is 100, which means the highest debugging level.
6881 Display short help message and exit.
6885 Display information about the program's name, version, origin and legal
6886 status, all on standard output, and then exit successfully.
6889 You should start the restore script with the media containing the
6890 first volume of the archive mounted. The script will prompt for other
6891 volumes as they are needed. If the archive is on tape, you don't need
6892 to rewind the tape to to its beginning---if the tape head is
6893 positioned past the beginning of the archive, the script will rewind
6894 the tape as needed. @xref{Tape Positioning}, for a discussion of tape
6898 @strong{Warning:} The script will delete files from the active file
6899 system if they were not in the file system when the archive was made.
6902 @xref{Incremental Dumps}, for an explanation of how the script makes
6906 @chapter Choosing Files and Names for @command{tar}
6908 Certain options to @command{tar} enable you to specify a name for your
6909 archive. Other options let you decide which files to include or exclude
6910 from the archive, based on when or whether files were modified, whether
6911 the file names do or don't match specified patterns, or whether files
6912 are in specified directories.
6914 This chapter discusses these options in detail.
6917 * file:: Choosing the Archive's Name
6918 * Selecting Archive Members::
6919 * files:: Reading Names from a File
6920 * exclude:: Excluding Some Files
6921 * wildcards:: Wildcards Patterns and Matching
6922 * quoting styles:: Ways of Quoting Special Characters in Names
6923 * transform:: Modifying File and Member Names
6924 * after:: Operating Only on New Files
6925 * recurse:: Descending into Directories
6926 * one:: Crossing File System Boundaries
6930 @section Choosing and Naming Archive Files
6932 @cindex Naming an archive
6933 @cindex Archive Name
6934 @cindex Choosing an archive file
6935 @cindex Where is the archive?
6937 By default, @command{tar} uses an archive file name that was compiled when
6938 it was built on the system; usually this name refers to some physical
6939 tape drive on the machine. However, the person who installed @command{tar}
6940 on the system may not have set the default to a meaningful value as far as
6941 most users are concerned. As a result, you will usually want to tell
6942 @command{tar} where to find (or create) the archive. The
6943 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
6944 option allows you to either specify or name a file to use as the archive
6945 instead of the default archive file location.
6948 @xopindex{file, short description}
6949 @item --file=@var{archive-name}
6950 @itemx -f @var{archive-name}
6951 Name the archive to create or operate on. Use in conjunction with
6955 For example, in this @command{tar} command,
6958 $ @kbd{tar -cvf collection.tar blues folk jazz}
6962 @file{collection.tar} is the name of the archive. It must directly
6963 follow the @option{-f} option, since whatever directly follows @option{-f}
6964 @emph{will} end up naming the archive. If you neglect to specify an
6965 archive name, you may end up overwriting a file in the working directory
6966 with the archive you create since @command{tar} will use this file's name
6967 for the archive name.
6969 An archive can be saved as a file in the file system, sent through a
6970 pipe or over a network, or written to an I/O device such as a tape,
6971 floppy disk, or CD write drive.
6973 @cindex Writing new archives
6974 @cindex Archive creation
6975 If you do not name the archive, @command{tar} uses the value of the
6976 environment variable @env{TAPE} as the file name for the archive. If
6977 that is not available, @command{tar} uses a default, compiled-in archive
6978 name, usually that for tape unit zero (i.e., @file{/dev/tu00}).
6980 @cindex Standard input and output
6981 @cindex tar to standard input and output
6982 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
6983 archive from standard input (when listing or extracting files), or
6984 writes it to standard output (when creating an archive). If you use
6985 @file{-} as an @var{archive-name} when modifying an archive,
6986 @command{tar} reads the original archive from its standard input and
6987 writes the entire new archive to its standard output.
6989 The following example is a convenient way of copying directory
6990 hierarchy from @file{sourcedir} to @file{targetdir}.
6993 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
6996 The @option{-C} option allows to avoid using subshells:
6999 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
7002 In both examples above, the leftmost @command{tar} invocation archives
7003 the contents of @file{sourcedir} to the standard output, while the
7004 rightmost one reads this archive from its standard input and
7005 extracts it. The @option{-p} option tells it to restore permissions
7006 of the extracted files.
7008 @cindex Remote devices
7009 @cindex tar to a remote device
7011 To specify an archive file on a device attached to a remote machine,
7015 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
7019 @command{tar} will set up the remote connection, if possible, and
7020 prompt you for a username and password. If you use
7021 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
7022 will attempt to set up the remote connection using your username
7023 as the username on the remote machine.
7025 @cindex Local and remote archives
7026 @anchor{local and remote archives}
7027 If the archive file name includes a colon (@samp{:}), then it is assumed
7028 to be a file on another machine. If the archive file is
7029 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
7030 host @var{host}. The remote host is accessed using the @command{rsh}
7031 program, with a username of @var{user}. If the username is omitted
7032 (along with the @samp{@@} sign), then your user name will be used.
7033 (This is the normal @command{rsh} behavior.) It is necessary for the
7034 remote machine, in addition to permitting your @command{rsh} access, to
7035 have the @file{rmt} program installed (this command is included in
7036 the @GNUTAR{} distribution and by default is installed under
7037 @file{@var{prefix}/libexec/rmt}, where @var{prefix} means your
7038 installation prefix). If you need to use a file whose name includes a
7039 colon, then the remote tape drive behavior
7040 can be inhibited by using the @option{--force-local} option.
7042 When the archive is being created to @file{/dev/null}, @GNUTAR{}
7043 tries to minimize input and output operations. The Amanda backup
7044 system, when used with @GNUTAR{}, has an initial sizing pass which
7047 @node Selecting Archive Members
7048 @section Selecting Archive Members
7049 @cindex Specifying files to act on
7050 @cindex Specifying archive members
7052 @dfn{File Name arguments} specify which files in the file system
7053 @command{tar} operates on, when creating or adding to an archive, or which
7054 archive members @command{tar} operates on, when reading or deleting from
7055 an archive. @xref{Operations}.
7057 To specify file names, you can include them as the last arguments on
7058 the command line, as follows:
7060 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
7063 If a file name begins with dash (@samp{-}), precede it with
7064 @option{--add-file} option to prevent it from being treated as an
7067 @anchor{input name quoting}
7068 By default @GNUTAR{} attempts to @dfn{unquote} each file or member
7069 name, replacing @dfn{escape sequences} according to the following
7072 @multitable @columnfractions 0.20 0.60
7073 @headitem Escape @tab Replaced with
7074 @item \a @tab Audible bell (@acronym{ASCII} 7)
7075 @item \b @tab Backspace (@acronym{ASCII} 8)
7076 @item \f @tab Form feed (@acronym{ASCII} 12)
7077 @item \n @tab New line (@acronym{ASCII} 10)
7078 @item \r @tab Carriage return (@acronym{ASCII} 13)
7079 @item \t @tab Horizontal tabulation (@acronym{ASCII} 9)
7080 @item \v @tab Vertical tabulation (@acronym{ASCII} 11)
7081 @item \? @tab @acronym{ASCII} 127
7082 @item \@var{n} @tab @acronym{ASCII} @var{n} (@var{n} should be an octal number
7086 A backslash followed by any other symbol is retained.
7088 This default behavior is controlled by the following command line
7094 Enable unquoting input file or member names (default).
7098 Disable unquoting input file or member names.
7101 If you specify a directory name as a file name argument, all the files
7102 in that directory are operated on by @command{tar}.
7104 If you do not specify files, @command{tar} behavior differs depending
7105 on the operation mode as described below:
7107 When @command{tar} is invoked with @option{--create} (@option{-c}),
7108 @command{tar} will stop immediately, reporting the following:
7112 $ @kbd{tar cf a.tar}
7113 tar: Cowardly refusing to create an empty archive
7114 Try 'tar --help' or 'tar --usage' for more information.
7118 If you specify either @option{--list} (@option{-t}) or
7119 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
7120 operates on all the archive members in the archive.
7122 If run with @option{--diff} option, tar will compare the archive with
7123 the contents of the current working directory.
7125 If you specify any other operation, @command{tar} does nothing.
7127 By default, @command{tar} takes file names from the command line. However,
7128 there are other ways to specify file or member names, or to modify the
7129 manner in which @command{tar} selects the files or members upon which to
7130 operate. In general, these methods work both for specifying the names
7131 of files and archive members.
7134 @section Reading Names from a File
7136 @cindex Reading file names from a file
7137 @cindex Lists of file names
7138 @cindex File Name arguments, alternatives
7139 @cindex @command{find}, using with @command{tar}
7140 Instead of giving the names of files or archive members on the command
7141 line, you can put the names into a file, and then use the
7142 @option{--files-from=@var{file-of-names}} (@option{-T
7143 @var{file-of-names}}) option to @command{tar}. Give the name of the
7144 file which contains the list of files to include as the argument to
7145 @option{--files-from}. In the list, the file names should be separated by
7146 newlines. You will frequently use this option when you have generated
7147 the list of files to archive with the @command{find} utility.
7151 @item --files-from=@var{file-name}
7152 @itemx -T @var{file-name}
7153 Get names to extract or create from file @var{file-name}.
7156 If you give a single dash as a file name for @option{--files-from}, (i.e.,
7157 you specify either @code{--files-from=-} or @code{-T -}), then the file
7158 names are read from standard input.
7160 Unless you are running @command{tar} with @option{--create}, you can not use
7161 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
7164 Any number of @option{-T} options can be given in the command line.
7166 The following example shows how to use @command{find} to generate a list of
7167 files smaller than 400K in length and put that list into a file
7168 called @file{small-files}. You can then use the @option{-T} option to
7169 @command{tar} to specify the files from that file, @file{small-files}, to
7170 create the archive @file{little.tgz}. (The @option{-z} option to
7171 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
7175 $ @kbd{find . -size -400 -print > small-files}
7176 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
7180 In the file list given by @option{-T} option, any file name beginning
7181 with @samp{-} character is considered a @command{tar} option and is
7182 processed accordingly@footnote{Versions of @GNUTAR{} up to 1.15.1
7183 recognized only @option{-C} option in file lists, and only if the
7184 option and its argument occupied two consecutive lines.}. For example,
7185 the common use of this feature is to change to another directory by
7186 specifying @option{-C} option:
7196 $ @kbd{tar -c -f foo.tar --files-from list}
7201 In this example, @command{tar} will first switch to @file{/etc}
7202 directory and add files @file{passwd} and @file{hosts} to the
7203 archive. Then it will change to @file{/lib} directory and will archive
7204 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
7209 $ @kbd{tar tf foo.tar}
7217 If you happen to have a file whose name starts with @samp{-},
7218 precede it with @option{--add-file} option to prevent it from
7219 being recognized as an option. For example: @code{--add-file=--my-file}.
7226 @subsection @code{NUL}-Terminated File Names
7228 @cindex File names, terminated by @code{NUL}
7229 @cindex @code{NUL}-terminated file names
7230 The @option{--null} option causes
7231 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
7232 to read file names terminated by a @code{NUL} instead of a newline, so
7233 files whose names contain newlines can be archived using
7234 @option{--files-from}.
7237 @xopindex{null, described}
7239 Only consider @code{NUL}-terminated file names, instead of files that
7240 terminate in a newline.
7242 @xopindex{no-null, described}
7244 Undo the effect of any previous @option{--null} option.
7247 The @option{--null} option is just like the one in @acronym{GNU}
7248 @command{xargs} and @command{cpio}, and is useful with the
7249 @option{-print0} predicate of @acronym{GNU} @command{find}. In
7250 @command{tar}, @option{--null} also disables special handling for
7251 file names that begin with dash.
7253 This example shows how to use @command{find} to generate a list of files
7254 larger than 800K in length and put that list into a file called
7255 @file{long-files}. The @option{-print0} option to @command{find} is just
7256 like @option{-print}, except that it separates files with a @code{NUL}
7257 rather than with a newline. You can then run @command{tar} with both the
7258 @option{--null} and @option{-T} options to specify that @command{tar} gets the
7259 files from that file, @file{long-files}, to create the archive
7260 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
7261 @command{tar} to recognize the @code{NUL} separator between files.
7264 $ @kbd{find . -size +800 -print0 > long-files}
7265 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
7268 The @option{--no-null} option can be used if you need to read both
7269 @code{NUL}-terminated and newline-terminated files on the same command line.
7270 For example, if @file{flist} is a newline-terminated file, then the
7271 following command can be used to combine it with the above command:
7275 $ @kbd{find . -size +800 -print0 |
7276 tar -c -f big.tar --null -T - --no-null -T flist}
7280 This example uses short options for typographic reasons, to avoid
7283 @GNUTAR is tries to automatically detect @code{NUL}-terminated file
7284 lists, so in many cases it is safe to use them even without the
7285 @option{--null} option. In this case @command{tar} will print a
7286 warning and continue reading such a file as if @option{--null} were
7291 $ @kbd{find . -size +800 -print0 | tar -c -f big.tar -T -}
7292 tar: -: file name read contains nul character
7296 The null terminator, however, remains in effect only for this
7297 particular file, any following @option{-T} options will assume
7298 newline termination. Of course, the null autodetection applies
7299 to these eventual surplus @option{-T} options as well.
7302 @section Excluding Some Files
7304 @cindex File names, excluding files by
7305 @cindex Excluding files by name and pattern
7306 @cindex Excluding files by file system
7308 @opindex exclude-from
7309 To avoid operating on files whose names match a particular pattern,
7310 use the @option{--exclude} or @option{--exclude-from} options.
7314 @item --exclude=@var{pattern}
7315 Causes @command{tar} to ignore files that match the @var{pattern}.
7319 The @option{--exclude=@var{pattern}} option prevents any file or
7320 member whose name matches the shell wildcard (@var{pattern}) from
7322 For example, to create an archive with all the contents of the directory
7323 @file{src} except for files whose names end in @file{.o}, use the
7324 command @samp{tar -cf src.tar --exclude='*.o' src}.
7326 You may give multiple @option{--exclude} options.
7329 @opindex exclude-from
7330 @item --exclude-from=@var{file}
7331 @itemx -X @var{file}
7332 Causes @command{tar} to ignore files that match the patterns listed in
7336 @findex exclude-from
7337 Use the @option{--exclude-from} option to read a
7338 list of patterns, one per line, from @var{file}; @command{tar} will
7339 ignore files matching those patterns. Thus if @command{tar} is
7340 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
7341 single line @file{*.o}, no files whose names end in @file{.o} will be
7342 added to the archive.
7344 Notice, that lines from @var{file} are read verbatim. One of the
7345 frequent errors is leaving some extra whitespace after a file name,
7346 which is difficult to catch using text editors.
7348 However, empty lines are OK.
7351 @cindex version control system, excluding files
7352 @cindex VCS, excluding files
7353 @cindex SCCS, excluding files
7354 @cindex RCS, excluding files
7355 @cindex CVS, excluding files
7356 @cindex SVN, excluding files
7357 @cindex git, excluding files
7358 @cindex Bazaar, excluding files
7359 @cindex Arch, excluding files
7360 @cindex Mercurial, excluding files
7361 @cindex Darcs, excluding files
7362 @opindex exclude-vcs
7364 Exclude files and directories used by following version control
7365 systems: @samp{CVS}, @samp{RCS}, @samp{SCCS}, @samp{SVN}, @samp{Arch},
7366 @samp{Bazaar}, @samp{Mercurial}, and @samp{Darcs}.
7368 As of version @value{VERSION}, the following files are excluded:
7371 @item @file{CVS/}, and everything under it
7372 @item @file{RCS/}, and everything under it
7373 @item @file{SCCS/}, and everything under it
7374 @item @file{.git/}, and everything under it
7375 @item @file{.gitignore}
7376 @item @file{.cvsignore}
7377 @item @file{.svn/}, and everything under it
7378 @item @file{.arch-ids/}, and everything under it
7379 @item @file{@{arch@}/}, and everything under it
7380 @item @file{=RELEASE-ID}
7381 @item @file{=meta-update}
7382 @item @file{=update}
7384 @item @file{.bzrignore}
7385 @item @file{.bzrtags}
7387 @item @file{.hgignore}
7388 @item @file{.hgrags}
7392 @opindex exclude-backups
7393 @item --exclude-backups
7394 Exclude backup and lock files. This option causes exclusion of files
7395 that match the following shell globbing patterns:
7405 @findex exclude-caches
7406 When creating an archive, the @option{--exclude-caches} option family
7407 causes @command{tar} to exclude all directories that contain a @dfn{cache
7408 directory tag}. A cache directory tag is a short file with the
7409 well-known name @file{CACHEDIR.TAG} and having a standard header
7410 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
7411 Various applications write cache directory tags into directories they
7412 use to hold regenerable, non-precious data, so that such data can be
7413 more easily excluded from backups.
7415 There are three @samp{exclude-caches} options, each providing a different
7416 exclusion semantics:
7419 @opindex exclude-caches
7420 @item --exclude-caches
7421 Do not archive the contents of the directory, but archive the
7422 directory itself and the @file{CACHEDIR.TAG} file.
7424 @opindex exclude-caches-under
7425 @item --exclude-caches-under
7426 Do not archive the contents of the directory, nor the
7427 @file{CACHEDIR.TAG} file, archive only the directory itself.
7429 @opindex exclude-caches-all
7430 @item --exclude-caches-all
7431 Omit directories containing @file{CACHEDIR.TAG} file entirely.
7435 Another option family, @option{--exclude-tag}, provides a generalization of
7436 this concept. It takes a single argument, a file name to look for.
7437 Any directory that contains this file will be excluded from the dump.
7438 Similarly to @samp{exclude-caches}, there are three options in this
7442 @opindex exclude-tag
7443 @item --exclude-tag=@var{file}
7444 Do not dump the contents of the directory, but dump the
7445 directory itself and the @var{file}.
7447 @opindex exclude-tag-under
7448 @item --exclude-tag-under=@var{file}
7449 Do not dump the contents of the directory, nor the
7450 @var{file}, archive only the directory itself.
7452 @opindex exclude-tag-all
7453 @item --exclude-tag-all=@var{file}
7454 Omit directories containing @var{file} file entirely.
7457 Multiple @option{--exclude-tag*} options can be given.
7459 For example, given this directory:
7474 The @option{--exclude-tag} will produce the following:
7477 $ @kbd{tar -cf archive.tar --exclude-tag=tagfile -v dir}
7482 tar: dir/folk/: contains a cache directory tag tagfile;
7487 Both the @file{dir/folk} directory and its tagfile are preserved in
7488 the archive, however the rest of files in this directory are not.
7490 Now, using the @option{--exclude-tag-under} option will exclude
7491 @file{tagfile} from the dump, while still preserving the directory
7492 itself, as shown in this example:
7495 $ @kbd{tar -cf archive.tar --exclude-tag-under=tagfile -v dir}
7500 ./tar: dir/folk/: contains a cache directory tag tagfile;
7504 Finally, using @option{--exclude-tag-all} omits the @file{dir/folk}
7508 $ @kbd{tar -cf archive.tar --exclude-tag-all=tagfile -v dir}
7512 ./tar: dir/folk/: contains a cache directory tag tagfile;
7513 directory not dumped
7517 * problems with exclude::
7520 @node problems with exclude
7521 @unnumberedsubsec Problems with Using the @code{exclude} Options
7523 @xopindex{exclude, potential problems with}
7524 Some users find @samp{exclude} options confusing. Here are some common
7529 The main operating mode of @command{tar} does not act on a file name
7530 explicitly listed on the command line, if one of its file name
7531 components is excluded. In the example above, if
7532 you create an archive and exclude files that end with @samp{*.o}, but
7533 explicitly name the file @samp{dir.o/foo} after all the options have been
7534 listed, @samp{dir.o/foo} will be excluded from the archive.
7537 You can sometimes confuse the meanings of @option{--exclude} and
7538 @option{--exclude-from}. Be careful: use @option{--exclude} when files
7539 to be excluded are given as a pattern on the command line. Use
7540 @option{--exclude-from} to introduce the name of a file which contains
7541 a list of patterns, one per line; each of these patterns can exclude
7542 zero, one, or many files.
7545 When you use @option{--exclude=@var{pattern}}, be sure to quote the
7546 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
7547 like @samp{*}. If you do not do this, the shell might expand the
7548 @samp{*} itself using files at hand, so @command{tar} might receive a
7549 list of files instead of one pattern, or none at all, making the
7550 command somewhat illegal. This might not correspond to what you want.
7555 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
7563 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
7567 You must use use shell syntax, or globbing, rather than @code{regexp}
7568 syntax, when using exclude options in @command{tar}. If you try to use
7569 @code{regexp} syntax to describe files to be excluded, your command
7573 @FIXME{The change in semantics must have occurred before 1.11,
7574 so I doubt if it is worth mentioning at all. Anyway, should at
7575 least specify in which version the semantics changed.}
7576 In earlier versions of @command{tar}, what is now the
7577 @option{--exclude-from} option was called @option{--exclude} instead.
7578 Now, @option{--exclude} applies to patterns listed on the command
7579 line and @option{--exclude-from} applies to patterns listed in a
7585 @section Wildcards Patterns and Matching
7587 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
7588 @samp{*} or @samp{?} for example, are replaced and expanded into all
7589 existing files matching the given pattern. @GNUTAR{} can use wildcard
7590 patterns for matching (or globbing) archive members when extracting
7591 from or listing an archive. Wildcard patterns are also used for
7592 verifying volume labels of @command{tar} archives. This section has the
7593 purpose of explaining wildcard syntax for @command{tar}.
7595 @FIXME{the next few paragraphs need work.}
7597 A @var{pattern} should be written according to shell syntax, using wildcard
7598 characters to effect globbing. Most characters in the pattern stand
7599 for themselves in the matched string, and case is significant: @samp{a}
7600 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
7601 pattern matches any single character in the matched string. The character
7602 @samp{*} in the pattern matches zero, one, or more single characters in
7603 the matched string. The character @samp{\} says to take the following
7604 character of the pattern @emph{literally}; it is useful when one needs to
7605 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
7607 The character @samp{[}, up to the matching @samp{]}, introduces a character
7608 class. A @dfn{character class} is a list of acceptable characters
7609 for the next single character of the matched string. For example,
7610 @samp{[abcde]} would match any of the first five letters of the alphabet.
7611 Note that within a character class, all of the ``special characters''
7612 listed above other than @samp{\} lose their special meaning; for example,
7613 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
7614 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
7615 the characters @samp{-} and @samp{]} must either come @emph{first} or
7616 @emph{last} in a character class.)
7618 @cindex Excluding characters from a character class
7619 @cindex Character class, excluding characters from
7620 If the first character of the class after the opening @samp{[}
7621 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
7622 Rather than listing character to match, it lists those characters which
7623 are @emph{forbidden} as the next single character of the matched string.
7625 Other characters of the class stand for themselves. The special
7626 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
7627 letters, is meant to represent all characters between @var{a} and
7630 @FIXME{need to add a sentence or so here to make this clear for those
7631 who don't have dan around.}
7633 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
7634 special for wildcard matches. However, if a pattern completely matches
7635 a directory prefix of a matched string, then it matches the full matched
7636 string: thus, excluding a directory also excludes all the files beneath it.
7639 * controlling pattern-matching::
7642 @node controlling pattern-matching
7643 @unnumberedsubsec Controlling Pattern-Matching
7645 For the purposes of this section, we call @dfn{exclusion members} all
7646 member names obtained while processing @option{--exclude} and
7647 @option{--exclude-from} options, and @dfn{inclusion members} those
7648 member names that were given in the command line or read from the file
7649 specified with @option{--files-from} option.
7651 These two pairs of member lists are used in the following operations:
7652 @option{--diff}, @option{--extract}, @option{--list},
7655 There are no inclusion members in create mode (@option{--create} and
7656 @option{--append}), since in this mode the names obtained from the
7657 command line refer to @emph{files}, not archive members.
7659 By default, inclusion members are compared with archive members
7660 literally @footnote{Notice that earlier @GNUTAR{} versions used
7661 globbing for inclusion members, which contradicted to UNIX98
7662 specification and was not documented. @xref{Changes}, for more
7663 information on this and other changes.} and exclusion members are
7664 treated as globbing patterns. For example:
7668 $ @kbd{tar tf foo.tar}
7673 # @i{Member names are used verbatim:}
7674 $ @kbd{tar -xf foo.tar -v '[remarks]'}
7676 # @i{Exclude member names are globbed:}
7677 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
7683 This behavior can be altered by using the following options:
7688 Treat all member names as wildcards.
7690 @opindex no-wildcards
7691 @item --no-wildcards
7692 Treat all member names as literal strings.
7695 Thus, to extract files whose names end in @samp{.c}, you can use:
7698 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
7704 Notice quoting of the pattern to prevent the shell from interpreting
7707 The effect of @option{--wildcards} option is canceled by
7708 @option{--no-wildcards}. This can be used to pass part of
7709 the command line arguments verbatim and other part as globbing
7710 patterns. For example, the following invocation:
7713 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
7717 instructs @command{tar} to extract from @file{foo.tar} all files whose
7718 names end in @samp{.txt} and the file named @file{[remarks]}.
7720 Normally, a pattern matches a name if an initial subsequence of the
7721 name's components matches the pattern, where @samp{*}, @samp{?}, and
7722 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
7723 and wildcards can match @samp{/}.
7725 Other than optionally stripping leading @samp{/} from names
7726 (@pxref{absolute}), patterns and names are used as-is. For
7727 example, trailing @samp{/} is not trimmed from a user-specified name
7728 before deciding whether to exclude it.
7730 However, this matching procedure can be altered by the options listed
7731 below. These options accumulate. For example:
7734 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
7738 ignores case when excluding @samp{makefile}, but not when excluding
7743 @opindex no-anchored
7745 @itemx --no-anchored
7746 If anchored, a pattern must match an initial subsequence
7747 of the name's components. Otherwise, the pattern can match any
7748 subsequence. Default is @option{--no-anchored} for exclusion members
7749 and @option{--anchored} inclusion members.
7751 @opindex ignore-case
7752 @opindex no-ignore-case
7754 @itemx --no-ignore-case
7755 When ignoring case, upper-case patterns match lower-case names and vice versa.
7756 When not ignoring case (the default), matching is case-sensitive.
7758 @opindex wildcards-match-slash
7759 @opindex no-wildcards-match-slash
7760 @item --wildcards-match-slash
7761 @itemx --no-wildcards-match-slash
7762 When wildcards match slash (the default for exclusion members), a
7763 wildcard like @samp{*} in the pattern can match a @samp{/} in the
7764 name. Otherwise, @samp{/} is matched only by @samp{/}.
7768 The @option{--recursion} and @option{--no-recursion} options
7769 (@pxref{recurse}) also affect how member patterns are interpreted. If
7770 recursion is in effect, a pattern matches a name if it matches any of
7771 the name's parent directories.
7773 The following table summarizes pattern-matching default values:
7775 @multitable @columnfractions .3 .7
7776 @headitem Members @tab Default settings
7777 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
7778 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
7781 @node quoting styles
7782 @section Quoting Member Names
7784 When displaying member names, @command{tar} takes care to avoid
7785 ambiguities caused by certain characters. This is called @dfn{name
7786 quoting}. The characters in question are:
7789 @item Non-printable control characters:
7790 @anchor{escape sequences}
7791 @multitable @columnfractions 0.20 0.10 0.60
7792 @headitem Character @tab @acronym{ASCII} @tab Character name
7793 @item \a @tab 7 @tab Audible bell
7794 @item \b @tab 8 @tab Backspace
7795 @item \f @tab 12 @tab Form feed
7796 @item \n @tab 10 @tab New line
7797 @item \r @tab 13 @tab Carriage return
7798 @item \t @tab 9 @tab Horizontal tabulation
7799 @item \v @tab 11 @tab Vertical tabulation
7802 @item Space (@acronym{ASCII} 32)
7804 @item Single and double quotes (@samp{'} and @samp{"})
7806 @item Backslash (@samp{\})
7809 The exact way @command{tar} uses to quote these characters depends on
7810 the @dfn{quoting style}. The default quoting style, called
7811 @dfn{escape} (see below), uses backslash notation to represent control
7812 characters, space and backslash. Using this quoting style, control
7813 characters are represented as listed in column @samp{Character} in the
7814 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
7816 @GNUTAR{} offers seven distinct quoting styles, which can be selected
7817 using @option{--quoting-style} option:
7820 @item --quoting-style=@var{style}
7821 @opindex quoting-style
7823 Sets quoting style. Valid values for @var{style} argument are:
7824 literal, shell, shell-always, c, escape, locale, clocale.
7827 These styles are described in detail below. To illustrate their
7828 effect, we will use an imaginary tar archive @file{arch.tar}
7829 containing the following members:
7833 # 1. Contains horizontal tabulation character.
7835 # 2. Contains newline character
7838 # 3. Contains a space
7840 # 4. Contains double quotes
7842 # 5. Contains single quotes
7844 # 6. Contains a backslash character:
7849 Here is how usual @command{ls} command would have listed them, if they
7850 had existed in the current working directory:
7868 No quoting, display each character as is:
7872 $ @kbd{tar tf arch.tar --quoting-style=literal}
7885 Display characters the same way Bourne shell does:
7886 control characters, except @samp{\t} and @samp{\n}, are printed using
7887 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
7888 single quote is printed as @samp{\'}. If a name contains any quoted
7889 characters, it is enclosed in single quotes. In particular, if a name
7890 contains single quotes, it is printed as several single-quoted strings:
7894 $ @kbd{tar tf arch.tar --quoting-style=shell}
7897 './a'\''single'\''quote'
7907 Same as @samp{shell}, but the names are always enclosed in single
7912 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
7915 './a'\''single'\''quote'
7925 Use the notation of the C programming language. All names are
7926 enclosed in double quotes. Control characters are quoted using
7927 backslash notations, double quotes are represented as @samp{\"},
7928 backslash characters are represented as @samp{\\}. Single quotes and
7929 spaces are not quoted:
7933 $ @kbd{tar tf arch.tar --quoting-style=c}
7937 "./a\"double\"quote"
7945 Control characters are printed using backslash notation, a space is
7946 printed as @samp{\ } and a backslash as @samp{\\}. This is the
7947 default quoting style, unless it was changed when configured the
7952 $ @kbd{tar tf arch.tar --quoting-style=escape}
7964 Control characters, single quote and backslash are printed using
7965 backslash notation. All names are quoted using left and right
7966 quotation marks, appropriate to the current locale. If it does not
7967 define quotation marks, use @samp{'} as left and as right
7968 quotation marks. Any occurrences of the right quotation mark in a
7969 name are escaped with @samp{\}, for example:
7975 $ @kbd{tar tf arch.tar --quoting-style=locale}
7978 './a\'single\'quote'
7987 Same as @samp{locale}, but @samp{"} is used for both left and right
7988 quotation marks, if not provided by the currently selected locale:
7992 $ @kbd{tar tf arch.tar --quoting-style=clocale}
7996 "./a\"double\"quote"
8004 You can specify which characters should be quoted in addition to those
8005 implied by the current quoting style:
8008 @item --quote-chars=@var{string}
8009 Always quote characters from @var{string}, even if the selected
8010 quoting style would not quote them.
8013 For example, using @samp{escape} quoting (compare with the usual
8014 escape listing above):
8018 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
8029 To disable quoting of such additional characters, use the following
8033 @item --no-quote-chars=@var{string}
8034 Remove characters listed in @var{string} from the list of quoted
8035 characters set by the previous @option{--quote-chars} option.
8038 This option is particularly useful if you have added
8039 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
8040 and wish to disable it for the current invocation.
8042 Note, that @option{--no-quote-chars} does @emph{not} disable those
8043 characters that are quoted by default in the selected quoting style.
8046 @section Modifying File and Member Names
8048 @command{Tar} archives contain detailed information about files stored
8049 in them and full file names are part of that information. When
8050 storing a file to an archive, its file name is recorded in it,
8051 along with the actual file contents. When restoring from an archive,
8052 a file is created on disk with exactly the same name as that stored
8053 in the archive. In the majority of cases this is the desired behavior
8054 of a file archiver. However, there are some cases when it is not.
8056 First of all, it is often unsafe to extract archive members with
8057 absolute file names or those that begin with a @file{../}. @GNUTAR{}
8058 takes special precautions when extracting such names and provides a
8059 special option for handling them, which is described in
8062 Secondly, you may wish to extract file names without some leading
8063 directory components, or with otherwise modified names. In other
8064 cases it is desirable to store files under differing names in the
8067 @GNUTAR{} provides several options for these needs.
8070 @opindex strip-components
8071 @item --strip-components=@var{number}
8072 Strip given @var{number} of leading components from file names before
8076 For example, suppose you have archived whole @file{/usr} hierarchy to
8077 a tar archive named @file{usr.tar}. Among other files, this archive
8078 contains @file{usr/include/stdlib.h}, which you wish to extract to
8079 the current working directory. To do so, you type:
8082 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
8085 The option @option{--strip=2} instructs @command{tar} to strip the
8086 two leading components (@file{usr/} and @file{include/}) off the file
8089 If you add the @option{--verbose} (@option{-v}) option to the invocation
8090 above, you will note that the verbose listing still contains the
8091 full file name, with the two removed components still in place. This
8092 can be inconvenient, so @command{tar} provides a special option for
8093 altering this behavior:
8095 @anchor{show-transformed-names}
8097 @opindex show-transformed-names
8098 @item --show-transformed-names
8099 Display file or member names with all requested transformations
8108 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
8109 usr/include/stdlib.h
8110 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
8115 Notice that in both cases the file @file{stdlib.h} is extracted to the
8116 current working directory, @option{--show-transformed-names} affects
8117 only the way its name is displayed.
8119 This option is especially useful for verifying whether the invocation
8120 will have the desired effect. Thus, before running
8123 $ @kbd{tar -x --strip=@var{n}}
8127 it is often advisable to run
8130 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
8134 to make sure the command will produce the intended results.
8136 In case you need to apply more complex modifications to the file name,
8137 @GNUTAR{} provides a general-purpose transformation option:
8142 @item --transform=@var{expression}
8143 @itemx --xform=@var{expression}
8144 Modify file names using supplied @var{expression}.
8148 The @var{expression} is a @command{sed}-like replace expression of the
8152 s/@var{regexp}/@var{replace}/[@var{flags}]
8156 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
8157 replacement for each file name part that matches @var{regexp}. Both
8158 @var{regexp} and @var{replace} are described in detail in
8159 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
8161 Any delimiter can be used in lieu of @samp{/}, the only requirement being
8162 that it be used consistently throughout the expression. For example,
8163 the following two expressions are equivalent:
8172 Changing delimiters is often useful when the @var{regex} contains
8173 slashes. For example, it is more convenient to write @code{s,/,-,} than
8176 As in @command{sed}, you can give several replace expressions,
8177 separated by a semicolon.
8179 Supported @var{flags} are:
8183 Apply the replacement to @emph{all} matches to the @var{regexp}, not
8187 Use case-insensitive matching.
8190 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
8191 regexps, Extended regular expressions, Extended regular expressions,
8195 Only replace the @var{number}th match of the @var{regexp}.
8197 Note: the @acronym{POSIX} standard does not specify what should happen
8198 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
8199 follows the GNU @command{sed} implementation in this regard, so
8200 the interaction is defined to be: ignore matches before the
8201 @var{number}th, and then match and replace all matches from the
8206 In addition, several @dfn{transformation scope} flags are supported,
8207 that control to what files transformations apply. These are:
8211 Apply transformation to regular archive members.
8214 Do not apply transformation to regular archive members.
8217 Apply transformation to symbolic link targets.
8220 Do not apply transformation to symbolic link targets.
8223 Apply transformation to hard link targets.
8226 Do not apply transformation to hard link targets.
8229 Default is @samp{rsh}, which means to apply tranformations to both archive
8230 members and targets of symbolic and hard links.
8232 Default scope flags can also be changed using @samp{flags=} statement
8233 in the transform expression. The flags set this way remain in force
8234 until next @samp{flags=} statement or end of expression, whichever
8235 occurs first. For example:
8238 --transform 'flags=S;s|^|/usr/local/|'
8241 Here are several examples of @option{--transform} usage:
8244 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
8247 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
8250 @item Strip two leading directory components (equivalent to
8251 @option{--strip-components=2}):
8254 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
8257 @item Convert each file name to lower case:
8260 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
8263 @item Prepend @file{/prefix/} to each file name:
8266 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
8269 @item Archive the @file{/lib} directory, prepending @samp{/usr/local}
8270 to each archive member:
8273 $ @kbd{tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib}
8277 Notice the use of flags in the last example. The @file{/lib}
8278 directory often contains many symbolic links to files within it.
8279 It may look, for example, like this:
8283 drwxr-xr-x root/root 0 2008-07-08 16:20 /lib/
8284 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
8285 lrwxrwxrwx root/root 0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
8289 Using the expression @samp{s,^,/usr/local/,} would mean adding
8290 @samp{/usr/local} to both regular archive members and to link
8291 targets. In this case, @file{/lib/libc.so.6} would become:
8294 /usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so
8297 This is definitely not desired. To avoid this, the @samp{S} flag
8298 is used, which excludes symbolic link targets from filename
8299 transformations. The result is:
8302 $ @kbd{tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \
8303 --show-transformed /lib}
8304 drwxr-xr-x root/root 0 2008-07-08 16:20 /usr/local/lib/
8305 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
8306 lrwxrwxrwx root/root 0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
8310 Unlike @option{--strip-components}, @option{--transform} can be used
8311 in any @GNUTAR{} operation mode. For example, the following command
8312 adds files to the archive while replacing the leading @file{usr/}
8313 component with @file{var/}:
8316 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
8319 To test @option{--transform} effect we suggest using
8320 @option{--show-transformed-names} option:
8323 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
8324 --verbose --show-transformed-names /}
8327 If both @option{--strip-components} and @option{--transform} are used
8328 together, then @option{--transform} is applied first, and the required
8329 number of components is then stripped from its result.
8331 You can use as many @option{--transform} options in a single command
8332 line as you want. The specified expressions will then be applied in
8333 order of their appearance. For example, the following two invocations
8337 $ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
8338 --transform='s,/usr/local,/usr/,'}
8339 $ @kbd{tar -cf arch.tar \
8340 --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
8344 @section Operating Only on New Files
8346 @cindex Excluding file by age
8347 @cindex Data Modification time, excluding files by
8348 @cindex Modification time, excluding files by
8349 @cindex Age, excluding files by
8350 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
8351 @option{-N @var{date}}) option causes @command{tar} to only work on
8352 files whose data modification or status change times are newer than
8353 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
8354 it is taken to be a file name; the data modification time of that file
8355 is used as the date. If you use this option when creating or appending
8356 to an archive, the archive will only include new files. If you use
8357 @option{--after-date} when extracting an archive, @command{tar} will
8358 only extract files newer than the @var{date} you specify.
8360 If you only want @command{tar} to make the date comparison based on
8361 modification of the file's data (rather than status
8362 changes), then use the @option{--newer-mtime=@var{date}} option.
8364 @cindex --after-date and --update compared
8365 @cindex --newer-mtime and --update compared
8366 You may use these options with any operation. Note that these options
8367 differ from the @option{--update} (@option{-u}) operation in that they
8368 allow you to specify a particular date against which @command{tar} can
8369 compare when deciding whether or not to archive the files.
8374 @item --after-date=@var{date}
8375 @itemx --newer=@var{date}
8376 @itemx -N @var{date}
8377 Only store files newer than @var{date}.
8379 Acts on files only if their data modification or status change times are
8380 later than @var{date}. Use in conjunction with any operation.
8382 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
8383 name; the data modification time of that file is used as the date.
8385 @opindex newer-mtime
8386 @item --newer-mtime=@var{date}
8387 Acts like @option{--after-date}, but only looks at data modification times.
8390 These options limit @command{tar} to operate only on files which have
8391 been modified after the date specified. A file's status is considered to have
8392 changed if its contents have been modified, or if its owner,
8393 permissions, and so forth, have been changed. (For more information on
8394 how to specify a date, see @ref{Date input formats}; remember that the
8395 entire date argument must be quoted if it contains any spaces.)
8397 Gurus would say that @option{--after-date} tests both the data
8398 modification time (@code{mtime}, the time the contents of the file
8399 were last modified) and the status change time (@code{ctime}, the time
8400 the file's status was last changed: owner, permissions, etc.@:)
8401 fields, while @option{--newer-mtime} tests only the @code{mtime}
8404 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
8405 @code{ctime} and processes the file if either one is more recent than
8406 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
8407 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
8408 contents of the file were looked at).
8410 Date specifiers can have embedded spaces. Because of this, you may need
8411 to quote date arguments to keep the shell from parsing them as separate
8412 arguments. For example, the following command will add to the archive
8413 all the files modified less than two days ago:
8416 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
8419 When any of these options is used with the option @option{--verbose}
8420 (@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
8421 date back to its textual representation and compare that with the
8422 one given with the option. If the two dates differ, @command{tar} will
8423 print a warning saying what date it will use. This is to help user
8424 ensure he is using the right date. For example:
8428 $ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
8429 tar: Option --after-date: Treating date '10 days ago' as 2006-06-11
8435 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
8436 should not be used for incremental backups. @xref{Incremental Dumps},
8437 for proper way of creating incremental backups.
8441 @section Descending into Directories
8442 @cindex Avoiding recursion in directories
8443 @cindex Descending directories, avoiding
8444 @cindex Directories, avoiding recursion
8445 @cindex Recursion in directories, avoiding
8447 Usually, @command{tar} will recursively explore all directories (either
8448 those given on the command line or through the @option{--files-from}
8449 option) for the various files they contain. However, you may not always
8450 want @command{tar} to act this way.
8452 @opindex no-recursion
8453 @cindex @command{find}, using with @command{tar}
8454 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
8455 into specified directories. If you specify @option{--no-recursion}, you can
8456 use the @command{find} (@pxref{Top,, find, find, GNU Find Manual})
8457 utility for hunting through levels of directories to
8458 construct a list of file names which you could then pass to @command{tar}.
8459 @command{find} allows you to be more selective when choosing which files to
8460 archive; see @ref{files}, for more information on using @command{find} with
8464 @item --no-recursion
8465 Prevents @command{tar} from recursively descending directories.
8469 Requires @command{tar} to recursively descend directories.
8470 This is the default.
8473 When you use @option{--no-recursion}, @GNUTAR{} grabs
8474 directory entries themselves, but does not descend on them
8475 recursively. Many people use @command{find} for locating files they
8476 want to back up, and since @command{tar} @emph{usually} recursively
8477 descends on directories, they have to use the @samp{@w{-not -type d}}
8478 test in their @command{find} invocation (@pxref{Type, Type, Type test,
8479 find, Finding Files}), as they usually do not want all the files in a
8480 directory. They then use the @option{--files-from} option to archive
8481 the files located via @command{find}.
8483 The problem when restoring files archived in this manner is that the
8484 directories themselves are not in the archive; so the
8485 @option{--same-permissions} (@option{--preserve-permissions},
8486 @option{-p}) option does not affect them---while users might really
8487 like it to. Specifying @option{--no-recursion} is a way to tell
8488 @command{tar} to grab only the directory entries given to it, adding
8489 no new files on its own. To summarize, if you use @command{find} to
8490 create a list of files to be stored in an archive, use it as follows:
8494 $ @kbd{find @var{dir} @var{tests} | \
8495 tar -cf @var{archive} -T - --no-recursion}
8499 The @option{--no-recursion} option also applies when extracting: it
8500 causes @command{tar} to extract only the matched directory entries, not
8501 the files under those directories.
8503 The @option{--no-recursion} option also affects how globbing patterns
8504 are interpreted (@pxref{controlling pattern-matching}).
8506 The @option{--no-recursion} and @option{--recursion} options apply to
8507 later options and operands, and can be overridden by later occurrences
8508 of @option{--no-recursion} and @option{--recursion}. For example:
8511 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
8515 creates an archive with one entry for @file{grape}, and the recursive
8516 contents of @file{grape/concord}, but no entries under @file{grape}
8517 other than @file{grape/concord}.
8520 @section Crossing File System Boundaries
8521 @cindex File system boundaries, not crossing
8523 @command{tar} will normally automatically cross file system boundaries in
8524 order to archive files which are part of a directory tree. You can
8525 change this behavior by running @command{tar} and specifying
8526 @option{--one-file-system}. This option only affects files that are
8527 archived because they are in a directory that is being archived;
8528 @command{tar} will still archive files explicitly named on the command line
8529 or through @option{--files-from}, regardless of where they reside.
8532 @opindex one-file-system
8533 @item --one-file-system
8534 Prevents @command{tar} from crossing file system boundaries when
8535 archiving. Use in conjunction with any write operation.
8538 The @option{--one-file-system} option causes @command{tar} to modify its
8539 normal behavior in archiving the contents of directories. If a file in
8540 a directory is not on the same file system as the directory itself, then
8541 @command{tar} will not archive that file. If the file is a directory
8542 itself, @command{tar} will not archive anything beneath it; in other words,
8543 @command{tar} will not cross mount points.
8545 This option is useful for making full or incremental archival backups of
8546 a file system. If this option is used in conjunction with
8547 @option{--verbose} (@option{-v}), files that are excluded are
8548 mentioned by name on the standard error.
8551 * directory:: Changing Directory
8552 * absolute:: Absolute File Names
8556 @subsection Changing the Working Directory
8558 @FIXME{need to read over this node now for continuity; i've switched
8559 things around some.}
8561 @cindex Changing directory mid-stream
8562 @cindex Directory, changing mid-stream
8563 @cindex Working directory, specifying
8564 To change the working directory in the middle of a list of file names,
8565 either on the command line or in a file specified using
8566 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
8567 This will change the working directory to the specified directory
8568 after that point in the list.
8572 @item --directory=@var{directory}
8573 @itemx -C @var{directory}
8574 Changes the working directory in the middle of a command line.
8580 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
8584 will place the files @file{grape} and @file{prune} from the current
8585 directory into the archive @file{jams.tar}, followed by the file
8586 @file{cherry} from the directory @file{food}. This option is especially
8587 useful when you have several widely separated files that you want to
8588 store in the same archive.
8590 Note that the file @file{cherry} is recorded in the archive under the
8591 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
8592 archive will contain three files that all appear to have come from the
8593 same directory; if the archive is extracted with plain @samp{tar
8594 --extract}, all three files will be written in the current directory.
8596 Contrast this with the command,
8599 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
8603 which records the third file in the archive under the name
8604 @file{red/cherry} so that, if the archive is extracted using
8605 @samp{tar --extract}, the third file will be written in a subdirectory
8608 You can use the @option{--directory} option to make the archive
8609 independent of the original name of the directory holding the files.
8610 The following command places the files @file{/etc/passwd},
8611 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
8615 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
8619 However, the names of the archive members will be exactly what they were
8620 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
8621 They will not appear to be related by file name to the original
8622 directories where those files were located.
8624 Note that @option{--directory} options are interpreted consecutively. If
8625 @option{--directory} specifies a relative file name, it is interpreted
8626 relative to the then current directory, which might not be the same as
8627 the original current working directory of @command{tar}, due to a previous
8628 @option{--directory} option.
8630 When using @option{--files-from} (@pxref{files}), you can put various
8631 @command{tar} options (including @option{-C}) in the file list. Notice,
8632 however, that in this case the option and its argument may not be
8633 separated by whitespace. If you use short option, its argument must
8634 either follow the option letter immediately, without any intervening
8635 whitespace, or occupy the next line. Otherwise, if you use long
8636 option, separate its argument by an equal sign.
8638 For instance, the file list for the above example will be:
8651 To use it, you would invoke @command{tar} as follows:
8654 $ @kbd{tar -c -f foo.tar --files-from list}
8657 The interpretation of @option{--directory} is disabled by
8658 @option{--null} option.
8661 @subsection Absolute File Names
8662 @cindex absolute file names
8663 @cindex file names, absolute
8665 By default, @GNUTAR{} drops a leading @samp{/} on
8666 input or output, and complains about file names containing a @file{..}
8667 component. There is an option that turns off this behavior:
8670 @opindex absolute-names
8671 @item --absolute-names
8673 Do not strip leading slashes from file names, and permit file names
8674 containing a @file{..} file name component.
8677 When @command{tar} extracts archive members from an archive, it strips any
8678 leading slashes (@samp{/}) from the member name. This causes absolute
8679 member names in the archive to be treated as relative file names. This
8680 allows you to have such members extracted wherever you want, instead of
8681 being restricted to extracting the member in the exact directory named
8682 in the archive. For example, if the archive member has the name
8683 @file{/etc/passwd}, @command{tar} will extract it as if the name were
8684 really @file{etc/passwd}.
8686 File names containing @file{..} can cause problems when extracting, so
8687 @command{tar} normally warns you about such files when creating an
8688 archive, and rejects attempts to extracts such files.
8690 Other @command{tar} programs do not do this. As a result, if you
8691 create an archive whose member names start with a slash, they will be
8692 difficult for other people with a non-@GNUTAR{}
8693 program to use. Therefore, @GNUTAR{} also strips
8694 leading slashes from member names when putting members into the
8695 archive. For example, if you ask @command{tar} to add the file
8696 @file{/bin/ls} to an archive, it will do so, but the member name will
8697 be @file{bin/ls}@footnote{A side effect of this is that when
8698 @option{--create} is used with @option{--verbose} the resulting output
8699 is not, generally speaking, the same as the one you'd get running
8700 @kbd{tar --list} command. This may be important if you use some
8701 scripts for comparing both outputs. @xref{listing member and file names},
8702 for the information on how to handle this case.}.
8704 Symbolic links containing @file{..} or leading @samp{/} can also cause
8705 problems when extracting, so @command{tar} normally extracts them last;
8706 it may create empty files as placeholders during extraction.
8708 If you use the @option{--absolute-names} (@option{-P}) option,
8709 @command{tar} will do none of these transformations.
8711 To archive or extract files relative to the root directory, specify
8712 the @option{--absolute-names} (@option{-P}) option.
8714 Normally, @command{tar} acts on files relative to the working
8715 directory---ignoring superior directory names when archiving, and
8716 ignoring leading slashes when extracting.
8718 When you specify @option{--absolute-names} (@option{-P}),
8719 @command{tar} stores file names including all superior directory
8720 names, and preserves leading slashes. If you only invoked
8721 @command{tar} from the root directory you would never need the
8722 @option{--absolute-names} option, but using this option
8723 may be more convenient than switching to root.
8725 @FIXME{Should be an example in the tutorial/wizardry section using this
8726 to transfer files between systems.}
8729 @item --absolute-names
8730 Preserves full file names (including superior directory names) when
8731 archiving and extracting files.
8735 @command{tar} prints out a message about removing the @samp{/} from
8736 file names. This message appears once per @GNUTAR{}
8737 invocation. It represents something which ought to be told; ignoring
8738 what it means can cause very serious surprises, later.
8740 Some people, nevertheless, do not want to see this message. Wanting to
8741 play really dangerously, one may of course redirect @command{tar} standard
8742 error to the sink. For example, under @command{sh}:
8745 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
8749 Another solution, both nicer and simpler, would be to change to
8750 the @file{/} directory first, and then avoid absolute notation.
8754 $ @kbd{tar -c -f archive.tar -C / home}
8757 @xref{Integrity}, for some of the security-related implications
8758 of using this option.
8760 @include parse-datetime.texi
8763 @chapter Controlling the Archive Format
8765 @cindex Tar archive formats
8766 Due to historical reasons, there are several formats of tar archives.
8767 All of them are based on the same principles, but have some subtle
8768 differences that often make them incompatible with each other.
8770 GNU tar is able to create and handle archives in a variety of formats.
8771 The most frequently used formats are (in alphabetical order):
8775 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
8776 from an early @acronym{POSIX} standard, adding some improvements such as
8777 sparse file handling and incremental archives. Unfortunately these
8778 features were implemented in a way incompatible with other archive
8781 Archives in @samp{gnu} format are able to hold file names of unlimited
8785 Format used by @GNUTAR{} of versions prior to 1.12.
8788 Archive format, compatible with the V7 implementation of tar. This
8789 format imposes a number of limitations. The most important of them
8793 @item The maximum length of a file name is limited to 99 characters.
8794 @item The maximum length of a symbolic link is limited to 99 characters.
8795 @item It is impossible to store special files (block and character
8796 devices, fifos etc.)
8797 @item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
8799 @item V7 archives do not contain symbolic ownership information (user
8800 and group name of the file owner).
8803 This format has traditionally been used by Automake when producing
8804 Makefiles. This practice will change in the future, in the meantime,
8805 however this means that projects containing file names more than 99
8806 characters long will not be able to use @GNUTAR{} @value{VERSION} and
8807 Automake prior to 1.9.
8810 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
8811 symbolic ownership information. It is also able to store
8812 special files. However, it imposes several restrictions as well:
8815 @item The maximum length of a file name is limited to 256 characters,
8816 provided that the file name can be split at a directory separator in
8817 two parts, first of them being at most 155 bytes long. So, in most
8818 cases the maximum file name length will be shorter than 256
8820 @item The maximum length of a symbolic link name is limited to
8822 @item Maximum size of a file the archive is able to accommodate
8824 @item Maximum value of UID/GID is 2097151.
8825 @item Maximum number of bits in device major and minor numbers is 21.
8829 Format used by J@"org Schilling @command{star}
8830 implementation. @GNUTAR{} is able to read @samp{star} archives but
8831 currently does not produce them.
8834 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
8835 most flexible and feature-rich format. It does not impose any
8836 restrictions on file sizes or file name lengths. This format is quite
8837 recent, so not all tar implementations are able to handle it properly.
8838 However, this format is designed in such a way that any tar
8839 implementation able to read @samp{ustar} archives will be able to read
8840 most @samp{posix} archives as well, with the only exception that any
8841 additional information (such as long file names etc.) will in such
8842 case be extracted as plain text files along with the files it refers to.
8844 This archive format will be the default format for future versions
8849 The following table summarizes the limitations of each of these
8852 @multitable @columnfractions .10 .20 .20 .20 .20
8853 @headitem Format @tab UID @tab File Size @tab File Name @tab Devn
8854 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8855 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8856 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
8857 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
8858 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
8861 The default format for @GNUTAR{} is defined at compilation
8862 time. You may check it by running @command{tar --help}, and examining
8863 the last lines of its output. Usually, @GNUTAR{} is configured
8864 to create archives in @samp{gnu} format, however, future version will
8865 switch to @samp{posix}.
8868 * Compression:: Using Less Space through Compression
8869 * Attributes:: Handling File Attributes
8870 * Portability:: Making @command{tar} Archives More Portable
8871 * cpio:: Comparison of @command{tar} and @command{cpio}
8875 @section Using Less Space through Compression
8878 * gzip:: Creating and Reading Compressed Archives
8879 * sparse:: Archiving Sparse Files
8883 @subsection Creating and Reading Compressed Archives
8884 @cindex Compressed archives
8885 @cindex Storing archives in compressed format
8893 @GNUTAR{} is able to create and read compressed archives. It supports
8894 a wide variety of compression programs, namely: @command{gzip},
8895 @command{bzip2}, @command{lzip}, @command{lzma}, @command{lzop},
8896 @command{xz} and traditional @command{compress}. The latter is
8897 supported mostly for backward compatibility, and we recommend
8898 against using it, because it is by far less effective than the other
8899 compression programs@footnote{It also had patent problems in the past.}.
8901 Creating a compressed archive is simple: you just specify a
8902 @dfn{compression option} along with the usual archive creation
8903 commands. The compression option is @option{-z} (@option{--gzip}) to
8904 create a @command{gzip} compressed archive, @option{-j}
8905 (@option{--bzip2}) to create a @command{bzip2} compressed archive,
8906 @option{--lzip} to create an @asis{lzip} compressed archive,
8907 @option{-J} (@option{--xz}) to create an @asis{XZ} archive,
8908 @option{--lzma} to create an @asis{LZMA} compressed
8909 archive, @option{--lzop} to create an @asis{LSOP} archive, and
8910 @option{-Z} (@option{--compress}) to use @command{compress} program.
8914 $ @kbd{tar czf archive.tar.gz .}
8917 You can also let @GNUTAR{} select the compression program based on
8918 the suffix of the archive file name. This is done using
8919 @option{--auto-compress} (@option{-a}) command line option. For
8920 example, the following invocation will use @command{bzip2} for
8924 $ @kbd{tar caf archive.tar.bz2 .}
8928 whereas the following one will use @command{lzma}:
8931 $ @kbd{tar caf archive.tar.lzma .}
8934 For a complete list of file name suffixes recognized by @GNUTAR{},
8935 see @ref{auto-compress}.
8937 Reading compressed archive is even simpler: you don't need to specify
8938 any additional options as @GNUTAR{} recognizes its format
8939 automatically. Thus, the following commands will list and extract the
8940 archive created in previous example:
8943 # List the compressed archive
8944 $ @kbd{tar tf archive.tar.gz}
8945 # Extract the compressed archive
8946 $ @kbd{tar xf archive.tar.gz}
8949 The format recognition algorithm is based on @dfn{signatures}, a
8950 special byte sequences in the beginning of file, that are specific for
8951 certain compression formats. If this approach fails, @command{tar}
8952 falls back to using archive name suffix to determine its format
8953 (@pxref{auto-compress}, for a list of recognized suffixes).
8955 @anchor{alternative decompression programs}
8956 @cindex alternative decompression programs
8957 Some compression programs are able to handle different compression
8958 formats. @GNUTAR{} uses this, if the principal decompressor for the
8959 given format is not available. For example, if @command{compress} is
8960 not installed, @command{tar} will try to use @command{gzip}. As of
8961 version @value{VERSION} the following alternatives are
8962 tried@footnote{To verbosely trace the decompressor selection, use the
8963 @option{--warning=decompress-program} option
8964 (@pxref{warnings,decompress-program}).}:
8966 @multitable @columnfractions 0.3 0.3 0.3
8967 @headitem Format @tab Main decompressor @tab Alternatives
8968 @item compress @tab compress @tab gzip
8969 @item lzma @tab lzma @tab xz
8970 @item bzip2 @tab bzip2 @tab lbzip2
8973 The only case when you have to specify a decompression option while
8974 reading the archive is when reading from a pipe or from a tape drive
8975 that does not support random access. However, in this case @GNUTAR{}
8976 will indicate which option you should use. For example:
8979 $ @kbd{cat archive.tar.gz | tar tf -}
8980 tar: Archive is compressed. Use -z option
8981 tar: Error is not recoverable: exiting now
8984 If you see such diagnostics, just add the suggested option to the
8985 invocation of @GNUTAR{}:
8988 $ @kbd{cat archive.tar.gz | tar tzf -}
8991 Notice also, that there are several restrictions on operations on
8992 compressed archives. First of all, compressed archives cannot be
8993 modified, i.e., you cannot update (@option{--update}, alias @option{-u})
8994 them or delete (@option{--delete}) members from them or
8995 add (@option{--append}, alias @option{-r}) members to them. Likewise, you
8996 cannot append another @command{tar} archive to a compressed archive using
8997 @option{--concatenate} (@option{-A}). Secondly, multi-volume
8998 archives cannot be compressed.
9000 The following options allow to select a particular compressor program:
9008 Filter the archive through @command{gzip}.
9013 Filter the archive through @code{xz}.
9017 Filter the archive through @code{bzip2}.
9021 Filter the archive through @command{lzip}.
9025 Filter the archive through @command{lzma}.
9029 Filter the archive through @command{lzop}.
9036 Filter the archive through @command{compress}.
9039 When any of these options is given, @GNUTAR{} searches the compressor
9040 binary in the current path and invokes it. The name of the compressor
9041 program is specified at compilation time using a corresponding
9042 @option{--with-@var{compname}} option to @command{configure}, e.g.
9043 @option{--with-bzip2} to select a specific @command{bzip2} binary.
9044 @xref{lbzip2}, for a detailed discussion.
9046 The output produced by @command{tar --help} shows the actual
9047 compressor names along with each of these options.
9049 You can use any of these options on physical devices (tape drives,
9050 etc.) and remote files as well as on normal files; data to or from
9051 such devices or remote files is reblocked by another copy of the
9052 @command{tar} program to enforce the specified (or default) record
9053 size. The default compression parameters are used. Most compression
9054 programs let you override these by setting a program-specific
9055 environment variable. For example, with @command{gzip} you can set
9059 $ @kbd{GZIP='-9 -n' tar czf archive.tar.gz subdir}
9061 Another way would be to use the @option{-I} option instead (see
9065 $ @kbd{tar -cf archive.tar.gz -I 'gzip -9 -n' subdir}
9069 Finally, the third, traditional, way to do this is to use a pipe:
9072 $ @kbd{tar cf - subdir | gzip -9 -n > archive.tar.gz}
9075 @cindex corrupted archives
9076 Compressed archives are easily corrupted, because compressed files
9077 have little redundancy. The adaptive nature of the
9078 compression scheme means that the compression tables are implicitly
9079 spread all over the archive. If you lose a few blocks, the dynamic
9080 construction of the compression tables becomes unsynchronized, and there
9081 is little chance that you could recover later in the archive.
9083 Other compression options provide better control over creating
9084 compressed archives. These are:
9087 @anchor{auto-compress}
9088 @opindex auto-compress
9089 @item --auto-compress
9091 Select a compression program to use by the archive file name
9092 suffix. The following suffixes are recognized:
9094 @multitable @columnfractions 0.3 0.6
9095 @headitem Suffix @tab Compression program
9096 @item @samp{.gz} @tab @command{gzip}
9097 @item @samp{.tgz} @tab @command{gzip}
9098 @item @samp{.taz} @tab @command{gzip}
9099 @item @samp{.Z} @tab @command{compress}
9100 @item @samp{.taZ} @tab @command{compress}
9101 @item @samp{.bz2} @tab @command{bzip2}
9102 @item @samp{.tz2} @tab @command{bzip2}
9103 @item @samp{.tbz2} @tab @command{bzip2}
9104 @item @samp{.tbz} @tab @command{bzip2}
9105 @item @samp{.lz} @tab @command{lzip}
9106 @item @samp{.lzma} @tab @command{lzma}
9107 @item @samp{.tlz} @tab @command{lzma}
9108 @item @samp{.lzo} @tab @command{lzop}
9109 @item @samp{.xz} @tab @command{xz}
9112 @anchor{use-compress-program}
9113 @opindex use-compress-program
9114 @item --use-compress-program=@var{command}
9115 @itemx -I=@var{command}
9116 Use external compression program @var{command}. Use this option if you
9117 are not happy with the compression program associated with the suffix
9118 at compile time or if you have a compression program that @GNUTAR{}
9119 does not support. The @var{command} argument is a valid command
9120 invocation, as you would type it at the command line prompt, with any
9121 additional options as needed. Enclose it in quotes if it contains
9122 white space (see @ref{external, Running External Commands}, for more detail).
9124 The @var{command} should follow two conventions:
9126 First, when invoked without additional options, it should read data
9127 from standard input, compress it and output it on standard output.
9129 Secondly, if invoked with the additional @option{-d} option, it should
9130 do exactly the opposite, i.e., read the compressed data from the
9131 standard input and produce uncompressed data on the standard output.
9133 The latter requirement means that you must not use the @option{-d}
9134 option as a part of the @var{command} itself.
9137 @cindex gpg, using with tar
9138 @cindex gnupg, using with tar
9139 @cindex Using encrypted archives
9140 The @option{--use-compress-program} option, in particular, lets you
9141 implement your own filters, not necessarily dealing with
9142 compression/decompression. For example, suppose you wish to implement
9143 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
9144 gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
9145 Manual}). The following script does that:
9151 -d) gpg --decrypt - | gzip -d -c;;
9152 '') gzip -c | gpg -s;;
9153 *) echo "Unknown option $1">&2; exit 1;;
9158 Suppose you name it @file{gpgz} and save it somewhere in your
9159 @env{PATH}. Then the following command will create a compressed
9160 archive signed with your private key:
9163 $ @kbd{tar -cf foo.tar.gpgz -Igpgz .}
9167 Likewise, the command below will list its contents:
9170 $ @kbd{tar -tf foo.tar.gpgz -Igpgz .}
9174 The above is based on the following discussion:
9176 I have one question, or maybe it's a suggestion if there isn't a way
9177 to do it now. I would like to use @option{--gzip}, but I'd also like
9178 the output to be fed through a program like @acronym{GNU}
9179 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
9180 to use :-)), basically adding ECC protection on top of compression.
9181 It seems as if this should be quite easy to do, but I can't work out
9182 exactly how to go about it. Of course, I can pipe the standard output
9183 of @command{tar} through @command{ecc}, but then I lose (though I
9184 haven't started using it yet, I confess) the ability to have
9185 @command{tar} use @command{rmt} for it's I/O (I think).
9187 I think the most straightforward thing would be to let me specify a
9188 general set of filters outboard of compression (preferably ordered,
9189 so the order can be automatically reversed on input operations, and
9190 with the options they require specifiable), but beggars shouldn't be
9191 choosers and anything you decide on would be fine with me.
9193 By the way, I like @command{ecc} but if (as the comments say) it can't
9194 deal with loss of block sync, I'm tempted to throw some time at adding
9195 that capability. Supposing I were to actually do such a thing and
9196 get it (apparently) working, do you accept contributed changes to
9197 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
9199 Isn't that exactly the role of the
9200 @option{--use-compress-prog=@var{program}} option?
9201 I never tried it myself, but I suspect you may want to write a
9202 @var{prog} script or program able to filter stdin to stdout to
9203 way you want. It should recognize the @option{-d} option, for when
9204 extraction is needed rather than creation.
9206 It has been reported that if one writes compressed data (through the
9207 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
9208 the DLT compression mode, the data will actually get bigger and one will
9209 end up with less space on the tape.
9213 * lbzip2:: Using lbzip2 with @GNUTAR{}.
9217 @subsubsection Using lbzip2 with @GNUTAR{}.
9219 @cindex Laszlo Ersek
9220 @command{Lbzip2} is a multithreaded utility for handling
9221 @samp{bzip2} compression, written by Laszlo Ersek. It makes use of
9222 multiple processors to speed up its operation and in general works
9223 considerably faster than @command{bzip2}. For a detailed description
9224 of @command{lbzip2} see @uref{http://freshmeat.net/@/projects/@/lbzip2} and
9225 @uref{http://www.linuxinsight.com/@/lbzip2-parallel-bzip2-utility.html,
9226 lbzip2: parallel bzip2 utility}.
9228 Recent versions of @command{lbzip2} are mostly command line compatible
9229 with @command{bzip2}, which makes it possible to automatically invoke
9230 it via the @option{--bzip2} @GNUTAR{} command line option. To do so,
9231 @GNUTAR{} must be configured with the @option{--with-bzip2} command
9232 line option, like this:
9235 $ @kbd{./configure --with-bzip2=lbzip2 [@var{other-options}]}
9238 Once configured and compiled this way, @command{tar --help} will show the
9243 $ @kbd{tar --help | grep -- --bzip2}
9244 -j, --bzip2 filter the archive through lbzip2
9249 which means that running @command{tar --bzip2} will invoke @command{lbzip2}.
9252 @subsection Archiving Sparse Files
9253 @cindex Sparse Files
9255 Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
9256 in a file is a section of the file's contents which was never written.
9257 The contents of a hole reads as all zeros. On many operating systems,
9258 actual disk storage is not allocated for holes, but they are counted
9259 in the length of the file. If you archive such a file, @command{tar}
9260 could create an archive longer than the original. To have @command{tar}
9261 attempt to recognize the holes in a file, use @option{--sparse}
9262 (@option{-S}). When you use this option, then, for any file using
9263 less disk space than would be expected from its length, @command{tar}
9264 searches the file for consecutive stretches of zeros. It then records
9265 in the archive for the file where the consecutive stretches of zeros
9266 are, and only archives the ``real contents'' of the file. On
9267 extraction (using @option{--sparse} is not needed on extraction) any
9268 such files have holes created wherever the continuous stretches of zeros
9269 were found. Thus, if you use @option{--sparse}, @command{tar} archives
9270 won't take more space than the original.
9276 This option instructs @command{tar} to test each file for sparseness
9277 before attempting to archive it. If the file is found to be sparse it
9278 is treated specially, thus allowing to decrease the amount of space
9279 used by its image in the archive.
9281 This option is meaningful only when creating or updating archives. It
9282 has no effect on extraction.
9285 Consider using @option{--sparse} when performing file system backups,
9286 to avoid archiving the expanded forms of files stored sparsely in the
9289 Even if your system has no sparse files currently, some may be
9290 created in the future. If you use @option{--sparse} while making file
9291 system backups as a matter of course, you can be assured the archive
9292 will never take more space on the media than the files take on disk
9293 (otherwise, archiving a disk filled with sparse files might take
9294 hundreds of tapes). @xref{Incremental Dumps}.
9296 However, be aware that @option{--sparse} option presents a serious
9297 drawback. Namely, in order to determine if the file is sparse
9298 @command{tar} has to read it before trying to archive it, so in total
9299 the file is read @strong{twice}. So, always bear in mind that the
9300 time needed to process all files with this option is roughly twice
9301 the time needed to archive them without it.
9302 @FIXME{A technical note:
9304 Programs like @command{dump} do not have to read the entire file; by
9305 examining the file system directly, they can determine in advance
9306 exactly where the holes are and thus avoid reading through them. The
9307 only data it need read are the actual allocated data blocks.
9308 @GNUTAR{} uses a more portable and straightforward
9309 archiving approach, it would be fairly difficult that it does
9310 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
9314 What I did say is that you cannot tell the difference between a hole and an
9315 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
9316 best tells you how many holes there are; it doesn't tell you @emph{where}.
9317 Just as programs may, conceivably, care what @code{st_blocks} is (care
9318 to name one that does?), they may also care where the holes are (I have
9319 no examples of this one either, but it's equally imaginable).
9321 I conclude from this that good archivers are not portable. One can
9322 arguably conclude that if you want a portable program, you can in good
9323 conscience restore files with as many holes as possible, since you can't
9328 @cindex sparse formats, defined
9329 When using @samp{POSIX} archive format, @GNUTAR{} is able to store
9330 sparse files using in three distinct ways, called @dfn{sparse
9331 formats}. A sparse format is identified by its @dfn{number},
9332 consisting, as usual of two decimal numbers, delimited by a dot. By
9333 default, format @samp{1.0} is used. If, for some reason, you wish to
9334 use an earlier format, you can select it using
9335 @option{--sparse-version} option.
9338 @opindex sparse-version
9339 @item --sparse-version=@var{version}
9341 Select the format to store sparse files in. Valid @var{version} values
9342 are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
9343 for a detailed description of each format.
9346 Using @option{--sparse-format} option implies @option{--sparse}.
9349 @section Handling File Attributes
9350 @cindex atrributes, files
9351 @cindex file attributes
9353 When @command{tar} reads files, it updates their access times. To
9354 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
9355 reset the access time retroactively or avoid changing it in the first
9359 @opindex atime-preserve
9360 @item --atime-preserve
9361 @itemx --atime-preserve=replace
9362 @itemx --atime-preserve=system
9363 Preserve the access times of files that are read. This works only for
9364 files that you own, unless you have superuser privileges.
9366 @option{--atime-preserve=replace} works on most systems, but it also
9367 restores the data modification time and updates the status change
9368 time. Hence it doesn't interact with incremental dumps nicely
9369 (@pxref{Incremental Dumps}), and it can set access or data modification times
9370 incorrectly if other programs access the file while @command{tar} is
9373 @option{--atime-preserve=system} avoids changing the access time in
9374 the first place, if the operating system supports this.
9375 Unfortunately, this may or may not work on any given operating system
9376 or file system. If @command{tar} knows for sure it won't work, it
9377 complains right away.
9379 Currently @option{--atime-preserve} with no operand defaults to
9380 @option{--atime-preserve=replace}, but this is intended to change to
9381 @option{--atime-preserve=system} when the latter is better-supported.
9386 Do not extract data modification time.
9388 When this option is used, @command{tar} leaves the data modification times
9389 of the files it extracts as the times when the files were extracted,
9390 instead of setting it to the times recorded in the archive.
9392 This option is meaningless with @option{--list} (@option{-t}).
9396 Create extracted files with the same ownership they have in the
9399 This is the default behavior for the superuser,
9400 so this option is meaningful only for non-root users, when @command{tar}
9401 is executed on those systems able to give files away. This is
9402 considered as a security flaw by many people, at least because it
9403 makes quite difficult to correctly account users for the disk space
9404 they occupy. Also, the @code{suid} or @code{sgid} attributes of
9405 files are easily and silently lost when files are given away.
9407 When writing an archive, @command{tar} writes the user @acronym{ID} and user name
9408 separately. If it can't find a user name (because the user @acronym{ID} is not
9409 in @file{/etc/passwd}), then it does not write one. When restoring,
9410 it tries to look the name (if one was written) up in
9411 @file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
9412 the archive instead.
9414 @opindex no-same-owner
9415 @item --no-same-owner
9417 Do not attempt to restore ownership when extracting. This is the
9418 default behavior for ordinary users, so this option has an effect
9419 only for the superuser.
9421 @opindex numeric-owner
9422 @item --numeric-owner
9423 The @option{--numeric-owner} option allows (ANSI) archives to be written
9424 without user/group name information or such information to be ignored
9425 when extracting. It effectively disables the generation and/or use
9426 of user/group name information. This option forces extraction using
9427 the numeric ids from the archive, ignoring the names.
9429 This is useful in certain circumstances, when restoring a backup from
9430 an emergency floppy with different passwd/group files for example.
9431 It is otherwise impossible to extract files with the right ownerships
9432 if the password file in use during the extraction does not match the
9433 one belonging to the file system(s) being extracted. This occurs,
9434 for example, if you are restoring your files after a major crash and
9435 had booted from an emergency floppy with no password file or put your
9436 disk into another machine to do the restore.
9438 The numeric ids are @emph{always} saved into @command{tar} archives.
9439 The identifying names are added at create time when provided by the
9440 system, unless @option{--format=oldgnu} is used. Numeric ids could be
9441 used when moving archives between a collection of machines using
9442 a centralized management for attribution of numeric ids to users
9443 and groups. This is often made through using the NIS capabilities.
9445 When making a @command{tar} file for distribution to other sites, it
9446 is sometimes cleaner to use a single owner for all files in the
9447 distribution, and nicer to specify the write permission bits of the
9448 files as stored in the archive independently of their actual value on
9449 the file system. The way to prepare a clean distribution is usually
9450 to have some Makefile rule creating a directory, copying all needed
9451 files in that directory, then setting ownership and permissions as
9452 wanted (there are a lot of possible schemes), and only then making a
9453 @command{tar} archive out of this directory, before cleaning
9454 everything out. Of course, we could add a lot of options to
9455 @GNUTAR{} for fine tuning permissions and ownership.
9456 This is not the good way, I think. @GNUTAR{} is
9457 already crowded with options and moreover, the approach just explained
9458 gives you a great deal of control already.
9460 @xopindex{same-permissions, short description}
9461 @xopindex{preserve-permissions, short description}
9463 @itemx --same-permissions
9464 @itemx --preserve-permissions
9465 Extract all protection information.
9467 This option causes @command{tar} to set the modes (access permissions) of
9468 extracted files exactly as recorded in the archive. If this option
9469 is not used, the current @code{umask} setting limits the permissions
9470 on extracted files. This option is by default enabled when
9471 @command{tar} is executed by a superuser.
9474 This option is meaningless with @option{--list} (@option{-t}).
9478 Same as both @option{--same-permissions} and @option{--same-order}.
9480 This option is deprecated, and will be removed in @GNUTAR{} version 1.23.
9485 @section Making @command{tar} Archives More Portable
9487 Creating a @command{tar} archive on a particular system that is meant to be
9488 useful later on many other machines and with other versions of @command{tar}
9489 is more challenging than you might think. @command{tar} archive formats
9490 have been evolving since the first versions of Unix. Many such formats
9491 are around, and are not always compatible with each other. This section
9492 discusses a few problems, and gives some advice about making @command{tar}
9493 archives more portable.
9495 One golden rule is simplicity. For example, limit your @command{tar}
9496 archives to contain only regular files and directories, avoiding
9497 other kind of special files. Do not attempt to save sparse files or
9498 contiguous files as such. Let's discuss a few more problems, in turn.
9500 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
9501 archives and archive labels) in GNU and PAX formats.}
9504 * Portable Names:: Portable Names
9505 * dereference:: Symbolic Links
9506 * hard links:: Hard Links
9507 * old:: Old V7 Archives
9508 * ustar:: Ustar Archives
9509 * gnu:: GNU and old GNU format archives.
9510 * posix:: @acronym{POSIX} archives
9511 * Checksumming:: Checksumming Problems
9512 * Large or Negative Values:: Large files, negative time stamps, etc.
9513 * Other Tars:: How to Extract GNU-Specific Data Using
9514 Other @command{tar} Implementations
9517 @node Portable Names
9518 @subsection Portable Names
9520 Use portable file and member names. A name is portable if it contains
9521 only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
9522 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
9523 contain @samp{/-}. Avoid deep directory nesting. For portability to
9524 old Unix hosts, limit your file name components to 14 characters or
9527 If you intend to have your @command{tar} archives to be read under
9528 MSDOS, you should not rely on case distinction for file names, and you
9529 might use the @acronym{GNU} @command{doschk} program for helping you
9530 further diagnosing illegal MSDOS names, which are even more limited
9534 @subsection Symbolic Links
9535 @cindex File names, using symbolic links
9536 @cindex Symbolic link as file name
9538 @opindex dereference
9539 Normally, when @command{tar} archives a symbolic link, it writes a
9540 block to the archive naming the target of the link. In that way, the
9541 @command{tar} archive is a faithful record of the file system contents.
9542 When @option{--dereference} (@option{-h}) is used with
9543 @option{--create} (@option{-c}), @command{tar} archives the files
9544 symbolic links point to, instead of
9545 the links themselves.
9547 When creating portable archives, use @option{--dereference}
9548 (@option{-h}): some systems do not support
9549 symbolic links, and moreover, your distribution might be unusable if
9550 it contains unresolved symbolic links.
9552 When reading from an archive, the @option{--dereference} (@option{-h})
9553 option causes @command{tar} to follow an already-existing symbolic
9554 link when @command{tar} writes or reads a file named in the archive.
9555 Ordinarily, @command{tar} does not follow such a link, though it may
9556 remove the link before writing a new file. @xref{Dealing with Old
9559 The @option{--dereference} option is unsafe if an untrusted user can
9560 modify directories while @command{tar} is running. @xref{Security}.
9563 @subsection Hard Links
9564 @cindex File names, using hard links
9565 @cindex hard links, dereferencing
9566 @cindex dereferencing hard links
9568 Normally, when @command{tar} archives a hard link, it writes a
9569 block to the archive naming the target of the link (a @samp{1} type
9570 block). In that way, the actual file contents is stored in file only
9571 once. For example, consider the following two files:
9576 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
9577 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
9581 Here, @file{jeden} is a link to @file{one}. When archiving this
9582 directory with a verbose level 2, you will get an output similar to
9586 $ tar cvvf ../archive.tar .
9587 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9588 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9589 hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
9592 The last line shows that, instead of storing two copies of the file,
9593 @command{tar} stored it only once, under the name @file{jeden}, and
9594 stored file @file{one} as a hard link to this file.
9596 It may be important to know that all hard links to the given file are
9597 stored in the archive. For example, this may be necessary for exact
9598 reproduction of the file system. The following option does that:
9601 @xopindex{check-links, described}
9604 Check the number of links dumped for each processed file. If this
9605 number does not match the total number of hard links for the file, print
9609 For example, trying to archive only file @file{jeden} with this option
9610 produces the following diagnostics:
9613 $ tar -c -f ../archive.tar -l jeden
9614 tar: Missing links to 'jeden'.
9617 Although creating special records for hard links helps keep a faithful
9618 record of the file system contents and makes archives more compact, it
9619 may present some difficulties when extracting individual members from
9620 the archive. For example, trying to extract file @file{one} from the
9621 archive created in previous examples produces, in the absense of file
9625 $ tar xf archive.tar ./one
9626 tar: ./one: Cannot hard link to './jeden': No such file or directory
9627 tar: Error exit delayed from previous errors
9630 The reason for this behavior is that @command{tar} cannot seek back in
9631 the archive to the previous member (in this case, @file{one}), to
9632 extract it@footnote{There are plans to fix this in future releases.}.
9633 If you wish to avoid such problems at the cost of a bigger archive,
9634 use the following option:
9637 @xopindex{hard-dereference, described}
9638 @item --hard-dereference
9639 Dereference hard links and store the files they refer to.
9642 For example, trying this option on our two sample files, we get two
9643 copies in the archive, each of which can then be extracted
9644 independently of the other:
9648 $ tar -c -vv -f ../archive.tar --hard-dereference .
9649 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9650 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9651 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
9656 @subsection Old V7 Archives
9657 @cindex Format, old style
9658 @cindex Old style format
9659 @cindex Old style archives
9660 @cindex v7 archive format
9662 Certain old versions of @command{tar} cannot handle additional
9663 information recorded by newer @command{tar} programs. To create an
9664 archive in V7 format (not ANSI), which can be read by these old
9665 versions, specify the @option{--format=v7} option in
9666 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
9667 accepts @option{--portability} or @option{--old-archive} for this
9668 option). When you specify it,
9669 @command{tar} leaves out information about directories, pipes, fifos,
9670 contiguous files, and device files, and specifies file ownership by
9671 group and user IDs instead of group and user names.
9673 When updating an archive, do not use @option{--format=v7}
9674 unless the archive was created using this option.
9676 In most cases, a @emph{new} format archive can be read by an @emph{old}
9677 @command{tar} program without serious trouble, so this option should
9678 seldom be needed. On the other hand, most modern @command{tar}s are
9679 able to read old format archives, so it might be safer for you to
9680 always use @option{--format=v7} for your distributions. Notice,
9681 however, that @samp{ustar} format is a better alternative, as it is
9682 free from many of @samp{v7}'s drawbacks.
9685 @subsection Ustar Archive Format
9687 @cindex ustar archive format
9688 Archive format defined by @acronym{POSIX}.1-1988 specification is called
9689 @code{ustar}. Although it is more flexible than the V7 format, it
9690 still has many restrictions (@pxref{Formats,ustar}, for the detailed
9691 description of @code{ustar} format). Along with V7 format,
9692 @code{ustar} format is a good choice for archives intended to be read
9693 with other implementations of @command{tar}.
9695 To create archive in @code{ustar} format, use @option{--format=ustar}
9696 option in conjunction with the @option{--create} (@option{-c}).
9699 @subsection @acronym{GNU} and old @GNUTAR{} format
9701 @cindex GNU archive format
9702 @cindex Old GNU archive format
9703 @GNUTAR{} was based on an early draft of the
9704 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
9705 @command{tar}, such as the support for file names longer than 100
9706 characters, use portions of the @command{tar} header record which were
9707 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
9708 @acronym{POSIX} have allocated the same parts of the header record for
9709 other purposes. As a result, @GNUTAR{} format is
9710 incompatible with the current @acronym{POSIX} specification, and with
9711 @command{tar} programs that follow it.
9713 In the majority of cases, @command{tar} will be configured to create
9714 this format by default. This will change in future releases, since
9715 we plan to make @samp{POSIX} format the default.
9717 To force creation a @GNUTAR{} archive, use option
9718 @option{--format=gnu}.
9721 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
9723 @cindex POSIX archive format
9724 @cindex PAX archive format
9725 Starting from version 1.14 @GNUTAR{} features full support for
9726 @acronym{POSIX.1-2001} archives.
9728 A @acronym{POSIX} conformant archive will be created if @command{tar}
9729 was given @option{--format=posix} (@option{--format=pax}) option. No
9730 special option is required to read and extract from a @acronym{POSIX}
9734 * PAX keywords:: Controlling Extended Header Keywords.
9738 @subsubsection Controlling Extended Header Keywords
9742 @item --pax-option=@var{keyword-list}
9743 Handle keywords in @acronym{PAX} extended headers. This option is
9744 equivalent to @option{-o} option of the @command{pax} utility.
9747 @var{Keyword-list} is a comma-separated
9748 list of keyword options, each keyword option taking one of
9749 the following forms:
9752 @item delete=@var{pattern}
9753 When used with one of archive-creation commands,
9754 this option instructs @command{tar} to omit from extended header records
9755 that it produces any keywords matching the string @var{pattern}.
9757 When used in extract or list mode, this option instructs tar
9758 to ignore any keywords matching the given @var{pattern} in the extended
9759 header records. In both cases, matching is performed using the pattern
9760 matching notation described in @acronym{POSIX 1003.2}, 3.13
9761 (@pxref{wildcards}). For example:
9764 --pax-option delete=security.*
9767 would suppress security-related information.
9769 @item exthdr.name=@var{string}
9771 This keyword allows user control over the name that is written into the
9772 ustar header blocks for the extended headers. The name is obtained
9773 from @var{string} after making the following substitutions:
9775 @multitable @columnfractions .25 .55
9776 @headitem Meta-character @tab Replaced By
9777 @item %d @tab The directory name of the file, equivalent to the
9778 result of the @command{dirname} utility on the translated file name.
9779 @item %f @tab The name of the file with the directory information
9780 stripped, equivalent to the result of the @command{basename} utility
9781 on the translated file name.
9782 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9783 @item %% @tab A @samp{%} character.
9786 Any other @samp{%} characters in @var{string} produce undefined
9789 If no option @samp{exthdr.name=string} is specified, @command{tar}
9790 will use the following default value:
9796 @item exthdr.mtime=@var{value}
9798 This keyword defines the value of the @samp{mtime} field that
9799 is written into the ustar header blocks for the extended headers.
9800 By default, the @samp{mtime} field is set to the modification time
9801 of the archive member described by that extended headers.
9803 @item globexthdr.name=@var{string}
9804 This keyword allows user control over the name that is written into
9805 the ustar header blocks for global extended header records. The name
9806 is obtained from the contents of @var{string}, after making
9807 the following substitutions:
9809 @multitable @columnfractions .25 .55
9810 @headitem Meta-character @tab Replaced By
9811 @item %n @tab An integer that represents the
9812 sequence number of the global extended header record in the archive,
9814 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9815 @item %% @tab A @samp{%} character.
9818 Any other @samp{%} characters in @var{string} produce undefined results.
9820 If no option @samp{globexthdr.name=string} is specified, @command{tar}
9821 will use the following default value:
9824 $TMPDIR/GlobalHead.%p.%n
9828 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
9829 environment variable. If @var{TMPDIR} is not set, @command{tar}
9832 @item globexthdr.mtime=@var{value}
9834 This keyword defines the value of the @samp{mtime} field that
9835 is written into the ustar header blocks for the global extended headers.
9836 By default, the @samp{mtime} field is set to the time when
9837 @command{tar} was invoked.
9839 @item @var{keyword}=@var{value}
9840 When used with one of archive-creation commands, these keyword/value pairs
9841 will be included at the beginning of the archive in a global extended
9842 header record. When used with one of archive-reading commands,
9843 @command{tar} will behave as if it has encountered these keyword/value
9844 pairs at the beginning of the archive in a global extended header
9847 @item @var{keyword}:=@var{value}
9848 When used with one of archive-creation commands, these keyword/value pairs
9849 will be included as records at the beginning of an extended header for
9850 each file. This is effectively equivalent to @var{keyword}=@var{value}
9851 form except that it creates no global extended header records.
9853 When used with one of archive-reading commands, @command{tar} will
9854 behave as if these keyword/value pairs were included as records at the
9855 end of each extended header; thus, they will override any global or
9856 file-specific extended header record keywords of the same names.
9857 For example, in the command:
9860 tar --format=posix --create \
9861 --file archive --pax-option gname:=user .
9864 the group name will be forced to a new value for all files
9865 stored in the archive.
9868 In any of the forms described above, the @var{value} may be
9869 a string enclosed in curly braces. In that case, the string
9870 between the braces is understood either as a textual time
9871 representation, as described in @ref{Date input formats}, or a name of
9872 the existing file, starting with @samp{/} or @samp{.}. In the latter
9873 case, the modification time of that file is used.
9875 For example, to set all modification times to the current date, you
9876 use the following option:
9879 --pax-option='mtime:=@{now@}'
9882 Note quoting of the option's argument.
9884 @cindex archives, binary equivalent
9885 @cindex binary equivalent archives, creating
9886 As another example, here is the option that ensures that any two
9887 archives created using it, will be binary equivalent if they have the
9891 --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0
9895 @subsection Checksumming Problems
9897 SunOS and HP-UX @command{tar} fail to accept archives created using
9898 @GNUTAR{} and containing non-@acronym{ASCII} file names, that
9899 is, file names having characters with the eight bit set, because they
9900 use signed checksums, while @GNUTAR{} uses unsigned
9901 checksums while creating archives, as per @acronym{POSIX} standards. On
9902 reading, @GNUTAR{} computes both checksums and
9903 accepts any. It is somewhat worrying that a lot of people may go
9904 around doing backup of their files using faulty (or at least
9905 non-standard) software, not learning about it until it's time to
9906 restore their missing files with an incompatible file extractor, or
9909 @GNUTAR{} computes checksums both ways, and accept
9910 any on read, so @acronym{GNU} tar can read Sun tapes even with their
9911 wrong checksums. @GNUTAR{} produces the standard
9912 checksum, however, raising incompatibilities with Sun. That is to
9913 say, @GNUTAR{} has not been modified to
9914 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
9915 I've been told that more recent Sun @command{tar} now read standard
9916 archives, so maybe Sun did a similar patch, after all?
9918 The story seems to be that when Sun first imported @command{tar}
9919 sources on their system, they recompiled it without realizing that
9920 the checksums were computed differently, because of a change in
9921 the default signing of @code{char}'s in their compiler. So they
9922 started computing checksums wrongly. When they later realized their
9923 mistake, they merely decided to stay compatible with it, and with
9924 themselves afterwards. Presumably, but I do not really know, HP-UX
9925 has chosen that their @command{tar} archives to be compatible with Sun's.
9926 The current standards do not favor Sun @command{tar} format. In any
9927 case, it now falls on the shoulders of SunOS and HP-UX users to get
9928 a @command{tar} able to read the good archives they receive.
9930 @node Large or Negative Values
9931 @subsection Large or Negative Values
9932 @cindex large values
9933 @cindex future time stamps
9934 @cindex negative time stamps
9937 The above sections suggest to use @samp{oldest possible} archive
9938 format if in doubt. However, sometimes it is not possible. If you
9939 attempt to archive a file whose metadata cannot be represented using
9940 required format, @GNUTAR{} will print error message and ignore such a
9941 file. You will than have to switch to a format that is able to
9942 handle such values. The format summary table (@pxref{Formats}) will
9945 In particular, when trying to archive files larger than 8GB or with
9946 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
9947 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
9948 @acronym{POSIX} archive formats. When considering which format to
9949 choose, bear in mind that the @acronym{GNU} format uses
9950 two's-complement base-256 notation to store values that do not fit
9951 into standard @acronym{ustar} range. Such archives can generally be
9952 read only by a @GNUTAR{} implementation. Moreover, they sometimes
9953 cannot be correctly restored on another hosts even by @GNUTAR{}. For
9954 example, using two's complement representation for negative time
9955 stamps that assumes a signed 32-bit @code{time_t} generates archives
9956 that are not portable to hosts with differing @code{time_t}
9959 On the other hand, @acronym{POSIX} archives, generally speaking, can
9960 be extracted by any tar implementation that understands older
9961 @acronym{ustar} format. The only exception are files larger than 8GB.
9963 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
9967 @subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations
9969 In previous sections you became acquainted with various quirks
9970 necessary to make your archives portable. Sometimes you may need to
9971 extract archives containing GNU-specific members using some
9972 third-party @command{tar} implementation or an older version of
9973 @GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed,
9974 but if it is for some reason impossible, this section will explain
9975 how to cope without it.
9977 When we speak about @dfn{GNU-specific} members we mean two classes of
9978 them: members split between the volumes of a multi-volume archive and
9979 sparse members. You will be able to always recover such members if
9980 the archive is in PAX format. In addition split members can be
9981 recovered from archives in old GNU format. The following subsections
9982 describe the required procedures in detail.
9985 * Split Recovery:: Members Split Between Volumes
9986 * Sparse Recovery:: Sparse Members
9989 @node Split Recovery
9990 @subsubsection Extracting Members Split Between Volumes
9992 @cindex Mutli-volume archives, extracting using non-GNU tars
9993 If a member is split between several volumes of an old GNU format archive
9994 most third party @command{tar} implementation will fail to extract
9995 it. To extract it, use @command{tarcat} program (@pxref{Tarcat}).
9996 This program is available from
9997 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat.html, @GNUTAR{}
9998 home page}. It concatenates several archive volumes into a single
9999 valid archive. For example, if you have three volumes named from
10000 @file{vol-1.tar} to @file{vol-3.tar}, you can do the following to
10001 extract them using a third-party @command{tar}:
10004 $ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -}
10007 @cindex Mutli-volume archives in PAX format, extracting using non-GNU tars
10008 You could use this approach for most (although not all) PAX
10009 format archives as well. However, extracting split members from a PAX
10010 archive is a much easier task, because PAX volumes are constructed in
10011 such a way that each part of a split member is extracted to a
10012 different file by @command{tar} implementations that are not aware of
10013 GNU extensions. More specifically, the very first part retains its
10014 original name, and all subsequent parts are named using the pattern:
10017 %d/GNUFileParts.%p/%f.%n
10021 where symbols preceeded by @samp{%} are @dfn{macro characters} that
10022 have the following meaning:
10024 @multitable @columnfractions .25 .55
10025 @headitem Meta-character @tab Replaced By
10026 @item %d @tab The directory name of the file, equivalent to the
10027 result of the @command{dirname} utility on its full name.
10028 @item %f @tab The file name of the file, equivalent to the result
10029 of the @command{basename} utility on its full name.
10030 @item %p @tab The process @acronym{ID} of the @command{tar} process that
10031 created the archive.
10032 @item %n @tab Ordinal number of this particular part.
10035 For example, if the file @file{var/longfile} was split during archive
10036 creation between three volumes, and the creator @command{tar} process
10037 had process @acronym{ID} @samp{27962}, then the member names will be:
10041 var/GNUFileParts.27962/longfile.1
10042 var/GNUFileParts.27962/longfile.2
10045 When you extract your archive using a third-party @command{tar}, these
10046 files will be created on your disk, and the only thing you will need
10047 to do to restore your file in its original form is concatenate them in
10048 the proper order, for example:
10053 $ @kbd{cat GNUFileParts.27962/longfile.1 \
10054 GNUFileParts.27962/longfile.2 >> longfile}
10055 $ rm -f GNUFileParts.27962
10059 Notice, that if the @command{tar} implementation you use supports PAX
10060 format archives, it will probably emit warnings about unknown keywords
10061 during extraction. They will look like this:
10066 Unknown extended header keyword 'GNU.volume.filename' ignored.
10067 Unknown extended header keyword 'GNU.volume.size' ignored.
10068 Unknown extended header keyword 'GNU.volume.offset' ignored.
10073 You can safely ignore these warnings.
10075 If your @command{tar} implementation is not PAX-aware, you will get
10076 more warnings and more files generated on your disk, e.g.:
10080 $ @kbd{tar xf vol-1.tar}
10081 var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
10083 Unexpected EOF in archive
10084 $ @kbd{tar xf vol-2.tar}
10085 tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
10086 GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
10087 'x', extracted as normal file
10091 Ignore these warnings. The @file{PaxHeaders.*} directories created
10092 will contain files with @dfn{extended header keywords} describing the
10093 extracted files. You can delete them, unless they describe sparse
10094 members. Read further to learn more about them.
10096 @node Sparse Recovery
10097 @subsubsection Extracting Sparse Members
10099 @cindex sparse files, extracting with non-GNU tars
10100 Any @command{tar} implementation will be able to extract sparse members from a
10101 PAX archive. However, the extracted files will be @dfn{condensed},
10102 i.e., any zero blocks will be removed from them. When we restore such
10103 a condensed file to its original form, by adding zero blocks (or
10104 @dfn{holes}) back to their original locations, we call this process
10105 @dfn{expanding} a compressed sparse file.
10108 To expand a file, you will need a simple auxiliary program called
10109 @command{xsparse}. It is available in source form from
10110 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse.html, @GNUTAR{}
10113 @cindex sparse files v.1.0, extracting with non-GNU tars
10114 Let's begin with archive members in @dfn{sparse format
10115 version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand.
10116 The condensed file will contain both file map and file data, so no
10117 additional data will be needed to restore it. If the original file
10118 name was @file{@var{dir}/@var{name}}, then the condensed file will be
10119 named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where
10120 @var{n} is a decimal number@footnote{Technically speaking, @var{n} is a
10121 @dfn{process @acronym{ID}} of the @command{tar} process which created the
10122 archive (@pxref{PAX keywords}).}.
10124 To expand a version 1.0 file, run @command{xsparse} as follows:
10127 $ @kbd{xsparse @file{cond-file}}
10131 where @file{cond-file} is the name of the condensed file. The utility
10132 will deduce the name for the resulting expanded file using the
10133 following algorithm:
10136 @item If @file{cond-file} does not contain any directories,
10137 @file{../cond-file} will be used;
10139 @item If @file{cond-file} has the form
10140 @file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name}
10141 are simple names, with no @samp{/} characters in them, the output file
10142 name will be @file{@var{dir}/@var{name}}.
10144 @item Otherwise, if @file{cond-file} has the form
10145 @file{@var{dir}/@var{name}}, the output file name will be
10149 In the unlikely case when this algorithm does not suit your needs,
10150 you can explicitly specify output file name as a second argument to
10154 $ @kbd{xsparse @file{cond-file} @file{out-file}}
10157 It is often a good idea to run @command{xsparse} in @dfn{dry run} mode
10158 first. In this mode, the command does not actually expand the file,
10159 but verbosely lists all actions it would be taking to do so. The dry
10160 run mode is enabled by @option{-n} command line argument:
10164 $ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile}
10165 Reading v.1.0 sparse map
10166 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10167 '/home/gray/sparsefile'
10172 To actually expand the file, you would run:
10175 $ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile}
10179 The program behaves the same way all UNIX utilities do: it will keep
10180 quiet unless it has simething important to tell you (e.g. an error
10181 condition or something). If you wish it to produce verbose output,
10182 similar to that from the dry run mode, use @option{-v} option:
10186 $ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile}
10187 Reading v.1.0 sparse map
10188 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10189 '/home/gray/sparsefile'
10194 Additionally, if your @command{tar} implementation has extracted the
10195 @dfn{extended headers} for this file, you can instruct @command{xstar}
10196 to use them in order to verify the integrity of the expanded file.
10197 The option @option{-x} sets the name of the extended header file to
10198 use. Continuing our example:
10202 $ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
10203 /home/gray/GNUSparseFile.6058/sparsefile}
10204 Reading extended header file
10205 Found variable GNU.sparse.major = 1
10206 Found variable GNU.sparse.minor = 0
10207 Found variable GNU.sparse.name = sparsefile
10208 Found variable GNU.sparse.realsize = 217481216
10209 Reading v.1.0 sparse map
10210 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10211 '/home/gray/sparsefile'
10216 @anchor{extracting sparse v.0.x}
10217 @cindex sparse files v.0.1, extracting with non-GNU tars
10218 @cindex sparse files v.0.0, extracting with non-GNU tars
10219 An @dfn{extended header} is a special @command{tar} archive header
10220 that precedes an archive member and contains a set of
10221 @dfn{variables}, describing the member properties that cannot be
10222 stored in the standard @code{ustar} header. While optional for
10223 expanding sparse version 1.0 members, the use of extended headers is
10224 mandatory when expanding sparse members in older sparse formats: v.0.0
10225 and v.0.1 (The sparse formats are described in detail in @ref{Sparse
10226 Formats}.) So, for these formats, the question is: how to obtain
10227 extended headers from the archive?
10229 If you use a @command{tar} implementation that does not support PAX
10230 format, extended headers for each member will be extracted as a
10231 separate file. If we represent the member name as
10232 @file{@var{dir}/@var{name}}, then the extended header file will be
10233 named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where
10234 @var{n} is an integer number.
10236 Things become more difficult if your @command{tar} implementation
10237 does support PAX headers, because in this case you will have to
10238 manually extract the headers. We recommend the following algorithm:
10242 Consult the documentation of your @command{tar} implementation for an
10243 option that prints @dfn{block numbers} along with the archive
10244 listing (analogous to @GNUTAR{}'s @option{-R} option). For example,
10245 @command{star} has @option{-block-number}.
10248 Obtain verbose listing using the @samp{block number} option, and
10249 find block numbers of the sparse member in question and the member
10250 immediately following it. For example, running @command{star} on our
10255 $ @kbd{star -t -v -block-number -f arc.tar}
10257 star: Unknown extended header keyword 'GNU.sparse.size' ignored.
10258 star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
10259 star: Unknown extended header keyword 'GNU.sparse.name' ignored.
10260 star: Unknown extended header keyword 'GNU.sparse.map' ignored.
10261 block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
10262 block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
10268 (as usual, ignore the warnings about unknown keywords.)
10271 Let @var{size} be the size of the sparse member, @var{Bs} be its block number
10272 and @var{Bn} be the block number of the next member.
10276 @var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2
10280 This number gives the size of the extended header part in tar @dfn{blocks}.
10281 In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2
10285 Use @command{dd} to extract the headers:
10288 @kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}}
10292 where @var{archive} is the archive name, @var{hname} is a name of the
10293 file to store the extended header in, @var{Bs} and @var{N} are
10294 computed in previous steps.
10296 In our example, this command will be
10299 $ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7}
10303 Finally, you can expand the condensed file, using the obtained header:
10307 $ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile}
10308 Reading extended header file
10309 Found variable GNU.sparse.size = 217481216
10310 Found variable GNU.sparse.numblocks = 208
10311 Found variable GNU.sparse.name = sparsefile
10312 Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{}
10313 Expanding file 'GNUSparseFile.28124/sparsefile' to 'sparsefile'
10319 @section Comparison of @command{tar} and @command{cpio}
10322 @FIXME{Reorganize the following material}
10324 The @command{cpio} archive formats, like @command{tar}, do have maximum
10325 file name lengths. The binary and old @acronym{ASCII} formats have a maximum file
10326 length of 256, and the new @acronym{ASCII} and @acronym{CRC ASCII} formats have a max
10327 file length of 1024. @acronym{GNU} @command{cpio} can read and write archives
10328 with arbitrary file name lengths, but other @command{cpio} implementations
10329 may crash unexplainedly trying to read them.
10331 @command{tar} handles symbolic links in the form in which it comes in @acronym{BSD};
10332 @command{cpio} doesn't handle symbolic links in the form in which it comes
10333 in System V prior to SVR4, and some vendors may have added symlinks
10334 to their system without enhancing @command{cpio} to know about them.
10335 Others may have enhanced it in a way other than the way I did it
10336 at Sun, and which was adopted by AT&T (and which is, I think, also
10337 present in the @command{cpio} that Berkeley picked up from AT&T and put
10338 into a later @acronym{BSD} release---I think I gave them my changes).
10340 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
10341 can handle @command{tar} format input, and write it on output, and it
10342 probably handles symbolic links. They may not have bothered doing
10343 anything to enhance @command{tar} as a result.)
10345 @command{cpio} handles special files; traditional @command{tar} doesn't.
10347 @command{tar} comes with V7, System III, System V, and @acronym{BSD} source;
10348 @command{cpio} comes only with System III, System V, and later @acronym{BSD}
10349 (4.3-tahoe and later).
10351 @command{tar}'s way of handling multiple hard links to a file can handle
10352 file systems that support 32-bit i-numbers (e.g., the @acronym{BSD} file system);
10353 @command{cpio}s way requires you to play some games (in its ``binary''
10354 format, i-numbers are only 16 bits, and in its ``portable @acronym{ASCII}'' format,
10355 they're 18 bits---it would have to play games with the "file system @acronym{ID}"
10356 field of the header to make sure that the file system @acronym{ID}/i-number pairs
10357 of different files were always different), and I don't know which
10358 @command{cpio}s, if any, play those games. Those that don't might get
10359 confused and think two files are the same file when they're not, and
10360 make hard links between them.
10362 @command{tar}s way of handling multiple hard links to a file places only
10363 one copy of the link on the tape, but the name attached to that copy
10364 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
10365 way puts one copy for every link, but you can retrieve it using any
10369 What type of check sum (if any) is used, and how is this calculated.
10372 See the attached manual pages for @command{tar} and @command{cpio} format.
10373 @command{tar} uses a checksum which is the sum of all the bytes in the
10374 @command{tar} header for a file; @command{cpio} uses no checksum.
10377 If anyone knows why @command{cpio} was made when @command{tar} was present
10381 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
10382 generally-available version of UNIX had @command{tar} at the time. I don't
10383 know whether any version that was generally available @emph{within AT&T}
10384 had @command{tar}, or, if so, whether the people within AT&T who did
10385 @command{cpio} knew about it.
10387 On restore, if there is a corruption on a tape @command{tar} will stop at
10388 that point, while @command{cpio} will skip over it and try to restore the
10391 The main difference is just in the command syntax and header format.
10393 @command{tar} is a little more tape-oriented in that everything is blocked
10394 to start on a record boundary.
10397 Is there any differences between the ability to recover crashed
10398 archives between the two of them. (Is there any chance of recovering
10399 crashed archives at all.)
10402 Theoretically it should be easier under @command{tar} since the blocking
10403 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
10404 However, modern @command{cpio}'s and variations have an option to just
10405 search for the next file header after an error with a reasonable chance
10406 of resyncing. However, lots of tape driver software won't allow you to
10407 continue past a media error which should be the only reason for getting
10408 out of sync unless a file changed sizes while you were writing the
10412 If anyone knows why @command{cpio} was made when @command{tar} was present
10413 at the unix scene, please tell me about this too.
10416 Probably because it is more media efficient (by not blocking everything
10417 and using only the space needed for the headers where @command{tar}
10418 always uses 512 bytes per file header) and it knows how to archive
10421 You might want to look at the freely available alternatives. The
10422 major ones are @command{afio}, @GNUTAR{}, and
10423 @command{pax}, each of which have their own extensions with some
10424 backwards compatibility.
10426 Sparse files were @command{tar}red as sparse files (which you can
10427 easily test, because the resulting archive gets smaller, and
10428 @acronym{GNU} @command{cpio} can no longer read it).
10431 @chapter Tapes and Other Archive Media
10434 A few special cases about tape handling warrant more detailed
10435 description. These special cases are discussed below.
10437 Many complexities surround the use of @command{tar} on tape drives. Since
10438 the creation and manipulation of archives located on magnetic tape was
10439 the original purpose of @command{tar}, it contains many features making
10440 such manipulation easier.
10442 Archives are usually written on dismountable media---tape cartridges,
10443 mag tapes, or floppy disks.
10445 The amount of data a tape or disk holds depends not only on its size,
10446 but also on how it is formatted. A 2400 foot long reel of mag tape
10447 holds 40 megabytes of data when formatted at 1600 bits per inch. The
10448 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
10450 Magnetic media are re-usable---once the archive on a tape is no longer
10451 needed, the archive can be erased and the tape or disk used over.
10452 Media quality does deteriorate with use, however. Most tapes or disks
10453 should be discarded when they begin to produce data errors. EXABYTE
10454 tape cartridges should be discarded when they generate an @dfn{error
10455 count} (number of non-usable bits) of more than 10k.
10457 Magnetic media are written and erased using magnetic fields, and
10458 should be protected from such fields to avoid damage to stored data.
10459 Sticking a floppy disk to a filing cabinet using a magnet is probably
10463 * Device:: Device selection and switching
10464 * Remote Tape Server::
10465 * Common Problems and Solutions::
10466 * Blocking:: Blocking
10467 * Many:: Many archives on one tape
10468 * Using Multiple Tapes:: Using Multiple Tapes
10469 * label:: Including a Label in the Archive
10471 * Write Protection::
10475 @section Device Selection and Switching
10479 @item -f [@var{hostname}:]@var{file}
10480 @itemx --file=[@var{hostname}:]@var{file}
10481 Use archive file or device @var{file} on @var{hostname}.
10484 This option is used to specify the file name of the archive @command{tar}
10487 If the file name is @samp{-}, @command{tar} reads the archive from standard
10488 input (when listing or extracting), or writes it to standard output
10489 (when creating). If the @samp{-} file name is given when updating an
10490 archive, @command{tar} will read the original archive from its standard
10491 input, and will write the entire new archive to its standard output.
10493 If the file name contains a @samp{:}, it is interpreted as
10494 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
10495 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
10496 either case, @command{tar} will invoke the command @command{rsh} (or
10497 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
10498 machine. If you give an alternate login name, it will be given to the
10500 Naturally, the remote machine must have an executable
10501 @command{/usr/libexec/rmt}. This program is free software from the
10502 University of California, and a copy of the source code can be found
10503 with the sources for @command{tar}; it's compiled and installed by default.
10504 The exact path to this utility is determined when configuring the package.
10505 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
10506 your installation prefix. This location may also be overridden at
10507 runtime by using the @option{--rmt-command=@var{command}} option (@xref{Option Summary,
10508 ---rmt-command}, for detailed description of this option. @xref{Remote
10509 Tape Server}, for the description of @command{rmt} command).
10511 If this option is not given, but the environment variable @env{TAPE}
10512 is set, its value is used; otherwise, old versions of @command{tar}
10513 used a default archive name (which was picked when @command{tar} was
10514 compiled). The default is normally set up to be the @dfn{first} tape
10515 drive or other transportable I/O medium on the system.
10517 Starting with version 1.11.5, @GNUTAR{} uses
10518 standard input and standard output as the default device, and I will
10519 not try anymore supporting automatic device detection at installation
10520 time. This was failing really in too many cases, it was hopeless.
10521 This is now completely left to the installer to override standard
10522 input and standard output for default device, if this seems
10523 preferable. Further, I think @emph{most} actual usages of
10524 @command{tar} are done with pipes or disks, not really tapes,
10525 cartridges or diskettes.
10527 Some users think that using standard input and output is running
10528 after trouble. This could lead to a nasty surprise on your screen if
10529 you forget to specify an output file name---especially if you are going
10530 through a network or terminal server capable of buffering large amounts
10531 of output. We had so many bug reports in that area of configuring
10532 default tapes automatically, and so many contradicting requests, that
10533 we finally consider the problem to be portably intractable. We could
10534 of course use something like @samp{/dev/tape} as a default, but this
10535 is @emph{also} running after various kind of trouble, going from hung
10536 processes to accidental destruction of real tapes. After having seen
10537 all this mess, using standard input and output as a default really
10538 sounds like the only clean choice left, and a very useful one too.
10540 @GNUTAR{} reads and writes archive in records, I
10541 suspect this is the main reason why block devices are preferred over
10542 character devices. Most probably, block devices are more efficient
10543 too. The installer could also check for @samp{DEFTAPE} in
10544 @file{<sys/mtio.h>}.
10547 @xopindex{force-local, short description}
10548 @item --force-local
10549 Archive file is local even if it contains a colon.
10551 @opindex rsh-command
10552 @item --rsh-command=@var{command}
10553 Use remote @var{command} instead of @command{rsh}. This option exists
10554 so that people who use something other than the standard @command{rsh}
10555 (e.g., a Kerberized @command{rsh}) can access a remote device.
10557 When this command is not used, the shell command found when
10558 the @command{tar} program was installed is used instead. This is
10559 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
10560 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
10561 The installer may have overridden this by defining the environment
10562 variable @env{RSH} @emph{at installation time}.
10565 Specify drive and density.
10567 @xopindex{multi-volume, short description}
10569 @itemx --multi-volume
10570 Create/list/extract multi-volume archive.
10572 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
10573 that may be larger than will fit on the medium used to hold it.
10574 @xref{Multi-Volume Archives}.
10576 @xopindex{tape-length, short description}
10578 @itemx --tape-length=@var{size}[@var{suf}]
10579 Change tape after writing @var{size} units of data. Unless @var{suf} is
10580 given, @var{size} is treated as kilobytes, i.e. @samp{@var{size} x
10581 1024} bytes. The following suffixes alter this behavior:
10583 @float Table, size-suffixes
10584 @caption{Size Suffixes}
10585 @multitable @columnfractions 0.2 0.3 0.3
10586 @headitem Suffix @tab Units @tab Byte Equivalent
10587 @item b @tab Blocks @tab @var{size} x 512
10588 @item B @tab Kilobytes @tab @var{size} x 1024
10589 @item c @tab Bytes @tab @var{size}
10590 @item G @tab Gigabytes @tab @var{size} x 1024^3
10591 @item K @tab Kilobytes @tab @var{size} x 1024
10592 @item k @tab Kilobytes @tab @var{size} x 1024
10593 @item M @tab Megabytes @tab @var{size} x 1024^2
10594 @item P @tab Petabytes @tab @var{size} x 1024^5
10595 @item T @tab Terabytes @tab @var{size} x 1024^4
10596 @item w @tab Words @tab @var{size} x 2
10600 This option might be useful when your tape drivers do not properly
10601 detect end of physical tapes. By being slightly conservative on the
10602 maximum tape length, you might avoid the problem entirely.
10604 @xopindex{info-script, short description}
10605 @xopindex{new-volume-script, short description}
10606 @item -F @var{command}
10607 @itemx --info-script=@var{command}
10608 @itemx --new-volume-script=@var{command}
10609 Execute @var{command} at end of each tape. This implies
10610 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
10611 description of this option.
10614 @node Remote Tape Server
10615 @section Remote Tape Server
10617 @cindex remote tape drive
10619 In order to access the tape drive on a remote machine, @command{tar}
10620 uses the remote tape server written at the University of California at
10621 Berkeley. The remote tape server must be installed as
10622 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
10623 want to use. @command{tar} calls @command{rmt} by running an
10624 @command{rsh} or @command{remsh} to the remote machine, optionally
10625 using a different login name if one is supplied.
10627 A copy of the source for the remote tape server is provided. Its
10628 source code can be freely distributed. It is compiled and
10629 installed by default.
10631 @cindex absolute file names
10632 Unless you use the @option{--absolute-names} (@option{-P}) option,
10633 @GNUTAR{} will not allow you to create an archive that contains
10634 absolute file names (a file name beginning with @samp{/}.) If you try,
10635 @command{tar} will automatically remove the leading @samp{/} from the
10636 file names it stores in the archive. It will also type a warning
10637 message telling you what it is doing.
10639 When reading an archive that was created with a different
10640 @command{tar} program, @GNUTAR{} automatically
10641 extracts entries in the archive which have absolute file names as if
10642 the file names were not absolute. This is an important feature. A
10643 visitor here once gave a @command{tar} tape to an operator to restore;
10644 the operator used Sun @command{tar} instead of @GNUTAR{},
10645 and the result was that it replaced large portions of
10646 our @file{/bin} and friends with versions from the tape; needless to
10647 say, we were unhappy about having to recover the file system from
10650 For example, if the archive contained a file @file{/usr/bin/computoy},
10651 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
10652 relative to the current directory. If you want to extract the files in
10653 an archive to the same absolute names that they had when the archive
10654 was created, you should do a @samp{cd /} before extracting the files
10655 from the archive, or you should either use the @option{--absolute-names}
10656 option, or use the command @samp{tar -C / @dots{}}.
10658 @cindex Ultrix 3.1 and write failure
10659 Some versions of Unix (Ultrix 3.1 is known to have this problem),
10660 can claim that a short write near the end of a tape succeeded,
10661 when it actually failed. This will result in the -M option not
10662 working correctly. The best workaround at the moment is to use a
10663 significantly larger blocking factor than the default 20.
10665 In order to update an archive, @command{tar} must be able to backspace the
10666 archive in order to reread or rewrite a record that was just read (or
10667 written). This is currently possible only on two kinds of files: normal
10668 disk files (or any other file that can be backspaced with @samp{lseek}),
10669 and industry-standard 9-track magnetic tape (or any other kind of tape
10670 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}).
10672 This means that the @option{--append}, @option{--concatenate}, and
10673 @option{--delete} commands will not work on any other kind of file.
10674 Some media simply cannot be backspaced, which means these commands and
10675 options will never be able to work on them. These non-backspacing
10676 media include pipes and cartridge tape drives.
10678 Some other media can be backspaced, and @command{tar} will work on them
10679 once @command{tar} is modified to do so.
10681 Archives created with the @option{--multi-volume}, @option{--label}, and
10682 @option{--incremental} (@option{-G}) options may not be readable by other version
10683 of @command{tar}. In particular, restoring a file that was split over
10684 a volume boundary will require some careful work with @command{dd}, if
10685 it can be done at all. Other versions of @command{tar} may also create
10686 an empty file whose name is that of the volume header. Some versions
10687 of @command{tar} may create normal files instead of directories archived
10688 with the @option{--incremental} (@option{-G}) option.
10690 @node Common Problems and Solutions
10691 @section Some Common Problems and their Solutions
10696 errors from system:
10698 no such file or directory
10701 errors from @command{tar}:
10702 directory checksum error
10703 header format error
10705 errors from media/system:
10717 @dfn{Block} and @dfn{record} terminology is rather confused, and it
10718 is also confusing to the expert reader. On the other hand, readers
10719 who are new to the field have a fresh mind, and they may safely skip
10720 the next two paragraphs, as the remainder of this manual uses those
10721 two terms in a quite consistent way.
10723 John Gilmore, the writer of the public domain @command{tar} from which
10724 @GNUTAR{} was originally derived, wrote (June 1995):
10727 The nomenclature of tape drives comes from IBM, where I believe
10728 they were invented for the IBM 650 or so. On IBM mainframes, what
10729 is recorded on tape are tape blocks. The logical organization of
10730 data is into records. There are various ways of putting records into
10731 blocks, including @code{F} (fixed sized records), @code{V} (variable
10732 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
10733 to a block), @code{VB} (variable size records, @var{n} to a block),
10734 @code{VSB} (variable spanned blocked: variable sized records that can
10735 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
10736 parameter specified this to the operating system.
10738 The Unix man page on @command{tar} was totally confused about this.
10739 When I wrote @code{PD TAR}, I used the historically correct terminology
10740 (@command{tar} writes data records, which are grouped into blocks).
10741 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
10742 here), and now Fran@,{c}ois has migrated that terminology back
10743 into the source code too.
10746 The term @dfn{physical block} means the basic transfer chunk from or
10747 to a device, after which reading or writing may stop without anything
10748 being lost. In this manual, the term @dfn{block} usually refers to
10749 a disk physical block, @emph{assuming} that each disk block is 512
10750 bytes in length. It is true that some disk devices have different
10751 physical blocks, but @command{tar} ignore these differences in its own
10752 format, which is meant to be portable, so a @command{tar} block is always
10753 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
10754 The term @dfn{logical block} often represents the basic chunk of
10755 allocation of many disk blocks as a single entity, which the operating
10756 system treats somewhat atomically; this concept is only barely used
10759 The term @dfn{physical record} is another way to speak of a physical
10760 block, those two terms are somewhat interchangeable. In this manual,
10761 the term @dfn{record} usually refers to a tape physical block,
10762 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
10763 It is true that archives may be put on disk or used with pipes,
10764 but nevertheless, @command{tar} tries to read and write the archive one
10765 @dfn{record} at a time, whatever the medium in use. One record is made
10766 up of an integral number of blocks, and this operation of putting many
10767 disk blocks into a single tape block is called @dfn{reblocking}, or
10768 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
10769 the logical organization of many characters into something meaningful
10770 to the application. The term @dfn{unit record} describes a small set
10771 of characters which are transmitted whole to or by the application,
10772 and often refers to a line of text. Those two last terms are unrelated
10773 to what we call a @dfn{record} in @GNUTAR{}.
10775 When writing to tapes, @command{tar} writes the contents of the archive
10776 in chunks known as @dfn{records}. To change the default blocking
10777 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
10778 @var{512-size}}) option. Each record will then be composed of
10779 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
10780 @xref{Standard}.) Each file written to the archive uses at least one
10781 full record. As a result, using a larger record size can result in
10782 more wasted space for small files. On the other hand, a larger record
10783 size can often be read and written much more efficiently.
10785 Further complicating the problem is that some tape drives ignore the
10786 blocking entirely. For these, a larger record size can still improve
10787 performance (because the software layers above the tape drive still
10788 honor the blocking), but not as dramatically as on tape drives that
10791 When reading an archive, @command{tar} can usually figure out the
10792 record size on itself. When this is the case, and a non-standard
10793 record size was used when the archive was created, @command{tar} will
10794 print a message about a non-standard blocking factor, and then operate
10795 normally@footnote{If this message is not needed, you can turn it off
10796 using the @option{--warning=no-record-size} option.}. On some tape
10797 devices, however, @command{tar} cannot figure out the record size
10798 itself. On most of those, you can specify a blocking factor (with
10799 @option{--blocking-factor}) larger than the actual blocking factor,
10800 and then use the @option{--read-full-records} (@option{-B}) option.
10801 (If you specify a blocking factor with @option{--blocking-factor} and
10802 don't use the @option{--read-full-records} option, then @command{tar}
10803 will not attempt to figure out the recording size itself.) On some
10804 devices, you must always specify the record size exactly with
10805 @option{--blocking-factor} when reading, because @command{tar} cannot
10806 figure it out. In any case, use @option{--list} (@option{-t}) before
10807 doing any extractions to see whether @command{tar} is reading the archive
10810 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
10811 putting them into records is to put a whole number of them (one or
10812 more) into each record. @command{tar} records are all the same size;
10813 at the end of the file there's a block containing all zeros, which
10814 is how you tell that the remainder of the last record(s) are garbage.
10816 In a standard @command{tar} file (no options), the block size is 512
10817 and the record size is 10240, for a blocking factor of 20. What the
10818 @option{--blocking-factor} option does is sets the blocking factor,
10819 changing the record size while leaving the block size at 512 bytes.
10820 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
10821 most tape drives these days prefer much bigger records in order to
10822 stream and not waste tape. When writing tapes for myself, some tend
10823 to use a factor of the order of 2048, say, giving a record size of
10824 around one megabyte.
10826 If you use a blocking factor larger than 20, older @command{tar}
10827 programs might not be able to read the archive, so we recommend this
10828 as a limit to use in practice. @GNUTAR{}, however,
10829 will support arbitrarily large record sizes, limited only by the
10830 amount of virtual memory or the physical characteristics of the tape
10834 * Format Variations:: Format Variations
10835 * Blocking Factor:: The Blocking Factor of an Archive
10838 @node Format Variations
10839 @subsection Format Variations
10840 @cindex Format Parameters
10841 @cindex Format Options
10842 @cindex Options, archive format specifying
10843 @cindex Options, format specifying
10846 Format parameters specify how an archive is written on the archive
10847 media. The best choice of format parameters will vary depending on
10848 the type and number of files being archived, and on the media used to
10851 To specify format parameters when accessing or creating an archive,
10852 you can use the options described in the following sections.
10853 If you do not specify any format parameters, @command{tar} uses
10854 default parameters. You cannot modify a compressed archive.
10855 If you create an archive with the @option{--blocking-factor} option
10856 specified (@pxref{Blocking Factor}), you must specify that
10857 blocking-factor when operating on the archive. @xref{Formats}, for other
10858 examples of format parameter considerations.
10860 @node Blocking Factor
10861 @subsection The Blocking Factor of an Archive
10862 @cindex Blocking Factor
10863 @cindex Record Size
10864 @cindex Number of blocks per record
10865 @cindex Number of bytes per record
10866 @cindex Bytes per record
10867 @cindex Blocks per record
10870 @opindex blocking-factor
10871 The data in an archive is grouped into blocks, which are 512 bytes.
10872 Blocks are read and written in whole number multiples called
10873 @dfn{records}. The number of blocks in a record (i.e., the size of a
10874 record in units of 512 bytes) is called the @dfn{blocking factor}.
10875 The @option{--blocking-factor=@var{512-size}} (@option{-b
10876 @var{512-size}}) option specifies the blocking factor of an archive.
10877 The default blocking factor is typically 20 (i.e., 10240 bytes), but
10878 can be specified at installation. To find out the blocking factor of
10879 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
10880 This may not work on some devices.
10882 Records are separated by gaps, which waste space on the archive media.
10883 If you are archiving on magnetic tape, using a larger blocking factor
10884 (and therefore larger records) provides faster throughput and allows you
10885 to fit more data on a tape (because there are fewer gaps). If you are
10886 archiving on cartridge, a very large blocking factor (say 126 or more)
10887 greatly increases performance. A smaller blocking factor, on the other
10888 hand, may be useful when archiving small files, to avoid archiving lots
10889 of nulls as @command{tar} fills out the archive to the end of the record.
10890 In general, the ideal record size depends on the size of the
10891 inter-record gaps on the tape you are using, and the average size of the
10892 files you are archiving. @xref{create}, for information on
10895 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
10897 Archives with blocking factors larger than 20 cannot be read
10898 by very old versions of @command{tar}, or by some newer versions
10899 of @command{tar} running on old machines with small address spaces.
10900 With @GNUTAR{}, the blocking factor of an archive is limited
10901 only by the maximum record size of the device containing the archive,
10902 or by the amount of available virtual memory.
10904 Also, on some systems, not using adequate blocking factors, as sometimes
10905 imposed by the device drivers, may yield unexpected diagnostics. For
10906 example, this has been reported:
10909 Cannot write to /dev/dlt: Invalid argument
10913 In such cases, it sometimes happen that the @command{tar} bundled by
10914 the system is aware of block size idiosyncrasies, while @GNUTAR{}
10915 requires an explicit specification for the block size,
10916 which it cannot guess. This yields some people to consider
10917 @GNUTAR{} is misbehaving, because by comparison,
10918 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
10919 for example, might resolve the problem.
10921 If you use a non-default blocking factor when you create an archive, you
10922 must specify the same blocking factor when you modify that archive. Some
10923 archive devices will also require you to specify the blocking factor when
10924 reading that archive, however this is not typically the case. Usually, you
10925 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
10926 reports a non-default record size and then lists the archive members as
10927 it would normally. To extract files from an archive with a non-standard
10928 blocking factor (particularly if you're not sure what the blocking factor
10929 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
10930 specifying a blocking factor larger then the blocking factor of the archive
10931 (i.e., @samp{tar --extract --read-full-records --blocking-factor=300}).
10932 @xref{list}, for more information on the @option{--list} (@option{-t})
10933 operation. @xref{Reading}, for a more detailed explanation of that option.
10936 @item --blocking-factor=@var{number}
10937 @itemx -b @var{number}
10938 Specifies the blocking factor of an archive. Can be used with any
10939 operation, but is usually not necessary with @option{--list} (@option{-t}).
10945 @item -b @var{blocks}
10946 @itemx --blocking-factor=@var{blocks}
10947 Set record size to @math{@var{blocks}*512} bytes.
10949 This option is used to specify a @dfn{blocking factor} for the archive.
10950 When reading or writing the archive, @command{tar}, will do reads and writes
10951 of the archive in records of @math{@var{block}*512} bytes. This is true
10952 even when the archive is compressed. Some devices requires that all
10953 write operations be a multiple of a certain size, and so, @command{tar}
10954 pads the archive out to the next record boundary.
10956 The default blocking factor is set when @command{tar} is compiled, and is
10957 typically 20. Blocking factors larger than 20 cannot be read by very
10958 old versions of @command{tar}, or by some newer versions of @command{tar}
10959 running on old machines with small address spaces.
10961 With a magnetic tape, larger records give faster throughput and fit
10962 more data on a tape (because there are fewer inter-record gaps).
10963 If the archive is in a disk file or a pipe, you may want to specify
10964 a smaller blocking factor, since a large one will result in a large
10965 number of null bytes at the end of the archive.
10967 When writing cartridge or other streaming tapes, a much larger
10968 blocking factor (say 126 or more) will greatly increase performance.
10969 However, you must specify the same blocking factor when reading or
10970 updating the archive.
10972 Apparently, Exabyte drives have a physical block size of 8K bytes.
10973 If we choose our blocksize as a multiple of 8k bytes, then the problem
10974 seems to disappear. Id est, we are using block size of 112 right
10975 now, and we haven't had the problem since we switched@dots{}
10977 With @GNUTAR{} the blocking factor is limited only
10978 by the maximum record size of the device containing the archive, or by
10979 the amount of available virtual memory.
10981 However, deblocking or reblocking is virtually avoided in a special
10982 case which often occurs in practice, but which requires all the
10983 following conditions to be simultaneously true:
10986 the archive is subject to a compression option,
10988 the archive is not handled through standard input or output, nor
10989 redirected nor piped,
10991 the archive is directly handled to a local disk, instead of any special
10994 @option{--blocking-factor} is not explicitly specified on the @command{tar}
10998 If the output goes directly to a local disk, and not through
10999 stdout, then the last write is not extended to a full record size.
11000 Otherwise, reblocking occurs. Here are a few other remarks on this
11006 @command{gzip} will complain about trailing garbage if asked to
11007 uncompress a compressed archive on tape, there is an option to turn
11008 the message off, but it breaks the regularity of simply having to use
11009 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
11010 silently ignoring any number of trailing zeros. I'll ask Jean-loup
11011 Gailly, by sending a copy of this message to him.
11014 @command{compress} does not show this problem, but as Jean-loup pointed
11015 out to Michael, @samp{compress -d} silently adds garbage after
11016 the result of decompression, which tar ignores because it already
11017 recognized its end-of-file indicator. So this bug may be safely
11021 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
11022 but will still return an exit status of 2 which tar reports in turn.
11023 @command{tar} might ignore the exit status returned, but I hate doing
11024 that, as it weakens the protection @command{tar} offers users against
11025 other possible problems at decompression time. If @command{gzip} was
11026 silently skipping trailing zeros @emph{and} also avoiding setting the
11027 exit status in this innocuous case, that would solve this situation.
11030 @command{tar} should become more solid at not stopping to read a pipe at
11031 the first null block encountered. This inelegantly breaks the pipe.
11032 @command{tar} should rather drain the pipe out before exiting itself.
11035 @xopindex{ignore-zeros, short description}
11037 @itemx --ignore-zeros
11038 Ignore blocks of zeros in archive (means EOF).
11040 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
11041 of zeros in the archive. Normally a block of zeros indicates the
11042 end of the archive, but when reading a damaged archive, or one which
11043 was created by concatenating several archives together, this option
11044 allows @command{tar} to read the entire archive. This option is not on
11045 by default because many versions of @command{tar} write garbage after
11048 Note that this option causes @command{tar} to read to the end of the
11049 archive file, which may sometimes avoid problems when multiple files
11050 are stored on a single physical tape.
11052 @xopindex{read-full-records, short description}
11054 @itemx --read-full-records
11055 Reblock as we read (for reading 4.2@acronym{BSD} pipes).
11057 If @option{--read-full-records} is used, @command{tar}
11058 will not panic if an attempt to read a record from the archive does
11059 not return a full record. Instead, @command{tar} will keep reading
11060 until it has obtained a full
11063 This option is turned on by default when @command{tar} is reading
11064 an archive from standard input, or from a remote machine. This is
11065 because on @acronym{BSD} Unix systems, a read of a pipe will return however
11066 much happens to be in the pipe, even if it is less than @command{tar}
11067 requested. If this option was not used, @command{tar} would fail as
11068 soon as it read an incomplete record from the pipe.
11070 This option is also useful with the commands for updating an archive.
11076 @FIXME{Appropriate options should be moved here from elsewhere.}
11078 @cindex blocking factor
11079 @cindex tape blocking
11081 When handling various tapes or cartridges, you have to take care of
11082 selecting a proper blocking, that is, the number of disk blocks you
11083 put together as a single tape block on the tape, without intervening
11084 tape gaps. A @dfn{tape gap} is a small landing area on the tape
11085 with no information on it, used for decelerating the tape to a
11086 full stop, and for later regaining the reading or writing speed.
11087 When the tape driver starts reading a record, the record has to
11088 be read whole without stopping, as a tape gap is needed to stop the
11089 tape motion without losing information.
11091 @cindex Exabyte blocking
11092 @cindex DAT blocking
11093 Using higher blocking (putting more disk blocks per tape block) will use
11094 the tape more efficiently as there will be less tape gaps. But reading
11095 such tapes may be more difficult for the system, as more memory will be
11096 required to receive at once the whole record. Further, if there is a
11097 reading error on a huge record, this is less likely that the system will
11098 succeed in recovering the information. So, blocking should not be too
11099 low, nor it should be too high. @command{tar} uses by default a blocking of
11100 20 for historical reasons, and it does not really matter when reading or
11101 writing to disk. Current tape technology would easily accommodate higher
11102 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
11103 We were told that for some DLT drives, the blocking should be a multiple
11104 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
11105 Other manufacturers may use different recommendations for the same tapes.
11106 This might also depends of the buffering techniques used inside modern
11107 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
11108 Others request blocking to be some exponent of two.
11110 So, there is no fixed rule for blocking. But blocking at read time
11111 should ideally be the same as blocking used at write time. At one place
11112 I know, with a wide variety of equipment, they found it best to use a
11113 blocking of 32 to guarantee that their tapes are fully interchangeable.
11115 I was also told that, for recycled tapes, prior erasure (by the same
11116 drive unit that will be used to create the archives) sometimes lowers
11117 the error rates observed at rewriting time.
11119 I might also use @option{--number-blocks} instead of
11120 @option{--block-number}, so @option{--block} will then expand to
11121 @option{--blocking-factor} unambiguously.
11124 @section Many Archives on One Tape
11126 @FIXME{Appropriate options should be moved here from elsewhere.}
11128 @findex ntape @r{device}
11129 Most tape devices have two entries in the @file{/dev} directory, or
11130 entries that come in pairs, which differ only in the minor number for
11131 this device. Let's take for example @file{/dev/tape}, which often
11132 points to the only or usual tape device of a given system. There might
11133 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
11134 name is the @emph{rewinding} version of the device, while the name
11135 having @samp{nr} in it is the @emph{no rewinding} version of the same
11138 A rewinding tape device will bring back the tape to its beginning point
11139 automatically when this device is opened or closed. Since @command{tar}
11140 opens the archive file before using it and closes it afterwards, this
11141 means that a simple:
11144 $ @kbd{tar cf /dev/tape @var{directory}}
11148 will reposition the tape to its beginning both prior and after saving
11149 @var{directory} contents to it, thus erasing prior tape contents and
11150 making it so that any subsequent write operation will destroy what has
11153 @cindex tape positioning
11154 So, a rewinding device is normally meant to hold one and only one file.
11155 If you want to put more than one @command{tar} archive on a given tape, you
11156 will need to avoid using the rewinding version of the tape device. You
11157 will also have to pay special attention to tape positioning. Errors in
11158 positioning may overwrite the valuable data already on your tape. Many
11159 people, burnt by past experiences, will only use rewinding devices and
11160 limit themselves to one file per tape, precisely to avoid the risk of
11161 such errors. Be fully aware that writing at the wrong position on a
11162 tape loses all information past this point and most probably until the
11163 end of the tape, and this destroyed information @emph{cannot} be
11166 To save @var{directory-1} as a first archive at the beginning of a
11167 tape, and leave that tape ready for a second archive, you should use:
11170 $ @kbd{mt -f /dev/nrtape rewind}
11171 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
11175 @dfn{Tape marks} are special magnetic patterns written on the tape
11176 media, which are later recognizable by the reading hardware. These
11177 marks are used after each file, when there are many on a single tape.
11178 An empty file (that is to say, two tape marks in a row) signal the
11179 logical end of the tape, after which no file exist. Usually,
11180 non-rewinding tape device drivers will react to the close request issued
11181 by @command{tar} by first writing two tape marks after your archive, and by
11182 backspacing over one of these. So, if you remove the tape at that time
11183 from the tape drive, it is properly terminated. But if you write
11184 another file at the current position, the second tape mark will be
11185 erased by the new information, leaving only one tape mark between files.
11187 So, you may now save @var{directory-2} as a second archive after the
11188 first on the same tape by issuing the command:
11191 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
11195 and so on for all the archives you want to put on the same tape.
11197 Another usual case is that you do not write all the archives the same
11198 day, and you need to remove and store the tape between two archive
11199 sessions. In general, you must remember how many files are already
11200 saved on your tape. Suppose your tape already has 16 files on it, and
11201 that you are ready to write the 17th. You have to take care of skipping
11202 the first 16 tape marks before saving @var{directory-17}, say, by using
11206 $ @kbd{mt -f /dev/nrtape rewind}
11207 $ @kbd{mt -f /dev/nrtape fsf 16}
11208 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
11211 In all the previous examples, we put aside blocking considerations, but
11212 you should do the proper things for that as well. @xref{Blocking}.
11215 * Tape Positioning:: Tape Positions and Tape Marks
11216 * mt:: The @command{mt} Utility
11219 @node Tape Positioning
11220 @subsection Tape Positions and Tape Marks
11223 Just as archives can store more than one file from the file system,
11224 tapes can store more than one archive file. To keep track of where
11225 archive files (or any other type of file stored on tape) begin and
11226 end, tape archive devices write magnetic @dfn{tape marks} on the
11227 archive media. Tape drives write one tape mark between files,
11228 two at the end of all the file entries.
11230 If you think of data as a series of records "rrrr"'s, and tape marks as
11231 "*"'s, a tape might look like the following:
11234 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
11237 Tape devices read and write tapes using a read/write @dfn{tape
11238 head}---a physical part of the device which can only access one
11239 point on the tape at a time. When you use @command{tar} to read or
11240 write archive data from a tape device, the device will begin reading
11241 or writing from wherever on the tape the tape head happens to be,
11242 regardless of which archive or what part of the archive the tape
11243 head is on. Before writing an archive, you should make sure that no
11244 data on the tape will be overwritten (unless it is no longer needed).
11245 Before reading an archive, you should make sure the tape head is at
11246 the beginning of the archive you want to read. You can do it manually
11247 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
11248 that automatically (@pxref{Scripted Restoration}).
11250 If you want to add new archive file entries to a tape, you should
11251 advance the tape to the end of the existing file entries, backspace
11252 over the last tape mark, and write the new archive file. If you were
11253 to add two archives to the example above, the tape might look like the
11257 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
11261 @subsection The @command{mt} Utility
11264 @FIXME{Is it true that this only works on non-block devices?
11265 should explain the difference, (fixed or variable).}
11266 @xref{Blocking Factor}.
11268 You can use the @command{mt} utility to advance or rewind a tape past a
11269 specified number of archive files on the tape. This will allow you
11270 to move to the beginning of an archive before extracting or reading
11271 it, or to the end of all the archives before writing a new one.
11272 @FIXME{Why isn't there an "advance 'til you find two tape marks
11275 The syntax of the @command{mt} command is:
11278 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
11281 where @var{tapename} is the name of the tape device, @var{number} is
11282 the number of times an operation is performed (with a default of one),
11283 and @var{operation} is one of the following:
11285 @FIXME{is there any use for record operations?}
11290 Writes @var{number} tape marks at the current position on the tape.
11293 Moves tape position forward @var{number} files.
11296 Moves tape position back @var{number} files.
11299 Rewinds the tape. (Ignores @var{number}.)
11303 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}.)
11306 Prints status information about the tape unit.
11310 If you don't specify a @var{tapename}, @command{mt} uses the environment
11311 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
11312 the default device specified in your @file{sys/mtio.h} file
11313 (@code{DEFTAPE} variable). If this is not defined, the program will
11314 display a descriptive error message and exit with code 1.
11316 @command{mt} returns a 0 exit status when the operation(s) were
11317 successful, 1 if the command was unrecognized, and 2 if an operation
11320 @node Using Multiple Tapes
11321 @section Using Multiple Tapes
11323 Often you might want to write a large archive, one larger than will fit
11324 on the actual tape you are using. In such a case, you can run multiple
11325 @command{tar} commands, but this can be inconvenient, particularly if you
11326 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
11327 Therefore, @command{tar} provides a special mode for creating
11328 multi-volume archives.
11330 @dfn{Multi-volume} archive is a single @command{tar} archive, stored
11331 on several media volumes of fixed size. Although in this section we will
11332 often call @samp{volume} a @dfn{tape}, there is absolutely no
11333 requirement for multi-volume archives to be stored on tapes. Instead,
11334 they can use whatever media type the user finds convenient, they can
11335 even be located on files.
11337 When creating a multi-volume archive, @GNUTAR{} continues to fill
11338 current volume until it runs out of space, then it switches to
11339 next volume (usually the operator is queried to replace the tape on
11340 this point), and continues working on the new volume. This operation
11341 continues until all requested files are dumped. If @GNUTAR{} detects
11342 end of media while dumping a file, such a file is archived in split
11343 form. Some very big files can even be split across several volumes.
11345 Each volume is itself a valid @GNUTAR{} archive, so it can be read
11346 without any special options. Consequently any file member residing
11347 entirely on one volume can be extracted or otherwise operated upon
11348 without needing the other volume. Sure enough, to extract a split
11349 member you would need all volumes its parts reside on.
11351 Multi-volume archives suffer from several limitations. In particular,
11352 they cannot be compressed.
11354 @GNUTAR{} is able to create multi-volume archives of two formats
11355 (@pxref{Formats}): @samp{GNU} and @samp{POSIX}.
11358 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
11359 * Tape Files:: Tape Files
11360 * Tarcat:: Concatenate Volumes into a Single Archive
11364 @node Multi-Volume Archives
11365 @subsection Archives Longer than One Tape or Disk
11366 @cindex Multi-volume archives
11368 @opindex multi-volume
11369 To create an archive that is larger than will fit on a single unit of
11370 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
11371 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
11372 archive can be manipulated like any other archive (provided the
11373 @option{--multi-volume} option is specified), but is stored on more
11374 than one tape or file.
11376 When you specify @option{--multi-volume}, @command{tar} does not report an
11377 error when it comes to the end of an archive volume (when reading), or
11378 the end of the media (when writing). Instead, it prompts you to load
11379 a new storage volume. If the archive is on a magnetic tape, you
11380 should change tapes when you see the prompt; if the archive is on a
11381 floppy disk, you should change disks; etc.
11384 @item --multi-volume
11386 Creates a multi-volume archive, when used in conjunction with
11387 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
11388 archive, specify @option{--multi-volume} in conjunction with that
11393 $ @kbd{tar --create --multi-volume --file=/dev/tape @var{files}}
11397 The method @command{tar} uses to detect end of tape is not perfect, and
11398 fails on some operating systems or on some devices. If @command{tar}
11399 cannot detect the end of the tape itself, you can use
11400 @option{--tape-length} option to inform it about the capacity of the
11403 @anchor{tape-length}
11405 @opindex tape-length
11406 @item --tape-length=@var{size}[@var{suf}]
11407 @itemx -L @var{size}[@var{suf}]
11408 Set maximum length of a volume. The @var{suf}, if given, specifies
11409 units in which @var{size} is expressed, e.g. @samp{2M} mean 2
11410 megabytes (@pxref{size-suffixes}, for a list of allowed size
11411 suffixes). Without @var{suf}, units of 1024 bytes (kilobyte) are
11414 This option selects @option{--multi-volume} automatically. For example:
11417 $ @kbd{tar --create --tape-length=41943040 --file=/dev/tape @var{files}}
11421 or, which is equivalent:
11424 $ @kbd{tar --create --tape-length=4G --file=/dev/tape @var{files}}
11428 @anchor{change volume prompt}
11429 When @GNUTAR{} comes to the end of a storage media, it asks you to
11430 change the volume. The built-in prompt for POSIX locale
11431 is@footnote{If you run @GNUTAR{} under a different locale, the
11432 translation to the locale's language will be used.}:
11435 Prepare volume #@var{n} for '@var{archive}' and hit return:
11439 where @var{n} is the ordinal number of the volume to be created and
11440 @var{archive} is archive file or device name.
11442 When prompting for a new tape, @command{tar} accepts any of the following
11447 Request @command{tar} to explain possible responses.
11449 Request @command{tar} to exit immediately.
11450 @item n @var{file-name}
11451 Request @command{tar} to write the next volume on the file @var{file-name}.
11453 Request @command{tar} to run a subshell. This option can be disabled
11454 by giving @option{--restrict} command line option to
11455 @command{tar}@footnote{@xref{--restrict}, for more information about
11458 Request @command{tar} to begin writing the next volume.
11461 (You should only type @samp{y} after you have changed the tape;
11462 otherwise @command{tar} will write over the volume it just finished.)
11464 @cindex Volume number file
11466 @anchor{volno-file}
11467 @opindex volno-file
11468 The volume number used by @command{tar} in its tape-changing prompt
11469 can be changed; if you give the
11470 @option{--volno-file=@var{file-of-number}} option, then
11471 @var{file-of-number} should be an non-existing file to be created, or
11472 else, a file already containing a decimal number. That number will be
11473 used as the volume number of the first volume written. When
11474 @command{tar} is finished, it will rewrite the file with the
11475 now-current volume number. (This does not change the volume number
11476 written on a tape label, as per @ref{label}, it @emph{only} affects
11477 the number used in the prompt.)
11479 @cindex End-of-archive info script
11480 @cindex Info script
11481 @anchor{info-script}
11482 @opindex info-script
11483 @opindex new-volume-script
11484 If you want more elaborate behavior than this, you can write a special
11485 @dfn{new volume script}, that will be responsible for changing the
11486 volume, and instruct @command{tar} to use it instead of its normal
11487 prompting procedure:
11490 @item --info-script=@var{command}
11491 @itemx --new-volume-script=@var{command}
11492 @itemx -F @var{command}
11493 Specify the command to invoke when switching volumes. The @var{command}
11494 can be used to eject cassettes, or to broadcast messages such as
11495 @samp{Someone please come change my tape} when performing unattended
11499 The @var{command} can contain additional options, if such are needed.
11500 @xref{external, Running External Commands}, for a detailed discussion
11501 of the way @GNUTAR{} runs external commands. It inherits
11502 @command{tar}'s shell environment. Additional data is passed to it
11503 via the following environment variables:
11506 @vrindex TAR_VERSION, info script environment variable
11508 @GNUTAR{} version number.
11510 @vrindex TAR_ARCHIVE, info script environment variable
11512 The name of the archive @command{tar} is processing.
11514 @vrindex TAR_BLOCKING_FACTOR, info script environment variable
11515 @item TAR_BLOCKING_FACTOR
11516 Current blocking factor (@pxref{Blocking}).
11518 @vrindex TAR_VOLUME, info script environment variable
11520 Ordinal number of the volume @command{tar} is about to start.
11522 @vrindex TAR_SUBCOMMAND, info script environment variable
11523 @item TAR_SUBCOMMAND
11524 A short option describing the operation @command{tar} is executing.
11525 @xref{Operations}, for a complete list of subcommand options.
11527 @vrindex TAR_FORMAT, info script environment variable
11529 Format of the archive being processed. @xref{Formats}, for a complete
11530 list of archive format names.
11532 @vrindex TAR_FD, info script environment variable
11534 File descriptor which can be used to communicate the new volume
11535 name to @command{tar}.
11538 These variables can be used in the @var{command} itself, provided that
11539 they are properly quoted to prevent them from being expanded by the
11540 shell that invokes @command{tar}.
11542 The volume script can instruct @command{tar} to use new archive name,
11543 by writing in to file descriptor @env{$TAR_FD} (see below for an example).
11545 If the info script fails, @command{tar} exits; otherwise, it begins
11546 writing the next volume.
11548 If you want @command{tar} to cycle through a series of files or tape
11549 drives, there are three approaches to choose from. First of all, you
11550 can give @command{tar} multiple @option{--file} options. In this case
11551 the specified files will be used, in sequence, as the successive
11552 volumes of the archive. Only when the first one in the sequence needs
11553 to be used again will @command{tar} prompt for a tape change (or run
11554 the info script). For example, suppose someone has two tape drives on
11555 a system named @file{/dev/tape0} and @file{/dev/tape1}. For having
11556 @GNUTAR{} to switch to the second drive when it needs to write the
11557 second tape, and then back to the first tape, etc., just do either of:
11560 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
11561 $ @kbd{tar -cM -f /dev/tape0 -f /dev/tape1 @var{files}}
11564 The second method is to use the @samp{n} response to the tape-change
11567 Finally, the most flexible approach is to use a volume script, that
11568 writes new archive name to the file descriptor @env{$TAR_FD}. For example, the
11569 following volume script will create a series of archive files, named
11570 @file{@var{archive}-@var{vol}}, where @var{archive} is the name of the
11571 archive being created (as given by @option{--file} option) and
11572 @var{vol} is the ordinal number of the archive being created:
11577 # For this script it's advisable to use a shell, such as Bash,
11578 # that supports a TAR_FD value greater than 9.
11580 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
11582 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
11583 case $TAR_SUBCOMMAND in
11585 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
11590 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&$TAR_FD
11594 The same script can be used while listing, comparing or extracting
11595 from the created archive. For example:
11599 # @r{Create a multi-volume archive:}
11600 $ @kbd{tar -c -L1024 -f archive.tar -F new-volume .}
11601 # @r{Extract from the created archive:}
11602 $ @kbd{tar -x -f archive.tar -F new-volume .}
11607 Notice, that the first command had to use @option{-L} option, since
11608 otherwise @GNUTAR{} will end up writing everything to file
11609 @file{archive.tar}.
11611 You can read each individual volume of a multi-volume archive as if it
11612 were an archive by itself. For example, to list the contents of one
11613 volume, use @option{--list}, without @option{--multi-volume} specified.
11614 To extract an archive member from one volume (assuming it is described
11615 that volume), use @option{--extract}, again without
11616 @option{--multi-volume}.
11618 If an archive member is split across volumes (i.e., its entry begins on
11619 one volume of the media and ends on another), you need to specify
11620 @option{--multi-volume} to extract it successfully. In this case, you
11621 should load the volume where the archive member starts, and use
11622 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
11623 volumes as it needs them. @xref{extracting archives}, for more
11624 information about extracting archives.
11626 Multi-volume archives can be modified like any other archive. To add
11627 files to a multi-volume archive, you need to only mount the last
11628 volume of the archive media (and new volumes, if needed). For all
11629 other operations, you need to use the entire archive.
11631 If a multi-volume archive was labeled using
11632 @option{--label=@var{archive-label}} (@pxref{label}) when it was
11633 created, @command{tar} will not automatically label volumes which are
11634 added later. To label subsequent volumes, specify
11635 @option{--label=@var{archive-label}} again in conjunction with the
11636 @option{--append}, @option{--update} or @option{--concatenate} operation.
11638 Notice that multi-volume support is a GNU extension and the archives
11639 created in this mode should be read only using @GNUTAR{}. If you
11640 absolutely have to process such archives using a third-party @command{tar}
11641 implementation, read @ref{Split Recovery}.
11644 @subsection Tape Files
11645 @cindex labeling archives
11649 To give the archive a name which will be recorded in it, use the
11650 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
11651 option. This will write a special block identifying
11652 @var{volume-label} as the name of the archive to the front of the
11653 archive which will be displayed when the archive is listed with
11654 @option{--list}. If you are creating a multi-volume archive with
11655 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
11656 volume label will have @samp{Volume @var{nnn}} appended to the name
11657 you give, where @var{nnn} is the number of the volume of the archive.
11658 If you use the @option{--label=@var{volume-label}} option when
11659 reading an archive, it checks to make sure the label on the tape
11660 matches the one you gave. @xref{label}.
11662 When @command{tar} writes an archive to tape, it creates a single
11663 tape file. If multiple archives are written to the same tape, one
11664 after the other, they each get written as separate tape files. When
11665 extracting, it is necessary to position the tape at the right place
11666 before running @command{tar}. To do this, use the @command{mt} command.
11667 For more information on the @command{mt} command and on the organization
11668 of tapes into a sequence of tape files, see @ref{mt}.
11670 People seem to often do:
11673 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
11676 or such, for pushing a common date in all volumes or an archive set.
11679 @subsection Concatenate Volumes into a Single Archive
11682 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
11683 archive to a single @command{tar} archive. Simply concatenating all
11684 volumes into one will not work, since each volume carries an additional
11685 information at the beginning. @GNUTAR{} is shipped with the shell
11686 script @command{tarcat} designed for this purpose.
11688 The script takes a list of files comprising a multi-volume archive
11689 and creates the resulting archive at the standard output. For example:
11692 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
11695 The script implements a simple heuristics to determine the format of
11696 the first volume file and to decide how to process the rest of the
11697 files. However, it makes no attempt to verify whether the files are
11698 given in order or even if they are valid @command{tar} archives.
11699 It uses @command{dd} and does not filter its standard error, so you
11700 will usually see lots of spurious messages.
11702 @FIXME{The script is not installed. Should we install it?}
11705 @section Including a Label in the Archive
11706 @cindex Labeling an archive
11707 @cindex Labels on the archive media
11708 @cindex Labeling multi-volume archives
11711 To avoid problems caused by misplaced paper labels on the archive
11712 media, you can include a @dfn{label} entry --- an archive member which
11713 contains the name of the archive --- in the archive itself. Use the
11714 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11715 option@footnote{Until version 1.10, that option was called
11716 @option{--volume}, but is not available under that name anymore.} in
11717 conjunction with the @option{--create} operation to include a label
11718 entry in the archive as it is being created.
11721 @item --label=@var{archive-label}
11722 @itemx -V @var{archive-label}
11723 Includes an @dfn{archive-label} at the beginning of the archive when
11724 the archive is being created, when used in conjunction with the
11725 @option{--create} operation. Checks to make sure the archive label
11726 matches the one specified (when used in conjunction with any other
11730 If you create an archive using both
11731 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11732 and @option{--multi-volume} (@option{-M}), each volume of the archive
11733 will have an archive label of the form @samp{@var{archive-label}
11734 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
11735 next, and so on. @xref{Using Multiple Tapes}, for information on
11736 creating multiple volume archives.
11738 @cindex Volume label, listing
11739 @cindex Listing volume label
11740 The volume label will be displayed by @option{--list} along with
11741 the file contents. If verbose display is requested, it will also be
11742 explicitly marked as in the example below:
11746 $ @kbd{tar --verbose --list --file=iamanarchive}
11747 V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header--
11748 -rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename
11752 @opindex test-label
11753 @anchor{--test-label option}
11754 However, @option{--list} option will cause listing entire
11755 contents of the archive, which may be undesirable (for example, if the
11756 archive is stored on a tape). You can request checking only the volume
11757 label by specifying @option{--test-label} option. This option reads only the
11758 first block of an archive, so it can be used with slow storage
11759 devices. For example:
11763 $ @kbd{tar --test-label --file=iamanarchive}
11768 If @option{--test-label} is used with one or more command line
11769 arguments, @command{tar} compares the volume label with each
11770 argument. It exits with code 0 if a match is found, and with code 1
11771 otherwise@footnote{Note that @GNUTAR{} versions up to 1.23 indicated
11772 mismatch with an exit code 2 and printed a spurious diagnostics on
11773 stderr.}. No output is displayed, unless you also used the
11774 @option{--verbose} option. For example:
11778 $ @kbd{tar --test-label --file=iamanarchive 'iamalabel'}
11780 $ @kbd{tar --test-label --file=iamanarchive 'alabel'}
11785 When used with the @option{--verbose} option, @command{tar}
11786 prints the actual volume label (if any), and a verbose diagnostics in
11787 case of a mismatch:
11791 $ @kbd{tar --test-label --verbose --file=iamanarchive 'iamalabel'}
11794 $ @kbd{tar --test-label --verbose --file=iamanarchive 'alabel'}
11796 tar: Archive label mismatch
11801 If you request any operation, other than @option{--create}, along
11802 with using @option{--label} option, @command{tar} will first check if
11803 the archive label matches the one specified and will refuse to proceed
11804 if it does not. Use this as a safety precaution to avoid accidentally
11805 overwriting existing archives. For example, if you wish to add files
11806 to @file{archive}, presumably labeled with string @samp{My volume},
11811 $ @kbd{tar -rf archive --label 'My volume' .}
11812 tar: Archive not labeled to match 'My volume'
11817 in case its label does not match. This will work even if
11818 @file{archive} is not labeled at all.
11820 Similarly, @command{tar} will refuse to list or extract the
11821 archive if its label doesn't match the @var{archive-label}
11822 specified. In those cases, @var{archive-label} argument is interpreted
11823 as a globbing-style pattern which must match the actual magnetic
11824 volume label. @xref{exclude}, for a precise description of how match
11825 is attempted@footnote{Previous versions of @command{tar} used full
11826 regular expression matching, or before that, only exact string
11827 matching, instead of wildcard matchers. We decided for the sake of
11828 simplicity to use a uniform matching device through
11829 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
11830 the volume label matcher will also suffix @var{archive-label} by
11831 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
11832 up. Since the volume numbering is automatically added in labels at
11833 creation time, it sounded logical to equally help the user taking care
11834 of it when the archive is being read.
11836 You can also use @option{--label} to get a common information on
11837 all tapes of a series. For having this information different in each
11838 series created through a single script used on a regular basis, just
11839 manage to get some date string as part of the label. For example:
11843 $ @kbd{tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"}
11844 $ @kbd{tar --create --file=/dev/tape --multi-volume \
11845 --label="Daily backup for `date +%Y-%m-%d`"}
11849 Some more notes about volume labels:
11852 @item Each label has its own date and time, which corresponds
11853 to the time when @GNUTAR{} initially attempted to write it,
11854 often soon after the operator launches @command{tar} or types the
11855 carriage return telling that the next tape is ready.
11857 @item Comparing date labels to get an idea of tape throughput is
11858 unreliable. It gives correct results only if the delays for rewinding
11859 tapes and the operator switching them were negligible, which is
11860 usually not the case.
11864 @section Verifying Data as It is Stored
11865 @cindex Verifying a write operation
11866 @cindex Double-checking a write operation
11871 @opindex verify, short description
11872 Attempt to verify the archive after writing.
11875 This option causes @command{tar} to verify the archive after writing it.
11876 Each volume is checked after it is written, and any discrepancies
11877 are recorded on the standard error output.
11879 Verification requires that the archive be on a back-space-able medium.
11880 This means pipes, some cartridge tape drives, and some other devices
11881 cannot be verified.
11883 You can insure the accuracy of an archive by comparing files in the
11884 system with archive members. @command{tar} can compare an archive to the
11885 file system as the archive is being written, to verify a write
11886 operation, or can compare a previously written archive, to insure that
11889 @xopindex{verify, using with @option{--create}}
11890 @xopindex{create, using with @option{--verify}}
11891 To check for discrepancies in an archive immediately after it is
11892 written, use the @option{--verify} (@option{-W}) option in conjunction with
11893 the @option{--create} operation. When this option is
11894 specified, @command{tar} checks archive members against their counterparts
11895 in the file system, and reports discrepancies on the standard error.
11897 To verify an archive, you must be able to read it from before the end
11898 of the last written entry. This option is useful for detecting data
11899 errors on some tapes. Archives written to pipes, some cartridge tape
11900 drives, and some other devices cannot be verified.
11902 One can explicitly compare an already made archive with the file
11903 system by using the @option{--compare} (@option{--diff}, @option{-d})
11904 option, instead of using the more automatic @option{--verify} option.
11907 Note that these two options have a slightly different intent. The
11908 @option{--compare} option checks how identical are the logical contents of some
11909 archive with what is on your disks, while the @option{--verify} option is
11910 really for checking if the physical contents agree and if the recording
11911 media itself is of dependable quality. So, for the @option{--verify}
11912 operation, @command{tar} tries to defeat all in-memory cache pertaining to
11913 the archive, while it lets the speed optimization undisturbed for the
11914 @option{--compare} option. If you nevertheless use @option{--compare} for
11915 media verification, you may have to defeat the in-memory cache yourself,
11916 maybe by opening and reclosing the door latch of your recording unit,
11917 forcing some doubt in your operating system about the fact this is really
11918 the same volume as the one just written or read.
11920 The @option{--verify} option would not be necessary if drivers were indeed
11921 able to detect dependably all write failures. This sometimes require many
11922 magnetic heads, some able to read after the writes occurred. One would
11923 not say that drivers unable to detect all cases are necessarily flawed,
11924 as long as programming is concerned.
11926 The @option{--verify} (@option{-W}) option will not work in
11927 conjunction with the @option{--multi-volume} (@option{-M}) option or
11928 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
11929 and @option{--delete} operations. @xref{Operations}, for more
11930 information on these operations.
11932 Also, since @command{tar} normally strips leading @samp{/} from file
11933 names (@pxref{absolute}), a command like @samp{tar --verify -cf
11934 /tmp/foo.tar /etc} will work as desired only if the working directory is
11935 @file{/}, as @command{tar} uses the archive's relative member names
11936 (e.g., @file{etc/motd}) when verifying the archive.
11938 @node Write Protection
11939 @section Write Protection
11941 Almost all tapes and diskettes, and in a few rare cases, even disks can
11942 be @dfn{write protected}, to protect data on them from being changed.
11943 Once an archive is written, you should write protect the media to prevent
11944 the archive from being accidentally overwritten or deleted. (This will
11945 protect the archive from being changed with a tape or floppy drive---it
11946 will not protect it from magnet fields or other physical hazards.)
11948 The write protection device itself is usually an integral part of the
11949 physical media, and can be a two position (write enabled/write
11950 disabled) switch, a notch which can be popped out or covered, a ring
11951 which can be removed from the center of a tape reel, or some other
11952 changeable feature.
11954 @node Reliability and security
11955 @chapter Reliability and Security
11957 The @command{tar} command reads and writes files as any other
11958 application does, and is subject to the usual caveats about
11959 reliability and security. This section contains some commonsense
11960 advice on the topic.
11968 @section Reliability
11970 Ideally, when @command{tar} is creating an archive, it reads from a
11971 file system that is not being modified, and encounters no errors or
11972 inconsistencies while reading and writing. If this is the case, the
11973 archive should faithfully reflect what was read. Similarly, when
11974 extracting from an archive, ideally @command{tar} ideally encounters
11975 no errors and the extracted files faithfully reflect what was in the
11978 However, when reading or writing real-world file systems, several
11979 things can go wrong; these include permissions problems, corruption of
11980 data, and race conditions.
11983 * Permissions problems::
11984 * Data corruption and repair::
11985 * Race conditions::
11988 @node Permissions problems
11989 @subsection Permissions Problems
11991 If @command{tar} encounters errors while reading or writing files, it
11992 normally reports an error and exits with nonzero status. The work it
11993 does may therefore be incomplete. For example, when creating an
11994 archive, if @command{tar} cannot read a file then it cannot copy the
11995 file into the archive.
11997 @node Data corruption and repair
11998 @subsection Data Corruption and Repair
12000 If an archive becomes corrupted by an I/O error, this may corrupt the
12001 data in an extracted file. Worse, it may corrupt the file's metadata,
12002 which may cause later parts of the archive to become misinterpreted.
12003 An tar-format archive contains a checksum that most likely will detect
12004 errors in the metadata, but it will not detect errors in the data.
12006 If data corruption is a concern, you can compute and check your own
12007 checksums of an archive by using other programs, such as
12010 When attempting to recover from a read error or data corruption in an
12011 archive, you may need to skip past the questionable data and read the
12012 rest of the archive. This requires some expertise in the archive
12013 format and in other software tools.
12015 @node Race conditions
12016 @subsection Race conditions
12018 If some other process is modifying the file system while @command{tar}
12019 is reading or writing files, the result may well be inconsistent due
12020 to race conditions. For example, if another process creates some
12021 files in a directory while @command{tar} is creating an archive
12022 containing the directory's files, @command{tar} may see some of the
12023 files but not others, or it may see a file that is in the process of
12024 being created. The resulting archive may not be a snapshot of the
12025 file system at any point in time. If an application such as a
12026 database system depends on an accurate snapshot, restoring from the
12027 @command{tar} archive of a live file system may therefore break that
12028 consistency and may break the application. The simplest way to avoid
12029 the consistency issues is to avoid making other changes to the file
12030 system while tar is reading it or writing it.
12032 When creating an archive, several options are available to avoid race
12033 conditions. Some hosts have a way of snapshotting a file system, or
12034 of temporarily suspending all changes to a file system, by (say)
12035 suspending the only virtual machine that can modify a file system; if
12036 you use these facilities and have @command{tar -c} read from a
12037 snapshot when creating an archive, you can avoid inconsistency
12038 problems. More drastically, before starting @command{tar} you could
12039 suspend or shut down all processes other than @command{tar} that have
12040 access to the file system, or you could unmount the file system and
12041 then mount it read-only.
12043 When extracting from an archive, one approach to avoid race conditions
12044 is to create a directory that no other process can write to, and
12050 In some cases @command{tar} may be used in an adversarial situation,
12051 where an untrusted user is attempting to gain information about or
12052 modify otherwise-inaccessible files. Dealing with untrusted data
12053 (that is, data generated by an untrusted user) typically requires
12054 extra care, because even the smallest mistake in the use of
12055 @command{tar} is more likely to be exploited by an adversary than by a
12061 * Live untrusted data::
12062 * Security rules of thumb::
12066 @subsection Privacy
12068 Standard privacy concerns apply when using @command{tar}. For
12069 example, suppose you are archiving your home directory into a file
12070 @file{/archive/myhome.tar}. Any secret information in your home
12071 directory, such as your SSH secret keys, are copied faithfully into
12072 the archive. Therefore, if your home directory contains any file that
12073 should not be read by some other user, the archive itself should be
12074 not be readable by that user. And even if the archive's data are
12075 inaccessible to untrusted users, its metadata (such as size or
12076 last-modified date) may reveal some information about your home
12077 directory; if the metadata are intended to be private, the archive's
12078 parent directory should also be inaccessible to untrusted users.
12080 One precaution is to create @file{/archive} so that it is not
12081 accessible to any user, unless that user also has permission to access
12082 all the files in your home directory.
12084 Similarly, when extracting from an archive, take care that the
12085 permissions of the extracted files are not more generous than what you
12086 want. Even if the archive itself is readable only to you, files
12087 extracted from it have their own permissions that may differ.
12090 @subsection Integrity
12092 When creating archives, take care that they are not writable by a
12093 untrusted user; otherwise, that user could modify the archive, and
12094 when you later extract from the archive you will get incorrect data.
12096 When @command{tar} extracts from an archive, by default it writes into
12097 files relative to the working directory. If the archive was generated
12098 by an untrusted user, that user therefore can write into any file
12099 under the working directory. If the working directory contains a
12100 symbolic link to another directory, the untrusted user can also write
12101 into any file under the referenced directory. When extracting from an
12102 untrusted archive, it is therefore good practice to create an empty
12103 directory and run @command{tar} in that directory.
12105 When extracting from two or more untrusted archives, each one should
12106 be extracted independently, into different empty directories.
12107 Otherwise, the first archive could create a symbolic link into an area
12108 outside the working directory, and the second one could follow the
12109 link and overwrite data that is not under the working directory. For
12110 example, when restoring from a series of incremental dumps, the
12111 archives should have been created by a trusted process, as otherwise
12112 the incremental restores might alter data outside the working
12115 If you use the @option{--absolute-names} (@option{-P}) option when
12116 extracting, @command{tar} respects any file names in the archive, even
12117 file names that begin with @file{/} or contain @file{..}. As this
12118 lets the archive overwrite any file in your system that you can write,
12119 the @option{--absolute-names} (@option{-P}) option should be used only
12120 for trusted archives.
12122 Conversely, with the @option{--keep-old-files} (@option{-k}) and
12123 @option{--skip-old-files} options, @command{tar} refuses to replace
12124 existing files when extracting. The difference between the two
12125 options is that the former treats existing files as errors whereas the
12126 latter just silently ignores them.
12128 Finally, with the @option{--no-overwrite-dir} option, @command{tar}
12129 refuses to replace the permissions or ownership of already-existing
12130 directories. These options may help when extracting from untrusted
12133 @node Live untrusted data
12134 @subsection Dealing with Live Untrusted Data
12136 Extra care is required when creating from or extracting into a file
12137 system that is accessible to untrusted users. For example, superusers
12138 who invoke @command{tar} must be wary about its actions being hijacked
12139 by an adversary who is reading or writing the file system at the same
12140 time that @command{tar} is operating.
12142 When creating an archive from a live file system, @command{tar} is
12143 vulnerable to denial-of-service attacks. For example, an adversarial
12144 user could create the illusion of an indefinitely-deep directory
12145 hierarchy @file{d/e/f/g/...} by creating directories one step ahead of
12146 @command{tar}, or the illusion of an indefinitely-long file by
12147 creating a sparse file but arranging for blocks to be allocated just
12148 before @command{tar} reads them. There is no easy way for
12149 @command{tar} to distinguish these scenarios from legitimate uses, so
12150 you may need to monitor @command{tar}, just as you'd need to monitor
12151 any other system service, to detect such attacks.
12153 While a superuser is extracting from an archive into a live file
12154 system, an untrusted user might replace a directory with a symbolic
12155 link, in hopes that @command{tar} will follow the symbolic link and
12156 extract data into files that the untrusted user does not have access
12157 to. Even if the archive was generated by the superuser, it may
12158 contain a file such as @file{d/etc/passwd} that the untrusted user
12159 earlier created in order to break in; if the untrusted user replaces
12160 the directory @file{d/etc} with a symbolic link to @file{/etc} while
12161 @command{tar} is running, @command{tar} will overwrite
12162 @file{/etc/passwd}. This attack can be prevented by extracting into a
12163 directory that is inaccessible to untrusted users.
12165 Similar attacks via symbolic links are also possible when creating an
12166 archive, if the untrusted user can modify an ancestor of a top-level
12167 argument of @command{tar}. For example, an untrusted user that can
12168 modify @file{/home/eve} can hijack a running instance of @samp{tar -cf
12169 - /home/eve/Documents/yesterday} by replacing
12170 @file{/home/eve/Documents} with a symbolic link to some other
12171 location. Attacks like these can be prevented by making sure that
12172 untrusted users cannot modify any files that are top-level arguments
12173 to @command{tar}, or any ancestor directories of these files.
12175 @node Security rules of thumb
12176 @subsection Security Rules of Thumb
12178 This section briefly summarizes rules of thumb for avoiding security
12184 Protect archives at least as much as you protect any of the files
12188 Extract from an untrusted archive only into an otherwise-empty
12189 directory. This directory and its parent should be accessible only to
12190 trusted users. For example:
12194 $ @kbd{chmod go-rwx .}
12195 $ @kbd{mkdir -m go-rwx dir}
12197 $ @kbd{tar -xvf /archives/got-it-off-the-net.tar.gz}
12201 As a corollary, do not do an incremental restore from an untrusted archive.
12204 Do not let untrusted users access files extracted from untrusted
12205 archives without checking first for problems such as setuid programs.
12208 Do not let untrusted users modify directories that are ancestors of
12209 top-level arguments of @command{tar}. For example, while you are
12210 executing @samp{tar -cf /archive/u-home.tar /u/home}, do not let an
12211 untrusted user modify @file{/}, @file{/archive}, or @file{/u}.
12214 Pay attention to the diagnostics and exit status of @command{tar}.
12217 When archiving live file systems, monitor running instances of
12218 @command{tar} to detect denial-of-service attacks.
12221 Avoid unusual options such as @option{--absolute-names} (@option{-P}),
12222 @option{--dereference} (@option{-h}), @option{--overwrite},
12223 @option{--recursive-unlink}, and @option{--remove-files} unless you
12224 understand their security implications.
12231 This appendix lists some important user-visible changes between
12232 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
12233 version of this document is available at
12234 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
12235 @GNUTAR{} documentation page}.
12238 @item Use of globbing patterns when listing and extracting.
12240 Previous versions of GNU tar assumed shell-style globbing when
12241 extracting from or listing an archive. For example:
12244 $ @kbd{tar xf foo.tar '*.c'}
12247 would extract all files whose names end in @samp{.c}. This behavior
12248 was not documented and was incompatible with traditional tar
12249 implementations. Therefore, starting from version 1.15.91, GNU tar
12250 no longer uses globbing by default. For example, the above invocation
12251 is now interpreted as a request to extract from the archive the file
12254 To facilitate transition to the new behavior for those users who got
12255 used to the previous incorrect one, @command{tar} will print a warning
12256 if it finds out that a requested member was not found in the archive
12257 and its name looks like a globbing pattern. For example:
12260 $ @kbd{tar xf foo.tar '*.c'}
12261 tar: Pattern matching characters used in file names. Please,
12262 tar: use --wildcards to enable pattern matching, or --no-wildcards to
12263 tar: suppress this warning.
12264 tar: *.c: Not found in archive
12265 tar: Error exit delayed from previous errors
12268 To treat member names as globbing patterns, use the @option{--wildcards} option.
12269 If you want to tar to mimic the behavior of versions prior to 1.15.91,
12270 add this option to your @env{TAR_OPTIONS} variable.
12272 @xref{wildcards}, for the detailed discussion of the use of globbing
12273 patterns by @GNUTAR{}.
12275 @item Use of short option @option{-o}.
12277 Earlier versions of @GNUTAR{} understood @option{-o} command line
12278 option as a synonym for @option{--old-archive}.
12280 @GNUTAR{} starting from version 1.13.90 understands this option as
12281 a synonym for @option{--no-same-owner}. This is compatible with
12282 UNIX98 @command{tar} implementations.
12284 However, to facilitate transition, @option{-o} option retains its
12285 old semantics when it is used with one of archive-creation commands.
12286 Users are encouraged to use @option{--format=oldgnu} instead.
12288 It is especially important, since versions of @acronym{GNU} Automake
12289 up to and including 1.8.4 invoke tar with this option to produce
12290 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
12291 of this issue and its implications.
12293 @xref{Options, tar-formats, Changing Automake's Behavior,
12294 automake, GNU Automake}, for a description on how to use various
12295 archive formats with @command{automake}.
12297 Future versions of @GNUTAR{} will understand @option{-o} only as a
12298 synonym for @option{--no-same-owner}.
12300 @item Use of short option @option{-l}
12302 Earlier versions of @GNUTAR{} understood @option{-l} option as a
12303 synonym for @option{--one-file-system}. Since such usage contradicted
12304 to UNIX98 specification and harmed compatibility with other
12305 implementations, it was declared deprecated in version 1.14. However,
12306 to facilitate transition to its new semantics, it was supported by
12307 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
12308 variant of @option{--check-links} was introduced in version 1.15.91.
12310 @item Use of options @option{--portability} and @option{--old-archive}
12312 These options are deprecated. Please use @option{--format=v7} instead.
12314 @item Use of option @option{--posix}
12316 This option is deprecated. Please use @option{--format=posix} instead.
12319 @node Configuring Help Summary
12320 @appendix Configuring Help Summary
12322 Running @kbd{tar --help} displays the short @command{tar} option
12323 summary (@pxref{help}). This summary is organized by @dfn{groups} of
12324 semantically close options. The options within each group are printed
12325 in the following order: a short option, eventually followed by a list
12326 of corresponding long option names, followed by a short description of
12327 the option. For example, here is an excerpt from the actual @kbd{tar
12331 Main operation mode:
12333 -A, --catenate, --concatenate append tar files to an archive
12334 -c, --create create a new archive
12335 -d, --diff, --compare find differences between archive and
12337 --delete delete from the archive
12340 @vrindex ARGP_HELP_FMT, environment variable
12341 The exact visual representation of the help output is configurable via
12342 @env{ARGP_HELP_FMT} environment variable. The value of this variable
12343 is a comma-separated list of @dfn{format variable} assignments. There
12344 are two kinds of format variables. An @dfn{offset variable} keeps the
12345 offset of some part of help output text from the leftmost column on
12346 the screen. A @dfn{boolean} variable is a flag that toggles some
12347 output feature on or off. Depending on the type of the corresponding
12348 variable, there are two kinds of assignments:
12351 @item Offset assignment
12353 The assignment to an offset variable has the following syntax:
12356 @var{variable}=@var{value}
12360 where @var{variable} is the variable name, and @var{value} is a
12361 numeric value to be assigned to the variable.
12363 @item Boolean assignment
12365 To assign @code{true} value to a variable, simply put this variable name. To
12366 assign @code{false} value, prefix the variable name with @samp{no-}. For
12371 # Assign @code{true} value:
12373 # Assign @code{false} value:
12379 Following variables are declared:
12381 @deftypevr {Help Output} boolean dup-args
12382 If true, arguments for an option are shown with both short and long
12383 options, even when a given option has both forms, for example:
12386 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
12389 If false, then if an option has both short and long forms, the
12390 argument is only shown with the long one, for example:
12393 -f, --file=ARCHIVE use archive file or device ARCHIVE
12397 and a message indicating that the argument is applicable to both
12398 forms is printed below the options. This message can be disabled
12399 using @code{dup-args-note} (see below).
12401 The default is false.
12404 @deftypevr {Help Output} boolean dup-args-note
12405 If this variable is true, which is the default, the following notice
12406 is displayed at the end of the help output:
12409 Mandatory or optional arguments to long options are also mandatory or
12410 optional for any corresponding short options.
12413 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
12414 variables @code{dup-args} or @code{dup-args-note} should be set.
12417 @deftypevr {Help Output} offset short-opt-col
12418 Column in which short options start. Default is 2.
12422 $ @kbd{tar --help|grep ARCHIVE}
12423 -f, --file=ARCHIVE use archive file or device ARCHIVE
12424 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
12425 -f, --file=ARCHIVE use archive file or device ARCHIVE
12430 @deftypevr {Help Output} offset long-opt-col
12431 Column in which long options start. Default is 6. For example:
12435 $ @kbd{tar --help|grep ARCHIVE}
12436 -f, --file=ARCHIVE use archive file or device ARCHIVE
12437 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
12438 -f, --file=ARCHIVE use archive file or device ARCHIVE
12443 @deftypevr {Help Output} offset doc-opt-col
12444 Column in which @dfn{doc options} start. A doc option isn't actually
12445 an option, but rather an arbitrary piece of documentation that is
12446 displayed in much the same manner as the options. For example, in
12447 the description of @option{--format} option:
12451 -H, --format=FORMAT create archive of the given format.
12453 FORMAT is one of the following:
12455 gnu GNU tar 1.13.x format
12456 oldgnu GNU format as per tar <= 1.12
12457 pax POSIX 1003.1-2001 (pax) format
12459 ustar POSIX 1003.1-1988 (ustar) format
12460 v7 old V7 tar format
12465 the format names are doc options. Thus, if you set
12466 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
12467 will look as follows:
12471 -H, --format=FORMAT create archive of the given format.
12473 FORMAT is one of the following:
12475 gnu GNU tar 1.13.x format
12476 oldgnu GNU format as per tar <= 1.12
12477 pax POSIX 1003.1-2001 (pax) format
12479 ustar POSIX 1003.1-1988 (ustar) format
12480 v7 old V7 tar format
12485 @deftypevr {Help Output} offset opt-doc-col
12486 Column in which option description starts. Default is 29.
12490 $ @kbd{tar --help|grep ARCHIVE}
12491 -f, --file=ARCHIVE use archive file or device ARCHIVE
12492 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
12493 -f, --file=ARCHIVE use archive file or device ARCHIVE
12494 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
12496 use archive file or device ARCHIVE
12501 Notice, that the description starts on a separate line if
12502 @code{opt-doc-col} value is too small.
12505 @deftypevr {Help Output} offset header-col
12506 Column in which @dfn{group headers} are printed. A group header is a
12507 descriptive text preceding an option group. For example, in the
12511 Main operation mode:
12513 -A, --catenate, --concatenate append tar files to
12515 -c, --create create a new archive
12518 @samp{Main operation mode:} is the group header.
12520 The default value is 1.
12523 @deftypevr {Help Output} offset usage-indent
12524 Indentation of wrapped usage lines. Affects @option{--usage}
12525 output. Default is 12.
12528 @deftypevr {Help Output} offset rmargin
12529 Right margin of the text output. Used for wrapping.
12532 @node Fixing Snapshot Files
12533 @appendix Fixing Snapshot Files
12534 @include tar-snapshot-edit.texi
12536 @node Tar Internals
12537 @appendix Tar Internals
12538 @include intern.texi
12542 @include genfile.texi
12544 @node Free Software Needs Free Documentation
12545 @appendix Free Software Needs Free Documentation
12546 @include freemanuals.texi
12548 @node GNU Free Documentation License
12549 @appendix GNU Free Documentation License
12553 @node Index of Command Line Options
12554 @appendix Index of Command Line Options
12556 This appendix contains an index of all @GNUTAR{} long command line
12557 options. The options are listed without the preceding double-dash.
12558 For a cross-reference of short command line options, see
12559 @ref{Short Option Summary}.
12572 @c Local variables:
12573 @c texinfo-column-for-description: 32