1 \input texinfo @c -*-texinfo-*-
2 @comment %**start of header
5 @settitle GNU tar @value{VERSION}
13 @include rendition.texi
16 @c Put everything in one index (arbitrarily chosen to be the concept index).
26 This manual is for @acronym{GNU} @command{tar} (version
27 @value{VERSION}, @value{UPDATED}), which creates and extracts files
30 Copyright @copyright{} 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001,
31 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
34 Permission is granted to copy, distribute and/or modify this document
35 under the terms of the GNU Free Documentation License, Version 1.1 or
36 any later version published by the Free Software Foundation; with no
37 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
38 and with the Back-Cover Texts as in (a) below. A copy of the license
39 is included in the section entitled "GNU Free Documentation License".
41 (a) The FSF's Back-Cover Text is: ``You are free to copy and modify
42 this GNU Manual. Buying copies from GNU Press supports the FSF in
43 developing GNU and promoting software freedom.''
47 @dircategory Archiving
49 * Tar: (tar). Making tape (or disk) archives.
52 @dircategory Individual utilities
54 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
57 @shorttitlepage @acronym{GNU} @command{tar}
60 @title @acronym{GNU} tar: an archiver tool
61 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
62 @author John Gilmore, Jay Fenlason et al.
65 @vskip 0pt plus 1filll
71 @top @acronym{GNU} tar: an archiver tool
76 @cindex archiving files
78 The first part of this master menu lists the major nodes in this Info
79 document. The rest of the menu lists all the lower level nodes.
82 @c The master menu, created with texinfo-master-menu, goes here.
83 @c (However, getdate.texi's menu is interpolated by hand.)
92 * Date input formats::
99 * Configuring Help Summary::
102 * Free Software Needs Free Documentation::
103 * Copying This Manual::
104 * Index of Command Line Options::
108 --- The Detailed Node Listing ---
112 * Book Contents:: What this Book Contains
113 * Definitions:: Some Definitions
114 * What tar Does:: What @command{tar} Does
115 * Naming tar Archives:: How @command{tar} Archives are Named
116 * Authors:: @GNUTAR{} Authors
117 * Reports:: Reporting bugs or suggestions
119 Tutorial Introduction to @command{tar}
122 * stylistic conventions::
123 * basic tar options:: Basic @command{tar} Operations and Options
124 * frequent operations::
125 * Two Frequent Options::
126 * create:: How to Create Archives
127 * list:: How to List Archives
128 * extract:: How to Extract Members from an Archive
131 Two Frequently Used Options
137 How to Create Archives
139 * prepare for examples::
140 * Creating the archive::
149 How to Extract Members from an Archive
151 * extracting archives::
159 * using tar options::
167 The Three Option Styles
169 * Mnemonic Options:: Mnemonic Option Style
170 * Short Options:: Short Option Style
171 * Old Options:: Old Option Style
172 * Mixing:: Mixing Option Styles
174 All @command{tar} Options
176 * Operation Summary::
178 * Short Option Summary::
190 Advanced @GNUTAR{} Operations
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 * after:: Operating Only on New Files
266 * recurse:: Descending into Directories
267 * one:: Crossing File System Boundaries
269 Reading Names from a File
275 * problems with exclude::
277 Crossing File System Boundaries
279 * directory:: Changing Directory
280 * absolute:: Absolute File Names
284 * General date syntax:: Common rules.
285 * Calendar date items:: 19 Dec 1994.
286 * Time of day items:: 9:20pm.
287 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}, ...
288 * Day of week items:: Monday and others.
289 * Relative items in date strings:: next tuesday, 2 years ago.
290 * Pure numbers in date strings:: 19931219, 1440.
291 * Seconds since the Epoch:: @@1078100502.
292 * Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al.
294 Controlling the Archive Format
296 * Portability:: Making @command{tar} Archives More Portable
297 * Compression:: Using Less Space through Compression
298 * Attributes:: Handling File Attributes
299 * Standard:: The Standard Format
300 * Extensions:: @acronym{GNU} Extensions to the Archive Format
301 * cpio:: Comparison of @command{tar} and @command{cpio}
303 Making @command{tar} Archives More Portable
305 * Portable Names:: Portable Names
306 * dereference:: Symbolic Links
307 * old:: Old V7 Archives
308 * posix:: @acronym{POSIX} archives
309 * Checksumming:: Checksumming Problems
310 * Large or Negative Values:: Large files, negative time stamps, etc.
312 Using Less Space through Compression
314 * gzip:: Creating and Reading Compressed Archives
315 * sparse:: Archiving Sparse Files
317 Tapes and Other Archive Media
319 * Device:: Device selection and switching
320 * Remote Tape Server::
321 * Common Problems and Solutions::
322 * Blocking:: Blocking
323 * Many:: Many archives on one tape
324 * Using Multiple Tapes:: Using Multiple Tapes
325 * label:: Including a Label in the Archive
331 * Format Variations:: Format Variations
332 * Blocking Factor:: The Blocking Factor of an Archive
334 Many Archives on One Tape
336 * Tape Positioning:: Tape Positions and Tape Marks
337 * mt:: The @command{mt} Utility
341 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
342 * Tape Files:: Tape Files
343 * Tarcat:: Concatenate Volumes into a Single Archive
345 GNU tar internals and development
352 * Free Software Needs Free Documentation::
353 * GNU Free Documentation License:: License for copying this manual
359 @chapter Introduction
362 and manipulates @dfn{archives} which are actually collections of
363 many other files; the program provides users with an organized and
364 systematic method for controlling a large amount of data.
365 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
366 archives need not (and these days, typically do not) reside on tapes.
369 * Book Contents:: What this Book Contains
370 * Definitions:: Some Definitions
371 * What tar Does:: What @command{tar} Does
372 * Naming tar Archives:: How @command{tar} Archives are Named
373 * Authors:: @GNUTAR{} Authors
374 * Reports:: Reporting bugs or suggestions
378 @section What this Book Contains
380 The first part of this chapter introduces you to various terms that will
381 recur throughout the book. It also tells you who has worked on @GNUTAR{}
382 and its documentation, and where you should send bug reports
385 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
386 gentle introduction for people who are new to using @command{tar}. It is
387 meant to be self contained, not requiring any reading from subsequent
388 chapters to make sense. It moves from topic to topic in a logical,
389 progressive order, building on information already explained.
391 Although the tutorial is paced and structured to allow beginners to
392 learn how to use @command{tar}, it is not intended solely for beginners.
393 The tutorial explains how to use the three most frequently used
394 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
395 two frequently used options (@samp{file} and @samp{verbose}). The other
396 chapters do not refer to the tutorial frequently; however, if a section
397 discusses something which is a complex variant of a basic concept, there
398 may be a cross reference to that basic concept. (The entire book,
399 including the tutorial, assumes that the reader understands some basic
400 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
402 The third chapter presents the remaining five operations, and
403 information about using @command{tar} options and option syntax.
405 @FIXME{this sounds more like a @acronym{GNU} Project Manuals Concept [tm] more
406 than the reality. should think about whether this makes sense to say
407 here, or not.} The other chapters are meant to be used as a
408 reference. Each chapter presents everything that needs to be said
409 about a specific topic.
411 One of the chapters (@pxref{Date input formats}) exists in its
412 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
413 In addition, one section of this manual (@pxref{Standard}) contains a
414 big quote which is taken directly from @command{tar} sources.
416 In general, we give both long and short (abbreviated) option names
417 at least once in each section where the relevant option is covered, so
418 that novice readers will become familiar with both styles. (A few
419 options have no short versions, and the relevant sections will
423 @section Some Definitions
427 The @command{tar} program is used to create and manipulate @command{tar}
428 archives. An @dfn{archive} is a single file which contains the contents
429 of many files, while still identifying the names of the files, their
430 owner(s), and so forth. (In addition, archives record access
431 permissions, user and group, size in bytes, and data modification time.
432 Some archives also record the file names in each archived directory, as
433 well as other file and directory information.) You can use @command{tar}
434 to @dfn{create} a new archive in a specified directory.
437 @cindex archive member
440 The files inside an archive are called @dfn{members}. Within this
441 manual, we use the term @dfn{file} to refer only to files accessible in
442 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
443 @dfn{member} to refer only to the members of an archive. Similarly, a
444 @dfn{file name} is the name of a file, as it resides in the file system,
445 and a @dfn{member name} is the name of an archive member within the
450 The term @dfn{extraction} refers to the process of copying an archive
451 member (or multiple members) into a file in the file system. Extracting
452 all the members of an archive is often called @dfn{extracting the
453 archive}. The term @dfn{unpack} can also be used to refer to the
454 extraction of many or all the members of an archive. Extracting an
455 archive does not destroy the archive's structure, just as creating an
456 archive does not destroy the copies of the files that exist outside of
457 the archive. You may also @dfn{list} the members in a given archive
458 (this is often thought of as ``printing'' them to the standard output,
459 or the command line), or @dfn{append} members to a pre-existing archive.
460 All of these operations can be performed using @command{tar}.
463 @section What @command{tar} Does
466 The @command{tar} program provides the ability to create @command{tar}
467 archives, as well as various other kinds of manipulation. For example,
468 you can use @command{tar} on previously created archives to extract files,
469 to store additional files, or to update or list files which were already
472 Initially, @command{tar} archives were used to store files conveniently on
473 magnetic tape. The name @command{tar} comes from this use; it stands for
474 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
475 direct its output to available devices, files, or other programs (using
476 pipes). @command{tar} may even access remote devices or files (as archives).
478 You can use @command{tar} archives in many ways. We want to stress a few
479 of them: storage, backup, and transportation.
481 @FIXME{the following table entries need a bit of work..}
484 Often, @command{tar} archives are used to store related files for
485 convenient file transfer over a network. For example, the
486 @acronym{GNU} Project distributes its software bundled into
487 @command{tar} archives, so that all the files relating to a particular
488 program (or set of related programs) can be transferred as a single
491 A magnetic tape can store several files in sequence. However, the tape
492 has no names for these files; it only knows their relative position on
493 the tape. One way to store several files on one tape and retain their
494 names is by creating a @command{tar} archive. Even when the basic transfer
495 mechanism can keep track of names, as FTP can, the nuisance of handling
496 multiple files, directories, and multiple links makes @command{tar}
499 Archive files are also used for long-term storage. You can think of
500 this as transportation from the present into the future. (It is a
501 science-fiction idiom that you can move through time as well as in
502 space; the idea here is that @command{tar} can be used to move archives in
503 all dimensions, even time!)
506 Because the archive created by @command{tar} is capable of preserving
507 file information and directory structure, @command{tar} is commonly
508 used for performing full and incremental backups of disks. A backup
509 puts a collection of files (possibly pertaining to many users and
510 projects) together on a disk or a tape. This guards against
511 accidental destruction of the information in those files.
512 @GNUTAR{} has special features that allow it to be
513 used to make incremental and full dumps of all the files in a
517 You can create an archive on one system, transfer it to another system,
518 and extract the contents there. This allows you to transport a group of
519 files from one system to another.
522 @node Naming tar Archives
523 @section How @command{tar} Archives are Named
525 Conventionally, @command{tar} archives are given names ending with
526 @samp{.tar}. This is not necessary for @command{tar} to operate properly,
527 but this manual follows that convention in order to accustom readers to
528 it and to make examples more clear.
533 Often, people refer to @command{tar} archives as ``@command{tar} files,'' and
534 archive members as ``files'' or ``entries''. For people familiar with
535 the operation of @command{tar}, this causes no difficulty. However, in
536 this manual, we consistently refer to ``archives'' and ``archive
537 members'' to make learning to use @command{tar} easier for novice users.
540 @section @GNUTAR{} Authors
542 @GNUTAR{} was originally written by John Gilmore,
543 and modified by many people. The @acronym{GNU} enhancements were
544 written by Jay Fenlason, then Joy Kendall, and the whole package has
545 been further maintained by Thomas Bushnell, n/BSG, Fran@,{c}ois
546 Pinard, Paul Eggert, and finally Sergey Poznyakoff with the help of
547 numerous and kind users.
549 We wish to stress that @command{tar} is a collective work, and owes much to
550 all those people who reported problems, offered solutions and other
551 insights, or shared their thoughts and suggestions. An impressive, yet
552 partial list of those contributors can be found in the @file{THANKS}
553 file from the @GNUTAR{} distribution.
555 @FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not
556 sure i want to spell out the history in this detail, at least not for
557 the printed book. i'm just not sure it needs to be said this way.
558 i'll think about it.}
560 @FIXME{History is more important, and surely more interesting, than
561 actual names. Quoting names without history would be meaningless. FP}
563 Jay Fenlason put together a draft of a @GNUTAR{}
564 manual, borrowing notes from the original man page from John Gilmore.
565 This was withdrawn in version 1.11. Thomas Bushnell, n/BSG and Amy
566 Gorin worked on a tutorial and manual for @GNUTAR{}.
567 Fran@,{c}ois Pinard put version 1.11.8 of the manual together by
568 taking information from all these sources and merging them. Melissa
569 Weisshaus finally edited and redesigned the book to create version
570 1.12. @FIXME{update version number as necessary; i'm being
571 optimistic!} @FIXME{Someone [maybe karl berry? maybe bob chassell?
572 maybe melissa? maybe julie sussman?] needs to properly index the
575 For version 1.12, Daniel Hagerty contributed a great deal of technical
576 consulting. In particular, he is the primary author of @ref{Backups}.
578 In July, 2003 @GNUTAR{} was put on CVS at savannah.gnu.org
579 (see @url{http://savannah.gnu.org/projects/tar}), and
580 active development and maintenance work has started
581 again. Currently @GNUTAR{} is being maintained by Paul Eggert, Sergey
582 Poznyakoff and Jeff Bailey.
584 Support for @acronym{POSIX} archives was added by Sergey Poznyakoff.
587 @section Reporting bugs or suggestions
590 @cindex reporting bugs
591 If you find problems or have suggestions about this program or manual,
592 please report them to @file{bug-tar@@gnu.org}.
594 When reporting a bug, please be sure to include as much detail as
595 possible, in order to reproduce it. @FIXME{Be more specific, I'd
596 like to make this node as detailed as 'Bug reporting' node in Emacs
600 @chapter Tutorial Introduction to @command{tar}
602 This chapter guides you through some basic examples of three @command{tar}
603 operations: @option{--create}, @option{--list}, and @option{--extract}. If
604 you already know how to use some other version of @command{tar}, then you
605 may not need to read this chapter. This chapter omits most complicated
606 details about how @command{tar} works.
610 * stylistic conventions::
611 * basic tar options:: Basic @command{tar} Operations and Options
612 * frequent operations::
613 * Two Frequent Options::
614 * create:: How to Create Archives
615 * list:: How to List Archives
616 * extract:: How to Extract Members from an Archive
621 @section Assumptions this Tutorial Makes
623 This chapter is paced to allow beginners to learn about @command{tar}
624 slowly. At the same time, we will try to cover all the basic aspects of
625 these three operations. In order to accomplish both of these tasks, we
626 have made certain assumptions about your knowledge before reading this
627 manual, and the hardware you will be using:
631 Before you start to work through this tutorial, you should understand
632 what the terms ``archive'' and ``archive member'' mean
633 (@pxref{Definitions}). In addition, you should understand something
634 about how Unix-type operating systems work, and you should know how to
635 use some basic utilities. For example, you should know how to create,
636 list, copy, rename, edit, and delete files and directories; how to
637 change between directories; and how to figure out where you are in the
638 file system. You should have some basic understanding of directory
639 structure and how files are named according to which directory they are
640 in. You should understand concepts such as standard output and standard
641 input, what various definitions of the term ``argument'' mean, and the
642 differences between relative and absolute path names. @FIXME{and what
646 This manual assumes that you are working from your own home directory
647 (unless we state otherwise). In this tutorial, you will create a
648 directory to practice @command{tar} commands in. When we show path names,
649 we will assume that those paths are relative to your home directory.
650 For example, my home directory path is @file{/home/fsf/melissa}. All of
651 my examples are in a subdirectory of the directory named by that path
652 name; the subdirectory is called @file{practice}.
655 In general, we show examples of archives which exist on (or can be
656 written to, or worked with from) a directory on a hard disk. In most
657 cases, you could write those archives to, or work with them on any other
658 device, such as a tape drive. However, some of the later examples in
659 the tutorial and next chapter will not work on tape drives.
660 Additionally, working with tapes is much more complicated than working
661 with hard disks. For these reasons, the tutorial does not cover working
662 with tape drives. @xref{Media}, for complete information on using
663 @command{tar} archives with tape drives.
665 @FIXME{this is a cop out. need to add some simple tape drive info.}
668 @node stylistic conventions
669 @section Stylistic Conventions
671 In the examples, @samp{$} represents a typical shell prompt. It
672 precedes lines you should type; to make this more clear, those lines are
673 shown in @kbd{this font}, as opposed to lines which represent the
674 computer's response; those lines are shown in @code{this font}, or
675 sometimes @samp{like this}.
677 @c When we have lines which are too long to be
678 @c displayed in any other way, we will show them like this:
680 @node basic tar options
681 @section Basic @command{tar} Operations and Options
683 @command{tar} can take a wide variety of arguments which specify and define
684 the actions it will have on the particular set of files or the archive.
685 The main types of arguments to @command{tar} fall into one of two classes:
686 operations, and options.
688 Some arguments fall into a class called @dfn{operations}; exactly one of
689 these is both allowed and required for any instance of using @command{tar};
690 you may @emph{not} specify more than one. People sometimes speak of
691 @dfn{operating modes}. You are in a particular operating mode when you
692 have specified the operation which specifies it; there are eight
693 operations in total, and thus there are eight operating modes.
695 The other arguments fall into the class known as @dfn{options}. You are
696 not required to specify any options, and you are allowed to specify more
697 than one at a time (depending on the way you are using @command{tar} at
698 that time). Some options are used so frequently, and are so useful for
699 helping you type commands more carefully that they are effectively
700 ``required''. We will discuss them in this chapter.
702 You can write most of the @command{tar} operations and options in any
703 of three forms: long (mnemonic) form, short form, and old style. Some
704 of the operations and options have no short or ``old'' forms; however,
705 the operations and options which we will cover in this tutorial have
706 corresponding abbreviations. @FIXME{make sure this is still the case,
707 at the end}We will indicate those abbreviations appropriately to get
708 you used to seeing them. (Note that the ``old style'' option forms
709 exist in @GNUTAR{} for compatibility with Unix
710 @command{tar}. In this book we present a full discussion of this way
711 of writing options and operations (@pxref{Old Options}), and we discuss
712 the other two styles of writing options (@xref{Mnemonic Options}, and
713 @pxref{Short Options}).
715 In the examples and in the text of this tutorial, we usually use the
716 long forms of operations and options; but the ``short'' forms produce
717 the same result and can make typing long @command{tar} commands easier.
718 For example, instead of typing
721 @kbd{tar --create --verbose --file=afiles.tar apple angst aspic}
727 @kbd{tar -c -v -f afiles.tar apple angst aspic}
733 @kbd{tar -cvf afiles.tar apple angst aspic}
737 For more information on option syntax, see @ref{Advanced tar}. In
738 discussions in the text, when we name an option by its long form, we
739 also give the corresponding short option in parentheses.
741 The term, ``option'', can be confusing at times, since ``operations''
742 are often lumped in with the actual, @emph{optional} ``options'' in certain
743 general class statements. For example, we just talked about ``short and
744 long forms of options and operations''. However, experienced @command{tar}
745 users often refer to these by shorthand terms such as, ``short and long
746 options''. This term assumes that the ``operations'' are included, also.
747 Context will help you determine which definition of ``options'' to use.
749 Similarly, the term ``command'' can be confusing, as it is often used in
750 two different ways. People sometimes refer to @command{tar} ``commands''.
751 A @command{tar} @dfn{command} is the entire command line of user input
752 which tells @command{tar} what to do --- including the operation, options,
753 and any arguments (file names, pipes, other commands, etc). However,
754 you will also sometimes hear the term ``the @command{tar} command''. When
755 the word ``command'' is used specifically like this, a person is usually
756 referring to the @command{tar} @emph{operation}, not the whole line.
757 Again, use context to figure out which of the meanings the speaker
760 @node frequent operations
761 @section The Three Most Frequently Used Operations
763 Here are the three most frequently used operations (both short and long
764 forms), as well as a brief description of their meanings. The rest of
765 this chapter will cover how to use these operations in detail. We will
766 present the rest of the operations in the next chapter.
771 Create a new @command{tar} archive.
774 List the contents of an archive.
777 Extract one or more members from an archive.
780 @node Two Frequent Options
781 @section Two Frequently Used Options
783 To understand how to run @command{tar} in the three operating modes listed
784 previously, you also need to understand how to use two of the options to
785 @command{tar}: @option{--file} (which takes an archive file as an argument)
786 and @option{--verbose}. (You are usually not @emph{required} to specify
787 either of these options when you run @command{tar}, but they can be very
788 useful in making things more clear and helping you avoid errors.)
797 @unnumberedsubsec The @option{--file} Option
800 @opindex file, tutorial
801 @item --file=@var{archive-name}
802 @itemx -f @var{archive-name}
803 Specify the name of an archive file.
806 You can specify an argument for the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option whenever you
807 use @command{tar}; this option determines the name of the archive file
808 that @command{tar} will work on.
811 If you don't specify this argument, then @command{tar} will examine
812 the environment variable @env{TAPE}. If it is set, its value will be
813 used as the archive name. Otherwise, @command{tar} will use the
814 default archive, determined at the compile time. Usually it is
815 standard output or some physical tape drive attached to your machine
816 (you can verify what the default is by running @kbd{tar
817 --show-defaults}, @pxref{defaults}). If there is no tape drive
818 attached, or the default is not meaningful, then @command{tar} will
819 print an error message. The error message might look roughly like one
823 tar: can't open /dev/rmt8 : No such device or address
824 tar: can't open /dev/rsmt0 : I/O error
828 To avoid confusion, we recommend that you always specify an archive file
829 name by using @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) when writing your @command{tar} commands.
830 For more information on using the @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option, see
833 @node verbose tutorial
834 @unnumberedsubsec The @option{--verbose} Option
837 @opindex verbose, introduced
840 Show the files being worked on as @command{tar} is running.
843 @option{--verbose} (@option{-v}) shows details about the results of running
844 @command{tar}. This can be especially useful when the results might not be
845 obvious. For example, if you want to see the progress of @command{tar} as
846 it writes files into the archive, you can use the @option{--verbose}
847 option. In the beginning, you may find it useful to use
848 @option{--verbose} at all times; when you are more accustomed to
849 @command{tar}, you will likely want to use it at certain times but not at
850 others. We will use @option{--verbose} at times to help make something
851 clear, and we will give many examples both using and not using
852 @option{--verbose} to show the differences.
854 Sometimes, a single instance of @option{--verbose} on the command line
855 will show a full, @samp{ls} style listing of an archive or files,
856 giving sizes, owners, and similar information. @FIXME{Describe the
857 exact output format, e.g., how hard links are displayed.}
858 Other times, @option{--verbose} will only show files or members that the particular
859 operation is operating on at the time. In the latter case, you can
860 use @option{--verbose} twice in a command to get a listing such as that
861 in the former case. For example, instead of saying
864 @kbd{tar -cvf afiles.tar apple angst aspic}
871 @kbd{tar -cvvf afiles.tar apple angst aspic}
875 This works equally well using short or long forms of options. Using
876 long forms, you would simply write out the mnemonic form of the option
880 $ @kbd{tar --create --verbose --verbose @dots{}}
884 Note that you must double the hyphens properly each time.
886 Later in the tutorial, we will give examples using @w{@option{--verbose
890 @unnumberedsubsec Getting Help: Using the @option{--help} Option
896 The @option{--help} option to @command{tar} prints out a very brief list of
897 all operations and option available for the current version of
898 @command{tar} available on your system.
902 @section How to Create Archives
905 @cindex Creation of the archive
906 @cindex Archive, creation of
907 One of the basic operations of @command{tar} is @option{--create} (@option{-c}), which
908 you use to create a @command{tar} archive. We will explain
909 @option{--create} first because, in order to learn about the other
910 operations, you will find it useful to have an archive available to
913 To make this easier, in this section you will first create a directory
914 containing three files. Then, we will show you how to create an
915 @emph{archive} (inside the new directory). Both the directory, and
916 the archive are specifically for you to practice on. The rest of this
917 chapter and the next chapter will show many examples using this
918 directory and the files you will create: some of those files may be
919 other directories and other archives.
921 The three files you will archive in this example are called
922 @file{blues}, @file{folk}, and @file{jazz}. The archive is called
923 @file{collection.tar}.
925 This section will proceed slowly, detailing how to use @option{--create}
926 in @code{verbose} mode, and showing examples using both short and long
927 forms. In the rest of the tutorial, and in the examples in the next
928 chapter, we will proceed at a slightly quicker pace. This section
929 moves more slowly to allow beginning users to understand how
933 * prepare for examples::
934 * Creating the archive::
940 @node prepare for examples
941 @subsection Preparing a Practice Directory for Examples
943 To follow along with this and future examples, create a new directory
944 called @file{practice} containing files called @file{blues}, @file{folk}
945 and @file{jazz}. The files can contain any information you like:
946 ideally, they should contain information which relates to their names,
947 and be of different lengths. Our examples assume that @file{practice}
948 is a subdirectory of your home directory.
950 Now @command{cd} to the directory named @file{practice}; @file{practice}
951 is now your @dfn{working directory}. (@emph{Please note}: Although
952 the full path name of this directory is
953 @file{/@var{homedir}/practice}, in our examples we will refer to
954 this directory as @file{practice}; the @var{homedir} is presumed.
956 In general, you should check that the files to be archived exist where
957 you think they do (in the working directory) by running @command{ls}.
958 Because you just created the directory and the files and have changed to
959 that directory, you probably don't need to do that this time.
961 It is very important to make sure there isn't already a file in the
962 working directory with the archive name you intend to use (in this case,
963 @samp{collection.tar}), or that you don't care about its contents.
964 Whenever you use @samp{create}, @command{tar} will erase the current
965 contents of the file named by @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) if it exists. @command{tar}
966 will not tell you if you are about to overwrite an archive unless you
967 specify an option which does this (@pxref{backup}, for the
968 information on how to do so). To add files to an existing archive,
969 you need to use a different option, such as @option{--append} (@option{-r}); see
970 @ref{append} for information on how to do this.
972 @node Creating the archive
973 @subsection Creating the Archive
975 @opindex create, introduced
976 To place the files @file{blues}, @file{folk}, and @file{jazz} into an
977 archive named @file{collection.tar}, use the following command:
980 $ @kbd{tar --create --file=collection.tar blues folk jazz}
983 The order of the arguments is not very important, @emph{when using long
984 option forms}. You could also say:
987 $ @kbd{tar blues --create folk --file=collection.tar jazz}
991 However, you can see that this order is harder to understand; this is
992 why we will list the arguments in the order that makes the commands
993 easiest to understand (and we encourage you to do the same when you use
994 @command{tar}, to avoid errors).
996 Note that the part of the command which says,
997 @w{@option{--file=collection.tar}} is considered to be @emph{one} argument.
998 If you substituted any other string of characters for
999 @kbd{collection.tar}, then that string would become the name of the
1000 archive file you create.
1002 The order of the options becomes more important when you begin to use
1003 short forms. With short forms, if you type commands in the wrong order
1004 (even if you type them correctly in all other ways), you may end up with
1005 results you don't expect. For this reason, it is a good idea to get
1006 into the habit of typing options in the order that makes inherent sense.
1007 @xref{short create}, for more information on this.
1009 In this example, you type the command as shown above: @option{--create}
1010 is the operation which creates the new archive
1011 (@file{collection.tar}), and @option{--file} is the option which lets
1012 you give it the name you chose. The files, @file{blues}, @file{folk},
1013 and @file{jazz}, are now members of the archive, @file{collection.tar}
1014 (they are @dfn{file name arguments} to the @option{--create} operation).
1015 @FIXME{xref here to the discussion of file name args?}Now that they are
1016 in the archive, they are called @emph{archive members}, not files.
1017 (@pxref{Definitions,members}).
1019 When you create an archive, you @emph{must} specify which files you
1020 want placed in the archive. If you do not specify any archive
1021 members, @GNUTAR{} will complain.
1023 If you now list the contents of the working directory (@command{ls}), you will
1024 find the archive file listed as well as the files you saw previously:
1027 blues folk jazz collection.tar
1031 Creating the archive @samp{collection.tar} did not destroy the copies of
1032 the files in the directory.
1034 Keep in mind that if you don't indicate an operation, @command{tar} will not
1035 run and will prompt you for one. If you don't name any files, @command{tar}
1036 will complain. You must have write access to the working directory,
1037 or else you will not be able to create an archive in that directory.
1039 @emph{Caution}: Do not attempt to use @option{--create} (@option{-c}) to add files to
1040 an existing archive; it will delete the archive and write a new one.
1041 Use @option{--append} (@option{-r}) instead. @xref{append}.
1043 @node create verbose
1044 @subsection Running @option{--create} with @option{--verbose}
1046 @opindex create, using with @option{--verbose}
1047 @opindex verbose, using with @option{--create}
1048 If you include the @option{--verbose} (@option{-v}) option on the command line,
1049 @command{tar} will list the files it is acting on as it is working. In
1050 verbose mode, the @code{create} example above would appear as:
1053 $ @kbd{tar --create --verbose --file=collection.tar blues folk jazz}
1059 This example is just like the example we showed which did not use
1060 @option{--verbose}, except that @command{tar} generated the remaining lines
1062 (note the different font styles).
1068 In the rest of the examples in this chapter, we will frequently use
1069 @code{verbose} mode so we can show actions or @command{tar} responses that
1070 you would otherwise not see, and which are important for you to
1074 @subsection Short Forms with @samp{create}
1076 As we said before, the @option{--create} (@option{-c}) operation is one of the most
1077 basic uses of @command{tar}, and you will use it countless times.
1078 Eventually, you will probably want to use abbreviated (or ``short'')
1079 forms of options. A full discussion of the three different forms that
1080 options can take appears in @ref{Styles}; for now, here is what the
1081 previous example (including the @option{--verbose} (@option{-v}) option) looks like
1082 using short option forms:
1085 $ @kbd{tar -cvf collection.tar blues folk jazz}
1092 As you can see, the system responds the same no matter whether you use
1093 long or short option forms.
1095 @FIXME{i don't like how this is worded:} One difference between using
1096 short and long option forms is that, although the exact placement of
1097 arguments following options is no more specific when using short forms,
1098 it is easier to become confused and make a mistake when using short
1099 forms. For example, suppose you attempted the above example in the
1103 $ @kbd{tar -cfv collection.tar blues folk jazz}
1107 In this case, @command{tar} will make an archive file called @file{v},
1108 containing the files @file{blues}, @file{folk}, and @file{jazz}, because
1109 the @samp{v} is the closest ``file name'' to the @option{-f} option, and
1110 is thus taken to be the chosen archive file name. @command{tar} will try
1111 to add a file called @file{collection.tar} to the @file{v} archive file;
1112 if the file @file{collection.tar} did not already exist, @command{tar} will
1113 report an error indicating that this file does not exist. If the file
1114 @file{collection.tar} does already exist (e.g., from a previous command
1115 you may have run), then @command{tar} will add this file to the archive.
1116 Because the @option{-v} option did not get registered, @command{tar} will not
1117 run under @samp{verbose} mode, and will not report its progress.
1119 The end result is that you may be quite confused about what happened,
1120 and possibly overwrite a file. To illustrate this further, we will show
1121 you how an example we showed previously would look using short forms.
1126 $ @kbd{tar blues --create folk --file=collection.tar jazz}
1130 is confusing as it is. When shown using short forms, however, it
1131 becomes much more so:
1134 $ @kbd{tar blues -c folk -f collection.tar jazz}
1138 It would be very easy to put the wrong string of characters
1139 immediately following the @option{-f}, but doing that could sacrifice
1142 For this reason, we recommend that you pay very careful attention to
1143 the order of options and placement of file and archive names,
1144 especially when using short option forms. Not having the option name
1145 written out mnemonically can affect how well you remember which option
1146 does what, and therefore where different names have to be placed.
1149 @subsection Archiving Directories
1151 @cindex Archiving Directories
1152 @cindex Directories, Archiving
1153 You can archive a directory by specifying its directory name as a
1154 file name argument to @command{tar}. The files in the directory will be
1155 archived relative to the working directory, and the directory will be
1156 re-created along with its contents when the archive is extracted.
1158 To archive a directory, first move to its superior directory. If you
1159 have followed the previous instructions in this tutorial, you should
1168 This will put you into the directory which contains @file{practice},
1169 i.e., your home directory. Once in the superior directory, you can
1170 specify the subdirectory, @file{practice}, as a file name argument. To
1171 store @file{practice} in the new archive file @file{music.tar}, type:
1174 $ @kbd{tar --create --verbose --file=music.tar practice}
1178 @command{tar} should output:
1185 practice/collection.tar
1188 Note that the archive thus created is not in the subdirectory
1189 @file{practice}, but rather in the current working directory---the
1190 directory from which @command{tar} was invoked. Before trying to archive a
1191 directory from its superior directory, you should make sure you have
1192 write access to the superior directory itself, not only the directory
1193 you are trying archive with @command{tar}. For example, you will probably
1194 not be able to store your home directory in an archive by invoking
1195 @command{tar} from the root directory; @xref{absolute}. (Note
1196 also that @file{collection.tar}, the original archive file, has itself
1197 been archived. @command{tar} will accept any file as a file to be
1198 archived, regardless of its content. When @file{music.tar} is
1199 extracted, the archive file @file{collection.tar} will be re-written
1200 into the file system).
1202 If you give @command{tar} a command such as
1205 $ @kbd{tar --create --file=foo.tar .}
1209 @command{tar} will report @samp{tar: ./foo.tar is the archive; not
1210 dumped}. This happens because @command{tar} creates the archive
1211 @file{foo.tar} in the current directory before putting any files into
1212 it. Then, when @command{tar} attempts to add all the files in the
1213 directory @file{.} to the archive, it notices that the file
1214 @file{./foo.tar} is the same as the archive @file{foo.tar}, and skips
1215 it. (It makes no sense to put an archive into itself.) @GNUTAR{}
1216 will continue in this case, and create the archive
1217 normally, except for the exclusion of that one file. (@emph{Please
1218 note:} Other implementations of @command{tar} may not be so clever;
1219 they will enter an infinite loop when this happens, so you should not
1220 depend on this behavior unless you are certain you are running
1221 @GNUTAR{}. In general, it is wise to always place the archive outside
1222 of the directory being dumped.
1225 @section How to List Archives
1228 Frequently, you will find yourself wanting to determine exactly what a
1229 particular archive contains. You can use the @option{--list} (@option{-t}) operation
1230 to get the member names as they currently appear in the archive, as well
1231 as various attributes of the files at the time they were archived. For
1232 example, you can examine the archive @file{collection.tar} that you
1233 created in the last section with the command,
1236 $ @kbd{tar --list --file=collection.tar}
1240 The output of @command{tar} would then be:
1249 The archive @file{bfiles.tar} would list as follows:
1258 Be sure to use a @option{--file=@var{archive-name}} (@option{-f
1259 @var{archive-name}}) option just as with @option{--create}
1260 (@option{-c}) to specify the name of the archive.
1262 @opindex list, using with @option{--verbose}
1263 @opindex verbose, using with @option{--list}
1264 If you use the @option{--verbose} (@option{-v}) option with
1265 @option{--list}, then @command{tar} will print out a listing
1266 reminiscent of @w{@samp{ls -l}}, showing owner, file size, and so forth.
1268 If you had used @option{--verbose} (@option{-v}) mode, the example
1269 above would look like:
1272 $ @kbd{tar --list --verbose --file=collection.tar folk}
1273 -rw-r--r-- myself user 62 1990-05-23 10:55 folk
1276 @cindex listing member and file names
1277 @anchor{listing member and file names}
1278 It is important to notice that the output of @kbd{tar --list
1279 --verbose} does not necessarily match that produced by @kbd{tar
1280 --create --verbose} while creating the archive. It is because
1281 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1282 prefixes from file names before storing them in the archive
1283 (@xref{absolute}, for more information). In other
1284 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1285 an archive and @dfn{member names} when listing it. Consider this
1290 $ @kbd{tar cfv archive /etc/mail}
1291 tar: Removing leading `/' from member names
1293 /etc/mail/sendmail.cf
1295 $ @kbd{tar tf archive}
1297 etc/mail/sendmail.cf
1302 @opindex show-stored-names
1303 This default behavior can sometimes be inconvenient. You can force
1304 @GNUTAR{} show member names when creating archive by supplying
1305 @option{--show-stored-names} option.
1308 @item --show-stored-names
1309 Print member (as opposed to @emph{file}) names when creating the archive.
1312 @cindex File name arguments, using @option{--list} with
1313 @opindex list, using with file name arguments
1314 You can specify one or more individual member names as arguments when
1315 using @samp{list}. In this case, @command{tar} will only list the
1316 names of members you identify. For example, @w{@kbd{tar --list
1317 --file=afiles.tar apple}} would only print @file{apple}.
1319 Because @command{tar} preserves paths, file names must be specified as
1320 they appear in the archive (ie., relative to the directory from which
1321 the archive was created). Therefore, it is essential when specifying
1322 member names to @command{tar} that you give the exact member names.
1323 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1324 error message something like @samp{tar: birds: Not found in archive},
1325 because there is no member named @file{birds}, only one named
1326 @file{./birds}. While the names @file{birds} and @file{./birds} name
1327 the same file, @emph{member} names by default are compared verbatim.
1329 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1330 with @file{baboon}, because this exact member name is in the archive file
1331 @file{bfiles.tar}. If you are not sure of the exact file name,
1332 use @dfn{globbing patterns}, for example:
1335 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1339 will list all members whose name contains @samp{b}. @xref{Wildcards},
1340 for a detailed discussion of globbing patterns and related
1341 @command{tar} command line options.
1348 @unnumberedsubsec Listing the Contents of a Stored Directory
1350 To get information about the contents of an archived directory,
1351 use the directory name as a file name argument in conjunction with
1352 @option{--list} (@option{-t}). To find out file attributes, include the
1353 @option{--verbose} (@option{-v}) option.
1355 For example, to find out about files in the directory @file{practice}, in
1356 the archive file @file{music.tar}, type:
1359 $ @kbd{tar --list --verbose --file=music.tar practice}
1362 @command{tar} responds:
1365 drwxrwxrwx myself user 0 1990-05-31 21:49 practice/
1366 -rw-r--r-- myself user 42 1990-05-21 13:29 practice/blues
1367 -rw-r--r-- myself user 62 1990-05-23 10:55 practice/folk
1368 -rw-r--r-- myself user 40 1990-05-21 13:30 practice/jazz
1369 -rw-r--r-- myself user 10240 1990-05-31 21:49 practice/collection.tar
1372 When you use a directory name as a file name argument, @command{tar} acts on
1373 all the files (including sub-directories) in that directory.
1376 @section How to Extract Members from an Archive
1379 @cindex Retrieving files from an archive
1380 @cindex Resurrecting files from an archive
1383 Creating an archive is only half the job---there is no point in storing
1384 files in an archive if you can't retrieve them. The act of retrieving
1385 members from an archive so they can be used and manipulated as
1386 unarchived files again is called @dfn{extraction}. To extract files
1387 from an archive, use the @option{--extract} (@option{--get} or
1388 @option{-x}) operation. As with @option{--create}, specify the name
1389 of the archive with @option{--file} (@option{-f}) option. Extracting
1390 an archive does not modify the archive in any way; you can extract it
1391 multiple times if you want or need to.
1393 Using @option{--extract}, you can extract an entire archive, or specific
1394 files. The files can be directories containing other files, or not. As
1395 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1396 long form of the operation without affecting the performance.
1399 * extracting archives::
1400 * extracting files::
1402 * extracting untrusted archives::
1403 * failing commands::
1406 @node extracting archives
1407 @subsection Extracting an Entire Archive
1409 To extract an entire archive, specify the archive file name only, with
1410 no individual file names as arguments. For example,
1413 $ @kbd{tar -xvf collection.tar}
1420 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
1421 -rw-r--r-- me user 21 1996-09-23 16:44 blues
1422 -rw-r--r-- me user 20 1996-09-23 16:44 folk
1425 @node extracting files
1426 @subsection Extracting Specific Files
1428 To extract specific archive members, give their exact member names as
1429 arguments, as printed by @option{--list} (@option{-t}). If you had
1430 mistakenly deleted one of the files you had placed in the archive
1431 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1432 from the archive without changing the archive's structure. Its
1433 contents will be identical to the original file @file{blues} that you
1436 First, make sure you are in the @file{practice} directory, and list the
1437 files in the directory. Now, delete the file, @samp{blues}, and list
1438 the files in the directory again.
1440 You can now extract the member @file{blues} from the archive file
1441 @file{collection.tar} like this:
1444 $ @kbd{tar --extract --file=collection.tar blues}
1448 If you list the files in the directory again, you will see that the file
1449 @file{blues} has been restored, with its original permissions, data
1450 modification times, and owner.@footnote{This is only accidentally
1451 true, but not in general. Whereas modification times are always
1452 restored, in most cases, one has to be root for restoring the owner,
1453 and use a special option for restoring permissions. Here, it just
1454 happens that the restoring user is also the owner of the archived
1455 members, and that the current @code{umask} is compatible with original
1456 permissions.} (These parameters will be identical to those which
1457 the file had when you originally placed it in the archive; any changes
1458 you may have made before deleting the file from the file system,
1459 however, will @emph{not} have been made to the archive member.) The
1460 archive file, @samp{collection.tar}, is the same as it was before you
1461 extracted @samp{blues}. You can confirm this by running @command{tar} with
1462 @option{--list} (@option{-t}).
1464 Remember that as with other operations, specifying the exact member
1465 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1466 will fail, because there is no member named @file{birds}. To extract
1467 the member named @file{./birds}, you must specify @w{@kbd{tar
1468 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1469 exact member names, use @option{--list} (@option{-t}) option
1470 (@pxref{list}). You can also extract those members that match a
1471 specific @dfn{globbing pattern}. For example, to extract from
1472 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1473 directory prefix, you could type:
1476 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1480 Here, @option{--wildcards} instructs @command{tar} to treat
1481 command line arguments as globbing patterns and @option{--no-anchored}
1482 informs it that the patterns apply to member names after any @samp{/}
1483 delimiter. The use of globbing patterns is discussed in detail in
1486 You can extract a file to standard output by combining the above options
1487 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1490 If you give the @option{--verbose} option, then @option{--extract}
1491 will print the names of the archive members as it extracts them.
1494 @subsection Extracting Files that are Directories
1496 Extracting directories which are members of an archive is similar to
1497 extracting other files. The main difference to be aware of is that if
1498 the extracted directory has the same name as any directory already in
1499 the working directory, then files in the extracted directory will be
1500 placed into the directory of the same name. Likewise, if there are
1501 files in the pre-existing directory with the same names as the members
1502 which you extract, the files from the extracted archive will replace
1503 the files already in the working directory (and possible
1504 subdirectories). This will happen regardless of whether or not the
1505 files in the working directory were more recent than those extracted
1506 (there exist, however, special options that alter this behavior
1509 However, if a file was stored with a directory name as part of its file
1510 name, and that directory does not exist under the working directory when
1511 the file is extracted, @command{tar} will create the directory.
1513 We can demonstrate how to use @option{--extract} to extract a directory
1514 file with an example. Change to the @file{practice} directory if you
1515 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1516 go back to the parent directory and extract the archive
1517 @file{music.tar}. You may either extract the entire archive, or you may
1518 extract only the files you just deleted. To extract the entire archive,
1519 don't give any file names as arguments after the archive name
1520 @file{music.tar}. To extract only the files you deleted, use the
1524 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1530 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1531 would have displayed more detail about the extracted files, as shown
1532 in the example below:
1535 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1536 -rw-r--r-- me user 28 1996-10-18 16:31 practice/jazz
1537 -rw-r--r-- me user 20 1996-09-23 16:44 practice/folk
1541 Because you created the directory with @file{practice} as part of the
1542 file names of each of the files by archiving the @file{practice}
1543 directory as @file{practice}, you must give @file{practice} as part
1544 of the file names when you extract those files from the archive.
1546 @node extracting untrusted archives
1547 @subsection Extracting Archives from Untrusted Sources
1549 Extracting files from archives can overwrite files that already exist.
1550 If you receive an archive from an untrusted source, you should make a
1551 new directory and extract into that directory, so that you don't have
1552 to worry about the extraction overwriting one of your existing files.
1553 For example, if @file{untrusted.tar} came from somewhere else on the
1554 Internet, and you don't necessarily trust its contents, you can
1555 extract it as follows:
1558 $ @kbd{mkdir newdir}
1560 $ @kbd{tar -xvf ../untrusted.tar}
1563 It is also a good practice to examine contents of the archive
1564 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1565 with @option{--verbose} (@option{-v}).
1567 @node failing commands
1568 @subsection Commands That Will Fail
1570 Here are some sample commands you might try which will not work, and why
1573 If you try to use this command,
1576 $ @kbd{tar -xvf music.tar folk jazz}
1580 you will get the following response:
1583 tar: folk: Not found in archive
1584 tar: jazz: Not found in archive
1589 This is because these files were not originally @emph{in} the parent
1590 directory @file{..}, where the archive is located; they were in the
1591 @file{practice} directory, and their file names reflect this:
1594 $ @kbd{tar -tvf music.tar}
1600 @FIXME{make sure the above works when going through the examples in
1604 Likewise, if you try to use this command,
1607 $ @kbd{tar -tvf music.tar folk jazz}
1611 you would get a similar response. Members with those names are not in the
1612 archive. You must use the correct member names, or wildcards, in order
1613 to extract the files from the archive.
1615 If you have forgotten the correct names of the files in the archive,
1616 use @w{@kbd{tar --list --verbose}} to list them correctly.
1618 @FIXME{more examples, here? hag thinks it's a good idea.}
1621 @section Going Further Ahead in this Manual
1623 @FIXME{need to write up a node here about the things that are going to
1624 be in the rest of the manual.}
1626 @node tar invocation
1627 @chapter Invoking @GNUTAR{}
1630 This chapter is about how one invokes the @GNUTAR{}
1631 command, from the command synopsis (@pxref{Synopsis}). There are
1632 numerous options, and many styles for writing them. One mandatory
1633 option specifies the operation @command{tar} should perform
1634 (@pxref{Operation Summary}), other options are meant to detail how
1635 this operation should be performed (@pxref{Option Summary}).
1636 Non-option arguments are not always interpreted the same way,
1637 depending on what the operation is.
1639 You will find in this chapter everything about option styles and rules for
1640 writing them (@pxref{Styles}). On the other hand, operations and options
1641 are fully described elsewhere, in other chapters. Here, you will find
1642 only synthetic descriptions for operations and options, together with
1643 pointers to other parts of the @command{tar} manual.
1645 Some options are so special they are fully described right in this
1646 chapter. They have the effect of inhibiting the normal operation of
1647 @command{tar} or else, they globally alter the amount of feedback the user
1648 receives about what is going on. These are the @option{--help} and
1649 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1650 and @option{--interactive} options (@pxref{interactive}).
1654 * using tar options::
1664 @section General Synopsis of @command{tar}
1666 The @GNUTAR{} program is invoked as either one of:
1669 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1670 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1673 The second form is for when old options are being used.
1675 You can use @command{tar} to store files in an archive, to extract them from
1676 an archive, and to do other types of archive manipulation. The primary
1677 argument to @command{tar}, which is called the @dfn{operation}, specifies
1678 which action to take. The other arguments to @command{tar} are either
1679 @dfn{options}, which change the way @command{tar} performs an operation,
1680 or file names or archive members, which specify the files or members
1681 @command{tar} is to act on.
1683 You can actually type in arguments in any order, even if in this manual
1684 the options always precede the other arguments, to make examples easier
1685 to understand. Further, the option stating the main operation mode
1686 (the @command{tar} main command) is usually given first.
1688 Each @var{name} in the synopsis above is interpreted as an archive member
1689 name when the main command is one of @option{--compare}
1690 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1691 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1692 @option{--update} (@option{-u}). When naming archive members, you
1693 must give the exact name of the member in the archive, as it is
1694 printed by @option{--list}. For @option{--append} (@option{-r}) and
1695 @option{--create} (@option{-c}), these @var{name} arguments specify
1696 the names of either files or directory hierarchies to place in the archive.
1697 These files or hierarchies should already exist in the file system,
1698 prior to the execution of the @command{tar} command.
1700 @command{tar} interprets relative file names as being relative to the
1701 working directory. @command{tar} will make all file names relative
1702 (by removing leading slashes when archiving or restoring files),
1703 unless you specify otherwise (using the @option{--absolute-names}
1704 option). @xref{absolute}, for more information about
1705 @option{--absolute-names}.
1707 If you give the name of a directory as either a file name or a member
1708 name, then @command{tar} acts recursively on all the files and directories
1709 beneath that directory. For example, the name @file{/} identifies all
1710 the files in the file system to @command{tar}.
1712 The distinction between file names and archive member names is especially
1713 important when shell globbing is used, and sometimes a source of confusion
1714 for newcomers. @xref{Wildcards}, for more information about globbing.
1715 The problem is that shells may only glob using existing files in the
1716 file system. Only @command{tar} itself may glob on archive members, so when
1717 needed, you must ensure that wildcard characters reach @command{tar} without
1718 being interpreted by the shell first. Using a backslash before @samp{*}
1719 or @samp{?}, or putting the whole argument between quotes, is usually
1720 sufficient for this.
1722 Even if @var{name}s are often specified on the command line, they
1723 can also be read from a text file in the file system, using the
1724 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1726 If you don't use any file name arguments, @option{--append} (@option{-r}),
1727 @option{--delete} and @option{--concatenate} (@option{--catenate},
1728 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1729 will usually yield a diagnostic and inhibit @command{tar} execution.
1730 The other operations of @command{tar} (@option{--list},
1731 @option{--extract}, @option{--compare}, and @option{--update})
1732 will act on the entire contents of the archive.
1735 @cindex return status
1736 Besides successful exits, @GNUTAR{} may fail for
1737 many reasons. Some reasons correspond to bad usage, that is, when the
1738 @command{tar} command is improperly written. Errors may be
1739 encountered later, while encountering an error processing the archive
1740 or the files. Some errors are recoverable, in which case the failure
1741 is delayed until @command{tar} has completed all its work. Some
1742 errors are such that it would not meaningful, or at least risky, to
1743 continue processing: @command{tar} then aborts processing immediately.
1744 All abnormal exits, whether immediate or delayed, should always be
1745 clearly diagnosed on @code{stderr}, after a line stating the nature of
1748 @GNUTAR{} returns only a few exit statuses. I'm really
1749 aiming simplicity in that area, for now. If you are not using the
1750 @option{--compare} @option{--diff}, @option{-d}) option, zero means
1751 that everything went well, besides maybe innocuous warnings. Nonzero
1752 means that something went wrong. Right now, as of today, ``nonzero''
1753 is almost always 2, except for remote operations, where it may be
1756 @node using tar options
1757 @section Using @command{tar} Options
1759 @GNUTAR{} has a total of eight operating modes which
1760 allow you to perform a variety of tasks. You are required to choose
1761 one operating mode each time you employ the @command{tar} program by
1762 specifying one, and only one operation as an argument to the
1763 @command{tar} command (two lists of four operations each may be found
1764 at @ref{frequent operations} and @ref{Operations}). Depending on
1765 circumstances, you may also wish to customize how the chosen operating
1766 mode behaves. For example, you may wish to change the way the output
1767 looks, or the format of the files that you wish to archive may require
1768 you to do something special in order to make the archive look right.
1770 You can customize and control @command{tar}'s performance by running
1771 @command{tar} with one or more options (such as @option{--verbose}
1772 (@option{-v}), which we used in the tutorial). As we said in the
1773 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1774 their name suggests) optional. Depending on the operating mode, you
1775 may specify one or more options. Different options will have different
1776 effects, but in general they all change details of the operation, such
1777 as archive format, archive name, or level of user interaction. Some
1778 options make sense with all operating modes, while others are
1779 meaningful only with particular modes. You will likely use some
1780 options frequently, while you will only use others infrequently, or
1781 not at all. (A full list of options is available in @pxref{All Options}.)
1783 @vrindex TAR_OPTIONS, environment variable
1784 @anchor{TAR_OPTIONS}
1785 The @env{TAR_OPTIONS} environment variable specifies default options to
1786 be placed in front of any explicit options. For example, if
1787 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1788 if the two options @option{-v} and @option{--unlink-first} had been
1789 specified before any explicit options. Option specifications are
1790 separated by whitespace. A backslash escapes the next character, so it
1791 can be used to specify an option containing whitespace or a backslash.
1793 Note that @command{tar} options are case sensitive. For example, the
1794 options @option{-T} and @option{-t} are different; the first requires an
1795 argument for stating the name of a file providing a list of @var{name}s,
1796 while the second does not require an argument and is another way to
1797 write @option{--list} (@option{-t}).
1799 In addition to the eight operations, there are many options to
1800 @command{tar}, and three different styles for writing both: long (mnemonic)
1801 form, short form, and old style. These styles are discussed below.
1802 Both the options and the operations can be written in any of these three
1805 @FIXME{menu at end of this node. need to think of an actual outline
1806 for this chapter; probably do that after stuff from chapter 4 is
1810 @section The Three Option Styles
1812 There are three styles for writing operations and options to the command
1813 line invoking @command{tar}. The different styles were developed at
1814 different times during the history of @command{tar}. These styles will be
1815 presented below, from the most recent to the oldest.
1817 Some options must take an argument. (For example, @option{--file}
1818 (@option{-f})) takes the name of an archive file as an argument. If
1819 you do not supply an archive file name, @command{tar} will use a
1820 default, but this can be confusing; thus, we recommend that you always
1821 supply a specific archive file name.) Where you @emph{place} the
1822 arguments generally depends on which style of options you choose. We
1823 will detail specific information relevant to each option style in the
1824 sections on the different option styles, below. The differences are
1825 subtle, yet can often be very important; incorrect option placement
1826 can cause you to overwrite a number of important files. We urge you
1827 to note these differences, and only use the option style(s) which
1828 makes the most sense to you until you feel comfortable with the others.
1830 Some options @emph{may} take an argument (currently, there are
1831 two such options: @option{--backup} and @option{--occurrence}). Such
1832 options may have at most long and short forms, they do not have old style
1833 equivalent. The rules for specifying an argument for such options
1834 are stricter than those for specifying mandatory arguments. Please,
1835 pay special attention to them.
1838 * Mnemonic Options:: Mnemonic Option Style
1839 * Short Options:: Short Option Style
1840 * Old Options:: Old Option Style
1841 * Mixing:: Mixing Option Styles
1844 @node Mnemonic Options
1845 @subsection Mnemonic Option Style
1847 @FIXME{have to decide whether or not to replace other occurrences of
1848 "mnemonic" with "long", or *ugh* vice versa.}
1850 Each option has at least one long (or mnemonic) name starting with two
1851 dashes in a row, e.g., @option{--list}. The long names are more clear than
1852 their corresponding short or old names. It sometimes happens that a
1853 single mnemonic option has many different different names which are
1854 synonymous, such as @option{--compare} and @option{--diff}. In addition,
1855 long option names can be given unique abbreviations. For example,
1856 @option{--cre} can be used in place of @option{--create} because there is no
1857 other mnemonic option which begins with @samp{cre}. (One way to find
1858 this out is by trying it and seeing what happens; if a particular
1859 abbreviation could represent more than one option, @command{tar} will tell
1860 you that that abbreviation is ambiguous and you'll know that that
1861 abbreviation won't work. You may also choose to run @samp{tar --help}
1862 to see a list of options. Be aware that if you run @command{tar} with a
1863 unique abbreviation for the long name of an option you didn't want to
1864 use, you are stuck; @command{tar} will perform the command as ordered.)
1866 Mnemonic options are meant to be obvious and easy to remember, and their
1867 meanings are generally easier to discern than those of their
1868 corresponding short options (see below). For example:
1871 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
1875 gives a fairly good set of hints about what the command does, even
1876 for those not fully acquainted with @command{tar}.
1878 Mnemonic options which require arguments take those arguments
1879 immediately following the option name. There are two ways of
1880 specifying a mandatory argument. It can be separated from the
1881 option name either by an equal sign, or by any amount of
1882 white space characters. For example, the @option{--file} option (which
1883 tells the name of the @command{tar} archive) is given a file such as
1884 @file{archive.tar} as argument by using any of the following notations:
1885 @option{--file=archive.tar} or @option{--file archive.tar}.
1887 In contrast, optional arguments must always be introduced using
1888 an equal sign. For example, the @option{--backup} option takes
1889 an optional argument specifying backup type. It must be used
1890 as @option{--backup=@var{backup-type}}.
1893 @subsection Short Option Style
1895 Most options also have a short option name. Short options start with
1896 a single dash, and are followed by a single character, e.g., @option{-t}
1897 (which is equivalent to @option{--list}). The forms are absolutely
1898 identical in function; they are interchangeable.
1900 The short option names are faster to type than long option names.
1902 Short options which require arguments take their arguments immediately
1903 following the option, usually separated by white space. It is also
1904 possible to stick the argument right after the short option name, using
1905 no intervening space. For example, you might write @w{@option{-f
1906 archive.tar}} or @option{-farchive.tar} instead of using
1907 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
1908 @w{@option{-f @var{archive-name}}} denote the option which indicates a
1909 specific archive, here named @file{archive.tar}.
1911 Short options which take optional arguments take their arguments
1912 immediately following the option letter, @emph{without any intervening
1913 white space characters}.
1915 Short options' letters may be clumped together, but you are not
1916 required to do this (as compared to old options; see below). When
1917 short options are clumped as a set, use one (single) dash for them
1918 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
1919 such a set is allowed to have an argument@footnote{Clustering many
1920 options, the last of which has an argument, is a rather opaque way to
1921 write options. Some wonder if @acronym{GNU} @code{getopt} should not
1922 even be made helpful enough for considering such usages as invalid.}.
1924 When the options are separated, the argument for each option which requires
1925 an argument directly follows that option, as is usual for Unix programs.
1929 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
1932 If you reorder short options' locations, be sure to move any arguments
1933 that belong to them. If you do not move the arguments properly, you may
1934 end up overwriting files.
1937 @subsection Old Option Style
1940 Like short options, old options are single letters. However, old options
1941 must be written together as a single clumped set, without spaces separating
1942 them or dashes preceding them@footnote{Beware that if you precede options
1943 with a dash, you are announcing the short option style instead of the
1944 old option style; short options are decoded differently.}. This set
1945 of letters must be the first to appear on the command line, after the
1946 @command{tar} program name and some white space; old options cannot appear
1947 anywhere else. The letter of an old option is exactly the same letter as
1948 the corresponding short option. For example, the old option @samp{t} is
1949 the same as the short option @option{-t}, and consequently, the same as the
1950 mnemonic option @option{--list}. So for example, the command @w{@samp{tar
1951 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
1953 When options that need arguments are given together with the command,
1954 all the associated arguments follow, in the same order as the options.
1955 Thus, the example given previously could also be written in the old
1959 $ @kbd{tar cvbf 20 /dev/rmt0}
1963 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
1964 the argument of @option{-f}.
1966 On the other hand, this old style syntax makes it difficult to match
1967 option letters with their corresponding arguments, and is often
1968 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
1969 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
1970 argument for @option{-f}, and @option{-v} does not have a corresponding
1971 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
1972 /dev/rmt0}} is clearer, putting all arguments next to the option they
1975 If you want to reorder the letters in the old option argument, be
1976 sure to reorder any corresponding argument appropriately.
1978 This old way of writing @command{tar} options can surprise even experienced
1979 users. For example, the two commands:
1982 @kbd{tar cfz archive.tar.gz file}
1983 @kbd{tar -cfz archive.tar.gz file}
1987 are quite different. The first example uses @file{archive.tar.gz} as
1988 the value for option @samp{f} and recognizes the option @samp{z}. The
1989 second example, however, uses @file{z} as the value for option
1990 @samp{f} --- probably not what was intended.
1992 Old options are kept for compatibility with old versions of @command{tar}.
1994 This second example could be corrected in many ways, among which the
1995 following are equivalent:
1998 @kbd{tar -czf archive.tar.gz file}
1999 @kbd{tar -cf archive.tar.gz -z file}
2000 @kbd{tar cf archive.tar.gz -z file}
2003 @cindex option syntax, traditional
2004 As far as we know, all @command{tar} programs, @acronym{GNU} and
2005 non-@acronym{GNU}, support old options. @GNUTAR{}
2006 supports them not only for historical reasons, but also because many
2007 people are used to them. For compatibility with Unix @command{tar},
2008 the first argument is always treated as containing command and option
2009 letters even if it doesn't start with @samp{-}. Thus, @samp{tar c} is
2010 equivalent to @w{@samp{tar -c}:} both of them specify the
2011 @option{--create} (@option{-c}) command to create an archive.
2014 @subsection Mixing Option Styles
2016 All three styles may be intermixed in a single @command{tar} command,
2017 so long as the rules for each style are fully
2018 respected@footnote{Before @GNUTAR{} version 1.11.6,
2019 a bug prevented intermixing old style options with mnemonic options in
2020 some cases.}. Old style options and either of the modern styles of
2021 options may be mixed within a single @command{tar} command. However,
2022 old style options must be introduced as the first arguments only,
2023 following the rule for old options (old options must appear directly
2024 after the @command{tar} command and some white space). Modern options
2025 may be given only after all arguments to the old options have been
2026 collected. If this rule is not respected, a modern option might be
2027 falsely interpreted as the value of the argument to one of the old
2030 For example, all the following commands are wholly equivalent, and
2031 illustrate the many combinations and orderings of option styles.
2034 @kbd{tar --create --file=archive.tar}
2035 @kbd{tar --create -f archive.tar}
2036 @kbd{tar --create -farchive.tar}
2037 @kbd{tar --file=archive.tar --create}
2038 @kbd{tar --file=archive.tar -c}
2039 @kbd{tar -c --file=archive.tar}
2040 @kbd{tar -c -f archive.tar}
2041 @kbd{tar -c -farchive.tar}
2042 @kbd{tar -cf archive.tar}
2043 @kbd{tar -cfarchive.tar}
2044 @kbd{tar -f archive.tar --create}
2045 @kbd{tar -f archive.tar -c}
2046 @kbd{tar -farchive.tar --create}
2047 @kbd{tar -farchive.tar -c}
2048 @kbd{tar c --file=archive.tar}
2049 @kbd{tar c -f archive.tar}
2050 @kbd{tar c -farchive.tar}
2051 @kbd{tar cf archive.tar}
2052 @kbd{tar f archive.tar --create}
2053 @kbd{tar f archive.tar -c}
2054 @kbd{tar fc archive.tar}
2057 On the other hand, the following commands are @emph{not} equivalent to
2061 @kbd{tar -f -c archive.tar}
2062 @kbd{tar -fc archive.tar}
2063 @kbd{tar -fcarchive.tar}
2064 @kbd{tar -farchive.tarc}
2065 @kbd{tar cfarchive.tar}
2069 These last examples mean something completely different from what the
2070 user intended (judging based on the example in the previous set which
2071 uses long options, whose intent is therefore very clear). The first
2072 four specify that the @command{tar} archive would be a file named
2073 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2074 respectively. The first two examples also specify a single non-option,
2075 @var{name} argument having the value @samp{archive.tar}. The last
2076 example contains only old style option letters (repeating option
2077 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2078 @samp{h}, or @samp{i}), with no argument value. @FIXME{not sure i liked
2079 the first sentence of this paragraph..}
2082 @section All @command{tar} Options
2084 The coming manual sections contain an alphabetical listing of all
2085 @command{tar} operations and options, with brief descriptions and cross
2086 references to more in-depth explanations in the body of the manual.
2087 They also contain an alphabetically arranged table of the short option
2088 forms with their corresponding long option. You can use this table as
2089 a reference for deciphering @command{tar} commands in scripts.
2092 * Operation Summary::
2094 * Short Option Summary::
2097 @node Operation Summary
2098 @subsection Operations
2102 @opindex append, summary
2106 Appends files to the end of the archive. @xref{append}.
2108 @opindex catenate, summary
2112 Same as @option{--concatenate}. @xref{concatenate}.
2114 @opindex compare, summary
2118 Compares archive members with their counterparts in the file
2119 system, and reports differences in file size, mode, owner,
2120 modification date and contents. @xref{compare}.
2122 @opindex concatenate, summary
2126 Appends other @command{tar} archives to the end of the archive.
2129 @opindex create, summary
2133 Creates a new @command{tar} archive. @xref{create}.
2135 @opindex delete, summary
2138 Deletes members from the archive. Don't try this on a archive on a
2139 tape! @xref{delete}.
2141 @opindex diff, summary
2145 Same @option{--compare}. @xref{compare}.
2147 @opindex extract, summary
2151 Extracts members from the archive into the file system. @xref{extract}.
2153 @opindex get, summary
2157 Same as @option{--extract}. @xref{extract}.
2159 @opindex list, summary
2163 Lists the members in an archive. @xref{list}.
2165 @opindex update, summary
2169 Adds files to the end of the archive, but only if they are newer than
2170 their counterparts already in the archive, or if they do not already
2171 exist in the archive. @xref{update}.
2175 @node Option Summary
2176 @subsection @command{tar} Options
2180 @opindex absolute-names, summary
2181 @item --absolute-names
2184 Normally when creating an archive, @command{tar} strips an initial
2185 @samp{/} from member names. This option disables that behavior.
2188 @opindex after-date, summary
2191 (See @option{--newer}, @pxref{after})
2193 @opindex anchored, summary
2195 A pattern must match an initial subsequence of the name's components.
2196 @xref{controlling pattern-matching}.
2198 @opindex atime-preserve, summary
2199 @item --atime-preserve
2200 @itemx --atime-preserve=replace
2201 @itemx --atime-preserve=system
2203 Attempt to preserve the access time of files when reading them. This
2204 option currently is effective only on files that you own, unless you
2205 have superuser privileges.
2207 @option{--atime-preserve=replace} remembers the access time of a file
2208 before reading it, and then restores the access time afterwards. This
2209 may cause problems if other programs are reading the file at the same
2210 time, as the times of their accesses will be lost. On most platforms
2211 restoring the access time also requires @command{tar} to restore the
2212 data modification time too, so this option may also cause problems if
2213 other programs are writing the file at the same time. (Tar attempts
2214 to detect this situation, but cannot do so reliably due to race
2215 conditions.) Worse, on most platforms restoring the access time also
2216 updates the status change time, which means that this option is
2217 incompatible with incremental backups.
2219 @option{--atime-preserve=system} avoids changing time stamps on files,
2220 without interfering with time stamp updates
2221 caused by other programs, so it works better with incremental backups.
2222 However, it requires a special @code{O_NOATIME} option from the
2223 underlying operating and file system implementation, and it also requires
2224 that searching directories does not update their access times. As of
2225 this writing (November 2005) this works only with Linux, and only with
2226 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2227 way to know whether this feature actually works. Sometimes
2228 @command{tar} knows that it does not work, and if you use
2229 @option{--atime-preserve=system} then @command{tar} complains and
2230 exits right away. But other times @command{tar} might think that the
2231 option works when it actually does not.
2233 Currently @option{--atime-preserve} with no operand defaults to
2234 @option{--atime-preserve=replace}, but this may change in the future
2235 as support for @option{--atime-preserve=system} improves.
2237 If your operating system does not support
2238 @option{--atime-preserve=system}, you might be able to preserve access
2239 times reliably by by using the @command{mount} command. For example,
2240 you can mount the file system read-only, or access the file system via
2241 a read-only loopback mount, or use the @samp{noatime} mount option
2242 available on some systems. However, mounting typically requires
2243 superuser privileges and can be a pain to manage.
2245 @opindex backup, summary
2246 @item --backup=@var{backup-type}
2248 Rather than deleting files from the file system, @command{tar} will
2249 back them up using simple or numbered backups, depending upon
2250 @var{backup-type}. @xref{backup}.
2252 @opindex block-number, summary
2253 @item --block-number
2256 With this option present, @command{tar} prints error messages for read errors
2257 with the block number in the archive file. @xref{block-number}.
2259 @opindex blocking-factor, summary
2260 @item --blocking-factor=@var{blocking}
2261 @itemx -b @var{blocking}
2263 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2264 record. @xref{Blocking Factor}.
2266 @opindex bzip2, summary
2270 This option tells @command{tar} to read or write archives through
2271 @code{bzip2}. @xref{gzip}.
2273 @opindex checkpoint, summary
2276 This option directs @command{tar} to print periodic checkpoint messages as it
2277 reads through the archive. It is intended for when you want a visual
2278 indication that @command{tar} is still running, but don't want to see
2279 @option{--verbose} output. @FIXME-xref{}
2281 @opindex check-links, summary
2284 If this option was given, @command{tar} will check the number of links
2285 dumped for each processed file. If this number does not match the
2286 total number of hard links for the file, a warning message will be
2287 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2288 synonym for @option{--one-file-system}. The current semantics, wich
2289 complies to UNIX98, was introduced with version
2290 1.15.91. @xref{Changes}, for more information.}.
2292 @opindex compress, summary
2293 @opindex uncompress, summary
2298 @command{tar} will use the @command{compress} program when reading or
2299 writing the archive. This allows you to directly act on archives
2300 while saving space. @xref{gzip}.
2302 @opindex confirmation, summary
2303 @item --confirmation
2305 (See @option{--interactive}.) @xref{interactive}.
2307 @opindex delay-directory-restore, summary
2308 @item --delay-directory-restore
2310 Delay setting modification times and permissions of extracted
2311 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2313 @opindex dereference, summary
2317 When creating a @command{tar} archive, @command{tar} will archive the
2318 file that a symbolic link points to, rather than archiving the
2319 symlink. @xref{dereference}.
2321 @opindex directory, summary
2322 @item --directory=@var{dir}
2325 When this option is specified, @command{tar} will change its current directory
2326 to @var{dir} before performing any operations. When this option is used
2327 during archive creation, it is order sensitive. @xref{directory}.
2329 @opindex exclude, summary
2330 @item --exclude=@var{pattern}
2332 When performing operations, @command{tar} will skip files that match
2333 @var{pattern}. @xref{exclude}.
2335 @opindex exclude-from, summary
2336 @item --exclude-from=@var{file}
2337 @itemx -X @var{file}
2339 Similar to @option{--exclude}, except @command{tar} will use the list of
2340 patterns in the file @var{file}. @xref{exclude}.
2342 @opindex exclude-caches, summary
2343 @item --exclude-caches
2345 Automatically excludes all directories
2346 containing a cache directory tag. @xref{exclude}.
2348 @opindex file, summary
2349 @item --file=@var{archive}
2350 @itemx -f @var{archive}
2352 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2353 performs operations on, rather than @command{tar}'s compilation dependent
2354 default. @xref{file tutorial}.
2356 @opindex files-from, summary
2357 @item --files-from=@var{file}
2358 @itemx -T @var{file}
2360 @command{tar} will use the contents of @var{file} as a list of archive members
2361 or files to operate on, in addition to those specified on the
2362 command-line. @xref{files}.
2364 @opindex force-local, summary
2367 Forces @command{tar} to interpret the filename given to @option{--file}
2368 as a local file, even if it looks like a remote tape drive name.
2369 @xref{local and remote archives}.
2371 @opindex format, summary
2372 @item --format=@var{format}
2374 Selects output archive format. @var{Format} may be one of the
2379 Creates an archive that is compatible with Unix V7 @command{tar}.
2382 Creates an archive that is compatible with GNU @command{tar} version
2386 Creates archive in GNU tar 1.13 format. Basically it is the same as
2387 @samp{oldgnu} with the only difference in the way it handles long
2391 Creates a @acronym{POSIX.1-1988} compatible archive.
2394 Creates a @acronym{POSIX.1-2001 archive}.
2398 @xref{Formats}, for a detailed discussion of these formats.
2400 @opindex group, summary
2401 @item --group=@var{group}
2403 Files added to the @command{tar} archive will have a group id of @var{group},
2404 rather than the group from the source file. @var{group} is first decoded
2405 as a group symbolic name, but if this interpretation fails, it has to be
2406 a decimal numeric group ID. @FIXME-xref{}
2408 Also see the comments for the @option{--owner=@var{user}} option.
2410 @opindex gzip, summary
2411 @opindex gunzip, summary
2412 @opindex ungzip, summary
2418 This option tells @command{tar} to read or write archives through
2419 @command{gzip}, allowing @command{tar} to directly operate on several
2420 kinds of compressed archives transparently. @xref{gzip}.
2422 @opindex help, summary
2425 @command{tar} will print out a short message summarizing the operations and
2426 options to @command{tar} and exit. @xref{help}.
2428 @opindex ignore-case, summary
2430 Ignore case when matching member or file names with
2431 patterns. @xref{controlling pattern-matching}.
2433 @opindex ignore-command-error, summary
2434 @item --ignore-command-error
2435 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2437 @opindex ignore-failed-read, summary
2438 @item --ignore-failed-read
2440 Do not exit unsuccessfully merely because an unreadable file was encountered.
2443 @opindex ignore-zeros, summary
2444 @item --ignore-zeros
2447 With this option, @command{tar} will ignore zeroed blocks in the
2448 archive, which normally signals EOF. @xref{Reading}.
2450 @opindex incremental, summary
2454 Used to inform @command{tar} that it is working with an old
2455 @acronym{GNU}-format incremental backup archive. It is intended
2456 primarily for backwards compatibility only. @xref{Incremental Dumps},
2457 for a detailed discussion of incremental archives.
2459 @opindex index-file, summary
2460 @item --index-file=@var{file}
2462 Send verbose output to @var{file} instead of to standard output.
2464 @opindex info-script, summary
2465 @opindex new-volume-script, summary
2466 @item --info-script=@var{script-file}
2467 @itemx --new-volume-script=@var{script-file}
2468 @itemx -F @var{script-file}
2470 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2471 at the end of each tape. If @var{script-file} exits with nonzero status,
2472 @command{tar} fails immediately. @xref{info-script}, for a detailed
2473 discussion of @var{script-file}.
2475 @opindex interactive, summary
2477 @itemx --confirmation
2480 Specifies that @command{tar} should ask the user for confirmation before
2481 performing potentially destructive options, such as overwriting files.
2484 @opindex keep-newer-files, summary
2485 @item --keep-newer-files
2487 Do not replace existing files that are newer than their archive copies
2488 when extracting files from an archive.
2490 @opindex keep-old-files, summary
2491 @item --keep-old-files
2494 Do not overwrite existing files when extracting files from an archive.
2495 @xref{Keep Old Files}.
2497 @opindex label, summary
2498 @item --label=@var{name}
2499 @itemx -V @var{name}
2501 When creating an archive, instructs @command{tar} to write @var{name}
2502 as a name record in the archive. When extracting or listing archives,
2503 @command{tar} will only operate on archives that have a label matching
2504 the pattern specified in @var{name}. @xref{Tape Files}.
2506 @opindex listed-incremental, summary
2507 @item --listed-incremental=@var{snapshot-file}
2508 @itemx -g @var{snapshot-file}
2510 During a @option{--create} operation, specifies that the archive that
2511 @command{tar} creates is a new @acronym{GNU}-format incremental
2512 backup, using @var{snapshot-file} to determine which files to backup.
2513 With other operations, informs @command{tar} that the archive is in
2514 incremental format. @xref{Incremental Dumps}.
2516 @opindex mode, summary
2517 @item --mode=@var{permissions}
2519 When adding files to an archive, @command{tar} will use
2520 @var{permissions} for the archive members, rather than the permissions
2521 from the files. The program @command{chmod} and this @command{tar}
2522 option share the same syntax for what @var{permissions} might be.
2523 @xref{File permissions, Permissions, File permissions, fileutils,
2524 @acronym{GNU} file utilities}. This reference also has useful
2525 information for those not being overly familiar with the Unix
2528 Of course, @var{permissions} might be plainly specified as an octal number.
2529 However, by using generic symbolic modifications to mode bits, this allows
2530 more flexibility. For example, the value @samp{a+rw} adds read and write
2531 permissions for everybody, while retaining executable bits on directories
2532 or on any other file already marked as executable.
2534 @opindex multi-volume, summary
2535 @item --multi-volume
2538 Informs @command{tar} that it should create or otherwise operate on a
2539 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2541 @opindex new-volume-script, summary
2542 @item --new-volume-script
2546 @opindex seek, summary
2550 Assume that the archive media supports seeks to arbitrary
2551 locations. Usually @command{tar} determines automatically whether
2552 the archive can be seeked or not. This option is intended for use
2553 in cases when such recognition fails.
2555 @opindex newer, summary
2556 @item --newer=@var{date}
2557 @itemx --after-date=@var{date}
2560 When creating an archive, @command{tar} will only add files that have changed
2561 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2562 is taken to be the name of a file whose data modification time specifies
2563 the date. @xref{after}.
2565 @opindex newer-mtime, summary
2566 @item --newer-mtime=@var{date}
2568 Like @option{--newer}, but add only files whose
2569 contents have changed (as opposed to just @option{--newer}, which will
2570 also back up files for which any status information has changed).
2572 @opindex no-anchored, summary
2574 An exclude pattern can match any subsequence of the name's components.
2575 @xref{controlling pattern-matching}.
2577 @opindex no-delay-directory-restore, summary
2578 @item --no-delay-directory-restore
2580 Setting modification times and permissions of extracted
2581 directories when all files from this directory has been
2582 extracted. This is the default. @xref{Directory Modification Times and Permissions}.
2584 @opindex no-ignore-case, summary
2585 @item --no-ignore-case
2586 Use case-sensitive matching.
2587 @xref{controlling pattern-matching}.
2589 @opindex no-ignore-command-error, summary
2590 @item --no-ignore-command-error
2591 Print warnings about subprocesses terminated with a non-zero exit
2592 code. @xref{Writing to an External Program}.
2594 @opindex no-quote-chars, summary
2595 @item --no-quote-chars=@var{string}
2596 Do not quote characters from @var{string}, even if the selected
2597 quoting style implies they should be quoted (@pxref{quoting styles}).
2599 @opindex no-recursion, summary
2600 @item --no-recursion
2602 With this option, @command{tar} will not recurse into directories.
2605 @opindex no-same-owner, summary
2606 @item --no-same-owner
2609 When extracting an archive, do not attempt to preserve the owner
2610 specified in the @command{tar} archive. This the default behavior
2613 @opindex no-same-permissions, summary
2614 @item --no-same-permissions
2616 When extracting an archive, subtract the user's umask from files from
2617 the permissions specified in the archive. This is the default behavior
2620 @opindex no-wildcards, summary
2621 @item --no-wildcards
2622 Do not use wildcards.
2623 @xref{controlling pattern-matching}.
2625 @opindex no-wildcards-match-slash, summary
2626 @item --no-wildcards-match-slash
2627 Wildcards do not match @samp{/}.
2628 @xref{controlling pattern-matching}.
2630 @opindex null, summary
2633 When @command{tar} is using the @option{--files-from} option, this option
2634 instructs @command{tar} to expect filenames terminated with @option{NUL}, so
2635 @command{tar} can correctly work with file names that contain newlines.
2638 @opindex numeric-owner, summary
2639 @item --numeric-owner
2641 This option will notify @command{tar} that it should use numeric user
2642 and group IDs when creating a @command{tar} file, rather than names.
2646 When extracting files, this option is a synonym for
2647 @option{--no-same-owner}, i.e. it prevents @command{tar} from
2648 restoring ownership of files being extracted.
2650 When creating an archive, @option{-o} is a synonym for
2651 @option{--old-archive}. This behavior is for compatibility
2652 with previous versions of @GNUTAR{}, and will be
2653 removed in the future releases.
2655 @xref{Changes}, for more information.
2657 @opindex occurrence, summary
2658 @item --occurrence[=@var{number}]
2660 This option can be used in conjunction with one of the subcommands
2661 @option{--delete}, @option{--diff}, @option{--extract} or
2662 @option{--list} when a list of files is given either on the command
2663 line or via @option{-T} option.
2665 This option instructs @command{tar} to process only the @var{number}th
2666 occurrence of each named file. @var{Number} defaults to 1, so
2669 tar -x -f archive.tar --occurrence filename
2673 will extract the first occurrence of @file{filename} from @file{archive.tar}
2674 and will terminate without scanning to the end of the archive.
2676 @opindex old-archive, summary
2678 Synonym for @option{--format=v7}.
2680 @opindex one-file-system, summary
2681 @item --one-file-system
2682 Used when creating an archive. Prevents @command{tar} from recursing into
2683 directories that are on different file systems from the current
2684 directory @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2685 synonym for @option{--one-file-system}. This has changed in version
2686 1.15.91. @xref{Changes}, for more information.}.
2688 @opindex overwrite, summary
2691 Overwrite existing files and directory metadata when extracting files
2692 from an archive. @xref{Overwrite Old Files}.
2694 @opindex overwrite-dir, summary
2695 @item --overwrite-dir
2697 Overwrite the metadata of existing directories when extracting files
2698 from an archive. @xref{Overwrite Old Files}.
2700 @opindex owner, summary
2701 @item --owner=@var{user}
2703 Specifies that @command{tar} should use @var{user} as the owner of members
2704 when creating archives, instead of the user associated with the source
2705 file. @var{user} is first decoded as a user symbolic name, but if
2706 this interpretation fails, it has to be a decimal numeric user ID.
2709 There is no value indicating a missing number, and @samp{0} usually means
2710 @code{root}. Some people like to force @samp{0} as the value to offer in
2711 their distributions for the owner of files, because the @code{root} user is
2712 anonymous anyway, so that might as well be the owner of anonymous archives.
2714 This option does not affect extraction from archives.
2716 @opindex quote-chars, summary
2717 @item --quote-chars=@var{string}
2718 Always quote characters from @var{string}, even if the selected
2719 quoting style would not quote them (@pxref{quoting styles}).
2721 @opindex quoting-style, summary
2722 @item --quoting-style=@var{style}
2723 Set quoting style to use when printing member and file names
2724 (@pxref{quoting styles}). Valid @var{style} values are:
2725 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
2726 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
2727 style is @code{escape}, unless overridden while configuring the
2730 @opindex pax-option, summary
2731 @item --pax-option=@var{keyword-list}
2732 @FIXME{Such a detailed description does not belong there, move it elsewhere.}
2733 This option is meaningful only with @acronym{POSIX.1-2001} archives
2734 (@pxref{posix}). It modifies the way @command{tar} handles the
2735 extended header keywords. @var{Keyword-list} is a comma-separated
2736 list of keyword options, each keyword option taking one of
2737 the following forms:
2740 @item delete=@var{pattern}
2741 When used with one of archive-creation commands,
2742 this option instructs @command{tar} to omit from extended header records
2743 that it produces any keywords matching the string @var{pattern}.
2745 When used in extract or list mode, this option instructs tar
2746 to ignore any keywords matching the given @var{pattern} in the extended
2747 header records. In both cases, matching is performed using the pattern
2748 matching notation described in @acronym{POSIX 1003.2}, 3.13
2749 (See @cite{glob(7)}). For example:
2752 --pax-option delete=security.*
2755 would suppress security-related information.
2757 @item exthdr.name=@var{string}
2759 This keyword allows user control over the name that is written into the
2760 ustar header blocks for the extended headers. The name is obtained
2761 from @var{string} after making the following substitutions:
2763 @multitable @columnfractions .30 .70
2764 @headitem Meta-character @tab Replaced By
2765 @item %d @tab The directory name of the file, equivalent to the
2766 result of the @command{dirname} utility on the translated pathname.
2767 @item %f @tab The filename of the file, equivalent to the result
2768 of the @command{basename} utility on the translated pathname.
2769 @item %p @tab The process ID of the @command{tar} process.
2770 @item %% @tab A @samp{%} character.
2773 Any other @samp{%} characters in @var{string} produce undefined
2776 If no option @samp{exthdr.name=string} is specified, @command{tar}
2777 will use the following default value:
2783 @item globexthdr.name=@var{string}
2784 This keyword allows user control over the name that is written into
2785 the ustar header blocks for global extended header records. The name
2786 is obtained from the contents of @var{string}, after making
2787 the following substitutions:
2789 @multitable @columnfractions .30 .70
2790 @headitem Meta-character @tab Replaced By
2791 @item %n @tab An integer that represents the
2792 sequence number of the global extended header record in the archive,
2794 @item %p @tab The process ID of the @command{tar} process.
2795 @item %% @tab A @samp{%} character.
2798 Any other @samp{%} characters in @var{string} produce undefined results.
2800 If no option @samp{globexthdr.name=string} is specified, @command{tar}
2801 will use the following default value:
2804 $TMPDIR/GlobalHead.%p.%n
2808 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
2809 environment variable. If @var{TMPDIR} is not set, @command{tar}
2812 @item @var{keyword}=@var{value}
2813 When used with one of archive-creation commands, these keyword/value pairs
2814 will be included at the beginning of the archive in a global extended
2815 header record. When used with one of archive-reading commands,
2816 @command{tar} will behave as if it has encountered these keyword/value
2817 pairs at the beginning of the archive in a global extended header
2820 @item @var{keyword}:=@var{value}
2821 When used with one of archive-creation commands, these keyword/value pairs
2822 will be included as records at the beginning of an extended header for
2823 each file. This is effectively equivalent to @var{keyword}=@var{value}
2824 form except that it creates no global extended header records.
2826 When used with one of archive-reading commands, @command{tar} will
2827 behave as if these keyword/value pairs were included as records at the
2828 end of each extended header; thus, they will override any global or
2829 file-specific extended header record keywords of the same names.
2830 For example, in the command:
2833 tar --format=posix --create \
2834 --file archive --pax-option gname:=user .
2837 the group name will be forced to a new value for all files
2838 stored in the archive.
2841 @opindex portability, summary
2843 @itemx --old-archive
2844 Synonym for @option{--format=v7}.
2846 @opindex posix, summary
2848 Same as @option{--format=posix}.
2850 @opindex preserve, summary
2853 Synonymous with specifying both @option{--preserve-permissions} and
2854 @option{--same-order}. @xref{Setting Access Permissions}.
2856 @opindex preserve-order, summary
2857 @item --preserve-order
2859 (See @option{--same-order}; @pxref{Reading}.)
2861 @opindex preserve-permissions, summary
2862 @opindex same-permissions, summary
2863 @item --preserve-permissions
2864 @itemx --same-permissions
2867 When @command{tar} is extracting an archive, it normally subtracts the
2868 users' umask from the permissions specified in the archive and uses
2869 that number as the permissions to create the destination file.
2870 Specifying this option instructs @command{tar} that it should use the
2871 permissions directly from the archive. @xref{Setting Access Permissions}.
2873 @opindex read-full-records, summary
2874 @item --read-full-records
2877 Specifies that @command{tar} should reblock its input, for reading
2878 from pipes on systems with buggy implementations. @xref{Reading}.
2880 @opindex record-size, summary
2881 @item --record-size=@var{size}
2883 Instructs @command{tar} to use @var{size} bytes per record when accessing the
2884 archive. @xref{Blocking Factor}.
2886 @opindex recursion, summary
2889 With this option, @command{tar} recurses into directories.
2892 @opindex recursive-unlink, summary
2893 @item --recursive-unlink
2896 directory hierarchies before extracting directories of the same name
2897 from the archive. @xref{Recursive Unlink}.
2899 @opindex remove-files, summary
2900 @item --remove-files
2902 Directs @command{tar} to remove the source file from the file system after
2903 appending it to an archive. @xref{remove files}.
2905 @opindex restrict, summary
2908 Disable use of some potentially harmful @command{tar} options.
2909 Currently this option disables shell invocaton from multi-volume menu
2910 (@pxref{Using Multiple Tapes}).
2912 @opindex rmt-command, summary
2913 @item --rmt-command=@var{cmd}
2915 Notifies @command{tar} that it should use @var{cmd} instead of
2916 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
2918 @opindex rsh-command, summary
2919 @item --rsh-command=@var{cmd}
2921 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
2922 devices. @xref{Device}.
2924 @opindex same-order, summary
2926 @itemx --preserve-order
2929 This option is an optimization for @command{tar} when running on machines with
2930 small amounts of memory. It informs @command{tar} that the list of file
2931 arguments has already been sorted to match the order of files in the
2932 archive. @xref{Reading}.
2934 @opindex same-owner, summary
2937 When extracting an archive, @command{tar} will attempt to preserve the owner
2938 specified in the @command{tar} archive with this option present.
2939 This is the default behavior for the superuser; this option has an
2940 effect only for ordinary users. @xref{Attributes}.
2942 @opindex same-permissions, summary
2943 @item --same-permissions
2945 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
2947 @opindex show-defaults, summary
2948 @item --show-defaults
2950 Displays the default options used by @command{tar} and exits
2951 successfully. This option is intended for use in shell scripts.
2952 Here is an example of what you can see using this option:
2955 $ tar --show-defaults
2956 --format=gnu -f- -b20 --quoting-style=escape \
2957 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
2960 @opindex show-omitted-dirs, summary
2961 @item --show-omitted-dirs
2963 Instructs @command{tar} to mention directories its skipping over when
2964 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
2966 @opindex show-stored-names, summary
2967 @item --show-stored-names
2969 This option has effect only when used in conjunction with one of
2970 archive creation operations. It instructs tar to list the member names
2971 stored in the archive, as opposed to the actual file
2972 names. @xref{listing member and file names}.
2974 @opindex sparse, summary
2978 Invokes a @acronym{GNU} extension when adding files to an archive that handles
2979 sparse files efficiently. @xref{sparse}.
2981 @opindex starting-file, summary
2982 @item --starting-file=@var{name}
2983 @itemx -K @var{name}
2985 This option affects extraction only; @command{tar} will skip extracting
2986 files in the archive until it finds one that matches @var{name}.
2989 @opindex strip-components, summary
2990 @item --strip-components=@var{number}
2991 Strip given @var{number} of leading components from file names before
2992 extraction.@footnote{This option was called @option{--strip-path} in
2993 version 1.14.} For example, if archive @file{archive.tar} contained
2994 @file{/some/file/name}, then running
2997 tar --extract --file archive.tar --strip-components=2
3001 would extracted this file to file @file{name}.
3003 @opindex suffix, summary
3004 @item --suffix=@var{suffix}
3006 Alters the suffix @command{tar} uses when backing up files from the default
3007 @samp{~}. @xref{backup}.
3009 @opindex tape-length, summary
3010 @item --tape-length=@var{num}
3013 Specifies the length of tapes that @command{tar} is writing as being
3014 @w{@var{num} x 1024} bytes long. @xref{Using Multiple Tapes}.
3016 @opindex test-label, summary
3019 Reads the volume label. If an argument is specified, test whether it
3020 matches the volume label. @xref{--test-label option}.
3022 @opindex to-command, summary
3023 @item --to-command=@var{command}
3025 During extraction @command{tar} will pipe extracted files to the
3026 standard input of @var{command}. @xref{Writing to an External Program}.
3028 @opindex to-stdout, summary
3032 During extraction, @command{tar} will extract files to stdout rather
3033 than to the file system. @xref{Writing to Standard Output}.
3035 @opindex totals, summary
3038 Displays the total number of bytes written after creating an archive.
3041 @opindex touch, summary
3045 Sets the data modification time of extracted files to the extraction time,
3046 rather than the data modification time stored in the archive.
3047 @xref{Data Modification Times}.
3049 @opindex uncompress, summary
3052 (See @option{--compress}. @pxref{gzip})
3054 @opindex ungzip, summary
3057 (See @option{--gzip}. @pxref{gzip})
3059 @opindex unlink-first, summary
3060 @item --unlink-first
3063 Directs @command{tar} to remove the corresponding file from the file
3064 system before extracting it from the archive. @xref{Unlink First}.
3066 @opindex use-compress-program, summary
3067 @item --use-compress-program=@var{prog}
3069 Instructs @command{tar} to access the archive through @var{prog}, which is
3070 presumed to be a compression program of some sort. @xref{gzip}.
3072 @opindex utc, summary
3075 Display file modification dates in @acronym{UTC}. This option implies
3078 @opindex verbose, summary
3082 Specifies that @command{tar} should be more verbose about the operations its
3083 performing. This option can be specified multiple times for some
3084 operations to increase the amount of information displayed.
3087 @opindex verify, summary
3091 Verifies that the archive was correctly written when creating an
3092 archive. @xref{verify}.
3094 @opindex version, summary
3097 Print information about the program's name, version, origin and legal
3098 status, all on standard output, and then exit successfully.
3101 @opindex volno-file, summary
3102 @item --volno-file=@var{file}
3104 Used in conjunction with @option{--multi-volume}. @command{tar} will keep track
3105 of which volume of a multi-volume archive its working in @var{file}.
3108 @opindex wildcards, summary
3110 Use wildcards when matching member names with patterns.
3111 @xref{controlling pattern-matching}.
3113 @opindex wildcards-match-slash, summary
3114 @item --wildcards-match-slash
3115 Wildcards match @samp{/}.
3116 @xref{controlling pattern-matching}.
3119 @node Short Option Summary
3120 @subsection Short Options Cross Reference
3122 Here is an alphabetized list of all of the short option forms, matching
3123 them with the equivalent long option.
3129 @option{--concatenate}
3133 @option{--read-full-records}
3137 @option{--directory}
3141 @option{--info-script}
3145 @option{--incremental}
3149 @option{--starting-file}
3153 @option{--tape-length}
3157 @option{--multi-volume}
3165 @option{--to-stdout}
3169 @option{--absolute-names}
3173 @option{--block-number}
3181 @option{--files-from}
3185 @option{--unlink-first}
3197 @option{--exclude-from}
3205 @option{--blocking-factor}
3221 @option{--listed-incremental}
3225 @option{--dereference}
3229 @option{--ignore-zeros}
3237 @option{--keep-old-files}
3241 @option{--one-file-system}. Use of this short option is deprecated. It
3242 is retained for compatibility with the earlier versions of GNU
3243 @command{tar}, and will be changed in future releases.
3245 @xref{Changes}, for more information.
3253 When creating --- @option{--no-same-owner}, when extracting ---
3254 @option{--portability}.
3256 The later usage is deprecated. It is retained for compatibility with
3257 the earlier versions of @GNUTAR{}. In the future releases
3258 @option{-o} will be equivalent to @option{--no-same-owner} only.
3262 @option{--preserve-permissions}
3270 @option{--same-order}
3286 @option{--interactive}
3299 @section @GNUTAR{} documentation
3301 @cindex Getting program version number
3303 @cindex Version of the @command{tar} program
3304 Being careful, the first thing is really checking that you are using
3305 @GNUTAR{}, indeed. The @option{--version} option
3306 causes @command{tar} to print information about its name, version,
3307 origin and legal status, all on standard output, and then exit
3308 successfully. For example, @w{@samp{tar --version}} might print:
3311 tar (GNU tar) 1.15.2
3312 Copyright (C) 2006 Free Software Foundation, Inc.
3313 This is free software. You may redistribute copies of it under the terms of
3314 the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
3315 There is NO WARRANTY, to the extent permitted by law.
3317 Written by John Gilmore and Jay Fenlason.
3321 The first occurrence of @samp{tar} in the result above is the program
3322 name in the package (for example, @command{rmt} is another program),
3323 while the second occurrence of @samp{tar} is the name of the package
3324 itself, containing possibly many programs. The package is currently
3325 named @samp{tar}, after the name of the main program it
3326 contains@footnote{There are plans to merge the @command{cpio} and
3327 @command{tar} packages into a single one which would be called
3328 @code{paxutils}. So, who knows if, one of this days, the
3329 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3332 @cindex Obtaining help
3333 @cindex Listing all @command{tar} options
3334 @opindex help, introduction
3335 Another thing you might want to do is checking the spelling or meaning
3336 of some particular @command{tar} option, without resorting to this
3337 manual, for once you have carefully read it. @GNUTAR{}
3338 has a short help feature, triggerable through the
3339 @option{--help} option. By using this option, @command{tar} will
3340 print a usage message listing all available options on standard
3341 output, then exit successfully, without doing anything else and
3342 ignoring all other options. Even if this is only a brief summary, it
3343 may be several screens long. So, if you are not using some kind of
3344 scrollable window, you might prefer to use something like:
3347 $ @kbd{tar --help | less}
3351 presuming, here, that you like using @command{less} for a pager. Other
3352 popular pagers are @command{more} and @command{pg}. If you know about some
3353 @var{keyword} which interests you and do not want to read all the
3354 @option{--help} output, another common idiom is doing:
3357 tar --help | grep @var{keyword}
3361 for getting only the pertinent lines. Notice, however, that some
3362 @command{tar} options have long description lines and the above
3363 command will list only the first of them.
3365 The exact look of the option summary displayed by @kbd{tar --help} is
3366 configurable. @xref{Configuring Help Summary}, for a detailed description.
3369 If you only wish to check the spelling of an option, running @kbd{tar
3370 --usage} may be a better choice. This will display a terse list of
3371 @command{tar} option without accompanying explanations.
3373 The short help output is quite succinct, and you might have to get
3374 back to the full documentation for precise points. If you are reading
3375 this paragraph, you already have the @command{tar} manual in some
3376 form. This manual is available in a variety of forms from
3377 @url{http://www.gnu.org/software/tar/manual}. It may printed out of the @GNUTAR{}
3378 distribution, provided you have @TeX{} already installed somewhere,
3379 and a laser printer around. Just configure the distribution, execute
3380 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3381 usual way (contact your local guru to know how). If @GNUTAR{}
3382 has been conveniently installed at your place, this
3383 manual is also available in interactive, hypertextual form as an Info
3384 file. Just call @w{@samp{info tar}} or, if you do not have the
3385 @command{info} program handy, use the Info reader provided within
3386 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3388 There is currently no @code{man} page for @GNUTAR{}.
3389 If you observe such a @code{man} page on the system you are running,
3390 either it does not belong to @GNUTAR{}, or it has not
3391 been produced by @acronym{GNU}. Some package maintainers convert
3392 @kbd{tar --help} output to a man page, using @command{help2man}. In
3393 any case, please bear in mind that the authoritative source of
3394 information about @GNUTAR{} is this Texinfo documentation.
3397 @section Obtaining @GNUTAR{} default values
3399 @opindex show-defaults
3400 @GNUTAR{} has some predefined defaults that are used when you do not
3401 explicitely specify another values. To obtain a list of such
3402 defaults, use @option{--show-defaults} option. This will output the
3403 values in the form of @command{tar} command line options:
3407 @kbd{tar --show-defaults}
3408 --format=gnu -f- -b20 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3413 The above output shows that this version of @GNUTAR{} defaults to
3414 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3415 output as the archive, if no @option{--file} option has been given
3416 (@pxref{file tutorial}), the default blocking factor is 20
3417 (@pxref{Blocking Factor}). It also shows the default locations where
3418 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3421 @section Checking @command{tar} progress
3423 Typically, @command{tar} performs most operations without reporting any
3424 information to the user except error messages. When using @command{tar}
3425 with many options, particularly ones with complicated or
3426 difficult-to-predict behavior, it is possible to make serious mistakes.
3427 @command{tar} provides several options that make observing @command{tar}
3428 easier. These options cause @command{tar} to print information as it
3429 progresses in its job, and you might want to use them just for being
3430 more careful about what is going on, or merely for entertaining
3431 yourself. If you have encountered a problem when operating on an
3432 archive, however, you may need more information than just an error
3433 message in order to solve the problem. The following options can be
3434 helpful diagnostic tools.
3436 @cindex Verbose operation
3438 Normally, the @option{--list} (@option{-t}) command to list an archive
3439 prints just the file names (one per line) and the other commands are
3440 silent. When used with most operations, the @option{--verbose}
3441 (@option{-v}) option causes @command{tar} to print the name of each
3442 file or archive member as it is processed. This and the other options
3443 which make @command{tar} print status information can be useful in
3444 monitoring @command{tar}.
3446 With @option{--create} or @option{--extract}, @option{--verbose} used
3447 once just prints the names of the files or members as they are processed.
3448 Using it twice causes @command{tar} to print a longer listing
3449 (reminiscent of @samp{ls -l}) for each member. Since @option{--list}
3450 already prints the names of the members, @option{--verbose} used once
3451 with @option{--list} causes @command{tar} to print an @samp{ls -l}
3452 type listing of the files in the archive. The following examples both
3453 extract members with long list output:
3456 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3457 $ @kbd{tar xvvf archive.tar}
3460 Verbose output appears on the standard output except when an archive is
3461 being written to the standard output, as with @samp{tar --create
3462 --file=- --verbose} (@samp{tar cfv -}, or even @samp{tar cv}---if the
3463 installer let standard output be the default archive). In that case
3464 @command{tar} writes verbose output to the standard error stream.
3466 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3467 verbose output to @var{file} rather than to standard output or standard
3470 @cindex Obtaining total status information
3472 The @option{--totals} option---which is only meaningful when used with
3473 @option{--create} (@option{-c})---causes @command{tar} to print the total
3474 amount written to the archive, after it has been fully created.
3476 @cindex Progress information
3478 The @option{--checkpoint} option prints an occasional message
3479 as @command{tar} reads or writes the archive. In fact, it prints
3480 a message each 10 records read or written. It is designed for
3481 those who don't need the more detailed (and voluminous) output of
3482 @option{--block-number} (@option{-R}), but do want visual confirmation
3483 that @command{tar} is actually making forward progress.
3485 @FIXME{There is some confusion here. It seems that -R once wrote a
3486 message at @samp{every} record read or written.}
3488 @opindex show-omitted-dirs
3489 @anchor{show-omitted-dirs}
3490 The @option{--show-omitted-dirs} option, when reading an archive---with
3491 @option{--list} or @option{--extract}, for example---causes a message
3492 to be printed for each directory in the archive which is skipped.
3493 This happens regardless of the reason for skipping: the directory might
3494 not have been named on the command line (implicitly or explicitly),
3495 it might be excluded by the use of the @option{--exclude=@var{pattern}} option, or
3498 @opindex block-number
3499 @cindex Block number where error occurred
3500 @anchor{block-number}
3501 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3502 every message it would normally produce, the block number within the
3503 archive where the message was triggered. Also, supplementary messages
3504 are triggered when reading blocks full of NULs, or when hitting end of
3505 file on the archive. As of now, if the archive if properly terminated
3506 with a NUL block, the reading of the file may stop before end of file
3507 is met, so the position of end of file will not usually show when
3508 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3509 drains the archive before exiting when reading the
3510 archive from a pipe.
3512 @cindex Error message, block number of
3513 This option is especially useful when reading damaged archives, since
3514 it helps pinpoint the damaged sections. It can also be used with
3515 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3516 choose among several backup tapes when retrieving a file later, in
3517 favor of the tape where the file appears earliest (closest to the
3518 front of the tape). @xref{backup}.
3521 @section Asking for Confirmation During Operations
3522 @cindex Interactive operation
3524 Typically, @command{tar} carries out a command without stopping for
3525 further instructions. In some situations however, you may want to
3526 exclude some files and archive members from the operation (for instance
3527 if disk or storage space is tight). You can do this by excluding
3528 certain files automatically (@pxref{Choosing}), or by performing
3529 an operation interactively, using the @option{--interactive} (@option{-w}) option.
3530 @command{tar} also accepts @option{--confirmation} for this option.
3532 @opindex interactive
3533 When the @option{--interactive} (@option{-w}) option is specified, before
3534 reading, writing, or deleting files, @command{tar} first prints a message
3535 for each such file, telling what operation it intends to take, then asks
3536 for confirmation on the terminal. The actions which require
3537 confirmation include adding a file to the archive, extracting a file
3538 from the archive, deleting a file from the archive, and deleting a file
3539 from disk. To confirm the action, you must type a line of input
3540 beginning with @samp{y}. If your input line begins with anything other
3541 than @samp{y}, @command{tar} skips that file.
3543 If @command{tar} is reading the archive from the standard input,
3544 @command{tar} opens the file @file{/dev/tty} to support the interactive
3547 Verbose output is normally sent to standard output, separate from
3548 other error messages. However, if the archive is produced directly
3549 on standard output, then verbose output is mixed with errors on
3550 @code{stderr}. Producing the archive on standard output may be used
3551 as a way to avoid using disk space, when the archive is soon to be
3552 consumed by another process reading it, say. Some people felt the need
3553 of producing an archive on stdout, still willing to segregate between
3554 verbose output and error output. A possible approach would be using a
3555 named pipe to receive the archive, and having the consumer process to
3556 read from that named pipe. This has the advantage of letting standard
3557 output free to receive verbose output, all separate from errors.
3560 @chapter @GNUTAR{} Operations
3573 @section Basic @GNUTAR{} Operations
3575 The basic @command{tar} operations, @option{--create} (@option{-c}),
3576 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
3577 @option{-x}), are currently presented and described in the tutorial
3578 chapter of this manual. This section provides some complementary notes
3579 for these operations.
3582 @opindex create, complementary notes
3586 Creating an empty archive would have some kind of elegance. One can
3587 initialize an empty archive and later use @option{--append}
3588 (@option{-r}) for adding all members. Some applications would not
3589 welcome making an exception in the way of adding the first archive
3590 member. On the other hand, many people reported that it is
3591 dangerously too easy for @command{tar} to destroy a magnetic tape with
3592 an empty archive@footnote{This is well described in @cite{Unix-haters
3593 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
3594 Books, ISBN 1-56884-203-1.}. The two most common errors are:
3598 Mistakingly using @code{create} instead of @code{extract}, when the
3599 intent was to extract the full contents of an archive. This error
3600 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
3601 the QWERTY keyboard. Instead of being unpacked, the archive then
3602 gets wholly destroyed. When users speak about @dfn{exploding} an
3603 archive, they usually mean something else :-).
3606 Forgetting the argument to @code{file}, when the intent was to create
3607 an archive with a single file in it. This error is likely because a
3608 tired user can easily add the @kbd{f} key to the cluster of option
3609 letters, by the mere force of habit, without realizing the full
3610 consequence of doing so. The usual consequence is that the single
3611 file, which was meant to be saved, is rather destroyed.
3614 So, recognizing the likelihood and the catastrophical nature of these
3615 errors, @GNUTAR{} now takes some distance from elegance, and
3616 cowardly refuses to create an archive when @option{--create} option is
3617 given, there are no arguments besides options, and
3618 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
3619 around the cautiousness of @GNUTAR{} and nevertheless create an
3620 archive with nothing in it, one may still use, as the value for the
3621 @option{--files-from} option, a file with no names in it, as shown in
3622 the following commands:
3625 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
3626 @kbd{tar cfT empty-archive.tar /dev/null}
3629 @opindex extract, complementary notes
3634 A socket is stored, within a @GNUTAR{} archive, as a pipe.
3636 @item @option{--list} (@option{-t})
3638 @GNUTAR{} now shows dates as @samp{1996-08-30},
3639 while it used to show them as @samp{Aug 30 1996}. Preferably,
3640 people should get used to ISO 8601 dates. Local American dates should
3641 be made available again with full date localization support, once
3642 ready. In the meantime, programs not being localizable for dates
3643 should prefer international dates, that's really the way to go.
3645 Look up @url{http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html} if you
3646 are curious, it contains a detailed explanation of the ISO 8601 standard.
3651 @section Advanced @GNUTAR{} Operations
3653 Now that you have learned the basics of using @GNUTAR{}, you may want
3654 to learn about further ways in which @command{tar} can help you.
3656 This chapter presents five, more advanced operations which you probably
3657 won't use on a daily basis, but which serve more specialized functions.
3658 We also explain the different styles of options and why you might want
3659 to use one or another, or a combination of them in your @command{tar}
3660 commands. Additionally, this chapter includes options which allow you to
3661 define the output from @command{tar} more carefully, and provide help and
3662 error correction in special circumstances.
3664 @FIXME{check this after the chapter is actually revised to make sure
3665 it still introduces the info in the chapter correctly : ).}
3678 @subsection The Five Advanced @command{tar} Operations
3681 In the last chapter, you learned about the first three operations to
3682 @command{tar}. This chapter presents the remaining five operations to
3683 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
3684 @option{--delete}, and @option{--compare}.
3686 You are not likely to use these operations as frequently as those
3687 covered in the last chapter; however, since they perform specialized
3688 functions, they are quite useful when you do need to use them. We
3689 will give examples using the same directory and files that you created
3690 in the last chapter. As you may recall, the directory is called
3691 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
3692 @samp{rock}, and the two archive files you created are
3693 @samp{collection.tar} and @samp{music.tar}.
3695 We will also use the archive files @samp{afiles.tar} and
3696 @samp{bfiles.tar}. @samp{afiles.tar} contains the members @samp{apple},
3697 @samp{angst}, and @samp{aspic}. @samp{bfiles.tar} contains the members
3698 @samp{./birds}, @samp{baboon}, and @samp{./box}.
3700 Unless we state otherwise, all practicing you do and examples you follow
3701 in this chapter will take place in the @file{practice} directory that
3702 you created in the previous chapter; see @ref{prepare for examples}.
3703 (Below in this section, we will remind you of the state of the examples
3704 where the last chapter left them.)
3706 The five operations that we will cover in this chapter are:
3711 Add new entries to an archive that already exists.
3714 Add more recent copies of archive members to the end of an archive, if
3719 Add one or more pre-existing archives to the end of another archive.
3721 Delete items from an archive (does not work on tapes).
3725 Compare archive members to their counterparts in the file system.
3729 @subsection How to Add Files to Existing Archives: @option{--append}
3733 If you want to add files to an existing archive, you don't need to
3734 create a new archive; you can use @option{--append} (@option{-r}).
3735 The archive must already exist in order to use @option{--append}. (A
3736 related operation is the @option{--update} operation; you can use this
3737 to add newer versions of archive members to an existing archive. To learn how to
3738 do this with @option{--update}, @pxref{update}.)
3740 If you use @option{--append} to add a file that has the same name as an
3741 archive member to an archive containing that archive member, then the
3742 old member is not deleted. What does happen, however, is somewhat
3743 complex. @command{tar} @emph{allows} you to have infinite number of files
3744 with the same name. Some operations treat these same-named members no
3745 differently than any other set of archive members: for example, if you
3746 view an archive with @option{--list} (@option{-t}), you will see all
3747 of those members listed, with their data modification times, owners, etc.
3749 Other operations don't deal with these members as perfectly as you might
3750 prefer; if you were to use @option{--extract} to extract the archive,
3751 only the most recently added copy of a member with the same name as four
3752 other members would end up in the working directory. This is because
3753 @option{--extract} extracts an archive in the order the members appeared
3754 in the archive; the most recently archived members will be extracted
3755 last. Additionally, an extracted member will @emph{replace} a file of
3756 the same name which existed in the directory already, and @command{tar}
3757 will not prompt you about this@footnote{Unless you give it
3758 @option{--keep-old-files} option, or the disk copy is newer than the
3759 the one in the archive and you invoke @command{tar} with
3760 @option{--keep-newer-files} option}. Thus, only the most recently archived
3761 member will end up being extracted, as it will replace the one
3762 extracted before it, and so on.
3764 There exists a special option that allows you to get around this
3765 behavior and extract (or list) only a particular copy of the file.
3766 This is @option{--occurrence} option. If you run @command{tar} with
3767 this option, it will extract only the first copy of the file. You
3768 may also give this option an argument specifying the number of
3769 copy to be extracted. Thus, for example if the archive
3770 @file{archive.tar} contained three copies of file @file{myfile}, then
3774 tar --extract --file archive.tar --occurrence=2 myfile
3778 would extract only the second copy. @xref{Option
3779 Summary,---occurrence}, for the description of @option{--occurrence}
3782 @FIXME{ hag -- you might want to incorporate some of the above into the
3783 MMwtSN node; not sure. i didn't know how to make it simpler...
3785 There are a few ways to get around this. (maybe xref Multiple Members
3786 with the Same Name.}
3788 @cindex Members, replacing with other members
3789 @cindex Replacing members with other members
3790 If you want to replace an archive member, use @option{--delete} to
3791 delete the member you want to remove from the archive, , and then use
3792 @option{--append} to add the member you want to be in the archive. Note
3793 that you can not change the order of the archive; the most recently
3794 added member will still appear last. In this sense, you cannot truly
3795 ``replace'' one member with another. (Replacing one member with another
3796 will not work on certain types of media, such as tapes; see @ref{delete}
3797 and @ref{Media}, for more information.)
3800 * appending files:: Appending Files to an Archive
3804 @node appending files
3805 @subsubsection Appending Files to an Archive
3807 @cindex Adding files to an Archive
3808 @cindex Appending files to an Archive
3809 @cindex Archives, Appending files to
3811 The simplest way to add a file to an already existing archive is the
3812 @option{--append} (@option{-r}) operation, which writes specified
3813 files into the archive whether or not they are already among the
3816 When you use @option{--append}, you @emph{must} specify file name
3817 arguments, as there is no default. If you specify a file that already
3818 exists in the archive, another copy of the file will be added to the
3819 end of the archive. As with other operations, the member names of the
3820 newly added files will be exactly the same as their names given on the
3821 command line. The @option{--verbose} (@option{-v}) option will print
3822 out the names of the files as they are written into the archive.
3824 @option{--append} cannot be performed on some tape drives, unfortunately,
3825 due to deficiencies in the formats those tape drives use. The archive
3826 must be a valid @command{tar} archive, or else the results of using this
3827 operation will be unpredictable. @xref{Media}.
3829 To demonstrate using @option{--append} to add a file to an archive,
3830 create a file called @file{rock} in the @file{practice} directory.
3831 Make sure you are in the @file{practice} directory. Then, run the
3832 following @command{tar} command to add @file{rock} to
3833 @file{collection.tar}:
3836 $ @kbd{tar --append --file=collection.tar rock}
3840 If you now use the @option{--list} (@option{-t}) operation, you will see that
3841 @file{rock} has been added to the archive:
3844 $ @kbd{tar --list --file=collection.tar}
3845 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
3846 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3847 -rw-r--r-- me user 20 1996-09-23 16:44 folk
3848 -rw-r--r-- me user 20 1996-09-23 16:44 rock
3852 @subsubsection Multiple Files with the Same Name
3854 You can use @option{--append} (@option{-r}) to add copies of files
3855 which have been updated since the archive was created. (However, we
3856 do not recommend doing this since there is another @command{tar}
3857 option called @option{--update}; @xref{update}, for more information.
3858 We describe this use of @option{--append} here for the sake of
3859 completeness.) When you extract the archive, the older version will
3860 be effectively lost. This works because files are extracted from an
3861 archive in the order in which they were archived. Thus, when the
3862 archive is extracted, a file archived later in time will replace a
3863 file of the same name which was archived earlier, even though the
3864 older version of the file will remain in the archive unless you delete
3865 all versions of the file.
3867 Supposing you change the file @file{blues} and then append the changed
3868 version to @file{collection.tar}. As you saw above, the original
3869 @file{blues} is in the archive @file{collection.tar}. If you change the
3870 file and append the new version of the file to the archive, there will
3871 be two copies in the archive. When you extract the archive, the older
3872 version of the file will be extracted first, and then replaced by the
3873 newer version when it is extracted.
3875 You can append the new, changed copy of the file @file{blues} to the
3876 archive in this way:
3879 $ @kbd{tar --append --verbose --file=collection.tar blues}
3884 Because you specified the @option{--verbose} option, @command{tar} has
3885 printed the name of the file being appended as it was acted on. Now
3886 list the contents of the archive:
3889 $ @kbd{tar --list --verbose --file=collection.tar}
3890 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
3891 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3892 -rw-r--r-- me user 20 1996-09-23 16:44 folk
3893 -rw-r--r-- me user 20 1996-09-23 16:44 rock
3894 -rw-r--r-- me user 58 1996-10-24 18:30 blues
3898 The newest version of @file{blues} is now at the end of the archive
3899 (note the different creation dates and file sizes). If you extract
3900 the archive, the older version of the file @file{blues} will be
3901 replaced by the newer version. You can confirm this by extracting
3902 the archive and running @samp{ls} on the directory.
3904 If you wish to extract the first occurrence of the file @file{blues}
3905 from the archive, use @option{--occurrence} option, as shown in
3906 the following example:
3909 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
3910 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3913 @xref{Writing}, for more information on @option{--extract} and
3914 @xref{Option Summary, --occurrence}, for the description of
3915 @option{--occurrence} option.
3918 @subsection Updating an Archive
3920 @cindex Updating an archive
3923 In the previous section, you learned how to use @option{--append} to
3924 add a file to an existing archive. A related operation is
3925 @option{--update} (@option{-u}). The @option{--update} operation
3926 updates a @command{tar} archive by comparing the date of the specified
3927 archive members against the date of the file with the same name. If
3928 the file has been modified more recently than the archive member, then
3929 the newer version of the file is added to the archive (as with
3932 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
3933 The operation will fail.
3935 @FIXME{other examples of media on which --update will fail? need to ask
3936 charles and/or mib/thomas/dave shevett..}
3938 Both @option{--update} and @option{--append} work by adding to the end
3939 of the archive. When you extract a file from the archive, only the
3940 version stored last will wind up in the file system, unless you use
3941 the @option{--backup} option. @FIXME-ref{Multiple Members with the
3949 @subsubsection How to Update an Archive Using @option{--update}
3951 You must use file name arguments with the @option{--update}
3952 (@option{-u}) operation. If you don't specify any files,
3953 @command{tar} won't act on any files and won't tell you that it didn't
3954 do anything (which may end up confusing you).
3956 @c note: the above parenthetical added because in fact, this
3957 @c behavior just confused the author. :-)
3959 To see the @option{--update} option at work, create a new file,
3960 @file{classical}, in your practice directory, and some extra text to the
3961 file @file{blues}, using any text editor. Then invoke @command{tar} with
3962 the @samp{update} operation and the @option{--verbose} (@option{-v})
3963 option specified, using the names of all the files in the practice
3964 directory as file name arguments:
3967 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
3974 Because we have specified verbose mode, @command{tar} prints out the names
3975 of the files it is working on, which in this case are the names of the
3976 files that needed to be updated. If you run @samp{tar --list} and look
3977 at the archive, you will see @file{blues} and @file{classical} at its
3978 end. There will be a total of two versions of the member @samp{blues};
3979 the one at the end will be newer and larger, since you added text before
3982 (The reason @command{tar} does not overwrite the older file when updating
3983 it is because writing to the middle of a section of tape is a difficult
3984 process. Tapes are not designed to go backward. @xref{Media}, for more
3985 information about tapes.
3987 @option{--update} (@option{-u}) is not suitable for performing backups for two
3988 reasons: it does not change directory content entries, and it
3989 lengthens the archive every time it is used. The @GNUTAR{}
3990 options intended specifically for backups are more
3991 efficient. If you need to run backups, please consult @ref{Backups}.
3994 @subsection Combining Archives with @option{--concatenate}
3996 @cindex Adding archives to an archive
3997 @cindex Concatenating Archives
3998 @opindex concatenate
4000 @c @cindex @option{-A} described
4001 Sometimes it may be convenient to add a second archive onto the end of
4002 an archive rather than adding individual files to the archive. To add
4003 one or more archives to the end of another archive, you should use the
4004 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4006 To use @option{--concatenate}, give the first archive with
4007 @option{--file} option and name the rest of archives to be
4008 concatenated on the command line. The members, and their member
4009 names, will be copied verbatim from those archives to the first one.
4010 @FIXME-ref{This can cause multiple members to have the same name, for
4011 information on how this affects reading the archive, Multiple
4012 Members with the Same Name.}
4013 The new, concatenated archive will be called by the same name as the
4014 one given with the @option{--file} option. As usual, if you omit
4015 @option{--file}, @command{tar} will use the value of the environment
4016 variable @env{TAPE}, or, if this has not been set, the default archive name.
4018 @FIXME{There is no way to specify a new name...}
4020 To demonstrate how @option{--concatenate} works, create two small archives
4021 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4022 files from @file{practice}:
4025 $ @kbd{tar -cvf bluesrock.tar blues rock}
4028 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4034 If you like, You can run @samp{tar --list} to make sure the archives
4035 contain what they are supposed to:
4038 $ @kbd{tar -tvf bluesrock.tar}
4039 -rw-r--r-- melissa user 105 1997-01-21 19:42 blues
4040 -rw-r--r-- melissa user 33 1997-01-20 15:34 rock
4041 $ @kbd{tar -tvf jazzfolk.tar}
4042 -rw-r--r-- melissa user 20 1996-09-23 16:44 folk
4043 -rw-r--r-- melissa user 65 1997-01-30 14:15 jazz
4046 We can concatenate these two archives with @command{tar}:
4050 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4053 If you now list the contents of the @file{bluesrock.tar}, you will see
4054 that now it also contains the archive members of @file{jazzfolk.tar}:
4057 $ @kbd{tar --list --file=bluesrock.tar}
4064 When you use @option{--concatenate}, the source and target archives must
4065 already exist and must have been created using compatible format
4066 parameters. Notice, that @command{tar} does not check whether the
4067 archives it concatenates have compatible formats, it does not
4068 even check if the files are really tar archives.
4070 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4071 tape drives, due to deficiencies in the formats those tape drives use.
4073 @cindex @code{concatenate} vs @command{cat}
4074 @cindex @command{cat} vs @code{concatenate}
4075 It may seem more intuitive to you to want or try to use @command{cat} to
4076 concatenate two archives instead of using the @option{--concatenate}
4077 operation; after all, @command{cat} is the utility for combining files.
4079 However, @command{tar} archives incorporate an end-of-file marker which
4080 must be removed if the concatenated archives are to be read properly as
4081 one archive. @option{--concatenate} removes the end-of-archive marker
4082 from the target archive before each new archive is appended. If you use
4083 @command{cat} to combine the archives, the result will not be a valid
4084 @command{tar} format archive. If you need to retrieve files from an
4085 archive that was added to using the @command{cat} utility, use the
4086 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4087 information on dealing with archives improperly combined using the
4088 @command{cat} shell utility.
4091 @subsection Removing Archive Members Using @option{--delete}
4093 @cindex Deleting files from an archive
4094 @cindex Removing files from an archive
4097 You can remove members from an archive by using the @option{--delete}
4098 option. Specify the name of the archive with @option{--file}
4099 (@option{-f}) and then specify the names of the members to be deleted;
4100 if you list no member names, nothing will be deleted. The
4101 @option{--verbose} option will cause @command{tar} to print the names
4102 of the members as they are deleted. As with @option{--extract}, you
4103 must give the exact member names when using @samp{tar --delete}.
4104 @option{--delete} will remove all versions of the named file from the
4105 archive. The @option{--delete} operation can run very slowly.
4107 Unlike other operations, @option{--delete} has no short form.
4109 @cindex Tapes, using @option{--delete} and
4110 @cindex Deleting from tape archives
4111 This operation will rewrite the archive. You can only use
4112 @option{--delete} on an archive if the archive device allows you to
4113 write to any point on the media, such as a disk; because of this, it
4114 does not work on magnetic tapes. Do not try to delete an archive member
4115 from a magnetic tape; the action will not succeed, and you will be
4116 likely to scramble the archive and damage your tape. There is no safe
4117 way (except by completely re-writing the archive) to delete files from
4118 most kinds of magnetic tape. @xref{Media}.
4120 To delete all versions of the file @file{blues} from the archive
4121 @file{collection.tar} in the @file{practice} directory, make sure you
4122 are in that directory, and then,
4125 $ @kbd{tar --list --file=collection.tar}
4130 $ @kbd{tar --delete --file=collection.tar blues}
4131 $ @kbd{tar --list --file=collection.tar}
4138 @FIXME{Check if the above listing is actually produced after running
4139 all the examples on collection.tar.}
4141 The @option{--delete} option has been reported to work properly when
4142 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4145 @subsection Comparing Archive Members with the File System
4146 @cindex Verifying the currency of an archive
4150 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4151 specified archive members against files with the same names, and then
4152 reports differences in file size, mode, owner, modification date and
4153 contents. You should @emph{only} specify archive member names, not file
4154 names. If you do not name any members, then @command{tar} will compare the
4155 entire archive. If a file is represented in the archive but does not
4156 exist in the file system, @command{tar} reports a difference.
4158 You have to specify the record size of the archive when modifying an
4159 archive with a non-default record size.
4161 @command{tar} ignores files in the file system that do not have
4162 corresponding members in the archive.
4164 The following example compares the archive members @file{rock},
4165 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4166 files of the same name in the file system. (Note that there is no file,
4167 @file{funk}; @command{tar} will report an error message.)
4170 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4173 tar: funk not found in archive
4176 The spirit behind the @option{--compare} (@option{--diff}, @option{-d}) option is to check whether the
4177 archive represents the current state of files on disk, more than validating
4178 the integrity of the archive media. For this later goal, @xref{verify}.
4180 @node quoting styles
4181 @subsection Quoting Member Names
4193 $ @kbd{tar cf arch .}
4199 $ @kbd{tar tf arch --quoting-style=literal}
4213 $ @kbd{tar tf arch --quoting-style=shell}
4216 './a'\''single'\''quote'
4227 $ @kbd{tar tf arch --quoting-style=shell-always}
4230 './a'\''single'\''quote'
4241 $ @kbd{tar tf arch --quoting-style=c}
4245 "./a\"double\"quote"
4254 $ @kbd{tar tf arch --quoting-style=escape}
4267 $ @kbd{tar tf arch --quoting-style=locale}
4270 `./a\'single\'quote'
4280 $ @kbd{tar tf arch --quoting-style=clocale}
4284 "./a\"double\"quote"
4291 @node create options
4292 @section Options Used by @option{--create}
4294 @opindex create, additional options
4295 The previous chapter described the basics of how to use
4296 @option{--create} (@option{-c}) to create an archive from a set of files.
4297 @xref{create}. This section described advanced options to be used with
4301 * Ignore Failed Read::
4304 @node Ignore Failed Read
4305 @subsection Ignore Fail Read
4308 @item --ignore-failed-read
4309 Do not exit with nonzero on unreadable files or directories.
4312 @node extract options
4313 @section Options Used by @option{--extract}
4316 @opindex extract, additional options
4317 The previous chapter showed how to use @option{--extract} to extract
4318 an archive into the file system. Various options cause @command{tar} to
4319 extract more information than just file contents, such as the owner,
4320 the permissions, the modification date, and so forth. This section
4321 presents options to be used with @option{--extract} when certain special
4322 considerations arise. You may review the information presented in
4323 @ref{extract} for more basic information about the
4324 @option{--extract} operation.
4327 * Reading:: Options to Help Read Archives
4328 * Writing:: Changing How @command{tar} Writes Files
4329 * Scarce:: Coping with Scarce Resources
4333 @subsection Options to Help Read Archives
4334 @cindex Options when reading archives
4337 @cindex Reading incomplete records
4338 @cindex Records, incomplete
4339 @opindex read-full-records
4340 Normally, @command{tar} will request data in full record increments from
4341 an archive storage device. If the device cannot return a full record,
4342 @command{tar} will report an error. However, some devices do not always
4343 return full records, or do not require the last record of an archive to
4344 be padded out to the next record boundary. To keep reading until you
4345 obtain a full record, or to accept an incomplete record if it contains
4346 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
4347 in conjunction with the @option{--extract} or @option{--list} operations.
4350 The @option{--read-full-records} (@option{-B}) option is turned on by default when
4351 @command{tar} reads an archive from standard input, or from a remote
4352 machine. This is because on BSD Unix systems, attempting to read a
4353 pipe returns however much happens to be in the pipe, even if it is
4354 less than was requested. If this option were not enabled, @command{tar}
4355 would fail as soon as it read an incomplete record from the pipe.
4357 If you're not sure of the blocking factor of an archive, you can
4358 read the archive by specifying @option{--read-full-records} (@option{-B}) and
4359 @option{--blocking-factor=@var{512-size}} (@option{-b
4360 @var{512-size}}), using a blocking factor larger than what the archive
4361 uses. This lets you avoid having to determine the blocking factor
4362 of an archive. @xref{Blocking Factor}.
4365 * read full records::
4369 @node read full records
4370 @unnumberedsubsubsec Reading Full Records
4372 @FIXME{need sentence or so of intro here}
4375 @opindex read-full-records
4376 @item --read-full-records
4378 Use in conjunction with @option{--extract} (@option{--get},
4379 @option{-x}) to read an archive which contains incomplete records, or
4380 one which has a blocking factor less than the one specified.
4384 @unnumberedsubsubsec Ignoring Blocks of Zeros
4386 @cindex End-of-archive blocks, ignoring
4387 @cindex Ignoring end-of-archive blocks
4388 @opindex ignore-zeros
4389 Normally, @command{tar} stops reading when it encounters a block of zeros
4390 between file entries (which usually indicates the end of the archive).
4391 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
4392 completely read an archive which contains a block of zeros before the
4393 end (i.e., a damaged archive, or one that was created by concatenating
4394 several archives together).
4396 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
4397 versions of @command{tar} write garbage after the end-of-archive entry,
4398 since that part of the media is never supposed to be read. @GNUTAR{}
4399 does not write after the end of an archive, but seeks to
4400 maintain compatiblity among archiving utilities.
4403 @item --ignore-zeros
4405 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
4406 encountered while reading an archive. Use in conjunction with
4407 @option{--extract} or @option{--list}.
4411 @subsection Changing How @command{tar} Writes Files
4414 @FIXME{Introductory paragraph}
4417 * Dealing with Old Files::
4418 * Overwrite Old Files::
4420 * Keep Newer Files::
4422 * Recursive Unlink::
4423 * Data Modification Times::
4424 * Setting Access Permissions::
4425 * Directory Modification Times and Permissions::
4426 * Writing to Standard Output::
4427 * Writing to an External Program::
4431 @node Dealing with Old Files
4432 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
4434 @opindex overwrite-dir, introduced
4435 When extracting files, if @command{tar} discovers that the extracted
4436 file already exists, it normally replaces the file by removing it before
4437 extracting it, to prevent confusion in the presence of hard or symbolic
4438 links. (If the existing file is a symbolic link, it is removed, not
4439 followed.) However, if a directory cannot be removed because it is
4440 nonempty, @command{tar} normally overwrites its metadata (ownership,
4441 permission, etc.). The @option{--overwrite-dir} option enables this
4442 default behavior. To be more cautious and preserve the metadata of
4443 such a directory, use the @option{--no-overwrite-dir} option.
4445 @cindex Overwriting old files, prevention
4446 @opindex keep-old-files, introduced
4447 To be even more cautious and prevent existing files from being replaced, use
4448 the @option{--keep-old-files} (@option{-k}) option. It causes @command{tar} to refuse
4449 to replace or update a file that already exists, i.e., a file with the
4450 same name as an archive member prevents extraction of that archive
4451 member. Instead, it reports an error.
4453 @opindex overwrite, introduced
4454 To be more aggressive about altering existing files, use the
4455 @option{--overwrite} option. It causes @command{tar} to overwrite
4456 existing files and to follow existing symbolic links when extracting.
4458 @cindex Protecting old files
4459 Some people argue that @GNUTAR{} should not hesitate
4460 to overwrite files with other files when extracting. When extracting
4461 a @command{tar} archive, they expect to see a faithful copy of the
4462 state of the file system when the archive was created. It is debatable
4463 that this would always be a proper behavior. For example, suppose one
4464 has an archive in which @file{usr/local} is a link to
4465 @file{usr/local2}. Since then, maybe the site removed the link and
4466 renamed the whole hierarchy from @file{/usr/local2} to
4467 @file{/usr/local}. Such things happen all the time. I guess it would
4468 not be welcome at all that @GNUTAR{} removes the
4469 whole hierarchy just to make room for the link to be reinstated
4470 (unless it @emph{also} simultaneously restores the full
4471 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
4472 able to remove a whole hierarchy to reestablish a symbolic link, for
4473 example, but @emph{only if} @option{--recursive-unlink} is specified
4474 to allow this behavior. In any case, single files are silently
4477 @opindex unlink-first, introduced
4478 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
4479 some cases by causing @command{tar} to remove files unconditionally
4480 before extracting them.
4482 @node Overwrite Old Files
4483 @unnumberedsubsubsec Overwrite Old Files
4488 Overwrite existing files and directory metadata when extracting files
4491 This causes @command{tar} to write extracted files into the file system without
4492 regard to the files already on the system; i.e., files with the same
4493 names as archive members are overwritten when the archive is extracted.
4494 It also causes @command{tar} to extract the ownership, permissions,
4495 and time stamps onto any preexisting files or directories.
4496 If the name of a corresponding file name is a symbolic link, the file
4497 pointed to by the symbolic link will be overwritten instead of the
4498 symbolic link itself (if this is possible). Moreover, special devices,
4499 empty directories and even symbolic links are automatically removed if
4500 they are in the way of extraction.
4502 Be careful when using the @option{--overwrite} option, particularly when
4503 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
4504 can change the contents, ownership or permissions of any file on your
4505 system. Also, many systems do not take kindly to overwriting files that
4506 are currently being executed.
4508 @opindex overwrite-dir
4509 @item --overwrite-dir
4510 Overwrite the metadata of directories when extracting files from an
4511 archive, but remove other files before extracting.
4514 @node Keep Old Files
4515 @unnumberedsubsubsec Keep Old Files
4518 @opindex keep-old-files
4519 @item --keep-old-files
4521 Do not replace existing files from archive. The
4522 @option{--keep-old-files} (@option{-k}) option prevents @command{tar}
4523 from replacing existing files with files with the same name from the
4524 archive. The @option{--keep-old-files} option is meaningless with
4525 @option{--list} (@option{-t}). Prevents @command{tar} from replacing
4526 files in the file system during extraction.
4529 @node Keep Newer Files
4530 @unnumberedsubsubsec Keep Newer Files
4533 @opindex keep-newer-files
4534 @item --keep-newer-files
4535 Do not replace existing files that are newer than their archive
4536 copies. This option is meaningless with @option{--list} (@option{-t}).
4540 @unnumberedsubsubsec Unlink First
4543 @opindex unlink-first
4544 @item --unlink-first
4546 Remove files before extracting over them.
4547 This can make @command{tar} run a bit faster if you know in advance
4548 that the extracted files all need to be removed. Normally this option
4549 slows @command{tar} down slightly, so it is disabled by default.
4552 @node Recursive Unlink
4553 @unnumberedsubsubsec Recursive Unlink
4556 @opindex recursive-unlink
4557 @item --recursive-unlink
4558 When this option is specified, try removing files and directory hierarchies
4559 before extracting over them. @emph{This is a dangerous option!}
4562 If you specify the @option{--recursive-unlink} option,
4563 @command{tar} removes @emph{anything} that keeps you from extracting a file
4564 as far as current permissions will allow it. This could include removal
4565 of the contents of a full directory hierarchy.
4567 @node Data Modification Times
4568 @unnumberedsubsubsec Setting Data Modification Times
4570 @cindex Data modification times of extracted files
4571 @cindex Modification times of extracted files
4572 Normally, @command{tar} sets the data modification times of extracted
4573 files to the corresponding times recorded for the files in the archive, but
4574 limits the permissions of extracted files by the current @code{umask}
4577 To set the data modification times of extracted files to the time when
4578 the files were extracted, use the @option{--touch} (@option{-m}) option in
4579 conjunction with @option{--extract} (@option{--get}, @option{-x}).
4585 Sets the data modification time of extracted archive members to the time
4586 they were extracted, not the time recorded for them in the archive.
4587 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
4590 @node Setting Access Permissions
4591 @unnumberedsubsubsec Setting Access Permissions
4593 @cindex Permissions of extracted files
4594 @cindex Modes of extracted files
4595 To set the modes (access permissions) of extracted files to those
4596 recorded for those files in the archive, use @option{--same-permissions}
4597 in conjunction with the @option{--extract} (@option{--get},
4598 @option{-x}) operation.
4601 @opindex preserve-permission
4602 @opindex same-permission
4603 @item --preserve-permission
4604 @itemx --same-permission
4605 @c @itemx --ignore-umask
4607 Set modes of extracted archive members to those recorded in the
4608 archive, instead of current umask settings. Use in conjunction with
4609 @option{--extract} (@option{--get}, @option{-x}).
4612 @node Directory Modification Times and Permissions
4613 @unnumberedsubsubsec Directory Modification Times and Permissions
4615 After sucessfully extracting a file member, @GNUTAR{} normally
4616 restores its permissions and modification times, as described in the
4617 previous sections. This cannot be done for directories, because
4618 after extracting a directory @command{tar} will almost certainly
4619 extract files into that directory and this will cause the directory
4620 modification time to be updated. Moreover, restoring that directory
4621 permissions may not permit file creation within it. Thus, restoring
4622 directory permissions and modification times must be delayed at least
4623 until all files have been extracted into that directory. @GNUTAR{}
4624 restores directories using the following approach.
4626 The extracted directories are created with the mode specified in the
4627 archive, as modified by the umask of the user, which gives sufficient
4628 permissions to allow file creation. The meta-information about the
4629 directory is recorded in the temporary list of directories. When
4630 preparing to extract next archive member, @GNUTAR{} checks if the
4631 directory prefix of this file contains the remembered directory. If
4632 it does not, the program assumes that all files have been extracted
4633 into that directory, restores its modification time and permissions
4634 and removes its entry from the internal list. This approach allows
4635 to correctly restore directory meta-information in the majority of
4636 cases, while keeping memory requirements sufficiently small. It is
4637 based on the fact, that most @command{tar} archives use the predefined
4638 order of members: first the directory, then all the files and
4639 subdirectories in that directory.
4641 However, this is not always true. The most important exception are
4642 incremental archives (@pxref{Incremental Dumps}). The member order in
4643 an incremental archive is reversed: first all directory members are
4644 stored, followed by other (non-directory) members. So, when extracting
4645 from incremental archives, @GNUTAR{} alters the above procedure. It
4646 remebers all restored directories, and restores their meta-data
4647 only after the entire archive has been processed. Notice, that you do
4648 not need to specity any special options for that, as @GNUTAR{}
4649 automatically detects archives in incremental format.
4651 There may be cases, when such processing is required for normal archives
4652 too. Consider the following example:
4656 $ @kbd{tar --no-recursion -cvf archive \
4657 foo foo/file1 bar bar/file foo/file2}
4666 During the normal operation, after encountering @file{bar}
4667 @GNUTAR{} will assume that all files from the directory @file{foo}
4668 were already extracted and will therefore restore its timestamp and
4669 permission bits. However, after extracting @file{foo/file2} the
4670 directory timestamp will be offset again.
4672 To correctly restore directory meta-information in such cases, use
4673 @option{delay-directory-restore} command line option:
4676 @opindex delay-directory-restore
4677 @item --delay-directory-restore
4678 Delays restoring of the modification times and permissions of extracted
4679 directories until the end of extraction. This way, correct
4680 meta-information is restored even if the archive has unusual member
4683 @opindex no-delay-directory-restore
4684 @item --no-delay-directory-restore
4685 Cancel the effect of the previous @option{--delay-directory-restore}.
4686 Use this option if you have used @option{--delay-directory-restore} in
4687 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
4688 temporarily disable it.
4691 @node Writing to Standard Output
4692 @unnumberedsubsubsec Writing to Standard Output
4694 @cindex Writing extracted files to standard output
4695 @cindex Standard output, writing extracted files to
4696 To write the extracted files to the standard output, instead of
4697 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
4698 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
4699 extracting files to send them through a pipe, and do not need to
4700 preserve them in the file system. If you extract multiple members,
4701 they appear on standard output concatenated, in the order they are
4702 found in the archive.
4708 Writes files to the standard output. Use only in conjunction with
4709 @option{--extract} (@option{--get}, @option{-x}). When this option is
4710 used, instead of creating the files specified, @command{tar} writes
4711 the contents of the files extracted to its standard output. This may
4712 be useful if you are only extracting the files in order to send them
4713 through a pipe. This option is meaningless with @option{--list}
4717 This can be useful, for example, if you have a tar archive containing
4718 a big file and don't want to store the file on disk before processing
4719 it. You can use a command like this:
4722 tar -xOzf foo.tgz bigfile | process
4725 or even like this if you want to process the concatenation of the files:
4728 tar -xOzf foo.tgz bigfile1 bigfile2 | process
4731 Hovewer, @option{--to-command} may be more convenient for use with
4732 multiple files. See the next section.
4734 @node Writing to an External Program
4735 @unnumberedsubsubsec Writing to an External Program
4737 You can instruct @command{tar} to send the contents of each extracted
4738 file to the standard input of an external program:
4742 @item --to-command=@var{command}
4743 Extract files and pipe their contents to the standard input of
4744 @var{command}. When this option is used, instead of creating the
4745 files specified, @command{tar} invokes @var{command} and pipes the
4746 contents of the files to its standard output. @var{Command} may
4747 contain command line arguments. The program is executed via
4748 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
4749 extracted. Non-regular files (directories, etc.) are ignored when this
4753 The command can obtain the information about the file it processes
4754 from the following environment variables:
4757 @vrindex TAR_FILETYPE, to-command environment
4759 Type of the file. It is a single letter with the following meaning:
4761 @multitable @columnfractions 0.10 0.90
4762 @item f @tab Regular file
4763 @item d @tab Directory
4764 @item l @tab Symbolic link
4765 @item h @tab Hard link
4766 @item b @tab Block device
4767 @item c @tab Character device
4770 Currently only regular files are supported.
4772 @vrindex TAR_MODE, to-command environment
4774 File mode, an octal number.
4776 @vrindex TAR_FILENAME, to-command environment
4778 The name of the file.
4780 @vrindex TAR_REALNAME, to-command environment
4782 Name of the file as stored in the archive.
4784 @vrindex TAR_UNAME, to-command environment
4786 Name of the file owner.
4788 @vrindex TAR_GNAME, to-command environment
4790 Name of the file owner group.
4792 @vrindex TAR_ATIME, to-command environment
4794 Time of last access. It is a decimal number, representing seconds
4795 since the epoch. If the archive provides times with nanosecond
4796 precision, the nanoseconds are appended to the timestamp after a
4799 @vrindex TAR_MTIME, to-command environment
4801 Time of last modification.
4803 @vrindex TAR_CTIME, to-command environment
4805 Time of last status change.
4807 @vrindex TAR_SIZE, to-command environment
4811 @vrindex TAR_UID, to-command environment
4813 UID of the file owner.
4815 @vrindex TAR_GID, to-command environment
4817 GID of the file owner.
4820 In addition to these variables, @env{TAR_VERSION} contains the
4821 @GNUTAR{} version number.
4823 If @var{command} exits with a non-0 status, @command{tar} will print
4824 an error message similar to the following:
4827 tar: 2345: Child returned status 1
4830 Here, @samp{2345} is the PID of the finished process.
4832 If this behavior is not wanted, use @option{--ignore-command-error}:
4835 @opindex ignore-command-error
4836 @item --ignore-command-error
4837 Ignore exit codes of subprocesses. Notice that if the program
4838 exits on signal or otherwise terminates abnormally, the error message
4839 will be printed even if this option is used.
4841 @opindex no-ignore-command-error
4842 @item --no-ignore-command-error
4843 Cancel the effect of any previous @option{--ignore-command-error}
4844 option. This option is useful if you have set
4845 @option{--ignore-command-error} in @env{TAR_OPTIONS}
4846 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
4850 @unnumberedsubsubsec Removing Files
4852 @FIXME{The section is too terse. Something more to add? An example,
4856 @opindex remove-files
4857 @item --remove-files
4858 Remove files after adding them to the archive.
4862 @subsection Coping with Scarce Resources
4865 @cindex Small memory
4866 @cindex Running out of space
4874 @unnumberedsubsubsec Starting File
4877 @opindex starting-file
4878 @item --starting-file=@var{name}
4879 @itemx -K @var{name}
4880 Starts an operation in the middle of an archive. Use in conjunction
4881 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
4884 @cindex Middle of the archive, starting in the
4885 If a previous attempt to extract files failed due to lack of disk
4886 space, you can use @option{--starting-file=@var{name}} (@option{-K
4887 @var{name}}) to start extracting only after member @var{name} of the
4888 archive. This assumes, of course, that there is now free space, or
4889 that you are now extracting into a different file system. (You could
4890 also choose to suspend @command{tar}, remove unnecessary files from
4891 the file system, and then restart the same @command{tar} operation.
4892 In this case, @option{--starting-file} is not necessary.
4893 @xref{Incremental Dumps}, @xref{interactive}, and @ref{exclude}.)
4896 @unnumberedsubsubsec Same Order
4899 @cindex Large lists of file names on small machines
4901 @opindex preserve-order
4903 @itemx --preserve-order
4905 To process large lists of file names on machines with small amounts of
4906 memory. Use in conjunction with @option{--compare} (@option{--diff},
4907 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
4908 (@option{--get}, @option{-x}).
4911 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
4912 names to be listed or extracted is sorted in the same order as the
4913 files in the archive. This allows a large list of names to be used,
4914 even on a small machine that would not otherwise be able to hold all
4915 the names in memory at the same time. Such a sorted list can easily be
4916 created by running @samp{tar -t} on the archive and editing its output.
4918 This option is probably never needed on modern computer systems.
4921 @section Backup options
4923 @cindex backup options
4925 @GNUTAR{} offers options for making backups of files
4926 before writing new versions. These options control the details of
4927 these backups. They may apply to the archive itself before it is
4928 created or rewritten, as well as individual extracted members. Other
4929 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
4930 and @command{mv}, for example) offer similar options.
4932 Backup options may prove unexpectedly useful when extracting archives
4933 containing many members having identical name, or when extracting archives
4934 on systems having file name limitations, making different members appear
4935 has having similar names through the side-effect of name truncation.
4936 (This is true only if we have a good scheme for truncated backup names,
4937 which I'm not sure at all: I suspect work is needed in this area.)
4938 When any existing file is backed up before being overwritten by extraction,
4939 then clashing files are automatically be renamed to be unique, and the
4940 true name is kept for only the last file of a series of clashing files.
4941 By using verbose mode, users may track exactly what happens.
4943 At the detail level, some decisions are still experimental, and may
4944 change in the future, we are waiting comments from our users. So, please
4945 do not learn to depend blindly on the details of the backup features.
4946 For example, currently, directories themselves are never renamed through
4947 using these options, so, extracting a file over a directory still has
4948 good chances to fail. Also, backup options apply to created archives,
4949 not only to extracted members. For created archives, backups will not
4950 be attempted when the archive is a block or character device, or when it
4951 refers to a remote file.
4953 For the sake of simplicity and efficiency, backups are made by renaming old
4954 files prior to creation or extraction, and not by copying. The original
4955 name is restored if the file creation fails. If a failure occurs after a
4956 partial extraction of a file, both the backup and the partially extracted
4960 @item --backup[=@var{method}]
4962 @vindex VERSION_CONTROL
4964 Back up files that are about to be overwritten or removed.
4965 Without this option, the original versions are destroyed.
4967 Use @var{method} to determine the type of backups made.
4968 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
4969 environment variable. And if @env{VERSION_CONTROL} is not set,
4970 use the @samp{existing} method.
4972 @vindex version-control @r{Emacs variable}
4973 This option corresponds to the Emacs variable @samp{version-control};
4974 the same values for @var{method} are accepted as in Emacs. This option
4975 also allows more descriptive names. The valid @var{method}s are:
4980 @cindex numbered @r{backup method}
4981 Always make numbered backups.
4985 @cindex existing @r{backup method}
4986 Make numbered backups of files that already have them, simple backups
4991 @cindex simple @r{backup method}
4992 Always make simple backups.
4996 @item --suffix=@var{suffix}
4998 @cindex backup suffix
4999 @vindex SIMPLE_BACKUP_SUFFIX
5000 Append @var{suffix} to each backup file made with @option{--backup}. If this
5001 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
5002 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
5003 set, the default is @samp{~}, just as in Emacs.
5007 Some people express the desire to @emph{always} use the @option{--backup}
5008 option, by defining some kind of alias or script. This is not as easy
5009 as one may think, due to the fact that old style options should appear first
5010 and consume arguments a bit unpredictably for an alias or script. But,
5011 if you are ready to give up using old style options, you may resort to
5012 using something like (a Bourne shell function here):
5015 tar () @{ /usr/local/bin/tar --backup $*; @}
5019 @section Notable @command{tar} Usages
5022 @FIXME{Using Unix file linking capability to recreate directory
5023 structures---linking files into one subdirectory and then
5024 @command{tar}ring that directory.}
5026 @FIXME{Nice hairy example using absolute-names, newer, etc.}
5029 You can easily use archive files to transport a group of files from
5030 one system to another: put all relevant files into an archive on one
5031 computer system, transfer the archive to another system, and extract
5032 the contents there. The basic transfer medium might be magnetic tape,
5033 Internet FTP, or even electronic mail (though you must encode the
5034 archive with @command{uuencode} in order to transport it properly by
5035 mail). Both machines do not have to use the same operating system, as
5036 long as they both support the @command{tar} program.
5038 For example, here is how you might copy a directory's contents from
5039 one disk to another, while preserving the dates, modes, owners and
5040 link-structure of all the files therein. In this case, the transfer
5041 medium is a @dfn{pipe}, which is one a Unix redirection mechanism:
5044 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
5048 You can avoid subshells by using @option{-C} option:
5051 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
5055 The command also works using short option forms:
5058 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
5059 | (cd targetdir; tar --extract --file=-)}
5061 $ @kbd{tar --directory sourcedir --create --file=- . ) \
5062 | tar --directory targetdir --extract --file=-}
5066 This is one of the easiest methods to transfer a @command{tar} archive.
5069 @section Looking Ahead: The Rest of this Manual
5071 You have now seen how to use all eight of the operations available to
5072 @command{tar}, and a number of the possible options. The next chapter
5073 explains how to choose and change file and archive names, how to use
5074 files to store names of other files which you can then call as
5075 arguments to @command{tar} (this can help you save time if you expect to
5076 archive the same list of files a number of times), and so forth.
5077 @FIXME{in case it's not obvious, i'm making this up in some sense
5078 based on my limited memory of what the next chapter *really* does. i
5079 just wanted to flesh out this final section a little bit so i'd
5080 remember to stick it in here. :-)}
5082 If there are too many files to conveniently list on the command line,
5083 you can list the names in a file, and @command{tar} will read that file.
5086 There are various ways of causing @command{tar} to skip over some files,
5087 and not archive them. @xref{Choosing}.
5090 @chapter Performing Backups and Restoring Files
5093 @GNUTAR{} is distributed along with the scripts
5094 which the Free Software Foundation uses for performing backups. There
5095 is no corresponding scripts available yet for doing restoration of
5096 files. Even if there is a good chance those scripts may be satisfying
5097 to you, they are not the only scripts or methods available for doing
5098 backups and restore. You may well create your own, or use more
5099 sophisticated packages dedicated to that purpose.
5101 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5102 Automatic Network Disk Archiver), a backup system developed by James
5103 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5104 This is free software, and it is available at these places:
5107 http://www.cs.umd.edu/projects/amanda/amanda.html
5108 ftp://ftp.cs.umd.edu/pub/amanda
5113 Here is a possible plan for a future documentation about the backuping
5114 scripts which are provided within the @GNUTAR{}
5120 @item what are dumps
5121 @item different levels of dumps
5123 @item full dump = dump everything
5124 @item level 1, level 2 dumps etc
5125 A level @var{n} dump dumps everything changed since the last level
5128 @item how to use scripts for dumps (ie, the concept)
5130 @item scripts to run after editing backup specs (details)
5132 @item Backup Specs, what is it.
5134 @item how to customize
5135 @item actual text of script [/sp/dump/backup-specs]
5139 @item rsh doesn't work
5140 @item rtape isn't installed
5143 @item the @option{--incremental} option of tar
5146 @item write protection
5147 @item types of media, different sizes and types, useful for different things
5148 @item files and tape marks
5149 one tape mark between files, two at end.
5150 @item positioning the tape
5151 MT writes two at end of write,
5152 backspaces over one when writing again.
5158 This chapter documents both the provided shell scripts and @command{tar}
5159 options which are more specific to usage as a backup tool.
5161 To @dfn{back up} a file system means to create archives that contain
5162 all the files in that file system. Those archives can then be used to
5163 restore any or all of those files (for instance if a disk crashes or a
5164 file is accidentally deleted). File system @dfn{backups} are also
5168 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5169 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5170 * Backup Levels:: Levels of Backups
5171 * Backup Parameters:: Setting Parameters for Backups and Restoration
5172 * Scripted Backups:: Using the Backup Scripts
5173 * Scripted Restoration:: Using the Restore Script
5177 @section Using @command{tar} to Perform Full Dumps
5183 @cindex corrupted archives
5184 Full dumps should only be made when no other people or programs
5185 are modifying files in the file system. If files are modified while
5186 @command{tar} is making the backup, they may not be stored properly in
5187 the archive, in which case you won't be able to restore them if you
5188 have to. (Files not being modified are written with no trouble, and do
5189 not corrupt the entire archive.)
5191 You will want to use the @option{--label=@var{archive-label}}
5192 (@option{-V @var{archive-label}}) option to give the archive a
5193 volume label, so you can tell what this archive is even if the label
5194 falls off the tape, or anything like that.
5196 Unless the file system you are dumping is guaranteed to fit on
5197 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5198 Make sure you have enough tapes on hand to complete the backup.
5200 If you want to dump each file system separately you will need to use
5201 the @option{--one-file-system} option to prevent
5202 @command{tar} from crossing file system boundaries when storing
5205 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5206 option is not needed, since this is a complete copy of everything in
5207 the file system, and a full restore from this backup would only be
5208 done onto a completely
5211 Unless you are in a hurry, and trust the @command{tar} program (and your
5212 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5213 option, to make sure your files really made it onto the dump properly.
5214 This will also detect cases where the file was modified while (or just
5215 after) it was being archived. Not all media (notably cartridge tapes)
5216 are capable of being verified, unfortunately.
5218 @node Incremental Dumps
5219 @section Using @command{tar} to Perform Incremental Dumps
5221 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5222 stores additional metadata so that exact state of the file system
5223 can be restored when extracting the archive.
5225 @GNUTAR{} currently offers two options for handling incremental
5226 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5227 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5229 @opindex listed-incremental
5230 The option @option{--listed-incremental} instructs tar to operate on
5231 an incremental archive with additional metadata stored in a standalone
5232 file, called a @dfn{snapshot file}. The purpose of this file is to help
5233 determine which files have been changed, added or deleted since the
5234 last backup, so that the next incremental backup will contain only
5235 modified files. The name of the snapshot file is given as an argument
5239 @item --listed-incremental=@var{file}
5240 @itemx -g @var{file}
5241 Handle incremental backups with snapshot data in @var{file}.
5244 To create an incremental backup, you would use
5245 @option{--listed-incremental} together with @option{--create}
5246 (@pxref{create}). For example:
5249 $ @kbd{tar --create \
5250 --file=archive.1.tar \
5251 --listed-incremental=/var/log/usr.snar \
5255 This will create in @file{archive.1.tar} an incremental backup of
5256 the @file{/usr} file system, storing additional metadata in the file
5257 @file{/var/log/usr.snar}. If this file does not exist, it will be
5258 created. The created archive will then be a @dfn{level 0 backup};
5259 please see the next section for more on backup levels.
5261 Otherwise, if the file @file{/var/log/usr.snar} exists, it
5262 determines which files are modified. In this case only these files will be
5263 stored in the archive. Suppose, for example, that after running the
5264 above command, you delete file @file{/usr/doc/old} and create
5265 directory @file{/usr/local/db} with the following contents:
5268 $ @kbd{ls /usr/local/db}
5273 Some time later you create another incremental backup. You will
5277 $ @kbd{tar --create \
5278 --file=archive.2.tar \
5279 --listed-incremental=/var/log/usr.snar \
5281 tar: usr/local/db: Directory is new
5288 The created archive @file{archive.2.tar} will contain only these
5289 three members. This archive is called a @dfn{level 1 backup}. Notice
5290 that @file{/var/log/usr.snar} will be updated with the new data, so if
5291 you plan to create more @samp{level 1} backups, it is necessary to
5292 create a working copy of the snapshot file before running
5293 @command{tar}. The above example will then be modified as follows:
5296 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
5297 $ @kbd{tar --create \
5298 --file=archive.2.tar \
5299 --listed-incremental=/var/log/usr.snar-1 \
5303 Incremental dumps depend crucially on time stamps, so the results are
5304 unreliable if you modify a file's time stamps during dumping (e.g.,
5305 with the @option{--atime-preserve=replace} option), or if you set the clock
5308 Metadata stored in snapshot files include device numbers, which,
5309 obviously is supposed to be a non-volatile value. However, it turns
5310 out that NFS devices have undependable values when an automounter
5311 gets in the picture. This can lead to a great deal of spurious
5312 redumping in incremental dumps, so it is somewhat useless to compare
5313 two NFS devices numbers over time. The solution implemented currently
5314 is to considers all NFS devices as being equal when it comes to
5315 comparing directories; this is fairly gross, but there does not seem
5316 to be a better way to go.
5318 Note that incremental archives use @command{tar} extensions and may
5319 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
5321 @opindex listed-incremental, using with @option{--extract}
5322 @opindex extract, using with @option{--listed-incremental}
5323 To extract from the incremental dumps, use
5324 @option{--listed-incremental} together with @option{--extract}
5325 option (@pxref{extracting files}). In this case, @command{tar} does
5326 not need to access snapshot file, since all the data necessary for
5327 extraction are stored in the archive itself. So, when extracting, you
5328 can give whatever argument to @option{--listed-incremental}, the usual
5329 practice is to use @option{--listed-incremental=/dev/null}.
5330 Alternatively, you can use @option{--incremental}, which needs no
5331 arguments. In general, @option{--incremental} (@option{-G}) can be
5332 used as a shortcut for @option{--listed-incremental} when listing or
5333 extracting incremental backups (for more information, regarding this
5334 option, @pxref{incremental-op}).
5336 When extracting from the incremental backup @GNUTAR{} attempts to
5337 restore the exact state the file system had when the archive was
5338 created. In particular, it will @emph{delete} those files in the file
5339 system that did not exist in their directories when the archive was
5340 created. If you have created several levels of incremental files,
5341 then in order to restore the exact contents the file system had when
5342 the last level was created, you will need to restore from all backups
5343 in turn. Continuing our example, to restore the state of @file{/usr}
5344 file system, one would do@footnote{Notice, that since both archives
5345 were created withouth @option{-P} option (@pxref{absolute}), these
5346 commands should be run from the root file system.}:
5349 $ @kbd{tar --extract \
5350 --listed-incremental=/dev/null \
5351 --file archive.1.tar}
5352 $ @kbd{tar --extract \
5353 --listed-incremental=/dev/null \
5354 --file archive.2.tar}
5357 To list the contents of an incremental archive, use @option{--list}
5358 (@pxref{list}), as usual. To obtain more information about the
5359 archive, use @option{--listed-incremental} or @option{--incremental}
5360 combined with two @option{--verbose} options@footnote{Two
5361 @option{--verbose} options were selected to avoid breaking usual
5362 verbose listing output (@option{--list --verbose}) when using in
5365 @opindex incremental, using with @option{--list}
5366 @opindex listed-incremental, using with @option{--list}
5367 @opindex list, using with @option{--incremental}
5368 @opindex list, using with @option{--listed-incremental}
5369 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
5370 contents of the DUMPDIR header (with terminating nulls) when
5371 @option{--incremental} or @option{--listed-incremental} option was
5372 given, no matter what the verbosity level. This behavior, and,
5373 especially, the binary output it produced were considered incovenient
5374 and were changed in version 1.16}:
5377 @kbd{tar --list --incremental --verbose --verbose archive.tar}
5380 This command will print, for each directory in the archive, the list
5381 of files in that directory at the time the archive was created. This
5382 information is put out in a format which is both human-readable and
5383 unambiguous for a program: each file name is printed as
5390 where @var{x} is a letter describing the status of the file: @samp{Y}
5391 if the file is present in the archive, @samp{N} if the file is not
5392 included in the archive, or a @samp{D} if the file is a directory (and
5393 is included in the archive).@FIXME-xref{dumpdir format}. Each such
5394 line is terminated by a newline character. The last line is followed
5395 by an additional newline to indicate the end of the data.
5397 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
5398 gives the same behavior as @option{--listed-incremental} when used
5399 with @option{--list} and @option{--extract} options. When used with
5400 @option{--create} option, it creates an incremental archive without
5401 creating snapshot file. Thus, it is impossible to create several
5402 levels of incremental backups with @option{--incremental} option.
5405 @section Levels of Backups
5407 An archive containing all the files in the file system is called a
5408 @dfn{full backup} or @dfn{full dump}. You could insure your data by
5409 creating a full dump every day. This strategy, however, would waste a
5410 substantial amount of archive media and user time, as unchanged files
5411 are daily re-archived.
5413 It is more efficient to do a full dump only occasionally. To back up
5414 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
5415 one} dump archives all the files that have changed since the last full
5418 A typical dump strategy would be to perform a full dump once a week,
5419 and a level one dump once a day. This means some versions of files
5420 will in fact be archived more than once, but this dump strategy makes
5421 it possible to restore a file system to within one day of accuracy by
5422 only extracting two archives---the last weekly (full) dump and the
5423 last daily (level one) dump. The only information lost would be in
5424 files changed or created since the last daily backup. (Doing dumps
5425 more than once a day is usually not worth the trouble).
5427 @GNUTAR{} comes with scripts you can use to do full
5428 and level-one (actually, even level-two and so on) dumps. Using
5429 scripts (shell programs) to perform backups and restoration is a
5430 convenient and reliable alternative to typing out file name lists
5431 and @command{tar} commands by hand.
5433 Before you use these scripts, you need to edit the file
5434 @file{backup-specs}, which specifies parameters used by the backup
5435 scripts and by the restore script. This file is usually located
5436 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
5437 detailed description. Once the backup parameters are set, you can
5438 perform backups or restoration by running the appropriate script.
5440 The name of the backup script is @code{backup}. The name of the
5441 restore script is @code{restore}. The following sections describe
5442 their use in detail.
5444 @emph{Please Note:} The backup and restoration scripts are
5445 designed to be used together. While it is possible to restore files by
5446 hand from an archive which was created using a backup script, and to create
5447 an archive by hand which could then be extracted using the restore script,
5448 it is easier to use the scripts. @xref{Incremental Dumps}, before
5449 making such an attempt.
5451 @node Backup Parameters
5452 @section Setting Parameters for Backups and Restoration
5454 The file @file{backup-specs} specifies backup parameters for the
5455 backup and restoration scripts provided with @command{tar}. You must
5456 edit @file{backup-specs} to fit your system configuration and schedule
5457 before using these scripts.
5459 Syntactically, @file{backup-specs} is a shell script, containing
5460 mainly variable assignments. However, any valid shell construct
5461 is allowed in this file. Particularly, you may wish to define
5462 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
5463 For more information about shell script syntax, please refer to
5464 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
5465 g_02, the definition of the Shell Command Language}. See also
5466 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
5468 The shell variables controlling behavior of @code{backup} and
5469 @code{restore} are described in the following subsections.
5472 * General-Purpose Variables::
5473 * Magnetic Tape Control::
5475 * backup-specs example:: An Example Text of @file{Backup-specs}
5478 @node General-Purpose Variables
5479 @subsection General-Purpose Variables
5481 @defvr {Backup variable} ADMINISTRATOR
5482 The user name of the backup administrator. @code{Backup} scripts
5483 sends a backup report to this address.
5486 @defvr {Backup variable} BACKUP_HOUR
5487 The hour at which the backups are done. This can be a number from 0
5488 to 23, or the time specification in form @var{hours}:@var{minutes},
5489 or the string @samp{now}.
5491 This variable is used by @code{backup}. Its value may be overridden
5492 using @option{--time} option (@pxref{Scripted Backups}).
5495 @defvr {Backup variable} TAPE_FILE
5497 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
5498 is a remote archive (@pxref{remote-dev}), backup script will suppose
5499 that your @command{mt} is able to access remote devices. If @var{RSH}
5500 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
5501 invocations of @command{mt}.
5504 @defvr {Backup variable} BLOCKING
5506 The blocking factor @command{tar} will use when writing the dump archive.
5507 @xref{Blocking Factor}.
5510 @defvr {Backup variable} BACKUP_DIRS
5512 A list of file systems to be dumped (for @code{backup}), or restored
5513 (for @code{restore}). You can include any directory
5514 name in the list --- subdirectories on that file system will be
5515 included, regardless of how they may look to other networked machines.
5516 Subdirectories on other file systems will be ignored.
5518 The host name specifies which host to run @command{tar} on, and should
5519 normally be the host that actually contains the file system. However,
5520 the host machine must have @GNUTAR{} installed, and
5521 must be able to access the directory containing the backup scripts and
5522 their support files using the same file name that is used on the
5523 machine where the scripts are run (ie. what @command{pwd} will print
5524 when in that directory on that machine). If the host that contains
5525 the file system does not have this capability, you can specify another
5526 host as long as it can access the file system through NFS.
5528 If the list of file systems is very long you may wish to put it
5529 in a separate file. This file is usually named
5530 @file{/etc/backup/dirs}, but this name may be overridden in
5531 @file{backup-specs} using @code{DIRLIST} variable.
5534 @defvr {Backup variable} DIRLIST
5536 A path to the file containing the list of the file systems to backup
5537 or restore. By default it is @file{/etc/backup/dirs}.
5540 @defvr {Backup variable} BACKUP_FILES
5542 A list of individual files to be dumped (for @code{backup}), or restored
5543 (for @code{restore}). These should be accessible from the machine on
5544 which the backup script is run.
5546 If the list of file systems is very long you may wish to store it
5547 in a separate file. This file is usually named
5548 @file{/etc/backup/files}, but this name may be overridden in
5549 @file{backup-specs} using @code{FILELIST} variable.
5552 @defvr {Backup variable} FILELIST
5554 A path to the file containing the list of the individual files to backup
5555 or restore. By default it is @file{/etc/backup/files}.
5558 @defvr {Backup variable} MT
5560 Full file name of @command{mt} binary.
5563 @defvr {Backup variable} RSH
5565 Full file name of @command{rsh} binary or its equivalent. You may wish to
5566 set it to @code{ssh}, to improve security. In this case you will have
5567 to use public key authentication.
5570 @defvr {Backup variable} RSH_COMMAND
5572 Full file name of @command{rsh} binary on remote mashines. This will
5573 be passed via @option{--rsh-command} option to the remote invocation
5577 @defvr {Backup variable} VOLNO_FILE
5579 Name of temporary file to hold volume numbers. This needs to be accessible
5580 by all the machines which have file systems to be dumped.
5583 @defvr {Backup variable} XLIST
5585 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
5586 located on the remote machine and containing the list of files to
5587 be excluded from the backup. Exclude file lists are searched in
5588 /etc/tar-backup directory. A common use for exclude file lists
5589 is to exclude files containing security-sensitive information
5590 (e.g., @file{/etc/shadow} from backups).
5592 This variable affects only @code{backup}.
5595 @defvr {Backup variable} SLEEP_TIME
5597 Time to sleep between dumps of any two successive file systems
5599 This variable affects only @code{backup}.
5602 @defvr {Backup variable} DUMP_REMIND_SCRIPT
5604 Script to be run when it's time to insert a new tape in for the next
5605 volume. Administrators may want to tailor this script for their site.
5606 If this variable isn't set, @GNUTAR{} will display its built-in prompt
5607 @FIXME-xref{describe it somewhere!}, and will expect confirmation from
5611 @defvr {Backup variable} SLEEP_MESSAGE
5613 Message to display on the terminal while waiting for dump time. Usually
5614 this will just be some literal text.
5617 @defvr {Backup variable} TAR
5619 Full file name of the @GNUTAR{} executable. If this is not set, backup
5620 scripts will search @command{tar} in the current shell path.
5623 @node Magnetic Tape Control
5624 @subsection Magnetic Tape Control
5626 Backup scripts access tape device using special @dfn{hook functions}.
5627 These functions take a single argument -- the name of the tape
5628 device. Their names are kept in the following variables:
5630 @defvr {Backup variable} MT_BEGIN
5631 The name of @dfn{begin} function. This function is called before
5632 accessing the drive. By default it retensions the tape:
5638 mt -f "$1" retension
5643 @defvr {Backup variable} MT_REWIND
5644 The name of @dfn{rewind} function. The default definition is as
5657 @defvr {Backup variable} MT_OFFLINE
5658 The name of the function switching the tape off line. By default
5659 it is defined as follows:
5662 MT_OFFLINE=mt_offline
5670 @defvr {Backup variable} MT_STATUS
5671 The name of the function used to obtain the status of the archive device,
5672 including error count. Default definition:
5684 @subsection User Hooks
5686 @dfn{User hooks} are shell functions executed before and after
5687 each @command{tar} invocation. Thus, there are @dfn{backup
5688 hooks}, which are executed before and after dumping each file
5689 system, and @dfn{restore hooks}, executed before and
5690 after restoring a file system. Each user hook is a shell function
5691 taking four arguments:
5693 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
5698 Current backup or restore level.
5701 Name or IP address of the host machine being dumped or restored.
5704 Full path name to the file system being dumped or restored.
5707 File system name with directory separators replaced with colons. This
5708 is useful, e.g., for creating unique files.
5712 Following variables keep the names of user hook functions
5714 @defvr {Backup variable} DUMP_BEGIN
5715 Dump begin function. It is executed before dumping the file system.
5718 @defvr {Backup variable} DUMP_END
5719 Executed after dumping the file system.
5722 @defvr {Backup variable} RESTORE_BEGIN
5723 Executed before restoring the file system.
5726 @defvr {Backup variable} RESTORE_END
5727 Executed after restoring the file system.
5730 @node backup-specs example
5731 @subsection An Example Text of @file{Backup-specs}
5733 The following is an example of @file{backup-specs}:
5736 # site-specific parameters for file system backup.
5738 ADMINISTRATOR=friedman
5740 TAPE_FILE=/dev/nrsmt0
5742 # Use @code{ssh} instead of the less secure @code{rsh}
5744 RSH_COMMAND=/usr/bin/ssh
5746 # Override MT_STATUS function:
5752 # Disable MT_OFFLINE function
5769 apple-gunkies:/com/mailer/gnu
5770 apple-gunkies:/com/archive/gnu"
5772 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
5776 @node Scripted Backups
5777 @section Using the Backup Scripts
5779 The syntax for running a backup script is:
5782 backup --level=@var{level} --time=@var{time}
5785 The @option{level} option requests the dump level. Thus, to produce
5786 a full dump, specify @code{--level=0} (this is the default, so
5787 @option{--level} may be omitted if its value is @code{0}).
5788 @footnote{For backward compatibility, the @code{backup} will also
5789 try to deduce the requested dump level from the name of the
5790 script itself. If the name consists of a string @samp{level-}
5791 followed by a single decimal digit, that digit is taken as
5792 the dump level number. Thus, you may create a link from @code{backup}
5793 to @code{level-1} and then run @code{level-1} whenever you need to
5794 create a level one dump.}
5796 The @option{--time} option determines when should the backup be
5797 run. @var{Time} may take three forms:
5800 @item @var{hh}:@var{mm}
5802 The dump must be run at @var{hh} hours @var{mm} minutes.
5806 The dump must be run at @var{hh} hours
5810 The dump must be run immediately.
5813 You should start a script with a tape or disk mounted. Once you
5814 start a script, it prompts you for new tapes or disks as it
5815 needs them. Media volumes don't have to correspond to archive
5816 files --- a multi-volume archive can be started in the middle of a
5817 tape that already contains the end of another multi-volume archive.
5818 The @code{restore} script prompts for media by its archive volume,
5819 so to avoid an error message you should keep track of which tape
5820 (or disk) contains which volume of the archive (@pxref{Scripted
5823 The backup scripts write two files on the file system. The first is a
5824 record file in @file{/etc/tar-backup/}, which is used by the scripts
5825 to store and retrieve information about which files were dumped. This
5826 file is not meant to be read by humans, and should not be deleted by
5827 them. @xref{Snapshot Files}, for a more detailed explanation of this
5830 The second file is a log file containing the names of the file systems
5831 and files dumped, what time the backup was made, and any error
5832 messages that were generated, as well as how much space was left in
5833 the media volume after the last volume of the archive was written.
5834 You should check this log file after every backup. The file name is
5835 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
5836 represents current date, and @var{n} represents current dump level number.
5838 The script also prints the name of each system being dumped to the
5841 Following is the full list of options accepted by @code{backup}
5845 @item -l @var{level}
5846 @itemx --level=@var{level}
5847 Do backup level @var{level} (default 0).
5851 Force backup even if today's log file already exists.
5853 @item -v[@var{level}]
5854 @itemx --verbose[=@var{level}]
5855 Set verbosity level. The higher the level is, the more debugging
5856 information will be output during execution. Devault @var{level}
5857 is 100, which means the highest debugging level.
5859 @item -t @var{start-time}
5860 @itemx --time=@var{start-time}
5861 Wait till @var{time}, then do backup.
5865 Display short help message and exit.
5869 Display information about the program's name, version, origin and legal
5870 status, all on standard output, and then exit successfully.
5874 @node Scripted Restoration
5875 @section Using the Restore Script
5877 To restore files that were archived using a scripted backup, use the
5878 @code{restore} script. Its usage is quite straightforward. In the
5879 simplest form, invoke @code{restore --all}, it will
5880 then restore all the file systems and files specified in
5881 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
5883 You may select the file systems (and/or files) to restore by
5884 giving @code{restore} list of @dfn{patterns} in its command
5885 line. For example, running
5892 will restore all file systems on the machine @samp{albert}. A more
5893 complicated example:
5896 restore 'albert:*' '*:/var'
5900 This command will restore all file systems on the machine @samp{albert}
5901 as well as @file{/var} file system on all machines.
5903 By default @code{restore} will start restoring files from the lowest
5904 available dump level (usually zero) and will continue through
5905 all available dump levels. There may be situations where such a
5906 thorough restore is not necessary. For example, you may wish to
5907 restore only files from the recent level one backup. To do so,
5908 use @option{--level} option, as shown in the example below:
5914 The full list of options accepted by @code{restore} follows:
5919 Restore all file systems and files specified in @file{backup-specs}
5921 @item -l @var{level}
5922 @itemx --level=@var{level}
5923 Start restoring from the given backup level, instead of the default 0.
5925 @item -v[@var{level}]
5926 @itemx --verbose[=@var{level}]
5927 Set verbosity level. The higher the level is, the more debugging
5928 information will be output during execution. Devault @var{level}
5929 is 100, which means the highest debugging level.
5933 Display short help message and exit.
5937 Display information about the program's name, version, origin and legal
5938 status, all on standard output, and then exit successfully.
5941 You should start the restore script with the media containing the
5942 first volume of the archive mounted. The script will prompt for other
5943 volumes as they are needed. If the archive is on tape, you don't need
5944 to rewind the tape to to its beginning---if the tape head is
5945 positioned past the beginning of the archive, the script will rewind
5946 the tape as needed. @FIXME-xref{Media, for a discussion of tape
5950 @strong{Warning:} The script will delete files from the active file
5951 system if they were not in the file system when the archive was made.
5954 @xref{Incremental Dumps}, for an explanation of how the script makes
5958 @chapter Choosing Files and Names for @command{tar}
5961 Certain options to @command{tar} enable you to specify a name for your
5962 archive. Other options let you decide which files to include or exclude
5963 from the archive, based on when or whether files were modified, whether
5964 the file names do or don't match specified patterns, or whether files
5965 are in specified directories.
5967 This chapter discusses these options in detail.
5970 * file:: Choosing the Archive's Name
5971 * Selecting Archive Members::
5972 * files:: Reading Names from a File
5973 * exclude:: Excluding Some Files
5974 * Wildcards:: Wildcards Patterns and Matching
5975 * after:: Operating Only on New Files
5976 * recurse:: Descending into Directories
5977 * one:: Crossing File System Boundaries
5981 @section Choosing and Naming Archive Files
5984 @cindex Naming an archive
5985 @cindex Archive Name
5986 @cindex Choosing an archive file
5987 @cindex Where is the archive?
5988 By default, @command{tar} uses an archive file name that was compiled when
5989 it was built on the system; usually this name refers to some physical
5990 tape drive on the machine. However, the person who installed @command{tar}
5991 on the system may not have set the default to a meaningful value as far as
5992 most users are concerned. As a result, you will usually want to tell
5993 @command{tar} where to find (or create) the archive. The
5994 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
5995 option allows you to either specify or name a file to use as the archive
5996 instead of the default archive file location.
5999 @opindex file, short description
6000 @item --file=@var{archive-name}
6001 @itemx -f @var{archive-name}
6002 Name the archive to create or operate on. Use in conjunction with
6006 For example, in this @command{tar} command,
6009 $ @kbd{tar -cvf collection.tar blues folk jazz}
6013 @file{collection.tar} is the name of the archive. It must directly
6014 follow the @option{-f} option, since whatever directly follows @option{-f}
6015 @emph{will} end up naming the archive. If you neglect to specify an
6016 archive name, you may end up overwriting a file in the working directory
6017 with the archive you create since @command{tar} will use this file's name
6018 for the archive name.
6020 An archive can be saved as a file in the file system, sent through a
6021 pipe or over a network, or written to an I/O device such as a tape,
6022 floppy disk, or CD write drive.
6024 @cindex Writing new archives
6025 @cindex Archive creation
6026 If you do not name the archive, @command{tar} uses the value of the
6027 environment variable @env{TAPE} as the file name for the archive. If
6028 that is not available, @command{tar} uses a default, compiled-in archive
6029 name, usually that for tape unit zero (ie. @file{/dev/tu00}).
6031 @cindex Standard input and output
6032 @cindex tar to standard input and output
6033 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
6034 archive from standard input (when listing or extracting files), or
6035 writes it to standard output (when creating an archive). If you use
6036 @file{-} as an @var{archive-name} when modifying an archive,
6037 @command{tar} reads the original archive from its standard input and
6038 writes the entire new archive to its standard output.
6040 The following example is a convenient way of copying directory
6041 hierarchy from @file{sourcedir} to @file{targetdir}.
6044 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
6047 The @option{-C} option allows to avoid using subshells:
6050 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
6053 In both examples above, the leftmost @command{tar} invocation archives
6054 the contents of @file{sourcedir} to the standard output, while the
6055 rightmost one reads this archive from its standard input and
6056 extracts it. The @option{-p} option tells it to restore permissions
6057 of the extracted files.
6059 @cindex Remote devices
6060 @cindex tar to a remote device
6062 To specify an archive file on a device attached to a remote machine,
6066 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
6070 @command{tar} will complete the remote connection, if possible, and
6071 prompt you for a username and password. If you use
6072 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
6073 will complete the remote connection, if possible, using your username
6074 as the username on the remote machine.
6076 @cindex Local and remote archives
6077 @anchor{local and remote archives}
6078 If the archive file name includes a colon (@samp{:}), then it is assumed
6079 to be a file on another machine. If the archive file is
6080 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6081 host @var{host}. The remote host is accessed using the @command{rsh}
6082 program, with a username of @var{user}. If the username is omitted
6083 (along with the @samp{@@} sign), then your user name will be used.
6084 (This is the normal @command{rsh} behavior.) It is necessary for the
6085 remote machine, in addition to permitting your @command{rsh} access, to
6086 have the @file{rmt} program installed (This command is included in
6087 the @GNUTAR{} distribution and by default is installed under
6088 @file{@var{prefix}/libexec/rmt}, were @var{prefix} means your
6089 installation prefix). If you need to use a file whose name includes a
6090 colon, then the remote tape drive behavior
6091 can be inhibited by using the @option{--force-local} option.
6093 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6094 tries to minimize input and output operations. The Amanda backup
6095 system, when used with @GNUTAR{}, has an initial sizing pass which
6098 @node Selecting Archive Members
6099 @section Selecting Archive Members
6100 @cindex Specifying files to act on
6101 @cindex Specifying archive members
6103 @dfn{File Name arguments} specify which files in the file system
6104 @command{tar} operates on, when creating or adding to an archive, or which
6105 archive members @command{tar} operates on, when reading or deleting from
6106 an archive. @xref{Operations}.
6108 To specify file names, you can include them as the last arguments on
6109 the command line, as follows:
6111 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6114 If a file name begins with dash (@samp{-}), precede it with
6115 @option{--add-file} option to prevent it from being treated as an
6118 If you specify a directory name as a file name argument, all the files
6119 in that directory are operated on by @command{tar}.
6121 If you do not specify files, @command{tar} behavior differs depending
6122 on the operation mode as described below:
6124 When @command{tar} is invoked with @option{--create} (@option{-c}),
6125 @command{tar} will stop immediately, reporting the following:
6129 $ @kbd{tar cf a.tar}
6130 tar: Cowardly refusing to create an empty archive
6131 Try `tar --help' or `tar --usage' for more information.
6135 If you specify either @option{--list} (@option{-t}) or
6136 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6137 operates on all the archive members in the archive.
6139 If run with @option{--diff} option, tar will compare the archive with
6140 the contents of the current working directory.
6142 If you specify any other operation, @command{tar} does nothing.
6144 By default, @command{tar} takes file names from the command line. However,
6145 there are other ways to specify file or member names, or to modify the
6146 manner in which @command{tar} selects the files or members upon which to
6147 operate. In general, these methods work both for specifying the names
6148 of files and archive members.
6151 @section Reading Names from a File
6153 @cindex Reading file names from a file
6154 @cindex Lists of file names
6155 @cindex File Name arguments, alternatives
6156 Instead of giving the names of files or archive members on the command
6157 line, you can put the names into a file, and then use the
6158 @option{--files-from=@var{file-of-names}} (@option{-T
6159 @var{file-of-names}}) option to @command{tar}. Give the name of the
6160 file which contains the list of files to include as the argument to
6161 @option{--files-from}. In the list, the file names should be separated by
6162 newlines. You will frequently use this option when you have generated
6163 the list of files to archive with the @command{find} utility.
6167 @item --files-from=@var{file-name}
6168 @itemx -T @var{file-name}
6169 Get names to extract or create from file @var{file-name}.
6172 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6173 you specify either @code{--files-from=-} or @code{-T -}), then the file
6174 names are read from standard input.
6176 Unless you are running @command{tar} with @option{--create}, you can not use
6177 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6180 Any number of @option{-T} options can be given in the command line.
6182 The following example shows how to use @command{find} to generate a list of
6183 files smaller than 400K in length and put that list into a file
6184 called @file{small-files}. You can then use the @option{-T} option to
6185 @command{tar} to specify the files from that file, @file{small-files}, to
6186 create the archive @file{little.tgz}. (The @option{-z} option to
6187 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
6191 $ @kbd{find . -size -400 -print > small-files}
6192 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
6196 In the file list given by @option{-T} option, any file name beginning
6197 with @samp{-} character is considered a @command{tar} option and is
6198 processed accordingly.@footnote{Versions of @GNUTAR{} up to 1.15.1
6199 recognized only @option{-C} option in file lists, and only if the
6200 option and its argument occupied two consecutive lines.} For example,
6201 the common use of this feature is to change to another directory by
6202 specifying @option{-C} option:
6212 $ @kbd{tar -c -f foo.tar --files-from list}
6217 In this example, @command{tar} will first switch to @file{/etc}
6218 directory and add files @file{passwd} and @file{hosts} to the
6219 archive. Then it will change to @file{/lib} directory and will archive
6220 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
6225 $ @kbd{tar tf foo.tar}
6233 @opindex directory, using in @option{--files-from} argument
6234 Notice that the option parsing algorithm used with @option{-T} is
6235 stricter than the one used by shell. Namely, when specifying option
6236 arguments, you should observe the following rules:
6240 When using short (single-letter) option form, its argument must
6241 immediately follow the option letter, without any intervening
6242 whitespace. For example: @code{-Cdir}.
6245 When using long option form, the option argument must be separated
6246 from the option by a single equal sign. No whitespace is allowed on
6247 any side of the equal sign. For example: @code{--directory=dir}.
6250 For both short and long option forms, the option argument can be given
6251 on the next line after the option name, e.g.:
6272 If you happen to have a file whose name starts with @samp{-},
6273 precede it with @option{--add-file} option to prevent it from
6274 being recognized as an option. For example: @code{--add-file=--my-file}.
6281 @subsection @code{NUL} Terminated File Names
6283 @cindex File names, terminated by @code{NUL}
6284 @cindex @code{NUL} terminated file names
6285 The @option{--null} option causes
6286 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
6287 to read file names terminated by a @code{NUL} instead of a newline, so
6288 files whose names contain newlines can be archived using
6289 @option{--files-from}.
6294 Only consider @code{NUL} terminated file names, instead of files that
6295 terminate in a newline.
6298 The @option{--null} option is just like the one in @acronym{GNU}
6299 @command{xargs} and @command{cpio}, and is useful with the
6300 @option{-print0} predicate of @acronym{GNU} @command{find}. In
6301 @command{tar}, @option{--null} also disables special handling for
6302 file names that begin with dash.
6304 This example shows how to use @command{find} to generate a list of files
6305 larger than 800K in length and put that list into a file called
6306 @file{long-files}. The @option{-print0} option to @command{find} is just
6307 like @option{-print}, except that it separates files with a @code{NUL}
6308 rather than with a newline. You can then run @command{tar} with both the
6309 @option{--null} and @option{-T} options to specify that @command{tar} get the
6310 files from that file, @file{long-files}, to create the archive
6311 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
6312 @command{tar} to recognize the @code{NUL} separator between files.
6315 $ @kbd{find . -size +800 -print0 > long-files}
6316 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
6319 @FIXME{say anything else here to conclude the section?}
6322 @section Excluding Some Files
6325 @cindex File names, excluding files by
6326 @cindex Excluding files by name and pattern
6327 @cindex Excluding files by file system
6328 To avoid operating on files whose names match a particular pattern,
6329 use the @option{--exclude} or @option{--exclude-from} options.
6333 @item --exclude=@var{pattern}
6334 Causes @command{tar} to ignore files that match the @var{pattern}.
6338 The @option{--exclude=@var{pattern}} option prevents any file or
6339 member whose name matches the shell wildcard (@var{pattern}) from
6341 For example, to create an archive with all the contents of the directory
6342 @file{src} except for files whose names end in @file{.o}, use the
6343 command @samp{tar -cf src.tar --exclude='*.o' src}.
6345 You may give multiple @option{--exclude} options.
6348 @opindex exclude-from
6349 @item --exclude-from=@var{file}
6350 @itemx -X @var{file}
6351 Causes @command{tar} to ignore files that match the patterns listed in
6355 @findex exclude-from
6356 Use the @option{--exclude-from} option to read a
6357 list of patterns, one per line, from @var{file}; @command{tar} will
6358 ignore files matching those patterns. Thus if @command{tar} is
6359 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
6360 single line @file{*.o}, no files whose names end in @file{.o} will be
6361 added to the archive.
6364 @opindex exclude-caches
6365 @item --exclude-caches
6366 Causes @command{tar} to ignore directories containing a cache directory tag.
6369 @findex exclude-caches
6370 When creating an archive, the @option{--exclude-caches} option causes
6371 @command{tar} to exclude all directories that contain a @dfn{cache
6372 directory tag}. A cache directory tag is a short file with the
6373 well-known name @file{CACHEDIR.TAG} and having a standard header
6374 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
6375 Various applications write cache directory tags into directories they
6376 use to hold regenerable, non-precious data, so that such data can be
6377 more easily excluded from backups.
6380 * problems with exclude::
6383 @node problems with exclude
6384 @unnumberedsubsec Problems with Using the @code{exclude} Options
6386 @opindex exclude, potential problems with
6387 Some users find @samp{exclude} options confusing. Here are some common
6392 The main operating mode of @command{tar} does not act on a path name
6393 explicitly listed on the command line if one of its file name
6394 components is excluded. In the example above, if
6395 you create an archive and exclude files that end with @samp{*.o}, but
6396 explicitly name the file @samp{dir.o/foo} after all the options have been
6397 listed, @samp{dir.o/foo} will be excluded from the archive.
6400 You can sometimes confuse the meanings of @option{--exclude} and
6401 @option{--exclude-from}. Be careful: use @option{--exclude} when files
6402 to be excluded are given as a pattern on the command line. Use
6403 @option{--exclude-from} to introduce the name of a file which contains
6404 a list of patterns, one per line; each of these patterns can exclude
6405 zero, one, or many files.
6408 When you use @option{--exclude=@var{pattern}}, be sure to quote the
6409 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
6410 like @samp{*}. If you do not do this, the shell might expand the
6411 @samp{*} itself using files at hand, so @command{tar} might receive a
6412 list of files instead of one pattern, or none at all, making the
6413 command somewhat illegal. This might not correspond to what you want.
6418 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
6426 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
6430 You must use use shell syntax, or globbing, rather than @code{regexp}
6431 syntax, when using exclude options in @command{tar}. If you try to use
6432 @code{regexp} syntax to describe files to be excluded, your command
6436 @FIXME{The change in semantics must have occurred before 1.11,
6437 so I doubt if it is worth mentioning at all. Anyway, should at
6438 least specify in which version the semantics changed.}
6439 In earlier versions of @command{tar}, what is now the
6440 @option{--exclude-from} option was called @option{--exclude} instead.
6441 Now, @option{--exclude} applies to patterns listed on the command
6442 line and @option{--exclude-from} applies to patterns listed in a
6448 @section Wildcards Patterns and Matching
6450 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
6451 @samp{*} or @samp{?} for example, are replaced and expanded into all
6452 existing files matching the given pattern. @GNUTAR{} can use wildcard
6453 patterns for matching (or globbing) archive members when extracting
6454 from or listing an archive. Wildcard patterns are also used for
6455 verifying volume labels of @command{tar} archives. This section has the
6456 purpose of explaining wildcard syntax for @command{tar}.
6458 @FIXME{the next few paragraphs need work.}
6460 A @var{pattern} should be written according to shell syntax, using wildcard
6461 characters to effect globbing. Most characters in the pattern stand
6462 for themselves in the matched string, and case is significant: @samp{a}
6463 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
6464 pattern matches any single character in the matched string. The character
6465 @samp{*} in the pattern matches zero, one, or more single characters in
6466 the matched string. The character @samp{\} says to take the following
6467 character of the pattern @emph{literally}; it is useful when one needs to
6468 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
6470 The character @samp{[}, up to the matching @samp{]}, introduces a character
6471 class. A @dfn{character class} is a list of acceptable characters
6472 for the next single character of the matched string. For example,
6473 @samp{[abcde]} would match any of the first five letters of the alphabet.
6474 Note that within a character class, all of the ``special characters''
6475 listed above other than @samp{\} lose their special meaning; for example,
6476 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
6477 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
6478 the characters @samp{-} and @samp{]} must either come @emph{first} or
6479 @emph{last} in a character class.)
6481 @cindex Excluding characters from a character class
6482 @cindex Character class, excluding characters from
6483 If the first character of the class after the opening @samp{[}
6484 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
6485 Rather than listing character to match, it lists those characters which
6486 are @emph{forbidden} as the next single character of the matched string.
6488 Other characters of the class stand for themselves. The special
6489 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
6490 letters, is meant to represent all characters between @var{a} and
6493 @FIXME{need to add a sentence or so here to make this clear for those
6494 who don't have dan around.}
6496 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
6497 special for wildcard matches. However, if a pattern completely matches
6498 a directory prefix of a matched string, then it matches the full matched
6499 string: thus, excluding a directory also excludes all the files beneath it.
6502 * controlling pattern-matching::
6505 @node controlling pattern-matching
6506 @unnumberedsubsec Controlling Pattern-Matching
6508 For the purposes of this section, we call @dfn{exclusion members} all
6509 member names obtained while processing @option{--exclude} and
6510 @option{--exclude-from} options, and @dfn{inclusion members} those
6511 member names that were given in the command line or read from the file
6512 specified with @option{--files-from} option.
6514 These two pairs of member lists are used in the following operations:
6515 @option{--diff}, @option{--extract}, @option{--list},
6518 There are no inclusion members in create mode (@option{--create} and
6519 @option{--append}), since in this mode the names obtained from the
6520 command line refer to @emph{files}, not archive members.
6522 By default, inclusion members are compared with archive members
6523 literally @footnote{Notice that earlier @GNUTAR{} versions used
6524 globbing for inclusion members, which contradicted to UNIX98
6525 specification and was not documented. @xref{Changes}, for more
6526 information on this and other changes} and exclusion members are
6527 treated as globbing patterns. For example:
6531 $ @kbd{tar tf foo.tar}
6536 # @i{Member names are used verbatim:}
6537 $ @kbd{tar -xf foo.tar -v '[remarks]'}
6539 # @i{Exclude member names are globbed:}
6540 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
6546 This behavior can be altered by using the following options:
6551 Treat all member names as wildcards.
6553 @opindex no-wildcards
6554 @item --no-wildcards
6555 Treat all member names as literal strings.
6558 Thus, to extract files whose names end in @samp{.c}, you can use:
6561 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
6567 Notice quoting of the pattern to prevent the shell from interpreting
6570 The effect of @option{--wildcards} option is cancelled by
6571 @option{--no-wildcards}. This can be used to pass part of
6572 the command line arguments verbatim and other part as globbing
6573 patterns. For example, the following invocation:
6576 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
6580 instructs @command{tar} to extract from @file{foo.tar} all files whose
6581 names end in @samp{.txt} and the file named @file{[remarks]}.
6583 Normally, a pattern matches a name if an initial subsequence of the
6584 name's components matches the pattern, where @samp{*}, @samp{?}, and
6585 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
6586 and wildcards can match @samp{/}.
6588 Other than optionally stripping leading @samp{/} from names
6589 (@pxref{absolute}), patterns and names are used as-is. For
6590 example, trailing @samp{/} is not trimmed from a user-specified name
6591 before deciding whether to exclude it.
6593 However, this matching procedure can be altered by the options listed
6594 below. These options accumulate. For example:
6597 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
6600 ignores case when excluding @samp{makefile}, but not when excluding
6605 @opindex no-anchored
6607 @itemx --no-anchored
6608 If anchored, a pattern must match an initial subsequence
6609 of the name's components. Otherwise, the pattern can match any
6610 subsequence. Default is @option{--no-anchored} for exclusion members
6611 and @option{--anchored} inclusion members.
6613 @opindex ignore-case
6614 @opindex no-ignore-case
6616 @itemx --no-ignore-case
6617 When ignoring case, upper-case patterns match lower-case names and vice versa.
6618 When not ignoring case (the default), matching is case-sensitive.
6620 @opindex wildcards-match-slash
6621 @opindex no-wildcards-match-slash
6622 @item --wildcards-match-slash
6623 @itemx --no-wildcards-match-slash
6624 When wildcards match slash (the default for exclusion members), a
6625 wildcard like @samp{*} in the pattern can match a @samp{/} in the
6626 name. Otherwise, @samp{/} is matched only by @samp{/}.
6630 The @option{--recursion} and @option{--no-recursion} options
6631 (@pxref{recurse}) also affect how member patterns are interpreted. If
6632 recursion is in effect, a pattern matches a name if it matches any of
6633 the name's parent directories.
6635 The following table summarizes pattern-matching default values:
6637 @multitable @columnfractions .3 .7
6638 @headitem Members @tab Default settings
6639 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
6640 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
6644 @section Operating Only on New Files
6647 @cindex Excluding file by age
6648 @cindex Data Modification time, excluding files by
6649 @cindex Modification time, excluding files by
6650 @cindex Age, excluding files by
6651 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
6652 @option{-N @var{date}}) option causes @command{tar} to only work on
6653 files whose data modification or status change times are newer than
6654 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
6655 it is taken to be a file name; the data modification time of that file
6656 is used as the date. If you use this option when creating or appending
6657 to an archive, the archive will only include new files. If you use
6658 @option{--after-date} when extracting an archive, @command{tar} will
6659 only extract files newer than the @var{date} you specify.
6661 If you only want @command{tar} to make the date comparison based on
6662 modification of the file's data (rather than status
6663 changes), then use the @option{--newer-mtime=@var{date}} option.
6665 You may use these options with any operation. Note that these options
6666 differ from the @option{--update} (@option{-u}) operation in that they
6667 allow you to specify a particular date against which @command{tar} can
6668 compare when deciding whether or not to archive the files.
6673 @item --after-date=@var{date}
6674 @itemx --newer=@var{date}
6675 @itemx -N @var{date}
6676 Only store files newer than @var{date}.
6678 Acts on files only if their data modification or status change times are
6679 later than @var{date}. Use in conjunction with any operation.
6681 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
6682 name; the data modification time of that file is used as the date.
6684 @opindex newer-mtime
6685 @item --newer-mtime=@var{date}
6686 Acts like @option{--after-date}, but only looks at data modification times.
6689 These options limit @command{tar} to operate only on files which have
6690 been modified after the date specified. A file's status is considered to have
6691 changed if its contents have been modified, or if its owner,
6692 permissions, and so forth, have been changed. (For more information on
6693 how to specify a date, see @ref{Date input formats}; remember that the
6694 entire date argument must be quoted if it contains any spaces.)
6696 Gurus would say that @option{--after-date} tests both the data
6697 modification time (@code{mtime}, the time the contents of the file
6698 were last modified) and the status change time (@code{ctime}, the time
6699 the file's status was last changed: owner, permissions, etc.@:)
6700 fields, while @option{--newer-mtime} tests only the @code{mtime}
6703 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
6704 @code{ctime} and processes the file if either one is more recent than
6705 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
6706 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
6707 contents of the file were looked at).
6709 Date specifiers can have embedded spaces. Because of this, you may need
6710 to quote date arguments to keep the shell from parsing them as separate
6711 arguments. For example, the following command will add to the archive
6712 all the files modified less than two days ago:
6715 $ @kbd{tar -cf foo.tar --newer-mtime '2 days ago'}
6719 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
6720 should not be used for incremental backups. @xref{Incremental Dumps},
6721 for proper way of creating incremental backups.
6725 @section Descending into Directories
6727 @cindex Avoiding recursion in directories
6728 @cindex Descending directories, avoiding
6729 @cindex Directories, avoiding recursion
6730 @cindex Recursion in directories, avoiding
6732 @FIXME{arrggh! this is still somewhat confusing to me. :-< }
6734 Usually, @command{tar} will recursively explore all directories (either
6735 those given on the command line or through the @option{--files-from}
6736 option) for the various files they contain. However, you may not always
6737 want @command{tar} to act this way.
6739 @opindex no-recursion
6740 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
6741 into specified directories. If you specify @option{--no-recursion}, you can
6742 use the @command{find} utility for hunting through levels of directories to
6743 construct a list of file names which you could then pass to @command{tar}.
6744 @command{find} allows you to be more selective when choosing which files to
6745 archive; see @ref{files}, for more information on using @command{find} with
6746 @command{tar}, or look.
6749 @item --no-recursion
6750 Prevents @command{tar} from recursively descending directories.
6754 Requires @command{tar} to recursively descend directories.
6755 This is the default.
6758 When you use @option{--no-recursion}, @GNUTAR{} grabs
6759 directory entries themselves, but does not descend on them
6760 recursively. Many people use @command{find} for locating files they
6761 want to back up, and since @command{tar} @emph{usually} recursively
6762 descends on directories, they have to use the @samp{@w{-not -type d}}
6763 test in their @command{find} invocation (@pxref{Type, Type, Type test,
6764 find, Finding Files}), as they usually do not want all the files in a
6765 directory. They then use the @option{--files-from} option to archive
6766 the files located via @command{find}.
6768 The problem when restoring files archived in this manner is that the
6769 directories themselves are not in the archive; so the
6770 @option{--same-permissions} (@option{--preserve-permissions},
6771 @option{-p}) option does not affect them---while users might really
6772 like it to. Specifying @option{--no-recursion} is a way to tell
6773 @command{tar} to grab only the directory entries given to it, adding
6774 no new files on its own. To summarize, if you use @command{find} to
6775 create a list of files to be stored in an archive, use it as follows:
6779 $ @kbd{find @var{dir} @var{tests} | \
6780 tar -cf @var{archive} -T - --no-recursion}
6784 The @option{--no-recursion} option also applies when extracting: it
6785 causes @command{tar} to extract only the matched directory entries, not
6786 the files under those directories.
6788 The @option{--no-recursion} option also affects how globbing patterns
6789 are interpreted (@pxref{controlling pattern-matching}).
6791 The @option{--no-recursion} and @option{--recursion} options apply to
6792 later options and operands, and can be overridden by later occurrences
6793 of @option{--no-recursion} and @option{--recursion}. For example:
6796 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
6800 creates an archive with one entry for @file{grape}, and the recursive
6801 contents of @file{grape/concord}, but no entries under @file{grape}
6802 other than @file{grape/concord}.
6805 @section Crossing File System Boundaries
6806 @cindex File system boundaries, not crossing
6809 @command{tar} will normally automatically cross file system boundaries in
6810 order to archive files which are part of a directory tree. You can
6811 change this behavior by running @command{tar} and specifying
6812 @option{--one-file-system}. This option only affects files that are
6813 archived because they are in a directory that is being archived;
6814 @command{tar} will still archive files explicitly named on the command line
6815 or through @option{--files-from}, regardless of where they reside.
6818 @opindex one-file-system
6819 @item --one-file-system
6820 Prevents @command{tar} from crossing file system boundaries when
6821 archiving. Use in conjunction with any write operation.
6824 The @option{--one-file-system} option causes @command{tar} to modify its
6825 normal behavior in archiving the contents of directories. If a file in
6826 a directory is not on the same file system as the directory itself, then
6827 @command{tar} will not archive that file. If the file is a directory
6828 itself, @command{tar} will not archive anything beneath it; in other words,
6829 @command{tar} will not cross mount points.
6831 This option is useful for making full or incremental archival backups of
6832 a file system. If this option is used in conjunction with
6833 @option{--verbose} (@option{-v}), files that are excluded are
6834 mentioned by name on the standard error.
6837 * directory:: Changing Directory
6838 * absolute:: Absolute File Names
6842 @subsection Changing the Working Directory
6845 @FIXME{need to read over this node now for continuity; i've switched
6846 things around some.}
6848 @cindex Changing directory mid-stream
6849 @cindex Directory, changing mid-stream
6850 @cindex Working directory, specifying
6851 To change the working directory in the middle of a list of file names,
6852 either on the command line or in a file specified using
6853 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
6854 This will change the working directory to the specified directory
6855 after that point in the list.
6859 @item --directory=@var{directory}
6860 @itemx -C @var{directory}
6861 Changes the working directory in the middle of a command line.
6867 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
6871 will place the files @file{grape} and @file{prune} from the current
6872 directory into the archive @file{jams.tar}, followed by the file
6873 @file{cherry} from the directory @file{food}. This option is especially
6874 useful when you have several widely separated files that you want to
6875 store in the same archive.
6877 Note that the file @file{cherry} is recorded in the archive under the
6878 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
6879 archive will contain three files that all appear to have come from the
6880 same directory; if the archive is extracted with plain @samp{tar
6881 --extract}, all three files will be written in the current directory.
6883 Contrast this with the command,
6886 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
6890 which records the third file in the archive under the name
6891 @file{red/cherry} so that, if the archive is extracted using
6892 @samp{tar --extract}, the third file will be written in a subdirectory
6893 named @file{orange-colored}.
6895 You can use the @option{--directory} option to make the archive
6896 independent of the original name of the directory holding the files.
6897 The following command places the files @file{/etc/passwd},
6898 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
6902 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
6906 However, the names of the archive members will be exactly what they were
6907 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
6908 They will not appear to be related by file name to the original
6909 directories where those files were located.
6911 Note that @option{--directory} options are interpreted consecutively. If
6912 @option{--directory} specifies a relative file name, it is interpreted
6913 relative to the then current directory, which might not be the same as
6914 the original current working directory of @command{tar}, due to a previous
6915 @option{--directory} option.
6917 When using @option{--files-from} (@pxref{files}), you can put various
6918 @command{tar} options (including @option{-C}) in the file list. Notice,
6919 however, that in this case the option and its argument may not be
6920 separated by whitespace. If you use short option, its argument must
6921 either follow the option letter immediately, without any intervening
6922 whitespace, or occupy the next line. Otherwise, if you use long
6923 option, separate its argument by an equal sign.
6925 For instance, the file list for the above example will be:
6940 To use it, you would invoke @command{tar} as follows:
6943 $ @kbd{tar -c -f foo.tar --files-from list}
6946 Notice also that you can only use the short option variant in the file
6947 list, i.e., always use @option{-C}, not @option{--directory}.
6949 The interpretation of @option{--directory} is disabled by
6950 @option{--null} option.
6953 @subsection Absolute File Names
6957 @opindex absolute-names
6958 @item --absolute-names
6960 Do not strip leading slashes from file names, and permit file names
6961 containing a @file{..} file name component.
6964 By default, @GNUTAR{} drops a leading @samp{/} on
6965 input or output, and complains about file names containing a @file{..}
6966 component. This option turns off this behavior.
6968 When @command{tar} extracts archive members from an archive, it strips any
6969 leading slashes (@samp{/}) from the member name. This causes absolute
6970 member names in the archive to be treated as relative file names. This
6971 allows you to have such members extracted wherever you want, instead of
6972 being restricted to extracting the member in the exact directory named
6973 in the archive. For example, if the archive member has the name
6974 @file{/etc/passwd}, @command{tar} will extract it as if the name were
6975 really @file{etc/passwd}.
6977 File names containing @file{..} can cause problems when extracting, so
6978 @command{tar} normally warns you about such files when creating an
6979 archive, and rejects attempts to extracts such files.
6981 Other @command{tar} programs do not do this. As a result, if you
6982 create an archive whose member names start with a slash, they will be
6983 difficult for other people with a non-@GNUTAR{}
6984 program to use. Therefore, @GNUTAR{} also strips
6985 leading slashes from member names when putting members into the
6986 archive. For example, if you ask @command{tar} to add the file
6987 @file{/bin/ls} to an archive, it will do so, but the member name will
6988 be @file{bin/ls}.@footnote{A side effect of this is that when
6989 @option{--create} is used with @option{--verbose} the resulting output
6990 is not, generally speaking, the same as the one you'd get running
6991 @kbd{tar --list} command. This may be important if you use some
6992 scripts for comparing both outputs. @xref{listing member and file names},
6993 for the information on how to handle this case.}
6995 If you use the @option{--absolute-names} (@option{-P}) option,
6996 @command{tar} will do none of these transformations.
6998 To archive or extract files relative to the root directory, specify
6999 the @option{--absolute-names} (@option{-P}) option.
7001 Normally, @command{tar} acts on files relative to the working
7002 directory---ignoring superior directory names when archiving, and
7003 ignoring leading slashes when extracting.
7005 When you specify @option{--absolute-names} (@option{-P}),
7006 @command{tar} stores file names including all superior directory
7007 names, and preserves leading slashes. If you only invoked
7008 @command{tar} from the root directory you would never need the
7009 @option{--absolute-names} option, but using this option
7010 may be more convenient than switching to root.
7012 @FIXME{Should be an example in the tutorial/wizardry section using this
7013 to transfer files between systems.}
7015 @FIXME{Is write access an issue?}
7018 @item --absolute-names
7019 Preserves full file names (including superior directory names) when
7020 archiving files. Preserves leading slash when extracting files.
7024 @FIXME{this is still horrible; need to talk with dan on monday.}
7026 @command{tar} prints out a message about removing the @samp{/} from
7027 file names. This message appears once per @GNUTAR{}
7028 invocation. It represents something which ought to be told; ignoring
7029 what it means can cause very serious surprises, later.
7031 Some people, nevertheless, do not want to see this message. Wanting to
7032 play really dangerously, one may of course redirect @command{tar} standard
7033 error to the sink. For example, under @command{sh}:
7036 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
7040 Another solution, both nicer and simpler, would be to change to
7041 the @file{/} directory first, and then avoid absolute notation.
7045 $ @kbd{(cd / && tar -c -f archive.tar home)}
7047 $ @kbd{tar -c -f archive.tar -C / home}
7050 @include getdate.texi
7053 @chapter Controlling the Archive Format
7055 @cindex Tar archive formats
7056 Due to historical reasons, there are several formats of tar archives.
7057 All of them are based on the same principles, but have some subtle
7058 differences that often make them incompatible with each other.
7060 GNU tar is able to create and handle archives in a variety of formats.
7061 The most frequently used formats are (in alphabetical order):
7065 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
7066 from an early @acronym{POSIX} standard, adding some improvements such as
7067 sparse file handling and incremental archives. Unfortunately these
7068 features were implemented in a way incompatible with other archive
7071 Archives in @samp{gnu} format are able to hold pathnames of unlimited
7075 Format used by @GNUTAR{} of versions prior to 1.12.
7078 Archive format, compatible with the V7 implementation of tar. This
7079 format imposes a number of limitations. The most important of them
7083 @item The maximum length of a file name is limited to 99 characters.
7084 @item The maximum length of a symbolic link is limited to 99 characters.
7085 @item It is impossible to store special files (block and character
7086 devices, fifos etc.)
7087 @item Maximum value of user or group ID is limited to 2097151 (7777777
7089 @item V7 archives do not contain symbolic ownership information (user
7090 and group name of the file owner).
7093 This format has traditionally been used by Automake when producing
7094 Makefiles. This practice will change in the future, in the meantime,
7095 however this means that projects containing filenames more than 99
7096 characters long will not be able to use @GNUTAR{} @value{VERSION} and
7097 Automake prior to 1.9.
7100 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
7101 symbolic ownership information. It is also able to store
7102 special files. However, it imposes several restrictions as well:
7105 @item The maximum length of a file name is limited to 256 characters,
7106 provided that the filename can be split at directory separator in
7107 two parts, first of them being at most 155 bytes long. So, in most
7108 cases the maximum file name length will be shorter than 256
7110 @item The maximum length of a symbolic link name is limited to
7112 @item Maximum size of a file the archive is able to accomodate
7114 @item Maximum value of UID/GID is 2097151.
7115 @item Maximum number of bits in device major and minor numbers is 21.
7119 Format used by J@"org Schilling @command{star}
7120 implementation. @GNUTAR{} is able to read @samp{star} archives but
7121 currently does not produce them.
7124 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
7125 most flexible and feature-rich format. It does not impose any
7126 restrictions on file sizes or filename lengths. This format is quite
7127 recent, so not all tar implementations are able to handle it properly.
7128 However, this format is designed in such a way that any tar
7129 implementation able to read @samp{ustar} archives will be able to read
7130 most @samp{posix} archives as well, with the only exception that any
7131 additional information (such as long file names etc.) will in such
7132 case be extracted as plain text files along with the files it refers to.
7134 This archive format will be the default format for future versions
7139 The following table summarizes the limitations of each of these
7142 @multitable @columnfractions .10 .20 .20 .20 .20
7143 @headitem Format @tab UID @tab File Size @tab Path Name @tab Devn
7144 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
7145 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
7146 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
7147 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
7148 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
7151 The default format for @GNUTAR{} is defined at compilation
7152 time. You may check it by running @command{tar --help}, and examining
7153 the last lines of its output. Usually, @GNUTAR{} is configured
7154 to create archives in @samp{gnu} format, however, future version will
7155 switch to @samp{posix}.
7158 * Portability:: Making @command{tar} Archives More Portable
7159 * Compression:: Using Less Space through Compression
7160 * Attributes:: Handling File Attributes
7161 * Standard:: The Standard Format
7162 * Extensions:: @acronym{GNU} Extensions to the Archive Format
7163 * cpio:: Comparison of @command{tar} and @command{cpio}
7167 @section Making @command{tar} Archives More Portable
7169 Creating a @command{tar} archive on a particular system that is meant to be
7170 useful later on many other machines and with other versions of @command{tar}
7171 is more challenging than you might think. @command{tar} archive formats
7172 have been evolving since the first versions of Unix. Many such formats
7173 are around, and are not always compatible with each other. This section
7174 discusses a few problems, and gives some advice about making @command{tar}
7175 archives more portable.
7177 One golden rule is simplicity. For example, limit your @command{tar}
7178 archives to contain only regular files and directories, avoiding
7179 other kind of special files. Do not attempt to save sparse files or
7180 contiguous files as such. Let's discuss a few more problems, in turn.
7182 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
7183 archives and archive labels) in GNU and PAX formats.}
7186 * Portable Names:: Portable Names
7187 * dereference:: Symbolic Links
7188 * old:: Old V7 Archives
7189 * ustar:: Ustar Archives
7190 * gnu:: GNU and old GNU format archives.
7191 * posix:: @acronym{POSIX} archives
7192 * Checksumming:: Checksumming Problems
7193 * Large or Negative Values:: Large files, negative time stamps, etc.
7196 @node Portable Names
7197 @subsection Portable Names
7199 Use portable file and member names. A name is portable if it contains
7200 only ASCII letters and digits, @samp{/}, @samp{.}, @samp{_}, and
7201 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
7202 contain @samp{/-}. Avoid deep directory nesting. For portability to
7203 old Unix hosts, limit your file name components to 14 characters or
7206 If you intend to have your @command{tar} archives to be read under
7207 MSDOS, you should not rely on case distinction for file names, and you
7208 might use the @acronym{GNU} @command{doschk} program for helping you
7209 further diagnosing illegal MSDOS names, which are even more limited
7213 @subsection Symbolic Links
7214 @cindex File names, using symbolic links
7215 @cindex Symbolic link as file name
7217 @opindex dereference
7218 Normally, when @command{tar} archives a symbolic link, it writes a
7219 block to the archive naming the target of the link. In that way, the
7220 @command{tar} archive is a faithful record of the file system contents.
7221 @option{--dereference} (@option{-h}) is used with @option{--create} (@option{-c}), and causes
7222 @command{tar} to archive the files symbolic links point to, instead of
7223 the links themselves. When this option is used, when @command{tar}
7224 encounters a symbolic link, it will archive the linked-to file,
7225 instead of simply recording the presence of a symbolic link.
7227 The name under which the file is stored in the file system is not
7228 recorded in the archive. To record both the symbolic link name and
7229 the file name in the system, archive the file under both names. If
7230 all links were recorded automatically by @command{tar}, an extracted file
7231 might be linked to a file name that no longer exists in the file
7234 If a linked-to file is encountered again by @command{tar} while creating
7235 the same archive, an entire second copy of it will be stored. (This
7236 @emph{might} be considered a bug.)
7238 So, for portable archives, do not archive symbolic links as such,
7239 and use @option{--dereference} (@option{-h}): many systems do not support
7240 symbolic links, and moreover, your distribution might be unusable if
7241 it contains unresolved symbolic links.
7244 @subsection Old V7 Archives
7245 @cindex Format, old style
7246 @cindex Old style format
7247 @cindex Old style archives
7248 @cindex v7 archive format
7250 Certain old versions of @command{tar} cannot handle additional
7251 information recorded by newer @command{tar} programs. To create an
7252 archive in V7 format (not ANSI), which can be read by these old
7253 versions, specify the @option{--format=v7} option in
7254 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
7255 accepts @option{--portability} or @samp{op-old-archive} for this
7256 option). When you specify it,
7257 @command{tar} leaves out information about directories, pipes, fifos,
7258 contiguous files, and device files, and specifies file ownership by
7259 group and user IDs instead of group and user names.
7261 When updating an archive, do not use @option{--format=v7}
7262 unless the archive was created using this option.
7264 In most cases, a @emph{new} format archive can be read by an @emph{old}
7265 @command{tar} program without serious trouble, so this option should
7266 seldom be needed. On the other hand, most modern @command{tar}s are
7267 able to read old format archives, so it might be safer for you to
7268 always use @option{--format=v7} for your distributions.
7271 @subsection Ustar Archive Format
7273 @cindex ustar archive format
7274 Archive format defined by @acronym{POSIX}.1-1988 specification is called
7275 @code{ustar}. Although it is more flexible than the V7 format, it
7276 still has many restrictions (@xref{Formats,ustar}, for the detailed
7277 description of @code{ustar} format). Along with V7 format,
7278 @code{ustar} format is a good choice for archives intended to be read
7279 with other implementations of @command{tar}.
7281 To create archive in @code{ustar} format, use @option{--format=ustar}
7282 option in conjunction with the @option{--create} (@option{-c}).
7285 @subsection @acronym{GNU} and old @GNUTAR{} format
7287 @cindex GNU archive format
7288 @cindex Old GNU archive format
7289 @GNUTAR{} was based on an early draft of the
7290 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
7291 @command{tar}, such as the support for file names longer than 100
7292 characters, use portions of the @command{tar} header record which were
7293 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
7294 @acronym{POSIX} have allocated the same parts of the header record for
7295 other purposes. As a result, @GNUTAR{} format is
7296 incompatible with the current @acronym{POSIX} specification, and with
7297 @command{tar} programs that follow it.
7299 In the majority of cases, @command{tar} will be configured to create
7300 this format by default. This will change in the future releases, since
7301 we plan to make @samp{posix} format the default.
7303 To force creation a @GNUTAR{} archive, use option
7304 @option{--format=gnu}.
7307 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
7309 @cindex POSIX archive format
7310 @cindex PAX archive format
7311 The version @value{VERSION} of @GNUTAR{} is able
7312 to read and create archives conforming to @acronym{POSIX.1-2001} standard.
7314 A @acronym{POSIX} conformant archive will be created if @command{tar}
7315 was given @option{--format=posix} option.
7318 @subsection Checksumming Problems
7320 SunOS and HP-UX @command{tar} fail to accept archives created using
7321 @GNUTAR{} and containing non-ASCII file names, that
7322 is, file names having characters with the eight bit set, because they
7323 use signed checksums, while @GNUTAR{} uses unsigned
7324 checksums while creating archives, as per @acronym{POSIX} standards. On
7325 reading, @GNUTAR{} computes both checksums and
7326 accept any. It is somewhat worrying that a lot of people may go
7327 around doing backup of their files using faulty (or at least
7328 non-standard) software, not learning about it until it's time to
7329 restore their missing files with an incompatible file extractor, or
7332 @GNUTAR{} compute checksums both ways, and accept
7333 any on read, so @acronym{GNU} tar can read Sun tapes even with their
7334 wrong checksums. @GNUTAR{} produces the standard
7335 checksum, however, raising incompatibilities with Sun. That is to
7336 say, @GNUTAR{} has not been modified to
7337 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
7338 I've been told that more recent Sun @command{tar} now read standard
7339 archives, so maybe Sun did a similar patch, after all?
7341 The story seems to be that when Sun first imported @command{tar}
7342 sources on their system, they recompiled it without realizing that
7343 the checksums were computed differently, because of a change in
7344 the default signing of @code{char}'s in their compiler. So they
7345 started computing checksums wrongly. When they later realized their
7346 mistake, they merely decided to stay compatible with it, and with
7347 themselves afterwards. Presumably, but I do not really know, HP-UX
7348 has chosen that their @command{tar} archives to be compatible with Sun's.
7349 The current standards do not favor Sun @command{tar} format. In any
7350 case, it now falls on the shoulders of SunOS and HP-UX users to get
7351 a @command{tar} able to read the good archives they receive.
7353 @node Large or Negative Values
7354 @subsection Large or Negative Values
7355 @cindex large values
7356 @cindex future time stamps
7357 @cindex negative time stamps
7360 The above sections suggest to use @samp{oldest possible} archive
7361 format if in doubt. However, sometimes it is not possible. If you
7362 attempt to archive a file whose metadata cannot be represented using
7363 required format, @GNUTAR{} will print error message and ignore such a
7364 file. You will than have to switch to a format that is able to
7365 handle such values. The format summary table (@pxref{Formats}) will
7368 In particular, when trying to archive files larger than 8GB or with
7369 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
7370 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
7371 @acronym{POSIX} archive formats. When considering which format to
7372 choose, bear in mind that the @acronym{GNU} format uses
7373 two's-complement base-256 notation to store values that do not fit
7374 into standard @acronym{ustar} range. Such archives can generally be
7375 read only by a @GNUTAR{} implementation. Moreover, they sometimes
7376 cannot be correctly restored on another hosts even by @GNUTAR{}. For
7377 example, using two's complement representation for negative time
7378 stamps that assumes a signed 32-bit @code{time_t} generates archives
7379 that are not portable to hosts with differing @code{time_t}
7382 On the other hand, @acronym{POSIX} archives, generally speaking, can
7383 be extracted by any tar implementation that understands older
7384 @acronym{ustar} format. The only exception are files larger than 8GB.
7386 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
7390 @section Using Less Space through Compression
7393 * gzip:: Creating and Reading Compressed Archives
7394 * sparse:: Archiving Sparse Files
7398 @subsection Creating and Reading Compressed Archives
7399 @cindex Compressed archives
7400 @cindex Storing archives in compressed format
7402 @GNUTAR{} is able to create and read compressed archives. It supports
7403 @command{gzip} and @command{bzip2} compression programs. For backward
7404 compatibilty, it also supports @command{compress} command, although
7405 we strongly recommend against using it, since there is a patent
7406 covering the algorithm it uses and you could be sued for patent
7407 infringement merely by running @command{compress}! Besides, it is less
7408 effective than @command{gzip} and @command{bzip2}.
7410 Creating a compressed archive is simple: you just specify a
7411 @dfn{compression option} along with the usual archive creation
7412 commands. The compression option is @option{-z} (@option{--gzip}) to
7413 create a @command{gzip} compressed archive, @option{-j}
7414 (@option{--bzip2}) to create a @command{bzip2} compressed archive, and
7415 @option{-Z} (@option{--compress}) to use @command{compress} program.
7419 $ @kbd{tar cfz archive.tar.gz .}
7422 Reading compressed archive is even simpler: you don't need to specify
7423 any additional options as @GNUTAR{} recognizes its format
7424 automatically. Thus, the following commands will list and extract the
7425 archive created in previous example:
7428 # List the compressed archive
7429 $ @kbd{tar tf archive.tar.gz}
7430 # Extract the compressed archive
7431 $ @kbd{tar xf archive.tar.gz}
7434 The only case when you have to specify a decompression option while
7435 reading the archive is when reading from a pipe or from a tape drive
7436 that does not support random access. However, in this case @GNUTAR{}
7437 will indicate which option you should use. For example:
7440 $ @kbd{cat archive.tar.gz | tar tf -}
7441 tar: Archive is compressed. Use -z option
7442 tar: Error is not recoverable: exiting now
7445 If you see such diagnostics, just add the suggested option to the
7446 invocation of @GNUTAR{}:
7449 $ @kbd{cat archive.tar.gz | tar tfz -}
7452 Notice also, that there are several restrictions on operations on
7453 compressed archives. First of all, compressed archives cannot be
7454 modified, i.e., you cannot update (@option{--update} (@option{-u})) them or delete
7455 (@option{--delete}) members from them. Likewise, you cannot append
7456 another @command{tar} archive to a compressed archive using
7457 @option{--append} (@option{-r})). Secondly, multi-volume archives cannot be
7460 The following table summarizes compression options used by @GNUTAR{}.
7468 Filter the archive through @command{gzip}.
7470 You can use @option{--gzip} and @option{--gunzip} on physical devices
7471 (tape drives, etc.) and remote files as well as on normal files; data
7472 to or from such devices or remote files is reblocked by another copy
7473 of the @command{tar} program to enforce the specified (or default) record
7474 size. The default compression parameters are used; if you need to
7475 override them, set @env{GZIP} environment variable, e.g.:
7478 $ @kbd{GZIP=--best tar cfz archive.tar.gz subdir}
7482 Another way would be to avoid the @option{--gzip} (@option{--gunzip}, @option{--ungzip}, @option{-z}) option and run
7483 @command{gzip} explicitly:
7486 $ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz}
7489 @cindex corrupted archives
7490 About corrupted compressed archives: @command{gzip}'ed files have no
7491 redundancy, for maximum compression. The adaptive nature of the
7492 compression scheme means that the compression tables are implicitly
7493 spread all over the archive. If you lose a few blocks, the dynamic
7494 construction of the compression tables becomes unsynchronized, and there
7495 is little chance that you could recover later in the archive.
7497 There are pending suggestions for having a per-volume or per-file
7498 compression in @GNUTAR{}. This would allow for viewing the
7499 contents without decompression, and for resynchronizing decompression at
7500 every volume or file, in case of corrupted archives. Doing so, we might
7501 lose some compressibility. But this would have make recovering easier.
7502 So, there are pros and cons. We'll see!
7507 Filter the archive through @code{bzip2}. Otherwise like @option{--gzip}.
7514 Filter the archive through @command{compress}. Otherwise like @option{--gzip}.
7516 The @acronym{GNU} Project recommends you not use
7517 @command{compress}, because there is a patent covering the algorithm it
7518 uses. You could be sued for patent infringement merely by running
7521 @opindex use-compress-program
7522 @item --use-compress-program=@var{prog}
7523 Use external compression program @var{prog}. Use this option if you
7524 have a compression program that @GNUTAR{} does not support. There
7525 are two requirements to which @var{prog} should comply:
7527 First, when called without options, it should read data from standard
7528 input, compress it and output it on standard output.
7530 Secondly, if called with @option{-d} argument, it should do exactly
7531 the opposite, i.e., read the compressed data from the standard input
7532 and produce uncompressed data on the standard output.
7535 @cindex gpg, using with tar
7536 @cindex gnupg, using with tar
7537 @cindex Using encrypted archives
7538 The @option{--use-compress-program} option, in particular, lets you
7539 implement your own filters, not necessarily dealing with
7540 compression/decomression. For example, suppose you wish to implement
7541 PGP encryption on top of compression, using @command{gpg} (@pxref{Top,
7542 gpg, gpg ---- encryption and signing tool, gpg}). The following
7549 -d) gpg --decrypt - | gzip -d -c;;
7550 '') gzip -c | gpg -s ;;
7551 *) echo "Unknown option $1">&2; exit 1;;
7556 Suppose you name it @file{gpgz} and save it somewhere in your
7557 @env{PATH}. Then the following command will create a commpressed
7558 archive signed with your private key:
7561 $ @kbd{tar -cf foo.tar.gpgz --use-compress=gpgz .}
7565 Likewise, the following command will list its contents:
7568 $ @kbd{tar -tf foo.tar.gpgz --use-compress=gpgz .}
7572 The above is based on the following discussion:
7574 I have one question, or maybe it's a suggestion if there isn't a way
7575 to do it now. I would like to use @option{--gzip}, but I'd also like
7576 the output to be fed through a program like @acronym{GNU}
7577 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
7578 to use :-)), basically adding ECC protection on top of compression.
7579 It seems as if this should be quite easy to do, but I can't work out
7580 exactly how to go about it. Of course, I can pipe the standard output
7581 of @command{tar} through @command{ecc}, but then I lose (though I
7582 haven't started using it yet, I confess) the ability to have
7583 @command{tar} use @command{rmt} for it's I/O (I think).
7585 I think the most straightforward thing would be to let me specify a
7586 general set of filters outboard of compression (preferably ordered,
7587 so the order can be automatically reversed on input operations, and
7588 with the options they require specifiable), but beggars shouldn't be
7589 choosers and anything you decide on would be fine with me.
7591 By the way, I like @command{ecc} but if (as the comments say) it can't
7592 deal with loss of block sync, I'm tempted to throw some time at adding
7593 that capability. Supposing I were to actually do such a thing and
7594 get it (apparently) working, do you accept contributed changes to
7595 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
7597 Isn't that exactly the role of the
7598 @option{--use-compress-prog=@var{program}} option?
7599 I never tried it myself, but I suspect you may want to write a
7600 @var{prog} script or program able to filter stdin to stdout to
7601 way you want. It should recognize the @option{-d} option, for when
7602 extraction is needed rather than creation.
7604 It has been reported that if one writes compressed data (through the
7605 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
7606 the DLT compression mode, the data will actually get bigger and one will
7607 end up with less space on the tape.
7611 @subsection Archiving Sparse Files
7612 @cindex Sparse Files
7619 Handle sparse files efficiently.
7622 This option causes all files to be put in the archive to be tested for
7623 sparseness, and handled specially if they are. The @option{--sparse}
7624 (@option{-S}) option is useful when many @code{dbm} files, for example, are being
7625 backed up. Using this option dramatically decreases the amount of
7626 space needed to store such a file.
7628 In later versions, this option may be removed, and the testing and
7629 treatment of sparse files may be done automatically with any special
7630 @acronym{GNU} options. For now, it is an option needing to be specified on
7631 the command line with the creation or updating of an archive.
7633 Files in the file system occasionally have ``holes.'' A hole in a file
7634 is a section of the file's contents which was never written. The
7635 contents of a hole read as all zeros. On many operating systems,
7636 actual disk storage is not allocated for holes, but they are counted
7637 in the length of the file. If you archive such a file, @command{tar}
7638 could create an archive longer than the original. To have @command{tar}
7639 attempt to recognize the holes in a file, use @option{--sparse} (@option{-S}). When
7640 you use this option, then, for any file using less disk space than
7641 would be expected from its length, @command{tar} searches the file for
7642 consecutive stretches of zeros. It then records in the archive for
7643 the file where the consecutive stretches of zeros are, and only
7644 archives the ``real contents'' of the file. On extraction (using
7645 @option{--sparse} is not needed on extraction) any such
7646 files have holes created wherever the continuous stretches of zeros
7647 were found. Thus, if you use @option{--sparse}, @command{tar} archives
7648 won't take more space than the original.
7650 A file is sparse if it contains blocks of zeros whose existence is
7651 recorded, but that have no space allocated on disk. When you specify
7652 the @option{--sparse} option in conjunction with the @option{--create}
7653 (@option{-c}) operation, @command{tar} tests all files for sparseness
7654 while archiving. If @command{tar} finds a file to be sparse, it uses a
7655 sparse representation of the file in the archive. @xref{create}, for
7656 more information about creating archives.
7658 @option{--sparse} is useful when archiving files, such as dbm files,
7659 likely to contain many nulls. This option dramatically
7660 decreases the amount of space needed to store such an archive.
7663 @strong{Please Note:} Always use @option{--sparse} when performing file
7664 system backups, to avoid archiving the expanded forms of files stored
7665 sparsely in the system.
7667 Even if your system has no sparse files currently, some may be
7668 created in the future. If you use @option{--sparse} while making file
7669 system backups as a matter of course, you can be assured the archive
7670 will never take more space on the media than the files take on disk
7671 (otherwise, archiving a disk filled with sparse files might take
7672 hundreds of tapes). @xref{Incremental Dumps}.
7675 @command{tar} ignores the @option{--sparse} option when reading an archive.
7680 Files stored sparsely in the file system are represented sparsely in
7681 the archive. Use in conjunction with write operations.
7684 However, users should be well aware that at archive creation time,
7685 @GNUTAR{} still has to read whole disk file to
7686 locate the @dfn{holes}, and so, even if sparse files use little space
7687 on disk and in the archive, they may sometimes require inordinate
7688 amount of time for reading and examining all-zero blocks of a file.
7689 Although it works, it's painfully slow for a large (sparse) file, even
7690 though the resulting tar archive may be small. (One user reports that
7691 dumping a @file{core} file of over 400 megabytes, but with only about
7692 3 megabytes of actual data, took about 9 minutes on a Sun Sparcstation
7693 ELC, with full CPU utilization.)
7695 This reading is required in all cases and is not related to the fact
7696 the @option{--sparse} option is used or not, so by merely @emph{not}
7697 using the option, you are not saving time@footnote{Well! We should say
7698 the whole truth, here. When @option{--sparse} is selected while creating
7699 an archive, the current @command{tar} algorithm requires sparse files to be
7700 read twice, not once. We hope to develop a new archive format for saving
7701 sparse files in which one pass will be sufficient.}.
7703 Programs like @command{dump} do not have to read the entire file; by
7704 examining the file system directly, they can determine in advance
7705 exactly where the holes are and thus avoid reading through them. The
7706 only data it need read are the actual allocated data blocks.
7707 @GNUTAR{} uses a more portable and straightforward
7708 archiving approach, it would be fairly difficult that it does
7709 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
7713 What I did say is that you cannot tell the difference between a hole and an
7714 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
7715 best tells you how many holes there are; it doesn't tell you @emph{where}.
7716 Just as programs may, conceivably, care what @code{st_blocks} is (care
7717 to name one that does?), they may also care where the holes are (I have
7718 no examples of this one either, but it's equally imaginable).
7720 I conclude from this that good archivers are not portable. One can
7721 arguably conclude that if you want a portable program, you can in good
7722 conscience restore files with as many holes as possible, since you can't
7727 @section Handling File Attributes
7730 When @command{tar} reads files, it updates their access times. To
7731 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
7732 reset the access time retroactively or avoid changing it in the first
7735 Handling of file attributes
7738 @opindex atime-preserve
7739 @item --atime-preserve
7740 @itemx --atime-preserve=replace
7741 @itemx --atime-preserve=system
7742 Preserve the access times of files that are read. This works only for
7743 files that you own, unless you have superuser privileges.
7745 @option{--atime-preserve=replace} works on most systems, but it also
7746 restores the data modification time and updates the status change
7747 time. Hence it doesn't interact with incremental dumps nicely
7748 (@pxref{Backups}), and it can set access or data modification times
7749 incorrectly if other programs access the file while @command{tar} is
7752 @option{--atime-preserve=system} avoids changing the access time in
7753 the first place, if the operating system supports this.
7754 Unfortunately, this may or may not work on any given operating system
7755 or file system. If @command{tar} knows for sure it won't work, it
7756 complains right away.
7758 Currently @option{--atime-preserve} with no operand defaults to
7759 @option{--atime-preserve=replace}, but this is intended to change to
7760 @option{--atime-preserve=system} when the latter is better-supported.
7765 Do not extract data modification time.
7767 When this option is used, @command{tar} leaves the data modification times
7768 of the files it extracts as the times when the files were extracted,
7769 instead of setting it to the times recorded in the archive.
7771 This option is meaningless with @option{--list} (@option{-t}).
7775 Create extracted files with the same ownership they have in the
7778 This is the default behavior for the superuser,
7779 so this option is meaningful only for non-root users, when @command{tar}
7780 is executed on those systems able to give files away. This is
7781 considered as a security flaw by many people, at least because it
7782 makes quite difficult to correctly account users for the disk space
7783 they occupy. Also, the @code{suid} or @code{sgid} attributes of
7784 files are easily and silently lost when files are given away.
7786 When writing an archive, @command{tar} writes the user id and user name
7787 separately. If it can't find a user name (because the user id is not
7788 in @file{/etc/passwd}), then it does not write one. When restoring,
7789 it tries to look the name (if one was written) up in
7790 @file{/etc/passwd}. If it fails, then it uses the user id stored in
7791 the archive instead.
7793 @opindex no-same-owner
7794 @item --no-same-owner
7796 Do not attempt to restore ownership when extracting. This is the
7797 default behavior for ordinary users, so this option has an effect
7798 only for the superuser.
7800 @opindex numeric-owner
7801 @item --numeric-owner
7802 The @option{--numeric-owner} option allows (ANSI) archives to be written
7803 without user/group name information or such information to be ignored
7804 when extracting. It effectively disables the generation and/or use
7805 of user/group name information. This option forces extraction using
7806 the numeric ids from the archive, ignoring the names.
7808 This is useful in certain circumstances, when restoring a backup from
7809 an emergency floppy with different passwd/group files for example.
7810 It is otherwise impossible to extract files with the right ownerships
7811 if the password file in use during the extraction does not match the
7812 one belonging to the file system(s) being extracted. This occurs,
7813 for example, if you are restoring your files after a major crash and
7814 had booted from an emergency floppy with no password file or put your
7815 disk into another machine to do the restore.
7817 The numeric ids are @emph{always} saved into @command{tar} archives.
7818 The identifying names are added at create time when provided by the
7819 system, unless @option{--old-archive} (@option{-o}) is used. Numeric ids could be
7820 used when moving archives between a collection of machines using
7821 a centralized management for attribution of numeric ids to users
7822 and groups. This is often made through using the NIS capabilities.
7824 When making a @command{tar} file for distribution to other sites, it
7825 is sometimes cleaner to use a single owner for all files in the
7826 distribution, and nicer to specify the write permission bits of the
7827 files as stored in the archive independently of their actual value on
7828 the file system. The way to prepare a clean distribution is usually
7829 to have some Makefile rule creating a directory, copying all needed
7830 files in that directory, then setting ownership and permissions as
7831 wanted (there are a lot of possible schemes), and only then making a
7832 @command{tar} archive out of this directory, before cleaning
7833 everything out. Of course, we could add a lot of options to
7834 @GNUTAR{} for fine tuning permissions and ownership.
7835 This is not the good way, I think. @GNUTAR{} is
7836 already crowded with options and moreover, the approach just explained
7837 gives you a great deal of control already.
7839 @opindex same-permissions, short description
7840 @opindex preserve-permissions, short description
7842 @itemx --same-permissions
7843 @itemx --preserve-permissions
7844 Extract all protection information.
7846 This option causes @command{tar} to set the modes (access permissions) of
7847 extracted files exactly as recorded in the archive. If this option
7848 is not used, the current @code{umask} setting limits the permissions
7849 on extracted files. This option is by default enabled when
7850 @command{tar} is executed by a superuser.
7853 This option is meaningless with @option{--list} (@option{-t}).
7857 Same as both @option{--same-permissions} and @option{--same-order}.
7859 The @option{--preserve} option has no equivalent short option name.
7860 It is equivalent to @option{--same-permissions} plus @option{--same-order}.
7862 @FIXME{I do not see the purpose of such an option. (Neither I. FP.)
7863 Neither do I. --Sergey}
7868 @section Basic Tar Format
7871 While an archive may contain many files, the archive itself is a
7872 single ordinary file. Like any other file, an archive file can be
7873 written to a storage device such as a tape or disk, sent through a
7874 pipe or over a network, saved on the active file system, or even
7875 stored in another archive. An archive file is not easy to read or
7876 manipulate without using the @command{tar} utility or Tar mode in
7877 @acronym{GNU} Emacs.
7879 Physically, an archive consists of a series of file entries terminated
7880 by an end-of-archive entry, which consists of two 512 blocks of zero
7882 entry usually describes one of the files in the archive (an
7883 @dfn{archive member}), and consists of a file header and the contents
7884 of the file. File headers contain file names and statistics, checksum
7885 information which @command{tar} uses to detect file corruption, and
7886 information about file types.
7888 Archives are permitted to have more than one member with the same
7889 member name. One way this situation can occur is if more than one
7890 version of a file has been stored in the archive. For information
7891 about adding new versions of a file to an archive, see @ref{update}.
7892 @FIXME-xref{To learn more about having more than one archive member with the
7893 same name, see -backup node, when it's written.}
7895 In addition to entries describing archive members, an archive may
7896 contain entries which @command{tar} itself uses to store information.
7897 @xref{label}, for an example of such an archive entry.
7899 A @command{tar} archive file contains a series of blocks. Each block
7900 contains @code{BLOCKSIZE} bytes. Although this format may be thought
7901 of as being on magnetic tape, other media are often used.
7903 Each file archived is represented by a header block which describes
7904 the file, followed by zero or more blocks which give the contents
7905 of the file. At the end of the archive file there are two 512-byte blocks
7906 filled with binary zeros as an end-of-file marker. A reasonable system
7907 should write such end-of-file marker at the end of an archive, but
7908 must not assume that such a block exists when reading an archive. In
7909 particular @GNUTAR{} always issues a warning if it does not encounter it.
7911 The blocks may be @dfn{blocked} for physical I/O operations.
7912 Each record of @var{n} blocks (where @var{n} is set by the
7913 @option{--blocking-factor=@var{512-size}} (@option{-b @var{512-size}}) option to @command{tar}) is written with a single
7914 @w{@samp{write ()}} operation. On magnetic tapes, the result of
7915 such a write is a single record. When writing an archive,
7916 the last record of blocks should be written at the full size, with
7917 blocks after the zero block containing all zeros. When reading
7918 an archive, a reasonable system should properly handle an archive
7919 whose last record is shorter than the rest, or which contains garbage
7920 records after a zero block.
7922 The header block is defined in C as follows. In the @GNUTAR{}
7923 distribution, this is part of file @file{src/tar.h}:
7926 @include header.texi
7929 All characters in header blocks are represented by using 8-bit
7930 characters in the local variant of ASCII. Each field within the
7931 structure is contiguous; that is, there is no padding used within
7932 the structure. Each character on the archive medium is stored
7935 Bytes representing the contents of files (after the header block
7936 of each file) are not translated in any way and are not constrained
7937 to represent characters in any character set. The @command{tar} format
7938 does not distinguish text files from binary files, and no translation
7939 of file contents is performed.
7941 The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and
7942 @code{gname} are null-terminated character strings. All other fields
7943 are zero-filled octal numbers in ASCII. Each numeric field of width
7944 @var{w} contains @var{w} minus 1 digits, and a null.
7946 The @code{name} field is the file name of the file, with directory names
7947 (if any) preceding the file name, separated by slashes.
7949 @FIXME{how big a name before field overflows?}
7951 The @code{mode} field provides nine bits specifying file permissions
7952 and three bits to specify the Set UID, Set GID, and Save Text
7953 (@dfn{sticky}) modes. Values for these bits are defined above.
7954 When special permissions are required to create a file with a given
7955 mode, and the user restoring files from the archive does not hold such
7956 permissions, the mode bit(s) specifying those special permissions
7957 are ignored. Modes which are not supported by the operating system
7958 restoring files from the archive will be ignored. Unsupported modes
7959 should be faked up when creating or updating an archive; e.g., the
7960 group permission could be copied from the @emph{other} permission.
7962 The @code{uid} and @code{gid} fields are the numeric user and group
7963 ID of the file owners, respectively. If the operating system does
7964 not support numeric user or group IDs, these fields should be ignored.
7966 The @code{size} field is the size of the file in bytes; linked files
7967 are archived with this field specified as zero. @FIXME-xref{Modifiers, in
7968 particular the @option{--incremental} (@option{-G}) option.}
7970 The @code{mtime} field is the data modification time of the file at
7971 the time it was archived. It is the ASCII representation of the octal
7972 value of the last time the file's contents were modified, represented
7973 as an integer number of
7974 seconds since January 1, 1970, 00:00 Coordinated Universal Time.
7976 The @code{chksum} field is the ASCII representation of the octal value
7977 of the simple sum of all bytes in the header block. Each 8-bit
7978 byte in the header is added to an unsigned integer, initialized to
7979 zero, the precision of which shall be no less than seventeen bits.
7980 When calculating the checksum, the @code{chksum} field is treated as
7981 if it were all blanks.
7983 The @code{typeflag} field specifies the type of file archived. If a
7984 particular implementation does not recognize or permit the specified
7985 type, the file will be extracted as if it were a regular file. As this
7986 action occurs, @command{tar} issues a warning to the standard error.
7988 The @code{atime} and @code{ctime} fields are used in making incremental
7989 backups; they store, respectively, the particular file's access and
7990 status change times.
7992 The @code{offset} is used by the @option{--multi-volume} (@option{-M}) option, when
7993 making a multi-volume archive. The offset is number of bytes into
7994 the file that we need to restart at to continue the file on the next
7995 tape, i.e., where we store the location that a continued file is
7998 The following fields were added to deal with sparse files. A file
7999 is @dfn{sparse} if it takes in unallocated blocks which end up being
8000 represented as zeros, i.e., no useful data. A test to see if a file
8001 is sparse is to look at the number blocks allocated for it versus the
8002 number of characters in the file; if there are fewer blocks allocated
8003 for the file than would normally be allocated for a file of that
8004 size, then the file is sparse. This is the method @command{tar} uses to
8005 detect a sparse file, and once such a file is detected, it is treated
8006 differently from non-sparse files.
8008 Sparse files are often @code{dbm} files, or other database-type files
8009 which have data at some points and emptiness in the greater part of
8010 the file. Such files can appear to be very large when an @samp{ls
8011 -l} is done on them, when in truth, there may be a very small amount
8012 of important data contained in the file. It is thus undesirable
8013 to have @command{tar} think that it must back up this entire file, as
8014 great quantities of room are wasted on empty blocks, which can lead
8015 to running out of room on a tape far earlier than is necessary.
8016 Thus, sparse files are dealt with so that these empty blocks are
8017 not written to the tape. Instead, what is written to the tape is a
8018 description, of sorts, of the sparse file: where the holes are, how
8019 big the holes are, and how much data is found at the end of the hole.
8020 This way, the file takes up potentially far less room on the tape,
8021 and when the file is extracted later on, it will look exactly the way
8022 it looked beforehand. The following is a description of the fields
8023 used to handle a sparse file:
8025 The @code{sp} is an array of @code{struct sparse}. Each @code{struct
8026 sparse} contains two 12-character strings which represent an offset
8027 into the file and a number of bytes to be written at that offset.
8028 The offset is absolute, and not relative to the offset in preceding
8031 The header can hold four of these @code{struct sparse} at the moment;
8032 if more are needed, they are not stored in the header.
8034 The @code{isextended} flag is set when an @code{extended_header}
8035 is needed to deal with a file. Note that this means that this flag
8036 can only be set when dealing with a sparse file, and it is only set
8037 in the event that the description of the file will not fit in the
8038 allotted room for sparse structures in the header. In other words,
8039 an extended_header is needed.
8041 The @code{extended_header} structure is used for sparse files which
8042 need more sparse structures than can fit in the header. The header can
8043 fit 4 such structures; if more are needed, the flag @code{isextended}
8044 gets set and the next block is an @code{extended_header}.
8046 Each @code{extended_header} structure contains an array of 21
8047 sparse structures, along with a similar @code{isextended} flag
8048 that the header had. There can be an indeterminate number of such
8049 @code{extended_header}s to describe a sparse file.
8053 @item @code{REGTYPE}
8054 @itemx @code{AREGTYPE}
8055 These flags represent a regular file. In order to be compatible
8056 with older versions of @command{tar}, a @code{typeflag} value of
8057 @code{AREGTYPE} should be silently recognized as a regular file.
8058 New archives should be created using @code{REGTYPE}. Also, for
8059 backward compatibility, @command{tar} treats a regular file whose name
8060 ends with a slash as a directory.
8062 @item @code{LNKTYPE}
8063 This flag represents a file linked to another file, of any type,
8064 previously archived. Such files are identified in Unix by each
8065 file having the same device and inode number. The linked-to name is
8066 specified in the @code{linkname} field with a trailing null.
8068 @item @code{SYMTYPE}
8069 This represents a symbolic link to another file. The linked-to name
8070 is specified in the @code{linkname} field with a trailing null.
8072 @item @code{CHRTYPE}
8073 @itemx @code{BLKTYPE}
8074 These represent character special files and block special files
8075 respectively. In this case the @code{devmajor} and @code{devminor}
8076 fields will contain the major and minor device numbers respectively.
8077 Operating systems may map the device specifications to their own
8078 local specification, or may ignore the entry.
8080 @item @code{DIRTYPE}
8081 This flag specifies a directory or sub-directory. The directory
8082 name in the @code{name} field should end with a slash. On systems where
8083 disk allocation is performed on a directory basis, the @code{size} field
8084 will contain the maximum number of bytes (which may be rounded to
8085 the nearest disk block allocation unit) which the directory may
8086 hold. A @code{size} field of zero indicates no such limiting. Systems
8087 which do not support limiting in this manner should ignore the
8090 @item @code{FIFOTYPE}
8091 This specifies a FIFO special file. Note that the archiving of a
8092 FIFO file archives the existence of this file and not its contents.
8094 @item @code{CONTTYPE}
8095 This specifies a contiguous file, which is the same as a normal
8096 file except that, in operating systems which support it, all its
8097 space is allocated contiguously on the disk. Operating systems
8098 which do not allow contiguous allocation should silently treat this
8099 type as a normal file.
8101 @item @code{A} @dots{} @code{Z}
8102 These are reserved for custom implementations. Some of these are
8103 used in the @acronym{GNU} modified format, as described below.
8107 Other values are reserved for specification in future revisions of
8108 the P1003 standard, and should not be used by any @command{tar} program.
8110 The @code{magic} field indicates that this archive was output in
8111 the P1003 archive format. If this field contains @code{TMAGIC},
8112 the @code{uname} and @code{gname} fields will contain the ASCII
8113 representation of the owner and group of the file respectively.
8114 If found, the user and group IDs are used rather than the values in
8115 the @code{uid} and @code{gid} fields.
8117 For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages
8118 169-173 (section 10.1) for @cite{Archive/Interchange File Format}; and
8119 IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940
8120 (section E.4.48) for @cite{pax - Portable archive interchange}.
8123 @section @acronym{GNU} Extensions to the Archive Format
8126 The @acronym{GNU} format uses additional file types to describe new types of
8127 files in an archive. These are listed below.
8130 @item GNUTYPE_DUMPDIR
8132 This represents a directory and a list of files created by the
8133 @option{--incremental} (@option{-G}) option. The @code{size} field gives the total
8134 size of the associated list of files. Each file name is preceded by
8135 either a @samp{Y} (the file should be in this archive) or an @samp{N}.
8136 (The file is a directory, or is not stored in the archive.) Each file
8137 name is terminated by a null. There is an additional null after the
8140 @item GNUTYPE_MULTIVOL
8142 This represents a file continued from another volume of a multi-volume
8143 archive created with the @option{--multi-volume} (@option{-M}) option. The original
8144 type of the file is not given here. The @code{size} field gives the
8145 maximum size of this piece of the file (assuming the volume does
8146 not end before the file is written out). The @code{offset} field
8147 gives the offset from the beginning of the file where this part of
8148 the file begins. Thus @code{size} plus @code{offset} should equal
8149 the original size of the file.
8151 @item GNUTYPE_SPARSE
8153 This flag indicates that we are dealing with a sparse file. Note
8154 that archiving a sparse file requires special operations to find
8155 holes in the file, which mark the positions of these holes, along
8156 with the number of bytes of data to be found after the hole.
8158 @item GNUTYPE_VOLHDR
8160 This file type is used to mark the volume header that was given with
8161 the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option when the archive was created. The @code{name}
8162 field contains the @code{name} given after the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option.
8163 The @code{size} field is zero. Only the first file in each volume
8164 of an archive should have this type.
8168 You may have trouble reading a @acronym{GNU} format archive on a
8169 non-@acronym{GNU} system if the options @option{--incremental} (@option{-G}),
8170 @option{--multi-volume} (@option{-M}), @option{--sparse} (@option{-S}), or @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) were
8171 used when writing the archive. In general, if @command{tar} does not
8172 use the @acronym{GNU}-added fields of the header, other versions of
8173 @command{tar} should be able to read the archive. Otherwise, the
8174 @command{tar} program will give an error, the most likely one being a
8178 @section Comparison of @command{tar} and @command{cpio}
8181 @FIXME{Reorganize the following material}
8183 The @command{cpio} archive formats, like @command{tar}, do have maximum
8184 pathname lengths. The binary and old ASCII formats have a max path
8185 length of 256, and the new ASCII and CRC ASCII formats have a max
8186 path length of 1024. @acronym{GNU} @command{cpio} can read and write archives
8187 with arbitrary pathname lengths, but other @command{cpio} implementations
8188 may crash unexplainedly trying to read them.
8190 @command{tar} handles symbolic links in the form in which it comes in BSD;
8191 @command{cpio} doesn't handle symbolic links in the form in which it comes
8192 in System V prior to SVR4, and some vendors may have added symlinks
8193 to their system without enhancing @command{cpio} to know about them.
8194 Others may have enhanced it in a way other than the way I did it
8195 at Sun, and which was adopted by AT&T (and which is, I think, also
8196 present in the @command{cpio} that Berkeley picked up from AT&T and put
8197 into a later BSD release---I think I gave them my changes).
8199 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
8200 can handle @command{tar} format input, and write it on output, and it
8201 probably handles symbolic links. They may not have bothered doing
8202 anything to enhance @command{tar} as a result.)
8204 @command{cpio} handles special files; traditional @command{tar} doesn't.
8206 @command{tar} comes with V7, System III, System V, and BSD source;
8207 @command{cpio} comes only with System III, System V, and later BSD
8208 (4.3-tahoe and later).
8210 @command{tar}'s way of handling multiple hard links to a file can handle
8211 file systems that support 32-bit inumbers (e.g., the BSD file system);
8212 @command{cpio}s way requires you to play some games (in its "binary"
8213 format, i-numbers are only 16 bits, and in its "portable ASCII" format,
8214 they're 18 bits---it would have to play games with the "file system ID"
8215 field of the header to make sure that the file system ID/i-number pairs
8216 of different files were always different), and I don't know which
8217 @command{cpio}s, if any, play those games. Those that don't might get
8218 confused and think two files are the same file when they're not, and
8219 make hard links between them.
8221 @command{tar}s way of handling multiple hard links to a file places only
8222 one copy of the link on the tape, but the name attached to that copy
8223 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
8224 way puts one copy for every link, but you can retrieve it using any
8228 What type of check sum (if any) is used, and how is this calculated.
8231 See the attached manual pages for @command{tar} and @command{cpio} format.
8232 @command{tar} uses a checksum which is the sum of all the bytes in the
8233 @command{tar} header for a file; @command{cpio} uses no checksum.
8236 If anyone knows why @command{cpio} was made when @command{tar} was present
8240 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
8241 generally-available version of UNIX had @command{tar} at the time. I don't
8242 know whether any version that was generally available @emph{within AT&T}
8243 had @command{tar}, or, if so, whether the people within AT&T who did
8244 @command{cpio} knew about it.
8246 On restore, if there is a corruption on a tape @command{tar} will stop at
8247 that point, while @command{cpio} will skip over it and try to restore the
8250 The main difference is just in the command syntax and header format.
8252 @command{tar} is a little more tape-oriented in that everything is blocked
8253 to start on a record boundary.
8256 Is there any differences between the ability to recover crashed
8257 archives between the two of them. (Is there any chance of recovering
8258 crashed archives at all.)
8261 Theoretically it should be easier under @command{tar} since the blocking
8262 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
8263 However, modern @command{cpio}'s and variations have an option to just
8264 search for the next file header after an error with a reasonable chance
8265 of resyncing. However, lots of tape driver software won't allow you to
8266 continue past a media error which should be the only reason for getting
8267 out of sync unless a file changed sizes while you were writing the
8271 If anyone knows why @command{cpio} was made when @command{tar} was present
8272 at the unix scene, please tell me about this too.
8275 Probably because it is more media efficient (by not blocking everything
8276 and using only the space needed for the headers where @command{tar}
8277 always uses 512 bytes per file header) and it knows how to archive
8280 You might want to look at the freely available alternatives. The
8281 major ones are @command{afio}, @GNUTAR{}, and
8282 @command{pax}, each of which have their own extensions with some
8283 backwards compatibility.
8285 Sparse files were @command{tar}red as sparse files (which you can
8286 easily test, because the resulting archive gets smaller, and
8287 @acronym{GNU} @command{cpio} can no longer read it).
8290 @chapter Tapes and Other Archive Media
8293 A few special cases about tape handling warrant more detailed
8294 description. These special cases are discussed below.
8296 Many complexities surround the use of @command{tar} on tape drives. Since
8297 the creation and manipulation of archives located on magnetic tape was
8298 the original purpose of @command{tar}, it contains many features making
8299 such manipulation easier.
8301 Archives are usually written on dismountable media---tape cartridges,
8302 mag tapes, or floppy disks.
8304 The amount of data a tape or disk holds depends not only on its size,
8305 but also on how it is formatted. A 2400 foot long reel of mag tape
8306 holds 40 megabytes of data when formatted at 1600 bits per inch. The
8307 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
8309 Magnetic media are re-usable---once the archive on a tape is no longer
8310 needed, the archive can be erased and the tape or disk used over.
8311 Media quality does deteriorate with use, however. Most tapes or disks
8312 should be discarded when they begin to produce data errors. EXABYTE
8313 tape cartridges should be discarded when they generate an @dfn{error
8314 count} (number of non-usable bits) of more than 10k.
8316 Magnetic media are written and erased using magnetic fields, and
8317 should be protected from such fields to avoid damage to stored data.
8318 Sticking a floppy disk to a filing cabinet using a magnet is probably
8322 * Device:: Device selection and switching
8323 * Remote Tape Server::
8324 * Common Problems and Solutions::
8325 * Blocking:: Blocking
8326 * Many:: Many archives on one tape
8327 * Using Multiple Tapes:: Using Multiple Tapes
8328 * label:: Including a Label in the Archive
8330 * Write Protection::
8334 @section Device Selection and Switching
8338 @item -f [@var{hostname}:]@var{file}
8339 @itemx --file=[@var{hostname}:]@var{file}
8340 Use archive file or device @var{file} on @var{hostname}.
8343 This option is used to specify the file name of the archive @command{tar}
8346 If the file name is @samp{-}, @command{tar} reads the archive from standard
8347 input (when listing or extracting), or writes it to standard output
8348 (when creating). If the @samp{-} file name is given when updating an
8349 archive, @command{tar} will read the original archive from its standard
8350 input, and will write the entire new archive to its standard output.
8352 If the file name contains a @samp{:}, it is interpreted as
8353 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
8354 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
8355 either case, @command{tar} will invoke the command @command{rsh} (or
8356 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
8357 machine. If you give an alternate login name, it will be given to the
8359 Naturally, the remote machine must have an executable
8360 @command{/usr/libexec/rmt}. This program is free software from the
8361 University of California, and a copy of the source code can be found
8362 with the sources for @command{tar}; it's compiled and installed by default.
8363 The exact path to this utility is determined when configuring the package.
8364 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
8365 your installation prefix. This location may also be overridden at
8366 runtime by using @option{rmt-command=@var{command}} option (@xref{Option Summary,
8367 ---rmt-command}, for detailed description of this option. @xref{Remote
8368 Tape Server}, for the description of @command{rmt} command).
8370 If this option is not given, but the environment variable @env{TAPE}
8371 is set, its value is used; otherwise, old versions of @command{tar}
8372 used a default archive name (which was picked when @command{tar} was
8373 compiled). The default is normally set up to be the @dfn{first} tape
8374 drive or other transportable I/O medium on the system.
8376 Starting with version 1.11.5, @GNUTAR{} uses
8377 standard input and standard output as the default device, and I will
8378 not try anymore supporting automatic device detection at installation
8379 time. This was failing really in too many cases, it was hopeless.
8380 This is now completely left to the installer to override standard
8381 input and standard output for default device, if this seems
8382 preferable. Further, I think @emph{most} actual usages of
8383 @command{tar} are done with pipes or disks, not really tapes,
8384 cartridges or diskettes.
8386 Some users think that using standard input and output is running
8387 after trouble. This could lead to a nasty surprise on your screen if
8388 you forget to specify an output file name---especially if you are going
8389 through a network or terminal server capable of buffering large amounts
8390 of output. We had so many bug reports in that area of configuring
8391 default tapes automatically, and so many contradicting requests, that
8392 we finally consider the problem to be portably intractable. We could
8393 of course use something like @samp{/dev/tape} as a default, but this
8394 is @emph{also} running after various kind of trouble, going from hung
8395 processes to accidental destruction of real tapes. After having seen
8396 all this mess, using standard input and output as a default really
8397 sounds like the only clean choice left, and a very useful one too.
8399 @GNUTAR{} reads and writes archive in records, I
8400 suspect this is the main reason why block devices are preferred over
8401 character devices. Most probably, block devices are more efficient
8402 too. The installer could also check for @samp{DEFTAPE} in
8403 @file{<sys/mtio.h>}.
8406 @opindex force-local, short description
8408 Archive file is local even if it contains a colon.
8410 @opindex rsh-command
8411 @item --rsh-command=@var{command}
8412 Use remote @var{command} instead of @command{rsh}. This option exists
8413 so that people who use something other than the standard @command{rsh}
8414 (e.g., a Kerberized @command{rsh}) can access a remote device.
8416 When this command is not used, the shell command found when
8417 the @command{tar} program was installed is used instead. This is
8418 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
8419 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
8420 The installer may have overridden this by defining the environment
8421 variable @env{RSH} @emph{at installation time}.
8424 Specify drive and density.
8426 @opindex multi-volume, short description
8428 @itemx --multi-volume
8429 Create/list/extract multi-volume archive.
8431 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
8432 that may be larger than will fit on the medium used to hold it.
8433 @xref{Multi-Volume Archives}.
8435 @opindex tape-length, short description
8437 @itemx --tape-length=@var{num}
8438 Change tape after writing @var{num} x 1024 bytes.
8440 This option might be useful when your tape drivers do not properly
8441 detect end of physical tapes. By being slightly conservative on the
8442 maximum tape length, you might avoid the problem entirely.
8444 @opindex info-script, short description
8445 @opindex new-volume-script, short description
8447 @itemx --info-script=@var{file}
8448 @itemx --new-volume-script=@var{file}
8449 Execute @file{file} at end of each tape. This implies
8450 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
8451 description of this option.
8454 @node Remote Tape Server
8455 @section The Remote Tape Server
8457 @cindex remote tape drive
8459 In order to access the tape drive on a remote machine, @command{tar}
8460 uses the remote tape server written at the University of California at
8461 Berkeley. The remote tape server must be installed as
8462 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
8463 want to use. @command{tar} calls @command{rmt} by running an
8464 @command{rsh} or @command{remsh} to the remote machine, optionally
8465 using a different login name if one is supplied.
8467 A copy of the source for the remote tape server is provided. It is
8468 Copyright @copyright{} 1983 by the Regents of the University of
8469 California, but can be freely distributed. It is compiled and
8470 installed by default.
8472 @cindex absolute file names
8473 Unless you use the @option{--absolute-names} (@option{-P}) option,
8474 @GNUTAR{} will not allow you to create an archive that contains
8475 absolute file names (a file name beginning with @samp{/}.) If you try,
8476 @command{tar} will automatically remove the leading @samp{/} from the
8477 file names it stores in the archive. It will also type a warning
8478 message telling you what it is doing.
8480 When reading an archive that was created with a different
8481 @command{tar} program, @GNUTAR{} automatically
8482 extracts entries in the archive which have absolute file names as if
8483 the file names were not absolute. This is an important feature. A
8484 visitor here once gave a @command{tar} tape to an operator to restore;
8485 the operator used Sun @command{tar} instead of @GNUTAR{},
8486 and the result was that it replaced large portions of
8487 our @file{/bin} and friends with versions from the tape; needless to
8488 say, we were unhappy about having to recover the file system from
8491 For example, if the archive contained a file @file{/usr/bin/computoy},
8492 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
8493 relative to the current directory. If you want to extract the files in
8494 an archive to the same absolute names that they had when the archive
8495 was created, you should do a @samp{cd /} before extracting the files
8496 from the archive, or you should either use the @option{--absolute-names}
8497 option, or use the command @samp{tar -C / @dots{}}.
8499 @cindex Ultrix 3.1 and write failure
8500 Some versions of Unix (Ultrix 3.1 is known to have this problem),
8501 can claim that a short write near the end of a tape succeeded,
8502 when it actually failed. This will result in the -M option not
8503 working correctly. The best workaround at the moment is to use a
8504 significantly larger blocking factor than the default 20.
8506 In order to update an archive, @command{tar} must be able to backspace the
8507 archive in order to reread or rewrite a record that was just read (or
8508 written). This is currently possible only on two kinds of files: normal
8509 disk files (or any other file that can be backspaced with @samp{lseek}),
8510 and industry-standard 9-track magnetic tape (or any other kind of tape
8511 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}.
8513 This means that the @option{--append}, @option{--concatenate}, and
8514 @option{--delete} commands will not work on any other kind of file.
8515 Some media simply cannot be backspaced, which means these commands and
8516 options will never be able to work on them. These non-backspacing
8517 media include pipes and cartridge tape drives.
8519 Some other media can be backspaced, and @command{tar} will work on them
8520 once @command{tar} is modified to do so.
8522 Archives created with the @option{--multi-volume}, @option{--label}, and
8523 @option{--incremental} (@option{-G}) options may not be readable by other version
8524 of @command{tar}. In particular, restoring a file that was split over
8525 a volume boundary will require some careful work with @command{dd}, if
8526 it can be done at all. Other versions of @command{tar} may also create
8527 an empty file whose name is that of the volume header. Some versions
8528 of @command{tar} may create normal files instead of directories archived
8529 with the @option{--incremental} (@option{-G}) option.
8531 @node Common Problems and Solutions
8532 @section Some Common Problems and their Solutions
8539 no such file or directory
8542 errors from @command{tar}:
8543 directory checksum error
8546 errors from media/system:
8557 @dfn{Block} and @dfn{record} terminology is rather confused, and it
8558 is also confusing to the expert reader. On the other hand, readers
8559 who are new to the field have a fresh mind, and they may safely skip
8560 the next two paragraphs, as the remainder of this manual uses those
8561 two terms in a quite consistent way.
8563 John Gilmore, the writer of the public domain @command{tar} from which
8564 @GNUTAR{} was originally derived, wrote (June 1995):
8567 The nomenclature of tape drives comes from IBM, where I believe
8568 they were invented for the IBM 650 or so. On IBM mainframes, what
8569 is recorded on tape are tape blocks. The logical organization of
8570 data is into records. There are various ways of putting records into
8571 blocks, including @code{F} (fixed sized records), @code{V} (variable
8572 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
8573 to a block), @code{VB} (variable size records, @var{n} to a block),
8574 @code{VSB} (variable spanned blocked: variable sized records that can
8575 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
8576 parameter specified this to the operating system.
8578 The Unix man page on @command{tar} was totally confused about this.
8579 When I wrote @code{PD TAR}, I used the historically correct terminology
8580 (@command{tar} writes data records, which are grouped into blocks).
8581 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
8582 here), and now Fran@,{c}ois has migrated that terminology back
8583 into the source code too.
8586 The term @dfn{physical block} means the basic transfer chunk from or
8587 to a device, after which reading or writing may stop without anything
8588 being lost. In this manual, the term @dfn{block} usually refers to
8589 a disk physical block, @emph{assuming} that each disk block is 512
8590 bytes in length. It is true that some disk devices have different
8591 physical blocks, but @command{tar} ignore these differences in its own
8592 format, which is meant to be portable, so a @command{tar} block is always
8593 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
8594 The term @dfn{logical block} often represents the basic chunk of
8595 allocation of many disk blocks as a single entity, which the operating
8596 system treats somewhat atomically; this concept is only barely used
8599 The term @dfn{physical record} is another way to speak of a physical
8600 block, those two terms are somewhat interchangeable. In this manual,
8601 the term @dfn{record} usually refers to a tape physical block,
8602 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
8603 It is true that archives may be put on disk or used with pipes,
8604 but nevertheless, @command{tar} tries to read and write the archive one
8605 @dfn{record} at a time, whatever the medium in use. One record is made
8606 up of an integral number of blocks, and this operation of putting many
8607 disk blocks into a single tape block is called @dfn{reblocking}, or
8608 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
8609 the logical organization of many characters into something meaningful
8610 to the application. The term @dfn{unit record} describes a small set
8611 of characters which are transmitted whole to or by the application,
8612 and often refers to a line of text. Those two last terms are unrelated
8613 to what we call a @dfn{record} in @GNUTAR{}.
8615 When writing to tapes, @command{tar} writes the contents of the archive
8616 in chunks known as @dfn{records}. To change the default blocking
8617 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
8618 @var{512-size}}) option. Each record will then be composed of
8619 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
8620 @xref{Standard}.) Each file written to the archive uses at least one
8621 full record. As a result, using a larger record size can result in
8622 more wasted space for small files. On the other hand, a larger record
8623 size can often be read and written much more efficiently.
8625 Further complicating the problem is that some tape drives ignore the
8626 blocking entirely. For these, a larger record size can still improve
8627 performance (because the software layers above the tape drive still
8628 honor the blocking), but not as dramatically as on tape drives that
8631 When reading an archive, @command{tar} can usually figure out the
8632 record size on itself. When this is the case, and a non-standard
8633 record size was used when the archive was created, @command{tar} will
8634 print a message about a non-standard blocking factor, and then operate
8635 normally. On some tape devices, however, @command{tar} cannot figure
8636 out the record size itself. On most of those, you can specify a
8637 blocking factor (with @option{--blocking-factor}) larger than the
8638 actual blocking factor, and then use the @option{--read-full-records}
8639 (@option{-B}) option. (If you specify a blocking factor with
8640 @option{--blocking-factor} and don't use the
8641 @option{--read-full-records} option, then @command{tar} will not
8642 attempt to figure out the recording size itself.) On some devices,
8643 you must always specify the record size exactly with
8644 @option{--blocking-factor} when reading, because @command{tar} cannot
8645 figure it out. In any case, use @option{--list} (@option{-t}) before
8646 doing any extractions to see whether @command{tar} is reading the archive
8649 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
8650 putting them into records is to put a whole number of them (one or
8651 more) into each record. @command{tar} records are all the same size;
8652 at the end of the file there's a block containing all zeros, which
8653 is how you tell that the remainder of the last record(s) are garbage.
8655 In a standard @command{tar} file (no options), the block size is 512
8656 and the record size is 10240, for a blocking factor of 20. What the
8657 @option{--blocking-factor} option does is sets the blocking factor,
8658 changing the record size while leaving the block size at 512 bytes.
8659 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
8660 most tape drives these days prefer much bigger records in order to
8661 stream and not waste tape. When writing tapes for myself, some tend
8662 to use a factor of the order of 2048, say, giving a record size of
8663 around one megabyte.
8665 If you use a blocking factor larger than 20, older @command{tar}
8666 programs might not be able to read the archive, so we recommend this
8667 as a limit to use in practice. @GNUTAR{}, however,
8668 will support arbitrarily large record sizes, limited only by the
8669 amount of virtual memory or the physical characteristics of the tape
8673 * Format Variations:: Format Variations
8674 * Blocking Factor:: The Blocking Factor of an Archive
8677 @node Format Variations
8678 @subsection Format Variations
8679 @cindex Format Parameters
8680 @cindex Format Options
8681 @cindex Options, archive format specifying
8682 @cindex Options, format specifying
8685 Format parameters specify how an archive is written on the archive
8686 media. The best choice of format parameters will vary depending on
8687 the type and number of files being archived, and on the media used to
8690 To specify format parameters when accessing or creating an archive,
8691 you can use the options described in the following sections.
8692 If you do not specify any format parameters, @command{tar} uses
8693 default parameters. You cannot modify a compressed archive.
8694 If you create an archive with the @option{--blocking-factor} option
8695 specified (@pxref{Blocking Factor}), you must specify that
8696 blocking-factor when operating on the archive. @xref{Formats}, for other
8697 examples of format parameter considerations.
8699 @node Blocking Factor
8700 @subsection The Blocking Factor of an Archive
8701 @cindex Blocking Factor
8703 @cindex Number of blocks per record
8704 @cindex Number of bytes per record
8705 @cindex Bytes per record
8706 @cindex Blocks per record
8709 @opindex blocking-factor
8710 The data in an archive is grouped into blocks, which are 512 bytes.
8711 Blocks are read and written in whole number multiples called
8712 @dfn{records}. The number of blocks in a record (ie. the size of a
8713 record in units of 512 bytes) is called the @dfn{blocking factor}.
8714 The @option{--blocking-factor=@var{512-size}} (@option{-b
8715 @var{512-size}}) option specifies the blocking factor of an archive.
8716 The default blocking factor is typically 20 (i.e., 10240 bytes), but
8717 can be specified at installation. To find out the blocking factor of
8718 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
8719 This may not work on some devices.
8721 Records are separated by gaps, which waste space on the archive media.
8722 If you are archiving on magnetic tape, using a larger blocking factor
8723 (and therefore larger records) provides faster throughput and allows you
8724 to fit more data on a tape (because there are fewer gaps). If you are
8725 archiving on cartridge, a very large blocking factor (say 126 or more)
8726 greatly increases performance. A smaller blocking factor, on the other
8727 hand, may be useful when archiving small files, to avoid archiving lots
8728 of nulls as @command{tar} fills out the archive to the end of the record.
8729 In general, the ideal record size depends on the size of the
8730 inter-record gaps on the tape you are using, and the average size of the
8731 files you are archiving. @xref{create}, for information on
8734 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
8736 Archives with blocking factors larger than 20 cannot be read
8737 by very old versions of @command{tar}, or by some newer versions
8738 of @command{tar} running on old machines with small address spaces.
8739 With @GNUTAR{}, the blocking factor of an archive is limited
8740 only by the maximum record size of the device containing the archive,
8741 or by the amount of available virtual memory.
8743 Also, on some systems, not using adequate blocking factors, as sometimes
8744 imposed by the device drivers, may yield unexpected diagnostics. For
8745 example, this has been reported:
8748 Cannot write to /dev/dlt: Invalid argument
8752 In such cases, it sometimes happen that the @command{tar} bundled by
8753 the system is aware of block size idiosyncrasies, while @GNUTAR{}
8754 requires an explicit specification for the block size,
8755 which it cannot guess. This yields some people to consider
8756 @GNUTAR{} is misbehaving, because by comparison,
8757 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
8758 for example, might resolve the problem.
8760 If you use a non-default blocking factor when you create an archive, you
8761 must specify the same blocking factor when you modify that archive. Some
8762 archive devices will also require you to specify the blocking factor when
8763 reading that archive, however this is not typically the case. Usually, you
8764 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
8765 reports a non-default record size and then lists the archive members as
8766 it would normally. To extract files from an archive with a non-standard
8767 blocking factor (particularly if you're not sure what the blocking factor
8768 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
8769 specifying a blocking factor larger then the blocking factor of the archive
8770 (ie. @samp{tar --extract --read-full-records --blocking-factor=300}.
8771 @xref{list}, for more information on the @option{--list} (@option{-t})
8772 operation. @xref{Reading}, for a more detailed explanation of that option.
8775 @item --blocking-factor=@var{number}
8776 @itemx -b @var{number}
8777 Specifies the blocking factor of an archive. Can be used with any
8778 operation, but is usually not necessary with @option{--list} (@option{-t}).
8784 @item -b @var{blocks}
8785 @itemx --blocking-factor=@var{blocks}
8786 Set record size to @math{@var{blocks} * 512} bytes.
8788 This option is used to specify a @dfn{blocking factor} for the archive.
8789 When reading or writing the archive, @command{tar}, will do reads and writes
8790 of the archive in records of @math{@var{block}*512} bytes. This is true
8791 even when the archive is compressed. Some devices requires that all
8792 write operations be a multiple of a certain size, and so, @command{tar}
8793 pads the archive out to the next record boundary.
8795 The default blocking factor is set when @command{tar} is compiled, and is
8796 typically 20. Blocking factors larger than 20 cannot be read by very
8797 old versions of @command{tar}, or by some newer versions of @command{tar}
8798 running on old machines with small address spaces.
8800 With a magnetic tape, larger records give faster throughput and fit
8801 more data on a tape (because there are fewer inter-record gaps).
8802 If the archive is in a disk file or a pipe, you may want to specify
8803 a smaller blocking factor, since a large one will result in a large
8804 number of null bytes at the end of the archive.
8806 When writing cartridge or other streaming tapes, a much larger
8807 blocking factor (say 126 or more) will greatly increase performance.
8808 However, you must specify the same blocking factor when reading or
8809 updating the archive.
8811 Apparently, Exabyte drives have a physical block size of 8K bytes.
8812 If we choose our blocksize as a multiple of 8k bytes, then the problem
8813 seems to disappear. Id est, we are using block size of 112 right
8814 now, and we haven't had the problem since we switched@dots{}
8816 With @GNUTAR{} the blocking factor is limited only
8817 by the maximum record size of the device containing the archive, or by
8818 the amount of available virtual memory.
8820 However, deblocking or reblocking is virtually avoided in a special
8821 case which often occurs in practice, but which requires all the
8822 following conditions to be simultaneously true:
8825 the archive is subject to a compression option,
8827 the archive is not handled through standard input or output, nor
8828 redirected nor piped,
8830 the archive is directly handled to a local disk, instead of any special
8833 @option{--blocking-factor} is not explicitly specified on the @command{tar}
8837 If the output goes directly to a local disk, and not through
8838 stdout, then the last write is not extended to a full record size.
8839 Otherwise, reblocking occurs. Here are a few other remarks on this
8845 @command{gzip} will complain about trailing garbage if asked to
8846 uncompress a compressed archive on tape, there is an option to turn
8847 the message off, but it breaks the regularity of simply having to use
8848 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
8849 silently ignoring any number of trailing zeros. I'll ask Jean-loup
8850 Gailly, by sending a copy of this message to him.
8853 @command{compress} does not show this problem, but as Jean-loup pointed
8854 out to Michael, @samp{compress -d} silently adds garbage after
8855 the result of decompression, which tar ignores because it already
8856 recognized its end-of-file indicator. So this bug may be safely
8860 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
8861 but will still return an exit status of 2 which tar reports in turn.
8862 @command{tar} might ignore the exit status returned, but I hate doing
8863 that, as it weakens the protection @command{tar} offers users against
8864 other possible problems at decompression time. If @command{gzip} was
8865 silently skipping trailing zeros @emph{and} also avoiding setting the
8866 exit status in this innocuous case, that would solve this situation.
8869 @command{tar} should become more solid at not stopping to read a pipe at
8870 the first null block encountered. This inelegantly breaks the pipe.
8871 @command{tar} should rather drain the pipe out before exiting itself.
8874 @opindex ignore-zeros, short description
8876 @itemx --ignore-zeros
8877 Ignore blocks of zeros in archive (means EOF).
8879 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
8880 of zeros in the archive. Normally a block of zeros indicates the
8881 end of the archive, but when reading a damaged archive, or one which
8882 was created by concatenating several archives together, this option
8883 allows @command{tar} to read the entire archive. This option is not on
8884 by default because many versions of @command{tar} write garbage after
8887 Note that this option causes @command{tar} to read to the end of the
8888 archive file, which may sometimes avoid problems when multiple files
8889 are stored on a single physical tape.
8891 @opindex read-full-records, short description
8893 @itemx --read-full-records
8894 Reblock as we read (for reading 4.2BSD pipes).
8896 If @option{--read-full-records} is used, @command{tar}
8897 will not panic if an attempt to read a record from the archive does
8898 not return a full record. Instead, @command{tar} will keep reading
8899 until it has obtained a full
8902 This option is turned on by default when @command{tar} is reading
8903 an archive from standard input, or from a remote machine. This is
8904 because on BSD Unix systems, a read of a pipe will return however
8905 much happens to be in the pipe, even if it is less than @command{tar}
8906 requested. If this option was not used, @command{tar} would fail as
8907 soon as it read an incomplete record from the pipe.
8909 This option is also useful with the commands for updating an archive.
8915 @FIXME{Appropriate options should be moved here from elsewhere.}
8917 @cindex blocking factor
8918 @cindex tape blocking
8920 When handling various tapes or cartridges, you have to take care of
8921 selecting a proper blocking, that is, the number of disk blocks you
8922 put together as a single tape block on the tape, without intervening
8923 tape gaps. A @dfn{tape gap} is a small landing area on the tape
8924 with no information on it, used for decelerating the tape to a
8925 full stop, and for later regaining the reading or writing speed.
8926 When the tape driver starts reading a record, the record has to
8927 be read whole without stopping, as a tape gap is needed to stop the
8928 tape motion without loosing information.
8930 @cindex Exabyte blocking
8931 @cindex DAT blocking
8932 Using higher blocking (putting more disk blocks per tape block) will use
8933 the tape more efficiently as there will be less tape gaps. But reading
8934 such tapes may be more difficult for the system, as more memory will be
8935 required to receive at once the whole record. Further, if there is a
8936 reading error on a huge record, this is less likely that the system will
8937 succeed in recovering the information. So, blocking should not be too
8938 low, nor it should be too high. @command{tar} uses by default a blocking of
8939 20 for historical reasons, and it does not really matter when reading or
8940 writing to disk. Current tape technology would easily accommodate higher
8941 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
8942 We were told that for some DLT drives, the blocking should be a multiple
8943 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
8944 Other manufacturers may use different recommendations for the same tapes.
8945 This might also depends of the buffering techniques used inside modern
8946 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
8947 Others request blocking to be some exponent of two.
8949 So, there is no fixed rule for blocking. But blocking at read time
8950 should ideally be the same as blocking used at write time. At one place
8951 I know, with a wide variety of equipment, they found it best to use a
8952 blocking of 32 to guarantee that their tapes are fully interchangeable.
8954 I was also told that, for recycled tapes, prior erasure (by the same
8955 drive unit that will be used to create the archives) sometimes lowers
8956 the error rates observed at rewriting time.
8958 I might also use @option{--number-blocks} instead of
8959 @option{--block-number}, so @option{--block} will then expand to
8960 @option{--blocking-factor} unambiguously.
8963 @section Many Archives on One Tape
8965 @FIXME{Appropriate options should be moved here from elsewhere.}
8967 @findex ntape @r{device}
8968 Most tape devices have two entries in the @file{/dev} directory, or
8969 entries that come in pairs, which differ only in the minor number for
8970 this device. Let's take for example @file{/dev/tape}, which often
8971 points to the only or usual tape device of a given system. There might
8972 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
8973 name is the @emph{rewinding} version of the device, while the name
8974 having @samp{nr} in it is the @emph{no rewinding} version of the same
8977 A rewinding tape device will bring back the tape to its beginning point
8978 automatically when this device is opened or closed. Since @command{tar}
8979 opens the archive file before using it and closes it afterwards, this
8980 means that a simple:
8983 $ @kbd{tar cf /dev/tape @var{directory}}
8987 will reposition the tape to its beginning both prior and after saving
8988 @var{directory} contents to it, thus erasing prior tape contents and
8989 making it so that any subsequent write operation will destroy what has
8992 @cindex tape positioning
8993 So, a rewinding device is normally meant to hold one and only one file.
8994 If you want to put more than one @command{tar} archive on a given tape, you
8995 will need to avoid using the rewinding version of the tape device. You
8996 will also have to pay special attention to tape positioning. Errors in
8997 positioning may overwrite the valuable data already on your tape. Many
8998 people, burnt by past experiences, will only use rewinding devices and
8999 limit themselves to one file per tape, precisely to avoid the risk of
9000 such errors. Be fully aware that writing at the wrong position on a
9001 tape loses all information past this point and most probably until the
9002 end of the tape, and this destroyed information @emph{cannot} be
9005 To save @var{directory-1} as a first archive at the beginning of a
9006 tape, and leave that tape ready for a second archive, you should use:
9009 $ @kbd{mt -f /dev/nrtape rewind}
9010 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
9014 @dfn{Tape marks} are special magnetic patterns written on the tape
9015 media, which are later recognizable by the reading hardware. These
9016 marks are used after each file, when there are many on a single tape.
9017 An empty file (that is to say, two tape marks in a row) signal the
9018 logical end of the tape, after which no file exist. Usually,
9019 non-rewinding tape device drivers will react to the close request issued
9020 by @command{tar} by first writing two tape marks after your archive, and by
9021 backspacing over one of these. So, if you remove the tape at that time
9022 from the tape drive, it is properly terminated. But if you write
9023 another file at the current position, the second tape mark will be
9024 erased by the new information, leaving only one tape mark between files.
9026 So, you may now save @var{directory-2} as a second archive after the
9027 first on the same tape by issuing the command:
9030 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
9034 and so on for all the archives you want to put on the same tape.
9036 Another usual case is that you do not write all the archives the same
9037 day, and you need to remove and store the tape between two archive
9038 sessions. In general, you must remember how many files are already
9039 saved on your tape. Suppose your tape already has 16 files on it, and
9040 that you are ready to write the 17th. You have to take care of skipping
9041 the first 16 tape marks before saving @var{directory-17}, say, by using
9045 $ @kbd{mt -f /dev/nrtape rewind}
9046 $ @kbd{mt -f /dev/nrtape fsf 16}
9047 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
9050 In all the previous examples, we put aside blocking considerations, but
9051 you should do the proper things for that as well. @xref{Blocking}.
9054 * Tape Positioning:: Tape Positions and Tape Marks
9055 * mt:: The @command{mt} Utility
9058 @node Tape Positioning
9059 @subsection Tape Positions and Tape Marks
9062 Just as archives can store more than one file from the file system,
9063 tapes can store more than one archive file. To keep track of where
9064 archive files (or any other type of file stored on tape) begin and
9065 end, tape archive devices write magnetic @dfn{tape marks} on the
9066 archive media. Tape drives write one tape mark between files,
9067 two at the end of all the file entries.
9069 If you think of data as a series of records "rrrr"'s, and tape marks as
9070 "*"'s, a tape might look like the following:
9073 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
9076 Tape devices read and write tapes using a read/write @dfn{tape
9077 head}---a physical part of the device which can only access one
9078 point on the tape at a time. When you use @command{tar} to read or
9079 write archive data from a tape device, the device will begin reading
9080 or writing from wherever on the tape the tape head happens to be,
9081 regardless of which archive or what part of the archive the tape
9082 head is on. Before writing an archive, you should make sure that no
9083 data on the tape will be overwritten (unless it is no longer needed).
9084 Before reading an archive, you should make sure the tape head is at
9085 the beginning of the archive you want to read. You can do it manually
9086 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
9087 that automatically (@pxref{Scripted Restoration}).
9089 If you want to add new archive file entries to a tape, you should
9090 advance the tape to the end of the existing file entries, backspace
9091 over the last tape mark, and write the new archive file. If you were
9092 to add two archives to the example above, the tape might look like the
9096 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
9100 @subsection The @command{mt} Utility
9103 @FIXME{Is it true that this only works on non-block devices?
9104 should explain the difference, (fixed or variable).}
9105 @xref{Blocking Factor}.
9107 You can use the @command{mt} utility to advance or rewind a tape past a
9108 specified number of archive files on the tape. This will allow you
9109 to move to the beginning of an archive before extracting or reading
9110 it, or to the end of all the archives before writing a new one.
9111 @FIXME{Why isn't there an "advance 'til you find two tape marks
9114 The syntax of the @command{mt} command is:
9117 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
9120 where @var{tapename} is the name of the tape device, @var{number} is
9121 the number of times an operation is performed (with a default of one),
9122 and @var{operation} is one of the following:
9124 @FIXME{is there any use for record operations?}
9129 Writes @var{number} tape marks at the current position on the tape.
9132 Moves tape position forward @var{number} files.
9135 Moves tape position back @var{number} files.
9138 Rewinds the tape. (Ignores @var{number}).
9142 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}).
9145 Prints status information about the tape unit.
9149 @FIXME{Is there a better way to frob the spacing on the list?}
9151 If you don't specify a @var{tapename}, @command{mt} uses the environment
9152 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} uses the device
9155 @command{mt} returns a 0 exit status when the operation(s) were
9156 successful, 1 if the command was unrecognized, and 2 if an operation
9159 @node Using Multiple Tapes
9160 @section Using Multiple Tapes
9163 Often you might want to write a large archive, one larger than will fit
9164 on the actual tape you are using. In such a case, you can run multiple
9165 @command{tar} commands, but this can be inconvenient, particularly if you
9166 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
9167 Therefore, @command{tar} supports multiple tapes automatically.
9169 Use @option{--multi-volume} (@option{-M}) on the command line, and
9170 then @command{tar} will, when it reaches the end of the tape, prompt
9171 for another tape, and continue the archive. Each tape will have an
9172 independent archive, and can be read without needing the other. (As
9173 an exception to this, the file that @command{tar} was archiving when
9174 it ran out of tape will usually be split between the two archives; in
9175 this case you need to extract from the first archive, using
9176 @option{--multi-volume}, and then put in the second tape when
9177 prompted, so @command{tar} can restore both halves of the file.)
9179 @GNUTAR{} multi-volume archives do not use a truly portable format.
9180 You need @GNUTAR{} at both ends to process them properly.
9182 When prompting for a new tape, @command{tar} accepts any of the following
9187 Request @command{tar} to explain possible responses
9189 Request @command{tar} to exit immediately.
9190 @item n @var{file-name}
9191 Request @command{tar} to write the next volume on the file @var{file-name}.
9193 Request @command{tar} to run a subshell. This option can be disabled
9194 by giving @option{--restrict} command line option to @command{tar}.
9196 Request @command{tar} to begin writing the next volume.
9199 (You should only type @samp{y} after you have changed the tape;
9200 otherwise @command{tar} will write over the volume it just finished.)
9202 @cindex End-of-archive info script
9204 @anchor{info-script}
9205 @opindex info-script
9206 @opindex new-volume-script
9207 If you want more elaborate behavior than this, give @command{tar} the
9208 @option{--info-script=@var{script-name}}
9209 (@option{--new-volume-script=@var{script-name}}, @option{-F
9210 @var{script-name}}) option. The file @var{script-name} is expected to
9211 be a program (or shell script) to be run instead of the normal
9212 prompting procedure. It is executed without any command line
9213 arguments. Additional data is passed to it via the following
9214 environment variables:
9217 @vrindex TAR_VERSION, info script environment variable
9219 @GNUTAR{} version number.
9221 @vrindex TAR_ARCHIVE, info script environment variable
9223 The name of the archive @command{tar} is processing.
9225 @vrindex TAR_VOLUME, info script environment variable
9227 Ordinal number of the volume @command{tar} is about to start.
9229 @vrindex TAR_SUBCOMMAND, info script environment variable
9230 @item TAR_SUBCOMMAND
9231 Short option describing the operation @command{tar} is executed.
9232 @xref{Operations}, for a complete list of subcommand options.
9234 @vrindex TAR_FORMAT, info script environment variable
9236 Format of the archive being processed. @xref{Formats}, for a complete
9237 list of archive format names.
9240 The info script can instruct @command{tar} to use new archive name,
9241 by writing in to file descriptor 3 (see below for an
9244 If the info script fails, @command{tar} exits; otherwise, it begins
9245 writing the next volume.
9247 The method @command{tar} uses to detect end of tape is not perfect, and
9248 fails on some operating systems or on some devices. You can use the
9249 @option{--tape-length=@var{size}} (@option{-L @var{size}}) option if
9250 @command{tar} can't detect the end of the tape itself. This option
9251 selects @option{--multi-volume} (@option{-M}) automatically. The
9252 @var{size} argument should then be the usable size of the tape in
9253 units of 1024 bytes. But for many devices, and floppy disks in
9254 particular, this option is never required for real, as far as we know.
9256 @cindex Volume number file
9260 The volume number used by @command{tar} in its tape-change prompt
9261 can be changed; if you give the
9262 @option{--volno-file=@var{file-of-number}} option, then
9263 @var{file-of-number} should be an unexisting file to be created, or
9264 else, a file already containing a decimal number. That number will be
9265 used as the volume number of the first volume written. When
9266 @command{tar} is finished, it will rewrite the file with the
9267 now-current volume number. (This does not change the volume number
9268 written on a tape label, as per @ref{label}, it @emph{only} affects
9269 the number used in the prompt.)
9271 If you want @command{tar} to cycle through a series of files or tape
9272 drives, there are three approaches to choose from. First of all, you
9273 can give @command{tar} multiple @option{--file} options. In this case
9274 the specified files will be used, in sequence, as the successive
9275 volumes of the archive. Only when the first one in the sequence needs
9276 to be used again will @command{tar} prompt for a tape change (or run
9277 the info script). Secondly, you can use the @samp{n} response to the
9278 tape-change prompt, and, finally, you can use an info script, that
9279 writes new archive name to file descriptor. The following example
9280 illustrates this approach:
9285 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
9287 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
9288 case $TAR_SUBCOMMAND in
9290 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
9295 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&3
9299 Each volume of a multi-volume archive is an independent @command{tar}
9300 archive, complete in itself. For example, you can list or extract any
9301 volume alone; just don't specify @option{--multi-volume}
9302 (@option{-M}). However, if one file in the archive is split across
9303 volumes, the only way to extract it successfully is with a
9304 multi-volume extract command @option{--extract --multi-volume}
9305 (@option{-xM}) starting on or before the volume where the file begins.
9307 For example, let's presume someone has two tape drives on a system
9308 named @file{/dev/tape0} and @file{/dev/tape1}. For having @GNUTAR{}
9309 to switch to the second drive when it needs to write the
9310 second tape, and then back to the first tape, etc., just do either of:
9313 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
9314 $ @kbd{tar cMff /dev/tape0 /dev/tape1 @var{files}}
9318 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
9319 * Tape Files:: Tape Files
9320 * Tarcat:: Concatenate Volumes into a Single Archive
9324 @node Multi-Volume Archives
9325 @subsection Archives Longer than One Tape or Disk
9326 @cindex Multi-volume archives
9329 @opindex multi-volume
9330 To create an archive that is larger than will fit on a single unit of
9331 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
9332 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
9333 archive can be manipulated like any other archive (provided the
9334 @option{--multi-volume} option is specified), but is stored on more
9335 than one tape or disk.
9337 When you specify @option{--multi-volume}, @command{tar} does not report an
9338 error when it comes to the end of an archive volume (when reading), or
9339 the end of the media (when writing). Instead, it prompts you to load
9340 a new storage volume. If the archive is on a magnetic tape, you
9341 should change tapes when you see the prompt; if the archive is on a
9342 floppy disk, you should change disks; etc.
9344 You can read each individual volume of a multi-volume archive as if it
9345 were an archive by itself. For example, to list the contents of one
9346 volume, use @option{--list}, without @option{--multi-volume} specified.
9347 To extract an archive member from one volume (assuming it is described
9348 that volume), use @option{--extract}, again without
9349 @option{--multi-volume}.
9351 If an archive member is split across volumes (ie. its entry begins on
9352 one volume of the media and ends on another), you need to specify
9353 @option{--multi-volume} to extract it successfully. In this case, you
9354 should load the volume where the archive member starts, and use
9355 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
9356 volumes as it needs them. @xref{extracting archives}, for more
9357 information about extracting archives.
9359 @option{--info-script=@var{script-name}}
9360 (@option{--new-volume-script=@var{script-name}}, @option{-F
9361 @var{script-name}}) (@pxref{info-script}) is like
9362 @option{--multi-volume} (@option{-M}), except that @command{tar} does
9363 not prompt you directly to change media volumes when a volume is
9364 full---instead, @command{tar} runs commands you have stored in
9365 @var{script-name}. For example, this option can be used to eject
9366 cassettes, or to broadcast messages such as @samp{Someone please come
9367 change my tape} when performing unattended backups. When
9368 @var{script-name} is done, @command{tar} will assume that the media
9371 Multi-volume archives can be modified like any other archive. To add
9372 files to a multi-volume archive, you need to only mount the last
9373 volume of the archive media (and new volumes, if needed). For all
9374 other operations, you need to use the entire archive.
9376 If a multi-volume archive was labeled using
9377 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9378 (@pxref{label}) when it was created, @command{tar} will not
9379 automatically label volumes which are added later. To label
9380 subsequent volumes, specify @option{--label=@var{archive-label}} again
9381 in conjunction with the @option{--append}, @option{--update} or
9382 @option{--concatenate} operation.
9384 @cindex Labeling multi-volume archives
9387 @FIXME{There should be a sample program here, including an exit
9388 before end. Is the exit status even checked in tar? :-(}
9391 @item --multi-volume
9393 Creates a multi-volume archive, when used in conjunction with
9394 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
9395 archive, specify @option{--multi-volume} in conjunction with that
9398 @item --info-script=@var{program-file}
9399 @itemx --new-volume-script=@var{program-file}
9400 @itemx -F @var{program-file}
9401 Creates a multi-volume archive via a script. Used in conjunction with
9402 @option{--create} (@option{-c}). @xref{info-script}, dor a detailed discussion.
9405 Beware that there is @emph{no} real standard about the proper way, for
9406 a @command{tar} archive, to span volume boundaries. If you have a
9407 multi-volume created by some vendor's @command{tar}, there is almost
9408 no chance you could read all the volumes with @GNUTAR{}.
9409 The converse is also true: you may not expect
9410 multi-volume archives created by @GNUTAR{} to be
9411 fully recovered by vendor's @command{tar}. Since there is little
9412 chance that, in mixed system configurations, some vendor's
9413 @command{tar} will work on another vendor's machine, and there is a
9414 great chance that @GNUTAR{} will work on most of
9415 them, your best bet is to install @GNUTAR{} on all
9416 machines between which you know exchange of files is possible.
9419 @subsection Tape Files
9422 To give the archive a name which will be recorded in it, use the
9423 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
9424 option. This will write a special block identifying
9425 @var{volume-label} as the name of the archive to the front of the
9426 archive which will be displayed when the archive is listed with
9427 @option{--list}. If you are creating a multi-volume archive with
9428 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
9429 volume label will have @samp{Volume @var{nnn}} appended to the name
9430 you give, where @var{nnn} is the number of the volume of the archive.
9431 (If you use the @option{--label=@var{volume-label}}) option when
9432 reading an archive, it checks to make sure the label on the tape
9433 matches the one you give. @xref{label}.
9435 When @command{tar} writes an archive to tape, it creates a single
9436 tape file. If multiple archives are written to the same tape, one
9437 after the other, they each get written as separate tape files. When
9438 extracting, it is necessary to position the tape at the right place
9439 before running @command{tar}. To do this, use the @command{mt} command.
9440 For more information on the @command{mt} command and on the organization
9441 of tapes into a sequence of tape files, see @ref{mt}.
9443 People seem to often do:
9446 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
9449 or such, for pushing a common date in all volumes or an archive set.
9452 @subsection Concatenate Volumes into a Single Archive
9455 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
9456 archive to a single @command{tar} archive. Simply concatenating all
9457 volumes into one will not work, since each volume carries an additional
9458 information at the beginning. @GNUTAR{} is shipped with the shell
9459 script @command{tarcat} designed for this purpose.
9461 The script takes a list of files comprising a multi-volume archive
9462 and creates the resulting archive at the standard output. For example:
9465 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
9468 The script implements a simple heuristics to determine the format of
9469 the first volume file and to decide how to process the rest of the
9470 files. However, it makes no attempt to verify whether the files are
9471 given in order or even if they are valid @command{tar} archives.
9472 It uses @command{dd} and does not filter its standard error, so you
9473 will usually see lots of spurious messages.
9475 @FIXME{The script is not installed. Should we install it?}
9478 @section Including a Label in the Archive
9479 @cindex Labeling an archive
9480 @cindex Labels on the archive media
9484 To avoid problems caused by misplaced paper labels on the archive
9485 media, you can include a @dfn{label} entry---an archive member which
9486 contains the name of the archive---in the archive itself. Use the
9487 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9488 option in conjunction with the @option{--create} operation to include
9489 a label entry in the archive as it is being created.
9492 @item --label=@var{archive-label}
9493 @itemx -V @var{archive-label}
9494 Includes an @dfn{archive-label} at the beginning of the archive when
9495 the archive is being created, when used in conjunction with the
9496 @option{--create} operation. Checks to make sure the archive label
9497 matches the one specified (when used in conjunction with any other
9501 If you create an archive using both
9502 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9503 and @option{--multi-volume} (@option{-M}), each volume of the archive
9504 will have an archive label of the form @samp{@var{archive-label}
9505 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
9506 next, and so on. @xref{Using Multiple Tapes}, for information on
9507 creating multiple volume archives.
9509 @cindex Volume label, listing
9510 @cindex Listing volume label
9511 The volume label will be displayed by @option{--list} along with
9512 the file contents. If verbose display is requested, it will also be
9513 explicitely marked as in the example below:
9517 $ @kbd{tar --verbose --list --file=iamanarchive}
9518 V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header--
9519 -rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename
9524 @anchor{--test-label option}
9525 However, @option{--list} option will cause listing entire
9526 contents of the archive, which may be undesirable (for example, if the
9527 archive is stored on a tape). You can request checking only the volume
9528 by specifying @option{--test-label} option. This option reads only the
9529 first block of an archive, so it can be used with slow storage
9530 devices. For example:
9534 $ @kbd{tar --test-label --file=iamanarchive}
9539 If @option{--test-label} is used with a single command line
9540 argument, @command{tar} compares the volume label with the
9541 argument. It exits with code 0 if the two strings match, and with code
9542 2 otherwise. In this case no output is displayed. For example:
9546 $ @kbd{tar --test-label --file=iamanarchive 'iamalable'}
9548 $ @kbd{tar --test-label --file=iamanarchive 'iamalable' alabel}
9553 If you request any operation, other than @option{--create}, along
9554 with using @option{--label} option, @command{tar} will first check if
9555 the archive label matches the one specified and will refuse to proceed
9556 if it does not. Use this as a safety precaution to avoid accidentally
9557 overwriting existing archives. For example, if you wish to add files
9558 to @file{archive}, presumably labelled with string @samp{My volume},
9563 $ @kbd{tar -rf archive --label 'My volume' .}
9564 tar: Archive not labeled to match `My volume'
9569 in case its label does not match. This will work even if
9570 @file{archive} is not labelled at all.
9572 Similarly, @command{tar} will refuse to list or extract the
9573 archive if its label doesn't match the @var{archive-label}
9574 specified. In those cases, @var{archive-label} argument is interpreted
9575 as a globbing-style pattern which must match the actual magnetic
9576 volume label. @xref{exclude}, for a precise description of how match
9577 is attempted@footnote{Previous versions of @command{tar} used full
9578 regular expression matching, or before that, only exact string
9579 matching, instead of wildcard matchers. We decided for the sake of
9580 simplicity to use a uniform matching device through
9581 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
9582 the volume label matcher will also suffix @var{archive-label} by
9583 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
9584 up. Since the volume numbering is automatically added in labels at
9585 creation time, it sounded logical to equally help the user taking care
9586 of it when the archive is being read.
9588 The @option{--label} was once called @option{--volume}, but is not
9589 available under that name anymore.
9591 You can also use @option{--label} to get a common information on
9592 all tapes of a series. For having this information different in each
9593 series created through a single script used on a regular basis, just
9594 manage to get some date string as part of the label. For example:
9598 $ @kbd{tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"}
9599 $ @kbd{tar --create --file=/dev/tape --multi-volume \
9600 --volume="Daily backup for `date +%Y-%m-%d`"}
9604 Also note that each label has its own date and time, which corresponds
9605 to when @GNUTAR{} initially attempted to write it,
9606 often soon after the operator launches @command{tar} or types the
9607 carriage return telling that the next tape is ready. Comparing date
9608 labels does give an idea of tape throughput only if the delays for
9609 rewinding tapes and the operator switching them were negligible, which
9610 is usually not the case.
9613 @section Verifying Data as It is Stored
9614 @cindex Verifying a write operation
9615 @cindex Double-checking a write operation
9620 @opindex verify, short description
9621 Attempt to verify the archive after writing.
9624 This option causes @command{tar} to verify the archive after writing it.
9625 Each volume is checked after it is written, and any discrepancies
9626 are recorded on the standard error output.
9628 Verification requires that the archive be on a back-space-able medium.
9629 This means pipes, some cartridge tape drives, and some other devices
9632 You can insure the accuracy of an archive by comparing files in the
9633 system with archive members. @command{tar} can compare an archive to the
9634 file system as the archive is being written, to verify a write
9635 operation, or can compare a previously written archive, to insure that
9638 @opindex verify, using with @option{--create}
9639 @opindex create, using with @option{--verify}
9640 To check for discrepancies in an archive immediately after it is
9641 written, use the @option{--verify} (@option{-W}) option in conjunction with
9642 the @option{--create} operation. When this option is
9643 specified, @command{tar} checks archive members against their counterparts
9644 in the file system, and reports discrepancies on the standard error.
9646 To verify an archive, you must be able to read it from before the end
9647 of the last written entry. This option is useful for detecting data
9648 errors on some tapes. Archives written to pipes, some cartridge tape
9649 drives, and some other devices cannot be verified.
9651 One can explicitly compare an already made archive with the file
9652 system by using the @option{--compare} (@option{--diff}, @option{-d})
9653 option, instead of using the more automatic @option{--verify} option.
9656 Note that these two options have a slightly different intent. The
9657 @option{--compare} option checks how identical are the logical contents of some
9658 archive with what is on your disks, while the @option{--verify} option is
9659 really for checking if the physical contents agree and if the recording
9660 media itself is of dependable quality. So, for the @option{--verify}
9661 operation, @command{tar} tries to defeat all in-memory cache pertaining to
9662 the archive, while it lets the speed optimization undisturbed for the
9663 @option{--compare} option. If you nevertheless use @option{--compare} for
9664 media verification, you may have to defeat the in-memory cache yourself,
9665 maybe by opening and reclosing the door latch of your recording unit,
9666 forcing some doubt in your operating system about the fact this is really
9667 the same volume as the one just written or read.
9669 The @option{--verify} option would not be necessary if drivers were indeed
9670 able to detect dependably all write failures. This sometimes require many
9671 magnetic heads, some able to read after the writes occurred. One would
9672 not say that drivers unable to detect all cases are necessarily flawed,
9673 as long as programming is concerned.
9675 The @option{--verify} (@option{-W}) option will not work in
9676 conjunction with the @option{--multi-volume} (@option{-M}) option or
9677 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
9678 and @option{--delete} operations. @xref{Operations}, for more
9679 information on these operations.
9681 Also, since @command{tar} normally strips leading @samp{/} from file
9682 names (@pxref{absolute}), a command like @samp{tar --verify -cf
9683 /tmp/foo.tar /etc} will work as desired only if the working directory is
9684 @file{/}, as @command{tar} uses the archive's relative member names
9685 (e.g., @file{etc/motd}) when verifying the archive.
9687 @node Write Protection
9688 @section Write Protection
9690 Almost all tapes and diskettes, and in a few rare cases, even disks can
9691 be @dfn{write protected}, to protect data on them from being changed.
9692 Once an archive is written, you should write protect the media to prevent
9693 the archive from being accidentally overwritten or deleted. (This will
9694 protect the archive from being changed with a tape or floppy drive---it
9695 will not protect it from magnet fields or other physical hazards).
9697 The write protection device itself is usually an integral part of the
9698 physical media, and can be a two position (write enabled/write
9699 disabled) switch, a notch which can be popped out or covered, a ring
9700 which can be removed from the center of a tape reel, or some other
9706 This appendix lists some important user-visible changes between
9707 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
9708 version of this document is available at
9709 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
9710 @GNUTAR{} documentation page}.
9713 @item Use of globbing patterns when listing and extracting.
9715 Previous versions of GNU tar assumed shell-style globbing when
9716 extracting from or listing an archive. For example:
9719 $ @kbd{tar xf foo.tar '*.c'}
9722 would extract all files whose names end in @samp{.c}. This behavior
9723 was not documented and was incompatible with traditional tar
9724 implementations. Therefore, starting from version 1.15.91, GNU tar
9725 no longer uses globbing by default. For example, the above invocation
9726 is now interpreted as a request to extract from the archive the file
9729 To facilitate transition to the new behavior for those users who got
9730 used to the previous incorrect one, @command{tar} will print a warning
9731 if it finds out that a requested member was not found in the archive
9732 and its name looks like a globbing pattern. For example:
9735 $ @kbd{tar xf foo.tar '*.c'}
9736 tar: Pattern matching characters used in file names. Please,
9737 tar: use --wildcards to enable pattern matching, or --no-wildcards to
9738 tar: suppress this warning.
9739 tar: *.c: Not found in archive
9740 tar: Error exit delayed from previous errors
9743 To treat member names as globbing patterns, use --wildcards option.
9744 If you want to tar to mimic the behavior of versions prior to 1.15.91,
9745 add this option to your @env{TAR_OPTIONS} variable.
9747 @xref{Wildcards}, for the detailed discussion of the use of globbing
9748 patterns by @GNUTAR{}.
9750 @item Use of short option @option{-o}.
9752 Earlier versions of @GNUTAR{} understood @option{-o} command line
9753 option as a synonym for @option{--old-archive}.
9755 @GNUTAR{} starting from version 1.13.90 understands this option as
9756 a synonym for @option{--no-same-owner}. This is compatible with
9757 UNIX98 @command{tar} implementations.
9759 However, to facilitate transition, @option{-o} option retains its
9760 old semantics when it is used with one of archive-creation commands.
9761 Users are encouraged to use @option{--format=oldgnu} instead.
9763 It is especially important, since versions of @acronym{GNU} Automake
9764 up to and including 1.8.4 invoke tar with this option to produce
9765 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
9766 of this issue and its implications.
9768 @FIXME{Change the first argument to tar-formats if and when Automake
9769 people accept my patch to the documentation, and the new Automake is
9770 out --Sergey 2006-05-25}.
9771 @xref{Options, tar-v7, Changing Automake's Behavior,
9772 automake, GNU Automake}, for a description on how to use various
9773 archive formats with @command{automake}.
9775 Future versions of @GNUTAR{} will understand @option{-o} only as a
9776 synonym for @option{--no-same-owner}.
9778 @item Use of short option @option{-l}
9780 Earlier versions of @GNUTAR{} understood @option{-l} option as a
9781 synonym for @option{--one-file-system}. Since such usage contradicted
9782 to UNIX98 specification and harmed compatibility with other
9783 implementation, it was declared deprecated in version 1.14. However,
9784 to facilitate transition to its new semantics, it was supported by
9785 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
9786 variant of @option{--check-links} was introduced in version 1.15.91.
9788 @item Use of options @option{--portability} and @option{--old-archive}
9790 These options are deprecated. Please use @option{--format=v7} instead.
9792 @item Use of option @option{--posix}
9794 This option is deprecated. Please use @option{--format=posix} instead.
9797 @node Configuring Help Summary
9798 @appendix Configuring Help Summary
9800 Running @kbd{tar --help} displays the short @command{tar} option
9801 summary (@pxref{help}). This summary is organised by @dfn{groups} of
9802 semantically close options. The options within each group are printed
9803 in the following order: a short option, eventually followed by a list
9804 of corresponding long option names, followed by a short description of
9805 the option. For example, here is an excerpt from the actual @kbd{tar
9809 Main operation mode:
9811 -A, --catenate, --concatenate append tar files to an archive
9812 -c, --create create a new archive
9813 -d, --diff, --compare find differences between archive and
9815 --delete delete from the archive
9818 @vrindex ARGP_HELP_FMT, environment variable
9819 The exact visual representation of the help output is configurable via
9820 @env{ARGP_HELP_FMT} environment variable. The value of this variable
9821 is a comma-separated list of @dfn{format variable} assignments. There
9822 are two kinds of format variables. An @dfn{offset variable} keeps the
9823 offset of some part of help output text from the leftmost column on
9824 the screen. A @dfn{boolean} variable is a flag that toggles some
9825 output feature on or off. Depending on the type of the corresponding
9826 variable, there are two kinds of assignments:
9829 @item Offset assignment
9831 The assignment to an offset variable has the following syntax:
9834 @var{variable}=@var{value}
9838 where @var{variable} is the variable name, and @var{value} is a
9839 numeric value to be assigned to the variable.
9841 @item Boolean assignment
9843 To assign @code{true} value to a variable, simply put this variable name. To
9844 assign @code{false} value, prefix the variable name with @samp{no-}. For
9849 # Assign @code{true} value:
9851 # Assign @code{false} value:
9857 Following variables are declared:
9859 @deftypevr {Help Output} boolean dup-args
9860 If true, arguments for an option are shown with both short and long
9861 options, even when a given option has both forms, for example:
9864 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
9867 If false, then if an option has both short and long forms, the
9868 argument is only shown with the long one, for example:
9871 -f, --file=ARCHIVE use archive file or device ARCHIVE
9875 and a message indicating that the argument is applicable to both
9876 forms is printed below the options. This message can be disabled
9877 using @code{dup-args-note} (see below).
9879 The default is false.
9882 @deftypevr {Help Output} boolean dup-args-note
9883 If this variable is true, which is the default, the following notice
9884 is displayed at the end of the help output:
9887 Mandatory or optional arguments to long options are also mandatory or
9888 optional for any corresponding short options.
9891 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
9892 variables @code{dup-args} or @code{dup-args-note} should be set.
9895 @deftypevr {Help Output} offset short-opt-col
9896 Column in which short options start. Default is 2.
9900 $ @kbd{tar --help|grep ARCHIVE}
9901 -f, --file=ARCHIVE use archive file or device ARCHIVE
9902 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
9903 -f, --file=ARCHIVE use archive file or device ARCHIVE
9908 @deftypevr {Help Output} offset long-opt-col
9909 Column in which long options start. Default is 6. For example:
9913 $ @kbd{tar --help|grep ARCHIVE}
9914 -f, --file=ARCHIVE use archive file or device ARCHIVE
9915 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
9916 -f, --file=ARCHIVE use archive file or device ARCHIVE
9921 @deftypevr {Help Output} offset doc-opt-col
9922 Column in which @dfn{doc options} start. A doc option isn't actually
9923 an option, but rather an arbitrary piece of documentation that is
9924 displayed in much the same manner as the options. For example, in
9925 the description of @option{--format} option:
9929 -H, --format=FORMAT create archive of the given format.
9931 FORMAT is one of the following:
9933 gnu GNU tar 1.13.x format
9934 oldgnu GNU format as per tar <= 1.12
9935 pax POSIX 1003.1-2001 (pax) format
9937 ustar POSIX 1003.1-1988 (ustar) format
9938 v7 old V7 tar format
9943 the format names are doc options. Thus, if you set
9944 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
9945 will look as follows:
9949 -H, --format=FORMAT create archive of the given format.
9951 FORMAT is one of the following:
9953 gnu GNU tar 1.13.x format
9954 oldgnu GNU format as per tar <= 1.12
9955 pax POSIX 1003.1-2001 (pax) format
9957 ustar POSIX 1003.1-1988 (ustar) format
9958 v7 old V7 tar format
9963 @deftypevr {Help Output} offset opt-doc-col
9964 Column in which option description starts. Default is 29.
9968 $ @kbd{tar --help|grep ARCHIVE}
9969 -f, --file=ARCHIVE use archive file or device ARCHIVE
9970 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
9971 -f, --file=ARCHIVE use archive file or device ARCHIVE
9972 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
9974 use archive file or device ARCHIVE
9979 Notice, that the description starts on a separate line if
9980 @code{opt-doc-col} value is too small.
9983 @deftypevr {Help Output} offset header-col
9984 Column in which @dfn{group headers} are printed. A group header is a
9985 descriptive text preceding an option group. For example, in the
9989 Main operation mode:
9991 -A, --catenate, --concatenate append tar files to
9993 -c, --create create a new archive
9996 @samp{Main operation mode:} is the group header.
9998 The default value is 1.
10001 @deftypevr {Help Output} offset usage-indent
10002 Indentation of wrapped usage lines. Affects @option{--usage}
10003 output. Default is 12.
10006 @deftypevr {Help Output} offset rmargin
10007 Right margin of the text output. Used for wrapping.
10012 @include genfile.texi
10014 @node Snapshot Files
10015 @appendix Format of the Incremental Snapshot Files
10016 @include snapshot.texi
10018 @node Free Software Needs Free Documentation
10019 @appendix Free Software Needs Free Documentation
10020 @include freemanuals.texi
10022 @node Copying This Manual
10023 @appendix Copying This Manual
10026 * GNU Free Documentation License:: License for copying this manual
10031 @node Index of Command Line Options
10032 @appendix Index of Command Line Options
10034 This appendix contains an index of all @GNUTAR{} long command line
10035 options. The options are listed without the preceeding double-dash.
10038 @item Make sure @emph{all} options are indexed.
10039 @item Provide an index of short options
10053 @c Local variables:
10054 @c texinfo-column-for-description: 32