From: Paul Eggert Date: Thu, 24 Apr 1997 13:19:40 +0000 (+0000) Subject: GNU tar 1.12 X-Git-Url: https://git.dogcows.com/gitweb?a=commitdiff_plain;h=093b8189300dcbdb8a7cd836a2986199664f3c89;p=chaz%2Ftar GNU tar 1.12 --- diff --git a/doc/tar.texi b/doc/tar.texi index 78cc8b1..68bb091 100644 --- a/doc/tar.texi +++ b/doc/tar.texi @@ -1,3849 +1,8725 @@ -\input texinfo @c -*-texinfo-*- +\input texinfo @c %**start of header @setfilename tar.info -@settitle The Tar Manual: DRAFT -@setchapternewpage odd +@settitle GNU tar +@finalout +@smallbook @c %**end of header -@c Note: the edition number and date is listed in *two* places; please update. -@c subtitle and top node; search for !!set - -@c Search for comments marked with !! or <<< (or >>>) - -@smallbook +@c ====================================================================== +@c This document has three levels of rendition: PUBLISH, DISTRIB or PROOF, +@c as decided by @set symbols. The PUBLISH rendition does not show +@c notes or marks asking for revision. Most users will prefer having more +@c information, even if this information is not fully revised for adequacy, +@c so DISTRIB is the default for tar distributions. The PROOF rendition +@c show all marks to the point of ugliness, but is nevertheless useful to +@c those working on the manual itself. +@c ====================================================================== + +@ifclear PUBLISH +@ifclear DISTRIB +@ifclear PROOF +@set DISTRIB +@end ifclear +@end ifclear +@end ifclear + +@ifset PUBLISH +@set RENDITION The book, version +@end ifset + +@ifset DISTRIB +@set RENDITION FTP release, version +@end ifset + +@ifset PROOF +@set RENDITION Proof reading version +@end ifset + +@c --------------------------------------------------------------------- +@c The @FIXME's, @UNREVISED and @c comments are part Fran@,{c}ois's work +@c plan. These annotations are somewhat precious to him; he asks that I +@c do not alter them inconsiderately. Much work is needed for GNU tar +@c internals (the sources, the programs themselves). Revising the +@c adequacy of the manual while revising the sources, and cleaning them +@c both at the same time, seems to him like a good way to proceed. +@c --------------------------------------------------------------------- + +@c Output marks for nodes needing revision, but not in PUBLISH rendition. + +@macro UNREVISED +@ifclear PUBLISH +@quotation +@emph{(This message will disappear, once this node revised.)} +@end quotation +@end ifclear +@end macro + +@c Output various FIXME information only in PROOF rendition. + +@macro FIXME{string} +@allow-recursion +@quote-arg +@ifset PROOF +@strong{} \string\ @strong{} +@end ifset + +@end macro + +@macro FIXME-ref{string} +@quote-arg +@ifset PROOF +@strong{} \string\ @strong{} +@end ifset + +@end macro + +@macro FIXME-pxref{string} +@quote-arg +@ifset PROOF +@strong{} \string\ @strong{} +@end ifset + +@end macro + +@macro FIXME-xref{string} +@quote-arg +@ifset PROOF +@strong{} \string\ @strong{} +@end ifset + +@end macro + +@c @macro option{entry} +@c @quote-arg +@c @opindex{--\entry\} +@c @value{\entry\} +@c @end macro + +@set op-absolute-names @kbd{--absolute-names} (@kbd{-P}) +@set ref-absolute-names @ref{absolute} +@set xref-absolute-names @xref{absolute} +@set pxref-absolute-names @pxref{absolute} + +@set op-after-date @kbd{--after-date=@var{date}} (@kbd{--newer=@var{date}}, @kbd{-N @var{date}}) +@set ref-after-date @ref{after} +@set xref-after-date @xref{after} +@set pxref-after-date @pxref{after} + +@set op-append @kbd{--append} (@kbd{-r}) +@set ref-append @ref{add} +@set xref-append @xref{add} +@set pxref-append @pxref{add} + +@set op-atime-preserve @kbd{--atime-preserve} +@set ref-atime-preserve @ref{Attributes} +@set xref-atime-preserve @xref{Attributes} +@set pxref-atime-preserve @pxref{Attributes} + +@set op-backup @kbd{--backup} +@set ref-backup @ref{Backup options} +@set xref-backup @xref{Backup options} +@set pxref-backup @pxref{Backup options} + +@set op-block-number @kbd{--block-number} (@kbd{-R}) +@set ref-block-number @ref{verbose} +@set xref-block-number @xref{verbose} +@set pxref-block-number @pxref{verbose} + +@set op-blocking-factor @kbd{--blocking-factor=@var{512-size}} (@kbd{-b @var{512-size}}) +@set ref-blocking-factor @ref{Blocking Factor} +@set xref-blocking-factor @xref{Blocking Factor} +@set pxref-blocking-factor @pxref{Blocking Factor} + +@set op-checkpoint @kbd{--checkpoint} +@set ref-checkpoint @ref{verbose} +@set xref-checkpoint @xref{verbose} +@set pxref-checkpoint @pxref{verbose} + +@set op-compare @kbd{--compare} (@kbd{--diff}, @kbd{-d}) +@set ref-compare @ref{compare} +@set xref-compare @xref{compare} +@set pxref-compare @pxref{compare} + +@set op-compress @kbd{--compress} (@kbd{--uncompress}, @kbd{-Z}) +@set ref-compress @ref{gzip} +@set xref-compress @xref{gzip} +@set pxref-compress @pxref{gzip} + +@set op-concatenate @kbd{--concatenate} (@kbd{--catenate}, @kbd{-A}) +@set ref-concatenate @ref{concatenate} +@set xref-concatenate @xref{concatenate} +@set pxref-concatenate @pxref{concatenate} + +@set op-create @kbd{--create} (@kbd{-c}) +@set ref-create @ref{create} +@set xref-create @xref{create} +@set pxref-create @pxref{create} + +@set op-delete @kbd{--delete} +@set ref-delete @ref{delete} +@set xref-delete @xref{delete} +@set pxref-delete @pxref{delete} + +@set op-dereference @kbd{--dereference} (@kbd{-h}) +@set ref-dereference @ref{dereference} +@set xref-dereference @xref{dereference} +@set pxref-dereference @pxref{dereference} + +@set op-directory @kbd{--directory=@var{directory}} (@kbd{-C @var{directory}}) +@set ref-directory @ref{directory} +@set xref-directory @xref{directory} +@set pxref-directory @pxref{directory} + +@set op-exclude @kbd{--exclude=@var{pattern}} +@set ref-exclude @ref{exclude} +@set xref-exclude @xref{exclude} +@set pxref-exclude @pxref{exclude} + +@set op-exclude-from @kbd{--exclude-from=@var{file-of-patterns}} (@kbd{-X @var{file-of-patterns}}) +@set ref-exclude-from @ref{exclude} +@set xref-exclude-from @xref{exclude} +@set pxref-exclude-from @pxref{exclude} + +@set op-extract @kbd{--extract} (@kbd{--get}, @kbd{-x}) +@set ref-extract @ref{extract} +@set xref-extract @xref{extract} +@set pxref-extract @pxref{extract} + +@set op-file @kbd{--file=@var{archive-name}} (@kbd{-f @var{archive-name}}) +@set ref-file @ref{file} +@set xref-file @xref{file} +@set pxref-file @pxref{file} + +@set op-files-from @kbd{--files-from=@var{file-of-names}} (@kbd{-T @var{file-of-names}}) +@set ref-files-from @ref{files} +@set xref-files-from @xref{files} +@set pxref-files-from @pxref{files} + +@set op-force-local @kbd{--force-local} +@set ref-force-local @ref{file} +@set xref-force-local @xref{file} +@set pxref-force-local @pxref{file} + +@set op-group @kbd{--group=@var{group}} +@set ref-group @ref{Option Summary} +@set xref-group @xref{Option Summary} +@set pxref-group @pxref{Option Summary} + +@set op-gzip @kbd{--gzip} (@kbd{--gunzip}, @kbd{--ungzip}, @kbd{-z}) +@set ref-gzip @ref{gzip} +@set xref-gzip @xref{gzip} +@set pxref-gzip @pxref{gzip} + +@set op-help @kbd{--help} +@set ref-help @ref{help} +@set xref-help @xref{help} +@set pxref-help @pxref{help} + +@set op-ignore-failed-read @kbd{--ignore-failed-read} +@set ref-ignore-failed-read @ref{Reading} +@set xref-ignore-failed-read @xref{Reading} +@set pxref-ignore-failed-read @pxref{Reading} + +@set op-ignore-zeros @kbd{--ignore-zeros} (@kbd{-i}) +@set ref-ignore-zeros @ref{Reading} +@set xref-ignore-zeros @xref{Reading} +@set pxref-ignore-zeros @pxref{Reading} + +@set op-incremental @kbd{--incremental} (@kbd{-G}) +@set ref-incremental @ref{Inc Dumps} +@set xref-incremental @xref{Inc Dumps} +@set pxref-incremental @pxref{Inc Dumps} + +@set op-info-script @kbd{--info-script=@var{script-name}} (@kbd{--new-volume-script=@var{script-name}}, @kbd{-F @var{script-name}}) +@set ref-info-script @ref{Multi-Volume Archives} +@set xref-info-script @xref{Multi-Volume Archives} +@set pxref-info-script @pxref{Multi-Volume Archives} + +@set op-interactive @kbd{--interactive} (@kbd{-w}) +@set ref-interactive @ref{interactive} +@set xref-interactive @xref{interactive} +@set pxref-interactive @pxref{interactive} + +@set op-keep-old-files @kbd{--keep-old-files} (@kbd{-k}) +@set ref-keep-old-files @ref{Writing} +@set xref-keep-old-files @xref{Writing} +@set pxref-keep-old-files @pxref{Writing} + +@set op-label @kbd{--label=@var{archive-label}} (@kbd{-V @var{archive-label}}) +@set ref-label @ref{label} +@set xref-label @xref{label} +@set pxref-label @pxref{label} + +@set op-list @kbd{--list} (@kbd{-t}) +@set ref-list @ref{list} +@set xref-list @xref{list} +@set pxref-list @pxref{list} + +@set op-listed-incremental @kbd{--listed-incremental=@var{snapshot-file}} (@kbd{-g @var{snapshot-file}}) +@set ref-listed-incremental @ref{Inc Dumps} +@set xref-listed-incremental @xref{Inc Dumps} +@set pxref-listed-incremental @pxref{Inc Dumps} + +@set op-mode @kbd{--mode=@var{permissions}} +@set ref-mode @ref{Option Summary} +@set xref-mode @xref{Option Summary} +@set pxref-mode @pxref{Option Summary} + +@set op-multi-volume @kbd{--multi-volume} (@kbd{-M}) +@set ref-multi-volume @ref{Multi-Volume Archives} +@set xref-multi-volume @xref{Multi-Volume Archives} +@set pxref-multi-volume @pxref{Multi-Volume Archives} + +@set op-newer-mtime @kbd{--newer-mtime=@var{date}} +@set ref-newer-mtime @ref{after} +@set xref-newer-mtime @xref{after} +@set pxref-newer-mtime @pxref{after} + +@set op-no-recursion @kbd{--no-recursion} +@set ref-no-recursion @ref{recurse} +@set xref-no-recursion @xref{recurse} +@set pxref-no-recursion @pxref{recurse} + +@set op-null @kbd{--null} +@set ref-null @ref{files} +@set xref-null @xref{files} +@set pxref-null @pxref{files} + +@set op-numeric-owner @kbd{--numeric-owner} +@set ref-numeric-owner @ref{Attributes} +@set xref-numeric-owner @xref{Attributes} +@set pxref-numeric-owner @pxref{Attributes} + +@set op-old-archive @kbd{--old-archive} (@kbd{-o}) +@set ref-old-archive @ref{old} +@set xref-old-archive @xref{old} +@set pxref-old-archive @pxref{old} + +@set op-one-file-system @kbd{--one-file-system} (@kbd{-l}) +@set ref-one-file-system @ref{one} +@set xref-one-file-system @xref{one} +@set pxref-one-file-system @pxref{one} + +@set op-owner @kbd{--owner=@var{user}} +@set ref-owner @ref{Option Summary} +@set xref-owner @xref{Option Summary} +@set pxref-owner @pxref{Option Summary} + +@set op-posix @kbd{--posix} +@set ref-posix @ref{posix} +@set xref-posix @xref{posix} +@set pxref-posix @pxref{posix} + +@set op-preserve @kbd{--preserve} +@set ref-preserve @ref{Attributes} +@set xref-preserve @xref{Attributes} +@set pxref-preserve @pxref{Attributes} + +@set op-record-size @kbd{--record-size=@var{size}} +@set ref-record-size @ref{Blocking} +@set xref-record-size @xref{Blocking} +@set pxref-record-size @pxref{Blocking} + +@set op-recursive-unlink @kbd{--recursive-unlink} +@set ref-recursive-unlink @ref{Writing} +@set xref-recursive-unlink @xref{Writing} +@set pxref-recursive-unlink @pxref{Writing} + +@set op-read-full-records @kbd{--read-full-records} (@kbd{-B}) +@set ref-read-full-records @ref{Blocking} +@set xref-read-full-records @xref{Blocking} +@set pxref-read-full-records @pxref{Blocking} +@c FIXME: or should it be Reading, or Blocking Factor + +@set op-remove-files @kbd{--remove-files} +@set ref-remove-files @ref{Writing} +@set xref-remove-files @xref{Writing} +@set pxref-remove-files @pxref{Writing} + +@set op-rsh-command @kbd{rsh-command=@var{command}} + +@set op-same-order @kbd{--same-order} (@kbd{--preserve-order}, @kbd{-s}) +@set ref-same-order @ref{Scarce} +@set xref-same-order @xref{Scarce} +@set pxref-same-order @pxref{Scarce} +@c FIXME: or should it be Reading, or Attributes? + +@set op-same-owner @kbd{--same-owner} +@set ref-same-owner @ref{Attributes} +@set xref-same-owner @xref{Attributes} +@set pxref-same-owner @pxref{Attributes} + +@set op-same-permissions @kbd{--same-permissions} (@kbd{--preserve-permissions}, @kbd{-p}) +@set ref-same-permissions @ref{Attributes} +@set xref-same-permissions @xref{Attributes} +@set pxref-same-permissions @pxref{Attributes} +@c FIXME: or should it be Writing? + +@set op-show-omitted-dirs @kbd{--show-omitted-dirs} +@set ref-show-omitted-dirs @ref{verbose} +@set xref-show-omitted-dirs @xref{verbose} +@set pxref-show-omitted-dirs @pxref{verbose} + +@set op-sparse @kbd{--sparse} (@kbd{-S}) +@set ref-sparse @ref{sparse} +@set xref-sparse @xref{sparse} +@set pxref-sparse @pxref{sparse} + +@set op-starting-file @kbd{--starting-file=@var{name}} (@kbd{-K @var{name}}) +@set ref-starting-file @ref{Scarce} +@set xref-starting-file @xref{Scarce} +@set pxref-starting-file @pxref{Scarce} + +@set op-suffix @kbd{--suffix=@var{suffix}} +@set ref-suffix @ref{Backup options} +@set xref-suffix @xref{Backup options} +@set pxref-suffix @pxref{Backup options} + +@set op-tape-length @kbd{--tape-length=@var{1024-size}} (@kbd{-L @var{1024-size}}) +@set ref-tape-length @ref{Using Multiple Tapes} +@set xref-tape-length @xref{Using Multiple Tapes} +@set pxref-tape-length @pxref{Using Multiple Tapes} + +@set op-to-stdout @kbd{--to-stdout} (@kbd{-O}) +@set ref-to-stdout @ref{Writing} +@set xref-to-stdout @xref{Writing} +@set pxref-to-stdout @pxref{Writing} + +@set op-totals @kbd{--totals} +@set ref-totals @ref{verbose} +@set xref-totals @xref{verbose} +@set pxref-totals @pxref{verbose} + +@set op-touch @kbd{--touch} (@kbd{-m}) +@set ref-touch @ref{Writing} +@set xref-touch @xref{Writing} +@set pxref-touch @pxref{Writing} + +@set op-unlink-first @kbd{--unlink-first} (@kbd{-U}) +@set ref-unlink-first @ref{Writing} +@set xref-unlink-first @xref{Writing} +@set pxref-unlink-first @pxref{Writing} + +@set op-update @kbd{--update} (@kbd{-u}) +@set ref-update @ref{update} +@set xref-update @xref{update} +@set pxref-update @pxref{update} + +@set op-use-compress-prog @kbd{--use-compress-prog=@var{program}} +@set ref-use-compress-prog @ref{gzip} +@set xref-use-compress-prog @xref{gzip} +@set pxref-use-compress-prog @pxref{gzip} + +@set op-verbose @kbd{--verbose} (@kbd{-v}) +@set ref-verbose @ref{verbose} +@set xref-verbose @xref{verbose} +@set pxref-verbose @pxref{verbose} + +@set op-verify @kbd{--verify} (@kbd{-W}) +@set ref-verify @ref{verify} +@set xref-verify @xref{verify} +@set pxref-verify @pxref{verify} + +@set op-version @kbd{--version} +@set ref-version @ref{help} +@set xref-version @xref{help} +@set pxref-version @pxref{help} + +@set op-version-control @kbd{--version-control=@var{method}} +@set ref-version-control @ref{Backup options} +@set xref-version-control @xref{Backup options} +@set pxref-version-control @pxref{Backup options} + +@set op-volno-file @kbd{--volno-file=@var{file-of-number}} +@set ref-volno-file @ref{Using Multiple Tapes} +@set xref-volno-file @xref{Using Multiple Tapes} +@set pxref-volno-file @pxref{Using Multiple Tapes} + +@include version.texi + +@c Put everything in one index (arbitrarily chosen to be the concept index). +@syncodeindex fn cp +@syncodeindex ky cp +@syncodeindex pg cp +@syncodeindex vr cp + +@defindex op +@syncodeindex op cp -@iftex -@c finalout -@end iftex +@ifinfo +@format +START-INFO-DIR-ENTRY +* tar: (tar). Making tape (or disk) archives. +END-INFO-DIR-ENTRY +@end format +@end ifinfo @ifinfo -This file documents @code{tar}, a utility used to store, backup, and +This file documents GNU @code{tar}, a utility used to store, backup, and transport files. -Copyright (C) 1992 Free Software Foundation, Inc. DRAFT! -@c Need to put distribution information here when ready. +Copyright (C) 1992, 1994, 1995, 1996, 1997 Free Software Foundation, Inc. + +Permission is granted to make and distribute verbatim copies of +this manual provided the copyright notice and this permission notice +are preserved on all copies. + +@ignore +Permission is granted to process this file through TeX and print the +results, provided the printed document carries copying permission +notice identical to this one except for the removal of this paragraph +(this paragraph not being relevant to the printed manual). + +@end ignore +Permission is granted to copy and distribute modified versions of this +manual under the conditions for verbatim copying, provided that the entire +resulting derived work is distributed under the terms of a permission +notice identical to this one. + +Permission is granted to copy and distribute translations of this manual +into another language, under the above conditions for modified versions, +except that this permission notice may be stated in a translation approved +by the Foundation. @end ifinfo -@c !!set edition number and date here -@titlepage -@title @code{tar} -@subtitle The GNU Tape Archiver -@subtitle Edition 0.01, for @code{tar} Version 1.10 -@subtitle @today{} -@c remove preceding today line when ready -@sp 1 -@subtitle DRAFT -@c subtitle insert month here when ready +@setchapternewpage odd -@author Michael I. Bushnell and Amy Gorin +@shorttitlepage GNU @code{tar} + +@titlepage +@title GNU tar: an archiver tool +@subtitle @value{RENDITION} @value{VERSION}, @value{UPDATED} +@author Melissa Weisshaus, Jay Fenlason, +@author Thomas Bushnell, n/BSG, Amy Gorin +@c he said to remove it: Fran@,{c}ois Pinard +@c i'm thinking about how the author page *should* look. -mew 2may96 @page @vskip 0pt plus 1filll -Copyright @copyright{} 1992 Free Software Foundation, Inc. - -@sp 2 -This draft is not yet ready for distribution. +Copyright @copyright{} 1992, 1994, 1995, 1996, 1997 Free Software +Foundation, Inc. + +Permission is granted to make and distribute verbatim copies of +this manual provided the copyright notice and this permission notice +are preserved on all copies. + +Permission is granted to copy and distribute modified versions of this +manual under the conditions for verbatim copying, provided that the entire +resulting derived work is distributed under the terms of a permission +notice identical to this one. + +Permission is granted to copy and distribute translations of this manual +into another language, under the above conditions for modified versions, +except that this permission notice may be stated in a translation approved +by the Foundation. @end titlepage @ifinfo -@node Top, Introduction, (dir), (dir) -@top @code{tar} -This file documents @code{tar}, a utility used to store, backup, and -transport files. +This file documents GNU @code{tar}, which is a utility used to store, +backup, and transport files. @code{tar} is a tape (or disk) archiver. +This manual documents the release @value{VERSION}. -@c !!set edition number and date here -This is DRAFT Edition 0.01 of the @code{tar} documentation, @today{}, for @code{tar} -version 1.12. @end ifinfo -@c <<< The menus need to be gone over, and node names fixed. +@node Top, Introduction, (dir), (dir) + @menu -* Introduction:: @code{tar}: The GNU Tape Archiver -* Invoking @code{tar}:: How to invoke @code{tar} -* Tutorial:: Getting started -* Wizardry:: Some More Advanced Uses for @code{tar} -* Archive Structure:: The structure of an archive -* Reading and Writing:: Reading and writing archives -* Insuring Accuracy:: How to insure the accuracy of an archive -* Selecting Archive Members:: How to select archive members -* User Interaction:: How @code{tar} interacts with people. -* Backups and Restoration:: How to restore files and perform backups -* Media:: Using tapes and other archive media -* Quick Reference:: A quick reference guide to - @code{tar} operations and options -* Data Format Details:: Details of the archive data format -* Concept Index:: Concept Index -@end menu +* Introduction:: +* Tutorial:: +* tar invocation:: +* operations:: +* Backups:: +* Choosing:: +* Date input formats:: +* Formats:: +* Media:: +* Index:: -@chapter Tutorial Introduction to @code{tar} + --- The Detailed Node Listing --- -This chapter guides you through some basic examples of @code{tar} -operations. If you already know how to use some other version of -@code{tar}, then you probably don't need to read this chapter. This -chapter omits complicated details about many of the ways @code{tar} -works. See later chapters for full information. +Introduction -@menu -* Creating Archives:: Creating Archives -* Extracting Files:: Extracting Files from an Archive -* Listing Archive Contents:: Listing the Contents of an Archive -* Comparing Files:: Comparing Archives with the File System -* Adding to Archives:: Adding Files to Existing Archives -* Concatenate:: Concatenating Archives -* Deleting Files:: Deleting Files From an Archive -@end menu +* Book Contents:: What this Book Contains +* Definitions:: Some Definitions +* What tar Does:: What @code{tar} Does +* Naming tar Archives:: How @code{tar} Archives are Named +* posix compliance:: +* Authors:: GNU @code{tar} Authors +* Reports:: Reporting bugs or suggestions -@section What @code{tar} Does +Tutorial Introduction to @code{tar} -The @code{tar} program is used to create and manipulate @code{tar} -archives. An @dfn{archive} is a single file which contains within it -the contents of many files. In addition, the archive identifies the -names of the files, their owner, and so forth. +* assumptions:: +* stylistic conventions:: +* basic tar options:: Basic @code{tar} Operations and Options +* frequent operations:: +* Two Frequent Options:: +* create:: How to Create Archives +* list:: How to List Archives +* extract:: How to Extract Members from an Archive +* going further:: -You can use @code{tar} archives in many ways. Initially, @code{tar} -archives were used to store files conveniently on magnetic tape. The -name @samp{tar} comes from this use; it stands for Tape ARchiver. -Often, @code{tar} archives are used to store related files for -convenient file transfer over a network. For example, the GNU Project -distributes its software bundled into @code{tar} archives, so that all -the files relating to a particular program (or set of related programs) -can be transferred as a single unit. +Two Frequently Used Options -The files inside an archive are called @dfn{members}. Within this -manual, we use the term @dfn{file} to refer only to files accessible in -the normal ways (by @code{ls}, @code{cat}, and so forth), and the term -@dfn{members} to refer only to the members of an archive. Similarly, a -@dfn{file name} is the name of a file, as it resides in the filesystem, -and a @dfn{member name} is the name of an archive member within the -archive. +* file tutorial:: +* verbose tutorial:: +* help tutorial:: -The @code{tar} program provides the ability to create @code{tar} -archives, as well as for various other kinds of manipulation. The term -@dfn{extraction} is used to refer to the process of copying an archive -member into a file in the filesystem. One might speak of extracting a -single member. Extracting all the members of an archive is often called -extracting the archive. Often the term @dfn{unpack} is used to refer to -the extraction of many or all the members of an archive. +How to Create Archives -Conventionally, @code{tar} archives are given names ending with -@samp{.tar}. This is not necessary for @code{tar} to operate properly, -but this manual follows the convention in order to get the reader used -to seeing it. +* prepare for examples:: +* Creating the archive:: +* create verbose:: +* short create:: +* create dir:: -Occasionally archive members are referred to as files. For people -familiar with the operation of @code{tar}, this causes no difficulty. -However, this manual consistently uses the terminology above in -referring to files and archive members, to make it easier to learn how -to use @code{tar}. +How to List Archives -@section How to Create Archives +* list dir:: -To create a new archive, use @samp{tar --create}. You should generally -use the @samp{--file} option to specify the name the tar archive will -have. Then specify the names of the files you wish to place in the new -archive. For example, to place the files @file{apple}, @file{angst}, -and @file{asparagus} into an archive named @file{afiles.tar}, use the -following command: +How to Extract Members from an Archive -@example -tar --create --file=afiles.tar apple angst asparagus -@end example +* extracting archives:: +* extracting files:: +* extract dir:: +* failing commands:: -The order of the arguments is not important. You could also say: +Invoking GNU @code{tar} -@example -tar apple --create angst --file=afiles.tar asparagus -@end example +* Synopsis:: +* using tar options:: +* Styles:: +* All Options:: +* help:: +* verbose:: +* interactive:: -This order is harder to understand however. In this manual, we will -list the arguments in a reasonable order to make the commands easier to -understand, but you can type them in any order you wish. +The Three Option Styles -If you don't specify the names of any files to put in the archive, then -tar will create an empty archive. So, the following command will create -an archive with nothing in it: +* Mnemonic Options:: Mnemonic Option Style +* Short Options:: Short Option Style +* Old Options:: Old Option Style +* Mixing:: Mixing Option Styles -@example -tar --create --file=empty-archive.tar -@end example +All @code{tar} Options -Whenever you use @samp{tar --create}, @code{tar} will erase the current -contents of the file named by @samp{--file} if it exists. To add files -to an existing archive, you need to use a different option. -@xref{Adding to Archives} for information on how to do this. +* Operation Summary:: +* Option Summary:: +* Short Option Summary:: -When @samp{tar --create} creates an archive, the member names of the -members of the archive are exactly the same as the file names as you -typed them in the @code{tar} command. So, the member names of -@file{afiles} (as created by the first example above) are @file{apple}, -@file{angst}, and @file{asparagus}. However, suppose an archive were -created with this command: +GNU @code{tar} Operations -@example -tar --create --file=bfiles.tar ./balloons baboon ./bodacious -@end example +* Basic tar:: +* Advanced tar:: +* extract options:: +* backup:: +* Applications:: +* looking ahead:: -Then, the three files @file{balloons}, @file{baboon}, and -@file{bodacious} would get placed in the archive (because @file{./} is a -synonym for the current directory), but their member names would be -@file{./balloons}, @file{baboon}, and @file{./bodacious}. +Advanced GNU @code{tar} Operations -If you want to see the progress of tar as it writes files into the -archive, you can use the @samp{--verbose} option. +* Operations:: +* current state:: +* append:: +* update:: +* concatenate:: +* delete:: +* compare:: -If one of the files named to @samp{tar --create} is a directory, then -the operation of tar is more complicated. @xref{Tar and Directories}, -the last section of this tutorial, for more information. +How to Add Files to Existing Archives: @code{--append} -If you don't specify the @samp{--file} option, then @code{tar} will use -a default. Usually this default is some physical tape drive attached to -your machine. If there is no tape drive attached, or the default is not -meaningful, then tar will print an error message. This error message -might look roughly like one of the following: +* appending files:: Appending Files to an Archive +* multiple:: -@example -tar: can't open /dev/rmt8 : No such device or address -tar: can't open /dev/rsmt0 : I/O error -@end example +Updating an Archive -If you get an error like this, mentioning a file you didn't specify -(@file{/dev/rmt8} or @file{/dev/rsmt0} in the examples above), then @code{tar} -is using a default value for @samp{--file}. You should generally specify a -@samp{--file} argument whenever you use @code{tar}, rather than relying -on a default. +* how to update:: -@section How to List Archives +Options Used by @code{--extract} -Use @samp{tar --list} to print the names of members stored in an -archive. Use a @samp{--file} option just as with @samp{tar --create} to -specify the name of the archive. For example, the archive -@file{afiles.tar} created in the last section could be examined with the -command @samp{tar --list --file=afiles.tar}. The output of tar would -then be: +* Reading:: Options to Help Read Archives +* Writing:: Changing How @code{tar} Writes Files +* Scarce:: Coping with Scarce Resources -@example -apple -angst -asparagus -@end example +Options to Help Read Archives -The archive @file{bfiles.tar} would list as follows: +* read full records:: +* Ignore Zeros:: +* Ignore Failed Read:: -@example -./baloons -baboon -./bodacious -@end example +Changing How @code{tar} Writes Files -(Of course, @samp{tar --list --file=empty-archive.tar} would produce no -output.) +* Prevention Overwriting:: +* Keep Old Files:: +* Unlink First:: +* Recursive Unlink:: +* Modification Times:: +* Setting Access Permissions:: +* Writing to Standard Output:: +* remove files:: -If you use the @samp{--verbose} option with @samp{tar --list}, then tar -will print out a listing reminiscent of @samp{ls -l}, showing owner, -file size, and so forth. +Options to Prevent Overwriting Files -You can also specify member names when using @samp{tar --list}. In this -case, tar will only list the names of members you identify. For -example, @samp{tar --list --file=afiles.tar apple} would only print -@samp{apple}. It is essential when specifying member names to tar that -you give the exact member names. For example, @samp{tar --list ---file=bfiles baloons} would produce no output, because there is no -member named @file{baloons}, only one named @file{./baloons}. While the -file names @file{baloons} and @file{./baloons} name the same file, -member names are compared using a simplistic name comparison, in which -an exact match is necessary. +* Keep Old Files:: +* Unlink First:: +* Recursive Unlink:: -@section How to Extract Members from an Archive +Coping with Scarce Resources -In order to extract members from an archive, use @samp{tar --extract}. -Specify the name of the archive with @samp{--file}. To extract specific -archive members, give their member names as arguments. It essential to -give their exact member name, as printed by @samp{tar --list}. This -will create a copy of the archive member, with a file name the same as -its name in the archive. - -Keeping the example of the two archives created at the beginning of this -tutorial, @samp{tar --extract --file=afiles.tar apple} would create a -file @file{apple} in the current directory with the contents of the -archive member @file{apple}. It would remove any file named -@file{apple} already present in the directory, but it would not change -the archive in any way. - -Remember that specifying the exact member name is important. @samp{tar ---extract --file=bfiles.tar baloons} will fail, because there is no -member named @file{baloons}. To extract the member named -@file{./baloons} you would need to specify @samp{tar --extract ---file=bfiles.tar ./baloons}. To find the exact member names of the -members of an archive, use @samp{tar --list} (@pxref{Listing -Archives}). - -If you do not list any archive member names, then @samp{tar --extract} -will extract all the members of the archive. - -If you give the @samp{--verbose} option, then @samp{tar --extract} will -print the names of the archive members as it extracts them. +* Starting File:: +* Same Order:: -@section How to Add Files to Existing Archives +Performing Backups and Restoring Files -If you want to add files to an existing archive, then don't use -@samp{tar --create}. That will erase the archive and create a new one -in its place. Instead, use @samp{tar --append}. The command @samp{tar ---append --file=afiles.tar arbalest} would add the file @file{arbalest} -to the existing archive @file{afiles.tar}. The archive must already -exist in order to use @samp{tar --append}. +* Full Dumps:: Using @code{tar} to Perform Full Dumps +* Inc Dumps:: Using @code{tar} to Perform Incremental Dumps +* incremental and listed-incremental:: The Incremental Options +* Backup Levels:: Levels of Backups +* Backup Parameters:: Setting Parameters for Backups and Restoration +* Scripted Backups:: Using the Backup Scripts +* Scripted Restoration:: Using the Restore Script -As with @samp{tar --create}, the member names of the newly added files -will be the exact same as their names given on the command line. The -@samp{--verbose} option will print out the names of the files as they -are written into the archive. +Setting Parameters for Backups and Restoration -If you add a file to an archive using @samp{tar --append} with the -same name as an archive member already present in the archive, then the -old member is not deleted. What does happen, however, is somewhat -complex. @xref{Multiple Members with the Same Name}. If you want to -replace an archive member, use @samp{tar --delete} first, and then use -@samp{tar --append}. - -@section How to Delete Members from Archives - -You can delete members from an archive using @samp{tar --delete}. -Specify the name of the archive with @samp{--file}. List the member -names of the members to be deleted. (If you list no member names, then -nothing will be deleted.) The @samp{--verbose} option will cause -@code{tar} to print the names of the members as they are deleted. As -with @samp{tar --extract}, it is important that you give the exact -member names when using @samp{tar --delete}. Use @samp{tar --list} to -find out the exact member names in an archive (@pxref{Listing -Archives}). - -The @samp{tar --delete} command only works with archives stored on disk. -You cannot delete members from an archive stored on a tape. - -@section How to Archive Directories - -When the names of files or members specify directories, the operation of -@code{tar} is more complex. Generally, when a directory is named, -@code{tar} also operates on all the contents of the directory, -recursively. Thus, to @code{tar}, the file name @file{/} names the -entire file system. - -To archive the entire contents of a directory, use @samp{tar --create} -(or @samp{tar --append}) as usual, and specify the name of the -directory. For example, to archive all the contents of the current -directory, use @samp{tar --create --file=@var{archive-name} .}. Doing -this will give the archive members names starting with @samp{./}. To -archive the contents of a directory named @file{foodir}, use @samp{tar ---create --file=@var{archive-name} foodir}. In this case, the member -names will all start with @samp{foodir/}. - -If you give @code{tar} a command such as @samp{tar --create ---file=foo.tar .}, it will report @samp{tar: foo.tar is the archive; not -dumped}. This happens because the archive @file{foo.tar} is created -before putting any files into it. Then, when @code{tar} attempts to add -all the files in the directory @file{.} to the archive, it notices that -the file @file{foo.tar} is the same as the archive, and skips it. (It -makes no sense to put an archive into itself.) GNU @code{tar} will -continue in this case, and create the archive as normal, except for the -exclusion of that one file. Other versions of @code{tar}, however, are -not so clever, and will enter an infinite loop when this happens, so you -should not depend on this behavior. In general, make sure that the -archive is not inside a directory being dumped. - -When extracting files, you can also name directory archive members on -the command line. In this case, @code{tar} extracts all the archive -members whose names begin with the name of the directory. As usual, -@code{tar} is not particularly clever about interpreting member names. -The command @samp{tar --extract --file=@var{archive-name} .} will not -extract all the contents of the archive, but only those members whose -member names begin with @samp{./}. - -@section Shorthand Names - -Most of the options to @code{tar} come in both long forms and short -forms. The options described in this tutorial have the following -abbreviations (except @samp{--delete}, which has no shorthand form): +* backup-specs example:: An Example Text of @file{Backup-specs} +* Script Syntax:: Syntax for @file{Backup-specs} -@table @samp -@item --create -@samp{-c} -@item --list -@samp{-t} -@item --extract -@samp{-x} -@item --append -@samp{-r} -@item --verbose -@samp{-v} -@item --file=@var{archive-name} -@samp{-f @var{archive-name}} -@end table +Choosing Files and Names for @code{tar} -These options make typing long @code{tar} commands easier. For example, -instead of typing -@example -tar --create --file=/tmp/afiles.tar --verbose apple angst asparagus -@end example -you can type -@example -tar -c -f /tmp/afiles.tar -v apple angst asparagus -@end example +* file:: Choosing the Archive's Name +* Selecting Archive Members:: +* files:: Reading Names from a File +* exclude:: Excluding Some Files +* Wildcards:: +* after:: Operating Only on New Files +* recurse:: Descending into Directories +* one:: Crossing Filesystem Boundaries -For more information on option syntax, @ref{Invoking @code{tar}}. In -the remainder of this manual, short forms and long forms are given -together when an option is discussed. +Reading Names from a File -@chapter Invoking @code{tar} +* nul:: -The usual way to invoke tar is +Excluding Some Files -@example -@code{tar} @var{options}... [@var{file-or-member-names}...] -@end example +* problems with exclude:: -All the options start with @samp{-}. You can actually type in arguments -in any order, but in this manual the options always precede the other -arguments, to make examples easier to understand. +Crossing Filesystem Boundaries -@menu -* Option Form:: The Forms of Arguments -* Argument Functions:: The Functions of Arguments -* Old Syntax for Commands:: An Old, but Still Supported, Syntax - for @code{tar} Commands -@end menu +* directory:: Changing Directory +* absolute:: Absolute File Names -@section The Forms of Arguments - -Most options of @code{tar} have a single letter form (a single letter -preceded by @samp{-}), and at least one mnemonic form (a word or -abbreviation preceded by @samp{--}). The forms are absolutely -identical in function. For example, you can use either @samp{tar -t} -or @samp{tar --list} to list the contents of an archive. In addition, -mnemonic names can be given unique abbreviations. For example, -@samp{--cre} can be used in place of @samp{--create} because there is -no other option which begins with @samp{cre}. - -Some options require an additional argument. Single letter options -which require arguments use the immediately following argument. -Mnemonic options are separated from their arguments by an @samp{=} -sign. For example, to create an an archive file named -@file{george.tar}, use either @samp{tar --create --file=george.tar} or -@samp{tar --create -f george.tar}. Both -@samp{--file=@var{archive-name}} and @samp{-f @var{archive-name}} denote -the option to give the archive a non-default name, which in the example -is @file{george.tar}. - -You can mix single letter and mnemonic forms in the same command. You -could type the above example as @samp{tar -c --file=george} or -@samp{tar --create -f george}. However, @code{tar} operations and -options are case sensitive. You would not type the above example as -@samp{tar -C --file=george}, because @samp{-C} is an option that -causes @code{tar} to change directories, not an operation that creates -an archive. In fact, @samp{-C} requires a further argument (the name -of the directory which to change to). In this case, tar would think -it needs to change to a directory named @samp{--file=george}, and -wouldn't interpret @samp{--file-george} as an option at all! - -@section The Functions of Arguments - -You must give exactly one option from the following list to tar. This -option specifies the basic operation for @code{tar} to perform. - -@table samp -@item --help -Print a summary of the options to @code{tar} and do nothing else +Date input formats -@item --create -@item -c -Create a new archive +* General date syntax:: Common rules. +* Calendar date item:: 19 Dec 1994. +* Time of day item:: 9:20pm. +* Timezone item:: EST, DST, BST, UCT, AHST, ... +* Day of week item:: Monday and others. +* Relative item in date strings:: next tuesday, 2 years ago. +* Pure numbers in date strings:: 19931219, 1440. +* Authors of getdate:: Bellovin, Salz, Berets, et al. -@item --catenate -@item --concatenate -@item -A -Add the contents of one or more archives to another archive +Controlling the Archive Format -@item --append -@item -a -Add files to an existing archive +* Portability:: Making @code{tar} Archives More Portable +* Compression:: Using Less Space through Compression +* Attributes:: Handling File Attributes +* Standard:: The Standard Format +* Extensions:: GNU Extensions to the Archive Format +* cpio:: Comparison of @code{tar} and @code{cpio} -@item --list -@item -t -List the members in an archive +Making @code{tar} Archives More Portable -@item --delete -Delete members from an archive +* Portable Names:: Portable Names +* dereference:: Symbolic Links +* old:: Old V7 Archives +* posix:: POSIX archives +* Checksumming:: Checksumming Problems -@item --extract -@item --get -@item -x -Extract members from an archive +Using Less Space through Compression -@item --compare -@item --diff -@item -d -Compare members in an archive with files in the file system +* gzip:: Creating and Reading Compressed Archives +* sparse:: Archiving Sparse Files -@item --update -@item -u -Update an archive by appending newer versions of already stored files -@end itemize +Tapes and Other Archive Media -The remaining options to @code{tar} change details of the operation, -such as archive format, archive name, or level of user interaction. -You can specify more than one option. - -The remaining arguments are interpreted either as file names or as -member names, depending on the basic operation @code{tar} is -performing. For @samp{--append} and @samp{--create} these arguments -specify the names of files (which must already exist) to place in the -archive. For the remaining operation types, the additional arguments -specify archive members to compare, delete, extract, list, or update. -When naming archive members, you must give the exact name of the member -in the archive, as it is printed by @code{tar --list}. When naming -files, the normal file name rules apply. - -If you don't use any additional arguments, @samp{--append}, -@samp{--catenate}, and @samp{--delete} will do nothing. Naturally, -@samp{--create} will make an empty archive if given no files to add. -The other operations of @code{tar} (@samp{--list}, @samp{--extract}, -@samp{--compare}, and @samp{--update}) will act on the entire contents -of the archive. +* Device:: Device selection and switching +* Remote Tape Server:: +* Common Problems and Solutions:: +* Blocking:: Blocking +* Many:: Many archives on one tape +* Using Multiple Tapes:: Using Multiple Tapes +* label:: Including a Label in the Archive +* verify:: +* Write Protection:: -If you give the name of a directory as either a file name or a member -name, then @code{tar} acts recursively on all the files and directories -beneath that directory. For example, the name @file{/} identifies all -the files in the filesystem to @code{tar}. +Blocking -@section An Old, but Still Supported, Syntax for @code{tar} Commands +* Format Variations:: Format Variations +* Blocking Factor:: The Blocking Factor of an Archive -For historical reasons, GNU @code{tar} also accepts a syntax for -commands which splits options that require additional arguments into -two parts. That syntax is of the form: +Many Archives on One Tape -@example -@code{tar} @var{option-letters}... [@var{option-arguments}...] [@var{file-names}...]@refill -@end example +* Tape Positioning:: Tape Positions and Tape Marks +* mt:: The @code{mt} Utility -@noindent -where arguments to the options appear in the same order as the letters -to which they correspond, and the operation and all the option letters -appear as a single argument, without separating spaces. +Using Multiple Tapes -This command syntax is useful because it lets you type the single -letter forms of the operation and options as a single argument to -@code{tar}, without writing preceding @samp{-}s or inserting spaces -between letters. @samp{tar cv} or @samp{tar -cv} are equivalent to -@samp{tar -c -v}. +* Multi-Volume Archives:: Archives Longer than One Tape or Disk +* Tape Files:: Tape Files +@end menu -On the other hand, this old style syntax makes it difficult to match -option letters with their corresponding arguments, and is often -confusing. In the command @samp{tar cvbf 20 /dev/rmt0}, for example, -@samp{20} is the argument for @samp{-b}, @samp{/dev/rmt0} is the -argument for @samp{-f}, and @samp{-v} does not have a corresponding -argument. The modern syntax---@samp{tar -c -v -b 20 -f -/dev/rmt0}---is clearer. +@node Introduction, Tutorial, Top, Top +@chapter Introduction -@chapter Basic @code{tar} Operations +Welcome to the GNU @code{tar} manual. GNU @code{tar} is used to create +and manipulate files (@dfn{archives}) which are actually collections of +many other files; the program provides users with an organized and +systematic method for controlling a large amount of data. + +@menu +* Book Contents:: What this Book Contains +* Definitions:: Some Definitions +* What tar Does:: What @code{tar} Does +* Naming tar Archives:: How @code{tar} Archives are Named +* posix compliance:: +* Authors:: GNU @code{tar} Authors +* Reports:: Reporting bugs or suggestions +@end menu -This chapter describes the basic operations supported by the @code{tar} -program. A given invocation of @code{tar} will do exactly one of these -operations. +@node Book Contents, Definitions, Introduction, Introduction +@ifinfo +@heading What this Book Contains +@end ifinfo -@section Creating a New Archive +The first part of this chapter introduces you to various terms that will +recur throughout the book. It also tells you who has worked on GNU +@code{tar} and its documentation, and where you should send bug reports +or comments. + +The second chapter is a tutorial (@pxref{Tutorial}) which provides a +gentle introduction for people who are new to using @code{tar}. It is +meant to be self contained, not requiring any reading from subsequent +chapters to make sense. It moves from topic to topic in a logical, +progressive order, building on information already explained. + +Although the tutorial is paced and structured to allow beginners to +learn how to use @code{tar}, it is not intended solely for beginners. +The tutorial explains how to use the three most frequently used +operations (@samp{create}, @samp{list}, and @samp{extract}) as well as +two frequently used options (@samp{file} and @samp{verbose}). The other +chapters do not refer to the tutorial frequently; however, if a section +discusses something which is a complex variant of a basic concept, there +may be a cross reference to that basic concept. (The entire book, +including the tutorial, assumes that the reader understands some basic +concepts of using a Unix-type operating system; @pxref{Tutorial}.) + +The third chapter presents the remaining five operations, and +information about using @code{tar} options and option syntax. + +@FIXME{this sounds more like a GNU Project Manuals Concept [tm] more +than the reality. should think about whether this makes sense to say +here, or not.} The other chapters are meant to be used as a +reference. Each chapter presents everything that needs to be said +about a specific topic. + +One of the chapters (@pxref{Date input formats}) exists in its entirety +in other GNU manuals, and is mostly self-contained. In addition, one +section of this manual (@pxref{Standard}) contains a big quote which is +taken directly from @code{tar} sources. + +In general, we give both the long and short (abbreviated) option names +at least once in each section where the relevant option is covered, so +that novice readers will become familiar with both styles. (A few +options have no short versions, and the relevant sections will +indicate this.) + +@node Definitions, What tar Does, Book Contents, Introduction +@section Some Definitions + +@cindex archive +@cindex tar archive +The @code{tar} program is used to create and manipulate @code{tar} +archives. An @dfn{archive} is a single file which contains the contents +of many files, while still identifying the names of the files, their +owner(s), and so forth. (In addition, archives record access +permissions, user and group, size in bytes, and last modification time. +Some archives also record the file names in each archived directory, as +well as other file and directory information.) You can use @code{tar} +to @dfn{create} a new archive in a specified directory. + +@cindex member +@cindex archive member +@cindex file name +@cindex member name +The files inside an archive are called @dfn{members}. Within this +manual, we use the term @dfn{file} to refer only to files accessible in +the normal ways (by @code{ls}, @code{cat}, and so forth), and the term +@dfn{member} to refer only to the members of an archive. Similarly, a +@dfn{file name} is the name of a file, as it resides in the filesystem, +and a @dfn{member name} is the name of an archive member within the +archive. -The @samp{--create} (@code{-c}) option causes @code{tar} to create a new -archive. The files to be archived are then named on the command line. -Each file will be added to the archive with a member name exactly the -same as the name given on the command line. (When you give an absolute -file name @code{tar} actually modifies it slightly, @ref{Absolute -Paths}.) If you list no files to be archived, then an empty archive is -created. +@cindex extraction +@cindex unpacking +The term @dfn{extraction} refers to the process of copying an archive +member (or multiple members) into a file in the filesystem. Extracting +all the members of an archive is often called @dfn{extracting the +archive}. The term @dfn{unpack} can also be used to refer to the +extraction of many or all the members of an archive. Extracting an +archive does not destroy the archive's structure, just as creating an +archive does not destroy the copies of the files that exist outside of +the archive. You may also @dfn{list} the members in a given archive +(this is often thought of as ``printing'' them to the standard output, +or the command line), or @dfn{append} members to a pre-existing archive. +All of these operations can be peformed using @code{tar}. + +@node What tar Does, Naming tar Archives, Definitions, Introduction +@section What @code{tar} Does -If there are two many files to conveniently list on the command line, -you can list the names in a file, and @code{tar} will read that file. -@xref{Reading Names from a File}. +@cindex tar +The @code{tar} program provides the ability to create @code{tar} +archives, as well as various other kinds of manipulation. For example, +you can use @code{tar} on previously created archives to extract files, +to store additional files, or to update or list files which were already +stored. -If you name a directory, then @code{tar} will archive not only the -directory, but all its contents, recursively. For example, if you name -@file{/}, then @code{tar} will archive the entire filesystem. +Initially, @code{tar} archives were used to store files conveniently on +magnetic tape. The name @samp{tar} comes from this use; it stands for +@code{t}ape @code{ar}chiver. Despite the utility's name, @code{tar} can +direct its output to available devices, files, or other programs (using +pipes). @code{tar} may even access remote devices or files (as archives). -Do not use the option to add files to an existing archive; it will -delete the archive and write a new one. Use @samp{--append} instead. -(@xref{Adding to an Existing Archive}.) +@FIXME{the following table entries need a bit of work..} -There are various ways of causing @code{tar} to skip over some files, -and not archive them. @xref{Specifying Names to @code{tar}}. +You can use @code{tar} archives in many ways. We want to stress a few +of them: storage, backup, and transportation. -@section Adding to an Existing Archive +@table @asis +@item Storage +Often, @code{tar} archives are used to store related files for +convenient file transfer over a network. For example, the GNU Project +distributes its software bundled into @code{tar} archives, so that +all the files relating to a particular program (or set of related +programs) can be transferred as a single unit. + +A magnetic tape can store several files in sequence. However, the tape +has no names for these files; it only knows their relative position on +the tape. One way to store several files on one tape and retain their +names is by creating a @code{tar} archive. Even when the basic transfer +mechanism can keep track of names, as FTP can, the nuisance of handling +multiple files, directories, and multiple links makes @code{tar} +archives useful. + +Archive files are also used for long-term storage. You can think of +this as transportation from the present into the future. (It is a +science-fiction idiom that you can move through time as well as in +space; the idea here is that @code{tar} can be used to move archives in +all dimensions, even time!) + +@item Backup +Because the archive created by @code{tar} is capable of preserving file +information and directory structure, @code{tar} is commonly used for +performing full and incremental backups of disks. A backup puts a +collection of files (possibly pertaining to many users and +projects) together on a disk or a tape. This guards against accidental +destruction of the information in those files. GNU @code{tar} has +special features that allow it to be used to make incremental and full +dumps of all the files in a filesystem. + +@item Transportation +You can create an archive on one system, transfer it to another system, +and extract the contents there. This allows you to transport a group of +files from one system to another. +@end table -The @samp{--append} (@code{-r}) option will case @code{tar} to add new -files to an existing archive. It interprets file names and member names -in exactly the same manner as @samp{--create}. Nothing happens if you -don't list any names. +@node Naming tar Archives, posix compliance, What tar Does, Introduction +@section How @code{tar} Archives are Named -This option never deletes members. If a new member is added under the -same name as an existing member, then both will be in the archive, with -the new member after the old one. For information on how this affects -reading the archive, @ref{Multiple Members with the Same Name}. +Conventionally, @code{tar} archives are given names ending with +@samp{.tar}. This is not necessary for @code{tar} to operate properly, +but this manual follows that convention in order to accustom readers to +it and to make examples more clear. -This operation cannot be performed on some tape drives, unfortunately, -due to deficiencies in the formats thoes tape drives use. +@cindex tar file +@cindex entry +@cindex tar entry +Often, people refer to @code{tar} archives as ``@code{tar} files,'' and +archive members as ``files'' or ``entries''. For people familiar with +the operation of @code{tar}, this causes no difficulty. However, in +this manual, we consistently refer to ``archives'' and ``archive +members'' to make learning to use @code{tar} easier for novice users. -@section Combining Archives +@node posix compliance, Authors, Naming tar Archives, Introduction +@section POSIX Compliance -The @samp{--catenate} (or @code{--concatenate}, or @code{-A}) causes -@code{tar} to add the contents of several archives to an existing -archive. +@noindent +@FIXME{must ask franc,ois about this. dan hagerty thinks this might +be an issue, but we're not really sure at this time. dan just tried a +test case of mixing up options' orders while the variable was set, and +there was no problem...} + +We make some of our recommendations throughout this book for one +reason in addition to what we think of as ``good sense''. The main +additional reason for a recommendation is to be compliant with the +POSIX standards. If you set the shell environment variable +@code{POSIXLY_CORRECT}, GNU @code{tar} will force you to adhere to +these standards. Therefore, if this variable is set and you violate +one of the POSIX standards in the way you phrase a command, for +example, GNU @code{tar} will not allow the command and will signal an +error message. You would then have to reorder the options or rephrase +the command to comply with the POSIX standards. + +There is a chance in the future that, if you set this environment +variable, your archives will be forced to comply with POSIX standards, +also. No GNU @code{tar} extensions will be allowed. + +@node Authors, Reports, posix compliance, Introduction +@section GNU @code{tar} Authors + +GNU @code{tar} was originally written by John Gilmore, and modified by +many people. The GNU enhancements were written by Jay Fenlason, then +Joy Kendall, and the whole package has been further maintained by +Thomas Bushnell, n/BSG, and finally Fran@,{c}ois Pinard, with +the help of numerous and kind users. + +We wish to stress that @code{tar} is a collective work, and owes much to +all those people who reported problems, offered solutions and other +insights, or shared their thoughts and suggestions. An impressive, yet +partial list of those contributors can be found in the @file{THANKS} +file from the GNU @code{tar} distribution. + +@FIXME{i want all of these names mentioned, Absolutely. BUT, i'm not +sure i want to spell out the history in this detail, at least not for +the printed book. i'm just not sure it needs to be said this way. +i'll think about it.} + +@FIXME{History is more important, and surely more interesting, than +actual names. Quoting names without history would be meaningless. FP} + +Jay Fenlason put together a draft of a GNU @code{tar} manual, +borrowing notes from the original man page from John Gilmore. This +draft has been distributed in @code{tar} versions 1.04 (or even +before?) @FIXME{huh? IMO, either we know or we don't; the +parenthetical is confusing.} through 1.10, then withdrawn in version +1.11. Thomas Bushnell, n/BSG and Amy Gorin worked on a tutorial and +manual for GNU @code{tar}. Fran@,{c}ois Pinard put version 1.11.8 +of the manual together by taking information from all these sources +and merging them. Melissa Weisshaus finally edited and redesigned the +book to create version 1.12. @FIXME{update version number as +necessary; i'm being optimistic!} @FIXME{Someone [maybe karl berry? +maybe bob chassell? maybe melissa? maybe julie sussman?] needs to +properly index the thing.} + +For version 1.12, Daniel Hagerty contributed a great deal of technical +consulting. In particular, he is the primary author of @ref{Backups}. + +@node Reports, , Authors, Introduction +@section Reporting bugs or suggestions + +@cindex bug reports +@cindex reporting bugs +If you find problems or have suggestions about this program or manual, +please report them to @file{bug-gnu-utils@@prep.ai.mit.edu}. + +@node Tutorial, tar invocation, Introduction, Top +@chapter Tutorial Introduction to @code{tar} -Name the archives to be catenated on the command line. (Nothing happens -if you don't list any.) The members, and their member names, will be -copied verbatim from those archives. If this causes multiple members to -have the same name, it does not delete either; all the members with the -same name coexist. For information on how this affects reading the -archive, @ref{Multiple Members with the Same Name}. +This chapter guides you through some basic examples of three @code{tar} +operations: @samp{--create}, @samp{--list}, and @samp{--extract}. If +you already know how to use some other version of @code{tar}, then you +may not need to read this chapter. This chapter omits most complicated +details about how @code{tar} works. -You must use this option to concatenate archives. If you just combine -them with @code{cat}, the result will not be a valid @code{tar} format -archive. +@menu +* assumptions:: +* stylistic conventions:: +* basic tar options:: Basic @code{tar} Operations and Options +* frequent operations:: +* Two Frequent Options:: +* create:: How to Create Archives +* list:: How to List Archives +* extract:: How to Extract Members from an Archive +* going further:: +@end menu -This operation cannot be performed on some tape drives, unfortunately, -due to deficiencies in the formats thoes tape drives use. +@node assumptions, stylistic conventions, Tutorial, Tutorial +@ifinfo +@heading Assumptions this Tutorial Makes +@end ifinfo -@section Removing Archive Members +This chapter is paced to allow beginners to learn about @code{tar} +slowly. At the same time, we will try to cover all the basic aspects of +these three operations. In order to accomplish both of these tasks, we +have made certain assumptions about your knowledge before reading this +manual, and the hardware you will be using: + +@itemize @bullet +@item +Before you start to work through this tutorial, you should understand +what the terms ``archive'' and ``archive member'' mean +(@pxref{Definitions}). In addition, you should understand something +about how Unix-type operating systems work, and you should know how to +use some basic utilities. For example, you should know how to create, +list, copy, rename, edit, and delete files and directories; how to +change between directories; and how to figure out where you are in the +filesystem. You should have some basic understanding of directory +structure and how files are named according to which directory they are +in. You should understand concepts such as standard output and standard +input, what various definitions of the term ``argument'' mean, the +differences between relative and absolute path names, and @FIXME{what +else?}. + +@item +This manual assumes that you are working from your own home directory +(unless we state otherwise). In this tutorial, you will create a +directory to practice @code{tar} commands in. When we show path names, +we will assume that those paths are relative to your home directory. +For example, my home directory path is @file{/home/fsf/melissa}. All of +my examples are in a subdirectory of the directory named by that path +name; the subdirectory is called @file{practice}. + +@item +In general, we show examples of archives which exist on (or can be +written to, or worked with from) a directory on a hard disk. In most +cases, you could write those archives to, or work with them on any other +device, such as a tape drive. However, some of the later examples in +the tutorial and next chapter will not work on tape drives. +Additionally, working with tapes is much more complicated than working +with hard disks. For these reasons, the tutorial does not cover working +with tape drives. @xref{Media} for complete information on using +@code{tar} archives with tape drives. + +@FIXME{this is a cop out. need to add some simple tape drive info.} +@end itemize -You can use the @samp{--delete} option to remove members from an -archive. Name the members on the command line to be deleted. This -option will rewrite the archive; because of this, it does not work on -tape drives. If you list no members to be deleted, nothing happens. +@node stylistic conventions, basic tar options, assumptions, Tutorial +@ifinfo +@heading Stylistic Conventions +@end ifinfo -@section Listing Archive Members +In the examples, @samp{$} represents a typical shell prompt. It +precedes lines you should type; to make this more clear, those lines are +shown in @kbd{this font}, as opposed to lines which represent the +computer's response; those lines are shown in @code{this font}, or +sometimes @samp{like this}. When we have lines which are too long to be +displayed in any other way, we will show them like this: + +@smallexample +This is an example of a line which would otherwise not fit in this space. +@end smallexample + +@FIXME{how often do we use smallexample?} + +@node basic tar options, frequent operations, stylistic conventions, Tutorial +@section Basic @code{tar} Operations and Options + +@code{tar} can take a wide variety of arguments which specify and define +the actions it will have on the particular set of files or the archive. +The main types of arguments to @code{tar} fall into one of two classes: +operations, and options. + +Some arguments fall into a class called @dfn{operations}; exactly one of +these is both allowed and required for any instance of using @code{tar}; +you may @emph{not} specify more than one. People sometimes speak of +@dfn{operating modes}. You are in a particular operating mode when you +have specified the operation which specifies it; there are eight +operations in total, and thus there are eight operating modes. + +The other arguments fall into the class known as @dfn{options}. You are +not required to specify any options, and you are allowed to specify more +than one at a time (depending on the way you are using @code{tar} at +that time). Some options are used so frequently, and are so useful for +helping you type commands more carefully that they are effectively +``required''. We will discuss them in this chapter. + +You can write most of the @code{tar} operations and options in any of +three forms: long (mnemonic) form, short form, and old style. Some of +the operations and options have no short or ``old'' forms; however, the +operations and options which we will cover in this tutorial have +corresponding abbreviations. @FIXME{make sure this is still the case, +at the end} We will indicate those abbreviations appropriately to get +you used to seeing them. (Note that the ``old style'' option forms +exist in GNU @code{tar} for compatibility with Unix @code{tar}. We +present a full discussion of this way of writing options and operations +appears in @ref{Old Options}, and we discuss the other two styles of +writing options in @ref{Mnemonic Options} and @ref{Short Options}.) + +In the examples and in the text of this tutorial, we usually use the +long forms of operations and options; but the ``short'' forms produce +the same result and can make typing long @code{tar} commands easier. +For example, instead of typing -The @samp{--list} (@samp{-t}) option will list the names of members of -the archive. Name the members to be listed on the command line (to -modify the way these names are interpreted, @pxref{Specifying Names to -@code{tar}}). If you name no members, then @samp{--list} will list the -names of all the members of the archive. +@example +@kbd{tar --create --verbose --file=afiles.tar apple angst aspic} +@end example -To see more than just the names of the members, use the @samp{--verbose} -option to cause @code{tar} to print out a listing similar to that of -@samp{ls -l}. +@noindent +you can type +@example +@kbd{tar -c -v -f afiles.tar apple angst aspic} +@end example -@section Extracting Archive Members +@noindent +or even +@example +@kbd{tar -cvf afiles.tar apple angst aspic} +@end example -Use @samp{--extract} (or @samp{--get}, or @samp{-x}) to extract members -from an archive. For each member named (or for the entire archive if no -members are named) on the command line (or with @samp{--files-from}) the -a file is created with the contents of the archive member. The name of -the file is the same as the member name. +@noindent +For more information on option syntax, see @ref{Advanced tar}. In +discussions in the text, when we name an option by its long form, we +also give the corresponding short option in parentheses. + +The term, ``option'', can be confusing at times, since ``operations'' +are often lumped in with the actual, @emph{optional} ``options'' in certain +general class statements. For example, we just talked about ``short and +long forms of options and operations''. However, experienced @code{tar} +users often refer to these by shorthand terms such as, ``short and long +options''. This term assumes that the ``operations'' are included, also. +Context will help you determine which definition of ``options'' to use. + +Similarly, the term ``command'' can be confusing, as it is often used in +two different ways. People sometimes refer to @code{tar} ``commands''. +A @code{tar} @dfn{command} is the entire command line of user input +which tells @code{tar} what to do --- including the operation, options, +and any arguments (file names, pipes, other commands, etc). However, +you will also sometimes hear the term ``the @code{tar} command''. When +the word ``command'' is used specifically like this, a person is usually +referring to the @code{tar} @emph{operation}, not the whole line. +Again, use context to figure out which of the meanings the speaker +intends. + +@node frequent operations, Two Frequent Options, basic tar options, Tutorial +@section The Three Most Frequently Used Operations + +Here are the three most frequently used operations (both short and long +forms), as well as a brief description of their meanings. The rest of +this chapter will cover how to use these operations in detail. We will +present the rest of the operations in the next chapter. + +@table @kbd +@item --create +@itemx -c +Create a new @code{tar} archive. +@item --list +@itemx -t +List the contents of an archive. +@item --extract +@itemx -x +Extract one or more members from an archive. +@end table -Various options cause @code{tar} to extract more than just file -contents, such as the owner, the permissions, the modification date, and -so forth. +@node Two Frequent Options, create, frequent operations, Tutorial +@section Two Frequently Used Options -XXX -The @samp{--same-permissions} (or @samp{--preserve-permissions}, or -@samp{-p}) options cause @code{tar} to cause the new file to have the -same permissions as the original file did when it was placed in the -archive. Without this option, the current @code{umask} is used to -affect the permissions. +To understand how to run @code{tar} in the three operating modes listed +previously, you also need to understand how to use two of the options to +@code{tar}: @samp{--file} (which takes an archive file as an argument) +and @samp{--verbose}. (You are usually not @emph{required} to specify +either of these options when you run @code{tar}, but they can be very +useful in making things more clear and helping you avoid errors.) -When extrating, @code{tar} normally sets the modification time of the -file to the value recorded in the archive. The -@samp{--modification-time} option causes @code{tar} to omit doing this. -XXX +@menu +* file tutorial:: +* verbose tutorial:: +* help tutorial:: +@end menu -@section Updating an Archive +@node file tutorial, verbose tutorial, Two Frequent Options, Two Frequent Options +@unnumberedsubsec The @samp{--file} Option -The @samp{--update} (or @samp{-u}) option updates a @code{tar} archive -by comparing the date of the specified archive members against the date -of the file with the same name. If the file has been modified more -recently than the archive member, then the archive member is deleted (as -with @samp{--delete}) and then the file is added to the archive (as with -@samp{--append}). On media where the @samp{--delete} option cannot be -performed (such as magnetic tapes), the @samp{--update} option similarly -fails. +@table @kbd +@item --file=@var{archive-name} +@itemx -f @var{archive-name} +Specify the name of an archive file. +@end table -If no archive members are named (either on the command line or via -@samp{--files-from}), then the entire archive is processed in this -manner. +You can specify an argument for the @value{op-file} option whenever you +use @code{tar}; this option determines the name of the archive file +that @code{tar} will work on. -@section Comparing Archives Members with Files +If you don't specify this argument, then @code{tar} will use a +default, usually some physical tape drive attached to your machine. +If there is no tape drive attached, or the default is not meaningful, +then @code{tar} will print an error message. The error message might +look roughly like one of the following: -The @samp{--compare} (or @samp{--diff}, or @samp{-d}) option compares -the contents of the specified archive members against the files with the -same names, and reports its findings. If no members are named on the -command line (or through @samp{--files-from}), then the entire archive -is so compared. +@example +tar: can't open /dev/rmt8 : No such device or address +tar: can't open /dev/rsmt0 : I/O error +@end example -@chapter Specifying Names to @code{tar} +@noindent +To avoid confusion, we recommend that you always specfiy an archive file +name by using @value{op-file} when writing your @code{tar} commands. +For more information on using the @value{op-file} option, see +@ref{file}. -When specifying the names of files or members to @code{tar}, it by -default takes the names of the files from the command line. There are -other ways, however, to specify file or member names, or to modify the -manner in which @code{tar} selects the files or members upon which to -operate. In general, these methods work both for specifying the names -of files and archive members. +@node verbose tutorial, help tutorial, file tutorial, Two Frequent Options +@unnumberedsubsec The @samp{--verbose} Option -@section Reading Names from a File +@table @kbd +@item --verbose +@itemx -v +Show the files being worked on as @code{tar} is running. +@end table -Instead of giving the names of files or archive members on the command -line, you can put the names into a file, and then use the -@samp{--files-from=@var{file-name-list}} (@samp{-T -@var{file-name-list}}) option to @code{tar}. Give the name of the file -which contains the list as the argument to @samp{--files-from}. The -file names should be separated by newlines in the list. If you give a -single dash as a filename for @samp{--files-from} (that is, you specify -@samp{--files-from=-} or @samp{-T -}), then the filenames are read from -standard input. - -If you want to specify names that might contain newlines, use the -@samp{--null} option. Then, the filenames should be separated by NUL -characters (ASCII 000) instead of newlines. In addition, the -@samp{--null} option turns off the @samp{-C} option (@pxref{Changing -Directory}). +@value{op-verbose} shows details about the results of running +@code{tar}. This can be especially useful when the results might not be +obvious. For example, if you want to see the progress of @code{tar} as +it writes files into the archive, you can use the @samp{--verbose} +option. In the beginning, you may find it useful to use +@samp{--verbose} at all times; when you are more accustomed to +@code{tar}, you will likely want to use it at certain times but not at +others. We will use @samp{--verbose} at times to help make something +clear, and we will give many examples both using and not using +@samp{--verbose} to show the differences. + +Sometimes, a single instance of @samp{--verbose} on the command line +will show a full, @samp{ls} style listing of an archive or files, +giving sizes, owners, and similar information. Other times, +@samp{--verbose} will only show files or members that the particular +operation is operating on at the time. In the latter case, you can +use @samp{--verbose} twice in a command to get a listing such as that +in the former case. For example, instead of saying -@section Excluding Some Files +@example +@kbd{tar -cvf afiles.tar apple angst aspic} +@end example -The @samp{--exclude=@var{pattern}} option will prevent any file or -member which matches the regular expression @var{pattern} from being -operated on. For example, if you want to create an archive with all the -contents of @file{/tmp} except the file @file{/tmp/foo}, you can use the -command @samp{tar --create --file=arch.tar --exclude=foo}. +@noindent +above, you might say -If there are many files you want to exclude, you can use the -@samp{--exclude-from=@var{exclude-list}} (@samp{-X @var{exclude-list}}) -option. This works just like the -@samp{--files-from=@var{file-name-list}} option: specify the name of a -file as @var{exclude-list} which contains the list of patterns you want -to exclude. +@example +@kbd{tar -cvvf afiles.tar apple angst aspic} +@end example -@xref{Regular Expressions} for more information on the syntax and -meaning of regular expressions. +@noindent +This works equally well using short or long forms of options. Using +long forms, you would simply write out the mnemonic form of the option +twice, like this: -@section Operating Only on New Files +@example +$ @kbd{tar --create --verbose --verbose @dots{}} +@end example -The @samp{--newer=@var{date}} (@samp{--after-date=@var{date}} or -@samp{-N @var{date}}) limits @code{tar} to only operating on files which -have been modified after the date specified. (For more information on -how to specify a date, @xref{Date Formats}.) A file is considered to -have changed if the contents have been modified, or if the owner, -permissions, and so forth, have been changed. +@noindent +Note that you must double the hyphens properly each time. -If you only want @code{tar} make the date comparison on the basis of the -actual contents of the file's modification, then use the -@samp{--newer-mtime=@var{date}} option. +Later in the tutorial, we will give examples using @w{@samp{--verbose +--verbose}}. -You should never use this option for making incremental dumps. To learn -how to use @code{tar} to make backups, @ref{Making Backups}. +@node help tutorial, , verbose tutorial, Two Frequent Options +@unnumberedsubsec Getting Help: Using the @code{--help} Option -@section Crossing Filesystem Boundaries +@table @kbd +@item --help -The @samp{--one-file-system} option causes @code{tar} to modify its -normal behavior in archiving the contents of directories. If a file in -a directory is not on the same filesystem as the directory itself -(because it is a mounted filesystem in its own right), then @code{tar} -will not archive that file, or (if it is a directory itself) anything -beneath it. +The @samp{--help} option to @code{tar} prints out a very brief list of +all operations and option available for the current version of +@code{tar} available on your system. +@end table -This does not necessarily limit @code{tar} to only archiving the -contents of a single filesystem, because all files named on the command -line (or through the @samp{--files-from} option) will always be -archived. +@node create, list, Two Frequent Options, Tutorial +@section How to Create Archives +@UNREVISED + +One of the basic operations of @code{tar} is @value{op-create}, which +you use to create a @code{tar} archive. We will explain +@samp{--create} first because, in order to learn about the other +operations, you will find it useful to have an archive available to +practice on. + +To make this easier, in this section you will first create a directory +containing three files. Then, we will show you how to create an +@emph{archive} (inside the new directory). Both the directory, and +the archive are specifically for you to practice on. The rest of this +chapter and the next chapter will show many examples using this +directory and the files you will create: some of those files may be +other directories and other archives. + +The three files you will archive in this example are called +@file{blues}, @file{folk}, and @file{jazz}. The archive is called +@file{collection.tar}. + +This section will proceed slowly, detailing how to use @samp{--create} +in @code{verbose} mode, and showing examples using both short and long +forms. In the rest of the tutorial, and in the examples in the next +chapter, we will proceed at a slightly quicker pace. This section +moves more slowly to allow beginning users to understand how +@code{tar} works. -@chapter Changing the Names of Members when Archiving +@menu +* prepare for examples:: +* Creating the archive:: +* create verbose:: +* short create:: +* create dir:: +@end menu -@section Changing Directory +@node prepare for examples, Creating the archive, create, create +@subsection Preparing a Practice Directory for Examples + +To follow along with this and future examples, create a new directory +called @file{practice} containing files called @file{blues}, @file{folk} +and @file{jazz}. The files can contain any information you like: +ideally, they should contain information which relates to their names, +and be of different lengths. Our examples assume that @file{practice} +is a subdirectory of your home directory. + +Now @code{cd} to the directory named @file{practice}; @file{practice} +is now your @dfn{working directory}. (@emph{Please note}: Although +the full path name of this directory is +@file{/@var{homedir}/practice}, in our examples we will refer to +this directory as @file{practice}; the @var{homedir} is presumed. + +In general, you should check that the files to be archived exist where +you think they do (in the working directory) by running @code{ls}. +Because you just created the directory and the files and have changed to +that directory, you probably don't need to do that this time. + +It is very important to make sure there isn't already a file in the +working directory with the archive name you intend to use (in this case, +@samp{collection.tar}), or that you don't care about its contents. +Whenever you use @samp{create}, @code{tar} will erase the current +contents of the file named by @value{op-file} if it exists. @code{tar} +will not tell you if you are about to overwrite a file unless you +specify an option which does this @FIXME{xref to the node for +--backup!}. To add files to an existing archive, you need to use a +different option, such as @value{op-append}; see @ref{append} for +information on how to do this. + +@node Creating the archive, create verbose, prepare for examples, create +@subsection Creating the Archive + +To place the files @file{blues}, @file{folk}, and @file{jazz} into an +archive named @file{collection.tar}, use the following command: -The @samp{--directory=@var{directory}} (@samp{-C @var{directory}}) -option causes @code{tar} to change its current working directory to -@var{directory}. Unlike most options, this one is processed at the -point it occurs within the list of files to be processed. Consider the -following command: @example -tar --create --file=foo.tar -C /etc passwd hosts -C /lib libc.a +$ @kbd{tar --create --file=collection.tar blues folk jazz} @end example -This command will place the files @file{/etc/passwd}, @file{/etc/hosts}, -and @file{/lib/libc.a} into the archive. However, the names of the -archive members will be exactly what they were on the command line: -@file{passwd}, @file{hosts}, and @file{libc.a}. The @samp{--directory} -option is frequently used to make the archive independent of the -original name of the directory holding the files. +The order of the arguments is not very important, @emph{when using long +option forms}. You could also say: -Note that @samp{--directory} options are interpreted consecutively. If -@samp{--directory} option specifies a relative pathname, it is -interpreted relative to the then current directory, which might not be -the same as the original current working directory of @code{tar}, due to -a previous @samp{--directory} option. +@example +$ @kbd{tar blues --create folk --file=collection.tar jazz} +@end example -When using @samp{--files-from} (@pxref{Reading Names from a File}), you -can put @samp{-C} options in the file list. Unfortunately, you cannot -put @samp{--directory} options in the file list. (This interpretation -can be disabled by using the @samp{--null} option.) +@noindent +However, you can see that this order is harder to understand; this is +why we will list the arguments in the order that makes the commands +easiest to understand (and we encourage you to do the same when you use +@code{tar}, to avoid errors). + +Note that the part of the command which says, +@w{@kbd{--file=collection.tar}} is considered to be @emph{one} argument. +If you substituted any other string of characters for +@kbd{`collection.tar'}, then that string would become the name of the +archive file you create. + +The order of the options becomes more important when you begin to use +short forms. With short forms, if you type commands in the wrong order +(even if you type them correctly in all other ways), you may end up with +results you don't expect. For this reason, it is a good idea to get +into the habit of typing options in the order that makes inherent sense. +@xref{short create} for more information on this. + +In this example, you type the command as shown above: @samp{--create} +is the operation which creates the new archive +(@file{collection.tar}), and @samp{--file} is the option which lets +you give it the name you chose. The files, @file{blues}, @file{folk}, +and @file{jazz}, are now members of the archive, @file{collection.tar} +(they are @dfn{file name arguments} to the @samp{--create} operation) +@FIXME{xref here to the discussion of file name args?}. Now that they +are are in the archive, they are called @emph{archive members}, not +files @FIXME{xref to definitions?}. + +When you create an archive, you @emph{must} specify which files you want +placed in the archive. If you do not specify any archive members, GNU +@code{tar} will complain. + +If you now list the contents of the working directory (@kbd{ls}), you will +find the archive file listed as well as the files you saw previously: -@section Absolute Path Names +@example +blues folk jazz collection.tar +@end example -When @code{tar} extracts archive members from an archive, it strips any -leading slashes (@code{/}) from the member name. This causes absolute -member names in the archive to be treated as relative file names. This -allows you to have such members extracted wherever you want, instead of -being restricted to extracting the member in the exact directory named -in the archive. For example, if the archive member has the name -@file{/etc/passwd}, @code{tar} will extract it as if the name were -really @file{etc/passwd}. +@noindent +Creating the archive @samp{collection.tar} did not destroy the copies of +the files in the directory. -Other @code{tar} programs do not do this. As a result, if you create an -archive whose member names start with a slash, they will be difficult -for other people with an inferior @code{tar} program to use. Therefore, -GNU @code{tar} also strips leading slashes from member names when -putting members into the archive. For example, if you ask @code{tar} to -add the file @file{/bin/ls} to an archive, it will do so, but the member -name will be @file{bin/ls}. +Keep in mind that if you don't indicate an operation, @code{tar} will not +run and will prompt you for one. If you don't name any files, @code{tar} +will complain. You must have write access to the working directory, +or else you will not be able to create an archive in that directory. -If you use the @samp{--absolute-paths} option, @code{tar} will do -neither of these transformations. +@emph{Caution}: Do not attempt to use @value{op-create} to add files to +an existing archive; it will delete the archive and write a new one. +Use @value{op-append} instead. @xref{append}. -@section Symbolic Links +@node create verbose, short create, Creating the archive, create +@subsection Running @samp{--create} with @samp{--verbose} -Normally, when @code{tar} archives a symbolic link, it writes a record -to the archive naming the target of the link. In that way, the -@code{tar} archive is a faithful record of the filesystem contents. -However, if you want @code{tar} to actually dump the contents of the -target of the symbolic link, then use the @samp{--dereference} option. +If you include the @value{op-verbose} option on the command line, +@code{tar} will list the files it is acting on as it is working. In +verbose mode, the @code{create} example above would appear as: -@chapter Making @code{tar} More Verbose +@example +$ @kbd{tar --create --verbose --file=collection.tar blues folk jazz} +blues +folk +jazz +@end example -Various options cause @code{tar} to print information as it progresses -in its job. +This example is just like the example we showed which did not use +@samp{--verbose}, except that @code{tar} generated the remaining lines +@iftex +(note the different font styles). +@end iftex +@ifinfo +. +@end ifinfo -The @samp{--verbose} (or @samp{-v}) option causes @code{tar} to print -the name of each archive member or file as it is processed. Since -@samp{--list} already prints the names of the members, @samp{--verbose} -used with @samp{--list} causes @code{tar} to print a longer listing -(reminiscent of @samp{ls -l}) for each member. +In the rest of the examples in this chapter, we will frequently use +@code{verbose} mode so we can show actions or @code{tar} responses that +you would otherwise not see, and which are important for you to +understand. -To see the progress of @code{tar} through the archive, the -@samp{--record-number} option prints a message for each record read or -writted. (@xref{Archive Structure}.) This option can be very helpful -when trying to figure out where in the archive an error occurs. +@node short create, create dir, create verbose, create +@subsection Short Forms with @samp{create} -The @samp{--totals} option (which is only meaningful when used with -@samp{--create}) causes @code{tar} to print the total amount written to -the archive, after it has been fully created. +As we said before, the @value{op-create} operation is one of the most +basic uses of @code{tar}, and you will use it countless times. +Eventually, you will probably want to use abbreviated (or ``short'') +forms of options. A full discussion of the three different forms that +options can take appears in @ref{Styles}; for now, here is what the +previous example (including the @value{op-verbose} option) looks like +using short option forms: -The @samp{--checkpoint} option prints an occasional message as -@code{tar} reads or writes the archive. It is designed for those who -don't need the more detailed (and voluminous) output of -@samp{--record-number}, but do want visual confirmation that @code{tar} -is actually making forward progress. +@example +$ @kbd{tar -cvf collection.tar blues folk jazz} +blues +folk +jazz +@end example -The @samp{--version} option will generate a message with the version of -GNU @code{tar} you are using. +@noindent +As you can see, the system responds the same no matter whether you use +long or short option forms. -@chapter Input and Output +@FIXME{i don't like how this is worded:} One difference between using +short and long option forms is that, although the exact placement of +arguments following options is no more specific when using short forms, +it is easier to become confused and make a mistake when using short +forms. For example, suppose you attempted the above example in the +following way: -@section Changing the Archive Name +@example +$ @kbd{tar -cfv collection.tar blues folk jazz} +@end example -By default, @code{tar} uses an archive file name compiled in when -@code{tar} was built. Usually this refers to some physical tape drive -on the machine. Often, the installer of @code{tar} didn't set the -default to anything meaningful at all. +@noindent +In this case, @code{tar} will make an archive file called @file{v}, +containing the files @file{blues}, @file{folk}, and @file{jazz}, because +the @samp{v} is the closest ``file name'' to the @samp{-f} option, and +is thus taken to be the chosen archive file name. @code{tar} will try +to add a file called @file{collection.tar} to the @file{v} archive file; +if the file @file{collection.tar} did not already exist, @code{tar} will +report an error indicating that this file does not exist. If the file +@file{collection.tar} does already exist (e.g., from a previous command +you may have run), then @code{tar} will add this file to the archive. +Because the @samp{-v} option did not get registered, @code{tar} will not +run under @samp{verbose} mode, and will not report its progress. + +The end result is that you may be quite confused about what happened, +and possibly overwrite a file. To illustrate this further, we will show +you how an example we showed previously would look using short forms. + +This example, -As a result, most uses of @code{tar} need to tell @code{tar} where to -find (or create) the archive. The @samp{--file=@var{archive-name}} (or -@samp{-f @var{archive-name}} option selects another file to use as the -archive. +@example +$ @kbd{tar blues --create folk --file=collection.tar jazz} +@end example -If the archive file name includes a colon (@samp{:}), then it is assumed -to be a file on another machine. If the archive file is -@samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the -host @var{host}. The remote host is accessed using the @code{rsh} -program, with a username of @var{user}. If the username is omitted -(along with the @samp{@@} sign), then your user name will be used. -(This is the normal @code{rsh} behavior.) It is necessary for the -remote machine, in addition to permitting your @code{rsh} access, to -have the @code{/usr/ucb/rmt} program installed. If you need to use a -file whose name includes a colon, then the remote tape drive behavior -can be inhibited by using the @samp{--force-local} option. +@noindent +is confusing as it is. When shown using short forms, however, it +becomes much more so: -If the filename you give to @samp{--file} is a single dash (@samp{-}), -then @code{tar} will read the archive from (or write it to) standard -input (or standard output). +@example +$ @kbd{tar blues -c folk -f collection.tar jazz} +@end example -@section Extracting Members to Standard Output +@noindent +It would be very easy to put the wrong string of characters +immediately following the @samp{-f}, but doing that could sacrifice +valuable data. + +For this reason, we recommend that you pay very careful attention to +the order of options and placement of file and archive names, +especially when using short option forms. Not having the option name +written out mnemonically can affect how well you remember which option +does what, and therefore where different names have to be placed. +(Placing options in an unusual order can also cause @code{tar} to +report an error if you have set the shell environment variable, +@code{POSIXLY_CORRECT}; @pxref{posix compliance} for more information +on this.) + +@node create dir, , short create, create +@subsection Archiving Directories + +@cindex Archiving Directories +@cindex Directories, Archiving +You can archive a directory by specifying its directory name as a +file name argument to @code{tar}. The files in the directory will be +archived relative to the working directory, and the directory will be +re-created along with its contents when the archive is extracted. + +To archive a directory, first move to its superior directory. If you +have followed the previous instructions in this tutorial, you should +type: -An archive member in normally extracted into a file with the same name -as the archive member. However, you can use the @samp{--to-stdout} to -cause @code{tar} to write extracted archive members to standard output. -If you extract multiple members, they appear on standard output -concatenated, in the order they are found in the archive. +@example +$ @kbd{cd ..} +$ +@end example -@section Dealing with Compressed Archives +@noindent +This will put you into the directory which contains @file{practice}, +i.e. your home directory. Once in the superior directory, you can +specify the subdirectory, @file{practice}, as a file name argument. To +store @file{practice} in the new archive file @file{music.tar}, type: -You can have archives be compressed by using the @samp{--gzip} (or -@samp{-z}) option. This will arrange for @code{tar} to use the -@code{gzip} program to be used to compress or uncompress the archive -wren writing or reading it. +@example +$ @kbd{tar --create --verbose --file=music.tar practice} +@end example -To use the older, obsolete, @code{compress} program, use the -@samp{--compress} (or @samp{-Z}) option. The GNU Project recommends you -not use @code{compress}, because there is a patent covering the -algorithm it uses. Merely by running @code{compress} you could be sued -for patent infringment. - -When using either @samp{--gzip} or @samp{--compress}, @code{tar} does -not do blocking (@pxref{Blocking}) correctly. Use @samp{--gzip-block} -or @samp{--compress-block} instead when using real tape drives. - -@chapter Being More Careful - -When using @code{tar} with many options, particularly ones with -complicated or difficult-to-predict behavior, it is possible to make -serious mistakes. As a result, @code{tar} provides several options that -make observing @code{tar} easier. - -The @samp{--verbose} option causes @code{tar} to print the name of each -file or archive member as it is processed. This and the other options -which make tar print status information can be useful in monitoring -@code{tar}. @xref{Making @code{tar} More Verbose}. - -If you use @samp{--interactive} (or {@samp--confirm}), then @code{tar} -will ask you for confirmation before each operation. For example, when -extracting, it will prompt you before each archive member is extracted, -and you can select that member for extraction or pass over to the next. - -The @samp{--verify} option, when using @samp{--create}, causes -@code{tar}, after having finished creating the archive, to go back over -it and compare its contents against the files that were placed in the -archive. - -The @samp{--show-omitted-dirs} option, when reading an archive (with -@samp{--list} or @samp{--extract}, for example), causes a message to be -printed for each directory in the archive which is skipped. This -happens regardless of the reason for skipping: the directory might not -have been named on the command line (implicitly or explicitly), it might -be excluded by the use of the @samp{--exclude} option, or some other -reason. - -@chapter Using Real Tape Drives +@noindent +@code{tar} should output: -Many complexities surround the use of @code{tar} on tape drives. Since -the creation and manipulation of archives located on magnetic tape was -the original purpose of @code{tar}, it contains many features making -such manipulation easier. +@example +practice/ +practice/blues +practice/folk +practice/jazz +practice/collection.tar +@end example -@section Blocking +Note that the archive thus created is not in the subdirectory +@file{practice}, but rather in the current working directory---the +directory from which @code{tar} was invoked. Before trying to archive a +directory from its superior directory, you should make sure you have +write access to the superior directory itself, not only the directory +you are trying archive with @code{tar}. For example, you will probably +not be able to store your home directory in an archive by invoking +@code{tar} from the root directory; @value{xref-absolute-names}. (Note +also that @file{collection.tar}, the original archive file, has itself +been archived. @code{tar} will accept any file as a file to be +archived, regardless of its content. When @file{music.tar} is +extracted, the archive file @file{collection.tar} will be re-written +into the file system). + +If you give @code{tar} a command such as -When writing to tapes, @code{tar} writes the contents of the archive in -chunks known as @dfn{blocks}. To change the default blocksize, use the -@samp{--block-size=@var{blocking-factor}} (@samp{-b -@var{blocking-factor}) option. Each block will then be composed of -@var{blocking-factor} records. (Each @code{tar} record is 512 bytes. -@xref{Archive Format}.) Each file written to the archive uses at least -one full block. As a result, using a larger block size can result in -more wasted space for small files. On the other hand, a larger block -size can ofter be read and written much more efficiently. +@example +$ @kbd{tar --create --file=foo.tar .} +@end example -Further complicating the problem is that some tape drives ignore the -blocking entirely. For these, a larger block size can still improve -performance (because the software layers above the tape drive still -honor the blocking), but not as dramatically as on tape drives that -honor blocking. +@noindent +@code{tar} will report @samp{tar: foo.tar is the archive; not dumped}. +This happens because @code{tar} creates the archive @file{foo.tar} in +the current directory before putting any files into it. Then, when +@code{tar} attempts to add all the files in the directory @file{.} to +the archive, it notices that the file @file{foo.tar} is the same as the +archive, and skips it. (It makes no sense to put an archive into +itself.) GNU @code{tar} will continue in this case, and create the +archive normally, except for the exclusion of that one file. +(@emph{Please note:} Other versions of @code{tar} are not so clever; +they will enter an infinite loop when this happens, so you should not +depend on this behavior unless you are certain you are running GNU +@code{tar}. @FIXME{bob doesn't like this sentence, since he does it +all the time, and we've been doing it in the editing passes for this +manual: In general, make sure that the archive is not inside a +directory being dumped.}) + +@node list, extract, create, Tutorial +@section How to List Archives -Wher reading an archive, @code{tar} can usually figure out the block -size on itself. When this is the case, and a non-standard block size -was used when the archive was created, @code{tar} will print a message -about a non-standard blocking factor, and then operate normally. On -some tape devices, however, @code{tar} cannot figure out the block size -itself. On most of those, you can specify a blocking factor (with -@samp{--block-size) larger than the actual blocking factor, and then use -the @samp{--read-full-blocks} option. (If you specify a blocking factor -with @samp{--block-size} and don't use the @samp{--read-full-blocks} -option, then @code{tar} will not attempt to figure out the blocking size -itself.) On some devices, you must always specify the block size -exactly with @samp{--block-size} when reading, because @code{tar} cannot -figure it out. In any case, use @samp{--list} before doing any -extractions to see whether @code{tar} is reading the archive correctly. +Frequently, you will find yourself wanting to determine exactly what a +particular archive contains. You can use the @value{op-list} operation +to get the member names as they currently appear in the archive, as well +as various attributes of the files at the time they were archived. For +example, you can examine the archive @file{collection.tar} that you +created in the last section with the command, -If you use a blocking factor larger than 20, older @code{tar} programs -might not be able to read the archive, so we recommend this as a limit -to use in practice. GNU @code{tar}, however, will support arbitrarily -large block sizes, limited only by the amount of virtual memory or the -physical characteristics of the tape device. +@example +$ @kbd{tar --list --file=collection.tar} +@end example -If you are writing a compressed archive to tape with @samp{--compress} -or @samp{--gzip} (@pxref{Input and Output}), @code{tar} will not block -the archive correctly. This doesn't matter if you are writing the -archive to a normal file or through a pipe, but if you are writing it to -a tape drive, then this causes problems. Use @samp{--compress-block} or -@samp{--gzip-block} instead, to cause @code{tar} to arrange to have -blocking work correctly. +@noindent +The output of @code{tar} would then be: -@section Using Multiple Tapes +@example +blues +folk +jazz +@end example -Often you might want to write a large archive, one larger than will fit -on the actual tape you are using. In such a case, you can run multiple -@code{tar} commands, but this can be inconvenient, particularly if you -are using options like @samp{--exclude} or dumping entire filesystems. -Therefore, @code{tar} supports multiple tapes automatically. +@FIXME{we hope this will change. if it doesn't, need to show the +creation of bfiles somewhere above!!! : } -Use @samp{--multi-volume} on the command line, and then @code{tar} will, -when it reaches the end of the tape, prompt for another tape, and -continue the archive. Each tape will have an independent archive, and -can be read without needing the other. (As an exception to this, the -file that @code{tar} was archiving when it ran out of tape will usually -be split between the two archives; in this case you need to extract from -the first archive, using @samp{--multi-volume}, and then put in the -second tape when prompted, so @code{tar} can restore both halves of the -file.) +@noindent +The archive @file{bfiles.tar} would list as follows: -When prompting for a new tape, @code{tar} accepts any of the following -responses: +@example +./birds +baboon +./box +@end example -@table @samp -@item ? -Request @code{tar} to explain possible responses -@item q -Request @code{tar} to exit immediately. -@item n @var{file-name} -Request @code{tar} to write the next volume on the file @var{file-name}. -@item ! -Request @code{tar} to run a subshell. -@item y -Request @code{tar} to begin writing the next volume. -@end table +@noindent +Be sure to use a @value{op-file} option just as with @value{op-create} +to specify the name of the archive. -(You should only type @samp{y} after you have changed the tape; -otherwise @code{tar} will write over the volume it just finished.) +If you use the @value{op-verbose} option with @samp{--list}, then +@code{tar} will print out a listing reminiscent of @w{@samp{ls -l}}, +showing owner, file size, and so forth. -If you want more elaborate behavior than this, give @code{tar} the -@samp{--info-script=@var{script-name}} option. The file -@var{script-name} is expected to be a program (or shell script) to be -run instead of the normal prompting procedure. When the program -finishes, @code{tar} will immediately begin writing the next volume. -(The behavior of the @samp{n} response to the normal tape-change prompt -is not available if you use @samp{--info-script}.) +If you had used @value{op-verbose} mode, the example above would look +like: -The method @code{tar} uses to detect end of tape is not perfect, and -fails on some operating systems or on some devices. You can use the -@samp{--tape-length=@var{size}} (or @samp{-L @var{size}}) option if -@code{tar} can't detect the end of the tape itself. The @var{size} -argument should be the size of the tape. - -The volume number used by @code{tar} in its tape-change prompt can be -changed; if you give the @samp{--volno-file=@var{file-name}} option, -then @var{file-name} should contain a decimal number. That number will -be used as the volume number of the first volume written. When -@code{tar} is finished, it will rewrite the file with the now--current -volume number. (This does not change the volume number written on a -tape label (@pxref{Special Options for Archiving}; it @emph{only} -affects the number used in the prompt.) +@example +$ @kbd{tar --list --verbose --file=collection.tar folk} +-rw-rw-rw- myself user 62 1990-05-23 10:55 folk +@end example -If you want @code{tar} to cycle through a series of tape drives, then -you can use the @samp{n} response to the tape-change prompt. This is -error prone, however, and doesn't work at all with @samp{--info-script}. -Therefore, if you give @code{tar} multiple @samp{--file} options, then -the specified files will be used, in sequence, as the successive volumes -of the archive. Only when the first one in the sequence needs to be -used again will @code{tar} prompt for a tape change (or run the info -script). +@cindex File name arguments, using @code{--list} with +@cindex @code{--list} with file name arguments +You can specify one or more individual member names as arguments when +using @samp{list}. In this case, @code{tar} will only list the +names of members you identify. For example, @w{@kbd{tar --list +--file=afiles.tar apple}} would only print @file{apple}. + +@FIXME{we hope the relevant aspects of this will change:}Because +@code{tar} preserves paths, file names must be specified as they appear +in the archive (ie., relative to the directory from which the archive +was created). Therefore, it is essential when specifying member names +to @code{tar} that you give the exact member names. For example, +@w{@kbd{tar --list --file=bfiles birds}} would produce an error message +something like @samp{tar: birds: Not found in archive}, because there is +no member named @file{birds}, only one named @file{./birds}. While the +names @file{birds} and @file{./birds} name the same file, @emph{member} +names are compared using a simplistic name comparison, in which an exact +match is necessary. @xref{absolute}. + +However, @w{@kbd{tar --list --file=collection.tar folk}} would respond +with @file{folk}, because @file{folk} is in the archive file +@file{collection.tar}. If you are not sure of the exact file name, try +listing all the files in the archive and searching for the one you +expect to find; remember that if you use @samp{--list} with no file +names as arguments, @code{tar} will print the names of all the members +stored in the specified archive. -@section Tape Files +@menu +* list dir:: +@end menu -When @code{tar} writes an archive to tape, it creates a single tape -file. If multiple archives are written to the same tape, one after the -other, they each get written as separate tape files. When extracting, -it is necessary to position the tape at the right place before running -@code{tar}. To do this, use the @code{mt} command. For more -information on the @code{mt} command and on the organization of tapes -into a sequence of tape files, see XXX. +@node list dir, , list, list +@unnumberedsubsec Listing the Contents of a Stored Directory +@UNREVISED -@chapter Special Options for Archiving +@FIXME{i changed the order of these nodes around and haven't had a +chance to play around with this node's example, yet. i have to play +with it and see what it actually does for my own satisfaction, even if +what it says *is* correct..} -To give the archive a name which will be recorded in it, use the -@samp{--label=@var{volume-label}} (or @samp{-V}) option. This will -write a special record identifying @var{volume-label} as the name of the -archive to the front of the archive which will be displayed when the -archive is listed with @samp{--list}. If you are creating a -multi-volume archive with @samp{--multi-volume} (@pxref{Using Multiple -Tapes}), then the volume label will have @same{ Volume @var{nnn}} -appended to the name you give, where @var{nnn} is the number of the -volume of the archive. (If you use the @samp{--label} option when -reading an archive, it checks to make sure the label on the tape matches -the one you give. @xref{Special Options for Archiving}.) +To get information about the contents of an archived directory, +use the directory name as a file name argument in conjunction with +@value{op-list}. To find out file attributes, include the +@value{op-verbose} option. -Files in the filesystem occasionally have ``holes.'' A hole in a file -is a section of the file's contents which was never written. The -contents of a hole read as all zeros. On many operating systems, actual@c -disk storage is not allocated for holes, but they are counted in the -length of the file. If you archive such a file, @code{tar} could create -an archive longer than the original. To have @code{tar} attempt to -recognize the holes in a file, use @samp{--sparse}. When you use the -@samp{--sparse} option, then, for any file using less disk space than -would be expected from its length, @code{tar} searches the file for -consecutive stretches of zeros. It then records in the archive for the -file where the consecutive stretches of zeros are, and only archives the -``real contents'' of the file. On extraction (using @samp{--sparse} is -not needed on extraction) any such files have hols created wherever the -continuous stretches of zeros were found. Thus, if you use -@samp{--sparse}, @code{tar} archives won't take more space than the -original. +For example, to find out about files in the directory @file{practice}, in +the archive file @file{music.tar}, type: -When @code{tar} reads files, this causes them to have the access times -updated. To have @code{tar} attempt to set the access times back to -what they were before they were read, use the @samp{--atime-preserve} -option. This doesn't work for files that you don't own, unless you're -root, and it doesn't interact with incremental dumps nicely -(@pxref{Making Backups}), but it is good enough for some purposes. +@example +$ @kbd{tar --list --verbose --file=music.tar practice} +@end example + +@code{tar} responds: -@chapter Special Options for Reading Archives +@example +drwxrwxrwx myself user 0 1990-05-31 21:49 practice/ +-rw-rw-rw- myself user 42 1990-05-21 13:29 practice/blues +-rw-rw-rw- myself user 62 1990-05-23 10:55 practice/folk +-rw-rw-rw- myself user 40 1990-05-21 13:30 practice/jazz +-rw-rw-rw- myself user 10240 1990-05-31 21:49 practice/collection.tar +@end example +When you use a directory name as a file name argument, @code{tar} acts on +all the files (including sub-directories) in that directory. +@node extract, going further, list, Tutorial +@section How to Extract Members from an Archive +@UNREVISED +@cindex Extraction +@cindex Retrieving files from an archive +@cindex Resurrecting files from an archive +Creating an archive is only half the job---there is no point in storing +files in an archive if you can't retrieve them. The act of retrieving +members from an archive so they can be used and manipulated as +unarchived files again is called @dfn{extraction}. To extract files +from an archive, use the @value{op-extract} operation. As with +@value{op-create}, specify the name of the archive with @value{op-file}. +Extracting an archive does not modify the archive in any way; you can +extract it multiple times if you want or need to. +Using @samp{--extract}, you can extract an entire archive, or specific +files. The files can be directories containing other files, or not. As +with @value{op-create} and @value{op-list}, you may use the short or the +long form of the operation without affecting the performance. +@menu +* extracting archives:: +* extracting files:: +* extract dir:: +* failing commands:: +@end menu +@node extracting archives, extracting files, extract, extract +@subsection Extracting an Entire Archive +To extract an entire archive, specify the archive file name only, with +no individual file names as arguments. For example, +@example +$ @kbd{tar -xvf collection.tar} +@end example -XXXX MIB XXXX +@noindent +produces this: +@example +-rw-rw-rw- me user 28 1996-10-18 16:31 jazz +-rw-rw-rw- me user 21 1996-09-23 16:44 blues +-rw-rw-rw- me user 20 1996-09-23 16:44 folk +@end example +@node extracting files, extract dir, extracting archives, extract +@subsection Extracting Specific Files -@node Wizardry, Archive Structure, Tutorial, Top -@chapter Wizardry +To extract specific archive members, give their exact member names as +arguments, as printed by @value{op-list}. If you had mistakenly deleted +one of the files you had placed in the archive @file{collection.tar} +earlier (say, @file{blues}), you can extract it from the archive without +changing the archive's structure. It will be identical to the original +file @file{blues} that you deleted. @FIXME{check this; will the times, +permissions, owner, etc be the same, also?} -<<>>>> +@example +tar: folk: Not found in archive +tar: jazz: Not found in archive +$ +@end example -@node Archive Structure, Reading and Writing, Wizardry, Top -@chapter The Structure of an Archive +@noindent +This is because these files were not originally @emph{in} the parent +directory @file{..}, where the archive is located; they were in the +@file{practice} directory, and their file names reflect this: -While an archive may contain many files, the archive itself is a -single ordinary file. Like any other file, an archive file can be -written to a storage device such as a tape or disk, sent through a -pipe or over a network, saved on the active file system, or even -stored in another archive. An archive file is not easy to read or -manipulate without using the @code{tar} utility or Tar mode in Emacs. +@example +$ @kbd{tar -tvf music.tar} +practice/folk +practice/jazz +practice/rock +@end example +@FIXME{make sure the above works when going through the examples in +order...} -Physically, an archive consists of a series of file entries terminated -by an end-of-archive entry, which consists of 512 zero bytes. A file -entry usually describes one of the files in the archive (an -@dfn{archive member}), and consists of a file header and the contents -of the file. File headers contain file names and statistics, checksum -information which @code{tar} uses to detect file corruption, and -information about file types. +@noindent +Likewise, if you try to use this command, -More than archive member can have the same file name. One way this -situation can occur is if more than one version of a file has been -stored in the archive. For information about adding new versions of a -file to an archive, @pxref{Modifying}. +@example +$ @kbd{tar -tvf music.tar folk jazz} +@end example -In addition to entries describing archive members, an archive may contain -entries which @code{tar} itself uses to store information. -@xref{Archive Label}, for an example of such an archive entry. +@noindent +you would get a similar response. Members with those names are not in the +archive. You must use the correct member names in order to extract the +files from the archive. + +If you have forgotten the correct names of the files in the archive, +use @w{@kbd{tar --list --verbose}} to list them correctly. + +@FIXME{more examples, here? hag thinks it's a good idea.} + +@node going further, , extract, Tutorial +@section Going Further Ahead in this Manual + +@FIXME{need to write up a node here about the things that are going to +be in the rest of the manual.} + +@node tar invocation, operations, Tutorial, Top +@chapter Invoking GNU @code{tar} +@UNREVISED + +This chapter is about how one invokes the GNU @code{tar} command, from +the command synopsis (@pxref{Synopsis}). There are numerous options, +and many styles for writing them. One mandatory option specifies +the operation @code{tar} should perform (@pxref{Operation Summary}), +other options are meant to detail how this operation should be performed +(@pxref{Option Summary}). Non-option arguments are not always interpreted +the same way, depending on what the operation is. + +You will find in this chapter everything about option styles and rules for +writing them (@pxref{Styles}). On the other hand, operations and options +are fully described elsewhere, in other chapters. Here, you will find +only synthetic descriptions for operations and options, together with +pointers to other parts of the @code{tar} manual. + +Some options are so special they are fully described right in this +chapter. They have the effect of inhibiting the normal operation of +@code{tar} or else, they globally alter the amount of feedback the user +receives about what is going on. These are the @value{op-help} and +@value{op-version} (@pxref{help}), @value{op-verbose} (@pxref{verbose}) +and @value{op-interactive} options (@pxref{interactive}). @menu -* Old Style File Information:: Old Style File Information -* Archive Label:: -* Format Variations:: +* Synopsis:: +* using tar options:: +* Styles:: +* All Options:: +* help:: +* verbose:: +* interactive:: @end menu -@node Old Style File Information, Archive Label, Archive Structure, Archive Structure -@section Old Style File Information -@cindex Format, old style -@cindex Old style format -@cindex Old style archives +@node Synopsis, using tar options, tar invocation, tar invocation +@section General Synopsis of @code{tar} -Archives record not only an archive member's contents, but also its -file name or names, its access permissions, user and group, size in -bytes, and last modification time. Some archives also record the file -names in each archived directory, as well as other file and directory -information. +The GNU @code{tar} program is invoked as either one of: -Certain old versions of @code{tar} cannot handle additional -information recorded by newer @code{tar} programs. To create an -archive which can be read by these old versions, specify the -@samp{--old-archive} option in conjunction with the @samp{tar --create} -operation. When you specify this option, @code{tar} leaves out -information about directories, pipes, fifos, contiguous files, and -device files, and specifies file ownership by group and user ids -instead of names. - -The @samp{--old-archive} option is needed only if the archive must be -readable by an older tape archive program which cannot handle the new format. -Most @code{tar} programs do not have this limitation, so this option -is seldom needed. +@example +@kbd{tar @var{option}@dots{} [@var{name}]@dots{}} +@kbd{tar @var{letter}@dots{} [@var{argument}]@dots{} [@var{option}]@dots{} [@var{name}]@dots{}} +@end example -@table @samp -@item --old-archive -@itemx -o -@itemx --old -@itemx --portable -@c has portability been changed to portable? -Creates an archive that can be read by an old @code{tar} program. -Used in conjunction with the @samp{tar --create} operation. -@end table +The second form is for when old options are being used. + +You can use @code{tar} to store files in an archive, to extract them from +an archive, and to do other types of archive manipulation. The primary +argument to @code{tar}, which is called the @dfn{operation}, specifies +which action to take. The other arguments to @code{tar} are either +@dfn{options}, which change the way @code{tar} performs an operation, +or file names or archive members, which specify the files or members +@code{tar} is to act on. + +You can actually type in arguments in any order, even if in this manual +the options always precede the other arguments, to make examples easier +to understand. Further, the option stating the main operation mode +(the @code{tar} main command) is usually given first. + +Each @var{name} in the synopsis above is interpreted as an archive member +name when the main command is one of @value{op-compare}, @value{op-delete}, +@value{op-extract}, @value{op-list} or @value{op-update}. When naming +archive members, you must give the exact name of the member in the +archive, as it is printed by @value{op-list}. For @value{op-append} +and @value{op-create}, these @var{name} arguments specify the names +of either files or directory hierarchies to place in the archive. +These files or hierarchies should already exist in the file system, +prior to the execution of the @code{tar} command. + +@code{tar} interprets relative file names as being relative to the +working directory. @code{tar} will make all file names relative +(by removing leading slashes when archiving or restoring files), +unless you specify otherwise (using the @value{op-absolute-names} +option). @value{xref-absolute-names}, for more information about +@value{op-absolute-names}. -@node Archive Label, Format Variations, Old Style File Information, Archive Structure -@section Including a Label in the Archive -@cindex Labeling an archive -@cindex Labels on the archive media +If you give the name of a directory as either a file name or a member +name, then @code{tar} acts recursively on all the files and directories +beneath that directory. For example, the name @file{/} identifies all +the files in the filesystem to @code{tar}. -@c !! Should the arg to --label be a quoted string?? no - ringo -To avoid problems caused by misplaced paper labels on the archive -media, you can include a @dfn{label} entry---an archive member which -contains the name of the archive---in the archive itself. Use the -@samp{--label=@var{archive-label}} option in conjunction with the -@samp{--create} operation to include a label entry in the archive as it -is being created. +The distinction between file names and archive member names is especially +important when shell globbing is used, and sometimes a source of confusion +for newcomers. @xref{Wildcards}, for more information about globbing. +The problem is that shells may only glob using existing files in the +file system. Only @code{tar} itself may glob on archive members, so when +needed, you must ensure that wildcard characters reach @code{tar} without +being interpreted by the shell first. Using a backslash before @samp{*} +or @samp{?}, or putting the whole argument between quotes, is usually +sufficient for this. + +Even if @var{name}s are often specified on the command line, they +can also be read from a text file in the file system, using the +@value{op-files-from} option. + +If you don't use any file name arguments, @value{op-append}, +@value{op-delete} and @value{op-concatenate} will do nothing, while +@value{op-create} will usually yield a diagnostic and inhibit @code{tar} +execution. The other operations of @code{tar} (@value{op-list}, +@value{op-extract}, @value{op-compare}, and @value{op-update}) will act +on the entire contents of the archive. + +@cindex exit status +@cindex return status +Besides successful exits, GNU @code{tar} may fail for many reasons. +Some reasons correspond to bad usage, that is, when the @code{tar} +command is improperly written. +Errors may be encountered later, while encountering an error +processing the archive or the files. Some errors are recoverable, +in which case the failure is delayed until @code{tar} has completed +all its work. Some errors are such that it would not meaningful, +or at least risky, to continue processing: @code{tar} then aborts +processing immediately. All abnormal exits, whether immediate or +delayed, should always be clearly diagnosed on @code{stderr}, after +a line stating the nature of the error. + +GNU @code{tar} returns only a few exit statuses. I'm really +aiming simplicity in that area, for now. If you are not using the +@value{op-compare} option, zero means that everything went well, besides +maybe innocuous warnings. Nonzero means that something went wrong. +Right now, as of today, ``nonzero'' is almost always 2, except for +remote operations, where it may be 128. + +@node using tar options, Styles, Synopsis, tar invocation +@section Using @code{tar} Options + +GNU @code{tar} has a total of eight operating modes which allow you to +perform a variety of tasks. You are required to choose one operating +mode each time you employ the @code{tar} program by specifying one, and +only one operation as an argument to the @code{tar} command (two lists +of four operations each may be found at @ref{frequent operations} and +@ref{Operations}). Depending on circumstances, you may also wish to +customize how the chosen operating mode behaves. For example, you may +wish to change the way the output looks, or the format of the files that +you wish to archive may require you to do something special in order to +make the archive look right. + +You can customize and control @code{tar}'s performance by running +@code{tar} with one or more options (such as @value{op-verbose}, which +we used in the tutorial). As we said in the tutorial, @dfn{options} are +arguments to @code{tar} which are (as their name suggests) optional. +Depending on the operating mode, you may specify one or more options. +Different options will have different effects, but in general they all +change details of the operation, such as archive format, archive name, +or level of user interaction. Some options make sense with all +operating modes, while others are meaningful only with particular modes. +You will likely use some options frequently, while you will only use +others infrequently, or not at all. (A full list of options is +available in @pxref{All Options}.) + +Note that @code{tar} options are case sensitive. For example, the +options @samp{-T} and @samp{-t} are different; the first requires an +argument for stating the name of a file providing a list of @var{name}s, +while the second does not require an argument and is another way to +write @value{op-list}. + +In addition to the eight operations, there are many options to +@code{tar}, and three different styles for writing both: long (mnemonic) +form, short form, and old style. These styles are discussed below. +Both the options and the operations can be written in any of these three +styles. + +@FIXME{menu at end of this node. need to think of an actual outline +for this chapter; probably do that after stuff from chap. 4 is +incorporated.} + +@node Styles, All Options, using tar options, tar invocation +@section The Three Option Styles + +There are three styles for writing operations and options to the command +line invoking @code{tar}. The different styles were developed at +different times during the history of @code{tar}. These styles will be +presented below, from the most recent to the oldest. + +Some options must take an argument. (For example, @value{op-file} takes +the name of an archive file as an argument. If you do not supply an +archive file name, @code{tar} will use a default, but this can be +confusing; thus, we recommend that you always supply a specific archive +file name.) Where you @emph{place} the arguments generally depends on +which style of options you choose. We will detail specific information +relevant to each option style in the sections on the different option +styles, below. The differences are subtle, yet can often be very +important; incorrect option placement can cause you to overwrite a +number of important files. We urge you to note these differences, and +only use the option style(s) which makes the most sense to you until you +feel comfortable with the others. + +@FIXME{hag to write a brief paragraph on the option(s) which can +optionally take an argument} -If you create an archive using both @samp{--label=@var{archive-label}} -and @samp{--multi-volume}, each volume of the archive will have an -archive label of the form @samp{@var{archive-label} Volume @var{n}}, -where @var{n} is 1 for the first volume, 2 for the next, and so on. -@xref{Multi-Volume Archives}, for information on creating multiple -volume archives. +@menu +* Mnemonic Options:: Mnemonic Option Style +* Short Options:: Short Option Style +* Old Options:: Old Option Style +* Mixing:: Mixing Option Styles +@end menu -If you extract an archive using @samp{--label=@var{archive-label}}, -@code{tar} will print an error if the archive label doesn't match the -@var{archive-label} specified, and will then not extract the archive. -You can include a regular expression in @var{archive-label}, in this -case only. -@c >>> why is a reg. exp. useful here? (to limit extraction to a -@c >>>specific group? ie for multi-volume??? -ringo +@node Mnemonic Options, Short Options, Styles, Styles +@subsection Mnemonic Option Style + +@FIXME{have to decide whether or ot to replace other occurrences of +"mnemonic" with "long", or *ugh* vice versa.} + +Each option has at least one long (or mnemonic) name starting with two +dashes in a row, e.g. @samp{list}. The long names are more clear than +their corresponding short or old names. It sometimes happens that a +single mnemonic option has many different different names which are +synonymous, such as @samp{--compare} and @samp{--diff}. In addition, +long option names can be given unique abbreviations. For example, +@samp{--cre} can be used in place of @samp{--create} because there is no +other mnemonic option which begins with @samp{cre}. (One way to find +this out is by trying it and seeing what happens; if a particular +abbreviation could represent more than one option, @code{tar} will tell +you that that abbreviation is ambiguous and you'll know that that +abbreviation won't work. You may also choose to run @samp{tar --help} +to see a list of options. Be aware that if you run @code{tar} with a +unique abbreviation for the long name of an option you didn't want to +use, you are stuck; @code{tar} will perform the command as ordered.) + +Mnemonic options are meant to be obvious and easy to remember, and their +meanings are generally easier to discern than those of their +corresponding short options (see below). For example: -To find out an archive's label entry (or to find out if an archive has -a label at all), use @samp{tar --list --verbose}. @code{tar} will print the -label first, and then print archive member information, as in the -example below: +@example +$ @kbd{tar --create --verbose --blocking-factor=20 --file=/dev/rmt0} +@end example + +@noindent +gives a fairly good set of hints about what the command does, even +for those not fully acquainted with @code{tar}. + +Mnemonic options which require arguments take those arguments +immediately following the option name; they are introduced by an equal +sign. For example, the @samp{--file} option (which tells the name +of the @code{tar} archive) is given a file such as @file{archive.tar} +as argument by using the notation @samp{--file=archive.tar} for the +mnemonic option. + +@node Short Options, Old Options, Mnemonic Options, Styles +@subsection Short Option Style + +Most options also have a short option name. Short options start with +a single dash, and are followed by a single character, e.g. @samp{-t} +(which is equivalent to @samp{--list}). The forms are absolutely +identical in function; they are interchangeable. + +The short option names are faster to type than long option names. + +Short options which require arguments take their arguments immediately +following the option, usually separated by white space. It is also +possible to stick the argument right after the short option name, using +no intervening space. For example, you might write @w{@samp{-f +archive.tar}} or @samp{-farchive.tar} instead of using +@samp{--file=archive.tar}. Both @samp{--file=@var{archive-name}} and +@w{@samp{-f @var{archive-name}}} denote the option which indicates a +specific archive, here named @file{archive.tar}. + +Short options' letters may be clumped together, but you are not +required to do this (as compared to old options; see below). When short +options are clumped as a set, use one (single) dash for them all, e.g. +@w{@samp{@code{tar} -cvf}}. Only the last option in such a set is allowed +to have an argument@footnote{Clustering many options, the last of which +has an argument, is a rather opaque way to write options. Some wonder if +GNU @code{getopt} should not even be made helpful enough for considering +such usages as invalid.}. + +When the options are separated, the argument for each option which requires +an argument directly follows that option, as is usual for Unix programs. +For example: @example -% tar --verbose --list --file=iamanarchive -V--------- 0/0 0 Mar 7 12:01 1992 iamalabel--Volume Header-- --rw-rw-rw- ringo/user 40 May 21 13:30 1990 iamafilename +$ @kbd{tar -c -v -b 20 -f /dev/rmt0} @end example -@table @samp -@item --label=@var{archive-label} -@itemx -V @var{archive-label} -Includes an @dfn{archive-label} at the beginning of the archive when -the archive is being created (when used in conjunction with the -@samp{tar --create} operation). Checks to make sure the archive label -matches the one specified (when used in conjunction with the @samp{tar ---extract} operation. -@end table -@c was --volume +If you reorder short options' locations, be sure to move any arguments +that belong to them. If you do not move the arguments properly, you may +end up overwriting files. + +@node Old Options, Mixing, Short Options, Styles +@subsection Old Option Style +@UNREVISED + +Like short options, old options are single letters. However, old options +must be written together as a single clumped set, without spaces separating +them or dashes preceding them@footnote{Beware that if you precede options +with a dash, you are announcing the short option style instead of the +old option style; short options are decoded differently.}. This set +of letters must be the first to appear on the command line, after the +@code{tar} program name and some whitespace; old options cannot appear +anywhere else. The letter of an old option is exactly the same letter as +the corresponding short option. For example, the old option @samp{t} is +the same as the short option @samp{-t}, and consequently, the same as the +mnemonic option @samp{--list}. So for example, the command @w{@samp{tar +cv}} specifies the option @samp{-v} in addition to the operation @samp{-c}. + +@FIXME{bob suggests having an uglier example. :-) } + +When options that need arguments are given together with the command, +all the associated arguments follow, in the same order as the options. +Thus, the example given previously could also be written in the old +style as follows: -@node Format Variations, , Archive Label, Archive Structure -@section Format Variations -@cindex Format Parameters -@cindex Format Options -@cindex Options to specify archive format. +@example +$ @kbd{tar cvbf 20 /dev/rmt0} +@end example -Format parameters specify how an archive is written on the archive -media. The best choice of format parameters will vary depending on -the type and number of files being archived, and on the media used to -store the archive. +@noindent +Here, @samp{20} is the argument of @samp{-b} and @samp{/dev/rmt0} is +the argument of @samp{-f}. -To specify format parameters when accessing or creating an archive, -you can use the options described in the following sections. If you -do not specify any format parameters, @code{tar} uses default -parameters. You cannot modify a compressed archive. If you create an -archive with the @samp{--block-size} option specified (@pxref{Blocking -Factor}), you must specify that block-size when operating on the -archive. @xref{Matching Format Parameters}, for other examples of -format parameter considerations. +On the other hand, this old style syntax makes it difficult to match +option letters with their corresponding arguments, and is often +confusing. In the command @w{@samp{tar cvbf 20 /dev/rmt0}}, for example, +@samp{20} is the argument for @samp{-b}, @samp{/dev/rmt0} is the +argument for @samp{-f}, and @samp{-v} does not have a corresponding +argument. Even using short options like in @w{@samp{tar -c -v -b 20 -f +/dev/rmt0}} is clearer, putting all arguments next to the option they +pertain to. +If you want to reorder the letters in the old option argument, be +sure to reorder any corresponding argument appropriately. -@menu -* Multi-Volume Archives:: -* Sparse Files:: -* Blocking Factor:: -* Compressed Archives:: -@end menu +This old way of writing @code{tar} options can surprise even experienced +users. For example, the two commands: -@node Multi-Volume Archives, Sparse Files, Format Variations, Format Variations -@subsection Archives Longer than One Tape or Disk -@cindex Multi-volume archives +@example +@kbd{tar cfz archive.tar.gz file} +@kbd{tar -cfz archive.tar.gz file} +@end example -To create an archive that is larger than will fit on a single unit of -the media, use the @samp{--multi-volume} option in conjunction with the -@samp{tar --create} operation (@pxref{Creating Archives}). A -@dfn{multi-volume} archive can be manipulated like any other archive -(provided the @samp{--multi-volume} option is specified), but is stored -on more than one tape or disk. +@noindent +are quite different. The first example uses @file{archive.tar.gz} as +the value for option @samp{f} and recognizes the option @samp{z}. The +second example, however, uses @file{z} as the value for option +@samp{f}---probably not what was intended. -When you specify @samp{--multi-volume}, @code{tar} does not report an -error when it comes to the end of an archive volume (when reading), or -the end of the media (when writing). Instead, it prompts you to load -a new storage volume. If the archive is on a magnetic tape, you -should change tapes when you see the prompt; if the archive is on a -floppy disk, you should change disks; etc. +Old options are kept for compatibility with old versions of @code{tar}. -You can read each individual volume of a multi-volume archive as if it -were an archive by itself. For example, to list the contents of one -volume, use @samp{tar --list}, without @samp{--multi-volume} specified. -To extract an archive member from one volume (assuming it is described -that volume), use @samp{tar --extract}, again without -@samp{--multi-volume}. +This second example could be corrected in many ways, among which the +following are equivalent: -If an archive member is split across volumes (ie. its entry begins on -one volume of the media and ends on another), you need to specify -@samp{--multi-volume} to extract it successfully. In this case, you -should load the volume where the archive member starts, and use -@samp{tar --extract --multi-volume}---@code{tar} will prompt for later -volumes as it needs them. @xref{Extracting From Archives} for more -information about extracting archives. +@example +@kbd{tar -czf archive.tar.gz file} +@kbd{tar -cf archive.tar.gz -z file} +@kbd{tar cf archive.tar.gz -z file} +@end example -@samp{--info-script=@var{program-file}} is like @samp{--multi-volume}, -except that @code{tar} does not prompt you directly to change media -volumes when a volume is full---instead, @code{tar} runs commands you -have stored in @var{program-file}. This option can be used to -broadcast messages such as @samp{someone please come change my tape} -when performing unattended backups. When @var{program-file} is done, -@code{tar} will assume that the media has been changed. +@FIXME{still could explain this better; it's redundant:} +@cindex option syntax, traditional +As far as we know, all @code{tar} programs, GNU and non-GNU, support +old options. GNU @code{tar} supports them not only for historical +reasons, but also because many people are used to them. For +compatibility with Unix @code{tar}, the first argument is always +treated as containing command and option letters even if it doesn't +start with @samp{-}. Thus, @samp{tar c} is equivalent to @w{@samp{tar +-c}:} both of them specify the @value{op-create} command to create an +archive. -<<< There should be a sample program here, including an exit before -<<< end. +@node Mixing, , Old Options, Styles +@subsection Mixing Option Styles -@table @samp -@item --multi-volume -@itemx -M -Creates a multi-volume archive, when used in conjunction with -@samp{tar --create}. To perform any other operation on a multi-volume -archive, specify @samp{--multi-volume} in conjunction with that -operation. +All three styles may be intermixed in a single @code{tar} command, so +long as the rules for each style are fully respected@footnote{Before GNU +@code{tar} version 1.11.6, a bug prevented intermixing old style options +with mnemonic options in some cases.}. Old style options and either of the +modern styles of options may be mixed within a single @code{tar} command. +However, old style options must be introduced as the first arguments only, +following the rule for old options (old options must appear directly +after the @code{tar} command and some whitespace). Modern options may +be given only after all arguments to the old options have been collected. +If this rule is not respected, a modern option might be falsely interpreted +as the value of the argument to one of the old style options. -@item --info-script=@var{program-file} -@itemx -F @var{program-file} -Creates a multi-volume archive via a script. Used in conjunction with -@samp{tar --create}. -@end table +For example, all the following commands are wholly equivalent, and +illustrate the many combinations and orderings of option styles. -@node Sparse Files, Blocking Factor, Multi-Volume Archives, Format Variations -@subsection Archiving Sparse Files -@cindex Sparse Files +@example +@kbd{tar --create --file=archive.tar} +@kbd{tar --create -f archive.tar} +@kbd{tar --create -farchive.tar} +@kbd{tar --file=archive.tar --create} +@kbd{tar --file=archive.tar -c} +@kbd{tar -c --file=archive.tar} +@kbd{tar -c -f archive.tar} +@kbd{tar -c -farchive.tar} +@kbd{tar -cf archive.tar} +@kbd{tar -cfarchive.tar} +@kbd{tar -f archive.tar --create} +@kbd{tar -f archive.tar -c} +@kbd{tar -farchive.tar --create} +@kbd{tar -farchive.tar -c} +@kbd{tar c --file=archive.tar} +@kbd{tar c -f archive.tar} +@kbd{tar c -farchive.tar} +@kbd{tar cf archive.tar} +@kbd{tar f archive.tar --create} +@kbd{tar f archive.tar -c} +@kbd{tar fc archive.tar} +@end example -A file is sparse if it contains blocks of zeros whose existance is -recorded, but that have no space allocated on disk. When you specify -the @samp{--sparse} option in conjunction with the @samp{--create} -operation, @code{tar} tests all files for sparseness while archiving. -If @code{tar} finds a file to be sparse, it uses a sparse -representation of the file in the archive. @xref{Creating Archives}, -for more information about creating archives. +On the other hand, the following commands are @emph{not} equivalent to +the previous set: -@samp{--sparse} is useful when archiving files, such as dbm files, -likely to contain many nulls. This option dramatically -decreases the amount of space needed to store such an archive. +@example +@kbd{tar -f -c archive.tar} +@kbd{tar -fc archive.tar} +@kbd{tar -fcarchive.tar} +@kbd{tar -farchive.tarc} +@kbd{tar cfarchive.tar} +@end example -@quotation -@strong{Please Note:} Always use @samp{--sparse} when performing file -system backups, to avoid archiving the expanded forms of files stored -sparsely in the system.@refill +@noindent +These last examples mean something completely different from what the +user intended (judging based on the example in the previous set which +uses long options, whose intent is therefore very clear). The first +four specify that the @code{tar} archive would be a file named +@samp{-c}, @samp{c}, @samp{carchive.tar} or @samp{archive.tarc}, +respectively. The first two examples also specify a single non-option, +@var{name} argument having the value @samp{archive.tar}. The last +example contains only old style option letters (repeating option +@samp{c} twice), not all of which are meaningful (eg., @samp{.}, +@samp{h}, or @samp{i}), with no argument value. @FIXME{not sure i liked +the first sentence of this paragraph..} + +@node All Options, help, Styles, tar invocation +@section All @code{tar} Options + +The coming manual sections contain an alphabetical listing of all +@code{tar} operations and options, with brief descriptions and cross +references to more in-depth explanations in the body of the manual. +They also contain an alphabetically arranged table of the short option +forms with their corresponding long option. You can use this table as +a reference for deciphering @code{tar} commands in scripts. -Even if your system has no no sparse files currently, some may be -created in the future. If you use @samp{--sparse} while making file -system backups as a matter of course, you can be assured the archive -will always take no more space on the media than the files take on -disk (otherwise, archiving a disk filled with sparse files might take -hundreds of tapes).@refill -<<< xref incremental when node name is set. -@end quotation +@menu +* Operation Summary:: +* Option Summary:: +* Short Option Summary:: +@end menu -@code{tar} ignores the @samp{--sparse} option when reading an archive. +@node Operation Summary, Option Summary, All Options, All Options +@subsection Operations -@table @samp -@item --sparse -@itemx -S -Files stored sparsely in the file system are represented sparsely in -the archive. Use in conjunction with write operations. -@end table +@table @kbd -@node Blocking Factor, Compressed Archives, Sparse Files, Format Variations -@subsection The Blocking Factor of an Archive -@cindex Blocking Factor -@cindex Block Size -@cindex Number of records per block -@cindex Number of bytes per block -@cindex Bytes per block -@cindex Records per block - -The data in an archive is grouped into records, which are 512 bytes. -Records are read and written in whole number multiples called -@dfn{blocks}. The number of records in a block (ie. the size of a -block in units of 512 bytes) is called the @dfn{blocking factor}. The -@samp{--block-size=@var{number}} option specifies the blocking factor -of an archive. The default blocking factor is typically 20 (ie.@: -10240 bytes), but can be specified at installation. To find out the -blocking factor of an existing archive, use @samp {tar --list ---file=@var{archive-name}}. This may not work on some devices. - -Blocks are seperated by gaps, which waste space on the archive media. -If you are archiving on magnetic tape, using a larger blocking factor -(and therefore larger blocks) provides faster throughput and allows -you to fit more data on a tape (because there are fewer gaps). If you -are archiving on cartridge, a very large blocking factor (say 126 or -more) greatly increases performance. A -smaller blocking factor, on the other hand, may be usefull when -archiving small files, to avoid archiving lots of nulls as @code{tar} -fills out the archive to the end of the block. In general, the ideal block size -depends on the size of the inter-block gaps on the tape you are using, -and the average size of the files you are archiving. @xref{Creating -Archives}, for information on writing archives. - -Archives with blocking factors larger than 20 cannot be read by very -old versions of @code{tar}, or by some newer versions of @code{tar} -running on old machines with small address spaces. With GNU -@code{tar}, the blocking factor of an archive is limited only by the -maximum block size of the device containing the archive, or by the -amount of available virtual memory. - -If you use a non-default blocking factor when you create an archive, -you must specify the same blocking factor when you modify that -archive. Some archive devices will also require you to specify the -blocking factor when reading that archive, however this is not -typically the case. Usually, you can use @samp{tar --list} without -specifying a blocking factor---@code{tar} reports a non-default block -size and then lists the archive members as it would normally. To -extract files from an archive with a non-standard blocking factor -(particularly if you're not sure what the blocking factor is), you can -usually use the {--read-full-blocks} option while specifying a blocking -factor larger then the blocking factor of the archive (ie. @samp{tar ---extract --read-full-blocks --block-size=300}. @xref{Listing Contents} -for more information on the @samp{--list} operation. -@xref{read-full-blocks} for a more detailed explanation of that -option. +@item --append +@itemx -r -@table @samp -@item --block-size=@var{number} -@itemx -b @var{number} -Specifies the blocking factor of an archive. Can be used with any -operation, but is usually not necessary with @samp{tar --list}. -@end table +Appends files to the end of the archive. @xref{append}. -@node Compressed Archives, , Blocking Factor, Format Variations -@subsection Creating and Reading Compressed Archives -@cindex Compressed archives -@cindex Storing archives in compressed format +@item --catenate +@itemx -A -@samp{--compress} indicates an archive stored in compressed format. -The @samp{--compress} option is useful in saving time over networks and -space in pipes, and when storage space is at a premium. -@samp{--compress} causes @code{tar} to compress when writing the -archive, or to uncompress when reading the archive. +Same as @samp{--concatenate}. @xref{concatenate}. -To perform compression and uncompression on the archive, @code{tar} -runs the @code{compress} utility. @code{tar} uses the default -compression parameters; if you need to override them, avoid the -@samp{--compress} option and run the @code{compress} utility -explicitly. It is useful to be able to call the @code{compress} -utility from within @code{tar} because the @code{compress} utility by -itself cannot access remote tape drives. +@item --compare +@itemx -d -The @samp{--compress} option will not work in conjunction with the -@samp{--multi-volume} option or the @samp{--add-file}, @samp{--update}, -@samp{--add-file} and @samp{--delete} operations. @xref{Modifying}, for -more information on these operations. +Compares archive members with their counterparts in the file +system, and reports differences in file size, mode, owner, +modification date and contents. @xref{compare}. -If there is no compress utility available, @code{tar} will report an -error. +@item --concatenate +@itemx -A -@samp{--compress-block} is like @samp{--compress}, but when used in -conjunction with @samp{--create} also causes @code{tar} to pad the last -block of the archive out to the next block boundary as it is written. -This is useful with certain devices which require all write operations -be a multiple of a specific size. +Appends other @code{tar} archives to the end of the archive. +@xref{concatenate}. -@quotation -@strong{Please Note:} The @code{compress} program may be covered by a patent, -and therefore we recommend you stop using it. We hope to have a -different compress program in the future. We may change the name of -this option at that time. -@end quotation - -@table @samp -@item --compress -@itemx --uncompress -@itemx -z -@itemx -Z -When this option is specified, @code{tar} will compress (when writing -an archive), or uncompress (when reading an archive). Used in -conjunction with the @samp{--create}, @samp{--extract}, @samp{--list} and -@samp{--compare} operations. - -@item --compress-block -@itemx -z -z -Acts like @samp{--compress}, but pads the archive out to the next block -boundary as it is written when used in conjunction with the -@samp{--create} operation. -@end table - -@c >>> MIB -- why not use -Z instead of -z -z ? -ringo +@item --create +@itemx -c -@node Reading and Writing, Insuring Accuracy, Archive Structure, Top -@chapter Reading and Writing Archives +Creates a new @code{tar} archive. @xref{create}. -The @samp{--create} operation writes a new archive, and the -@samp{--extract} operation reads files from an archive and writes them -into the file system. You can use other @code{tar} operations to -write new information into an existing archive (adding files to it, -adding another archive to it, or deleting files from it), and you can -read a list of the files in an archive without extracting it using the -@samp{--list} operation. +@item --delete -@menu -* Archive Name:: The name of an archive -* Creating in Detail:: Creating in detail -* Modifying:: Modifying archives -* Listing Contents:: Listing the contents of an archive -* Extracting From Archives:: Extracting files from an archive -@end menu +Deletes members from the archive. Don't try this on a archive on a +tape! @xref{delete}. -@node Archive Name, Creating in Detail, Reading and Writing, Reading and Writing -@section The Name of an Archive -@cindex Naming an archive -@cindex Archive Name -@cindex Directing output -@cindex Where is the archive? +@item --diff +@itemx -d -An archive can be saved as a file in the file system, sent through a -pipe or over a network, or written to an I/O device such as a tape or -disk drive. To specify the name of the archive, use the -@samp{--file=@var{archive-name}} option. +Same @samp{--compare}. @xref{compare}. -An archive name can be the name of an ordinary file or the name of an -I/O device. @code{tar} always needs an archive name---if you do not -specify an archive name, the archive name comes from the environment -variable @code{TAPE} or, if that variable is not specified, a default -archive name, which is usually the name of tape unit zero (ie. -/dev/tu00). +@item --extract +@itemx -x -If you use @file{-} as an @var{archive-name}, @code{tar} reads the -archive from standard input (when listing or extracting files), or -writes it to standard output (when creating an archive). If you use -@file{-} as an @var{archive-name} when modifying an archive, -@code{tar} reads the original archive from its standard input and -writes the entire new archive to its standard output. +Extracts members from the archive into the file system. @xref{extract}. -@c >>> MIB--does standard input and output redirection work with all -@c >>> operations? -@c >>> need example for standard input and output (screen and keyboard?) +@item --get +@itemx -x -@cindex Standard input and output -@cindex tar to standard input and output +Same as @samp{--extract}. @xref{extract}. -To specify an archive file on a device attached to a remote machine, -use the following: +@item --list +@itemx -t -@example ---file=@var{hostname}:/@var{dev}/@var{file name} -@end example +Lists the members in an archive. @xref{list}. -@noindent -@code{tar} will complete the remote connection, if possible, and -prompt you for a username and password. If you use -@samp{--file=@@@var{hostname}:/@var{dev}/@var{file-name}}, @code{tar} -will complete the remote connection, if possible, using your username -as the username on the remote machine. +@item --update +@itemx -u -@c >>>MIB --- is this clear? +@FIXME{It was: A combination of the @samp{--compare} and @samp{--append} operations. +This is not true and rather misleading, as @value{op-compare} +does a lot more than @value{op-update} for ensuring files are identical.} +Adds files to the end of the archive, but only if they are newer than +their counterparts already in the archive, or if they do not already +exist in the archive. +@xref{update}. -@table @samp -@item --file=@var{archive-name} -@itemx -f @var{archive-name} -Names the archive to create or operate on. Use in conjunction with -any operation. @end table -@node Creating in Detail, Modifying, Archive Name, Reading and Writing -@section Creating in Detail -@c operations should probably have examples, not tables. -@cindex Writing new archives -@cindex Archive creation +@node Option Summary, Short Option Summary, Operation Summary, All Options +@subsection @code{tar} Options -To create an archive, use @samp{tar --create}. To name the archive, -use @samp{--file=@var{archive-name}} in conjunction with the -@samp{--create} operation (@pxref{Archive Name}). If you do not name -the archive, @code{tar} uses the value of the environment variable -@code{TAPE} as the file name for the archive, or, if that is not -available, @code{tar} uses a default archive name, usually that for tape -unit zero. @xref{Archive Name}, for more information about specifying -an archive name. +@table @kbd -The following example creates an archive named @file{stooges}, -containing the files @file{larry}, @file{moe} and @file{curley}: +@item --absolute-names +@itemx -P -@example -tar --create --file=stooges larry moe curley -@end example +Normally when creating an archive, @code{tar} strips an initial @samp{/} from +member names. This option disables that behavior. @FIXME-xref{}. -If you specify a directory name as a file-name argument, @code{tar} -will archive all the files in that directory. The following example -creates an archive named @file{hail/hail/fredonia}, containing the -contents of the directory @file{marx}: +@item --after-date -@example -tar --create --file=hail/hail/fredonia marx -@end example +(See @samp{--newer}; @FIXME-pxref{}.) -If you don't specify files to put in the archive, @code{tar} archives -all the files in the working directory. The following example creates -an archive named @file{home} containing all the files in the working -directory: +@item --atime-preserve -@example -tar --create --file=home -@end example +Tells @code{tar} to preserve the access time field in a file's inode when +dumping it. @FIXME-xref{}. -@xref{File Name Lists}, for other ways to specify files to archive. +@item --backup=@var{backup-type} -Note: In the example above, an archive containing all the files in the -working directory is being written to the working directory. GNU -@code{tar} stores files in the working directory in an archive which -is itself in the working directory without falling into an infinite -loop. Other versions of @code{tar} may fall into this trap. +Rather than deleting files from the file system, @code{tar} will back them up +using simple or numbered backups, depending upon @var{backup-type}. +@FIXME-xref{}. -@node Modifying, Listing Contents, Creating in Detail, Reading and Writing -@section Modifying Archives -@cindex Modifying archives +@item --block-number +@itemx -R -Once an archive is created, you can add new archive members to it, add -the contents of another archive, add newer versions of members already -stored, or delete archive members already stored. +With this option present, @code{tar} prints error messages for read errors +with the block number in the archive file. @FIXME-xref{}. -To find out what files are already stored in an archive, use @samp{tar ---list --file=@var{archive-name}}. @xref{Listing Contents}. +@item --blocking-factor=@var{blocking} +@itemx -b @var{blocking} -@menu -* Adding Files:: -* Appending Archives:: -* Deleting Archive Files:: Deleting Files From an Archive -* Matching Format Parameters:: -@end menu +Sets the blocking factor @code{tar} uses to @var{blocking} x 512 bytes per +record. @FIXME-xref{}. -@node Adding Files, Appending Archives, Modifying, Modifying -@subsection Adding Files to an Archive -@cindex Adding files to an archive -@cindex Updating an archive +@item --checkpoint -To add files to an archive, use @samp{tar --add-file}. The archive to -be added to must already exist and be in proper archive format (which -normally means it was created previously using @code{tar}). If the -archive was created with a different block size than now specified, -@code{tar} will report an error (@pxref{Blocking Factor}). If the -archive is not a valid @code{tar} archive, the results will be -unpredictable. You cannot add files to a compressed archive, however -you can add files to the last volume of a multi-volume archive. -@xref{Matching Format Parameters}. +This option directs @code{tar} to print periodic checkpoint messages as it +reads through the archive. Its intended for when you want a visual +indication that @code{tar} is still running, but don't want to see +@samp{--verbose} output. @FIXME-xref{}. -The following example adds the file @file{shemp} to the archive -@file{stooges} created above: +@item --compress +@itemx --uncompress +@itemx -Z -@example -tar --add-file --file=stooges shemp -@end example +@code{tar} will use the @code{compress} program when reading or writing the +archive. This allows you to directly act on archives while saving +space. @FIXME-xref{}. -You must specify the files to be added; there is no default. +@item --confirmation -@samp{tar --update} acts like @samp{tar --add-file}, but does not add -files to the archive if there is already a file entry with that name -in the archive that has the same modification time. +(See @samp{--interactive}; @FIXME-pxref{}.) -Both @samp{--update} and @samp{--add-file} work by adding to the end of -the archive. When you extract a file from the archive, only the -version stored last will wind up in the file system. Because -@samp{tar --extract} extracts files from an archive in sequence, and -overwrites files with the same name in the file system, if a file name -appears more than once in an archive the last version of the file will -overwrite the previous versions which have just been extracted. You -should avoid storing older versions of a file later in the archive. +@item --dereference +@itemx -h -Note: @samp{--update} is not suitable for performing backups, because -it doesn't change directory content entries, and because it lengthens -the archive every time it is used. -@c <<< xref to scripted backup, listed incremental, for info on backups. +When creating a @code{tar} archive, @code{tar} will archive the file that a symbolic +link points to, rather than archiving the symlink. @FIXME-xref{}. -@node Appending Archives, Deleting Archive Files, Adding Files, Modifying -@subsection Appending One Archive's Contents to Another Archive -@cindex Adding archives to an archive -@cindex Concatenating Archives +@item --directory=@var{dir} +@itemx -C @var{dir} -To append copies of an archive or archives to the end of another -archive, use @samp{tar --add-archive}. The source and target archives -must already exist and have been created using compatable format -parameters (@pxref{Matching Format Parameters}). - -@code{tar} will stop reading an archive if it encounters an -end-of-archive marker. The @code{cat} utility does not remove -end-of-archive markers, and is therefore unsuitable for concatenating -archives. @samp{tar --add-archive} removes the end-of-archive marker -from the target archive before each new archive is appended. -@c <<< xref ignore-zeros - -You must specify the source archives using -@samp{--file=@var{archive-name}} (@pxref{Archive Name}). If you do not -specify the target archive , @code{tar} uses the value of the -environment variable @code{TAPE}, or, if this has not been set, the -default archive name. +When this option is specified, @code{tar} will change its current directory +to @var{dir} before performing any operations. When this option is used +during archive creation, it is order sensitive. @FIXME-xref{}. -The following example adds the contents of the archive -@file{hail/hail/fredonia} to the archive @file{stooges} (both archives -were created in examples above): +@item --exclude=@var{pattern} -@example -tar --add-archive --file=stooges hail/hail/fredonia -@end example +When performing operations, @code{tar} will skip files that match +@var{pattern}. @FIXME-xref{}. -If you need to retrieve files from an archive that was added to using -the @code{cat} utility, use the @samp{--ignore-zeros} option -(@pxref{Archive Reading Options}). +@item --exclude-from=@var{file} +@itemx -X @var{file} -@node Deleting Archive Files, Matching Format Parameters, Appending Archives, Modifying -@subsection Deleting Files From an Archive -@cindex Deleting files from an archive -@cindex Removing files from an archive +Similar to @samp{--exclude}, except @code{tar} will use the list of patterns +in the file @var{file}. @FIXME-xref{}. -To delete archive members from an archive, use @samp{tar --delete}. -You must specify the file names of the members to be deleted. All -archive members with the specified file names will be removed from the -archive. +@item --file=@var{archive} +@itemx -f @var{archive} -The following example removes the file @file{curley} from the archive -@file{stooges}: +@code{tar} will use the file @var{archive} as the @code{tar} archive it +performs operations on, rather than @code{tar}'s compilation dependent +default. @FIXME-xref{}. -@example -tar --delete --file=stooges curley -@end example +@item --files-from=@var{file} +@itemx -T @var{file} -You can only use @samp{tar --delete} on an archive if the archive -device allows you to write to any point on the media. +@code{tar} will use the contents of @var{file} as a list of archive members +or files to operate on, in addition to those specified on the +command-line. @FIXME-xref{}. -@quotation -@strong{Warning:} Don't try to delete an archive member from a -magnetic tape, lest you scramble the archive. There is no safe way -(except by completely re-writing the archive) to delete files from -most kinds of magnetic tape. -@end quotation +@item --force-local -@c <<< MIB -- how about automatic detection of archive media? give error -@c <<< unless the archive device is either an ordinary file or different -@c <<< input and output (--file=-). +Forces @code{tar} to interpret the filename given to @samp{--file} as a local +file, even if it looks like a remote tape drive name. @FIXME-xref{}. -@node Matching Format Parameters, , Deleting Archive Files, Modifying -@subsection Matching the Format Parameters +@item --group=@var{group} -Some format parameters must be taken into consideration when modifying -an archive: +Files added to the @code{tar} archive will have a group id of @var{group}, +rather than the group from the source file. @var{group} is first decoded +as a group symbolic name, but if this interpretation fails, it has to be +a decimal numeric group ID. @FIXME-xref{}. -Compressed archives cannot be modified. +Also see the comments for the @value{op-owner} option. -You have to specify the block size of the archive when modifying an -archive with a non-default block size. +@item --gunzip -Multi-volume archives can be modified like any other archive. To add -files to a multi-volume archive, you need to only mount the last -volume of the archive media (and new volumes, if needed). For all -other operations, you need to use the entire archive. - -If a multi-volume archive was labeled using @samp{--label} -(@pxref{Archive Label}) when it was created, @code{tar} will not -automatically label volumes which are added later. To label -subsequent volumes, specify @samp{--label=@var{archive-label}} again in -conjunction with the @samp{--add-file}, @samp{--update} or -@samp{--add-archive} operation. -@cindex Labelling multi-volume archives -@c <<< example +(See @samp{--gzip}; @FIXME-pxref{}.) -@c <<< xref somewhere, for more information about format parameters. +@item --gzip +@itemx --gunzip +@itemx --ungzip +@itemx -z -@node Listing Contents, Extracting From Archives, Modifying, Reading and Writing -@section Listing the Contents of an Archive -@cindex Names of the files in an archive -@cindex Archive contents, list of -@cindex Archive members, list of +This option tells @code{tar} to read or write archives through @code{gzip}, +allowing @code{tar} to directly operate on several kinds of compressed +archives transparently. @FIXME-xref{}. -@samp{tar --list} prints a list of the file names of the archive -members on the standard output. If you specify @var{file-name} -arguments on the command line (or using the @samp{--files-from} option, -@pxref{File Name Lists}), only the files you specify will be listed, -and only if they exist in the archive. Files not specified will be -ignored, unless they are under a specific directory. +@item --help -If you include the @samp{--verbose} option, @code{tar} prints an -@samp{ls -l} type listing for the archive. @pxref{Additional -Information}, for a description of the @samp{--verbose} option. +@code{tar} will print out a short message summarizing the operations and +options to @code{tar} and exit. @FIXME-xref{}. -If the blocking factor of the archive differs from the default, -@code{tar} reports this. @xref{Blocking Factor}. +@item --ignore-failed-read -@xref{Archive Reading Options} for a list of options which can be used -to modify @samp{--list}'s operation. +Instructs @code{tar} to exit successfully if it encounters an +unreadable file. @xref{Reading}. -This example prints a list of the archive members of the archive -@file{stooges}: +@item --ignore-umask +@FIXME{does this exist?} -@example -tar --list --file=stooges -@end example +(See @samp{--preserve-permissions}; @pxref{Writing}.) -@noindent -@code{tar} responds: +@item --ignore-zeros +@itemx -i -@example -larry -moe -shemp -marx/julius -marx/alexander -marx/karl -@end example +With this option, @code{tar} will ignore zeroed blocks in the archive, which +normally signals EOF. @xref{Reading}. -This example generates a verbose list of the archive members of the -archive file @file{dwarves}, which has a blocking factor of two: +@item --incremental +@itemx -G -@example -tar --list -v --file=blocks -@end example +Used to inform @code{tar} that it is working with an old GNU-format +incremental backup archive. It is intended primarily for backwards +compatibility only. @FIXME-xref{}. -@noindent -@code{tar} responds: +@item --info-script=@var{script-file} +@itemx --new-volume-script=@var{script-file} +@itemx -F @var{script-file} -@example -tar: Blocksize = 2 records --rw------- ringo/user 42 May 1 13:29 1990 .bashful --rw-rw-rw- ringo/user 42 Oct 4 13:29 1990 doc --rw-rw-rw- ringo/user 42 Jul 20 18:01 1969 dopey --rw-rw---- ringo/user 42 Nov 26 13:42 1963 grumpy --rw-rw-rw- ringo/user 42 May 5 13:29 1990 happy --rw-rw-rw- ringo/user 42 May 1 12:00 1868 sleepy --rw-rw-rw- ringo/user 42 Jul 4 17:29 1776 sneezy -@end example +When @code{tar} is performing multi-tape backups, @var{script-file} is run +at the end of each tape. @FIXME-xref{}. -@node Extracting From Archives, , Listing Contents, Reading and Writing -@section Extracting Files from an Archive -@cindex Extraction -@cindex Retrieving files from an archive -@cindex Resurrecting files from an archive +@item --interactive +@itemx --confirmation +@itemx -w -To read archive members from the archive and write them into the file -system, use @samp{tar --extract}. The archive itself is left -unchanged. +Specifies that @code{tar} should ask the user for confirmation before +performing potentially destructive options, such as overwriting files. +@FIXME-xref{}. -If you do not specify the files to extract, @code{tar} extracts all -the files in the archive. If you specify the name of a directory as a -file-name argument, @code{tar} will extract all files which have been -stored as part of that directory. If a file was stored with a -directory name as part of its file name, and that directory does not -exist under the working directory when the file is extracted, -@code{tar} will create the directory. @xref{Selecting Archive -Members}, for information on specifying files to extract. +@item --keep-old-files +@itemx -k -The following example shows the extraction of the archive -@file{stooges} into an empty directory: +When extracting files from an archive, @code{tar} will not overwrite existing +files if this option is present. @xref{Writing}. -@example -tar --extract --file=stooges -@end example +@item --label=@var{name} +@itemx -V @var{name} -@noindent -Generating a listing of the directory (@samp{ls}) produces: +When creating an archive, instructs @code{tar} to write @var{name} as a name +record in the archive. When extracting or listing archives, @code{tar} will +only operate on archives that have a label matching the pattern +specified in @var{name}. @FIXME-xref{}. -@example -larry -moe -shemp -marx -@end example +@item --listed-incremental=@var{snapshot-file} +@itemx -g @var{snapshot-file} -@noindent -The subdirectory @file{marx} contains the files @file{julius}, -@file{alexander} and @file{karl}. +During a @samp{--create} operation, specifies that the archive that +@code{tar} creates is a new GNU-format incremental backup, using +@var{snapshot-file} to determine which files to backup. +With other operations, informs @code{tar} that the archive is in incremental +format. @FIXME-xref{}. -If you wanted to just extract the files in the subdirectory -@file{marx}, you could specify that directory as a file-name argument -in conjunction with the @samp{--extract} operation: +@item --mode=@var{permissions} -@example -tar --extract --file=stooges marx -@end example +When adding files to an archive, @code{tar} will use @var{permissions} +for the archive members, rather than the permissions from the files. +The program @code{chmod} and this @code{tar} option share the same syntax +for what @var{permissions} might be. @xref{File permissions, Permissions, +File permissions, filetutils, GNU file utilities}. This reference also +has useful information for those not being overly familiar with the Unix +permission system. -@quotation -@strong{Warning:} Extraction can overwrite files in the file system. -To avoid losing files in the file system when extracting files from -the archive with the same name, use the @samp{--keep-old-files} option -(@pxref{File Writing Options}). -@end quotation +Of course, @var{permissions} might be plainly specified as an octal number. +However, by using generic symbolic modifications to mode bits, this allows +more flexibility. For example, the value @samp{a+rw} adds read and write +permissions for everybody, while retaining executable bits on directories +or on any other file already marked as executable. -If the archive was created using @samp{--block-size}, @samp{--compress} -or @samp{--multi-volume}, you must specify those format options again -when extracting files from the archive (@pxref{Format Variations}). +@item --multi-volume +@itemx -M -@menu -* Archive Reading Options:: -* File Writing Options:: -* Scarce Disk Space:: Recovering From Scarce Disk Space -@end menu +Informs @code{tar} that it should create or otherwise operate on a +multi-volume @code{tar} archive. @FIXME-xref{}. -@node Archive Reading Options, File Writing Options, Extracting From Archives, Extracting From Archives -@subsection Options to Help Read Archives -@cindex Options when reading archives -@cindex Reading incomplete blocks -@cindex Blocks, incomplete -@cindex End of archive markers, ignoring -@cindex Ignoring end of archive markers -@cindex Large lists of file names on small machines -@cindex Small memory -@cindex Running out of space +@item --new-volume-script -@c <<< each option wants its own node. summary after menu +(see --info-script) -Normally, @code{tar} will request data in full block increments from -an archive storage device. If the device cannot return a full block, -@code{tar} will report an error. However, some devices do not always -return full blocks, or do not require the last block of an archive to -be padded out to the next block boundary. To keep reading until you -obtain a full block, or to accept an incomplete block if it contains -an end-of-archive marker, specify the @samp{--read-full-blocks} option -in conjunction with the @samp{--extract} or @samp{--list} operations. -@xref{Listing Contents}. - -The @samp{--read-full-blocks} option is turned on by default when -@code{tar} reads an archive from standard input, or from a remote -machine. This is because on BSD Unix systems, attempting to read a -pipe returns however much happens to be in the pipe, even if it is -less than was requested. If this option were not enabled, @code{tar} -would fail as soon as it read an incomplete block from the pipe. +@item --newer=@var{date} +@itemx --after-date=@var{date} +@itemx -N -If you're not sure of the blocking factor of an archive, you can read -the archive by specifying @samp{--read-full-blocks} and -@samp{--block-size=@var{n}}, where @var{n} is a blocking factor larger -than the blocking factor of the archive. This lets you avoid having -to determine the blocking factor of an archive. @xref{Blocking -Factor}. +When creating an archive, @code{tar} will only add files that have changed +since @var{date}. @FIXME-xref{}. -@table @samp -@item --read-full-blocks -@item -B -Use in conjunction with @samp{tar --extract} to read an archive which -contains incomplete blocks, or one which has a blocking factor less -than the one specified. -@end table +@item --newer-mtime -Normally @code{tar} stops reading when it encounters a block of zeros -between file entries (which usually indicates the end of the archive). -@samp{--ignore-zeros} allows @code{tar} to completely read an archive -which contains a block of zeros before the end (i.e.@: a damaged -archive, or one which was created by @code{cat}-ing several archives -together). +In conjunction with @samp{--newer}, @code{tar} will only add files whose +contents have changed (as opposed to just @samp{--newer}, which will +also back up files for which any status information has changed). -The @samp{--ignore-zeros} option is turned off by default because many -versions of @code{tar} write garbage after the end of archive entry, -since that part of the media is never supposed to be read. GNU -@code{tar} does not write after the end of an archive, but seeks to -maintain compatablity among archiving utilities. +@item --no-recursion -@table @samp -@item --ignore-zeros -@itemx -i -To ignore blocks of zeros (ie.@: end-of-archive entries) which may be -encountered while reading an archive. Use in conjunction with -@samp{tar --extract} or @samp{tar --list}. -@end table +With this option, @code{tar} will not recurse into directories unless a +directory is explicitly named as an argument to @code{tar}. @FIXME-xref{}. -If you are using a machine with a small amount of memory, and you need -to process large list of file-names, you can reduce the amount of -space @code{tar} needs to process the list. To do so, specify the -@samp{--same-order} option and provide an ordered list of file names. -This option tells @code{tar} that the @file{file-name} arguments -(provided on the command line, or read from a file using the -@samp{--files-from} option) are listed in the same order as the files -in the archive. +@item --null -You can create a file containing an ordered list of files in the -archive by storing the output produced by @samp{tar --list ---file=@var{archive-name}}. @xref{Listing Contents}, for information -on the @samp{--list} operation. +When @code{tar} is using the @samp{--files-from} option, this option +instructs @code{tar} to expect filenames terminated with @kbd{NUL}, so +@code{tar} can correctly work with file names that contain newlines. +@FIXME-xref{}. -This option is probably never needed on modern computer systems. +@item --numeric-owner -@table @samp -@item --same-order -@itemx --preserve-order -@itemx -s -To process large lists of file-names on machines with small amounts of -memory. Use in conjunction with @samp{tar --compare}, @samp{tar --list} -or @samp{tar --extract}. -@end table +This option will notify @code{tar} that it should use numeric user and group +IDs when creating a @code{tar} file, rather than names. @FIXME-xref{}. -@c we don't need/want --preserve to exist any more +@item --old-archive -@node File Writing Options, Scarce Disk Space, Archive Reading Options, Extracting From Archives -@subsection Changing How @code{tar} Writes Files -@c <<< find a better title -@cindex Overwriting old files, prevention -@cindex Protecting old files -@cindex Modification times of extracted files -@cindex Permissions of extracted files -@cindex Modes of extracted files -@cindex Writing extracted files to standard output -@cindex Standard output, writing extracted files to +(See @samp{--portability}; @FIXME-pxref{}.) -Normally, @code{tar} writes extracted files into the file system -without regard to the files already on the system---files with the -same name as archive members are overwritten. To prevent @code{tar} -from extracting an archive member from an archive, if doing so will -overwrite a file in the file system, use @samp{--keep-old-files} in -conjunction with the @samp{--extract} operation. When this option is -specified, @code{tar} reports an error stating the name of the files -in conflict, instead of writing the file from the archive. +@item --one-file-system +@itemx -l -@table @samp -@item --keep-old files -@itemx -k -Prevents @code{tar} from overwriting files in the file system during -extraction. -@end table +Used when creating an archive. Prevents @code{tar} from recursing into +directories that are on different file systems from the current +directory. @FIXME-xref{}. -Normally, @code{tar} sets the modification times of extracted files to -the modification times recorded for the files in the archive, but -limits the permissions of extracted files by the current @code{umask} -setting. +@item --owner=@var{user} -To set the modification times of extracted files to the time when -the files were extracted, use the @samp{--modification-time} option in -conjunction with @samp{tar --extract}. +Specifies that @code{tar} should use @var{user} as the owner of members +when creating archives, instead of the user associated with the source +file. @var{user} is first decoded as a user symbolic name, but if +this interpretation fails, it has to be a decimal numeric user ID. +@FIXME-xref{}. -@table @samp -@item --modification-time -@itemx -m -Sets the modification time of extracted archive members to the time -they were extracted, not the time recorded for them in the archive. -Use in conjunction with @samp{--extract}. -@end table +There is no value indicating a missing number, and @samp{0} usually means +@code{root}. Some people like to force @samp{0} as the value to offer in +their distributions for the owner of files, because the @code{root} user is +anonymous anyway, so that might as well be the owner of anonymous archives. -To set the modes (access permissions) of extracted files to those -recorded for those files in the archive, use the -@samp{--preserve-permissions} option in conjunction with the -@samp{--extract} operation. -@c <<>> should be an example in the tutorial/wizardry section using this -@c >>> to transfer files between systems. +Sets the modification time of extracted files to the extraction time, +rather than the modification time stored in the archive. +@xref{Writing}. -@c >>> is write access an issue? +@item --uncompress -@table @samp -@item --absolute-paths -Preserves full file names (inclusing superior dirctory names) when -archiving files. Preserves leading slash when extracting files. -@end table +(See @samp{--compress}; @FIXME-pxref{}.) -@node Changing Working Directory, Archiving with Symbolic Links, Absolute File Names, File Name Interpretation -@subsection Changing the Working Directory Within a List of File-names -@cindex Directory, changing in mid-stream -@cindex Working directory, specifying +@item --ungzip -To change working directory in the middle of a list of file names, -(either on the command line or in a file specified using -@samp{--files-from}), use @samp{--directory=@var{directory}}. This will -change the working directory to the directory @var{directory} after -that point in the list. For example, +(See @samp{--gzip}; @FIXME-pxref{}.) -@example -tar --create iggy ziggy --directory=baz melvin -@end example +@item --unlink-first +@itemx -U -@noindent -will place the files @file{iggy} and @file{ziggy} from the current -directory into the archive, followed by the file @file{melvin} from -the directory @file{baz}. This option is especially useful when you -have several widely separated files that you want to store in the same -directory in the archive. - -Note that the file @file{melvin} is recorded in the archive under the -precise name @file{melvin}, @emph{not} @file{baz/melvin}. Thus, the -archive will contain three files that all appear to have come from the -same directory; if the archive is extracted with plain @samp{tar ---extract}, all three files will be written in the current directory. +Directs @code{tar} to remove the corresponding file from the file system +before extracting it from the archive. @xref{Writing}. -Contrast this with the command +@item --use-compress-program=@var{prog} -@example -tar -c iggy ziggy bar/melvin -@end example +Instructs @code{tar} to access the archive through @var{prog}, which is +presumed to be a compression program of some sort. @FIXME-xref{}. -@noindent -which records the third file in the archive under the name -@file{bar/melvin} so that, if the archive is extracted using @samp{tar ---extract}, the third file will be written in a subdirectory named -@file{bar}. +@item --verbose +@itemx -v -@table @samp -@item --directory=@file{directory} -@itemx -C @file{directory} -Changes the working directory. -@end table +Specifies that @code{tar} should be more verbose about the operations its +performing. This option can be specified multiple times for some +operations to increase the amount of information displayed. @FIXME-xref{}. -@c <<>> +@samp{--to-stdout} -@node User Interaction, Backups and Restoration, Selecting Archive Members, Top -@chapter User Interaction -@cindex Getting more information during the operation -@cindex Information during operation -@cindex Feedback from @code{tar} +@item -P -Once you have typed a @code{tar}command, it is usually performed -without any further information required of the user, or provided by -@code{tar}. The following options allow you to generate progress and -status information during an operation, or to confirm operations on -files as they are performed. +@samp{--absolute-names} -@menu -* Additional Information:: -* Interactive Operation:: -@end menu +@item -R -@node Additional Information, Interactive Operation, User Interaction, User Interaction -@section Progress and Status Information -@cindex Progress information -@cindex Status information -@cindex Information on progress and status of operations -@cindex Verbose operation -@cindex Record number where error occured -@cindex Error message, record number of -@cindex Version of the @code{tar} program +@samp{--block-number} -Typically, @code{tar} performs most operations without reporting any -information to the user except error messages. If you have -encountered a problem when operating on an archive, however, you may -need more information than just an error message in order to solve the -problem. The following options can be helpful diagnostic tools. - -When used with most operations, @samp{--verbose} causes @code{tar} to -print the file names of the files or archive members it is operating -on. When used with @samp{tar --list}, the verbose option causes -@code{tar} to print out an @samp{ls -l} type listing of the files in -the archive. - -Verbose output appears on the standard output except when an archive -is being written to the standard output (as with @samp{tar --create ---file=- --verbose}). In that case @code{tar} writes verbose output to -the standard error stream. +@item -S -@table @samp -@item --verbose -@itemx -v -Prints the names of files or archive members as they are being -operated on. Can be used in conjunction with any operation. When -used with @samp{--list}, generates an @samp{ls -l} type listing. -@end table +@samp{--sparse} -To find out where in an archive a message was triggered, use -@samp{--record-number}. @samp{--record-number} causes @code{tar} to -print, along with every message it produces, the record number within -the archive where the message was triggered. +@item -T -This option is especially useful when reading damaged archives, since -it helps pinpoint the damaged sections. It can also be used with -@samp{tar --list} when listing a file-system backup tape, allowing you -to choose among several backup tapes when retrieving a file later, in -favor of the tape where the file appears earliest (closest to the -front of the tape). -@c <<< xref when the node name is set and the backup section written +@samp{--files-from} -@table @samp -@item --record-number -@itemx -R -Prints the record number whenever a message is generated by -@code{tar}. Use in conjunction with any operation. -@end table +@item -U -@c rewrite below -To print the version number of the @code{tar} program, use @samp{tar ---version}. @code{tar} prints the version number to the standard -error. For example: +@samp{--unlink-first} -@example -tar --version -@end example +@item -V -@noindent -might return: +@samp{--label} -@example -GNU tar version 1.09 -@end example -@c used to be an option. has been fixed. +@item -W -@node Interactive Operation, , Additional Information, User Interaction -@section Asking for Confirmation During Operations -@cindex Interactive operation +@samp{--verify} -Typically, @code{tar} carries out a command without stopping for -further instructions. In some situations however, you -may want to exclude some files and archive members from the operation -(for instance if disk or storage space is tight). You can do this by -excluding certain files automatically (@pxref{File Exclusion}), or by -performing an operation interactively, using the @samp{--interactive} -operation. - -When the @samp{--interactive} option is specified, @code{tar} asks for -confirmation before reading, writing, or deleting each file it -encounters while carrying out an operation. To confirm the action you -must type a line of input beginning with @samp{y}. If your input line -begins with anything other than @samp{y}, @code{tar} skips that file. - -Commands which might be useful to perform interactively include -appending files to an archive, extracting files from an archive, -deleting a file from an archive, and deleting a file from disk during -an incremental restore. +@item -X -If @code{tar} is reading the archive from the standard input, -@code{tar} opens the file @file{/dev/tty} to support the interactive -communications. -<<< this aborts if you won't OK the working directory. this is a bug. -ringo +@samp{--exclude-from} -@table @samp -@item --interactive -@itemx --confirmation -@itemx -w -Asks for confirmation before reading, writing or deleting an archive -member (when listing, comparing or writing an archive or deleting -archive members), or before writing or deleting a file (when -extracting an archive). -@end table +@item -Z -@node Backups and Restoration, Media, User Interaction, Top -@chapter Performing Backups and Restoring Files +@samp{--compress} -To @dfn{back up} a file system means to create archives that contain -all the files in that file system. Those archives can then be used to -restore any or all of those files (for instance if a disk crashes or a -file is accidently deleted). File system @dfn{backups} are also -called @dfn{dumps}. +@item -b -@menu -* Backup Levels:: Levels of backups -* Backup Scripts:: Using scripts to perform backups - and restoration -* incremental and listed-incremental:: The --incremental - and --listed-incremental Options -* Problems:: Some common problems and their solutions -@end menu +@samp{--blocking-factor} -@node Backup Levels, Backup Scripts, Backups and Restoration, Backups and Restoration -@section Levels of Backups +@item -c -An archive containing all the files in the file system is called a -@dfn{full backup} or @dfn{full dump}. You could insure your data by -creating a full dump every day. This strategy, however, would waste a -substantial amount of archive media and user time, as unchanged files -are daily re-archived. +@samp{--create} -It is more efficient to do a full dump only occasionally. To back up -files between full dumps, you can a incremental dump. A @dfn{level -one} dump archives all the files that have changed since the last full -dump. +@item -d -A typical dump strategy would be to perform a full dump once a week, -and a level one dump once a day. This means some versions of files -will in fact be archived more than once, but this dump strategy makes -it possible to restore a file system to within one day of accuracy by -only extracting two archives---the last weekly (full) dump and the -last daily (level one) dump. The only information lost would be in -files changed or created since the last daily backup. (Doing dumps -more than once a day is usually not worth the trouble). +@samp{--compare} -@node Backup Scripts, incremental and listed-incremental, Backup Levels, Backups and Restoration -@section Using Scripts to Perform Backups and Restoration +@item -f -GNU @code{tar} comes with scripts you can use to do full and level-one -dumps. Using scripts (shell programs) to perform backups and -restoration is a convenient and reliable alternative to typing out -file name lists and @code{tar} commands by hand. +@samp{--file} -Before you use these scripts, you need to edit the file -@file{backup-specs}, which specifies parameters used by the backup -scripts and by the restore script. @xref{Script Syntax}. -Once the backup parameters are set, you can perform backups or -restoration by running the appropriate script. - -The name of the restore script is @code{restore}. The names of the -level one and full backup scripts are, respectively, @code{level-1} and -@code{level-0}. The @code{level-0} script also exists under the name -@code{weekly}, and the @code{level-1} under the name -@code{daily}---these additional names can be changed according to your -backup schedule. @xref{Scripted Restoration}, for more information -on running the restoration script. @xref{Scripted Backups}, for more -information on running the backup scripts. +@item -g -@emph{Please Note:} The backup scripts and the restoration scripts are -designed to be used together. While it is possible to restore files -by hand from an archive which was created using a backup script, and -to create an archive by hand which could then be extracted using the -restore script, it is easier to use the scripts. @xref{incremental -and listed-incremental}, before making such an attempt. +@samp{--listed-incremental} -@c shorten node names -@menu -* Backup Parameters:: Setting parameters for backups and restoration -* Scripted Backups:: Using the backup scripts -* Scripted Restoration:: Using the restore script -@end menu +@item -h -@node Backup Parameters, Scripted Backups, Backup Scripts, Backup Scripts -@subsection Setting Parameters for Backups and Restoration +@samp{--dereference} -The file @file{backup-specs} specifies backup parameters for the -backup and restoration scripts provided with @code{tar}. You must -edit @file{backup-specs} to fit your system configuration and schedule -before using these scripts. +@item -i -@c <<< This about backup scripts needs to be written: -@c << date.nfs.dump +mt -f /dev/rtu20 rew +@end example + +Output from this script is stored in a file, for the operator to +read later. + +This script uses the file @file{date.nfs.dump} to store the date/time +of the last dump. + +Since this is a streaming tape drive, no attempt to verify the archive +is done. This is also why the high blocking factor (126) is used. +The tape drive must also be rewound by the @code{mt} command after +the dump is made. + +@node incremental and listed-incremental, Backup Levels, Inc Dumps, Backups +@section The Incremental Options +@UNREVISED + +@value{op-incremental} is used in conjunction with @value{op-create}, +@value{op-extract} or @value{op-list} when backing up and restoring file +systems. An archive cannot be extracted or listed with the +@value{op-incremental} option specified unless it was created with the +option specified. This option should only be used by a script, not by +the user, and is usually disregarded in favor of +@value{op-listed-incremental}, which is described below. + +@value{op-incremental} in conjunction with @value{op-create} causes +@code{tar} to write, at the beginning of the archive, an entry for +each of the directories that will be archived. The entry for a +directory includes a list of all the files in the directory at the +time the archive was created and a flag for each file indicating +whether or not the file is going to be put in the archive. + +Note that this option causes @code{tar} to create a non-standard +archive that may not be readable by non-GNU versions of the @code{tar} +program. + +@value{op-incremental} in conjunction with @value{op-extract} causes +@code{tar} to read the lists of directory contents previously stored +in the archive, @emph{delete} files in the file system that did not +exist in their directories when the archive was created, and then +extract the files in the archive. + +This behavior is convenient when restoring a damaged file system from +a succession of incremental backups: it restores the entire state of +the file system to that which obtained when the backup was made. If +@value{op-incremental} isn't specified, the file system will probably +fill up with files that shouldn't exist any more. + +@value{op-incremental} in conjunction with @value{op-list}, causes +@code{tar} to print, for each directory in the archive, the list of +files in that directory at the time the archive was created. This +information is put out in a format that is not easy for humans to +read, but which is unambiguous for a program: each file name is +preceded by either a @samp{Y} if the file is present in the archive, +an @samp{N} if the file is not included in the archive, or a @samp{D} +if the file is a directory (and is included in the archive). Each +file name is terminated by a null character. The last file is followed +by an additional null and a newline to indicate the end of the data. + +@value{op-listed-incremental} acts like @value{op-incremental}, but when +used in conjunction with @value{op-create} will also cause @code{tar} +to use the file @var{snapshot-file}, which contains information about +the state of the file system at the time of the last backup, to decide +which files to include in the archive being created. That file will +then be updated by @code{tar}. If the file @var{file} does not exist +when this option is specified, @code{tar} will create it, and include +all appropriate files in the archive. + +The file @var{file}, which is archive independent, contains the date +it was last modified and a list of devices, inode numbers and +directory names. @code{tar} will archive files with newer mod dates +or inode change times, and directories with an unchanged inode number +and device but a changed directory name. The file is updated after +the files to be archived are determined, but before the new archive is +actually created. + +Despite it should be obvious that a device has a non-volatile value, NFS +devices have non-dependable values when an automounter gets in the picture. +This led to a great deal of spurious redumping in incremental dumps, +so it is somewhat useless to compare two NFS devices numbers over time. +So @code{tar} now considers all NFS devices as being equal when it comes +to comparing directories; this is fairly gross, but there does not seem +to be a better way to go. + +@FIXME{this section needs to be written} + +@node Backup Levels, Backup Parameters, incremental and listed-incremental, Backups +@section Levels of Backups +@UNREVISED + +An archive containing all the files in the file system is called a +@dfn{full backup} or @dfn{full dump}. You could insure your data by +creating a full dump every day. This strategy, however, would waste a +substantial amount of archive media and user time, as unchanged files +are daily re-archived. + +It is more efficient to do a full dump only occasionally. To back up +files between full dumps, you can a incremental dump. A @dfn{level +one} dump archives all the files that have changed since the last full +dump. + +A typical dump strategy would be to perform a full dump once a week, +and a level one dump once a day. This means some versions of files +will in fact be archived more than once, but this dump strategy makes +it possible to restore a file system to within one day of accuracy by +only extracting two archives---the last weekly (full) dump and the +last daily (level one) dump. The only information lost would be in +files changed or created since the last daily backup. (Doing dumps +more than once a day is usually not worth the trouble). + +GNU @code{tar} comes with scripts you can use to do full and level-one +dumps. Using scripts (shell programs) to perform backups and +restoration is a convenient and reliable alternative to typing out +file name lists and @code{tar} commands by hand. + +Before you use these scripts, you need to edit the file +@file{backup-specs}, which specifies parameters used by the backup +scripts and by the restore script. @FIXME{There is no such restore +script!}. @FIXME-xref{Script Syntax}. Once the backup parameters +are set, you can perform backups or restoration by running the +appropriate script. + +The name of the restore script is @code{restore}. @FIXME{There is +no such restore script!}. The names of the level one and full backup +scripts are, respectively, @code{level-1} and @code{level-0}. +The @code{level-0} script also exists under the name @code{weekly}, and +the @code{level-1} under the name @code{daily}---these additional names +can be changed according to your backup schedule. @FIXME-xref{Scripted +Restoration}, for more information on running the restoration script. +@FIXME-xref{Scripted Backups}, for more information on running the +backup scripts. + +@emph{Please Note:} The backup scripts and the restoration scripts are +designed to be used together. While it is possible to restore files by +hand from an archive which was created using a backup script, and to create +an archive by hand which could then be extracted using the restore script, +it is easier to use the scripts. @FIXME{There is no such restore script!}. +@value{xref-incremental}, and @value{xref-listed-incremental}, +before making such an attempt. + +@FIXME{shorten node names} + +@node Backup Parameters, Scripted Backups, Backup Levels, Backups +@section Setting Parameters for Backups and Restoration +@UNREVISED + +The file @file{backup-specs} specifies backup parameters for the +backup and restoration scripts provided with @code{tar}. You must +edit @file{backup-specs} to fit your system configuration and schedule +before using these scripts. + +@FIXME{This about backup scripts needs to be written: BS is a shell +script .... thus ... @file{backup-specs} is in shell script syntax.} + +@FIXME-xref{Script Syntax}, for an explanation of this syntax. + +@FIXME{Whats a parameter .... looked at by the backup scripts +... which will be expecting to find ... now syntax ... value is linked +to lame ... @file{backup-specs} specifies the following parameters:} + +@table @samp +@item ADMINISTRATOR +The user name of the backup administrator. + +@item BACKUP_HOUR +The hour at which the backups are done. This can be a number from 0 +to 23, or the string @samp{now}. + +@item TAPE_FILE +The device @code{tar} writes the archive to. This device should be +attached to the host on which the dump scripts are run. + +@FIXME{examples for all ...} + +@item TAPE_STATUS +The command to use to obtain the status of the archive device, +including error count. On some tape drives there may not be such a +command; in that case, simply use `TAPE_STATUS=false'. + +@item BLOCKING +The blocking factor @code{tar} will use when writing the dump archive. +@value{xref-blocking-factor}. + +@item BACKUP_DIRS +A list of file systems to be dumped. You can include any directory +name in the list---subdirectories on that file system will be +included, regardless of how they may look to other networked machines. +Subdirectories on other file systems will be ignored. + +The host name specifies which host to run @code{tar} on, and should +normally be the host that actually contains the file system. However, +the host machine must have GNU @code{tar} installed, and must be able +to access the directory containing the backup scripts and their +support files using the same file name that is used on the machine +where the scripts are run (ie. what @code{pwd} will print when in that +directory on that machine). If the host that contains the file system +does not have this capability, you can specify another host as long as +it can access the file system through NFS. + +@item BACKUP_FILES +A list of individual files to be dumped. These should be accessible +from the machine on which the backup script is run. + +@FIXME{Same file name, be specific. Through NFS ...} + +@end table + +@menu +* backup-specs example:: An Example Text of @file{Backup-specs} +* Script Syntax:: Syntax for @file{Backup-specs} +@end menu + +@node backup-specs example, Script Syntax, Backup Parameters, Backup Parameters +@subsection An Example Text of @file{Backup-specs} +@UNREVISED + +The following is the text of @file{backup-specs} as it appears at FSF: + +@example +# site-specific parameters for file system backup. + +ADMINISTRATOR=friedman +BACKUP_HOUR=1 +TAPE_FILE=/dev/nrsmt0 +TAPE_STATUS="mts -t $TAPE_FILE" +BLOCKING=124 +BACKUP_DIRS=" + albert:/fs/fsf + apple-gunkies:/gd + albert:/fs/gd2 + albert:/fs/gp + geech:/usr/jla + churchy:/usr/roland + albert:/ + albert:/usr + apple-gunkies:/ + apple-gunkies:/usr + gnu:/hack + gnu:/u + apple-gunkies:/com/mailer/gnu + apple-gunkies:/com/archive/gnu" + +BACKUP_FILES="/com/mailer/aliases /com/mailer/league*[a-z]" + +@end example + +@node Script Syntax, , backup-specs example, Backup Parameters +@subsection Syntax for @file{Backup-specs} +@UNREVISED + +@file{backup-specs} is in shell script syntax. The following +conventions should be considered when editing the script: +@FIXME{"conventions?"} + +A quoted string is considered to be contiguous, even if it is on more +than one line. Therefore, you cannot include commented-out lines +within a multi-line quoted string. BACKUP_FILES and BACKUP_DIRS are +the two most likely parameters to be multi-line. + +A quoted string typically cannot contain wildcards. In +@file{backup-specs}, however, the parameters BACKUP_DIRS and +BACKUP_FILES can contain wildcards. + +@node Scripted Backups, Scripted Restoration, Backup Parameters, Backups +@section Using the Backup Scripts +@UNREVISED + +The syntax for running a backup script is: + +@example +@file{script-name} [@var{time-to-be-run}] +@end example + +where @var{time-to-be-run} can be a specific system time, or can be +@kbd{now}. If you do not specify a time, the script runs at the time +specified in @file{backup-specs} (@FIXME-pxref{Script Syntax}). + +You should start a script with a tape or disk mounted. Once you +start a script, it prompts you for new tapes or disks as it +needs them. Media volumes don't have to correspond to archive +files---a multi-volume archive can be started in the middle of a +tape that already contains the end of another multi-volume archive. +The @code{restore} script prompts for media by its archive volume, +so to avoid an error message you should keep track of which tape +(or disk) contains which volume of the archive. @FIXME{There is +no such restore script!}. @FIXME-xref{Scripted Restoration}. +@FIXME{Have file names changed?} + +The backup scripts write two files on the file system. The first is a +record file in @file{/etc/tar-backup/}, which is used by the scripts +to store and retrieve information about which files were dumped. This +file is not meant to be read by humans, and should not be deleted by +them. @FIXME-xref{incremental and listed-incremental}, for a more +detailed explanation of this file. + +The second file is a log file containing the names of the file systems +and files dumped, what time the backup was made, and any error +messages that were generated, as well as how much space was left in +the media volume after the last volume of the archive was written. +You should check this log file after every backup. The file name is +@file{log-@var{mmm-ddd-yyyy}-level-1} or +@file{log-@var{mmm-ddd-yyyy}-full}. + +The script also prints the name of each system being dumped to the +standard output. + +@node Scripted Restoration, , Scripted Backups, Backups +@section Using the Restore Script +@UNREVISED + +@ifset PUBLISH + +The @code{tar} distribution does not provide restoring scripts. + +@end ifset + +@ifclear PUBLISH + +@quotation +@strong{Warning:} The GNU @code{tar} distribution does @emph{not} +provide any such @code{restore} script yet. This section is only +listed here for documentation maintenance purposes. In any case, +all contents is subject to change as things develop. +@end quotation + +@FIXME{A section on non-scripted restore may be a good idea.} + +To restore files that were archived using a scripted backup, use the +@code{restore} script. The syntax for the script is: + +where ***** are the file systems to restore from, and +***** is a regular expression which specifies which files to +restore. If you specify --all, the script restores all the files +in the file system. + +You should start the restore script with the media containing the +first volume of the archive mounted. The script will prompt for other +volumes as they are needed. If the archive is on tape, you don't need +to rewind the tape to to its beginning---if the tape head is +positioned past the beginning of the archive, the script will rewind +the tape as needed. @FIXME-xref{Media}, for a discussion of tape +positioning. + +If you specify @samp{--all} as the @var{files} argument, the +@code{restore} script extracts all the files in the archived file +system into the active file system. + +@quotation +@strong{Warning:} The script will delete files from the active file +system if they were not in the file system when the archive was made. +@end quotation + +@value{xref-incremental}, and @value{ref-listed-incremental}, +for an explanation of how the script makes that determination. + +@FIXME{this may be an option, not a given} + +@end ifclear + +@node Choosing, Date input formats, Backups, Top +@chapter Choosing Files and Names for @code{tar} +@UNREVISED + +@FIXME{Melissa (still) Doesn't Really Like This ``Intro'' Paragraph!!!} + +Certain options to @code{tar} enable you to specify a name for your +archive. Other options let you decide which files to include or exclude +from the archive, based on when or whether files were modified, whether +the file names do or don't match specified patterns, or whether files +are in specified directories. + +@menu +* file:: Choosing the Archive's Name +* Selecting Archive Members:: +* files:: Reading Names from a File +* exclude:: Excluding Some Files +* Wildcards:: +* after:: Operating Only on New Files +* recurse:: Descending into Directories +* one:: Crossing Filesystem Boundaries +@end menu + +@node file, Selecting Archive Members, Choosing, Choosing +@section Choosing and Naming Archive Files +@cindex Naming an archive +@cindex Archive Name +@cindex Directing output +@cindex Choosing an archive file +@cindex Where is the archive? +@UNREVISED + +@FIXME{should the title of this section actually be, "naming an +archive"?} + +By default, @code{tar} uses an archive file name that was compiled when +it was built on the system; usually this name refers to some physical +tape drive on the machine. However, the person who installed @code{tar} +on the system may not set the default to a meaningful value as far as +most users are concerned. As a result, you will usually want to tell +@code{tar} where to find (or create) the archive. The @value{op-file} +option allows you to either specify or name a file to use as the archive +instead of the default archive file location. + +@table @kbd +@item --file=@var{archive-name} +@itemx -f @var{archive-name} +Name the archive to create or operate on. Use in conjunction with +any operation. +@end table + +For example, in this @code{tar} command, + +@example +$ @kbd{tar -cvf collection.tar blues folk jazz} +@end example + +@noindent +@file{collection.tar} is the name of the archive. It must directly +follow the @samp{-f} option, since whatever directly follows @samp{-f} +@emph{will} end up naming the archive. If you neglect to specify an +archive name, you may end up overwriting a file in the working directory +with the archive you create since @code{tar} will use this file's name +for the archive name. + +An archive can be saved as a file in the file system, sent through a +pipe or over a network, or written to an I/O device such as a tape, +floppy disk, or CD write drive. + +@cindex Writing new archives +@cindex Archive creation +If you do not name the archive, @code{tar} uses the value of the +environment variable @code{TAPE} as the file name for the archive. If +that is not available, @code{tar} uses a default, compiled-in archive +name, usually that for tape unit zero (ie. @file{/dev/tu00}). +@code{tar} always needs an archive name. + +If you use @file{-} as an @var{archive-name}, @code{tar} reads the +archive from standard input (when listing or extracting files), or +writes it to standard output (when creating an archive). If you use +@file{-} as an @var{archive-name} when modifying an archive, +@code{tar} reads the original archive from its standard input and +writes the entire new archive to its standard output. + +@FIXME{might want a different example here; this is already used in +"notable tar usages".} + +@example +$ @kbd{cd sourcedir; tar -cf - . | (cd targetdir; tar -xf -)} +@end example + +@FIXME{help!} + +@cindex Standard input and output +@cindex tar to standard input and output +To specify an archive file on a device attached to a remote machine, +use the following: + +@example +@kbd{--file=@var{hostname}:/@var{dev}/@var{file name}} +@end example + +@noindent +@code{tar} will complete the remote connection, if possible, and +prompt you for a username and password. If you use +@samp{--file=@@@var{hostname}:/@var{dev}/@var{file name}}, @code{tar} +will complete the remote connection, if possible, using your username +as the username on the remote machine. + +If the archive file name includes a colon (@samp{:}), then it is assumed +to be a file on another machine. If the archive file is +@samp{@var{user}@@@var{host}:@var{file}}, then @var{file} is used on the +host @var{host}. The remote host is accessed using the @code{rsh} +program, with a username of @var{user}. If the username is omitted +(along with the @samp{@@} sign), then your user name will be used. +(This is the normal @code{rsh} behavior.) It is necessary for the +remote machine, in addition to permitting your @code{rsh} access, to +have the @file{/usr/ucb/rmt} program installed. If you need to use a +file whose name includes a colon, then the remote tape drive behavior +can be inhibited by using the @value{op-force-local} option. + +@FIXME{i know we went over this yesterday, but bob (and now i do again, +too) thinks it's out of the middle of nowhere. it doesn't seem to tie +into what came before it well enough <>. bob also comments that if Amanda isn't free software, we +shouldn't mention it..} + +When the archive is being created to @file{/dev/null}, GNU @code{tar} +tries to minimize input and output operations. The Amanda backup +system, when used with GNU @code{tar}, has an initial sizing pass which +uses this feature. + +@node Selecting Archive Members, files, file, Choosing +@section Selecting Archive Members +@cindex Specifying files to act on +@cindex Specifying archive members + +@dfn{File Name arguments} specify which files in the file system +@code{tar} operates on, when creating or adding to an archive, or which +archive members @code{tar} operates on, when reading or deleting from +an archive. @xref{Operations}. + +To specify file names, you can include them as the last arguments on +the command line, as follows: +@smallexample +@kbd{tar} @var{operation} [@var{option1} @var{option2} @dots{}] [@var{file name-1} @var{file name-2} @dots{}] +@end smallexample + +If you specify a directory name as a file name argument, all the files +in that directory are operated on by @code{tar}. + +If you do not specify files when @code{tar} is invoked with +@value{op-create}, @code{tar} operates on all the non-directory files in +the working directory. If you specify either @value{op-list} or +@value{op-extract}, @code{tar} operates on all the archive members in the +archive. If you specify any operation other than one of these three, +@code{tar} does nothing. + +By default, @code{tar} takes file names from the command line. However, +there are other ways to specify file or member names, or to modify the +manner in which @code{tar} selects the files or members upon which to +operate; @FIXME{add xref here}. In general, these methods work both for +specifying the names of files and archive members. + +@node files, exclude, Selecting Archive Members, Choosing +@section Reading Names from a File +@UNREVISED + +@cindex Reading file names from a file +@cindex Lists of file names +@cindex File Name arguments, alternatives +Instead of giving the names of files or archive members on the command +line, you can put the names into a file, and then use the +@value{op-files-from} option to @code{tar}. Give the name of the file +which contains the list of files to include as the argument to +@samp{--files-from}. In the list, the file names should be separated by +newlines. You will frequently use this option when you have generated +the list of files to archive with the @code{find} utility. + +@table @kbd +@item --files-from=@var{file name} +@itemx -T @var{file name} +Get names to extract or create from file @var{file name}. +@end table + +If you give a single dash as a file name for @samp{--files-from}, (i.e., +you specify either @samp{--files-from=-} or @samp{-T -}), then the file +names are read from standard input. + +Unless you are running @code{tar} with @samp{--create}, you can not use +both @samp{--files-from=-} and @samp{--file=-} (@samp{-f -}) in the same +command. + +@FIXME{add bob's example, from his message on 2-10-97} + +The following example shows how to use @code{find} to generate a list of +files smaller than 400K in length and put that list into a file +called @file{small-files}. You can then use the @samp{-T} option to +@code{tar} to specify the files from that file, @file{small-files}, to +create the archive @file{little.tgz}. (The @samp{-z} option to +@code{tar} compresses the archive with @code{gzip}; @pxref{gzip} for +more information.) + +@example +$ @kbd{find . -size -400 -print > small-files} +$ @kbd{tar -c -v -z -T small-files -f little.tgz} +@end example + +@noindent +@FIXME{say more here to conclude the example/section?} + +@menu +* nul:: +@end menu + +@node nul, , files, files +@ifinfo +@unnumberedsubsec @kbd{NUL} Terminated File Names +@end ifinfo + +@cindex File names, terminated by @kbd{NUL} +@cindex @kbd{NUL} terminated file names +The @value{op-null} option causes @value{op-files-from} to read file +names terminated by a @code{NUL} instead of a newline, so files whose +names contain newlines can be archived using @samp{--files-from}. + +@table @kbd +@item --null +Only consider @kbd{NUL} terminated file names, instead of files that +terminate in a newline. +@end table + +The @samp{--null} option is just like the one in GNU @code{xargs} and +@code{cpio}, and is useful with the @samp{-print0} predicate of GNU +@code{find}. In @code{tar}, @samp{--null} also causes +@value{op-directory} options to be treated as file names to archive, in +case there are any files out there called @file{-C}. + +This example shows how to use @code{find} to generate a list of files +larger than 800K in length and put that list into a file called +@file{long-files}. The @samp{-print0} option to @code{find} just just +like @samp{-print}, except that it separates files with a @kbd{NUL} +rather than with a newline. You can then run @code{tar} with both the +@samp{--null} and @samp{-T} options to specify that @code{tar} get the +files from that file, @file{long-files}, to create the archive +@file{big.tgz}. The @samp{--null} option to @code{tar} will cause +@code{tar} to recognize the @kbd{NUL} separator between files. + +@example +$ @kbd{find . -size +800 -print0 > long-files} +$ @kbd{tar -c -v --null --files-from=long-files --file=big.tar} +@end example + +@FIXME{say anything else here to conclude the section?} + +@node exclude, Wildcards, files, Choosing +@section Excluding Some Files +@cindex File names, excluding files by +@cindex Excluding files by name and pattern +@cindex Excluding files by file system +@UNREVISED + +To avoid operating on files whose names match a particular pattern, +use the @value{op-exclude} or @value{op-exclude-from} options. + +@table @kbd +@item --exclude=@var{pattern} +Causes @code{tar} to ignore files that match the @var{pattern}. +@end table + +@findex exclude +The @value{op-exclude} option will prevent any file or member which +matches the shell wildcards (@var{pattern}) from being operated on +(@var{pattern} can be a single file name or a more complex expression). +For example, if you want to create an archive with all the contents of +@file{/tmp} except the file @file{/tmp/foo}, you can use the command +@samp{tar --create --file=arch.tar --exclude=foo}. You may give +multiple @samp{--exclude} options. + +@table @kbd +@item --exclude-from=@var{file} +@itemx -X @var{file} +Causes @code{tar} to ignore files that match the patterns listed in +@var{file}. +@end table + +@findex exclude-from +Use the @samp{--exclude-from=@var{file-of-patterns}} option to read a +list of shell wildcards, one per line, from @var{file}; @code{tar} will +ignore files matching those regular expressions. Thus if @code{tar} is +called as @w{@samp{tar -c -X foo .}} and the file @file{foo} contains a +single line @file{*.o}, no files whose names end in @file{.o} will be +added to the archive. + +@FIXME{do the exclude options files need to have stuff separated by +newlines the same as the files-from option does?} + +@menu +* problems with exclude:: +@end menu + +@node problems with exclude, , exclude, exclude +@unnumberedsubsec Problems with Using the @code{exclude} Options + +@FIXME{put in for the editor's/editors' amusement, but should be taken +out in the final draft, just in case! : } + +@ignore +subtitled: getting screwed using exclewed +@end ignore + +Some users find @samp{exclude} options confusing. Here are some common +pitfalls: + +@itemize @bullet +@item +The main operating mode of @code{tar} will always act on file names +listed on the command line, no matter whether or not there is an +exclusion which would otherwise affect them. In the example above, if +you create an archive and exclude files that end with @samp{*.o}, but +explicitly name the file @samp{catc.o} after all the options have been +listed, @samp{catc.o} @emph{will} be included in the archive. + +@item +You can sometimes confuse the meanings of @value{op-exclude} and +@value{op-exclude-from}. Be careful: use @value{op-exclude} when files +to be excluded are given as a pattern on the command line. Use +@samp{--exclude-from=@var{file-of-patterns}} to introduce the name of a +file which contains a list of patterns, one per line; each of these +patterns can exclude zero, one, or many files. + +@item +When you use @value{op-exclude}, be sure to quote the @var{pattern} +parameter, so GNU @code{tar} sees wildcard characters like @samp{*}. +If you do not do this, the shell might expand the @samp{*} itself +using files at hand, so @code{tar} might receive a list of files +instead of one pattern, or none at all, making the command somewhat +illegal. This might not correspond to what you want. + +For example, write: + +@example +$ @kbd{tar -c -f @var{archive.tar} -X '*/tmp/*' @var{directory}} +@end example + +@noindent +rather than: + +@example +$ @kbd{tar -c -f @var{archive.tar} -X */tmp/* @var{directory}} +@end example + +@item +You must use use shell syntax, or globbing, rather than @code{regexp} +syntax, when using exclude options in @code{tar}. If you try to use +@code{regexp} syntax to describe files to be excluded, your command +might fail. + +@item +In earlier versions of @code{tar}, what is now the +@samp{--exclude-from=@var{file-of-patterns}} option was called +@samp{--exclude-@var{pattern}} instead. Now, +@samp{--exclude=@var{pattern}} applies to patterns listed on the command +line and @samp{--exclude-from=@var{file-of-patterns}} applies to +patterns listed in a file. + +@end itemize + +@node Wildcards, after, exclude, Choosing +@section Wildcards Patterns and Matching + +@dfn{Globbing} is the operation by which @dfn{wildcard} characters, +@samp{*} or @samp{?} for example, are replaced and expanded into all +existing files matching the given pattern. However, @code{tar} often +uses wildcard patterns for matching (or globbing) archive members instead +of actual files in the filesystem. Wildcard patterns are also used for +verifying volume labels of @code{tar} archives. This section has the +purpose of explaining wildcard syntax for @code{tar}. + +@FIXME{the next few paragraphs need work.} + +A @var{pattern} should be written according to shell syntax, using wildcard +characters to effect globbing. Most characters in the pattern stand +for themselves in the matched string, and case is significant: @samp{a} +will match only @samp{a}, and not @samp{A}. The character @samp{?} in the +pattern matches any single character in the matched string. The character +@samp{*} in the pattern matches zero, one, or more single characters in +the matched string. The character @samp{\} says to take the following +character of the pattern @emph{literally}; it is useful when one needs to +match the @samp{?}, @samp{*}, @samp{[} or @samp{\} characters, themselves. + +The character @samp{[}, up to the matching @samp{]}, introduces a character +class. A @dfn{character class} is a list of acceptable characters +for the next single character of the matched string. For example, +@samp{[abcde]} would match any of the first five letters of the alphabet. +Note that within a character class, all of the ``special characters'' +listed above other than @samp{\} lose their special meaning; for example, +@samp{[-\\[*?]]} would match any of the characters, @samp{-}, @samp{\}, +@samp{[}, @samp{*}, @samp{?}, or @samp{]}. (Due to parsing constraints, +the characters @samp{-} and @samp{]} must either come @emph{first} or +@emph{last} in a character class.) + +@cindex Excluding characters from a character class +@cindex Character class, excluding characters from +If the first character of the class after the opening @samp{[} +is @samp{!} or @samp{^}, then the meaning of the class is reversed. +Rather than listing character to match, it lists those characters which +are @emph{forbidden} as the next single character of the matched string. + +Other characters of the class stand for themselves. The special +construction @samp{[@var{a}-@var{e}]}, using an hyphen between two +letters, is meant to represent all characters between @var{a} and +@var{e}, inclusive. + +@FIXME{need to add a sentence or so here to make this clear for those +who don't have dan around.} + +Periods (@samp{.}) or forward slashes (@samp{/}) are not considered +special for wildcard matches. However, if a pattern completely matches +a directory prefix of a matched string, then it matches the full matched +string: excluding a directory also excludes all the files beneath it. + +There are some discussions floating in the air and asking for modifications +in the way GNU @code{tar} accomplishes wildcard matches. We perceive +any change of semantics in this area as a delicate thing to impose on +GNU @code{tar} users. On the other hand, the GNU project should be +progressive enough to correct any ill design: compatibility at all price +is not always a good attitude. In conclusion, it is @emph{possible} +that slight amendments be later brought to the previous description. +Your opinions on the matter are welcome. + +@node after, recurse, Wildcards, Choosing +@section Operating Only on New Files +@cindex Excluding file by age +@cindex Modification time, excluding files by +@cindex Age, excluding files by +@UNREVISED + +The @value{op-after-date} option causes @code{tar} to only work on files +whose modification or inode-changed times are newer than the @var{date} +given. If you use this option when creating or appending to an archive, +the archive will only include new files. If you use @samp{--after-date} +when extracting an archive, @code{tar} will only extract files newer +than the @var{date} you specify. + +If you only want @code{tar} to make the date comparison based on +modification of the actual contents of the file (rather than inode +changes), then use the @value{op-newer-mtime} option. + +You may use these options with any operation. Note that these options +differ from the @value{op-update} operation in that they allow you to +specify a particular date against which @code{tar} can compare when +deciding whether or not to archive the files. + +@table @kbd +@item --after-date=@var{date} +@itemx --newer=@var{date} +@itemx -N @var{date} +Only store files newer than @var{date}. + +Acts on files only if their modification or inode-changed times are +later than @var{date}. Use in conjunction with any operation. + +@item --newer-mtime=@var{date} +Acts like @value{op-after-date}, but only looks at modification times. +@end table + +These options limit @code{tar} to only operating on files which have +been modified after the date specified. A file is considered to have +changed if the contents have been modified, or if the owner, +permissions, and so forth, have been changed. (For more information on +how to specify a date, see @ref{Date input formats}; remember that the +entire date argument must be quoted if it contains any spaces.) + +Gurus would say that @value{op-after-date} tests both the @code{mtime} +(time the contents of the file were last modified) and @code{ctime} +(time the file's status was last changed: owner, permissions, etc) +fields, while @value{op-newer-mtime} tests only @code{mtime} field. + +To be precise, @value{op-after-date} checks @emph{both} @code{mtime} and +@code{ctime} and processes the file if either one is more recent than +@var{date}, while @value{op-newer-mtime} only checks @code{mtime} and +disregards @code{ctime}. Neither uses @code{atime} (the last time the +contents of the file were looked at). + +Date specifiers can have embedded spaces. Because of this, you may need +to quote date arguments to keep the shell from parsing them as separate +arguments. + +@FIXME{Need example of --newer-mtime with quoted argument.} + +@quotation +@strong{Please Note:} @value{op-after-date} and @value{op-newer-mtime} +should not be used for incremental backups. Some files (such as those +in renamed directories) are not selected properly by these options. +@xref{incremental and listed-incremental}. +@end quotation + +To select files newer than the modification time of a file that already +exists, you can use the @samp{--reference} (@samp{-r}) option of GNU +@code{date}, available in GNU shell utilities 1.13 or later. It returns +the timestamp of that already existing file; this timestamp expands to +become the referent date which @samp{--newer} uses to determine which +files to archive. For example, you could say, + +@example +$ @kbd{tar -cf @var{archive.tar} --newer="`date -r @var{file}`" /home} +@end example + +@noindent +which tells @FIXME{need to fill this in!}. + +@node recurse, one, after, Choosing +@section Descending into Directories +@cindex Avoiding recursion in directories +@cindex Descending directories, avoiding +@cindex Directories, avoiding recursion +@cindex Recursion in directories, avoiding +@UNREVISED + +@FIXME{arrggh! this is still somewhat confusing to me. :-< } + +@FIXME{show dan bob's comments, from 2-10-97} + +Usually, @code{tar} will recursively explore all directories (either +those given on the command line or through the @value{op-files-from} +option) for the various files they contain. However, you may not always +want @code{tar} to act this way. + +The @value{op-no-recursion} option inhibits @code{tar}'s recursive descent +into specified directories. If you specify @samp{--no-recursion}, you can +use the @code{find} utility for hunting through levels of directories to +construct a list of file names which you could then pass to @code{tar}. +@code{find} allows you to be more selective when choosing which files to +archive; see @ref{files} for more information on using @code{find} with +@code{tar}, or look. + +@table @kbd +@item --no-recursion +Prevents @code{tar} from recursively descending directories. +@end table + +When you use @samp{--no-recursion}, GNU @code{tar} grabs directory entries +themselves, but does not descend on them recursively. Many people use +@code{find} for locating files they want to back up, and since +@code{tar} @emph{usually} recursively descends on directories, they have +to use the @samp{@w{! -d}} option to @code{find} @FIXME{needs more +explanation or a cite to another info file} as they usually do not want +all the files in a directory. They then use the @value{op-file-from} +option to archive the files located via @code{find}. + +The problem when restoring files archived in this manner is that the +directories themselves are not in the archive; so the +@value{op-same-permissions} option does not affect them---while users +might really like it to. Specifying @value{op-no-recursion} is a way to +tell @code{tar} to grab only the directory entries given to it, adding +no new files on its own. + +@FIXME{example here} + +@node one, , recurse, Choosing +@section Crossing Filesystem Boundaries +@cindex File system boundaries, not crossing +@UNREVISED + +@code{tar} will normally automatically cross file system boundaries in +order to archive files which are part of a directory tree. You can +change this behavior by running @code{tar} and specifying +@value{op-one-file-system}. This option only affects files that are +archived because they are in a directory that is being archived; +@code{tar} will still archive files explicitly named on the command line +or through @value{op-files-from}, regardless of where they reside. + +@table @kbd +@item --one-file-system +@itemx -l +Prevents @code{tar} from crossing file system boundaries when +archiving. Use in conjunction with any write operation. +@end table + +The @samp{--one-file-system} option causes @code{tar} to modify its +normal behavior in archiving the contents of directories. If a file in +a directory is not on the same filesystem as the directory itself, then +@code{tar} will not archive that file. If the file is a directory +itself, @code{tar} will not archive anything beneath it; in other words, +@code{tar} will not cross mount points. + +It is reported that using this option, the mount point is is archived, +but nothing under it. + +This option is useful for making full or incremental archival backups of +a file system. If this option is used in conjunction with +@value{op-verbose}, files that are excluded are mentioned by name on the +standard error. + +@menu +* directory:: Changing Directory +* absolute:: Absolute File Names +@end menu + +@node directory, absolute, one, one +@subsection Changing the Working Directory + +@FIXME{need to read over this node now for continuity; i've switched +things around some.} + +@cindex Changing directory mid-stream +@cindex Directory, changing mid-stream +@cindex Working directory, specifying +@UNREVISED + +To change the working directory in the middle of a list of file names, +either on the command line or in a file specified using +@value{op-files-from}, use @value{op-directory}. This will change the +working directory to the directory @var{directory} after that point in +the list. + +@table @kbd +@item --directory=@var{directory} +@itemx -C @var{directory} +Changes the working directory in the middle of a command line. +@end table + +For example, + +@example +$ @kbd{tar -c -f jams.tar grape prune -C food cherry} +@end example + +@noindent +will place the files @file{grape} and @file{prune} from the current +directory into the archive @file{jams.tar}, followed by the file +@file{cherry} from the directory @file{food}. This option is especially +useful when you have several widely separated files that you want to +store in the same archive. + +Note that the file @file{cherry} is recorded in the archive under the +precise name @file{cherry}, @emph{not} @file{food/cherry}. Thus, the +archive will contain three files that all appear to have come from the +same directory; if the archive is extracted with plain @samp{tar +--extract}, all three files will be written in the current directory. + +Contrast this with the command, + +@example +$ @kbd{tar -c -f jams.tar grape prune -C food red/cherry} +@end example + +@noindent +which records the third file in the archive under the name +@file{red/cherry} so that, if the archive is extracted using +@samp{tar --extract}, the third file will be written in a subdirectory +named @file{orange-colored}. + +You can use the @samp{--directory} option to make the archive +independent of the original name of the directory holding the files. +The following command places the files @file{/etc/passwd}, +@file{/etc/hosts}, and @file{/lib/libc.a} into the archive +@file{foo.tar}: + +@example +$ @kbd{tar -c -f foo.tar -C /etc passwd hosts -C /lib libc.a} +@end example + +@noindent +However, the names of the archive members will be exactly what they were +on the command line: @file{passwd}, @file{hosts}, and @file{libc.a}. +They will not appear to be related by file name to the original +directories where those files were located. + +Note that @samp{--directory} options are interpreted consecutively. If +@samp{--directory} specifies a relative file name, it is interpreted +relative to the then current directory, which might not be the same as +the original current working directory of @code{tar}, due to a previous +@samp{--directory} option. + +@FIXME{dan: does this mean that you *can* use the short option form, but +you can *not* use the long option form with --files-from? or is this +totally screwed?} + +When using @samp{--files-from} (@pxref{files}), you can put @samp{-C} +options in the file list. Unfortunately, you cannot put +@samp{--directory} options in the file list. (This interpretation can +be disabled by using the @value{op-null} option.) + +@node absolute, , directory, one +@subsection Absolute File Names +@UNREVISED + +@table @kbd +@item -P +@itemx --absolute-names +Do not strip leading slashes from file names. +@end table + +By default, GNU @code{tar} drops a leading @samp{/} on input or output. +This option turns off this behavior; it is equivalent to changing to the +root directory before running @code{tar} (except it also turns off the +usual warning message). + +When @code{tar} extracts archive members from an archive, it strips any +leading slashes (@samp{/}) from the member name. This causes absolute +member names in the archive to be treated as relative file names. This +allows you to have such members extracted wherever you want, instead of +being restricted to extracting the member in the exact directory named +in the archive. For example, if the archive member has the name +@file{/etc/passwd}, @code{tar} will extract it as if the name were +really @file{etc/passwd}. + +Other @code{tar} programs do not do this. As a result, if you create an +archive whose member names start with a slash, they will be difficult +for other people with a non-GNU @code{tar} program to use. Therefore, +GNU @code{tar} also strips leading slashes from member names when +putting members into the archive. For example, if you ask @code{tar} to +add the file @file{/bin/ls} to an archive, it will do so, but the member +name will be @file{bin/ls}. + +If you use the @value{op-absolute-names} option, @code{tar} will do +neither of these transformations. + +To archive or extract files relative to the root directory, specify +the @value{op-absolute-names} option. + +Normally, @code{tar} acts on files relative to the working +directory---ignoring superior directory names when archiving, and +ignoring leading slashes when extracting. + +When you specify @value{op-absolute-names}, @code{tar} stores file names +including all superior directory names, and preserves leading slashes. +If you only invoked @code{tar} from the root directory you would never +need the @value{op-absolute-names} option, but using this option may be +more convenient than switching to root. + +@FIXME{Should be an example in the tutorial/wizardry section using this +to transfer files between systems.} + +@FIXME{Is write access an issue?} + +@table @kbd +@item --absolute-names +Preserves full file names (inclusing superior dirctory names) when +archiving files. Preserves leading slash when extracting files. + +@end table + +@FIXME{this is still horrible; need to talk with dan on monday.} + +@code{tar} prints out a message about removing the @samp{/} from file +names. This message appears once per GNU @code{tar} invocation. It +represents something which ought to be told; ignoring what it means can +cause very serious surprises, later. + +Some people, nevertheless, do not want to see this message. Wanting to +play really dangerously, one may of course redirect @code{tar} standard +error to the sink. For example, under @code{sh}: + +@example +$ @kbd{tar -c -f archive.tar /home 2> /dev/null} +@end example + +@noindent +Another solution, both nicer and simpler, would be to change to +the @file{/} directory first, and then avoid absolute notation. +For example: + +@example +$ @kbd{(cd / && tar -c -f archive.tar home)} +$ @kbd{tar -c -f archive.tar -C / home} +@end example + +@node Date input formats, Formats, Choosing, Top +@chapter Date input formats + +@cindex date input formats +@findex getdate + +@quotation +Our units of temporal measurement, from seconds on up to months, are so +complicated, asymmetrical and disjunctive so as to make coherent mental +reckoning in time all but impossible. Indeed, had some tyrannical god +contrived to enslave our minds to time, to make it all but impossible +for us to escape subjection to sodden routines and unpleasant surprises, +he could hardly have done better than handing down our present system. +It is like a set of trapezoidal building blocks, with no vertical or +horizontal surfaces, like a language in which the simplest thought +demands ornate constructions, useless particles and lengthy +circumlocutions. Unlike the more successful patterns of language and +science, which enable us to face experience boldly or at least +level-headedly, our system of temporal calculation silently and +persistently encourages our terror of time. + +@dots{} It is as though architects had to measure length in feet, width +in meters and height in ells; as though basic instruction manuals +demanded a knowledge of five different languages. It is no wonder then +that we often look into our own immediate past or future, last Tuesday +or a week from Sunday, with feelings of helpless confusion. @dots{} + +--- Robert Grudin, @cite{Time and the Art of Living}. +@end quotation + +This section describes the textual date representations that GNU +programs accept. These are the strings you, as a user, can supply as +arguments to the various programs. The C interface (via the +@code{getdate} function) is not described here. + +@cindex beginning of time, for Unix +@cindex epoch, for Unix +Although the date syntax here can represent any possible time since zero +A.D., computer integers are not big enough for such a (comparatively) +long time. The earliest date semantically allowed on Unix systems is +midnight, 1 January 1970 UCT. + +@menu +* General date syntax:: Common rules. +* Calendar date item:: 19 Dec 1994. +* Time of day item:: 9:20pm. +* Timezone item:: EST, DST, BST, UCT, AHST, ... +* Day of week item:: Monday and others. +* Relative item in date strings:: next tuesday, 2 years ago. +* Pure numbers in date strings:: 19931219, 1440. +* Authors of getdate:: Bellovin, Salz, Berets, et al. +@end menu + + +@node General date syntax, Calendar date item, Date input formats, Date input formats +@section General date syntax + +@cindex general date syntax + +@cindex items in date strings +A @dfn{date} is a string, possibly empty, containing many items +separated by whitespace. The whitespace may be omitted when no +ambiguity arises. The empty string means the beginning of today (i.e., +midnight). Order of the items is immaterial. A date string may contain +many flavors of items: + +@itemize @bullet +@item calendar date items +@item time of the day items +@item time zone items +@item day of the week items +@item relative items +@item pure numbers. +@end itemize + +@noindent We describe each of these item types in turn, below. + +@cindex numbers, written-out +@cindex ordinal numbers +@findex first @r{in date strings} +@findex next @r{in date strings} +@findex last @r{in date strings} +A few numbers may be written out in words in most contexts. This is +most useful for specifying day of the week items or relative items (see +below). Here is the list: @samp{first} for 1, @samp{next} for 2, +@samp{third} for 3, @samp{fourth} for 4, @samp{fifth} for 5, +@samp{sixth} for 6, @samp{seventh} for 7, @samp{eighth} for 8, +@samp{ninth} for 9, @samp{tenth} for 10, @samp{eleventh} for 11 and +@samp{twelfth} for 12. Also, @samp{last} means exactly @math{-1}. + +@cindex months, written-out +When a month is written this way, it is still considered to be written +numerically, instead of being ``spelled in full''; this changes the +allowed strings. + +@cindex case, ignored in dates +@cindex comments, in dates +Alphabetic case is completely ignored in dates. Comments may be introduced +between round parentheses, as long as included parentheses are properly +nested. Hyphens not followed by a digit are currently ignored. Leading +zeros on numbers are ignored. + + +@node Calendar date item, Time of day item, General date syntax, Date input formats +@section Calendar date item + +@cindex calendar date item + +A @dfn{calendar date item} specifies a day of the year. It is +specified differently, depending on whether the month is specified +numerically or literally. All these strings specify the same calendar date: + +@example +1970-09-17 # ISO 8601. +70-9-17 # This century assumed by default. +70-09-17 # Leading zeros are ignored. +9/17/72 # Common U.S. writing. +24 September 1972 +24 Sept 72 # September has a special abbreviation. +24 Sep 72 # Three-letter abbreviations always allowed. +Sep 24, 1972 +24-sep-72 +24sep72 +@end example + +The year can also be omitted. In this case, the last specified year is +used, or the current year if none. For example: + +@example +9/17 +sep 17 +@end example + +Here are the rules. + +@cindex ISO 8601 date format +@cindex date format, ISO 8601 +For numeric months, the ISO 8601 format +@samp{@var{year}-@var{month}-@var{day}} is allowed, where @var{year} is +any positive number, @var{month} is a number between 01 and 12, and +@var{day} is a number between 01 and 31. A leading zero must be present +if a number is less than ten. If @var{year} is less than 100, then 1900 +is added to it to force a date in this century. The construct +@samp{@var{month}/@var{day}/@var{year}}, popular in the United States, +is accepted. Also @samp{@var{month}/@var{day}}, omitting the year. + +@cindex month names in date strings +@cindex abbreviations for months +Literal months may be spelled out in full: @samp{January}, +@samp{February}, @samp{March}, @samp{April}, @samp{May}, @samp{June}, +@samp{July}, @samp{August}, @samp{September}, @samp{October}, +@samp{November} or @samp{December}. Literal months may be abbreviated +to their first three letters, possibly followed by an abbreviating dot. +It is also permitted to write @samp{Sept} instead of @samp{September}. + +When months are written literally, the calendar date may be given as any +of the following: + +@example +@var{day} @var{month} @var{year} +@var{day} @var{month} +@var{month} @var{day} @var{year} +@var{day}-@var{month}-@var{year} +@end example + +Or, omitting the year: + +@example +@var{month} @var{day} +@end example + + +@node Time of day item, Timezone item, Calendar date item, Date input formats +@section Time of day item + +@cindex time of day item + +A @dfn{time of day item} in date strings specifies the time on a given +day. Here are some examples, all of which represent the same time: + +@example +20:02:0 +20:02 +8:02pm +20:02-0500 # In EST (Eastern U.S. Standard Time). +@end example + +More generally, the time of the day may be given as +@samp{@var{hour}:@var{minute}:@var{second}}, where @var{hour} is +a number between 0 and 23, @var{minute} is a number between 0 and +59, and @var{second} is a number between 0 and 59. Alternatively, +@samp{:@var{second}} can be omitted, in which case it is taken to +be zero. + +@findex am @r{in date strings} +@findex pm @r{in date strings} +@findex midnight @r{in date strings} +@findex noon @r{in date strings} +If the time is followed by @samp{am} or @samp{pm} (or @samp{a.m.} +or @samp{p.m.}), @var{hour} is restricted to run from 1 to 12, and +@samp{:@var{minute}} may be omitted (taken to be zero). @samp{am} +indicates the first half of the day, @samp{pm} indicates the second +half of the day. In this notation, 12 is the predecessor of 1: +midnight is @samp{12am} while noon is @samp{12pm}. + +@cindex timezone correction +@cindex minutes, timezone correction by +The time may alternatively be followed by a timezone correction, +expressed as @samp{@var{s}@var{hh}@var{mm}}, where @var{s} is @samp{+} +or @samp{-}, @var{hh} is a number of zone hours and @var{mm} is a number +of zone minutes. When a timezone correction is given this way, it +forces interpretation of the time in UTC, overriding any previous +specification for the timezone or the local timezone. The @var{minute} +part of the time of the day may not be elided when a timezone correction +is used. This is the only way to specify a timezone correction by +fractional parts of an hour. + +Either @samp{am}/@samp{pm} or a timezone correction may be specified, +but not both. + + +@node Timezone item, Day of week item, Time of day item, Date input formats +@section Timezone item + +@cindex timezone item + +A @dfn{timezone item} specifies an international timezone, indicated by +a small set of letters. Any included period is ignored. Military +timezone designations use a single letter. Currently, only integral +zone hours may be represented in a timezone item. See the previous +section for a finer control over the timezone correction. + +Here are many non-daylight-savings-time timezones, indexed by the zone +hour value. + +@table @asis +@item +000 +@cindex Greenwich Mean Time +@cindex Universal Coordinated Time +@cindex Western European Time +@samp{GMT} for Greenwich Mean, @samp{UT} or @samp{UTC} for Universal +(Coordinated), @samp{WET} for Western European and @samp{Z} for +militaries. +@item +100 +@cindex West African Time +@samp{WAT} for West Africa and +@samp{A} for militaries. +@item +200 +@cindex Azores Time +@samp{AT} for Azores and @samp{B} for militaries. +@item +300 +@samp{C} for militaries. +@item +400 +@cindex Atlantic Standard Time +@samp{AST} for Atlantic Standard and @samp{D} for militaries. +@item +500 +@cindex Eastern Standard Time +@samp{E} for militaries and @samp{EST} for Eastern Standard. +@item +600 +@cindex Central Standard Time +@samp{CST} for Central Standard and @samp{F} for militaries. +@item +700 +@cindex Mountain Standard Time +@samp{G} for militaries and @samp{MST} for Mountain Standard. +@item +800 +@cindex Pacific Standard Time +@samp{H} for militaries and @samp{PST} for Pacific Standard. +@item +900 +@cindex Yukon Standard Time +@samp{I} for militaries and @samp{YST} for Yukon Standard. +@item +1000 +@cindex Alaska-Hawaii Time +@cindex Central Alaska Time +@cindex Hawaii Standard Time +@samp{AHST} for Alaska-Hawaii Standard, @samp{CAT} for Central Alaska, +@samp{HST} for Hawaii Standard and @samp{K} for militaries. +@item +1100 +@cindex Nome Standard Time +@samp{L} for militaries and @samp{NT} for Nome. +@item +1200 +@cindex International Date Line West +@samp{IDLW} for International Date Line West and @samp{M} for +militaries. +@item -100 +@cindex Central European Time +@cindex Middle European Time +@cindex Middle European Winter Time +@cindex French Winter Time +@cindex Swedish Winter Time +@samp{CET} for Central European, @samp{FWT} for French Winter, +@samp{MET} for Middle European, @samp{MEWT} for Middle European +Winter, @samp{N} for militaries and @samp{SWT} for Swedish Winter. +@item -200 +@cindex Eastern European Time +@cindex USSR Zone +@samp{EET} for Eastern European, USSR Zone 1 and @samp{O} for militaries. +@item -300 +@cindex Baghdad Time +@samp{BT} for Baghdad, USSR Zone 2 and @samp{P} for militaries. +@item -400 +@samp{Q} for militaries and @samp{ZP4} for USSR Zone 3. +@item -500 +@samp{R} for militaries and @samp{ZP5} for USSR Zone 4. +@item -600 +@samp{S} for militaries and @samp{ZP6} for USSR Zone 5. +@item -700 +@cindex West Australian Standard Time +@samp{T} for militaries and @samp{WAST} for West Australian Standard. +@item -800 +@cindex China Coast Time +@samp{CCT} for China Coast, USSR Zone 7 and @samp{U} for militaries. +@item -900 +@cindex Japan Standard Time +@samp{JST} for Japan Standard, USSR Zone 8 and @samp{V} for militaries. +@item -1000 +@cindex East Australian Standard Time +@cindex Guam Standard Time +@samp{EAST} for East Australian Standard, @samp{GST} for Guam +Standard, USSR Zone 9 and @samp{W} for militaries. +@item -1100 +@samp{X} for militaries. +@item -1200 +@cindex International Date Line East +@cindex New Zealand Standard Time +@samp{IDLE} for International Date Line East, @samp{NZST} for +New Zealand Standard, @samp{NZT} for New Zealand and @samp{Y} for +militaries. +@end table + +@cindex daylight savings time +Here are many DST timezones, indexed by the zone hour value. Also, by +following a non-DST timezone by the string @samp{DST} in a separate word +(that is, separated by some whitespace), the corresponding DST timezone +may be specified. + +@table @asis +@item 0 +@samp{BST} for British Summer. +@item +400 +@samp{ADT} for Atlantic Daylight. +@item +500 +@samp{EDT} for Eastern Daylight. +@item +600 +@samp{CDT} for Central Daylight. +@item +700 +@samp{MDT} for Mountain Daylight. +@item +800 +@samp{PDT} for Pacific Daylight. +@item +900 +@samp{YDT} for Yukon Daylight. +@item +1000 +@samp{HDT} for Hawaii Daylight. +@item -100 +@samp{MEST} for Middle European Summer, @samp{MESZ} for Middle European +Summer, @samp{SST} for Swedish Summer and @samp{FST} for French Summer. +@item -700 +@samp{WADT} for West Australian Daylight. +@item -1000 +@samp{EADT} for Eastern Australian Daylight. +@item -1200 +@samp{NZDT} for New Zealand Daylight. +@end table + + +@node Day of week item, Relative item in date strings, Timezone item, Date input formats +@section Day of week item + +@cindex day of week item + +The explicit mention of a day of the week will forward the date +(only if necessary) to reach that day of the week in the future. + +Days of the week may be spelled out in full: @samp{Sunday}, +@samp{Monday}, @samp{Tuesday}, @samp{Wednesday}, @samp{Thursday}, +@samp{Friday} or @samp{Saturday}. Days may be abbreviated to their +first three letters, optionally followed by a period. The special +abbreviations @samp{Tues} for @samp{Tuesday}, @samp{Wednes} for +@samp{Wednesday} and @samp{Thur} or @samp{Thurs} for @samp{Thursday} are +also allowed. + +@findex next @var{day} +@findex last @var{day} +A number may precede a day of the week item to move forward +supplementary weeks. It is best used in expression like @samp{third +monday}. In this context, @samp{last @var{day}} or @samp{next +@var{day}} is also acceptable; they move one week before or after +the day that @var{day} by itself would represent. + +A comma following a day of the week item is ignored. + + +@node Relative item in date strings, Pure numbers in date strings, Day of week item, Date input formats +@section Relative item in date strings + +@cindex relative items in date strings +@cindex displacement of dates + +@dfn{Relative items} adjust a date (or the current date if none) forward +or backward. The effects of relative items accumulate. Here are some +examples: + +@example +1 year +1 year ago +3 years +2 days +@end example + +@findex year @r{in date strings} +@findex month @r{in date strings} +@findex fortnight @r{in date strings} +@findex week @r{in date strings} +@findex day @r{in date strings} +@findex hour @r{in date strings} +@findex minute @r{in date strings} +The unit of time displacement may be selected by the string @samp{year} +or @samp{month} for moving by whole years or months. These are fuzzy +units, as years and months are not all of equal duration. More precise +units are @samp{fortnight} which is worth 14 days, @samp{week} worth 7 +days, @samp{day} worth 24 hours, @samp{hour} worth 60 minutes, +@samp{minute} or @samp{min} worth 60 seconds, and @samp{second} or +@samp{sec} worth one second. An @samp{s} suffix on these units is +accepted and ignored. + +@findex ago @r{in date strings} +The unit of time may be preceded by a multiplier, given as an optionally +signed number. Unsigned numbers are taken as positively signed. No +number at all implies 1 for a multiplier. Following a relative item by +the string @samp{ago} is equivalent to preceding the unit by a +multiplicator with value @math{-1}. + +@findex day @r{in date strings} +@findex tomorrow @r{in date strings} +@findex yesterday @r{in date strings} +The string @samp{tomorrow} is worth one day in the future (equivalent +to @samp{day}), the string @samp{yesterday} is worth +one day in the past (equivalent to @samp{day ago}). + +@findex now @r{in date strings} +@findex today @r{in date strings} +@findex this @r{in date strings} +The strings @samp{now} or @samp{today} are relative items corresponding +to zero-valued time displacement, these strings come from the fact +a zero-valued time displacement represents the current time when not +otherwise change by previous items. They may be used to stress other +items, like in @samp{12:00 today}. The string @samp{this} also has +the meaning of a zero-valued time displacement, but is preferred in +date strings like @samp{this thursday}. + +When a relative item makes the resulting date to cross the boundary +between DST and non-DST (or vice-versa), the hour is adjusted according +to the local time. + + +@node Pure numbers in date strings, Authors of getdate, Relative item in date strings, Date input formats +@section Pure numbers in date strings + +@cindex pure numbers in date strings + +The precise intepretation of a pure decimal number is dependent of +the context in the date string. + +If the decimal number is of the form @var{yyyy}@var{mm}@var{dd} and no +other calendar date item (@pxref{Calendar date item}) appears before it +in the date string, then @var{yyyy} is read as the year, @var{mm} as the +month number and @var{dd} as the day of the month, for the specified +calendar date. + +If the decimal number is of the form @var{hh}@var{mm} and no other time +of day item appears before it in the date string, then @var{hh} is read +as the hour of the day and @var{mm} as the minute of the hour, for the +specified time of the day. @var{mm} can also be omitted. + +If both a calendar date and a time of day appear to the left of a number +in the date string, but no relative item, then the number overrides the +year. + + +@node Authors of getdate, , Pure numbers in date strings, Date input formats +@section Authors of @code{getdate} + +@cindex authors of @code{getdate} + +@cindex Bellovin, Steven M. +@cindex Salz, Rich +@cindex Berets, Jim +@cindex MacKenzie, David +@cindex Meyering, Jim +@code{getdate} was originally implemented by Steven M. Bellovin +(@samp{smb@@research.att.com}) while at the University of North Carolina +at Chapel Hill. The code was later tweaked by a couple of people on +Usenet, then completely overhauled by Rich $alz (@samp{rsalz@@bbn.com}) +and Jim Berets (@samp{jberets@@bbn.com}) in August, 1990. Various +revisions for the GNU system were made by David MacKenzie, Jim Meyering, +and others. + +@cindex Pinard, F. +@cindex Berry, K. +This chapter was originally produced by Fran@,{c}ois Pinard +(@samp{pinard@@iro.umontreal.ca}) from the @file{getdate.y} source code, +and then edited by K.@: Berry (@samp{kb@@cs.umb.edu}). + +@node Formats, Media, Date input formats, Top +@chapter Controlling the Archive Format + +@FIXME{need an intro here} + +@menu +* Portability:: Making @code{tar} Archives More Portable +* Compression:: Using Less Space through Compression +* Attributes:: Handling File Attributes +* Standard:: The Standard Format +* Extensions:: GNU Extensions to the Archive Format +* cpio:: Comparison of @code{tar} and @code{cpio} +@end menu + +@node Portability, Compression, Formats, Formats +@section Making @code{tar} Archives More Portable + +Creating a @code{tar} archive on a particular system that is meant to be +useful later on many other machines and with other versions of @code{tar} +is more challenging than you might think. @code{tar} archive formats +have been evolving since the first versions of Unix. Many such formats +are around, and are not always comptible with each other. This section +discusses a few problems, and gives some advice about making @code{tar} +archives more portable. + +One golden rule is simplicity. For example, limit your @code{tar} +archives to contain only regular files and directories, avoiding +other kind of special files. Do not attempt to save sparse files or +contiguous files as such. Let's discuss a few more problems, in turn. + +@menu +* Portable Names:: Portable Names +* dereference:: Symbolic Links +* old:: Old V7 Archives +* posix:: POSIX archives +* Checksumming:: Checksumming Problems +@end menu + +@node Portable Names, dereference, Portability, Portability +@subsection Portable Names + +Use @emph{straight} file and directory names, made up of printable +ASCII characters, avoiding colons, slashes, backslashes, spaces, and +other @emph{dangerous} characters. Avoid deep directory nesting. +Accounting for oldish System V machines, limit your file and directory +names to 14 characters or less. + +If you intend to have your @code{tar} archives to be read under MSDOS, +you should not rely on case distinction for file names, and you might +use the GNU @code{doschk} program for helping you further diagnosing +illegal MSDOS names, which are even more limited than System V's. + +@node dereference, old, Portable Names, Portability +@subsection Symbolic Links +@cindex File names, using symbolic links +@cindex Symbolic link as file name + +Normally, when @code{tar} archives a symbolic link, it writes a +block to the archive naming the target of the link. In that way, the +@code{tar} archive is a faithful record of the filesystem contents. +@value{op-dereference} is used with @value{op-create}, and causes @code{tar} +to archive the files symbolic links point to, instead of the links +themselves. When this option is used, when @code{tar} encounters a +symbolic link, it will archive the linked-to file, instead of simply +recording the presence of a symbolic link. + +The name under which the file is stored in the file system is not +recorded in the archive. To record both the symbolic link name and +the file name in the system, archive the file under both names. If +all links were recorded automatically by @code{tar}, an extracted file +might be linked to a file name that no longer exists in the file system. -@item --help -Prints a list of @code{tar} operations and options. +If a linked-to file is encountered again by @code{tar} while creating +the same archive, an entire second copy of it will be stored. (This +@emph{might} be considered a bug.) + +So, for portable archives, do not archive symbolic links as such, +and use @value{op-dereference}: many systems do not support +symbolic links, and moreover, your distribution might be unusable if +it contains unresolved symbolic links. + +@node old, posix, dereference, Portability +@subsection Old V7 Archives +@cindex Format, old style +@cindex Old style format +@cindex Old style archives + +Certain old versions of @code{tar} cannot handle additional +information recorded by newer @code{tar} programs. To create an +archive in V7 format (not ANSI), which can be read by these old +versions, specify the @value{op-old-archive} option in +conjunction with the @value{op-create}. @code{tar} also +accepts @samp{--portability} for this option. When you specify it, +@code{tar} leaves out information about directories, pipes, fifos, +contiguous files, and device files, and specifies file ownership by +group and user IDs instead of group and user names. + +When updating an archive, do not use @value{op-old-archive} +unless the archive was created with using this option. + +In most cases, a @emph{new} format archive can be read by an @emph{old} +@code{tar} program without serious trouble, so this option should +seldom be needed. On the other hand, most modern @code{tar}s are +able to read old format archives, so it might be safer for you to +always use @value{op-old-archive} for your distributions. + +@node posix, Checksumming, old, Portability +@subsection GNU @code{tar} and POSIX @code{tar} + +GNU @code{tar} was based on an early draft of the POSIX 1003.1 +@code{ustar} standard. GNU extensions to @code{tar}, such as the +support for file names longer than 100 characters, use portions of the +@code{tar} header record which were specified in that POSIX draft as +unused. Subsequent changes in POSIX have allocated the same parts of +the header record for other purposes. As a result, GNU @code{tar} is +incompatible with the current POSIX spec, and with @code{tar} programs +that follow it. + +We plan to reimplement these GNU extensions in a new way which is +upward compatible with the latest POSIX @code{tar} format, but we +don't know when this will be done. + +In the mean time, there is simply no telling what might happen if you +read a GNU @code{tar} archive, which uses the GNU extensions, using +some other @code{tar} program. So if you want to read the archive +with another @code{tar} program, be sure to write it using the +@samp{--old-archive} option (@samp{-o}). + +@FIXME{is there a way to tell which flavor of tar was used to write a +particular archive before you try to read it?} + +Traditionally, old @code{tar}s have a limit of 100 characters. GNU +@code{tar} attempted two different approaches to overcome this limit, +using and extending a format specified by a draft of some P1003.1. +The first way was not that successful, and involved @file{@@MaNgLeD@@} +file names, or such; while a second approach used @file{././@@LongLink} +and other tricks, yielding better success. In theory, GNU @code{tar} +should be able to handle file names of practically unlimited length. +So, if GNU @code{tar} fails to dump and retrieve files having more +than 100 characters, then there is a bug in GNU @code{tar}, indeed. + +But, being strictly POSIX, the limit was still 100 characters. +For various other purposes, GNU @code{tar} used areas left unassigned +in the POSIX draft. POSIX later revised P1003.1 @code{ustar} format by +assigning previously unused header fields, in such a way that the upper +limit for file name length was raised to 256 characters. However, the +actual POSIX limit oscillates between 100 and 256, depending on the +precise location of slashes in full file name (this is rather ugly). +Since GNU @code{tar} use the same fields for quite other purposes, +it became incompatible with the latest POSIX standards. + +For longer or non-fitting file names, we plan to use yet another set +of GNU extensions, but this time, complying with the provisions POSIX +offers for extending the format, rather than conflicting with it. +Whenever an archive uses old GNU @code{tar} extension format or POSIX +extensions, would it be for very long file names or other specialities, +this archive becomes non-portable to other @code{tar} implementations. +In fact, anything can happen. The most forgiving @code{tar}s will +merely unpack the file using a wrong name, and maybe create another +file named something like @file{@@LongName}, with the true file name +in it. @code{tar}s not protecting themselves may segment violate! + +Compatibility concerns make all this thing more difficult, as we +will have to support @emph{all} these things together, for a while. +GNU @code{tar} should be able to produce and read true POSIX format +files, while being able to detect old GNU @code{tar} formats, besides +old V7 format, and process them conveniently. It would take years +before this whole area stabilizes@dots{} + +There are plans to raise this 100 limit to 256, and yet produce POSIX +conformant archives. Past 256, I do not know yet if GNU @code{tar} +will go non-POSIX again, or merely refuse to archive the file. + +There are plans so GNU @code{tar} support more fully the latest POSIX +format, while being able to read old V7 format, GNU (semi-POSIX plus +extension), as well as full POSIX. One may ask if there is part of +the POSIX format that we still cannot support. This simple question +has a complex answer. Maybe that, on intimate look, some strong +limitations will pop up, but until now, nothing sounds too difficult +(but see below). I only have these few pages of POSIX telling about +`Extended tar Format' (P1003.1-1990 -- section 10.1.1), and there are +references to other parts of the standard I do not have, which should +normally enforce limitations on stored file names (I suspect things +like fixing what @kbd{/} and @kbd{@key{NUL}} means). There are also +some points which the standard does not make clear, Existing practice +will then drive what I should do. + +POSIX mandates that, when a file name cannot fit within 100 to +256 characters (the variance comes from the fact a @kbd{/} is +ideally needed as the 156'th character), or a link name cannot +fit within 100 characters, a warning should be issued and the file +@emph{not} be stored. Unless some @value{op-posix} option is given +(or @code{POSIXLY_CORRECT} is set), I suspect that GNU @code{tar} +should disobey this specification, and automatically switch to using +GNU extensions to overcome file name or link name length limitations. + +There is a problem, however, which I did not intimately studied yet. +Given a truly POSIX archive with names having more than 100 characters, +I guess that GNU @code{tar} up to 1.11.8 will process it as if it were an +old V7 archive, and be fooled by some fields which are coded differently. +So, the question is to decide if the next generation of GNU @code{tar} +should produce POSIX format by default, whenever possible, producing +archives older versions of GNU @code{tar} might not be able to read +correctly. I fear that we will have to suffer such a choice one of these +days, if we want GNU @code{tar} to go closer to POSIX. We can rush it. +Another possibility is to produce the current GNU @code{tar} format +by default for a few years, but have GNU @code{tar} versions from some +1.@var{POSIX} and up able to recognize all three formats, and let older +GNU @code{tar} fade out slowly. Then, we could switch to producing POSIX +format by default, with not much harm to those still having (very old at +that time) GNU @code{tar} versions prior to 1.@var{POSIX}. + +POSIX format cannot represent very long names, volume headers, +splitting of files in multi-volumes, sparse files, and incremental +dumps; these would be all disallowed if @value{op-posix} or +@code{POSIXLY_CORRECT}. Otherwise, if @code{tar} is given long +names, or @samp{-[VMSgG]}, then it should automatically go non-POSIX. +I think this is easily granted without much discussion. + +Another point is that only @code{mtime} is stored in POSIX +archives, while GNU @code{tar} currently also store @code{atime} +and @code{ctime}. If we want GNU @code{tar} to go closer to POSIX, +my choice would be to drop @code{atime} and @code{ctime} support on +average. On the other hand, I perceive that full dumps or incremental +dumps need @code{atime} and @code{ctime} support, so for those special +applications, POSIX has to be avoided altogether. + +A few users requested that @value{op-sparse} be always active by +default, I think that before replying to them, we have to decide +if we want GNU @code{tar} to go closer to POSIX on average, while +producing files. My choice would be to go closer to POSIX in the +long run. Besides possible double reading, I do not see any point +of not trying to save files as sparse when creating archives which +are neither POSIX nor old-V7, so the actual @value{op-sparse} would +become selected by default when producing such archives, whatever +the reason is. So, @value{op-sparse} alone might be redefined to force +GNU-format archives, and recover its previous meaning from this fact. + +GNU-format as it exists now can easily fool other POSIX @code{tar}, +as it uses fields which POSIX considers to be part of the file name +prefix. I wonder if it would not be a good idea, in the long run, +to try changing GNU-format so any added field (like @code{ctime}, +@code{atime}, file offset in subsequent volumes, or sparse file +descriptions) be wholly and always pushed into an extension block, +instead of using space in the POSIX header block. I could manage +to do that portably between future GNU @code{tar}s. So other POSIX +@code{tar}s might be at least able to provide kind of correct listings +for the archives produced by GNU @code{tar}, if not able to process +them otherwise. + +Using these projected extensions might induce older @code{tar}s to fail. +We would use the same approach as for POSIX. I'll put out a @code{tar} +capable of reading POSIXier, yet extended archives, but will not produce +this format by default, in GNU mode. In a few years, when newer GNU +@code{tar}s will have flooded out @code{tar} 1.11.X and previous, we +could switch to producing POSIXier extended archives, with no real harm +to users, as almost all existing GNU @code{tar}s will be ready to read +POSIXier format. In fact, I'll do both changes at the same time, in a +few years, and just prepare @code{tar} for both changes, without effecting +them, from 1.@var{POSIX}. (Both changes: 1---using POSIX convention for +getting over 100 characters; 2---avoiding mangling POSIX headers for GNU +extensions, using only POSIX mandated extension techniques). + +So, a future @code{tar} will have a @value{op-posix} +flag forcing the usage of truly POSIX headers, and so, producing +archives previous GNU @code{tar} will not be able to read. +So, @emph{once} pretest will announce that feature, it would be +particularly useful that users test how exchangeable will be archives +between GNU @code{tar} with @value{op-posix} and other POSIX @code{tar}. + +In a few years, when GNU @code{tar} will produce POSIX headers by +default, @value{op-posix} will have a strong meaning and will disallow +GNU extensions. But in the meantime, for a long while, @value{op-posix} +in GNU tar will not disallow GNU extensions like @value{op-label}, +@value{op-multi-volume}, @value{op-sparse}, or very long file or link names. +However, @value{op-posix} with GNU extensions will use POSIX +headers with reserved-for-users extensions to headers, and I will be +curious to know how well or bad POSIX @code{tar}s will react to these. + +GNU @code{tar} prior to 1.@var{POSIX}, and after 1.@var{POSIX} without +@value{op-posix}, generates and checks @samp{ustar@w{ }@w{ }}, with two +suffixed spaces. This is sufficient for older GNU @code{tar} not to +recognize POSIX archives, and consequently, wrongly decide those archives +are in old V7 format. It is a useful bug for me, because GNU @code{tar} +has other POSIX incompatibilities, and I need to segregate GNU @code{tar} +semi-POSIX archives from truly POSIX archives, for GNU @code{tar} should +be somewhat compatible with itself, while migrating closer to latest +POSIX standards. So, I'll be very careful about how and when I will do +the correction. + +@node Checksumming, , posix, Portability +@subsection Checksumming Problems + +SunOS and HP-UX @code{tar} fail to accept archives created using GNU +@code{tar} and containing non-ASCII file names, that is, file names +having characters with the eight bit set, because they use signed +checksums, while GNU @code{tar} uses unsigned checksums while creating +archives, as per POSIX standards. On reading, GNU @code{tar} computes +both checksums and accept any. It is somewhat worrying that a lot of +people may go around doing backup of their files using faulty (or at +least non-standard) software, not learning about it until it's time +to restore their missing files with an incompatible file extractor, +or vice versa. + +GNU @code{tar} compute checksums both ways, and accept any on read, +so GNU tar can read Sun tapes even with their wrong checksums. +GNU @code{tar} produces the standard checksum, however, raising +incompatibilities with Sun. That is to say, GNU @code{tar} has not +been modified to @emph{produce} incorrect archives to be read by buggy +@code{tar}'s. I've been told that more recent Sun @code{tar} now +read standard archives, so maybe Sun did a similar patch, after all? + +The story seems to be that when Sun first imported @code{tar} +sources on their system, they recompiled it without realizing that +the checksums were computed differently, because of a change in +the default signing of @code{char}'s in their compiler. So they +started computing checksums wrongly. When they later realized their +mistake, they merely decided to stay compatible with it, and with +themselves afterwards. Presumably, but I do not really know, HP-UX +has chosen that their @code{tar} archives to be compatible with Sun's. +The current standards do not favor Sun @code{tar} format. In any +case, it now falls on the shoulders of SunOS and HP-UX users to get +a @code{tar} able to read the good archives they receive. + +@node Compression, Attributes, Portability, Formats +@section Using Less Space through Compression + +@menu +* gzip:: Creating and Reading Compressed Archives +* sparse:: Archiving Sparse Files +@end menu + +@node gzip, sparse, Compression, Compression +@subsection Creating and Reading Compressed Archives +@cindex Compressed archives +@cindex Storing archives in compressed format +@UNREVISED + +@table @kbd +@item -z +@itemx --gzip +@itemx --ungzip +Filter the archive through @code{gzip}. +@end table + +@FIXME{ach; these two bits orig from "compare" (?). where to put?} Some +format parameters must be taken into consideration when modifying an +archive: @FIXME{???}. Compressed archives cannot be modified. + +You can use @samp{--gzip} and @samp{--gunzip} on physical devices +(tape drives, etc.) and remote files as well as on normal files; data +to or from such devices or remote files is reblocked by another copy +of the @code{tar} program to enforce the specified (or default) record +size. The default compression parameters are used; if you need to +override them, avoid the @value{op-gzip} option and run @code{gzip} +explicitly. (Or set the @samp{GZIP} environment variable.) + +The @value{op-gzip} option does not work with the @value{op-multi-volume} +option, or with the @value{op-update}, @value{op-append}, +@value{op-concatenate}, or @value{op-delete} operations. + +It is not exact to say that GNU @code{tar} is to work in concert +with @code{gzip} in a way similar to @code{zip}, say. Surely, it is +possible that @code{tar} and @code{gzip} be done with a single call, +like in: + +@example +$ @kbd{tar cfz archive.tar.gz subdir} +@end example + +@noindent +to save all of @samp{subdir} into a @code{gzip}'ed archive. Later you +can do: + +@example +$ @kbd{tar xfz archive.tar.gz} +@end example + +@noindent +to explode and unpack. + +The difference is that the whole archive is compressed. With +@code{zip}, archive members are archived individually. @code{tar}'s +method yields better compression. On the other hand, one can view the +contents of a @code{zip} archive without having to decompress it. As +for the @code{tar} and @code{gzip} tandem, you need to decompress the +archive to see its contents. However, this may be done without needing +disk space, by using pipes internally: + +@example +$ @kbd{tar tfz archive.tar.gz} +@end example + +@cindex corrupted archives +About corrupted compressed archives: @code{gzip}'ed files have no +redundancy, for maximum compression. The adaptive nature of the +compression scheme means that the compression tables are implicitly +spread all over the archive. If you lose a few blocks, the dynamic +construction of the compression tables becomes unsychronized, and there +is little chance that you could recover later in the archive. + +There are pending suggestions for having a per-volume or per-file +compression in GNU @code{tar}. This would allow for viewing the +contents without decompression, and for resynchronizing decompression at +every volume or file, in case of corrupted archives. Doing so, we might +loose some compressibility. But this would have make recovering easier. +So, there are pros and cons. We'll see! + +@table @kbd +@item -Z +@itemx --compress +@itemx --uncompress +Filter the archive through @code{compress}. Otherwise like @value{op-gzip}. + +@item --use-compress-program=@var{prog} +Filter through @var{prog} (must accept @samp{-d}). +@end table + +@value{op-compress} stores an archive in compressed format. This +option is useful in saving time over networks and space in pipes, and +when storage space is at a premium. @value{op-compress} causes +@code{tar} to compress when writing the archive, or to uncompress when +reading the archive. + +To perform compression and uncompression on the archive, @code{tar} +runs the @code{compress} utility. @code{tar} uses the default +compression parameters; if you need to override them, avoid the +@value{op-compress} option and run the @code{compress} utility +explicitly. It is useful to be able to call the @code{compress} +utility from within @code{tar} because the @code{compress} utility by +itself cannot access remote tape drives. + +The @value{op-compress} option will not work in conjunction with the +@value{op-multi-volume} option or the @value{op-append}, @value{op-update}, +@value{op-append} and @value{op-delete} operations. @xref{Operations}, for +more information on these operations. + +If there is no compress utility available, @code{tar} will report an error. +@strong{Please note} that the @code{compress} program may be covered by +a patent, and therefore we recommend you stop using it. + +@table @kbd +@item --compress +@itemx --uncompress +@itemx -z +@itemx -Z +When this option is specified, @code{tar} will compress (when writing +an archive), or uncompress (when reading an archive). Used in +conjunction with the @value{op-create}, @value{op-extract}, @value{op-list} and +@value{op-compare} operations. +@end table + +You can have archives be compressed by using the @value{op-gzip} option. +This will arrange for @code{tar} to use the @code{gzip} program to be +used to compress or uncompress the archive wren writing or reading it. + +To use the older, obsolete, @code{compress} program, use the +@value{op-compress} option. The GNU Project recommends you not use +@code{compress}, because there is a patent covering the algorithm it +uses. You could be sued for patent infringment merely by running +@code{compress}. + +I have one question, or maybe it's a suggestion if there isn't a way +to do it now. I would like to use @value{op-gzip}, but I'd also like the +output to be fed through a program like GNU @code{ecc} (actually, right +now that's @samp{exactly} what I'd like to use :-)), basically adding +ECC protection on top of compression. It seems as if this should be +quite easy to do, but I can't work out exactly how to go about it. +Of course, I can pipe the standard output of @code{tar} through +@code{ecc}, but then I lose (though I haven't started using it yet, +I confess) the ability to have @code{tar} use @code{rmt} for it's I/O +(I think). + +I think the most straightforward thing would be to let me specify a +general set of filters outboard of compression (preferably ordered, +so the order can be automatically reversed on input operations, and +with the options they require specifiable), but beggars shouldn't be +choosers and anything you decide on would be fine with me. + +By the way, I like @code{ecc} but if (as the comments say) it can't +deal with loss of block sync, I'm tempted to throw some time at adding +that capability. Supposing I were to actually do such a thing and +get it (apparantly) working, do you accept contributed changes to +utilities like that? (Leigh Clayton @file{loc@@soliton.com}, May 1995). + +Isn't that exactly the role of the @value{op-use-compress-prog} option? +I never tried it myself, but I suspect you may want to write a +@var{prog} script or program able to filter stdin to stdout to +way you want. It should recognize the @samp{-d} option, for when +extraction is needed rather than creation. + +It has been reported that if one writes compressed data (through the +@value{op-gzip} or @value{op-compress} options) to a DLT and tries to use +the DLT compression mode, the data will actually get bigger and one will +end up with less space on the tape. + +@node sparse, , gzip, Compression +@subsection Archiving Sparse Files +@cindex Sparse Files +@UNREVISED + +@table @kbd +@item -S +@itemx --sparse +Handle sparse files efficiently. +@end table + +This option causes all files to be put in the archive to be tested for +sparseness, and handled specially if they are. The @value{op-sparse} +option is useful when many @code{dbm} files, for example, are being +backed up. Using this option dramatically decreases the amount of +space needed to store such a file. + +In later versions, this option may be removed, and the testing and +treatment of sparse files may be done automatically with any special +GNU options. For now, it is an option needing to be specified on +the command line with the creation or updating of an archive. + +Files in the filesystem occasionally have ``holes.'' A hole in a file +is a section of the file's contents which was never written. The +contents of a hole read as all zeros. On many operating systems, +actual disk storage is not allocated for holes, but they are counted +in the length of the file. If you archive such a file, @code{tar} +could create an archive longer than the original. To have @code{tar} +attempt to recognize the holes in a file, use @value{op-sparse}. When +you use the @value{op-sparse} option, then, for any file using less +disk space than would be expected from its length, @code{tar} searches +the file for consecutive stretches of zeros. It then records in the +archive for the file where the consecutive stretches of zeros are, and +only archives the ``real contents'' of the file. On extraction (using +@value{op-sparse} is not needed on extraction) any such files have +hols created wherever the continuous stretches of zeros were found. +Thus, if you use @value{op-sparse}, @code{tar} archives won't take +more space than the original. + +A file is sparse if it contains blocks of zeros whose existence is +recorded, but that have no space allocated on disk. When you specify +the @value{op-sparse} option in conjunction with the @value{op-create} +operation, @code{tar} tests all files for sparseness while archiving. +If @code{tar} finds a file to be sparse, it uses a sparse representation of +the file in the archive. @value{xref-create}, for more information +about creating archives. + +@value{op-sparse} is useful when archiving files, such as dbm files, +likely to contain many nulls. This option dramatically +decreases the amount of space needed to store such an archive. + +@quotation +@strong{Please Note:} Always use @value{op-sparse} when performing file +system backups, to avoid archiving the expanded forms of files stored +sparsely in the system. + +Even if your system has no sparse files currently, some may be +created in the future. If you use @value{op-sparse} while making file +system backups as a matter of course, you can be assured the archive +will never take more space on the media than the files take on disk +(otherwise, archiving a disk filled with sparse files might take +hundreds of tapes). @FIXME-xref{incremental when node name is set.} +@end quotation + +@code{tar} ignores the @value{op-sparse} option when reading an archive. + +@table @kbd +@item --sparse +@itemx -S +Files stored sparsely in the file system are represented sparsely in +the archive. Use in conjunction with write operations. +@end table + +However, users should be well aware that at archive creation time, GNU +@code{tar} still has to read whole disk file to locate the @dfn{holes}, and +so, even if sparse files use little space on disk and in the archive, they +may sometimes require inordinate amount of time for reading and examining +all-zero blocks of a file. Although it works, it's painfully slow for a +large (sparse) file, even though the resulting tar archive may be small. +(One user reports that dumping a @file{core} file of over 400 megabytes, +but with only about 3 megabytes of actual data, took about 9 minutes on +a Sun Sparstation ELC, with full CPU utilisation.) + +This reading is required in all cases and is not related to the fact +the @value{op-sparse} option is used or not, so by merely @emph{not} +using the option, you are not saving time@footnote{Well! We should say +the whole truth, here. When @value{op-sparse} is selected while creating +an archive, the current @code{tar} algorithm requires sparse files to be +read twice, not once. We hope to develop a new archive format for saving +sparse files in which one pass will be sufficient.}. + +Programs like @code{dump} do not have to read the entire file; by examining +the file system directly, they can determine in advance exactly where the +holes are and thus avoid reading through them. The only data it need read +are the actual allocated data blocks. GNU @code{tar} uses a more portable +and straightforward archiving approach, it would be fairly difficult that +it does otherwise. Elizabeth Zwicky writes to @file{comp.unix.internals}, +on 1990-12-10: + +@quotation +What I did say is that you cannot tell the difference between a hole and an +equivalent number of nulls without reading raw blocks. @code{st_blocks} at +best tells you how many holes there are; it doesn't tell you @emph{where}. +Just as programs may, conceivably, care what @code{st_blocks} is (care +to name one that does?), they may also care where the holes are (I have +no examples of this one either, but it's equally imaginable). + +I conclude from this that good archivers are not portable. One can +arguably conclude that if you want a portable program, you can in good +conscience restore files with as many holes as possible, since you can't +get it right. +@end quotation + +@node Attributes, Standard, Compression, Formats +@section Handling File Attributes +@UNREVISED + +When @code{tar} reads files, this causes them to have the access times +updated. To have @code{tar} attempt to set the access times back to +what they were before they were read, use the @value{op-atime-preserve} +option. This doesn't work for files that you don't own, unless +you're root, and it doesn't interact with incremental dumps nicely +(@pxref{Backups}), but it is good enough for some purposes. + +Handling of file attributes + +@table @kbd +@item --atime-preserve +Do not change access times on dumped files. + +@item -m +@itemx --touch +Do not extract file modified time. + +When this option is used, @code{tar} leaves the modification times +of the files it extracts as the time when the files were extracted, +instead of setting it to the time recorded in the archive. + +This option is meaningless with @value{op-list}. + +@item --same-owner +Create extracted files with the same ownership they have in the +archive. + +When using super-user at extraction time, ownership is always restored. +So, this option is meaningful only for non-root users, when @code{tar} +is executed on those systems able to give files away. This is +considered as a security flaw by many people, at least because it +makes quite difficult to correctly account users for the disk space +they occupy. Also, the @code{suid} or @code{sgid} attributes of +files are easily and silently lost when files are given away. + +When writing an archive, @code{tar} writes the user id and user name +separately. If it can't find a user name (because the user id is not +in @file{/etc/passwd}), then it does not write one. When restoring, +and doing a @code{chmod} like when you use @value{op-same-permissions} +(@FIXME{same-owner?}), it tries to look the name (if one was written) +up in @file{/etc/passwd}. If it fails, then it uses the user id +stored in the archive instead. + +@item --numeric-owner +The @value{op-numeric-owner} option allows (ANSI) archives to be written +without user/group name information or such information to be ignored +when extracting. It effectively disables the generation and/or use +of user/group name information. This option forces extraction using +the numeric ids from the archive, ignoring the names. + +This is useful in certain circumstances, when restoring a backup from +an emergency floppy with different passwd/group files for example. +It is otherwise impossible to extract files with the right ownerships +if the password file in use during the extraction does not match the +one belonging to the filesystem(s) being extracted. This occurs, +for example, if you are restoring your files after a major crash and +had booted from an emergency floppy with no password file or put your +disk into another machine to do the restore. + +The numeric ids are @emph{always} saved into @code{tar} archives. +The identifying names are added at create time when provided by the +system, unless @value{op-old-archive} is used. Numeric ids could be +used when moving archives between a collection of machines using +a centralized management for attribution of numeric ids to users +and groups. This is often made through using the NIS capabilities. + +When making a @code{tar} file for distribution to other sites, it +is sometimes cleaner to use a single owner for all files in the +distribution, and nicer to specify the write permission bits of the +files as stored in the archive independently of their actual value on +the file system. The way to prepare a clean distribution is usually +to have some Makefile rule creating a directory, copying all needed +files in that directory, then setting ownership and permissions as +wanted (there are a lot of possible schemes), and only then making a +@code{tar} archive out of this directory, before cleaning everything +out. Of course, we could add a lot of options to GNU @code{tar} for +fine tuning permissions and ownership. This is not the good way, +I think. GNU @code{tar} is already crowded with options and moreover, +the approach just explained gives you a great deal of control already. + +@item -p +@itemx --same-permissions +@itemx --preserve-permissions +Extract all protection information. + +This option causes @code{tar} to set the modes (access permissions) of +extracted files exactly as recorded in the archive. If this option +is not used, the current @code{umask} setting limits the permissions +on extracted files. + +This option is meaningless with @value{op-list}. + +@item --preserve +Same as both @value{op-same-permissions} and @value{op-same-order}. + +The @value{op-preserve} option has no equivalent short option name. +It is equivalent to @value{op-same-permissions} plus @value{op-same-order}. + +@FIXME{I do not see the purpose of such an option. (Neither I. FP.)} + +@end table + +@node Standard, Extensions, Attributes, Formats +@section The Standard Format +@UNREVISED + +While an archive may contain many files, the archive itself is a +single ordinary file. Like any other file, an archive file can be +written to a storage device such as a tape or disk, sent through a +pipe or over a network, saved on the active file system, or even +stored in another archive. An archive file is not easy to read or +manipulate without using the @code{tar} utility or Tar mode in GNU +Emacs. + +Physically, an archive consists of a series of file entries terminated +by an end-of-archive entry, which consists of 512 zero bytes. A file +entry usually describes one of the files in the archive (an +@dfn{archive member}), and consists of a file header and the contents +of the file. File headers contain file names and statistics, checksum +information which @code{tar} uses to detect file corruption, and +information about file types. + +Archives are permitted to have more than one member with the same +member name. One way this situation can occur is if more than one +version of a file has been stored in the archive. For information +about adding new versions of a file to an archive, see @ref{update}, +and to learn more about having more than one archive member with the +same name, see @FIXME-xref{-backup node, when it's written}. + +In addition to entries describing archive members, an archive may +contain entries which @code{tar} itself uses to store information. +@value{xref-label}, for an example of such an archive entry. + +A @code{tar} archive file contains a series of blocks. Each block +contains @code{BLOCKSIZE} bytes. Although this format may be thought +of as being on magnetic tape, other media are often used. + +Each file archived is represented by a header block which describes +the file, followed by zero or more blocks which give the contents +of the file. At the end of the archive file there may be a block +filled with binary zeros as an end-of-file marker. A reasonable system +should write a block of zeros at the end, but must not assume that +such a block exists when reading an archive. + +The blocks may be @dfn{blocked} for physical I/O operations. +Each record of @var{n} blocks (where @var{n} is set by the +@value{op-blocking-factor} option to @code{tar}) is written with a single +@w{@samp{write ()}} operation. On magnetic tapes, the result of +such a write is a single record. When writing an archive, +the last record of blocks should be written at the full size, with +blocks after the zero block containing all zeros. When reading +an archive, a reasonable system should properly handle an archive +whose last record is shorter than the rest, or which contains garbage +records after a zero block. + +The header block is defined in C as follows. In the GNU @code{tar} +distribution, this is part of file @file{src/tar.h}: + +@example +@include header.texi +@end example + +All characters in header blocks are represented by using 8-bit +characters in the local variant of ASCII. Each field within the +structure is contiguous; that is, there is no padding used within +the structure. Each character on the archive medium is stored +contiguously. + +Bytes representing the contents of files (after the header block +of each file) are not translated in any way and are not constrained +to represent characters in any character set. The @code{tar} format +does not distinguish text files from binary files, and no translation +of file contents is performed. + +The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and +@code{gname} are null-terminated character strings. All other fileds +are zero-filled octal numbers in ASCII. Each numeric field of width +@var{w} contains @var{w} minus 2 digits, a space, and a null, except +@code{size}, and @code{mtime}, which do not contain the trailing null. + +The @code{name} field is the file name of the file, with directory names +(if any) preceding the file name, separated by slashes. + +@FIXME{how big a name before field overflows?} + +The @code{mode} field provides nine bits specifying file permissions +and three bits to specify the Set UID, Set GID, and Save Text +(@dfn{sticky}) modes. Values for these bits are defined above. +When special permissions are required to create a file with a given +mode, and the user restoring files from the archive does not hold such +permissions, the mode bit(s) specifying those special permissions +are ignored. Modes which are not supported by the operating system +restoring files from the archive will be ignored. Unsupported modes +should be faked up when creating or updating an archive; e.g. the +group permission could be copied from the @emph{other} permission. + +The @code{uid} and @code{gid} fields are the numeric user and group +ID of the file owners, respectively. If the operating system does +not support numeric user or group IDs, these fields should be ignored. + +The @code{size} field is the size of the file in bytes; linked files +are archived with this field specified as zero. @FIXME-xref{Modifiers}, in +particular the @value{op-incremental} option. + +The @code{mtime} field is the modification time of the file at the time +it was archived. It is the ASCII representation of the octal value of +the last time the file was modified, represented as an integer number of +seconds since January 1, 1970, 00:00 Coordinated Universal Time. + +The @code{chksum} field is the ASCII representation of the octal value +of the simple sum of all bytes in the header block. Each 8-bit +byte in the header is added to an unsigned integer, initialized to +zero, the precision of which shall be no less than seventeen bits. +When calculating the checksum, the @code{chksum} field is treated as +if it were all blanks. + +The @code{typeflag} field specifies the type of file archived. If a +particular implementation does not recognize or permit the specified +type, the file will be extracted as if it were a regular file. As this +action occurs, @code{tar} issues a warning to the standard error. + +The @code{atime} and @code{ctime} fields are used in making incremental +backups; they store, respectively, the particular file's access time +and last inode-change time. + +The @code{offset} is used by the @value{op-multi-volume} option, when +making a multi-volume archive. The offset is number of bytes into +the file that we need to restart at to continue the file on the next +tape, i.e., where we store the location that a continued file is +continued at. + +The following fields were added to deal with sparse files. A file +is @dfn{sparse} if it takes in unallocated blocks which end up being +represented as zeros, i.e., no useful data. A test to see if a file +is sparse is to look at the number blocks allocated for it versus the +number of characters in the file; if there are fewer blocks allocated +for the file than would normally be allocated for a file of that +size, then the file is sparse. This is the method @code{tar} uses to +detect a sparse file, and once such a file is detected, it is treated +differently from non-sparse files. + +Sparse files are often @code{dbm} files, or other database-type files +which have data at some points and emptiness in the greater part of +the file. Such files can appear to be very large when an @samp{ls +-l} is done on them, when in truth, there may be a very small amount +of important data contained in the file. It is thus undesirable +to have @code{tar} think that it must back up this entire file, as +great quantities of room are wasted on empty blocks, which can lead +to running out of room on a tape far earlier than is necessary. +Thus, sparse files are dealt with so that these empty blocks are +not written to the tape. Instead, what is written to the tape is a +description, of sorts, of the sparse file: where the holes are, how +big the holes are, and how much data is found at the end of the hole. +This way, the file takes up potentially far less room on the tape, +and when the file is extracted later on, it will look exactly the way +it looked beforehand. The following is a description of the fields +used to handle a sparse file: + +The @code{sp} is an array of @code{struct sparse}. Each @code{struct +sparse} contains two 12-character strings which represent an offset +into the file and a number of bytes to be written at that offset. +The offset is absolute, and not relative to the offset in preceding +array element. + +The header can hold four of these @code{struct sparse} at the moment; +if more are needed, they are not stored in the header. + +The @code{isextended} flag is set when an @code{extended_header} +is needed to deal with a file. Note that this means that this flag +can only be set when dealing with a sparse file, and it is only set +in the event that the description of the file will not fit in the +alloted room for sparse structures in the header. In other words, +an extended_header is needed. + +The @code{extended_header} structure is used for sparse files which +need more sparse structures than can fit in the header. The header can +fit 4 such structures; if more are needed, the flag @code{isextended} +gets set and the next block is an @code{extended_header}. + +Each @code{extended_header} structure contains an array of 21 +sparse structures, along with a similar @code{isextended} flag +that the header had. There can be an indeterminate number of such +@code{extended_header}s to describe a sparse file. + +@table @asis + +@item @code{REGTYPE} +@itemx @code{AREGTYPE} +These flags represent a regular file. In order to be compatible +with older versions of @code{tar}, a @code{typeflag} value of +@code{AREGTYPE} should be silently recognized as a regular file. +New archives should be created using @code{REGTYPE}. Also, for +backward compatibility, @code{tar} treats a regular file whose name +ends with a slash as a directory. + +@item @code{LNKTYPE} +This flag represents a file linked to another file, of any type, +previously archived. Such files are identified in Unix by each +file having the same device and inode number. The linked-to name is +specified in the @code{linkname} field with a trailing null. + +@item @code{SYMTYPE} +This represents a symbolic link to another file. The linked-to name +is specified in the @code{linkname} field with a trailing null. + +@item @code{CHRTYPE} +@itemx @code{BLKTYPE} +These represent character special files and block special files +respectively. In this case the @code{devmajor} and @code{devminor} +fields will contain the major and minor device numbers respectively. +Operating systems may map the device specifications to their own +local specification, or may ignore the entry. + +@item @code{DIRTYPE} +This flag specifies a directory or sub-directory. The directory +name in the @code{name} field should end with a slash. On systems where +disk allocation is performed on a directory basis, the @code{size} field +will contain the maximum number of bytes (which may be rounded to +the nearest disk block allocation unit) which the directory may +hold. A @code{size} field of zero indicates no such limiting. Systems +which do not support limiting in this manner should ignore the +@code{size} field. + +@item @code{FIFOTYPE} +This specifies a FIFO special file. Note that the archiving of a +FIFO file archives the existence of this file and not its contents. + +@item @code{CONTTYPE} +This specifies a contiguous file, which is the same as a normal +file except that, in operating systems which support it, all its +space is allocated contiguously on the disk. Operating systems +which do not allow contiguous allocation should silently treat this +type as a normal file. + +@item @code{A} @dots{} @code{Z} +These are reserved for custom implementations. Some of these are +used in the GNU modified format, as described below. + +@end table + +Other values are reserved for specification in future revisions of +the P1003 standard, and should not be used by any @code{tar} program. + +The @code{magic} field indicates that this archive was output in +the P1003 archive format. If this field contains @code{TMAGIC}, +the @code{uname} and @code{gname} fields will contain the ASCII +representation of the owner and group of the file respectively. +If found, the user and group IDs are used rather than the values in +the @code{uid} and @code{gid} fields. + +For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages +169-173 (section 10.1) for @cite{Archive/Interchange File Format}; and +IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940 +(section E.4.48) for @cite{pax - Portable archive interchange}. + +@node Extensions, cpio, Standard, Formats +@section GNU Extensions to the Archive Format +@UNREVISED + +The GNU format uses additional file types to describe new types of +files in an archive. These are listed below. + +@table @code +@item GNUTYPE_DUMPDIR +@itemx 'D' +This represents a directory and a list of files created by the +@value{op-incremental} option. The @code{size} field gives the total +size of the associated list of files. Each file name is preceded by +either a @samp{Y} (the file should be in this archive) or an @samp{N}. +(The file is a directory, or is not stored in the archive.) Each file +name is terminated by a null. There is an additional null after the +last file name. + +@item GNUTYPE_MULTIVOL +@itemx 'M' +This represents a file continued from another volume of a multi-volume +archive created with the @value{op-multi-volume} option. The original +type of the file is not given here. The @code{size} field gives the +maximum size of this piece of the file (assuming the volume does +not end before the file is written out). The @code{offset} field +gives the offset from the beginning of the file where this part of +the file begins. Thus @code{size} plus @code{offset} should equal +the original size of the file. + +@item GNUTYPE_SPARSE +@itemx 'S' +This flag indicates that we are dealing with a sparse file. Note +that archiving a sparse file requires special operations to find +holes in the file, which mark the positions of these holes, along +with the number of bytes of data to be found after the hole. + +@item GNUTYPE_VOLHDR +@itemx 'V' +This file type is used to mark the volume header that was given with +the @value{op-label} option when the archive was created. The @code{name} +field contains the @code{name} given after the @value{op-label} option. +The @code{size} field is zero. Only the first file in each volume +of an archive should have this type. + +@end table + +You may have trouble reading a GNU format archive on a non-GNU +system if the options @value{op-incremental}, @value{op-multi-volume}, +@value{op-sparse}, or @value{op-label} were used when writing the archive. +In general, if @code{tar} does not use the GNU-added fields of the +header, other versions of @code{tar} should be able to read the +archive. Otherwise, the @code{tar} program will give an error, the +most likely one being a checksum error. + +@node cpio, , Extensions, Formats +@section Comparison of @code{tar} and @code{cpio} +@UNREVISED + +@FIXME{Reorganize the following material} + +The @code{cpio} archive formats, like @code{tar}, do have maximum +pathname lengths. The binary and old ASCII formats have a max path +length of 256, and the new ASCII and CRC ASCII formats have a max +path length of 1024. GNU @code{cpio} can read and write archives +with arbitrary pathname lengths, but other @code{cpio} implementations +may crash unexplainedly trying to read them. + +@code{tar} handles symbolic links in the form in which it comes in BSD; +@code{cpio} doesn't handle symbolic links in the form in which it comes +in System V prior to SVR4, and some vendors may have added symlinks +to their system without enhancing @code{cpio} to know about them. +Others may have enhanced it in a way other than the way I did it +at Sun, and which was adopted by AT&T (and which is, I think, also +present in the @code{cpio} that Berkeley picked up from AT&T and put +into a later BSD release---I think I gave them my changes). + +(SVR4 does some funny stuff with @code{tar}; basically, its @code{cpio} +can handle @code{tar} format input, and write it on output, and it +probably handles symbolic links. They may not have bothered doing +anything to enhance @code{tar} as a result.) + +@code{cpio} handles special files; traditional @code{tar} doesn't. + +@code{tar} comes with V7, System III, System V, and BSD source; +@code{cpio} comes only with System III, System V, and later BSD +(4.3-tahoe and later). + +@code{tar}'s way of handling multiple hard links to a file can handle +file systems that support 32-bit inumbers (e.g., the BSD file system); +@code{cpio}s way requires you to play some games (in its "binary" +format, i-numbers are only 16 bits, and in its "portable ASCII" format, +they're 18 bits---it would have to play games with the "file system ID" +field of the header to make sure that the file system ID/i-number pairs +of different files were always different), and I don't know which +@code{cpio}s, if any, play those games. Those that don't might get +confused and think two files are the same file when they're not, and +make hard links between them. + +@code{tar}s way of handling multiple hard links to a file places only +one copy of the link on the tape, but the name attached to that copy +is the @emph{only} one you can use to retrieve the file; @code{cpio}s +way puts one copy for every link, but you can retrieve it using any +of the names. + +@quotation +What type of check sum (if any) is used, and how is this calculated. +@end quotation + +See the attached manual pages for @code{tar} and @code{cpio} format. +@code{tar} uses a checksum which is the sum of all the bytes in the +@code{tar} header for a file; @code{cpio} uses no checksum. + +@quotation +If anyone knows why @code{cpio} was made when @code{tar} was present +at the unix scene, +@end quotation + +It wasn't. @code{cpio} first showed up in PWB/UNIX 1.0; no +generally-available version of UNIX had @code{tar} at the time. I don't +know whether any version that was generally available @emph{within AT&T} +had @code{tar}, or, if so, whether the people within AT&T who did +@code{cpio} knew about it. + +On restore, if there is a corruption on a tape @code{tar} will stop at +that point, while @code{cpio} will skip over it and try to restore the +rest of the files. + +The main difference is just in the command syntax and header format. + +@code{tar} is a little more tape-oriented in that everything is blocked +to start on a record boundary. + +@quotation +Is there any differences between the ability to recover crashed +archives between the two of them. (Is there any chance of recovering +crashed archives at all.) +@end quotation + +Theoretically it should be easier under @code{tar} since the blocking +lets you find a header with some variation of @samp{dd skip=@var{nn}}. +However, modern @code{cpio}'s and variations have an option to just +search for the next file header after an error with a reasonable chance +of re-syncing. However, lots of tape driver software won't allow you to +continue past a media error which should be the only reason for getting +out of sync unless a file changed sizes while you were writing the +archive. + +@quotation +If anyone knows why @code{cpio} was made when @code{tar} was present +at the unix scene, please tell me about this too. +@end quotation + +Probably because it is more media efficient (by not blocking everything +and using only the space needed for the headers where @code{tar} +always uses 512 bytes per file header) and it knows how to archive +special files. + +You might want to look at the freely available alternatives. The major +ones are @code{afio}, GNU @code{tar}, and @code{pax}, each of which +have their own extensions with some backwards compatibility. + +Sparse files were @code{tar}red as sparse files (which you can easily +test, because the resulting archive gets smaller, and GNU @code{cpio} +can no longer read it). + +@node Media, Index, Formats, Top +@chapter Tapes and Other Archive Media +@UNREVISED + +A few special cases about tape handling warrant more detailed +description. These special cases are discussed below. + +Many complexities surround the use of @code{tar} on tape drives. Since +the creation and manipulation of archives located on magnetic tape was +the original purpose of @code{tar}, it contains many features making +such manipulation easier. + +Archives are usually written on dismountable media---tape cartridges, +mag tapes, or floppy disks. + +The amount of data a tape or disk holds depends not only on its size, +but also on how it is formatted. A 2400 foot long reel of mag tape +holds 40 megabytes of data when formated at 1600 bits per inch. The +physically smaller EXABYTE tape cartridge holds 2.3 gigabytes. + +Magnetic media are re-usable---once the archive on a tape is no longer +needed, the archive can be erased and the tape or disk used over. +Media quality does deteriorate with use, however. Most tapes or disks +should be disgarded when they begin to produce data errors. EXABYTE +tape cartridges should be disgarded when they generate an @dfn{error +count} (number of non-usable bits) of more than 10k. + +Magnetic media are written and erased using magnetic fields, and +should be protected from such fields to avoid damage to stored data. +Sticking a floppy disk to a filing cabinet using a magnet is probably +not a good idea. + +@menu +* Device:: Device selection and switching +* Remote Tape Server:: +* Common Problems and Solutions:: +* Blocking:: Blocking +* Many:: Many archives on one tape +* Using Multiple Tapes:: Using Multiple Tapes +* label:: Including a Label in the Archive +* verify:: +* Write Protection:: +@end menu + +@node Device, Remote Tape Server, Media, Media +@section Device Selection and Switching +@UNREVISED + +@table @kbd +@item -f [@var{hostname}:]@var{file} +@itemx --file=[@var{hostname}:]@var{file} +Use archive file or device @var{file} on @var{hostname}. +@end table + +This option is used to specify the file name of the archive @code{tar} +works on. + +If the file name is @samp{-}, @code{tar} reads the archive from standard +input (when listing or extracting), or writes it to standard output +(when creating). If the @samp{-} file name is given when updating an +archive, @code{tar} will read the original archive from its standard +input, and will write the entire new archive to its standard output. + +If the file name contains a @samp{:}, it is interpreted as +@samp{hostname:file name}. If the @var{hostname} contains an @dfn{at} +sign (@kbd{@@}), it is treated as @samp{user@@hostname:file name}. In +either case, @code{tar} will invoke the command @code{rsh} (or +@code{remsh}) to start up an @file{/etc/rmt} on the remote machine. If +you give an alternate login name, it will be given to the @code{rsh}. +Naturally, the remote machine must have an executable @file{/etc/rmt}. +This program is free software from the University of California, and a +copy of the source code can be found with the sources for @code{tar}; +it's compiled and installed by default. + +If this option is not given, but the environment variable @code{TAPE} is +set, its value is used; otherwise, old versions of @code{tar} used a default +archive name (which was picked when @code{tar} was compiled). The +default is normally set up to be the @dfn{first} tape drive or other +transportable I/O medium on the system. + +Starting with version 1.11.5, GNU @code{tar} uses standard input and +standard output as the default device, and I will not try anymore +supporting automatic device detection at installation time. This was +failing really in too many cases, it was hopeless. This is now +completely left to the installer to override standard input and standard +output for default device, if this seems preferrable to him/her. +Further, I think @emph{most} actual usages of @code{tar} are done with +pipes or disks, not really tapes, cartridges or diskettes. + +Some users think that using standard input and output is running +after trouble. This could lead to a nasty surprise on your screen if +you forget to specify an output file name---especially if you are going +through a network or terminal server capable of buffering large amounts +of output. We had so many bug reports in that area of configuring +default tapes automatically, and so many contradicting requests, that +we finally consider the problem to be portably intractable. We could +of course use something like @samp{/dev/tape} as a default, but this +is @emph{also} running after various kind of trouble, going from hung +processes to accidental destruction of real tapes. After having seen +all this mess, using standard input and output as a default really +sounds like the only clean choice left, and a very useful one too. + +GNU @code{tar} reads and writes archive in records, I suspect this is the +main reason why block devices are preferred over character devices. +Most probably, block devices are more efficient too. The installer +could also check for @samp{DEFTAPE} in @file{}. + +@table @kbd +@item --force-local +Archive file is local even if it contains a colon. + +@item --rsh-command=@var{command} +Use remote @var{command} instead of @code{rsh}. This option exists +so that people who use something other than the standard @code{rsh} +(e.g., a Kerberized @code{rsh}) can access a remote device. + +When this command is not used, the shell command found when +the @code{tar} program was installed is used instead. This is +the first found of @file{/usr/ucb/rsh}, @file{/usr/bin/remsh}, +@file{/usr/bin/rsh}, @file{/usr/bsd/rsh} or @file{/usr/bin/nsh}. +The installer may have overriden this by defining the environment +variable @code{RSH} @emph{at installation time}. + +@item -[0-7][lmh] +Specify drive and density. + +@item -M +@itemx --multi-volume +Create/list/extract multi-volume archive. + +This option causes @code{tar} to write a @dfn{multi-volume} archive---one +that may be larger than will fit on the medium used to hold it. +@xref{Multi-Volume Archives}. + +@item -L @var{num} +@itemx --tape-length=@var{num} +Change tape after writing @var{num} x 1024 bytes. + +This option might be useful when your tape drivers do not properly +detect end of physical tapes. By being slightly conservative on the +maximum tape length, you might avoid the problem entirely. + +@item -F @var{file} +@itemx --info-script=@var{file} +@itemx --new-volume-script=@var{file} +Execute @file{file} at end of each tape. This implies +@value{op-multi-volume}. +@end table + +@node Remote Tape Server, Common Problems and Solutions, Device, Media +@section The Remote Tape Server + +@cindex remote tape drive +@pindex rmt +In order to access the tape drive on a remote machine, @code{tar} +uses the remote tape server written at the University of California at +Berkeley. The remote tape server must be installed as @file{/etc/rmt} +on any machine whose tape drive you want to use. @code{tar} calls +@file{/etc/rmt} by running an @code{rsh} or @code{remsh} to the remote +machine, optionally using a different login name if one is supplied. + +A copy of the source for the remote tape server is provided. It is +Copyright @copyright{} 1983 by the Regents of the University of +California, but can be freely distributed. Instructions for compiling +and installing it are included in the @file{Makefile}. + +@cindex absolute file names +Unless you use the @value{op-absolute-names} option, GNU @code{tar} will +not allow you to create an archive that contains absolute file names +(a file name beginning with @samp{/}.) If you try, @code{tar} will +automatically remove the leading @samp{/} from the file names it +stores in the archive. It will also type a warning message telling +you what it is doing. + +When reading an archive that was created with a different @code{tar} +program, GNU @code{tar} automatically extracts entries in the archive +which have absolute file names as if the file names were not absolute. +This is an important feature. A visitor here once gave a +@code{tar} tape to an operator to restore; the operator used Sun @code{tar} +instead of GNU @code{tar}, and the result was that it replaced large +portions of our @file{/bin} and friends with versions from the tape; +needless to say, we were unhappy about having to recover the file system +from backup tapes. + +For example, if the archive contained a file @file{/usr/bin/computoy}, +GNU @code{tar} would extract the file to @file{usr/bin/computoy}, +relative to the current directory. If you want to extract the files in +an archive to the same absolute names that they had when the archive +was created, you should do a @samp{cd /} before extracting the files +from the archive, or you should either use the @value{op-absolute-names} +option, or use the command @samp{tar -C / @dots{}}. + +@cindex Ultrix 3.1 and write failure +Some versions of Unix (Ultrix 3.1 is know to have this problem), +can claim that a short write near the end of a tape succeeded, +when it actually failed. This will result in the -M option not +working correctly. The best workaround at the moment is to use a +significantly larger blocking factor than the default 20. + +In order to update an archive, @code{tar} must be able to backspace the +archive in order to reread or rewrite a record that was just read (or +written). This is currently possible only on two kinds of files: normal +disk files (or any other file that can be backspaced with @samp{lseek}), +and industry-standard 9-track magnetic tape (or any other kind of tape +that can be backspaced with the @code{MTIOCTOP} @code{ioctl}. + +This means that the @value{op-append}, @value{op-update}, +@value{op-concatenate}, and @value{op-delete} commands will not work on any +other kind of file. Some media simply cannot be backspaced, which +means these commands and options will never be able to work on them. +These non-backspacing media include pipes and cartridge tape drives. + +Some other media can be backspaced, and @code{tar} will work on them +once @code{tar} is modified to do so. + +Archives created with the @value{op-multi-volume}, @value{op-label}, and +@value{op-incremental} options may not be readable by other version +of @code{tar}. In particular, restoring a file that was split over +a volume boundary will require some careful work with @code{dd}, if +it can be done at all. Other versions of @code{tar} may also create +an empty file whose name is that of the volume header. Some versions +of @code{tar} may create normal files instead of directories archived +with the @value{op-incremental} option. + +@node Common Problems and Solutions, Blocking, Remote Tape Server, Media +@section Some Common Problems and their Solutions + +@ifclear PUBLISH + +@format +errors from system: +permission denied +no such file or directory +not owner + +errors from @code{tar}: +directory checksum error +header format error + +errors from media/system: +i/o error +device busy +@end format + +@end ifclear + +@node Blocking, Many, Common Problems and Solutions, Media +@section Blocking +@UNREVISED + +@dfn{Block} and @dfn{record} terminology is rather confused, and it +is also confusing to the expert reader. On the other hand, readers +who are new to the field have a fresh mind, and they may safely skip +the next two paragraphs, as the remainder of this manual uses those +two terms in a quite consistent way. + +John Gilmore, the writer of the public domain @code{tar} from which +GNU @code{tar} was originally derived, wrote (June 1995): + +@quotation +The nomenclature of tape drives comes from IBM, where I believe +they were invented for the IBM 650 or so. On IBM mainframes, what +is recorded on tape are tape blocks. The logical organization of +data is into records. There are various ways of putting records into +blocks, including @code{F} (fixed sized records), @code{V} (variable +sized records), @code{FB} (fixed blocked: fixed size records, @var{n} +to a block), @code{VB} (variable size records, @var{n} to a block), +@code{VSB} (variable spanned blocked: variable sized records that can +occupy more than one block), etc. The @code{JCL} @samp{DD RECFORM=} +parameter specified this to the operating system. + +The Unix man page on @code{tar} was totally confused about this. +When I wrote @code{PD TAR}, I used the historically correct terminology +(@code{tar} writes data records, which are grouped into blocks). +It appears that the bogus terminology made it into POSIX (no surprise +here), and now Fran@,{c}ois has migrated that terminology back +into the source code too. +@end quotation + +The term @dfn{physical block} means the basic transfer chunk from or +to a device, after which reading or writing may stop without anything +being lost. In this manual, the term @dfn{block} usually refers to +a disk physical block, @emph{assuming} that each disk block is 512 +bytes in length. It is true that some disk devices have different +physical blocks, but @code{tar} ignore these differences in its own +format, which is meant to be portable, so a @code{tar} block is always +512 bytes in length, and @dfn{block} always mean a @code{tar} block. +The term @dfn{logical block} often represents the basic chunk of +allocation of many disk blocks as a single entity, which the operating +system treats somewhat atomically; this concept is only barely used +in GNU @code{tar}. + +The term @dfn{physical record} is another way to speak of a physical +block, those two terms are somewhat interchangeable. In this manual, +the term @dfn{record} usually refers to a tape physical block, +@emph{assuming} that the @code{tar} archive is kept on magnetic tape. +It is true that archives may be put on disk or used with pipes, +but nevertheless, @code{tar} tries to read and write the archive one +@dfn{record} at a time, whatever the medium in use. One record is made +up of an integral number of blocks, and this operation of putting many +disk blocks into a single tape block is called @dfn{reblocking}, or +more simply, @dfn{blocking}. The term @dfn{logical record} refers to +the logical organization of many characters into something meaningful +to the application. The term @dfn{unit record} describes a small set +of characters which are transmitted whole to or by the application, +and often refers to a line of text. Those two last terms are unrelated +to what we call a @dfn{record} in GNU @code{tar}. + +When writing to tapes, @code{tar} writes the contents of the archive +in chunks known as @dfn{records}. To change the default blocking +factor, use the @value{op-blocking-factor} option. Each record will +then be composed of @var{512-size} blocks. (Each @code{tar} block is +512 bytes. @xref{Standard}.) Each file written to the archive uses +at least one full record. As a result, using a larger record size +can result in more wasted space for small files. On the other hand, a +larger record size can often be read and written much more efficiently. + +Further complicating the problem is that some tape drives ignore the +blocking entirely. For these, a larger record size can still improve +performance (because the software layers above the tape drive still +honor the blocking), but not as dramatically as on tape drives that +honor blocking. + +When reading an archive, @code{tar} can usually figure out the record +size on itself. When this is the case, and a non-standard record size +was used when the archive was created, @code{tar} will print a message +about a non-standard blocking factor, and then operate normally. On +some tape devices, however, @code{tar} cannot figure out the record size +itself. On most of those, you can specify a blocking factor (with +@value{op-blocking-factor}) larger than the actual blocking factor, and then use +the @value{op-read-full-records} option. (If you specify a blocking factor +with @value{op-blocking-factor} and don't use the @value{op-read-full-records} +option, then @code{tar} will not attempt to figure out the recording size +itself.) On some devices, you must always specify the record size +exactly with @value{op-blocking-factor} when reading, because @code{tar} cannot +figure it out. In any case, use @value{op-list} before doing any +extractions to see whether @code{tar} is reading the archive correctly. + +@code{tar} blocks are all fixed size (512 bytes), and its scheme for +putting them into records is to put a whole number of them (one or +more) into each record. @code{tar} records are all the same size; +at the end of the file there's a block containing all zeros, which +is how you tell that the remainder of the last record(s) are garbage. + +In a standard @code{tar} file (no options), the block size is 512 +and the record size is 10240, for a blocking factor of 20. What the +@value{op-blocking-factor} option does is sets the blocking factor, +changing the record size while leaving the block size at 512 bytes. +20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives; +most tape drives these days prefer much bigger records in order to +stream and not waste tape. When writing tapes for myself, some tend +to use a factor of the order of 2048, say, giving a record size of +around one megabyte. + +If you use a blocking factor larger than 20, older @code{tar} programs +might not be able to read the archive, so we recommend this as a limit +to use in practice. GNU @code{tar}, however, will support arbitrarily +large record sizes, limited only by the amount of virtual memory or the +physical characteristics of the tape device. + +@menu +* Format Variations:: Format Variations +* Blocking Factor:: The Blocking Factor of an Archive +@end menu + +@node Format Variations, Blocking Factor, Blocking, Blocking +@subsection Format Variations +@cindex Format Parameters +@cindex Format Options +@cindex Options, archive format specifying +@cindex Options, format specifying +@UNREVISED + +Format parameters specify how an archive is written on the archive +media. The best choice of format parameters will vary depending on +the type and number of files being archived, and on the media used to +store the archive. -@item --list -Prints a list of the contents of the archive. +To specify format parameters when accessing or creating an archive, +you can use the options described in the following sections. +If you do not specify any format parameters, @code{tar} uses +default parameters. You cannot modify a compressed archive. +If you create an archive with the @value{op-blocking-factor} option +specified (@value{pxref-blocking-factor}), you must specify that +blocking-factor when operating on the archive. @xref{Formats}, for other +examples of format parameter considerations. + +@node Blocking Factor, , Format Variations, Blocking +@subsection The Blocking Factor of an Archive +@cindex Blocking Factor +@cindex Record Size +@cindex Number of blocks per record +@cindex Number of bytes per record +@cindex Bytes per record +@cindex Blocks per record +@UNREVISED + +The data in an archive is grouped into blocks, which are 512 bytes. +Blocks are read and written in whole number multiples called +@dfn{records}. The number of blocks in a record (ie. the size of a +record in units of 512 bytes) is called the @dfn{blocking factor}. +The @value{op-blocking-factor} option specifies the blocking factor of +an archive. The default blocking factor is typically 20 (ie.@: +10240 bytes), but can be specified at installation. To find out +the blocking factor of an existing archive, use @samp{tar --list +--file=@var{archive-name}}. This may not work on some devices. + +Records are separated by gaps, which waste space on the archive media. +If you are archiving on magnetic tape, using a larger blocking factor +(and therefore larger records) provides faster throughput and allows you +to fit more data on a tape (because there are fewer gaps). If you are +archiving on cartridge, a very large blocking factor (say 126 or more) +greatly increases performance. A smaller blocking factor, on the other +hand, may be usefull when archiving small files, to avoid archiving lots +of nulls as @code{tar} fills out the archive to the end of the record. +In general, the ideal record size depends on the size of the +inter-record gaps on the tape you are using, and the average size of the +files you are archiving. @xref{create}, for information on +writing archives. + +@FIXME{Need example of using a cartridge with blocking factor=126 or more.} + +Archives with blocking factors larger than 20 cannot be read +by very old versions of @code{tar}, or by some newer versions +of @code{tar} running on old machines with small address spaces. +With GNU @code{tar}, the blocking factor of an archive is limited +only by the maximum record size of the device containing the archive, +or by the amount of available virtual memory. + +Also, on some systems, not using adequate blocking factors, as sometimes +imposed by the device drivers, may yield unexpected diagnostics. For +example, this has been reported: -@item --update -Adds files to the end of the archive, but only if they are newer than -their counterparts already in the archive, or if they do not already -exist in the archive. +@example +Cannot write to /dev/dlt: Invalid argument +@end example -@item --version -Prints the version number of the @code{tar} program to the standard -error. +@noindent +In such cases, it sometimes happen that the @code{tar} bundled by the +system is aware of block size idiosyncrasies, while GNU @code{tar} requires +an explicit specification for the block size, which it cannot guess. +This yields some people to consider GNU @code{tar} is misbehaving, because +by comparison, @cite{the bundle @code{tar} works OK}. Adding @w{@kbd{-b +256}}, for example, might resolve the problem. + +If you use a non-default blocking factor when you create an archive, you +must specify the same blocking factor when you modify that archive. Some +archive devices will also require you to specify the blocking factor when +reading that archive, however this is not typically the case. Usually, you +can use @value{op-list} without specifying a blocking factor---@code{tar} +reports a non-default record size and then lists the archive members as +it would normally. To extract files from an archive with a non-standard +blocking factor (particularly if you're not sure what the blocking factor +is), you can usually use the @value{op-read-full-records} option while +specifying a blocking factor larger then the blocking factor of the archive +(ie. @samp{tar --extract --read-full-records --blocking-factor=300}. +@xref{list}, for more information on the @value{op-list} +operation. @xref{Reading}, for a more detailed explanation of that option. + +@table @kbd +@item --blocking-factor=@var{number} +@itemx -b @var{number} +Specifies the blocking factor of an archive. Can be used with any +operation, but is usually not necessary with @value{op-list}. @end table -@node Options, , Operations, Quick Reference -@appendixsec Table of Options +Device blocking -Options change the way @code{tar} performs an operation. +@table @kbd +@item -b @var{blocks} +@itemx --blocking-factor=@var{blocks} +Set record size to @math{@var{blocks} * 512} bytes. -@table @samp -@item --absolute-paths -WILL BE INPUT WHEN QUESTION IS RESOLVED +This option is used to specify a @dfn{blocking factor} for the archive. +When reading or writing the archive, @code{tar}, will do reads and writes +of the archive in records of @math{@var{block}*512} bytes. This is true +even when the archive is compressed. Some devices requires that all +write operations be a multiple of a certain size, and so, @code{tar} +pads the archive out to the next record boundary. -@item --after-date=@var{date} -Limit the operation to files changed after the given date. -@xref{File Exclusion}. +The default blocking factor is set when @code{tar} is compiled, and is +typically 20. Blocking factors larger than 20 cannot be read by very +old versions of @code{tar}, or by some newer versions of @code{tar} +running on old machines with small address spaces. + +With a magnetic tape, larger records give faster throughput and fit +more data on a tape (because there are fewer inter-record gaps). +If the archive is in a disk file or a pipe, you may want to specify +a smaller blocking factor, since a large one will result in a large +number of null bytes at the end of the archive. + +When writing cartridge or other streaming tapes, a much larger +blocking factor (say 126 or more) will greatly increase performance. +However, you must specify the same blocking factor when reading or +updating the archive. + +Apparently, Exabyte drives have a physical block size of 8K bytes. +If we choose our blocksize as a multiple of 8k bytes, then the problem +seems to dissapper. Id est, we are using block size of 112 right +now, and we haven't had the problem since we switched@dots{} + +With GNU @code{tar} the blocking factor is limited only by the maximum +record size of the device containing the archive, or by the amount of +available virtual memory. + +However, deblocking or reblocking is virtually avoided in a special +case which often occurs in practice, but which requires all the +following conditions to be simultaneously true: +@itemize @bullet +@item +the archive is subject to a compression option, +@item +the archive is not handled through standard input or output, nor +redirected nor piped, +@item +the archive is directly handled to a local disk, instead of any special +device, +@item +@value{op-blocking-factor} is not explicitely specified on the @code{tar} +invocation. +@end itemize -@item --block-size=@var{number} -Specify the blocking factor of an archive. @xref{Blocking Factor}. +In previous versions of GNU @code{tar}, the @samp{--compress-block} +option (or even older: @samp{--block-compress}) was necessary to +reblock compressed archives. It is now a dummy option just asking +not to be used, and otherwise ignored. If the output goes directly +to a local disk, and not through stdout, then the last write is +not extended to a full record size. Otherwise, reblocking occurs. +Here are a few other remarks on this topic: + +@itemize @bullet + +@item +@code{gzip} will complain about trailing garbage if asked to +uncompress a compressed archive on tape, there is an option to turn +the message off, but it breaks the regularity of simply having to use +@samp{@var{prog} -d} for decompression. It would be nice if gzip was +silently ignoring any number of trailing zeros. I'll ask Jean-loup +Gailly, by sending a copy of this message to him. + +@item +@code{compress} does not show this problem, but as Jean-loup pointed +out to Michael, @samp{compress -d} silently adds garbage after +the result of decompression, which tar ignores because it already +recognized its end-of-file indicator. So this bug may be safely +ignored. + +@item +@samp{gzip -d -q} will be silent about the trailing zeros indeed, +but will still return an exit status of 2 which tar reports in turn. +@code{tar} might ignore the exit status returned, but I hate doing +that, as it weakens the protection @code{tar} offers users against +other possible problems at decompression time. If @code{gzip} was +silently skipping trailing zeros @emph{and} also avoiding setting the +exit status in this innocuous case, that would solve this situation. + +@item +@code{tar} should become more solid at not stopping to read a pipe at +the first null block encountered. This inelegantly breaks the pipe. +@code{tar} should rather drain the pipe out before exiting itself. +@end itemize -@item --compress -Specify a compressed archive. @xref{Compressed Archives}. +@item -i +@itemx --ignore-zeros +Ignore blocks of zeros in archive (means EOF). + +The @value{op-ignore-zeros} option causes @code{tar} to ignore blocks +of zeros in the archive. Normally a block of zeros indicates the +end of the archive, but when reading a damaged archive, or one which +was created by @code{cat}-ing several archives together, this option +allows @code{tar} to read the entire archive. This option is not on +by default because many versions of @code{tar} write garbage after +the zeroed blocks. + +Note that this option causes @code{tar} to read to the end of the +archive file, which may sometimes avoid problems when multiple files +are stored on a single physical tape. + +@item -B +@itemx --read-full-records +Reblock as we read (for reading 4.2BSD pipes). + +If @value{op-read-full-records} is used, @code{tar} will not panic if an +attempt to read a record from the archive does not return a full record. +Instead, @code{tar} will keep reading until it has obtained a full +record. + +This option is turned on by default when @code{tar} is reading +an archive from standard input, or from a remote machine. This is +because on BSD Unix systems, a read of a pipe will return however +much happens to be in the pipe, even if it is less than @code{tar} +requested. If this option was not used, @code{tar} would fail as +soon as it read an incomplete record from the pipe. + +This option is also useful with the commands for updating an archive. -@item --compress-block. -Create a whole block sized compressed archive. @xref{Compressed Archives}. +@end table -@item --confirmation -Solicit confirmation for each file. @xref{Interactive Operation} -<<< --selective should be a synonym. +Tape blocking + +@FIXME{Appropriate options should be moved here from elsewhere.} + +@cindex blocking factor +@cindex tape blocking + +When handling various tapes or cartridges, you have to take care of +selecting a proper blocking, that is, the number of disk blocks you +put together as a single tape block on the tape, without intervening +tape gaps. A @dfn{tape gap} is a small landing area on the tape +with no information on it, used for decelerating the tape to a +full stop, and for later regaining the reading or writing speed. +When the tape driver starts reading a record, the record has to +be read whole without stopping, as a tape gap is needed to stop the +tape motion without loosing information. + +@cindex Exabyte blocking +@cindex DAT blocking +Using higher blocking (putting more disk blocks per tape block) will use +the tape more efficiently as there will be less tape gaps. But reading +such tapes may be more difficult for the system, as more memory will be +required to receive at once the whole record. Further, if there is a +reading error on a huge record, this is less likely that the system will +succeed in recovering the information. So, blocking should not be too +low, nor it should be too high. @code{tar} uses by default a blocking of +20 for historical reasons, and it does not really matter when reading or +writing to disk. Current tape technology would easily accomodate higher +blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs. +We were told that for some DLT drives, the blocking should be a multiple +of 4Kb, preferably 64Kb (@w{@kbd{-b 128}}) or 256 for decent performance. +Other manufacturers may use different recommendations for the same tapes. +This might also depends of the buffering techniques used inside modern +tape controllers. Some imposes a minimum blocking, or a maximum blocking. +Others request blocking to be some exponent of two. + +So, there is no fixed rule for blocking. But blocking at read time +should ideally be the same as blocking used at write time. At one place +I know, with a wide variety of equipment, they found it best to use a +blocking of 32 to guarantee that their tapes are fully interchangeable. + +I was also told that, for recycled tapes, prior erasure (by the same +drive unit that will be used to create the archives) sometimes lowers +the error rates observed at rewriting time. + +I might also use @samp{--number-blocks} instead of +@samp{--block-number}, so @samp{--block} will then expand to +@samp{--blocking-factor} unambiguously. + +@node Many, Using Multiple Tapes, Blocking, Media +@section Many Archives on One Tape + +@FIXME{Appropriate options should be moved here from elsewhere.} + +@findex ntape @r{device} +Most tape devices have two entries in the @file{/dev} directory, or +entries that come in pairs, which differ only in the minor number for +this device. Let's take for example @file{/dev/tape}, which often +points to the only or usual tape device of a given system. There might +be a corresponding @file{/dev/nrtape} or @file{/dev/ntape}. The simpler +name is the @emph{rewinding} version of the device, while the name +having @samp{nr} in it is the @emph{no rewinding} version of the same +device. + +A rewinding tape device will bring back the tape to its beginning point +automatically when this device is opened or closed. Since @code{tar} +opens the archive file before using it and closes it afterwards, this +means that a simple: -@item --dereference -Treat a symbolic link as an alternate name for the file the link -points to. @xref{Symbolic Links}. +@example +$ @kbd{tar cf /dev/tape @var{directory}} +@end example -@item --directory=@file{directory} -Change the working directory. @xref{Changing Working Directory}. +@noindent +will reposition the tape to its beginning both prior and after saving +@var{directory} contents to it, thus erasing prior tape contents and +making it so that any subsequent write operation will destroy what has +just been saved. + +@cindex tape positioning +So, a rewinding device is normally meant to hold one and only one file. +If you want to put more than one @code{tar} archive on a given tape, you +will need to avoid using the rewinding version of the tape device. You +will also have to pay special attention to tape positioning. Errors in +positionning may overwrite the valuable data already on your tape. Many +people, burnt by past experiences, will only use rewinding devices and +limit themselves to one file per tape, precisely to avoid the risk of +such errors. Be fully aware that writing at the wrong position on a +tape loses all information past this point and most probably until the +end of the tape, and this destroyed information @emph{cannot} be +recovered. + +To save @var{directory-1} as a first archive at the beginning of a +tape, and leave that tape ready for a second archive, you should use: -@item --exclude=@var{pattern} -Exclude files which match the regular expression @var{pattern}. -@xref{File Exclusion}. +@example +$ @kbd{mt -f /dev/nrtape rewind} +$ @kbd{tar cf /dev/nrtape @var{directory-1}} +@end example -@item --exclude-from=@file{file} -Exclude files which match any of the regular expressions listed in -the file @file{file}. @xref{File Exclusion}. +@cindex tape marks +@dfn{Tape marks} are special magnetic patterns written on the tape +media, which are later recognizable by the reading hardware. These +marks are used after each file, when there are many on a single tape. +An empty file (that is to say, two tape marks in a row) signal the +logical end of the tape, after which no file exist. Usually, +non-rewinding tape device drivers will react to the close request issued +by @code{tar} by first writing two tape marks after your archive, and by +backspacing over one of these. So, if you remove the tape at that time +from the tape drive, it is properly terminated. But if you write +another file at the current position, the second tape mark will be +erased by the new information, leaving only one tape mark between files. + +So, you may now save @var{directory-2} as a second archive after the +first on the same tape by issuing the command: -@item --file=@var{archive-name} -Name the archive. @xref{Archive Name}). +@example +$ @kbd{tar cf /dev/nrtape @var{directory-2}} +@end example -@item --files-from=@file{file} -Read file-name arguments from a file on the file system. -@xref{File Name Lists}. +@noindent +and so on for all the archives you want to put on the same tape. -@item --ignore-umask -Set modes of extracted files to those recorded in the archive. -@xref{File Writing Options}. +Another usual case is that you do not write all the archives the same +day, and you need to remove and store the tape between two archive +sessions. In general, you must remember how many files are already +saved on your tape. Suppose your tape already has 16 files on it, and +that you are ready to write the 17th. You have to take care of skipping +the first 16 tape marks before saving @var{directory-17}, say, by using +these commands: -@item --ignore-zeros -Ignore end-of-archive entries. @xref{Archive Reading Options}. -<<< this should be changed to --ignore-end +@example +$ @kbd{mt -f /dev/nrtape rewind} +$ @kbd{mt -f /dev/nrtape fsf 16} +$ @kbd{tar cf /dev/nrtape @var{directory-17}} +@end example -@item --listed-incremental=@var{file-name} (-g) -Take a file name argument always. If the file doesn't exist, run a level -zero dump, creating the file. If the file exists, uses that file to see -what has changed. +In all the previous examples, we put aside blocking considerations, but +you should do the proper things for that as well. @xref{Blocking}. -@item --incremental (-G) -@c <<>> +@menu +* Tape Positioning:: Tape Positions and Tape Marks +* mt:: The @code{mt} Utility +@end menu -@item --tape-length=@var{n} (-L) -@c <<>> -@c <<< this needs to be written into main body as well -ringo +@node Tape Positioning, mt, Many, Many +@subsection Tape Positions and Tape Marks +@UNREVISED -@item --info-script=@var{program-file} -Create a multi-volume archive via a script. @xref{Multi-Volume Archives}. +Just as archives can store more than one file from the file system, +tapes can store more than one archive file. To keep track of where +archive files (or any other type of file stored on tape) begin and +end, tape archive devices write magnetic @dfn{tape marks} on the +archive media. Tape drives write one tape mark between files, +two at the end of all the file entries. -@item --interactive -Ask for confirmation before performing any operation on a file or -archive member. +If you think of data as a series of records "rrrr"'s, and tape marks as +"*"'s, a tape might look like the following: -@item --keep-old-files -Prevent overwriting during extraction. @xref{File Writing Options}. +@example +rrrr*rrrrrr*rrrrr*rr*rrrrr**------------------------- +@end example -@item --label=@var{archive-label} -Include an archive-label in the archive being created. @xref{Archive -Label}. +Tape devices read and write tapes using a read/write @dfn{tape +head}---a physical part of the device which can only access one +point on the tape at a time. When you use @code{tar} to read or +write archive data from a tape device, the device will begin reading +or writing from wherever on the tape the tape head happens to be, +regardless of which archive or what part of the archive the tape +head is on. Before writing an archive, you should make sure that no +data on the tape will be overwritten (unless it is no longer needed). +Before reading an archive, you should make sure the tape head is at +the beginning of the archive you want to read. (The @code{restore} +script will find the archive automatically. @FIXME{There is no such +restore script!}. @FIXME-xref{Scripted Restoration}). @xref{mt}, for +an explanation of the tape moving utility. -@item --modification-time -Set the modification time of extracted files to the time they were -extracted. @xref{File Writing Options}. +If you want to add new archive file entries to a tape, you should +advance the tape to the end of the existing file entries, backspace +over the last tape mark, and write the new archive file. If you were +to add two archives to the example above, the tape might look like the +following: -@item --multi-volume -Specify a multi-volume archive. @xref{Multi-Volume Archives}. +@example +rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**---------------- +@end example -@item --newer=@var{date} -Limit the operation to files changed after the given date. -@xref{File Exclusion}. +@node mt, , Tape Positioning, Many +@subsection The @code{mt} Utility +@UNREVISED -@item --newer-mtime=@var{date} -Limit the operation to files modified after the given date. @xref{File -Exclusion}. +@FIXME{Is it true that this only works on non-block devices? +should explain the difference, (fixed or variable).} +@value{xref-blocking-factor}. -@item --old -Create an old format archive. @xref{Old Style File Information}. -@c <<< did we agree this should go away as a synonym? +You can use the @code{mt} utility to advance or rewind a tape past a +specified number of archive files on the tape. This will allow you +to move to the beginning of an archive before extracting or reading +it, or to the end of all the archives before writing a new one. +@FIXME{Why isn't there an "advance 'til you find two tape marks +together"?} -@item --old-archive -Create an old format archive. @xref{Old Style File Information}. +The syntax of the @code{mt} command is: -@item --one-file-system -Prevent @code{tar} from crossing file system boundaries when -archiving. @xref{File Exclusion}. +@example +@kbd{mt [-f @var{tapename}] @var{operation} [@var{number}]} +@end example -@item --portable -Create an old format archive. @xref{Old Style File Information}. -@c <<< was portability, may still need to be changed +where @var{tapename} is the name of the tape device, @var{number} is +the number of times an operation is performed (with a default of one), +and @var{operation} is one of the following: -@item --preserve-order -Help process large lists of file-names on machines with small amounts of -memory. @xref{Archive Reading Options}. +@FIXME{is there any use for record operations?} -@item --preserve-permission -Set modes of extracted files to those recorded in the archive. -@xref{File Writing Options}. - -@item --read-full-blocks -Read an archive with a smaller than specified block size or which -contains incomplete blocks. @xref{Archive Reading Options}). -@c should be --partial-blocks (!!!) - -@item --record-number -Print the record number where a message is generated. -@xref{Additional Information}. +@table @kbd +@item eof +@itemx weof +Writes @var{number} tape marks at the current position on the tape. -@item --same-order -Help process large lists of file-names on machines with small amounts of -memory. @xref{Archive Reading Options}. +@item fsf +Moves tape position forward @var{number} files. -@item --same-permission -Set the modes of extracted files to those recorded in the archive. -@xref{File Writing Options}. +@item bsf +Moves tape position back @var{number} files. -@item --sparse -Archive sparse files sparsely. @xref{Sparse Files}. +@item rewind +Rewinds the tape. (Ignores @var{number}). -@item --starting-file=@var{file-name} -Begin reading in the middle of an archive. @xref{Scarce Disk Space}. +@item offline +@itemx rewoff1 +Rewinds the tape and takes the tape device off-line. (Ignores @var{number}). -@item --to-stdout -Write files to the standard output. @xref{File Writing Options}. +@item status +Prints status information about the tape unit. -@item --uncompress -Specifdo a compressed archive. @xref{Compressed Archives}. +@end table -@item -V @var{archive-label} -Include an archive-label in the archive being created. @xref{Archive -Label}. -@c was --volume +@FIXME{Is there a better way to frob the spacing on the list?} -@item --verbose -Print the names of files or archive members as they are being -operated on. @xref{Additional Information}. +If you don't specify a @var{tapename}, @code{mt} uses the environment +variable TAPE; if TAPE does not exist, @code{mt} uses the device +@file{/dev/rmt12}. -@item --verify -Check for discrepancies in the archive immediately after it is -written. @xref{Write Verification}. +@code{mt} returns a 0 exit status when the operation(s) were +successful, 1 if the command was unrecognized, and 2 if an operation +failed. -@item -B -Read an archive with a smaller than specified block size or which -contains incomplete blocks. @xref{Archive Reading Options}). +@FIXME{New node on how to find an archive?} -@item -K @var{file-name} -Begin reading in the middle of an archive. @xref{Scarce Disk Space}. +If you use @value{op-extract} with the @value{op-label} option specified, +@code{tar} will read an archive label (the tape head has to be positioned +on it) and print an error if the archive label doesn't match the +@var{archive-name} specified. @var{archive-name} can be any regular +expression. If the labels match, @code{tar} extracts the archive. +@value{xref-label}. @FIXME-xref{Matching Format Parameters}. +@FIXME{fix cross references} @samp{tar --list --label} will cause +@code{tar} to print the label. -@item -M -Specify a multi-volume archive. @xref{Multi-Volume Archives}. +@FIXME{Program to list all the labels on a tape?} -@item -N @var{date} -Limit operation to files changed after the given date. @xref{File Exclusion}. +@node Using Multiple Tapes, label, Many, Media +@section Using Multiple Tapes +@UNREVISED -@item -O -Write files to the standard output. @xref{File Writing Options}. +Often you might want to write a large archive, one larger than will fit +on the actual tape you are using. In such a case, you can run multiple +@code{tar} commands, but this can be inconvenient, particularly if you +are using options like @value{op-exclude} or dumping entire filesystems. +Therefore, @code{tar} supports multiple tapes automatically. -@c <<<<- P is absolute paths, add when resolved. -ringo>>> +Use @value{op-multi-volume} on the command line, and then @code{tar} will, +when it reaches the end of the tape, prompt for another tape, and +continue the archive. Each tape will have an independent archive, and +can be read without needing the other. (As an exception to this, the +file that @code{tar} was archiving when it ran out of tape will usually +be split between the two archives; in this case you need to extract from +the first archive, using @value{op-multi-volume}, and then put in the +second tape when prompted, so @code{tar} can restore both halves of the +file.) -@item -R -Print the record number where a message is generated. -@xref{Additional Information}. +GNU @code{tar} multi-volume archives do not use a truly portable format. +You need GNU @code{tar} at both end to process them properly. -@item -S -Archive sparse files sparsely. @xref{Sparse Files}. +When prompting for a new tape, @code{tar} accepts any of the following +responses: -@item -T @var{file} -Read file-name arguments from a file on the file system. -@xref{File Name Lists}. +@table @kbd +@item ? +Request @code{tar} to explain possible responses +@item q +Request @code{tar} to exit immediately. +@item n @var{file name} +Request @code{tar} to write the next volume on the file @var{file name}. +@item ! +Request @code{tar} to run a subshell. +@item y +Request @code{tar} to begin writing the next volume. +@end table -@item -W -Check for discrepancies in the archive immediately after it is -written. @xref{Write Verification}. +(You should only type @samp{y} after you have changed the tape; +otherwise @code{tar} will write over the volume it just finished.) -@item -Z -Specify a compressed archive. @xref{Compressed Archives}. +If you want more elaborate behavior than this, give @code{tar} the +@value{op-info-script} option. The file @var{script-name} is expected +to be a program (or shell script) to be run instead of the normal +prompting procedure. When the program finishes, @code{tar} will +immediately begin writing the next volume. The behavior of the +@samp{n} response to the normal tape-change prompt is not available +if you use @value{op-info-script}. -@item -b @var{number} -Specify the blocking factor of an archive. @xref{Blocking Factor}. +The method @code{tar} uses to detect end of tape is not perfect, and +fails on some operating systems or on some devices. You can use the +@value{op-tape-length} option if @code{tar} can't detect the end of the +tape itself. This option selects @value{op-multi-volume} automatically. +The @var{size} argument should then be the usable size of the tape. +But for many devices, and floppy disks in particular, this option is +never required for real, as far as we know. + +The volume number used by @code{tar} in its tape-change prompt +can be changed; if you give the @value{op-volno-file} option, then +@var{file-of-number} should be an unexisting file to be created, or else, +a file already containing a decimal number. That number will be used +as the volume number of the first volume written. When @code{tar} is +finished, it will rewrite the file with the now-current volume number. +(This does not change the volume number written on a tape label, as +per @value{ref-label}, it @emph{only} affects the number used in +the prompt.) -@item -f @var{archive-name} -Name the archive. @xref{Archive Name}). +If you want @code{tar} to cycle through a series of tape drives, then +you can use the @samp{n} response to the tape-change prompt. This is +error prone, however, and doesn't work at all with @value{op-info-script}. +Therefore, if you give @code{tar} multiple @value{op-file} options, then +the specified files will be used, in sequence, as the successive volumes +of the archive. Only when the first one in the sequence needs to be +used again will @code{tar} prompt for a tape change (or run the info +script). -@item -h -Treat a symbolic link as an alternate name for the file the link -points to. @xref{Symbolic Links}. +Multi-volume archives -@item -i -Ignore end-of-archive entries. @xref{Archive Reading Options}. +With @value{op-multi-volume}, @code{tar} will not abort when it cannot +read or write any more data. Instead, it will ask you to prepare a new +volume. If the archive is on a magnetic tape, you should change tapes +now; if the archive is on a floppy disk, you should change disks, etc. -@item -k -Prevent overwriting during extraction. @xref{File Writing Options}. +Each volume of a multi-volume archive is an independent @code{tar} +archive, complete in itself. For example, you can list or extract any +volume alone; just don't specify @value{op-multi-volume}. However, if one +file in the archive is split across volumes, the only way to extract +it successfully is with a multi-volume extract command @samp{--extract +--multi-volume} (@samp{-xM}) starting on or before the volume where +the file begins. -@item -l -Prevent @code{tar} from crossing file system boundaries when -archiving. @xref{File Exclusion}. +For example, let's presume someone has two tape drives on a system +named @file{/dev/tape0} and @file{/dev/tape1}. For having GNU +@code{tar} to switch to the second drive when it needs to write the +second tape, and then back to the first tape, etc., just do either of: -@item -m -Set the modification time of extracted files to the time they were -extracted. @xref{File Writing Options}. +@smallexample +$ @kbd{tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 @var{files}} +$ @kbd{tar cMff /dev/tape0 /dev/tape1 @var{files}} +@end smallexample -@item -o -Create an old format archive. @xref{Old Style File Information}. +@menu +* Multi-Volume Archives:: Archives Longer than One Tape or Disk +* Tape Files:: Tape Files +@end menu -@item -p -Set the modes of extracted files to those recorded in the archive. -@xref{File Writing Options}. +@node Multi-Volume Archives, Tape Files, Using Multiple Tapes, Using Multiple Tapes +@subsection Archives Longer than One Tape or Disk +@cindex Multi-volume archives +@UNREVISED -@item -s -Help process large lists of file-names on machines with small amounts of -memory. @xref{Archive Reading Options}. +To create an archive that is larger than will fit on a single unit of +the media, use the @value{op-multi-volume} option in conjunction with +the @value{op-create} option (@pxref{create}). A +@dfn{multi-volume} archive can be manipulated like any other archive +(provided the @value{op-multi-volume} option is specified), but is +stored on more than one tape or disk. -@item -v -Print the names of files or archive members they are being operated -on. @xref{Additional Information}. +When you specify @value{op-multi-volume}, @code{tar} does not report an +error when it comes to the end of an archive volume (when reading), or +the end of the media (when writing). Instead, it prompts you to load +a new storage volume. If the archive is on a magnetic tape, you +should change tapes when you see the prompt; if the archive is on a +floppy disk, you should change disks; etc. -@item -w -@c <<>> +You can read each individual volume of a multi-volume archive as if it +were an archive by itself. For example, to list the contents of one +volume, use @value{op-list}, without @value{op-multi-volume} specified. +To extract an archive member from one volume (assuming it is described +that volume), use @value{op-extract}, again without +@value{op-multi-volume}. -@item -z -Specify a compressed archive. @xref{Compressed Archives}. +If an archive member is split across volumes (ie. its entry begins on +one volume of the media and ends on another), you need to specify +@value{op-multi-volume} to extract it successfully. In this case, you +should load the volume where the archive member starts, and use +@samp{tar --extract --multi-volume}---@code{tar} will prompt for later +volumes as it needs them. @xref{extracting archives}, for more +information about extracting archives. -@item -z -z -Create a whole block sized compressed archive. @xref{Compressed Archives}. -@c I would rather this were -Z. it is the only double letter short -@c form. +@value{op-info-script} is like @value{op-multi-volume}, except that +@code{tar} does not prompt you directly to change media volumes when +a volume is full---instead, @code{tar} runs commands you have stored +in @var{script-name}. For example, this option can be used to eject +cassettes, or to broadcast messages such as @samp{Someone please come +change my tape} when performing unattended backups. When @var{script-name} +is done, @code{tar} will assume that the media has been changed. -@item -C @file{directory} -Change the working directory. @xref{Changing Working Directory}. +Multi-volume archives can be modified like any other archive. To add +files to a multi-volume archive, you need to only mount the last +volume of the archive media (and new volumes, if needed). For all +other operations, you need to use the entire archive. -@item -F @var{program-file} -Create a multi-volume archive via a script. @xref{Multi-Volume Archives}. +If a multi-volume archive was labeled using @value{op-label} +(@value{pxref-label}) when it was created, @code{tar} will not +automatically label volumes which are added later. To label subsequent +volumes, specify @value{op-label} again in conjunction with the +@value{op-append}, @value{op-update} or @value{op-concatenate} operation. -@item -X @file{file} -Exclude files which match any of the regular expressions listed in -the file @file{file}. @xref{File Exclusion}. -@end table +@cindex Labelling multi-volume archives +@FIXME{example} -@node Data Format Details, Concept Index, Quick Reference, Top -@appendix Details of the Archive Data Format +@FIXME{There should be a sample program here, including an exit +before end. Is the exit status even checked in tar? :-(} -This chapter is based heavily on John Gilmore's @i{tar}(5) manual page -for the public domain @code{tar} that GNU @code{tar} is based on. -@c it's been majorly edited since, we may be able to lose this. +@table @kbd +@item --multi-volume +@itemx -M +Creates a multi-volume archive, when used in conjunction with +@value{op-create}. To perform any other operation on a multi-volume +archive, specify @value{op-multi-volume} in conjunction with that +operation. -The archive media contains a series of records, each of which contains -512 bytes. Each archive member is represented by a header record, -which describes the file, followed by zero or more records which -represent the contents of the file. At the end of the archive file -there may be a record consisting of a series of binary zeros, as an -end-of-archive marker. GNU @code{tar} writes a record of zeros at the -end of an archive, but does not assume that such a record exists when -reading an archive. +@item --info-script=@var{program-file} +@itemx -F @var{program-file} +Creates a multi-volume archive via a script. Used in conjunction with +@value{op-create}. +@end table -Records may be grouped into @dfn{blocks} for I/O operations. A block -of records is written with a single @code{write()} operation. The -number of records in a block is specified using the @samp{--block-size} -option. @xref{Blocking Factor}, for more information about specifying -block size. +Beware that there is @emph{no} real standard about the proper way, for a +@code{tar} archive, to span volume boundaries. If you have a multi-volume +created by some vendor's @code{tar}, there is almost no chance you could +read all the volumes with GNU @code{tar}. The converse is also true: +you may not expect multi-volume archives created by GNU @code{tar} to +be fully recovered by vendor's @code{tar}. Since there is little chance +that, in mixed system configurations, some vendor's @code{tar} will work on +another vendor's machine, and there is a great chance that GNU @code{tar} +will work on most of them, your best bet is to install GNU @code{tar} +on all machines between which you know exchange of files is possible. + +@node Tape Files, , Multi-Volume Archives, Using Multiple Tapes +@subsection Tape Files +@UNREVISED -@menu -* Header Data:: The Distribution of Data in the Header -* Header Fields:: The Meaning of Header Fields -* Sparse File Handling:: Fields to Handle Sparse Files -@end menu +To give the archive a name which will be recorded in it, use the +@value{op-label} option. This will write a special block identifying +@var{volume-label} as the name of the archive to the front of the archive +which will be displayed when the archive is listed with @value{op-list}. +If you are creating a multi-volume archive with @value{op-multi-volume} +(@FIXME-pxref{Using Multiple Tapes}), then the volume label will have +@samp{Volume @var{nnn}} appended to the name you give, where @var{nnn} is +the number of the volume of the archive. (If you use the @value{op-label} +option when reading an archive, it checks to make sure the label on the +tape matches the one you give. @value{xref-label}. + +When @code{tar} writes an archive to tape, it creates a single +tape file. If multiple archives are written to the same tape, one +after the other, they each get written as separate tape files. When +extracting, it is necessary to position the tape at the right place +before running @code{tar}. To do this, use the @code{mt} command. +For more information on the @code{mt} command and on the organization +of tapes into a sequence of tape files, see @ref{mt}. + +People seem to often do: -@node Header Data, Header Fields, Data Format Details, Data Format Details -@appendixsec The Distribution of Data in the Header - -The header record is defined in C as follows: -@c I am taking the following code on faith. - -@example -@r{Standard Archive Format - Standard TAR - USTAR} - -#define RECORDSIZE 512 -#define NAMSIZ 100 -#define TUNMLEN 32 -#define TGNMLEN 32 -#define SPARSE_EXT_HDR 21 -#define SPARSE_IN_HDR 4 - -struct sparse @{ - char offset[12]; - char numbytes[12]; -@}; - -union record @{ - char charptr[RECORDSIZE]; - struct header @{ - char name[NAMSIZ]; - char mode[8]; - char uid[8]; - char gid[8]; - char size[12]; - char mtime[12]; - char chksum[8]; - char linkflag; - char linkname[NAMSIZ]; - char magic[8]; - char uname[TUNMLEN]; - char gname[TGNMLEN]; - char devmajor[8]; - char devminor[8]; - -@r{The following fields were added by gnu and are not used by other} -@r{versions of @code{tar}}. - char atime[12]; - char ctime[12]; - char offset[12]; - char longnames[4]; -@r{The next three fields were added by gnu to deal with shrinking down} -@r{sparse files.} - struct sparse sp[SPARSE_IN_HDR]; - char isextended; -@r{This is the number of nulls at the end of the file, if any.} - char ending_blanks[12]; - - @} header; - - struct extended_header @{ - struct sparse sp[21]; - char isextended; - @} ext_hdr; - -@}; -@c <<< this whole thing needs to be put into better english - -@r{The checksum field is filled with this while the checksum is computed.} -#define CHKBLANKS " " @r{8 blanks, no null} - -@r{Inclusion of this field marks an archive as being in standard} -@r{Posix format (though GNU tar itself is not Posix conforming). GNU} -@r{tar puts "ustar" in this field if uname and gname are valid.} -#define TMAGIC "ustar " @r{7 chars and a null} - -@r{The magic field is filled with this if this is a GNU format dump entry.} -#define GNUMAGIC "GNUtar " @r{7 chars and a null} - -@r{The linkflag defines the type of file.} -#define LF_OLDNORMAL '\0' @r{Normal disk file, Unix compatible} -#define LF_NORMAL '0' @r{Normal disk file} -#define LF_LINK '1' @r{Link to previously dumped file} -#define LF_SYMLINK '2' @r{Symbolic link} -#define LF_CHR '3' @r{Character special file} -#define LF_BLK '4' @r{Block special file} -#define LF_DIR '5' @r{Directory} -#define LF_FIFO '6' @r{FIFO special file} -#define LF_CONTIG '7' @r{Contiguous file} - -@r{hhe following are further link types which were defined later.} - -@r{This is a dir entry that contains the names of files that were in} -@r{the dir at the time the dump was made.} -#define LF_DUMPDIR 'D' - -@r{This is the continuation of a file that began on another volume} -#define LF_MULTIVOL 'M' - -@r{This is for sparse files} -#define LF_SPARSE 'S' - -@r{This file is a tape/volume header. Ignore it on extraction.} -#define LF_VOLHDR 'V' - -@r{These are bits used in the mode field - the values are in octal} -#define TSUID 04000 @r{Set UID on execution} -#define TSGID 02000 @r{Set GID on execution} -#define TSVTX 01000 @r{Save text (sticky bit)} - -@r{These are file permissions} -#define TUREAD 00400 @r{read by owner} -#define TUWRITE 00200 @r{write by owner} -#define TUEXEC 00100 @r{execute/search by owner} -#define TGREAD 00040 @r{read by group} -#define TGWRITE 00020 @r{write by group} -#define TGEXEC 00010 @r{execute/search by group} -#define TOREAD 00004 @r{read by other} -#define TOWRITE 00002 @r{write by other} -#define TOEXEC 00001 @r{execute/search by other} -@end example - - -All characters in headers are 8-bit characters in the local variant of -ASCII. Each field in the header is contiguous; that is, there is no -padding in the header format. - -Data representing the contents of files is not translated in any way -and is not constrained to represent characters in any character set. -@code{tar} does not distinguish between text files and binary files. +@example +@kbd{--label="@var{some-prefix} `date +@var{some-format}`"} +@end example -The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and -@code{gname} fields contain null-terminated character strings. All -other fields contain zero-filled octal numbers in ASCII. Each numeric -field of width @var{w} contains @var{w} @minus{} 2 digits, a space, and a -null, except @code{size} and @code{mtime}, which do not contain the -trailing null. - -@node Header Fields, Sparse File Handling, Header Data, Data Format Details -@appendixsec The Meaning of Header Fields - -The @code{name} field contains the name of the file. -<<< how big a name before field overflows? - -The @code{mode} field contains nine bits which specify file -permissions, and three bits which specify the Set UID, Set GID, and -Save Text (``stick'') modes. Values for these bits are defined above. -@xref{File Writing Options}, for information on how file permissions -and modes are used by @code{tar}. - -The @code{uid} and @code{gid} fields contain the numeric user and -group IDs of the file owners. If the operating system does not -support numeric user or group IDs, these fields should be ignored. -@c but are they? - -The @code{size} field contains the size of the file in bytes; this -field contains a zero if the header describes a link to a file. - -The @code{mtime} field contains the modification time of the file. -This is the ASCII representation of the octal value of the last time -the file was modified, represented as an integer number of seconds -since January 1, 1970, 00:00 Coordinated Universal Time. -@xref{File Writing Options}, for a description of how @code{tar} uses -this information. - -The @code{chksum} field contains the ASCII representation of the octal -value of the simple sum of all bytes in the header record. To -generate this sum, each 8-bit byte in the header is added to an -unsigned integer, which has been initialized to zero. The precision -of the integer is seventeen bits. When calculating the checksum, the -@code{chksum} field itself is treated as blank. - -The @code{atime} and @code{ctime} fields are used when making -incremental backups; they store, respectively, the file's access time -and last inode-change time. +or such, for pushing a common date in all volumes or an archive set. -The value in the @code{offset} field is used when making a -multi-volume archive. The offset is number of bytes into the file -that we need to go to pick up where we left off in the previous -volume, i.e the location that a continued file is continued from. +@node label, verify, Using Multiple Tapes, Media +@section Including a Label in the Archive +@cindex Labeling an archive +@cindex Labels on the archive media +@UNREVISED -The @code{longnames} field supports a feature that is not yet -implemented. This field should be empty. +@table @kbd +@item -V @var{name} +@itemx --label=@var{name} +Create archive with volume name @var{name}. +@end table -The @code{magic} field indicates that this archive was output in the -P1003 archive format. If this field contains @code{TMAGIC}, the -@code{uname} and @code{gname} fields will contain the ASCII -representation of the owner and group of the file respectively. If -found, the user and group IDs are used rather than the values in the -@code{uid} and @code{gid} fields. +This option causes @code{tar} to write out a @dfn{volume header} at +the beginning of the archive. If @value{op-multi-volume} is used, each +volume of the archive will have a volume header of @samp{@var{name} +Volume @var{n}}, where @var{n} is 1 for the first volume, 2 for the +next, and so on. -The @code{sp} field is used to archive sparse files efficiently. -@xref{Sparse File Handling}, for a description of this field, and -other fields it may imply. +@FIXME{Should the arg to --label be a quoted string?? No.} -The @code{typeflag} field specifies the file's type. If a particular -implementation does not recognize or permit the specified type, -@code{tar} extracts the file as if it were a regular file, and reports -the discrepancy on the standard error. @xref{File Types}. @xref{GNU -File Types}. +To avoid problems caused by misplaced paper labels on the archive +media, you can include a @dfn{label} entry---an archive member which +contains the name of the archive---in the archive itself. Use the +@value{op-label} option in conjunction with the @value{op-create} operation +to include a label entry in the archive as it is being created. -@menu -* File Types:: File Types -* GNU File Types:: Additional File Types Supported by GNU -@end menu +If you create an archive using both @value{op-label} and +@value{op-multi-volume}, each volume of the archive will have an +archive label of the form @samp{@var{archive-label} Volume @var{n}}, +where @var{n} is 1 for the first volume, 2 for the next, and so on. +@FIXME-xref{Multi-Volume Archives}, for information on creating multiple +volume archives. -@node File Types, GNU File Types, Header Fields, Header Fields -@appendixsubsec File Types +If you list or extract an archive using @value{op-label}, @code{tar} will +print an error if the archive label doesn't match the @var{archive-label} +specified, and will then not list nor extract the archive. In those cases, +@var{archive-label} argument is interpreted as a globbing-style pattern +which must match the actual magnetic volume label. @xref{exclude}, for +a precise description of how match is attempted@footnote{Previous versions +of @code{tar} used full regular expression matching, or before that, only +exact string matching, instead of wildcard matchers. We decided for the +sake of simplicity to use a uniform matching device through @code{tar}.}. +If the switch @value{op-multi-volume} is being used, the volume label +matcher will also suffix @var{archive-label} by @w{@samp{ Volume [1-9]*}} +if the initial match fails, before giving up. Since the volume numbering +is automatically added in labels at creation time, it sounded logical to +equally help the user taking care of it when the archive is being read. + +The @value{op-label} was once called @samp{--volume}, but is not available +under that name anymore. -The following flags are used to describe file types: +To find out an archive's label entry (or to find out if an archive has +a label at all), use @samp{tar --list --verbose}. @code{tar} will print the +label first, and then print archive member information, as in the +example below: -@table @code -@item LF_NORMAL -@itemx LF_OLDNORMAL -Indicates a regular file. In order to be compatible with older -versions of @code{tar}, a @code{typeflag} value of @code{LF_OLDNORMAL} -should be silently recognized as a regular file. New archives should -be created using @code{LF_NORMAL} for regular files. For backward -compatibility, @code{tar} treats a regular file whose name ends with a -slash as a directory. - -@item LF_LINK -Indicates a link to another file, of any type, which has been -previously archived. @code{tar} identifies linked files in Unix by -matching device and inode numbers. The linked-to name is specified in -the @code{linkname} field with a trailing null. - -@item LF_SYMLINK -Indicates a symbolic link to another file. The linked-to -name is specified in the @code{linkname} field with a trailing null. -@xref{File Writing Options}, for information on archiving files -referenced by a symbolic link. - -@item LF_CHR -@itemx LF_BLK -Indicate character special files and block special files, -respectively. In this case the @code{devmajor} and @code{devminor} -fields will contain the major and minor device numbers. Operating -systems may map the device specifications to their own local -specification, or may ignore the entry. - -@item LF_DIR -Indicates a directory or sub-directory. The directory name in the -@code{name} field should end with a slash. On systems where disk -allocation is performed on a directory basis, the @code{size} field -will contain the maximum number of bytes (which may be rounded to the -nearest disk block allocation unit) that the directory can hold. A -@code{size} field of zero indicates no size limitations. Systems that -do not support size limiting in this manner should ignore the -@code{size} field. +@example +$ @kbd{tar --verbose --list --file=iamanarchive} +V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header-- +-rw-rw-rw- ringo user 40 1990-05-21 13:30 iamafilename +@end example -@item LF_FIFO -Indicates a FIFO special file. Note that archiving a FIFO file -archives the existence of the file and not its contents. - -@item LF_CONTIG -Indicates a contiguous file. Contiguous files are the same as normal -files except that, in operating systems that support it, all the -files' disk space is allocated contiguously. Operating systems which -do not allow contiguous allocation should silently treat this type as -a normal file. - -@item 'A' @dots{} -@itemx 'Z' -These are reserved for custom implementations. Some of these are used -in the GNU modified format, which is described below. @xref{GNU File -Types}. +@table @kbd +@item --label=@var{archive-label} +@itemx -V @var{archive-label} +Includes an @dfn{archive-label} at the beginning of the archive when +the archive is being created, when used in conjunction with the +@value{op-create} option. Checks to make sure the archive label +matches the one specified (when used in conjunction with the +@value{op-extract} option. @end table -Certain other flag values are reserved for specification in future -revisions of the P1003 standard, and should not be used by any -@code{tar} program. +To get a common information on all tapes of a series, use the +@value{op-label} option. For having this information different in each +series created through a single script used on a regular basis, just +manage to get some date string as part of the label. For example: + +@example +$ @kbd{tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`"} +$ @kbd{tar --create --file=/dev/tape --multi-volume \ + --volume="Daily backup for `date +%Y-%m-%d`"} +@end example -@node GNU File Types, , File Types, Header Fields -@appendixsubsec Additional File Types Supported by GNU +Also note that each label has its own date and time, which corresponds +to when GNU @code{tar} initially attempted to write it, often soon +after the operator launches @code{tar} or types the carriage return +telling that the next tape is ready. Comparing date labels does give +an idea of tape throughput only if the delays for rewinding tapes +and the operator switching them were negligible, which is ususally +not the case. -GNU @code{tar} uses additional file types to describe new types of -files in an archive. These are listed below. +@FIXME{was --volume} -@table @code -@item LF_DUMPDIR -@itemx 'D' -Indicates a directory and a list of files created by the -@samp{--incremental} option. The @code{size} field gives the total -size of the associated list of files. Each file name is preceded by -either a @code{'Y'} (the file should be in this archive) or an -@code{'N'} (the file is a directory, or is not stored in the archive). -Each file name is terminated by a null. There is an additional null -after the last file name. +@node verify, Write Protection, label, Media +@section Verifying Data as It is Stored +@cindex Verifying a write operation +@cindex Double-checking a write operation -@item LF_MULTIVOL -@itemx 'M' -Indicates a file continued from another volume of a multi-volume -archive (@pxref{Multi-Volume Archives}). The original type of the file is not -given here. The @code{size} field gives the maximum size of this -piece of the file (assuming the volume does not end before the file is -written out). The @code{offset} field gives the offset from the -beginning of the file where this part of the file begins. Thus -@code{size} plus @code{offset} should equal the original size of the -file. - -@item LF_SPARSE -@itemx 'S' -Indicates a sparse file. @xref{Sparse Files}. @xref{Sparse File -Handling}. - -@item LF_VOLHDR -@itemx 'V' -Marks an archive label that was created using the @samp{--label} option -when the archive was created (@pxref{Archive Label}. The @code{name} -field contains the argument to the option. The @code{size} field is -zero. Only the first file in each volume of an archive should have -this type. +@table @kbd +@item -W +@itemx --verify +Attempt to verify the archive after writing. @end table -@node Sparse File Handling, , Header Fields, Data Format Details -@appendixsec Fields to Handle Sparse Files +This option causes @code{tar} to verify the archive after writing it. +Each volume is checked after it is written, and any discrepancies +are recorded on the standard error output. -The following header information was added to deal with sparse files -(@pxref{Sparse Files}): +Verification requires that the archive be on a back-space-able medium. +This means pipes, some cartridge tape drives, and some other devices +cannot be verified. + +You can insure the accuracy of an archive by comparing files in the +system with archive members. @code{tar} can compare an archive to the +file system as the archive is being written, to verify a write +operation, or can compare a previously written archive, to insure that +it is up to date. -@c TALK TO MIB -The @code{sp} field (fields? something else?) is an array of -@code{struct sparse}. Each @code{struct sparse} contains two -12-character strings, which represent the offset into the file and the -number of bytes to be written at that offset. The offset is absolute, -and not relative to the offset in preceding array elements. +To check for discrepancies in an archive immediately after it is +written, use the @value{op-verify} option in conjunction with +the @value{op-create} operation. When this option is +specified, @code{tar} checks archive members against their counterparts +in the file system, and reports discrepancies on the standard error. In +multi-volume archives, each volume is verified after it is written, +before the next volume is written. -The header can contain four of these @code{struct sparse}; if more are -needed, they are not stored in the header, instead, the flag -@code{isextended} is set and the next record is an -@code{extended_header}. -@c @code{extended_header} or @dfn{extended_header} ??? the next -@c record after the header, or in the middle of it. +To verify an archive, you must be able to read it from before the end +of the last written entry. This option is useful for detecting data +errors on some tapes. Archives written to pipes, some cartridge tape +drives, and some other devices cannot be verified. -The @code{isextended} flag is only set for sparse files, and then only -if extended header records are needed when archiving the file. +One can explicitely compare an already made archive with the file system +by using the @value{op-compare} option, instead of using the more automatic +@value{op-verify} option. @value{xref-compare}. + +Note that these two options have a slightly different intent. The +@value{op-compare} option how identical are the logical contents of some +archive with what is on your disks, while the @value{op-verify} option is +really for checking if the physical contents agree and if the recording +media itself is of dependable quality. So, for the @value{op-verify} +operation, @code{tar} tries to defeat all in-memory cache pertaining to +the archive, while it lets the speed optimization undisturbed for the +@value{op-compare} option. If you nevertheless use @value{op-compare} for +media verification, you may have to defeat the in-memory cache yourself, +maybe by opening and reclosing the door latch of your recording unit, +forcing some doubt in your operating system about the fact this is really +the same volume as the one just written or read. + +The @value{op-verify} option would not be necessary if drivers were indeed +able to detect dependably all write failures. This sometimes require many +magnetic heads, some able to read after the writes occurred. One would +not say that drivers unable to detect all cases are necessarily flawed, +as long as programming is concerned. + +@node Write Protection, , verify, Media +@section Write Protection -Each extended header record can contain an array of 21 sparse -structures, as well as another @code{isextended} flag. There is no -limit (except that implied by the archive media) on the number of -extended header records that can be used to describe a sparse file. +Almost all tapes and diskettes, and in a few rare cases, even disks can +be @dfn{write protected}, to protect data on them from being changed. +Once an archive is written, you should write protect the media to prevent +the archive from being accidently overwritten or deleted. (This will +protect the archive from being changed with a tape or floppy drive---it +will not protect it from magnet fields or other physical hazards). -@c so is @code{extended_header} the right way to write this? +The write protection device itself is usually an integral part of the +physical media, and can be a two position (write enabled/write +disabled) switch, a notch which can be popped out or covered, a ring +which can be removed from the center of a tape reel, or some other +changeable feature. -@node Concept Index, , Data Format Details, Top -@unnumbered Concept Index +@node Index, , Media, Top +@unnumbered Index @printindex cp @@ -3851,4 +8727,6 @@ extended header records that can be used to describe a sparse file. @contents @bye - +@c Local variables: +@c texinfo-column-for-description: 32 +@c End: