1 \input texinfo @c -*-texinfo-*-
2 @comment %**start of header
5 @settitle GNU tar @value{VERSION}
14 @c 1. Pay attention to @FIXME{}s and @UNREVISED{}s
15 @c 2. Before creating final variant:
16 @c 2.1. Run 'make check-options' to make sure all options are properly
18 @c 2.2. Run 'make master-menu' (see comment before the master menu).
20 @include rendition.texi
26 @c Put everything in one index (arbitrarily chosen to be the concept index).
35 This manual is for @acronym{GNU} @command{tar} (version
36 @value{VERSION}, @value{UPDATED}), which creates and extracts files
39 Copyright @copyright{} 1992, 1994--1997, 1999--2001, 2003--2013 Free
40 Software Foundation, Inc.
43 Permission is granted to copy, distribute and/or modify this document
44 under the terms of the GNU Free Documentation License, Version 1.3 or
45 any later version published by the Free Software Foundation; with the
46 Invariant Sections being ``GNU General Public License'', with the
47 Front-Cover Texts being ``A GNU Manual'', and with the Back-Cover Texts
48 as in (a) below. A copy of the license is included in the section
49 entitled ``GNU Free Documentation License''.
51 (a) The FSF's Back-Cover Text is: ``You have the freedom to
52 copy and modify this GNU manual.''
56 @dircategory Archiving
58 * Tar: (tar). Making tape (or disk) archives.
61 @dircategory Individual utilities
63 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
66 @shorttitlepage @acronym{GNU} @command{tar}
69 @title @acronym{GNU} tar: an archiver tool
70 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
71 @author John Gilmore, Jay Fenlason et al.
74 @vskip 0pt plus 1filll
80 @top @acronym{GNU} tar: an archiver tool
85 @cindex archiving files
87 The first part of this master menu lists the major nodes in this Info
88 document. The rest of the menu lists all the lower level nodes.
91 @c The master menu goes here.
93 @c NOTE: To update it from within Emacs, make sure mastermenu.el is
94 @c loaded and run texinfo-master-menu.
95 @c To update it from the command line, run
106 * Date input formats::
109 * Reliability and security::
114 * Configuring Help Summary::
115 * Fixing Snapshot Files::
118 * Free Software Needs Free Documentation::
119 * GNU Free Documentation License::
120 * Index of Command Line Options::
124 --- The Detailed Node Listing ---
128 * Book Contents:: What this Book Contains
129 * Definitions:: Some Definitions
130 * What tar Does:: What @command{tar} Does
131 * Naming tar Archives:: How @command{tar} Archives are Named
132 * Authors:: @GNUTAR{} Authors
133 * Reports:: Reporting bugs or suggestions
135 Tutorial Introduction to @command{tar}
138 * stylistic conventions::
139 * basic tar options:: Basic @command{tar} Operations and Options
140 * frequent operations::
141 * Two Frequent Options::
142 * create:: How to Create Archives
143 * list:: How to List Archives
144 * extract:: How to Extract Members from an Archive
147 Two Frequently Used Options
153 How to Create Archives
155 * prepare for examples::
156 * Creating the archive::
165 How to Extract Members from an Archive
167 * extracting archives::
170 * extracting untrusted archives::
176 * using tar options::
186 The Three Option Styles
188 * Long Options:: Long Option Style
189 * Short Options:: Short Option Style
190 * Old Options:: Old Option Style
191 * Mixing:: Mixing Option Styles
193 All @command{tar} Options
195 * Operation Summary::
197 * Short Option Summary::
209 Advanced @GNUTAR{} Operations
218 How to Add Files to Existing Archives: @option{--append}
220 * appending files:: Appending Files to an Archive
227 Options Used by @option{--create}
229 * override:: Overriding File Metadata.
230 * Ignore Failed Read::
232 Options Used by @option{--extract}
234 * Reading:: Options to Help Read Archives
235 * Writing:: Changing How @command{tar} Writes Files
236 * Scarce:: Coping with Scarce Resources
238 Options to Help Read Archives
240 * read full records::
243 Changing How @command{tar} Writes Files
245 * Dealing with Old Files::
246 * Overwrite Old Files::
251 * Data Modification Times::
252 * Setting Access Permissions::
253 * Directory Modification Times and Permissions::
254 * Writing to Standard Output::
255 * Writing to an External Program::
258 Coping with Scarce Resources
263 Performing Backups and Restoring Files
265 * Full Dumps:: Using @command{tar} to Perform Full Dumps
266 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
267 * Backup Levels:: Levels of Backups
268 * Backup Parameters:: Setting Parameters for Backups and Restoration
269 * Scripted Backups:: Using the Backup Scripts
270 * Scripted Restoration:: Using the Restore Script
272 Setting Parameters for Backups and Restoration
274 * General-Purpose Variables::
275 * Magnetic Tape Control::
277 * backup-specs example:: An Example Text of @file{Backup-specs}
279 Choosing Files and Names for @command{tar}
281 * file:: Choosing the Archive's Name
282 * Selecting Archive Members::
283 * files:: Reading Names from a File
284 * exclude:: Excluding Some Files
285 * wildcards:: Wildcards Patterns and Matching
286 * quoting styles:: Ways of Quoting Special Characters in Names
287 * transform:: Modifying File and Member Names
288 * after:: Operating Only on New Files
289 * recurse:: Descending into Directories
290 * one:: Crossing File System Boundaries
292 Reading Names from a File
298 * problems with exclude::
300 Wildcards Patterns and Matching
302 * controlling pattern-matching::
304 Crossing File System Boundaries
306 * directory:: Changing Directory
307 * absolute:: Absolute File Names
311 * General date syntax:: Common rules.
312 * Calendar date items:: 19 Dec 1994.
313 * Time of day items:: 9:20pm.
314 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}.
315 * Day of week items:: Monday and others.
316 * Relative items in date strings:: next tuesday, 2 years ago.
317 * Pure numbers in date strings:: 19931219, 1440.
318 * Seconds since the Epoch:: @@1078100502.
319 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0".
320 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al.
322 Controlling the Archive Format
324 * Compression:: Using Less Space through Compression
325 * Attributes:: Handling File Attributes
326 * Portability:: Making @command{tar} Archives More Portable
327 * cpio:: Comparison of @command{tar} and @command{cpio}
329 Using Less Space through Compression
331 * gzip:: Creating and Reading Compressed Archives
332 * sparse:: Archiving Sparse Files
334 Creating and Reading Compressed Archives
336 * lbzip2:: Using lbzip2 with @GNUTAR{}.
338 Making @command{tar} Archives More Portable
340 * Portable Names:: Portable Names
341 * dereference:: Symbolic Links
342 * hard links:: Hard Links
343 * old:: Old V7 Archives
344 * ustar:: Ustar Archives
345 * gnu:: GNU and old GNU format archives.
346 * posix:: @acronym{POSIX} archives
347 * Checksumming:: Checksumming Problems
348 * Large or Negative Values:: Large files, negative time stamps, etc.
349 * Other Tars:: How to Extract GNU-Specific Data Using
350 Other @command{tar} Implementations
352 @GNUTAR{} and @acronym{POSIX} @command{tar}
354 * PAX keywords:: Controlling Extended Header Keywords.
356 How to Extract GNU-Specific Data Using Other @command{tar} Implementations
358 * Split Recovery:: Members Split Between Volumes
359 * Sparse Recovery:: Sparse Members
361 Tapes and Other Archive Media
363 * Device:: Device selection and switching
364 * Remote Tape Server::
365 * Common Problems and Solutions::
366 * Blocking:: Blocking
367 * Many:: Many archives on one tape
368 * Using Multiple Tapes:: Using Multiple Tapes
369 * label:: Including a Label in the Archive
375 * Format Variations:: Format Variations
376 * Blocking Factor:: The Blocking Factor of an Archive
378 Many Archives on One Tape
380 * Tape Positioning:: Tape Positions and Tape Marks
381 * mt:: The @command{mt} Utility
385 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
386 * Tape Files:: Tape Files
387 * Tarcat:: Concatenate Volumes into a Single Archive
392 * Standard:: Basic Tar Format
393 * Extensions:: @acronym{GNU} Extensions to the Archive Format
394 * Sparse Formats:: Storing Sparse Files
401 * PAX 0:: PAX Format, Versions 0.0 and 0.1
402 * PAX 1:: PAX Format, Version 1.0
406 * Generate Mode:: File Generation Mode.
407 * Status Mode:: File Status Mode.
408 * Exec Mode:: Synchronous Execution mode.
412 * GNU Free Documentation License:: License for copying this manual
418 @chapter Introduction
421 and manipulates @dfn{archives} which are actually collections of
422 many other files; the program provides users with an organized and
423 systematic method for controlling a large amount of data.
424 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
425 archives need not (and these days, typically do not) reside on tapes.
428 * Book Contents:: What this Book Contains
429 * Definitions:: Some Definitions
430 * What tar Does:: What @command{tar} Does
431 * Naming tar Archives:: How @command{tar} Archives are Named
432 * Authors:: @GNUTAR{} Authors
433 * Reports:: Reporting bugs or suggestions
437 @section What this Book Contains
439 The first part of this chapter introduces you to various terms that will
440 recur throughout the book. It also tells you who has worked on @GNUTAR{}
441 and its documentation, and where you should send bug reports
444 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
445 gentle introduction for people who are new to using @command{tar}. It is
446 meant to be self-contained, not requiring any reading from subsequent
447 chapters to make sense. It moves from topic to topic in a logical,
448 progressive order, building on information already explained.
450 Although the tutorial is paced and structured to allow beginners to
451 learn how to use @command{tar}, it is not intended solely for beginners.
452 The tutorial explains how to use the three most frequently used
453 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
454 two frequently used options (@samp{file} and @samp{verbose}). The other
455 chapters do not refer to the tutorial frequently; however, if a section
456 discusses something which is a complex variant of a basic concept, there
457 may be a cross-reference to that basic concept. (The entire book,
458 including the tutorial, assumes that the reader understands some basic
459 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
461 The third chapter presents the remaining five operations, and
462 information about using @command{tar} options and option syntax.
464 The other chapters are meant to be used as a reference. Each chapter
465 presents everything that needs to be said about a specific topic.
467 One of the chapters (@pxref{Date input formats}) exists in its
468 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
469 In addition, one section of this manual (@pxref{Standard}) contains a
470 big quote which is taken directly from @command{tar} sources.
472 In general, we give both long and short (abbreviated) option names
473 at least once in each section where the relevant option is covered, so
474 that novice readers will become familiar with both styles. (A few
475 options have no short versions, and the relevant sections will
479 @section Some Definitions
483 The @command{tar} program is used to create and manipulate @command{tar}
484 archives. An @dfn{archive} is a single file which contains the contents
485 of many files, while still identifying the names of the files, their
486 owner(s), and so forth. (In addition, archives record access
487 permissions, user and group, size in bytes, and data modification time.
488 Some archives also record the file names in each archived directory, as
489 well as other file and directory information.) You can use @command{tar}
490 to @dfn{create} a new archive in a specified directory.
493 @cindex archive member
496 The files inside an archive are called @dfn{members}. Within this
497 manual, we use the term @dfn{file} to refer only to files accessible in
498 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
499 @dfn{member} to refer only to the members of an archive. Similarly, a
500 @dfn{file name} is the name of a file, as it resides in the file system,
501 and a @dfn{member name} is the name of an archive member within the
506 The term @dfn{extraction} refers to the process of copying an archive
507 member (or multiple members) into a file in the file system. Extracting
508 all the members of an archive is often called @dfn{extracting the
509 archive}. The term @dfn{unpack} can also be used to refer to the
510 extraction of many or all the members of an archive. Extracting an
511 archive does not destroy the archive's structure, just as creating an
512 archive does not destroy the copies of the files that exist outside of
513 the archive. You may also @dfn{list} the members in a given archive
514 (this is often thought of as ``printing'' them to the standard output,
515 or the command line), or @dfn{append} members to a pre-existing archive.
516 All of these operations can be performed using @command{tar}.
519 @section What @command{tar} Does
522 The @command{tar} program provides the ability to create @command{tar}
523 archives, as well as various other kinds of manipulation. For example,
524 you can use @command{tar} on previously created archives to extract files,
525 to store additional files, or to update or list files which were already
528 Initially, @command{tar} archives were used to store files conveniently on
529 magnetic tape. The name @command{tar} comes from this use; it stands for
530 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
531 direct its output to available devices, files, or other programs (using
532 pipes). @command{tar} may even access remote devices or files (as archives).
534 You can use @command{tar} archives in many ways. We want to stress a few
535 of them: storage, backup, and transportation.
537 @FIXME{the following table entries need a bit of work.}
540 Often, @command{tar} archives are used to store related files for
541 convenient file transfer over a network. For example, the
542 @acronym{GNU} Project distributes its software bundled into
543 @command{tar} archives, so that all the files relating to a particular
544 program (or set of related programs) can be transferred as a single
547 A magnetic tape can store several files in sequence. However, the tape
548 has no names for these files; it only knows their relative position on
549 the tape. One way to store several files on one tape and retain their
550 names is by creating a @command{tar} archive. Even when the basic transfer
551 mechanism can keep track of names, as FTP can, the nuisance of handling
552 multiple files, directories, and multiple links makes @command{tar}
555 Archive files are also used for long-term storage. You can think of
556 this as transportation from the present into the future. (It is a
557 science-fiction idiom that you can move through time as well as in
558 space; the idea here is that @command{tar} can be used to move archives in
559 all dimensions, even time!)
562 Because the archive created by @command{tar} is capable of preserving
563 file information and directory structure, @command{tar} is commonly
564 used for performing full and incremental backups of disks. A backup
565 puts a collection of files (possibly pertaining to many users and
566 projects) together on a disk or a tape. This guards against
567 accidental destruction of the information in those files.
568 @GNUTAR{} has special features that allow it to be
569 used to make incremental and full dumps of all the files in a
573 You can create an archive on one system, transfer it to another system,
574 and extract the contents there. This allows you to transport a group of
575 files from one system to another.
578 @node Naming tar Archives
579 @section How @command{tar} Archives are Named
581 Conventionally, @command{tar} archives are given names ending with
582 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
583 but this manual follows that convention in order to accustom readers to
584 it and to make examples more clear.
589 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
590 archive members as ``files'' or ``entries''. For people familiar with
591 the operation of @command{tar}, this causes no difficulty. However, in
592 this manual, we consistently refer to ``archives'' and ``archive
593 members'' to make learning to use @command{tar} easier for novice users.
596 @section @GNUTAR{} Authors
598 @GNUTAR{} was originally written by John Gilmore,
599 and modified by many people. The @acronym{GNU} enhancements were
600 written by Jay Fenlason, then Joy Kendall, and the whole package has
601 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
602 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
603 numerous and kind users.
605 We wish to stress that @command{tar} is a collective work, and owes much to
606 all those people who reported problems, offered solutions and other
607 insights, or shared their thoughts and suggestions. An impressive, yet
608 partial list of those contributors can be found in the @file{THANKS}
609 file from the @GNUTAR{} distribution.
611 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
612 sure i want to spell out the history in this detail, at least not for
613 the printed book. i'm just not sure it needs to be said this way.
614 i'll think about it.}
616 @FIXME{History is more important, and surely more interesting, than
617 actual names. Quoting names without history would be meaningless. FP}
619 Jay Fenlason put together a draft of a @GNUTAR{}
620 manual, borrowing notes from the original man page from John Gilmore.
621 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
622 Gorin worked on a tutorial and manual for @GNUTAR{}.
623 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
624 taking information from all these sources and merging them. Melissa
625 Weisshaus finally edited and redesigned the book to create version
626 1.12. The book for versions from 1.14 up to @value{VERSION} were edited
627 by the current maintainer, Sergey Poznyakoff.
629 For version 1.12, Daniel Hagerty contributed a great deal of technical
630 consulting. In particular, he is the primary author of @ref{Backups}.
632 In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org
633 (see @url{http://savannah.gnu.org/projects/tar}), and
634 active development and maintenance work has started
635 again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey
636 Poznyakoff and Jeff Bailey.
638 Support for @acronym{POSIX} archives was added by Sergey Poznyakoff.
641 @section Reporting bugs or suggestions
644 @cindex reporting bugs
645 If you find problems or have suggestions about this program or manual,
646 please report them to @file{bug-tar@@gnu.org}.
648 When reporting a bug, please be sure to include as much detail as
649 possible, in order to reproduce it.
650 @FIXME{Be more specific, I'd like to make this node as detailed as
651 'Bug reporting' node in Emacs manual.}
654 @chapter Tutorial Introduction to @command{tar}
656 This chapter guides you through some basic examples of three @command{tar}
657 operations: @option{--create}, @option{--list}, and @option{--extract}. If
658 you already know how to use some other version of @command{tar}, then you
659 may not need to read this chapter. This chapter omits most complicated
660 details about how @command{tar} works.
664 * stylistic conventions::
665 * basic tar options:: Basic @command{tar} Operations and Options
666 * frequent operations::
667 * Two Frequent Options::
668 * create:: How to Create Archives
669 * list:: How to List Archives
670 * extract:: How to Extract Members from an Archive
675 @section Assumptions this Tutorial Makes
677 This chapter is paced to allow beginners to learn about @command{tar}
678 slowly. At the same time, we will try to cover all the basic aspects of
679 these three operations. In order to accomplish both of these tasks, we
680 have made certain assumptions about your knowledge before reading this
681 manual, and the hardware you will be using:
685 Before you start to work through this tutorial, you should understand
686 what the terms ``archive'' and ``archive member'' mean
687 (@pxref{Definitions}). In addition, you should understand something
688 about how Unix-type operating systems work, and you should know how to
689 use some basic utilities. For example, you should know how to create,
690 list, copy, rename, edit, and delete files and directories; how to
691 change between directories; and how to figure out where you are in the
692 file system. You should have some basic understanding of directory
693 structure and how files are named according to which directory they are
694 in. You should understand concepts such as standard output and standard
695 input, what various definitions of the term @samp{argument} mean, and the
696 differences between relative and absolute file names.
697 @FIXME{and what else?}
700 This manual assumes that you are working from your own home directory
701 (unless we state otherwise). In this tutorial, you will create a
702 directory to practice @command{tar} commands in. When we show file names,
703 we will assume that those names are relative to your home directory.
704 For example, my home directory is @file{/home/fsf/melissa}. All of
705 my examples are in a subdirectory of the directory named by that file
706 name; the subdirectory is called @file{practice}.
709 In general, we show examples of archives which exist on (or can be
710 written to, or worked with from) a directory on a hard disk. In most
711 cases, you could write those archives to, or work with them on any other
712 device, such as a tape drive. However, some of the later examples in
713 the tutorial and next chapter will not work on tape drives.
714 Additionally, working with tapes is much more complicated than working
715 with hard disks. For these reasons, the tutorial does not cover working
716 with tape drives. @xref{Media}, for complete information on using
717 @command{tar} archives with tape drives.
719 @FIXME{this is a cop out. need to add some simple tape drive info.}
722 @node stylistic conventions
723 @section Stylistic Conventions
725 In the examples, @samp{$} represents a typical shell prompt. It
726 precedes lines you should type; to make this more clear, those lines are
727 shown in @kbd{this font}, as opposed to lines which represent the
728 computer's response; those lines are shown in @code{this font}, or
729 sometimes @samp{like this}.
731 @c When we have lines which are too long to be
732 @c displayed in any other way, we will show them like this:
734 @node basic tar options
735 @section Basic @command{tar} Operations and Options
737 @command{tar} can take a wide variety of arguments which specify and define
738 the actions it will have on the particular set of files or the archive.
739 The main types of arguments to @command{tar} fall into one of two classes:
740 operations, and options.
742 Some arguments fall into a class called @dfn{operations}; exactly one of
743 these is both allowed and required for any instance of using @command{tar};
744 you may @emph{not} specify more than one. People sometimes speak of
745 @dfn{operating modes}. You are in a particular operating mode when you
746 have specified the operation which specifies it; there are eight
747 operations in total, and thus there are eight operating modes.
749 The other arguments fall into the class known as @dfn{options}. You are
750 not required to specify any options, and you are allowed to specify more
751 than one at a time (depending on the way you are using @command{tar} at
752 that time). Some options are used so frequently, and are so useful for
753 helping you type commands more carefully that they are effectively
754 ``required''. We will discuss them in this chapter.
756 You can write most of the @command{tar} operations and options in any
757 of three forms: long (mnemonic) form, short form, and old style. Some
758 of the operations and options have no short or ``old'' forms; however,
759 the operations and options which we will cover in this tutorial have
760 corresponding abbreviations. We will indicate those abbreviations
761 appropriately to get you used to seeing them. Note, that the ``old
762 style'' option forms exist in @GNUTAR{} for compatibility with Unix
763 @command{tar}. In this book we present a full discussion of this way
764 of writing options and operations (@pxref{Old Options}), and we discuss
765 the other two styles of writing options (@xref{Long Options}, and
766 @pxref{Short Options}).
768 In the examples and in the text of this tutorial, we usually use the
769 long forms of operations and options; but the ``short'' forms produce
770 the same result and can make typing long @command{tar} commands easier.
771 For example, instead of typing
774 @kbd{tar --create --verbose --file=afiles.tar apple angst aspic}
780 @kbd{tar -c -v -f afiles.tar apple angst aspic}
786 @kbd{tar -cvf afiles.tar apple angst aspic}
790 For more information on option syntax, see @ref{Advanced tar}. In
791 discussions in the text, when we name an option by its long form, we
792 also give the corresponding short option in parentheses.
794 The term, ``option'', can be confusing at times, since ``operations''
795 are often lumped in with the actual, @emph{optional} ``options'' in certain
796 general class statements. For example, we just talked about ``short and
797 long forms of options and operations''. However, experienced @command{tar}
798 users often refer to these by shorthand terms such as, ``short and long
799 options''. This term assumes that the ``operations'' are included, also.
800 Context will help you determine which definition of ``options'' to use.
802 Similarly, the term ``command'' can be confusing, as it is often used in
803 two different ways. People sometimes refer to @command{tar} ``commands''.
804 A @command{tar} @dfn{command} is the entire command line of user input
805 which tells @command{tar} what to do --- including the operation, options,
806 and any arguments (file names, pipes, other commands, etc.). However,
807 you will also sometimes hear the term ``the @command{tar} command''. When
808 the word ``command'' is used specifically like this, a person is usually
809 referring to the @command{tar} @emph{operation}, not the whole line.
810 Again, use context to figure out which of the meanings the speaker
813 @node frequent operations
814 @section The Three Most Frequently Used Operations
816 Here are the three most frequently used operations (both short and long
817 forms), as well as a brief description of their meanings. The rest of
818 this chapter will cover how to use these operations in detail. We will
819 present the rest of the operations in the next chapter.
824 Create a new @command{tar} archive.
827 List the contents of an archive.
830 Extract one or more members from an archive.
833 @node Two Frequent Options
834 @section Two Frequently Used Options
836 To understand how to run @command{tar} in the three operating modes listed
837 previously, you also need to understand how to use two of the options to
838 @command{tar}: @option{--file} (which takes an archive file as an argument)
839 and @option{--verbose}. (You are usually not @emph{required} to specify
840 either of these options when you run @command{tar}, but they can be very
841 useful in making things more clear and helping you avoid errors.)
850 @unnumberedsubsec The @option{--file} Option
853 @xopindex{file, tutorial}
854 @item --file=@var{archive-name}
855 @itemx -f @var{archive-name}
856 Specify the name of an archive file.
859 You can specify an argument for the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option whenever you
860 use @command{tar}; this option determines the name of the archive file
861 that @command{tar} will work on.
864 If you don't specify this argument, then @command{tar} will examine
865 the environment variable @env{TAPE}. If it is set, its value will be
866 used as the archive name. Otherwise, @command{tar} will use the
867 default archive, determined at compile time. Usually it is
868 standard output or some physical tape drive attached to your machine
869 (you can verify what the default is by running @kbd{tar
870 --show-defaults}, @pxref{defaults}). If there is no tape drive
871 attached, or the default is not meaningful, then @command{tar} will
872 print an error message. The error message might look roughly like one
876 tar: can't open /dev/rmt8 : No such device or address
877 tar: can't open /dev/rsmt0 : I/O error
881 To avoid confusion, we recommend that you always specify an archive file
882 name by using @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) when writing your @command{tar} commands.
883 For more information on using the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option, see
886 @node verbose tutorial
887 @unnumberedsubsec The @option{--verbose} Option
890 @xopindex{verbose, introduced}
893 Show the files being worked on as @command{tar} is running.
896 @option{--verbose} (@option{-v}) shows details about the results of running
897 @command{tar}. This can be especially useful when the results might not be
898 obvious. For example, if you want to see the progress of @command{tar} as
899 it writes files into the archive, you can use the @option{--verbose}
900 option. In the beginning, you may find it useful to use
901 @option{--verbose} at all times; when you are more accustomed to
902 @command{tar}, you will likely want to use it at certain times but not at
903 others. We will use @option{--verbose} at times to help make something
904 clear, and we will give many examples both using and not using
905 @option{--verbose} to show the differences.
907 Each instance of @option{--verbose} on the command line increases the
908 verbosity level by one, so if you need more details on the output,
911 When reading archives (@option{--list}, @option{--extract},
912 @option{--diff}), @command{tar} by default prints only the names of
913 the members being extracted. Using @option{--verbose} will show a full,
914 @command{ls} style member listing.
916 In contrast, when writing archives (@option{--create}, @option{--append},
917 @option{--update}), @command{tar} does not print file names by
918 default. So, a single @option{--verbose} option shows the file names
919 being added to the archive, while two @option{--verbose} options
920 enable the full listing.
922 For example, to create an archive in verbose mode:
925 $ @kbd{tar -cvf afiles.tar apple angst aspic}
932 Creating the same archive with the verbosity level 2 could give:
935 $ @kbd{tar -cvvf afiles.tar apple angst aspic}
936 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
937 -rw-r--r-- gray/staff 11481 2006-06-09 12:06 angst
938 -rw-r--r-- gray/staff 23152 2006-06-09 12:06 aspic
942 This works equally well using short or long forms of options. Using
943 long forms, you would simply write out the mnemonic form of the option
947 $ @kbd{tar --create --verbose --verbose @dots{}}
951 Note that you must double the hyphens properly each time.
953 Later in the tutorial, we will give examples using @w{@option{--verbose
956 @anchor{verbose member listing}
957 The full output consists of six fields:
960 @item File type and permissions in symbolic form.
961 These are displayed in the same format as the first column of
962 @command{ls -l} output (@pxref{What information is listed,
963 format=verbose, Verbose listing, fileutils, GNU file utilities}).
965 @item Owner name and group separated by a slash character.
966 If these data are not available (for example, when listing a @samp{v7} format
967 archive), numeric @acronym{ID} values are printed instead.
969 @item Size of the file, in bytes.
971 @item File modification date in ISO 8601 format.
973 @item File modification time.
976 If the name contains any special characters (white space, newlines,
977 etc.) these are displayed in an unambiguous form using so called
978 @dfn{quoting style}. For the detailed discussion of available styles
979 and on how to use them, see @ref{quoting styles}.
981 Depending on the file type, the name can be followed by some
982 additional information, described in the following table:
985 @item -> @var{link-name}
986 The file or archive member is a @dfn{symbolic link} and
987 @var{link-name} is the name of file it links to.
989 @item link to @var{link-name}
990 The file or archive member is a @dfn{hard link} and @var{link-name} is
991 the name of file it links to.
994 The archive member is an old GNU format long link. You will normally
998 The archive member is an old GNU format long name. You will normally
1001 @item --Volume Header--
1002 The archive member is a GNU @dfn{volume header} (@pxref{Tape Files}).
1004 @item --Continued at byte @var{n}--
1005 Encountered only at the beginning of a multi-volume archive
1006 (@pxref{Using Multiple Tapes}). This archive member is a continuation
1007 from the previous volume. The number @var{n} gives the offset where
1008 the original file was split.
1010 @item unknown file type @var{c}
1011 An archive member of unknown type. @var{c} is the type character from
1012 the archive header. If you encounter such a message, it means that
1013 either your archive contains proprietary member types @GNUTAR{} is not
1014 able to handle, or the archive is corrupted.
1019 For example, here is an archive listing containing most of the special
1020 suffixes explained above:
1024 V--------- 0/0 1536 2006-06-09 13:07 MyVolume--Volume Header--
1025 -rw-r--r-- gray/staff 456783 2006-06-09 12:06 aspic--Continued at byte 32456--
1026 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
1027 lrwxrwxrwx gray/staff 0 2006-06-09 13:01 angst -> apple
1028 -rw-r--r-- gray/staff 35793 2006-06-09 12:06 blues
1029 hrw-r--r-- gray/staff 0 2006-06-09 12:06 music link to blues
1037 @unnumberedsubsec Getting Help: Using the @option{--help} Option
1043 The @option{--help} option to @command{tar} prints out a very brief list of
1044 all operations and option available for the current version of
1045 @command{tar} available on your system.
1049 @section How to Create Archives
1052 @cindex Creation of the archive
1053 @cindex Archive, creation of
1054 One of the basic operations of @command{tar} is @option{--create} (@option{-c}), which
1055 you use to create a @command{tar} archive. We will explain
1056 @option{--create} first because, in order to learn about the other
1057 operations, you will find it useful to have an archive available to
1060 To make this easier, in this section you will first create a directory
1061 containing three files. Then, we will show you how to create an
1062 @emph{archive} (inside the new directory). Both the directory, and
1063 the archive are specifically for you to practice on. The rest of this
1064 chapter and the next chapter will show many examples using this
1065 directory and the files you will create: some of those files may be
1066 other directories and other archives.
1068 The three files you will archive in this example are called
1069 @file{blues}, @file{folk}, and @file{jazz}. The archive is called
1070 @file{collection.tar}.
1072 This section will proceed slowly, detailing how to use @option{--create}
1073 in @code{verbose} mode, and showing examples using both short and long
1074 forms. In the rest of the tutorial, and in the examples in the next
1075 chapter, we will proceed at a slightly quicker pace. This section
1076 moves more slowly to allow beginning users to understand how
1077 @command{tar} works.
1080 * prepare for examples::
1081 * Creating the archive::
1087 @node prepare for examples
1088 @subsection Preparing a Practice Directory for Examples
1090 To follow along with this and future examples, create a new directory
1091 called @file{practice} containing files called @file{blues}, @file{folk}
1092 and @file{jazz}. The files can contain any information you like:
1093 ideally, they should contain information which relates to their names,
1094 and be of different lengths. Our examples assume that @file{practice}
1095 is a subdirectory of your home directory.
1097 Now @command{cd} to the directory named @file{practice}; @file{practice}
1098 is now your @dfn{working directory}. (@emph{Please note}: Although
1099 the full file name of this directory is
1100 @file{/@var{homedir}/practice}, in our examples we will refer to
1101 this directory as @file{practice}; the @var{homedir} is presumed.)
1103 In general, you should check that the files to be archived exist where
1104 you think they do (in the working directory) by running @command{ls}.
1105 Because you just created the directory and the files and have changed to
1106 that directory, you probably don't need to do that this time.
1108 It is very important to make sure there isn't already a file in the
1109 working directory with the archive name you intend to use (in this case,
1110 @samp{collection.tar}), or that you don't care about its contents.
1111 Whenever you use @samp{create}, @command{tar} will erase the current
1112 contents of the file named by @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) if it exists. @command{tar}
1113 will not tell you if you are about to overwrite an archive unless you
1114 specify an option which does this (@pxref{backup}, for the
1115 information on how to do so). To add files to an existing archive,
1116 you need to use a different option, such as @option{--append} (@option{-r}); see
1117 @ref{append} for information on how to do this.
1119 @node Creating the archive
1120 @subsection Creating the Archive
1122 @xopindex{create, introduced}
1123 To place the files @file{blues}, @file{folk}, and @file{jazz} into an
1124 archive named @file{collection.tar}, use the following command:
1127 $ @kbd{tar --create --file=collection.tar blues folk jazz}
1130 The order of the arguments is not very important, @emph{when using long
1131 option forms}. You could also say:
1134 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1138 However, you can see that this order is harder to understand; this is
1139 why we will list the arguments in the order that makes the commands
1140 easiest to understand (and we encourage you to do the same when you use
1141 @command{tar}, to avoid errors).
1143 Note that the sequence
1144 @option{--file=@-collection.tar} is considered to be @emph{one} argument.
1145 If you substituted any other string of characters for
1146 @kbd{collection.tar}, then that string would become the name of the
1147 archive file you create.
1149 The order of the options becomes more important when you begin to use
1150 short forms. With short forms, if you type commands in the wrong order
1151 (even if you type them correctly in all other ways), you may end up with
1152 results you don't expect. For this reason, it is a good idea to get
1153 into the habit of typing options in the order that makes inherent sense.
1154 @xref{short create}, for more information on this.
1156 In this example, you type the command as shown above: @option{--create}
1157 is the operation which creates the new archive
1158 (@file{collection.tar}), and @option{--file} is the option which lets
1159 you give it the name you chose. The files, @file{blues}, @file{folk},
1160 and @file{jazz}, are now members of the archive, @file{collection.tar}
1161 (they are @dfn{file name arguments} to the @option{--create} operation.
1162 @xref{Choosing}, for the detailed discussion on these.) Now that they are
1163 in the archive, they are called @emph{archive members}, not files.
1164 (@pxref{Definitions,members}).
1166 When you create an archive, you @emph{must} specify which files you
1167 want placed in the archive. If you do not specify any archive
1168 members, @GNUTAR{} will complain.
1170 If you now list the contents of the working directory (@command{ls}), you will
1171 find the archive file listed as well as the files you saw previously:
1174 blues folk jazz collection.tar
1178 Creating the archive @samp{collection.tar} did not destroy the copies of
1179 the files in the directory.
1181 Keep in mind that if you don't indicate an operation, @command{tar} will not
1182 run and will prompt you for one. If you don't name any files, @command{tar}
1183 will complain. You must have write access to the working directory,
1184 or else you will not be able to create an archive in that directory.
1186 @emph{Caution}: Do not attempt to use @option{--create} (@option{-c}) to add files to
1187 an existing archive; it will delete the archive and write a new one.
1188 Use @option{--append} (@option{-r}) instead. @xref{append}.
1190 @node create verbose
1191 @subsection Running @option{--create} with @option{--verbose}
1193 @xopindex{create, using with @option{--verbose}}
1194 @xopindex{verbose, using with @option{--create}}
1195 If you include the @option{--verbose} (@option{-v}) option on the command line,
1196 @command{tar} will list the files it is acting on as it is working. In
1197 verbose mode, the @code{create} example above would appear as:
1200 $ @kbd{tar --create --verbose --file=collection.tar blues folk jazz}
1206 This example is just like the example we showed which did not use
1207 @option{--verbose}, except that @command{tar} generated the remaining
1209 lines (note the different font styles).
1215 In the rest of the examples in this chapter, we will frequently use
1216 @code{verbose} mode so we can show actions or @command{tar} responses that
1217 you would otherwise not see, and which are important for you to
1221 @subsection Short Forms with @samp{create}
1223 As we said before, the @option{--create} (@option{-c}) operation is one of the most
1224 basic uses of @command{tar}, and you will use it countless times.
1225 Eventually, you will probably want to use abbreviated (or ``short'')
1226 forms of options. A full discussion of the three different forms that
1227 options can take appears in @ref{Styles}; for now, here is what the
1228 previous example (including the @option{--verbose} (@option{-v}) option) looks like
1229 using short option forms:
1232 $ @kbd{tar -cvf collection.tar blues folk jazz}
1239 As you can see, the system responds the same no matter whether you use
1240 long or short option forms.
1242 @FIXME{i don't like how this is worded:} One difference between using
1243 short and long option forms is that, although the exact placement of
1244 arguments following options is no more specific when using short forms,
1245 it is easier to become confused and make a mistake when using short
1246 forms. For example, suppose you attempted the above example in the
1250 $ @kbd{tar -cfv collection.tar blues folk jazz}
1254 In this case, @command{tar} will make an archive file called @file{v},
1255 containing the files @file{blues}, @file{folk}, and @file{jazz}, because
1256 the @samp{v} is the closest ``file name'' to the @option{-f} option, and
1257 is thus taken to be the chosen archive file name. @command{tar} will try
1258 to add a file called @file{collection.tar} to the @file{v} archive file;
1259 if the file @file{collection.tar} did not already exist, @command{tar} will
1260 report an error indicating that this file does not exist. If the file
1261 @file{collection.tar} does already exist (e.g., from a previous command
1262 you may have run), then @command{tar} will add this file to the archive.
1263 Because the @option{-v} option did not get registered, @command{tar} will not
1264 run under @samp{verbose} mode, and will not report its progress.
1266 The end result is that you may be quite confused about what happened,
1267 and possibly overwrite a file. To illustrate this further, we will show
1268 you how an example we showed previously would look using short forms.
1273 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1277 is confusing as it is. When shown using short forms, however, it
1278 becomes much more so:
1281 $ @kbd{tar blues -c folk -f collection.tar jazz}
1285 It would be very easy to put the wrong string of characters
1286 immediately following the @option{-f}, but doing that could sacrifice
1289 For this reason, we recommend that you pay very careful attention to
1290 the order of options and placement of file and archive names,
1291 especially when using short option forms. Not having the option name
1292 written out mnemonically can affect how well you remember which option
1293 does what, and therefore where different names have to be placed.
1296 @subsection Archiving Directories
1298 @cindex Archiving Directories
1299 @cindex Directories, Archiving
1300 You can archive a directory by specifying its directory name as a
1301 file name argument to @command{tar}. The files in the directory will be
1302 archived relative to the working directory, and the directory will be
1303 re-created along with its contents when the archive is extracted.
1305 To archive a directory, first move to its superior directory. If you
1306 have followed the previous instructions in this tutorial, you should
1315 This will put you into the directory which contains @file{practice},
1316 i.e., your home directory. Once in the superior directory, you can
1317 specify the subdirectory, @file{practice}, as a file name argument. To
1318 store @file{practice} in the new archive file @file{music.tar}, type:
1321 $ @kbd{tar --create --verbose --file=music.tar practice}
1325 @command{tar} should output:
1332 practice/collection.tar
1335 Note that the archive thus created is not in the subdirectory
1336 @file{practice}, but rather in the current working directory---the
1337 directory from which @command{tar} was invoked. Before trying to archive a
1338 directory from its superior directory, you should make sure you have
1339 write access to the superior directory itself, not only the directory
1340 you are trying archive with @command{tar}. For example, you will probably
1341 not be able to store your home directory in an archive by invoking
1342 @command{tar} from the root directory; @xref{absolute}. (Note
1343 also that @file{collection.tar}, the original archive file, has itself
1344 been archived. @command{tar} will accept any file as a file to be
1345 archived, regardless of its content. When @file{music.tar} is
1346 extracted, the archive file @file{collection.tar} will be re-written
1347 into the file system).
1349 If you give @command{tar} a command such as
1352 $ @kbd{tar --create --file=foo.tar .}
1356 @command{tar} will report @samp{tar: ./foo.tar is the archive; not
1357 dumped}. This happens because @command{tar} creates the archive
1358 @file{foo.tar} in the current directory before putting any files into
1359 it. Then, when @command{tar} attempts to add all the files in the
1360 directory @file{.} to the archive, it notices that the file
1361 @file{./foo.tar} is the same as the archive @file{foo.tar}, and skips
1362 it. (It makes no sense to put an archive into itself.) @GNUTAR{}
1363 will continue in this case, and create the archive
1364 normally, except for the exclusion of that one file. (@emph{Please
1365 note:} Other implementations of @command{tar} may not be so clever;
1366 they will enter an infinite loop when this happens, so you should not
1367 depend on this behavior unless you are certain you are running
1368 @GNUTAR{}. In general, it is wise to always place the archive outside
1369 of the directory being dumped.)
1372 @section How to List Archives
1375 Frequently, you will find yourself wanting to determine exactly what a
1376 particular archive contains. You can use the @option{--list}
1377 (@option{-t}) operation to get the member names as they currently
1378 appear in the archive, as well as various attributes of the files at
1379 the time they were archived. For example, you can examine the archive
1380 @file{collection.tar} that you created in the last section with the
1384 $ @kbd{tar --list --file=collection.tar}
1388 The output of @command{tar} would then be:
1397 The archive @file{bfiles.tar} would list as follows:
1406 Be sure to use a @option{--file=@var{archive-name}} (@option{-f
1407 @var{archive-name}}) option just as with @option{--create}
1408 (@option{-c}) to specify the name of the archive.
1410 @xopindex{list, using with @option{--verbose}}
1411 @xopindex{verbose, using with @option{--list}}
1412 If you use the @option{--verbose} (@option{-v}) option with
1413 @option{--list}, then @command{tar} will print out a listing
1414 reminiscent of @w{@samp{ls -l}}, showing owner, file size, and so
1415 forth. This output is described in detail in @ref{verbose member listing}.
1417 If you had used @option{--verbose} (@option{-v}) mode, the example
1418 above would look like:
1421 $ @kbd{tar --list --verbose --file=collection.tar folk}
1422 -rw-r--r-- myself/user 62 1990-05-23 10:55 folk
1425 @cindex listing member and file names
1426 @anchor{listing member and file names}
1427 It is important to notice that the output of @kbd{tar --list
1428 --verbose} does not necessarily match that produced by @kbd{tar
1429 --create --verbose} while creating the archive. It is because
1430 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1431 prefixes from file names before storing them in the archive
1432 (@xref{absolute}, for more information). In other
1433 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1434 an archive and @dfn{member names} when listing it. Consider this
1439 $ @kbd{tar --create --verbose --file archive /etc/mail}
1440 tar: Removing leading '/' from member names
1442 /etc/mail/sendmail.cf
1444 $ @kbd{tar --test --file archive}
1446 etc/mail/sendmail.cf
1451 @opindex show-stored-names
1452 This default behavior can sometimes be inconvenient. You can force
1453 @GNUTAR{} show member names when creating archive by supplying
1454 @option{--show-stored-names} option.
1457 @item --show-stored-names
1458 Print member (as opposed to @emph{file}) names when creating the archive.
1461 @cindex File name arguments, using @option{--list} with
1462 @xopindex{list, using with file name arguments}
1463 You can specify one or more individual member names as arguments when
1464 using @samp{list}. In this case, @command{tar} will only list the
1465 names of members you identify. For example, @w{@kbd{tar --list
1466 --file=afiles.tar apple}} would only print @file{apple}.
1468 Because @command{tar} preserves file names, these must be specified as
1469 they appear in the archive (i.e., relative to the directory from which
1470 the archive was created). Therefore, it is essential when specifying
1471 member names to @command{tar} that you give the exact member names.
1472 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1473 error message something like @samp{tar: birds: Not found in archive},
1474 because there is no member named @file{birds}, only one named
1475 @file{./birds}. While the names @file{birds} and @file{./birds} name
1476 the same file, @emph{member} names by default are compared verbatim.
1478 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1479 with @file{baboon}, because this exact member name is in the archive file
1480 @file{bfiles.tar}. If you are not sure of the exact file name,
1481 use @dfn{globbing patterns}, for example:
1484 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1488 will list all members whose name contains @samp{b}. @xref{wildcards},
1489 for a detailed discussion of globbing patterns and related
1490 @command{tar} command line options.
1497 @unnumberedsubsec Listing the Contents of a Stored Directory
1499 To get information about the contents of an archived directory,
1500 use the directory name as a file name argument in conjunction with
1501 @option{--list} (@option{-t}). To find out file attributes, include the
1502 @option{--verbose} (@option{-v}) option.
1504 For example, to find out about files in the directory @file{practice}, in
1505 the archive file @file{music.tar}, type:
1508 $ @kbd{tar --list --verbose --file=music.tar practice}
1511 @command{tar} responds:
1514 drwxrwxrwx myself/user 0 1990-05-31 21:49 practice/
1515 -rw-r--r-- myself/user 42 1990-05-21 13:29 practice/blues
1516 -rw-r--r-- myself/user 62 1990-05-23 10:55 practice/folk
1517 -rw-r--r-- myself/user 40 1990-05-21 13:30 practice/jazz
1518 -rw-r--r-- myself/user 10240 1990-05-31 21:49 practice/collection.tar
1521 When you use a directory name as a file name argument, @command{tar} acts on
1522 all the files (including sub-directories) in that directory.
1525 @section How to Extract Members from an Archive
1527 @cindex Retrieving files from an archive
1528 @cindex Resurrecting files from an archive
1531 Creating an archive is only half the job---there is no point in storing
1532 files in an archive if you can't retrieve them. The act of retrieving
1533 members from an archive so they can be used and manipulated as
1534 unarchived files again is called @dfn{extraction}. To extract files
1535 from an archive, use the @option{--extract} (@option{--get} or
1536 @option{-x}) operation. As with @option{--create}, specify the name
1537 of the archive with @option{--file} (@option{-f}) option. Extracting
1538 an archive does not modify the archive in any way; you can extract it
1539 multiple times if you want or need to.
1541 Using @option{--extract}, you can extract an entire archive, or specific
1542 files. The files can be directories containing other files, or not. As
1543 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1544 long form of the operation without affecting the performance.
1547 * extracting archives::
1548 * extracting files::
1550 * extracting untrusted archives::
1551 * failing commands::
1554 @node extracting archives
1555 @subsection Extracting an Entire Archive
1557 To extract an entire archive, specify the archive file name only, with
1558 no individual file names as arguments. For example,
1561 $ @kbd{tar -xvf collection.tar}
1568 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
1569 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
1570 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
1573 @node extracting files
1574 @subsection Extracting Specific Files
1576 To extract specific archive members, give their exact member names as
1577 arguments, as printed by @option{--list} (@option{-t}). If you had
1578 mistakenly deleted one of the files you had placed in the archive
1579 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1580 from the archive without changing the archive's structure. Its
1581 contents will be identical to the original file @file{blues} that you
1584 First, make sure you are in the @file{practice} directory, and list the
1585 files in the directory. Now, delete the file, @samp{blues}, and list
1586 the files in the directory again.
1588 You can now extract the member @file{blues} from the archive file
1589 @file{collection.tar} like this:
1592 $ @kbd{tar --extract --file=collection.tar blues}
1596 If you list the files in the directory again, you will see that the file
1597 @file{blues} has been restored, with its original permissions, data
1598 modification times, and owner.@footnote{This is only accidentally
1599 true, but not in general. Whereas modification times are always
1600 restored, in most cases, one has to be root for restoring the owner,
1601 and use a special option for restoring permissions. Here, it just
1602 happens that the restoring user is also the owner of the archived
1603 members, and that the current @code{umask} is compatible with original
1604 permissions.} (These parameters will be identical to those which
1605 the file had when you originally placed it in the archive; any changes
1606 you may have made before deleting the file from the file system,
1607 however, will @emph{not} have been made to the archive member.) The
1608 archive file, @samp{collection.tar}, is the same as it was before you
1609 extracted @samp{blues}. You can confirm this by running @command{tar} with
1610 @option{--list} (@option{-t}).
1612 Remember that as with other operations, specifying the exact member
1613 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1614 will fail, because there is no member named @file{birds}. To extract
1615 the member named @file{./birds}, you must specify @w{@kbd{tar
1616 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1617 exact member names, use @option{--list} (@option{-t}) option
1618 (@pxref{list}). You can also extract those members that match a
1619 specific @dfn{globbing pattern}. For example, to extract from
1620 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1621 directory prefix, you could type:
1624 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1628 Here, @option{--wildcards} instructs @command{tar} to treat
1629 command line arguments as globbing patterns and @option{--no-anchored}
1630 informs it that the patterns apply to member names after any @samp{/}
1631 delimiter. The use of globbing patterns is discussed in detail in
1634 You can extract a file to standard output by combining the above options
1635 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1638 If you give the @option{--verbose} option, then @option{--extract}
1639 will print the names of the archive members as it extracts them.
1642 @subsection Extracting Files that are Directories
1644 Extracting directories which are members of an archive is similar to
1645 extracting other files. The main difference to be aware of is that if
1646 the extracted directory has the same name as any directory already in
1647 the working directory, then files in the extracted directory will be
1648 placed into the directory of the same name. Likewise, if there are
1649 files in the pre-existing directory with the same names as the members
1650 which you extract, the files from the extracted archive will replace
1651 the files already in the working directory (and possible
1652 subdirectories). This will happen regardless of whether or not the
1653 files in the working directory were more recent than those extracted
1654 (there exist, however, special options that alter this behavior
1657 However, if a file was stored with a directory name as part of its file
1658 name, and that directory does not exist under the working directory when
1659 the file is extracted, @command{tar} will create the directory.
1661 We can demonstrate how to use @option{--extract} to extract a directory
1662 file with an example. Change to the @file{practice} directory if you
1663 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1664 go back to the parent directory and extract the archive
1665 @file{music.tar}. You may either extract the entire archive, or you may
1666 extract only the files you just deleted. To extract the entire archive,
1667 don't give any file names as arguments after the archive name
1668 @file{music.tar}. To extract only the files you deleted, use the
1672 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1678 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1679 would have displayed more detail about the extracted files, as shown
1680 in the example below:
1683 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1684 -rw-r--r-- me/user 28 1996-10-18 16:31 practice/jazz
1685 -rw-r--r-- me/user 20 1996-09-23 16:44 practice/folk
1689 Because you created the directory with @file{practice} as part of the
1690 file names of each of the files by archiving the @file{practice}
1691 directory as @file{practice}, you must give @file{practice} as part
1692 of the file names when you extract those files from the archive.
1694 @node extracting untrusted archives
1695 @subsection Extracting Archives from Untrusted Sources
1697 Extracting files from archives can overwrite files that already exist.
1698 If you receive an archive from an untrusted source, you should make a
1699 new directory and extract into that directory, so that you don't have
1700 to worry about the extraction overwriting one of your existing files.
1701 For example, if @file{untrusted.tar} came from somewhere else on the
1702 Internet, and you don't necessarily trust its contents, you can
1703 extract it as follows:
1706 $ @kbd{mkdir newdir}
1708 $ @kbd{tar -xvf ../untrusted.tar}
1711 It is also a good practice to examine contents of the archive
1712 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1713 with @option{--verbose} (@option{-v}).
1715 @node failing commands
1716 @subsection Commands That Will Fail
1718 Here are some sample commands you might try which will not work, and why
1721 If you try to use this command,
1724 $ @kbd{tar -xvf music.tar folk jazz}
1728 you will get the following response:
1731 tar: folk: Not found in archive
1732 tar: jazz: Not found in archive
1736 This is because these files were not originally @emph{in} the parent
1737 directory @file{..}, where the archive is located; they were in the
1738 @file{practice} directory, and their file names reflect this:
1741 $ @kbd{tar -tvf music.tar}
1747 @FIXME{make sure the above works when going through the examples in
1751 Likewise, if you try to use this command,
1754 $ @kbd{tar -tvf music.tar folk jazz}
1758 you would get a similar response. Members with those names are not in the
1759 archive. You must use the correct member names, or wildcards, in order
1760 to extract the files from the archive.
1762 If you have forgotten the correct names of the files in the archive,
1763 use @w{@kbd{tar --list --verbose}} to list them correctly.
1765 @FIXME{more examples, here? hag thinks it's a good idea.}
1768 @section Going Further Ahead in this Manual
1771 @FIXME{need to write up a node here about the things that are going to
1772 be in the rest of the manual.}
1774 @node tar invocation
1775 @chapter Invoking @GNUTAR{}
1777 This chapter is about how one invokes the @GNUTAR{}
1778 command, from the command synopsis (@pxref{Synopsis}). There are
1779 numerous options, and many styles for writing them. One mandatory
1780 option specifies the operation @command{tar} should perform
1781 (@pxref{Operation Summary}), other options are meant to detail how
1782 this operation should be performed (@pxref{Option Summary}).
1783 Non-option arguments are not always interpreted the same way,
1784 depending on what the operation is.
1786 You will find in this chapter everything about option styles and rules for
1787 writing them (@pxref{Styles}). On the other hand, operations and options
1788 are fully described elsewhere, in other chapters. Here, you will find
1789 only synthetic descriptions for operations and options, together with
1790 pointers to other parts of the @command{tar} manual.
1792 Some options are so special they are fully described right in this
1793 chapter. They have the effect of inhibiting the normal operation of
1794 @command{tar} or else, they globally alter the amount of feedback the user
1795 receives about what is going on. These are the @option{--help} and
1796 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1797 and @option{--interactive} options (@pxref{interactive}).
1801 * using tar options::
1803 * All Options:: All @command{tar} Options.
1804 * help:: Where to Get Help.
1805 * defaults:: What are the Default Values.
1806 * verbose:: Checking @command{tar} progress.
1807 * checkpoints:: Checkpoints.
1808 * warnings:: Controlling Warning Messages.
1809 * interactive:: Asking for Confirmation During Operations.
1810 * external:: Running External Commands.
1814 @section General Synopsis of @command{tar}
1816 The @GNUTAR{} program is invoked as either one of:
1819 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1820 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1823 The second form is for when old options are being used.
1825 You can use @command{tar} to store files in an archive, to extract them from
1826 an archive, and to do other types of archive manipulation. The primary
1827 argument to @command{tar}, which is called the @dfn{operation}, specifies
1828 which action to take. The other arguments to @command{tar} are either
1829 @dfn{options}, which change the way @command{tar} performs an operation,
1830 or file names or archive members, which specify the files or members
1831 @command{tar} is to act on.
1833 You can actually type in arguments in any order, even if in this manual
1834 the options always precede the other arguments, to make examples easier
1835 to understand. Further, the option stating the main operation mode
1836 (the @command{tar} main command) is usually given first.
1838 Each @var{name} in the synopsis above is interpreted as an archive member
1839 name when the main command is one of @option{--compare}
1840 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1841 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1842 @option{--update} (@option{-u}). When naming archive members, you
1843 must give the exact name of the member in the archive, as it is
1844 printed by @option{--list}. For @option{--append} (@option{-r}) and
1845 @option{--create} (@option{-c}), these @var{name} arguments specify
1846 the names of either files or directory hierarchies to place in the archive.
1847 These files or hierarchies should already exist in the file system,
1848 prior to the execution of the @command{tar} command.
1850 @command{tar} interprets relative file names as being relative to the
1851 working directory. @command{tar} will make all file names relative
1852 (by removing leading slashes when archiving or restoring files),
1853 unless you specify otherwise (using the @option{--absolute-names}
1854 option). @xref{absolute}, for more information about
1855 @option{--absolute-names}.
1857 If you give the name of a directory as either a file name or a member
1858 name, then @command{tar} acts recursively on all the files and directories
1859 beneath that directory. For example, the name @file{/} identifies all
1860 the files in the file system to @command{tar}.
1862 The distinction between file names and archive member names is especially
1863 important when shell globbing is used, and sometimes a source of confusion
1864 for newcomers. @xref{wildcards}, for more information about globbing.
1865 The problem is that shells may only glob using existing files in the
1866 file system. Only @command{tar} itself may glob on archive members, so when
1867 needed, you must ensure that wildcard characters reach @command{tar} without
1868 being interpreted by the shell first. Using a backslash before @samp{*}
1869 or @samp{?}, or putting the whole argument between quotes, is usually
1870 sufficient for this.
1872 Even if @var{name}s are often specified on the command line, they
1873 can also be read from a text file in the file system, using the
1874 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1876 If you don't use any file name arguments, @option{--append} (@option{-r}),
1877 @option{--delete} and @option{--concatenate} (@option{--catenate},
1878 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1879 will usually yield a diagnostic and inhibit @command{tar} execution.
1880 The other operations of @command{tar} (@option{--list},
1881 @option{--extract}, @option{--compare}, and @option{--update})
1882 will act on the entire contents of the archive.
1884 @anchor{exit status}
1886 @cindex return status
1887 Besides successful exits, @GNUTAR{} may fail for
1888 many reasons. Some reasons correspond to bad usage, that is, when the
1889 @command{tar} command line is improperly written. Errors may be
1890 encountered later, while processing the archive or the files. Some
1891 errors are recoverable, in which case the failure is delayed until
1892 @command{tar} has completed all its work. Some errors are such that
1893 it would be not meaningful, or at least risky, to continue processing:
1894 @command{tar} then aborts processing immediately. All abnormal exits,
1895 whether immediate or delayed, should always be clearly diagnosed on
1896 @code{stderr}, after a line stating the nature of the error.
1898 Possible exit codes of @GNUTAR{} are summarized in the following
1903 @samp{Successful termination}.
1906 @samp{Some files differ}. If tar was invoked with @option{--compare}
1907 (@option{--diff}, @option{-d}) command line option, this means that
1908 some files in the archive differ from their disk counterparts
1909 (@pxref{compare}). If tar was given @option{--create},
1910 @option{--append} or @option{--update} option, this exit code means
1911 that some files were changed while being archived and so the resulting
1912 archive does not contain the exact copy of the file set.
1915 @samp{Fatal error}. This means that some fatal, unrecoverable error
1919 If @command{tar} has invoked a subprocess and that subprocess exited with a
1920 nonzero exit code, @command{tar} exits with that code as well.
1921 This can happen, for example, if @command{tar} was given some
1922 compression option (@pxref{gzip}) and the external compressor program
1923 failed. Another example is @command{rmt} failure during backup to the
1924 remote device (@pxref{Remote Tape Server}).
1926 @node using tar options
1927 @section Using @command{tar} Options
1929 @GNUTAR{} has a total of eight operating modes which
1930 allow you to perform a variety of tasks. You are required to choose
1931 one operating mode each time you employ the @command{tar} program by
1932 specifying one, and only one operation as an argument to the
1933 @command{tar} command (the corresponding options may be found
1934 at @ref{frequent operations} and @ref{Operations}). Depending on
1935 circumstances, you may also wish to customize how the chosen operating
1936 mode behaves. For example, you may wish to change the way the output
1937 looks, or the format of the files that you wish to archive may require
1938 you to do something special in order to make the archive look right.
1940 You can customize and control @command{tar}'s performance by running
1941 @command{tar} with one or more options (such as @option{--verbose}
1942 (@option{-v}), which we used in the tutorial). As we said in the
1943 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1944 their name suggests) optional. Depending on the operating mode, you
1945 may specify one or more options. Different options will have different
1946 effects, but in general they all change details of the operation, such
1947 as archive format, archive name, or level of user interaction. Some
1948 options make sense with all operating modes, while others are
1949 meaningful only with particular modes. You will likely use some
1950 options frequently, while you will only use others infrequently, or
1951 not at all. (A full list of options is available in @pxref{All Options}.)
1953 @vrindex TAR_OPTIONS, environment variable
1954 @anchor{TAR_OPTIONS}
1955 The @env{TAR_OPTIONS} environment variable specifies default options to
1956 be placed in front of any explicit options. For example, if
1957 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1958 if the two options @option{-v} and @option{--unlink-first} had been
1959 specified before any explicit options. Option specifications are
1960 separated by whitespace. A backslash escapes the next character, so it
1961 can be used to specify an option containing whitespace or a backslash.
1963 Note that @command{tar} options are case sensitive. For example, the
1964 options @option{-T} and @option{-t} are different; the first requires an
1965 argument for stating the name of a file providing a list of @var{name}s,
1966 while the second does not require an argument and is another way to
1967 write @option{--list} (@option{-t}).
1969 In addition to the eight operations, there are many options to
1970 @command{tar}, and three different styles for writing both: long (mnemonic)
1971 form, short form, and old style. These styles are discussed below.
1972 Both the options and the operations can be written in any of these three
1975 @FIXME{menu at end of this node. need to think of an actual outline
1976 for this chapter; probably do that after stuff from chapter 4 is
1980 @section The Three Option Styles
1982 There are three styles for writing operations and options to the command
1983 line invoking @command{tar}. The different styles were developed at
1984 different times during the history of @command{tar}. These styles will be
1985 presented below, from the most recent to the oldest.
1987 Some options must take an argument@footnote{For example, @option{--file}
1988 (@option{-f}) takes the name of an archive file as an argument. If
1989 you do not supply an archive file name, @command{tar} will use a
1990 default, but this can be confusing; thus, we recommend that you always
1991 supply a specific archive file name.}. Where you @emph{place} the
1992 arguments generally depends on which style of options you choose. We
1993 will detail specific information relevant to each option style in the
1994 sections on the different option styles, below. The differences are
1995 subtle, yet can often be very important; incorrect option placement
1996 can cause you to overwrite a number of important files. We urge you
1997 to note these differences, and only use the option style(s) which
1998 makes the most sense to you until you feel comfortable with the others.
2000 Some options @emph{may} take an argument. Such options may have at
2001 most long and short forms, they do not have old style equivalent. The
2002 rules for specifying an argument for such options are stricter than
2003 those for specifying mandatory arguments. Please, pay special
2007 * Long Options:: Long Option Style
2008 * Short Options:: Short Option Style
2009 * Old Options:: Old Option Style
2010 * Mixing:: Mixing Option Styles
2014 @subsection Long Option Style
2016 @cindex long options
2017 @cindex options, long style
2018 @cindex options, GNU style
2019 @cindex options, mnemonic names
2020 Each option has at least one @dfn{long} (or @dfn{mnemonic}) name starting with two
2021 dashes in a row, e.g., @option{--list}. The long names are more clear than
2022 their corresponding short or old names. It sometimes happens that a
2023 single long option has many different names which are
2024 synonymous, such as @option{--compare} and @option{--diff}. In addition,
2025 long option names can be given unique abbreviations. For example,
2026 @option{--cre} can be used in place of @option{--create} because there is no
2027 other long option which begins with @samp{cre}. (One way to find
2028 this out is by trying it and seeing what happens; if a particular
2029 abbreviation could represent more than one option, @command{tar} will tell
2030 you that that abbreviation is ambiguous and you'll know that that
2031 abbreviation won't work. You may also choose to run @samp{tar --help}
2032 to see a list of options. Be aware that if you run @command{tar} with a
2033 unique abbreviation for the long name of an option you didn't want to
2034 use, you are stuck; @command{tar} will perform the command as ordered.)
2036 Long options are meant to be obvious and easy to remember, and their
2037 meanings are generally easier to discern than those of their
2038 corresponding short options (see below). For example:
2041 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
2045 gives a fairly good set of hints about what the command does, even
2046 for those not fully acquainted with @command{tar}.
2048 @cindex arguments to long options
2049 @cindex long options with mandatory arguments
2050 Long options which require arguments take those arguments
2051 immediately following the option name. There are two ways of
2052 specifying a mandatory argument. It can be separated from the
2053 option name either by an equal sign, or by any amount of
2054 white space characters. For example, the @option{--file} option (which
2055 tells the name of the @command{tar} archive) is given a file such as
2056 @file{archive.tar} as argument by using any of the following notations:
2057 @option{--file=archive.tar} or @option{--file archive.tar}.
2059 @cindex optional arguments to long options
2060 @cindex long options with optional arguments
2061 In contrast, optional arguments must always be introduced using
2062 an equal sign. For example, the @option{--backup} option takes
2063 an optional argument specifying backup type. It must be used
2064 as @option{--backup=@var{backup-type}}.
2067 @subsection Short Option Style
2069 @cindex short options
2070 @cindex options, short style
2071 @cindex options, traditional
2072 Most options also have a @dfn{short option} name. Short options start with
2073 a single dash, and are followed by a single character, e.g., @option{-t}
2074 (which is equivalent to @option{--list}). The forms are absolutely
2075 identical in function; they are interchangeable.
2077 The short option names are faster to type than long option names.
2079 @cindex arguments to short options
2080 @cindex short options with mandatory arguments
2081 Short options which require arguments take their arguments immediately
2082 following the option, usually separated by white space. It is also
2083 possible to stick the argument right after the short option name, using
2084 no intervening space. For example, you might write @w{@option{-f
2085 archive.tar}} or @option{-farchive.tar} instead of using
2086 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
2087 @w{@option{-f @var{archive-name}}} denote the option which indicates a
2088 specific archive, here named @file{archive.tar}.
2090 @cindex optional arguments to short options
2091 @cindex short options with optional arguments
2092 Short options which take optional arguments take their arguments
2093 immediately following the option letter, @emph{without any intervening
2094 white space characters}.
2096 Short options' letters may be clumped together, but you are not
2097 required to do this (as compared to old options; see below). When
2098 short options are clumped as a set, use one (single) dash for them
2099 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
2100 such a set is allowed to have an argument@footnote{Clustering many
2101 options, the last of which has an argument, is a rather opaque way to
2102 write options. Some wonder if @acronym{GNU} @code{getopt} should not
2103 even be made helpful enough for considering such usages as invalid.}.
2105 When the options are separated, the argument for each option which requires
2106 an argument directly follows that option, as is usual for Unix programs.
2110 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
2113 If you reorder short options' locations, be sure to move any arguments
2114 that belong to them. If you do not move the arguments properly, you may
2115 end up overwriting files.
2118 @subsection Old Option Style
2119 @cindex options, old style
2120 @cindex old option style
2121 @cindex option syntax, traditional
2123 As far as we know, all @command{tar} programs, @acronym{GNU} and
2124 non-@acronym{GNU}, support @dfn{old options}: that is, if the first
2125 argument does not start with @samp{-}, it is assumed to specify option
2126 letters. @GNUTAR{} supports old options not only for historical
2127 reasons, but also because many people are used to them. If the first
2128 argument does not start with a dash, you are announcing the old option
2129 style instead of the short option style; old options are decoded
2132 Like short options, old options are single letters. However, old options
2133 must be written together as a single clumped set, without spaces separating
2134 them or dashes preceding them. This set
2135 of letters must be the first to appear on the command line, after the
2136 @command{tar} program name and some white space; old options cannot appear
2137 anywhere else. The letter of an old option is exactly the same letter as
2138 the corresponding short option. For example, the old option @samp{t} is
2139 the same as the short option @option{-t}, and consequently, the same as the
2140 long option @option{--list}. So for example, the command @w{@samp{tar
2141 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
2143 @cindex arguments to old options
2144 @cindex old options with mandatory arguments
2145 When options that need arguments are given together with the command,
2146 all the associated arguments follow, in the same order as the options.
2147 Thus, the example given previously could also be written in the old
2151 $ @kbd{tar cvbf 20 /dev/rmt0}
2155 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
2156 the argument of @option{-f}.
2158 The old style syntax can make it difficult to match
2159 option letters with their corresponding arguments, and is often
2160 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
2161 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
2162 argument for @option{-f}, and @option{-v} does not have a corresponding
2163 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
2164 /dev/rmt0}} is clearer, putting all arguments next to the option they
2167 If you want to reorder the letters in the old option argument, be
2168 sure to reorder any corresponding argument appropriately.
2170 This old way of writing @command{tar} options can surprise even experienced
2171 users. For example, the two commands:
2174 @kbd{tar cfz archive.tar.gz file}
2175 @kbd{tar -cfz archive.tar.gz file}
2179 are quite different. The first example uses @file{archive.tar.gz} as
2180 the value for option @samp{f} and recognizes the option @samp{z}. The
2181 second example, however, uses @file{z} as the value for option
2182 @samp{f} --- probably not what was intended.
2184 This second example could be corrected in many ways, among which the
2185 following are equivalent:
2188 @kbd{tar -czf archive.tar.gz file}
2189 @kbd{tar -cf archive.tar.gz -z file}
2190 @kbd{tar cf archive.tar.gz -z file}
2194 @subsection Mixing Option Styles
2196 @cindex options, mixing different styles
2197 All three styles may be intermixed in a single @command{tar} command,
2198 so long as the rules for each style are fully
2199 respected@footnote{Before @GNUTAR{} version 1.11.6,
2200 a bug prevented intermixing old style options with long options in
2201 some cases.}. Old style options and either of the modern styles of
2202 options may be mixed within a single @command{tar} command. However,
2203 old style options must be introduced as the first arguments only,
2204 following the rule for old options (old options must appear directly
2205 after the @command{tar} command and some white space). Modern options
2206 may be given only after all arguments to the old options have been
2207 collected. If this rule is not respected, a modern option might be
2208 falsely interpreted as the value of the argument to one of the old
2211 For example, all the following commands are wholly equivalent, and
2212 illustrate the many combinations and orderings of option styles.
2215 @kbd{tar --create --file=archive.tar}
2216 @kbd{tar --create -f archive.tar}
2217 @kbd{tar --create -farchive.tar}
2218 @kbd{tar --file=archive.tar --create}
2219 @kbd{tar --file=archive.tar -c}
2220 @kbd{tar -c --file=archive.tar}
2221 @kbd{tar -c -f archive.tar}
2222 @kbd{tar -c -farchive.tar}
2223 @kbd{tar -cf archive.tar}
2224 @kbd{tar -cfarchive.tar}
2225 @kbd{tar -f archive.tar --create}
2226 @kbd{tar -f archive.tar -c}
2227 @kbd{tar -farchive.tar --create}
2228 @kbd{tar -farchive.tar -c}
2229 @kbd{tar c --file=archive.tar}
2230 @kbd{tar c -f archive.tar}
2231 @kbd{tar c -farchive.tar}
2232 @kbd{tar cf archive.tar}
2233 @kbd{tar f archive.tar --create}
2234 @kbd{tar f archive.tar -c}
2235 @kbd{tar fc archive.tar}
2238 On the other hand, the following commands are @emph{not} equivalent to
2242 @kbd{tar -f -c archive.tar}
2243 @kbd{tar -fc archive.tar}
2244 @kbd{tar -fcarchive.tar}
2245 @kbd{tar -farchive.tarc}
2246 @kbd{tar cfarchive.tar}
2250 These last examples mean something completely different from what the
2251 user intended (judging based on the example in the previous set which
2252 uses long options, whose intent is therefore very clear). The first
2253 four specify that the @command{tar} archive would be a file named
2254 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2255 respectively. The first two examples also specify a single non-option,
2256 @var{name} argument having the value @samp{archive.tar}. The last
2257 example contains only old style option letters (repeating option
2258 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2259 @samp{h}, or @samp{i}), with no argument value.
2260 @FIXME{not sure i liked
2261 the first sentence of this paragraph..}
2264 @section All @command{tar} Options
2266 The coming manual sections contain an alphabetical listing of all
2267 @command{tar} operations and options, with brief descriptions and
2268 cross-references to more in-depth explanations in the body of the manual.
2269 They also contain an alphabetically arranged table of the short option
2270 forms with their corresponding long option. You can use this table as
2271 a reference for deciphering @command{tar} commands in scripts.
2274 * Operation Summary::
2276 * Short Option Summary::
2279 @node Operation Summary
2280 @subsection Operations
2288 Appends files to the end of the archive. @xref{append}.
2290 @opsummary{catenate}
2294 Same as @option{--concatenate}. @xref{concatenate}.
2300 Compares archive members with their counterparts in the file
2301 system, and reports differences in file size, mode, owner,
2302 modification date and contents. @xref{compare}.
2304 @opsummary{concatenate}
2308 Appends other @command{tar} archives to the end of the archive.
2315 Creates a new @command{tar} archive. @xref{create}.
2320 Deletes members from the archive. Don't try this on an archive on a
2321 tape! @xref{delete}.
2327 Same @option{--compare}. @xref{compare}.
2333 Extracts members from the archive into the file system. @xref{extract}.
2339 Same as @option{--extract}. @xref{extract}.
2345 Lists the members in an archive. @xref{list}.
2351 Adds files to the end of the archive, but only if they are newer than
2352 their counterparts already in the archive, or if they do not already
2353 exist in the archive. @xref{update}.
2357 @node Option Summary
2358 @subsection @command{tar} Options
2362 @opsummary{absolute-names}
2363 @item --absolute-names
2366 Normally when creating an archive, @command{tar} strips an initial
2367 @samp{/} from member names, and when extracting from an archive @command{tar}
2368 treats names specially if they have initial @samp{/} or internal
2369 @samp{..}. This option disables that behavior. @xref{absolute}.
2371 @opsummary{after-date}
2374 (See @option{--newer}, @pxref{after})
2376 @opsummary{anchored}
2378 A pattern must match an initial subsequence of the name's components.
2379 @xref{controlling pattern-matching}.
2381 @opsummary{atime-preserve}
2382 @item --atime-preserve
2383 @itemx --atime-preserve=replace
2384 @itemx --atime-preserve=system
2386 Attempt to preserve the access time of files when reading them. This
2387 option currently is effective only on files that you own, unless you
2388 have superuser privileges.
2390 @option{--atime-preserve=replace} remembers the access time of a file
2391 before reading it, and then restores the access time afterwards. This
2392 may cause problems if other programs are reading the file at the same
2393 time, as the times of their accesses will be lost. On most platforms
2394 restoring the access time also requires @command{tar} to restore the
2395 data modification time too, so this option may also cause problems if
2396 other programs are writing the file at the same time (@command{tar} attempts
2397 to detect this situation, but cannot do so reliably due to race
2398 conditions). Worse, on most platforms restoring the access time also
2399 updates the status change time, which means that this option is
2400 incompatible with incremental backups.
2402 @option{--atime-preserve=system} avoids changing time stamps on files,
2403 without interfering with time stamp updates
2404 caused by other programs, so it works better with incremental backups.
2405 However, it requires a special @code{O_NOATIME} option from the
2406 underlying operating and file system implementation, and it also requires
2407 that searching directories does not update their access times. As of
2408 this writing (November 2005) this works only with Linux, and only with
2409 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2410 way to know whether this feature actually works. Sometimes
2411 @command{tar} knows that it does not work, and if you use
2412 @option{--atime-preserve=system} then @command{tar} complains and
2413 exits right away. But other times @command{tar} might think that the
2414 option works when it actually does not.
2416 Currently @option{--atime-preserve} with no operand defaults to
2417 @option{--atime-preserve=replace}, but this may change in the future
2418 as support for @option{--atime-preserve=system} improves.
2420 If your operating or file system does not support
2421 @option{--atime-preserve=@-system}, you might be able to preserve access
2422 times reliably by using the @command{mount} command. For example,
2423 you can mount the file system read-only, or access the file system via
2424 a read-only loopback mount, or use the @samp{noatime} mount option
2425 available on some systems. However, mounting typically requires
2426 superuser privileges and can be a pain to manage.
2428 @opsummary{auto-compress}
2429 @item --auto-compress
2432 During a @option{--create} operation, enables automatic compressed
2433 format recognition based on the archive suffix. The effect of this
2434 option is cancelled by @option{--no-auto-compress}. @xref{gzip}.
2437 @item --backup=@var{backup-type}
2439 Rather than deleting files from the file system, @command{tar} will
2440 back them up using simple or numbered backups, depending upon
2441 @var{backup-type}. @xref{backup}.
2443 @opsummary{block-number}
2444 @item --block-number
2447 With this option present, @command{tar} prints error messages for read errors
2448 with the block number in the archive file. @xref{block-number}.
2450 @opsummary{blocking-factor}
2451 @item --blocking-factor=@var{blocking}
2452 @itemx -b @var{blocking}
2454 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2455 record. @xref{Blocking Factor}.
2461 This option tells @command{tar} to read or write archives through
2462 @code{bzip2}. @xref{gzip}.
2464 @opsummary{check-device}
2465 @item --check-device
2466 Check device numbers when creating a list of modified files for
2467 incremental archiving. This is the default. @xref{device numbers},
2468 for a detailed description.
2470 @opsummary{checkpoint}
2471 @item --checkpoint[=@var{number}]
2473 This option directs @command{tar} to print periodic checkpoint
2474 messages as it reads through the archive. It is intended for when you
2475 want a visual indication that @command{tar} is still running, but
2476 don't want to see @option{--verbose} output. You can also instruct
2477 @command{tar} to execute a list of actions on each checkpoint, see
2478 @option{--checkpoint-action} below. For a detailed description, see
2481 @opsummary{checkpoint-action}
2482 @item --checkpoint-action=@var{action}
2483 Instruct @command{tar} to execute an action upon hitting a
2484 breakpoint. Here we give only a brief outline. @xref{checkpoints},
2485 for a complete description.
2487 The @var{action} argument can be one of the following:
2491 Produce an audible bell on the console.
2495 Print a single dot on the standard listing stream.
2498 Display a textual message on the standard error, with the status and
2499 number of the checkpoint. This is the default.
2501 @item echo=@var{string}
2502 Display @var{string} on the standard error. Before output, the string
2503 is subject to meta-character expansion.
2505 @item exec=@var{command}
2506 Execute the given @var{command}.
2508 @item sleep=@var{time}
2509 Wait for @var{time} seconds.
2511 @item ttyout=@var{string}
2512 Output @var{string} on the current console (@file{/dev/tty}).
2515 Several @option{--checkpoint-action} options can be specified. The
2516 supplied actions will be executed in order of their appearance in the
2519 Using @option{--checkpoint-action} without @option{--checkpoint}
2520 assumes default checkpoint frequency of one checkpoint per 10 records.
2522 @opsummary{check-links}
2525 If this option was given, @command{tar} will check the number of links
2526 dumped for each processed file. If this number does not match the
2527 total number of hard links for the file, a warning message will be
2528 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2529 synonym for @option{--one-file-system}. The current semantics, which
2530 complies to UNIX98, was introduced with version
2531 1.15.91. @xref{Changes}, for more information.}.
2535 @opsummary{compress}
2536 @opsummary{uncompress}
2541 @command{tar} will use the @command{compress} program when reading or
2542 writing the archive. This allows you to directly act on archives
2543 while saving space. @xref{gzip}.
2545 @opsummary{confirmation}
2546 @item --confirmation
2548 (See @option{--interactive}.) @xref{interactive}.
2550 @opsummary{delay-directory-restore}
2551 @item --delay-directory-restore
2553 Delay setting modification times and permissions of extracted
2554 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2556 @opsummary{dereference}
2560 When reading or writing a file to be archived, @command{tar} accesses
2561 the file that a symbolic link points to, rather than the symlink
2562 itself. @xref{dereference}.
2564 @opsummary{directory}
2565 @item --directory=@var{dir}
2568 When this option is specified, @command{tar} will change its current directory
2569 to @var{dir} before performing any operations. When this option is used
2570 during archive creation, it is order sensitive. @xref{directory}.
2573 @item --exclude=@var{pattern}
2575 When performing operations, @command{tar} will skip files that match
2576 @var{pattern}. @xref{exclude}.
2578 @opsummary{exclude-backups}
2579 @item --exclude-backups
2580 Exclude backup and lock files. @xref{exclude,, exclude-backups}.
2582 @opsummary{exclude-from}
2583 @item --exclude-from=@var{file}
2584 @itemx -X @var{file}
2586 Similar to @option{--exclude}, except @command{tar} will use the list of
2587 patterns in the file @var{file}. @xref{exclude}.
2589 @opsummary{exclude-caches}
2590 @item --exclude-caches
2592 Exclude from dump any directory containing a valid cache directory
2593 tag file, but still dump the directory node and the tag file itself.
2595 @xref{exclude,, exclude-caches}.
2597 @opsummary{exclude-caches-under}
2598 @item --exclude-caches-under
2600 Exclude from dump any directory containing a valid cache directory
2601 tag file, but still dump the directory node itself.
2605 @opsummary{exclude-caches-all}
2606 @item --exclude-caches-all
2608 Exclude from dump any directory containing a valid cache directory
2609 tag file. @xref{exclude}.
2611 @opsummary{exclude-tag}
2612 @item --exclude-tag=@var{file}
2614 Exclude from dump any directory containing file named @var{file}, but
2615 dump the directory node and @var{file} itself. @xref{exclude,, exclude-tag}.
2617 @opsummary{exclude-tag-under}
2618 @item --exclude-tag-under=@var{file}
2620 Exclude from dump the contents of any directory containing file
2621 named @var{file}, but dump the directory node itself. @xref{exclude,,
2624 @opsummary{exclude-tag-all}
2625 @item --exclude-tag-all=@var{file}
2627 Exclude from dump any directory containing file named @var{file}.
2628 @xref{exclude,,exclude-tag-all}.
2630 @opsummary{exclude-vcs}
2633 Exclude from dump directories and files, that are internal for some
2634 widely used version control systems.
2636 @xref{exclude,,exclude-vcs}.
2639 @item --file=@var{archive}
2640 @itemx -f @var{archive}
2642 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2643 performs operations on, rather than @command{tar}'s compilation dependent
2644 default. @xref{file tutorial}.
2646 @opsummary{files-from}
2647 @item --files-from=@var{file}
2648 @itemx -T @var{file}
2650 @command{tar} will use the contents of @var{file} as a list of archive members
2651 or files to operate on, in addition to those specified on the
2652 command-line. @xref{files}.
2654 @opsummary{force-local}
2657 Forces @command{tar} to interpret the file name given to @option{--file}
2658 as a local file, even if it looks like a remote tape drive name.
2659 @xref{local and remote archives}.
2662 @item --format=@var{format}
2663 @itemx -H @var{format}
2665 Selects output archive format. @var{Format} may be one of the
2670 Creates an archive that is compatible with Unix V7 @command{tar}.
2673 Creates an archive that is compatible with GNU @command{tar} version
2677 Creates archive in GNU tar 1.13 format. Basically it is the same as
2678 @samp{oldgnu} with the only difference in the way it handles long
2682 Creates a @acronym{POSIX.1-1988} compatible archive.
2685 Creates a @acronym{POSIX.1-2001 archive}.
2689 @xref{Formats}, for a detailed discussion of these formats.
2691 @opsummary{full-time}
2693 This option instructs @command{tar} to print file times to their full
2694 resolution. Usually this means 1-second resolution, but that depends
2695 on the underlying file system. The @option{--full-time} option takes
2696 effect only when detailed output (verbosity level 2 or higher) has
2697 been requested using the @option{--verbose} option, e.g., when listing
2698 or extracting archives:
2701 $ @kbd{tar -t -v --full-time -f archive.tar}
2705 or, when creating an archive:
2708 $ @kbd{tar -c -vv --full-time -f archive.tar .}
2711 Notice, thar when creating the archive you need to specify
2712 @option{--verbose} twice to get a detailed output (@pxref{verbose
2716 @item --group=@var{group}
2718 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
2719 rather than the group from the source file. @var{group} can specify a
2720 symbolic name, or a numeric @acronym{ID}, or both as
2721 @var{name}:@var{id}. @xref{override}.
2723 Also see the comments for the @option{--owner=@var{user}} option.
2733 This option tells @command{tar} to read or write archives through
2734 @command{gzip}, allowing @command{tar} to directly operate on several
2735 kinds of compressed archives transparently. @xref{gzip}.
2737 @opsummary{hard-dereference}
2738 @item --hard-dereference
2739 When creating an archive, dereference hard links and store the files
2740 they refer to, instead of creating usual hard link members.
2748 @command{tar} will print out a short message summarizing the operations and
2749 options to @command{tar} and exit. @xref{help}.
2751 @opsummary{ignore-case}
2753 Ignore case when matching member or file names with
2754 patterns. @xref{controlling pattern-matching}.
2756 @opsummary{ignore-command-error}
2757 @item --ignore-command-error
2758 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2760 @opsummary{ignore-failed-read}
2761 @item --ignore-failed-read
2763 Do not exit unsuccessfully merely because an unreadable file was encountered.
2764 @xref{Ignore Failed Read}.
2766 @opsummary{ignore-zeros}
2767 @item --ignore-zeros
2770 With this option, @command{tar} will ignore zeroed blocks in the
2771 archive, which normally signals EOF. @xref{Reading}.
2773 @opsummary{incremental}
2777 Informs @command{tar} that it is working with an old
2778 @acronym{GNU}-format incremental backup archive. It is intended
2779 primarily for backwards compatibility only. @xref{Incremental Dumps},
2780 for a detailed discussion of incremental archives.
2782 @opsummary{index-file}
2783 @item --index-file=@var{file}
2785 Send verbose output to @var{file} instead of to standard output.
2787 @opsummary{info-script}
2788 @opsummary{new-volume-script}
2789 @item --info-script=@var{command}
2790 @itemx --new-volume-script=@var{command}
2791 @itemx -F @var{command}
2793 When @command{tar} is performing multi-tape backups, @var{command} is run
2794 at the end of each tape. If it exits with nonzero status,
2795 @command{tar} fails immediately. @xref{info-script}, for a detailed
2796 discussion of this feature.
2798 @opsummary{interactive}
2800 @itemx --confirmation
2803 Specifies that @command{tar} should ask the user for confirmation before
2804 performing potentially destructive options, such as overwriting files.
2807 @opsummary{--keep-directory-symlink}
2808 @item --keep-directory-symlink
2810 This option changes the behavior of tar when it encounters a symlink
2811 with the same name as the directory that it is about to extract. By
2812 default, in this case tar would first remove the symlink and then
2813 proceed extracting the directory.
2815 The @option{--keep-directory-symlink} option disables this behavior
2816 and instructs tar to follow symlinks to directories when extracting
2819 It is mainly intended to provide compatibility with the Slackware
2820 installation scripts.
2822 @opsummary{keep-newer-files}
2823 @item --keep-newer-files
2825 Do not replace existing files that are newer than their archive copies
2826 when extracting files from an archive.
2828 @opsummary{keep-old-files}
2829 @item --keep-old-files
2832 Do not overwrite existing files when extracting files from an
2833 archive. Return error if such files exist. See also
2834 @ref{--skip-old-files}.
2836 @xref{Keep Old Files}.
2839 @item --label=@var{name}
2840 @itemx -V @var{name}
2842 When creating an archive, instructs @command{tar} to write @var{name}
2843 as a name record in the archive. When extracting or listing archives,
2844 @command{tar} will only operate on archives that have a label matching
2845 the pattern specified in @var{name}. @xref{Tape Files}.
2848 @item --level=@var{n}
2849 Force incremental backup of level @var{n}. As of @GNUTAR version
2850 @value{VERSION}, the option @option{--level=0} truncates the snapshot
2851 file, thereby forcing the level 0 dump. Other values of @var{n} are
2852 effectively ignored. @xref{--level=0}, for details and examples.
2854 The use of this option is valid only in conjunction with the
2855 @option{--listed-incremental} option. @xref{Incremental Dumps},
2856 for a detailed description.
2858 @opsummary{listed-incremental}
2859 @item --listed-incremental=@var{snapshot-file}
2860 @itemx -g @var{snapshot-file}
2862 During a @option{--create} operation, specifies that the archive that
2863 @command{tar} creates is a new @acronym{GNU}-format incremental
2864 backup, using @var{snapshot-file} to determine which files to backup.
2865 With other operations, informs @command{tar} that the archive is in
2866 incremental format. @xref{Incremental Dumps}.
2871 This option tells @command{tar} to read or write archives through
2872 @command{lzip}. @xref{gzip}.
2877 This option tells @command{tar} to read or write archives through
2878 @command{lzma}. @xref{gzip}.
2882 This option tells @command{tar} to read or write archives through
2883 @command{lzop}. @xref{gzip}.
2886 @item --mode=@var{permissions}
2888 When adding files to an archive, @command{tar} will use
2889 @var{permissions} for the archive members, rather than the permissions
2890 from the files. @var{permissions} can be specified either as an octal
2891 number or as symbolic permissions, like with
2892 @command{chmod}. @xref{override}.
2895 @item --mtime=@var{date}
2897 When adding files to an archive, @command{tar} will use @var{date} as
2898 the modification time of members when creating archives, instead of
2899 their actual modification times. The value of @var{date} can be
2900 either a textual date representation (@pxref{Date input formats}) or a
2901 name of the existing file, starting with @samp{/} or @samp{.}. In the
2902 latter case, the modification time of that file is used. @xref{override}.
2904 @opsummary{multi-volume}
2905 @item --multi-volume
2908 Informs @command{tar} that it should create or otherwise operate on a
2909 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2911 @opsummary{new-volume-script}
2912 @item --new-volume-script
2914 (see @option{--info-script})
2917 @item --newer=@var{date}
2918 @itemx --after-date=@var{date}
2921 When creating an archive, @command{tar} will only add files that have changed
2922 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2923 is taken to be the name of a file whose data modification time specifies
2924 the date. @xref{after}.
2926 @opsummary{newer-mtime}
2927 @item --newer-mtime=@var{date}
2929 Like @option{--newer}, but add only files whose
2930 contents have changed (as opposed to just @option{--newer}, which will
2931 also back up files for which any status information has
2932 changed). @xref{after}.
2934 @opsummary{no-anchored}
2936 An exclude pattern can match any subsequence of the name's components.
2937 @xref{controlling pattern-matching}.
2939 @opsummary{no-auto-compress}
2940 @item --no-auto-compress
2942 Disables automatic compressed format recognition based on the archive
2943 suffix. @xref{--auto-compress}. @xref{gzip}.
2945 @opsummary{no-check-device}
2946 @item --no-check-device
2947 Do not check device numbers when creating a list of modified files
2948 for incremental archiving. @xref{device numbers}, for
2949 a detailed description.
2951 @opsummary{no-delay-directory-restore}
2952 @item --no-delay-directory-restore
2954 Modification times and permissions of extracted
2955 directories are set when all files from this directory have been
2956 extracted. This is the default.
2957 @xref{Directory Modification Times and Permissions}.
2959 @opsummary{no-ignore-case}
2960 @item --no-ignore-case
2961 Use case-sensitive matching.
2962 @xref{controlling pattern-matching}.
2964 @opsummary{no-ignore-command-error}
2965 @item --no-ignore-command-error
2966 Print warnings about subprocesses that terminated with a nonzero exit
2967 code. @xref{Writing to an External Program}.
2972 If the @option{--null} option was given previously, this option
2973 cancels its effect, so that any following @option{--files-from}
2974 options will expect their file lists to be newline-terminated.
2976 @opsummary{no-overwrite-dir}
2977 @item --no-overwrite-dir
2979 Preserve metadata of existing directories when extracting files
2980 from an archive. @xref{Overwrite Old Files}.
2982 @opsummary{no-quote-chars}
2983 @item --no-quote-chars=@var{string}
2984 Remove characters listed in @var{string} from the list of quoted
2985 characters set by the previous @option{--quote-chars} option
2986 (@pxref{quoting styles}).
2988 @opsummary{no-recursion}
2989 @item --no-recursion
2991 With this option, @command{tar} will not recurse into directories.
2994 @opsummary{no-same-owner}
2995 @item --no-same-owner
2998 When extracting an archive, do not attempt to preserve the owner
2999 specified in the @command{tar} archive. This the default behavior
3002 @opsummary{no-same-permissions}
3003 @item --no-same-permissions
3005 When extracting an archive, subtract the user's umask from files from
3006 the permissions specified in the archive. This is the default behavior
3012 The archive media does not support seeks to arbitrary
3013 locations. Usually @command{tar} determines automatically whether
3014 the archive can be seeked or not. Use this option to disable this
3017 @opsummary{no-unquote}
3019 Treat all input file or member names literally, do not interpret
3020 escape sequences. @xref{input name quoting}.
3022 @opsummary{no-wildcards}
3023 @item --no-wildcards
3024 Do not use wildcards.
3025 @xref{controlling pattern-matching}.
3027 @opsummary{no-wildcards-match-slash}
3028 @item --no-wildcards-match-slash
3029 Wildcards do not match @samp{/}.
3030 @xref{controlling pattern-matching}.
3035 When @command{tar} is using the @option{--files-from} option, this option
3036 instructs @command{tar} to expect file names terminated with @acronym{NUL}, so
3037 @command{tar} can correctly work with file names that contain newlines.
3040 @opsummary{numeric-owner}
3041 @item --numeric-owner
3043 This option will notify @command{tar} that it should use numeric user
3044 and group IDs when creating a @command{tar} file, rather than names.
3048 The function of this option depends on the action @command{tar} is
3049 performing. When extracting files, @option{-o} is a synonym for
3050 @option{--no-same-owner}, i.e., it prevents @command{tar} from
3051 restoring ownership of files being extracted.
3053 When creating an archive, it is a synonym for
3054 @option{--old-archive}. This behavior is for compatibility
3055 with previous versions of @GNUTAR{}, and will be
3056 removed in future releases.
3058 @xref{Changes}, for more information.
3060 @opsummary{occurrence}
3061 @item --occurrence[=@var{number}]
3063 This option can be used in conjunction with one of the subcommands
3064 @option{--delete}, @option{--diff}, @option{--extract} or
3065 @option{--list} when a list of files is given either on the command
3066 line or via @option{-T} option.
3068 This option instructs @command{tar} to process only the @var{number}th
3069 occurrence of each named file. @var{Number} defaults to 1, so
3072 tar -x -f archive.tar --occurrence filename
3076 will extract the first occurrence of the member @file{filename} from @file{archive.tar}
3077 and will terminate without scanning to the end of the archive.
3079 @opsummary{old-archive}
3081 Synonym for @option{--format=v7}.
3083 @opsummary{one-file-system}
3084 @item --one-file-system
3085 Used when creating an archive. Prevents @command{tar} from recursing into
3086 directories that are on different file systems from the current
3089 @opsummary{one-top-level}
3090 @item --one-top-level[=@var{dir}]
3091 Tells @command{tar} to create a new directory beneath the extraction directory
3092 (or the one passed to @option{-C}) and use it to guard against
3093 tarbombs. In the absence of @var{dir} argument, the name of the new directory
3094 will be equal to the base name of the archive (file name minus the
3095 archive suffix, if recognized). Any member names that do not begin
3096 with that directory name (after
3097 transformations from @option{--transform} and
3098 @option{--strip-components}) will be prefixed with it. Recognized
3099 file name suffixes are @samp{.tar}, and any compression suffixes
3100 recognizable by @xref{--auto-compress}.
3102 @opsummary{overwrite}
3105 Overwrite existing files and directory metadata when extracting files
3106 from an archive. @xref{Overwrite Old Files}.
3108 @opsummary{overwrite-dir}
3109 @item --overwrite-dir
3111 Overwrite the metadata of existing directories when extracting files
3112 from an archive. @xref{Overwrite Old Files}.
3115 @item --owner=@var{user}
3117 Specifies that @command{tar} should use @var{user} as the owner of members
3118 when creating archives, instead of the user associated with the source
3119 file. @var{user} can specify a symbolic name, or a numeric
3120 @acronym{ID}, or both as @var{name}:@var{id}.
3123 This option does not affect extraction from archives.
3125 @opsummary{pax-option}
3126 @item --pax-option=@var{keyword-list}
3127 This option enables creation of the archive in @acronym{POSIX.1-2001}
3128 format (@pxref{posix}) and modifies the way @command{tar} handles the
3129 extended header keywords. @var{Keyword-list} is a comma-separated
3130 list of keyword options. @xref{PAX keywords}, for a detailed
3133 @opsummary{portability}
3135 @itemx --old-archive
3136 Synonym for @option{--format=v7}.
3140 Same as @option{--format=posix}.
3142 @opsummary{preserve}
3145 Synonymous with specifying both @option{--preserve-permissions} and
3146 @option{--same-order}. @xref{Setting Access Permissions}.
3148 @opsummary{preserve-order}
3149 @item --preserve-order
3151 (See @option{--same-order}; @pxref{Reading}.)
3153 @opsummary{preserve-permissions}
3154 @opsummary{same-permissions}
3155 @item --preserve-permissions
3156 @itemx --same-permissions
3159 When @command{tar} is extracting an archive, it normally subtracts the
3160 users' umask from the permissions specified in the archive and uses
3161 that number as the permissions to create the destination file.
3162 Specifying this option instructs @command{tar} that it should use the
3163 permissions directly from the archive. @xref{Setting Access Permissions}.
3165 @opsummary{quote-chars}
3166 @item --quote-chars=@var{string}
3167 Always quote characters from @var{string}, even if the selected
3168 quoting style would not quote them (@pxref{quoting styles}).
3170 @opsummary{quoting-style}
3171 @item --quoting-style=@var{style}
3172 Set quoting style to use when printing member and file names
3173 (@pxref{quoting styles}). Valid @var{style} values are:
3174 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
3175 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
3176 style is @code{escape}, unless overridden while configuring the
3179 @opsummary{read-full-records}
3180 @item --read-full-records
3183 Specifies that @command{tar} should reblock its input, for reading
3184 from pipes on systems with buggy implementations. @xref{Reading}.
3186 @opsummary{record-size}
3187 @item --record-size=@var{size}[@var{suf}]
3189 Instructs @command{tar} to use @var{size} bytes per record when accessing the
3190 archive. The argument can be suffixed with a @dfn{size suffix}, e.g.
3191 @option{--record-size=10K} for 10 Kilobytes. @xref{size-suffixes},
3192 for a list of valid suffixes. @xref{Blocking Factor}, for a detailed
3193 description of this option.
3195 @opsummary{recursion}
3198 With this option, @command{tar} recurses into directories (default).
3201 @opsummary{recursive-unlink}
3202 @item --recursive-unlink
3205 directory hierarchies before extracting directories of the same name
3206 from the archive. @xref{Recursive Unlink}.
3208 @opsummary{remove-files}
3209 @item --remove-files
3211 Directs @command{tar} to remove the source file from the file system after
3212 appending it to an archive. @xref{remove files}.
3214 @opsummary{restrict}
3217 Disable use of some potentially harmful @command{tar} options.
3218 Currently this option disables shell invocation from multi-volume menu
3219 (@pxref{Using Multiple Tapes}).
3221 @opsummary{rmt-command}
3222 @item --rmt-command=@var{cmd}
3224 Notifies @command{tar} that it should use @var{cmd} instead of
3225 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
3227 @opsummary{rsh-command}
3228 @item --rsh-command=@var{cmd}
3230 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
3231 devices. @xref{Device}.
3233 @opsummary{same-order}
3235 @itemx --preserve-order
3238 This option is an optimization for @command{tar} when running on machines with
3239 small amounts of memory. It informs @command{tar} that the list of file
3240 arguments has already been sorted to match the order of files in the
3241 archive. @xref{Reading}.
3243 @opsummary{same-owner}
3246 When extracting an archive, @command{tar} will attempt to preserve the owner
3247 specified in the @command{tar} archive with this option present.
3248 This is the default behavior for the superuser; this option has an
3249 effect only for ordinary users. @xref{Attributes}.
3251 @opsummary{same-permissions}
3252 @item --same-permissions
3254 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
3260 Assume that the archive media supports seeks to arbitrary
3261 locations. Usually @command{tar} determines automatically whether
3262 the archive can be seeked or not. This option is intended for use
3263 in cases when such recognition fails. It takes effect only if the
3264 archive is open for reading (e.g. with @option{--list} or
3265 @option{--extract} options).
3267 @opsummary{show-defaults}
3268 @item --show-defaults
3270 Displays the default options used by @command{tar} and exits
3271 successfully. This option is intended for use in shell scripts.
3272 Here is an example of what you can see using this option:
3275 $ @kbd{tar --show-defaults}
3276 --format=gnu -f- -b20 --quoting-style=escape
3277 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
3281 Notice, that this option outputs only one line. The example output
3282 above has been split to fit page boundaries. @xref{defaults}.
3284 @opsummary{show-omitted-dirs}
3285 @item --show-omitted-dirs
3287 Instructs @command{tar} to mention the directories it is skipping when
3288 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
3290 @opsummary{show-snapshot-field-ranges}
3291 @item --show-snapshot-field-ranges
3293 Displays the range of values allowed by this version of @command{tar}
3294 for each field in the snapshot file, then exits successfully.
3295 @xref{Snapshot Files}.
3297 @opsummary{show-transformed-names}
3298 @opsummary{show-stored-names}
3299 @item --show-transformed-names
3300 @itemx --show-stored-names
3302 Display file or member names after applying any transformations
3303 (@pxref{transform}). In particular, when used in conjunction with one of
3304 the archive creation operations it instructs @command{tar} to list the
3305 member names stored in the archive, as opposed to the actual file
3306 names. @xref{listing member and file names}.
3308 @opsummary{skip-old-files}
3309 @item --skip-old-files
3311 Do not overwrite existing files when extracting files from an
3312 archive. @xref{Keep Old Files}.
3314 This option differs from @option{--keep-old-files} in that it does not
3315 treat such files as an error, instead it just silently avoids
3318 The @option{--warning=existing-file} option can be used together with
3319 this option to produce warning messages about existing old files
3323 @item --sort=@var{order}
3324 Specify the directory sorting order when reading directories.
3325 @var{Order} may be one of the following:
3329 No directory sorting is performed. This is the default.
3332 Sort the directory entries on name. The operating system may deliver
3333 directory entries in a more or less random order, and sorting them
3334 makes archive creation reproducible.
3337 Sort the directory entries on inode number. Sorting directories on
3338 inode number may reduce the amount of disk seek operations when
3339 creating an archive for some file systems.
3347 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3348 sparse files efficiently. @xref{sparse}.
3350 @opsummary{sparse-version}
3351 @item --sparse-version=@var{version}
3353 Specifies the @dfn{format version} to use when archiving sparse
3354 files. Implies @option{--sparse}. @xref{sparse}. For the description
3355 of the supported sparse formats, @xref{Sparse Formats}.
3357 @opsummary{starting-file}
3358 @item --starting-file=@var{name}
3359 @itemx -K @var{name}
3361 This option affects extraction only; @command{tar} will skip extracting
3362 files in the archive until it finds one that matches @var{name}.
3365 @opsummary{strip-components}
3366 @item --strip-components=@var{number}
3367 Strip given @var{number} of leading components from file names before
3368 extraction. For example, if archive @file{archive.tar} contained
3369 @file{/some/file/name}, then running
3372 tar --extract --file archive.tar --strip-components=2
3376 would extract this file to file @file{name}.
3379 @item --suffix=@var{suffix}
3381 Alters the suffix @command{tar} uses when backing up files from the default
3382 @samp{~}. @xref{backup}.
3384 @opsummary{tape-length}
3385 @item --tape-length=@var{num}[@var{suf}]
3386 @itemx -L @var{num}[@var{suf}]
3388 Specifies the length of tapes that @command{tar} is writing as being
3389 @w{@var{num} x 1024} bytes long. If optional @var{suf} is given, it
3390 specifies a multiplicative factor to be used instead of 1024. For
3391 example, @samp{-L2M} means 2 megabytes. @xref{size-suffixes}, for a
3392 list of allowed suffixes. @xref{Using Multiple Tapes}, for a detailed
3393 discussion of this option.
3395 @opsummary{test-label}
3398 Reads the volume label. If an argument is specified, test whether it
3399 matches the volume label. @xref{--test-label option}.
3401 @opsummary{to-command}
3402 @item --to-command=@var{command}
3404 During extraction @command{tar} will pipe extracted files to the
3405 standard input of @var{command}. @xref{Writing to an External Program}.
3407 @opsummary{to-stdout}
3411 During extraction, @command{tar} will extract files to stdout rather
3412 than to the file system. @xref{Writing to Standard Output}.
3415 @item --totals[=@var{signo}]
3417 Displays the total number of bytes transferred when processing an
3418 archive. If an argument is given, these data are displayed on
3419 request, when signal @var{signo} is delivered to @command{tar}.
3426 Sets the data modification time of extracted files to the extraction time,
3427 rather than the data modification time stored in the archive.
3428 @xref{Data Modification Times}.
3430 @opsummary{transform}
3432 @item --transform=@var{sed-expr}
3433 @itemx --xform=@var{sed-expr}
3434 Transform file or member names using @command{sed} replacement expression
3435 @var{sed-expr}. For example,
3438 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
3442 will add to @file{archive} files from the current working directory,
3443 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
3444 discussion, @xref{transform}.
3446 To see transformed member names in verbose listings, use
3447 @option{--show-transformed-names} option
3448 (@pxref{show-transformed-names}).
3450 @opsummary{uncompress}
3453 (See @option{--compress}, @pxref{gzip})
3458 (See @option{--gzip}, @pxref{gzip})
3460 @opsummary{unlink-first}
3461 @item --unlink-first
3464 Directs @command{tar} to remove the corresponding file from the file
3465 system before extracting it from the archive. @xref{Unlink First}.
3469 Enable unquoting input file or member names (default). @xref{input
3472 @opsummary{use-compress-program}
3473 @item --use-compress-program=@var{prog}
3474 @itemx -I=@var{prog}
3476 Instructs @command{tar} to access the archive through @var{prog}, which is
3477 presumed to be a compression program of some sort. @xref{gzip}.
3482 Display file modification dates in @acronym{UTC}. This option implies
3489 Specifies that @command{tar} should be more verbose about the
3490 operations it is performing. This option can be specified multiple
3491 times for some operations to increase the amount of information displayed.
3498 Verifies that the archive was correctly written when creating an
3499 archive. @xref{verify}.
3504 Print information about the program's name, version, origin and legal
3505 status, all on standard output, and then exit successfully.
3508 @opsummary{volno-file}
3509 @item --volno-file=@var{file}
3511 Used in conjunction with @option{--multi-volume}. @command{tar} will
3512 keep track of which volume of a multi-volume archive it is working in
3513 @var{file}. @xref{volno-file}.
3516 @item --warning=@var{keyword}
3518 Enable or disable warning messages identified by @var{keyword}. The
3519 messages are suppressed if @var{keyword} is prefixed with @samp{no-}.
3522 @opsummary{wildcards}
3524 Use wildcards when matching member names with patterns.
3525 @xref{controlling pattern-matching}.
3527 @opsummary{wildcards-match-slash}
3528 @item --wildcards-match-slash
3529 Wildcards match @samp{/}.
3530 @xref{controlling pattern-matching}.
3535 Use @command{xz} for compressing or decompressing the archives. @xref{gzip}.
3539 @node Short Option Summary
3540 @subsection Short Options Cross Reference
3542 Here is an alphabetized list of all of the short option forms, matching
3543 them with the equivalent long option.
3545 @multitable @columnfractions 0.20 0.80
3546 @headitem Short Option @tab Reference
3548 @item -A @tab @ref{--concatenate}.
3550 @item -B @tab @ref{--read-full-records}.
3552 @item -C @tab @ref{--directory}.
3554 @item -F @tab @ref{--info-script}.
3556 @item -G @tab @ref{--incremental}.
3558 @item -J @tab @ref{--xz}.
3560 @item -K @tab @ref{--starting-file}.
3562 @item -L @tab @ref{--tape-length}.
3564 @item -M @tab @ref{--multi-volume}.
3566 @item -N @tab @ref{--newer}.
3568 @item -O @tab @ref{--to-stdout}.
3570 @item -P @tab @ref{--absolute-names}.
3572 @item -R @tab @ref{--block-number}.
3574 @item -S @tab @ref{--sparse}.
3576 @item -T @tab @ref{--files-from}.
3578 @item -U @tab @ref{--unlink-first}.
3580 @item -V @tab @ref{--label}.
3582 @item -W @tab @ref{--verify}.
3584 @item -X @tab @ref{--exclude-from}.
3586 @item -Z @tab @ref{--compress}.
3588 @item -b @tab @ref{--blocking-factor}.
3590 @item -c @tab @ref{--create}.
3592 @item -d @tab @ref{--compare}.
3594 @item -f @tab @ref{--file}.
3596 @item -g @tab @ref{--listed-incremental}.
3598 @item -h @tab @ref{--dereference}.
3600 @item -i @tab @ref{--ignore-zeros}.
3602 @item -j @tab @ref{--bzip2}.
3604 @item -k @tab @ref{--keep-old-files}.
3606 @item -l @tab @ref{--check-links}.
3608 @item -m @tab @ref{--touch}.
3610 @item -o @tab When creating, @ref{--no-same-owner}, when extracting ---
3611 @ref{--portability}.
3613 The latter usage is deprecated. It is retained for compatibility with
3614 the earlier versions of @GNUTAR{}. In future releases
3615 @option{-o} will be equivalent to @option{--no-same-owner} only.
3617 @item -p @tab @ref{--preserve-permissions}.
3619 @item -r @tab @ref{--append}.
3621 @item -s @tab @ref{--same-order}.
3623 @item -t @tab @ref{--list}.
3625 @item -u @tab @ref{--update}.
3627 @item -v @tab @ref{--verbose}.
3629 @item -w @tab @ref{--interactive}.
3631 @item -x @tab @ref{--extract}.
3633 @item -z @tab @ref{--gzip}.
3638 @section @GNUTAR{} documentation
3640 @cindex Getting program version number
3642 @cindex Version of the @command{tar} program
3643 Being careful, the first thing is really checking that you are using
3644 @GNUTAR{}, indeed. The @option{--version} option
3645 causes @command{tar} to print information about its name, version,
3646 origin and legal status, all on standard output, and then exit
3647 successfully. For example, @w{@samp{tar --version}} might print:
3650 tar (GNU tar) @value{VERSION}
3651 Copyright (C) 2013-2014 Free Software Foundation, Inc.
3652 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
3653 This is free software: you are free to change and redistribute it.
3654 There is NO WARRANTY, to the extent permitted by law.
3656 Written by John Gilmore and Jay Fenlason.
3660 The first occurrence of @samp{tar} in the result above is the program
3661 name in the package (for example, @command{rmt} is another program),
3662 while the second occurrence of @samp{tar} is the name of the package
3663 itself, containing possibly many programs. The package is currently
3664 named @samp{tar}, after the name of the main program it
3665 contains@footnote{There are plans to merge the @command{cpio} and
3666 @command{tar} packages into a single one which would be called
3667 @code{paxutils}. So, who knows if, one of this days, the
3668 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3671 @cindex Obtaining help
3672 @cindex Listing all @command{tar} options
3673 @xopindex{help, introduction}
3674 Another thing you might want to do is checking the spelling or meaning
3675 of some particular @command{tar} option, without resorting to this
3676 manual, for once you have carefully read it. @GNUTAR{}
3677 has a short help feature, triggerable through the
3678 @option{--help} option. By using this option, @command{tar} will
3679 print a usage message listing all available options on standard
3680 output, then exit successfully, without doing anything else and
3681 ignoring all other options. Even if this is only a brief summary, it
3682 may be several screens long. So, if you are not using some kind of
3683 scrollable window, you might prefer to use something like:
3686 $ @kbd{tar --help | less}
3690 presuming, here, that you like using @command{less} for a pager. Other
3691 popular pagers are @command{more} and @command{pg}. If you know about some
3692 @var{keyword} which interests you and do not want to read all the
3693 @option{--help} output, another common idiom is doing:
3696 tar --help | grep @var{keyword}
3700 for getting only the pertinent lines. Notice, however, that some
3701 @command{tar} options have long description lines and the above
3702 command will list only the first of them.
3704 The exact look of the option summary displayed by @kbd{tar --help} is
3705 configurable. @xref{Configuring Help Summary}, for a detailed description.
3708 If you only wish to check the spelling of an option, running @kbd{tar
3709 --usage} may be a better choice. This will display a terse list of
3710 @command{tar} options without accompanying explanations.
3712 The short help output is quite succinct, and you might have to get
3713 back to the full documentation for precise points. If you are reading
3714 this paragraph, you already have the @command{tar} manual in some
3715 form. This manual is available in a variety of forms from
3716 @url{http://www.gnu.org/software/tar/manual}. It may be printed out of the @GNUTAR{}
3717 distribution, provided you have @TeX{} already installed somewhere,
3718 and a laser printer around. Just configure the distribution, execute
3719 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3720 usual way (contact your local guru to know how). If @GNUTAR{}
3721 has been conveniently installed at your place, this
3722 manual is also available in interactive, hypertextual form as an Info
3723 file. Just call @w{@samp{info tar}} or, if you do not have the
3724 @command{info} program handy, use the Info reader provided within
3725 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3727 There is currently no @code{man} page for @GNUTAR{}.
3728 If you observe such a @code{man} page on the system you are running,
3729 either it does not belong to @GNUTAR{}, or it has not
3730 been produced by @acronym{GNU}. Some package maintainers convert
3731 @kbd{tar --help} output to a man page, using @command{help2man}. In
3732 any case, please bear in mind that the authoritative source of
3733 information about @GNUTAR{} is this Texinfo documentation.
3736 @section Obtaining @GNUTAR{} default values
3738 @opindex show-defaults
3739 @GNUTAR{} has some predefined defaults that are used when you do not
3740 explicitly specify another values. To obtain a list of such
3741 defaults, use @option{--show-defaults} option. This will output the
3742 values in the form of @command{tar} command line options:
3746 $ @kbd{tar --show-defaults}
3747 --format=gnu -f- -b20 --quoting-style=escape
3748 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3753 Notice, that this option outputs only one line. The example output above
3754 has been split to fit page boundaries.
3757 The above output shows that this version of @GNUTAR{} defaults to
3758 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3759 output as the archive, if no @option{--file} option has been given
3760 (@pxref{file tutorial}), the default blocking factor is 20
3761 (@pxref{Blocking Factor}). It also shows the default locations where
3762 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3765 @section Checking @command{tar} progress
3767 Typically, @command{tar} performs most operations without reporting any
3768 information to the user except error messages. When using @command{tar}
3769 with many options, particularly ones with complicated or
3770 difficult-to-predict behavior, it is possible to make serious mistakes.
3771 @command{tar} provides several options that make observing @command{tar}
3772 easier. These options cause @command{tar} to print information as it
3773 progresses in its job, and you might want to use them just for being
3774 more careful about what is going on, or merely for entertaining
3775 yourself. If you have encountered a problem when operating on an
3776 archive, however, you may need more information than just an error
3777 message in order to solve the problem. The following options can be
3778 helpful diagnostic tools.
3780 @cindex Verbose operation
3782 Normally, the @option{--list} (@option{-t}) command to list an archive
3783 prints just the file names (one per line) and the other commands are
3784 silent. When used with most operations, the @option{--verbose}
3785 (@option{-v}) option causes @command{tar} to print the name of each
3786 file or archive member as it is processed. This and the other options
3787 which make @command{tar} print status information can be useful in
3788 monitoring @command{tar}.
3790 With @option{--create} or @option{--extract}, @option{--verbose} used
3791 once just prints the names of the files or members as they are processed.
3792 Using it twice causes @command{tar} to print a longer listing
3793 (@xref{verbose member listing}, for the description) for each member.
3794 Since @option{--list} already prints the names of the members,
3795 @option{--verbose} used once with @option{--list} causes @command{tar}
3796 to print an @samp{ls -l} type listing of the files in the archive.
3797 The following examples both extract members with long list output:
3800 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3801 $ @kbd{tar xvvf archive.tar}
3804 Verbose output appears on the standard output except when an archive is
3805 being written to the standard output, as with @samp{tar --create
3806 --file=- --verbose} (@samp{tar cvf -}, or even @samp{tar cv}---if the
3807 installer let standard output be the default archive). In that case
3808 @command{tar} writes verbose output to the standard error stream.
3810 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3811 verbose output to @var{file} rather than to standard output or standard
3815 @cindex Obtaining total status information
3817 The @option{--totals} option causes @command{tar} to print on the
3818 standard error the total amount of bytes transferred when processing
3819 an archive. When creating or appending to an archive, this option
3820 prints the number of bytes written to the archive and the average
3821 speed at which they have been written, e.g.:
3825 $ @kbd{tar -c -f archive.tar --totals /home}
3826 Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
3830 When reading an archive, this option displays the number of bytes
3835 $ @kbd{tar -x -f archive.tar --totals}
3836 Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
3840 Finally, when deleting from an archive, the @option{--totals} option
3841 displays both numbers plus number of bytes removed from the archive:
3845 $ @kbd{tar --delete -f foo.tar --totals --wildcards '*~'}
3846 Total bytes read: 9543680 (9.2MiB, 201MiB/s)
3847 Total bytes written: 3829760 (3.7MiB, 81MiB/s)
3848 Total bytes deleted: 1474048
3852 You can also obtain this information on request. When
3853 @option{--totals} is used with an argument, this argument is
3854 interpreted as a symbolic name of a signal, upon delivery of which the
3855 statistics is to be printed:
3858 @item --totals=@var{signo}
3859 Print statistics upon delivery of signal @var{signo}. Valid arguments
3860 are: @code{SIGHUP}, @code{SIGQUIT}, @code{SIGINT}, @code{SIGUSR1} and
3861 @code{SIGUSR2}. Shortened names without @samp{SIG} prefix are also
3865 Both forms of @option{--totals} option can be used simultaneously.
3866 Thus, @kbd{tar -x --totals --totals=USR1} instructs @command{tar} to
3867 extract all members from its default archive and print statistics
3868 after finishing the extraction, as well as when receiving signal
3871 @anchor{Progress information}
3872 @cindex Progress information
3873 The @option{--checkpoint} option prints an occasional message
3874 as @command{tar} reads or writes the archive. It is designed for
3875 those who don't need the more detailed (and voluminous) output of
3876 @option{--block-number} (@option{-R}), but do want visual confirmation
3877 that @command{tar} is actually making forward progress. By default it
3878 prints a message each 10 records read or written. This can be changed
3879 by giving it a numeric argument after an equal sign:
3882 $ @kbd{tar -c --checkpoint=1000} /var
3883 tar: Write checkpoint 1000
3884 tar: Write checkpoint 2000
3885 tar: Write checkpoint 3000
3888 This example shows the default checkpoint message used by
3889 @command{tar}. If you place a dot immediately after the equal
3890 sign, it will print a @samp{.} at each checkpoint@footnote{This is
3891 actually a shortcut for @option{--checkpoint=@var{n}
3892 --checkpoint-action=dot}. @xref{checkpoints, dot}.}. For example:
3895 $ @kbd{tar -c --checkpoint=.1000} /var
3899 The @option{--checkpoint} option provides a flexible mechanism for
3900 executing arbitrary actions upon hitting checkpoints, see the next
3901 section (@pxref{checkpoints}), for more information on it.
3903 @opindex show-omitted-dirs
3904 @anchor{show-omitted-dirs}
3905 The @option{--show-omitted-dirs} option, when reading an archive---with
3906 @option{--list} or @option{--extract}, for example---causes a message
3907 to be printed for each directory in the archive which is skipped.
3908 This happens regardless of the reason for skipping: the directory might
3909 not have been named on the command line (implicitly or explicitly),
3910 it might be excluded by the use of the
3911 @option{--exclude=@var{pattern}} option, or some other reason.
3913 @opindex block-number
3914 @cindex Block number where error occurred
3915 @anchor{block-number}
3916 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3917 every message it would normally produce, the block number within the
3918 archive where the message was triggered. Also, supplementary messages
3919 are triggered when reading blocks full of NULs, or when hitting end of
3920 file on the archive. As of now, if the archive is properly terminated
3921 with a NUL block, the reading of the file may stop before end of file
3922 is met, so the position of end of file will not usually show when
3923 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3924 drains the archive before exiting when reading the
3925 archive from a pipe.
3927 @cindex Error message, block number of
3928 This option is especially useful when reading damaged archives, since
3929 it helps pinpoint the damaged sections. It can also be used with
3930 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3931 choose among several backup tapes when retrieving a file later, in
3932 favor of the tape where the file appears earliest (closest to the
3933 front of the tape). @xref{backup}.
3936 @section Checkpoints
3937 @cindex checkpoints, defined
3939 @opindex checkpoint-action
3941 A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
3942 the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
3943 from the archive (a @dfn{read checkpoint}). Checkpoints allow to
3944 periodically execute arbitrary actions.
3946 The checkpoint facility is enabled using the following option:
3949 @xopindex{checkpoint, defined}
3950 @item --checkpoint[=@var{n}]
3951 Schedule checkpoints before writing or reading each @var{n}th record.
3952 The default value for @var{n} is 10.
3955 A list of arbitrary @dfn{actions} can be executed at each checkpoint.
3956 These actions include: pausing, displaying textual messages, and
3957 executing arbitrary external programs. Actions are defined using
3958 the @option{--checkpoint-action} option.
3961 @xopindex{checkpoint-action, defined}
3962 @item --checkpoint-action=@var{action}
3963 Execute an @var{action} at each checkpoint.
3966 @cindex @code{echo}, checkpoint action
3967 The simplest value of @var{action} is @samp{echo}. It instructs
3968 @command{tar} to display the default message on the standard error
3969 stream upon arriving at each checkpoint. The default message is (in
3970 @acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
3971 checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
3972 Here, @var{n} represents ordinal number of the checkpoint.
3974 In another locales, translated versions of this message are used.
3976 This is the default action, so running:
3979 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
3986 $ @kbd{tar -c --checkpoint=1000} /var
3989 The @samp{echo} action also allows to supply a customized message.
3990 You do so by placing an equals sign and the message right after it,
3994 --checkpoint-action="echo=Hit %s checkpoint #%u"
3997 The @samp{%s} and @samp{%u} in the above example are
3998 @dfn{format specifiers}. The @samp{%s} specifier is replaced with
3999 the @dfn{type} of the checkpoint: @samp{write} or
4000 @samp{read} (or a corresponding translated version in locales other
4001 than @acronym{POSIX}). The @samp{%u} specifier is replaced with
4002 the ordinal number of the checkpoint. Thus, the above example could
4003 produce the following output when used with the @option{--create}
4007 tar: Hit write checkpoint #10
4008 tar: Hit write checkpoint #20
4009 tar: Hit write checkpoint #30
4012 The complete list of available format specifiers follows. Some of
4013 them can take optional arguments. These arguments, if given, are
4014 supplied in curly braces between the percent sign and the specifier
4019 Print type of the checkpoint (@samp{write} or @samp{read}).
4022 Print number of the checkpoint.
4025 Print number of bytes transferred so far and approximate transfer
4026 speed. Optional arguments supply prefixes to be used before number
4027 of bytes read, written and deleted, correspondingly. If absent,
4028 they default to @samp{R}. @samp{W}, @samp{D}. Any or all of them can
4029 be omitted, so, that e.g. @samp{%@{@}T} means to print corresponding
4030 statistics without any prefixes. Any surplus arguments, if present,
4031 are silently ignored.
4034 $ @kbd{tar --delete -f f.tar --checkpoint-action=echo="#%u: %T" main.c}
4035 tar: #1: R: 0 (0B, 0B/s),W: 0 (0B, 0B/s),D: 0
4036 tar: #2: R: 10240 (10KiB, 19MiB/s),W: 0 (0B, 0B/s),D: 10240
4040 See also the @samp{totals} action, described below.
4042 @item %@{@var{fmt}@}t
4043 Output current local time using @var{fmt} as format for @command{strftime}
4044 (@pxref{strftime, strftime,,strftime(3), strftime(3) man page}). The
4045 @samp{@{@var{fmt}@}} part is optional. If not present, the default
4046 format is @samp{%c}, i.e. the preferred date and time representation
4047 for the current locale.
4050 Pad output with spaces to the @var{n}th column. If the
4051 @samp{@{@var{n}@}} part is omitted, the current screen width
4055 This is a shortcut for @samp{%@{%Y-%m-%d %H:%M:%S@}t: %ds, %@{read,wrote@}T%*\r},
4056 intended mainly for use with @samp{ttyout} action (see below).
4059 Aside from format expansion, the message string is subject to
4060 @dfn{unquoting}, during which the backslash @dfn{escape sequences} are
4061 replaced with their corresponding @acronym{ASCII} characters
4062 (@pxref{escape sequences}). E.g. the following action will produce an
4063 audible bell and the message described above at each checkpoint:
4066 --checkpoint-action='echo=\aHit %s checkpoint #%u'
4069 @cindex @code{bell}, checkpoint action
4070 There is also a special action which produces an audible signal:
4071 @samp{bell}. It is not equivalent to @samp{echo='\a'}, because
4072 @samp{bell} sends the bell directly to the console (@file{/dev/tty}),
4073 whereas @samp{echo='\a'} sends it to the standard error.
4075 @cindex @code{ttyout}, checkpoint action
4076 The @samp{ttyout=@var{string}} action outputs @var{string} to
4077 @file{/dev/tty}, so it can be used even if the standard output is
4078 redirected elsewhere. The @var{string} is subject to the same
4079 modifications as with @samp{echo} action. In contrast to the latter,
4080 @samp{ttyout} does not prepend @command{tar} executable name to the
4081 string, nor does it output a newline after it. For example, the
4082 following action will print the checkpoint message at the same screen
4083 line, overwriting any previous message:
4086 --checkpoint-action="ttyout=Hit %s checkpoint #%u%*\r"
4090 Notice the use of @samp{%*} specifier to clear out any eventual
4091 remains of the prior output line. As as more complex example,
4095 --checkpoint-action=ttyout='%@{%Y-%m-%d %H:%M:%S@}t (%d sec): #%u, %T%*\r'
4099 This prints the current local time, number of seconds expired since
4100 tar was started, the checkpoint ordinal number, transferred bytes and
4101 average computed I/O speed.
4103 @cindex @code{dot}, checkpoint action
4104 Another available checkpoint action is @samp{dot} (or @samp{.}). It
4105 instructs @command{tar} to print a single dot on the standard listing
4109 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
4113 For compatibility with previous @GNUTAR{} versions, this action can
4114 be abbreviated by placing a dot in front of the checkpoint frequency,
4115 as shown in the previous section.
4117 @cindex @code{totals}, checkpoint action
4118 The @samp{totals} action prints the total number of bytes transferred
4119 so far. The format of the data is the same as for the
4120 @option{--totals} option (@pxref{totals}). See also @samp{%T} format
4121 specifier of the @samp{echo} or @samp{ttyout} action.
4123 @cindex @code{sleep}, checkpoint action
4124 Yet another action, @samp{sleep}, pauses @command{tar} for a specified
4125 amount of seconds. The following example will stop for 30 seconds at each
4129 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
4132 @anchor{checkpoint exec}
4133 @cindex @code{exec}, checkpoint action
4134 Finally, the @code{exec} action executes a given external command.
4138 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
4141 The supplied command can be any valid command invocation, with or
4142 without additional command line arguments. If it does contain
4143 arguments, don't forget to quote it to prevent it from being split by
4144 the shell. @xref{external, Running External Commands}, for more detail.
4146 The command gets a copy of @command{tar}'s environment plus the
4147 following variables:
4150 @vrindex TAR_VERSION, checkpoint script environment
4152 @GNUTAR{} version number.
4154 @vrindex TAR_ARCHIVE, checkpoint script environment
4156 The name of the archive @command{tar} is processing.
4158 @vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
4159 @item TAR_BLOCKING_FACTOR
4160 Current blocking factor (@pxref{Blocking}).
4162 @vrindex TAR_CHECKPOINT, checkpoint script environment
4163 @item TAR_CHECKPOINT
4164 Number of the checkpoint.
4166 @vrindex TAR_SUBCOMMAND, checkpoint script environment
4167 @item TAR_SUBCOMMAND
4168 A short option describing the operation @command{tar} is executing.
4169 @xref{Operations}, for a complete list of subcommand options.
4171 @vrindex TAR_FORMAT, checkpoint script environment
4173 Format of the archive being processed. @xref{Formats}, for a complete
4174 list of archive format names.
4177 These environment variables can also be passed as arguments to the
4178 command, provided that they are properly escaped, for example:
4181 @kbd{tar -c -f arc.tar \
4182 --checkpoint-action='exec=/sbin/cpoint $TAR_FILENAME'}
4186 Notice single quotes to prevent variable names from being expanded by
4187 the shell when invoking @command{tar}.
4189 Any number of actions can be defined, by supplying several
4190 @option{--checkpoint-action} options in the command line. For
4191 example, the command below displays two messages, pauses
4192 execution for 30 seconds and executes the @file{/sbin/cpoint} script:
4196 $ @kbd{tar -c -f arc.tar \
4197 --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
4198 --checkpoint-action='echo=Sleeping for 30 seconds' \
4199 --checkpoint-action='sleep=30' \
4200 --checkpoint-action='exec=/sbin/cpoint'}
4204 This example also illustrates the fact that
4205 @option{--checkpoint-action} can be used without
4206 @option{--checkpoint}. In this case, the default checkpoint frequency
4207 (at each 10th record) is assumed.
4210 @section Controlling Warning Messages
4212 Sometimes, while performing the requested task, @GNUTAR{} notices
4213 some conditions that are not exactly errors, but which the user
4214 should be aware of. When this happens, @command{tar} issues a
4215 @dfn{warning message} describing the condition. Warning messages
4216 are output to the standard error and they do not affect the exit
4217 code of @command{tar} command.
4219 @xopindex{warning, explained}
4220 @GNUTAR{} allows the user to suppress some or all of its warning
4224 @item --warning=@var{keyword}
4225 Control display of the warning messages identified by @var{keyword}.
4226 If @var{keyword} starts with the prefix @samp{no-}, such messages are
4227 suppressed. Otherwise, they are enabled.
4229 Multiple @option{--warning} messages accumulate.
4231 The tables below list allowed values for @var{keyword} along with the
4232 warning messages they control.
4235 @subheading Keywords controlling @command{tar} operation
4239 Enable all warning messages. This is the default.
4242 Disable all warning messages.
4243 @kwindex filename-with-nuls
4244 @cindex @samp{file name read contains nul character}, warning message
4245 @item filename-with-nuls
4246 @samp{%s: file name read contains nul character}
4247 @kwindex alone-zero-block
4248 @cindex @samp{A lone zero block at}, warning message
4249 @item alone-zero-block
4250 @samp{A lone zero block at %s}
4253 @subheading Keywords applicable for @command{tar --create}
4256 @cindex @samp{contains a cache directory tag}, warning message
4258 @samp{%s: contains a cache directory tag %s; %s}
4259 @kwindex file-shrank
4260 @cindex @samp{File shrank by %s bytes}, warning message
4262 @samp{%s: File shrank by %s bytes; padding with zeros}
4264 @cindex @samp{file is on a different filesystem}, warning message
4266 @samp{%s: file is on a different filesystem; not dumped}
4267 @kwindex file-ignored
4268 @cindex @samp{Unknown file type; file ignored}, warning message
4269 @cindex @samp{socket ignored}, warning message
4270 @cindex @samp{door ignored}, warning message
4272 @samp{%s: Unknown file type; file ignored}
4273 @*@samp{%s: socket ignored}
4274 @*@samp{%s: door ignored}
4275 @kwindex file-unchanged
4276 @cindex @samp{file is unchanged; not dumped}, warning message
4277 @item file-unchanged
4278 @samp{%s: file is unchanged; not dumped}
4279 @kwindex ignore-archive
4280 @cindex @samp{file is the archive; not dumped}, warning message
4281 @kwindex ignore-archive
4282 @cindex @samp{file is the archive; not dumped}, warning message
4283 @item ignore-archive
4284 @samp{%s: file is the archive; not dumped}
4285 @kwindex file-removed
4286 @cindex @samp{File removed before we read it}, warning message
4288 @samp{%s: File removed before we read it}
4289 @kwindex file-changed
4290 @cindex @samp{file changed as we read it}, warning message
4292 @samp{%s: file changed as we read it}
4295 @subheading Keywords applicable for @command{tar --extract}
4298 @cindex @samp{implausibly old time stamp %s}, warning message
4299 @cindex @samp{time stamp %s is %s s in the future}, warning message
4301 @samp{%s: implausibly old time stamp %s}
4302 @*@samp{%s: time stamp %s is %s s in the future}
4303 @kwindex contiguous-cast
4304 @cindex @samp{Extracting contiguous files as regular files}, warning message
4305 @item contiguous-cast
4306 @samp{Extracting contiguous files as regular files}
4307 @kwindex symlink-cast
4308 @cindex @samp{Attempting extraction of symbolic links as hard links}, warning message
4310 @samp{Attempting extraction of symbolic links as hard links}
4311 @kwindex unknown-cast
4312 @cindex @samp{Unknown file type '%c', extracted as normal file}, warning message
4314 @samp{%s: Unknown file type '%c', extracted as normal file}
4315 @kwindex ignore-newer
4316 @cindex @samp{Current %s is newer or same age}, warning message
4318 @samp{Current %s is newer or same age}
4319 @kwindex unknown-keyword
4320 @cindex @samp{Ignoring unknown extended header keyword '%s'}, warning message
4321 @item unknown-keyword
4322 @samp{Ignoring unknown extended header keyword '%s'}
4323 @kwindex decompress-program
4324 @item decompress-program
4325 Controls verbose description of failures occurring when trying to run
4326 alternative decompressor programs (@pxref{alternative decompression
4327 programs}). This warning is disabled by default (unless
4328 @option{--verbose} is used). A common example of what you can get
4329 when using this warning is:
4332 $ @kbd{tar --warning=decompress-program -x -f archive.Z}
4333 tar (child): cannot run compress: No such file or directory
4334 tar (child): trying gzip
4337 This means that @command{tar} first tried to decompress
4338 @file{archive.Z} using @command{compress}, and, when that
4339 failed, switched to @command{gzip}.
4340 @kwindex record-size
4341 @cindex @samp{Record size = %lu blocks}, warning message
4343 @samp{Record size = %lu blocks}
4346 @subheading Keywords controlling incremental extraction:
4348 @kwindex rename-directory
4349 @cindex @samp{%s: Directory has been renamed from %s}, warning message
4350 @cindex @samp{%s: Directory has been renamed}, warning message
4351 @item rename-directory
4352 @samp{%s: Directory has been renamed from %s}
4353 @*@samp{%s: Directory has been renamed}
4354 @kwindex new-directory
4355 @cindex @samp{%s: Directory is new}, warning message
4357 @samp{%s: Directory is new}
4359 @cindex @samp{%s: directory is on a different device: not purging}, warning message
4361 @samp{%s: directory is on a different device: not purging}
4362 @kwindex bad-dumpdir
4363 @cindex @samp{Malformed dumpdir: 'X' never used}, warning message
4365 @samp{Malformed dumpdir: 'X' never used}
4369 @section Asking for Confirmation During Operations
4370 @cindex Interactive operation
4372 Typically, @command{tar} carries out a command without stopping for
4373 further instructions. In some situations however, you may want to
4374 exclude some files and archive members from the operation (for instance
4375 if disk or storage space is tight). You can do this by excluding
4376 certain files automatically (@pxref{Choosing}), or by performing
4377 an operation interactively, using the @option{--interactive} (@option{-w}) option.
4378 @command{tar} also accepts @option{--confirmation} for this option.
4380 @opindex interactive
4381 When the @option{--interactive} (@option{-w}) option is specified, before
4382 reading, writing, or deleting files, @command{tar} first prints a message
4383 for each such file, telling what operation it intends to take, then asks
4384 for confirmation on the terminal. The actions which require
4385 confirmation include adding a file to the archive, extracting a file
4386 from the archive, deleting a file from the archive, and deleting a file
4387 from disk. To confirm the action, you must type a line of input
4388 beginning with @samp{y}. If your input line begins with anything other
4389 than @samp{y}, @command{tar} skips that file.
4391 If @command{tar} is reading the archive from the standard input,
4392 @command{tar} opens the file @file{/dev/tty} to support the interactive
4395 Verbose output is normally sent to standard output, separate from
4396 other error messages. However, if the archive is produced directly
4397 on standard output, then verbose output is mixed with errors on
4398 @code{stderr}. Producing the archive on standard output may be used
4399 as a way to avoid using disk space, when the archive is soon to be
4400 consumed by another process reading it, say. Some people felt the need
4401 of producing an archive on stdout, still willing to segregate between
4402 verbose output and error output. A possible approach would be using a
4403 named pipe to receive the archive, and having the consumer process to
4404 read from that named pipe. This has the advantage of letting standard
4405 output free to receive verbose output, all separate from errors.
4408 @section Running External Commands
4410 Certain @GNUTAR{} operations imply running external commands that you
4411 supply on the command line. One of such operations is checkpointing,
4412 described above (@pxref{checkpoint exec}). Another example of this
4413 feature is the @option{-I} option, which allows you to supply the
4414 program to use for compressing or decompressing the archive
4415 (@pxref{use-compress-program}).
4417 Whenever such operation is requested, @command{tar} first splits the
4418 supplied command into words much like the shell does. It then treats
4419 the first word as the name of the program or the shell script to execute
4420 and the rest of words as its command line arguments. The program,
4421 unless given as an absolute file name, is searched in the shell's
4424 Any additional information is normally supplied to external commands
4425 in environment variables, specific to each particular operation. For
4426 example, the @option{--checkpoint-action=exec} option, defines the
4427 @env{TAR_ARCHIVE} variable to the name of the archive being worked
4428 upon. You can, should the need be, use these variables in the
4429 command line of the external command. For example:
4432 $ @kbd{tar -x -f archive.tar \
4433 --checkpoint=exec='printf "%04d in %32s\r" $TAR_CHECKPOINT $TAR_ARCHIVE'}
4437 This command prints for each checkpoint its number and the name of the
4438 archive, using the same output line on the screen.
4440 Notice the use of single quotes to prevent variable names from being
4441 expanded by the shell when invoking @command{tar}.
4444 @chapter @GNUTAR{} Operations
4457 @section Basic @GNUTAR{} Operations
4459 The basic @command{tar} operations, @option{--create} (@option{-c}),
4460 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
4461 @option{-x}), are currently presented and described in the tutorial
4462 chapter of this manual. This section provides some complementary notes
4463 for these operations.
4466 @xopindex{create, complementary notes}
4470 Creating an empty archive would have some kind of elegance. One can
4471 initialize an empty archive and later use @option{--append}
4472 (@option{-r}) for adding all members. Some applications would not
4473 welcome making an exception in the way of adding the first archive
4474 member. On the other hand, many people reported that it is
4475 dangerously too easy for @command{tar} to destroy a magnetic tape with
4476 an empty archive@footnote{This is well described in @cite{Unix-haters
4477 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
4478 Books, ISBN 1-56884-203-1.}. The two most common errors are:
4482 Mistakingly using @code{create} instead of @code{extract}, when the
4483 intent was to extract the full contents of an archive. This error
4484 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
4485 the QWERTY keyboard. Instead of being unpacked, the archive then
4486 gets wholly destroyed. When users speak about @dfn{exploding} an
4487 archive, they usually mean something else :-).
4490 Forgetting the argument to @code{file}, when the intent was to create
4491 an archive with a single file in it. This error is likely because a
4492 tired user can easily add the @kbd{f} key to the cluster of option
4493 letters, by the mere force of habit, without realizing the full
4494 consequence of doing so. The usual consequence is that the single
4495 file, which was meant to be saved, is rather destroyed.
4498 So, recognizing the likelihood and the catastrophic nature of these
4499 errors, @GNUTAR{} now takes some distance from elegance, and
4500 cowardly refuses to create an archive when @option{--create} option is
4501 given, there are no arguments besides options, and
4502 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
4503 around the cautiousness of @GNUTAR{} and nevertheless create an
4504 archive with nothing in it, one may still use, as the value for the
4505 @option{--files-from} option, a file with no names in it, as shown in
4506 the following commands:
4509 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
4510 @kbd{tar -cf empty-archive.tar -T /dev/null}
4513 @xopindex{extract, complementary notes}
4518 A socket is stored, within a @GNUTAR{} archive, as a pipe.
4520 @item @option{--list} (@option{-t})
4522 @GNUTAR{} now shows dates as @samp{1996-08-30},
4523 while it used to show them as @samp{Aug 30 1996}. Preferably,
4524 people should get used to ISO 8601 dates. Local American dates should
4525 be made available again with full date localization support, once
4526 ready. In the meantime, programs not being localizable for dates
4527 should prefer international dates, that's really the way to go.
4529 Look up @url{http://www.cl.cam.ac.uk/@/~mgk25/@/iso-time.html} if you
4530 are curious, it contains a detailed explanation of the ISO 8601 standard.
4535 @section Advanced @GNUTAR{} Operations
4537 Now that you have learned the basics of using @GNUTAR{}, you may want
4538 to learn about further ways in which @command{tar} can help you.
4540 This chapter presents five, more advanced operations which you probably
4541 won't use on a daily basis, but which serve more specialized functions.
4542 We also explain the different styles of options and why you might want
4543 to use one or another, or a combination of them in your @command{tar}
4544 commands. Additionally, this chapter includes options which allow you to
4545 define the output from @command{tar} more carefully, and provide help and
4546 error correction in special circumstances.
4548 @FIXME{check this after the chapter is actually revised to make sure
4549 it still introduces the info in the chapter correctly : ).}
4561 @subsection The Five Advanced @command{tar} Operations
4563 @cindex basic operations
4564 In the last chapter, you learned about the first three operations to
4565 @command{tar}. This chapter presents the remaining five operations to
4566 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
4567 @option{--delete}, and @option{--compare}.
4569 You are not likely to use these operations as frequently as those
4570 covered in the last chapter; however, since they perform specialized
4571 functions, they are quite useful when you do need to use them. We
4572 will give examples using the same directory and files that you created
4573 in the last chapter. As you may recall, the directory is called
4574 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
4575 and the two archive files you created are
4576 @samp{collection.tar} and @samp{music.tar}.
4578 We will also use the archive files @samp{afiles.tar} and
4579 @samp{bfiles.tar}. The archive @samp{afiles.tar} contains the members @samp{apple},
4580 @samp{angst}, and @samp{aspic}; @samp{bfiles.tar} contains the members
4581 @samp{./birds}, @samp{baboon}, and @samp{./box}.
4583 Unless we state otherwise, all practicing you do and examples you follow
4584 in this chapter will take place in the @file{practice} directory that
4585 you created in the previous chapter; see @ref{prepare for examples}.
4586 (Below in this section, we will remind you of the state of the examples
4587 where the last chapter left them.)
4589 The five operations that we will cover in this chapter are:
4594 Add new entries to an archive that already exists.
4597 Add more recent copies of archive members to the end of an archive, if
4602 Add one or more pre-existing archives to the end of another archive.
4604 Delete items from an archive (does not work on tapes).
4608 Compare archive members to their counterparts in the file system.
4612 @subsection How to Add Files to Existing Archives: @option{--append}
4614 @cindex appending files to existing archive
4616 If you want to add files to an existing archive, you don't need to
4617 create a new archive; you can use @option{--append} (@option{-r}).
4618 The archive must already exist in order to use @option{--append}. (A
4619 related operation is the @option{--update} operation; you can use this
4620 to add newer versions of archive members to an existing archive. To learn how to
4621 do this with @option{--update}, @pxref{update}.)
4623 If you use @option{--append} to add a file that has the same name as an
4624 archive member to an archive containing that archive member, then the
4625 old member is not deleted. What does happen, however, is somewhat
4626 complex. @command{tar} @emph{allows} you to have infinite number of files
4627 with the same name. Some operations treat these same-named members no
4628 differently than any other set of archive members: for example, if you
4629 view an archive with @option{--list} (@option{-t}), you will see all
4630 of those members listed, with their data modification times, owners, etc.
4632 Other operations don't deal with these members as perfectly as you might
4633 prefer; if you were to use @option{--extract} to extract the archive,
4634 only the most recently added copy of a member with the same name as
4635 other members would end up in the working directory. This is because
4636 @option{--extract} extracts an archive in the order the members appeared
4637 in the archive; the most recently archived members will be extracted
4638 last. Additionally, an extracted member will @emph{replace} a file of
4639 the same name which existed in the directory already, and @command{tar}
4640 will not prompt you about this@footnote{Unless you give it
4641 @option{--keep-old-files} (or @option{--skip-old-files}) option, or
4642 the disk copy is newer than the one in the archive and you invoke
4643 @command{tar} with @option{--keep-newer-files} option.}. Thus, only
4644 the most recently archived member will end up being extracted, as it
4645 will replace the one extracted before it, and so on.
4647 @cindex extracting @var{n}th copy of the file
4648 @xopindex{occurrence, described}
4649 There exists a special option that allows you to get around this
4650 behavior and extract (or list) only a particular copy of the file.
4651 This is @option{--occurrence} option. If you run @command{tar} with
4652 this option, it will extract only the first copy of the file. You
4653 may also give this option an argument specifying the number of
4654 copy to be extracted. Thus, for example if the archive
4655 @file{archive.tar} contained three copies of file @file{myfile}, then
4659 tar --extract --file archive.tar --occurrence=2 myfile
4663 would extract only the second copy. @xref{Option
4664 Summary,---occurrence}, for the description of @option{--occurrence}
4667 @FIXME{ hag -- you might want to incorporate some of the above into the
4668 MMwtSN node; not sure. i didn't know how to make it simpler...
4670 There are a few ways to get around this. Xref to Multiple Members
4671 with the Same Name, maybe.}
4673 @cindex Members, replacing with other members
4674 @cindex Replacing members with other members
4675 @xopindex{delete, using before --append}
4676 If you want to replace an archive member, use @option{--delete} to
4677 delete the member you want to remove from the archive, and then use
4678 @option{--append} to add the member you want to be in the archive. Note
4679 that you can not change the order of the archive; the most recently
4680 added member will still appear last. In this sense, you cannot truly
4681 ``replace'' one member with another. (Replacing one member with another
4682 will not work on certain types of media, such as tapes; see @ref{delete}
4683 and @ref{Media}, for more information.)
4686 * appending files:: Appending Files to an Archive
4690 @node appending files
4691 @subsubsection Appending Files to an Archive
4692 @cindex Adding files to an Archive
4693 @cindex Appending files to an Archive
4694 @cindex Archives, Appending files to
4697 The simplest way to add a file to an already existing archive is the
4698 @option{--append} (@option{-r}) operation, which writes specified
4699 files into the archive whether or not they are already among the
4702 When you use @option{--append}, you @emph{must} specify file name
4703 arguments, as there is no default. If you specify a file that already
4704 exists in the archive, another copy of the file will be added to the
4705 end of the archive. As with other operations, the member names of the
4706 newly added files will be exactly the same as their names given on the
4707 command line. The @option{--verbose} (@option{-v}) option will print
4708 out the names of the files as they are written into the archive.
4710 @option{--append} cannot be performed on some tape drives, unfortunately,
4711 due to deficiencies in the formats those tape drives use. The archive
4712 must be a valid @command{tar} archive, or else the results of using this
4713 operation will be unpredictable. @xref{Media}.
4715 To demonstrate using @option{--append} to add a file to an archive,
4716 create a file called @file{rock} in the @file{practice} directory.
4717 Make sure you are in the @file{practice} directory. Then, run the
4718 following @command{tar} command to add @file{rock} to
4719 @file{collection.tar}:
4722 $ @kbd{tar --append --file=collection.tar rock}
4726 If you now use the @option{--list} (@option{-t}) operation, you will see that
4727 @file{rock} has been added to the archive:
4730 $ @kbd{tar --list --file=collection.tar}
4731 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4732 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4733 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4734 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4738 @subsubsection Multiple Members with the Same Name
4739 @cindex members, multiple
4740 @cindex multiple members
4742 You can use @option{--append} (@option{-r}) to add copies of files
4743 which have been updated since the archive was created. (However, we
4744 do not recommend doing this since there is another @command{tar}
4745 option called @option{--update}; @xref{update}, for more information.
4746 We describe this use of @option{--append} here for the sake of
4747 completeness.) When you extract the archive, the older version will
4748 be effectively lost. This works because files are extracted from an
4749 archive in the order in which they were archived. Thus, when the
4750 archive is extracted, a file archived later in time will replace a
4751 file of the same name which was archived earlier, even though the
4752 older version of the file will remain in the archive unless you delete
4753 all versions of the file.
4755 Supposing you change the file @file{blues} and then append the changed
4756 version to @file{collection.tar}. As you saw above, the original
4757 @file{blues} is in the archive @file{collection.tar}. If you change the
4758 file and append the new version of the file to the archive, there will
4759 be two copies in the archive. When you extract the archive, the older
4760 version of the file will be extracted first, and then replaced by the
4761 newer version when it is extracted.
4763 You can append the new, changed copy of the file @file{blues} to the
4764 archive in this way:
4767 $ @kbd{tar --append --verbose --file=collection.tar blues}
4772 Because you specified the @option{--verbose} option, @command{tar} has
4773 printed the name of the file being appended as it was acted on. Now
4774 list the contents of the archive:
4777 $ @kbd{tar --list --verbose --file=collection.tar}
4778 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4779 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4780 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4781 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4782 -rw-r--r-- me/user 58 1996-10-24 18:30 blues
4786 The newest version of @file{blues} is now at the end of the archive
4787 (note the different creation dates and file sizes). If you extract
4788 the archive, the older version of the file @file{blues} will be
4789 replaced by the newer version. You can confirm this by extracting
4790 the archive and running @samp{ls} on the directory.
4792 If you wish to extract the first occurrence of the file @file{blues}
4793 from the archive, use @option{--occurrence} option, as shown in
4794 the following example:
4797 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
4798 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4801 @xref{Writing}, for more information on @option{--extract} and
4802 see @ref{Option Summary, --occurrence}, for a description of
4803 @option{--occurrence} option.
4806 @subsection Updating an Archive
4807 @cindex Updating an archive
4810 In the previous section, you learned how to use @option{--append} to
4811 add a file to an existing archive. A related operation is
4812 @option{--update} (@option{-u}). The @option{--update} operation
4813 updates a @command{tar} archive by comparing the date of the specified
4814 archive members against the date of the file with the same name. If
4815 the file has been modified more recently than the archive member, then
4816 the newer version of the file is added to the archive (as with
4819 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
4820 The operation will fail.
4822 @FIXME{other examples of media on which --update will fail? need to ask
4823 charles and/or mib/thomas/dave shevett..}
4825 Both @option{--update} and @option{--append} work by adding to the end
4826 of the archive. When you extract a file from the archive, only the
4827 version stored last will wind up in the file system, unless you use
4828 the @option{--backup} option. @xref{multiple}, for a detailed discussion.
4835 @subsubsection How to Update an Archive Using @option{--update}
4838 You must use file name arguments with the @option{--update}
4839 (@option{-u}) operation. If you don't specify any files,
4840 @command{tar} won't act on any files and won't tell you that it didn't
4841 do anything (which may end up confusing you).
4843 @c note: the above parenthetical added because in fact, this
4844 @c behavior just confused the author. :-)
4846 To see the @option{--update} option at work, create a new file,
4847 @file{classical}, in your practice directory, and some extra text to the
4848 file @file{blues}, using any text editor. Then invoke @command{tar} with
4849 the @samp{update} operation and the @option{--verbose} (@option{-v})
4850 option specified, using the names of all the files in the @file{practice}
4851 directory as file name arguments:
4854 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4861 Because we have specified verbose mode, @command{tar} prints out the names
4862 of the files it is working on, which in this case are the names of the
4863 files that needed to be updated. If you run @samp{tar --list} and look
4864 at the archive, you will see @file{blues} and @file{classical} at its
4865 end. There will be a total of two versions of the member @samp{blues};
4866 the one at the end will be newer and larger, since you added text before
4869 The reason @command{tar} does not overwrite the older file when updating
4870 it is because writing to the middle of a section of tape is a difficult
4871 process. Tapes are not designed to go backward. @xref{Media}, for more
4872 information about tapes.
4874 @option{--update} (@option{-u}) is not suitable for performing backups for two
4875 reasons: it does not change directory content entries, and it
4876 lengthens the archive every time it is used. The @GNUTAR{}
4877 options intended specifically for backups are more
4878 efficient. If you need to run backups, please consult @ref{Backups}.
4881 @subsection Combining Archives with @option{--concatenate}
4883 @cindex Adding archives to an archive
4884 @cindex Concatenating Archives
4885 @opindex concatenate
4887 @c @cindex @option{-A} described
4888 Sometimes it may be convenient to add a second archive onto the end of
4889 an archive rather than adding individual files to the archive. To add
4890 one or more archives to the end of another archive, you should use the
4891 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4893 To use @option{--concatenate}, give the first archive with
4894 @option{--file} option and name the rest of archives to be
4895 concatenated on the command line. The members, and their member
4896 names, will be copied verbatim from those archives to the first
4897 one@footnote{This can cause multiple members to have the same name. For
4898 information on how this affects reading the archive, see @ref{multiple}.}.
4899 The new, concatenated archive will be called by the same name as the
4900 one given with the @option{--file} option. As usual, if you omit
4901 @option{--file}, @command{tar} will use the value of the environment
4902 variable @env{TAPE}, or, if this has not been set, the default archive name.
4904 @FIXME{There is no way to specify a new name...}
4906 To demonstrate how @option{--concatenate} works, create two small archives
4907 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4908 files from @file{practice}:
4911 $ @kbd{tar -cvf bluesrock.tar blues rock}
4914 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4920 If you like, You can run @samp{tar --list} to make sure the archives
4921 contain what they are supposed to:
4924 $ @kbd{tar -tvf bluesrock.tar}
4925 -rw-r--r-- melissa/user 105 1997-01-21 19:42 blues
4926 -rw-r--r-- melissa/user 33 1997-01-20 15:34 rock
4927 $ @kbd{tar -tvf jazzfolk.tar}
4928 -rw-r--r-- melissa/user 20 1996-09-23 16:44 folk
4929 -rw-r--r-- melissa/user 65 1997-01-30 14:15 jazz
4932 We can concatenate these two archives with @command{tar}:
4936 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4939 If you now list the contents of the @file{bluesrock.tar}, you will see
4940 that now it also contains the archive members of @file{jazzfolk.tar}:
4943 $ @kbd{tar --list --file=bluesrock.tar}
4950 When you use @option{--concatenate}, the source and target archives must
4951 already exist and must have been created using compatible format
4952 parameters. Notice, that @command{tar} does not check whether the
4953 archives it concatenates have compatible formats, it does not
4954 even check if the files are really tar archives.
4956 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4957 tape drives, due to deficiencies in the formats those tape drives use.
4959 @cindex @code{concatenate} vs @command{cat}
4960 @cindex @command{cat} vs @code{concatenate}
4961 It may seem more intuitive to you to want or try to use @command{cat} to
4962 concatenate two archives instead of using the @option{--concatenate}
4963 operation; after all, @command{cat} is the utility for combining files.
4965 However, @command{tar} archives incorporate an end-of-file marker which
4966 must be removed if the concatenated archives are to be read properly as
4967 one archive. @option{--concatenate} removes the end-of-archive marker
4968 from the target archive before each new archive is appended. If you use
4969 @command{cat} to combine the archives, the result will not be a valid
4970 @command{tar} format archive. If you need to retrieve files from an
4971 archive that was added to using the @command{cat} utility, use the
4972 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4973 information on dealing with archives improperly combined using the
4974 @command{cat} shell utility.
4977 @subsection Removing Archive Members Using @option{--delete}
4978 @cindex Deleting files from an archive
4979 @cindex Removing files from an archive
4982 You can remove members from an archive by using the @option{--delete}
4983 option. Specify the name of the archive with @option{--file}
4984 (@option{-f}) and then specify the names of the members to be deleted;
4985 if you list no member names, nothing will be deleted. The
4986 @option{--verbose} option will cause @command{tar} to print the names
4987 of the members as they are deleted. As with @option{--extract}, you
4988 must give the exact member names when using @samp{tar --delete}.
4989 @option{--delete} will remove all versions of the named file from the
4990 archive. The @option{--delete} operation can run very slowly.
4992 Unlike other operations, @option{--delete} has no short form.
4994 @cindex Tapes, using @option{--delete} and
4995 @cindex Deleting from tape archives
4996 This operation will rewrite the archive. You can only use
4997 @option{--delete} on an archive if the archive device allows you to
4998 write to any point on the media, such as a disk; because of this, it
4999 does not work on magnetic tapes. Do not try to delete an archive member
5000 from a magnetic tape; the action will not succeed, and you will be
5001 likely to scramble the archive and damage your tape. There is no safe
5002 way (except by completely re-writing the archive) to delete files from
5003 most kinds of magnetic tape. @xref{Media}.
5005 To delete all versions of the file @file{blues} from the archive
5006 @file{collection.tar} in the @file{practice} directory, make sure you
5007 are in that directory, and then,
5010 $ @kbd{tar --list --file=collection.tar}
5015 $ @kbd{tar --delete --file=collection.tar blues}
5016 $ @kbd{tar --list --file=collection.tar}
5022 @FIXME{Check if the above listing is actually produced after running
5023 all the examples on collection.tar.}
5025 The @option{--delete} option has been reported to work properly when
5026 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
5029 @subsection Comparing Archive Members with the File System
5030 @cindex Verifying the currency of an archive
5033 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
5034 specified archive members against files with the same names, and then
5035 reports differences in file size, mode, owner, modification date and
5036 contents. You should @emph{only} specify archive member names, not file
5037 names. If you do not name any members, then @command{tar} will compare the
5038 entire archive. If a file is represented in the archive but does not
5039 exist in the file system, @command{tar} reports a difference.
5041 You have to specify the record size of the archive when modifying an
5042 archive with a non-default record size.
5044 @command{tar} ignores files in the file system that do not have
5045 corresponding members in the archive.
5047 The following example compares the archive members @file{rock},
5048 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
5049 files of the same name in the file system. (Note that there is no file,
5050 @file{funk}; @command{tar} will report an error message.)
5053 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
5056 tar: funk not found in archive
5059 The spirit behind the @option{--compare} (@option{--diff},
5060 @option{-d}) option is to check whether the archive represents the
5061 current state of files on disk, more than validating the integrity of
5062 the archive media. For this latter goal, see @ref{verify}.
5064 @node create options
5065 @section Options Used by @option{--create}
5067 @xopindex{create, additional options}
5068 The previous chapter described the basics of how to use
5069 @option{--create} (@option{-c}) to create an archive from a set of files.
5070 @xref{create}. This section described advanced options to be used with
5074 * override:: Overriding File Metadata.
5075 * Ignore Failed Read::
5079 @subsection Overriding File Metadata
5081 As described above, a @command{tar} archive keeps, for each member it contains,
5082 its @dfn{metadata}, such as modification time, mode and ownership of
5083 the file. @GNUTAR{} allows to replace these data with other values
5084 when adding files to the archive. The options described in this
5085 section affect creation of archives of any type. For POSIX archives,
5086 see also @ref{PAX keywords}, for additional ways of controlling
5087 metadata, stored in the archive.
5091 @item --mode=@var{permissions}
5093 When adding files to an archive, @command{tar} will use
5094 @var{permissions} for the archive members, rather than the permissions
5095 from the files. @var{permissions} can be specified either as an octal
5096 number or as symbolic permissions, like with
5097 @command{chmod} (@xref{File permissions, Permissions, File
5098 permissions, fileutils, @acronym{GNU} file utilities}. This reference
5099 also has useful information for those not being overly familiar with
5100 the UNIX permission system). Using latter syntax allows for
5101 more flexibility. For example, the value @samp{a+rw} adds read and write
5102 permissions for everybody, while retaining executable bits on directories
5103 or on any other file already marked as executable:
5106 $ @kbd{tar -c -f archive.tar --mode='a+rw' .}
5109 @item --mtime=@var{date}
5112 When adding files to an archive, @command{tar} will use @var{date} as
5113 the modification time of members when creating archives, instead of
5114 their actual modification times. The argument @var{date} can be
5115 either a textual date representation in almost arbitrary format
5116 (@pxref{Date input formats}) or a name of an existing file, starting
5117 with @samp{/} or @samp{.}. In the latter case, the modification time
5118 of that file will be used.
5120 The following example will set the modification date to 00:00:00,
5124 $ @kbd{tar -c -f archive.tar --mtime='1970-01-01' .}
5128 When used with @option{--verbose} (@pxref{verbose tutorial}) @GNUTAR{}
5129 will try to convert the specified date back to its textual
5130 representation and compare it with the one given with
5131 @option{--mtime} options. If the two dates differ, @command{tar} will
5132 print a warning saying what date it will use. This is to help user
5133 ensure he is using the right date.
5138 $ @kbd{tar -c -f archive.tar -v --mtime=yesterday .}
5139 tar: Option --mtime: Treating date 'yesterday' as 2006-06-20
5144 @item --owner=@var{user}
5147 Specifies that @command{tar} should use @var{user} as the owner of members
5148 when creating archives, instead of the user associated with the source
5151 If @var{user} contains a colon, it is taken to be of the form
5152 @var{name}:@var{id} where a nonempty @var{name} specifies the user
5153 name and a nonempty @var{id} specifies the decimal numeric user
5154 @acronym{ID}. If @var{user} does not contain a colon, it is taken to
5155 be a user number if it is one or more decimal digits; otherwise it is
5156 taken to be a user name.
5158 If a name is given but no number, the number is inferred from the
5159 current host's user database if possible, and the file's user number
5160 is used otherwise. If a number is given but no name, the name is
5161 inferred from the number if possible, and an empty name is used
5162 otherwise. If both name and number are given, the user database is
5163 not consulted, and the name and number need not be valid on the
5166 There is no value indicating a missing number, and @samp{0} usually means
5167 @code{root}. Some people like to force @samp{0} as the value to offer in
5168 their distributions for the owner of files, because the @code{root} user is
5169 anonymous anyway, so that might as well be the owner of anonymous
5170 archives. For example:
5173 $ @kbd{tar -c -f archive.tar --owner=0 .}
5180 $ @kbd{tar -c -f archive.tar --owner=root .}
5183 @item --group=@var{group}
5186 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
5187 rather than the group from the source file. As with @option{--owner},
5188 the argument @var{group} can be an existing group symbolic name, or a
5189 decimal numeric group @acronym{ID}, or @var{name}:@var{id}.
5192 @node Ignore Failed Read
5193 @subsection Ignore Fail Read
5196 @item --ignore-failed-read
5197 @opindex ignore-failed-read
5198 Do not exit with nonzero on unreadable files or directories.
5201 @node extract options
5202 @section Options Used by @option{--extract}
5203 @cindex options for use with @option{--extract}
5205 @xopindex{extract, additional options}
5206 The previous chapter showed how to use @option{--extract} to extract
5207 an archive into the file system. Various options cause @command{tar} to
5208 extract more information than just file contents, such as the owner,
5209 the permissions, the modification date, and so forth. This section
5210 presents options to be used with @option{--extract} when certain special
5211 considerations arise. You may review the information presented in
5212 @ref{extract} for more basic information about the
5213 @option{--extract} operation.
5216 * Reading:: Options to Help Read Archives
5217 * Writing:: Changing How @command{tar} Writes Files
5218 * Scarce:: Coping with Scarce Resources
5222 @subsection Options to Help Read Archives
5223 @cindex Options when reading archives
5225 @cindex Reading incomplete records
5226 @cindex Records, incomplete
5227 @opindex read-full-records
5228 Normally, @command{tar} will request data in full record increments from
5229 an archive storage device. If the device cannot return a full record,
5230 @command{tar} will report an error. However, some devices do not always
5231 return full records, or do not require the last record of an archive to
5232 be padded out to the next record boundary. To keep reading until you
5233 obtain a full record, or to accept an incomplete record if it contains
5234 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
5235 in conjunction with the @option{--extract} or @option{--list} operations.
5238 The @option{--read-full-records} (@option{-B}) option is turned on by default when
5239 @command{tar} reads an archive from standard input, or from a remote
5240 machine. This is because on @acronym{BSD} Unix systems, attempting to read a
5241 pipe returns however much happens to be in the pipe, even if it is
5242 less than was requested. If this option were not enabled, @command{tar}
5243 would fail as soon as it read an incomplete record from the pipe.
5245 If you're not sure of the blocking factor of an archive, you can
5246 read the archive by specifying @option{--read-full-records} (@option{-B}) and
5247 @option{--blocking-factor=@var{512-size}} (@option{-b
5248 @var{512-size}}), using a blocking factor larger than what the archive
5249 uses. This lets you avoid having to determine the blocking factor
5250 of an archive. @xref{Blocking Factor}.
5253 * read full records::
5257 @node read full records
5258 @unnumberedsubsubsec Reading Full Records
5260 @FIXME{need sentence or so of intro here}
5263 @opindex read-full-records
5264 @item --read-full-records
5266 Use in conjunction with @option{--extract} (@option{--get},
5267 @option{-x}) to read an archive which contains incomplete records, or
5268 one which has a blocking factor less than the one specified.
5272 @unnumberedsubsubsec Ignoring Blocks of Zeros
5274 @cindex End-of-archive blocks, ignoring
5275 @cindex Ignoring end-of-archive blocks
5276 @opindex ignore-zeros
5277 Normally, @command{tar} stops reading when it encounters a block of zeros
5278 between file entries (which usually indicates the end of the archive).
5279 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
5280 completely read an archive which contains a block of zeros before the
5281 end (i.e., a damaged archive, or one that was created by concatenating
5282 several archives together).
5284 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
5285 versions of @command{tar} write garbage after the end-of-archive entry,
5286 since that part of the media is never supposed to be read. @GNUTAR{}
5287 does not write after the end of an archive, but seeks to
5288 maintain compatibility among archiving utilities.
5291 @item --ignore-zeros
5293 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
5294 encountered while reading an archive. Use in conjunction with
5295 @option{--extract} or @option{--list}.
5299 @subsection Changing How @command{tar} Writes Files
5302 @FIXME{Introductory paragraph}
5305 * Dealing with Old Files::
5306 * Overwrite Old Files::
5308 * Keep Newer Files::
5310 * Recursive Unlink::
5311 * Data Modification Times::
5312 * Setting Access Permissions::
5313 * Directory Modification Times and Permissions::
5314 * Writing to Standard Output::
5315 * Writing to an External Program::
5319 @node Dealing with Old Files
5320 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
5322 @xopindex{overwrite-dir, introduced}
5323 When extracting files, if @command{tar} discovers that the extracted
5324 file already exists, it normally replaces the file by removing it before
5325 extracting it, to prevent confusion in the presence of hard or symbolic
5326 links. (If the existing file is a symbolic link, it is removed, not
5327 followed.) However, if a directory cannot be removed because it is
5328 nonempty, @command{tar} normally overwrites its metadata (ownership,
5329 permission, etc.). The @option{--overwrite-dir} option enables this
5330 default behavior. To be more cautious and preserve the metadata of
5331 such a directory, use the @option{--no-overwrite-dir} option.
5333 @cindex Overwriting old files, prevention
5334 @xopindex{keep-old-files, introduced}
5335 To be even more cautious and prevent existing files from being replaced, use
5336 the @option{--keep-old-files} (@option{-k}) option. It causes
5337 @command{tar} to refuse to replace or update a file that already
5338 exists, i.e., a file with the same name as an archive member prevents
5339 extraction of that archive member. Instead, it reports an error. For
5345 $ @kbd{tar -x -k -f archive.tar}
5346 tar: blues: Cannot open: File exists
5347 tar: Exiting with failure status due to previous errors
5350 @xopindex{skip-old-files, introduced}
5351 If you wish to preserve old files untouched, but don't want
5352 @command{tar} to treat them as errors, use the
5353 @option{--skip-old-files} option. This option causes @command{tar} to
5354 silently skip extracting over existing files.
5356 @xopindex{overwrite, introduced}
5357 To be more aggressive about altering existing files, use the
5358 @option{--overwrite} option. It causes @command{tar} to overwrite
5359 existing files and to follow existing symbolic links when extracting.
5361 @cindex Protecting old files
5362 Some people argue that @GNUTAR{} should not hesitate
5363 to overwrite files with other files when extracting. When extracting
5364 a @command{tar} archive, they expect to see a faithful copy of the
5365 state of the file system when the archive was created. It is debatable
5366 that this would always be a proper behavior. For example, suppose one
5367 has an archive in which @file{usr/local} is a link to
5368 @file{usr/local2}. Since then, maybe the site removed the link and
5369 renamed the whole hierarchy from @file{/usr/local2} to
5370 @file{/usr/local}. Such things happen all the time. I guess it would
5371 not be welcome at all that @GNUTAR{} removes the
5372 whole hierarchy just to make room for the link to be reinstated
5373 (unless it @emph{also} simultaneously restores the full
5374 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
5375 able to remove a whole hierarchy to reestablish a symbolic link, for
5376 example, but @emph{only if} @option{--recursive-unlink} is specified
5377 to allow this behavior. In any case, single files are silently
5380 @xopindex{unlink-first, introduced}
5381 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
5382 some cases by causing @command{tar} to remove files unconditionally
5383 before extracting them.
5385 @node Overwrite Old Files
5386 @unnumberedsubsubsec Overwrite Old Files
5391 Overwrite existing files and directory metadata when extracting files
5394 This causes @command{tar} to write extracted files into the file system without
5395 regard to the files already on the system; i.e., files with the same
5396 names as archive members are overwritten when the archive is extracted.
5397 It also causes @command{tar} to extract the ownership, permissions,
5398 and time stamps onto any preexisting files or directories.
5399 If the name of a corresponding file name is a symbolic link, the file
5400 pointed to by the symbolic link will be overwritten instead of the
5401 symbolic link itself (if this is possible). Moreover, special devices,
5402 empty directories and even symbolic links are automatically removed if
5403 they are in the way of extraction.
5405 Be careful when using the @option{--overwrite} option, particularly when
5406 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
5407 can change the contents, ownership or permissions of any file on your
5408 system. Also, many systems do not take kindly to overwriting files that
5409 are currently being executed.
5411 @opindex overwrite-dir
5412 @item --overwrite-dir
5413 Overwrite the metadata of directories when extracting files from an
5414 archive, but remove other files before extracting.
5417 @node Keep Old Files
5418 @unnumberedsubsubsec Keep Old Files
5420 @GNUTAR{} provides two options to control its actions in a situation
5421 when it is about to extract a file which already exists on disk.
5424 @opindex keep-old-files
5425 @item --keep-old-files
5427 Do not replace existing files from archive. When such a file is
5428 encountered, @command{tar} issues an error message. Upon end of
5429 extraction, @command{tar} exits with code 2 (@pxref{exit status}).
5431 @item --skip-old-files
5432 Do not replace existing files from archive, but do not treat that
5433 as error. Such files are silently skipped and do not affect
5434 @command{tar} exit status.
5436 Additional verbosity can be obtained using @option{--warning=existing-file}
5437 together with that option (@pxref{warnings}).
5440 @node Keep Newer Files
5441 @unnumberedsubsubsec Keep Newer Files
5444 @opindex keep-newer-files
5445 @item --keep-newer-files
5446 Do not replace existing files that are newer than their archive
5447 copies. This option is meaningless with @option{--list} (@option{-t}).
5451 @unnumberedsubsubsec Unlink First
5454 @opindex unlink-first
5455 @item --unlink-first
5457 Remove files before extracting over them.
5458 This can make @command{tar} run a bit faster if you know in advance
5459 that the extracted files all need to be removed. Normally this option
5460 slows @command{tar} down slightly, so it is disabled by default.
5463 @node Recursive Unlink
5464 @unnumberedsubsubsec Recursive Unlink
5467 @opindex recursive-unlink
5468 @item --recursive-unlink
5469 When this option is specified, try removing files and directory hierarchies
5470 before extracting over them. @emph{This is a dangerous option!}
5473 If you specify the @option{--recursive-unlink} option,
5474 @command{tar} removes @emph{anything} that keeps you from extracting a file
5475 as far as current permissions will allow it. This could include removal
5476 of the contents of a full directory hierarchy.
5478 @node Data Modification Times
5479 @unnumberedsubsubsec Setting Data Modification Times
5481 @cindex Data modification times of extracted files
5482 @cindex Modification times of extracted files
5483 Normally, @command{tar} sets the data modification times of extracted
5484 files to the corresponding times recorded for the files in the archive, but
5485 limits the permissions of extracted files by the current @code{umask}
5488 To set the data modification times of extracted files to the time when
5489 the files were extracted, use the @option{--touch} (@option{-m}) option in
5490 conjunction with @option{--extract} (@option{--get}, @option{-x}).
5496 Sets the data modification time of extracted archive members to the time
5497 they were extracted, not the time recorded for them in the archive.
5498 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
5501 @node Setting Access Permissions
5502 @unnumberedsubsubsec Setting Access Permissions
5504 @cindex Permissions of extracted files
5505 @cindex Modes of extracted files
5506 To set the modes (access permissions) of extracted files to those
5507 recorded for those files in the archive, use @option{--same-permissions}
5508 in conjunction with the @option{--extract} (@option{--get},
5509 @option{-x}) operation.
5512 @opindex preserve-permissions
5513 @opindex same-permissions
5514 @item --preserve-permissions
5515 @itemx --same-permissions
5516 @c @itemx --ignore-umask
5518 Set modes of extracted archive members to those recorded in the
5519 archive, instead of current umask settings. Use in conjunction with
5520 @option{--extract} (@option{--get}, @option{-x}).
5523 @node Directory Modification Times and Permissions
5524 @unnumberedsubsubsec Directory Modification Times and Permissions
5526 After successfully extracting a file member, @GNUTAR{} normally
5527 restores its permissions and modification times, as described in the
5528 previous sections. This cannot be done for directories, because
5529 after extracting a directory @command{tar} will almost certainly
5530 extract files into that directory and this will cause the directory
5531 modification time to be updated. Moreover, restoring that directory
5532 permissions may not permit file creation within it. Thus, restoring
5533 directory permissions and modification times must be delayed at least
5534 until all files have been extracted into that directory. @GNUTAR{}
5535 restores directories using the following approach.
5537 The extracted directories are created with the mode specified in the
5538 archive, as modified by the umask of the user, which gives sufficient
5539 permissions to allow file creation. The meta-information about the
5540 directory is recorded in the temporary list of directories. When
5541 preparing to extract next archive member, @GNUTAR{} checks if the
5542 directory prefix of this file contains the remembered directory. If
5543 it does not, the program assumes that all files have been extracted
5544 into that directory, restores its modification time and permissions
5545 and removes its entry from the internal list. This approach allows
5546 to correctly restore directory meta-information in the majority of
5547 cases, while keeping memory requirements sufficiently small. It is
5548 based on the fact, that most @command{tar} archives use the predefined
5549 order of members: first the directory, then all the files and
5550 subdirectories in that directory.
5552 However, this is not always true. The most important exception are
5553 incremental archives (@pxref{Incremental Dumps}). The member order in
5554 an incremental archive is reversed: first all directory members are
5555 stored, followed by other (non-directory) members. So, when extracting
5556 from incremental archives, @GNUTAR{} alters the above procedure. It
5557 remembers all restored directories, and restores their meta-data
5558 only after the entire archive has been processed. Notice, that you do
5559 not need to specify any special options for that, as @GNUTAR{}
5560 automatically detects archives in incremental format.
5562 There may be cases, when such processing is required for normal archives
5563 too. Consider the following example:
5567 $ @kbd{tar --no-recursion -cvf archive \
5568 foo foo/file1 bar bar/file foo/file2}
5577 During the normal operation, after encountering @file{bar}
5578 @GNUTAR{} will assume that all files from the directory @file{foo}
5579 were already extracted and will therefore restore its timestamp and
5580 permission bits. However, after extracting @file{foo/file2} the
5581 directory timestamp will be offset again.
5583 To correctly restore directory meta-information in such cases, use
5584 the @option{--delay-directory-restore} command line option:
5587 @opindex delay-directory-restore
5588 @item --delay-directory-restore
5589 Delays restoring of the modification times and permissions of extracted
5590 directories until the end of extraction. This way, correct
5591 meta-information is restored even if the archive has unusual member
5594 @opindex no-delay-directory-restore
5595 @item --no-delay-directory-restore
5596 Cancel the effect of the previous @option{--delay-directory-restore}.
5597 Use this option if you have used @option{--delay-directory-restore} in
5598 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
5599 temporarily disable it.
5602 @node Writing to Standard Output
5603 @unnumberedsubsubsec Writing to Standard Output
5605 @cindex Writing extracted files to standard output
5606 @cindex Standard output, writing extracted files to
5607 To write the extracted files to the standard output, instead of
5608 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
5609 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
5610 extracting files to send them through a pipe, and do not need to
5611 preserve them in the file system. If you extract multiple members,
5612 they appear on standard output concatenated, in the order they are
5613 found in the archive.
5619 Writes files to the standard output. Use only in conjunction with
5620 @option{--extract} (@option{--get}, @option{-x}). When this option is
5621 used, instead of creating the files specified, @command{tar} writes
5622 the contents of the files extracted to its standard output. This may
5623 be useful if you are only extracting the files in order to send them
5624 through a pipe. This option is meaningless with @option{--list}
5628 This can be useful, for example, if you have a tar archive containing
5629 a big file and don't want to store the file on disk before processing
5630 it. You can use a command like this:
5633 tar -xOzf foo.tgz bigfile | process
5636 or even like this if you want to process the concatenation of the files:
5639 tar -xOzf foo.tgz bigfile1 bigfile2 | process
5642 However, @option{--to-command} may be more convenient for use with
5643 multiple files. See the next section.
5645 @node Writing to an External Program
5646 @unnumberedsubsubsec Writing to an External Program
5648 You can instruct @command{tar} to send the contents of each extracted
5649 file to the standard input of an external program:
5653 @item --to-command=@var{command}
5654 Extract files and pipe their contents to the standard input of
5655 @var{command}. When this option is used, instead of creating the
5656 files specified, @command{tar} invokes @var{command} and pipes the
5657 contents of the files to its standard output. The @var{command} may
5658 contain command line arguments (see @ref{external, Running External Commands},
5661 Notice, that @var{command} is executed once for each regular file
5662 extracted. Non-regular files (directories, etc.) are ignored when this
5666 The command can obtain the information about the file it processes
5667 from the following environment variables:
5670 @vrindex TAR_FILETYPE, to-command environment
5672 Type of the file. It is a single letter with the following meaning:
5674 @multitable @columnfractions 0.10 0.90
5675 @item f @tab Regular file
5676 @item d @tab Directory
5677 @item l @tab Symbolic link
5678 @item h @tab Hard link
5679 @item b @tab Block device
5680 @item c @tab Character device
5683 Currently only regular files are supported.
5685 @vrindex TAR_MODE, to-command environment
5687 File mode, an octal number.
5689 @vrindex TAR_FILENAME, to-command environment
5691 The name of the file.
5693 @vrindex TAR_REALNAME, to-command environment
5695 Name of the file as stored in the archive.
5697 @vrindex TAR_UNAME, to-command environment
5699 Name of the file owner.
5701 @vrindex TAR_GNAME, to-command environment
5703 Name of the file owner group.
5705 @vrindex TAR_ATIME, to-command environment
5707 Time of last access. It is a decimal number, representing seconds
5708 since the Epoch. If the archive provides times with nanosecond
5709 precision, the nanoseconds are appended to the timestamp after a
5712 @vrindex TAR_MTIME, to-command environment
5714 Time of last modification.
5716 @vrindex TAR_CTIME, to-command environment
5718 Time of last status change.
5720 @vrindex TAR_SIZE, to-command environment
5724 @vrindex TAR_UID, to-command environment
5726 UID of the file owner.
5728 @vrindex TAR_GID, to-command environment
5730 GID of the file owner.
5733 Additionally, the following variables contain information about
5734 tar mode and the archive being processed:
5737 @vrindex TAR_VERSION, to-command environment
5739 @GNUTAR{} version number.
5741 @vrindex TAR_ARCHIVE, to-command environment
5743 The name of the archive @command{tar} is processing.
5745 @vrindex TAR_BLOCKING_FACTOR, to-command environment
5746 @item TAR_BLOCKING_FACTOR
5747 Current blocking factor (@pxref{Blocking}).
5749 @vrindex TAR_VOLUME, to-command environment
5751 Ordinal number of the volume @command{tar} is processing.
5753 @vrindex TAR_FORMAT, to-command environment
5755 Format of the archive being processed. @xref{Formats}, for a complete
5756 list of archive format names.
5759 These variables are defined prior to executing the command, so you can
5760 pass them as arguments, if you prefer. For example, if the command
5761 @var{proc} takes the member name and size as its arguments, then you
5765 $ @kbd{tar -x -f archive.tar \
5766 --to-command='proc $TAR_FILENAME $TAR_SIZE'}
5770 Notice single quotes to prevent variable names from being expanded by
5771 the shell when invoking @command{tar}.
5773 If @var{command} exits with a non-0 status, @command{tar} will print
5774 an error message similar to the following:
5777 tar: 2345: Child returned status 1
5780 Here, @samp{2345} is the PID of the finished process.
5782 If this behavior is not wanted, use @option{--ignore-command-error}:
5785 @opindex ignore-command-error
5786 @item --ignore-command-error
5787 Ignore exit codes of subprocesses. Notice that if the program
5788 exits on signal or otherwise terminates abnormally, the error message
5789 will be printed even if this option is used.
5791 @opindex no-ignore-command-error
5792 @item --no-ignore-command-error
5793 Cancel the effect of any previous @option{--ignore-command-error}
5794 option. This option is useful if you have set
5795 @option{--ignore-command-error} in @env{TAR_OPTIONS}
5796 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
5800 @unnumberedsubsubsec Removing Files
5802 @FIXME{The section is too terse. Something more to add? An example,
5806 @opindex remove-files
5807 @item --remove-files
5808 Remove files after adding them to the archive.
5812 @subsection Coping with Scarce Resources
5815 @cindex Small memory
5816 @cindex Running out of space
5824 @unnumberedsubsubsec Starting File
5827 @opindex starting-file
5828 @item --starting-file=@var{name}
5829 @itemx -K @var{name}
5830 Starts an operation in the middle of an archive. Use in conjunction
5831 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
5834 @cindex Middle of the archive, starting in the
5835 If a previous attempt to extract files failed due to lack of disk
5836 space, you can use @option{--starting-file=@var{name}} (@option{-K
5837 @var{name}}) to start extracting only after member @var{name} of the
5838 archive. This assumes, of course, that there is now free space, or
5839 that you are now extracting into a different file system. (You could
5840 also choose to suspend @command{tar}, remove unnecessary files from
5841 the file system, and then resume the same @command{tar} operation.
5842 In this case, @option{--starting-file} is not necessary.) See also
5843 @ref{interactive}, and @ref{exclude}.
5846 @unnumberedsubsubsec Same Order
5849 @cindex Large lists of file names on small machines
5851 @opindex preserve-order
5853 @itemx --preserve-order
5855 To process large lists of file names on machines with small amounts of
5856 memory. Use in conjunction with @option{--compare} (@option{--diff},
5857 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
5858 (@option{--get}, @option{-x}).
5861 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
5862 names to be listed or extracted is sorted in the same order as the
5863 files in the archive. This allows a large list of names to be used,
5864 even on a small machine that would not otherwise be able to hold all
5865 the names in memory at the same time. Such a sorted list can easily be
5866 created by running @samp{tar -t} on the archive and editing its output.
5868 This option is probably never needed on modern computer systems.
5871 @section Backup options
5873 @cindex backup options
5875 @GNUTAR{} offers options for making backups of files
5876 before writing new versions. These options control the details of
5877 these backups. They may apply to the archive itself before it is
5878 created or rewritten, as well as individual extracted members. Other
5879 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
5880 and @command{mv}, for example) offer similar options.
5882 Backup options may prove unexpectedly useful when extracting archives
5883 containing many members having identical name, or when extracting archives
5884 on systems having file name limitations, making different members appear
5885 as having similar names through the side-effect of name truncation.
5886 @FIXME{This is true only if we have a good scheme for truncated backup names,
5887 which I'm not sure at all: I suspect work is needed in this area.}
5888 When any existing file is backed up before being overwritten by extraction,
5889 then clashing files are automatically be renamed to be unique, and the
5890 true name is kept for only the last file of a series of clashing files.
5891 By using verbose mode, users may track exactly what happens.
5893 At the detail level, some decisions are still experimental, and may
5894 change in the future, we are waiting comments from our users. So, please
5895 do not learn to depend blindly on the details of the backup features.
5896 For example, currently, directories themselves are never renamed through
5897 using these options, so, extracting a file over a directory still has
5898 good chances to fail. Also, backup options apply to created archives,
5899 not only to extracted members. For created archives, backups will not
5900 be attempted when the archive is a block or character device, or when it
5901 refers to a remote file.
5903 For the sake of simplicity and efficiency, backups are made by renaming old
5904 files prior to creation or extraction, and not by copying. The original
5905 name is restored if the file creation fails. If a failure occurs after a
5906 partial extraction of a file, both the backup and the partially extracted
5910 @item --backup[=@var{method}]
5912 @vindex VERSION_CONTROL
5914 Back up files that are about to be overwritten or removed.
5915 Without this option, the original versions are destroyed.
5917 Use @var{method} to determine the type of backups made.
5918 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
5919 environment variable. And if @env{VERSION_CONTROL} is not set,
5920 use the @samp{existing} method.
5922 @vindex version-control @r{Emacs variable}
5923 This option corresponds to the Emacs variable @samp{version-control};
5924 the same values for @var{method} are accepted as in Emacs. This option
5925 also allows more descriptive names. The valid @var{method}s are:
5930 @cindex numbered @r{backup method}
5931 Always make numbered backups.
5935 @cindex existing @r{backup method}
5936 Make numbered backups of files that already have them, simple backups
5941 @cindex simple @r{backup method}
5942 Always make simple backups.
5946 @item --suffix=@var{suffix}
5948 @cindex backup suffix
5949 @vindex SIMPLE_BACKUP_SUFFIX
5950 Append @var{suffix} to each backup file made with @option{--backup}. If this
5951 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5952 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5953 set, the default is @samp{~}, just as in Emacs.
5958 @section Notable @command{tar} Usages
5961 @FIXME{Using Unix file linking capability to recreate directory
5962 structures---linking files into one subdirectory and then
5963 @command{tar}ring that directory.}
5965 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5968 You can easily use archive files to transport a group of files from
5969 one system to another: put all relevant files into an archive on one
5970 computer system, transfer the archive to another system, and extract
5971 the contents there. The basic transfer medium might be magnetic tape,
5972 Internet FTP, or even electronic mail (though you must encode the
5973 archive with @command{uuencode} in order to transport it properly by
5974 mail). Both machines do not have to use the same operating system, as
5975 long as they both support the @command{tar} program.
5977 For example, here is how you might copy a directory's contents from
5978 one disk to another, while preserving the dates, modes, owners and
5979 link-structure of all the files therein. In this case, the transfer
5980 medium is a @dfn{pipe}:
5983 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5987 You can avoid subshells by using @option{-C} option:
5990 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5994 The command also works using long option forms:
5998 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5999 | (cd targetdir; tar --extract --file=-)}
6008 $ @kbd{tar --directory sourcedir --create --file=- . \
6009 | tar --directory targetdir --extract --file=-}
6014 This is one of the easiest methods to transfer a @command{tar} archive.
6017 @section Looking Ahead: The Rest of this Manual
6019 You have now seen how to use all eight of the operations available to
6020 @command{tar}, and a number of the possible options. The next chapter
6021 explains how to choose and change file and archive names, how to use
6022 files to store names of other files which you can then call as
6023 arguments to @command{tar} (this can help you save time if you expect to
6024 archive the same list of files a number of times), and so forth.
6025 @FIXME{in case it's not obvious, i'm making this up in some sense
6026 based on my limited memory of what the next chapter *really* does. i
6027 just wanted to flesh out this final section a little bit so i'd
6028 remember to stick it in here. :-)}
6030 If there are too many files to conveniently list on the command line,
6031 you can list the names in a file, and @command{tar} will read that file.
6034 There are various ways of causing @command{tar} to skip over some files,
6035 and not archive them. @xref{Choosing}.
6038 @chapter Performing Backups and Restoring Files
6041 @GNUTAR{} is distributed along with the scripts for performing backups
6042 and restores. Even if there is a good chance those scripts may be
6043 satisfying to you, they are not the only scripts or methods available for doing
6044 backups and restore. You may well create your own, or use more
6045 sophisticated packages dedicated to that purpose.
6047 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
6048 Automatic Network Disk Archiver), a backup system developed by James
6049 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
6050 This is free software, and it is available from @uref{http://www.amanda.org}.
6054 Here is a possible plan for a future documentation about the backuping
6055 scripts which are provided within the @GNUTAR{}
6061 @item what are dumps
6062 @item different levels of dumps
6064 @item full dump = dump everything
6065 @item level 1, level 2 dumps etc
6066 A level @var{n} dump dumps everything changed since the last level
6069 @item how to use scripts for dumps (ie, the concept)
6071 @item scripts to run after editing backup specs (details)
6073 @item Backup Specs, what is it.
6075 @item how to customize
6076 @item actual text of script [/sp/dump/backup-specs]
6080 @item rsh doesn't work
6081 @item rtape isn't installed
6084 @item the @option{--incremental} option of tar
6087 @item write protection
6088 @item types of media, different sizes and types, useful for different things
6089 @item files and tape marks
6090 one tape mark between files, two at end.
6091 @item positioning the tape
6092 MT writes two at end of write,
6093 backspaces over one when writing again.
6099 This chapter documents both the provided shell scripts and @command{tar}
6100 options which are more specific to usage as a backup tool.
6102 To @dfn{back up} a file system means to create archives that contain
6103 all the files in that file system. Those archives can then be used to
6104 restore any or all of those files (for instance if a disk crashes or a
6105 file is accidentally deleted). File system @dfn{backups} are also
6109 * Full Dumps:: Using @command{tar} to Perform Full Dumps
6110 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
6111 * Backup Levels:: Levels of Backups
6112 * Backup Parameters:: Setting Parameters for Backups and Restoration
6113 * Scripted Backups:: Using the Backup Scripts
6114 * Scripted Restoration:: Using the Restore Script
6118 @section Using @command{tar} to Perform Full Dumps
6124 @cindex corrupted archives
6125 Full dumps should only be made when no other people or programs
6126 are modifying files in the file system. If files are modified while
6127 @command{tar} is making the backup, they may not be stored properly in
6128 the archive, in which case you won't be able to restore them if you
6129 have to. (Files not being modified are written with no trouble, and do
6130 not corrupt the entire archive.)
6132 You will want to use the @option{--label=@var{archive-label}}
6133 (@option{-V @var{archive-label}}) option to give the archive a
6134 volume label, so you can tell what this archive is even if the label
6135 falls off the tape, or anything like that.
6137 Unless the file system you are dumping is guaranteed to fit on
6138 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
6139 Make sure you have enough tapes on hand to complete the backup.
6141 If you want to dump each file system separately you will need to use
6142 the @option{--one-file-system} option to prevent
6143 @command{tar} from crossing file system boundaries when storing
6146 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
6147 option is not needed, since this is a complete copy of everything in
6148 the file system, and a full restore from this backup would only be
6149 done onto a completely
6152 Unless you are in a hurry, and trust the @command{tar} program (and your
6153 tapes), it is a good idea to use the @option{--verify} (@option{-W})
6154 option, to make sure your files really made it onto the dump properly.
6155 This will also detect cases where the file was modified while (or just
6156 after) it was being archived. Not all media (notably cartridge tapes)
6157 are capable of being verified, unfortunately.
6159 @node Incremental Dumps
6160 @section Using @command{tar} to Perform Incremental Dumps
6162 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
6163 stores additional metadata so that exact state of the file system
6164 can be restored when extracting the archive.
6166 @GNUTAR{} currently offers two options for handling incremental
6167 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
6168 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
6170 @xopindex{listed-incremental, described}
6171 The option @option{--listed-incremental} instructs tar to operate on
6172 an incremental archive with additional metadata stored in a standalone
6173 file, called a @dfn{snapshot file}. The purpose of this file is to help
6174 determine which files have been changed, added or deleted since the
6175 last backup, so that the next incremental backup will contain only
6176 modified files. The name of the snapshot file is given as an argument
6180 @item --listed-incremental=@var{file}
6181 @itemx -g @var{file}
6182 Handle incremental backups with snapshot data in @var{file}.
6185 To create an incremental backup, you would use
6186 @option{--listed-incremental} together with @option{--create}
6187 (@pxref{create}). For example:
6190 $ @kbd{tar --create \
6191 --file=archive.1.tar \
6192 --listed-incremental=/var/log/usr.snar \
6196 This will create in @file{archive.1.tar} an incremental backup of
6197 the @file{/usr} file system, storing additional metadata in the file
6198 @file{/var/log/usr.snar}. If this file does not exist, it will be
6199 created. The created archive will then be a @dfn{level 0 backup};
6200 please see the next section for more on backup levels.
6202 Otherwise, if the file @file{/var/log/usr.snar} exists, it
6203 determines which files are modified. In this case only these files will be
6204 stored in the archive. Suppose, for example, that after running the
6205 above command, you delete file @file{/usr/doc/old} and create
6206 directory @file{/usr/local/db} with the following contents:
6209 $ @kbd{ls /usr/local/db}
6214 Some time later you create another incremental backup. You will
6218 $ @kbd{tar --create \
6219 --file=archive.2.tar \
6220 --listed-incremental=/var/log/usr.snar \
6222 tar: usr/local/db: Directory is new
6229 The created archive @file{archive.2.tar} will contain only these
6230 three members. This archive is called a @dfn{level 1 backup}. Notice
6231 that @file{/var/log/usr.snar} will be updated with the new data, so if
6232 you plan to create more @samp{level 1} backups, it is necessary to
6233 create a working copy of the snapshot file before running
6234 @command{tar}. The above example will then be modified as follows:
6237 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
6238 $ @kbd{tar --create \
6239 --file=archive.2.tar \
6240 --listed-incremental=/var/log/usr.snar-1 \
6245 @xopindex{level, described}
6246 You can force @samp{level 0} backups either by removing the snapshot
6247 file before running @command{tar}, or by supplying the
6248 @option{--level=0} option, e.g.:
6251 $ @kbd{tar --create \
6252 --file=archive.2.tar \
6253 --listed-incremental=/var/log/usr.snar-0 \
6258 Incremental dumps depend crucially on time stamps, so the results are
6259 unreliable if you modify a file's time stamps during dumping (e.g.,
6260 with the @option{--atime-preserve=replace} option), or if you set the clock
6263 @anchor{device numbers}
6264 @cindex Device numbers, using in incremental backups
6265 Metadata stored in snapshot files include device numbers, which,
6266 obviously are supposed to be non-volatile values. However, it turns
6267 out that @acronym{NFS} devices have undependable values when an automounter
6268 gets in the picture. This can lead to a great deal of spurious
6269 redumping in incremental dumps, so it is somewhat useless to compare
6270 two @acronym{NFS} devices numbers over time. The solution implemented
6271 currently is to consider all @acronym{NFS} devices as being equal
6272 when it comes to comparing directories; this is fairly gross, but
6273 there does not seem to be a better way to go.
6275 Apart from using @acronym{NFS}, there are a number of cases where
6276 relying on device numbers can cause spurious redumping of unmodified
6277 files. For example, this occurs when archiving @acronym{LVM} snapshot
6278 volumes. To avoid this, use @option{--no-check-device} option:
6281 @xopindex{no-check-device, described}
6282 @item --no-check-device
6283 Do not rely on device numbers when preparing a list of changed files
6284 for an incremental dump.
6286 @xopindex{check-device, described}
6287 @item --check-device
6288 Use device numbers when preparing a list of changed files
6289 for an incremental dump. This is the default behavior. The purpose
6290 of this option is to undo the effect of the @option{--no-check-device}
6291 if it was given in @env{TAR_OPTIONS} environment variable
6292 (@pxref{TAR_OPTIONS}).
6295 There is also another way to cope with changing device numbers. It is
6296 described in detail in @ref{Fixing Snapshot Files}.
6298 Note that incremental archives use @command{tar} extensions and may
6299 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
6301 @xopindex{listed-incremental, using with @option{--extract}}
6302 @xopindex{extract, using with @option{--listed-incremental}}
6303 To extract from the incremental dumps, use
6304 @option{--listed-incremental} together with @option{--extract}
6305 option (@pxref{extracting files}). In this case, @command{tar} does
6306 not need to access snapshot file, since all the data necessary for
6307 extraction are stored in the archive itself. So, when extracting, you
6308 can give whatever argument to @option{--listed-incremental}, the usual
6309 practice is to use @option{--listed-incremental=/dev/null}.
6310 Alternatively, you can use @option{--incremental}, which needs no
6311 arguments. In general, @option{--incremental} (@option{-G}) can be
6312 used as a shortcut for @option{--listed-incremental} when listing or
6313 extracting incremental backups (for more information regarding this
6314 option, @pxref{incremental-op}).
6316 When extracting from the incremental backup @GNUTAR{} attempts to
6317 restore the exact state the file system had when the archive was
6318 created. In particular, it will @emph{delete} those files in the file
6319 system that did not exist in their directories when the archive was
6320 created. If you have created several levels of incremental files,
6321 then in order to restore the exact contents the file system had when
6322 the last level was created, you will need to restore from all backups
6323 in turn. Continuing our example, to restore the state of @file{/usr}
6324 file system, one would do@footnote{Notice, that since both archives
6325 were created without @option{-P} option (@pxref{absolute}), these
6326 commands should be run from the root file system.}:
6329 $ @kbd{tar --extract \
6330 --listed-incremental=/dev/null \
6331 --file archive.1.tar}
6332 $ @kbd{tar --extract \
6333 --listed-incremental=/dev/null \
6334 --file archive.2.tar}
6337 To list the contents of an incremental archive, use @option{--list}
6338 (@pxref{list}), as usual. To obtain more information about the
6339 archive, use @option{--listed-incremental} or @option{--incremental}
6340 combined with two @option{--verbose} options@footnote{Two
6341 @option{--verbose} options were selected to avoid breaking usual
6342 verbose listing output (@option{--list --verbose}) when using in
6345 @xopindex{incremental, using with @option{--list}}
6346 @xopindex{listed-incremental, using with @option{--list}}
6347 @xopindex{list, using with @option{--incremental}}
6348 @xopindex{list, using with @option{--listed-incremental}}
6349 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
6350 contents of the DUMPDIR header (with terminating nulls) when
6351 @option{--incremental} or @option{--listed-incremental} option was
6352 given, no matter what the verbosity level. This behavior, and,
6353 especially, the binary output it produced were considered inconvenient
6354 and were changed in version 1.16.}:
6357 @kbd{tar --list --incremental --verbose --verbose archive.tar}
6360 This command will print, for each directory in the archive, the list
6361 of files in that directory at the time the archive was created. This
6362 information is put out in a format which is both human-readable and
6363 unambiguous for a program: each file name is printed as
6370 where @var{x} is a letter describing the status of the file: @samp{Y}
6371 if the file is present in the archive, @samp{N} if the file is not
6372 included in the archive, or a @samp{D} if the file is a directory (and
6373 is included in the archive). @xref{Dumpdir}, for the detailed
6374 description of dumpdirs and status codes. Each such
6375 line is terminated by a newline character. The last line is followed
6376 by an additional newline to indicate the end of the data.
6378 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
6379 gives the same behavior as @option{--listed-incremental} when used
6380 with @option{--list} and @option{--extract} options. When used with
6381 @option{--create} option, it creates an incremental archive without
6382 creating snapshot file. Thus, it is impossible to create several
6383 levels of incremental backups with @option{--incremental} option.
6386 @section Levels of Backups
6388 An archive containing all the files in the file system is called a
6389 @dfn{full backup} or @dfn{full dump}. You could insure your data by
6390 creating a full dump every day. This strategy, however, would waste a
6391 substantial amount of archive media and user time, as unchanged files
6392 are daily re-archived.
6394 It is more efficient to do a full dump only occasionally. To back up
6395 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
6396 one} dump archives all the files that have changed since the last full
6399 A typical dump strategy would be to perform a full dump once a week,
6400 and a level one dump once a day. This means some versions of files
6401 will in fact be archived more than once, but this dump strategy makes
6402 it possible to restore a file system to within one day of accuracy by
6403 only extracting two archives---the last weekly (full) dump and the
6404 last daily (level one) dump. The only information lost would be in
6405 files changed or created since the last daily backup. (Doing dumps
6406 more than once a day is usually not worth the trouble.)
6408 @GNUTAR{} comes with scripts you can use to do full
6409 and level-one (actually, even level-two and so on) dumps. Using
6410 scripts (shell programs) to perform backups and restoration is a
6411 convenient and reliable alternative to typing out file name lists
6412 and @command{tar} commands by hand.
6414 Before you use these scripts, you need to edit the file
6415 @file{backup-specs}, which specifies parameters used by the backup
6416 scripts and by the restore script. This file is usually located
6417 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
6418 detailed description. Once the backup parameters are set, you can
6419 perform backups or restoration by running the appropriate script.
6421 The name of the backup script is @code{backup}. The name of the
6422 restore script is @code{restore}. The following sections describe
6423 their use in detail.
6425 @emph{Please Note:} The backup and restoration scripts are
6426 designed to be used together. While it is possible to restore files by
6427 hand from an archive which was created using a backup script, and to create
6428 an archive by hand which could then be extracted using the restore script,
6429 it is easier to use the scripts. @xref{Incremental Dumps}, before
6430 making such an attempt.
6432 @node Backup Parameters
6433 @section Setting Parameters for Backups and Restoration
6435 The file @file{backup-specs} specifies backup parameters for the
6436 backup and restoration scripts provided with @command{tar}. You must
6437 edit @file{backup-specs} to fit your system configuration and schedule
6438 before using these scripts.
6440 Syntactically, @file{backup-specs} is a shell script, containing
6441 mainly variable assignments. However, any valid shell construct
6442 is allowed in this file. Particularly, you may wish to define
6443 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
6444 For more information about shell script syntax, please refer to
6445 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
6446 g_02, the definition of the Shell Command Language}. See also
6447 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
6449 The shell variables controlling behavior of @code{backup} and
6450 @code{restore} are described in the following subsections.
6453 * General-Purpose Variables::
6454 * Magnetic Tape Control::
6456 * backup-specs example:: An Example Text of @file{Backup-specs}
6459 @node General-Purpose Variables
6460 @subsection General-Purpose Variables
6462 @defvr {Backup variable} ADMINISTRATOR
6463 The user name of the backup administrator. @code{Backup} scripts
6464 sends a backup report to this address.
6467 @defvr {Backup variable} BACKUP_HOUR
6468 The hour at which the backups are done. This can be a number from 0
6469 to 23, or the time specification in form @var{hours}:@var{minutes},
6470 or the string @samp{now}.
6472 This variable is used by @code{backup}. Its value may be overridden
6473 using @option{--time} option (@pxref{Scripted Backups}).
6476 @defvr {Backup variable} TAPE_FILE
6478 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
6479 is a remote archive (@pxref{remote-dev}), backup script will suppose
6480 that your @command{mt} is able to access remote devices. If @var{RSH}
6481 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
6482 invocations of @command{mt}.
6485 @defvr {Backup variable} BLOCKING
6487 The blocking factor @command{tar} will use when writing the dump archive.
6488 @xref{Blocking Factor}.
6491 @defvr {Backup variable} BACKUP_DIRS
6493 A list of file systems to be dumped (for @code{backup}), or restored
6494 (for @code{restore}). You can include any directory
6495 name in the list --- subdirectories on that file system will be
6496 included, regardless of how they may look to other networked machines.
6497 Subdirectories on other file systems will be ignored.
6499 The host name specifies which host to run @command{tar} on, and should
6500 normally be the host that actually contains the file system. However,
6501 the host machine must have @GNUTAR{} installed, and
6502 must be able to access the directory containing the backup scripts and
6503 their support files using the same file name that is used on the
6504 machine where the scripts are run (i.e., what @command{pwd} will print
6505 when in that directory on that machine). If the host that contains
6506 the file system does not have this capability, you can specify another
6507 host as long as it can access the file system through @acronym{NFS}.
6509 If the list of file systems is very long you may wish to put it
6510 in a separate file. This file is usually named
6511 @file{/etc/backup/dirs}, but this name may be overridden in
6512 @file{backup-specs} using @code{DIRLIST} variable.
6515 @defvr {Backup variable} DIRLIST
6517 The name of the file that contains a list of file systems to backup
6518 or restore. By default it is @file{/etc/backup/dirs}.
6521 @defvr {Backup variable} BACKUP_FILES
6523 A list of individual files to be dumped (for @code{backup}), or restored
6524 (for @code{restore}). These should be accessible from the machine on
6525 which the backup script is run.
6527 If the list of individual files is very long you may wish to store it
6528 in a separate file. This file is usually named
6529 @file{/etc/backup/files}, but this name may be overridden in
6530 @file{backup-specs} using @code{FILELIST} variable.
6533 @defvr {Backup variable} FILELIST
6535 The name of the file that contains a list of individual files to backup
6536 or restore. By default it is @file{/etc/backup/files}.
6539 @defvr {Backup variable} MT
6541 Full file name of @command{mt} binary.
6544 @defvr {Backup variable} RSH
6546 Full file name of @command{rsh} binary or its equivalent. You may wish to
6547 set it to @code{ssh}, to improve security. In this case you will have
6548 to use public key authentication.
6551 @defvr {Backup variable} RSH_COMMAND
6553 Full file name of @command{rsh} binary on remote machines. This will
6554 be passed via @option{--rsh-command} option to the remote invocation
6558 @defvr {Backup variable} VOLNO_FILE
6560 Name of temporary file to hold volume numbers. This needs to be accessible
6561 by all the machines which have file systems to be dumped.
6564 @defvr {Backup variable} XLIST
6566 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
6567 located on the remote machine and containing the list of files to
6568 be excluded from the backup. Exclude file lists are searched in
6569 /etc/tar-backup directory. A common use for exclude file lists
6570 is to exclude files containing security-sensitive information
6571 (e.g., @file{/etc/shadow} from backups).
6573 This variable affects only @code{backup}.
6576 @defvr {Backup variable} SLEEP_TIME
6578 Time to sleep between dumps of any two successive file systems
6580 This variable affects only @code{backup}.
6583 @defvr {Backup variable} DUMP_REMIND_SCRIPT
6585 Script to be run when it's time to insert a new tape in for the next
6586 volume. Administrators may want to tailor this script for their site.
6587 If this variable isn't set, @GNUTAR{} will display its built-in
6588 prompt, and will expect confirmation from the console. For the
6589 description of the default prompt, see @ref{change volume prompt}.
6593 @defvr {Backup variable} SLEEP_MESSAGE
6595 Message to display on the terminal while waiting for dump time. Usually
6596 this will just be some literal text.
6599 @defvr {Backup variable} TAR
6601 Full file name of the @GNUTAR{} executable. If this is not set, backup
6602 scripts will search @command{tar} in the current shell path.
6605 @node Magnetic Tape Control
6606 @subsection Magnetic Tape Control
6608 Backup scripts access tape device using special @dfn{hook functions}.
6609 These functions take a single argument --- the name of the tape
6610 device. Their names are kept in the following variables:
6612 @defvr {Backup variable} MT_BEGIN
6613 The name of @dfn{begin} function. This function is called before
6614 accessing the drive. By default it retensions the tape:
6620 mt -f "$1" retension
6625 @defvr {Backup variable} MT_REWIND
6626 The name of @dfn{rewind} function. The default definition is as
6639 @defvr {Backup variable} MT_OFFLINE
6640 The name of the function switching the tape off line. By default
6641 it is defined as follows:
6644 MT_OFFLINE=mt_offline
6652 @defvr {Backup variable} MT_STATUS
6653 The name of the function used to obtain the status of the archive device,
6654 including error count. Default definition:
6666 @subsection User Hooks
6668 @dfn{User hooks} are shell functions executed before and after
6669 each @command{tar} invocation. Thus, there are @dfn{backup
6670 hooks}, which are executed before and after dumping each file
6671 system, and @dfn{restore hooks}, executed before and
6672 after restoring a file system. Each user hook is a shell function
6673 taking four arguments:
6675 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
6680 Current backup or restore level.
6683 Name or IP address of the host machine being dumped or restored.
6686 Full file name of the file system being dumped or restored.
6689 File system name with directory separators replaced with colons. This
6690 is useful, e.g., for creating unique files.
6694 Following variables keep the names of user hook functions:
6696 @defvr {Backup variable} DUMP_BEGIN
6697 Dump begin function. It is executed before dumping the file system.
6700 @defvr {Backup variable} DUMP_END
6701 Executed after dumping the file system.
6704 @defvr {Backup variable} RESTORE_BEGIN
6705 Executed before restoring the file system.
6708 @defvr {Backup variable} RESTORE_END
6709 Executed after restoring the file system.
6712 @node backup-specs example
6713 @subsection An Example Text of @file{Backup-specs}
6715 The following is an example of @file{backup-specs}:
6718 # site-specific parameters for file system backup.
6720 ADMINISTRATOR=friedman
6722 TAPE_FILE=/dev/nrsmt0
6724 # Use @code{ssh} instead of the less secure @code{rsh}
6726 RSH_COMMAND=/usr/bin/ssh
6728 # Override MT_STATUS function:
6734 # Disable MT_OFFLINE function
6751 apple-gunkies:/com/mailer/gnu
6752 apple-gunkies:/com/archive/gnu"
6754 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
6758 @node Scripted Backups
6759 @section Using the Backup Scripts
6761 The syntax for running a backup script is:
6764 backup --level=@var{level} --time=@var{time}
6767 The @option{--level} option requests the dump level. Thus, to produce
6768 a full dump, specify @code{--level=0} (this is the default, so
6769 @option{--level} may be omitted if its value is
6770 @code{0})@footnote{For backward compatibility, the @code{backup} will also
6771 try to deduce the requested dump level from the name of the
6772 script itself. If the name consists of a string @samp{level-}
6773 followed by a single decimal digit, that digit is taken as
6774 the dump level number. Thus, you may create a link from @code{backup}
6775 to @code{level-1} and then run @code{level-1} whenever you need to
6776 create a level one dump.}.
6778 The @option{--time} option determines when should the backup be
6779 run. @var{Time} may take three forms:
6782 @item @var{hh}:@var{mm}
6784 The dump must be run at @var{hh} hours @var{mm} minutes.
6788 The dump must be run at @var{hh} hours.
6792 The dump must be run immediately.
6795 You should start a script with a tape or disk mounted. Once you
6796 start a script, it prompts you for new tapes or disks as it
6797 needs them. Media volumes don't have to correspond to archive
6798 files --- a multi-volume archive can be started in the middle of a
6799 tape that already contains the end of another multi-volume archive.
6800 The @code{restore} script prompts for media by its archive volume,
6801 so to avoid an error message you should keep track of which tape
6802 (or disk) contains which volume of the archive (@pxref{Scripted
6805 The backup scripts write two files on the file system. The first is a
6806 record file in @file{/etc/tar-backup/}, which is used by the scripts
6807 to store and retrieve information about which files were dumped. This
6808 file is not meant to be read by humans, and should not be deleted by
6809 them. @xref{Snapshot Files}, for a more detailed explanation of this
6812 The second file is a log file containing the names of the file systems
6813 and files dumped, what time the backup was made, and any error
6814 messages that were generated, as well as how much space was left in
6815 the media volume after the last volume of the archive was written.
6816 You should check this log file after every backup. The file name is
6817 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
6818 represents current date, and @var{n} represents current dump level number.
6820 The script also prints the name of each system being dumped to the
6823 Following is the full list of options accepted by @code{backup}
6827 @item -l @var{level}
6828 @itemx --level=@var{level}
6829 Do backup level @var{level} (default 0).
6833 Force backup even if today's log file already exists.
6835 @item -v[@var{level}]
6836 @itemx --verbose[=@var{level}]
6837 Set verbosity level. The higher the level is, the more debugging
6838 information will be output during execution. Default @var{level}
6839 is 100, which means the highest debugging level.
6841 @item -t @var{start-time}
6842 @itemx --time=@var{start-time}
6843 Wait till @var{time}, then do backup.
6847 Display short help message and exit.
6851 Display information about the program's name, version, origin and legal
6852 status, all on standard output, and then exit successfully.
6856 @node Scripted Restoration
6857 @section Using the Restore Script
6859 To restore files that were archived using a scripted backup, use the
6860 @code{restore} script. Its usage is quite straightforward. In the
6861 simplest form, invoke @code{restore --all}, it will
6862 then restore all the file systems and files specified in
6863 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
6865 You may select the file systems (and/or files) to restore by
6866 giving @code{restore} a list of @dfn{patterns} in its command
6867 line. For example, running
6874 will restore all file systems on the machine @samp{albert}. A more
6875 complicated example:
6878 restore 'albert:*' '*:/var'
6882 This command will restore all file systems on the machine @samp{albert}
6883 as well as @file{/var} file system on all machines.
6885 By default @code{restore} will start restoring files from the lowest
6886 available dump level (usually zero) and will continue through
6887 all available dump levels. There may be situations where such a
6888 thorough restore is not necessary. For example, you may wish to
6889 restore only files from the recent level one backup. To do so,
6890 use @option{--level} option, as shown in the example below:
6896 The full list of options accepted by @code{restore} follows:
6901 Restore all file systems and files specified in @file{backup-specs}.
6903 @item -l @var{level}
6904 @itemx --level=@var{level}
6905 Start restoring from the given backup level, instead of the default 0.
6907 @item -v[@var{level}]
6908 @itemx --verbose[=@var{level}]
6909 Set verbosity level. The higher the level is, the more debugging
6910 information will be output during execution. Default @var{level}
6911 is 100, which means the highest debugging level.
6915 Display short help message and exit.
6919 Display information about the program's name, version, origin and legal
6920 status, all on standard output, and then exit successfully.
6923 You should start the restore script with the media containing the
6924 first volume of the archive mounted. The script will prompt for other
6925 volumes as they are needed. If the archive is on tape, you don't need
6926 to rewind the tape to to its beginning---if the tape head is
6927 positioned past the beginning of the archive, the script will rewind
6928 the tape as needed. @xref{Tape Positioning}, for a discussion of tape
6932 @strong{Warning:} The script will delete files from the active file
6933 system if they were not in the file system when the archive was made.
6936 @xref{Incremental Dumps}, for an explanation of how the script makes
6940 @chapter Choosing Files and Names for @command{tar}
6942 Certain options to @command{tar} enable you to specify a name for your
6943 archive. Other options let you decide which files to include or exclude
6944 from the archive, based on when or whether files were modified, whether
6945 the file names do or don't match specified patterns, or whether files
6946 are in specified directories.
6948 This chapter discusses these options in detail.
6951 * file:: Choosing the Archive's Name
6952 * Selecting Archive Members::
6953 * files:: Reading Names from a File
6954 * exclude:: Excluding Some Files
6955 * wildcards:: Wildcards Patterns and Matching
6956 * quoting styles:: Ways of Quoting Special Characters in Names
6957 * transform:: Modifying File and Member Names
6958 * after:: Operating Only on New Files
6959 * recurse:: Descending into Directories
6960 * one:: Crossing File System Boundaries
6964 @section Choosing and Naming Archive Files
6966 @cindex Naming an archive
6967 @cindex Archive Name
6968 @cindex Choosing an archive file
6969 @cindex Where is the archive?
6971 By default, @command{tar} uses an archive file name that was compiled when
6972 it was built on the system; usually this name refers to some physical
6973 tape drive on the machine. However, the person who installed @command{tar}
6974 on the system may not have set the default to a meaningful value as far as
6975 most users are concerned. As a result, you will usually want to tell
6976 @command{tar} where to find (or create) the archive. The
6977 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
6978 option allows you to either specify or name a file to use as the archive
6979 instead of the default archive file location.
6982 @xopindex{file, short description}
6983 @item --file=@var{archive-name}
6984 @itemx -f @var{archive-name}
6985 Name the archive to create or operate on. Use in conjunction with
6989 For example, in this @command{tar} command,
6992 $ @kbd{tar -cvf collection.tar blues folk jazz}
6996 @file{collection.tar} is the name of the archive. It must directly
6997 follow the @option{-f} option, since whatever directly follows @option{-f}
6998 @emph{will} end up naming the archive. If you neglect to specify an
6999 archive name, you may end up overwriting a file in the working directory
7000 with the archive you create since @command{tar} will use this file's name
7001 for the archive name.
7003 An archive can be saved as a file in the file system, sent through a
7004 pipe or over a network, or written to an I/O device such as a tape,
7005 floppy disk, or CD write drive.
7007 @cindex Writing new archives
7008 @cindex Archive creation
7009 If you do not name the archive, @command{tar} uses the value of the
7010 environment variable @env{TAPE} as the file name for the archive. If
7011 that is not available, @command{tar} uses a default, compiled-in archive
7012 name, usually that for tape unit zero (i.e., @file{/dev/tu00}).
7014 @cindex Standard input and output
7015 @cindex tar to standard input and output
7016 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
7017 archive from standard input (when listing or extracting files), or
7018 writes it to standard output (when creating an archive). If you use
7019 @file{-} as an @var{archive-name} when modifying an archive,
7020 @command{tar} reads the original archive from its standard input and
7021 writes the entire new archive to its standard output.
7023 The following example is a convenient way of copying directory
7024 hierarchy from @file{sourcedir} to @file{targetdir}.
7027 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
7030 The @option{-C} option allows to avoid using subshells:
7033 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
7036 In both examples above, the leftmost @command{tar} invocation archives
7037 the contents of @file{sourcedir} to the standard output, while the
7038 rightmost one reads this archive from its standard input and
7039 extracts it. The @option{-p} option tells it to restore permissions
7040 of the extracted files.
7042 @cindex Remote devices
7043 @cindex tar to a remote device
7045 To specify an archive file on a device attached to a remote machine,
7049 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
7053 @command{tar} will set up the remote connection, if possible, and
7054 prompt you for a username and password. If you use
7055 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
7056 will attempt to set up the remote connection using your username
7057 as the username on the remote machine.
7059 @cindex Local and remote archives
7060 @anchor{local and remote archives}
7061 If the archive file name includes a colon (@samp{:}), then it is assumed
7062 to be a file on another machine. If the archive file is
7063 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
7064 host @var{host}. The remote host is accessed using the @command{rsh}
7065 program, with a username of @var{user}. If the username is omitted
7066 (along with the @samp{@@} sign), then your user name will be used.
7067 (This is the normal @command{rsh} behavior.) It is necessary for the
7068 remote machine, in addition to permitting your @command{rsh} access, to
7069 have the @file{rmt} program installed (this command is included in
7070 the @GNUTAR{} distribution and by default is installed under
7071 @file{@var{prefix}/libexec/rmt}, where @var{prefix} means your
7072 installation prefix). If you need to use a file whose name includes a
7073 colon, then the remote tape drive behavior
7074 can be inhibited by using the @option{--force-local} option.
7076 When the archive is being created to @file{/dev/null}, @GNUTAR{}
7077 tries to minimize input and output operations. The Amanda backup
7078 system, when used with @GNUTAR{}, has an initial sizing pass which
7081 @node Selecting Archive Members
7082 @section Selecting Archive Members
7083 @cindex Specifying files to act on
7084 @cindex Specifying archive members
7086 @dfn{File Name arguments} specify which files in the file system
7087 @command{tar} operates on, when creating or adding to an archive, or which
7088 archive members @command{tar} operates on, when reading or deleting from
7089 an archive. @xref{Operations}.
7091 To specify file names, you can include them as the last arguments on
7092 the command line, as follows:
7094 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
7097 If a file name begins with dash (@samp{-}), precede it with
7098 @option{--add-file} option to prevent it from being treated as an
7101 @anchor{input name quoting}
7102 By default @GNUTAR{} attempts to @dfn{unquote} each file or member
7103 name, replacing @dfn{escape sequences} according to the following
7106 @multitable @columnfractions 0.20 0.60
7107 @headitem Escape @tab Replaced with
7108 @item \a @tab Audible bell (@acronym{ASCII} 7)
7109 @item \b @tab Backspace (@acronym{ASCII} 8)
7110 @item \f @tab Form feed (@acronym{ASCII} 12)
7111 @item \n @tab New line (@acronym{ASCII} 10)
7112 @item \r @tab Carriage return (@acronym{ASCII} 13)
7113 @item \t @tab Horizontal tabulation (@acronym{ASCII} 9)
7114 @item \v @tab Vertical tabulation (@acronym{ASCII} 11)
7115 @item \? @tab @acronym{ASCII} 127
7116 @item \@var{n} @tab @acronym{ASCII} @var{n} (@var{n} should be an octal number
7120 A backslash followed by any other symbol is retained.
7122 This default behavior is controlled by the following command line
7128 Enable unquoting input file or member names (default).
7132 Disable unquoting input file or member names.
7135 If you specify a directory name as a file name argument, all the files
7136 in that directory are operated on by @command{tar}.
7138 If you do not specify files, @command{tar} behavior differs depending
7139 on the operation mode as described below:
7141 When @command{tar} is invoked with @option{--create} (@option{-c}),
7142 @command{tar} will stop immediately, reporting the following:
7146 $ @kbd{tar cf a.tar}
7147 tar: Cowardly refusing to create an empty archive
7148 Try 'tar --help' or 'tar --usage' for more information.
7152 If you specify either @option{--list} (@option{-t}) or
7153 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
7154 operates on all the archive members in the archive.
7156 If run with @option{--diff} option, tar will compare the archive with
7157 the contents of the current working directory.
7159 If you specify any other operation, @command{tar} does nothing.
7161 By default, @command{tar} takes file names from the command line. However,
7162 there are other ways to specify file or member names, or to modify the
7163 manner in which @command{tar} selects the files or members upon which to
7164 operate. In general, these methods work both for specifying the names
7165 of files and archive members.
7168 @section Reading Names from a File
7170 @cindex Reading file names from a file
7171 @cindex Lists of file names
7172 @cindex File Name arguments, alternatives
7173 @cindex @command{find}, using with @command{tar}
7174 Instead of giving the names of files or archive members on the command
7175 line, you can put the names into a file, and then use the
7176 @option{--files-from=@var{file-of-names}} (@option{-T
7177 @var{file-of-names}}) option to @command{tar}. Give the name of the
7178 file which contains the list of files to include as the argument to
7179 @option{--files-from}. In the list, the file names should be separated by
7180 newlines. You will frequently use this option when you have generated
7181 the list of files to archive with the @command{find} utility.
7185 @item --files-from=@var{file-name}
7186 @itemx -T @var{file-name}
7187 Get names to extract or create from file @var{file-name}.
7190 If you give a single dash as a file name for @option{--files-from}, (i.e.,
7191 you specify either @code{--files-from=-} or @code{-T -}), then the file
7192 names are read from standard input.
7194 Unless you are running @command{tar} with @option{--create}, you can not use
7195 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
7198 Any number of @option{-T} options can be given in the command line.
7200 The following example shows how to use @command{find} to generate a list of
7201 files smaller than 400K in length and put that list into a file
7202 called @file{small-files}. You can then use the @option{-T} option to
7203 @command{tar} to specify the files from that file, @file{small-files}, to
7204 create the archive @file{little.tgz}. (The @option{-z} option to
7205 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
7209 $ @kbd{find . -size -400 -print > small-files}
7210 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
7214 In the file list given by @option{-T} option, any file name beginning
7215 with @samp{-} character is considered a @command{tar} option and is
7216 processed accordingly@footnote{Versions of @GNUTAR{} up to 1.15.1
7217 recognized only @option{-C} option in file lists, and only if the
7218 option and its argument occupied two consecutive lines.}. For example,
7219 the common use of this feature is to change to another directory by
7220 specifying @option{-C} option:
7230 $ @kbd{tar -c -f foo.tar --files-from list}
7235 In this example, @command{tar} will first switch to @file{/etc}
7236 directory and add files @file{passwd} and @file{hosts} to the
7237 archive. Then it will change to @file{/lib} directory and will archive
7238 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
7243 $ @kbd{tar tf foo.tar}
7251 If you happen to have a file whose name starts with @samp{-},
7252 precede it with @option{--add-file} option to prevent it from
7253 being recognized as an option. For example: @code{--add-file=--my-file}.
7260 @subsection @code{NUL}-Terminated File Names
7262 @cindex File names, terminated by @code{NUL}
7263 @cindex @code{NUL}-terminated file names
7264 The @option{--null} option causes
7265 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
7266 to read file names terminated by a @code{NUL} instead of a newline, so
7267 files whose names contain newlines can be archived using
7268 @option{--files-from}.
7271 @xopindex{null, described}
7273 Only consider @code{NUL}-terminated file names, instead of files that
7274 terminate in a newline.
7276 @xopindex{no-null, described}
7278 Undo the effect of any previous @option{--null} option.
7281 The @option{--null} option is just like the one in @acronym{GNU}
7282 @command{xargs} and @command{cpio}, and is useful with the
7283 @option{-print0} predicate of @acronym{GNU} @command{find}. In
7284 @command{tar}, @option{--null} also disables special handling for
7285 file names that begin with dash.
7287 This example shows how to use @command{find} to generate a list of files
7288 larger than 800K in length and put that list into a file called
7289 @file{long-files}. The @option{-print0} option to @command{find} is just
7290 like @option{-print}, except that it separates files with a @code{NUL}
7291 rather than with a newline. You can then run @command{tar} with both the
7292 @option{--null} and @option{-T} options to specify that @command{tar} gets the
7293 files from that file, @file{long-files}, to create the archive
7294 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
7295 @command{tar} to recognize the @code{NUL} separator between files.
7298 $ @kbd{find . -size +800 -print0 > long-files}
7299 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
7302 The @option{--no-null} option can be used if you need to read both
7303 @code{NUL}-terminated and newline-terminated files on the same command line.
7304 For example, if @file{flist} is a newline-terminated file, then the
7305 following command can be used to combine it with the above command:
7309 $ @kbd{find . -size +800 -print0 |
7310 tar -c -f big.tar --null -T - --no-null -T flist}
7314 This example uses short options for typographic reasons, to avoid
7317 @GNUTAR is tries to automatically detect @code{NUL}-terminated file
7318 lists, so in many cases it is safe to use them even without the
7319 @option{--null} option. In this case @command{tar} will print a
7320 warning and continue reading such a file as if @option{--null} were
7325 $ @kbd{find . -size +800 -print0 | tar -c -f big.tar -T -}
7326 tar: -: file name read contains nul character
7330 The null terminator, however, remains in effect only for this
7331 particular file, any following @option{-T} options will assume
7332 newline termination. Of course, the null autodetection applies
7333 to these eventual surplus @option{-T} options as well.
7336 @section Excluding Some Files
7338 @cindex File names, excluding files by
7339 @cindex Excluding files by name and pattern
7340 @cindex Excluding files by file system
7342 @opindex exclude-from
7343 To avoid operating on files whose names match a particular pattern,
7344 use the @option{--exclude} or @option{--exclude-from} options.
7348 @item --exclude=@var{pattern}
7349 Causes @command{tar} to ignore files that match the @var{pattern}.
7353 The @option{--exclude=@var{pattern}} option prevents any file or
7354 member whose name matches the shell wildcard (@var{pattern}) from
7356 For example, to create an archive with all the contents of the directory
7357 @file{src} except for files whose names end in @file{.o}, use the
7358 command @samp{tar -cf src.tar --exclude='*.o' src}.
7360 You may give multiple @option{--exclude} options.
7363 @opindex exclude-from
7364 @item --exclude-from=@var{file}
7365 @itemx -X @var{file}
7366 Causes @command{tar} to ignore files that match the patterns listed in
7370 @findex exclude-from
7371 Use the @option{--exclude-from} option to read a
7372 list of patterns, one per line, from @var{file}; @command{tar} will
7373 ignore files matching those patterns. Thus if @command{tar} is
7374 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
7375 single line @file{*.o}, no files whose names end in @file{.o} will be
7376 added to the archive.
7378 Notice, that lines from @var{file} are read verbatim. One of the
7379 frequent errors is leaving some extra whitespace after a file name,
7380 which is difficult to catch using text editors.
7382 However, empty lines are OK.
7385 @cindex version control system, excluding files
7386 @cindex VCS, excluding files
7387 @cindex SCCS, excluding files
7388 @cindex RCS, excluding files
7389 @cindex CVS, excluding files
7390 @cindex SVN, excluding files
7391 @cindex git, excluding files
7392 @cindex Bazaar, excluding files
7393 @cindex Arch, excluding files
7394 @cindex Mercurial, excluding files
7395 @cindex Darcs, excluding files
7396 @opindex exclude-vcs
7398 Exclude files and directories used by following version control
7399 systems: @samp{CVS}, @samp{RCS}, @samp{SCCS}, @samp{SVN}, @samp{Arch},
7400 @samp{Bazaar}, @samp{Mercurial}, and @samp{Darcs}.
7402 As of version @value{VERSION}, the following files are excluded:
7405 @item @file{CVS/}, and everything under it
7406 @item @file{RCS/}, and everything under it
7407 @item @file{SCCS/}, and everything under it
7408 @item @file{.git/}, and everything under it
7409 @item @file{.gitignore}
7410 @item @file{.cvsignore}
7411 @item @file{.svn/}, and everything under it
7412 @item @file{.arch-ids/}, and everything under it
7413 @item @file{@{arch@}/}, and everything under it
7414 @item @file{=RELEASE-ID}
7415 @item @file{=meta-update}
7416 @item @file{=update}
7418 @item @file{.bzrignore}
7419 @item @file{.bzrtags}
7421 @item @file{.hgignore}
7422 @item @file{.hgrags}
7426 @opindex exclude-backups
7427 @item --exclude-backups
7428 Exclude backup and lock files. This option causes exclusion of files
7429 that match the following shell globbing patterns:
7439 @findex exclude-caches
7440 When creating an archive, the @option{--exclude-caches} option family
7441 causes @command{tar} to exclude all directories that contain a @dfn{cache
7442 directory tag}. A cache directory tag is a short file with the
7443 well-known name @file{CACHEDIR.TAG} and having a standard header
7444 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
7445 Various applications write cache directory tags into directories they
7446 use to hold regenerable, non-precious data, so that such data can be
7447 more easily excluded from backups.
7449 There are three @samp{exclude-caches} options, each providing a different
7450 exclusion semantics:
7453 @opindex exclude-caches
7454 @item --exclude-caches
7455 Do not archive the contents of the directory, but archive the
7456 directory itself and the @file{CACHEDIR.TAG} file.
7458 @opindex exclude-caches-under
7459 @item --exclude-caches-under
7460 Do not archive the contents of the directory, nor the
7461 @file{CACHEDIR.TAG} file, archive only the directory itself.
7463 @opindex exclude-caches-all
7464 @item --exclude-caches-all
7465 Omit directories containing @file{CACHEDIR.TAG} file entirely.
7469 Another option family, @option{--exclude-tag}, provides a generalization of
7470 this concept. It takes a single argument, a file name to look for.
7471 Any directory that contains this file will be excluded from the dump.
7472 Similarly to @samp{exclude-caches}, there are three options in this
7476 @opindex exclude-tag
7477 @item --exclude-tag=@var{file}
7478 Do not dump the contents of the directory, but dump the
7479 directory itself and the @var{file}.
7481 @opindex exclude-tag-under
7482 @item --exclude-tag-under=@var{file}
7483 Do not dump the contents of the directory, nor the
7484 @var{file}, archive only the directory itself.
7486 @opindex exclude-tag-all
7487 @item --exclude-tag-all=@var{file}
7488 Omit directories containing @var{file} file entirely.
7491 Multiple @option{--exclude-tag*} options can be given.
7493 For example, given this directory:
7508 The @option{--exclude-tag} will produce the following:
7511 $ @kbd{tar -cf archive.tar --exclude-tag=tagfile -v dir}
7516 tar: dir/folk/: contains a cache directory tag tagfile;
7521 Both the @file{dir/folk} directory and its tagfile are preserved in
7522 the archive, however the rest of files in this directory are not.
7524 Now, using the @option{--exclude-tag-under} option will exclude
7525 @file{tagfile} from the dump, while still preserving the directory
7526 itself, as shown in this example:
7529 $ @kbd{tar -cf archive.tar --exclude-tag-under=tagfile -v dir}
7534 ./tar: dir/folk/: contains a cache directory tag tagfile;
7538 Finally, using @option{--exclude-tag-all} omits the @file{dir/folk}
7542 $ @kbd{tar -cf archive.tar --exclude-tag-all=tagfile -v dir}
7546 ./tar: dir/folk/: contains a cache directory tag tagfile;
7547 directory not dumped
7551 * problems with exclude::
7554 @node problems with exclude
7555 @unnumberedsubsec Problems with Using the @code{exclude} Options
7557 @xopindex{exclude, potential problems with}
7558 Some users find @samp{exclude} options confusing. Here are some common
7563 The main operating mode of @command{tar} does not act on a file name
7564 explicitly listed on the command line, if one of its file name
7565 components is excluded. In the example above, if
7566 you create an archive and exclude files that end with @samp{*.o}, but
7567 explicitly name the file @samp{dir.o/foo} after all the options have been
7568 listed, @samp{dir.o/foo} will be excluded from the archive.
7571 You can sometimes confuse the meanings of @option{--exclude} and
7572 @option{--exclude-from}. Be careful: use @option{--exclude} when files
7573 to be excluded are given as a pattern on the command line. Use
7574 @option{--exclude-from} to introduce the name of a file which contains
7575 a list of patterns, one per line; each of these patterns can exclude
7576 zero, one, or many files.
7579 When you use @option{--exclude=@var{pattern}}, be sure to quote the
7580 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
7581 like @samp{*}. If you do not do this, the shell might expand the
7582 @samp{*} itself using files at hand, so @command{tar} might receive a
7583 list of files instead of one pattern, or none at all, making the
7584 command somewhat illegal. This might not correspond to what you want.
7589 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
7597 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
7601 You must use use shell syntax, or globbing, rather than @code{regexp}
7602 syntax, when using exclude options in @command{tar}. If you try to use
7603 @code{regexp} syntax to describe files to be excluded, your command
7607 @FIXME{The change in semantics must have occurred before 1.11,
7608 so I doubt if it is worth mentioning at all. Anyway, should at
7609 least specify in which version the semantics changed.}
7610 In earlier versions of @command{tar}, what is now the
7611 @option{--exclude-from} option was called @option{--exclude} instead.
7612 Now, @option{--exclude} applies to patterns listed on the command
7613 line and @option{--exclude-from} applies to patterns listed in a
7619 @section Wildcards Patterns and Matching
7621 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
7622 @samp{*} or @samp{?} for example, are replaced and expanded into all
7623 existing files matching the given pattern. @GNUTAR{} can use wildcard
7624 patterns for matching (or globbing) archive members when extracting
7625 from or listing an archive. Wildcard patterns are also used for
7626 verifying volume labels of @command{tar} archives. This section has the
7627 purpose of explaining wildcard syntax for @command{tar}.
7629 @FIXME{the next few paragraphs need work.}
7631 A @var{pattern} should be written according to shell syntax, using wildcard
7632 characters to effect globbing. Most characters in the pattern stand
7633 for themselves in the matched string, and case is significant: @samp{a}
7634 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
7635 pattern matches any single character in the matched string. The character
7636 @samp{*} in the pattern matches zero, one, or more single characters in
7637 the matched string. The character @samp{\} says to take the following
7638 character of the pattern @emph{literally}; it is useful when one needs to
7639 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
7641 The character @samp{[}, up to the matching @samp{]}, introduces a character
7642 class. A @dfn{character class} is a list of acceptable characters
7643 for the next single character of the matched string. For example,
7644 @samp{[abcde]} would match any of the first five letters of the alphabet.
7645 Note that within a character class, all of the ``special characters''
7646 listed above other than @samp{\} lose their special meaning; for example,
7647 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
7648 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
7649 the characters @samp{-} and @samp{]} must either come @emph{first} or
7650 @emph{last} in a character class.)
7652 @cindex Excluding characters from a character class
7653 @cindex Character class, excluding characters from
7654 If the first character of the class after the opening @samp{[}
7655 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
7656 Rather than listing character to match, it lists those characters which
7657 are @emph{forbidden} as the next single character of the matched string.
7659 Other characters of the class stand for themselves. The special
7660 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
7661 letters, is meant to represent all characters between @var{a} and
7664 @FIXME{need to add a sentence or so here to make this clear for those
7665 who don't have dan around.}
7667 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
7668 special for wildcard matches. However, if a pattern completely matches
7669 a directory prefix of a matched string, then it matches the full matched
7670 string: thus, excluding a directory also excludes all the files beneath it.
7673 * controlling pattern-matching::
7676 @node controlling pattern-matching
7677 @unnumberedsubsec Controlling Pattern-Matching
7679 For the purposes of this section, we call @dfn{exclusion members} all
7680 member names obtained while processing @option{--exclude} and
7681 @option{--exclude-from} options, and @dfn{inclusion members} those
7682 member names that were given in the command line or read from the file
7683 specified with @option{--files-from} option.
7685 These two pairs of member lists are used in the following operations:
7686 @option{--diff}, @option{--extract}, @option{--list},
7689 There are no inclusion members in create mode (@option{--create} and
7690 @option{--append}), since in this mode the names obtained from the
7691 command line refer to @emph{files}, not archive members.
7693 By default, inclusion members are compared with archive members
7694 literally @footnote{Notice that earlier @GNUTAR{} versions used
7695 globbing for inclusion members, which contradicted to UNIX98
7696 specification and was not documented. @xref{Changes}, for more
7697 information on this and other changes.} and exclusion members are
7698 treated as globbing patterns. For example:
7702 $ @kbd{tar tf foo.tar}
7707 # @i{Member names are used verbatim:}
7708 $ @kbd{tar -xf foo.tar -v '[remarks]'}
7710 # @i{Exclude member names are globbed:}
7711 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
7717 This behavior can be altered by using the following options:
7722 Treat all member names as wildcards.
7724 @opindex no-wildcards
7725 @item --no-wildcards
7726 Treat all member names as literal strings.
7729 Thus, to extract files whose names end in @samp{.c}, you can use:
7732 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
7738 Notice quoting of the pattern to prevent the shell from interpreting
7741 The effect of @option{--wildcards} option is canceled by
7742 @option{--no-wildcards}. This can be used to pass part of
7743 the command line arguments verbatim and other part as globbing
7744 patterns. For example, the following invocation:
7747 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
7751 instructs @command{tar} to extract from @file{foo.tar} all files whose
7752 names end in @samp{.txt} and the file named @file{[remarks]}.
7754 Normally, a pattern matches a name if an initial subsequence of the
7755 name's components matches the pattern, where @samp{*}, @samp{?}, and
7756 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
7757 and wildcards can match @samp{/}.
7759 Other than optionally stripping leading @samp{/} from names
7760 (@pxref{absolute}), patterns and names are used as-is. For
7761 example, trailing @samp{/} is not trimmed from a user-specified name
7762 before deciding whether to exclude it.
7764 However, this matching procedure can be altered by the options listed
7765 below. These options accumulate. For example:
7768 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
7772 ignores case when excluding @samp{makefile}, but not when excluding
7777 @opindex no-anchored
7779 @itemx --no-anchored
7780 If anchored, a pattern must match an initial subsequence
7781 of the name's components. Otherwise, the pattern can match any
7782 subsequence. Default is @option{--no-anchored} for exclusion members
7783 and @option{--anchored} inclusion members.
7785 @opindex ignore-case
7786 @opindex no-ignore-case
7788 @itemx --no-ignore-case
7789 When ignoring case, upper-case patterns match lower-case names and vice versa.
7790 When not ignoring case (the default), matching is case-sensitive.
7792 @opindex wildcards-match-slash
7793 @opindex no-wildcards-match-slash
7794 @item --wildcards-match-slash
7795 @itemx --no-wildcards-match-slash
7796 When wildcards match slash (the default for exclusion members), a
7797 wildcard like @samp{*} in the pattern can match a @samp{/} in the
7798 name. Otherwise, @samp{/} is matched only by @samp{/}.
7802 The @option{--recursion} and @option{--no-recursion} options
7803 (@pxref{recurse}) also affect how member patterns are interpreted. If
7804 recursion is in effect, a pattern matches a name if it matches any of
7805 the name's parent directories.
7807 The following table summarizes pattern-matching default values:
7809 @multitable @columnfractions .3 .7
7810 @headitem Members @tab Default settings
7811 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
7812 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
7815 @node quoting styles
7816 @section Quoting Member Names
7818 When displaying member names, @command{tar} takes care to avoid
7819 ambiguities caused by certain characters. This is called @dfn{name
7820 quoting}. The characters in question are:
7823 @item Non-printable control characters:
7824 @anchor{escape sequences}
7825 @multitable @columnfractions 0.20 0.10 0.60
7826 @headitem Character @tab @acronym{ASCII} @tab Character name
7827 @item \a @tab 7 @tab Audible bell
7828 @item \b @tab 8 @tab Backspace
7829 @item \f @tab 12 @tab Form feed
7830 @item \n @tab 10 @tab New line
7831 @item \r @tab 13 @tab Carriage return
7832 @item \t @tab 9 @tab Horizontal tabulation
7833 @item \v @tab 11 @tab Vertical tabulation
7836 @item Space (@acronym{ASCII} 32)
7838 @item Single and double quotes (@samp{'} and @samp{"})
7840 @item Backslash (@samp{\})
7843 The exact way @command{tar} uses to quote these characters depends on
7844 the @dfn{quoting style}. The default quoting style, called
7845 @dfn{escape} (see below), uses backslash notation to represent control
7846 characters, space and backslash. Using this quoting style, control
7847 characters are represented as listed in column @samp{Character} in the
7848 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
7850 @GNUTAR{} offers seven distinct quoting styles, which can be selected
7851 using @option{--quoting-style} option:
7854 @item --quoting-style=@var{style}
7855 @opindex quoting-style
7857 Sets quoting style. Valid values for @var{style} argument are:
7858 literal, shell, shell-always, c, escape, locale, clocale.
7861 These styles are described in detail below. To illustrate their
7862 effect, we will use an imaginary tar archive @file{arch.tar}
7863 containing the following members:
7867 # 1. Contains horizontal tabulation character.
7869 # 2. Contains newline character
7872 # 3. Contains a space
7874 # 4. Contains double quotes
7876 # 5. Contains single quotes
7878 # 6. Contains a backslash character:
7883 Here is how usual @command{ls} command would have listed them, if they
7884 had existed in the current working directory:
7902 No quoting, display each character as is:
7906 $ @kbd{tar tf arch.tar --quoting-style=literal}
7919 Display characters the same way Bourne shell does:
7920 control characters, except @samp{\t} and @samp{\n}, are printed using
7921 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
7922 single quote is printed as @samp{\'}. If a name contains any quoted
7923 characters, it is enclosed in single quotes. In particular, if a name
7924 contains single quotes, it is printed as several single-quoted strings:
7928 $ @kbd{tar tf arch.tar --quoting-style=shell}
7931 './a'\''single'\''quote'
7941 Same as @samp{shell}, but the names are always enclosed in single
7946 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
7949 './a'\''single'\''quote'
7959 Use the notation of the C programming language. All names are
7960 enclosed in double quotes. Control characters are quoted using
7961 backslash notations, double quotes are represented as @samp{\"},
7962 backslash characters are represented as @samp{\\}. Single quotes and
7963 spaces are not quoted:
7967 $ @kbd{tar tf arch.tar --quoting-style=c}
7971 "./a\"double\"quote"
7979 Control characters are printed using backslash notation, a space is
7980 printed as @samp{\ } and a backslash as @samp{\\}. This is the
7981 default quoting style, unless it was changed when configured the
7986 $ @kbd{tar tf arch.tar --quoting-style=escape}
7998 Control characters, single quote and backslash are printed using
7999 backslash notation. All names are quoted using left and right
8000 quotation marks, appropriate to the current locale. If it does not
8001 define quotation marks, use @samp{'} as left and as right
8002 quotation marks. Any occurrences of the right quotation mark in a
8003 name are escaped with @samp{\}, for example:
8009 $ @kbd{tar tf arch.tar --quoting-style=locale}
8012 './a\'single\'quote'
8021 Same as @samp{locale}, but @samp{"} is used for both left and right
8022 quotation marks, if not provided by the currently selected locale:
8026 $ @kbd{tar tf arch.tar --quoting-style=clocale}
8030 "./a\"double\"quote"
8038 You can specify which characters should be quoted in addition to those
8039 implied by the current quoting style:
8042 @item --quote-chars=@var{string}
8043 Always quote characters from @var{string}, even if the selected
8044 quoting style would not quote them.
8047 For example, using @samp{escape} quoting (compare with the usual
8048 escape listing above):
8052 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
8063 To disable quoting of such additional characters, use the following
8067 @item --no-quote-chars=@var{string}
8068 Remove characters listed in @var{string} from the list of quoted
8069 characters set by the previous @option{--quote-chars} option.
8072 This option is particularly useful if you have added
8073 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
8074 and wish to disable it for the current invocation.
8076 Note, that @option{--no-quote-chars} does @emph{not} disable those
8077 characters that are quoted by default in the selected quoting style.
8080 @section Modifying File and Member Names
8082 @command{Tar} archives contain detailed information about files stored
8083 in them and full file names are part of that information. When
8084 storing a file to an archive, its file name is recorded in it,
8085 along with the actual file contents. When restoring from an archive,
8086 a file is created on disk with exactly the same name as that stored
8087 in the archive. In the majority of cases this is the desired behavior
8088 of a file archiver. However, there are some cases when it is not.
8090 First of all, it is often unsafe to extract archive members with
8091 absolute file names or those that begin with a @file{../}. @GNUTAR{}
8092 takes special precautions when extracting such names and provides a
8093 special option for handling them, which is described in
8096 Secondly, you may wish to extract file names without some leading
8097 directory components, or with otherwise modified names. In other
8098 cases it is desirable to store files under differing names in the
8101 @GNUTAR{} provides several options for these needs.
8104 @opindex strip-components
8105 @item --strip-components=@var{number}
8106 Strip given @var{number} of leading components from file names before
8110 For example, suppose you have archived whole @file{/usr} hierarchy to
8111 a tar archive named @file{usr.tar}. Among other files, this archive
8112 contains @file{usr/include/stdlib.h}, which you wish to extract to
8113 the current working directory. To do so, you type:
8116 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
8119 The option @option{--strip=2} instructs @command{tar} to strip the
8120 two leading components (@file{usr/} and @file{include/}) off the file
8123 If you add the @option{--verbose} (@option{-v}) option to the invocation
8124 above, you will note that the verbose listing still contains the
8125 full file name, with the two removed components still in place. This
8126 can be inconvenient, so @command{tar} provides a special option for
8127 altering this behavior:
8129 @anchor{show-transformed-names}
8131 @opindex show-transformed-names
8132 @item --show-transformed-names
8133 Display file or member names with all requested transformations
8142 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
8143 usr/include/stdlib.h
8144 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
8149 Notice that in both cases the file @file{stdlib.h} is extracted to the
8150 current working directory, @option{--show-transformed-names} affects
8151 only the way its name is displayed.
8153 This option is especially useful for verifying whether the invocation
8154 will have the desired effect. Thus, before running
8157 $ @kbd{tar -x --strip=@var{n}}
8161 it is often advisable to run
8164 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
8168 to make sure the command will produce the intended results.
8170 In case you need to apply more complex modifications to the file name,
8171 @GNUTAR{} provides a general-purpose transformation option:
8176 @item --transform=@var{expression}
8177 @itemx --xform=@var{expression}
8178 Modify file names using supplied @var{expression}.
8182 The @var{expression} is a @command{sed}-like replace expression of the
8186 s/@var{regexp}/@var{replace}/[@var{flags}]
8190 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
8191 replacement for each file name part that matches @var{regexp}. Both
8192 @var{regexp} and @var{replace} are described in detail in
8193 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
8195 Any delimiter can be used in lieu of @samp{/}, the only requirement being
8196 that it be used consistently throughout the expression. For example,
8197 the following two expressions are equivalent:
8206 Changing delimiters is often useful when the @var{regex} contains
8207 slashes. For example, it is more convenient to write @code{s,/,-,} than
8210 As in @command{sed}, you can give several replace expressions,
8211 separated by a semicolon.
8213 Supported @var{flags} are:
8217 Apply the replacement to @emph{all} matches to the @var{regexp}, not
8221 Use case-insensitive matching.
8224 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
8225 regexps, Extended regular expressions, Extended regular expressions,
8229 Only replace the @var{number}th match of the @var{regexp}.
8231 Note: the @acronym{POSIX} standard does not specify what should happen
8232 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
8233 follows the GNU @command{sed} implementation in this regard, so
8234 the interaction is defined to be: ignore matches before the
8235 @var{number}th, and then match and replace all matches from the
8240 In addition, several @dfn{transformation scope} flags are supported,
8241 that control to what files transformations apply. These are:
8245 Apply transformation to regular archive members.
8248 Do not apply transformation to regular archive members.
8251 Apply transformation to symbolic link targets.
8254 Do not apply transformation to symbolic link targets.
8257 Apply transformation to hard link targets.
8260 Do not apply transformation to hard link targets.
8263 Default is @samp{rsh}, which means to apply tranformations to both archive
8264 members and targets of symbolic and hard links.
8266 Default scope flags can also be changed using @samp{flags=} statement
8267 in the transform expression. The flags set this way remain in force
8268 until next @samp{flags=} statement or end of expression, whichever
8269 occurs first. For example:
8272 --transform 'flags=S;s|^|/usr/local/|'
8275 Here are several examples of @option{--transform} usage:
8278 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
8281 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
8284 @item Strip two leading directory components (equivalent to
8285 @option{--strip-components=2}):
8288 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
8291 @item Convert each file name to lower case:
8294 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
8297 @item Prepend @file{/prefix/} to each file name:
8300 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
8303 @item Archive the @file{/lib} directory, prepending @samp{/usr/local}
8304 to each archive member:
8307 $ @kbd{tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib}
8311 Notice the use of flags in the last example. The @file{/lib}
8312 directory often contains many symbolic links to files within it.
8313 It may look, for example, like this:
8317 drwxr-xr-x root/root 0 2008-07-08 16:20 /lib/
8318 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
8319 lrwxrwxrwx root/root 0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
8323 Using the expression @samp{s,^,/usr/local/,} would mean adding
8324 @samp{/usr/local} to both regular archive members and to link
8325 targets. In this case, @file{/lib/libc.so.6} would become:
8328 /usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so
8331 This is definitely not desired. To avoid this, the @samp{S} flag
8332 is used, which excludes symbolic link targets from filename
8333 transformations. The result is:
8336 $ @kbd{tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \
8337 --show-transformed /lib}
8338 drwxr-xr-x root/root 0 2008-07-08 16:20 /usr/local/lib/
8339 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
8340 lrwxrwxrwx root/root 0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
8344 Unlike @option{--strip-components}, @option{--transform} can be used
8345 in any @GNUTAR{} operation mode. For example, the following command
8346 adds files to the archive while replacing the leading @file{usr/}
8347 component with @file{var/}:
8350 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
8353 To test @option{--transform} effect we suggest using
8354 @option{--show-transformed-names} option:
8357 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
8358 --verbose --show-transformed-names /}
8361 If both @option{--strip-components} and @option{--transform} are used
8362 together, then @option{--transform} is applied first, and the required
8363 number of components is then stripped from its result.
8365 You can use as many @option{--transform} options in a single command
8366 line as you want. The specified expressions will then be applied in
8367 order of their appearance. For example, the following two invocations
8371 $ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
8372 --transform='s,/usr/local,/usr/,'}
8373 $ @kbd{tar -cf arch.tar \
8374 --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
8378 @section Operating Only on New Files
8380 @cindex Excluding file by age
8381 @cindex Data Modification time, excluding files by
8382 @cindex Modification time, excluding files by
8383 @cindex Age, excluding files by
8384 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
8385 @option{-N @var{date}}) option causes @command{tar} to only work on
8386 files whose data modification or status change times are newer than
8387 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
8388 it is taken to be a file name; the data modification time of that file
8389 is used as the date. If you use this option when creating or appending
8390 to an archive, the archive will only include new files. If you use
8391 @option{--after-date} when extracting an archive, @command{tar} will
8392 only extract files newer than the @var{date} you specify.
8394 If you only want @command{tar} to make the date comparison based on
8395 modification of the file's data (rather than status
8396 changes), then use the @option{--newer-mtime=@var{date}} option.
8398 @cindex --after-date and --update compared
8399 @cindex --newer-mtime and --update compared
8400 You may use these options with any operation. Note that these options
8401 differ from the @option{--update} (@option{-u}) operation in that they
8402 allow you to specify a particular date against which @command{tar} can
8403 compare when deciding whether or not to archive the files.
8408 @item --after-date=@var{date}
8409 @itemx --newer=@var{date}
8410 @itemx -N @var{date}
8411 Only store files newer than @var{date}.
8413 Acts on files only if their data modification or status change times are
8414 later than @var{date}. Use in conjunction with any operation.
8416 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
8417 name; the data modification time of that file is used as the date.
8419 @opindex newer-mtime
8420 @item --newer-mtime=@var{date}
8421 Acts like @option{--after-date}, but only looks at data modification times.
8424 These options limit @command{tar} to operate only on files which have
8425 been modified after the date specified. A file's status is considered to have
8426 changed if its contents have been modified, or if its owner,
8427 permissions, and so forth, have been changed. (For more information on
8428 how to specify a date, see @ref{Date input formats}; remember that the
8429 entire date argument must be quoted if it contains any spaces.)
8431 Gurus would say that @option{--after-date} tests both the data
8432 modification time (@code{mtime}, the time the contents of the file
8433 were last modified) and the status change time (@code{ctime}, the time
8434 the file's status was last changed: owner, permissions, etc.@:)
8435 fields, while @option{--newer-mtime} tests only the @code{mtime}
8438 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
8439 @code{ctime} and processes the file if either one is more recent than
8440 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
8441 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
8442 contents of the file were looked at).
8444 Date specifiers can have embedded spaces. Because of this, you may need
8445 to quote date arguments to keep the shell from parsing them as separate
8446 arguments. For example, the following command will add to the archive
8447 all the files modified less than two days ago:
8450 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
8453 When any of these options is used with the option @option{--verbose}
8454 (@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
8455 date back to its textual representation and compare that with the
8456 one given with the option. If the two dates differ, @command{tar} will
8457 print a warning saying what date it will use. This is to help user
8458 ensure he is using the right date. For example:
8462 $ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
8463 tar: Option --after-date: Treating date '10 days ago' as 2006-06-11
8469 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
8470 should not be used for incremental backups. @xref{Incremental Dumps},
8471 for proper way of creating incremental backups.
8475 @section Descending into Directories
8476 @cindex Avoiding recursion in directories
8477 @cindex Descending directories, avoiding
8478 @cindex Directories, avoiding recursion
8479 @cindex Recursion in directories, avoiding
8481 Usually, @command{tar} will recursively explore all directories (either
8482 those given on the command line or through the @option{--files-from}
8483 option) for the various files they contain. However, you may not always
8484 want @command{tar} to act this way.
8486 @opindex no-recursion
8487 @cindex @command{find}, using with @command{tar}
8488 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
8489 into specified directories. If you specify @option{--no-recursion}, you can
8490 use the @command{find} (@pxref{Top,, find, find, GNU Find Manual})
8491 utility for hunting through levels of directories to
8492 construct a list of file names which you could then pass to @command{tar}.
8493 @command{find} allows you to be more selective when choosing which files to
8494 archive; see @ref{files}, for more information on using @command{find} with
8498 @item --no-recursion
8499 Prevents @command{tar} from recursively descending directories.
8503 Requires @command{tar} to recursively descend directories.
8504 This is the default.
8507 When you use @option{--no-recursion}, @GNUTAR{} grabs
8508 directory entries themselves, but does not descend on them
8509 recursively. Many people use @command{find} for locating files they
8510 want to back up, and since @command{tar} @emph{usually} recursively
8511 descends on directories, they have to use the @samp{@w{-not -type d}}
8512 test in their @command{find} invocation (@pxref{Type, Type, Type test,
8513 find, Finding Files}), as they usually do not want all the files in a
8514 directory. They then use the @option{--files-from} option to archive
8515 the files located via @command{find}.
8517 The problem when restoring files archived in this manner is that the
8518 directories themselves are not in the archive; so the
8519 @option{--same-permissions} (@option{--preserve-permissions},
8520 @option{-p}) option does not affect them---while users might really
8521 like it to. Specifying @option{--no-recursion} is a way to tell
8522 @command{tar} to grab only the directory entries given to it, adding
8523 no new files on its own. To summarize, if you use @command{find} to
8524 create a list of files to be stored in an archive, use it as follows:
8528 $ @kbd{find @var{dir} @var{tests} | \
8529 tar -cf @var{archive} -T - --no-recursion}
8533 The @option{--no-recursion} option also applies when extracting: it
8534 causes @command{tar} to extract only the matched directory entries, not
8535 the files under those directories.
8537 The @option{--no-recursion} option also affects how globbing patterns
8538 are interpreted (@pxref{controlling pattern-matching}).
8540 The @option{--no-recursion} and @option{--recursion} options apply to
8541 later options and operands, and can be overridden by later occurrences
8542 of @option{--no-recursion} and @option{--recursion}. For example:
8545 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
8549 creates an archive with one entry for @file{grape}, and the recursive
8550 contents of @file{grape/concord}, but no entries under @file{grape}
8551 other than @file{grape/concord}.
8554 @section Crossing File System Boundaries
8555 @cindex File system boundaries, not crossing
8557 @command{tar} will normally automatically cross file system boundaries in
8558 order to archive files which are part of a directory tree. You can
8559 change this behavior by running @command{tar} and specifying
8560 @option{--one-file-system}. This option only affects files that are
8561 archived because they are in a directory that is being archived;
8562 @command{tar} will still archive files explicitly named on the command line
8563 or through @option{--files-from}, regardless of where they reside.
8566 @opindex one-file-system
8567 @item --one-file-system
8568 Prevents @command{tar} from crossing file system boundaries when
8569 archiving. Use in conjunction with any write operation.
8572 The @option{--one-file-system} option causes @command{tar} to modify its
8573 normal behavior in archiving the contents of directories. If a file in
8574 a directory is not on the same file system as the directory itself, then
8575 @command{tar} will not archive that file. If the file is a directory
8576 itself, @command{tar} will not archive anything beneath it; in other words,
8577 @command{tar} will not cross mount points.
8579 This option is useful for making full or incremental archival backups of
8580 a file system. If this option is used in conjunction with
8581 @option{--verbose} (@option{-v}), files that are excluded are
8582 mentioned by name on the standard error.
8585 * directory:: Changing Directory
8586 * absolute:: Absolute File Names
8590 @subsection Changing the Working Directory
8592 @FIXME{need to read over this node now for continuity; i've switched
8593 things around some.}
8595 @cindex Changing directory mid-stream
8596 @cindex Directory, changing mid-stream
8597 @cindex Working directory, specifying
8598 To change the working directory in the middle of a list of file names,
8599 either on the command line or in a file specified using
8600 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
8601 This will change the working directory to the specified directory
8602 after that point in the list.
8606 @item --directory=@var{directory}
8607 @itemx -C @var{directory}
8608 Changes the working directory in the middle of a command line.
8614 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
8618 will place the files @file{grape} and @file{prune} from the current
8619 directory into the archive @file{jams.tar}, followed by the file
8620 @file{cherry} from the directory @file{food}. This option is especially
8621 useful when you have several widely separated files that you want to
8622 store in the same archive.
8624 Note that the file @file{cherry} is recorded in the archive under the
8625 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
8626 archive will contain three files that all appear to have come from the
8627 same directory; if the archive is extracted with plain @samp{tar
8628 --extract}, all three files will be written in the current directory.
8630 Contrast this with the command,
8633 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
8637 which records the third file in the archive under the name
8638 @file{red/cherry} so that, if the archive is extracted using
8639 @samp{tar --extract}, the third file will be written in a subdirectory
8642 You can use the @option{--directory} option to make the archive
8643 independent of the original name of the directory holding the files.
8644 The following command places the files @file{/etc/passwd},
8645 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
8649 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
8653 However, the names of the archive members will be exactly what they were
8654 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
8655 They will not appear to be related by file name to the original
8656 directories where those files were located.
8658 Note that @option{--directory} options are interpreted consecutively. If
8659 @option{--directory} specifies a relative file name, it is interpreted
8660 relative to the then current directory, which might not be the same as
8661 the original current working directory of @command{tar}, due to a previous
8662 @option{--directory} option.
8664 When using @option{--files-from} (@pxref{files}), you can put various
8665 @command{tar} options (including @option{-C}) in the file list. Notice,
8666 however, that in this case the option and its argument may not be
8667 separated by whitespace. If you use short option, its argument must
8668 either follow the option letter immediately, without any intervening
8669 whitespace, or occupy the next line. Otherwise, if you use long
8670 option, separate its argument by an equal sign.
8672 For instance, the file list for the above example will be:
8685 To use it, you would invoke @command{tar} as follows:
8688 $ @kbd{tar -c -f foo.tar --files-from list}
8691 The interpretation of @option{--directory} is disabled by
8692 @option{--null} option.
8695 @subsection Absolute File Names
8696 @cindex absolute file names
8697 @cindex file names, absolute
8699 By default, @GNUTAR{} drops a leading @samp{/} on
8700 input or output, and complains about file names containing a @file{..}
8701 component. There is an option that turns off this behavior:
8704 @opindex absolute-names
8705 @item --absolute-names
8707 Do not strip leading slashes from file names, and permit file names
8708 containing a @file{..} file name component.
8711 When @command{tar} extracts archive members from an archive, it strips any
8712 leading slashes (@samp{/}) from the member name. This causes absolute
8713 member names in the archive to be treated as relative file names. This
8714 allows you to have such members extracted wherever you want, instead of
8715 being restricted to extracting the member in the exact directory named
8716 in the archive. For example, if the archive member has the name
8717 @file{/etc/passwd}, @command{tar} will extract it as if the name were
8718 really @file{etc/passwd}.
8720 File names containing @file{..} can cause problems when extracting, so
8721 @command{tar} normally warns you about such files when creating an
8722 archive, and rejects attempts to extracts such files.
8724 Other @command{tar} programs do not do this. As a result, if you
8725 create an archive whose member names start with a slash, they will be
8726 difficult for other people with a non-@GNUTAR{}
8727 program to use. Therefore, @GNUTAR{} also strips
8728 leading slashes from member names when putting members into the
8729 archive. For example, if you ask @command{tar} to add the file
8730 @file{/bin/ls} to an archive, it will do so, but the member name will
8731 be @file{bin/ls}@footnote{A side effect of this is that when
8732 @option{--create} is used with @option{--verbose} the resulting output
8733 is not, generally speaking, the same as the one you'd get running
8734 @kbd{tar --list} command. This may be important if you use some
8735 scripts for comparing both outputs. @xref{listing member and file names},
8736 for the information on how to handle this case.}.
8738 Symbolic links containing @file{..} or leading @samp{/} can also cause
8739 problems when extracting, so @command{tar} normally extracts them last;
8740 it may create empty files as placeholders during extraction.
8742 If you use the @option{--absolute-names} (@option{-P}) option,
8743 @command{tar} will do none of these transformations.
8745 To archive or extract files relative to the root directory, specify
8746 the @option{--absolute-names} (@option{-P}) option.
8748 Normally, @command{tar} acts on files relative to the working
8749 directory---ignoring superior directory names when archiving, and
8750 ignoring leading slashes when extracting.
8752 When you specify @option{--absolute-names} (@option{-P}),
8753 @command{tar} stores file names including all superior directory
8754 names, and preserves leading slashes. If you only invoked
8755 @command{tar} from the root directory you would never need the
8756 @option{--absolute-names} option, but using this option
8757 may be more convenient than switching to root.
8759 @FIXME{Should be an example in the tutorial/wizardry section using this
8760 to transfer files between systems.}
8763 @item --absolute-names
8764 Preserves full file names (including superior directory names) when
8765 archiving and extracting files.
8769 @command{tar} prints out a message about removing the @samp{/} from
8770 file names. This message appears once per @GNUTAR{}
8771 invocation. It represents something which ought to be told; ignoring
8772 what it means can cause very serious surprises, later.
8774 Some people, nevertheless, do not want to see this message. Wanting to
8775 play really dangerously, one may of course redirect @command{tar} standard
8776 error to the sink. For example, under @command{sh}:
8779 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
8783 Another solution, both nicer and simpler, would be to change to
8784 the @file{/} directory first, and then avoid absolute notation.
8788 $ @kbd{tar -c -f archive.tar -C / home}
8791 @xref{Integrity}, for some of the security-related implications
8792 of using this option.
8794 @include parse-datetime.texi
8797 @chapter Controlling the Archive Format
8799 @cindex Tar archive formats
8800 Due to historical reasons, there are several formats of tar archives.
8801 All of them are based on the same principles, but have some subtle
8802 differences that often make them incompatible with each other.
8804 GNU tar is able to create and handle archives in a variety of formats.
8805 The most frequently used formats are (in alphabetical order):
8809 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
8810 from an early @acronym{POSIX} standard, adding some improvements such as
8811 sparse file handling and incremental archives. Unfortunately these
8812 features were implemented in a way incompatible with other archive
8815 Archives in @samp{gnu} format are able to hold file names of unlimited
8819 Format used by @GNUTAR{} of versions prior to 1.12.
8822 Archive format, compatible with the V7 implementation of tar. This
8823 format imposes a number of limitations. The most important of them
8827 @item The maximum length of a file name is limited to 99 characters.
8828 @item The maximum length of a symbolic link is limited to 99 characters.
8829 @item It is impossible to store special files (block and character
8830 devices, fifos etc.)
8831 @item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
8833 @item V7 archives do not contain symbolic ownership information (user
8834 and group name of the file owner).
8837 This format has traditionally been used by Automake when producing
8838 Makefiles. This practice will change in the future, in the meantime,
8839 however this means that projects containing file names more than 99
8840 characters long will not be able to use @GNUTAR{} @value{VERSION} and
8841 Automake prior to 1.9.
8844 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
8845 symbolic ownership information. It is also able to store
8846 special files. However, it imposes several restrictions as well:
8849 @item The maximum length of a file name is limited to 256 characters,
8850 provided that the file name can be split at a directory separator in
8851 two parts, first of them being at most 155 bytes long. So, in most
8852 cases the maximum file name length will be shorter than 256
8854 @item The maximum length of a symbolic link name is limited to
8856 @item Maximum size of a file the archive is able to accommodate
8858 @item Maximum value of UID/GID is 2097151.
8859 @item Maximum number of bits in device major and minor numbers is 21.
8863 Format used by J@"org Schilling @command{star}
8864 implementation. @GNUTAR{} is able to read @samp{star} archives but
8865 currently does not produce them.
8868 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
8869 most flexible and feature-rich format. It does not impose any
8870 restrictions on file sizes or file name lengths. This format is quite
8871 recent, so not all tar implementations are able to handle it properly.
8872 However, this format is designed in such a way that any tar
8873 implementation able to read @samp{ustar} archives will be able to read
8874 most @samp{posix} archives as well, with the only exception that any
8875 additional information (such as long file names etc.) will in such
8876 case be extracted as plain text files along with the files it refers to.
8878 This archive format will be the default format for future versions
8883 The following table summarizes the limitations of each of these
8886 @multitable @columnfractions .10 .20 .20 .20 .20
8887 @headitem Format @tab UID @tab File Size @tab File Name @tab Devn
8888 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8889 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8890 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
8891 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
8892 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
8895 The default format for @GNUTAR{} is defined at compilation
8896 time. You may check it by running @command{tar --help}, and examining
8897 the last lines of its output. Usually, @GNUTAR{} is configured
8898 to create archives in @samp{gnu} format, however, future version will
8899 switch to @samp{posix}.
8902 * Compression:: Using Less Space through Compression
8903 * Attributes:: Handling File Attributes
8904 * Portability:: Making @command{tar} Archives More Portable
8905 * cpio:: Comparison of @command{tar} and @command{cpio}
8909 @section Using Less Space through Compression
8912 * gzip:: Creating and Reading Compressed Archives
8913 * sparse:: Archiving Sparse Files
8917 @subsection Creating and Reading Compressed Archives
8918 @cindex Compressed archives
8919 @cindex Storing archives in compressed format
8927 @GNUTAR{} is able to create and read compressed archives. It supports
8928 a wide variety of compression programs, namely: @command{gzip},
8929 @command{bzip2}, @command{lzip}, @command{lzma}, @command{lzop},
8930 @command{xz} and traditional @command{compress}. The latter is
8931 supported mostly for backward compatibility, and we recommend
8932 against using it, because it is by far less effective than the other
8933 compression programs@footnote{It also had patent problems in the past.}.
8935 Creating a compressed archive is simple: you just specify a
8936 @dfn{compression option} along with the usual archive creation
8937 commands. The compression option is @option{-z} (@option{--gzip}) to
8938 create a @command{gzip} compressed archive, @option{-j}
8939 (@option{--bzip2}) to create a @command{bzip2} compressed archive,
8940 @option{--lzip} to create an @asis{lzip} compressed archive,
8941 @option{-J} (@option{--xz}) to create an @asis{XZ} archive,
8942 @option{--lzma} to create an @asis{LZMA} compressed
8943 archive, @option{--lzop} to create an @asis{LSOP} archive, and
8944 @option{-Z} (@option{--compress}) to use @command{compress} program.
8948 $ @kbd{tar czf archive.tar.gz .}
8951 You can also let @GNUTAR{} select the compression program based on
8952 the suffix of the archive file name. This is done using
8953 @option{--auto-compress} (@option{-a}) command line option. For
8954 example, the following invocation will use @command{bzip2} for
8958 $ @kbd{tar caf archive.tar.bz2 .}
8962 whereas the following one will use @command{lzma}:
8965 $ @kbd{tar caf archive.tar.lzma .}
8968 For a complete list of file name suffixes recognized by @GNUTAR{},
8969 see @ref{auto-compress}.
8971 Reading compressed archive is even simpler: you don't need to specify
8972 any additional options as @GNUTAR{} recognizes its format
8973 automatically. Thus, the following commands will list and extract the
8974 archive created in previous example:
8977 # List the compressed archive
8978 $ @kbd{tar tf archive.tar.gz}
8979 # Extract the compressed archive
8980 $ @kbd{tar xf archive.tar.gz}
8983 The format recognition algorithm is based on @dfn{signatures}, a
8984 special byte sequences in the beginning of file, that are specific for
8985 certain compression formats. If this approach fails, @command{tar}
8986 falls back to using archive name suffix to determine its format
8987 (@pxref{auto-compress}, for a list of recognized suffixes).
8989 @anchor{alternative decompression programs}
8990 @cindex alternative decompression programs
8991 Some compression programs are able to handle different compression
8992 formats. @GNUTAR{} uses this, if the principal decompressor for the
8993 given format is not available. For example, if @command{compress} is
8994 not installed, @command{tar} will try to use @command{gzip}. As of
8995 version @value{VERSION} the following alternatives are
8996 tried@footnote{To verbosely trace the decompressor selection, use the
8997 @option{--warning=decompress-program} option
8998 (@pxref{warnings,decompress-program}).}:
9000 @multitable @columnfractions 0.3 0.3 0.3
9001 @headitem Format @tab Main decompressor @tab Alternatives
9002 @item compress @tab compress @tab gzip
9003 @item lzma @tab lzma @tab xz
9004 @item bzip2 @tab bzip2 @tab lbzip2
9007 The only case when you have to specify a decompression option while
9008 reading the archive is when reading from a pipe or from a tape drive
9009 that does not support random access. However, in this case @GNUTAR{}
9010 will indicate which option you should use. For example:
9013 $ @kbd{cat archive.tar.gz | tar tf -}
9014 tar: Archive is compressed. Use -z option
9015 tar: Error is not recoverable: exiting now
9018 If you see such diagnostics, just add the suggested option to the
9019 invocation of @GNUTAR{}:
9022 $ @kbd{cat archive.tar.gz | tar tzf -}
9025 Notice also, that there are several restrictions on operations on
9026 compressed archives. First of all, compressed archives cannot be
9027 modified, i.e., you cannot update (@option{--update}, alias @option{-u})
9028 them or delete (@option{--delete}) members from them or
9029 add (@option{--append}, alias @option{-r}) members to them. Likewise, you
9030 cannot append another @command{tar} archive to a compressed archive using
9031 @option{--concatenate} (@option{-A}). Secondly, multi-volume
9032 archives cannot be compressed.
9034 The following options allow to select a particular compressor program:
9042 Filter the archive through @command{gzip}.
9047 Filter the archive through @code{xz}.
9051 Filter the archive through @code{bzip2}.
9055 Filter the archive through @command{lzip}.
9059 Filter the archive through @command{lzma}.
9063 Filter the archive through @command{lzop}.
9070 Filter the archive through @command{compress}.
9073 When any of these options is given, @GNUTAR{} searches the compressor
9074 binary in the current path and invokes it. The name of the compressor
9075 program is specified at compilation time using a corresponding
9076 @option{--with-@var{compname}} option to @command{configure}, e.g.
9077 @option{--with-bzip2} to select a specific @command{bzip2} binary.
9078 @xref{lbzip2}, for a detailed discussion.
9080 The output produced by @command{tar --help} shows the actual
9081 compressor names along with each of these options.
9083 You can use any of these options on physical devices (tape drives,
9084 etc.) and remote files as well as on normal files; data to or from
9085 such devices or remote files is reblocked by another copy of the
9086 @command{tar} program to enforce the specified (or default) record
9087 size. The default compression parameters are used. Most compression
9088 programs let you override these by setting a program-specific
9089 environment variable. For example, with @command{gzip} you can set
9093 $ @kbd{GZIP='-9 -n' tar czf archive.tar.gz subdir}
9095 Another way would be to use the @option{-I} option instead (see
9099 $ @kbd{tar -cf archive.tar.gz -I 'gzip -9 -n' subdir}
9103 Finally, the third, traditional, way to do this is to use a pipe:
9106 $ @kbd{tar cf - subdir | gzip -9 -n > archive.tar.gz}
9109 @cindex corrupted archives
9110 Compressed archives are easily corrupted, because compressed files
9111 have little redundancy. The adaptive nature of the
9112 compression scheme means that the compression tables are implicitly
9113 spread all over the archive. If you lose a few blocks, the dynamic
9114 construction of the compression tables becomes unsynchronized, and there
9115 is little chance that you could recover later in the archive.
9117 Other compression options provide better control over creating
9118 compressed archives. These are:
9121 @anchor{auto-compress}
9122 @opindex auto-compress
9123 @item --auto-compress
9125 Select a compression program to use by the archive file name
9126 suffix. The following suffixes are recognized:
9128 @multitable @columnfractions 0.3 0.6
9129 @headitem Suffix @tab Compression program
9130 @item @samp{.gz} @tab @command{gzip}
9131 @item @samp{.tgz} @tab @command{gzip}
9132 @item @samp{.taz} @tab @command{gzip}
9133 @item @samp{.Z} @tab @command{compress}
9134 @item @samp{.taZ} @tab @command{compress}
9135 @item @samp{.bz2} @tab @command{bzip2}
9136 @item @samp{.tz2} @tab @command{bzip2}
9137 @item @samp{.tbz2} @tab @command{bzip2}
9138 @item @samp{.tbz} @tab @command{bzip2}
9139 @item @samp{.lz} @tab @command{lzip}
9140 @item @samp{.lzma} @tab @command{lzma}
9141 @item @samp{.tlz} @tab @command{lzma}
9142 @item @samp{.lzo} @tab @command{lzop}
9143 @item @samp{.xz} @tab @command{xz}
9146 @anchor{use-compress-program}
9147 @opindex use-compress-program
9148 @item --use-compress-program=@var{command}
9149 @itemx -I=@var{command}
9150 Use external compression program @var{command}. Use this option if you
9151 are not happy with the compression program associated with the suffix
9152 at compile time or if you have a compression program that @GNUTAR{}
9153 does not support. The @var{command} argument is a valid command
9154 invocation, as you would type it at the command line prompt, with any
9155 additional options as needed. Enclose it in quotes if it contains
9156 white space (see @ref{external, Running External Commands}, for more detail).
9158 The @var{command} should follow two conventions:
9160 First, when invoked without additional options, it should read data
9161 from standard input, compress it and output it on standard output.
9163 Secondly, if invoked with the additional @option{-d} option, it should
9164 do exactly the opposite, i.e., read the compressed data from the
9165 standard input and produce uncompressed data on the standard output.
9167 The latter requirement means that you must not use the @option{-d}
9168 option as a part of the @var{command} itself.
9171 @cindex gpg, using with tar
9172 @cindex gnupg, using with tar
9173 @cindex Using encrypted archives
9174 The @option{--use-compress-program} option, in particular, lets you
9175 implement your own filters, not necessarily dealing with
9176 compression/decompression. For example, suppose you wish to implement
9177 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
9178 gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
9179 Manual}). The following script does that:
9185 -d) gpg --decrypt - | gzip -d -c;;
9186 '') gzip -c | gpg -s;;
9187 *) echo "Unknown option $1">&2; exit 1;;
9192 Suppose you name it @file{gpgz} and save it somewhere in your
9193 @env{PATH}. Then the following command will create a compressed
9194 archive signed with your private key:
9197 $ @kbd{tar -cf foo.tar.gpgz -Igpgz .}
9201 Likewise, the command below will list its contents:
9204 $ @kbd{tar -tf foo.tar.gpgz -Igpgz .}
9208 The above is based on the following discussion:
9210 I have one question, or maybe it's a suggestion if there isn't a way
9211 to do it now. I would like to use @option{--gzip}, but I'd also like
9212 the output to be fed through a program like @acronym{GNU}
9213 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
9214 to use :-)), basically adding ECC protection on top of compression.
9215 It seems as if this should be quite easy to do, but I can't work out
9216 exactly how to go about it. Of course, I can pipe the standard output
9217 of @command{tar} through @command{ecc}, but then I lose (though I
9218 haven't started using it yet, I confess) the ability to have
9219 @command{tar} use @command{rmt} for it's I/O (I think).
9221 I think the most straightforward thing would be to let me specify a
9222 general set of filters outboard of compression (preferably ordered,
9223 so the order can be automatically reversed on input operations, and
9224 with the options they require specifiable), but beggars shouldn't be
9225 choosers and anything you decide on would be fine with me.
9227 By the way, I like @command{ecc} but if (as the comments say) it can't
9228 deal with loss of block sync, I'm tempted to throw some time at adding
9229 that capability. Supposing I were to actually do such a thing and
9230 get it (apparently) working, do you accept contributed changes to
9231 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
9233 Isn't that exactly the role of the
9234 @option{--use-compress-prog=@var{program}} option?
9235 I never tried it myself, but I suspect you may want to write a
9236 @var{prog} script or program able to filter stdin to stdout to
9237 way you want. It should recognize the @option{-d} option, for when
9238 extraction is needed rather than creation.
9240 It has been reported that if one writes compressed data (through the
9241 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
9242 the DLT compression mode, the data will actually get bigger and one will
9243 end up with less space on the tape.
9247 * lbzip2:: Using lbzip2 with @GNUTAR{}.
9251 @subsubsection Using lbzip2 with @GNUTAR{}.
9253 @cindex Laszlo Ersek
9254 @command{Lbzip2} is a multithreaded utility for handling
9255 @samp{bzip2} compression, written by Laszlo Ersek. It makes use of
9256 multiple processors to speed up its operation and in general works
9257 considerably faster than @command{bzip2}. For a detailed description
9258 of @command{lbzip2} see @uref{http://freshmeat.net/@/projects/@/lbzip2} and
9259 @uref{http://www.linuxinsight.com/@/lbzip2-parallel-bzip2-utility.html,
9260 lbzip2: parallel bzip2 utility}.
9262 Recent versions of @command{lbzip2} are mostly command line compatible
9263 with @command{bzip2}, which makes it possible to automatically invoke
9264 it via the @option{--bzip2} @GNUTAR{} command line option. To do so,
9265 @GNUTAR{} must be configured with the @option{--with-bzip2} command
9266 line option, like this:
9269 $ @kbd{./configure --with-bzip2=lbzip2 [@var{other-options}]}
9272 Once configured and compiled this way, @command{tar --help} will show the
9277 $ @kbd{tar --help | grep -- --bzip2}
9278 -j, --bzip2 filter the archive through lbzip2
9283 which means that running @command{tar --bzip2} will invoke @command{lbzip2}.
9286 @subsection Archiving Sparse Files
9287 @cindex Sparse Files
9289 Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
9290 in a file is a section of the file's contents which was never written.
9291 The contents of a hole reads as all zeros. On many operating systems,
9292 actual disk storage is not allocated for holes, but they are counted
9293 in the length of the file. If you archive such a file, @command{tar}
9294 could create an archive longer than the original. To have @command{tar}
9295 attempt to recognize the holes in a file, use @option{--sparse}
9296 (@option{-S}). When you use this option, then, for any file using
9297 less disk space than would be expected from its length, @command{tar}
9298 searches the file for consecutive stretches of zeros. It then records
9299 in the archive for the file where the consecutive stretches of zeros
9300 are, and only archives the ``real contents'' of the file. On
9301 extraction (using @option{--sparse} is not needed on extraction) any
9302 such files have holes created wherever the continuous stretches of zeros
9303 were found. Thus, if you use @option{--sparse}, @command{tar} archives
9304 won't take more space than the original.
9310 This option instructs @command{tar} to test each file for sparseness
9311 before attempting to archive it. If the file is found to be sparse it
9312 is treated specially, thus allowing to decrease the amount of space
9313 used by its image in the archive.
9315 This option is meaningful only when creating or updating archives. It
9316 has no effect on extraction.
9319 Consider using @option{--sparse} when performing file system backups,
9320 to avoid archiving the expanded forms of files stored sparsely in the
9323 Even if your system has no sparse files currently, some may be
9324 created in the future. If you use @option{--sparse} while making file
9325 system backups as a matter of course, you can be assured the archive
9326 will never take more space on the media than the files take on disk
9327 (otherwise, archiving a disk filled with sparse files might take
9328 hundreds of tapes). @xref{Incremental Dumps}.
9330 However, be aware that @option{--sparse} option presents a serious
9331 drawback. Namely, in order to determine if the file is sparse
9332 @command{tar} has to read it before trying to archive it, so in total
9333 the file is read @strong{twice}. So, always bear in mind that the
9334 time needed to process all files with this option is roughly twice
9335 the time needed to archive them without it.
9336 @FIXME{A technical note:
9338 Programs like @command{dump} do not have to read the entire file; by
9339 examining the file system directly, they can determine in advance
9340 exactly where the holes are and thus avoid reading through them. The
9341 only data it need read are the actual allocated data blocks.
9342 @GNUTAR{} uses a more portable and straightforward
9343 archiving approach, it would be fairly difficult that it does
9344 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
9348 What I did say is that you cannot tell the difference between a hole and an
9349 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
9350 best tells you how many holes there are; it doesn't tell you @emph{where}.
9351 Just as programs may, conceivably, care what @code{st_blocks} is (care
9352 to name one that does?), they may also care where the holes are (I have
9353 no examples of this one either, but it's equally imaginable).
9355 I conclude from this that good archivers are not portable. One can
9356 arguably conclude that if you want a portable program, you can in good
9357 conscience restore files with as many holes as possible, since you can't
9362 @cindex sparse formats, defined
9363 When using @samp{POSIX} archive format, @GNUTAR{} is able to store
9364 sparse files using in three distinct ways, called @dfn{sparse
9365 formats}. A sparse format is identified by its @dfn{number},
9366 consisting, as usual of two decimal numbers, delimited by a dot. By
9367 default, format @samp{1.0} is used. If, for some reason, you wish to
9368 use an earlier format, you can select it using
9369 @option{--sparse-version} option.
9372 @opindex sparse-version
9373 @item --sparse-version=@var{version}
9375 Select the format to store sparse files in. Valid @var{version} values
9376 are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
9377 for a detailed description of each format.
9380 Using @option{--sparse-format} option implies @option{--sparse}.
9383 @section Handling File Attributes
9384 @cindex atrributes, files
9385 @cindex file attributes
9387 When @command{tar} reads files, it updates their access times. To
9388 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
9389 reset the access time retroactively or avoid changing it in the first
9393 @opindex atime-preserve
9394 @item --atime-preserve
9395 @itemx --atime-preserve=replace
9396 @itemx --atime-preserve=system
9397 Preserve the access times of files that are read. This works only for
9398 files that you own, unless you have superuser privileges.
9400 @option{--atime-preserve=replace} works on most systems, but it also
9401 restores the data modification time and updates the status change
9402 time. Hence it doesn't interact with incremental dumps nicely
9403 (@pxref{Incremental Dumps}), and it can set access or data modification times
9404 incorrectly if other programs access the file while @command{tar} is
9407 @option{--atime-preserve=system} avoids changing the access time in
9408 the first place, if the operating system supports this.
9409 Unfortunately, this may or may not work on any given operating system
9410 or file system. If @command{tar} knows for sure it won't work, it
9411 complains right away.
9413 Currently @option{--atime-preserve} with no operand defaults to
9414 @option{--atime-preserve=replace}, but this is intended to change to
9415 @option{--atime-preserve=system} when the latter is better-supported.
9420 Do not extract data modification time.
9422 When this option is used, @command{tar} leaves the data modification times
9423 of the files it extracts as the times when the files were extracted,
9424 instead of setting it to the times recorded in the archive.
9426 This option is meaningless with @option{--list} (@option{-t}).
9430 Create extracted files with the same ownership they have in the
9433 This is the default behavior for the superuser,
9434 so this option is meaningful only for non-root users, when @command{tar}
9435 is executed on those systems able to give files away. This is
9436 considered as a security flaw by many people, at least because it
9437 makes quite difficult to correctly account users for the disk space
9438 they occupy. Also, the @code{suid} or @code{sgid} attributes of
9439 files are easily and silently lost when files are given away.
9441 When writing an archive, @command{tar} writes the user @acronym{ID} and user name
9442 separately. If it can't find a user name (because the user @acronym{ID} is not
9443 in @file{/etc/passwd}), then it does not write one. When restoring,
9444 it tries to look the name (if one was written) up in
9445 @file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
9446 the archive instead.
9448 @opindex no-same-owner
9449 @item --no-same-owner
9451 Do not attempt to restore ownership when extracting. This is the
9452 default behavior for ordinary users, so this option has an effect
9453 only for the superuser.
9455 @opindex numeric-owner
9456 @item --numeric-owner
9457 The @option{--numeric-owner} option allows (ANSI) archives to be written
9458 without user/group name information or such information to be ignored
9459 when extracting. It effectively disables the generation and/or use
9460 of user/group name information. This option forces extraction using
9461 the numeric ids from the archive, ignoring the names.
9463 This is useful in certain circumstances, when restoring a backup from
9464 an emergency floppy with different passwd/group files for example.
9465 It is otherwise impossible to extract files with the right ownerships
9466 if the password file in use during the extraction does not match the
9467 one belonging to the file system(s) being extracted. This occurs,
9468 for example, if you are restoring your files after a major crash and
9469 had booted from an emergency floppy with no password file or put your
9470 disk into another machine to do the restore.
9472 The numeric ids are @emph{always} saved into @command{tar} archives.
9473 The identifying names are added at create time when provided by the
9474 system, unless @option{--format=oldgnu} is used. Numeric ids could be
9475 used when moving archives between a collection of machines using
9476 a centralized management for attribution of numeric ids to users
9477 and groups. This is often made through using the NIS capabilities.
9479 When making a @command{tar} file for distribution to other sites, it
9480 is sometimes cleaner to use a single owner for all files in the
9481 distribution, and nicer to specify the write permission bits of the
9482 files as stored in the archive independently of their actual value on
9483 the file system. The way to prepare a clean distribution is usually
9484 to have some Makefile rule creating a directory, copying all needed
9485 files in that directory, then setting ownership and permissions as
9486 wanted (there are a lot of possible schemes), and only then making a
9487 @command{tar} archive out of this directory, before cleaning
9488 everything out. Of course, we could add a lot of options to
9489 @GNUTAR{} for fine tuning permissions and ownership.
9490 This is not the good way, I think. @GNUTAR{} is
9491 already crowded with options and moreover, the approach just explained
9492 gives you a great deal of control already.
9494 @xopindex{same-permissions, short description}
9495 @xopindex{preserve-permissions, short description}
9497 @itemx --same-permissions
9498 @itemx --preserve-permissions
9499 Extract all protection information.
9501 This option causes @command{tar} to set the modes (access permissions) of
9502 extracted files exactly as recorded in the archive. If this option
9503 is not used, the current @code{umask} setting limits the permissions
9504 on extracted files. This option is by default enabled when
9505 @command{tar} is executed by a superuser.
9508 This option is meaningless with @option{--list} (@option{-t}).
9512 Same as both @option{--same-permissions} and @option{--same-order}.
9514 This option is deprecated, and will be removed in @GNUTAR{} version 1.23.
9519 @section Making @command{tar} Archives More Portable
9521 Creating a @command{tar} archive on a particular system that is meant to be
9522 useful later on many other machines and with other versions of @command{tar}
9523 is more challenging than you might think. @command{tar} archive formats
9524 have been evolving since the first versions of Unix. Many such formats
9525 are around, and are not always compatible with each other. This section
9526 discusses a few problems, and gives some advice about making @command{tar}
9527 archives more portable.
9529 One golden rule is simplicity. For example, limit your @command{tar}
9530 archives to contain only regular files and directories, avoiding
9531 other kind of special files. Do not attempt to save sparse files or
9532 contiguous files as such. Let's discuss a few more problems, in turn.
9534 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
9535 archives and archive labels) in GNU and PAX formats.}
9538 * Portable Names:: Portable Names
9539 * dereference:: Symbolic Links
9540 * hard links:: Hard Links
9541 * old:: Old V7 Archives
9542 * ustar:: Ustar Archives
9543 * gnu:: GNU and old GNU format archives.
9544 * posix:: @acronym{POSIX} archives
9545 * Checksumming:: Checksumming Problems
9546 * Large or Negative Values:: Large files, negative time stamps, etc.
9547 * Other Tars:: How to Extract GNU-Specific Data Using
9548 Other @command{tar} Implementations
9551 @node Portable Names
9552 @subsection Portable Names
9554 Use portable file and member names. A name is portable if it contains
9555 only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
9556 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
9557 contain @samp{/-}. Avoid deep directory nesting. For portability to
9558 old Unix hosts, limit your file name components to 14 characters or
9561 If you intend to have your @command{tar} archives to be read under
9562 MSDOS, you should not rely on case distinction for file names, and you
9563 might use the @acronym{GNU} @command{doschk} program for helping you
9564 further diagnosing illegal MSDOS names, which are even more limited
9568 @subsection Symbolic Links
9569 @cindex File names, using symbolic links
9570 @cindex Symbolic link as file name
9572 @opindex dereference
9573 Normally, when @command{tar} archives a symbolic link, it writes a
9574 block to the archive naming the target of the link. In that way, the
9575 @command{tar} archive is a faithful record of the file system contents.
9576 When @option{--dereference} (@option{-h}) is used with
9577 @option{--create} (@option{-c}), @command{tar} archives the files
9578 symbolic links point to, instead of
9579 the links themselves.
9581 When creating portable archives, use @option{--dereference}
9582 (@option{-h}): some systems do not support
9583 symbolic links, and moreover, your distribution might be unusable if
9584 it contains unresolved symbolic links.
9586 When reading from an archive, the @option{--dereference} (@option{-h})
9587 option causes @command{tar} to follow an already-existing symbolic
9588 link when @command{tar} writes or reads a file named in the archive.
9589 Ordinarily, @command{tar} does not follow such a link, though it may
9590 remove the link before writing a new file. @xref{Dealing with Old
9593 The @option{--dereference} option is unsafe if an untrusted user can
9594 modify directories while @command{tar} is running. @xref{Security}.
9597 @subsection Hard Links
9598 @cindex File names, using hard links
9599 @cindex hard links, dereferencing
9600 @cindex dereferencing hard links
9602 Normally, when @command{tar} archives a hard link, it writes a
9603 block to the archive naming the target of the link (a @samp{1} type
9604 block). In that way, the actual file contents is stored in file only
9605 once. For example, consider the following two files:
9610 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
9611 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
9615 Here, @file{jeden} is a link to @file{one}. When archiving this
9616 directory with a verbose level 2, you will get an output similar to
9620 $ tar cvvf ../archive.tar .
9621 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9622 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9623 hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
9626 The last line shows that, instead of storing two copies of the file,
9627 @command{tar} stored it only once, under the name @file{jeden}, and
9628 stored file @file{one} as a hard link to this file.
9630 It may be important to know that all hard links to the given file are
9631 stored in the archive. For example, this may be necessary for exact
9632 reproduction of the file system. The following option does that:
9635 @xopindex{check-links, described}
9638 Check the number of links dumped for each processed file. If this
9639 number does not match the total number of hard links for the file, print
9643 For example, trying to archive only file @file{jeden} with this option
9644 produces the following diagnostics:
9647 $ tar -c -f ../archive.tar -l jeden
9648 tar: Missing links to 'jeden'.
9651 Although creating special records for hard links helps keep a faithful
9652 record of the file system contents and makes archives more compact, it
9653 may present some difficulties when extracting individual members from
9654 the archive. For example, trying to extract file @file{one} from the
9655 archive created in previous examples produces, in the absense of file
9659 $ tar xf archive.tar ./one
9660 tar: ./one: Cannot hard link to './jeden': No such file or directory
9661 tar: Error exit delayed from previous errors
9664 The reason for this behavior is that @command{tar} cannot seek back in
9665 the archive to the previous member (in this case, @file{one}), to
9666 extract it@footnote{There are plans to fix this in future releases.}.
9667 If you wish to avoid such problems at the cost of a bigger archive,
9668 use the following option:
9671 @xopindex{hard-dereference, described}
9672 @item --hard-dereference
9673 Dereference hard links and store the files they refer to.
9676 For example, trying this option on our two sample files, we get two
9677 copies in the archive, each of which can then be extracted
9678 independently of the other:
9682 $ tar -c -vv -f ../archive.tar --hard-dereference .
9683 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9684 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9685 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
9690 @subsection Old V7 Archives
9691 @cindex Format, old style
9692 @cindex Old style format
9693 @cindex Old style archives
9694 @cindex v7 archive format
9696 Certain old versions of @command{tar} cannot handle additional
9697 information recorded by newer @command{tar} programs. To create an
9698 archive in V7 format (not ANSI), which can be read by these old
9699 versions, specify the @option{--format=v7} option in
9700 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
9701 accepts @option{--portability} or @option{--old-archive} for this
9702 option). When you specify it,
9703 @command{tar} leaves out information about directories, pipes, fifos,
9704 contiguous files, and device files, and specifies file ownership by
9705 group and user IDs instead of group and user names.
9707 When updating an archive, do not use @option{--format=v7}
9708 unless the archive was created using this option.
9710 In most cases, a @emph{new} format archive can be read by an @emph{old}
9711 @command{tar} program without serious trouble, so this option should
9712 seldom be needed. On the other hand, most modern @command{tar}s are
9713 able to read old format archives, so it might be safer for you to
9714 always use @option{--format=v7} for your distributions. Notice,
9715 however, that @samp{ustar} format is a better alternative, as it is
9716 free from many of @samp{v7}'s drawbacks.
9719 @subsection Ustar Archive Format
9721 @cindex ustar archive format
9722 Archive format defined by @acronym{POSIX}.1-1988 specification is called
9723 @code{ustar}. Although it is more flexible than the V7 format, it
9724 still has many restrictions (@pxref{Formats,ustar}, for the detailed
9725 description of @code{ustar} format). Along with V7 format,
9726 @code{ustar} format is a good choice for archives intended to be read
9727 with other implementations of @command{tar}.
9729 To create archive in @code{ustar} format, use @option{--format=ustar}
9730 option in conjunction with the @option{--create} (@option{-c}).
9733 @subsection @acronym{GNU} and old @GNUTAR{} format
9735 @cindex GNU archive format
9736 @cindex Old GNU archive format
9737 @GNUTAR{} was based on an early draft of the
9738 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
9739 @command{tar}, such as the support for file names longer than 100
9740 characters, use portions of the @command{tar} header record which were
9741 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
9742 @acronym{POSIX} have allocated the same parts of the header record for
9743 other purposes. As a result, @GNUTAR{} format is
9744 incompatible with the current @acronym{POSIX} specification, and with
9745 @command{tar} programs that follow it.
9747 In the majority of cases, @command{tar} will be configured to create
9748 this format by default. This will change in future releases, since
9749 we plan to make @samp{POSIX} format the default.
9751 To force creation a @GNUTAR{} archive, use option
9752 @option{--format=gnu}.
9755 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
9757 @cindex POSIX archive format
9758 @cindex PAX archive format
9759 Starting from version 1.14 @GNUTAR{} features full support for
9760 @acronym{POSIX.1-2001} archives.
9762 A @acronym{POSIX} conformant archive will be created if @command{tar}
9763 was given @option{--format=posix} (@option{--format=pax}) option. No
9764 special option is required to read and extract from a @acronym{POSIX}
9768 * PAX keywords:: Controlling Extended Header Keywords.
9772 @subsubsection Controlling Extended Header Keywords
9776 @item --pax-option=@var{keyword-list}
9777 Handle keywords in @acronym{PAX} extended headers. This option is
9778 equivalent to @option{-o} option of the @command{pax} utility.
9781 @var{Keyword-list} is a comma-separated
9782 list of keyword options, each keyword option taking one of
9783 the following forms:
9786 @item delete=@var{pattern}
9787 When used with one of archive-creation commands,
9788 this option instructs @command{tar} to omit from extended header records
9789 that it produces any keywords matching the string @var{pattern}.
9791 When used in extract or list mode, this option instructs tar
9792 to ignore any keywords matching the given @var{pattern} in the extended
9793 header records. In both cases, matching is performed using the pattern
9794 matching notation described in @acronym{POSIX 1003.2}, 3.13
9795 (@pxref{wildcards}). For example:
9798 --pax-option delete=security.*
9801 would suppress security-related information.
9803 @item exthdr.name=@var{string}
9805 This keyword allows user control over the name that is written into the
9806 ustar header blocks for the extended headers. The name is obtained
9807 from @var{string} after making the following substitutions:
9809 @multitable @columnfractions .25 .55
9810 @headitem Meta-character @tab Replaced By
9811 @item %d @tab The directory name of the file, equivalent to the
9812 result of the @command{dirname} utility on the translated file name.
9813 @item %f @tab The name of the file with the directory information
9814 stripped, equivalent to the result of the @command{basename} utility
9815 on the translated file name.
9816 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9817 @item %% @tab A @samp{%} character.
9820 Any other @samp{%} characters in @var{string} produce undefined
9823 If no option @samp{exthdr.name=string} is specified, @command{tar}
9824 will use the following default value:
9830 @item exthdr.mtime=@var{value}
9832 This keyword defines the value of the @samp{mtime} field that
9833 is written into the ustar header blocks for the extended headers.
9834 By default, the @samp{mtime} field is set to the modification time
9835 of the archive member described by that extended headers.
9837 @item globexthdr.name=@var{string}
9838 This keyword allows user control over the name that is written into
9839 the ustar header blocks for global extended header records. The name
9840 is obtained from the contents of @var{string}, after making
9841 the following substitutions:
9843 @multitable @columnfractions .25 .55
9844 @headitem Meta-character @tab Replaced By
9845 @item %n @tab An integer that represents the
9846 sequence number of the global extended header record in the archive,
9848 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9849 @item %% @tab A @samp{%} character.
9852 Any other @samp{%} characters in @var{string} produce undefined results.
9854 If no option @samp{globexthdr.name=string} is specified, @command{tar}
9855 will use the following default value:
9858 $TMPDIR/GlobalHead.%p.%n
9862 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
9863 environment variable. If @var{TMPDIR} is not set, @command{tar}
9866 @item globexthdr.mtime=@var{value}
9868 This keyword defines the value of the @samp{mtime} field that
9869 is written into the ustar header blocks for the global extended headers.
9870 By default, the @samp{mtime} field is set to the time when
9871 @command{tar} was invoked.
9873 @item @var{keyword}=@var{value}
9874 When used with one of archive-creation commands, these keyword/value pairs
9875 will be included at the beginning of the archive in a global extended
9876 header record. When used with one of archive-reading commands,
9877 @command{tar} will behave as if it has encountered these keyword/value
9878 pairs at the beginning of the archive in a global extended header
9881 @item @var{keyword}:=@var{value}
9882 When used with one of archive-creation commands, these keyword/value pairs
9883 will be included as records at the beginning of an extended header for
9884 each file. This is effectively equivalent to @var{keyword}=@var{value}
9885 form except that it creates no global extended header records.
9887 When used with one of archive-reading commands, @command{tar} will
9888 behave as if these keyword/value pairs were included as records at the
9889 end of each extended header; thus, they will override any global or
9890 file-specific extended header record keywords of the same names.
9891 For example, in the command:
9894 tar --format=posix --create \
9895 --file archive --pax-option gname:=user .
9898 the group name will be forced to a new value for all files
9899 stored in the archive.
9902 In any of the forms described above, the @var{value} may be
9903 a string enclosed in curly braces. In that case, the string
9904 between the braces is understood either as a textual time
9905 representation, as described in @ref{Date input formats}, or a name of
9906 the existing file, starting with @samp{/} or @samp{.}. In the latter
9907 case, the modification time of that file is used.
9909 For example, to set all modification times to the current date, you
9910 use the following option:
9913 --pax-option='mtime:=@{now@}'
9916 Note quoting of the option's argument.
9918 @cindex archives, binary equivalent
9919 @cindex binary equivalent archives, creating
9920 As another example, here is the option that ensures that any two
9921 archives created using it, will be binary equivalent if they have the
9925 --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0
9929 @subsection Checksumming Problems
9931 SunOS and HP-UX @command{tar} fail to accept archives created using
9932 @GNUTAR{} and containing non-@acronym{ASCII} file names, that
9933 is, file names having characters with the eight bit set, because they
9934 use signed checksums, while @GNUTAR{} uses unsigned
9935 checksums while creating archives, as per @acronym{POSIX} standards. On
9936 reading, @GNUTAR{} computes both checksums and
9937 accepts any. It is somewhat worrying that a lot of people may go
9938 around doing backup of their files using faulty (or at least
9939 non-standard) software, not learning about it until it's time to
9940 restore their missing files with an incompatible file extractor, or
9943 @GNUTAR{} computes checksums both ways, and accept
9944 any on read, so @acronym{GNU} tar can read Sun tapes even with their
9945 wrong checksums. @GNUTAR{} produces the standard
9946 checksum, however, raising incompatibilities with Sun. That is to
9947 say, @GNUTAR{} has not been modified to
9948 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
9949 I've been told that more recent Sun @command{tar} now read standard
9950 archives, so maybe Sun did a similar patch, after all?
9952 The story seems to be that when Sun first imported @command{tar}
9953 sources on their system, they recompiled it without realizing that
9954 the checksums were computed differently, because of a change in
9955 the default signing of @code{char}'s in their compiler. So they
9956 started computing checksums wrongly. When they later realized their
9957 mistake, they merely decided to stay compatible with it, and with
9958 themselves afterwards. Presumably, but I do not really know, HP-UX
9959 has chosen that their @command{tar} archives to be compatible with Sun's.
9960 The current standards do not favor Sun @command{tar} format. In any
9961 case, it now falls on the shoulders of SunOS and HP-UX users to get
9962 a @command{tar} able to read the good archives they receive.
9964 @node Large or Negative Values
9965 @subsection Large or Negative Values
9966 @cindex large values
9967 @cindex future time stamps
9968 @cindex negative time stamps
9971 The above sections suggest to use @samp{oldest possible} archive
9972 format if in doubt. However, sometimes it is not possible. If you
9973 attempt to archive a file whose metadata cannot be represented using
9974 required format, @GNUTAR{} will print error message and ignore such a
9975 file. You will than have to switch to a format that is able to
9976 handle such values. The format summary table (@pxref{Formats}) will
9979 In particular, when trying to archive files larger than 8GB or with
9980 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
9981 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
9982 @acronym{POSIX} archive formats. When considering which format to
9983 choose, bear in mind that the @acronym{GNU} format uses
9984 two's-complement base-256 notation to store values that do not fit
9985 into standard @acronym{ustar} range. Such archives can generally be
9986 read only by a @GNUTAR{} implementation. Moreover, they sometimes
9987 cannot be correctly restored on another hosts even by @GNUTAR{}. For
9988 example, using two's complement representation for negative time
9989 stamps that assumes a signed 32-bit @code{time_t} generates archives
9990 that are not portable to hosts with differing @code{time_t}
9993 On the other hand, @acronym{POSIX} archives, generally speaking, can
9994 be extracted by any tar implementation that understands older
9995 @acronym{ustar} format. The only exception are files larger than 8GB.
9997 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
10001 @subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations
10003 In previous sections you became acquainted with various quirks
10004 necessary to make your archives portable. Sometimes you may need to
10005 extract archives containing GNU-specific members using some
10006 third-party @command{tar} implementation or an older version of
10007 @GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed,
10008 but if it is for some reason impossible, this section will explain
10009 how to cope without it.
10011 When we speak about @dfn{GNU-specific} members we mean two classes of
10012 them: members split between the volumes of a multi-volume archive and
10013 sparse members. You will be able to always recover such members if
10014 the archive is in PAX format. In addition split members can be
10015 recovered from archives in old GNU format. The following subsections
10016 describe the required procedures in detail.
10019 * Split Recovery:: Members Split Between Volumes
10020 * Sparse Recovery:: Sparse Members
10023 @node Split Recovery
10024 @subsubsection Extracting Members Split Between Volumes
10026 @cindex Mutli-volume archives, extracting using non-GNU tars
10027 If a member is split between several volumes of an old GNU format archive
10028 most third party @command{tar} implementation will fail to extract
10029 it. To extract it, use @command{tarcat} program (@pxref{Tarcat}).
10030 This program is available from
10031 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat.html, @GNUTAR{}
10032 home page}. It concatenates several archive volumes into a single
10033 valid archive. For example, if you have three volumes named from
10034 @file{vol-1.tar} to @file{vol-3.tar}, you can do the following to
10035 extract them using a third-party @command{tar}:
10038 $ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -}
10041 @cindex Mutli-volume archives in PAX format, extracting using non-GNU tars
10042 You could use this approach for most (although not all) PAX
10043 format archives as well. However, extracting split members from a PAX
10044 archive is a much easier task, because PAX volumes are constructed in
10045 such a way that each part of a split member is extracted to a
10046 different file by @command{tar} implementations that are not aware of
10047 GNU extensions. More specifically, the very first part retains its
10048 original name, and all subsequent parts are named using the pattern:
10051 %d/GNUFileParts.%p/%f.%n
10055 where symbols preceeded by @samp{%} are @dfn{macro characters} that
10056 have the following meaning:
10058 @multitable @columnfractions .25 .55
10059 @headitem Meta-character @tab Replaced By
10060 @item %d @tab The directory name of the file, equivalent to the
10061 result of the @command{dirname} utility on its full name.
10062 @item %f @tab The file name of the file, equivalent to the result
10063 of the @command{basename} utility on its full name.
10064 @item %p @tab The process @acronym{ID} of the @command{tar} process that
10065 created the archive.
10066 @item %n @tab Ordinal number of this particular part.
10069 For example, if the file @file{var/longfile} was split during archive
10070 creation between three volumes, and the creator @command{tar} process
10071 had process @acronym{ID} @samp{27962}, then the member names will be:
10075 var/GNUFileParts.27962/longfile.1
10076 var/GNUFileParts.27962/longfile.2
10079 When you extract your archive using a third-party @command{tar}, these
10080 files will be created on your disk, and the only thing you will need
10081 to do to restore your file in its original form is concatenate them in
10082 the proper order, for example:
10087 $ @kbd{cat GNUFileParts.27962/longfile.1 \
10088 GNUFileParts.27962/longfile.2 >> longfile}
10089 $ rm -f GNUFileParts.27962
10093 Notice, that if the @command{tar} implementation you use supports PAX
10094 format archives, it will probably emit warnings about unknown keywords
10095 during extraction. They will look like this:
10100 Unknown extended header keyword 'GNU.volume.filename' ignored.
10101 Unknown extended header keyword 'GNU.volume.size' ignored.
10102 Unknown extended header keyword 'GNU.volume.offset' ignored.
10107 You can safely ignore these warnings.
10109 If your @command{tar} implementation is not PAX-aware, you will get
10110 more warnings and more files generated on your disk, e.g.:
10114 $ @kbd{tar xf vol-1.tar}
10115 var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
10117 Unexpected EOF in archive
10118 $ @kbd{tar xf vol-2.tar}
10119 tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
10120 GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
10121 'x', extracted as normal file
10125 Ignore these warnings. The @file{PaxHeaders.*} directories created
10126 will contain files with @dfn{extended header keywords} describing the
10127 extracted files. You can delete them, unless they describe sparse
10128 members. Read further to learn more about them.
10130 @node Sparse Recovery
10131 @subsubsection Extracting Sparse Members
10133 @cindex sparse files, extracting with non-GNU tars
10134 Any @command{tar} implementation will be able to extract sparse members from a
10135 PAX archive. However, the extracted files will be @dfn{condensed},
10136 i.e., any zero blocks will be removed from them. When we restore such
10137 a condensed file to its original form, by adding zero blocks (or
10138 @dfn{holes}) back to their original locations, we call this process
10139 @dfn{expanding} a compressed sparse file.
10142 To expand a file, you will need a simple auxiliary program called
10143 @command{xsparse}. It is available in source form from
10144 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse.html, @GNUTAR{}
10147 @cindex sparse files v.1.0, extracting with non-GNU tars
10148 Let's begin with archive members in @dfn{sparse format
10149 version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand.
10150 The condensed file will contain both file map and file data, so no
10151 additional data will be needed to restore it. If the original file
10152 name was @file{@var{dir}/@var{name}}, then the condensed file will be
10153 named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where
10154 @var{n} is a decimal number@footnote{Technically speaking, @var{n} is a
10155 @dfn{process @acronym{ID}} of the @command{tar} process which created the
10156 archive (@pxref{PAX keywords}).}.
10158 To expand a version 1.0 file, run @command{xsparse} as follows:
10161 $ @kbd{xsparse @file{cond-file}}
10165 where @file{cond-file} is the name of the condensed file. The utility
10166 will deduce the name for the resulting expanded file using the
10167 following algorithm:
10170 @item If @file{cond-file} does not contain any directories,
10171 @file{../cond-file} will be used;
10173 @item If @file{cond-file} has the form
10174 @file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name}
10175 are simple names, with no @samp{/} characters in them, the output file
10176 name will be @file{@var{dir}/@var{name}}.
10178 @item Otherwise, if @file{cond-file} has the form
10179 @file{@var{dir}/@var{name}}, the output file name will be
10183 In the unlikely case when this algorithm does not suit your needs,
10184 you can explicitly specify output file name as a second argument to
10188 $ @kbd{xsparse @file{cond-file} @file{out-file}}
10191 It is often a good idea to run @command{xsparse} in @dfn{dry run} mode
10192 first. In this mode, the command does not actually expand the file,
10193 but verbosely lists all actions it would be taking to do so. The dry
10194 run mode is enabled by @option{-n} command line argument:
10198 $ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile}
10199 Reading v.1.0 sparse map
10200 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10201 '/home/gray/sparsefile'
10206 To actually expand the file, you would run:
10209 $ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile}
10213 The program behaves the same way all UNIX utilities do: it will keep
10214 quiet unless it has simething important to tell you (e.g. an error
10215 condition or something). If you wish it to produce verbose output,
10216 similar to that from the dry run mode, use @option{-v} option:
10220 $ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile}
10221 Reading v.1.0 sparse map
10222 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10223 '/home/gray/sparsefile'
10228 Additionally, if your @command{tar} implementation has extracted the
10229 @dfn{extended headers} for this file, you can instruct @command{xstar}
10230 to use them in order to verify the integrity of the expanded file.
10231 The option @option{-x} sets the name of the extended header file to
10232 use. Continuing our example:
10236 $ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
10237 /home/gray/GNUSparseFile.6058/sparsefile}
10238 Reading extended header file
10239 Found variable GNU.sparse.major = 1
10240 Found variable GNU.sparse.minor = 0
10241 Found variable GNU.sparse.name = sparsefile
10242 Found variable GNU.sparse.realsize = 217481216
10243 Reading v.1.0 sparse map
10244 Expanding file '/home/gray/GNUSparseFile.6058/sparsefile' to
10245 '/home/gray/sparsefile'
10250 @anchor{extracting sparse v.0.x}
10251 @cindex sparse files v.0.1, extracting with non-GNU tars
10252 @cindex sparse files v.0.0, extracting with non-GNU tars
10253 An @dfn{extended header} is a special @command{tar} archive header
10254 that precedes an archive member and contains a set of
10255 @dfn{variables}, describing the member properties that cannot be
10256 stored in the standard @code{ustar} header. While optional for
10257 expanding sparse version 1.0 members, the use of extended headers is
10258 mandatory when expanding sparse members in older sparse formats: v.0.0
10259 and v.0.1 (The sparse formats are described in detail in @ref{Sparse
10260 Formats}.) So, for these formats, the question is: how to obtain
10261 extended headers from the archive?
10263 If you use a @command{tar} implementation that does not support PAX
10264 format, extended headers for each member will be extracted as a
10265 separate file. If we represent the member name as
10266 @file{@var{dir}/@var{name}}, then the extended header file will be
10267 named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where
10268 @var{n} is an integer number.
10270 Things become more difficult if your @command{tar} implementation
10271 does support PAX headers, because in this case you will have to
10272 manually extract the headers. We recommend the following algorithm:
10276 Consult the documentation of your @command{tar} implementation for an
10277 option that prints @dfn{block numbers} along with the archive
10278 listing (analogous to @GNUTAR{}'s @option{-R} option). For example,
10279 @command{star} has @option{-block-number}.
10282 Obtain verbose listing using the @samp{block number} option, and
10283 find block numbers of the sparse member in question and the member
10284 immediately following it. For example, running @command{star} on our
10289 $ @kbd{star -t -v -block-number -f arc.tar}
10291 star: Unknown extended header keyword 'GNU.sparse.size' ignored.
10292 star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
10293 star: Unknown extended header keyword 'GNU.sparse.name' ignored.
10294 star: Unknown extended header keyword 'GNU.sparse.map' ignored.
10295 block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
10296 block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
10302 (as usual, ignore the warnings about unknown keywords.)
10305 Let @var{size} be the size of the sparse member, @var{Bs} be its block number
10306 and @var{Bn} be the block number of the next member.
10310 @var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2
10314 This number gives the size of the extended header part in tar @dfn{blocks}.
10315 In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2
10319 Use @command{dd} to extract the headers:
10322 @kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}}
10326 where @var{archive} is the archive name, @var{hname} is a name of the
10327 file to store the extended header in, @var{Bs} and @var{N} are
10328 computed in previous steps.
10330 In our example, this command will be
10333 $ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7}
10337 Finally, you can expand the condensed file, using the obtained header:
10341 $ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile}
10342 Reading extended header file
10343 Found variable GNU.sparse.size = 217481216
10344 Found variable GNU.sparse.numblocks = 208
10345 Found variable GNU.sparse.name = sparsefile
10346 Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{}
10347 Expanding file 'GNUSparseFile.28124/sparsefile' to 'sparsefile'
10353 @section Comparison of @command{tar} and @command{cpio}
10356 @FIXME{Reorganize the following material}
10358 The @command{cpio} archive formats, like @command{tar}, do have maximum
10359 file name lengths. The binary and old @acronym{ASCII} formats have a maximum file
10360 length of 256, and the new @acronym{ASCII} and @acronym{CRC ASCII} formats have a max
10361 file length of 1024. @acronym{GNU} @command{cpio} can read and write archives
10362 with arbitrary file name lengths, but other @command{cpio} implementations
10363 may crash unexplainedly trying to read them.
10365 @command{tar} handles symbolic links in the form in which it comes in @acronym{BSD};
10366 @command{cpio} doesn't handle symbolic links in the form in which it comes
10367 in System V prior to SVR4, and some vendors may have added symlinks
10368 to their system without enhancing @command{cpio} to know about them.
10369 Others may have enhanced it in a way other than the way I did it
10370 at Sun, and which was adopted by AT&T (and which is, I think, also
10371 present in the @command{cpio} that Berkeley picked up from AT&T and put
10372 into a later @acronym{BSD} release---I think I gave them my changes).
10374 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
10375 can handle @command{tar} format input, and write it on output, and it
10376 probably handles symbolic links. They may not have bothered doing
10377 anything to enhance @command{tar} as a result.)
10379 @command{cpio} handles special files; traditional @command{tar} doesn't.
10381 @command{tar} comes with V7, System III, System V, and @acronym{BSD} source;
10382 @command{cpio} comes only with System III, System V, and later @acronym{BSD}
10383 (4.3-tahoe and later).
10385 @command{tar}'s way of handling multiple hard links to a file can handle
10386 file systems that support 32-bit i-numbers (e.g., the @acronym{BSD} file system);
10387 @command{cpio}s way requires you to play some games (in its ``binary''
10388 format, i-numbers are only 16 bits, and in its ``portable @acronym{ASCII}'' format,
10389 they're 18 bits---it would have to play games with the "file system @acronym{ID}"
10390 field of the header to make sure that the file system @acronym{ID}/i-number pairs
10391 of different files were always different), and I don't know which
10392 @command{cpio}s, if any, play those games. Those that don't might get
10393 confused and think two files are the same file when they're not, and
10394 make hard links between them.
10396 @command{tar}s way of handling multiple hard links to a file places only
10397 one copy of the link on the tape, but the name attached to that copy
10398 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
10399 way puts one copy for every link, but you can retrieve it using any
10403 What type of check sum (if any) is used, and how is this calculated.
10406 See the attached manual pages for @command{tar} and @command{cpio} format.
10407 @command{tar} uses a checksum which is the sum of all the bytes in the
10408 @command{tar} header for a file; @command{cpio} uses no checksum.
10411 If anyone knows why @command{cpio} was made when @command{tar} was present
10415 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
10416 generally-available version of UNIX had @command{tar} at the time. I don't
10417 know whether any version that was generally available @emph{within AT&T}
10418 had @command{tar}, or, if so, whether the people within AT&T who did
10419 @command{cpio} knew about it.
10421 On restore, if there is a corruption on a tape @command{tar} will stop at
10422 that point, while @command{cpio} will skip over it and try to restore the
10425 The main difference is just in the command syntax and header format.
10427 @command{tar} is a little more tape-oriented in that everything is blocked
10428 to start on a record boundary.
10431 Is there any differences between the ability to recover crashed
10432 archives between the two of them. (Is there any chance of recovering
10433 crashed archives at all.)
10436 Theoretically it should be easier under @command{tar} since the blocking
10437 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
10438 However, modern @command{cpio}'s and variations have an option to just
10439 search for the next file header after an error with a reasonable chance
10440 of resyncing. However, lots of tape driver software won't allow you to
10441 continue past a media error which should be the only reason for getting
10442 out of sync unless a file changed sizes while you were writing the
10446 If anyone knows why @command{cpio} was made when @command{tar} was present
10447 at the unix scene, please tell me about this too.
10450 Probably because it is more media efficient (by not blocking everything
10451 and using only the space needed for the headers where @command{tar}
10452 always uses 512 bytes per file header) and it knows how to archive
10455 You might want to look at the freely available alternatives. The
10456 major ones are @command{afio}, @GNUTAR{}, and
10457 @command{pax}, each of which have their own extensions with some
10458 backwards compatibility.
10460 Sparse files were @command{tar}red as sparse files (which you can
10461 easily test, because the resulting archive gets smaller, and
10462 @acronym{GNU} @command{cpio} can no longer read it).
10465 @chapter Tapes and Other Archive Media
10468 A few special cases about tape handling warrant more detailed
10469 description. These special cases are discussed below.
10471 Many complexities surround the use of @command{tar} on tape drives. Since
10472 the creation and manipulation of archives located on magnetic tape was
10473 the original purpose of @command{tar}, it contains many features making
10474 such manipulation easier.
10476 Archives are usually written on dismountable media---tape cartridges,
10477 mag tapes, or floppy disks.
10479 The amount of data a tape or disk holds depends not only on its size,
10480 but also on how it is formatted. A 2400 foot long reel of mag tape
10481 holds 40 megabytes of data when formatted at 1600 bits per inch. The
10482 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
10484 Magnetic media are re-usable---once the archive on a tape is no longer
10485 needed, the archive can be erased and the tape or disk used over.
10486 Media quality does deteriorate with use, however. Most tapes or disks
10487 should be discarded when they begin to produce data errors. EXABYTE
10488 tape cartridges should be discarded when they generate an @dfn{error
10489 count} (number of non-usable bits) of more than 10k.
10491 Magnetic media are written and erased using magnetic fields, and
10492 should be protected from such fields to avoid damage to stored data.
10493 Sticking a floppy disk to a filing cabinet using a magnet is probably
10497 * Device:: Device selection and switching
10498 * Remote Tape Server::
10499 * Common Problems and Solutions::
10500 * Blocking:: Blocking
10501 * Many:: Many archives on one tape
10502 * Using Multiple Tapes:: Using Multiple Tapes
10503 * label:: Including a Label in the Archive
10505 * Write Protection::
10509 @section Device Selection and Switching
10513 @item -f [@var{hostname}:]@var{file}
10514 @itemx --file=[@var{hostname}:]@var{file}
10515 Use archive file or device @var{file} on @var{hostname}.
10518 This option is used to specify the file name of the archive @command{tar}
10521 If the file name is @samp{-}, @command{tar} reads the archive from standard
10522 input (when listing or extracting), or writes it to standard output
10523 (when creating). If the @samp{-} file name is given when updating an
10524 archive, @command{tar} will read the original archive from its standard
10525 input, and will write the entire new archive to its standard output.
10527 If the file name contains a @samp{:}, it is interpreted as
10528 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
10529 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
10530 either case, @command{tar} will invoke the command @command{rsh} (or
10531 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
10532 machine. If you give an alternate login name, it will be given to the
10534 Naturally, the remote machine must have an executable
10535 @command{/usr/libexec/rmt}. This program is free software from the
10536 University of California, and a copy of the source code can be found
10537 with the sources for @command{tar}; it's compiled and installed by default.
10538 The exact path to this utility is determined when configuring the package.
10539 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
10540 your installation prefix. This location may also be overridden at
10541 runtime by using the @option{--rmt-command=@var{command}} option (@xref{Option Summary,
10542 ---rmt-command}, for detailed description of this option. @xref{Remote
10543 Tape Server}, for the description of @command{rmt} command).
10545 If this option is not given, but the environment variable @env{TAPE}
10546 is set, its value is used; otherwise, old versions of @command{tar}
10547 used a default archive name (which was picked when @command{tar} was
10548 compiled). The default is normally set up to be the @dfn{first} tape
10549 drive or other transportable I/O medium on the system.
10551 Starting with version 1.11.5, @GNUTAR{} uses
10552 standard input and standard output as the default device, and I will
10553 not try anymore supporting automatic device detection at installation
10554 time. This was failing really in too many cases, it was hopeless.
10555 This is now completely left to the installer to override standard
10556 input and standard output for default device, if this seems
10557 preferable. Further, I think @emph{most} actual usages of
10558 @command{tar} are done with pipes or disks, not really tapes,
10559 cartridges or diskettes.
10561 Some users think that using standard input and output is running
10562 after trouble. This could lead to a nasty surprise on your screen if
10563 you forget to specify an output file name---especially if you are going
10564 through a network or terminal server capable of buffering large amounts
10565 of output. We had so many bug reports in that area of configuring
10566 default tapes automatically, and so many contradicting requests, that
10567 we finally consider the problem to be portably intractable. We could
10568 of course use something like @samp{/dev/tape} as a default, but this
10569 is @emph{also} running after various kind of trouble, going from hung
10570 processes to accidental destruction of real tapes. After having seen
10571 all this mess, using standard input and output as a default really
10572 sounds like the only clean choice left, and a very useful one too.
10574 @GNUTAR{} reads and writes archive in records, I
10575 suspect this is the main reason why block devices are preferred over
10576 character devices. Most probably, block devices are more efficient
10577 too. The installer could also check for @samp{DEFTAPE} in
10578 @file{<sys/mtio.h>}.
10581 @xopindex{force-local, short description}
10582 @item --force-local
10583 Archive file is local even if it contains a colon.
10585 @opindex rsh-command
10586 @item --rsh-command=@var{command}
10587 Use remote @var{command} instead of @command{rsh}. This option exists
10588 so that people who use something other than the standard @command{rsh}
10589 (e.g., a Kerberized @command{rsh}) can access a remote device.
10591 When this command is not used, the shell command found when
10592 the @command{tar} program was installed is used instead. This is
10593 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
10594 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
10595 The installer may have overridden this by defining the environment
10596 variable @env{RSH} @emph{at installation time}.
10599 Specify drive and density.
10601 @xopindex{multi-volume, short description}
10603 @itemx --multi-volume
10604 Create/list/extract multi-volume archive.
10606 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
10607 that may be larger than will fit on the medium used to hold it.
10608 @xref{Multi-Volume Archives}.
10610 @xopindex{tape-length, short description}
10612 @itemx --tape-length=@var{size}[@var{suf}]
10613 Change tape after writing @var{size} units of data. Unless @var{suf} is
10614 given, @var{size} is treated as kilobytes, i.e. @samp{@var{size} x
10615 1024} bytes. The following suffixes alter this behavior:
10617 @float Table, size-suffixes
10618 @caption{Size Suffixes}
10619 @multitable @columnfractions 0.2 0.3 0.3
10620 @headitem Suffix @tab Units @tab Byte Equivalent
10621 @item b @tab Blocks @tab @var{size} x 512
10622 @item B @tab Kilobytes @tab @var{size} x 1024
10623 @item c @tab Bytes @tab @var{size}
10624 @item G @tab Gigabytes @tab @var{size} x 1024^3
10625 @item K @tab Kilobytes @tab @var{size} x 1024
10626 @item k @tab Kilobytes @tab @var{size} x 1024
10627 @item M @tab Megabytes @tab @var{size} x 1024^2
10628 @item P @tab Petabytes @tab @var{size} x 1024^5
10629 @item T @tab Terabytes @tab @var{size} x 1024^4
10630 @item w @tab Words @tab @var{size} x 2
10634 This option might be useful when your tape drivers do not properly
10635 detect end of physical tapes. By being slightly conservative on the
10636 maximum tape length, you might avoid the problem entirely.
10638 @xopindex{info-script, short description}
10639 @xopindex{new-volume-script, short description}
10640 @item -F @var{command}
10641 @itemx --info-script=@var{command}
10642 @itemx --new-volume-script=@var{command}
10643 Execute @var{command} at end of each tape. This implies
10644 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
10645 description of this option.
10648 @node Remote Tape Server
10649 @section Remote Tape Server
10651 @cindex remote tape drive
10653 In order to access the tape drive on a remote machine, @command{tar}
10654 uses the remote tape server written at the University of California at
10655 Berkeley. The remote tape server must be installed as
10656 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
10657 want to use. @command{tar} calls @command{rmt} by running an
10658 @command{rsh} or @command{remsh} to the remote machine, optionally
10659 using a different login name if one is supplied.
10661 A copy of the source for the remote tape server is provided. Its
10662 source code can be freely distributed. It is compiled and
10663 installed by default.
10665 @cindex absolute file names
10666 Unless you use the @option{--absolute-names} (@option{-P}) option,
10667 @GNUTAR{} will not allow you to create an archive that contains
10668 absolute file names (a file name beginning with @samp{/}.) If you try,
10669 @command{tar} will automatically remove the leading @samp{/} from the
10670 file names it stores in the archive. It will also type a warning
10671 message telling you what it is doing.
10673 When reading an archive that was created with a different
10674 @command{tar} program, @GNUTAR{} automatically
10675 extracts entries in the archive which have absolute file names as if
10676 the file names were not absolute. This is an important feature. A
10677 visitor here once gave a @command{tar} tape to an operator to restore;
10678 the operator used Sun @command{tar} instead of @GNUTAR{},
10679 and the result was that it replaced large portions of
10680 our @file{/bin} and friends with versions from the tape; needless to
10681 say, we were unhappy about having to recover the file system from
10684 For example, if the archive contained a file @file{/usr/bin/computoy},
10685 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
10686 relative to the current directory. If you want to extract the files in
10687 an archive to the same absolute names that they had when the archive
10688 was created, you should do a @samp{cd /} before extracting the files
10689 from the archive, or you should either use the @option{--absolute-names}
10690 option, or use the command @samp{tar -C / @dots{}}.
10692 @cindex Ultrix 3.1 and write failure
10693 Some versions of Unix (Ultrix 3.1 is known to have this problem),
10694 can claim that a short write near the end of a tape succeeded,
10695 when it actually failed. This will result in the -M option not
10696 working correctly. The best workaround at the moment is to use a
10697 significantly larger blocking factor than the default 20.
10699 In order to update an archive, @command{tar} must be able to backspace the
10700 archive in order to reread or rewrite a record that was just read (or
10701 written). This is currently possible only on two kinds of files: normal
10702 disk files (or any other file that can be backspaced with @samp{lseek}),
10703 and industry-standard 9-track magnetic tape (or any other kind of tape
10704 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}).
10706 This means that the @option{--append}, @option{--concatenate}, and
10707 @option{--delete} commands will not work on any other kind of file.
10708 Some media simply cannot be backspaced, which means these commands and
10709 options will never be able to work on them. These non-backspacing
10710 media include pipes and cartridge tape drives.
10712 Some other media can be backspaced, and @command{tar} will work on them
10713 once @command{tar} is modified to do so.
10715 Archives created with the @option{--multi-volume}, @option{--label}, and
10716 @option{--incremental} (@option{-G}) options may not be readable by other version
10717 of @command{tar}. In particular, restoring a file that was split over
10718 a volume boundary will require some careful work with @command{dd}, if
10719 it can be done at all. Other versions of @command{tar} may also create
10720 an empty file whose name is that of the volume header. Some versions
10721 of @command{tar} may create normal files instead of directories archived
10722 with the @option{--incremental} (@option{-G}) option.
10724 @node Common Problems and Solutions
10725 @section Some Common Problems and their Solutions
10730 errors from system:
10732 no such file or directory
10735 errors from @command{tar}:
10736 directory checksum error
10737 header format error
10739 errors from media/system:
10751 @dfn{Block} and @dfn{record} terminology is rather confused, and it
10752 is also confusing to the expert reader. On the other hand, readers
10753 who are new to the field have a fresh mind, and they may safely skip
10754 the next two paragraphs, as the remainder of this manual uses those
10755 two terms in a quite consistent way.
10757 John Gilmore, the writer of the public domain @command{tar} from which
10758 @GNUTAR{} was originally derived, wrote (June 1995):
10761 The nomenclature of tape drives comes from IBM, where I believe
10762 they were invented for the IBM 650 or so. On IBM mainframes, what
10763 is recorded on tape are tape blocks. The logical organization of
10764 data is into records. There are various ways of putting records into
10765 blocks, including @code{F} (fixed sized records), @code{V} (variable
10766 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
10767 to a block), @code{VB} (variable size records, @var{n} to a block),
10768 @code{VSB} (variable spanned blocked: variable sized records that can
10769 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
10770 parameter specified this to the operating system.
10772 The Unix man page on @command{tar} was totally confused about this.
10773 When I wrote @code{PD TAR}, I used the historically correct terminology
10774 (@command{tar} writes data records, which are grouped into blocks).
10775 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
10776 here), and now Fran@,{c}ois has migrated that terminology back
10777 into the source code too.
10780 The term @dfn{physical block} means the basic transfer chunk from or
10781 to a device, after which reading or writing may stop without anything
10782 being lost. In this manual, the term @dfn{block} usually refers to
10783 a disk physical block, @emph{assuming} that each disk block is 512
10784 bytes in length. It is true that some disk devices have different
10785 physical blocks, but @command{tar} ignore these differences in its own
10786 format, which is meant to be portable, so a @command{tar} block is always
10787 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
10788 The term @dfn{logical block} often represents the basic chunk of
10789 allocation of many disk blocks as a single entity, which the operating
10790 system treats somewhat atomically; this concept is only barely used
10793 The term @dfn{physical record} is another way to speak of a physical
10794 block, those two terms are somewhat interchangeable. In this manual,
10795 the term @dfn{record} usually refers to a tape physical block,
10796 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
10797 It is true that archives may be put on disk or used with pipes,
10798 but nevertheless, @command{tar} tries to read and write the archive one
10799 @dfn{record} at a time, whatever the medium in use. One record is made
10800 up of an integral number of blocks, and this operation of putting many
10801 disk blocks into a single tape block is called @dfn{reblocking}, or
10802 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
10803 the logical organization of many characters into something meaningful
10804 to the application. The term @dfn{unit record} describes a small set
10805 of characters which are transmitted whole to or by the application,
10806 and often refers to a line of text. Those two last terms are unrelated
10807 to what we call a @dfn{record} in @GNUTAR{}.
10809 When writing to tapes, @command{tar} writes the contents of the archive
10810 in chunks known as @dfn{records}. To change the default blocking
10811 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
10812 @var{512-size}}) option. Each record will then be composed of
10813 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
10814 @xref{Standard}.) Each file written to the archive uses at least one
10815 full record. As a result, using a larger record size can result in
10816 more wasted space for small files. On the other hand, a larger record
10817 size can often be read and written much more efficiently.
10819 Further complicating the problem is that some tape drives ignore the
10820 blocking entirely. For these, a larger record size can still improve
10821 performance (because the software layers above the tape drive still
10822 honor the blocking), but not as dramatically as on tape drives that
10825 When reading an archive, @command{tar} can usually figure out the
10826 record size on itself. When this is the case, and a non-standard
10827 record size was used when the archive was created, @command{tar} will
10828 print a message about a non-standard blocking factor, and then operate
10829 normally@footnote{If this message is not needed, you can turn it off
10830 using the @option{--warning=no-record-size} option.}. On some tape
10831 devices, however, @command{tar} cannot figure out the record size
10832 itself. On most of those, you can specify a blocking factor (with
10833 @option{--blocking-factor}) larger than the actual blocking factor,
10834 and then use the @option{--read-full-records} (@option{-B}) option.
10835 (If you specify a blocking factor with @option{--blocking-factor} and
10836 don't use the @option{--read-full-records} option, then @command{tar}
10837 will not attempt to figure out the recording size itself.) On some
10838 devices, you must always specify the record size exactly with
10839 @option{--blocking-factor} when reading, because @command{tar} cannot
10840 figure it out. In any case, use @option{--list} (@option{-t}) before
10841 doing any extractions to see whether @command{tar} is reading the archive
10844 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
10845 putting them into records is to put a whole number of them (one or
10846 more) into each record. @command{tar} records are all the same size;
10847 at the end of the file there's a block containing all zeros, which
10848 is how you tell that the remainder of the last record(s) are garbage.
10850 In a standard @command{tar} file (no options), the block size is 512
10851 and the record size is 10240, for a blocking factor of 20. What the
10852 @option{--blocking-factor} option does is sets the blocking factor,
10853 changing the record size while leaving the block size at 512 bytes.
10854 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
10855 most tape drives these days prefer much bigger records in order to
10856 stream and not waste tape. When writing tapes for myself, some tend
10857 to use a factor of the order of 2048, say, giving a record size of
10858 around one megabyte.
10860 If you use a blocking factor larger than 20, older @command{tar}
10861 programs might not be able to read the archive, so we recommend this
10862 as a limit to use in practice. @GNUTAR{}, however,
10863 will support arbitrarily large record sizes, limited only by the
10864 amount of virtual memory or the physical characteristics of the tape
10868 * Format Variations:: Format Variations
10869 * Blocking Factor:: The Blocking Factor of an Archive
10872 @node Format Variations
10873 @subsection Format Variations
10874 @cindex Format Parameters
10875 @cindex Format Options
10876 @cindex Options, archive format specifying
10877 @cindex Options, format specifying
10880 Format parameters specify how an archive is written on the archive
10881 media. The best choice of format parameters will vary depending on
10882 the type and number of files being archived, and on the media used to
10885 To specify format parameters when accessing or creating an archive,
10886 you can use the options described in the following sections.
10887 If you do not specify any format parameters, @command{tar} uses
10888 default parameters. You cannot modify a compressed archive.
10889 If you create an archive with the @option{--blocking-factor} option
10890 specified (@pxref{Blocking Factor}), you must specify that
10891 blocking-factor when operating on the archive. @xref{Formats}, for other
10892 examples of format parameter considerations.
10894 @node Blocking Factor
10895 @subsection The Blocking Factor of an Archive
10896 @cindex Blocking Factor
10897 @cindex Record Size
10898 @cindex Number of blocks per record
10899 @cindex Number of bytes per record
10900 @cindex Bytes per record
10901 @cindex Blocks per record
10904 @opindex blocking-factor
10905 The data in an archive is grouped into blocks, which are 512 bytes.
10906 Blocks are read and written in whole number multiples called
10907 @dfn{records}. The number of blocks in a record (i.e., the size of a
10908 record in units of 512 bytes) is called the @dfn{blocking factor}.
10909 The @option{--blocking-factor=@var{512-size}} (@option{-b
10910 @var{512-size}}) option specifies the blocking factor of an archive.
10911 The default blocking factor is typically 20 (i.e., 10240 bytes), but
10912 can be specified at installation. To find out the blocking factor of
10913 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
10914 This may not work on some devices.
10916 Records are separated by gaps, which waste space on the archive media.
10917 If you are archiving on magnetic tape, using a larger blocking factor
10918 (and therefore larger records) provides faster throughput and allows you
10919 to fit more data on a tape (because there are fewer gaps). If you are
10920 archiving on cartridge, a very large blocking factor (say 126 or more)
10921 greatly increases performance. A smaller blocking factor, on the other
10922 hand, may be useful when archiving small files, to avoid archiving lots
10923 of nulls as @command{tar} fills out the archive to the end of the record.
10924 In general, the ideal record size depends on the size of the
10925 inter-record gaps on the tape you are using, and the average size of the
10926 files you are archiving. @xref{create}, for information on
10929 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
10931 Archives with blocking factors larger than 20 cannot be read
10932 by very old versions of @command{tar}, or by some newer versions
10933 of @command{tar} running on old machines with small address spaces.
10934 With @GNUTAR{}, the blocking factor of an archive is limited
10935 only by the maximum record size of the device containing the archive,
10936 or by the amount of available virtual memory.
10938 Also, on some systems, not using adequate blocking factors, as sometimes
10939 imposed by the device drivers, may yield unexpected diagnostics. For
10940 example, this has been reported:
10943 Cannot write to /dev/dlt: Invalid argument
10947 In such cases, it sometimes happen that the @command{tar} bundled by
10948 the system is aware of block size idiosyncrasies, while @GNUTAR{}
10949 requires an explicit specification for the block size,
10950 which it cannot guess. This yields some people to consider
10951 @GNUTAR{} is misbehaving, because by comparison,
10952 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
10953 for example, might resolve the problem.
10955 If you use a non-default blocking factor when you create an archive, you
10956 must specify the same blocking factor when you modify that archive. Some
10957 archive devices will also require you to specify the blocking factor when
10958 reading that archive, however this is not typically the case. Usually, you
10959 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
10960 reports a non-default record size and then lists the archive members as
10961 it would normally. To extract files from an archive with a non-standard
10962 blocking factor (particularly if you're not sure what the blocking factor
10963 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
10964 specifying a blocking factor larger then the blocking factor of the archive
10965 (i.e., @samp{tar --extract --read-full-records --blocking-factor=300}).
10966 @xref{list}, for more information on the @option{--list} (@option{-t})
10967 operation. @xref{Reading}, for a more detailed explanation of that option.
10970 @item --blocking-factor=@var{number}
10971 @itemx -b @var{number}
10972 Specifies the blocking factor of an archive. Can be used with any
10973 operation, but is usually not necessary with @option{--list} (@option{-t}).
10979 @item -b @var{blocks}
10980 @itemx --blocking-factor=@var{blocks}
10981 Set record size to @math{@var{blocks}*512} bytes.
10983 This option is used to specify a @dfn{blocking factor} for the archive.
10984 When reading or writing the archive, @command{tar}, will do reads and writes
10985 of the archive in records of @math{@var{block}*512} bytes. This is true
10986 even when the archive is compressed. Some devices requires that all
10987 write operations be a multiple of a certain size, and so, @command{tar}
10988 pads the archive out to the next record boundary.
10990 The default blocking factor is set when @command{tar} is compiled, and is
10991 typically 20. Blocking factors larger than 20 cannot be read by very
10992 old versions of @command{tar}, or by some newer versions of @command{tar}
10993 running on old machines with small address spaces.
10995 With a magnetic tape, larger records give faster throughput and fit
10996 more data on a tape (because there are fewer inter-record gaps).
10997 If the archive is in a disk file or a pipe, you may want to specify
10998 a smaller blocking factor, since a large one will result in a large
10999 number of null bytes at the end of the archive.
11001 When writing cartridge or other streaming tapes, a much larger
11002 blocking factor (say 126 or more) will greatly increase performance.
11003 However, you must specify the same blocking factor when reading or
11004 updating the archive.
11006 Apparently, Exabyte drives have a physical block size of 8K bytes.
11007 If we choose our blocksize as a multiple of 8k bytes, then the problem
11008 seems to disappear. Id est, we are using block size of 112 right
11009 now, and we haven't had the problem since we switched@dots{}
11011 With @GNUTAR{} the blocking factor is limited only
11012 by the maximum record size of the device containing the archive, or by
11013 the amount of available virtual memory.
11015 However, deblocking or reblocking is virtually avoided in a special
11016 case which often occurs in practice, but which requires all the
11017 following conditions to be simultaneously true:
11020 the archive is subject to a compression option,
11022 the archive is not handled through standard input or output, nor
11023 redirected nor piped,
11025 the archive is directly handled to a local disk, instead of any special
11028 @option{--blocking-factor} is not explicitly specified on the @command{tar}
11032 If the output goes directly to a local disk, and not through
11033 stdout, then the last write is not extended to a full record size.
11034 Otherwise, reblocking occurs. Here are a few other remarks on this
11040 @command{gzip} will complain about trailing garbage if asked to
11041 uncompress a compressed archive on tape, there is an option to turn
11042 the message off, but it breaks the regularity of simply having to use
11043 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
11044 silently ignoring any number of trailing zeros. I'll ask Jean-loup
11045 Gailly, by sending a copy of this message to him.
11048 @command{compress} does not show this problem, but as Jean-loup pointed
11049 out to Michael, @samp{compress -d} silently adds garbage after
11050 the result of decompression, which tar ignores because it already
11051 recognized its end-of-file indicator. So this bug may be safely
11055 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
11056 but will still return an exit status of 2 which tar reports in turn.
11057 @command{tar} might ignore the exit status returned, but I hate doing
11058 that, as it weakens the protection @command{tar} offers users against
11059 other possible problems at decompression time. If @command{gzip} was
11060 silently skipping trailing zeros @emph{and} also avoiding setting the
11061 exit status in this innocuous case, that would solve this situation.
11064 @command{tar} should become more solid at not stopping to read a pipe at
11065 the first null block encountered. This inelegantly breaks the pipe.
11066 @command{tar} should rather drain the pipe out before exiting itself.
11069 @xopindex{ignore-zeros, short description}
11071 @itemx --ignore-zeros
11072 Ignore blocks of zeros in archive (means EOF).
11074 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
11075 of zeros in the archive. Normally a block of zeros indicates the
11076 end of the archive, but when reading a damaged archive, or one which
11077 was created by concatenating several archives together, this option
11078 allows @command{tar} to read the entire archive. This option is not on
11079 by default because many versions of @command{tar} write garbage after
11082 Note that this option causes @command{tar} to read to the end of the
11083 archive file, which may sometimes avoid problems when multiple files
11084 are stored on a single physical tape.
11086 @xopindex{read-full-records, short description}
11088 @itemx --read-full-records
11089 Reblock as we read (for reading 4.2@acronym{BSD} pipes).
11091 If @option{--read-full-records} is used, @command{tar}
11092 will not panic if an attempt to read a record from the archive does
11093 not return a full record. Instead, @command{tar} will keep reading
11094 until it has obtained a full
11097 This option is turned on by default when @command{tar} is reading
11098 an archive from standard input, or from a remote machine. This is
11099 because on @acronym{BSD} Unix systems, a read of a pipe will return however
11100 much happens to be in the pipe, even if it is less than @command{tar}
11101 requested. If this option was not used, @command{tar} would fail as
11102 soon as it read an incomplete record from the pipe.
11104 This option is also useful with the commands for updating an archive.
11110 @FIXME{Appropriate options should be moved here from elsewhere.}
11112 @cindex blocking factor
11113 @cindex tape blocking
11115 When handling various tapes or cartridges, you have to take care of
11116 selecting a proper blocking, that is, the number of disk blocks you
11117 put together as a single tape block on the tape, without intervening
11118 tape gaps. A @dfn{tape gap} is a small landing area on the tape
11119 with no information on it, used for decelerating the tape to a
11120 full stop, and for later regaining the reading or writing speed.
11121 When the tape driver starts reading a record, the record has to
11122 be read whole without stopping, as a tape gap is needed to stop the
11123 tape motion without losing information.
11125 @cindex Exabyte blocking
11126 @cindex DAT blocking
11127 Using higher blocking (putting more disk blocks per tape block) will use
11128 the tape more efficiently as there will be less tape gaps. But reading
11129 such tapes may be more difficult for the system, as more memory will be
11130 required to receive at once the whole record. Further, if there is a
11131 reading error on a huge record, this is less likely that the system will
11132 succeed in recovering the information. So, blocking should not be too
11133 low, nor it should be too high. @command{tar} uses by default a blocking of
11134 20 for historical reasons, and it does not really matter when reading or
11135 writing to disk. Current tape technology would easily accommodate higher
11136 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
11137 We were told that for some DLT drives, the blocking should be a multiple
11138 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
11139 Other manufacturers may use different recommendations for the same tapes.
11140 This might also depends of the buffering techniques used inside modern
11141 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
11142 Others request blocking to be some exponent of two.
11144 So, there is no fixed rule for blocking. But blocking at read time
11145 should ideally be the same as blocking used at write time. At one place
11146 I know, with a wide variety of equipment, they found it best to use a
11147 blocking of 32 to guarantee that their tapes are fully interchangeable.
11149 I was also told that, for recycled tapes, prior erasure (by the same
11150 drive unit that will be used to create the archives) sometimes lowers
11151 the error rates observed at rewriting time.
11153 I might also use @option{--number-blocks} instead of
11154 @option{--block-number}, so @option{--block} will then expand to
11155 @option{--blocking-factor} unambiguously.
11158 @section Many Archives on One Tape
11160 @FIXME{Appropriate options should be moved here from elsewhere.}
11162 @findex ntape @r{device}
11163 Most tape devices have two entries in the @file{/dev} directory, or
11164 entries that come in pairs, which differ only in the minor number for
11165 this device. Let's take for example @file{/dev/tape}, which often
11166 points to the only or usual tape device of a given system. There might
11167 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
11168 name is the @emph{rewinding} version of the device, while the name
11169 having @samp{nr} in it is the @emph{no rewinding} version of the same
11172 A rewinding tape device will bring back the tape to its beginning point
11173 automatically when this device is opened or closed. Since @command{tar}
11174 opens the archive file before using it and closes it afterwards, this
11175 means that a simple:
11178 $ @kbd{tar cf /dev/tape @var{directory}}
11182 will reposition the tape to its beginning both prior and after saving
11183 @var{directory} contents to it, thus erasing prior tape contents and
11184 making it so that any subsequent write operation will destroy what has
11187 @cindex tape positioning
11188 So, a rewinding device is normally meant to hold one and only one file.
11189 If you want to put more than one @command{tar} archive on a given tape, you
11190 will need to avoid using the rewinding version of the tape device. You
11191 will also have to pay special attention to tape positioning. Errors in
11192 positioning may overwrite the valuable data already on your tape. Many
11193 people, burnt by past experiences, will only use rewinding devices and
11194 limit themselves to one file per tape, precisely to avoid the risk of
11195 such errors. Be fully aware that writing at the wrong position on a
11196 tape loses all information past this point and most probably until the
11197 end of the tape, and this destroyed information @emph{cannot} be
11200 To save @var{directory-1} as a first archive at the beginning of a
11201 tape, and leave that tape ready for a second archive, you should use:
11204 $ @kbd{mt -f /dev/nrtape rewind}
11205 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
11209 @dfn{Tape marks} are special magnetic patterns written on the tape
11210 media, which are later recognizable by the reading hardware. These
11211 marks are used after each file, when there are many on a single tape.
11212 An empty file (that is to say, two tape marks in a row) signal the
11213 logical end of the tape, after which no file exist. Usually,
11214 non-rewinding tape device drivers will react to the close request issued
11215 by @command{tar} by first writing two tape marks after your archive, and by
11216 backspacing over one of these. So, if you remove the tape at that time
11217 from the tape drive, it is properly terminated. But if you write
11218 another file at the current position, the second tape mark will be
11219 erased by the new information, leaving only one tape mark between files.
11221 So, you may now save @var{directory-2} as a second archive after the
11222 first on the same tape by issuing the command:
11225 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
11229 and so on for all the archives you want to put on the same tape.
11231 Another usual case is that you do not write all the archives the same
11232 day, and you need to remove and store the tape between two archive
11233 sessions. In general, you must remember how many files are already
11234 saved on your tape. Suppose your tape already has 16 files on it, and
11235 that you are ready to write the 17th. You have to take care of skipping
11236 the first 16 tape marks before saving @var{directory-17}, say, by using
11240 $ @kbd{mt -f /dev/nrtape rewind}
11241 $ @kbd{mt -f /dev/nrtape fsf 16}
11242 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
11245 In all the previous examples, we put aside blocking considerations, but
11246 you should do the proper things for that as well. @xref{Blocking}.
11249 * Tape Positioning:: Tape Positions and Tape Marks
11250 * mt:: The @command{mt} Utility
11253 @node Tape Positioning
11254 @subsection Tape Positions and Tape Marks
11257 Just as archives can store more than one file from the file system,
11258 tapes can store more than one archive file. To keep track of where
11259 archive files (or any other type of file stored on tape) begin and
11260 end, tape archive devices write magnetic @dfn{tape marks} on the
11261 archive media. Tape drives write one tape mark between files,
11262 two at the end of all the file entries.
11264 If you think of data as a series of records "rrrr"'s, and tape marks as
11265 "*"'s, a tape might look like the following:
11268 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
11271 Tape devices read and write tapes using a read/write @dfn{tape
11272 head}---a physical part of the device which can only access one
11273 point on the tape at a time. When you use @command{tar} to read or
11274 write archive data from a tape device, the device will begin reading
11275 or writing from wherever on the tape the tape head happens to be,
11276 regardless of which archive or what part of the archive the tape
11277 head is on. Before writing an archive, you should make sure that no
11278 data on the tape will be overwritten (unless it is no longer needed).
11279 Before reading an archive, you should make sure the tape head is at
11280 the beginning of the archive you want to read. You can do it manually
11281 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
11282 that automatically (@pxref{Scripted Restoration}).
11284 If you want to add new archive file entries to a tape, you should
11285 advance the tape to the end of the existing file entries, backspace
11286 over the last tape mark, and write the new archive file. If you were
11287 to add two archives to the example above, the tape might look like the
11291 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
11295 @subsection The @command{mt} Utility
11298 @FIXME{Is it true that this only works on non-block devices?
11299 should explain the difference, (fixed or variable).}
11300 @xref{Blocking Factor}.
11302 You can use the @command{mt} utility to advance or rewind a tape past a
11303 specified number of archive files on the tape. This will allow you
11304 to move to the beginning of an archive before extracting or reading
11305 it, or to the end of all the archives before writing a new one.
11306 @FIXME{Why isn't there an "advance 'til you find two tape marks
11309 The syntax of the @command{mt} command is:
11312 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
11315 where @var{tapename} is the name of the tape device, @var{number} is
11316 the number of times an operation is performed (with a default of one),
11317 and @var{operation} is one of the following:
11319 @FIXME{is there any use for record operations?}
11324 Writes @var{number} tape marks at the current position on the tape.
11327 Moves tape position forward @var{number} files.
11330 Moves tape position back @var{number} files.
11333 Rewinds the tape. (Ignores @var{number}.)
11337 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}.)
11340 Prints status information about the tape unit.
11344 If you don't specify a @var{tapename}, @command{mt} uses the environment
11345 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
11346 the default device specified in your @file{sys/mtio.h} file
11347 (@code{DEFTAPE} variable). If this is not defined, the program will
11348 display a descriptive error message and exit with code 1.
11350 @command{mt} returns a 0 exit status when the operation(s) were
11351 successful, 1 if the command was unrecognized, and 2 if an operation
11354 @node Using Multiple Tapes
11355 @section Using Multiple Tapes
11357 Often you might want to write a large archive, one larger than will fit
11358 on the actual tape you are using. In such a case, you can run multiple
11359 @command{tar} commands, but this can be inconvenient, particularly if you
11360 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
11361 Therefore, @command{tar} provides a special mode for creating
11362 multi-volume archives.
11364 @dfn{Multi-volume} archive is a single @command{tar} archive, stored
11365 on several media volumes of fixed size. Although in this section we will
11366 often call @samp{volume} a @dfn{tape}, there is absolutely no
11367 requirement for multi-volume archives to be stored on tapes. Instead,
11368 they can use whatever media type the user finds convenient, they can
11369 even be located on files.
11371 When creating a multi-volume archive, @GNUTAR{} continues to fill
11372 current volume until it runs out of space, then it switches to
11373 next volume (usually the operator is queried to replace the tape on
11374 this point), and continues working on the new volume. This operation
11375 continues until all requested files are dumped. If @GNUTAR{} detects
11376 end of media while dumping a file, such a file is archived in split
11377 form. Some very big files can even be split across several volumes.
11379 Each volume is itself a valid @GNUTAR{} archive, so it can be read
11380 without any special options. Consequently any file member residing
11381 entirely on one volume can be extracted or otherwise operated upon
11382 without needing the other volume. Sure enough, to extract a split
11383 member you would need all volumes its parts reside on.
11385 Multi-volume archives suffer from several limitations. In particular,
11386 they cannot be compressed.
11388 @GNUTAR{} is able to create multi-volume archives of two formats
11389 (@pxref{Formats}): @samp{GNU} and @samp{POSIX}.
11392 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
11393 * Tape Files:: Tape Files
11394 * Tarcat:: Concatenate Volumes into a Single Archive
11398 @node Multi-Volume Archives
11399 @subsection Archives Longer than One Tape or Disk
11400 @cindex Multi-volume archives
11402 @opindex multi-volume
11403 To create an archive that is larger than will fit on a single unit of
11404 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
11405 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
11406 archive can be manipulated like any other archive (provided the
11407 @option{--multi-volume} option is specified), but is stored on more
11408 than one tape or file.
11410 When you specify @option{--multi-volume}, @command{tar} does not report an
11411 error when it comes to the end of an archive volume (when reading), or
11412 the end of the media (when writing). Instead, it prompts you to load
11413 a new storage volume. If the archive is on a magnetic tape, you
11414 should change tapes when you see the prompt; if the archive is on a
11415 floppy disk, you should change disks; etc.
11418 @item --multi-volume
11420 Creates a multi-volume archive, when used in conjunction with
11421 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
11422 archive, specify @option{--multi-volume} in conjunction with that
11427 $ @kbd{tar --create --multi-volume --file=/dev/tape @var{files}}
11431 The method @command{tar} uses to detect end of tape is not perfect, and
11432 fails on some operating systems or on some devices. If @command{tar}
11433 cannot detect the end of the tape itself, you can use
11434 @option{--tape-length} option to inform it about the capacity of the
11437 @anchor{tape-length}
11439 @opindex tape-length
11440 @item --tape-length=@var{size}[@var{suf}]
11441 @itemx -L @var{size}[@var{suf}]
11442 Set maximum length of a volume. The @var{suf}, if given, specifies
11443 units in which @var{size} is expressed, e.g. @samp{2M} mean 2
11444 megabytes (@pxref{size-suffixes}, for a list of allowed size
11445 suffixes). Without @var{suf}, units of 1024 bytes (kilobyte) are
11448 This option selects @option{--multi-volume} automatically. For example:
11451 $ @kbd{tar --create --tape-length=41943040 --file=/dev/tape @var{files}}
11455 or, which is equivalent:
11458 $ @kbd{tar --create --tape-length=4G --file=/dev/tape @var{files}}
11462 @anchor{change volume prompt}
11463 When @GNUTAR{} comes to the end of a storage media, it asks you to
11464 change the volume. The built-in prompt for POSIX locale
11465 is@footnote{If you run @GNUTAR{} under a different locale, the
11466 translation to the locale's language will be used.}:
11469 Prepare volume #@var{n} for '@var{archive}' and hit return:
11473 where @var{n} is the ordinal number of the volume to be created and
11474 @var{archive} is archive file or device name.
11476 When prompting for a new tape, @command{tar} accepts any of the following
11481 Request @command{tar} to explain possible responses.
11483 Request @command{tar} to exit immediately.
11484 @item n @var{file-name}
11485 Request @command{tar} to write the next volume on the file @var{file-name}.
11487 Request @command{tar} to run a subshell. This option can be disabled
11488 by giving @option{--restrict} command line option to
11489 @command{tar}@footnote{@xref{--restrict}, for more information about
11492 Request @command{tar} to begin writing the next volume.
11495 (You should only type @samp{y} after you have changed the tape;
11496 otherwise @command{tar} will write over the volume it just finished.)
11498 @cindex Volume number file
11500 @anchor{volno-file}
11501 @opindex volno-file
11502 The volume number used by @command{tar} in its tape-changing prompt
11503 can be changed; if you give the
11504 @option{--volno-file=@var{file-of-number}} option, then
11505 @var{file-of-number} should be an non-existing file to be created, or
11506 else, a file already containing a decimal number. That number will be
11507 used as the volume number of the first volume written. When
11508 @command{tar} is finished, it will rewrite the file with the
11509 now-current volume number. (This does not change the volume number
11510 written on a tape label, as per @ref{label}, it @emph{only} affects
11511 the number used in the prompt.)
11513 @cindex End-of-archive info script
11514 @cindex Info script
11515 @anchor{info-script}
11516 @opindex info-script
11517 @opindex new-volume-script
11518 If you want more elaborate behavior than this, you can write a special
11519 @dfn{new volume script}, that will be responsible for changing the
11520 volume, and instruct @command{tar} to use it instead of its normal
11521 prompting procedure:
11524 @item --info-script=@var{command}
11525 @itemx --new-volume-script=@var{command}
11526 @itemx -F @var{command}
11527 Specify the command to invoke when switching volumes. The @var{command}
11528 can be used to eject cassettes, or to broadcast messages such as
11529 @samp{Someone please come change my tape} when performing unattended
11533 The @var{command} can contain additional options, if such are needed.
11534 @xref{external, Running External Commands}, for a detailed discussion
11535 of the way @GNUTAR{} runs external commands. It inherits
11536 @command{tar}'s shell environment. Additional data is passed to it
11537 via the following environment variables:
11540 @vrindex TAR_VERSION, info script environment variable
11542 @GNUTAR{} version number.
11544 @vrindex TAR_ARCHIVE, info script environment variable
11546 The name of the archive @command{tar} is processing.
11548 @vrindex TAR_BLOCKING_FACTOR, info script environment variable
11549 @item TAR_BLOCKING_FACTOR
11550 Current blocking factor (@pxref{Blocking}).
11552 @vrindex TAR_VOLUME, info script environment variable
11554 Ordinal number of the volume @command{tar} is about to start.
11556 @vrindex TAR_SUBCOMMAND, info script environment variable
11557 @item TAR_SUBCOMMAND
11558 A short option describing the operation @command{tar} is executing.
11559 @xref{Operations}, for a complete list of subcommand options.
11561 @vrindex TAR_FORMAT, info script environment variable
11563 Format of the archive being processed. @xref{Formats}, for a complete
11564 list of archive format names.
11566 @vrindex TAR_FD, info script environment variable
11568 File descriptor which can be used to communicate the new volume
11569 name to @command{tar}.
11572 These variables can be used in the @var{command} itself, provided that
11573 they are properly quoted to prevent them from being expanded by the
11574 shell that invokes @command{tar}.
11576 The volume script can instruct @command{tar} to use new archive name,
11577 by writing in to file descriptor @env{$TAR_FD} (see below for an example).
11579 If the info script fails, @command{tar} exits; otherwise, it begins
11580 writing the next volume.
11582 If you want @command{tar} to cycle through a series of files or tape
11583 drives, there are three approaches to choose from. First of all, you
11584 can give @command{tar} multiple @option{--file} options. In this case
11585 the specified files will be used, in sequence, as the successive
11586 volumes of the archive. Only when the first one in the sequence needs
11587 to be used again will @command{tar} prompt for a tape change (or run
11588 the info script). For example, suppose someone has two tape drives on
11589 a system named @file{/dev/tape0} and @file{/dev/tape1}. For having
11590 @GNUTAR{} to switch to the second drive when it needs to write the
11591 second tape, and then back to the first tape, etc., just do either of:
11594 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
11595 $ @kbd{tar -cM -f /dev/tape0 -f /dev/tape1 @var{files}}
11598 The second method is to use the @samp{n} response to the tape-change
11601 Finally, the most flexible approach is to use a volume script, that
11602 writes new archive name to the file descriptor @env{$TAR_FD}. For example, the
11603 following volume script will create a series of archive files, named
11604 @file{@var{archive}-@var{vol}}, where @var{archive} is the name of the
11605 archive being created (as given by @option{--file} option) and
11606 @var{vol} is the ordinal number of the archive being created:
11611 # For this script it's advisable to use a shell, such as Bash,
11612 # that supports a TAR_FD value greater than 9.
11614 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
11616 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
11617 case $TAR_SUBCOMMAND in
11619 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
11624 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&$TAR_FD
11628 The same script can be used while listing, comparing or extracting
11629 from the created archive. For example:
11633 # @r{Create a multi-volume archive:}
11634 $ @kbd{tar -c -L1024 -f archive.tar -F new-volume .}
11635 # @r{Extract from the created archive:}
11636 $ @kbd{tar -x -f archive.tar -F new-volume .}
11641 Notice, that the first command had to use @option{-L} option, since
11642 otherwise @GNUTAR{} will end up writing everything to file
11643 @file{archive.tar}.
11645 You can read each individual volume of a multi-volume archive as if it
11646 were an archive by itself. For example, to list the contents of one
11647 volume, use @option{--list}, without @option{--multi-volume} specified.
11648 To extract an archive member from one volume (assuming it is described
11649 that volume), use @option{--extract}, again without
11650 @option{--multi-volume}.
11652 If an archive member is split across volumes (i.e., its entry begins on
11653 one volume of the media and ends on another), you need to specify
11654 @option{--multi-volume} to extract it successfully. In this case, you
11655 should load the volume where the archive member starts, and use
11656 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
11657 volumes as it needs them. @xref{extracting archives}, for more
11658 information about extracting archives.
11660 Multi-volume archives can be modified like any other archive. To add
11661 files to a multi-volume archive, you need to only mount the last
11662 volume of the archive media (and new volumes, if needed). For all
11663 other operations, you need to use the entire archive.
11665 If a multi-volume archive was labeled using
11666 @option{--label=@var{archive-label}} (@pxref{label}) when it was
11667 created, @command{tar} will not automatically label volumes which are
11668 added later. To label subsequent volumes, specify
11669 @option{--label=@var{archive-label}} again in conjunction with the
11670 @option{--append}, @option{--update} or @option{--concatenate} operation.
11672 Notice that multi-volume support is a GNU extension and the archives
11673 created in this mode should be read only using @GNUTAR{}. If you
11674 absolutely have to process such archives using a third-party @command{tar}
11675 implementation, read @ref{Split Recovery}.
11678 @subsection Tape Files
11679 @cindex labeling archives
11683 To give the archive a name which will be recorded in it, use the
11684 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
11685 option. This will write a special block identifying
11686 @var{volume-label} as the name of the archive to the front of the
11687 archive which will be displayed when the archive is listed with
11688 @option{--list}. If you are creating a multi-volume archive with
11689 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
11690 volume label will have @samp{Volume @var{nnn}} appended to the name
11691 you give, where @var{nnn} is the number of the volume of the archive.
11692 If you use the @option{--label=@var{volume-label}} option when
11693 reading an archive, it checks to make sure the label on the tape
11694 matches the one you gave. @xref{label}.
11696 When @command{tar} writes an archive to tape, it creates a single
11697 tape file. If multiple archives are written to the same tape, one
11698 after the other, they each get written as separate tape files. When
11699 extracting, it is necessary to position the tape at the right place
11700 before running @command{tar}. To do this, use the @command{mt} command.
11701 For more information on the @command{mt} command and on the organization
11702 of tapes into a sequence of tape files, see @ref{mt}.
11704 People seem to often do:
11707 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
11710 or such, for pushing a common date in all volumes or an archive set.
11713 @subsection Concatenate Volumes into a Single Archive
11716 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
11717 archive to a single @command{tar} archive. Simply concatenating all
11718 volumes into one will not work, since each volume carries an additional
11719 information at the beginning. @GNUTAR{} is shipped with the shell
11720 script @command{tarcat} designed for this purpose.
11722 The script takes a list of files comprising a multi-volume archive
11723 and creates the resulting archive at the standard output. For example:
11726 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
11729 The script implements a simple heuristics to determine the format of
11730 the first volume file and to decide how to process the rest of the
11731 files. However, it makes no attempt to verify whether the files are
11732 given in order or even if they are valid @command{tar} archives.
11733 It uses @command{dd} and does not filter its standard error, so you
11734 will usually see lots of spurious messages.
11736 @FIXME{The script is not installed. Should we install it?}
11739 @section Including a Label in the Archive
11740 @cindex Labeling an archive
11741 @cindex Labels on the archive media
11742 @cindex Labeling multi-volume archives
11745 To avoid problems caused by misplaced paper labels on the archive
11746 media, you can include a @dfn{label} entry --- an archive member which
11747 contains the name of the archive --- in the archive itself. Use the
11748 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11749 option@footnote{Until version 1.10, that option was called
11750 @option{--volume}, but is not available under that name anymore.} in
11751 conjunction with the @option{--create} operation to include a label
11752 entry in the archive as it is being created.
11755 @item --label=@var{archive-label}
11756 @itemx -V @var{archive-label}
11757 Includes an @dfn{archive-label} at the beginning of the archive when
11758 the archive is being created, when used in conjunction with the
11759 @option{--create} operation. Checks to make sure the archive label
11760 matches the one specified (when used in conjunction with any other
11764 If you create an archive using both
11765 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11766 and @option{--multi-volume} (@option{-M}), each volume of the archive
11767 will have an archive label of the form @samp{@var{archive-label}
11768 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
11769 next, and so on. @xref{Using Multiple Tapes}, for information on
11770 creating multiple volume archives.
11772 @cindex Volume label, listing
11773 @cindex Listing volume label
11774 The volume label will be displayed by @option{--list} along with
11775 the file contents. If verbose display is requested, it will also be
11776 explicitly marked as in the example below:
11780 $ @kbd{tar --verbose --list --file=iamanarchive}
11781 V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header--
11782 -rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename
11786 @opindex test-label
11787 @anchor{--test-label option}
11788 However, @option{--list} option will cause listing entire
11789 contents of the archive, which may be undesirable (for example, if the
11790 archive is stored on a tape). You can request checking only the volume
11791 label by specifying @option{--test-label} option. This option reads only the
11792 first block of an archive, so it can be used with slow storage
11793 devices. For example:
11797 $ @kbd{tar --test-label --file=iamanarchive}
11802 If @option{--test-label} is used with one or more command line
11803 arguments, @command{tar} compares the volume label with each
11804 argument. It exits with code 0 if a match is found, and with code 1
11805 otherwise@footnote{Note that @GNUTAR{} versions up to 1.23 indicated
11806 mismatch with an exit code 2 and printed a spurious diagnostics on
11807 stderr.}. No output is displayed, unless you also used the
11808 @option{--verbose} option. For example:
11812 $ @kbd{tar --test-label --file=iamanarchive 'iamalabel'}
11814 $ @kbd{tar --test-label --file=iamanarchive 'alabel'}
11819 When used with the @option{--verbose} option, @command{tar}
11820 prints the actual volume label (if any), and a verbose diagnostics in
11821 case of a mismatch:
11825 $ @kbd{tar --test-label --verbose --file=iamanarchive 'iamalabel'}
11828 $ @kbd{tar --test-label --verbose --file=iamanarchive 'alabel'}
11830 tar: Archive label mismatch
11835 If you request any operation, other than @option{--create}, along
11836 with using @option{--label} option, @command{tar} will first check if
11837 the archive label matches the one specified and will refuse to proceed
11838 if it does not. Use this as a safety precaution to avoid accidentally
11839 overwriting existing archives. For example, if you wish to add files
11840 to @file{archive}, presumably labeled with string @samp{My volume},
11845 $ @kbd{tar -rf archive --label 'My volume' .}
11846 tar: Archive not labeled to match 'My volume'
11851 in case its label does not match. This will work even if
11852 @file{archive} is not labeled at all.
11854 Similarly, @command{tar} will refuse to list or extract the
11855 archive if its label doesn't match the @var{archive-label}
11856 specified. In those cases, @var{archive-label} argument is interpreted
11857 as a globbing-style pattern which must match the actual magnetic
11858 volume label. @xref{exclude}, for a precise description of how match
11859 is attempted@footnote{Previous versions of @command{tar} used full
11860 regular expression matching, or before that, only exact string
11861 matching, instead of wildcard matchers. We decided for the sake of
11862 simplicity to use a uniform matching device through
11863 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
11864 the volume label matcher will also suffix @var{archive-label} by
11865 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
11866 up. Since the volume numbering is automatically added in labels at
11867 creation time, it sounded logical to equally help the user taking care
11868 of it when the archive is being read.
11870 You can also use @option{--label} to get a common information on
11871 all tapes of a series. For having this information different in each
11872 series created through a single script used on a regular basis, just
11873 manage to get some date string as part of the label. For example:
11877 $ @kbd{tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"}
11878 $ @kbd{tar --create --file=/dev/tape --multi-volume \
11879 --label="Daily backup for `date +%Y-%m-%d`"}
11883 Some more notes about volume labels:
11886 @item Each label has its own date and time, which corresponds
11887 to the time when @GNUTAR{} initially attempted to write it,
11888 often soon after the operator launches @command{tar} or types the
11889 carriage return telling that the next tape is ready.
11891 @item Comparing date labels to get an idea of tape throughput is
11892 unreliable. It gives correct results only if the delays for rewinding
11893 tapes and the operator switching them were negligible, which is
11894 usually not the case.
11898 @section Verifying Data as It is Stored
11899 @cindex Verifying a write operation
11900 @cindex Double-checking a write operation
11905 @opindex verify, short description
11906 Attempt to verify the archive after writing.
11909 This option causes @command{tar} to verify the archive after writing it.
11910 Each volume is checked after it is written, and any discrepancies
11911 are recorded on the standard error output.
11913 Verification requires that the archive be on a back-space-able medium.
11914 This means pipes, some cartridge tape drives, and some other devices
11915 cannot be verified.
11917 You can insure the accuracy of an archive by comparing files in the
11918 system with archive members. @command{tar} can compare an archive to the
11919 file system as the archive is being written, to verify a write
11920 operation, or can compare a previously written archive, to insure that
11923 @xopindex{verify, using with @option{--create}}
11924 @xopindex{create, using with @option{--verify}}
11925 To check for discrepancies in an archive immediately after it is
11926 written, use the @option{--verify} (@option{-W}) option in conjunction with
11927 the @option{--create} operation. When this option is
11928 specified, @command{tar} checks archive members against their counterparts
11929 in the file system, and reports discrepancies on the standard error.
11931 To verify an archive, you must be able to read it from before the end
11932 of the last written entry. This option is useful for detecting data
11933 errors on some tapes. Archives written to pipes, some cartridge tape
11934 drives, and some other devices cannot be verified.
11936 One can explicitly compare an already made archive with the file
11937 system by using the @option{--compare} (@option{--diff}, @option{-d})
11938 option, instead of using the more automatic @option{--verify} option.
11941 Note that these two options have a slightly different intent. The
11942 @option{--compare} option checks how identical are the logical contents of some
11943 archive with what is on your disks, while the @option{--verify} option is
11944 really for checking if the physical contents agree and if the recording
11945 media itself is of dependable quality. So, for the @option{--verify}
11946 operation, @command{tar} tries to defeat all in-memory cache pertaining to
11947 the archive, while it lets the speed optimization undisturbed for the
11948 @option{--compare} option. If you nevertheless use @option{--compare} for
11949 media verification, you may have to defeat the in-memory cache yourself,
11950 maybe by opening and reclosing the door latch of your recording unit,
11951 forcing some doubt in your operating system about the fact this is really
11952 the same volume as the one just written or read.
11954 The @option{--verify} option would not be necessary if drivers were indeed
11955 able to detect dependably all write failures. This sometimes require many
11956 magnetic heads, some able to read after the writes occurred. One would
11957 not say that drivers unable to detect all cases are necessarily flawed,
11958 as long as programming is concerned.
11960 The @option{--verify} (@option{-W}) option will not work in
11961 conjunction with the @option{--multi-volume} (@option{-M}) option or
11962 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
11963 and @option{--delete} operations. @xref{Operations}, for more
11964 information on these operations.
11966 Also, since @command{tar} normally strips leading @samp{/} from file
11967 names (@pxref{absolute}), a command like @samp{tar --verify -cf
11968 /tmp/foo.tar /etc} will work as desired only if the working directory is
11969 @file{/}, as @command{tar} uses the archive's relative member names
11970 (e.g., @file{etc/motd}) when verifying the archive.
11972 @node Write Protection
11973 @section Write Protection
11975 Almost all tapes and diskettes, and in a few rare cases, even disks can
11976 be @dfn{write protected}, to protect data on them from being changed.
11977 Once an archive is written, you should write protect the media to prevent
11978 the archive from being accidentally overwritten or deleted. (This will
11979 protect the archive from being changed with a tape or floppy drive---it
11980 will not protect it from magnet fields or other physical hazards.)
11982 The write protection device itself is usually an integral part of the
11983 physical media, and can be a two position (write enabled/write
11984 disabled) switch, a notch which can be popped out or covered, a ring
11985 which can be removed from the center of a tape reel, or some other
11986 changeable feature.
11988 @node Reliability and security
11989 @chapter Reliability and Security
11991 The @command{tar} command reads and writes files as any other
11992 application does, and is subject to the usual caveats about
11993 reliability and security. This section contains some commonsense
11994 advice on the topic.
12002 @section Reliability
12004 Ideally, when @command{tar} is creating an archive, it reads from a
12005 file system that is not being modified, and encounters no errors or
12006 inconsistencies while reading and writing. If this is the case, the
12007 archive should faithfully reflect what was read. Similarly, when
12008 extracting from an archive, ideally @command{tar} ideally encounters
12009 no errors and the extracted files faithfully reflect what was in the
12012 However, when reading or writing real-world file systems, several
12013 things can go wrong; these include permissions problems, corruption of
12014 data, and race conditions.
12017 * Permissions problems::
12018 * Data corruption and repair::
12019 * Race conditions::
12022 @node Permissions problems
12023 @subsection Permissions Problems
12025 If @command{tar} encounters errors while reading or writing files, it
12026 normally reports an error and exits with nonzero status. The work it
12027 does may therefore be incomplete. For example, when creating an
12028 archive, if @command{tar} cannot read a file then it cannot copy the
12029 file into the archive.
12031 @node Data corruption and repair
12032 @subsection Data Corruption and Repair
12034 If an archive becomes corrupted by an I/O error, this may corrupt the
12035 data in an extracted file. Worse, it may corrupt the file's metadata,
12036 which may cause later parts of the archive to become misinterpreted.
12037 An tar-format archive contains a checksum that most likely will detect
12038 errors in the metadata, but it will not detect errors in the data.
12040 If data corruption is a concern, you can compute and check your own
12041 checksums of an archive by using other programs, such as
12044 When attempting to recover from a read error or data corruption in an
12045 archive, you may need to skip past the questionable data and read the
12046 rest of the archive. This requires some expertise in the archive
12047 format and in other software tools.
12049 @node Race conditions
12050 @subsection Race conditions
12052 If some other process is modifying the file system while @command{tar}
12053 is reading or writing files, the result may well be inconsistent due
12054 to race conditions. For example, if another process creates some
12055 files in a directory while @command{tar} is creating an archive
12056 containing the directory's files, @command{tar} may see some of the
12057 files but not others, or it may see a file that is in the process of
12058 being created. The resulting archive may not be a snapshot of the
12059 file system at any point in time. If an application such as a
12060 database system depends on an accurate snapshot, restoring from the
12061 @command{tar} archive of a live file system may therefore break that
12062 consistency and may break the application. The simplest way to avoid
12063 the consistency issues is to avoid making other changes to the file
12064 system while tar is reading it or writing it.
12066 When creating an archive, several options are available to avoid race
12067 conditions. Some hosts have a way of snapshotting a file system, or
12068 of temporarily suspending all changes to a file system, by (say)
12069 suspending the only virtual machine that can modify a file system; if
12070 you use these facilities and have @command{tar -c} read from a
12071 snapshot when creating an archive, you can avoid inconsistency
12072 problems. More drastically, before starting @command{tar} you could
12073 suspend or shut down all processes other than @command{tar} that have
12074 access to the file system, or you could unmount the file system and
12075 then mount it read-only.
12077 When extracting from an archive, one approach to avoid race conditions
12078 is to create a directory that no other process can write to, and
12084 In some cases @command{tar} may be used in an adversarial situation,
12085 where an untrusted user is attempting to gain information about or
12086 modify otherwise-inaccessible files. Dealing with untrusted data
12087 (that is, data generated by an untrusted user) typically requires
12088 extra care, because even the smallest mistake in the use of
12089 @command{tar} is more likely to be exploited by an adversary than by a
12095 * Live untrusted data::
12096 * Security rules of thumb::
12100 @subsection Privacy
12102 Standard privacy concerns apply when using @command{tar}. For
12103 example, suppose you are archiving your home directory into a file
12104 @file{/archive/myhome.tar}. Any secret information in your home
12105 directory, such as your SSH secret keys, are copied faithfully into
12106 the archive. Therefore, if your home directory contains any file that
12107 should not be read by some other user, the archive itself should be
12108 not be readable by that user. And even if the archive's data are
12109 inaccessible to untrusted users, its metadata (such as size or
12110 last-modified date) may reveal some information about your home
12111 directory; if the metadata are intended to be private, the archive's
12112 parent directory should also be inaccessible to untrusted users.
12114 One precaution is to create @file{/archive} so that it is not
12115 accessible to any user, unless that user also has permission to access
12116 all the files in your home directory.
12118 Similarly, when extracting from an archive, take care that the
12119 permissions of the extracted files are not more generous than what you
12120 want. Even if the archive itself is readable only to you, files
12121 extracted from it have their own permissions that may differ.
12124 @subsection Integrity
12126 When creating archives, take care that they are not writable by a
12127 untrusted user; otherwise, that user could modify the archive, and
12128 when you later extract from the archive you will get incorrect data.
12130 When @command{tar} extracts from an archive, by default it writes into
12131 files relative to the working directory. If the archive was generated
12132 by an untrusted user, that user therefore can write into any file
12133 under the working directory. If the working directory contains a
12134 symbolic link to another directory, the untrusted user can also write
12135 into any file under the referenced directory. When extracting from an
12136 untrusted archive, it is therefore good practice to create an empty
12137 directory and run @command{tar} in that directory.
12139 When extracting from two or more untrusted archives, each one should
12140 be extracted independently, into different empty directories.
12141 Otherwise, the first archive could create a symbolic link into an area
12142 outside the working directory, and the second one could follow the
12143 link and overwrite data that is not under the working directory. For
12144 example, when restoring from a series of incremental dumps, the
12145 archives should have been created by a trusted process, as otherwise
12146 the incremental restores might alter data outside the working
12149 If you use the @option{--absolute-names} (@option{-P}) option when
12150 extracting, @command{tar} respects any file names in the archive, even
12151 file names that begin with @file{/} or contain @file{..}. As this
12152 lets the archive overwrite any file in your system that you can write,
12153 the @option{--absolute-names} (@option{-P}) option should be used only
12154 for trusted archives.
12156 Conversely, with the @option{--keep-old-files} (@option{-k}) and
12157 @option{--skip-old-files} options, @command{tar} refuses to replace
12158 existing files when extracting. The difference between the two
12159 options is that the former treats existing files as errors whereas the
12160 latter just silently ignores them.
12162 Finally, with the @option{--no-overwrite-dir} option, @command{tar}
12163 refuses to replace the permissions or ownership of already-existing
12164 directories. These options may help when extracting from untrusted
12167 @node Live untrusted data
12168 @subsection Dealing with Live Untrusted Data
12170 Extra care is required when creating from or extracting into a file
12171 system that is accessible to untrusted users. For example, superusers
12172 who invoke @command{tar} must be wary about its actions being hijacked
12173 by an adversary who is reading or writing the file system at the same
12174 time that @command{tar} is operating.
12176 When creating an archive from a live file system, @command{tar} is
12177 vulnerable to denial-of-service attacks. For example, an adversarial
12178 user could create the illusion of an indefinitely-deep directory
12179 hierarchy @file{d/e/f/g/...} by creating directories one step ahead of
12180 @command{tar}, or the illusion of an indefinitely-long file by
12181 creating a sparse file but arranging for blocks to be allocated just
12182 before @command{tar} reads them. There is no easy way for
12183 @command{tar} to distinguish these scenarios from legitimate uses, so
12184 you may need to monitor @command{tar}, just as you'd need to monitor
12185 any other system service, to detect such attacks.
12187 While a superuser is extracting from an archive into a live file
12188 system, an untrusted user might replace a directory with a symbolic
12189 link, in hopes that @command{tar} will follow the symbolic link and
12190 extract data into files that the untrusted user does not have access
12191 to. Even if the archive was generated by the superuser, it may
12192 contain a file such as @file{d/etc/passwd} that the untrusted user
12193 earlier created in order to break in; if the untrusted user replaces
12194 the directory @file{d/etc} with a symbolic link to @file{/etc} while
12195 @command{tar} is running, @command{tar} will overwrite
12196 @file{/etc/passwd}. This attack can be prevented by extracting into a
12197 directory that is inaccessible to untrusted users.
12199 Similar attacks via symbolic links are also possible when creating an
12200 archive, if the untrusted user can modify an ancestor of a top-level
12201 argument of @command{tar}. For example, an untrusted user that can
12202 modify @file{/home/eve} can hijack a running instance of @samp{tar -cf
12203 - /home/eve/Documents/yesterday} by replacing
12204 @file{/home/eve/Documents} with a symbolic link to some other
12205 location. Attacks like these can be prevented by making sure that
12206 untrusted users cannot modify any files that are top-level arguments
12207 to @command{tar}, or any ancestor directories of these files.
12209 @node Security rules of thumb
12210 @subsection Security Rules of Thumb
12212 This section briefly summarizes rules of thumb for avoiding security
12218 Protect archives at least as much as you protect any of the files
12222 Extract from an untrusted archive only into an otherwise-empty
12223 directory. This directory and its parent should be accessible only to
12224 trusted users. For example:
12228 $ @kbd{chmod go-rwx .}
12229 $ @kbd{mkdir -m go-rwx dir}
12231 $ @kbd{tar -xvf /archives/got-it-off-the-net.tar.gz}
12235 As a corollary, do not do an incremental restore from an untrusted archive.
12238 Do not let untrusted users access files extracted from untrusted
12239 archives without checking first for problems such as setuid programs.
12242 Do not let untrusted users modify directories that are ancestors of
12243 top-level arguments of @command{tar}. For example, while you are
12244 executing @samp{tar -cf /archive/u-home.tar /u/home}, do not let an
12245 untrusted user modify @file{/}, @file{/archive}, or @file{/u}.
12248 Pay attention to the diagnostics and exit status of @command{tar}.
12251 When archiving live file systems, monitor running instances of
12252 @command{tar} to detect denial-of-service attacks.
12255 Avoid unusual options such as @option{--absolute-names} (@option{-P}),
12256 @option{--dereference} (@option{-h}), @option{--overwrite},
12257 @option{--recursive-unlink}, and @option{--remove-files} unless you
12258 understand their security implications.
12265 This appendix lists some important user-visible changes between
12266 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
12267 version of this document is available at
12268 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
12269 @GNUTAR{} documentation page}.
12272 @item Use of globbing patterns when listing and extracting.
12274 Previous versions of GNU tar assumed shell-style globbing when
12275 extracting from or listing an archive. For example:
12278 $ @kbd{tar xf foo.tar '*.c'}
12281 would extract all files whose names end in @samp{.c}. This behavior
12282 was not documented and was incompatible with traditional tar
12283 implementations. Therefore, starting from version 1.15.91, GNU tar
12284 no longer uses globbing by default. For example, the above invocation
12285 is now interpreted as a request to extract from the archive the file
12288 To facilitate transition to the new behavior for those users who got
12289 used to the previous incorrect one, @command{tar} will print a warning
12290 if it finds out that a requested member was not found in the archive
12291 and its name looks like a globbing pattern. For example:
12294 $ @kbd{tar xf foo.tar '*.c'}
12295 tar: Pattern matching characters used in file names. Please,
12296 tar: use --wildcards to enable pattern matching, or --no-wildcards to
12297 tar: suppress this warning.
12298 tar: *.c: Not found in archive
12299 tar: Error exit delayed from previous errors
12302 To treat member names as globbing patterns, use the @option{--wildcards} option.
12303 If you want to tar to mimic the behavior of versions prior to 1.15.91,
12304 add this option to your @env{TAR_OPTIONS} variable.
12306 @xref{wildcards}, for the detailed discussion of the use of globbing
12307 patterns by @GNUTAR{}.
12309 @item Use of short option @option{-o}.
12311 Earlier versions of @GNUTAR{} understood @option{-o} command line
12312 option as a synonym for @option{--old-archive}.
12314 @GNUTAR{} starting from version 1.13.90 understands this option as
12315 a synonym for @option{--no-same-owner}. This is compatible with
12316 UNIX98 @command{tar} implementations.
12318 However, to facilitate transition, @option{-o} option retains its
12319 old semantics when it is used with one of archive-creation commands.
12320 Users are encouraged to use @option{--format=oldgnu} instead.
12322 It is especially important, since versions of @acronym{GNU} Automake
12323 up to and including 1.8.4 invoke tar with this option to produce
12324 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
12325 of this issue and its implications.
12327 @xref{Options, tar-formats, Changing Automake's Behavior,
12328 automake, GNU Automake}, for a description on how to use various
12329 archive formats with @command{automake}.
12331 Future versions of @GNUTAR{} will understand @option{-o} only as a
12332 synonym for @option{--no-same-owner}.
12334 @item Use of short option @option{-l}
12336 Earlier versions of @GNUTAR{} understood @option{-l} option as a
12337 synonym for @option{--one-file-system}. Since such usage contradicted
12338 to UNIX98 specification and harmed compatibility with other
12339 implementations, it was declared deprecated in version 1.14. However,
12340 to facilitate transition to its new semantics, it was supported by
12341 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
12342 variant of @option{--check-links} was introduced in version 1.15.91.
12344 @item Use of options @option{--portability} and @option{--old-archive}
12346 These options are deprecated. Please use @option{--format=v7} instead.
12348 @item Use of option @option{--posix}
12350 This option is deprecated. Please use @option{--format=posix} instead.
12353 @node Configuring Help Summary
12354 @appendix Configuring Help Summary
12356 Running @kbd{tar --help} displays the short @command{tar} option
12357 summary (@pxref{help}). This summary is organized by @dfn{groups} of
12358 semantically close options. The options within each group are printed
12359 in the following order: a short option, eventually followed by a list
12360 of corresponding long option names, followed by a short description of
12361 the option. For example, here is an excerpt from the actual @kbd{tar
12365 Main operation mode:
12367 -A, --catenate, --concatenate append tar files to an archive
12368 -c, --create create a new archive
12369 -d, --diff, --compare find differences between archive and
12371 --delete delete from the archive
12374 @vrindex ARGP_HELP_FMT, environment variable
12375 The exact visual representation of the help output is configurable via
12376 @env{ARGP_HELP_FMT} environment variable. The value of this variable
12377 is a comma-separated list of @dfn{format variable} assignments. There
12378 are two kinds of format variables. An @dfn{offset variable} keeps the
12379 offset of some part of help output text from the leftmost column on
12380 the screen. A @dfn{boolean} variable is a flag that toggles some
12381 output feature on or off. Depending on the type of the corresponding
12382 variable, there are two kinds of assignments:
12385 @item Offset assignment
12387 The assignment to an offset variable has the following syntax:
12390 @var{variable}=@var{value}
12394 where @var{variable} is the variable name, and @var{value} is a
12395 numeric value to be assigned to the variable.
12397 @item Boolean assignment
12399 To assign @code{true} value to a variable, simply put this variable name. To
12400 assign @code{false} value, prefix the variable name with @samp{no-}. For
12405 # Assign @code{true} value:
12407 # Assign @code{false} value:
12413 Following variables are declared:
12415 @deftypevr {Help Output} boolean dup-args
12416 If true, arguments for an option are shown with both short and long
12417 options, even when a given option has both forms, for example:
12420 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
12423 If false, then if an option has both short and long forms, the
12424 argument is only shown with the long one, for example:
12427 -f, --file=ARCHIVE use archive file or device ARCHIVE
12431 and a message indicating that the argument is applicable to both
12432 forms is printed below the options. This message can be disabled
12433 using @code{dup-args-note} (see below).
12435 The default is false.
12438 @deftypevr {Help Output} boolean dup-args-note
12439 If this variable is true, which is the default, the following notice
12440 is displayed at the end of the help output:
12443 Mandatory or optional arguments to long options are also mandatory or
12444 optional for any corresponding short options.
12447 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
12448 variables @code{dup-args} or @code{dup-args-note} should be set.
12451 @deftypevr {Help Output} offset short-opt-col
12452 Column in which short options start. Default is 2.
12456 $ @kbd{tar --help|grep ARCHIVE}
12457 -f, --file=ARCHIVE use archive file or device ARCHIVE
12458 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
12459 -f, --file=ARCHIVE use archive file or device ARCHIVE
12464 @deftypevr {Help Output} offset long-opt-col
12465 Column in which long options start. Default is 6. For example:
12469 $ @kbd{tar --help|grep ARCHIVE}
12470 -f, --file=ARCHIVE use archive file or device ARCHIVE
12471 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
12472 -f, --file=ARCHIVE use archive file or device ARCHIVE
12477 @deftypevr {Help Output} offset doc-opt-col
12478 Column in which @dfn{doc options} start. A doc option isn't actually
12479 an option, but rather an arbitrary piece of documentation that is
12480 displayed in much the same manner as the options. For example, in
12481 the description of @option{--format} option:
12485 -H, --format=FORMAT create archive of the given format.
12487 FORMAT is one of the following:
12489 gnu GNU tar 1.13.x format
12490 oldgnu GNU format as per tar <= 1.12
12491 pax POSIX 1003.1-2001 (pax) format
12493 ustar POSIX 1003.1-1988 (ustar) format
12494 v7 old V7 tar format
12499 the format names are doc options. Thus, if you set
12500 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
12501 will look as follows:
12505 -H, --format=FORMAT create archive of the given format.
12507 FORMAT is one of the following:
12509 gnu GNU tar 1.13.x format
12510 oldgnu GNU format as per tar <= 1.12
12511 pax POSIX 1003.1-2001 (pax) format
12513 ustar POSIX 1003.1-1988 (ustar) format
12514 v7 old V7 tar format
12519 @deftypevr {Help Output} offset opt-doc-col
12520 Column in which option description starts. Default is 29.
12524 $ @kbd{tar --help|grep ARCHIVE}
12525 -f, --file=ARCHIVE use archive file or device ARCHIVE
12526 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
12527 -f, --file=ARCHIVE use archive file or device ARCHIVE
12528 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
12530 use archive file or device ARCHIVE
12535 Notice, that the description starts on a separate line if
12536 @code{opt-doc-col} value is too small.
12539 @deftypevr {Help Output} offset header-col
12540 Column in which @dfn{group headers} are printed. A group header is a
12541 descriptive text preceding an option group. For example, in the
12545 Main operation mode:
12547 -A, --catenate, --concatenate append tar files to
12549 -c, --create create a new archive
12552 @samp{Main operation mode:} is the group header.
12554 The default value is 1.
12557 @deftypevr {Help Output} offset usage-indent
12558 Indentation of wrapped usage lines. Affects @option{--usage}
12559 output. Default is 12.
12562 @deftypevr {Help Output} offset rmargin
12563 Right margin of the text output. Used for wrapping.
12566 @node Fixing Snapshot Files
12567 @appendix Fixing Snapshot Files
12568 @include tar-snapshot-edit.texi
12570 @node Tar Internals
12571 @appendix Tar Internals
12572 @include intern.texi
12576 @include genfile.texi
12578 @node Free Software Needs Free Documentation
12579 @appendix Free Software Needs Free Documentation
12580 @include freemanuals.texi
12582 @node GNU Free Documentation License
12583 @appendix GNU Free Documentation License
12587 @node Index of Command Line Options
12588 @appendix Index of Command Line Options
12590 This appendix contains an index of all @GNUTAR{} long command line
12591 options. The options are listed without the preceding double-dash.
12592 For a cross-reference of short command line options, see
12593 @ref{Short Option Summary}.
12606 @c Local variables:
12607 @c texinfo-column-for-description: 32