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-newer-files}
2808 @item --keep-newer-files
2810 Do not replace existing files that are newer than their archive copies
2811 when extracting files from an archive.
2813 @opsummary{keep-old-files}
2814 @item --keep-old-files
2817 Do not overwrite existing files when extracting files from an
2818 archive. Return error if such files exist. See also
2819 @ref{--skip-old-files}.
2821 @xref{Keep Old Files}.
2824 @item --label=@var{name}
2825 @itemx -V @var{name}
2827 When creating an archive, instructs @command{tar} to write @var{name}
2828 as a name record in the archive. When extracting or listing archives,
2829 @command{tar} will only operate on archives that have a label matching
2830 the pattern specified in @var{name}. @xref{Tape Files}.
2833 @item --level=@var{n}
2834 Force incremental backup of level @var{n}. As of @GNUTAR version
2835 @value{VERSION}, the option @option{--level=0} truncates the snapshot
2836 file, thereby forcing the level 0 dump. Other values of @var{n} are
2837 effectively ignored. @xref{--level=0}, for details and examples.
2839 The use of this option is valid only in conjunction with the
2840 @option{--listed-incremental} option. @xref{Incremental Dumps},
2841 for a detailed description.
2843 @opsummary{listed-incremental}
2844 @item --listed-incremental=@var{snapshot-file}
2845 @itemx -g @var{snapshot-file}
2847 During a @option{--create} operation, specifies that the archive that
2848 @command{tar} creates is a new @acronym{GNU}-format incremental
2849 backup, using @var{snapshot-file} to determine which files to backup.
2850 With other operations, informs @command{tar} that the archive is in
2851 incremental format. @xref{Incremental Dumps}.
2856 This option tells @command{tar} to read or write archives through
2857 @command{lzip}. @xref{gzip}.
2862 This option tells @command{tar} to read or write archives through
2863 @command{lzma}. @xref{gzip}.
2867 This option tells @command{tar} to read or write archives through
2868 @command{lzop}. @xref{gzip}.
2871 @item --mode=@var{permissions}
2873 When adding files to an archive, @command{tar} will use
2874 @var{permissions} for the archive members, rather than the permissions
2875 from the files. @var{permissions} can be specified either as an octal
2876 number or as symbolic permissions, like with
2877 @command{chmod}. @xref{override}.
2880 @item --mtime=@var{date}
2882 When adding files to an archive, @command{tar} will use @var{date} as
2883 the modification time of members when creating archives, instead of
2884 their actual modification times. The value of @var{date} can be
2885 either a textual date representation (@pxref{Date input formats}) or a
2886 name of the existing file, starting with @samp{/} or @samp{.}. In the
2887 latter case, the modification time of that file is used. @xref{override}.
2889 @opsummary{multi-volume}
2890 @item --multi-volume
2893 Informs @command{tar} that it should create or otherwise operate on a
2894 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2896 @opsummary{new-volume-script}
2897 @item --new-volume-script
2899 (see @option{--info-script})
2902 @item --newer=@var{date}
2903 @itemx --after-date=@var{date}
2906 When creating an archive, @command{tar} will only add files that have changed
2907 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2908 is taken to be the name of a file whose data modification time specifies
2909 the date. @xref{after}.
2911 @opsummary{newer-mtime}
2912 @item --newer-mtime=@var{date}
2914 Like @option{--newer}, but add only files whose
2915 contents have changed (as opposed to just @option{--newer}, which will
2916 also back up files for which any status information has
2917 changed). @xref{after}.
2919 @opsummary{no-anchored}
2921 An exclude pattern can match any subsequence of the name's components.
2922 @xref{controlling pattern-matching}.
2924 @opsummary{no-auto-compress}
2925 @item --no-auto-compress
2927 Disables automatic compressed format recognition based on the archive
2928 suffix. @xref{--auto-compress}. @xref{gzip}.
2930 @opsummary{no-check-device}
2931 @item --no-check-device
2932 Do not check device numbers when creating a list of modified files
2933 for incremental archiving. @xref{device numbers}, for
2934 a detailed description.
2936 @opsummary{no-delay-directory-restore}
2937 @item --no-delay-directory-restore
2939 Modification times and permissions of extracted
2940 directories are set when all files from this directory have been
2941 extracted. This is the default.
2942 @xref{Directory Modification Times and Permissions}.
2944 @opsummary{no-ignore-case}
2945 @item --no-ignore-case
2946 Use case-sensitive matching.
2947 @xref{controlling pattern-matching}.
2949 @opsummary{no-ignore-command-error}
2950 @item --no-ignore-command-error
2951 Print warnings about subprocesses that terminated with a nonzero exit
2952 code. @xref{Writing to an External Program}.
2957 If the @option{--null} option was given previously, this option
2958 cancels its effect, so that any following @option{--files-from}
2959 options will expect their file lists to be newline-terminated.
2961 @opsummary{no-overwrite-dir}
2962 @item --no-overwrite-dir
2964 Preserve metadata of existing directories when extracting files
2965 from an archive. @xref{Overwrite Old Files}.
2967 @opsummary{no-quote-chars}
2968 @item --no-quote-chars=@var{string}
2969 Remove characters listed in @var{string} from the list of quoted
2970 characters set by the previous @option{--quote-chars} option
2971 (@pxref{quoting styles}).
2973 @opsummary{no-recursion}
2974 @item --no-recursion
2976 With this option, @command{tar} will not recurse into directories.
2979 @opsummary{no-same-owner}
2980 @item --no-same-owner
2983 When extracting an archive, do not attempt to preserve the owner
2984 specified in the @command{tar} archive. This the default behavior
2987 @opsummary{no-same-permissions}
2988 @item --no-same-permissions
2990 When extracting an archive, subtract the user's umask from files from
2991 the permissions specified in the archive. This is the default behavior
2997 The archive media does not support seeks to arbitrary
2998 locations. Usually @command{tar} determines automatically whether
2999 the archive can be seeked or not. Use this option to disable this
3002 @opsummary{no-unquote}
3004 Treat all input file or member names literally, do not interpret
3005 escape sequences. @xref{input name quoting}.
3007 @opsummary{no-wildcards}
3008 @item --no-wildcards
3009 Do not use wildcards.
3010 @xref{controlling pattern-matching}.
3012 @opsummary{no-wildcards-match-slash}
3013 @item --no-wildcards-match-slash
3014 Wildcards do not match @samp{/}.
3015 @xref{controlling pattern-matching}.
3020 When @command{tar} is using the @option{--files-from} option, this option
3021 instructs @command{tar} to expect file names terminated with @acronym{NUL}, so
3022 @command{tar} can correctly work with file names that contain newlines.
3025 @opsummary{numeric-owner}
3026 @item --numeric-owner
3028 This option will notify @command{tar} that it should use numeric user
3029 and group IDs when creating a @command{tar} file, rather than names.
3033 The function of this option depends on the action @command{tar} is
3034 performing. When extracting files, @option{-o} is a synonym for
3035 @option{--no-same-owner}, i.e., it prevents @command{tar} from
3036 restoring ownership of files being extracted.
3038 When creating an archive, it is a synonym for
3039 @option{--old-archive}. This behavior is for compatibility
3040 with previous versions of @GNUTAR{}, and will be
3041 removed in future releases.
3043 @xref{Changes}, for more information.
3045 @opsummary{occurrence}
3046 @item --occurrence[=@var{number}]
3048 This option can be used in conjunction with one of the subcommands
3049 @option{--delete}, @option{--diff}, @option{--extract} or
3050 @option{--list} when a list of files is given either on the command
3051 line or via @option{-T} option.
3053 This option instructs @command{tar} to process only the @var{number}th
3054 occurrence of each named file. @var{Number} defaults to 1, so
3057 tar -x -f archive.tar --occurrence filename
3061 will extract the first occurrence of the member @file{filename} from @file{archive.tar}
3062 and will terminate without scanning to the end of the archive.
3064 @opsummary{old-archive}
3066 Synonym for @option{--format=v7}.
3068 @opsummary{one-file-system}
3069 @item --one-file-system
3070 Used when creating an archive. Prevents @command{tar} from recursing into
3071 directories that are on different file systems from the current
3074 @opsummary{overwrite}
3077 Overwrite existing files and directory metadata when extracting files
3078 from an archive. @xref{Overwrite Old Files}.
3080 @opsummary{overwrite-dir}
3081 @item --overwrite-dir
3083 Overwrite the metadata of existing directories when extracting files
3084 from an archive. @xref{Overwrite Old Files}.
3087 @item --owner=@var{user}
3089 Specifies that @command{tar} should use @var{user} as the owner of members
3090 when creating archives, instead of the user associated with the source
3091 file. @var{user} can specify a symbolic name, or a numeric
3092 @acronym{ID}, or both as @var{name}:@var{id}.
3095 This option does not affect extraction from archives.
3097 @opsummary{pax-option}
3098 @item --pax-option=@var{keyword-list}
3099 This option enables creation of the archive in @acronym{POSIX.1-2001}
3100 format (@pxref{posix}) and modifies the way @command{tar} handles the
3101 extended header keywords. @var{Keyword-list} is a comma-separated
3102 list of keyword options. @xref{PAX keywords}, for a detailed
3105 @opsummary{portability}
3107 @itemx --old-archive
3108 Synonym for @option{--format=v7}.
3112 Same as @option{--format=posix}.
3114 @opsummary{preserve}
3117 Synonymous with specifying both @option{--preserve-permissions} and
3118 @option{--same-order}. @xref{Setting Access Permissions}.
3120 @opsummary{preserve-order}
3121 @item --preserve-order
3123 (See @option{--same-order}; @pxref{Reading}.)
3125 @opsummary{preserve-permissions}
3126 @opsummary{same-permissions}
3127 @item --preserve-permissions
3128 @itemx --same-permissions
3131 When @command{tar} is extracting an archive, it normally subtracts the
3132 users' umask from the permissions specified in the archive and uses
3133 that number as the permissions to create the destination file.
3134 Specifying this option instructs @command{tar} that it should use the
3135 permissions directly from the archive. @xref{Setting Access Permissions}.
3137 @opsummary{quote-chars}
3138 @item --quote-chars=@var{string}
3139 Always quote characters from @var{string}, even if the selected
3140 quoting style would not quote them (@pxref{quoting styles}).
3142 @opsummary{quoting-style}
3143 @item --quoting-style=@var{style}
3144 Set quoting style to use when printing member and file names
3145 (@pxref{quoting styles}). Valid @var{style} values are:
3146 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
3147 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
3148 style is @code{escape}, unless overridden while configuring the
3151 @opsummary{read-full-records}
3152 @item --read-full-records
3155 Specifies that @command{tar} should reblock its input, for reading
3156 from pipes on systems with buggy implementations. @xref{Reading}.
3158 @opsummary{record-size}
3159 @item --record-size=@var{size}[@var{suf}]
3161 Instructs @command{tar} to use @var{size} bytes per record when accessing the
3162 archive. The argument can be suffixed with a @dfn{size suffix}, e.g.
3163 @option{--record-size=10K} for 10 Kilobytes. @xref{size-suffixes},
3164 for a list of valid suffixes. @xref{Blocking Factor}, for a detailed
3165 description of this option.
3167 @opsummary{recursion}
3170 With this option, @command{tar} recurses into directories (default).
3173 @opsummary{recursive-unlink}
3174 @item --recursive-unlink
3177 directory hierarchies before extracting directories of the same name
3178 from the archive. @xref{Recursive Unlink}.
3180 @opsummary{remove-files}
3181 @item --remove-files
3183 Directs @command{tar} to remove the source file from the file system after
3184 appending it to an archive. @xref{remove files}.
3186 @opsummary{restrict}
3189 Disable use of some potentially harmful @command{tar} options.
3190 Currently this option disables shell invocation from multi-volume menu
3191 (@pxref{Using Multiple Tapes}).
3193 @opsummary{rmt-command}
3194 @item --rmt-command=@var{cmd}
3196 Notifies @command{tar} that it should use @var{cmd} instead of
3197 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
3199 @opsummary{rsh-command}
3200 @item --rsh-command=@var{cmd}
3202 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
3203 devices. @xref{Device}.
3205 @opsummary{same-order}
3207 @itemx --preserve-order
3210 This option is an optimization for @command{tar} when running on machines with
3211 small amounts of memory. It informs @command{tar} that the list of file
3212 arguments has already been sorted to match the order of files in the
3213 archive. @xref{Reading}.
3215 @opsummary{same-owner}
3218 When extracting an archive, @command{tar} will attempt to preserve the owner
3219 specified in the @command{tar} archive with this option present.
3220 This is the default behavior for the superuser; this option has an
3221 effect only for ordinary users. @xref{Attributes}.
3223 @opsummary{same-permissions}
3224 @item --same-permissions
3226 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
3232 Assume that the archive media supports seeks to arbitrary
3233 locations. Usually @command{tar} determines automatically whether
3234 the archive can be seeked or not. This option is intended for use
3235 in cases when such recognition fails. It takes effect only if the
3236 archive is open for reading (e.g. with @option{--list} or
3237 @option{--extract} options).
3239 @opsummary{show-defaults}
3240 @item --show-defaults
3242 Displays the default options used by @command{tar} and exits
3243 successfully. This option is intended for use in shell scripts.
3244 Here is an example of what you can see using this option:
3247 $ @kbd{tar --show-defaults}
3248 --format=gnu -f- -b20 --quoting-style=escape
3249 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
3253 Notice, that this option outputs only one line. The example output
3254 above has been split to fit page boundaries.
3256 @opsummary{show-omitted-dirs}
3257 @item --show-omitted-dirs
3259 Instructs @command{tar} to mention the directories it is skipping when
3260 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
3262 @opsummary{show-transformed-names}
3263 @opsummary{show-stored-names}
3264 @item --show-transformed-names
3265 @itemx --show-stored-names
3267 Display file or member names after applying any transformations
3268 (@pxref{transform}). In particular, when used in conjunction with one of
3269 the archive creation operations it instructs @command{tar} to list the
3270 member names stored in the archive, as opposed to the actual file
3271 names. @xref{listing member and file names}.
3273 @opsummary{skip-old-files}
3274 @item --skip-old-files
3276 Do not overwrite existing files when extracting files from an
3277 archive. @xref{Keep Old Files}.
3279 This option differs from @option{--keep-old-files} in that it does not
3280 treat such files as an error, instead it just silently avoids
3283 The @option{--warning=existing-file} option can be used together with
3284 this option to produce warning messages about existing old files
3291 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3292 sparse files efficiently. @xref{sparse}.
3294 @opsummary{sparse-version}
3295 @item --sparse-version=@var{version}
3297 Specifies the @dfn{format version} to use when archiving sparse
3298 files. Implies @option{--sparse}. @xref{sparse}. For the description
3299 of the supported sparse formats, @xref{Sparse Formats}.
3301 @opsummary{starting-file}
3302 @item --starting-file=@var{name}
3303 @itemx -K @var{name}
3305 This option affects extraction only; @command{tar} will skip extracting
3306 files in the archive until it finds one that matches @var{name}.
3309 @opsummary{strip-components}
3310 @item --strip-components=@var{number}
3311 Strip given @var{number} of leading components from file names before
3312 extraction. For example, if archive @file{archive.tar} contained
3313 @file{/some/file/name}, then running
3316 tar --extract --file archive.tar --strip-components=2
3320 would extract this file to file @file{name}.
3323 @item --suffix=@var{suffix}
3325 Alters the suffix @command{tar} uses when backing up files from the default
3326 @samp{~}. @xref{backup}.
3328 @opsummary{tape-length}
3329 @item --tape-length=@var{num}[@var{suf}]
3330 @itemx -L @var{num}[@var{suf}]
3332 Specifies the length of tapes that @command{tar} is writing as being
3333 @w{@var{num} x 1024} bytes long. If optional @var{suf} is given, it
3334 specifies a multiplicative factor to be used instead of 1024. For
3335 example, @samp{-L2M} means 2 megabytes. @xref{size-suffixes}, for a
3336 list of allowed suffixes. @xref{Using Multiple Tapes}, for a detailed
3337 discussion of this option.
3339 @opsummary{test-label}
3342 Reads the volume label. If an argument is specified, test whether it
3343 matches the volume label. @xref{--test-label option}.
3345 @opsummary{to-command}
3346 @item --to-command=@var{command}
3348 During extraction @command{tar} will pipe extracted files to the
3349 standard input of @var{command}. @xref{Writing to an External Program}.
3351 @opsummary{to-stdout}
3355 During extraction, @command{tar} will extract files to stdout rather
3356 than to the file system. @xref{Writing to Standard Output}.
3359 @item --totals[=@var{signo}]
3361 Displays the total number of bytes transferred when processing an
3362 archive. If an argument is given, these data are displayed on
3363 request, when signal @var{signo} is delivered to @command{tar}.
3370 Sets the data modification time of extracted files to the extraction time,
3371 rather than the data modification time stored in the archive.
3372 @xref{Data Modification Times}.
3374 @opsummary{transform}
3376 @item --transform=@var{sed-expr}
3377 @itemx --xform=@var{sed-expr}
3378 Transform file or member names using @command{sed} replacement expression
3379 @var{sed-expr}. For example,
3382 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
3386 will add to @file{archive} files from the current working directory,
3387 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
3388 discussion, @xref{transform}.
3390 To see transformed member names in verbose listings, use
3391 @option{--show-transformed-names} option
3392 (@pxref{show-transformed-names}).
3394 @opsummary{uncompress}
3397 (See @option{--compress}, @pxref{gzip})
3402 (See @option{--gzip}, @pxref{gzip})
3404 @opsummary{unlink-first}
3405 @item --unlink-first
3408 Directs @command{tar} to remove the corresponding file from the file
3409 system before extracting it from the archive. @xref{Unlink First}.
3413 Enable unquoting input file or member names (default). @xref{input
3416 @opsummary{use-compress-program}
3417 @item --use-compress-program=@var{prog}
3418 @itemx -I=@var{prog}
3420 Instructs @command{tar} to access the archive through @var{prog}, which is
3421 presumed to be a compression program of some sort. @xref{gzip}.
3426 Display file modification dates in @acronym{UTC}. This option implies
3433 Specifies that @command{tar} should be more verbose about the
3434 operations it is performing. This option can be specified multiple
3435 times for some operations to increase the amount of information displayed.
3442 Verifies that the archive was correctly written when creating an
3443 archive. @xref{verify}.
3448 Print information about the program's name, version, origin and legal
3449 status, all on standard output, and then exit successfully.
3452 @opsummary{volno-file}
3453 @item --volno-file=@var{file}
3455 Used in conjunction with @option{--multi-volume}. @command{tar} will
3456 keep track of which volume of a multi-volume archive it is working in
3457 @var{file}. @xref{volno-file}.
3460 @item --warning=@var{keyword}
3462 Enable or disable warning messages identified by @var{keyword}. The
3463 messages are suppressed if @var{keyword} is prefixed with @samp{no-}.
3466 @opsummary{wildcards}
3468 Use wildcards when matching member names with patterns.
3469 @xref{controlling pattern-matching}.
3471 @opsummary{wildcards-match-slash}
3472 @item --wildcards-match-slash
3473 Wildcards match @samp{/}.
3474 @xref{controlling pattern-matching}.
3479 Use @command{xz} for compressing or decompressing the archives. @xref{gzip}.
3483 @node Short Option Summary
3484 @subsection Short Options Cross Reference
3486 Here is an alphabetized list of all of the short option forms, matching
3487 them with the equivalent long option.
3489 @multitable @columnfractions 0.20 0.80
3490 @headitem Short Option @tab Reference
3492 @item -A @tab @ref{--concatenate}.
3494 @item -B @tab @ref{--read-full-records}.
3496 @item -C @tab @ref{--directory}.
3498 @item -F @tab @ref{--info-script}.
3500 @item -G @tab @ref{--incremental}.
3502 @item -J @tab @ref{--xz}.
3504 @item -K @tab @ref{--starting-file}.
3506 @item -L @tab @ref{--tape-length}.
3508 @item -M @tab @ref{--multi-volume}.
3510 @item -N @tab @ref{--newer}.
3512 @item -O @tab @ref{--to-stdout}.
3514 @item -P @tab @ref{--absolute-names}.
3516 @item -R @tab @ref{--block-number}.
3518 @item -S @tab @ref{--sparse}.
3520 @item -T @tab @ref{--files-from}.
3522 @item -U @tab @ref{--unlink-first}.
3524 @item -V @tab @ref{--label}.
3526 @item -W @tab @ref{--verify}.
3528 @item -X @tab @ref{--exclude-from}.
3530 @item -Z @tab @ref{--compress}.
3532 @item -b @tab @ref{--blocking-factor}.
3534 @item -c @tab @ref{--create}.
3536 @item -d @tab @ref{--compare}.
3538 @item -f @tab @ref{--file}.
3540 @item -g @tab @ref{--listed-incremental}.
3542 @item -h @tab @ref{--dereference}.
3544 @item -i @tab @ref{--ignore-zeros}.
3546 @item -j @tab @ref{--bzip2}.
3548 @item -k @tab @ref{--keep-old-files}.
3550 @item -l @tab @ref{--check-links}.
3552 @item -m @tab @ref{--touch}.
3554 @item -o @tab When creating, @ref{--no-same-owner}, when extracting ---
3555 @ref{--portability}.
3557 The latter usage is deprecated. It is retained for compatibility with
3558 the earlier versions of @GNUTAR{}. In future releases
3559 @option{-o} will be equivalent to @option{--no-same-owner} only.
3561 @item -p @tab @ref{--preserve-permissions}.
3563 @item -r @tab @ref{--append}.
3565 @item -s @tab @ref{--same-order}.
3567 @item -t @tab @ref{--list}.
3569 @item -u @tab @ref{--update}.
3571 @item -v @tab @ref{--verbose}.
3573 @item -w @tab @ref{--interactive}.
3575 @item -x @tab @ref{--extract}.
3577 @item -z @tab @ref{--gzip}.
3582 @section @GNUTAR{} documentation
3584 @cindex Getting program version number
3586 @cindex Version of the @command{tar} program
3587 Being careful, the first thing is really checking that you are using
3588 @GNUTAR{}, indeed. The @option{--version} option
3589 causes @command{tar} to print information about its name, version,
3590 origin and legal status, all on standard output, and then exit
3591 successfully. For example, @w{@samp{tar --version}} might print:
3594 tar (GNU tar) @value{VERSION}
3595 Copyright (C) 2013 Free Software Foundation, Inc.
3596 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
3597 This is free software: you are free to change and redistribute it.
3598 There is NO WARRANTY, to the extent permitted by law.
3600 Written by John Gilmore and Jay Fenlason.
3604 The first occurrence of @samp{tar} in the result above is the program
3605 name in the package (for example, @command{rmt} is another program),
3606 while the second occurrence of @samp{tar} is the name of the package
3607 itself, containing possibly many programs. The package is currently
3608 named @samp{tar}, after the name of the main program it
3609 contains@footnote{There are plans to merge the @command{cpio} and
3610 @command{tar} packages into a single one which would be called
3611 @code{paxutils}. So, who knows if, one of this days, the
3612 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3615 @cindex Obtaining help
3616 @cindex Listing all @command{tar} options
3617 @xopindex{help, introduction}
3618 Another thing you might want to do is checking the spelling or meaning
3619 of some particular @command{tar} option, without resorting to this
3620 manual, for once you have carefully read it. @GNUTAR{}
3621 has a short help feature, triggerable through the
3622 @option{--help} option. By using this option, @command{tar} will
3623 print a usage message listing all available options on standard
3624 output, then exit successfully, without doing anything else and
3625 ignoring all other options. Even if this is only a brief summary, it
3626 may be several screens long. So, if you are not using some kind of
3627 scrollable window, you might prefer to use something like:
3630 $ @kbd{tar --help | less}
3634 presuming, here, that you like using @command{less} for a pager. Other
3635 popular pagers are @command{more} and @command{pg}. If you know about some
3636 @var{keyword} which interests you and do not want to read all the
3637 @option{--help} output, another common idiom is doing:
3640 tar --help | grep @var{keyword}
3644 for getting only the pertinent lines. Notice, however, that some
3645 @command{tar} options have long description lines and the above
3646 command will list only the first of them.
3648 The exact look of the option summary displayed by @kbd{tar --help} is
3649 configurable. @xref{Configuring Help Summary}, for a detailed description.
3652 If you only wish to check the spelling of an option, running @kbd{tar
3653 --usage} may be a better choice. This will display a terse list of
3654 @command{tar} options without accompanying explanations.
3656 The short help output is quite succinct, and you might have to get
3657 back to the full documentation for precise points. If you are reading
3658 this paragraph, you already have the @command{tar} manual in some
3659 form. This manual is available in a variety of forms from
3660 @url{http://www.gnu.org/software/tar/manual}. It may be printed out of the @GNUTAR{}
3661 distribution, provided you have @TeX{} already installed somewhere,
3662 and a laser printer around. Just configure the distribution, execute
3663 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3664 usual way (contact your local guru to know how). If @GNUTAR{}
3665 has been conveniently installed at your place, this
3666 manual is also available in interactive, hypertextual form as an Info
3667 file. Just call @w{@samp{info tar}} or, if you do not have the
3668 @command{info} program handy, use the Info reader provided within
3669 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3671 There is currently no @code{man} page for @GNUTAR{}.
3672 If you observe such a @code{man} page on the system you are running,
3673 either it does not belong to @GNUTAR{}, or it has not
3674 been produced by @acronym{GNU}. Some package maintainers convert
3675 @kbd{tar --help} output to a man page, using @command{help2man}. In
3676 any case, please bear in mind that the authoritative source of
3677 information about @GNUTAR{} is this Texinfo documentation.
3680 @section Obtaining @GNUTAR{} default values
3682 @opindex show-defaults
3683 @GNUTAR{} has some predefined defaults that are used when you do not
3684 explicitly specify another values. To obtain a list of such
3685 defaults, use @option{--show-defaults} option. This will output the
3686 values in the form of @command{tar} command line options:
3690 $ @kbd{tar --show-defaults}
3691 --format=gnu -f- -b20 --quoting-style=escape
3692 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3697 Notice, that this option outputs only one line. The example output above
3698 has been split to fit page boundaries.
3701 The above output shows that this version of @GNUTAR{} defaults to
3702 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3703 output as the archive, if no @option{--file} option has been given
3704 (@pxref{file tutorial}), the default blocking factor is 20
3705 (@pxref{Blocking Factor}). It also shows the default locations where
3706 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3709 @section Checking @command{tar} progress
3711 Typically, @command{tar} performs most operations without reporting any
3712 information to the user except error messages. When using @command{tar}
3713 with many options, particularly ones with complicated or
3714 difficult-to-predict behavior, it is possible to make serious mistakes.
3715 @command{tar} provides several options that make observing @command{tar}
3716 easier. These options cause @command{tar} to print information as it
3717 progresses in its job, and you might want to use them just for being
3718 more careful about what is going on, or merely for entertaining
3719 yourself. If you have encountered a problem when operating on an
3720 archive, however, you may need more information than just an error
3721 message in order to solve the problem. The following options can be
3722 helpful diagnostic tools.
3724 @cindex Verbose operation
3726 Normally, the @option{--list} (@option{-t}) command to list an archive
3727 prints just the file names (one per line) and the other commands are
3728 silent. When used with most operations, the @option{--verbose}
3729 (@option{-v}) option causes @command{tar} to print the name of each
3730 file or archive member as it is processed. This and the other options
3731 which make @command{tar} print status information can be useful in
3732 monitoring @command{tar}.
3734 With @option{--create} or @option{--extract}, @option{--verbose} used
3735 once just prints the names of the files or members as they are processed.
3736 Using it twice causes @command{tar} to print a longer listing
3737 (@xref{verbose member listing}, for the description) for each member.
3738 Since @option{--list} already prints the names of the members,
3739 @option{--verbose} used once with @option{--list} causes @command{tar}
3740 to print an @samp{ls -l} type listing of the files in the archive.
3741 The following examples both extract members with long list output:
3744 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3745 $ @kbd{tar xvvf archive.tar}
3748 Verbose output appears on the standard output except when an archive is
3749 being written to the standard output, as with @samp{tar --create
3750 --file=- --verbose} (@samp{tar cvf -}, or even @samp{tar cv}---if the
3751 installer let standard output be the default archive). In that case
3752 @command{tar} writes verbose output to the standard error stream.
3754 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3755 verbose output to @var{file} rather than to standard output or standard
3759 @cindex Obtaining total status information
3761 The @option{--totals} option causes @command{tar} to print on the
3762 standard error the total amount of bytes transferred when processing
3763 an archive. When creating or appending to an archive, this option
3764 prints the number of bytes written to the archive and the average
3765 speed at which they have been written, e.g.:
3769 $ @kbd{tar -c -f archive.tar --totals /home}
3770 Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
3774 When reading an archive, this option displays the number of bytes
3779 $ @kbd{tar -x -f archive.tar --totals}
3780 Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
3784 Finally, when deleting from an archive, the @option{--totals} option
3785 displays both numbers plus number of bytes removed from the archive:
3789 $ @kbd{tar --delete -f foo.tar --totals --wildcards '*~'}
3790 Total bytes read: 9543680 (9.2MiB, 201MiB/s)
3791 Total bytes written: 3829760 (3.7MiB, 81MiB/s)
3792 Total bytes deleted: 1474048
3796 You can also obtain this information on request. When
3797 @option{--totals} is used with an argument, this argument is
3798 interpreted as a symbolic name of a signal, upon delivery of which the
3799 statistics is to be printed:
3802 @item --totals=@var{signo}
3803 Print statistics upon delivery of signal @var{signo}. Valid arguments
3804 are: @code{SIGHUP}, @code{SIGQUIT}, @code{SIGINT}, @code{SIGUSR1} and
3805 @code{SIGUSR2}. Shortened names without @samp{SIG} prefix are also
3809 Both forms of @option{--totals} option can be used simultaneously.
3810 Thus, @kbd{tar -x --totals --totals=USR1} instructs @command{tar} to
3811 extract all members from its default archive and print statistics
3812 after finishing the extraction, as well as when receiving signal
3815 @anchor{Progress information}
3816 @cindex Progress information
3817 The @option{--checkpoint} option prints an occasional message
3818 as @command{tar} reads or writes the archive. It is designed for
3819 those who don't need the more detailed (and voluminous) output of
3820 @option{--block-number} (@option{-R}), but do want visual confirmation
3821 that @command{tar} is actually making forward progress. By default it
3822 prints a message each 10 records read or written. This can be changed
3823 by giving it a numeric argument after an equal sign:
3826 $ @kbd{tar -c --checkpoint=1000} /var
3827 tar: Write checkpoint 1000
3828 tar: Write checkpoint 2000
3829 tar: Write checkpoint 3000
3832 This example shows the default checkpoint message used by
3833 @command{tar}. If you place a dot immediately after the equal
3834 sign, it will print a @samp{.} at each checkpoint@footnote{This is
3835 actually a shortcut for @option{--checkpoint=@var{n}
3836 --checkpoint-action=dot}. @xref{checkpoints, dot}.}. For example:
3839 $ @kbd{tar -c --checkpoint=.1000} /var
3843 The @option{--checkpoint} option provides a flexible mechanism for
3844 executing arbitrary actions upon hitting checkpoints, see the next
3845 section (@pxref{checkpoints}), for more information on it.
3847 @opindex show-omitted-dirs
3848 @anchor{show-omitted-dirs}
3849 The @option{--show-omitted-dirs} option, when reading an archive---with
3850 @option{--list} or @option{--extract}, for example---causes a message
3851 to be printed for each directory in the archive which is skipped.
3852 This happens regardless of the reason for skipping: the directory might
3853 not have been named on the command line (implicitly or explicitly),
3854 it might be excluded by the use of the
3855 @option{--exclude=@var{pattern}} option, or some other reason.
3857 @opindex block-number
3858 @cindex Block number where error occurred
3859 @anchor{block-number}
3860 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3861 every message it would normally produce, the block number within the
3862 archive where the message was triggered. Also, supplementary messages
3863 are triggered when reading blocks full of NULs, or when hitting end of
3864 file on the archive. As of now, if the archive is properly terminated
3865 with a NUL block, the reading of the file may stop before end of file
3866 is met, so the position of end of file will not usually show when
3867 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3868 drains the archive before exiting when reading the
3869 archive from a pipe.
3871 @cindex Error message, block number of
3872 This option is especially useful when reading damaged archives, since
3873 it helps pinpoint the damaged sections. It can also be used with
3874 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3875 choose among several backup tapes when retrieving a file later, in
3876 favor of the tape where the file appears earliest (closest to the
3877 front of the tape). @xref{backup}.
3880 @section Checkpoints
3881 @cindex checkpoints, defined
3883 @opindex checkpoint-action
3885 A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
3886 the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
3887 from the archive (a @dfn{read checkpoint}). Checkpoints allow to
3888 periodically execute arbitrary actions.
3890 The checkpoint facility is enabled using the following option:
3893 @xopindex{checkpoint, defined}
3894 @item --checkpoint[=@var{n}]
3895 Schedule checkpoints before writing or reading each @var{n}th record.
3896 The default value for @var{n} is 10.
3899 A list of arbitrary @dfn{actions} can be executed at each checkpoint.
3900 These actions include: pausing, displaying textual messages, and
3901 executing arbitrary external programs. Actions are defined using
3902 the @option{--checkpoint-action} option.
3905 @xopindex{checkpoint-action, defined}
3906 @item --checkpoint-action=@var{action}
3907 Execute an @var{action} at each checkpoint.
3910 @cindex @code{echo}, checkpoint action
3911 The simplest value of @var{action} is @samp{echo}. It instructs
3912 @command{tar} to display the default message on the standard error
3913 stream upon arriving at each checkpoint. The default message is (in
3914 @acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
3915 checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
3916 Here, @var{n} represents ordinal number of the checkpoint.
3918 In another locales, translated versions of this message are used.
3920 This is the default action, so running:
3923 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
3930 $ @kbd{tar -c --checkpoint=1000} /var
3933 The @samp{echo} action also allows to supply a customized message.
3934 You do so by placing an equals sign and the message right after it,
3938 --checkpoint-action="echo=Hit %s checkpoint #%u"
3941 The @samp{%s} and @samp{%u} in the above example are
3942 @dfn{meta-characters}. The @samp{%s} meta-character is replaced with
3943 the @dfn{type} of the checkpoint: @samp{write} or
3944 @samp{read} (or a corresponding translated version in locales other
3945 than @acronym{POSIX}). The @samp{%u} meta-character is replaced with
3946 the ordinal number of the checkpoint. Thus, the above example could
3947 produce the following output when used with the @option{--create}
3951 tar: Hit write checkpoint #10
3952 tar: Hit write checkpoint #20
3953 tar: Hit write checkpoint #30
3956 Aside from meta-character expansion, the message string is subject to
3957 @dfn{unquoting}, during which the backslash @dfn{escape sequences} are
3958 replaced with their corresponding @acronym{ASCII} characters
3959 (@pxref{escape sequences}). E.g. the following action will produce an
3960 audible bell and the message described above at each checkpoint:
3963 --checkpoint-action='echo=\aHit %s checkpoint #%u'
3966 @cindex @code{bell}, checkpoint action
3967 There is also a special action which produces an audible signal:
3968 @samp{bell}. It is not equivalent to @samp{echo='\a'}, because
3969 @samp{bell} sends the bell directly to the console (@file{/dev/tty}),
3970 whereas @samp{echo='\a'} sends it to the standard error.
3972 @cindex @code{ttyout}, checkpoint action
3973 The @samp{ttyout=@var{string}} action outputs @var{string} to
3974 @file{/dev/tty}, so it can be used even if the standard output is
3975 redirected elsewhere. The @var{string} is subject to the same
3976 modifications as with @samp{echo} action. In contrast to the latter,
3977 @samp{ttyout} does not prepend @command{tar} executable name to the
3978 string, nor does it output a newline after it. For example, the
3979 following action will print the checkpoint message at the same screen
3980 line, overwriting any previous message:
3983 --checkpoint-action="ttyout=\rHit %s checkpoint #%u"
3986 @cindex @code{dot}, checkpoint action
3987 Another available checkpoint action is @samp{dot} (or @samp{.}). It
3988 instructs @command{tar} to print a single dot on the standard listing
3992 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
3996 For compatibility with previous @GNUTAR{} versions, this action can
3997 be abbreviated by placing a dot in front of the checkpoint frequency,
3998 as shown in the previous section.
4000 @cindex @code{sleep}, checkpoint action
4001 Yet another action, @samp{sleep}, pauses @command{tar} for a specified
4002 amount of seconds. The following example will stop for 30 seconds at each
4006 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
4009 @anchor{checkpoint exec}
4010 @cindex @code{exec}, checkpoint action
4011 Finally, the @code{exec} action executes a given external command.
4015 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
4018 The supplied command can be any valid command invocation, with or
4019 without additional command line arguments. If it does contain
4020 arguments, don't forget to quote it to prevent it from being split by
4021 the shell. @xref{external, Running External Commands}, for more detail.
4023 The command gets a copy of @command{tar}'s environment plus the
4024 following variables:
4027 @vrindex TAR_VERSION, checkpoint script environment
4029 @GNUTAR{} version number.
4031 @vrindex TAR_ARCHIVE, checkpoint script environment
4033 The name of the archive @command{tar} is processing.
4035 @vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
4036 @item TAR_BLOCKING_FACTOR
4037 Current blocking factor (@pxref{Blocking}).
4039 @vrindex TAR_CHECKPOINT, checkpoint script environment
4040 @item TAR_CHECKPOINT
4041 Number of the checkpoint.
4043 @vrindex TAR_SUBCOMMAND, checkpoint script environment
4044 @item TAR_SUBCOMMAND
4045 A short option describing the operation @command{tar} is executing.
4046 @xref{Operations}, for a complete list of subcommand options.
4048 @vrindex TAR_FORMAT, checkpoint script environment
4050 Format of the archive being processed. @xref{Formats}, for a complete
4051 list of archive format names.
4054 These environment variables can also be passed as arguments to the
4055 command, provided that they are properly escaped, for example:
4058 @kbd{tar -c -f arc.tar \
4059 --checkpoint-action='exec=/sbin/cpoint $TAR_FILENAME'}
4063 Notice single quotes to prevent variable names from being expanded by
4064 the shell when invoking @command{tar}.
4066 Any number of actions can be defined, by supplying several
4067 @option{--checkpoint-action} options in the command line. For
4068 example, the command below displays two messages, pauses
4069 execution for 30 seconds and executes the @file{/sbin/cpoint} script:
4073 $ @kbd{tar -c -f arc.tar \
4074 --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
4075 --checkpoint-action='echo=Sleeping for 30 seconds' \
4076 --checkpoint-action='sleep=30' \
4077 --checkpoint-action='exec=/sbin/cpoint'}
4081 This example also illustrates the fact that
4082 @option{--checkpoint-action} can be used without
4083 @option{--checkpoint}. In this case, the default checkpoint frequency
4084 (at each 10th record) is assumed.
4087 @section Controlling Warning Messages
4089 Sometimes, while performing the requested task, @GNUTAR{} notices
4090 some conditions that are not exactly errors, but which the user
4091 should be aware of. When this happens, @command{tar} issues a
4092 @dfn{warning message} describing the condition. Warning messages
4093 are output to the standard error and they do not affect the exit
4094 code of @command{tar} command.
4096 @xopindex{warning, explained}
4097 @GNUTAR{} allows the user to suppress some or all of its warning
4101 @item --warning=@var{keyword}
4102 Control display of the warning messages identified by @var{keyword}.
4103 If @var{keyword} starts with the prefix @samp{no-}, such messages are
4104 suppressed. Otherwise, they are enabled.
4106 Multiple @option{--warning} messages accumulate.
4108 The tables below list allowed values for @var{keyword} along with the
4109 warning messages they control.
4112 @subheading Keywords controlling @command{tar} operation
4116 Enable all warning messages. This is the default.
4119 Disable all warning messages.
4120 @kwindex filename-with-nuls
4121 @cindex @samp{file name read contains nul character}, warning message
4122 @item filename-with-nuls
4123 @samp{%s: file name read contains nul character}
4124 @kwindex alone-zero-block
4125 @cindex @samp{A lone zero block at}, warning message
4126 @item alone-zero-block
4127 @samp{A lone zero block at %s}
4130 @subheading Keywords applicable for @command{tar --create}
4133 @cindex @samp{contains a cache directory tag}, warning message
4135 @samp{%s: contains a cache directory tag %s; %s}
4136 @kwindex file-shrank
4137 @cindex @samp{File shrank by %s bytes}, warning message
4139 @samp{%s: File shrank by %s bytes; padding with zeros}
4141 @cindex @samp{file is on a different filesystem}, warning message
4143 @samp{%s: file is on a different filesystem; not dumped}
4144 @kwindex file-ignored
4145 @cindex @samp{Unknown file type; file ignored}, warning message
4146 @cindex @samp{socket ignored}, warning message
4147 @cindex @samp{door ignored}, warning message
4149 @samp{%s: Unknown file type; file ignored}
4150 @*@samp{%s: socket ignored}
4151 @*@samp{%s: door ignored}
4152 @kwindex file-unchanged
4153 @cindex @samp{file is unchanged; not dumped}, warning message
4154 @item file-unchanged
4155 @samp{%s: file is unchanged; not dumped}
4156 @kwindex ignore-archive
4157 @cindex @samp{file is the archive; not dumped}, warning message
4158 @kwindex ignore-archive
4159 @cindex @samp{file is the archive; not dumped}, warning message
4160 @item ignore-archive
4161 @samp{%s: file is the archive; not dumped}
4162 @kwindex file-removed
4163 @cindex @samp{File removed before we read it}, warning message
4165 @samp{%s: File removed before we read it}
4166 @kwindex file-changed
4167 @cindex @samp{file changed as we read it}, warning message
4169 @samp{%s: file changed as we read it}
4172 @subheading Keywords applicable for @command{tar --extract}
4175 @cindex @samp{implausibly old time stamp %s}, warning message
4176 @cindex @samp{time stamp %s is %s s in the future}, warning message
4178 @samp{%s: implausibly old time stamp %s}
4179 @*@samp{%s: time stamp %s is %s s in the future}
4180 @kwindex contiguous-cast
4181 @cindex @samp{Extracting contiguous files as regular files}, warning message
4182 @item contiguous-cast
4183 @samp{Extracting contiguous files as regular files}
4184 @kwindex symlink-cast
4185 @cindex @samp{Attempting extraction of symbolic links as hard links}, warning message
4187 @samp{Attempting extraction of symbolic links as hard links}
4188 @kwindex unknown-cast
4189 @cindex @samp{Unknown file type '%c', extracted as normal file}, warning message
4191 @samp{%s: Unknown file type '%c', extracted as normal file}
4192 @kwindex ignore-newer
4193 @cindex @samp{Current %s is newer or same age}, warning message
4195 @samp{Current %s is newer or same age}
4196 @kwindex unknown-keyword
4197 @cindex @samp{Ignoring unknown extended header keyword '%s'}, warning message
4198 @item unknown-keyword
4199 @samp{Ignoring unknown extended header keyword '%s'}
4200 @kwindex decompress-program
4201 @item decompress-program
4202 Controls verbose description of failures occurring when trying to run
4203 alternative decompressor programs (@pxref{alternative decompression
4204 programs}). This warning is disabled by default (unless
4205 @option{--verbose} is used). A common example of what you can get
4206 when using this warning is:
4209 $ @kbd{tar --warning=decompress-program -x -f archive.Z}
4210 tar (child): cannot run compress: No such file or directory
4211 tar (child): trying gzip
4214 This means that @command{tar} first tried to decompress
4215 @file{archive.Z} using @command{compress}, and, when that
4216 failed, switched to @command{gzip}.
4219 @subheading Keywords controlling incremental extraction:
4221 @kwindex rename-directory
4222 @cindex @samp{%s: Directory has been renamed from %s}, warning message
4223 @cindex @samp{%s: Directory has been renamed}, warning message
4224 @item rename-directory
4225 @samp{%s: Directory has been renamed from %s}
4226 @*@samp{%s: Directory has been renamed}
4227 @kwindex new-directory
4228 @cindex @samp{%s: Directory is new}, warning message
4230 @samp{%s: Directory is new}
4232 @cindex @samp{%s: directory is on a different device: not purging}, warning message
4234 @samp{%s: directory is on a different device: not purging}
4235 @kwindex bad-dumpdir
4236 @cindex @samp{Malformed dumpdir: 'X' never used}, warning message
4238 @samp{Malformed dumpdir: 'X' never used}
4242 @section Asking for Confirmation During Operations
4243 @cindex Interactive operation
4245 Typically, @command{tar} carries out a command without stopping for
4246 further instructions. In some situations however, you may want to
4247 exclude some files and archive members from the operation (for instance
4248 if disk or storage space is tight). You can do this by excluding
4249 certain files automatically (@pxref{Choosing}), or by performing
4250 an operation interactively, using the @option{--interactive} (@option{-w}) option.
4251 @command{tar} also accepts @option{--confirmation} for this option.
4253 @opindex interactive
4254 When the @option{--interactive} (@option{-w}) option is specified, before
4255 reading, writing, or deleting files, @command{tar} first prints a message
4256 for each such file, telling what operation it intends to take, then asks
4257 for confirmation on the terminal. The actions which require
4258 confirmation include adding a file to the archive, extracting a file
4259 from the archive, deleting a file from the archive, and deleting a file
4260 from disk. To confirm the action, you must type a line of input
4261 beginning with @samp{y}. If your input line begins with anything other
4262 than @samp{y}, @command{tar} skips that file.
4264 If @command{tar} is reading the archive from the standard input,
4265 @command{tar} opens the file @file{/dev/tty} to support the interactive
4268 Verbose output is normally sent to standard output, separate from
4269 other error messages. However, if the archive is produced directly
4270 on standard output, then verbose output is mixed with errors on
4271 @code{stderr}. Producing the archive on standard output may be used
4272 as a way to avoid using disk space, when the archive is soon to be
4273 consumed by another process reading it, say. Some people felt the need
4274 of producing an archive on stdout, still willing to segregate between
4275 verbose output and error output. A possible approach would be using a
4276 named pipe to receive the archive, and having the consumer process to
4277 read from that named pipe. This has the advantage of letting standard
4278 output free to receive verbose output, all separate from errors.
4281 @section Running External Commands
4283 Certain @GNUTAR{} operations imply running external commands that you
4284 supply on the command line. One of such operations is checkpointing,
4285 described above (@pxref{checkpoint exec}). Another example of this
4286 feature is the @option{-I} option, which allows you to supply the
4287 program to use for compressing or decompressing the archive
4288 (@pxref{use-compress-program}).
4290 Whenever such operation is requested, @command{tar} first splits the
4291 supplied command into words much like the shell does. It then treats
4292 the first word as the name of the program or the shell script to execute
4293 and the rest of words as its command line arguments. The program,
4294 unless given as an absolute file name, is searched in the shell's
4297 Any additional information is normally supplied to external commands
4298 in environment variables, specific to each particular operation. For
4299 example, the @option{--checkpoint-action=exec} option, defines the
4300 @env{TAR_ARCHIVE} variable to the name of the archive being worked
4301 upon. You can, should the need be, use these variables in the
4302 command line of the external command. For example:
4305 $ @kbd{tar -x -f archive.tar \
4306 --checkpoint=exec='printf "%04d in %32s\r" $TAR_CHECKPOINT $TAR_ARCHIVE'}
4310 This command prints for each checkpoint its number and the name of the
4311 archive, using the same output line on the screen.
4313 Notice the use of single quotes to prevent variable names from being
4314 expanded by the shell when invoking @command{tar}.
4317 @chapter @GNUTAR{} Operations
4330 @section Basic @GNUTAR{} Operations
4332 The basic @command{tar} operations, @option{--create} (@option{-c}),
4333 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
4334 @option{-x}), are currently presented and described in the tutorial
4335 chapter of this manual. This section provides some complementary notes
4336 for these operations.
4339 @xopindex{create, complementary notes}
4343 Creating an empty archive would have some kind of elegance. One can
4344 initialize an empty archive and later use @option{--append}
4345 (@option{-r}) for adding all members. Some applications would not
4346 welcome making an exception in the way of adding the first archive
4347 member. On the other hand, many people reported that it is
4348 dangerously too easy for @command{tar} to destroy a magnetic tape with
4349 an empty archive@footnote{This is well described in @cite{Unix-haters
4350 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
4351 Books, ISBN 1-56884-203-1.}. The two most common errors are:
4355 Mistakingly using @code{create} instead of @code{extract}, when the
4356 intent was to extract the full contents of an archive. This error
4357 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
4358 the QWERTY keyboard. Instead of being unpacked, the archive then
4359 gets wholly destroyed. When users speak about @dfn{exploding} an
4360 archive, they usually mean something else :-).
4363 Forgetting the argument to @code{file}, when the intent was to create
4364 an archive with a single file in it. This error is likely because a
4365 tired user can easily add the @kbd{f} key to the cluster of option
4366 letters, by the mere force of habit, without realizing the full
4367 consequence of doing so. The usual consequence is that the single
4368 file, which was meant to be saved, is rather destroyed.
4371 So, recognizing the likelihood and the catastrophic nature of these
4372 errors, @GNUTAR{} now takes some distance from elegance, and
4373 cowardly refuses to create an archive when @option{--create} option is
4374 given, there are no arguments besides options, and
4375 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
4376 around the cautiousness of @GNUTAR{} and nevertheless create an
4377 archive with nothing in it, one may still use, as the value for the
4378 @option{--files-from} option, a file with no names in it, as shown in
4379 the following commands:
4382 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
4383 @kbd{tar -cf empty-archive.tar -T /dev/null}
4386 @xopindex{extract, complementary notes}
4391 A socket is stored, within a @GNUTAR{} archive, as a pipe.
4393 @item @option{--list} (@option{-t})
4395 @GNUTAR{} now shows dates as @samp{1996-08-30},
4396 while it used to show them as @samp{Aug 30 1996}. Preferably,
4397 people should get used to ISO 8601 dates. Local American dates should
4398 be made available again with full date localization support, once
4399 ready. In the meantime, programs not being localizable for dates
4400 should prefer international dates, that's really the way to go.
4402 Look up @url{http://www.cl.cam.ac.uk/@/~mgk25/@/iso-time.html} if you
4403 are curious, it contains a detailed explanation of the ISO 8601 standard.
4408 @section Advanced @GNUTAR{} Operations
4410 Now that you have learned the basics of using @GNUTAR{}, you may want
4411 to learn about further ways in which @command{tar} can help you.
4413 This chapter presents five, more advanced operations which you probably
4414 won't use on a daily basis, but which serve more specialized functions.
4415 We also explain the different styles of options and why you might want
4416 to use one or another, or a combination of them in your @command{tar}
4417 commands. Additionally, this chapter includes options which allow you to
4418 define the output from @command{tar} more carefully, and provide help and
4419 error correction in special circumstances.
4421 @FIXME{check this after the chapter is actually revised to make sure
4422 it still introduces the info in the chapter correctly : ).}
4434 @subsection The Five Advanced @command{tar} Operations
4436 @cindex basic operations
4437 In the last chapter, you learned about the first three operations to
4438 @command{tar}. This chapter presents the remaining five operations to
4439 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
4440 @option{--delete}, and @option{--compare}.
4442 You are not likely to use these operations as frequently as those
4443 covered in the last chapter; however, since they perform specialized
4444 functions, they are quite useful when you do need to use them. We
4445 will give examples using the same directory and files that you created
4446 in the last chapter. As you may recall, the directory is called
4447 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
4448 and the two archive files you created are
4449 @samp{collection.tar} and @samp{music.tar}.
4451 We will also use the archive files @samp{afiles.tar} and
4452 @samp{bfiles.tar}. The archive @samp{afiles.tar} contains the members @samp{apple},
4453 @samp{angst}, and @samp{aspic}; @samp{bfiles.tar} contains the members
4454 @samp{./birds}, @samp{baboon}, and @samp{./box}.
4456 Unless we state otherwise, all practicing you do and examples you follow
4457 in this chapter will take place in the @file{practice} directory that
4458 you created in the previous chapter; see @ref{prepare for examples}.
4459 (Below in this section, we will remind you of the state of the examples
4460 where the last chapter left them.)
4462 The five operations that we will cover in this chapter are:
4467 Add new entries to an archive that already exists.
4470 Add more recent copies of archive members to the end of an archive, if
4475 Add one or more pre-existing archives to the end of another archive.
4477 Delete items from an archive (does not work on tapes).
4481 Compare archive members to their counterparts in the file system.
4485 @subsection How to Add Files to Existing Archives: @option{--append}
4487 @cindex appending files to existing archive
4489 If you want to add files to an existing archive, you don't need to
4490 create a new archive; you can use @option{--append} (@option{-r}).
4491 The archive must already exist in order to use @option{--append}. (A
4492 related operation is the @option{--update} operation; you can use this
4493 to add newer versions of archive members to an existing archive. To learn how to
4494 do this with @option{--update}, @pxref{update}.)
4496 If you use @option{--append} to add a file that has the same name as an
4497 archive member to an archive containing that archive member, then the
4498 old member is not deleted. What does happen, however, is somewhat
4499 complex. @command{tar} @emph{allows} you to have infinite number of files
4500 with the same name. Some operations treat these same-named members no
4501 differently than any other set of archive members: for example, if you
4502 view an archive with @option{--list} (@option{-t}), you will see all
4503 of those members listed, with their data modification times, owners, etc.
4505 Other operations don't deal with these members as perfectly as you might
4506 prefer; if you were to use @option{--extract} to extract the archive,
4507 only the most recently added copy of a member with the same name as
4508 other members would end up in the working directory. This is because
4509 @option{--extract} extracts an archive in the order the members appeared
4510 in the archive; the most recently archived members will be extracted
4511 last. Additionally, an extracted member will @emph{replace} a file of
4512 the same name which existed in the directory already, and @command{tar}
4513 will not prompt you about this@footnote{Unless you give it
4514 @option{--keep-old-files} (or @option{--skip-old-files}) option, or
4515 the disk copy is newer than the one in the archive and you invoke
4516 @command{tar} with @option{--keep-newer-files} option.}. Thus, only
4517 the most recently archived member will end up being extracted, as it
4518 will replace the one extracted before it, and so on.
4520 @cindex extracting @var{n}th copy of the file
4521 @xopindex{occurrence, described}
4522 There exists a special option that allows you to get around this
4523 behavior and extract (or list) only a particular copy of the file.
4524 This is @option{--occurrence} option. If you run @command{tar} with
4525 this option, it will extract only the first copy of the file. You
4526 may also give this option an argument specifying the number of
4527 copy to be extracted. Thus, for example if the archive
4528 @file{archive.tar} contained three copies of file @file{myfile}, then
4532 tar --extract --file archive.tar --occurrence=2 myfile
4536 would extract only the second copy. @xref{Option
4537 Summary,---occurrence}, for the description of @option{--occurrence}
4540 @FIXME{ hag -- you might want to incorporate some of the above into the
4541 MMwtSN node; not sure. i didn't know how to make it simpler...
4543 There are a few ways to get around this. Xref to Multiple Members
4544 with the Same Name, maybe.}
4546 @cindex Members, replacing with other members
4547 @cindex Replacing members with other members
4548 @xopindex{delete, using before --append}
4549 If you want to replace an archive member, use @option{--delete} to
4550 delete the member you want to remove from the archive, and then use
4551 @option{--append} to add the member you want to be in the archive. Note
4552 that you can not change the order of the archive; the most recently
4553 added member will still appear last. In this sense, you cannot truly
4554 ``replace'' one member with another. (Replacing one member with another
4555 will not work on certain types of media, such as tapes; see @ref{delete}
4556 and @ref{Media}, for more information.)
4559 * appending files:: Appending Files to an Archive
4563 @node appending files
4564 @subsubsection Appending Files to an Archive
4565 @cindex Adding files to an Archive
4566 @cindex Appending files to an Archive
4567 @cindex Archives, Appending files to
4570 The simplest way to add a file to an already existing archive is the
4571 @option{--append} (@option{-r}) operation, which writes specified
4572 files into the archive whether or not they are already among the
4575 When you use @option{--append}, you @emph{must} specify file name
4576 arguments, as there is no default. If you specify a file that already
4577 exists in the archive, another copy of the file will be added to the
4578 end of the archive. As with other operations, the member names of the
4579 newly added files will be exactly the same as their names given on the
4580 command line. The @option{--verbose} (@option{-v}) option will print
4581 out the names of the files as they are written into the archive.
4583 @option{--append} cannot be performed on some tape drives, unfortunately,
4584 due to deficiencies in the formats those tape drives use. The archive
4585 must be a valid @command{tar} archive, or else the results of using this
4586 operation will be unpredictable. @xref{Media}.
4588 To demonstrate using @option{--append} to add a file to an archive,
4589 create a file called @file{rock} in the @file{practice} directory.
4590 Make sure you are in the @file{practice} directory. Then, run the
4591 following @command{tar} command to add @file{rock} to
4592 @file{collection.tar}:
4595 $ @kbd{tar --append --file=collection.tar rock}
4599 If you now use the @option{--list} (@option{-t}) operation, you will see that
4600 @file{rock} has been added to the archive:
4603 $ @kbd{tar --list --file=collection.tar}
4604 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4605 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4606 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4607 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4611 @subsubsection Multiple Members with the Same Name
4612 @cindex members, multiple
4613 @cindex multiple members
4615 You can use @option{--append} (@option{-r}) to add copies of files
4616 which have been updated since the archive was created. (However, we
4617 do not recommend doing this since there is another @command{tar}
4618 option called @option{--update}; @xref{update}, for more information.
4619 We describe this use of @option{--append} here for the sake of
4620 completeness.) When you extract the archive, the older version will
4621 be effectively lost. This works because files are extracted from an
4622 archive in the order in which they were archived. Thus, when the
4623 archive is extracted, a file archived later in time will replace a
4624 file of the same name which was archived earlier, even though the
4625 older version of the file will remain in the archive unless you delete
4626 all versions of the file.
4628 Supposing you change the file @file{blues} and then append the changed
4629 version to @file{collection.tar}. As you saw above, the original
4630 @file{blues} is in the archive @file{collection.tar}. If you change the
4631 file and append the new version of the file to the archive, there will
4632 be two copies in the archive. When you extract the archive, the older
4633 version of the file will be extracted first, and then replaced by the
4634 newer version when it is extracted.
4636 You can append the new, changed copy of the file @file{blues} to the
4637 archive in this way:
4640 $ @kbd{tar --append --verbose --file=collection.tar blues}
4645 Because you specified the @option{--verbose} option, @command{tar} has
4646 printed the name of the file being appended as it was acted on. Now
4647 list the contents of the archive:
4650 $ @kbd{tar --list --verbose --file=collection.tar}
4651 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4652 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4653 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4654 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4655 -rw-r--r-- me/user 58 1996-10-24 18:30 blues
4659 The newest version of @file{blues} is now at the end of the archive
4660 (note the different creation dates and file sizes). If you extract
4661 the archive, the older version of the file @file{blues} will be
4662 replaced by the newer version. You can confirm this by extracting
4663 the archive and running @samp{ls} on the directory.
4665 If you wish to extract the first occurrence of the file @file{blues}
4666 from the archive, use @option{--occurrence} option, as shown in
4667 the following example:
4670 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
4671 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4674 @xref{Writing}, for more information on @option{--extract} and
4675 see @ref{Option Summary, --occurrence}, for a description of
4676 @option{--occurrence} option.
4679 @subsection Updating an Archive
4680 @cindex Updating an archive
4683 In the previous section, you learned how to use @option{--append} to
4684 add a file to an existing archive. A related operation is
4685 @option{--update} (@option{-u}). The @option{--update} operation
4686 updates a @command{tar} archive by comparing the date of the specified
4687 archive members against the date of the file with the same name. If
4688 the file has been modified more recently than the archive member, then
4689 the newer version of the file is added to the archive (as with
4692 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
4693 The operation will fail.
4695 @FIXME{other examples of media on which --update will fail? need to ask
4696 charles and/or mib/thomas/dave shevett..}
4698 Both @option{--update} and @option{--append} work by adding to the end
4699 of the archive. When you extract a file from the archive, only the
4700 version stored last will wind up in the file system, unless you use
4701 the @option{--backup} option. @xref{multiple}, for a detailed discussion.
4708 @subsubsection How to Update an Archive Using @option{--update}
4711 You must use file name arguments with the @option{--update}
4712 (@option{-u}) operation. If you don't specify any files,
4713 @command{tar} won't act on any files and won't tell you that it didn't
4714 do anything (which may end up confusing you).
4716 @c note: the above parenthetical added because in fact, this
4717 @c behavior just confused the author. :-)
4719 To see the @option{--update} option at work, create a new file,
4720 @file{classical}, in your practice directory, and some extra text to the
4721 file @file{blues}, using any text editor. Then invoke @command{tar} with
4722 the @samp{update} operation and the @option{--verbose} (@option{-v})
4723 option specified, using the names of all the files in the @file{practice}
4724 directory as file name arguments:
4727 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4734 Because we have specified verbose mode, @command{tar} prints out the names
4735 of the files it is working on, which in this case are the names of the
4736 files that needed to be updated. If you run @samp{tar --list} and look
4737 at the archive, you will see @file{blues} and @file{classical} at its
4738 end. There will be a total of two versions of the member @samp{blues};
4739 the one at the end will be newer and larger, since you added text before
4742 The reason @command{tar} does not overwrite the older file when updating
4743 it is because writing to the middle of a section of tape is a difficult
4744 process. Tapes are not designed to go backward. @xref{Media}, for more
4745 information about tapes.
4747 @option{--update} (@option{-u}) is not suitable for performing backups for two
4748 reasons: it does not change directory content entries, and it
4749 lengthens the archive every time it is used. The @GNUTAR{}
4750 options intended specifically for backups are more
4751 efficient. If you need to run backups, please consult @ref{Backups}.
4754 @subsection Combining Archives with @option{--concatenate}
4756 @cindex Adding archives to an archive
4757 @cindex Concatenating Archives
4758 @opindex concatenate
4760 @c @cindex @option{-A} described
4761 Sometimes it may be convenient to add a second archive onto the end of
4762 an archive rather than adding individual files to the archive. To add
4763 one or more archives to the end of another archive, you should use the
4764 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4766 To use @option{--concatenate}, give the first archive with
4767 @option{--file} option and name the rest of archives to be
4768 concatenated on the command line. The members, and their member
4769 names, will be copied verbatim from those archives to the first
4770 one@footnote{This can cause multiple members to have the same name. For
4771 information on how this affects reading the archive, see @ref{multiple}.}.
4772 The new, concatenated archive will be called by the same name as the
4773 one given with the @option{--file} option. As usual, if you omit
4774 @option{--file}, @command{tar} will use the value of the environment
4775 variable @env{TAPE}, or, if this has not been set, the default archive name.
4777 @FIXME{There is no way to specify a new name...}
4779 To demonstrate how @option{--concatenate} works, create two small archives
4780 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4781 files from @file{practice}:
4784 $ @kbd{tar -cvf bluesrock.tar blues rock}
4787 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4793 If you like, You can run @samp{tar --list} to make sure the archives
4794 contain what they are supposed to:
4797 $ @kbd{tar -tvf bluesrock.tar}
4798 -rw-r--r-- melissa/user 105 1997-01-21 19:42 blues
4799 -rw-r--r-- melissa/user 33 1997-01-20 15:34 rock
4800 $ @kbd{tar -tvf jazzfolk.tar}
4801 -rw-r--r-- melissa/user 20 1996-09-23 16:44 folk
4802 -rw-r--r-- melissa/user 65 1997-01-30 14:15 jazz
4805 We can concatenate these two archives with @command{tar}:
4809 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4812 If you now list the contents of the @file{bluesrock.tar}, you will see
4813 that now it also contains the archive members of @file{jazzfolk.tar}:
4816 $ @kbd{tar --list --file=bluesrock.tar}
4823 When you use @option{--concatenate}, the source and target archives must
4824 already exist and must have been created using compatible format
4825 parameters. Notice, that @command{tar} does not check whether the
4826 archives it concatenates have compatible formats, it does not
4827 even check if the files are really tar archives.
4829 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4830 tape drives, due to deficiencies in the formats those tape drives use.
4832 @cindex @code{concatenate} vs @command{cat}
4833 @cindex @command{cat} vs @code{concatenate}
4834 It may seem more intuitive to you to want or try to use @command{cat} to
4835 concatenate two archives instead of using the @option{--concatenate}
4836 operation; after all, @command{cat} is the utility for combining files.
4838 However, @command{tar} archives incorporate an end-of-file marker which
4839 must be removed if the concatenated archives are to be read properly as
4840 one archive. @option{--concatenate} removes the end-of-archive marker
4841 from the target archive before each new archive is appended. If you use
4842 @command{cat} to combine the archives, the result will not be a valid
4843 @command{tar} format archive. If you need to retrieve files from an
4844 archive that was added to using the @command{cat} utility, use the
4845 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4846 information on dealing with archives improperly combined using the
4847 @command{cat} shell utility.
4850 @subsection Removing Archive Members Using @option{--delete}
4851 @cindex Deleting files from an archive
4852 @cindex Removing files from an archive
4855 You can remove members from an archive by using the @option{--delete}
4856 option. Specify the name of the archive with @option{--file}
4857 (@option{-f}) and then specify the names of the members to be deleted;
4858 if you list no member names, nothing will be deleted. The
4859 @option{--verbose} option will cause @command{tar} to print the names
4860 of the members as they are deleted. As with @option{--extract}, you
4861 must give the exact member names when using @samp{tar --delete}.
4862 @option{--delete} will remove all versions of the named file from the
4863 archive. The @option{--delete} operation can run very slowly.
4865 Unlike other operations, @option{--delete} has no short form.
4867 @cindex Tapes, using @option{--delete} and
4868 @cindex Deleting from tape archives
4869 This operation will rewrite the archive. You can only use
4870 @option{--delete} on an archive if the archive device allows you to
4871 write to any point on the media, such as a disk; because of this, it
4872 does not work on magnetic tapes. Do not try to delete an archive member
4873 from a magnetic tape; the action will not succeed, and you will be
4874 likely to scramble the archive and damage your tape. There is no safe
4875 way (except by completely re-writing the archive) to delete files from
4876 most kinds of magnetic tape. @xref{Media}.
4878 To delete all versions of the file @file{blues} from the archive
4879 @file{collection.tar} in the @file{practice} directory, make sure you
4880 are in that directory, and then,
4883 $ @kbd{tar --list --file=collection.tar}
4888 $ @kbd{tar --delete --file=collection.tar blues}
4889 $ @kbd{tar --list --file=collection.tar}
4895 @FIXME{Check if the above listing is actually produced after running
4896 all the examples on collection.tar.}
4898 The @option{--delete} option has been reported to work properly when
4899 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4902 @subsection Comparing Archive Members with the File System
4903 @cindex Verifying the currency of an archive
4906 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4907 specified archive members against files with the same names, and then
4908 reports differences in file size, mode, owner, modification date and
4909 contents. You should @emph{only} specify archive member names, not file
4910 names. If you do not name any members, then @command{tar} will compare the
4911 entire archive. If a file is represented in the archive but does not
4912 exist in the file system, @command{tar} reports a difference.
4914 You have to specify the record size of the archive when modifying an
4915 archive with a non-default record size.
4917 @command{tar} ignores files in the file system that do not have
4918 corresponding members in the archive.
4920 The following example compares the archive members @file{rock},
4921 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4922 files of the same name in the file system. (Note that there is no file,
4923 @file{funk}; @command{tar} will report an error message.)
4926 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4929 tar: funk not found in archive
4932 The spirit behind the @option{--compare} (@option{--diff},
4933 @option{-d}) option is to check whether the archive represents the
4934 current state of files on disk, more than validating the integrity of
4935 the archive media. For this latter goal, see @ref{verify}.
4937 @node create options
4938 @section Options Used by @option{--create}
4940 @xopindex{create, additional options}
4941 The previous chapter described the basics of how to use
4942 @option{--create} (@option{-c}) to create an archive from a set of files.
4943 @xref{create}. This section described advanced options to be used with
4947 * override:: Overriding File Metadata.
4948 * Ignore Failed Read::
4952 @subsection Overriding File Metadata
4954 As described above, a @command{tar} archive keeps, for each member it contains,
4955 its @dfn{metadata}, such as modification time, mode and ownership of
4956 the file. @GNUTAR{} allows to replace these data with other values
4957 when adding files to the archive. The options described in this
4958 section affect creation of archives of any type. For POSIX archives,
4959 see also @ref{PAX keywords}, for additional ways of controlling
4960 metadata, stored in the archive.
4964 @item --mode=@var{permissions}
4966 When adding files to an archive, @command{tar} will use
4967 @var{permissions} for the archive members, rather than the permissions
4968 from the files. @var{permissions} can be specified either as an octal
4969 number or as symbolic permissions, like with
4970 @command{chmod} (@xref{File permissions, Permissions, File
4971 permissions, fileutils, @acronym{GNU} file utilities}. This reference
4972 also has useful information for those not being overly familiar with
4973 the UNIX permission system). Using latter syntax allows for
4974 more flexibility. For example, the value @samp{a+rw} adds read and write
4975 permissions for everybody, while retaining executable bits on directories
4976 or on any other file already marked as executable:
4979 $ @kbd{tar -c -f archive.tar --mode='a+rw' .}
4982 @item --mtime=@var{date}
4985 When adding files to an archive, @command{tar} will use @var{date} as
4986 the modification time of members when creating archives, instead of
4987 their actual modification times. The argument @var{date} can be
4988 either a textual date representation in almost arbitrary format
4989 (@pxref{Date input formats}) or a name of an existing file, starting
4990 with @samp{/} or @samp{.}. In the latter case, the modification time
4991 of that file will be used.
4993 The following example will set the modification date to 00:00:00,
4997 $ @kbd{tar -c -f archive.tar --mtime='1970-01-01' .}
5001 When used with @option{--verbose} (@pxref{verbose tutorial}) @GNUTAR{}
5002 will try to convert the specified date back to its textual
5003 representation and compare it with the one given with
5004 @option{--mtime} options. If the two dates differ, @command{tar} will
5005 print a warning saying what date it will use. This is to help user
5006 ensure he is using the right date.
5011 $ @kbd{tar -c -f archive.tar -v --mtime=yesterday .}
5012 tar: Option --mtime: Treating date 'yesterday' as 2006-06-20
5017 @item --owner=@var{user}
5020 Specifies that @command{tar} should use @var{user} as the owner of members
5021 when creating archives, instead of the user associated with the source
5024 If @var{user} contains a colon, it is taken to be of the form
5025 @var{name}:@var{id} where a nonempty @var{name} specifies the user
5026 name and a nonempty @var{id} specifies the decimal numeric user
5027 @acronym{ID}. If @var{user} does not contain a colon, it is taken to
5028 be a user number if it is one or more decimal digits; otherwise it is
5029 taken to be a user name.
5031 If a name is given but no number, the number is inferred from the
5032 current host's user database if possible, and the file's user number
5033 is used otherwise. If a number is given but no name, the name is
5034 inferred from the number if possible, and an empty name is used
5035 otherwise. If both name and number are given, the user database is
5036 not consulted, and the name and number need not be valid on the
5039 There is no value indicating a missing number, and @samp{0} usually means
5040 @code{root}. Some people like to force @samp{0} as the value to offer in
5041 their distributions for the owner of files, because the @code{root} user is
5042 anonymous anyway, so that might as well be the owner of anonymous
5043 archives. For example:
5046 $ @kbd{tar -c -f archive.tar --owner=0 .}
5053 $ @kbd{tar -c -f archive.tar --owner=root .}
5056 @item --group=@var{group}
5059 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
5060 rather than the group from the source file. As with @option{--owner},
5061 the argument @var{group} can be an existing group symbolic name, or a
5062 decimal numeric group @acronym{ID}, or @var{name}:@var{id}.
5065 @node Ignore Failed Read
5066 @subsection Ignore Fail Read
5069 @item --ignore-failed-read
5070 @opindex ignore-failed-read
5071 Do not exit with nonzero on unreadable files or directories.
5074 @node extract options
5075 @section Options Used by @option{--extract}
5076 @cindex options for use with @option{--extract}
5078 @xopindex{extract, additional options}
5079 The previous chapter showed how to use @option{--extract} to extract
5080 an archive into the file system. Various options cause @command{tar} to
5081 extract more information than just file contents, such as the owner,
5082 the permissions, the modification date, and so forth. This section
5083 presents options to be used with @option{--extract} when certain special
5084 considerations arise. You may review the information presented in
5085 @ref{extract} for more basic information about the
5086 @option{--extract} operation.
5089 * Reading:: Options to Help Read Archives
5090 * Writing:: Changing How @command{tar} Writes Files
5091 * Scarce:: Coping with Scarce Resources
5095 @subsection Options to Help Read Archives
5096 @cindex Options when reading archives
5098 @cindex Reading incomplete records
5099 @cindex Records, incomplete
5100 @opindex read-full-records
5101 Normally, @command{tar} will request data in full record increments from
5102 an archive storage device. If the device cannot return a full record,
5103 @command{tar} will report an error. However, some devices do not always
5104 return full records, or do not require the last record of an archive to
5105 be padded out to the next record boundary. To keep reading until you
5106 obtain a full record, or to accept an incomplete record if it contains
5107 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
5108 in conjunction with the @option{--extract} or @option{--list} operations.
5111 The @option{--read-full-records} (@option{-B}) option is turned on by default when
5112 @command{tar} reads an archive from standard input, or from a remote
5113 machine. This is because on @acronym{BSD} Unix systems, attempting to read a
5114 pipe returns however much happens to be in the pipe, even if it is
5115 less than was requested. If this option were not enabled, @command{tar}
5116 would fail as soon as it read an incomplete record from the pipe.
5118 If you're not sure of the blocking factor of an archive, you can
5119 read the archive by specifying @option{--read-full-records} (@option{-B}) and
5120 @option{--blocking-factor=@var{512-size}} (@option{-b
5121 @var{512-size}}), using a blocking factor larger than what the archive
5122 uses. This lets you avoid having to determine the blocking factor
5123 of an archive. @xref{Blocking Factor}.
5126 * read full records::
5130 @node read full records
5131 @unnumberedsubsubsec Reading Full Records
5133 @FIXME{need sentence or so of intro here}
5136 @opindex read-full-records
5137 @item --read-full-records
5139 Use in conjunction with @option{--extract} (@option{--get},
5140 @option{-x}) to read an archive which contains incomplete records, or
5141 one which has a blocking factor less than the one specified.
5145 @unnumberedsubsubsec Ignoring Blocks of Zeros
5147 @cindex End-of-archive blocks, ignoring
5148 @cindex Ignoring end-of-archive blocks
5149 @opindex ignore-zeros
5150 Normally, @command{tar} stops reading when it encounters a block of zeros
5151 between file entries (which usually indicates the end of the archive).
5152 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
5153 completely read an archive which contains a block of zeros before the
5154 end (i.e., a damaged archive, or one that was created by concatenating
5155 several archives together).
5157 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
5158 versions of @command{tar} write garbage after the end-of-archive entry,
5159 since that part of the media is never supposed to be read. @GNUTAR{}
5160 does not write after the end of an archive, but seeks to
5161 maintain compatibility among archiving utilities.
5164 @item --ignore-zeros
5166 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
5167 encountered while reading an archive. Use in conjunction with
5168 @option{--extract} or @option{--list}.
5172 @subsection Changing How @command{tar} Writes Files
5175 @FIXME{Introductory paragraph}
5178 * Dealing with Old Files::
5179 * Overwrite Old Files::
5181 * Keep Newer Files::
5183 * Recursive Unlink::
5184 * Data Modification Times::
5185 * Setting Access Permissions::
5186 * Directory Modification Times and Permissions::
5187 * Writing to Standard Output::
5188 * Writing to an External Program::
5192 @node Dealing with Old Files
5193 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
5195 @xopindex{overwrite-dir, introduced}
5196 When extracting files, if @command{tar} discovers that the extracted
5197 file already exists, it normally replaces the file by removing it before
5198 extracting it, to prevent confusion in the presence of hard or symbolic
5199 links. (If the existing file is a symbolic link, it is removed, not
5200 followed.) However, if a directory cannot be removed because it is
5201 nonempty, @command{tar} normally overwrites its metadata (ownership,
5202 permission, etc.). The @option{--overwrite-dir} option enables this
5203 default behavior. To be more cautious and preserve the metadata of
5204 such a directory, use the @option{--no-overwrite-dir} option.
5206 @cindex Overwriting old files, prevention
5207 @xopindex{keep-old-files, introduced}
5208 To be even more cautious and prevent existing files from being replaced, use
5209 the @option{--keep-old-files} (@option{-k}) option. It causes
5210 @command{tar} to refuse to replace or update a file that already
5211 exists, i.e., a file with the same name as an archive member prevents
5212 extraction of that archive member. Instead, it reports an error. For
5218 $ @kbd{tar -x -k -f archive.tar}
5219 tar: blues: Cannot open: File exists
5220 tar: Exiting with failure status due to previous errors
5223 @xopindex{skip-old-files, introduced}
5224 If you wish to preserve old files untouched, but don't want
5225 @command{tar} to treat them as errors, use the
5226 @option{--skip-old-files} option. This option causes @command{tar} to
5227 silently skip extracting over existing files.
5229 @xopindex{overwrite, introduced}
5230 To be more aggressive about altering existing files, use the
5231 @option{--overwrite} option. It causes @command{tar} to overwrite
5232 existing files and to follow existing symbolic links when extracting.
5234 @cindex Protecting old files
5235 Some people argue that @GNUTAR{} should not hesitate
5236 to overwrite files with other files when extracting. When extracting
5237 a @command{tar} archive, they expect to see a faithful copy of the
5238 state of the file system when the archive was created. It is debatable
5239 that this would always be a proper behavior. For example, suppose one
5240 has an archive in which @file{usr/local} is a link to
5241 @file{usr/local2}. Since then, maybe the site removed the link and
5242 renamed the whole hierarchy from @file{/usr/local2} to
5243 @file{/usr/local}. Such things happen all the time. I guess it would
5244 not be welcome at all that @GNUTAR{} removes the
5245 whole hierarchy just to make room for the link to be reinstated
5246 (unless it @emph{also} simultaneously restores the full
5247 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
5248 able to remove a whole hierarchy to reestablish a symbolic link, for
5249 example, but @emph{only if} @option{--recursive-unlink} is specified
5250 to allow this behavior. In any case, single files are silently
5253 @xopindex{unlink-first, introduced}
5254 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
5255 some cases by causing @command{tar} to remove files unconditionally
5256 before extracting them.
5258 @node Overwrite Old Files
5259 @unnumberedsubsubsec Overwrite Old Files
5264 Overwrite existing files and directory metadata when extracting files
5267 This causes @command{tar} to write extracted files into the file system without
5268 regard to the files already on the system; i.e., files with the same
5269 names as archive members are overwritten when the archive is extracted.
5270 It also causes @command{tar} to extract the ownership, permissions,
5271 and time stamps onto any preexisting files or directories.
5272 If the name of a corresponding file name is a symbolic link, the file
5273 pointed to by the symbolic link will be overwritten instead of the
5274 symbolic link itself (if this is possible). Moreover, special devices,
5275 empty directories and even symbolic links are automatically removed if
5276 they are in the way of extraction.
5278 Be careful when using the @option{--overwrite} option, particularly when
5279 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
5280 can change the contents, ownership or permissions of any file on your
5281 system. Also, many systems do not take kindly to overwriting files that
5282 are currently being executed.
5284 @opindex overwrite-dir
5285 @item --overwrite-dir
5286 Overwrite the metadata of directories when extracting files from an
5287 archive, but remove other files before extracting.
5290 @node Keep Old Files
5291 @unnumberedsubsubsec Keep Old Files
5293 @GNUTAR{} provides two options to control its actions in a situation
5294 when it is about to extract a file which already exists on disk.
5297 @opindex keep-old-files
5298 @item --keep-old-files
5300 Do not replace existing files from archive. When such a file is
5301 encountered, @command{tar} issues an error message. Upon end of
5302 extraction, @command{tar} exits with code 2 (@pxref{exit status}).
5304 @item --skip-old-files
5305 Do not replace existing files from archive, but do not treat that
5306 as error. Such files are silently skipped and do not affect
5307 @command{tar} exit status.
5309 Additional verbosity can be obtained using @option{--warning=existing-file}
5310 together with that option (@pxref{warnings}).
5313 @node Keep Newer Files
5314 @unnumberedsubsubsec Keep Newer Files
5317 @opindex keep-newer-files
5318 @item --keep-newer-files
5319 Do not replace existing files that are newer than their archive
5320 copies. This option is meaningless with @option{--list} (@option{-t}).
5324 @unnumberedsubsubsec Unlink First
5327 @opindex unlink-first
5328 @item --unlink-first
5330 Remove files before extracting over them.
5331 This can make @command{tar} run a bit faster if you know in advance
5332 that the extracted files all need to be removed. Normally this option
5333 slows @command{tar} down slightly, so it is disabled by default.
5336 @node Recursive Unlink
5337 @unnumberedsubsubsec Recursive Unlink
5340 @opindex recursive-unlink
5341 @item --recursive-unlink
5342 When this option is specified, try removing files and directory hierarchies
5343 before extracting over them. @emph{This is a dangerous option!}
5346 If you specify the @option{--recursive-unlink} option,
5347 @command{tar} removes @emph{anything} that keeps you from extracting a file
5348 as far as current permissions will allow it. This could include removal
5349 of the contents of a full directory hierarchy.
5351 @node Data Modification Times
5352 @unnumberedsubsubsec Setting Data Modification Times
5354 @cindex Data modification times of extracted files
5355 @cindex Modification times of extracted files
5356 Normally, @command{tar} sets the data modification times of extracted
5357 files to the corresponding times recorded for the files in the archive, but
5358 limits the permissions of extracted files by the current @code{umask}
5361 To set the data modification times of extracted files to the time when
5362 the files were extracted, use the @option{--touch} (@option{-m}) option in
5363 conjunction with @option{--extract} (@option{--get}, @option{-x}).
5369 Sets the data modification time of extracted archive members to the time
5370 they were extracted, not the time recorded for them in the archive.
5371 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
5374 @node Setting Access Permissions
5375 @unnumberedsubsubsec Setting Access Permissions
5377 @cindex Permissions of extracted files
5378 @cindex Modes of extracted files
5379 To set the modes (access permissions) of extracted files to those
5380 recorded for those files in the archive, use @option{--same-permissions}
5381 in conjunction with the @option{--extract} (@option{--get},
5382 @option{-x}) operation.
5385 @opindex preserve-permissions
5386 @opindex same-permissions
5387 @item --preserve-permissions
5388 @itemx --same-permissions
5389 @c @itemx --ignore-umask
5391 Set modes of extracted archive members to those recorded in the
5392 archive, instead of current umask settings. Use in conjunction with
5393 @option{--extract} (@option{--get}, @option{-x}).
5396 @node Directory Modification Times and Permissions
5397 @unnumberedsubsubsec Directory Modification Times and Permissions
5399 After successfully extracting a file member, @GNUTAR{} normally
5400 restores its permissions and modification times, as described in the
5401 previous sections. This cannot be done for directories, because
5402 after extracting a directory @command{tar} will almost certainly
5403 extract files into that directory and this will cause the directory
5404 modification time to be updated. Moreover, restoring that directory
5405 permissions may not permit file creation within it. Thus, restoring
5406 directory permissions and modification times must be delayed at least
5407 until all files have been extracted into that directory. @GNUTAR{}
5408 restores directories using the following approach.
5410 The extracted directories are created with the mode specified in the
5411 archive, as modified by the umask of the user, which gives sufficient
5412 permissions to allow file creation. The meta-information about the
5413 directory is recorded in the temporary list of directories. When
5414 preparing to extract next archive member, @GNUTAR{} checks if the
5415 directory prefix of this file contains the remembered directory. If
5416 it does not, the program assumes that all files have been extracted
5417 into that directory, restores its modification time and permissions
5418 and removes its entry from the internal list. This approach allows
5419 to correctly restore directory meta-information in the majority of
5420 cases, while keeping memory requirements sufficiently small. It is
5421 based on the fact, that most @command{tar} archives use the predefined
5422 order of members: first the directory, then all the files and
5423 subdirectories in that directory.
5425 However, this is not always true. The most important exception are
5426 incremental archives (@pxref{Incremental Dumps}). The member order in
5427 an incremental archive is reversed: first all directory members are
5428 stored, followed by other (non-directory) members. So, when extracting
5429 from incremental archives, @GNUTAR{} alters the above procedure. It
5430 remembers all restored directories, and restores their meta-data
5431 only after the entire archive has been processed. Notice, that you do
5432 not need to specify any special options for that, as @GNUTAR{}
5433 automatically detects archives in incremental format.
5435 There may be cases, when such processing is required for normal archives
5436 too. Consider the following example:
5440 $ @kbd{tar --no-recursion -cvf archive \
5441 foo foo/file1 bar bar/file foo/file2}
5450 During the normal operation, after encountering @file{bar}
5451 @GNUTAR{} will assume that all files from the directory @file{foo}
5452 were already extracted and will therefore restore its timestamp and
5453 permission bits. However, after extracting @file{foo/file2} the
5454 directory timestamp will be offset again.
5456 To correctly restore directory meta-information in such cases, use
5457 the @option{--delay-directory-restore} command line option:
5460 @opindex delay-directory-restore
5461 @item --delay-directory-restore
5462 Delays restoring of the modification times and permissions of extracted
5463 directories until the end of extraction. This way, correct
5464 meta-information is restored even if the archive has unusual member
5467 @opindex no-delay-directory-restore
5468 @item --no-delay-directory-restore
5469 Cancel the effect of the previous @option{--delay-directory-restore}.
5470 Use this option if you have used @option{--delay-directory-restore} in
5471 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
5472 temporarily disable it.
5475 @node Writing to Standard Output
5476 @unnumberedsubsubsec Writing to Standard Output
5478 @cindex Writing extracted files to standard output
5479 @cindex Standard output, writing extracted files to
5480 To write the extracted files to the standard output, instead of
5481 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
5482 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
5483 extracting files to send them through a pipe, and do not need to
5484 preserve them in the file system. If you extract multiple members,
5485 they appear on standard output concatenated, in the order they are
5486 found in the archive.
5492 Writes files to the standard output. Use only in conjunction with
5493 @option{--extract} (@option{--get}, @option{-x}). When this option is
5494 used, instead of creating the files specified, @command{tar} writes
5495 the contents of the files extracted to its standard output. This may
5496 be useful if you are only extracting the files in order to send them
5497 through a pipe. This option is meaningless with @option{--list}
5501 This can be useful, for example, if you have a tar archive containing
5502 a big file and don't want to store the file on disk before processing
5503 it. You can use a command like this:
5506 tar -xOzf foo.tgz bigfile | process
5509 or even like this if you want to process the concatenation of the files:
5512 tar -xOzf foo.tgz bigfile1 bigfile2 | process
5515 However, @option{--to-command} may be more convenient for use with
5516 multiple files. See the next section.
5518 @node Writing to an External Program
5519 @unnumberedsubsubsec Writing to an External Program
5521 You can instruct @command{tar} to send the contents of each extracted
5522 file to the standard input of an external program:
5526 @item --to-command=@var{command}
5527 Extract files and pipe their contents to the standard input of
5528 @var{command}. When this option is used, instead of creating the
5529 files specified, @command{tar} invokes @var{command} and pipes the
5530 contents of the files to its standard output. The @var{command} may
5531 contain command line arguments (see @ref{external, Running External Commands},
5534 Notice, that @var{command} is executed once for each regular file
5535 extracted. Non-regular files (directories, etc.) are ignored when this
5539 The command can obtain the information about the file it processes
5540 from the following environment variables:
5543 @vrindex TAR_FILETYPE, to-command environment
5545 Type of the file. It is a single letter with the following meaning:
5547 @multitable @columnfractions 0.10 0.90
5548 @item f @tab Regular file
5549 @item d @tab Directory
5550 @item l @tab Symbolic link
5551 @item h @tab Hard link
5552 @item b @tab Block device
5553 @item c @tab Character device
5556 Currently only regular files are supported.
5558 @vrindex TAR_MODE, to-command environment
5560 File mode, an octal number.
5562 @vrindex TAR_FILENAME, to-command environment
5564 The name of the file.
5566 @vrindex TAR_REALNAME, to-command environment
5568 Name of the file as stored in the archive.
5570 @vrindex TAR_UNAME, to-command environment
5572 Name of the file owner.
5574 @vrindex TAR_GNAME, to-command environment
5576 Name of the file owner group.
5578 @vrindex TAR_ATIME, to-command environment
5580 Time of last access. It is a decimal number, representing seconds
5581 since the Epoch. If the archive provides times with nanosecond
5582 precision, the nanoseconds are appended to the timestamp after a
5585 @vrindex TAR_MTIME, to-command environment
5587 Time of last modification.
5589 @vrindex TAR_CTIME, to-command environment
5591 Time of last status change.
5593 @vrindex TAR_SIZE, to-command environment
5597 @vrindex TAR_UID, to-command environment
5599 UID of the file owner.
5601 @vrindex TAR_GID, to-command environment
5603 GID of the file owner.
5606 Additionally, the following variables contain information about
5607 tar mode and the archive being processed:
5610 @vrindex TAR_VERSION, to-command environment
5612 @GNUTAR{} version number.
5614 @vrindex TAR_ARCHIVE, to-command environment
5616 The name of the archive @command{tar} is processing.
5618 @vrindex TAR_BLOCKING_FACTOR, to-command environment
5619 @item TAR_BLOCKING_FACTOR
5620 Current blocking factor (@pxref{Blocking}).
5622 @vrindex TAR_VOLUME, to-command environment
5624 Ordinal number of the volume @command{tar} is processing.
5626 @vrindex TAR_FORMAT, to-command environment
5628 Format of the archive being processed. @xref{Formats}, for a complete
5629 list of archive format names.
5632 These variables are defined prior to executing the command, so you can
5633 pass them as arguments, if you prefer. For example, if the command
5634 @var{proc} takes the member name and size as its arguments, then you
5638 $ @kbd{tar -x -f archive.tar \
5639 --to-command='proc $TAR_FILENAME $TAR_SIZE'}
5643 Notice single quotes to prevent variable names from being expanded by
5644 the shell when invoking @command{tar}.
5646 If @var{command} exits with a non-0 status, @command{tar} will print
5647 an error message similar to the following:
5650 tar: 2345: Child returned status 1
5653 Here, @samp{2345} is the PID of the finished process.
5655 If this behavior is not wanted, use @option{--ignore-command-error}:
5658 @opindex ignore-command-error
5659 @item --ignore-command-error
5660 Ignore exit codes of subprocesses. Notice that if the program
5661 exits on signal or otherwise terminates abnormally, the error message
5662 will be printed even if this option is used.
5664 @opindex no-ignore-command-error
5665 @item --no-ignore-command-error
5666 Cancel the effect of any previous @option{--ignore-command-error}
5667 option. This option is useful if you have set
5668 @option{--ignore-command-error} in @env{TAR_OPTIONS}
5669 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
5673 @unnumberedsubsubsec Removing Files
5675 @FIXME{The section is too terse. Something more to add? An example,
5679 @opindex remove-files
5680 @item --remove-files
5681 Remove files after adding them to the archive.
5685 @subsection Coping with Scarce Resources
5688 @cindex Small memory
5689 @cindex Running out of space
5697 @unnumberedsubsubsec Starting File
5700 @opindex starting-file
5701 @item --starting-file=@var{name}
5702 @itemx -K @var{name}
5703 Starts an operation in the middle of an archive. Use in conjunction
5704 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
5707 @cindex Middle of the archive, starting in the
5708 If a previous attempt to extract files failed due to lack of disk
5709 space, you can use @option{--starting-file=@var{name}} (@option{-K
5710 @var{name}}) to start extracting only after member @var{name} of the
5711 archive. This assumes, of course, that there is now free space, or
5712 that you are now extracting into a different file system. (You could
5713 also choose to suspend @command{tar}, remove unnecessary files from
5714 the file system, and then resume the same @command{tar} operation.
5715 In this case, @option{--starting-file} is not necessary.) See also
5716 @ref{interactive}, and @ref{exclude}.
5719 @unnumberedsubsubsec Same Order
5722 @cindex Large lists of file names on small machines
5724 @opindex preserve-order
5726 @itemx --preserve-order
5728 To process large lists of file names on machines with small amounts of
5729 memory. Use in conjunction with @option{--compare} (@option{--diff},
5730 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
5731 (@option{--get}, @option{-x}).
5734 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
5735 names to be listed or extracted is sorted in the same order as the
5736 files in the archive. This allows a large list of names to be used,
5737 even on a small machine that would not otherwise be able to hold all
5738 the names in memory at the same time. Such a sorted list can easily be
5739 created by running @samp{tar -t} on the archive and editing its output.
5741 This option is probably never needed on modern computer systems.
5744 @section Backup options
5746 @cindex backup options
5748 @GNUTAR{} offers options for making backups of files
5749 before writing new versions. These options control the details of
5750 these backups. They may apply to the archive itself before it is
5751 created or rewritten, as well as individual extracted members. Other
5752 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
5753 and @command{mv}, for example) offer similar options.
5755 Backup options may prove unexpectedly useful when extracting archives
5756 containing many members having identical name, or when extracting archives
5757 on systems having file name limitations, making different members appear
5758 as having similar names through the side-effect of name truncation.
5759 @FIXME{This is true only if we have a good scheme for truncated backup names,
5760 which I'm not sure at all: I suspect work is needed in this area.}
5761 When any existing file is backed up before being overwritten by extraction,
5762 then clashing files are automatically be renamed to be unique, and the
5763 true name is kept for only the last file of a series of clashing files.
5764 By using verbose mode, users may track exactly what happens.
5766 At the detail level, some decisions are still experimental, and may
5767 change in the future, we are waiting comments from our users. So, please
5768 do not learn to depend blindly on the details of the backup features.
5769 For example, currently, directories themselves are never renamed through
5770 using these options, so, extracting a file over a directory still has
5771 good chances to fail. Also, backup options apply to created archives,
5772 not only to extracted members. For created archives, backups will not
5773 be attempted when the archive is a block or character device, or when it
5774 refers to a remote file.
5776 For the sake of simplicity and efficiency, backups are made by renaming old
5777 files prior to creation or extraction, and not by copying. The original
5778 name is restored if the file creation fails. If a failure occurs after a
5779 partial extraction of a file, both the backup and the partially extracted
5783 @item --backup[=@var{method}]
5785 @vindex VERSION_CONTROL
5787 Back up files that are about to be overwritten or removed.
5788 Without this option, the original versions are destroyed.
5790 Use @var{method} to determine the type of backups made.
5791 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
5792 environment variable. And if @env{VERSION_CONTROL} is not set,
5793 use the @samp{existing} method.
5795 @vindex version-control @r{Emacs variable}
5796 This option corresponds to the Emacs variable @samp{version-control};
5797 the same values for @var{method} are accepted as in Emacs. This option
5798 also allows more descriptive names. The valid @var{method}s are:
5803 @cindex numbered @r{backup method}
5804 Always make numbered backups.
5808 @cindex existing @r{backup method}
5809 Make numbered backups of files that already have them, simple backups
5814 @cindex simple @r{backup method}
5815 Always make simple backups.
5819 @item --suffix=@var{suffix}
5821 @cindex backup suffix
5822 @vindex SIMPLE_BACKUP_SUFFIX
5823 Append @var{suffix} to each backup file made with @option{--backup}. If this
5824 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5825 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5826 set, the default is @samp{~}, just as in Emacs.
5831 @section Notable @command{tar} Usages
5834 @FIXME{Using Unix file linking capability to recreate directory
5835 structures---linking files into one subdirectory and then
5836 @command{tar}ring that directory.}
5838 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5841 You can easily use archive files to transport a group of files from
5842 one system to another: put all relevant files into an archive on one
5843 computer system, transfer the archive to another system, and extract
5844 the contents there. The basic transfer medium might be magnetic tape,
5845 Internet FTP, or even electronic mail (though you must encode the
5846 archive with @command{uuencode} in order to transport it properly by
5847 mail). Both machines do not have to use the same operating system, as
5848 long as they both support the @command{tar} program.
5850 For example, here is how you might copy a directory's contents from
5851 one disk to another, while preserving the dates, modes, owners and
5852 link-structure of all the files therein. In this case, the transfer
5853 medium is a @dfn{pipe}:
5856 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5860 You can avoid subshells by using @option{-C} option:
5863 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5867 The command also works using long option forms:
5871 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5872 | (cd targetdir; tar --extract --file=-)}
5881 $ @kbd{tar --directory sourcedir --create --file=- . \
5882 | tar --directory targetdir --extract --file=-}
5887 This is one of the easiest methods to transfer a @command{tar} archive.
5890 @section Looking Ahead: The Rest of this Manual
5892 You have now seen how to use all eight of the operations available to
5893 @command{tar}, and a number of the possible options. The next chapter
5894 explains how to choose and change file and archive names, how to use
5895 files to store names of other files which you can then call as
5896 arguments to @command{tar} (this can help you save time if you expect to
5897 archive the same list of files a number of times), and so forth.
5898 @FIXME{in case it's not obvious, i'm making this up in some sense
5899 based on my limited memory of what the next chapter *really* does. i
5900 just wanted to flesh out this final section a little bit so i'd
5901 remember to stick it in here. :-)}
5903 If there are too many files to conveniently list on the command line,
5904 you can list the names in a file, and @command{tar} will read that file.
5907 There are various ways of causing @command{tar} to skip over some files,
5908 and not archive them. @xref{Choosing}.
5911 @chapter Performing Backups and Restoring Files
5914 @GNUTAR{} is distributed along with the scripts for performing backups
5915 and restores. Even if there is a good chance those scripts may be
5916 satisfying to you, they are not the only scripts or methods available for doing
5917 backups and restore. You may well create your own, or use more
5918 sophisticated packages dedicated to that purpose.
5920 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5921 Automatic Network Disk Archiver), a backup system developed by James
5922 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5923 This is free software, and it is available from @uref{http://www.amanda.org}.
5927 Here is a possible plan for a future documentation about the backuping
5928 scripts which are provided within the @GNUTAR{}
5934 @item what are dumps
5935 @item different levels of dumps
5937 @item full dump = dump everything
5938 @item level 1, level 2 dumps etc
5939 A level @var{n} dump dumps everything changed since the last level
5942 @item how to use scripts for dumps (ie, the concept)
5944 @item scripts to run after editing backup specs (details)
5946 @item Backup Specs, what is it.
5948 @item how to customize
5949 @item actual text of script [/sp/dump/backup-specs]
5953 @item rsh doesn't work
5954 @item rtape isn't installed
5957 @item the @option{--incremental} option of tar
5960 @item write protection
5961 @item types of media, different sizes and types, useful for different things
5962 @item files and tape marks
5963 one tape mark between files, two at end.
5964 @item positioning the tape
5965 MT writes two at end of write,
5966 backspaces over one when writing again.
5972 This chapter documents both the provided shell scripts and @command{tar}
5973 options which are more specific to usage as a backup tool.
5975 To @dfn{back up} a file system means to create archives that contain
5976 all the files in that file system. Those archives can then be used to
5977 restore any or all of those files (for instance if a disk crashes or a
5978 file is accidentally deleted). File system @dfn{backups} are also
5982 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5983 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5984 * Backup Levels:: Levels of Backups
5985 * Backup Parameters:: Setting Parameters for Backups and Restoration
5986 * Scripted Backups:: Using the Backup Scripts
5987 * Scripted Restoration:: Using the Restore Script
5991 @section Using @command{tar} to Perform Full Dumps
5997 @cindex corrupted archives
5998 Full dumps should only be made when no other people or programs
5999 are modifying files in the file system. If files are modified while
6000 @command{tar} is making the backup, they may not be stored properly in
6001 the archive, in which case you won't be able to restore them if you
6002 have to. (Files not being modified are written with no trouble, and do
6003 not corrupt the entire archive.)
6005 You will want to use the @option{--label=@var{archive-label}}
6006 (@option{-V @var{archive-label}}) option to give the archive a
6007 volume label, so you can tell what this archive is even if the label
6008 falls off the tape, or anything like that.
6010 Unless the file system you are dumping is guaranteed to fit on
6011 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
6012 Make sure you have enough tapes on hand to complete the backup.
6014 If you want to dump each file system separately you will need to use
6015 the @option{--one-file-system} option to prevent
6016 @command{tar} from crossing file system boundaries when storing
6019 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
6020 option is not needed, since this is a complete copy of everything in
6021 the file system, and a full restore from this backup would only be
6022 done onto a completely
6025 Unless you are in a hurry, and trust the @command{tar} program (and your
6026 tapes), it is a good idea to use the @option{--verify} (@option{-W})
6027 option, to make sure your files really made it onto the dump properly.
6028 This will also detect cases where the file was modified while (or just
6029 after) it was being archived. Not all media (notably cartridge tapes)
6030 are capable of being verified, unfortunately.
6032 @node Incremental Dumps
6033 @section Using @command{tar} to Perform Incremental Dumps
6035 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
6036 stores additional metadata so that exact state of the file system
6037 can be restored when extracting the archive.
6039 @GNUTAR{} currently offers two options for handling incremental
6040 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
6041 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
6043 @xopindex{listed-incremental, described}
6044 The option @option{--listed-incremental} instructs tar to operate on
6045 an incremental archive with additional metadata stored in a standalone
6046 file, called a @dfn{snapshot file}. The purpose of this file is to help
6047 determine which files have been changed, added or deleted since the
6048 last backup, so that the next incremental backup will contain only
6049 modified files. The name of the snapshot file is given as an argument
6053 @item --listed-incremental=@var{file}
6054 @itemx -g @var{file}
6055 Handle incremental backups with snapshot data in @var{file}.
6058 To create an incremental backup, you would use
6059 @option{--listed-incremental} together with @option{--create}
6060 (@pxref{create}). For example:
6063 $ @kbd{tar --create \
6064 --file=archive.1.tar \
6065 --listed-incremental=/var/log/usr.snar \
6069 This will create in @file{archive.1.tar} an incremental backup of
6070 the @file{/usr} file system, storing additional metadata in the file
6071 @file{/var/log/usr.snar}. If this file does not exist, it will be
6072 created. The created archive will then be a @dfn{level 0 backup};
6073 please see the next section for more on backup levels.
6075 Otherwise, if the file @file{/var/log/usr.snar} exists, it
6076 determines which files are modified. In this case only these files will be
6077 stored in the archive. Suppose, for example, that after running the
6078 above command, you delete file @file{/usr/doc/old} and create
6079 directory @file{/usr/local/db} with the following contents:
6082 $ @kbd{ls /usr/local/db}
6087 Some time later you create another incremental backup. You will
6091 $ @kbd{tar --create \
6092 --file=archive.2.tar \
6093 --listed-incremental=/var/log/usr.snar \
6095 tar: usr/local/db: Directory is new
6102 The created archive @file{archive.2.tar} will contain only these
6103 three members. This archive is called a @dfn{level 1 backup}. Notice
6104 that @file{/var/log/usr.snar} will be updated with the new data, so if
6105 you plan to create more @samp{level 1} backups, it is necessary to
6106 create a working copy of the snapshot file before running
6107 @command{tar}. The above example will then be modified as follows:
6110 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
6111 $ @kbd{tar --create \
6112 --file=archive.2.tar \
6113 --listed-incremental=/var/log/usr.snar-1 \
6118 @xopindex{level, described}
6119 You can force @samp{level 0} backups either by removing the snapshot
6120 file before running @command{tar}, or by supplying the
6121 @option{--level=0} option, e.g.:
6124 $ @kbd{tar --create \
6125 --file=archive.2.tar \
6126 --listed-incremental=/var/log/usr.snar-0 \
6131 Incremental dumps depend crucially on time stamps, so the results are
6132 unreliable if you modify a file's time stamps during dumping (e.g.,
6133 with the @option{--atime-preserve=replace} option), or if you set the clock
6136 @anchor{device numbers}
6137 @cindex Device numbers, using in incremental backups
6138 Metadata stored in snapshot files include device numbers, which,
6139 obviously are supposed to be non-volatile values. However, it turns
6140 out that @acronym{NFS} devices have undependable values when an automounter
6141 gets in the picture. This can lead to a great deal of spurious
6142 redumping in incremental dumps, so it is somewhat useless to compare
6143 two @acronym{NFS} devices numbers over time. The solution implemented
6144 currently is to consider all @acronym{NFS} devices as being equal
6145 when it comes to comparing directories; this is fairly gross, but
6146 there does not seem to be a better way to go.
6148 Apart from using @acronym{NFS}, there are a number of cases where
6149 relying on device numbers can cause spurious redumping of unmodified
6150 files. For example, this occurs when archiving @acronym{LVM} snapshot
6151 volumes. To avoid this, use @option{--no-check-device} option:
6154 @xopindex{no-check-device, described}
6155 @item --no-check-device
6156 Do not rely on device numbers when preparing a list of changed files
6157 for an incremental dump.
6159 @xopindex{check-device, described}
6160 @item --check-device
6161 Use device numbers when preparing a list of changed files
6162 for an incremental dump. This is the default behavior. The purpose
6163 of this option is to undo the effect of the @option{--no-check-device}
6164 if it was given in @env{TAR_OPTIONS} environment variable
6165 (@pxref{TAR_OPTIONS}).
6168 There is also another way to cope with changing device numbers. It is
6169 described in detail in @ref{Fixing Snapshot Files}.
6171 Note that incremental archives use @command{tar} extensions and may
6172 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
6174 @xopindex{listed-incremental, using with @option{--extract}}
6175 @xopindex{extract, using with @option{--listed-incremental}}
6176 To extract from the incremental dumps, use
6177 @option{--listed-incremental} together with @option{--extract}
6178 option (@pxref{extracting files}). In this case, @command{tar} does
6179 not need to access snapshot file, since all the data necessary for
6180 extraction are stored in the archive itself. So, when extracting, you
6181 can give whatever argument to @option{--listed-incremental}, the usual
6182 practice is to use @option{--listed-incremental=/dev/null}.
6183 Alternatively, you can use @option{--incremental}, which needs no
6184 arguments. In general, @option{--incremental} (@option{-G}) can be
6185 used as a shortcut for @option{--listed-incremental} when listing or
6186 extracting incremental backups (for more information regarding this
6187 option, @pxref{incremental-op}).
6189 When extracting from the incremental backup @GNUTAR{} attempts to
6190 restore the exact state the file system had when the archive was
6191 created. In particular, it will @emph{delete} those files in the file
6192 system that did not exist in their directories when the archive was
6193 created. If you have created several levels of incremental files,
6194 then in order to restore the exact contents the file system had when
6195 the last level was created, you will need to restore from all backups
6196 in turn. Continuing our example, to restore the state of @file{/usr}
6197 file system, one would do@footnote{Notice, that since both archives
6198 were created without @option{-P} option (@pxref{absolute}), these
6199 commands should be run from the root file system.}:
6202 $ @kbd{tar --extract \
6203 --listed-incremental=/dev/null \
6204 --file archive.1.tar}
6205 $ @kbd{tar --extract \
6206 --listed-incremental=/dev/null \
6207 --file archive.2.tar}
6210 To list the contents of an incremental archive, use @option{--list}
6211 (@pxref{list}), as usual. To obtain more information about the
6212 archive, use @option{--listed-incremental} or @option{--incremental}
6213 combined with two @option{--verbose} options@footnote{Two
6214 @option{--verbose} options were selected to avoid breaking usual
6215 verbose listing output (@option{--list --verbose}) when using in
6218 @xopindex{incremental, using with @option{--list}}
6219 @xopindex{listed-incremental, using with @option{--list}}
6220 @xopindex{list, using with @option{--incremental}}
6221 @xopindex{list, using with @option{--listed-incremental}}
6222 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
6223 contents of the DUMPDIR header (with terminating nulls) when
6224 @option{--incremental} or @option{--listed-incremental} option was
6225 given, no matter what the verbosity level. This behavior, and,
6226 especially, the binary output it produced were considered inconvenient
6227 and were changed in version 1.16.}:
6230 @kbd{tar --list --incremental --verbose --verbose archive.tar}
6233 This command will print, for each directory in the archive, the list
6234 of files in that directory at the time the archive was created. This
6235 information is put out in a format which is both human-readable and
6236 unambiguous for a program: each file name is printed as
6243 where @var{x} is a letter describing the status of the file: @samp{Y}
6244 if the file is present in the archive, @samp{N} if the file is not
6245 included in the archive, or a @samp{D} if the file is a directory (and
6246 is included in the archive). @xref{Dumpdir}, for the detailed
6247 description of dumpdirs and status codes. Each such
6248 line is terminated by a newline character. The last line is followed
6249 by an additional newline to indicate the end of the data.
6251 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
6252 gives the same behavior as @option{--listed-incremental} when used
6253 with @option{--list} and @option{--extract} options. When used with
6254 @option{--create} option, it creates an incremental archive without
6255 creating snapshot file. Thus, it is impossible to create several
6256 levels of incremental backups with @option{--incremental} option.
6259 @section Levels of Backups
6261 An archive containing all the files in the file system is called a
6262 @dfn{full backup} or @dfn{full dump}. You could insure your data by
6263 creating a full dump every day. This strategy, however, would waste a
6264 substantial amount of archive media and user time, as unchanged files
6265 are daily re-archived.
6267 It is more efficient to do a full dump only occasionally. To back up
6268 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
6269 one} dump archives all the files that have changed since the last full
6272 A typical dump strategy would be to perform a full dump once a week,
6273 and a level one dump once a day. This means some versions of files
6274 will in fact be archived more than once, but this dump strategy makes
6275 it possible to restore a file system to within one day of accuracy by
6276 only extracting two archives---the last weekly (full) dump and the
6277 last daily (level one) dump. The only information lost would be in
6278 files changed or created since the last daily backup. (Doing dumps
6279 more than once a day is usually not worth the trouble.)
6281 @GNUTAR{} comes with scripts you can use to do full
6282 and level-one (actually, even level-two and so on) dumps. Using
6283 scripts (shell programs) to perform backups and restoration is a
6284 convenient and reliable alternative to typing out file name lists
6285 and @command{tar} commands by hand.
6287 Before you use these scripts, you need to edit the file
6288 @file{backup-specs}, which specifies parameters used by the backup
6289 scripts and by the restore script. This file is usually located
6290 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
6291 detailed description. Once the backup parameters are set, you can
6292 perform backups or restoration by running the appropriate script.
6294 The name of the backup script is @code{backup}. The name of the
6295 restore script is @code{restore}. The following sections describe
6296 their use in detail.
6298 @emph{Please Note:} The backup and restoration scripts are
6299 designed to be used together. While it is possible to restore files by
6300 hand from an archive which was created using a backup script, and to create
6301 an archive by hand which could then be extracted using the restore script,
6302 it is easier to use the scripts. @xref{Incremental Dumps}, before
6303 making such an attempt.
6305 @node Backup Parameters
6306 @section Setting Parameters for Backups and Restoration
6308 The file @file{backup-specs} specifies backup parameters for the
6309 backup and restoration scripts provided with @command{tar}. You must
6310 edit @file{backup-specs} to fit your system configuration and schedule
6311 before using these scripts.
6313 Syntactically, @file{backup-specs} is a shell script, containing
6314 mainly variable assignments. However, any valid shell construct
6315 is allowed in this file. Particularly, you may wish to define
6316 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
6317 For more information about shell script syntax, please refer to
6318 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
6319 g_02, the definition of the Shell Command Language}. See also
6320 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
6322 The shell variables controlling behavior of @code{backup} and
6323 @code{restore} are described in the following subsections.
6326 * General-Purpose Variables::
6327 * Magnetic Tape Control::
6329 * backup-specs example:: An Example Text of @file{Backup-specs}
6332 @node General-Purpose Variables
6333 @subsection General-Purpose Variables
6335 @defvr {Backup variable} ADMINISTRATOR
6336 The user name of the backup administrator. @code{Backup} scripts
6337 sends a backup report to this address.
6340 @defvr {Backup variable} BACKUP_HOUR
6341 The hour at which the backups are done. This can be a number from 0
6342 to 23, or the time specification in form @var{hours}:@var{minutes},
6343 or the string @samp{now}.
6345 This variable is used by @code{backup}. Its value may be overridden
6346 using @option{--time} option (@pxref{Scripted Backups}).
6349 @defvr {Backup variable} TAPE_FILE
6351 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
6352 is a remote archive (@pxref{remote-dev}), backup script will suppose
6353 that your @command{mt} is able to access remote devices. If @var{RSH}
6354 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
6355 invocations of @command{mt}.
6358 @defvr {Backup variable} BLOCKING
6360 The blocking factor @command{tar} will use when writing the dump archive.
6361 @xref{Blocking Factor}.
6364 @defvr {Backup variable} BACKUP_DIRS
6366 A list of file systems to be dumped (for @code{backup}), or restored
6367 (for @code{restore}). You can include any directory
6368 name in the list --- subdirectories on that file system will be
6369 included, regardless of how they may look to other networked machines.
6370 Subdirectories on other file systems will be ignored.
6372 The host name specifies which host to run @command{tar} on, and should
6373 normally be the host that actually contains the file system. However,
6374 the host machine must have @GNUTAR{} installed, and
6375 must be able to access the directory containing the backup scripts and
6376 their support files using the same file name that is used on the
6377 machine where the scripts are run (i.e., what @command{pwd} will print
6378 when in that directory on that machine). If the host that contains
6379 the file system does not have this capability, you can specify another
6380 host as long as it can access the file system through @acronym{NFS}.
6382 If the list of file systems is very long you may wish to put it
6383 in a separate file. This file is usually named
6384 @file{/etc/backup/dirs}, but this name may be overridden in
6385 @file{backup-specs} using @code{DIRLIST} variable.
6388 @defvr {Backup variable} DIRLIST
6390 The name of the file that contains a list of file systems to backup
6391 or restore. By default it is @file{/etc/backup/dirs}.
6394 @defvr {Backup variable} BACKUP_FILES
6396 A list of individual files to be dumped (for @code{backup}), or restored
6397 (for @code{restore}). These should be accessible from the machine on
6398 which the backup script is run.
6400 If the list of individual files is very long you may wish to store it
6401 in a separate file. This file is usually named
6402 @file{/etc/backup/files}, but this name may be overridden in
6403 @file{backup-specs} using @code{FILELIST} variable.
6406 @defvr {Backup variable} FILELIST
6408 The name of the file that contains a list of individual files to backup
6409 or restore. By default it is @file{/etc/backup/files}.
6412 @defvr {Backup variable} MT
6414 Full file name of @command{mt} binary.
6417 @defvr {Backup variable} RSH
6419 Full file name of @command{rsh} binary or its equivalent. You may wish to
6420 set it to @code{ssh}, to improve security. In this case you will have
6421 to use public key authentication.
6424 @defvr {Backup variable} RSH_COMMAND
6426 Full file name of @command{rsh} binary on remote machines. This will
6427 be passed via @option{--rsh-command} option to the remote invocation
6431 @defvr {Backup variable} VOLNO_FILE
6433 Name of temporary file to hold volume numbers. This needs to be accessible
6434 by all the machines which have file systems to be dumped.
6437 @defvr {Backup variable} XLIST
6439 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
6440 located on the remote machine and containing the list of files to
6441 be excluded from the backup. Exclude file lists are searched in
6442 /etc/tar-backup directory. A common use for exclude file lists
6443 is to exclude files containing security-sensitive information
6444 (e.g., @file{/etc/shadow} from backups).
6446 This variable affects only @code{backup}.
6449 @defvr {Backup variable} SLEEP_TIME
6451 Time to sleep between dumps of any two successive file systems
6453 This variable affects only @code{backup}.
6456 @defvr {Backup variable} DUMP_REMIND_SCRIPT
6458 Script to be run when it's time to insert a new tape in for the next
6459 volume. Administrators may want to tailor this script for their site.
6460 If this variable isn't set, @GNUTAR{} will display its built-in
6461 prompt, and will expect confirmation from the console. For the
6462 description of the default prompt, see @ref{change volume prompt}.
6466 @defvr {Backup variable} SLEEP_MESSAGE
6468 Message to display on the terminal while waiting for dump time. Usually
6469 this will just be some literal text.
6472 @defvr {Backup variable} TAR
6474 Full file name of the @GNUTAR{} executable. If this is not set, backup
6475 scripts will search @command{tar} in the current shell path.
6478 @node Magnetic Tape Control
6479 @subsection Magnetic Tape Control
6481 Backup scripts access tape device using special @dfn{hook functions}.
6482 These functions take a single argument --- the name of the tape
6483 device. Their names are kept in the following variables:
6485 @defvr {Backup variable} MT_BEGIN
6486 The name of @dfn{begin} function. This function is called before
6487 accessing the drive. By default it retensions the tape:
6493 mt -f "$1" retension
6498 @defvr {Backup variable} MT_REWIND
6499 The name of @dfn{rewind} function. The default definition is as
6512 @defvr {Backup variable} MT_OFFLINE
6513 The name of the function switching the tape off line. By default
6514 it is defined as follows:
6517 MT_OFFLINE=mt_offline
6525 @defvr {Backup variable} MT_STATUS
6526 The name of the function used to obtain the status of the archive device,
6527 including error count. Default definition:
6539 @subsection User Hooks
6541 @dfn{User hooks} are shell functions executed before and after
6542 each @command{tar} invocation. Thus, there are @dfn{backup
6543 hooks}, which are executed before and after dumping each file
6544 system, and @dfn{restore hooks}, executed before and
6545 after restoring a file system. Each user hook is a shell function
6546 taking four arguments:
6548 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
6553 Current backup or restore level.
6556 Name or IP address of the host machine being dumped or restored.
6559 Full file name of the file system being dumped or restored.
6562 File system name with directory separators replaced with colons. This
6563 is useful, e.g., for creating unique files.
6567 Following variables keep the names of user hook functions:
6569 @defvr {Backup variable} DUMP_BEGIN
6570 Dump begin function. It is executed before dumping the file system.
6573 @defvr {Backup variable} DUMP_END
6574 Executed after dumping the file system.
6577 @defvr {Backup variable} RESTORE_BEGIN
6578 Executed before restoring the file system.
6581 @defvr {Backup variable} RESTORE_END
6582 Executed after restoring the file system.
6585 @node backup-specs example
6586 @subsection An Example Text of @file{Backup-specs}
6588 The following is an example of @file{backup-specs}:
6591 # site-specific parameters for file system backup.
6593 ADMINISTRATOR=friedman
6595 TAPE_FILE=/dev/nrsmt0
6597 # Use @code{ssh} instead of the less secure @code{rsh}
6599 RSH_COMMAND=/usr/bin/ssh
6601 # Override MT_STATUS function:
6607 # Disable MT_OFFLINE function
6624 apple-gunkies:/com/mailer/gnu
6625 apple-gunkies:/com/archive/gnu"
6627 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
6631 @node Scripted Backups
6632 @section Using the Backup Scripts
6634 The syntax for running a backup script is:
6637 backup --level=@var{level} --time=@var{time}
6640 The @option{--level} option requests the dump level. Thus, to produce
6641 a full dump, specify @code{--level=0} (this is the default, so
6642 @option{--level} may be omitted if its value is
6643 @code{0})@footnote{For backward compatibility, the @code{backup} will also
6644 try to deduce the requested dump level from the name of the
6645 script itself. If the name consists of a string @samp{level-}
6646 followed by a single decimal digit, that digit is taken as
6647 the dump level number. Thus, you may create a link from @code{backup}
6648 to @code{level-1} and then run @code{level-1} whenever you need to
6649 create a level one dump.}.
6651 The @option{--time} option determines when should the backup be
6652 run. @var{Time} may take three forms:
6655 @item @var{hh}:@var{mm}
6657 The dump must be run at @var{hh} hours @var{mm} minutes.
6661 The dump must be run at @var{hh} hours.
6665 The dump must be run immediately.
6668 You should start a script with a tape or disk mounted. Once you
6669 start a script, it prompts you for new tapes or disks as it
6670 needs them. Media volumes don't have to correspond to archive
6671 files --- a multi-volume archive can be started in the middle of a
6672 tape that already contains the end of another multi-volume archive.
6673 The @code{restore} script prompts for media by its archive volume,
6674 so to avoid an error message you should keep track of which tape
6675 (or disk) contains which volume of the archive (@pxref{Scripted
6678 The backup scripts write two files on the file system. The first is a
6679 record file in @file{/etc/tar-backup/}, which is used by the scripts
6680 to store and retrieve information about which files were dumped. This
6681 file is not meant to be read by humans, and should not be deleted by
6682 them. @xref{Snapshot Files}, for a more detailed explanation of this
6685 The second file is a log file containing the names of the file systems
6686 and files dumped, what time the backup was made, and any error
6687 messages that were generated, as well as how much space was left in
6688 the media volume after the last volume of the archive was written.
6689 You should check this log file after every backup. The file name is
6690 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
6691 represents current date, and @var{n} represents current dump level number.
6693 The script also prints the name of each system being dumped to the
6696 Following is the full list of options accepted by @code{backup}
6700 @item -l @var{level}
6701 @itemx --level=@var{level}
6702 Do backup level @var{level} (default 0).
6706 Force backup even if today's log file already exists.
6708 @item -v[@var{level}]
6709 @itemx --verbose[=@var{level}]
6710 Set verbosity level. The higher the level is, the more debugging
6711 information will be output during execution. Default @var{level}
6712 is 100, which means the highest debugging level.
6714 @item -t @var{start-time}
6715 @itemx --time=@var{start-time}
6716 Wait till @var{time}, then do backup.
6720 Display short help message and exit.
6724 Display information about the program's name, version, origin and legal
6725 status, all on standard output, and then exit successfully.
6729 @node Scripted Restoration
6730 @section Using the Restore Script
6732 To restore files that were archived using a scripted backup, use the
6733 @code{restore} script. Its usage is quite straightforward. In the
6734 simplest form, invoke @code{restore --all}, it will
6735 then restore all the file systems and files specified in
6736 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
6738 You may select the file systems (and/or files) to restore by
6739 giving @code{restore} a list of @dfn{patterns} in its command
6740 line. For example, running
6747 will restore all file systems on the machine @samp{albert}. A more
6748 complicated example:
6751 restore 'albert:*' '*:/var'
6755 This command will restore all file systems on the machine @samp{albert}
6756 as well as @file{/var} file system on all machines.
6758 By default @code{restore} will start restoring files from the lowest
6759 available dump level (usually zero) and will continue through
6760 all available dump levels. There may be situations where such a
6761 thorough restore is not necessary. For example, you may wish to
6762 restore only files from the recent level one backup. To do so,
6763 use @option{--level} option, as shown in the example below:
6769 The full list of options accepted by @code{restore} follows:
6774 Restore all file systems and files specified in @file{backup-specs}.
6776 @item -l @var{level}
6777 @itemx --level=@var{level}
6778 Start restoring from the given backup level, instead of the default 0.
6780 @item -v[@var{level}]
6781 @itemx --verbose[=@var{level}]
6782 Set verbosity level. The higher the level is, the more debugging
6783 information will be output during execution. Default @var{level}
6784 is 100, which means the highest debugging level.
6788 Display short help message and exit.
6792 Display information about the program's name, version, origin and legal
6793 status, all on standard output, and then exit successfully.
6796 You should start the restore script with the media containing the
6797 first volume of the archive mounted. The script will prompt for other
6798 volumes as they are needed. If the archive is on tape, you don't need
6799 to rewind the tape to to its beginning---if the tape head is
6800 positioned past the beginning of the archive, the script will rewind
6801 the tape as needed. @xref{Tape Positioning}, for a discussion of tape
6805 @strong{Warning:} The script will delete files from the active file
6806 system if they were not in the file system when the archive was made.
6809 @xref{Incremental Dumps}, for an explanation of how the script makes
6813 @chapter Choosing Files and Names for @command{tar}
6815 Certain options to @command{tar} enable you to specify a name for your
6816 archive. Other options let you decide which files to include or exclude
6817 from the archive, based on when or whether files were modified, whether
6818 the file names do or don't match specified patterns, or whether files
6819 are in specified directories.
6821 This chapter discusses these options in detail.
6824 * file:: Choosing the Archive's Name
6825 * Selecting Archive Members::
6826 * files:: Reading Names from a File
6827 * exclude:: Excluding Some Files
6828 * wildcards:: Wildcards Patterns and Matching
6829 * quoting styles:: Ways of Quoting Special Characters in Names
6830 * transform:: Modifying File and Member Names
6831 * after:: Operating Only on New Files
6832 * recurse:: Descending into Directories
6833 * one:: Crossing File System Boundaries
6837 @section Choosing and Naming Archive Files
6839 @cindex Naming an archive
6840 @cindex Archive Name
6841 @cindex Choosing an archive file
6842 @cindex Where is the archive?
6844 By default, @command{tar} uses an archive file name that was compiled when
6845 it was built on the system; usually this name refers to some physical
6846 tape drive on the machine. However, the person who installed @command{tar}
6847 on the system may not have set the default to a meaningful value as far as
6848 most users are concerned. As a result, you will usually want to tell
6849 @command{tar} where to find (or create) the archive. The
6850 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
6851 option allows you to either specify or name a file to use as the archive
6852 instead of the default archive file location.
6855 @xopindex{file, short description}
6856 @item --file=@var{archive-name}
6857 @itemx -f @var{archive-name}
6858 Name the archive to create or operate on. Use in conjunction with
6862 For example, in this @command{tar} command,
6865 $ @kbd{tar -cvf collection.tar blues folk jazz}
6869 @file{collection.tar} is the name of the archive. It must directly
6870 follow the @option{-f} option, since whatever directly follows @option{-f}
6871 @emph{will} end up naming the archive. If you neglect to specify an
6872 archive name, you may end up overwriting a file in the working directory
6873 with the archive you create since @command{tar} will use this file's name
6874 for the archive name.
6876 An archive can be saved as a file in the file system, sent through a
6877 pipe or over a network, or written to an I/O device such as a tape,
6878 floppy disk, or CD write drive.
6880 @cindex Writing new archives
6881 @cindex Archive creation
6882 If you do not name the archive, @command{tar} uses the value of the
6883 environment variable @env{TAPE} as the file name for the archive. If
6884 that is not available, @command{tar} uses a default, compiled-in archive
6885 name, usually that for tape unit zero (i.e., @file{/dev/tu00}).
6887 @cindex Standard input and output
6888 @cindex tar to standard input and output
6889 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
6890 archive from standard input (when listing or extracting files), or
6891 writes it to standard output (when creating an archive). If you use
6892 @file{-} as an @var{archive-name} when modifying an archive,
6893 @command{tar} reads the original archive from its standard input and
6894 writes the entire new archive to its standard output.
6896 The following example is a convenient way of copying directory
6897 hierarchy from @file{sourcedir} to @file{targetdir}.
6900 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
6903 The @option{-C} option allows to avoid using subshells:
6906 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
6909 In both examples above, the leftmost @command{tar} invocation archives
6910 the contents of @file{sourcedir} to the standard output, while the
6911 rightmost one reads this archive from its standard input and
6912 extracts it. The @option{-p} option tells it to restore permissions
6913 of the extracted files.
6915 @cindex Remote devices
6916 @cindex tar to a remote device
6918 To specify an archive file on a device attached to a remote machine,
6922 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
6926 @command{tar} will set up the remote connection, if possible, and
6927 prompt you for a username and password. If you use
6928 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
6929 will attempt to set up the remote connection using your username
6930 as the username on the remote machine.
6932 @cindex Local and remote archives
6933 @anchor{local and remote archives}
6934 If the archive file name includes a colon (@samp{:}), then it is assumed
6935 to be a file on another machine. If the archive file is
6936 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6937 host @var{host}. The remote host is accessed using the @command{rsh}
6938 program, with a username of @var{user}. If the username is omitted
6939 (along with the @samp{@@} sign), then your user name will be used.
6940 (This is the normal @command{rsh} behavior.) It is necessary for the
6941 remote machine, in addition to permitting your @command{rsh} access, to
6942 have the @file{rmt} program installed (this command is included in
6943 the @GNUTAR{} distribution and by default is installed under
6944 @file{@var{prefix}/libexec/rmt}, where @var{prefix} means your
6945 installation prefix). If you need to use a file whose name includes a
6946 colon, then the remote tape drive behavior
6947 can be inhibited by using the @option{--force-local} option.
6949 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6950 tries to minimize input and output operations. The Amanda backup
6951 system, when used with @GNUTAR{}, has an initial sizing pass which
6954 @node Selecting Archive Members
6955 @section Selecting Archive Members
6956 @cindex Specifying files to act on
6957 @cindex Specifying archive members
6959 @dfn{File Name arguments} specify which files in the file system
6960 @command{tar} operates on, when creating or adding to an archive, or which
6961 archive members @command{tar} operates on, when reading or deleting from
6962 an archive. @xref{Operations}.
6964 To specify file names, you can include them as the last arguments on
6965 the command line, as follows:
6967 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6970 If a file name begins with dash (@samp{-}), precede it with
6971 @option{--add-file} option to prevent it from being treated as an
6974 @anchor{input name quoting}
6975 By default @GNUTAR{} attempts to @dfn{unquote} each file or member
6976 name, replacing @dfn{escape sequences} according to the following
6979 @multitable @columnfractions 0.20 0.60
6980 @headitem Escape @tab Replaced with
6981 @item \a @tab Audible bell (@acronym{ASCII} 7)
6982 @item \b @tab Backspace (@acronym{ASCII} 8)
6983 @item \f @tab Form feed (@acronym{ASCII} 12)
6984 @item \n @tab New line (@acronym{ASCII} 10)
6985 @item \r @tab Carriage return (@acronym{ASCII} 13)
6986 @item \t @tab Horizontal tabulation (@acronym{ASCII} 9)
6987 @item \v @tab Vertical tabulation (@acronym{ASCII} 11)
6988 @item \? @tab @acronym{ASCII} 127
6989 @item \@var{n} @tab @acronym{ASCII} @var{n} (@var{n} should be an octal number
6993 A backslash followed by any other symbol is retained.
6995 This default behavior is controlled by the following command line
7001 Enable unquoting input file or member names (default).
7005 Disable unquoting input file or member names.
7008 If you specify a directory name as a file name argument, all the files
7009 in that directory are operated on by @command{tar}.
7011 If you do not specify files, @command{tar} behavior differs depending
7012 on the operation mode as described below:
7014 When @command{tar} is invoked with @option{--create} (@option{-c}),
7015 @command{tar} will stop immediately, reporting the following:
7019 $ @kbd{tar cf a.tar}
7020 tar: Cowardly refusing to create an empty archive
7021 Try 'tar --help' or 'tar --usage' for more information.
7025 If you specify either @option{--list} (@option{-t}) or
7026 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
7027 operates on all the archive members in the archive.
7029 If run with @option{--diff} option, tar will compare the archive with
7030 the contents of the current working directory.
7032 If you specify any other operation, @command{tar} does nothing.
7034 By default, @command{tar} takes file names from the command line. However,
7035 there are other ways to specify file or member names, or to modify the
7036 manner in which @command{tar} selects the files or members upon which to
7037 operate. In general, these methods work both for specifying the names
7038 of files and archive members.
7041 @section Reading Names from a File
7043 @cindex Reading file names from a file
7044 @cindex Lists of file names
7045 @cindex File Name arguments, alternatives
7046 @cindex @command{find}, using with @command{tar}
7047 Instead of giving the names of files or archive members on the command
7048 line, you can put the names into a file, and then use the
7049 @option{--files-from=@var{file-of-names}} (@option{-T
7050 @var{file-of-names}}) option to @command{tar}. Give the name of the
7051 file which contains the list of files to include as the argument to
7052 @option{--files-from}. In the list, the file names should be separated by
7053 newlines. You will frequently use this option when you have generated
7054 the list of files to archive with the @command{find} utility.
7058 @item --files-from=@var{file-name}
7059 @itemx -T @var{file-name}
7060 Get names to extract or create from file @var{file-name}.
7063 If you give a single dash as a file name for @option{--files-from}, (i.e.,
7064 you specify either @code{--files-from=-} or @code{-T -}), then the file
7065 names are read from standard input.
7067 Unless you are running @command{tar} with @option{--create}, you can not use
7068 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
7071 Any number of @option{-T} options can be given in the command line.
7073 The following example shows how to use @command{find} to generate a list of
7074 files smaller than 400K in length and put that list into a file
7075 called @file{small-files}. You can then use the @option{-T} option to
7076 @command{tar} to specify the files from that file, @file{small-files}, to
7077 create the archive @file{little.tgz}. (The @option{-z} option to
7078 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
7082 $ @kbd{find . -size -400 -print > small-files}
7083 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
7087 In the file list given by @option{-T} option, any file name beginning
7088 with @samp{-} character is considered a @command{tar} option and is
7089 processed accordingly@footnote{Versions of @GNUTAR{} up to 1.15.1
7090 recognized only @option{-C} option in file lists, and only if the
7091 option and its argument occupied two consecutive lines.}. For example,
7092 the common use of this feature is to change to another directory by
7093 specifying @option{-C} option:
7103 $ @kbd{tar -c -f foo.tar --files-from list}
7108 In this example, @command{tar} will first switch to @file{/etc}
7109 directory and add files @file{passwd} and @file{hosts} to the
7110 archive. Then it will change to @file{/lib} directory and will archive
7111 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
7116 $ @kbd{tar tf foo.tar}
7124 If you happen to have a file whose name starts with @samp{-},
7125 precede it with @option{--add-file} option to prevent it from
7126 being recognized as an option. For example: @code{--add-file=--my-file}.
7133 @subsection @code{NUL}-Terminated File Names
7135 @cindex File names, terminated by @code{NUL}
7136 @cindex @code{NUL}-terminated file names
7137 The @option{--null} option causes
7138 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
7139 to read file names terminated by a @code{NUL} instead of a newline, so
7140 files whose names contain newlines can be archived using
7141 @option{--files-from}.
7144 @xopindex{null, described}
7146 Only consider @code{NUL}-terminated file names, instead of files that
7147 terminate in a newline.
7149 @xopindex{no-null, described}
7151 Undo the effect of any previous @option{--null} option.
7154 The @option{--null} option is just like the one in @acronym{GNU}
7155 @command{xargs} and @command{cpio}, and is useful with the
7156 @option{-print0} predicate of @acronym{GNU} @command{find}. In
7157 @command{tar}, @option{--null} also disables special handling for
7158 file names that begin with dash.
7160 This example shows how to use @command{find} to generate a list of files
7161 larger than 800K in length and put that list into a file called
7162 @file{long-files}. The @option{-print0} option to @command{find} is just
7163 like @option{-print}, except that it separates files with a @code{NUL}
7164 rather than with a newline. You can then run @command{tar} with both the
7165 @option{--null} and @option{-T} options to specify that @command{tar} gets the
7166 files from that file, @file{long-files}, to create the archive
7167 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
7168 @command{tar} to recognize the @code{NUL} separator between files.
7171 $ @kbd{find . -size +800 -print0 > long-files}
7172 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
7175 The @option{--no-null} option can be used if you need to read both
7176 @code{NUL}-terminated and newline-terminated files on the same command line.
7177 For example, if @file{flist} is a newline-terminated file, then the
7178 following command can be used to combine it with the above command:
7182 $ @kbd{find . -size +800 -print0 |
7183 tar -c -f big.tar --null -T - --no-null -T flist}
7187 This example uses short options for typographic reasons, to avoid
7190 @GNUTAR is tries to automatically detect @code{NUL}-terminated file
7191 lists, so in many cases it is safe to use them even without the
7192 @option{--null} option. In this case @command{tar} will print a
7193 warning and continue reading such a file as if @option{--null} were
7198 $ @kbd{find . -size +800 -print0 | tar -c -f big.tar -T -}
7199 tar: -: file name read contains nul character
7203 The null terminator, however, remains in effect only for this
7204 particular file, any following @option{-T} options will assume
7205 newline termination. Of course, the null autodetection applies
7206 to these eventual surplus @option{-T} options as well.
7209 @section Excluding Some Files
7211 @cindex File names, excluding files by
7212 @cindex Excluding files by name and pattern
7213 @cindex Excluding files by file system
7215 @opindex exclude-from
7216 To avoid operating on files whose names match a particular pattern,
7217 use the @option{--exclude} or @option{--exclude-from} options.
7221 @item --exclude=@var{pattern}
7222 Causes @command{tar} to ignore files that match the @var{pattern}.
7226 The @option{--exclude=@var{pattern}} option prevents any file or
7227 member whose name matches the shell wildcard (@var{pattern}) from
7229 For example, to create an archive with all the contents of the directory
7230 @file{src} except for files whose names end in @file{.o}, use the
7231 command @samp{tar -cf src.tar --exclude='*.o' src}.
7233 You may give multiple @option{--exclude} options.
7236 @opindex exclude-from
7237 @item --exclude-from=@var{file}
7238 @itemx -X @var{file}
7239 Causes @command{tar} to ignore files that match the patterns listed in
7243 @findex exclude-from
7244 Use the @option{--exclude-from} option to read a
7245 list of patterns, one per line, from @var{file}; @command{tar} will
7246 ignore files matching those patterns. Thus if @command{tar} is
7247 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
7248 single line @file{*.o}, no files whose names end in @file{.o} will be
7249 added to the archive.
7251 Notice, that lines from @var{file} are read verbatim. One of the
7252 frequent errors is leaving some extra whitespace after a file name,
7253 which is difficult to catch using text editors.
7255 However, empty lines are OK.
7258 @cindex version control system, excluding files
7259 @cindex VCS, excluding files
7260 @cindex SCCS, excluding files
7261 @cindex RCS, excluding files
7262 @cindex CVS, excluding files
7263 @cindex SVN, excluding files
7264 @cindex git, excluding files
7265 @cindex Bazaar, excluding files
7266 @cindex Arch, excluding files
7267 @cindex Mercurial, excluding files
7268 @cindex Darcs, excluding files
7269 @opindex exclude-vcs
7271 Exclude files and directories used by following version control
7272 systems: @samp{CVS}, @samp{RCS}, @samp{SCCS}, @samp{SVN}, @samp{Arch},
7273 @samp{Bazaar}, @samp{Mercurial}, and @samp{Darcs}.
7275 As of version @value{VERSION}, the following files are excluded:
7278 @item @file{CVS/}, and everything under it
7279 @item @file{RCS/}, and everything under it
7280 @item @file{SCCS/}, and everything under it
7281 @item @file{.git/}, and everything under it
7282 @item @file{.gitignore}
7283 @item @file{.cvsignore}
7284 @item @file{.svn/}, and everything under it
7285 @item @file{.arch-ids/}, and everything under it
7286 @item @file{@{arch@}/}, and everything under it
7287 @item @file{=RELEASE-ID}
7288 @item @file{=meta-update}
7289 @item @file{=update}
7291 @item @file{.bzrignore}
7292 @item @file{.bzrtags}
7294 @item @file{.hgignore}
7295 @item @file{.hgrags}
7299 @opindex exclude-backups
7300 @item --exclude-backups
7301 Exclude backup and lock files. This option causes exclusion of files
7302 that match the following shell globbing patterns:
7312 @findex exclude-caches
7313 When creating an archive, the @option{--exclude-caches} option family
7314 causes @command{tar} to exclude all directories that contain a @dfn{cache
7315 directory tag}. A cache directory tag is a short file with the
7316 well-known name @file{CACHEDIR.TAG} and having a standard header
7317 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
7318 Various applications write cache directory tags into directories they
7319 use to hold regenerable, non-precious data, so that such data can be
7320 more easily excluded from backups.
7322 There are three @samp{exclude-caches} options, each providing a different
7323 exclusion semantics:
7326 @opindex exclude-caches
7327 @item --exclude-caches
7328 Do not archive the contents of the directory, but archive the
7329 directory itself and the @file{CACHEDIR.TAG} file.
7331 @opindex exclude-caches-under
7332 @item --exclude-caches-under
7333 Do not archive the contents of the directory, nor the
7334 @file{CACHEDIR.TAG} file, archive only the directory itself.
7336 @opindex exclude-caches-all
7337 @item --exclude-caches-all
7338 Omit directories containing @file{CACHEDIR.TAG} file entirely.
7342 Another option family, @option{--exclude-tag}, provides a generalization of
7343 this concept. It takes a single argument, a file name to look for.
7344 Any directory that contains this file will be excluded from the dump.
7345 Similarly to @samp{exclude-caches}, there are three options in this
7349 @opindex exclude-tag
7350 @item --exclude-tag=@var{file}
7351 Do not dump the contents of the directory, but dump the
7352 directory itself and the @var{file}.
7354 @opindex exclude-tag-under
7355 @item --exclude-tag-under=@var{file}
7356 Do not dump the contents of the directory, nor the
7357 @var{file}, archive only the directory itself.
7359 @opindex exclude-tag-all
7360 @item --exclude-tag-all=@var{file}
7361 Omit directories containing @var{file} file entirely.
7364 Multiple @option{--exclude-tag*} options can be given.
7366 For example, given this directory:
7381 The @option{--exclude-tag} will produce the following:
7384 $ @kbd{tar -cf archive.tar --exclude-tag=tagfile -v dir}
7389 tar: dir/folk/: contains a cache directory tag tagfile;
7394 Both the @file{dir/folk} directory and its tagfile are preserved in
7395 the archive, however the rest of files in this directory are not.
7397 Now, using the @option{--exclude-tag-under} option will exclude
7398 @file{tagfile} from the dump, while still preserving the directory
7399 itself, as shown in this example:
7402 $ @kbd{tar -cf archive.tar --exclude-tag-under=tagfile -v dir}
7407 ./tar: dir/folk/: contains a cache directory tag tagfile;
7411 Finally, using @option{--exclude-tag-all} omits the @file{dir/folk}
7415 $ @kbd{tar -cf archive.tar --exclude-tag-all=tagfile -v dir}
7419 ./tar: dir/folk/: contains a cache directory tag tagfile;
7420 directory not dumped
7424 * problems with exclude::
7427 @node problems with exclude
7428 @unnumberedsubsec Problems with Using the @code{exclude} Options
7430 @xopindex{exclude, potential problems with}
7431 Some users find @samp{exclude} options confusing. Here are some common
7436 The main operating mode of @command{tar} does not act on a file name
7437 explicitly listed on the command line, if one of its file name
7438 components is excluded. In the example above, if
7439 you create an archive and exclude files that end with @samp{*.o}, but
7440 explicitly name the file @samp{dir.o/foo} after all the options have been
7441 listed, @samp{dir.o/foo} will be excluded from the archive.
7444 You can sometimes confuse the meanings of @option{--exclude} and
7445 @option{--exclude-from}. Be careful: use @option{--exclude} when files
7446 to be excluded are given as a pattern on the command line. Use
7447 @option{--exclude-from} to introduce the name of a file which contains
7448 a list of patterns, one per line; each of these patterns can exclude
7449 zero, one, or many files.
7452 When you use @option{--exclude=@var{pattern}}, be sure to quote the
7453 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
7454 like @samp{*}. If you do not do this, the shell might expand the
7455 @samp{*} itself using files at hand, so @command{tar} might receive a
7456 list of files instead of one pattern, or none at all, making the
7457 command somewhat illegal. This might not correspond to what you want.
7462 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
7470 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
7474 You must use use shell syntax, or globbing, rather than @code{regexp}
7475 syntax, when using exclude options in @command{tar}. If you try to use
7476 @code{regexp} syntax to describe files to be excluded, your command
7480 @FIXME{The change in semantics must have occurred before 1.11,
7481 so I doubt if it is worth mentioning at all. Anyway, should at
7482 least specify in which version the semantics changed.}
7483 In earlier versions of @command{tar}, what is now the
7484 @option{--exclude-from} option was called @option{--exclude} instead.
7485 Now, @option{--exclude} applies to patterns listed on the command
7486 line and @option{--exclude-from} applies to patterns listed in a
7492 @section Wildcards Patterns and Matching
7494 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
7495 @samp{*} or @samp{?} for example, are replaced and expanded into all
7496 existing files matching the given pattern. @GNUTAR{} can use wildcard
7497 patterns for matching (or globbing) archive members when extracting
7498 from or listing an archive. Wildcard patterns are also used for
7499 verifying volume labels of @command{tar} archives. This section has the
7500 purpose of explaining wildcard syntax for @command{tar}.
7502 @FIXME{the next few paragraphs need work.}
7504 A @var{pattern} should be written according to shell syntax, using wildcard
7505 characters to effect globbing. Most characters in the pattern stand
7506 for themselves in the matched string, and case is significant: @samp{a}
7507 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
7508 pattern matches any single character in the matched string. The character
7509 @samp{*} in the pattern matches zero, one, or more single characters in
7510 the matched string. The character @samp{\} says to take the following
7511 character of the pattern @emph{literally}; it is useful when one needs to
7512 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
7514 The character @samp{[}, up to the matching @samp{]}, introduces a character
7515 class. A @dfn{character class} is a list of acceptable characters
7516 for the next single character of the matched string. For example,
7517 @samp{[abcde]} would match any of the first five letters of the alphabet.
7518 Note that within a character class, all of the ``special characters''
7519 listed above other than @samp{\} lose their special meaning; for example,
7520 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
7521 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
7522 the characters @samp{-} and @samp{]} must either come @emph{first} or
7523 @emph{last} in a character class.)
7525 @cindex Excluding characters from a character class
7526 @cindex Character class, excluding characters from
7527 If the first character of the class after the opening @samp{[}
7528 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
7529 Rather than listing character to match, it lists those characters which
7530 are @emph{forbidden} as the next single character of the matched string.
7532 Other characters of the class stand for themselves. The special
7533 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
7534 letters, is meant to represent all characters between @var{a} and
7537 @FIXME{need to add a sentence or so here to make this clear for those
7538 who don't have dan around.}
7540 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
7541 special for wildcard matches. However, if a pattern completely matches
7542 a directory prefix of a matched string, then it matches the full matched
7543 string: thus, excluding a directory also excludes all the files beneath it.
7546 * controlling pattern-matching::
7549 @node controlling pattern-matching
7550 @unnumberedsubsec Controlling Pattern-Matching
7552 For the purposes of this section, we call @dfn{exclusion members} all
7553 member names obtained while processing @option{--exclude} and
7554 @option{--exclude-from} options, and @dfn{inclusion members} those
7555 member names that were given in the command line or read from the file
7556 specified with @option{--files-from} option.
7558 These two pairs of member lists are used in the following operations:
7559 @option{--diff}, @option{--extract}, @option{--list},
7562 There are no inclusion members in create mode (@option{--create} and
7563 @option{--append}), since in this mode the names obtained from the
7564 command line refer to @emph{files}, not archive members.
7566 By default, inclusion members are compared with archive members
7567 literally @footnote{Notice that earlier @GNUTAR{} versions used
7568 globbing for inclusion members, which contradicted to UNIX98
7569 specification and was not documented. @xref{Changes}, for more
7570 information on this and other changes.} and exclusion members are
7571 treated as globbing patterns. For example:
7575 $ @kbd{tar tf foo.tar}
7580 # @i{Member names are used verbatim:}
7581 $ @kbd{tar -xf foo.tar -v '[remarks]'}
7583 # @i{Exclude member names are globbed:}
7584 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
7590 This behavior can be altered by using the following options:
7595 Treat all member names as wildcards.
7597 @opindex no-wildcards
7598 @item --no-wildcards
7599 Treat all member names as literal strings.
7602 Thus, to extract files whose names end in @samp{.c}, you can use:
7605 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
7611 Notice quoting of the pattern to prevent the shell from interpreting
7614 The effect of @option{--wildcards} option is canceled by
7615 @option{--no-wildcards}. This can be used to pass part of
7616 the command line arguments verbatim and other part as globbing
7617 patterns. For example, the following invocation:
7620 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
7624 instructs @command{tar} to extract from @file{foo.tar} all files whose
7625 names end in @samp{.txt} and the file named @file{[remarks]}.
7627 Normally, a pattern matches a name if an initial subsequence of the
7628 name's components matches the pattern, where @samp{*}, @samp{?}, and
7629 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
7630 and wildcards can match @samp{/}.
7632 Other than optionally stripping leading @samp{/} from names
7633 (@pxref{absolute}), patterns and names are used as-is. For
7634 example, trailing @samp{/} is not trimmed from a user-specified name
7635 before deciding whether to exclude it.
7637 However, this matching procedure can be altered by the options listed
7638 below. These options accumulate. For example:
7641 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
7645 ignores case when excluding @samp{makefile}, but not when excluding
7650 @opindex no-anchored
7652 @itemx --no-anchored
7653 If anchored, a pattern must match an initial subsequence
7654 of the name's components. Otherwise, the pattern can match any
7655 subsequence. Default is @option{--no-anchored} for exclusion members
7656 and @option{--anchored} inclusion members.
7658 @opindex ignore-case
7659 @opindex no-ignore-case
7661 @itemx --no-ignore-case
7662 When ignoring case, upper-case patterns match lower-case names and vice versa.
7663 When not ignoring case (the default), matching is case-sensitive.
7665 @opindex wildcards-match-slash
7666 @opindex no-wildcards-match-slash
7667 @item --wildcards-match-slash
7668 @itemx --no-wildcards-match-slash
7669 When wildcards match slash (the default for exclusion members), a
7670 wildcard like @samp{*} in the pattern can match a @samp{/} in the
7671 name. Otherwise, @samp{/} is matched only by @samp{/}.
7675 The @option{--recursion} and @option{--no-recursion} options
7676 (@pxref{recurse}) also affect how member patterns are interpreted. If
7677 recursion is in effect, a pattern matches a name if it matches any of
7678 the name's parent directories.
7680 The following table summarizes pattern-matching default values:
7682 @multitable @columnfractions .3 .7
7683 @headitem Members @tab Default settings
7684 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
7685 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
7688 @node quoting styles
7689 @section Quoting Member Names
7691 When displaying member names, @command{tar} takes care to avoid
7692 ambiguities caused by certain characters. This is called @dfn{name
7693 quoting}. The characters in question are:
7696 @item Non-printable control characters:
7697 @anchor{escape sequences}
7698 @multitable @columnfractions 0.20 0.10 0.60
7699 @headitem Character @tab @acronym{ASCII} @tab Character name
7700 @item \a @tab 7 @tab Audible bell
7701 @item \b @tab 8 @tab Backspace
7702 @item \f @tab 12 @tab Form feed
7703 @item \n @tab 10 @tab New line
7704 @item \r @tab 13 @tab Carriage return
7705 @item \t @tab 9 @tab Horizontal tabulation
7706 @item \v @tab 11 @tab Vertical tabulation
7709 @item Space (@acronym{ASCII} 32)
7711 @item Single and double quotes (@samp{'} and @samp{"})
7713 @item Backslash (@samp{\})
7716 The exact way @command{tar} uses to quote these characters depends on
7717 the @dfn{quoting style}. The default quoting style, called
7718 @dfn{escape} (see below), uses backslash notation to represent control
7719 characters, space and backslash. Using this quoting style, control
7720 characters are represented as listed in column @samp{Character} in the
7721 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
7723 @GNUTAR{} offers seven distinct quoting styles, which can be selected
7724 using @option{--quoting-style} option:
7727 @item --quoting-style=@var{style}
7728 @opindex quoting-style
7730 Sets quoting style. Valid values for @var{style} argument are:
7731 literal, shell, shell-always, c, escape, locale, clocale.
7734 These styles are described in detail below. To illustrate their
7735 effect, we will use an imaginary tar archive @file{arch.tar}
7736 containing the following members:
7740 # 1. Contains horizontal tabulation character.
7742 # 2. Contains newline character
7745 # 3. Contains a space
7747 # 4. Contains double quotes
7749 # 5. Contains single quotes
7751 # 6. Contains a backslash character:
7756 Here is how usual @command{ls} command would have listed them, if they
7757 had existed in the current working directory:
7775 No quoting, display each character as is:
7779 $ @kbd{tar tf arch.tar --quoting-style=literal}
7792 Display characters the same way Bourne shell does:
7793 control characters, except @samp{\t} and @samp{\n}, are printed using
7794 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
7795 single quote is printed as @samp{\'}. If a name contains any quoted
7796 characters, it is enclosed in single quotes. In particular, if a name
7797 contains single quotes, it is printed as several single-quoted strings:
7801 $ @kbd{tar tf arch.tar --quoting-style=shell}
7804 './a'\''single'\''quote'
7814 Same as @samp{shell}, but the names are always enclosed in single
7819 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
7822 './a'\''single'\''quote'
7832 Use the notation of the C programming language. All names are
7833 enclosed in double quotes. Control characters are quoted using
7834 backslash notations, double quotes are represented as @samp{\"},
7835 backslash characters are represented as @samp{\\}. Single quotes and
7836 spaces are not quoted:
7840 $ @kbd{tar tf arch.tar --quoting-style=c}
7844 "./a\"double\"quote"
7852 Control characters are printed using backslash notation, a space is
7853 printed as @samp{\ } and a backslash as @samp{\\}. This is the
7854 default quoting style, unless it was changed when configured the
7859 $ @kbd{tar tf arch.tar --quoting-style=escape}
7871 Control characters, single quote and backslash are printed using
7872 backslash notation. All names are quoted using left and right
7873 quotation marks, appropriate to the current locale. If it does not
7874 define quotation marks, use @samp{'} as left and as right
7875 quotation marks. Any occurrences of the right quotation mark in a
7876 name are escaped with @samp{\}, for example:
7882 $ @kbd{tar tf arch.tar --quoting-style=locale}
7885 './a\'single\'quote'
7894 Same as @samp{locale}, but @samp{"} is used for both left and right
7895 quotation marks, if not provided by the currently selected locale:
7899 $ @kbd{tar tf arch.tar --quoting-style=clocale}
7903 "./a\"double\"quote"
7911 You can specify which characters should be quoted in addition to those
7912 implied by the current quoting style:
7915 @item --quote-chars=@var{string}
7916 Always quote characters from @var{string}, even if the selected
7917 quoting style would not quote them.
7920 For example, using @samp{escape} quoting (compare with the usual
7921 escape listing above):
7925 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
7936 To disable quoting of such additional characters, use the following
7940 @item --no-quote-chars=@var{string}
7941 Remove characters listed in @var{string} from the list of quoted
7942 characters set by the previous @option{--quote-chars} option.
7945 This option is particularly useful if you have added
7946 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
7947 and wish to disable it for the current invocation.
7949 Note, that @option{--no-quote-chars} does @emph{not} disable those
7950 characters that are quoted by default in the selected quoting style.
7953 @section Modifying File and Member Names
7955 @command{Tar} archives contain detailed information about files stored
7956 in them and full file names are part of that information. When
7957 storing a file to an archive, its file name is recorded in it,
7958 along with the actual file contents. When restoring from an archive,
7959 a file is created on disk with exactly the same name as that stored
7960 in the archive. In the majority of cases this is the desired behavior
7961 of a file archiver. However, there are some cases when it is not.
7963 First of all, it is often unsafe to extract archive members with
7964 absolute file names or those that begin with a @file{../}. @GNUTAR{}
7965 takes special precautions when extracting such names and provides a
7966 special option for handling them, which is described in
7969 Secondly, you may wish to extract file names without some leading
7970 directory components, or with otherwise modified names. In other
7971 cases it is desirable to store files under differing names in the
7974 @GNUTAR{} provides several options for these needs.
7977 @opindex strip-components
7978 @item --strip-components=@var{number}
7979 Strip given @var{number} of leading components from file names before
7983 For example, suppose you have archived whole @file{/usr} hierarchy to
7984 a tar archive named @file{usr.tar}. Among other files, this archive
7985 contains @file{usr/include/stdlib.h}, which you wish to extract to
7986 the current working directory. To do so, you type:
7989 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
7992 The option @option{--strip=2} instructs @command{tar} to strip the
7993 two leading components (@file{usr/} and @file{include/}) off the file
7996 If you add the @option{--verbose} (@option{-v}) option to the invocation
7997 above, you will note that the verbose listing still contains the
7998 full file name, with the two removed components still in place. This
7999 can be inconvenient, so @command{tar} provides a special option for
8000 altering this behavior:
8002 @anchor{show-transformed-names}
8004 @opindex show-transformed-names
8005 @item --show-transformed-names
8006 Display file or member names with all requested transformations
8015 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
8016 usr/include/stdlib.h
8017 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
8022 Notice that in both cases the file @file{stdlib.h} is extracted to the
8023 current working directory, @option{--show-transformed-names} affects
8024 only the way its name is displayed.
8026 This option is especially useful for verifying whether the invocation
8027 will have the desired effect. Thus, before running
8030 $ @kbd{tar -x --strip=@var{n}}
8034 it is often advisable to run
8037 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
8041 to make sure the command will produce the intended results.
8043 In case you need to apply more complex modifications to the file name,
8044 @GNUTAR{} provides a general-purpose transformation option:
8049 @item --transform=@var{expression}
8050 @itemx --xform=@var{expression}
8051 Modify file names using supplied @var{expression}.
8055 The @var{expression} is a @command{sed}-like replace expression of the
8059 s/@var{regexp}/@var{replace}/[@var{flags}]
8063 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
8064 replacement for each file name part that matches @var{regexp}. Both
8065 @var{regexp} and @var{replace} are described in detail in
8066 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
8068 Any delimiter can be used in lieu of @samp{/}, the only requirement being
8069 that it be used consistently throughout the expression. For example,
8070 the following two expressions are equivalent:
8079 Changing delimiters is often useful when the @var{regex} contains
8080 slashes. For example, it is more convenient to write @code{s,/,-,} than
8083 As in @command{sed}, you can give several replace expressions,
8084 separated by a semicolon.
8086 Supported @var{flags} are:
8090 Apply the replacement to @emph{all} matches to the @var{regexp}, not
8094 Use case-insensitive matching.
8097 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
8098 regexps, Extended regular expressions, Extended regular expressions,
8102 Only replace the @var{number}th match of the @var{regexp}.
8104 Note: the @acronym{POSIX} standard does not specify what should happen
8105 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
8106 follows the GNU @command{sed} implementation in this regard, so
8107 the interaction is defined to be: ignore matches before the
8108 @var{number}th, and then match and replace all matches from the
8113 In addition, several @dfn{transformation scope} flags are supported,
8114 that control to what files transformations apply. These are:
8118 Apply transformation to regular archive members.
8121 Do not apply transformation to regular archive members.
8124 Apply transformation to symbolic link targets.
8127 Do not apply transformation to symbolic link targets.
8130 Apply transformation to hard link targets.
8133 Do not apply transformation to hard link targets.
8136 Default is @samp{rsh}, which means to apply tranformations to both archive
8137 members and targets of symbolic and hard links.
8139 Default scope flags can also be changed using @samp{flags=} statement
8140 in the transform expression. The flags set this way remain in force
8141 until next @samp{flags=} statement or end of expression, whichever
8142 occurs first. For example:
8145 --transform 'flags=S;s|^|/usr/local/|'
8148 Here are several examples of @option{--transform} usage:
8151 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
8154 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
8157 @item Strip two leading directory components (equivalent to
8158 @option{--strip-components=2}):
8161 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
8164 @item Convert each file name to lower case:
8167 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
8170 @item Prepend @file{/prefix/} to each file name:
8173 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
8176 @item Archive the @file{/lib} directory, prepending @samp{/usr/local}
8177 to each archive member:
8180 $ @kbd{tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib}
8184 Notice the use of flags in the last example. The @file{/lib}
8185 directory often contains many symbolic links to files within it.
8186 It may look, for example, like this:
8190 drwxr-xr-x root/root 0 2008-07-08 16:20 /lib/
8191 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
8192 lrwxrwxrwx root/root 0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
8196 Using the expression @samp{s,^,/usr/local/,} would mean adding
8197 @samp{/usr/local} to both regular archive members and to link
8198 targets. In this case, @file{/lib/libc.so.6} would become:
8201 /usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so
8204 This is definitely not desired. To avoid this, the @samp{S} flag
8205 is used, which excludes symbolic link targets from filename
8206 transformations. The result is:
8209 $ @kbd{tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \
8210 --show-transformed /lib}
8211 drwxr-xr-x root/root 0 2008-07-08 16:20 /usr/local/lib/
8212 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
8213 lrwxrwxrwx root/root 0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
8217 Unlike @option{--strip-components}, @option{--transform} can be used
8218 in any @GNUTAR{} operation mode. For example, the following command
8219 adds files to the archive while replacing the leading @file{usr/}
8220 component with @file{var/}:
8223 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
8226 To test @option{--transform} effect we suggest using
8227 @option{--show-transformed-names} option:
8230 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
8231 --verbose --show-transformed-names /}
8234 If both @option{--strip-components} and @option{--transform} are used
8235 together, then @option{--transform} is applied first, and the required
8236 number of components is then stripped from its result.
8238 You can use as many @option{--transform} options in a single command
8239 line as you want. The specified expressions will then be applied in
8240 order of their appearance. For example, the following two invocations
8244 $ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
8245 --transform='s,/usr/local,/usr/,'}
8246 $ @kbd{tar -cf arch.tar \
8247 --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
8251 @section Operating Only on New Files
8253 @cindex Excluding file by age
8254 @cindex Data Modification time, excluding files by
8255 @cindex Modification time, excluding files by
8256 @cindex Age, excluding files by
8257 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
8258 @option{-N @var{date}}) option causes @command{tar} to only work on
8259 files whose data modification or status change times are newer than
8260 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
8261 it is taken to be a file name; the data modification time of that file
8262 is used as the date. If you use this option when creating or appending
8263 to an archive, the archive will only include new files. If you use
8264 @option{--after-date} when extracting an archive, @command{tar} will
8265 only extract files newer than the @var{date} you specify.
8267 If you only want @command{tar} to make the date comparison based on
8268 modification of the file's data (rather than status
8269 changes), then use the @option{--newer-mtime=@var{date}} option.
8271 @cindex --after-date and --update compared
8272 @cindex --newer-mtime and --update compared
8273 You may use these options with any operation. Note that these options
8274 differ from the @option{--update} (@option{-u}) operation in that they
8275 allow you to specify a particular date against which @command{tar} can
8276 compare when deciding whether or not to archive the files.
8281 @item --after-date=@var{date}
8282 @itemx --newer=@var{date}
8283 @itemx -N @var{date}
8284 Only store files newer than @var{date}.
8286 Acts on files only if their data modification or status change times are
8287 later than @var{date}. Use in conjunction with any operation.
8289 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
8290 name; the data modification time of that file is used as the date.
8292 @opindex newer-mtime
8293 @item --newer-mtime=@var{date}
8294 Acts like @option{--after-date}, but only looks at data modification times.
8297 These options limit @command{tar} to operate only on files which have
8298 been modified after the date specified. A file's status is considered to have
8299 changed if its contents have been modified, or if its owner,
8300 permissions, and so forth, have been changed. (For more information on
8301 how to specify a date, see @ref{Date input formats}; remember that the
8302 entire date argument must be quoted if it contains any spaces.)
8304 Gurus would say that @option{--after-date} tests both the data
8305 modification time (@code{mtime}, the time the contents of the file
8306 were last modified) and the status change time (@code{ctime}, the time
8307 the file's status was last changed: owner, permissions, etc.@:)
8308 fields, while @option{--newer-mtime} tests only the @code{mtime}
8311 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
8312 @code{ctime} and processes the file if either one is more recent than
8313 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
8314 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
8315 contents of the file were looked at).
8317 Date specifiers can have embedded spaces. Because of this, you may need
8318 to quote date arguments to keep the shell from parsing them as separate
8319 arguments. For example, the following command will add to the archive
8320 all the files modified less than two days ago:
8323 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
8326 When any of these options is used with the option @option{--verbose}
8327 (@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
8328 date back to its textual representation and compare that with the
8329 one given with the option. If the two dates differ, @command{tar} will
8330 print a warning saying what date it will use. This is to help user
8331 ensure he is using the right date. For example:
8335 $ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
8336 tar: Option --after-date: Treating date '10 days ago' as 2006-06-11
8342 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
8343 should not be used for incremental backups. @xref{Incremental Dumps},
8344 for proper way of creating incremental backups.
8348 @section Descending into Directories
8349 @cindex Avoiding recursion in directories
8350 @cindex Descending directories, avoiding
8351 @cindex Directories, avoiding recursion
8352 @cindex Recursion in directories, avoiding
8354 Usually, @command{tar} will recursively explore all directories (either
8355 those given on the command line or through the @option{--files-from}
8356 option) for the various files they contain. However, you may not always
8357 want @command{tar} to act this way.
8359 @opindex no-recursion
8360 @cindex @command{find}, using with @command{tar}
8361 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
8362 into specified directories. If you specify @option{--no-recursion}, you can
8363 use the @command{find} (@pxref{Top,, find, find, GNU Find Manual})
8364 utility for hunting through levels of directories to
8365 construct a list of file names which you could then pass to @command{tar}.
8366 @command{find} allows you to be more selective when choosing which files to
8367 archive; see @ref{files}, for more information on using @command{find} with
8371 @item --no-recursion
8372 Prevents @command{tar} from recursively descending directories.
8376 Requires @command{tar} to recursively descend directories.
8377 This is the default.
8380 When you use @option{--no-recursion}, @GNUTAR{} grabs
8381 directory entries themselves, but does not descend on them
8382 recursively. Many people use @command{find} for locating files they
8383 want to back up, and since @command{tar} @emph{usually} recursively
8384 descends on directories, they have to use the @samp{@w{-not -type d}}
8385 test in their @command{find} invocation (@pxref{Type, Type, Type test,
8386 find, Finding Files}), as they usually do not want all the files in a
8387 directory. They then use the @option{--files-from} option to archive
8388 the files located via @command{find}.
8390 The problem when restoring files archived in this manner is that the
8391 directories themselves are not in the archive; so the
8392 @option{--same-permissions} (@option{--preserve-permissions},
8393 @option{-p}) option does not affect them---while users might really
8394 like it to. Specifying @option{--no-recursion} is a way to tell
8395 @command{tar} to grab only the directory entries given to it, adding
8396 no new files on its own. To summarize, if you use @command{find} to
8397 create a list of files to be stored in an archive, use it as follows:
8401 $ @kbd{find @var{dir} @var{tests} | \
8402 tar -cf @var{archive} -T - --no-recursion}
8406 The @option{--no-recursion} option also applies when extracting: it
8407 causes @command{tar} to extract only the matched directory entries, not
8408 the files under those directories.
8410 The @option{--no-recursion} option also affects how globbing patterns
8411 are interpreted (@pxref{controlling pattern-matching}).
8413 The @option{--no-recursion} and @option{--recursion} options apply to
8414 later options and operands, and can be overridden by later occurrences
8415 of @option{--no-recursion} and @option{--recursion}. For example:
8418 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
8422 creates an archive with one entry for @file{grape}, and the recursive
8423 contents of @file{grape/concord}, but no entries under @file{grape}
8424 other than @file{grape/concord}.
8427 @section Crossing File System Boundaries
8428 @cindex File system boundaries, not crossing
8430 @command{tar} will normally automatically cross file system boundaries in
8431 order to archive files which are part of a directory tree. You can
8432 change this behavior by running @command{tar} and specifying
8433 @option{--one-file-system}. This option only affects files that are
8434 archived because they are in a directory that is being archived;
8435 @command{tar} will still archive files explicitly named on the command line
8436 or through @option{--files-from}, regardless of where they reside.
8439 @opindex one-file-system
8440 @item --one-file-system
8441 Prevents @command{tar} from crossing file system boundaries when
8442 archiving. Use in conjunction with any write operation.
8445 The @option{--one-file-system} option causes @command{tar} to modify its
8446 normal behavior in archiving the contents of directories. If a file in
8447 a directory is not on the same file system as the directory itself, then
8448 @command{tar} will not archive that file. If the file is a directory
8449 itself, @command{tar} will not archive anything beneath it; in other words,
8450 @command{tar} will not cross mount points.
8452 This option is useful for making full or incremental archival backups of
8453 a file system. If this option is used in conjunction with
8454 @option{--verbose} (@option{-v}), files that are excluded are
8455 mentioned by name on the standard error.
8458 * directory:: Changing Directory
8459 * absolute:: Absolute File Names
8463 @subsection Changing the Working Directory
8465 @FIXME{need to read over this node now for continuity; i've switched
8466 things around some.}
8468 @cindex Changing directory mid-stream
8469 @cindex Directory, changing mid-stream
8470 @cindex Working directory, specifying
8471 To change the working directory in the middle of a list of file names,
8472 either on the command line or in a file specified using
8473 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
8474 This will change the working directory to the specified directory
8475 after that point in the list.
8479 @item --directory=@var{directory}
8480 @itemx -C @var{directory}
8481 Changes the working directory in the middle of a command line.
8487 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
8491 will place the files @file{grape} and @file{prune} from the current
8492 directory into the archive @file{jams.tar}, followed by the file
8493 @file{cherry} from the directory @file{food}. This option is especially
8494 useful when you have several widely separated files that you want to
8495 store in the same archive.
8497 Note that the file @file{cherry} is recorded in the archive under the
8498 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
8499 archive will contain three files that all appear to have come from the
8500 same directory; if the archive is extracted with plain @samp{tar
8501 --extract}, all three files will be written in the current directory.
8503 Contrast this with the command,
8506 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
8510 which records the third file in the archive under the name
8511 @file{red/cherry} so that, if the archive is extracted using
8512 @samp{tar --extract}, the third file will be written in a subdirectory
8515 You can use the @option{--directory} option to make the archive
8516 independent of the original name of the directory holding the files.
8517 The following command places the files @file{/etc/passwd},
8518 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
8522 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
8526 However, the names of the archive members will be exactly what they were
8527 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
8528 They will not appear to be related by file name to the original
8529 directories where those files were located.
8531 Note that @option{--directory} options are interpreted consecutively. If
8532 @option{--directory} specifies a relative file name, it is interpreted
8533 relative to the then current directory, which might not be the same as
8534 the original current working directory of @command{tar}, due to a previous
8535 @option{--directory} option.
8537 When using @option{--files-from} (@pxref{files}), you can put various
8538 @command{tar} options (including @option{-C}) in the file list. Notice,
8539 however, that in this case the option and its argument may not be
8540 separated by whitespace. If you use short option, its argument must
8541 either follow the option letter immediately, without any intervening
8542 whitespace, or occupy the next line. Otherwise, if you use long
8543 option, separate its argument by an equal sign.
8545 For instance, the file list for the above example will be:
8558 To use it, you would invoke @command{tar} as follows:
8561 $ @kbd{tar -c -f foo.tar --files-from list}
8564 The interpretation of @option{--directory} is disabled by
8565 @option{--null} option.
8568 @subsection Absolute File Names
8569 @cindex absolute file names
8570 @cindex file names, absolute
8572 By default, @GNUTAR{} drops a leading @samp{/} on
8573 input or output, and complains about file names containing a @file{..}
8574 component. There is an option that turns off this behavior:
8577 @opindex absolute-names
8578 @item --absolute-names
8580 Do not strip leading slashes from file names, and permit file names
8581 containing a @file{..} file name component.
8584 When @command{tar} extracts archive members from an archive, it strips any
8585 leading slashes (@samp{/}) from the member name. This causes absolute
8586 member names in the archive to be treated as relative file names. This
8587 allows you to have such members extracted wherever you want, instead of
8588 being restricted to extracting the member in the exact directory named
8589 in the archive. For example, if the archive member has the name
8590 @file{/etc/passwd}, @command{tar} will extract it as if the name were
8591 really @file{etc/passwd}.
8593 File names containing @file{..} can cause problems when extracting, so
8594 @command{tar} normally warns you about such files when creating an
8595 archive, and rejects attempts to extracts such files.
8597 Other @command{tar} programs do not do this. As a result, if you
8598 create an archive whose member names start with a slash, they will be
8599 difficult for other people with a non-@GNUTAR{}
8600 program to use. Therefore, @GNUTAR{} also strips
8601 leading slashes from member names when putting members into the
8602 archive. For example, if you ask @command{tar} to add the file
8603 @file{/bin/ls} to an archive, it will do so, but the member name will
8604 be @file{bin/ls}@footnote{A side effect of this is that when
8605 @option{--create} is used with @option{--verbose} the resulting output
8606 is not, generally speaking, the same as the one you'd get running
8607 @kbd{tar --list} command. This may be important if you use some
8608 scripts for comparing both outputs. @xref{listing member and file names},
8609 for the information on how to handle this case.}.
8611 Symbolic links containing @file{..} or leading @samp{/} can also cause
8612 problems when extracting, so @command{tar} normally extracts them last;
8613 it may create empty files as placeholders during extraction.
8615 If you use the @option{--absolute-names} (@option{-P}) option,
8616 @command{tar} will do none of these transformations.
8618 To archive or extract files relative to the root directory, specify
8619 the @option{--absolute-names} (@option{-P}) option.
8621 Normally, @command{tar} acts on files relative to the working
8622 directory---ignoring superior directory names when archiving, and
8623 ignoring leading slashes when extracting.
8625 When you specify @option{--absolute-names} (@option{-P}),
8626 @command{tar} stores file names including all superior directory
8627 names, and preserves leading slashes. If you only invoked
8628 @command{tar} from the root directory you would never need the
8629 @option{--absolute-names} option, but using this option
8630 may be more convenient than switching to root.
8632 @FIXME{Should be an example in the tutorial/wizardry section using this
8633 to transfer files between systems.}
8636 @item --absolute-names
8637 Preserves full file names (including superior directory names) when
8638 archiving and extracting files.
8642 @command{tar} prints out a message about removing the @samp{/} from
8643 file names. This message appears once per @GNUTAR{}
8644 invocation. It represents something which ought to be told; ignoring
8645 what it means can cause very serious surprises, later.
8647 Some people, nevertheless, do not want to see this message. Wanting to
8648 play really dangerously, one may of course redirect @command{tar} standard
8649 error to the sink. For example, under @command{sh}:
8652 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
8656 Another solution, both nicer and simpler, would be to change to
8657 the @file{/} directory first, and then avoid absolute notation.
8661 $ @kbd{tar -c -f archive.tar -C / home}
8664 @xref{Integrity}, for some of the security-related implications
8665 of using this option.
8667 @include parse-datetime.texi
8670 @chapter Controlling the Archive Format
8672 @cindex Tar archive formats
8673 Due to historical reasons, there are several formats of tar archives.
8674 All of them are based on the same principles, but have some subtle
8675 differences that often make them incompatible with each other.
8677 GNU tar is able to create and handle archives in a variety of formats.
8678 The most frequently used formats are (in alphabetical order):
8682 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
8683 from an early @acronym{POSIX} standard, adding some improvements such as
8684 sparse file handling and incremental archives. Unfortunately these
8685 features were implemented in a way incompatible with other archive
8688 Archives in @samp{gnu} format are able to hold file names of unlimited
8692 Format used by @GNUTAR{} of versions prior to 1.12.
8695 Archive format, compatible with the V7 implementation of tar. This
8696 format imposes a number of limitations. The most important of them
8700 @item The maximum length of a file name is limited to 99 characters.
8701 @item The maximum length of a symbolic link is limited to 99 characters.
8702 @item It is impossible to store special files (block and character
8703 devices, fifos etc.)
8704 @item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
8706 @item V7 archives do not contain symbolic ownership information (user
8707 and group name of the file owner).
8710 This format has traditionally been used by Automake when producing
8711 Makefiles. This practice will change in the future, in the meantime,
8712 however this means that projects containing file names more than 99
8713 characters long will not be able to use @GNUTAR{} @value{VERSION} and
8714 Automake prior to 1.9.
8717 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
8718 symbolic ownership information. It is also able to store
8719 special files. However, it imposes several restrictions as well:
8722 @item The maximum length of a file name is limited to 256 characters,
8723 provided that the file name can be split at a directory separator in
8724 two parts, first of them being at most 155 bytes long. So, in most
8725 cases the maximum file name length will be shorter than 256
8727 @item The maximum length of a symbolic link name is limited to
8729 @item Maximum size of a file the archive is able to accommodate
8731 @item Maximum value of UID/GID is 2097151.
8732 @item Maximum number of bits in device major and minor numbers is 21.
8736 Format used by J@"org Schilling @command{star}
8737 implementation. @GNUTAR{} is able to read @samp{star} archives but
8738 currently does not produce them.
8741 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
8742 most flexible and feature-rich format. It does not impose any
8743 restrictions on file sizes or file name lengths. This format is quite
8744 recent, so not all tar implementations are able to handle it properly.
8745 However, this format is designed in such a way that any tar
8746 implementation able to read @samp{ustar} archives will be able to read
8747 most @samp{posix} archives as well, with the only exception that any
8748 additional information (such as long file names etc.) will in such
8749 case be extracted as plain text files along with the files it refers to.
8751 This archive format will be the default format for future versions
8756 The following table summarizes the limitations of each of these
8759 @multitable @columnfractions .10 .20 .20 .20 .20
8760 @headitem Format @tab UID @tab File Size @tab File Name @tab Devn
8761 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8762 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8763 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
8764 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
8765 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
8768 The default format for @GNUTAR{} is defined at compilation
8769 time. You may check it by running @command{tar --help}, and examining
8770 the last lines of its output. Usually, @GNUTAR{} is configured
8771 to create archives in @samp{gnu} format, however, future version will
8772 switch to @samp{posix}.
8775 * Compression:: Using Less Space through Compression
8776 * Attributes:: Handling File Attributes
8777 * Portability:: Making @command{tar} Archives More Portable
8778 * cpio:: Comparison of @command{tar} and @command{cpio}
8782 @section Using Less Space through Compression
8785 * gzip:: Creating and Reading Compressed Archives
8786 * sparse:: Archiving Sparse Files
8790 @subsection Creating and Reading Compressed Archives
8791 @cindex Compressed archives
8792 @cindex Storing archives in compressed format
8800 @GNUTAR{} is able to create and read compressed archives. It supports
8801 a wide variety of compression programs, namely: @command{gzip},
8802 @command{bzip2}, @command{lzip}, @command{lzma}, @command{lzop},
8803 @command{xz} and traditional @command{compress}. The latter is
8804 supported mostly for backward compatibility, and we recommend
8805 against using it, because it is by far less effective than the other
8806 compression programs@footnote{It also had patent problems in the past.}.
8808 Creating a compressed archive is simple: you just specify a
8809 @dfn{compression option} along with the usual archive creation
8810 commands. The compression option is @option{-z} (@option{--gzip}) to
8811 create a @command{gzip} compressed archive, @option{-j}
8812 (@option{--bzip2}) to create a @command{bzip2} compressed archive,
8813 @option{--lzip} to create an @asis{lzip} compressed archive,
8814 @option{-J} (@option{--xz}) to create an @asis{XZ} archive,
8815 @option{--lzma} to create an @asis{LZMA} compressed
8816 archive, @option{--lzop} to create an @asis{LSOP} archive, and
8817 @option{-Z} (@option{--compress}) to use @command{compress} program.
8821 $ @kbd{tar czf archive.tar.gz .}
8824 You can also let @GNUTAR{} select the compression program based on
8825 the suffix of the archive file name. This is done using
8826 @option{--auto-compress} (@option{-a}) command line option. For
8827 example, the following invocation will use @command{bzip2} for
8831 $ @kbd{tar caf archive.tar.bz2 .}
8835 whereas the following one will use @command{lzma}:
8838 $ @kbd{tar caf archive.tar.lzma .}
8841 For a complete list of file name suffixes recognized by @GNUTAR{},
8842 see @ref{auto-compress}.
8844 Reading compressed archive is even simpler: you don't need to specify
8845 any additional options as @GNUTAR{} recognizes its format
8846 automatically. Thus, the following commands will list and extract the
8847 archive created in previous example:
8850 # List the compressed archive
8851 $ @kbd{tar tf archive.tar.gz}
8852 # Extract the compressed archive
8853 $ @kbd{tar xf archive.tar.gz}
8856 The format recognition algorithm is based on @dfn{signatures}, a
8857 special byte sequences in the beginning of file, that are specific for
8858 certain compression formats. If this approach fails, @command{tar}
8859 falls back to using archive name suffix to determine its format
8860 (@pxref{auto-compress}, for a list of recognized suffixes).
8862 @anchor{alternative decompression programs}
8863 @cindex alternative decompression programs
8864 Some compression programs are able to handle different compression
8865 formats. @GNUTAR{} uses this, if the principal decompressor for the
8866 given format is not available. For example, if @command{compress} is
8867 not installed, @command{tar} will try to use @command{gzip}. As of
8868 version @value{VERSION} the following alternatives are
8869 tried@footnote{To verbosely trace the decompressor selection, use the
8870 @option{--warning=decompress-program} option
8871 (@pxref{warnings,decompress-program}).}:
8873 @multitable @columnfractions 0.3 0.3 0.3
8874 @headitem Format @tab Main decompressor @tab Alternatives
8875 @item compress @tab compress @tab gzip
8876 @item lzma @tab lzma @tab xz
8877 @item bzip2 @tab bzip2 @tab lbzip2
8880 The only case when you have to specify a decompression option while
8881 reading the archive is when reading from a pipe or from a tape drive
8882 that does not support random access. However, in this case @GNUTAR{}
8883 will indicate which option you should use. For example:
8886 $ @kbd{cat archive.tar.gz | tar tf -}
8887 tar: Archive is compressed. Use -z option
8888 tar: Error is not recoverable: exiting now
8891 If you see such diagnostics, just add the suggested option to the
8892 invocation of @GNUTAR{}:
8895 $ @kbd{cat archive.tar.gz | tar tzf -}
8898 Notice also, that there are several restrictions on operations on
8899 compressed archives. First of all, compressed archives cannot be
8900 modified, i.e., you cannot update (@option{--update}, alias @option{-u})
8901 them or delete (@option{--delete}) members from them or
8902 add (@option{--append}, alias @option{-r}) members to them. Likewise, you
8903 cannot append another @command{tar} archive to a compressed archive using
8904 @option{--concatenate} (@option{-A}). Secondly, multi-volume
8905 archives cannot be compressed.
8907 The following options allow to select a particular compressor program:
8915 Filter the archive through @command{gzip}.
8920 Filter the archive through @code{xz}.
8924 Filter the archive through @code{bzip2}.
8928 Filter the archive through @command{lzip}.
8932 Filter the archive through @command{lzma}.
8936 Filter the archive through @command{lzop}.
8943 Filter the archive through @command{compress}.
8946 When any of these options is given, @GNUTAR{} searches the compressor
8947 binary in the current path and invokes it. The name of the compressor
8948 program is specified at compilation time using a corresponding
8949 @option{--with-@var{compname}} option to @command{configure}, e.g.
8950 @option{--with-bzip2} to select a specific @command{bzip2} binary.
8951 @xref{lbzip2}, for a detailed discussion.
8953 The output produced by @command{tar --help} shows the actual
8954 compressor names along with each of these options.
8956 You can use any of these options on physical devices (tape drives,
8957 etc.) and remote files as well as on normal files; data to or from
8958 such devices or remote files is reblocked by another copy of the
8959 @command{tar} program to enforce the specified (or default) record
8960 size. The default compression parameters are used. Most compression
8961 programs let you override these by setting a program-specific
8962 environment variable. For example, with @command{gzip} you can set
8966 $ @kbd{GZIP='-9 -n' tar czf archive.tar.gz subdir}
8968 Another way would be to use the @option{-I} option instead (see
8972 $ @kbd{tar -cf archive.tar.gz -I 'gzip -9 -n' subdir}
8976 Finally, the third, traditional, way to do this is to use a pipe:
8979 $ @kbd{tar cf - subdir | gzip -9 -n > archive.tar.gz}
8982 @cindex corrupted archives
8983 Compressed archives are easily corrupted, because compressed files
8984 have little redundancy. The adaptive nature of the
8985 compression scheme means that the compression tables are implicitly
8986 spread all over the archive. If you lose a few blocks, the dynamic
8987 construction of the compression tables becomes unsynchronized, and there
8988 is little chance that you could recover later in the archive.
8990 Other compression options provide better control over creating
8991 compressed archives. These are:
8994 @anchor{auto-compress}
8995 @opindex auto-compress
8996 @item --auto-compress
8998 Select a compression program to use by the archive file name
8999 suffix. The following suffixes are recognized:
9001 @multitable @columnfractions 0.3 0.6
9002 @headitem Suffix @tab Compression program
9003 @item @samp{.gz} @tab @command{gzip}
9004 @item @samp{.tgz} @tab @command{gzip}
9005 @item @samp{.taz} @tab @command{gzip}
9006 @item @samp{.Z} @tab @command{compress}
9007 @item @samp{.taZ} @tab @command{compress}
9008 @item @samp{.bz2} @tab @command{bzip2}
9009 @item @samp{.tz2} @tab @command{bzip2}
9010 @item @samp{.tbz2} @tab @command{bzip2}
9011 @item @samp{.tbz} @tab @command{bzip2}
9012 @item @samp{.lz} @tab @command{lzip}
9013 @item @samp{.lzma} @tab @command{lzma}
9014 @item @samp{.tlz} @tab @command{lzma}
9015 @item @samp{.lzo} @tab @command{lzop}
9016 @item @samp{.xz} @tab @command{xz}
9019 @anchor{use-compress-program}
9020 @opindex use-compress-program
9021 @item --use-compress-program=@var{command}
9022 @itemx -I=@var{command}
9023 Use external compression program @var{command}. Use this option if you
9024 are not happy with the compression program associated with the suffix
9025 at compile time or if you have a compression program that @GNUTAR{}
9026 does not support. The @var{command} argument is a valid command
9027 invocation, as you would type it at the command line prompt, with any
9028 additional options as needed. Enclose it in quotes if it contains
9029 white space (see @ref{external, Running External Commands}, for more detail).
9031 The @var{command} should follow two conventions:
9033 First, when invoked without additional options, it should read data
9034 from standard input, compress it and output it on standard output.
9036 Secondly, if invoked with the additional @option{-d} option, it should
9037 do exactly the opposite, i.e., read the compressed data from the
9038 standard input and produce uncompressed data on the standard output.
9040 The latter requirement means that you must not use the @option{-d}
9041 option as a part of the @var{command} itself.
9044 @cindex gpg, using with tar
9045 @cindex gnupg, using with tar
9046 @cindex Using encrypted archives
9047 The @option{--use-compress-program} option, in particular, lets you
9048 implement your own filters, not necessarily dealing with
9049 compression/decompression. For example, suppose you wish to implement
9050 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
9051 gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
9052 Manual}). The following script does that:
9058 -d) gpg --decrypt - | gzip -d -c;;
9059 '') gzip -c | gpg -s;;
9060 *) echo "Unknown option $1">&2; exit 1;;
9065 Suppose you name it @file{gpgz} and save it somewhere in your
9066 @env{PATH}. Then the following command will create a compressed
9067 archive signed with your private key:
9070 $ @kbd{tar -cf foo.tar.gpgz -Igpgz .}
9074 Likewise, the command below will list its contents:
9077 $ @kbd{tar -tf foo.tar.gpgz -Igpgz .}
9081 The above is based on the following discussion:
9083 I have one question, or maybe it's a suggestion if there isn't a way
9084 to do it now. I would like to use @option{--gzip}, but I'd also like
9085 the output to be fed through a program like @acronym{GNU}
9086 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
9087 to use :-)), basically adding ECC protection on top of compression.
9088 It seems as if this should be quite easy to do, but I can't work out
9089 exactly how to go about it. Of course, I can pipe the standard output
9090 of @command{tar} through @command{ecc}, but then I lose (though I
9091 haven't started using it yet, I confess) the ability to have
9092 @command{tar} use @command{rmt} for it's I/O (I think).
9094 I think the most straightforward thing would be to let me specify a
9095 general set of filters outboard of compression (preferably ordered,
9096 so the order can be automatically reversed on input operations, and
9097 with the options they require specifiable), but beggars shouldn't be
9098 choosers and anything you decide on would be fine with me.
9100 By the way, I like @command{ecc} but if (as the comments say) it can't
9101 deal with loss of block sync, I'm tempted to throw some time at adding
9102 that capability. Supposing I were to actually do such a thing and
9103 get it (apparently) working, do you accept contributed changes to
9104 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
9106 Isn't that exactly the role of the
9107 @option{--use-compress-prog=@var{program}} option?
9108 I never tried it myself, but I suspect you may want to write a
9109 @var{prog} script or program able to filter stdin to stdout to
9110 way you want. It should recognize the @option{-d} option, for when
9111 extraction is needed rather than creation.
9113 It has been reported that if one writes compressed data (through the
9114 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
9115 the DLT compression mode, the data will actually get bigger and one will
9116 end up with less space on the tape.
9120 * lbzip2:: Using lbzip2 with @GNUTAR{}.
9124 @subsubsection Using lbzip2 with @GNUTAR{}.
9126 @cindex Laszlo Ersek
9127 @command{Lbzip2} is a multithreaded utility for handling
9128 @samp{bzip2} compression, written by Laszlo Ersek. It makes use of
9129 multiple processors to speed up its operation and in general works
9130 considerably faster than @command{bzip2}. For a detailed description
9131 of @command{lbzip2} see @uref{http://freshmeat.net/@/projects/@/lbzip2} and
9132 @uref{http://www.linuxinsight.com/@/lbzip2-parallel-bzip2-utility.html,
9133 lbzip2: parallel bzip2 utility}.
9135 Recent versions of @command{lbzip2} are mostly command line compatible
9136 with @command{bzip2}, which makes it possible to automatically invoke
9137 it via the @option{--bzip2} @GNUTAR{} command line option. To do so,
9138 @GNUTAR{} must be configured with the @option{--with-bzip2} command
9139 line option, like this:
9142 $ @kbd{./configure --with-bzip2=lbzip2 [@var{other-options}]}
9145 Once configured and compiled this way, @command{tar --help} will show the
9150 $ @kbd{tar --help | grep -- --bzip2}
9151 -j, --bzip2 filter the archive through lbzip2
9156 which means that running @command{tar --bzip2} will invoke @command{lbzip2}.
9159 @subsection Archiving Sparse Files
9160 @cindex Sparse Files
9162 Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
9163 in a file is a section of the file's contents which was never written.
9164 The contents of a hole reads as all zeros. On many operating systems,
9165 actual disk storage is not allocated for holes, but they are counted
9166 in the length of the file. If you archive such a file, @command{tar}
9167 could create an archive longer than the original. To have @command{tar}
9168 attempt to recognize the holes in a file, use @option{--sparse}
9169 (@option{-S}). When you use this option, then, for any file using
9170 less disk space than would be expected from its length, @command{tar}
9171 searches the file for consecutive stretches of zeros. It then records
9172 in the archive for the file where the consecutive stretches of zeros
9173 are, and only archives the ``real contents'' of the file. On
9174 extraction (using @option{--sparse} is not needed on extraction) any
9175 such files have holes created wherever the continuous stretches of zeros
9176 were found. Thus, if you use @option{--sparse}, @command{tar} archives
9177 won't take more space than the original.
9183 This option instructs @command{tar} to test each file for sparseness
9184 before attempting to archive it. If the file is found to be sparse it
9185 is treated specially, thus allowing to decrease the amount of space
9186 used by its image in the archive.
9188 This option is meaningful only when creating or updating archives. It
9189 has no effect on extraction.
9192 Consider using @option{--sparse} when performing file system backups,
9193 to avoid archiving the expanded forms of files stored sparsely in the
9196 Even if your system has no sparse files currently, some may be
9197 created in the future. If you use @option{--sparse} while making file
9198 system backups as a matter of course, you can be assured the archive
9199 will never take more space on the media than the files take on disk
9200 (otherwise, archiving a disk filled with sparse files might take
9201 hundreds of tapes). @xref{Incremental Dumps}.
9203 However, be aware that @option{--sparse} option presents a serious
9204 drawback. Namely, in order to determine if the file is sparse
9205 @command{tar} has to read it before trying to archive it, so in total
9206 the file is read @strong{twice}. So, always bear in mind that the
9207 time needed to process all files with this option is roughly twice
9208 the time needed to archive them without it.
9209 @FIXME{A technical note:
9211 Programs like @command{dump} do not have to read the entire file; by
9212 examining the file system directly, they can determine in advance
9213 exactly where the holes are and thus avoid reading through them. The
9214 only data it need read are the actual allocated data blocks.
9215 @GNUTAR{} uses a more portable and straightforward
9216 archiving approach, it would be fairly difficult that it does
9217 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
9221 What I did say is that you cannot tell the difference between a hole and an
9222 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
9223 best tells you how many holes there are; it doesn't tell you @emph{where}.
9224 Just as programs may, conceivably, care what @code{st_blocks} is (care
9225 to name one that does?), they may also care where the holes are (I have
9226 no examples of this one either, but it's equally imaginable).
9228 I conclude from this that good archivers are not portable. One can
9229 arguably conclude that if you want a portable program, you can in good
9230 conscience restore files with as many holes as possible, since you can't
9235 @cindex sparse formats, defined
9236 When using @samp{POSIX} archive format, @GNUTAR{} is able to store
9237 sparse files using in three distinct ways, called @dfn{sparse
9238 formats}. A sparse format is identified by its @dfn{number},
9239 consisting, as usual of two decimal numbers, delimited by a dot. By
9240 default, format @samp{1.0} is used. If, for some reason, you wish to
9241 use an earlier format, you can select it using
9242 @option{--sparse-version} option.
9245 @opindex sparse-version
9246 @item --sparse-version=@var{version}
9248 Select the format to store sparse files in. Valid @var{version} values
9249 are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
9250 for a detailed description of each format.
9253 Using @option{--sparse-format} option implies @option{--sparse}.
9256 @section Handling File Attributes
9257 @cindex atrributes, files
9258 @cindex file attributes
9260 When @command{tar} reads files, it updates their access times. To
9261 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
9262 reset the access time retroactively or avoid changing it in the first
9266 @opindex atime-preserve
9267 @item --atime-preserve
9268 @itemx --atime-preserve=replace
9269 @itemx --atime-preserve=system
9270 Preserve the access times of files that are read. This works only for
9271 files that you own, unless you have superuser privileges.
9273 @option{--atime-preserve=replace} works on most systems, but it also
9274 restores the data modification time and updates the status change
9275 time. Hence it doesn't interact with incremental dumps nicely
9276 (@pxref{Incremental Dumps}), and it can set access or data modification times
9277 incorrectly if other programs access the file while @command{tar} is
9280 @option{--atime-preserve=system} avoids changing the access time in
9281 the first place, if the operating system supports this.
9282 Unfortunately, this may or may not work on any given operating system
9283 or file system. If @command{tar} knows for sure it won't work, it
9284 complains right away.
9286 Currently @option{--atime-preserve} with no operand defaults to
9287 @option{--atime-preserve=replace}, but this is intended to change to
9288 @option{--atime-preserve=system} when the latter is better-supported.
9293 Do not extract data modification time.
9295 When this option is used, @command{tar} leaves the data modification times
9296 of the files it extracts as the times when the files were extracted,
9297 instead of setting it to the times recorded in the archive.
9299 This option is meaningless with @option{--list} (@option{-t}).
9303 Create extracted files with the same ownership they have in the
9306 This is the default behavior for the superuser,
9307 so this option is meaningful only for non-root users, when @command{tar}
9308 is executed on those systems able to give files away. This is
9309 considered as a security flaw by many people, at least because it
9310 makes quite difficult to correctly account users for the disk space
9311 they occupy. Also, the @code{suid} or @code{sgid} attributes of
9312 files are easily and silently lost when files are given away.
9314 When writing an archive, @command{tar} writes the user @acronym{ID} and user name
9315 separately. If it can't find a user name (because the user @acronym{ID} is not
9316 in @file{/etc/passwd}), then it does not write one. When restoring,
9317 it tries to look the name (if one was written) up in
9318 @file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
9319 the archive instead.
9321 @opindex no-same-owner
9322 @item --no-same-owner
9324 Do not attempt to restore ownership when extracting. This is the
9325 default behavior for ordinary users, so this option has an effect
9326 only for the superuser.
9328 @opindex numeric-owner
9329 @item --numeric-owner
9330 The @option{--numeric-owner} option allows (ANSI) archives to be written
9331 without user/group name information or such information to be ignored
9332 when extracting. It effectively disables the generation and/or use
9333 of user/group name information. This option forces extraction using
9334 the numeric ids from the archive, ignoring the names.
9336 This is useful in certain circumstances, when restoring a backup from
9337 an emergency floppy with different passwd/group files for example.
9338 It is otherwise impossible to extract files with the right ownerships
9339 if the password file in use during the extraction does not match the
9340 one belonging to the file system(s) being extracted. This occurs,
9341 for example, if you are restoring your files after a major crash and
9342 had booted from an emergency floppy with no password file or put your
9343 disk into another machine to do the restore.
9345 The numeric ids are @emph{always} saved into @command{tar} archives.
9346 The identifying names are added at create time when provided by the
9347 system, unless @option{--format=oldgnu} is used. Numeric ids could be
9348 used when moving archives between a collection of machines using
9349 a centralized management for attribution of numeric ids to users
9350 and groups. This is often made through using the NIS capabilities.
9352 When making a @command{tar} file for distribution to other sites, it
9353 is sometimes cleaner to use a single owner for all files in the
9354 distribution, and nicer to specify the write permission bits of the
9355 files as stored in the archive independently of their actual value on
9356 the file system. The way to prepare a clean distribution is usually
9357 to have some Makefile rule creating a directory, copying all needed
9358 files in that directory, then setting ownership and permissions as
9359 wanted (there are a lot of possible schemes), and only then making a
9360 @command{tar} archive out of this directory, before cleaning
9361 everything out. Of course, we could add a lot of options to
9362 @GNUTAR{} for fine tuning permissions and ownership.
9363 This is not the good way, I think. @GNUTAR{} is
9364 already crowded with options and moreover, the approach just explained
9365 gives you a great deal of control already.
9367 @xopindex{same-permissions, short description}
9368 @xopindex{preserve-permissions, short description}
9370 @itemx --same-permissions
9371 @itemx --preserve-permissions
9372 Extract all protection information.
9374 This option causes @command{tar} to set the modes (access permissions) of
9375 extracted files exactly as recorded in the archive. If this option
9376 is not used, the current @code{umask} setting limits the permissions
9377 on extracted files. This option is by default enabled when
9378 @command{tar} is executed by a superuser.
9381 This option is meaningless with @option{--list} (@option{-t}).
9385 Same as both @option{--same-permissions} and @option{--same-order}.
9387 This option is deprecated, and will be removed in @GNUTAR{} version 1.23.
9392 @section Making @command{tar} Archives More Portable
9394 Creating a @command{tar} archive on a particular system that is meant to be
9395 useful later on many other machines and with other versions of @command{tar}
9396 is more challenging than you might think. @command{tar} archive formats
9397 have been evolving since the first versions of Unix. Many such formats
9398 are around, and are not always compatible with each other. This section
9399 discusses a few problems, and gives some advice about making @command{tar}
9400 archives more portable.
9402 One golden rule is simplicity. For example, limit your @command{tar}
9403 archives to contain only regular files and directories, avoiding
9404 other kind of special files. Do not attempt to save sparse files or
9405 contiguous files as such. Let's discuss a few more problems, in turn.
9407 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
9408 archives and archive labels) in GNU and PAX formats.}
9411 * Portable Names:: Portable Names
9412 * dereference:: Symbolic Links
9413 * hard links:: Hard Links
9414 * old:: Old V7 Archives
9415 * ustar:: Ustar Archives
9416 * gnu:: GNU and old GNU format archives.
9417 * posix:: @acronym{POSIX} archives
9418 * Checksumming:: Checksumming Problems
9419 * Large or Negative Values:: Large files, negative time stamps, etc.
9420 * Other Tars:: How to Extract GNU-Specific Data Using
9421 Other @command{tar} Implementations
9424 @node Portable Names
9425 @subsection Portable Names
9427 Use portable file and member names. A name is portable if it contains
9428 only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
9429 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
9430 contain @samp{/-}. Avoid deep directory nesting. For portability to
9431 old Unix hosts, limit your file name components to 14 characters or
9434 If you intend to have your @command{tar} archives to be read under
9435 MSDOS, you should not rely on case distinction for file names, and you
9436 might use the @acronym{GNU} @command{doschk} program for helping you
9437 further diagnosing illegal MSDOS names, which are even more limited
9441 @subsection Symbolic Links
9442 @cindex File names, using symbolic links
9443 @cindex Symbolic link as file name
9445 @opindex dereference
9446 Normally, when @command{tar} archives a symbolic link, it writes a
9447 block to the archive naming the target of the link. In that way, the
9448 @command{tar} archive is a faithful record of the file system contents.
9449 When @option{--dereference} (@option{-h}) is used with
9450 @option{--create} (@option{-c}), @command{tar} archives the files
9451 symbolic links point to, instead of
9452 the links themselves.
9454 When creating portable archives, use @option{--dereference}
9455 (@option{-h}): some systems do not support
9456 symbolic links, and moreover, your distribution might be unusable if
9457 it contains unresolved symbolic links.
9459 When reading from an archive, the @option{--dereference} (@option{-h})
9460 option causes @command{tar} to follow an already-existing symbolic
9461 link when @command{tar} writes or reads a file named in the archive.
9462 Ordinarily, @command{tar} does not follow such a link, though it may
9463 remove the link before writing a new file. @xref{Dealing with Old
9466 The @option{--dereference} option is unsafe if an untrusted user can
9467 modify directories while @command{tar} is running. @xref{Security}.
9470 @subsection Hard Links
9471 @cindex File names, using hard links
9472 @cindex hard links, dereferencing
9473 @cindex dereferencing hard links
9475 Normally, when @command{tar} archives a hard link, it writes a
9476 block to the archive naming the target of the link (a @samp{1} type
9477 block). In that way, the actual file contents is stored in file only
9478 once. For example, consider the following two files:
9483 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
9484 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
9488 Here, @file{jeden} is a link to @file{one}. When archiving this
9489 directory with a verbose level 2, you will get an output similar to
9493 $ tar cvvf ../archive.tar .
9494 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9495 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9496 hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
9499 The last line shows that, instead of storing two copies of the file,
9500 @command{tar} stored it only once, under the name @file{jeden}, and
9501 stored file @file{one} as a hard link to this file.
9503 It may be important to know that all hard links to the given file are
9504 stored in the archive. For example, this may be necessary for exact
9505 reproduction of the file system. The following option does that:
9508 @xopindex{check-links, described}
9511 Check the number of links dumped for each processed file. If this
9512 number does not match the total number of hard links for the file, print
9516 For example, trying to archive only file @file{jeden} with this option
9517 produces the following diagnostics:
9520 $ tar -c -f ../archive.tar -l jeden
9521 tar: Missing links to 'jeden'.
9524 Although creating special records for hard links helps keep a faithful
9525 record of the file system contents and makes archives more compact, it
9526 may present some difficulties when extracting individual members from
9527 the archive. For example, trying to extract file @file{one} from the
9528 archive created in previous examples produces, in the absense of file
9532 $ tar xf archive.tar ./one
9533 tar: ./one: Cannot hard link to './jeden': No such file or directory
9534 tar: Error exit delayed from previous errors
9537 The reason for this behavior is that @command{tar} cannot seek back in
9538 the archive to the previous member (in this case, @file{one}), to
9539 extract it@footnote{There are plans to fix this in future releases.}.
9540 If you wish to avoid such problems at the cost of a bigger archive,
9541 use the following option:
9544 @xopindex{hard-dereference, described}
9545 @item --hard-dereference
9546 Dereference hard links and store the files they refer to.
9549 For example, trying this option on our two sample files, we get two
9550 copies in the archive, each of which can then be extracted
9551 independently of the other:
9555 $ tar -c -vv -f ../archive.tar --hard-dereference .
9556 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9557 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9558 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
9563 @subsection Old V7 Archives
9564 @cindex Format, old style
9565 @cindex Old style format
9566 @cindex Old style archives
9567 @cindex v7 archive format
9569 Certain old versions of @command{tar} cannot handle additional
9570 information recorded by newer @command{tar} programs. To create an
9571 archive in V7 format (not ANSI), which can be read by these old
9572 versions, specify the @option{--format=v7} option in
9573 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
9574 accepts @option{--portability} or @option{--old-archive} for this
9575 option). When you specify it,
9576 @command{tar} leaves out information about directories, pipes, fifos,
9577 contiguous files, and device files, and specifies file ownership by
9578 group and user IDs instead of group and user names.
9580 When updating an archive, do not use @option{--format=v7}
9581 unless the archive was created using this option.
9583 In most cases, a @emph{new} format archive can be read by an @emph{old}
9584 @command{tar} program without serious trouble, so this option should
9585 seldom be needed. On the other hand, most modern @command{tar}s are
9586 able to read old format archives, so it might be safer for you to
9587 always use @option{--format=v7} for your distributions. Notice,
9588 however, that @samp{ustar} format is a better alternative, as it is
9589 free from many of @samp{v7}'s drawbacks.
9592 @subsection Ustar Archive Format
9594 @cindex ustar archive format
9595 Archive format defined by @acronym{POSIX}.1-1988 specification is called
9596 @code{ustar}. Although it is more flexible than the V7 format, it
9597 still has many restrictions (@pxref{Formats,ustar}, for the detailed
9598 description of @code{ustar} format). Along with V7 format,
9599 @code{ustar} format is a good choice for archives intended to be read
9600 with other implementations of @command{tar}.
9602 To create archive in @code{ustar} format, use @option{--format=ustar}
9603 option in conjunction with the @option{--create} (@option{-c}).
9606 @subsection @acronym{GNU} and old @GNUTAR{} format
9608 @cindex GNU archive format
9609 @cindex Old GNU archive format
9610 @GNUTAR{} was based on an early draft of the
9611 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
9612 @command{tar}, such as the support for file names longer than 100
9613 characters, use portions of the @command{tar} header record which were
9614 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
9615 @acronym{POSIX} have allocated the same parts of the header record for
9616 other purposes. As a result, @GNUTAR{} format is
9617 incompatible with the current @acronym{POSIX} specification, and with
9618 @command{tar} programs that follow it.
9620 In the majority of cases, @command{tar} will be configured to create
9621 this format by default. This will change in future releases, since
9622 we plan to make @samp{POSIX} format the default.
9624 To force creation a @GNUTAR{} archive, use option
9625 @option{--format=gnu}.
9628 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
9630 @cindex POSIX archive format
9631 @cindex PAX archive format
9632 Starting from version 1.14 @GNUTAR{} features full support for
9633 @acronym{POSIX.1-2001} archives.
9635 A @acronym{POSIX} conformant archive will be created if @command{tar}
9636 was given @option{--format=posix} (@option{--format=pax}) option. No
9637 special option is required to read and extract from a @acronym{POSIX}
9641 * PAX keywords:: Controlling Extended Header Keywords.
9645 @subsubsection Controlling Extended Header Keywords
9649 @item --pax-option=@var{keyword-list}
9650 Handle keywords in @acronym{PAX} extended headers. This option is
9651 equivalent to @option{-o} option of the @command{pax} utility.
9654 @var{Keyword-list} is a comma-separated
9655 list of keyword options, each keyword option taking one of
9656 the following forms:
9659 @item delete=@var{pattern}
9660 When used with one of archive-creation commands,
9661 this option instructs @command{tar} to omit from extended header records
9662 that it produces any keywords matching the string @var{pattern}.
9664 When used in extract or list mode, this option instructs tar
9665 to ignore any keywords matching the given @var{pattern} in the extended
9666 header records. In both cases, matching is performed using the pattern
9667 matching notation described in @acronym{POSIX 1003.2}, 3.13
9668 (@pxref{wildcards}). For example:
9671 --pax-option delete=security.*
9674 would suppress security-related information.
9676 @item exthdr.name=@var{string}
9678 This keyword allows user control over the name that is written into the
9679 ustar header blocks for the extended headers. The name is obtained
9680 from @var{string} after making the following substitutions:
9682 @multitable @columnfractions .25 .55
9683 @headitem Meta-character @tab Replaced By
9684 @item %d @tab The directory name of the file, equivalent to the
9685 result of the @command{dirname} utility on the translated file name.
9686 @item %f @tab The name of the file with the directory information
9687 stripped, equivalent to the result of the @command{basename} utility
9688 on the translated file name.
9689 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9690 @item %% @tab A @samp{%} character.
9693 Any other @samp{%} characters in @var{string} produce undefined
9696 If no option @samp{exthdr.name=string} is specified, @command{tar}
9697 will use the following default value:
9703 @item exthdr.mtime=@var{value}
9705 This keyword defines the value of the @samp{mtime} field that
9706 is written into the ustar header blocks for the extended headers.
9707 By default, the @samp{mtime} field is set to the modification time
9708 of the archive member described by that extended headers.
9710 @item globexthdr.name=@var{string}
9711 This keyword allows user control over the name that is written into
9712 the ustar header blocks for global extended header records. The name
9713 is obtained from the contents of @var{string}, after making
9714 the following substitutions:
9716 @multitable @columnfractions .25 .55
9717 @headitem Meta-character @tab Replaced By
9718 @item %n @tab An integer that represents the
9719 sequence number of the global extended header record in the archive,
9721 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9722 @item %% @tab A @samp{%} character.
9725 Any other @samp{%} characters in @var{string} produce undefined results.
9727 If no option @samp{globexthdr.name=string} is specified, @command{tar}
9728 will use the following default value:
9731 $TMPDIR/GlobalHead.%p.%n
9735 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
9736 environment variable. If @var{TMPDIR} is not set, @command{tar}
9739 @item globexthdr.mtime=@var{value}
9741 This keyword defines the value of the @samp{mtime} field that
9742 is written into the ustar header blocks for the global extended headers.
9743 By default, the @samp{mtime} field is set to the time when
9744 @command{tar} was invoked.
9746 @item @var{keyword}=@var{value}
9747 When used with one of archive-creation commands, these keyword/value pairs
9748 will be included at the beginning of the archive in a global extended
9749 header record. When used with one of archive-reading commands,
9750 @command{tar} will behave as if it has encountered these keyword/value
9751 pairs at the beginning of the archive in a global extended header
9754 @item @var{keyword}:=@var{value}
9755 When used with one of archive-creation commands, these keyword/value pairs
9756 will be included as records at the beginning of an extended header for
9757 each file. This is effectively equivalent to @var{keyword}=@var{value}
9758 form except that it creates no global extended header records.
9760 When used with one of archive-reading commands, @command{tar} will
9761 behave as if these keyword/value pairs were included as records at the
9762 end of each extended header; thus, they will override any global or
9763 file-specific extended header record keywords of the same names.
9764 For example, in the command:
9767 tar --format=posix --create \
9768 --file archive --pax-option gname:=user .
9771 the group name will be forced to a new value for all files
9772 stored in the archive.
9775 In any of the forms described above, the @var{value} may be
9776 a string enclosed in curly braces. In that case, the string
9777 between the braces is understood either as a textual time
9778 representation, as described in @ref{Date input formats}, or a name of
9779 the existing file, starting with @samp{/} or @samp{.}. In the latter
9780 case, the modification time of that file is used.
9782 For example, to set all modification times to the current date, you
9783 use the following option:
9786 --pax-option='mtime:=@{now@}'
9789 Note quoting of the option's argument.
9791 @cindex archives, binary equivalent
9792 @cindex binary equivalent archives, creating
9793 As another example, here is the option that ensures that any two
9794 archives created using it, will be binary equivalent if they have the
9798 --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0
9802 @subsection Checksumming Problems
9804 SunOS and HP-UX @command{tar} fail to accept archives created using
9805 @GNUTAR{} and containing non-@acronym{ASCII} file names, that
9806 is, file names having characters with the eight bit set, because they
9807 use signed checksums, while @GNUTAR{} uses unsigned
9808 checksums while creating archives, as per @acronym{POSIX} standards. On
9809 reading, @GNUTAR{} computes both checksums and
9810 accepts any. It is somewhat worrying that a lot of people may go
9811 around doing backup of their files using faulty (or at least
9812 non-standard) software, not learning about it until it's time to
9813 restore their missing files with an incompatible file extractor, or
9816 @GNUTAR{} computes checksums both ways, and accept
9817 any on read, so @acronym{GNU} tar can read Sun tapes even with their
9818 wrong checksums. @GNUTAR{} produces the standard
9819 checksum, however, raising incompatibilities with Sun. That is to
9820 say, @GNUTAR{} has not been modified to
9821 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
9822 I've been told that more recent Sun @command{tar} now read standard
9823 archives, so maybe Sun did a similar patch, after all?
9825 The story seems to be that when Sun first imported @command{tar}
9826 sources on their system, they recompiled it without realizing that
9827 the checksums were computed differently, because of a change in
9828 the default signing of @code{char}'s in their compiler. So they
9829 started computing checksums wrongly. When they later realized their
9830 mistake, they merely decided to stay compatible with it, and with
9831 themselves afterwards. Presumably, but I do not really know, HP-UX
9832 has chosen that their @command{tar} archives to be compatible with Sun's.
9833 The current standards do not favor Sun @command{tar} format. In any
9834 case, it now falls on the shoulders of SunOS and HP-UX users to get
9835 a @command{tar} able to read the good archives they receive.
9837 @node Large or Negative Values
9838 @subsection Large or Negative Values
9839 @cindex large values
9840 @cindex future time stamps
9841 @cindex negative time stamps
9844 The above sections suggest to use @samp{oldest possible} archive
9845 format if in doubt. However, sometimes it is not possible. If you
9846 attempt to archive a file whose metadata cannot be represented using
9847 required format, @GNUTAR{} will print error message and ignore such a
9848 file. You will than have to switch to a format that is able to
9849 handle such values. The format summary table (@pxref{Formats}) will
9852 In particular, when trying to archive files larger than 8GB or with
9853 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
9854 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
9855 @acronym{POSIX} archive formats. When considering which format to
9856 choose, bear in mind that the @acronym{GNU} format uses
9857 two's-complement base-256 notation to store values that do not fit
9858 into standard @acronym{ustar} range. Such archives can generally be
9859 read only by a @GNUTAR{} implementation. Moreover, they sometimes
9860 cannot be correctly restored on another hosts even by @GNUTAR{}. For
9861 example, using two's complement representation for negative time
9862 stamps that assumes a signed 32-bit @code{time_t} generates archives
9863 that are not portable to hosts with differing @code{time_t}
9866 On the other hand, @acronym{POSIX} archives, generally speaking, can
9867 be extracted by any tar implementation that understands older
9868 @acronym{ustar} format. The only exception are files larger than 8GB.
9870 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
9874 @subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations
9876 In previous sections you became acquainted with various quirks
9877 necessary to make your archives portable. Sometimes you may need to
9878 extract archives containing GNU-specific members using some
9879 third-party @command{tar} implementation or an older version of
9880 @GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed,
9881 but if it is for some reason impossible, this section will explain
9882 how to cope without it.
9884 When we speak about @dfn{GNU-specific} members we mean two classes of
9885 them: members split between the volumes of a multi-volume archive and
9886 sparse members. You will be able to always recover such members if
9887 the archive is in PAX format. In addition split members can be
9888 recovered from archives in old GNU format. The following subsections
9889 describe the required procedures in detail.
9892 * Split Recovery:: Members Split Between Volumes
9893 * Sparse Recovery:: Sparse Members
9896 @node Split Recovery
9897 @subsubsection Extracting Members Split Between Volumes
9899 @cindex Mutli-volume archives, extracting using non-GNU tars
9900 If a member is split between several volumes of an old GNU format archive
9901 most third party @command{tar} implementation will fail to extract
9902 it. To extract it, use @command{tarcat} program (@pxref{Tarcat}).
9903 This program is available from
9904 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat.html, @GNUTAR{}
9905 home page}. It concatenates several archive volumes into a single
9906 valid archive. For example, if you have three volumes named from
9907 @file{vol-1.tar} to @file{vol-3.tar}, you can do the following to
9908 extract them using a third-party @command{tar}:
9911 $ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -}
9914 @cindex Mutli-volume archives in PAX format, extracting using non-GNU tars
9915 You could use this approach for most (although not all) PAX
9916 format archives as well. However, extracting split members from a PAX
9917 archive is a much easier task, because PAX volumes are constructed in
9918 such a way that each part of a split member is extracted to a
9919 different file by @command{tar} implementations that are not aware of
9920 GNU extensions. More specifically, the very first part retains its
9921 original name, and all subsequent parts are named using the pattern:
9924 %d/GNUFileParts.%p/%f.%n
9928 where symbols preceeded by @samp{%} are @dfn{macro characters} that
9929 have the following meaning:
9931 @multitable @columnfractions .25 .55
9932 @headitem Meta-character @tab Replaced By
9933 @item %d @tab The directory name of the file, equivalent to the
9934 result of the @command{dirname} utility on its full name.
9935 @item %f @tab The file name of the file, equivalent to the result
9936 of the @command{basename} utility on its full name.
9937 @item %p @tab The process @acronym{ID} of the @command{tar} process that
9938 created the archive.
9939 @item %n @tab Ordinal number of this particular part.
9942 For example, if the file @file{var/longfile} was split during archive
9943 creation between three volumes, and the creator @command{tar} process
9944 had process @acronym{ID} @samp{27962}, then the member names will be:
9948 var/GNUFileParts.27962/longfile.1
9949 var/GNUFileParts.27962/longfile.2
9952 When you extract your archive using a third-party @command{tar}, these
9953 files will be created on your disk, and the only thing you will need
9954 to do to restore your file in its original form is concatenate them in
9955 the proper order, for example:
9960 $ @kbd{cat GNUFileParts.27962/longfile.1 \
9961 GNUFileParts.27962/longfile.2 >> longfile}
9962 $ rm -f GNUFileParts.27962
9966 Notice, that if the @command{tar} implementation you use supports PAX
9967 format archives, it will probably emit warnings about unknown keywords
9968 during extraction. They will look like this:
9973 Unknown extended header keyword 'GNU.volume.filename' ignored.
9974 Unknown extended header keyword 'GNU.volume.size' ignored.
9975 Unknown extended header keyword 'GNU.volume.offset' ignored.
9980 You can safely ignore these warnings.
9982 If your @command{tar} implementation is not PAX-aware, you will get
9983 more warnings and more files generated on your disk, e.g.:
9987 $ @kbd{tar xf vol-1.tar}
9988 var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
9990 Unexpected EOF in archive
9991 $ @kbd{tar xf vol-2.tar}
9992 tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
9993 GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
9994 'x', extracted as normal file
9998 Ignore these warnings. The @file{PaxHeaders.*} directories created
9999 will contain files with @dfn{extended header keywords} describing the
10000 extracted files. You can delete them, unless they describe sparse
10001 members. Read further to learn more about them.
10003 @node Sparse Recovery
10004 @subsubsection Extracting Sparse Members
10006 @cindex sparse files, extracting with non-GNU tars
10007 Any @command{tar} implementation will be able to extract sparse members from a
10008 PAX archive. However, the extracted files will be @dfn{condensed},
10009 i.e., any zero blocks will be removed from them. When we restore such
10010 a condensed file to its original form, by adding zero blocks (or
10011 @dfn{holes}) back to their original locations, we call this process
10012 @dfn{expanding} a compressed sparse file.
10015 To expand a file, you will need a simple auxiliary program called
10016 @command{xsparse}. It is available in source form from
10017 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse.html, @GNUTAR{}
10020 @cindex sparse files v.1.0, extracting with non-GNU tars
10021 Let's begin with archive members in @dfn{sparse format
10022 version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand.
10023 The condensed file will contain both file map and file data, so no
10024 additional data will be needed to restore it. If the original file
10025 name was @file{@var{dir}/@var{name}}, then the condensed file will be
10026 named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where
10027 @var{n} is a decimal number@footnote{Technically speaking, @var{n} is a
10028 @dfn{process @acronym{ID}} of the @command{tar} process which created the
10029 archive (@pxref{PAX keywords}).}.
10031 To expand a version 1.0 file, run @command{xsparse} as follows:
10034 $ @kbd{xsparse @file{cond-file}}
10038 where @file{cond-file} is the name of the condensed file. The utility
10039 will deduce the name for the resulting expanded file using the
10040 following algorithm:
10043 @item If @file{cond-file} does not contain any directories,
10044 @file{../cond-file} will be used;
10046 @item If @file{cond-file} has the form
10047 @file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name}
10048 are simple names, with no @samp{/} characters in them, the output file
10049 name will be @file{@var{dir}/@var{name}}.
10051 @item Otherwise, if @file{cond-file} has the form
10052 @file{@var{dir}/@var{name}}, the output file name will be
10056 In the unlikely case when this algorithm does not suit your needs,
10057 you can explicitly specify output file name as a second argument to
10061 $ @kbd{xsparse @file{cond-file} @file{out-file}}
10064 It is often a good idea to run @command{xsparse} in @dfn{dry run} mode
10065 first. In this mode, the command does not actually expand the file,
10066 but verbosely lists all actions it would be taking to do so. The dry
10067 run mode is enabled by @option{-n} command line argument:
10071 $ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile}
10072 Reading v.1.0 sparse map
10073 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10074 '/home/gray/sparsefile'
10079 To actually expand the file, you would run:
10082 $ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile}
10086 The program behaves the same way all UNIX utilities do: it will keep
10087 quiet unless it has simething important to tell you (e.g. an error
10088 condition or something). If you wish it to produce verbose output,
10089 similar to that from the dry run mode, use @option{-v} option:
10093 $ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile}
10094 Reading v.1.0 sparse map
10095 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10096 '/home/gray/sparsefile'
10101 Additionally, if your @command{tar} implementation has extracted the
10102 @dfn{extended headers} for this file, you can instruct @command{xstar}
10103 to use them in order to verify the integrity of the expanded file.
10104 The option @option{-x} sets the name of the extended header file to
10105 use. Continuing our example:
10109 $ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
10110 /home/gray/GNUSparseFile.6058/sparsefile}
10111 Reading extended header file
10112 Found variable GNU.sparse.major = 1
10113 Found variable GNU.sparse.minor = 0
10114 Found variable GNU.sparse.name = sparsefile
10115 Found variable GNU.sparse.realsize = 217481216
10116 Reading v.1.0 sparse map
10117 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10118 '/home/gray/sparsefile'
10123 @anchor{extracting sparse v.0.x}
10124 @cindex sparse files v.0.1, extracting with non-GNU tars
10125 @cindex sparse files v.0.0, extracting with non-GNU tars
10126 An @dfn{extended header} is a special @command{tar} archive header
10127 that precedes an archive member and contains a set of
10128 @dfn{variables}, describing the member properties that cannot be
10129 stored in the standard @code{ustar} header. While optional for
10130 expanding sparse version 1.0 members, the use of extended headers is
10131 mandatory when expanding sparse members in older sparse formats: v.0.0
10132 and v.0.1 (The sparse formats are described in detail in @ref{Sparse
10133 Formats}.) So, for these formats, the question is: how to obtain
10134 extended headers from the archive?
10136 If you use a @command{tar} implementation that does not support PAX
10137 format, extended headers for each member will be extracted as a
10138 separate file. If we represent the member name as
10139 @file{@var{dir}/@var{name}}, then the extended header file will be
10140 named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where
10141 @var{n} is an integer number.
10143 Things become more difficult if your @command{tar} implementation
10144 does support PAX headers, because in this case you will have to
10145 manually extract the headers. We recommend the following algorithm:
10149 Consult the documentation of your @command{tar} implementation for an
10150 option that prints @dfn{block numbers} along with the archive
10151 listing (analogous to @GNUTAR{}'s @option{-R} option). For example,
10152 @command{star} has @option{-block-number}.
10155 Obtain verbose listing using the @samp{block number} option, and
10156 find block numbers of the sparse member in question and the member
10157 immediately following it. For example, running @command{star} on our
10162 $ @kbd{star -t -v -block-number -f arc.tar}
10164 star: Unknown extended header keyword 'GNU.sparse.size' ignored.
10165 star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
10166 star: Unknown extended header keyword 'GNU.sparse.name' ignored.
10167 star: Unknown extended header keyword 'GNU.sparse.map' ignored.
10168 block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
10169 block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
10175 (as usual, ignore the warnings about unknown keywords.)
10178 Let @var{size} be the size of the sparse member, @var{Bs} be its block number
10179 and @var{Bn} be the block number of the next member.
10183 @var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2
10187 This number gives the size of the extended header part in tar @dfn{blocks}.
10188 In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2
10192 Use @command{dd} to extract the headers:
10195 @kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}}
10199 where @var{archive} is the archive name, @var{hname} is a name of the
10200 file to store the extended header in, @var{Bs} and @var{N} are
10201 computed in previous steps.
10203 In our example, this command will be
10206 $ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7}
10210 Finally, you can expand the condensed file, using the obtained header:
10214 $ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile}
10215 Reading extended header file
10216 Found variable GNU.sparse.size = 217481216
10217 Found variable GNU.sparse.numblocks = 208
10218 Found variable GNU.sparse.name = sparsefile
10219 Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{}
10220 Expanding file 'GNUSparseFile.28124/sparsefile' to 'sparsefile'
10226 @section Comparison of @command{tar} and @command{cpio}
10229 @FIXME{Reorganize the following material}
10231 The @command{cpio} archive formats, like @command{tar}, do have maximum
10232 file name lengths. The binary and old @acronym{ASCII} formats have a maximum file
10233 length of 256, and the new @acronym{ASCII} and @acronym{CRC ASCII} formats have a max
10234 file length of 1024. @acronym{GNU} @command{cpio} can read and write archives
10235 with arbitrary file name lengths, but other @command{cpio} implementations
10236 may crash unexplainedly trying to read them.
10238 @command{tar} handles symbolic links in the form in which it comes in @acronym{BSD};
10239 @command{cpio} doesn't handle symbolic links in the form in which it comes
10240 in System V prior to SVR4, and some vendors may have added symlinks
10241 to their system without enhancing @command{cpio} to know about them.
10242 Others may have enhanced it in a way other than the way I did it
10243 at Sun, and which was adopted by AT&T (and which is, I think, also
10244 present in the @command{cpio} that Berkeley picked up from AT&T and put
10245 into a later @acronym{BSD} release---I think I gave them my changes).
10247 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
10248 can handle @command{tar} format input, and write it on output, and it
10249 probably handles symbolic links. They may not have bothered doing
10250 anything to enhance @command{tar} as a result.)
10252 @command{cpio} handles special files; traditional @command{tar} doesn't.
10254 @command{tar} comes with V7, System III, System V, and @acronym{BSD} source;
10255 @command{cpio} comes only with System III, System V, and later @acronym{BSD}
10256 (4.3-tahoe and later).
10258 @command{tar}'s way of handling multiple hard links to a file can handle
10259 file systems that support 32-bit i-numbers (e.g., the @acronym{BSD} file system);
10260 @command{cpio}s way requires you to play some games (in its ``binary''
10261 format, i-numbers are only 16 bits, and in its ``portable @acronym{ASCII}'' format,
10262 they're 18 bits---it would have to play games with the "file system @acronym{ID}"
10263 field of the header to make sure that the file system @acronym{ID}/i-number pairs
10264 of different files were always different), and I don't know which
10265 @command{cpio}s, if any, play those games. Those that don't might get
10266 confused and think two files are the same file when they're not, and
10267 make hard links between them.
10269 @command{tar}s way of handling multiple hard links to a file places only
10270 one copy of the link on the tape, but the name attached to that copy
10271 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
10272 way puts one copy for every link, but you can retrieve it using any
10276 What type of check sum (if any) is used, and how is this calculated.
10279 See the attached manual pages for @command{tar} and @command{cpio} format.
10280 @command{tar} uses a checksum which is the sum of all the bytes in the
10281 @command{tar} header for a file; @command{cpio} uses no checksum.
10284 If anyone knows why @command{cpio} was made when @command{tar} was present
10288 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
10289 generally-available version of UNIX had @command{tar} at the time. I don't
10290 know whether any version that was generally available @emph{within AT&T}
10291 had @command{tar}, or, if so, whether the people within AT&T who did
10292 @command{cpio} knew about it.
10294 On restore, if there is a corruption on a tape @command{tar} will stop at
10295 that point, while @command{cpio} will skip over it and try to restore the
10298 The main difference is just in the command syntax and header format.
10300 @command{tar} is a little more tape-oriented in that everything is blocked
10301 to start on a record boundary.
10304 Is there any differences between the ability to recover crashed
10305 archives between the two of them. (Is there any chance of recovering
10306 crashed archives at all.)
10309 Theoretically it should be easier under @command{tar} since the blocking
10310 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
10311 However, modern @command{cpio}'s and variations have an option to just
10312 search for the next file header after an error with a reasonable chance
10313 of resyncing. However, lots of tape driver software won't allow you to
10314 continue past a media error which should be the only reason for getting
10315 out of sync unless a file changed sizes while you were writing the
10319 If anyone knows why @command{cpio} was made when @command{tar} was present
10320 at the unix scene, please tell me about this too.
10323 Probably because it is more media efficient (by not blocking everything
10324 and using only the space needed for the headers where @command{tar}
10325 always uses 512 bytes per file header) and it knows how to archive
10328 You might want to look at the freely available alternatives. The
10329 major ones are @command{afio}, @GNUTAR{}, and
10330 @command{pax}, each of which have their own extensions with some
10331 backwards compatibility.
10333 Sparse files were @command{tar}red as sparse files (which you can
10334 easily test, because the resulting archive gets smaller, and
10335 @acronym{GNU} @command{cpio} can no longer read it).
10338 @chapter Tapes and Other Archive Media
10341 A few special cases about tape handling warrant more detailed
10342 description. These special cases are discussed below.
10344 Many complexities surround the use of @command{tar} on tape drives. Since
10345 the creation and manipulation of archives located on magnetic tape was
10346 the original purpose of @command{tar}, it contains many features making
10347 such manipulation easier.
10349 Archives are usually written on dismountable media---tape cartridges,
10350 mag tapes, or floppy disks.
10352 The amount of data a tape or disk holds depends not only on its size,
10353 but also on how it is formatted. A 2400 foot long reel of mag tape
10354 holds 40 megabytes of data when formatted at 1600 bits per inch. The
10355 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
10357 Magnetic media are re-usable---once the archive on a tape is no longer
10358 needed, the archive can be erased and the tape or disk used over.
10359 Media quality does deteriorate with use, however. Most tapes or disks
10360 should be discarded when they begin to produce data errors. EXABYTE
10361 tape cartridges should be discarded when they generate an @dfn{error
10362 count} (number of non-usable bits) of more than 10k.
10364 Magnetic media are written and erased using magnetic fields, and
10365 should be protected from such fields to avoid damage to stored data.
10366 Sticking a floppy disk to a filing cabinet using a magnet is probably
10370 * Device:: Device selection and switching
10371 * Remote Tape Server::
10372 * Common Problems and Solutions::
10373 * Blocking:: Blocking
10374 * Many:: Many archives on one tape
10375 * Using Multiple Tapes:: Using Multiple Tapes
10376 * label:: Including a Label in the Archive
10378 * Write Protection::
10382 @section Device Selection and Switching
10386 @item -f [@var{hostname}:]@var{file}
10387 @itemx --file=[@var{hostname}:]@var{file}
10388 Use archive file or device @var{file} on @var{hostname}.
10391 This option is used to specify the file name of the archive @command{tar}
10394 If the file name is @samp{-}, @command{tar} reads the archive from standard
10395 input (when listing or extracting), or writes it to standard output
10396 (when creating). If the @samp{-} file name is given when updating an
10397 archive, @command{tar} will read the original archive from its standard
10398 input, and will write the entire new archive to its standard output.
10400 If the file name contains a @samp{:}, it is interpreted as
10401 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
10402 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
10403 either case, @command{tar} will invoke the command @command{rsh} (or
10404 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
10405 machine. If you give an alternate login name, it will be given to the
10407 Naturally, the remote machine must have an executable
10408 @command{/usr/libexec/rmt}. This program is free software from the
10409 University of California, and a copy of the source code can be found
10410 with the sources for @command{tar}; it's compiled and installed by default.
10411 The exact path to this utility is determined when configuring the package.
10412 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
10413 your installation prefix. This location may also be overridden at
10414 runtime by using the @option{--rmt-command=@var{command}} option (@xref{Option Summary,
10415 ---rmt-command}, for detailed description of this option. @xref{Remote
10416 Tape Server}, for the description of @command{rmt} command).
10418 If this option is not given, but the environment variable @env{TAPE}
10419 is set, its value is used; otherwise, old versions of @command{tar}
10420 used a default archive name (which was picked when @command{tar} was
10421 compiled). The default is normally set up to be the @dfn{first} tape
10422 drive or other transportable I/O medium on the system.
10424 Starting with version 1.11.5, @GNUTAR{} uses
10425 standard input and standard output as the default device, and I will
10426 not try anymore supporting automatic device detection at installation
10427 time. This was failing really in too many cases, it was hopeless.
10428 This is now completely left to the installer to override standard
10429 input and standard output for default device, if this seems
10430 preferable. Further, I think @emph{most} actual usages of
10431 @command{tar} are done with pipes or disks, not really tapes,
10432 cartridges or diskettes.
10434 Some users think that using standard input and output is running
10435 after trouble. This could lead to a nasty surprise on your screen if
10436 you forget to specify an output file name---especially if you are going
10437 through a network or terminal server capable of buffering large amounts
10438 of output. We had so many bug reports in that area of configuring
10439 default tapes automatically, and so many contradicting requests, that
10440 we finally consider the problem to be portably intractable. We could
10441 of course use something like @samp{/dev/tape} as a default, but this
10442 is @emph{also} running after various kind of trouble, going from hung
10443 processes to accidental destruction of real tapes. After having seen
10444 all this mess, using standard input and output as a default really
10445 sounds like the only clean choice left, and a very useful one too.
10447 @GNUTAR{} reads and writes archive in records, I
10448 suspect this is the main reason why block devices are preferred over
10449 character devices. Most probably, block devices are more efficient
10450 too. The installer could also check for @samp{DEFTAPE} in
10451 @file{<sys/mtio.h>}.
10454 @xopindex{force-local, short description}
10455 @item --force-local
10456 Archive file is local even if it contains a colon.
10458 @opindex rsh-command
10459 @item --rsh-command=@var{command}
10460 Use remote @var{command} instead of @command{rsh}. This option exists
10461 so that people who use something other than the standard @command{rsh}
10462 (e.g., a Kerberized @command{rsh}) can access a remote device.
10464 When this command is not used, the shell command found when
10465 the @command{tar} program was installed is used instead. This is
10466 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
10467 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
10468 The installer may have overridden this by defining the environment
10469 variable @env{RSH} @emph{at installation time}.
10472 Specify drive and density.
10474 @xopindex{multi-volume, short description}
10476 @itemx --multi-volume
10477 Create/list/extract multi-volume archive.
10479 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
10480 that may be larger than will fit on the medium used to hold it.
10481 @xref{Multi-Volume Archives}.
10483 @xopindex{tape-length, short description}
10485 @itemx --tape-length=@var{size}[@var{suf}]
10486 Change tape after writing @var{size} units of data. Unless @var{suf} is
10487 given, @var{size} is treated as kilobytes, i.e. @samp{@var{size} x
10488 1024} bytes. The following suffixes alter this behavior:
10490 @float Table, size-suffixes
10491 @caption{Size Suffixes}
10492 @multitable @columnfractions 0.2 0.3 0.3
10493 @headitem Suffix @tab Units @tab Byte Equivalent
10494 @item b @tab Blocks @tab @var{size} x 512
10495 @item B @tab Kilobytes @tab @var{size} x 1024
10496 @item c @tab Bytes @tab @var{size}
10497 @item G @tab Gigabytes @tab @var{size} x 1024^3
10498 @item K @tab Kilobytes @tab @var{size} x 1024
10499 @item k @tab Kilobytes @tab @var{size} x 1024
10500 @item M @tab Megabytes @tab @var{size} x 1024^2
10501 @item P @tab Petabytes @tab @var{size} x 1024^5
10502 @item T @tab Terabytes @tab @var{size} x 1024^4
10503 @item w @tab Words @tab @var{size} x 2
10507 This option might be useful when your tape drivers do not properly
10508 detect end of physical tapes. By being slightly conservative on the
10509 maximum tape length, you might avoid the problem entirely.
10511 @xopindex{info-script, short description}
10512 @xopindex{new-volume-script, short description}
10513 @item -F @var{command}
10514 @itemx --info-script=@var{command}
10515 @itemx --new-volume-script=@var{command}
10516 Execute @var{command} at end of each tape. This implies
10517 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
10518 description of this option.
10521 @node Remote Tape Server
10522 @section Remote Tape Server
10524 @cindex remote tape drive
10526 In order to access the tape drive on a remote machine, @command{tar}
10527 uses the remote tape server written at the University of California at
10528 Berkeley. The remote tape server must be installed as
10529 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
10530 want to use. @command{tar} calls @command{rmt} by running an
10531 @command{rsh} or @command{remsh} to the remote machine, optionally
10532 using a different login name if one is supplied.
10534 A copy of the source for the remote tape server is provided. Its
10535 source code can be freely distributed. It is compiled and
10536 installed by default.
10538 @cindex absolute file names
10539 Unless you use the @option{--absolute-names} (@option{-P}) option,
10540 @GNUTAR{} will not allow you to create an archive that contains
10541 absolute file names (a file name beginning with @samp{/}.) If you try,
10542 @command{tar} will automatically remove the leading @samp{/} from the
10543 file names it stores in the archive. It will also type a warning
10544 message telling you what it is doing.
10546 When reading an archive that was created with a different
10547 @command{tar} program, @GNUTAR{} automatically
10548 extracts entries in the archive which have absolute file names as if
10549 the file names were not absolute. This is an important feature. A
10550 visitor here once gave a @command{tar} tape to an operator to restore;
10551 the operator used Sun @command{tar} instead of @GNUTAR{},
10552 and the result was that it replaced large portions of
10553 our @file{/bin} and friends with versions from the tape; needless to
10554 say, we were unhappy about having to recover the file system from
10557 For example, if the archive contained a file @file{/usr/bin/computoy},
10558 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
10559 relative to the current directory. If you want to extract the files in
10560 an archive to the same absolute names that they had when the archive
10561 was created, you should do a @samp{cd /} before extracting the files
10562 from the archive, or you should either use the @option{--absolute-names}
10563 option, or use the command @samp{tar -C / @dots{}}.
10565 @cindex Ultrix 3.1 and write failure
10566 Some versions of Unix (Ultrix 3.1 is known to have this problem),
10567 can claim that a short write near the end of a tape succeeded,
10568 when it actually failed. This will result in the -M option not
10569 working correctly. The best workaround at the moment is to use a
10570 significantly larger blocking factor than the default 20.
10572 In order to update an archive, @command{tar} must be able to backspace the
10573 archive in order to reread or rewrite a record that was just read (or
10574 written). This is currently possible only on two kinds of files: normal
10575 disk files (or any other file that can be backspaced with @samp{lseek}),
10576 and industry-standard 9-track magnetic tape (or any other kind of tape
10577 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}).
10579 This means that the @option{--append}, @option{--concatenate}, and
10580 @option{--delete} commands will not work on any other kind of file.
10581 Some media simply cannot be backspaced, which means these commands and
10582 options will never be able to work on them. These non-backspacing
10583 media include pipes and cartridge tape drives.
10585 Some other media can be backspaced, and @command{tar} will work on them
10586 once @command{tar} is modified to do so.
10588 Archives created with the @option{--multi-volume}, @option{--label}, and
10589 @option{--incremental} (@option{-G}) options may not be readable by other version
10590 of @command{tar}. In particular, restoring a file that was split over
10591 a volume boundary will require some careful work with @command{dd}, if
10592 it can be done at all. Other versions of @command{tar} may also create
10593 an empty file whose name is that of the volume header. Some versions
10594 of @command{tar} may create normal files instead of directories archived
10595 with the @option{--incremental} (@option{-G}) option.
10597 @node Common Problems and Solutions
10598 @section Some Common Problems and their Solutions
10603 errors from system:
10605 no such file or directory
10608 errors from @command{tar}:
10609 directory checksum error
10610 header format error
10612 errors from media/system:
10624 @dfn{Block} and @dfn{record} terminology is rather confused, and it
10625 is also confusing to the expert reader. On the other hand, readers
10626 who are new to the field have a fresh mind, and they may safely skip
10627 the next two paragraphs, as the remainder of this manual uses those
10628 two terms in a quite consistent way.
10630 John Gilmore, the writer of the public domain @command{tar} from which
10631 @GNUTAR{} was originally derived, wrote (June 1995):
10634 The nomenclature of tape drives comes from IBM, where I believe
10635 they were invented for the IBM 650 or so. On IBM mainframes, what
10636 is recorded on tape are tape blocks. The logical organization of
10637 data is into records. There are various ways of putting records into
10638 blocks, including @code{F} (fixed sized records), @code{V} (variable
10639 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
10640 to a block), @code{VB} (variable size records, @var{n} to a block),
10641 @code{VSB} (variable spanned blocked: variable sized records that can
10642 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
10643 parameter specified this to the operating system.
10645 The Unix man page on @command{tar} was totally confused about this.
10646 When I wrote @code{PD TAR}, I used the historically correct terminology
10647 (@command{tar} writes data records, which are grouped into blocks).
10648 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
10649 here), and now Fran@,{c}ois has migrated that terminology back
10650 into the source code too.
10653 The term @dfn{physical block} means the basic transfer chunk from or
10654 to a device, after which reading or writing may stop without anything
10655 being lost. In this manual, the term @dfn{block} usually refers to
10656 a disk physical block, @emph{assuming} that each disk block is 512
10657 bytes in length. It is true that some disk devices have different
10658 physical blocks, but @command{tar} ignore these differences in its own
10659 format, which is meant to be portable, so a @command{tar} block is always
10660 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
10661 The term @dfn{logical block} often represents the basic chunk of
10662 allocation of many disk blocks as a single entity, which the operating
10663 system treats somewhat atomically; this concept is only barely used
10666 The term @dfn{physical record} is another way to speak of a physical
10667 block, those two terms are somewhat interchangeable. In this manual,
10668 the term @dfn{record} usually refers to a tape physical block,
10669 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
10670 It is true that archives may be put on disk or used with pipes,
10671 but nevertheless, @command{tar} tries to read and write the archive one
10672 @dfn{record} at a time, whatever the medium in use. One record is made
10673 up of an integral number of blocks, and this operation of putting many
10674 disk blocks into a single tape block is called @dfn{reblocking}, or
10675 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
10676 the logical organization of many characters into something meaningful
10677 to the application. The term @dfn{unit record} describes a small set
10678 of characters which are transmitted whole to or by the application,
10679 and often refers to a line of text. Those two last terms are unrelated
10680 to what we call a @dfn{record} in @GNUTAR{}.
10682 When writing to tapes, @command{tar} writes the contents of the archive
10683 in chunks known as @dfn{records}. To change the default blocking
10684 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
10685 @var{512-size}}) option. Each record will then be composed of
10686 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
10687 @xref{Standard}.) Each file written to the archive uses at least one
10688 full record. As a result, using a larger record size can result in
10689 more wasted space for small files. On the other hand, a larger record
10690 size can often be read and written much more efficiently.
10692 Further complicating the problem is that some tape drives ignore the
10693 blocking entirely. For these, a larger record size can still improve
10694 performance (because the software layers above the tape drive still
10695 honor the blocking), but not as dramatically as on tape drives that
10698 When reading an archive, @command{tar} can usually figure out the
10699 record size on itself. When this is the case, and a non-standard
10700 record size was used when the archive was created, @command{tar} will
10701 print a message about a non-standard blocking factor, and then operate
10702 normally. On some tape devices, however, @command{tar} cannot figure
10703 out the record size itself. On most of those, you can specify a
10704 blocking factor (with @option{--blocking-factor}) larger than the
10705 actual blocking factor, and then use the @option{--read-full-records}
10706 (@option{-B}) option. (If you specify a blocking factor with
10707 @option{--blocking-factor} and don't use the
10708 @option{--read-full-records} option, then @command{tar} will not
10709 attempt to figure out the recording size itself.) On some devices,
10710 you must always specify the record size exactly with
10711 @option{--blocking-factor} when reading, because @command{tar} cannot
10712 figure it out. In any case, use @option{--list} (@option{-t}) before
10713 doing any extractions to see whether @command{tar} is reading the archive
10716 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
10717 putting them into records is to put a whole number of them (one or
10718 more) into each record. @command{tar} records are all the same size;
10719 at the end of the file there's a block containing all zeros, which
10720 is how you tell that the remainder of the last record(s) are garbage.
10722 In a standard @command{tar} file (no options), the block size is 512
10723 and the record size is 10240, for a blocking factor of 20. What the
10724 @option{--blocking-factor} option does is sets the blocking factor,
10725 changing the record size while leaving the block size at 512 bytes.
10726 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
10727 most tape drives these days prefer much bigger records in order to
10728 stream and not waste tape. When writing tapes for myself, some tend
10729 to use a factor of the order of 2048, say, giving a record size of
10730 around one megabyte.
10732 If you use a blocking factor larger than 20, older @command{tar}
10733 programs might not be able to read the archive, so we recommend this
10734 as a limit to use in practice. @GNUTAR{}, however,
10735 will support arbitrarily large record sizes, limited only by the
10736 amount of virtual memory or the physical characteristics of the tape
10740 * Format Variations:: Format Variations
10741 * Blocking Factor:: The Blocking Factor of an Archive
10744 @node Format Variations
10745 @subsection Format Variations
10746 @cindex Format Parameters
10747 @cindex Format Options
10748 @cindex Options, archive format specifying
10749 @cindex Options, format specifying
10752 Format parameters specify how an archive is written on the archive
10753 media. The best choice of format parameters will vary depending on
10754 the type and number of files being archived, and on the media used to
10757 To specify format parameters when accessing or creating an archive,
10758 you can use the options described in the following sections.
10759 If you do not specify any format parameters, @command{tar} uses
10760 default parameters. You cannot modify a compressed archive.
10761 If you create an archive with the @option{--blocking-factor} option
10762 specified (@pxref{Blocking Factor}), you must specify that
10763 blocking-factor when operating on the archive. @xref{Formats}, for other
10764 examples of format parameter considerations.
10766 @node Blocking Factor
10767 @subsection The Blocking Factor of an Archive
10768 @cindex Blocking Factor
10769 @cindex Record Size
10770 @cindex Number of blocks per record
10771 @cindex Number of bytes per record
10772 @cindex Bytes per record
10773 @cindex Blocks per record
10776 @opindex blocking-factor
10777 The data in an archive is grouped into blocks, which are 512 bytes.
10778 Blocks are read and written in whole number multiples called
10779 @dfn{records}. The number of blocks in a record (i.e., the size of a
10780 record in units of 512 bytes) is called the @dfn{blocking factor}.
10781 The @option{--blocking-factor=@var{512-size}} (@option{-b
10782 @var{512-size}}) option specifies the blocking factor of an archive.
10783 The default blocking factor is typically 20 (i.e., 10240 bytes), but
10784 can be specified at installation. To find out the blocking factor of
10785 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
10786 This may not work on some devices.
10788 Records are separated by gaps, which waste space on the archive media.
10789 If you are archiving on magnetic tape, using a larger blocking factor
10790 (and therefore larger records) provides faster throughput and allows you
10791 to fit more data on a tape (because there are fewer gaps). If you are
10792 archiving on cartridge, a very large blocking factor (say 126 or more)
10793 greatly increases performance. A smaller blocking factor, on the other
10794 hand, may be useful when archiving small files, to avoid archiving lots
10795 of nulls as @command{tar} fills out the archive to the end of the record.
10796 In general, the ideal record size depends on the size of the
10797 inter-record gaps on the tape you are using, and the average size of the
10798 files you are archiving. @xref{create}, for information on
10801 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
10803 Archives with blocking factors larger than 20 cannot be read
10804 by very old versions of @command{tar}, or by some newer versions
10805 of @command{tar} running on old machines with small address spaces.
10806 With @GNUTAR{}, the blocking factor of an archive is limited
10807 only by the maximum record size of the device containing the archive,
10808 or by the amount of available virtual memory.
10810 Also, on some systems, not using adequate blocking factors, as sometimes
10811 imposed by the device drivers, may yield unexpected diagnostics. For
10812 example, this has been reported:
10815 Cannot write to /dev/dlt: Invalid argument
10819 In such cases, it sometimes happen that the @command{tar} bundled by
10820 the system is aware of block size idiosyncrasies, while @GNUTAR{}
10821 requires an explicit specification for the block size,
10822 which it cannot guess. This yields some people to consider
10823 @GNUTAR{} is misbehaving, because by comparison,
10824 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
10825 for example, might resolve the problem.
10827 If you use a non-default blocking factor when you create an archive, you
10828 must specify the same blocking factor when you modify that archive. Some
10829 archive devices will also require you to specify the blocking factor when
10830 reading that archive, however this is not typically the case. Usually, you
10831 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
10832 reports a non-default record size and then lists the archive members as
10833 it would normally. To extract files from an archive with a non-standard
10834 blocking factor (particularly if you're not sure what the blocking factor
10835 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
10836 specifying a blocking factor larger then the blocking factor of the archive
10837 (i.e., @samp{tar --extract --read-full-records --blocking-factor=300}).
10838 @xref{list}, for more information on the @option{--list} (@option{-t})
10839 operation. @xref{Reading}, for a more detailed explanation of that option.
10842 @item --blocking-factor=@var{number}
10843 @itemx -b @var{number}
10844 Specifies the blocking factor of an archive. Can be used with any
10845 operation, but is usually not necessary with @option{--list} (@option{-t}).
10851 @item -b @var{blocks}
10852 @itemx --blocking-factor=@var{blocks}
10853 Set record size to @math{@var{blocks}*512} bytes.
10855 This option is used to specify a @dfn{blocking factor} for the archive.
10856 When reading or writing the archive, @command{tar}, will do reads and writes
10857 of the archive in records of @math{@var{block}*512} bytes. This is true
10858 even when the archive is compressed. Some devices requires that all
10859 write operations be a multiple of a certain size, and so, @command{tar}
10860 pads the archive out to the next record boundary.
10862 The default blocking factor is set when @command{tar} is compiled, and is
10863 typically 20. Blocking factors larger than 20 cannot be read by very
10864 old versions of @command{tar}, or by some newer versions of @command{tar}
10865 running on old machines with small address spaces.
10867 With a magnetic tape, larger records give faster throughput and fit
10868 more data on a tape (because there are fewer inter-record gaps).
10869 If the archive is in a disk file or a pipe, you may want to specify
10870 a smaller blocking factor, since a large one will result in a large
10871 number of null bytes at the end of the archive.
10873 When writing cartridge or other streaming tapes, a much larger
10874 blocking factor (say 126 or more) will greatly increase performance.
10875 However, you must specify the same blocking factor when reading or
10876 updating the archive.
10878 Apparently, Exabyte drives have a physical block size of 8K bytes.
10879 If we choose our blocksize as a multiple of 8k bytes, then the problem
10880 seems to disappear. Id est, we are using block size of 112 right
10881 now, and we haven't had the problem since we switched@dots{}
10883 With @GNUTAR{} the blocking factor is limited only
10884 by the maximum record size of the device containing the archive, or by
10885 the amount of available virtual memory.
10887 However, deblocking or reblocking is virtually avoided in a special
10888 case which often occurs in practice, but which requires all the
10889 following conditions to be simultaneously true:
10892 the archive is subject to a compression option,
10894 the archive is not handled through standard input or output, nor
10895 redirected nor piped,
10897 the archive is directly handled to a local disk, instead of any special
10900 @option{--blocking-factor} is not explicitly specified on the @command{tar}
10904 If the output goes directly to a local disk, and not through
10905 stdout, then the last write is not extended to a full record size.
10906 Otherwise, reblocking occurs. Here are a few other remarks on this
10912 @command{gzip} will complain about trailing garbage if asked to
10913 uncompress a compressed archive on tape, there is an option to turn
10914 the message off, but it breaks the regularity of simply having to use
10915 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
10916 silently ignoring any number of trailing zeros. I'll ask Jean-loup
10917 Gailly, by sending a copy of this message to him.
10920 @command{compress} does not show this problem, but as Jean-loup pointed
10921 out to Michael, @samp{compress -d} silently adds garbage after
10922 the result of decompression, which tar ignores because it already
10923 recognized its end-of-file indicator. So this bug may be safely
10927 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
10928 but will still return an exit status of 2 which tar reports in turn.
10929 @command{tar} might ignore the exit status returned, but I hate doing
10930 that, as it weakens the protection @command{tar} offers users against
10931 other possible problems at decompression time. If @command{gzip} was
10932 silently skipping trailing zeros @emph{and} also avoiding setting the
10933 exit status in this innocuous case, that would solve this situation.
10936 @command{tar} should become more solid at not stopping to read a pipe at
10937 the first null block encountered. This inelegantly breaks the pipe.
10938 @command{tar} should rather drain the pipe out before exiting itself.
10941 @xopindex{ignore-zeros, short description}
10943 @itemx --ignore-zeros
10944 Ignore blocks of zeros in archive (means EOF).
10946 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
10947 of zeros in the archive. Normally a block of zeros indicates the
10948 end of the archive, but when reading a damaged archive, or one which
10949 was created by concatenating several archives together, this option
10950 allows @command{tar} to read the entire archive. This option is not on
10951 by default because many versions of @command{tar} write garbage after
10954 Note that this option causes @command{tar} to read to the end of the
10955 archive file, which may sometimes avoid problems when multiple files
10956 are stored on a single physical tape.
10958 @xopindex{read-full-records, short description}
10960 @itemx --read-full-records
10961 Reblock as we read (for reading 4.2@acronym{BSD} pipes).
10963 If @option{--read-full-records} is used, @command{tar}
10964 will not panic if an attempt to read a record from the archive does
10965 not return a full record. Instead, @command{tar} will keep reading
10966 until it has obtained a full
10969 This option is turned on by default when @command{tar} is reading
10970 an archive from standard input, or from a remote machine. This is
10971 because on @acronym{BSD} Unix systems, a read of a pipe will return however
10972 much happens to be in the pipe, even if it is less than @command{tar}
10973 requested. If this option was not used, @command{tar} would fail as
10974 soon as it read an incomplete record from the pipe.
10976 This option is also useful with the commands for updating an archive.
10982 @FIXME{Appropriate options should be moved here from elsewhere.}
10984 @cindex blocking factor
10985 @cindex tape blocking
10987 When handling various tapes or cartridges, you have to take care of
10988 selecting a proper blocking, that is, the number of disk blocks you
10989 put together as a single tape block on the tape, without intervening
10990 tape gaps. A @dfn{tape gap} is a small landing area on the tape
10991 with no information on it, used for decelerating the tape to a
10992 full stop, and for later regaining the reading or writing speed.
10993 When the tape driver starts reading a record, the record has to
10994 be read whole without stopping, as a tape gap is needed to stop the
10995 tape motion without losing information.
10997 @cindex Exabyte blocking
10998 @cindex DAT blocking
10999 Using higher blocking (putting more disk blocks per tape block) will use
11000 the tape more efficiently as there will be less tape gaps. But reading
11001 such tapes may be more difficult for the system, as more memory will be
11002 required to receive at once the whole record. Further, if there is a
11003 reading error on a huge record, this is less likely that the system will
11004 succeed in recovering the information. So, blocking should not be too
11005 low, nor it should be too high. @command{tar} uses by default a blocking of
11006 20 for historical reasons, and it does not really matter when reading or
11007 writing to disk. Current tape technology would easily accommodate higher
11008 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
11009 We were told that for some DLT drives, the blocking should be a multiple
11010 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
11011 Other manufacturers may use different recommendations for the same tapes.
11012 This might also depends of the buffering techniques used inside modern
11013 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
11014 Others request blocking to be some exponent of two.
11016 So, there is no fixed rule for blocking. But blocking at read time
11017 should ideally be the same as blocking used at write time. At one place
11018 I know, with a wide variety of equipment, they found it best to use a
11019 blocking of 32 to guarantee that their tapes are fully interchangeable.
11021 I was also told that, for recycled tapes, prior erasure (by the same
11022 drive unit that will be used to create the archives) sometimes lowers
11023 the error rates observed at rewriting time.
11025 I might also use @option{--number-blocks} instead of
11026 @option{--block-number}, so @option{--block} will then expand to
11027 @option{--blocking-factor} unambiguously.
11030 @section Many Archives on One Tape
11032 @FIXME{Appropriate options should be moved here from elsewhere.}
11034 @findex ntape @r{device}
11035 Most tape devices have two entries in the @file{/dev} directory, or
11036 entries that come in pairs, which differ only in the minor number for
11037 this device. Let's take for example @file{/dev/tape}, which often
11038 points to the only or usual tape device of a given system. There might
11039 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
11040 name is the @emph{rewinding} version of the device, while the name
11041 having @samp{nr} in it is the @emph{no rewinding} version of the same
11044 A rewinding tape device will bring back the tape to its beginning point
11045 automatically when this device is opened or closed. Since @command{tar}
11046 opens the archive file before using it and closes it afterwards, this
11047 means that a simple:
11050 $ @kbd{tar cf /dev/tape @var{directory}}
11054 will reposition the tape to its beginning both prior and after saving
11055 @var{directory} contents to it, thus erasing prior tape contents and
11056 making it so that any subsequent write operation will destroy what has
11059 @cindex tape positioning
11060 So, a rewinding device is normally meant to hold one and only one file.
11061 If you want to put more than one @command{tar} archive on a given tape, you
11062 will need to avoid using the rewinding version of the tape device. You
11063 will also have to pay special attention to tape positioning. Errors in
11064 positioning may overwrite the valuable data already on your tape. Many
11065 people, burnt by past experiences, will only use rewinding devices and
11066 limit themselves to one file per tape, precisely to avoid the risk of
11067 such errors. Be fully aware that writing at the wrong position on a
11068 tape loses all information past this point and most probably until the
11069 end of the tape, and this destroyed information @emph{cannot} be
11072 To save @var{directory-1} as a first archive at the beginning of a
11073 tape, and leave that tape ready for a second archive, you should use:
11076 $ @kbd{mt -f /dev/nrtape rewind}
11077 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
11081 @dfn{Tape marks} are special magnetic patterns written on the tape
11082 media, which are later recognizable by the reading hardware. These
11083 marks are used after each file, when there are many on a single tape.
11084 An empty file (that is to say, two tape marks in a row) signal the
11085 logical end of the tape, after which no file exist. Usually,
11086 non-rewinding tape device drivers will react to the close request issued
11087 by @command{tar} by first writing two tape marks after your archive, and by
11088 backspacing over one of these. So, if you remove the tape at that time
11089 from the tape drive, it is properly terminated. But if you write
11090 another file at the current position, the second tape mark will be
11091 erased by the new information, leaving only one tape mark between files.
11093 So, you may now save @var{directory-2} as a second archive after the
11094 first on the same tape by issuing the command:
11097 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
11101 and so on for all the archives you want to put on the same tape.
11103 Another usual case is that you do not write all the archives the same
11104 day, and you need to remove and store the tape between two archive
11105 sessions. In general, you must remember how many files are already
11106 saved on your tape. Suppose your tape already has 16 files on it, and
11107 that you are ready to write the 17th. You have to take care of skipping
11108 the first 16 tape marks before saving @var{directory-17}, say, by using
11112 $ @kbd{mt -f /dev/nrtape rewind}
11113 $ @kbd{mt -f /dev/nrtape fsf 16}
11114 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
11117 In all the previous examples, we put aside blocking considerations, but
11118 you should do the proper things for that as well. @xref{Blocking}.
11121 * Tape Positioning:: Tape Positions and Tape Marks
11122 * mt:: The @command{mt} Utility
11125 @node Tape Positioning
11126 @subsection Tape Positions and Tape Marks
11129 Just as archives can store more than one file from the file system,
11130 tapes can store more than one archive file. To keep track of where
11131 archive files (or any other type of file stored on tape) begin and
11132 end, tape archive devices write magnetic @dfn{tape marks} on the
11133 archive media. Tape drives write one tape mark between files,
11134 two at the end of all the file entries.
11136 If you think of data as a series of records "rrrr"'s, and tape marks as
11137 "*"'s, a tape might look like the following:
11140 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
11143 Tape devices read and write tapes using a read/write @dfn{tape
11144 head}---a physical part of the device which can only access one
11145 point on the tape at a time. When you use @command{tar} to read or
11146 write archive data from a tape device, the device will begin reading
11147 or writing from wherever on the tape the tape head happens to be,
11148 regardless of which archive or what part of the archive the tape
11149 head is on. Before writing an archive, you should make sure that no
11150 data on the tape will be overwritten (unless it is no longer needed).
11151 Before reading an archive, you should make sure the tape head is at
11152 the beginning of the archive you want to read. You can do it manually
11153 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
11154 that automatically (@pxref{Scripted Restoration}).
11156 If you want to add new archive file entries to a tape, you should
11157 advance the tape to the end of the existing file entries, backspace
11158 over the last tape mark, and write the new archive file. If you were
11159 to add two archives to the example above, the tape might look like the
11163 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
11167 @subsection The @command{mt} Utility
11170 @FIXME{Is it true that this only works on non-block devices?
11171 should explain the difference, (fixed or variable).}
11172 @xref{Blocking Factor}.
11174 You can use the @command{mt} utility to advance or rewind a tape past a
11175 specified number of archive files on the tape. This will allow you
11176 to move to the beginning of an archive before extracting or reading
11177 it, or to the end of all the archives before writing a new one.
11178 @FIXME{Why isn't there an "advance 'til you find two tape marks
11181 The syntax of the @command{mt} command is:
11184 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
11187 where @var{tapename} is the name of the tape device, @var{number} is
11188 the number of times an operation is performed (with a default of one),
11189 and @var{operation} is one of the following:
11191 @FIXME{is there any use for record operations?}
11196 Writes @var{number} tape marks at the current position on the tape.
11199 Moves tape position forward @var{number} files.
11202 Moves tape position back @var{number} files.
11205 Rewinds the tape. (Ignores @var{number}.)
11209 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}.)
11212 Prints status information about the tape unit.
11216 If you don't specify a @var{tapename}, @command{mt} uses the environment
11217 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
11218 the default device specified in your @file{sys/mtio.h} file
11219 (@code{DEFTAPE} variable). If this is not defined, the program will
11220 display a descriptive error message and exit with code 1.
11222 @command{mt} returns a 0 exit status when the operation(s) were
11223 successful, 1 if the command was unrecognized, and 2 if an operation
11226 @node Using Multiple Tapes
11227 @section Using Multiple Tapes
11229 Often you might want to write a large archive, one larger than will fit
11230 on the actual tape you are using. In such a case, you can run multiple
11231 @command{tar} commands, but this can be inconvenient, particularly if you
11232 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
11233 Therefore, @command{tar} provides a special mode for creating
11234 multi-volume archives.
11236 @dfn{Multi-volume} archive is a single @command{tar} archive, stored
11237 on several media volumes of fixed size. Although in this section we will
11238 often call @samp{volume} a @dfn{tape}, there is absolutely no
11239 requirement for multi-volume archives to be stored on tapes. Instead,
11240 they can use whatever media type the user finds convenient, they can
11241 even be located on files.
11243 When creating a multi-volume archive, @GNUTAR{} continues to fill
11244 current volume until it runs out of space, then it switches to
11245 next volume (usually the operator is queried to replace the tape on
11246 this point), and continues working on the new volume. This operation
11247 continues until all requested files are dumped. If @GNUTAR{} detects
11248 end of media while dumping a file, such a file is archived in split
11249 form. Some very big files can even be split across several volumes.
11251 Each volume is itself a valid @GNUTAR{} archive, so it can be read
11252 without any special options. Consequently any file member residing
11253 entirely on one volume can be extracted or otherwise operated upon
11254 without needing the other volume. Sure enough, to extract a split
11255 member you would need all volumes its parts reside on.
11257 Multi-volume archives suffer from several limitations. In particular,
11258 they cannot be compressed.
11260 @GNUTAR{} is able to create multi-volume archives of two formats
11261 (@pxref{Formats}): @samp{GNU} and @samp{POSIX}.
11264 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
11265 * Tape Files:: Tape Files
11266 * Tarcat:: Concatenate Volumes into a Single Archive
11270 @node Multi-Volume Archives
11271 @subsection Archives Longer than One Tape or Disk
11272 @cindex Multi-volume archives
11274 @opindex multi-volume
11275 To create an archive that is larger than will fit on a single unit of
11276 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
11277 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
11278 archive can be manipulated like any other archive (provided the
11279 @option{--multi-volume} option is specified), but is stored on more
11280 than one tape or file.
11282 When you specify @option{--multi-volume}, @command{tar} does not report an
11283 error when it comes to the end of an archive volume (when reading), or
11284 the end of the media (when writing). Instead, it prompts you to load
11285 a new storage volume. If the archive is on a magnetic tape, you
11286 should change tapes when you see the prompt; if the archive is on a
11287 floppy disk, you should change disks; etc.
11290 @item --multi-volume
11292 Creates a multi-volume archive, when used in conjunction with
11293 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
11294 archive, specify @option{--multi-volume} in conjunction with that
11299 $ @kbd{tar --create --multi-volume --file=/dev/tape @var{files}}
11303 The method @command{tar} uses to detect end of tape is not perfect, and
11304 fails on some operating systems or on some devices. If @command{tar}
11305 cannot detect the end of the tape itself, you can use
11306 @option{--tape-length} option to inform it about the capacity of the
11309 @anchor{tape-length}
11311 @opindex tape-length
11312 @item --tape-length=@var{size}[@var{suf}]
11313 @itemx -L @var{size}[@var{suf}]
11314 Set maximum length of a volume. The @var{suf}, if given, specifies
11315 units in which @var{size} is expressed, e.g. @samp{2M} mean 2
11316 megabytes (@pxref{size-suffixes}, for a list of allowed size
11317 suffixes). Without @var{suf}, units of 1024 bytes (kilobyte) are
11320 This option selects @option{--multi-volume} automatically. For example:
11323 $ @kbd{tar --create --tape-length=41943040 --file=/dev/tape @var{files}}
11327 or, which is equivalent:
11330 $ @kbd{tar --create --tape-length=4G --file=/dev/tape @var{files}}
11334 @anchor{change volume prompt}
11335 When @GNUTAR{} comes to the end of a storage media, it asks you to
11336 change the volume. The built-in prompt for POSIX locale
11337 is@footnote{If you run @GNUTAR{} under a different locale, the
11338 translation to the locale's language will be used.}:
11341 Prepare volume #@var{n} for '@var{archive}' and hit return:
11345 where @var{n} is the ordinal number of the volume to be created and
11346 @var{archive} is archive file or device name.
11348 When prompting for a new tape, @command{tar} accepts any of the following
11353 Request @command{tar} to explain possible responses.
11355 Request @command{tar} to exit immediately.
11356 @item n @var{file-name}
11357 Request @command{tar} to write the next volume on the file @var{file-name}.
11359 Request @command{tar} to run a subshell. This option can be disabled
11360 by giving @option{--restrict} command line option to
11361 @command{tar}@footnote{@xref{--restrict}, for more information about
11364 Request @command{tar} to begin writing the next volume.
11367 (You should only type @samp{y} after you have changed the tape;
11368 otherwise @command{tar} will write over the volume it just finished.)
11370 @cindex Volume number file
11372 @anchor{volno-file}
11373 @opindex volno-file
11374 The volume number used by @command{tar} in its tape-changing prompt
11375 can be changed; if you give the
11376 @option{--volno-file=@var{file-of-number}} option, then
11377 @var{file-of-number} should be an non-existing file to be created, or
11378 else, a file already containing a decimal number. That number will be
11379 used as the volume number of the first volume written. When
11380 @command{tar} is finished, it will rewrite the file with the
11381 now-current volume number. (This does not change the volume number
11382 written on a tape label, as per @ref{label}, it @emph{only} affects
11383 the number used in the prompt.)
11385 @cindex End-of-archive info script
11386 @cindex Info script
11387 @anchor{info-script}
11388 @opindex info-script
11389 @opindex new-volume-script
11390 If you want more elaborate behavior than this, you can write a special
11391 @dfn{new volume script}, that will be responsible for changing the
11392 volume, and instruct @command{tar} to use it instead of its normal
11393 prompting procedure:
11396 @item --info-script=@var{command}
11397 @itemx --new-volume-script=@var{command}
11398 @itemx -F @var{command}
11399 Specify the command to invoke when switching volumes. The @var{command}
11400 can be used to eject cassettes, or to broadcast messages such as
11401 @samp{Someone please come change my tape} when performing unattended
11405 The @var{command} can contain additional options, if such are needed.
11406 @xref{external, Running External Commands}, for a detailed discussion
11407 of the way @GNUTAR{} runs external commands. It inherits
11408 @command{tar}'s shell environment. Additional data is passed to it
11409 via the following environment variables:
11412 @vrindex TAR_VERSION, info script environment variable
11414 @GNUTAR{} version number.
11416 @vrindex TAR_ARCHIVE, info script environment variable
11418 The name of the archive @command{tar} is processing.
11420 @vrindex TAR_BLOCKING_FACTOR, info script environment variable
11421 @item TAR_BLOCKING_FACTOR
11422 Current blocking factor (@pxref{Blocking}).
11424 @vrindex TAR_VOLUME, info script environment variable
11426 Ordinal number of the volume @command{tar} is about to start.
11428 @vrindex TAR_SUBCOMMAND, info script environment variable
11429 @item TAR_SUBCOMMAND
11430 A short option describing the operation @command{tar} is executing.
11431 @xref{Operations}, for a complete list of subcommand options.
11433 @vrindex TAR_FORMAT, info script environment variable
11435 Format of the archive being processed. @xref{Formats}, for a complete
11436 list of archive format names.
11438 @vrindex TAR_FD, info script environment variable
11440 File descriptor which can be used to communicate the new volume
11441 name to @command{tar}.
11444 These variables can be used in the @var{command} itself, provided that
11445 they are properly quoted to prevent them from being expanded by the
11446 shell that invokes @command{tar}.
11448 The volume script can instruct @command{tar} to use new archive name,
11449 by writing in to file descriptor @env{$TAR_FD} (see below for an example).
11451 If the info script fails, @command{tar} exits; otherwise, it begins
11452 writing the next volume.
11454 If you want @command{tar} to cycle through a series of files or tape
11455 drives, there are three approaches to choose from. First of all, you
11456 can give @command{tar} multiple @option{--file} options. In this case
11457 the specified files will be used, in sequence, as the successive
11458 volumes of the archive. Only when the first one in the sequence needs
11459 to be used again will @command{tar} prompt for a tape change (or run
11460 the info script). For example, suppose someone has two tape drives on
11461 a system named @file{/dev/tape0} and @file{/dev/tape1}. For having
11462 @GNUTAR{} to switch to the second drive when it needs to write the
11463 second tape, and then back to the first tape, etc., just do either of:
11466 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
11467 $ @kbd{tar -cM -f /dev/tape0 -f /dev/tape1 @var{files}}
11470 The second method is to use the @samp{n} response to the tape-change
11473 Finally, the most flexible approach is to use a volume script, that
11474 writes new archive name to the file descriptor @env{$TAR_FD}. For example, the
11475 following volume script will create a series of archive files, named
11476 @file{@var{archive}-@var{vol}}, where @var{archive} is the name of the
11477 archive being created (as given by @option{--file} option) and
11478 @var{vol} is the ordinal number of the archive being created:
11483 # For this script it's advisable to use a shell, such as Bash,
11484 # that supports a TAR_FD value greater than 9.
11486 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
11488 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
11489 case $TAR_SUBCOMMAND in
11491 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
11496 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&$TAR_FD
11500 The same script can be used while listing, comparing or extracting
11501 from the created archive. For example:
11505 # @r{Create a multi-volume archive:}
11506 $ @kbd{tar -c -L1024 -f archive.tar -F new-volume .}
11507 # @r{Extract from the created archive:}
11508 $ @kbd{tar -x -f archive.tar -F new-volume .}
11513 Notice, that the first command had to use @option{-L} option, since
11514 otherwise @GNUTAR{} will end up writing everything to file
11515 @file{archive.tar}.
11517 You can read each individual volume of a multi-volume archive as if it
11518 were an archive by itself. For example, to list the contents of one
11519 volume, use @option{--list}, without @option{--multi-volume} specified.
11520 To extract an archive member from one volume (assuming it is described
11521 that volume), use @option{--extract}, again without
11522 @option{--multi-volume}.
11524 If an archive member is split across volumes (i.e., its entry begins on
11525 one volume of the media and ends on another), you need to specify
11526 @option{--multi-volume} to extract it successfully. In this case, you
11527 should load the volume where the archive member starts, and use
11528 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
11529 volumes as it needs them. @xref{extracting archives}, for more
11530 information about extracting archives.
11532 Multi-volume archives can be modified like any other archive. To add
11533 files to a multi-volume archive, you need to only mount the last
11534 volume of the archive media (and new volumes, if needed). For all
11535 other operations, you need to use the entire archive.
11537 If a multi-volume archive was labeled using
11538 @option{--label=@var{archive-label}} (@pxref{label}) when it was
11539 created, @command{tar} will not automatically label volumes which are
11540 added later. To label subsequent volumes, specify
11541 @option{--label=@var{archive-label}} again in conjunction with the
11542 @option{--append}, @option{--update} or @option{--concatenate} operation.
11544 Notice that multi-volume support is a GNU extension and the archives
11545 created in this mode should be read only using @GNUTAR{}. If you
11546 absolutely have to process such archives using a third-party @command{tar}
11547 implementation, read @ref{Split Recovery}.
11550 @subsection Tape Files
11551 @cindex labeling archives
11555 To give the archive a name which will be recorded in it, use the
11556 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
11557 option. This will write a special block identifying
11558 @var{volume-label} as the name of the archive to the front of the
11559 archive which will be displayed when the archive is listed with
11560 @option{--list}. If you are creating a multi-volume archive with
11561 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
11562 volume label will have @samp{Volume @var{nnn}} appended to the name
11563 you give, where @var{nnn} is the number of the volume of the archive.
11564 If you use the @option{--label=@var{volume-label}} option when
11565 reading an archive, it checks to make sure the label on the tape
11566 matches the one you gave. @xref{label}.
11568 When @command{tar} writes an archive to tape, it creates a single
11569 tape file. If multiple archives are written to the same tape, one
11570 after the other, they each get written as separate tape files. When
11571 extracting, it is necessary to position the tape at the right place
11572 before running @command{tar}. To do this, use the @command{mt} command.
11573 For more information on the @command{mt} command and on the organization
11574 of tapes into a sequence of tape files, see @ref{mt}.
11576 People seem to often do:
11579 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
11582 or such, for pushing a common date in all volumes or an archive set.
11585 @subsection Concatenate Volumes into a Single Archive
11588 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
11589 archive to a single @command{tar} archive. Simply concatenating all
11590 volumes into one will not work, since each volume carries an additional
11591 information at the beginning. @GNUTAR{} is shipped with the shell
11592 script @command{tarcat} designed for this purpose.
11594 The script takes a list of files comprising a multi-volume archive
11595 and creates the resulting archive at the standard output. For example:
11598 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
11601 The script implements a simple heuristics to determine the format of
11602 the first volume file and to decide how to process the rest of the
11603 files. However, it makes no attempt to verify whether the files are
11604 given in order or even if they are valid @command{tar} archives.
11605 It uses @command{dd} and does not filter its standard error, so you
11606 will usually see lots of spurious messages.
11608 @FIXME{The script is not installed. Should we install it?}
11611 @section Including a Label in the Archive
11612 @cindex Labeling an archive
11613 @cindex Labels on the archive media
11614 @cindex Labeling multi-volume archives
11617 To avoid problems caused by misplaced paper labels on the archive
11618 media, you can include a @dfn{label} entry --- an archive member which
11619 contains the name of the archive --- in the archive itself. Use the
11620 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11621 option@footnote{Until version 1.10, that option was called
11622 @option{--volume}, but is not available under that name anymore.} in
11623 conjunction with the @option{--create} operation to include a label
11624 entry in the archive as it is being created.
11627 @item --label=@var{archive-label}
11628 @itemx -V @var{archive-label}
11629 Includes an @dfn{archive-label} at the beginning of the archive when
11630 the archive is being created, when used in conjunction with the
11631 @option{--create} operation. Checks to make sure the archive label
11632 matches the one specified (when used in conjunction with any other
11636 If you create an archive using both
11637 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11638 and @option{--multi-volume} (@option{-M}), each volume of the archive
11639 will have an archive label of the form @samp{@var{archive-label}
11640 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
11641 next, and so on. @xref{Using Multiple Tapes}, for information on
11642 creating multiple volume archives.
11644 @cindex Volume label, listing
11645 @cindex Listing volume label
11646 The volume label will be displayed by @option{--list} along with
11647 the file contents. If verbose display is requested, it will also be
11648 explicitly marked as in the example below:
11652 $ @kbd{tar --verbose --list --file=iamanarchive}
11653 V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header--
11654 -rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename
11658 @opindex test-label
11659 @anchor{--test-label option}
11660 However, @option{--list} option will cause listing entire
11661 contents of the archive, which may be undesirable (for example, if the
11662 archive is stored on a tape). You can request checking only the volume
11663 label by specifying @option{--test-label} option. This option reads only the
11664 first block of an archive, so it can be used with slow storage
11665 devices. For example:
11669 $ @kbd{tar --test-label --file=iamanarchive}
11674 If @option{--test-label} is used with one or more command line
11675 arguments, @command{tar} compares the volume label with each
11676 argument. It exits with code 0 if a match is found, and with code 1
11677 otherwise@footnote{Note that @GNUTAR{} versions up to 1.23 indicated
11678 mismatch with an exit code 2 and printed a spurious diagnostics on
11679 stderr.}. No output is displayed, unless you also used the
11680 @option{--verbose} option. For example:
11684 $ @kbd{tar --test-label --file=iamanarchive 'iamalabel'}
11686 $ @kbd{tar --test-label --file=iamanarchive 'alabel'}
11691 When used with the @option{--verbose} option, @command{tar}
11692 prints the actual volume label (if any), and a verbose diagnostics in
11693 case of a mismatch:
11697 $ @kbd{tar --test-label --verbose --file=iamanarchive 'iamalabel'}
11700 $ @kbd{tar --test-label --verbose --file=iamanarchive 'alabel'}
11702 tar: Archive label mismatch
11707 If you request any operation, other than @option{--create}, along
11708 with using @option{--label} option, @command{tar} will first check if
11709 the archive label matches the one specified and will refuse to proceed
11710 if it does not. Use this as a safety precaution to avoid accidentally
11711 overwriting existing archives. For example, if you wish to add files
11712 to @file{archive}, presumably labeled with string @samp{My volume},
11717 $ @kbd{tar -rf archive --label 'My volume' .}
11718 tar: Archive not labeled to match 'My volume'
11723 in case its label does not match. This will work even if
11724 @file{archive} is not labeled at all.
11726 Similarly, @command{tar} will refuse to list or extract the
11727 archive if its label doesn't match the @var{archive-label}
11728 specified. In those cases, @var{archive-label} argument is interpreted
11729 as a globbing-style pattern which must match the actual magnetic
11730 volume label. @xref{exclude}, for a precise description of how match
11731 is attempted@footnote{Previous versions of @command{tar} used full
11732 regular expression matching, or before that, only exact string
11733 matching, instead of wildcard matchers. We decided for the sake of
11734 simplicity to use a uniform matching device through
11735 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
11736 the volume label matcher will also suffix @var{archive-label} by
11737 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
11738 up. Since the volume numbering is automatically added in labels at
11739 creation time, it sounded logical to equally help the user taking care
11740 of it when the archive is being read.
11742 You can also use @option{--label} to get a common information on
11743 all tapes of a series. For having this information different in each
11744 series created through a single script used on a regular basis, just
11745 manage to get some date string as part of the label. For example:
11749 $ @kbd{tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"}
11750 $ @kbd{tar --create --file=/dev/tape --multi-volume \
11751 --label="Daily backup for `date +%Y-%m-%d`"}
11755 Some more notes about volume labels:
11758 @item Each label has its own date and time, which corresponds
11759 to the time when @GNUTAR{} initially attempted to write it,
11760 often soon after the operator launches @command{tar} or types the
11761 carriage return telling that the next tape is ready.
11763 @item Comparing date labels to get an idea of tape throughput is
11764 unreliable. It gives correct results only if the delays for rewinding
11765 tapes and the operator switching them were negligible, which is
11766 usually not the case.
11770 @section Verifying Data as It is Stored
11771 @cindex Verifying a write operation
11772 @cindex Double-checking a write operation
11777 @opindex verify, short description
11778 Attempt to verify the archive after writing.
11781 This option causes @command{tar} to verify the archive after writing it.
11782 Each volume is checked after it is written, and any discrepancies
11783 are recorded on the standard error output.
11785 Verification requires that the archive be on a back-space-able medium.
11786 This means pipes, some cartridge tape drives, and some other devices
11787 cannot be verified.
11789 You can insure the accuracy of an archive by comparing files in the
11790 system with archive members. @command{tar} can compare an archive to the
11791 file system as the archive is being written, to verify a write
11792 operation, or can compare a previously written archive, to insure that
11795 @xopindex{verify, using with @option{--create}}
11796 @xopindex{create, using with @option{--verify}}
11797 To check for discrepancies in an archive immediately after it is
11798 written, use the @option{--verify} (@option{-W}) option in conjunction with
11799 the @option{--create} operation. When this option is
11800 specified, @command{tar} checks archive members against their counterparts
11801 in the file system, and reports discrepancies on the standard error.
11803 To verify an archive, you must be able to read it from before the end
11804 of the last written entry. This option is useful for detecting data
11805 errors on some tapes. Archives written to pipes, some cartridge tape
11806 drives, and some other devices cannot be verified.
11808 One can explicitly compare an already made archive with the file
11809 system by using the @option{--compare} (@option{--diff}, @option{-d})
11810 option, instead of using the more automatic @option{--verify} option.
11813 Note that these two options have a slightly different intent. The
11814 @option{--compare} option checks how identical are the logical contents of some
11815 archive with what is on your disks, while the @option{--verify} option is
11816 really for checking if the physical contents agree and if the recording
11817 media itself is of dependable quality. So, for the @option{--verify}
11818 operation, @command{tar} tries to defeat all in-memory cache pertaining to
11819 the archive, while it lets the speed optimization undisturbed for the
11820 @option{--compare} option. If you nevertheless use @option{--compare} for
11821 media verification, you may have to defeat the in-memory cache yourself,
11822 maybe by opening and reclosing the door latch of your recording unit,
11823 forcing some doubt in your operating system about the fact this is really
11824 the same volume as the one just written or read.
11826 The @option{--verify} option would not be necessary if drivers were indeed
11827 able to detect dependably all write failures. This sometimes require many
11828 magnetic heads, some able to read after the writes occurred. One would
11829 not say that drivers unable to detect all cases are necessarily flawed,
11830 as long as programming is concerned.
11832 The @option{--verify} (@option{-W}) option will not work in
11833 conjunction with the @option{--multi-volume} (@option{-M}) option or
11834 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
11835 and @option{--delete} operations. @xref{Operations}, for more
11836 information on these operations.
11838 Also, since @command{tar} normally strips leading @samp{/} from file
11839 names (@pxref{absolute}), a command like @samp{tar --verify -cf
11840 /tmp/foo.tar /etc} will work as desired only if the working directory is
11841 @file{/}, as @command{tar} uses the archive's relative member names
11842 (e.g., @file{etc/motd}) when verifying the archive.
11844 @node Write Protection
11845 @section Write Protection
11847 Almost all tapes and diskettes, and in a few rare cases, even disks can
11848 be @dfn{write protected}, to protect data on them from being changed.
11849 Once an archive is written, you should write protect the media to prevent
11850 the archive from being accidentally overwritten or deleted. (This will
11851 protect the archive from being changed with a tape or floppy drive---it
11852 will not protect it from magnet fields or other physical hazards.)
11854 The write protection device itself is usually an integral part of the
11855 physical media, and can be a two position (write enabled/write
11856 disabled) switch, a notch which can be popped out or covered, a ring
11857 which can be removed from the center of a tape reel, or some other
11858 changeable feature.
11860 @node Reliability and security
11861 @chapter Reliability and Security
11863 The @command{tar} command reads and writes files as any other
11864 application does, and is subject to the usual caveats about
11865 reliability and security. This section contains some commonsense
11866 advice on the topic.
11874 @section Reliability
11876 Ideally, when @command{tar} is creating an archive, it reads from a
11877 file system that is not being modified, and encounters no errors or
11878 inconsistencies while reading and writing. If this is the case, the
11879 archive should faithfully reflect what was read. Similarly, when
11880 extracting from an archive, ideally @command{tar} ideally encounters
11881 no errors and the extracted files faithfully reflect what was in the
11884 However, when reading or writing real-world file systems, several
11885 things can go wrong; these include permissions problems, corruption of
11886 data, and race conditions.
11889 * Permissions problems::
11890 * Data corruption and repair::
11891 * Race conditions::
11894 @node Permissions problems
11895 @subsection Permissions Problems
11897 If @command{tar} encounters errors while reading or writing files, it
11898 normally reports an error and exits with nonzero status. The work it
11899 does may therefore be incomplete. For example, when creating an
11900 archive, if @command{tar} cannot read a file then it cannot copy the
11901 file into the archive.
11903 @node Data corruption and repair
11904 @subsection Data Corruption and Repair
11906 If an archive becomes corrupted by an I/O error, this may corrupt the
11907 data in an extracted file. Worse, it may corrupt the file's metadata,
11908 which may cause later parts of the archive to become misinterpreted.
11909 An tar-format archive contains a checksum that most likely will detect
11910 errors in the metadata, but it will not detect errors in the data.
11912 If data corruption is a concern, you can compute and check your own
11913 checksums of an archive by using other programs, such as
11916 When attempting to recover from a read error or data corruption in an
11917 archive, you may need to skip past the questionable data and read the
11918 rest of the archive. This requires some expertise in the archive
11919 format and in other software tools.
11921 @node Race conditions
11922 @subsection Race conditions
11924 If some other process is modifying the file system while @command{tar}
11925 is reading or writing files, the result may well be inconsistent due
11926 to race conditions. For example, if another process creates some
11927 files in a directory while @command{tar} is creating an archive
11928 containing the directory's files, @command{tar} may see some of the
11929 files but not others, or it may see a file that is in the process of
11930 being created. The resulting archive may not be a snapshot of the
11931 file system at any point in time. If an application such as a
11932 database system depends on an accurate snapshot, restoring from the
11933 @command{tar} archive of a live file system may therefore break that
11934 consistency and may break the application. The simplest way to avoid
11935 the consistency issues is to avoid making other changes to the file
11936 system while tar is reading it or writing it.
11938 When creating an archive, several options are available to avoid race
11939 conditions. Some hosts have a way of snapshotting a file system, or
11940 of temporarily suspending all changes to a file system, by (say)
11941 suspending the only virtual machine that can modify a file system; if
11942 you use these facilities and have @command{tar -c} read from a
11943 snapshot when creating an archive, you can avoid inconsistency
11944 problems. More drastically, before starting @command{tar} you could
11945 suspend or shut down all processes other than @command{tar} that have
11946 access to the file system, or you could unmount the file system and
11947 then mount it read-only.
11949 When extracting from an archive, one approach to avoid race conditions
11950 is to create a directory that no other process can write to, and
11956 In some cases @command{tar} may be used in an adversarial situation,
11957 where an untrusted user is attempting to gain information about or
11958 modify otherwise-inaccessible files. Dealing with untrusted data
11959 (that is, data generated by an untrusted user) typically requires
11960 extra care, because even the smallest mistake in the use of
11961 @command{tar} is more likely to be exploited by an adversary than by a
11967 * Live untrusted data::
11968 * Security rules of thumb::
11972 @subsection Privacy
11974 Standard privacy concerns apply when using @command{tar}. For
11975 example, suppose you are archiving your home directory into a file
11976 @file{/archive/myhome.tar}. Any secret information in your home
11977 directory, such as your SSH secret keys, are copied faithfully into
11978 the archive. Therefore, if your home directory contains any file that
11979 should not be read by some other user, the archive itself should be
11980 not be readable by that user. And even if the archive's data are
11981 inaccessible to untrusted users, its metadata (such as size or
11982 last-modified date) may reveal some information about your home
11983 directory; if the metadata are intended to be private, the archive's
11984 parent directory should also be inaccessible to untrusted users.
11986 One precaution is to create @file{/archive} so that it is not
11987 accessible to any user, unless that user also has permission to access
11988 all the files in your home directory.
11990 Similarly, when extracting from an archive, take care that the
11991 permissions of the extracted files are not more generous than what you
11992 want. Even if the archive itself is readable only to you, files
11993 extracted from it have their own permissions that may differ.
11996 @subsection Integrity
11998 When creating archives, take care that they are not writable by a
11999 untrusted user; otherwise, that user could modify the archive, and
12000 when you later extract from the archive you will get incorrect data.
12002 When @command{tar} extracts from an archive, by default it writes into
12003 files relative to the working directory. If the archive was generated
12004 by an untrusted user, that user therefore can write into any file
12005 under the working directory. If the working directory contains a
12006 symbolic link to another directory, the untrusted user can also write
12007 into any file under the referenced directory. When extracting from an
12008 untrusted archive, it is therefore good practice to create an empty
12009 directory and run @command{tar} in that directory.
12011 When extracting from two or more untrusted archives, each one should
12012 be extracted independently, into different empty directories.
12013 Otherwise, the first archive could create a symbolic link into an area
12014 outside the working directory, and the second one could follow the
12015 link and overwrite data that is not under the working directory. For
12016 example, when restoring from a series of incremental dumps, the
12017 archives should have been created by a trusted process, as otherwise
12018 the incremental restores might alter data outside the working
12021 If you use the @option{--absolute-names} (@option{-P}) option when
12022 extracting, @command{tar} respects any file names in the archive, even
12023 file names that begin with @file{/} or contain @file{..}. As this
12024 lets the archive overwrite any file in your system that you can write,
12025 the @option{--absolute-names} (@option{-P}) option should be used only
12026 for trusted archives.
12028 Conversely, with the @option{--keep-old-files} (@option{-k}) and
12029 @option{--skip-old-files} options, @command{tar} refuses to replace
12030 existing files when extracting. The difference between the two
12031 options is that the former treats existing files as errors whereas the
12032 latter just silently ignores them.
12034 Finally, with the @option{--no-overwrite-dir} option, @command{tar}
12035 refuses to replace the permissions or ownership of already-existing
12036 directories. These options may help when extracting from untrusted
12039 @node Live untrusted data
12040 @subsection Dealing with Live Untrusted Data
12042 Extra care is required when creating from or extracting into a file
12043 system that is accessible to untrusted users. For example, superusers
12044 who invoke @command{tar} must be wary about its actions being hijacked
12045 by an adversary who is reading or writing the file system at the same
12046 time that @command{tar} is operating.
12048 When creating an archive from a live file system, @command{tar} is
12049 vulnerable to denial-of-service attacks. For example, an adversarial
12050 user could create the illusion of an indefinitely-deep directory
12051 hierarchy @file{d/e/f/g/...} by creating directories one step ahead of
12052 @command{tar}, or the illusion of an indefinitely-long file by
12053 creating a sparse file but arranging for blocks to be allocated just
12054 before @command{tar} reads them. There is no easy way for
12055 @command{tar} to distinguish these scenarios from legitimate uses, so
12056 you may need to monitor @command{tar}, just as you'd need to monitor
12057 any other system service, to detect such attacks.
12059 While a superuser is extracting from an archive into a live file
12060 system, an untrusted user might replace a directory with a symbolic
12061 link, in hopes that @command{tar} will follow the symbolic link and
12062 extract data into files that the untrusted user does not have access
12063 to. Even if the archive was generated by the superuser, it may
12064 contain a file such as @file{d/etc/passwd} that the untrusted user
12065 earlier created in order to break in; if the untrusted user replaces
12066 the directory @file{d/etc} with a symbolic link to @file{/etc} while
12067 @command{tar} is running, @command{tar} will overwrite
12068 @file{/etc/passwd}. This attack can be prevented by extracting into a
12069 directory that is inaccessible to untrusted users.
12071 Similar attacks via symbolic links are also possible when creating an
12072 archive, if the untrusted user can modify an ancestor of a top-level
12073 argument of @command{tar}. For example, an untrusted user that can
12074 modify @file{/home/eve} can hijack a running instance of @samp{tar -cf
12075 - /home/eve/Documents/yesterday} by replacing
12076 @file{/home/eve/Documents} with a symbolic link to some other
12077 location. Attacks like these can be prevented by making sure that
12078 untrusted users cannot modify any files that are top-level arguments
12079 to @command{tar}, or any ancestor directories of these files.
12081 @node Security rules of thumb
12082 @subsection Security Rules of Thumb
12084 This section briefly summarizes rules of thumb for avoiding security
12090 Protect archives at least as much as you protect any of the files
12094 Extract from an untrusted archive only into an otherwise-empty
12095 directory. This directory and its parent should be accessible only to
12096 trusted users. For example:
12100 $ @kbd{chmod go-rwx .}
12101 $ @kbd{mkdir -m go-rwx dir}
12103 $ @kbd{tar -xvf /archives/got-it-off-the-net.tar.gz}
12107 As a corollary, do not do an incremental restore from an untrusted archive.
12110 Do not let untrusted users access files extracted from untrusted
12111 archives without checking first for problems such as setuid programs.
12114 Do not let untrusted users modify directories that are ancestors of
12115 top-level arguments of @command{tar}. For example, while you are
12116 executing @samp{tar -cf /archive/u-home.tar /u/home}, do not let an
12117 untrusted user modify @file{/}, @file{/archive}, or @file{/u}.
12120 Pay attention to the diagnostics and exit status of @command{tar}.
12123 When archiving live file systems, monitor running instances of
12124 @command{tar} to detect denial-of-service attacks.
12127 Avoid unusual options such as @option{--absolute-names} (@option{-P}),
12128 @option{--dereference} (@option{-h}), @option{--overwrite},
12129 @option{--recursive-unlink}, and @option{--remove-files} unless you
12130 understand their security implications.
12137 This appendix lists some important user-visible changes between
12138 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
12139 version of this document is available at
12140 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
12141 @GNUTAR{} documentation page}.
12144 @item Use of globbing patterns when listing and extracting.
12146 Previous versions of GNU tar assumed shell-style globbing when
12147 extracting from or listing an archive. For example:
12150 $ @kbd{tar xf foo.tar '*.c'}
12153 would extract all files whose names end in @samp{.c}. This behavior
12154 was not documented and was incompatible with traditional tar
12155 implementations. Therefore, starting from version 1.15.91, GNU tar
12156 no longer uses globbing by default. For example, the above invocation
12157 is now interpreted as a request to extract from the archive the file
12160 To facilitate transition to the new behavior for those users who got
12161 used to the previous incorrect one, @command{tar} will print a warning
12162 if it finds out that a requested member was not found in the archive
12163 and its name looks like a globbing pattern. For example:
12166 $ @kbd{tar xf foo.tar '*.c'}
12167 tar: Pattern matching characters used in file names. Please,
12168 tar: use --wildcards to enable pattern matching, or --no-wildcards to
12169 tar: suppress this warning.
12170 tar: *.c: Not found in archive
12171 tar: Error exit delayed from previous errors
12174 To treat member names as globbing patterns, use the @option{--wildcards} option.
12175 If you want to tar to mimic the behavior of versions prior to 1.15.91,
12176 add this option to your @env{TAR_OPTIONS} variable.
12178 @xref{wildcards}, for the detailed discussion of the use of globbing
12179 patterns by @GNUTAR{}.
12181 @item Use of short option @option{-o}.
12183 Earlier versions of @GNUTAR{} understood @option{-o} command line
12184 option as a synonym for @option{--old-archive}.
12186 @GNUTAR{} starting from version 1.13.90 understands this option as
12187 a synonym for @option{--no-same-owner}. This is compatible with
12188 UNIX98 @command{tar} implementations.
12190 However, to facilitate transition, @option{-o} option retains its
12191 old semantics when it is used with one of archive-creation commands.
12192 Users are encouraged to use @option{--format=oldgnu} instead.
12194 It is especially important, since versions of @acronym{GNU} Automake
12195 up to and including 1.8.4 invoke tar with this option to produce
12196 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
12197 of this issue and its implications.
12199 @xref{Options, tar-formats, Changing Automake's Behavior,
12200 automake, GNU Automake}, for a description on how to use various
12201 archive formats with @command{automake}.
12203 Future versions of @GNUTAR{} will understand @option{-o} only as a
12204 synonym for @option{--no-same-owner}.
12206 @item Use of short option @option{-l}
12208 Earlier versions of @GNUTAR{} understood @option{-l} option as a
12209 synonym for @option{--one-file-system}. Since such usage contradicted
12210 to UNIX98 specification and harmed compatibility with other
12211 implementations, it was declared deprecated in version 1.14. However,
12212 to facilitate transition to its new semantics, it was supported by
12213 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
12214 variant of @option{--check-links} was introduced in version 1.15.91.
12216 @item Use of options @option{--portability} and @option{--old-archive}
12218 These options are deprecated. Please use @option{--format=v7} instead.
12220 @item Use of option @option{--posix}
12222 This option is deprecated. Please use @option{--format=posix} instead.
12225 @node Configuring Help Summary
12226 @appendix Configuring Help Summary
12228 Running @kbd{tar --help} displays the short @command{tar} option
12229 summary (@pxref{help}). This summary is organized by @dfn{groups} of
12230 semantically close options. The options within each group are printed
12231 in the following order: a short option, eventually followed by a list
12232 of corresponding long option names, followed by a short description of
12233 the option. For example, here is an excerpt from the actual @kbd{tar
12237 Main operation mode:
12239 -A, --catenate, --concatenate append tar files to an archive
12240 -c, --create create a new archive
12241 -d, --diff, --compare find differences between archive and
12243 --delete delete from the archive
12246 @vrindex ARGP_HELP_FMT, environment variable
12247 The exact visual representation of the help output is configurable via
12248 @env{ARGP_HELP_FMT} environment variable. The value of this variable
12249 is a comma-separated list of @dfn{format variable} assignments. There
12250 are two kinds of format variables. An @dfn{offset variable} keeps the
12251 offset of some part of help output text from the leftmost column on
12252 the screen. A @dfn{boolean} variable is a flag that toggles some
12253 output feature on or off. Depending on the type of the corresponding
12254 variable, there are two kinds of assignments:
12257 @item Offset assignment
12259 The assignment to an offset variable has the following syntax:
12262 @var{variable}=@var{value}
12266 where @var{variable} is the variable name, and @var{value} is a
12267 numeric value to be assigned to the variable.
12269 @item Boolean assignment
12271 To assign @code{true} value to a variable, simply put this variable name. To
12272 assign @code{false} value, prefix the variable name with @samp{no-}. For
12277 # Assign @code{true} value:
12279 # Assign @code{false} value:
12285 Following variables are declared:
12287 @deftypevr {Help Output} boolean dup-args
12288 If true, arguments for an option are shown with both short and long
12289 options, even when a given option has both forms, for example:
12292 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
12295 If false, then if an option has both short and long forms, the
12296 argument is only shown with the long one, for example:
12299 -f, --file=ARCHIVE use archive file or device ARCHIVE
12303 and a message indicating that the argument is applicable to both
12304 forms is printed below the options. This message can be disabled
12305 using @code{dup-args-note} (see below).
12307 The default is false.
12310 @deftypevr {Help Output} boolean dup-args-note
12311 If this variable is true, which is the default, the following notice
12312 is displayed at the end of the help output:
12315 Mandatory or optional arguments to long options are also mandatory or
12316 optional for any corresponding short options.
12319 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
12320 variables @code{dup-args} or @code{dup-args-note} should be set.
12323 @deftypevr {Help Output} offset short-opt-col
12324 Column in which short options start. Default is 2.
12328 $ @kbd{tar --help|grep ARCHIVE}
12329 -f, --file=ARCHIVE use archive file or device ARCHIVE
12330 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
12331 -f, --file=ARCHIVE use archive file or device ARCHIVE
12336 @deftypevr {Help Output} offset long-opt-col
12337 Column in which long options start. Default is 6. For example:
12341 $ @kbd{tar --help|grep ARCHIVE}
12342 -f, --file=ARCHIVE use archive file or device ARCHIVE
12343 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
12344 -f, --file=ARCHIVE use archive file or device ARCHIVE
12349 @deftypevr {Help Output} offset doc-opt-col
12350 Column in which @dfn{doc options} start. A doc option isn't actually
12351 an option, but rather an arbitrary piece of documentation that is
12352 displayed in much the same manner as the options. For example, in
12353 the description of @option{--format} option:
12357 -H, --format=FORMAT create archive of the given format.
12359 FORMAT is one of the following:
12361 gnu GNU tar 1.13.x format
12362 oldgnu GNU format as per tar <= 1.12
12363 pax POSIX 1003.1-2001 (pax) format
12365 ustar POSIX 1003.1-1988 (ustar) format
12366 v7 old V7 tar format
12371 the format names are doc options. Thus, if you set
12372 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
12373 will look as follows:
12377 -H, --format=FORMAT create archive of the given format.
12379 FORMAT is one of the following:
12381 gnu GNU tar 1.13.x format
12382 oldgnu GNU format as per tar <= 1.12
12383 pax POSIX 1003.1-2001 (pax) format
12385 ustar POSIX 1003.1-1988 (ustar) format
12386 v7 old V7 tar format
12391 @deftypevr {Help Output} offset opt-doc-col
12392 Column in which option description starts. Default is 29.
12396 $ @kbd{tar --help|grep ARCHIVE}
12397 -f, --file=ARCHIVE use archive file or device ARCHIVE
12398 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
12399 -f, --file=ARCHIVE use archive file or device ARCHIVE
12400 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
12402 use archive file or device ARCHIVE
12407 Notice, that the description starts on a separate line if
12408 @code{opt-doc-col} value is too small.
12411 @deftypevr {Help Output} offset header-col
12412 Column in which @dfn{group headers} are printed. A group header is a
12413 descriptive text preceding an option group. For example, in the
12417 Main operation mode:
12419 -A, --catenate, --concatenate append tar files to
12421 -c, --create create a new archive
12424 @samp{Main operation mode:} is the group header.
12426 The default value is 1.
12429 @deftypevr {Help Output} offset usage-indent
12430 Indentation of wrapped usage lines. Affects @option{--usage}
12431 output. Default is 12.
12434 @deftypevr {Help Output} offset rmargin
12435 Right margin of the text output. Used for wrapping.
12438 @node Fixing Snapshot Files
12439 @appendix Fixing Snapshot Files
12440 @include tar-snapshot-edit.texi
12442 @node Tar Internals
12443 @appendix Tar Internals
12444 @include intern.texi
12448 @include genfile.texi
12450 @node Free Software Needs Free Documentation
12451 @appendix Free Software Needs Free Documentation
12452 @include freemanuals.texi
12454 @node GNU Free Documentation License
12455 @appendix GNU Free Documentation License
12459 @node Index of Command Line Options
12460 @appendix Index of Command Line Options
12462 This appendix contains an index of all @GNUTAR{} long command line
12463 options. The options are listed without the preceding double-dash.
12464 For a cross-reference of short command line options, see
12465 @ref{Short Option Summary}.
12478 @c Local variables:
12479 @c texinfo-column-for-description: 32