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::
103 * Free Software Needs Free Documentation::
104 * Copying This Manual::
105 * Index of Command Line Options::
109 --- The Detailed Node Listing ---
113 * Book Contents:: What this Book Contains
114 * Definitions:: Some Definitions
115 * What tar Does:: What @command{tar} Does
116 * Naming tar Archives:: How @command{tar} Archives are Named
117 * Authors:: @GNUTAR{} Authors
118 * Reports:: Reporting bugs or suggestions
120 Tutorial Introduction to @command{tar}
123 * stylistic conventions::
124 * basic tar options:: Basic @command{tar} Operations and Options
125 * frequent operations::
126 * Two Frequent Options::
127 * create:: How to Create Archives
128 * list:: How to List Archives
129 * extract:: How to Extract Members from an Archive
132 Two Frequently Used Options
138 How to Create Archives
140 * prepare for examples::
141 * Creating the archive::
150 How to Extract Members from an Archive
152 * extracting archives::
160 * using tar options::
168 The Three Option Styles
170 * Mnemonic Options:: Mnemonic Option Style
171 * Short Options:: Short Option Style
172 * Old Options:: Old Option Style
173 * Mixing:: Mixing Option Styles
175 All @command{tar} Options
177 * Operation Summary::
179 * Short Option Summary::
191 Advanced @GNUTAR{} Operations
200 How to Add Files to Existing Archives: @option{--append}
202 * appending files:: Appending Files to an Archive
209 Options Used by @option{--create}
211 * Ignore Failed Read::
213 Options Used by @option{--extract}
215 * Reading:: Options to Help Read Archives
216 * Writing:: Changing How @command{tar} Writes Files
217 * Scarce:: Coping with Scarce Resources
219 Options to Help Read Archives
221 * read full records::
224 Changing How @command{tar} Writes Files
226 * Dealing with Old Files::
227 * Overwrite Old Files::
232 * Data Modification Times::
233 * Setting Access Permissions::
234 * Writing to Standard Output::
237 Coping with Scarce Resources
242 Performing Backups and Restoring Files
244 * Full Dumps:: Using @command{tar} to Perform Full Dumps
245 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
246 * Backup Levels:: Levels of Backups
247 * Backup Parameters:: Setting Parameters for Backups and Restoration
248 * Scripted Backups:: Using the Backup Scripts
249 * Scripted Restoration:: Using the Restore Script
251 Setting Parameters for Backups and Restoration
253 * General-Purpose Variables::
254 * Magnetic Tape Control::
256 * backup-specs example:: An Example Text of @file{Backup-specs}
258 Choosing Files and Names for @command{tar}
260 * file:: Choosing the Archive's Name
261 * Selecting Archive Members::
262 * files:: Reading Names from a File
263 * exclude:: Excluding Some Files
264 * wildcards:: Wildcards Patterns and Matching
265 * quoting styles:: Ways of Quoting Special Characters in Names
266 * transform:: Modifying File and Member Names
267 * after:: Operating Only on New Files
268 * recurse:: Descending into Directories
269 * one:: Crossing File System Boundaries
271 Reading Names from a File
277 * problems with exclude::
279 Crossing File System Boundaries
281 * directory:: Changing Directory
282 * absolute:: Absolute File Names
286 * General date syntax:: Common rules.
287 * Calendar date items:: 19 Dec 1994.
288 * Time of day items:: 9:20pm.
289 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}, ...
290 * Day of week items:: Monday and others.
291 * Relative items in date strings:: next tuesday, 2 years ago.
292 * Pure numbers in date strings:: 19931219, 1440.
293 * Seconds since the Epoch:: @@1078100502.
294 * Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al.
296 Controlling the Archive Format
298 * Portability:: Making @command{tar} Archives More Portable
299 * Compression:: Using Less Space through Compression
300 * Attributes:: Handling File Attributes
301 * Standard:: The Standard Format
302 * Extensions:: @acronym{GNU} Extensions to the Archive Format
303 * cpio:: Comparison of @command{tar} and @command{cpio}
305 Making @command{tar} Archives More Portable
307 * Portable Names:: Portable Names
308 * dereference:: Symbolic Links
309 * old:: Old V7 Archives
310 * posix:: @acronym{POSIX} archives
311 * Checksumming:: Checksumming Problems
312 * Large or Negative Values:: Large files, negative time stamps, etc.
314 Using Less Space through Compression
316 * gzip:: Creating and Reading Compressed Archives
317 * sparse:: Archiving Sparse Files
319 Tapes and Other Archive Media
321 * Device:: Device selection and switching
322 * Remote Tape Server::
323 * Common Problems and Solutions::
324 * Blocking:: Blocking
325 * Many:: Many archives on one tape
326 * Using Multiple Tapes:: Using Multiple Tapes
327 * label:: Including a Label in the Archive
333 * Format Variations:: Format Variations
334 * Blocking Factor:: The Blocking Factor of an Archive
336 Many Archives on One Tape
338 * Tape Positioning:: Tape Positions and Tape Marks
339 * mt:: The @command{mt} Utility
343 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
344 * Tape Files:: Tape Files
345 * Tarcat:: Concatenate Volumes into a Single Archive
347 GNU tar internals and development
355 * Free Software Needs Free Documentation::
356 * GNU Free Documentation License:: License for copying this manual
362 @chapter Introduction
365 and manipulates @dfn{archives} which are actually collections of
366 many other files; the program provides users with an organized and
367 systematic method for controlling a large amount of data.
368 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
369 archives need not (and these days, typically do not) reside on tapes.
372 * Book Contents:: What this Book Contains
373 * Definitions:: Some Definitions
374 * What tar Does:: What @command{tar} Does
375 * Naming tar Archives:: How @command{tar} Archives are Named
376 * Authors:: @GNUTAR{} Authors
377 * Reports:: Reporting bugs or suggestions
381 @section What this Book Contains
383 The first part of this chapter introduces you to various terms that will
384 recur throughout the book. It also tells you who has worked on @GNUTAR{}
385 and its documentation, and where you should send bug reports
388 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
389 gentle introduction for people who are new to using @command{tar}. It is
390 meant to be self contained, not requiring any reading from subsequent
391 chapters to make sense. It moves from topic to topic in a logical,
392 progressive order, building on information already explained.
394 Although the tutorial is paced and structured to allow beginners to
395 learn how to use @command{tar}, it is not intended solely for beginners.
396 The tutorial explains how to use the three most frequently used
397 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
398 two frequently used options (@samp{file} and @samp{verbose}). The other
399 chapters do not refer to the tutorial frequently; however, if a section
400 discusses something which is a complex variant of a basic concept, there
401 may be a cross reference to that basic concept. (The entire book,
402 including the tutorial, assumes that the reader understands some basic
403 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
405 The third chapter presents the remaining five operations, and
406 information about using @command{tar} options and option syntax.
408 @FIXME{this sounds more like a @acronym{GNU} Project Manuals Concept [tm] more
409 than the reality. should think about whether this makes sense to say
410 here, or not.} The other chapters are meant to be used as a
411 reference. Each chapter presents everything that needs to be said
412 about a specific topic.
414 One of the chapters (@pxref{Date input formats}) exists in its
415 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
416 In addition, one section of this manual (@pxref{Standard}) contains a
417 big quote which is taken directly from @command{tar} sources.
419 In general, we give both long and short (abbreviated) option names
420 at least once in each section where the relevant option is covered, so
421 that novice readers will become familiar with both styles. (A few
422 options have no short versions, and the relevant sections will
426 @section Some Definitions
430 The @command{tar} program is used to create and manipulate @command{tar}
431 archives. An @dfn{archive} is a single file which contains the contents
432 of many files, while still identifying the names of the files, their
433 owner(s), and so forth. (In addition, archives record access
434 permissions, user and group, size in bytes, and data modification time.
435 Some archives also record the file names in each archived directory, as
436 well as other file and directory information.) You can use @command{tar}
437 to @dfn{create} a new archive in a specified directory.
440 @cindex archive member
443 The files inside an archive are called @dfn{members}. Within this
444 manual, we use the term @dfn{file} to refer only to files accessible in
445 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
446 @dfn{member} to refer only to the members of an archive. Similarly, a
447 @dfn{file name} is the name of a file, as it resides in the file system,
448 and a @dfn{member name} is the name of an archive member within the
453 The term @dfn{extraction} refers to the process of copying an archive
454 member (or multiple members) into a file in the file system. Extracting
455 all the members of an archive is often called @dfn{extracting the
456 archive}. The term @dfn{unpack} can also be used to refer to the
457 extraction of many or all the members of an archive. Extracting an
458 archive does not destroy the archive's structure, just as creating an
459 archive does not destroy the copies of the files that exist outside of
460 the archive. You may also @dfn{list} the members in a given archive
461 (this is often thought of as ``printing'' them to the standard output,
462 or the command line), or @dfn{append} members to a pre-existing archive.
463 All of these operations can be performed using @command{tar}.
466 @section What @command{tar} Does
469 The @command{tar} program provides the ability to create @command{tar}
470 archives, as well as various other kinds of manipulation. For example,
471 you can use @command{tar} on previously created archives to extract files,
472 to store additional files, or to update or list files which were already
475 Initially, @command{tar} archives were used to store files conveniently on
476 magnetic tape. The name @command{tar} comes from this use; it stands for
477 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
478 direct its output to available devices, files, or other programs (using
479 pipes). @command{tar} may even access remote devices or files (as archives).
481 You can use @command{tar} archives in many ways. We want to stress a few
482 of them: storage, backup, and transportation.
484 @FIXME{the following table entries need a bit of work..}
487 Often, @command{tar} archives are used to store related files for
488 convenient file transfer over a network. For example, the
489 @acronym{GNU} Project distributes its software bundled into
490 @command{tar} archives, so that all the files relating to a particular
491 program (or set of related programs) can be transferred as a single
494 A magnetic tape can store several files in sequence. However, the tape
495 has no names for these files; it only knows their relative position on
496 the tape. One way to store several files on one tape and retain their
497 names is by creating a @command{tar} archive. Even when the basic transfer
498 mechanism can keep track of names, as FTP can, the nuisance of handling
499 multiple files, directories, and multiple links makes @command{tar}
502 Archive files are also used for long-term storage. You can think of
503 this as transportation from the present into the future. (It is a
504 science-fiction idiom that you can move through time as well as in
505 space; the idea here is that @command{tar} can be used to move archives in
506 all dimensions, even time!)
509 Because the archive created by @command{tar} is capable of preserving
510 file information and directory structure, @command{tar} is commonly
511 used for performing full and incremental backups of disks. A backup
512 puts a collection of files (possibly pertaining to many users and
513 projects) together on a disk or a tape. This guards against
514 accidental destruction of the information in those files.
515 @GNUTAR{} has special features that allow it to be
516 used to make incremental and full dumps of all the files in a
520 You can create an archive on one system, transfer it to another system,
521 and extract the contents there. This allows you to transport a group of
522 files from one system to another.
525 @node Naming tar Archives
526 @section How @command{tar} Archives are Named
528 Conventionally, @command{tar} archives are given names ending with
529 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
530 but this manual follows that convention in order to accustom readers to
531 it and to make examples more clear.
536 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
537 archive members as ``files'' or ``entries''. For people familiar with
538 the operation of @command{tar}, this causes no difficulty. However, in
539 this manual, we consistently refer to ``archives'' and ``archive
540 members'' to make learning to use @command{tar} easier for novice users.
543 @section @GNUTAR{} Authors
545 @GNUTAR{} was originally written by John Gilmore,
546 and modified by many people. The @acronym{GNU} enhancements were
547 written by Jay Fenlason, then Joy Kendall, and the whole package has
548 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
549 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
550 numerous and kind users.
552 We wish to stress that @command{tar} is a collective work, and owes much to
553 all those people who reported problems, offered solutions and other
554 insights, or shared their thoughts and suggestions. An impressive, yet
555 partial list of those contributors can be found in the @file{THANKS}
556 file from the @GNUTAR{} distribution.
558 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
559 sure i want to spell out the history in this detail, at least not for
560 the printed book. i'm just not sure it needs to be said this way.
561 i'll think about it.}
563 @FIXME{History is more important, and surely more interesting, than
564 actual names. Quoting names without history would be meaningless. FP}
566 Jay Fenlason put together a draft of a @GNUTAR{}
567 manual, borrowing notes from the original man page from John Gilmore.
568 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
569 Gorin worked on a tutorial and manual for @GNUTAR{}.
570 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
571 taking information from all these sources and merging them. Melissa
572 Weisshaus finally edited and redesigned the book to create version
573 1.12. The book for versions from 1.14 up to @value{VERSION} were edited
574 by the current maintainer, Sergey Poznyakoff.
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 @xref{Choosing}, for the detailed discussion on these.) 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
2275 @item --checkpoint[=@var{number}]
2277 This option directs @command{tar} to print periodic checkpoint
2278 messages as it reads through the archive. It is intended for when you
2279 want a visual indication that @command{tar} is still running, but
2280 don't want to see @option{--verbose} output. For a detailed
2281 description, see @ref{Progress information}.
2283 @opindex check-links, summary
2286 If this option was given, @command{tar} will check the number of links
2287 dumped for each processed file. If this number does not match the
2288 total number of hard links for the file, a warning message will be
2289 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2290 synonym for @option{--one-file-system}. The current semantics, wich
2291 complies to UNIX98, was introduced with version
2292 1.15.91. @xref{Changes}, for more information.}.
2294 @opindex compress, summary
2295 @opindex uncompress, summary
2300 @command{tar} will use the @command{compress} program when reading or
2301 writing the archive. This allows you to directly act on archives
2302 while saving space. @xref{gzip}.
2304 @opindex confirmation, summary
2305 @item --confirmation
2307 (See @option{--interactive}.) @xref{interactive}.
2309 @opindex delay-directory-restore, summary
2310 @item --delay-directory-restore
2312 Delay setting modification times and permissions of extracted
2313 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2315 @opindex dereference, summary
2319 When creating a @command{tar} archive, @command{tar} will archive the
2320 file that a symbolic link points to, rather than archiving the
2321 symlink. @xref{dereference}.
2323 @opindex directory, summary
2324 @item --directory=@var{dir}
2327 When this option is specified, @command{tar} will change its current directory
2328 to @var{dir} before performing any operations. When this option is used
2329 during archive creation, it is order sensitive. @xref{directory}.
2331 @opindex exclude, summary
2332 @item --exclude=@var{pattern}
2334 When performing operations, @command{tar} will skip files that match
2335 @var{pattern}. @xref{exclude}.
2337 @opindex exclude-from, summary
2338 @item --exclude-from=@var{file}
2339 @itemx -X @var{file}
2341 Similar to @option{--exclude}, except @command{tar} will use the list of
2342 patterns in the file @var{file}. @xref{exclude}.
2344 @opindex exclude-caches, summary
2345 @item --exclude-caches
2347 Automatically excludes all directories
2348 containing a cache directory tag. @xref{exclude}.
2350 @opindex file, summary
2351 @item --file=@var{archive}
2352 @itemx -f @var{archive}
2354 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2355 performs operations on, rather than @command{tar}'s compilation dependent
2356 default. @xref{file tutorial}.
2358 @opindex files-from, summary
2359 @item --files-from=@var{file}
2360 @itemx -T @var{file}
2362 @command{tar} will use the contents of @var{file} as a list of archive members
2363 or files to operate on, in addition to those specified on the
2364 command-line. @xref{files}.
2366 @opindex force-local, summary
2369 Forces @command{tar} to interpret the filename given to @option{--file}
2370 as a local file, even if it looks like a remote tape drive name.
2371 @xref{local and remote archives}.
2373 @opindex format, summary
2374 @item --format=@var{format}
2376 Selects output archive format. @var{Format} may be one of the
2381 Creates an archive that is compatible with Unix V7 @command{tar}.
2384 Creates an archive that is compatible with GNU @command{tar} version
2388 Creates archive in GNU tar 1.13 format. Basically it is the same as
2389 @samp{oldgnu} with the only difference in the way it handles long
2393 Creates a @acronym{POSIX.1-1988} compatible archive.
2396 Creates a @acronym{POSIX.1-2001 archive}.
2400 @xref{Formats}, for a detailed discussion of these formats.
2402 @opindex group, summary
2403 @item --group=@var{group}
2405 Files added to the @command{tar} archive will have a group id of @var{group},
2406 rather than the group from the source file. @var{group} is first decoded
2407 as a group symbolic name, but if this interpretation fails, it has to be
2408 a decimal numeric group ID. @FIXME-xref{}
2410 Also see the comments for the @option{--owner=@var{user}} option.
2412 @opindex gzip, summary
2413 @opindex gunzip, summary
2414 @opindex ungzip, summary
2420 This option tells @command{tar} to read or write archives through
2421 @command{gzip}, allowing @command{tar} to directly operate on several
2422 kinds of compressed archives transparently. @xref{gzip}.
2424 @opindex help, summary
2427 @command{tar} will print out a short message summarizing the operations and
2428 options to @command{tar} and exit. @xref{help}.
2430 @opindex ignore-case, summary
2432 Ignore case when matching member or file names with
2433 patterns. @xref{controlling pattern-matching}.
2435 @opindex ignore-command-error, summary
2436 @item --ignore-command-error
2437 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2439 @opindex ignore-failed-read, summary
2440 @item --ignore-failed-read
2442 Do not exit unsuccessfully merely because an unreadable file was encountered.
2445 @opindex ignore-zeros, summary
2446 @item --ignore-zeros
2449 With this option, @command{tar} will ignore zeroed blocks in the
2450 archive, which normally signals EOF. @xref{Reading}.
2452 @opindex incremental, summary
2456 Used to inform @command{tar} that it is working with an old
2457 @acronym{GNU}-format incremental backup archive. It is intended
2458 primarily for backwards compatibility only. @xref{Incremental Dumps},
2459 for a detailed discussion of incremental archives.
2461 @opindex index-file, summary
2462 @item --index-file=@var{file}
2464 Send verbose output to @var{file} instead of to standard output.
2466 @opindex info-script, summary
2467 @opindex new-volume-script, summary
2468 @item --info-script=@var{script-file}
2469 @itemx --new-volume-script=@var{script-file}
2470 @itemx -F @var{script-file}
2472 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2473 at the end of each tape. If @var{script-file} exits with nonzero status,
2474 @command{tar} fails immediately. @xref{info-script}, for a detailed
2475 discussion of @var{script-file}.
2477 @opindex interactive, summary
2479 @itemx --confirmation
2482 Specifies that @command{tar} should ask the user for confirmation before
2483 performing potentially destructive options, such as overwriting files.
2486 @opindex keep-newer-files, summary
2487 @item --keep-newer-files
2489 Do not replace existing files that are newer than their archive copies
2490 when extracting files from an archive.
2492 @opindex keep-old-files, summary
2493 @item --keep-old-files
2496 Do not overwrite existing files when extracting files from an archive.
2497 @xref{Keep Old Files}.
2499 @opindex label, summary
2500 @item --label=@var{name}
2501 @itemx -V @var{name}
2503 When creating an archive, instructs @command{tar} to write @var{name}
2504 as a name record in the archive. When extracting or listing archives,
2505 @command{tar} will only operate on archives that have a label matching
2506 the pattern specified in @var{name}. @xref{Tape Files}.
2508 @opindex listed-incremental, summary
2509 @item --listed-incremental=@var{snapshot-file}
2510 @itemx -g @var{snapshot-file}
2512 During a @option{--create} operation, specifies that the archive that
2513 @command{tar} creates is a new @acronym{GNU}-format incremental
2514 backup, using @var{snapshot-file} to determine which files to backup.
2515 With other operations, informs @command{tar} that the archive is in
2516 incremental format. @xref{Incremental Dumps}.
2518 @opindex mode, summary
2519 @item --mode=@var{permissions}
2521 When adding files to an archive, @command{tar} will use
2522 @var{permissions} for the archive members, rather than the permissions
2523 from the files. The program @command{chmod} and this @command{tar}
2524 option share the same syntax for what @var{permissions} might be.
2525 @xref{File permissions, Permissions, File permissions, fileutils,
2526 @acronym{GNU} file utilities}. This reference also has useful
2527 information for those not being overly familiar with the Unix
2530 Of course, @var{permissions} might be plainly specified as an octal number.
2531 However, by using generic symbolic modifications to mode bits, this allows
2532 more flexibility. For example, the value @samp{a+rw} adds read and write
2533 permissions for everybody, while retaining executable bits on directories
2534 or on any other file already marked as executable.
2536 @opindex multi-volume, summary
2537 @item --multi-volume
2540 Informs @command{tar} that it should create or otherwise operate on a
2541 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2543 @opindex new-volume-script, summary
2544 @item --new-volume-script
2548 @opindex seek, summary
2552 Assume that the archive media supports seeks to arbitrary
2553 locations. Usually @command{tar} determines automatically whether
2554 the archive can be seeked or not. This option is intended for use
2555 in cases when such recognition fails.
2557 @opindex newer, summary
2558 @item --newer=@var{date}
2559 @itemx --after-date=@var{date}
2562 When creating an archive, @command{tar} will only add files that have changed
2563 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2564 is taken to be the name of a file whose data modification time specifies
2565 the date. @xref{after}.
2567 @opindex newer-mtime, summary
2568 @item --newer-mtime=@var{date}
2570 Like @option{--newer}, but add only files whose
2571 contents have changed (as opposed to just @option{--newer}, which will
2572 also back up files for which any status information has changed).
2574 @opindex no-anchored, summary
2576 An exclude pattern can match any subsequence of the name's components.
2577 @xref{controlling pattern-matching}.
2579 @opindex no-delay-directory-restore, summary
2580 @item --no-delay-directory-restore
2582 Setting modification times and permissions of extracted
2583 directories when all files from this directory has been
2584 extracted. This is the default. @xref{Directory Modification Times and Permissions}.
2586 @opindex no-ignore-case, summary
2587 @item --no-ignore-case
2588 Use case-sensitive matching.
2589 @xref{controlling pattern-matching}.
2591 @opindex no-ignore-command-error, summary
2592 @item --no-ignore-command-error
2593 Print warnings about subprocesses terminated with a non-zero exit
2594 code. @xref{Writing to an External Program}.
2596 @opindex no-quote-chars, summary
2597 @item --no-quote-chars=@var{string}
2598 Remove characters listed in @var{string} from the list of quoted
2599 characters set by the previous @option{--quote-chars} option
2600 (@pxref{quoting styles}).
2602 @opindex no-recursion, summary
2603 @item --no-recursion
2605 With this option, @command{tar} will not recurse into directories.
2608 @opindex no-same-owner, summary
2609 @item --no-same-owner
2612 When extracting an archive, do not attempt to preserve the owner
2613 specified in the @command{tar} archive. This the default behavior
2616 @opindex no-same-permissions, summary
2617 @item --no-same-permissions
2619 When extracting an archive, subtract the user's umask from files from
2620 the permissions specified in the archive. This is the default behavior
2623 @opindex no-wildcards, summary
2624 @item --no-wildcards
2625 Do not use wildcards.
2626 @xref{controlling pattern-matching}.
2628 @opindex no-wildcards-match-slash, summary
2629 @item --no-wildcards-match-slash
2630 Wildcards do not match @samp{/}.
2631 @xref{controlling pattern-matching}.
2633 @opindex null, summary
2636 When @command{tar} is using the @option{--files-from} option, this option
2637 instructs @command{tar} to expect filenames terminated with @option{NUL}, so
2638 @command{tar} can correctly work with file names that contain newlines.
2641 @opindex numeric-owner, summary
2642 @item --numeric-owner
2644 This option will notify @command{tar} that it should use numeric user
2645 and group IDs when creating a @command{tar} file, rather than names.
2649 When extracting files, this option is a synonym for
2650 @option{--no-same-owner}, i.e. it prevents @command{tar} from
2651 restoring ownership of files being extracted.
2653 When creating an archive, @option{-o} is a synonym for
2654 @option{--old-archive}. This behavior is for compatibility
2655 with previous versions of @GNUTAR{}, and will be
2656 removed in the future releases.
2658 @xref{Changes}, for more information.
2660 @opindex occurrence, summary
2661 @item --occurrence[=@var{number}]
2663 This option can be used in conjunction with one of the subcommands
2664 @option{--delete}, @option{--diff}, @option{--extract} or
2665 @option{--list} when a list of files is given either on the command
2666 line or via @option{-T} option.
2668 This option instructs @command{tar} to process only the @var{number}th
2669 occurrence of each named file. @var{Number} defaults to 1, so
2672 tar -x -f archive.tar --occurrence filename
2676 will extract the first occurrence of @file{filename} from @file{archive.tar}
2677 and will terminate without scanning to the end of the archive.
2679 @opindex old-archive, summary
2681 Synonym for @option{--format=v7}.
2683 @opindex one-file-system, summary
2684 @item --one-file-system
2685 Used when creating an archive. Prevents @command{tar} from recursing into
2686 directories that are on different file systems from the current
2687 directory @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2688 synonym for @option{--one-file-system}. This has changed in version
2689 1.15.91. @xref{Changes}, for more information.}.
2691 @opindex overwrite, summary
2694 Overwrite existing files and directory metadata when extracting files
2695 from an archive. @xref{Overwrite Old Files}.
2697 @opindex overwrite-dir, summary
2698 @item --overwrite-dir
2700 Overwrite the metadata of existing directories when extracting files
2701 from an archive. @xref{Overwrite Old Files}.
2703 @opindex owner, summary
2704 @item --owner=@var{user}
2706 Specifies that @command{tar} should use @var{user} as the owner of members
2707 when creating archives, instead of the user associated with the source
2708 file. @var{user} is first decoded as a user symbolic name, but if
2709 this interpretation fails, it has to be a decimal numeric user ID.
2712 There is no value indicating a missing number, and @samp{0} usually means
2713 @code{root}. Some people like to force @samp{0} as the value to offer in
2714 their distributions for the owner of files, because the @code{root} user is
2715 anonymous anyway, so that might as well be the owner of anonymous archives.
2717 This option does not affect extraction from archives.
2719 @opindex transform, summary
2720 @item --transform=@var{sed-expr}
2722 Transform file or member names using @command{sed} replacement expression
2723 @var{sed-expr}. For example,
2726 $ @kbd{tar cf archive.tar --transform 's,^\./,usr/,' .}
2730 will add to @file{archive} files from the current working directory,
2731 replacing initial @samp{./} prefix with @samp{usr/}. For the detailed
2732 discussion, @xref{transform}.
2734 To see transformed member names in verbose listings, use
2735 @option{--show-transformed-names} option
2736 (@pxref{show-transformed-names}).
2738 @opindex quote-chars, summary
2739 @item --quote-chars=@var{string}
2740 Always quote characters from @var{string}, even if the selected
2741 quoting style would not quote them (@pxref{quoting styles}).
2743 @opindex quoting-style, summary
2744 @item --quoting-style=@var{style}
2745 Set quoting style to use when printing member and file names
2746 (@pxref{quoting styles}). Valid @var{style} values are:
2747 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
2748 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
2749 style is @code{escape}, unless overridden while configuring the
2752 @opindex pax-option, summary
2753 @item --pax-option=@var{keyword-list}
2754 @FIXME{Such a detailed description does not belong there, move it elsewhere.}
2755 This option is meaningful only with @acronym{POSIX.1-2001} archives
2756 (@pxref{posix}). It modifies the way @command{tar} handles the
2757 extended header keywords. @var{Keyword-list} is a comma-separated
2758 list of keyword options, each keyword option taking one of
2759 the following forms:
2762 @item delete=@var{pattern}
2763 When used with one of archive-creation commands,
2764 this option instructs @command{tar} to omit from extended header records
2765 that it produces any keywords matching the string @var{pattern}.
2767 When used in extract or list mode, this option instructs tar
2768 to ignore any keywords matching the given @var{pattern} in the extended
2769 header records. In both cases, matching is performed using the pattern
2770 matching notation described in @acronym{POSIX 1003.2}, 3.13
2771 (See @cite{glob(7)}). For example:
2774 --pax-option delete=security.*
2777 would suppress security-related information.
2779 @item exthdr.name=@var{string}
2781 This keyword allows user control over the name that is written into the
2782 ustar header blocks for the extended headers. The name is obtained
2783 from @var{string} after making the following substitutions:
2785 @multitable @columnfractions .30 .70
2786 @headitem Meta-character @tab Replaced By
2787 @item %d @tab The directory name of the file, equivalent to the
2788 result of the @command{dirname} utility on the translated pathname.
2789 @item %f @tab The filename of the file, equivalent to the result
2790 of the @command{basename} utility on the translated pathname.
2791 @item %p @tab The process ID of the @command{tar} process.
2792 @item %% @tab A @samp{%} character.
2795 Any other @samp{%} characters in @var{string} produce undefined
2798 If no option @samp{exthdr.name=string} is specified, @command{tar}
2799 will use the following default value:
2805 @item globexthdr.name=@var{string}
2806 This keyword allows user control over the name that is written into
2807 the ustar header blocks for global extended header records. The name
2808 is obtained from the contents of @var{string}, after making
2809 the following substitutions:
2811 @multitable @columnfractions .30 .70
2812 @headitem Meta-character @tab Replaced By
2813 @item %n @tab An integer that represents the
2814 sequence number of the global extended header record in the archive,
2816 @item %p @tab The process ID of the @command{tar} process.
2817 @item %% @tab A @samp{%} character.
2820 Any other @samp{%} characters in @var{string} produce undefined results.
2822 If no option @samp{globexthdr.name=string} is specified, @command{tar}
2823 will use the following default value:
2826 $TMPDIR/GlobalHead.%p.%n
2830 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
2831 environment variable. If @var{TMPDIR} is not set, @command{tar}
2834 @item @var{keyword}=@var{value}
2835 When used with one of archive-creation commands, these keyword/value pairs
2836 will be included at the beginning of the archive in a global extended
2837 header record. When used with one of archive-reading commands,
2838 @command{tar} will behave as if it has encountered these keyword/value
2839 pairs at the beginning of the archive in a global extended header
2842 @item @var{keyword}:=@var{value}
2843 When used with one of archive-creation commands, these keyword/value pairs
2844 will be included as records at the beginning of an extended header for
2845 each file. This is effectively equivalent to @var{keyword}=@var{value}
2846 form except that it creates no global extended header records.
2848 When used with one of archive-reading commands, @command{tar} will
2849 behave as if these keyword/value pairs were included as records at the
2850 end of each extended header; thus, they will override any global or
2851 file-specific extended header record keywords of the same names.
2852 For example, in the command:
2855 tar --format=posix --create \
2856 --file archive --pax-option gname:=user .
2859 the group name will be forced to a new value for all files
2860 stored in the archive.
2863 @opindex portability, summary
2865 @itemx --old-archive
2866 Synonym for @option{--format=v7}.
2868 @opindex posix, summary
2870 Same as @option{--format=posix}.
2872 @opindex preserve, summary
2875 Synonymous with specifying both @option{--preserve-permissions} and
2876 @option{--same-order}. @xref{Setting Access Permissions}.
2878 @opindex preserve-order, summary
2879 @item --preserve-order
2881 (See @option{--same-order}; @pxref{Reading}.)
2883 @opindex preserve-permissions, summary
2884 @opindex same-permissions, summary
2885 @item --preserve-permissions
2886 @itemx --same-permissions
2889 When @command{tar} is extracting an archive, it normally subtracts the
2890 users' umask from the permissions specified in the archive and uses
2891 that number as the permissions to create the destination file.
2892 Specifying this option instructs @command{tar} that it should use the
2893 permissions directly from the archive. @xref{Setting Access Permissions}.
2895 @opindex read-full-records, summary
2896 @item --read-full-records
2899 Specifies that @command{tar} should reblock its input, for reading
2900 from pipes on systems with buggy implementations. @xref{Reading}.
2902 @opindex record-size, summary
2903 @item --record-size=@var{size}
2905 Instructs @command{tar} to use @var{size} bytes per record when accessing the
2906 archive. @xref{Blocking Factor}.
2908 @opindex recursion, summary
2911 With this option, @command{tar} recurses into directories.
2914 @opindex recursive-unlink, summary
2915 @item --recursive-unlink
2918 directory hierarchies before extracting directories of the same name
2919 from the archive. @xref{Recursive Unlink}.
2921 @opindex remove-files, summary
2922 @item --remove-files
2924 Directs @command{tar} to remove the source file from the file system after
2925 appending it to an archive. @xref{remove files}.
2927 @opindex restrict, summary
2930 Disable use of some potentially harmful @command{tar} options.
2931 Currently this option disables shell invocaton from multi-volume menu
2932 (@pxref{Using Multiple Tapes}).
2934 @opindex rmt-command, summary
2935 @item --rmt-command=@var{cmd}
2937 Notifies @command{tar} that it should use @var{cmd} instead of
2938 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
2940 @opindex rsh-command, summary
2941 @item --rsh-command=@var{cmd}
2943 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
2944 devices. @xref{Device}.
2946 @opindex same-order, summary
2948 @itemx --preserve-order
2951 This option is an optimization for @command{tar} when running on machines with
2952 small amounts of memory. It informs @command{tar} that the list of file
2953 arguments has already been sorted to match the order of files in the
2954 archive. @xref{Reading}.
2956 @opindex same-owner, summary
2959 When extracting an archive, @command{tar} will attempt to preserve the owner
2960 specified in the @command{tar} archive with this option present.
2961 This is the default behavior for the superuser; this option has an
2962 effect only for ordinary users. @xref{Attributes}.
2964 @opindex same-permissions, summary
2965 @item --same-permissions
2967 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
2969 @opindex show-defaults, summary
2970 @item --show-defaults
2972 Displays the default options used by @command{tar} and exits
2973 successfully. This option is intended for use in shell scripts.
2974 Here is an example of what you can see using this option:
2977 $ tar --show-defaults
2978 --format=gnu -f- -b20 --quoting-style=escape \
2979 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
2982 @opindex show-omitted-dirs, summary
2983 @item --show-omitted-dirs
2985 Instructs @command{tar} to mention directories its skipping over when
2986 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
2988 @opindex show-transformed-names, summary
2989 @opindex show-stored-names, summary
2990 @item --show-transformed-names
2991 @itemx --show-stored-names
2993 Display file or member names after applying any transformations
2994 (@pxref{transform}). In particular, when used in conjunction with one of
2995 archive creation operations it instructs tar to list the member names
2996 stored in the archive, as opposed to the actual file
2997 names. @xref{listing member and file names}.
2999 @opindex sparse, summary
3003 Invokes a @acronym{GNU} extension when adding files to an archive that handles
3004 sparse files efficiently. @xref{sparse}.
3006 @opindex starting-file, summary
3007 @item --starting-file=@var{name}
3008 @itemx -K @var{name}
3010 This option affects extraction only; @command{tar} will skip extracting
3011 files in the archive until it finds one that matches @var{name}.
3014 @opindex strip-components, summary
3015 @item --strip-components=@var{number}
3016 Strip given @var{number} of leading components from file names before
3017 extraction.@footnote{This option was called @option{--strip-path} in
3018 version 1.14.} For example, if archive @file{archive.tar} contained
3019 @file{/some/file/name}, then running
3022 tar --extract --file archive.tar --strip-components=2
3026 would extract this file to file @file{name}.
3028 @opindex suffix, summary
3029 @item --suffix=@var{suffix}
3031 Alters the suffix @command{tar} uses when backing up files from the default
3032 @samp{~}. @xref{backup}.
3034 @opindex tape-length, summary
3035 @item --tape-length=@var{num}
3038 Specifies the length of tapes that @command{tar} is writing as being
3039 @w{@var{num} x 1024} bytes long. @xref{Using Multiple Tapes}.
3041 @opindex test-label, summary
3044 Reads the volume label. If an argument is specified, test whether it
3045 matches the volume label. @xref{--test-label option}.
3047 @opindex to-command, summary
3048 @item --to-command=@var{command}
3050 During extraction @command{tar} will pipe extracted files to the
3051 standard input of @var{command}. @xref{Writing to an External Program}.
3053 @opindex to-stdout, summary
3057 During extraction, @command{tar} will extract files to stdout rather
3058 than to the file system. @xref{Writing to Standard Output}.
3060 @opindex totals, summary
3063 Displays the total number of bytes written after creating an archive.
3066 @opindex touch, summary
3070 Sets the data modification time of extracted files to the extraction time,
3071 rather than the data modification time stored in the archive.
3072 @xref{Data Modification Times}.
3074 @opindex uncompress, summary
3077 (See @option{--compress}. @pxref{gzip})
3079 @opindex ungzip, summary
3082 (See @option{--gzip}. @pxref{gzip})
3084 @opindex unlink-first, summary
3085 @item --unlink-first
3088 Directs @command{tar} to remove the corresponding file from the file
3089 system before extracting it from the archive. @xref{Unlink First}.
3091 @opindex use-compress-program, summary
3092 @item --use-compress-program=@var{prog}
3094 Instructs @command{tar} to access the archive through @var{prog}, which is
3095 presumed to be a compression program of some sort. @xref{gzip}.
3097 @opindex utc, summary
3100 Display file modification dates in @acronym{UTC}. This option implies
3103 @opindex verbose, summary
3107 Specifies that @command{tar} should be more verbose about the operations its
3108 performing. This option can be specified multiple times for some
3109 operations to increase the amount of information displayed.
3112 @opindex verify, summary
3116 Verifies that the archive was correctly written when creating an
3117 archive. @xref{verify}.
3119 @opindex version, summary
3122 Print information about the program's name, version, origin and legal
3123 status, all on standard output, and then exit successfully.
3126 @opindex volno-file, summary
3127 @item --volno-file=@var{file}
3129 Used in conjunction with @option{--multi-volume}. @command{tar} will keep track
3130 of which volume of a multi-volume archive its working in @var{file}.
3133 @opindex wildcards, summary
3135 Use wildcards when matching member names with patterns.
3136 @xref{controlling pattern-matching}.
3138 @opindex wildcards-match-slash, summary
3139 @item --wildcards-match-slash
3140 Wildcards match @samp{/}.
3141 @xref{controlling pattern-matching}.
3144 @node Short Option Summary
3145 @subsection Short Options Cross Reference
3147 Here is an alphabetized list of all of the short option forms, matching
3148 them with the equivalent long option.
3154 @option{--concatenate}
3158 @option{--read-full-records}
3162 @option{--directory}
3166 @option{--info-script}
3170 @option{--incremental}
3174 @option{--starting-file}
3178 @option{--tape-length}
3182 @option{--multi-volume}
3190 @option{--to-stdout}
3194 @option{--absolute-names}
3198 @option{--block-number}
3206 @option{--files-from}
3210 @option{--unlink-first}
3222 @option{--exclude-from}
3230 @option{--blocking-factor}
3246 @option{--listed-incremental}
3250 @option{--dereference}
3254 @option{--ignore-zeros}
3262 @option{--keep-old-files}
3266 @option{--one-file-system}. Use of this short option is deprecated. It
3267 is retained for compatibility with the earlier versions of GNU
3268 @command{tar}, and will be changed in future releases.
3270 @xref{Changes}, for more information.
3278 When creating --- @option{--no-same-owner}, when extracting ---
3279 @option{--portability}.
3281 The later usage is deprecated. It is retained for compatibility with
3282 the earlier versions of @GNUTAR{}. In the future releases
3283 @option{-o} will be equivalent to @option{--no-same-owner} only.
3287 @option{--preserve-permissions}
3295 @option{--same-order}
3311 @option{--interactive}
3324 @section @GNUTAR{} documentation
3326 @cindex Getting program version number
3328 @cindex Version of the @command{tar} program
3329 Being careful, the first thing is really checking that you are using
3330 @GNUTAR{}, indeed. The @option{--version} option
3331 causes @command{tar} to print information about its name, version,
3332 origin and legal status, all on standard output, and then exit
3333 successfully. For example, @w{@samp{tar --version}} might print:
3336 tar (GNU tar) 1.15.2
3337 Copyright (C) 2006 Free Software Foundation, Inc.
3338 This is free software. You may redistribute copies of it under the terms of
3339 the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
3340 There is NO WARRANTY, to the extent permitted by law.
3342 Written by John Gilmore and Jay Fenlason.
3346 The first occurrence of @samp{tar} in the result above is the program
3347 name in the package (for example, @command{rmt} is another program),
3348 while the second occurrence of @samp{tar} is the name of the package
3349 itself, containing possibly many programs. The package is currently
3350 named @samp{tar}, after the name of the main program it
3351 contains@footnote{There are plans to merge the @command{cpio} and
3352 @command{tar} packages into a single one which would be called
3353 @code{paxutils}. So, who knows if, one of this days, the
3354 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3357 @cindex Obtaining help
3358 @cindex Listing all @command{tar} options
3359 @opindex help, introduction
3360 Another thing you might want to do is checking the spelling or meaning
3361 of some particular @command{tar} option, without resorting to this
3362 manual, for once you have carefully read it. @GNUTAR{}
3363 has a short help feature, triggerable through the
3364 @option{--help} option. By using this option, @command{tar} will
3365 print a usage message listing all available options on standard
3366 output, then exit successfully, without doing anything else and
3367 ignoring all other options. Even if this is only a brief summary, it
3368 may be several screens long. So, if you are not using some kind of
3369 scrollable window, you might prefer to use something like:
3372 $ @kbd{tar --help | less}
3376 presuming, here, that you like using @command{less} for a pager. Other
3377 popular pagers are @command{more} and @command{pg}. If you know about some
3378 @var{keyword} which interests you and do not want to read all the
3379 @option{--help} output, another common idiom is doing:
3382 tar --help | grep @var{keyword}
3386 for getting only the pertinent lines. Notice, however, that some
3387 @command{tar} options have long description lines and the above
3388 command will list only the first of them.
3390 The exact look of the option summary displayed by @kbd{tar --help} is
3391 configurable. @xref{Configuring Help Summary}, for a detailed description.
3394 If you only wish to check the spelling of an option, running @kbd{tar
3395 --usage} may be a better choice. This will display a terse list of
3396 @command{tar} option without accompanying explanations.
3398 The short help output is quite succinct, and you might have to get
3399 back to the full documentation for precise points. If you are reading
3400 this paragraph, you already have the @command{tar} manual in some
3401 form. This manual is available in a variety of forms from
3402 @url{http://www.gnu.org/software/tar/manual}. It may printed out of the @GNUTAR{}
3403 distribution, provided you have @TeX{} already installed somewhere,
3404 and a laser printer around. Just configure the distribution, execute
3405 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3406 usual way (contact your local guru to know how). If @GNUTAR{}
3407 has been conveniently installed at your place, this
3408 manual is also available in interactive, hypertextual form as an Info
3409 file. Just call @w{@samp{info tar}} or, if you do not have the
3410 @command{info} program handy, use the Info reader provided within
3411 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3413 There is currently no @code{man} page for @GNUTAR{}.
3414 If you observe such a @code{man} page on the system you are running,
3415 either it does not belong to @GNUTAR{}, or it has not
3416 been produced by @acronym{GNU}. Some package maintainers convert
3417 @kbd{tar --help} output to a man page, using @command{help2man}. In
3418 any case, please bear in mind that the authoritative source of
3419 information about @GNUTAR{} is this Texinfo documentation.
3422 @section Obtaining @GNUTAR{} default values
3424 @opindex show-defaults
3425 @GNUTAR{} has some predefined defaults that are used when you do not
3426 explicitely specify another values. To obtain a list of such
3427 defaults, use @option{--show-defaults} option. This will output the
3428 values in the form of @command{tar} command line options:
3432 @kbd{tar --show-defaults}
3433 --format=gnu -f- -b20 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3438 The above output shows that this version of @GNUTAR{} defaults to
3439 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3440 output as the archive, if no @option{--file} option has been given
3441 (@pxref{file tutorial}), the default blocking factor is 20
3442 (@pxref{Blocking Factor}). It also shows the default locations where
3443 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3446 @section Checking @command{tar} progress
3448 Typically, @command{tar} performs most operations without reporting any
3449 information to the user except error messages. When using @command{tar}
3450 with many options, particularly ones with complicated or
3451 difficult-to-predict behavior, it is possible to make serious mistakes.
3452 @command{tar} provides several options that make observing @command{tar}
3453 easier. These options cause @command{tar} to print information as it
3454 progresses in its job, and you might want to use them just for being
3455 more careful about what is going on, or merely for entertaining
3456 yourself. If you have encountered a problem when operating on an
3457 archive, however, you may need more information than just an error
3458 message in order to solve the problem. The following options can be
3459 helpful diagnostic tools.
3461 @cindex Verbose operation
3463 Normally, the @option{--list} (@option{-t}) command to list an archive
3464 prints just the file names (one per line) and the other commands are
3465 silent. When used with most operations, the @option{--verbose}
3466 (@option{-v}) option causes @command{tar} to print the name of each
3467 file or archive member as it is processed. This and the other options
3468 which make @command{tar} print status information can be useful in
3469 monitoring @command{tar}.
3471 With @option{--create} or @option{--extract}, @option{--verbose} used
3472 once just prints the names of the files or members as they are processed.
3473 Using it twice causes @command{tar} to print a longer listing
3474 (reminiscent of @samp{ls -l}) for each member. Since @option{--list}
3475 already prints the names of the members, @option{--verbose} used once
3476 with @option{--list} causes @command{tar} to print an @samp{ls -l}
3477 type listing of the files in the archive. The following examples both
3478 extract members with long list output:
3481 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3482 $ @kbd{tar xvvf archive.tar}
3485 Verbose output appears on the standard output except when an archive is
3486 being written to the standard output, as with @samp{tar --create
3487 --file=- --verbose} (@samp{tar cfv -}, or even @samp{tar cv}---if the
3488 installer let standard output be the default archive). In that case
3489 @command{tar} writes verbose output to the standard error stream.
3491 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3492 verbose output to @var{file} rather than to standard output or standard
3495 @cindex Obtaining total status information
3497 The @option{--totals} option---which is only meaningful when used with
3498 @option{--create} (@option{-c})---causes @command{tar} to print the total
3499 amount written to the archive, after it has been fully created.
3501 @anchor{Progress information}
3502 @cindex Progress information
3504 The @option{--checkpoint} option prints an occasional message
3505 as @command{tar} reads or writes the archive. It is designed for
3506 those who don't need the more detailed (and voluminous) output of
3507 @option{--block-number} (@option{-R}), but do want visual confirmation
3508 that @command{tar} is actually making forward progress. By default it
3509 prints a message each 10 records read or written. This can be changed
3510 by giving it a numeric argument after an equal sign:
3513 $ @kbd{tar -c --checkpoint=1000} /var
3514 tar: Write checkpoint 1000
3515 tar: Write checkpoint 2000
3516 tar: Write checkpoint 3000
3519 This example shows the default checkpoint message used by
3520 @command{tar}. If you place a dot immediately after the equal
3521 sign, it will print a @samp{.} at each checkpoint. For example:
3524 $ @kbd{tar -c --checkpoint=.1000} /var
3528 @opindex show-omitted-dirs
3529 @anchor{show-omitted-dirs}
3530 The @option{--show-omitted-dirs} option, when reading an archive---with
3531 @option{--list} or @option{--extract}, for example---causes a message
3532 to be printed for each directory in the archive which is skipped.
3533 This happens regardless of the reason for skipping: the directory might
3534 not have been named on the command line (implicitly or explicitly),
3535 it might be excluded by the use of the
3536 @option{--exclude=@var{pattern}} option, or some other reason.
3538 @opindex block-number
3539 @cindex Block number where error occurred
3540 @anchor{block-number}
3541 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3542 every message it would normally produce, the block number within the
3543 archive where the message was triggered. Also, supplementary messages
3544 are triggered when reading blocks full of NULs, or when hitting end of
3545 file on the archive. As of now, if the archive if properly terminated
3546 with a NUL block, the reading of the file may stop before end of file
3547 is met, so the position of end of file will not usually show when
3548 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3549 drains the archive before exiting when reading the
3550 archive from a pipe.
3552 @cindex Error message, block number of
3553 This option is especially useful when reading damaged archives, since
3554 it helps pinpoint the damaged sections. It can also be used with
3555 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3556 choose among several backup tapes when retrieving a file later, in
3557 favor of the tape where the file appears earliest (closest to the
3558 front of the tape). @xref{backup}.
3561 @section Asking for Confirmation During Operations
3562 @cindex Interactive operation
3564 Typically, @command{tar} carries out a command without stopping for
3565 further instructions. In some situations however, you may want to
3566 exclude some files and archive members from the operation (for instance
3567 if disk or storage space is tight). You can do this by excluding
3568 certain files automatically (@pxref{Choosing}), or by performing
3569 an operation interactively, using the @option{--interactive} (@option{-w}) option.
3570 @command{tar} also accepts @option{--confirmation} for this option.
3572 @opindex interactive
3573 When the @option{--interactive} (@option{-w}) option is specified, before
3574 reading, writing, or deleting files, @command{tar} first prints a message
3575 for each such file, telling what operation it intends to take, then asks
3576 for confirmation on the terminal. The actions which require
3577 confirmation include adding a file to the archive, extracting a file
3578 from the archive, deleting a file from the archive, and deleting a file
3579 from disk. To confirm the action, you must type a line of input
3580 beginning with @samp{y}. If your input line begins with anything other
3581 than @samp{y}, @command{tar} skips that file.
3583 If @command{tar} is reading the archive from the standard input,
3584 @command{tar} opens the file @file{/dev/tty} to support the interactive
3587 Verbose output is normally sent to standard output, separate from
3588 other error messages. However, if the archive is produced directly
3589 on standard output, then verbose output is mixed with errors on
3590 @code{stderr}. Producing the archive on standard output may be used
3591 as a way to avoid using disk space, when the archive is soon to be
3592 consumed by another process reading it, say. Some people felt the need
3593 of producing an archive on stdout, still willing to segregate between
3594 verbose output and error output. A possible approach would be using a
3595 named pipe to receive the archive, and having the consumer process to
3596 read from that named pipe. This has the advantage of letting standard
3597 output free to receive verbose output, all separate from errors.
3600 @chapter @GNUTAR{} Operations
3613 @section Basic @GNUTAR{} Operations
3615 The basic @command{tar} operations, @option{--create} (@option{-c}),
3616 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
3617 @option{-x}), are currently presented and described in the tutorial
3618 chapter of this manual. This section provides some complementary notes
3619 for these operations.
3622 @opindex create, complementary notes
3626 Creating an empty archive would have some kind of elegance. One can
3627 initialize an empty archive and later use @option{--append}
3628 (@option{-r}) for adding all members. Some applications would not
3629 welcome making an exception in the way of adding the first archive
3630 member. On the other hand, many people reported that it is
3631 dangerously too easy for @command{tar} to destroy a magnetic tape with
3632 an empty archive@footnote{This is well described in @cite{Unix-haters
3633 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
3634 Books, ISBN 1-56884-203-1.}. The two most common errors are:
3638 Mistakingly using @code{create} instead of @code{extract}, when the
3639 intent was to extract the full contents of an archive. This error
3640 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
3641 the QWERTY keyboard. Instead of being unpacked, the archive then
3642 gets wholly destroyed. When users speak about @dfn{exploding} an
3643 archive, they usually mean something else :-).
3646 Forgetting the argument to @code{file}, when the intent was to create
3647 an archive with a single file in it. This error is likely because a
3648 tired user can easily add the @kbd{f} key to the cluster of option
3649 letters, by the mere force of habit, without realizing the full
3650 consequence of doing so. The usual consequence is that the single
3651 file, which was meant to be saved, is rather destroyed.
3654 So, recognizing the likelihood and the catastrophical nature of these
3655 errors, @GNUTAR{} now takes some distance from elegance, and
3656 cowardly refuses to create an archive when @option{--create} option is
3657 given, there are no arguments besides options, and
3658 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
3659 around the cautiousness of @GNUTAR{} and nevertheless create an
3660 archive with nothing in it, one may still use, as the value for the
3661 @option{--files-from} option, a file with no names in it, as shown in
3662 the following commands:
3665 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
3666 @kbd{tar cfT empty-archive.tar /dev/null}
3669 @opindex extract, complementary notes
3674 A socket is stored, within a @GNUTAR{} archive, as a pipe.
3676 @item @option{--list} (@option{-t})
3678 @GNUTAR{} now shows dates as @samp{1996-08-30},
3679 while it used to show them as @samp{Aug 30 1996}. Preferably,
3680 people should get used to ISO 8601 dates. Local American dates should
3681 be made available again with full date localization support, once
3682 ready. In the meantime, programs not being localizable for dates
3683 should prefer international dates, that's really the way to go.
3685 Look up @url{http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html} if you
3686 are curious, it contains a detailed explanation of the ISO 8601 standard.
3691 @section Advanced @GNUTAR{} Operations
3693 Now that you have learned the basics of using @GNUTAR{}, you may want
3694 to learn about further ways in which @command{tar} can help you.
3696 This chapter presents five, more advanced operations which you probably
3697 won't use on a daily basis, but which serve more specialized functions.
3698 We also explain the different styles of options and why you might want
3699 to use one or another, or a combination of them in your @command{tar}
3700 commands. Additionally, this chapter includes options which allow you to
3701 define the output from @command{tar} more carefully, and provide help and
3702 error correction in special circumstances.
3704 @FIXME{check this after the chapter is actually revised to make sure
3705 it still introduces the info in the chapter correctly : ).}
3717 @subsection The Five Advanced @command{tar} Operations
3720 In the last chapter, you learned about the first three operations to
3721 @command{tar}. This chapter presents the remaining five operations to
3722 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
3723 @option{--delete}, and @option{--compare}.
3725 You are not likely to use these operations as frequently as those
3726 covered in the last chapter; however, since they perform specialized
3727 functions, they are quite useful when you do need to use them. We
3728 will give examples using the same directory and files that you created
3729 in the last chapter. As you may recall, the directory is called
3730 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
3731 @samp{rock}, and the two archive files you created are
3732 @samp{collection.tar} and @samp{music.tar}.
3734 We will also use the archive files @samp{afiles.tar} and
3735 @samp{bfiles.tar}. @samp{afiles.tar} contains the members @samp{apple},
3736 @samp{angst}, and @samp{aspic}. @samp{bfiles.tar} contains the members
3737 @samp{./birds}, @samp{baboon}, and @samp{./box}.
3739 Unless we state otherwise, all practicing you do and examples you follow
3740 in this chapter will take place in the @file{practice} directory that
3741 you created in the previous chapter; see @ref{prepare for examples}.
3742 (Below in this section, we will remind you of the state of the examples
3743 where the last chapter left them.)
3745 The five operations that we will cover in this chapter are:
3750 Add new entries to an archive that already exists.
3753 Add more recent copies of archive members to the end of an archive, if
3758 Add one or more pre-existing archives to the end of another archive.
3760 Delete items from an archive (does not work on tapes).
3764 Compare archive members to their counterparts in the file system.
3768 @subsection How to Add Files to Existing Archives: @option{--append}
3772 If you want to add files to an existing archive, you don't need to
3773 create a new archive; you can use @option{--append} (@option{-r}).
3774 The archive must already exist in order to use @option{--append}. (A
3775 related operation is the @option{--update} operation; you can use this
3776 to add newer versions of archive members to an existing archive. To learn how to
3777 do this with @option{--update}, @pxref{update}.)
3779 If you use @option{--append} to add a file that has the same name as an
3780 archive member to an archive containing that archive member, then the
3781 old member is not deleted. What does happen, however, is somewhat
3782 complex. @command{tar} @emph{allows} you to have infinite number of files
3783 with the same name. Some operations treat these same-named members no
3784 differently than any other set of archive members: for example, if you
3785 view an archive with @option{--list} (@option{-t}), you will see all
3786 of those members listed, with their data modification times, owners, etc.
3788 Other operations don't deal with these members as perfectly as you might
3789 prefer; if you were to use @option{--extract} to extract the archive,
3790 only the most recently added copy of a member with the same name as four
3791 other members would end up in the working directory. This is because
3792 @option{--extract} extracts an archive in the order the members appeared
3793 in the archive; the most recently archived members will be extracted
3794 last. Additionally, an extracted member will @emph{replace} a file of
3795 the same name which existed in the directory already, and @command{tar}
3796 will not prompt you about this@footnote{Unless you give it
3797 @option{--keep-old-files} option, or the disk copy is newer than the
3798 the one in the archive and you invoke @command{tar} with
3799 @option{--keep-newer-files} option}. Thus, only the most recently archived
3800 member will end up being extracted, as it will replace the one
3801 extracted before it, and so on.
3803 There exists a special option that allows you to get around this
3804 behavior and extract (or list) only a particular copy of the file.
3805 This is @option{--occurrence} option. If you run @command{tar} with
3806 this option, it will extract only the first copy of the file. You
3807 may also give this option an argument specifying the number of
3808 copy to be extracted. Thus, for example if the archive
3809 @file{archive.tar} contained three copies of file @file{myfile}, then
3813 tar --extract --file archive.tar --occurrence=2 myfile
3817 would extract only the second copy. @xref{Option
3818 Summary,---occurrence}, for the description of @option{--occurrence}
3821 @FIXME{ hag -- you might want to incorporate some of the above into the
3822 MMwtSN node; not sure. i didn't know how to make it simpler...
3824 There are a few ways to get around this. (maybe xref Multiple Members
3825 with the Same Name.}
3827 @cindex Members, replacing with other members
3828 @cindex Replacing members with other members
3829 If you want to replace an archive member, use @option{--delete} to
3830 delete the member you want to remove from the archive, , and then use
3831 @option{--append} to add the member you want to be in the archive. Note
3832 that you can not change the order of the archive; the most recently
3833 added member will still appear last. In this sense, you cannot truly
3834 ``replace'' one member with another. (Replacing one member with another
3835 will not work on certain types of media, such as tapes; see @ref{delete}
3836 and @ref{Media}, for more information.)
3839 * appending files:: Appending Files to an Archive
3843 @node appending files
3844 @subsubsection Appending Files to an Archive
3846 @cindex Adding files to an Archive
3847 @cindex Appending files to an Archive
3848 @cindex Archives, Appending files to
3850 The simplest way to add a file to an already existing archive is the
3851 @option{--append} (@option{-r}) operation, which writes specified
3852 files into the archive whether or not they are already among the
3855 When you use @option{--append}, you @emph{must} specify file name
3856 arguments, as there is no default. If you specify a file that already
3857 exists in the archive, another copy of the file will be added to the
3858 end of the archive. As with other operations, the member names of the
3859 newly added files will be exactly the same as their names given on the
3860 command line. The @option{--verbose} (@option{-v}) option will print
3861 out the names of the files as they are written into the archive.
3863 @option{--append} cannot be performed on some tape drives, unfortunately,
3864 due to deficiencies in the formats those tape drives use. The archive
3865 must be a valid @command{tar} archive, or else the results of using this
3866 operation will be unpredictable. @xref{Media}.
3868 To demonstrate using @option{--append} to add a file to an archive,
3869 create a file called @file{rock} in the @file{practice} directory.
3870 Make sure you are in the @file{practice} directory. Then, run the
3871 following @command{tar} command to add @file{rock} to
3872 @file{collection.tar}:
3875 $ @kbd{tar --append --file=collection.tar rock}
3879 If you now use the @option{--list} (@option{-t}) operation, you will see that
3880 @file{rock} has been added to the archive:
3883 $ @kbd{tar --list --file=collection.tar}
3884 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
3885 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3886 -rw-r--r-- me user 20 1996-09-23 16:44 folk
3887 -rw-r--r-- me user 20 1996-09-23 16:44 rock
3891 @subsubsection Multiple Files with the Same Name
3893 You can use @option{--append} (@option{-r}) to add copies of files
3894 which have been updated since the archive was created. (However, we
3895 do not recommend doing this since there is another @command{tar}
3896 option called @option{--update}; @xref{update}, for more information.
3897 We describe this use of @option{--append} here for the sake of
3898 completeness.) When you extract the archive, the older version will
3899 be effectively lost. This works because files are extracted from an
3900 archive in the order in which they were archived. Thus, when the
3901 archive is extracted, a file archived later in time will replace a
3902 file of the same name which was archived earlier, even though the
3903 older version of the file will remain in the archive unless you delete
3904 all versions of the file.
3906 Supposing you change the file @file{blues} and then append the changed
3907 version to @file{collection.tar}. As you saw above, the original
3908 @file{blues} is in the archive @file{collection.tar}. If you change the
3909 file and append the new version of the file to the archive, there will
3910 be two copies in the archive. When you extract the archive, the older
3911 version of the file will be extracted first, and then replaced by the
3912 newer version when it is extracted.
3914 You can append the new, changed copy of the file @file{blues} to the
3915 archive in this way:
3918 $ @kbd{tar --append --verbose --file=collection.tar blues}
3923 Because you specified the @option{--verbose} option, @command{tar} has
3924 printed the name of the file being appended as it was acted on. Now
3925 list the contents of the archive:
3928 $ @kbd{tar --list --verbose --file=collection.tar}
3929 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
3930 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3931 -rw-r--r-- me user 20 1996-09-23 16:44 folk
3932 -rw-r--r-- me user 20 1996-09-23 16:44 rock
3933 -rw-r--r-- me user 58 1996-10-24 18:30 blues
3937 The newest version of @file{blues} is now at the end of the archive
3938 (note the different creation dates and file sizes). If you extract
3939 the archive, the older version of the file @file{blues} will be
3940 replaced by the newer version. You can confirm this by extracting
3941 the archive and running @samp{ls} on the directory.
3943 If you wish to extract the first occurrence of the file @file{blues}
3944 from the archive, use @option{--occurrence} option, as shown in
3945 the following example:
3948 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
3949 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3952 @xref{Writing}, for more information on @option{--extract} and
3953 @xref{Option Summary, --occurrence}, for the description of
3954 @option{--occurrence} option.
3957 @subsection Updating an Archive
3959 @cindex Updating an archive
3962 In the previous section, you learned how to use @option{--append} to
3963 add a file to an existing archive. A related operation is
3964 @option{--update} (@option{-u}). The @option{--update} operation
3965 updates a @command{tar} archive by comparing the date of the specified
3966 archive members against the date of the file with the same name. If
3967 the file has been modified more recently than the archive member, then
3968 the newer version of the file is added to the archive (as with
3971 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
3972 The operation will fail.
3974 @FIXME{other examples of media on which --update will fail? need to ask
3975 charles and/or mib/thomas/dave shevett..}
3977 Both @option{--update} and @option{--append} work by adding to the end
3978 of the archive. When you extract a file from the archive, only the
3979 version stored last will wind up in the file system, unless you use
3980 the @option{--backup} option. @FIXME-ref{Multiple Members with the
3988 @subsubsection How to Update an Archive Using @option{--update}
3990 You must use file name arguments with the @option{--update}
3991 (@option{-u}) operation. If you don't specify any files,
3992 @command{tar} won't act on any files and won't tell you that it didn't
3993 do anything (which may end up confusing you).
3995 @c note: the above parenthetical added because in fact, this
3996 @c behavior just confused the author. :-)
3998 To see the @option{--update} option at work, create a new file,
3999 @file{classical}, in your practice directory, and some extra text to the
4000 file @file{blues}, using any text editor. Then invoke @command{tar} with
4001 the @samp{update} operation and the @option{--verbose} (@option{-v})
4002 option specified, using the names of all the files in the practice
4003 directory as file name arguments:
4006 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
4013 Because we have specified verbose mode, @command{tar} prints out the names
4014 of the files it is working on, which in this case are the names of the
4015 files that needed to be updated. If you run @samp{tar --list} and look
4016 at the archive, you will see @file{blues} and @file{classical} at its
4017 end. There will be a total of two versions of the member @samp{blues};
4018 the one at the end will be newer and larger, since you added text before
4021 (The reason @command{tar} does not overwrite the older file when updating
4022 it is because writing to the middle of a section of tape is a difficult
4023 process. Tapes are not designed to go backward. @xref{Media}, for more
4024 information about tapes.
4026 @option{--update} (@option{-u}) is not suitable for performing backups for two
4027 reasons: it does not change directory content entries, and it
4028 lengthens the archive every time it is used. The @GNUTAR{}
4029 options intended specifically for backups are more
4030 efficient. If you need to run backups, please consult @ref{Backups}.
4033 @subsection Combining Archives with @option{--concatenate}
4035 @cindex Adding archives to an archive
4036 @cindex Concatenating Archives
4037 @opindex concatenate
4039 @c @cindex @option{-A} described
4040 Sometimes it may be convenient to add a second archive onto the end of
4041 an archive rather than adding individual files to the archive. To add
4042 one or more archives to the end of another archive, you should use the
4043 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4045 To use @option{--concatenate}, give the first archive with
4046 @option{--file} option and name the rest of archives to be
4047 concatenated on the command line. The members, and their member
4048 names, will be copied verbatim from those archives to the first one.
4049 @FIXME-ref{This can cause multiple members to have the same name, for
4050 information on how this affects reading the archive, Multiple
4051 Members with the Same Name.}
4052 The new, concatenated archive will be called by the same name as the
4053 one given with the @option{--file} option. As usual, if you omit
4054 @option{--file}, @command{tar} will use the value of the environment
4055 variable @env{TAPE}, or, if this has not been set, the default archive name.
4057 @FIXME{There is no way to specify a new name...}
4059 To demonstrate how @option{--concatenate} works, create two small archives
4060 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4061 files from @file{practice}:
4064 $ @kbd{tar -cvf bluesrock.tar blues rock}
4067 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4073 If you like, You can run @samp{tar --list} to make sure the archives
4074 contain what they are supposed to:
4077 $ @kbd{tar -tvf bluesrock.tar}
4078 -rw-r--r-- melissa user 105 1997-01-21 19:42 blues
4079 -rw-r--r-- melissa user 33 1997-01-20 15:34 rock
4080 $ @kbd{tar -tvf jazzfolk.tar}
4081 -rw-r--r-- melissa user 20 1996-09-23 16:44 folk
4082 -rw-r--r-- melissa user 65 1997-01-30 14:15 jazz
4085 We can concatenate these two archives with @command{tar}:
4089 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4092 If you now list the contents of the @file{bluesrock.tar}, you will see
4093 that now it also contains the archive members of @file{jazzfolk.tar}:
4096 $ @kbd{tar --list --file=bluesrock.tar}
4103 When you use @option{--concatenate}, the source and target archives must
4104 already exist and must have been created using compatible format
4105 parameters. Notice, that @command{tar} does not check whether the
4106 archives it concatenates have compatible formats, it does not
4107 even check if the files are really tar archives.
4109 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4110 tape drives, due to deficiencies in the formats those tape drives use.
4112 @cindex @code{concatenate} vs @command{cat}
4113 @cindex @command{cat} vs @code{concatenate}
4114 It may seem more intuitive to you to want or try to use @command{cat} to
4115 concatenate two archives instead of using the @option{--concatenate}
4116 operation; after all, @command{cat} is the utility for combining files.
4118 However, @command{tar} archives incorporate an end-of-file marker which
4119 must be removed if the concatenated archives are to be read properly as
4120 one archive. @option{--concatenate} removes the end-of-archive marker
4121 from the target archive before each new archive is appended. If you use
4122 @command{cat} to combine the archives, the result will not be a valid
4123 @command{tar} format archive. If you need to retrieve files from an
4124 archive that was added to using the @command{cat} utility, use the
4125 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4126 information on dealing with archives improperly combined using the
4127 @command{cat} shell utility.
4130 @subsection Removing Archive Members Using @option{--delete}
4132 @cindex Deleting files from an archive
4133 @cindex Removing files from an archive
4136 You can remove members from an archive by using the @option{--delete}
4137 option. Specify the name of the archive with @option{--file}
4138 (@option{-f}) and then specify the names of the members to be deleted;
4139 if you list no member names, nothing will be deleted. The
4140 @option{--verbose} option will cause @command{tar} to print the names
4141 of the members as they are deleted. As with @option{--extract}, you
4142 must give the exact member names when using @samp{tar --delete}.
4143 @option{--delete} will remove all versions of the named file from the
4144 archive. The @option{--delete} operation can run very slowly.
4146 Unlike other operations, @option{--delete} has no short form.
4148 @cindex Tapes, using @option{--delete} and
4149 @cindex Deleting from tape archives
4150 This operation will rewrite the archive. You can only use
4151 @option{--delete} on an archive if the archive device allows you to
4152 write to any point on the media, such as a disk; because of this, it
4153 does not work on magnetic tapes. Do not try to delete an archive member
4154 from a magnetic tape; the action will not succeed, and you will be
4155 likely to scramble the archive and damage your tape. There is no safe
4156 way (except by completely re-writing the archive) to delete files from
4157 most kinds of magnetic tape. @xref{Media}.
4159 To delete all versions of the file @file{blues} from the archive
4160 @file{collection.tar} in the @file{practice} directory, make sure you
4161 are in that directory, and then,
4164 $ @kbd{tar --list --file=collection.tar}
4169 $ @kbd{tar --delete --file=collection.tar blues}
4170 $ @kbd{tar --list --file=collection.tar}
4177 @FIXME{Check if the above listing is actually produced after running
4178 all the examples on collection.tar.}
4180 The @option{--delete} option has been reported to work properly when
4181 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4184 @subsection Comparing Archive Members with the File System
4185 @cindex Verifying the currency of an archive
4189 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4190 specified archive members against files with the same names, and then
4191 reports differences in file size, mode, owner, modification date and
4192 contents. You should @emph{only} specify archive member names, not file
4193 names. If you do not name any members, then @command{tar} will compare the
4194 entire archive. If a file is represented in the archive but does not
4195 exist in the file system, @command{tar} reports a difference.
4197 You have to specify the record size of the archive when modifying an
4198 archive with a non-default record size.
4200 @command{tar} ignores files in the file system that do not have
4201 corresponding members in the archive.
4203 The following example compares the archive members @file{rock},
4204 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4205 files of the same name in the file system. (Note that there is no file,
4206 @file{funk}; @command{tar} will report an error message.)
4209 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4212 tar: funk not found in archive
4215 The spirit behind the @option{--compare} (@option{--diff},
4216 @option{-d}) option is to check whether the archive represents the
4217 current state of files on disk, more than validating the integrity of
4218 the archive media. For this later goal, @xref{verify}.
4220 @node create options
4221 @section Options Used by @option{--create}
4223 @opindex create, additional options
4224 The previous chapter described the basics of how to use
4225 @option{--create} (@option{-c}) to create an archive from a set of files.
4226 @xref{create}. This section described advanced options to be used with
4230 * Ignore Failed Read::
4233 @node Ignore Failed Read
4234 @subsection Ignore Fail Read
4237 @item --ignore-failed-read
4238 Do not exit with nonzero on unreadable files or directories.
4241 @node extract options
4242 @section Options Used by @option{--extract}
4245 @opindex extract, additional options
4246 The previous chapter showed how to use @option{--extract} to extract
4247 an archive into the file system. Various options cause @command{tar} to
4248 extract more information than just file contents, such as the owner,
4249 the permissions, the modification date, and so forth. This section
4250 presents options to be used with @option{--extract} when certain special
4251 considerations arise. You may review the information presented in
4252 @ref{extract} for more basic information about the
4253 @option{--extract} operation.
4256 * Reading:: Options to Help Read Archives
4257 * Writing:: Changing How @command{tar} Writes Files
4258 * Scarce:: Coping with Scarce Resources
4262 @subsection Options to Help Read Archives
4263 @cindex Options when reading archives
4266 @cindex Reading incomplete records
4267 @cindex Records, incomplete
4268 @opindex read-full-records
4269 Normally, @command{tar} will request data in full record increments from
4270 an archive storage device. If the device cannot return a full record,
4271 @command{tar} will report an error. However, some devices do not always
4272 return full records, or do not require the last record of an archive to
4273 be padded out to the next record boundary. To keep reading until you
4274 obtain a full record, or to accept an incomplete record if it contains
4275 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
4276 in conjunction with the @option{--extract} or @option{--list} operations.
4279 The @option{--read-full-records} (@option{-B}) option is turned on by default when
4280 @command{tar} reads an archive from standard input, or from a remote
4281 machine. This is because on BSD Unix systems, attempting to read a
4282 pipe returns however much happens to be in the pipe, even if it is
4283 less than was requested. If this option were not enabled, @command{tar}
4284 would fail as soon as it read an incomplete record from the pipe.
4286 If you're not sure of the blocking factor of an archive, you can
4287 read the archive by specifying @option{--read-full-records} (@option{-B}) and
4288 @option{--blocking-factor=@var{512-size}} (@option{-b
4289 @var{512-size}}), using a blocking factor larger than what the archive
4290 uses. This lets you avoid having to determine the blocking factor
4291 of an archive. @xref{Blocking Factor}.
4294 * read full records::
4298 @node read full records
4299 @unnumberedsubsubsec Reading Full Records
4301 @FIXME{need sentence or so of intro here}
4304 @opindex read-full-records
4305 @item --read-full-records
4307 Use in conjunction with @option{--extract} (@option{--get},
4308 @option{-x}) to read an archive which contains incomplete records, or
4309 one which has a blocking factor less than the one specified.
4313 @unnumberedsubsubsec Ignoring Blocks of Zeros
4315 @cindex End-of-archive blocks, ignoring
4316 @cindex Ignoring end-of-archive blocks
4317 @opindex ignore-zeros
4318 Normally, @command{tar} stops reading when it encounters a block of zeros
4319 between file entries (which usually indicates the end of the archive).
4320 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
4321 completely read an archive which contains a block of zeros before the
4322 end (i.e., a damaged archive, or one that was created by concatenating
4323 several archives together).
4325 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
4326 versions of @command{tar} write garbage after the end-of-archive entry,
4327 since that part of the media is never supposed to be read. @GNUTAR{}
4328 does not write after the end of an archive, but seeks to
4329 maintain compatiblity among archiving utilities.
4332 @item --ignore-zeros
4334 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
4335 encountered while reading an archive. Use in conjunction with
4336 @option{--extract} or @option{--list}.
4340 @subsection Changing How @command{tar} Writes Files
4343 @FIXME{Introductory paragraph}
4346 * Dealing with Old Files::
4347 * Overwrite Old Files::
4349 * Keep Newer Files::
4351 * Recursive Unlink::
4352 * Data Modification Times::
4353 * Setting Access Permissions::
4354 * Directory Modification Times and Permissions::
4355 * Writing to Standard Output::
4356 * Writing to an External Program::
4360 @node Dealing with Old Files
4361 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
4363 @opindex overwrite-dir, introduced
4364 When extracting files, if @command{tar} discovers that the extracted
4365 file already exists, it normally replaces the file by removing it before
4366 extracting it, to prevent confusion in the presence of hard or symbolic
4367 links. (If the existing file is a symbolic link, it is removed, not
4368 followed.) However, if a directory cannot be removed because it is
4369 nonempty, @command{tar} normally overwrites its metadata (ownership,
4370 permission, etc.). The @option{--overwrite-dir} option enables this
4371 default behavior. To be more cautious and preserve the metadata of
4372 such a directory, use the @option{--no-overwrite-dir} option.
4374 @cindex Overwriting old files, prevention
4375 @opindex keep-old-files, introduced
4376 To be even more cautious and prevent existing files from being replaced, use
4377 the @option{--keep-old-files} (@option{-k}) option. It causes @command{tar} to refuse
4378 to replace or update a file that already exists, i.e., a file with the
4379 same name as an archive member prevents extraction of that archive
4380 member. Instead, it reports an error.
4382 @opindex overwrite, introduced
4383 To be more aggressive about altering existing files, use the
4384 @option{--overwrite} option. It causes @command{tar} to overwrite
4385 existing files and to follow existing symbolic links when extracting.
4387 @cindex Protecting old files
4388 Some people argue that @GNUTAR{} should not hesitate
4389 to overwrite files with other files when extracting. When extracting
4390 a @command{tar} archive, they expect to see a faithful copy of the
4391 state of the file system when the archive was created. It is debatable
4392 that this would always be a proper behavior. For example, suppose one
4393 has an archive in which @file{usr/local} is a link to
4394 @file{usr/local2}. Since then, maybe the site removed the link and
4395 renamed the whole hierarchy from @file{/usr/local2} to
4396 @file{/usr/local}. Such things happen all the time. I guess it would
4397 not be welcome at all that @GNUTAR{} removes the
4398 whole hierarchy just to make room for the link to be reinstated
4399 (unless it @emph{also} simultaneously restores the full
4400 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
4401 able to remove a whole hierarchy to reestablish a symbolic link, for
4402 example, but @emph{only if} @option{--recursive-unlink} is specified
4403 to allow this behavior. In any case, single files are silently
4406 @opindex unlink-first, introduced
4407 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
4408 some cases by causing @command{tar} to remove files unconditionally
4409 before extracting them.
4411 @node Overwrite Old Files
4412 @unnumberedsubsubsec Overwrite Old Files
4417 Overwrite existing files and directory metadata when extracting files
4420 This causes @command{tar} to write extracted files into the file system without
4421 regard to the files already on the system; i.e., files with the same
4422 names as archive members are overwritten when the archive is extracted.
4423 It also causes @command{tar} to extract the ownership, permissions,
4424 and time stamps onto any preexisting files or directories.
4425 If the name of a corresponding file name is a symbolic link, the file
4426 pointed to by the symbolic link will be overwritten instead of the
4427 symbolic link itself (if this is possible). Moreover, special devices,
4428 empty directories and even symbolic links are automatically removed if
4429 they are in the way of extraction.
4431 Be careful when using the @option{--overwrite} option, particularly when
4432 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
4433 can change the contents, ownership or permissions of any file on your
4434 system. Also, many systems do not take kindly to overwriting files that
4435 are currently being executed.
4437 @opindex overwrite-dir
4438 @item --overwrite-dir
4439 Overwrite the metadata of directories when extracting files from an
4440 archive, but remove other files before extracting.
4443 @node Keep Old Files
4444 @unnumberedsubsubsec Keep Old Files
4447 @opindex keep-old-files
4448 @item --keep-old-files
4450 Do not replace existing files from archive. The
4451 @option{--keep-old-files} (@option{-k}) option prevents @command{tar}
4452 from replacing existing files with files with the same name from the
4453 archive. The @option{--keep-old-files} option is meaningless with
4454 @option{--list} (@option{-t}). Prevents @command{tar} from replacing
4455 files in the file system during extraction.
4458 @node Keep Newer Files
4459 @unnumberedsubsubsec Keep Newer Files
4462 @opindex keep-newer-files
4463 @item --keep-newer-files
4464 Do not replace existing files that are newer than their archive
4465 copies. This option is meaningless with @option{--list} (@option{-t}).
4469 @unnumberedsubsubsec Unlink First
4472 @opindex unlink-first
4473 @item --unlink-first
4475 Remove files before extracting over them.
4476 This can make @command{tar} run a bit faster if you know in advance
4477 that the extracted files all need to be removed. Normally this option
4478 slows @command{tar} down slightly, so it is disabled by default.
4481 @node Recursive Unlink
4482 @unnumberedsubsubsec Recursive Unlink
4485 @opindex recursive-unlink
4486 @item --recursive-unlink
4487 When this option is specified, try removing files and directory hierarchies
4488 before extracting over them. @emph{This is a dangerous option!}
4491 If you specify the @option{--recursive-unlink} option,
4492 @command{tar} removes @emph{anything} that keeps you from extracting a file
4493 as far as current permissions will allow it. This could include removal
4494 of the contents of a full directory hierarchy.
4496 @node Data Modification Times
4497 @unnumberedsubsubsec Setting Data Modification Times
4499 @cindex Data modification times of extracted files
4500 @cindex Modification times of extracted files
4501 Normally, @command{tar} sets the data modification times of extracted
4502 files to the corresponding times recorded for the files in the archive, but
4503 limits the permissions of extracted files by the current @code{umask}
4506 To set the data modification times of extracted files to the time when
4507 the files were extracted, use the @option{--touch} (@option{-m}) option in
4508 conjunction with @option{--extract} (@option{--get}, @option{-x}).
4514 Sets the data modification time of extracted archive members to the time
4515 they were extracted, not the time recorded for them in the archive.
4516 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
4519 @node Setting Access Permissions
4520 @unnumberedsubsubsec Setting Access Permissions
4522 @cindex Permissions of extracted files
4523 @cindex Modes of extracted files
4524 To set the modes (access permissions) of extracted files to those
4525 recorded for those files in the archive, use @option{--same-permissions}
4526 in conjunction with the @option{--extract} (@option{--get},
4527 @option{-x}) operation.
4530 @opindex preserve-permission
4531 @opindex same-permission
4532 @item --preserve-permission
4533 @itemx --same-permission
4534 @c @itemx --ignore-umask
4536 Set modes of extracted archive members to those recorded in the
4537 archive, instead of current umask settings. Use in conjunction with
4538 @option{--extract} (@option{--get}, @option{-x}).
4541 @node Directory Modification Times and Permissions
4542 @unnumberedsubsubsec Directory Modification Times and Permissions
4544 After sucessfully extracting a file member, @GNUTAR{} normally
4545 restores its permissions and modification times, as described in the
4546 previous sections. This cannot be done for directories, because
4547 after extracting a directory @command{tar} will almost certainly
4548 extract files into that directory and this will cause the directory
4549 modification time to be updated. Moreover, restoring that directory
4550 permissions may not permit file creation within it. Thus, restoring
4551 directory permissions and modification times must be delayed at least
4552 until all files have been extracted into that directory. @GNUTAR{}
4553 restores directories using the following approach.
4555 The extracted directories are created with the mode specified in the
4556 archive, as modified by the umask of the user, which gives sufficient
4557 permissions to allow file creation. The meta-information about the
4558 directory is recorded in the temporary list of directories. When
4559 preparing to extract next archive member, @GNUTAR{} checks if the
4560 directory prefix of this file contains the remembered directory. If
4561 it does not, the program assumes that all files have been extracted
4562 into that directory, restores its modification time and permissions
4563 and removes its entry from the internal list. This approach allows
4564 to correctly restore directory meta-information in the majority of
4565 cases, while keeping memory requirements sufficiently small. It is
4566 based on the fact, that most @command{tar} archives use the predefined
4567 order of members: first the directory, then all the files and
4568 subdirectories in that directory.
4570 However, this is not always true. The most important exception are
4571 incremental archives (@pxref{Incremental Dumps}). The member order in
4572 an incremental archive is reversed: first all directory members are
4573 stored, followed by other (non-directory) members. So, when extracting
4574 from incremental archives, @GNUTAR{} alters the above procedure. It
4575 remebers all restored directories, and restores their meta-data
4576 only after the entire archive has been processed. Notice, that you do
4577 not need to specity any special options for that, as @GNUTAR{}
4578 automatically detects archives in incremental format.
4580 There may be cases, when such processing is required for normal archives
4581 too. Consider the following example:
4585 $ @kbd{tar --no-recursion -cvf archive \
4586 foo foo/file1 bar bar/file foo/file2}
4595 During the normal operation, after encountering @file{bar}
4596 @GNUTAR{} will assume that all files from the directory @file{foo}
4597 were already extracted and will therefore restore its timestamp and
4598 permission bits. However, after extracting @file{foo/file2} the
4599 directory timestamp will be offset again.
4601 To correctly restore directory meta-information in such cases, use
4602 @option{delay-directory-restore} command line option:
4605 @opindex delay-directory-restore
4606 @item --delay-directory-restore
4607 Delays restoring of the modification times and permissions of extracted
4608 directories until the end of extraction. This way, correct
4609 meta-information is restored even if the archive has unusual member
4612 @opindex no-delay-directory-restore
4613 @item --no-delay-directory-restore
4614 Cancel the effect of the previous @option{--delay-directory-restore}.
4615 Use this option if you have used @option{--delay-directory-restore} in
4616 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
4617 temporarily disable it.
4620 @node Writing to Standard Output
4621 @unnumberedsubsubsec Writing to Standard Output
4623 @cindex Writing extracted files to standard output
4624 @cindex Standard output, writing extracted files to
4625 To write the extracted files to the standard output, instead of
4626 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
4627 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
4628 extracting files to send them through a pipe, and do not need to
4629 preserve them in the file system. If you extract multiple members,
4630 they appear on standard output concatenated, in the order they are
4631 found in the archive.
4637 Writes files to the standard output. Use only in conjunction with
4638 @option{--extract} (@option{--get}, @option{-x}). When this option is
4639 used, instead of creating the files specified, @command{tar} writes
4640 the contents of the files extracted to its standard output. This may
4641 be useful if you are only extracting the files in order to send them
4642 through a pipe. This option is meaningless with @option{--list}
4646 This can be useful, for example, if you have a tar archive containing
4647 a big file and don't want to store the file on disk before processing
4648 it. You can use a command like this:
4651 tar -xOzf foo.tgz bigfile | process
4654 or even like this if you want to process the concatenation of the files:
4657 tar -xOzf foo.tgz bigfile1 bigfile2 | process
4660 Hovewer, @option{--to-command} may be more convenient for use with
4661 multiple files. See the next section.
4663 @node Writing to an External Program
4664 @unnumberedsubsubsec Writing to an External Program
4666 You can instruct @command{tar} to send the contents of each extracted
4667 file to the standard input of an external program:
4671 @item --to-command=@var{command}
4672 Extract files and pipe their contents to the standard input of
4673 @var{command}. When this option is used, instead of creating the
4674 files specified, @command{tar} invokes @var{command} and pipes the
4675 contents of the files to its standard output. @var{Command} may
4676 contain command line arguments. The program is executed via
4677 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
4678 extracted. Non-regular files (directories, etc.) are ignored when this
4682 The command can obtain the information about the file it processes
4683 from the following environment variables:
4686 @vrindex TAR_FILETYPE, to-command environment
4688 Type of the file. It is a single letter with the following meaning:
4690 @multitable @columnfractions 0.10 0.90
4691 @item f @tab Regular file
4692 @item d @tab Directory
4693 @item l @tab Symbolic link
4694 @item h @tab Hard link
4695 @item b @tab Block device
4696 @item c @tab Character device
4699 Currently only regular files are supported.
4701 @vrindex TAR_MODE, to-command environment
4703 File mode, an octal number.
4705 @vrindex TAR_FILENAME, to-command environment
4707 The name of the file.
4709 @vrindex TAR_REALNAME, to-command environment
4711 Name of the file as stored in the archive.
4713 @vrindex TAR_UNAME, to-command environment
4715 Name of the file owner.
4717 @vrindex TAR_GNAME, to-command environment
4719 Name of the file owner group.
4721 @vrindex TAR_ATIME, to-command environment
4723 Time of last access. It is a decimal number, representing seconds
4724 since the epoch. If the archive provides times with nanosecond
4725 precision, the nanoseconds are appended to the timestamp after a
4728 @vrindex TAR_MTIME, to-command environment
4730 Time of last modification.
4732 @vrindex TAR_CTIME, to-command environment
4734 Time of last status change.
4736 @vrindex TAR_SIZE, to-command environment
4740 @vrindex TAR_UID, to-command environment
4742 UID of the file owner.
4744 @vrindex TAR_GID, to-command environment
4746 GID of the file owner.
4749 In addition to these variables, @env{TAR_VERSION} contains the
4750 @GNUTAR{} version number.
4752 If @var{command} exits with a non-0 status, @command{tar} will print
4753 an error message similar to the following:
4756 tar: 2345: Child returned status 1
4759 Here, @samp{2345} is the PID of the finished process.
4761 If this behavior is not wanted, use @option{--ignore-command-error}:
4764 @opindex ignore-command-error
4765 @item --ignore-command-error
4766 Ignore exit codes of subprocesses. Notice that if the program
4767 exits on signal or otherwise terminates abnormally, the error message
4768 will be printed even if this option is used.
4770 @opindex no-ignore-command-error
4771 @item --no-ignore-command-error
4772 Cancel the effect of any previous @option{--ignore-command-error}
4773 option. This option is useful if you have set
4774 @option{--ignore-command-error} in @env{TAR_OPTIONS}
4775 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
4779 @unnumberedsubsubsec Removing Files
4781 @FIXME{The section is too terse. Something more to add? An example,
4785 @opindex remove-files
4786 @item --remove-files
4787 Remove files after adding them to the archive.
4791 @subsection Coping with Scarce Resources
4794 @cindex Small memory
4795 @cindex Running out of space
4803 @unnumberedsubsubsec Starting File
4806 @opindex starting-file
4807 @item --starting-file=@var{name}
4808 @itemx -K @var{name}
4809 Starts an operation in the middle of an archive. Use in conjunction
4810 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
4813 @cindex Middle of the archive, starting in the
4814 If a previous attempt to extract files failed due to lack of disk
4815 space, you can use @option{--starting-file=@var{name}} (@option{-K
4816 @var{name}}) to start extracting only after member @var{name} of the
4817 archive. This assumes, of course, that there is now free space, or
4818 that you are now extracting into a different file system. (You could
4819 also choose to suspend @command{tar}, remove unnecessary files from
4820 the file system, and then restart the same @command{tar} operation.
4821 In this case, @option{--starting-file} is not necessary.
4822 @xref{Incremental Dumps}, @xref{interactive}, and @ref{exclude}.)
4825 @unnumberedsubsubsec Same Order
4828 @cindex Large lists of file names on small machines
4830 @opindex preserve-order
4832 @itemx --preserve-order
4834 To process large lists of file names on machines with small amounts of
4835 memory. Use in conjunction with @option{--compare} (@option{--diff},
4836 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
4837 (@option{--get}, @option{-x}).
4840 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
4841 names to be listed or extracted is sorted in the same order as the
4842 files in the archive. This allows a large list of names to be used,
4843 even on a small machine that would not otherwise be able to hold all
4844 the names in memory at the same time. Such a sorted list can easily be
4845 created by running @samp{tar -t} on the archive and editing its output.
4847 This option is probably never needed on modern computer systems.
4850 @section Backup options
4852 @cindex backup options
4854 @GNUTAR{} offers options for making backups of files
4855 before writing new versions. These options control the details of
4856 these backups. They may apply to the archive itself before it is
4857 created or rewritten, as well as individual extracted members. Other
4858 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
4859 and @command{mv}, for example) offer similar options.
4861 Backup options may prove unexpectedly useful when extracting archives
4862 containing many members having identical name, or when extracting archives
4863 on systems having file name limitations, making different members appear
4864 has having similar names through the side-effect of name truncation.
4865 (This is true only if we have a good scheme for truncated backup names,
4866 which I'm not sure at all: I suspect work is needed in this area.)
4867 When any existing file is backed up before being overwritten by extraction,
4868 then clashing files are automatically be renamed to be unique, and the
4869 true name is kept for only the last file of a series of clashing files.
4870 By using verbose mode, users may track exactly what happens.
4872 At the detail level, some decisions are still experimental, and may
4873 change in the future, we are waiting comments from our users. So, please
4874 do not learn to depend blindly on the details of the backup features.
4875 For example, currently, directories themselves are never renamed through
4876 using these options, so, extracting a file over a directory still has
4877 good chances to fail. Also, backup options apply to created archives,
4878 not only to extracted members. For created archives, backups will not
4879 be attempted when the archive is a block or character device, or when it
4880 refers to a remote file.
4882 For the sake of simplicity and efficiency, backups are made by renaming old
4883 files prior to creation or extraction, and not by copying. The original
4884 name is restored if the file creation fails. If a failure occurs after a
4885 partial extraction of a file, both the backup and the partially extracted
4889 @item --backup[=@var{method}]
4891 @vindex VERSION_CONTROL
4893 Back up files that are about to be overwritten or removed.
4894 Without this option, the original versions are destroyed.
4896 Use @var{method} to determine the type of backups made.
4897 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
4898 environment variable. And if @env{VERSION_CONTROL} is not set,
4899 use the @samp{existing} method.
4901 @vindex version-control @r{Emacs variable}
4902 This option corresponds to the Emacs variable @samp{version-control};
4903 the same values for @var{method} are accepted as in Emacs. This option
4904 also allows more descriptive names. The valid @var{method}s are:
4909 @cindex numbered @r{backup method}
4910 Always make numbered backups.
4914 @cindex existing @r{backup method}
4915 Make numbered backups of files that already have them, simple backups
4920 @cindex simple @r{backup method}
4921 Always make simple backups.
4925 @item --suffix=@var{suffix}
4927 @cindex backup suffix
4928 @vindex SIMPLE_BACKUP_SUFFIX
4929 Append @var{suffix} to each backup file made with @option{--backup}. If this
4930 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
4931 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
4932 set, the default is @samp{~}, just as in Emacs.
4936 Some people express the desire to @emph{always} use the @option{--backup}
4937 option, by defining some kind of alias or script. This is not as easy
4938 as one may think, due to the fact that old style options should appear first
4939 and consume arguments a bit unpredictably for an alias or script. But,
4940 if you are ready to give up using old style options, you may resort to
4941 using something like (a Bourne shell function here):
4944 tar () @{ /usr/local/bin/tar --backup $*; @}
4948 @section Notable @command{tar} Usages
4951 @FIXME{Using Unix file linking capability to recreate directory
4952 structures---linking files into one subdirectory and then
4953 @command{tar}ring that directory.}
4955 @FIXME{Nice hairy example using absolute-names, newer, etc.}
4958 You can easily use archive files to transport a group of files from
4959 one system to another: put all relevant files into an archive on one
4960 computer system, transfer the archive to another system, and extract
4961 the contents there. The basic transfer medium might be magnetic tape,
4962 Internet FTP, or even electronic mail (though you must encode the
4963 archive with @command{uuencode} in order to transport it properly by
4964 mail). Both machines do not have to use the same operating system, as
4965 long as they both support the @command{tar} program.
4967 For example, here is how you might copy a directory's contents from
4968 one disk to another, while preserving the dates, modes, owners and
4969 link-structure of all the files therein. In this case, the transfer
4970 medium is a @dfn{pipe}, which is one a Unix redirection mechanism:
4973 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
4977 You can avoid subshells by using @option{-C} option:
4980 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
4984 The command also works using short option forms:
4987 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
4988 | (cd targetdir; tar --extract --file=-)}
4990 $ @kbd{tar --directory sourcedir --create --file=- . ) \
4991 | tar --directory targetdir --extract --file=-}
4995 This is one of the easiest methods to transfer a @command{tar} archive.
4998 @section Looking Ahead: The Rest of this Manual
5000 You have now seen how to use all eight of the operations available to
5001 @command{tar}, and a number of the possible options. The next chapter
5002 explains how to choose and change file and archive names, how to use
5003 files to store names of other files which you can then call as
5004 arguments to @command{tar} (this can help you save time if you expect to
5005 archive the same list of files a number of times), and so forth.
5006 @FIXME{in case it's not obvious, i'm making this up in some sense
5007 based on my limited memory of what the next chapter *really* does. i
5008 just wanted to flesh out this final section a little bit so i'd
5009 remember to stick it in here. :-)}
5011 If there are too many files to conveniently list on the command line,
5012 you can list the names in a file, and @command{tar} will read that file.
5015 There are various ways of causing @command{tar} to skip over some files,
5016 and not archive them. @xref{Choosing}.
5019 @chapter Performing Backups and Restoring Files
5022 @GNUTAR{} is distributed along with the scripts
5023 which the Free Software Foundation uses for performing backups. There
5024 is no corresponding scripts available yet for doing restoration of
5025 files. Even if there is a good chance those scripts may be satisfying
5026 to you, they are not the only scripts or methods available for doing
5027 backups and restore. You may well create your own, or use more
5028 sophisticated packages dedicated to that purpose.
5030 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5031 Automatic Network Disk Archiver), a backup system developed by James
5032 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5033 This is free software, and it is available at these places:
5036 http://www.cs.umd.edu/projects/amanda/amanda.html
5037 ftp://ftp.cs.umd.edu/pub/amanda
5042 Here is a possible plan for a future documentation about the backuping
5043 scripts which are provided within the @GNUTAR{}
5049 @item what are dumps
5050 @item different levels of dumps
5052 @item full dump = dump everything
5053 @item level 1, level 2 dumps etc
5054 A level @var{n} dump dumps everything changed since the last level
5057 @item how to use scripts for dumps (ie, the concept)
5059 @item scripts to run after editing backup specs (details)
5061 @item Backup Specs, what is it.
5063 @item how to customize
5064 @item actual text of script [/sp/dump/backup-specs]
5068 @item rsh doesn't work
5069 @item rtape isn't installed
5072 @item the @option{--incremental} option of tar
5075 @item write protection
5076 @item types of media, different sizes and types, useful for different things
5077 @item files and tape marks
5078 one tape mark between files, two at end.
5079 @item positioning the tape
5080 MT writes two at end of write,
5081 backspaces over one when writing again.
5087 This chapter documents both the provided shell scripts and @command{tar}
5088 options which are more specific to usage as a backup tool.
5090 To @dfn{back up} a file system means to create archives that contain
5091 all the files in that file system. Those archives can then be used to
5092 restore any or all of those files (for instance if a disk crashes or a
5093 file is accidentally deleted). File system @dfn{backups} are also
5097 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5098 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5099 * Backup Levels:: Levels of Backups
5100 * Backup Parameters:: Setting Parameters for Backups and Restoration
5101 * Scripted Backups:: Using the Backup Scripts
5102 * Scripted Restoration:: Using the Restore Script
5106 @section Using @command{tar} to Perform Full Dumps
5112 @cindex corrupted archives
5113 Full dumps should only be made when no other people or programs
5114 are modifying files in the file system. If files are modified while
5115 @command{tar} is making the backup, they may not be stored properly in
5116 the archive, in which case you won't be able to restore them if you
5117 have to. (Files not being modified are written with no trouble, and do
5118 not corrupt the entire archive.)
5120 You will want to use the @option{--label=@var{archive-label}}
5121 (@option{-V @var{archive-label}}) option to give the archive a
5122 volume label, so you can tell what this archive is even if the label
5123 falls off the tape, or anything like that.
5125 Unless the file system you are dumping is guaranteed to fit on
5126 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5127 Make sure you have enough tapes on hand to complete the backup.
5129 If you want to dump each file system separately you will need to use
5130 the @option{--one-file-system} option to prevent
5131 @command{tar} from crossing file system boundaries when storing
5134 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5135 option is not needed, since this is a complete copy of everything in
5136 the file system, and a full restore from this backup would only be
5137 done onto a completely
5140 Unless you are in a hurry, and trust the @command{tar} program (and your
5141 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5142 option, to make sure your files really made it onto the dump properly.
5143 This will also detect cases where the file was modified while (or just
5144 after) it was being archived. Not all media (notably cartridge tapes)
5145 are capable of being verified, unfortunately.
5147 @node Incremental Dumps
5148 @section Using @command{tar} to Perform Incremental Dumps
5150 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5151 stores additional metadata so that exact state of the file system
5152 can be restored when extracting the archive.
5154 @GNUTAR{} currently offers two options for handling incremental
5155 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5156 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5158 @opindex listed-incremental
5159 The option @option{--listed-incremental} instructs tar to operate on
5160 an incremental archive with additional metadata stored in a standalone
5161 file, called a @dfn{snapshot file}. The purpose of this file is to help
5162 determine which files have been changed, added or deleted since the
5163 last backup, so that the next incremental backup will contain only
5164 modified files. The name of the snapshot file is given as an argument
5168 @item --listed-incremental=@var{file}
5169 @itemx -g @var{file}
5170 Handle incremental backups with snapshot data in @var{file}.
5173 To create an incremental backup, you would use
5174 @option{--listed-incremental} together with @option{--create}
5175 (@pxref{create}). For example:
5178 $ @kbd{tar --create \
5179 --file=archive.1.tar \
5180 --listed-incremental=/var/log/usr.snar \
5184 This will create in @file{archive.1.tar} an incremental backup of
5185 the @file{/usr} file system, storing additional metadata in the file
5186 @file{/var/log/usr.snar}. If this file does not exist, it will be
5187 created. The created archive will then be a @dfn{level 0 backup};
5188 please see the next section for more on backup levels.
5190 Otherwise, if the file @file{/var/log/usr.snar} exists, it
5191 determines which files are modified. In this case only these files will be
5192 stored in the archive. Suppose, for example, that after running the
5193 above command, you delete file @file{/usr/doc/old} and create
5194 directory @file{/usr/local/db} with the following contents:
5197 $ @kbd{ls /usr/local/db}
5202 Some time later you create another incremental backup. You will
5206 $ @kbd{tar --create \
5207 --file=archive.2.tar \
5208 --listed-incremental=/var/log/usr.snar \
5210 tar: usr/local/db: Directory is new
5217 The created archive @file{archive.2.tar} will contain only these
5218 three members. This archive is called a @dfn{level 1 backup}. Notice
5219 that @file{/var/log/usr.snar} will be updated with the new data, so if
5220 you plan to create more @samp{level 1} backups, it is necessary to
5221 create a working copy of the snapshot file before running
5222 @command{tar}. The above example will then be modified as follows:
5225 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
5226 $ @kbd{tar --create \
5227 --file=archive.2.tar \
5228 --listed-incremental=/var/log/usr.snar-1 \
5232 Incremental dumps depend crucially on time stamps, so the results are
5233 unreliable if you modify a file's time stamps during dumping (e.g.,
5234 with the @option{--atime-preserve=replace} option), or if you set the clock
5237 Metadata stored in snapshot files include device numbers, which,
5238 obviously is supposed to be a non-volatile value. However, it turns
5239 out that NFS devices have undependable values when an automounter
5240 gets in the picture. This can lead to a great deal of spurious
5241 redumping in incremental dumps, so it is somewhat useless to compare
5242 two NFS devices numbers over time. The solution implemented currently
5243 is to considers all NFS devices as being equal when it comes to
5244 comparing directories; this is fairly gross, but there does not seem
5245 to be a better way to go.
5247 Note that incremental archives use @command{tar} extensions and may
5248 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
5250 @opindex listed-incremental, using with @option{--extract}
5251 @opindex extract, using with @option{--listed-incremental}
5252 To extract from the incremental dumps, use
5253 @option{--listed-incremental} together with @option{--extract}
5254 option (@pxref{extracting files}). In this case, @command{tar} does
5255 not need to access snapshot file, since all the data necessary for
5256 extraction are stored in the archive itself. So, when extracting, you
5257 can give whatever argument to @option{--listed-incremental}, the usual
5258 practice is to use @option{--listed-incremental=/dev/null}.
5259 Alternatively, you can use @option{--incremental}, which needs no
5260 arguments. In general, @option{--incremental} (@option{-G}) can be
5261 used as a shortcut for @option{--listed-incremental} when listing or
5262 extracting incremental backups (for more information, regarding this
5263 option, @pxref{incremental-op}).
5265 When extracting from the incremental backup @GNUTAR{} attempts to
5266 restore the exact state the file system had when the archive was
5267 created. In particular, it will @emph{delete} those files in the file
5268 system that did not exist in their directories when the archive was
5269 created. If you have created several levels of incremental files,
5270 then in order to restore the exact contents the file system had when
5271 the last level was created, you will need to restore from all backups
5272 in turn. Continuing our example, to restore the state of @file{/usr}
5273 file system, one would do@footnote{Notice, that since both archives
5274 were created withouth @option{-P} option (@pxref{absolute}), these
5275 commands should be run from the root file system.}:
5278 $ @kbd{tar --extract \
5279 --listed-incremental=/dev/null \
5280 --file archive.1.tar}
5281 $ @kbd{tar --extract \
5282 --listed-incremental=/dev/null \
5283 --file archive.2.tar}
5286 To list the contents of an incremental archive, use @option{--list}
5287 (@pxref{list}), as usual. To obtain more information about the
5288 archive, use @option{--listed-incremental} or @option{--incremental}
5289 combined with two @option{--verbose} options@footnote{Two
5290 @option{--verbose} options were selected to avoid breaking usual
5291 verbose listing output (@option{--list --verbose}) when using in
5294 @opindex incremental, using with @option{--list}
5295 @opindex listed-incremental, using with @option{--list}
5296 @opindex list, using with @option{--incremental}
5297 @opindex list, using with @option{--listed-incremental}
5298 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
5299 contents of the DUMPDIR header (with terminating nulls) when
5300 @option{--incremental} or @option{--listed-incremental} option was
5301 given, no matter what the verbosity level. This behavior, and,
5302 especially, the binary output it produced were considered incovenient
5303 and were changed in version 1.16}:
5306 @kbd{tar --list --incremental --verbose --verbose archive.tar}
5309 This command will print, for each directory in the archive, the list
5310 of files in that directory at the time the archive was created. This
5311 information is put out in a format which is both human-readable and
5312 unambiguous for a program: each file name is printed as
5319 where @var{x} is a letter describing the status of the file: @samp{Y}
5320 if the file is present in the archive, @samp{N} if the file is not
5321 included in the archive, or a @samp{D} if the file is a directory (and
5322 is included in the archive). @xref{Dumpdir}, for the detailed
5323 description of dumpdirs and status codes. Each such
5324 line is terminated by a newline character. The last line is followed
5325 by an additional newline to indicate the end of the data.
5327 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
5328 gives the same behavior as @option{--listed-incremental} when used
5329 with @option{--list} and @option{--extract} options. When used with
5330 @option{--create} option, it creates an incremental archive without
5331 creating snapshot file. Thus, it is impossible to create several
5332 levels of incremental backups with @option{--incremental} option.
5335 @section Levels of Backups
5337 An archive containing all the files in the file system is called a
5338 @dfn{full backup} or @dfn{full dump}. You could insure your data by
5339 creating a full dump every day. This strategy, however, would waste a
5340 substantial amount of archive media and user time, as unchanged files
5341 are daily re-archived.
5343 It is more efficient to do a full dump only occasionally. To back up
5344 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
5345 one} dump archives all the files that have changed since the last full
5348 A typical dump strategy would be to perform a full dump once a week,
5349 and a level one dump once a day. This means some versions of files
5350 will in fact be archived more than once, but this dump strategy makes
5351 it possible to restore a file system to within one day of accuracy by
5352 only extracting two archives---the last weekly (full) dump and the
5353 last daily (level one) dump. The only information lost would be in
5354 files changed or created since the last daily backup. (Doing dumps
5355 more than once a day is usually not worth the trouble).
5357 @GNUTAR{} comes with scripts you can use to do full
5358 and level-one (actually, even level-two and so on) dumps. Using
5359 scripts (shell programs) to perform backups and restoration is a
5360 convenient and reliable alternative to typing out file name lists
5361 and @command{tar} commands by hand.
5363 Before you use these scripts, you need to edit the file
5364 @file{backup-specs}, which specifies parameters used by the backup
5365 scripts and by the restore script. This file is usually located
5366 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
5367 detailed description. Once the backup parameters are set, you can
5368 perform backups or restoration by running the appropriate script.
5370 The name of the backup script is @code{backup}. The name of the
5371 restore script is @code{restore}. The following sections describe
5372 their use in detail.
5374 @emph{Please Note:} The backup and restoration scripts are
5375 designed to be used together. While it is possible to restore files by
5376 hand from an archive which was created using a backup script, and to create
5377 an archive by hand which could then be extracted using the restore script,
5378 it is easier to use the scripts. @xref{Incremental Dumps}, before
5379 making such an attempt.
5381 @node Backup Parameters
5382 @section Setting Parameters for Backups and Restoration
5384 The file @file{backup-specs} specifies backup parameters for the
5385 backup and restoration scripts provided with @command{tar}. You must
5386 edit @file{backup-specs} to fit your system configuration and schedule
5387 before using these scripts.
5389 Syntactically, @file{backup-specs} is a shell script, containing
5390 mainly variable assignments. However, any valid shell construct
5391 is allowed in this file. Particularly, you may wish to define
5392 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
5393 For more information about shell script syntax, please refer to
5394 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
5395 g_02, the definition of the Shell Command Language}. See also
5396 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
5398 The shell variables controlling behavior of @code{backup} and
5399 @code{restore} are described in the following subsections.
5402 * General-Purpose Variables::
5403 * Magnetic Tape Control::
5405 * backup-specs example:: An Example Text of @file{Backup-specs}
5408 @node General-Purpose Variables
5409 @subsection General-Purpose Variables
5411 @defvr {Backup variable} ADMINISTRATOR
5412 The user name of the backup administrator. @code{Backup} scripts
5413 sends a backup report to this address.
5416 @defvr {Backup variable} BACKUP_HOUR
5417 The hour at which the backups are done. This can be a number from 0
5418 to 23, or the time specification in form @var{hours}:@var{minutes},
5419 or the string @samp{now}.
5421 This variable is used by @code{backup}. Its value may be overridden
5422 using @option{--time} option (@pxref{Scripted Backups}).
5425 @defvr {Backup variable} TAPE_FILE
5427 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
5428 is a remote archive (@pxref{remote-dev}), backup script will suppose
5429 that your @command{mt} is able to access remote devices. If @var{RSH}
5430 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
5431 invocations of @command{mt}.
5434 @defvr {Backup variable} BLOCKING
5436 The blocking factor @command{tar} will use when writing the dump archive.
5437 @xref{Blocking Factor}.
5440 @defvr {Backup variable} BACKUP_DIRS
5442 A list of file systems to be dumped (for @code{backup}), or restored
5443 (for @code{restore}). You can include any directory
5444 name in the list --- subdirectories on that file system will be
5445 included, regardless of how they may look to other networked machines.
5446 Subdirectories on other file systems will be ignored.
5448 The host name specifies which host to run @command{tar} on, and should
5449 normally be the host that actually contains the file system. However,
5450 the host machine must have @GNUTAR{} installed, and
5451 must be able to access the directory containing the backup scripts and
5452 their support files using the same file name that is used on the
5453 machine where the scripts are run (ie. what @command{pwd} will print
5454 when in that directory on that machine). If the host that contains
5455 the file system does not have this capability, you can specify another
5456 host as long as it can access the file system through NFS.
5458 If the list of file systems is very long you may wish to put it
5459 in a separate file. This file is usually named
5460 @file{/etc/backup/dirs}, but this name may be overridden in
5461 @file{backup-specs} using @code{DIRLIST} variable.
5464 @defvr {Backup variable} DIRLIST
5466 A path to the file containing the list of the file systems to backup
5467 or restore. By default it is @file{/etc/backup/dirs}.
5470 @defvr {Backup variable} BACKUP_FILES
5472 A list of individual files to be dumped (for @code{backup}), or restored
5473 (for @code{restore}). These should be accessible from the machine on
5474 which the backup script is run.
5476 If the list of file systems is very long you may wish to store it
5477 in a separate file. This file is usually named
5478 @file{/etc/backup/files}, but this name may be overridden in
5479 @file{backup-specs} using @code{FILELIST} variable.
5482 @defvr {Backup variable} FILELIST
5484 A path to the file containing the list of the individual files to backup
5485 or restore. By default it is @file{/etc/backup/files}.
5488 @defvr {Backup variable} MT
5490 Full file name of @command{mt} binary.
5493 @defvr {Backup variable} RSH
5495 Full file name of @command{rsh} binary or its equivalent. You may wish to
5496 set it to @code{ssh}, to improve security. In this case you will have
5497 to use public key authentication.
5500 @defvr {Backup variable} RSH_COMMAND
5502 Full file name of @command{rsh} binary on remote mashines. This will
5503 be passed via @option{--rsh-command} option to the remote invocation
5507 @defvr {Backup variable} VOLNO_FILE
5509 Name of temporary file to hold volume numbers. This needs to be accessible
5510 by all the machines which have file systems to be dumped.
5513 @defvr {Backup variable} XLIST
5515 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
5516 located on the remote machine and containing the list of files to
5517 be excluded from the backup. Exclude file lists are searched in
5518 /etc/tar-backup directory. A common use for exclude file lists
5519 is to exclude files containing security-sensitive information
5520 (e.g., @file{/etc/shadow} from backups).
5522 This variable affects only @code{backup}.
5525 @defvr {Backup variable} SLEEP_TIME
5527 Time to sleep between dumps of any two successive file systems
5529 This variable affects only @code{backup}.
5532 @defvr {Backup variable} DUMP_REMIND_SCRIPT
5534 Script to be run when it's time to insert a new tape in for the next
5535 volume. Administrators may want to tailor this script for their site.
5536 If this variable isn't set, @GNUTAR{} will display its built-in
5537 prompt, and will expect confirmation from the console.
5539 The built-in prompt for POSIX locale is:
5542 Prepare volume #@var{n} for `@var{archive}' and hit return:
5546 where @var{n} is the ordinal number of the volume to be created and
5547 @var{archive} is archive file or device name.
5549 If you run @GNUTAR{} under a different locale, the translation of
5550 the above prompt to the locale's language will be used.
5554 @defvr {Backup variable} SLEEP_MESSAGE
5556 Message to display on the terminal while waiting for dump time. Usually
5557 this will just be some literal text.
5560 @defvr {Backup variable} TAR
5562 Full file name of the @GNUTAR{} executable. If this is not set, backup
5563 scripts will search @command{tar} in the current shell path.
5566 @node Magnetic Tape Control
5567 @subsection Magnetic Tape Control
5569 Backup scripts access tape device using special @dfn{hook functions}.
5570 These functions take a single argument -- the name of the tape
5571 device. Their names are kept in the following variables:
5573 @defvr {Backup variable} MT_BEGIN
5574 The name of @dfn{begin} function. This function is called before
5575 accessing the drive. By default it retensions the tape:
5581 mt -f "$1" retension
5586 @defvr {Backup variable} MT_REWIND
5587 The name of @dfn{rewind} function. The default definition is as
5600 @defvr {Backup variable} MT_OFFLINE
5601 The name of the function switching the tape off line. By default
5602 it is defined as follows:
5605 MT_OFFLINE=mt_offline
5613 @defvr {Backup variable} MT_STATUS
5614 The name of the function used to obtain the status of the archive device,
5615 including error count. Default definition:
5627 @subsection User Hooks
5629 @dfn{User hooks} are shell functions executed before and after
5630 each @command{tar} invocation. Thus, there are @dfn{backup
5631 hooks}, which are executed before and after dumping each file
5632 system, and @dfn{restore hooks}, executed before and
5633 after restoring a file system. Each user hook is a shell function
5634 taking four arguments:
5636 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
5641 Current backup or restore level.
5644 Name or IP address of the host machine being dumped or restored.
5647 Full path name to the file system being dumped or restored.
5650 File system name with directory separators replaced with colons. This
5651 is useful, e.g., for creating unique files.
5655 Following variables keep the names of user hook functions
5657 @defvr {Backup variable} DUMP_BEGIN
5658 Dump begin function. It is executed before dumping the file system.
5661 @defvr {Backup variable} DUMP_END
5662 Executed after dumping the file system.
5665 @defvr {Backup variable} RESTORE_BEGIN
5666 Executed before restoring the file system.
5669 @defvr {Backup variable} RESTORE_END
5670 Executed after restoring the file system.
5673 @node backup-specs example
5674 @subsection An Example Text of @file{Backup-specs}
5676 The following is an example of @file{backup-specs}:
5679 # site-specific parameters for file system backup.
5681 ADMINISTRATOR=friedman
5683 TAPE_FILE=/dev/nrsmt0
5685 # Use @code{ssh} instead of the less secure @code{rsh}
5687 RSH_COMMAND=/usr/bin/ssh
5689 # Override MT_STATUS function:
5695 # Disable MT_OFFLINE function
5712 apple-gunkies:/com/mailer/gnu
5713 apple-gunkies:/com/archive/gnu"
5715 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
5719 @node Scripted Backups
5720 @section Using the Backup Scripts
5722 The syntax for running a backup script is:
5725 backup --level=@var{level} --time=@var{time}
5728 The @option{level} option requests the dump level. Thus, to produce
5729 a full dump, specify @code{--level=0} (this is the default, so
5730 @option{--level} may be omitted if its value is @code{0}).
5731 @footnote{For backward compatibility, the @code{backup} will also
5732 try to deduce the requested dump level from the name of the
5733 script itself. If the name consists of a string @samp{level-}
5734 followed by a single decimal digit, that digit is taken as
5735 the dump level number. Thus, you may create a link from @code{backup}
5736 to @code{level-1} and then run @code{level-1} whenever you need to
5737 create a level one dump.}
5739 The @option{--time} option determines when should the backup be
5740 run. @var{Time} may take three forms:
5743 @item @var{hh}:@var{mm}
5745 The dump must be run at @var{hh} hours @var{mm} minutes.
5749 The dump must be run at @var{hh} hours
5753 The dump must be run immediately.
5756 You should start a script with a tape or disk mounted. Once you
5757 start a script, it prompts you for new tapes or disks as it
5758 needs them. Media volumes don't have to correspond to archive
5759 files --- a multi-volume archive can be started in the middle of a
5760 tape that already contains the end of another multi-volume archive.
5761 The @code{restore} script prompts for media by its archive volume,
5762 so to avoid an error message you should keep track of which tape
5763 (or disk) contains which volume of the archive (@pxref{Scripted
5766 The backup scripts write two files on the file system. The first is a
5767 record file in @file{/etc/tar-backup/}, which is used by the scripts
5768 to store and retrieve information about which files were dumped. This
5769 file is not meant to be read by humans, and should not be deleted by
5770 them. @xref{Snapshot Files}, for a more detailed explanation of this
5773 The second file is a log file containing the names of the file systems
5774 and files dumped, what time the backup was made, and any error
5775 messages that were generated, as well as how much space was left in
5776 the media volume after the last volume of the archive was written.
5777 You should check this log file after every backup. The file name is
5778 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
5779 represents current date, and @var{n} represents current dump level number.
5781 The script also prints the name of each system being dumped to the
5784 Following is the full list of options accepted by @code{backup}
5788 @item -l @var{level}
5789 @itemx --level=@var{level}
5790 Do backup level @var{level} (default 0).
5794 Force backup even if today's log file already exists.
5796 @item -v[@var{level}]
5797 @itemx --verbose[=@var{level}]
5798 Set verbosity level. The higher the level is, the more debugging
5799 information will be output during execution. Devault @var{level}
5800 is 100, which means the highest debugging level.
5802 @item -t @var{start-time}
5803 @itemx --time=@var{start-time}
5804 Wait till @var{time}, then do backup.
5808 Display short help message and exit.
5812 Display information about the program's name, version, origin and legal
5813 status, all on standard output, and then exit successfully.
5817 @node Scripted Restoration
5818 @section Using the Restore Script
5820 To restore files that were archived using a scripted backup, use the
5821 @code{restore} script. Its usage is quite straightforward. In the
5822 simplest form, invoke @code{restore --all}, it will
5823 then restore all the file systems and files specified in
5824 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
5826 You may select the file systems (and/or files) to restore by
5827 giving @code{restore} list of @dfn{patterns} in its command
5828 line. For example, running
5835 will restore all file systems on the machine @samp{albert}. A more
5836 complicated example:
5839 restore 'albert:*' '*:/var'
5843 This command will restore all file systems on the machine @samp{albert}
5844 as well as @file{/var} file system on all machines.
5846 By default @code{restore} will start restoring files from the lowest
5847 available dump level (usually zero) and will continue through
5848 all available dump levels. There may be situations where such a
5849 thorough restore is not necessary. For example, you may wish to
5850 restore only files from the recent level one backup. To do so,
5851 use @option{--level} option, as shown in the example below:
5857 The full list of options accepted by @code{restore} follows:
5862 Restore all file systems and files specified in @file{backup-specs}
5864 @item -l @var{level}
5865 @itemx --level=@var{level}
5866 Start restoring from the given backup level, instead of the default 0.
5868 @item -v[@var{level}]
5869 @itemx --verbose[=@var{level}]
5870 Set verbosity level. The higher the level is, the more debugging
5871 information will be output during execution. Devault @var{level}
5872 is 100, which means the highest debugging level.
5876 Display short help message and exit.
5880 Display information about the program's name, version, origin and legal
5881 status, all on standard output, and then exit successfully.
5884 You should start the restore script with the media containing the
5885 first volume of the archive mounted. The script will prompt for other
5886 volumes as they are needed. If the archive is on tape, you don't need
5887 to rewind the tape to to its beginning---if the tape head is
5888 positioned past the beginning of the archive, the script will rewind
5889 the tape as needed. @FIXME-xref{Media, for a discussion of tape
5893 @strong{Warning:} The script will delete files from the active file
5894 system if they were not in the file system when the archive was made.
5897 @xref{Incremental Dumps}, for an explanation of how the script makes
5901 @chapter Choosing Files and Names for @command{tar}
5904 Certain options to @command{tar} enable you to specify a name for your
5905 archive. Other options let you decide which files to include or exclude
5906 from the archive, based on when or whether files were modified, whether
5907 the file names do or don't match specified patterns, or whether files
5908 are in specified directories.
5910 This chapter discusses these options in detail.
5913 * file:: Choosing the Archive's Name
5914 * Selecting Archive Members::
5915 * files:: Reading Names from a File
5916 * exclude:: Excluding Some Files
5917 * wildcards:: Wildcards Patterns and Matching
5918 * quoting styles:: Ways of Quoting Special Characters in Names
5919 * transform:: Modifying File and Member Names
5920 * after:: Operating Only on New Files
5921 * recurse:: Descending into Directories
5922 * one:: Crossing File System Boundaries
5926 @section Choosing and Naming Archive Files
5929 @cindex Naming an archive
5930 @cindex Archive Name
5931 @cindex Choosing an archive file
5932 @cindex Where is the archive?
5933 By default, @command{tar} uses an archive file name that was compiled when
5934 it was built on the system; usually this name refers to some physical
5935 tape drive on the machine. However, the person who installed @command{tar}
5936 on the system may not have set the default to a meaningful value as far as
5937 most users are concerned. As a result, you will usually want to tell
5938 @command{tar} where to find (or create) the archive. The
5939 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
5940 option allows you to either specify or name a file to use as the archive
5941 instead of the default archive file location.
5944 @opindex file, short description
5945 @item --file=@var{archive-name}
5946 @itemx -f @var{archive-name}
5947 Name the archive to create or operate on. Use in conjunction with
5951 For example, in this @command{tar} command,
5954 $ @kbd{tar -cvf collection.tar blues folk jazz}
5958 @file{collection.tar} is the name of the archive. It must directly
5959 follow the @option{-f} option, since whatever directly follows @option{-f}
5960 @emph{will} end up naming the archive. If you neglect to specify an
5961 archive name, you may end up overwriting a file in the working directory
5962 with the archive you create since @command{tar} will use this file's name
5963 for the archive name.
5965 An archive can be saved as a file in the file system, sent through a
5966 pipe or over a network, or written to an I/O device such as a tape,
5967 floppy disk, or CD write drive.
5969 @cindex Writing new archives
5970 @cindex Archive creation
5971 If you do not name the archive, @command{tar} uses the value of the
5972 environment variable @env{TAPE} as the file name for the archive. If
5973 that is not available, @command{tar} uses a default, compiled-in archive
5974 name, usually that for tape unit zero (ie. @file{/dev/tu00}).
5976 @cindex Standard input and output
5977 @cindex tar to standard input and output
5978 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
5979 archive from standard input (when listing or extracting files), or
5980 writes it to standard output (when creating an archive). If you use
5981 @file{-} as an @var{archive-name} when modifying an archive,
5982 @command{tar} reads the original archive from its standard input and
5983 writes the entire new archive to its standard output.
5985 The following example is a convenient way of copying directory
5986 hierarchy from @file{sourcedir} to @file{targetdir}.
5989 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
5992 The @option{-C} option allows to avoid using subshells:
5995 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
5998 In both examples above, the leftmost @command{tar} invocation archives
5999 the contents of @file{sourcedir} to the standard output, while the
6000 rightmost one reads this archive from its standard input and
6001 extracts it. The @option{-p} option tells it to restore permissions
6002 of the extracted files.
6004 @cindex Remote devices
6005 @cindex tar to a remote device
6007 To specify an archive file on a device attached to a remote machine,
6011 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
6015 @command{tar} will complete the remote connection, if possible, and
6016 prompt you for a username and password. If you use
6017 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
6018 will complete the remote connection, if possible, using your username
6019 as the username on the remote machine.
6021 @cindex Local and remote archives
6022 @anchor{local and remote archives}
6023 If the archive file name includes a colon (@samp{:}), then it is assumed
6024 to be a file on another machine. If the archive file is
6025 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6026 host @var{host}. The remote host is accessed using the @command{rsh}
6027 program, with a username of @var{user}. If the username is omitted
6028 (along with the @samp{@@} sign), then your user name will be used.
6029 (This is the normal @command{rsh} behavior.) It is necessary for the
6030 remote machine, in addition to permitting your @command{rsh} access, to
6031 have the @file{rmt} program installed (This command is included in
6032 the @GNUTAR{} distribution and by default is installed under
6033 @file{@var{prefix}/libexec/rmt}, were @var{prefix} means your
6034 installation prefix). If you need to use a file whose name includes a
6035 colon, then the remote tape drive behavior
6036 can be inhibited by using the @option{--force-local} option.
6038 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6039 tries to minimize input and output operations. The Amanda backup
6040 system, when used with @GNUTAR{}, has an initial sizing pass which
6043 @node Selecting Archive Members
6044 @section Selecting Archive Members
6045 @cindex Specifying files to act on
6046 @cindex Specifying archive members
6048 @dfn{File Name arguments} specify which files in the file system
6049 @command{tar} operates on, when creating or adding to an archive, or which
6050 archive members @command{tar} operates on, when reading or deleting from
6051 an archive. @xref{Operations}.
6053 To specify file names, you can include them as the last arguments on
6054 the command line, as follows:
6056 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6059 If a file name begins with dash (@samp{-}), precede it with
6060 @option{--add-file} option to prevent it from being treated as an
6063 If you specify a directory name as a file name argument, all the files
6064 in that directory are operated on by @command{tar}.
6066 If you do not specify files, @command{tar} behavior differs depending
6067 on the operation mode as described below:
6069 When @command{tar} is invoked with @option{--create} (@option{-c}),
6070 @command{tar} will stop immediately, reporting the following:
6074 $ @kbd{tar cf a.tar}
6075 tar: Cowardly refusing to create an empty archive
6076 Try `tar --help' or `tar --usage' for more information.
6080 If you specify either @option{--list} (@option{-t}) or
6081 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6082 operates on all the archive members in the archive.
6084 If run with @option{--diff} option, tar will compare the archive with
6085 the contents of the current working directory.
6087 If you specify any other operation, @command{tar} does nothing.
6089 By default, @command{tar} takes file names from the command line. However,
6090 there are other ways to specify file or member names, or to modify the
6091 manner in which @command{tar} selects the files or members upon which to
6092 operate. In general, these methods work both for specifying the names
6093 of files and archive members.
6096 @section Reading Names from a File
6098 @cindex Reading file names from a file
6099 @cindex Lists of file names
6100 @cindex File Name arguments, alternatives
6101 Instead of giving the names of files or archive members on the command
6102 line, you can put the names into a file, and then use the
6103 @option{--files-from=@var{file-of-names}} (@option{-T
6104 @var{file-of-names}}) option to @command{tar}. Give the name of the
6105 file which contains the list of files to include as the argument to
6106 @option{--files-from}. In the list, the file names should be separated by
6107 newlines. You will frequently use this option when you have generated
6108 the list of files to archive with the @command{find} utility.
6112 @item --files-from=@var{file-name}
6113 @itemx -T @var{file-name}
6114 Get names to extract or create from file @var{file-name}.
6117 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6118 you specify either @code{--files-from=-} or @code{-T -}), then the file
6119 names are read from standard input.
6121 Unless you are running @command{tar} with @option{--create}, you can not use
6122 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6125 Any number of @option{-T} options can be given in the command line.
6127 The following example shows how to use @command{find} to generate a list of
6128 files smaller than 400K in length and put that list into a file
6129 called @file{small-files}. You can then use the @option{-T} option to
6130 @command{tar} to specify the files from that file, @file{small-files}, to
6131 create the archive @file{little.tgz}. (The @option{-z} option to
6132 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
6136 $ @kbd{find . -size -400 -print > small-files}
6137 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
6141 In the file list given by @option{-T} option, any file name beginning
6142 with @samp{-} character is considered a @command{tar} option and is
6143 processed accordingly.@footnote{Versions of @GNUTAR{} up to 1.15.1
6144 recognized only @option{-C} option in file lists, and only if the
6145 option and its argument occupied two consecutive lines.} For example,
6146 the common use of this feature is to change to another directory by
6147 specifying @option{-C} option:
6157 $ @kbd{tar -c -f foo.tar --files-from list}
6162 In this example, @command{tar} will first switch to @file{/etc}
6163 directory and add files @file{passwd} and @file{hosts} to the
6164 archive. Then it will change to @file{/lib} directory and will archive
6165 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
6170 $ @kbd{tar tf foo.tar}
6178 @opindex directory, using in @option{--files-from} argument
6179 Notice that the option parsing algorithm used with @option{-T} is
6180 stricter than the one used by shell. Namely, when specifying option
6181 arguments, you should observe the following rules:
6185 When using short (single-letter) option form, its argument must
6186 immediately follow the option letter, without any intervening
6187 whitespace. For example: @code{-Cdir}.
6190 When using long option form, the option argument must be separated
6191 from the option by a single equal sign. No whitespace is allowed on
6192 any side of the equal sign. For example: @code{--directory=dir}.
6195 For both short and long option forms, the option argument can be given
6196 on the next line after the option name, e.g.:
6217 If you happen to have a file whose name starts with @samp{-},
6218 precede it with @option{--add-file} option to prevent it from
6219 being recognized as an option. For example: @code{--add-file=--my-file}.
6226 @subsection @code{NUL} Terminated File Names
6228 @cindex File names, terminated by @code{NUL}
6229 @cindex @code{NUL} terminated file names
6230 The @option{--null} option causes
6231 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
6232 to read file names terminated by a @code{NUL} instead of a newline, so
6233 files whose names contain newlines can be archived using
6234 @option{--files-from}.
6239 Only consider @code{NUL} terminated file names, instead of files that
6240 terminate in a newline.
6243 The @option{--null} option is just like the one in @acronym{GNU}
6244 @command{xargs} and @command{cpio}, and is useful with the
6245 @option{-print0} predicate of @acronym{GNU} @command{find}. In
6246 @command{tar}, @option{--null} also disables special handling for
6247 file names that begin with dash.
6249 This example shows how to use @command{find} to generate a list of files
6250 larger than 800K in length and put that list into a file called
6251 @file{long-files}. The @option{-print0} option to @command{find} is just
6252 like @option{-print}, except that it separates files with a @code{NUL}
6253 rather than with a newline. You can then run @command{tar} with both the
6254 @option{--null} and @option{-T} options to specify that @command{tar} get the
6255 files from that file, @file{long-files}, to create the archive
6256 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
6257 @command{tar} to recognize the @code{NUL} separator between files.
6260 $ @kbd{find . -size +800 -print0 > long-files}
6261 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
6264 @FIXME{say anything else here to conclude the section?}
6267 @section Excluding Some Files
6270 @cindex File names, excluding files by
6271 @cindex Excluding files by name and pattern
6272 @cindex Excluding files by file system
6273 To avoid operating on files whose names match a particular pattern,
6274 use the @option{--exclude} or @option{--exclude-from} options.
6278 @item --exclude=@var{pattern}
6279 Causes @command{tar} to ignore files that match the @var{pattern}.
6283 The @option{--exclude=@var{pattern}} option prevents any file or
6284 member whose name matches the shell wildcard (@var{pattern}) from
6286 For example, to create an archive with all the contents of the directory
6287 @file{src} except for files whose names end in @file{.o}, use the
6288 command @samp{tar -cf src.tar --exclude='*.o' src}.
6290 You may give multiple @option{--exclude} options.
6293 @opindex exclude-from
6294 @item --exclude-from=@var{file}
6295 @itemx -X @var{file}
6296 Causes @command{tar} to ignore files that match the patterns listed in
6300 @findex exclude-from
6301 Use the @option{--exclude-from} option to read a
6302 list of patterns, one per line, from @var{file}; @command{tar} will
6303 ignore files matching those patterns. Thus if @command{tar} is
6304 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
6305 single line @file{*.o}, no files whose names end in @file{.o} will be
6306 added to the archive.
6309 @opindex exclude-caches
6310 @item --exclude-caches
6311 Causes @command{tar} to ignore directories containing a cache directory tag.
6314 @findex exclude-caches
6315 When creating an archive, the @option{--exclude-caches} option causes
6316 @command{tar} to exclude all directories that contain a @dfn{cache
6317 directory tag}. A cache directory tag is a short file with the
6318 well-known name @file{CACHEDIR.TAG} and having a standard header
6319 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
6320 Various applications write cache directory tags into directories they
6321 use to hold regenerable, non-precious data, so that such data can be
6322 more easily excluded from backups.
6325 * problems with exclude::
6328 @node problems with exclude
6329 @unnumberedsubsec Problems with Using the @code{exclude} Options
6331 @opindex exclude, potential problems with
6332 Some users find @samp{exclude} options confusing. Here are some common
6337 The main operating mode of @command{tar} does not act on a path name
6338 explicitly listed on the command line if one of its file name
6339 components is excluded. In the example above, if
6340 you create an archive and exclude files that end with @samp{*.o}, but
6341 explicitly name the file @samp{dir.o/foo} after all the options have been
6342 listed, @samp{dir.o/foo} will be excluded from the archive.
6345 You can sometimes confuse the meanings of @option{--exclude} and
6346 @option{--exclude-from}. Be careful: use @option{--exclude} when files
6347 to be excluded are given as a pattern on the command line. Use
6348 @option{--exclude-from} to introduce the name of a file which contains
6349 a list of patterns, one per line; each of these patterns can exclude
6350 zero, one, or many files.
6353 When you use @option{--exclude=@var{pattern}}, be sure to quote the
6354 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
6355 like @samp{*}. If you do not do this, the shell might expand the
6356 @samp{*} itself using files at hand, so @command{tar} might receive a
6357 list of files instead of one pattern, or none at all, making the
6358 command somewhat illegal. This might not correspond to what you want.
6363 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
6371 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
6375 You must use use shell syntax, or globbing, rather than @code{regexp}
6376 syntax, when using exclude options in @command{tar}. If you try to use
6377 @code{regexp} syntax to describe files to be excluded, your command
6381 @FIXME{The change in semantics must have occurred before 1.11,
6382 so I doubt if it is worth mentioning at all. Anyway, should at
6383 least specify in which version the semantics changed.}
6384 In earlier versions of @command{tar}, what is now the
6385 @option{--exclude-from} option was called @option{--exclude} instead.
6386 Now, @option{--exclude} applies to patterns listed on the command
6387 line and @option{--exclude-from} applies to patterns listed in a
6393 @section Wildcards Patterns and Matching
6395 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
6396 @samp{*} or @samp{?} for example, are replaced and expanded into all
6397 existing files matching the given pattern. @GNUTAR{} can use wildcard
6398 patterns for matching (or globbing) archive members when extracting
6399 from or listing an archive. Wildcard patterns are also used for
6400 verifying volume labels of @command{tar} archives. This section has the
6401 purpose of explaining wildcard syntax for @command{tar}.
6403 @FIXME{the next few paragraphs need work.}
6405 A @var{pattern} should be written according to shell syntax, using wildcard
6406 characters to effect globbing. Most characters in the pattern stand
6407 for themselves in the matched string, and case is significant: @samp{a}
6408 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
6409 pattern matches any single character in the matched string. The character
6410 @samp{*} in the pattern matches zero, one, or more single characters in
6411 the matched string. The character @samp{\} says to take the following
6412 character of the pattern @emph{literally}; it is useful when one needs to
6413 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
6415 The character @samp{[}, up to the matching @samp{]}, introduces a character
6416 class. A @dfn{character class} is a list of acceptable characters
6417 for the next single character of the matched string. For example,
6418 @samp{[abcde]} would match any of the first five letters of the alphabet.
6419 Note that within a character class, all of the ``special characters''
6420 listed above other than @samp{\} lose their special meaning; for example,
6421 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
6422 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
6423 the characters @samp{-} and @samp{]} must either come @emph{first} or
6424 @emph{last} in a character class.)
6426 @cindex Excluding characters from a character class
6427 @cindex Character class, excluding characters from
6428 If the first character of the class after the opening @samp{[}
6429 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
6430 Rather than listing character to match, it lists those characters which
6431 are @emph{forbidden} as the next single character of the matched string.
6433 Other characters of the class stand for themselves. The special
6434 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
6435 letters, is meant to represent all characters between @var{a} and
6438 @FIXME{need to add a sentence or so here to make this clear for those
6439 who don't have dan around.}
6441 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
6442 special for wildcard matches. However, if a pattern completely matches
6443 a directory prefix of a matched string, then it matches the full matched
6444 string: thus, excluding a directory also excludes all the files beneath it.
6447 * controlling pattern-matching::
6450 @node controlling pattern-matching
6451 @unnumberedsubsec Controlling Pattern-Matching
6453 For the purposes of this section, we call @dfn{exclusion members} all
6454 member names obtained while processing @option{--exclude} and
6455 @option{--exclude-from} options, and @dfn{inclusion members} those
6456 member names that were given in the command line or read from the file
6457 specified with @option{--files-from} option.
6459 These two pairs of member lists are used in the following operations:
6460 @option{--diff}, @option{--extract}, @option{--list},
6463 There are no inclusion members in create mode (@option{--create} and
6464 @option{--append}), since in this mode the names obtained from the
6465 command line refer to @emph{files}, not archive members.
6467 By default, inclusion members are compared with archive members
6468 literally @footnote{Notice that earlier @GNUTAR{} versions used
6469 globbing for inclusion members, which contradicted to UNIX98
6470 specification and was not documented. @xref{Changes}, for more
6471 information on this and other changes} and exclusion members are
6472 treated as globbing patterns. For example:
6476 $ @kbd{tar tf foo.tar}
6481 # @i{Member names are used verbatim:}
6482 $ @kbd{tar -xf foo.tar -v '[remarks]'}
6484 # @i{Exclude member names are globbed:}
6485 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
6491 This behavior can be altered by using the following options:
6496 Treat all member names as wildcards.
6498 @opindex no-wildcards
6499 @item --no-wildcards
6500 Treat all member names as literal strings.
6503 Thus, to extract files whose names end in @samp{.c}, you can use:
6506 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
6512 Notice quoting of the pattern to prevent the shell from interpreting
6515 The effect of @option{--wildcards} option is cancelled by
6516 @option{--no-wildcards}. This can be used to pass part of
6517 the command line arguments verbatim and other part as globbing
6518 patterns. For example, the following invocation:
6521 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
6525 instructs @command{tar} to extract from @file{foo.tar} all files whose
6526 names end in @samp{.txt} and the file named @file{[remarks]}.
6528 Normally, a pattern matches a name if an initial subsequence of the
6529 name's components matches the pattern, where @samp{*}, @samp{?}, and
6530 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
6531 and wildcards can match @samp{/}.
6533 Other than optionally stripping leading @samp{/} from names
6534 (@pxref{absolute}), patterns and names are used as-is. For
6535 example, trailing @samp{/} is not trimmed from a user-specified name
6536 before deciding whether to exclude it.
6538 However, this matching procedure can be altered by the options listed
6539 below. These options accumulate. For example:
6542 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
6545 ignores case when excluding @samp{makefile}, but not when excluding
6550 @opindex no-anchored
6552 @itemx --no-anchored
6553 If anchored, a pattern must match an initial subsequence
6554 of the name's components. Otherwise, the pattern can match any
6555 subsequence. Default is @option{--no-anchored} for exclusion members
6556 and @option{--anchored} inclusion members.
6558 @opindex ignore-case
6559 @opindex no-ignore-case
6561 @itemx --no-ignore-case
6562 When ignoring case, upper-case patterns match lower-case names and vice versa.
6563 When not ignoring case (the default), matching is case-sensitive.
6565 @opindex wildcards-match-slash
6566 @opindex no-wildcards-match-slash
6567 @item --wildcards-match-slash
6568 @itemx --no-wildcards-match-slash
6569 When wildcards match slash (the default for exclusion members), a
6570 wildcard like @samp{*} in the pattern can match a @samp{/} in the
6571 name. Otherwise, @samp{/} is matched only by @samp{/}.
6575 The @option{--recursion} and @option{--no-recursion} options
6576 (@pxref{recurse}) also affect how member patterns are interpreted. If
6577 recursion is in effect, a pattern matches a name if it matches any of
6578 the name's parent directories.
6580 The following table summarizes pattern-matching default values:
6582 @multitable @columnfractions .3 .7
6583 @headitem Members @tab Default settings
6584 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
6585 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
6588 @node quoting styles
6589 @section Quoting Member Names
6591 When displaying member names, @command{tar} takes care to avoid
6592 ambiguities caused by certain characters. This is called @dfn{name
6593 quoting}. The characters in question are:
6596 @item Non-printable control characters:
6598 @multitable @columnfractions 0.20 0.10 0.60
6599 @headitem Character @tab ASCII @tab Character name
6600 @item \a @tab 7 @tab Audible bell
6601 @item \b @tab 8 @tab Backspace
6602 @item \f @tab 12 @tab Form feed
6603 @item \n @tab 10 @tab New line
6604 @item \r @tab 13 @tab Carriage return
6605 @item \t @tab 9 @tab Horizontal tabulation
6606 @item \v @tab 11 @tab Vertical tabulation
6609 @item Space (ASCII 32)
6611 @item Single and double quotes (@samp{'} and @samp{"})
6613 @item Backslash (@samp{\})
6616 The exact way @command{tar} uses to quote these characters depends on
6617 the @dfn{quoting style}. The default quoting style, called
6618 @dfn{escape} (see below), uses backslash notation to represent control
6619 characters, space and backslash. Using this quoting style, control
6620 characters are represented as listed in column @samp{Character} in the
6621 above table, a space is printed as @samp{\ } and a backslash as @samp{\\}.
6623 @GNUTAR{} offers seven distinct quoting styles, which can be selected
6624 using @option{--quoting-style} option:
6627 @item --quoting-style=@var{style}
6628 @opindex quoting-style
6630 Sets quoting style. Valid values for @var{style} argument are:
6631 literal, shell, shell-always, c, escape, locale, clocale.
6634 These styles are described in detail below. To illustrate their
6635 effect, we will use an imaginary tar archive @file{arch.tar}
6636 containing the following members:
6640 # 1. Contains horizontal tabulation character.
6642 # 2. Contains newline character
6645 # 3. Contains a space
6647 # 4. Contains double quotes
6649 # 5. Contains single quotes
6651 # 6. Contains a backslash character:
6656 Here is how usual @command{ls} command would have listed them, if they
6657 had existed in the current working directory:
6675 No quoting, display each character as is:
6679 $ @kbd{tar tf arch.tar --quoting-style=literal}
6692 Display characters the same way Bourne shell does:
6693 control characters, except @samp{\t} and @samp{\n}, are printed using
6694 backslash escapes, @samp{\t} and @samp{\n} are printed as is, and a
6695 single quote is printed as @samp{\'}. If a name contains any quoted
6696 characters, it is enclosed in single quotes. In particular, if a name
6697 contains single quotes, it is printed as several single-quoted strings:
6701 $ @kbd{tar tf arch.tar --quoting-style=shell}
6704 './a'\''single'\''quote'
6714 Same as @samp{shell}, but the names are always enclosed in single
6719 $ @kbd{tar tf arch.tar --quoting-style=shell-always}
6722 './a'\''single'\''quote'
6732 Use the notation of the C programming language. All names are
6733 enclosed in double quotes. Control characters are quoted using
6734 backslash notations, double quotes are represented as @samp{\"},
6735 backslash characters are represented as @samp{\\}. Single quotes and
6736 spaces are not quoted:
6740 $ @kbd{tar tf arch.tar --quoting-style=c}
6744 "./a\"double\"quote"
6752 Control characters are printed using backslash notation, a space is
6753 printed as @samp{\ } and a backslash as @samp{\\}. This is the
6754 default quoting style, unless it was changed when configured the
6759 $ @kbd{tar tf arch.tar --quoting-style=escape}
6771 Control characters, single quote and backslash are printed using
6772 backslash notation. All names are quoted using left and right
6773 quotation marks, appropriate to the current locale. If it does not
6774 define quotation marks, use @samp{`} as left and @samp{'} as right
6775 quotation marks. Any occurrences of the right quotation mark in a
6776 name are escaped with @samp{\}, for example:
6782 $ @kbd{tar tf arch.tar --quoting-style=locale}
6785 `./a\'single\'quote'
6794 Same as @samp{locale}, but @samp{"} is used for both left and right
6795 quotation marks, if not provided by the currently selected locale:
6799 $ @kbd{tar tf arch.tar --quoting-style=clocale}
6803 "./a\"double\"quote"
6811 You can specify which characters should be quoted in addition to those
6812 implied by the current quoting style:
6815 @item --quote-chars=@var{string}
6816 Always quote characters from @var{string}, even if the selected
6817 quoting style would not quote them.
6820 For example, using @samp{escape} quoting (compare with the usual
6821 escape listing above):
6825 $ @kbd{tar tf arch.tar --quoting-style=escape --quote-chars=' "'}
6836 To disable quoting of such additional characters, use the following
6840 @item --no-quote-chars=@var{string}
6841 Remove characters listed in @var{string} from the list of quoted
6842 characters set by the previous @option{--quote-chars} option.
6845 This option is particularly useful if you have added
6846 @option{--quote-chars} to your @env{TAR_OPTIONS} (@pxref{TAR_OPTIONS})
6847 and wish to disable it for the current invocation.
6849 Note, that @option{--no-quote-chars} does @emph{not} disable those
6850 characters that are quoted by default in the selected quoting style.
6853 @section Modifying File and Member Names
6855 @command{Tar} archives contain detailed information about files stored
6856 in them and full file names are part of that information. When
6857 storing file to an archive, its file name is recorded in the archive
6858 along with the actual file contents. When restoring from an archive,
6859 a file is created on disk with exactly the same name as that stored
6860 in the archive. In the majority of cases this is the desired behavior
6861 of a file archiver. However, there are some cases when it is not.
6863 First of all, it is often unsafe to extract archive members with
6864 absolute file names or those that begin with a @file{../}. @GNUTAR{}
6865 takes special precautions when extracting such names and provides a
6866 special option for handling them, which is described in
6869 Secondly, you may wish to extract file names without some leading
6870 directory components, or with otherwise modified names. In other
6871 cases it is desirable to store files under differing names in the
6874 @GNUTAR{} provides two options for these needs.
6877 @opindex strip-components
6878 @item --strip-components=@var{number}
6879 Strip given @var{number} of leading components from file names before
6883 For example, suppose you have archived whole @file{/usr} hierarchy to
6884 a tar archive named @file{usr.tar}. Among other files, this archive
6885 contains @file{usr/include/stdlib.h}, which you wish to extract to
6886 the current working directory. To do so, you type:
6889 $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
6892 The option @option{--strip=2} instructs @command{tar} to strip the
6893 two leading components (@file{usr/} and @file{include/}) off the file
6896 If you add to the above invocation @option{--verbose} (@option{-v})
6897 option, you will note that the verbose listing still contains the
6898 full file name, with the two removed components still in place. This
6899 can be inconvenient, so @command{tar} provides a special option for
6900 altering this behavior:
6902 @anchor{show-transformed-names}
6904 @opindex --show-transformed-names
6905 @item --show-transformed-names
6906 Display file or member names with all requested transformations
6914 $ @kbd{tar -xf usr.tar -v --strip=2 usr/include/stdlib.h}
6915 usr/include/stdlib.h
6916 $ @kbd{tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h}
6921 Notice that in both cases the file is @file{stdlib.h} extracted to the
6922 current working directory, @option{--show-transformed-names} affects
6923 only the way its name is displayed.
6925 This option is especially useful for verifying whether the invocation
6926 will have the desired effect. Thus, before running
6929 $ @kbd{tar -x --strip=@var{n}}
6933 it is often advisable to run
6936 $ @kbd{tar -t -v --show-transformed --strip=@var{n}}
6940 to make sure the command will produce the intended results.
6942 In case you need to apply more complex modifications to the file name,
6943 @GNUTAR{} provides a general-purpose transformation option:
6946 @opindex --transform
6947 @item --transform=@var{expression}
6948 Modify file names using supplied @var{expression}.
6952 The @var{expression} is a @command{sed}-like replace expression of the
6956 s/@var{regexp}/@var{replace}/[@var{flags}]
6960 where @var{regexp} is a @dfn{regular expression}, @var{replace} is a
6961 replacement for each file name part that matches @var{regexp}. Both
6962 @var{regexp} and @var{replace} are described in detail in
6963 @ref{The "s" Command, The "s" Command, The `s' Command, sed, GNU sed}.
6965 Supported @var{flags} are:
6969 Apply the replacement to @emph{all} matches to the @var{regexp}, not
6973 Use case-insensitive matching
6976 @var{regexp} is an @dfn{extended regular expression} (@pxref{Extended
6977 regexps, Extended regular expressions, Extended regular expressions,
6981 Only replace the @var{number}th match of the @var{regexp}.
6983 Note: the @var{posix} standard does not specify what should happen
6984 when you mix the @samp{g} and @var{number} modifiers. @GNUTAR{}
6985 follows the GNU @command{sed} implementation in this regard, so
6986 the the interaction is defined to be: ignore matches before the
6987 @var{number}th, and then match and replace all matches from the
6992 Any delimiter can be used in lieue of @samp{/}, the only requirement being
6993 that it be used consistently throughout the expression. For example,
6994 the following two expressions are equivalent:
7003 Changing of delimiter is often useful when the @var{regex} contains
7004 slashes. For example, it is more convenient to write:
7017 Here are several examples of @option{--transform} usage:
7020 @item Extract @file{usr/} hierarchy into @file{usr/local/}:
7023 $ @kbd{tar --transform='s,usr/,usr/local/,' -x -f arch.tar}
7026 @item Strip two leading directory components (equivalent to
7027 @option{--strip-components=2}):
7030 $ @kbd{tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar}
7033 @item Prepend @file{/prefix/} to each file name:
7036 $ @kbd{tar --transform 's,^,/prefix/,' -x -f arch.tar}
7039 @item Convert each file name to lower case:
7042 $ @kbd{tar --transform 's/.*/\L&/' -x -f arch.tar}
7047 Unlike @option{--strip-components}, @option{--transform} can be used
7048 in any @GNUTAR{} operation mode. For example, the following command
7049 adds files to the archive while replacing the leading @file{usr/}
7050 component with @file{var/}:
7053 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' /}
7056 To test @option{--transform} effect we suggest to use
7057 @option{--show-transformed-names}:
7060 $ @kbd{tar -cf arch.tar --transform='s,^usr/,var/,' \
7061 --verbose --show-transformed-names /}
7064 If both @option{--strip-components} and @option{--transform} are used
7065 together, then @option{--transform} is applied first, and the required
7066 number of components is then stripped from its result.
7069 @section Operating Only on New Files
7072 @cindex Excluding file by age
7073 @cindex Data Modification time, excluding files by
7074 @cindex Modification time, excluding files by
7075 @cindex Age, excluding files by
7076 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
7077 @option{-N @var{date}}) option causes @command{tar} to only work on
7078 files whose data modification or status change times are newer than
7079 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
7080 it is taken to be a file name; the data modification time of that file
7081 is used as the date. If you use this option when creating or appending
7082 to an archive, the archive will only include new files. If you use
7083 @option{--after-date} when extracting an archive, @command{tar} will
7084 only extract files newer than the @var{date} you specify.
7086 If you only want @command{tar} to make the date comparison based on
7087 modification of the file's data (rather than status
7088 changes), then use the @option{--newer-mtime=@var{date}} option.
7090 You may use these options with any operation. Note that these options
7091 differ from the @option{--update} (@option{-u}) operation in that they
7092 allow you to specify a particular date against which @command{tar} can
7093 compare when deciding whether or not to archive the files.
7098 @item --after-date=@var{date}
7099 @itemx --newer=@var{date}
7100 @itemx -N @var{date}
7101 Only store files newer than @var{date}.
7103 Acts on files only if their data modification or status change times are
7104 later than @var{date}. Use in conjunction with any operation.
7106 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
7107 name; the data modification time of that file is used as the date.
7109 @opindex newer-mtime
7110 @item --newer-mtime=@var{date}
7111 Acts like @option{--after-date}, but only looks at data modification times.
7114 These options limit @command{tar} to operate only on files which have
7115 been modified after the date specified. A file's status is considered to have
7116 changed if its contents have been modified, or if its owner,
7117 permissions, and so forth, have been changed. (For more information on
7118 how to specify a date, see @ref{Date input formats}; remember that the
7119 entire date argument must be quoted if it contains any spaces.)
7121 Gurus would say that @option{--after-date} tests both the data
7122 modification time (@code{mtime}, the time the contents of the file
7123 were last modified) and the status change time (@code{ctime}, the time
7124 the file's status was last changed: owner, permissions, etc.@:)
7125 fields, while @option{--newer-mtime} tests only the @code{mtime}
7128 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
7129 @code{ctime} and processes the file if either one is more recent than
7130 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
7131 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
7132 contents of the file were looked at).
7134 Date specifiers can have embedded spaces. Because of this, you may need
7135 to quote date arguments to keep the shell from parsing them as separate
7136 arguments. For example, the following command will add to the archive
7137 all the files modified less than two days ago:
7140 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
7144 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
7145 should not be used for incremental backups. @xref{Incremental Dumps},
7146 for proper way of creating incremental backups.
7150 @section Descending into Directories
7152 @cindex Avoiding recursion in directories
7153 @cindex Descending directories, avoiding
7154 @cindex Directories, avoiding recursion
7155 @cindex Recursion in directories, avoiding
7157 @FIXME{arrggh! this is still somewhat confusing to me. :-< }
7159 Usually, @command{tar} will recursively explore all directories (either
7160 those given on the command line or through the @option{--files-from}
7161 option) for the various files they contain. However, you may not always
7162 want @command{tar} to act this way.
7164 @opindex no-recursion
7165 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
7166 into specified directories. If you specify @option{--no-recursion}, you can
7167 use the @command{find} utility for hunting through levels of directories to
7168 construct a list of file names which you could then pass to @command{tar}.
7169 @command{find} allows you to be more selective when choosing which files to
7170 archive; see @ref{files}, for more information on using @command{find} with
7171 @command{tar}, or look.
7174 @item --no-recursion
7175 Prevents @command{tar} from recursively descending directories.
7179 Requires @command{tar} to recursively descend directories.
7180 This is the default.
7183 When you use @option{--no-recursion}, @GNUTAR{} grabs
7184 directory entries themselves, but does not descend on them
7185 recursively. Many people use @command{find} for locating files they
7186 want to back up, and since @command{tar} @emph{usually} recursively
7187 descends on directories, they have to use the @samp{@w{-not -type d}}
7188 test in their @command{find} invocation (@pxref{Type, Type, Type test,
7189 find, Finding Files}), as they usually do not want all the files in a
7190 directory. They then use the @option{--files-from} option to archive
7191 the files located via @command{find}.
7193 The problem when restoring files archived in this manner is that the
7194 directories themselves are not in the archive; so the
7195 @option{--same-permissions} (@option{--preserve-permissions},
7196 @option{-p}) option does not affect them---while users might really
7197 like it to. Specifying @option{--no-recursion} is a way to tell
7198 @command{tar} to grab only the directory entries given to it, adding
7199 no new files on its own. To summarize, if you use @command{find} to
7200 create a list of files to be stored in an archive, use it as follows:
7204 $ @kbd{find @var{dir} @var{tests} | \
7205 tar -cf @var{archive} -T - --no-recursion}
7209 The @option{--no-recursion} option also applies when extracting: it
7210 causes @command{tar} to extract only the matched directory entries, not
7211 the files under those directories.
7213 The @option{--no-recursion} option also affects how globbing patterns
7214 are interpreted (@pxref{controlling pattern-matching}).
7216 The @option{--no-recursion} and @option{--recursion} options apply to
7217 later options and operands, and can be overridden by later occurrences
7218 of @option{--no-recursion} and @option{--recursion}. For example:
7221 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
7225 creates an archive with one entry for @file{grape}, and the recursive
7226 contents of @file{grape/concord}, but no entries under @file{grape}
7227 other than @file{grape/concord}.
7230 @section Crossing File System Boundaries
7231 @cindex File system boundaries, not crossing
7234 @command{tar} will normally automatically cross file system boundaries in
7235 order to archive files which are part of a directory tree. You can
7236 change this behavior by running @command{tar} and specifying
7237 @option{--one-file-system}. This option only affects files that are
7238 archived because they are in a directory that is being archived;
7239 @command{tar} will still archive files explicitly named on the command line
7240 or through @option{--files-from}, regardless of where they reside.
7243 @opindex one-file-system
7244 @item --one-file-system
7245 Prevents @command{tar} from crossing file system boundaries when
7246 archiving. Use in conjunction with any write operation.
7249 The @option{--one-file-system} option causes @command{tar} to modify its
7250 normal behavior in archiving the contents of directories. If a file in
7251 a directory is not on the same file system as the directory itself, then
7252 @command{tar} will not archive that file. If the file is a directory
7253 itself, @command{tar} will not archive anything beneath it; in other words,
7254 @command{tar} will not cross mount points.
7256 This option is useful for making full or incremental archival backups of
7257 a file system. If this option is used in conjunction with
7258 @option{--verbose} (@option{-v}), files that are excluded are
7259 mentioned by name on the standard error.
7262 * directory:: Changing Directory
7263 * absolute:: Absolute File Names
7267 @subsection Changing the Working Directory
7270 @FIXME{need to read over this node now for continuity; i've switched
7271 things around some.}
7273 @cindex Changing directory mid-stream
7274 @cindex Directory, changing mid-stream
7275 @cindex Working directory, specifying
7276 To change the working directory in the middle of a list of file names,
7277 either on the command line or in a file specified using
7278 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
7279 This will change the working directory to the specified directory
7280 after that point in the list.
7284 @item --directory=@var{directory}
7285 @itemx -C @var{directory}
7286 Changes the working directory in the middle of a command line.
7292 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
7296 will place the files @file{grape} and @file{prune} from the current
7297 directory into the archive @file{jams.tar}, followed by the file
7298 @file{cherry} from the directory @file{food}. This option is especially
7299 useful when you have several widely separated files that you want to
7300 store in the same archive.
7302 Note that the file @file{cherry} is recorded in the archive under the
7303 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
7304 archive will contain three files that all appear to have come from the
7305 same directory; if the archive is extracted with plain @samp{tar
7306 --extract}, all three files will be written in the current directory.
7308 Contrast this with the command,
7311 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
7315 which records the third file in the archive under the name
7316 @file{red/cherry} so that, if the archive is extracted using
7317 @samp{tar --extract}, the third file will be written in a subdirectory
7318 named @file{orange-colored}.
7320 You can use the @option{--directory} option to make the archive
7321 independent of the original name of the directory holding the files.
7322 The following command places the files @file{/etc/passwd},
7323 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
7327 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
7331 However, the names of the archive members will be exactly what they were
7332 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
7333 They will not appear to be related by file name to the original
7334 directories where those files were located.
7336 Note that @option{--directory} options are interpreted consecutively. If
7337 @option{--directory} specifies a relative file name, it is interpreted
7338 relative to the then current directory, which might not be the same as
7339 the original current working directory of @command{tar}, due to a previous
7340 @option{--directory} option.
7342 When using @option{--files-from} (@pxref{files}), you can put various
7343 @command{tar} options (including @option{-C}) in the file list. Notice,
7344 however, that in this case the option and its argument may not be
7345 separated by whitespace. If you use short option, its argument must
7346 either follow the option letter immediately, without any intervening
7347 whitespace, or occupy the next line. Otherwise, if you use long
7348 option, separate its argument by an equal sign.
7350 For instance, the file list for the above example will be:
7365 To use it, you would invoke @command{tar} as follows:
7368 $ @kbd{tar -c -f foo.tar --files-from list}
7371 Notice also that you can only use the short option variant in the file
7372 list, i.e., always use @option{-C}, not @option{--directory}.
7374 The interpretation of @option{--directory} is disabled by
7375 @option{--null} option.
7378 @subsection Absolute File Names
7382 @opindex absolute-names
7383 @item --absolute-names
7385 Do not strip leading slashes from file names, and permit file names
7386 containing a @file{..} file name component.
7389 By default, @GNUTAR{} drops a leading @samp{/} on
7390 input or output, and complains about file names containing a @file{..}
7391 component. This option turns off this behavior.
7393 When @command{tar} extracts archive members from an archive, it strips any
7394 leading slashes (@samp{/}) from the member name. This causes absolute
7395 member names in the archive to be treated as relative file names. This
7396 allows you to have such members extracted wherever you want, instead of
7397 being restricted to extracting the member in the exact directory named
7398 in the archive. For example, if the archive member has the name
7399 @file{/etc/passwd}, @command{tar} will extract it as if the name were
7400 really @file{etc/passwd}.
7402 File names containing @file{..} can cause problems when extracting, so
7403 @command{tar} normally warns you about such files when creating an
7404 archive, and rejects attempts to extracts such files.
7406 Other @command{tar} programs do not do this. As a result, if you
7407 create an archive whose member names start with a slash, they will be
7408 difficult for other people with a non-@GNUTAR{}
7409 program to use. Therefore, @GNUTAR{} also strips
7410 leading slashes from member names when putting members into the
7411 archive. For example, if you ask @command{tar} to add the file
7412 @file{/bin/ls} to an archive, it will do so, but the member name will
7413 be @file{bin/ls}.@footnote{A side effect of this is that when
7414 @option{--create} is used with @option{--verbose} the resulting output
7415 is not, generally speaking, the same as the one you'd get running
7416 @kbd{tar --list} command. This may be important if you use some
7417 scripts for comparing both outputs. @xref{listing member and file names},
7418 for the information on how to handle this case.}
7420 If you use the @option{--absolute-names} (@option{-P}) option,
7421 @command{tar} will do none of these transformations.
7423 To archive or extract files relative to the root directory, specify
7424 the @option{--absolute-names} (@option{-P}) option.
7426 Normally, @command{tar} acts on files relative to the working
7427 directory---ignoring superior directory names when archiving, and
7428 ignoring leading slashes when extracting.
7430 When you specify @option{--absolute-names} (@option{-P}),
7431 @command{tar} stores file names including all superior directory
7432 names, and preserves leading slashes. If you only invoked
7433 @command{tar} from the root directory you would never need the
7434 @option{--absolute-names} option, but using this option
7435 may be more convenient than switching to root.
7437 @FIXME{Should be an example in the tutorial/wizardry section using this
7438 to transfer files between systems.}
7440 @FIXME{Is write access an issue?}
7443 @item --absolute-names
7444 Preserves full file names (including superior directory names) when
7445 archiving files. Preserves leading slash when extracting files.
7449 @FIXME{this is still horrible; need to talk with dan on monday.}
7451 @command{tar} prints out a message about removing the @samp{/} from
7452 file names. This message appears once per @GNUTAR{}
7453 invocation. It represents something which ought to be told; ignoring
7454 what it means can cause very serious surprises, later.
7456 Some people, nevertheless, do not want to see this message. Wanting to
7457 play really dangerously, one may of course redirect @command{tar} standard
7458 error to the sink. For example, under @command{sh}:
7461 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
7465 Another solution, both nicer and simpler, would be to change to
7466 the @file{/} directory first, and then avoid absolute notation.
7470 $ @kbd{(cd / && tar -c -f archive.tar home)}
7472 $ @kbd{tar -c -f archive.tar -C / home}
7475 @include getdate.texi
7478 @chapter Controlling the Archive Format
7480 @cindex Tar archive formats
7481 Due to historical reasons, there are several formats of tar archives.
7482 All of them are based on the same principles, but have some subtle
7483 differences that often make them incompatible with each other.
7485 GNU tar is able to create and handle archives in a variety of formats.
7486 The most frequently used formats are (in alphabetical order):
7490 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
7491 from an early @acronym{POSIX} standard, adding some improvements such as
7492 sparse file handling and incremental archives. Unfortunately these
7493 features were implemented in a way incompatible with other archive
7496 Archives in @samp{gnu} format are able to hold pathnames of unlimited
7500 Format used by @GNUTAR{} of versions prior to 1.12.
7503 Archive format, compatible with the V7 implementation of tar. This
7504 format imposes a number of limitations. The most important of them
7508 @item The maximum length of a file name is limited to 99 characters.
7509 @item The maximum length of a symbolic link is limited to 99 characters.
7510 @item It is impossible to store special files (block and character
7511 devices, fifos etc.)
7512 @item Maximum value of user or group ID is limited to 2097151 (7777777
7514 @item V7 archives do not contain symbolic ownership information (user
7515 and group name of the file owner).
7518 This format has traditionally been used by Automake when producing
7519 Makefiles. This practice will change in the future, in the meantime,
7520 however this means that projects containing filenames more than 99
7521 characters long will not be able to use @GNUTAR{} @value{VERSION} and
7522 Automake prior to 1.9.
7525 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
7526 symbolic ownership information. It is also able to store
7527 special files. However, it imposes several restrictions as well:
7530 @item The maximum length of a file name is limited to 256 characters,
7531 provided that the filename can be split at directory separator in
7532 two parts, first of them being at most 155 bytes long. So, in most
7533 cases the maximum file name length will be shorter than 256
7535 @item The maximum length of a symbolic link name is limited to
7537 @item Maximum size of a file the archive is able to accomodate
7539 @item Maximum value of UID/GID is 2097151.
7540 @item Maximum number of bits in device major and minor numbers is 21.
7544 Format used by J@"org Schilling @command{star}
7545 implementation. @GNUTAR{} is able to read @samp{star} archives but
7546 currently does not produce them.
7549 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
7550 most flexible and feature-rich format. It does not impose any
7551 restrictions on file sizes or filename lengths. This format is quite
7552 recent, so not all tar implementations are able to handle it properly.
7553 However, this format is designed in such a way that any tar
7554 implementation able to read @samp{ustar} archives will be able to read
7555 most @samp{posix} archives as well, with the only exception that any
7556 additional information (such as long file names etc.) will in such
7557 case be extracted as plain text files along with the files it refers to.
7559 This archive format will be the default format for future versions
7564 The following table summarizes the limitations of each of these
7567 @multitable @columnfractions .10 .20 .20 .20 .20
7568 @headitem Format @tab UID @tab File Size @tab Path Name @tab Devn
7569 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
7570 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
7571 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
7572 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
7573 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
7576 The default format for @GNUTAR{} is defined at compilation
7577 time. You may check it by running @command{tar --help}, and examining
7578 the last lines of its output. Usually, @GNUTAR{} is configured
7579 to create archives in @samp{gnu} format, however, future version will
7580 switch to @samp{posix}.
7583 * Portability:: Making @command{tar} Archives More Portable
7584 * Compression:: Using Less Space through Compression
7585 * Attributes:: Handling File Attributes
7586 * Standard:: The Standard Format
7587 * Extensions:: @acronym{GNU} Extensions to the Archive Format
7588 * cpio:: Comparison of @command{tar} and @command{cpio}
7592 @section Making @command{tar} Archives More Portable
7594 Creating a @command{tar} archive on a particular system that is meant to be
7595 useful later on many other machines and with other versions of @command{tar}
7596 is more challenging than you might think. @command{tar} archive formats
7597 have been evolving since the first versions of Unix. Many such formats
7598 are around, and are not always compatible with each other. This section
7599 discusses a few problems, and gives some advice about making @command{tar}
7600 archives more portable.
7602 One golden rule is simplicity. For example, limit your @command{tar}
7603 archives to contain only regular files and directories, avoiding
7604 other kind of special files. Do not attempt to save sparse files or
7605 contiguous files as such. Let's discuss a few more problems, in turn.
7607 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
7608 archives and archive labels) in GNU and PAX formats.}
7611 * Portable Names:: Portable Names
7612 * dereference:: Symbolic Links
7613 * old:: Old V7 Archives
7614 * ustar:: Ustar Archives
7615 * gnu:: GNU and old GNU format archives.
7616 * posix:: @acronym{POSIX} archives
7617 * Checksumming:: Checksumming Problems
7618 * Large or Negative Values:: Large files, negative time stamps, etc.
7621 @node Portable Names
7622 @subsection Portable Names
7624 Use portable file and member names. A name is portable if it contains
7625 only ASCII letters and digits, @samp{/}, @samp{.}, @samp{_}, and
7626 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
7627 contain @samp{/-}. Avoid deep directory nesting. For portability to
7628 old Unix hosts, limit your file name components to 14 characters or
7631 If you intend to have your @command{tar} archives to be read under
7632 MSDOS, you should not rely on case distinction for file names, and you
7633 might use the @acronym{GNU} @command{doschk} program for helping you
7634 further diagnosing illegal MSDOS names, which are even more limited
7638 @subsection Symbolic Links
7639 @cindex File names, using symbolic links
7640 @cindex Symbolic link as file name
7642 @opindex dereference
7643 Normally, when @command{tar} archives a symbolic link, it writes a
7644 block to the archive naming the target of the link. In that way, the
7645 @command{tar} archive is a faithful record of the file system contents.
7646 @option{--dereference} (@option{-h}) is used with @option{--create} (@option{-c}), and causes
7647 @command{tar} to archive the files symbolic links point to, instead of
7648 the links themselves. When this option is used, when @command{tar}
7649 encounters a symbolic link, it will archive the linked-to file,
7650 instead of simply recording the presence of a symbolic link.
7652 The name under which the file is stored in the file system is not
7653 recorded in the archive. To record both the symbolic link name and
7654 the file name in the system, archive the file under both names. If
7655 all links were recorded automatically by @command{tar}, an extracted file
7656 might be linked to a file name that no longer exists in the file
7659 If a linked-to file is encountered again by @command{tar} while creating
7660 the same archive, an entire second copy of it will be stored. (This
7661 @emph{might} be considered a bug.)
7663 So, for portable archives, do not archive symbolic links as such,
7664 and use @option{--dereference} (@option{-h}): many systems do not support
7665 symbolic links, and moreover, your distribution might be unusable if
7666 it contains unresolved symbolic links.
7669 @subsection Old V7 Archives
7670 @cindex Format, old style
7671 @cindex Old style format
7672 @cindex Old style archives
7673 @cindex v7 archive format
7675 Certain old versions of @command{tar} cannot handle additional
7676 information recorded by newer @command{tar} programs. To create an
7677 archive in V7 format (not ANSI), which can be read by these old
7678 versions, specify the @option{--format=v7} option in
7679 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
7680 accepts @option{--portability} or @samp{op-old-archive} for this
7681 option). When you specify it,
7682 @command{tar} leaves out information about directories, pipes, fifos,
7683 contiguous files, and device files, and specifies file ownership by
7684 group and user IDs instead of group and user names.
7686 When updating an archive, do not use @option{--format=v7}
7687 unless the archive was created using this option.
7689 In most cases, a @emph{new} format archive can be read by an @emph{old}
7690 @command{tar} program without serious trouble, so this option should
7691 seldom be needed. On the other hand, most modern @command{tar}s are
7692 able to read old format archives, so it might be safer for you to
7693 always use @option{--format=v7} for your distributions.
7696 @subsection Ustar Archive Format
7698 @cindex ustar archive format
7699 Archive format defined by @acronym{POSIX}.1-1988 specification is called
7700 @code{ustar}. Although it is more flexible than the V7 format, it
7701 still has many restrictions (@xref{Formats,ustar}, for the detailed
7702 description of @code{ustar} format). Along with V7 format,
7703 @code{ustar} format is a good choice for archives intended to be read
7704 with other implementations of @command{tar}.
7706 To create archive in @code{ustar} format, use @option{--format=ustar}
7707 option in conjunction with the @option{--create} (@option{-c}).
7710 @subsection @acronym{GNU} and old @GNUTAR{} format
7712 @cindex GNU archive format
7713 @cindex Old GNU archive format
7714 @GNUTAR{} was based on an early draft of the
7715 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
7716 @command{tar}, such as the support for file names longer than 100
7717 characters, use portions of the @command{tar} header record which were
7718 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
7719 @acronym{POSIX} have allocated the same parts of the header record for
7720 other purposes. As a result, @GNUTAR{} format is
7721 incompatible with the current @acronym{POSIX} specification, and with
7722 @command{tar} programs that follow it.
7724 In the majority of cases, @command{tar} will be configured to create
7725 this format by default. This will change in the future releases, since
7726 we plan to make @samp{posix} format the default.
7728 To force creation a @GNUTAR{} archive, use option
7729 @option{--format=gnu}.
7732 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
7734 @cindex POSIX archive format
7735 @cindex PAX archive format
7736 The version @value{VERSION} of @GNUTAR{} is able
7737 to read and create archives conforming to @acronym{POSIX.1-2001} standard.
7739 A @acronym{POSIX} conformant archive will be created if @command{tar}
7740 was given @option{--format=posix} option.
7743 @subsection Checksumming Problems
7745 SunOS and HP-UX @command{tar} fail to accept archives created using
7746 @GNUTAR{} and containing non-ASCII file names, that
7747 is, file names having characters with the eight bit set, because they
7748 use signed checksums, while @GNUTAR{} uses unsigned
7749 checksums while creating archives, as per @acronym{POSIX} standards. On
7750 reading, @GNUTAR{} computes both checksums and
7751 accept any. It is somewhat worrying that a lot of people may go
7752 around doing backup of their files using faulty (or at least
7753 non-standard) software, not learning about it until it's time to
7754 restore their missing files with an incompatible file extractor, or
7757 @GNUTAR{} compute checksums both ways, and accept
7758 any on read, so @acronym{GNU} tar can read Sun tapes even with their
7759 wrong checksums. @GNUTAR{} produces the standard
7760 checksum, however, raising incompatibilities with Sun. That is to
7761 say, @GNUTAR{} has not been modified to
7762 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
7763 I've been told that more recent Sun @command{tar} now read standard
7764 archives, so maybe Sun did a similar patch, after all?
7766 The story seems to be that when Sun first imported @command{tar}
7767 sources on their system, they recompiled it without realizing that
7768 the checksums were computed differently, because of a change in
7769 the default signing of @code{char}'s in their compiler. So they
7770 started computing checksums wrongly. When they later realized their
7771 mistake, they merely decided to stay compatible with it, and with
7772 themselves afterwards. Presumably, but I do not really know, HP-UX
7773 has chosen that their @command{tar} archives to be compatible with Sun's.
7774 The current standards do not favor Sun @command{tar} format. In any
7775 case, it now falls on the shoulders of SunOS and HP-UX users to get
7776 a @command{tar} able to read the good archives they receive.
7778 @node Large or Negative Values
7779 @subsection Large or Negative Values
7780 @cindex large values
7781 @cindex future time stamps
7782 @cindex negative time stamps
7785 The above sections suggest to use @samp{oldest possible} archive
7786 format if in doubt. However, sometimes it is not possible. If you
7787 attempt to archive a file whose metadata cannot be represented using
7788 required format, @GNUTAR{} will print error message and ignore such a
7789 file. You will than have to switch to a format that is able to
7790 handle such values. The format summary table (@pxref{Formats}) will
7793 In particular, when trying to archive files larger than 8GB or with
7794 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
7795 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
7796 @acronym{POSIX} archive formats. When considering which format to
7797 choose, bear in mind that the @acronym{GNU} format uses
7798 two's-complement base-256 notation to store values that do not fit
7799 into standard @acronym{ustar} range. Such archives can generally be
7800 read only by a @GNUTAR{} implementation. Moreover, they sometimes
7801 cannot be correctly restored on another hosts even by @GNUTAR{}. For
7802 example, using two's complement representation for negative time
7803 stamps that assumes a signed 32-bit @code{time_t} generates archives
7804 that are not portable to hosts with differing @code{time_t}
7807 On the other hand, @acronym{POSIX} archives, generally speaking, can
7808 be extracted by any tar implementation that understands older
7809 @acronym{ustar} format. The only exception are files larger than 8GB.
7811 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
7815 @section Using Less Space through Compression
7818 * gzip:: Creating and Reading Compressed Archives
7819 * sparse:: Archiving Sparse Files
7823 @subsection Creating and Reading Compressed Archives
7824 @cindex Compressed archives
7825 @cindex Storing archives in compressed format
7827 @GNUTAR{} is able to create and read compressed archives. It supports
7828 @command{gzip} and @command{bzip2} compression programs. For backward
7829 compatibilty, it also supports @command{compress} command, although
7830 we strongly recommend against using it, since there is a patent
7831 covering the algorithm it uses and you could be sued for patent
7832 infringement merely by running @command{compress}! Besides, it is less
7833 effective than @command{gzip} and @command{bzip2}.
7835 Creating a compressed archive is simple: you just specify a
7836 @dfn{compression option} along with the usual archive creation
7837 commands. The compression option is @option{-z} (@option{--gzip}) to
7838 create a @command{gzip} compressed archive, @option{-j}
7839 (@option{--bzip2}) to create a @command{bzip2} compressed archive, and
7840 @option{-Z} (@option{--compress}) to use @command{compress} program.
7844 $ @kbd{tar cfz archive.tar.gz .}
7847 Reading compressed archive is even simpler: you don't need to specify
7848 any additional options as @GNUTAR{} recognizes its format
7849 automatically. Thus, the following commands will list and extract the
7850 archive created in previous example:
7853 # List the compressed archive
7854 $ @kbd{tar tf archive.tar.gz}
7855 # Extract the compressed archive
7856 $ @kbd{tar xf archive.tar.gz}
7859 The only case when you have to specify a decompression option while
7860 reading the archive is when reading from a pipe or from a tape drive
7861 that does not support random access. However, in this case @GNUTAR{}
7862 will indicate which option you should use. For example:
7865 $ @kbd{cat archive.tar.gz | tar tf -}
7866 tar: Archive is compressed. Use -z option
7867 tar: Error is not recoverable: exiting now
7870 If you see such diagnostics, just add the suggested option to the
7871 invocation of @GNUTAR{}:
7874 $ @kbd{cat archive.tar.gz | tar tfz -}
7877 Notice also, that there are several restrictions on operations on
7878 compressed archives. First of all, compressed archives cannot be
7879 modified, i.e., you cannot update (@option{--update} (@option{-u})) them or delete
7880 (@option{--delete}) members from them. Likewise, you cannot append
7881 another @command{tar} archive to a compressed archive using
7882 @option{--append} (@option{-r})). Secondly, multi-volume archives cannot be
7885 The following table summarizes compression options used by @GNUTAR{}.
7893 Filter the archive through @command{gzip}.
7895 You can use @option{--gzip} and @option{--gunzip} on physical devices
7896 (tape drives, etc.) and remote files as well as on normal files; data
7897 to or from such devices or remote files is reblocked by another copy
7898 of the @command{tar} program to enforce the specified (or default) record
7899 size. The default compression parameters are used; if you need to
7900 override them, set @env{GZIP} environment variable, e.g.:
7903 $ @kbd{GZIP=--best tar cfz archive.tar.gz subdir}
7907 Another way would be to avoid the @option{--gzip} (@option{--gunzip}, @option{--ungzip}, @option{-z}) option and run
7908 @command{gzip} explicitly:
7911 $ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz}
7914 @cindex corrupted archives
7915 About corrupted compressed archives: @command{gzip}'ed files have no
7916 redundancy, for maximum compression. The adaptive nature of the
7917 compression scheme means that the compression tables are implicitly
7918 spread all over the archive. If you lose a few blocks, the dynamic
7919 construction of the compression tables becomes unsynchronized, and there
7920 is little chance that you could recover later in the archive.
7922 There are pending suggestions for having a per-volume or per-file
7923 compression in @GNUTAR{}. This would allow for viewing the
7924 contents without decompression, and for resynchronizing decompression at
7925 every volume or file, in case of corrupted archives. Doing so, we might
7926 lose some compressibility. But this would have make recovering easier.
7927 So, there are pros and cons. We'll see!
7932 Filter the archive through @code{bzip2}. Otherwise like @option{--gzip}.
7939 Filter the archive through @command{compress}. Otherwise like @option{--gzip}.
7941 The @acronym{GNU} Project recommends you not use
7942 @command{compress}, because there is a patent covering the algorithm it
7943 uses. You could be sued for patent infringement merely by running
7946 @opindex use-compress-program
7947 @item --use-compress-program=@var{prog}
7948 Use external compression program @var{prog}. Use this option if you
7949 have a compression program that @GNUTAR{} does not support. There
7950 are two requirements to which @var{prog} should comply:
7952 First, when called without options, it should read data from standard
7953 input, compress it and output it on standard output.
7955 Secondly, if called with @option{-d} argument, it should do exactly
7956 the opposite, i.e., read the compressed data from the standard input
7957 and produce uncompressed data on the standard output.
7960 @cindex gpg, using with tar
7961 @cindex gnupg, using with tar
7962 @cindex Using encrypted archives
7963 The @option{--use-compress-program} option, in particular, lets you
7964 implement your own filters, not necessarily dealing with
7965 compression/decomression. For example, suppose you wish to implement
7966 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
7967 gpg, gpg ---- encryption and signing tool, gpg}). The following
7974 -d) gpg --decrypt - | gzip -d -c;;
7975 '') gzip -c | gpg -s ;;
7976 *) echo "Unknown option $1">&2; exit 1;;
7981 Suppose you name it @file{gpgz} and save it somewhere in your
7982 @env{PATH}. Then the following command will create a commpressed
7983 archive signed with your private key:
7986 $ @kbd{tar -cf foo.tar.gpgz --use-compress=gpgz .}
7990 Likewise, the following command will list its contents:
7993 $ @kbd{tar -tf foo.tar.gpgz --use-compress=gpgz .}
7997 The above is based on the following discussion:
7999 I have one question, or maybe it's a suggestion if there isn't a way
8000 to do it now. I would like to use @option{--gzip}, but I'd also like
8001 the output to be fed through a program like @acronym{GNU}
8002 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
8003 to use :-)), basically adding ECC protection on top of compression.
8004 It seems as if this should be quite easy to do, but I can't work out
8005 exactly how to go about it. Of course, I can pipe the standard output
8006 of @command{tar} through @command{ecc}, but then I lose (though I
8007 haven't started using it yet, I confess) the ability to have
8008 @command{tar} use @command{rmt} for it's I/O (I think).
8010 I think the most straightforward thing would be to let me specify a
8011 general set of filters outboard of compression (preferably ordered,
8012 so the order can be automatically reversed on input operations, and
8013 with the options they require specifiable), but beggars shouldn't be
8014 choosers and anything you decide on would be fine with me.
8016 By the way, I like @command{ecc} but if (as the comments say) it can't
8017 deal with loss of block sync, I'm tempted to throw some time at adding
8018 that capability. Supposing I were to actually do such a thing and
8019 get it (apparently) working, do you accept contributed changes to
8020 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
8022 Isn't that exactly the role of the
8023 @option{--use-compress-prog=@var{program}} option?
8024 I never tried it myself, but I suspect you may want to write a
8025 @var{prog} script or program able to filter stdin to stdout to
8026 way you want. It should recognize the @option{-d} option, for when
8027 extraction is needed rather than creation.
8029 It has been reported that if one writes compressed data (through the
8030 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
8031 the DLT compression mode, the data will actually get bigger and one will
8032 end up with less space on the tape.
8036 @subsection Archiving Sparse Files
8037 @cindex Sparse Files
8044 Handle sparse files efficiently.
8047 This option causes all files to be put in the archive to be tested for
8048 sparseness, and handled specially if they are. The @option{--sparse}
8049 (@option{-S}) option is useful when many @code{dbm} files, for example, are being
8050 backed up. Using this option dramatically decreases the amount of
8051 space needed to store such a file.
8053 In later versions, this option may be removed, and the testing and
8054 treatment of sparse files may be done automatically with any special
8055 @acronym{GNU} options. For now, it is an option needing to be specified on
8056 the command line with the creation or updating of an archive.
8058 Files in the file system occasionally have ``holes.'' A hole in a file
8059 is a section of the file's contents which was never written. The
8060 contents of a hole read as all zeros. On many operating systems,
8061 actual disk storage is not allocated for holes, but they are counted
8062 in the length of the file. If you archive such a file, @command{tar}
8063 could create an archive longer than the original. To have @command{tar}
8064 attempt to recognize the holes in a file, use @option{--sparse} (@option{-S}). When
8065 you use this option, then, for any file using less disk space than
8066 would be expected from its length, @command{tar} searches the file for
8067 consecutive stretches of zeros. It then records in the archive for
8068 the file where the consecutive stretches of zeros are, and only
8069 archives the ``real contents'' of the file. On extraction (using
8070 @option{--sparse} is not needed on extraction) any such
8071 files have holes created wherever the continuous stretches of zeros
8072 were found. Thus, if you use @option{--sparse}, @command{tar} archives
8073 won't take more space than the original.
8075 A file is sparse if it contains blocks of zeros whose existence is
8076 recorded, but that have no space allocated on disk. When you specify
8077 the @option{--sparse} option in conjunction with the @option{--create}
8078 (@option{-c}) operation, @command{tar} tests all files for sparseness
8079 while archiving. If @command{tar} finds a file to be sparse, it uses a
8080 sparse representation of the file in the archive. @xref{create}, for
8081 more information about creating archives.
8083 @option{--sparse} is useful when archiving files, such as dbm files,
8084 likely to contain many nulls. This option dramatically
8085 decreases the amount of space needed to store such an archive.
8088 @strong{Please Note:} Always use @option{--sparse} when performing file
8089 system backups, to avoid archiving the expanded forms of files stored
8090 sparsely in the system.
8092 Even if your system has no sparse files currently, some may be
8093 created in the future. If you use @option{--sparse} while making file
8094 system backups as a matter of course, you can be assured the archive
8095 will never take more space on the media than the files take on disk
8096 (otherwise, archiving a disk filled with sparse files might take
8097 hundreds of tapes). @xref{Incremental Dumps}.
8100 @command{tar} ignores the @option{--sparse} option when reading an archive.
8105 Files stored sparsely in the file system are represented sparsely in
8106 the archive. Use in conjunction with write operations.
8109 However, users should be well aware that at archive creation time,
8110 @GNUTAR{} still has to read whole disk file to
8111 locate the @dfn{holes}, and so, even if sparse files use little space
8112 on disk and in the archive, they may sometimes require inordinate
8113 amount of time for reading and examining all-zero blocks of a file.
8114 Although it works, it's painfully slow for a large (sparse) file, even
8115 though the resulting tar archive may be small. (One user reports that
8116 dumping a @file{core} file of over 400 megabytes, but with only about
8117 3 megabytes of actual data, took about 9 minutes on a Sun Sparcstation
8118 ELC, with full CPU utilization.)
8120 This reading is required in all cases and is not related to the fact
8121 the @option{--sparse} option is used or not, so by merely @emph{not}
8122 using the option, you are not saving time@footnote{Well! We should say
8123 the whole truth, here. When @option{--sparse} is selected while creating
8124 an archive, the current @command{tar} algorithm requires sparse files to be
8125 read twice, not once. We hope to develop a new archive format for saving
8126 sparse files in which one pass will be sufficient.}.
8128 Programs like @command{dump} do not have to read the entire file; by
8129 examining the file system directly, they can determine in advance
8130 exactly where the holes are and thus avoid reading through them. The
8131 only data it need read are the actual allocated data blocks.
8132 @GNUTAR{} uses a more portable and straightforward
8133 archiving approach, it would be fairly difficult that it does
8134 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
8138 What I did say is that you cannot tell the difference between a hole and an
8139 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
8140 best tells you how many holes there are; it doesn't tell you @emph{where}.
8141 Just as programs may, conceivably, care what @code{st_blocks} is (care
8142 to name one that does?), they may also care where the holes are (I have
8143 no examples of this one either, but it's equally imaginable).
8145 I conclude from this that good archivers are not portable. One can
8146 arguably conclude that if you want a portable program, you can in good
8147 conscience restore files with as many holes as possible, since you can't
8152 @section Handling File Attributes
8155 When @command{tar} reads files, it updates their access times. To
8156 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
8157 reset the access time retroactively or avoid changing it in the first
8160 Handling of file attributes
8163 @opindex atime-preserve
8164 @item --atime-preserve
8165 @itemx --atime-preserve=replace
8166 @itemx --atime-preserve=system
8167 Preserve the access times of files that are read. This works only for
8168 files that you own, unless you have superuser privileges.
8170 @option{--atime-preserve=replace} works on most systems, but it also
8171 restores the data modification time and updates the status change
8172 time. Hence it doesn't interact with incremental dumps nicely
8173 (@pxref{Backups}), and it can set access or data modification times
8174 incorrectly if other programs access the file while @command{tar} is
8177 @option{--atime-preserve=system} avoids changing the access time in
8178 the first place, if the operating system supports this.
8179 Unfortunately, this may or may not work on any given operating system
8180 or file system. If @command{tar} knows for sure it won't work, it
8181 complains right away.
8183 Currently @option{--atime-preserve} with no operand defaults to
8184 @option{--atime-preserve=replace}, but this is intended to change to
8185 @option{--atime-preserve=system} when the latter is better-supported.
8190 Do not extract data modification time.
8192 When this option is used, @command{tar} leaves the data modification times
8193 of the files it extracts as the times when the files were extracted,
8194 instead of setting it to the times recorded in the archive.
8196 This option is meaningless with @option{--list} (@option{-t}).
8200 Create extracted files with the same ownership they have in the
8203 This is the default behavior for the superuser,
8204 so this option is meaningful only for non-root users, when @command{tar}
8205 is executed on those systems able to give files away. This is
8206 considered as a security flaw by many people, at least because it
8207 makes quite difficult to correctly account users for the disk space
8208 they occupy. Also, the @code{suid} or @code{sgid} attributes of
8209 files are easily and silently lost when files are given away.
8211 When writing an archive, @command{tar} writes the user id and user name
8212 separately. If it can't find a user name (because the user id is not
8213 in @file{/etc/passwd}), then it does not write one. When restoring,
8214 it tries to look the name (if one was written) up in
8215 @file{/etc/passwd}. If it fails, then it uses the user id stored in
8216 the archive instead.
8218 @opindex no-same-owner
8219 @item --no-same-owner
8221 Do not attempt to restore ownership when extracting. This is the
8222 default behavior for ordinary users, so this option has an effect
8223 only for the superuser.
8225 @opindex numeric-owner
8226 @item --numeric-owner
8227 The @option{--numeric-owner} option allows (ANSI) archives to be written
8228 without user/group name information or such information to be ignored
8229 when extracting. It effectively disables the generation and/or use
8230 of user/group name information. This option forces extraction using
8231 the numeric ids from the archive, ignoring the names.
8233 This is useful in certain circumstances, when restoring a backup from
8234 an emergency floppy with different passwd/group files for example.
8235 It is otherwise impossible to extract files with the right ownerships
8236 if the password file in use during the extraction does not match the
8237 one belonging to the file system(s) being extracted. This occurs,
8238 for example, if you are restoring your files after a major crash and
8239 had booted from an emergency floppy with no password file or put your
8240 disk into another machine to do the restore.
8242 The numeric ids are @emph{always} saved into @command{tar} archives.
8243 The identifying names are added at create time when provided by the
8244 system, unless @option{--old-archive} (@option{-o}) is used. Numeric ids could be
8245 used when moving archives between a collection of machines using
8246 a centralized management for attribution of numeric ids to users
8247 and groups. This is often made through using the NIS capabilities.
8249 When making a @command{tar} file for distribution to other sites, it
8250 is sometimes cleaner to use a single owner for all files in the
8251 distribution, and nicer to specify the write permission bits of the
8252 files as stored in the archive independently of their actual value on
8253 the file system. The way to prepare a clean distribution is usually
8254 to have some Makefile rule creating a directory, copying all needed
8255 files in that directory, then setting ownership and permissions as
8256 wanted (there are a lot of possible schemes), and only then making a
8257 @command{tar} archive out of this directory, before cleaning
8258 everything out. Of course, we could add a lot of options to
8259 @GNUTAR{} for fine tuning permissions and ownership.
8260 This is not the good way, I think. @GNUTAR{} is
8261 already crowded with options and moreover, the approach just explained
8262 gives you a great deal of control already.
8264 @opindex same-permissions, short description
8265 @opindex preserve-permissions, short description
8267 @itemx --same-permissions
8268 @itemx --preserve-permissions
8269 Extract all protection information.
8271 This option causes @command{tar} to set the modes (access permissions) of
8272 extracted files exactly as recorded in the archive. If this option
8273 is not used, the current @code{umask} setting limits the permissions
8274 on extracted files. This option is by default enabled when
8275 @command{tar} is executed by a superuser.
8278 This option is meaningless with @option{--list} (@option{-t}).
8282 Same as both @option{--same-permissions} and @option{--same-order}.
8284 The @option{--preserve} option has no equivalent short option name.
8285 It is equivalent to @option{--same-permissions} plus @option{--same-order}.
8287 @FIXME{I do not see the purpose of such an option. (Neither I. FP.)
8288 Neither do I. --Sergey}
8293 @section Basic Tar Format
8296 While an archive may contain many files, the archive itself is a
8297 single ordinary file. Like any other file, an archive file can be
8298 written to a storage device such as a tape or disk, sent through a
8299 pipe or over a network, saved on the active file system, or even
8300 stored in another archive. An archive file is not easy to read or
8301 manipulate without using the @command{tar} utility or Tar mode in
8302 @acronym{GNU} Emacs.
8304 Physically, an archive consists of a series of file entries terminated
8305 by an end-of-archive entry, which consists of two 512 blocks of zero
8307 entry usually describes one of the files in the archive (an
8308 @dfn{archive member}), and consists of a file header and the contents
8309 of the file. File headers contain file names and statistics, checksum
8310 information which @command{tar} uses to detect file corruption, and
8311 information about file types.
8313 Archives are permitted to have more than one member with the same
8314 member name. One way this situation can occur is if more than one
8315 version of a file has been stored in the archive. For information
8316 about adding new versions of a file to an archive, see @ref{update}.
8317 @FIXME-xref{To learn more about having more than one archive member with the
8318 same name, see -backup node, when it's written.}
8320 In addition to entries describing archive members, an archive may
8321 contain entries which @command{tar} itself uses to store information.
8322 @xref{label}, for an example of such an archive entry.
8324 A @command{tar} archive file contains a series of blocks. Each block
8325 contains @code{BLOCKSIZE} bytes. Although this format may be thought
8326 of as being on magnetic tape, other media are often used.
8328 Each file archived is represented by a header block which describes
8329 the file, followed by zero or more blocks which give the contents
8330 of the file. At the end of the archive file there are two 512-byte blocks
8331 filled with binary zeros as an end-of-file marker. A reasonable system
8332 should write such end-of-file marker at the end of an archive, but
8333 must not assume that such a block exists when reading an archive. In
8334 particular @GNUTAR{} always issues a warning if it does not encounter it.
8336 The blocks may be @dfn{blocked} for physical I/O operations.
8337 Each record of @var{n} blocks (where @var{n} is set by the
8338 @option{--blocking-factor=@var{512-size}} (@option{-b @var{512-size}}) option to @command{tar}) is written with a single
8339 @w{@samp{write ()}} operation. On magnetic tapes, the result of
8340 such a write is a single record. When writing an archive,
8341 the last record of blocks should be written at the full size, with
8342 blocks after the zero block containing all zeros. When reading
8343 an archive, a reasonable system should properly handle an archive
8344 whose last record is shorter than the rest, or which contains garbage
8345 records after a zero block.
8347 The header block is defined in C as follows. In the @GNUTAR{}
8348 distribution, this is part of file @file{src/tar.h}:
8351 @include header.texi
8354 All characters in header blocks are represented by using 8-bit
8355 characters in the local variant of ASCII. Each field within the
8356 structure is contiguous; that is, there is no padding used within
8357 the structure. Each character on the archive medium is stored
8360 Bytes representing the contents of files (after the header block
8361 of each file) are not translated in any way and are not constrained
8362 to represent characters in any character set. The @command{tar} format
8363 does not distinguish text files from binary files, and no translation
8364 of file contents is performed.
8366 The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and
8367 @code{gname} are null-terminated character strings. All other fields
8368 are zero-filled octal numbers in ASCII. Each numeric field of width
8369 @var{w} contains @var{w} minus 1 digits, and a null.
8371 The @code{name} field is the file name of the file, with directory names
8372 (if any) preceding the file name, separated by slashes.
8374 @FIXME{how big a name before field overflows?}
8376 The @code{mode} field provides nine bits specifying file permissions
8377 and three bits to specify the Set UID, Set GID, and Save Text
8378 (@dfn{sticky}) modes. Values for these bits are defined above.
8379 When special permissions are required to create a file with a given
8380 mode, and the user restoring files from the archive does not hold such
8381 permissions, the mode bit(s) specifying those special permissions
8382 are ignored. Modes which are not supported by the operating system
8383 restoring files from the archive will be ignored. Unsupported modes
8384 should be faked up when creating or updating an archive; e.g., the
8385 group permission could be copied from the @emph{other} permission.
8387 The @code{uid} and @code{gid} fields are the numeric user and group
8388 ID of the file owners, respectively. If the operating system does
8389 not support numeric user or group IDs, these fields should be ignored.
8391 The @code{size} field is the size of the file in bytes; linked files
8392 are archived with this field specified as zero. @FIXME-xref{Modifiers, in
8393 particular the @option{--incremental} (@option{-G}) option.}
8395 The @code{mtime} field is the data modification time of the file at
8396 the time it was archived. It is the ASCII representation of the octal
8397 value of the last time the file's contents were modified, represented
8398 as an integer number of
8399 seconds since January 1, 1970, 00:00 Coordinated Universal Time.
8401 The @code{chksum} field is the ASCII representation of the octal value
8402 of the simple sum of all bytes in the header block. Each 8-bit
8403 byte in the header is added to an unsigned integer, initialized to
8404 zero, the precision of which shall be no less than seventeen bits.
8405 When calculating the checksum, the @code{chksum} field is treated as
8406 if it were all blanks.
8408 The @code{typeflag} field specifies the type of file archived. If a
8409 particular implementation does not recognize or permit the specified
8410 type, the file will be extracted as if it were a regular file. As this
8411 action occurs, @command{tar} issues a warning to the standard error.
8413 The @code{atime} and @code{ctime} fields are used in making incremental
8414 backups; they store, respectively, the particular file's access and
8415 status change times.
8417 The @code{offset} is used by the @option{--multi-volume} (@option{-M}) option, when
8418 making a multi-volume archive. The offset is number of bytes into
8419 the file that we need to restart at to continue the file on the next
8420 tape, i.e., where we store the location that a continued file is
8423 The following fields were added to deal with sparse files. A file
8424 is @dfn{sparse} if it takes in unallocated blocks which end up being
8425 represented as zeros, i.e., no useful data. A test to see if a file
8426 is sparse is to look at the number blocks allocated for it versus the
8427 number of characters in the file; if there are fewer blocks allocated
8428 for the file than would normally be allocated for a file of that
8429 size, then the file is sparse. This is the method @command{tar} uses to
8430 detect a sparse file, and once such a file is detected, it is treated
8431 differently from non-sparse files.
8433 Sparse files are often @code{dbm} files, or other database-type files
8434 which have data at some points and emptiness in the greater part of
8435 the file. Such files can appear to be very large when an @samp{ls
8436 -l} is done on them, when in truth, there may be a very small amount
8437 of important data contained in the file. It is thus undesirable
8438 to have @command{tar} think that it must back up this entire file, as
8439 great quantities of room are wasted on empty blocks, which can lead
8440 to running out of room on a tape far earlier than is necessary.
8441 Thus, sparse files are dealt with so that these empty blocks are
8442 not written to the tape. Instead, what is written to the tape is a
8443 description, of sorts, of the sparse file: where the holes are, how
8444 big the holes are, and how much data is found at the end of the hole.
8445 This way, the file takes up potentially far less room on the tape,
8446 and when the file is extracted later on, it will look exactly the way
8447 it looked beforehand. The following is a description of the fields
8448 used to handle a sparse file:
8450 The @code{sp} is an array of @code{struct sparse}. Each @code{struct
8451 sparse} contains two 12-character strings which represent an offset
8452 into the file and a number of bytes to be written at that offset.
8453 The offset is absolute, and not relative to the offset in preceding
8456 The header can hold four of these @code{struct sparse} at the moment;
8457 if more are needed, they are not stored in the header.
8459 The @code{isextended} flag is set when an @code{extended_header}
8460 is needed to deal with a file. Note that this means that this flag
8461 can only be set when dealing with a sparse file, and it is only set
8462 in the event that the description of the file will not fit in the
8463 allotted room for sparse structures in the header. In other words,
8464 an extended_header is needed.
8466 The @code{extended_header} structure is used for sparse files which
8467 need more sparse structures than can fit in the header. The header can
8468 fit 4 such structures; if more are needed, the flag @code{isextended}
8469 gets set and the next block is an @code{extended_header}.
8471 Each @code{extended_header} structure contains an array of 21
8472 sparse structures, along with a similar @code{isextended} flag
8473 that the header had. There can be an indeterminate number of such
8474 @code{extended_header}s to describe a sparse file.
8478 @item @code{REGTYPE}
8479 @itemx @code{AREGTYPE}
8480 These flags represent a regular file. In order to be compatible
8481 with older versions of @command{tar}, a @code{typeflag} value of
8482 @code{AREGTYPE} should be silently recognized as a regular file.
8483 New archives should be created using @code{REGTYPE}. Also, for
8484 backward compatibility, @command{tar} treats a regular file whose name
8485 ends with a slash as a directory.
8487 @item @code{LNKTYPE}
8488 This flag represents a file linked to another file, of any type,
8489 previously archived. Such files are identified in Unix by each
8490 file having the same device and inode number. The linked-to name is
8491 specified in the @code{linkname} field with a trailing null.
8493 @item @code{SYMTYPE}
8494 This represents a symbolic link to another file. The linked-to name
8495 is specified in the @code{linkname} field with a trailing null.
8497 @item @code{CHRTYPE}
8498 @itemx @code{BLKTYPE}
8499 These represent character special files and block special files
8500 respectively. In this case the @code{devmajor} and @code{devminor}
8501 fields will contain the major and minor device numbers respectively.
8502 Operating systems may map the device specifications to their own
8503 local specification, or may ignore the entry.
8505 @item @code{DIRTYPE}
8506 This flag specifies a directory or sub-directory. The directory
8507 name in the @code{name} field should end with a slash. On systems where
8508 disk allocation is performed on a directory basis, the @code{size} field
8509 will contain the maximum number of bytes (which may be rounded to
8510 the nearest disk block allocation unit) which the directory may
8511 hold. A @code{size} field of zero indicates no such limiting. Systems
8512 which do not support limiting in this manner should ignore the
8515 @item @code{FIFOTYPE}
8516 This specifies a FIFO special file. Note that the archiving of a
8517 FIFO file archives the existence of this file and not its contents.
8519 @item @code{CONTTYPE}
8520 This specifies a contiguous file, which is the same as a normal
8521 file except that, in operating systems which support it, all its
8522 space is allocated contiguously on the disk. Operating systems
8523 which do not allow contiguous allocation should silently treat this
8524 type as a normal file.
8526 @item @code{A} @dots{} @code{Z}
8527 These are reserved for custom implementations. Some of these are
8528 used in the @acronym{GNU} modified format, as described below.
8532 Other values are reserved for specification in future revisions of
8533 the P1003 standard, and should not be used by any @command{tar} program.
8535 The @code{magic} field indicates that this archive was output in
8536 the P1003 archive format. If this field contains @code{TMAGIC},
8537 the @code{uname} and @code{gname} fields will contain the ASCII
8538 representation of the owner and group of the file respectively.
8539 If found, the user and group IDs are used rather than the values in
8540 the @code{uid} and @code{gid} fields.
8542 For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages
8543 169-173 (section 10.1) for @cite{Archive/Interchange File Format}; and
8544 IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940
8545 (section E.4.48) for @cite{pax - Portable archive interchange}.
8548 @section @acronym{GNU} Extensions to the Archive Format
8551 The @acronym{GNU} format uses additional file types to describe new types of
8552 files in an archive. These are listed below.
8555 @item GNUTYPE_DUMPDIR
8557 This represents a directory and a list of files created by the
8558 @option{--incremental} (@option{-G}) option. The @code{size} field gives the total
8559 size of the associated list of files. Each file name is preceded by
8560 either a @samp{Y} (the file should be in this archive) or an @samp{N}.
8561 (The file is a directory, or is not stored in the archive.) Each file
8562 name is terminated by a null. There is an additional null after the
8565 @item GNUTYPE_MULTIVOL
8567 This represents a file continued from another volume of a multi-volume
8568 archive created with the @option{--multi-volume} (@option{-M}) option. The original
8569 type of the file is not given here. The @code{size} field gives the
8570 maximum size of this piece of the file (assuming the volume does
8571 not end before the file is written out). The @code{offset} field
8572 gives the offset from the beginning of the file where this part of
8573 the file begins. Thus @code{size} plus @code{offset} should equal
8574 the original size of the file.
8576 @item GNUTYPE_SPARSE
8578 This flag indicates that we are dealing with a sparse file. Note
8579 that archiving a sparse file requires special operations to find
8580 holes in the file, which mark the positions of these holes, along
8581 with the number of bytes of data to be found after the hole.
8583 @item GNUTYPE_VOLHDR
8585 This file type is used to mark the volume header that was given with
8586 the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option when the archive was created. The @code{name}
8587 field contains the @code{name} given after the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option.
8588 The @code{size} field is zero. Only the first file in each volume
8589 of an archive should have this type.
8593 You may have trouble reading a @acronym{GNU} format archive on a
8594 non-@acronym{GNU} system if the options @option{--incremental} (@option{-G}),
8595 @option{--multi-volume} (@option{-M}), @option{--sparse} (@option{-S}), or @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) were
8596 used when writing the archive. In general, if @command{tar} does not
8597 use the @acronym{GNU}-added fields of the header, other versions of
8598 @command{tar} should be able to read the archive. Otherwise, the
8599 @command{tar} program will give an error, the most likely one being a
8603 @section Comparison of @command{tar} and @command{cpio}
8606 @FIXME{Reorganize the following material}
8608 The @command{cpio} archive formats, like @command{tar}, do have maximum
8609 pathname lengths. The binary and old ASCII formats have a max path
8610 length of 256, and the new ASCII and CRC ASCII formats have a max
8611 path length of 1024. @acronym{GNU} @command{cpio} can read and write archives
8612 with arbitrary pathname lengths, but other @command{cpio} implementations
8613 may crash unexplainedly trying to read them.
8615 @command{tar} handles symbolic links in the form in which it comes in BSD;
8616 @command{cpio} doesn't handle symbolic links in the form in which it comes
8617 in System V prior to SVR4, and some vendors may have added symlinks
8618 to their system without enhancing @command{cpio} to know about them.
8619 Others may have enhanced it in a way other than the way I did it
8620 at Sun, and which was adopted by AT&T (and which is, I think, also
8621 present in the @command{cpio} that Berkeley picked up from AT&T and put
8622 into a later BSD release---I think I gave them my changes).
8624 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
8625 can handle @command{tar} format input, and write it on output, and it
8626 probably handles symbolic links. They may not have bothered doing
8627 anything to enhance @command{tar} as a result.)
8629 @command{cpio} handles special files; traditional @command{tar} doesn't.
8631 @command{tar} comes with V7, System III, System V, and BSD source;
8632 @command{cpio} comes only with System III, System V, and later BSD
8633 (4.3-tahoe and later).
8635 @command{tar}'s way of handling multiple hard links to a file can handle
8636 file systems that support 32-bit inumbers (e.g., the BSD file system);
8637 @command{cpio}s way requires you to play some games (in its "binary"
8638 format, i-numbers are only 16 bits, and in its "portable ASCII" format,
8639 they're 18 bits---it would have to play games with the "file system ID"
8640 field of the header to make sure that the file system ID/i-number pairs
8641 of different files were always different), and I don't know which
8642 @command{cpio}s, if any, play those games. Those that don't might get
8643 confused and think two files are the same file when they're not, and
8644 make hard links between them.
8646 @command{tar}s way of handling multiple hard links to a file places only
8647 one copy of the link on the tape, but the name attached to that copy
8648 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
8649 way puts one copy for every link, but you can retrieve it using any
8653 What type of check sum (if any) is used, and how is this calculated.
8656 See the attached manual pages for @command{tar} and @command{cpio} format.
8657 @command{tar} uses a checksum which is the sum of all the bytes in the
8658 @command{tar} header for a file; @command{cpio} uses no checksum.
8661 If anyone knows why @command{cpio} was made when @command{tar} was present
8665 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
8666 generally-available version of UNIX had @command{tar} at the time. I don't
8667 know whether any version that was generally available @emph{within AT&T}
8668 had @command{tar}, or, if so, whether the people within AT&T who did
8669 @command{cpio} knew about it.
8671 On restore, if there is a corruption on a tape @command{tar} will stop at
8672 that point, while @command{cpio} will skip over it and try to restore the
8675 The main difference is just in the command syntax and header format.
8677 @command{tar} is a little more tape-oriented in that everything is blocked
8678 to start on a record boundary.
8681 Is there any differences between the ability to recover crashed
8682 archives between the two of them. (Is there any chance of recovering
8683 crashed archives at all.)
8686 Theoretically it should be easier under @command{tar} since the blocking
8687 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
8688 However, modern @command{cpio}'s and variations have an option to just
8689 search for the next file header after an error with a reasonable chance
8690 of resyncing. However, lots of tape driver software won't allow you to
8691 continue past a media error which should be the only reason for getting
8692 out of sync unless a file changed sizes while you were writing the
8696 If anyone knows why @command{cpio} was made when @command{tar} was present
8697 at the unix scene, please tell me about this too.
8700 Probably because it is more media efficient (by not blocking everything
8701 and using only the space needed for the headers where @command{tar}
8702 always uses 512 bytes per file header) and it knows how to archive
8705 You might want to look at the freely available alternatives. The
8706 major ones are @command{afio}, @GNUTAR{}, and
8707 @command{pax}, each of which have their own extensions with some
8708 backwards compatibility.
8710 Sparse files were @command{tar}red as sparse files (which you can
8711 easily test, because the resulting archive gets smaller, and
8712 @acronym{GNU} @command{cpio} can no longer read it).
8715 @chapter Tapes and Other Archive Media
8718 A few special cases about tape handling warrant more detailed
8719 description. These special cases are discussed below.
8721 Many complexities surround the use of @command{tar} on tape drives. Since
8722 the creation and manipulation of archives located on magnetic tape was
8723 the original purpose of @command{tar}, it contains many features making
8724 such manipulation easier.
8726 Archives are usually written on dismountable media---tape cartridges,
8727 mag tapes, or floppy disks.
8729 The amount of data a tape or disk holds depends not only on its size,
8730 but also on how it is formatted. A 2400 foot long reel of mag tape
8731 holds 40 megabytes of data when formatted at 1600 bits per inch. The
8732 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
8734 Magnetic media are re-usable---once the archive on a tape is no longer
8735 needed, the archive can be erased and the tape or disk used over.
8736 Media quality does deteriorate with use, however. Most tapes or disks
8737 should be discarded when they begin to produce data errors. EXABYTE
8738 tape cartridges should be discarded when they generate an @dfn{error
8739 count} (number of non-usable bits) of more than 10k.
8741 Magnetic media are written and erased using magnetic fields, and
8742 should be protected from such fields to avoid damage to stored data.
8743 Sticking a floppy disk to a filing cabinet using a magnet is probably
8747 * Device:: Device selection and switching
8748 * Remote Tape Server::
8749 * Common Problems and Solutions::
8750 * Blocking:: Blocking
8751 * Many:: Many archives on one tape
8752 * Using Multiple Tapes:: Using Multiple Tapes
8753 * label:: Including a Label in the Archive
8755 * Write Protection::
8759 @section Device Selection and Switching
8763 @item -f [@var{hostname}:]@var{file}
8764 @itemx --file=[@var{hostname}:]@var{file}
8765 Use archive file or device @var{file} on @var{hostname}.
8768 This option is used to specify the file name of the archive @command{tar}
8771 If the file name is @samp{-}, @command{tar} reads the archive from standard
8772 input (when listing or extracting), or writes it to standard output
8773 (when creating). If the @samp{-} file name is given when updating an
8774 archive, @command{tar} will read the original archive from its standard
8775 input, and will write the entire new archive to its standard output.
8777 If the file name contains a @samp{:}, it is interpreted as
8778 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
8779 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
8780 either case, @command{tar} will invoke the command @command{rsh} (or
8781 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
8782 machine. If you give an alternate login name, it will be given to the
8784 Naturally, the remote machine must have an executable
8785 @command{/usr/libexec/rmt}. This program is free software from the
8786 University of California, and a copy of the source code can be found
8787 with the sources for @command{tar}; it's compiled and installed by default.
8788 The exact path to this utility is determined when configuring the package.
8789 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
8790 your installation prefix. This location may also be overridden at
8791 runtime by using @option{rmt-command=@var{command}} option (@xref{Option Summary,
8792 ---rmt-command}, for detailed description of this option. @xref{Remote
8793 Tape Server}, for the description of @command{rmt} command).
8795 If this option is not given, but the environment variable @env{TAPE}
8796 is set, its value is used; otherwise, old versions of @command{tar}
8797 used a default archive name (which was picked when @command{tar} was
8798 compiled). The default is normally set up to be the @dfn{first} tape
8799 drive or other transportable I/O medium on the system.
8801 Starting with version 1.11.5, @GNUTAR{} uses
8802 standard input and standard output as the default device, and I will
8803 not try anymore supporting automatic device detection at installation
8804 time. This was failing really in too many cases, it was hopeless.
8805 This is now completely left to the installer to override standard
8806 input and standard output for default device, if this seems
8807 preferable. Further, I think @emph{most} actual usages of
8808 @command{tar} are done with pipes or disks, not really tapes,
8809 cartridges or diskettes.
8811 Some users think that using standard input and output is running
8812 after trouble. This could lead to a nasty surprise on your screen if
8813 you forget to specify an output file name---especially if you are going
8814 through a network or terminal server capable of buffering large amounts
8815 of output. We had so many bug reports in that area of configuring
8816 default tapes automatically, and so many contradicting requests, that
8817 we finally consider the problem to be portably intractable. We could
8818 of course use something like @samp{/dev/tape} as a default, but this
8819 is @emph{also} running after various kind of trouble, going from hung
8820 processes to accidental destruction of real tapes. After having seen
8821 all this mess, using standard input and output as a default really
8822 sounds like the only clean choice left, and a very useful one too.
8824 @GNUTAR{} reads and writes archive in records, I
8825 suspect this is the main reason why block devices are preferred over
8826 character devices. Most probably, block devices are more efficient
8827 too. The installer could also check for @samp{DEFTAPE} in
8828 @file{<sys/mtio.h>}.
8831 @opindex force-local, short description
8833 Archive file is local even if it contains a colon.
8835 @opindex rsh-command
8836 @item --rsh-command=@var{command}
8837 Use remote @var{command} instead of @command{rsh}. This option exists
8838 so that people who use something other than the standard @command{rsh}
8839 (e.g., a Kerberized @command{rsh}) can access a remote device.
8841 When this command is not used, the shell command found when
8842 the @command{tar} program was installed is used instead. This is
8843 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
8844 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
8845 The installer may have overridden this by defining the environment
8846 variable @env{RSH} @emph{at installation time}.
8849 Specify drive and density.
8851 @opindex multi-volume, short description
8853 @itemx --multi-volume
8854 Create/list/extract multi-volume archive.
8856 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
8857 that may be larger than will fit on the medium used to hold it.
8858 @xref{Multi-Volume Archives}.
8860 @opindex tape-length, short description
8862 @itemx --tape-length=@var{num}
8863 Change tape after writing @var{num} x 1024 bytes.
8865 This option might be useful when your tape drivers do not properly
8866 detect end of physical tapes. By being slightly conservative on the
8867 maximum tape length, you might avoid the problem entirely.
8869 @opindex info-script, short description
8870 @opindex new-volume-script, short description
8872 @itemx --info-script=@var{file}
8873 @itemx --new-volume-script=@var{file}
8874 Execute @file{file} at end of each tape. This implies
8875 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
8876 description of this option.
8879 @node Remote Tape Server
8880 @section The Remote Tape Server
8882 @cindex remote tape drive
8884 In order to access the tape drive on a remote machine, @command{tar}
8885 uses the remote tape server written at the University of California at
8886 Berkeley. The remote tape server must be installed as
8887 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
8888 want to use. @command{tar} calls @command{rmt} by running an
8889 @command{rsh} or @command{remsh} to the remote machine, optionally
8890 using a different login name if one is supplied.
8892 A copy of the source for the remote tape server is provided. It is
8893 Copyright @copyright{} 1983 by the Regents of the University of
8894 California, but can be freely distributed. It is compiled and
8895 installed by default.
8897 @cindex absolute file names
8898 Unless you use the @option{--absolute-names} (@option{-P}) option,
8899 @GNUTAR{} will not allow you to create an archive that contains
8900 absolute file names (a file name beginning with @samp{/}.) If you try,
8901 @command{tar} will automatically remove the leading @samp{/} from the
8902 file names it stores in the archive. It will also type a warning
8903 message telling you what it is doing.
8905 When reading an archive that was created with a different
8906 @command{tar} program, @GNUTAR{} automatically
8907 extracts entries in the archive which have absolute file names as if
8908 the file names were not absolute. This is an important feature. A
8909 visitor here once gave a @command{tar} tape to an operator to restore;
8910 the operator used Sun @command{tar} instead of @GNUTAR{},
8911 and the result was that it replaced large portions of
8912 our @file{/bin} and friends with versions from the tape; needless to
8913 say, we were unhappy about having to recover the file system from
8916 For example, if the archive contained a file @file{/usr/bin/computoy},
8917 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
8918 relative to the current directory. If you want to extract the files in
8919 an archive to the same absolute names that they had when the archive
8920 was created, you should do a @samp{cd /} before extracting the files
8921 from the archive, or you should either use the @option{--absolute-names}
8922 option, or use the command @samp{tar -C / @dots{}}.
8924 @cindex Ultrix 3.1 and write failure
8925 Some versions of Unix (Ultrix 3.1 is known to have this problem),
8926 can claim that a short write near the end of a tape succeeded,
8927 when it actually failed. This will result in the -M option not
8928 working correctly. The best workaround at the moment is to use a
8929 significantly larger blocking factor than the default 20.
8931 In order to update an archive, @command{tar} must be able to backspace the
8932 archive in order to reread or rewrite a record that was just read (or
8933 written). This is currently possible only on two kinds of files: normal
8934 disk files (or any other file that can be backspaced with @samp{lseek}),
8935 and industry-standard 9-track magnetic tape (or any other kind of tape
8936 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}.
8938 This means that the @option{--append}, @option{--concatenate}, and
8939 @option{--delete} commands will not work on any other kind of file.
8940 Some media simply cannot be backspaced, which means these commands and
8941 options will never be able to work on them. These non-backspacing
8942 media include pipes and cartridge tape drives.
8944 Some other media can be backspaced, and @command{tar} will work on them
8945 once @command{tar} is modified to do so.
8947 Archives created with the @option{--multi-volume}, @option{--label}, and
8948 @option{--incremental} (@option{-G}) options may not be readable by other version
8949 of @command{tar}. In particular, restoring a file that was split over
8950 a volume boundary will require some careful work with @command{dd}, if
8951 it can be done at all. Other versions of @command{tar} may also create
8952 an empty file whose name is that of the volume header. Some versions
8953 of @command{tar} may create normal files instead of directories archived
8954 with the @option{--incremental} (@option{-G}) option.
8956 @node Common Problems and Solutions
8957 @section Some Common Problems and their Solutions
8964 no such file or directory
8967 errors from @command{tar}:
8968 directory checksum error
8971 errors from media/system:
8982 @dfn{Block} and @dfn{record} terminology is rather confused, and it
8983 is also confusing to the expert reader. On the other hand, readers
8984 who are new to the field have a fresh mind, and they may safely skip
8985 the next two paragraphs, as the remainder of this manual uses those
8986 two terms in a quite consistent way.
8988 John Gilmore, the writer of the public domain @command{tar} from which
8989 @GNUTAR{} was originally derived, wrote (June 1995):
8992 The nomenclature of tape drives comes from IBM, where I believe
8993 they were invented for the IBM 650 or so. On IBM mainframes, what
8994 is recorded on tape are tape blocks. The logical organization of
8995 data is into records. There are various ways of putting records into
8996 blocks, including @code{F} (fixed sized records), @code{V} (variable
8997 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
8998 to a block), @code{VB} (variable size records, @var{n} to a block),
8999 @code{VSB} (variable spanned blocked: variable sized records that can
9000 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
9001 parameter specified this to the operating system.
9003 The Unix man page on @command{tar} was totally confused about this.
9004 When I wrote @code{PD TAR}, I used the historically correct terminology
9005 (@command{tar} writes data records, which are grouped into blocks).
9006 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
9007 here), and now Fran@,{c}ois has migrated that terminology back
9008 into the source code too.
9011 The term @dfn{physical block} means the basic transfer chunk from or
9012 to a device, after which reading or writing may stop without anything
9013 being lost. In this manual, the term @dfn{block} usually refers to
9014 a disk physical block, @emph{assuming} that each disk block is 512
9015 bytes in length. It is true that some disk devices have different
9016 physical blocks, but @command{tar} ignore these differences in its own
9017 format, which is meant to be portable, so a @command{tar} block is always
9018 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
9019 The term @dfn{logical block} often represents the basic chunk of
9020 allocation of many disk blocks as a single entity, which the operating
9021 system treats somewhat atomically; this concept is only barely used
9024 The term @dfn{physical record} is another way to speak of a physical
9025 block, those two terms are somewhat interchangeable. In this manual,
9026 the term @dfn{record} usually refers to a tape physical block,
9027 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
9028 It is true that archives may be put on disk or used with pipes,
9029 but nevertheless, @command{tar} tries to read and write the archive one
9030 @dfn{record} at a time, whatever the medium in use. One record is made
9031 up of an integral number of blocks, and this operation of putting many
9032 disk blocks into a single tape block is called @dfn{reblocking}, or
9033 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
9034 the logical organization of many characters into something meaningful
9035 to the application. The term @dfn{unit record} describes a small set
9036 of characters which are transmitted whole to or by the application,
9037 and often refers to a line of text. Those two last terms are unrelated
9038 to what we call a @dfn{record} in @GNUTAR{}.
9040 When writing to tapes, @command{tar} writes the contents of the archive
9041 in chunks known as @dfn{records}. To change the default blocking
9042 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
9043 @var{512-size}}) option. Each record will then be composed of
9044 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
9045 @xref{Standard}.) Each file written to the archive uses at least one
9046 full record. As a result, using a larger record size can result in
9047 more wasted space for small files. On the other hand, a larger record
9048 size can often be read and written much more efficiently.
9050 Further complicating the problem is that some tape drives ignore the
9051 blocking entirely. For these, a larger record size can still improve
9052 performance (because the software layers above the tape drive still
9053 honor the blocking), but not as dramatically as on tape drives that
9056 When reading an archive, @command{tar} can usually figure out the
9057 record size on itself. When this is the case, and a non-standard
9058 record size was used when the archive was created, @command{tar} will
9059 print a message about a non-standard blocking factor, and then operate
9060 normally. On some tape devices, however, @command{tar} cannot figure
9061 out the record size itself. On most of those, you can specify a
9062 blocking factor (with @option{--blocking-factor}) larger than the
9063 actual blocking factor, and then use the @option{--read-full-records}
9064 (@option{-B}) option. (If you specify a blocking factor with
9065 @option{--blocking-factor} and don't use the
9066 @option{--read-full-records} option, then @command{tar} will not
9067 attempt to figure out the recording size itself.) On some devices,
9068 you must always specify the record size exactly with
9069 @option{--blocking-factor} when reading, because @command{tar} cannot
9070 figure it out. In any case, use @option{--list} (@option{-t}) before
9071 doing any extractions to see whether @command{tar} is reading the archive
9074 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
9075 putting them into records is to put a whole number of them (one or
9076 more) into each record. @command{tar} records are all the same size;
9077 at the end of the file there's a block containing all zeros, which
9078 is how you tell that the remainder of the last record(s) are garbage.
9080 In a standard @command{tar} file (no options), the block size is 512
9081 and the record size is 10240, for a blocking factor of 20. What the
9082 @option{--blocking-factor} option does is sets the blocking factor,
9083 changing the record size while leaving the block size at 512 bytes.
9084 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
9085 most tape drives these days prefer much bigger records in order to
9086 stream and not waste tape. When writing tapes for myself, some tend
9087 to use a factor of the order of 2048, say, giving a record size of
9088 around one megabyte.
9090 If you use a blocking factor larger than 20, older @command{tar}
9091 programs might not be able to read the archive, so we recommend this
9092 as a limit to use in practice. @GNUTAR{}, however,
9093 will support arbitrarily large record sizes, limited only by the
9094 amount of virtual memory or the physical characteristics of the tape
9098 * Format Variations:: Format Variations
9099 * Blocking Factor:: The Blocking Factor of an Archive
9102 @node Format Variations
9103 @subsection Format Variations
9104 @cindex Format Parameters
9105 @cindex Format Options
9106 @cindex Options, archive format specifying
9107 @cindex Options, format specifying
9110 Format parameters specify how an archive is written on the archive
9111 media. The best choice of format parameters will vary depending on
9112 the type and number of files being archived, and on the media used to
9115 To specify format parameters when accessing or creating an archive,
9116 you can use the options described in the following sections.
9117 If you do not specify any format parameters, @command{tar} uses
9118 default parameters. You cannot modify a compressed archive.
9119 If you create an archive with the @option{--blocking-factor} option
9120 specified (@pxref{Blocking Factor}), you must specify that
9121 blocking-factor when operating on the archive. @xref{Formats}, for other
9122 examples of format parameter considerations.
9124 @node Blocking Factor
9125 @subsection The Blocking Factor of an Archive
9126 @cindex Blocking Factor
9128 @cindex Number of blocks per record
9129 @cindex Number of bytes per record
9130 @cindex Bytes per record
9131 @cindex Blocks per record
9134 @opindex blocking-factor
9135 The data in an archive is grouped into blocks, which are 512 bytes.
9136 Blocks are read and written in whole number multiples called
9137 @dfn{records}. The number of blocks in a record (ie. the size of a
9138 record in units of 512 bytes) is called the @dfn{blocking factor}.
9139 The @option{--blocking-factor=@var{512-size}} (@option{-b
9140 @var{512-size}}) option specifies the blocking factor of an archive.
9141 The default blocking factor is typically 20 (i.e., 10240 bytes), but
9142 can be specified at installation. To find out the blocking factor of
9143 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
9144 This may not work on some devices.
9146 Records are separated by gaps, which waste space on the archive media.
9147 If you are archiving on magnetic tape, using a larger blocking factor
9148 (and therefore larger records) provides faster throughput and allows you
9149 to fit more data on a tape (because there are fewer gaps). If you are
9150 archiving on cartridge, a very large blocking factor (say 126 or more)
9151 greatly increases performance. A smaller blocking factor, on the other
9152 hand, may be useful when archiving small files, to avoid archiving lots
9153 of nulls as @command{tar} fills out the archive to the end of the record.
9154 In general, the ideal record size depends on the size of the
9155 inter-record gaps on the tape you are using, and the average size of the
9156 files you are archiving. @xref{create}, for information on
9159 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
9161 Archives with blocking factors larger than 20 cannot be read
9162 by very old versions of @command{tar}, or by some newer versions
9163 of @command{tar} running on old machines with small address spaces.
9164 With @GNUTAR{}, the blocking factor of an archive is limited
9165 only by the maximum record size of the device containing the archive,
9166 or by the amount of available virtual memory.
9168 Also, on some systems, not using adequate blocking factors, as sometimes
9169 imposed by the device drivers, may yield unexpected diagnostics. For
9170 example, this has been reported:
9173 Cannot write to /dev/dlt: Invalid argument
9177 In such cases, it sometimes happen that the @command{tar} bundled by
9178 the system is aware of block size idiosyncrasies, while @GNUTAR{}
9179 requires an explicit specification for the block size,
9180 which it cannot guess. This yields some people to consider
9181 @GNUTAR{} is misbehaving, because by comparison,
9182 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
9183 for example, might resolve the problem.
9185 If you use a non-default blocking factor when you create an archive, you
9186 must specify the same blocking factor when you modify that archive. Some
9187 archive devices will also require you to specify the blocking factor when
9188 reading that archive, however this is not typically the case. Usually, you
9189 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
9190 reports a non-default record size and then lists the archive members as
9191 it would normally. To extract files from an archive with a non-standard
9192 blocking factor (particularly if you're not sure what the blocking factor
9193 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
9194 specifying a blocking factor larger then the blocking factor of the archive
9195 (ie. @samp{tar --extract --read-full-records --blocking-factor=300}.
9196 @xref{list}, for more information on the @option{--list} (@option{-t})
9197 operation. @xref{Reading}, for a more detailed explanation of that option.
9200 @item --blocking-factor=@var{number}
9201 @itemx -b @var{number}
9202 Specifies the blocking factor of an archive. Can be used with any
9203 operation, but is usually not necessary with @option{--list} (@option{-t}).
9209 @item -b @var{blocks}
9210 @itemx --blocking-factor=@var{blocks}
9211 Set record size to @math{@var{blocks} * 512} bytes.
9213 This option is used to specify a @dfn{blocking factor} for the archive.
9214 When reading or writing the archive, @command{tar}, will do reads and writes
9215 of the archive in records of @math{@var{block}*512} bytes. This is true
9216 even when the archive is compressed. Some devices requires that all
9217 write operations be a multiple of a certain size, and so, @command{tar}
9218 pads the archive out to the next record boundary.
9220 The default blocking factor is set when @command{tar} is compiled, and is
9221 typically 20. Blocking factors larger than 20 cannot be read by very
9222 old versions of @command{tar}, or by some newer versions of @command{tar}
9223 running on old machines with small address spaces.
9225 With a magnetic tape, larger records give faster throughput and fit
9226 more data on a tape (because there are fewer inter-record gaps).
9227 If the archive is in a disk file or a pipe, you may want to specify
9228 a smaller blocking factor, since a large one will result in a large
9229 number of null bytes at the end of the archive.
9231 When writing cartridge or other streaming tapes, a much larger
9232 blocking factor (say 126 or more) will greatly increase performance.
9233 However, you must specify the same blocking factor when reading or
9234 updating the archive.
9236 Apparently, Exabyte drives have a physical block size of 8K bytes.
9237 If we choose our blocksize as a multiple of 8k bytes, then the problem
9238 seems to disappear. Id est, we are using block size of 112 right
9239 now, and we haven't had the problem since we switched@dots{}
9241 With @GNUTAR{} the blocking factor is limited only
9242 by the maximum record size of the device containing the archive, or by
9243 the amount of available virtual memory.
9245 However, deblocking or reblocking is virtually avoided in a special
9246 case which often occurs in practice, but which requires all the
9247 following conditions to be simultaneously true:
9250 the archive is subject to a compression option,
9252 the archive is not handled through standard input or output, nor
9253 redirected nor piped,
9255 the archive is directly handled to a local disk, instead of any special
9258 @option{--blocking-factor} is not explicitly specified on the @command{tar}
9262 If the output goes directly to a local disk, and not through
9263 stdout, then the last write is not extended to a full record size.
9264 Otherwise, reblocking occurs. Here are a few other remarks on this
9270 @command{gzip} will complain about trailing garbage if asked to
9271 uncompress a compressed archive on tape, there is an option to turn
9272 the message off, but it breaks the regularity of simply having to use
9273 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
9274 silently ignoring any number of trailing zeros. I'll ask Jean-loup
9275 Gailly, by sending a copy of this message to him.
9278 @command{compress} does not show this problem, but as Jean-loup pointed
9279 out to Michael, @samp{compress -d} silently adds garbage after
9280 the result of decompression, which tar ignores because it already
9281 recognized its end-of-file indicator. So this bug may be safely
9285 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
9286 but will still return an exit status of 2 which tar reports in turn.
9287 @command{tar} might ignore the exit status returned, but I hate doing
9288 that, as it weakens the protection @command{tar} offers users against
9289 other possible problems at decompression time. If @command{gzip} was
9290 silently skipping trailing zeros @emph{and} also avoiding setting the
9291 exit status in this innocuous case, that would solve this situation.
9294 @command{tar} should become more solid at not stopping to read a pipe at
9295 the first null block encountered. This inelegantly breaks the pipe.
9296 @command{tar} should rather drain the pipe out before exiting itself.
9299 @opindex ignore-zeros, short description
9301 @itemx --ignore-zeros
9302 Ignore blocks of zeros in archive (means EOF).
9304 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
9305 of zeros in the archive. Normally a block of zeros indicates the
9306 end of the archive, but when reading a damaged archive, or one which
9307 was created by concatenating several archives together, this option
9308 allows @command{tar} to read the entire archive. This option is not on
9309 by default because many versions of @command{tar} write garbage after
9312 Note that this option causes @command{tar} to read to the end of the
9313 archive file, which may sometimes avoid problems when multiple files
9314 are stored on a single physical tape.
9316 @opindex read-full-records, short description
9318 @itemx --read-full-records
9319 Reblock as we read (for reading 4.2BSD pipes).
9321 If @option{--read-full-records} is used, @command{tar}
9322 will not panic if an attempt to read a record from the archive does
9323 not return a full record. Instead, @command{tar} will keep reading
9324 until it has obtained a full
9327 This option is turned on by default when @command{tar} is reading
9328 an archive from standard input, or from a remote machine. This is
9329 because on BSD Unix systems, a read of a pipe will return however
9330 much happens to be in the pipe, even if it is less than @command{tar}
9331 requested. If this option was not used, @command{tar} would fail as
9332 soon as it read an incomplete record from the pipe.
9334 This option is also useful with the commands for updating an archive.
9340 @FIXME{Appropriate options should be moved here from elsewhere.}
9342 @cindex blocking factor
9343 @cindex tape blocking
9345 When handling various tapes or cartridges, you have to take care of
9346 selecting a proper blocking, that is, the number of disk blocks you
9347 put together as a single tape block on the tape, without intervening
9348 tape gaps. A @dfn{tape gap} is a small landing area on the tape
9349 with no information on it, used for decelerating the tape to a
9350 full stop, and for later regaining the reading or writing speed.
9351 When the tape driver starts reading a record, the record has to
9352 be read whole without stopping, as a tape gap is needed to stop the
9353 tape motion without loosing information.
9355 @cindex Exabyte blocking
9356 @cindex DAT blocking
9357 Using higher blocking (putting more disk blocks per tape block) will use
9358 the tape more efficiently as there will be less tape gaps. But reading
9359 such tapes may be more difficult for the system, as more memory will be
9360 required to receive at once the whole record. Further, if there is a
9361 reading error on a huge record, this is less likely that the system will
9362 succeed in recovering the information. So, blocking should not be too
9363 low, nor it should be too high. @command{tar} uses by default a blocking of
9364 20 for historical reasons, and it does not really matter when reading or
9365 writing to disk. Current tape technology would easily accommodate higher
9366 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
9367 We were told that for some DLT drives, the blocking should be a multiple
9368 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
9369 Other manufacturers may use different recommendations for the same tapes.
9370 This might also depends of the buffering techniques used inside modern
9371 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
9372 Others request blocking to be some exponent of two.
9374 So, there is no fixed rule for blocking. But blocking at read time
9375 should ideally be the same as blocking used at write time. At one place
9376 I know, with a wide variety of equipment, they found it best to use a
9377 blocking of 32 to guarantee that their tapes are fully interchangeable.
9379 I was also told that, for recycled tapes, prior erasure (by the same
9380 drive unit that will be used to create the archives) sometimes lowers
9381 the error rates observed at rewriting time.
9383 I might also use @option{--number-blocks} instead of
9384 @option{--block-number}, so @option{--block} will then expand to
9385 @option{--blocking-factor} unambiguously.
9388 @section Many Archives on One Tape
9390 @FIXME{Appropriate options should be moved here from elsewhere.}
9392 @findex ntape @r{device}
9393 Most tape devices have two entries in the @file{/dev} directory, or
9394 entries that come in pairs, which differ only in the minor number for
9395 this device. Let's take for example @file{/dev/tape}, which often
9396 points to the only or usual tape device of a given system. There might
9397 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
9398 name is the @emph{rewinding} version of the device, while the name
9399 having @samp{nr} in it is the @emph{no rewinding} version of the same
9402 A rewinding tape device will bring back the tape to its beginning point
9403 automatically when this device is opened or closed. Since @command{tar}
9404 opens the archive file before using it and closes it afterwards, this
9405 means that a simple:
9408 $ @kbd{tar cf /dev/tape @var{directory}}
9412 will reposition the tape to its beginning both prior and after saving
9413 @var{directory} contents to it, thus erasing prior tape contents and
9414 making it so that any subsequent write operation will destroy what has
9417 @cindex tape positioning
9418 So, a rewinding device is normally meant to hold one and only one file.
9419 If you want to put more than one @command{tar} archive on a given tape, you
9420 will need to avoid using the rewinding version of the tape device. You
9421 will also have to pay special attention to tape positioning. Errors in
9422 positioning may overwrite the valuable data already on your tape. Many
9423 people, burnt by past experiences, will only use rewinding devices and
9424 limit themselves to one file per tape, precisely to avoid the risk of
9425 such errors. Be fully aware that writing at the wrong position on a
9426 tape loses all information past this point and most probably until the
9427 end of the tape, and this destroyed information @emph{cannot} be
9430 To save @var{directory-1} as a first archive at the beginning of a
9431 tape, and leave that tape ready for a second archive, you should use:
9434 $ @kbd{mt -f /dev/nrtape rewind}
9435 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
9439 @dfn{Tape marks} are special magnetic patterns written on the tape
9440 media, which are later recognizable by the reading hardware. These
9441 marks are used after each file, when there are many on a single tape.
9442 An empty file (that is to say, two tape marks in a row) signal the
9443 logical end of the tape, after which no file exist. Usually,
9444 non-rewinding tape device drivers will react to the close request issued
9445 by @command{tar} by first writing two tape marks after your archive, and by
9446 backspacing over one of these. So, if you remove the tape at that time
9447 from the tape drive, it is properly terminated. But if you write
9448 another file at the current position, the second tape mark will be
9449 erased by the new information, leaving only one tape mark between files.
9451 So, you may now save @var{directory-2} as a second archive after the
9452 first on the same tape by issuing the command:
9455 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
9459 and so on for all the archives you want to put on the same tape.
9461 Another usual case is that you do not write all the archives the same
9462 day, and you need to remove and store the tape between two archive
9463 sessions. In general, you must remember how many files are already
9464 saved on your tape. Suppose your tape already has 16 files on it, and
9465 that you are ready to write the 17th. You have to take care of skipping
9466 the first 16 tape marks before saving @var{directory-17}, say, by using
9470 $ @kbd{mt -f /dev/nrtape rewind}
9471 $ @kbd{mt -f /dev/nrtape fsf 16}
9472 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
9475 In all the previous examples, we put aside blocking considerations, but
9476 you should do the proper things for that as well. @xref{Blocking}.
9479 * Tape Positioning:: Tape Positions and Tape Marks
9480 * mt:: The @command{mt} Utility
9483 @node Tape Positioning
9484 @subsection Tape Positions and Tape Marks
9487 Just as archives can store more than one file from the file system,
9488 tapes can store more than one archive file. To keep track of where
9489 archive files (or any other type of file stored on tape) begin and
9490 end, tape archive devices write magnetic @dfn{tape marks} on the
9491 archive media. Tape drives write one tape mark between files,
9492 two at the end of all the file entries.
9494 If you think of data as a series of records "rrrr"'s, and tape marks as
9495 "*"'s, a tape might look like the following:
9498 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
9501 Tape devices read and write tapes using a read/write @dfn{tape
9502 head}---a physical part of the device which can only access one
9503 point on the tape at a time. When you use @command{tar} to read or
9504 write archive data from a tape device, the device will begin reading
9505 or writing from wherever on the tape the tape head happens to be,
9506 regardless of which archive or what part of the archive the tape
9507 head is on. Before writing an archive, you should make sure that no
9508 data on the tape will be overwritten (unless it is no longer needed).
9509 Before reading an archive, you should make sure the tape head is at
9510 the beginning of the archive you want to read. You can do it manually
9511 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
9512 that automatically (@pxref{Scripted Restoration}).
9514 If you want to add new archive file entries to a tape, you should
9515 advance the tape to the end of the existing file entries, backspace
9516 over the last tape mark, and write the new archive file. If you were
9517 to add two archives to the example above, the tape might look like the
9521 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
9525 @subsection The @command{mt} Utility
9528 @FIXME{Is it true that this only works on non-block devices?
9529 should explain the difference, (fixed or variable).}
9530 @xref{Blocking Factor}.
9532 You can use the @command{mt} utility to advance or rewind a tape past a
9533 specified number of archive files on the tape. This will allow you
9534 to move to the beginning of an archive before extracting or reading
9535 it, or to the end of all the archives before writing a new one.
9536 @FIXME{Why isn't there an "advance 'til you find two tape marks
9539 The syntax of the @command{mt} command is:
9542 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
9545 where @var{tapename} is the name of the tape device, @var{number} is
9546 the number of times an operation is performed (with a default of one),
9547 and @var{operation} is one of the following:
9549 @FIXME{is there any use for record operations?}
9554 Writes @var{number} tape marks at the current position on the tape.
9557 Moves tape position forward @var{number} files.
9560 Moves tape position back @var{number} files.
9563 Rewinds the tape. (Ignores @var{number}).
9567 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}).
9570 Prints status information about the tape unit.
9574 @FIXME{Is there a better way to frob the spacing on the list?}
9576 If you don't specify a @var{tapename}, @command{mt} uses the environment
9577 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} will use
9578 the default device specified in your @file{sys/mtio.h} file
9579 (@code{DEFTAPE} variable). If this is not defined, the program will
9580 display a descriptive error message and exit with code 1.
9582 @command{mt} returns a 0 exit status when the operation(s) were
9583 successful, 1 if the command was unrecognized, and 2 if an operation
9586 @node Using Multiple Tapes
9587 @section Using Multiple Tapes
9590 Often you might want to write a large archive, one larger than will fit
9591 on the actual tape you are using. In such a case, you can run multiple
9592 @command{tar} commands, but this can be inconvenient, particularly if you
9593 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
9594 Therefore, @command{tar} supports multiple tapes automatically.
9596 Use @option{--multi-volume} (@option{-M}) on the command line, and
9597 then @command{tar} will, when it reaches the end of the tape, prompt
9598 for another tape, and continue the archive. Each tape will have an
9599 independent archive, and can be read without needing the other. (As
9600 an exception to this, the file that @command{tar} was archiving when
9601 it ran out of tape will usually be split between the two archives; in
9602 this case you need to extract from the first archive, using
9603 @option{--multi-volume}, and then put in the second tape when
9604 prompted, so @command{tar} can restore both halves of the file.)
9606 @GNUTAR{} multi-volume archives do not use a truly portable format.
9607 You need @GNUTAR{} at both ends to process them properly.
9609 When prompting for a new tape, @command{tar} accepts any of the following
9614 Request @command{tar} to explain possible responses
9616 Request @command{tar} to exit immediately.
9617 @item n @var{file-name}
9618 Request @command{tar} to write the next volume on the file @var{file-name}.
9620 Request @command{tar} to run a subshell. This option can be disabled
9621 by giving @option{--restrict} command line option to @command{tar}.
9623 Request @command{tar} to begin writing the next volume.
9626 (You should only type @samp{y} after you have changed the tape;
9627 otherwise @command{tar} will write over the volume it just finished.)
9629 @cindex End-of-archive info script
9631 @anchor{info-script}
9632 @opindex info-script
9633 @opindex new-volume-script
9634 If you want more elaborate behavior than this, give @command{tar} the
9635 @option{--info-script=@var{script-name}}
9636 (@option{--new-volume-script=@var{script-name}}, @option{-F
9637 @var{script-name}}) option. The file @var{script-name} is expected to
9638 be a program (or shell script) to be run instead of the normal
9639 prompting procedure. It is executed without any command line
9640 arguments. Additional data is passed to it via the following
9641 environment variables:
9644 @vrindex TAR_VERSION, info script environment variable
9646 @GNUTAR{} version number.
9648 @vrindex TAR_ARCHIVE, info script environment variable
9650 The name of the archive @command{tar} is processing.
9652 @vrindex TAR_VOLUME, info script environment variable
9654 Ordinal number of the volume @command{tar} is about to start.
9656 @vrindex TAR_SUBCOMMAND, info script environment variable
9657 @item TAR_SUBCOMMAND
9658 Short option describing the operation @command{tar} is executed.
9659 @xref{Operations}, for a complete list of subcommand options.
9661 @vrindex TAR_FORMAT, info script environment variable
9663 Format of the archive being processed. @xref{Formats}, for a complete
9664 list of archive format names.
9667 The info script can instruct @command{tar} to use new archive name,
9668 by writing in to file descriptor 3 (see below for an
9671 If the info script fails, @command{tar} exits; otherwise, it begins
9672 writing the next volume.
9674 The method @command{tar} uses to detect end of tape is not perfect, and
9675 fails on some operating systems or on some devices. You can use the
9676 @option{--tape-length=@var{size}} (@option{-L @var{size}}) option if
9677 @command{tar} can't detect the end of the tape itself. This option
9678 selects @option{--multi-volume} (@option{-M}) automatically. The
9679 @var{size} argument should then be the usable size of the tape in
9680 units of 1024 bytes. But for many devices, and floppy disks in
9681 particular, this option is never required for real, as far as we know.
9683 @cindex Volume number file
9687 The volume number used by @command{tar} in its tape-change prompt
9688 can be changed; if you give the
9689 @option{--volno-file=@var{file-of-number}} option, then
9690 @var{file-of-number} should be an unexisting file to be created, or
9691 else, a file already containing a decimal number. That number will be
9692 used as the volume number of the first volume written. When
9693 @command{tar} is finished, it will rewrite the file with the
9694 now-current volume number. (This does not change the volume number
9695 written on a tape label, as per @ref{label}, it @emph{only} affects
9696 the number used in the prompt.)
9698 If you want @command{tar} to cycle through a series of files or tape
9699 drives, there are three approaches to choose from. First of all, you
9700 can give @command{tar} multiple @option{--file} options. In this case
9701 the specified files will be used, in sequence, as the successive
9702 volumes of the archive. Only when the first one in the sequence needs
9703 to be used again will @command{tar} prompt for a tape change (or run
9704 the info script). Secondly, you can use the @samp{n} response to the
9705 tape-change prompt, and, finally, you can use an info script, that
9706 writes new archive name to file descriptor. The following example
9707 illustrates this approach:
9712 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
9714 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
9715 case $TAR_SUBCOMMAND in
9717 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
9722 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&3
9726 Each volume of a multi-volume archive is an independent @command{tar}
9727 archive, complete in itself. For example, you can list or extract any
9728 volume alone; just don't specify @option{--multi-volume}
9729 (@option{-M}). However, if one file in the archive is split across
9730 volumes, the only way to extract it successfully is with a
9731 multi-volume extract command @option{--extract --multi-volume}
9732 (@option{-xM}) starting on or before the volume where the file begins.
9734 For example, let's presume someone has two tape drives on a system
9735 named @file{/dev/tape0} and @file{/dev/tape1}. For having @GNUTAR{}
9736 to switch to the second drive when it needs to write the
9737 second tape, and then back to the first tape, etc., just do either of:
9740 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
9741 $ @kbd{tar cMff /dev/tape0 /dev/tape1 @var{files}}
9745 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
9746 * Tape Files:: Tape Files
9747 * Tarcat:: Concatenate Volumes into a Single Archive
9751 @node Multi-Volume Archives
9752 @subsection Archives Longer than One Tape or Disk
9753 @cindex Multi-volume archives
9756 @opindex multi-volume
9757 To create an archive that is larger than will fit on a single unit of
9758 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
9759 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
9760 archive can be manipulated like any other archive (provided the
9761 @option{--multi-volume} option is specified), but is stored on more
9762 than one tape or disk.
9764 When you specify @option{--multi-volume}, @command{tar} does not report an
9765 error when it comes to the end of an archive volume (when reading), or
9766 the end of the media (when writing). Instead, it prompts you to load
9767 a new storage volume. If the archive is on a magnetic tape, you
9768 should change tapes when you see the prompt; if the archive is on a
9769 floppy disk, you should change disks; etc.
9771 You can read each individual volume of a multi-volume archive as if it
9772 were an archive by itself. For example, to list the contents of one
9773 volume, use @option{--list}, without @option{--multi-volume} specified.
9774 To extract an archive member from one volume (assuming it is described
9775 that volume), use @option{--extract}, again without
9776 @option{--multi-volume}.
9778 If an archive member is split across volumes (ie. its entry begins on
9779 one volume of the media and ends on another), you need to specify
9780 @option{--multi-volume} to extract it successfully. In this case, you
9781 should load the volume where the archive member starts, and use
9782 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
9783 volumes as it needs them. @xref{extracting archives}, for more
9784 information about extracting archives.
9786 @option{--info-script=@var{script-name}}
9787 (@option{--new-volume-script=@var{script-name}}, @option{-F
9788 @var{script-name}}) (@pxref{info-script}) is like
9789 @option{--multi-volume} (@option{-M}), except that @command{tar} does
9790 not prompt you directly to change media volumes when a volume is
9791 full---instead, @command{tar} runs commands you have stored in
9792 @var{script-name}. For example, this option can be used to eject
9793 cassettes, or to broadcast messages such as @samp{Someone please come
9794 change my tape} when performing unattended backups. When
9795 @var{script-name} is done, @command{tar} will assume that the media
9798 Multi-volume archives can be modified like any other archive. To add
9799 files to a multi-volume archive, you need to only mount the last
9800 volume of the archive media (and new volumes, if needed). For all
9801 other operations, you need to use the entire archive.
9803 If a multi-volume archive was labeled using
9804 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9805 (@pxref{label}) when it was created, @command{tar} will not
9806 automatically label volumes which are added later. To label
9807 subsequent volumes, specify @option{--label=@var{archive-label}} again
9808 in conjunction with the @option{--append}, @option{--update} or
9809 @option{--concatenate} operation.
9811 @cindex Labeling multi-volume archives
9814 @FIXME{There should be a sample program here, including an exit
9815 before end. Is the exit status even checked in tar? :-(}
9818 @item --multi-volume
9820 Creates a multi-volume archive, when used in conjunction with
9821 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
9822 archive, specify @option{--multi-volume} in conjunction with that
9825 @item --info-script=@var{program-file}
9826 @itemx --new-volume-script=@var{program-file}
9827 @itemx -F @var{program-file}
9828 Creates a multi-volume archive via a script. Used in conjunction with
9829 @option{--create} (@option{-c}). @xref{info-script}, dor a detailed discussion.
9832 Beware that there is @emph{no} real standard about the proper way, for
9833 a @command{tar} archive, to span volume boundaries. If you have a
9834 multi-volume created by some vendor's @command{tar}, there is almost
9835 no chance you could read all the volumes with @GNUTAR{}.
9836 The converse is also true: you may not expect
9837 multi-volume archives created by @GNUTAR{} to be
9838 fully recovered by vendor's @command{tar}. Since there is little
9839 chance that, in mixed system configurations, some vendor's
9840 @command{tar} will work on another vendor's machine, and there is a
9841 great chance that @GNUTAR{} will work on most of
9842 them, your best bet is to install @GNUTAR{} on all
9843 machines between which you know exchange of files is possible.
9846 @subsection Tape Files
9849 To give the archive a name which will be recorded in it, use the
9850 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
9851 option. This will write a special block identifying
9852 @var{volume-label} as the name of the archive to the front of the
9853 archive which will be displayed when the archive is listed with
9854 @option{--list}. If you are creating a multi-volume archive with
9855 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
9856 volume label will have @samp{Volume @var{nnn}} appended to the name
9857 you give, where @var{nnn} is the number of the volume of the archive.
9858 (If you use the @option{--label=@var{volume-label}}) option when
9859 reading an archive, it checks to make sure the label on the tape
9860 matches the one you give. @xref{label}.
9862 When @command{tar} writes an archive to tape, it creates a single
9863 tape file. If multiple archives are written to the same tape, one
9864 after the other, they each get written as separate tape files. When
9865 extracting, it is necessary to position the tape at the right place
9866 before running @command{tar}. To do this, use the @command{mt} command.
9867 For more information on the @command{mt} command and on the organization
9868 of tapes into a sequence of tape files, see @ref{mt}.
9870 People seem to often do:
9873 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
9876 or such, for pushing a common date in all volumes or an archive set.
9879 @subsection Concatenate Volumes into a Single Archive
9882 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
9883 archive to a single @command{tar} archive. Simply concatenating all
9884 volumes into one will not work, since each volume carries an additional
9885 information at the beginning. @GNUTAR{} is shipped with the shell
9886 script @command{tarcat} designed for this purpose.
9888 The script takes a list of files comprising a multi-volume archive
9889 and creates the resulting archive at the standard output. For example:
9892 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
9895 The script implements a simple heuristics to determine the format of
9896 the first volume file and to decide how to process the rest of the
9897 files. However, it makes no attempt to verify whether the files are
9898 given in order or even if they are valid @command{tar} archives.
9899 It uses @command{dd} and does not filter its standard error, so you
9900 will usually see lots of spurious messages.
9902 @FIXME{The script is not installed. Should we install it?}
9905 @section Including a Label in the Archive
9906 @cindex Labeling an archive
9907 @cindex Labels on the archive media
9911 To avoid problems caused by misplaced paper labels on the archive
9912 media, you can include a @dfn{label} entry---an archive member which
9913 contains the name of the archive---in the archive itself. Use the
9914 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9915 option in conjunction with the @option{--create} operation to include
9916 a label entry in the archive as it is being created.
9919 @item --label=@var{archive-label}
9920 @itemx -V @var{archive-label}
9921 Includes an @dfn{archive-label} at the beginning of the archive when
9922 the archive is being created, when used in conjunction with the
9923 @option{--create} operation. Checks to make sure the archive label
9924 matches the one specified (when used in conjunction with any other
9928 If you create an archive using both
9929 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9930 and @option{--multi-volume} (@option{-M}), each volume of the archive
9931 will have an archive label of the form @samp{@var{archive-label}
9932 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
9933 next, and so on. @xref{Using Multiple Tapes}, for information on
9934 creating multiple volume archives.
9936 @cindex Volume label, listing
9937 @cindex Listing volume label
9938 The volume label will be displayed by @option{--list} along with
9939 the file contents. If verbose display is requested, it will also be
9940 explicitely marked as in the example below:
9944 $ @kbd{tar --verbose --list --file=iamanarchive}
9945 V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header--
9946 -rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename
9951 @anchor{--test-label option}
9952 However, @option{--list} option will cause listing entire
9953 contents of the archive, which may be undesirable (for example, if the
9954 archive is stored on a tape). You can request checking only the volume
9955 by specifying @option{--test-label} option. This option reads only the
9956 first block of an archive, so it can be used with slow storage
9957 devices. For example:
9961 $ @kbd{tar --test-label --file=iamanarchive}
9966 If @option{--test-label} is used with a single command line
9967 argument, @command{tar} compares the volume label with the
9968 argument. It exits with code 0 if the two strings match, and with code
9969 2 otherwise. In this case no output is displayed. For example:
9973 $ @kbd{tar --test-label --file=iamanarchive 'iamalable'}
9975 $ @kbd{tar --test-label --file=iamanarchive 'iamalable' alabel}
9980 If you request any operation, other than @option{--create}, along
9981 with using @option{--label} option, @command{tar} will first check if
9982 the archive label matches the one specified and will refuse to proceed
9983 if it does not. Use this as a safety precaution to avoid accidentally
9984 overwriting existing archives. For example, if you wish to add files
9985 to @file{archive}, presumably labelled with string @samp{My volume},
9990 $ @kbd{tar -rf archive --label 'My volume' .}
9991 tar: Archive not labeled to match `My volume'
9996 in case its label does not match. This will work even if
9997 @file{archive} is not labelled at all.
9999 Similarly, @command{tar} will refuse to list or extract the
10000 archive if its label doesn't match the @var{archive-label}
10001 specified. In those cases, @var{archive-label} argument is interpreted
10002 as a globbing-style pattern which must match the actual magnetic
10003 volume label. @xref{exclude}, for a precise description of how match
10004 is attempted@footnote{Previous versions of @command{tar} used full
10005 regular expression matching, or before that, only exact string
10006 matching, instead of wildcard matchers. We decided for the sake of
10007 simplicity to use a uniform matching device through
10008 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
10009 the volume label matcher will also suffix @var{archive-label} by
10010 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
10011 up. Since the volume numbering is automatically added in labels at
10012 creation time, it sounded logical to equally help the user taking care
10013 of it when the archive is being read.
10015 The @option{--label} was once called @option{--volume}, but is not
10016 available under that name anymore.
10018 You can also use @option{--label} to get a common information on
10019 all tapes of a series. For having this information different in each
10020 series created through a single script used on a regular basis, just
10021 manage to get some date string as part of the label. For example:
10025 $ @kbd{tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"}
10026 $ @kbd{tar --create --file=/dev/tape --multi-volume \
10027 --volume="Daily backup for `date +%Y-%m-%d`"}
10031 Also note that each label has its own date and time, which corresponds
10032 to when @GNUTAR{} initially attempted to write it,
10033 often soon after the operator launches @command{tar} or types the
10034 carriage return telling that the next tape is ready. Comparing date
10035 labels does give an idea of tape throughput only if the delays for
10036 rewinding tapes and the operator switching them were negligible, which
10037 is usually not the case.
10040 @section Verifying Data as It is Stored
10041 @cindex Verifying a write operation
10042 @cindex Double-checking a write operation
10047 @opindex verify, short description
10048 Attempt to verify the archive after writing.
10051 This option causes @command{tar} to verify the archive after writing it.
10052 Each volume is checked after it is written, and any discrepancies
10053 are recorded on the standard error output.
10055 Verification requires that the archive be on a back-space-able medium.
10056 This means pipes, some cartridge tape drives, and some other devices
10057 cannot be verified.
10059 You can insure the accuracy of an archive by comparing files in the
10060 system with archive members. @command{tar} can compare an archive to the
10061 file system as the archive is being written, to verify a write
10062 operation, or can compare a previously written archive, to insure that
10065 @opindex verify, using with @option{--create}
10066 @opindex create, using with @option{--verify}
10067 To check for discrepancies in an archive immediately after it is
10068 written, use the @option{--verify} (@option{-W}) option in conjunction with
10069 the @option{--create} operation. When this option is
10070 specified, @command{tar} checks archive members against their counterparts
10071 in the file system, and reports discrepancies on the standard error.
10073 To verify an archive, you must be able to read it from before the end
10074 of the last written entry. This option is useful for detecting data
10075 errors on some tapes. Archives written to pipes, some cartridge tape
10076 drives, and some other devices cannot be verified.
10078 One can explicitly compare an already made archive with the file
10079 system by using the @option{--compare} (@option{--diff}, @option{-d})
10080 option, instead of using the more automatic @option{--verify} option.
10083 Note that these two options have a slightly different intent. The
10084 @option{--compare} option checks how identical are the logical contents of some
10085 archive with what is on your disks, while the @option{--verify} option is
10086 really for checking if the physical contents agree and if the recording
10087 media itself is of dependable quality. So, for the @option{--verify}
10088 operation, @command{tar} tries to defeat all in-memory cache pertaining to
10089 the archive, while it lets the speed optimization undisturbed for the
10090 @option{--compare} option. If you nevertheless use @option{--compare} for
10091 media verification, you may have to defeat the in-memory cache yourself,
10092 maybe by opening and reclosing the door latch of your recording unit,
10093 forcing some doubt in your operating system about the fact this is really
10094 the same volume as the one just written or read.
10096 The @option{--verify} option would not be necessary if drivers were indeed
10097 able to detect dependably all write failures. This sometimes require many
10098 magnetic heads, some able to read after the writes occurred. One would
10099 not say that drivers unable to detect all cases are necessarily flawed,
10100 as long as programming is concerned.
10102 The @option{--verify} (@option{-W}) option will not work in
10103 conjunction with the @option{--multi-volume} (@option{-M}) option or
10104 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
10105 and @option{--delete} operations. @xref{Operations}, for more
10106 information on these operations.
10108 Also, since @command{tar} normally strips leading @samp{/} from file
10109 names (@pxref{absolute}), a command like @samp{tar --verify -cf
10110 /tmp/foo.tar /etc} will work as desired only if the working directory is
10111 @file{/}, as @command{tar} uses the archive's relative member names
10112 (e.g., @file{etc/motd}) when verifying the archive.
10114 @node Write Protection
10115 @section Write Protection
10117 Almost all tapes and diskettes, and in a few rare cases, even disks can
10118 be @dfn{write protected}, to protect data on them from being changed.
10119 Once an archive is written, you should write protect the media to prevent
10120 the archive from being accidentally overwritten or deleted. (This will
10121 protect the archive from being changed with a tape or floppy drive---it
10122 will not protect it from magnet fields or other physical hazards).
10124 The write protection device itself is usually an integral part of the
10125 physical media, and can be a two position (write enabled/write
10126 disabled) switch, a notch which can be popped out or covered, a ring
10127 which can be removed from the center of a tape reel, or some other
10128 changeable feature.
10133 This appendix lists some important user-visible changes between
10134 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
10135 version of this document is available at
10136 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
10137 @GNUTAR{} documentation page}.
10140 @item Use of globbing patterns when listing and extracting.
10142 Previous versions of GNU tar assumed shell-style globbing when
10143 extracting from or listing an archive. For example:
10146 $ @kbd{tar xf foo.tar '*.c'}
10149 would extract all files whose names end in @samp{.c}. This behavior
10150 was not documented and was incompatible with traditional tar
10151 implementations. Therefore, starting from version 1.15.91, GNU tar
10152 no longer uses globbing by default. For example, the above invocation
10153 is now interpreted as a request to extract from the archive the file
10156 To facilitate transition to the new behavior for those users who got
10157 used to the previous incorrect one, @command{tar} will print a warning
10158 if it finds out that a requested member was not found in the archive
10159 and its name looks like a globbing pattern. For example:
10162 $ @kbd{tar xf foo.tar '*.c'}
10163 tar: Pattern matching characters used in file names. Please,
10164 tar: use --wildcards to enable pattern matching, or --no-wildcards to
10165 tar: suppress this warning.
10166 tar: *.c: Not found in archive
10167 tar: Error exit delayed from previous errors
10170 To treat member names as globbing patterns, use --wildcards option.
10171 If you want to tar to mimic the behavior of versions prior to 1.15.91,
10172 add this option to your @env{TAR_OPTIONS} variable.
10174 @xref{wildcards}, for the detailed discussion of the use of globbing
10175 patterns by @GNUTAR{}.
10177 @item Use of short option @option{-o}.
10179 Earlier versions of @GNUTAR{} understood @option{-o} command line
10180 option as a synonym for @option{--old-archive}.
10182 @GNUTAR{} starting from version 1.13.90 understands this option as
10183 a synonym for @option{--no-same-owner}. This is compatible with
10184 UNIX98 @command{tar} implementations.
10186 However, to facilitate transition, @option{-o} option retains its
10187 old semantics when it is used with one of archive-creation commands.
10188 Users are encouraged to use @option{--format=oldgnu} instead.
10190 It is especially important, since versions of @acronym{GNU} Automake
10191 up to and including 1.8.4 invoke tar with this option to produce
10192 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
10193 of this issue and its implications.
10195 @FIXME{Change the first argument to tar-formats when the new Automake is
10196 out. The proposition to add @anchor{} to the appropriate place of its
10197 docs was accepted by Automake people --Sergey 2006-05-25}.
10198 @xref{Options, tar-v7, Changing Automake's Behavior,
10199 automake, GNU Automake}, for a description on how to use various
10200 archive formats with @command{automake}.
10202 Future versions of @GNUTAR{} will understand @option{-o} only as a
10203 synonym for @option{--no-same-owner}.
10205 @item Use of short option @option{-l}
10207 Earlier versions of @GNUTAR{} understood @option{-l} option as a
10208 synonym for @option{--one-file-system}. Since such usage contradicted
10209 to UNIX98 specification and harmed compatibility with other
10210 implementation, it was declared deprecated in version 1.14. However,
10211 to facilitate transition to its new semantics, it was supported by
10212 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
10213 variant of @option{--check-links} was introduced in version 1.15.91.
10215 @item Use of options @option{--portability} and @option{--old-archive}
10217 These options are deprecated. Please use @option{--format=v7} instead.
10219 @item Use of option @option{--posix}
10221 This option is deprecated. Please use @option{--format=posix} instead.
10224 @node Configuring Help Summary
10225 @appendix Configuring Help Summary
10227 Running @kbd{tar --help} displays the short @command{tar} option
10228 summary (@pxref{help}). This summary is organised by @dfn{groups} of
10229 semantically close options. The options within each group are printed
10230 in the following order: a short option, eventually followed by a list
10231 of corresponding long option names, followed by a short description of
10232 the option. For example, here is an excerpt from the actual @kbd{tar
10236 Main operation mode:
10238 -A, --catenate, --concatenate append tar files to an archive
10239 -c, --create create a new archive
10240 -d, --diff, --compare find differences between archive and
10242 --delete delete from the archive
10245 @vrindex ARGP_HELP_FMT, environment variable
10246 The exact visual representation of the help output is configurable via
10247 @env{ARGP_HELP_FMT} environment variable. The value of this variable
10248 is a comma-separated list of @dfn{format variable} assignments. There
10249 are two kinds of format variables. An @dfn{offset variable} keeps the
10250 offset of some part of help output text from the leftmost column on
10251 the screen. A @dfn{boolean} variable is a flag that toggles some
10252 output feature on or off. Depending on the type of the corresponding
10253 variable, there are two kinds of assignments:
10256 @item Offset assignment
10258 The assignment to an offset variable has the following syntax:
10261 @var{variable}=@var{value}
10265 where @var{variable} is the variable name, and @var{value} is a
10266 numeric value to be assigned to the variable.
10268 @item Boolean assignment
10270 To assign @code{true} value to a variable, simply put this variable name. To
10271 assign @code{false} value, prefix the variable name with @samp{no-}. For
10276 # Assign @code{true} value:
10278 # Assign @code{false} value:
10284 Following variables are declared:
10286 @deftypevr {Help Output} boolean dup-args
10287 If true, arguments for an option are shown with both short and long
10288 options, even when a given option has both forms, for example:
10291 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
10294 If false, then if an option has both short and long forms, the
10295 argument is only shown with the long one, for example:
10298 -f, --file=ARCHIVE use archive file or device ARCHIVE
10302 and a message indicating that the argument is applicable to both
10303 forms is printed below the options. This message can be disabled
10304 using @code{dup-args-note} (see below).
10306 The default is false.
10309 @deftypevr {Help Output} boolean dup-args-note
10310 If this variable is true, which is the default, the following notice
10311 is displayed at the end of the help output:
10314 Mandatory or optional arguments to long options are also mandatory or
10315 optional for any corresponding short options.
10318 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
10319 variables @code{dup-args} or @code{dup-args-note} should be set.
10322 @deftypevr {Help Output} offset short-opt-col
10323 Column in which short options start. Default is 2.
10327 $ @kbd{tar --help|grep ARCHIVE}
10328 -f, --file=ARCHIVE use archive file or device ARCHIVE
10329 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
10330 -f, --file=ARCHIVE use archive file or device ARCHIVE
10335 @deftypevr {Help Output} offset long-opt-col
10336 Column in which long options start. Default is 6. For example:
10340 $ @kbd{tar --help|grep ARCHIVE}
10341 -f, --file=ARCHIVE use archive file or device ARCHIVE
10342 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
10343 -f, --file=ARCHIVE use archive file or device ARCHIVE
10348 @deftypevr {Help Output} offset doc-opt-col
10349 Column in which @dfn{doc options} start. A doc option isn't actually
10350 an option, but rather an arbitrary piece of documentation that is
10351 displayed in much the same manner as the options. For example, in
10352 the description of @option{--format} option:
10356 -H, --format=FORMAT create archive of the given format.
10358 FORMAT is one of the following:
10360 gnu GNU tar 1.13.x format
10361 oldgnu GNU format as per tar <= 1.12
10362 pax POSIX 1003.1-2001 (pax) format
10364 ustar POSIX 1003.1-1988 (ustar) format
10365 v7 old V7 tar format
10370 the format names are doc options. Thus, if you set
10371 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
10372 will look as follows:
10376 -H, --format=FORMAT create archive of the given format.
10378 FORMAT is one of the following:
10380 gnu GNU tar 1.13.x format
10381 oldgnu GNU format as per tar <= 1.12
10382 pax POSIX 1003.1-2001 (pax) format
10384 ustar POSIX 1003.1-1988 (ustar) format
10385 v7 old V7 tar format
10390 @deftypevr {Help Output} offset opt-doc-col
10391 Column in which option description starts. Default is 29.
10395 $ @kbd{tar --help|grep ARCHIVE}
10396 -f, --file=ARCHIVE use archive file or device ARCHIVE
10397 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
10398 -f, --file=ARCHIVE use archive file or device ARCHIVE
10399 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
10401 use archive file or device ARCHIVE
10406 Notice, that the description starts on a separate line if
10407 @code{opt-doc-col} value is too small.
10410 @deftypevr {Help Output} offset header-col
10411 Column in which @dfn{group headers} are printed. A group header is a
10412 descriptive text preceding an option group. For example, in the
10416 Main operation mode:
10418 -A, --catenate, --concatenate append tar files to
10420 -c, --create create a new archive
10423 @samp{Main operation mode:} is the group header.
10425 The default value is 1.
10428 @deftypevr {Help Output} offset usage-indent
10429 Indentation of wrapped usage lines. Affects @option{--usage}
10430 output. Default is 12.
10433 @deftypevr {Help Output} offset rmargin
10434 Right margin of the text output. Used for wrapping.
10439 @include genfile.texi
10441 @node Snapshot Files
10442 @appendix Format of the Incremental Snapshot Files
10443 @include snapshot.texi
10447 @include dumpdir.texi
10449 @node Free Software Needs Free Documentation
10450 @appendix Free Software Needs Free Documentation
10451 @include freemanuals.texi
10453 @node Copying This Manual
10454 @appendix Copying This Manual
10457 * GNU Free Documentation License:: License for copying this manual
10462 @node Index of Command Line Options
10463 @appendix Index of Command Line Options
10465 This appendix contains an index of all @GNUTAR{} long command line
10466 options. The options are listed without the preceeding double-dash.
10469 @item Make sure @emph{all} options are indexed.
10470 @item Provide an index of short options
10484 @c Local variables:
10485 @c texinfo-column-for-description: 32