1 \input texinfo @c -*-texinfo-*-
2 @comment %**start of header
5 @settitle GNU tar @value{VERSION}
13 @include rendition.texi
16 @c Put everything in one index (arbitrarily chosen to be the concept index).
26 This manual is for @acronym{GNU} @command{tar} (version
27 @value{VERSION}, @value{UPDATED}), which creates and extracts files
30 Copyright @copyright{} 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001,
31 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
34 Permission is granted to copy, distribute and/or modify this document
35 under the terms of the GNU Free Documentation License, Version 1.1 or
36 any later version published by the Free Software Foundation; with no
37 Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
38 and with the Back-Cover Texts as in (a) below. A copy of the license
39 is included in the section entitled "GNU Free Documentation License".
41 (a) The FSF's Back-Cover Text is: ``You are free to copy and modify
42 this GNU Manual. Buying copies from GNU Press supports the FSF in
43 developing GNU and promoting software freedom.''
47 @dircategory Archiving
49 * Tar: (tar). Making tape (or disk) archives.
52 @dircategory Individual utilities
54 * tar: (tar)tar invocation. Invoking @GNUTAR{}.
57 @shorttitlepage @acronym{GNU} @command{tar}
60 @title @acronym{GNU} tar: an archiver tool
61 @subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED}
62 @author John Gilmore, Jay Fenlason et al.
65 @vskip 0pt plus 1filll
71 @top @acronym{GNU} tar: an archiver tool
76 @cindex archiving files
78 The first part of this master menu lists the major nodes in this Info
79 document. The rest of the menu lists all the lower level nodes.
82 @c The master menu, created with texinfo-master-menu, goes here.
83 @c (However, getdate.texi's menu is interpolated by hand.)
92 * Date input formats::
99 * Configuring Help Summary::
102 * Free Software Needs Free Documentation::
103 * Copying This Manual::
104 * Index of Command Line Options::
108 --- The Detailed Node Listing ---
112 * Book Contents:: What this Book Contains
113 * Definitions:: Some Definitions
114 * What tar Does:: What @command{tar} Does
115 * Naming tar Archives:: How @command{tar} Archives are Named
116 * Authors:: @GNUTAR{} Authors
117 * Reports:: Reporting bugs or suggestions
119 Tutorial Introduction to @command{tar}
122 * stylistic conventions::
123 * basic tar options:: Basic @command{tar} Operations and Options
124 * frequent operations::
125 * Two Frequent Options::
126 * create:: How to Create Archives
127 * list:: How to List Archives
128 * extract:: How to Extract Members from an Archive
131 Two Frequently Used Options
137 How to Create Archives
139 * prepare for examples::
140 * Creating the archive::
149 How to Extract Members from an Archive
151 * extracting archives::
159 * using tar options::
167 The Three Option Styles
169 * Mnemonic Options:: Mnemonic Option Style
170 * Short Options:: Short Option Style
171 * Old Options:: Old Option Style
172 * Mixing:: Mixing Option Styles
174 All @command{tar} Options
176 * Operation Summary::
178 * Short Option Summary::
190 Advanced @GNUTAR{} Operations
199 How to Add Files to Existing Archives: @option{--append}
201 * appending files:: Appending Files to an Archive
208 Options Used by @option{--create}
210 * Ignore Failed Read::
212 Options Used by @option{--extract}
214 * Reading:: Options to Help Read Archives
215 * Writing:: Changing How @command{tar} Writes Files
216 * Scarce:: Coping with Scarce Resources
218 Options to Help Read Archives
220 * read full records::
223 Changing How @command{tar} Writes Files
225 * Dealing with Old Files::
226 * Overwrite Old Files::
231 * Data Modification Times::
232 * Setting Access Permissions::
233 * Writing to Standard Output::
236 Coping with Scarce Resources
241 Performing Backups and Restoring Files
243 * Full Dumps:: Using @command{tar} to Perform Full Dumps
244 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
245 * Backup Levels:: Levels of Backups
246 * Backup Parameters:: Setting Parameters for Backups and Restoration
247 * Scripted Backups:: Using the Backup Scripts
248 * Scripted Restoration:: Using the Restore Script
250 Setting Parameters for Backups and Restoration
252 * General-Purpose Variables::
253 * Magnetic Tape Control::
255 * backup-specs example:: An Example Text of @file{Backup-specs}
257 Choosing Files and Names for @command{tar}
259 * file:: Choosing the Archive's Name
260 * Selecting Archive Members::
261 * files:: Reading Names from a File
262 * exclude:: Excluding Some Files
263 * Wildcards:: Wildcards Patterns and Matching
264 * after:: Operating Only on New Files
265 * recurse:: Descending into Directories
266 * one:: Crossing File System Boundaries
268 Reading Names from a File
274 * problems with exclude::
276 Crossing File System Boundaries
278 * directory:: Changing Directory
279 * absolute:: Absolute File Names
283 * General date syntax:: Common rules.
284 * Calendar date items:: 19 Dec 1994.
285 * Time of day items:: 9:20pm.
286 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}, ...
287 * Day of week items:: Monday and others.
288 * Relative items in date strings:: next tuesday, 2 years ago.
289 * Pure numbers in date strings:: 19931219, 1440.
290 * Seconds since the Epoch:: @@1078100502.
291 * Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al.
293 Controlling the Archive Format
295 * Portability:: Making @command{tar} Archives More Portable
296 * Compression:: Using Less Space through Compression
297 * Attributes:: Handling File Attributes
298 * Standard:: The Standard Format
299 * Extensions:: @acronym{GNU} Extensions to the Archive Format
300 * cpio:: Comparison of @command{tar} and @command{cpio}
302 Making @command{tar} Archives More Portable
304 * Portable Names:: Portable Names
305 * dereference:: Symbolic Links
306 * old:: Old V7 Archives
307 * posix:: @acronym{POSIX} archives
308 * Checksumming:: Checksumming Problems
309 * Large or Negative Values:: Large files, negative time stamps, etc.
311 Using Less Space through Compression
313 * gzip:: Creating and Reading Compressed Archives
314 * sparse:: Archiving Sparse Files
316 Tapes and Other Archive Media
318 * Device:: Device selection and switching
319 * Remote Tape Server::
320 * Common Problems and Solutions::
321 * Blocking:: Blocking
322 * Many:: Many archives on one tape
323 * Using Multiple Tapes:: Using Multiple Tapes
324 * label:: Including a Label in the Archive
330 * Format Variations:: Format Variations
331 * Blocking Factor:: The Blocking Factor of an Archive
333 Many Archives on One Tape
335 * Tape Positioning:: Tape Positions and Tape Marks
336 * mt:: The @command{mt} Utility
340 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
341 * Tape Files:: Tape Files
342 * Tarcat:: Concatenate Volumes into a Single Archive
344 GNU tar internals and development
351 * Free Software Needs Free Documentation::
352 * GNU Free Documentation License:: License for copying this manual
358 @chapter Introduction
361 and manipulates @dfn{archives} which are actually collections of
362 many other files; the program provides users with an organized and
363 systematic method for controlling a large amount of data.
364 The name ``tar'' originally came from the phrase ``Tape ARchive'', but
365 archives need not (and these days, typically do not) reside on tapes.
368 * Book Contents:: What this Book Contains
369 * Definitions:: Some Definitions
370 * What tar Does:: What @command{tar} Does
371 * Naming tar Archives:: How @command{tar} Archives are Named
372 * Authors:: @GNUTAR{} Authors
373 * Reports:: Reporting bugs or suggestions
377 @section What this Book Contains
379 The first part of this chapter introduces you to various terms that will
380 recur throughout the book. It also tells you who has worked on @GNUTAR{}
381 and its documentation, and where you should send bug reports
384 The second chapter is a tutorial (@pxref{Tutorial}) which provides a
385 gentle introduction for people who are new to using @command{tar}. It is
386 meant to be self contained, not requiring any reading from subsequent
387 chapters to make sense. It moves from topic to topic in a logical,
388 progressive order, building on information already explained.
390 Although the tutorial is paced and structured to allow beginners to
391 learn how to use @command{tar}, it is not intended solely for beginners.
392 The tutorial explains how to use the three most frequently used
393 operations (@samp{create}, @samp{list}, and @samp{extract}) as well as
394 two frequently used options (@samp{file} and @samp{verbose}). The other
395 chapters do not refer to the tutorial frequently; however, if a section
396 discusses something which is a complex variant of a basic concept, there
397 may be a cross reference to that basic concept. (The entire book,
398 including the tutorial, assumes that the reader understands some basic
399 concepts of using a Unix-type operating system; @pxref{Tutorial}.)
401 The third chapter presents the remaining five operations, and
402 information about using @command{tar} options and option syntax.
404 @FIXME{this sounds more like a @acronym{GNU} Project Manuals Concept [tm] more
405 than the reality. should think about whether this makes sense to say
406 here, or not.} The other chapters are meant to be used as a
407 reference. Each chapter presents everything that needs to be said
408 about a specific topic.
410 One of the chapters (@pxref{Date input formats}) exists in its
411 entirety in other @acronym{GNU} manuals, and is mostly self-contained.
412 In addition, one section of this manual (@pxref{Standard}) contains a
413 big quote which is taken directly from @command{tar} sources.
415 In general, we give both long and short (abbreviated) option names
416 at least once in each section where the relevant option is covered, so
417 that novice readers will become familiar with both styles. (A few
418 options have no short versions, and the relevant sections will
422 @section Some Definitions
426 The @command{tar} program is used to create and manipulate @command{tar}
427 archives. An @dfn{archive} is a single file which contains the contents
428 of many files, while still identifying the names of the files, their
429 owner(s), and so forth. (In addition, archives record access
430 permissions, user and group, size in bytes, and data modification time.
431 Some archives also record the file names in each archived directory, as
432 well as other file and directory information.) You can use @command{tar}
433 to @dfn{create} a new archive in a specified directory.
436 @cindex archive member
439 The files inside an archive are called @dfn{members}. Within this
440 manual, we use the term @dfn{file} to refer only to files accessible in
441 the normal ways (by @command{ls}, @command{cat}, and so forth), and the term
442 @dfn{member} to refer only to the members of an archive. Similarly, a
443 @dfn{file name} is the name of a file, as it resides in the file system,
444 and a @dfn{member name} is the name of an archive member within the
449 The term @dfn{extraction} refers to the process of copying an archive
450 member (or multiple members) into a file in the file system. Extracting
451 all the members of an archive is often called @dfn{extracting the
452 archive}. The term @dfn{unpack} can also be used to refer to the
453 extraction of many or all the members of an archive. Extracting an
454 archive does not destroy the archive's structure, just as creating an
455 archive does not destroy the copies of the files that exist outside of
456 the archive. You may also @dfn{list} the members in a given archive
457 (this is often thought of as ``printing'' them to the standard output,
458 or the command line), or @dfn{append} members to a pre-existing archive.
459 All of these operations can be performed using @command{tar}.
462 @section What @command{tar} Does
465 The @command{tar} program provides the ability to create @command{tar}
466 archives, as well as various other kinds of manipulation. For example,
467 you can use @command{tar} on previously created archives to extract files,
468 to store additional files, or to update or list files which were already
471 Initially, @command{tar} archives were used to store files conveniently on
472 magnetic tape. The name @command{tar} comes from this use; it stands for
473 @code{t}ape @code{ar}chiver. Despite the utility's name, @command{tar} can
474 direct its output to available devices, files, or other programs (using
475 pipes). @command{tar} may even access remote devices or files (as archives).
477 @FIXME{the following table entries need a bit of work..}
479 You can use @command{tar} archives in many ways. We want to stress a few
480 of them: storage, backup, and transportation.
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 versions of @command{tar} are not so clever; they will
1219 enter an infinite loop when this happens, so you should not depend on
1220 this behavior unless you are certain you are running @GNUTAR{}.)
1221 @FIXME{bob doesn't like this sentence, since he does
1222 it all the time, and we've been doing it in the editing passes for
1223 this manual: In general, make sure that the archive is not inside a
1224 directory being dumped.}
1227 @section How to List Archives
1230 Frequently, you will find yourself wanting to determine exactly what a
1231 particular archive contains. You can use the @option{--list} (@option{-t}) operation
1232 to get the member names as they currently appear in the archive, as well
1233 as various attributes of the files at the time they were archived. For
1234 example, you can examine the archive @file{collection.tar} that you
1235 created in the last section with the command,
1238 $ @kbd{tar --list --file=collection.tar}
1242 The output of @command{tar} would then be:
1250 @FIXME{we hope this will change. if it doesn't, need to show the
1251 creation of bfiles somewhere above!!! : }
1254 The archive @file{bfiles.tar} would list as follows:
1263 Be sure to use a @option{--file=@var{archive-name}} (@option{-f @var{archive-name}}) option just as with @option{--create} (@option{-c})
1264 to specify the name of the archive.
1266 @opindex list, using with @option{--verbose}
1267 @opindex verbose, using with @option{--list}
1268 If you use the @option{--verbose} (@option{-v}) option with @option{--list}, then
1269 @command{tar} will print out a listing reminiscent of @w{@samp{ls -l}},
1270 showing owner, file size, and so forth.
1272 If you had used @option{--verbose} (@option{-v}) mode, the example above would look
1276 $ @kbd{tar --list --verbose --file=collection.tar folk}
1277 -rw-r--r-- myself user 62 1990-05-23 10:55 folk
1280 @cindex listing member and file names
1281 @anchor{listing member and file names}
1282 It is important to notice that the output of @kbd{tar --list
1283 --verbose} does not necessarily match that produced by @kbd{tar
1284 --create --verbose} while creating the archive. It is because
1285 @GNUTAR{}, unless told explicitly not to do so, removes some directory
1286 prefixes from file names before storing them in the archive
1287 (@xref{absolute}, for more information). In other
1288 words, in verbose mode @GNUTAR{} shows @dfn{file names} when creating
1289 an archive and @dfn{member names} when listing it. Consider this
1294 $ @kbd{tar cfv archive /etc/mail}
1295 tar: Removing leading `/' from member names
1297 /etc/mail/sendmail.cf
1299 $ @kbd{tar tf archive}
1301 etc/mail/sendmail.cf
1306 @opindex show-stored-names
1307 This default behavior can sometimes be inconvenient. You can force
1308 @GNUTAR{} show member names when creating archive by supplying
1309 @option{--show-stored-names} option.
1312 @item --show-stored-names
1313 Print member (as opposed to @emph{file}) names when creating the archive.
1316 @cindex File name arguments, using @option{--list} with
1317 @opindex list, using with file name arguments
1318 You can specify one or more individual member names as arguments when
1319 using @samp{list}. In this case, @command{tar} will only list the
1320 names of members you identify. For example, @w{@kbd{tar --list
1321 --file=afiles.tar apple}} would only print @file{apple}.
1323 Because @command{tar} preserves paths, file names must be specified as
1324 they appear in the archive (ie., relative to the directory from which
1325 the archive was created). Therefore, it is essential when specifying
1326 member names to @command{tar} that you give the exact member names.
1327 For example, @w{@kbd{tar --list --file=bfiles.tar birds}} would produce an
1328 error message something like @samp{tar: birds: Not found in archive},
1329 because there is no member named @file{birds}, only one named
1330 @file{./birds}. While the names @file{birds} and @file{./birds} name
1331 the same file, @emph{member} names by default are compared verbatim.
1333 However, @w{@kbd{tar --list --file=bfiles.tar baboon}} would respond
1334 with @file{baboon}, because this exact member name is in the archive file
1335 @file{bfiles.tar}. If you are not sure of the exact file name,
1336 use @dfn{globbing patterns}, for example:
1339 $ @kbd{tar --list --file=bfiles.tar --wildcards '*b*'}
1343 will list all members whose name contains @samp{b}. @xref{Wildcards},
1344 for a detailed discussion of globbing patterns and related
1345 @command{tar} command line options.
1352 @unnumberedsubsec Listing the Contents of a Stored Directory
1354 To get information about the contents of an archived directory,
1355 use the directory name as a file name argument in conjunction with
1356 @option{--list} (@option{-t}). To find out file attributes, include the
1357 @option{--verbose} (@option{-v}) option.
1359 For example, to find out about files in the directory @file{practice}, in
1360 the archive file @file{music.tar}, type:
1363 $ @kbd{tar --list --verbose --file=music.tar practice}
1366 @command{tar} responds:
1369 drwxrwxrwx myself user 0 1990-05-31 21:49 practice/
1370 -rw-r--r-- myself user 42 1990-05-21 13:29 practice/blues
1371 -rw-r--r-- myself user 62 1990-05-23 10:55 practice/folk
1372 -rw-r--r-- myself user 40 1990-05-21 13:30 practice/jazz
1373 -rw-r--r-- myself user 10240 1990-05-31 21:49 practice/collection.tar
1376 When you use a directory name as a file name argument, @command{tar} acts on
1377 all the files (including sub-directories) in that directory.
1380 @section How to Extract Members from an Archive
1383 @cindex Retrieving files from an archive
1384 @cindex Resurrecting files from an archive
1387 Creating an archive is only half the job---there is no point in storing
1388 files in an archive if you can't retrieve them. The act of retrieving
1389 members from an archive so they can be used and manipulated as
1390 unarchived files again is called @dfn{extraction}. To extract files
1391 from an archive, use the @option{--extract} (@option{--get} or
1392 @option{-x}) operation. As with @option{--create}, specify the name
1393 of the archive with @option{--file} (@option{-f}) option. Extracting
1394 an archive does not modify the archive in any way; you can extract it
1395 multiple times if you want or need to.
1397 Using @option{--extract}, you can extract an entire archive, or specific
1398 files. The files can be directories containing other files, or not. As
1399 with @option{--create} (@option{-c}) and @option{--list} (@option{-t}), you may use the short or the
1400 long form of the operation without affecting the performance.
1403 * extracting archives::
1404 * extracting files::
1406 * extracting untrusted archives::
1407 * failing commands::
1410 @node extracting archives
1411 @subsection Extracting an Entire Archive
1413 To extract an entire archive, specify the archive file name only, with
1414 no individual file names as arguments. For example,
1417 $ @kbd{tar -xvf collection.tar}
1424 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
1425 -rw-r--r-- me user 21 1996-09-23 16:44 blues
1426 -rw-r--r-- me user 20 1996-09-23 16:44 folk
1429 @node extracting files
1430 @subsection Extracting Specific Files
1432 To extract specific archive members, give their exact member names as
1433 arguments, as printed by @option{--list} (@option{-t}). If you had
1434 mistakenly deleted one of the files you had placed in the archive
1435 @file{collection.tar} earlier (say, @file{blues}), you can extract it
1436 from the archive without changing the archive's structure. Its
1437 contents will be identical to the original file @file{blues} that you
1440 First, make sure you are in the @file{practice} directory, and list the
1441 files in the directory. Now, delete the file, @samp{blues}, and list
1442 the files in the directory again.
1444 You can now extract the member @file{blues} from the archive file
1445 @file{collection.tar} like this:
1448 $ @kbd{tar --extract --file=collection.tar blues}
1452 If you list the files in the directory again, you will see that the file
1453 @file{blues} has been restored, with its original permissions, data
1454 modification times, and owner.@FIXME{This is only accidentally true, but not in
1455 general. In most cases, one has to be root for restoring the owner, and
1456 use a special option for restoring permissions. Here, it just happens
1457 that the restoring user is also the owner of the archived members, and
1458 that the current @code{umask} is compatible with original permissions.}
1459 (These parameters will be identical to those which
1460 the file had when you originally placed it in the archive; any changes
1461 you may have made before deleting the file from the file system,
1462 however, will @emph{not} have been made to the archive member.) The
1463 archive file, @samp{collection.tar}, is the same as it was before you
1464 extracted @samp{blues}. You can confirm this by running @command{tar} with
1465 @option{--list} (@option{-t}).
1467 Remember that as with other operations, specifying the exact member
1468 name is important. @w{@kbd{tar --extract --file=bfiles.tar birds}}
1469 will fail, because there is no member named @file{birds}. To extract
1470 the member named @file{./birds}, you must specify @w{@kbd{tar
1471 --extract --file=bfiles.tar ./birds}}. If you don't remember the
1472 exact member names, use @option{--list} (@option{-t}) option
1473 (@pxref{list}). You can also extract those members that match a
1474 specific @dfn{globbing pattern}. For example, to extract from
1475 @file{bfiles.tar} all files that begin with @samp{b}, no matter their
1476 directory prefix, you could type:
1479 $ @kbd{tar -x -f bfiles.tar --wildcards --no-anchored 'b*'}
1483 Here, @option{--wildcards} instructs @command{tar} to treat
1484 command line arguments as globbing patterns and @option{--no-anchored}
1485 informs it that the patterns apply to member names after any @samp{/}
1486 delimiter. The use of globbing patterns is discussed in detail in
1489 You can extract a file to standard output by combining the above options
1490 with the @option{--to-stdout} (@option{-O}) option (@pxref{Writing to Standard
1493 If you give the @option{--verbose} option, then @option{--extract}
1494 will print the names of the archive members as it extracts them.
1497 @subsection Extracting Files that are Directories
1499 Extracting directories which are members of an archive is similar to
1500 extracting other files. The main difference to be aware of is that if
1501 the extracted directory has the same name as any directory already in
1502 the working directory, then files in the extracted directory will be
1503 placed into the directory of the same name. Likewise, if there are
1504 files in the pre-existing directory with the same names as the members
1505 which you extract, the files from the extracted archive will replace
1506 the files already in the working directory (and possible
1507 subdirectories). This will happen regardless of whether or not the
1508 files in the working directory were more recent than those extracted
1509 (there exist, however, special options that alter this behavior
1512 However, if a file was stored with a directory name as part of its file
1513 name, and that directory does not exist under the working directory when
1514 the file is extracted, @command{tar} will create the directory.
1516 We can demonstrate how to use @option{--extract} to extract a directory
1517 file with an example. Change to the @file{practice} directory if you
1518 weren't there, and remove the files @file{folk} and @file{jazz}. Then,
1519 go back to the parent directory and extract the archive
1520 @file{music.tar}. You may either extract the entire archive, or you may
1521 extract only the files you just deleted. To extract the entire archive,
1522 don't give any file names as arguments after the archive name
1523 @file{music.tar}. To extract only the files you deleted, use the
1527 $ @kbd{tar -xvf music.tar practice/folk practice/jazz}
1533 If you were to specify two @option{--verbose} (@option{-v}) options, @command{tar}
1534 would have displayed more detail about the extracted files, as shown
1535 in the example below:
1538 $ @kbd{tar -xvvf music.tar practice/folk practice/jazz}
1539 -rw-r--r-- me user 28 1996-10-18 16:31 practice/jazz
1540 -rw-r--r-- me user 20 1996-09-23 16:44 practice/folk
1544 Because you created the directory with @file{practice} as part of the
1545 file names of each of the files by archiving the @file{practice}
1546 directory as @file{practice}, you must give @file{practice} as part
1547 of the file names when you extract those files from the archive.
1549 @FIXME{IMPORTANT! show the final structure, here. figure out what it
1552 @node extracting untrusted archives
1553 @subsection Extracting Archives from Untrusted Sources
1555 Extracting files from archives can overwrite files that already exist.
1556 If you receive an archive from an untrusted source, you should make a
1557 new directory and extract into that directory, so that you don't have
1558 to worry about the extraction overwriting one of your existing files.
1559 For example, if @file{untrusted.tar} came from somewhere else on the
1560 Internet, and you don't necessarily trust its contents, you can
1561 extract it as follows:
1564 $ @kbd{mkdir newdir}
1566 $ @kbd{tar -xvf ../untrusted.tar}
1569 It is also a good practice to examine contents of the archive
1570 before extracting it, using @option{--list} (@option{-t}) option, possibly combined
1571 with @option{--verbose} (@option{-v}).
1573 @node failing commands
1574 @subsection Commands That Will Fail
1576 Here are some sample commands you might try which will not work, and why
1579 If you try to use this command,
1582 $ @kbd{tar -xvf music.tar folk jazz}
1586 you will get the following response:
1589 tar: folk: Not found in archive
1590 tar: jazz: Not found in archive
1595 This is because these files were not originally @emph{in} the parent
1596 directory @file{..}, where the archive is located; they were in the
1597 @file{practice} directory, and their file names reflect this:
1600 $ @kbd{tar -tvf music.tar}
1606 @FIXME{make sure the above works when going through the examples in
1610 Likewise, if you try to use this command,
1613 $ @kbd{tar -tvf music.tar folk jazz}
1617 you would get a similar response. Members with those names are not in the
1618 archive. You must use the correct member names, or wildcards, in order
1619 to extract the files from the archive.
1621 If you have forgotten the correct names of the files in the archive,
1622 use @w{@kbd{tar --list --verbose}} to list them correctly.
1624 @FIXME{more examples, here? hag thinks it's a good idea.}
1627 @section Going Further Ahead in this Manual
1629 @FIXME{need to write up a node here about the things that are going to
1630 be in the rest of the manual.}
1632 @node tar invocation
1633 @chapter Invoking @GNUTAR{}
1636 This chapter is about how one invokes the @GNUTAR{}
1637 command, from the command synopsis (@pxref{Synopsis}). There are
1638 numerous options, and many styles for writing them. One mandatory
1639 option specifies the operation @command{tar} should perform
1640 (@pxref{Operation Summary}), other options are meant to detail how
1641 this operation should be performed (@pxref{Option Summary}).
1642 Non-option arguments are not always interpreted the same way,
1643 depending on what the operation is.
1645 You will find in this chapter everything about option styles and rules for
1646 writing them (@pxref{Styles}). On the other hand, operations and options
1647 are fully described elsewhere, in other chapters. Here, you will find
1648 only synthetic descriptions for operations and options, together with
1649 pointers to other parts of the @command{tar} manual.
1651 Some options are so special they are fully described right in this
1652 chapter. They have the effect of inhibiting the normal operation of
1653 @command{tar} or else, they globally alter the amount of feedback the user
1654 receives about what is going on. These are the @option{--help} and
1655 @option{--version} (@pxref{help}), @option{--verbose} (@pxref{verbose})
1656 and @option{--interactive} options (@pxref{interactive}).
1660 * using tar options::
1670 @section General Synopsis of @command{tar}
1672 The @GNUTAR{} program is invoked as either one of:
1675 @kbd{tar @var{option}@dots{} [@var{name}]@dots{}}
1676 @kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}}
1679 The second form is for when old options are being used.
1681 You can use @command{tar} to store files in an archive, to extract them from
1682 an archive, and to do other types of archive manipulation. The primary
1683 argument to @command{tar}, which is called the @dfn{operation}, specifies
1684 which action to take. The other arguments to @command{tar} are either
1685 @dfn{options}, which change the way @command{tar} performs an operation,
1686 or file names or archive members, which specify the files or members
1687 @command{tar} is to act on.
1689 You can actually type in arguments in any order, even if in this manual
1690 the options always precede the other arguments, to make examples easier
1691 to understand. Further, the option stating the main operation mode
1692 (the @command{tar} main command) is usually given first.
1694 Each @var{name} in the synopsis above is interpreted as an archive member
1695 name when the main command is one of @option{--compare}
1696 (@option{--diff}, @option{-d}), @option{--delete}, @option{--extract}
1697 (@option{--get}, @option{-x}), @option{--list} (@option{-t}) or
1698 @option{--update} (@option{-u}). When naming archive members, you
1699 must give the exact name of the member in the archive, as it is
1700 printed by @option{--list}. For @option{--append} (@option{-r}) and
1701 @option{--create} (@option{-c}), these @var{name} arguments specify
1702 the names of either files or directory hierarchies to place in the archive.
1703 These files or hierarchies should already exist in the file system,
1704 prior to the execution of the @command{tar} command.
1706 @command{tar} interprets relative file names as being relative to the
1707 working directory. @command{tar} will make all file names relative
1708 (by removing leading slashes when archiving or restoring files),
1709 unless you specify otherwise (using the @option{--absolute-names}
1710 option). @xref{absolute}, for more information about
1711 @option{--absolute-names}.
1713 If you give the name of a directory as either a file name or a member
1714 name, then @command{tar} acts recursively on all the files and directories
1715 beneath that directory. For example, the name @file{/} identifies all
1716 the files in the file system to @command{tar}.
1718 The distinction between file names and archive member names is especially
1719 important when shell globbing is used, and sometimes a source of confusion
1720 for newcomers. @xref{Wildcards}, for more information about globbing.
1721 The problem is that shells may only glob using existing files in the
1722 file system. Only @command{tar} itself may glob on archive members, so when
1723 needed, you must ensure that wildcard characters reach @command{tar} without
1724 being interpreted by the shell first. Using a backslash before @samp{*}
1725 or @samp{?}, or putting the whole argument between quotes, is usually
1726 sufficient for this.
1728 Even if @var{name}s are often specified on the command line, they
1729 can also be read from a text file in the file system, using the
1730 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}}) option.
1732 If you don't use any file name arguments, @option{--append} (@option{-r}),
1733 @option{--delete} and @option{--concatenate} (@option{--catenate},
1734 @option{-A}) will do nothing, while @option{--create} (@option{-c})
1735 will usually yield a diagnostic and inhibit @command{tar} execution.
1736 The other operations of @command{tar} (@option{--list},
1737 @option{--extract}, @option{--compare}, and @option{--update})
1738 will act on the entire contents of the archive.
1741 @cindex return status
1742 Besides successful exits, @GNUTAR{} may fail for
1743 many reasons. Some reasons correspond to bad usage, that is, when the
1744 @command{tar} command is improperly written. Errors may be
1745 encountered later, while encountering an error processing the archive
1746 or the files. Some errors are recoverable, in which case the failure
1747 is delayed until @command{tar} has completed all its work. Some
1748 errors are such that it would not meaningful, or at least risky, to
1749 continue processing: @command{tar} then aborts processing immediately.
1750 All abnormal exits, whether immediate or delayed, should always be
1751 clearly diagnosed on @code{stderr}, after a line stating the nature of
1754 @GNUTAR{} returns only a few exit statuses. I'm really
1755 aiming simplicity in that area, for now. If you are not using the
1756 @option{--compare} @option{--diff}, @option{-d}) option, zero means
1757 that everything went well, besides maybe innocuous warnings. Nonzero
1758 means that something went wrong. Right now, as of today, ``nonzero''
1759 is almost always 2, except for remote operations, where it may be
1762 @node using tar options
1763 @section Using @command{tar} Options
1765 @GNUTAR{} has a total of eight operating modes which
1766 allow you to perform a variety of tasks. You are required to choose
1767 one operating mode each time you employ the @command{tar} program by
1768 specifying one, and only one operation as an argument to the
1769 @command{tar} command (two lists of four operations each may be found
1770 at @ref{frequent operations} and @ref{Operations}). Depending on
1771 circumstances, you may also wish to customize how the chosen operating
1772 mode behaves. For example, you may wish to change the way the output
1773 looks, or the format of the files that you wish to archive may require
1774 you to do something special in order to make the archive look right.
1776 You can customize and control @command{tar}'s performance by running
1777 @command{tar} with one or more options (such as @option{--verbose}
1778 (@option{-v}), which we used in the tutorial). As we said in the
1779 tutorial, @dfn{options} are arguments to @command{tar} which are (as
1780 their name suggests) optional. Depending on the operating mode, you
1781 may specify one or more options. Different options will have different
1782 effects, but in general they all change details of the operation, such
1783 as archive format, archive name, or level of user interaction. Some
1784 options make sense with all operating modes, while others are
1785 meaningful only with particular modes. You will likely use some
1786 options frequently, while you will only use others infrequently, or
1787 not at all. (A full list of options is available in @pxref{All Options}.)
1789 @vrindex TAR_OPTIONS, environment variable
1790 @anchor{TAR_OPTIONS}
1791 The @env{TAR_OPTIONS} environment variable specifies default options to
1792 be placed in front of any explicit options. For example, if
1793 @code{TAR_OPTIONS} is @samp{-v --unlink-first}, @command{tar} behaves as
1794 if the two options @option{-v} and @option{--unlink-first} had been
1795 specified before any explicit options. Option specifications are
1796 separated by whitespace. A backslash escapes the next character, so it
1797 can be used to specify an option containing whitespace or a backslash.
1799 Note that @command{tar} options are case sensitive. For example, the
1800 options @option{-T} and @option{-t} are different; the first requires an
1801 argument for stating the name of a file providing a list of @var{name}s,
1802 while the second does not require an argument and is another way to
1803 write @option{--list} (@option{-t}).
1805 In addition to the eight operations, there are many options to
1806 @command{tar}, and three different styles for writing both: long (mnemonic)
1807 form, short form, and old style. These styles are discussed below.
1808 Both the options and the operations can be written in any of these three
1811 @FIXME{menu at end of this node. need to think of an actual outline
1812 for this chapter; probably do that after stuff from chapter 4 is
1816 @section The Three Option Styles
1818 There are three styles for writing operations and options to the command
1819 line invoking @command{tar}. The different styles were developed at
1820 different times during the history of @command{tar}. These styles will be
1821 presented below, from the most recent to the oldest.
1823 Some options must take an argument. (For example, @option{--file}
1824 (@option{-f})) takes the name of an archive file as an argument. If
1825 you do not supply an archive file name, @command{tar} will use a
1826 default, but this can be confusing; thus, we recommend that you always
1827 supply a specific archive file name.) Where you @emph{place} the
1828 arguments generally depends on which style of options you choose. We
1829 will detail specific information relevant to each option style in the
1830 sections on the different option styles, below. The differences are
1831 subtle, yet can often be very important; incorrect option placement
1832 can cause you to overwrite a number of important files. We urge you
1833 to note these differences, and only use the option style(s) which
1834 makes the most sense to you until you feel comfortable with the others.
1836 Some options @emph{may} take an argument (currently, there are
1837 two such options: @option{--backup} and @option{--occurrence}). Such
1838 options may have at most long and short forms, they do not have old style
1839 equivalent. The rules for specifying an argument for such options
1840 are stricter than those for specifying mandatory arguments. Please,
1841 pay special attention to them.
1844 * Mnemonic Options:: Mnemonic Option Style
1845 * Short Options:: Short Option Style
1846 * Old Options:: Old Option Style
1847 * Mixing:: Mixing Option Styles
1850 @node Mnemonic Options
1851 @subsection Mnemonic Option Style
1853 @FIXME{have to decide whether or not to replace other occurrences of
1854 "mnemonic" with "long", or *ugh* vice versa.}
1856 Each option has at least one long (or mnemonic) name starting with two
1857 dashes in a row, e.g., @option{--list}. The long names are more clear than
1858 their corresponding short or old names. It sometimes happens that a
1859 single mnemonic option has many different different names which are
1860 synonymous, such as @option{--compare} and @option{--diff}. In addition,
1861 long option names can be given unique abbreviations. For example,
1862 @option{--cre} can be used in place of @option{--create} because there is no
1863 other mnemonic option which begins with @samp{cre}. (One way to find
1864 this out is by trying it and seeing what happens; if a particular
1865 abbreviation could represent more than one option, @command{tar} will tell
1866 you that that abbreviation is ambiguous and you'll know that that
1867 abbreviation won't work. You may also choose to run @samp{tar --help}
1868 to see a list of options. Be aware that if you run @command{tar} with a
1869 unique abbreviation for the long name of an option you didn't want to
1870 use, you are stuck; @command{tar} will perform the command as ordered.)
1872 Mnemonic options are meant to be obvious and easy to remember, and their
1873 meanings are generally easier to discern than those of their
1874 corresponding short options (see below). For example:
1877 $ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0}
1881 gives a fairly good set of hints about what the command does, even
1882 for those not fully acquainted with @command{tar}.
1884 Mnemonic options which require arguments take those arguments
1885 immediately following the option name. There are two ways of
1886 specifying a mandatory argument. It can be separated from the
1887 option name either by an equal sign, or by any amount of
1888 white space characters. For example, the @option{--file} option (which
1889 tells the name of the @command{tar} archive) is given a file such as
1890 @file{archive.tar} as argument by using any of the following notations:
1891 @option{--file=archive.tar} or @option{--file archive.tar}.
1893 In contrast, optional arguments must always be introduced using
1894 an equal sign. For example, the @option{--backup} option takes
1895 an optional argument specifying backup type. It must be used
1896 as @option{--backup=@var{backup-type}}.
1899 @subsection Short Option Style
1901 Most options also have a short option name. Short options start with
1902 a single dash, and are followed by a single character, e.g., @option{-t}
1903 (which is equivalent to @option{--list}). The forms are absolutely
1904 identical in function; they are interchangeable.
1906 The short option names are faster to type than long option names.
1908 Short options which require arguments take their arguments immediately
1909 following the option, usually separated by white space. It is also
1910 possible to stick the argument right after the short option name, using
1911 no intervening space. For example, you might write @w{@option{-f
1912 archive.tar}} or @option{-farchive.tar} instead of using
1913 @option{--file=archive.tar}. Both @option{--file=@var{archive-name}} and
1914 @w{@option{-f @var{archive-name}}} denote the option which indicates a
1915 specific archive, here named @file{archive.tar}.
1917 Short options which take optional arguments take their arguments
1918 immediately following the option letter, @emph{without any intervening
1919 white space characters}.
1921 Short options' letters may be clumped together, but you are not
1922 required to do this (as compared to old options; see below). When
1923 short options are clumped as a set, use one (single) dash for them
1924 all, e.g., @w{@samp{@command{tar} -cvf}}. Only the last option in
1925 such a set is allowed to have an argument@footnote{Clustering many
1926 options, the last of which has an argument, is a rather opaque way to
1927 write options. Some wonder if @acronym{GNU} @code{getopt} should not
1928 even be made helpful enough for considering such usages as invalid.}.
1930 When the options are separated, the argument for each option which requires
1931 an argument directly follows that option, as is usual for Unix programs.
1935 $ @kbd{tar -c -v -b 20 -f /dev/rmt0}
1938 If you reorder short options' locations, be sure to move any arguments
1939 that belong to them. If you do not move the arguments properly, you may
1940 end up overwriting files.
1943 @subsection Old Option Style
1946 Like short options, old options are single letters. However, old options
1947 must be written together as a single clumped set, without spaces separating
1948 them or dashes preceding them@footnote{Beware that if you precede options
1949 with a dash, you are announcing the short option style instead of the
1950 old option style; short options are decoded differently.}. This set
1951 of letters must be the first to appear on the command line, after the
1952 @command{tar} program name and some white space; old options cannot appear
1953 anywhere else. The letter of an old option is exactly the same letter as
1954 the corresponding short option. For example, the old option @samp{t} is
1955 the same as the short option @option{-t}, and consequently, the same as the
1956 mnemonic option @option{--list}. So for example, the command @w{@samp{tar
1957 cv}} specifies the option @option{-v} in addition to the operation @option{-c}.
1959 @FIXME{bob suggests having an uglier example. :-) }
1961 When options that need arguments are given together with the command,
1962 all the associated arguments follow, in the same order as the options.
1963 Thus, the example given previously could also be written in the old
1967 $ @kbd{tar cvbf 20 /dev/rmt0}
1971 Here, @samp{20} is the argument of @option{-b} and @samp{/dev/rmt0} is
1972 the argument of @option{-f}.
1974 On the other hand, this old style syntax makes it difficult to match
1975 option letters with their corresponding arguments, and is often
1976 confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example,
1977 @samp{20} is the argument for @option{-b}, @samp{/dev/rmt0} is the
1978 argument for @option{-f}, and @option{-v} does not have a corresponding
1979 argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f
1980 /dev/rmt0}} is clearer, putting all arguments next to the option they
1983 If you want to reorder the letters in the old option argument, be
1984 sure to reorder any corresponding argument appropriately.
1986 This old way of writing @command{tar} options can surprise even experienced
1987 users. For example, the two commands:
1990 @kbd{tar cfz archive.tar.gz file}
1991 @kbd{tar -cfz archive.tar.gz file}
1995 are quite different. The first example uses @file{archive.tar.gz} as
1996 the value for option @samp{f} and recognizes the option @samp{z}. The
1997 second example, however, uses @file{z} as the value for option
1998 @samp{f} --- probably not what was intended.
2000 Old options are kept for compatibility with old versions of @command{tar}.
2002 This second example could be corrected in many ways, among which the
2003 following are equivalent:
2006 @kbd{tar -czf archive.tar.gz file}
2007 @kbd{tar -cf archive.tar.gz -z file}
2008 @kbd{tar cf archive.tar.gz -z file}
2011 @FIXME{still could explain this better; it's redundant:}
2013 @cindex option syntax, traditional
2014 As far as we know, all @command{tar} programs, @acronym{GNU} and
2015 non-@acronym{GNU}, support old options. @GNUTAR{}
2016 supports them not only for historical reasons, but also because many
2017 people are used to them. For compatibility with Unix @command{tar},
2018 the first argument is always treated as containing command and option
2019 letters even if it doesn't start with @samp{-}. Thus, @samp{tar c} is
2020 equivalent to @w{@samp{tar -c}:} both of them specify the
2021 @option{--create} (@option{-c}) command to create an archive.
2024 @subsection Mixing Option Styles
2026 All three styles may be intermixed in a single @command{tar} command,
2027 so long as the rules for each style are fully
2028 respected@footnote{Before @GNUTAR{} version 1.11.6,
2029 a bug prevented intermixing old style options with mnemonic options in
2030 some cases.}. Old style options and either of the modern styles of
2031 options may be mixed within a single @command{tar} command. However,
2032 old style options must be introduced as the first arguments only,
2033 following the rule for old options (old options must appear directly
2034 after the @command{tar} command and some white space). Modern options
2035 may be given only after all arguments to the old options have been
2036 collected. If this rule is not respected, a modern option might be
2037 falsely interpreted as the value of the argument to one of the old
2040 For example, all the following commands are wholly equivalent, and
2041 illustrate the many combinations and orderings of option styles.
2044 @kbd{tar --create --file=archive.tar}
2045 @kbd{tar --create -f archive.tar}
2046 @kbd{tar --create -farchive.tar}
2047 @kbd{tar --file=archive.tar --create}
2048 @kbd{tar --file=archive.tar -c}
2049 @kbd{tar -c --file=archive.tar}
2050 @kbd{tar -c -f archive.tar}
2051 @kbd{tar -c -farchive.tar}
2052 @kbd{tar -cf archive.tar}
2053 @kbd{tar -cfarchive.tar}
2054 @kbd{tar -f archive.tar --create}
2055 @kbd{tar -f archive.tar -c}
2056 @kbd{tar -farchive.tar --create}
2057 @kbd{tar -farchive.tar -c}
2058 @kbd{tar c --file=archive.tar}
2059 @kbd{tar c -f archive.tar}
2060 @kbd{tar c -farchive.tar}
2061 @kbd{tar cf archive.tar}
2062 @kbd{tar f archive.tar --create}
2063 @kbd{tar f archive.tar -c}
2064 @kbd{tar fc archive.tar}
2067 On the other hand, the following commands are @emph{not} equivalent to
2071 @kbd{tar -f -c archive.tar}
2072 @kbd{tar -fc archive.tar}
2073 @kbd{tar -fcarchive.tar}
2074 @kbd{tar -farchive.tarc}
2075 @kbd{tar cfarchive.tar}
2079 These last examples mean something completely different from what the
2080 user intended (judging based on the example in the previous set which
2081 uses long options, whose intent is therefore very clear). The first
2082 four specify that the @command{tar} archive would be a file named
2083 @option{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc},
2084 respectively. The first two examples also specify a single non-option,
2085 @var{name} argument having the value @samp{archive.tar}. The last
2086 example contains only old style option letters (repeating option
2087 @samp{c} twice), not all of which are meaningful (eg., @samp{.},
2088 @samp{h}, or @samp{i}), with no argument value. @FIXME{not sure i liked
2089 the first sentence of this paragraph..}
2092 @section All @command{tar} Options
2094 The coming manual sections contain an alphabetical listing of all
2095 @command{tar} operations and options, with brief descriptions and cross
2096 references to more in-depth explanations in the body of the manual.
2097 They also contain an alphabetically arranged table of the short option
2098 forms with their corresponding long option. You can use this table as
2099 a reference for deciphering @command{tar} commands in scripts.
2102 * Operation Summary::
2104 * Short Option Summary::
2107 @node Operation Summary
2108 @subsection Operations
2112 @opindex append, summary
2116 Appends files to the end of the archive. @xref{append}.
2118 @opindex catenate, summary
2122 Same as @option{--concatenate}. @xref{concatenate}.
2124 @opindex compare, summary
2128 Compares archive members with their counterparts in the file
2129 system, and reports differences in file size, mode, owner,
2130 modification date and contents. @xref{compare}.
2132 @opindex concatenate, summary
2136 Appends other @command{tar} archives to the end of the archive.
2139 @opindex create, summary
2143 Creates a new @command{tar} archive. @xref{create}.
2145 @opindex delete, summary
2148 Deletes members from the archive. Don't try this on a archive on a
2149 tape! @xref{delete}.
2151 @opindex diff, summary
2155 Same @option{--compare}. @xref{compare}.
2157 @opindex extract, summary
2161 Extracts members from the archive into the file system. @xref{extract}.
2163 @opindex get, summary
2167 Same as @option{--extract}. @xref{extract}.
2169 @opindex list, summary
2173 Lists the members in an archive. @xref{list}.
2175 @opindex update, summary
2179 @FIXME{It was: A combination of the @option{--compare} and
2180 @option{--append} operations. This is not true and rather misleading,
2181 as @option{--compare} does a lot more than @option{--update} for
2182 ensuring files are identical.} Adds files to the end of the archive,
2183 but only if they are newer than their counterparts already in the
2184 archive, or if they do not already exist in the archive.
2189 @node Option Summary
2190 @subsection @command{tar} Options
2194 @opindex absolute-names, summary
2195 @item --absolute-names
2198 Normally when creating an archive, @command{tar} strips an initial
2199 @samp{/} from member names. This option disables that behavior.
2202 @opindex after-date, summary
2205 (See @option{--newer}, @pxref{after})
2207 @opindex anchored, summary
2209 A pattern must match an initial subsequence of the name's components.
2210 @xref{controlling pattern-matching}.
2212 @opindex atime-preserve, summary
2213 @item --atime-preserve
2214 @itemx --atime-preserve=replace
2215 @itemx --atime-preserve=system
2217 Attempt to preserve the access time of files when reading them. This
2218 option currently is effective only on files that you own, unless you
2219 have superuser privileges.
2221 @option{--atime-preserve=replace} remembers the access time of a file
2222 before reading it, and then restores the access time afterwards. This
2223 may cause problems if other programs are reading the file at the same
2224 time, as the times of their accesses will be lost. On most platforms
2225 restoring the access time also requires @command{tar} to restore the
2226 data modification time too, so this option may also cause problems if
2227 other programs are writing the file at the same time. (Tar attempts
2228 to detect this situation, but cannot do so reliably due to race
2229 conditions.) Worse, on most platforms restoring the access time also
2230 updates the status change time, which means that this option is
2231 incompatible with incremental backups.
2233 @option{--atime-preserve=system} avoids changing time stamps on files,
2234 without interfering with time stamp updates
2235 caused by other programs, so it works better with incremental backups.
2236 However, it requires a special @code{O_NOATIME} option from the
2237 underlying operating and file system implementation, and it also requires
2238 that searching directories does not update their access times. As of
2239 this writing (November 2005) this works only with Linux, and only with
2240 Linux kernels 2.6.8 and later. Worse, there is currently no reliable
2241 way to know whether this feature actually works. Sometimes
2242 @command{tar} knows that it does not work, and if you use
2243 @option{--atime-preserve=system} then @command{tar} complains and
2244 exits right away. But other times @command{tar} might think that the
2245 option works when it actually does not.
2247 Currently @option{--atime-preserve} with no operand defaults to
2248 @option{--atime-preserve=replace}, but this may change in the future
2249 as support for @option{--atime-preserve=system} improves.
2251 If your operating system does not support
2252 @option{--atime-preserve=system}, you might be able to preserve access
2253 times reliably by by using the @command{mount} command. For example,
2254 you can mount the file system read-only, or access the file system via
2255 a read-only loopback mount, or use the @samp{noatime} mount option
2256 available on some systems. However, mounting typically requires
2257 superuser privileges and can be a pain to manage.
2259 @opindex backup, summary
2260 @item --backup=@var{backup-type}
2262 Rather than deleting files from the file system, @command{tar} will
2263 back them up using simple or numbered backups, depending upon
2264 @var{backup-type}. @xref{backup}.
2266 @opindex block-number, summary
2267 @item --block-number
2270 With this option present, @command{tar} prints error messages for read errors
2271 with the block number in the archive file. @xref{block-number}.
2273 @opindex blocking-factor, summary
2274 @item --blocking-factor=@var{blocking}
2275 @itemx -b @var{blocking}
2277 Sets the blocking factor @command{tar} uses to @var{blocking} x 512 bytes per
2278 record. @xref{Blocking Factor}.
2280 @opindex bzip2, summary
2284 This option tells @command{tar} to read or write archives through
2285 @code{bzip2}. @xref{gzip}.
2287 @opindex checkpoint, summary
2290 This option directs @command{tar} to print periodic checkpoint messages as it
2291 reads through the archive. It is intended for when you want a visual
2292 indication that @command{tar} is still running, but don't want to see
2293 @option{--verbose} output. @FIXME-xref{}
2295 @opindex check-links, summary
2298 If this option was given, @command{tar} will check the number of links
2299 dumped for each processed file. If this number does not match the
2300 total number of hard links for the file, a warning message will be
2301 output @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2302 synonym for @option{--one-file-system}. The current semantics, wich
2303 complies to UNIX98, was introduced with version
2304 1.15.91. @xref{Changes}, for more information.}.
2306 @opindex compress, summary
2307 @opindex uncompress, summary
2312 @command{tar} will use the @command{compress} program when reading or
2313 writing the archive. This allows you to directly act on archives
2314 while saving space. @xref{gzip}.
2316 @opindex confirmation, summary
2317 @item --confirmation
2319 (See @option{--interactive}.) @xref{interactive}.
2321 @opindex delay-directory-restore, summary
2322 @item --delay-directory-restore
2324 Delay setting modification times and permissions of extracted
2325 directories until the end of extraction. @xref{Directory Modification Times and Permissions}.
2327 @opindex dereference, summary
2331 When creating a @command{tar} archive, @command{tar} will archive the
2332 file that a symbolic link points to, rather than archiving the
2333 symlink. @xref{dereference}.
2335 @opindex directory, summary
2336 @item --directory=@var{dir}
2339 When this option is specified, @command{tar} will change its current directory
2340 to @var{dir} before performing any operations. When this option is used
2341 during archive creation, it is order sensitive. @xref{directory}.
2343 @opindex exclude, summary
2344 @item --exclude=@var{pattern}
2346 When performing operations, @command{tar} will skip files that match
2347 @var{pattern}. @xref{exclude}.
2349 @opindex exclude-from, summary
2350 @item --exclude-from=@var{file}
2351 @itemx -X @var{file}
2353 Similar to @option{--exclude}, except @command{tar} will use the list of
2354 patterns in the file @var{file}. @xref{exclude}.
2356 @opindex exclude-caches, summary
2357 @item --exclude-caches
2359 Automatically excludes all directories
2360 containing a cache directory tag. @xref{exclude}.
2362 @opindex file, summary
2363 @item --file=@var{archive}
2364 @itemx -f @var{archive}
2366 @command{tar} will use the file @var{archive} as the @command{tar} archive it
2367 performs operations on, rather than @command{tar}'s compilation dependent
2368 default. @xref{file tutorial}.
2370 @opindex files-from, summary
2371 @item --files-from=@var{file}
2372 @itemx -T @var{file}
2374 @command{tar} will use the contents of @var{file} as a list of archive members
2375 or files to operate on, in addition to those specified on the
2376 command-line. @xref{files}.
2378 @opindex force-local, summary
2381 Forces @command{tar} to interpret the filename given to @option{--file}
2382 as a local file, even if it looks like a remote tape drive name.
2383 @xref{local and remote archives}.
2385 @opindex format, summary
2386 @item --format=@var{format}
2388 Selects output archive format. @var{Format} may be one of the
2393 Creates an archive that is compatible with Unix V7 @command{tar}.
2396 Creates an archive that is compatible with GNU @command{tar} version
2400 Creates archive in GNU tar 1.13 format. Basically it is the same as
2401 @samp{oldgnu} with the only difference in the way it handles long
2405 Creates a @acronym{POSIX.1-1988} compatible archive.
2408 Creates a @acronym{POSIX.1-2001 archive}.
2412 @xref{Formats}, for a detailed discussion of these formats.
2414 @opindex group, summary
2415 @item --group=@var{group}
2417 Files added to the @command{tar} archive will have a group id of @var{group},
2418 rather than the group from the source file. @var{group} is first decoded
2419 as a group symbolic name, but if this interpretation fails, it has to be
2420 a decimal numeric group ID. @FIXME-xref{}
2422 Also see the comments for the @option{--owner=@var{user}} option.
2424 @opindex gzip, summary
2425 @opindex gunzip, summary
2426 @opindex ungzip, summary
2432 This option tells @command{tar} to read or write archives through
2433 @command{gzip}, allowing @command{tar} to directly operate on several
2434 kinds of compressed archives transparently. @xref{gzip}.
2436 @opindex help, summary
2439 @command{tar} will print out a short message summarizing the operations and
2440 options to @command{tar} and exit. @xref{help}.
2442 @opindex ignore-case, summary
2444 Ignore case when matching member or file names with
2445 patterns. @xref{controlling pattern-matching}.
2447 @opindex ignore-command-error, summary
2448 @item --ignore-command-error
2449 Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
2451 @opindex ignore-failed-read, summary
2452 @item --ignore-failed-read
2454 Do not exit unsuccessfully merely because an unreadable file was encountered.
2457 @opindex ignore-zeros, summary
2458 @item --ignore-zeros
2461 With this option, @command{tar} will ignore zeroed blocks in the
2462 archive, which normally signals EOF. @xref{Reading}.
2464 @opindex incremental, summary
2468 Used to inform @command{tar} that it is working with an old
2469 @acronym{GNU}-format incremental backup archive. It is intended
2470 primarily for backwards compatibility only. @xref{Incremental Dumps},
2471 for a detailed discussion of incremental archives.
2473 @opindex index-file, summary
2474 @item --index-file=@var{file}
2476 Send verbose output to @var{file} instead of to standard output.
2478 @opindex info-script, summary
2479 @opindex new-volume-script, summary
2480 @item --info-script=@var{script-file}
2481 @itemx --new-volume-script=@var{script-file}
2482 @itemx -F @var{script-file}
2484 When @command{tar} is performing multi-tape backups, @var{script-file} is run
2485 at the end of each tape. If @var{script-file} exits with nonzero status,
2486 @command{tar} fails immediately. @xref{info-script}, for a detailed
2487 discussion of @var{script-file}.
2489 @opindex interactive, summary
2491 @itemx --confirmation
2494 Specifies that @command{tar} should ask the user for confirmation before
2495 performing potentially destructive options, such as overwriting files.
2498 @opindex keep-newer-files, summary
2499 @item --keep-newer-files
2501 Do not replace existing files that are newer than their archive copies
2502 when extracting files from an archive.
2504 @opindex keep-old-files, summary
2505 @item --keep-old-files
2508 Do not overwrite existing files when extracting files from an archive.
2509 @xref{Keep Old Files}.
2511 @opindex label, summary
2512 @item --label=@var{name}
2513 @itemx -V @var{name}
2515 When creating an archive, instructs @command{tar} to write @var{name}
2516 as a name record in the archive. When extracting or listing archives,
2517 @command{tar} will only operate on archives that have a label matching
2518 the pattern specified in @var{name}. @xref{Tape Files}.
2520 @opindex listed-incremental, summary
2521 @item --listed-incremental=@var{snapshot-file}
2522 @itemx -g @var{snapshot-file}
2524 During a @option{--create} operation, specifies that the archive that
2525 @command{tar} creates is a new @acronym{GNU}-format incremental
2526 backup, using @var{snapshot-file} to determine which files to backup.
2527 With other operations, informs @command{tar} that the archive is in
2528 incremental format. @xref{Incremental Dumps}.
2530 @opindex mode, summary
2531 @item --mode=@var{permissions}
2533 When adding files to an archive, @command{tar} will use
2534 @var{permissions} for the archive members, rather than the permissions
2535 from the files. The program @command{chmod} and this @command{tar}
2536 option share the same syntax for what @var{permissions} might be.
2537 @xref{File permissions, Permissions, File permissions, fileutils,
2538 @acronym{GNU} file utilities}. This reference also has useful
2539 information for those not being overly familiar with the Unix
2542 Of course, @var{permissions} might be plainly specified as an octal number.
2543 However, by using generic symbolic modifications to mode bits, this allows
2544 more flexibility. For example, the value @samp{a+rw} adds read and write
2545 permissions for everybody, while retaining executable bits on directories
2546 or on any other file already marked as executable.
2548 @opindex multi-volume, summary
2549 @item --multi-volume
2552 Informs @command{tar} that it should create or otherwise operate on a
2553 multi-volume @command{tar} archive. @xref{Using Multiple Tapes}.
2555 @opindex new-volume-script, summary
2556 @item --new-volume-script
2560 @opindex seek, summary
2564 Assume that the archive media supports seeks to arbitrary
2565 locations. Usually @command{tar} determines automatically whether
2566 the archive can be seeked or not. This option is intended for use
2567 in cases when such recognition fails.
2569 @opindex newer, summary
2570 @item --newer=@var{date}
2571 @itemx --after-date=@var{date}
2574 When creating an archive, @command{tar} will only add files that have changed
2575 since @var{date}. If @var{date} begins with @samp{/} or @samp{.}, it
2576 is taken to be the name of a file whose data modification time specifies
2577 the date. @xref{after}.
2579 @opindex newer-mtime, summary
2580 @item --newer-mtime=@var{date}
2582 Like @option{--newer}, but add only files whose
2583 contents have changed (as opposed to just @option{--newer}, which will
2584 also back up files for which any status information has changed).
2586 @opindex no-anchored, summary
2588 An exclude pattern can match any subsequence of the name's components.
2589 @xref{controlling pattern-matching}.
2591 @opindex no-delay-directory-restore, summary
2592 @item --no-delay-directory-restore
2594 Setting modification times and permissions of extracted
2595 directories when all files from this directory has been
2596 extracted. This is the default. @xref{Directory Modification Times and Permissions}.
2598 @opindex no-ignore-case, summary
2599 @item --no-ignore-case
2600 Use case-sensitive matching.
2601 @xref{controlling pattern-matching}.
2603 @opindex no-ignore-command-error, summary
2604 @item --no-ignore-command-error
2605 Print warnings about subprocesses terminated with a non-zero exit
2606 code. @xref{Writing to an External Program}.
2608 @opindex no-quote-chars, summary
2609 @item --no-quote-chars=@var{string}
2610 Do not quote characters from @var{string}, even if the selected
2611 quoting style implies they should be quoted (@FIXME-pxref{Quoting Styles}).
2613 @opindex no-recursion, summary
2614 @item --no-recursion
2616 With this option, @command{tar} will not recurse into directories.
2619 @opindex no-same-owner, summary
2620 @item --no-same-owner
2623 When extracting an archive, do not attempt to preserve the owner
2624 specified in the @command{tar} archive. This the default behavior
2627 @opindex no-same-permissions, summary
2628 @item --no-same-permissions
2630 When extracting an archive, subtract the user's umask from files from
2631 the permissions specified in the archive. This is the default behavior
2634 @opindex no-wildcards, summary
2635 @item --no-wildcards
2636 Do not use wildcards.
2637 @xref{controlling pattern-matching}.
2639 @opindex no-wildcards-match-slash, summary
2640 @item --no-wildcards-match-slash
2641 Wildcards do not match @samp{/}.
2642 @xref{controlling pattern-matching}.
2644 @opindex null, summary
2647 When @command{tar} is using the @option{--files-from} option, this option
2648 instructs @command{tar} to expect filenames terminated with @option{NUL}, so
2649 @command{tar} can correctly work with file names that contain newlines.
2652 @opindex numeric-owner, summary
2653 @item --numeric-owner
2655 This option will notify @command{tar} that it should use numeric user
2656 and group IDs when creating a @command{tar} file, rather than names.
2660 When extracting files, this option is a synonym for
2661 @option{--no-same-owner}, i.e. it prevents @command{tar} from
2662 restoring ownership of files being extracted.
2664 When creating an archive, @option{-o} is a synonym for
2665 @option{--old-archive}. This behavior is for compatibility
2666 with previous versions of @GNUTAR{}, and will be
2667 removed in the future releases.
2669 @xref{Changes}, for more information.
2671 @opindex occurrence, summary
2672 @item --occurrence[=@var{number}]
2674 This option can be used in conjunction with one of the subcommands
2675 @option{--delete}, @option{--diff}, @option{--extract} or
2676 @option{--list} when a list of files is given either on the command
2677 line or via @option{-T} option.
2679 This option instructs @command{tar} to process only the @var{number}th
2680 occurrence of each named file. @var{Number} defaults to 1, so
2683 tar -x -f archive.tar --occurrence filename
2687 will extract the first occurrence of @file{filename} from @file{archive.tar}
2688 and will terminate without scanning to the end of the archive.
2690 @opindex old-archive, summary
2692 Synonym for @option{--format=v7}.
2694 @opindex one-file-system, summary
2695 @item --one-file-system
2696 Used when creating an archive. Prevents @command{tar} from recursing into
2697 directories that are on different file systems from the current
2698 directory @footnote{Earlier versions of @GNUTAR{} understood @option{-l} as a
2699 synonym for @option{--one-file-system}. This has changed in version
2700 1.15.91. @xref{Changes}, for more information.}.
2702 @opindex overwrite, summary
2705 Overwrite existing files and directory metadata when extracting files
2706 from an archive. @xref{Overwrite Old Files}.
2708 @opindex overwrite-dir, summary
2709 @item --overwrite-dir
2711 Overwrite the metadata of existing directories when extracting files
2712 from an archive. @xref{Overwrite Old Files}.
2714 @opindex owner, summary
2715 @item --owner=@var{user}
2717 Specifies that @command{tar} should use @var{user} as the owner of members
2718 when creating archives, instead of the user associated with the source
2719 file. @var{user} is first decoded as a user symbolic name, but if
2720 this interpretation fails, it has to be a decimal numeric user ID.
2723 There is no value indicating a missing number, and @samp{0} usually means
2724 @code{root}. Some people like to force @samp{0} as the value to offer in
2725 their distributions for the owner of files, because the @code{root} user is
2726 anonymous anyway, so that might as well be the owner of anonymous archives.
2728 This option does not affect extraction from archives.
2730 @opindex quote-chars, summary
2731 @item --quote-chars=@var{string}
2732 Always quote characters from @var{string}, even if the selected
2733 quoting style would not quote them (@FIXME-pxref{Quoting Styles}).
2735 @opindex quoting-style, summary
2736 @item --quoting-style=@var{style}
2737 Set quoting style to use when printing member and file names
2738 (@FIXME-pxref{Quoting Styles}). Valid @var{style} values are:
2739 @code{literal}, @code{shell}, @code{shell-always}, @code{c},
2740 @code{escape}, @code{locale}, and @code{clocale}. Default quoting
2741 style is @code{escape}, unless overridden while configuring the
2744 @opindex pax-option, summary
2745 @item --pax-option=@var{keyword-list}
2746 @FIXME{Such a detailed description does not belong there, move it elsewhere.}
2747 This option is meaningful only with @acronym{POSIX.1-2001} archives
2748 (@pxref{posix}). It modifies the way @command{tar} handles the
2749 extended header keywords. @var{Keyword-list} is a comma-separated
2750 list of keyword options, each keyword option taking one of
2751 the following forms:
2754 @item delete=@var{pattern}
2755 When used with one of archive-creation commands,
2756 this option instructs @command{tar} to omit from extended header records
2757 that it produces any keywords matching the string @var{pattern}.
2759 When used in extract or list mode, this option instructs tar
2760 to ignore any keywords matching the given @var{pattern} in the extended
2761 header records. In both cases, matching is performed using the pattern
2762 matching notation described in @acronym{POSIX 1003.2}, 3.13
2763 (See @cite{glob(7)}). For example:
2766 --pax-option delete=security.*
2769 would suppress security-related information.
2771 @item exthdr.name=@var{string}
2773 This keyword allows user control over the name that is written into the
2774 ustar header blocks for the extended headers. The name is obtained
2775 from @var{string} after making the following substitutions:
2777 @multitable @columnfractions .30 .70
2778 @headitem Meta-character @tab Replaced By
2779 @item %d @tab The directory name of the file, equivalent to the
2780 result of the @command{dirname} utility on the translated pathname.
2781 @item %f @tab The filename of the file, equivalent to the result
2782 of the @command{basename} utility on the translated pathname.
2783 @item %p @tab The process ID of the @command{tar} process.
2784 @item %% @tab A @samp{%} character.
2787 Any other @samp{%} characters in @var{string} produce undefined
2790 If no option @samp{exthdr.name=string} is specified, @command{tar}
2791 will use the following default value:
2797 @item globexthdr.name=@var{string}
2798 This keyword allows user control over the name that is written into
2799 the ustar header blocks for global extended header records. The name
2800 is obtained from the contents of @var{string}, after making
2801 the following substitutions:
2803 @multitable @columnfractions .30 .70
2804 @headitem Meta-character @tab Replaced By
2805 @item %n @tab An integer that represents the
2806 sequence number of the global extended header record in the archive,
2808 @item %p @tab The process ID of the @command{tar} process.
2809 @item %% @tab A @samp{%} character.
2812 Any other @samp{%} characters in @var{string} produce undefined results.
2814 If no option @samp{globexthdr.name=string} is specified, @command{tar}
2815 will use the following default value:
2818 $TMPDIR/GlobalHead.%p.%n
2822 where @samp{$TMPDIR} represents the value of the @var{TMPDIR}
2823 environment variable. If @var{TMPDIR} is not set, @command{tar}
2826 @item @var{keyword}=@var{value}
2827 When used with one of archive-creation commands, these keyword/value pairs
2828 will be included at the beginning of the archive in a global extended
2829 header record. When used with one of archive-reading commands,
2830 @command{tar} will behave as if it has encountered these keyword/value
2831 pairs at the beginning of the archive in a global extended header
2834 @item @var{keyword}:=@var{value}
2835 When used with one of archive-creation commands, these keyword/value pairs
2836 will be included as records at the beginning of an extended header for
2837 each file. This is effectively equivalent to @var{keyword}=@var{value}
2838 form except that it creates no global extended header records.
2840 When used with one of archive-reading commands, @command{tar} will
2841 behave as if these keyword/value pairs were included as records at the
2842 end of each extended header; thus, they will override any global or
2843 file-specific extended header record keywords of the same names.
2844 For example, in the command:
2847 tar --format=posix --create \
2848 --file archive --pax-option gname:=user .
2851 the group name will be forced to a new value for all files
2852 stored in the archive.
2855 @opindex portability, summary
2857 @itemx --old-archive
2858 Synonym for @option{--format=v7}.
2860 @opindex posix, summary
2862 Same as @option{--format=posix}.
2864 @opindex preserve, summary
2867 Synonymous with specifying both @option{--preserve-permissions} and
2868 @option{--same-order}. @xref{Setting Access Permissions}.
2870 @opindex preserve-order, summary
2871 @item --preserve-order
2873 (See @option{--same-order}; @pxref{Reading}.)
2875 @opindex preserve-permissions, summary
2876 @opindex same-permissions, summary
2877 @item --preserve-permissions
2878 @itemx --same-permissions
2881 When @command{tar} is extracting an archive, it normally subtracts the
2882 users' umask from the permissions specified in the archive and uses
2883 that number as the permissions to create the destination file.
2884 Specifying this option instructs @command{tar} that it should use the
2885 permissions directly from the archive. @xref{Setting Access Permissions}.
2887 @opindex read-full-records, summary
2888 @item --read-full-records
2891 Specifies that @command{tar} should reblock its input, for reading
2892 from pipes on systems with buggy implementations. @xref{Reading}.
2894 @opindex record-size, summary
2895 @item --record-size=@var{size}
2897 Instructs @command{tar} to use @var{size} bytes per record when accessing the
2898 archive. @xref{Blocking Factor}.
2900 @opindex recursion, summary
2903 With this option, @command{tar} recurses into directories.
2906 @opindex recursive-unlink, summary
2907 @item --recursive-unlink
2910 directory hierarchies before extracting directories of the same name
2911 from the archive. @xref{Recursive Unlink}.
2913 @opindex remove-files, summary
2914 @item --remove-files
2916 Directs @command{tar} to remove the source file from the file system after
2917 appending it to an archive. @xref{remove files}.
2919 @opindex restrict, summary
2922 Disable use of some potentially harmful @command{tar} options.
2923 Currently this option disables shell invocaton from multi-volume menu
2924 (@pxref{Using Multiple Tapes}).
2926 @opindex rmt-command, summary
2927 @item --rmt-command=@var{cmd}
2929 Notifies @command{tar} that it should use @var{cmd} instead of
2930 the default @file{/usr/libexec/rmt} (@pxref{Remote Tape Server}).
2932 @opindex rsh-command, summary
2933 @item --rsh-command=@var{cmd}
2935 Notifies @command{tar} that is should use @var{cmd} to communicate with remote
2936 devices. @xref{Device}.
2938 @opindex same-order, summary
2940 @itemx --preserve-order
2943 This option is an optimization for @command{tar} when running on machines with
2944 small amounts of memory. It informs @command{tar} that the list of file
2945 arguments has already been sorted to match the order of files in the
2946 archive. @xref{Reading}.
2948 @opindex same-owner, summary
2951 When extracting an archive, @command{tar} will attempt to preserve the owner
2952 specified in the @command{tar} archive with this option present.
2953 This is the default behavior for the superuser; this option has an
2954 effect only for ordinary users. @xref{Attributes}.
2956 @opindex same-permissions, summary
2957 @item --same-permissions
2959 (See @option{--preserve-permissions}; @pxref{Setting Access Permissions}.)
2961 @opindex show-defaults, summary
2962 @item --show-defaults
2964 Displays the default options used by @command{tar} and exits
2965 successfully. This option is intended for use in shell scripts.
2966 Here is an example of what you can see using this option:
2969 $ tar --show-defaults
2970 --format=gnu -f- -b20 --quoting-style=escape \
2971 --rmt-command=/usr/libexec/rmt --rsh-command=/usr/bin/rsh
2974 @opindex show-omitted-dirs, summary
2975 @item --show-omitted-dirs
2977 Instructs @command{tar} to mention directories its skipping over when
2978 operating on a @command{tar} archive. @xref{show-omitted-dirs}.
2980 @opindex show-stored-names, summary
2981 @item --show-stored-names
2983 This option has effect only when used in conjunction with one of
2984 archive creation operations. It instructs tar to list the member names
2985 stored in the archive, as opposed to the actual file
2986 names. @xref{listing member and file names}.
2988 @opindex sparse, summary
2992 Invokes a @acronym{GNU} extension when adding files to an archive that handles
2993 sparse files efficiently. @xref{sparse}.
2995 @opindex starting-file, summary
2996 @item --starting-file=@var{name}
2997 @itemx -K @var{name}
2999 This option affects extraction only; @command{tar} will skip extracting
3000 files in the archive until it finds one that matches @var{name}.
3003 @opindex strip-components, summary
3004 @item --strip-components=@var{number}
3005 Strip given @var{number} of leading components from file names before
3006 extraction.@footnote{This option was called @option{--strip-path} in
3007 version 1.14.} For example, if archive @file{archive.tar} contained
3008 @file{/some/file/name}, then running
3011 tar --extract --file archive.tar --strip-components=2
3015 would extracted this file to file @file{name}.
3017 @opindex suffix, summary
3018 @item --suffix=@var{suffix}
3020 Alters the suffix @command{tar} uses when backing up files from the default
3021 @samp{~}. @xref{backup}.
3023 @opindex tape-length, summary
3024 @item --tape-length=@var{num}
3027 Specifies the length of tapes that @command{tar} is writing as being
3028 @w{@var{num} x 1024} bytes long. @xref{Using Multiple Tapes}.
3030 @opindex test-label, summary
3033 Reads the volume label. If an argument is specified, test whether it
3034 matches the volume label. @xref{--test-label option}.
3036 @opindex to-command, summary
3037 @item --to-command=@var{command}
3039 During extraction @command{tar} will pipe extracted files to the
3040 standard input of @var{command}. @xref{Writing to an External Program}.
3042 @opindex to-stdout, summary
3046 During extraction, @command{tar} will extract files to stdout rather
3047 than to the file system. @xref{Writing to Standard Output}.
3049 @opindex totals, summary
3052 Displays the total number of bytes written after creating an archive.
3055 @opindex touch, summary
3059 Sets the data modification time of extracted files to the extraction time,
3060 rather than the data modification time stored in the archive.
3061 @xref{Data Modification Times}.
3063 @opindex uncompress, summary
3066 (See @option{--compress}. @pxref{gzip})
3068 @opindex ungzip, summary
3071 (See @option{--gzip}. @pxref{gzip})
3073 @opindex unlink-first, summary
3074 @item --unlink-first
3077 Directs @command{tar} to remove the corresponding file from the file
3078 system before extracting it from the archive. @xref{Unlink First}.
3080 @opindex use-compress-program, summary
3081 @item --use-compress-program=@var{prog}
3083 Instructs @command{tar} to access the archive through @var{prog}, which is
3084 presumed to be a compression program of some sort. @xref{gzip}.
3086 @opindex utc, summary
3089 Display file modification dates in @acronym{UTC}. This option implies
3092 @opindex verbose, summary
3096 Specifies that @command{tar} should be more verbose about the operations its
3097 performing. This option can be specified multiple times for some
3098 operations to increase the amount of information displayed.
3101 @opindex verify, summary
3105 Verifies that the archive was correctly written when creating an
3106 archive. @xref{verify}.
3108 @opindex version, summary
3111 Print information about the program's name, version, origin and legal
3112 status, all on standard output, and then exit successfully.
3115 @opindex volno-file, summary
3116 @item --volno-file=@var{file}
3118 Used in conjunction with @option{--multi-volume}. @command{tar} will keep track
3119 of which volume of a multi-volume archive its working in @var{file}.
3122 @opindex wildcards, summary
3124 Use wildcards when matching member names with patterns.
3125 @xref{controlling pattern-matching}.
3127 @opindex wildcards-match-slash, summary
3128 @item --wildcards-match-slash
3129 Wildcards match @samp{/}.
3130 @xref{controlling pattern-matching}.
3133 @node Short Option Summary
3134 @subsection Short Options Cross Reference
3136 Here is an alphabetized list of all of the short option forms, matching
3137 them with the equivalent long option.
3143 @option{--concatenate}
3147 @option{--read-full-records}
3151 @option{--directory}
3155 @option{--info-script}
3159 @option{--incremental}
3163 @option{--starting-file}
3167 @option{--tape-length}
3171 @option{--multi-volume}
3179 @option{--to-stdout}
3183 @option{--absolute-names}
3187 @option{--block-number}
3195 @option{--files-from}
3199 @option{--unlink-first}
3211 @option{--exclude-from}
3219 @option{--blocking-factor}
3235 @option{--listed-incremental}
3239 @option{--dereference}
3243 @option{--ignore-zeros}
3251 @option{--keep-old-files}
3255 @option{--one-file-system}. Use of this short option is deprecated. It
3256 is retained for compatibility with the earlier versions of GNU
3257 @command{tar}, and will be changed in future releases.
3259 @xref{Changes}, for more information.
3267 When creating --- @option{--no-same-owner}, when extracting ---
3268 @option{--portability}.
3270 The later usage is deprecated. It is retained for compatibility with
3271 the earlier versions of @GNUTAR{}. In the future releases
3272 @option{-o} will be equivalent to @option{--no-same-owner} only.
3276 @option{--preserve-permissions}
3284 @option{--same-order}
3300 @option{--interactive}
3313 @section @GNUTAR{} documentation
3315 @cindex Getting program version number
3317 @cindex Version of the @command{tar} program
3318 Being careful, the first thing is really checking that you are using
3319 @GNUTAR{}, indeed. The @option{--version} option
3320 causes @command{tar} to print information about its name, version,
3321 origin and legal status, all on standard output, and then exit
3322 successfully. For example, @w{@samp{tar --version}} might print:
3325 tar (GNU tar) 1.15.2
3326 Copyright (C) 2006 Free Software Foundation, Inc.
3327 This is free software. You may redistribute copies of it under the terms of
3328 the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
3329 There is NO WARRANTY, to the extent permitted by law.
3331 Written by John Gilmore and Jay Fenlason.
3335 The first occurrence of @samp{tar} in the result above is the program
3336 name in the package (for example, @command{rmt} is another program),
3337 while the second occurrence of @samp{tar} is the name of the package
3338 itself, containing possibly many programs. The package is currently
3339 named @samp{tar}, after the name of the main program it
3340 contains@footnote{There are plans to merge the @command{cpio} and
3341 @command{tar} packages into a single one which would be called
3342 @code{paxutils}. So, who knows if, one of this days, the
3343 @option{--version} would not output @w{@samp{tar (@acronym{GNU}
3346 @cindex Obtaining help
3347 @cindex Listing all @command{tar} options
3348 @opindex help, introduction
3349 Another thing you might want to do is checking the spelling or meaning
3350 of some particular @command{tar} option, without resorting to this
3351 manual, for once you have carefully read it. @GNUTAR{}
3352 has a short help feature, triggerable through the
3353 @option{--help} option. By using this option, @command{tar} will
3354 print a usage message listing all available options on standard
3355 output, then exit successfully, without doing anything else and
3356 ignoring all other options. Even if this is only a brief summary, it
3357 may be several screens long. So, if you are not using some kind of
3358 scrollable window, you might prefer to use something like:
3361 $ @kbd{tar --help | less}
3365 presuming, here, that you like using @command{less} for a pager. Other
3366 popular pagers are @command{more} and @command{pg}. If you know about some
3367 @var{keyword} which interests you and do not want to read all the
3368 @option{--help} output, another common idiom is doing:
3371 tar --help | grep @var{keyword}
3375 for getting only the pertinent lines. Notice, however, that some
3376 @command{tar} options have long description lines and the above
3377 command will list only the first of them.
3379 The exact look of the option summary displayed by @kbd{tar --help} is
3380 configurable. @xref{Configuring Help Summary}, for a detailed description.
3383 If you only wish to check the spelling of an option, running @kbd{tar
3384 --usage} may be a better choice. This will display a terse list of
3385 @command{tar} option without accompanying explanations.
3387 The short help output is quite succinct, and you might have to get
3388 back to the full documentation for precise points. If you are reading
3389 this paragraph, you already have the @command{tar} manual in some
3390 form. This manual is available in a variety of forms from
3391 @url{http://www.gnu.org/software/tar/manual}. It may printed out of the @GNUTAR{}
3392 distribution, provided you have @TeX{} already installed somewhere,
3393 and a laser printer around. Just configure the distribution, execute
3394 the command @w{@samp{make dvi}}, then print @file{doc/tar.dvi} the
3395 usual way (contact your local guru to know how). If @GNUTAR{}
3396 has been conveniently installed at your place, this
3397 manual is also available in interactive, hypertextual form as an Info
3398 file. Just call @w{@samp{info tar}} or, if you do not have the
3399 @command{info} program handy, use the Info reader provided within
3400 @acronym{GNU} Emacs, calling @samp{tar} from the main Info menu.
3402 There is currently no @code{man} page for @GNUTAR{}.
3403 If you observe such a @code{man} page on the system you are running,
3404 either it does not belong to @GNUTAR{}, or it has not
3405 been produced by @acronym{GNU}. Some package maintainers convert
3406 @kbd{tar --help} output to a man page, using @command{help2man}. In
3407 any case, please bear in mind that the authoritative source of
3408 information about @GNUTAR{} is this Texinfo documentation.
3411 @section Obtaining @GNUTAR{} default values
3413 @opindex show-defaults
3414 @GNUTAR{} has some predefined defaults that are used when you do not
3415 explicitely specify another values. To obtain a list of such
3416 defaults, use @option{--show-defaults} option. This will output the
3417 values in the form of @command{tar} command line options:
3421 @kbd{tar --show-defaults}
3422 --format=gnu -f- -b20 --rmt-command=/etc/rmt --rsh-command=/usr/bin/rsh
3427 The above output shows that this version of @GNUTAR{} defaults to
3428 using @samp{gnu} archive format (@pxref{Formats}), it uses standard
3429 output as the archive, if no @option{--file} option has been given
3430 (@pxref{file tutorial}), the default blocking factor is 20
3431 (@pxref{Blocking Factor}). It also shows the default locations where
3432 @command{tar} will look for @command{rmt} and @command{rsh} binaries.
3435 @section Checking @command{tar} progress
3437 Typically, @command{tar} performs most operations without reporting any
3438 information to the user except error messages. When using @command{tar}
3439 with many options, particularly ones with complicated or
3440 difficult-to-predict behavior, it is possible to make serious mistakes.
3441 @command{tar} provides several options that make observing @command{tar}
3442 easier. These options cause @command{tar} to print information as it
3443 progresses in its job, and you might want to use them just for being
3444 more careful about what is going on, or merely for entertaining
3445 yourself. If you have encountered a problem when operating on an
3446 archive, however, you may need more information than just an error
3447 message in order to solve the problem. The following options can be
3448 helpful diagnostic tools.
3450 @cindex Verbose operation
3452 Normally, the @option{--list} (@option{-t}) command to list an archive
3453 prints just the file names (one per line) and the other commands are
3454 silent. When used with most operations, the @option{--verbose}
3455 (@option{-v}) option causes @command{tar} to print the name of each
3456 file or archive member as it is processed. This and the other options
3457 which make @command{tar} print status information can be useful in
3458 monitoring @command{tar}.
3460 With @option{--create} or @option{--extract}, @option{--verbose} used
3461 once just prints the names of the files or members as they are processed.
3462 Using it twice causes @command{tar} to print a longer listing
3463 (reminiscent of @samp{ls -l}) for each member. Since @option{--list}
3464 already prints the names of the members, @option{--verbose} used once
3465 with @option{--list} causes @command{tar} to print an @samp{ls -l}
3466 type listing of the files in the archive. The following examples both
3467 extract members with long list output:
3470 $ @kbd{tar --extract --file=archive.tar --verbose --verbose}
3471 $ @kbd{tar xvvf archive.tar}
3474 Verbose output appears on the standard output except when an archive is
3475 being written to the standard output, as with @samp{tar --create
3476 --file=- --verbose} (@samp{tar cfv -}, or even @samp{tar cv}---if the
3477 installer let standard output be the default archive). In that case
3478 @command{tar} writes verbose output to the standard error stream.
3480 If @option{--index-file=@var{file}} is specified, @command{tar} sends
3481 verbose output to @var{file} rather than to standard output or standard
3484 @cindex Obtaining total status information
3486 The @option{--totals} option---which is only meaningful when used with
3487 @option{--create} (@option{-c})---causes @command{tar} to print the total
3488 amount written to the archive, after it has been fully created.
3490 @cindex Progress information
3492 The @option{--checkpoint} option prints an occasional message
3493 as @command{tar} reads or writes the archive. In fact, it prints
3494 a message each 10 records read or written. It is designed for
3495 those who don't need the more detailed (and voluminous) output of
3496 @option{--block-number} (@option{-R}), but do want visual confirmation that @command{tar}
3497 is actually making forward progress.
3499 @FIXME{There is some confusion here. It seems that -R once wrote a
3500 message at @samp{every} record read or written.}
3502 @opindex show-omitted-dirs
3503 @anchor{show-omitted-dirs}
3504 The @option{--show-omitted-dirs} option, when reading an archive---with
3505 @option{--list} or @option{--extract}, for example---causes a message
3506 to be printed for each directory in the archive which is skipped.
3507 This happens regardless of the reason for skipping: the directory might
3508 not have been named on the command line (implicitly or explicitly),
3509 it might be excluded by the use of the @option{--exclude=@var{pattern}} option, or
3512 @opindex block-number
3513 @cindex Block number where error occurred
3514 @anchor{block-number}
3515 If @option{--block-number} (@option{-R}) is used, @command{tar} prints, along with
3516 every message it would normally produce, the block number within the
3517 archive where the message was triggered. Also, supplementary messages
3518 are triggered when reading blocks full of NULs, or when hitting end of
3519 file on the archive. As of now, if the archive if properly terminated
3520 with a NUL block, the reading of the file may stop before end of file
3521 is met, so the position of end of file will not usually show when
3522 @option{--block-number} (@option{-R}) is used. Note that @GNUTAR{}
3523 drains the archive before exiting when reading the
3524 archive from a pipe.
3526 @cindex Error message, block number of
3527 This option is especially useful when reading damaged archives, since
3528 it helps pinpoint the damaged sections. It can also be used with
3529 @option{--list} (@option{-t}) when listing a file-system backup tape, allowing you to
3530 choose among several backup tapes when retrieving a file later, in
3531 favor of the tape where the file appears earliest (closest to the
3532 front of the tape). @xref{backup}.
3535 @section Asking for Confirmation During Operations
3536 @cindex Interactive operation
3538 Typically, @command{tar} carries out a command without stopping for
3539 further instructions. In some situations however, you may want to
3540 exclude some files and archive members from the operation (for instance
3541 if disk or storage space is tight). You can do this by excluding
3542 certain files automatically (@pxref{Choosing}), or by performing
3543 an operation interactively, using the @option{--interactive} (@option{-w}) option.
3544 @command{tar} also accepts @option{--confirmation} for this option.
3546 @opindex interactive
3547 When the @option{--interactive} (@option{-w}) option is specified, before
3548 reading, writing, or deleting files, @command{tar} first prints a message
3549 for each such file, telling what operation it intends to take, then asks
3550 for confirmation on the terminal. The actions which require
3551 confirmation include adding a file to the archive, extracting a file
3552 from the archive, deleting a file from the archive, and deleting a file
3553 from disk. To confirm the action, you must type a line of input
3554 beginning with @samp{y}. If your input line begins with anything other
3555 than @samp{y}, @command{tar} skips that file.
3557 If @command{tar} is reading the archive from the standard input,
3558 @command{tar} opens the file @file{/dev/tty} to support the interactive
3561 Verbose output is normally sent to standard output, separate from
3562 other error messages. However, if the archive is produced directly
3563 on standard output, then verbose output is mixed with errors on
3564 @code{stderr}. Producing the archive on standard output may be used
3565 as a way to avoid using disk space, when the archive is soon to be
3566 consumed by another process reading it, say. Some people felt the need
3567 of producing an archive on stdout, still willing to segregate between
3568 verbose output and error output. A possible approach would be using a
3569 named pipe to receive the archive, and having the consumer process to
3570 read from that named pipe. This has the advantage of letting standard
3571 output free to receive verbose output, all separate from errors.
3574 @chapter @GNUTAR{} Operations
3587 @section Basic @GNUTAR{} Operations
3589 The basic @command{tar} operations, @option{--create} (@option{-c}),
3590 @option{--list} (@option{-t}) and @option{--extract} (@option{--get},
3591 @option{-x}), are currently presented and described in the tutorial
3592 chapter of this manual. This section provides some complementary notes
3593 for these operations.
3596 @opindex create, complementary notes
3600 Creating an empty archive would have some kind of elegance. One can
3601 initialize an empty archive and later use @option{--append}
3602 (@option{-r}) for adding all members. Some applications would not
3603 welcome making an exception in the way of adding the first archive
3604 member. On the other hand, many people reported that it is
3605 dangerously too easy for @command{tar} to destroy a magnetic tape with
3606 an empty archive@footnote{This is well described in @cite{Unix-haters
3607 Handbook}, by Simson Garfinkel, Daniel Weise & Steven Strassmann, IDG
3608 Books, ISBN 1-56884-203-1.}. The two most common errors are:
3612 Mistakingly using @code{create} instead of @code{extract}, when the
3613 intent was to extract the full contents of an archive. This error
3614 is likely: keys @kbd{c} and @kbd{x} are right next to each other on
3615 the QWERTY keyboard. Instead of being unpacked, the archive then
3616 gets wholly destroyed. When users speak about @dfn{exploding} an
3617 archive, they usually mean something else :-).
3620 Forgetting the argument to @code{file}, when the intent was to create
3621 an archive with a single file in it. This error is likely because a
3622 tired user can easily add the @kbd{f} key to the cluster of option
3623 letters, by the mere force of habit, without realizing the full
3624 consequence of doing so. The usual consequence is that the single
3625 file, which was meant to be saved, is rather destroyed.
3628 So, recognizing the likelihood and the catastrophical nature of these
3629 errors, @GNUTAR{} now takes some distance from elegance, and
3630 cowardly refuses to create an archive when @option{--create} option is
3631 given, there are no arguments besides options, and
3632 @option{--files-from} (@option{-T}) option is @emph{not} used. To get
3633 around the cautiousness of @GNUTAR{} and nevertheless create an
3634 archive with nothing in it, one may still use, as the value for the
3635 @option{--files-from} option, a file with no names in it, as shown in
3636 the following commands:
3639 @kbd{tar --create --file=empty-archive.tar --files-from=/dev/null}
3640 @kbd{tar cfT empty-archive.tar /dev/null}
3643 @opindex extract, complementary notes
3648 A socket is stored, within a @GNUTAR{} archive, as a pipe.
3650 @item @option{--list} (@option{-t})
3652 @GNUTAR{} now shows dates as @samp{1996-08-30},
3653 while it used to show them as @samp{Aug 30 1996}. Preferably,
3654 people should get used to ISO 8601 dates. Local American dates should
3655 be made available again with full date localization support, once
3656 ready. In the meantime, programs not being localizable for dates
3657 should prefer international dates, that's really the way to go.
3659 Look up @url{http://www.ft.uni-erlangen.de/~mskuhn/iso-time.html} if you
3660 are curious, it contains a detailed explanation of the ISO 8601 standard.
3665 @section Advanced @GNUTAR{} Operations
3667 Now that you have learned the basics of using @GNUTAR{}, you may want
3668 to learn about further ways in which @command{tar} can help you.
3670 This chapter presents five, more advanced operations which you probably
3671 won't use on a daily basis, but which serve more specialized functions.
3672 We also explain the different styles of options and why you might want
3673 to use one or another, or a combination of them in your @command{tar}
3674 commands. Additionally, this chapter includes options which allow you to
3675 define the output from @command{tar} more carefully, and provide help and
3676 error correction in special circumstances.
3678 @FIXME{check this after the chapter is actually revised to make sure
3679 it still introduces the info in the chapter correctly : ).}
3691 @subsection The Five Advanced @command{tar} Operations
3694 In the last chapter, you learned about the first three operations to
3695 @command{tar}. This chapter presents the remaining five operations to
3696 @command{tar}: @option{--append}, @option{--update}, @option{--concatenate},
3697 @option{--delete}, and @option{--compare}.
3699 You are not likely to use these operations as frequently as those
3700 covered in the last chapter; however, since they perform specialized
3701 functions, they are quite useful when you do need to use them. We
3702 will give examples using the same directory and files that you created
3703 in the last chapter. As you may recall, the directory is called
3704 @file{practice}, the files are @samp{jazz}, @samp{blues}, @samp{folk},
3705 @samp{rock}, and the two archive files you created are
3706 @samp{collection.tar} and @samp{music.tar}.
3708 We will also use the archive files @samp{afiles.tar} and
3709 @samp{bfiles.tar}. @samp{afiles.tar} contains the members @samp{apple},
3710 @samp{angst}, and @samp{aspic}. @samp{bfiles.tar} contains the members
3711 @samp{./birds}, @samp{baboon}, and @samp{./box}.
3713 Unless we state otherwise, all practicing you do and examples you follow
3714 in this chapter will take place in the @file{practice} directory that
3715 you created in the previous chapter; see @ref{prepare for examples}.
3716 (Below in this section, we will remind you of the state of the examples
3717 where the last chapter left them.)
3719 The five operations that we will cover in this chapter are:
3724 Add new entries to an archive that already exists.
3727 Add more recent copies of archive members to the end of an archive, if
3732 Add one or more pre-existing archives to the end of another archive.
3734 Delete items from an archive (does not work on tapes).
3738 Compare archive members to their counterparts in the file system.
3742 @subsection How to Add Files to Existing Archives: @option{--append}
3746 If you want to add files to an existing archive, you don't need to
3747 create a new archive; you can use @option{--append} (@option{-r}).
3748 The archive must already exist in order to use @option{--append}. (A
3749 related operation is the @option{--update} operation; you can use this
3750 to add newer versions of archive members to an existing archive. To learn how to
3751 do this with @option{--update}, @pxref{update}.)
3753 If you use @option{--append} to add a file that has the same name as an
3754 archive member to an archive containing that archive member, then the
3755 old member is not deleted. What does happen, however, is somewhat
3756 complex. @command{tar} @emph{allows} you to have infinite number of files
3757 with the same name. Some operations treat these same-named members no
3758 differently than any other set of archive members: for example, if you
3759 view an archive with @option{--list} (@option{-t}), you will see all
3760 of those members listed, with their data modification times, owners, etc.
3762 Other operations don't deal with these members as perfectly as you might
3763 prefer; if you were to use @option{--extract} to extract the archive,
3764 only the most recently added copy of a member with the same name as four
3765 other members would end up in the working directory. This is because
3766 @option{--extract} extracts an archive in the order the members appeared
3767 in the archive; the most recently archived members will be extracted
3768 last. Additionally, an extracted member will @emph{replace} a file of
3769 the same name which existed in the directory already, and @command{tar}
3770 will not prompt you about this@footnote{Unless you give it
3771 @option{--keep-old-files} option, or the disk copy is newer than the
3772 the one in the archive and you invoke @command{tar} with
3773 @option{--keep-newer-files} option}. Thus, only the most recently archived
3774 member will end up being extracted, as it will replace the one
3775 extracted before it, and so on.
3777 There exists a special option that allows you to get around this
3778 behavior and extract (or list) only a particular copy of the file.
3779 This is @option{--occurrence} option. If you run @command{tar} with
3780 this option, it will extract only the first copy of the file. You
3781 may also give this option an argument specifying the number of
3782 copy to be extracted. Thus, for example if the archive
3783 @file{archive.tar} contained three copies of file @file{myfile}, then
3787 tar --extract --file archive.tar --occurrence=2 myfile
3791 would extract only the second copy. @xref{Option
3792 Summary,---occurrence}, for the description of @option{--occurrence}
3795 @FIXME{ hag -- you might want to incorporate some of the above into the
3796 MMwtSN node; not sure. i didn't know how to make it simpler...
3798 There are a few ways to get around this. (maybe xref Multiple Members
3799 with the Same Name.}
3801 @cindex Members, replacing with other members
3802 @cindex Replacing members with other members
3803 If you want to replace an archive member, use @option{--delete} to
3804 delete the member you want to remove from the archive, , and then use
3805 @option{--append} to add the member you want to be in the archive. Note
3806 that you can not change the order of the archive; the most recently
3807 added member will still appear last. In this sense, you cannot truly
3808 ``replace'' one member with another. (Replacing one member with another
3809 will not work on certain types of media, such as tapes; see @ref{delete}
3810 and @ref{Media}, for more information.)
3813 * appending files:: Appending Files to an Archive
3817 @node appending files
3818 @subsubsection Appending Files to an Archive
3820 @cindex Adding files to an Archive
3821 @cindex Appending files to an Archive
3822 @cindex Archives, Appending files to
3824 The simplest way to add a file to an already existing archive is the
3825 @option{--append} (@option{-r}) operation, which writes specified files into the
3826 archive whether or not they are already among the archived files.
3827 When you use @option{--append}, you @emph{must} specify file name
3828 arguments, as there is no default. If you specify a file that already
3829 exists in the archive, another copy of the file will be added to the
3830 end of the archive. As with other operations, the member names of the
3831 newly added files will be exactly the same as their names given on the
3832 command line. The @option{--verbose} (@option{-v}) option will print
3833 out the names of the files as they are written into the archive.
3835 @option{--append} cannot be performed on some tape drives, unfortunately,
3836 due to deficiencies in the formats those tape drives use. The archive
3837 must be a valid @command{tar} archive, or else the results of using this
3838 operation will be unpredictable. @xref{Media}.
3840 To demonstrate using @option{--append} to add a file to an archive,
3841 create a file called @file{rock} in the @file{practice} directory.
3842 Make sure you are in the @file{practice} directory. Then, run the
3843 following @command{tar} command to add @file{rock} to
3844 @file{collection.tar}:
3847 $ @kbd{tar --append --file=collection.tar rock}
3851 If you now use the @option{--list} (@option{-t}) operation, you will see that
3852 @file{rock} has been added to the archive:
3855 $ @kbd{tar --list --file=collection.tar}
3856 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
3857 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3858 -rw-r--r-- me user 20 1996-09-23 16:44 folk
3859 -rw-r--r-- me user 20 1996-09-23 16:44 rock
3862 @FIXME{in theory, dan will (soon) try to turn this node into what it's
3863 title claims it will become...}
3866 @subsubsection Multiple Files with the Same Name
3868 You can use @option{--append} (@option{-r}) to add copies of files which have been
3869 updated since the archive was created. (However, we do not recommend
3870 doing this since there is another @command{tar} option called
3871 @option{--update}; @pxref{update} for more information. We describe this
3872 use of @option{--append} here for the sake of completeness.) @FIXME{is
3873 this really a good idea, to give this whole description for something
3874 which i believe is basically a Stupid way of doing something? certain
3875 aspects of it show ways in which tar is more broken than i'd personally
3876 like to admit to, specifically the last sentence. On the other hand, i
3877 don't think it's a good idea to be saying that we explicitly don't
3878 recommend using something, but i can't see any better way to deal with
3879 the situation.}When you extract the archive, the older version will be
3880 effectively lost. This works because files are extracted from an
3881 archive in the order in which they were archived. Thus, when the
3882 archive is extracted, a file archived later in time will replace a
3883 file of the same name which was archived earlier, even though the older
3884 version of the file will remain in the archive unless you delete all
3885 versions of the file.
3887 Supposing you change the file @file{blues} and then append the changed
3888 version to @file{collection.tar}. As you saw above, the original
3889 @file{blues} is in the archive @file{collection.tar}. If you change the
3890 file and append the new version of the file to the archive, there will
3891 be two copies in the archive. When you extract the archive, the older
3892 version of the file will be extracted first, and then replaced by the
3893 newer version when it is extracted.
3895 You can append the new, changed copy of the file @file{blues} to the
3896 archive in this way:
3899 $ @kbd{tar --append --verbose --file=collection.tar blues}
3904 Because you specified the @option{--verbose} option, @command{tar} has
3905 printed the name of the file being appended as it was acted on. Now
3906 list the contents of the archive:
3909 $ @kbd{tar --list --verbose --file=collection.tar}
3910 -rw-r--r-- me user 28 1996-10-18 16:31 jazz
3911 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3912 -rw-r--r-- me user 20 1996-09-23 16:44 folk
3913 -rw-r--r-- me user 20 1996-09-23 16:44 rock
3914 -rw-r--r-- me user 58 1996-10-24 18:30 blues
3918 The newest version of @file{blues} is now at the end of the archive
3919 (note the different creation dates and file sizes). If you extract
3920 the archive, the older version of the file @file{blues} will be
3921 replaced by the newer version. You can confirm this by extracting
3922 the archive and running @samp{ls} on the directory.
3924 If you wish to extract the first occurrence of the file @file{blues}
3925 from the archive, use @option{--occurrence} option, as shown in
3926 the following example:
3929 $ @kbd{tar --extract -vv --occurrence --file=collection.tar blues}
3930 -rw-r--r-- me user 21 1996-09-23 16:44 blues
3933 @xref{Writing}, for more information on @option{--extract} and
3934 @xref{Option Summary, --occurrence}, for the description of
3935 @option{--occurrence} option.
3938 @subsection Updating an Archive
3940 @cindex Updating an archive
3943 In the previous section, you learned how to use @option{--append} to
3944 add a file to an existing archive. A related operation is
3945 @option{--update} (@option{-u}). The @option{--update} operation
3946 updates a @command{tar} archive by comparing the date of the specified
3947 archive members against the date of the file with the same name. If
3948 the file has been modified more recently than the archive member, then
3949 the newer version of the file is added to the archive (as with
3952 Unfortunately, you cannot use @option{--update} with magnetic tape drives.
3953 The operation will fail.
3955 @FIXME{other examples of media on which --update will fail? need to ask
3956 charles and/or mib/thomas/dave shevett..}
3958 Both @option{--update} and @option{--append} work by adding to the end
3959 of the archive. When you extract a file from the archive, only the
3960 version stored last will wind up in the file system, unless you use
3961 the @option{--backup} option. @FIXME-ref{Multiple Members with the
3969 @subsubsection How to Update an Archive Using @option{--update}
3971 You must use file name arguments with the @option{--update} (@option{-u}) operation.
3972 If you don't specify any files, @command{tar} won't act on any files and
3973 won't tell you that it didn't do anything (which may end up confusing
3976 @FIXME{note: the above parenthetical added because in fact, this
3977 behavior just confused the author. :-) }
3979 To see the @option{--update} option at work, create a new file,
3980 @file{classical}, in your practice directory, and some extra text to the
3981 file @file{blues}, using any text editor. Then invoke @command{tar} with
3982 the @samp{update} operation and the @option{--verbose} (@option{-v}) option specified,
3983 using the names of all the files in the practice directory as file name
3987 $ @kbd{tar --update -v -f collection.tar blues folk rock classical}
3994 Because we have specified verbose mode, @command{tar} prints out the names
3995 of the files it is working on, which in this case are the names of the
3996 files that needed to be updated. If you run @samp{tar --list} and look
3997 at the archive, you will see @file{blues} and @file{classical} at its
3998 end. There will be a total of two versions of the member @samp{blues};
3999 the one at the end will be newer and larger, since you added text before
4002 (The reason @command{tar} does not overwrite the older file when updating
4003 it is because writing to the middle of a section of tape is a difficult
4004 process. Tapes are not designed to go backward. @xref{Media}, for more
4005 information about tapes.
4007 @option{--update} (@option{-u}) is not suitable for performing backups for two
4008 reasons: it does not change directory content entries, and it
4009 lengthens the archive every time it is used. The @GNUTAR{}
4010 options intended specifically for backups are more
4011 efficient. If you need to run backups, please consult @ref{Backups}.
4014 @subsection Combining Archives with @option{--concatenate}
4016 @cindex Adding archives to an archive
4017 @cindex Concatenating Archives
4018 @opindex concatenate
4020 @c @cindex @option{-A} described
4021 Sometimes it may be convenient to add a second archive onto the end of
4022 an archive rather than adding individual files to the archive. To add
4023 one or more archives to the end of another archive, you should use the
4024 @option{--concatenate} (@option{--catenate}, @option{-A}) operation.
4026 To use @option{--concatenate}, give the first archive with
4027 @option{--file} option and name the rest of archives to be
4028 concatenated on the command line. The members, and their member
4029 names, will be copied verbatim from those archives to the first one.
4030 @FIXME-ref{This can cause multiple members to have the same name, for
4031 information on how this affects reading the archive, Multiple
4032 Members with the Same Name.}
4033 The new, concatenated archive will be called by the same name as the
4034 one given with the @option{--file} option. As usual, if you omit
4035 @option{--file}, @command{tar} will use the value of the environment
4036 variable @env{TAPE}, or, if this has not been set, the default archive name.
4038 @FIXME{There is no way to specify a new name...}
4040 To demonstrate how @option{--concatenate} works, create two small archives
4041 called @file{bluesrock.tar} and @file{folkjazz.tar}, using the relevant
4042 files from @file{practice}:
4045 $ @kbd{tar -cvf bluesrock.tar blues rock}
4048 $ @kbd{tar -cvf folkjazz.tar folk jazz}
4054 If you like, You can run @samp{tar --list} to make sure the archives
4055 contain what they are supposed to:
4058 $ @kbd{tar -tvf bluesrock.tar}
4059 -rw-r--r-- melissa user 105 1997-01-21 19:42 blues
4060 -rw-r--r-- melissa user 33 1997-01-20 15:34 rock
4061 $ @kbd{tar -tvf jazzfolk.tar}
4062 -rw-r--r-- melissa user 20 1996-09-23 16:44 folk
4063 -rw-r--r-- melissa user 65 1997-01-30 14:15 jazz
4066 We can concatenate these two archives with @command{tar}:
4070 $ @kbd{tar --concatenate --file=bluesrock.tar jazzfolk.tar}
4073 If you now list the contents of the @file{bluesrock.tar}, you will see
4074 that now it also contains the archive members of @file{jazzfolk.tar}:
4077 $ @kbd{tar --list --file=bluesrock.tar}
4084 When you use @option{--concatenate}, the source and target archives must
4085 already exist and must have been created using compatible format
4086 parameters. Notice, that @command{tar} does not check whether the
4087 archives it concatenates have compatible formats, it does not
4088 even check if the files are really tar archives.
4090 Like @option{--append} (@option{-r}), this operation cannot be performed on some
4091 tape drives, due to deficiencies in the formats those tape drives use.
4093 @cindex @code{concatenate} vs @command{cat}
4094 @cindex @command{cat} vs @code{concatenate}
4095 It may seem more intuitive to you to want or try to use @command{cat} to
4096 concatenate two archives instead of using the @option{--concatenate}
4097 operation; after all, @command{cat} is the utility for combining files.
4099 However, @command{tar} archives incorporate an end-of-file marker which
4100 must be removed if the concatenated archives are to be read properly as
4101 one archive. @option{--concatenate} removes the end-of-archive marker
4102 from the target archive before each new archive is appended. If you use
4103 @command{cat} to combine the archives, the result will not be a valid
4104 @command{tar} format archive. If you need to retrieve files from an
4105 archive that was added to using the @command{cat} utility, use the
4106 @option{--ignore-zeros} (@option{-i}) option. @xref{Ignore Zeros}, for further
4107 information on dealing with archives improperly combined using the
4108 @command{cat} shell utility.
4111 @subsection Removing Archive Members Using @option{--delete}
4113 @cindex Deleting files from an archive
4114 @cindex Removing files from an archive
4117 You can remove members from an archive by using the @option{--delete}
4118 option. Specify the name of the archive with @option{--file}
4119 (@option{-f}) and then specify the names of the members to be deleted;
4120 if you list no member names, nothing will be deleted. The
4121 @option{--verbose} option will cause @command{tar} to print the names
4122 of the members as they are deleted. As with @option{--extract}, you
4123 must give the exact member names when using @samp{tar --delete}.
4124 @option{--delete} will remove all versions of the named file from the
4125 archive. The @option{--delete} operation can run very slowly.
4127 Unlike other operations, @option{--delete} has no short form.
4129 @cindex Tapes, using @option{--delete} and
4130 @cindex Deleting from tape archives
4131 This operation will rewrite the archive. You can only use
4132 @option{--delete} on an archive if the archive device allows you to
4133 write to any point on the media, such as a disk; because of this, it
4134 does not work on magnetic tapes. Do not try to delete an archive member
4135 from a magnetic tape; the action will not succeed, and you will be
4136 likely to scramble the archive and damage your tape. There is no safe
4137 way (except by completely re-writing the archive) to delete files from
4138 most kinds of magnetic tape. @xref{Media}.
4140 To delete all versions of the file @file{blues} from the archive
4141 @file{collection.tar} in the @file{practice} directory, make sure you
4142 are in that directory, and then,
4145 $ @kbd{tar --list --file=collection.tar}
4155 $ @kbd{tar --delete --file=collection.tar blues}
4156 $ @kbd{tar --list --file=collection.tar}
4163 @FIXME{I changed the order of these nodes around and haven't had a chance
4164 to fix the above example's results, yet. I have to play with this and
4165 follow it and see what it actually does!}
4167 The @option{--delete} option has been reported to work properly when
4168 @command{tar} acts as a filter from @code{stdin} to @code{stdout}.
4171 @subsection Comparing Archive Members with the File System
4172 @cindex Verifying the currency of an archive
4176 The @option{--compare} (@option{-d}), or @option{--diff} operation compares
4177 specified archive members against files with the same names, and then
4178 reports differences in file size, mode, owner, modification date and
4179 contents. You should @emph{only} specify archive member names, not file
4180 names. If you do not name any members, then @command{tar} will compare the
4181 entire archive. If a file is represented in the archive but does not
4182 exist in the file system, @command{tar} reports a difference.
4184 You have to specify the record size of the archive when modifying an
4185 archive with a non-default record size.
4187 @command{tar} ignores files in the file system that do not have
4188 corresponding members in the archive.
4190 The following example compares the archive members @file{rock},
4191 @file{blues} and @file{funk} in the archive @file{bluesrock.tar} with
4192 files of the same name in the file system. (Note that there is no file,
4193 @file{funk}; @command{tar} will report an error message.)
4196 $ @kbd{tar --compare --file=bluesrock.tar rock blues funk}
4199 tar: funk not found in archive
4202 The spirit behind the @option{--compare} (@option{--diff}, @option{-d}) option is to check whether the
4203 archive represents the current state of files on disk, more than validating
4204 the integrity of the archive media. For this later goal, @xref{verify}.
4206 @node create options
4207 @section Options Used by @option{--create}
4209 @opindex create, additional options
4210 The previous chapter described the basics of how to use
4211 @option{--create} (@option{-c}) to create an archive from a set of files.
4212 @xref{create}. This section described advanced options to be used with
4216 * Ignore Failed Read::
4219 @node Ignore Failed Read
4220 @subsection Ignore Fail Read
4223 @item --ignore-failed-read
4224 Do not exit with nonzero on unreadable files or directories.
4227 @node extract options
4228 @section Options Used by @option{--extract}
4231 @FIXME{i need to get dan to go over these options with me and see if
4232 there's a better way of organizing them.}
4234 @opindex extract, additional options
4235 The previous chapter showed how to use @option{--extract} to extract
4236 an archive into the file system. Various options cause @command{tar} to
4237 extract more information than just file contents, such as the owner,
4238 the permissions, the modification date, and so forth. This section
4239 presents options to be used with @option{--extract} when certain special
4240 considerations arise. You may review the information presented in
4241 @ref{extract} for more basic information about the
4242 @option{--extract} operation.
4245 * Reading:: Options to Help Read Archives
4246 * Writing:: Changing How @command{tar} Writes Files
4247 * Scarce:: Coping with Scarce Resources
4251 @subsection Options to Help Read Archives
4252 @cindex Options when reading archives
4255 @cindex Reading incomplete records
4256 @cindex Records, incomplete
4257 @opindex read-full-records
4258 Normally, @command{tar} will request data in full record increments from
4259 an archive storage device. If the device cannot return a full record,
4260 @command{tar} will report an error. However, some devices do not always
4261 return full records, or do not require the last record of an archive to
4262 be padded out to the next record boundary. To keep reading until you
4263 obtain a full record, or to accept an incomplete record if it contains
4264 an end-of-archive marker, specify the @option{--read-full-records} (@option{-B}) option
4265 in conjunction with the @option{--extract} or @option{--list} operations.
4268 The @option{--read-full-records} (@option{-B}) option is turned on by default when
4269 @command{tar} reads an archive from standard input, or from a remote
4270 machine. This is because on BSD Unix systems, attempting to read a
4271 pipe returns however much happens to be in the pipe, even if it is
4272 less than was requested. If this option were not enabled, @command{tar}
4273 would fail as soon as it read an incomplete record from the pipe.
4275 If you're not sure of the blocking factor of an archive, you can
4276 read the archive by specifying @option{--read-full-records} (@option{-B}) and
4277 @option{--blocking-factor=@var{512-size}} (@option{-b
4278 @var{512-size}}), using a blocking factor larger than what the archive
4279 uses. This lets you avoid having to determine the blocking factor
4280 of an archive. @xref{Blocking Factor}.
4283 * read full records::
4287 @node read full records
4288 @unnumberedsubsubsec Reading Full Records
4290 @FIXME{need sentence or so of intro here}
4293 @opindex read-full-records
4294 @item --read-full-records
4296 Use in conjunction with @option{--extract} (@option{--get},
4297 @option{-x}) to read an archive which contains incomplete records, or
4298 one which has a blocking factor less than the one specified.
4302 @unnumberedsubsubsec Ignoring Blocks of Zeros
4304 @cindex End-of-archive blocks, ignoring
4305 @cindex Ignoring end-of-archive blocks
4306 @opindex ignore-zeros
4307 Normally, @command{tar} stops reading when it encounters a block of zeros
4308 between file entries (which usually indicates the end of the archive).
4309 @option{--ignore-zeros} (@option{-i}) allows @command{tar} to
4310 completely read an archive which contains a block of zeros before the
4311 end (i.e., a damaged archive, or one that was created by concatenating
4312 several archives together).
4314 The @option{--ignore-zeros} (@option{-i}) option is turned off by default because many
4315 versions of @command{tar} write garbage after the end-of-archive entry,
4316 since that part of the media is never supposed to be read. @GNUTAR{}
4317 does not write after the end of an archive, but seeks to
4318 maintain compatiblity among archiving utilities.
4321 @item --ignore-zeros
4323 To ignore blocks of zeros (i.e., end-of-archive entries) which may be
4324 encountered while reading an archive. Use in conjunction with
4325 @option{--extract} or @option{--list}.
4329 @subsection Changing How @command{tar} Writes Files
4332 @FIXME{Introductory paragraph}
4335 * Dealing with Old Files::
4336 * Overwrite Old Files::
4338 * Keep Newer Files::
4340 * Recursive Unlink::
4341 * Data Modification Times::
4342 * Setting Access Permissions::
4343 * Directory Modification Times and Permissions::
4344 * Writing to Standard Output::
4345 * Writing to an External Program::
4349 @node Dealing with Old Files
4350 @unnumberedsubsubsec Options Controlling the Overwriting of Existing Files
4352 @opindex overwrite-dir, introduced
4353 When extracting files, if @command{tar} discovers that the extracted
4354 file already exists, it normally replaces the file by removing it before
4355 extracting it, to prevent confusion in the presence of hard or symbolic
4356 links. (If the existing file is a symbolic link, it is removed, not
4357 followed.) However, if a directory cannot be removed because it is
4358 nonempty, @command{tar} normally overwrites its metadata (ownership,
4359 permission, etc.). The @option{--overwrite-dir} option enables this
4360 default behavior. To be more cautious and preserve the metadata of
4361 such a directory, use the @option{--no-overwrite-dir} option.
4363 @cindex Overwriting old files, prevention
4364 @opindex keep-old-files, introduced
4365 To be even more cautious and prevent existing files from being replaced, use
4366 the @option{--keep-old-files} (@option{-k}) option. It causes @command{tar} to refuse
4367 to replace or update a file that already exists, i.e., a file with the
4368 same name as an archive member prevents extraction of that archive
4369 member. Instead, it reports an error.
4371 @opindex overwrite, introduced
4372 To be more aggressive about altering existing files, use the
4373 @option{--overwrite} option. It causes @command{tar} to overwrite
4374 existing files and to follow existing symbolic links when extracting.
4376 @cindex Protecting old files
4377 Some people argue that @GNUTAR{} should not hesitate
4378 to overwrite files with other files when extracting. When extracting
4379 a @command{tar} archive, they expect to see a faithful copy of the
4380 state of the file system when the archive was created. It is debatable
4381 that this would always be a proper behavior. For example, suppose one
4382 has an archive in which @file{usr/local} is a link to
4383 @file{usr/local2}. Since then, maybe the site removed the link and
4384 renamed the whole hierarchy from @file{/usr/local2} to
4385 @file{/usr/local}. Such things happen all the time. I guess it would
4386 not be welcome at all that @GNUTAR{} removes the
4387 whole hierarchy just to make room for the link to be reinstated
4388 (unless it @emph{also} simultaneously restores the full
4389 @file{/usr/local2}, of course!) @GNUTAR{} is indeed
4390 able to remove a whole hierarchy to reestablish a symbolic link, for
4391 example, but @emph{only if} @option{--recursive-unlink} is specified
4392 to allow this behavior. In any case, single files are silently
4395 @opindex unlink-first, introduced
4396 Finally, the @option{--unlink-first} (@option{-U}) option can improve performance in
4397 some cases by causing @command{tar} to remove files unconditionally
4398 before extracting them.
4400 @node Overwrite Old Files
4401 @unnumberedsubsubsec Overwrite Old Files
4406 Overwrite existing files and directory metadata when extracting files
4409 This causes @command{tar} to write extracted files into the file system without
4410 regard to the files already on the system; i.e., files with the same
4411 names as archive members are overwritten when the archive is extracted.
4412 It also causes @command{tar} to extract the ownership, permissions,
4413 and time stamps onto any preexisting files or directories.
4414 If the name of a corresponding file name is a symbolic link, the file
4415 pointed to by the symbolic link will be overwritten instead of the
4416 symbolic link itself (if this is possible). Moreover, special devices,
4417 empty directories and even symbolic links are automatically removed if
4418 they are in the way of extraction.
4420 Be careful when using the @option{--overwrite} option, particularly when
4421 combined with the @option{--absolute-names} (@option{-P}) option, as this combination
4422 can change the contents, ownership or permissions of any file on your
4423 system. Also, many systems do not take kindly to overwriting files that
4424 are currently being executed.
4426 @opindex overwrite-dir
4427 @item --overwrite-dir
4428 Overwrite the metadata of directories when extracting files from an
4429 archive, but remove other files before extracting.
4432 @node Keep Old Files
4433 @unnumberedsubsubsec Keep Old Files
4436 @opindex keep-old-files
4437 @item --keep-old-files
4439 Do not replace existing files from archive. The
4440 @option{--keep-old-files} (@option{-k}) option prevents @command{tar}
4441 from replacing existing files with files with the same name from the
4442 archive. The @option{--keep-old-files} option is meaningless with
4443 @option{--list} (@option{-t}). Prevents @command{tar} from replacing
4444 files in the file system during extraction.
4447 @node Keep Newer Files
4448 @unnumberedsubsubsec Keep Newer Files
4451 @opindex keep-newer-files
4452 @item --keep-newer-files
4453 Do not replace existing files that are newer than their archive
4454 copies. This option is meaningless with @option{--list} (@option{-t}).
4458 @unnumberedsubsubsec Unlink First
4461 @opindex unlink-first
4462 @item --unlink-first
4464 Remove files before extracting over them.
4465 This can make @command{tar} run a bit faster if you know in advance
4466 that the extracted files all need to be removed. Normally this option
4467 slows @command{tar} down slightly, so it is disabled by default.
4470 @node Recursive Unlink
4471 @unnumberedsubsubsec Recursive Unlink
4474 @opindex recursive-unlink
4475 @item --recursive-unlink
4476 When this option is specified, try removing files and directory hierarchies
4477 before extracting over them. @emph{This is a dangerous option!}
4480 If you specify the @option{--recursive-unlink} option,
4481 @command{tar} removes @emph{anything} that keeps you from extracting a file
4482 as far as current permissions will allow it. This could include removal
4483 of the contents of a full directory hierarchy.
4485 @node Data Modification Times
4486 @unnumberedsubsubsec Setting Data Modification Times
4488 @cindex Data modification times of extracted files
4489 @cindex Modification times of extracted files
4490 Normally, @command{tar} sets the data modification times of extracted
4491 files to the corresponding times recorded for the files in the archive, but
4492 limits the permissions of extracted files by the current @code{umask}
4495 To set the data modification times of extracted files to the time when
4496 the files were extracted, use the @option{--touch} (@option{-m}) option in
4497 conjunction with @option{--extract} (@option{--get}, @option{-x}).
4503 Sets the data modification time of extracted archive members to the time
4504 they were extracted, not the time recorded for them in the archive.
4505 Use in conjunction with @option{--extract} (@option{--get}, @option{-x}).
4508 @node Setting Access Permissions
4509 @unnumberedsubsubsec Setting Access Permissions
4511 @cindex Permissions of extracted files
4512 @cindex Modes of extracted files
4513 To set the modes (access permissions) of extracted files to those
4514 recorded for those files in the archive, use @option{--same-permissions}
4515 in conjunction with the @option{--extract} (@option{--get},
4516 @option{-x}) operation. @FIXME{Should be aliased to ignore-umask.}
4519 @opindex preserve-permission
4520 @opindex same-permission
4521 @item --preserve-permission
4522 @itemx --same-permission
4523 @c @itemx --ignore-umask
4525 Set modes of extracted archive members to those recorded in the
4526 archive, instead of current umask settings. Use in conjunction with
4527 @option{--extract} (@option{--get}, @option{-x}).
4530 @node Directory Modification Times and Permissions
4531 @unnumberedsubsubsec Directory Modification Times and Permissions
4533 After sucessfully extracting a file member, @GNUTAR{} normally
4534 restores its permissions and modification times, as described in the
4535 previous sections. This cannot be done for directories, because
4536 after extracting a directory @command{tar} will almost certainly
4537 extract files into that directory and this will cause the directory
4538 modification time to be updated. Moreover, restoring that directory
4539 permissions may not permit file creation within it. Thus, restoring
4540 directory permissions and modification times must be delayed at least
4541 until all files have been extracted into that directory. @GNUTAR{}
4542 restores directories using the following approach.
4544 The extracted directories are created with the mode specified in the
4545 archive, as modified by the umask of the user, which gives sufficient
4546 permissions to allow file creation. The meta-information about the
4547 directory is recorded in the temporary list of directories. When
4548 preparing to extract next archive member, @GNUTAR{} checks if the
4549 directory prefix of this file contains the remembered directory. If
4550 it does not, the program assumes that all files have been extracted
4551 into that directory, restores its modification time and permissions
4552 and removes its entry from the internal list. This approach allows
4553 to correctly restore directory meta-information in the majority of
4554 cases, while keeping memory requirements sufficiently small. It is
4555 based on the fact, that most @command{tar} archives use the predefined
4556 order of members: first the directory, then all the files and
4557 subdirectories in that directory.
4559 However, this is not always true. The most important exception are
4560 incremental archives (@pxref{Incremental Dumps}). The member order in
4561 an incremental archive is reversed: first all directory members are
4562 stored, followed by other (non-directory) members. So, when extracting
4563 from incremental archives, @GNUTAR{} alters the above procedure. It
4564 remebers all restored directories, and restores their meta-data
4565 only after the entire archive has been processed. Notice, that you do
4566 not need to specity any special options for that, as @GNUTAR{}
4567 automatically detects archives in incremental format.
4569 There may be cases, when such processing is required for normal archives
4570 too. Consider the following example:
4574 $ @kbd{tar --no-recursion -cvf archive \
4575 foo foo/file1 bar bar/file foo/file2}
4584 During the normal operation, after encountering @file{bar}
4585 @GNUTAR{} will assume that all files from the directory @file{foo}
4586 were already extracted and will therefore restore its timestamp and
4587 permission bits. However, after extracting @file{foo/file2} the
4588 directory timestamp will be offset again.
4590 To correctly restore directory meta-information in such cases, use
4591 @option{delay-directory-restore} command line option:
4594 @opindex delay-directory-restore
4595 @item --delay-directory-restore
4596 Delays restoring of the modification times and permissions of extracted
4597 directories until the end of extraction. This way, correct
4598 meta-information is restored even if the archive has unusual member
4601 @opindex no-delay-directory-restore
4602 @item --no-delay-directory-restore
4603 Cancel the effect of the previous @option{--delay-directory-restore}.
4604 Use this option if you have used @option{--delay-directory-restore} in
4605 @env{TAR_OPTIONS} variable (@pxref{TAR_OPTIONS}) and wish to
4606 temporarily disable it.
4609 @node Writing to Standard Output
4610 @unnumberedsubsubsec Writing to Standard Output
4612 @cindex Writing extracted files to standard output
4613 @cindex Standard output, writing extracted files to
4614 To write the extracted files to the standard output, instead of
4615 creating the files on the file system, use @option{--to-stdout} (@option{-O}) in
4616 conjunction with @option{--extract} (@option{--get}, @option{-x}). This option is useful if you are
4617 extracting files to send them through a pipe, and do not need to
4618 preserve them in the file system. If you extract multiple members,
4619 they appear on standard output concatenated, in the order they are
4620 found in the archive.
4626 Writes files to the standard output. Use only in conjunction with
4627 @option{--extract} (@option{--get}, @option{-x}). When this option is
4628 used, instead of creating the files specified, @command{tar} writes
4629 the contents of the files extracted to its standard output. This may
4630 be useful if you are only extracting the files in order to send them
4631 through a pipe. This option is meaningless with @option{--list}
4635 This can be useful, for example, if you have a tar archive containing
4636 a big file and don't want to store the file on disk before processing
4637 it. You can use a command like this:
4640 tar -xOzf foo.tgz bigfile | process
4643 or even like this if you want to process the concatenation of the files:
4646 tar -xOzf foo.tgz bigfile1 bigfile2 | process
4649 Hovewer, @option{--to-command} may be more convenient for use with
4650 multiple files. See the next section.
4652 @node Writing to an External Program
4653 @unnumberedsubsubsec Writing to an External Program
4655 You can instruct @command{tar} to send the contents of each extracted
4656 file to the standard input of an external program:
4660 @item --to-command=@var{command}
4661 Extract files and pipe their contents to the standard input of
4662 @var{command}. When this option is used, instead of creating the
4663 files specified, @command{tar} invokes @var{command} and pipes the
4664 contents of the files to its standard output. @var{Command} may
4665 contain command line arguments. The program is executed via
4666 @code{sh -c}. Notice, that @var{command} is executed once for each regular file
4667 extracted. Non-regular files (directories, etc.) are ignored when this
4671 The command can obtain the information about the file it processes
4672 from the following environment variables:
4675 @vrindex TAR_FILETYPE, to-command environment
4677 Type of the file. It is a single letter with the following meaning:
4679 @multitable @columnfractions 0.10 0.90
4680 @item f @tab Regular file
4681 @item d @tab Directory
4682 @item l @tab Symbolic link
4683 @item h @tab Hard link
4684 @item b @tab Block device
4685 @item c @tab Character device
4688 Currently only regular files are supported.
4690 @vrindex TAR_MODE, to-command environment
4692 File mode, an octal number.
4694 @vrindex TAR_FILENAME, to-command environment
4696 The name of the file.
4698 @vrindex TAR_REALNAME, to-command environment
4700 Name of the file as stored in the archive.
4702 @vrindex TAR_UNAME, to-command environment
4704 Name of the file owner.
4706 @vrindex TAR_GNAME, to-command environment
4708 Name of the file owner group.
4710 @vrindex TAR_ATIME, to-command environment
4712 Time of last access. It is a decimal number, representing seconds
4713 since the epoch. If the archive provides times with nanosecond
4714 precision, the nanoseconds are appended to the timestamp after a
4717 @vrindex TAR_MTIME, to-command environment
4719 Time of last modification.
4721 @vrindex TAR_CTIME, to-command environment
4723 Time of last status change.
4725 @vrindex TAR_SIZE, to-command environment
4729 @vrindex TAR_UID, to-command environment
4731 UID of the file owner.
4733 @vrindex TAR_GID, to-command environment
4735 GID of the file owner.
4738 In addition to these variables, @env{TAR_VERSION} contains the
4739 @GNUTAR{} version number.
4741 If @var{command} exits with a non-0 status, @command{tar} will print
4742 an error message similar to the following:
4745 tar: 2345: Child returned status 1
4748 Here, @samp{2345} is the PID of the finished process.
4750 If this behavior is not wanted, use @option{--ignore-command-error}:
4753 @opindex ignore-command-error
4754 @item --ignore-command-error
4755 Ignore exit codes of subprocesses. Notice that if the program
4756 exits on signal or otherwise terminates abnormally, the error message
4757 will be printed even if this option is used.
4759 @opindex no-ignore-command-error
4760 @item --no-ignore-command-error
4761 Cancel the effect of any previous @option{--ignore-command-error}
4762 option. This option is useful if you have set
4763 @option{--ignore-command-error} in @env{TAR_OPTIONS}
4764 (@pxref{TAR_OPTIONS}) and wish to temporarily cancel it.
4768 @unnumberedsubsubsec Removing Files
4770 @FIXME{the various macros in the front of the manual think that this
4771 option goes in this section. i have no idea; i only know it's nowhere
4772 else in the book...}
4775 @opindex remove-files
4776 @item --remove-files
4777 Remove files after adding them to the archive.
4781 @subsection Coping with Scarce Resources
4784 @cindex Small memory
4785 @cindex Running out of space
4793 @unnumberedsubsubsec Starting File
4796 @opindex starting-file
4797 @item --starting-file=@var{name}
4798 @itemx -K @var{name}
4799 Starts an operation in the middle of an archive. Use in conjunction
4800 with @option{--extract} (@option{--get}, @option{-x}) or @option{--list} (@option{-t}).
4803 @cindex Middle of the archive, starting in the
4804 If a previous attempt to extract files failed due to lack of disk
4805 space, you can use @option{--starting-file=@var{name}} (@option{-K
4806 @var{name}}) to start extracting only after member @var{name} of the
4807 archive. This assumes, of course, that there is now free space, or
4808 that you are now extracting into a different file system. (You could
4809 also choose to suspend @command{tar}, remove unnecessary files from
4810 the file system, and then restart the same @command{tar} operation.
4811 In this case, @option{--starting-file} is not necessary.
4812 @xref{Incremental Dumps}, @xref{interactive}, and @ref{exclude}.)
4815 @unnumberedsubsubsec Same Order
4818 @cindex Large lists of file names on small machines
4820 @opindex preserve-order
4822 @itemx --preserve-order
4824 To process large lists of file names on machines with small amounts of
4825 memory. Use in conjunction with @option{--compare} (@option{--diff},
4826 @option{-d}), @option{--list} (@option{-t}) or @option{--extract}
4827 (@option{--get}, @option{-x}).
4830 The @option{--same-order} (@option{--preserve-order}, @option{-s}) option tells @command{tar} that the list of file
4831 names to be listed or extracted is sorted in the same order as the
4832 files in the archive. This allows a large list of names to be used,
4833 even on a small machine that would not otherwise be able to hold all
4834 the names in memory at the same time. Such a sorted list can easily be
4835 created by running @samp{tar -t} on the archive and editing its output.
4837 This option is probably never needed on modern computer systems.
4840 @section Backup options
4842 @cindex backup options
4844 @GNUTAR{} offers options for making backups of files
4845 before writing new versions. These options control the details of
4846 these backups. They may apply to the archive itself before it is
4847 created or rewritten, as well as individual extracted members. Other
4848 @acronym{GNU} programs (@command{cp}, @command{install}, @command{ln},
4849 and @command{mv}, for example) offer similar options.
4851 Backup options may prove unexpectedly useful when extracting archives
4852 containing many members having identical name, or when extracting archives
4853 on systems having file name limitations, making different members appear
4854 has having similar names through the side-effect of name truncation.
4855 (This is true only if we have a good scheme for truncated backup names,
4856 which I'm not sure at all: I suspect work is needed in this area.)
4857 When any existing file is backed up before being overwritten by extraction,
4858 then clashing files are automatically be renamed to be unique, and the
4859 true name is kept for only the last file of a series of clashing files.
4860 By using verbose mode, users may track exactly what happens.
4862 At the detail level, some decisions are still experimental, and may
4863 change in the future, we are waiting comments from our users. So, please
4864 do not learn to depend blindly on the details of the backup features.
4865 For example, currently, directories themselves are never renamed through
4866 using these options, so, extracting a file over a directory still has
4867 good chances to fail. Also, backup options apply to created archives,
4868 not only to extracted members. For created archives, backups will not
4869 be attempted when the archive is a block or character device, or when it
4870 refers to a remote file.
4872 For the sake of simplicity and efficiency, backups are made by renaming old
4873 files prior to creation or extraction, and not by copying. The original
4874 name is restored if the file creation fails. If a failure occurs after a
4875 partial extraction of a file, both the backup and the partially extracted
4879 @item --backup[=@var{method}]
4881 @vindex VERSION_CONTROL
4883 Back up files that are about to be overwritten or removed.
4884 Without this option, the original versions are destroyed.
4886 Use @var{method} to determine the type of backups made.
4887 If @var{method} is not specified, use the value of the @env{VERSION_CONTROL}
4888 environment variable. And if @env{VERSION_CONTROL} is not set,
4889 use the @samp{existing} method.
4891 @vindex version-control @r{Emacs variable}
4892 This option corresponds to the Emacs variable @samp{version-control};
4893 the same values for @var{method} are accepted as in Emacs. This option
4894 also allows more descriptive names. The valid @var{method}s are:
4899 @cindex numbered @r{backup method}
4900 Always make numbered backups.
4904 @cindex existing @r{backup method}
4905 Make numbered backups of files that already have them, simple backups
4910 @cindex simple @r{backup method}
4911 Always make simple backups.
4915 @item --suffix=@var{suffix}
4917 @cindex backup suffix
4918 @vindex SIMPLE_BACKUP_SUFFIX
4919 Append @var{suffix} to each backup file made with @option{--backup}. If this
4920 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
4921 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
4922 set, the default is @samp{~}, just as in Emacs.
4926 Some people express the desire to @emph{always} use the @option{--backup}
4927 option, by defining some kind of alias or script. This is not as easy
4928 as one may think, due to the fact that old style options should appear first
4929 and consume arguments a bit unpredictably for an alias or script. But,
4930 if you are ready to give up using old style options, you may resort to
4931 using something like (a Bourne shell function here):
4934 tar () @{ /usr/local/bin/tar --backup $*; @}
4938 @section Notable @command{tar} Usages
4941 @FIXME{Using Unix file linking capability to recreate directory
4942 structures---linking files into one subdirectory and then
4943 @command{tar}ring that directory.}
4945 @FIXME{Nice hairy example using absolute-names, newer, etc.}
4948 You can easily use archive files to transport a group of files from
4949 one system to another: put all relevant files into an archive on one
4950 computer system, transfer the archive to another system, and extract
4951 the contents there. The basic transfer medium might be magnetic tape,
4952 Internet FTP, or even electronic mail (though you must encode the
4953 archive with @command{uuencode} in order to transport it properly by
4954 mail). Both machines do not have to use the same operating system, as
4955 long as they both support the @command{tar} program.
4957 For example, here is how you might copy a directory's contents from
4958 one disk to another, while preserving the dates, modes, owners and
4959 link-structure of all the files therein. In this case, the transfer
4960 medium is a @dfn{pipe}, which is one a Unix redirection mechanism:
4963 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xf -)}
4967 You can avoid subshells by using @option{-C} option:
4970 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xf -}
4974 The command also works using short option forms:
4977 $ @kbd{(cd sourcedir; tar --create --file=- . ) \
4978 | (cd targetdir; tar --extract --file=-)}
4980 $ @kbd{tar --directory sourcedir --create --file=- . ) \
4981 | tar --directory targetdir --extract --file=-}
4985 This is one of the easiest methods to transfer a @command{tar} archive.
4988 @section Looking Ahead: The Rest of this Manual
4990 You have now seen how to use all eight of the operations available to
4991 @command{tar}, and a number of the possible options. The next chapter
4992 explains how to choose and change file and archive names, how to use
4993 files to store names of other files which you can then call as
4994 arguments to @command{tar} (this can help you save time if you expect to
4995 archive the same list of files a number of times), and so forth.
4996 @FIXME{in case it's not obvious, i'm making this up in some sense
4997 based on my limited memory of what the next chapter *really* does. i
4998 just wanted to flesh out this final section a little bit so i'd
4999 remember to stick it in here. :-)}
5001 If there are too many files to conveniently list on the command line,
5002 you can list the names in a file, and @command{tar} will read that file.
5005 There are various ways of causing @command{tar} to skip over some files,
5006 and not archive them. @xref{Choosing}.
5009 @chapter Performing Backups and Restoring Files
5012 @GNUTAR{} is distributed along with the scripts
5013 which the Free Software Foundation uses for performing backups. There
5014 is no corresponding scripts available yet for doing restoration of
5015 files. Even if there is a good chance those scripts may be satisfying
5016 to you, they are not the only scripts or methods available for doing
5017 backups and restore. You may well create your own, or use more
5018 sophisticated packages dedicated to that purpose.
5020 Some users are enthusiastic about @code{Amanda} (The Advanced Maryland
5021 Automatic Network Disk Archiver), a backup system developed by James
5022 da Silva @file{jds@@cs.umd.edu} and available on many Unix systems.
5023 This is free software, and it is available at these places:
5026 http://www.cs.umd.edu/projects/amanda/amanda.html
5027 ftp://ftp.cs.umd.edu/pub/amanda
5032 Here is a possible plan for a future documentation about the backuping
5033 scripts which are provided within the @GNUTAR{}
5039 @item what are dumps
5040 @item different levels of dumps
5042 @item full dump = dump everything
5043 @item level 1, level 2 dumps etc
5044 A level @var{n} dump dumps everything changed since the last level
5047 @item how to use scripts for dumps (ie, the concept)
5049 @item scripts to run after editing backup specs (details)
5051 @item Backup Specs, what is it.
5053 @item how to customize
5054 @item actual text of script [/sp/dump/backup-specs]
5058 @item rsh doesn't work
5059 @item rtape isn't installed
5062 @item the @option{--incremental} option of tar
5065 @item write protection
5066 @item types of media, different sizes and types, useful for different things
5067 @item files and tape marks
5068 one tape mark between files, two at end.
5069 @item positioning the tape
5070 MT writes two at end of write,
5071 backspaces over one when writing again.
5077 This chapter documents both the provided shell scripts and @command{tar}
5078 options which are more specific to usage as a backup tool.
5080 To @dfn{back up} a file system means to create archives that contain
5081 all the files in that file system. Those archives can then be used to
5082 restore any or all of those files (for instance if a disk crashes or a
5083 file is accidentally deleted). File system @dfn{backups} are also
5087 * Full Dumps:: Using @command{tar} to Perform Full Dumps
5088 * Incremental Dumps:: Using @command{tar} to Perform Incremental Dumps
5089 * Backup Levels:: Levels of Backups
5090 * Backup Parameters:: Setting Parameters for Backups and Restoration
5091 * Scripted Backups:: Using the Backup Scripts
5092 * Scripted Restoration:: Using the Restore Script
5096 @section Using @command{tar} to Perform Full Dumps
5102 @cindex corrupted archives
5103 Full dumps should only be made when no other people or programs
5104 are modifying files in the file system. If files are modified while
5105 @command{tar} is making the backup, they may not be stored properly in
5106 the archive, in which case you won't be able to restore them if you
5107 have to. (Files not being modified are written with no trouble, and do
5108 not corrupt the entire archive.)
5110 You will want to use the @option{--label=@var{archive-label}}
5111 (@option{-V @var{archive-label}}) option to give the archive a
5112 volume label, so you can tell what this archive is even if the label
5113 falls off the tape, or anything like that.
5115 Unless the file system you are dumping is guaranteed to fit on
5116 one volume, you will need to use the @option{--multi-volume} (@option{-M}) option.
5117 Make sure you have enough tapes on hand to complete the backup.
5119 If you want to dump each file system separately you will need to use
5120 the @option{--one-file-system} option to prevent
5121 @command{tar} from crossing file system boundaries when storing
5124 The @option{--incremental} (@option{-G}) (@pxref{Incremental Dumps})
5125 option is not needed, since this is a complete copy of everything in
5126 the file system, and a full restore from this backup would only be
5127 done onto a completely
5130 Unless you are in a hurry, and trust the @command{tar} program (and your
5131 tapes), it is a good idea to use the @option{--verify} (@option{-W})
5132 option, to make sure your files really made it onto the dump properly.
5133 This will also detect cases where the file was modified while (or just
5134 after) it was being archived. Not all media (notably cartridge tapes)
5135 are capable of being verified, unfortunately.
5137 @node Incremental Dumps
5138 @section Using @command{tar} to Perform Incremental Dumps
5140 @dfn{Incremental backup} is a special form of @GNUTAR{} archive that
5141 stores additional metadata so that exact state of the file system
5142 can be restored when extracting the archive.
5144 @GNUTAR{} currently offers two options for handling incremental
5145 backups: @option{--listed-incremental=@var{snapshot-file}} (@option{-g
5146 @var{snapshot-file}}) and @option{--incremental} (@option{-G}).
5148 @opindex listed-incremental
5149 The option @option{--listed-incremental} instructs tar to operate on
5150 an incremental archive with additional metadata stored in a standalone
5151 file, called a @dfn{snapshot file}. The purpose of this file is to help
5152 determine which files have been changed, added or deleted since the
5153 last backup, so that the next incremental backup will contain only
5154 modified files. The name of the snapshot file is given as an argument
5158 @item --listed-incremental=@var{file}
5159 @itemx -g @var{file}
5160 Handle incremental backups with snapshot data in @var{file}.
5163 To create an incremental backup, you would use
5164 @option{--listed-incremental} together with @option{--create}
5165 (@pxref{create}). For example:
5168 $ @kbd{tar --create \
5169 --file=archive.1.tar \
5170 --listed-incremental=/var/log/usr.snar \
5174 This will create in @file{archive.1.tar} an incremental backup of
5175 the @file{/usr} file system, storing additional metadata in the file
5176 @file{/var/log/usr.snar}. If this file does not exist, it will be
5177 created. The created archive will then be a @dfn{level 0 backup};
5178 please see the next section for more on backup levels.
5180 Otherwise, if the file @file{/var/log/usr.snar} exists, it
5181 determines which files are modified. In this case only these files will be
5182 stored in the archive. Suppose, for example, that after running the
5183 above command, you delete file @file{/usr/doc/old} and create
5184 directory @file{/usr/local/db} with the following contents:
5187 $ @kbd{ls /usr/local/db}
5192 Some time later you create another incremental backup. You will
5196 $ @kbd{tar --create \
5197 --file=archive.2.tar \
5198 --listed-incremental=/var/log/usr.snar \
5200 tar: usr/local/db: Directory is new
5207 The created archive @file{archive.2.tar} will contain only these
5208 three members. This archive is called a @dfn{level 1 backup}. Notice
5209 that @file{/var/log/usr.snar} will be updated with the new data, so if
5210 you plan to create more @samp{level 1} backups, it is necessary to
5211 create a working copy of the snapshot file before running
5212 @command{tar}. The above example will then be modified as follows:
5215 $ @kbd{cp /var/log/usr.snar /var/log/usr.snar-1}
5216 $ @kbd{tar --create \
5217 --file=archive.2.tar \
5218 --listed-incremental=/var/log/usr.snar-1 \
5222 Incremental dumps depend crucially on time stamps, so the results are
5223 unreliable if you modify a file's time stamps during dumping (e.g.,
5224 with the @option{--atime-preserve=replace} option), or if you set the clock
5227 Metadata stored in snapshot files include device numbers, which,
5228 obviously is supposed to be a non-volatile value. However, it turns
5229 out that NFS devices have undependable values when an automounter
5230 gets in the picture. This can lead to a great deal of spurious
5231 redumping in incremental dumps, so it is somewhat useless to compare
5232 two NFS devices numbers over time. The solution implemented currently
5233 is to considers all NFS devices as being equal when it comes to
5234 comparing directories; this is fairly gross, but there does not seem
5235 to be a better way to go.
5237 Note that incremental archives use @command{tar} extensions and may
5238 not be readable by non-@acronym{GNU} versions of the @command{tar} program.
5240 @opindex listed-incremental, using with @option{--extract}
5241 @opindex extract, using with @option{--listed-incremental}
5242 To extract from the incremental dumps, use
5243 @option{--listed-incremental} together with @option{--extract}
5244 option (@pxref{extracting files}). In this case, @command{tar} does
5245 not need to access snapshot file, since all the data necessary for
5246 extraction are stored in the archive itself. So, when extracting, you
5247 can give whatever argument to @option{--listed-incremental}, the usual
5248 practice is to use @option{--listed-incremental=/dev/null}.
5249 Alternatively, you can use @option{--incremental}, which needs no
5250 arguments. In general, @option{--incremental} (@option{-G}) can be
5251 used as a shortcut for @option{--listed-incremental} when listing or
5252 extracting incremental backups (for more information, regarding this
5253 option, @pxref{incremental-op}).
5255 When extracting from the incremental backup @GNUTAR{} attempts to
5256 restore the exact state the file system had when the archive was
5257 created. In particular, it will @emph{delete} those files in the file
5258 system that did not exist in their directories when the archive was
5259 created. If you have created several levels of incremental files,
5260 then in order to restore the exact contents the file system had when
5261 the last level was created, you will need to restore from all backups
5262 in turn. Continuing our example, to restore the state of @file{/usr}
5263 file system, one would do@footnote{Notice, that since both archives
5264 were created withouth @option{-P} option (@pxref{absolute}), these
5265 commands should be run from the root file system.}:
5268 $ @kbd{tar --extract \
5269 --listed-incremental=/dev/null \
5270 --file archive.1.tar}
5271 $ @kbd{tar --extract \
5272 --listed-incremental=/dev/null \
5273 --file archive.2.tar}
5276 To list the contents of an incremental archive, use @option{--list}
5277 (@pxref{list}), as usual. To obtain more information about the
5278 archive, use @option{--listed-incremental} or @option{--incremental}
5279 combined with two @option{--verbose} options@footnote{Two
5280 @option{--verbose} options were selected to avoid breaking usual
5281 verbose listing output (@option{--list --verbose}) when using in
5284 @opindex incremental, using with @option{--list}
5285 @opindex listed-incremental, using with @option{--list}
5286 @opindex list, using with @option{--incremental}
5287 @opindex list, using with @option{--listed-incremental}
5288 Versions of @GNUTAR{} up to 1.15.1 used to dump verbatim binary
5289 contents of the DUMPDIR header (with terminating nulls) when
5290 @option{--incremental} or @option{--listed-incremental} option was
5291 given, no matter what the verbosity level. This behavior, and,
5292 especially, the binary output it produced were considered incovenient
5293 and were changed in version 1.16}:
5296 @kbd{tar --list --incremental --verbose --verbose archive.tar}
5299 This command will print, for each directory in the archive, the list
5300 of files in that directory at the time the archive was created. This
5301 information is put out in a format which is both human-readable and
5302 unambiguous for a program: each file name is printed as
5309 where @var{x} is a letter describing the status of the file: @samp{Y}
5310 if the file is present in the archive, @samp{N} if the file is not
5311 included in the archive, or a @samp{D} if the file is a directory (and
5312 is included in the archive).@FIXME-xref{dumpdir format}. Each such
5313 line is terminated by a newline character. The last line is followed
5314 by an additional newline to indicate the end of the data.
5316 @anchor{incremental-op}The option @option{--incremental} (@option{-G})
5317 gives the same behavior as @option{--listed-incremental} when used
5318 with @option{--list} and @option{--extract} options. When used with
5319 @option{--create} option, it creates an incremental archive without
5320 creating snapshot file. Thus, it is impossible to create several
5321 levels of incremental backups with @option{--incremental} option.
5324 @section Levels of Backups
5326 An archive containing all the files in the file system is called a
5327 @dfn{full backup} or @dfn{full dump}. You could insure your data by
5328 creating a full dump every day. This strategy, however, would waste a
5329 substantial amount of archive media and user time, as unchanged files
5330 are daily re-archived.
5332 It is more efficient to do a full dump only occasionally. To back up
5333 files between full dumps, you can use @dfn{incremental dumps}. A @dfn{level
5334 one} dump archives all the files that have changed since the last full
5337 A typical dump strategy would be to perform a full dump once a week,
5338 and a level one dump once a day. This means some versions of files
5339 will in fact be archived more than once, but this dump strategy makes
5340 it possible to restore a file system to within one day of accuracy by
5341 only extracting two archives---the last weekly (full) dump and the
5342 last daily (level one) dump. The only information lost would be in
5343 files changed or created since the last daily backup. (Doing dumps
5344 more than once a day is usually not worth the trouble).
5346 @GNUTAR{} comes with scripts you can use to do full
5347 and level-one (actually, even level-two and so on) dumps. Using
5348 scripts (shell programs) to perform backups and restoration is a
5349 convenient and reliable alternative to typing out file name lists
5350 and @command{tar} commands by hand.
5352 Before you use these scripts, you need to edit the file
5353 @file{backup-specs}, which specifies parameters used by the backup
5354 scripts and by the restore script. This file is usually located
5355 in @file{/etc/backup} directory. @xref{Backup Parameters}, for its
5356 detailed description. Once the backup parameters are set, you can
5357 perform backups or restoration by running the appropriate script.
5359 The name of the backup script is @code{backup}. The name of the
5360 restore script is @code{restore}. The following sections describe
5361 their use in detail.
5363 @emph{Please Note:} The backup and restoration scripts are
5364 designed to be used together. While it is possible to restore files by
5365 hand from an archive which was created using a backup script, and to create
5366 an archive by hand which could then be extracted using the restore script,
5367 it is easier to use the scripts. @xref{Incremental Dumps}, before
5368 making such an attempt.
5370 @node Backup Parameters
5371 @section Setting Parameters for Backups and Restoration
5373 The file @file{backup-specs} specifies backup parameters for the
5374 backup and restoration scripts provided with @command{tar}. You must
5375 edit @file{backup-specs} to fit your system configuration and schedule
5376 before using these scripts.
5378 Syntactically, @file{backup-specs} is a shell script, containing
5379 mainly variable assignments. However, any valid shell construct
5380 is allowed in this file. Particularly, you may wish to define
5381 functions within that script (e.g., see @code{RESTORE_BEGIN} below).
5382 For more information about shell script syntax, please refer to
5383 @url{http://www.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#ta
5384 g_02, the definition of the Shell Command Language}. See also
5385 @ref{Top,,Bash Features,bashref,Bash Reference Manual}.
5387 The shell variables controlling behavior of @code{backup} and
5388 @code{restore} are described in the following subsections.
5391 * General-Purpose Variables::
5392 * Magnetic Tape Control::
5394 * backup-specs example:: An Example Text of @file{Backup-specs}
5397 @node General-Purpose Variables
5398 @subsection General-Purpose Variables
5400 @defvr {Backup variable} ADMINISTRATOR
5401 The user name of the backup administrator. @code{Backup} scripts
5402 sends a backup report to this address.
5405 @defvr {Backup variable} BACKUP_HOUR
5406 The hour at which the backups are done. This can be a number from 0
5407 to 23, or the time specification in form @var{hours}:@var{minutes},
5408 or the string @samp{now}.
5410 This variable is used by @code{backup}. Its value may be overridden
5411 using @option{--time} option (@pxref{Scripted Backups}).
5414 @defvr {Backup variable} TAPE_FILE
5416 The device @command{tar} writes the archive to. If @var{TAPE_FILE}
5417 is a remote archive (@pxref{remote-dev}), backup script will suppose
5418 that your @command{mt} is able to access remote devices. If @var{RSH}
5419 (@pxref{RSH}) is set, @option{--rsh-command} option will be added to
5420 invocations of @command{mt}.
5423 @defvr {Backup variable} BLOCKING
5425 The blocking factor @command{tar} will use when writing the dump archive.
5426 @xref{Blocking Factor}.
5429 @defvr {Backup variable} BACKUP_DIRS
5431 A list of file systems to be dumped (for @code{backup}), or restored
5432 (for @code{restore}). You can include any directory
5433 name in the list --- subdirectories on that file system will be
5434 included, regardless of how they may look to other networked machines.
5435 Subdirectories on other file systems will be ignored.
5437 The host name specifies which host to run @command{tar} on, and should
5438 normally be the host that actually contains the file system. However,
5439 the host machine must have @GNUTAR{} installed, and
5440 must be able to access the directory containing the backup scripts and
5441 their support files using the same file name that is used on the
5442 machine where the scripts are run (ie. what @command{pwd} will print
5443 when in that directory on that machine). If the host that contains
5444 the file system does not have this capability, you can specify another
5445 host as long as it can access the file system through NFS.
5447 If the list of file systems is very long you may wish to put it
5448 in a separate file. This file is usually named
5449 @file{/etc/backup/dirs}, but this name may be overridden in
5450 @file{backup-specs} using @code{DIRLIST} variable.
5453 @defvr {Backup variable} DIRLIST
5455 A path to the file containing the list of the file systems to backup
5456 or restore. By default it is @file{/etc/backup/dirs}.
5459 @defvr {Backup variable} BACKUP_FILES
5461 A list of individual files to be dumped (for @code{backup}), or restored
5462 (for @code{restore}). These should be accessible from the machine on
5463 which the backup script is run.
5465 If the list of file systems is very long you may wish to store it
5466 in a separate file. This file is usually named
5467 @file{/etc/backup/files}, but this name may be overridden in
5468 @file{backup-specs} using @code{FILELIST} variable.
5471 @defvr {Backup variable} FILELIST
5473 A path to the file containing the list of the individual files to backup
5474 or restore. By default it is @file{/etc/backup/files}.
5477 @defvr {Backup variable} MT
5479 Full file name of @command{mt} binary.
5482 @defvr {Backup variable} RSH
5484 Full file name of @command{rsh} binary or its equivalent. You may wish to
5485 set it to @code{ssh}, to improve security. In this case you will have
5486 to use public key authentication.
5489 @defvr {Backup variable} RSH_COMMAND
5491 Full file name of @command{rsh} binary on remote mashines. This will
5492 be passed via @option{--rsh-command} option to the remote invocation
5496 @defvr {Backup variable} VOLNO_FILE
5498 Name of temporary file to hold volume numbers. This needs to be accessible
5499 by all the machines which have file systems to be dumped.
5502 @defvr {Backup variable} XLIST
5504 Name of @dfn{exclude file list}. An @dfn{exclude file list} is a file
5505 located on the remote machine and containing the list of files to
5506 be excluded from the backup. Exclude file lists are searched in
5507 /etc/tar-backup directory. A common use for exclude file lists
5508 is to exclude files containing security-sensitive information
5509 (e.g., @file{/etc/shadow} from backups).
5511 This variable affects only @code{backup}.
5514 @defvr {Backup variable} SLEEP_TIME
5516 Time to sleep between dumps of any two successive file systems
5518 This variable affects only @code{backup}.
5521 @defvr {Backup variable} DUMP_REMIND_SCRIPT
5523 Script to be run when it's time to insert a new tape in for the next
5524 volume. Administrators may want to tailor this script for their site.
5525 If this variable isn't set, @GNUTAR{} will display its built-in prompt
5526 @FIXME-xref{describe it somewhere!}, and will expect confirmation from
5530 @defvr {Backup variable} SLEEP_MESSAGE
5532 Message to display on the terminal while waiting for dump time. Usually
5533 this will just be some literal text.
5536 @defvr {Backup variable} TAR
5538 Full file name of the @GNUTAR{} executable. If this is not set, backup
5539 scripts will search @command{tar} in the current shell path.
5542 @node Magnetic Tape Control
5543 @subsection Magnetic Tape Control
5545 Backup scripts access tape device using special @dfn{hook functions}.
5546 These functions take a single argument -- the name of the tape
5547 device. Their names are kept in the following variables:
5549 @defvr {Backup variable} MT_BEGIN
5550 The name of @dfn{begin} function. This function is called before
5551 accessing the drive. By default it retensions the tape:
5557 mt -f "$1" retension
5562 @defvr {Backup variable} MT_REWIND
5563 The name of @dfn{rewind} function. The default definition is as
5576 @defvr {Backup variable} MT_OFFLINE
5577 The name of the function switching the tape off line. By default
5578 it is defined as follows:
5581 MT_OFFLINE=mt_offline
5589 @defvr {Backup variable} MT_STATUS
5590 The name of the function used to obtain the status of the archive device,
5591 including error count. Default definition:
5603 @subsection User Hooks
5605 @dfn{User hooks} are shell functions executed before and after
5606 each @command{tar} invocation. Thus, there are @dfn{backup
5607 hooks}, which are executed before and after dumping each file
5608 system, and @dfn{restore hooks}, executed before and
5609 after restoring a file system. Each user hook is a shell function
5610 taking four arguments:
5612 @deffn {User Hook Function} hook @var{level} @var{host} @var{fs} @var{fsname}
5617 Current backup or restore level.
5620 Name or IP address of the host machine being dumped or restored.
5623 Full path name to the file system being dumped or restored.
5626 File system name with directory separators replaced with colons. This
5627 is useful, e.g., for creating unique files.
5631 Following variables keep the names of user hook functions
5633 @defvr {Backup variable} DUMP_BEGIN
5634 Dump begin function. It is executed before dumping the file system.
5637 @defvr {Backup variable} DUMP_END
5638 Executed after dumping the file system.
5641 @defvr {Backup variable} RESTORE_BEGIN
5642 Executed before restoring the file system.
5645 @defvr {Backup variable} RESTORE_END
5646 Executed after restoring the file system.
5649 @node backup-specs example
5650 @subsection An Example Text of @file{Backup-specs}
5652 The following is an example of @file{backup-specs}:
5655 # site-specific parameters for file system backup.
5657 ADMINISTRATOR=friedman
5659 TAPE_FILE=/dev/nrsmt0
5661 # Use @code{ssh} instead of the less secure @code{rsh}
5663 RSH_COMMAND=/usr/bin/ssh
5665 # Override MT_STATUS function:
5671 # Disable MT_OFFLINE function
5688 apple-gunkies:/com/mailer/gnu
5689 apple-gunkies:/com/archive/gnu"
5691 BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]"
5695 @node Scripted Backups
5696 @section Using the Backup Scripts
5698 The syntax for running a backup script is:
5701 backup --level=@var{level} --time=@var{time}
5704 The @option{level} option requests the dump level. Thus, to produce
5705 a full dump, specify @code{--level=0} (this is the default, so
5706 @option{--level} may be omitted if its value is @code{0}).
5707 @footnote{For backward compatibility, the @code{backup} will also
5708 try to deduce the requested dump level from the name of the
5709 script itself. If the name consists of a string @samp{level-}
5710 followed by a single decimal digit, that digit is taken as
5711 the dump level number. Thus, you may create a link from @code{backup}
5712 to @code{level-1} and then run @code{level-1} whenever you need to
5713 create a level one dump.}
5715 The @option{--time} option determines when should the backup be
5716 run. @var{Time} may take three forms:
5719 @item @var{hh}:@var{mm}
5721 The dump must be run at @var{hh} hours @var{mm} minutes.
5725 The dump must be run at @var{hh} hours
5729 The dump must be run immediately.
5732 You should start a script with a tape or disk mounted. Once you
5733 start a script, it prompts you for new tapes or disks as it
5734 needs them. Media volumes don't have to correspond to archive
5735 files --- a multi-volume archive can be started in the middle of a
5736 tape that already contains the end of another multi-volume archive.
5737 The @code{restore} script prompts for media by its archive volume,
5738 so to avoid an error message you should keep track of which tape
5739 (or disk) contains which volume of the archive (@pxref{Scripted
5742 The backup scripts write two files on the file system. The first is a
5743 record file in @file{/etc/tar-backup/}, which is used by the scripts
5744 to store and retrieve information about which files were dumped. This
5745 file is not meant to be read by humans, and should not be deleted by
5746 them. @xref{Snapshot Files}, for a more detailed explanation of this
5749 The second file is a log file containing the names of the file systems
5750 and files dumped, what time the backup was made, and any error
5751 messages that were generated, as well as how much space was left in
5752 the media volume after the last volume of the archive was written.
5753 You should check this log file after every backup. The file name is
5754 @file{log-@var{mm-dd-yyyy}-level-@var{n}}, where @var{mm-dd-yyyy}
5755 represents current date, and @var{n} represents current dump level number.
5757 The script also prints the name of each system being dumped to the
5760 Following is the full list of options accepted by @code{backup}
5764 @item -l @var{level}
5765 @itemx --level=@var{level}
5766 Do backup level @var{level} (default 0).
5770 Force backup even if today's log file already exists.
5772 @item -v[@var{level}]
5773 @itemx --verbose[=@var{level}]
5774 Set verbosity level. The higher the level is, the more debugging
5775 information will be output during execution. Devault @var{level}
5776 is 100, which means the highest debugging level.
5778 @item -t @var{start-time}
5779 @itemx --time=@var{start-time}
5780 Wait till @var{time}, then do backup.
5784 Display short help message and exit.
5788 Display information about the program's name, version, origin and legal
5789 status, all on standard output, and then exit successfully.
5793 @node Scripted Restoration
5794 @section Using the Restore Script
5796 To restore files that were archived using a scripted backup, use the
5797 @code{restore} script. Its usage is quite straightforward. In the
5798 simplest form, invoke @code{restore --all}, it will
5799 then restore all the file systems and files specified in
5800 @file{backup-specs} (@pxref{General-Purpose Variables,BACKUP_DIRS}).
5802 You may select the file systems (and/or files) to restore by
5803 giving @code{restore} list of @dfn{patterns} in its command
5804 line. For example, running
5811 will restore all file systems on the machine @samp{albert}. A more
5812 complicated example:
5815 restore 'albert:*' '*:/var'
5819 This command will restore all file systems on the machine @samp{albert}
5820 as well as @file{/var} file system on all machines.
5822 By default @code{restore} will start restoring files from the lowest
5823 available dump level (usually zero) and will continue through
5824 all available dump levels. There may be situations where such a
5825 thorough restore is not necessary. For example, you may wish to
5826 restore only files from the recent level one backup. To do so,
5827 use @option{--level} option, as shown in the example below:
5833 The full list of options accepted by @code{restore} follows:
5838 Restore all file systems and files specified in @file{backup-specs}
5840 @item -l @var{level}
5841 @itemx --level=@var{level}
5842 Start restoring from the given backup level, instead of the default 0.
5844 @item -v[@var{level}]
5845 @itemx --verbose[=@var{level}]
5846 Set verbosity level. The higher the level is, the more debugging
5847 information will be output during execution. Devault @var{level}
5848 is 100, which means the highest debugging level.
5852 Display short help message and exit.
5856 Display information about the program's name, version, origin and legal
5857 status, all on standard output, and then exit successfully.
5860 You should start the restore script with the media containing the
5861 first volume of the archive mounted. The script will prompt for other
5862 volumes as they are needed. If the archive is on tape, you don't need
5863 to rewind the tape to to its beginning---if the tape head is
5864 positioned past the beginning of the archive, the script will rewind
5865 the tape as needed. @FIXME-xref{Media, for a discussion of tape
5869 @strong{Warning:} The script will delete files from the active file
5870 system if they were not in the file system when the archive was made.
5873 @xref{Incremental Dumps}, for an explanation of how the script makes
5877 @chapter Choosing Files and Names for @command{tar}
5880 Certain options to @command{tar} enable you to specify a name for your
5881 archive. Other options let you decide which files to include or exclude
5882 from the archive, based on when or whether files were modified, whether
5883 the file names do or don't match specified patterns, or whether files
5884 are in specified directories.
5886 This chapter discusses these options in detail.
5889 * file:: Choosing the Archive's Name
5890 * Selecting Archive Members::
5891 * files:: Reading Names from a File
5892 * exclude:: Excluding Some Files
5893 * Wildcards:: Wildcards Patterns and Matching
5894 * after:: Operating Only on New Files
5895 * recurse:: Descending into Directories
5896 * one:: Crossing File System Boundaries
5900 @section Choosing and Naming Archive Files
5903 @FIXME{should the title of this section actually be, "naming an
5906 @cindex Naming an archive
5907 @cindex Archive Name
5908 @cindex Choosing an archive file
5909 @cindex Where is the archive?
5910 By default, @command{tar} uses an archive file name that was compiled when
5911 it was built on the system; usually this name refers to some physical
5912 tape drive on the machine. However, the person who installed @command{tar}
5913 on the system may not have set the default to a meaningful value as far as
5914 most users are concerned. As a result, you will usually want to tell
5915 @command{tar} where to find (or create) the archive. The
5916 @option{--file=@var{archive-name}} (@option{-f @var{archive-name}})
5917 option allows you to either specify or name a file to use as the archive
5918 instead of the default archive file location.
5921 @opindex file, short description
5922 @item --file=@var{archive-name}
5923 @itemx -f @var{archive-name}
5924 Name the archive to create or operate on. Use in conjunction with
5928 For example, in this @command{tar} command,
5931 $ @kbd{tar -cvf collection.tar blues folk jazz}
5935 @file{collection.tar} is the name of the archive. It must directly
5936 follow the @option{-f} option, since whatever directly follows @option{-f}
5937 @emph{will} end up naming the archive. If you neglect to specify an
5938 archive name, you may end up overwriting a file in the working directory
5939 with the archive you create since @command{tar} will use this file's name
5940 for the archive name.
5942 An archive can be saved as a file in the file system, sent through a
5943 pipe or over a network, or written to an I/O device such as a tape,
5944 floppy disk, or CD write drive.
5946 @cindex Writing new archives
5947 @cindex Archive creation
5948 If you do not name the archive, @command{tar} uses the value of the
5949 environment variable @env{TAPE} as the file name for the archive. If
5950 that is not available, @command{tar} uses a default, compiled-in archive
5951 name, usually that for tape unit zero (ie. @file{/dev/tu00}).
5953 @cindex Standard input and output
5954 @cindex tar to standard input and output
5955 If you use @file{-} as an @var{archive-name}, @command{tar} reads the
5956 archive from standard input (when listing or extracting files), or
5957 writes it to standard output (when creating an archive). If you use
5958 @file{-} as an @var{archive-name} when modifying an archive,
5959 @command{tar} reads the original archive from its standard input and
5960 writes the entire new archive to its standard output.
5962 The following example is a convenient way of copying directory
5963 hierarchy from @file{sourcedir} to @file{targetdir}.
5966 $ @kbd{(cd sourcedir; tar -cf - .) | (cd targetdir; tar -xpf -)}
5969 The @option{-C} option allows to avoid using subshells:
5972 $ @kbd{tar -C sourcedir -cf - . | tar -C targetdir -xpf -}
5975 In both examples above, the leftmost @command{tar} invocation archives
5976 the contents of @file{sourcedir} to the standard output, while the
5977 rightmost one reads this archive from its standard input and
5978 extracts it. The @option{-p} option tells it to restore permissions
5979 of the extracted files.
5981 @cindex Remote devices
5982 @cindex tar to a remote device
5984 To specify an archive file on a device attached to a remote machine,
5988 @kbd{--file=@var{hostname}:/@var{dev}/@var{file-name}}
5992 @command{tar} will complete the remote connection, if possible, and
5993 prompt you for a username and password. If you use
5994 @option{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @command{tar}
5995 will complete the remote connection, if possible, using your username
5996 as the username on the remote machine.
5998 @cindex Local and remote archives
5999 @anchor{local and remote archives}
6000 If the archive file name includes a colon (@samp{:}), then it is assumed
6001 to be a file on another machine. If the archive file is
6002 @samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the
6003 host @var{host}. The remote host is accessed using the @command{rsh}
6004 program, with a username of @var{user}. If the username is omitted
6005 (along with the @samp{@@} sign), then your user name will be used.
6006 (This is the normal @command{rsh} behavior.) It is necessary for the
6007 remote machine, in addition to permitting your @command{rsh} access, to
6008 have the @file{rmt} program installed (This command is included in
6009 the @GNUTAR{} distribution and by default is installed under
6010 @file{@var{prefix}/libexec/rmt}, were @var{prefix} means your
6011 installation prefix). If you need to use a file whose name includes a
6012 colon, then the remote tape drive behavior
6013 can be inhibited by using the @option{--force-local} option.
6015 When the archive is being created to @file{/dev/null}, @GNUTAR{}
6016 tries to minimize input and output operations. The Amanda backup
6017 system, when used with @GNUTAR{}, has an initial sizing pass which
6020 @node Selecting Archive Members
6021 @section Selecting Archive Members
6022 @cindex Specifying files to act on
6023 @cindex Specifying archive members
6025 @dfn{File Name arguments} specify which files in the file system
6026 @command{tar} operates on, when creating or adding to an archive, or which
6027 archive members @command{tar} operates on, when reading or deleting from
6028 an archive. @xref{Operations}.
6030 To specify file names, you can include them as the last arguments on
6031 the command line, as follows:
6033 @kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}]
6036 If a file name begins with dash (@samp{-}), precede it with
6037 @option{--add-file} option to prevent it from being treated as an
6040 If you specify a directory name as a file name argument, all the files
6041 in that directory are operated on by @command{tar}.
6043 If you do not specify files, @command{tar} behavior differs depending
6044 on the operation mode as described below:
6046 When @command{tar} is invoked with @option{--create} (@option{-c}),
6047 @command{tar} will stop immediately, reporting the following:
6051 $ @kbd{tar cf a.tar}
6052 tar: Cowardly refusing to create an empty archive
6053 Try `tar --help' or `tar --usage' for more information.
6057 If you specify either @option{--list} (@option{-t}) or
6058 @option{--extract} (@option{--get}, @option{-x}), @command{tar}
6059 operates on all the archive members in the archive.
6061 If run with @option{--diff} option, tar will compare the archive with
6062 the contents of the current working directory.
6064 If you specify any other operation, @command{tar} does nothing.
6066 By default, @command{tar} takes file names from the command line. However,
6067 there are other ways to specify file or member names, or to modify the
6068 manner in which @command{tar} selects the files or members upon which to
6069 operate. @FIXME{add xref here}In general, these methods work both for
6070 specifying the names of files and archive members.
6073 @section Reading Names from a File
6075 @cindex Reading file names from a file
6076 @cindex Lists of file names
6077 @cindex File Name arguments, alternatives
6078 Instead of giving the names of files or archive members on the command
6079 line, you can put the names into a file, and then use the
6080 @option{--files-from=@var{file-of-names}} (@option{-T
6081 @var{file-of-names}}) option to @command{tar}. Give the name of the
6082 file which contains the list of files to include as the argument to
6083 @option{--files-from}. In the list, the file names should be separated by
6084 newlines. You will frequently use this option when you have generated
6085 the list of files to archive with the @command{find} utility.
6089 @item --files-from=@var{file-name}
6090 @itemx -T @var{file-name}
6091 Get names to extract or create from file @var{file-name}.
6094 If you give a single dash as a file name for @option{--files-from}, (i.e.,
6095 you specify either @code{--files-from=-} or @code{-T -}), then the file
6096 names are read from standard input.
6098 Unless you are running @command{tar} with @option{--create}, you can not use
6099 both @code{--files-from=-} and @code{--file=-} (@code{-f -}) in the same
6102 Any number of @option{-T} options can be given in the command line.
6104 The following example shows how to use @command{find} to generate a list of
6105 files smaller than 400K in length and put that list into a file
6106 called @file{small-files}. You can then use the @option{-T} option to
6107 @command{tar} to specify the files from that file, @file{small-files}, to
6108 create the archive @file{little.tgz}. (The @option{-z} option to
6109 @command{tar} compresses the archive with @command{gzip}; @pxref{gzip} for
6113 $ @kbd{find . -size -400 -print > small-files}
6114 $ @kbd{tar -c -v -z -T small-files -f little.tgz}
6118 In the file list given by @option{-T} option, any file name beginning
6119 with @samp{-} character is considered a @command{tar} option and is
6120 processed accordingly.@footnote{Versions of @GNUTAR{} up to 1.15.1
6121 recognized only @option{-C} option in file lists, and only if the
6122 option and its argument occupied two consecutive lines.} For example,
6123 the common use of this feature is to change to another directory by
6124 specifying @option{-C} option:
6134 $ @kbd{tar -c -f foo.tar --files-from list}
6139 In this example, @command{tar} will first switch to @file{/etc}
6140 directory and add files @file{passwd} and @file{hosts} to the
6141 archive. Then it will change to @file{/lib} directory and will archive
6142 the file @file{libc.a}. Thus, the resulting archive @file{foo.tar} will
6147 $ @kbd{tar tf foo.tar}
6155 @opindex directory, using in @option{--files-from} argument
6156 Notice that the option parsing algorithm used with @option{-T} is
6157 stricter than the one used by shell. Namely, when specifying option
6158 arguments, you should observe the following rules:
6162 When using short (single-letter) option form, its argument must
6163 immediately follow the option letter, without any intervening
6164 whitespace. For example: @code{-Cdir}.
6167 When using long option form, the option argument must be separated
6168 from the option by a single equal sign. No whitespace is allowed on
6169 any side of the equal sign. For example: @code{--directory=dir}.
6172 For both short and long option forms, the option argument can be given
6173 on the next line after the option name, e.g.:
6194 If you happen to have a file whose name starts with @samp{-},
6195 precede it with @option{--add-file} option to prevent it from
6196 being recognized as an option. For example: @code{--add-file=--my-file}.
6203 @subsection @code{NUL} Terminated File Names
6205 @cindex File names, terminated by @code{NUL}
6206 @cindex @code{NUL} terminated file names
6207 The @option{--null} option causes
6208 @option{--files-from=@var{file-of-names}} (@option{-T @var{file-of-names}})
6209 to read file names terminated by a @code{NUL} instead of a newline, so
6210 files whose names contain newlines can be archived using
6211 @option{--files-from}.
6216 Only consider @code{NUL} terminated file names, instead of files that
6217 terminate in a newline.
6220 The @option{--null} option is just like the one in @acronym{GNU}
6221 @command{xargs} and @command{cpio}, and is useful with the
6222 @option{-print0} predicate of @acronym{GNU} @command{find}. In
6223 @command{tar}, @option{--null} also disables special handling for
6224 file names that begin with dash.
6226 This example shows how to use @command{find} to generate a list of files
6227 larger than 800K in length and put that list into a file called
6228 @file{long-files}. The @option{-print0} option to @command{find} is just
6229 like @option{-print}, except that it separates files with a @code{NUL}
6230 rather than with a newline. You can then run @command{tar} with both the
6231 @option{--null} and @option{-T} options to specify that @command{tar} get the
6232 files from that file, @file{long-files}, to create the archive
6233 @file{big.tgz}. The @option{--null} option to @command{tar} will cause
6234 @command{tar} to recognize the @code{NUL} separator between files.
6237 $ @kbd{find . -size +800 -print0 > long-files}
6238 $ @kbd{tar -c -v --null --files-from=long-files --file=big.tar}
6241 @FIXME{say anything else here to conclude the section?}
6244 @section Excluding Some Files
6247 @cindex File names, excluding files by
6248 @cindex Excluding files by name and pattern
6249 @cindex Excluding files by file system
6250 To avoid operating on files whose names match a particular pattern,
6251 use the @option{--exclude} or @option{--exclude-from} options.
6255 @item --exclude=@var{pattern}
6256 Causes @command{tar} to ignore files that match the @var{pattern}.
6260 The @option{--exclude=@var{pattern}} option prevents any file or
6261 member whose name matches the shell wildcard (@var{pattern}) from
6263 For example, to create an archive with all the contents of the directory
6264 @file{src} except for files whose names end in @file{.o}, use the
6265 command @samp{tar -cf src.tar --exclude='*.o' src}.
6267 You may give multiple @option{--exclude} options.
6270 @opindex exclude-from
6271 @item --exclude-from=@var{file}
6272 @itemx -X @var{file}
6273 Causes @command{tar} to ignore files that match the patterns listed in
6277 @findex exclude-from
6278 Use the @option{--exclude-from} option to read a
6279 list of patterns, one per line, from @var{file}; @command{tar} will
6280 ignore files matching those patterns. Thus if @command{tar} is
6281 called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a
6282 single line @file{*.o}, no files whose names end in @file{.o} will be
6283 added to the archive.
6286 @opindex exclude-caches
6287 @item --exclude-caches
6288 Causes @command{tar} to ignore directories containing a cache directory tag.
6291 @findex exclude-caches
6292 When creating an archive, the @option{--exclude-caches} option causes
6293 @command{tar} to exclude all directories that contain a @dfn{cache
6294 directory tag}. A cache directory tag is a short file with the
6295 well-known name @file{CACHEDIR.TAG} and having a standard header
6296 specified in @url{http://www.brynosaurus.com/cachedir/spec.html}.
6297 Various applications write cache directory tags into directories they
6298 use to hold regenerable, non-precious data, so that such data can be
6299 more easily excluded from backups.
6302 * problems with exclude::
6305 @node problems with exclude
6306 @unnumberedsubsec Problems with Using the @code{exclude} Options
6308 @opindex exclude, potential problems with
6309 Some users find @samp{exclude} options confusing. Here are some common
6314 The main operating mode of @command{tar} does not act on a path name
6315 explicitly listed on the command line if one of its file name
6316 components is excluded. In the example above, if
6317 you create an archive and exclude files that end with @samp{*.o}, but
6318 explicitly name the file @samp{dir.o/foo} after all the options have been
6319 listed, @samp{dir.o/foo} will be excluded from the archive.
6322 You can sometimes confuse the meanings of @option{--exclude} and
6323 @option{--exclude-from}. Be careful: use @option{--exclude} when files
6324 to be excluded are given as a pattern on the command line. Use
6325 @option{--exclude-from} to introduce the name of a file which contains
6326 a list of patterns, one per line; each of these patterns can exclude
6327 zero, one, or many files.
6330 When you use @option{--exclude=@var{pattern}}, be sure to quote the
6331 @var{pattern} parameter, so @GNUTAR{} sees wildcard characters
6332 like @samp{*}. If you do not do this, the shell might expand the
6333 @samp{*} itself using files at hand, so @command{tar} might receive a
6334 list of files instead of one pattern, or none at all, making the
6335 command somewhat illegal. This might not correspond to what you want.
6340 $ @kbd{tar -c -f @var{archive.tar} --exclude '*.o' @var{directory}}
6348 $ @kbd{tar -c -f @var{archive.tar} --exclude *.o @var{directory}}
6352 You must use use shell syntax, or globbing, rather than @code{regexp}
6353 syntax, when using exclude options in @command{tar}. If you try to use
6354 @code{regexp} syntax to describe files to be excluded, your command
6358 @FIXME{The change in semantics must have occurred before 1.11,
6359 so I doubt if it is worth mentioning at all. Anyway, should at
6360 least specify in which version the semantics changed.}
6361 In earlier versions of @command{tar}, what is now the
6362 @option{--exclude-from} option was called @option{--exclude} instead.
6363 Now, @option{--exclude} applies to patterns listed on the command
6364 line and @option{--exclude-from} applies to patterns listed in a
6370 @section Wildcards Patterns and Matching
6372 @dfn{Globbing} is the operation by which @dfn{wildcard} characters,
6373 @samp{*} or @samp{?} for example, are replaced and expanded into all
6374 existing files matching the given pattern. @GNUTAR{} can use wildcard
6375 patterns for matching (or globbing) archive members when extracting
6376 from or listing an archive. Wildcard patterns are also used for
6377 verifying volume labels of @command{tar} archives. This section has the
6378 purpose of explaining wildcard syntax for @command{tar}.
6380 @FIXME{the next few paragraphs need work.}
6382 A @var{pattern} should be written according to shell syntax, using wildcard
6383 characters to effect globbing. Most characters in the pattern stand
6384 for themselves in the matched string, and case is significant: @samp{a}
6385 will match only @samp{a}, and not @samp{A}. The character @samp{?} in the
6386 pattern matches any single character in the matched string. The character
6387 @samp{*} in the pattern matches zero, one, or more single characters in
6388 the matched string. The character @samp{\} says to take the following
6389 character of the pattern @emph{literally}; it is useful when one needs to
6390 match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves.
6392 The character @samp{[}, up to the matching @samp{]}, introduces a character
6393 class. A @dfn{character class} is a list of acceptable characters
6394 for the next single character of the matched string. For example,
6395 @samp{[abcde]} would match any of the first five letters of the alphabet.
6396 Note that within a character class, all of the ``special characters''
6397 listed above other than @samp{\} lose their special meaning; for example,
6398 @samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\},
6399 @samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints,
6400 the characters @samp{-} and @samp{]} must either come @emph{first} or
6401 @emph{last} in a character class.)
6403 @cindex Excluding characters from a character class
6404 @cindex Character class, excluding characters from
6405 If the first character of the class after the opening @samp{[}
6406 is @samp{!} or @samp{^}, then the meaning of the class is reversed.
6407 Rather than listing character to match, it lists those characters which
6408 are @emph{forbidden} as the next single character of the matched string.
6410 Other characters of the class stand for themselves. The special
6411 construction @samp{[@var{a}-@var{e}]}, using an hyphen between two
6412 letters, is meant to represent all characters between @var{a} and
6415 @FIXME{need to add a sentence or so here to make this clear for those
6416 who don't have dan around.}
6418 Periods (@samp{.}) or forward slashes (@samp{/}) are not considered
6419 special for wildcard matches. However, if a pattern completely matches
6420 a directory prefix of a matched string, then it matches the full matched
6421 string: thus, excluding a directory also excludes all the files beneath it.
6424 * controlling pattern-matching::
6427 @node controlling pattern-matching
6428 @unnumberedsubsec Controlling Pattern-Matching
6430 For the purposes of this section, we call @dfn{exclusion members} all
6431 member names obtained while processing @option{--exclude} and
6432 @option{--exclude-from} options, and @dfn{inclusion members} those
6433 member names that were given in the command line or read from the file
6434 specified with @option{--files-from} option.
6436 These two pairs of member lists are used in the following operations:
6437 @option{--diff}, @option{--extract}, @option{--list},
6440 There are no inclusion members in create mode (@option{--create} and
6441 @option{--append}), since in this mode the names obtained from the
6442 command line refer to @emph{files}, not archive members.
6444 By default, inclusion members are compared with archive members
6445 literally @footnote{Notice that earlier @GNUTAR{} versions used
6446 globbing for inclusion members, which contradicted to UNIX98
6447 specification and was not documented. @xref{Changes}, for more
6448 information on this and other changes} and exclusion members are
6449 treated as globbing patterns. For example:
6453 $ @kbd{tar tf foo.tar}
6458 # @i{Member names are used verbatim:}
6459 $ @kbd{tar -xf foo.tar -v '[remarks]'}
6461 # @i{Exclude member names are globbed:}
6462 $ @kbd{tar -xf foo.tar -v --exclude '*.c'}
6468 This behavior can be altered by using the following options:
6473 Treat all member names as wildcards.
6475 @opindex no-wildcards
6476 @item --no-wildcards
6477 Treat all member names as literal strings.
6480 Thus, to extract files whose names end in @samp{.c}, you can use:
6483 $ @kbd{tar -xf foo.tar -v --wildcards '*.c'}
6489 Notice quoting of the pattern to prevent the shell from interpreting
6492 The effect of @option{--wildcards} option is cancelled by
6493 @option{--no-wildcards}. This can be used to pass part of
6494 the command line arguments verbatim and other part as globbing
6495 patterns. For example, the following invocation:
6498 $ @kbd{tar -xf foo.tar --wildcards '*.txt' --no-wildcards '[remarks]'}
6502 instructs @command{tar} to extract from @file{foo.tar} all files whose
6503 names end in @samp{.txt} and the file named @file{[remarks]}.
6505 Normally, a pattern matches a name if an initial subsequence of the
6506 name's components matches the pattern, where @samp{*}, @samp{?}, and
6507 @samp{[...]} are the usual shell wildcards, @samp{\} escapes wildcards,
6508 and wildcards can match @samp{/}.
6510 Other than optionally stripping leading @samp{/} from names
6511 (@pxref{absolute}), patterns and names are used as-is. For
6512 example, trailing @samp{/} is not trimmed from a user-specified name
6513 before deciding whether to exclude it.
6515 However, this matching procedure can be altered by the options listed
6516 below. These options accumulate. For example:
6519 --ignore-case --exclude='makefile' --no-ignore-case ---exclude='readme'
6522 ignores case when excluding @samp{makefile}, but not when excluding
6527 @opindex no-anchored
6529 @itemx --no-anchored
6530 If anchored, a pattern must match an initial subsequence
6531 of the name's components. Otherwise, the pattern can match any
6532 subsequence. Default is @option{--no-anchored} for exclusion members
6533 and @option{--anchored} inclusion members.
6535 @opindex ignore-case
6536 @opindex no-ignore-case
6538 @itemx --no-ignore-case
6539 When ignoring case, upper-case patterns match lower-case names and vice versa.
6540 When not ignoring case (the default), matching is case-sensitive.
6542 @opindex wildcards-match-slash
6543 @opindex no-wildcards-match-slash
6544 @item --wildcards-match-slash
6545 @itemx --no-wildcards-match-slash
6546 When wildcards match slash (the default for exclusion members), a
6547 wildcard like @samp{*} in the pattern can match a @samp{/} in the
6548 name. Otherwise, @samp{/} is matched only by @samp{/}.
6552 The @option{--recursion} and @option{--no-recursion} options
6553 (@pxref{recurse}) also affect how member patterns are interpreted. If
6554 recursion is in effect, a pattern matches a name if it matches any of
6555 the name's parent directories.
6557 The following table summarizes pattern-matching default values:
6559 @multitable @columnfractions .3 .7
6560 @headitem Members @tab Default settings
6561 @item Inclusion @tab @option{--no-wildcards --anchored --no-wildcards-match-slash}
6562 @item Exclusion @tab @option{--wildcards --no-anchored --wildcards-match-slash}
6566 @section Operating Only on New Files
6569 @cindex Excluding file by age
6570 @cindex Data Modification time, excluding files by
6571 @cindex Modification time, excluding files by
6572 @cindex Age, excluding files by
6573 The @option{--after-date=@var{date}} (@option{--newer=@var{date}},
6574 @option{-N @var{date}}) option causes @command{tar} to only work on
6575 files whose data modification or status change times are newer than
6576 the @var{date} given. If @var{date} starts with @samp{/} or @samp{.},
6577 it is taken to be a file name; the data modification time of that file
6578 is used as the date. If you use this option when creating or appending
6579 to an archive, the archive will only include new files. If you use
6580 @option{--after-date} when extracting an archive, @command{tar} will
6581 only extract files newer than the @var{date} you specify.
6583 If you only want @command{tar} to make the date comparison based on
6584 modification of the file's data (rather than status
6585 changes), then use the @option{--newer-mtime=@var{date}} option.
6587 You may use these options with any operation. Note that these options
6588 differ from the @option{--update} (@option{-u}) operation in that they
6589 allow you to specify a particular date against which @command{tar} can
6590 compare when deciding whether or not to archive the files.
6595 @item --after-date=@var{date}
6596 @itemx --newer=@var{date}
6597 @itemx -N @var{date}
6598 Only store files newer than @var{date}.
6600 Acts on files only if their data modification or status change times are
6601 later than @var{date}. Use in conjunction with any operation.
6603 If @var{date} starts with @samp{/} or @samp{.}, it is taken to be a file
6604 name; the data modification time of that file is used as the date.
6606 @opindex newer-mtime
6607 @item --newer-mtime=@var{date}
6608 Acts like @option{--after-date}, but only looks at data modification times.
6611 These options limit @command{tar} to operate only on files which have
6612 been modified after the date specified. A file's status is considered to have
6613 changed if its contents have been modified, or if its owner,
6614 permissions, and so forth, have been changed. (For more information on
6615 how to specify a date, see @ref{Date input formats}; remember that the
6616 entire date argument must be quoted if it contains any spaces.)
6618 Gurus would say that @option{--after-date} tests both the data
6619 modification time (@code{mtime}, the time the contents of the file
6620 were last modified) and the status change time (@code{ctime}, the time
6621 the file's status was last changed: owner, permissions, etc.@:)
6622 fields, while @option{--newer-mtime} tests only the @code{mtime}
6625 To be precise, @option{--after-date} checks @emph{both} @code{mtime} and
6626 @code{ctime} and processes the file if either one is more recent than
6627 @var{date}, while @option{--newer-mtime} only checks @code{mtime} and
6628 disregards @code{ctime}. Neither does it use @code{atime} (the last time the
6629 contents of the file were looked at).
6631 Date specifiers can have embedded spaces. Because of this, you may need
6632 to quote date arguments to keep the shell from parsing them as separate
6635 @FIXME{Need example of --newer-mtime with quoted argument.}
6638 @strong{Please Note:} @option{--after-date} and @option{--newer-mtime}
6639 should not be used for incremental backups. Some files (such as those
6640 in renamed directories) are not selected properly by these options.
6641 @xref{Incremental Dumps}.
6645 @FIXME{which tells -- need to fill this in!}
6648 @section Descending into Directories
6650 @cindex Avoiding recursion in directories
6651 @cindex Descending directories, avoiding
6652 @cindex Directories, avoiding recursion
6653 @cindex Recursion in directories, avoiding
6655 @FIXME{arrggh! this is still somewhat confusing to me. :-< }
6657 @FIXME{show dan bob's comments, from 2-10-97}
6659 Usually, @command{tar} will recursively explore all directories (either
6660 those given on the command line or through the @option{--files-from}
6661 option) for the various files they contain. However, you may not always
6662 want @command{tar} to act this way.
6664 @opindex no-recursion
6665 The @option{--no-recursion} option inhibits @command{tar}'s recursive descent
6666 into specified directories. If you specify @option{--no-recursion}, you can
6667 use the @command{find} utility for hunting through levels of directories to
6668 construct a list of file names which you could then pass to @command{tar}.
6669 @command{find} allows you to be more selective when choosing which files to
6670 archive; see @ref{files} for more information on using @command{find} with
6671 @command{tar}, or look.
6674 @item --no-recursion
6675 Prevents @command{tar} from recursively descending directories.
6679 Requires @command{tar} to recursively descend directories.
6680 This is the default.
6683 When you use @option{--no-recursion}, @GNUTAR{} grabs
6684 directory entries themselves, but does not descend on them
6685 recursively. Many people use @command{find} for locating files they
6686 want to back up, and since @command{tar} @emph{usually} recursively
6687 descends on directories, they have to use the @samp{@w{! -d}} option
6688 to @command{find} @FIXME{needs more explanation or a cite to another
6689 info file}as they usually do not want all the files in a directory.
6690 They then use the @option{--files-from} option to archive the files
6691 located via @command{find}.
6693 The problem when restoring files archived in this manner is that the
6694 directories themselves are not in the archive; so the
6695 @option{--same-permissions} (@option{--preserve-permissions},
6696 @option{-p}) option does not affect them---while users might really
6697 like it to. Specifying @option{--no-recursion} is a way to tell
6698 @command{tar} to grab only the directory entries given to it, adding
6699 no new files on its own.
6701 The @option{--no-recursion} option also applies when extracting: it
6702 causes @command{tar} to extract only the matched directory entries, not
6703 the files under those directories.
6705 The @option{--no-recursion} option also affects how globbing patterns
6706 are interpreted (@pxref{controlling pattern-matching}).
6708 The @option{--no-recursion} and @option{--recursion} options apply to
6709 later options and operands, and can be overridden by later occurrences
6710 of @option{--no-recursion} and @option{--recursion}. For example:
6713 $ @kbd{tar -cf jams.tar --no-recursion grape --recursion grape/concord}
6717 creates an archive with one entry for @file{grape}, and the recursive
6718 contents of @file{grape/concord}, but no entries under @file{grape}
6719 other than @file{grape/concord}.
6722 @section Crossing File System Boundaries
6723 @cindex File system boundaries, not crossing
6726 @command{tar} will normally automatically cross file system boundaries in
6727 order to archive files which are part of a directory tree. You can
6728 change this behavior by running @command{tar} and specifying
6729 @option{--one-file-system}. This option only affects files that are
6730 archived because they are in a directory that is being archived;
6731 @command{tar} will still archive files explicitly named on the command line
6732 or through @option{--files-from}, regardless of where they reside.
6735 @opindex one-file-system
6736 @item --one-file-system
6738 Prevents @command{tar} from crossing file system boundaries when
6739 archiving. Use in conjunction with any write operation.
6742 The @option{--one-file-system} option causes @command{tar} to modify its
6743 normal behavior in archiving the contents of directories. If a file in
6744 a directory is not on the same file system as the directory itself, then
6745 @command{tar} will not archive that file. If the file is a directory
6746 itself, @command{tar} will not archive anything beneath it; in other words,
6747 @command{tar} will not cross mount points.
6749 It is reported that using this option, the mount point is is archived,
6750 but nothing under it.
6752 This option is useful for making full or incremental archival backups of
6753 a file system. If this option is used in conjunction with
6754 @option{--verbose} (@option{-v}), files that are excluded are mentioned by name on the
6758 * directory:: Changing Directory
6759 * absolute:: Absolute File Names
6763 @subsection Changing the Working Directory
6766 @FIXME{need to read over this node now for continuity; i've switched
6767 things around some.}
6769 @cindex Changing directory mid-stream
6770 @cindex Directory, changing mid-stream
6771 @cindex Working directory, specifying
6772 To change the working directory in the middle of a list of file names,
6773 either on the command line or in a file specified using
6774 @option{--files-from} (@option{-T}), use @option{--directory} (@option{-C}).
6775 This will change the working directory to the specified directory
6776 after that point in the list.
6780 @item --directory=@var{directory}
6781 @itemx -C @var{directory}
6782 Changes the working directory in the middle of a command line.
6788 $ @kbd{tar -c -f jams.tar grape prune -C food cherry}
6792 will place the files @file{grape} and @file{prune} from the current
6793 directory into the archive @file{jams.tar}, followed by the file
6794 @file{cherry} from the directory @file{food}. This option is especially
6795 useful when you have several widely separated files that you want to
6796 store in the same archive.
6798 Note that the file @file{cherry} is recorded in the archive under the
6799 precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the
6800 archive will contain three files that all appear to have come from the
6801 same directory; if the archive is extracted with plain @samp{tar
6802 --extract}, all three files will be written in the current directory.
6804 Contrast this with the command,
6807 $ @kbd{tar -c -f jams.tar grape prune -C food red/cherry}
6811 which records the third file in the archive under the name
6812 @file{red/cherry} so that, if the archive is extracted using
6813 @samp{tar --extract}, the third file will be written in a subdirectory
6814 named @file{orange-colored}.
6816 You can use the @option{--directory} option to make the archive
6817 independent of the original name of the directory holding the files.
6818 The following command places the files @file{/etc/passwd},
6819 @file{/etc/hosts}, and @file{/lib/libc.a} into the archive
6823 $ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a}
6827 However, the names of the archive members will be exactly what they were
6828 on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}.
6829 They will not appear to be related by file name to the original
6830 directories where those files were located.
6832 Note that @option{--directory} options are interpreted consecutively. If
6833 @option{--directory} specifies a relative file name, it is interpreted
6834 relative to the then current directory, which might not be the same as
6835 the original current working directory of @command{tar}, due to a previous
6836 @option{--directory} option.
6838 When using @option{--files-from} (@pxref{files}), you can put various
6839 @command{tar} options (including @option{-C}) in the file list. Notice,
6840 however, that in this case the option and its argument may not be
6841 separated by whitespace. If you use short option, its argument must
6842 either follow the option letter immediately, without any intervening
6843 whitespace, or occupy the next line. Otherwise, if you use long
6844 option, separate its argument by an equal sign.
6846 For instance, the file list for the above example will be:
6861 To use it, you would invoke @command{tar} as follows:
6864 $ @kbd{tar -c -f foo.tar --files-from list}
6867 Notice also that you can only use the short option variant in the file
6868 list, i.e., always use @option{-C}, not @option{--directory}.
6870 The interpretation of @option{--directory} is disabled by
6871 @option{--null} option.
6874 @subsection Absolute File Names
6878 @opindex absolute-names
6879 @item --absolute-names
6881 Do not strip leading slashes from file names, and permit file names
6882 containing a @file{..} file name component.
6885 By default, @GNUTAR{} drops a leading @samp{/} on
6886 input or output, and complains about file names containing a @file{..}
6887 component. This option turns off this behavior.
6889 When @command{tar} extracts archive members from an archive, it strips any
6890 leading slashes (@samp{/}) from the member name. This causes absolute
6891 member names in the archive to be treated as relative file names. This
6892 allows you to have such members extracted wherever you want, instead of
6893 being restricted to extracting the member in the exact directory named
6894 in the archive. For example, if the archive member has the name
6895 @file{/etc/passwd}, @command{tar} will extract it as if the name were
6896 really @file{etc/passwd}.
6898 File names containing @file{..} can cause problems when extracting, so
6899 @command{tar} normally warns you about such files when creating an
6900 archive, and rejects attempts to extracts such files.
6902 Other @command{tar} programs do not do this. As a result, if you
6903 create an archive whose member names start with a slash, they will be
6904 difficult for other people with a non-@GNUTAR{}
6905 program to use. Therefore, @GNUTAR{} also strips
6906 leading slashes from member names when putting members into the
6907 archive. For example, if you ask @command{tar} to add the file
6908 @file{/bin/ls} to an archive, it will do so, but the member name will
6909 be @file{bin/ls}.@footnote{A side effect of this is that when
6910 @option{--create} is used with @option{--verbose} the resulting output
6911 is not, generally speaking, the same as the one you'd get running
6912 @kbd{tar --list} command. This may be important if you use some
6913 scripts for comparing both outputs. @xref{listing member and file names},
6914 for the information on how to handle this case.}
6916 If you use the @option{--absolute-names} (@option{-P}) option,
6917 @command{tar} will do none of these transformations.
6919 To archive or extract files relative to the root directory, specify
6920 the @option{--absolute-names} (@option{-P}) option.
6922 Normally, @command{tar} acts on files relative to the working
6923 directory---ignoring superior directory names when archiving, and
6924 ignoring leading slashes when extracting.
6926 When you specify @option{--absolute-names} (@option{-P}),
6927 @command{tar} stores file names including all superior directory
6928 names, and preserves leading slashes. If you only invoked
6929 @command{tar} from the root directory you would never need the
6930 @option{--absolute-names} option, but using this option
6931 may be more convenient than switching to root.
6933 @FIXME{Should be an example in the tutorial/wizardry section using this
6934 to transfer files between systems.}
6936 @FIXME{Is write access an issue?}
6939 @item --absolute-names
6940 Preserves full file names (including superior directory names) when
6941 archiving files. Preserves leading slash when extracting files.
6945 @FIXME{this is still horrible; need to talk with dan on monday.}
6947 @command{tar} prints out a message about removing the @samp{/} from
6948 file names. This message appears once per @GNUTAR{}
6949 invocation. It represents something which ought to be told; ignoring
6950 what it means can cause very serious surprises, later.
6952 Some people, nevertheless, do not want to see this message. Wanting to
6953 play really dangerously, one may of course redirect @command{tar} standard
6954 error to the sink. For example, under @command{sh}:
6957 $ @kbd{tar -c -f archive.tar /home 2> /dev/null}
6961 Another solution, both nicer and simpler, would be to change to
6962 the @file{/} directory first, and then avoid absolute notation.
6966 $ @kbd{(cd / && tar -c -f archive.tar home)}
6967 $ @kbd{tar -c -f archive.tar -C / home}
6970 @include getdate.texi
6973 @chapter Controlling the Archive Format
6975 @cindex Tar archive formats
6976 Due to historical reasons, there are several formats of tar archives.
6977 All of them are based on the same principles, but have some subtle
6978 differences that often make them incompatible with each other.
6980 GNU tar is able to create and handle archives in a variety of formats.
6981 The most frequently used formats are (in alphabetical order):
6985 Format used by @GNUTAR{} versions up to 1.13.25. This format derived
6986 from an early @acronym{POSIX} standard, adding some improvements such as
6987 sparse file handling and incremental archives. Unfortunately these
6988 features were implemented in a way incompatible with other archive
6991 Archives in @samp{gnu} format are able to hold pathnames of unlimited
6995 Format used by @GNUTAR{} of versions prior to 1.12.
6998 Archive format, compatible with the V7 implementation of tar. This
6999 format imposes a number of limitations. The most important of them
7003 @item The maximum length of a file name is limited to 99 characters.
7004 @item The maximum length of a symbolic link is limited to 99 characters.
7005 @item It is impossible to store special files (block and character
7006 devices, fifos etc.)
7007 @item Maximum value of user or group ID is limited to 2097151 (7777777
7009 @item V7 archives do not contain symbolic ownership information (user
7010 and group name of the file owner).
7013 This format has traditionally been used by Automake when producing
7014 Makefiles. This practice will change in the future, in the meantime,
7015 however this means that projects containing filenames more than 99
7016 characters long will not be able to use @GNUTAR{} @value{VERSION} and
7017 Automake prior to 1.9.
7020 Archive format defined by @acronym{POSIX.1-1988} specification. It stores
7021 symbolic ownership information. It is also able to store
7022 special files. However, it imposes several restrictions as well:
7025 @item The maximum length of a file name is limited to 256 characters,
7026 provided that the filename can be split at directory separator in
7027 two parts, first of them being at most 155 bytes long. So, in most
7028 cases the maximum file name length will be shorter than 256
7030 @item The maximum length of a symbolic link name is limited to
7032 @item Maximum size of a file the archive is able to accomodate
7034 @item Maximum value of UID/GID is 2097151.
7035 @item Maximum number of bits in device major and minor numbers is 21.
7039 Format used by J@"org Schilling @command{star}
7040 implementation. @GNUTAR{} is able to read @samp{star} archives but
7041 currently does not produce them.
7044 Archive format defined by @acronym{POSIX.1-2001} specification. This is the
7045 most flexible and feature-rich format. It does not impose any
7046 restrictions on file sizes or filename lengths. This format is quite
7047 recent, so not all tar implementations are able to handle it properly.
7048 However, this format is designed in such a way that any tar
7049 implementation able to read @samp{ustar} archives will be able to read
7050 most @samp{posix} archives as well, with the only exception that any
7051 additional information (such as long file names etc.) will in such
7052 case be extracted as plain text files along with the files it refers to.
7054 This archive format will be the default format for future versions
7059 The following table summarizes the limitations of each of these
7062 @multitable @columnfractions .10 .20 .20 .20 .20
7063 @headitem Format @tab UID @tab File Size @tab Path Name @tab Devn
7064 @item gnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
7065 @item oldgnu @tab 1.8e19 @tab Unlimited @tab Unlimited @tab 63
7066 @item v7 @tab 2097151 @tab 8GB @tab 99 @tab n/a
7067 @item ustar @tab 2097151 @tab 8GB @tab 256 @tab 21
7068 @item posix @tab Unlimited @tab Unlimited @tab Unlimited @tab Unlimited
7071 The default format for @GNUTAR{} is defined at compilation
7072 time. You may check it by running @command{tar --help}, and examining
7073 the last lines of its output. Usually, @GNUTAR{} is configured
7074 to create archives in @samp{gnu} format, however, future version will
7075 switch to @samp{posix}.
7078 * Portability:: Making @command{tar} Archives More Portable
7079 * Compression:: Using Less Space through Compression
7080 * Attributes:: Handling File Attributes
7081 * Standard:: The Standard Format
7082 * Extensions:: @acronym{GNU} Extensions to the Archive Format
7083 * cpio:: Comparison of @command{tar} and @command{cpio}
7087 @section Making @command{tar} Archives More Portable
7089 Creating a @command{tar} archive on a particular system that is meant to be
7090 useful later on many other machines and with other versions of @command{tar}
7091 is more challenging than you might think. @command{tar} archive formats
7092 have been evolving since the first versions of Unix. Many such formats
7093 are around, and are not always compatible with each other. This section
7094 discusses a few problems, and gives some advice about making @command{tar}
7095 archives more portable.
7097 One golden rule is simplicity. For example, limit your @command{tar}
7098 archives to contain only regular files and directories, avoiding
7099 other kind of special files. Do not attempt to save sparse files or
7100 contiguous files as such. Let's discuss a few more problems, in turn.
7102 @FIXME{Discuss GNU extensions (incremental backups, multi-volume
7103 archives and archive labels) in GNU and PAX formats.}
7106 * Portable Names:: Portable Names
7107 * dereference:: Symbolic Links
7108 * old:: Old V7 Archives
7109 * ustar:: Ustar Archives
7110 * gnu:: GNU and old GNU format archives.
7111 * posix:: @acronym{POSIX} archives
7112 * Checksumming:: Checksumming Problems
7113 * Large or Negative Values:: Large files, negative time stamps, etc.
7116 @node Portable Names
7117 @subsection Portable Names
7119 Use portable file and member names. A name is portable if it contains
7120 only ASCII letters and digits, @samp{/}, @samp{.}, @samp{_}, and
7121 @samp{-}; it cannot be empty, start with @samp{-} or @samp{//}, or
7122 contain @samp{/-}. Avoid deep directory nesting. For portability to
7123 old Unix hosts, limit your file name components to 14 characters or
7126 If you intend to have your @command{tar} archives to be read under
7127 MSDOS, you should not rely on case distinction for file names, and you
7128 might use the @acronym{GNU} @command{doschk} program for helping you
7129 further diagnosing illegal MSDOS names, which are even more limited
7133 @subsection Symbolic Links
7134 @cindex File names, using symbolic links
7135 @cindex Symbolic link as file name
7137 @opindex dereference
7138 Normally, when @command{tar} archives a symbolic link, it writes a
7139 block to the archive naming the target of the link. In that way, the
7140 @command{tar} archive is a faithful record of the file system contents.
7141 @option{--dereference} (@option{-h}) is used with @option{--create} (@option{-c}), and causes
7142 @command{tar} to archive the files symbolic links point to, instead of
7143 the links themselves. When this option is used, when @command{tar}
7144 encounters a symbolic link, it will archive the linked-to file,
7145 instead of simply recording the presence of a symbolic link.
7147 The name under which the file is stored in the file system is not
7148 recorded in the archive. To record both the symbolic link name and
7149 the file name in the system, archive the file under both names. If
7150 all links were recorded automatically by @command{tar}, an extracted file
7151 might be linked to a file name that no longer exists in the file
7154 If a linked-to file is encountered again by @command{tar} while creating
7155 the same archive, an entire second copy of it will be stored. (This
7156 @emph{might} be considered a bug.)
7158 So, for portable archives, do not archive symbolic links as such,
7159 and use @option{--dereference} (@option{-h}): many systems do not support
7160 symbolic links, and moreover, your distribution might be unusable if
7161 it contains unresolved symbolic links.
7164 @subsection Old V7 Archives
7165 @cindex Format, old style
7166 @cindex Old style format
7167 @cindex Old style archives
7168 @cindex v7 archive format
7170 Certain old versions of @command{tar} cannot handle additional
7171 information recorded by newer @command{tar} programs. To create an
7172 archive in V7 format (not ANSI), which can be read by these old
7173 versions, specify the @option{--format=v7} option in
7174 conjunction with the @option{--create} (@option{-c}) (@command{tar} also
7175 accepts @option{--portability} or @samp{op-old-archive} for this
7176 option). When you specify it,
7177 @command{tar} leaves out information about directories, pipes, fifos,
7178 contiguous files, and device files, and specifies file ownership by
7179 group and user IDs instead of group and user names.
7181 When updating an archive, do not use @option{--format=v7}
7182 unless the archive was created using this option.
7184 In most cases, a @emph{new} format archive can be read by an @emph{old}
7185 @command{tar} program without serious trouble, so this option should
7186 seldom be needed. On the other hand, most modern @command{tar}s are
7187 able to read old format archives, so it might be safer for you to
7188 always use @option{--format=v7} for your distributions.
7191 @subsection Ustar Archive Format
7193 @cindex ustar archive format
7194 Archive format defined by @acronym{POSIX}.1-1988 specification is called
7195 @code{ustar}. Although it is more flexible than the V7 format, it
7196 still has many restrictions (@xref{Formats,ustar}, for the detailed
7197 description of @code{ustar} format). Along with V7 format,
7198 @code{ustar} format is a good choice for archives intended to be read
7199 with other implementations of @command{tar}.
7201 To create archive in @code{ustar} format, use @option{--format=ustar}
7202 option in conjunction with the @option{--create} (@option{-c}).
7205 @subsection @acronym{GNU} and old @GNUTAR{} format
7207 @cindex GNU archive format
7208 @cindex Old GNU archive format
7209 @GNUTAR{} was based on an early draft of the
7210 @acronym{POSIX} 1003.1 @code{ustar} standard. @acronym{GNU} extensions to
7211 @command{tar}, such as the support for file names longer than 100
7212 characters, use portions of the @command{tar} header record which were
7213 specified in that @acronym{POSIX} draft as unused. Subsequent changes in
7214 @acronym{POSIX} have allocated the same parts of the header record for
7215 other purposes. As a result, @GNUTAR{} format is
7216 incompatible with the current @acronym{POSIX} specification, and with
7217 @command{tar} programs that follow it.
7219 In the majority of cases, @command{tar} will be configured to create
7220 this format by default. This will change in the future releases, since
7221 we plan to make @samp{posix} format the default.
7223 To force creation a @GNUTAR{} archive, use option
7224 @option{--format=gnu}.
7227 @subsection @GNUTAR{} and @acronym{POSIX} @command{tar}
7229 @cindex POSIX archive format
7230 @cindex PAX archive format
7231 The version @value{VERSION} of @GNUTAR{} is able
7232 to read and create archives conforming to @acronym{POSIX.1-2001} standard.
7234 A @acronym{POSIX} conformant archive will be created if @command{tar}
7235 was given @option{--format=posix} option.
7238 @subsection Checksumming Problems
7240 SunOS and HP-UX @command{tar} fail to accept archives created using
7241 @GNUTAR{} and containing non-ASCII file names, that
7242 is, file names having characters with the eight bit set, because they
7243 use signed checksums, while @GNUTAR{} uses unsigned
7244 checksums while creating archives, as per @acronym{POSIX} standards. On
7245 reading, @GNUTAR{} computes both checksums and
7246 accept any. It is somewhat worrying that a lot of people may go
7247 around doing backup of their files using faulty (or at least
7248 non-standard) software, not learning about it until it's time to
7249 restore their missing files with an incompatible file extractor, or
7252 @GNUTAR{} compute checksums both ways, and accept
7253 any on read, so @acronym{GNU} tar can read Sun tapes even with their
7254 wrong checksums. @GNUTAR{} produces the standard
7255 checksum, however, raising incompatibilities with Sun. That is to
7256 say, @GNUTAR{} has not been modified to
7257 @emph{produce} incorrect archives to be read by buggy @command{tar}'s.
7258 I've been told that more recent Sun @command{tar} now read standard
7259 archives, so maybe Sun did a similar patch, after all?
7261 The story seems to be that when Sun first imported @command{tar}
7262 sources on their system, they recompiled it without realizing that
7263 the checksums were computed differently, because of a change in
7264 the default signing of @code{char}'s in their compiler. So they
7265 started computing checksums wrongly. When they later realized their
7266 mistake, they merely decided to stay compatible with it, and with
7267 themselves afterwards. Presumably, but I do not really know, HP-UX
7268 has chosen that their @command{tar} archives to be compatible with Sun's.
7269 The current standards do not favor Sun @command{tar} format. In any
7270 case, it now falls on the shoulders of SunOS and HP-UX users to get
7271 a @command{tar} able to read the good archives they receive.
7273 @node Large or Negative Values
7274 @subsection Large or Negative Values
7275 @cindex large values
7276 @cindex future time stamps
7277 @cindex negative time stamps
7280 The above sections suggest to use @samp{oldest possible} archive
7281 format if in doubt. However, sometimes it is not possible. If you
7282 attempt to archive a file whose metadata cannot be represented using
7283 required format, @GNUTAR{} will print error message and ignore such a
7284 file. You will than have to switch to a format that is able to
7285 handle such values. The format summary table (@pxref{Formats}) will
7288 In particular, when trying to archive files larger than 8GB or with
7289 timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16
7290 12:56:31 @sc{utc}, you will have to chose between @acronym{GNU} and
7291 @acronym{POSIX} archive formats. When considering which format to
7292 choose, bear in mind that the @acronym{GNU} format uses
7293 two's-complement base-256 notation to store values that do not fit
7294 into standard @acronym{ustar} range. Such archives can generally be
7295 read only by a @GNUTAR{} implementation. Moreover, they sometimes
7296 cannot be correctly restored on another hosts even by @GNUTAR{}. For
7297 example, using two's complement representation for negative time
7298 stamps that assumes a signed 32-bit @code{time_t} generates archives
7299 that are not portable to hosts with differing @code{time_t}
7302 On the other hand, @acronym{POSIX} archives, generally speaking, can
7303 be extracted by any tar implementation that understands older
7304 @acronym{ustar} format. The only exception are files larger than 8GB.
7306 @FIXME{Describe how @acronym{POSIX} archives are extracted by non
7310 @section Using Less Space through Compression
7313 * gzip:: Creating and Reading Compressed Archives
7314 * sparse:: Archiving Sparse Files
7318 @subsection Creating and Reading Compressed Archives
7319 @cindex Compressed archives
7320 @cindex Storing archives in compressed format
7322 @GNUTAR{} is able to create and read compressed archives. It supports
7323 @command{gzip} and @command{bzip2} compression programs. For backward
7324 compatibilty, it also supports @command{compress} command, although
7325 we strongly recommend against using it, since there is a patent
7326 covering the algorithm it uses and you could be sued for patent
7327 infringement merely by running @command{compress}! Besides, it is less
7328 effective than @command{gzip} and @command{bzip2}.
7330 Creating a compressed archive is simple: you just specify a
7331 @dfn{compression option} along with the usual archive creation
7332 commands. The compression option is @option{-z} (@option{--gzip}) to
7333 create a @command{gzip} compressed archive, @option{-j}
7334 (@option{--bzip2}) to create a @command{bzip2} compressed archive, and
7335 @option{-Z} (@option{--compress}) to use @command{compress} program.
7339 $ @kbd{tar cfz archive.tar.gz .}
7342 Reading compressed archive is even simpler: you don't need to specify
7343 any additional options as @GNUTAR{} recognizes its format
7344 automatically. Thus, the following commands will list and extract the
7345 archive created in previous example:
7348 # List the compressed archive
7349 $ @kbd{tar tf archive.tar.gz}
7350 # Extract the compressed archive
7351 $ @kbd{tar xf archive.tar.gz}
7354 The only case when you have to specify a decompression option while
7355 reading the archive is when reading from a pipe or from a tape drive
7356 that does not support random access. However, in this case @GNUTAR{}
7357 will indicate which option you should use. For example:
7360 $ @kbd{cat archive.tar.gz | tar tf -}
7361 tar: Archive is compressed. Use -z option
7362 tar: Error is not recoverable: exiting now
7365 If you see such diagnostics, just add the suggested option to the
7366 invocation of @GNUTAR{}:
7369 $ @kbd{cat archive.tar.gz | tar tfz -}
7372 Notice also, that there are several restrictions on operations on
7373 compressed archives. First of all, compressed archives cannot be
7374 modified, i.e., you cannot update (@option{--update} (@option{-u})) them or delete
7375 (@option{--delete}) members from them. Likewise, you cannot append
7376 another @command{tar} archive to a compressed archive using
7377 @option{--append} (@option{-r})). Secondly, multi-volume archives cannot be
7380 The following table summarizes compression options used by @GNUTAR{}.
7388 Filter the archive through @command{gzip}.
7390 You can use @option{--gzip} and @option{--gunzip} on physical devices
7391 (tape drives, etc.) and remote files as well as on normal files; data
7392 to or from such devices or remote files is reblocked by another copy
7393 of the @command{tar} program to enforce the specified (or default) record
7394 size. The default compression parameters are used; if you need to
7395 override them, set @env{GZIP} environment variable, e.g.:
7398 $ @kbd{GZIP=--best tar cfz archive.tar.gz subdir}
7402 Another way would be to avoid the @option{--gzip} (@option{--gunzip}, @option{--ungzip}, @option{-z}) option and run
7403 @command{gzip} explicitly:
7406 $ @kbd{tar cf - subdir | gzip --best -c - > archive.tar.gz}
7409 @cindex corrupted archives
7410 About corrupted compressed archives: @command{gzip}'ed files have no
7411 redundancy, for maximum compression. The adaptive nature of the
7412 compression scheme means that the compression tables are implicitly
7413 spread all over the archive. If you lose a few blocks, the dynamic
7414 construction of the compression tables becomes unsynchronized, and there
7415 is little chance that you could recover later in the archive.
7417 There are pending suggestions for having a per-volume or per-file
7418 compression in @GNUTAR{}. This would allow for viewing the
7419 contents without decompression, and for resynchronizing decompression at
7420 every volume or file, in case of corrupted archives. Doing so, we might
7421 lose some compressibility. But this would have make recovering easier.
7422 So, there are pros and cons. We'll see!
7427 Filter the archive through @code{bzip2}. Otherwise like @option{--gzip}.
7434 Filter the archive through @command{compress}. Otherwise like @option{--gzip}.
7436 The @acronym{GNU} Project recommends you not use
7437 @command{compress}, because there is a patent covering the algorithm it
7438 uses. You could be sued for patent infringement merely by running
7441 @opindex use-compress-program
7442 @item --use-compress-program=@var{prog}
7443 Use external compression program @var{prog}. Use this option if you
7444 have a compression program that @GNUTAR{} does not support. There
7445 are two requirements to which @var{prog} should comply:
7447 First, when called without options, it should read data from standard
7448 input, compress it and output it on standard output.
7450 Secondly, if called with @option{-d} argument, it should do exactly
7451 the opposite, i.e., read the compressed data from the standard input
7452 and produce uncompressed data on the standard output.
7455 @FIXME{I have one question, or maybe it's a suggestion if there isn't a way
7456 to do it now. I would like to use @option{--gzip}, but I'd also like
7457 the output to be fed through a program like @acronym{GNU}
7458 @command{ecc} (actually, right now that's @samp{exactly} what I'd like
7459 to use :-)), basically adding ECC protection on top of compression.
7460 It seems as if this should be quite easy to do, but I can't work out
7461 exactly how to go about it. Of course, I can pipe the standard output
7462 of @command{tar} through @command{ecc}, but then I lose (though I
7463 haven't started using it yet, I confess) the ability to have
7464 @command{tar} use @command{rmt} for it's I/O (I think).
7466 I think the most straightforward thing would be to let me specify a
7467 general set of filters outboard of compression (preferably ordered,
7468 so the order can be automatically reversed on input operations, and
7469 with the options they require specifiable), but beggars shouldn't be
7470 choosers and anything you decide on would be fine with me.
7472 By the way, I like @command{ecc} but if (as the comments say) it can't
7473 deal with loss of block sync, I'm tempted to throw some time at adding
7474 that capability. Supposing I were to actually do such a thing and
7475 get it (apparently) working, do you accept contributed changes to
7476 utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995).
7478 Isn't that exactly the role of the @option{--use-compress-prog=@var{program}} option?
7479 I never tried it myself, but I suspect you may want to write a
7480 @var{prog} script or program able to filter stdin to stdout to
7481 way you want. It should recognize the @option{-d} option, for when
7482 extraction is needed rather than creation.
7484 It has been reported that if one writes compressed data (through the
7485 @option{--gzip} or @option{--compress} options) to a DLT and tries to use
7486 the DLT compression mode, the data will actually get bigger and one will
7487 end up with less space on the tape.}
7490 @subsection Archiving Sparse Files
7491 @cindex Sparse Files
7498 Handle sparse files efficiently.
7501 This option causes all files to be put in the archive to be tested for
7502 sparseness, and handled specially if they are. The @option{--sparse}
7503 (@option{-S}) option is useful when many @code{dbm} files, for example, are being
7504 backed up. Using this option dramatically decreases the amount of
7505 space needed to store such a file.
7507 In later versions, this option may be removed, and the testing and
7508 treatment of sparse files may be done automatically with any special
7509 @acronym{GNU} options. For now, it is an option needing to be specified on
7510 the command line with the creation or updating of an archive.
7512 Files in the file system occasionally have ``holes.'' A hole in a file
7513 is a section of the file's contents which was never written. The
7514 contents of a hole read as all zeros. On many operating systems,
7515 actual disk storage is not allocated for holes, but they are counted
7516 in the length of the file. If you archive such a file, @command{tar}
7517 could create an archive longer than the original. To have @command{tar}
7518 attempt to recognize the holes in a file, use @option{--sparse} (@option{-S}). When
7519 you use this option, then, for any file using less disk space than
7520 would be expected from its length, @command{tar} searches the file for
7521 consecutive stretches of zeros. It then records in the archive for
7522 the file where the consecutive stretches of zeros are, and only
7523 archives the ``real contents'' of the file. On extraction (using
7524 @option{--sparse} is not needed on extraction) any such
7525 files have holes created wherever the continuous stretches of zeros
7526 were found. Thus, if you use @option{--sparse}, @command{tar} archives
7527 won't take more space than the original.
7529 A file is sparse if it contains blocks of zeros whose existence is
7530 recorded, but that have no space allocated on disk. When you specify
7531 the @option{--sparse} option in conjunction with the @option{--create}
7532 (@option{-c}) operation, @command{tar} tests all files for sparseness
7533 while archiving. If @command{tar} finds a file to be sparse, it uses a
7534 sparse representation of the file in the archive. @xref{create}, for
7535 more information about creating archives.
7537 @option{--sparse} is useful when archiving files, such as dbm files,
7538 likely to contain many nulls. This option dramatically
7539 decreases the amount of space needed to store such an archive.
7542 @strong{Please Note:} Always use @option{--sparse} when performing file
7543 system backups, to avoid archiving the expanded forms of files stored
7544 sparsely in the system.
7546 Even if your system has no sparse files currently, some may be
7547 created in the future. If you use @option{--sparse} while making file
7548 system backups as a matter of course, you can be assured the archive
7549 will never take more space on the media than the files take on disk
7550 (otherwise, archiving a disk filled with sparse files might take
7551 hundreds of tapes). @FIXME-xref{incremental when node name is set.}
7554 @command{tar} ignores the @option{--sparse} option when reading an archive.
7559 Files stored sparsely in the file system are represented sparsely in
7560 the archive. Use in conjunction with write operations.
7563 However, users should be well aware that at archive creation time,
7564 @GNUTAR{} still has to read whole disk file to
7565 locate the @dfn{holes}, and so, even if sparse files use little space
7566 on disk and in the archive, they may sometimes require inordinate
7567 amount of time for reading and examining all-zero blocks of a file.
7568 Although it works, it's painfully slow for a large (sparse) file, even
7569 though the resulting tar archive may be small. (One user reports that
7570 dumping a @file{core} file of over 400 megabytes, but with only about
7571 3 megabytes of actual data, took about 9 minutes on a Sun Sparcstation
7572 ELC, with full CPU utilization.)
7574 This reading is required in all cases and is not related to the fact
7575 the @option{--sparse} option is used or not, so by merely @emph{not}
7576 using the option, you are not saving time@footnote{Well! We should say
7577 the whole truth, here. When @option{--sparse} is selected while creating
7578 an archive, the current @command{tar} algorithm requires sparse files to be
7579 read twice, not once. We hope to develop a new archive format for saving
7580 sparse files in which one pass will be sufficient.}.
7582 Programs like @command{dump} do not have to read the entire file; by
7583 examining the file system directly, they can determine in advance
7584 exactly where the holes are and thus avoid reading through them. The
7585 only data it need read are the actual allocated data blocks.
7586 @GNUTAR{} uses a more portable and straightforward
7587 archiving approach, it would be fairly difficult that it does
7588 otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, on
7592 What I did say is that you cannot tell the difference between a hole and an
7593 equivalent number of nulls without reading raw blocks. @code{st_blocks} at
7594 best tells you how many holes there are; it doesn't tell you @emph{where}.
7595 Just as programs may, conceivably, care what @code{st_blocks} is (care
7596 to name one that does?), they may also care where the holes are (I have
7597 no examples of this one either, but it's equally imaginable).
7599 I conclude from this that good archivers are not portable. One can
7600 arguably conclude that if you want a portable program, you can in good
7601 conscience restore files with as many holes as possible, since you can't
7606 @section Handling File Attributes
7609 When @command{tar} reads files, it updates their access times. To
7610 avoid this, use the @option{--atime-preserve[=METHOD]} option, which can either
7611 reset the access time retroactively or avoid changing it in the first
7614 Handling of file attributes
7617 @opindex atime-preserve
7618 @item --atime-preserve
7619 @itemx --atime-preserve=replace
7620 @itemx --atime-preserve=system
7621 Preserve the access times of files that are read. This works only for
7622 files that you own, unless you have superuser privileges.
7624 @option{--atime-preserve=replace} works on most systems, but it also
7625 restores the data modification time and updates the status change
7626 time. Hence it doesn't interact with incremental dumps nicely
7627 (@pxref{Backups}), and it can set access or data modification times
7628 incorrectly if other programs access the file while @command{tar} is
7631 @option{--atime-preserve=system} avoids changing the access time in
7632 the first place, if the operating system supports this.
7633 Unfortunately, this may or may not work on any given operating system
7634 or file system. If @command{tar} knows for sure it won't work, it
7635 complains right away.
7637 Currently @option{--atime-preserve} with no operand defaults to
7638 @option{--atime-preserve=replace}, but this is intended to change to
7639 @option{--atime-preserve=system} when the latter is better-supported.
7644 Do not extract data modification time.
7646 When this option is used, @command{tar} leaves the data modification times
7647 of the files it extracts as the times when the files were extracted,
7648 instead of setting it to the times recorded in the archive.
7650 This option is meaningless with @option{--list} (@option{-t}).
7654 Create extracted files with the same ownership they have in the
7657 This is the default behavior for the superuser,
7658 so this option is meaningful only for non-root users, when @command{tar}
7659 is executed on those systems able to give files away. This is
7660 considered as a security flaw by many people, at least because it
7661 makes quite difficult to correctly account users for the disk space
7662 they occupy. Also, the @code{suid} or @code{sgid} attributes of
7663 files are easily and silently lost when files are given away.
7665 When writing an archive, @command{tar} writes the user id and user name
7666 separately. If it can't find a user name (because the user id is not
7667 in @file{/etc/passwd}), then it does not write one. When restoring,
7668 and doing a @code{chmod} like when you use @option{--same-permissions},
7669 @FIXME{same-owner?}it tries to look the name (if one was written)
7670 up in @file{/etc/passwd}. If it fails, then it uses the user id
7671 stored in the archive instead.
7673 @opindex no-same-owner
7674 @item --no-same-owner
7676 Do not attempt to restore ownership when extracting. This is the
7677 default behavior for ordinary users, so this option has an effect
7678 only for the superuser.
7680 @opindex numeric-owner
7681 @item --numeric-owner
7682 The @option{--numeric-owner} option allows (ANSI) archives to be written
7683 without user/group name information or such information to be ignored
7684 when extracting. It effectively disables the generation and/or use
7685 of user/group name information. This option forces extraction using
7686 the numeric ids from the archive, ignoring the names.
7688 This is useful in certain circumstances, when restoring a backup from
7689 an emergency floppy with different passwd/group files for example.
7690 It is otherwise impossible to extract files with the right ownerships
7691 if the password file in use during the extraction does not match the
7692 one belonging to the file system(s) being extracted. This occurs,
7693 for example, if you are restoring your files after a major crash and
7694 had booted from an emergency floppy with no password file or put your
7695 disk into another machine to do the restore.
7697 The numeric ids are @emph{always} saved into @command{tar} archives.
7698 The identifying names are added at create time when provided by the
7699 system, unless @option{--old-archive} (@option{-o}) is used. Numeric ids could be
7700 used when moving archives between a collection of machines using
7701 a centralized management for attribution of numeric ids to users
7702 and groups. This is often made through using the NIS capabilities.
7704 When making a @command{tar} file for distribution to other sites, it
7705 is sometimes cleaner to use a single owner for all files in the
7706 distribution, and nicer to specify the write permission bits of the
7707 files as stored in the archive independently of their actual value on
7708 the file system. The way to prepare a clean distribution is usually
7709 to have some Makefile rule creating a directory, copying all needed
7710 files in that directory, then setting ownership and permissions as
7711 wanted (there are a lot of possible schemes), and only then making a
7712 @command{tar} archive out of this directory, before cleaning
7713 everything out. Of course, we could add a lot of options to
7714 @GNUTAR{} for fine tuning permissions and ownership.
7715 This is not the good way, I think. @GNUTAR{} is
7716 already crowded with options and moreover, the approach just explained
7717 gives you a great deal of control already.
7719 @opindex same-permissions, short description
7720 @opindex preserve-permissions, short description
7722 @itemx --same-permissions
7723 @itemx --preserve-permissions
7724 Extract all protection information.
7726 This option causes @command{tar} to set the modes (access permissions) of
7727 extracted files exactly as recorded in the archive. If this option
7728 is not used, the current @code{umask} setting limits the permissions
7729 on extracted files. This option is by default enabled when
7730 @command{tar} is executed by a superuser.
7733 This option is meaningless with @option{--list} (@option{-t}).
7737 Same as both @option{--same-permissions} and @option{--same-order}.
7739 The @option{--preserve} option has no equivalent short option name.
7740 It is equivalent to @option{--same-permissions} plus @option{--same-order}.
7742 @FIXME{I do not see the purpose of such an option. (Neither I. FP.)}
7747 @section Basic Tar Format
7750 While an archive may contain many files, the archive itself is a
7751 single ordinary file. Like any other file, an archive file can be
7752 written to a storage device such as a tape or disk, sent through a
7753 pipe or over a network, saved on the active file system, or even
7754 stored in another archive. An archive file is not easy to read or
7755 manipulate without using the @command{tar} utility or Tar mode in
7756 @acronym{GNU} Emacs.
7758 Physically, an archive consists of a series of file entries terminated
7759 by an end-of-archive entry, which consists of two 512 blocks of zero
7761 entry usually describes one of the files in the archive (an
7762 @dfn{archive member}), and consists of a file header and the contents
7763 of the file. File headers contain file names and statistics, checksum
7764 information which @command{tar} uses to detect file corruption, and
7765 information about file types.
7767 Archives are permitted to have more than one member with the same
7768 member name. One way this situation can occur is if more than one
7769 version of a file has been stored in the archive. For information
7770 about adding new versions of a file to an archive, see @ref{update}.
7771 @FIXME-xref{To learn more about having more than one archive member with the
7772 same name, see -backup node, when it's written.}
7774 In addition to entries describing archive members, an archive may
7775 contain entries which @command{tar} itself uses to store information.
7776 @xref{label}, for an example of such an archive entry.
7778 A @command{tar} archive file contains a series of blocks. Each block
7779 contains @code{BLOCKSIZE} bytes. Although this format may be thought
7780 of as being on magnetic tape, other media are often used.
7782 Each file archived is represented by a header block which describes
7783 the file, followed by zero or more blocks which give the contents
7784 of the file. At the end of the archive file there are two 512-byte blocks
7785 filled with binary zeros as an end-of-file marker. A reasonable system
7786 should write such end-of-file marker at the end of an archive, but
7787 must not assume that such a block exists when reading an archive. In
7788 particular @GNUTAR{} always issues a warning if it does not encounter it.
7790 The blocks may be @dfn{blocked} for physical I/O operations.
7791 Each record of @var{n} blocks (where @var{n} is set by the
7792 @option{--blocking-factor=@var{512-size}} (@option{-b @var{512-size}}) option to @command{tar}) is written with a single
7793 @w{@samp{write ()}} operation. On magnetic tapes, the result of
7794 such a write is a single record. When writing an archive,
7795 the last record of blocks should be written at the full size, with
7796 blocks after the zero block containing all zeros. When reading
7797 an archive, a reasonable system should properly handle an archive
7798 whose last record is shorter than the rest, or which contains garbage
7799 records after a zero block.
7801 The header block is defined in C as follows. In the @GNUTAR{}
7802 distribution, this is part of file @file{src/tar.h}:
7805 @include header.texi
7808 All characters in header blocks are represented by using 8-bit
7809 characters in the local variant of ASCII. Each field within the
7810 structure is contiguous; that is, there is no padding used within
7811 the structure. Each character on the archive medium is stored
7814 Bytes representing the contents of files (after the header block
7815 of each file) are not translated in any way and are not constrained
7816 to represent characters in any character set. The @command{tar} format
7817 does not distinguish text files from binary files, and no translation
7818 of file contents is performed.
7820 The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and
7821 @code{gname} are null-terminated character strings. All other fields
7822 are zero-filled octal numbers in ASCII. Each numeric field of width
7823 @var{w} contains @var{w} minus 1 digits, and a null.
7825 The @code{name} field is the file name of the file, with directory names
7826 (if any) preceding the file name, separated by slashes.
7828 @FIXME{how big a name before field overflows?}
7830 The @code{mode} field provides nine bits specifying file permissions
7831 and three bits to specify the Set UID, Set GID, and Save Text
7832 (@dfn{sticky}) modes. Values for these bits are defined above.
7833 When special permissions are required to create a file with a given
7834 mode, and the user restoring files from the archive does not hold such
7835 permissions, the mode bit(s) specifying those special permissions
7836 are ignored. Modes which are not supported by the operating system
7837 restoring files from the archive will be ignored. Unsupported modes
7838 should be faked up when creating or updating an archive; e.g., the
7839 group permission could be copied from the @emph{other} permission.
7841 The @code{uid} and @code{gid} fields are the numeric user and group
7842 ID of the file owners, respectively. If the operating system does
7843 not support numeric user or group IDs, these fields should be ignored.
7845 The @code{size} field is the size of the file in bytes; linked files
7846 are archived with this field specified as zero. @FIXME-xref{Modifiers, in
7847 particular the @option{--incremental} (@option{-G}) option.}
7849 The @code{mtime} field is the data modification time of the file at
7850 the time it was archived. It is the ASCII representation of the octal
7851 value of the last time the file's contents were modified, represented
7852 as an integer number of
7853 seconds since January 1, 1970, 00:00 Coordinated Universal Time.
7855 The @code{chksum} field is the ASCII representation of the octal value
7856 of the simple sum of all bytes in the header block. Each 8-bit
7857 byte in the header is added to an unsigned integer, initialized to
7858 zero, the precision of which shall be no less than seventeen bits.
7859 When calculating the checksum, the @code{chksum} field is treated as
7860 if it were all blanks.
7862 The @code{typeflag} field specifies the type of file archived. If a
7863 particular implementation does not recognize or permit the specified
7864 type, the file will be extracted as if it were a regular file. As this
7865 action occurs, @command{tar} issues a warning to the standard error.
7867 The @code{atime} and @code{ctime} fields are used in making incremental
7868 backups; they store, respectively, the particular file's access and
7869 status change times.
7871 The @code{offset} is used by the @option{--multi-volume} (@option{-M}) option, when
7872 making a multi-volume archive. The offset is number of bytes into
7873 the file that we need to restart at to continue the file on the next
7874 tape, i.e., where we store the location that a continued file is
7877 The following fields were added to deal with sparse files. A file
7878 is @dfn{sparse} if it takes in unallocated blocks which end up being
7879 represented as zeros, i.e., no useful data. A test to see if a file
7880 is sparse is to look at the number blocks allocated for it versus the
7881 number of characters in the file; if there are fewer blocks allocated
7882 for the file than would normally be allocated for a file of that
7883 size, then the file is sparse. This is the method @command{tar} uses to
7884 detect a sparse file, and once such a file is detected, it is treated
7885 differently from non-sparse files.
7887 Sparse files are often @code{dbm} files, or other database-type files
7888 which have data at some points and emptiness in the greater part of
7889 the file. Such files can appear to be very large when an @samp{ls
7890 -l} is done on them, when in truth, there may be a very small amount
7891 of important data contained in the file. It is thus undesirable
7892 to have @command{tar} think that it must back up this entire file, as
7893 great quantities of room are wasted on empty blocks, which can lead
7894 to running out of room on a tape far earlier than is necessary.
7895 Thus, sparse files are dealt with so that these empty blocks are
7896 not written to the tape. Instead, what is written to the tape is a
7897 description, of sorts, of the sparse file: where the holes are, how
7898 big the holes are, and how much data is found at the end of the hole.
7899 This way, the file takes up potentially far less room on the tape,
7900 and when the file is extracted later on, it will look exactly the way
7901 it looked beforehand. The following is a description of the fields
7902 used to handle a sparse file:
7904 The @code{sp} is an array of @code{struct sparse}. Each @code{struct
7905 sparse} contains two 12-character strings which represent an offset
7906 into the file and a number of bytes to be written at that offset.
7907 The offset is absolute, and not relative to the offset in preceding
7910 The header can hold four of these @code{struct sparse} at the moment;
7911 if more are needed, they are not stored in the header.
7913 The @code{isextended} flag is set when an @code{extended_header}
7914 is needed to deal with a file. Note that this means that this flag
7915 can only be set when dealing with a sparse file, and it is only set
7916 in the event that the description of the file will not fit in the
7917 allotted room for sparse structures in the header. In other words,
7918 an extended_header is needed.
7920 The @code{extended_header} structure is used for sparse files which
7921 need more sparse structures than can fit in the header. The header can
7922 fit 4 such structures; if more are needed, the flag @code{isextended}
7923 gets set and the next block is an @code{extended_header}.
7925 Each @code{extended_header} structure contains an array of 21
7926 sparse structures, along with a similar @code{isextended} flag
7927 that the header had. There can be an indeterminate number of such
7928 @code{extended_header}s to describe a sparse file.
7932 @item @code{REGTYPE}
7933 @itemx @code{AREGTYPE}
7934 These flags represent a regular file. In order to be compatible
7935 with older versions of @command{tar}, a @code{typeflag} value of
7936 @code{AREGTYPE} should be silently recognized as a regular file.
7937 New archives should be created using @code{REGTYPE}. Also, for
7938 backward compatibility, @command{tar} treats a regular file whose name
7939 ends with a slash as a directory.
7941 @item @code{LNKTYPE}
7942 This flag represents a file linked to another file, of any type,
7943 previously archived. Such files are identified in Unix by each
7944 file having the same device and inode number. The linked-to name is
7945 specified in the @code{linkname} field with a trailing null.
7947 @item @code{SYMTYPE}
7948 This represents a symbolic link to another file. The linked-to name
7949 is specified in the @code{linkname} field with a trailing null.
7951 @item @code{CHRTYPE}
7952 @itemx @code{BLKTYPE}
7953 These represent character special files and block special files
7954 respectively. In this case the @code{devmajor} and @code{devminor}
7955 fields will contain the major and minor device numbers respectively.
7956 Operating systems may map the device specifications to their own
7957 local specification, or may ignore the entry.
7959 @item @code{DIRTYPE}
7960 This flag specifies a directory or sub-directory. The directory
7961 name in the @code{name} field should end with a slash. On systems where
7962 disk allocation is performed on a directory basis, the @code{size} field
7963 will contain the maximum number of bytes (which may be rounded to
7964 the nearest disk block allocation unit) which the directory may
7965 hold. A @code{size} field of zero indicates no such limiting. Systems
7966 which do not support limiting in this manner should ignore the
7969 @item @code{FIFOTYPE}
7970 This specifies a FIFO special file. Note that the archiving of a
7971 FIFO file archives the existence of this file and not its contents.
7973 @item @code{CONTTYPE}
7974 This specifies a contiguous file, which is the same as a normal
7975 file except that, in operating systems which support it, all its
7976 space is allocated contiguously on the disk. Operating systems
7977 which do not allow contiguous allocation should silently treat this
7978 type as a normal file.
7980 @item @code{A} @dots{} @code{Z}
7981 These are reserved for custom implementations. Some of these are
7982 used in the @acronym{GNU} modified format, as described below.
7986 Other values are reserved for specification in future revisions of
7987 the P1003 standard, and should not be used by any @command{tar} program.
7989 The @code{magic} field indicates that this archive was output in
7990 the P1003 archive format. If this field contains @code{TMAGIC},
7991 the @code{uname} and @code{gname} fields will contain the ASCII
7992 representation of the owner and group of the file respectively.
7993 If found, the user and group IDs are used rather than the values in
7994 the @code{uid} and @code{gid} fields.
7996 For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages
7997 169-173 (section 10.1) for @cite{Archive/Interchange File Format}; and
7998 IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940
7999 (section E.4.48) for @cite{pax - Portable archive interchange}.
8002 @section @acronym{GNU} Extensions to the Archive Format
8005 The @acronym{GNU} format uses additional file types to describe new types of
8006 files in an archive. These are listed below.
8009 @item GNUTYPE_DUMPDIR
8011 This represents a directory and a list of files created by the
8012 @option{--incremental} (@option{-G}) option. The @code{size} field gives the total
8013 size of the associated list of files. Each file name is preceded by
8014 either a @samp{Y} (the file should be in this archive) or an @samp{N}.
8015 (The file is a directory, or is not stored in the archive.) Each file
8016 name is terminated by a null. There is an additional null after the
8019 @item GNUTYPE_MULTIVOL
8021 This represents a file continued from another volume of a multi-volume
8022 archive created with the @option{--multi-volume} (@option{-M}) option. The original
8023 type of the file is not given here. The @code{size} field gives the
8024 maximum size of this piece of the file (assuming the volume does
8025 not end before the file is written out). The @code{offset} field
8026 gives the offset from the beginning of the file where this part of
8027 the file begins. Thus @code{size} plus @code{offset} should equal
8028 the original size of the file.
8030 @item GNUTYPE_SPARSE
8032 This flag indicates that we are dealing with a sparse file. Note
8033 that archiving a sparse file requires special operations to find
8034 holes in the file, which mark the positions of these holes, along
8035 with the number of bytes of data to be found after the hole.
8037 @item GNUTYPE_VOLHDR
8039 This file type is used to mark the volume header that was given with
8040 the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option when the archive was created. The @code{name}
8041 field contains the @code{name} given after the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option.
8042 The @code{size} field is zero. Only the first file in each volume
8043 of an archive should have this type.
8047 You may have trouble reading a @acronym{GNU} format archive on a
8048 non-@acronym{GNU} system if the options @option{--incremental} (@option{-G}),
8049 @option{--multi-volume} (@option{-M}), @option{--sparse} (@option{-S}), or @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) were
8050 used when writing the archive. In general, if @command{tar} does not
8051 use the @acronym{GNU}-added fields of the header, other versions of
8052 @command{tar} should be able to read the archive. Otherwise, the
8053 @command{tar} program will give an error, the most likely one being a
8057 @section Comparison of @command{tar} and @command{cpio}
8060 @FIXME{Reorganize the following material}
8062 The @command{cpio} archive formats, like @command{tar}, do have maximum
8063 pathname lengths. The binary and old ASCII formats have a max path
8064 length of 256, and the new ASCII and CRC ASCII formats have a max
8065 path length of 1024. @acronym{GNU} @command{cpio} can read and write archives
8066 with arbitrary pathname lengths, but other @command{cpio} implementations
8067 may crash unexplainedly trying to read them.
8069 @command{tar} handles symbolic links in the form in which it comes in BSD;
8070 @command{cpio} doesn't handle symbolic links in the form in which it comes
8071 in System V prior to SVR4, and some vendors may have added symlinks
8072 to their system without enhancing @command{cpio} to know about them.
8073 Others may have enhanced it in a way other than the way I did it
8074 at Sun, and which was adopted by AT&T (and which is, I think, also
8075 present in the @command{cpio} that Berkeley picked up from AT&T and put
8076 into a later BSD release---I think I gave them my changes).
8078 (SVR4 does some funny stuff with @command{tar}; basically, its @command{cpio}
8079 can handle @command{tar} format input, and write it on output, and it
8080 probably handles symbolic links. They may not have bothered doing
8081 anything to enhance @command{tar} as a result.)
8083 @command{cpio} handles special files; traditional @command{tar} doesn't.
8085 @command{tar} comes with V7, System III, System V, and BSD source;
8086 @command{cpio} comes only with System III, System V, and later BSD
8087 (4.3-tahoe and later).
8089 @command{tar}'s way of handling multiple hard links to a file can handle
8090 file systems that support 32-bit inumbers (e.g., the BSD file system);
8091 @command{cpio}s way requires you to play some games (in its "binary"
8092 format, i-numbers are only 16 bits, and in its "portable ASCII" format,
8093 they're 18 bits---it would have to play games with the "file system ID"
8094 field of the header to make sure that the file system ID/i-number pairs
8095 of different files were always different), and I don't know which
8096 @command{cpio}s, if any, play those games. Those that don't might get
8097 confused and think two files are the same file when they're not, and
8098 make hard links between them.
8100 @command{tar}s way of handling multiple hard links to a file places only
8101 one copy of the link on the tape, but the name attached to that copy
8102 is the @emph{only} one you can use to retrieve the file; @command{cpio}s
8103 way puts one copy for every link, but you can retrieve it using any
8107 What type of check sum (if any) is used, and how is this calculated.
8110 See the attached manual pages for @command{tar} and @command{cpio} format.
8111 @command{tar} uses a checksum which is the sum of all the bytes in the
8112 @command{tar} header for a file; @command{cpio} uses no checksum.
8115 If anyone knows why @command{cpio} was made when @command{tar} was present
8119 It wasn't. @command{cpio} first showed up in PWB/UNIX 1.0; no
8120 generally-available version of UNIX had @command{tar} at the time. I don't
8121 know whether any version that was generally available @emph{within AT&T}
8122 had @command{tar}, or, if so, whether the people within AT&T who did
8123 @command{cpio} knew about it.
8125 On restore, if there is a corruption on a tape @command{tar} will stop at
8126 that point, while @command{cpio} will skip over it and try to restore the
8129 The main difference is just in the command syntax and header format.
8131 @command{tar} is a little more tape-oriented in that everything is blocked
8132 to start on a record boundary.
8135 Is there any differences between the ability to recover crashed
8136 archives between the two of them. (Is there any chance of recovering
8137 crashed archives at all.)
8140 Theoretically it should be easier under @command{tar} since the blocking
8141 lets you find a header with some variation of @samp{dd skip=@var{nn}}.
8142 However, modern @command{cpio}'s and variations have an option to just
8143 search for the next file header after an error with a reasonable chance
8144 of resyncing. However, lots of tape driver software won't allow you to
8145 continue past a media error which should be the only reason for getting
8146 out of sync unless a file changed sizes while you were writing the
8150 If anyone knows why @command{cpio} was made when @command{tar} was present
8151 at the unix scene, please tell me about this too.
8154 Probably because it is more media efficient (by not blocking everything
8155 and using only the space needed for the headers where @command{tar}
8156 always uses 512 bytes per file header) and it knows how to archive
8159 You might want to look at the freely available alternatives. The
8160 major ones are @command{afio}, @GNUTAR{}, and
8161 @command{pax}, each of which have their own extensions with some
8162 backwards compatibility.
8164 Sparse files were @command{tar}red as sparse files (which you can
8165 easily test, because the resulting archive gets smaller, and
8166 @acronym{GNU} @command{cpio} can no longer read it).
8169 @chapter Tapes and Other Archive Media
8172 A few special cases about tape handling warrant more detailed
8173 description. These special cases are discussed below.
8175 Many complexities surround the use of @command{tar} on tape drives. Since
8176 the creation and manipulation of archives located on magnetic tape was
8177 the original purpose of @command{tar}, it contains many features making
8178 such manipulation easier.
8180 Archives are usually written on dismountable media---tape cartridges,
8181 mag tapes, or floppy disks.
8183 The amount of data a tape or disk holds depends not only on its size,
8184 but also on how it is formatted. A 2400 foot long reel of mag tape
8185 holds 40 megabytes of data when formatted at 1600 bits per inch. The
8186 physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
8188 Magnetic media are re-usable---once the archive on a tape is no longer
8189 needed, the archive can be erased and the tape or disk used over.
8190 Media quality does deteriorate with use, however. Most tapes or disks
8191 should be discarded when they begin to produce data errors. EXABYTE
8192 tape cartridges should be discarded when they generate an @dfn{error
8193 count} (number of non-usable bits) of more than 10k.
8195 Magnetic media are written and erased using magnetic fields, and
8196 should be protected from such fields to avoid damage to stored data.
8197 Sticking a floppy disk to a filing cabinet using a magnet is probably
8201 * Device:: Device selection and switching
8202 * Remote Tape Server::
8203 * Common Problems and Solutions::
8204 * Blocking:: Blocking
8205 * Many:: Many archives on one tape
8206 * Using Multiple Tapes:: Using Multiple Tapes
8207 * label:: Including a Label in the Archive
8209 * Write Protection::
8213 @section Device Selection and Switching
8217 @item -f [@var{hostname}:]@var{file}
8218 @itemx --file=[@var{hostname}:]@var{file}
8219 Use archive file or device @var{file} on @var{hostname}.
8222 This option is used to specify the file name of the archive @command{tar}
8225 If the file name is @samp{-}, @command{tar} reads the archive from standard
8226 input (when listing or extracting), or writes it to standard output
8227 (when creating). If the @samp{-} file name is given when updating an
8228 archive, @command{tar} will read the original archive from its standard
8229 input, and will write the entire new archive to its standard output.
8231 If the file name contains a @samp{:}, it is interpreted as
8232 @samp{hostname:file name}. If the @var{hostname} contains an @dfn{at}
8233 sign (@samp{@@}), it is treated as @samp{user@@hostname:file name}. In
8234 either case, @command{tar} will invoke the command @command{rsh} (or
8235 @command{remsh}) to start up an @command{/usr/libexec/rmt} on the remote
8236 machine. If you give an alternate login name, it will be given to the
8238 Naturally, the remote machine must have an executable
8239 @command{/usr/libexec/rmt}. This program is free software from the
8240 University of California, and a copy of the source code can be found
8241 with the sources for @command{tar}; it's compiled and installed by default.
8242 The exact path to this utility is determined when configuring the package.
8243 It is @file{@var{prefix}/libexec/rmt}, where @var{prefix} stands for
8244 your installation prefix. This location may also be overridden at
8245 runtime by using @option{rmt-command=@var{command}} option (@xref{Option Summary,
8246 ---rmt-command}, for detailed description of this option. @xref{Remote
8247 Tape Server}, for the description of @command{rmt} command).
8249 If this option is not given, but the environment variable @env{TAPE}
8250 is set, its value is used; otherwise, old versions of @command{tar}
8251 used a default archive name (which was picked when @command{tar} was
8252 compiled). The default is normally set up to be the @dfn{first} tape
8253 drive or other transportable I/O medium on the system.
8255 Starting with version 1.11.5, @GNUTAR{} uses
8256 standard input and standard output as the default device, and I will
8257 not try anymore supporting automatic device detection at installation
8258 time. This was failing really in too many cases, it was hopeless.
8259 This is now completely left to the installer to override standard
8260 input and standard output for default device, if this seems
8261 preferable. Further, I think @emph{most} actual usages of
8262 @command{tar} are done with pipes or disks, not really tapes,
8263 cartridges or diskettes.
8265 Some users think that using standard input and output is running
8266 after trouble. This could lead to a nasty surprise on your screen if
8267 you forget to specify an output file name---especially if you are going
8268 through a network or terminal server capable of buffering large amounts
8269 of output. We had so many bug reports in that area of configuring
8270 default tapes automatically, and so many contradicting requests, that
8271 we finally consider the problem to be portably intractable. We could
8272 of course use something like @samp{/dev/tape} as a default, but this
8273 is @emph{also} running after various kind of trouble, going from hung
8274 processes to accidental destruction of real tapes. After having seen
8275 all this mess, using standard input and output as a default really
8276 sounds like the only clean choice left, and a very useful one too.
8278 @GNUTAR{} reads and writes archive in records, I
8279 suspect this is the main reason why block devices are preferred over
8280 character devices. Most probably, block devices are more efficient
8281 too. The installer could also check for @samp{DEFTAPE} in
8282 @file{<sys/mtio.h>}.
8285 @opindex force-local, short description
8287 Archive file is local even if it contains a colon.
8289 @opindex rsh-command
8290 @item --rsh-command=@var{command}
8291 Use remote @var{command} instead of @command{rsh}. This option exists
8292 so that people who use something other than the standard @command{rsh}
8293 (e.g., a Kerberized @command{rsh}) can access a remote device.
8295 When this command is not used, the shell command found when
8296 the @command{tar} program was installed is used instead. This is
8297 the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh},
8298 @file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}.
8299 The installer may have overridden this by defining the environment
8300 variable @env{RSH} @emph{at installation time}.
8303 Specify drive and density.
8305 @opindex multi-volume, short description
8307 @itemx --multi-volume
8308 Create/list/extract multi-volume archive.
8310 This option causes @command{tar} to write a @dfn{multi-volume} archive---one
8311 that may be larger than will fit on the medium used to hold it.
8312 @xref{Multi-Volume Archives}.
8314 @opindex tape-length, short description
8316 @itemx --tape-length=@var{num}
8317 Change tape after writing @var{num} x 1024 bytes.
8319 This option might be useful when your tape drivers do not properly
8320 detect end of physical tapes. By being slightly conservative on the
8321 maximum tape length, you might avoid the problem entirely.
8323 @opindex info-script, short description
8324 @opindex new-volume-script, short description
8326 @itemx --info-script=@var{file}
8327 @itemx --new-volume-script=@var{file}
8328 Execute @file{file} at end of each tape. This implies
8329 @option{--multi-volume} (@option{-M}). @xref{info-script}, for a detailed
8330 description of this option.
8333 @node Remote Tape Server
8334 @section The Remote Tape Server
8336 @cindex remote tape drive
8338 In order to access the tape drive on a remote machine, @command{tar}
8339 uses the remote tape server written at the University of California at
8340 Berkeley. The remote tape server must be installed as
8341 @file{@var{prefix}/libexec/rmt} on any machine whose tape drive you
8342 want to use. @command{tar} calls @command{rmt} by running an
8343 @command{rsh} or @command{remsh} to the remote machine, optionally
8344 using a different login name if one is supplied.
8346 A copy of the source for the remote tape server is provided. It is
8347 Copyright @copyright{} 1983 by the Regents of the University of
8348 California, but can be freely distributed. It is compiled and
8349 installed by default.
8351 @cindex absolute file names
8352 Unless you use the @option{--absolute-names} (@option{-P}) option,
8353 @GNUTAR{} will not allow you to create an archive that contains
8354 absolute file names (a file name beginning with @samp{/}.) If you try,
8355 @command{tar} will automatically remove the leading @samp{/} from the
8356 file names it stores in the archive. It will also type a warning
8357 message telling you what it is doing.
8359 When reading an archive that was created with a different
8360 @command{tar} program, @GNUTAR{} automatically
8361 extracts entries in the archive which have absolute file names as if
8362 the file names were not absolute. This is an important feature. A
8363 visitor here once gave a @command{tar} tape to an operator to restore;
8364 the operator used Sun @command{tar} instead of @GNUTAR{},
8365 and the result was that it replaced large portions of
8366 our @file{/bin} and friends with versions from the tape; needless to
8367 say, we were unhappy about having to recover the file system from
8370 For example, if the archive contained a file @file{/usr/bin/computoy},
8371 @GNUTAR{} would extract the file to @file{usr/bin/computoy},
8372 relative to the current directory. If you want to extract the files in
8373 an archive to the same absolute names that they had when the archive
8374 was created, you should do a @samp{cd /} before extracting the files
8375 from the archive, or you should either use the @option{--absolute-names}
8376 option, or use the command @samp{tar -C / @dots{}}.
8378 @cindex Ultrix 3.1 and write failure
8379 Some versions of Unix (Ultrix 3.1 is known to have this problem),
8380 can claim that a short write near the end of a tape succeeded,
8381 when it actually failed. This will result in the -M option not
8382 working correctly. The best workaround at the moment is to use a
8383 significantly larger blocking factor than the default 20.
8385 In order to update an archive, @command{tar} must be able to backspace the
8386 archive in order to reread or rewrite a record that was just read (or
8387 written). This is currently possible only on two kinds of files: normal
8388 disk files (or any other file that can be backspaced with @samp{lseek}),
8389 and industry-standard 9-track magnetic tape (or any other kind of tape
8390 that can be backspaced with the @code{MTIOCTOP} @code{ioctl}.
8392 This means that the @option{--append}, @option{--concatenate}, and
8393 @option{--delete} commands will not work on any other kind of file.
8394 Some media simply cannot be backspaced, which means these commands and
8395 options will never be able to work on them. These non-backspacing
8396 media include pipes and cartridge tape drives.
8398 Some other media can be backspaced, and @command{tar} will work on them
8399 once @command{tar} is modified to do so.
8401 Archives created with the @option{--multi-volume}, @option{--label}, and
8402 @option{--incremental} (@option{-G}) options may not be readable by other version
8403 of @command{tar}. In particular, restoring a file that was split over
8404 a volume boundary will require some careful work with @command{dd}, if
8405 it can be done at all. Other versions of @command{tar} may also create
8406 an empty file whose name is that of the volume header. Some versions
8407 of @command{tar} may create normal files instead of directories archived
8408 with the @option{--incremental} (@option{-G}) option.
8410 @node Common Problems and Solutions
8411 @section Some Common Problems and their Solutions
8418 no such file or directory
8421 errors from @command{tar}:
8422 directory checksum error
8425 errors from media/system:
8436 @dfn{Block} and @dfn{record} terminology is rather confused, and it
8437 is also confusing to the expert reader. On the other hand, readers
8438 who are new to the field have a fresh mind, and they may safely skip
8439 the next two paragraphs, as the remainder of this manual uses those
8440 two terms in a quite consistent way.
8442 John Gilmore, the writer of the public domain @command{tar} from which
8443 @GNUTAR{} was originally derived, wrote (June 1995):
8446 The nomenclature of tape drives comes from IBM, where I believe
8447 they were invented for the IBM 650 or so. On IBM mainframes, what
8448 is recorded on tape are tape blocks. The logical organization of
8449 data is into records. There are various ways of putting records into
8450 blocks, including @code{F} (fixed sized records), @code{V} (variable
8451 sized records), @code{FB} (fixed blocked: fixed size records, @var{n}
8452 to a block), @code{VB} (variable size records, @var{n} to a block),
8453 @code{VSB} (variable spanned blocked: variable sized records that can
8454 occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=}
8455 parameter specified this to the operating system.
8457 The Unix man page on @command{tar} was totally confused about this.
8458 When I wrote @code{PD TAR}, I used the historically correct terminology
8459 (@command{tar} writes data records, which are grouped into blocks).
8460 It appears that the bogus terminology made it into @acronym{POSIX} (no surprise
8461 here), and now Fran@,{c}ois has migrated that terminology back
8462 into the source code too.
8465 The term @dfn{physical block} means the basic transfer chunk from or
8466 to a device, after which reading or writing may stop without anything
8467 being lost. In this manual, the term @dfn{block} usually refers to
8468 a disk physical block, @emph{assuming} that each disk block is 512
8469 bytes in length. It is true that some disk devices have different
8470 physical blocks, but @command{tar} ignore these differences in its own
8471 format, which is meant to be portable, so a @command{tar} block is always
8472 512 bytes in length, and @dfn{block} always mean a @command{tar} block.
8473 The term @dfn{logical block} often represents the basic chunk of
8474 allocation of many disk blocks as a single entity, which the operating
8475 system treats somewhat atomically; this concept is only barely used
8478 The term @dfn{physical record} is another way to speak of a physical
8479 block, those two terms are somewhat interchangeable. In this manual,
8480 the term @dfn{record} usually refers to a tape physical block,
8481 @emph{assuming} that the @command{tar} archive is kept on magnetic tape.
8482 It is true that archives may be put on disk or used with pipes,
8483 but nevertheless, @command{tar} tries to read and write the archive one
8484 @dfn{record} at a time, whatever the medium in use. One record is made
8485 up of an integral number of blocks, and this operation of putting many
8486 disk blocks into a single tape block is called @dfn{reblocking}, or
8487 more simply, @dfn{blocking}. The term @dfn{logical record} refers to
8488 the logical organization of many characters into something meaningful
8489 to the application. The term @dfn{unit record} describes a small set
8490 of characters which are transmitted whole to or by the application,
8491 and often refers to a line of text. Those two last terms are unrelated
8492 to what we call a @dfn{record} in @GNUTAR{}.
8494 When writing to tapes, @command{tar} writes the contents of the archive
8495 in chunks known as @dfn{records}. To change the default blocking
8496 factor, use the @option{--blocking-factor=@var{512-size}} (@option{-b
8497 @var{512-size}}) option. Each record will then be composed of
8498 @var{512-size} blocks. (Each @command{tar} block is 512 bytes.
8499 @xref{Standard}.) Each file written to the archive uses at least one
8500 full record. As a result, using a larger record size can result in
8501 more wasted space for small files. On the other hand, a larger record
8502 size can often be read and written much more efficiently.
8504 Further complicating the problem is that some tape drives ignore the
8505 blocking entirely. For these, a larger record size can still improve
8506 performance (because the software layers above the tape drive still
8507 honor the blocking), but not as dramatically as on tape drives that
8510 When reading an archive, @command{tar} can usually figure out the
8511 record size on itself. When this is the case, and a non-standard
8512 record size was used when the archive was created, @command{tar} will
8513 print a message about a non-standard blocking factor, and then operate
8514 normally. On some tape devices, however, @command{tar} cannot figure
8515 out the record size itself. On most of those, you can specify a
8516 blocking factor (with @option{--blocking-factor}) larger than the
8517 actual blocking factor, and then use the @option{--read-full-records}
8518 (@option{-B}) option. (If you specify a blocking factor with
8519 @option{--blocking-factor} and don't use the
8520 @option{--read-full-records} option, then @command{tar} will not
8521 attempt to figure out the recording size itself.) On some devices,
8522 you must always specify the record size exactly with
8523 @option{--blocking-factor} when reading, because @command{tar} cannot
8524 figure it out. In any case, use @option{--list} (@option{-t}) before
8525 doing any extractions to see whether @command{tar} is reading the archive
8528 @command{tar} blocks are all fixed size (512 bytes), and its scheme for
8529 putting them into records is to put a whole number of them (one or
8530 more) into each record. @command{tar} records are all the same size;
8531 at the end of the file there's a block containing all zeros, which
8532 is how you tell that the remainder of the last record(s) are garbage.
8534 In a standard @command{tar} file (no options), the block size is 512
8535 and the record size is 10240, for a blocking factor of 20. What the
8536 @option{--blocking-factor} option does is sets the blocking factor,
8537 changing the record size while leaving the block size at 512 bytes.
8538 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives;
8539 most tape drives these days prefer much bigger records in order to
8540 stream and not waste tape. When writing tapes for myself, some tend
8541 to use a factor of the order of 2048, say, giving a record size of
8542 around one megabyte.
8544 If you use a blocking factor larger than 20, older @command{tar}
8545 programs might not be able to read the archive, so we recommend this
8546 as a limit to use in practice. @GNUTAR{}, however,
8547 will support arbitrarily large record sizes, limited only by the
8548 amount of virtual memory or the physical characteristics of the tape
8552 * Format Variations:: Format Variations
8553 * Blocking Factor:: The Blocking Factor of an Archive
8556 @node Format Variations
8557 @subsection Format Variations
8558 @cindex Format Parameters
8559 @cindex Format Options
8560 @cindex Options, archive format specifying
8561 @cindex Options, format specifying
8564 Format parameters specify how an archive is written on the archive
8565 media. The best choice of format parameters will vary depending on
8566 the type and number of files being archived, and on the media used to
8569 To specify format parameters when accessing or creating an archive,
8570 you can use the options described in the following sections.
8571 If you do not specify any format parameters, @command{tar} uses
8572 default parameters. You cannot modify a compressed archive.
8573 If you create an archive with the @option{--blocking-factor} option
8574 specified (@pxref{Blocking Factor}), you must specify that
8575 blocking-factor when operating on the archive. @xref{Formats}, for other
8576 examples of format parameter considerations.
8578 @node Blocking Factor
8579 @subsection The Blocking Factor of an Archive
8580 @cindex Blocking Factor
8582 @cindex Number of blocks per record
8583 @cindex Number of bytes per record
8584 @cindex Bytes per record
8585 @cindex Blocks per record
8588 @opindex blocking-factor
8589 The data in an archive is grouped into blocks, which are 512 bytes.
8590 Blocks are read and written in whole number multiples called
8591 @dfn{records}. The number of blocks in a record (ie. the size of a
8592 record in units of 512 bytes) is called the @dfn{blocking factor}.
8593 The @option{--blocking-factor=@var{512-size}} (@option{-b
8594 @var{512-size}}) option specifies the blocking factor of an archive.
8595 The default blocking factor is typically 20 (i.e., 10240 bytes), but
8596 can be specified at installation. To find out the blocking factor of
8597 an existing archive, use @samp{tar --list --file=@var{archive-name}}.
8598 This may not work on some devices.
8600 Records are separated by gaps, which waste space on the archive media.
8601 If you are archiving on magnetic tape, using a larger blocking factor
8602 (and therefore larger records) provides faster throughput and allows you
8603 to fit more data on a tape (because there are fewer gaps). If you are
8604 archiving on cartridge, a very large blocking factor (say 126 or more)
8605 greatly increases performance. A smaller blocking factor, on the other
8606 hand, may be useful when archiving small files, to avoid archiving lots
8607 of nulls as @command{tar} fills out the archive to the end of the record.
8608 In general, the ideal record size depends on the size of the
8609 inter-record gaps on the tape you are using, and the average size of the
8610 files you are archiving. @xref{create}, for information on
8613 @FIXME{Need example of using a cartridge with blocking factor=126 or more.}
8615 Archives with blocking factors larger than 20 cannot be read
8616 by very old versions of @command{tar}, or by some newer versions
8617 of @command{tar} running on old machines with small address spaces.
8618 With @GNUTAR{}, the blocking factor of an archive is limited
8619 only by the maximum record size of the device containing the archive,
8620 or by the amount of available virtual memory.
8622 Also, on some systems, not using adequate blocking factors, as sometimes
8623 imposed by the device drivers, may yield unexpected diagnostics. For
8624 example, this has been reported:
8627 Cannot write to /dev/dlt: Invalid argument
8631 In such cases, it sometimes happen that the @command{tar} bundled by
8632 the system is aware of block size idiosyncrasies, while @GNUTAR{}
8633 requires an explicit specification for the block size,
8634 which it cannot guess. This yields some people to consider
8635 @GNUTAR{} is misbehaving, because by comparison,
8636 @cite{the bundle @command{tar} works OK}. Adding @w{@kbd{-b 256}},
8637 for example, might resolve the problem.
8639 If you use a non-default blocking factor when you create an archive, you
8640 must specify the same blocking factor when you modify that archive. Some
8641 archive devices will also require you to specify the blocking factor when
8642 reading that archive, however this is not typically the case. Usually, you
8643 can use @option{--list} (@option{-t}) without specifying a blocking factor---@command{tar}
8644 reports a non-default record size and then lists the archive members as
8645 it would normally. To extract files from an archive with a non-standard
8646 blocking factor (particularly if you're not sure what the blocking factor
8647 is), you can usually use the @option{--read-full-records} (@option{-B}) option while
8648 specifying a blocking factor larger then the blocking factor of the archive
8649 (ie. @samp{tar --extract --read-full-records --blocking-factor=300}.
8650 @xref{list}, for more information on the @option{--list} (@option{-t})
8651 operation. @xref{Reading}, for a more detailed explanation of that option.
8654 @item --blocking-factor=@var{number}
8655 @itemx -b @var{number}
8656 Specifies the blocking factor of an archive. Can be used with any
8657 operation, but is usually not necessary with @option{--list} (@option{-t}).
8663 @item -b @var{blocks}
8664 @itemx --blocking-factor=@var{blocks}
8665 Set record size to @math{@var{blocks} * 512} bytes.
8667 This option is used to specify a @dfn{blocking factor} for the archive.
8668 When reading or writing the archive, @command{tar}, will do reads and writes
8669 of the archive in records of @math{@var{block}*512} bytes. This is true
8670 even when the archive is compressed. Some devices requires that all
8671 write operations be a multiple of a certain size, and so, @command{tar}
8672 pads the archive out to the next record boundary.
8674 The default blocking factor is set when @command{tar} is compiled, and is
8675 typically 20. Blocking factors larger than 20 cannot be read by very
8676 old versions of @command{tar}, or by some newer versions of @command{tar}
8677 running on old machines with small address spaces.
8679 With a magnetic tape, larger records give faster throughput and fit
8680 more data on a tape (because there are fewer inter-record gaps).
8681 If the archive is in a disk file or a pipe, you may want to specify
8682 a smaller blocking factor, since a large one will result in a large
8683 number of null bytes at the end of the archive.
8685 When writing cartridge or other streaming tapes, a much larger
8686 blocking factor (say 126 or more) will greatly increase performance.
8687 However, you must specify the same blocking factor when reading or
8688 updating the archive.
8690 Apparently, Exabyte drives have a physical block size of 8K bytes.
8691 If we choose our blocksize as a multiple of 8k bytes, then the problem
8692 seems to disappear. Id est, we are using block size of 112 right
8693 now, and we haven't had the problem since we switched@dots{}
8695 With @GNUTAR{} the blocking factor is limited only
8696 by the maximum record size of the device containing the archive, or by
8697 the amount of available virtual memory.
8699 However, deblocking or reblocking is virtually avoided in a special
8700 case which often occurs in practice, but which requires all the
8701 following conditions to be simultaneously true:
8704 the archive is subject to a compression option,
8706 the archive is not handled through standard input or output, nor
8707 redirected nor piped,
8709 the archive is directly handled to a local disk, instead of any special
8712 @option{--blocking-factor} is not explicitly specified on the @command{tar}
8716 If the output goes directly to a local disk, and not through
8717 stdout, then the last write is not extended to a full record size.
8718 Otherwise, reblocking occurs. Here are a few other remarks on this
8724 @command{gzip} will complain about trailing garbage if asked to
8725 uncompress a compressed archive on tape, there is an option to turn
8726 the message off, but it breaks the regularity of simply having to use
8727 @samp{@var{prog} -d} for decompression. It would be nice if gzip was
8728 silently ignoring any number of trailing zeros. I'll ask Jean-loup
8729 Gailly, by sending a copy of this message to him.
8732 @command{compress} does not show this problem, but as Jean-loup pointed
8733 out to Michael, @samp{compress -d} silently adds garbage after
8734 the result of decompression, which tar ignores because it already
8735 recognized its end-of-file indicator. So this bug may be safely
8739 @samp{gzip -d -q} will be silent about the trailing zeros indeed,
8740 but will still return an exit status of 2 which tar reports in turn.
8741 @command{tar} might ignore the exit status returned, but I hate doing
8742 that, as it weakens the protection @command{tar} offers users against
8743 other possible problems at decompression time. If @command{gzip} was
8744 silently skipping trailing zeros @emph{and} also avoiding setting the
8745 exit status in this innocuous case, that would solve this situation.
8748 @command{tar} should become more solid at not stopping to read a pipe at
8749 the first null block encountered. This inelegantly breaks the pipe.
8750 @command{tar} should rather drain the pipe out before exiting itself.
8753 @opindex ignore-zeros, short description
8755 @itemx --ignore-zeros
8756 Ignore blocks of zeros in archive (means EOF).
8758 The @option{--ignore-zeros} (@option{-i}) option causes @command{tar} to ignore blocks
8759 of zeros in the archive. Normally a block of zeros indicates the
8760 end of the archive, but when reading a damaged archive, or one which
8761 was created by concatenating several archives together, this option
8762 allows @command{tar} to read the entire archive. This option is not on
8763 by default because many versions of @command{tar} write garbage after
8766 Note that this option causes @command{tar} to read to the end of the
8767 archive file, which may sometimes avoid problems when multiple files
8768 are stored on a single physical tape.
8770 @opindex read-full-records, short description
8772 @itemx --read-full-records
8773 Reblock as we read (for reading 4.2BSD pipes).
8775 If @option{--read-full-records} is used, @command{tar}
8776 will not panic if an attempt to read a record from the archive does
8777 not return a full record. Instead, @command{tar} will keep reading
8778 until it has obtained a full
8781 This option is turned on by default when @command{tar} is reading
8782 an archive from standard input, or from a remote machine. This is
8783 because on BSD Unix systems, a read of a pipe will return however
8784 much happens to be in the pipe, even if it is less than @command{tar}
8785 requested. If this option was not used, @command{tar} would fail as
8786 soon as it read an incomplete record from the pipe.
8788 This option is also useful with the commands for updating an archive.
8794 @FIXME{Appropriate options should be moved here from elsewhere.}
8796 @cindex blocking factor
8797 @cindex tape blocking
8799 When handling various tapes or cartridges, you have to take care of
8800 selecting a proper blocking, that is, the number of disk blocks you
8801 put together as a single tape block on the tape, without intervening
8802 tape gaps. A @dfn{tape gap} is a small landing area on the tape
8803 with no information on it, used for decelerating the tape to a
8804 full stop, and for later regaining the reading or writing speed.
8805 When the tape driver starts reading a record, the record has to
8806 be read whole without stopping, as a tape gap is needed to stop the
8807 tape motion without loosing information.
8809 @cindex Exabyte blocking
8810 @cindex DAT blocking
8811 Using higher blocking (putting more disk blocks per tape block) will use
8812 the tape more efficiently as there will be less tape gaps. But reading
8813 such tapes may be more difficult for the system, as more memory will be
8814 required to receive at once the whole record. Further, if there is a
8815 reading error on a huge record, this is less likely that the system will
8816 succeed in recovering the information. So, blocking should not be too
8817 low, nor it should be too high. @command{tar} uses by default a blocking of
8818 20 for historical reasons, and it does not really matter when reading or
8819 writing to disk. Current tape technology would easily accommodate higher
8820 blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs.
8821 We were told that for some DLT drives, the blocking should be a multiple
8822 of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance.
8823 Other manufacturers may use different recommendations for the same tapes.
8824 This might also depends of the buffering techniques used inside modern
8825 tape controllers. Some imposes a minimum blocking, or a maximum blocking.
8826 Others request blocking to be some exponent of two.
8828 So, there is no fixed rule for blocking. But blocking at read time
8829 should ideally be the same as blocking used at write time. At one place
8830 I know, with a wide variety of equipment, they found it best to use a
8831 blocking of 32 to guarantee that their tapes are fully interchangeable.
8833 I was also told that, for recycled tapes, prior erasure (by the same
8834 drive unit that will be used to create the archives) sometimes lowers
8835 the error rates observed at rewriting time.
8837 I might also use @option{--number-blocks} instead of
8838 @option{--block-number}, so @option{--block} will then expand to
8839 @option{--blocking-factor} unambiguously.
8842 @section Many Archives on One Tape
8844 @FIXME{Appropriate options should be moved here from elsewhere.}
8846 @findex ntape @r{device}
8847 Most tape devices have two entries in the @file{/dev} directory, or
8848 entries that come in pairs, which differ only in the minor number for
8849 this device. Let's take for example @file{/dev/tape}, which often
8850 points to the only or usual tape device of a given system. There might
8851 be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler
8852 name is the @emph{rewinding} version of the device, while the name
8853 having @samp{nr} in it is the @emph{no rewinding} version of the same
8856 A rewinding tape device will bring back the tape to its beginning point
8857 automatically when this device is opened or closed. Since @command{tar}
8858 opens the archive file before using it and closes it afterwards, this
8859 means that a simple:
8862 $ @kbd{tar cf /dev/tape @var{directory}}
8866 will reposition the tape to its beginning both prior and after saving
8867 @var{directory} contents to it, thus erasing prior tape contents and
8868 making it so that any subsequent write operation will destroy what has
8871 @cindex tape positioning
8872 So, a rewinding device is normally meant to hold one and only one file.
8873 If you want to put more than one @command{tar} archive on a given tape, you
8874 will need to avoid using the rewinding version of the tape device. You
8875 will also have to pay special attention to tape positioning. Errors in
8876 positioning may overwrite the valuable data already on your tape. Many
8877 people, burnt by past experiences, will only use rewinding devices and
8878 limit themselves to one file per tape, precisely to avoid the risk of
8879 such errors. Be fully aware that writing at the wrong position on a
8880 tape loses all information past this point and most probably until the
8881 end of the tape, and this destroyed information @emph{cannot} be
8884 To save @var{directory-1} as a first archive at the beginning of a
8885 tape, and leave that tape ready for a second archive, you should use:
8888 $ @kbd{mt -f /dev/nrtape rewind}
8889 $ @kbd{tar cf /dev/nrtape @var{directory-1}}
8893 @dfn{Tape marks} are special magnetic patterns written on the tape
8894 media, which are later recognizable by the reading hardware. These
8895 marks are used after each file, when there are many on a single tape.
8896 An empty file (that is to say, two tape marks in a row) signal the
8897 logical end of the tape, after which no file exist. Usually,
8898 non-rewinding tape device drivers will react to the close request issued
8899 by @command{tar} by first writing two tape marks after your archive, and by
8900 backspacing over one of these. So, if you remove the tape at that time
8901 from the tape drive, it is properly terminated. But if you write
8902 another file at the current position, the second tape mark will be
8903 erased by the new information, leaving only one tape mark between files.
8905 So, you may now save @var{directory-2} as a second archive after the
8906 first on the same tape by issuing the command:
8909 $ @kbd{tar cf /dev/nrtape @var{directory-2}}
8913 and so on for all the archives you want to put on the same tape.
8915 Another usual case is that you do not write all the archives the same
8916 day, and you need to remove and store the tape between two archive
8917 sessions. In general, you must remember how many files are already
8918 saved on your tape. Suppose your tape already has 16 files on it, and
8919 that you are ready to write the 17th. You have to take care of skipping
8920 the first 16 tape marks before saving @var{directory-17}, say, by using
8924 $ @kbd{mt -f /dev/nrtape rewind}
8925 $ @kbd{mt -f /dev/nrtape fsf 16}
8926 $ @kbd{tar cf /dev/nrtape @var{directory-17}}
8929 In all the previous examples, we put aside blocking considerations, but
8930 you should do the proper things for that as well. @xref{Blocking}.
8933 * Tape Positioning:: Tape Positions and Tape Marks
8934 * mt:: The @command{mt} Utility
8937 @node Tape Positioning
8938 @subsection Tape Positions and Tape Marks
8941 Just as archives can store more than one file from the file system,
8942 tapes can store more than one archive file. To keep track of where
8943 archive files (or any other type of file stored on tape) begin and
8944 end, tape archive devices write magnetic @dfn{tape marks} on the
8945 archive media. Tape drives write one tape mark between files,
8946 two at the end of all the file entries.
8948 If you think of data as a series of records "rrrr"'s, and tape marks as
8949 "*"'s, a tape might look like the following:
8952 rrrr*rrrrrr*rrrrr*rr*rrrrr**-------------------------
8955 Tape devices read and write tapes using a read/write @dfn{tape
8956 head}---a physical part of the device which can only access one
8957 point on the tape at a time. When you use @command{tar} to read or
8958 write archive data from a tape device, the device will begin reading
8959 or writing from wherever on the tape the tape head happens to be,
8960 regardless of which archive or what part of the archive the tape
8961 head is on. Before writing an archive, you should make sure that no
8962 data on the tape will be overwritten (unless it is no longer needed).
8963 Before reading an archive, you should make sure the tape head is at
8964 the beginning of the archive you want to read. You can do it manually
8965 via @code{mt} utility (@pxref{mt}). The @code{restore} script does
8966 that automatically (@pxref{Scripted Restoration}).
8968 If you want to add new archive file entries to a tape, you should
8969 advance the tape to the end of the existing file entries, backspace
8970 over the last tape mark, and write the new archive file. If you were
8971 to add two archives to the example above, the tape might look like the
8975 rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------
8979 @subsection The @command{mt} Utility
8982 @FIXME{Is it true that this only works on non-block devices?
8983 should explain the difference, (fixed or variable).}
8984 @xref{Blocking Factor}.
8986 You can use the @command{mt} utility to advance or rewind a tape past a
8987 specified number of archive files on the tape. This will allow you
8988 to move to the beginning of an archive before extracting or reading
8989 it, or to the end of all the archives before writing a new one.
8990 @FIXME{Why isn't there an "advance 'til you find two tape marks
8993 The syntax of the @command{mt} command is:
8996 @kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]}
8999 where @var{tapename} is the name of the tape device, @var{number} is
9000 the number of times an operation is performed (with a default of one),
9001 and @var{operation} is one of the following:
9003 @FIXME{is there any use for record operations?}
9008 Writes @var{number} tape marks at the current position on the tape.
9011 Moves tape position forward @var{number} files.
9014 Moves tape position back @var{number} files.
9017 Rewinds the tape. (Ignores @var{number}).
9021 Rewinds the tape and takes the tape device off-line. (Ignores @var{number}).
9024 Prints status information about the tape unit.
9028 @FIXME{Is there a better way to frob the spacing on the list?}
9030 If you don't specify a @var{tapename}, @command{mt} uses the environment
9031 variable @env{TAPE}; if @env{TAPE} is not set, @command{mt} uses the device
9034 @command{mt} returns a 0 exit status when the operation(s) were
9035 successful, 1 if the command was unrecognized, and 2 if an operation
9038 @node Using Multiple Tapes
9039 @section Using Multiple Tapes
9042 Often you might want to write a large archive, one larger than will fit
9043 on the actual tape you are using. In such a case, you can run multiple
9044 @command{tar} commands, but this can be inconvenient, particularly if you
9045 are using options like @option{--exclude=@var{pattern}} or dumping entire file systems.
9046 Therefore, @command{tar} supports multiple tapes automatically.
9048 Use @option{--multi-volume} (@option{-M}) on the command line, and
9049 then @command{tar} will, when it reaches the end of the tape, prompt
9050 for another tape, and continue the archive. Each tape will have an
9051 independent archive, and can be read without needing the other. (As
9052 an exception to this, the file that @command{tar} was archiving when
9053 it ran out of tape will usually be split between the two archives; in
9054 this case you need to extract from the first archive, using
9055 @option{--multi-volume}, and then put in the second tape when
9056 prompted, so @command{tar} can restore both halves of the file.)
9058 @GNUTAR{} multi-volume archives do not use a truly portable format.
9059 You need @GNUTAR{} at both ends to process them properly.
9061 When prompting for a new tape, @command{tar} accepts any of the following
9066 Request @command{tar} to explain possible responses
9068 Request @command{tar} to exit immediately.
9069 @item n @var{file-name}
9070 Request @command{tar} to write the next volume on the file @var{file-name}.
9072 Request @command{tar} to run a subshell. This option can be disabled
9073 by giving @option{--restrict} command line option to @command{tar}.
9075 Request @command{tar} to begin writing the next volume.
9078 (You should only type @samp{y} after you have changed the tape;
9079 otherwise @command{tar} will write over the volume it just finished.)
9081 @cindex End-of-archive info script
9083 @anchor{info-script}
9084 @opindex info-script
9085 @opindex new-volume-script
9086 If you want more elaborate behavior than this, give @command{tar} the
9087 @option{--info-script=@var{script-name}}
9088 (@option{--new-volume-script=@var{script-name}}, @option{-F
9089 @var{script-name}}) option. The file @var{script-name} is expected to
9090 be a program (or shell script) to be run instead of the normal
9091 prompting procedure. It is executed without any command line
9092 arguments. Additional data is passed to it via the following
9093 environment variables:
9096 @vrindex TAR_VERSION, info script environment variable
9098 @GNUTAR{} version number.
9100 @vrindex TAR_ARCHIVE, info script environment variable
9102 The name of the archive @command{tar} is processing.
9104 @vrindex TAR_VOLUME, info script environment variable
9106 Ordinal number of the volume @command{tar} is about to start.
9108 @vrindex TAR_SUBCOMMAND, info script environment variable
9109 @item TAR_SUBCOMMAND
9110 Short option describing the operation @command{tar} is executed.
9111 @xref{Operations}, for a complete list of subcommand options.
9113 @vrindex TAR_FORMAT, info script environment variable
9115 Format of the archive being processed. @xref{Formats}, for a complete
9116 list of archive format names.
9119 The info script can instruct @command{tar} to use new archive name,
9120 by writing in to file descriptor 3 (see below for an
9123 If the info script fails, @command{tar} exits; otherwise, it begins
9124 writing the next volume.
9126 The method @command{tar} uses to detect end of tape is not perfect, and
9127 fails on some operating systems or on some devices. You can use the
9128 @option{--tape-length=@var{size}} (@option{-L @var{size}}) option if
9129 @command{tar} can't detect the end of the tape itself. This option
9130 selects @option{--multi-volume} (@option{-M}) automatically. The
9131 @var{size} argument should then be the usable size of the tape in
9132 units of 1024 bytes. But for many devices, and floppy disks in
9133 particular, this option is never required for real, as far as we know.
9135 @cindex Volume number file
9139 The volume number used by @command{tar} in its tape-change prompt
9140 can be changed; if you give the
9141 @option{--volno-file=@var{file-of-number}} option, then
9142 @var{file-of-number} should be an unexisting file to be created, or
9143 else, a file already containing a decimal number. That number will be
9144 used as the volume number of the first volume written. When
9145 @command{tar} is finished, it will rewrite the file with the
9146 now-current volume number. (This does not change the volume number
9147 written on a tape label, as per @ref{label}, it @emph{only} affects
9148 the number used in the prompt.)
9150 If you want @command{tar} to cycle through a series of files or tape
9151 drives, there are three approaches to choose from. First of all, you
9152 can give @command{tar} multiple @option{--file} options. In this case
9153 the specified files will be used, in sequence, as the successive
9154 volumes of the archive. Only when the first one in the sequence needs
9155 to be used again will @command{tar} prompt for a tape change (or run
9156 the info script). Secondly, you can use the @samp{n} response to the
9157 tape-change prompt, and, finally, you can use an info script, that
9158 writes new archive name to file descriptor. The following example
9159 illustrates this approach:
9164 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
9166 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`
9167 case $TAR_SUBCOMMAND in
9169 -d|-x|-t) test -r $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME || exit 1
9174 echo $@{name:-$TAR_ARCHIVE@}-$TAR_VOLUME >&3
9178 Each volume of a multi-volume archive is an independent @command{tar}
9179 archive, complete in itself. For example, you can list or extract any
9180 volume alone; just don't specify @option{--multi-volume}
9181 (@option{-M}). However, if one file in the archive is split across
9182 volumes, the only way to extract it successfully is with a
9183 multi-volume extract command @option{--extract --multi-volume}
9184 (@option{-xM}) starting on or before the volume where the file begins.
9186 For example, let's presume someone has two tape drives on a system
9187 named @file{/dev/tape0} and @file{/dev/tape1}. For having @GNUTAR{}
9188 to switch to the second drive when it needs to write the
9189 second tape, and then back to the first tape, etc., just do either of:
9192 $ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}}
9193 $ @kbd{tar cMff /dev/tape0 /dev/tape1 @var{files}}
9197 * Multi-Volume Archives:: Archives Longer than One Tape or Disk
9198 * Tape Files:: Tape Files
9199 * Tarcat:: Concatenate Volumes into a Single Archive
9203 @node Multi-Volume Archives
9204 @subsection Archives Longer than One Tape or Disk
9205 @cindex Multi-volume archives
9208 @opindex multi-volume
9209 To create an archive that is larger than will fit on a single unit of
9210 the media, use the @option{--multi-volume} (@option{-M}) option in conjunction with
9211 the @option{--create} option (@pxref{create}). A @dfn{multi-volume}
9212 archive can be manipulated like any other archive (provided the
9213 @option{--multi-volume} option is specified), but is stored on more
9214 than one tape or disk.
9216 When you specify @option{--multi-volume}, @command{tar} does not report an
9217 error when it comes to the end of an archive volume (when reading), or
9218 the end of the media (when writing). Instead, it prompts you to load
9219 a new storage volume. If the archive is on a magnetic tape, you
9220 should change tapes when you see the prompt; if the archive is on a
9221 floppy disk, you should change disks; etc.
9223 You can read each individual volume of a multi-volume archive as if it
9224 were an archive by itself. For example, to list the contents of one
9225 volume, use @option{--list}, without @option{--multi-volume} specified.
9226 To extract an archive member from one volume (assuming it is described
9227 that volume), use @option{--extract}, again without
9228 @option{--multi-volume}.
9230 If an archive member is split across volumes (ie. its entry begins on
9231 one volume of the media and ends on another), you need to specify
9232 @option{--multi-volume} to extract it successfully. In this case, you
9233 should load the volume where the archive member starts, and use
9234 @samp{tar --extract --multi-volume}---@command{tar} will prompt for later
9235 volumes as it needs them. @xref{extracting archives}, for more
9236 information about extracting archives.
9238 @option{--info-script=@var{script-name}}
9239 (@option{--new-volume-script=@var{script-name}}, @option{-F
9240 @var{script-name}}) (@pxref{info-script}) is like
9241 @option{--multi-volume} (@option{-M}), except that @command{tar} does
9242 not prompt you directly to change media volumes when a volume is
9243 full---instead, @command{tar} runs commands you have stored in
9244 @var{script-name}. For example, this option can be used to eject
9245 cassettes, or to broadcast messages such as @samp{Someone please come
9246 change my tape} when performing unattended backups. When
9247 @var{script-name} is done, @command{tar} will assume that the media
9250 Multi-volume archives can be modified like any other archive. To add
9251 files to a multi-volume archive, you need to only mount the last
9252 volume of the archive media (and new volumes, if needed). For all
9253 other operations, you need to use the entire archive.
9255 If a multi-volume archive was labeled using
9256 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9257 (@pxref{label}) when it was created, @command{tar} will not
9258 automatically label volumes which are added later. To label
9259 subsequent volumes, specify @option{--label=@var{archive-label}} again
9260 in conjunction with the @option{--append}, @option{--update} or
9261 @option{--concatenate} operation.
9263 @cindex Labeling multi-volume archives
9266 @FIXME{There should be a sample program here, including an exit
9267 before end. Is the exit status even checked in tar? :-(}
9270 @item --multi-volume
9272 Creates a multi-volume archive, when used in conjunction with
9273 @option{--create} (@option{-c}). To perform any other operation on a multi-volume
9274 archive, specify @option{--multi-volume} in conjunction with that
9277 @item --info-script=@var{program-file}
9278 @itemx --new-volume-script=@var{program-file}
9279 @itemx -F @var{program-file}
9280 Creates a multi-volume archive via a script. Used in conjunction with
9281 @option{--create} (@option{-c}). @xref{info-script}, dor a detailed discussion.
9284 Beware that there is @emph{no} real standard about the proper way, for
9285 a @command{tar} archive, to span volume boundaries. If you have a
9286 multi-volume created by some vendor's @command{tar}, there is almost
9287 no chance you could read all the volumes with @GNUTAR{}.
9288 The converse is also true: you may not expect
9289 multi-volume archives created by @GNUTAR{} to be
9290 fully recovered by vendor's @command{tar}. Since there is little
9291 chance that, in mixed system configurations, some vendor's
9292 @command{tar} will work on another vendor's machine, and there is a
9293 great chance that @GNUTAR{} will work on most of
9294 them, your best bet is to install @GNUTAR{} on all
9295 machines between which you know exchange of files is possible.
9298 @subsection Tape Files
9301 To give the archive a name which will be recorded in it, use the
9302 @option{--label=@var{volume-label}} (@option{-V @var{volume-label}})
9303 option. This will write a special block identifying
9304 @var{volume-label} as the name of the archive to the front of the
9305 archive which will be displayed when the archive is listed with
9306 @option{--list}. If you are creating a multi-volume archive with
9307 @option{--multi-volume} (@pxref{Using Multiple Tapes}), then the
9308 volume label will have @samp{Volume @var{nnn}} appended to the name
9309 you give, where @var{nnn} is the number of the volume of the archive.
9310 (If you use the @option{--label=@var{volume-label}}) option when
9311 reading an archive, it checks to make sure the label on the tape
9312 matches the one you give. @xref{label}.
9314 When @command{tar} writes an archive to tape, it creates a single
9315 tape file. If multiple archives are written to the same tape, one
9316 after the other, they each get written as separate tape files. When
9317 extracting, it is necessary to position the tape at the right place
9318 before running @command{tar}. To do this, use the @command{mt} command.
9319 For more information on the @command{mt} command and on the organization
9320 of tapes into a sequence of tape files, see @ref{mt}.
9322 People seem to often do:
9325 @kbd{--label="@var{some-prefix} `date +@var{some-format}`"}
9328 or such, for pushing a common date in all volumes or an archive set.
9331 @subsection Concatenate Volumes into a Single Archive
9334 Sometimes it is necessary to convert existing @GNUTAR{} multi-volume
9335 archive to a single @command{tar} archive. Simply concatenating all
9336 volumes into one will not work, since each volume carries an additional
9337 information at the beginning. @GNUTAR{} is shipped with the shell
9338 script @command{tarcat} designed for this purpose.
9340 The script takes a list of files comprising a multi-volume archive
9341 and creates the resulting archive at the standard output. For example:
9344 @kbd{tarcat vol.1 vol.2 vol.3 | tar tf -}
9347 The script implements a simple heuristics to determine the format of
9348 the first volume file and to decide how to process the rest of the
9349 files. However, it makes no attempt to verify whether the files are
9350 given in order or even if they are valid @command{tar} archives.
9351 It uses @command{dd} and does not filter its standard error, so you
9352 will usually see lots of spurious messages.
9354 @FIXME{The script is not installed. Should we install it?}
9357 @section Including a Label in the Archive
9358 @cindex Labeling an archive
9359 @cindex Labels on the archive media
9363 To avoid problems caused by misplaced paper labels on the archive
9364 media, you can include a @dfn{label} entry---an archive member which
9365 contains the name of the archive---in the archive itself. Use the
9366 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9367 option in conjunction with the @option{--create} operation to include
9368 a label entry in the archive as it is being created.
9371 @item --label=@var{archive-label}
9372 @itemx -V @var{archive-label}
9373 Includes an @dfn{archive-label} at the beginning of the archive when
9374 the archive is being created, when used in conjunction with the
9375 @option{--create} operation. Checks to make sure the archive label
9376 matches the one specified (when used in conjunction with any other
9380 If you create an archive using both
9381 @option{--label=@var{archive-label}} (@option{-V @var{archive-label}})
9382 and @option{--multi-volume} (@option{-M}), each volume of the archive
9383 will have an archive label of the form @samp{@var{archive-label}
9384 Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the
9385 next, and so on. @xref{Using Multiple Tapes}, for information on
9386 creating multiple volume archives.
9388 @cindex Volume label, listing
9389 @cindex Listing volume label
9390 The volume label will be displayed by @option{--list} along with
9391 the file contents. If verbose display is requested, it will also be
9392 explicitely marked as in the example below:
9396 $ @kbd{tar --verbose --list --file=iamanarchive}
9397 V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header--
9398 -rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename
9403 @anchor{--test-label option}
9404 However, @option{--list} option will cause listing entire
9405 contents of the archive, which may be undesirable (for example, if the
9406 archive is stored on a tape). You can request checking only the volume
9407 by specifying @option{--test-label} option. This option reads only the
9408 first block of an archive, so it can be used with slow storage
9409 devices. For example:
9413 $ @kbd{tar --test-label --file=iamanarchive}
9418 If @option{--test-label} is used with a single command line
9419 argument, @command{tar} compares the volume label with the
9420 argument. It exits with code 0 if the two strings match, and with code
9421 2 otherwise. In this case no output is displayed. For example:
9425 $ @kbd{tar --test-label --file=iamanarchive 'iamalable'}
9427 $ @kbd{tar --test-label --file=iamanarchive 'iamalable' alabel}
9432 If you request any operation, other than @option{--create}, along
9433 with using @option{--label} option, @command{tar} will first check if
9434 the archive label matches the one specified and will refuse to proceed
9435 if it does not. Use this as a safety precaution to avoid accidentally
9436 overwriting existing archives. For example, if you wish to add files
9437 to @file{archive}, presumably labelled with string @samp{My volume},
9442 $ @kbd{tar -rf archive --label 'My volume' .}
9443 tar: Archive not labeled to match `My volume'
9448 in case its label does not match. This will work even if
9449 @file{archive} is not labelled at all.
9451 Similarly, @command{tar} will refuse to list or extract the
9452 archive if its label doesn't match the @var{archive-label}
9453 specified. In those cases, @var{archive-label} argument is interpreted
9454 as a globbing-style pattern which must match the actual magnetic
9455 volume label. @xref{exclude}, for a precise description of how match
9456 is attempted@footnote{Previous versions of @command{tar} used full
9457 regular expression matching, or before that, only exact string
9458 matching, instead of wildcard matchers. We decided for the sake of
9459 simplicity to use a uniform matching device through
9460 @command{tar}.}. If the switch @option{--multi-volume} (@option{-M}) is being used,
9461 the volume label matcher will also suffix @var{archive-label} by
9462 @w{@samp{ Volume [1-9]*}} if the initial match fails, before giving
9463 up. Since the volume numbering is automatically added in labels at
9464 creation time, it sounded logical to equally help the user taking care
9465 of it when the archive is being read.
9467 The @option{--label} was once called @option{--volume}, but is not
9468 available under that name anymore.
9470 You can also use @option{--label} to get a common information on
9471 all tapes of a series. For having this information different in each
9472 series created through a single script used on a regular basis, just
9473 manage to get some date string as part of the label. For example:
9477 $ @kbd{tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"}
9478 $ @kbd{tar --create --file=/dev/tape --multi-volume \
9479 --volume="Daily backup for `date +%Y-%m-%d`"}
9483 Also note that each label has its own date and time, which corresponds
9484 to when @GNUTAR{} initially attempted to write it,
9485 often soon after the operator launches @command{tar} or types the
9486 carriage return telling that the next tape is ready. Comparing date
9487 labels does give an idea of tape throughput only if the delays for
9488 rewinding tapes and the operator switching them were negligible, which
9489 is usually not the case.
9492 @section Verifying Data as It is Stored
9493 @cindex Verifying a write operation
9494 @cindex Double-checking a write operation
9499 @opindex verify, short description
9500 Attempt to verify the archive after writing.
9503 This option causes @command{tar} to verify the archive after writing it.
9504 Each volume is checked after it is written, and any discrepancies
9505 are recorded on the standard error output.
9507 Verification requires that the archive be on a back-space-able medium.
9508 This means pipes, some cartridge tape drives, and some other devices
9511 You can insure the accuracy of an archive by comparing files in the
9512 system with archive members. @command{tar} can compare an archive to the
9513 file system as the archive is being written, to verify a write
9514 operation, or can compare a previously written archive, to insure that
9517 @opindex verify, using with @option{--create}
9518 @opindex create, using with @option{--verify}
9519 To check for discrepancies in an archive immediately after it is
9520 written, use the @option{--verify} (@option{-W}) option in conjunction with
9521 the @option{--create} operation. When this option is
9522 specified, @command{tar} checks archive members against their counterparts
9523 in the file system, and reports discrepancies on the standard error.
9525 To verify an archive, you must be able to read it from before the end
9526 of the last written entry. This option is useful for detecting data
9527 errors on some tapes. Archives written to pipes, some cartridge tape
9528 drives, and some other devices cannot be verified.
9530 One can explicitly compare an already made archive with the file
9531 system by using the @option{--compare} (@option{--diff}, @option{-d})
9532 option, instead of using the more automatic @option{--verify} option.
9535 Note that these two options have a slightly different intent. The
9536 @option{--compare} option checks how identical are the logical contents of some
9537 archive with what is on your disks, while the @option{--verify} option is
9538 really for checking if the physical contents agree and if the recording
9539 media itself is of dependable quality. So, for the @option{--verify}
9540 operation, @command{tar} tries to defeat all in-memory cache pertaining to
9541 the archive, while it lets the speed optimization undisturbed for the
9542 @option{--compare} option. If you nevertheless use @option{--compare} for
9543 media verification, you may have to defeat the in-memory cache yourself,
9544 maybe by opening and reclosing the door latch of your recording unit,
9545 forcing some doubt in your operating system about the fact this is really
9546 the same volume as the one just written or read.
9548 The @option{--verify} option would not be necessary if drivers were indeed
9549 able to detect dependably all write failures. This sometimes require many
9550 magnetic heads, some able to read after the writes occurred. One would
9551 not say that drivers unable to detect all cases are necessarily flawed,
9552 as long as programming is concerned.
9554 The @option{--verify} (@option{-W}) option will not work in
9555 conjunction with the @option{--multi-volume} (@option{-M}) option or
9556 the @option{--append} (@option{-r}), @option{--update} (@option{-u})
9557 and @option{--delete} operations. @xref{Operations}, for more
9558 information on these operations.
9560 Also, since @command{tar} normally strips leading @samp{/} from file
9561 names (@pxref{absolute}), a command like @samp{tar --verify -cf
9562 /tmp/foo.tar /etc} will work as desired only if the working directory is
9563 @file{/}, as @command{tar} uses the archive's relative member names
9564 (e.g., @file{etc/motd}) when verifying the archive.
9566 @node Write Protection
9567 @section Write Protection
9569 Almost all tapes and diskettes, and in a few rare cases, even disks can
9570 be @dfn{write protected}, to protect data on them from being changed.
9571 Once an archive is written, you should write protect the media to prevent
9572 the archive from being accidentally overwritten or deleted. (This will
9573 protect the archive from being changed with a tape or floppy drive---it
9574 will not protect it from magnet fields or other physical hazards).
9576 The write protection device itself is usually an integral part of the
9577 physical media, and can be a two position (write enabled/write
9578 disabled) switch, a notch which can be popped out or covered, a ring
9579 which can be removed from the center of a tape reel, or some other
9585 This appendix lists some important user-visible changes between
9586 version @GNUTAR{} @value{VERSION} and previous versions. An up-to-date
9587 version of this document is available at
9588 @uref{http://www.gnu.org/@/software/@/tar/manual/changes.html,the
9589 @GNUTAR{} documentation page}.
9592 @item Use of globbing patterns when listing and extracting.
9594 Previous versions of GNU tar assumed shell-style globbing when
9595 extracting from or listing an archive. For example:
9598 $ @kbd{tar xf foo.tar '*.c'}
9601 would extract all files whose names end in @samp{.c}. This behavior
9602 was not documented and was incompatible with traditional tar
9603 implementations. Therefore, starting from version 1.15.91, GNU tar
9604 no longer uses globbing by default. For example, the above invocation
9605 is now interpreted as a request to extract from the archive the file
9608 To facilitate transition to the new behavior for those users who got
9609 used to the previous incorrect one, @command{tar} will print a warning
9610 if it finds out that a requested member was not found in the archive
9611 and its name looks like a globbing pattern. For example:
9614 $ @kbd{tar xf foo.tar '*.c'}
9615 tar: Pattern matching characters used in file names. Please,
9616 tar: use --wildcards to enable pattern matching, or --no-wildcards to
9617 tar: suppress this warning.
9618 tar: *.c: Not found in archive
9619 tar: Error exit delayed from previous errors
9622 To treat member names as globbing patterns, use --wildcards option.
9623 If you want to tar to mimic the behavior of versions prior to 1.15.91,
9624 add this option to your @env{TAR_OPTIONS} variable.
9626 @xref{Wildcards}, for the detailed discussion of the use of globbing
9627 patterns by @GNUTAR{}.
9629 @item Use of short option @option{-o}.
9631 Earlier versions of @GNUTAR{} understood @option{-o} command line
9632 option as a synonym for @option{--old-archive}.
9634 @GNUTAR{} starting from version 1.13.90 understands this option as
9635 a synonym for @option{--no-same-owner}. This is compatible with
9636 UNIX98 @command{tar} implementations.
9638 However, to facilitate transition, @option{-o} option retains its
9639 old semantics when it is used with one of archive-creation commands.
9640 Users are encouraged to use @option{--format=oldgnu} instead.
9642 It is especially important, since versions of @acronym{GNU} Automake
9643 up to and including 1.8.4 invoke tar with this option to produce
9644 distribution tarballs. @xref{Formats,v7}, for the detailed discussion
9645 of this issue and its implications.
9647 @FIXME{Change the first argument to tar-formats if and when Automake
9648 people accept my patch to the documentation, and the new Automake is
9649 out --Sergey 2006-05-25}.
9650 @xref{Options, tar-v7, Changing Automake's Behavior,
9651 automake, GNU Automake}, for a description on how to use various
9652 archive formats with @command{automake}.
9654 Future versions of @GNUTAR{} will understand @option{-o} only as a
9655 synonym for @option{--no-same-owner}.
9657 @item Use of short option @option{-l}
9659 Earlier versions of @GNUTAR{} understood @option{-l} option as a
9660 synonym for @option{--one-file-system}. Since such usage contradicted
9661 to UNIX98 specification and harmed compatibility with other
9662 implementation, it was declared deprecated in version 1.14. However,
9663 to facilitate transition to its new semantics, it was supported by
9664 versions 1.15 and 1.15.90. The present use of @option{-l} as a short
9665 variant of @option{--check-links} was introduced in version 1.15.91.
9667 @item Use of options @option{--portability} and @option{--old-archive}
9669 These options are deprecated. Please use @option{--format=v7} instead.
9671 @item Use of option @option{--posix}
9673 This option is deprecated. Please use @option{--format=posix} instead.
9676 @node Configuring Help Summary
9677 @appendix Configuring Help Summary
9679 Running @kbd{tar --help} displays the short @command{tar} option
9680 summary (@pxref{help}). This summary is organised by @dfn{groups} of
9681 semantically close options. The options within each group are printed
9682 in the following order: a short option, eventually followed by a list
9683 of corresponding long option names, followed by a short description of
9684 the option. For example, here is an excerpt from the actual @kbd{tar
9688 Main operation mode:
9690 -A, --catenate, --concatenate append tar files to an archive
9691 -c, --create create a new archive
9692 -d, --diff, --compare find differences between archive and
9694 --delete delete from the archive
9697 @vrindex ARGP_HELP_FMT, environment variable
9698 The exact visual representation of the help output is configurable via
9699 @env{ARGP_HELP_FMT} environment variable. The value of this variable
9700 is a comma-separated list of @dfn{format variable} assignments. There
9701 are two kinds of format variables. An @dfn{offset variable} keeps the
9702 offset of some part of help output text from the leftmost column on
9703 the screen. A @dfn{boolean} variable is a flag that toggles some
9704 output feature on or off. Depending on the type of the corresponding
9705 variable, there are two kinds of assignments:
9708 @item Offset assignment
9710 The assignment to an offset variable has the following syntax:
9713 @var{variable}=@var{value}
9717 where @var{variable} is the variable name, and @var{value} is a
9718 numeric value to be assigned to the variable.
9720 @item Boolean assignment
9722 To assign @code{true} value to a variable, simply put this variable name. To
9723 assign @code{false} value, prefix the variable name with @samp{no-}. For
9728 # Assign @code{true} value:
9730 # Assign @code{false} value:
9736 Following variables are declared:
9738 @deftypevr {Help Output} boolean dup-args
9739 If true, arguments for an option are shown with both short and long
9740 options, even when a given option has both forms, for example:
9743 -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE
9746 If false, then if an option has both short and long forms, the
9747 argument is only shown with the long one, for example:
9750 -f, --file=ARCHIVE use archive file or device ARCHIVE
9754 and a message indicating that the argument is applicable to both
9755 forms is printed below the options. This message can be disabled
9756 using @code{dup-args-note} (see below).
9758 The default is false.
9761 @deftypevr {Help Output} boolean dup-args-note
9762 If this variable is true, which is the default, the following notice
9763 is displayed at the end of the help output:
9766 Mandatory or optional arguments to long options are also mandatory or
9767 optional for any corresponding short options.
9770 Setting @code{no-dup-args-note} inhibits this message. Normally, only one of
9771 variables @code{dup-args} or @code{dup-args-note} should be set.
9774 @deftypevr {Help Output} offset short-opt-col
9775 Column in which short options start. Default is 2.
9779 $ @kbd{tar --help|grep ARCHIVE}
9780 -f, --file=ARCHIVE use archive file or device ARCHIVE
9781 $ @kbd{ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE}
9782 -f, --file=ARCHIVE use archive file or device ARCHIVE
9787 @deftypevr {Help Output} offset long-opt-col
9788 Column in which long options start. Default is 6. For example:
9792 $ @kbd{tar --help|grep ARCHIVE}
9793 -f, --file=ARCHIVE use archive file or device ARCHIVE
9794 $ @kbd{ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE}
9795 -f, --file=ARCHIVE use archive file or device ARCHIVE
9800 @deftypevr {Help Output} offset doc-opt-col
9801 Column in which @dfn{doc options} start. A doc option isn't actually
9802 an option, but rather an arbitrary piece of documentation that is
9803 displayed in much the same manner as the options. For example, in
9804 the description of @option{--format} option:
9808 -H, --format=FORMAT create archive of the given format.
9810 FORMAT is one of the following:
9812 gnu GNU tar 1.13.x format
9813 oldgnu GNU format as per tar <= 1.12
9814 pax POSIX 1003.1-2001 (pax) format
9816 ustar POSIX 1003.1-1988 (ustar) format
9817 v7 old V7 tar format
9822 the format names are doc options. Thus, if you set
9823 @kbd{ARGP_HELP_FMT=doc-opt-col=6} the above part of the help output
9824 will look as follows:
9828 -H, --format=FORMAT create archive of the given format.
9830 FORMAT is one of the following:
9832 gnu GNU tar 1.13.x format
9833 oldgnu GNU format as per tar <= 1.12
9834 pax POSIX 1003.1-2001 (pax) format
9836 ustar POSIX 1003.1-1988 (ustar) format
9837 v7 old V7 tar format
9842 @deftypevr {Help Output} offset opt-doc-col
9843 Column in which option description starts. Default is 29.
9847 $ @kbd{tar --help|grep ARCHIVE}
9848 -f, --file=ARCHIVE use archive file or device ARCHIVE
9849 $ @kbd{ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE}
9850 -f, --file=ARCHIVE use archive file or device ARCHIVE
9851 $ @kbd{ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE}
9853 use archive file or device ARCHIVE
9858 Notice, that the description starts on a separate line if
9859 @code{opt-doc-col} value is too small.
9862 @deftypevr {Help Output} offset header-col
9863 Column in which @dfn{group headers} are printed. A group header is a
9864 descriptive text preceding an option group. For example, in the
9868 Main operation mode:
9870 -A, --catenate, --concatenate append tar files to
9872 -c, --create create a new archive
9875 @samp{Main operation mode:} is the group header.
9877 The default value is 1.
9880 @deftypevr {Help Output} offset usage-indent
9881 Indentation of wrapped usage lines. Affects @option{--usage}
9882 output. Default is 12.
9885 @deftypevr {Help Output} offset rmargin
9886 Right margin of the text output. Used for wrapping.
9891 @include genfile.texi
9893 @node Snapshot Files
9894 @appendix Format of the Incremental Snapshot Files
9895 @include snapshot.texi
9897 @node Free Software Needs Free Documentation
9898 @appendix Free Software Needs Free Documentation
9899 @include freemanuals.texi
9901 @node Copying This Manual
9902 @appendix Copying This Manual
9905 * GNU Free Documentation License:: License for copying this manual
9910 @node Index of Command Line Options
9911 @appendix Index of Command Line Options
9913 This appendix contains an index of all @GNUTAR{} long command line
9914 options. The options are listed without the preceeding double-dash.
9917 @item Make sure @emph{all} options are indexed.
9918 @item Provide an index of short options
9933 @c texinfo-column-for-description: 32