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, 1995, 1996, 1997, 1999, 2000, 2001,
40 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Free Software
44 Permission is granted to copy, distribute and/or modify this document
45 under the terms of the GNU Free Documentation License, Version 1.3 or
46 any later version published by the Free Software Foundation; with no
47 Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
48 A copy of the license is included in the section entitled ``GNU Free
49 Documentation License''.
53 @dircategory Archiving
55 * Tar: (tar). Making tape (or disk) archives.
58 @dircategory Individual utilities
60 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
63 @shorttitlepage @acronym{GNU} @command{tar}
66 @title @acronym{GNU} tar: an archiver tool
67 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
68 @author John Gilmore, Jay Fenlason et al.
71 @vskip 0pt plus 1filll
77 @top @acronym{GNU} tar: an archiver tool
82 @cindex archiving files
84 The first part of this master menu lists the major nodes in this Info
85 document. The rest of the menu lists all the lower level nodes.
88 @c The master menu goes here.
90 @c NOTE: To update it from within Emacs, make sure mastermenu.el is
91 @c loaded and run texinfo-master-menu.
92 @c To update it from the command line, run
103 * Date input formats::
106 * Reliability and security::
111 * Configuring Help Summary::
112 * Fixing Snapshot Files::
115 * Free Software Needs Free Documentation::
116 * GNU Free Documentation License::
117 * Index of Command Line Options::
121 --- The Detailed Node Listing ---
125 * Book Contents:: What this Book Contains
126 * Definitions:: Some Definitions
127 * What tar Does:: What @command{tar} Does
128 * Naming tar Archives:: How @command{tar} Archives are Named
129 * Authors:: @GNUTAR{} Authors
130 * Reports:: Reporting bugs or suggestions
132 Tutorial Introduction to @command{tar}
135 * stylistic conventions::
136 * basic tar options:: Basic @command{tar} Operations and Options
137 * frequent operations::
138 * Two Frequent Options::
139 * create:: How to Create Archives
140 * list:: How to List Archives
141 * extract:: How to Extract Members from an Archive
144 Two Frequently Used Options
150 How to Create Archives
152 * prepare for examples::
153 * Creating the archive::
162 How to Extract Members from an Archive
164 * extracting archives::
167 * extracting untrusted archives::
173 * using tar options::
183 The Three Option Styles
185 * Long Options:: Long Option Style
186 * Short Options:: Short Option Style
187 * Old Options:: Old Option Style
188 * Mixing:: Mixing Option Styles
190 All @command{tar} Options
192 * Operation Summary::
194 * Short Option Summary::
206 Advanced @GNUTAR{} Operations
215 How to Add Files to Existing Archives: @option{--append}
217 * appending files:: Appending Files to an Archive
224 Options Used by @option{--create}
226 * override:: Overriding File Metadata.
227 * Ignore Failed Read::
229 Options Used by @option{--extract}
231 * Reading:: Options to Help Read Archives
232 * Writing:: Changing How @command{tar} Writes Files
233 * Scarce:: Coping with Scarce Resources
235 Options to Help Read Archives
237 * read full records::
240 Changing How @command{tar} Writes Files
242 * Dealing with Old Files::
243 * Overwrite Old Files::
248 * Data Modification Times::
249 * Setting Access Permissions::
250 * Directory Modification Times and Permissions::
251 * Writing to Standard Output::
252 * Writing to an External Program::
255 Coping with Scarce Resources
260 Performing Backups and Restoring Files
262 * Full Dumps:: Using @command{tar} to Perform Full Dumps
263 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
264 * Backup Levels:: Levels of Backups
265 * Backup Parameters:: Setting Parameters for Backups and Restoration
266 * Scripted Backups:: Using the Backup Scripts
267 * Scripted Restoration:: Using the Restore Script
269 Setting Parameters for Backups and Restoration
271 * General-Purpose Variables::
272 * Magnetic Tape Control::
274 * backup-specs example:: An Example Text of @file{Backup-specs}
276 Choosing Files and Names for @command{tar}
278 * file:: Choosing the Archive's Name
279 * Selecting Archive Members::
280 * files:: Reading Names from a File
281 * exclude:: Excluding Some Files
282 * wildcards:: Wildcards Patterns and Matching
283 * quoting styles:: Ways of Quoting Special Characters in Names
284 * transform:: Modifying File and Member Names
285 * after:: Operating Only on New Files
286 * recurse:: Descending into Directories
287 * one:: Crossing File System Boundaries
289 Reading Names from a File
295 * problems with exclude::
297 Wildcards Patterns and Matching
299 * controlling pattern-matching::
301 Crossing File System Boundaries
303 * directory:: Changing Directory
304 * absolute:: Absolute File Names
308 * General date syntax:: Common rules.
309 * Calendar date items:: 19 Dec 1994.
310 * Time of day items:: 9:20pm.
311 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}.
312 * Day of week items:: Monday and others.
313 * Relative items in date strings:: next tuesday, 2 years ago.
314 * Pure numbers in date strings:: 19931219, 1440.
315 * Seconds since the Epoch:: @@1078100502.
316 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0".
317 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al.
319 Controlling the Archive Format
321 * Compression:: Using Less Space through Compression
322 * Attributes:: Handling File Attributes
323 * Portability:: Making @command{tar} Archives More Portable
324 * cpio:: Comparison of @command{tar} and @command{cpio}
326 Using Less Space through Compression
328 * gzip:: Creating and Reading Compressed Archives
329 * sparse:: Archiving Sparse Files
331 Creating and Reading Compressed Archives
333 * lbzip2:: Using lbzip2 with @GNUTAR{}.
335 Making @command{tar} Archives More Portable
337 * Portable Names:: Portable Names
338 * dereference:: Symbolic Links
339 * hard links:: Hard Links
340 * old:: Old V7 Archives
341 * ustar:: Ustar Archives
342 * gnu:: GNU and old GNU format archives.
343 * posix:: @acronym{POSIX} archives
344 * Checksumming:: Checksumming Problems
345 * Large or Negative Values:: Large files, negative time stamps, etc.
346 * Other Tars:: How to Extract GNU-Specific Data Using
347 Other @command{tar} Implementations
349 @GNUTAR{} and @acronym{POSIX} @command{tar}
351 * PAX keywords:: Controlling Extended Header Keywords.
353 How to Extract GNU-Specific Data Using Other @command{tar} Implementations
355 * Split Recovery:: Members Split Between Volumes
356 * Sparse Recovery:: Sparse Members
358 Tapes and Other Archive Media
360 * Device:: Device selection and switching
361 * Remote Tape Server::
362 * Common Problems and Solutions::
363 * Blocking:: Blocking
364 * Many:: Many archives on one tape
365 * Using Multiple Tapes:: Using Multiple Tapes
366 * label:: Including a Label in the Archive
372 * Format Variations:: Format Variations
373 * Blocking Factor:: The Blocking Factor of an Archive
375 Many Archives on One Tape
377 * Tape Positioning:: Tape Positions and Tape Marks
378 * mt:: The @command{mt} Utility
382 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
383 * Tape Files:: Tape Files
384 * Tarcat:: Concatenate Volumes into a Single Archive
389 * Standard:: Basic Tar Format
390 * Extensions:: @acronym{GNU} Extensions to the Archive Format
391 * Sparse Formats:: Storing Sparse Files
398 * PAX 0:: PAX Format, Versions 0.0 and 0.1
399 * PAX 1:: PAX Format, Version 1.0
403 * Generate Mode:: File Generation Mode.
404 * Status Mode:: File Status Mode.
405 * Exec Mode:: Synchronous Execution mode.
409 * GNU Free Documentation License:: License for copying this manual
415 @chapter Introduction
418 and manipulates @dfn{archives} which are actually collections of
419 many other files; the program provides users with an organized and
420 systematic method for controlling a large amount of data.
421 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
422 archives need not (and these days, typically do not) reside on tapes.
425 * Book Contents:: What this Book Contains
426 * Definitions:: Some Definitions
427 * What tar Does:: What @command{tar} Does
428 * Naming tar Archives:: How @command{tar} Archives are Named
429 * Authors:: @GNUTAR{} Authors
430 * Reports:: Reporting bugs or suggestions
434 @section What this Book Contains
436 The first part of this chapter introduces you to various terms that will
437 recur throughout the book. It also tells you who has worked on @GNUTAR{}
438 and its documentation, and where you should send bug reports
441 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
442 gentle introduction for people who are new to using @command{tar}. It is
443 meant to be self-contained, not requiring any reading from subsequent
444 chapters to make sense. It moves from topic to topic in a logical,
445 progressive order, building on information already explained.
447 Although the tutorial is paced and structured to allow beginners to
448 learn how to use @command{tar}, it is not intended solely for beginners.
449 The tutorial explains how to use the three most frequently used
450 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
451 two frequently used options (@samp{file} and @samp{verbose}). The other
452 chapters do not refer to the tutorial frequently; however, if a section
453 discusses something which is a complex variant of a basic concept, there
454 may be a cross-reference to that basic concept. (The entire book,
455 including the tutorial, assumes that the reader understands some basic
456 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
458 The third chapter presents the remaining five operations, and
459 information about using @command{tar} options and option syntax.
461 The other chapters are meant to be used as a reference. Each chapter
462 presents everything that needs to be said about a specific topic.
464 One of the chapters (@pxref{Date input formats}) exists in its
465 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
466 In addition, one section of this manual (@pxref{Standard}) contains a
467 big quote which is taken directly from @command{tar} sources.
469 In general, we give both long and short (abbreviated) option names
470 at least once in each section where the relevant option is covered, so
471 that novice readers will become familiar with both styles. (A few
472 options have no short versions, and the relevant sections will
476 @section Some Definitions
480 The @command{tar} program is used to create and manipulate @command{tar}
481 archives. An @dfn{archive} is a single file which contains the contents
482 of many files, while still identifying the names of the files, their
483 owner(s), and so forth. (In addition, archives record access
484 permissions, user and group, size in bytes, and data modification time.
485 Some archives also record the file names in each archived directory, as
486 well as other file and directory information.) You can use @command{tar}
487 to @dfn{create} a new archive in a specified directory.
490 @cindex archive member
493 The files inside an archive are called @dfn{members}. Within this
494 manual, we use the term @dfn{file} to refer only to files accessible in
495 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
496 @dfn{member} to refer only to the members of an archive. Similarly, a
497 @dfn{file name} is the name of a file, as it resides in the file system,
498 and a @dfn{member name} is the name of an archive member within the
503 The term @dfn{extraction} refers to the process of copying an archive
504 member (or multiple members) into a file in the file system. Extracting
505 all the members of an archive is often called @dfn{extracting the
506 archive}. The term @dfn{unpack} can also be used to refer to the
507 extraction of many or all the members of an archive. Extracting an
508 archive does not destroy the archive's structure, just as creating an
509 archive does not destroy the copies of the files that exist outside of
510 the archive. You may also @dfn{list} the members in a given archive
511 (this is often thought of as ``printing'' them to the standard output,
512 or the command line), or @dfn{append} members to a pre-existing archive.
513 All of these operations can be performed using @command{tar}.
516 @section What @command{tar} Does
519 The @command{tar} program provides the ability to create @command{tar}
520 archives, as well as various other kinds of manipulation. For example,
521 you can use @command{tar} on previously created archives to extract files,
522 to store additional files, or to update or list files which were already
525 Initially, @command{tar} archives were used to store files conveniently on
526 magnetic tape. The name @command{tar} comes from this use; it stands for
527 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
528 direct its output to available devices, files, or other programs (using
529 pipes). @command{tar} may even access remote devices or files (as archives).
531 You can use @command{tar} archives in many ways. We want to stress a few
532 of them: storage, backup, and transportation.
534 @FIXME{the following table entries need a bit of work.}
537 Often, @command{tar} archives are used to store related files for
538 convenient file transfer over a network. For example, the
539 @acronym{GNU} Project distributes its software bundled into
540 @command{tar} archives, so that all the files relating to a particular
541 program (or set of related programs) can be transferred as a single
544 A magnetic tape can store several files in sequence. However, the tape
545 has no names for these files; it only knows their relative position on
546 the tape. One way to store several files on one tape and retain their
547 names is by creating a @command{tar} archive. Even when the basic transfer
548 mechanism can keep track of names, as FTP can, the nuisance of handling
549 multiple files, directories, and multiple links makes @command{tar}
552 Archive files are also used for long-term storage. You can think of
553 this as transportation from the present into the future. (It is a
554 science-fiction idiom that you can move through time as well as in
555 space; the idea here is that @command{tar} can be used to move archives in
556 all dimensions, even time!)
559 Because the archive created by @command{tar} is capable of preserving
560 file information and directory structure, @command{tar} is commonly
561 used for performing full and incremental backups of disks. A backup
562 puts a collection of files (possibly pertaining to many users and
563 projects) together on a disk or a tape. This guards against
564 accidental destruction of the information in those files.
565 @GNUTAR{} has special features that allow it to be
566 used to make incremental and full dumps of all the files in a
570 You can create an archive on one system, transfer it to another system,
571 and extract the contents there. This allows you to transport a group of
572 files from one system to another.
575 @node Naming tar Archives
576 @section How @command{tar} Archives are Named
578 Conventionally, @command{tar} archives are given names ending with
579 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
580 but this manual follows that convention in order to accustom readers to
581 it and to make examples more clear.
586 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
587 archive members as ``files'' or ``entries''. For people familiar with
588 the operation of @command{tar}, this causes no difficulty. However, in
589 this manual, we consistently refer to ``archives'' and ``archive
590 members'' to make learning to use @command{tar} easier for novice users.
593 @section @GNUTAR{} Authors
595 @GNUTAR{} was originally written by John Gilmore,
596 and modified by many people. The @acronym{GNU} enhancements were
597 written by Jay Fenlason, then Joy Kendall, and the whole package has
598 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
599 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
600 numerous and kind users.
602 We wish to stress that @command{tar} is a collective work, and owes much to
603 all those people who reported problems, offered solutions and other
604 insights, or shared their thoughts and suggestions. An impressive, yet
605 partial list of those contributors can be found in the @file{THANKS}
606 file from the @GNUTAR{} distribution.
608 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
609 sure i want to spell out the history in this detail, at least not for
610 the printed book. i'm just not sure it needs to be said this way.
611 i'll think about it.}
613 @FIXME{History is more important, and surely more interesting, than
614 actual names. Quoting names without history would be meaningless. FP}
616 Jay Fenlason put together a draft of a @GNUTAR{}
617 manual, borrowing notes from the original man page from John Gilmore.
618 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
619 Gorin worked on a tutorial and manual for @GNUTAR{}.
620 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
621 taking information from all these sources and merging them. Melissa
622 Weisshaus finally edited and redesigned the book to create version
623 1.12. The book for versions from 1.14 up to @value{VERSION} were edited
624 by the current maintainer, Sergey Poznyakoff.
626 For version 1.12, Daniel Hagerty contributed a great deal of technical
627 consulting. In particular, he is the primary author of @ref{Backups}.
629 In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org
630 (see @url{http://savannah.gnu.org/projects/tar}), and
631 active development and maintenance work has started
632 again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey
633 Poznyakoff and Jeff Bailey.
635 Support for @acronym{POSIX} archives was added by Sergey Poznyakoff.
638 @section Reporting bugs or suggestions
641 @cindex reporting bugs
642 If you find problems or have suggestions about this program or manual,
643 please report them to @file{bug-tar@@gnu.org}.
645 When reporting a bug, please be sure to include as much detail as
646 possible, in order to reproduce it. @FIXME{Be more specific, I'd
647 like to make this node as detailed as 'Bug reporting' node in Emacs
651 @chapter Tutorial Introduction to @command{tar}
653 This chapter guides you through some basic examples of three @command{tar}
654 operations: @option{--create}, @option{--list}, and @option{--extract}. If
655 you already know how to use some other version of @command{tar}, then you
656 may not need to read this chapter. This chapter omits most complicated
657 details about how @command{tar} works.
661 * stylistic conventions::
662 * basic tar options:: Basic @command{tar} Operations and Options
663 * frequent operations::
664 * Two Frequent Options::
665 * create:: How to Create Archives
666 * list:: How to List Archives
667 * extract:: How to Extract Members from an Archive
672 @section Assumptions this Tutorial Makes
674 This chapter is paced to allow beginners to learn about @command{tar}
675 slowly. At the same time, we will try to cover all the basic aspects of
676 these three operations. In order to accomplish both of these tasks, we
677 have made certain assumptions about your knowledge before reading this
678 manual, and the hardware you will be using:
682 Before you start to work through this tutorial, you should understand
683 what the terms ``archive'' and ``archive member'' mean
684 (@pxref{Definitions}). In addition, you should understand something
685 about how Unix-type operating systems work, and you should know how to
686 use some basic utilities. For example, you should know how to create,
687 list, copy, rename, edit, and delete files and directories; how to
688 change between directories; and how to figure out where you are in the
689 file system. You should have some basic understanding of directory
690 structure and how files are named according to which directory they are
691 in. You should understand concepts such as standard output and standard
692 input, what various definitions of the term @samp{argument} mean, and the
693 differences between relative and absolute file names. @FIXME{and what
697 This manual assumes that you are working from your own home directory
698 (unless we state otherwise). In this tutorial, you will create a
699 directory to practice @command{tar} commands in. When we show file names,
700 we will assume that those names are relative to your home directory.
701 For example, my home directory is @file{/home/fsf/melissa}. All of
702 my examples are in a subdirectory of the directory named by that file
703 name; the subdirectory is called @file{practice}.
706 In general, we show examples of archives which exist on (or can be
707 written to, or worked with from) a directory on a hard disk. In most
708 cases, you could write those archives to, or work with them on any other
709 device, such as a tape drive. However, some of the later examples in
710 the tutorial and next chapter will not work on tape drives.
711 Additionally, working with tapes is much more complicated than working
712 with hard disks. For these reasons, the tutorial does not cover working
713 with tape drives. @xref{Media}, for complete information on using
714 @command{tar} archives with tape drives.
716 @FIXME{this is a cop out. need to add some simple tape drive info.}
719 @node stylistic conventions
720 @section Stylistic Conventions
722 In the examples, @samp{$} represents a typical shell prompt. It
723 precedes lines you should type; to make this more clear, those lines are
724 shown in @kbd{this font}, as opposed to lines which represent the
725 computer's response; those lines are shown in @code{this font}, or
726 sometimes @samp{like this}.
728 @c When we have lines which are too long to be
729 @c displayed in any other way, we will show them like this:
731 @node basic tar options
732 @section Basic @command{tar} Operations and Options
734 @command{tar} can take a wide variety of arguments which specify and define
735 the actions it will have on the particular set of files or the archive.
736 The main types of arguments to @command{tar} fall into one of two classes:
737 operations, and options.
739 Some arguments fall into a class called @dfn{operations}; exactly one of
740 these is both allowed and required for any instance of using @command{tar};
741 you may @emph{not} specify more than one. People sometimes speak of
742 @dfn{operating modes}. You are in a particular operating mode when you
743 have specified the operation which specifies it; there are eight
744 operations in total, and thus there are eight operating modes.
746 The other arguments fall into the class known as @dfn{options}. You are
747 not required to specify any options, and you are allowed to specify more
748 than one at a time (depending on the way you are using @command{tar} at
749 that time). Some options are used so frequently, and are so useful for
750 helping you type commands more carefully that they are effectively
751 ``required''. We will discuss them in this chapter.
753 You can write most of the @command{tar} operations and options in any
754 of three forms: long (mnemonic) form, short form, and old style. Some
755 of the operations and options have no short or ``old'' forms; however,
756 the operations and options which we will cover in this tutorial have
757 corresponding abbreviations. We will indicate those abbreviations
758 appropriately to get you used to seeing them. Note, that the ``old
759 style'' option forms exist in @GNUTAR{} for compatibility with Unix
760 @command{tar}. In this book we present a full discussion of this way
761 of writing options and operations (@pxref{Old Options}), and we discuss
762 the other two styles of writing options (@xref{Long Options}, and
763 @pxref{Short Options}).
765 In the examples and in the text of this tutorial, we usually use the
766 long forms of operations and options; but the ``short'' forms produce
767 the same result and can make typing long @command{tar} commands easier.
768 For example, instead of typing
771 @kbd{tar --create --verbose --file=afiles.tar apple angst aspic}
777 @kbd{tar -c -v -f afiles.tar apple angst aspic}
783 @kbd{tar -cvf afiles.tar apple angst aspic}
787 For more information on option syntax, see @ref{Advanced tar}. In
788 discussions in the text, when we name an option by its long form, we
789 also give the corresponding short option in parentheses.
791 The term, ``option'', can be confusing at times, since ``operations''
792 are often lumped in with the actual, @emph{optional} ``options'' in certain
793 general class statements. For example, we just talked about ``short and
794 long forms of options and operations''. However, experienced @command{tar}
795 users often refer to these by shorthand terms such as, ``short and long
796 options''. This term assumes that the ``operations'' are included, also.
797 Context will help you determine which definition of ``options'' to use.
799 Similarly, the term ``command'' can be confusing, as it is often used in
800 two different ways. People sometimes refer to @command{tar} ``commands''.
801 A @command{tar} @dfn{command} is the entire command line of user input
802 which tells @command{tar} what to do --- including the operation, options,
803 and any arguments (file names, pipes, other commands, etc.). However,
804 you will also sometimes hear the term ``the @command{tar} command''. When
805 the word ``command'' is used specifically like this, a person is usually
806 referring to the @command{tar} @emph{operation}, not the whole line.
807 Again, use context to figure out which of the meanings the speaker
810 @node frequent operations
811 @section The Three Most Frequently Used Operations
813 Here are the three most frequently used operations (both short and long
814 forms), as well as a brief description of their meanings. The rest of
815 this chapter will cover how to use these operations in detail. We will
816 present the rest of the operations in the next chapter.
821 Create a new @command{tar} archive.
824 List the contents of an archive.
827 Extract one or more members from an archive.
830 @node Two Frequent Options
831 @section Two Frequently Used Options
833 To understand how to run @command{tar} in the three operating modes listed
834 previously, you also need to understand how to use two of the options to
835 @command{tar}: @option{--file} (which takes an archive file as an argument)
836 and @option{--verbose}. (You are usually not @emph{required} to specify
837 either of these options when you run @command{tar}, but they can be very
838 useful in making things more clear and helping you avoid errors.)
847 @unnumberedsubsec The @option{--file} Option
850 @xopindex{file, tutorial}
851 @item --file=@var{archive-name}
852 @itemx -f @var{archive-name}
853 Specify the name of an archive file.
856 You can specify an argument for the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option whenever you
857 use @command{tar}; this option determines the name of the archive file
858 that @command{tar} will work on.
861 If you don't specify this argument, then @command{tar} will examine
862 the environment variable @env{TAPE}. If it is set, its value will be
863 used as the archive name. Otherwise, @command{tar} will use the
864 default archive, determined at compile time. Usually it is
865 standard output or some physical tape drive attached to your machine
866 (you can verify what the default is by running @kbd{tar
867 --show-defaults}, @pxref{defaults}). If there is no tape drive
868 attached, or the default is not meaningful, then @command{tar} will
869 print an error message. The error message might look roughly like one
873 tar: can't open /dev/rmt8 : No such device or address
874 tar: can't open /dev/rsmt0 : I/O error
878 To avoid confusion, we recommend that you always specify an archive file
879 name by using @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) when writing your @command{tar} commands.
880 For more information on using the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option, see
883 @node verbose tutorial
884 @unnumberedsubsec The @option{--verbose} Option
887 @xopindex{verbose, introduced}
890 Show the files being worked on as @command{tar} is running.
893 @option{--verbose} (@option{-v}) shows details about the results of running
894 @command{tar}. This can be especially useful when the results might not be
895 obvious. For example, if you want to see the progress of @command{tar} as
896 it writes files into the archive, you can use the @option{--verbose}
897 option. In the beginning, you may find it useful to use
898 @option{--verbose} at all times; when you are more accustomed to
899 @command{tar}, you will likely want to use it at certain times but not at
900 others. We will use @option{--verbose} at times to help make something
901 clear, and we will give many examples both using and not using
902 @option{--verbose} to show the differences.
904 Each instance of @option{--verbose} on the command line increases the
905 verbosity level by one, so if you need more details on the output,
908 When reading archives (@option{--list}, @option{--extract},
909 @option{--diff}), @command{tar} by default prints only the names of
910 the members being extracted. Using @option{--verbose} will show a full,
911 @command{ls} style member listing.
913 In contrast, when writing archives (@option{--create}, @option{--append},
914 @option{--update}), @command{tar} does not print file names by
915 default. So, a single @option{--verbose} option shows the file names
916 being added to the archive, while two @option{--verbose} options
917 enable the full listing.
919 For example, to create an archive in verbose mode:
922 $ @kbd{tar -cvf afiles.tar apple angst aspic}
929 Creating the same archive with the verbosity level 2 could give:
932 $ @kbd{tar -cvvf afiles.tar apple angst aspic}
933 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
934 -rw-r--r-- gray/staff 11481 2006-06-09 12:06 angst
935 -rw-r--r-- gray/staff 23152 2006-06-09 12:06 aspic
939 This works equally well using short or long forms of options. Using
940 long forms, you would simply write out the mnemonic form of the option
944 $ @kbd{tar --create --verbose --verbose @dots{}}
948 Note that you must double the hyphens properly each time.
950 Later in the tutorial, we will give examples using @w{@option{--verbose
953 @anchor{verbose member listing}
954 The full output consists of six fields:
957 @item File type and permissions in symbolic form.
958 These are displayed in the same format as the first column of
959 @command{ls -l} output (@pxref{What information is listed,
960 format=verbose, Verbose listing, fileutils, GNU file utilities}).
962 @item Owner name and group separated by a slash character.
963 If these data are not available (for example, when listing a @samp{v7} format
964 archive), numeric @acronym{ID} values are printed instead.
966 @item Size of the file, in bytes.
968 @item File modification date in ISO 8601 format.
970 @item File modification time.
973 If the name contains any special characters (white space, newlines,
974 etc.) these are displayed in an unambiguous form using so called
975 @dfn{quoting style}. For the detailed discussion of available styles
976 and on how to use them, see @ref{quoting styles}.
978 Depending on the file type, the name can be followed by some
979 additional information, described in the following table:
982 @item -> @var{link-name}
983 The file or archive member is a @dfn{symbolic link} and
984 @var{link-name} is the name of file it links to.
986 @item link to @var{link-name}
987 The file or archive member is a @dfn{hard link} and @var{link-name} is
988 the name of file it links to.
991 The archive member is an old GNU format long link. You will normally
995 The archive member is an old GNU format long name. You will normally
998 @item --Volume Header--
999 The archive member is a GNU @dfn{volume header} (@pxref{Tape Files}).
1001 @item --Continued at byte @var{n}--
1002 Encountered only at the beginning of a multi-volume archive
1003 (@pxref{Using Multiple Tapes}). This archive member is a continuation
1004 from the previous volume. The number @var{n} gives the offset where
1005 the original file was split.
1007 @item unknown file type @var{c}
1008 An archive member of unknown type. @var{c} is the type character from
1009 the archive header. If you encounter such a message, it means that
1010 either your archive contains proprietary member types @GNUTAR{} is not
1011 able to handle, or the archive is corrupted.
1016 For example, here is an archive listing containing most of the special
1017 suffixes explained above:
1021 V--------- 0/0 1536 2006-06-09 13:07 MyVolume--Volume Header--
1022 -rw-r--r-- gray/staff 456783 2006-06-09 12:06 aspic--Continued at byte 32456--
1023 -rw-r--r-- gray/staff 62373 2006-06-09 12:06 apple
1024 lrwxrwxrwx gray/staff 0 2006-06-09 13:01 angst -> apple
1025 -rw-r--r-- gray/staff 35793 2006-06-09 12:06 blues
1026 hrw-r--r-- gray/staff 0 2006-06-09 12:06 music link to blues
1034 @unnumberedsubsec Getting Help: Using the @option{--help} Option
1040 The @option{--help} option to @command{tar} prints out a very brief list of
1041 all operations and option available for the current version of
1042 @command{tar} available on your system.
1046 @section How to Create Archives
1049 @cindex Creation of the archive
1050 @cindex Archive, creation of
1051 One of the basic operations of @command{tar} is @option{--create} (@option{-c}), which
1052 you use to create a @command{tar} archive. We will explain
1053 @option{--create} first because, in order to learn about the other
1054 operations, you will find it useful to have an archive available to
1057 To make this easier, in this section you will first create a directory
1058 containing three files. Then, we will show you how to create an
1059 @emph{archive} (inside the new directory). Both the directory, and
1060 the archive are specifically for you to practice on. The rest of this
1061 chapter and the next chapter will show many examples using this
1062 directory and the files you will create: some of those files may be
1063 other directories and other archives.
1065 The three files you will archive in this example are called
1066 @file{blues}, @file{folk}, and @file{jazz}. The archive is called
1067 @file{collection.tar}.
1069 This section will proceed slowly, detailing how to use @option{--create}
1070 in @code{verbose} mode, and showing examples using both short and long
1071 forms. In the rest of the tutorial, and in the examples in the next
1072 chapter, we will proceed at a slightly quicker pace. This section
1073 moves more slowly to allow beginning users to understand how
1074 @command{tar} works.
1077 * prepare for examples::
1078 * Creating the archive::
1084 @node prepare for examples
1085 @subsection Preparing a Practice Directory for Examples
1087 To follow along with this and future examples, create a new directory
1088 called @file{practice} containing files called @file{blues}, @file{folk}
1089 and @file{jazz}. The files can contain any information you like:
1090 ideally, they should contain information which relates to their names,
1091 and be of different lengths. Our examples assume that @file{practice}
1092 is a subdirectory of your home directory.
1094 Now @command{cd} to the directory named @file{practice}; @file{practice}
1095 is now your @dfn{working directory}. (@emph{Please note}: Although
1096 the full file name of this directory is
1097 @file{/@var{homedir}/practice}, in our examples we will refer to
1098 this directory as @file{practice}; the @var{homedir} is presumed.)
1100 In general, you should check that the files to be archived exist where
1101 you think they do (in the working directory) by running @command{ls}.
1102 Because you just created the directory and the files and have changed to
1103 that directory, you probably don't need to do that this time.
1105 It is very important to make sure there isn't already a file in the
1106 working directory with the archive name you intend to use (in this case,
1107 @samp{collection.tar}), or that you don't care about its contents.
1108 Whenever you use @samp{create}, @command{tar} will erase the current
1109 contents of the file named by @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) if it exists. @command{tar}
1110 will not tell you if you are about to overwrite an archive unless you
1111 specify an option which does this (@pxref{backup}, for the
1112 information on how to do so). To add files to an existing archive,
1113 you need to use a different option, such as @option{--append} (@option{-r}); see
1114 @ref{append} for information on how to do this.
1116 @node Creating the archive
1117 @subsection Creating the Archive
1119 @xopindex{create, introduced}
1120 To place the files @file{blues}, @file{folk}, and @file{jazz} into an
1121 archive named @file{collection.tar}, use the following command:
1124 $ @kbd{tar --create --file=collection.tar blues folk jazz}
1127 The order of the arguments is not very important, @emph{when using long
1128 option forms}. You could also say:
1131 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1135 However, you can see that this order is harder to understand; this is
1136 why we will list the arguments in the order that makes the commands
1137 easiest to understand (and we encourage you to do the same when you use
1138 @command{tar}, to avoid errors).
1140 Note that the sequence
1141 @option{--file=@-collection.tar} is considered to be @emph{one} argument.
1142 If you substituted any other string of characters for
1143 @kbd{collection.tar}, then that string would become the name of the
1144 archive file you create.
1146 The order of the options becomes more important when you begin to use
1147 short forms. With short forms, if you type commands in the wrong order
1148 (even if you type them correctly in all other ways), you may end up with
1149 results you don't expect. For this reason, it is a good idea to get
1150 into the habit of typing options in the order that makes inherent sense.
1151 @xref{short create}, for more information on this.
1153 In this example, you type the command as shown above: @option{--create}
1154 is the operation which creates the new archive
1155 (@file{collection.tar}), and @option{--file} is the option which lets
1156 you give it the name you chose. The files, @file{blues}, @file{folk},
1157 and @file{jazz}, are now members of the archive, @file{collection.tar}
1158 (they are @dfn{file name arguments} to the @option{--create} operation.
1159 @xref{Choosing}, for the detailed discussion on these.) Now that they are
1160 in the archive, they are called @emph{archive members}, not files.
1161 (@pxref{Definitions,members}).
1163 When you create an archive, you @emph{must} specify which files you
1164 want placed in the archive. If you do not specify any archive
1165 members, @GNUTAR{} will complain.
1167 If you now list the contents of the working directory (@command{ls}), you will
1168 find the archive file listed as well as the files you saw previously:
1171 blues folk jazz collection.tar
1175 Creating the archive @samp{collection.tar} did not destroy the copies of
1176 the files in the directory.
1178 Keep in mind that if you don't indicate an operation, @command{tar} will not
1179 run and will prompt you for one. If you don't name any files, @command{tar}
1180 will complain. You must have write access to the working directory,
1181 or else you will not be able to create an archive in that directory.
1183 @emph{Caution}: Do not attempt to use @option{--create} (@option{-c}) to add files to
1184 an existing archive; it will delete the archive and write a new one.
1185 Use @option{--append} (@option{-r}) instead. @xref{append}.
1187 @node create verbose
1188 @subsection Running @option{--create} with @option{--verbose}
1190 @xopindex{create, using with @option{--verbose}}
1191 @xopindex{verbose, using with @option{--create}}
1192 If you include the @option{--verbose} (@option{-v}) option on the command line,
1193 @command{tar} will list the files it is acting on as it is working. In
1194 verbose mode, the @code{create} example above would appear as:
1197 $ @kbd{tar --create --verbose --file=collection.tar blues folk jazz}
1203 This example is just like the example we showed which did not use
1204 @option{--verbose}, except that @command{tar} generated the remaining
1206 lines (note the different font styles).
1212 In the rest of the examples in this chapter, we will frequently use
1213 @code{verbose} mode so we can show actions or @command{tar} responses that
1214 you would otherwise not see, and which are important for you to
1218 @subsection Short Forms with @samp{create}
1220 As we said before, the @option{--create} (@option{-c}) operation is one of the most
1221 basic uses of @command{tar}, and you will use it countless times.
1222 Eventually, you will probably want to use abbreviated (or ``short'')
1223 forms of options. A full discussion of the three different forms that
1224 options can take appears in @ref{Styles}; for now, here is what the
1225 previous example (including the @option{--verbose} (@option{-v}) option) looks like
1226 using short option forms:
1229 $ @kbd{tar -cvf collection.tar blues folk jazz}
1236 As you can see, the system responds the same no matter whether you use
1237 long or short option forms.
1239 @FIXME{i don't like how this is worded:} One difference between using
1240 short and long option forms is that, although the exact placement of
1241 arguments following options is no more specific when using short forms,
1242 it is easier to become confused and make a mistake when using short
1243 forms. For example, suppose you attempted the above example in the
1247 $ @kbd{tar -cfv collection.tar blues folk jazz}
1251 In this case, @command{tar} will make an archive file called @file{v},
1252 containing the files @file{blues}, @file{folk}, and @file{jazz}, because
1253 the @samp{v} is the closest ``file name'' to the @option{-f} option, and
1254 is thus taken to be the chosen archive file name. @command{tar} will try
1255 to add a file called @file{collection.tar} to the @file{v} archive file;
1256 if the file @file{collection.tar} did not already exist, @command{tar} will
1257 report an error indicating that this file does not exist. If the file
1258 @file{collection.tar} does already exist (e.g., from a previous command
1259 you may have run), then @command{tar} will add this file to the archive.
1260 Because the @option{-v} option did not get registered, @command{tar} will not
1261 run under @samp{verbose} mode, and will not report its progress.
1263 The end result is that you may be quite confused about what happened,
1264 and possibly overwrite a file. To illustrate this further, we will show
1265 you how an example we showed previously would look using short forms.
1270 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1274 is confusing as it is. When shown using short forms, however, it
1275 becomes much more so:
1278 $ @kbd{tar blues -c folk -f collection.tar jazz}
1282 It would be very easy to put the wrong string of characters
1283 immediately following the @option{-f}, but doing that could sacrifice
1286 For this reason, we recommend that you pay very careful attention to
1287 the order of options and placement of file and archive names,
1288 especially when using short option forms. Not having the option name
1289 written out mnemonically can affect how well you remember which option
1290 does what, and therefore where different names have to be placed.
1293 @subsection Archiving Directories
1295 @cindex Archiving Directories
1296 @cindex Directories, Archiving
1297 You can archive a directory by specifying its directory name as a
1298 file name argument to @command{tar}. The files in the directory will be
1299 archived relative to the working directory, and the directory will be
1300 re-created along with its contents when the archive is extracted.
1302 To archive a directory, first move to its superior directory. If you
1303 have followed the previous instructions in this tutorial, you should
1312 This will put you into the directory which contains @file{practice},
1313 i.e., your home directory. Once in the superior directory, you can
1314 specify the subdirectory, @file{practice}, as a file name argument. To
1315 store @file{practice} in the new archive file @file{music.tar}, type:
1318 $ @kbd{tar --create --verbose --file=music.tar practice}
1322 @command{tar} should output:
1329 practice/collection.tar
1332 Note that the archive thus created is not in the subdirectory
1333 @file{practice}, but rather in the current working directory---the
1334 directory from which @command{tar} was invoked. Before trying to archive a
1335 directory from its superior directory, you should make sure you have
1336 write access to the superior directory itself, not only the directory
1337 you are trying archive with @command{tar}. For example, you will probably
1338 not be able to store your home directory in an archive by invoking
1339 @command{tar} from the root directory; @xref{absolute}. (Note
1340 also that @file{collection.tar}, the original archive file, has itself
1341 been archived. @command{tar} will accept any file as a file to be
1342 archived, regardless of its content. When @file{music.tar} is
1343 extracted, the archive file @file{collection.tar} will be re-written
1344 into the file system).
1346 If you give @command{tar} a command such as
1349 $ @kbd{tar --create --file=foo.tar .}
1353 @command{tar} will report @samp{tar: ./foo.tar is the archive; not
1354 dumped}. This happens because @command{tar} creates the archive
1355 @file{foo.tar} in the current directory before putting any files into
1356 it. Then, when @command{tar} attempts to add all the files in the
1357 directory @file{.} to the archive, it notices that the file
1358 @file{./foo.tar} is the same as the archive @file{foo.tar}, and skips
1359 it. (It makes no sense to put an archive into itself.) @GNUTAR{}
1360 will continue in this case, and create the archive
1361 normally, except for the exclusion of that one file. (@emph{Please
1362 note:} Other implementations of @command{tar} may not be so clever;
1363 they will enter an infinite loop when this happens, so you should not
1364 depend on this behavior unless you are certain you are running
1365 @GNUTAR{}. In general, it is wise to always place the archive outside
1366 of the directory being dumped.)
1369 @section How to List Archives
1372 Frequently, you will find yourself wanting to determine exactly what a
1373 particular archive contains. You can use the @option{--list}
1374 (@option{-t}) operation to get the member names as they currently
1375 appear in the archive, as well as various attributes of the files at
1376 the time they were archived. For example, you can examine the archive
1377 @file{collection.tar} that you created in the last section with the
1381 $ @kbd{tar --list --file=collection.tar}
1385 The output of @command{tar} would then be:
1394 The archive @file{bfiles.tar} would list as follows:
1403 Be sure to use a @option{--file=@var{archive-name}} (@option{-f
1404 @var{archive-name}}) option just as with @option{--create}
1405 (@option{-c}) to specify the name of the archive.
1407 @xopindex{list, using with @option{--verbose}}
1408 @xopindex{verbose, using with @option{--list}}
1409 If you use the @option{--verbose} (@option{-v}) option with
1410 @option{--list}, then @command{tar} will print out a listing
1411 reminiscent of @w{@samp{ls -l}}, showing owner, file size, and so
1412 forth. This output is described in detail in @ref{verbose member listing}.
1414 If you had used @option{--verbose} (@option{-v}) mode, the example
1415 above would look like:
1418 $ @kbd{tar --list --verbose --file=collection.tar folk}
1419 -rw-r--r-- myself/user 62 1990-05-23 10:55 folk
1422 @cindex listing member and file names
1423 @anchor{listing member and file names}
1424 It is important to notice that the output of @kbd{tar --list
1425 --verbose} does not necessarily match that produced by @kbd{tar
1426 --create --verbose} while creating the archive. It is because
1427 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1428 prefixes from file names before storing them in the archive
1429 (@xref{absolute}, for more information). In other
1430 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1431 an archive and @dfn{member names} when listing it. Consider this
1436 $ @kbd{tar --create --verbose --file archive /etc/mail}
1437 tar: Removing leading `/' from member names
1439 /etc/mail/sendmail.cf
1441 $ @kbd{tar --test --file archive}
1443 etc/mail/sendmail.cf
1448 @opindex show-stored-names
1449 This default behavior can sometimes be inconvenient. You can force
1450 @GNUTAR{} show member names when creating archive by supplying
1451 @option{--show-stored-names} option.
1454 @item --show-stored-names
1455 Print member (as opposed to @emph{file}) names when creating the archive.
1458 @cindex File name arguments, using @option{--list} with
1459 @xopindex{list, using with file name arguments}
1460 You can specify one or more individual member names as arguments when
1461 using @samp{list}. In this case, @command{tar} will only list the
1462 names of members you identify. For example, @w{@kbd{tar --list
1463 --file=afiles.tar apple}} would only print @file{apple}.
1465 Because @command{tar} preserves file names, these must be specified as
1466 they appear in the archive (i.e., relative to the directory from which
1467 the archive was created). Therefore, it is essential when specifying
1468 member names to @command{tar} that you give the exact member names.
1469 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1470 error message something like @samp{tar: birds: Not found in archive},
1471 because there is no member named @file{birds}, only one named
1472 @file{./birds}. While the names @file{birds} and @file{./birds} name
1473 the same file, @emph{member} names by default are compared verbatim.
1475 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1476 with @file{baboon}, because this exact member name is in the archive file
1477 @file{bfiles.tar}. If you are not sure of the exact file name,
1478 use @dfn{globbing patterns}, for example:
1481 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1485 will list all members whose name contains @samp{b}. @xref{wildcards},
1486 for a detailed discussion of globbing patterns and related
1487 @command{tar} command line options.
1494 @unnumberedsubsec Listing the Contents of a Stored Directory
1496 To get information about the contents of an archived directory,
1497 use the directory name as a file name argument in conjunction with
1498 @option{--list} (@option{-t}). To find out file attributes, include the
1499 @option{--verbose} (@option{-v}) option.
1501 For example, to find out about files in the directory @file{practice}, in
1502 the archive file @file{music.tar}, type:
1505 $ @kbd{tar --list --verbose --file=music.tar practice}
1508 @command{tar} responds:
1511 drwxrwxrwx myself/user 0 1990-05-31 21:49 practice/
1512 -rw-r--r-- myself/user 42 1990-05-21 13:29 practice/blues
1513 -rw-r--r-- myself/user 62 1990-05-23 10:55 practice/folk
1514 -rw-r--r-- myself/user 40 1990-05-21 13:30 practice/jazz
1515 -rw-r--r-- myself/user 10240 1990-05-31 21:49 practice/collection.tar
1518 When you use a directory name as a file name argument, @command{tar} acts on
1519 all the files (including sub-directories) in that directory.
1522 @section How to Extract Members from an Archive
1524 @cindex Retrieving files from an archive
1525 @cindex Resurrecting files from an archive
1528 Creating an archive is only half the job---there is no point in storing
1529 files in an archive if you can't retrieve them. The act of retrieving
1530 members from an archive so they can be used and manipulated as
1531 unarchived files again is called @dfn{extraction}. To extract files
1532 from an archive, use the @option{--extract} (@option{--get} or
1533 @option{-x}) operation. As with @option{--create}, specify the name
1534 of the archive with @option{--file} (@option{-f}) option. Extracting
1535 an archive does not modify the archive in any way; you can extract it
1536 multiple times if you want or need to.
1538 Using @option{--extract}, you can extract an entire archive, or specific
1539 files. The files can be directories containing other files, or not. As
1540 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1541 long form of the operation without affecting the performance.
1544 * extracting archives::
1545 * extracting files::
1547 * extracting untrusted archives::
1548 * failing commands::
1551 @node extracting archives
1552 @subsection Extracting an Entire Archive
1554 To extract an entire archive, specify the archive file name only, with
1555 no individual file names as arguments. For example,
1558 $ @kbd{tar -xvf collection.tar}
1565 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
1566 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
1567 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
1570 @node extracting files
1571 @subsection Extracting Specific Files
1573 To extract specific archive members, give their exact member names as
1574 arguments, as printed by @option{--list} (@option{-t}). If you had
1575 mistakenly deleted one of the files you had placed in the archive
1576 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1577 from the archive without changing the archive's structure. Its
1578 contents will be identical to the original file @file{blues} that you
1581 First, make sure you are in the @file{practice} directory, and list the
1582 files in the directory. Now, delete the file, @samp{blues}, and list
1583 the files in the directory again.
1585 You can now extract the member @file{blues} from the archive file
1586 @file{collection.tar} like this:
1589 $ @kbd{tar --extract --file=collection.tar blues}
1593 If you list the files in the directory again, you will see that the file
1594 @file{blues} has been restored, with its original permissions, data
1595 modification times, and owner.@footnote{This is only accidentally
1596 true, but not in general. Whereas modification times are always
1597 restored, in most cases, one has to be root for restoring the owner,
1598 and use a special option for restoring permissions. Here, it just
1599 happens that the restoring user is also the owner of the archived
1600 members, and that the current @code{umask} is compatible with original
1601 permissions.} (These parameters will be identical to those which
1602 the file had when you originally placed it in the archive; any changes
1603 you may have made before deleting the file from the file system,
1604 however, will @emph{not} have been made to the archive member.) The
1605 archive file, @samp{collection.tar}, is the same as it was before you
1606 extracted @samp{blues}. You can confirm this by running @command{tar} with
1607 @option{--list} (@option{-t}).
1609 Remember that as with other operations, specifying the exact member
1610 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1611 will fail, because there is no member named @file{birds}. To extract
1612 the member named @file{./birds}, you must specify @w{@kbd{tar
1613 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1614 exact member names, use @option{--list} (@option{-t}) option
1615 (@pxref{list}). You can also extract those members that match a
1616 specific @dfn{globbing pattern}. For example, to extract from
1617 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1618 directory prefix, you could type:
1621 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1625 Here, @option{--wildcards} instructs @command{tar} to treat
1626 command line arguments as globbing patterns and @option{--no-anchored}
1627 informs it that the patterns apply to member names after any @samp{/}
1628 delimiter. The use of globbing patterns is discussed in detail in
1631 You can extract a file to standard output by combining the above options
1632 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1635 If you give the @option{--verbose} option, then @option{--extract}
1636 will print the names of the archive members as it extracts them.
1639 @subsection Extracting Files that are Directories
1641 Extracting directories which are members of an archive is similar to
1642 extracting other files. The main difference to be aware of is that if
1643 the extracted directory has the same name as any directory already in
1644 the working directory, then files in the extracted directory will be
1645 placed into the directory of the same name. Likewise, if there are
1646 files in the pre-existing directory with the same names as the members
1647 which you extract, the files from the extracted archive will replace
1648 the files already in the working directory (and possible
1649 subdirectories). This will happen regardless of whether or not the
1650 files in the working directory were more recent than those extracted
1651 (there exist, however, special options that alter this behavior
1654 However, if a file was stored with a directory name as part of its file
1655 name, and that directory does not exist under the working directory when
1656 the file is extracted, @command{tar} will create the directory.
1658 We can demonstrate how to use @option{--extract} to extract a directory
1659 file with an example. Change to the @file{practice} directory if you
1660 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1661 go back to the parent directory and extract the archive
1662 @file{music.tar}. You may either extract the entire archive, or you may
1663 extract only the files you just deleted. To extract the entire archive,
1664 don't give any file names as arguments after the archive name
1665 @file{music.tar}. To extract only the files you deleted, use the
1669 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1675 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1676 would have displayed more detail about the extracted files, as shown
1677 in the example below:
1680 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1681 -rw-r--r-- me/user 28 1996-10-18 16:31 practice/jazz
1682 -rw-r--r-- me/user 20 1996-09-23 16:44 practice/folk
1686 Because you created the directory with @file{practice} as part of the
1687 file names of each of the files by archiving the @file{practice}
1688 directory as @file{practice}, you must give @file{practice} as part
1689 of the file names when you extract those files from the archive.
1691 @node extracting untrusted archives
1692 @subsection Extracting Archives from Untrusted Sources
1694 Extracting files from archives can overwrite files that already exist.
1695 If you receive an archive from an untrusted source, you should make a
1696 new directory and extract into that directory, so that you don't have
1697 to worry about the extraction overwriting one of your existing files.
1698 For example, if @file{untrusted.tar} came from somewhere else on the
1699 Internet, and you don't necessarily trust its contents, you can
1700 extract it as follows:
1703 $ @kbd{mkdir newdir}
1705 $ @kbd{tar -xvf ../untrusted.tar}
1708 It is also a good practice to examine contents of the archive
1709 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1710 with @option{--verbose} (@option{-v}).
1712 @node failing commands
1713 @subsection Commands That Will Fail
1715 Here are some sample commands you might try which will not work, and why
1718 If you try to use this command,
1721 $ @kbd{tar -xvf music.tar folk jazz}
1725 you will get the following response:
1728 tar: folk: Not found in archive
1729 tar: jazz: Not found in archive
1733 This is because these files were not originally @emph{in} the parent
1734 directory @file{..}, where the archive is located; they were in the
1735 @file{practice} directory, and their file names reflect this:
1738 $ @kbd{tar -tvf music.tar}
1744 @FIXME{make sure the above works when going through the examples in
1748 Likewise, if you try to use this command,
1751 $ @kbd{tar -tvf music.tar folk jazz}
1755 you would get a similar response. Members with those names are not in the
1756 archive. You must use the correct member names, or wildcards, in order
1757 to extract the files from the archive.
1759 If you have forgotten the correct names of the files in the archive,
1760 use @w{@kbd{tar --list --verbose}} to list them correctly.
1762 @FIXME{more examples, here? hag thinks it's a good idea.}
1765 @section Going Further Ahead in this Manual
1768 @FIXME{need to write up a node here about the things that are going to
1769 be in the rest of the manual.}
1771 @node tar invocation
1772 @chapter Invoking @GNUTAR{}
1774 This chapter is about how one invokes the @GNUTAR{}
1775 command, from the command synopsis (@pxref{Synopsis}). There are
1776 numerous options, and many styles for writing them. One mandatory
1777 option specifies the operation @command{tar} should perform
1778 (@pxref{Operation Summary}), other options are meant to detail how
1779 this operation should be performed (@pxref{Option Summary}).
1780 Non-option arguments are not always interpreted the same way,
1781 depending on what the operation is.
1783 You will find in this chapter everything about option styles and rules for
1784 writing them (@pxref{Styles}). On the other hand, operations and options
1785 are fully described elsewhere, in other chapters. Here, you will find
1786 only synthetic descriptions for operations and options, together with
1787 pointers to other parts of the @command{tar} manual.
1789 Some options are so special they are fully described right in this
1790 chapter. They have the effect of inhibiting the normal operation of
1791 @command{tar} or else, they globally alter the amount of feedback the user
1792 receives about what is going on. These are the @option{--help} and
1793 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1794 and @option{--interactive} options (@pxref{interactive}).
1798 * using tar options::
1810 @section General Synopsis of @command{tar}
1812 The @GNUTAR{} program is invoked as either one of:
1815 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1816 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1819 The second form is for when old options are being used.
1821 You can use @command{tar} to store files in an archive, to extract them from
1822 an archive, and to do other types of archive manipulation. The primary
1823 argument to @command{tar}, which is called the @dfn{operation}, specifies
1824 which action to take. The other arguments to @command{tar} are either
1825 @dfn{options}, which change the way @command{tar} performs an operation,
1826 or file names or archive members, which specify the files or members
1827 @command{tar} is to act on.
1829 You can actually type in arguments in any order, even if in this manual
1830 the options always precede the other arguments, to make examples easier
1831 to understand. Further, the option stating the main operation mode
1832 (the @command{tar} main command) is usually given first.
1834 Each @var{name} in the synopsis above is interpreted as an archive member
1835 name when the main command is one of @option{--compare}
1836 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1837 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1838 @option{--update} (@option{-u}). When naming archive members, you
1839 must give the exact name of the member in the archive, as it is
1840 printed by @option{--list}. For @option{--append} (@option{-r}) and
1841 @option{--create} (@option{-c}), these @var{name} arguments specify
1842 the names of either files or directory hierarchies to place in the archive.
1843 These files or hierarchies should already exist in the file system,
1844 prior to the execution of the @command{tar} command.
1846 @command{tar} interprets relative file names as being relative to the
1847 working directory. @command{tar} will make all file names relative
1848 (by removing leading slashes when archiving or restoring files),
1849 unless you specify otherwise (using the @option{--absolute-names}
1850 option). @xref{absolute}, for more information about
1851 @option{--absolute-names}.
1853 If you give the name of a directory as either a file name or a member
1854 name, then @command{tar} acts recursively on all the files and directories
1855 beneath that directory. For example, the name @file{/} identifies all
1856 the files in the file system to @command{tar}.
1858 The distinction between file names and archive member names is especially
1859 important when shell globbing is used, and sometimes a source of confusion
1860 for newcomers. @xref{wildcards}, for more information about globbing.
1861 The problem is that shells may only glob using existing files in the
1862 file system. Only @command{tar} itself may glob on archive members, so when
1863 needed, you must ensure that wildcard characters reach @command{tar} without
1864 being interpreted by the shell first. Using a backslash before @samp{*}
1865 or @samp{?}, or putting the whole argument between quotes, is usually
1866 sufficient for this.
1868 Even if @var{name}s are often specified on the command line, they
1869 can also be read from a text file in the file system, using the
1870 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1872 If you don't use any file name arguments, @option{--append} (@option{-r}),
1873 @option{--delete} and @option{--concatenate} (@option{--catenate},
1874 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1875 will usually yield a diagnostic and inhibit @command{tar} execution.
1876 The other operations of @command{tar} (@option{--list},
1877 @option{--extract}, @option{--compare}, and @option{--update})
1878 will act on the entire contents of the archive.
1881 @cindex return status
1882 Besides successful exits, @GNUTAR{} may fail for
1883 many reasons. Some reasons correspond to bad usage, that is, when the
1884 @command{tar} command line is improperly written. Errors may be
1885 encountered later, while processing the archive or the files. Some
1886 errors are recoverable, in which case the failure is delayed until
1887 @command{tar} has completed all its work. Some errors are such that
1888 it would be not meaningful, or at least risky, to continue processing:
1889 @command{tar} then aborts processing immediately. All abnormal exits,
1890 whether immediate or delayed, should always be clearly diagnosed on
1891 @code{stderr}, after a line stating the nature of the error.
1893 Possible exit codes of @GNUTAR{} are summarized in the following
1898 @samp{Successful termination}.
1901 @samp{Some files differ}. If tar was invoked with @option{--compare}
1902 (@option{--diff}, @option{-d}) command line option, this means that
1903 some files in the archive differ from their disk counterparts
1904 (@pxref{compare}). If tar was given @option{--create},
1905 @option{--append} or @option{--update} option, this exit code means
1906 that some files were changed while being archived and so the resulting
1907 archive does not contain the exact copy of the file set.
1910 @samp{Fatal error}. This means that some fatal, unrecoverable error
1914 If @command{tar} has invoked a subprocess and that subprocess exited with a
1915 nonzero exit code, @command{tar} exits with that code as well.
1916 This can happen, for example, if @command{tar} was given some
1917 compression option (@pxref{gzip}) and the external compressor program
1918 failed. Another example is @command{rmt} failure during backup to the
1919 remote device (@pxref{Remote Tape Server}).
1921 @node using tar options
1922 @section Using @command{tar} Options
1924 @GNUTAR{} has a total of eight operating modes which
1925 allow you to perform a variety of tasks. You are required to choose
1926 one operating mode each time you employ the @command{tar} program by
1927 specifying one, and only one operation as an argument to the
1928 @command{tar} command (the corresponding options may be found
1929 at @ref{frequent operations} and @ref{Operations}). Depending on
1930 circumstances, you may also wish to customize how the chosen operating
1931 mode behaves. For example, you may wish to change the way the output
1932 looks, or the format of the files that you wish to archive may require
1933 you to do something special in order to make the archive look right.
1935 You can customize and control @command{tar}'s performance by running
1936 @command{tar} with one or more options (such as @option{--verbose}
1937 (@option{-v}), which we used in the tutorial). As we said in the
1938 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1939 their name suggests) optional. Depending on the operating mode, you
1940 may specify one or more options. Different options will have different
1941 effects, but in general they all change details of the operation, such
1942 as archive format, archive name, or level of user interaction. Some
1943 options make sense with all operating modes, while others are
1944 meaningful only with particular modes. You will likely use some
1945 options frequently, while you will only use others infrequently, or
1946 not at all. (A full list of options is available in @pxref{All Options}.)
1948 @vrindex TAR_OPTIONS, environment variable
1949 @anchor{TAR_OPTIONS}
1950 The @env{TAR_OPTIONS} environment variable specifies default options to
1951 be placed in front of any explicit options. For example, if
1952 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1953 if the two options @option{-v} and @option{--unlink-first} had been
1954 specified before any explicit options. Option specifications are
1955 separated by whitespace. A backslash escapes the next character, so it
1956 can be used to specify an option containing whitespace or a backslash.
1958 Note that @command{tar} options are case sensitive. For example, the
1959 options @option{-T} and @option{-t} are different; the first requires an
1960 argument for stating the name of a file providing a list of @var{name}s,
1961 while the second does not require an argument and is another way to
1962 write @option{--list} (@option{-t}).
1964 In addition to the eight operations, there are many options to
1965 @command{tar}, and three different styles for writing both: long (mnemonic)
1966 form, short form, and old style. These styles are discussed below.
1967 Both the options and the operations can be written in any of these three
1970 @FIXME{menu at end of this node. need to think of an actual outline
1971 for this chapter; probably do that after stuff from chapter 4 is
1975 @section The Three Option Styles
1977 There are three styles for writing operations and options to the command
1978 line invoking @command{tar}. The different styles were developed at
1979 different times during the history of @command{tar}. These styles will be
1980 presented below, from the most recent to the oldest.
1982 Some options must take an argument@footnote{For example, @option{--file}
1983 (@option{-f}) takes the name of an archive file as an argument. If
1984 you do not supply an archive file name, @command{tar} will use a
1985 default, but this can be confusing; thus, we recommend that you always
1986 supply a specific archive file name.}. Where you @emph{place} the
1987 arguments generally depends on which style of options you choose. We
1988 will detail specific information relevant to each option style in the
1989 sections on the different option styles, below. The differences are
1990 subtle, yet can often be very important; incorrect option placement
1991 can cause you to overwrite a number of important files. We urge you
1992 to note these differences, and only use the option style(s) which
1993 makes the most sense to you until you feel comfortable with the others.
1995 Some options @emph{may} take an argument. Such options may have at
1996 most long and short forms, they do not have old style equivalent. The
1997 rules for specifying an argument for such options are stricter than
1998 those for specifying mandatory arguments. Please, pay special
2002 * Long Options:: Long Option Style
2003 * Short Options:: Short Option Style
2004 * Old Options:: Old Option Style
2005 * Mixing:: Mixing Option Styles
2009 @subsection Long Option Style
2011 @cindex long options
2012 @cindex options, long style
2013 @cindex options, GNU style
2014 @cindex options, mnemonic names
2015 Each option has at least one @dfn{long} (or @dfn{mnemonic}) name starting with two
2016 dashes in a row, e.g., @option{--list}. The long names are more clear than
2017 their corresponding short or old names. It sometimes happens that a
2018 single long option has many different names which are
2019 synonymous, such as @option{--compare} and @option{--diff}. In addition,
2020 long option names can be given unique abbreviations. For example,
2021 @option{--cre} can be used in place of @option{--create} because there is no
2022 other long option which begins with @samp{cre}. (One way to find
2023 this out is by trying it and seeing what happens; if a particular
2024 abbreviation could represent more than one option, @command{tar} will tell
2025 you that that abbreviation is ambiguous and you'll know that that
2026 abbreviation won't work. You may also choose to run @samp{tar --help}
2027 to see a list of options. Be aware that if you run @command{tar} with a
2028 unique abbreviation for the long name of an option you didn't want to
2029 use, you are stuck; @command{tar} will perform the command as ordered.)
2031 Long options are meant to be obvious and easy to remember, and their
2032 meanings are generally easier to discern than those of their
2033 corresponding short options (see below). For example:
2036 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
2040 gives a fairly good set of hints about what the command does, even
2041 for those not fully acquainted with @command{tar}.
2043 @cindex arguments to long options
2044 @cindex long options with mandatory arguments
2045 Long options which require arguments take those arguments
2046 immediately following the option name. There are two ways of
2047 specifying a mandatory argument. It can be separated from the
2048 option name either by an equal sign, or by any amount of
2049 white space characters. For example, the @option{--file} option (which
2050 tells the name of the @command{tar} archive) is given a file such as
2051 @file{archive.tar} as argument by using any of the following notations:
2052 @option{--file=archive.tar} or @option{--file archive.tar}.
2054 @cindex optional arguments to long options
2055 @cindex long options with optional arguments
2056 In contrast, optional arguments must always be introduced using
2057 an equal sign. For example, the @option{--backup} option takes
2058 an optional argument specifying backup type. It must be used
2059 as @option{--backup=@var{backup-type}}.
2062 @subsection Short Option Style
2064 @cindex short options
2065 @cindex options, short style
2066 @cindex options, traditional
2067 Most options also have a @dfn{short option} name. Short options start with
2068 a single dash, and are followed by a single character, e.g., @option{-t}
2069 (which is equivalent to @option{--list}). The forms are absolutely
2070 identical in function; they are interchangeable.
2072 The short option names are faster to type than long option names.
2074 @cindex arguments to short options
2075 @cindex short options with mandatory arguments
2076 Short options which require arguments take their arguments immediately
2077 following the option, usually separated by white space. It is also
2078 possible to stick the argument right after the short option name, using
2079 no intervening space. For example, you might write @w{@option{-f
2080 archive.tar}} or @option{-farchive.tar} instead of using
2081 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
2082 @w{@option{-f @var{archive-name}}} denote the option which indicates a
2083 specific archive, here named @file{archive.tar}.
2085 @cindex optional arguments to short options
2086 @cindex short options with optional arguments
2087 Short options which take optional arguments take their arguments
2088 immediately following the option letter, @emph{without any intervening
2089 white space characters}.
2091 Short options' letters may be clumped together, but you are not
2092 required to do this (as compared to old options; see below). When
2093 short options are clumped as a set, use one (single) dash for them
2094 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
2095 such a set is allowed to have an argument@footnote{Clustering many
2096 options, the last of which has an argument, is a rather opaque way to
2097 write options. Some wonder if @acronym{GNU} @code{getopt} should not
2098 even be made helpful enough for considering such usages as invalid.}.
2100 When the options are separated, the argument for each option which requires
2101 an argument directly follows that option, as is usual for Unix programs.
2105 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
2108 If you reorder short options' locations, be sure to move any arguments
2109 that belong to them. If you do not move the arguments properly, you may
2110 end up overwriting files.
2113 @subsection Old Option Style
2114 @cindex options, old style
2115 @cindex old option style
2116 @cindex option syntax, traditional
2118 As far as we know, all @command{tar} programs, @acronym{GNU} and
2119 non-@acronym{GNU}, support @dfn{old options}: that is, if the first
2120 argument does not start with @samp{-}, it is assumed to specify option
2121 letters. @GNUTAR{} supports old options not only for historical
2122 reasons, but also because many people are used to them. If the first
2123 argument does not start with a dash, you are announcing the old option
2124 style instead of the short option style; old options are decoded
2127 Like short options, old options are single letters. However, old options
2128 must be written together as a single clumped set, without spaces separating
2129 them or dashes preceding them. This set
2130 of letters must be the first to appear on the command line, after the
2131 @command{tar} program name and some white space; old options cannot appear
2132 anywhere else. The letter of an old option is exactly the same letter as
2133 the corresponding short option. For example, the old option @samp{t} is
2134 the same as the short option @option{-t}, and consequently, the same as the
2135 long option @option{--list}. So for example, the command @w{@samp{tar
2136 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
2138 @cindex arguments to old options
2139 @cindex old options with mandatory arguments
2140 When options that need arguments are given together with the command,
2141 all the associated arguments follow, in the same order as the options.
2142 Thus, the example given previously could also be written in the old
2146 $ @kbd{tar cvbf 20 /dev/rmt0}
2150 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
2151 the argument of @option{-f}.
2153 The old style syntax can make it difficult to match
2154 option letters with their corresponding arguments, and is often
2155 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
2156 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
2157 argument for @option{-f}, and @option{-v} does not have a corresponding
2158 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
2159 /dev/rmt0}} is clearer, putting all arguments next to the option they
2162 If you want to reorder the letters in the old option argument, be
2163 sure to reorder any corresponding argument appropriately.
2165 This old way of writing @command{tar} options can surprise even experienced
2166 users. For example, the two commands:
2169 @kbd{tar cfz archive.tar.gz file}
2170 @kbd{tar -cfz archive.tar.gz file}
2174 are quite different. The first example uses @file{archive.tar.gz} as
2175 the value for option @samp{f} and recognizes the option @samp{z}. The
2176 second example, however, uses @file{z} as the value for option
2177 @samp{f} --- probably not what was intended.
2179 This second example could be corrected in many ways, among which the
2180 following are equivalent:
2183 @kbd{tar -czf archive.tar.gz file}
2184 @kbd{tar -cf archive.tar.gz -z file}
2185 @kbd{tar cf archive.tar.gz -z file}
2189 @subsection Mixing Option Styles
2191 @cindex options, mixing different styles
2192 All three styles may be intermixed in a single @command{tar} command,
2193 so long as the rules for each style are fully
2194 respected@footnote{Before @GNUTAR{} version 1.11.6,
2195 a bug prevented intermixing old style options with long options in
2196 some cases.}. Old style options and either of the modern styles of
2197 options may be mixed within a single @command{tar} command. However,
2198 old style options must be introduced as the first arguments only,
2199 following the rule for old options (old options must appear directly
2200 after the @command{tar} command and some white space). Modern options
2201 may be given only after all arguments to the old options have been
2202 collected. If this rule is not respected, a modern option might be
2203 falsely interpreted as the value of the argument to one of the old
2206 For example, all the following commands are wholly equivalent, and
2207 illustrate the many combinations and orderings of option styles.
2210 @kbd{tar --create --file=archive.tar}
2211 @kbd{tar --create -f archive.tar}
2212 @kbd{tar --create -farchive.tar}
2213 @kbd{tar --file=archive.tar --create}
2214 @kbd{tar --file=archive.tar -c}
2215 @kbd{tar -c --file=archive.tar}
2216 @kbd{tar -c -f archive.tar}
2217 @kbd{tar -c -farchive.tar}
2218 @kbd{tar -cf archive.tar}
2219 @kbd{tar -cfarchive.tar}
2220 @kbd{tar -f archive.tar --create}
2221 @kbd{tar -f archive.tar -c}
2222 @kbd{tar -farchive.tar --create}
2223 @kbd{tar -farchive.tar -c}
2224 @kbd{tar c --file=archive.tar}
2225 @kbd{tar c -f archive.tar}
2226 @kbd{tar c -farchive.tar}
2227 @kbd{tar cf archive.tar}
2228 @kbd{tar f archive.tar --create}
2229 @kbd{tar f archive.tar -c}
2230 @kbd{tar fc archive.tar}
2233 On the other hand, the following commands are @emph{not} equivalent to
2237 @kbd{tar -f -c archive.tar}
2238 @kbd{tar -fc archive.tar}
2239 @kbd{tar -fcarchive.tar}
2240 @kbd{tar -farchive.tarc}
2241 @kbd{tar cfarchive.tar}
2245 These last examples mean something completely different from what the
2246 user intended (judging based on the example in the previous set which
2247 uses long options, whose intent is therefore very clear). The first
2248 four specify that the @command{tar} archive would be a file named
2249 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2250 respectively. The first two examples also specify a single non-option,
2251 @var{name} argument having the value @samp{archive.tar}. The last
2252 example contains only old style option letters (repeating option
2253 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2254 @samp{h}, or @samp{i}), with no argument value. @FIXME{not sure i liked
2255 the first sentence of this paragraph..}
2258 @section All @command{tar} Options
2260 The coming manual sections contain an alphabetical listing of all
2261 @command{tar} operations and options, with brief descriptions and
2262 cross-references to more in-depth explanations in the body of the manual.
2263 They also contain an alphabetically arranged table of the short option
2264 forms with their corresponding long option. You can use this table as
2265 a reference for deciphering @command{tar} commands in scripts.
2268 * Operation Summary::
2270 * Short Option Summary::
2273 @node Operation Summary
2274 @subsection Operations
2282 Appends files to the end of the archive. @xref{append}.
2284 @opsummary{catenate}
2288 Same as @option{--concatenate}. @xref{concatenate}.
2294 Compares archive members with their counterparts in the file
2295 system, and reports differences in file size, mode, owner,
2296 modification date and contents. @xref{compare}.
2298 @opsummary{concatenate}
2302 Appends other @command{tar} archives to the end of the archive.
2309 Creates a new @command{tar} archive. @xref{create}.
2314 Deletes members from the archive. Don't try this on an archive on a
2315 tape! @xref{delete}.
2321 Same @option{--compare}. @xref{compare}.
2327 Extracts members from the archive into the file system. @xref{extract}.
2333 Same as @option{--extract}. @xref{extract}.
2339 Lists the members in an archive. @xref{list}.
2345 Adds files to the end of the archive, but only if they are newer than
2346 their counterparts already in the archive, or if they do not already
2347 exist in the archive. @xref{update}.
2351 @node Option Summary
2352 @subsection @command{tar} Options
2356 @opsummary{absolute-names}
2357 @item --absolute-names
2360 Normally when creating an archive, @command{tar} strips an initial
2361 @samp{/} from member names. This option disables that behavior.
2364 @opsummary{after-date}
2367 (See @option{--newer}, @pxref{after})
2369 @opsummary{anchored}
2371 A pattern must match an initial subsequence of the name's components.
2372 @xref{controlling pattern-matching}.
2374 @opsummary{atime-preserve}
2375 @item --atime-preserve
2376 @itemx --atime-preserve=replace
2377 @itemx --atime-preserve=system
2379 Attempt to preserve the access time of files when reading them. This
2380 option currently is effective only on files that you own, unless you
2381 have superuser privileges.
2383 @option{--atime-preserve=replace} remembers the access time of a file
2384 before reading it, and then restores the access time afterwards. This
2385 may cause problems if other programs are reading the file at the same
2386 time, as the times of their accesses will be lost. On most platforms
2387 restoring the access time also requires @command{tar} to restore the
2388 data modification time too, so this option may also cause problems if
2389 other programs are writing the file at the same time (@command{tar} attempts
2390 to detect this situation, but cannot do so reliably due to race
2391 conditions). Worse, on most platforms restoring the access time also
2392 updates the status change time, which means that this option is
2393 incompatible with incremental backups.
2395 @option{--atime-preserve=system} avoids changing time stamps on files,
2396 without interfering with time stamp updates
2397 caused by other programs, so it works better with incremental backups.
2398 However, it requires a special @code{O_NOATIME} option from the
2399 underlying operating and file system implementation, and it also requires
2400 that searching directories does not update their access times. As of
2401 this writing (November 2005) this works only with Linux, and only with
2402 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2403 way to know whether this feature actually works. Sometimes
2404 @command{tar} knows that it does not work, and if you use
2405 @option{--atime-preserve=system} then @command{tar} complains and
2406 exits right away. But other times @command{tar} might think that the
2407 option works when it actually does not.
2409 Currently @option{--atime-preserve} with no operand defaults to
2410 @option{--atime-preserve=replace}, but this may change in the future
2411 as support for @option{--atime-preserve=system} improves.
2413 If your operating or file system does not support
2414 @option{--atime-preserve=@-system}, you might be able to preserve access
2415 times reliably by using the @command{mount} command. For example,
2416 you can mount the file system read-only, or access the file system via
2417 a read-only loopback mount, or use the @samp{noatime} mount option
2418 available on some systems. However, mounting typically requires
2419 superuser privileges and can be a pain to manage.
2421 @opsummary{auto-compress}
2422 @item --auto-compress
2425 During a @option{--create} operation, enables automatic compressed
2426 format recognition based on the archive suffix. The effect of this
2427 option is cancelled by @option{--no-auto-compress}. @xref{gzip}.
2430 @item --backup=@var{backup-type}
2432 Rather than deleting files from the file system, @command{tar} will
2433 back them up using simple or numbered backups, depending upon
2434 @var{backup-type}. @xref{backup}.
2436 @opsummary{block-number}
2437 @item --block-number
2440 With this option present, @command{tar} prints error messages for read errors
2441 with the block number in the archive file. @xref{block-number}.
2443 @opsummary{blocking-factor}
2444 @item --blocking-factor=@var{blocking}
2445 @itemx -b @var{blocking}
2447 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2448 record. @xref{Blocking Factor}.
2454 This option tells @command{tar} to read or write archives through
2455 @code{bzip2}. @xref{gzip}.
2457 @opsummary{check-device}
2458 @item --check-device
2459 Check device numbers when creating a list of modified files for
2460 incremental archiving. This is the default. @xref{device numbers},
2461 for a detailed description.
2463 @opsummary{checkpoint}
2464 @item --checkpoint[=@var{number}]
2466 This option directs @command{tar} to print periodic checkpoint
2467 messages as it reads through the archive. It is intended for when you
2468 want a visual indication that @command{tar} is still running, but
2469 don't want to see @option{--verbose} output. You can also instruct
2470 @command{tar} to execute a list of actions on each checkpoint, see
2471 @option{--checkpoint-action} below. For a detailed description, see
2474 @opsummary{checkpoint-action}
2475 @item --checkpoint-action=@var{action}
2476 Instruct @command{tar} to execute an action upon hitting a
2477 breakpoint. Here we give only a brief outline. @xref{checkpoints},
2478 for a complete description.
2480 The @var{action} argument can be one of the following:
2484 Produce an audible bell on the console.
2488 Print a single dot on the standard listing stream.
2491 Display a textual message on the standard error, with the status and
2492 number of the checkpoint. This is the default.
2494 @item echo=@var{string}
2495 Display @var{string} on the standard error. Before output, the string
2496 is subject to meta-character expansion.
2498 @item exec=@var{command}
2499 Execute the given @var{command}.
2501 @item sleep=@var{time}
2502 Wait for @var{time} seconds.
2504 @item ttyout=@var{string}
2505 Output @var{string} on the current console (@file{/dev/tty}).
2508 Several @option{--checkpoint-action} options can be specified. The
2509 supplied actions will be executed in order of their appearance in the
2512 Using @option{--checkpoint-action} without @option{--checkpoint}
2513 assumes default checkpoint frequency of one checkpoint per 10 records.
2515 @opsummary{check-links}
2518 If this option was given, @command{tar} will check the number of links
2519 dumped for each processed file. If this number does not match the
2520 total number of hard links for the file, a warning message will be
2521 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2522 synonym for @option{--one-file-system}. The current semantics, which
2523 complies to UNIX98, was introduced with version
2524 1.15.91. @xref{Changes}, for more information.}.
2528 @opsummary{compress}
2529 @opsummary{uncompress}
2534 @command{tar} will use the @command{compress} program when reading or
2535 writing the archive. This allows you to directly act on archives
2536 while saving space. @xref{gzip}.
2538 @opsummary{confirmation}
2539 @item --confirmation
2541 (See @option{--interactive}.) @xref{interactive}.
2543 @opsummary{delay-directory-restore}
2544 @item --delay-directory-restore
2546 Delay setting modification times and permissions of extracted
2547 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2549 @opsummary{dereference}
2553 When reading or writing a file to be archived, @command{tar} accesses
2554 the file that a symbolic link points to, rather than the symlink
2555 itself. @xref{dereference}.
2557 @opsummary{directory}
2558 @item --directory=@var{dir}
2561 When this option is specified, @command{tar} will change its current directory
2562 to @var{dir} before performing any operations. When this option is used
2563 during archive creation, it is order sensitive. @xref{directory}.
2566 @item --exclude=@var{pattern}
2568 When performing operations, @command{tar} will skip files that match
2569 @var{pattern}. @xref{exclude}.
2571 @opsummary{exclude-backups}
2572 @item --exclude-backups
2573 Exclude backup and lock files. @xref{exclude,, exclude-backups}.
2575 @opsummary{exclude-from}
2576 @item --exclude-from=@var{file}
2577 @itemx -X @var{file}
2579 Similar to @option{--exclude}, except @command{tar} will use the list of
2580 patterns in the file @var{file}. @xref{exclude}.
2582 @opsummary{exclude-caches}
2583 @item --exclude-caches
2585 Exclude from dump any directory containing a valid cache directory
2586 tag file, but still dump the directory node and the tag file itself.
2588 @xref{exclude,, exclude-caches}.
2590 @opsummary{exclude-caches-under}
2591 @item --exclude-caches-under
2593 Exclude from dump any directory containing a valid cache directory
2594 tag file, but still dump the directory node itself.
2598 @opsummary{exclude-caches-all}
2599 @item --exclude-caches-all
2601 Exclude from dump any directory containing a valid cache directory
2602 tag file. @xref{exclude}.
2604 @opsummary{exclude-tag}
2605 @item --exclude-tag=@var{file}
2607 Exclude from dump any directory containing file named @var{file}, but
2608 dump the directory node and @var{file} itself. @xref{exclude,, exclude-tag}.
2610 @opsummary{exclude-tag-under}
2611 @item --exclude-tag-under=@var{file}
2613 Exclude from dump the contents of any directory containing file
2614 named @var{file}, but dump the directory node itself. @xref{exclude,,
2617 @opsummary{exclude-tag-all}
2618 @item --exclude-tag-all=@var{file}
2620 Exclude from dump any directory containing file named @var{file}.
2621 @xref{exclude,,exclude-tag-all}.
2623 @opsummary{exclude-vcs}
2626 Exclude from dump directories and files, that are internal for some
2627 widely used version control systems.
2629 @xref{exclude,,exclude-vcs}.
2632 @item --file=@var{archive}
2633 @itemx -f @var{archive}
2635 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2636 performs operations on, rather than @command{tar}'s compilation dependent
2637 default. @xref{file tutorial}.
2639 @opsummary{files-from}
2640 @item --files-from=@var{file}
2641 @itemx -T @var{file}
2643 @command{tar} will use the contents of @var{file} as a list of archive members
2644 or files to operate on, in addition to those specified on the
2645 command-line. @xref{files}.
2647 @opsummary{force-local}
2650 Forces @command{tar} to interpret the file name given to @option{--file}
2651 as a local file, even if it looks like a remote tape drive name.
2652 @xref{local and remote archives}.
2655 @item --format=@var{format}
2656 @itemx -H @var{format}
2658 Selects output archive format. @var{Format} may be one of the
2663 Creates an archive that is compatible with Unix V7 @command{tar}.
2666 Creates an archive that is compatible with GNU @command{tar} version
2670 Creates archive in GNU tar 1.13 format. Basically it is the same as
2671 @samp{oldgnu} with the only difference in the way it handles long
2675 Creates a @acronym{POSIX.1-1988} compatible archive.
2678 Creates a @acronym{POSIX.1-2001 archive}.
2682 @xref{Formats}, for a detailed discussion of these formats.
2684 @opsummary{full-time}
2686 This option instructs @command{tar} to print file times to their full
2687 resolution. Usually this means 1-second resolution, but that depends
2688 on the underlying file system. The @option{--full-time} option takes
2689 effect only when detailed output (verbosity level 2 or higher) has
2690 been requested using the @option{--verbose} option, e.g., when listing
2691 or extracting archives:
2694 $ @kbd{tar -t -v --full-time -f archive.tar}
2698 or, when creating an archive:
2701 $ @kbd{tar -c -vv --full-time -f archive.tar .}
2704 Notice, thar when creating the archive you need to specify
2705 @option{--verbose} twice to get a detailed output (@pxref{verbose
2709 @item --group=@var{group}
2711 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
2712 rather than the group from the source file. @var{group} can specify a
2713 symbolic name, or a numeric @acronym{ID}, or both as
2714 @var{name}:@var{id}. @xref{override}.
2716 Also see the comments for the @option{--owner=@var{user}} option.
2726 This option tells @command{tar} to read or write archives through
2727 @command{gzip}, allowing @command{tar} to directly operate on several
2728 kinds of compressed archives transparently. @xref{gzip}.
2730 @opsummary{hard-dereference}
2731 @item --hard-dereference
2732 When creating an archive, dereference hard links and store the files
2733 they refer to, instead of creating usual hard link members.
2741 @command{tar} will print out a short message summarizing the operations and
2742 options to @command{tar} and exit. @xref{help}.
2744 @opsummary{ignore-case}
2746 Ignore case when matching member or file names with
2747 patterns. @xref{controlling pattern-matching}.
2749 @opsummary{ignore-command-error}
2750 @item --ignore-command-error
2751 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2753 @opsummary{ignore-failed-read}
2754 @item --ignore-failed-read
2756 Do not exit unsuccessfully merely because an unreadable file was encountered.
2759 @opsummary{ignore-zeros}
2760 @item --ignore-zeros
2763 With this option, @command{tar} will ignore zeroed blocks in the
2764 archive, which normally signals EOF. @xref{Reading}.
2766 @opsummary{incremental}
2770 Informs @command{tar} that it is working with an old
2771 @acronym{GNU}-format incremental backup archive. It is intended
2772 primarily for backwards compatibility only. @xref{Incremental Dumps},
2773 for a detailed discussion of incremental archives.
2775 @opsummary{index-file}
2776 @item --index-file=@var{file}
2778 Send verbose output to @var{file} instead of to standard output.
2780 @opsummary{info-script}
2781 @opsummary{new-volume-script}
2782 @item --info-script=@var{script-file}
2783 @itemx --new-volume-script=@var{script-file}
2784 @itemx -F @var{script-file}
2786 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2787 at the end of each tape. If @var{script-file} exits with nonzero status,
2788 @command{tar} fails immediately. @xref{info-script}, for a detailed
2789 discussion of @var{script-file}.
2791 @opsummary{interactive}
2793 @itemx --confirmation
2796 Specifies that @command{tar} should ask the user for confirmation before
2797 performing potentially destructive options, such as overwriting files.
2800 @opsummary{keep-newer-files}
2801 @item --keep-newer-files
2803 Do not replace existing files that are newer than their archive copies
2804 when extracting files from an archive.
2806 @opsummary{keep-old-files}
2807 @item --keep-old-files
2810 Do not overwrite existing files when extracting files from an archive.
2811 @xref{Keep Old Files}.
2814 @item --label=@var{name}
2815 @itemx -V @var{name}
2817 When creating an archive, instructs @command{tar} to write @var{name}
2818 as a name record in the archive. When extracting or listing archives,
2819 @command{tar} will only operate on archives that have a label matching
2820 the pattern specified in @var{name}. @xref{Tape Files}.
2823 @item --level=@var{n}
2824 Force incremental backup of level @var{n}. As of @GNUTAR version
2825 @value{VERSION}, the option @option{--level=0} truncates the snapshot
2826 file, thereby forcing the level 0 dump. Other values of @var{n} are
2827 effectively ignored. @xref{--level=0}, for details and examples.
2829 The use of this option is valid only in conjunction with the
2830 @option{--listed-incremental} option. @xref{Incremental Dumps},
2831 for a detailed description.
2833 @opsummary{listed-incremental}
2834 @item --listed-incremental=@var{snapshot-file}
2835 @itemx -g @var{snapshot-file}
2837 During a @option{--create} operation, specifies that the archive that
2838 @command{tar} creates is a new @acronym{GNU}-format incremental
2839 backup, using @var{snapshot-file} to determine which files to backup.
2840 With other operations, informs @command{tar} that the archive is in
2841 incremental format. @xref{Incremental Dumps}.
2846 This option tells @command{tar} to read or write archives through
2847 @command{lzip}. @xref{gzip}.
2852 This option tells @command{tar} to read or write archives through
2853 @command{lzma}. @xref{gzip}.
2857 This option tells @command{tar} to read or write archives through
2858 @command{lzop}. @xref{gzip}.
2861 @item --mode=@var{permissions}
2863 When adding files to an archive, @command{tar} will use
2864 @var{permissions} for the archive members, rather than the permissions
2865 from the files. @var{permissions} can be specified either as an octal
2866 number or as symbolic permissions, like with
2867 @command{chmod}. @xref{override}.
2870 @item --mtime=@var{date}
2872 When adding files to an archive, @command{tar} will use @var{date} as
2873 the modification time of members when creating archives, instead of
2874 their actual modification times. The value of @var{date} can be
2875 either a textual date representation (@pxref{Date input formats}) or a
2876 name of the existing file, starting with @samp{/} or @samp{.}. In the
2877 latter case, the modification time of that file is used. @xref{override}.
2879 @opsummary{multi-volume}
2880 @item --multi-volume
2883 Informs @command{tar} that it should create or otherwise operate on a
2884 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2886 @opsummary{new-volume-script}
2887 @item --new-volume-script
2889 (see @option{--info-script})
2892 @item --newer=@var{date}
2893 @itemx --after-date=@var{date}
2896 When creating an archive, @command{tar} will only add files that have changed
2897 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2898 is taken to be the name of a file whose data modification time specifies
2899 the date. @xref{after}.
2901 @opsummary{newer-mtime}
2902 @item --newer-mtime=@var{date}
2904 Like @option{--newer}, but add only files whose
2905 contents have changed (as opposed to just @option{--newer}, which will
2906 also back up files for which any status information has
2907 changed). @xref{after}.
2909 @opsummary{no-anchored}
2911 An exclude pattern can match any subsequence of the name's components.
2912 @xref{controlling pattern-matching}.
2914 @opsummary{no-auto-compress}
2915 @item --no-auto-compress
2917 Disables automatic compressed format recognition based on the archive
2918 suffix. @xref{--auto-compress}. @xref{gzip}.
2920 @opsummary{no-check-device}
2921 @item --no-check-device
2922 Do not check device numbers when creating a list of modified files
2923 for incremental archiving. @xref{device numbers}, for
2924 a detailed description.
2926 @opsummary{no-delay-directory-restore}
2927 @item --no-delay-directory-restore
2929 Modification times and permissions of extracted
2930 directories are set when all files from this directory have been
2931 extracted. This is the default.
2932 @xref{Directory Modification Times and Permissions}.
2934 @opsummary{no-ignore-case}
2935 @item --no-ignore-case
2936 Use case-sensitive matching.
2937 @xref{controlling pattern-matching}.
2939 @opsummary{no-ignore-command-error}
2940 @item --no-ignore-command-error
2941 Print warnings about subprocesses that terminated with a nonzero exit
2942 code. @xref{Writing to an External Program}.
2947 If the @option{--null} option was given previously, this option
2948 cancels its effect, so that any following @option{--files-from}
2949 options will expect their file lists to be newline-terminated.
2951 @opsummary{no-overwrite-dir}
2952 @item --no-overwrite-dir
2954 Preserve metadata of existing directories when extracting files
2955 from an archive. @xref{Overwrite Old Files}.
2957 @opsummary{no-quote-chars}
2958 @item --no-quote-chars=@var{string}
2959 Remove characters listed in @var{string} from the list of quoted
2960 characters set by the previous @option{--quote-chars} option
2961 (@pxref{quoting styles}).
2963 @opsummary{no-recursion}
2964 @item --no-recursion
2966 With this option, @command{tar} will not recurse into directories.
2969 @opsummary{no-same-owner}
2970 @item --no-same-owner
2973 When extracting an archive, do not attempt to preserve the owner
2974 specified in the @command{tar} archive. This the default behavior
2977 @opsummary{no-same-permissions}
2978 @item --no-same-permissions
2980 When extracting an archive, subtract the user's umask from files from
2981 the permissions specified in the archive. This is the default behavior
2987 The archive media does not support seeks to arbitrary
2988 locations. Usually @command{tar} determines automatically whether
2989 the archive can be seeked or not. Use this option to disable this
2992 @opsummary{no-unquote}
2994 Treat all input file or member names literally, do not interpret
2995 escape sequences. @xref{input name quoting}.
2997 @opsummary{no-wildcards}
2998 @item --no-wildcards
2999 Do not use wildcards.
3000 @xref{controlling pattern-matching}.
3002 @opsummary{no-wildcards-match-slash}
3003 @item --no-wildcards-match-slash
3004 Wildcards do not match @samp{/}.
3005 @xref{controlling pattern-matching}.
3010 When @command{tar} is using the @option{--files-from} option, this option
3011 instructs @command{tar} to expect file names terminated with @acronym{NUL}, so
3012 @command{tar} can correctly work with file names that contain newlines.
3015 @opsummary{numeric-owner}
3016 @item --numeric-owner
3018 This option will notify @command{tar} that it should use numeric user
3019 and group IDs when creating a @command{tar} file, rather than names.
3023 The function of this option depends on the action @command{tar} is
3024 performing. When extracting files, @option{-o} is a synonym for
3025 @option{--no-same-owner}, i.e., it prevents @command{tar} from
3026 restoring ownership of files being extracted.
3028 When creating an archive, it is a synonym for
3029 @option{--old-archive}. This behavior is for compatibility
3030 with previous versions of @GNUTAR{}, and will be
3031 removed in future releases.
3033 @xref{Changes}, for more information.
3035 @opsummary{occurrence}
3036 @item --occurrence[=@var{number}]
3038 This option can be used in conjunction with one of the subcommands
3039 @option{--delete}, @option{--diff}, @option{--extract} or
3040 @option{--list} when a list of files is given either on the command
3041 line or via @option{-T} option.
3043 This option instructs @command{tar} to process only the @var{number}th
3044 occurrence of each named file. @var{Number} defaults to 1, so
3047 tar -x -f archive.tar --occurrence filename
3051 will extract the first occurrence of the member @file{filename} from @file{archive.tar}
3052 and will terminate without scanning to the end of the archive.
3054 @opsummary{old-archive}
3056 Synonym for @option{--format=v7}.
3058 @opsummary{one-file-system}
3059 @item --one-file-system
3060 Used when creating an archive. Prevents @command{tar} from recursing into
3061 directories that are on different file systems from the current
3064 @opsummary{overwrite}
3067 Overwrite existing files and directory metadata when extracting files
3068 from an archive. @xref{Overwrite Old Files}.
3070 @opsummary{overwrite-dir}
3071 @item --overwrite-dir
3073 Overwrite the metadata of existing directories when extracting files
3074 from an archive. @xref{Overwrite Old Files}.
3077 @item --owner=@var{user}
3079 Specifies that @command{tar} should use @var{user} as the owner of members
3080 when creating archives, instead of the user associated with the source
3081 file. @var{user} can specify a symbolic name, or a numeric
3082 @acronym{ID}, or both as @var{name}:@var{id}.
3085 This option does not affect extraction from archives.
3087 @opsummary{pax-option}
3088 @item --pax-option=@var{keyword-list}
3089 This option enables creation of the archive in @acronym{POSIX.1-2001}
3090 format (@pxref{posix}) and modifies the way @command{tar} handles the
3091 extended header keywords. @var{Keyword-list} is a comma-separated
3092 list of keyword options. @xref{PAX keywords}, for a detailed
3095 @opsummary{portability}
3097 @itemx --old-archive
3098 Synonym for @option{--format=v7}.
3102 Same as @option{--format=posix}.
3104 @opsummary{preserve}
3107 Synonymous with specifying both @option{--preserve-permissions} and
3108 @option{--same-order}. @xref{Setting Access Permissions}.
3110 @opsummary{preserve-order}
3111 @item --preserve-order
3113 (See @option{--same-order}; @pxref{Reading}.)
3115 @opsummary{preserve-permissions}
3116 @opsummary{same-permissions}
3117 @item --preserve-permissions
3118 @itemx --same-permissions
3121 When @command{tar} is extracting an archive, it normally subtracts the
3122 users' umask from the permissions specified in the archive and uses
3123 that number as the permissions to create the destination file.
3124 Specifying this option instructs @command{tar} that it should use the
3125 permissions directly from the archive. @xref{Setting Access Permissions}.
3127 @opsummary{quote-chars}
3128 @item --quote-chars=@var{string}
3129 Always quote characters from @var{string}, even if the selected
3130 quoting style would not quote them (@pxref{quoting styles}).
3132 @opsummary{quoting-style}
3133 @item --quoting-style=@var{style}
3134 Set quoting style to use when printing member and file names
3135 (@pxref{quoting styles}). Valid @var{style} values are:
3136 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
3137 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
3138 style is @code{escape}, unless overridden while configuring the
3141 @opsummary{read-full-records}
3142 @item --read-full-records
3145 Specifies that @command{tar} should reblock its input, for reading
3146 from pipes on systems with buggy implementations. @xref{Reading}.
3148 @opsummary{record-size}
3149 @item --record-size=@var{size}[@var{suf}]
3151 Instructs @command{tar} to use @var{size} bytes per record when accessing the
3152 archive. The argument can be suffixed with a @dfn{size suffix}, e.g.
3153 @option{--record-size=10K} for 10 Kilobytes. @xref{size-suffixes},
3154 for a list of valid suffixes. @xref{Blocking Factor}, for a detailed
3155 description of this option.
3157 @opsummary{recursion}
3160 With this option, @command{tar} recurses into directories (default).
3163 @opsummary{recursive-unlink}
3164 @item --recursive-unlink
3167 directory hierarchies before extracting directories of the same name
3168 from the archive. @xref{Recursive Unlink}.
3170 @opsummary{remove-files}
3171 @item --remove-files
3173 Directs @command{tar} to remove the source file from the file system after
3174 appending it to an archive. @xref{remove files}.
3176 @opsummary{restrict}
3179 Disable use of some potentially harmful @command{tar} options.
3180 Currently this option disables shell invocation from multi-volume menu
3181 (@pxref{Using Multiple Tapes}).
3183 @opsummary{rmt-command}
3184 @item --rmt-command=@var{cmd}
3186 Notifies @command{tar} that it should use @var{cmd} instead of
3187 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
3189 @opsummary{rsh-command}
3190 @item --rsh-command=@var{cmd}
3192 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
3193 devices. @xref{Device}.
3195 @opsummary{same-order}
3197 @itemx --preserve-order
3200 This option is an optimization for @command{tar} when running on machines with
3201 small amounts of memory. It informs @command{tar} that the list of file
3202 arguments has already been sorted to match the order of files in the
3203 archive. @xref{Reading}.
3205 @opsummary{same-owner}
3208 When extracting an archive, @command{tar} will attempt to preserve the owner
3209 specified in the @command{tar} archive with this option present.
3210 This is the default behavior for the superuser; this option has an
3211 effect only for ordinary users. @xref{Attributes}.
3213 @opsummary{same-permissions}
3214 @item --same-permissions
3216 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
3222 Assume that the archive media supports seeks to arbitrary
3223 locations. Usually @command{tar} determines automatically whether
3224 the archive can be seeked or not. This option is intended for use
3225 in cases when such recognition fails. It takes effect only if the
3226 archive is open for reading (e.g. with @option{--list} or
3227 @option{--extract} options).
3229 @opsummary{show-defaults}
3230 @item --show-defaults
3232 Displays the default options used by @command{tar} and exits
3233 successfully. This option is intended for use in shell scripts.
3234 Here is an example of what you can see using this option:
3237 $ @kbd{tar --show-defaults}
3238 --format=gnu -f- -b20 --quoting-style=escape
3239 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
3243 Notice, that this option outputs only one line. The example output
3244 above has been split to fit page boundaries.
3246 @opsummary{show-omitted-dirs}
3247 @item --show-omitted-dirs
3249 Instructs @command{tar} to mention the directories it is skipping when
3250 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
3252 @opsummary{show-transformed-names}
3253 @opsummary{show-stored-names}
3254 @item --show-transformed-names
3255 @itemx --show-stored-names
3257 Display file or member names after applying any transformations
3258 (@pxref{transform}). In particular, when used in conjunction with one of
3259 the archive creation operations it instructs @command{tar} to list the
3260 member names stored in the archive, as opposed to the actual file
3261 names. @xref{listing member and file names}.
3267 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3268 sparse files efficiently. @xref{sparse}.
3270 @opsummary{sparse-version}
3271 @item --sparse-version=@var{version}
3273 Specifies the @dfn{format version} to use when archiving sparse
3274 files. Implies @option{--sparse}. @xref{sparse}. For the description
3275 of the supported sparse formats, @xref{Sparse Formats}.
3277 @opsummary{starting-file}
3278 @item --starting-file=@var{name}
3279 @itemx -K @var{name}
3281 This option affects extraction only; @command{tar} will skip extracting
3282 files in the archive until it finds one that matches @var{name}.
3285 @opsummary{strip-components}
3286 @item --strip-components=@var{number}
3287 Strip given @var{number} of leading components from file names before
3288 extraction. For example, if archive @file{archive.tar} contained
3289 @file{/some/file/name}, then running
3292 tar --extract --file archive.tar --strip-components=2
3296 would extract this file to file @file{name}.
3299 @item --suffix=@var{suffix}
3301 Alters the suffix @command{tar} uses when backing up files from the default
3302 @samp{~}. @xref{backup}.
3304 @opsummary{tape-length}
3305 @item --tape-length=@var{num}[@var{suf}]
3306 @itemx -L @var{num}[@var{suf}]
3308 Specifies the length of tapes that @command{tar} is writing as being
3309 @w{@var{num} x 1024} bytes long. If optional @var{suf} is given, it
3310 specifies a multiplicative factor to be used instead of 1024. For
3311 example, @samp{-L2M} means 2 megabytes. @xref{size-suffixes}, for a
3312 list of allowed suffixes. @xref{Using Multiple Tapes}, for a detailed
3313 discussion of this option.
3315 @opsummary{test-label}
3318 Reads the volume label. If an argument is specified, test whether it
3319 matches the volume label. @xref{--test-label option}.
3321 @opsummary{to-command}
3322 @item --to-command=@var{command}
3324 During extraction @command{tar} will pipe extracted files to the
3325 standard input of @var{command}. @xref{Writing to an External Program}.
3327 @opsummary{to-stdout}
3331 During extraction, @command{tar} will extract files to stdout rather
3332 than to the file system. @xref{Writing to Standard Output}.
3335 @item --totals[=@var{signo}]
3337 Displays the total number of bytes transferred when processing an
3338 archive. If an argument is given, these data are displayed on
3339 request, when signal @var{signo} is delivered to @command{tar}.
3346 Sets the data modification time of extracted files to the extraction time,
3347 rather than the data modification time stored in the archive.
3348 @xref{Data Modification Times}.
3350 @opsummary{transform}
3352 @item --transform=@var{sed-expr}
3353 @itemx --xform=@var{sed-expr}
3354 Transform file or member names using @command{sed} replacement expression
3355 @var{sed-expr}. For example,
3358 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
3362 will add to @file{archive} files from the current working directory,
3363 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
3364 discussion, @xref{transform}.
3366 To see transformed member names in verbose listings, use
3367 @option{--show-transformed-names} option
3368 (@pxref{show-transformed-names}).
3370 @opsummary{uncompress}
3373 (See @option{--compress}, @pxref{gzip})
3378 (See @option{--gzip}, @pxref{gzip})
3380 @opsummary{unlink-first}
3381 @item --unlink-first
3384 Directs @command{tar} to remove the corresponding file from the file
3385 system before extracting it from the archive. @xref{Unlink First}.
3389 Enable unquoting input file or member names (default). @xref{input
3392 @opsummary{use-compress-program}
3393 @item --use-compress-program=@var{prog}
3394 @itemx -I=@var{prog}
3396 Instructs @command{tar} to access the archive through @var{prog}, which is
3397 presumed to be a compression program of some sort. @xref{gzip}.
3402 Display file modification dates in @acronym{UTC}. This option implies
3409 Specifies that @command{tar} should be more verbose about the
3410 operations it is performing. This option can be specified multiple
3411 times for some operations to increase the amount of information displayed.
3418 Verifies that the archive was correctly written when creating an
3419 archive. @xref{verify}.
3424 Print information about the program's name, version, origin and legal
3425 status, all on standard output, and then exit successfully.
3428 @opsummary{volno-file}
3429 @item --volno-file=@var{file}
3431 Used in conjunction with @option{--multi-volume}. @command{tar} will
3432 keep track of which volume of a multi-volume archive it is working in
3433 @var{file}. @xref{volno-file}.
3436 @item --warning=@var{keyword}
3438 Enable or disable warning messages identified by @var{keyword}. The
3439 messages are suppressed if @var{keyword} is prefixed with @samp{no-}.
3442 @opsummary{wildcards}
3444 Use wildcards when matching member names with patterns.
3445 @xref{controlling pattern-matching}.
3447 @opsummary{wildcards-match-slash}
3448 @item --wildcards-match-slash
3449 Wildcards match @samp{/}.
3450 @xref{controlling pattern-matching}.
3455 Use @command{xz} for compressing or decompressing the archives. @xref{gzip}.
3459 @node Short Option Summary
3460 @subsection Short Options Cross Reference
3462 Here is an alphabetized list of all of the short option forms, matching
3463 them with the equivalent long option.
3465 @multitable @columnfractions 0.20 0.80
3466 @headitem Short Option @tab Reference
3468 @item -A @tab @ref{--concatenate}.
3470 @item -B @tab @ref{--read-full-records}.
3472 @item -C @tab @ref{--directory}.
3474 @item -F @tab @ref{--info-script}.
3476 @item -G @tab @ref{--incremental}.
3478 @item -J @tab @ref{--xz}.
3480 @item -K @tab @ref{--starting-file}.
3482 @item -L @tab @ref{--tape-length}.
3484 @item -M @tab @ref{--multi-volume}.
3486 @item -N @tab @ref{--newer}.
3488 @item -O @tab @ref{--to-stdout}.
3490 @item -P @tab @ref{--absolute-names}.
3492 @item -R @tab @ref{--block-number}.
3494 @item -S @tab @ref{--sparse}.
3496 @item -T @tab @ref{--files-from}.
3498 @item -U @tab @ref{--unlink-first}.
3500 @item -V @tab @ref{--label}.
3502 @item -W @tab @ref{--verify}.
3504 @item -X @tab @ref{--exclude-from}.
3506 @item -Z @tab @ref{--compress}.
3508 @item -b @tab @ref{--blocking-factor}.
3510 @item -c @tab @ref{--create}.
3512 @item -d @tab @ref{--compare}.
3514 @item -f @tab @ref{--file}.
3516 @item -g @tab @ref{--listed-incremental}.
3518 @item -h @tab @ref{--dereference}.
3520 @item -i @tab @ref{--ignore-zeros}.
3522 @item -j @tab @ref{--bzip2}.
3524 @item -k @tab @ref{--keep-old-files}.
3526 @item -l @tab @ref{--check-links}.
3528 @item -m @tab @ref{--touch}.
3530 @item -o @tab When creating, @ref{--no-same-owner}, when extracting ---
3531 @ref{--portability}.
3533 The latter usage is deprecated. It is retained for compatibility with
3534 the earlier versions of @GNUTAR{}. In future releases
3535 @option{-o} will be equivalent to @option{--no-same-owner} only.
3537 @item -p @tab @ref{--preserve-permissions}.
3539 @item -r @tab @ref{--append}.
3541 @item -s @tab @ref{--same-order}.
3543 @item -t @tab @ref{--list}.
3545 @item -u @tab @ref{--update}.
3547 @item -v @tab @ref{--verbose}.
3549 @item -w @tab @ref{--interactive}.
3551 @item -x @tab @ref{--extract}.
3553 @item -z @tab @ref{--gzip}.
3558 @section @GNUTAR{} documentation
3560 @cindex Getting program version number
3562 @cindex Version of the @command{tar} program
3563 Being careful, the first thing is really checking that you are using
3564 @GNUTAR{}, indeed. The @option{--version} option
3565 causes @command{tar} to print information about its name, version,
3566 origin and legal status, all on standard output, and then exit
3567 successfully. For example, @w{@samp{tar --version}} might print:
3570 tar (GNU tar) @value{VERSION}
3571 Copyright (C) 2011 Free Software Foundation, Inc.
3572 Copyright (C) 2011 Free Software Foundation, Inc.
3573 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
3574 This is free software: you are free to change and redistribute it.
3575 There is NO WARRANTY, to the extent permitted by law.
3577 Written by John Gilmore and Jay Fenlason.
3581 The first occurrence of @samp{tar} in the result above is the program
3582 name in the package (for example, @command{rmt} is another program),
3583 while the second occurrence of @samp{tar} is the name of the package
3584 itself, containing possibly many programs. The package is currently
3585 named @samp{tar}, after the name of the main program it
3586 contains@footnote{There are plans to merge the @command{cpio} and
3587 @command{tar} packages into a single one which would be called
3588 @code{paxutils}. So, who knows if, one of this days, the
3589 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3592 @cindex Obtaining help
3593 @cindex Listing all @command{tar} options
3594 @xopindex{help, introduction}
3595 Another thing you might want to do is checking the spelling or meaning
3596 of some particular @command{tar} option, without resorting to this
3597 manual, for once you have carefully read it. @GNUTAR{}
3598 has a short help feature, triggerable through the
3599 @option{--help} option. By using this option, @command{tar} will
3600 print a usage message listing all available options on standard
3601 output, then exit successfully, without doing anything else and
3602 ignoring all other options. Even if this is only a brief summary, it
3603 may be several screens long. So, if you are not using some kind of
3604 scrollable window, you might prefer to use something like:
3607 $ @kbd{tar --help | less}
3611 presuming, here, that you like using @command{less} for a pager. Other
3612 popular pagers are @command{more} and @command{pg}. If you know about some
3613 @var{keyword} which interests you and do not want to read all the
3614 @option{--help} output, another common idiom is doing:
3617 tar --help | grep @var{keyword}
3621 for getting only the pertinent lines. Notice, however, that some
3622 @command{tar} options have long description lines and the above
3623 command will list only the first of them.
3625 The exact look of the option summary displayed by @kbd{tar --help} is
3626 configurable. @xref{Configuring Help Summary}, for a detailed description.
3629 If you only wish to check the spelling of an option, running @kbd{tar
3630 --usage} may be a better choice. This will display a terse list of
3631 @command{tar} options without accompanying explanations.
3633 The short help output is quite succinct, and you might have to get
3634 back to the full documentation for precise points. If you are reading
3635 this paragraph, you already have the @command{tar} manual in some
3636 form. This manual is available in a variety of forms from
3637 @url{http://www.gnu.org/software/tar/manual}. It may be printed out of the @GNUTAR{}
3638 distribution, provided you have @TeX{} already installed somewhere,
3639 and a laser printer around. Just configure the distribution, execute
3640 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3641 usual way (contact your local guru to know how). If @GNUTAR{}
3642 has been conveniently installed at your place, this
3643 manual is also available in interactive, hypertextual form as an Info
3644 file. Just call @w{@samp{info tar}} or, if you do not have the
3645 @command{info} program handy, use the Info reader provided within
3646 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3648 There is currently no @code{man} page for @GNUTAR{}.
3649 If you observe such a @code{man} page on the system you are running,
3650 either it does not belong to @GNUTAR{}, or it has not
3651 been produced by @acronym{GNU}. Some package maintainers convert
3652 @kbd{tar --help} output to a man page, using @command{help2man}. In
3653 any case, please bear in mind that the authoritative source of
3654 information about @GNUTAR{} is this Texinfo documentation.
3657 @section Obtaining @GNUTAR{} default values
3659 @opindex show-defaults
3660 @GNUTAR{} has some predefined defaults that are used when you do not
3661 explicitly specify another values. To obtain a list of such
3662 defaults, use @option{--show-defaults} option. This will output the
3663 values in the form of @command{tar} command line options:
3667 $ @kbd{tar --show-defaults}
3668 --format=gnu -f- -b20 --quoting-style=escape
3669 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3674 Notice, that this option outputs only one line. The example output above
3675 has been split to fit page boundaries.
3678 The above output shows that this version of @GNUTAR{} defaults to
3679 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3680 output as the archive, if no @option{--file} option has been given
3681 (@pxref{file tutorial}), the default blocking factor is 20
3682 (@pxref{Blocking Factor}). It also shows the default locations where
3683 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3686 @section Checking @command{tar} progress
3688 Typically, @command{tar} performs most operations without reporting any
3689 information to the user except error messages. When using @command{tar}
3690 with many options, particularly ones with complicated or
3691 difficult-to-predict behavior, it is possible to make serious mistakes.
3692 @command{tar} provides several options that make observing @command{tar}
3693 easier. These options cause @command{tar} to print information as it
3694 progresses in its job, and you might want to use them just for being
3695 more careful about what is going on, or merely for entertaining
3696 yourself. If you have encountered a problem when operating on an
3697 archive, however, you may need more information than just an error
3698 message in order to solve the problem. The following options can be
3699 helpful diagnostic tools.
3701 @cindex Verbose operation
3703 Normally, the @option{--list} (@option{-t}) command to list an archive
3704 prints just the file names (one per line) and the other commands are
3705 silent. When used with most operations, the @option{--verbose}
3706 (@option{-v}) option causes @command{tar} to print the name of each
3707 file or archive member as it is processed. This and the other options
3708 which make @command{tar} print status information can be useful in
3709 monitoring @command{tar}.
3711 With @option{--create} or @option{--extract}, @option{--verbose} used
3712 once just prints the names of the files or members as they are processed.
3713 Using it twice causes @command{tar} to print a longer listing
3714 (@xref{verbose member listing}, for the description) for each member.
3715 Since @option{--list} already prints the names of the members,
3716 @option{--verbose} used once with @option{--list} causes @command{tar}
3717 to print an @samp{ls -l} type listing of the files in the archive.
3718 The following examples both extract members with long list output:
3721 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3722 $ @kbd{tar xvvf archive.tar}
3725 Verbose output appears on the standard output except when an archive is
3726 being written to the standard output, as with @samp{tar --create
3727 --file=- --verbose} (@samp{tar cvf -}, or even @samp{tar cv}---if the
3728 installer let standard output be the default archive). In that case
3729 @command{tar} writes verbose output to the standard error stream.
3731 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3732 verbose output to @var{file} rather than to standard output or standard
3736 @cindex Obtaining total status information
3738 The @option{--totals} option causes @command{tar} to print on the
3739 standard error the total amount of bytes transferred when processing
3740 an archive. When creating or appending to an archive, this option
3741 prints the number of bytes written to the archive and the average
3742 speed at which they have been written, e.g.:
3746 $ @kbd{tar -c -f archive.tar --totals /home}
3747 Total bytes written: 7924664320 (7.4GiB, 85MiB/s)
3751 When reading an archive, this option displays the number of bytes
3756 $ @kbd{tar -x -f archive.tar --totals}
3757 Total bytes read: 7924664320 (7.4GiB, 95MiB/s)
3761 Finally, when deleting from an archive, the @option{--totals} option
3762 displays both numbers plus number of bytes removed from the archive:
3766 $ @kbd{tar --delete -f foo.tar --totals --wildcards '*~'}
3767 Total bytes read: 9543680 (9.2MiB, 201MiB/s)
3768 Total bytes written: 3829760 (3.7MiB, 81MiB/s)
3769 Total bytes deleted: 1474048
3773 You can also obtain this information on request. When
3774 @option{--totals} is used with an argument, this argument is
3775 interpreted as a symbolic name of a signal, upon delivery of which the
3776 statistics is to be printed:
3779 @item --totals=@var{signo}
3780 Print statistics upon delivery of signal @var{signo}. Valid arguments
3781 are: @code{SIGHUP}, @code{SIGQUIT}, @code{SIGINT}, @code{SIGUSR1} and
3782 @code{SIGUSR2}. Shortened names without @samp{SIG} prefix are also
3786 Both forms of @option{--totals} option can be used simultaneously.
3787 Thus, @kbd{tar -x --totals --totals=USR1} instructs @command{tar} to
3788 extract all members from its default archive and print statistics
3789 after finishing the extraction, as well as when receiving signal
3792 @anchor{Progress information}
3793 @cindex Progress information
3794 The @option{--checkpoint} option prints an occasional message
3795 as @command{tar} reads or writes the archive. It is designed for
3796 those who don't need the more detailed (and voluminous) output of
3797 @option{--block-number} (@option{-R}), but do want visual confirmation
3798 that @command{tar} is actually making forward progress. By default it
3799 prints a message each 10 records read or written. This can be changed
3800 by giving it a numeric argument after an equal sign:
3803 $ @kbd{tar -c --checkpoint=1000} /var
3804 tar: Write checkpoint 1000
3805 tar: Write checkpoint 2000
3806 tar: Write checkpoint 3000
3809 This example shows the default checkpoint message used by
3810 @command{tar}. If you place a dot immediately after the equal
3811 sign, it will print a @samp{.} at each checkpoint@footnote{This is
3812 actually a shortcut for @option{--checkpoint=@var{n}
3813 --checkpoint-action=dot}. @xref{checkpoints, dot}.}. For example:
3816 $ @kbd{tar -c --checkpoint=.1000} /var
3820 The @option{--checkpoint} option provides a flexible mechanism for
3821 executing arbitrary actions upon hitting checkpoints, see the next
3822 section (@pxref{checkpoints}), for more information on it.
3824 @opindex show-omitted-dirs
3825 @anchor{show-omitted-dirs}
3826 The @option{--show-omitted-dirs} option, when reading an archive---with
3827 @option{--list} or @option{--extract}, for example---causes a message
3828 to be printed for each directory in the archive which is skipped.
3829 This happens regardless of the reason for skipping: the directory might
3830 not have been named on the command line (implicitly or explicitly),
3831 it might be excluded by the use of the
3832 @option{--exclude=@var{pattern}} option, or some other reason.
3834 @opindex block-number
3835 @cindex Block number where error occurred
3836 @anchor{block-number}
3837 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3838 every message it would normally produce, the block number within the
3839 archive where the message was triggered. Also, supplementary messages
3840 are triggered when reading blocks full of NULs, or when hitting end of
3841 file on the archive. As of now, if the archive is properly terminated
3842 with a NUL block, the reading of the file may stop before end of file
3843 is met, so the position of end of file will not usually show when
3844 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3845 drains the archive before exiting when reading the
3846 archive from a pipe.
3848 @cindex Error message, block number of
3849 This option is especially useful when reading damaged archives, since
3850 it helps pinpoint the damaged sections. It can also be used with
3851 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3852 choose among several backup tapes when retrieving a file later, in
3853 favor of the tape where the file appears earliest (closest to the
3854 front of the tape). @xref{backup}.
3857 @section Checkpoints
3858 @cindex checkpoints, defined
3860 @opindex checkpoint-action
3862 A @dfn{checkpoint} is a moment of time before writing @var{n}th record to
3863 the archive (a @dfn{write checkpoint}), or before reading @var{n}th record
3864 from the archive (a @dfn{read checkpoint}). Checkpoints allow to
3865 periodically execute arbitrary actions.
3867 The checkpoint facility is enabled using the following option:
3870 @xopindex{checkpoint, defined}
3871 @item --checkpoint[=@var{n}]
3872 Schedule checkpoints before writing or reading each @var{n}th record.
3873 The default value for @var{n} is 10.
3876 A list of arbitrary @dfn{actions} can be executed at each checkpoint.
3877 These actions include: pausing, displaying textual messages, and
3878 executing arbitrary external programs. Actions are defined using
3879 the @option{--checkpoint-action} option.
3882 @xopindex{checkpoint-action, defined}
3883 @item --checkpoint-action=@var{action}
3884 Execute an @var{action} at each checkpoint.
3887 @cindex @code{echo}, checkpoint action
3888 The simplest value of @var{action} is @samp{echo}. It instructs
3889 @command{tar} to display the default message on the standard error
3890 stream upon arriving at each checkpoint. The default message is (in
3891 @acronym{POSIX} locale) @samp{Write checkpoint @var{n}}, for write
3892 checkpoints, and @samp{Read checkpoint @var{n}}, for read checkpoints.
3893 Here, @var{n} represents ordinal number of the checkpoint.
3895 In another locales, translated versions of this message are used.
3897 This is the default action, so running:
3900 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=echo} /var
3907 $ @kbd{tar -c --checkpoint=1000} /var
3910 The @samp{echo} action also allows to supply a customized message.
3911 You do so by placing an equals sign and the message right after it,
3915 --checkpoint-action="echo=Hit %s checkpoint #%u"
3918 The @samp{%s} and @samp{%u} in the above example are
3919 @dfn{meta-characters}. The @samp{%s} meta-character is replaced with
3920 the @dfn{type} of the checkpoint: @samp{write} or
3921 @samp{read} (or a corresponding translated version in locales other
3922 than @acronym{POSIX}). The @samp{%u} meta-character is replaced with
3923 the ordinal number of the checkpoint. Thus, the above example could
3924 produce the following output when used with the @option{--create}
3928 tar: Hit write checkpoint #10
3929 tar: Hit write checkpoint #20
3930 tar: Hit write checkpoint #30
3933 Aside from meta-character expansion, the message string is subject to
3934 @dfn{unquoting}, during which the backslash @dfn{escape sequences} are
3935 replaced with their corresponding @acronym{ASCII} characters
3936 (@pxref{escape sequences}). E.g. the following action will produce an
3937 audible bell and the message described above at each checkpoint:
3940 --checkpoint-action='echo=\aHit %s checkpoint #%u'
3943 @cindex @code{bell}, checkpoint action
3944 There is also a special action which produces an audible signal:
3945 @samp{bell}. It is not equivalent to @samp{echo='\a'}, because
3946 @samp{bell} sends the bell directly to the console (@file{/dev/tty}),
3947 whereas @samp{echo='\a'} sends it to the standard error.
3949 @cindex @code{ttyout}, checkpoint action
3950 The @samp{ttyout=@var{string}} action outputs @var{string} to
3951 @file{/dev/tty}, so it can be used even if the standard output is
3952 redirected elsewhere. The @var{string} is subject to the same
3953 modifications as with @samp{echo} action. In contrast to the latter,
3954 @samp{ttyout} does not prepend @command{tar} executable name to the
3955 string, nor does it output a newline after it. For example, the
3956 following action will print the checkpoint message at the same screen
3957 line, overwriting any previous message:
3960 --checkpoint-action="ttyout=\rHit %s checkpoint #%u"
3963 @cindex @code{dot}, checkpoint action
3964 Another available checkpoint action is @samp{dot} (or @samp{.}). It
3965 instructs @command{tar} to print a single dot on the standard listing
3969 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=dot} /var
3973 For compatibility with previous @GNUTAR{} versions, this action can
3974 be abbreviated by placing a dot in front of the checkpoint frequency,
3975 as shown in the previous section.
3977 @cindex @code{sleep}, checkpoint action
3978 Yet another action, @samp{sleep}, pauses @command{tar} for a specified
3979 amount of seconds. The following example will stop for 30 seconds at each
3983 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=sleep=30}
3986 @cindex @code{exec}, checkpoint action
3987 Finally, the @code{exec} action executes a given external program.
3991 $ @kbd{tar -c --checkpoint=1000 --checkpoint-action=exec=/sbin/cpoint}
3994 This program is executed using @command{/bin/sh -c}, with no
3995 additional arguments. Its exit code is ignored. It gets a copy of
3996 @command{tar}'s environment plus the following variables:
3999 @vrindex TAR_VERSION, checkpoint script environment
4001 @GNUTAR{} version number.
4003 @vrindex TAR_ARCHIVE, checkpoint script environment
4005 The name of the archive @command{tar} is processing.
4007 @vrindex TAR_BLOCKING_FACTOR, checkpoint script environment
4008 @item TAR_BLOCKING_FACTOR
4009 Current blocking factor (@pxref{Blocking}).
4011 @vrindex TAR_CHECKPOINT, checkpoint script environment
4012 @item TAR_CHECKPOINT
4013 Number of the checkpoint.
4015 @vrindex TAR_SUBCOMMAND, checkpoint script environment
4016 @item TAR_SUBCOMMAND
4017 A short option describing the operation @command{tar} is executing.
4018 @xref{Operations}, for a complete list of subcommand options.
4020 @vrindex TAR_FORMAT, checkpoint script environment
4022 Format of the archive being processed. @xref{Formats}, for a complete
4023 list of archive format names.
4026 Any number of actions can be defined, by supplying several
4027 @option{--checkpoint-action} options in the command line. For
4028 example, the command below displays two messages, pauses
4029 execution for 30 seconds and executes the @file{/sbin/cpoint} script:
4033 $ @kbd{tar -c -f arc.tar \
4034 --checkpoint-action='\aecho=Hit %s checkpoint #%u' \
4035 --checkpoint-action='echo=Sleeping for 30 seconds' \
4036 --checkpoint-action='sleep=30' \
4037 --checkpoint-action='exec=/sbin/cpoint'}
4041 This example also illustrates the fact that
4042 @option{--checkpoint-action} can be used without
4043 @option{--checkpoint}. In this case, the default checkpoint frequency
4044 (at each 10th record) is assumed.
4047 @section Controlling Warning Messages
4049 Sometimes, while performing the requested task, @GNUTAR{} notices
4050 some conditions that are not exactly errors, but which the user
4051 should be aware of. When this happens, @command{tar} issues a
4052 @dfn{warning message} describing the condition. Warning messages
4053 are output to the standard error and they do not affect the exit
4054 code of @command{tar} command.
4056 @xopindex{warning, explained}
4057 @GNUTAR{} allows the user to suppress some or all of its warning
4061 @item --warning=@var{keyword}
4062 Control display of the warning messages identified by @var{keyword}.
4063 If @var{keyword} starts with the prefix @samp{no-}, such messages are
4064 suppressed. Otherwise, they are enabled.
4066 Multiple @option{--warning} messages accumulate.
4068 The tables below list allowed values for @var{keyword} along with the
4069 warning messages they control.
4072 @subheading Keywords controlling @command{tar} operation
4076 Enable all warning messages. This is the default.
4079 Disable all warning messages.
4080 @kwindex filename-with-nuls
4081 @cindex @samp{file name read contains nul character}, warning message
4082 @item filename-with-nuls
4083 @samp{%s: file name read contains nul character}
4084 @kwindex alone-zero-block
4085 @cindex @samp{A lone zero block at}, warning message
4086 @item alone-zero-block
4087 @samp{A lone zero block at %s}
4090 @subheading Keywords applicable for @command{tar --create}
4093 @cindex @samp{contains a cache directory tag}, warning message
4095 @samp{%s: contains a cache directory tag %s; %s}
4096 @kwindex file-shrank
4097 @cindex @samp{File shrank by %s bytes}, warning message
4099 @samp{%s: File shrank by %s bytes; padding with zeros}
4101 @cindex @samp{file is on a different filesystem}, warning message
4103 @samp{%s: file is on a different filesystem; not dumped}
4104 @kwindex file-ignored
4105 @cindex @samp{Unknown file type; file ignored}, warning message
4106 @cindex @samp{socket ignored}, warning message
4107 @cindex @samp{door ignored}, warning message
4109 @samp{%s: Unknown file type; file ignored}
4110 @*@samp{%s: socket ignored}
4111 @*@samp{%s: door ignored}
4112 @kwindex file-unchanged
4113 @cindex @samp{file is unchanged; not dumped}, warning message
4114 @item file-unchanged
4115 @samp{%s: file is unchanged; not dumped}
4116 @kwindex ignore-archive
4117 @cindex @samp{file is the archive; not dumped}, warning message
4118 @kwindex ignore-archive
4119 @cindex @samp{file is the archive; not dumped}, warning message
4120 @item ignore-archive
4121 @samp{%s: file is the archive; not dumped}
4122 @kwindex file-removed
4123 @cindex @samp{File removed before we read it}, warning message
4125 @samp{%s: File removed before we read it}
4126 @kwindex file-changed
4127 @cindex @samp{file changed as we read it}, warning message
4129 @samp{%s: file changed as we read it}
4132 @subheading Keywords applicable for @command{tar --extract}
4135 @cindex @samp{implausibly old time stamp %s}, warning message
4136 @cindex @samp{time stamp %s is %s s in the future}, warning message
4138 @samp{%s: implausibly old time stamp %s}
4139 @*@samp{%s: time stamp %s is %s s in the future}
4140 @kwindex contiguous-cast
4141 @cindex @samp{Extracting contiguous files as regular files}, warning message
4142 @item contiguous-cast
4143 @samp{Extracting contiguous files as regular files}
4144 @kwindex symlink-cast
4145 @cindex @samp{Attempting extraction of symbolic links as hard links}, warning message
4147 @samp{Attempting extraction of symbolic links as hard links}
4148 @kwindex unknown-cast
4149 @cindex @samp{Unknown file type `%c', extracted as normal file}, warning message
4151 @samp{%s: Unknown file type `%c', extracted as normal file}
4152 @kwindex ignore-newer
4153 @cindex @samp{Current %s is newer or same age}, warning message
4155 @samp{Current %s is newer or same age}
4156 @kwindex unknown-keyword
4157 @cindex @samp{Ignoring unknown extended header keyword `%s'}, warning message
4158 @item unknown-keyword
4159 @samp{Ignoring unknown extended header keyword `%s'}
4160 @kwindex decompress-program
4161 @item decompress-program
4162 Controls verbose description of failures occurring when trying to run
4163 alternative decompressor programs (@pxref{alternative decompression
4164 programs}). This warning is disabled by default (unless
4165 @option{--verbose} is used). A common example of what you can get
4166 when using this warning is:
4169 $ @kbd{tar --warning=decompress-program -x -f archive.Z}
4170 tar (child): cannot run compress: No such file or directory
4171 tar (child): trying gzip
4174 This means that @command{tar} first tried to decompress
4175 @file{archive.Z} using @command{compress}, and, when that
4176 failed, switched to @command{gzip}.
4179 @subheading Keywords controlling incremental extraction:
4181 @kwindex rename-directory
4182 @cindex @samp{%s: Directory has been renamed from %s}, warning message
4183 @cindex @samp{%s: Directory has been renamed}, warning message
4184 @item rename-directory
4185 @samp{%s: Directory has been renamed from %s}
4186 @*@samp{%s: Directory has been renamed}
4187 @kwindex new-directory
4188 @cindex @samp{%s: Directory is new}, warning message
4190 @samp{%s: Directory is new}
4192 @cindex @samp{%s: directory is on a different device: not purging}, warning message
4194 @samp{%s: directory is on a different device: not purging}
4195 @kwindex bad-dumpdir
4196 @cindex @samp{Malformed dumpdir: 'X' never used}, warning message
4198 @samp{Malformed dumpdir: 'X' never used}
4202 @section Asking for Confirmation During Operations
4203 @cindex Interactive operation
4205 Typically, @command{tar} carries out a command without stopping for
4206 further instructions. In some situations however, you may want to
4207 exclude some files and archive members from the operation (for instance
4208 if disk or storage space is tight). You can do this by excluding
4209 certain files automatically (@pxref{Choosing}), or by performing
4210 an operation interactively, using the @option{--interactive} (@option{-w}) option.
4211 @command{tar} also accepts @option{--confirmation} for this option.
4213 @opindex interactive
4214 When the @option{--interactive} (@option{-w}) option is specified, before
4215 reading, writing, or deleting files, @command{tar} first prints a message
4216 for each such file, telling what operation it intends to take, then asks
4217 for confirmation on the terminal. The actions which require
4218 confirmation include adding a file to the archive, extracting a file
4219 from the archive, deleting a file from the archive, and deleting a file
4220 from disk. To confirm the action, you must type a line of input
4221 beginning with @samp{y}. If your input line begins with anything other
4222 than @samp{y}, @command{tar} skips that file.
4224 If @command{tar} is reading the archive from the standard input,
4225 @command{tar} opens the file @file{/dev/tty} to support the interactive
4228 Verbose output is normally sent to standard output, separate from
4229 other error messages. However, if the archive is produced directly
4230 on standard output, then verbose output is mixed with errors on
4231 @code{stderr}. Producing the archive on standard output may be used
4232 as a way to avoid using disk space, when the archive is soon to be
4233 consumed by another process reading it, say. Some people felt the need
4234 of producing an archive on stdout, still willing to segregate between
4235 verbose output and error output. A possible approach would be using a
4236 named pipe to receive the archive, and having the consumer process to
4237 read from that named pipe. This has the advantage of letting standard
4238 output free to receive verbose output, all separate from errors.
4241 @chapter @GNUTAR{} Operations
4254 @section Basic @GNUTAR{} Operations
4256 The basic @command{tar} operations, @option{--create} (@option{-c}),
4257 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
4258 @option{-x}), are currently presented and described in the tutorial
4259 chapter of this manual. This section provides some complementary notes
4260 for these operations.
4263 @xopindex{create, complementary notes}
4267 Creating an empty archive would have some kind of elegance. One can
4268 initialize an empty archive and later use @option{--append}
4269 (@option{-r}) for adding all members. Some applications would not
4270 welcome making an exception in the way of adding the first archive
4271 member. On the other hand, many people reported that it is
4272 dangerously too easy for @command{tar} to destroy a magnetic tape with
4273 an empty archive@footnote{This is well described in @cite{Unix-haters
4274 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
4275 Books, ISBN 1-56884-203-1.}. The two most common errors are:
4279 Mistakingly using @code{create} instead of @code{extract}, when the
4280 intent was to extract the full contents of an archive. This error
4281 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
4282 the QWERTY keyboard. Instead of being unpacked, the archive then
4283 gets wholly destroyed. When users speak about @dfn{exploding} an
4284 archive, they usually mean something else :-).
4287 Forgetting the argument to @code{file}, when the intent was to create
4288 an archive with a single file in it. This error is likely because a
4289 tired user can easily add the @kbd{f} key to the cluster of option
4290 letters, by the mere force of habit, without realizing the full
4291 consequence of doing so. The usual consequence is that the single
4292 file, which was meant to be saved, is rather destroyed.
4295 So, recognizing the likelihood and the catastrophic nature of these
4296 errors, @GNUTAR{} now takes some distance from elegance, and
4297 cowardly refuses to create an archive when @option{--create} option is
4298 given, there are no arguments besides options, and
4299 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
4300 around the cautiousness of @GNUTAR{} and nevertheless create an
4301 archive with nothing in it, one may still use, as the value for the
4302 @option{--files-from} option, a file with no names in it, as shown in
4303 the following commands:
4306 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
4307 @kbd{tar -cf empty-archive.tar -T /dev/null}
4310 @xopindex{extract, complementary notes}
4315 A socket is stored, within a @GNUTAR{} archive, as a pipe.
4317 @item @option{--list} (@option{-t})
4319 @GNUTAR{} now shows dates as @samp{1996-08-30},
4320 while it used to show them as @samp{Aug 30 1996}. Preferably,
4321 people should get used to ISO 8601 dates. Local American dates should
4322 be made available again with full date localization support, once
4323 ready. In the meantime, programs not being localizable for dates
4324 should prefer international dates, that's really the way to go.
4326 Look up @url{http://www.cl.cam.ac.uk/@/~mgk25/@/iso-time.html} if you
4327 are curious, it contains a detailed explanation of the ISO 8601 standard.
4332 @section Advanced @GNUTAR{} Operations
4334 Now that you have learned the basics of using @GNUTAR{}, you may want
4335 to learn about further ways in which @command{tar} can help you.
4337 This chapter presents five, more advanced operations which you probably
4338 won't use on a daily basis, but which serve more specialized functions.
4339 We also explain the different styles of options and why you might want
4340 to use one or another, or a combination of them in your @command{tar}
4341 commands. Additionally, this chapter includes options which allow you to
4342 define the output from @command{tar} more carefully, and provide help and
4343 error correction in special circumstances.
4345 @FIXME{check this after the chapter is actually revised to make sure
4346 it still introduces the info in the chapter correctly : ).}
4358 @subsection The Five Advanced @command{tar} Operations
4360 @cindex basic operations
4361 In the last chapter, you learned about the first three operations to
4362 @command{tar}. This chapter presents the remaining five operations to
4363 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
4364 @option{--delete}, and @option{--compare}.
4366 You are not likely to use these operations as frequently as those
4367 covered in the last chapter; however, since they perform specialized
4368 functions, they are quite useful when you do need to use them. We
4369 will give examples using the same directory and files that you created
4370 in the last chapter. As you may recall, the directory is called
4371 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
4372 and the two archive files you created are
4373 @samp{collection.tar} and @samp{music.tar}.
4375 We will also use the archive files @samp{afiles.tar} and
4376 @samp{bfiles.tar}. The archive @samp{afiles.tar} contains the members @samp{apple},
4377 @samp{angst}, and @samp{aspic}; @samp{bfiles.tar} contains the members
4378 @samp{./birds}, @samp{baboon}, and @samp{./box}.
4380 Unless we state otherwise, all practicing you do and examples you follow
4381 in this chapter will take place in the @file{practice} directory that
4382 you created in the previous chapter; see @ref{prepare for examples}.
4383 (Below in this section, we will remind you of the state of the examples
4384 where the last chapter left them.)
4386 The five operations that we will cover in this chapter are:
4391 Add new entries to an archive that already exists.
4394 Add more recent copies of archive members to the end of an archive, if
4399 Add one or more pre-existing archives to the end of another archive.
4401 Delete items from an archive (does not work on tapes).
4405 Compare archive members to their counterparts in the file system.
4409 @subsection How to Add Files to Existing Archives: @option{--append}
4411 @cindex appending files to existing archive
4413 If you want to add files to an existing archive, you don't need to
4414 create a new archive; you can use @option{--append} (@option{-r}).
4415 The archive must already exist in order to use @option{--append}. (A
4416 related operation is the @option{--update} operation; you can use this
4417 to add newer versions of archive members to an existing archive. To learn how to
4418 do this with @option{--update}, @pxref{update}.)
4420 If you use @option{--append} to add a file that has the same name as an
4421 archive member to an archive containing that archive member, then the
4422 old member is not deleted. What does happen, however, is somewhat
4423 complex. @command{tar} @emph{allows} you to have infinite number of files
4424 with the same name. Some operations treat these same-named members no
4425 differently than any other set of archive members: for example, if you
4426 view an archive with @option{--list} (@option{-t}), you will see all
4427 of those members listed, with their data modification times, owners, etc.
4429 Other operations don't deal with these members as perfectly as you might
4430 prefer; if you were to use @option{--extract} to extract the archive,
4431 only the most recently added copy of a member with the same name as
4432 other members would end up in the working directory. This is because
4433 @option{--extract} extracts an archive in the order the members appeared
4434 in the archive; the most recently archived members will be extracted
4435 last. Additionally, an extracted member will @emph{replace} a file of
4436 the same name which existed in the directory already, and @command{tar}
4437 will not prompt you about this@footnote{Unless you give it
4438 @option{--keep-old-files} option, or the disk copy is newer than
4439 the one in the archive and you invoke @command{tar} with
4440 @option{--keep-newer-files} option.}. Thus, only the most recently archived
4441 member will end up being extracted, as it will replace the one
4442 extracted before it, and so on.
4444 @cindex extracting @var{n}th copy of the file
4445 @xopindex{occurrence, described}
4446 There exists a special option that allows you to get around this
4447 behavior and extract (or list) only a particular copy of the file.
4448 This is @option{--occurrence} option. If you run @command{tar} with
4449 this option, it will extract only the first copy of the file. You
4450 may also give this option an argument specifying the number of
4451 copy to be extracted. Thus, for example if the archive
4452 @file{archive.tar} contained three copies of file @file{myfile}, then
4456 tar --extract --file archive.tar --occurrence=2 myfile
4460 would extract only the second copy. @xref{Option
4461 Summary,---occurrence}, for the description of @option{--occurrence}
4464 @FIXME{ hag -- you might want to incorporate some of the above into the
4465 MMwtSN node; not sure. i didn't know how to make it simpler...
4467 There are a few ways to get around this. Xref to Multiple Members
4468 with the Same Name, maybe.}
4470 @cindex Members, replacing with other members
4471 @cindex Replacing members with other members
4472 @xopindex{delete, using before --append}
4473 If you want to replace an archive member, use @option{--delete} to
4474 delete the member you want to remove from the archive, and then use
4475 @option{--append} to add the member you want to be in the archive. Note
4476 that you can not change the order of the archive; the most recently
4477 added member will still appear last. In this sense, you cannot truly
4478 ``replace'' one member with another. (Replacing one member with another
4479 will not work on certain types of media, such as tapes; see @ref{delete}
4480 and @ref{Media}, for more information.)
4483 * appending files:: Appending Files to an Archive
4487 @node appending files
4488 @subsubsection Appending Files to an Archive
4489 @cindex Adding files to an Archive
4490 @cindex Appending files to an Archive
4491 @cindex Archives, Appending files to
4494 The simplest way to add a file to an already existing archive is the
4495 @option{--append} (@option{-r}) operation, which writes specified
4496 files into the archive whether or not they are already among the
4499 When you use @option{--append}, you @emph{must} specify file name
4500 arguments, as there is no default. If you specify a file that already
4501 exists in the archive, another copy of the file will be added to the
4502 end of the archive. As with other operations, the member names of the
4503 newly added files will be exactly the same as their names given on the
4504 command line. The @option{--verbose} (@option{-v}) option will print
4505 out the names of the files as they are written into the archive.
4507 @option{--append} cannot be performed on some tape drives, unfortunately,
4508 due to deficiencies in the formats those tape drives use. The archive
4509 must be a valid @command{tar} archive, or else the results of using this
4510 operation will be unpredictable. @xref{Media}.
4512 To demonstrate using @option{--append} to add a file to an archive,
4513 create a file called @file{rock} in the @file{practice} directory.
4514 Make sure you are in the @file{practice} directory. Then, run the
4515 following @command{tar} command to add @file{rock} to
4516 @file{collection.tar}:
4519 $ @kbd{tar --append --file=collection.tar rock}
4523 If you now use the @option{--list} (@option{-t}) operation, you will see that
4524 @file{rock} has been added to the archive:
4527 $ @kbd{tar --list --file=collection.tar}
4528 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4529 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4530 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4531 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4535 @subsubsection Multiple Members with the Same Name
4536 @cindex members, multiple
4537 @cindex multiple members
4539 You can use @option{--append} (@option{-r}) to add copies of files
4540 which have been updated since the archive was created. (However, we
4541 do not recommend doing this since there is another @command{tar}
4542 option called @option{--update}; @xref{update}, for more information.
4543 We describe this use of @option{--append} here for the sake of
4544 completeness.) When you extract the archive, the older version will
4545 be effectively lost. This works because files are extracted from an
4546 archive in the order in which they were archived. Thus, when the
4547 archive is extracted, a file archived later in time will replace a
4548 file of the same name which was archived earlier, even though the
4549 older version of the file will remain in the archive unless you delete
4550 all versions of the file.
4552 Supposing you change the file @file{blues} and then append the changed
4553 version to @file{collection.tar}. As you saw above, the original
4554 @file{blues} is in the archive @file{collection.tar}. If you change the
4555 file and append the new version of the file to the archive, there will
4556 be two copies in the archive. When you extract the archive, the older
4557 version of the file will be extracted first, and then replaced by the
4558 newer version when it is extracted.
4560 You can append the new, changed copy of the file @file{blues} to the
4561 archive in this way:
4564 $ @kbd{tar --append --verbose --file=collection.tar blues}
4569 Because you specified the @option{--verbose} option, @command{tar} has
4570 printed the name of the file being appended as it was acted on. Now
4571 list the contents of the archive:
4574 $ @kbd{tar --list --verbose --file=collection.tar}
4575 -rw-r--r-- me/user 28 1996-10-18 16:31 jazz
4576 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4577 -rw-r--r-- me/user 20 1996-09-23 16:44 folk
4578 -rw-r--r-- me/user 20 1996-09-23 16:44 rock
4579 -rw-r--r-- me/user 58 1996-10-24 18:30 blues
4583 The newest version of @file{blues} is now at the end of the archive
4584 (note the different creation dates and file sizes). If you extract
4585 the archive, the older version of the file @file{blues} will be
4586 replaced by the newer version. You can confirm this by extracting
4587 the archive and running @samp{ls} on the directory.
4589 If you wish to extract the first occurrence of the file @file{blues}
4590 from the archive, use @option{--occurrence} option, as shown in
4591 the following example:
4594 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
4595 -rw-r--r-- me/user 21 1996-09-23 16:44 blues
4598 @xref{Writing}, for more information on @option{--extract} and
4599 see @ref{Option Summary, --occurrence}, for a description of
4600 @option{--occurrence} option.
4603 @subsection Updating an Archive
4604 @cindex Updating an archive
4607 In the previous section, you learned how to use @option{--append} to
4608 add a file to an existing archive. A related operation is
4609 @option{--update} (@option{-u}). The @option{--update} operation
4610 updates a @command{tar} archive by comparing the date of the specified
4611 archive members against the date of the file with the same name. If
4612 the file has been modified more recently than the archive member, then
4613 the newer version of the file is added to the archive (as with
4616 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
4617 The operation will fail.
4619 @FIXME{other examples of media on which --update will fail? need to ask
4620 charles and/or mib/thomas/dave shevett..}
4622 Both @option{--update} and @option{--append} work by adding to the end
4623 of the archive. When you extract a file from the archive, only the
4624 version stored last will wind up in the file system, unless you use
4625 the @option{--backup} option. @xref{multiple}, for a detailed discussion.
4632 @subsubsection How to Update an Archive Using @option{--update}
4635 You must use file name arguments with the @option{--update}
4636 (@option{-u}) operation. If you don't specify any files,
4637 @command{tar} won't act on any files and won't tell you that it didn't
4638 do anything (which may end up confusing you).
4640 @c note: the above parenthetical added because in fact, this
4641 @c behavior just confused the author. :-)
4643 To see the @option{--update} option at work, create a new file,
4644 @file{classical}, in your practice directory, and some extra text to the
4645 file @file{blues}, using any text editor. Then invoke @command{tar} with
4646 the @samp{update} operation and the @option{--verbose} (@option{-v})
4647 option specified, using the names of all the files in the @file{practice}
4648 directory as file name arguments:
4651 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4658 Because we have specified verbose mode, @command{tar} prints out the names
4659 of the files it is working on, which in this case are the names of the
4660 files that needed to be updated. If you run @samp{tar --list} and look
4661 at the archive, you will see @file{blues} and @file{classical} at its
4662 end. There will be a total of two versions of the member @samp{blues};
4663 the one at the end will be newer and larger, since you added text before
4666 The reason @command{tar} does not overwrite the older file when updating
4667 it is because writing to the middle of a section of tape is a difficult
4668 process. Tapes are not designed to go backward. @xref{Media}, for more
4669 information about tapes.
4671 @option{--update} (@option{-u}) is not suitable for performing backups for two
4672 reasons: it does not change directory content entries, and it
4673 lengthens the archive every time it is used. The @GNUTAR{}
4674 options intended specifically for backups are more
4675 efficient. If you need to run backups, please consult @ref{Backups}.
4678 @subsection Combining Archives with @option{--concatenate}
4680 @cindex Adding archives to an archive
4681 @cindex Concatenating Archives
4682 @opindex concatenate
4684 @c @cindex @option{-A} described
4685 Sometimes it may be convenient to add a second archive onto the end of
4686 an archive rather than adding individual files to the archive. To add
4687 one or more archives to the end of another archive, you should use the
4688 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4690 To use @option{--concatenate}, give the first archive with
4691 @option{--file} option and name the rest of archives to be
4692 concatenated on the command line. The members, and their member
4693 names, will be copied verbatim from those archives to the first
4694 one@footnote{This can cause multiple members to have the same name. For
4695 information on how this affects reading the archive, see @ref{multiple}.}.
4696 The new, concatenated archive will be called by the same name as the
4697 one given with the @option{--file} option. As usual, if you omit
4698 @option{--file}, @command{tar} will use the value of the environment
4699 variable @env{TAPE}, or, if this has not been set, the default archive name.
4701 @FIXME{There is no way to specify a new name...}
4703 To demonstrate how @option{--concatenate} works, create two small archives
4704 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4705 files from @file{practice}:
4708 $ @kbd{tar -cvf bluesrock.tar blues rock}
4711 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4717 If you like, You can run @samp{tar --list} to make sure the archives
4718 contain what they are supposed to:
4721 $ @kbd{tar -tvf bluesrock.tar}
4722 -rw-r--r-- melissa/user 105 1997-01-21 19:42 blues
4723 -rw-r--r-- melissa/user 33 1997-01-20 15:34 rock
4724 $ @kbd{tar -tvf jazzfolk.tar}
4725 -rw-r--r-- melissa/user 20 1996-09-23 16:44 folk
4726 -rw-r--r-- melissa/user 65 1997-01-30 14:15 jazz
4729 We can concatenate these two archives with @command{tar}:
4733 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4736 If you now list the contents of the @file{bluesrock.tar}, you will see
4737 that now it also contains the archive members of @file{jazzfolk.tar}:
4740 $ @kbd{tar --list --file=bluesrock.tar}
4747 When you use @option{--concatenate}, the source and target archives must
4748 already exist and must have been created using compatible format
4749 parameters. Notice, that @command{tar} does not check whether the
4750 archives it concatenates have compatible formats, it does not
4751 even check if the files are really tar archives.
4753 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4754 tape drives, due to deficiencies in the formats those tape drives use.
4756 @cindex @code{concatenate} vs @command{cat}
4757 @cindex @command{cat} vs @code{concatenate}
4758 It may seem more intuitive to you to want or try to use @command{cat} to
4759 concatenate two archives instead of using the @option{--concatenate}
4760 operation; after all, @command{cat} is the utility for combining files.
4762 However, @command{tar} archives incorporate an end-of-file marker which
4763 must be removed if the concatenated archives are to be read properly as
4764 one archive. @option{--concatenate} removes the end-of-archive marker
4765 from the target archive before each new archive is appended. If you use
4766 @command{cat} to combine the archives, the result will not be a valid
4767 @command{tar} format archive. If you need to retrieve files from an
4768 archive that was added to using the @command{cat} utility, use the
4769 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4770 information on dealing with archives improperly combined using the
4771 @command{cat} shell utility.
4774 @subsection Removing Archive Members Using @option{--delete}
4775 @cindex Deleting files from an archive
4776 @cindex Removing files from an archive
4779 You can remove members from an archive by using the @option{--delete}
4780 option. Specify the name of the archive with @option{--file}
4781 (@option{-f}) and then specify the names of the members to be deleted;
4782 if you list no member names, nothing will be deleted. The
4783 @option{--verbose} option will cause @command{tar} to print the names
4784 of the members as they are deleted. As with @option{--extract}, you
4785 must give the exact member names when using @samp{tar --delete}.
4786 @option{--delete} will remove all versions of the named file from the
4787 archive. The @option{--delete} operation can run very slowly.
4789 Unlike other operations, @option{--delete} has no short form.
4791 @cindex Tapes, using @option{--delete} and
4792 @cindex Deleting from tape archives
4793 This operation will rewrite the archive. You can only use
4794 @option{--delete} on an archive if the archive device allows you to
4795 write to any point on the media, such as a disk; because of this, it
4796 does not work on magnetic tapes. Do not try to delete an archive member
4797 from a magnetic tape; the action will not succeed, and you will be
4798 likely to scramble the archive and damage your tape. There is no safe
4799 way (except by completely re-writing the archive) to delete files from
4800 most kinds of magnetic tape. @xref{Media}.
4802 To delete all versions of the file @file{blues} from the archive
4803 @file{collection.tar} in the @file{practice} directory, make sure you
4804 are in that directory, and then,
4807 $ @kbd{tar --list --file=collection.tar}
4812 $ @kbd{tar --delete --file=collection.tar blues}
4813 $ @kbd{tar --list --file=collection.tar}
4819 @FIXME{Check if the above listing is actually produced after running
4820 all the examples on collection.tar.}
4822 The @option{--delete} option has been reported to work properly when
4823 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4826 @subsection Comparing Archive Members with the File System
4827 @cindex Verifying the currency of an archive
4830 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4831 specified archive members against files with the same names, and then
4832 reports differences in file size, mode, owner, modification date and
4833 contents. You should @emph{only} specify archive member names, not file
4834 names. If you do not name any members, then @command{tar} will compare the
4835 entire archive. If a file is represented in the archive but does not
4836 exist in the file system, @command{tar} reports a difference.
4838 You have to specify the record size of the archive when modifying an
4839 archive with a non-default record size.
4841 @command{tar} ignores files in the file system that do not have
4842 corresponding members in the archive.
4844 The following example compares the archive members @file{rock},
4845 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4846 files of the same name in the file system. (Note that there is no file,
4847 @file{funk}; @command{tar} will report an error message.)
4850 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4853 tar: funk not found in archive
4856 The spirit behind the @option{--compare} (@option{--diff},
4857 @option{-d}) option is to check whether the archive represents the
4858 current state of files on disk, more than validating the integrity of
4859 the archive media. For this latter goal, see @ref{verify}.
4861 @node create options
4862 @section Options Used by @option{--create}
4864 @xopindex{create, additional options}
4865 The previous chapter described the basics of how to use
4866 @option{--create} (@option{-c}) to create an archive from a set of files.
4867 @xref{create}. This section described advanced options to be used with
4871 * override:: Overriding File Metadata.
4872 * Ignore Failed Read::
4876 @subsection Overriding File Metadata
4878 As described above, a @command{tar} archive keeps, for each member it contains,
4879 its @dfn{metadata}, such as modification time, mode and ownership of
4880 the file. @GNUTAR{} allows to replace these data with other values
4881 when adding files to the archive. The options described in this
4882 section affect creation of archives of any type. For POSIX archives,
4883 see also @ref{PAX keywords}, for additional ways of controlling
4884 metadata, stored in the archive.
4888 @item --mode=@var{permissions}
4890 When adding files to an archive, @command{tar} will use
4891 @var{permissions} for the archive members, rather than the permissions
4892 from the files. @var{permissions} can be specified either as an octal
4893 number or as symbolic permissions, like with
4894 @command{chmod} (@xref{File permissions, Permissions, File
4895 permissions, fileutils, @acronym{GNU} file utilities}. This reference
4896 also has useful information for those not being overly familiar with
4897 the UNIX permission system). Using latter syntax allows for
4898 more flexibility. For example, the value @samp{a+rw} adds read and write
4899 permissions for everybody, while retaining executable bits on directories
4900 or on any other file already marked as executable:
4903 $ @kbd{tar -c -f archive.tar --mode='a+rw' .}
4906 @item --mtime=@var{date}
4909 When adding files to an archive, @command{tar} will use @var{date} as
4910 the modification time of members when creating archives, instead of
4911 their actual modification times. The argument @var{date} can be
4912 either a textual date representation in almost arbitrary format
4913 (@pxref{Date input formats}) or a name of an existing file, starting
4914 with @samp{/} or @samp{.}. In the latter case, the modification time
4915 of that file will be used.
4917 The following example will set the modification date to 00:00:00,
4921 $ @kbd{tar -c -f archive.tar --mtime='1970-01-01' .}
4925 When used with @option{--verbose} (@pxref{verbose tutorial}) @GNUTAR{}
4926 will try to convert the specified date back to its textual
4927 representation and compare it with the one given with
4928 @option{--mtime} options. If the two dates differ, @command{tar} will
4929 print a warning saying what date it will use. This is to help user
4930 ensure he is using the right date.
4935 $ @kbd{tar -c -f archive.tar -v --mtime=yesterday .}
4936 tar: Option --mtime: Treating date `yesterday' as 2006-06-20
4941 @item --owner=@var{user}
4944 Specifies that @command{tar} should use @var{user} as the owner of members
4945 when creating archives, instead of the user associated with the source
4948 If @var{user} contains a colon, it is taken to be of the form
4949 @var{name}:@var{id} where a nonempty @var{name} specifies the user
4950 name and a nonempty @var{id} specifies the decimal numeric user
4951 @acronym{ID}. If @var{user} does not contain a colon, it is taken to
4952 be a user number if it is one or more decimal digits; otherwise it is
4953 taken to be a user name.
4955 If a name is given but no number, the number is inferred from the
4956 current host's user database if possible, and the file's user number
4957 is used otherwise. If a number is given but no name, the name is
4958 inferred from the number if possible, and an empty name is used
4959 otherwise. If both name and number are given, the user database is
4960 not consulted, and the name and number need not be valid on the
4963 There is no value indicating a missing number, and @samp{0} usually means
4964 @code{root}. Some people like to force @samp{0} as the value to offer in
4965 their distributions for the owner of files, because the @code{root} user is
4966 anonymous anyway, so that might as well be the owner of anonymous
4967 archives. For example:
4970 $ @kbd{tar -c -f archive.tar --owner=0 .}
4977 $ @kbd{tar -c -f archive.tar --owner=root .}
4980 @item --group=@var{group}
4983 Files added to the @command{tar} archive will have a group @acronym{ID} of @var{group},
4984 rather than the group from the source file. As with @option{--owner},
4985 the argument @var{group} can be an existing group symbolic name, or a
4986 decimal numeric group @acronym{ID}, or @var{name}:@var{id}.
4989 @node Ignore Failed Read
4990 @subsection Ignore Fail Read
4993 @item --ignore-failed-read
4994 @opindex ignore-failed-read
4995 Do not exit with nonzero on unreadable files or directories.
4998 @node extract options
4999 @section Options Used by @option{--extract}
5000 @cindex options for use with @option{--extract}
5002 @xopindex{extract, additional options}
5003 The previous chapter showed how to use @option{--extract} to extract
5004 an archive into the file system. Various options cause @command{tar} to
5005 extract more information than just file contents, such as the owner,
5006 the permissions, the modification date, and so forth. This section
5007 presents options to be used with @option{--extract} when certain special
5008 considerations arise. You may review the information presented in
5009 @ref{extract} for more basic information about the
5010 @option{--extract} operation.
5013 * Reading:: Options to Help Read Archives
5014 * Writing:: Changing How @command{tar} Writes Files
5015 * Scarce:: Coping with Scarce Resources
5019 @subsection Options to Help Read Archives
5020 @cindex Options when reading archives
5022 @cindex Reading incomplete records
5023 @cindex Records, incomplete
5024 @opindex read-full-records
5025 Normally, @command{tar} will request data in full record increments from
5026 an archive storage device. If the device cannot return a full record,
5027 @command{tar} will report an error. However, some devices do not always
5028 return full records, or do not require the last record of an archive to
5029 be padded out to the next record boundary. To keep reading until you
5030 obtain a full record, or to accept an incomplete record if it contains
5031 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
5032 in conjunction with the @option{--extract} or @option{--list} operations.
5035 The @option{--read-full-records} (@option{-B}) option is turned on by default when
5036 @command{tar} reads an archive from standard input, or from a remote
5037 machine. This is because on @acronym{BSD} Unix systems, attempting to read a
5038 pipe returns however much happens to be in the pipe, even if it is
5039 less than was requested. If this option were not enabled, @command{tar}
5040 would fail as soon as it read an incomplete record from the pipe.
5042 If you're not sure of the blocking factor of an archive, you can
5043 read the archive by specifying @option{--read-full-records} (@option{-B}) and
5044 @option{--blocking-factor=@var{512-size}} (@option{-b
5045 @var{512-size}}), using a blocking factor larger than what the archive
5046 uses. This lets you avoid having to determine the blocking factor
5047 of an archive. @xref{Blocking Factor}.
5050 * read full records::
5054 @node read full records
5055 @unnumberedsubsubsec Reading Full Records
5057 @FIXME{need sentence or so of intro here}
5060 @opindex read-full-records
5061 @item --read-full-records
5063 Use in conjunction with @option{--extract} (@option{--get},
5064 @option{-x}) to read an archive which contains incomplete records, or
5065 one which has a blocking factor less than the one specified.
5069 @unnumberedsubsubsec Ignoring Blocks of Zeros
5071 @cindex End-of-archive blocks, ignoring
5072 @cindex Ignoring end-of-archive blocks
5073 @opindex ignore-zeros
5074 Normally, @command{tar} stops reading when it encounters a block of zeros
5075 between file entries (which usually indicates the end of the archive).
5076 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
5077 completely read an archive which contains a block of zeros before the
5078 end (i.e., a damaged archive, or one that was created by concatenating
5079 several archives together).
5081 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
5082 versions of @command{tar} write garbage after the end-of-archive entry,
5083 since that part of the media is never supposed to be read. @GNUTAR{}
5084 does not write after the end of an archive, but seeks to
5085 maintain compatibility among archiving utilities.
5088 @item --ignore-zeros
5090 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
5091 encountered while reading an archive. Use in conjunction with
5092 @option{--extract} or @option{--list}.
5096 @subsection Changing How @command{tar} Writes Files
5099 @FIXME{Introductory paragraph}
5102 * Dealing with Old Files::
5103 * Overwrite Old Files::
5105 * Keep Newer Files::
5107 * Recursive Unlink::
5108 * Data Modification Times::
5109 * Setting Access Permissions::
5110 * Directory Modification Times and Permissions::
5111 * Writing to Standard Output::
5112 * Writing to an External Program::
5116 @node Dealing with Old Files
5117 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
5119 @xopindex{overwrite-dir, introduced}
5120 When extracting files, if @command{tar} discovers that the extracted
5121 file already exists, it normally replaces the file by removing it before
5122 extracting it, to prevent confusion in the presence of hard or symbolic
5123 links. (If the existing file is a symbolic link, it is removed, not
5124 followed.) However, if a directory cannot be removed because it is
5125 nonempty, @command{tar} normally overwrites its metadata (ownership,
5126 permission, etc.). The @option{--overwrite-dir} option enables this
5127 default behavior. To be more cautious and preserve the metadata of
5128 such a directory, use the @option{--no-overwrite-dir} option.
5130 @cindex Overwriting old files, prevention
5131 @xopindex{keep-old-files, introduced}
5132 To be even more cautious and prevent existing files from being replaced, use
5133 the @option{--keep-old-files} (@option{-k}) option. It causes @command{tar} to refuse
5134 to replace or update a file that already exists, i.e., a file with the
5135 same name as an archive member prevents extraction of that archive
5136 member. Instead, it reports an error.
5138 @xopindex{overwrite, introduced}
5139 To be more aggressive about altering existing files, use the
5140 @option{--overwrite} option. It causes @command{tar} to overwrite
5141 existing files and to follow existing symbolic links when extracting.
5143 @cindex Protecting old files
5144 Some people argue that @GNUTAR{} should not hesitate
5145 to overwrite files with other files when extracting. When extracting
5146 a @command{tar} archive, they expect to see a faithful copy of the
5147 state of the file system when the archive was created. It is debatable
5148 that this would always be a proper behavior. For example, suppose one
5149 has an archive in which @file{usr/local} is a link to
5150 @file{usr/local2}. Since then, maybe the site removed the link and
5151 renamed the whole hierarchy from @file{/usr/local2} to
5152 @file{/usr/local}. Such things happen all the time. I guess it would
5153 not be welcome at all that @GNUTAR{} removes the
5154 whole hierarchy just to make room for the link to be reinstated
5155 (unless it @emph{also} simultaneously restores the full
5156 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
5157 able to remove a whole hierarchy to reestablish a symbolic link, for
5158 example, but @emph{only if} @option{--recursive-unlink} is specified
5159 to allow this behavior. In any case, single files are silently
5162 @xopindex{unlink-first, introduced}
5163 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
5164 some cases by causing @command{tar} to remove files unconditionally
5165 before extracting them.
5167 @node Overwrite Old Files
5168 @unnumberedsubsubsec Overwrite Old Files
5173 Overwrite existing files and directory metadata when extracting files
5176 This causes @command{tar} to write extracted files into the file system without
5177 regard to the files already on the system; i.e., files with the same
5178 names as archive members are overwritten when the archive is extracted.
5179 It also causes @command{tar} to extract the ownership, permissions,
5180 and time stamps onto any preexisting files or directories.
5181 If the name of a corresponding file name is a symbolic link, the file
5182 pointed to by the symbolic link will be overwritten instead of the
5183 symbolic link itself (if this is possible). Moreover, special devices,
5184 empty directories and even symbolic links are automatically removed if
5185 they are in the way of extraction.
5187 Be careful when using the @option{--overwrite} option, particularly when
5188 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
5189 can change the contents, ownership or permissions of any file on your
5190 system. Also, many systems do not take kindly to overwriting files that
5191 are currently being executed.
5193 @opindex overwrite-dir
5194 @item --overwrite-dir
5195 Overwrite the metadata of directories when extracting files from an
5196 archive, but remove other files before extracting.
5199 @node Keep Old Files
5200 @unnumberedsubsubsec Keep Old Files
5203 @opindex keep-old-files
5204 @item --keep-old-files
5206 Do not replace existing files from archive. The
5207 @option{--keep-old-files} (@option{-k}) option prevents @command{tar}
5208 from replacing existing files with files with the same name from the
5209 archive. The @option{--keep-old-files} option is meaningless with
5210 @option{--list} (@option{-t}). Prevents @command{tar} from replacing
5211 files in the file system during extraction.
5214 @node Keep Newer Files
5215 @unnumberedsubsubsec Keep Newer Files
5218 @opindex keep-newer-files
5219 @item --keep-newer-files
5220 Do not replace existing files that are newer than their archive
5221 copies. This option is meaningless with @option{--list} (@option{-t}).
5225 @unnumberedsubsubsec Unlink First
5228 @opindex unlink-first
5229 @item --unlink-first
5231 Remove files before extracting over them.
5232 This can make @command{tar} run a bit faster if you know in advance
5233 that the extracted files all need to be removed. Normally this option
5234 slows @command{tar} down slightly, so it is disabled by default.
5237 @node Recursive Unlink
5238 @unnumberedsubsubsec Recursive Unlink
5241 @opindex recursive-unlink
5242 @item --recursive-unlink
5243 When this option is specified, try removing files and directory hierarchies
5244 before extracting over them. @emph{This is a dangerous option!}
5247 If you specify the @option{--recursive-unlink} option,
5248 @command{tar} removes @emph{anything} that keeps you from extracting a file
5249 as far as current permissions will allow it. This could include removal
5250 of the contents of a full directory hierarchy.
5252 @node Data Modification Times
5253 @unnumberedsubsubsec Setting Data Modification Times
5255 @cindex Data modification times of extracted files
5256 @cindex Modification times of extracted files
5257 Normally, @command{tar} sets the data modification times of extracted
5258 files to the corresponding times recorded for the files in the archive, but
5259 limits the permissions of extracted files by the current @code{umask}
5262 To set the data modification times of extracted files to the time when
5263 the files were extracted, use the @option{--touch} (@option{-m}) option in
5264 conjunction with @option{--extract} (@option{--get}, @option{-x}).
5270 Sets the data modification time of extracted archive members to the time
5271 they were extracted, not the time recorded for them in the archive.
5272 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
5275 @node Setting Access Permissions
5276 @unnumberedsubsubsec Setting Access Permissions
5278 @cindex Permissions of extracted files
5279 @cindex Modes of extracted files
5280 To set the modes (access permissions) of extracted files to those
5281 recorded for those files in the archive, use @option{--same-permissions}
5282 in conjunction with the @option{--extract} (@option{--get},
5283 @option{-x}) operation.
5286 @opindex preserve-permissions
5287 @opindex same-permissions
5288 @item --preserve-permissions
5289 @itemx --same-permissions
5290 @c @itemx --ignore-umask
5292 Set modes of extracted archive members to those recorded in the
5293 archive, instead of current umask settings. Use in conjunction with
5294 @option{--extract} (@option{--get}, @option{-x}).
5297 @node Directory Modification Times and Permissions
5298 @unnumberedsubsubsec Directory Modification Times and Permissions
5300 After successfully extracting a file member, @GNUTAR{} normally
5301 restores its permissions and modification times, as described in the
5302 previous sections. This cannot be done for directories, because
5303 after extracting a directory @command{tar} will almost certainly
5304 extract files into that directory and this will cause the directory
5305 modification time to be updated. Moreover, restoring that directory
5306 permissions may not permit file creation within it. Thus, restoring
5307 directory permissions and modification times must be delayed at least
5308 until all files have been extracted into that directory. @GNUTAR{}
5309 restores directories using the following approach.
5311 The extracted directories are created with the mode specified in the
5312 archive, as modified by the umask of the user, which gives sufficient
5313 permissions to allow file creation. The meta-information about the
5314 directory is recorded in the temporary list of directories. When
5315 preparing to extract next archive member, @GNUTAR{} checks if the
5316 directory prefix of this file contains the remembered directory. If
5317 it does not, the program assumes that all files have been extracted
5318 into that directory, restores its modification time and permissions
5319 and removes its entry from the internal list. This approach allows
5320 to correctly restore directory meta-information in the majority of
5321 cases, while keeping memory requirements sufficiently small. It is
5322 based on the fact, that most @command{tar} archives use the predefined
5323 order of members: first the directory, then all the files and
5324 subdirectories in that directory.
5326 However, this is not always true. The most important exception are
5327 incremental archives (@pxref{Incremental Dumps}). The member order in
5328 an incremental archive is reversed: first all directory members are
5329 stored, followed by other (non-directory) members. So, when extracting
5330 from incremental archives, @GNUTAR{} alters the above procedure. It
5331 remembers all restored directories, and restores their meta-data
5332 only after the entire archive has been processed. Notice, that you do
5333 not need to specify any special options for that, as @GNUTAR{}
5334 automatically detects archives in incremental format.
5336 There may be cases, when such processing is required for normal archives
5337 too. Consider the following example:
5341 $ @kbd{tar --no-recursion -cvf archive \
5342 foo foo/file1 bar bar/file foo/file2}
5351 During the normal operation, after encountering @file{bar}
5352 @GNUTAR{} will assume that all files from the directory @file{foo}
5353 were already extracted and will therefore restore its timestamp and
5354 permission bits. However, after extracting @file{foo/file2} the
5355 directory timestamp will be offset again.
5357 To correctly restore directory meta-information in such cases, use
5358 the @option{--delay-directory-restore} command line option:
5361 @opindex delay-directory-restore
5362 @item --delay-directory-restore
5363 Delays restoring of the modification times and permissions of extracted
5364 directories until the end of extraction. This way, correct
5365 meta-information is restored even if the archive has unusual member
5368 @opindex no-delay-directory-restore
5369 @item --no-delay-directory-restore
5370 Cancel the effect of the previous @option{--delay-directory-restore}.
5371 Use this option if you have used @option{--delay-directory-restore} in
5372 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
5373 temporarily disable it.
5376 @node Writing to Standard Output
5377 @unnumberedsubsubsec Writing to Standard Output
5379 @cindex Writing extracted files to standard output
5380 @cindex Standard output, writing extracted files to
5381 To write the extracted files to the standard output, instead of
5382 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
5383 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
5384 extracting files to send them through a pipe, and do not need to
5385 preserve them in the file system. If you extract multiple members,
5386 they appear on standard output concatenated, in the order they are
5387 found in the archive.
5393 Writes files to the standard output. Use only in conjunction with
5394 @option{--extract} (@option{--get}, @option{-x}). When this option is
5395 used, instead of creating the files specified, @command{tar} writes
5396 the contents of the files extracted to its standard output. This may
5397 be useful if you are only extracting the files in order to send them
5398 through a pipe. This option is meaningless with @option{--list}
5402 This can be useful, for example, if you have a tar archive containing
5403 a big file and don't want to store the file on disk before processing
5404 it. You can use a command like this:
5407 tar -xOzf foo.tgz bigfile | process
5410 or even like this if you want to process the concatenation of the files:
5413 tar -xOzf foo.tgz bigfile1 bigfile2 | process
5416 However, @option{--to-command} may be more convenient for use with
5417 multiple files. See the next section.
5419 @node Writing to an External Program
5420 @unnumberedsubsubsec Writing to an External Program
5422 You can instruct @command{tar} to send the contents of each extracted
5423 file to the standard input of an external program:
5427 @item --to-command=@var{command}
5428 Extract files and pipe their contents to the standard input of
5429 @var{command}. When this option is used, instead of creating the
5430 files specified, @command{tar} invokes @var{command} and pipes the
5431 contents of the files to its standard output. The @var{command} may
5432 contain command line arguments. The program is executed via
5433 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
5434 extracted. Non-regular files (directories, etc.) are ignored when this
5438 The command can obtain the information about the file it processes
5439 from the following environment variables:
5442 @vrindex TAR_FILETYPE, to-command environment
5444 Type of the file. It is a single letter with the following meaning:
5446 @multitable @columnfractions 0.10 0.90
5447 @item f @tab Regular file
5448 @item d @tab Directory
5449 @item l @tab Symbolic link
5450 @item h @tab Hard link
5451 @item b @tab Block device
5452 @item c @tab Character device
5455 Currently only regular files are supported.
5457 @vrindex TAR_MODE, to-command environment
5459 File mode, an octal number.
5461 @vrindex TAR_FILENAME, to-command environment
5463 The name of the file.
5465 @vrindex TAR_REALNAME, to-command environment
5467 Name of the file as stored in the archive.
5469 @vrindex TAR_UNAME, to-command environment
5471 Name of the file owner.
5473 @vrindex TAR_GNAME, to-command environment
5475 Name of the file owner group.
5477 @vrindex TAR_ATIME, to-command environment
5479 Time of last access. It is a decimal number, representing seconds
5480 since the Epoch. If the archive provides times with nanosecond
5481 precision, the nanoseconds are appended to the timestamp after a
5484 @vrindex TAR_MTIME, to-command environment
5486 Time of last modification.
5488 @vrindex TAR_CTIME, to-command environment
5490 Time of last status change.
5492 @vrindex TAR_SIZE, to-command environment
5496 @vrindex TAR_UID, to-command environment
5498 UID of the file owner.
5500 @vrindex TAR_GID, to-command environment
5502 GID of the file owner.
5505 Additionally, the following variables contain information about
5506 tar mode and the archive being processed:
5509 @vrindex TAR_VERSION, to-command environment
5511 @GNUTAR{} version number.
5513 @vrindex TAR_ARCHIVE, to-command environment
5515 The name of the archive @command{tar} is processing.
5517 @vrindex TAR_BLOCKING_FACTOR, to-command environment
5518 @item TAR_BLOCKING_FACTOR
5519 Current blocking factor (@pxref{Blocking}).
5521 @vrindex TAR_VOLUME, to-command environment
5523 Ordinal number of the volume @command{tar} is processing.
5525 @vrindex TAR_FORMAT, to-command environment
5527 Format of the archive being processed. @xref{Formats}, for a complete
5528 list of archive format names.
5531 If @var{command} exits with a non-0 status, @command{tar} will print
5532 an error message similar to the following:
5535 tar: 2345: Child returned status 1
5538 Here, @samp{2345} is the PID of the finished process.
5540 If this behavior is not wanted, use @option{--ignore-command-error}:
5543 @opindex ignore-command-error
5544 @item --ignore-command-error
5545 Ignore exit codes of subprocesses. Notice that if the program
5546 exits on signal or otherwise terminates abnormally, the error message
5547 will be printed even if this option is used.
5549 @opindex no-ignore-command-error
5550 @item --no-ignore-command-error
5551 Cancel the effect of any previous @option{--ignore-command-error}
5552 option. This option is useful if you have set
5553 @option{--ignore-command-error} in @env{TAR_OPTIONS}
5554 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
5558 @unnumberedsubsubsec Removing Files
5560 @FIXME{The section is too terse. Something more to add? An example,
5564 @opindex remove-files
5565 @item --remove-files
5566 Remove files after adding them to the archive.
5570 @subsection Coping with Scarce Resources
5573 @cindex Small memory
5574 @cindex Running out of space
5582 @unnumberedsubsubsec Starting File
5585 @opindex starting-file
5586 @item --starting-file=@var{name}
5587 @itemx -K @var{name}
5588 Starts an operation in the middle of an archive. Use in conjunction
5589 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
5592 @cindex Middle of the archive, starting in the
5593 If a previous attempt to extract files failed due to lack of disk
5594 space, you can use @option{--starting-file=@var{name}} (@option{-K
5595 @var{name}}) to start extracting only after member @var{name} of the
5596 archive. This assumes, of course, that there is now free space, or
5597 that you are now extracting into a different file system. (You could
5598 also choose to suspend @command{tar}, remove unnecessary files from
5599 the file system, and then resume the same @command{tar} operation.
5600 In this case, @option{--starting-file} is not necessary.) See also
5601 @ref{interactive}, and @ref{exclude}.
5604 @unnumberedsubsubsec Same Order
5607 @cindex Large lists of file names on small machines
5609 @opindex preserve-order
5611 @itemx --preserve-order
5613 To process large lists of file names on machines with small amounts of
5614 memory. Use in conjunction with @option{--compare} (@option{--diff},
5615 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
5616 (@option{--get}, @option{-x}).
5619 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
5620 names to be listed or extracted is sorted in the same order as the
5621 files in the archive. This allows a large list of names to be used,
5622 even on a small machine that would not otherwise be able to hold all
5623 the names in memory at the same time. Such a sorted list can easily be
5624 created by running @samp{tar -t} on the archive and editing its output.
5626 This option is probably never needed on modern computer systems.
5629 @section Backup options
5631 @cindex backup options
5633 @GNUTAR{} offers options for making backups of files
5634 before writing new versions. These options control the details of
5635 these backups. They may apply to the archive itself before it is
5636 created or rewritten, as well as individual extracted members. Other
5637 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
5638 and @command{mv}, for example) offer similar options.
5640 Backup options may prove unexpectedly useful when extracting archives
5641 containing many members having identical name, or when extracting archives
5642 on systems having file name limitations, making different members appear
5643 as having similar names through the side-effect of name truncation.
5644 @FIXME{This is true only if we have a good scheme for truncated backup names,
5645 which I'm not sure at all: I suspect work is needed in this area.}
5646 When any existing file is backed up before being overwritten by extraction,
5647 then clashing files are automatically be renamed to be unique, and the
5648 true name is kept for only the last file of a series of clashing files.
5649 By using verbose mode, users may track exactly what happens.
5651 At the detail level, some decisions are still experimental, and may
5652 change in the future, we are waiting comments from our users. So, please
5653 do not learn to depend blindly on the details of the backup features.
5654 For example, currently, directories themselves are never renamed through
5655 using these options, so, extracting a file over a directory still has
5656 good chances to fail. Also, backup options apply to created archives,
5657 not only to extracted members. For created archives, backups will not
5658 be attempted when the archive is a block or character device, or when it
5659 refers to a remote file.
5661 For the sake of simplicity and efficiency, backups are made by renaming old
5662 files prior to creation or extraction, and not by copying. The original
5663 name is restored if the file creation fails. If a failure occurs after a
5664 partial extraction of a file, both the backup and the partially extracted
5668 @item --backup[=@var{method}]
5670 @vindex VERSION_CONTROL
5672 Back up files that are about to be overwritten or removed.
5673 Without this option, the original versions are destroyed.
5675 Use @var{method} to determine the type of backups made.
5676 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
5677 environment variable. And if @env{VERSION_CONTROL} is not set,
5678 use the @samp{existing} method.
5680 @vindex version-control @r{Emacs variable}
5681 This option corresponds to the Emacs variable @samp{version-control};
5682 the same values for @var{method} are accepted as in Emacs. This option
5683 also allows more descriptive names. The valid @var{method}s are:
5688 @cindex numbered @r{backup method}
5689 Always make numbered backups.
5693 @cindex existing @r{backup method}
5694 Make numbered backups of files that already have them, simple backups
5699 @cindex simple @r{backup method}
5700 Always make simple backups.
5704 @item --suffix=@var{suffix}
5706 @cindex backup suffix
5707 @vindex SIMPLE_BACKUP_SUFFIX
5708 Append @var{suffix} to each backup file made with @option{--backup}. If this
5709 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5710 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5711 set, the default is @samp{~}, just as in Emacs.
5716 @section Notable @command{tar} Usages
5719 @FIXME{Using Unix file linking capability to recreate directory
5720 structures---linking files into one subdirectory and then
5721 @command{tar}ring that directory.}
5723 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5726 You can easily use archive files to transport a group of files from
5727 one system to another: put all relevant files into an archive on one
5728 computer system, transfer the archive to another system, and extract
5729 the contents there. The basic transfer medium might be magnetic tape,
5730 Internet FTP, or even electronic mail (though you must encode the
5731 archive with @command{uuencode} in order to transport it properly by
5732 mail). Both machines do not have to use the same operating system, as
5733 long as they both support the @command{tar} program.
5735 For example, here is how you might copy a directory's contents from
5736 one disk to another, while preserving the dates, modes, owners and
5737 link-structure of all the files therein. In this case, the transfer
5738 medium is a @dfn{pipe}:
5741 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5745 You can avoid subshells by using @option{-C} option:
5748 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5752 The command also works using long option forms:
5756 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5757 | (cd targetdir; tar --extract --file=-)}
5766 $ @kbd{tar --directory sourcedir --create --file=- . \
5767 | tar --directory targetdir --extract --file=-}
5772 This is one of the easiest methods to transfer a @command{tar} archive.
5775 @section Looking Ahead: The Rest of this Manual
5777 You have now seen how to use all eight of the operations available to
5778 @command{tar}, and a number of the possible options. The next chapter
5779 explains how to choose and change file and archive names, how to use
5780 files to store names of other files which you can then call as
5781 arguments to @command{tar} (this can help you save time if you expect to
5782 archive the same list of files a number of times), and so forth.
5783 @FIXME{in case it's not obvious, i'm making this up in some sense
5784 based on my limited memory of what the next chapter *really* does. i
5785 just wanted to flesh out this final section a little bit so i'd
5786 remember to stick it in here. :-)}
5788 If there are too many files to conveniently list on the command line,
5789 you can list the names in a file, and @command{tar} will read that file.
5792 There are various ways of causing @command{tar} to skip over some files,
5793 and not archive them. @xref{Choosing}.
5796 @chapter Performing Backups and Restoring Files
5799 @GNUTAR{} is distributed along with the scripts for performing backups
5800 and restores. Even if there is a good chance those scripts may be
5801 satisfying to you, they are not the only scripts or methods available for doing
5802 backups and restore. You may well create your own, or use more
5803 sophisticated packages dedicated to that purpose.
5805 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5806 Automatic Network Disk Archiver), a backup system developed by James
5807 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5808 This is free software, and it is available from @uref{http://www.amanda.org}.
5812 Here is a possible plan for a future documentation about the backuping
5813 scripts which are provided within the @GNUTAR{}
5819 @item what are dumps
5820 @item different levels of dumps
5822 @item full dump = dump everything
5823 @item level 1, level 2 dumps etc
5824 A level @var{n} dump dumps everything changed since the last level
5827 @item how to use scripts for dumps (ie, the concept)
5829 @item scripts to run after editing backup specs (details)
5831 @item Backup Specs, what is it.
5833 @item how to customize
5834 @item actual text of script [/sp/dump/backup-specs]
5838 @item rsh doesn't work
5839 @item rtape isn't installed
5842 @item the @option{--incremental} option of tar
5845 @item write protection
5846 @item types of media, different sizes and types, useful for different things
5847 @item files and tape marks
5848 one tape mark between files, two at end.
5849 @item positioning the tape
5850 MT writes two at end of write,
5851 backspaces over one when writing again.
5857 This chapter documents both the provided shell scripts and @command{tar}
5858 options which are more specific to usage as a backup tool.
5860 To @dfn{back up} a file system means to create archives that contain
5861 all the files in that file system. Those archives can then be used to
5862 restore any or all of those files (for instance if a disk crashes or a
5863 file is accidentally deleted). File system @dfn{backups} are also
5867 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5868 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5869 * Backup Levels:: Levels of Backups
5870 * Backup Parameters:: Setting Parameters for Backups and Restoration
5871 * Scripted Backups:: Using the Backup Scripts
5872 * Scripted Restoration:: Using the Restore Script
5876 @section Using @command{tar} to Perform Full Dumps
5882 @cindex corrupted archives
5883 Full dumps should only be made when no other people or programs
5884 are modifying files in the file system. If files are modified while
5885 @command{tar} is making the backup, they may not be stored properly in
5886 the archive, in which case you won't be able to restore them if you
5887 have to. (Files not being modified are written with no trouble, and do
5888 not corrupt the entire archive.)
5890 You will want to use the @option{--label=@var{archive-label}}
5891 (@option{-V @var{archive-label}}) option to give the archive a
5892 volume label, so you can tell what this archive is even if the label
5893 falls off the tape, or anything like that.
5895 Unless the file system you are dumping is guaranteed to fit on
5896 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5897 Make sure you have enough tapes on hand to complete the backup.
5899 If you want to dump each file system separately you will need to use
5900 the @option{--one-file-system} option to prevent
5901 @command{tar} from crossing file system boundaries when storing
5904 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5905 option is not needed, since this is a complete copy of everything in
5906 the file system, and a full restore from this backup would only be
5907 done onto a completely
5910 Unless you are in a hurry, and trust the @command{tar} program (and your
5911 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5912 option, to make sure your files really made it onto the dump properly.
5913 This will also detect cases where the file was modified while (or just
5914 after) it was being archived. Not all media (notably cartridge tapes)
5915 are capable of being verified, unfortunately.
5917 @node Incremental Dumps
5918 @section Using @command{tar} to Perform Incremental Dumps
5920 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5921 stores additional metadata so that exact state of the file system
5922 can be restored when extracting the archive.
5924 @GNUTAR{} currently offers two options for handling incremental
5925 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5926 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5928 @xopindex{listed-incremental, described}
5929 The option @option{--listed-incremental} instructs tar to operate on
5930 an incremental archive with additional metadata stored in a standalone
5931 file, called a @dfn{snapshot file}. The purpose of this file is to help
5932 determine which files have been changed, added or deleted since the
5933 last backup, so that the next incremental backup will contain only
5934 modified files. The name of the snapshot file is given as an argument
5938 @item --listed-incremental=@var{file}
5939 @itemx -g @var{file}
5940 Handle incremental backups with snapshot data in @var{file}.
5943 To create an incremental backup, you would use
5944 @option{--listed-incremental} together with @option{--create}
5945 (@pxref{create}). For example:
5948 $ @kbd{tar --create \
5949 --file=archive.1.tar \
5950 --listed-incremental=/var/log/usr.snar \
5954 This will create in @file{archive.1.tar} an incremental backup of
5955 the @file{/usr} file system, storing additional metadata in the file
5956 @file{/var/log/usr.snar}. If this file does not exist, it will be
5957 created. The created archive will then be a @dfn{level 0 backup};
5958 please see the next section for more on backup levels.
5960 Otherwise, if the file @file{/var/log/usr.snar} exists, it
5961 determines which files are modified. In this case only these files will be
5962 stored in the archive. Suppose, for example, that after running the
5963 above command, you delete file @file{/usr/doc/old} and create
5964 directory @file{/usr/local/db} with the following contents:
5967 $ @kbd{ls /usr/local/db}
5972 Some time later you create another incremental backup. You will
5976 $ @kbd{tar --create \
5977 --file=archive.2.tar \
5978 --listed-incremental=/var/log/usr.snar \
5980 tar: usr/local/db: Directory is new
5987 The created archive @file{archive.2.tar} will contain only these
5988 three members. This archive is called a @dfn{level 1 backup}. Notice
5989 that @file{/var/log/usr.snar} will be updated with the new data, so if
5990 you plan to create more @samp{level 1} backups, it is necessary to
5991 create a working copy of the snapshot file before running
5992 @command{tar}. The above example will then be modified as follows:
5995 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
5996 $ @kbd{tar --create \
5997 --file=archive.2.tar \
5998 --listed-incremental=/var/log/usr.snar-1 \
6003 @xopindex{level, described}
6004 You can force @samp{level 0} backups either by removing the snapshot
6005 file before running @command{tar}, or by supplying the
6006 @option{--level=0} option, e.g.:
6009 $ @kbd{tar --create \
6010 --file=archive.2.tar \
6011 --listed-incremental=/var/log/usr.snar-0 \
6016 Incremental dumps depend crucially on time stamps, so the results are
6017 unreliable if you modify a file's time stamps during dumping (e.g.,
6018 with the @option{--atime-preserve=replace} option), or if you set the clock
6021 @anchor{device numbers}
6022 @cindex Device numbers, using in incremental backups
6023 Metadata stored in snapshot files include device numbers, which,
6024 obviously are supposed to be non-volatile values. However, it turns
6025 out that @acronym{NFS} devices have undependable values when an automounter
6026 gets in the picture. This can lead to a great deal of spurious
6027 redumping in incremental dumps, so it is somewhat useless to compare
6028 two @acronym{NFS} devices numbers over time. The solution implemented
6029 currently is to consider all @acronym{NFS} devices as being equal
6030 when it comes to comparing directories; this is fairly gross, but
6031 there does not seem to be a better way to go.
6033 Apart from using @acronym{NFS}, there are a number of cases where
6034 relying on device numbers can cause spurious redumping of unmodified
6035 files. For example, this occurs when archiving @acronym{LVM} snapshot
6036 volumes. To avoid this, use @option{--no-check-device} option:
6039 @xopindex{no-check-device, described}
6040 @item --no-check-device
6041 Do not rely on device numbers when preparing a list of changed files
6042 for an incremental dump.
6044 @xopindex{check-device, described}
6045 @item --check-device
6046 Use device numbers when preparing a list of changed files
6047 for an incremental dump. This is the default behavior. The purpose
6048 of this option is to undo the effect of the @option{--no-check-device}
6049 if it was given in @env{TAR_OPTIONS} environment variable
6050 (@pxref{TAR_OPTIONS}).
6053 There is also another way to cope with changing device numbers. It is
6054 described in detail in @ref{Fixing Snapshot Files}.
6056 Note that incremental archives use @command{tar} extensions and may
6057 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
6059 @xopindex{listed-incremental, using with @option{--extract}}
6060 @xopindex{extract, using with @option{--listed-incremental}}
6061 To extract from the incremental dumps, use
6062 @option{--listed-incremental} together with @option{--extract}
6063 option (@pxref{extracting files}). In this case, @command{tar} does
6064 not need to access snapshot file, since all the data necessary for
6065 extraction are stored in the archive itself. So, when extracting, you
6066 can give whatever argument to @option{--listed-incremental}, the usual
6067 practice is to use @option{--listed-incremental=/dev/null}.
6068 Alternatively, you can use @option{--incremental}, which needs no
6069 arguments. In general, @option{--incremental} (@option{-G}) can be
6070 used as a shortcut for @option{--listed-incremental} when listing or
6071 extracting incremental backups (for more information regarding this
6072 option, @pxref{incremental-op}).
6074 When extracting from the incremental backup @GNUTAR{} attempts to
6075 restore the exact state the file system had when the archive was
6076 created. In particular, it will @emph{delete} those files in the file
6077 system that did not exist in their directories when the archive was
6078 created. If you have created several levels of incremental files,
6079 then in order to restore the exact contents the file system had when
6080 the last level was created, you will need to restore from all backups
6081 in turn. Continuing our example, to restore the state of @file{/usr}
6082 file system, one would do@footnote{Notice, that since both archives
6083 were created without @option{-P} option (@pxref{absolute}), these
6084 commands should be run from the root file system.}:
6087 $ @kbd{tar --extract \
6088 --listed-incremental=/dev/null \
6089 --file archive.1.tar}
6090 $ @kbd{tar --extract \
6091 --listed-incremental=/dev/null \
6092 --file archive.2.tar}
6095 To list the contents of an incremental archive, use @option{--list}
6096 (@pxref{list}), as usual. To obtain more information about the
6097 archive, use @option{--listed-incremental} or @option{--incremental}
6098 combined with two @option{--verbose} options@footnote{Two
6099 @option{--verbose} options were selected to avoid breaking usual
6100 verbose listing output (@option{--list --verbose}) when using in
6103 @xopindex{incremental, using with @option{--list}}
6104 @xopindex{listed-incremental, using with @option{--list}}
6105 @xopindex{list, using with @option{--incremental}}
6106 @xopindex{list, using with @option{--listed-incremental}}
6107 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
6108 contents of the DUMPDIR header (with terminating nulls) when
6109 @option{--incremental} or @option{--listed-incremental} option was
6110 given, no matter what the verbosity level. This behavior, and,
6111 especially, the binary output it produced were considered inconvenient
6112 and were changed in version 1.16.}:
6115 @kbd{tar --list --incremental --verbose --verbose archive.tar}
6118 This command will print, for each directory in the archive, the list
6119 of files in that directory at the time the archive was created. This
6120 information is put out in a format which is both human-readable and
6121 unambiguous for a program: each file name is printed as
6128 where @var{x} is a letter describing the status of the file: @samp{Y}
6129 if the file is present in the archive, @samp{N} if the file is not
6130 included in the archive, or a @samp{D} if the file is a directory (and
6131 is included in the archive). @xref{Dumpdir}, for the detailed
6132 description of dumpdirs and status codes. Each such
6133 line is terminated by a newline character. The last line is followed
6134 by an additional newline to indicate the end of the data.
6136 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
6137 gives the same behavior as @option{--listed-incremental} when used
6138 with @option{--list} and @option{--extract} options. When used with
6139 @option{--create} option, it creates an incremental archive without
6140 creating snapshot file. Thus, it is impossible to create several
6141 levels of incremental backups with @option{--incremental} option.
6144 @section Levels of Backups
6146 An archive containing all the files in the file system is called a
6147 @dfn{full backup} or @dfn{full dump}. You could insure your data by
6148 creating a full dump every day. This strategy, however, would waste a
6149 substantial amount of archive media and user time, as unchanged files
6150 are daily re-archived.
6152 It is more efficient to do a full dump only occasionally. To back up
6153 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
6154 one} dump archives all the files that have changed since the last full
6157 A typical dump strategy would be to perform a full dump once a week,
6158 and a level one dump once a day. This means some versions of files
6159 will in fact be archived more than once, but this dump strategy makes
6160 it possible to restore a file system to within one day of accuracy by
6161 only extracting two archives---the last weekly (full) dump and the
6162 last daily (level one) dump. The only information lost would be in
6163 files changed or created since the last daily backup. (Doing dumps
6164 more than once a day is usually not worth the trouble.)
6166 @GNUTAR{} comes with scripts you can use to do full
6167 and level-one (actually, even level-two and so on) dumps. Using
6168 scripts (shell programs) to perform backups and restoration is a
6169 convenient and reliable alternative to typing out file name lists
6170 and @command{tar} commands by hand.
6172 Before you use these scripts, you need to edit the file
6173 @file{backup-specs}, which specifies parameters used by the backup
6174 scripts and by the restore script. This file is usually located
6175 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
6176 detailed description. Once the backup parameters are set, you can
6177 perform backups or restoration by running the appropriate script.
6179 The name of the backup script is @code{backup}. The name of the
6180 restore script is @code{restore}. The following sections describe
6181 their use in detail.
6183 @emph{Please Note:} The backup and restoration scripts are
6184 designed to be used together. While it is possible to restore files by
6185 hand from an archive which was created using a backup script, and to create
6186 an archive by hand which could then be extracted using the restore script,
6187 it is easier to use the scripts. @xref{Incremental Dumps}, before
6188 making such an attempt.
6190 @node Backup Parameters
6191 @section Setting Parameters for Backups and Restoration
6193 The file @file{backup-specs} specifies backup parameters for the
6194 backup and restoration scripts provided with @command{tar}. You must
6195 edit @file{backup-specs} to fit your system configuration and schedule
6196 before using these scripts.
6198 Syntactically, @file{backup-specs} is a shell script, containing
6199 mainly variable assignments. However, any valid shell construct
6200 is allowed in this file. Particularly, you may wish to define
6201 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
6202 For more information about shell script syntax, please refer to
6203 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
6204 g_02, the definition of the Shell Command Language}. See also
6205 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
6207 The shell variables controlling behavior of @code{backup} and
6208 @code{restore} are described in the following subsections.
6211 * General-Purpose Variables::
6212 * Magnetic Tape Control::
6214 * backup-specs example:: An Example Text of @file{Backup-specs}
6217 @node General-Purpose Variables
6218 @subsection General-Purpose Variables
6220 @defvr {Backup variable} ADMINISTRATOR
6221 The user name of the backup administrator. @code{Backup} scripts
6222 sends a backup report to this address.
6225 @defvr {Backup variable} BACKUP_HOUR
6226 The hour at which the backups are done. This can be a number from 0
6227 to 23, or the time specification in form @var{hours}:@var{minutes},
6228 or the string @samp{now}.
6230 This variable is used by @code{backup}. Its value may be overridden
6231 using @option{--time} option (@pxref{Scripted Backups}).
6234 @defvr {Backup variable} TAPE_FILE
6236 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
6237 is a remote archive (@pxref{remote-dev}), backup script will suppose
6238 that your @command{mt} is able to access remote devices. If @var{RSH}
6239 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
6240 invocations of @command{mt}.
6243 @defvr {Backup variable} BLOCKING
6245 The blocking factor @command{tar} will use when writing the dump archive.
6246 @xref{Blocking Factor}.
6249 @defvr {Backup variable} BACKUP_DIRS
6251 A list of file systems to be dumped (for @code{backup}), or restored
6252 (for @code{restore}). You can include any directory
6253 name in the list --- subdirectories on that file system will be
6254 included, regardless of how they may look to other networked machines.
6255 Subdirectories on other file systems will be ignored.
6257 The host name specifies which host to run @command{tar} on, and should
6258 normally be the host that actually contains the file system. However,
6259 the host machine must have @GNUTAR{} installed, and
6260 must be able to access the directory containing the backup scripts and
6261 their support files using the same file name that is used on the
6262 machine where the scripts are run (i.e., what @command{pwd} will print
6263 when in that directory on that machine). If the host that contains
6264 the file system does not have this capability, you can specify another
6265 host as long as it can access the file system through @acronym{NFS}.
6267 If the list of file systems is very long you may wish to put it
6268 in a separate file. This file is usually named
6269 @file{/etc/backup/dirs}, but this name may be overridden in
6270 @file{backup-specs} using @code{DIRLIST} variable.
6273 @defvr {Backup variable} DIRLIST
6275 The name of the file that contains a list of file systems to backup
6276 or restore. By default it is @file{/etc/backup/dirs}.
6279 @defvr {Backup variable} BACKUP_FILES
6281 A list of individual files to be dumped (for @code{backup}), or restored
6282 (for @code{restore}). These should be accessible from the machine on
6283 which the backup script is run.
6285 If the list of individual files is very long you may wish to store it
6286 in a separate file. This file is usually named
6287 @file{/etc/backup/files}, but this name may be overridden in
6288 @file{backup-specs} using @code{FILELIST} variable.
6291 @defvr {Backup variable} FILELIST
6293 The name of the file that contains a list of individual files to backup
6294 or restore. By default it is @file{/etc/backup/files}.
6297 @defvr {Backup variable} MT
6299 Full file name of @command{mt} binary.
6302 @defvr {Backup variable} RSH
6304 Full file name of @command{rsh} binary or its equivalent. You may wish to
6305 set it to @code{ssh}, to improve security. In this case you will have
6306 to use public key authentication.
6309 @defvr {Backup variable} RSH_COMMAND
6311 Full file name of @command{rsh} binary on remote machines. This will
6312 be passed via @option{--rsh-command} option to the remote invocation
6316 @defvr {Backup variable} VOLNO_FILE
6318 Name of temporary file to hold volume numbers. This needs to be accessible
6319 by all the machines which have file systems to be dumped.
6322 @defvr {Backup variable} XLIST
6324 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
6325 located on the remote machine and containing the list of files to
6326 be excluded from the backup. Exclude file lists are searched in
6327 /etc/tar-backup directory. A common use for exclude file lists
6328 is to exclude files containing security-sensitive information
6329 (e.g., @file{/etc/shadow} from backups).
6331 This variable affects only @code{backup}.
6334 @defvr {Backup variable} SLEEP_TIME
6336 Time to sleep between dumps of any two successive file systems
6338 This variable affects only @code{backup}.
6341 @defvr {Backup variable} DUMP_REMIND_SCRIPT
6343 Script to be run when it's time to insert a new tape in for the next
6344 volume. Administrators may want to tailor this script for their site.
6345 If this variable isn't set, @GNUTAR{} will display its built-in
6346 prompt, and will expect confirmation from the console. For the
6347 description of the default prompt, see @ref{change volume prompt}.
6351 @defvr {Backup variable} SLEEP_MESSAGE
6353 Message to display on the terminal while waiting for dump time. Usually
6354 this will just be some literal text.
6357 @defvr {Backup variable} TAR
6359 Full file name of the @GNUTAR{} executable. If this is not set, backup
6360 scripts will search @command{tar} in the current shell path.
6363 @node Magnetic Tape Control
6364 @subsection Magnetic Tape Control
6366 Backup scripts access tape device using special @dfn{hook functions}.
6367 These functions take a single argument --- the name of the tape
6368 device. Their names are kept in the following variables:
6370 @defvr {Backup variable} MT_BEGIN
6371 The name of @dfn{begin} function. This function is called before
6372 accessing the drive. By default it retensions the tape:
6378 mt -f "$1" retension
6383 @defvr {Backup variable} MT_REWIND
6384 The name of @dfn{rewind} function. The default definition is as
6397 @defvr {Backup variable} MT_OFFLINE
6398 The name of the function switching the tape off line. By default
6399 it is defined as follows:
6402 MT_OFFLINE=mt_offline
6410 @defvr {Backup variable} MT_STATUS
6411 The name of the function used to obtain the status of the archive device,
6412 including error count. Default definition:
6424 @subsection User Hooks
6426 @dfn{User hooks} are shell functions executed before and after
6427 each @command{tar} invocation. Thus, there are @dfn{backup
6428 hooks}, which are executed before and after dumping each file
6429 system, and @dfn{restore hooks}, executed before and
6430 after restoring a file system. Each user hook is a shell function
6431 taking four arguments:
6433 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
6438 Current backup or restore level.
6441 Name or IP address of the host machine being dumped or restored.
6444 Full file name of the file system being dumped or restored.
6447 File system name with directory separators replaced with colons. This
6448 is useful, e.g., for creating unique files.
6452 Following variables keep the names of user hook functions:
6454 @defvr {Backup variable} DUMP_BEGIN
6455 Dump begin function. It is executed before dumping the file system.
6458 @defvr {Backup variable} DUMP_END
6459 Executed after dumping the file system.
6462 @defvr {Backup variable} RESTORE_BEGIN
6463 Executed before restoring the file system.
6466 @defvr {Backup variable} RESTORE_END
6467 Executed after restoring the file system.
6470 @node backup-specs example
6471 @subsection An Example Text of @file{Backup-specs}
6473 The following is an example of @file{backup-specs}:
6476 # site-specific parameters for file system backup.
6478 ADMINISTRATOR=friedman
6480 TAPE_FILE=/dev/nrsmt0
6482 # Use @code{ssh} instead of the less secure @code{rsh}
6484 RSH_COMMAND=/usr/bin/ssh
6486 # Override MT_STATUS function:
6492 # Disable MT_OFFLINE function
6509 apple-gunkies:/com/mailer/gnu
6510 apple-gunkies:/com/archive/gnu"
6512 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
6516 @node Scripted Backups
6517 @section Using the Backup Scripts
6519 The syntax for running a backup script is:
6522 backup --level=@var{level} --time=@var{time}
6525 The @option{--level} option requests the dump level. Thus, to produce
6526 a full dump, specify @code{--level=0} (this is the default, so
6527 @option{--level} may be omitted if its value is
6528 @code{0})@footnote{For backward compatibility, the @code{backup} will also
6529 try to deduce the requested dump level from the name of the
6530 script itself. If the name consists of a string @samp{level-}
6531 followed by a single decimal digit, that digit is taken as
6532 the dump level number. Thus, you may create a link from @code{backup}
6533 to @code{level-1} and then run @code{level-1} whenever you need to
6534 create a level one dump.}.
6536 The @option{--time} option determines when should the backup be
6537 run. @var{Time} may take three forms:
6540 @item @var{hh}:@var{mm}
6542 The dump must be run at @var{hh} hours @var{mm} minutes.
6546 The dump must be run at @var{hh} hours.
6550 The dump must be run immediately.
6553 You should start a script with a tape or disk mounted. Once you
6554 start a script, it prompts you for new tapes or disks as it
6555 needs them. Media volumes don't have to correspond to archive
6556 files --- a multi-volume archive can be started in the middle of a
6557 tape that already contains the end of another multi-volume archive.
6558 The @code{restore} script prompts for media by its archive volume,
6559 so to avoid an error message you should keep track of which tape
6560 (or disk) contains which volume of the archive (@pxref{Scripted
6563 The backup scripts write two files on the file system. The first is a
6564 record file in @file{/etc/tar-backup/}, which is used by the scripts
6565 to store and retrieve information about which files were dumped. This
6566 file is not meant to be read by humans, and should not be deleted by
6567 them. @xref{Snapshot Files}, for a more detailed explanation of this
6570 The second file is a log file containing the names of the file systems
6571 and files dumped, what time the backup was made, and any error
6572 messages that were generated, as well as how much space was left in
6573 the media volume after the last volume of the archive was written.
6574 You should check this log file after every backup. The file name is
6575 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
6576 represents current date, and @var{n} represents current dump level number.
6578 The script also prints the name of each system being dumped to the
6581 Following is the full list of options accepted by @code{backup}
6585 @item -l @var{level}
6586 @itemx --level=@var{level}
6587 Do backup level @var{level} (default 0).
6591 Force backup even if today's log file already exists.
6593 @item -v[@var{level}]
6594 @itemx --verbose[=@var{level}]
6595 Set verbosity level. The higher the level is, the more debugging
6596 information will be output during execution. Default @var{level}
6597 is 100, which means the highest debugging level.
6599 @item -t @var{start-time}
6600 @itemx --time=@var{start-time}
6601 Wait till @var{time}, then do backup.
6605 Display short help message and exit.
6609 Display information about the program's name, version, origin and legal
6610 status, all on standard output, and then exit successfully.
6614 @node Scripted Restoration
6615 @section Using the Restore Script
6617 To restore files that were archived using a scripted backup, use the
6618 @code{restore} script. Its usage is quite straightforward. In the
6619 simplest form, invoke @code{restore --all}, it will
6620 then restore all the file systems and files specified in
6621 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
6623 You may select the file systems (and/or files) to restore by
6624 giving @code{restore} a list of @dfn{patterns} in its command
6625 line. For example, running
6632 will restore all file systems on the machine @samp{albert}. A more
6633 complicated example:
6636 restore 'albert:*' '*:/var'
6640 This command will restore all file systems on the machine @samp{albert}
6641 as well as @file{/var} file system on all machines.
6643 By default @code{restore} will start restoring files from the lowest
6644 available dump level (usually zero) and will continue through
6645 all available dump levels. There may be situations where such a
6646 thorough restore is not necessary. For example, you may wish to
6647 restore only files from the recent level one backup. To do so,
6648 use @option{--level} option, as shown in the example below:
6654 The full list of options accepted by @code{restore} follows:
6659 Restore all file systems and files specified in @file{backup-specs}.
6661 @item -l @var{level}
6662 @itemx --level=@var{level}
6663 Start restoring from the given backup level, instead of the default 0.
6665 @item -v[@var{level}]
6666 @itemx --verbose[=@var{level}]
6667 Set verbosity level. The higher the level is, the more debugging
6668 information will be output during execution. Default @var{level}
6669 is 100, which means the highest debugging level.
6673 Display short help message and exit.
6677 Display information about the program's name, version, origin and legal
6678 status, all on standard output, and then exit successfully.
6681 You should start the restore script with the media containing the
6682 first volume of the archive mounted. The script will prompt for other
6683 volumes as they are needed. If the archive is on tape, you don't need
6684 to rewind the tape to to its beginning---if the tape head is
6685 positioned past the beginning of the archive, the script will rewind
6686 the tape as needed. @xref{Tape Positioning}, for a discussion of tape
6690 @strong{Warning:} The script will delete files from the active file
6691 system if they were not in the file system when the archive was made.
6694 @xref{Incremental Dumps}, for an explanation of how the script makes
6698 @chapter Choosing Files and Names for @command{tar}
6700 Certain options to @command{tar} enable you to specify a name for your
6701 archive. Other options let you decide which files to include or exclude
6702 from the archive, based on when or whether files were modified, whether
6703 the file names do or don't match specified patterns, or whether files
6704 are in specified directories.
6706 This chapter discusses these options in detail.
6709 * file:: Choosing the Archive's Name
6710 * Selecting Archive Members::
6711 * files:: Reading Names from a File
6712 * exclude:: Excluding Some Files
6713 * wildcards:: Wildcards Patterns and Matching
6714 * quoting styles:: Ways of Quoting Special Characters in Names
6715 * transform:: Modifying File and Member Names
6716 * after:: Operating Only on New Files
6717 * recurse:: Descending into Directories
6718 * one:: Crossing File System Boundaries
6722 @section Choosing and Naming Archive Files
6724 @cindex Naming an archive
6725 @cindex Archive Name
6726 @cindex Choosing an archive file
6727 @cindex Where is the archive?
6729 By default, @command{tar} uses an archive file name that was compiled when
6730 it was built on the system; usually this name refers to some physical
6731 tape drive on the machine. However, the person who installed @command{tar}
6732 on the system may not have set the default to a meaningful value as far as
6733 most users are concerned. As a result, you will usually want to tell
6734 @command{tar} where to find (or create) the archive. The
6735 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
6736 option allows you to either specify or name a file to use as the archive
6737 instead of the default archive file location.
6740 @xopindex{file, short description}
6741 @item --file=@var{archive-name}
6742 @itemx -f @var{archive-name}
6743 Name the archive to create or operate on. Use in conjunction with
6747 For example, in this @command{tar} command,
6750 $ @kbd{tar -cvf collection.tar blues folk jazz}
6754 @file{collection.tar} is the name of the archive. It must directly
6755 follow the @option{-f} option, since whatever directly follows @option{-f}
6756 @emph{will} end up naming the archive. If you neglect to specify an
6757 archive name, you may end up overwriting a file in the working directory
6758 with the archive you create since @command{tar} will use this file's name
6759 for the archive name.
6761 An archive can be saved as a file in the file system, sent through a
6762 pipe or over a network, or written to an I/O device such as a tape,
6763 floppy disk, or CD write drive.
6765 @cindex Writing new archives
6766 @cindex Archive creation
6767 If you do not name the archive, @command{tar} uses the value of the
6768 environment variable @env{TAPE} as the file name for the archive. If
6769 that is not available, @command{tar} uses a default, compiled-in archive
6770 name, usually that for tape unit zero (i.e., @file{/dev/tu00}).
6772 @cindex Standard input and output
6773 @cindex tar to standard input and output
6774 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
6775 archive from standard input (when listing or extracting files), or
6776 writes it to standard output (when creating an archive). If you use
6777 @file{-} as an @var{archive-name} when modifying an archive,
6778 @command{tar} reads the original archive from its standard input and
6779 writes the entire new archive to its standard output.
6781 The following example is a convenient way of copying directory
6782 hierarchy from @file{sourcedir} to @file{targetdir}.
6785 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
6788 The @option{-C} option allows to avoid using subshells:
6791 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
6794 In both examples above, the leftmost @command{tar} invocation archives
6795 the contents of @file{sourcedir} to the standard output, while the
6796 rightmost one reads this archive from its standard input and
6797 extracts it. The @option{-p} option tells it to restore permissions
6798 of the extracted files.
6800 @cindex Remote devices
6801 @cindex tar to a remote device
6803 To specify an archive file on a device attached to a remote machine,
6807 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
6811 @command{tar} will set up the remote connection, if possible, and
6812 prompt you for a username and password. If you use
6813 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
6814 will attempt to set up the remote connection using your username
6815 as the username on the remote machine.
6817 @cindex Local and remote archives
6818 @anchor{local and remote archives}
6819 If the archive file name includes a colon (@samp{:}), then it is assumed
6820 to be a file on another machine. If the archive file is
6821 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6822 host @var{host}. The remote host is accessed using the @command{rsh}
6823 program, with a username of @var{user}. If the username is omitted
6824 (along with the @samp{@@} sign), then your user name will be used.
6825 (This is the normal @command{rsh} behavior.) It is necessary for the
6826 remote machine, in addition to permitting your @command{rsh} access, to
6827 have the @file{rmt} program installed (this command is included in
6828 the @GNUTAR{} distribution and by default is installed under
6829 @file{@var{prefix}/libexec/rmt}, where @var{prefix} means your
6830 installation prefix). If you need to use a file whose name includes a
6831 colon, then the remote tape drive behavior
6832 can be inhibited by using the @option{--force-local} option.
6834 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6835 tries to minimize input and output operations. The Amanda backup
6836 system, when used with @GNUTAR{}, has an initial sizing pass which
6839 @node Selecting Archive Members
6840 @section Selecting Archive Members
6841 @cindex Specifying files to act on
6842 @cindex Specifying archive members
6844 @dfn{File Name arguments} specify which files in the file system
6845 @command{tar} operates on, when creating or adding to an archive, or which
6846 archive members @command{tar} operates on, when reading or deleting from
6847 an archive. @xref{Operations}.
6849 To specify file names, you can include them as the last arguments on
6850 the command line, as follows:
6852 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6855 If a file name begins with dash (@samp{-}), precede it with
6856 @option{--add-file} option to prevent it from being treated as an
6859 @anchor{input name quoting}
6860 By default @GNUTAR{} attempts to @dfn{unquote} each file or member
6861 name, replacing @dfn{escape sequences} according to the following
6864 @multitable @columnfractions 0.20 0.60
6865 @headitem Escape @tab Replaced with
6866 @item \a @tab Audible bell (@acronym{ASCII} 7)
6867 @item \b @tab Backspace (@acronym{ASCII} 8)
6868 @item \f @tab Form feed (@acronym{ASCII} 12)
6869 @item \n @tab New line (@acronym{ASCII} 10)
6870 @item \r @tab Carriage return (@acronym{ASCII} 13)
6871 @item \t @tab Horizontal tabulation (@acronym{ASCII} 9)
6872 @item \v @tab Vertical tabulation (@acronym{ASCII} 11)
6873 @item \? @tab @acronym{ASCII} 127
6874 @item \@var{n} @tab @acronym{ASCII} @var{n} (@var{n} should be an octal number
6878 A backslash followed by any other symbol is retained.
6880 This default behavior is controlled by the following command line
6886 Enable unquoting input file or member names (default).
6890 Disable unquoting input file or member names.
6893 If you specify a directory name as a file name argument, all the files
6894 in that directory are operated on by @command{tar}.
6896 If you do not specify files, @command{tar} behavior differs depending
6897 on the operation mode as described below:
6899 When @command{tar} is invoked with @option{--create} (@option{-c}),
6900 @command{tar} will stop immediately, reporting the following:
6904 $ @kbd{tar cf a.tar}
6905 tar: Cowardly refusing to create an empty archive
6906 Try `tar --help' or `tar --usage' for more information.
6910 If you specify either @option{--list} (@option{-t}) or
6911 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6912 operates on all the archive members in the archive.
6914 If run with @option{--diff} option, tar will compare the archive with
6915 the contents of the current working directory.
6917 If you specify any other operation, @command{tar} does nothing.
6919 By default, @command{tar} takes file names from the command line. However,
6920 there are other ways to specify file or member names, or to modify the
6921 manner in which @command{tar} selects the files or members upon which to
6922 operate. In general, these methods work both for specifying the names
6923 of files and archive members.
6926 @section Reading Names from a File
6928 @cindex Reading file names from a file
6929 @cindex Lists of file names
6930 @cindex File Name arguments, alternatives
6931 @cindex @command{find}, using with @command{tar}
6932 Instead of giving the names of files or archive members on the command
6933 line, you can put the names into a file, and then use the
6934 @option{--files-from=@var{file-of-names}} (@option{-T
6935 @var{file-of-names}}) option to @command{tar}. Give the name of the
6936 file which contains the list of files to include as the argument to
6937 @option{--files-from}. In the list, the file names should be separated by
6938 newlines. You will frequently use this option when you have generated
6939 the list of files to archive with the @command{find} utility.
6943 @item --files-from=@var{file-name}
6944 @itemx -T @var{file-name}
6945 Get names to extract or create from file @var{file-name}.
6948 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6949 you specify either @code{--files-from=-} or @code{-T -}), then the file
6950 names are read from standard input.
6952 Unless you are running @command{tar} with @option{--create}, you can not use
6953 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6956 Any number of @option{-T} options can be given in the command line.
6958 The following example shows how to use @command{find} to generate a list of
6959 files smaller than 400K in length and put that list into a file
6960 called @file{small-files}. You can then use the @option{-T} option to
6961 @command{tar} to specify the files from that file, @file{small-files}, to
6962 create the archive @file{little.tgz}. (The @option{-z} option to
6963 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
6967 $ @kbd{find . -size -400 -print > small-files}
6968 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
6972 In the file list given by @option{-T} option, any file name beginning
6973 with @samp{-} character is considered a @command{tar} option and is
6974 processed accordingly@footnote{Versions of @GNUTAR{} up to 1.15.1
6975 recognized only @option{-C} option in file lists, and only if the
6976 option and its argument occupied two consecutive lines.}. For example,
6977 the common use of this feature is to change to another directory by
6978 specifying @option{-C} option:
6988 $ @kbd{tar -c -f foo.tar --files-from list}
6993 In this example, @command{tar} will first switch to @file{/etc}
6994 directory and add files @file{passwd} and @file{hosts} to the
6995 archive. Then it will change to @file{/lib} directory and will archive
6996 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
7001 $ @kbd{tar tf foo.tar}
7009 @xopindex{directory, using in @option{--files-from} argument}
7010 Notice that the option parsing algorithm used with @option{-T} is
7011 stricter than the one used by shell. Namely, when specifying option
7012 arguments, you should observe the following rules:
7016 When using short (single-letter) option form, its argument must
7017 immediately follow the option letter, without any intervening
7018 whitespace. For example: @code{-Cdir}.
7021 When using long option form, the option argument must be separated
7022 from the option by a single equal sign. No whitespace is allowed on
7023 any side of the equal sign. For example: @code{--directory=dir}.
7026 For both short and long option forms, the option argument can be given
7027 on the next line after the option name, e.g.:
7048 If you happen to have a file whose name starts with @samp{-},
7049 precede it with @option{--add-file} option to prevent it from
7050 being recognized as an option. For example: @code{--add-file=--my-file}.
7057 @subsection @code{NUL}-Terminated File Names
7059 @cindex File names, terminated by @code{NUL}
7060 @cindex @code{NUL}-terminated file names
7061 The @option{--null} option causes
7062 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
7063 to read file names terminated by a @code{NUL} instead of a newline, so
7064 files whose names contain newlines can be archived using
7065 @option{--files-from}.
7068 @xopindex{null, described}
7070 Only consider @code{NUL}-terminated file names, instead of files that
7071 terminate in a newline.
7073 @xopindex{no-null, described}
7075 Undo the effect of any previous @option{--null} option.
7078 The @option{--null} option is just like the one in @acronym{GNU}
7079 @command{xargs} and @command{cpio}, and is useful with the
7080 @option{-print0} predicate of @acronym{GNU} @command{find}. In
7081 @command{tar}, @option{--null} also disables special handling for
7082 file names that begin with dash.
7084 This example shows how to use @command{find} to generate a list of files
7085 larger than 800K in length and put that list into a file called
7086 @file{long-files}. The @option{-print0} option to @command{find} is just
7087 like @option{-print}, except that it separates files with a @code{NUL}
7088 rather than with a newline. You can then run @command{tar} with both the
7089 @option{--null} and @option{-T} options to specify that @command{tar} gets the
7090 files from that file, @file{long-files}, to create the archive
7091 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
7092 @command{tar} to recognize the @code{NUL} separator between files.
7095 $ @kbd{find . -size +800 -print0 > long-files}
7096 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
7099 The @option{--no-null} option can be used if you need to read both
7100 @code{NUL}-terminated and newline-terminated files on the same command line.
7101 For example, if @file{flist} is a newline-terminated file, then the
7102 following command can be used to combine it with the above command:
7106 $ @kbd{find . -size +800 -print0 |
7107 tar -c -f big.tar --null -T - --no-null -T flist}
7111 This example uses short options for typographic reasons, to avoid
7114 @GNUTAR is able to automatically detect @code{NUL}-terminated file lists, so
7115 it is safe to use them even without the @option{--null} option. In
7116 this case @command{tar} will print a warning and continue reading such
7117 a file as if @option{--null} were actually given:
7121 $ @kbd{find . -size +800 -print0 | tar -c -f big.tar -T -}
7122 tar: -: file name read contains nul character
7126 The null terminator, however, remains in effect only for this
7127 particular file, any following @option{-T} options will assume
7128 newline termination. Of course, the null autodetection applies
7129 to these eventual surplus @option{-T} options as well.
7132 @section Excluding Some Files
7134 @cindex File names, excluding files by
7135 @cindex Excluding files by name and pattern
7136 @cindex Excluding files by file system
7138 @opindex exclude-from
7139 To avoid operating on files whose names match a particular pattern,
7140 use the @option{--exclude} or @option{--exclude-from} options.
7144 @item --exclude=@var{pattern}
7145 Causes @command{tar} to ignore files that match the @var{pattern}.
7149 The @option{--exclude=@var{pattern}} option prevents any file or
7150 member whose name matches the shell wildcard (@var{pattern}) from
7152 For example, to create an archive with all the contents of the directory
7153 @file{src} except for files whose names end in @file{.o}, use the
7154 command @samp{tar -cf src.tar --exclude='*.o' src}.
7156 You may give multiple @option{--exclude} options.
7159 @opindex exclude-from
7160 @item --exclude-from=@var{file}
7161 @itemx -X @var{file}
7162 Causes @command{tar} to ignore files that match the patterns listed in
7166 @findex exclude-from
7167 Use the @option{--exclude-from} option to read a
7168 list of patterns, one per line, from @var{file}; @command{tar} will
7169 ignore files matching those patterns. Thus if @command{tar} is
7170 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
7171 single line @file{*.o}, no files whose names end in @file{.o} will be
7172 added to the archive.
7174 Notice, that lines from @var{file} are read verbatim. One of the
7175 frequent errors is leaving some extra whitespace after a file name,
7176 which is difficult to catch using text editors.
7178 However, empty lines are OK.
7181 @cindex version control system, excluding files
7182 @cindex VCS, excluding files
7183 @cindex SCCS, excluding files
7184 @cindex RCS, excluding files
7185 @cindex CVS, excluding files
7186 @cindex SVN, excluding files
7187 @cindex git, excluding files
7188 @cindex Bazaar, excluding files
7189 @cindex Arch, excluding files
7190 @cindex Mercurial, excluding files
7191 @cindex Darcs, excluding files
7192 @opindex exclude-vcs
7194 Exclude files and directories used by following version control
7195 systems: @samp{CVS}, @samp{RCS}, @samp{SCCS}, @samp{SVN}, @samp{Arch},
7196 @samp{Bazaar}, @samp{Mercurial}, and @samp{Darcs}.
7198 As of version @value{VERSION}, the following files are excluded:
7201 @item @file{CVS/}, and everything under it
7202 @item @file{RCS/}, and everything under it
7203 @item @file{SCCS/}, and everything under it
7204 @item @file{.git/}, and everything under it
7205 @item @file{.gitignore}
7206 @item @file{.cvsignore}
7207 @item @file{.svn/}, and everything under it
7208 @item @file{.arch-ids/}, and everything under it
7209 @item @file{@{arch@}/}, and everything under it
7210 @item @file{=RELEASE-ID}
7211 @item @file{=meta-update}
7212 @item @file{=update}
7214 @item @file{.bzrignore}
7215 @item @file{.bzrtags}
7217 @item @file{.hgignore}
7218 @item @file{.hgrags}
7222 @opindex exclude-backups
7223 @item --exclude-backups
7224 Exclude backup and lock files. This option causes exclusion of files
7225 that match the following shell globbing patterns:
7235 @findex exclude-caches
7236 When creating an archive, the @option{--exclude-caches} option family
7237 causes @command{tar} to exclude all directories that contain a @dfn{cache
7238 directory tag}. A cache directory tag is a short file with the
7239 well-known name @file{CACHEDIR.TAG} and having a standard header
7240 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
7241 Various applications write cache directory tags into directories they
7242 use to hold regenerable, non-precious data, so that such data can be
7243 more easily excluded from backups.
7245 There are three @samp{exclude-caches} options, each providing a different
7246 exclusion semantics:
7249 @opindex exclude-caches
7250 @item --exclude-caches
7251 Do not archive the contents of the directory, but archive the
7252 directory itself and the @file{CACHEDIR.TAG} file.
7254 @opindex exclude-caches-under
7255 @item --exclude-caches-under
7256 Do not archive the contents of the directory, nor the
7257 @file{CACHEDIR.TAG} file, archive only the directory itself.
7259 @opindex exclude-caches-all
7260 @item --exclude-caches-all
7261 Omit directories containing @file{CACHEDIR.TAG} file entirely.
7265 Another option family, @option{--exclude-tag}, provides a generalization of
7266 this concept. It takes a single argument, a file name to look for.
7267 Any directory that contains this file will be excluded from the dump.
7268 Similarly to @samp{exclude-caches}, there are three options in this
7272 @opindex exclude-tag
7273 @item --exclude-tag=@var{file}
7274 Do not dump the contents of the directory, but dump the
7275 directory itself and the @var{file}.
7277 @opindex exclude-tag-under
7278 @item --exclude-tag-under=@var{file}
7279 Do not dump the contents of the directory, nor the
7280 @var{file}, archive only the directory itself.
7282 @opindex exclude-tag-all
7283 @item --exclude-tag-all=@var{file}
7284 Omit directories containing @var{file} file entirely.
7287 Multiple @option{--exclude-tag*} options can be given.
7289 For example, given this directory:
7304 The @option{--exclude-tag} will produce the following:
7307 $ @kbd{tar -cf archive.tar --exclude-tag=tagfile -v dir}
7312 tar: dir/folk/: contains a cache directory tag tagfile;
7317 Both the @file{dir/folk} directory and its tagfile are preserved in
7318 the archive, however the rest of files in this directory are not.
7320 Now, using the @option{--exclude-tag-under} option will exclude
7321 @file{tagfile} from the dump, while still preserving the directory
7322 itself, as shown in this example:
7325 $ @kbd{tar -cf archive.tar --exclude-tag-under=tagfile -v dir}
7330 ./tar: dir/folk/: contains a cache directory tag tagfile;
7334 Finally, using @option{--exclude-tag-all} omits the @file{dir/folk}
7338 $ @kbd{tar -cf archive.tar --exclude-tag-all=tagfile -v dir}
7342 ./tar: dir/folk/: contains a cache directory tag tagfile;
7343 directory not dumped
7347 * problems with exclude::
7350 @node problems with exclude
7351 @unnumberedsubsec Problems with Using the @code{exclude} Options
7353 @xopindex{exclude, potential problems with}
7354 Some users find @samp{exclude} options confusing. Here are some common
7359 The main operating mode of @command{tar} does not act on a file name
7360 explicitly listed on the command line, if one of its file name
7361 components is excluded. In the example above, if
7362 you create an archive and exclude files that end with @samp{*.o}, but
7363 explicitly name the file @samp{dir.o/foo} after all the options have been
7364 listed, @samp{dir.o/foo} will be excluded from the archive.
7367 You can sometimes confuse the meanings of @option{--exclude} and
7368 @option{--exclude-from}. Be careful: use @option{--exclude} when files
7369 to be excluded are given as a pattern on the command line. Use
7370 @option{--exclude-from} to introduce the name of a file which contains
7371 a list of patterns, one per line; each of these patterns can exclude
7372 zero, one, or many files.
7375 When you use @option{--exclude=@var{pattern}}, be sure to quote the
7376 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
7377 like @samp{*}. If you do not do this, the shell might expand the
7378 @samp{*} itself using files at hand, so @command{tar} might receive a
7379 list of files instead of one pattern, or none at all, making the
7380 command somewhat illegal. This might not correspond to what you want.
7385 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
7393 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
7397 You must use use shell syntax, or globbing, rather than @code{regexp}
7398 syntax, when using exclude options in @command{tar}. If you try to use
7399 @code{regexp} syntax to describe files to be excluded, your command
7403 @FIXME{The change in semantics must have occurred before 1.11,
7404 so I doubt if it is worth mentioning at all. Anyway, should at
7405 least specify in which version the semantics changed.}
7406 In earlier versions of @command{tar}, what is now the
7407 @option{--exclude-from} option was called @option{--exclude} instead.
7408 Now, @option{--exclude} applies to patterns listed on the command
7409 line and @option{--exclude-from} applies to patterns listed in a
7415 @section Wildcards Patterns and Matching
7417 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
7418 @samp{*} or @samp{?} for example, are replaced and expanded into all
7419 existing files matching the given pattern. @GNUTAR{} can use wildcard
7420 patterns for matching (or globbing) archive members when extracting
7421 from or listing an archive. Wildcard patterns are also used for
7422 verifying volume labels of @command{tar} archives. This section has the
7423 purpose of explaining wildcard syntax for @command{tar}.
7425 @FIXME{the next few paragraphs need work.}
7427 A @var{pattern} should be written according to shell syntax, using wildcard
7428 characters to effect globbing. Most characters in the pattern stand
7429 for themselves in the matched string, and case is significant: @samp{a}
7430 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
7431 pattern matches any single character in the matched string. The character
7432 @samp{*} in the pattern matches zero, one, or more single characters in
7433 the matched string. The character @samp{\} says to take the following
7434 character of the pattern @emph{literally}; it is useful when one needs to
7435 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
7437 The character @samp{[}, up to the matching @samp{]}, introduces a character
7438 class. A @dfn{character class} is a list of acceptable characters
7439 for the next single character of the matched string. For example,
7440 @samp{[abcde]} would match any of the first five letters of the alphabet.
7441 Note that within a character class, all of the ``special characters''
7442 listed above other than @samp{\} lose their special meaning; for example,
7443 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
7444 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
7445 the characters @samp{-} and @samp{]} must either come @emph{first} or
7446 @emph{last} in a character class.)
7448 @cindex Excluding characters from a character class
7449 @cindex Character class, excluding characters from
7450 If the first character of the class after the opening @samp{[}
7451 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
7452 Rather than listing character to match, it lists those characters which
7453 are @emph{forbidden} as the next single character of the matched string.
7455 Other characters of the class stand for themselves. The special
7456 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
7457 letters, is meant to represent all characters between @var{a} and
7460 @FIXME{need to add a sentence or so here to make this clear for those
7461 who don't have dan around.}
7463 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
7464 special for wildcard matches. However, if a pattern completely matches
7465 a directory prefix of a matched string, then it matches the full matched
7466 string: thus, excluding a directory also excludes all the files beneath it.
7469 * controlling pattern-matching::
7472 @node controlling pattern-matching
7473 @unnumberedsubsec Controlling Pattern-Matching
7475 For the purposes of this section, we call @dfn{exclusion members} all
7476 member names obtained while processing @option{--exclude} and
7477 @option{--exclude-from} options, and @dfn{inclusion members} those
7478 member names that were given in the command line or read from the file
7479 specified with @option{--files-from} option.
7481 These two pairs of member lists are used in the following operations:
7482 @option{--diff}, @option{--extract}, @option{--list},
7485 There are no inclusion members in create mode (@option{--create} and
7486 @option{--append}), since in this mode the names obtained from the
7487 command line refer to @emph{files}, not archive members.
7489 By default, inclusion members are compared with archive members
7490 literally @footnote{Notice that earlier @GNUTAR{} versions used
7491 globbing for inclusion members, which contradicted to UNIX98
7492 specification and was not documented. @xref{Changes}, for more
7493 information on this and other changes.} and exclusion members are
7494 treated as globbing patterns. For example:
7498 $ @kbd{tar tf foo.tar}
7503 # @i{Member names are used verbatim:}
7504 $ @kbd{tar -xf foo.tar -v '[remarks]'}
7506 # @i{Exclude member names are globbed:}
7507 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
7513 This behavior can be altered by using the following options:
7518 Treat all member names as wildcards.
7520 @opindex no-wildcards
7521 @item --no-wildcards
7522 Treat all member names as literal strings.
7525 Thus, to extract files whose names end in @samp{.c}, you can use:
7528 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
7534 Notice quoting of the pattern to prevent the shell from interpreting
7537 The effect of @option{--wildcards} option is canceled by
7538 @option{--no-wildcards}. This can be used to pass part of
7539 the command line arguments verbatim and other part as globbing
7540 patterns. For example, the following invocation:
7543 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
7547 instructs @command{tar} to extract from @file{foo.tar} all files whose
7548 names end in @samp{.txt} and the file named @file{[remarks]}.
7550 Normally, a pattern matches a name if an initial subsequence of the
7551 name's components matches the pattern, where @samp{*}, @samp{?}, and
7552 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
7553 and wildcards can match @samp{/}.
7555 Other than optionally stripping leading @samp{/} from names
7556 (@pxref{absolute}), patterns and names are used as-is. For
7557 example, trailing @samp{/} is not trimmed from a user-specified name
7558 before deciding whether to exclude it.
7560 However, this matching procedure can be altered by the options listed
7561 below. These options accumulate. For example:
7564 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
7568 ignores case when excluding @samp{makefile}, but not when excluding
7573 @opindex no-anchored
7575 @itemx --no-anchored
7576 If anchored, a pattern must match an initial subsequence
7577 of the name's components. Otherwise, the pattern can match any
7578 subsequence. Default is @option{--no-anchored} for exclusion members
7579 and @option{--anchored} inclusion members.
7581 @opindex ignore-case
7582 @opindex no-ignore-case
7584 @itemx --no-ignore-case
7585 When ignoring case, upper-case patterns match lower-case names and vice versa.
7586 When not ignoring case (the default), matching is case-sensitive.
7588 @opindex wildcards-match-slash
7589 @opindex no-wildcards-match-slash
7590 @item --wildcards-match-slash
7591 @itemx --no-wildcards-match-slash
7592 When wildcards match slash (the default for exclusion members), a
7593 wildcard like @samp{*} in the pattern can match a @samp{/} in the
7594 name. Otherwise, @samp{/} is matched only by @samp{/}.
7598 The @option{--recursion} and @option{--no-recursion} options
7599 (@pxref{recurse}) also affect how member patterns are interpreted. If
7600 recursion is in effect, a pattern matches a name if it matches any of
7601 the name's parent directories.
7603 The following table summarizes pattern-matching default values:
7605 @multitable @columnfractions .3 .7
7606 @headitem Members @tab Default settings
7607 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
7608 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
7611 @node quoting styles
7612 @section Quoting Member Names
7614 When displaying member names, @command{tar} takes care to avoid
7615 ambiguities caused by certain characters. This is called @dfn{name
7616 quoting}. The characters in question are:
7619 @item Non-printable control characters:
7620 @anchor{escape sequences}
7621 @multitable @columnfractions 0.20 0.10 0.60
7622 @headitem Character @tab @acronym{ASCII} @tab Character name
7623 @item \a @tab 7 @tab Audible bell
7624 @item \b @tab 8 @tab Backspace
7625 @item \f @tab 12 @tab Form feed
7626 @item \n @tab 10 @tab New line
7627 @item \r @tab 13 @tab Carriage return
7628 @item \t @tab 9 @tab Horizontal tabulation
7629 @item \v @tab 11 @tab Vertical tabulation
7632 @item Space (@acronym{ASCII} 32)
7634 @item Single and double quotes (@samp{'} and @samp{"})
7636 @item Backslash (@samp{\})
7639 The exact way @command{tar} uses to quote these characters depends on
7640 the @dfn{quoting style}. The default quoting style, called
7641 @dfn{escape} (see below), uses backslash notation to represent control
7642 characters, space and backslash. Using this quoting style, control
7643 characters are represented as listed in column @samp{Character} in the
7644 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
7646 @GNUTAR{} offers seven distinct quoting styles, which can be selected
7647 using @option{--quoting-style} option:
7650 @item --quoting-style=@var{style}
7651 @opindex quoting-style
7653 Sets quoting style. Valid values for @var{style} argument are:
7654 literal, shell, shell-always, c, escape, locale, clocale.
7657 These styles are described in detail below. To illustrate their
7658 effect, we will use an imaginary tar archive @file{arch.tar}
7659 containing the following members:
7663 # 1. Contains horizontal tabulation character.
7665 # 2. Contains newline character
7668 # 3. Contains a space
7670 # 4. Contains double quotes
7672 # 5. Contains single quotes
7674 # 6. Contains a backslash character:
7679 Here is how usual @command{ls} command would have listed them, if they
7680 had existed in the current working directory:
7698 No quoting, display each character as is:
7702 $ @kbd{tar tf arch.tar --quoting-style=literal}
7715 Display characters the same way Bourne shell does:
7716 control characters, except @samp{\t} and @samp{\n}, are printed using
7717 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
7718 single quote is printed as @samp{\'}. If a name contains any quoted
7719 characters, it is enclosed in single quotes. In particular, if a name
7720 contains single quotes, it is printed as several single-quoted strings:
7724 $ @kbd{tar tf arch.tar --quoting-style=shell}
7727 './a'\''single'\''quote'
7737 Same as @samp{shell}, but the names are always enclosed in single
7742 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
7745 './a'\''single'\''quote'
7755 Use the notation of the C programming language. All names are
7756 enclosed in double quotes. Control characters are quoted using
7757 backslash notations, double quotes are represented as @samp{\"},
7758 backslash characters are represented as @samp{\\}. Single quotes and
7759 spaces are not quoted:
7763 $ @kbd{tar tf arch.tar --quoting-style=c}
7767 "./a\"double\"quote"
7775 Control characters are printed using backslash notation, a space is
7776 printed as @samp{\ } and a backslash as @samp{\\}. This is the
7777 default quoting style, unless it was changed when configured the
7782 $ @kbd{tar tf arch.tar --quoting-style=escape}
7794 Control characters, single quote and backslash are printed using
7795 backslash notation. All names are quoted using left and right
7796 quotation marks, appropriate to the current locale. If it does not
7797 define quotation marks, use @samp{`} as left and @samp{'} as right
7798 quotation marks. Any occurrences of the right quotation mark in a
7799 name are escaped with @samp{\}, for example:
7805 $ @kbd{tar tf arch.tar --quoting-style=locale}
7808 `./a\'single\'quote'
7817 Same as @samp{locale}, but @samp{"} is used for both left and right
7818 quotation marks, if not provided by the currently selected locale:
7822 $ @kbd{tar tf arch.tar --quoting-style=clocale}
7826 "./a\"double\"quote"
7834 You can specify which characters should be quoted in addition to those
7835 implied by the current quoting style:
7838 @item --quote-chars=@var{string}
7839 Always quote characters from @var{string}, even if the selected
7840 quoting style would not quote them.
7843 For example, using @samp{escape} quoting (compare with the usual
7844 escape listing above):
7848 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
7859 To disable quoting of such additional characters, use the following
7863 @item --no-quote-chars=@var{string}
7864 Remove characters listed in @var{string} from the list of quoted
7865 characters set by the previous @option{--quote-chars} option.
7868 This option is particularly useful if you have added
7869 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
7870 and wish to disable it for the current invocation.
7872 Note, that @option{--no-quote-chars} does @emph{not} disable those
7873 characters that are quoted by default in the selected quoting style.
7876 @section Modifying File and Member Names
7878 @command{Tar} archives contain detailed information about files stored
7879 in them and full file names are part of that information. When
7880 storing a file to an archive, its file name is recorded in it,
7881 along with the actual file contents. When restoring from an archive,
7882 a file is created on disk with exactly the same name as that stored
7883 in the archive. In the majority of cases this is the desired behavior
7884 of a file archiver. However, there are some cases when it is not.
7886 First of all, it is often unsafe to extract archive members with
7887 absolute file names or those that begin with a @file{../}. @GNUTAR{}
7888 takes special precautions when extracting such names and provides a
7889 special option for handling them, which is described in
7892 Secondly, you may wish to extract file names without some leading
7893 directory components, or with otherwise modified names. In other
7894 cases it is desirable to store files under differing names in the
7897 @GNUTAR{} provides several options for these needs.
7900 @opindex strip-components
7901 @item --strip-components=@var{number}
7902 Strip given @var{number} of leading components from file names before
7906 For example, suppose you have archived whole @file{/usr} hierarchy to
7907 a tar archive named @file{usr.tar}. Among other files, this archive
7908 contains @file{usr/include/stdlib.h}, which you wish to extract to
7909 the current working directory. To do so, you type:
7912 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
7915 The option @option{--strip=2} instructs @command{tar} to strip the
7916 two leading components (@file{usr/} and @file{include/}) off the file
7919 If you add the @option{--verbose} (@option{-v}) option to the invocation
7920 above, you will note that the verbose listing still contains the
7921 full file name, with the two removed components still in place. This
7922 can be inconvenient, so @command{tar} provides a special option for
7923 altering this behavior:
7925 @anchor{show-transformed-names}
7927 @opindex show-transformed-names
7928 @item --show-transformed-names
7929 Display file or member names with all requested transformations
7938 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
7939 usr/include/stdlib.h
7940 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
7945 Notice that in both cases the file @file{stdlib.h} is extracted to the
7946 current working directory, @option{--show-transformed-names} affects
7947 only the way its name is displayed.
7949 This option is especially useful for verifying whether the invocation
7950 will have the desired effect. Thus, before running
7953 $ @kbd{tar -x --strip=@var{n}}
7957 it is often advisable to run
7960 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
7964 to make sure the command will produce the intended results.
7966 In case you need to apply more complex modifications to the file name,
7967 @GNUTAR{} provides a general-purpose transformation option:
7972 @item --transform=@var{expression}
7973 @itemx --xform=@var{expression}
7974 Modify file names using supplied @var{expression}.
7978 The @var{expression} is a @command{sed}-like replace expression of the
7982 s/@var{regexp}/@var{replace}/[@var{flags}]
7986 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
7987 replacement for each file name part that matches @var{regexp}. Both
7988 @var{regexp} and @var{replace} are described in detail in
7989 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
7991 Any delimiter can be used in lieu of @samp{/}, the only requirement being
7992 that it be used consistently throughout the expression. For example,
7993 the following two expressions are equivalent:
8002 Changing delimiters is often useful when the @var{regex} contains
8003 slashes. For example, it is more convenient to write @code{s,/,-,} than
8006 As in @command{sed}, you can give several replace expressions,
8007 separated by a semicolon.
8009 Supported @var{flags} are:
8013 Apply the replacement to @emph{all} matches to the @var{regexp}, not
8017 Use case-insensitive matching.
8020 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
8021 regexps, Extended regular expressions, Extended regular expressions,
8025 Only replace the @var{number}th match of the @var{regexp}.
8027 Note: the @acronym{POSIX} standard does not specify what should happen
8028 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
8029 follows the GNU @command{sed} implementation in this regard, so
8030 the interaction is defined to be: ignore matches before the
8031 @var{number}th, and then match and replace all matches from the
8036 In addition, several @dfn{transformation scope} flags are supported,
8037 that control to what files transformations apply. These are:
8041 Apply transformation to regular archive members.
8044 Do not apply transformation to regular archive members.
8047 Apply transformation to symbolic link targets.
8050 Do not apply transformation to symbolic link targets.
8053 Apply transformation to hard link targets.
8056 Do not apply transformation to hard link targets.
8059 Default is @samp{rsh}, which means to apply tranformations to both archive
8060 members and targets of symbolic and hard links.
8062 Default scope flags can also be changed using @samp{flags=} statement
8063 in the transform expression. The flags set this way remain in force
8064 until next @samp{flags=} statement or end of expression, whichever
8065 occurs first. For example:
8068 --transform 'flags=S;s|^|/usr/local/|'
8071 Here are several examples of @option{--transform} usage:
8074 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
8077 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
8080 @item Strip two leading directory components (equivalent to
8081 @option{--strip-components=2}):
8084 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
8087 @item Convert each file name to lower case:
8090 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
8093 @item Prepend @file{/prefix/} to each file name:
8096 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
8099 @item Archive the @file{/lib} directory, prepending @samp{/usr/local}
8100 to each archive member:
8103 $ @kbd{tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib}
8107 Notice the use of flags in the last example. The @file{/lib}
8108 directory often contains many symbolic links to files within it.
8109 It may look, for example, like this:
8113 drwxr-xr-x root/root 0 2008-07-08 16:20 /lib/
8114 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so
8115 lrwxrwxrwx root/root 0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so
8119 Using the expression @samp{s,^,/usr/local/,} would mean adding
8120 @samp{/usr/local} to both regular archive members and to link
8121 targets. In this case, @file{/lib/libc.so.6} would become:
8124 /usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so
8127 This is definitely not desired. To avoid this, the @samp{S} flag
8128 is used, which excludes symbolic link targets from filename
8129 transformations. The result is:
8132 $ @kbd{tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \
8133 --show-transformed /lib}
8134 drwxr-xr-x root/root 0 2008-07-08 16:20 /usr/local/lib/
8135 -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so
8136 lrwxrwxrwx root/root 0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \
8140 Unlike @option{--strip-components}, @option{--transform} can be used
8141 in any @GNUTAR{} operation mode. For example, the following command
8142 adds files to the archive while replacing the leading @file{usr/}
8143 component with @file{var/}:
8146 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
8149 To test @option{--transform} effect we suggest using
8150 @option{--show-transformed-names} option:
8153 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
8154 --verbose --show-transformed-names /}
8157 If both @option{--strip-components} and @option{--transform} are used
8158 together, then @option{--transform} is applied first, and the required
8159 number of components is then stripped from its result.
8161 You can use as many @option{--transform} options in a single command
8162 line as you want. The specified expressions will then be applied in
8163 order of their appearance. For example, the following two invocations
8167 $ @kbd{tar -cf arch.tar --transform='s,/usr/var,/var/' \
8168 --transform='s,/usr/local,/usr/,'}
8169 $ @kbd{tar -cf arch.tar \
8170 --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
8174 @section Operating Only on New Files
8176 @cindex Excluding file by age
8177 @cindex Data Modification time, excluding files by
8178 @cindex Modification time, excluding files by
8179 @cindex Age, excluding files by
8180 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
8181 @option{-N @var{date}}) option causes @command{tar} to only work on
8182 files whose data modification or status change times are newer than
8183 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
8184 it is taken to be a file name; the data modification time of that file
8185 is used as the date. If you use this option when creating or appending
8186 to an archive, the archive will only include new files. If you use
8187 @option{--after-date} when extracting an archive, @command{tar} will
8188 only extract files newer than the @var{date} you specify.
8190 If you only want @command{tar} to make the date comparison based on
8191 modification of the file's data (rather than status
8192 changes), then use the @option{--newer-mtime=@var{date}} option.
8194 @cindex --after-date and --update compared
8195 @cindex --newer-mtime and --update compared
8196 You may use these options with any operation. Note that these options
8197 differ from the @option{--update} (@option{-u}) operation in that they
8198 allow you to specify a particular date against which @command{tar} can
8199 compare when deciding whether or not to archive the files.
8204 @item --after-date=@var{date}
8205 @itemx --newer=@var{date}
8206 @itemx -N @var{date}
8207 Only store files newer than @var{date}.
8209 Acts on files only if their data modification or status change times are
8210 later than @var{date}. Use in conjunction with any operation.
8212 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
8213 name; the data modification time of that file is used as the date.
8215 @opindex newer-mtime
8216 @item --newer-mtime=@var{date}
8217 Acts like @option{--after-date}, but only looks at data modification times.
8220 These options limit @command{tar} to operate only on files which have
8221 been modified after the date specified. A file's status is considered to have
8222 changed if its contents have been modified, or if its owner,
8223 permissions, and so forth, have been changed. (For more information on
8224 how to specify a date, see @ref{Date input formats}; remember that the
8225 entire date argument must be quoted if it contains any spaces.)
8227 Gurus would say that @option{--after-date} tests both the data
8228 modification time (@code{mtime}, the time the contents of the file
8229 were last modified) and the status change time (@code{ctime}, the time
8230 the file's status was last changed: owner, permissions, etc.@:)
8231 fields, while @option{--newer-mtime} tests only the @code{mtime}
8234 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
8235 @code{ctime} and processes the file if either one is more recent than
8236 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
8237 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
8238 contents of the file were looked at).
8240 Date specifiers can have embedded spaces. Because of this, you may need
8241 to quote date arguments to keep the shell from parsing them as separate
8242 arguments. For example, the following command will add to the archive
8243 all the files modified less than two days ago:
8246 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
8249 When any of these options is used with the option @option{--verbose}
8250 (@pxref{verbose tutorial}) @GNUTAR{} will try to convert the specified
8251 date back to its textual representation and compare that with the
8252 one given with the option. If the two dates differ, @command{tar} will
8253 print a warning saying what date it will use. This is to help user
8254 ensure he is using the right date. For example:
8258 $ @kbd{tar -c -f archive.tar --after-date='10 days ago' .}
8259 tar: Option --after-date: Treating date `10 days ago' as 2006-06-11
8265 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
8266 should not be used for incremental backups. @xref{Incremental Dumps},
8267 for proper way of creating incremental backups.
8271 @section Descending into Directories
8272 @cindex Avoiding recursion in directories
8273 @cindex Descending directories, avoiding
8274 @cindex Directories, avoiding recursion
8275 @cindex Recursion in directories, avoiding
8277 Usually, @command{tar} will recursively explore all directories (either
8278 those given on the command line or through the @option{--files-from}
8279 option) for the various files they contain. However, you may not always
8280 want @command{tar} to act this way.
8282 @opindex no-recursion
8283 @cindex @command{find}, using with @command{tar}
8284 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
8285 into specified directories. If you specify @option{--no-recursion}, you can
8286 use the @command{find} (@pxref{Top,, find, find, GNU Find Manual})
8287 utility for hunting through levels of directories to
8288 construct a list of file names which you could then pass to @command{tar}.
8289 @command{find} allows you to be more selective when choosing which files to
8290 archive; see @ref{files}, for more information on using @command{find} with
8294 @item --no-recursion
8295 Prevents @command{tar} from recursively descending directories.
8299 Requires @command{tar} to recursively descend directories.
8300 This is the default.
8303 When you use @option{--no-recursion}, @GNUTAR{} grabs
8304 directory entries themselves, but does not descend on them
8305 recursively. Many people use @command{find} for locating files they
8306 want to back up, and since @command{tar} @emph{usually} recursively
8307 descends on directories, they have to use the @samp{@w{-not -type d}}
8308 test in their @command{find} invocation (@pxref{Type, Type, Type test,
8309 find, Finding Files}), as they usually do not want all the files in a
8310 directory. They then use the @option{--files-from} option to archive
8311 the files located via @command{find}.
8313 The problem when restoring files archived in this manner is that the
8314 directories themselves are not in the archive; so the
8315 @option{--same-permissions} (@option{--preserve-permissions},
8316 @option{-p}) option does not affect them---while users might really
8317 like it to. Specifying @option{--no-recursion} is a way to tell
8318 @command{tar} to grab only the directory entries given to it, adding
8319 no new files on its own. To summarize, if you use @command{find} to
8320 create a list of files to be stored in an archive, use it as follows:
8324 $ @kbd{find @var{dir} @var{tests} | \
8325 tar -cf @var{archive} -T - --no-recursion}
8329 The @option{--no-recursion} option also applies when extracting: it
8330 causes @command{tar} to extract only the matched directory entries, not
8331 the files under those directories.
8333 The @option{--no-recursion} option also affects how globbing patterns
8334 are interpreted (@pxref{controlling pattern-matching}).
8336 The @option{--no-recursion} and @option{--recursion} options apply to
8337 later options and operands, and can be overridden by later occurrences
8338 of @option{--no-recursion} and @option{--recursion}. For example:
8341 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
8345 creates an archive with one entry for @file{grape}, and the recursive
8346 contents of @file{grape/concord}, but no entries under @file{grape}
8347 other than @file{grape/concord}.
8350 @section Crossing File System Boundaries
8351 @cindex File system boundaries, not crossing
8353 @command{tar} will normally automatically cross file system boundaries in
8354 order to archive files which are part of a directory tree. You can
8355 change this behavior by running @command{tar} and specifying
8356 @option{--one-file-system}. This option only affects files that are
8357 archived because they are in a directory that is being archived;
8358 @command{tar} will still archive files explicitly named on the command line
8359 or through @option{--files-from}, regardless of where they reside.
8362 @opindex one-file-system
8363 @item --one-file-system
8364 Prevents @command{tar} from crossing file system boundaries when
8365 archiving. Use in conjunction with any write operation.
8368 The @option{--one-file-system} option causes @command{tar} to modify its
8369 normal behavior in archiving the contents of directories. If a file in
8370 a directory is not on the same file system as the directory itself, then
8371 @command{tar} will not archive that file. If the file is a directory
8372 itself, @command{tar} will not archive anything beneath it; in other words,
8373 @command{tar} will not cross mount points.
8375 This option is useful for making full or incremental archival backups of
8376 a file system. If this option is used in conjunction with
8377 @option{--verbose} (@option{-v}), files that are excluded are
8378 mentioned by name on the standard error.
8381 * directory:: Changing Directory
8382 * absolute:: Absolute File Names
8386 @subsection Changing the Working Directory
8388 @FIXME{need to read over this node now for continuity; i've switched
8389 things around some.}
8391 @cindex Changing directory mid-stream
8392 @cindex Directory, changing mid-stream
8393 @cindex Working directory, specifying
8394 To change the working directory in the middle of a list of file names,
8395 either on the command line or in a file specified using
8396 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
8397 This will change the working directory to the specified directory
8398 after that point in the list.
8402 @item --directory=@var{directory}
8403 @itemx -C @var{directory}
8404 Changes the working directory in the middle of a command line.
8410 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
8414 will place the files @file{grape} and @file{prune} from the current
8415 directory into the archive @file{jams.tar}, followed by the file
8416 @file{cherry} from the directory @file{food}. This option is especially
8417 useful when you have several widely separated files that you want to
8418 store in the same archive.
8420 Note that the file @file{cherry} is recorded in the archive under the
8421 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
8422 archive will contain three files that all appear to have come from the
8423 same directory; if the archive is extracted with plain @samp{tar
8424 --extract}, all three files will be written in the current directory.
8426 Contrast this with the command,
8429 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
8433 which records the third file in the archive under the name
8434 @file{red/cherry} so that, if the archive is extracted using
8435 @samp{tar --extract}, the third file will be written in a subdirectory
8438 You can use the @option{--directory} option to make the archive
8439 independent of the original name of the directory holding the files.
8440 The following command places the files @file{/etc/passwd},
8441 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
8445 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
8449 However, the names of the archive members will be exactly what they were
8450 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
8451 They will not appear to be related by file name to the original
8452 directories where those files were located.
8454 Note that @option{--directory} options are interpreted consecutively. If
8455 @option{--directory} specifies a relative file name, it is interpreted
8456 relative to the then current directory, which might not be the same as
8457 the original current working directory of @command{tar}, due to a previous
8458 @option{--directory} option.
8460 When using @option{--files-from} (@pxref{files}), you can put various
8461 @command{tar} options (including @option{-C}) in the file list. Notice,
8462 however, that in this case the option and its argument may not be
8463 separated by whitespace. If you use short option, its argument must
8464 either follow the option letter immediately, without any intervening
8465 whitespace, or occupy the next line. Otherwise, if you use long
8466 option, separate its argument by an equal sign.
8468 For instance, the file list for the above example will be:
8481 To use it, you would invoke @command{tar} as follows:
8484 $ @kbd{tar -c -f foo.tar --files-from list}
8487 The interpretation of @option{--directory} is disabled by
8488 @option{--null} option.
8491 @subsection Absolute File Names
8492 @cindex absolute file names
8493 @cindex file names, absolute
8495 By default, @GNUTAR{} drops a leading @samp{/} on
8496 input or output, and complains about file names containing a @file{..}
8497 component. There is an option that turns off this behavior:
8500 @opindex absolute-names
8501 @item --absolute-names
8503 Do not strip leading slashes from file names, and permit file names
8504 containing a @file{..} file name component.
8507 When @command{tar} extracts archive members from an archive, it strips any
8508 leading slashes (@samp{/}) from the member name. This causes absolute
8509 member names in the archive to be treated as relative file names. This
8510 allows you to have such members extracted wherever you want, instead of
8511 being restricted to extracting the member in the exact directory named
8512 in the archive. For example, if the archive member has the name
8513 @file{/etc/passwd}, @command{tar} will extract it as if the name were
8514 really @file{etc/passwd}.
8516 File names containing @file{..} can cause problems when extracting, so
8517 @command{tar} normally warns you about such files when creating an
8518 archive, and rejects attempts to extracts such files.
8520 Other @command{tar} programs do not do this. As a result, if you
8521 create an archive whose member names start with a slash, they will be
8522 difficult for other people with a non-@GNUTAR{}
8523 program to use. Therefore, @GNUTAR{} also strips
8524 leading slashes from member names when putting members into the
8525 archive. For example, if you ask @command{tar} to add the file
8526 @file{/bin/ls} to an archive, it will do so, but the member name will
8527 be @file{bin/ls}@footnote{A side effect of this is that when
8528 @option{--create} is used with @option{--verbose} the resulting output
8529 is not, generally speaking, the same as the one you'd get running
8530 @kbd{tar --list} command. This may be important if you use some
8531 scripts for comparing both outputs. @xref{listing member and file names},
8532 for the information on how to handle this case.}.
8534 If you use the @option{--absolute-names} (@option{-P}) option,
8535 @command{tar} will do none of these transformations.
8537 To archive or extract files relative to the root directory, specify
8538 the @option{--absolute-names} (@option{-P}) option.
8540 Normally, @command{tar} acts on files relative to the working
8541 directory---ignoring superior directory names when archiving, and
8542 ignoring leading slashes when extracting.
8544 When you specify @option{--absolute-names} (@option{-P}),
8545 @command{tar} stores file names including all superior directory
8546 names, and preserves leading slashes. If you only invoked
8547 @command{tar} from the root directory you would never need the
8548 @option{--absolute-names} option, but using this option
8549 may be more convenient than switching to root.
8551 @FIXME{Should be an example in the tutorial/wizardry section using this
8552 to transfer files between systems.}
8555 @item --absolute-names
8556 Preserves full file names (including superior directory names) when
8557 archiving files. Preserves leading slash when extracting files.
8561 @command{tar} prints out a message about removing the @samp{/} from
8562 file names. This message appears once per @GNUTAR{}
8563 invocation. It represents something which ought to be told; ignoring
8564 what it means can cause very serious surprises, later.
8566 Some people, nevertheless, do not want to see this message. Wanting to
8567 play really dangerously, one may of course redirect @command{tar} standard
8568 error to the sink. For example, under @command{sh}:
8571 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
8575 Another solution, both nicer and simpler, would be to change to
8576 the @file{/} directory first, and then avoid absolute notation.
8580 $ @kbd{tar -c -f archive.tar -C / home}
8583 @xref{Integrity}, for some of the security-related implications
8584 of using this option.
8586 @include parse-datetime.texi
8589 @chapter Controlling the Archive Format
8591 @cindex Tar archive formats
8592 Due to historical reasons, there are several formats of tar archives.
8593 All of them are based on the same principles, but have some subtle
8594 differences that often make them incompatible with each other.
8596 GNU tar is able to create and handle archives in a variety of formats.
8597 The most frequently used formats are (in alphabetical order):
8601 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
8602 from an early @acronym{POSIX} standard, adding some improvements such as
8603 sparse file handling and incremental archives. Unfortunately these
8604 features were implemented in a way incompatible with other archive
8607 Archives in @samp{gnu} format are able to hold file names of unlimited
8611 Format used by @GNUTAR{} of versions prior to 1.12.
8614 Archive format, compatible with the V7 implementation of tar. This
8615 format imposes a number of limitations. The most important of them
8619 @item The maximum length of a file name is limited to 99 characters.
8620 @item The maximum length of a symbolic link is limited to 99 characters.
8621 @item It is impossible to store special files (block and character
8622 devices, fifos etc.)
8623 @item Maximum value of user or group @acronym{ID} is limited to 2097151 (7777777
8625 @item V7 archives do not contain symbolic ownership information (user
8626 and group name of the file owner).
8629 This format has traditionally been used by Automake when producing
8630 Makefiles. This practice will change in the future, in the meantime,
8631 however this means that projects containing file names more than 99
8632 characters long will not be able to use @GNUTAR{} @value{VERSION} and
8633 Automake prior to 1.9.
8636 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
8637 symbolic ownership information. It is also able to store
8638 special files. However, it imposes several restrictions as well:
8641 @item The maximum length of a file name is limited to 256 characters,
8642 provided that the file name can be split at a directory separator in
8643 two parts, first of them being at most 155 bytes long. So, in most
8644 cases the maximum file name length will be shorter than 256
8646 @item The maximum length of a symbolic link name is limited to
8648 @item Maximum size of a file the archive is able to accommodate
8650 @item Maximum value of UID/GID is 2097151.
8651 @item Maximum number of bits in device major and minor numbers is 21.
8655 Format used by J@"org Schilling @command{star}
8656 implementation. @GNUTAR{} is able to read @samp{star} archives but
8657 currently does not produce them.
8660 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
8661 most flexible and feature-rich format. It does not impose any
8662 restrictions on file sizes or file name lengths. This format is quite
8663 recent, so not all tar implementations are able to handle it properly.
8664 However, this format is designed in such a way that any tar
8665 implementation able to read @samp{ustar} archives will be able to read
8666 most @samp{posix} archives as well, with the only exception that any
8667 additional information (such as long file names etc.) will in such
8668 case be extracted as plain text files along with the files it refers to.
8670 This archive format will be the default format for future versions
8675 The following table summarizes the limitations of each of these
8678 @multitable @columnfractions .10 .20 .20 .20 .20
8679 @headitem Format @tab UID @tab File Size @tab File Name @tab Devn
8680 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8681 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
8682 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
8683 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
8684 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
8687 The default format for @GNUTAR{} is defined at compilation
8688 time. You may check it by running @command{tar --help}, and examining
8689 the last lines of its output. Usually, @GNUTAR{} is configured
8690 to create archives in @samp{gnu} format, however, future version will
8691 switch to @samp{posix}.
8694 * Compression:: Using Less Space through Compression
8695 * Attributes:: Handling File Attributes
8696 * Portability:: Making @command{tar} Archives More Portable
8697 * cpio:: Comparison of @command{tar} and @command{cpio}
8701 @section Using Less Space through Compression
8704 * gzip:: Creating and Reading Compressed Archives
8705 * sparse:: Archiving Sparse Files
8709 @subsection Creating and Reading Compressed Archives
8710 @cindex Compressed archives
8711 @cindex Storing archives in compressed format
8719 @GNUTAR{} is able to create and read compressed archives. It supports
8720 a wide variety of compression programs, namely: @command{gzip},
8721 @command{bzip2}, @command{lzip}, @command{lzma}, @command{lzop},
8722 @command{xz} and traditional @command{compress}. The latter is
8723 supported mostly for backward compatibility, and we recommend
8724 against using it, because it is by far less effective than the other
8725 compression programs@footnote{It also had patent problems in the past.}.
8727 Creating a compressed archive is simple: you just specify a
8728 @dfn{compression option} along with the usual archive creation
8729 commands. The compression option is @option{-z} (@option{--gzip}) to
8730 create a @command{gzip} compressed archive, @option{-j}
8731 (@option{--bzip2}) to create a @command{bzip2} compressed archive,
8732 @option{--lzip} to create an @asis{lzip} compressed archive,
8733 @option{-J} (@option{--xz}) to create an @asis{XZ} archive,
8734 @option{--lzma} to create an @asis{LZMA} compressed
8735 archive, @option{--lzop} to create an @asis{LSOP} archive, and
8736 @option{-Z} (@option{--compress}) to use @command{compress} program.
8740 $ @kbd{tar czf archive.tar.gz .}
8743 You can also let @GNUTAR{} select the compression program based on
8744 the suffix of the archive file name. This is done using
8745 @option{--auto-compress} (@option{-a}) command line option. For
8746 example, the following invocation will use @command{bzip2} for
8750 $ @kbd{tar caf archive.tar.bz2 .}
8754 whereas the following one will use @command{lzma}:
8757 $ @kbd{tar caf archive.tar.lzma .}
8760 For a complete list of file name suffixes recognized by @GNUTAR{},
8761 see @ref{auto-compress}.
8763 Reading compressed archive is even simpler: you don't need to specify
8764 any additional options as @GNUTAR{} recognizes its format
8765 automatically. Thus, the following commands will list and extract the
8766 archive created in previous example:
8769 # List the compressed archive
8770 $ @kbd{tar tf archive.tar.gz}
8771 # Extract the compressed archive
8772 $ @kbd{tar xf archive.tar.gz}
8775 The format recognition algorithm is based on @dfn{signatures}, a
8776 special byte sequences in the beginning of file, that are specific for
8777 certain compression formats. If this approach fails, @command{tar}
8778 falls back to using archive name suffix to determine its format
8779 (@pxref{auto-compress}, for a list of recognized suffixes).
8781 @anchor{alternative decompression programs}
8782 @cindex alternative decompression programs
8783 Some compression programs are able to handle different compression
8784 formats. @GNUTAR{} uses this, if the principal decompressor for the
8785 given format is not available. For example, if @command{compress} is
8786 not installed, @command{tar} will try to use @command{gzip}. As of
8787 version @value{VERSION} the following alternatives are
8788 tried@footnote{To verbosely trace the decompressor selection, use the
8789 @option{--warning=decompress-program} option
8790 (@pxref{warnings,decompress-program}).}:
8792 @multitable @columnfractions 0.3 0.3 0.3
8793 @headitem Format @tab Main decompressor @tab Alternatives
8794 @item compress @tab compress @tab gzip
8795 @item lzma @tab lzma @tab xz
8796 @item bzip2 @tab bzip2 @tab lbzip2
8799 The only case when you have to specify a decompression option while
8800 reading the archive is when reading from a pipe or from a tape drive
8801 that does not support random access. However, in this case @GNUTAR{}
8802 will indicate which option you should use. For example:
8805 $ @kbd{cat archive.tar.gz | tar tf -}
8806 tar: Archive is compressed. Use -z option
8807 tar: Error is not recoverable: exiting now
8810 If you see such diagnostics, just add the suggested option to the
8811 invocation of @GNUTAR{}:
8814 $ @kbd{cat archive.tar.gz | tar tzf -}
8817 Notice also, that there are several restrictions on operations on
8818 compressed archives. First of all, compressed archives cannot be
8819 modified, i.e., you cannot update (@option{--update}, alias @option{-u})
8820 them or delete (@option{--delete}) members from them or
8821 add (@option{--append}, alias @option{-r}) members to them. Likewise, you
8822 cannot append another @command{tar} archive to a compressed archive using
8823 @option{--concatenate} (@option{-A}). Secondly, multi-volume
8824 archives cannot be compressed.
8826 The following options allow to select a particular compressor program:
8834 Filter the archive through @command{gzip}.
8839 Filter the archive through @code{xz}.
8843 Filter the archive through @code{bzip2}.
8847 Filter the archive through @command{lzip}.
8851 Filter the archive through @command{lzma}.
8855 Filter the archive through @command{lzop}.
8862 Filter the archive through @command{compress}.
8865 When any of these options is given, @GNUTAR{} searches the compressor
8866 binary in the current path and invokes it. The name of the compressor
8867 program is specified at compilation time using a corresponding
8868 @option{--with-@var{compname}} option to @command{configure}, e.g.
8869 @option{--with-bzip2} to select a specific @command{bzip2} binary.
8870 @xref{lbzip2}, for a detailed discussion.
8872 The output produced by @command{tar --help} shows the actual
8873 compressor names along with each of these options.
8875 You can use any of these options on physical devices (tape drives,
8876 etc.) and remote files as well as on normal files; data to or from
8877 such devices or remote files is reblocked by another copy of the
8878 @command{tar} program to enforce the specified (or default) record
8879 size. The default compression parameters are used. Most compression
8880 programs allow to override these by setting a program-specific
8881 environment variable. For example, when using @command{gzip} you can
8882 use @env{GZIP} as in the example below:
8885 $ @kbd{GZIP=--best tar czf archive.tar.gz subdir}
8889 Another way would be to use the @option{-I} option instead (see
8893 $ @kbd{tar -cf archive.tar.gz -I 'gzip --best' subdir}
8897 Finally, the third, traditional, way to achieve the same result is to
8901 $ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz}
8904 @cindex corrupted archives
8905 About corrupted compressed archives: compressed files have no
8906 redundancy, for maximum compression. The adaptive nature of the
8907 compression scheme means that the compression tables are implicitly
8908 spread all over the archive. If you lose a few blocks, the dynamic
8909 construction of the compression tables becomes unsynchronized, and there
8910 is little chance that you could recover later in the archive.
8912 Another compression options provide a better control over creating
8913 compressed archives. These are:
8916 @anchor{auto-compress}
8917 @opindex auto-compress
8918 @item --auto-compress
8920 Select a compression program to use by the archive file name
8921 suffix. The following suffixes are recognized:
8923 @multitable @columnfractions 0.3 0.6
8924 @headitem Suffix @tab Compression program
8925 @item @samp{.gz} @tab @command{gzip}
8926 @item @samp{.tgz} @tab @command{gzip}
8927 @item @samp{.taz} @tab @command{gzip}
8928 @item @samp{.Z} @tab @command{compress}
8929 @item @samp{.taZ} @tab @command{compress}
8930 @item @samp{.bz2} @tab @command{bzip2}
8931 @item @samp{.tz2} @tab @command{bzip2}
8932 @item @samp{.tbz2} @tab @command{bzip2}
8933 @item @samp{.tbz} @tab @command{bzip2}
8934 @item @samp{.lz} @tab @command{lzip}
8935 @item @samp{.lzma} @tab @command{lzma}
8936 @item @samp{.tlz} @tab @command{lzma}
8937 @item @samp{.lzo} @tab @command{lzop}
8938 @item @samp{.xz} @tab @command{xz}
8941 @opindex use-compress-program
8942 @item --use-compress-program=@var{prog}
8943 @itemx -I=@var{prog}
8944 Use external compression program @var{prog}. Use this option if you
8945 are not happy with the compression program associated with the suffix
8946 at compile time or if you have a compression program that @GNUTAR{}
8947 does not support. There are two requirements to which @var{prog}
8950 First, when called without options, it should read data from standard
8951 input, compress it and output it on standard output.
8953 Secondly, if called with @option{-d} argument, it should do exactly
8954 the opposite, i.e., read the compressed data from the standard input
8955 and produce uncompressed data on the standard output.
8958 @cindex gpg, using with tar
8959 @cindex gnupg, using with tar
8960 @cindex Using encrypted archives
8961 The @option{--use-compress-program} option, in particular, lets you
8962 implement your own filters, not necessarily dealing with
8963 compression/decompression. For example, suppose you wish to implement
8964 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
8965 gpg, gpg ---- encryption and signing tool, gpg, GNU Privacy Guard
8966 Manual}). The following script does that:
8972 -d) gpg --decrypt - | gzip -d -c;;
8973 '') gzip -c | gpg -s;;
8974 *) echo "Unknown option $1">&2; exit 1;;
8979 Suppose you name it @file{gpgz} and save it somewhere in your
8980 @env{PATH}. Then the following command will create a compressed
8981 archive signed with your private key:
8984 $ @kbd{tar -cf foo.tar.gpgz -Igpgz .}
8988 Likewise, the command below will list its contents:
8991 $ @kbd{tar -tf foo.tar.gpgz -Igpgz .}
8995 The above is based on the following discussion:
8997 I have one question, or maybe it's a suggestion if there isn't a way
8998 to do it now. I would like to use @option{--gzip}, but I'd also like
8999 the output to be fed through a program like @acronym{GNU}
9000 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
9001 to use :-)), basically adding ECC protection on top of compression.
9002 It seems as if this should be quite easy to do, but I can't work out
9003 exactly how to go about it. Of course, I can pipe the standard output
9004 of @command{tar} through @command{ecc}, but then I lose (though I
9005 haven't started using it yet, I confess) the ability to have
9006 @command{tar} use @command{rmt} for it's I/O (I think).
9008 I think the most straightforward thing would be to let me specify a
9009 general set of filters outboard of compression (preferably ordered,
9010 so the order can be automatically reversed on input operations, and
9011 with the options they require specifiable), but beggars shouldn't be
9012 choosers and anything you decide on would be fine with me.
9014 By the way, I like @command{ecc} but if (as the comments say) it can't
9015 deal with loss of block sync, I'm tempted to throw some time at adding
9016 that capability. Supposing I were to actually do such a thing and
9017 get it (apparently) working, do you accept contributed changes to
9018 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
9020 Isn't that exactly the role of the
9021 @option{--use-compress-prog=@var{program}} option?
9022 I never tried it myself, but I suspect you may want to write a
9023 @var{prog} script or program able to filter stdin to stdout to
9024 way you want. It should recognize the @option{-d} option, for when
9025 extraction is needed rather than creation.
9027 It has been reported that if one writes compressed data (through the
9028 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
9029 the DLT compression mode, the data will actually get bigger and one will
9030 end up with less space on the tape.
9034 * lbzip2:: Using lbzip2 with @GNUTAR{}.
9038 @subsubsection Using lbzip2 with @GNUTAR{}.
9040 @cindex Laszlo Ersek
9041 @command{Lbzip2} is a multithreaded utility for handling
9042 @samp{bzip2} compression, written by Laszlo Ersek. It makes use of
9043 multiple processors to speed up its operation and in general works
9044 considerably faster than @command{bzip2}. For a detailed description
9045 of @command{lbzip2} see @uref{http://freshmeat.net/@/projects/@/lbzip2} and
9046 @uref{http://www.linuxinsight.com/@/lbzip2-parallel-bzip2-utility.html,
9047 lbzip2: parallel bzip2 utility}.
9049 Recent versions of @command{lbzip2} are mostly command line compatible
9050 with @command{bzip2}, which makes it possible to automatically invoke
9051 it via the @option{--bzip2} @GNUTAR{} command line option. To do so,
9052 @GNUTAR{} must be configured with the @option{--with-bzip2} command
9053 line option, like this:
9056 $ @kbd{./configure --with-bzip2=lbzip2 [@var{other-options}]}
9059 Once configured and compiled this way, @command{tar --help} will show the
9064 $ @kbd{tar --help | grep -- --bzip2}
9065 -j, --bzip2 filter the archive through lbzip2
9070 which means that running @command{tar --bzip2} will invoke @command{lbzip2}.
9073 @subsection Archiving Sparse Files
9074 @cindex Sparse Files
9076 Files in the file system occasionally have @dfn{holes}. A @dfn{hole}
9077 in a file is a section of the file's contents which was never written.
9078 The contents of a hole reads as all zeros. On many operating systems,
9079 actual disk storage is not allocated for holes, but they are counted
9080 in the length of the file. If you archive such a file, @command{tar}
9081 could create an archive longer than the original. To have @command{tar}
9082 attempt to recognize the holes in a file, use @option{--sparse}
9083 (@option{-S}). When you use this option, then, for any file using
9084 less disk space than would be expected from its length, @command{tar}
9085 searches the file for consecutive stretches of zeros. It then records
9086 in the archive for the file where the consecutive stretches of zeros
9087 are, and only archives the ``real contents'' of the file. On
9088 extraction (using @option{--sparse} is not needed on extraction) any
9089 such files have holes created wherever the continuous stretches of zeros
9090 were found. Thus, if you use @option{--sparse}, @command{tar} archives
9091 won't take more space than the original.
9097 This option instructs @command{tar} to test each file for sparseness
9098 before attempting to archive it. If the file is found to be sparse it
9099 is treated specially, thus allowing to decrease the amount of space
9100 used by its image in the archive.
9102 This option is meaningful only when creating or updating archives. It
9103 has no effect on extraction.
9106 Consider using @option{--sparse} when performing file system backups,
9107 to avoid archiving the expanded forms of files stored sparsely in the
9110 Even if your system has no sparse files currently, some may be
9111 created in the future. If you use @option{--sparse} while making file
9112 system backups as a matter of course, you can be assured the archive
9113 will never take more space on the media than the files take on disk
9114 (otherwise, archiving a disk filled with sparse files might take
9115 hundreds of tapes). @xref{Incremental Dumps}.
9117 However, be aware that @option{--sparse} option presents a serious
9118 drawback. Namely, in order to determine if the file is sparse
9119 @command{tar} has to read it before trying to archive it, so in total
9120 the file is read @strong{twice}. So, always bear in mind that the
9121 time needed to process all files with this option is roughly twice
9122 the time needed to archive them without it.
9123 @FIXME{A technical note:
9125 Programs like @command{dump} do not have to read the entire file; by
9126 examining the file system directly, they can determine in advance
9127 exactly where the holes are and thus avoid reading through them. The
9128 only data it need read are the actual allocated data blocks.
9129 @GNUTAR{} uses a more portable and straightforward
9130 archiving approach, it would be fairly difficult that it does
9131 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
9135 What I did say is that you cannot tell the difference between a hole and an
9136 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
9137 best tells you how many holes there are; it doesn't tell you @emph{where}.
9138 Just as programs may, conceivably, care what @code{st_blocks} is (care
9139 to name one that does?), they may also care where the holes are (I have
9140 no examples of this one either, but it's equally imaginable).
9142 I conclude from this that good archivers are not portable. One can
9143 arguably conclude that if you want a portable program, you can in good
9144 conscience restore files with as many holes as possible, since you can't
9149 @cindex sparse formats, defined
9150 When using @samp{POSIX} archive format, @GNUTAR{} is able to store
9151 sparse files using in three distinct ways, called @dfn{sparse
9152 formats}. A sparse format is identified by its @dfn{number},
9153 consisting, as usual of two decimal numbers, delimited by a dot. By
9154 default, format @samp{1.0} is used. If, for some reason, you wish to
9155 use an earlier format, you can select it using
9156 @option{--sparse-version} option.
9159 @opindex sparse-version
9160 @item --sparse-version=@var{version}
9162 Select the format to store sparse files in. Valid @var{version} values
9163 are: @samp{0.0}, @samp{0.1} and @samp{1.0}. @xref{Sparse Formats},
9164 for a detailed description of each format.
9167 Using @option{--sparse-format} option implies @option{--sparse}.
9170 @section Handling File Attributes
9171 @cindex atrributes, files
9172 @cindex file attributes
9174 When @command{tar} reads files, it updates their access times. To
9175 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
9176 reset the access time retroactively or avoid changing it in the first
9180 @opindex atime-preserve
9181 @item --atime-preserve
9182 @itemx --atime-preserve=replace
9183 @itemx --atime-preserve=system
9184 Preserve the access times of files that are read. This works only for
9185 files that you own, unless you have superuser privileges.
9187 @option{--atime-preserve=replace} works on most systems, but it also
9188 restores the data modification time and updates the status change
9189 time. Hence it doesn't interact with incremental dumps nicely
9190 (@pxref{Incremental Dumps}), and it can set access or data modification times
9191 incorrectly if other programs access the file while @command{tar} is
9194 @option{--atime-preserve=system} avoids changing the access time in
9195 the first place, if the operating system supports this.
9196 Unfortunately, this may or may not work on any given operating system
9197 or file system. If @command{tar} knows for sure it won't work, it
9198 complains right away.
9200 Currently @option{--atime-preserve} with no operand defaults to
9201 @option{--atime-preserve=replace}, but this is intended to change to
9202 @option{--atime-preserve=system} when the latter is better-supported.
9207 Do not extract data modification time.
9209 When this option is used, @command{tar} leaves the data modification times
9210 of the files it extracts as the times when the files were extracted,
9211 instead of setting it to the times recorded in the archive.
9213 This option is meaningless with @option{--list} (@option{-t}).
9217 Create extracted files with the same ownership they have in the
9220 This is the default behavior for the superuser,
9221 so this option is meaningful only for non-root users, when @command{tar}
9222 is executed on those systems able to give files away. This is
9223 considered as a security flaw by many people, at least because it
9224 makes quite difficult to correctly account users for the disk space
9225 they occupy. Also, the @code{suid} or @code{sgid} attributes of
9226 files are easily and silently lost when files are given away.
9228 When writing an archive, @command{tar} writes the user @acronym{ID} and user name
9229 separately. If it can't find a user name (because the user @acronym{ID} is not
9230 in @file{/etc/passwd}), then it does not write one. When restoring,
9231 it tries to look the name (if one was written) up in
9232 @file{/etc/passwd}. If it fails, then it uses the user @acronym{ID} stored in
9233 the archive instead.
9235 @opindex no-same-owner
9236 @item --no-same-owner
9238 Do not attempt to restore ownership when extracting. This is the
9239 default behavior for ordinary users, so this option has an effect
9240 only for the superuser.
9242 @opindex numeric-owner
9243 @item --numeric-owner
9244 The @option{--numeric-owner} option allows (ANSI) archives to be written
9245 without user/group name information or such information to be ignored
9246 when extracting. It effectively disables the generation and/or use
9247 of user/group name information. This option forces extraction using
9248 the numeric ids from the archive, ignoring the names.
9250 This is useful in certain circumstances, when restoring a backup from
9251 an emergency floppy with different passwd/group files for example.
9252 It is otherwise impossible to extract files with the right ownerships
9253 if the password file in use during the extraction does not match the
9254 one belonging to the file system(s) being extracted. This occurs,
9255 for example, if you are restoring your files after a major crash and
9256 had booted from an emergency floppy with no password file or put your
9257 disk into another machine to do the restore.
9259 The numeric ids are @emph{always} saved into @command{tar} archives.
9260 The identifying names are added at create time when provided by the
9261 system, unless @option{--format=oldgnu} is used. Numeric ids could be
9262 used when moving archives between a collection of machines using
9263 a centralized management for attribution of numeric ids to users
9264 and groups. This is often made through using the NIS capabilities.
9266 When making a @command{tar} file for distribution to other sites, it
9267 is sometimes cleaner to use a single owner for all files in the
9268 distribution, and nicer to specify the write permission bits of the
9269 files as stored in the archive independently of their actual value on
9270 the file system. The way to prepare a clean distribution is usually
9271 to have some Makefile rule creating a directory, copying all needed
9272 files in that directory, then setting ownership and permissions as
9273 wanted (there are a lot of possible schemes), and only then making a
9274 @command{tar} archive out of this directory, before cleaning
9275 everything out. Of course, we could add a lot of options to
9276 @GNUTAR{} for fine tuning permissions and ownership.
9277 This is not the good way, I think. @GNUTAR{} is
9278 already crowded with options and moreover, the approach just explained
9279 gives you a great deal of control already.
9281 @xopindex{same-permissions, short description}
9282 @xopindex{preserve-permissions, short description}
9284 @itemx --same-permissions
9285 @itemx --preserve-permissions
9286 Extract all protection information.
9288 This option causes @command{tar} to set the modes (access permissions) of
9289 extracted files exactly as recorded in the archive. If this option
9290 is not used, the current @code{umask} setting limits the permissions
9291 on extracted files. This option is by default enabled when
9292 @command{tar} is executed by a superuser.
9295 This option is meaningless with @option{--list} (@option{-t}).
9299 Same as both @option{--same-permissions} and @option{--same-order}.
9301 This option is deprecated, and will be removed in @GNUTAR{} version 1.23.
9306 @section Making @command{tar} Archives More Portable
9308 Creating a @command{tar} archive on a particular system that is meant to be
9309 useful later on many other machines and with other versions of @command{tar}
9310 is more challenging than you might think. @command{tar} archive formats
9311 have been evolving since the first versions of Unix. Many such formats
9312 are around, and are not always compatible with each other. This section
9313 discusses a few problems, and gives some advice about making @command{tar}
9314 archives more portable.
9316 One golden rule is simplicity. For example, limit your @command{tar}
9317 archives to contain only regular files and directories, avoiding
9318 other kind of special files. Do not attempt to save sparse files or
9319 contiguous files as such. Let's discuss a few more problems, in turn.
9321 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
9322 archives and archive labels) in GNU and PAX formats.}
9325 * Portable Names:: Portable Names
9326 * dereference:: Symbolic Links
9327 * hard links:: Hard Links
9328 * old:: Old V7 Archives
9329 * ustar:: Ustar Archives
9330 * gnu:: GNU and old GNU format archives.
9331 * posix:: @acronym{POSIX} archives
9332 * Checksumming:: Checksumming Problems
9333 * Large or Negative Values:: Large files, negative time stamps, etc.
9334 * Other Tars:: How to Extract GNU-Specific Data Using
9335 Other @command{tar} Implementations
9338 @node Portable Names
9339 @subsection Portable Names
9341 Use portable file and member names. A name is portable if it contains
9342 only @acronym{ASCII} letters and digits, @samp{/}, @samp{.}, @samp{_}, and
9343 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
9344 contain @samp{/-}. Avoid deep directory nesting. For portability to
9345 old Unix hosts, limit your file name components to 14 characters or
9348 If you intend to have your @command{tar} archives to be read under
9349 MSDOS, you should not rely on case distinction for file names, and you
9350 might use the @acronym{GNU} @command{doschk} program for helping you
9351 further diagnosing illegal MSDOS names, which are even more limited
9355 @subsection Symbolic Links
9356 @cindex File names, using symbolic links
9357 @cindex Symbolic link as file name
9359 @opindex dereference
9360 Normally, when @command{tar} archives a symbolic link, it writes a
9361 block to the archive naming the target of the link. In that way, the
9362 @command{tar} archive is a faithful record of the file system contents.
9363 When @option{--dereference} (@option{-h}) is used with
9364 @option{--create} (@option{-c}), @command{tar} archives the files
9365 symbolic links point to, instead of
9366 the links themselves.
9368 When creating portable archives, use @option{--dereference}
9369 (@option{-h}): some systems do not support
9370 symbolic links, and moreover, your distribution might be unusable if
9371 it contains unresolved symbolic links.
9373 When reading from an archive, the @option{--dereference} (@option{-h})
9374 option causes @command{tar} to follow an already-existing symbolic
9375 link when @command{tar} writes or reads a file named in the archive.
9376 Ordinarily, @command{tar} does not follow such a link, though it may
9377 remove the link before writing a new file. @xref{Dealing with Old
9380 The @option{--dereference} option is unsafe if an untrusted user can
9381 modify directories while @command{tar} is running. @xref{Security}.
9384 @subsection Hard Links
9385 @cindex File names, using hard links
9386 @cindex hard links, dereferencing
9387 @cindex dereferencing hard links
9389 Normally, when @command{tar} archives a hard link, it writes a
9390 block to the archive naming the target of the link (a @samp{1} type
9391 block). In that way, the actual file contents is stored in file only
9392 once. For example, consider the following two files:
9397 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 one
9398 -rw-r--r-- 2 gray staff 4 2007-10-30 15:11 jeden
9402 Here, @file{jeden} is a link to @file{one}. When archiving this
9403 directory with a verbose level 2, you will get an output similar to
9407 $ tar cvvf ../archive.tar .
9408 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9409 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9410 hrw-r--r-- gray/staff 0 2007-10-30 15:11 ./one link to ./jeden
9413 The last line shows that, instead of storing two copies of the file,
9414 @command{tar} stored it only once, under the name @file{jeden}, and
9415 stored file @file{one} as a hard link to this file.
9417 It may be important to know that all hard links to the given file are
9418 stored in the archive. For example, this may be necessary for exact
9419 reproduction of the file system. The following option does that:
9422 @xopindex{check-links, described}
9425 Check the number of links dumped for each processed file. If this
9426 number does not match the total number of hard links for the file, print
9430 For example, trying to archive only file @file{jeden} with this option
9431 produces the following diagnostics:
9434 $ tar -c -f ../archive.tar -l jeden
9435 tar: Missing links to `jeden'.
9438 Although creating special records for hard links helps keep a faithful
9439 record of the file system contents and makes archives more compact, it
9440 may present some difficulties when extracting individual members from
9441 the archive. For example, trying to extract file @file{one} from the
9442 archive created in previous examples produces, in the absense of file
9446 $ tar xf archive.tar ./one
9447 tar: ./one: Cannot hard link to `./jeden': No such file or directory
9448 tar: Error exit delayed from previous errors
9451 The reason for this behavior is that @command{tar} cannot seek back in
9452 the archive to the previous member (in this case, @file{one}), to
9453 extract it@footnote{There are plans to fix this in future releases.}.
9454 If you wish to avoid such problems at the cost of a bigger archive,
9455 use the following option:
9458 @xopindex{hard-dereference, described}
9459 @item --hard-dereference
9460 Dereference hard links and store the files they refer to.
9463 For example, trying this option on our two sample files, we get two
9464 copies in the archive, each of which can then be extracted
9465 independently of the other:
9469 $ tar -c -vv -f ../archive.tar --hard-dereference .
9470 drwxr-xr-x gray/staff 0 2007-10-30 15:13 ./
9471 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./jeden
9472 -rw-r--r-- gray/staff 4 2007-10-30 15:11 ./one
9477 @subsection Old V7 Archives
9478 @cindex Format, old style
9479 @cindex Old style format
9480 @cindex Old style archives
9481 @cindex v7 archive format
9483 Certain old versions of @command{tar} cannot handle additional
9484 information recorded by newer @command{tar} programs. To create an
9485 archive in V7 format (not ANSI), which can be read by these old
9486 versions, specify the @option{--format=v7} option in
9487 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
9488 accepts @option{--portability} or @option{--old-archive} for this
9489 option). When you specify it,
9490 @command{tar} leaves out information about directories, pipes, fifos,
9491 contiguous files, and device files, and specifies file ownership by
9492 group and user IDs instead of group and user names.
9494 When updating an archive, do not use @option{--format=v7}
9495 unless the archive was created using this option.
9497 In most cases, a @emph{new} format archive can be read by an @emph{old}
9498 @command{tar} program without serious trouble, so this option should
9499 seldom be needed. On the other hand, most modern @command{tar}s are
9500 able to read old format archives, so it might be safer for you to
9501 always use @option{--format=v7} for your distributions. Notice,
9502 however, that @samp{ustar} format is a better alternative, as it is
9503 free from many of @samp{v7}'s drawbacks.
9506 @subsection Ustar Archive Format
9508 @cindex ustar archive format
9509 Archive format defined by @acronym{POSIX}.1-1988 specification is called
9510 @code{ustar}. Although it is more flexible than the V7 format, it
9511 still has many restrictions (@pxref{Formats,ustar}, for the detailed
9512 description of @code{ustar} format). Along with V7 format,
9513 @code{ustar} format is a good choice for archives intended to be read
9514 with other implementations of @command{tar}.
9516 To create archive in @code{ustar} format, use @option{--format=ustar}
9517 option in conjunction with the @option{--create} (@option{-c}).
9520 @subsection @acronym{GNU} and old @GNUTAR{} format
9522 @cindex GNU archive format
9523 @cindex Old GNU archive format
9524 @GNUTAR{} was based on an early draft of the
9525 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
9526 @command{tar}, such as the support for file names longer than 100
9527 characters, use portions of the @command{tar} header record which were
9528 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
9529 @acronym{POSIX} have allocated the same parts of the header record for
9530 other purposes. As a result, @GNUTAR{} format is
9531 incompatible with the current @acronym{POSIX} specification, and with
9532 @command{tar} programs that follow it.
9534 In the majority of cases, @command{tar} will be configured to create
9535 this format by default. This will change in future releases, since
9536 we plan to make @samp{POSIX} format the default.
9538 To force creation a @GNUTAR{} archive, use option
9539 @option{--format=gnu}.
9542 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
9544 @cindex POSIX archive format
9545 @cindex PAX archive format
9546 Starting from version 1.14 @GNUTAR{} features full support for
9547 @acronym{POSIX.1-2001} archives.
9549 A @acronym{POSIX} conformant archive will be created if @command{tar}
9550 was given @option{--format=posix} (@option{--format=pax}) option. No
9551 special option is required to read and extract from a @acronym{POSIX}
9555 * PAX keywords:: Controlling Extended Header Keywords.
9559 @subsubsection Controlling Extended Header Keywords
9563 @item --pax-option=@var{keyword-list}
9564 Handle keywords in @acronym{PAX} extended headers. This option is
9565 equivalent to @option{-o} option of the @command{pax} utility.
9568 @var{Keyword-list} is a comma-separated
9569 list of keyword options, each keyword option taking one of
9570 the following forms:
9573 @item delete=@var{pattern}
9574 When used with one of archive-creation commands,
9575 this option instructs @command{tar} to omit from extended header records
9576 that it produces any keywords matching the string @var{pattern}.
9578 When used in extract or list mode, this option instructs tar
9579 to ignore any keywords matching the given @var{pattern} in the extended
9580 header records. In both cases, matching is performed using the pattern
9581 matching notation described in @acronym{POSIX 1003.2}, 3.13
9582 (@pxref{wildcards}). For example:
9585 --pax-option delete=security.*
9588 would suppress security-related information.
9590 @item exthdr.name=@var{string}
9592 This keyword allows user control over the name that is written into the
9593 ustar header blocks for the extended headers. The name is obtained
9594 from @var{string} after making the following substitutions:
9596 @multitable @columnfractions .25 .55
9597 @headitem Meta-character @tab Replaced By
9598 @item %d @tab The directory name of the file, equivalent to the
9599 result of the @command{dirname} utility on the translated file name.
9600 @item %f @tab The name of the file with the directory information
9601 stripped, equivalent to the result of the @command{basename} utility
9602 on the translated file name.
9603 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9604 @item %% @tab A @samp{%} character.
9607 Any other @samp{%} characters in @var{string} produce undefined
9610 If no option @samp{exthdr.name=string} is specified, @command{tar}
9611 will use the following default value:
9617 @item exthdr.mtime=@var{value}
9619 This keyword defines the value of the @samp{mtime} field that
9620 is written into the ustar header blocks for the extended headers.
9621 By default, the @samp{mtime} field is set to the modification time
9622 of the archive member described by that extended headers.
9624 @item globexthdr.name=@var{string}
9625 This keyword allows user control over the name that is written into
9626 the ustar header blocks for global extended header records. The name
9627 is obtained from the contents of @var{string}, after making
9628 the following substitutions:
9630 @multitable @columnfractions .25 .55
9631 @headitem Meta-character @tab Replaced By
9632 @item %n @tab An integer that represents the
9633 sequence number of the global extended header record in the archive,
9635 @item %p @tab The process @acronym{ID} of the @command{tar} process.
9636 @item %% @tab A @samp{%} character.
9639 Any other @samp{%} characters in @var{string} produce undefined results.
9641 If no option @samp{globexthdr.name=string} is specified, @command{tar}
9642 will use the following default value:
9645 $TMPDIR/GlobalHead.%p.%n
9649 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
9650 environment variable. If @var{TMPDIR} is not set, @command{tar}
9653 @item globexthdr.mtime=@var{value}
9655 This keyword defines the value of the @samp{mtime} field that
9656 is written into the ustar header blocks for the global extended headers.
9657 By default, the @samp{mtime} field is set to the time when
9658 @command{tar} was invoked.
9660 @item @var{keyword}=@var{value}
9661 When used with one of archive-creation commands, these keyword/value pairs
9662 will be included at the beginning of the archive in a global extended
9663 header record. When used with one of archive-reading commands,
9664 @command{tar} will behave as if it has encountered these keyword/value
9665 pairs at the beginning of the archive in a global extended header
9668 @item @var{keyword}:=@var{value}
9669 When used with one of archive-creation commands, these keyword/value pairs
9670 will be included as records at the beginning of an extended header for
9671 each file. This is effectively equivalent to @var{keyword}=@var{value}
9672 form except that it creates no global extended header records.
9674 When used with one of archive-reading commands, @command{tar} will
9675 behave as if these keyword/value pairs were included as records at the
9676 end of each extended header; thus, they will override any global or
9677 file-specific extended header record keywords of the same names.
9678 For example, in the command:
9681 tar --format=posix --create \
9682 --file archive --pax-option gname:=user .
9685 the group name will be forced to a new value for all files
9686 stored in the archive.
9689 In any of the forms described above, the @var{value} may be
9690 a string enclosed in curly braces. In that case, the string
9691 between the braces is understood either as a textual time
9692 representation, as described in @ref{Date input formats}, or a name of
9693 the existing file, starting with @samp{/} or @samp{.}. In the latter
9694 case, the modification time of that file is used.
9696 For example, to set all modification times to the current date, you
9697 use the following option:
9700 --pax-option='mtime:=@{now@}'
9703 Note quoting of the option's argument.
9705 @cindex archives, binary equivalent
9706 @cindex binary equivalent archives, creating
9707 As another example, here is the option that ensures that any two
9708 archives created using it, will be binary equivalent if they have the
9712 --pax-option=exthdr.name=%d/PaxHeaders/%f,atime:=0
9716 @subsection Checksumming Problems
9718 SunOS and HP-UX @command{tar} fail to accept archives created using
9719 @GNUTAR{} and containing non-@acronym{ASCII} file names, that
9720 is, file names having characters with the eight bit set, because they
9721 use signed checksums, while @GNUTAR{} uses unsigned
9722 checksums while creating archives, as per @acronym{POSIX} standards. On
9723 reading, @GNUTAR{} computes both checksums and
9724 accepts any. It is somewhat worrying that a lot of people may go
9725 around doing backup of their files using faulty (or at least
9726 non-standard) software, not learning about it until it's time to
9727 restore their missing files with an incompatible file extractor, or
9730 @GNUTAR{} computes checksums both ways, and accept
9731 any on read, so @acronym{GNU} tar can read Sun tapes even with their
9732 wrong checksums. @GNUTAR{} produces the standard
9733 checksum, however, raising incompatibilities with Sun. That is to
9734 say, @GNUTAR{} has not been modified to
9735 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
9736 I've been told that more recent Sun @command{tar} now read standard
9737 archives, so maybe Sun did a similar patch, after all?
9739 The story seems to be that when Sun first imported @command{tar}
9740 sources on their system, they recompiled it without realizing that
9741 the checksums were computed differently, because of a change in
9742 the default signing of @code{char}'s in their compiler. So they
9743 started computing checksums wrongly. When they later realized their
9744 mistake, they merely decided to stay compatible with it, and with
9745 themselves afterwards. Presumably, but I do not really know, HP-UX
9746 has chosen that their @command{tar} archives to be compatible with Sun's.
9747 The current standards do not favor Sun @command{tar} format. In any
9748 case, it now falls on the shoulders of SunOS and HP-UX users to get
9749 a @command{tar} able to read the good archives they receive.
9751 @node Large or Negative Values
9752 @subsection Large or Negative Values
9753 @cindex large values
9754 @cindex future time stamps
9755 @cindex negative time stamps
9758 The above sections suggest to use @samp{oldest possible} archive
9759 format if in doubt. However, sometimes it is not possible. If you
9760 attempt to archive a file whose metadata cannot be represented using
9761 required format, @GNUTAR{} will print error message and ignore such a
9762 file. You will than have to switch to a format that is able to
9763 handle such values. The format summary table (@pxref{Formats}) will
9766 In particular, when trying to archive files larger than 8GB or with
9767 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
9768 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
9769 @acronym{POSIX} archive formats. When considering which format to
9770 choose, bear in mind that the @acronym{GNU} format uses
9771 two's-complement base-256 notation to store values that do not fit
9772 into standard @acronym{ustar} range. Such archives can generally be
9773 read only by a @GNUTAR{} implementation. Moreover, they sometimes
9774 cannot be correctly restored on another hosts even by @GNUTAR{}. For
9775 example, using two's complement representation for negative time
9776 stamps that assumes a signed 32-bit @code{time_t} generates archives
9777 that are not portable to hosts with differing @code{time_t}
9780 On the other hand, @acronym{POSIX} archives, generally speaking, can
9781 be extracted by any tar implementation that understands older
9782 @acronym{ustar} format. The only exception are files larger than 8GB.
9784 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
9788 @subsection How to Extract GNU-Specific Data Using Other @command{tar} Implementations
9790 In previous sections you became acquainted with various quirks
9791 necessary to make your archives portable. Sometimes you may need to
9792 extract archives containing GNU-specific members using some
9793 third-party @command{tar} implementation or an older version of
9794 @GNUTAR{}. Of course your best bet is to have @GNUTAR{} installed,
9795 but if it is for some reason impossible, this section will explain
9796 how to cope without it.
9798 When we speak about @dfn{GNU-specific} members we mean two classes of
9799 them: members split between the volumes of a multi-volume archive and
9800 sparse members. You will be able to always recover such members if
9801 the archive is in PAX format. In addition split members can be
9802 recovered from archives in old GNU format. The following subsections
9803 describe the required procedures in detail.
9806 * Split Recovery:: Members Split Between Volumes
9807 * Sparse Recovery:: Sparse Members
9810 @node Split Recovery
9811 @subsubsection Extracting Members Split Between Volumes
9813 @cindex Mutli-volume archives, extracting using non-GNU tars
9814 If a member is split between several volumes of an old GNU format archive
9815 most third party @command{tar} implementation will fail to extract
9816 it. To extract it, use @command{tarcat} program (@pxref{Tarcat}).
9817 This program is available from
9818 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/tarcat.html, @GNUTAR{}
9819 home page}. It concatenates several archive volumes into a single
9820 valid archive. For example, if you have three volumes named from
9821 @file{vol-1.tar} to @file{vol-3.tar}, you can do the following to
9822 extract them using a third-party @command{tar}:
9825 $ @kbd{tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf -}
9828 @cindex Mutli-volume archives in PAX format, extracting using non-GNU tars
9829 You could use this approach for most (although not all) PAX
9830 format archives as well. However, extracting split members from a PAX
9831 archive is a much easier task, because PAX volumes are constructed in
9832 such a way that each part of a split member is extracted to a
9833 different file by @command{tar} implementations that are not aware of
9834 GNU extensions. More specifically, the very first part retains its
9835 original name, and all subsequent parts are named using the pattern:
9838 %d/GNUFileParts.%p/%f.%n
9842 where symbols preceeded by @samp{%} are @dfn{macro characters} that
9843 have the following meaning:
9845 @multitable @columnfractions .25 .55
9846 @headitem Meta-character @tab Replaced By
9847 @item %d @tab The directory name of the file, equivalent to the
9848 result of the @command{dirname} utility on its full name.
9849 @item %f @tab The file name of the file, equivalent to the result
9850 of the @command{basename} utility on its full name.
9851 @item %p @tab The process @acronym{ID} of the @command{tar} process that
9852 created the archive.
9853 @item %n @tab Ordinal number of this particular part.
9856 For example, if the file @file{var/longfile} was split during archive
9857 creation between three volumes, and the creator @command{tar} process
9858 had process @acronym{ID} @samp{27962}, then the member names will be:
9862 var/GNUFileParts.27962/longfile.1
9863 var/GNUFileParts.27962/longfile.2
9866 When you extract your archive using a third-party @command{tar}, these
9867 files will be created on your disk, and the only thing you will need
9868 to do to restore your file in its original form is concatenate them in
9869 the proper order, for example:
9874 $ @kbd{cat GNUFileParts.27962/longfile.1 \
9875 GNUFileParts.27962/longfile.2 >> longfile}
9876 $ rm -f GNUFileParts.27962
9880 Notice, that if the @command{tar} implementation you use supports PAX
9881 format archives, it will probably emit warnings about unknown keywords
9882 during extraction. They will look like this:
9887 Unknown extended header keyword 'GNU.volume.filename' ignored.
9888 Unknown extended header keyword 'GNU.volume.size' ignored.
9889 Unknown extended header keyword 'GNU.volume.offset' ignored.
9894 You can safely ignore these warnings.
9896 If your @command{tar} implementation is not PAX-aware, you will get
9897 more warnings and more files generated on your disk, e.g.:
9901 $ @kbd{tar xf vol-1.tar}
9902 var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as
9904 Unexpected EOF in archive
9905 $ @kbd{tar xf vol-2.tar}
9906 tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file
9907 GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type
9908 'x', extracted as normal file
9912 Ignore these warnings. The @file{PaxHeaders.*} directories created
9913 will contain files with @dfn{extended header keywords} describing the
9914 extracted files. You can delete them, unless they describe sparse
9915 members. Read further to learn more about them.
9917 @node Sparse Recovery
9918 @subsubsection Extracting Sparse Members
9920 @cindex sparse files, extracting with non-GNU tars
9921 Any @command{tar} implementation will be able to extract sparse members from a
9922 PAX archive. However, the extracted files will be @dfn{condensed},
9923 i.e., any zero blocks will be removed from them. When we restore such
9924 a condensed file to its original form, by adding zero blocks (or
9925 @dfn{holes}) back to their original locations, we call this process
9926 @dfn{expanding} a compressed sparse file.
9929 To expand a file, you will need a simple auxiliary program called
9930 @command{xsparse}. It is available in source form from
9931 @uref{http://www.gnu.org/@/software/@/tar/@/utils/@/xsparse.html, @GNUTAR{}
9934 @cindex sparse files v.1.0, extracting with non-GNU tars
9935 Let's begin with archive members in @dfn{sparse format
9936 version 1.0}@footnote{@xref{PAX 1}.}, which are the easiest to expand.
9937 The condensed file will contain both file map and file data, so no
9938 additional data will be needed to restore it. If the original file
9939 name was @file{@var{dir}/@var{name}}, then the condensed file will be
9940 named @file{@var{dir}/@/GNUSparseFile.@var{n}/@/@var{name}}, where
9941 @var{n} is a decimal number@footnote{Technically speaking, @var{n} is a
9942 @dfn{process @acronym{ID}} of the @command{tar} process which created the
9943 archive (@pxref{PAX keywords}).}.
9945 To expand a version 1.0 file, run @command{xsparse} as follows:
9948 $ @kbd{xsparse @file{cond-file}}
9952 where @file{cond-file} is the name of the condensed file. The utility
9953 will deduce the name for the resulting expanded file using the
9954 following algorithm:
9957 @item If @file{cond-file} does not contain any directories,
9958 @file{../cond-file} will be used;
9960 @item If @file{cond-file} has the form
9961 @file{@var{dir}/@var{t}/@var{name}}, where both @var{t} and @var{name}
9962 are simple names, with no @samp{/} characters in them, the output file
9963 name will be @file{@var{dir}/@var{name}}.
9965 @item Otherwise, if @file{cond-file} has the form
9966 @file{@var{dir}/@var{name}}, the output file name will be
9970 In the unlikely case when this algorithm does not suit your needs,
9971 you can explicitly specify output file name as a second argument to
9975 $ @kbd{xsparse @file{cond-file} @file{out-file}}
9978 It is often a good idea to run @command{xsparse} in @dfn{dry run} mode
9979 first. In this mode, the command does not actually expand the file,
9980 but verbosely lists all actions it would be taking to do so. The dry
9981 run mode is enabled by @option{-n} command line argument:
9985 $ @kbd{xsparse -n /home/gray/GNUSparseFile.6058/sparsefile}
9986 Reading v.1.0 sparse map
9987 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
9988 `/home/gray/sparsefile'
9993 To actually expand the file, you would run:
9996 $ @kbd{xsparse /home/gray/GNUSparseFile.6058/sparsefile}
10000 The program behaves the same way all UNIX utilities do: it will keep
10001 quiet unless it has simething important to tell you (e.g. an error
10002 condition or something). If you wish it to produce verbose output,
10003 similar to that from the dry run mode, use @option{-v} option:
10007 $ @kbd{xsparse -v /home/gray/GNUSparseFile.6058/sparsefile}
10008 Reading v.1.0 sparse map
10009 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
10010 `/home/gray/sparsefile'
10015 Additionally, if your @command{tar} implementation has extracted the
10016 @dfn{extended headers} for this file, you can instruct @command{xstar}
10017 to use them in order to verify the integrity of the expanded file.
10018 The option @option{-x} sets the name of the extended header file to
10019 use. Continuing our example:
10023 $ @kbd{xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \
10024 /home/gray/GNUSparseFile.6058/sparsefile}
10025 Reading extended header file
10026 Found variable GNU.sparse.major = 1
10027 Found variable GNU.sparse.minor = 0
10028 Found variable GNU.sparse.name = sparsefile
10029 Found variable GNU.sparse.realsize = 217481216
10030 Reading v.1.0 sparse map
10031 Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to
10032 `/home/gray/sparsefile'
10037 @anchor{extracting sparse v.0.x}
10038 @cindex sparse files v.0.1, extracting with non-GNU tars
10039 @cindex sparse files v.0.0, extracting with non-GNU tars
10040 An @dfn{extended header} is a special @command{tar} archive header
10041 that precedes an archive member and contains a set of
10042 @dfn{variables}, describing the member properties that cannot be
10043 stored in the standard @code{ustar} header. While optional for
10044 expanding sparse version 1.0 members, the use of extended headers is
10045 mandatory when expanding sparse members in older sparse formats: v.0.0
10046 and v.0.1 (The sparse formats are described in detail in @ref{Sparse
10047 Formats}.) So, for these formats, the question is: how to obtain
10048 extended headers from the archive?
10050 If you use a @command{tar} implementation that does not support PAX
10051 format, extended headers for each member will be extracted as a
10052 separate file. If we represent the member name as
10053 @file{@var{dir}/@var{name}}, then the extended header file will be
10054 named @file{@var{dir}/@/PaxHeaders.@var{n}/@/@var{name}}, where
10055 @var{n} is an integer number.
10057 Things become more difficult if your @command{tar} implementation
10058 does support PAX headers, because in this case you will have to
10059 manually extract the headers. We recommend the following algorithm:
10063 Consult the documentation of your @command{tar} implementation for an
10064 option that prints @dfn{block numbers} along with the archive
10065 listing (analogous to @GNUTAR{}'s @option{-R} option). For example,
10066 @command{star} has @option{-block-number}.
10069 Obtain verbose listing using the @samp{block number} option, and
10070 find block numbers of the sparse member in question and the member
10071 immediately following it. For example, running @command{star} on our
10076 $ @kbd{star -t -v -block-number -f arc.tar}
10078 star: Unknown extended header keyword 'GNU.sparse.size' ignored.
10079 star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored.
10080 star: Unknown extended header keyword 'GNU.sparse.name' ignored.
10081 star: Unknown extended header keyword 'GNU.sparse.map' ignored.
10082 block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile
10083 block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README
10089 (as usual, ignore the warnings about unknown keywords.)
10092 Let @var{size} be the size of the sparse member, @var{Bs} be its block number
10093 and @var{Bn} be the block number of the next member.
10097 @var{N} = @var{Bs} - @var{Bn} - @var{size}/512 - 2
10101 This number gives the size of the extended header part in tar @dfn{blocks}.
10102 In our example, this formula gives: @code{897 - 56 - 425984 / 512 - 2
10106 Use @command{dd} to extract the headers:
10109 @kbd{dd if=@var{archive} of=@var{hname} bs=512 skip=@var{Bs} count=@var{N}}
10113 where @var{archive} is the archive name, @var{hname} is a name of the
10114 file to store the extended header in, @var{Bs} and @var{N} are
10115 computed in previous steps.
10117 In our example, this command will be
10120 $ @kbd{dd if=arc.tar of=xhdr bs=512 skip=56 count=7}
10124 Finally, you can expand the condensed file, using the obtained header:
10128 $ @kbd{xsparse -v -x xhdr GNUSparseFile.6058/sparsefile}
10129 Reading extended header file
10130 Found variable GNU.sparse.size = 217481216
10131 Found variable GNU.sparse.numblocks = 208
10132 Found variable GNU.sparse.name = sparsefile
10133 Found variable GNU.sparse.map = 0,2048,1050624,2048,@dots{}
10134 Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile'
10140 @section Comparison of @command{tar} and @command{cpio}
10143 @FIXME{Reorganize the following material}
10145 The @command{cpio} archive formats, like @command{tar}, do have maximum
10146 file name lengths. The binary and old @acronym{ASCII} formats have a maximum file
10147 length of 256, and the new @acronym{ASCII} and @acronym{CRC ASCII} formats have a max
10148 file length of 1024. @acronym{GNU} @command{cpio} can read and write archives
10149 with arbitrary file name lengths, but other @command{cpio} implementations
10150 may crash unexplainedly trying to read them.
10152 @command{tar} handles symbolic links in the form in which it comes in @acronym{BSD};
10153 @command{cpio} doesn't handle symbolic links in the form in which it comes
10154 in System V prior to SVR4, and some vendors may have added symlinks
10155 to their system without enhancing @command{cpio} to know about them.
10156 Others may have enhanced it in a way other than the way I did it
10157 at Sun, and which was adopted by AT&T (and which is, I think, also
10158 present in the @command{cpio} that Berkeley picked up from AT&T and put
10159 into a later @acronym{BSD} release---I think I gave them my changes).
10161 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
10162 can handle @command{tar} format input, and write it on output, and it
10163 probably handles symbolic links. They may not have bothered doing
10164 anything to enhance @command{tar} as a result.)
10166 @command{cpio} handles special files; traditional @command{tar} doesn't.
10168 @command{tar} comes with V7, System III, System V, and @acronym{BSD} source;
10169 @command{cpio} comes only with System III, System V, and later @acronym{BSD}
10170 (4.3-tahoe and later).
10172 @command{tar}'s way of handling multiple hard links to a file can handle
10173 file systems that support 32-bit i-numbers (e.g., the @acronym{BSD} file system);
10174 @command{cpio}s way requires you to play some games (in its ``binary''
10175 format, i-numbers are only 16 bits, and in its ``portable @acronym{ASCII}'' format,
10176 they're 18 bits---it would have to play games with the "file system @acronym{ID}"
10177 field of the header to make sure that the file system @acronym{ID}/i-number pairs
10178 of different files were always different), and I don't know which
10179 @command{cpio}s, if any, play those games. Those that don't might get
10180 confused and think two files are the same file when they're not, and
10181 make hard links between them.
10183 @command{tar}s way of handling multiple hard links to a file places only
10184 one copy of the link on the tape, but the name attached to that copy
10185 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
10186 way puts one copy for every link, but you can retrieve it using any
10190 What type of check sum (if any) is used, and how is this calculated.
10193 See the attached manual pages for @command{tar} and @command{cpio} format.
10194 @command{tar} uses a checksum which is the sum of all the bytes in the
10195 @command{tar} header for a file; @command{cpio} uses no checksum.
10198 If anyone knows why @command{cpio} was made when @command{tar} was present
10202 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
10203 generally-available version of UNIX had @command{tar} at the time. I don't
10204 know whether any version that was generally available @emph{within AT&T}
10205 had @command{tar}, or, if so, whether the people within AT&T who did
10206 @command{cpio} knew about it.
10208 On restore, if there is a corruption on a tape @command{tar} will stop at
10209 that point, while @command{cpio} will skip over it and try to restore the
10212 The main difference is just in the command syntax and header format.
10214 @command{tar} is a little more tape-oriented in that everything is blocked
10215 to start on a record boundary.
10218 Is there any differences between the ability to recover crashed
10219 archives between the two of them. (Is there any chance of recovering
10220 crashed archives at all.)
10223 Theoretically it should be easier under @command{tar} since the blocking
10224 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
10225 However, modern @command{cpio}'s and variations have an option to just
10226 search for the next file header after an error with a reasonable chance
10227 of resyncing. However, lots of tape driver software won't allow you to
10228 continue past a media error which should be the only reason for getting
10229 out of sync unless a file changed sizes while you were writing the
10233 If anyone knows why @command{cpio} was made when @command{tar} was present
10234 at the unix scene, please tell me about this too.
10237 Probably because it is more media efficient (by not blocking everything
10238 and using only the space needed for the headers where @command{tar}
10239 always uses 512 bytes per file header) and it knows how to archive
10242 You might want to look at the freely available alternatives. The
10243 major ones are @command{afio}, @GNUTAR{}, and
10244 @command{pax}, each of which have their own extensions with some
10245 backwards compatibility.
10247 Sparse files were @command{tar}red as sparse files (which you can
10248 easily test, because the resulting archive gets smaller, and
10249 @acronym{GNU} @command{cpio} can no longer read it).
10252 @chapter Tapes and Other Archive Media
10255 A few special cases about tape handling warrant more detailed
10256 description. These special cases are discussed below.
10258 Many complexities surround the use of @command{tar} on tape drives. Since
10259 the creation and manipulation of archives located on magnetic tape was
10260 the original purpose of @command{tar}, it contains many features making
10261 such manipulation easier.
10263 Archives are usually written on dismountable media---tape cartridges,
10264 mag tapes, or floppy disks.
10266 The amount of data a tape or disk holds depends not only on its size,
10267 but also on how it is formatted. A 2400 foot long reel of mag tape
10268 holds 40 megabytes of data when formatted at 1600 bits per inch. The
10269 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
10271 Magnetic media are re-usable---once the archive on a tape is no longer
10272 needed, the archive can be erased and the tape or disk used over.
10273 Media quality does deteriorate with use, however. Most tapes or disks
10274 should be discarded when they begin to produce data errors. EXABYTE
10275 tape cartridges should be discarded when they generate an @dfn{error
10276 count} (number of non-usable bits) of more than 10k.
10278 Magnetic media are written and erased using magnetic fields, and
10279 should be protected from such fields to avoid damage to stored data.
10280 Sticking a floppy disk to a filing cabinet using a magnet is probably
10284 * Device:: Device selection and switching
10285 * Remote Tape Server::
10286 * Common Problems and Solutions::
10287 * Blocking:: Blocking
10288 * Many:: Many archives on one tape
10289 * Using Multiple Tapes:: Using Multiple Tapes
10290 * label:: Including a Label in the Archive
10292 * Write Protection::
10296 @section Device Selection and Switching
10300 @item -f [@var{hostname}:]@var{file}
10301 @itemx --file=[@var{hostname}:]@var{file}
10302 Use archive file or device @var{file} on @var{hostname}.
10305 This option is used to specify the file name of the archive @command{tar}
10308 If the file name is @samp{-}, @command{tar} reads the archive from standard
10309 input (when listing or extracting), or writes it to standard output
10310 (when creating). If the @samp{-} file name is given when updating an
10311 archive, @command{tar} will read the original archive from its standard
10312 input, and will write the entire new archive to its standard output.
10314 If the file name contains a @samp{:}, it is interpreted as
10315 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
10316 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
10317 either case, @command{tar} will invoke the command @command{rsh} (or
10318 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
10319 machine. If you give an alternate login name, it will be given to the
10321 Naturally, the remote machine must have an executable
10322 @command{/usr/libexec/rmt}. This program is free software from the
10323 University of California, and a copy of the source code can be found
10324 with the sources for @command{tar}; it's compiled and installed by default.
10325 The exact path to this utility is determined when configuring the package.
10326 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
10327 your installation prefix. This location may also be overridden at
10328 runtime by using the @option{--rmt-command=@var{command}} option (@xref{Option Summary,
10329 ---rmt-command}, for detailed description of this option. @xref{Remote
10330 Tape Server}, for the description of @command{rmt} command).
10332 If this option is not given, but the environment variable @env{TAPE}
10333 is set, its value is used; otherwise, old versions of @command{tar}
10334 used a default archive name (which was picked when @command{tar} was
10335 compiled). The default is normally set up to be the @dfn{first} tape
10336 drive or other transportable I/O medium on the system.
10338 Starting with version 1.11.5, @GNUTAR{} uses
10339 standard input and standard output as the default device, and I will
10340 not try anymore supporting automatic device detection at installation
10341 time. This was failing really in too many cases, it was hopeless.
10342 This is now completely left to the installer to override standard
10343 input and standard output for default device, if this seems
10344 preferable. Further, I think @emph{most} actual usages of
10345 @command{tar} are done with pipes or disks, not really tapes,
10346 cartridges or diskettes.
10348 Some users think that using standard input and output is running
10349 after trouble. This could lead to a nasty surprise on your screen if
10350 you forget to specify an output file name---especially if you are going
10351 through a network or terminal server capable of buffering large amounts
10352 of output. We had so many bug reports in that area of configuring
10353 default tapes automatically, and so many contradicting requests, that
10354 we finally consider the problem to be portably intractable. We could
10355 of course use something like @samp{/dev/tape} as a default, but this
10356 is @emph{also} running after various kind of trouble, going from hung
10357 processes to accidental destruction of real tapes. After having seen
10358 all this mess, using standard input and output as a default really
10359 sounds like the only clean choice left, and a very useful one too.
10361 @GNUTAR{} reads and writes archive in records, I
10362 suspect this is the main reason why block devices are preferred over
10363 character devices. Most probably, block devices are more efficient
10364 too. The installer could also check for @samp{DEFTAPE} in
10365 @file{<sys/mtio.h>}.
10368 @xopindex{force-local, short description}
10369 @item --force-local
10370 Archive file is local even if it contains a colon.
10372 @opindex rsh-command
10373 @item --rsh-command=@var{command}
10374 Use remote @var{command} instead of @command{rsh}. This option exists
10375 so that people who use something other than the standard @command{rsh}
10376 (e.g., a Kerberized @command{rsh}) can access a remote device.
10378 When this command is not used, the shell command found when
10379 the @command{tar} program was installed is used instead. This is
10380 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
10381 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
10382 The installer may have overridden this by defining the environment
10383 variable @env{RSH} @emph{at installation time}.
10386 Specify drive and density.
10388 @xopindex{multi-volume, short description}
10390 @itemx --multi-volume
10391 Create/list/extract multi-volume archive.
10393 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
10394 that may be larger than will fit on the medium used to hold it.
10395 @xref{Multi-Volume Archives}.
10397 @xopindex{tape-length, short description}
10399 @itemx --tape-length=@var{size}[@var{suf}]
10400 Change tape after writing @var{size} units of data. Unless @var{suf} is
10401 given, @var{size} is treated as kilobytes, i.e. @samp{@var{size} x
10402 1024} bytes. The following suffixes alter this behavior:
10404 @float Table, size-suffixes
10405 @caption{Size Suffixes}
10406 @multitable @columnfractions 0.2 0.3 0.3
10407 @headitem Suffix @tab Units @tab Byte Equivalent
10408 @item b @tab Blocks @tab @var{size} x 512
10409 @item B @tab Kilobytes @tab @var{size} x 1024
10410 @item c @tab Bytes @tab @var{size}
10411 @item G @tab Gigabytes @tab @var{size} x 1024^3
10412 @item K @tab Kilobytes @tab @var{size} x 1024
10413 @item k @tab Kilobytes @tab @var{size} x 1024
10414 @item M @tab Megabytes @tab @var{size} x 1024^2
10415 @item P @tab Petabytes @tab @var{size} x 1024^5
10416 @item T @tab Terabytes @tab @var{size} x 1024^4
10417 @item w @tab Words @tab @var{size} x 2
10421 This option might be useful when your tape drivers do not properly
10422 detect end of physical tapes. By being slightly conservative on the
10423 maximum tape length, you might avoid the problem entirely.
10425 @xopindex{info-script, short description}
10426 @xopindex{new-volume-script, short description}
10427 @item -F @var{file}
10428 @itemx --info-script=@var{file}
10429 @itemx --new-volume-script=@var{file}
10430 Execute @file{file} at end of each tape. This implies
10431 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
10432 description of this option.
10435 @node Remote Tape Server
10436 @section Remote Tape Server
10438 @cindex remote tape drive
10440 In order to access the tape drive on a remote machine, @command{tar}
10441 uses the remote tape server written at the University of California at
10442 Berkeley. The remote tape server must be installed as
10443 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
10444 want to use. @command{tar} calls @command{rmt} by running an
10445 @command{rsh} or @command{remsh} to the remote machine, optionally
10446 using a different login name if one is supplied.
10448 A copy of the source for the remote tape server is provided. It is
10449 Copyright @copyright{} 1983 by the Regents of the University of
10450 California, but can be freely distributed. It is compiled and
10451 installed by default.
10453 @cindex absolute file names
10454 Unless you use the @option{--absolute-names} (@option{-P}) option,
10455 @GNUTAR{} will not allow you to create an archive that contains
10456 absolute file names (a file name beginning with @samp{/}.) If you try,
10457 @command{tar} will automatically remove the leading @samp{/} from the
10458 file names it stores in the archive. It will also type a warning
10459 message telling you what it is doing.
10461 When reading an archive that was created with a different
10462 @command{tar} program, @GNUTAR{} automatically
10463 extracts entries in the archive which have absolute file names as if
10464 the file names were not absolute. This is an important feature. A
10465 visitor here once gave a @command{tar} tape to an operator to restore;
10466 the operator used Sun @command{tar} instead of @GNUTAR{},
10467 and the result was that it replaced large portions of
10468 our @file{/bin} and friends with versions from the tape; needless to
10469 say, we were unhappy about having to recover the file system from
10472 For example, if the archive contained a file @file{/usr/bin/computoy},
10473 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
10474 relative to the current directory. If you want to extract the files in
10475 an archive to the same absolute names that they had when the archive
10476 was created, you should do a @samp{cd /} before extracting the files
10477 from the archive, or you should either use the @option{--absolute-names}
10478 option, or use the command @samp{tar -C / @dots{}}.
10480 @cindex Ultrix 3.1 and write failure
10481 Some versions of Unix (Ultrix 3.1 is known to have this problem),
10482 can claim that a short write near the end of a tape succeeded,
10483 when it actually failed. This will result in the -M option not
10484 working correctly. The best workaround at the moment is to use a
10485 significantly larger blocking factor than the default 20.
10487 In order to update an archive, @command{tar} must be able to backspace the
10488 archive in order to reread or rewrite a record that was just read (or
10489 written). This is currently possible only on two kinds of files: normal
10490 disk files (or any other file that can be backspaced with @samp{lseek}),
10491 and industry-standard 9-track magnetic tape (or any other kind of tape
10492 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}).
10494 This means that the @option{--append}, @option{--concatenate}, and
10495 @option{--delete} commands will not work on any other kind of file.
10496 Some media simply cannot be backspaced, which means these commands and
10497 options will never be able to work on them. These non-backspacing
10498 media include pipes and cartridge tape drives.
10500 Some other media can be backspaced, and @command{tar} will work on them
10501 once @command{tar} is modified to do so.
10503 Archives created with the @option{--multi-volume}, @option{--label}, and
10504 @option{--incremental} (@option{-G}) options may not be readable by other version
10505 of @command{tar}. In particular, restoring a file that was split over
10506 a volume boundary will require some careful work with @command{dd}, if
10507 it can be done at all. Other versions of @command{tar} may also create
10508 an empty file whose name is that of the volume header. Some versions
10509 of @command{tar} may create normal files instead of directories archived
10510 with the @option{--incremental} (@option{-G}) option.
10512 @node Common Problems and Solutions
10513 @section Some Common Problems and their Solutions
10518 errors from system:
10520 no such file or directory
10523 errors from @command{tar}:
10524 directory checksum error
10525 header format error
10527 errors from media/system:
10539 @dfn{Block} and @dfn{record} terminology is rather confused, and it
10540 is also confusing to the expert reader. On the other hand, readers
10541 who are new to the field have a fresh mind, and they may safely skip
10542 the next two paragraphs, as the remainder of this manual uses those
10543 two terms in a quite consistent way.
10545 John Gilmore, the writer of the public domain @command{tar} from which
10546 @GNUTAR{} was originally derived, wrote (June 1995):
10549 The nomenclature of tape drives comes from IBM, where I believe
10550 they were invented for the IBM 650 or so. On IBM mainframes, what
10551 is recorded on tape are tape blocks. The logical organization of
10552 data is into records. There are various ways of putting records into
10553 blocks, including @code{F} (fixed sized records), @code{V} (variable
10554 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
10555 to a block), @code{VB} (variable size records, @var{n} to a block),
10556 @code{VSB} (variable spanned blocked: variable sized records that can
10557 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
10558 parameter specified this to the operating system.
10560 The Unix man page on @command{tar} was totally confused about this.
10561 When I wrote @code{PD TAR}, I used the historically correct terminology
10562 (@command{tar} writes data records, which are grouped into blocks).
10563 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
10564 here), and now Fran@,{c}ois has migrated that terminology back
10565 into the source code too.
10568 The term @dfn{physical block} means the basic transfer chunk from or
10569 to a device, after which reading or writing may stop without anything
10570 being lost. In this manual, the term @dfn{block} usually refers to
10571 a disk physical block, @emph{assuming} that each disk block is 512
10572 bytes in length. It is true that some disk devices have different
10573 physical blocks, but @command{tar} ignore these differences in its own
10574 format, which is meant to be portable, so a @command{tar} block is always
10575 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
10576 The term @dfn{logical block} often represents the basic chunk of
10577 allocation of many disk blocks as a single entity, which the operating
10578 system treats somewhat atomically; this concept is only barely used
10581 The term @dfn{physical record} is another way to speak of a physical
10582 block, those two terms are somewhat interchangeable. In this manual,
10583 the term @dfn{record} usually refers to a tape physical block,
10584 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
10585 It is true that archives may be put on disk or used with pipes,
10586 but nevertheless, @command{tar} tries to read and write the archive one
10587 @dfn{record} at a time, whatever the medium in use. One record is made
10588 up of an integral number of blocks, and this operation of putting many
10589 disk blocks into a single tape block is called @dfn{reblocking}, or
10590 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
10591 the logical organization of many characters into something meaningful
10592 to the application. The term @dfn{unit record} describes a small set
10593 of characters which are transmitted whole to or by the application,
10594 and often refers to a line of text. Those two last terms are unrelated
10595 to what we call a @dfn{record} in @GNUTAR{}.
10597 When writing to tapes, @command{tar} writes the contents of the archive
10598 in chunks known as @dfn{records}. To change the default blocking
10599 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
10600 @var{512-size}}) option. Each record will then be composed of
10601 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
10602 @xref{Standard}.) Each file written to the archive uses at least one
10603 full record. As a result, using a larger record size can result in
10604 more wasted space for small files. On the other hand, a larger record
10605 size can often be read and written much more efficiently.
10607 Further complicating the problem is that some tape drives ignore the
10608 blocking entirely. For these, a larger record size can still improve
10609 performance (because the software layers above the tape drive still
10610 honor the blocking), but not as dramatically as on tape drives that
10613 When reading an archive, @command{tar} can usually figure out the
10614 record size on itself. When this is the case, and a non-standard
10615 record size was used when the archive was created, @command{tar} will
10616 print a message about a non-standard blocking factor, and then operate
10617 normally. On some tape devices, however, @command{tar} cannot figure
10618 out the record size itself. On most of those, you can specify a
10619 blocking factor (with @option{--blocking-factor}) larger than the
10620 actual blocking factor, and then use the @option{--read-full-records}
10621 (@option{-B}) option. (If you specify a blocking factor with
10622 @option{--blocking-factor} and don't use the
10623 @option{--read-full-records} option, then @command{tar} will not
10624 attempt to figure out the recording size itself.) On some devices,
10625 you must always specify the record size exactly with
10626 @option{--blocking-factor} when reading, because @command{tar} cannot
10627 figure it out. In any case, use @option{--list} (@option{-t}) before
10628 doing any extractions to see whether @command{tar} is reading the archive
10631 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
10632 putting them into records is to put a whole number of them (one or
10633 more) into each record. @command{tar} records are all the same size;
10634 at the end of the file there's a block containing all zeros, which
10635 is how you tell that the remainder of the last record(s) are garbage.
10637 In a standard @command{tar} file (no options), the block size is 512
10638 and the record size is 10240, for a blocking factor of 20. What the
10639 @option{--blocking-factor} option does is sets the blocking factor,
10640 changing the record size while leaving the block size at 512 bytes.
10641 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
10642 most tape drives these days prefer much bigger records in order to
10643 stream and not waste tape. When writing tapes for myself, some tend
10644 to use a factor of the order of 2048, say, giving a record size of
10645 around one megabyte.
10647 If you use a blocking factor larger than 20, older @command{tar}
10648 programs might not be able to read the archive, so we recommend this
10649 as a limit to use in practice. @GNUTAR{}, however,
10650 will support arbitrarily large record sizes, limited only by the
10651 amount of virtual memory or the physical characteristics of the tape
10655 * Format Variations:: Format Variations
10656 * Blocking Factor:: The Blocking Factor of an Archive
10659 @node Format Variations
10660 @subsection Format Variations
10661 @cindex Format Parameters
10662 @cindex Format Options
10663 @cindex Options, archive format specifying
10664 @cindex Options, format specifying
10667 Format parameters specify how an archive is written on the archive
10668 media. The best choice of format parameters will vary depending on
10669 the type and number of files being archived, and on the media used to
10672 To specify format parameters when accessing or creating an archive,
10673 you can use the options described in the following sections.
10674 If you do not specify any format parameters, @command{tar} uses
10675 default parameters. You cannot modify a compressed archive.
10676 If you create an archive with the @option{--blocking-factor} option
10677 specified (@pxref{Blocking Factor}), you must specify that
10678 blocking-factor when operating on the archive. @xref{Formats}, for other
10679 examples of format parameter considerations.
10681 @node Blocking Factor
10682 @subsection The Blocking Factor of an Archive
10683 @cindex Blocking Factor
10684 @cindex Record Size
10685 @cindex Number of blocks per record
10686 @cindex Number of bytes per record
10687 @cindex Bytes per record
10688 @cindex Blocks per record
10691 @opindex blocking-factor
10692 The data in an archive is grouped into blocks, which are 512 bytes.
10693 Blocks are read and written in whole number multiples called
10694 @dfn{records}. The number of blocks in a record (i.e., the size of a
10695 record in units of 512 bytes) is called the @dfn{blocking factor}.
10696 The @option{--blocking-factor=@var{512-size}} (@option{-b
10697 @var{512-size}}) option specifies the blocking factor of an archive.
10698 The default blocking factor is typically 20 (i.e., 10240 bytes), but
10699 can be specified at installation. To find out the blocking factor of
10700 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
10701 This may not work on some devices.
10703 Records are separated by gaps, which waste space on the archive media.
10704 If you are archiving on magnetic tape, using a larger blocking factor
10705 (and therefore larger records) provides faster throughput and allows you
10706 to fit more data on a tape (because there are fewer gaps). If you are
10707 archiving on cartridge, a very large blocking factor (say 126 or more)
10708 greatly increases performance. A smaller blocking factor, on the other
10709 hand, may be useful when archiving small files, to avoid archiving lots
10710 of nulls as @command{tar} fills out the archive to the end of the record.
10711 In general, the ideal record size depends on the size of the
10712 inter-record gaps on the tape you are using, and the average size of the
10713 files you are archiving. @xref{create}, for information on
10716 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
10718 Archives with blocking factors larger than 20 cannot be read
10719 by very old versions of @command{tar}, or by some newer versions
10720 of @command{tar} running on old machines with small address spaces.
10721 With @GNUTAR{}, the blocking factor of an archive is limited
10722 only by the maximum record size of the device containing the archive,
10723 or by the amount of available virtual memory.
10725 Also, on some systems, not using adequate blocking factors, as sometimes
10726 imposed by the device drivers, may yield unexpected diagnostics. For
10727 example, this has been reported:
10730 Cannot write to /dev/dlt: Invalid argument
10734 In such cases, it sometimes happen that the @command{tar} bundled by
10735 the system is aware of block size idiosyncrasies, while @GNUTAR{}
10736 requires an explicit specification for the block size,
10737 which it cannot guess. This yields some people to consider
10738 @GNUTAR{} is misbehaving, because by comparison,
10739 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
10740 for example, might resolve the problem.
10742 If you use a non-default blocking factor when you create an archive, you
10743 must specify the same blocking factor when you modify that archive. Some
10744 archive devices will also require you to specify the blocking factor when
10745 reading that archive, however this is not typically the case. Usually, you
10746 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
10747 reports a non-default record size and then lists the archive members as
10748 it would normally. To extract files from an archive with a non-standard
10749 blocking factor (particularly if you're not sure what the blocking factor
10750 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
10751 specifying a blocking factor larger then the blocking factor of the archive
10752 (i.e., @samp{tar --extract --read-full-records --blocking-factor=300}).
10753 @xref{list}, for more information on the @option{--list} (@option{-t})
10754 operation. @xref{Reading}, for a more detailed explanation of that option.
10757 @item --blocking-factor=@var{number}
10758 @itemx -b @var{number}
10759 Specifies the blocking factor of an archive. Can be used with any
10760 operation, but is usually not necessary with @option{--list} (@option{-t}).
10766 @item -b @var{blocks}
10767 @itemx --blocking-factor=@var{blocks}
10768 Set record size to @math{@var{blocks}*512} bytes.
10770 This option is used to specify a @dfn{blocking factor} for the archive.
10771 When reading or writing the archive, @command{tar}, will do reads and writes
10772 of the archive in records of @math{@var{block}*512} bytes. This is true
10773 even when the archive is compressed. Some devices requires that all
10774 write operations be a multiple of a certain size, and so, @command{tar}
10775 pads the archive out to the next record boundary.
10777 The default blocking factor is set when @command{tar} is compiled, and is
10778 typically 20. Blocking factors larger than 20 cannot be read by very
10779 old versions of @command{tar}, or by some newer versions of @command{tar}
10780 running on old machines with small address spaces.
10782 With a magnetic tape, larger records give faster throughput and fit
10783 more data on a tape (because there are fewer inter-record gaps).
10784 If the archive is in a disk file or a pipe, you may want to specify
10785 a smaller blocking factor, since a large one will result in a large
10786 number of null bytes at the end of the archive.
10788 When writing cartridge or other streaming tapes, a much larger
10789 blocking factor (say 126 or more) will greatly increase performance.
10790 However, you must specify the same blocking factor when reading or
10791 updating the archive.
10793 Apparently, Exabyte drives have a physical block size of 8K bytes.
10794 If we choose our blocksize as a multiple of 8k bytes, then the problem
10795 seems to disappear. Id est, we are using block size of 112 right
10796 now, and we haven't had the problem since we switched@dots{}
10798 With @GNUTAR{} the blocking factor is limited only
10799 by the maximum record size of the device containing the archive, or by
10800 the amount of available virtual memory.
10802 However, deblocking or reblocking is virtually avoided in a special
10803 case which often occurs in practice, but which requires all the
10804 following conditions to be simultaneously true:
10807 the archive is subject to a compression option,
10809 the archive is not handled through standard input or output, nor
10810 redirected nor piped,
10812 the archive is directly handled to a local disk, instead of any special
10815 @option{--blocking-factor} is not explicitly specified on the @command{tar}
10819 If the output goes directly to a local disk, and not through
10820 stdout, then the last write is not extended to a full record size.
10821 Otherwise, reblocking occurs. Here are a few other remarks on this
10827 @command{gzip} will complain about trailing garbage if asked to
10828 uncompress a compressed archive on tape, there is an option to turn
10829 the message off, but it breaks the regularity of simply having to use
10830 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
10831 silently ignoring any number of trailing zeros. I'll ask Jean-loup
10832 Gailly, by sending a copy of this message to him.
10835 @command{compress} does not show this problem, but as Jean-loup pointed
10836 out to Michael, @samp{compress -d} silently adds garbage after
10837 the result of decompression, which tar ignores because it already
10838 recognized its end-of-file indicator. So this bug may be safely
10842 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
10843 but will still return an exit status of 2 which tar reports in turn.
10844 @command{tar} might ignore the exit status returned, but I hate doing
10845 that, as it weakens the protection @command{tar} offers users against
10846 other possible problems at decompression time. If @command{gzip} was
10847 silently skipping trailing zeros @emph{and} also avoiding setting the
10848 exit status in this innocuous case, that would solve this situation.
10851 @command{tar} should become more solid at not stopping to read a pipe at
10852 the first null block encountered. This inelegantly breaks the pipe.
10853 @command{tar} should rather drain the pipe out before exiting itself.
10856 @xopindex{ignore-zeros, short description}
10858 @itemx --ignore-zeros
10859 Ignore blocks of zeros in archive (means EOF).
10861 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
10862 of zeros in the archive. Normally a block of zeros indicates the
10863 end of the archive, but when reading a damaged archive, or one which
10864 was created by concatenating several archives together, this option
10865 allows @command{tar} to read the entire archive. This option is not on
10866 by default because many versions of @command{tar} write garbage after
10869 Note that this option causes @command{tar} to read to the end of the
10870 archive file, which may sometimes avoid problems when multiple files
10871 are stored on a single physical tape.
10873 @xopindex{read-full-records, short description}
10875 @itemx --read-full-records
10876 Reblock as we read (for reading 4.2@acronym{BSD} pipes).
10878 If @option{--read-full-records} is used, @command{tar}
10879 will not panic if an attempt to read a record from the archive does
10880 not return a full record. Instead, @command{tar} will keep reading
10881 until it has obtained a full
10884 This option is turned on by default when @command{tar} is reading
10885 an archive from standard input, or from a remote machine. This is
10886 because on @acronym{BSD} Unix systems, a read of a pipe will return however
10887 much happens to be in the pipe, even if it is less than @command{tar}
10888 requested. If this option was not used, @command{tar} would fail as
10889 soon as it read an incomplete record from the pipe.
10891 This option is also useful with the commands for updating an archive.
10897 @FIXME{Appropriate options should be moved here from elsewhere.}
10899 @cindex blocking factor
10900 @cindex tape blocking
10902 When handling various tapes or cartridges, you have to take care of
10903 selecting a proper blocking, that is, the number of disk blocks you
10904 put together as a single tape block on the tape, without intervening
10905 tape gaps. A @dfn{tape gap} is a small landing area on the tape
10906 with no information on it, used for decelerating the tape to a
10907 full stop, and for later regaining the reading or writing speed.
10908 When the tape driver starts reading a record, the record has to
10909 be read whole without stopping, as a tape gap is needed to stop the
10910 tape motion without losing information.
10912 @cindex Exabyte blocking
10913 @cindex DAT blocking
10914 Using higher blocking (putting more disk blocks per tape block) will use
10915 the tape more efficiently as there will be less tape gaps. But reading
10916 such tapes may be more difficult for the system, as more memory will be
10917 required to receive at once the whole record. Further, if there is a
10918 reading error on a huge record, this is less likely that the system will
10919 succeed in recovering the information. So, blocking should not be too
10920 low, nor it should be too high. @command{tar} uses by default a blocking of
10921 20 for historical reasons, and it does not really matter when reading or
10922 writing to disk. Current tape technology would easily accommodate higher
10923 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
10924 We were told that for some DLT drives, the blocking should be a multiple
10925 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
10926 Other manufacturers may use different recommendations for the same tapes.
10927 This might also depends of the buffering techniques used inside modern
10928 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
10929 Others request blocking to be some exponent of two.
10931 So, there is no fixed rule for blocking. But blocking at read time
10932 should ideally be the same as blocking used at write time. At one place
10933 I know, with a wide variety of equipment, they found it best to use a
10934 blocking of 32 to guarantee that their tapes are fully interchangeable.
10936 I was also told that, for recycled tapes, prior erasure (by the same
10937 drive unit that will be used to create the archives) sometimes lowers
10938 the error rates observed at rewriting time.
10940 I might also use @option{--number-blocks} instead of
10941 @option{--block-number}, so @option{--block} will then expand to
10942 @option{--blocking-factor} unambiguously.
10945 @section Many Archives on One Tape
10947 @FIXME{Appropriate options should be moved here from elsewhere.}
10949 @findex ntape @r{device}
10950 Most tape devices have two entries in the @file{/dev} directory, or
10951 entries that come in pairs, which differ only in the minor number for
10952 this device. Let's take for example @file{/dev/tape}, which often
10953 points to the only or usual tape device of a given system. There might
10954 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
10955 name is the @emph{rewinding} version of the device, while the name
10956 having @samp{nr} in it is the @emph{no rewinding} version of the same
10959 A rewinding tape device will bring back the tape to its beginning point
10960 automatically when this device is opened or closed. Since @command{tar}
10961 opens the archive file before using it and closes it afterwards, this
10962 means that a simple:
10965 $ @kbd{tar cf /dev/tape @var{directory}}
10969 will reposition the tape to its beginning both prior and after saving
10970 @var{directory} contents to it, thus erasing prior tape contents and
10971 making it so that any subsequent write operation will destroy what has
10974 @cindex tape positioning
10975 So, a rewinding device is normally meant to hold one and only one file.
10976 If you want to put more than one @command{tar} archive on a given tape, you
10977 will need to avoid using the rewinding version of the tape device. You
10978 will also have to pay special attention to tape positioning. Errors in
10979 positioning may overwrite the valuable data already on your tape. Many
10980 people, burnt by past experiences, will only use rewinding devices and
10981 limit themselves to one file per tape, precisely to avoid the risk of
10982 such errors. Be fully aware that writing at the wrong position on a
10983 tape loses all information past this point and most probably until the
10984 end of the tape, and this destroyed information @emph{cannot} be
10987 To save @var{directory-1} as a first archive at the beginning of a
10988 tape, and leave that tape ready for a second archive, you should use:
10991 $ @kbd{mt -f /dev/nrtape rewind}
10992 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
10996 @dfn{Tape marks} are special magnetic patterns written on the tape
10997 media, which are later recognizable by the reading hardware. These
10998 marks are used after each file, when there are many on a single tape.
10999 An empty file (that is to say, two tape marks in a row) signal the
11000 logical end of the tape, after which no file exist. Usually,
11001 non-rewinding tape device drivers will react to the close request issued
11002 by @command{tar} by first writing two tape marks after your archive, and by
11003 backspacing over one of these. So, if you remove the tape at that time
11004 from the tape drive, it is properly terminated. But if you write
11005 another file at the current position, the second tape mark will be
11006 erased by the new information, leaving only one tape mark between files.
11008 So, you may now save @var{directory-2} as a second archive after the
11009 first on the same tape by issuing the command:
11012 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
11016 and so on for all the archives you want to put on the same tape.
11018 Another usual case is that you do not write all the archives the same
11019 day, and you need to remove and store the tape between two archive
11020 sessions. In general, you must remember how many files are already
11021 saved on your tape. Suppose your tape already has 16 files on it, and
11022 that you are ready to write the 17th. You have to take care of skipping
11023 the first 16 tape marks before saving @var{directory-17}, say, by using
11027 $ @kbd{mt -f /dev/nrtape rewind}
11028 $ @kbd{mt -f /dev/nrtape fsf 16}
11029 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
11032 In all the previous examples, we put aside blocking considerations, but
11033 you should do the proper things for that as well. @xref{Blocking}.
11036 * Tape Positioning:: Tape Positions and Tape Marks
11037 * mt:: The @command{mt} Utility
11040 @node Tape Positioning
11041 @subsection Tape Positions and Tape Marks
11044 Just as archives can store more than one file from the file system,
11045 tapes can store more than one archive file. To keep track of where
11046 archive files (or any other type of file stored on tape) begin and
11047 end, tape archive devices write magnetic @dfn{tape marks} on the
11048 archive media. Tape drives write one tape mark between files,
11049 two at the end of all the file entries.
11051 If you think of data as a series of records "rrrr"'s, and tape marks as
11052 "*"'s, a tape might look like the following:
11055 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
11058 Tape devices read and write tapes using a read/write @dfn{tape
11059 head}---a physical part of the device which can only access one
11060 point on the tape at a time. When you use @command{tar} to read or
11061 write archive data from a tape device, the device will begin reading
11062 or writing from wherever on the tape the tape head happens to be,
11063 regardless of which archive or what part of the archive the tape
11064 head is on. Before writing an archive, you should make sure that no
11065 data on the tape will be overwritten (unless it is no longer needed).
11066 Before reading an archive, you should make sure the tape head is at
11067 the beginning of the archive you want to read. You can do it manually
11068 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
11069 that automatically (@pxref{Scripted Restoration}).
11071 If you want to add new archive file entries to a tape, you should
11072 advance the tape to the end of the existing file entries, backspace
11073 over the last tape mark, and write the new archive file. If you were
11074 to add two archives to the example above, the tape might look like the
11078 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
11082 @subsection The @command{mt} Utility
11085 @FIXME{Is it true that this only works on non-block devices?
11086 should explain the difference, (fixed or variable).}
11087 @xref{Blocking Factor}.
11089 You can use the @command{mt} utility to advance or rewind a tape past a
11090 specified number of archive files on the tape. This will allow you
11091 to move to the beginning of an archive before extracting or reading
11092 it, or to the end of all the archives before writing a new one.
11093 @FIXME{Why isn't there an "advance 'til you find two tape marks
11096 The syntax of the @command{mt} command is:
11099 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
11102 where @var{tapename} is the name of the tape device, @var{number} is
11103 the number of times an operation is performed (with a default of one),
11104 and @var{operation} is one of the following:
11106 @FIXME{is there any use for record operations?}
11111 Writes @var{number} tape marks at the current position on the tape.
11114 Moves tape position forward @var{number} files.
11117 Moves tape position back @var{number} files.
11120 Rewinds the tape. (Ignores @var{number}.)
11124 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}.)
11127 Prints status information about the tape unit.
11131 If you don't specify a @var{tapename}, @command{mt} uses the environment
11132 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
11133 the default device specified in your @file{sys/mtio.h} file
11134 (@code{DEFTAPE} variable). If this is not defined, the program will
11135 display a descriptive error message and exit with code 1.
11137 @command{mt} returns a 0 exit status when the operation(s) were
11138 successful, 1 if the command was unrecognized, and 2 if an operation
11141 @node Using Multiple Tapes
11142 @section Using Multiple Tapes
11144 Often you might want to write a large archive, one larger than will fit
11145 on the actual tape you are using. In such a case, you can run multiple
11146 @command{tar} commands, but this can be inconvenient, particularly if you
11147 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
11148 Therefore, @command{tar} provides a special mode for creating
11149 multi-volume archives.
11151 @dfn{Multi-volume} archive is a single @command{tar} archive, stored
11152 on several media volumes of fixed size. Although in this section we will
11153 often call @samp{volume} a @dfn{tape}, there is absolutely no
11154 requirement for multi-volume archives to be stored on tapes. Instead,
11155 they can use whatever media type the user finds convenient, they can
11156 even be located on files.
11158 When creating a multi-volume archive, @GNUTAR{} continues to fill
11159 current volume until it runs out of space, then it switches to
11160 next volume (usually the operator is queried to replace the tape on
11161 this point), and continues working on the new volume. This operation
11162 continues until all requested files are dumped. If @GNUTAR{} detects
11163 end of media while dumping a file, such a file is archived in split
11164 form. Some very big files can even be split across several volumes.
11166 Each volume is itself a valid @GNUTAR{} archive, so it can be read
11167 without any special options. Consequently any file member residing
11168 entirely on one volume can be extracted or otherwise operated upon
11169 without needing the other volume. Sure enough, to extract a split
11170 member you would need all volumes its parts reside on.
11172 Multi-volume archives suffer from several limitations. In particular,
11173 they cannot be compressed.
11175 @GNUTAR{} is able to create multi-volume archives of two formats
11176 (@pxref{Formats}): @samp{GNU} and @samp{POSIX}.
11179 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
11180 * Tape Files:: Tape Files
11181 * Tarcat:: Concatenate Volumes into a Single Archive
11185 @node Multi-Volume Archives
11186 @subsection Archives Longer than One Tape or Disk
11187 @cindex Multi-volume archives
11189 @opindex multi-volume
11190 To create an archive that is larger than will fit on a single unit of
11191 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
11192 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
11193 archive can be manipulated like any other archive (provided the
11194 @option{--multi-volume} option is specified), but is stored on more
11195 than one tape or file.
11197 When you specify @option{--multi-volume}, @command{tar} does not report an
11198 error when it comes to the end of an archive volume (when reading), or
11199 the end of the media (when writing). Instead, it prompts you to load
11200 a new storage volume. If the archive is on a magnetic tape, you
11201 should change tapes when you see the prompt; if the archive is on a
11202 floppy disk, you should change disks; etc.
11205 @item --multi-volume
11207 Creates a multi-volume archive, when used in conjunction with
11208 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
11209 archive, specify @option{--multi-volume} in conjunction with that
11214 $ @kbd{tar --create --multi-volume --file=/dev/tape @var{files}}
11218 The method @command{tar} uses to detect end of tape is not perfect, and
11219 fails on some operating systems or on some devices. If @command{tar}
11220 cannot detect the end of the tape itself, you can use
11221 @option{--tape-length} option to inform it about the capacity of the
11224 @anchor{tape-length}
11226 @opindex tape-length
11227 @item --tape-length=@var{size}[@var{suf}]
11228 @itemx -L @var{size}[@var{suf}]
11229 Set maximum length of a volume. The @var{suf}, if given, specifies
11230 units in which @var{size} is expressed, e.g. @samp{2M} mean 2
11231 megabytes (@pxref{size-suffixes}, for a list of allowed size
11232 suffixes). Without @var{suf}, units of 1024 bytes (kilobyte) are
11235 This option selects @option{--multi-volume} automatically. For example:
11238 $ @kbd{tar --create --tape-length=41943040 --file=/dev/tape @var{files}}
11242 or, which is equivalent:
11245 $ @kbd{tar --create --tape-length=4G --file=/dev/tape @var{files}}
11249 @anchor{change volume prompt}
11250 When @GNUTAR{} comes to the end of a storage media, it asks you to
11251 change the volume. The built-in prompt for POSIX locale
11252 is@footnote{If you run @GNUTAR{} under a different locale, the
11253 translation to the locale's language will be used.}:
11256 Prepare volume #@var{n} for `@var{archive}' and hit return:
11260 where @var{n} is the ordinal number of the volume to be created and
11261 @var{archive} is archive file or device name.
11263 When prompting for a new tape, @command{tar} accepts any of the following
11268 Request @command{tar} to explain possible responses.
11270 Request @command{tar} to exit immediately.
11271 @item n @var{file-name}
11272 Request @command{tar} to write the next volume on the file @var{file-name}.
11274 Request @command{tar} to run a subshell. This option can be disabled
11275 by giving @option{--restrict} command line option to
11276 @command{tar}@footnote{@xref{--restrict}, for more information about
11279 Request @command{tar} to begin writing the next volume.
11282 (You should only type @samp{y} after you have changed the tape;
11283 otherwise @command{tar} will write over the volume it just finished.)
11285 @cindex Volume number file
11287 @anchor{volno-file}
11288 @opindex volno-file
11289 The volume number used by @command{tar} in its tape-changing prompt
11290 can be changed; if you give the
11291 @option{--volno-file=@var{file-of-number}} option, then
11292 @var{file-of-number} should be an non-existing file to be created, or
11293 else, a file already containing a decimal number. That number will be
11294 used as the volume number of the first volume written. When
11295 @command{tar} is finished, it will rewrite the file with the
11296 now-current volume number. (This does not change the volume number
11297 written on a tape label, as per @ref{label}, it @emph{only} affects
11298 the number used in the prompt.)
11300 @cindex End-of-archive info script
11301 @cindex Info script
11302 @anchor{info-script}
11303 @opindex info-script
11304 @opindex new-volume-script
11305 If you want more elaborate behavior than this, you can write a special
11306 @dfn{new volume script}, that will be responsible for changing the
11307 volume, and instruct @command{tar} to use it instead of its normal
11308 prompting procedure:
11311 @item --info-script=@var{script-name}
11312 @itemx --new-volume-script=@var{script-name}
11313 @itemx -F @var{script-name}
11314 Specify the full name of the volume script to use. The script can be
11315 used to eject cassettes, or to broadcast messages such as
11316 @samp{Someone please come change my tape} when performing unattended
11320 The @var{script-name} is executed without any command line
11321 arguments. It inherits @command{tar}'s shell environment.
11322 Additional data is passed to it via the following
11323 environment variables:
11326 @vrindex TAR_VERSION, info script environment variable
11328 @GNUTAR{} version number.
11330 @vrindex TAR_ARCHIVE, info script environment variable
11332 The name of the archive @command{tar} is processing.
11334 @vrindex TAR_BLOCKING_FACTOR, info script environment variable
11335 @item TAR_BLOCKING_FACTOR
11336 Current blocking factor (@pxref{Blocking}).
11338 @vrindex TAR_VOLUME, info script environment variable
11340 Ordinal number of the volume @command{tar} is about to start.
11342 @vrindex TAR_SUBCOMMAND, info script environment variable
11343 @item TAR_SUBCOMMAND
11344 A short option describing the operation @command{tar} is executing.
11345 @xref{Operations}, for a complete list of subcommand options.
11347 @vrindex TAR_FORMAT, info script environment variable
11349 Format of the archive being processed. @xref{Formats}, for a complete
11350 list of archive format names.
11352 @vrindex TAR_FD, info script environment variable
11354 File descriptor which can be used to communicate the new volume
11355 name to @command{tar}.
11358 The volume script can instruct @command{tar} to use new archive name,
11359 by writing in to file descriptor @env{$TAR_FD} (see below for an example).
11361 If the info script fails, @command{tar} exits; otherwise, it begins
11362 writing the next volume.
11364 If you want @command{tar} to cycle through a series of files or tape
11365 drives, there are three approaches to choose from. First of all, you
11366 can give @command{tar} multiple @option{--file} options. In this case
11367 the specified files will be used, in sequence, as the successive
11368 volumes of the archive. Only when the first one in the sequence needs
11369 to be used again will @command{tar} prompt for a tape change (or run
11370 the info script). For example, suppose someone has two tape drives on
11371 a system named @file{/dev/tape0} and @file{/dev/tape1}. For having
11372 @GNUTAR{} to switch to the second drive when it needs to write the
11373 second tape, and then back to the first tape, etc., just do either of:
11376 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
11377 $ @kbd{tar -cM -f /dev/tape0 -f /dev/tape1 @var{files}}
11380 The second method is to use the @samp{n} response to the tape-change
11383 Finally, the most flexible approach is to use a volume script, that
11384 writes new archive name to the file descriptor @env{$TAR_FD}. For example, the
11385 following volume script will create a series of archive files, named
11386 @file{@var{archive}-@var{vol}}, where @var{archive} is the name of the
11387 archive being created (as given by @option{--file} option) and
11388 @var{vol} is the ordinal number of the archive being created:
11393 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
11395 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
11396 case $TAR_SUBCOMMAND in
11398 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
11403 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&$TAR_FD
11407 The same script can be used while listing, comparing or extracting
11408 from the created archive. For example:
11412 # @r{Create a multi-volume archive:}
11413 $ @kbd{tar -c -L1024 -f archive.tar -F new-volume .}
11414 # @r{Extract from the created archive:}
11415 $ @kbd{tar -x -f archive.tar -F new-volume .}
11420 Notice, that the first command had to use @option{-L} option, since
11421 otherwise @GNUTAR{} will end up writing everything to file
11422 @file{archive.tar}.
11424 You can read each individual volume of a multi-volume archive as if it
11425 were an archive by itself. For example, to list the contents of one
11426 volume, use @option{--list}, without @option{--multi-volume} specified.
11427 To extract an archive member from one volume (assuming it is described
11428 that volume), use @option{--extract}, again without
11429 @option{--multi-volume}.
11431 If an archive member is split across volumes (i.e., its entry begins on
11432 one volume of the media and ends on another), you need to specify
11433 @option{--multi-volume} to extract it successfully. In this case, you
11434 should load the volume where the archive member starts, and use
11435 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
11436 volumes as it needs them. @xref{extracting archives}, for more
11437 information about extracting archives.
11439 Multi-volume archives can be modified like any other archive. To add
11440 files to a multi-volume archive, you need to only mount the last
11441 volume of the archive media (and new volumes, if needed). For all
11442 other operations, you need to use the entire archive.
11444 If a multi-volume archive was labeled using
11445 @option{--label=@var{archive-label}} (@pxref{label}) when it was
11446 created, @command{tar} will not automatically label volumes which are
11447 added later. To label subsequent volumes, specify
11448 @option{--label=@var{archive-label}} again in conjunction with the
11449 @option{--append}, @option{--update} or @option{--concatenate} operation.
11451 Notice that multi-volume support is a GNU extension and the archives
11452 created in this mode should be read only using @GNUTAR{}. If you
11453 absolutely have to process such archives using a third-party @command{tar}
11454 implementation, read @ref{Split Recovery}.
11457 @subsection Tape Files
11458 @cindex labeling archives
11462 To give the archive a name which will be recorded in it, use the
11463 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
11464 option. This will write a special block identifying
11465 @var{volume-label} as the name of the archive to the front of the
11466 archive which will be displayed when the archive is listed with
11467 @option{--list}. If you are creating a multi-volume archive with
11468 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
11469 volume label will have @samp{Volume @var{nnn}} appended to the name
11470 you give, where @var{nnn} is the number of the volume of the archive.
11471 If you use the @option{--label=@var{volume-label}} option when
11472 reading an archive, it checks to make sure the label on the tape
11473 matches the one you gave. @xref{label}.
11475 When @command{tar} writes an archive to tape, it creates a single
11476 tape file. If multiple archives are written to the same tape, one
11477 after the other, they each get written as separate tape files. When
11478 extracting, it is necessary to position the tape at the right place
11479 before running @command{tar}. To do this, use the @command{mt} command.
11480 For more information on the @command{mt} command and on the organization
11481 of tapes into a sequence of tape files, see @ref{mt}.
11483 People seem to often do:
11486 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
11489 or such, for pushing a common date in all volumes or an archive set.
11492 @subsection Concatenate Volumes into a Single Archive
11495 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
11496 archive to a single @command{tar} archive. Simply concatenating all
11497 volumes into one will not work, since each volume carries an additional
11498 information at the beginning. @GNUTAR{} is shipped with the shell
11499 script @command{tarcat} designed for this purpose.
11501 The script takes a list of files comprising a multi-volume archive
11502 and creates the resulting archive at the standard output. For example:
11505 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
11508 The script implements a simple heuristics to determine the format of
11509 the first volume file and to decide how to process the rest of the
11510 files. However, it makes no attempt to verify whether the files are
11511 given in order or even if they are valid @command{tar} archives.
11512 It uses @command{dd} and does not filter its standard error, so you
11513 will usually see lots of spurious messages.
11515 @FIXME{The script is not installed. Should we install it?}
11518 @section Including a Label in the Archive
11519 @cindex Labeling an archive
11520 @cindex Labels on the archive media
11521 @cindex Labeling multi-volume archives
11524 To avoid problems caused by misplaced paper labels on the archive
11525 media, you can include a @dfn{label} entry --- an archive member which
11526 contains the name of the archive --- in the archive itself. Use the
11527 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11528 option@footnote{Until version 1.10, that option was called
11529 @option{--volume}, but is not available under that name anymore.} in
11530 conjunction with the @option{--create} operation to include a label
11531 entry in the archive as it is being created.
11534 @item --label=@var{archive-label}
11535 @itemx -V @var{archive-label}
11536 Includes an @dfn{archive-label} at the beginning of the archive when
11537 the archive is being created, when used in conjunction with the
11538 @option{--create} operation. Checks to make sure the archive label
11539 matches the one specified (when used in conjunction with any other
11543 If you create an archive using both
11544 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
11545 and @option{--multi-volume} (@option{-M}), each volume of the archive
11546 will have an archive label of the form @samp{@var{archive-label}
11547 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
11548 next, and so on. @xref{Using Multiple Tapes}, for information on
11549 creating multiple volume archives.
11551 @cindex Volume label, listing
11552 @cindex Listing volume label
11553 The volume label will be displayed by @option{--list} along with
11554 the file contents. If verbose display is requested, it will also be
11555 explicitly marked as in the example below:
11559 $ @kbd{tar --verbose --list --file=iamanarchive}
11560 V--------- 0/0 0 1992-03-07 12:01 iamalabel--Volume Header--
11561 -rw-r--r-- ringo/user 40 1990-05-21 13:30 iamafilename
11565 @opindex test-label
11566 @anchor{--test-label option}
11567 However, @option{--list} option will cause listing entire
11568 contents of the archive, which may be undesirable (for example, if the
11569 archive is stored on a tape). You can request checking only the volume
11570 label by specifying @option{--test-label} option. This option reads only the
11571 first block of an archive, so it can be used with slow storage
11572 devices. For example:
11576 $ @kbd{tar --test-label --file=iamanarchive}
11581 If @option{--test-label} is used with one or more command line
11582 arguments, @command{tar} compares the volume label with each
11583 argument. It exits with code 0 if a match is found, and with code 1
11584 otherwise@footnote{Note that @GNUTAR{} versions up to 1.23 indicated
11585 mismatch with an exit code 2 and printed a spurious diagnostics on
11586 stderr.}. No output is displayed, unless you also used the
11587 @option{--verbose} option. For example:
11591 $ @kbd{tar --test-label --file=iamanarchive 'iamalabel'}
11593 $ @kbd{tar --test-label --file=iamanarchive 'alabel'}
11598 When used with the @option{--verbose} option, @command{tar}
11599 prints the actual volume label (if any), and a verbose diagnostics in
11600 case of a mismatch:
11604 $ @kbd{tar --test-label --verbose --file=iamanarchive 'iamalabel'}
11607 $ @kbd{tar --test-label --verbose --file=iamanarchive 'alabel'}
11609 tar: Archive label mismatch
11614 If you request any operation, other than @option{--create}, along
11615 with using @option{--label} option, @command{tar} will first check if
11616 the archive label matches the one specified and will refuse to proceed
11617 if it does not. Use this as a safety precaution to avoid accidentally
11618 overwriting existing archives. For example, if you wish to add files
11619 to @file{archive}, presumably labeled with string @samp{My volume},
11624 $ @kbd{tar -rf archive --label 'My volume' .}
11625 tar: Archive not labeled to match `My volume'
11630 in case its label does not match. This will work even if
11631 @file{archive} is not labeled at all.
11633 Similarly, @command{tar} will refuse to list or extract the
11634 archive if its label doesn't match the @var{archive-label}
11635 specified. In those cases, @var{archive-label} argument is interpreted
11636 as a globbing-style pattern which must match the actual magnetic
11637 volume label. @xref{exclude}, for a precise description of how match
11638 is attempted@footnote{Previous versions of @command{tar} used full
11639 regular expression matching, or before that, only exact string
11640 matching, instead of wildcard matchers. We decided for the sake of
11641 simplicity to use a uniform matching device through
11642 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
11643 the volume label matcher will also suffix @var{archive-label} by
11644 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
11645 up. Since the volume numbering is automatically added in labels at
11646 creation time, it sounded logical to equally help the user taking care
11647 of it when the archive is being read.
11649 You can also use @option{--label} to get a common information on
11650 all tapes of a series. For having this information different in each
11651 series created through a single script used on a regular basis, just
11652 manage to get some date string as part of the label. For example:
11656 $ @kbd{tar -cM -f /dev/tape -V "Daily backup for `date +%Y-%m-%d`"}
11657 $ @kbd{tar --create --file=/dev/tape --multi-volume \
11658 --label="Daily backup for `date +%Y-%m-%d`"}
11662 Some more notes about volume labels:
11665 @item Each label has its own date and time, which corresponds
11666 to the time when @GNUTAR{} initially attempted to write it,
11667 often soon after the operator launches @command{tar} or types the
11668 carriage return telling that the next tape is ready.
11670 @item Comparing date labels to get an idea of tape throughput is
11671 unreliable. It gives correct results only if the delays for rewinding
11672 tapes and the operator switching them were negligible, which is
11673 usually not the case.
11677 @section Verifying Data as It is Stored
11678 @cindex Verifying a write operation
11679 @cindex Double-checking a write operation
11684 @opindex verify, short description
11685 Attempt to verify the archive after writing.
11688 This option causes @command{tar} to verify the archive after writing it.
11689 Each volume is checked after it is written, and any discrepancies
11690 are recorded on the standard error output.
11692 Verification requires that the archive be on a back-space-able medium.
11693 This means pipes, some cartridge tape drives, and some other devices
11694 cannot be verified.
11696 You can insure the accuracy of an archive by comparing files in the
11697 system with archive members. @command{tar} can compare an archive to the
11698 file system as the archive is being written, to verify a write
11699 operation, or can compare a previously written archive, to insure that
11702 @xopindex{verify, using with @option{--create}}
11703 @xopindex{create, using with @option{--verify}}
11704 To check for discrepancies in an archive immediately after it is
11705 written, use the @option{--verify} (@option{-W}) option in conjunction with
11706 the @option{--create} operation. When this option is
11707 specified, @command{tar} checks archive members against their counterparts
11708 in the file system, and reports discrepancies on the standard error.
11710 To verify an archive, you must be able to read it from before the end
11711 of the last written entry. This option is useful for detecting data
11712 errors on some tapes. Archives written to pipes, some cartridge tape
11713 drives, and some other devices cannot be verified.
11715 One can explicitly compare an already made archive with the file
11716 system by using the @option{--compare} (@option{--diff}, @option{-d})
11717 option, instead of using the more automatic @option{--verify} option.
11720 Note that these two options have a slightly different intent. The
11721 @option{--compare} option checks how identical are the logical contents of some
11722 archive with what is on your disks, while the @option{--verify} option is
11723 really for checking if the physical contents agree and if the recording
11724 media itself is of dependable quality. So, for the @option{--verify}
11725 operation, @command{tar} tries to defeat all in-memory cache pertaining to
11726 the archive, while it lets the speed optimization undisturbed for the
11727 @option{--compare} option. If you nevertheless use @option{--compare} for
11728 media verification, you may have to defeat the in-memory cache yourself,
11729 maybe by opening and reclosing the door latch of your recording unit,
11730 forcing some doubt in your operating system about the fact this is really
11731 the same volume as the one just written or read.
11733 The @option{--verify} option would not be necessary if drivers were indeed
11734 able to detect dependably all write failures. This sometimes require many
11735 magnetic heads, some able to read after the writes occurred. One would
11736 not say that drivers unable to detect all cases are necessarily flawed,
11737 as long as programming is concerned.
11739 The @option{--verify} (@option{-W}) option will not work in
11740 conjunction with the @option{--multi-volume} (@option{-M}) option or
11741 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
11742 and @option{--delete} operations. @xref{Operations}, for more
11743 information on these operations.
11745 Also, since @command{tar} normally strips leading @samp{/} from file
11746 names (@pxref{absolute}), a command like @samp{tar --verify -cf
11747 /tmp/foo.tar /etc} will work as desired only if the working directory is
11748 @file{/}, as @command{tar} uses the archive's relative member names
11749 (e.g., @file{etc/motd}) when verifying the archive.
11751 @node Write Protection
11752 @section Write Protection
11754 Almost all tapes and diskettes, and in a few rare cases, even disks can
11755 be @dfn{write protected}, to protect data on them from being changed.
11756 Once an archive is written, you should write protect the media to prevent
11757 the archive from being accidentally overwritten or deleted. (This will
11758 protect the archive from being changed with a tape or floppy drive---it
11759 will not protect it from magnet fields or other physical hazards.)
11761 The write protection device itself is usually an integral part of the
11762 physical media, and can be a two position (write enabled/write
11763 disabled) switch, a notch which can be popped out or covered, a ring
11764 which can be removed from the center of a tape reel, or some other
11765 changeable feature.
11767 @node Reliability and security
11768 @chapter Reliability and Security
11770 The @command{tar} command reads and writes files as any other
11771 application does, and is subject to the usual caveats about
11772 reliability and security. This section contains some commonsense
11773 advice on the topic.
11781 @section Reliability
11783 Ideally, when @command{tar} is creating an archive, it reads from a
11784 file system that is not being modified, and encounters no errors or
11785 inconsistencies while reading and writing. If this is the case, the
11786 archive should faithfully reflect what was read. Similarly, when
11787 extracting from an archive, ideally @command{tar} ideally encounters
11788 no errors and the extracted files faithfully reflect what was in the
11791 However, when reading or writing real-world file systems, several
11792 things can go wrong; these include permissions problems, corruption of
11793 data, and race conditions.
11796 * Permissions problems::
11797 * Data corruption and repair::
11798 * Race conditions::
11801 @node Permissions problems
11802 @subsection Permissions Problems
11804 If @command{tar} encounters errors while reading or writing files, it
11805 normally reports an error and exits with nonzero status. The work it
11806 does may therefore be incomplete. For example, when creating an
11807 archive, if @command{tar} cannot read a file then it cannot copy the
11808 file into the archive.
11810 @node Data corruption and repair
11811 @subsection Data Corruption and Repair
11813 If an archive becomes corrupted by an I/O error, this may corrupt the
11814 data in an extracted file. Worse, it may corrupt the file's metadata,
11815 which may cause later parts of the archive to become misinterpreted.
11816 An tar-format archive contains a checksum that most likely will detect
11817 errors in the metadata, but it will not detect errors in the data.
11819 If data corruption is a concern, you can compute and check your own
11820 checksums of an archive by using other programs, such as
11823 When attempting to recover from a read error or data corruption in an
11824 archive, you may need to skip past the questionable data and read the
11825 rest of the archive. This requires some expertise in the archive
11826 format and in other software tools.
11828 @node Race conditions
11829 @subsection Race conditions
11831 If some other process is modifying the file system while @command{tar}
11832 is reading or writing files, the result may well be inconsistent due
11833 to race conditions. For example, if another process creates some
11834 files in a directory while @command{tar} is creating an archive
11835 containing the directory's files, @command{tar} may see some of the
11836 files but not others, or it may see a file that is in the process of
11837 being created. The resulting archive may not be a snapshot of the
11838 file system at any point in time. If an application such as a
11839 database system depends on an accurate snapshot, restoring from the
11840 @command{tar} archive of a live file system may therefore break that
11841 consistency and may break the application. The simplest way to avoid
11842 the consistency issues is to avoid making other changes to the file
11843 system while tar is reading it or writing it.
11845 When creating an archive, several options are available to avoid race
11846 conditions. Some hosts have a way of snapshotting a file system, or
11847 of temporarily suspending all changes to a file system, by (say)
11848 suspending the only virtual machine that can modify a file system; if
11849 you use these facilities and have @command{tar -c} read from a
11850 snapshot when creating an archive, you can avoid inconsistency
11851 problems. More drastically, before starting @command{tar} you could
11852 suspend or shut down all processes other than @command{tar} that have
11853 access to the file system, or you could unmount the file system and
11854 then mount it read-only.
11856 When extracting from an archive, one approach to avoid race conditions
11857 is to create a directory that no other process can write to, and
11863 In some cases @command{tar} may be used in an adversarial situation,
11864 where an untrusted user is attempting to gain information about or
11865 modify otherwise-inaccessible files. Dealing with untrusted data
11866 (that is, data generated by an untrusted user) typically requires
11867 extra care, because even the smallest mistake in the use of
11868 @command{tar} is more likely to be exploited by an adversary than by a
11874 * Live untrusted data::
11875 * Security rules of thumb::
11879 @subsection Privacy
11881 Standard privacy concerns apply when using @command{tar}. For
11882 example, suppose you are archiving your home directory into a file
11883 @file{/archive/myhome.tar}. Any secret information in your home
11884 directory, such as your SSH secret keys, are copied faithfully into
11885 the archive. Therefore, if your home directory contains any file that
11886 should not be read by some other user, the archive itself should be
11887 not be readable by that user. And even if the archive's data are
11888 inaccessible to untrusted users, its metadata (such as size or
11889 last-modified date) may reveal some information about your home
11890 directory; if the metadata are intended to be private, the archive's
11891 parent directory should also be inaccessible to untrusted users.
11893 One precaution is to create @file{/archive} so that it is not
11894 accessible to any user, unless that user also has permission to access
11895 all the files in your home directory.
11897 Similarly, when extracting from an archive, take care that the
11898 permissions of the extracted files are not more generous than what you
11899 want. Even if the archive itself is readable only to you, files
11900 extracted from it have their own permissions that may differ.
11903 @subsection Integrity
11905 When creating archives, take care that they are not writable by a
11906 untrusted user; otherwise, that user could modify the archive, and
11907 when you later extract from the archive you will get incorrect data.
11909 When @command{tar} extracts from an archive, by default it writes into
11910 files relative to the working directory. If the archive was generated
11911 by an untrusted user, that user therefore can write into any file
11912 under the working directory. If the working directory contains a
11913 symbolic link to another directory, the untrusted user can also write
11914 into any file under the referenced directory. When extracting from an
11915 untrusted archive, it is therefore good practice to create an empty
11916 directory and run @command{tar} in that directory.
11918 When extracting from two or more untrusted archives, each one should
11919 be extracted independently, into different empty directories.
11920 Otherwise, the first archive could create a symbolic link into an area
11921 outside the working directory, and the second one could follow the
11922 link and overwrite data that is not under the working directory. For
11923 example, when restoring from a series of incremental dumps, the
11924 archives should have been created by a trusted process, as otherwise
11925 the incremental restores might alter data outside the working
11928 If you use the @option{--absolute-names} (@option{-P}) option when
11929 extracting, @command{tar} respects any file names in the archive, even
11930 file names that begin with @file{/} or contain @file{..}. As this
11931 lets the archive overwrite any file in your system that you can write,
11932 the @option{--absolute-names} (@option{-P}) option should be used only
11933 for trusted archives.
11935 Conversely, with the @option{--keep-old-files} (@option{-k}) option,
11936 @command{tar} refuses to replace existing files when extracting; and
11937 with the @option{--no-overwrite-dir} option, @command{tar} refuses to
11938 replace the permissions or ownership of already-existing directories.
11939 These options may help when extracting from untrusted archives.
11941 @node Live untrusted data
11942 @subsection Dealing with Live Untrusted Data
11944 Extra care is required when creating from or extracting into a file
11945 system that is accessible to untrusted users. For example, superusers
11946 who invoke @command{tar} must be wary about its actions being hijacked
11947 by an adversary who is reading or writing the file system at the same
11948 time that @command{tar} is operating.
11950 When creating an archive from a live file system, @command{tar} is
11951 vulnerable to denial-of-service attacks. For example, an adversarial
11952 user could create the illusion of an indefinitely-deep directory
11953 hierarchy @file{d/e/f/g/...} by creating directories one step ahead of
11954 @command{tar}, or the illusion of an indefinitely-long file by
11955 creating a sparse file but arranging for blocks to be allocated just
11956 before @command{tar} reads them. There is no easy way for
11957 @command{tar} to distinguish these scenarios from legitimate uses, so
11958 you may need to monitor @command{tar}, just as you'd need to monitor
11959 any other system service, to detect such attacks.
11961 While a superuser is extracting from an archive into a live file
11962 system, an untrusted user might replace a directory with a symbolic
11963 link, in hopes that @command{tar} will follow the symbolic link and
11964 extract data into files that the untrusted user does not have access
11965 to. Even if the archive was generated by the superuser, it may
11966 contain a file such as @file{d/etc/passwd} that the untrusted user
11967 earlier created in order to break in; if the untrusted user replaces
11968 the directory @file{d/etc} with a symbolic link to @file{/etc} while
11969 @command{tar} is running, @command{tar} will overwrite
11970 @file{/etc/passwd}. This attack can be prevented by extracting into a
11971 directory that is inaccessible to untrusted users.
11973 Similar attacks via symbolic links are also possible when creating an
11974 archive, if the untrusted user can modify an ancestor of a top-level
11975 argument of @command{tar}. For example, an untrusted user that can
11976 modify @file{/home/eve} can hijack a running instance of @samp{tar -cf
11977 - /home/eve/Documents/yesterday} by replacing
11978 @file{/home/eve/Documents} with a symbolic link to some other
11979 location. Attacks like these can be prevented by making sure that
11980 untrusted users cannot modify any files that are top-level arguments
11981 to @command{tar}, or any ancestor directories of these files.
11983 @node Security rules of thumb
11984 @subsection Security Rules of Thumb
11986 This section briefly summarizes rules of thumb for avoiding security
11992 Protect archives at least as much as you protect any of the files
11996 Extract from an untrusted archive only into an otherwise-empty
11997 directory. This directory and its parent should be accessible only to
11998 trusted users. For example:
12002 $ @kbd{chmod go-rwx .}
12003 $ @kbd{mkdir -m go-rwx dir}
12005 $ @kbd{tar -xvf /archives/got-it-off-the-net.tar.gz}
12009 As a corollary, do not do an incremental restore from an untrusted archive.
12012 Do not let untrusted users access files extracted from untrusted
12013 archives without checking first for problems such as setuid programs.
12016 Do not let untrusted users modify directories that are ancestors of
12017 top-level arguments of @command{tar}. For example, while you are
12018 executing @samp{tar -cf /archive/u-home.tar /u/home}, do not let an
12019 untrusted user modify @file{/}, @file{/archive}, or @file{/u}.
12022 Pay attention to the diagnostics and exit status of @command{tar}.
12025 When archiving live file systems, monitor running instances of
12026 @command{tar} to detect denial-of-service attacks.
12029 Avoid unusual options such as @option{--absolute-names} (@option{-P}),
12030 @option{--dereference} (@option{-h}), @option{--overwrite},
12031 @option{--recursive-unlink}, and @option{--remove-files} unless you
12032 understand their security implications.
12039 This appendix lists some important user-visible changes between
12040 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
12041 version of this document is available at
12042 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
12043 @GNUTAR{} documentation page}.
12046 @item Use of globbing patterns when listing and extracting.
12048 Previous versions of GNU tar assumed shell-style globbing when
12049 extracting from or listing an archive. For example:
12052 $ @kbd{tar xf foo.tar '*.c'}
12055 would extract all files whose names end in @samp{.c}. This behavior
12056 was not documented and was incompatible with traditional tar
12057 implementations. Therefore, starting from version 1.15.91, GNU tar
12058 no longer uses globbing by default. For example, the above invocation
12059 is now interpreted as a request to extract from the archive the file
12062 To facilitate transition to the new behavior for those users who got
12063 used to the previous incorrect one, @command{tar} will print a warning
12064 if it finds out that a requested member was not found in the archive
12065 and its name looks like a globbing pattern. For example:
12068 $ @kbd{tar xf foo.tar '*.c'}
12069 tar: Pattern matching characters used in file names. Please,
12070 tar: use --wildcards to enable pattern matching, or --no-wildcards to
12071 tar: suppress this warning.
12072 tar: *.c: Not found in archive
12073 tar: Error exit delayed from previous errors
12076 To treat member names as globbing patterns, use the @option{--wildcards} option.
12077 If you want to tar to mimic the behavior of versions prior to 1.15.91,
12078 add this option to your @env{TAR_OPTIONS} variable.
12080 @xref{wildcards}, for the detailed discussion of the use of globbing
12081 patterns by @GNUTAR{}.
12083 @item Use of short option @option{-o}.
12085 Earlier versions of @GNUTAR{} understood @option{-o} command line
12086 option as a synonym for @option{--old-archive}.
12088 @GNUTAR{} starting from version 1.13.90 understands this option as
12089 a synonym for @option{--no-same-owner}. This is compatible with
12090 UNIX98 @command{tar} implementations.
12092 However, to facilitate transition, @option{-o} option retains its
12093 old semantics when it is used with one of archive-creation commands.
12094 Users are encouraged to use @option{--format=oldgnu} instead.
12096 It is especially important, since versions of @acronym{GNU} Automake
12097 up to and including 1.8.4 invoke tar with this option to produce
12098 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
12099 of this issue and its implications.
12101 @xref{Options, tar-formats, Changing Automake's Behavior,
12102 automake, GNU Automake}, for a description on how to use various
12103 archive formats with @command{automake}.
12105 Future versions of @GNUTAR{} will understand @option{-o} only as a
12106 synonym for @option{--no-same-owner}.
12108 @item Use of short option @option{-l}
12110 Earlier versions of @GNUTAR{} understood @option{-l} option as a
12111 synonym for @option{--one-file-system}. Since such usage contradicted
12112 to UNIX98 specification and harmed compatibility with other
12113 implementations, it was declared deprecated in version 1.14. However,
12114 to facilitate transition to its new semantics, it was supported by
12115 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
12116 variant of @option{--check-links} was introduced in version 1.15.91.
12118 @item Use of options @option{--portability} and @option{--old-archive}
12120 These options are deprecated. Please use @option{--format=v7} instead.
12122 @item Use of option @option{--posix}
12124 This option is deprecated. Please use @option{--format=posix} instead.
12127 @node Configuring Help Summary
12128 @appendix Configuring Help Summary
12130 Running @kbd{tar --help} displays the short @command{tar} option
12131 summary (@pxref{help}). This summary is organized by @dfn{groups} of
12132 semantically close options. The options within each group are printed
12133 in the following order: a short option, eventually followed by a list
12134 of corresponding long option names, followed by a short description of
12135 the option. For example, here is an excerpt from the actual @kbd{tar
12139 Main operation mode:
12141 -A, --catenate, --concatenate append tar files to an archive
12142 -c, --create create a new archive
12143 -d, --diff, --compare find differences between archive and
12145 --delete delete from the archive
12148 @vrindex ARGP_HELP_FMT, environment variable
12149 The exact visual representation of the help output is configurable via
12150 @env{ARGP_HELP_FMT} environment variable. The value of this variable
12151 is a comma-separated list of @dfn{format variable} assignments. There
12152 are two kinds of format variables. An @dfn{offset variable} keeps the
12153 offset of some part of help output text from the leftmost column on
12154 the screen. A @dfn{boolean} variable is a flag that toggles some
12155 output feature on or off. Depending on the type of the corresponding
12156 variable, there are two kinds of assignments:
12159 @item Offset assignment
12161 The assignment to an offset variable has the following syntax:
12164 @var{variable}=@var{value}
12168 where @var{variable} is the variable name, and @var{value} is a
12169 numeric value to be assigned to the variable.
12171 @item Boolean assignment
12173 To assign @code{true} value to a variable, simply put this variable name. To
12174 assign @code{false} value, prefix the variable name with @samp{no-}. For
12179 # Assign @code{true} value:
12181 # Assign @code{false} value:
12187 Following variables are declared:
12189 @deftypevr {Help Output} boolean dup-args
12190 If true, arguments for an option are shown with both short and long
12191 options, even when a given option has both forms, for example:
12194 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
12197 If false, then if an option has both short and long forms, the
12198 argument is only shown with the long one, for example:
12201 -f, --file=ARCHIVE use archive file or device ARCHIVE
12205 and a message indicating that the argument is applicable to both
12206 forms is printed below the options. This message can be disabled
12207 using @code{dup-args-note} (see below).
12209 The default is false.
12212 @deftypevr {Help Output} boolean dup-args-note
12213 If this variable is true, which is the default, the following notice
12214 is displayed at the end of the help output:
12217 Mandatory or optional arguments to long options are also mandatory or
12218 optional for any corresponding short options.
12221 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
12222 variables @code{dup-args} or @code{dup-args-note} should be set.
12225 @deftypevr {Help Output} offset short-opt-col
12226 Column in which short options start. Default is 2.
12230 $ @kbd{tar --help|grep ARCHIVE}
12231 -f, --file=ARCHIVE use archive file or device ARCHIVE
12232 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
12233 -f, --file=ARCHIVE use archive file or device ARCHIVE
12238 @deftypevr {Help Output} offset long-opt-col
12239 Column in which long options start. Default is 6. For example:
12243 $ @kbd{tar --help|grep ARCHIVE}
12244 -f, --file=ARCHIVE use archive file or device ARCHIVE
12245 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
12246 -f, --file=ARCHIVE use archive file or device ARCHIVE
12251 @deftypevr {Help Output} offset doc-opt-col
12252 Column in which @dfn{doc options} start. A doc option isn't actually
12253 an option, but rather an arbitrary piece of documentation that is
12254 displayed in much the same manner as the options. For example, in
12255 the description of @option{--format} option:
12259 -H, --format=FORMAT create archive of the given format.
12261 FORMAT is one of the following:
12263 gnu GNU tar 1.13.x format
12264 oldgnu GNU format as per tar <= 1.12
12265 pax POSIX 1003.1-2001 (pax) format
12267 ustar POSIX 1003.1-1988 (ustar) format
12268 v7 old V7 tar format
12273 the format names are doc options. Thus, if you set
12274 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
12275 will look as follows:
12279 -H, --format=FORMAT create archive of the given format.
12281 FORMAT is one of the following:
12283 gnu GNU tar 1.13.x format
12284 oldgnu GNU format as per tar <= 1.12
12285 pax POSIX 1003.1-2001 (pax) format
12287 ustar POSIX 1003.1-1988 (ustar) format
12288 v7 old V7 tar format
12293 @deftypevr {Help Output} offset opt-doc-col
12294 Column in which option description starts. Default is 29.
12298 $ @kbd{tar --help|grep ARCHIVE}
12299 -f, --file=ARCHIVE use archive file or device ARCHIVE
12300 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
12301 -f, --file=ARCHIVE use archive file or device ARCHIVE
12302 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
12304 use archive file or device ARCHIVE
12309 Notice, that the description starts on a separate line if
12310 @code{opt-doc-col} value is too small.
12313 @deftypevr {Help Output} offset header-col
12314 Column in which @dfn{group headers} are printed. A group header is a
12315 descriptive text preceding an option group. For example, in the
12319 Main operation mode:
12321 -A, --catenate, --concatenate append tar files to
12323 -c, --create create a new archive
12326 @samp{Main operation mode:} is the group header.
12328 The default value is 1.
12331 @deftypevr {Help Output} offset usage-indent
12332 Indentation of wrapped usage lines. Affects @option{--usage}
12333 output. Default is 12.
12336 @deftypevr {Help Output} offset rmargin
12337 Right margin of the text output. Used for wrapping.
12340 @node Fixing Snapshot Files
12341 @appendix Fixing Snapshot Files
12342 @include tar-snapshot-edit.texi
12344 @node Tar Internals
12345 @appendix Tar Internals
12346 @include intern.texi
12350 @include genfile.texi
12352 @node Free Software Needs Free Documentation
12353 @appendix Free Software Needs Free Documentation
12354 @include freemanuals.texi
12356 @node GNU Free Documentation License
12357 @appendix GNU Free Documentation License
12361 @node Index of Command Line Options
12362 @appendix Index of Command Line Options
12364 This appendix contains an index of all @GNUTAR{} long command line
12365 options. The options are listed without the preceding double-dash.
12366 For a cross-reference of short command line options, see
12367 @ref{Short Option Summary}.
12380 @c Local variables:
12381 @c texinfo-column-for-description: 32
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