1 \input texinfo @c -*-texinfo-*-
2 @comment %**start of header
5 @settitle GNU tar @value{VERSION}
13 @include rendition.texi
16 @c Put everything in one index (arbitrarily chosen to be the concept index).
26 This manual is for @acronym{GNU} @command{tar} (version
27 @value{VERSION}, @value{UPDATED}), which creates and extracts files
30 Copyright @copyright{} 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001,
31 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
34 Permission is granted to copy, distribute and/or modify this document
35 under the terms of the GNU Free Documentation License, Version 1.1 or
36 any later version published by the Free Software Foundation; with no
37 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
38 and with the Back-Cover Texts as in (a) below. A copy of the license
39 is included in the section entitled "GNU Free Documentation License".
41 (a) The FSF's Back-Cover Text is: ``You are free to copy and modify
42 this GNU Manual. Buying copies from GNU Press supports the FSF in
43 developing GNU and promoting software freedom.''
47 @dircategory Archiving
49 * Tar: (tar). Making tape (or disk) archives.
52 @dircategory Individual utilities
54 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
57 @shorttitlepage @acronym{GNU} @command{tar}
60 @title @acronym{GNU} tar: an archiver tool
61 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
62 @author John Gilmore, Jay Fenlason et al.
65 @vskip 0pt plus 1filll
71 @top @acronym{GNU} tar: an archiver tool
76 @cindex archiving files
78 The first part of this master menu lists the major nodes in this Info
79 document. The rest of the menu lists all the lower level nodes.
82 @c The master menu, created with texinfo-master-menu, goes here.
83 @c (However, getdate.texi's menu is interpolated by hand.)
92 * Date input formats::
99 * Configuring Help Summary::
102 * Free Software Needs Free Documentation::
103 * Copying This Manual::
104 * Index of Command Line Options::
108 --- The Detailed Node Listing ---
112 * Book Contents:: What this Book Contains
113 * Definitions:: Some Definitions
114 * What tar Does:: What @command{tar} Does
115 * Naming tar Archives:: How @command{tar} Archives are Named
116 * Authors:: @GNUTAR{} Authors
117 * Reports:: Reporting bugs or suggestions
119 Tutorial Introduction to @command{tar}
122 * stylistic conventions::
123 * basic tar options:: Basic @command{tar} Operations and Options
124 * frequent operations::
125 * Two Frequent Options::
126 * create:: How to Create Archives
127 * list:: How to List Archives
128 * extract:: How to Extract Members from an Archive
131 Two Frequently Used Options
137 How to Create Archives
139 * prepare for examples::
140 * Creating the archive::
149 How to Extract Members from an Archive
151 * extracting archives::
159 * using tar options::
167 The Three Option Styles
169 * Mnemonic Options:: Mnemonic Option Style
170 * Short Options:: Short Option Style
171 * Old Options:: Old Option Style
172 * Mixing:: Mixing Option Styles
174 All @command{tar} Options
176 * Operation Summary::
178 * Short Option Summary::
190 Advanced @GNUTAR{} Operations
199 How to Add Files to Existing Archives: @option{--append}
201 * appending files:: Appending Files to an Archive
208 Options Used by @option{--create}
210 * Ignore Failed Read::
212 Options Used by @option{--extract}
214 * Reading:: Options to Help Read Archives
215 * Writing:: Changing How @command{tar} Writes Files
216 * Scarce:: Coping with Scarce Resources
218 Options to Help Read Archives
220 * read full records::
223 Changing How @command{tar} Writes Files
225 * Dealing with Old Files::
226 * Overwrite Old Files::
231 * Data Modification Times::
232 * Setting Access Permissions::
233 * Writing to Standard Output::
236 Coping with Scarce Resources
241 Performing Backups and Restoring Files
243 * Full Dumps:: Using @command{tar} to Perform Full Dumps
244 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
245 * Backup Levels:: Levels of Backups
246 * Backup Parameters:: Setting Parameters for Backups and Restoration
247 * Scripted Backups:: Using the Backup Scripts
248 * Scripted Restoration:: Using the Restore Script
250 Setting Parameters for Backups and Restoration
252 * General-Purpose Variables::
253 * Magnetic Tape Control::
255 * backup-specs example:: An Example Text of @file{Backup-specs}
257 Choosing Files and Names for @command{tar}
259 * file:: Choosing the Archive's Name
260 * Selecting Archive Members::
261 * files:: Reading Names from a File
262 * exclude:: Excluding Some Files
263 * wildcards:: Wildcards Patterns and Matching
264 * quoting styles:: Ways of Quoting Special Characters in Names
265 * transform:: Modifying File and Member Names
266 * after:: Operating Only on New Files
267 * recurse:: Descending into Directories
268 * one:: Crossing File System Boundaries
270 Reading Names from a File
276 * problems with exclude::
278 Crossing File System Boundaries
280 * directory:: Changing Directory
281 * absolute:: Absolute File Names
285 * General date syntax:: Common rules.
286 * Calendar date items:: 19 Dec 1994.
287 * Time of day items:: 9:20pm.
288 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}, ...
289 * Day of week items:: Monday and others.
290 * Relative items in date strings:: next tuesday, 2 years ago.
291 * Pure numbers in date strings:: 19931219, 1440.
292 * Seconds since the Epoch:: @@1078100502.
293 * Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al.
295 Controlling the Archive Format
297 * Portability:: Making @command{tar} Archives More Portable
298 * Compression:: Using Less Space through Compression
299 * Attributes:: Handling File Attributes
300 * Standard:: The Standard Format
301 * Extensions:: @acronym{GNU} Extensions to the Archive Format
302 * cpio:: Comparison of @command{tar} and @command{cpio}
304 Making @command{tar} Archives More Portable
306 * Portable Names:: Portable Names
307 * dereference:: Symbolic Links
308 * old:: Old V7 Archives
309 * posix:: @acronym{POSIX} archives
310 * Checksumming:: Checksumming Problems
311 * Large or Negative Values:: Large files, negative time stamps, etc.
313 Using Less Space through Compression
315 * gzip:: Creating and Reading Compressed Archives
316 * sparse:: Archiving Sparse Files
318 Tapes and Other Archive Media
320 * Device:: Device selection and switching
321 * Remote Tape Server::
322 * Common Problems and Solutions::
323 * Blocking:: Blocking
324 * Many:: Many archives on one tape
325 * Using Multiple Tapes:: Using Multiple Tapes
326 * label:: Including a Label in the Archive
332 * Format Variations:: Format Variations
333 * Blocking Factor:: The Blocking Factor of an Archive
335 Many Archives on One Tape
337 * Tape Positioning:: Tape Positions and Tape Marks
338 * mt:: The @command{mt} Utility
342 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
343 * Tape Files:: Tape Files
344 * Tarcat:: Concatenate Volumes into a Single Archive
346 GNU tar internals and development
353 * Free Software Needs Free Documentation::
354 * GNU Free Documentation License:: License for copying this manual
360 @chapter Introduction
363 and manipulates @dfn{archives} which are actually collections of
364 many other files; the program provides users with an organized and
365 systematic method for controlling a large amount of data.
366 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
367 archives need not (and these days, typically do not) reside on tapes.
370 * Book Contents:: What this Book Contains
371 * Definitions:: Some Definitions
372 * What tar Does:: What @command{tar} Does
373 * Naming tar Archives:: How @command{tar} Archives are Named
374 * Authors:: @GNUTAR{} Authors
375 * Reports:: Reporting bugs or suggestions
379 @section What this Book Contains
381 The first part of this chapter introduces you to various terms that will
382 recur throughout the book. It also tells you who has worked on @GNUTAR{}
383 and its documentation, and where you should send bug reports
386 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
387 gentle introduction for people who are new to using @command{tar}. It is
388 meant to be self contained, not requiring any reading from subsequent
389 chapters to make sense. It moves from topic to topic in a logical,
390 progressive order, building on information already explained.
392 Although the tutorial is paced and structured to allow beginners to
393 learn how to use @command{tar}, it is not intended solely for beginners.
394 The tutorial explains how to use the three most frequently used
395 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
396 two frequently used options (@samp{file} and @samp{verbose}). The other
397 chapters do not refer to the tutorial frequently; however, if a section
398 discusses something which is a complex variant of a basic concept, there
399 may be a cross reference to that basic concept. (The entire book,
400 including the tutorial, assumes that the reader understands some basic
401 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
403 The third chapter presents the remaining five operations, and
404 information about using @command{tar} options and option syntax.
406 @FIXME{this sounds more like a @acronym{GNU} Project Manuals Concept [tm] more
407 than the reality. should think about whether this makes sense to say
408 here, or not.} The other chapters are meant to be used as a
409 reference. Each chapter presents everything that needs to be said
410 about a specific topic.
412 One of the chapters (@pxref{Date input formats}) exists in its
413 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
414 In addition, one section of this manual (@pxref{Standard}) contains a
415 big quote which is taken directly from @command{tar} sources.
417 In general, we give both long and short (abbreviated) option names
418 at least once in each section where the relevant option is covered, so
419 that novice readers will become familiar with both styles. (A few
420 options have no short versions, and the relevant sections will
424 @section Some Definitions
428 The @command{tar} program is used to create and manipulate @command{tar}
429 archives. An @dfn{archive} is a single file which contains the contents
430 of many files, while still identifying the names of the files, their
431 owner(s), and so forth. (In addition, archives record access
432 permissions, user and group, size in bytes, and data modification time.
433 Some archives also record the file names in each archived directory, as
434 well as other file and directory information.) You can use @command{tar}
435 to @dfn{create} a new archive in a specified directory.
438 @cindex archive member
441 The files inside an archive are called @dfn{members}. Within this
442 manual, we use the term @dfn{file} to refer only to files accessible in
443 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
444 @dfn{member} to refer only to the members of an archive. Similarly, a
445 @dfn{file name} is the name of a file, as it resides in the file system,
446 and a @dfn{member name} is the name of an archive member within the
451 The term @dfn{extraction} refers to the process of copying an archive
452 member (or multiple members) into a file in the file system. Extracting
453 all the members of an archive is often called @dfn{extracting the
454 archive}. The term @dfn{unpack} can also be used to refer to the
455 extraction of many or all the members of an archive. Extracting an
456 archive does not destroy the archive's structure, just as creating an
457 archive does not destroy the copies of the files that exist outside of
458 the archive. You may also @dfn{list} the members in a given archive
459 (this is often thought of as ``printing'' them to the standard output,
460 or the command line), or @dfn{append} members to a pre-existing archive.
461 All of these operations can be performed using @command{tar}.
464 @section What @command{tar} Does
467 The @command{tar} program provides the ability to create @command{tar}
468 archives, as well as various other kinds of manipulation. For example,
469 you can use @command{tar} on previously created archives to extract files,
470 to store additional files, or to update or list files which were already
473 Initially, @command{tar} archives were used to store files conveniently on
474 magnetic tape. The name @command{tar} comes from this use; it stands for
475 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
476 direct its output to available devices, files, or other programs (using
477 pipes). @command{tar} may even access remote devices or files (as archives).
479 You can use @command{tar} archives in many ways. We want to stress a few
480 of them: storage, backup, and transportation.
482 @FIXME{the following table entries need a bit of work..}
485 Often, @command{tar} archives are used to store related files for
486 convenient file transfer over a network. For example, the
487 @acronym{GNU} Project distributes its software bundled into
488 @command{tar} archives, so that all the files relating to a particular
489 program (or set of related programs) can be transferred as a single
492 A magnetic tape can store several files in sequence. However, the tape
493 has no names for these files; it only knows their relative position on
494 the tape. One way to store several files on one tape and retain their
495 names is by creating a @command{tar} archive. Even when the basic transfer
496 mechanism can keep track of names, as FTP can, the nuisance of handling
497 multiple files, directories, and multiple links makes @command{tar}
500 Archive files are also used for long-term storage. You can think of
501 this as transportation from the present into the future. (It is a
502 science-fiction idiom that you can move through time as well as in
503 space; the idea here is that @command{tar} can be used to move archives in
504 all dimensions, even time!)
507 Because the archive created by @command{tar} is capable of preserving
508 file information and directory structure, @command{tar} is commonly
509 used for performing full and incremental backups of disks. A backup
510 puts a collection of files (possibly pertaining to many users and
511 projects) together on a disk or a tape. This guards against
512 accidental destruction of the information in those files.
513 @GNUTAR{} has special features that allow it to be
514 used to make incremental and full dumps of all the files in a
518 You can create an archive on one system, transfer it to another system,
519 and extract the contents there. This allows you to transport a group of
520 files from one system to another.
523 @node Naming tar Archives
524 @section How @command{tar} Archives are Named
526 Conventionally, @command{tar} archives are given names ending with
527 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
528 but this manual follows that convention in order to accustom readers to
529 it and to make examples more clear.
534 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
535 archive members as ``files'' or ``entries''. For people familiar with
536 the operation of @command{tar}, this causes no difficulty. However, in
537 this manual, we consistently refer to ``archives'' and ``archive
538 members'' to make learning to use @command{tar} easier for novice users.
541 @section @GNUTAR{} Authors
543 @GNUTAR{} was originally written by John Gilmore,
544 and modified by many people. The @acronym{GNU} enhancements were
545 written by Jay Fenlason, then Joy Kendall, and the whole package has
546 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
547 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
548 numerous and kind users.
550 We wish to stress that @command{tar} is a collective work, and owes much to
551 all those people who reported problems, offered solutions and other
552 insights, or shared their thoughts and suggestions. An impressive, yet
553 partial list of those contributors can be found in the @file{THANKS}
554 file from the @GNUTAR{} distribution.
556 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
557 sure i want to spell out the history in this detail, at least not for
558 the printed book. i'm just not sure it needs to be said this way.
559 i'll think about it.}
561 @FIXME{History is more important, and surely more interesting, than
562 actual names. Quoting names without history would be meaningless. FP}
564 Jay Fenlason put together a draft of a @GNUTAR{}
565 manual, borrowing notes from the original man page from John Gilmore.
566 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
567 Gorin worked on a tutorial and manual for @GNUTAR{}.
568 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
569 taking information from all these sources and merging them. Melissa
570 Weisshaus finally edited and redesigned the book to create version
571 1.12. @FIXME{update version number as necessary; i'm being
572 optimistic!} @FIXME{Someone [maybe karl berry? maybe bob chassell?
573 maybe melissa? maybe julie sussman?] needs to properly index the
576 For version 1.12, Daniel Hagerty contributed a great deal of technical
577 consulting. In particular, he is the primary author of @ref{Backups}.
579 In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org
580 (see @url{http://savannah.gnu.org/projects/tar}), and
581 active development and maintenance work has started
582 again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey
583 Poznyakoff and Jeff Bailey.
585 Support for @acronym{POSIX} archives was added by Sergey Poznyakoff.
588 @section Reporting bugs or suggestions
591 @cindex reporting bugs
592 If you find problems or have suggestions about this program or manual,
593 please report them to @file{bug-tar@@gnu.org}.
595 When reporting a bug, please be sure to include as much detail as
596 possible, in order to reproduce it. @FIXME{Be more specific, I'd
597 like to make this node as detailed as 'Bug reporting' node in Emacs
601 @chapter Tutorial Introduction to @command{tar}
603 This chapter guides you through some basic examples of three @command{tar}
604 operations: @option{--create}, @option{--list}, and @option{--extract}. If
605 you already know how to use some other version of @command{tar}, then you
606 may not need to read this chapter. This chapter omits most complicated
607 details about how @command{tar} works.
611 * stylistic conventions::
612 * basic tar options:: Basic @command{tar} Operations and Options
613 * frequent operations::
614 * Two Frequent Options::
615 * create:: How to Create Archives
616 * list:: How to List Archives
617 * extract:: How to Extract Members from an Archive
622 @section Assumptions this Tutorial Makes
624 This chapter is paced to allow beginners to learn about @command{tar}
625 slowly. At the same time, we will try to cover all the basic aspects of
626 these three operations. In order to accomplish both of these tasks, we
627 have made certain assumptions about your knowledge before reading this
628 manual, and the hardware you will be using:
632 Before you start to work through this tutorial, you should understand
633 what the terms ``archive'' and ``archive member'' mean
634 (@pxref{Definitions}). In addition, you should understand something
635 about how Unix-type operating systems work, and you should know how to
636 use some basic utilities. For example, you should know how to create,
637 list, copy, rename, edit, and delete files and directories; how to
638 change between directories; and how to figure out where you are in the
639 file system. You should have some basic understanding of directory
640 structure and how files are named according to which directory they are
641 in. You should understand concepts such as standard output and standard
642 input, what various definitions of the term ``argument'' mean, and the
643 differences between relative and absolute path names. @FIXME{and what
647 This manual assumes that you are working from your own home directory
648 (unless we state otherwise). In this tutorial, you will create a
649 directory to practice @command{tar} commands in. When we show path names,
650 we will assume that those paths are relative to your home directory.
651 For example, my home directory path is @file{/home/fsf/melissa}. All of
652 my examples are in a subdirectory of the directory named by that path
653 name; the subdirectory is called @file{practice}.
656 In general, we show examples of archives which exist on (or can be
657 written to, or worked with from) a directory on a hard disk. In most
658 cases, you could write those archives to, or work with them on any other
659 device, such as a tape drive. However, some of the later examples in
660 the tutorial and next chapter will not work on tape drives.
661 Additionally, working with tapes is much more complicated than working
662 with hard disks. For these reasons, the tutorial does not cover working
663 with tape drives. @xref{Media}, for complete information on using
664 @command{tar} archives with tape drives.
666 @FIXME{this is a cop out. need to add some simple tape drive info.}
669 @node stylistic conventions
670 @section Stylistic Conventions
672 In the examples, @samp{$} represents a typical shell prompt. It
673 precedes lines you should type; to make this more clear, those lines are
674 shown in @kbd{this font}, as opposed to lines which represent the
675 computer's response; those lines are shown in @code{this font}, or
676 sometimes @samp{like this}.
678 @c When we have lines which are too long to be
679 @c displayed in any other way, we will show them like this:
681 @node basic tar options
682 @section Basic @command{tar} Operations and Options
684 @command{tar} can take a wide variety of arguments which specify and define
685 the actions it will have on the particular set of files or the archive.
686 The main types of arguments to @command{tar} fall into one of two classes:
687 operations, and options.
689 Some arguments fall into a class called @dfn{operations}; exactly one of
690 these is both allowed and required for any instance of using @command{tar};
691 you may @emph{not} specify more than one. People sometimes speak of
692 @dfn{operating modes}. You are in a particular operating mode when you
693 have specified the operation which specifies it; there are eight
694 operations in total, and thus there are eight operating modes.
696 The other arguments fall into the class known as @dfn{options}. You are
697 not required to specify any options, and you are allowed to specify more
698 than one at a time (depending on the way you are using @command{tar} at
699 that time). Some options are used so frequently, and are so useful for
700 helping you type commands more carefully that they are effectively
701 ``required''. We will discuss them in this chapter.
703 You can write most of the @command{tar} operations and options in any
704 of three forms: long (mnemonic) form, short form, and old style. Some
705 of the operations and options have no short or ``old'' forms; however,
706 the operations and options which we will cover in this tutorial have
707 corresponding abbreviations. @FIXME{make sure this is still the case,
708 at the end}We will indicate those abbreviations appropriately to get
709 you used to seeing them. (Note that the ``old style'' option forms
710 exist in @GNUTAR{} for compatibility with Unix
711 @command{tar}. In this book we present a full discussion of this way
712 of writing options and operations (@pxref{Old Options}), and we discuss
713 the other two styles of writing options (@xref{Mnemonic Options}, and
714 @pxref{Short Options}).
716 In the examples and in the text of this tutorial, we usually use the
717 long forms of operations and options; but the ``short'' forms produce
718 the same result and can make typing long @command{tar} commands easier.
719 For example, instead of typing
722 @kbd{tar --create --verbose --file=afiles.tar apple angst aspic}
728 @kbd{tar -c -v -f afiles.tar apple angst aspic}
734 @kbd{tar -cvf afiles.tar apple angst aspic}
738 For more information on option syntax, see @ref{Advanced tar}. In
739 discussions in the text, when we name an option by its long form, we
740 also give the corresponding short option in parentheses.
742 The term, ``option'', can be confusing at times, since ``operations''
743 are often lumped in with the actual, @emph{optional} ``options'' in certain
744 general class statements. For example, we just talked about ``short and
745 long forms of options and operations''. However, experienced @command{tar}
746 users often refer to these by shorthand terms such as, ``short and long
747 options''. This term assumes that the ``operations'' are included, also.
748 Context will help you determine which definition of ``options'' to use.
750 Similarly, the term ``command'' can be confusing, as it is often used in
751 two different ways. People sometimes refer to @command{tar} ``commands''.
752 A @command{tar} @dfn{command} is the entire command line of user input
753 which tells @command{tar} what to do --- including the operation, options,
754 and any arguments (file names, pipes, other commands, etc). However,
755 you will also sometimes hear the term ``the @command{tar} command''. When
756 the word ``command'' is used specifically like this, a person is usually
757 referring to the @command{tar} @emph{operation}, not the whole line.
758 Again, use context to figure out which of the meanings the speaker
761 @node frequent operations
762 @section The Three Most Frequently Used Operations
764 Here are the three most frequently used operations (both short and long
765 forms), as well as a brief description of their meanings. The rest of
766 this chapter will cover how to use these operations in detail. We will
767 present the rest of the operations in the next chapter.
772 Create a new @command{tar} archive.
775 List the contents of an archive.
778 Extract one or more members from an archive.
781 @node Two Frequent Options
782 @section Two Frequently Used Options
784 To understand how to run @command{tar} in the three operating modes listed
785 previously, you also need to understand how to use two of the options to
786 @command{tar}: @option{--file} (which takes an archive file as an argument)
787 and @option{--verbose}. (You are usually not @emph{required} to specify
788 either of these options when you run @command{tar}, but they can be very
789 useful in making things more clear and helping you avoid errors.)
798 @unnumberedsubsec The @option{--file} Option
801 @opindex file, tutorial
802 @item --file=@var{archive-name}
803 @itemx -f @var{archive-name}
804 Specify the name of an archive file.
807 You can specify an argument for the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option whenever you
808 use @command{tar}; this option determines the name of the archive file
809 that @command{tar} will work on.
812 If you don't specify this argument, then @command{tar} will examine
813 the environment variable @env{TAPE}. If it is set, its value will be
814 used as the archive name. Otherwise, @command{tar} will use the
815 default archive, determined at the compile time. Usually it is
816 standard output or some physical tape drive attached to your machine
817 (you can verify what the default is by running @kbd{tar
818 --show-defaults}, @pxref{defaults}). If there is no tape drive
819 attached, or the default is not meaningful, then @command{tar} will
820 print an error message. The error message might look roughly like one
824 tar: can't open /dev/rmt8 : No such device or address
825 tar: can't open /dev/rsmt0 : I/O error
829 To avoid confusion, we recommend that you always specify an archive file
830 name by using @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) when writing your @command{tar} commands.
831 For more information on using the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option, see
834 @node verbose tutorial
835 @unnumberedsubsec The @option{--verbose} Option
838 @opindex verbose, introduced
841 Show the files being worked on as @command{tar} is running.
844 @option{--verbose} (@option{-v}) shows details about the results of running
845 @command{tar}. This can be especially useful when the results might not be
846 obvious. For example, if you want to see the progress of @command{tar} as
847 it writes files into the archive, you can use the @option{--verbose}
848 option. In the beginning, you may find it useful to use
849 @option{--verbose} at all times; when you are more accustomed to
850 @command{tar}, you will likely want to use it at certain times but not at
851 others. We will use @option{--verbose} at times to help make something
852 clear, and we will give many examples both using and not using
853 @option{--verbose} to show the differences.
855 Sometimes, a single instance of @option{--verbose} on the command line
856 will show a full, @samp{ls} style listing of an archive or files,
857 giving sizes, owners, and similar information. @FIXME{Describe the
858 exact output format, e.g., how hard links are displayed.}
859 Other times, @option{--verbose} will only show files or members that the particular
860 operation is operating on at the time. In the latter case, you can
861 use @option{--verbose} twice in a command to get a listing such as that
862 in the former case. For example, instead of saying
865 @kbd{tar -cvf afiles.tar apple angst aspic}
872 @kbd{tar -cvvf afiles.tar apple angst aspic}
876 This works equally well using short or long forms of options. Using
877 long forms, you would simply write out the mnemonic form of the option
881 $ @kbd{tar --create --verbose --verbose @dots{}}
885 Note that you must double the hyphens properly each time.
887 Later in the tutorial, we will give examples using @w{@option{--verbose
891 @unnumberedsubsec Getting Help: Using the @option{--help} Option
897 The @option{--help} option to @command{tar} prints out a very brief list of
898 all operations and option available for the current version of
899 @command{tar} available on your system.
903 @section How to Create Archives
906 @cindex Creation of the archive
907 @cindex Archive, creation of
908 One of the basic operations of @command{tar} is @option{--create} (@option{-c}), which
909 you use to create a @command{tar} archive. We will explain
910 @option{--create} first because, in order to learn about the other
911 operations, you will find it useful to have an archive available to
914 To make this easier, in this section you will first create a directory
915 containing three files. Then, we will show you how to create an
916 @emph{archive} (inside the new directory). Both the directory, and
917 the archive are specifically for you to practice on. The rest of this
918 chapter and the next chapter will show many examples using this
919 directory and the files you will create: some of those files may be
920 other directories and other archives.
922 The three files you will archive in this example are called
923 @file{blues}, @file{folk}, and @file{jazz}. The archive is called
924 @file{collection.tar}.
926 This section will proceed slowly, detailing how to use @option{--create}
927 in @code{verbose} mode, and showing examples using both short and long
928 forms. In the rest of the tutorial, and in the examples in the next
929 chapter, we will proceed at a slightly quicker pace. This section
930 moves more slowly to allow beginning users to understand how
934 * prepare for examples::
935 * Creating the archive::
941 @node prepare for examples
942 @subsection Preparing a Practice Directory for Examples
944 To follow along with this and future examples, create a new directory
945 called @file{practice} containing files called @file{blues}, @file{folk}
946 and @file{jazz}. The files can contain any information you like:
947 ideally, they should contain information which relates to their names,
948 and be of different lengths. Our examples assume that @file{practice}
949 is a subdirectory of your home directory.
951 Now @command{cd} to the directory named @file{practice}; @file{practice}
952 is now your @dfn{working directory}. (@emph{Please note}: Although
953 the full path name of this directory is
954 @file{/@var{homedir}/practice}, in our examples we will refer to
955 this directory as @file{practice}; the @var{homedir} is presumed.
957 In general, you should check that the files to be archived exist where
958 you think they do (in the working directory) by running @command{ls}.
959 Because you just created the directory and the files and have changed to
960 that directory, you probably don't need to do that this time.
962 It is very important to make sure there isn't already a file in the
963 working directory with the archive name you intend to use (in this case,
964 @samp{collection.tar}), or that you don't care about its contents.
965 Whenever you use @samp{create}, @command{tar} will erase the current
966 contents of the file named by @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) if it exists. @command{tar}
967 will not tell you if you are about to overwrite an archive unless you
968 specify an option which does this (@pxref{backup}, for the
969 information on how to do so). To add files to an existing archive,
970 you need to use a different option, such as @option{--append} (@option{-r}); see
971 @ref{append} for information on how to do this.
973 @node Creating the archive
974 @subsection Creating the Archive
976 @opindex create, introduced
977 To place the files @file{blues}, @file{folk}, and @file{jazz} into an
978 archive named @file{collection.tar}, use the following command:
981 $ @kbd{tar --create --file=collection.tar blues folk jazz}
984 The order of the arguments is not very important, @emph{when using long
985 option forms}. You could also say:
988 $ @kbd{tar blues --create folk --file=collection.tar jazz}
992 However, you can see that this order is harder to understand; this is
993 why we will list the arguments in the order that makes the commands
994 easiest to understand (and we encourage you to do the same when you use
995 @command{tar}, to avoid errors).
997 Note that the part of the command which says,
998 @w{@option{--file=collection.tar}} is considered to be @emph{one} argument.
999 If you substituted any other string of characters for
1000 @kbd{collection.tar}, then that string would become the name of the
1001 archive file you create.
1003 The order of the options becomes more important when you begin to use
1004 short forms. With short forms, if you type commands in the wrong order
1005 (even if you type them correctly in all other ways), you may end up with
1006 results you don't expect. For this reason, it is a good idea to get
1007 into the habit of typing options in the order that makes inherent sense.
1008 @xref{short create}, for more information on this.
1010 In this example, you type the command as shown above: @option{--create}
1011 is the operation which creates the new archive
1012 (@file{collection.tar}), and @option{--file} is the option which lets
1013 you give it the name you chose. The files, @file{blues}, @file{folk},
1014 and @file{jazz}, are now members of the archive, @file{collection.tar}
1015 (they are @dfn{file name arguments} to the @option{--create} operation).
1016 @FIXME{xref here to the discussion of file name args?}Now that they are
1017 in the archive, they are called @emph{archive members}, not files.
1018 (@pxref{Definitions,members}).
1020 When you create an archive, you @emph{must} specify which files you
1021 want placed in the archive. If you do not specify any archive
1022 members, @GNUTAR{} will complain.
1024 If you now list the contents of the working directory (@command{ls}), you will
1025 find the archive file listed as well as the files you saw previously:
1028 blues folk jazz collection.tar
1032 Creating the archive @samp{collection.tar} did not destroy the copies of
1033 the files in the directory.
1035 Keep in mind that if you don't indicate an operation, @command{tar} will not
1036 run and will prompt you for one. If you don't name any files, @command{tar}
1037 will complain. You must have write access to the working directory,
1038 or else you will not be able to create an archive in that directory.
1040 @emph{Caution}: Do not attempt to use @option{--create} (@option{-c}) to add files to
1041 an existing archive; it will delete the archive and write a new one.
1042 Use @option{--append} (@option{-r}) instead. @xref{append}.
1044 @node create verbose
1045 @subsection Running @option{--create} with @option{--verbose}
1047 @opindex create, using with @option{--verbose}
1048 @opindex verbose, using with @option{--create}
1049 If you include the @option{--verbose} (@option{-v}) option on the command line,
1050 @command{tar} will list the files it is acting on as it is working. In
1051 verbose mode, the @code{create} example above would appear as:
1054 $ @kbd{tar --create --verbose --file=collection.tar blues folk jazz}
1060 This example is just like the example we showed which did not use
1061 @option{--verbose}, except that @command{tar} generated the remaining lines
1063 (note the different font styles).
1069 In the rest of the examples in this chapter, we will frequently use
1070 @code{verbose} mode so we can show actions or @command{tar} responses that
1071 you would otherwise not see, and which are important for you to
1075 @subsection Short Forms with @samp{create}
1077 As we said before, the @option{--create} (@option{-c}) operation is one of the most
1078 basic uses of @command{tar}, and you will use it countless times.
1079 Eventually, you will probably want to use abbreviated (or ``short'')
1080 forms of options. A full discussion of the three different forms that
1081 options can take appears in @ref{Styles}; for now, here is what the
1082 previous example (including the @option{--verbose} (@option{-v}) option) looks like
1083 using short option forms:
1086 $ @kbd{tar -cvf collection.tar blues folk jazz}
1093 As you can see, the system responds the same no matter whether you use
1094 long or short option forms.
1096 @FIXME{i don't like how this is worded:} One difference between using
1097 short and long option forms is that, although the exact placement of
1098 arguments following options is no more specific when using short forms,
1099 it is easier to become confused and make a mistake when using short
1100 forms. For example, suppose you attempted the above example in the
1104 $ @kbd{tar -cfv collection.tar blues folk jazz}
1108 In this case, @command{tar} will make an archive file called @file{v},
1109 containing the files @file{blues}, @file{folk}, and @file{jazz}, because
1110 the @samp{v} is the closest ``file name'' to the @option{-f} option, and
1111 is thus taken to be the chosen archive file name. @command{tar} will try
1112 to add a file called @file{collection.tar} to the @file{v} archive file;
1113 if the file @file{collection.tar} did not already exist, @command{tar} will
1114 report an error indicating that this file does not exist. If the file
1115 @file{collection.tar} does already exist (e.g., from a previous command
1116 you may have run), then @command{tar} will add this file to the archive.
1117 Because the @option{-v} option did not get registered, @command{tar} will not
1118 run under @samp{verbose} mode, and will not report its progress.
1120 The end result is that you may be quite confused about what happened,
1121 and possibly overwrite a file. To illustrate this further, we will show
1122 you how an example we showed previously would look using short forms.
1127 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1131 is confusing as it is. When shown using short forms, however, it
1132 becomes much more so:
1135 $ @kbd{tar blues -c folk -f collection.tar jazz}
1139 It would be very easy to put the wrong string of characters
1140 immediately following the @option{-f}, but doing that could sacrifice
1143 For this reason, we recommend that you pay very careful attention to
1144 the order of options and placement of file and archive names,
1145 especially when using short option forms. Not having the option name
1146 written out mnemonically can affect how well you remember which option
1147 does what, and therefore where different names have to be placed.
1150 @subsection Archiving Directories
1152 @cindex Archiving Directories
1153 @cindex Directories, Archiving
1154 You can archive a directory by specifying its directory name as a
1155 file name argument to @command{tar}. The files in the directory will be
1156 archived relative to the working directory, and the directory will be
1157 re-created along with its contents when the archive is extracted.
1159 To archive a directory, first move to its superior directory. If you
1160 have followed the previous instructions in this tutorial, you should
1169 This will put you into the directory which contains @file{practice},
1170 i.e., your home directory. Once in the superior directory, you can
1171 specify the subdirectory, @file{practice}, as a file name argument. To
1172 store @file{practice} in the new archive file @file{music.tar}, type:
1175 $ @kbd{tar --create --verbose --file=music.tar practice}
1179 @command{tar} should output:
1186 practice/collection.tar
1189 Note that the archive thus created is not in the subdirectory
1190 @file{practice}, but rather in the current working directory---the
1191 directory from which @command{tar} was invoked. Before trying to archive a
1192 directory from its superior directory, you should make sure you have
1193 write access to the superior directory itself, not only the directory
1194 you are trying archive with @command{tar}. For example, you will probably
1195 not be able to store your home directory in an archive by invoking
1196 @command{tar} from the root directory; @xref{absolute}. (Note
1197 also that @file{collection.tar}, the original archive file, has itself
1198 been archived. @command{tar} will accept any file as a file to be
1199 archived, regardless of its content. When @file{music.tar} is
1200 extracted, the archive file @file{collection.tar} will be re-written
1201 into the file system).
1203 If you give @command{tar} a command such as
1206 $ @kbd{tar --create --file=foo.tar .}
1210 @command{tar} will report @samp{tar: ./foo.tar is the archive; not
1211 dumped}. This happens because @command{tar} creates the archive
1212 @file{foo.tar} in the current directory before putting any files into
1213 it. Then, when @command{tar} attempts to add all the files in the
1214 directory @file{.} to the archive, it notices that the file
1215 @file{./foo.tar} is the same as the archive @file{foo.tar}, and skips
1216 it. (It makes no sense to put an archive into itself.) @GNUTAR{}
1217 will continue in this case, and create the archive
1218 normally, except for the exclusion of that one file. (@emph{Please
1219 note:} Other implementations of @command{tar} may not be so clever;
1220 they will enter an infinite loop when this happens, so you should not
1221 depend on this behavior unless you are certain you are running
1222 @GNUTAR{}. In general, it is wise to always place the archive outside
1223 of the directory being dumped.
1226 @section How to List Archives
1229 Frequently, you will find yourself wanting to determine exactly what a
1230 particular archive contains. You can use the @option{--list} (@option{-t}) operation
1231 to get the member names as they currently appear in the archive, as well
1232 as various attributes of the files at the time they were archived. For
1233 example, you can examine the archive @file{collection.tar} that you
1234 created in the last section with the command,
1237 $ @kbd{tar --list --file=collection.tar}
1241 The output of @command{tar} would then be:
1250 The archive @file{bfiles.tar} would list as follows:
1259 Be sure to use a @option{--file=@var{archive-name}} (@option{-f
1260 @var{archive-name}}) option just as with @option{--create}
1261 (@option{-c}) to specify the name of the archive.
1263 @opindex list, using with @option{--verbose}
1264 @opindex verbose, using with @option{--list}
1265 If you use the @option{--verbose} (@option{-v}) option with
1266 @option{--list}, then @command{tar} will print out a listing
1267 reminiscent of @w{@samp{ls -l}}, showing owner, file size, and so forth.
1269 If you had used @option{--verbose} (@option{-v}) mode, the example
1270 above would look like:
1273 $ @kbd{tar --list --verbose --file=collection.tar folk}
1274 -rw-r--r-- myself user 62 1990-05-23 10:55 folk
1277 @cindex listing member and file names
1278 @anchor{listing member and file names}
1279 It is important to notice that the output of @kbd{tar --list
1280 --verbose} does not necessarily match that produced by @kbd{tar
1281 --create --verbose} while creating the archive. It is because
1282 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1283 prefixes from file names before storing them in the archive
1284 (@xref{absolute}, for more information). In other
1285 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1286 an archive and @dfn{member names} when listing it. Consider this
1291 $ @kbd{tar cfv archive /etc/mail}
1292 tar: Removing leading `/' from member names
1294 /etc/mail/sendmail.cf
1296 $ @kbd{tar tf archive}
1298 etc/mail/sendmail.cf
1303 @opindex show-stored-names
1304 This default behavior can sometimes be inconvenient. You can force
1305 @GNUTAR{} show member names when creating archive by supplying
1306 @option{--show-stored-names} option.
1309 @item --show-stored-names
1310 Print member (as opposed to @emph{file}) names when creating the archive.
1313 @cindex File name arguments, using @option{--list} with
1314 @opindex list, using with file name arguments
1315 You can specify one or more individual member names as arguments when
1316 using @samp{list}. In this case, @command{tar} will only list the
1317 names of members you identify. For example, @w{@kbd{tar --list
1318 --file=afiles.tar apple}} would only print @file{apple}.
1320 Because @command{tar} preserves paths, file names must be specified as
1321 they appear in the archive (ie., relative to the directory from which
1322 the archive was created). Therefore, it is essential when specifying
1323 member names to @command{tar} that you give the exact member names.
1324 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1325 error message something like @samp{tar: birds: Not found in archive},
1326 because there is no member named @file{birds}, only one named
1327 @file{./birds}. While the names @file{birds} and @file{./birds} name
1328 the same file, @emph{member} names by default are compared verbatim.
1330 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1331 with @file{baboon}, because this exact member name is in the archive file
1332 @file{bfiles.tar}. If you are not sure of the exact file name,
1333 use @dfn{globbing patterns}, for example:
1336 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1340 will list all members whose name contains @samp{b}. @xref{wildcards},
1341 for a detailed discussion of globbing patterns and related
1342 @command{tar} command line options.
1349 @unnumberedsubsec Listing the Contents of a Stored Directory
1351 To get information about the contents of an archived directory,
1352 use the directory name as a file name argument in conjunction with
1353 @option{--list} (@option{-t}). To find out file attributes, include the
1354 @option{--verbose} (@option{-v}) option.
1356 For example, to find out about files in the directory @file{practice}, in
1357 the archive file @file{music.tar}, type:
1360 $ @kbd{tar --list --verbose --file=music.tar practice}
1363 @command{tar} responds:
1366 drwxrwxrwx myself user 0 1990-05-31 21:49 practice/
1367 -rw-r--r-- myself user 42 1990-05-21 13:29 practice/blues
1368 -rw-r--r-- myself user 62 1990-05-23 10:55 practice/folk
1369 -rw-r--r-- myself user 40 1990-05-21 13:30 practice/jazz
1370 -rw-r--r-- myself user 10240 1990-05-31 21:49 practice/collection.tar
1373 When you use a directory name as a file name argument, @command{tar} acts on
1374 all the files (including sub-directories) in that directory.
1377 @section How to Extract Members from an Archive
1380 @cindex Retrieving files from an archive
1381 @cindex Resurrecting files from an archive
1384 Creating an archive is only half the job---there is no point in storing
1385 files in an archive if you can't retrieve them. The act of retrieving
1386 members from an archive so they can be used and manipulated as
1387 unarchived files again is called @dfn{extraction}. To extract files
1388 from an archive, use the @option{--extract} (@option{--get} or
1389 @option{-x}) operation. As with @option{--create}, specify the name
1390 of the archive with @option{--file} (@option{-f}) option. Extracting
1391 an archive does not modify the archive in any way; you can extract it
1392 multiple times if you want or need to.
1394 Using @option{--extract}, you can extract an entire archive, or specific
1395 files. The files can be directories containing other files, or not. As
1396 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1397 long form of the operation without affecting the performance.
1400 * extracting archives::
1401 * extracting files::
1403 * extracting untrusted archives::
1404 * failing commands::
1407 @node extracting archives
1408 @subsection Extracting an Entire Archive
1410 To extract an entire archive, specify the archive file name only, with
1411 no individual file names as arguments. For example,
1414 $ @kbd{tar -xvf collection.tar}
1421 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
1422 -rw-r--r-- me user 21 1996-09-23 16:44 blues
1423 -rw-r--r-- me user 20 1996-09-23 16:44 folk
1426 @node extracting files
1427 @subsection Extracting Specific Files
1429 To extract specific archive members, give their exact member names as
1430 arguments, as printed by @option{--list} (@option{-t}). If you had
1431 mistakenly deleted one of the files you had placed in the archive
1432 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1433 from the archive without changing the archive's structure. Its
1434 contents will be identical to the original file @file{blues} that you
1437 First, make sure you are in the @file{practice} directory, and list the
1438 files in the directory. Now, delete the file, @samp{blues}, and list
1439 the files in the directory again.
1441 You can now extract the member @file{blues} from the archive file
1442 @file{collection.tar} like this:
1445 $ @kbd{tar --extract --file=collection.tar blues}
1449 If you list the files in the directory again, you will see that the file
1450 @file{blues} has been restored, with its original permissions, data
1451 modification times, and owner.@footnote{This is only accidentally
1452 true, but not in general. Whereas modification times are always
1453 restored, in most cases, one has to be root for restoring the owner,
1454 and use a special option for restoring permissions. Here, it just
1455 happens that the restoring user is also the owner of the archived
1456 members, and that the current @code{umask} is compatible with original
1457 permissions.} (These parameters will be identical to those which
1458 the file had when you originally placed it in the archive; any changes
1459 you may have made before deleting the file from the file system,
1460 however, will @emph{not} have been made to the archive member.) The
1461 archive file, @samp{collection.tar}, is the same as it was before you
1462 extracted @samp{blues}. You can confirm this by running @command{tar} with
1463 @option{--list} (@option{-t}).
1465 Remember that as with other operations, specifying the exact member
1466 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1467 will fail, because there is no member named @file{birds}. To extract
1468 the member named @file{./birds}, you must specify @w{@kbd{tar
1469 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1470 exact member names, use @option{--list} (@option{-t}) option
1471 (@pxref{list}). You can also extract those members that match a
1472 specific @dfn{globbing pattern}. For example, to extract from
1473 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1474 directory prefix, you could type:
1477 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1481 Here, @option{--wildcards} instructs @command{tar} to treat
1482 command line arguments as globbing patterns and @option{--no-anchored}
1483 informs it that the patterns apply to member names after any @samp{/}
1484 delimiter. The use of globbing patterns is discussed in detail in
1487 You can extract a file to standard output by combining the above options
1488 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1491 If you give the @option{--verbose} option, then @option{--extract}
1492 will print the names of the archive members as it extracts them.
1495 @subsection Extracting Files that are Directories
1497 Extracting directories which are members of an archive is similar to
1498 extracting other files. The main difference to be aware of is that if
1499 the extracted directory has the same name as any directory already in
1500 the working directory, then files in the extracted directory will be
1501 placed into the directory of the same name. Likewise, if there are
1502 files in the pre-existing directory with the same names as the members
1503 which you extract, the files from the extracted archive will replace
1504 the files already in the working directory (and possible
1505 subdirectories). This will happen regardless of whether or not the
1506 files in the working directory were more recent than those extracted
1507 (there exist, however, special options that alter this behavior
1510 However, if a file was stored with a directory name as part of its file
1511 name, and that directory does not exist under the working directory when
1512 the file is extracted, @command{tar} will create the directory.
1514 We can demonstrate how to use @option{--extract} to extract a directory
1515 file with an example. Change to the @file{practice} directory if you
1516 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1517 go back to the parent directory and extract the archive
1518 @file{music.tar}. You may either extract the entire archive, or you may
1519 extract only the files you just deleted. To extract the entire archive,
1520 don't give any file names as arguments after the archive name
1521 @file{music.tar}. To extract only the files you deleted, use the
1525 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1531 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1532 would have displayed more detail about the extracted files, as shown
1533 in the example below:
1536 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1537 -rw-r--r-- me user 28 1996-10-18 16:31 practice/jazz
1538 -rw-r--r-- me user 20 1996-09-23 16:44 practice/folk
1542 Because you created the directory with @file{practice} as part of the
1543 file names of each of the files by archiving the @file{practice}
1544 directory as @file{practice}, you must give @file{practice} as part
1545 of the file names when you extract those files from the archive.
1547 @node extracting untrusted archives
1548 @subsection Extracting Archives from Untrusted Sources
1550 Extracting files from archives can overwrite files that already exist.
1551 If you receive an archive from an untrusted source, you should make a
1552 new directory and extract into that directory, so that you don't have
1553 to worry about the extraction overwriting one of your existing files.
1554 For example, if @file{untrusted.tar} came from somewhere else on the
1555 Internet, and you don't necessarily trust its contents, you can
1556 extract it as follows:
1559 $ @kbd{mkdir newdir}
1561 $ @kbd{tar -xvf ../untrusted.tar}
1564 It is also a good practice to examine contents of the archive
1565 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1566 with @option{--verbose} (@option{-v}).
1568 @node failing commands
1569 @subsection Commands That Will Fail
1571 Here are some sample commands you might try which will not work, and why
1574 If you try to use this command,
1577 $ @kbd{tar -xvf music.tar folk jazz}
1581 you will get the following response:
1584 tar: folk: Not found in archive
1585 tar: jazz: Not found in archive
1590 This is because these files were not originally @emph{in} the parent
1591 directory @file{..}, where the archive is located; they were in the
1592 @file{practice} directory, and their file names reflect this:
1595 $ @kbd{tar -tvf music.tar}
1601 @FIXME{make sure the above works when going through the examples in
1605 Likewise, if you try to use this command,
1608 $ @kbd{tar -tvf music.tar folk jazz}
1612 you would get a similar response. Members with those names are not in the
1613 archive. You must use the correct member names, or wildcards, in order
1614 to extract the files from the archive.
1616 If you have forgotten the correct names of the files in the archive,
1617 use @w{@kbd{tar --list --verbose}} to list them correctly.
1619 @FIXME{more examples, here? hag thinks it's a good idea.}
1622 @section Going Further Ahead in this Manual
1624 @FIXME{need to write up a node here about the things that are going to
1625 be in the rest of the manual.}
1627 @node tar invocation
1628 @chapter Invoking @GNUTAR{}
1631 This chapter is about how one invokes the @GNUTAR{}
1632 command, from the command synopsis (@pxref{Synopsis}). There are
1633 numerous options, and many styles for writing them. One mandatory
1634 option specifies the operation @command{tar} should perform
1635 (@pxref{Operation Summary}), other options are meant to detail how
1636 this operation should be performed (@pxref{Option Summary}).
1637 Non-option arguments are not always interpreted the same way,
1638 depending on what the operation is.
1640 You will find in this chapter everything about option styles and rules for
1641 writing them (@pxref{Styles}). On the other hand, operations and options
1642 are fully described elsewhere, in other chapters. Here, you will find
1643 only synthetic descriptions for operations and options, together with
1644 pointers to other parts of the @command{tar} manual.
1646 Some options are so special they are fully described right in this
1647 chapter. They have the effect of inhibiting the normal operation of
1648 @command{tar} or else, they globally alter the amount of feedback the user
1649 receives about what is going on. These are the @option{--help} and
1650 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1651 and @option{--interactive} options (@pxref{interactive}).
1655 * using tar options::
1665 @section General Synopsis of @command{tar}
1667 The @GNUTAR{} program is invoked as either one of:
1670 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1671 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1674 The second form is for when old options are being used.
1676 You can use @command{tar} to store files in an archive, to extract them from
1677 an archive, and to do other types of archive manipulation. The primary
1678 argument to @command{tar}, which is called the @dfn{operation}, specifies
1679 which action to take. The other arguments to @command{tar} are either
1680 @dfn{options}, which change the way @command{tar} performs an operation,
1681 or file names or archive members, which specify the files or members
1682 @command{tar} is to act on.
1684 You can actually type in arguments in any order, even if in this manual
1685 the options always precede the other arguments, to make examples easier
1686 to understand. Further, the option stating the main operation mode
1687 (the @command{tar} main command) is usually given first.
1689 Each @var{name} in the synopsis above is interpreted as an archive member
1690 name when the main command is one of @option{--compare}
1691 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1692 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1693 @option{--update} (@option{-u}). When naming archive members, you
1694 must give the exact name of the member in the archive, as it is
1695 printed by @option{--list}. For @option{--append} (@option{-r}) and
1696 @option{--create} (@option{-c}), these @var{name} arguments specify
1697 the names of either files or directory hierarchies to place in the archive.
1698 These files or hierarchies should already exist in the file system,
1699 prior to the execution of the @command{tar} command.
1701 @command{tar} interprets relative file names as being relative to the
1702 working directory. @command{tar} will make all file names relative
1703 (by removing leading slashes when archiving or restoring files),
1704 unless you specify otherwise (using the @option{--absolute-names}
1705 option). @xref{absolute}, for more information about
1706 @option{--absolute-names}.
1708 If you give the name of a directory as either a file name or a member
1709 name, then @command{tar} acts recursively on all the files and directories
1710 beneath that directory. For example, the name @file{/} identifies all
1711 the files in the file system to @command{tar}.
1713 The distinction between file names and archive member names is especially
1714 important when shell globbing is used, and sometimes a source of confusion
1715 for newcomers. @xref{wildcards}, for more information about globbing.
1716 The problem is that shells may only glob using existing files in the
1717 file system. Only @command{tar} itself may glob on archive members, so when
1718 needed, you must ensure that wildcard characters reach @command{tar} without
1719 being interpreted by the shell first. Using a backslash before @samp{*}
1720 or @samp{?}, or putting the whole argument between quotes, is usually
1721 sufficient for this.
1723 Even if @var{name}s are often specified on the command line, they
1724 can also be read from a text file in the file system, using the
1725 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1727 If you don't use any file name arguments, @option{--append} (@option{-r}),
1728 @option{--delete} and @option{--concatenate} (@option{--catenate},
1729 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1730 will usually yield a diagnostic and inhibit @command{tar} execution.
1731 The other operations of @command{tar} (@option{--list},
1732 @option{--extract}, @option{--compare}, and @option{--update})
1733 will act on the entire contents of the archive.
1736 @cindex return status
1737 Besides successful exits, @GNUTAR{} may fail for
1738 many reasons. Some reasons correspond to bad usage, that is, when the
1739 @command{tar} command is improperly written. Errors may be
1740 encountered later, while encountering an error processing the archive
1741 or the files. Some errors are recoverable, in which case the failure
1742 is delayed until @command{tar} has completed all its work. Some
1743 errors are such that it would not meaningful, or at least risky, to
1744 continue processing: @command{tar} then aborts processing immediately.
1745 All abnormal exits, whether immediate or delayed, should always be
1746 clearly diagnosed on @code{stderr}, after a line stating the nature of
1749 @GNUTAR{} returns only a few exit statuses. I'm really
1750 aiming simplicity in that area, for now. If you are not using the
1751 @option{--compare} @option{--diff}, @option{-d}) option, zero means
1752 that everything went well, besides maybe innocuous warnings. Nonzero
1753 means that something went wrong. Right now, as of today, ``nonzero''
1754 is almost always 2, except for remote operations, where it may be
1757 @node using tar options
1758 @section Using @command{tar} Options
1760 @GNUTAR{} has a total of eight operating modes which
1761 allow you to perform a variety of tasks. You are required to choose
1762 one operating mode each time you employ the @command{tar} program by
1763 specifying one, and only one operation as an argument to the
1764 @command{tar} command (two lists of four operations each may be found
1765 at @ref{frequent operations} and @ref{Operations}). Depending on
1766 circumstances, you may also wish to customize how the chosen operating
1767 mode behaves. For example, you may wish to change the way the output
1768 looks, or the format of the files that you wish to archive may require
1769 you to do something special in order to make the archive look right.
1771 You can customize and control @command{tar}'s performance by running
1772 @command{tar} with one or more options (such as @option{--verbose}
1773 (@option{-v}), which we used in the tutorial). As we said in the
1774 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1775 their name suggests) optional. Depending on the operating mode, you
1776 may specify one or more options. Different options will have different
1777 effects, but in general they all change details of the operation, such
1778 as archive format, archive name, or level of user interaction. Some
1779 options make sense with all operating modes, while others are
1780 meaningful only with particular modes. You will likely use some
1781 options frequently, while you will only use others infrequently, or
1782 not at all. (A full list of options is available in @pxref{All Options}.)
1784 @vrindex TAR_OPTIONS, environment variable
1785 @anchor{TAR_OPTIONS}
1786 The @env{TAR_OPTIONS} environment variable specifies default options to
1787 be placed in front of any explicit options. For example, if
1788 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1789 if the two options @option{-v} and @option{--unlink-first} had been
1790 specified before any explicit options. Option specifications are
1791 separated by whitespace. A backslash escapes the next character, so it
1792 can be used to specify an option containing whitespace or a backslash.
1794 Note that @command{tar} options are case sensitive. For example, the
1795 options @option{-T} and @option{-t} are different; the first requires an
1796 argument for stating the name of a file providing a list of @var{name}s,
1797 while the second does not require an argument and is another way to
1798 write @option{--list} (@option{-t}).
1800 In addition to the eight operations, there are many options to
1801 @command{tar}, and three different styles for writing both: long (mnemonic)
1802 form, short form, and old style. These styles are discussed below.
1803 Both the options and the operations can be written in any of these three
1806 @FIXME{menu at end of this node. need to think of an actual outline
1807 for this chapter; probably do that after stuff from chapter 4 is
1811 @section The Three Option Styles
1813 There are three styles for writing operations and options to the command
1814 line invoking @command{tar}. The different styles were developed at
1815 different times during the history of @command{tar}. These styles will be
1816 presented below, from the most recent to the oldest.
1818 Some options must take an argument. (For example, @option{--file}
1819 (@option{-f})) takes the name of an archive file as an argument. If
1820 you do not supply an archive file name, @command{tar} will use a
1821 default, but this can be confusing; thus, we recommend that you always
1822 supply a specific archive file name.) Where you @emph{place} the
1823 arguments generally depends on which style of options you choose. We
1824 will detail specific information relevant to each option style in the
1825 sections on the different option styles, below. The differences are
1826 subtle, yet can often be very important; incorrect option placement
1827 can cause you to overwrite a number of important files. We urge you
1828 to note these differences, and only use the option style(s) which
1829 makes the most sense to you until you feel comfortable with the others.
1831 Some options @emph{may} take an argument (currently, there are
1832 two such options: @option{--backup} and @option{--occurrence}). Such
1833 options may have at most long and short forms, they do not have old style
1834 equivalent. The rules for specifying an argument for such options
1835 are stricter than those for specifying mandatory arguments. Please,
1836 pay special attention to them.
1839 * Mnemonic Options:: Mnemonic Option Style
1840 * Short Options:: Short Option Style
1841 * Old Options:: Old Option Style
1842 * Mixing:: Mixing Option Styles
1845 @node Mnemonic Options
1846 @subsection Mnemonic Option Style
1848 @FIXME{have to decide whether or not to replace other occurrences of
1849 "mnemonic" with "long", or *ugh* vice versa.}
1851 Each option has at least one long (or mnemonic) name starting with two
1852 dashes in a row, e.g., @option{--list}. The long names are more clear than
1853 their corresponding short or old names. It sometimes happens that a
1854 single mnemonic option has many different different names which are
1855 synonymous, such as @option{--compare} and @option{--diff}. In addition,
1856 long option names can be given unique abbreviations. For example,
1857 @option{--cre} can be used in place of @option{--create} because there is no
1858 other mnemonic option which begins with @samp{cre}. (One way to find
1859 this out is by trying it and seeing what happens; if a particular
1860 abbreviation could represent more than one option, @command{tar} will tell
1861 you that that abbreviation is ambiguous and you'll know that that
1862 abbreviation won't work. You may also choose to run @samp{tar --help}
1863 to see a list of options. Be aware that if you run @command{tar} with a
1864 unique abbreviation for the long name of an option you didn't want to
1865 use, you are stuck; @command{tar} will perform the command as ordered.)
1867 Mnemonic options are meant to be obvious and easy to remember, and their
1868 meanings are generally easier to discern than those of their
1869 corresponding short options (see below). For example:
1872 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
1876 gives a fairly good set of hints about what the command does, even
1877 for those not fully acquainted with @command{tar}.
1879 Mnemonic options which require arguments take those arguments
1880 immediately following the option name. There are two ways of
1881 specifying a mandatory argument. It can be separated from the
1882 option name either by an equal sign, or by any amount of
1883 white space characters. For example, the @option{--file} option (which
1884 tells the name of the @command{tar} archive) is given a file such as
1885 @file{archive.tar} as argument by using any of the following notations:
1886 @option{--file=archive.tar} or @option{--file archive.tar}.
1888 In contrast, optional arguments must always be introduced using
1889 an equal sign. For example, the @option{--backup} option takes
1890 an optional argument specifying backup type. It must be used
1891 as @option{--backup=@var{backup-type}}.
1894 @subsection Short Option Style
1896 Most options also have a short option name. Short options start with
1897 a single dash, and are followed by a single character, e.g., @option{-t}
1898 (which is equivalent to @option{--list}). The forms are absolutely
1899 identical in function; they are interchangeable.
1901 The short option names are faster to type than long option names.
1903 Short options which require arguments take their arguments immediately
1904 following the option, usually separated by white space. It is also
1905 possible to stick the argument right after the short option name, using
1906 no intervening space. For example, you might write @w{@option{-f
1907 archive.tar}} or @option{-farchive.tar} instead of using
1908 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
1909 @w{@option{-f @var{archive-name}}} denote the option which indicates a
1910 specific archive, here named @file{archive.tar}.
1912 Short options which take optional arguments take their arguments
1913 immediately following the option letter, @emph{without any intervening
1914 white space characters}.
1916 Short options' letters may be clumped together, but you are not
1917 required to do this (as compared to old options; see below). When
1918 short options are clumped as a set, use one (single) dash for them
1919 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
1920 such a set is allowed to have an argument@footnote{Clustering many
1921 options, the last of which has an argument, is a rather opaque way to
1922 write options. Some wonder if @acronym{GNU} @code{getopt} should not
1923 even be made helpful enough for considering such usages as invalid.}.
1925 When the options are separated, the argument for each option which requires
1926 an argument directly follows that option, as is usual for Unix programs.
1930 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
1933 If you reorder short options' locations, be sure to move any arguments
1934 that belong to them. If you do not move the arguments properly, you may
1935 end up overwriting files.
1938 @subsection Old Option Style
1941 Like short options, old options are single letters. However, old options
1942 must be written together as a single clumped set, without spaces separating
1943 them or dashes preceding them@footnote{Beware that if you precede options
1944 with a dash, you are announcing the short option style instead of the
1945 old option style; short options are decoded differently.}. This set
1946 of letters must be the first to appear on the command line, after the
1947 @command{tar} program name and some white space; old options cannot appear
1948 anywhere else. The letter of an old option is exactly the same letter as
1949 the corresponding short option. For example, the old option @samp{t} is
1950 the same as the short option @option{-t}, and consequently, the same as the
1951 mnemonic option @option{--list}. So for example, the command @w{@samp{tar
1952 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
1954 When options that need arguments are given together with the command,
1955 all the associated arguments follow, in the same order as the options.
1956 Thus, the example given previously could also be written in the old
1960 $ @kbd{tar cvbf 20 /dev/rmt0}
1964 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
1965 the argument of @option{-f}.
1967 On the other hand, this old style syntax makes it difficult to match
1968 option letters with their corresponding arguments, and is often
1969 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
1970 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
1971 argument for @option{-f}, and @option{-v} does not have a corresponding
1972 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
1973 /dev/rmt0}} is clearer, putting all arguments next to the option they
1976 If you want to reorder the letters in the old option argument, be
1977 sure to reorder any corresponding argument appropriately.
1979 This old way of writing @command{tar} options can surprise even experienced
1980 users. For example, the two commands:
1983 @kbd{tar cfz archive.tar.gz file}
1984 @kbd{tar -cfz archive.tar.gz file}
1988 are quite different. The first example uses @file{archive.tar.gz} as
1989 the value for option @samp{f} and recognizes the option @samp{z}. The
1990 second example, however, uses @file{z} as the value for option
1991 @samp{f} --- probably not what was intended.
1993 Old options are kept for compatibility with old versions of @command{tar}.
1995 This second example could be corrected in many ways, among which the
1996 following are equivalent:
1999 @kbd{tar -czf archive.tar.gz file}
2000 @kbd{tar -cf archive.tar.gz -z file}
2001 @kbd{tar cf archive.tar.gz -z file}
2004 @cindex option syntax, traditional
2005 As far as we know, all @command{tar} programs, @acronym{GNU} and
2006 non-@acronym{GNU}, support old options. @GNUTAR{}
2007 supports them not only for historical reasons, but also because many
2008 people are used to them. For compatibility with Unix @command{tar},
2009 the first argument is always treated as containing command and option
2010 letters even if it doesn't start with @samp{-}. Thus, @samp{tar c} is
2011 equivalent to @w{@samp{tar -c}:} both of them specify the
2012 @option{--create} (@option{-c}) command to create an archive.
2015 @subsection Mixing Option Styles
2017 All three styles may be intermixed in a single @command{tar} command,
2018 so long as the rules for each style are fully
2019 respected@footnote{Before @GNUTAR{} version 1.11.6,
2020 a bug prevented intermixing old style options with mnemonic options in
2021 some cases.}. Old style options and either of the modern styles of
2022 options may be mixed within a single @command{tar} command. However,
2023 old style options must be introduced as the first arguments only,
2024 following the rule for old options (old options must appear directly
2025 after the @command{tar} command and some white space). Modern options
2026 may be given only after all arguments to the old options have been
2027 collected. If this rule is not respected, a modern option might be
2028 falsely interpreted as the value of the argument to one of the old
2031 For example, all the following commands are wholly equivalent, and
2032 illustrate the many combinations and orderings of option styles.
2035 @kbd{tar --create --file=archive.tar}
2036 @kbd{tar --create -f archive.tar}
2037 @kbd{tar --create -farchive.tar}
2038 @kbd{tar --file=archive.tar --create}
2039 @kbd{tar --file=archive.tar -c}
2040 @kbd{tar -c --file=archive.tar}
2041 @kbd{tar -c -f archive.tar}
2042 @kbd{tar -c -farchive.tar}
2043 @kbd{tar -cf archive.tar}
2044 @kbd{tar -cfarchive.tar}
2045 @kbd{tar -f archive.tar --create}
2046 @kbd{tar -f archive.tar -c}
2047 @kbd{tar -farchive.tar --create}
2048 @kbd{tar -farchive.tar -c}
2049 @kbd{tar c --file=archive.tar}
2050 @kbd{tar c -f archive.tar}
2051 @kbd{tar c -farchive.tar}
2052 @kbd{tar cf archive.tar}
2053 @kbd{tar f archive.tar --create}
2054 @kbd{tar f archive.tar -c}
2055 @kbd{tar fc archive.tar}
2058 On the other hand, the following commands are @emph{not} equivalent to
2062 @kbd{tar -f -c archive.tar}
2063 @kbd{tar -fc archive.tar}
2064 @kbd{tar -fcarchive.tar}
2065 @kbd{tar -farchive.tarc}
2066 @kbd{tar cfarchive.tar}
2070 These last examples mean something completely different from what the
2071 user intended (judging based on the example in the previous set which
2072 uses long options, whose intent is therefore very clear). The first
2073 four specify that the @command{tar} archive would be a file named
2074 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2075 respectively. The first two examples also specify a single non-option,
2076 @var{name} argument having the value @samp{archive.tar}. The last
2077 example contains only old style option letters (repeating option
2078 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2079 @samp{h}, or @samp{i}), with no argument value. @FIXME{not sure i liked
2080 the first sentence of this paragraph..}
2083 @section All @command{tar} Options
2085 The coming manual sections contain an alphabetical listing of all
2086 @command{tar} operations and options, with brief descriptions and cross
2087 references to more in-depth explanations in the body of the manual.
2088 They also contain an alphabetically arranged table of the short option
2089 forms with their corresponding long option. You can use this table as
2090 a reference for deciphering @command{tar} commands in scripts.
2093 * Operation Summary::
2095 * Short Option Summary::
2098 @node Operation Summary
2099 @subsection Operations
2103 @opindex append, summary
2107 Appends files to the end of the archive. @xref{append}.
2109 @opindex catenate, summary
2113 Same as @option{--concatenate}. @xref{concatenate}.
2115 @opindex compare, summary
2119 Compares archive members with their counterparts in the file
2120 system, and reports differences in file size, mode, owner,
2121 modification date and contents. @xref{compare}.
2123 @opindex concatenate, summary
2127 Appends other @command{tar} archives to the end of the archive.
2130 @opindex create, summary
2134 Creates a new @command{tar} archive. @xref{create}.
2136 @opindex delete, summary
2139 Deletes members from the archive. Don't try this on a archive on a
2140 tape! @xref{delete}.
2142 @opindex diff, summary
2146 Same @option{--compare}. @xref{compare}.
2148 @opindex extract, summary
2152 Extracts members from the archive into the file system. @xref{extract}.
2154 @opindex get, summary
2158 Same as @option{--extract}. @xref{extract}.
2160 @opindex list, summary
2164 Lists the members in an archive. @xref{list}.
2166 @opindex update, summary
2170 Adds files to the end of the archive, but only if they are newer than
2171 their counterparts already in the archive, or if they do not already
2172 exist in the archive. @xref{update}.
2176 @node Option Summary
2177 @subsection @command{tar} Options
2181 @opindex absolute-names, summary
2182 @item --absolute-names
2185 Normally when creating an archive, @command{tar} strips an initial
2186 @samp{/} from member names. This option disables that behavior.
2189 @opindex after-date, summary
2192 (See @option{--newer}, @pxref{after})
2194 @opindex anchored, summary
2196 A pattern must match an initial subsequence of the name's components.
2197 @xref{controlling pattern-matching}.
2199 @opindex atime-preserve, summary
2200 @item --atime-preserve
2201 @itemx --atime-preserve=replace
2202 @itemx --atime-preserve=system
2204 Attempt to preserve the access time of files when reading them. This
2205 option currently is effective only on files that you own, unless you
2206 have superuser privileges.
2208 @option{--atime-preserve=replace} remembers the access time of a file
2209 before reading it, and then restores the access time afterwards. This
2210 may cause problems if other programs are reading the file at the same
2211 time, as the times of their accesses will be lost. On most platforms
2212 restoring the access time also requires @command{tar} to restore the
2213 data modification time too, so this option may also cause problems if
2214 other programs are writing the file at the same time. (Tar attempts
2215 to detect this situation, but cannot do so reliably due to race
2216 conditions.) Worse, on most platforms restoring the access time also
2217 updates the status change time, which means that this option is
2218 incompatible with incremental backups.
2220 @option{--atime-preserve=system} avoids changing time stamps on files,
2221 without interfering with time stamp updates
2222 caused by other programs, so it works better with incremental backups.
2223 However, it requires a special @code{O_NOATIME} option from the
2224 underlying operating and file system implementation, and it also requires
2225 that searching directories does not update their access times. As of
2226 this writing (November 2005) this works only with Linux, and only with
2227 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2228 way to know whether this feature actually works. Sometimes
2229 @command{tar} knows that it does not work, and if you use
2230 @option{--atime-preserve=system} then @command{tar} complains and
2231 exits right away. But other times @command{tar} might think that the
2232 option works when it actually does not.
2234 Currently @option{--atime-preserve} with no operand defaults to
2235 @option{--atime-preserve=replace}, but this may change in the future
2236 as support for @option{--atime-preserve=system} improves.
2238 If your operating system does not support
2239 @option{--atime-preserve=system}, you might be able to preserve access
2240 times reliably by by using the @command{mount} command. For example,
2241 you can mount the file system read-only, or access the file system via
2242 a read-only loopback mount, or use the @samp{noatime} mount option
2243 available on some systems. However, mounting typically requires
2244 superuser privileges and can be a pain to manage.
2246 @opindex backup, summary
2247 @item --backup=@var{backup-type}
2249 Rather than deleting files from the file system, @command{tar} will
2250 back them up using simple or numbered backups, depending upon
2251 @var{backup-type}. @xref{backup}.
2253 @opindex block-number, summary
2254 @item --block-number
2257 With this option present, @command{tar} prints error messages for read errors
2258 with the block number in the archive file. @xref{block-number}.
2260 @opindex blocking-factor, summary
2261 @item --blocking-factor=@var{blocking}
2262 @itemx -b @var{blocking}
2264 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2265 record. @xref{Blocking Factor}.
2267 @opindex bzip2, summary
2271 This option tells @command{tar} to read or write archives through
2272 @code{bzip2}. @xref{gzip}.
2274 @opindex checkpoint, summary
2277 This option directs @command{tar} to print periodic checkpoint messages as it
2278 reads through the archive. It is intended for when you want a visual
2279 indication that @command{tar} is still running, but don't want to see
2280 @option{--verbose} output. @FIXME-xref{}
2282 @opindex check-links, summary
2285 If this option was given, @command{tar} will check the number of links
2286 dumped for each processed file. If this number does not match the
2287 total number of hard links for the file, a warning message will be
2288 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2289 synonym for @option{--one-file-system}. The current semantics, wich
2290 complies to UNIX98, was introduced with version
2291 1.15.91. @xref{Changes}, for more information.}.
2293 @opindex compress, summary
2294 @opindex uncompress, summary
2299 @command{tar} will use the @command{compress} program when reading or
2300 writing the archive. This allows you to directly act on archives
2301 while saving space. @xref{gzip}.
2303 @opindex confirmation, summary
2304 @item --confirmation
2306 (See @option{--interactive}.) @xref{interactive}.
2308 @opindex delay-directory-restore, summary
2309 @item --delay-directory-restore
2311 Delay setting modification times and permissions of extracted
2312 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2314 @opindex dereference, summary
2318 When creating a @command{tar} archive, @command{tar} will archive the
2319 file that a symbolic link points to, rather than archiving the
2320 symlink. @xref{dereference}.
2322 @opindex directory, summary
2323 @item --directory=@var{dir}
2326 When this option is specified, @command{tar} will change its current directory
2327 to @var{dir} before performing any operations. When this option is used
2328 during archive creation, it is order sensitive. @xref{directory}.
2330 @opindex exclude, summary
2331 @item --exclude=@var{pattern}
2333 When performing operations, @command{tar} will skip files that match
2334 @var{pattern}. @xref{exclude}.
2336 @opindex exclude-from, summary
2337 @item --exclude-from=@var{file}
2338 @itemx -X @var{file}
2340 Similar to @option{--exclude}, except @command{tar} will use the list of
2341 patterns in the file @var{file}. @xref{exclude}.
2343 @opindex exclude-caches, summary
2344 @item --exclude-caches
2346 Automatically excludes all directories
2347 containing a cache directory tag. @xref{exclude}.
2349 @opindex file, summary
2350 @item --file=@var{archive}
2351 @itemx -f @var{archive}
2353 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2354 performs operations on, rather than @command{tar}'s compilation dependent
2355 default. @xref{file tutorial}.
2357 @opindex files-from, summary
2358 @item --files-from=@var{file}
2359 @itemx -T @var{file}
2361 @command{tar} will use the contents of @var{file} as a list of archive members
2362 or files to operate on, in addition to those specified on the
2363 command-line. @xref{files}.
2365 @opindex force-local, summary
2368 Forces @command{tar} to interpret the filename given to @option{--file}
2369 as a local file, even if it looks like a remote tape drive name.
2370 @xref{local and remote archives}.
2372 @opindex format, summary
2373 @item --format=@var{format}
2375 Selects output archive format. @var{Format} may be one of the
2380 Creates an archive that is compatible with Unix V7 @command{tar}.
2383 Creates an archive that is compatible with GNU @command{tar} version
2387 Creates archive in GNU tar 1.13 format. Basically it is the same as
2388 @samp{oldgnu} with the only difference in the way it handles long
2392 Creates a @acronym{POSIX.1-1988} compatible archive.
2395 Creates a @acronym{POSIX.1-2001 archive}.
2399 @xref{Formats}, for a detailed discussion of these formats.
2401 @opindex group, summary
2402 @item --group=@var{group}
2404 Files added to the @command{tar} archive will have a group id of @var{group},
2405 rather than the group from the source file. @var{group} is first decoded
2406 as a group symbolic name, but if this interpretation fails, it has to be
2407 a decimal numeric group ID. @FIXME-xref{}
2409 Also see the comments for the @option{--owner=@var{user}} option.
2411 @opindex gzip, summary
2412 @opindex gunzip, summary
2413 @opindex ungzip, summary
2419 This option tells @command{tar} to read or write archives through
2420 @command{gzip}, allowing @command{tar} to directly operate on several
2421 kinds of compressed archives transparently. @xref{gzip}.
2423 @opindex help, summary
2426 @command{tar} will print out a short message summarizing the operations and
2427 options to @command{tar} and exit. @xref{help}.
2429 @opindex ignore-case, summary
2431 Ignore case when matching member or file names with
2432 patterns. @xref{controlling pattern-matching}.
2434 @opindex ignore-command-error, summary
2435 @item --ignore-command-error
2436 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2438 @opindex ignore-failed-read, summary
2439 @item --ignore-failed-read
2441 Do not exit unsuccessfully merely because an unreadable file was encountered.
2444 @opindex ignore-zeros, summary
2445 @item --ignore-zeros
2448 With this option, @command{tar} will ignore zeroed blocks in the
2449 archive, which normally signals EOF. @xref{Reading}.
2451 @opindex incremental, summary
2455 Used to inform @command{tar} that it is working with an old
2456 @acronym{GNU}-format incremental backup archive. It is intended
2457 primarily for backwards compatibility only. @xref{Incremental Dumps},
2458 for a detailed discussion of incremental archives.
2460 @opindex index-file, summary
2461 @item --index-file=@var{file}
2463 Send verbose output to @var{file} instead of to standard output.
2465 @opindex info-script, summary
2466 @opindex new-volume-script, summary
2467 @item --info-script=@var{script-file}
2468 @itemx --new-volume-script=@var{script-file}
2469 @itemx -F @var{script-file}
2471 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2472 at the end of each tape. If @var{script-file} exits with nonzero status,
2473 @command{tar} fails immediately. @xref{info-script}, for a detailed
2474 discussion of @var{script-file}.
2476 @opindex interactive, summary
2478 @itemx --confirmation
2481 Specifies that @command{tar} should ask the user for confirmation before
2482 performing potentially destructive options, such as overwriting files.
2485 @opindex keep-newer-files, summary
2486 @item --keep-newer-files
2488 Do not replace existing files that are newer than their archive copies
2489 when extracting files from an archive.
2491 @opindex keep-old-files, summary
2492 @item --keep-old-files
2495 Do not overwrite existing files when extracting files from an archive.
2496 @xref{Keep Old Files}.
2498 @opindex label, summary
2499 @item --label=@var{name}
2500 @itemx -V @var{name}
2502 When creating an archive, instructs @command{tar} to write @var{name}
2503 as a name record in the archive. When extracting or listing archives,
2504 @command{tar} will only operate on archives that have a label matching
2505 the pattern specified in @var{name}. @xref{Tape Files}.
2507 @opindex listed-incremental, summary
2508 @item --listed-incremental=@var{snapshot-file}
2509 @itemx -g @var{snapshot-file}
2511 During a @option{--create} operation, specifies that the archive that
2512 @command{tar} creates is a new @acronym{GNU}-format incremental
2513 backup, using @var{snapshot-file} to determine which files to backup.
2514 With other operations, informs @command{tar} that the archive is in
2515 incremental format. @xref{Incremental Dumps}.
2517 @opindex mode, summary
2518 @item --mode=@var{permissions}
2520 When adding files to an archive, @command{tar} will use
2521 @var{permissions} for the archive members, rather than the permissions
2522 from the files. The program @command{chmod} and this @command{tar}
2523 option share the same syntax for what @var{permissions} might be.
2524 @xref{File permissions, Permissions, File permissions, fileutils,
2525 @acronym{GNU} file utilities}. This reference also has useful
2526 information for those not being overly familiar with the Unix
2529 Of course, @var{permissions} might be plainly specified as an octal number.
2530 However, by using generic symbolic modifications to mode bits, this allows
2531 more flexibility. For example, the value @samp{a+rw} adds read and write
2532 permissions for everybody, while retaining executable bits on directories
2533 or on any other file already marked as executable.
2535 @opindex multi-volume, summary
2536 @item --multi-volume
2539 Informs @command{tar} that it should create or otherwise operate on a
2540 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2542 @opindex new-volume-script, summary
2543 @item --new-volume-script
2547 @opindex seek, summary
2551 Assume that the archive media supports seeks to arbitrary
2552 locations. Usually @command{tar} determines automatically whether
2553 the archive can be seeked or not. This option is intended for use
2554 in cases when such recognition fails.
2556 @opindex newer, summary
2557 @item --newer=@var{date}
2558 @itemx --after-date=@var{date}
2561 When creating an archive, @command{tar} will only add files that have changed
2562 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2563 is taken to be the name of a file whose data modification time specifies
2564 the date. @xref{after}.
2566 @opindex newer-mtime, summary
2567 @item --newer-mtime=@var{date}
2569 Like @option{--newer}, but add only files whose
2570 contents have changed (as opposed to just @option{--newer}, which will
2571 also back up files for which any status information has changed).
2573 @opindex no-anchored, summary
2575 An exclude pattern can match any subsequence of the name's components.
2576 @xref{controlling pattern-matching}.
2578 @opindex no-delay-directory-restore, summary
2579 @item --no-delay-directory-restore
2581 Setting modification times and permissions of extracted
2582 directories when all files from this directory has been
2583 extracted. This is the default. @xref{Directory Modification Times and Permissions}.
2585 @opindex no-ignore-case, summary
2586 @item --no-ignore-case
2587 Use case-sensitive matching.
2588 @xref{controlling pattern-matching}.
2590 @opindex no-ignore-command-error, summary
2591 @item --no-ignore-command-error
2592 Print warnings about subprocesses terminated with a non-zero exit
2593 code. @xref{Writing to an External Program}.
2595 @opindex no-quote-chars, summary
2596 @item --no-quote-chars=@var{string}
2597 Remove characters listed in @var{string} from the list of quoted
2598 characters set by the previous @option{--quote-chars} option
2599 (@pxref{quoting styles}).
2601 @opindex no-recursion, summary
2602 @item --no-recursion
2604 With this option, @command{tar} will not recurse into directories.
2607 @opindex no-same-owner, summary
2608 @item --no-same-owner
2611 When extracting an archive, do not attempt to preserve the owner
2612 specified in the @command{tar} archive. This the default behavior
2615 @opindex no-same-permissions, summary
2616 @item --no-same-permissions
2618 When extracting an archive, subtract the user's umask from files from
2619 the permissions specified in the archive. This is the default behavior
2622 @opindex no-wildcards, summary
2623 @item --no-wildcards
2624 Do not use wildcards.
2625 @xref{controlling pattern-matching}.
2627 @opindex no-wildcards-match-slash, summary
2628 @item --no-wildcards-match-slash
2629 Wildcards do not match @samp{/}.
2630 @xref{controlling pattern-matching}.
2632 @opindex null, summary
2635 When @command{tar} is using the @option{--files-from} option, this option
2636 instructs @command{tar} to expect filenames terminated with @option{NUL}, so
2637 @command{tar} can correctly work with file names that contain newlines.
2640 @opindex numeric-owner, summary
2641 @item --numeric-owner
2643 This option will notify @command{tar} that it should use numeric user
2644 and group IDs when creating a @command{tar} file, rather than names.
2648 When extracting files, this option is a synonym for
2649 @option{--no-same-owner}, i.e. it prevents @command{tar} from
2650 restoring ownership of files being extracted.
2652 When creating an archive, @option{-o} is a synonym for
2653 @option{--old-archive}. This behavior is for compatibility
2654 with previous versions of @GNUTAR{}, and will be
2655 removed in the future releases.
2657 @xref{Changes}, for more information.
2659 @opindex occurrence, summary
2660 @item --occurrence[=@var{number}]
2662 This option can be used in conjunction with one of the subcommands
2663 @option{--delete}, @option{--diff}, @option{--extract} or
2664 @option{--list} when a list of files is given either on the command
2665 line or via @option{-T} option.
2667 This option instructs @command{tar} to process only the @var{number}th
2668 occurrence of each named file. @var{Number} defaults to 1, so
2671 tar -x -f archive.tar --occurrence filename
2675 will extract the first occurrence of @file{filename} from @file{archive.tar}
2676 and will terminate without scanning to the end of the archive.
2678 @opindex old-archive, summary
2680 Synonym for @option{--format=v7}.
2682 @opindex one-file-system, summary
2683 @item --one-file-system
2684 Used when creating an archive. Prevents @command{tar} from recursing into
2685 directories that are on different file systems from the current
2686 directory @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2687 synonym for @option{--one-file-system}. This has changed in version
2688 1.15.91. @xref{Changes}, for more information.}.
2690 @opindex overwrite, summary
2693 Overwrite existing files and directory metadata when extracting files
2694 from an archive. @xref{Overwrite Old Files}.
2696 @opindex overwrite-dir, summary
2697 @item --overwrite-dir
2699 Overwrite the metadata of existing directories when extracting files
2700 from an archive. @xref{Overwrite Old Files}.
2702 @opindex owner, summary
2703 @item --owner=@var{user}
2705 Specifies that @command{tar} should use @var{user} as the owner of members
2706 when creating archives, instead of the user associated with the source
2707 file. @var{user} is first decoded as a user symbolic name, but if
2708 this interpretation fails, it has to be a decimal numeric user ID.
2711 There is no value indicating a missing number, and @samp{0} usually means
2712 @code{root}. Some people like to force @samp{0} as the value to offer in
2713 their distributions for the owner of files, because the @code{root} user is
2714 anonymous anyway, so that might as well be the owner of anonymous archives.
2716 This option does not affect extraction from archives.
2718 @opindex transform, summary
2719 @item --transform=@var{sed-expr}
2721 Transform file or member names using @command{sed} replacement expression
2722 @var{sed-expr}. For example,
2725 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
2729 will add to @file{archive} files from the current working directory,
2730 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
2731 discussion, @xref{transform}.
2733 To see transformed member names in verbose listings, use
2734 @option{--show-transformed-names} option
2735 (@pxref{show-transformed-names}).
2737 @opindex quote-chars, summary
2738 @item --quote-chars=@var{string}
2739 Always quote characters from @var{string}, even if the selected
2740 quoting style would not quote them (@pxref{quoting styles}).
2742 @opindex quoting-style, summary
2743 @item --quoting-style=@var{style}
2744 Set quoting style to use when printing member and file names
2745 (@pxref{quoting styles}). Valid @var{style} values are:
2746 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
2747 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
2748 style is @code{escape}, unless overridden while configuring the
2751 @opindex pax-option, summary
2752 @item --pax-option=@var{keyword-list}
2753 @FIXME{Such a detailed description does not belong there, move it elsewhere.}
2754 This option is meaningful only with @acronym{POSIX.1-2001} archives
2755 (@pxref{posix}). It modifies the way @command{tar} handles the
2756 extended header keywords. @var{Keyword-list} is a comma-separated
2757 list of keyword options, each keyword option taking one of
2758 the following forms:
2761 @item delete=@var{pattern}
2762 When used with one of archive-creation commands,
2763 this option instructs @command{tar} to omit from extended header records
2764 that it produces any keywords matching the string @var{pattern}.
2766 When used in extract or list mode, this option instructs tar
2767 to ignore any keywords matching the given @var{pattern} in the extended
2768 header records. In both cases, matching is performed using the pattern
2769 matching notation described in @acronym{POSIX 1003.2}, 3.13
2770 (See @cite{glob(7)}). For example:
2773 --pax-option delete=security.*
2776 would suppress security-related information.
2778 @item exthdr.name=@var{string}
2780 This keyword allows user control over the name that is written into the
2781 ustar header blocks for the extended headers. The name is obtained
2782 from @var{string} after making the following substitutions:
2784 @multitable @columnfractions .30 .70
2785 @headitem Meta-character @tab Replaced By
2786 @item %d @tab The directory name of the file, equivalent to the
2787 result of the @command{dirname} utility on the translated pathname.
2788 @item %f @tab The filename of the file, equivalent to the result
2789 of the @command{basename} utility on the translated pathname.
2790 @item %p @tab The process ID of the @command{tar} process.
2791 @item %% @tab A @samp{%} character.
2794 Any other @samp{%} characters in @var{string} produce undefined
2797 If no option @samp{exthdr.name=string} is specified, @command{tar}
2798 will use the following default value:
2804 @item globexthdr.name=@var{string}
2805 This keyword allows user control over the name that is written into
2806 the ustar header blocks for global extended header records. The name
2807 is obtained from the contents of @var{string}, after making
2808 the following substitutions:
2810 @multitable @columnfractions .30 .70
2811 @headitem Meta-character @tab Replaced By
2812 @item %n @tab An integer that represents the
2813 sequence number of the global extended header record in the archive,
2815 @item %p @tab The process ID of the @command{tar} process.
2816 @item %% @tab A @samp{%} character.
2819 Any other @samp{%} characters in @var{string} produce undefined results.
2821 If no option @samp{globexthdr.name=string} is specified, @command{tar}
2822 will use the following default value:
2825 $TMPDIR/GlobalHead.%p.%n
2829 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
2830 environment variable. If @var{TMPDIR} is not set, @command{tar}
2833 @item @var{keyword}=@var{value}
2834 When used with one of archive-creation commands, these keyword/value pairs
2835 will be included at the beginning of the archive in a global extended
2836 header record. When used with one of archive-reading commands,
2837 @command{tar} will behave as if it has encountered these keyword/value
2838 pairs at the beginning of the archive in a global extended header
2841 @item @var{keyword}:=@var{value}
2842 When used with one of archive-creation commands, these keyword/value pairs
2843 will be included as records at the beginning of an extended header for
2844 each file. This is effectively equivalent to @var{keyword}=@var{value}
2845 form except that it creates no global extended header records.
2847 When used with one of archive-reading commands, @command{tar} will
2848 behave as if these keyword/value pairs were included as records at the
2849 end of each extended header; thus, they will override any global or
2850 file-specific extended header record keywords of the same names.
2851 For example, in the command:
2854 tar --format=posix --create \
2855 --file archive --pax-option gname:=user .
2858 the group name will be forced to a new value for all files
2859 stored in the archive.
2862 @opindex portability, summary
2864 @itemx --old-archive
2865 Synonym for @option{--format=v7}.
2867 @opindex posix, summary
2869 Same as @option{--format=posix}.
2871 @opindex preserve, summary
2874 Synonymous with specifying both @option{--preserve-permissions} and
2875 @option{--same-order}. @xref{Setting Access Permissions}.
2877 @opindex preserve-order, summary
2878 @item --preserve-order
2880 (See @option{--same-order}; @pxref{Reading}.)
2882 @opindex preserve-permissions, summary
2883 @opindex same-permissions, summary
2884 @item --preserve-permissions
2885 @itemx --same-permissions
2888 When @command{tar} is extracting an archive, it normally subtracts the
2889 users' umask from the permissions specified in the archive and uses
2890 that number as the permissions to create the destination file.
2891 Specifying this option instructs @command{tar} that it should use the
2892 permissions directly from the archive. @xref{Setting Access Permissions}.
2894 @opindex read-full-records, summary
2895 @item --read-full-records
2898 Specifies that @command{tar} should reblock its input, for reading
2899 from pipes on systems with buggy implementations. @xref{Reading}.
2901 @opindex record-size, summary
2902 @item --record-size=@var{size}
2904 Instructs @command{tar} to use @var{size} bytes per record when accessing the
2905 archive. @xref{Blocking Factor}.
2907 @opindex recursion, summary
2910 With this option, @command{tar} recurses into directories.
2913 @opindex recursive-unlink, summary
2914 @item --recursive-unlink
2917 directory hierarchies before extracting directories of the same name
2918 from the archive. @xref{Recursive Unlink}.
2920 @opindex remove-files, summary
2921 @item --remove-files
2923 Directs @command{tar} to remove the source file from the file system after
2924 appending it to an archive. @xref{remove files}.
2926 @opindex restrict, summary
2929 Disable use of some potentially harmful @command{tar} options.
2930 Currently this option disables shell invocaton from multi-volume menu
2931 (@pxref{Using Multiple Tapes}).
2933 @opindex rmt-command, summary
2934 @item --rmt-command=@var{cmd}
2936 Notifies @command{tar} that it should use @var{cmd} instead of
2937 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
2939 @opindex rsh-command, summary
2940 @item --rsh-command=@var{cmd}
2942 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
2943 devices. @xref{Device}.
2945 @opindex same-order, summary
2947 @itemx --preserve-order
2950 This option is an optimization for @command{tar} when running on machines with
2951 small amounts of memory. It informs @command{tar} that the list of file
2952 arguments has already been sorted to match the order of files in the
2953 archive. @xref{Reading}.
2955 @opindex same-owner, summary
2958 When extracting an archive, @command{tar} will attempt to preserve the owner
2959 specified in the @command{tar} archive with this option present.
2960 This is the default behavior for the superuser; this option has an
2961 effect only for ordinary users. @xref{Attributes}.
2963 @opindex same-permissions, summary
2964 @item --same-permissions
2966 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
2968 @opindex show-defaults, summary
2969 @item --show-defaults
2971 Displays the default options used by @command{tar} and exits
2972 successfully. This option is intended for use in shell scripts.
2973 Here is an example of what you can see using this option:
2976 $ tar --show-defaults
2977 --format=gnu -f- -b20 --quoting-style=escape \
2978 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
2981 @opindex show-omitted-dirs, summary
2982 @item --show-omitted-dirs
2984 Instructs @command{tar} to mention directories its skipping over when
2985 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
2987 @opindex show-transformed-names, summary
2988 @opindex show-stored-names, summary
2989 @item --show-transformed-names
2990 @itemx --show-stored-names
2992 Display file or member names after applying any transformations
2993 (@FIXME-pxref{}). In particular, when used in conjunction with one of
2994 archive creation operations it instructs tar to list the member names
2995 stored in the archive, as opposed to the actual file
2996 names. @xref{listing member and file names}.
2998 @opindex sparse, summary
3002 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3003 sparse files efficiently. @xref{sparse}.
3005 @opindex starting-file, summary
3006 @item --starting-file=@var{name}
3007 @itemx -K @var{name}
3009 This option affects extraction only; @command{tar} will skip extracting
3010 files in the archive until it finds one that matches @var{name}.
3013 @opindex strip-components, summary
3014 @item --strip-components=@var{number}
3015 Strip given @var{number} of leading components from file names before
3016 extraction.@footnote{This option was called @option{--strip-path} in
3017 version 1.14.} For example, if archive @file{archive.tar} contained
3018 @file{/some/file/name}, then running
3021 tar --extract --file archive.tar --strip-components=2
3025 would extract this file to file @file{name}.
3027 @opindex suffix, summary
3028 @item --suffix=@var{suffix}
3030 Alters the suffix @command{tar} uses when backing up files from the default
3031 @samp{~}. @xref{backup}.
3033 @opindex tape-length, summary
3034 @item --tape-length=@var{num}
3037 Specifies the length of tapes that @command{tar} is writing as being
3038 @w{@var{num} x 1024} bytes long. @xref{Using Multiple Tapes}.
3040 @opindex test-label, summary
3043 Reads the volume label. If an argument is specified, test whether it
3044 matches the volume label. @xref{--test-label option}.
3046 @opindex to-command, summary
3047 @item --to-command=@var{command}
3049 During extraction @command{tar} will pipe extracted files to the
3050 standard input of @var{command}. @xref{Writing to an External Program}.
3052 @opindex to-stdout, summary
3056 During extraction, @command{tar} will extract files to stdout rather
3057 than to the file system. @xref{Writing to Standard Output}.
3059 @opindex totals, summary
3062 Displays the total number of bytes written after creating an archive.
3065 @opindex touch, summary
3069 Sets the data modification time of extracted files to the extraction time,
3070 rather than the data modification time stored in the archive.
3071 @xref{Data Modification Times}.
3073 @opindex uncompress, summary
3076 (See @option{--compress}. @pxref{gzip})
3078 @opindex ungzip, summary
3081 (See @option{--gzip}. @pxref{gzip})
3083 @opindex unlink-first, summary
3084 @item --unlink-first
3087 Directs @command{tar} to remove the corresponding file from the file
3088 system before extracting it from the archive. @xref{Unlink First}.
3090 @opindex use-compress-program, summary
3091 @item --use-compress-program=@var{prog}
3093 Instructs @command{tar} to access the archive through @var{prog}, which is
3094 presumed to be a compression program of some sort. @xref{gzip}.
3096 @opindex utc, summary
3099 Display file modification dates in @acronym{UTC}. This option implies
3102 @opindex verbose, summary
3106 Specifies that @command{tar} should be more verbose about the operations its
3107 performing. This option can be specified multiple times for some
3108 operations to increase the amount of information displayed.
3111 @opindex verify, summary
3115 Verifies that the archive was correctly written when creating an
3116 archive. @xref{verify}.
3118 @opindex version, summary
3121 Print information about the program's name, version, origin and legal
3122 status, all on standard output, and then exit successfully.
3125 @opindex volno-file, summary
3126 @item --volno-file=@var{file}
3128 Used in conjunction with @option{--multi-volume}. @command{tar} will keep track
3129 of which volume of a multi-volume archive its working in @var{file}.
3132 @opindex wildcards, summary
3134 Use wildcards when matching member names with patterns.
3135 @xref{controlling pattern-matching}.
3137 @opindex wildcards-match-slash, summary
3138 @item --wildcards-match-slash
3139 Wildcards match @samp{/}.
3140 @xref{controlling pattern-matching}.
3143 @node Short Option Summary
3144 @subsection Short Options Cross Reference
3146 Here is an alphabetized list of all of the short option forms, matching
3147 them with the equivalent long option.
3153 @option{--concatenate}
3157 @option{--read-full-records}
3161 @option{--directory}
3165 @option{--info-script}
3169 @option{--incremental}
3173 @option{--starting-file}
3177 @option{--tape-length}
3181 @option{--multi-volume}
3189 @option{--to-stdout}
3193 @option{--absolute-names}
3197 @option{--block-number}
3205 @option{--files-from}
3209 @option{--unlink-first}
3221 @option{--exclude-from}
3229 @option{--blocking-factor}
3245 @option{--listed-incremental}
3249 @option{--dereference}
3253 @option{--ignore-zeros}
3261 @option{--keep-old-files}
3265 @option{--one-file-system}. Use of this short option is deprecated. It
3266 is retained for compatibility with the earlier versions of GNU
3267 @command{tar}, and will be changed in future releases.
3269 @xref{Changes}, for more information.
3277 When creating --- @option{--no-same-owner}, when extracting ---
3278 @option{--portability}.
3280 The later usage is deprecated. It is retained for compatibility with
3281 the earlier versions of @GNUTAR{}. In the future releases
3282 @option{-o} will be equivalent to @option{--no-same-owner} only.
3286 @option{--preserve-permissions}
3294 @option{--same-order}
3310 @option{--interactive}
3323 @section @GNUTAR{} documentation
3325 @cindex Getting program version number
3327 @cindex Version of the @command{tar} program
3328 Being careful, the first thing is really checking that you are using
3329 @GNUTAR{}, indeed. The @option{--version} option
3330 causes @command{tar} to print information about its name, version,
3331 origin and legal status, all on standard output, and then exit
3332 successfully. For example, @w{@samp{tar --version}} might print:
3335 tar (GNU tar) 1.15.2
3336 Copyright (C) 2006 Free Software Foundation, Inc.
3337 This is free software. You may redistribute copies of it under the terms of
3338 the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
3339 There is NO WARRANTY, to the extent permitted by law.
3341 Written by John Gilmore and Jay Fenlason.
3345 The first occurrence of @samp{tar} in the result above is the program
3346 name in the package (for example, @command{rmt} is another program),
3347 while the second occurrence of @samp{tar} is the name of the package
3348 itself, containing possibly many programs. The package is currently
3349 named @samp{tar}, after the name of the main program it
3350 contains@footnote{There are plans to merge the @command{cpio} and
3351 @command{tar} packages into a single one which would be called
3352 @code{paxutils}. So, who knows if, one of this days, the
3353 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3356 @cindex Obtaining help
3357 @cindex Listing all @command{tar} options
3358 @opindex help, introduction
3359 Another thing you might want to do is checking the spelling or meaning
3360 of some particular @command{tar} option, without resorting to this
3361 manual, for once you have carefully read it. @GNUTAR{}
3362 has a short help feature, triggerable through the
3363 @option{--help} option. By using this option, @command{tar} will
3364 print a usage message listing all available options on standard
3365 output, then exit successfully, without doing anything else and
3366 ignoring all other options. Even if this is only a brief summary, it
3367 may be several screens long. So, if you are not using some kind of
3368 scrollable window, you might prefer to use something like:
3371 $ @kbd{tar --help | less}
3375 presuming, here, that you like using @command{less} for a pager. Other
3376 popular pagers are @command{more} and @command{pg}. If you know about some
3377 @var{keyword} which interests you and do not want to read all the
3378 @option{--help} output, another common idiom is doing:
3381 tar --help | grep @var{keyword}
3385 for getting only the pertinent lines. Notice, however, that some
3386 @command{tar} options have long description lines and the above
3387 command will list only the first of them.
3389 The exact look of the option summary displayed by @kbd{tar --help} is
3390 configurable. @xref{Configuring Help Summary}, for a detailed description.
3393 If you only wish to check the spelling of an option, running @kbd{tar
3394 --usage} may be a better choice. This will display a terse list of
3395 @command{tar} option without accompanying explanations.
3397 The short help output is quite succinct, and you might have to get
3398 back to the full documentation for precise points. If you are reading
3399 this paragraph, you already have the @command{tar} manual in some
3400 form. This manual is available in a variety of forms from
3401 @url{http://www.gnu.org/software/tar/manual}. It may printed out of the @GNUTAR{}
3402 distribution, provided you have @TeX{} already installed somewhere,
3403 and a laser printer around. Just configure the distribution, execute
3404 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3405 usual way (contact your local guru to know how). If @GNUTAR{}
3406 has been conveniently installed at your place, this
3407 manual is also available in interactive, hypertextual form as an Info
3408 file. Just call @w{@samp{info tar}} or, if you do not have the
3409 @command{info} program handy, use the Info reader provided within
3410 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3412 There is currently no @code{man} page for @GNUTAR{}.
3413 If you observe such a @code{man} page on the system you are running,
3414 either it does not belong to @GNUTAR{}, or it has not
3415 been produced by @acronym{GNU}. Some package maintainers convert
3416 @kbd{tar --help} output to a man page, using @command{help2man}. In
3417 any case, please bear in mind that the authoritative source of
3418 information about @GNUTAR{} is this Texinfo documentation.
3421 @section Obtaining @GNUTAR{} default values
3423 @opindex show-defaults
3424 @GNUTAR{} has some predefined defaults that are used when you do not
3425 explicitely specify another values. To obtain a list of such
3426 defaults, use @option{--show-defaults} option. This will output the
3427 values in the form of @command{tar} command line options:
3431 @kbd{tar --show-defaults}
3432 --format=gnu -f- -b20 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3437 The above output shows that this version of @GNUTAR{} defaults to
3438 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3439 output as the archive, if no @option{--file} option has been given
3440 (@pxref{file tutorial}), the default blocking factor is 20
3441 (@pxref{Blocking Factor}). It also shows the default locations where
3442 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3445 @section Checking @command{tar} progress
3447 Typically, @command{tar} performs most operations without reporting any
3448 information to the user except error messages. When using @command{tar}
3449 with many options, particularly ones with complicated or
3450 difficult-to-predict behavior, it is possible to make serious mistakes.
3451 @command{tar} provides several options that make observing @command{tar}
3452 easier. These options cause @command{tar} to print information as it
3453 progresses in its job, and you might want to use them just for being
3454 more careful about what is going on, or merely for entertaining
3455 yourself. If you have encountered a problem when operating on an
3456 archive, however, you may need more information than just an error
3457 message in order to solve the problem. The following options can be
3458 helpful diagnostic tools.
3460 @cindex Verbose operation
3462 Normally, the @option{--list} (@option{-t}) command to list an archive
3463 prints just the file names (one per line) and the other commands are
3464 silent. When used with most operations, the @option{--verbose}
3465 (@option{-v}) option causes @command{tar} to print the name of each
3466 file or archive member as it is processed. This and the other options
3467 which make @command{tar} print status information can be useful in
3468 monitoring @command{tar}.
3470 With @option{--create} or @option{--extract}, @option{--verbose} used
3471 once just prints the names of the files or members as they are processed.
3472 Using it twice causes @command{tar} to print a longer listing
3473 (reminiscent of @samp{ls -l}) for each member. Since @option{--list}
3474 already prints the names of the members, @option{--verbose} used once
3475 with @option{--list} causes @command{tar} to print an @samp{ls -l}
3476 type listing of the files in the archive. The following examples both
3477 extract members with long list output:
3480 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3481 $ @kbd{tar xvvf archive.tar}
3484 Verbose output appears on the standard output except when an archive is
3485 being written to the standard output, as with @samp{tar --create
3486 --file=- --verbose} (@samp{tar cfv -}, or even @samp{tar cv}---if the
3487 installer let standard output be the default archive). In that case
3488 @command{tar} writes verbose output to the standard error stream.
3490 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3491 verbose output to @var{file} rather than to standard output or standard
3494 @cindex Obtaining total status information
3496 The @option{--totals} option---which is only meaningful when used with
3497 @option{--create} (@option{-c})---causes @command{tar} to print the total
3498 amount written to the archive, after it has been fully created.
3500 @cindex Progress information
3502 The @option{--checkpoint} option prints an occasional message
3503 as @command{tar} reads or writes the archive. In fact, it prints
3504 a message each 10 records read or written. It is designed for
3505 those who don't need the more detailed (and voluminous) output of
3506 @option{--block-number} (@option{-R}), but do want visual confirmation
3507 that @command{tar} is actually making forward progress.
3509 @FIXME{There is some confusion here. It seems that -R once wrote a
3510 message at @samp{every} record read or written.}
3512 @opindex show-omitted-dirs
3513 @anchor{show-omitted-dirs}
3514 The @option{--show-omitted-dirs} option, when reading an archive---with
3515 @option{--list} or @option{--extract}, for example---causes a message
3516 to be printed for each directory in the archive which is skipped.
3517 This happens regardless of the reason for skipping: the directory might
3518 not have been named on the command line (implicitly or explicitly),
3519 it might be excluded by the use of the @option{--exclude=@var{pattern}} option, or
3522 @opindex block-number
3523 @cindex Block number where error occurred
3524 @anchor{block-number}
3525 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3526 every message it would normally produce, the block number within the
3527 archive where the message was triggered. Also, supplementary messages
3528 are triggered when reading blocks full of NULs, or when hitting end of
3529 file on the archive. As of now, if the archive if properly terminated
3530 with a NUL block, the reading of the file may stop before end of file
3531 is met, so the position of end of file will not usually show when
3532 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3533 drains the archive before exiting when reading the
3534 archive from a pipe.
3536 @cindex Error message, block number of
3537 This option is especially useful when reading damaged archives, since
3538 it helps pinpoint the damaged sections. It can also be used with
3539 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3540 choose among several backup tapes when retrieving a file later, in
3541 favor of the tape where the file appears earliest (closest to the
3542 front of the tape). @xref{backup}.
3545 @section Asking for Confirmation During Operations
3546 @cindex Interactive operation
3548 Typically, @command{tar} carries out a command without stopping for
3549 further instructions. In some situations however, you may want to
3550 exclude some files and archive members from the operation (for instance
3551 if disk or storage space is tight). You can do this by excluding
3552 certain files automatically (@pxref{Choosing}), or by performing
3553 an operation interactively, using the @option{--interactive} (@option{-w}) option.
3554 @command{tar} also accepts @option{--confirmation} for this option.
3556 @opindex interactive
3557 When the @option{--interactive} (@option{-w}) option is specified, before
3558 reading, writing, or deleting files, @command{tar} first prints a message
3559 for each such file, telling what operation it intends to take, then asks
3560 for confirmation on the terminal. The actions which require
3561 confirmation include adding a file to the archive, extracting a file
3562 from the archive, deleting a file from the archive, and deleting a file
3563 from disk. To confirm the action, you must type a line of input
3564 beginning with @samp{y}. If your input line begins with anything other
3565 than @samp{y}, @command{tar} skips that file.
3567 If @command{tar} is reading the archive from the standard input,
3568 @command{tar} opens the file @file{/dev/tty} to support the interactive
3571 Verbose output is normally sent to standard output, separate from
3572 other error messages. However, if the archive is produced directly
3573 on standard output, then verbose output is mixed with errors on
3574 @code{stderr}. Producing the archive on standard output may be used
3575 as a way to avoid using disk space, when the archive is soon to be
3576 consumed by another process reading it, say. Some people felt the need
3577 of producing an archive on stdout, still willing to segregate between
3578 verbose output and error output. A possible approach would be using a
3579 named pipe to receive the archive, and having the consumer process to
3580 read from that named pipe. This has the advantage of letting standard
3581 output free to receive verbose output, all separate from errors.
3584 @chapter @GNUTAR{} Operations
3597 @section Basic @GNUTAR{} Operations
3599 The basic @command{tar} operations, @option{--create} (@option{-c}),
3600 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
3601 @option{-x}), are currently presented and described in the tutorial
3602 chapter of this manual. This section provides some complementary notes
3603 for these operations.
3606 @opindex create, complementary notes
3610 Creating an empty archive would have some kind of elegance. One can
3611 initialize an empty archive and later use @option{--append}
3612 (@option{-r}) for adding all members. Some applications would not
3613 welcome making an exception in the way of adding the first archive
3614 member. On the other hand, many people reported that it is
3615 dangerously too easy for @command{tar} to destroy a magnetic tape with
3616 an empty archive@footnote{This is well described in @cite{Unix-haters
3617 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
3618 Books, ISBN 1-56884-203-1.}. The two most common errors are:
3622 Mistakingly using @code{create} instead of @code{extract}, when the
3623 intent was to extract the full contents of an archive. This error
3624 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
3625 the QWERTY keyboard. Instead of being unpacked, the archive then
3626 gets wholly destroyed. When users speak about @dfn{exploding} an
3627 archive, they usually mean something else :-).
3630 Forgetting the argument to @code{file}, when the intent was to create
3631 an archive with a single file in it. This error is likely because a
3632 tired user can easily add the @kbd{f} key to the cluster of option
3633 letters, by the mere force of habit, without realizing the full
3634 consequence of doing so. The usual consequence is that the single
3635 file, which was meant to be saved, is rather destroyed.
3638 So, recognizing the likelihood and the catastrophical nature of these
3639 errors, @GNUTAR{} now takes some distance from elegance, and
3640 cowardly refuses to create an archive when @option{--create} option is
3641 given, there are no arguments besides options, and
3642 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
3643 around the cautiousness of @GNUTAR{} and nevertheless create an
3644 archive with nothing in it, one may still use, as the value for the
3645 @option{--files-from} option, a file with no names in it, as shown in
3646 the following commands:
3649 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
3650 @kbd{tar cfT empty-archive.tar /dev/null}
3653 @opindex extract, complementary notes
3658 A socket is stored, within a @GNUTAR{} archive, as a pipe.
3660 @item @option{--list} (@option{-t})
3662 @GNUTAR{} now shows dates as @samp{1996-08-30},
3663 while it used to show them as @samp{Aug 30 1996}. Preferably,
3664 people should get used to ISO 8601 dates. Local American dates should
3665 be made available again with full date localization support, once
3666 ready. In the meantime, programs not being localizable for dates
3667 should prefer international dates, that's really the way to go.
3669 Look up @url{http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html} if you
3670 are curious, it contains a detailed explanation of the ISO 8601 standard.
3675 @section Advanced @GNUTAR{} Operations
3677 Now that you have learned the basics of using @GNUTAR{}, you may want
3678 to learn about further ways in which @command{tar} can help you.
3680 This chapter presents five, more advanced operations which you probably
3681 won't use on a daily basis, but which serve more specialized functions.
3682 We also explain the different styles of options and why you might want
3683 to use one or another, or a combination of them in your @command{tar}
3684 commands. Additionally, this chapter includes options which allow you to
3685 define the output from @command{tar} more carefully, and provide help and
3686 error correction in special circumstances.
3688 @FIXME{check this after the chapter is actually revised to make sure
3689 it still introduces the info in the chapter correctly : ).}
3701 @subsection The Five Advanced @command{tar} Operations
3704 In the last chapter, you learned about the first three operations to
3705 @command{tar}. This chapter presents the remaining five operations to
3706 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
3707 @option{--delete}, and @option{--compare}.
3709 You are not likely to use these operations as frequently as those
3710 covered in the last chapter; however, since they perform specialized
3711 functions, they are quite useful when you do need to use them. We
3712 will give examples using the same directory and files that you created
3713 in the last chapter. As you may recall, the directory is called
3714 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
3715 @samp{rock}, and the two archive files you created are
3716 @samp{collection.tar} and @samp{music.tar}.
3718 We will also use the archive files @samp{afiles.tar} and
3719 @samp{bfiles.tar}. @samp{afiles.tar} contains the members @samp{apple},
3720 @samp{angst}, and @samp{aspic}. @samp{bfiles.tar} contains the members
3721 @samp{./birds}, @samp{baboon}, and @samp{./box}.
3723 Unless we state otherwise, all practicing you do and examples you follow
3724 in this chapter will take place in the @file{practice} directory that
3725 you created in the previous chapter; see @ref{prepare for examples}.
3726 (Below in this section, we will remind you of the state of the examples
3727 where the last chapter left them.)
3729 The five operations that we will cover in this chapter are:
3734 Add new entries to an archive that already exists.
3737 Add more recent copies of archive members to the end of an archive, if
3742 Add one or more pre-existing archives to the end of another archive.
3744 Delete items from an archive (does not work on tapes).
3748 Compare archive members to their counterparts in the file system.
3752 @subsection How to Add Files to Existing Archives: @option{--append}
3756 If you want to add files to an existing archive, you don't need to
3757 create a new archive; you can use @option{--append} (@option{-r}).
3758 The archive must already exist in order to use @option{--append}. (A
3759 related operation is the @option{--update} operation; you can use this
3760 to add newer versions of archive members to an existing archive. To learn how to
3761 do this with @option{--update}, @pxref{update}.)
3763 If you use @option{--append} to add a file that has the same name as an
3764 archive member to an archive containing that archive member, then the
3765 old member is not deleted. What does happen, however, is somewhat
3766 complex. @command{tar} @emph{allows} you to have infinite number of files
3767 with the same name. Some operations treat these same-named members no
3768 differently than any other set of archive members: for example, if you
3769 view an archive with @option{--list} (@option{-t}), you will see all
3770 of those members listed, with their data modification times, owners, etc.
3772 Other operations don't deal with these members as perfectly as you might
3773 prefer; if you were to use @option{--extract} to extract the archive,
3774 only the most recently added copy of a member with the same name as four
3775 other members would end up in the working directory. This is because
3776 @option{--extract} extracts an archive in the order the members appeared
3777 in the archive; the most recently archived members will be extracted
3778 last. Additionally, an extracted member will @emph{replace} a file of
3779 the same name which existed in the directory already, and @command{tar}
3780 will not prompt you about this@footnote{Unless you give it
3781 @option{--keep-old-files} option, or the disk copy is newer than the
3782 the one in the archive and you invoke @command{tar} with
3783 @option{--keep-newer-files} option}. Thus, only the most recently archived
3784 member will end up being extracted, as it will replace the one
3785 extracted before it, and so on.
3787 There exists a special option that allows you to get around this
3788 behavior and extract (or list) only a particular copy of the file.
3789 This is @option{--occurrence} option. If you run @command{tar} with
3790 this option, it will extract only the first copy of the file. You
3791 may also give this option an argument specifying the number of
3792 copy to be extracted. Thus, for example if the archive
3793 @file{archive.tar} contained three copies of file @file{myfile}, then
3797 tar --extract --file archive.tar --occurrence=2 myfile
3801 would extract only the second copy. @xref{Option
3802 Summary,---occurrence}, for the description of @option{--occurrence}
3805 @FIXME{ hag -- you might want to incorporate some of the above into the
3806 MMwtSN node; not sure. i didn't know how to make it simpler...
3808 There are a few ways to get around this. (maybe xref Multiple Members
3809 with the Same Name.}
3811 @cindex Members, replacing with other members
3812 @cindex Replacing members with other members
3813 If you want to replace an archive member, use @option{--delete} to
3814 delete the member you want to remove from the archive, , and then use
3815 @option{--append} to add the member you want to be in the archive. Note
3816 that you can not change the order of the archive; the most recently
3817 added member will still appear last. In this sense, you cannot truly
3818 ``replace'' one member with another. (Replacing one member with another
3819 will not work on certain types of media, such as tapes; see @ref{delete}
3820 and @ref{Media}, for more information.)
3823 * appending files:: Appending Files to an Archive
3827 @node appending files
3828 @subsubsection Appending Files to an Archive
3830 @cindex Adding files to an Archive
3831 @cindex Appending files to an Archive
3832 @cindex Archives, Appending files to
3834 The simplest way to add a file to an already existing archive is the
3835 @option{--append} (@option{-r}) operation, which writes specified
3836 files into the archive whether or not they are already among the
3839 When you use @option{--append}, you @emph{must} specify file name
3840 arguments, as there is no default. If you specify a file that already
3841 exists in the archive, another copy of the file will be added to the
3842 end of the archive. As with other operations, the member names of the
3843 newly added files will be exactly the same as their names given on the
3844 command line. The @option{--verbose} (@option{-v}) option will print
3845 out the names of the files as they are written into the archive.
3847 @option{--append} cannot be performed on some tape drives, unfortunately,
3848 due to deficiencies in the formats those tape drives use. The archive
3849 must be a valid @command{tar} archive, or else the results of using this
3850 operation will be unpredictable. @xref{Media}.
3852 To demonstrate using @option{--append} to add a file to an archive,
3853 create a file called @file{rock} in the @file{practice} directory.
3854 Make sure you are in the @file{practice} directory. Then, run the
3855 following @command{tar} command to add @file{rock} to
3856 @file{collection.tar}:
3859 $ @kbd{tar --append --file=collection.tar rock}
3863 If you now use the @option{--list} (@option{-t}) operation, you will see that
3864 @file{rock} has been added to the archive:
3867 $ @kbd{tar --list --file=collection.tar}
3868 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
3869 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3870 -rw-r--r-- me user 20 1996-09-23 16:44 folk
3871 -rw-r--r-- me user 20 1996-09-23 16:44 rock
3875 @subsubsection Multiple Files with the Same Name
3877 You can use @option{--append} (@option{-r}) to add copies of files
3878 which have been updated since the archive was created. (However, we
3879 do not recommend doing this since there is another @command{tar}
3880 option called @option{--update}; @xref{update}, for more information.
3881 We describe this use of @option{--append} here for the sake of
3882 completeness.) When you extract the archive, the older version will
3883 be effectively lost. This works because files are extracted from an
3884 archive in the order in which they were archived. Thus, when the
3885 archive is extracted, a file archived later in time will replace a
3886 file of the same name which was archived earlier, even though the
3887 older version of the file will remain in the archive unless you delete
3888 all versions of the file.
3890 Supposing you change the file @file{blues} and then append the changed
3891 version to @file{collection.tar}. As you saw above, the original
3892 @file{blues} is in the archive @file{collection.tar}. If you change the
3893 file and append the new version of the file to the archive, there will
3894 be two copies in the archive. When you extract the archive, the older
3895 version of the file will be extracted first, and then replaced by the
3896 newer version when it is extracted.
3898 You can append the new, changed copy of the file @file{blues} to the
3899 archive in this way:
3902 $ @kbd{tar --append --verbose --file=collection.tar blues}
3907 Because you specified the @option{--verbose} option, @command{tar} has
3908 printed the name of the file being appended as it was acted on. Now
3909 list the contents of the archive:
3912 $ @kbd{tar --list --verbose --file=collection.tar}
3913 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
3914 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3915 -rw-r--r-- me user 20 1996-09-23 16:44 folk
3916 -rw-r--r-- me user 20 1996-09-23 16:44 rock
3917 -rw-r--r-- me user 58 1996-10-24 18:30 blues
3921 The newest version of @file{blues} is now at the end of the archive
3922 (note the different creation dates and file sizes). If you extract
3923 the archive, the older version of the file @file{blues} will be
3924 replaced by the newer version. You can confirm this by extracting
3925 the archive and running @samp{ls} on the directory.
3927 If you wish to extract the first occurrence of the file @file{blues}
3928 from the archive, use @option{--occurrence} option, as shown in
3929 the following example:
3932 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
3933 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3936 @xref{Writing}, for more information on @option{--extract} and
3937 @xref{Option Summary, --occurrence}, for the description of
3938 @option{--occurrence} option.
3941 @subsection Updating an Archive
3943 @cindex Updating an archive
3946 In the previous section, you learned how to use @option{--append} to
3947 add a file to an existing archive. A related operation is
3948 @option{--update} (@option{-u}). The @option{--update} operation
3949 updates a @command{tar} archive by comparing the date of the specified
3950 archive members against the date of the file with the same name. If
3951 the file has been modified more recently than the archive member, then
3952 the newer version of the file is added to the archive (as with
3955 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
3956 The operation will fail.
3958 @FIXME{other examples of media on which --update will fail? need to ask
3959 charles and/or mib/thomas/dave shevett..}
3961 Both @option{--update} and @option{--append} work by adding to the end
3962 of the archive. When you extract a file from the archive, only the
3963 version stored last will wind up in the file system, unless you use
3964 the @option{--backup} option. @FIXME-ref{Multiple Members with the
3972 @subsubsection How to Update an Archive Using @option{--update}
3974 You must use file name arguments with the @option{--update}
3975 (@option{-u}) operation. If you don't specify any files,
3976 @command{tar} won't act on any files and won't tell you that it didn't
3977 do anything (which may end up confusing you).
3979 @c note: the above parenthetical added because in fact, this
3980 @c behavior just confused the author. :-)
3982 To see the @option{--update} option at work, create a new file,
3983 @file{classical}, in your practice directory, and some extra text to the
3984 file @file{blues}, using any text editor. Then invoke @command{tar} with
3985 the @samp{update} operation and the @option{--verbose} (@option{-v})
3986 option specified, using the names of all the files in the practice
3987 directory as file name arguments:
3990 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
3997 Because we have specified verbose mode, @command{tar} prints out the names
3998 of the files it is working on, which in this case are the names of the
3999 files that needed to be updated. If you run @samp{tar --list} and look
4000 at the archive, you will see @file{blues} and @file{classical} at its
4001 end. There will be a total of two versions of the member @samp{blues};
4002 the one at the end will be newer and larger, since you added text before
4005 (The reason @command{tar} does not overwrite the older file when updating
4006 it is because writing to the middle of a section of tape is a difficult
4007 process. Tapes are not designed to go backward. @xref{Media}, for more
4008 information about tapes.
4010 @option{--update} (@option{-u}) is not suitable for performing backups for two
4011 reasons: it does not change directory content entries, and it
4012 lengthens the archive every time it is used. The @GNUTAR{}
4013 options intended specifically for backups are more
4014 efficient. If you need to run backups, please consult @ref{Backups}.
4017 @subsection Combining Archives with @option{--concatenate}
4019 @cindex Adding archives to an archive
4020 @cindex Concatenating Archives
4021 @opindex concatenate
4023 @c @cindex @option{-A} described
4024 Sometimes it may be convenient to add a second archive onto the end of
4025 an archive rather than adding individual files to the archive. To add
4026 one or more archives to the end of another archive, you should use the
4027 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4029 To use @option{--concatenate}, give the first archive with
4030 @option{--file} option and name the rest of archives to be
4031 concatenated on the command line. The members, and their member
4032 names, will be copied verbatim from those archives to the first one.
4033 @FIXME-ref{This can cause multiple members to have the same name, for
4034 information on how this affects reading the archive, Multiple
4035 Members with the Same Name.}
4036 The new, concatenated archive will be called by the same name as the
4037 one given with the @option{--file} option. As usual, if you omit
4038 @option{--file}, @command{tar} will use the value of the environment
4039 variable @env{TAPE}, or, if this has not been set, the default archive name.
4041 @FIXME{There is no way to specify a new name...}
4043 To demonstrate how @option{--concatenate} works, create two small archives
4044 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4045 files from @file{practice}:
4048 $ @kbd{tar -cvf bluesrock.tar blues rock}
4051 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4057 If you like, You can run @samp{tar --list} to make sure the archives
4058 contain what they are supposed to:
4061 $ @kbd{tar -tvf bluesrock.tar}
4062 -rw-r--r-- melissa user 105 1997-01-21 19:42 blues
4063 -rw-r--r-- melissa user 33 1997-01-20 15:34 rock
4064 $ @kbd{tar -tvf jazzfolk.tar}
4065 -rw-r--r-- melissa user 20 1996-09-23 16:44 folk
4066 -rw-r--r-- melissa user 65 1997-01-30 14:15 jazz
4069 We can concatenate these two archives with @command{tar}:
4073 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4076 If you now list the contents of the @file{bluesrock.tar}, you will see
4077 that now it also contains the archive members of @file{jazzfolk.tar}:
4080 $ @kbd{tar --list --file=bluesrock.tar}
4087 When you use @option{--concatenate}, the source and target archives must
4088 already exist and must have been created using compatible format
4089 parameters. Notice, that @command{tar} does not check whether the
4090 archives it concatenates have compatible formats, it does not
4091 even check if the files are really tar archives.
4093 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4094 tape drives, due to deficiencies in the formats those tape drives use.
4096 @cindex @code{concatenate} vs @command{cat}
4097 @cindex @command{cat} vs @code{concatenate}
4098 It may seem more intuitive to you to want or try to use @command{cat} to
4099 concatenate two archives instead of using the @option{--concatenate}
4100 operation; after all, @command{cat} is the utility for combining files.
4102 However, @command{tar} archives incorporate an end-of-file marker which
4103 must be removed if the concatenated archives are to be read properly as
4104 one archive. @option{--concatenate} removes the end-of-archive marker
4105 from the target archive before each new archive is appended. If you use
4106 @command{cat} to combine the archives, the result will not be a valid
4107 @command{tar} format archive. If you need to retrieve files from an
4108 archive that was added to using the @command{cat} utility, use the
4109 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4110 information on dealing with archives improperly combined using the
4111 @command{cat} shell utility.
4114 @subsection Removing Archive Members Using @option{--delete}
4116 @cindex Deleting files from an archive
4117 @cindex Removing files from an archive
4120 You can remove members from an archive by using the @option{--delete}
4121 option. Specify the name of the archive with @option{--file}
4122 (@option{-f}) and then specify the names of the members to be deleted;
4123 if you list no member names, nothing will be deleted. The
4124 @option{--verbose} option will cause @command{tar} to print the names
4125 of the members as they are deleted. As with @option{--extract}, you
4126 must give the exact member names when using @samp{tar --delete}.
4127 @option{--delete} will remove all versions of the named file from the
4128 archive. The @option{--delete} operation can run very slowly.
4130 Unlike other operations, @option{--delete} has no short form.
4132 @cindex Tapes, using @option{--delete} and
4133 @cindex Deleting from tape archives
4134 This operation will rewrite the archive. You can only use
4135 @option{--delete} on an archive if the archive device allows you to
4136 write to any point on the media, such as a disk; because of this, it
4137 does not work on magnetic tapes. Do not try to delete an archive member
4138 from a magnetic tape; the action will not succeed, and you will be
4139 likely to scramble the archive and damage your tape. There is no safe
4140 way (except by completely re-writing the archive) to delete files from
4141 most kinds of magnetic tape. @xref{Media}.
4143 To delete all versions of the file @file{blues} from the archive
4144 @file{collection.tar} in the @file{practice} directory, make sure you
4145 are in that directory, and then,
4148 $ @kbd{tar --list --file=collection.tar}
4153 $ @kbd{tar --delete --file=collection.tar blues}
4154 $ @kbd{tar --list --file=collection.tar}
4161 @FIXME{Check if the above listing is actually produced after running
4162 all the examples on collection.tar.}
4164 The @option{--delete} option has been reported to work properly when
4165 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4168 @subsection Comparing Archive Members with the File System
4169 @cindex Verifying the currency of an archive
4173 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4174 specified archive members against files with the same names, and then
4175 reports differences in file size, mode, owner, modification date and
4176 contents. You should @emph{only} specify archive member names, not file
4177 names. If you do not name any members, then @command{tar} will compare the
4178 entire archive. If a file is represented in the archive but does not
4179 exist in the file system, @command{tar} reports a difference.
4181 You have to specify the record size of the archive when modifying an
4182 archive with a non-default record size.
4184 @command{tar} ignores files in the file system that do not have
4185 corresponding members in the archive.
4187 The following example compares the archive members @file{rock},
4188 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4189 files of the same name in the file system. (Note that there is no file,
4190 @file{funk}; @command{tar} will report an error message.)
4193 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4196 tar: funk not found in archive
4199 The spirit behind the @option{--compare} (@option{--diff},
4200 @option{-d}) option is to check whether the archive represents the
4201 current state of files on disk, more than validating the integrity of
4202 the archive media. For this later goal, @xref{verify}.
4204 @node create options
4205 @section Options Used by @option{--create}
4207 @opindex create, additional options
4208 The previous chapter described the basics of how to use
4209 @option{--create} (@option{-c}) to create an archive from a set of files.
4210 @xref{create}. This section described advanced options to be used with
4214 * Ignore Failed Read::
4217 @node Ignore Failed Read
4218 @subsection Ignore Fail Read
4221 @item --ignore-failed-read
4222 Do not exit with nonzero on unreadable files or directories.
4225 @node extract options
4226 @section Options Used by @option{--extract}
4229 @opindex extract, additional options
4230 The previous chapter showed how to use @option{--extract} to extract
4231 an archive into the file system. Various options cause @command{tar} to
4232 extract more information than just file contents, such as the owner,
4233 the permissions, the modification date, and so forth. This section
4234 presents options to be used with @option{--extract} when certain special
4235 considerations arise. You may review the information presented in
4236 @ref{extract} for more basic information about the
4237 @option{--extract} operation.
4240 * Reading:: Options to Help Read Archives
4241 * Writing:: Changing How @command{tar} Writes Files
4242 * Scarce:: Coping with Scarce Resources
4246 @subsection Options to Help Read Archives
4247 @cindex Options when reading archives
4250 @cindex Reading incomplete records
4251 @cindex Records, incomplete
4252 @opindex read-full-records
4253 Normally, @command{tar} will request data in full record increments from
4254 an archive storage device. If the device cannot return a full record,
4255 @command{tar} will report an error. However, some devices do not always
4256 return full records, or do not require the last record of an archive to
4257 be padded out to the next record boundary. To keep reading until you
4258 obtain a full record, or to accept an incomplete record if it contains
4259 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
4260 in conjunction with the @option{--extract} or @option{--list} operations.
4263 The @option{--read-full-records} (@option{-B}) option is turned on by default when
4264 @command{tar} reads an archive from standard input, or from a remote
4265 machine. This is because on BSD Unix systems, attempting to read a
4266 pipe returns however much happens to be in the pipe, even if it is
4267 less than was requested. If this option were not enabled, @command{tar}
4268 would fail as soon as it read an incomplete record from the pipe.
4270 If you're not sure of the blocking factor of an archive, you can
4271 read the archive by specifying @option{--read-full-records} (@option{-B}) and
4272 @option{--blocking-factor=@var{512-size}} (@option{-b
4273 @var{512-size}}), using a blocking factor larger than what the archive
4274 uses. This lets you avoid having to determine the blocking factor
4275 of an archive. @xref{Blocking Factor}.
4278 * read full records::
4282 @node read full records
4283 @unnumberedsubsubsec Reading Full Records
4285 @FIXME{need sentence or so of intro here}
4288 @opindex read-full-records
4289 @item --read-full-records
4291 Use in conjunction with @option{--extract} (@option{--get},
4292 @option{-x}) to read an archive which contains incomplete records, or
4293 one which has a blocking factor less than the one specified.
4297 @unnumberedsubsubsec Ignoring Blocks of Zeros
4299 @cindex End-of-archive blocks, ignoring
4300 @cindex Ignoring end-of-archive blocks
4301 @opindex ignore-zeros
4302 Normally, @command{tar} stops reading when it encounters a block of zeros
4303 between file entries (which usually indicates the end of the archive).
4304 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
4305 completely read an archive which contains a block of zeros before the
4306 end (i.e., a damaged archive, or one that was created by concatenating
4307 several archives together).
4309 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
4310 versions of @command{tar} write garbage after the end-of-archive entry,
4311 since that part of the media is never supposed to be read. @GNUTAR{}
4312 does not write after the end of an archive, but seeks to
4313 maintain compatiblity among archiving utilities.
4316 @item --ignore-zeros
4318 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
4319 encountered while reading an archive. Use in conjunction with
4320 @option{--extract} or @option{--list}.
4324 @subsection Changing How @command{tar} Writes Files
4327 @FIXME{Introductory paragraph}
4330 * Dealing with Old Files::
4331 * Overwrite Old Files::
4333 * Keep Newer Files::
4335 * Recursive Unlink::
4336 * Data Modification Times::
4337 * Setting Access Permissions::
4338 * Directory Modification Times and Permissions::
4339 * Writing to Standard Output::
4340 * Writing to an External Program::
4344 @node Dealing with Old Files
4345 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
4347 @opindex overwrite-dir, introduced
4348 When extracting files, if @command{tar} discovers that the extracted
4349 file already exists, it normally replaces the file by removing it before
4350 extracting it, to prevent confusion in the presence of hard or symbolic
4351 links. (If the existing file is a symbolic link, it is removed, not
4352 followed.) However, if a directory cannot be removed because it is
4353 nonempty, @command{tar} normally overwrites its metadata (ownership,
4354 permission, etc.). The @option{--overwrite-dir} option enables this
4355 default behavior. To be more cautious and preserve the metadata of
4356 such a directory, use the @option{--no-overwrite-dir} option.
4358 @cindex Overwriting old files, prevention
4359 @opindex keep-old-files, introduced
4360 To be even more cautious and prevent existing files from being replaced, use
4361 the @option{--keep-old-files} (@option{-k}) option. It causes @command{tar} to refuse
4362 to replace or update a file that already exists, i.e., a file with the
4363 same name as an archive member prevents extraction of that archive
4364 member. Instead, it reports an error.
4366 @opindex overwrite, introduced
4367 To be more aggressive about altering existing files, use the
4368 @option{--overwrite} option. It causes @command{tar} to overwrite
4369 existing files and to follow existing symbolic links when extracting.
4371 @cindex Protecting old files
4372 Some people argue that @GNUTAR{} should not hesitate
4373 to overwrite files with other files when extracting. When extracting
4374 a @command{tar} archive, they expect to see a faithful copy of the
4375 state of the file system when the archive was created. It is debatable
4376 that this would always be a proper behavior. For example, suppose one
4377 has an archive in which @file{usr/local} is a link to
4378 @file{usr/local2}. Since then, maybe the site removed the link and
4379 renamed the whole hierarchy from @file{/usr/local2} to
4380 @file{/usr/local}. Such things happen all the time. I guess it would
4381 not be welcome at all that @GNUTAR{} removes the
4382 whole hierarchy just to make room for the link to be reinstated
4383 (unless it @emph{also} simultaneously restores the full
4384 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
4385 able to remove a whole hierarchy to reestablish a symbolic link, for
4386 example, but @emph{only if} @option{--recursive-unlink} is specified
4387 to allow this behavior. In any case, single files are silently
4390 @opindex unlink-first, introduced
4391 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
4392 some cases by causing @command{tar} to remove files unconditionally
4393 before extracting them.
4395 @node Overwrite Old Files
4396 @unnumberedsubsubsec Overwrite Old Files
4401 Overwrite existing files and directory metadata when extracting files
4404 This causes @command{tar} to write extracted files into the file system without
4405 regard to the files already on the system; i.e., files with the same
4406 names as archive members are overwritten when the archive is extracted.
4407 It also causes @command{tar} to extract the ownership, permissions,
4408 and time stamps onto any preexisting files or directories.
4409 If the name of a corresponding file name is a symbolic link, the file
4410 pointed to by the symbolic link will be overwritten instead of the
4411 symbolic link itself (if this is possible). Moreover, special devices,
4412 empty directories and even symbolic links are automatically removed if
4413 they are in the way of extraction.
4415 Be careful when using the @option{--overwrite} option, particularly when
4416 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
4417 can change the contents, ownership or permissions of any file on your
4418 system. Also, many systems do not take kindly to overwriting files that
4419 are currently being executed.
4421 @opindex overwrite-dir
4422 @item --overwrite-dir
4423 Overwrite the metadata of directories when extracting files from an
4424 archive, but remove other files before extracting.
4427 @node Keep Old Files
4428 @unnumberedsubsubsec Keep Old Files
4431 @opindex keep-old-files
4432 @item --keep-old-files
4434 Do not replace existing files from archive. The
4435 @option{--keep-old-files} (@option{-k}) option prevents @command{tar}
4436 from replacing existing files with files with the same name from the
4437 archive. The @option{--keep-old-files} option is meaningless with
4438 @option{--list} (@option{-t}). Prevents @command{tar} from replacing
4439 files in the file system during extraction.
4442 @node Keep Newer Files
4443 @unnumberedsubsubsec Keep Newer Files
4446 @opindex keep-newer-files
4447 @item --keep-newer-files
4448 Do not replace existing files that are newer than their archive
4449 copies. This option is meaningless with @option{--list} (@option{-t}).
4453 @unnumberedsubsubsec Unlink First
4456 @opindex unlink-first
4457 @item --unlink-first
4459 Remove files before extracting over them.
4460 This can make @command{tar} run a bit faster if you know in advance
4461 that the extracted files all need to be removed. Normally this option
4462 slows @command{tar} down slightly, so it is disabled by default.
4465 @node Recursive Unlink
4466 @unnumberedsubsubsec Recursive Unlink
4469 @opindex recursive-unlink
4470 @item --recursive-unlink
4471 When this option is specified, try removing files and directory hierarchies
4472 before extracting over them. @emph{This is a dangerous option!}
4475 If you specify the @option{--recursive-unlink} option,
4476 @command{tar} removes @emph{anything} that keeps you from extracting a file
4477 as far as current permissions will allow it. This could include removal
4478 of the contents of a full directory hierarchy.
4480 @node Data Modification Times
4481 @unnumberedsubsubsec Setting Data Modification Times
4483 @cindex Data modification times of extracted files
4484 @cindex Modification times of extracted files
4485 Normally, @command{tar} sets the data modification times of extracted
4486 files to the corresponding times recorded for the files in the archive, but
4487 limits the permissions of extracted files by the current @code{umask}
4490 To set the data modification times of extracted files to the time when
4491 the files were extracted, use the @option{--touch} (@option{-m}) option in
4492 conjunction with @option{--extract} (@option{--get}, @option{-x}).
4498 Sets the data modification time of extracted archive members to the time
4499 they were extracted, not the time recorded for them in the archive.
4500 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
4503 @node Setting Access Permissions
4504 @unnumberedsubsubsec Setting Access Permissions
4506 @cindex Permissions of extracted files
4507 @cindex Modes of extracted files
4508 To set the modes (access permissions) of extracted files to those
4509 recorded for those files in the archive, use @option{--same-permissions}
4510 in conjunction with the @option{--extract} (@option{--get},
4511 @option{-x}) operation.
4514 @opindex preserve-permission
4515 @opindex same-permission
4516 @item --preserve-permission
4517 @itemx --same-permission
4518 @c @itemx --ignore-umask
4520 Set modes of extracted archive members to those recorded in the
4521 archive, instead of current umask settings. Use in conjunction with
4522 @option{--extract} (@option{--get}, @option{-x}).
4525 @node Directory Modification Times and Permissions
4526 @unnumberedsubsubsec Directory Modification Times and Permissions
4528 After sucessfully extracting a file member, @GNUTAR{} normally
4529 restores its permissions and modification times, as described in the
4530 previous sections. This cannot be done for directories, because
4531 after extracting a directory @command{tar} will almost certainly
4532 extract files into that directory and this will cause the directory
4533 modification time to be updated. Moreover, restoring that directory
4534 permissions may not permit file creation within it. Thus, restoring
4535 directory permissions and modification times must be delayed at least
4536 until all files have been extracted into that directory. @GNUTAR{}
4537 restores directories using the following approach.
4539 The extracted directories are created with the mode specified in the
4540 archive, as modified by the umask of the user, which gives sufficient
4541 permissions to allow file creation. The meta-information about the
4542 directory is recorded in the temporary list of directories. When
4543 preparing to extract next archive member, @GNUTAR{} checks if the
4544 directory prefix of this file contains the remembered directory. If
4545 it does not, the program assumes that all files have been extracted
4546 into that directory, restores its modification time and permissions
4547 and removes its entry from the internal list. This approach allows
4548 to correctly restore directory meta-information in the majority of
4549 cases, while keeping memory requirements sufficiently small. It is
4550 based on the fact, that most @command{tar} archives use the predefined
4551 order of members: first the directory, then all the files and
4552 subdirectories in that directory.
4554 However, this is not always true. The most important exception are
4555 incremental archives (@pxref{Incremental Dumps}). The member order in
4556 an incremental archive is reversed: first all directory members are
4557 stored, followed by other (non-directory) members. So, when extracting
4558 from incremental archives, @GNUTAR{} alters the above procedure. It
4559 remebers all restored directories, and restores their meta-data
4560 only after the entire archive has been processed. Notice, that you do
4561 not need to specity any special options for that, as @GNUTAR{}
4562 automatically detects archives in incremental format.
4564 There may be cases, when such processing is required for normal archives
4565 too. Consider the following example:
4569 $ @kbd{tar --no-recursion -cvf archive \
4570 foo foo/file1 bar bar/file foo/file2}
4579 During the normal operation, after encountering @file{bar}
4580 @GNUTAR{} will assume that all files from the directory @file{foo}
4581 were already extracted and will therefore restore its timestamp and
4582 permission bits. However, after extracting @file{foo/file2} the
4583 directory timestamp will be offset again.
4585 To correctly restore directory meta-information in such cases, use
4586 @option{delay-directory-restore} command line option:
4589 @opindex delay-directory-restore
4590 @item --delay-directory-restore
4591 Delays restoring of the modification times and permissions of extracted
4592 directories until the end of extraction. This way, correct
4593 meta-information is restored even if the archive has unusual member
4596 @opindex no-delay-directory-restore
4597 @item --no-delay-directory-restore
4598 Cancel the effect of the previous @option{--delay-directory-restore}.
4599 Use this option if you have used @option{--delay-directory-restore} in
4600 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
4601 temporarily disable it.
4604 @node Writing to Standard Output
4605 @unnumberedsubsubsec Writing to Standard Output
4607 @cindex Writing extracted files to standard output
4608 @cindex Standard output, writing extracted files to
4609 To write the extracted files to the standard output, instead of
4610 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
4611 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
4612 extracting files to send them through a pipe, and do not need to
4613 preserve them in the file system. If you extract multiple members,
4614 they appear on standard output concatenated, in the order they are
4615 found in the archive.
4621 Writes files to the standard output. Use only in conjunction with
4622 @option{--extract} (@option{--get}, @option{-x}). When this option is
4623 used, instead of creating the files specified, @command{tar} writes
4624 the contents of the files extracted to its standard output. This may
4625 be useful if you are only extracting the files in order to send them
4626 through a pipe. This option is meaningless with @option{--list}
4630 This can be useful, for example, if you have a tar archive containing
4631 a big file and don't want to store the file on disk before processing
4632 it. You can use a command like this:
4635 tar -xOzf foo.tgz bigfile | process
4638 or even like this if you want to process the concatenation of the files:
4641 tar -xOzf foo.tgz bigfile1 bigfile2 | process
4644 Hovewer, @option{--to-command} may be more convenient for use with
4645 multiple files. See the next section.
4647 @node Writing to an External Program
4648 @unnumberedsubsubsec Writing to an External Program
4650 You can instruct @command{tar} to send the contents of each extracted
4651 file to the standard input of an external program:
4655 @item --to-command=@var{command}
4656 Extract files and pipe their contents to the standard input of
4657 @var{command}. When this option is used, instead of creating the
4658 files specified, @command{tar} invokes @var{command} and pipes the
4659 contents of the files to its standard output. @var{Command} may
4660 contain command line arguments. The program is executed via
4661 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
4662 extracted. Non-regular files (directories, etc.) are ignored when this
4666 The command can obtain the information about the file it processes
4667 from the following environment variables:
4670 @vrindex TAR_FILETYPE, to-command environment
4672 Type of the file. It is a single letter with the following meaning:
4674 @multitable @columnfractions 0.10 0.90
4675 @item f @tab Regular file
4676 @item d @tab Directory
4677 @item l @tab Symbolic link
4678 @item h @tab Hard link
4679 @item b @tab Block device
4680 @item c @tab Character device
4683 Currently only regular files are supported.
4685 @vrindex TAR_MODE, to-command environment
4687 File mode, an octal number.
4689 @vrindex TAR_FILENAME, to-command environment
4691 The name of the file.
4693 @vrindex TAR_REALNAME, to-command environment
4695 Name of the file as stored in the archive.
4697 @vrindex TAR_UNAME, to-command environment
4699 Name of the file owner.
4701 @vrindex TAR_GNAME, to-command environment
4703 Name of the file owner group.
4705 @vrindex TAR_ATIME, to-command environment
4707 Time of last access. It is a decimal number, representing seconds
4708 since the epoch. If the archive provides times with nanosecond
4709 precision, the nanoseconds are appended to the timestamp after a
4712 @vrindex TAR_MTIME, to-command environment
4714 Time of last modification.
4716 @vrindex TAR_CTIME, to-command environment
4718 Time of last status change.
4720 @vrindex TAR_SIZE, to-command environment
4724 @vrindex TAR_UID, to-command environment
4726 UID of the file owner.
4728 @vrindex TAR_GID, to-command environment
4730 GID of the file owner.
4733 In addition to these variables, @env{TAR_VERSION} contains the
4734 @GNUTAR{} version number.
4736 If @var{command} exits with a non-0 status, @command{tar} will print
4737 an error message similar to the following:
4740 tar: 2345: Child returned status 1
4743 Here, @samp{2345} is the PID of the finished process.
4745 If this behavior is not wanted, use @option{--ignore-command-error}:
4748 @opindex ignore-command-error
4749 @item --ignore-command-error
4750 Ignore exit codes of subprocesses. Notice that if the program
4751 exits on signal or otherwise terminates abnormally, the error message
4752 will be printed even if this option is used.
4754 @opindex no-ignore-command-error
4755 @item --no-ignore-command-error
4756 Cancel the effect of any previous @option{--ignore-command-error}
4757 option. This option is useful if you have set
4758 @option{--ignore-command-error} in @env{TAR_OPTIONS}
4759 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
4763 @unnumberedsubsubsec Removing Files
4765 @FIXME{The section is too terse. Something more to add? An example,
4769 @opindex remove-files
4770 @item --remove-files
4771 Remove files after adding them to the archive.
4775 @subsection Coping with Scarce Resources
4778 @cindex Small memory
4779 @cindex Running out of space
4787 @unnumberedsubsubsec Starting File
4790 @opindex starting-file
4791 @item --starting-file=@var{name}
4792 @itemx -K @var{name}
4793 Starts an operation in the middle of an archive. Use in conjunction
4794 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
4797 @cindex Middle of the archive, starting in the
4798 If a previous attempt to extract files failed due to lack of disk
4799 space, you can use @option{--starting-file=@var{name}} (@option{-K
4800 @var{name}}) to start extracting only after member @var{name} of the
4801 archive. This assumes, of course, that there is now free space, or
4802 that you are now extracting into a different file system. (You could
4803 also choose to suspend @command{tar}, remove unnecessary files from
4804 the file system, and then restart the same @command{tar} operation.
4805 In this case, @option{--starting-file} is not necessary.
4806 @xref{Incremental Dumps}, @xref{interactive}, and @ref{exclude}.)
4809 @unnumberedsubsubsec Same Order
4812 @cindex Large lists of file names on small machines
4814 @opindex preserve-order
4816 @itemx --preserve-order
4818 To process large lists of file names on machines with small amounts of
4819 memory. Use in conjunction with @option{--compare} (@option{--diff},
4820 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
4821 (@option{--get}, @option{-x}).
4824 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
4825 names to be listed or extracted is sorted in the same order as the
4826 files in the archive. This allows a large list of names to be used,
4827 even on a small machine that would not otherwise be able to hold all
4828 the names in memory at the same time. Such a sorted list can easily be
4829 created by running @samp{tar -t} on the archive and editing its output.
4831 This option is probably never needed on modern computer systems.
4834 @section Backup options
4836 @cindex backup options
4838 @GNUTAR{} offers options for making backups of files
4839 before writing new versions. These options control the details of
4840 these backups. They may apply to the archive itself before it is
4841 created or rewritten, as well as individual extracted members. Other
4842 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
4843 and @command{mv}, for example) offer similar options.
4845 Backup options may prove unexpectedly useful when extracting archives
4846 containing many members having identical name, or when extracting archives
4847 on systems having file name limitations, making different members appear
4848 has having similar names through the side-effect of name truncation.
4849 (This is true only if we have a good scheme for truncated backup names,
4850 which I'm not sure at all: I suspect work is needed in this area.)
4851 When any existing file is backed up before being overwritten by extraction,
4852 then clashing files are automatically be renamed to be unique, and the
4853 true name is kept for only the last file of a series of clashing files.
4854 By using verbose mode, users may track exactly what happens.
4856 At the detail level, some decisions are still experimental, and may
4857 change in the future, we are waiting comments from our users. So, please
4858 do not learn to depend blindly on the details of the backup features.
4859 For example, currently, directories themselves are never renamed through
4860 using these options, so, extracting a file over a directory still has
4861 good chances to fail. Also, backup options apply to created archives,
4862 not only to extracted members. For created archives, backups will not
4863 be attempted when the archive is a block or character device, or when it
4864 refers to a remote file.
4866 For the sake of simplicity and efficiency, backups are made by renaming old
4867 files prior to creation or extraction, and not by copying. The original
4868 name is restored if the file creation fails. If a failure occurs after a
4869 partial extraction of a file, both the backup and the partially extracted
4873 @item --backup[=@var{method}]
4875 @vindex VERSION_CONTROL
4877 Back up files that are about to be overwritten or removed.
4878 Without this option, the original versions are destroyed.
4880 Use @var{method} to determine the type of backups made.
4881 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
4882 environment variable. And if @env{VERSION_CONTROL} is not set,
4883 use the @samp{existing} method.
4885 @vindex version-control @r{Emacs variable}
4886 This option corresponds to the Emacs variable @samp{version-control};
4887 the same values for @var{method} are accepted as in Emacs. This option
4888 also allows more descriptive names. The valid @var{method}s are:
4893 @cindex numbered @r{backup method}
4894 Always make numbered backups.
4898 @cindex existing @r{backup method}
4899 Make numbered backups of files that already have them, simple backups
4904 @cindex simple @r{backup method}
4905 Always make simple backups.
4909 @item --suffix=@var{suffix}
4911 @cindex backup suffix
4912 @vindex SIMPLE_BACKUP_SUFFIX
4913 Append @var{suffix} to each backup file made with @option{--backup}. If this
4914 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
4915 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
4916 set, the default is @samp{~}, just as in Emacs.
4920 Some people express the desire to @emph{always} use the @option{--backup}
4921 option, by defining some kind of alias or script. This is not as easy
4922 as one may think, due to the fact that old style options should appear first
4923 and consume arguments a bit unpredictably for an alias or script. But,
4924 if you are ready to give up using old style options, you may resort to
4925 using something like (a Bourne shell function here):
4928 tar () @{ /usr/local/bin/tar --backup $*; @}
4932 @section Notable @command{tar} Usages
4935 @FIXME{Using Unix file linking capability to recreate directory
4936 structures---linking files into one subdirectory and then
4937 @command{tar}ring that directory.}
4939 @FIXME{Nice hairy example using absolute-names, newer, etc.}
4942 You can easily use archive files to transport a group of files from
4943 one system to another: put all relevant files into an archive on one
4944 computer system, transfer the archive to another system, and extract
4945 the contents there. The basic transfer medium might be magnetic tape,
4946 Internet FTP, or even electronic mail (though you must encode the
4947 archive with @command{uuencode} in order to transport it properly by
4948 mail). Both machines do not have to use the same operating system, as
4949 long as they both support the @command{tar} program.
4951 For example, here is how you might copy a directory's contents from
4952 one disk to another, while preserving the dates, modes, owners and
4953 link-structure of all the files therein. In this case, the transfer
4954 medium is a @dfn{pipe}, which is one a Unix redirection mechanism:
4957 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
4961 You can avoid subshells by using @option{-C} option:
4964 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
4968 The command also works using short option forms:
4971 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
4972 | (cd targetdir; tar --extract --file=-)}
4974 $ @kbd{tar --directory sourcedir --create --file=- . ) \
4975 | tar --directory targetdir --extract --file=-}
4979 This is one of the easiest methods to transfer a @command{tar} archive.
4982 @section Looking Ahead: The Rest of this Manual
4984 You have now seen how to use all eight of the operations available to
4985 @command{tar}, and a number of the possible options. The next chapter
4986 explains how to choose and change file and archive names, how to use
4987 files to store names of other files which you can then call as
4988 arguments to @command{tar} (this can help you save time if you expect to
4989 archive the same list of files a number of times), and so forth.
4990 @FIXME{in case it's not obvious, i'm making this up in some sense
4991 based on my limited memory of what the next chapter *really* does. i
4992 just wanted to flesh out this final section a little bit so i'd
4993 remember to stick it in here. :-)}
4995 If there are too many files to conveniently list on the command line,
4996 you can list the names in a file, and @command{tar} will read that file.
4999 There are various ways of causing @command{tar} to skip over some files,
5000 and not archive them. @xref{Choosing}.
5003 @chapter Performing Backups and Restoring Files
5006 @GNUTAR{} is distributed along with the scripts
5007 which the Free Software Foundation uses for performing backups. There
5008 is no corresponding scripts available yet for doing restoration of
5009 files. Even if there is a good chance those scripts may be satisfying
5010 to you, they are not the only scripts or methods available for doing
5011 backups and restore. You may well create your own, or use more
5012 sophisticated packages dedicated to that purpose.
5014 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5015 Automatic Network Disk Archiver), a backup system developed by James
5016 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5017 This is free software, and it is available at these places:
5020 http://www.cs.umd.edu/projects/amanda/amanda.html
5021 ftp://ftp.cs.umd.edu/pub/amanda
5026 Here is a possible plan for a future documentation about the backuping
5027 scripts which are provided within the @GNUTAR{}
5033 @item what are dumps
5034 @item different levels of dumps
5036 @item full dump = dump everything
5037 @item level 1, level 2 dumps etc
5038 A level @var{n} dump dumps everything changed since the last level
5041 @item how to use scripts for dumps (ie, the concept)
5043 @item scripts to run after editing backup specs (details)
5045 @item Backup Specs, what is it.
5047 @item how to customize
5048 @item actual text of script [/sp/dump/backup-specs]
5052 @item rsh doesn't work
5053 @item rtape isn't installed
5056 @item the @option{--incremental} option of tar
5059 @item write protection
5060 @item types of media, different sizes and types, useful for different things
5061 @item files and tape marks
5062 one tape mark between files, two at end.
5063 @item positioning the tape
5064 MT writes two at end of write,
5065 backspaces over one when writing again.
5071 This chapter documents both the provided shell scripts and @command{tar}
5072 options which are more specific to usage as a backup tool.
5074 To @dfn{back up} a file system means to create archives that contain
5075 all the files in that file system. Those archives can then be used to
5076 restore any or all of those files (for instance if a disk crashes or a
5077 file is accidentally deleted). File system @dfn{backups} are also
5081 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5082 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5083 * Backup Levels:: Levels of Backups
5084 * Backup Parameters:: Setting Parameters for Backups and Restoration
5085 * Scripted Backups:: Using the Backup Scripts
5086 * Scripted Restoration:: Using the Restore Script
5090 @section Using @command{tar} to Perform Full Dumps
5096 @cindex corrupted archives
5097 Full dumps should only be made when no other people or programs
5098 are modifying files in the file system. If files are modified while
5099 @command{tar} is making the backup, they may not be stored properly in
5100 the archive, in which case you won't be able to restore them if you
5101 have to. (Files not being modified are written with no trouble, and do
5102 not corrupt the entire archive.)
5104 You will want to use the @option{--label=@var{archive-label}}
5105 (@option{-V @var{archive-label}}) option to give the archive a
5106 volume label, so you can tell what this archive is even if the label
5107 falls off the tape, or anything like that.
5109 Unless the file system you are dumping is guaranteed to fit on
5110 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5111 Make sure you have enough tapes on hand to complete the backup.
5113 If you want to dump each file system separately you will need to use
5114 the @option{--one-file-system} option to prevent
5115 @command{tar} from crossing file system boundaries when storing
5118 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5119 option is not needed, since this is a complete copy of everything in
5120 the file system, and a full restore from this backup would only be
5121 done onto a completely
5124 Unless you are in a hurry, and trust the @command{tar} program (and your
5125 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5126 option, to make sure your files really made it onto the dump properly.
5127 This will also detect cases where the file was modified while (or just
5128 after) it was being archived. Not all media (notably cartridge tapes)
5129 are capable of being verified, unfortunately.
5131 @node Incremental Dumps
5132 @section Using @command{tar} to Perform Incremental Dumps
5134 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5135 stores additional metadata so that exact state of the file system
5136 can be restored when extracting the archive.
5138 @GNUTAR{} currently offers two options for handling incremental
5139 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5140 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5142 @opindex listed-incremental
5143 The option @option{--listed-incremental} instructs tar to operate on
5144 an incremental archive with additional metadata stored in a standalone
5145 file, called a @dfn{snapshot file}. The purpose of this file is to help
5146 determine which files have been changed, added or deleted since the
5147 last backup, so that the next incremental backup will contain only
5148 modified files. The name of the snapshot file is given as an argument
5152 @item --listed-incremental=@var{file}
5153 @itemx -g @var{file}
5154 Handle incremental backups with snapshot data in @var{file}.
5157 To create an incremental backup, you would use
5158 @option{--listed-incremental} together with @option{--create}
5159 (@pxref{create}). For example:
5162 $ @kbd{tar --create \
5163 --file=archive.1.tar \
5164 --listed-incremental=/var/log/usr.snar \
5168 This will create in @file{archive.1.tar} an incremental backup of
5169 the @file{/usr} file system, storing additional metadata in the file
5170 @file{/var/log/usr.snar}. If this file does not exist, it will be
5171 created. The created archive will then be a @dfn{level 0 backup};
5172 please see the next section for more on backup levels.
5174 Otherwise, if the file @file{/var/log/usr.snar} exists, it
5175 determines which files are modified. In this case only these files will be
5176 stored in the archive. Suppose, for example, that after running the
5177 above command, you delete file @file{/usr/doc/old} and create
5178 directory @file{/usr/local/db} with the following contents:
5181 $ @kbd{ls /usr/local/db}
5186 Some time later you create another incremental backup. You will
5190 $ @kbd{tar --create \
5191 --file=archive.2.tar \
5192 --listed-incremental=/var/log/usr.snar \
5194 tar: usr/local/db: Directory is new
5201 The created archive @file{archive.2.tar} will contain only these
5202 three members. This archive is called a @dfn{level 1 backup}. Notice
5203 that @file{/var/log/usr.snar} will be updated with the new data, so if
5204 you plan to create more @samp{level 1} backups, it is necessary to
5205 create a working copy of the snapshot file before running
5206 @command{tar}. The above example will then be modified as follows:
5209 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
5210 $ @kbd{tar --create \
5211 --file=archive.2.tar \
5212 --listed-incremental=/var/log/usr.snar-1 \
5216 Incremental dumps depend crucially on time stamps, so the results are
5217 unreliable if you modify a file's time stamps during dumping (e.g.,
5218 with the @option{--atime-preserve=replace} option), or if you set the clock
5221 Metadata stored in snapshot files include device numbers, which,
5222 obviously is supposed to be a non-volatile value. However, it turns
5223 out that NFS devices have undependable values when an automounter
5224 gets in the picture. This can lead to a great deal of spurious
5225 redumping in incremental dumps, so it is somewhat useless to compare
5226 two NFS devices numbers over time. The solution implemented currently
5227 is to considers all NFS devices as being equal when it comes to
5228 comparing directories; this is fairly gross, but there does not seem
5229 to be a better way to go.
5231 Note that incremental archives use @command{tar} extensions and may
5232 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
5234 @opindex listed-incremental, using with @option{--extract}
5235 @opindex extract, using with @option{--listed-incremental}
5236 To extract from the incremental dumps, use
5237 @option{--listed-incremental} together with @option{--extract}
5238 option (@pxref{extracting files}). In this case, @command{tar} does
5239 not need to access snapshot file, since all the data necessary for
5240 extraction are stored in the archive itself. So, when extracting, you
5241 can give whatever argument to @option{--listed-incremental}, the usual
5242 practice is to use @option{--listed-incremental=/dev/null}.
5243 Alternatively, you can use @option{--incremental}, which needs no
5244 arguments. In general, @option{--incremental} (@option{-G}) can be
5245 used as a shortcut for @option{--listed-incremental} when listing or
5246 extracting incremental backups (for more information, regarding this
5247 option, @pxref{incremental-op}).
5249 When extracting from the incremental backup @GNUTAR{} attempts to
5250 restore the exact state the file system had when the archive was
5251 created. In particular, it will @emph{delete} those files in the file
5252 system that did not exist in their directories when the archive was
5253 created. If you have created several levels of incremental files,
5254 then in order to restore the exact contents the file system had when
5255 the last level was created, you will need to restore from all backups
5256 in turn. Continuing our example, to restore the state of @file{/usr}
5257 file system, one would do@footnote{Notice, that since both archives
5258 were created withouth @option{-P} option (@pxref{absolute}), these
5259 commands should be run from the root file system.}:
5262 $ @kbd{tar --extract \
5263 --listed-incremental=/dev/null \
5264 --file archive.1.tar}
5265 $ @kbd{tar --extract \
5266 --listed-incremental=/dev/null \
5267 --file archive.2.tar}
5270 To list the contents of an incremental archive, use @option{--list}
5271 (@pxref{list}), as usual. To obtain more information about the
5272 archive, use @option{--listed-incremental} or @option{--incremental}
5273 combined with two @option{--verbose} options@footnote{Two
5274 @option{--verbose} options were selected to avoid breaking usual
5275 verbose listing output (@option{--list --verbose}) when using in
5278 @opindex incremental, using with @option{--list}
5279 @opindex listed-incremental, using with @option{--list}
5280 @opindex list, using with @option{--incremental}
5281 @opindex list, using with @option{--listed-incremental}
5282 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
5283 contents of the DUMPDIR header (with terminating nulls) when
5284 @option{--incremental} or @option{--listed-incremental} option was
5285 given, no matter what the verbosity level. This behavior, and,
5286 especially, the binary output it produced were considered incovenient
5287 and were changed in version 1.16}:
5290 @kbd{tar --list --incremental --verbose --verbose archive.tar}
5293 This command will print, for each directory in the archive, the list
5294 of files in that directory at the time the archive was created. This
5295 information is put out in a format which is both human-readable and
5296 unambiguous for a program: each file name is printed as
5303 where @var{x} is a letter describing the status of the file: @samp{Y}
5304 if the file is present in the archive, @samp{N} if the file is not
5305 included in the archive, or a @samp{D} if the file is a directory (and
5306 is included in the archive).@FIXME-xref{dumpdir format}. Each such
5307 line is terminated by a newline character. The last line is followed
5308 by an additional newline to indicate the end of the data.
5310 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
5311 gives the same behavior as @option{--listed-incremental} when used
5312 with @option{--list} and @option{--extract} options. When used with
5313 @option{--create} option, it creates an incremental archive without
5314 creating snapshot file. Thus, it is impossible to create several
5315 levels of incremental backups with @option{--incremental} option.
5318 @section Levels of Backups
5320 An archive containing all the files in the file system is called a
5321 @dfn{full backup} or @dfn{full dump}. You could insure your data by
5322 creating a full dump every day. This strategy, however, would waste a
5323 substantial amount of archive media and user time, as unchanged files
5324 are daily re-archived.
5326 It is more efficient to do a full dump only occasionally. To back up
5327 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
5328 one} dump archives all the files that have changed since the last full
5331 A typical dump strategy would be to perform a full dump once a week,
5332 and a level one dump once a day. This means some versions of files
5333 will in fact be archived more than once, but this dump strategy makes
5334 it possible to restore a file system to within one day of accuracy by
5335 only extracting two archives---the last weekly (full) dump and the
5336 last daily (level one) dump. The only information lost would be in
5337 files changed or created since the last daily backup. (Doing dumps
5338 more than once a day is usually not worth the trouble).
5340 @GNUTAR{} comes with scripts you can use to do full
5341 and level-one (actually, even level-two and so on) dumps. Using
5342 scripts (shell programs) to perform backups and restoration is a
5343 convenient and reliable alternative to typing out file name lists
5344 and @command{tar} commands by hand.
5346 Before you use these scripts, you need to edit the file
5347 @file{backup-specs}, which specifies parameters used by the backup
5348 scripts and by the restore script. This file is usually located
5349 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
5350 detailed description. Once the backup parameters are set, you can
5351 perform backups or restoration by running the appropriate script.
5353 The name of the backup script is @code{backup}. The name of the
5354 restore script is @code{restore}. The following sections describe
5355 their use in detail.
5357 @emph{Please Note:} The backup and restoration scripts are
5358 designed to be used together. While it is possible to restore files by
5359 hand from an archive which was created using a backup script, and to create
5360 an archive by hand which could then be extracted using the restore script,
5361 it is easier to use the scripts. @xref{Incremental Dumps}, before
5362 making such an attempt.
5364 @node Backup Parameters
5365 @section Setting Parameters for Backups and Restoration
5367 The file @file{backup-specs} specifies backup parameters for the
5368 backup and restoration scripts provided with @command{tar}. You must
5369 edit @file{backup-specs} to fit your system configuration and schedule
5370 before using these scripts.
5372 Syntactically, @file{backup-specs} is a shell script, containing
5373 mainly variable assignments. However, any valid shell construct
5374 is allowed in this file. Particularly, you may wish to define
5375 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
5376 For more information about shell script syntax, please refer to
5377 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
5378 g_02, the definition of the Shell Command Language}. See also
5379 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
5381 The shell variables controlling behavior of @code{backup} and
5382 @code{restore} are described in the following subsections.
5385 * General-Purpose Variables::
5386 * Magnetic Tape Control::
5388 * backup-specs example:: An Example Text of @file{Backup-specs}
5391 @node General-Purpose Variables
5392 @subsection General-Purpose Variables
5394 @defvr {Backup variable} ADMINISTRATOR
5395 The user name of the backup administrator. @code{Backup} scripts
5396 sends a backup report to this address.
5399 @defvr {Backup variable} BACKUP_HOUR
5400 The hour at which the backups are done. This can be a number from 0
5401 to 23, or the time specification in form @var{hours}:@var{minutes},
5402 or the string @samp{now}.
5404 This variable is used by @code{backup}. Its value may be overridden
5405 using @option{--time} option (@pxref{Scripted Backups}).
5408 @defvr {Backup variable} TAPE_FILE
5410 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
5411 is a remote archive (@pxref{remote-dev}), backup script will suppose
5412 that your @command{mt} is able to access remote devices. If @var{RSH}
5413 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
5414 invocations of @command{mt}.
5417 @defvr {Backup variable} BLOCKING
5419 The blocking factor @command{tar} will use when writing the dump archive.
5420 @xref{Blocking Factor}.
5423 @defvr {Backup variable} BACKUP_DIRS
5425 A list of file systems to be dumped (for @code{backup}), or restored
5426 (for @code{restore}). You can include any directory
5427 name in the list --- subdirectories on that file system will be
5428 included, regardless of how they may look to other networked machines.
5429 Subdirectories on other file systems will be ignored.
5431 The host name specifies which host to run @command{tar} on, and should
5432 normally be the host that actually contains the file system. However,
5433 the host machine must have @GNUTAR{} installed, and
5434 must be able to access the directory containing the backup scripts and
5435 their support files using the same file name that is used on the
5436 machine where the scripts are run (ie. what @command{pwd} will print
5437 when in that directory on that machine). If the host that contains
5438 the file system does not have this capability, you can specify another
5439 host as long as it can access the file system through NFS.
5441 If the list of file systems is very long you may wish to put it
5442 in a separate file. This file is usually named
5443 @file{/etc/backup/dirs}, but this name may be overridden in
5444 @file{backup-specs} using @code{DIRLIST} variable.
5447 @defvr {Backup variable} DIRLIST
5449 A path to the file containing the list of the file systems to backup
5450 or restore. By default it is @file{/etc/backup/dirs}.
5453 @defvr {Backup variable} BACKUP_FILES
5455 A list of individual files to be dumped (for @code{backup}), or restored
5456 (for @code{restore}). These should be accessible from the machine on
5457 which the backup script is run.
5459 If the list of file systems is very long you may wish to store it
5460 in a separate file. This file is usually named
5461 @file{/etc/backup/files}, but this name may be overridden in
5462 @file{backup-specs} using @code{FILELIST} variable.
5465 @defvr {Backup variable} FILELIST
5467 A path to the file containing the list of the individual files to backup
5468 or restore. By default it is @file{/etc/backup/files}.
5471 @defvr {Backup variable} MT
5473 Full file name of @command{mt} binary.
5476 @defvr {Backup variable} RSH
5478 Full file name of @command{rsh} binary or its equivalent. You may wish to
5479 set it to @code{ssh}, to improve security. In this case you will have
5480 to use public key authentication.
5483 @defvr {Backup variable} RSH_COMMAND
5485 Full file name of @command{rsh} binary on remote mashines. This will
5486 be passed via @option{--rsh-command} option to the remote invocation
5490 @defvr {Backup variable} VOLNO_FILE
5492 Name of temporary file to hold volume numbers. This needs to be accessible
5493 by all the machines which have file systems to be dumped.
5496 @defvr {Backup variable} XLIST
5498 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
5499 located on the remote machine and containing the list of files to
5500 be excluded from the backup. Exclude file lists are searched in
5501 /etc/tar-backup directory. A common use for exclude file lists
5502 is to exclude files containing security-sensitive information
5503 (e.g., @file{/etc/shadow} from backups).
5505 This variable affects only @code{backup}.
5508 @defvr {Backup variable} SLEEP_TIME
5510 Time to sleep between dumps of any two successive file systems
5512 This variable affects only @code{backup}.
5515 @defvr {Backup variable} DUMP_REMIND_SCRIPT
5517 Script to be run when it's time to insert a new tape in for the next
5518 volume. Administrators may want to tailor this script for their site.
5519 If this variable isn't set, @GNUTAR{} will display its built-in prompt
5520 @FIXME-xref{describe it somewhere!}, and will expect confirmation from
5524 @defvr {Backup variable} SLEEP_MESSAGE
5526 Message to display on the terminal while waiting for dump time. Usually
5527 this will just be some literal text.
5530 @defvr {Backup variable} TAR
5532 Full file name of the @GNUTAR{} executable. If this is not set, backup
5533 scripts will search @command{tar} in the current shell path.
5536 @node Magnetic Tape Control
5537 @subsection Magnetic Tape Control
5539 Backup scripts access tape device using special @dfn{hook functions}.
5540 These functions take a single argument -- the name of the tape
5541 device. Their names are kept in the following variables:
5543 @defvr {Backup variable} MT_BEGIN
5544 The name of @dfn{begin} function. This function is called before
5545 accessing the drive. By default it retensions the tape:
5551 mt -f "$1" retension
5556 @defvr {Backup variable} MT_REWIND
5557 The name of @dfn{rewind} function. The default definition is as
5570 @defvr {Backup variable} MT_OFFLINE
5571 The name of the function switching the tape off line. By default
5572 it is defined as follows:
5575 MT_OFFLINE=mt_offline
5583 @defvr {Backup variable} MT_STATUS
5584 The name of the function used to obtain the status of the archive device,
5585 including error count. Default definition:
5597 @subsection User Hooks
5599 @dfn{User hooks} are shell functions executed before and after
5600 each @command{tar} invocation. Thus, there are @dfn{backup
5601 hooks}, which are executed before and after dumping each file
5602 system, and @dfn{restore hooks}, executed before and
5603 after restoring a file system. Each user hook is a shell function
5604 taking four arguments:
5606 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
5611 Current backup or restore level.
5614 Name or IP address of the host machine being dumped or restored.
5617 Full path name to the file system being dumped or restored.
5620 File system name with directory separators replaced with colons. This
5621 is useful, e.g., for creating unique files.
5625 Following variables keep the names of user hook functions
5627 @defvr {Backup variable} DUMP_BEGIN
5628 Dump begin function. It is executed before dumping the file system.
5631 @defvr {Backup variable} DUMP_END
5632 Executed after dumping the file system.
5635 @defvr {Backup variable} RESTORE_BEGIN
5636 Executed before restoring the file system.
5639 @defvr {Backup variable} RESTORE_END
5640 Executed after restoring the file system.
5643 @node backup-specs example
5644 @subsection An Example Text of @file{Backup-specs}
5646 The following is an example of @file{backup-specs}:
5649 # site-specific parameters for file system backup.
5651 ADMINISTRATOR=friedman
5653 TAPE_FILE=/dev/nrsmt0
5655 # Use @code{ssh} instead of the less secure @code{rsh}
5657 RSH_COMMAND=/usr/bin/ssh
5659 # Override MT_STATUS function:
5665 # Disable MT_OFFLINE function
5682 apple-gunkies:/com/mailer/gnu
5683 apple-gunkies:/com/archive/gnu"
5685 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
5689 @node Scripted Backups
5690 @section Using the Backup Scripts
5692 The syntax for running a backup script is:
5695 backup --level=@var{level} --time=@var{time}
5698 The @option{level} option requests the dump level. Thus, to produce
5699 a full dump, specify @code{--level=0} (this is the default, so
5700 @option{--level} may be omitted if its value is @code{0}).
5701 @footnote{For backward compatibility, the @code{backup} will also
5702 try to deduce the requested dump level from the name of the
5703 script itself. If the name consists of a string @samp{level-}
5704 followed by a single decimal digit, that digit is taken as
5705 the dump level number. Thus, you may create a link from @code{backup}
5706 to @code{level-1} and then run @code{level-1} whenever you need to
5707 create a level one dump.}
5709 The @option{--time} option determines when should the backup be
5710 run. @var{Time} may take three forms:
5713 @item @var{hh}:@var{mm}
5715 The dump must be run at @var{hh} hours @var{mm} minutes.
5719 The dump must be run at @var{hh} hours
5723 The dump must be run immediately.
5726 You should start a script with a tape or disk mounted. Once you
5727 start a script, it prompts you for new tapes or disks as it
5728 needs them. Media volumes don't have to correspond to archive
5729 files --- a multi-volume archive can be started in the middle of a
5730 tape that already contains the end of another multi-volume archive.
5731 The @code{restore} script prompts for media by its archive volume,
5732 so to avoid an error message you should keep track of which tape
5733 (or disk) contains which volume of the archive (@pxref{Scripted
5736 The backup scripts write two files on the file system. The first is a
5737 record file in @file{/etc/tar-backup/}, which is used by the scripts
5738 to store and retrieve information about which files were dumped. This
5739 file is not meant to be read by humans, and should not be deleted by
5740 them. @xref{Snapshot Files}, for a more detailed explanation of this
5743 The second file is a log file containing the names of the file systems
5744 and files dumped, what time the backup was made, and any error
5745 messages that were generated, as well as how much space was left in
5746 the media volume after the last volume of the archive was written.
5747 You should check this log file after every backup. The file name is
5748 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
5749 represents current date, and @var{n} represents current dump level number.
5751 The script also prints the name of each system being dumped to the
5754 Following is the full list of options accepted by @code{backup}
5758 @item -l @var{level}
5759 @itemx --level=@var{level}
5760 Do backup level @var{level} (default 0).
5764 Force backup even if today's log file already exists.
5766 @item -v[@var{level}]
5767 @itemx --verbose[=@var{level}]
5768 Set verbosity level. The higher the level is, the more debugging
5769 information will be output during execution. Devault @var{level}
5770 is 100, which means the highest debugging level.
5772 @item -t @var{start-time}
5773 @itemx --time=@var{start-time}
5774 Wait till @var{time}, then do backup.
5778 Display short help message and exit.
5782 Display information about the program's name, version, origin and legal
5783 status, all on standard output, and then exit successfully.
5787 @node Scripted Restoration
5788 @section Using the Restore Script
5790 To restore files that were archived using a scripted backup, use the
5791 @code{restore} script. Its usage is quite straightforward. In the
5792 simplest form, invoke @code{restore --all}, it will
5793 then restore all the file systems and files specified in
5794 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
5796 You may select the file systems (and/or files) to restore by
5797 giving @code{restore} list of @dfn{patterns} in its command
5798 line. For example, running
5805 will restore all file systems on the machine @samp{albert}. A more
5806 complicated example:
5809 restore 'albert:*' '*:/var'
5813 This command will restore all file systems on the machine @samp{albert}
5814 as well as @file{/var} file system on all machines.
5816 By default @code{restore} will start restoring files from the lowest
5817 available dump level (usually zero) and will continue through
5818 all available dump levels. There may be situations where such a
5819 thorough restore is not necessary. For example, you may wish to
5820 restore only files from the recent level one backup. To do so,
5821 use @option{--level} option, as shown in the example below:
5827 The full list of options accepted by @code{restore} follows:
5832 Restore all file systems and files specified in @file{backup-specs}
5834 @item -l @var{level}
5835 @itemx --level=@var{level}
5836 Start restoring from the given backup level, instead of the default 0.
5838 @item -v[@var{level}]
5839 @itemx --verbose[=@var{level}]
5840 Set verbosity level. The higher the level is, the more debugging
5841 information will be output during execution. Devault @var{level}
5842 is 100, which means the highest debugging level.
5846 Display short help message and exit.
5850 Display information about the program's name, version, origin and legal
5851 status, all on standard output, and then exit successfully.
5854 You should start the restore script with the media containing the
5855 first volume of the archive mounted. The script will prompt for other
5856 volumes as they are needed. If the archive is on tape, you don't need
5857 to rewind the tape to to its beginning---if the tape head is
5858 positioned past the beginning of the archive, the script will rewind
5859 the tape as needed. @FIXME-xref{Media, for a discussion of tape
5863 @strong{Warning:} The script will delete files from the active file
5864 system if they were not in the file system when the archive was made.
5867 @xref{Incremental Dumps}, for an explanation of how the script makes
5871 @chapter Choosing Files and Names for @command{tar}
5874 Certain options to @command{tar} enable you to specify a name for your
5875 archive. Other options let you decide which files to include or exclude
5876 from the archive, based on when or whether files were modified, whether
5877 the file names do or don't match specified patterns, or whether files
5878 are in specified directories.
5880 This chapter discusses these options in detail.
5883 * file:: Choosing the Archive's Name
5884 * Selecting Archive Members::
5885 * files:: Reading Names from a File
5886 * exclude:: Excluding Some Files
5887 * wildcards:: Wildcards Patterns and Matching
5888 * quoting styles:: Ways of Quoting Special Characters in Names
5889 * transform:: Modifying File and Member Names
5890 * after:: Operating Only on New Files
5891 * recurse:: Descending into Directories
5892 * one:: Crossing File System Boundaries
5896 @section Choosing and Naming Archive Files
5899 @cindex Naming an archive
5900 @cindex Archive Name
5901 @cindex Choosing an archive file
5902 @cindex Where is the archive?
5903 By default, @command{tar} uses an archive file name that was compiled when
5904 it was built on the system; usually this name refers to some physical
5905 tape drive on the machine. However, the person who installed @command{tar}
5906 on the system may not have set the default to a meaningful value as far as
5907 most users are concerned. As a result, you will usually want to tell
5908 @command{tar} where to find (or create) the archive. The
5909 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
5910 option allows you to either specify or name a file to use as the archive
5911 instead of the default archive file location.
5914 @opindex file, short description
5915 @item --file=@var{archive-name}
5916 @itemx -f @var{archive-name}
5917 Name the archive to create or operate on. Use in conjunction with
5921 For example, in this @command{tar} command,
5924 $ @kbd{tar -cvf collection.tar blues folk jazz}
5928 @file{collection.tar} is the name of the archive. It must directly
5929 follow the @option{-f} option, since whatever directly follows @option{-f}
5930 @emph{will} end up naming the archive. If you neglect to specify an
5931 archive name, you may end up overwriting a file in the working directory
5932 with the archive you create since @command{tar} will use this file's name
5933 for the archive name.
5935 An archive can be saved as a file in the file system, sent through a
5936 pipe or over a network, or written to an I/O device such as a tape,
5937 floppy disk, or CD write drive.
5939 @cindex Writing new archives
5940 @cindex Archive creation
5941 If you do not name the archive, @command{tar} uses the value of the
5942 environment variable @env{TAPE} as the file name for the archive. If
5943 that is not available, @command{tar} uses a default, compiled-in archive
5944 name, usually that for tape unit zero (ie. @file{/dev/tu00}).
5946 @cindex Standard input and output
5947 @cindex tar to standard input and output
5948 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
5949 archive from standard input (when listing or extracting files), or
5950 writes it to standard output (when creating an archive). If you use
5951 @file{-} as an @var{archive-name} when modifying an archive,
5952 @command{tar} reads the original archive from its standard input and
5953 writes the entire new archive to its standard output.
5955 The following example is a convenient way of copying directory
5956 hierarchy from @file{sourcedir} to @file{targetdir}.
5959 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
5962 The @option{-C} option allows to avoid using subshells:
5965 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
5968 In both examples above, the leftmost @command{tar} invocation archives
5969 the contents of @file{sourcedir} to the standard output, while the
5970 rightmost one reads this archive from its standard input and
5971 extracts it. The @option{-p} option tells it to restore permissions
5972 of the extracted files.
5974 @cindex Remote devices
5975 @cindex tar to a remote device
5977 To specify an archive file on a device attached to a remote machine,
5981 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
5985 @command{tar} will complete the remote connection, if possible, and
5986 prompt you for a username and password. If you use
5987 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
5988 will complete the remote connection, if possible, using your username
5989 as the username on the remote machine.
5991 @cindex Local and remote archives
5992 @anchor{local and remote archives}
5993 If the archive file name includes a colon (@samp{:}), then it is assumed
5994 to be a file on another machine. If the archive file is
5995 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
5996 host @var{host}. The remote host is accessed using the @command{rsh}
5997 program, with a username of @var{user}. If the username is omitted
5998 (along with the @samp{@@} sign), then your user name will be used.
5999 (This is the normal @command{rsh} behavior.) It is necessary for the
6000 remote machine, in addition to permitting your @command{rsh} access, to
6001 have the @file{rmt} program installed (This command is included in
6002 the @GNUTAR{} distribution and by default is installed under
6003 @file{@var{prefix}/libexec/rmt}, were @var{prefix} means your
6004 installation prefix). If you need to use a file whose name includes a
6005 colon, then the remote tape drive behavior
6006 can be inhibited by using the @option{--force-local} option.
6008 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6009 tries to minimize input and output operations. The Amanda backup
6010 system, when used with @GNUTAR{}, has an initial sizing pass which
6013 @node Selecting Archive Members
6014 @section Selecting Archive Members
6015 @cindex Specifying files to act on
6016 @cindex Specifying archive members
6018 @dfn{File Name arguments} specify which files in the file system
6019 @command{tar} operates on, when creating or adding to an archive, or which
6020 archive members @command{tar} operates on, when reading or deleting from
6021 an archive. @xref{Operations}.
6023 To specify file names, you can include them as the last arguments on
6024 the command line, as follows:
6026 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6029 If a file name begins with dash (@samp{-}), precede it with
6030 @option{--add-file} option to prevent it from being treated as an
6033 If you specify a directory name as a file name argument, all the files
6034 in that directory are operated on by @command{tar}.
6036 If you do not specify files, @command{tar} behavior differs depending
6037 on the operation mode as described below:
6039 When @command{tar} is invoked with @option{--create} (@option{-c}),
6040 @command{tar} will stop immediately, reporting the following:
6044 $ @kbd{tar cf a.tar}
6045 tar: Cowardly refusing to create an empty archive
6046 Try `tar --help' or `tar --usage' for more information.
6050 If you specify either @option{--list} (@option{-t}) or
6051 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6052 operates on all the archive members in the archive.
6054 If run with @option{--diff} option, tar will compare the archive with
6055 the contents of the current working directory.
6057 If you specify any other operation, @command{tar} does nothing.
6059 By default, @command{tar} takes file names from the command line. However,
6060 there are other ways to specify file or member names, or to modify the
6061 manner in which @command{tar} selects the files or members upon which to
6062 operate. In general, these methods work both for specifying the names
6063 of files and archive members.
6066 @section Reading Names from a File
6068 @cindex Reading file names from a file
6069 @cindex Lists of file names
6070 @cindex File Name arguments, alternatives
6071 Instead of giving the names of files or archive members on the command
6072 line, you can put the names into a file, and then use the
6073 @option{--files-from=@var{file-of-names}} (@option{-T
6074 @var{file-of-names}}) option to @command{tar}. Give the name of the
6075 file which contains the list of files to include as the argument to
6076 @option{--files-from}. In the list, the file names should be separated by
6077 newlines. You will frequently use this option when you have generated
6078 the list of files to archive with the @command{find} utility.
6082 @item --files-from=@var{file-name}
6083 @itemx -T @var{file-name}
6084 Get names to extract or create from file @var{file-name}.
6087 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6088 you specify either @code{--files-from=-} or @code{-T -}), then the file
6089 names are read from standard input.
6091 Unless you are running @command{tar} with @option{--create}, you can not use
6092 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6095 Any number of @option{-T} options can be given in the command line.
6097 The following example shows how to use @command{find} to generate a list of
6098 files smaller than 400K in length and put that list into a file
6099 called @file{small-files}. You can then use the @option{-T} option to
6100 @command{tar} to specify the files from that file, @file{small-files}, to
6101 create the archive @file{little.tgz}. (The @option{-z} option to
6102 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
6106 $ @kbd{find . -size -400 -print > small-files}
6107 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
6111 In the file list given by @option{-T} option, any file name beginning
6112 with @samp{-} character is considered a @command{tar} option and is
6113 processed accordingly.@footnote{Versions of @GNUTAR{} up to 1.15.1
6114 recognized only @option{-C} option in file lists, and only if the
6115 option and its argument occupied two consecutive lines.} For example,
6116 the common use of this feature is to change to another directory by
6117 specifying @option{-C} option:
6127 $ @kbd{tar -c -f foo.tar --files-from list}
6132 In this example, @command{tar} will first switch to @file{/etc}
6133 directory and add files @file{passwd} and @file{hosts} to the
6134 archive. Then it will change to @file{/lib} directory and will archive
6135 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
6140 $ @kbd{tar tf foo.tar}
6148 @opindex directory, using in @option{--files-from} argument
6149 Notice that the option parsing algorithm used with @option{-T} is
6150 stricter than the one used by shell. Namely, when specifying option
6151 arguments, you should observe the following rules:
6155 When using short (single-letter) option form, its argument must
6156 immediately follow the option letter, without any intervening
6157 whitespace. For example: @code{-Cdir}.
6160 When using long option form, the option argument must be separated
6161 from the option by a single equal sign. No whitespace is allowed on
6162 any side of the equal sign. For example: @code{--directory=dir}.
6165 For both short and long option forms, the option argument can be given
6166 on the next line after the option name, e.g.:
6187 If you happen to have a file whose name starts with @samp{-},
6188 precede it with @option{--add-file} option to prevent it from
6189 being recognized as an option. For example: @code{--add-file=--my-file}.
6196 @subsection @code{NUL} Terminated File Names
6198 @cindex File names, terminated by @code{NUL}
6199 @cindex @code{NUL} terminated file names
6200 The @option{--null} option causes
6201 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
6202 to read file names terminated by a @code{NUL} instead of a newline, so
6203 files whose names contain newlines can be archived using
6204 @option{--files-from}.
6209 Only consider @code{NUL} terminated file names, instead of files that
6210 terminate in a newline.
6213 The @option{--null} option is just like the one in @acronym{GNU}
6214 @command{xargs} and @command{cpio}, and is useful with the
6215 @option{-print0} predicate of @acronym{GNU} @command{find}. In
6216 @command{tar}, @option{--null} also disables special handling for
6217 file names that begin with dash.
6219 This example shows how to use @command{find} to generate a list of files
6220 larger than 800K in length and put that list into a file called
6221 @file{long-files}. The @option{-print0} option to @command{find} is just
6222 like @option{-print}, except that it separates files with a @code{NUL}
6223 rather than with a newline. You can then run @command{tar} with both the
6224 @option{--null} and @option{-T} options to specify that @command{tar} get the
6225 files from that file, @file{long-files}, to create the archive
6226 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
6227 @command{tar} to recognize the @code{NUL} separator between files.
6230 $ @kbd{find . -size +800 -print0 > long-files}
6231 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
6234 @FIXME{say anything else here to conclude the section?}
6237 @section Excluding Some Files
6240 @cindex File names, excluding files by
6241 @cindex Excluding files by name and pattern
6242 @cindex Excluding files by file system
6243 To avoid operating on files whose names match a particular pattern,
6244 use the @option{--exclude} or @option{--exclude-from} options.
6248 @item --exclude=@var{pattern}
6249 Causes @command{tar} to ignore files that match the @var{pattern}.
6253 The @option{--exclude=@var{pattern}} option prevents any file or
6254 member whose name matches the shell wildcard (@var{pattern}) from
6256 For example, to create an archive with all the contents of the directory
6257 @file{src} except for files whose names end in @file{.o}, use the
6258 command @samp{tar -cf src.tar --exclude='*.o' src}.
6260 You may give multiple @option{--exclude} options.
6263 @opindex exclude-from
6264 @item --exclude-from=@var{file}
6265 @itemx -X @var{file}
6266 Causes @command{tar} to ignore files that match the patterns listed in
6270 @findex exclude-from
6271 Use the @option{--exclude-from} option to read a
6272 list of patterns, one per line, from @var{file}; @command{tar} will
6273 ignore files matching those patterns. Thus if @command{tar} is
6274 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
6275 single line @file{*.o}, no files whose names end in @file{.o} will be
6276 added to the archive.
6279 @opindex exclude-caches
6280 @item --exclude-caches
6281 Causes @command{tar} to ignore directories containing a cache directory tag.
6284 @findex exclude-caches
6285 When creating an archive, the @option{--exclude-caches} option causes
6286 @command{tar} to exclude all directories that contain a @dfn{cache
6287 directory tag}. A cache directory tag is a short file with the
6288 well-known name @file{CACHEDIR.TAG} and having a standard header
6289 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
6290 Various applications write cache directory tags into directories they
6291 use to hold regenerable, non-precious data, so that such data can be
6292 more easily excluded from backups.
6295 * problems with exclude::
6298 @node problems with exclude
6299 @unnumberedsubsec Problems with Using the @code{exclude} Options
6301 @opindex exclude, potential problems with
6302 Some users find @samp{exclude} options confusing. Here are some common
6307 The main operating mode of @command{tar} does not act on a path name
6308 explicitly listed on the command line if one of its file name
6309 components is excluded. In the example above, if
6310 you create an archive and exclude files that end with @samp{*.o}, but
6311 explicitly name the file @samp{dir.o/foo} after all the options have been
6312 listed, @samp{dir.o/foo} will be excluded from the archive.
6315 You can sometimes confuse the meanings of @option{--exclude} and
6316 @option{--exclude-from}. Be careful: use @option{--exclude} when files
6317 to be excluded are given as a pattern on the command line. Use
6318 @option{--exclude-from} to introduce the name of a file which contains
6319 a list of patterns, one per line; each of these patterns can exclude
6320 zero, one, or many files.
6323 When you use @option{--exclude=@var{pattern}}, be sure to quote the
6324 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
6325 like @samp{*}. If you do not do this, the shell might expand the
6326 @samp{*} itself using files at hand, so @command{tar} might receive a
6327 list of files instead of one pattern, or none at all, making the
6328 command somewhat illegal. This might not correspond to what you want.
6333 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
6341 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
6345 You must use use shell syntax, or globbing, rather than @code{regexp}
6346 syntax, when using exclude options in @command{tar}. If you try to use
6347 @code{regexp} syntax to describe files to be excluded, your command
6351 @FIXME{The change in semantics must have occurred before 1.11,
6352 so I doubt if it is worth mentioning at all. Anyway, should at
6353 least specify in which version the semantics changed.}
6354 In earlier versions of @command{tar}, what is now the
6355 @option{--exclude-from} option was called @option{--exclude} instead.
6356 Now, @option{--exclude} applies to patterns listed on the command
6357 line and @option{--exclude-from} applies to patterns listed in a
6363 @section Wildcards Patterns and Matching
6365 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
6366 @samp{*} or @samp{?} for example, are replaced and expanded into all
6367 existing files matching the given pattern. @GNUTAR{} can use wildcard
6368 patterns for matching (or globbing) archive members when extracting
6369 from or listing an archive. Wildcard patterns are also used for
6370 verifying volume labels of @command{tar} archives. This section has the
6371 purpose of explaining wildcard syntax for @command{tar}.
6373 @FIXME{the next few paragraphs need work.}
6375 A @var{pattern} should be written according to shell syntax, using wildcard
6376 characters to effect globbing. Most characters in the pattern stand
6377 for themselves in the matched string, and case is significant: @samp{a}
6378 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
6379 pattern matches any single character in the matched string. The character
6380 @samp{*} in the pattern matches zero, one, or more single characters in
6381 the matched string. The character @samp{\} says to take the following
6382 character of the pattern @emph{literally}; it is useful when one needs to
6383 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
6385 The character @samp{[}, up to the matching @samp{]}, introduces a character
6386 class. A @dfn{character class} is a list of acceptable characters
6387 for the next single character of the matched string. For example,
6388 @samp{[abcde]} would match any of the first five letters of the alphabet.
6389 Note that within a character class, all of the ``special characters''
6390 listed above other than @samp{\} lose their special meaning; for example,
6391 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
6392 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
6393 the characters @samp{-} and @samp{]} must either come @emph{first} or
6394 @emph{last} in a character class.)
6396 @cindex Excluding characters from a character class
6397 @cindex Character class, excluding characters from
6398 If the first character of the class after the opening @samp{[}
6399 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
6400 Rather than listing character to match, it lists those characters which
6401 are @emph{forbidden} as the next single character of the matched string.
6403 Other characters of the class stand for themselves. The special
6404 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
6405 letters, is meant to represent all characters between @var{a} and
6408 @FIXME{need to add a sentence or so here to make this clear for those
6409 who don't have dan around.}
6411 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
6412 special for wildcard matches. However, if a pattern completely matches
6413 a directory prefix of a matched string, then it matches the full matched
6414 string: thus, excluding a directory also excludes all the files beneath it.
6417 * controlling pattern-matching::
6420 @node controlling pattern-matching
6421 @unnumberedsubsec Controlling Pattern-Matching
6423 For the purposes of this section, we call @dfn{exclusion members} all
6424 member names obtained while processing @option{--exclude} and
6425 @option{--exclude-from} options, and @dfn{inclusion members} those
6426 member names that were given in the command line or read from the file
6427 specified with @option{--files-from} option.
6429 These two pairs of member lists are used in the following operations:
6430 @option{--diff}, @option{--extract}, @option{--list},
6433 There are no inclusion members in create mode (@option{--create} and
6434 @option{--append}), since in this mode the names obtained from the
6435 command line refer to @emph{files}, not archive members.
6437 By default, inclusion members are compared with archive members
6438 literally @footnote{Notice that earlier @GNUTAR{} versions used
6439 globbing for inclusion members, which contradicted to UNIX98
6440 specification and was not documented. @xref{Changes}, for more
6441 information on this and other changes} and exclusion members are
6442 treated as globbing patterns. For example:
6446 $ @kbd{tar tf foo.tar}
6451 # @i{Member names are used verbatim:}
6452 $ @kbd{tar -xf foo.tar -v '[remarks]'}
6454 # @i{Exclude member names are globbed:}
6455 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
6461 This behavior can be altered by using the following options:
6466 Treat all member names as wildcards.
6468 @opindex no-wildcards
6469 @item --no-wildcards
6470 Treat all member names as literal strings.
6473 Thus, to extract files whose names end in @samp{.c}, you can use:
6476 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
6482 Notice quoting of the pattern to prevent the shell from interpreting
6485 The effect of @option{--wildcards} option is cancelled by
6486 @option{--no-wildcards}. This can be used to pass part of
6487 the command line arguments verbatim and other part as globbing
6488 patterns. For example, the following invocation:
6491 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
6495 instructs @command{tar} to extract from @file{foo.tar} all files whose
6496 names end in @samp{.txt} and the file named @file{[remarks]}.
6498 Normally, a pattern matches a name if an initial subsequence of the
6499 name's components matches the pattern, where @samp{*}, @samp{?}, and
6500 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
6501 and wildcards can match @samp{/}.
6503 Other than optionally stripping leading @samp{/} from names
6504 (@pxref{absolute}), patterns and names are used as-is. For
6505 example, trailing @samp{/} is not trimmed from a user-specified name
6506 before deciding whether to exclude it.
6508 However, this matching procedure can be altered by the options listed
6509 below. These options accumulate. For example:
6512 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
6515 ignores case when excluding @samp{makefile}, but not when excluding
6520 @opindex no-anchored
6522 @itemx --no-anchored
6523 If anchored, a pattern must match an initial subsequence
6524 of the name's components. Otherwise, the pattern can match any
6525 subsequence. Default is @option{--no-anchored} for exclusion members
6526 and @option{--anchored} inclusion members.
6528 @opindex ignore-case
6529 @opindex no-ignore-case
6531 @itemx --no-ignore-case
6532 When ignoring case, upper-case patterns match lower-case names and vice versa.
6533 When not ignoring case (the default), matching is case-sensitive.
6535 @opindex wildcards-match-slash
6536 @opindex no-wildcards-match-slash
6537 @item --wildcards-match-slash
6538 @itemx --no-wildcards-match-slash
6539 When wildcards match slash (the default for exclusion members), a
6540 wildcard like @samp{*} in the pattern can match a @samp{/} in the
6541 name. Otherwise, @samp{/} is matched only by @samp{/}.
6545 The @option{--recursion} and @option{--no-recursion} options
6546 (@pxref{recurse}) also affect how member patterns are interpreted. If
6547 recursion is in effect, a pattern matches a name if it matches any of
6548 the name's parent directories.
6550 The following table summarizes pattern-matching default values:
6552 @multitable @columnfractions .3 .7
6553 @headitem Members @tab Default settings
6554 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
6555 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
6558 @node quoting styles
6559 @section Quoting Member Names
6561 When displaying member names, @command{tar} takes care to avoid
6562 ambiguities caused by certain characters. This is called @dfn{name
6563 quoting}. The characters in question are:
6566 @item Non-printable control characters:
6568 @multitable @columnfractions 0.20 0.10 0.60
6569 @headitem Character @tab ASCII @tab Character name
6570 @item \a @tab 7 @tab Audible bell
6571 @item \b @tab 8 @tab Backspace
6572 @item \f @tab 12 @tab Form feed
6573 @item \n @tab 10 @tab New line
6574 @item \r @tab 13 @tab Carriage return
6575 @item \t @tab 9 @tab Horizontal tabulation
6576 @item \v @tab 11 @tab Vertical tabulation
6579 @item Space (ASCII 32)
6581 @item Single and double quotes (@samp{'} and @samp{"})
6583 @item Backslash (@samp{\})
6586 The exact way @command{tar} uses to quote these characters depends on
6587 the @dfn{quoting style}. The default quoting style, called
6588 @dfn{escape} (see below), uses backslash notation to represent control
6589 characters, space and backslash. Using this quoting style, control
6590 characters are represented as listed in column @samp{Character} in the
6591 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
6593 @GNUTAR{} offers seven distinct quoting styles, which can be selected
6594 using @option{--quoting-style} option:
6597 @item --quoting-style=@var{style}
6598 @opindex quoting-style
6600 Sets quoting style. Valid values for @var{style} argument are:
6601 literal, shell, shell-always, c, escape, locale, clocale.
6604 These styles are described in detail below. To illustrate their
6605 effect, we will use an imaginary tar archive @file{arch.tar}
6606 containing the following members:
6610 # 1. Contains horizontal tabulation character.
6612 # 2. Contains newline character
6615 # 3. Contains a space
6617 # 4. Contains double quotes
6619 # 5. Contains single quotes
6621 # 6. Contains a backslash character:
6626 Here is how usual @command{ls} command would have listed them, if they
6627 had existed in the current working directory:
6645 No quoting, display each character as is:
6649 $ @kbd{tar tf arch.tar --quoting-style=literal}
6662 Display characters the same way Bourne shell does:
6663 control characters, except @samp{\t} and @samp{\n}, are printed using
6664 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
6665 single quote is printed as @samp{\'}. If a name contains any quoted
6666 characters, it is enclosed in single quotes. In particular, if a name
6667 contains single quotes, it is printed as several single-quoted strings:
6671 $ @kbd{tar tf arch.tar --quoting-style=shell}
6674 './a'\''single'\''quote'
6684 Same as @samp{shell}, but the names are always enclosed in single
6689 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
6692 './a'\''single'\''quote'
6702 Use the notation of the C programming language. All names are
6703 enclosed in double quotes. Control characters are quoted using
6704 backslash notations, double quotes are represented as @samp{\"},
6705 backslash characters are represented as @samp{\\}. Single quotes and
6706 spaces are not quoted:
6710 $ @kbd{tar tf arch.tar --quoting-style=c}
6714 "./a\"double\"quote"
6722 Control characters are printed using backslash notation, a space is
6723 printed as @samp{\ } and a backslash as @samp{\\}. This is the
6724 default quoting style, unless it was changed when configured the
6729 $ @kbd{tar tf arch.tar --quoting-style=escape}
6741 Control characters, single quote and backslash are printed using
6742 backslash notation. All names are quoted using left and right
6743 quotation marks, appropriate to the current locale. If it does not
6744 define quotation marks, use @samp{`} as left and @samp{'} as right
6745 quotation marks. Any occurrences of the right quotation mark in a
6746 name are escaped with @samp{\}, for example:
6752 $ @kbd{tar tf arch.tar --quoting-style=locale}
6755 `./a\'single\'quote'
6764 Same as @samp{locale}, but @samp{"} is used for both left and right
6765 quotation marks, if not provided by the currently selected locale:
6769 $ @kbd{tar tf arch.tar --quoting-style=clocale}
6773 "./a\"double\"quote"
6781 You can specify which characters should be quoted in addition to those
6782 implied by the current quoting style:
6785 @item --quote-chars=@var{string}
6786 Always quote characters from @var{string}, even if the selected
6787 quoting style would not quote them.
6790 For example, using @samp{escape} quoting (compare with the usual
6791 escape listing above):
6795 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
6806 To disable quoting of such additional characters, use the following
6810 @item --no-quote-chars=@var{string}
6811 Remove characters listed in @var{string} from the list of quoted
6812 characters set by the previous @option{--quote-chars} option.
6815 This option is particularly useful if you have added
6816 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
6817 and wish to disable it for the current invocation.
6819 Note, that @option{--no-quote-chars} does @emph{not} disable those
6820 characters that are quoted by default in the selected quoting style.
6823 @section Modifying File and Member Names
6825 @command{Tar} archives contain detailed information about files stored
6826 in them and full file names are part of that information. When
6827 storing file to an archive, its file name is recorded in the archive
6828 along with the actual file contents. When restoring from an archive,
6829 a file is created on disk with exactly the same name as that stored
6830 in the archive. In the majority of cases this is the desired behavior
6831 of a file archiver. However, there are some cases when it is not.
6833 First of all, it is often unsafe to extract archive members with
6834 absolute file names or those that begin with a @file{../}. @GNUTAR{}
6835 takes special precautions when extracting such names and provides a
6836 special option for handling them, which is described in
6839 Secondly, you may wish to extract file names without some leading
6840 directory components, or with otherwise modified names. In other
6841 cases it is desirable to store files under differing names in the
6844 @GNUTAR{} provides two options for these needs.
6847 @opindex strip-components
6848 @item --strip-components=@var{number}
6849 Strip given @var{number} of leading components from file names before
6853 For example, suppose you have archived whole @file{/usr} hierarchy to
6854 a tar archive named @file{usr.tar}. Among other files, this archive
6855 contains @file{usr/include/stdlib.h}, which you wish to extract to
6856 the current working directory. To do so, you type:
6859 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
6862 The option @option{--strip=2} instructs @command{tar} to strip the
6863 two leading components (@file{usr/} and @file{include/}) off the file
6866 If you add to the above invocation @option{--verbose} (@option{-v})
6867 option, you will note that the verbose listing still contains the
6868 full file name, with the two removed components still in place. This
6869 can be inconvenient, so @command{tar} provides a special option for
6870 altering this behavior:
6872 @anchor{show-transformed-names}
6874 @opindex --show-transformed-names
6875 @item --show-transformed-names
6876 Display file or member names with all requested transformations
6884 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
6885 usr/include/stdlib.h
6886 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
6891 Notice that in both cases the file is @file{stdlib.h} extracted to the
6892 current working directory, @option{--show-transformed-names} affects
6893 only the way its name is displayed.
6895 This option is especially useful for verifying whether the invocation
6896 will have the desired effect. Thus, before running
6899 $ @kbd{tar -x --strip=@var{n}}
6903 it is often advisable to run
6906 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
6910 to make sure the command will produce the intended results.
6912 In case you need to apply more complex modifications to the file name,
6913 @GNUTAR{} provides a general-purpose transformation option:
6916 @opindex --transform
6917 @item --transform=@var{expression}
6918 Modify file names using supplied @var{expression}.
6922 The @var{expression} is a @command{sed}-like replace expression of the
6926 s/@var{regexp}/@var{replace}/[@var{flags}]
6930 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
6931 replacement for each file name part that matches @var{regexp}. Both
6932 @var{regexp} and @var{replace} are described in detail in
6933 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
6935 Notice, however, that the following @command{sed}-specific escapes
6936 are not supported in @var{replace}: @samp{\L}, @samp{\l}, @samp{\U},
6937 @samp{\u}, @samp{\E}.
6939 The supported @var{flags} are:
6943 Apply the replacement to @emph{all} matches to the @var{regexp}, not
6947 Use case-insensitive matching
6950 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
6951 regexps, Extended regular expressions, Extended regular expressions,
6955 Any delimiter can be used in lieue of @samp{/}, the only requirement being
6956 that it be used consistently throughout the expression. For example,
6957 the following two expressions are equivalent:
6966 Changing of delimiter is often useful when the @var{regex} contains
6967 slashes. For example, it is more convenient to write:
6980 Here are several examples of @option{--transform} usage:
6983 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
6986 $ @kbd{tar --transform='s,usr/,usr/local/,' -x arch.tar}
6989 @item Strip two leading directory components (equivalent to
6990 @option{--strip-components=2}):
6993 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x arch.tar}
6996 @item Prepend @file{/prefix/} to each file name:
6999 $ @kbd{tar --transform 's,^,/prefix/,' -x arch.tar}
7004 Unlike @option{--strip-components}, @option{--transform} can be used
7005 in any @GNUTAR{} operation mode. For example, the following command
7006 adds files to the archive while replacing the leading @file{usr/}
7007 component with @file{var/}:
7010 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
7013 To test @option{--transform} effect we suggest to use
7014 @option{--show-transformed-names}:
7017 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
7018 --verbose --show-transformed-names /}
7022 @section Operating Only on New Files
7025 @cindex Excluding file by age
7026 @cindex Data Modification time, excluding files by
7027 @cindex Modification time, excluding files by
7028 @cindex Age, excluding files by
7029 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
7030 @option{-N @var{date}}) option causes @command{tar} to only work on
7031 files whose data modification or status change times are newer than
7032 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
7033 it is taken to be a file name; the data modification time of that file
7034 is used as the date. If you use this option when creating or appending
7035 to an archive, the archive will only include new files. If you use
7036 @option{--after-date} when extracting an archive, @command{tar} will
7037 only extract files newer than the @var{date} you specify.
7039 If you only want @command{tar} to make the date comparison based on
7040 modification of the file's data (rather than status
7041 changes), then use the @option{--newer-mtime=@var{date}} option.
7043 You may use these options with any operation. Note that these options
7044 differ from the @option{--update} (@option{-u}) operation in that they
7045 allow you to specify a particular date against which @command{tar} can
7046 compare when deciding whether or not to archive the files.
7051 @item --after-date=@var{date}
7052 @itemx --newer=@var{date}
7053 @itemx -N @var{date}
7054 Only store files newer than @var{date}.
7056 Acts on files only if their data modification or status change times are
7057 later than @var{date}. Use in conjunction with any operation.
7059 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
7060 name; the data modification time of that file is used as the date.
7062 @opindex newer-mtime
7063 @item --newer-mtime=@var{date}
7064 Acts like @option{--after-date}, but only looks at data modification times.
7067 These options limit @command{tar} to operate only on files which have
7068 been modified after the date specified. A file's status is considered to have
7069 changed if its contents have been modified, or if its owner,
7070 permissions, and so forth, have been changed. (For more information on
7071 how to specify a date, see @ref{Date input formats}; remember that the
7072 entire date argument must be quoted if it contains any spaces.)
7074 Gurus would say that @option{--after-date} tests both the data
7075 modification time (@code{mtime}, the time the contents of the file
7076 were last modified) and the status change time (@code{ctime}, the time
7077 the file's status was last changed: owner, permissions, etc.@:)
7078 fields, while @option{--newer-mtime} tests only the @code{mtime}
7081 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
7082 @code{ctime} and processes the file if either one is more recent than
7083 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
7084 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
7085 contents of the file were looked at).
7087 Date specifiers can have embedded spaces. Because of this, you may need
7088 to quote date arguments to keep the shell from parsing them as separate
7089 arguments. For example, the following command will add to the archive
7090 all the files modified less than two days ago:
7093 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
7097 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
7098 should not be used for incremental backups. @xref{Incremental Dumps},
7099 for proper way of creating incremental backups.
7103 @section Descending into Directories
7105 @cindex Avoiding recursion in directories
7106 @cindex Descending directories, avoiding
7107 @cindex Directories, avoiding recursion
7108 @cindex Recursion in directories, avoiding
7110 @FIXME{arrggh! this is still somewhat confusing to me. :-< }
7112 Usually, @command{tar} will recursively explore all directories (either
7113 those given on the command line or through the @option{--files-from}
7114 option) for the various files they contain. However, you may not always
7115 want @command{tar} to act this way.
7117 @opindex no-recursion
7118 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
7119 into specified directories. If you specify @option{--no-recursion}, you can
7120 use the @command{find} utility for hunting through levels of directories to
7121 construct a list of file names which you could then pass to @command{tar}.
7122 @command{find} allows you to be more selective when choosing which files to
7123 archive; see @ref{files}, for more information on using @command{find} with
7124 @command{tar}, or look.
7127 @item --no-recursion
7128 Prevents @command{tar} from recursively descending directories.
7132 Requires @command{tar} to recursively descend directories.
7133 This is the default.
7136 When you use @option{--no-recursion}, @GNUTAR{} grabs
7137 directory entries themselves, but does not descend on them
7138 recursively. Many people use @command{find} for locating files they
7139 want to back up, and since @command{tar} @emph{usually} recursively
7140 descends on directories, they have to use the @samp{@w{-not -type d}}
7141 test in their @command{find} invocation (@pxref{Type, Type, Type test,
7142 find, Finding Files}), as they usually do not want all the files in a
7143 directory. They then use the @option{--files-from} option to archive
7144 the files located via @command{find}.
7146 The problem when restoring files archived in this manner is that the
7147 directories themselves are not in the archive; so the
7148 @option{--same-permissions} (@option{--preserve-permissions},
7149 @option{-p}) option does not affect them---while users might really
7150 like it to. Specifying @option{--no-recursion} is a way to tell
7151 @command{tar} to grab only the directory entries given to it, adding
7152 no new files on its own. To summarize, if you use @command{find} to
7153 create a list of files to be stored in an archive, use it as follows:
7157 $ @kbd{find @var{dir} @var{tests} | \
7158 tar -cf @var{archive} -T - --no-recursion}
7162 The @option{--no-recursion} option also applies when extracting: it
7163 causes @command{tar} to extract only the matched directory entries, not
7164 the files under those directories.
7166 The @option{--no-recursion} option also affects how globbing patterns
7167 are interpreted (@pxref{controlling pattern-matching}).
7169 The @option{--no-recursion} and @option{--recursion} options apply to
7170 later options and operands, and can be overridden by later occurrences
7171 of @option{--no-recursion} and @option{--recursion}. For example:
7174 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
7178 creates an archive with one entry for @file{grape}, and the recursive
7179 contents of @file{grape/concord}, but no entries under @file{grape}
7180 other than @file{grape/concord}.
7183 @section Crossing File System Boundaries
7184 @cindex File system boundaries, not crossing
7187 @command{tar} will normally automatically cross file system boundaries in
7188 order to archive files which are part of a directory tree. You can
7189 change this behavior by running @command{tar} and specifying
7190 @option{--one-file-system}. This option only affects files that are
7191 archived because they are in a directory that is being archived;
7192 @command{tar} will still archive files explicitly named on the command line
7193 or through @option{--files-from}, regardless of where they reside.
7196 @opindex one-file-system
7197 @item --one-file-system
7198 Prevents @command{tar} from crossing file system boundaries when
7199 archiving. Use in conjunction with any write operation.
7202 The @option{--one-file-system} option causes @command{tar} to modify its
7203 normal behavior in archiving the contents of directories. If a file in
7204 a directory is not on the same file system as the directory itself, then
7205 @command{tar} will not archive that file. If the file is a directory
7206 itself, @command{tar} will not archive anything beneath it; in other words,
7207 @command{tar} will not cross mount points.
7209 This option is useful for making full or incremental archival backups of
7210 a file system. If this option is used in conjunction with
7211 @option{--verbose} (@option{-v}), files that are excluded are
7212 mentioned by name on the standard error.
7215 * directory:: Changing Directory
7216 * absolute:: Absolute File Names
7220 @subsection Changing the Working Directory
7223 @FIXME{need to read over this node now for continuity; i've switched
7224 things around some.}
7226 @cindex Changing directory mid-stream
7227 @cindex Directory, changing mid-stream
7228 @cindex Working directory, specifying
7229 To change the working directory in the middle of a list of file names,
7230 either on the command line or in a file specified using
7231 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
7232 This will change the working directory to the specified directory
7233 after that point in the list.
7237 @item --directory=@var{directory}
7238 @itemx -C @var{directory}
7239 Changes the working directory in the middle of a command line.
7245 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
7249 will place the files @file{grape} and @file{prune} from the current
7250 directory into the archive @file{jams.tar}, followed by the file
7251 @file{cherry} from the directory @file{food}. This option is especially
7252 useful when you have several widely separated files that you want to
7253 store in the same archive.
7255 Note that the file @file{cherry} is recorded in the archive under the
7256 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
7257 archive will contain three files that all appear to have come from the
7258 same directory; if the archive is extracted with plain @samp{tar
7259 --extract}, all three files will be written in the current directory.
7261 Contrast this with the command,
7264 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
7268 which records the third file in the archive under the name
7269 @file{red/cherry} so that, if the archive is extracted using
7270 @samp{tar --extract}, the third file will be written in a subdirectory
7271 named @file{orange-colored}.
7273 You can use the @option{--directory} option to make the archive
7274 independent of the original name of the directory holding the files.
7275 The following command places the files @file{/etc/passwd},
7276 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
7280 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
7284 However, the names of the archive members will be exactly what they were
7285 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
7286 They will not appear to be related by file name to the original
7287 directories where those files were located.
7289 Note that @option{--directory} options are interpreted consecutively. If
7290 @option{--directory} specifies a relative file name, it is interpreted
7291 relative to the then current directory, which might not be the same as
7292 the original current working directory of @command{tar}, due to a previous
7293 @option{--directory} option.
7295 When using @option{--files-from} (@pxref{files}), you can put various
7296 @command{tar} options (including @option{-C}) in the file list. Notice,
7297 however, that in this case the option and its argument may not be
7298 separated by whitespace. If you use short option, its argument must
7299 either follow the option letter immediately, without any intervening
7300 whitespace, or occupy the next line. Otherwise, if you use long
7301 option, separate its argument by an equal sign.
7303 For instance, the file list for the above example will be:
7318 To use it, you would invoke @command{tar} as follows:
7321 $ @kbd{tar -c -f foo.tar --files-from list}
7324 Notice also that you can only use the short option variant in the file
7325 list, i.e., always use @option{-C}, not @option{--directory}.
7327 The interpretation of @option{--directory} is disabled by
7328 @option{--null} option.
7331 @subsection Absolute File Names
7335 @opindex absolute-names
7336 @item --absolute-names
7338 Do not strip leading slashes from file names, and permit file names
7339 containing a @file{..} file name component.
7342 By default, @GNUTAR{} drops a leading @samp{/} on
7343 input or output, and complains about file names containing a @file{..}
7344 component. This option turns off this behavior.
7346 When @command{tar} extracts archive members from an archive, it strips any
7347 leading slashes (@samp{/}) from the member name. This causes absolute
7348 member names in the archive to be treated as relative file names. This
7349 allows you to have such members extracted wherever you want, instead of
7350 being restricted to extracting the member in the exact directory named
7351 in the archive. For example, if the archive member has the name
7352 @file{/etc/passwd}, @command{tar} will extract it as if the name were
7353 really @file{etc/passwd}.
7355 File names containing @file{..} can cause problems when extracting, so
7356 @command{tar} normally warns you about such files when creating an
7357 archive, and rejects attempts to extracts such files.
7359 Other @command{tar} programs do not do this. As a result, if you
7360 create an archive whose member names start with a slash, they will be
7361 difficult for other people with a non-@GNUTAR{}
7362 program to use. Therefore, @GNUTAR{} also strips
7363 leading slashes from member names when putting members into the
7364 archive. For example, if you ask @command{tar} to add the file
7365 @file{/bin/ls} to an archive, it will do so, but the member name will
7366 be @file{bin/ls}.@footnote{A side effect of this is that when
7367 @option{--create} is used with @option{--verbose} the resulting output
7368 is not, generally speaking, the same as the one you'd get running
7369 @kbd{tar --list} command. This may be important if you use some
7370 scripts for comparing both outputs. @xref{listing member and file names},
7371 for the information on how to handle this case.}
7373 If you use the @option{--absolute-names} (@option{-P}) option,
7374 @command{tar} will do none of these transformations.
7376 To archive or extract files relative to the root directory, specify
7377 the @option{--absolute-names} (@option{-P}) option.
7379 Normally, @command{tar} acts on files relative to the working
7380 directory---ignoring superior directory names when archiving, and
7381 ignoring leading slashes when extracting.
7383 When you specify @option{--absolute-names} (@option{-P}),
7384 @command{tar} stores file names including all superior directory
7385 names, and preserves leading slashes. If you only invoked
7386 @command{tar} from the root directory you would never need the
7387 @option{--absolute-names} option, but using this option
7388 may be more convenient than switching to root.
7390 @FIXME{Should be an example in the tutorial/wizardry section using this
7391 to transfer files between systems.}
7393 @FIXME{Is write access an issue?}
7396 @item --absolute-names
7397 Preserves full file names (including superior directory names) when
7398 archiving files. Preserves leading slash when extracting files.
7402 @FIXME{this is still horrible; need to talk with dan on monday.}
7404 @command{tar} prints out a message about removing the @samp{/} from
7405 file names. This message appears once per @GNUTAR{}
7406 invocation. It represents something which ought to be told; ignoring
7407 what it means can cause very serious surprises, later.
7409 Some people, nevertheless, do not want to see this message. Wanting to
7410 play really dangerously, one may of course redirect @command{tar} standard
7411 error to the sink. For example, under @command{sh}:
7414 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
7418 Another solution, both nicer and simpler, would be to change to
7419 the @file{/} directory first, and then avoid absolute notation.
7423 $ @kbd{(cd / && tar -c -f archive.tar home)}
7425 $ @kbd{tar -c -f archive.tar -C / home}
7428 @include getdate.texi
7431 @chapter Controlling the Archive Format
7433 @cindex Tar archive formats
7434 Due to historical reasons, there are several formats of tar archives.
7435 All of them are based on the same principles, but have some subtle
7436 differences that often make them incompatible with each other.
7438 GNU tar is able to create and handle archives in a variety of formats.
7439 The most frequently used formats are (in alphabetical order):
7443 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
7444 from an early @acronym{POSIX} standard, adding some improvements such as
7445 sparse file handling and incremental archives. Unfortunately these
7446 features were implemented in a way incompatible with other archive
7449 Archives in @samp{gnu} format are able to hold pathnames of unlimited
7453 Format used by @GNUTAR{} of versions prior to 1.12.
7456 Archive format, compatible with the V7 implementation of tar. This
7457 format imposes a number of limitations. The most important of them
7461 @item The maximum length of a file name is limited to 99 characters.
7462 @item The maximum length of a symbolic link is limited to 99 characters.
7463 @item It is impossible to store special files (block and character
7464 devices, fifos etc.)
7465 @item Maximum value of user or group ID is limited to 2097151 (7777777
7467 @item V7 archives do not contain symbolic ownership information (user
7468 and group name of the file owner).
7471 This format has traditionally been used by Automake when producing
7472 Makefiles. This practice will change in the future, in the meantime,
7473 however this means that projects containing filenames more than 99
7474 characters long will not be able to use @GNUTAR{} @value{VERSION} and
7475 Automake prior to 1.9.
7478 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
7479 symbolic ownership information. It is also able to store
7480 special files. However, it imposes several restrictions as well:
7483 @item The maximum length of a file name is limited to 256 characters,
7484 provided that the filename can be split at directory separator in
7485 two parts, first of them being at most 155 bytes long. So, in most
7486 cases the maximum file name length will be shorter than 256
7488 @item The maximum length of a symbolic link name is limited to
7490 @item Maximum size of a file the archive is able to accomodate
7492 @item Maximum value of UID/GID is 2097151.
7493 @item Maximum number of bits in device major and minor numbers is 21.
7497 Format used by J@"org Schilling @command{star}
7498 implementation. @GNUTAR{} is able to read @samp{star} archives but
7499 currently does not produce them.
7502 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
7503 most flexible and feature-rich format. It does not impose any
7504 restrictions on file sizes or filename lengths. This format is quite
7505 recent, so not all tar implementations are able to handle it properly.
7506 However, this format is designed in such a way that any tar
7507 implementation able to read @samp{ustar} archives will be able to read
7508 most @samp{posix} archives as well, with the only exception that any
7509 additional information (such as long file names etc.) will in such
7510 case be extracted as plain text files along with the files it refers to.
7512 This archive format will be the default format for future versions
7517 The following table summarizes the limitations of each of these
7520 @multitable @columnfractions .10 .20 .20 .20 .20
7521 @headitem Format @tab UID @tab File Size @tab Path Name @tab Devn
7522 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
7523 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
7524 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
7525 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
7526 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
7529 The default format for @GNUTAR{} is defined at compilation
7530 time. You may check it by running @command{tar --help}, and examining
7531 the last lines of its output. Usually, @GNUTAR{} is configured
7532 to create archives in @samp{gnu} format, however, future version will
7533 switch to @samp{posix}.
7536 * Portability:: Making @command{tar} Archives More Portable
7537 * Compression:: Using Less Space through Compression
7538 * Attributes:: Handling File Attributes
7539 * Standard:: The Standard Format
7540 * Extensions:: @acronym{GNU} Extensions to the Archive Format
7541 * cpio:: Comparison of @command{tar} and @command{cpio}
7545 @section Making @command{tar} Archives More Portable
7547 Creating a @command{tar} archive on a particular system that is meant to be
7548 useful later on many other machines and with other versions of @command{tar}
7549 is more challenging than you might think. @command{tar} archive formats
7550 have been evolving since the first versions of Unix. Many such formats
7551 are around, and are not always compatible with each other. This section
7552 discusses a few problems, and gives some advice about making @command{tar}
7553 archives more portable.
7555 One golden rule is simplicity. For example, limit your @command{tar}
7556 archives to contain only regular files and directories, avoiding
7557 other kind of special files. Do not attempt to save sparse files or
7558 contiguous files as such. Let's discuss a few more problems, in turn.
7560 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
7561 archives and archive labels) in GNU and PAX formats.}
7564 * Portable Names:: Portable Names
7565 * dereference:: Symbolic Links
7566 * old:: Old V7 Archives
7567 * ustar:: Ustar Archives
7568 * gnu:: GNU and old GNU format archives.
7569 * posix:: @acronym{POSIX} archives
7570 * Checksumming:: Checksumming Problems
7571 * Large or Negative Values:: Large files, negative time stamps, etc.
7574 @node Portable Names
7575 @subsection Portable Names
7577 Use portable file and member names. A name is portable if it contains
7578 only ASCII letters and digits, @samp{/}, @samp{.}, @samp{_}, and
7579 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
7580 contain @samp{/-}. Avoid deep directory nesting. For portability to
7581 old Unix hosts, limit your file name components to 14 characters or
7584 If you intend to have your @command{tar} archives to be read under
7585 MSDOS, you should not rely on case distinction for file names, and you
7586 might use the @acronym{GNU} @command{doschk} program for helping you
7587 further diagnosing illegal MSDOS names, which are even more limited
7591 @subsection Symbolic Links
7592 @cindex File names, using symbolic links
7593 @cindex Symbolic link as file name
7595 @opindex dereference
7596 Normally, when @command{tar} archives a symbolic link, it writes a
7597 block to the archive naming the target of the link. In that way, the
7598 @command{tar} archive is a faithful record of the file system contents.
7599 @option{--dereference} (@option{-h}) is used with @option{--create} (@option{-c}), and causes
7600 @command{tar} to archive the files symbolic links point to, instead of
7601 the links themselves. When this option is used, when @command{tar}
7602 encounters a symbolic link, it will archive the linked-to file,
7603 instead of simply recording the presence of a symbolic link.
7605 The name under which the file is stored in the file system is not
7606 recorded in the archive. To record both the symbolic link name and
7607 the file name in the system, archive the file under both names. If
7608 all links were recorded automatically by @command{tar}, an extracted file
7609 might be linked to a file name that no longer exists in the file
7612 If a linked-to file is encountered again by @command{tar} while creating
7613 the same archive, an entire second copy of it will be stored. (This
7614 @emph{might} be considered a bug.)
7616 So, for portable archives, do not archive symbolic links as such,
7617 and use @option{--dereference} (@option{-h}): many systems do not support
7618 symbolic links, and moreover, your distribution might be unusable if
7619 it contains unresolved symbolic links.
7622 @subsection Old V7 Archives
7623 @cindex Format, old style
7624 @cindex Old style format
7625 @cindex Old style archives
7626 @cindex v7 archive format
7628 Certain old versions of @command{tar} cannot handle additional
7629 information recorded by newer @command{tar} programs. To create an
7630 archive in V7 format (not ANSI), which can be read by these old
7631 versions, specify the @option{--format=v7} option in
7632 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
7633 accepts @option{--portability} or @samp{op-old-archive} for this
7634 option). When you specify it,
7635 @command{tar} leaves out information about directories, pipes, fifos,
7636 contiguous files, and device files, and specifies file ownership by
7637 group and user IDs instead of group and user names.
7639 When updating an archive, do not use @option{--format=v7}
7640 unless the archive was created using this option.
7642 In most cases, a @emph{new} format archive can be read by an @emph{old}
7643 @command{tar} program without serious trouble, so this option should
7644 seldom be needed. On the other hand, most modern @command{tar}s are
7645 able to read old format archives, so it might be safer for you to
7646 always use @option{--format=v7} for your distributions.
7649 @subsection Ustar Archive Format
7651 @cindex ustar archive format
7652 Archive format defined by @acronym{POSIX}.1-1988 specification is called
7653 @code{ustar}. Although it is more flexible than the V7 format, it
7654 still has many restrictions (@xref{Formats,ustar}, for the detailed
7655 description of @code{ustar} format). Along with V7 format,
7656 @code{ustar} format is a good choice for archives intended to be read
7657 with other implementations of @command{tar}.
7659 To create archive in @code{ustar} format, use @option{--format=ustar}
7660 option in conjunction with the @option{--create} (@option{-c}).
7663 @subsection @acronym{GNU} and old @GNUTAR{} format
7665 @cindex GNU archive format
7666 @cindex Old GNU archive format
7667 @GNUTAR{} was based on an early draft of the
7668 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
7669 @command{tar}, such as the support for file names longer than 100
7670 characters, use portions of the @command{tar} header record which were
7671 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
7672 @acronym{POSIX} have allocated the same parts of the header record for
7673 other purposes. As a result, @GNUTAR{} format is
7674 incompatible with the current @acronym{POSIX} specification, and with
7675 @command{tar} programs that follow it.
7677 In the majority of cases, @command{tar} will be configured to create
7678 this format by default. This will change in the future releases, since
7679 we plan to make @samp{posix} format the default.
7681 To force creation a @GNUTAR{} archive, use option
7682 @option{--format=gnu}.
7685 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
7687 @cindex POSIX archive format
7688 @cindex PAX archive format
7689 The version @value{VERSION} of @GNUTAR{} is able
7690 to read and create archives conforming to @acronym{POSIX.1-2001} standard.
7692 A @acronym{POSIX} conformant archive will be created if @command{tar}
7693 was given @option{--format=posix} option.
7696 @subsection Checksumming Problems
7698 SunOS and HP-UX @command{tar} fail to accept archives created using
7699 @GNUTAR{} and containing non-ASCII file names, that
7700 is, file names having characters with the eight bit set, because they
7701 use signed checksums, while @GNUTAR{} uses unsigned
7702 checksums while creating archives, as per @acronym{POSIX} standards. On
7703 reading, @GNUTAR{} computes both checksums and
7704 accept any. It is somewhat worrying that a lot of people may go
7705 around doing backup of their files using faulty (or at least
7706 non-standard) software, not learning about it until it's time to
7707 restore their missing files with an incompatible file extractor, or
7710 @GNUTAR{} compute checksums both ways, and accept
7711 any on read, so @acronym{GNU} tar can read Sun tapes even with their
7712 wrong checksums. @GNUTAR{} produces the standard
7713 checksum, however, raising incompatibilities with Sun. That is to
7714 say, @GNUTAR{} has not been modified to
7715 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
7716 I've been told that more recent Sun @command{tar} now read standard
7717 archives, so maybe Sun did a similar patch, after all?
7719 The story seems to be that when Sun first imported @command{tar}
7720 sources on their system, they recompiled it without realizing that
7721 the checksums were computed differently, because of a change in
7722 the default signing of @code{char}'s in their compiler. So they
7723 started computing checksums wrongly. When they later realized their
7724 mistake, they merely decided to stay compatible with it, and with
7725 themselves afterwards. Presumably, but I do not really know, HP-UX
7726 has chosen that their @command{tar} archives to be compatible with Sun's.
7727 The current standards do not favor Sun @command{tar} format. In any
7728 case, it now falls on the shoulders of SunOS and HP-UX users to get
7729 a @command{tar} able to read the good archives they receive.
7731 @node Large or Negative Values
7732 @subsection Large or Negative Values
7733 @cindex large values
7734 @cindex future time stamps
7735 @cindex negative time stamps
7738 The above sections suggest to use @samp{oldest possible} archive
7739 format if in doubt. However, sometimes it is not possible. If you
7740 attempt to archive a file whose metadata cannot be represented using
7741 required format, @GNUTAR{} will print error message and ignore such a
7742 file. You will than have to switch to a format that is able to
7743 handle such values. The format summary table (@pxref{Formats}) will
7746 In particular, when trying to archive files larger than 8GB or with
7747 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
7748 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
7749 @acronym{POSIX} archive formats. When considering which format to
7750 choose, bear in mind that the @acronym{GNU} format uses
7751 two's-complement base-256 notation to store values that do not fit
7752 into standard @acronym{ustar} range. Such archives can generally be
7753 read only by a @GNUTAR{} implementation. Moreover, they sometimes
7754 cannot be correctly restored on another hosts even by @GNUTAR{}. For
7755 example, using two's complement representation for negative time
7756 stamps that assumes a signed 32-bit @code{time_t} generates archives
7757 that are not portable to hosts with differing @code{time_t}
7760 On the other hand, @acronym{POSIX} archives, generally speaking, can
7761 be extracted by any tar implementation that understands older
7762 @acronym{ustar} format. The only exception are files larger than 8GB.
7764 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
7768 @section Using Less Space through Compression
7771 * gzip:: Creating and Reading Compressed Archives
7772 * sparse:: Archiving Sparse Files
7776 @subsection Creating and Reading Compressed Archives
7777 @cindex Compressed archives
7778 @cindex Storing archives in compressed format
7780 @GNUTAR{} is able to create and read compressed archives. It supports
7781 @command{gzip} and @command{bzip2} compression programs. For backward
7782 compatibilty, it also supports @command{compress} command, although
7783 we strongly recommend against using it, since there is a patent
7784 covering the algorithm it uses and you could be sued for patent
7785 infringement merely by running @command{compress}! Besides, it is less
7786 effective than @command{gzip} and @command{bzip2}.
7788 Creating a compressed archive is simple: you just specify a
7789 @dfn{compression option} along with the usual archive creation
7790 commands. The compression option is @option{-z} (@option{--gzip}) to
7791 create a @command{gzip} compressed archive, @option{-j}
7792 (@option{--bzip2}) to create a @command{bzip2} compressed archive, and
7793 @option{-Z} (@option{--compress}) to use @command{compress} program.
7797 $ @kbd{tar cfz archive.tar.gz .}
7800 Reading compressed archive is even simpler: you don't need to specify
7801 any additional options as @GNUTAR{} recognizes its format
7802 automatically. Thus, the following commands will list and extract the
7803 archive created in previous example:
7806 # List the compressed archive
7807 $ @kbd{tar tf archive.tar.gz}
7808 # Extract the compressed archive
7809 $ @kbd{tar xf archive.tar.gz}
7812 The only case when you have to specify a decompression option while
7813 reading the archive is when reading from a pipe or from a tape drive
7814 that does not support random access. However, in this case @GNUTAR{}
7815 will indicate which option you should use. For example:
7818 $ @kbd{cat archive.tar.gz | tar tf -}
7819 tar: Archive is compressed. Use -z option
7820 tar: Error is not recoverable: exiting now
7823 If you see such diagnostics, just add the suggested option to the
7824 invocation of @GNUTAR{}:
7827 $ @kbd{cat archive.tar.gz | tar tfz -}
7830 Notice also, that there are several restrictions on operations on
7831 compressed archives. First of all, compressed archives cannot be
7832 modified, i.e., you cannot update (@option{--update} (@option{-u})) them or delete
7833 (@option{--delete}) members from them. Likewise, you cannot append
7834 another @command{tar} archive to a compressed archive using
7835 @option{--append} (@option{-r})). Secondly, multi-volume archives cannot be
7838 The following table summarizes compression options used by @GNUTAR{}.
7846 Filter the archive through @command{gzip}.
7848 You can use @option{--gzip} and @option{--gunzip} on physical devices
7849 (tape drives, etc.) and remote files as well as on normal files; data
7850 to or from such devices or remote files is reblocked by another copy
7851 of the @command{tar} program to enforce the specified (or default) record
7852 size. The default compression parameters are used; if you need to
7853 override them, set @env{GZIP} environment variable, e.g.:
7856 $ @kbd{GZIP=--best tar cfz archive.tar.gz subdir}
7860 Another way would be to avoid the @option{--gzip} (@option{--gunzip}, @option{--ungzip}, @option{-z}) option and run
7861 @command{gzip} explicitly:
7864 $ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz}
7867 @cindex corrupted archives
7868 About corrupted compressed archives: @command{gzip}'ed files have no
7869 redundancy, for maximum compression. The adaptive nature of the
7870 compression scheme means that the compression tables are implicitly
7871 spread all over the archive. If you lose a few blocks, the dynamic
7872 construction of the compression tables becomes unsynchronized, and there
7873 is little chance that you could recover later in the archive.
7875 There are pending suggestions for having a per-volume or per-file
7876 compression in @GNUTAR{}. This would allow for viewing the
7877 contents without decompression, and for resynchronizing decompression at
7878 every volume or file, in case of corrupted archives. Doing so, we might
7879 lose some compressibility. But this would have make recovering easier.
7880 So, there are pros and cons. We'll see!
7885 Filter the archive through @code{bzip2}. Otherwise like @option{--gzip}.
7892 Filter the archive through @command{compress}. Otherwise like @option{--gzip}.
7894 The @acronym{GNU} Project recommends you not use
7895 @command{compress}, because there is a patent covering the algorithm it
7896 uses. You could be sued for patent infringement merely by running
7899 @opindex use-compress-program
7900 @item --use-compress-program=@var{prog}
7901 Use external compression program @var{prog}. Use this option if you
7902 have a compression program that @GNUTAR{} does not support. There
7903 are two requirements to which @var{prog} should comply:
7905 First, when called without options, it should read data from standard
7906 input, compress it and output it on standard output.
7908 Secondly, if called with @option{-d} argument, it should do exactly
7909 the opposite, i.e., read the compressed data from the standard input
7910 and produce uncompressed data on the standard output.
7913 @cindex gpg, using with tar
7914 @cindex gnupg, using with tar
7915 @cindex Using encrypted archives
7916 The @option{--use-compress-program} option, in particular, lets you
7917 implement your own filters, not necessarily dealing with
7918 compression/decomression. For example, suppose you wish to implement
7919 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
7920 gpg, gpg ---- encryption and signing tool, gpg}). The following
7927 -d) gpg --decrypt - | gzip -d -c;;
7928 '') gzip -c | gpg -s ;;
7929 *) echo "Unknown option $1">&2; exit 1;;
7934 Suppose you name it @file{gpgz} and save it somewhere in your
7935 @env{PATH}. Then the following command will create a commpressed
7936 archive signed with your private key:
7939 $ @kbd{tar -cf foo.tar.gpgz --use-compress=gpgz .}
7943 Likewise, the following command will list its contents:
7946 $ @kbd{tar -tf foo.tar.gpgz --use-compress=gpgz .}
7950 The above is based on the following discussion:
7952 I have one question, or maybe it's a suggestion if there isn't a way
7953 to do it now. I would like to use @option{--gzip}, but I'd also like
7954 the output to be fed through a program like @acronym{GNU}
7955 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
7956 to use :-)), basically adding ECC protection on top of compression.
7957 It seems as if this should be quite easy to do, but I can't work out
7958 exactly how to go about it. Of course, I can pipe the standard output
7959 of @command{tar} through @command{ecc}, but then I lose (though I
7960 haven't started using it yet, I confess) the ability to have
7961 @command{tar} use @command{rmt} for it's I/O (I think).
7963 I think the most straightforward thing would be to let me specify a
7964 general set of filters outboard of compression (preferably ordered,
7965 so the order can be automatically reversed on input operations, and
7966 with the options they require specifiable), but beggars shouldn't be
7967 choosers and anything you decide on would be fine with me.
7969 By the way, I like @command{ecc} but if (as the comments say) it can't
7970 deal with loss of block sync, I'm tempted to throw some time at adding
7971 that capability. Supposing I were to actually do such a thing and
7972 get it (apparently) working, do you accept contributed changes to
7973 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
7975 Isn't that exactly the role of the
7976 @option{--use-compress-prog=@var{program}} option?
7977 I never tried it myself, but I suspect you may want to write a
7978 @var{prog} script or program able to filter stdin to stdout to
7979 way you want. It should recognize the @option{-d} option, for when
7980 extraction is needed rather than creation.
7982 It has been reported that if one writes compressed data (through the
7983 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
7984 the DLT compression mode, the data will actually get bigger and one will
7985 end up with less space on the tape.
7989 @subsection Archiving Sparse Files
7990 @cindex Sparse Files
7997 Handle sparse files efficiently.
8000 This option causes all files to be put in the archive to be tested for
8001 sparseness, and handled specially if they are. The @option{--sparse}
8002 (@option{-S}) option is useful when many @code{dbm} files, for example, are being
8003 backed up. Using this option dramatically decreases the amount of
8004 space needed to store such a file.
8006 In later versions, this option may be removed, and the testing and
8007 treatment of sparse files may be done automatically with any special
8008 @acronym{GNU} options. For now, it is an option needing to be specified on
8009 the command line with the creation or updating of an archive.
8011 Files in the file system occasionally have ``holes.'' A hole in a file
8012 is a section of the file's contents which was never written. The
8013 contents of a hole read as all zeros. On many operating systems,
8014 actual disk storage is not allocated for holes, but they are counted
8015 in the length of the file. If you archive such a file, @command{tar}
8016 could create an archive longer than the original. To have @command{tar}
8017 attempt to recognize the holes in a file, use @option{--sparse} (@option{-S}). When
8018 you use this option, then, for any file using less disk space than
8019 would be expected from its length, @command{tar} searches the file for
8020 consecutive stretches of zeros. It then records in the archive for
8021 the file where the consecutive stretches of zeros are, and only
8022 archives the ``real contents'' of the file. On extraction (using
8023 @option{--sparse} is not needed on extraction) any such
8024 files have holes created wherever the continuous stretches of zeros
8025 were found. Thus, if you use @option{--sparse}, @command{tar} archives
8026 won't take more space than the original.
8028 A file is sparse if it contains blocks of zeros whose existence is
8029 recorded, but that have no space allocated on disk. When you specify
8030 the @option{--sparse} option in conjunction with the @option{--create}
8031 (@option{-c}) operation, @command{tar} tests all files for sparseness
8032 while archiving. If @command{tar} finds a file to be sparse, it uses a
8033 sparse representation of the file in the archive. @xref{create}, for
8034 more information about creating archives.
8036 @option{--sparse} is useful when archiving files, such as dbm files,
8037 likely to contain many nulls. This option dramatically
8038 decreases the amount of space needed to store such an archive.
8041 @strong{Please Note:} Always use @option{--sparse} when performing file
8042 system backups, to avoid archiving the expanded forms of files stored
8043 sparsely in the system.
8045 Even if your system has no sparse files currently, some may be
8046 created in the future. If you use @option{--sparse} while making file
8047 system backups as a matter of course, you can be assured the archive
8048 will never take more space on the media than the files take on disk
8049 (otherwise, archiving a disk filled with sparse files might take
8050 hundreds of tapes). @xref{Incremental Dumps}.
8053 @command{tar} ignores the @option{--sparse} option when reading an archive.
8058 Files stored sparsely in the file system are represented sparsely in
8059 the archive. Use in conjunction with write operations.
8062 However, users should be well aware that at archive creation time,
8063 @GNUTAR{} still has to read whole disk file to
8064 locate the @dfn{holes}, and so, even if sparse files use little space
8065 on disk and in the archive, they may sometimes require inordinate
8066 amount of time for reading and examining all-zero blocks of a file.
8067 Although it works, it's painfully slow for a large (sparse) file, even
8068 though the resulting tar archive may be small. (One user reports that
8069 dumping a @file{core} file of over 400 megabytes, but with only about
8070 3 megabytes of actual data, took about 9 minutes on a Sun Sparcstation
8071 ELC, with full CPU utilization.)
8073 This reading is required in all cases and is not related to the fact
8074 the @option{--sparse} option is used or not, so by merely @emph{not}
8075 using the option, you are not saving time@footnote{Well! We should say
8076 the whole truth, here. When @option{--sparse} is selected while creating
8077 an archive, the current @command{tar} algorithm requires sparse files to be
8078 read twice, not once. We hope to develop a new archive format for saving
8079 sparse files in which one pass will be sufficient.}.
8081 Programs like @command{dump} do not have to read the entire file; by
8082 examining the file system directly, they can determine in advance
8083 exactly where the holes are and thus avoid reading through them. The
8084 only data it need read are the actual allocated data blocks.
8085 @GNUTAR{} uses a more portable and straightforward
8086 archiving approach, it would be fairly difficult that it does
8087 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
8091 What I did say is that you cannot tell the difference between a hole and an
8092 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
8093 best tells you how many holes there are; it doesn't tell you @emph{where}.
8094 Just as programs may, conceivably, care what @code{st_blocks} is (care
8095 to name one that does?), they may also care where the holes are (I have
8096 no examples of this one either, but it's equally imaginable).
8098 I conclude from this that good archivers are not portable. One can
8099 arguably conclude that if you want a portable program, you can in good
8100 conscience restore files with as many holes as possible, since you can't
8105 @section Handling File Attributes
8108 When @command{tar} reads files, it updates their access times. To
8109 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
8110 reset the access time retroactively or avoid changing it in the first
8113 Handling of file attributes
8116 @opindex atime-preserve
8117 @item --atime-preserve
8118 @itemx --atime-preserve=replace
8119 @itemx --atime-preserve=system
8120 Preserve the access times of files that are read. This works only for
8121 files that you own, unless you have superuser privileges.
8123 @option{--atime-preserve=replace} works on most systems, but it also
8124 restores the data modification time and updates the status change
8125 time. Hence it doesn't interact with incremental dumps nicely
8126 (@pxref{Backups}), and it can set access or data modification times
8127 incorrectly if other programs access the file while @command{tar} is
8130 @option{--atime-preserve=system} avoids changing the access time in
8131 the first place, if the operating system supports this.
8132 Unfortunately, this may or may not work on any given operating system
8133 or file system. If @command{tar} knows for sure it won't work, it
8134 complains right away.
8136 Currently @option{--atime-preserve} with no operand defaults to
8137 @option{--atime-preserve=replace}, but this is intended to change to
8138 @option{--atime-preserve=system} when the latter is better-supported.
8143 Do not extract data modification time.
8145 When this option is used, @command{tar} leaves the data modification times
8146 of the files it extracts as the times when the files were extracted,
8147 instead of setting it to the times recorded in the archive.
8149 This option is meaningless with @option{--list} (@option{-t}).
8153 Create extracted files with the same ownership they have in the
8156 This is the default behavior for the superuser,
8157 so this option is meaningful only for non-root users, when @command{tar}
8158 is executed on those systems able to give files away. This is
8159 considered as a security flaw by many people, at least because it
8160 makes quite difficult to correctly account users for the disk space
8161 they occupy. Also, the @code{suid} or @code{sgid} attributes of
8162 files are easily and silently lost when files are given away.
8164 When writing an archive, @command{tar} writes the user id and user name
8165 separately. If it can't find a user name (because the user id is not
8166 in @file{/etc/passwd}), then it does not write one. When restoring,
8167 it tries to look the name (if one was written) up in
8168 @file{/etc/passwd}. If it fails, then it uses the user id stored in
8169 the archive instead.
8171 @opindex no-same-owner
8172 @item --no-same-owner
8174 Do not attempt to restore ownership when extracting. This is the
8175 default behavior for ordinary users, so this option has an effect
8176 only for the superuser.
8178 @opindex numeric-owner
8179 @item --numeric-owner
8180 The @option{--numeric-owner} option allows (ANSI) archives to be written
8181 without user/group name information or such information to be ignored
8182 when extracting. It effectively disables the generation and/or use
8183 of user/group name information. This option forces extraction using
8184 the numeric ids from the archive, ignoring the names.
8186 This is useful in certain circumstances, when restoring a backup from
8187 an emergency floppy with different passwd/group files for example.
8188 It is otherwise impossible to extract files with the right ownerships
8189 if the password file in use during the extraction does not match the
8190 one belonging to the file system(s) being extracted. This occurs,
8191 for example, if you are restoring your files after a major crash and
8192 had booted from an emergency floppy with no password file or put your
8193 disk into another machine to do the restore.
8195 The numeric ids are @emph{always} saved into @command{tar} archives.
8196 The identifying names are added at create time when provided by the
8197 system, unless @option{--old-archive} (@option{-o}) is used. Numeric ids could be
8198 used when moving archives between a collection of machines using
8199 a centralized management for attribution of numeric ids to users
8200 and groups. This is often made through using the NIS capabilities.
8202 When making a @command{tar} file for distribution to other sites, it
8203 is sometimes cleaner to use a single owner for all files in the
8204 distribution, and nicer to specify the write permission bits of the
8205 files as stored in the archive independently of their actual value on
8206 the file system. The way to prepare a clean distribution is usually
8207 to have some Makefile rule creating a directory, copying all needed
8208 files in that directory, then setting ownership and permissions as
8209 wanted (there are a lot of possible schemes), and only then making a
8210 @command{tar} archive out of this directory, before cleaning
8211 everything out. Of course, we could add a lot of options to
8212 @GNUTAR{} for fine tuning permissions and ownership.
8213 This is not the good way, I think. @GNUTAR{} is
8214 already crowded with options and moreover, the approach just explained
8215 gives you a great deal of control already.
8217 @opindex same-permissions, short description
8218 @opindex preserve-permissions, short description
8220 @itemx --same-permissions
8221 @itemx --preserve-permissions
8222 Extract all protection information.
8224 This option causes @command{tar} to set the modes (access permissions) of
8225 extracted files exactly as recorded in the archive. If this option
8226 is not used, the current @code{umask} setting limits the permissions
8227 on extracted files. This option is by default enabled when
8228 @command{tar} is executed by a superuser.
8231 This option is meaningless with @option{--list} (@option{-t}).
8235 Same as both @option{--same-permissions} and @option{--same-order}.
8237 The @option{--preserve} option has no equivalent short option name.
8238 It is equivalent to @option{--same-permissions} plus @option{--same-order}.
8240 @FIXME{I do not see the purpose of such an option. (Neither I. FP.)
8241 Neither do I. --Sergey}
8246 @section Basic Tar Format
8249 While an archive may contain many files, the archive itself is a
8250 single ordinary file. Like any other file, an archive file can be
8251 written to a storage device such as a tape or disk, sent through a
8252 pipe or over a network, saved on the active file system, or even
8253 stored in another archive. An archive file is not easy to read or
8254 manipulate without using the @command{tar} utility or Tar mode in
8255 @acronym{GNU} Emacs.
8257 Physically, an archive consists of a series of file entries terminated
8258 by an end-of-archive entry, which consists of two 512 blocks of zero
8260 entry usually describes one of the files in the archive (an
8261 @dfn{archive member}), and consists of a file header and the contents
8262 of the file. File headers contain file names and statistics, checksum
8263 information which @command{tar} uses to detect file corruption, and
8264 information about file types.
8266 Archives are permitted to have more than one member with the same
8267 member name. One way this situation can occur is if more than one
8268 version of a file has been stored in the archive. For information
8269 about adding new versions of a file to an archive, see @ref{update}.
8270 @FIXME-xref{To learn more about having more than one archive member with the
8271 same name, see -backup node, when it's written.}
8273 In addition to entries describing archive members, an archive may
8274 contain entries which @command{tar} itself uses to store information.
8275 @xref{label}, for an example of such an archive entry.
8277 A @command{tar} archive file contains a series of blocks. Each block
8278 contains @code{BLOCKSIZE} bytes. Although this format may be thought
8279 of as being on magnetic tape, other media are often used.
8281 Each file archived is represented by a header block which describes
8282 the file, followed by zero or more blocks which give the contents
8283 of the file. At the end of the archive file there are two 512-byte blocks
8284 filled with binary zeros as an end-of-file marker. A reasonable system
8285 should write such end-of-file marker at the end of an archive, but
8286 must not assume that such a block exists when reading an archive. In
8287 particular @GNUTAR{} always issues a warning if it does not encounter it.
8289 The blocks may be @dfn{blocked} for physical I/O operations.
8290 Each record of @var{n} blocks (where @var{n} is set by the
8291 @option{--blocking-factor=@var{512-size}} (@option{-b @var{512-size}}) option to @command{tar}) is written with a single
8292 @w{@samp{write ()}} operation. On magnetic tapes, the result of
8293 such a write is a single record. When writing an archive,
8294 the last record of blocks should be written at the full size, with
8295 blocks after the zero block containing all zeros. When reading
8296 an archive, a reasonable system should properly handle an archive
8297 whose last record is shorter than the rest, or which contains garbage
8298 records after a zero block.
8300 The header block is defined in C as follows. In the @GNUTAR{}
8301 distribution, this is part of file @file{src/tar.h}:
8304 @include header.texi
8307 All characters in header blocks are represented by using 8-bit
8308 characters in the local variant of ASCII. Each field within the
8309 structure is contiguous; that is, there is no padding used within
8310 the structure. Each character on the archive medium is stored
8313 Bytes representing the contents of files (after the header block
8314 of each file) are not translated in any way and are not constrained
8315 to represent characters in any character set. The @command{tar} format
8316 does not distinguish text files from binary files, and no translation
8317 of file contents is performed.
8319 The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and
8320 @code{gname} are null-terminated character strings. All other fields
8321 are zero-filled octal numbers in ASCII. Each numeric field of width
8322 @var{w} contains @var{w} minus 1 digits, and a null.
8324 The @code{name} field is the file name of the file, with directory names
8325 (if any) preceding the file name, separated by slashes.
8327 @FIXME{how big a name before field overflows?}
8329 The @code{mode} field provides nine bits specifying file permissions
8330 and three bits to specify the Set UID, Set GID, and Save Text
8331 (@dfn{sticky}) modes. Values for these bits are defined above.
8332 When special permissions are required to create a file with a given
8333 mode, and the user restoring files from the archive does not hold such
8334 permissions, the mode bit(s) specifying those special permissions
8335 are ignored. Modes which are not supported by the operating system
8336 restoring files from the archive will be ignored. Unsupported modes
8337 should be faked up when creating or updating an archive; e.g., the
8338 group permission could be copied from the @emph{other} permission.
8340 The @code{uid} and @code{gid} fields are the numeric user and group
8341 ID of the file owners, respectively. If the operating system does
8342 not support numeric user or group IDs, these fields should be ignored.
8344 The @code{size} field is the size of the file in bytes; linked files
8345 are archived with this field specified as zero. @FIXME-xref{Modifiers, in
8346 particular the @option{--incremental} (@option{-G}) option.}
8348 The @code{mtime} field is the data modification time of the file at
8349 the time it was archived. It is the ASCII representation of the octal
8350 value of the last time the file's contents were modified, represented
8351 as an integer number of
8352 seconds since January 1, 1970, 00:00 Coordinated Universal Time.
8354 The @code{chksum} field is the ASCII representation of the octal value
8355 of the simple sum of all bytes in the header block. Each 8-bit
8356 byte in the header is added to an unsigned integer, initialized to
8357 zero, the precision of which shall be no less than seventeen bits.
8358 When calculating the checksum, the @code{chksum} field is treated as
8359 if it were all blanks.
8361 The @code{typeflag} field specifies the type of file archived. If a
8362 particular implementation does not recognize or permit the specified
8363 type, the file will be extracted as if it were a regular file. As this
8364 action occurs, @command{tar} issues a warning to the standard error.
8366 The @code{atime} and @code{ctime} fields are used in making incremental
8367 backups; they store, respectively, the particular file's access and
8368 status change times.
8370 The @code{offset} is used by the @option{--multi-volume} (@option{-M}) option, when
8371 making a multi-volume archive. The offset is number of bytes into
8372 the file that we need to restart at to continue the file on the next
8373 tape, i.e., where we store the location that a continued file is
8376 The following fields were added to deal with sparse files. A file
8377 is @dfn{sparse} if it takes in unallocated blocks which end up being
8378 represented as zeros, i.e., no useful data. A test to see if a file
8379 is sparse is to look at the number blocks allocated for it versus the
8380 number of characters in the file; if there are fewer blocks allocated
8381 for the file than would normally be allocated for a file of that
8382 size, then the file is sparse. This is the method @command{tar} uses to
8383 detect a sparse file, and once such a file is detected, it is treated
8384 differently from non-sparse files.
8386 Sparse files are often @code{dbm} files, or other database-type files
8387 which have data at some points and emptiness in the greater part of
8388 the file. Such files can appear to be very large when an @samp{ls
8389 -l} is done on them, when in truth, there may be a very small amount
8390 of important data contained in the file. It is thus undesirable
8391 to have @command{tar} think that it must back up this entire file, as
8392 great quantities of room are wasted on empty blocks, which can lead
8393 to running out of room on a tape far earlier than is necessary.
8394 Thus, sparse files are dealt with so that these empty blocks are
8395 not written to the tape. Instead, what is written to the tape is a
8396 description, of sorts, of the sparse file: where the holes are, how
8397 big the holes are, and how much data is found at the end of the hole.
8398 This way, the file takes up potentially far less room on the tape,
8399 and when the file is extracted later on, it will look exactly the way
8400 it looked beforehand. The following is a description of the fields
8401 used to handle a sparse file:
8403 The @code{sp} is an array of @code{struct sparse}. Each @code{struct
8404 sparse} contains two 12-character strings which represent an offset
8405 into the file and a number of bytes to be written at that offset.
8406 The offset is absolute, and not relative to the offset in preceding
8409 The header can hold four of these @code{struct sparse} at the moment;
8410 if more are needed, they are not stored in the header.
8412 The @code{isextended} flag is set when an @code{extended_header}
8413 is needed to deal with a file. Note that this means that this flag
8414 can only be set when dealing with a sparse file, and it is only set
8415 in the event that the description of the file will not fit in the
8416 allotted room for sparse structures in the header. In other words,
8417 an extended_header is needed.
8419 The @code{extended_header} structure is used for sparse files which
8420 need more sparse structures than can fit in the header. The header can
8421 fit 4 such structures; if more are needed, the flag @code{isextended}
8422 gets set and the next block is an @code{extended_header}.
8424 Each @code{extended_header} structure contains an array of 21
8425 sparse structures, along with a similar @code{isextended} flag
8426 that the header had. There can be an indeterminate number of such
8427 @code{extended_header}s to describe a sparse file.
8431 @item @code{REGTYPE}
8432 @itemx @code{AREGTYPE}
8433 These flags represent a regular file. In order to be compatible
8434 with older versions of @command{tar}, a @code{typeflag} value of
8435 @code{AREGTYPE} should be silently recognized as a regular file.
8436 New archives should be created using @code{REGTYPE}. Also, for
8437 backward compatibility, @command{tar} treats a regular file whose name
8438 ends with a slash as a directory.
8440 @item @code{LNKTYPE}
8441 This flag represents a file linked to another file, of any type,
8442 previously archived. Such files are identified in Unix by each
8443 file having the same device and inode number. The linked-to name is
8444 specified in the @code{linkname} field with a trailing null.
8446 @item @code{SYMTYPE}
8447 This represents a symbolic link to another file. The linked-to name
8448 is specified in the @code{linkname} field with a trailing null.
8450 @item @code{CHRTYPE}
8451 @itemx @code{BLKTYPE}
8452 These represent character special files and block special files
8453 respectively. In this case the @code{devmajor} and @code{devminor}
8454 fields will contain the major and minor device numbers respectively.
8455 Operating systems may map the device specifications to their own
8456 local specification, or may ignore the entry.
8458 @item @code{DIRTYPE}
8459 This flag specifies a directory or sub-directory. The directory
8460 name in the @code{name} field should end with a slash. On systems where
8461 disk allocation is performed on a directory basis, the @code{size} field
8462 will contain the maximum number of bytes (which may be rounded to
8463 the nearest disk block allocation unit) which the directory may
8464 hold. A @code{size} field of zero indicates no such limiting. Systems
8465 which do not support limiting in this manner should ignore the
8468 @item @code{FIFOTYPE}
8469 This specifies a FIFO special file. Note that the archiving of a
8470 FIFO file archives the existence of this file and not its contents.
8472 @item @code{CONTTYPE}
8473 This specifies a contiguous file, which is the same as a normal
8474 file except that, in operating systems which support it, all its
8475 space is allocated contiguously on the disk. Operating systems
8476 which do not allow contiguous allocation should silently treat this
8477 type as a normal file.
8479 @item @code{A} @dots{} @code{Z}
8480 These are reserved for custom implementations. Some of these are
8481 used in the @acronym{GNU} modified format, as described below.
8485 Other values are reserved for specification in future revisions of
8486 the P1003 standard, and should not be used by any @command{tar} program.
8488 The @code{magic} field indicates that this archive was output in
8489 the P1003 archive format. If this field contains @code{TMAGIC},
8490 the @code{uname} and @code{gname} fields will contain the ASCII
8491 representation of the owner and group of the file respectively.
8492 If found, the user and group IDs are used rather than the values in
8493 the @code{uid} and @code{gid} fields.
8495 For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages
8496 169-173 (section 10.1) for @cite{Archive/Interchange File Format}; and
8497 IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940
8498 (section E.4.48) for @cite{pax - Portable archive interchange}.
8501 @section @acronym{GNU} Extensions to the Archive Format
8504 The @acronym{GNU} format uses additional file types to describe new types of
8505 files in an archive. These are listed below.
8508 @item GNUTYPE_DUMPDIR
8510 This represents a directory and a list of files created by the
8511 @option{--incremental} (@option{-G}) option. The @code{size} field gives the total
8512 size of the associated list of files. Each file name is preceded by
8513 either a @samp{Y} (the file should be in this archive) or an @samp{N}.
8514 (The file is a directory, or is not stored in the archive.) Each file
8515 name is terminated by a null. There is an additional null after the
8518 @item GNUTYPE_MULTIVOL
8520 This represents a file continued from another volume of a multi-volume
8521 archive created with the @option{--multi-volume} (@option{-M}) option. The original
8522 type of the file is not given here. The @code{size} field gives the
8523 maximum size of this piece of the file (assuming the volume does
8524 not end before the file is written out). The @code{offset} field
8525 gives the offset from the beginning of the file where this part of
8526 the file begins. Thus @code{size} plus @code{offset} should equal
8527 the original size of the file.
8529 @item GNUTYPE_SPARSE
8531 This flag indicates that we are dealing with a sparse file. Note
8532 that archiving a sparse file requires special operations to find
8533 holes in the file, which mark the positions of these holes, along
8534 with the number of bytes of data to be found after the hole.
8536 @item GNUTYPE_VOLHDR
8538 This file type is used to mark the volume header that was given with
8539 the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option when the archive was created. The @code{name}
8540 field contains the @code{name} given after the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option.
8541 The @code{size} field is zero. Only the first file in each volume
8542 of an archive should have this type.
8546 You may have trouble reading a @acronym{GNU} format archive on a
8547 non-@acronym{GNU} system if the options @option{--incremental} (@option{-G}),
8548 @option{--multi-volume} (@option{-M}), @option{--sparse} (@option{-S}), or @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) were
8549 used when writing the archive. In general, if @command{tar} does not
8550 use the @acronym{GNU}-added fields of the header, other versions of
8551 @command{tar} should be able to read the archive. Otherwise, the
8552 @command{tar} program will give an error, the most likely one being a
8556 @section Comparison of @command{tar} and @command{cpio}
8559 @FIXME{Reorganize the following material}
8561 The @command{cpio} archive formats, like @command{tar}, do have maximum
8562 pathname lengths. The binary and old ASCII formats have a max path
8563 length of 256, and the new ASCII and CRC ASCII formats have a max
8564 path length of 1024. @acronym{GNU} @command{cpio} can read and write archives
8565 with arbitrary pathname lengths, but other @command{cpio} implementations
8566 may crash unexplainedly trying to read them.
8568 @command{tar} handles symbolic links in the form in which it comes in BSD;
8569 @command{cpio} doesn't handle symbolic links in the form in which it comes
8570 in System V prior to SVR4, and some vendors may have added symlinks
8571 to their system without enhancing @command{cpio} to know about them.
8572 Others may have enhanced it in a way other than the way I did it
8573 at Sun, and which was adopted by AT&T (and which is, I think, also
8574 present in the @command{cpio} that Berkeley picked up from AT&T and put
8575 into a later BSD release---I think I gave them my changes).
8577 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
8578 can handle @command{tar} format input, and write it on output, and it
8579 probably handles symbolic links. They may not have bothered doing
8580 anything to enhance @command{tar} as a result.)
8582 @command{cpio} handles special files; traditional @command{tar} doesn't.
8584 @command{tar} comes with V7, System III, System V, and BSD source;
8585 @command{cpio} comes only with System III, System V, and later BSD
8586 (4.3-tahoe and later).
8588 @command{tar}'s way of handling multiple hard links to a file can handle
8589 file systems that support 32-bit inumbers (e.g., the BSD file system);
8590 @command{cpio}s way requires you to play some games (in its "binary"
8591 format, i-numbers are only 16 bits, and in its "portable ASCII" format,
8592 they're 18 bits---it would have to play games with the "file system ID"
8593 field of the header to make sure that the file system ID/i-number pairs
8594 of different files were always different), and I don't know which
8595 @command{cpio}s, if any, play those games. Those that don't might get
8596 confused and think two files are the same file when they're not, and
8597 make hard links between them.
8599 @command{tar}s way of handling multiple hard links to a file places only
8600 one copy of the link on the tape, but the name attached to that copy
8601 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
8602 way puts one copy for every link, but you can retrieve it using any
8606 What type of check sum (if any) is used, and how is this calculated.
8609 See the attached manual pages for @command{tar} and @command{cpio} format.
8610 @command{tar} uses a checksum which is the sum of all the bytes in the
8611 @command{tar} header for a file; @command{cpio} uses no checksum.
8614 If anyone knows why @command{cpio} was made when @command{tar} was present
8618 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
8619 generally-available version of UNIX had @command{tar} at the time. I don't
8620 know whether any version that was generally available @emph{within AT&T}
8621 had @command{tar}, or, if so, whether the people within AT&T who did
8622 @command{cpio} knew about it.
8624 On restore, if there is a corruption on a tape @command{tar} will stop at
8625 that point, while @command{cpio} will skip over it and try to restore the
8628 The main difference is just in the command syntax and header format.
8630 @command{tar} is a little more tape-oriented in that everything is blocked
8631 to start on a record boundary.
8634 Is there any differences between the ability to recover crashed
8635 archives between the two of them. (Is there any chance of recovering
8636 crashed archives at all.)
8639 Theoretically it should be easier under @command{tar} since the blocking
8640 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
8641 However, modern @command{cpio}'s and variations have an option to just
8642 search for the next file header after an error with a reasonable chance
8643 of resyncing. However, lots of tape driver software won't allow you to
8644 continue past a media error which should be the only reason for getting
8645 out of sync unless a file changed sizes while you were writing the
8649 If anyone knows why @command{cpio} was made when @command{tar} was present
8650 at the unix scene, please tell me about this too.
8653 Probably because it is more media efficient (by not blocking everything
8654 and using only the space needed for the headers where @command{tar}
8655 always uses 512 bytes per file header) and it knows how to archive
8658 You might want to look at the freely available alternatives. The
8659 major ones are @command{afio}, @GNUTAR{}, and
8660 @command{pax}, each of which have their own extensions with some
8661 backwards compatibility.
8663 Sparse files were @command{tar}red as sparse files (which you can
8664 easily test, because the resulting archive gets smaller, and
8665 @acronym{GNU} @command{cpio} can no longer read it).
8668 @chapter Tapes and Other Archive Media
8671 A few special cases about tape handling warrant more detailed
8672 description. These special cases are discussed below.
8674 Many complexities surround the use of @command{tar} on tape drives. Since
8675 the creation and manipulation of archives located on magnetic tape was
8676 the original purpose of @command{tar}, it contains many features making
8677 such manipulation easier.
8679 Archives are usually written on dismountable media---tape cartridges,
8680 mag tapes, or floppy disks.
8682 The amount of data a tape or disk holds depends not only on its size,
8683 but also on how it is formatted. A 2400 foot long reel of mag tape
8684 holds 40 megabytes of data when formatted at 1600 bits per inch. The
8685 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
8687 Magnetic media are re-usable---once the archive on a tape is no longer
8688 needed, the archive can be erased and the tape or disk used over.
8689 Media quality does deteriorate with use, however. Most tapes or disks
8690 should be discarded when they begin to produce data errors. EXABYTE
8691 tape cartridges should be discarded when they generate an @dfn{error
8692 count} (number of non-usable bits) of more than 10k.
8694 Magnetic media are written and erased using magnetic fields, and
8695 should be protected from such fields to avoid damage to stored data.
8696 Sticking a floppy disk to a filing cabinet using a magnet is probably
8700 * Device:: Device selection and switching
8701 * Remote Tape Server::
8702 * Common Problems and Solutions::
8703 * Blocking:: Blocking
8704 * Many:: Many archives on one tape
8705 * Using Multiple Tapes:: Using Multiple Tapes
8706 * label:: Including a Label in the Archive
8708 * Write Protection::
8712 @section Device Selection and Switching
8716 @item -f [@var{hostname}:]@var{file}
8717 @itemx --file=[@var{hostname}:]@var{file}
8718 Use archive file or device @var{file} on @var{hostname}.
8721 This option is used to specify the file name of the archive @command{tar}
8724 If the file name is @samp{-}, @command{tar} reads the archive from standard
8725 input (when listing or extracting), or writes it to standard output
8726 (when creating). If the @samp{-} file name is given when updating an
8727 archive, @command{tar} will read the original archive from its standard
8728 input, and will write the entire new archive to its standard output.
8730 If the file name contains a @samp{:}, it is interpreted as
8731 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
8732 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
8733 either case, @command{tar} will invoke the command @command{rsh} (or
8734 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
8735 machine. If you give an alternate login name, it will be given to the
8737 Naturally, the remote machine must have an executable
8738 @command{/usr/libexec/rmt}. This program is free software from the
8739 University of California, and a copy of the source code can be found
8740 with the sources for @command{tar}; it's compiled and installed by default.
8741 The exact path to this utility is determined when configuring the package.
8742 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
8743 your installation prefix. This location may also be overridden at
8744 runtime by using @option{rmt-command=@var{command}} option (@xref{Option Summary,
8745 ---rmt-command}, for detailed description of this option. @xref{Remote
8746 Tape Server}, for the description of @command{rmt} command).
8748 If this option is not given, but the environment variable @env{TAPE}
8749 is set, its value is used; otherwise, old versions of @command{tar}
8750 used a default archive name (which was picked when @command{tar} was
8751 compiled). The default is normally set up to be the @dfn{first} tape
8752 drive or other transportable I/O medium on the system.
8754 Starting with version 1.11.5, @GNUTAR{} uses
8755 standard input and standard output as the default device, and I will
8756 not try anymore supporting automatic device detection at installation
8757 time. This was failing really in too many cases, it was hopeless.
8758 This is now completely left to the installer to override standard
8759 input and standard output for default device, if this seems
8760 preferable. Further, I think @emph{most} actual usages of
8761 @command{tar} are done with pipes or disks, not really tapes,
8762 cartridges or diskettes.
8764 Some users think that using standard input and output is running
8765 after trouble. This could lead to a nasty surprise on your screen if
8766 you forget to specify an output file name---especially if you are going
8767 through a network or terminal server capable of buffering large amounts
8768 of output. We had so many bug reports in that area of configuring
8769 default tapes automatically, and so many contradicting requests, that
8770 we finally consider the problem to be portably intractable. We could
8771 of course use something like @samp{/dev/tape} as a default, but this
8772 is @emph{also} running after various kind of trouble, going from hung
8773 processes to accidental destruction of real tapes. After having seen
8774 all this mess, using standard input and output as a default really
8775 sounds like the only clean choice left, and a very useful one too.
8777 @GNUTAR{} reads and writes archive in records, I
8778 suspect this is the main reason why block devices are preferred over
8779 character devices. Most probably, block devices are more efficient
8780 too. The installer could also check for @samp{DEFTAPE} in
8781 @file{<sys/mtio.h>}.
8784 @opindex force-local, short description
8786 Archive file is local even if it contains a colon.
8788 @opindex rsh-command
8789 @item --rsh-command=@var{command}
8790 Use remote @var{command} instead of @command{rsh}. This option exists
8791 so that people who use something other than the standard @command{rsh}
8792 (e.g., a Kerberized @command{rsh}) can access a remote device.
8794 When this command is not used, the shell command found when
8795 the @command{tar} program was installed is used instead. This is
8796 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
8797 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
8798 The installer may have overridden this by defining the environment
8799 variable @env{RSH} @emph{at installation time}.
8802 Specify drive and density.
8804 @opindex multi-volume, short description
8806 @itemx --multi-volume
8807 Create/list/extract multi-volume archive.
8809 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
8810 that may be larger than will fit on the medium used to hold it.
8811 @xref{Multi-Volume Archives}.
8813 @opindex tape-length, short description
8815 @itemx --tape-length=@var{num}
8816 Change tape after writing @var{num} x 1024 bytes.
8818 This option might be useful when your tape drivers do not properly
8819 detect end of physical tapes. By being slightly conservative on the
8820 maximum tape length, you might avoid the problem entirely.
8822 @opindex info-script, short description
8823 @opindex new-volume-script, short description
8825 @itemx --info-script=@var{file}
8826 @itemx --new-volume-script=@var{file}
8827 Execute @file{file} at end of each tape. This implies
8828 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
8829 description of this option.
8832 @node Remote Tape Server
8833 @section The Remote Tape Server
8835 @cindex remote tape drive
8837 In order to access the tape drive on a remote machine, @command{tar}
8838 uses the remote tape server written at the University of California at
8839 Berkeley. The remote tape server must be installed as
8840 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
8841 want to use. @command{tar} calls @command{rmt} by running an
8842 @command{rsh} or @command{remsh} to the remote machine, optionally
8843 using a different login name if one is supplied.
8845 A copy of the source for the remote tape server is provided. It is
8846 Copyright @copyright{} 1983 by the Regents of the University of
8847 California, but can be freely distributed. It is compiled and
8848 installed by default.
8850 @cindex absolute file names
8851 Unless you use the @option{--absolute-names} (@option{-P}) option,
8852 @GNUTAR{} will not allow you to create an archive that contains
8853 absolute file names (a file name beginning with @samp{/}.) If you try,
8854 @command{tar} will automatically remove the leading @samp{/} from the
8855 file names it stores in the archive. It will also type a warning
8856 message telling you what it is doing.
8858 When reading an archive that was created with a different
8859 @command{tar} program, @GNUTAR{} automatically
8860 extracts entries in the archive which have absolute file names as if
8861 the file names were not absolute. This is an important feature. A
8862 visitor here once gave a @command{tar} tape to an operator to restore;
8863 the operator used Sun @command{tar} instead of @GNUTAR{},
8864 and the result was that it replaced large portions of
8865 our @file{/bin} and friends with versions from the tape; needless to
8866 say, we were unhappy about having to recover the file system from
8869 For example, if the archive contained a file @file{/usr/bin/computoy},
8870 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
8871 relative to the current directory. If you want to extract the files in
8872 an archive to the same absolute names that they had when the archive
8873 was created, you should do a @samp{cd /} before extracting the files
8874 from the archive, or you should either use the @option{--absolute-names}
8875 option, or use the command @samp{tar -C / @dots{}}.
8877 @cindex Ultrix 3.1 and write failure
8878 Some versions of Unix (Ultrix 3.1 is known to have this problem),
8879 can claim that a short write near the end of a tape succeeded,
8880 when it actually failed. This will result in the -M option not
8881 working correctly. The best workaround at the moment is to use a
8882 significantly larger blocking factor than the default 20.
8884 In order to update an archive, @command{tar} must be able to backspace the
8885 archive in order to reread or rewrite a record that was just read (or
8886 written). This is currently possible only on two kinds of files: normal
8887 disk files (or any other file that can be backspaced with @samp{lseek}),
8888 and industry-standard 9-track magnetic tape (or any other kind of tape
8889 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}.
8891 This means that the @option{--append}, @option{--concatenate}, and
8892 @option{--delete} commands will not work on any other kind of file.
8893 Some media simply cannot be backspaced, which means these commands and
8894 options will never be able to work on them. These non-backspacing
8895 media include pipes and cartridge tape drives.
8897 Some other media can be backspaced, and @command{tar} will work on them
8898 once @command{tar} is modified to do so.
8900 Archives created with the @option{--multi-volume}, @option{--label}, and
8901 @option{--incremental} (@option{-G}) options may not be readable by other version
8902 of @command{tar}. In particular, restoring a file that was split over
8903 a volume boundary will require some careful work with @command{dd}, if
8904 it can be done at all. Other versions of @command{tar} may also create
8905 an empty file whose name is that of the volume header. Some versions
8906 of @command{tar} may create normal files instead of directories archived
8907 with the @option{--incremental} (@option{-G}) option.
8909 @node Common Problems and Solutions
8910 @section Some Common Problems and their Solutions
8917 no such file or directory
8920 errors from @command{tar}:
8921 directory checksum error
8924 errors from media/system:
8935 @dfn{Block} and @dfn{record} terminology is rather confused, and it
8936 is also confusing to the expert reader. On the other hand, readers
8937 who are new to the field have a fresh mind, and they may safely skip
8938 the next two paragraphs, as the remainder of this manual uses those
8939 two terms in a quite consistent way.
8941 John Gilmore, the writer of the public domain @command{tar} from which
8942 @GNUTAR{} was originally derived, wrote (June 1995):
8945 The nomenclature of tape drives comes from IBM, where I believe
8946 they were invented for the IBM 650 or so. On IBM mainframes, what
8947 is recorded on tape are tape blocks. The logical organization of
8948 data is into records. There are various ways of putting records into
8949 blocks, including @code{F} (fixed sized records), @code{V} (variable
8950 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
8951 to a block), @code{VB} (variable size records, @var{n} to a block),
8952 @code{VSB} (variable spanned blocked: variable sized records that can
8953 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
8954 parameter specified this to the operating system.
8956 The Unix man page on @command{tar} was totally confused about this.
8957 When I wrote @code{PD TAR}, I used the historically correct terminology
8958 (@command{tar} writes data records, which are grouped into blocks).
8959 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
8960 here), and now Fran@,{c}ois has migrated that terminology back
8961 into the source code too.
8964 The term @dfn{physical block} means the basic transfer chunk from or
8965 to a device, after which reading or writing may stop without anything
8966 being lost. In this manual, the term @dfn{block} usually refers to
8967 a disk physical block, @emph{assuming} that each disk block is 512
8968 bytes in length. It is true that some disk devices have different
8969 physical blocks, but @command{tar} ignore these differences in its own
8970 format, which is meant to be portable, so a @command{tar} block is always
8971 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
8972 The term @dfn{logical block} often represents the basic chunk of
8973 allocation of many disk blocks as a single entity, which the operating
8974 system treats somewhat atomically; this concept is only barely used
8977 The term @dfn{physical record} is another way to speak of a physical
8978 block, those two terms are somewhat interchangeable. In this manual,
8979 the term @dfn{record} usually refers to a tape physical block,
8980 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
8981 It is true that archives may be put on disk or used with pipes,
8982 but nevertheless, @command{tar} tries to read and write the archive one
8983 @dfn{record} at a time, whatever the medium in use. One record is made
8984 up of an integral number of blocks, and this operation of putting many
8985 disk blocks into a single tape block is called @dfn{reblocking}, or
8986 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
8987 the logical organization of many characters into something meaningful
8988 to the application. The term @dfn{unit record} describes a small set
8989 of characters which are transmitted whole to or by the application,
8990 and often refers to a line of text. Those two last terms are unrelated
8991 to what we call a @dfn{record} in @GNUTAR{}.
8993 When writing to tapes, @command{tar} writes the contents of the archive
8994 in chunks known as @dfn{records}. To change the default blocking
8995 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
8996 @var{512-size}}) option. Each record will then be composed of
8997 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
8998 @xref{Standard}.) Each file written to the archive uses at least one
8999 full record. As a result, using a larger record size can result in
9000 more wasted space for small files. On the other hand, a larger record
9001 size can often be read and written much more efficiently.
9003 Further complicating the problem is that some tape drives ignore the
9004 blocking entirely. For these, a larger record size can still improve
9005 performance (because the software layers above the tape drive still
9006 honor the blocking), but not as dramatically as on tape drives that
9009 When reading an archive, @command{tar} can usually figure out the
9010 record size on itself. When this is the case, and a non-standard
9011 record size was used when the archive was created, @command{tar} will
9012 print a message about a non-standard blocking factor, and then operate
9013 normally. On some tape devices, however, @command{tar} cannot figure
9014 out the record size itself. On most of those, you can specify a
9015 blocking factor (with @option{--blocking-factor}) larger than the
9016 actual blocking factor, and then use the @option{--read-full-records}
9017 (@option{-B}) option. (If you specify a blocking factor with
9018 @option{--blocking-factor} and don't use the
9019 @option{--read-full-records} option, then @command{tar} will not
9020 attempt to figure out the recording size itself.) On some devices,
9021 you must always specify the record size exactly with
9022 @option{--blocking-factor} when reading, because @command{tar} cannot
9023 figure it out. In any case, use @option{--list} (@option{-t}) before
9024 doing any extractions to see whether @command{tar} is reading the archive
9027 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
9028 putting them into records is to put a whole number of them (one or
9029 more) into each record. @command{tar} records are all the same size;
9030 at the end of the file there's a block containing all zeros, which
9031 is how you tell that the remainder of the last record(s) are garbage.
9033 In a standard @command{tar} file (no options), the block size is 512
9034 and the record size is 10240, for a blocking factor of 20. What the
9035 @option{--blocking-factor} option does is sets the blocking factor,
9036 changing the record size while leaving the block size at 512 bytes.
9037 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
9038 most tape drives these days prefer much bigger records in order to
9039 stream and not waste tape. When writing tapes for myself, some tend
9040 to use a factor of the order of 2048, say, giving a record size of
9041 around one megabyte.
9043 If you use a blocking factor larger than 20, older @command{tar}
9044 programs might not be able to read the archive, so we recommend this
9045 as a limit to use in practice. @GNUTAR{}, however,
9046 will support arbitrarily large record sizes, limited only by the
9047 amount of virtual memory or the physical characteristics of the tape
9051 * Format Variations:: Format Variations
9052 * Blocking Factor:: The Blocking Factor of an Archive
9055 @node Format Variations
9056 @subsection Format Variations
9057 @cindex Format Parameters
9058 @cindex Format Options
9059 @cindex Options, archive format specifying
9060 @cindex Options, format specifying
9063 Format parameters specify how an archive is written on the archive
9064 media. The best choice of format parameters will vary depending on
9065 the type and number of files being archived, and on the media used to
9068 To specify format parameters when accessing or creating an archive,
9069 you can use the options described in the following sections.
9070 If you do not specify any format parameters, @command{tar} uses
9071 default parameters. You cannot modify a compressed archive.
9072 If you create an archive with the @option{--blocking-factor} option
9073 specified (@pxref{Blocking Factor}), you must specify that
9074 blocking-factor when operating on the archive. @xref{Formats}, for other
9075 examples of format parameter considerations.
9077 @node Blocking Factor
9078 @subsection The Blocking Factor of an Archive
9079 @cindex Blocking Factor
9081 @cindex Number of blocks per record
9082 @cindex Number of bytes per record
9083 @cindex Bytes per record
9084 @cindex Blocks per record
9087 @opindex blocking-factor
9088 The data in an archive is grouped into blocks, which are 512 bytes.
9089 Blocks are read and written in whole number multiples called
9090 @dfn{records}. The number of blocks in a record (ie. the size of a
9091 record in units of 512 bytes) is called the @dfn{blocking factor}.
9092 The @option{--blocking-factor=@var{512-size}} (@option{-b
9093 @var{512-size}}) option specifies the blocking factor of an archive.
9094 The default blocking factor is typically 20 (i.e., 10240 bytes), but
9095 can be specified at installation. To find out the blocking factor of
9096 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
9097 This may not work on some devices.
9099 Records are separated by gaps, which waste space on the archive media.
9100 If you are archiving on magnetic tape, using a larger blocking factor
9101 (and therefore larger records) provides faster throughput and allows you
9102 to fit more data on a tape (because there are fewer gaps). If you are
9103 archiving on cartridge, a very large blocking factor (say 126 or more)
9104 greatly increases performance. A smaller blocking factor, on the other
9105 hand, may be useful when archiving small files, to avoid archiving lots
9106 of nulls as @command{tar} fills out the archive to the end of the record.
9107 In general, the ideal record size depends on the size of the
9108 inter-record gaps on the tape you are using, and the average size of the
9109 files you are archiving. @xref{create}, for information on
9112 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
9114 Archives with blocking factors larger than 20 cannot be read
9115 by very old versions of @command{tar}, or by some newer versions
9116 of @command{tar} running on old machines with small address spaces.
9117 With @GNUTAR{}, the blocking factor of an archive is limited
9118 only by the maximum record size of the device containing the archive,
9119 or by the amount of available virtual memory.
9121 Also, on some systems, not using adequate blocking factors, as sometimes
9122 imposed by the device drivers, may yield unexpected diagnostics. For
9123 example, this has been reported:
9126 Cannot write to /dev/dlt: Invalid argument
9130 In such cases, it sometimes happen that the @command{tar} bundled by
9131 the system is aware of block size idiosyncrasies, while @GNUTAR{}
9132 requires an explicit specification for the block size,
9133 which it cannot guess. This yields some people to consider
9134 @GNUTAR{} is misbehaving, because by comparison,
9135 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
9136 for example, might resolve the problem.
9138 If you use a non-default blocking factor when you create an archive, you
9139 must specify the same blocking factor when you modify that archive. Some
9140 archive devices will also require you to specify the blocking factor when
9141 reading that archive, however this is not typically the case. Usually, you
9142 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
9143 reports a non-default record size and then lists the archive members as
9144 it would normally. To extract files from an archive with a non-standard
9145 blocking factor (particularly if you're not sure what the blocking factor
9146 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
9147 specifying a blocking factor larger then the blocking factor of the archive
9148 (ie. @samp{tar --extract --read-full-records --blocking-factor=300}.
9149 @xref{list}, for more information on the @option{--list} (@option{-t})
9150 operation. @xref{Reading}, for a more detailed explanation of that option.
9153 @item --blocking-factor=@var{number}
9154 @itemx -b @var{number}
9155 Specifies the blocking factor of an archive. Can be used with any
9156 operation, but is usually not necessary with @option{--list} (@option{-t}).
9162 @item -b @var{blocks}
9163 @itemx --blocking-factor=@var{blocks}
9164 Set record size to @math{@var{blocks} * 512} bytes.
9166 This option is used to specify a @dfn{blocking factor} for the archive.
9167 When reading or writing the archive, @command{tar}, will do reads and writes
9168 of the archive in records of @math{@var{block}*512} bytes. This is true
9169 even when the archive is compressed. Some devices requires that all
9170 write operations be a multiple of a certain size, and so, @command{tar}
9171 pads the archive out to the next record boundary.
9173 The default blocking factor is set when @command{tar} is compiled, and is
9174 typically 20. Blocking factors larger than 20 cannot be read by very
9175 old versions of @command{tar}, or by some newer versions of @command{tar}
9176 running on old machines with small address spaces.
9178 With a magnetic tape, larger records give faster throughput and fit
9179 more data on a tape (because there are fewer inter-record gaps).
9180 If the archive is in a disk file or a pipe, you may want to specify
9181 a smaller blocking factor, since a large one will result in a large
9182 number of null bytes at the end of the archive.
9184 When writing cartridge or other streaming tapes, a much larger
9185 blocking factor (say 126 or more) will greatly increase performance.
9186 However, you must specify the same blocking factor when reading or
9187 updating the archive.
9189 Apparently, Exabyte drives have a physical block size of 8K bytes.
9190 If we choose our blocksize as a multiple of 8k bytes, then the problem
9191 seems to disappear. Id est, we are using block size of 112 right
9192 now, and we haven't had the problem since we switched@dots{}
9194 With @GNUTAR{} the blocking factor is limited only
9195 by the maximum record size of the device containing the archive, or by
9196 the amount of available virtual memory.
9198 However, deblocking or reblocking is virtually avoided in a special
9199 case which often occurs in practice, but which requires all the
9200 following conditions to be simultaneously true:
9203 the archive is subject to a compression option,
9205 the archive is not handled through standard input or output, nor
9206 redirected nor piped,
9208 the archive is directly handled to a local disk, instead of any special
9211 @option{--blocking-factor} is not explicitly specified on the @command{tar}
9215 If the output goes directly to a local disk, and not through
9216 stdout, then the last write is not extended to a full record size.
9217 Otherwise, reblocking occurs. Here are a few other remarks on this
9223 @command{gzip} will complain about trailing garbage if asked to
9224 uncompress a compressed archive on tape, there is an option to turn
9225 the message off, but it breaks the regularity of simply having to use
9226 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
9227 silently ignoring any number of trailing zeros. I'll ask Jean-loup
9228 Gailly, by sending a copy of this message to him.
9231 @command{compress} does not show this problem, but as Jean-loup pointed
9232 out to Michael, @samp{compress -d} silently adds garbage after
9233 the result of decompression, which tar ignores because it already
9234 recognized its end-of-file indicator. So this bug may be safely
9238 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
9239 but will still return an exit status of 2 which tar reports in turn.
9240 @command{tar} might ignore the exit status returned, but I hate doing
9241 that, as it weakens the protection @command{tar} offers users against
9242 other possible problems at decompression time. If @command{gzip} was
9243 silently skipping trailing zeros @emph{and} also avoiding setting the
9244 exit status in this innocuous case, that would solve this situation.
9247 @command{tar} should become more solid at not stopping to read a pipe at
9248 the first null block encountered. This inelegantly breaks the pipe.
9249 @command{tar} should rather drain the pipe out before exiting itself.
9252 @opindex ignore-zeros, short description
9254 @itemx --ignore-zeros
9255 Ignore blocks of zeros in archive (means EOF).
9257 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
9258 of zeros in the archive. Normally a block of zeros indicates the
9259 end of the archive, but when reading a damaged archive, or one which
9260 was created by concatenating several archives together, this option
9261 allows @command{tar} to read the entire archive. This option is not on
9262 by default because many versions of @command{tar} write garbage after
9265 Note that this option causes @command{tar} to read to the end of the
9266 archive file, which may sometimes avoid problems when multiple files
9267 are stored on a single physical tape.
9269 @opindex read-full-records, short description
9271 @itemx --read-full-records
9272 Reblock as we read (for reading 4.2BSD pipes).
9274 If @option{--read-full-records} is used, @command{tar}
9275 will not panic if an attempt to read a record from the archive does
9276 not return a full record. Instead, @command{tar} will keep reading
9277 until it has obtained a full
9280 This option is turned on by default when @command{tar} is reading
9281 an archive from standard input, or from a remote machine. This is
9282 because on BSD Unix systems, a read of a pipe will return however
9283 much happens to be in the pipe, even if it is less than @command{tar}
9284 requested. If this option was not used, @command{tar} would fail as
9285 soon as it read an incomplete record from the pipe.
9287 This option is also useful with the commands for updating an archive.
9293 @FIXME{Appropriate options should be moved here from elsewhere.}
9295 @cindex blocking factor
9296 @cindex tape blocking
9298 When handling various tapes or cartridges, you have to take care of
9299 selecting a proper blocking, that is, the number of disk blocks you
9300 put together as a single tape block on the tape, without intervening
9301 tape gaps. A @dfn{tape gap} is a small landing area on the tape
9302 with no information on it, used for decelerating the tape to a
9303 full stop, and for later regaining the reading or writing speed.
9304 When the tape driver starts reading a record, the record has to
9305 be read whole without stopping, as a tape gap is needed to stop the
9306 tape motion without loosing information.
9308 @cindex Exabyte blocking
9309 @cindex DAT blocking
9310 Using higher blocking (putting more disk blocks per tape block) will use
9311 the tape more efficiently as there will be less tape gaps. But reading
9312 such tapes may be more difficult for the system, as more memory will be
9313 required to receive at once the whole record. Further, if there is a
9314 reading error on a huge record, this is less likely that the system will
9315 succeed in recovering the information. So, blocking should not be too
9316 low, nor it should be too high. @command{tar} uses by default a blocking of
9317 20 for historical reasons, and it does not really matter when reading or
9318 writing to disk. Current tape technology would easily accommodate higher
9319 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
9320 We were told that for some DLT drives, the blocking should be a multiple
9321 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
9322 Other manufacturers may use different recommendations for the same tapes.
9323 This might also depends of the buffering techniques used inside modern
9324 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
9325 Others request blocking to be some exponent of two.
9327 So, there is no fixed rule for blocking. But blocking at read time
9328 should ideally be the same as blocking used at write time. At one place
9329 I know, with a wide variety of equipment, they found it best to use a
9330 blocking of 32 to guarantee that their tapes are fully interchangeable.
9332 I was also told that, for recycled tapes, prior erasure (by the same
9333 drive unit that will be used to create the archives) sometimes lowers
9334 the error rates observed at rewriting time.
9336 I might also use @option{--number-blocks} instead of
9337 @option{--block-number}, so @option{--block} will then expand to
9338 @option{--blocking-factor} unambiguously.
9341 @section Many Archives on One Tape
9343 @FIXME{Appropriate options should be moved here from elsewhere.}
9345 @findex ntape @r{device}
9346 Most tape devices have two entries in the @file{/dev} directory, or
9347 entries that come in pairs, which differ only in the minor number for
9348 this device. Let's take for example @file{/dev/tape}, which often
9349 points to the only or usual tape device of a given system. There might
9350 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
9351 name is the @emph{rewinding} version of the device, while the name
9352 having @samp{nr} in it is the @emph{no rewinding} version of the same
9355 A rewinding tape device will bring back the tape to its beginning point
9356 automatically when this device is opened or closed. Since @command{tar}
9357 opens the archive file before using it and closes it afterwards, this
9358 means that a simple:
9361 $ @kbd{tar cf /dev/tape @var{directory}}
9365 will reposition the tape to its beginning both prior and after saving
9366 @var{directory} contents to it, thus erasing prior tape contents and
9367 making it so that any subsequent write operation will destroy what has
9370 @cindex tape positioning
9371 So, a rewinding device is normally meant to hold one and only one file.
9372 If you want to put more than one @command{tar} archive on a given tape, you
9373 will need to avoid using the rewinding version of the tape device. You
9374 will also have to pay special attention to tape positioning. Errors in
9375 positioning may overwrite the valuable data already on your tape. Many
9376 people, burnt by past experiences, will only use rewinding devices and
9377 limit themselves to one file per tape, precisely to avoid the risk of
9378 such errors. Be fully aware that writing at the wrong position on a
9379 tape loses all information past this point and most probably until the
9380 end of the tape, and this destroyed information @emph{cannot} be
9383 To save @var{directory-1} as a first archive at the beginning of a
9384 tape, and leave that tape ready for a second archive, you should use:
9387 $ @kbd{mt -f /dev/nrtape rewind}
9388 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
9392 @dfn{Tape marks} are special magnetic patterns written on the tape
9393 media, which are later recognizable by the reading hardware. These
9394 marks are used after each file, when there are many on a single tape.
9395 An empty file (that is to say, two tape marks in a row) signal the
9396 logical end of the tape, after which no file exist. Usually,
9397 non-rewinding tape device drivers will react to the close request issued
9398 by @command{tar} by first writing two tape marks after your archive, and by
9399 backspacing over one of these. So, if you remove the tape at that time
9400 from the tape drive, it is properly terminated. But if you write
9401 another file at the current position, the second tape mark will be
9402 erased by the new information, leaving only one tape mark between files.
9404 So, you may now save @var{directory-2} as a second archive after the
9405 first on the same tape by issuing the command:
9408 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
9412 and so on for all the archives you want to put on the same tape.
9414 Another usual case is that you do not write all the archives the same
9415 day, and you need to remove and store the tape between two archive
9416 sessions. In general, you must remember how many files are already
9417 saved on your tape. Suppose your tape already has 16 files on it, and
9418 that you are ready to write the 17th. You have to take care of skipping
9419 the first 16 tape marks before saving @var{directory-17}, say, by using
9423 $ @kbd{mt -f /dev/nrtape rewind}
9424 $ @kbd{mt -f /dev/nrtape fsf 16}
9425 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
9428 In all the previous examples, we put aside blocking considerations, but
9429 you should do the proper things for that as well. @xref{Blocking}.
9432 * Tape Positioning:: Tape Positions and Tape Marks
9433 * mt:: The @command{mt} Utility
9436 @node Tape Positioning
9437 @subsection Tape Positions and Tape Marks
9440 Just as archives can store more than one file from the file system,
9441 tapes can store more than one archive file. To keep track of where
9442 archive files (or any other type of file stored on tape) begin and
9443 end, tape archive devices write magnetic @dfn{tape marks} on the
9444 archive media. Tape drives write one tape mark between files,
9445 two at the end of all the file entries.
9447 If you think of data as a series of records "rrrr"'s, and tape marks as
9448 "*"'s, a tape might look like the following:
9451 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
9454 Tape devices read and write tapes using a read/write @dfn{tape
9455 head}---a physical part of the device which can only access one
9456 point on the tape at a time. When you use @command{tar} to read or
9457 write archive data from a tape device, the device will begin reading
9458 or writing from wherever on the tape the tape head happens to be,
9459 regardless of which archive or what part of the archive the tape
9460 head is on. Before writing an archive, you should make sure that no
9461 data on the tape will be overwritten (unless it is no longer needed).
9462 Before reading an archive, you should make sure the tape head is at
9463 the beginning of the archive you want to read. You can do it manually
9464 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
9465 that automatically (@pxref{Scripted Restoration}).
9467 If you want to add new archive file entries to a tape, you should
9468 advance the tape to the end of the existing file entries, backspace
9469 over the last tape mark, and write the new archive file. If you were
9470 to add two archives to the example above, the tape might look like the
9474 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
9478 @subsection The @command{mt} Utility
9481 @FIXME{Is it true that this only works on non-block devices?
9482 should explain the difference, (fixed or variable).}
9483 @xref{Blocking Factor}.
9485 You can use the @command{mt} utility to advance or rewind a tape past a
9486 specified number of archive files on the tape. This will allow you
9487 to move to the beginning of an archive before extracting or reading
9488 it, or to the end of all the archives before writing a new one.
9489 @FIXME{Why isn't there an "advance 'til you find two tape marks
9492 The syntax of the @command{mt} command is:
9495 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
9498 where @var{tapename} is the name of the tape device, @var{number} is
9499 the number of times an operation is performed (with a default of one),
9500 and @var{operation} is one of the following:
9502 @FIXME{is there any use for record operations?}
9507 Writes @var{number} tape marks at the current position on the tape.
9510 Moves tape position forward @var{number} files.
9513 Moves tape position back @var{number} files.
9516 Rewinds the tape. (Ignores @var{number}).
9520 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}).
9523 Prints status information about the tape unit.
9527 @FIXME{Is there a better way to frob the spacing on the list?}
9529 If you don't specify a @var{tapename}, @command{mt} uses the environment
9530 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} uses the device
9533 @command{mt} returns a 0 exit status when the operation(s) were
9534 successful, 1 if the command was unrecognized, and 2 if an operation
9537 @node Using Multiple Tapes
9538 @section Using Multiple Tapes
9541 Often you might want to write a large archive, one larger than will fit
9542 on the actual tape you are using. In such a case, you can run multiple
9543 @command{tar} commands, but this can be inconvenient, particularly if you
9544 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
9545 Therefore, @command{tar} supports multiple tapes automatically.
9547 Use @option{--multi-volume} (@option{-M}) on the command line, and
9548 then @command{tar} will, when it reaches the end of the tape, prompt
9549 for another tape, and continue the archive. Each tape will have an
9550 independent archive, and can be read without needing the other. (As
9551 an exception to this, the file that @command{tar} was archiving when
9552 it ran out of tape will usually be split between the two archives; in
9553 this case you need to extract from the first archive, using
9554 @option{--multi-volume}, and then put in the second tape when
9555 prompted, so @command{tar} can restore both halves of the file.)
9557 @GNUTAR{} multi-volume archives do not use a truly portable format.
9558 You need @GNUTAR{} at both ends to process them properly.
9560 When prompting for a new tape, @command{tar} accepts any of the following
9565 Request @command{tar} to explain possible responses
9567 Request @command{tar} to exit immediately.
9568 @item n @var{file-name}
9569 Request @command{tar} to write the next volume on the file @var{file-name}.
9571 Request @command{tar} to run a subshell. This option can be disabled
9572 by giving @option{--restrict} command line option to @command{tar}.
9574 Request @command{tar} to begin writing the next volume.
9577 (You should only type @samp{y} after you have changed the tape;
9578 otherwise @command{tar} will write over the volume it just finished.)
9580 @cindex End-of-archive info script
9582 @anchor{info-script}
9583 @opindex info-script
9584 @opindex new-volume-script
9585 If you want more elaborate behavior than this, give @command{tar} the
9586 @option{--info-script=@var{script-name}}
9587 (@option{--new-volume-script=@var{script-name}}, @option{-F
9588 @var{script-name}}) option. The file @var{script-name} is expected to
9589 be a program (or shell script) to be run instead of the normal
9590 prompting procedure. It is executed without any command line
9591 arguments. Additional data is passed to it via the following
9592 environment variables:
9595 @vrindex TAR_VERSION, info script environment variable
9597 @GNUTAR{} version number.
9599 @vrindex TAR_ARCHIVE, info script environment variable
9601 The name of the archive @command{tar} is processing.
9603 @vrindex TAR_VOLUME, info script environment variable
9605 Ordinal number of the volume @command{tar} is about to start.
9607 @vrindex TAR_SUBCOMMAND, info script environment variable
9608 @item TAR_SUBCOMMAND
9609 Short option describing the operation @command{tar} is executed.
9610 @xref{Operations}, for a complete list of subcommand options.
9612 @vrindex TAR_FORMAT, info script environment variable
9614 Format of the archive being processed. @xref{Formats}, for a complete
9615 list of archive format names.
9618 The info script can instruct @command{tar} to use new archive name,
9619 by writing in to file descriptor 3 (see below for an
9622 If the info script fails, @command{tar} exits; otherwise, it begins
9623 writing the next volume.
9625 The method @command{tar} uses to detect end of tape is not perfect, and
9626 fails on some operating systems or on some devices. You can use the
9627 @option{--tape-length=@var{size}} (@option{-L @var{size}}) option if
9628 @command{tar} can't detect the end of the tape itself. This option
9629 selects @option{--multi-volume} (@option{-M}) automatically. The
9630 @var{size} argument should then be the usable size of the tape in
9631 units of 1024 bytes. But for many devices, and floppy disks in
9632 particular, this option is never required for real, as far as we know.
9634 @cindex Volume number file
9638 The volume number used by @command{tar} in its tape-change prompt
9639 can be changed; if you give the
9640 @option{--volno-file=@var{file-of-number}} option, then
9641 @var{file-of-number} should be an unexisting file to be created, or
9642 else, a file already containing a decimal number. That number will be
9643 used as the volume number of the first volume written. When
9644 @command{tar} is finished, it will rewrite the file with the
9645 now-current volume number. (This does not change the volume number
9646 written on a tape label, as per @ref{label}, it @emph{only} affects
9647 the number used in the prompt.)
9649 If you want @command{tar} to cycle through a series of files or tape
9650 drives, there are three approaches to choose from. First of all, you
9651 can give @command{tar} multiple @option{--file} options. In this case
9652 the specified files will be used, in sequence, as the successive
9653 volumes of the archive. Only when the first one in the sequence needs
9654 to be used again will @command{tar} prompt for a tape change (or run
9655 the info script). Secondly, you can use the @samp{n} response to the
9656 tape-change prompt, and, finally, you can use an info script, that
9657 writes new archive name to file descriptor. The following example
9658 illustrates this approach:
9663 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
9665 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
9666 case $TAR_SUBCOMMAND in
9668 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
9673 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&3
9677 Each volume of a multi-volume archive is an independent @command{tar}
9678 archive, complete in itself. For example, you can list or extract any
9679 volume alone; just don't specify @option{--multi-volume}
9680 (@option{-M}). However, if one file in the archive is split across
9681 volumes, the only way to extract it successfully is with a
9682 multi-volume extract command @option{--extract --multi-volume}
9683 (@option{-xM}) starting on or before the volume where the file begins.
9685 For example, let's presume someone has two tape drives on a system
9686 named @file{/dev/tape0} and @file{/dev/tape1}. For having @GNUTAR{}
9687 to switch to the second drive when it needs to write the
9688 second tape, and then back to the first tape, etc., just do either of:
9691 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
9692 $ @kbd{tar cMff /dev/tape0 /dev/tape1 @var{files}}
9696 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
9697 * Tape Files:: Tape Files
9698 * Tarcat:: Concatenate Volumes into a Single Archive
9702 @node Multi-Volume Archives
9703 @subsection Archives Longer than One Tape or Disk
9704 @cindex Multi-volume archives
9707 @opindex multi-volume
9708 To create an archive that is larger than will fit on a single unit of
9709 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
9710 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
9711 archive can be manipulated like any other archive (provided the
9712 @option{--multi-volume} option is specified), but is stored on more
9713 than one tape or disk.
9715 When you specify @option{--multi-volume}, @command{tar} does not report an
9716 error when it comes to the end of an archive volume (when reading), or
9717 the end of the media (when writing). Instead, it prompts you to load
9718 a new storage volume. If the archive is on a magnetic tape, you
9719 should change tapes when you see the prompt; if the archive is on a
9720 floppy disk, you should change disks; etc.
9722 You can read each individual volume of a multi-volume archive as if it
9723 were an archive by itself. For example, to list the contents of one
9724 volume, use @option{--list}, without @option{--multi-volume} specified.
9725 To extract an archive member from one volume (assuming it is described
9726 that volume), use @option{--extract}, again without
9727 @option{--multi-volume}.
9729 If an archive member is split across volumes (ie. its entry begins on
9730 one volume of the media and ends on another), you need to specify
9731 @option{--multi-volume} to extract it successfully. In this case, you
9732 should load the volume where the archive member starts, and use
9733 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
9734 volumes as it needs them. @xref{extracting archives}, for more
9735 information about extracting archives.
9737 @option{--info-script=@var{script-name}}
9738 (@option{--new-volume-script=@var{script-name}}, @option{-F
9739 @var{script-name}}) (@pxref{info-script}) is like
9740 @option{--multi-volume} (@option{-M}), except that @command{tar} does
9741 not prompt you directly to change media volumes when a volume is
9742 full---instead, @command{tar} runs commands you have stored in
9743 @var{script-name}. For example, this option can be used to eject
9744 cassettes, or to broadcast messages such as @samp{Someone please come
9745 change my tape} when performing unattended backups. When
9746 @var{script-name} is done, @command{tar} will assume that the media
9749 Multi-volume archives can be modified like any other archive. To add
9750 files to a multi-volume archive, you need to only mount the last
9751 volume of the archive media (and new volumes, if needed). For all
9752 other operations, you need to use the entire archive.
9754 If a multi-volume archive was labeled using
9755 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9756 (@pxref{label}) when it was created, @command{tar} will not
9757 automatically label volumes which are added later. To label
9758 subsequent volumes, specify @option{--label=@var{archive-label}} again
9759 in conjunction with the @option{--append}, @option{--update} or
9760 @option{--concatenate} operation.
9762 @cindex Labeling multi-volume archives
9765 @FIXME{There should be a sample program here, including an exit
9766 before end. Is the exit status even checked in tar? :-(}
9769 @item --multi-volume
9771 Creates a multi-volume archive, when used in conjunction with
9772 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
9773 archive, specify @option{--multi-volume} in conjunction with that
9776 @item --info-script=@var{program-file}
9777 @itemx --new-volume-script=@var{program-file}
9778 @itemx -F @var{program-file}
9779 Creates a multi-volume archive via a script. Used in conjunction with
9780 @option{--create} (@option{-c}). @xref{info-script}, dor a detailed discussion.
9783 Beware that there is @emph{no} real standard about the proper way, for
9784 a @command{tar} archive, to span volume boundaries. If you have a
9785 multi-volume created by some vendor's @command{tar}, there is almost
9786 no chance you could read all the volumes with @GNUTAR{}.
9787 The converse is also true: you may not expect
9788 multi-volume archives created by @GNUTAR{} to be
9789 fully recovered by vendor's @command{tar}. Since there is little
9790 chance that, in mixed system configurations, some vendor's
9791 @command{tar} will work on another vendor's machine, and there is a
9792 great chance that @GNUTAR{} will work on most of
9793 them, your best bet is to install @GNUTAR{} on all
9794 machines between which you know exchange of files is possible.
9797 @subsection Tape Files
9800 To give the archive a name which will be recorded in it, use the
9801 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
9802 option. This will write a special block identifying
9803 @var{volume-label} as the name of the archive to the front of the
9804 archive which will be displayed when the archive is listed with
9805 @option{--list}. If you are creating a multi-volume archive with
9806 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
9807 volume label will have @samp{Volume @var{nnn}} appended to the name
9808 you give, where @var{nnn} is the number of the volume of the archive.
9809 (If you use the @option{--label=@var{volume-label}}) option when
9810 reading an archive, it checks to make sure the label on the tape
9811 matches the one you give. @xref{label}.
9813 When @command{tar} writes an archive to tape, it creates a single
9814 tape file. If multiple archives are written to the same tape, one
9815 after the other, they each get written as separate tape files. When
9816 extracting, it is necessary to position the tape at the right place
9817 before running @command{tar}. To do this, use the @command{mt} command.
9818 For more information on the @command{mt} command and on the organization
9819 of tapes into a sequence of tape files, see @ref{mt}.
9821 People seem to often do:
9824 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
9827 or such, for pushing a common date in all volumes or an archive set.
9830 @subsection Concatenate Volumes into a Single Archive
9833 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
9834 archive to a single @command{tar} archive. Simply concatenating all
9835 volumes into one will not work, since each volume carries an additional
9836 information at the beginning. @GNUTAR{} is shipped with the shell
9837 script @command{tarcat} designed for this purpose.
9839 The script takes a list of files comprising a multi-volume archive
9840 and creates the resulting archive at the standard output. For example:
9843 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
9846 The script implements a simple heuristics to determine the format of
9847 the first volume file and to decide how to process the rest of the
9848 files. However, it makes no attempt to verify whether the files are
9849 given in order or even if they are valid @command{tar} archives.
9850 It uses @command{dd} and does not filter its standard error, so you
9851 will usually see lots of spurious messages.
9853 @FIXME{The script is not installed. Should we install it?}
9856 @section Including a Label in the Archive
9857 @cindex Labeling an archive
9858 @cindex Labels on the archive media
9862 To avoid problems caused by misplaced paper labels on the archive
9863 media, you can include a @dfn{label} entry---an archive member which
9864 contains the name of the archive---in the archive itself. Use the
9865 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9866 option in conjunction with the @option{--create} operation to include
9867 a label entry in the archive as it is being created.
9870 @item --label=@var{archive-label}
9871 @itemx -V @var{archive-label}
9872 Includes an @dfn{archive-label} at the beginning of the archive when
9873 the archive is being created, when used in conjunction with the
9874 @option{--create} operation. Checks to make sure the archive label
9875 matches the one specified (when used in conjunction with any other
9879 If you create an archive using both
9880 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9881 and @option{--multi-volume} (@option{-M}), each volume of the archive
9882 will have an archive label of the form @samp{@var{archive-label}
9883 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
9884 next, and so on. @xref{Using Multiple Tapes}, for information on
9885 creating multiple volume archives.
9887 @cindex Volume label, listing
9888 @cindex Listing volume label
9889 The volume label will be displayed by @option{--list} along with
9890 the file contents. If verbose display is requested, it will also be
9891 explicitely marked as in the example below:
9895 $ @kbd{tar --verbose --list --file=iamanarchive}
9896 V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header--
9897 -rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename
9902 @anchor{--test-label option}
9903 However, @option{--list} option will cause listing entire
9904 contents of the archive, which may be undesirable (for example, if the
9905 archive is stored on a tape). You can request checking only the volume
9906 by specifying @option{--test-label} option. This option reads only the
9907 first block of an archive, so it can be used with slow storage
9908 devices. For example:
9912 $ @kbd{tar --test-label --file=iamanarchive}
9917 If @option{--test-label} is used with a single command line
9918 argument, @command{tar} compares the volume label with the
9919 argument. It exits with code 0 if the two strings match, and with code
9920 2 otherwise. In this case no output is displayed. For example:
9924 $ @kbd{tar --test-label --file=iamanarchive 'iamalable'}
9926 $ @kbd{tar --test-label --file=iamanarchive 'iamalable' alabel}
9931 If you request any operation, other than @option{--create}, along
9932 with using @option{--label} option, @command{tar} will first check if
9933 the archive label matches the one specified and will refuse to proceed
9934 if it does not. Use this as a safety precaution to avoid accidentally
9935 overwriting existing archives. For example, if you wish to add files
9936 to @file{archive}, presumably labelled with string @samp{My volume},
9941 $ @kbd{tar -rf archive --label 'My volume' .}
9942 tar: Archive not labeled to match `My volume'
9947 in case its label does not match. This will work even if
9948 @file{archive} is not labelled at all.
9950 Similarly, @command{tar} will refuse to list or extract the
9951 archive if its label doesn't match the @var{archive-label}
9952 specified. In those cases, @var{archive-label} argument is interpreted
9953 as a globbing-style pattern which must match the actual magnetic
9954 volume label. @xref{exclude}, for a precise description of how match
9955 is attempted@footnote{Previous versions of @command{tar} used full
9956 regular expression matching, or before that, only exact string
9957 matching, instead of wildcard matchers. We decided for the sake of
9958 simplicity to use a uniform matching device through
9959 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
9960 the volume label matcher will also suffix @var{archive-label} by
9961 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
9962 up. Since the volume numbering is automatically added in labels at
9963 creation time, it sounded logical to equally help the user taking care
9964 of it when the archive is being read.
9966 The @option{--label} was once called @option{--volume}, but is not
9967 available under that name anymore.
9969 You can also use @option{--label} to get a common information on
9970 all tapes of a series. For having this information different in each
9971 series created through a single script used on a regular basis, just
9972 manage to get some date string as part of the label. For example:
9976 $ @kbd{tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"}
9977 $ @kbd{tar --create --file=/dev/tape --multi-volume \
9978 --volume="Daily backup for `date +%Y-%m-%d`"}
9982 Also note that each label has its own date and time, which corresponds
9983 to when @GNUTAR{} initially attempted to write it,
9984 often soon after the operator launches @command{tar} or types the
9985 carriage return telling that the next tape is ready. Comparing date
9986 labels does give an idea of tape throughput only if the delays for
9987 rewinding tapes and the operator switching them were negligible, which
9988 is usually not the case.
9991 @section Verifying Data as It is Stored
9992 @cindex Verifying a write operation
9993 @cindex Double-checking a write operation
9998 @opindex verify, short description
9999 Attempt to verify the archive after writing.
10002 This option causes @command{tar} to verify the archive after writing it.
10003 Each volume is checked after it is written, and any discrepancies
10004 are recorded on the standard error output.
10006 Verification requires that the archive be on a back-space-able medium.
10007 This means pipes, some cartridge tape drives, and some other devices
10008 cannot be verified.
10010 You can insure the accuracy of an archive by comparing files in the
10011 system with archive members. @command{tar} can compare an archive to the
10012 file system as the archive is being written, to verify a write
10013 operation, or can compare a previously written archive, to insure that
10016 @opindex verify, using with @option{--create}
10017 @opindex create, using with @option{--verify}
10018 To check for discrepancies in an archive immediately after it is
10019 written, use the @option{--verify} (@option{-W}) option in conjunction with
10020 the @option{--create} operation. When this option is
10021 specified, @command{tar} checks archive members against their counterparts
10022 in the file system, and reports discrepancies on the standard error.
10024 To verify an archive, you must be able to read it from before the end
10025 of the last written entry. This option is useful for detecting data
10026 errors on some tapes. Archives written to pipes, some cartridge tape
10027 drives, and some other devices cannot be verified.
10029 One can explicitly compare an already made archive with the file
10030 system by using the @option{--compare} (@option{--diff}, @option{-d})
10031 option, instead of using the more automatic @option{--verify} option.
10034 Note that these two options have a slightly different intent. The
10035 @option{--compare} option checks how identical are the logical contents of some
10036 archive with what is on your disks, while the @option{--verify} option is
10037 really for checking if the physical contents agree and if the recording
10038 media itself is of dependable quality. So, for the @option{--verify}
10039 operation, @command{tar} tries to defeat all in-memory cache pertaining to
10040 the archive, while it lets the speed optimization undisturbed for the
10041 @option{--compare} option. If you nevertheless use @option{--compare} for
10042 media verification, you may have to defeat the in-memory cache yourself,
10043 maybe by opening and reclosing the door latch of your recording unit,
10044 forcing some doubt in your operating system about the fact this is really
10045 the same volume as the one just written or read.
10047 The @option{--verify} option would not be necessary if drivers were indeed
10048 able to detect dependably all write failures. This sometimes require many
10049 magnetic heads, some able to read after the writes occurred. One would
10050 not say that drivers unable to detect all cases are necessarily flawed,
10051 as long as programming is concerned.
10053 The @option{--verify} (@option{-W}) option will not work in
10054 conjunction with the @option{--multi-volume} (@option{-M}) option or
10055 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
10056 and @option{--delete} operations. @xref{Operations}, for more
10057 information on these operations.
10059 Also, since @command{tar} normally strips leading @samp{/} from file
10060 names (@pxref{absolute}), a command like @samp{tar --verify -cf
10061 /tmp/foo.tar /etc} will work as desired only if the working directory is
10062 @file{/}, as @command{tar} uses the archive's relative member names
10063 (e.g., @file{etc/motd}) when verifying the archive.
10065 @node Write Protection
10066 @section Write Protection
10068 Almost all tapes and diskettes, and in a few rare cases, even disks can
10069 be @dfn{write protected}, to protect data on them from being changed.
10070 Once an archive is written, you should write protect the media to prevent
10071 the archive from being accidentally overwritten or deleted. (This will
10072 protect the archive from being changed with a tape or floppy drive---it
10073 will not protect it from magnet fields or other physical hazards).
10075 The write protection device itself is usually an integral part of the
10076 physical media, and can be a two position (write enabled/write
10077 disabled) switch, a notch which can be popped out or covered, a ring
10078 which can be removed from the center of a tape reel, or some other
10079 changeable feature.
10084 This appendix lists some important user-visible changes between
10085 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
10086 version of this document is available at
10087 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
10088 @GNUTAR{} documentation page}.
10091 @item Use of globbing patterns when listing and extracting.
10093 Previous versions of GNU tar assumed shell-style globbing when
10094 extracting from or listing an archive. For example:
10097 $ @kbd{tar xf foo.tar '*.c'}
10100 would extract all files whose names end in @samp{.c}. This behavior
10101 was not documented and was incompatible with traditional tar
10102 implementations. Therefore, starting from version 1.15.91, GNU tar
10103 no longer uses globbing by default. For example, the above invocation
10104 is now interpreted as a request to extract from the archive the file
10107 To facilitate transition to the new behavior for those users who got
10108 used to the previous incorrect one, @command{tar} will print a warning
10109 if it finds out that a requested member was not found in the archive
10110 and its name looks like a globbing pattern. For example:
10113 $ @kbd{tar xf foo.tar '*.c'}
10114 tar: Pattern matching characters used in file names. Please,
10115 tar: use --wildcards to enable pattern matching, or --no-wildcards to
10116 tar: suppress this warning.
10117 tar: *.c: Not found in archive
10118 tar: Error exit delayed from previous errors
10121 To treat member names as globbing patterns, use --wildcards option.
10122 If you want to tar to mimic the behavior of versions prior to 1.15.91,
10123 add this option to your @env{TAR_OPTIONS} variable.
10125 @xref{wildcards}, for the detailed discussion of the use of globbing
10126 patterns by @GNUTAR{}.
10128 @item Use of short option @option{-o}.
10130 Earlier versions of @GNUTAR{} understood @option{-o} command line
10131 option as a synonym for @option{--old-archive}.
10133 @GNUTAR{} starting from version 1.13.90 understands this option as
10134 a synonym for @option{--no-same-owner}. This is compatible with
10135 UNIX98 @command{tar} implementations.
10137 However, to facilitate transition, @option{-o} option retains its
10138 old semantics when it is used with one of archive-creation commands.
10139 Users are encouraged to use @option{--format=oldgnu} instead.
10141 It is especially important, since versions of @acronym{GNU} Automake
10142 up to and including 1.8.4 invoke tar with this option to produce
10143 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
10144 of this issue and its implications.
10146 @FIXME{Change the first argument to tar-formats if and when Automake
10147 people accept my patch to the documentation, and the new Automake is
10148 out --Sergey 2006-05-25}.
10149 @xref{Options, tar-v7, Changing Automake's Behavior,
10150 automake, GNU Automake}, for a description on how to use various
10151 archive formats with @command{automake}.
10153 Future versions of @GNUTAR{} will understand @option{-o} only as a
10154 synonym for @option{--no-same-owner}.
10156 @item Use of short option @option{-l}
10158 Earlier versions of @GNUTAR{} understood @option{-l} option as a
10159 synonym for @option{--one-file-system}. Since such usage contradicted
10160 to UNIX98 specification and harmed compatibility with other
10161 implementation, it was declared deprecated in version 1.14. However,
10162 to facilitate transition to its new semantics, it was supported by
10163 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
10164 variant of @option{--check-links} was introduced in version 1.15.91.
10166 @item Use of options @option{--portability} and @option{--old-archive}
10168 These options are deprecated. Please use @option{--format=v7} instead.
10170 @item Use of option @option{--posix}
10172 This option is deprecated. Please use @option{--format=posix} instead.
10175 @node Configuring Help Summary
10176 @appendix Configuring Help Summary
10178 Running @kbd{tar --help} displays the short @command{tar} option
10179 summary (@pxref{help}). This summary is organised by @dfn{groups} of
10180 semantically close options. The options within each group are printed
10181 in the following order: a short option, eventually followed by a list
10182 of corresponding long option names, followed by a short description of
10183 the option. For example, here is an excerpt from the actual @kbd{tar
10187 Main operation mode:
10189 -A, --catenate, --concatenate append tar files to an archive
10190 -c, --create create a new archive
10191 -d, --diff, --compare find differences between archive and
10193 --delete delete from the archive
10196 @vrindex ARGP_HELP_FMT, environment variable
10197 The exact visual representation of the help output is configurable via
10198 @env{ARGP_HELP_FMT} environment variable. The value of this variable
10199 is a comma-separated list of @dfn{format variable} assignments. There
10200 are two kinds of format variables. An @dfn{offset variable} keeps the
10201 offset of some part of help output text from the leftmost column on
10202 the screen. A @dfn{boolean} variable is a flag that toggles some
10203 output feature on or off. Depending on the type of the corresponding
10204 variable, there are two kinds of assignments:
10207 @item Offset assignment
10209 The assignment to an offset variable has the following syntax:
10212 @var{variable}=@var{value}
10216 where @var{variable} is the variable name, and @var{value} is a
10217 numeric value to be assigned to the variable.
10219 @item Boolean assignment
10221 To assign @code{true} value to a variable, simply put this variable name. To
10222 assign @code{false} value, prefix the variable name with @samp{no-}. For
10227 # Assign @code{true} value:
10229 # Assign @code{false} value:
10235 Following variables are declared:
10237 @deftypevr {Help Output} boolean dup-args
10238 If true, arguments for an option are shown with both short and long
10239 options, even when a given option has both forms, for example:
10242 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
10245 If false, then if an option has both short and long forms, the
10246 argument is only shown with the long one, for example:
10249 -f, --file=ARCHIVE use archive file or device ARCHIVE
10253 and a message indicating that the argument is applicable to both
10254 forms is printed below the options. This message can be disabled
10255 using @code{dup-args-note} (see below).
10257 The default is false.
10260 @deftypevr {Help Output} boolean dup-args-note
10261 If this variable is true, which is the default, the following notice
10262 is displayed at the end of the help output:
10265 Mandatory or optional arguments to long options are also mandatory or
10266 optional for any corresponding short options.
10269 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
10270 variables @code{dup-args} or @code{dup-args-note} should be set.
10273 @deftypevr {Help Output} offset short-opt-col
10274 Column in which short options start. Default is 2.
10278 $ @kbd{tar --help|grep ARCHIVE}
10279 -f, --file=ARCHIVE use archive file or device ARCHIVE
10280 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
10281 -f, --file=ARCHIVE use archive file or device ARCHIVE
10286 @deftypevr {Help Output} offset long-opt-col
10287 Column in which long options start. Default is 6. For example:
10291 $ @kbd{tar --help|grep ARCHIVE}
10292 -f, --file=ARCHIVE use archive file or device ARCHIVE
10293 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
10294 -f, --file=ARCHIVE use archive file or device ARCHIVE
10299 @deftypevr {Help Output} offset doc-opt-col
10300 Column in which @dfn{doc options} start. A doc option isn't actually
10301 an option, but rather an arbitrary piece of documentation that is
10302 displayed in much the same manner as the options. For example, in
10303 the description of @option{--format} option:
10307 -H, --format=FORMAT create archive of the given format.
10309 FORMAT is one of the following:
10311 gnu GNU tar 1.13.x format
10312 oldgnu GNU format as per tar <= 1.12
10313 pax POSIX 1003.1-2001 (pax) format
10315 ustar POSIX 1003.1-1988 (ustar) format
10316 v7 old V7 tar format
10321 the format names are doc options. Thus, if you set
10322 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
10323 will look as follows:
10327 -H, --format=FORMAT create archive of the given format.
10329 FORMAT is one of the following:
10331 gnu GNU tar 1.13.x format
10332 oldgnu GNU format as per tar <= 1.12
10333 pax POSIX 1003.1-2001 (pax) format
10335 ustar POSIX 1003.1-1988 (ustar) format
10336 v7 old V7 tar format
10341 @deftypevr {Help Output} offset opt-doc-col
10342 Column in which option description starts. Default is 29.
10346 $ @kbd{tar --help|grep ARCHIVE}
10347 -f, --file=ARCHIVE use archive file or device ARCHIVE
10348 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
10349 -f, --file=ARCHIVE use archive file or device ARCHIVE
10350 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
10352 use archive file or device ARCHIVE
10357 Notice, that the description starts on a separate line if
10358 @code{opt-doc-col} value is too small.
10361 @deftypevr {Help Output} offset header-col
10362 Column in which @dfn{group headers} are printed. A group header is a
10363 descriptive text preceding an option group. For example, in the
10367 Main operation mode:
10369 -A, --catenate, --concatenate append tar files to
10371 -c, --create create a new archive
10374 @samp{Main operation mode:} is the group header.
10376 The default value is 1.
10379 @deftypevr {Help Output} offset usage-indent
10380 Indentation of wrapped usage lines. Affects @option{--usage}
10381 output. Default is 12.
10384 @deftypevr {Help Output} offset rmargin
10385 Right margin of the text output. Used for wrapping.
10390 @include genfile.texi
10392 @node Snapshot Files
10393 @appendix Format of the Incremental Snapshot Files
10394 @include snapshot.texi
10396 @node Free Software Needs Free Documentation
10397 @appendix Free Software Needs Free Documentation
10398 @include freemanuals.texi
10400 @node Copying This Manual
10401 @appendix Copying This Manual
10404 * GNU Free Documentation License:: License for copying this manual
10409 @node Index of Command Line Options
10410 @appendix Index of Command Line Options
10412 This appendix contains an index of all @GNUTAR{} long command line
10413 options. The options are listed without the preceeding double-dash.
10416 @item Make sure @emph{all} options are indexed.
10417 @item Provide an index of short options
10431 @c Local variables:
10432 @c texinfo-column-for-description: 32